### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

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 standards 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 a nd 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 to set a new multi

16 pollutant secondary standard in this review

. In addition , EPA has decided to undertake a field 17 pilot program to gather and anal yz e additional relevant data so as to enhance the Agency ’s 18 understanding of the degree of protectiveness that a new multi

- pollutant approach , defined in 19 term s of an aquatic acidification index (AAI), would afford and to support development of an 20 appropriate monitoring network for such a standard. The EPA solicits comment on the 21 framework of such a standard and on the design of the field pilot program. 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

2 **DATES:** Written comments on this proposed rule must be received b y **[insert date 60**  **days** 1 **after date of publication in the Federal Register]**

. 2 Public Hearings : The EPA intends to hold a public hearing around the end of August to 3 earl y September and will announce in a separate Federal Register notice the date , time, and 4 address of the public hearing on this proposed rule. 5 **ADDRESSES:** Submit your comments, identified by Docket ID No. EPA

- HQ

- OAR

- 2007

- 6 1145, by

one of the following methods: 7

 www.regulations.gov : Follow the on

- line instructions for submitting comments. 8

 Email: a - and

- r - Docket@epa.gov

. 9

 Fax: 202

- 566

- 1741. 10

 Mail: Docket No. EPA

- HQ

- OAR

- 2007

- 1145, Environmental Protection Agen cy , Mail 11 code 6102T, 1200 Pennsylvania Ave., NW., Washington, DC 20460. Please include a 12 total of two copies. 13

 Hand Delivery : Docket No. EPA

- HQ

- OAR

- 2007

- 1145, Environmental Protection 14 Agency , EPA West, Room 3334, 1301 Constitution Ave., NW, Washington, D C. Such 15 deliveries are onl y accepted during the Docket’s normal hours of operation, and special 16 arrangements should be made for deliveries of boxed information. 17 Instructions: Direct your comments to Docket ID No. EPA

- HQ

- OAR

- 2007

- 1145. The 18 EPA’s policy

i s that all comments received will be included in the public docket without change 19 and may

be made available online at www.regulations.gov , including any personal information 20 provided, unless the comment includes inf ormation claimed to be Confidential Business 21 In formation (CBI) or other information whose disclosure is restricted b y statute. Do not submit 22 information that you consider to be CBI or otherwise protected through www .regulations.gov or 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

3 email. The www.regulations.gov website is an “anony mous access” sy stem, which means EPA 1 will not know your identity or contact information unless you provide it in the body of your 2 comment. If you send an email comment directl y to EPA without going through 3 www.regulations.gov , your email address will be automatically captured and included as part of 4 the comment that is placed in the public docket and mad e available on the Internet. If you 5 submit an electronic comment, EPA recommends that you include your name and other contact 6 information in the body of your comment and with any disk or CD

- ROM you submit. I f EPA 7 cannot read your comment due to technical difficulties and cannot contact you for clarification, 8 EPA may

not be able to consider your comment. Electronic files should avoid the use of special 9 characters, any form of encry ption, and be free of any

defects or viruses. For additional 10 information a

bout EPA’s public docket, visit the EPA Docket Center homepage at 11 http://www.epa.gov/epahome/dockets.htm

. 12 Docket : All documents in the docket are listed in the www.regulations.gov index. 13 Although listed in the index, some information is not publicly available, e.g., CBI or other 14 information whose disclosure is restricted b y statute. Certain other material, such as cop yri ghted 15 material, will be publicly availab le onl y in hard copy . Publicly available docket materials are 16 available either electronically in www.regulations.gov or in hard cop y at the Air and Radiation 17 Docket and Information Center, EPA/DC, EPA West, Room 33 34, 1301 Constitution Ave., NW, 18 Washington, DC. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday 19 through Friday , excluding legal holiday s. The telephone number for the Public Reading Room is 20 (202) 566

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

- 1742. 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

4 **FOR FURTHER INFORMATION CONTACT:** Dr. Richard Scheffe, Office of Air Quality 1 Planning and Standards, U.S. Environmental Protection Agency , Mail code C304

- 02, Research 2 Triangle Park, NC 27711; telephone: 919

- 541

- 4650; fax: 919

- 541

- 2357; email: 3 scheffe.rich@epa.gov. 4 **SUPPLEMENTARY INFORMATION:** 5 **General Information** 6 What Should IConsider as I Prepare M y Comments for EPA? 7 1. Submitting CBI. Do not submit this information to EPA through www.regulations.gov or 8 email. Clearl y mark the part or all of the information that you claim to be CBI. For CBI 9 information in a disk or CD ROM that you mail to EPA, mark the outside of the disk or 10 CD ROM as CBI and th en identify electronically

within the disk or CD ROM the specific 11 information that is claimed as CBI. In addition to one complete version of the comment 12 that includes information claimed as CBI, a cop y of the comment that does not contain 13 the information claimed as CBI must be submitted for inclusion in the public docket. 14 In formation so marked will not be disclosed except in accordance with procedures set 15 forth in 40 CFR part 2. 16 2. Tips for Preparing Your Comments . When submitting comments, remember to: 17

 Id e ntify

the rulemaking by

docket number and other identifying information (subject 18 heading, Federal Register date and page number). 19

 Follow directions

– The Agency may ask you to respond to specific questions or 20 organize comments by referencing a Code of Fed eral Regulations (CFR) part or section 21 number. 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

5  Explain why

you agree or disagree, suggest alternatives, and substitute language for your 1 requested changes. 2

 Describe any assumptions and provide any technical information and/or data that you 3 used. 4

 If you est imate potential costs or burdens, explain how you arrived at your estimate in 5 sufficient detail to allow for it to be reproduced. 6

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

 Explain your views as clearl y as possible

. Make sure to submit your comments by the 8 comment period deadline identified. 9 Availability

of Related Information 10 A number of documents relevant to this rulemaking are available on EPA web sites. The 11 Integrated Science Assessment for Oxides of Nitrogen and Sulfur

- Ecological Criteria: FINAL 12 REPORT (I SA) is available on EPAs National Center for Environmental Assessment web site. 13 To obtain this document, go to http://www .epa.gov/ncea , and click on Air Quality then click on 14 Oxides of Nitrogen and Sulfur. The Policy Assessment, Risk and Exposure Assessment (REA) , 15 and other related technical documents are available on EPA’s Office of Air Quality Planning and 16 Standards (OAQ PS) Technology Transfer Network (TTN) web site. The Policy Assessment is 17 available at http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_pa.html, and the exposure and 18 risk assessments and other related technical documents are available at 19 http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_rea.html

. These and other related 20 documents are also available for inspection and copying in the EPA docket identified above. 21 **Table of Contents** 22 The following topics are discussed in this preamble: 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

6 I. Background 1 A. Legislative Requirements 2 B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides 3 1. NAAQS for Oxides of Nitrogen 4 2. NAAQS for Oxides of Sulfur 5 C. History of Related Assessments and Agency Actions 6 D. History of the Current Review 7 E. Scope of the Current Review 8 II. Rationale for Proposed Decision on the Adequacy of the Current Secondary Standards 9 A. Ecological Effects 10 1. Effects Associated with Gas

- Phase Oxides of Nitrogen and Sulfur 11 a. Nature of ecos ys tem responses to gas
- phase nitrogen and sulfur 12 b. Magnitude of ecos ys tem response to gas
- phase nitrogen and sulfur 13 2. Acidification Effects Associated with Deposition of Oxides of N itrogen 14 and Sulfur 15 a. Nature of Acidification

–relate d Ecosy stem Responses 16 i. Aquatic ecos ys tems 17 ii. Terrestrial ecos ys tems 18 iii. Ecosy stem sensitivity 19 b. Magnitude of Acidification

- related Ecos ys tem Responses 20 i. Aquatic acidification 21 ii. Terrestrial acidification 22 c. Key Uncertainties Associated with Acidification 23 i. Aquatic acidificati on 24 ii. Terrestrial acidification 25 3. Nutrient Enrichment Effects Associated with Deposition of Oxides of 26 Nitrogen 27 a. Nature of Nutrient Enrichment

- related Ecos ys tem Responses 28 i. Aquatic ecos ys tems 29 ii. Terrestrial ecos ys tems 30 iii. Ecosy stem sensitiv ity to nutrient enrichment 31 b. Magnitude of Nutrient Enrichment

- related Ecos ys tem Responses 32 i. Aquatic ecos ys tems 33 ii. Terrestrial ecos ys tems 34 c. Key Uncertainties Associated with Nutrient Enrichment 35 i. Aquatic ecos ys tems 36 ii. Terrestrial ecos ys tems 37 4. Other Ecological Effects 38 B. Risk and Exposure Assessment 39 1. Overview of Risk and Exposure Assessment 40 2. Key Findings 41 a. Air quality anal ys es 42 b. Deposition

- related aquatic acidification 43 c. Deposition

- related terrestrial acidification 44 d. Deposition

- related aquatic nutrient enrichment 45 e. Deposition

- related terrestrial nutrient enrichment 46

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

7 f. Additional effects 1 3. Conclusions on Effects 2 C. Adversity of Effects to Public Welfare 3 1. Ecosy stem Services 4 2. Effects on Ecosy stem Services 5 a. Aquatic Acidification 6 b. Terrestrial Acidification 7 c. Nutrient Enrichment 8 3. Summary 9 D. Adequacy of the Current S tandards 10 1. Adequacy of the Current Standards for Direct Effects 11 2. Appropriateness and Adequacy of the Current Standards for Deposition

12 related Effects 13 a. Appropriateness 14 b. Adequacy of Protection 15 i. Aquatic Acidification 16 ii. Terrestrial acidification 17 iii. Terrestrial nutrient enrichment 18 iv. Aquatic nutrient enrichment 19 v. Other effects 20 3. CASAC Views 21 4. Administrator’s Proposed Conclusions Concerning Adequacy of Current 22 Standard 23 III. Rationale for Proposed Decision on Alternative Multi

- pollutant Approach to Secondary 24 Standard s for Aquatic Acidific ation 25 A. Ambient Air Indicators 26 1. Oxides of Sulfur 27 2. Oxides of Nitrogen 28 B. Form 29 1. Ecological Indicator 30 2. Linking ANC to Deposition 31 3. Linking Deposition to Ambient Air Indicators 32 4. Aquatic Acidification Index 33 5. Spatial Aggregation 34 a. Ecoregion Sensitivity 35 b. Representative Ecoregion

- specific Factors 36 i. Factor F1 37 a) Acid

- sensitive Ecoregions 38 b) Non
- acid sensitive Ecoregions 39 ii. Factor F2 40 iii. Factors F3 and F4 41 c. Factors in Data

- limited Ecoregions 42 d. Application to Hawaii, Alaska, and the U.S. Territories 43 6. Summary of the AAI Form 44 C. Averaging Time 45 D. Level 46

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

8 1. Associat ion Between pH Levels and Target ANC Levels 1 2. ANC Levels Related to Effects on Aquatic Ecos ys tems 2 3. Consideration of Episodic Acidity 3 4. Consideration of Ecos ys tem Response Time 4 5. Prior Examples of Target ANC Levels 5 6. Consideration of Public Welfare Benefits 6 7. Summary of Alternative Levels 7 E. Combined Alternative Levels and Forms 8 F. Characterization of Uncertainties 9 1. Overview of Uncertainty 10 2. Uncertainties Associated with Data Gaps 11 3. Uncertainties in Modeled Processes 12 G. CASAC Advice 13 H. Administrator’s Proposed Conclusions 14 IV
. Field Pi lot Program and Ambient Monitoring 15 A. Field Pilot Program 16 a. Objectives 17 b. Overview of Field Pilot Program 18 c. Complementary Measurements 19 d. Complementary Research Efforts 20 e. Implementation Challenges 21 f. Final Monitoring Plan Development and Stakeholder Participation 22 B. Evaluatio n of Monitoring Methods 23 1. Potential FRMs for SO 2 and p

- SO 4 24 2. Potential FRM for NO y 25 V. Statutory

and Executive Order Reviews 26 A. Executive Order 12866: Regulatory Planning and Review 27 B. Paperwork Reduction Act 28 C. Regulatory Flexibility

Act 29 D. Unfunded Mandates Reform Act 30 E. Exe cutive Order 13132: Federalism 31 F. Executive Order 13175: Consultation and Coordination with Indian Tribal 32 Governments 33 G. Executive Order 13045: Protection of Children From Environmental Health and 34 Safety Risks 35 H. Executive Order 13211: Actions that Significantl y Affect Energy Suppl y, 36 Distribution, or Use 37 I. National Technology Transfer and Advancement Act 38 J. Executive Order 12898: Federal Actions to Address Environmental Justice in 39 Minority Populations and L ow - Income Populations 40 References 41 42

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

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

standard, as defined in section 109(b)(2), must “specify a level of air quality the 17 attainment and maintenance of which, in the judgment of the Administrator, based o n such 18 criteria, is requisite to protect the public welfare from any known or anticipated adverse effects 19 associated with the presence of [the] pollutant in the ambient air.” Welfare effects as defined in 20 section 302(h) (42 U.S.C. § 7602(h)) include, but are not limited to, “effects on soils, water, 21

1 The legislative history of section 109 indicates 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 persons comprising the sensitive group rather than to a single person in such a group” S. Rep. No. 91

- 1196, 91st Cong., 2d Sess. 10 (1970).

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

10 crops, vegetation, man

- made materials, animals, wildlife, weather, visibility and climate, damage 1 to and deterioration of property , and hazards to transportation, as well as effects on economic 2 values and on per sonal comfort and well

- being.” 3 In setting standards that are “requisite” to protect public health and welfare, as provided 4 in section 109(b), EPA’s task is to establish standards that are neither more nor less stringent 5 than necessary

for these purposes. In so doing, EPA may not consider the costs of implementing 6 the standards. See generally , Whitman v. American Trucking Associations, 531 U.S. 457, 465

- 7 472, 475

- 76 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant 8 considerations in the promulgation of national ambient air quality standards.” American 9 Petroleum Institute v. Costle, 665 F. 2d at 1185. Section 109(d)(1) requires that “not later than 10 December 31, 1980, and at 5

- ye ar intervals thereafter, the Administrator shal l complete a 11 thorough review of the criteria published under section 108 and the national ambient air quality 12 standards . . . and shall make such revisions in such criteria and standards and promulgate such 13 new standards as may be appropriate . . . .” Sec tion 109(d)(2) requires that an independent 14 scientific review committee “shall complete a review of the criteria . . . and the national primary 15 and secondary

ambient air quality

standards . . . and shall recommend to the Administrator any 16 new . . . standards and revisions of existing criteria and standards as may be appropriate . . . .” 17 Since the earl y 1980's, this independent review function has been performed by the Clean Air 18 Scientific Advisory Committee (CASAC). 19 B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides 20 1. NAAQS for Oxides of Nitrogen 21 After reviewing the relevant science on the public health and welfare effects associated 22 with oxides of nitrogen, EPA promulgated identical primary and secondary NAAQS for NO2 in 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

11 April 1971. Th ese standards were set at a level of 0.053 parts per million (ppm) as an annual 1 average (36 FR 8186). In 1982, EPA published Air Quality Criteria for Oxides of Nitrogen (US 2 EPA, 1982), which updated the scientific criteria upon which the initial standards were based. In 3 February

1984 EPA proposed to retain these standards (49 FR 6866). After taking into account 4 public comments, EPA published the final decision to retain these standards in June 1985 (50 FR 5 25532). 6 The EPA review ed the oxides of nitrogen secondary standards in 1987. InNovember 7 1991 EPA released an updated draft air quality criteria document (AQCD ) for CASAC and 8 public review and comment (56 FR 59285), which provided a comprehensive assessment of the 9 avai lable scientific and technical information on health and welfare effects associated with NO2 10 and other oxides of nitrogen. The CASAC reviewed the draft document at a meeting held on 11 July

1, 1993 and concluded in a closure letter to the Administrator that the document “provides a 12 scientifically

balanced and defensible summary of current knowledge of the effects of this 13 pollutant and provides an adequate basis for EPA to make a decision as to the appropriate 14 NAAQS for NO2” (Wolff, 1993). The Air Quality Cri teria for Oxides of Nitrogen was then 15 finalized (US EPA, 199 5 a). EPA’s OAQPS also prepared a Staff Paper that summarized and 16 integrated the key

studies and scientific evidence contained in the revised AQCD for oxides of 17 nitrogen and identified the critical elements to be considered in the review of the NO2 NAAQS. 18 CASAC reviewed two drafts of the Staff Paper and concluded in a closure letter to the 19 Administrator that the document provided a “scientifically adequate basis for regulatory 20 decisions on nitrogen dioxide” (Wolff, 1995). 21 In October 1995 the Administrator announced her proposed decision not to revise either 22 the primary

or secondary NAAQS for NO2 (60 FR 52874; October 11, 1995). A year later, the 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

12 Administrator made a final determination not to rev ise the NAAQS for NO2 after careful 1 evaluation of the comments received on the proposal (61 FR 52852; October 8, 1996). While the 2 primary

NO2 standard was revised in January 2010 by supplementing the existing annual 3 standard with the establishment of a new 1

- hour standard (75 FR 6474), the secondary NAAQS 4 for NO2 remains 0.053 ppm (100 micrograms per cubic meter [μg/m3] of air), annual arithmetic 5 average, calculated as the arithmetic mean of the 1

- hour NO2 concentrations. 6 2. NAAQS for Oxides of Sulfur 7 EPA promulgated primary and secondary NAAQS for SO2 in April 1971 (36 FR 8186). 8 The secondary

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

- 9 hour average standard set at 0.5 ppm, not to be exceeded more than once per year. Th ese 10 secondary

standards were established solel y on the basis of evidence of adverse effects on 11 vegetation. In 1973, revisions made to Chapter 5 (“Effects of Sulfur Oxide in the Atmosphere on 12 Vegetation”) of Air Quality Criteria for Sulfur Oxides (US EPA, 1973) indicated that it could not 13 properl y be concluded that the vegetation injury reported resulted from the average SO2 14 exposure over the growing season, rather than from short

- term peak concentrations. Therefore, 15 EPA proposed (38 FR 11355) and then fin alized (38 FR 25678) a revocation of the annual mean 16 secondary

standard. At that time, EPA was aware that then

- current concentrations of oxides of 17 sulfur in the ambient air had other public welfare effects, including effects on materials, 18 visibility , soil s, and water. However, the available data were considered insufficient to establish 19 a quantitative relationship between specific ambient concentrations of oxides of sulfur and such 20 public welfare effects (38 FR 25679). 21 In 1979, EPA announced that it was revising the AQCD for oxides of sulfur concurrentl y 22 with that for particulate matter (PM) and would produce a combined PM and oxides of sulfur 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

13 criteria document. Following its review of a draft revised criteria document in August 1980, 1 CASAC concluded that acid deposition was a topic of extreme scientific complexity because of 2 the difficulty

in establishing firm quantitative relationships among (1) emissions of relevant 3 pollutants (e.g., SO2 and oxides of nitrogen), (2) formation of acidic wet and dry depos ition 4 products, and (3) effects on terrestrial and aquatic ecos ys tems. CASAC also noted that acid 5 deposition involves, at a minimum, several different criteria pollutants: oxides of sulfur, oxides 6 of nitrogen, and the fine particulate fraction of suspend ed particles. CASAC felt that any 7 document on this subject should address both wet and dry deposition, since dry

deposition was 8 believed to account for a substantial portion of the total acid deposition problem. 9 For these reasons, CASAC recommended that a separate, comprehensive document on 10 acid deposition be prepared prior to any consideration of using the NAAQS as a regulatory 11 mechanism for the control of acid deposition. CASAC also suggested that a discussion of acid 12 deposition be included in the AQCD s for oxides of nitrogen and PM and oxides of sulfur. 13 Following CASAC closure on the AQCD for oxides of sulfur in December 1981, EPA’s OAQPS 14 published a Staff Paper in November 1982, although the paper did not directly assess the issue of 15 acid deposition. Instead, EPA subsequently prepared the following documents to address acid 16 deposition: The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Review 17 Papers, Volumes I and II (US EPA, 1984a, b) and The Acidic Deposition Phenomenon and Its 18 Effects: Critical Assessment Document (US EPA, 1985) (53 FR 14935

- 14936). These 19 documents, though they were not considered criteria documents and did not undergo CASAC 20 review, represented the most comprehensive summary of scientific information relevant to acid 21 deposition completed by EPA at that point. 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

14 In April 1988 (53 FR 14926), EPA proposed not to revise the existing primary and 1 secondary

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

- current scientific 3 understanding of the acid deposition problem, it would be premature and unwise to prescribe any 4 regulatory

control program at that time and (2) when the fundamental scientific uncertainties had 5 be en decreased through ongoing research efforts, EPA would draft and support an appropriate set 6 of control measures. Although EPA revised the primary SO 2 standard in June 2010 by 7 establishing a new 1

- hour standard and revoking the existing 24

- hour and annual standards (75 8 FR 35520), no further decisions on the secondary SO 2 standard have been published. 9 C. History of Related Assessments and Agency Actions 10 In 1980, the Congress created the National Acid Precipitation Assessment Program 11 (NAPAP) in response to growing concern about acidic deposition. The NAPAP was given a 12 broad 10

- ye ar mandate to examine the causes and effects of acidic deposition and to explore 13 alternative control options to alleviate acidic deposition and its effects. During the course of the 14 program, the NAPAP issued a series of publicl y available interim reports prior to the completion 15 of a final report in 1990 (NAPAP, 1990). 16 In spite of the complexities and significant remaining uncertainties associated with the 17 acid deposition problem , it soon became clear that a program to address acid deposition was 18 needed. The Clean Air Act Amendments of 1990 included numerous separate provisions related 19 to the acid deposition problem. The primary and most important of the provisions, the 20 amendments to Title IV of the Act, established the Acid Rain Program to reduce emissions of 21 SO 2 by

10 million tons and emissions of nitrogen oxides by 2 million tons from 1980 emission 22 levels in order to achieve reductions over broad geographic regions. In this p rovision, Congress 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

15 included a statement of findings that led them to take action, concluding that (1) the presence of 1 acid compounds and their precursors in the atmosphere and in deposition from the atmosphere 2 represents a threat to natural resources, ecos ys tems, materials, visibility , and public health; (2) 3 the problem of acid deposition is of national and international significance; and (3) current and 4 future generations of Americans will be adversely affected by

delaying measures to remedy the 5 problem. 6 Second, Congress authorized the continuation of the NAPAP in order to assure that the 7 research and monitoring efforts already undertaken would continue to be coordinated and would 8 provide the basis for an impartial assessment of the effectiveness of the T itle IV program. 9 Third, Congress considered that further action might be necessary in the long term to 10 address an y problems remaining after implementation of the Title IV program and, reserving 11 judgment on the form that action could take, included Section 404 of the 1990 Amendments 12 (Clean Air Act Amendments of 1990, Pub. L . 101

- 549, § 404) requiring EPA to conduct a study 13 on the feasibility

and effectiveness of an acid deposition standard or standards to protect 14 “sensitive and critically sensitive aquatic and terrestrial resources.” At the conclusion of the 15 study , EPA was to submit a report to Congress. Five years later, EPA submitted its report, 16 entitled Acid Deposition Standard Feasibility Study : Report to Congress (US EPA, 1995 b ) in 17 fulfillment of this requirement. That Report concluded that establishing acid deposition 18 standards for sulfur and nitrogen deposition may at some point in the future be technically 19 feasible, although appropriate deposition loads for these acidifying chemicals could not be 20 d efined with reasonable certainty at that time. 21 Fourth, the 1990 Amendments also added new language to sections of the CAA 22 pertaining to the scope and application of the secondary NAAQS designed to protect the public 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

16 welfare. Specifically , the definition of “effects on welfare” in Section 302(h) was expanded to 1 state that the welfare effects include effects “...whether caused by transformation, conversion, or 2 combination with other air pollutants.” 3 In 1999, seven Northeastern states cited this amended lan guage in Section 302(h) in a 4 petition asking EPA to use its authority under the NAAQS program to promulgate secondary 5 NAAQS for the criteria pollutants associated with the formation of acid rain. The petition stated 6 that this language “clearly references the transformation of pollutants resulting in the inevitable 7 formation of sulfate and nitrate aerosols and/or their ultimate environmental impacts as wet and 8 dry deposition, clearl y signaling Congressional intent that the welfare damage occasioned b y 9 sulfur and nitrogen oxides be addressed through the secondary standard provisions of Section 10 109 of the Act.” The petition further stated that “recent federal studies, including the NAPAP 11 Biennial Report to Congress: An Integrated Assessment, document the cont inued and increasing 12 damage being inflicted by acid deposition to the lakes and forests of New York, New England 13 and other parts of our nation, demonstrating that the Title IV program had proven insufficient.” 14 The petition also listed other adverse welfare effects associated with the transformation of these 15 criteria pollutants, including impaired visibility , eutrophication of coastal estuaries, global 16 warming, and tropospheric ozone and stratospheric ozone depletion. 17 In a related matter, the Office of the Secretary of the U.S. Department of Interior (DOI) 18 requested in 2000 that EPA initiate a rulemaking proceeding to enhance the air quality in 19 national parks and wilderness areas in order to protect resources and values that are being 20 adversel y affected b y air pollution. Included among the effects of concern identified in the 21 request were the acidification of streams, surface waters, and/or soils; eutrophication of coastal 22 waters; visibility

impairment; and foliar injury from ozone. 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

17 In a Federal Register notice in 2001, EPA announced receipt of these requests and asked 1 for comment on the issues raised in them. EPA stated that it would consider any relevant 2 comments and information submitted, along with the information provided by the petitioners and 3 DOI, bef

ore making any

decision concerning a response to these requests for rulemaking (65 FR 4 48699). 5 The 2005 NAPAP report states that “... scientific studies indicate that the emission 6 reductions achieved b y Title IV are not sufficient to allow recovery of acid

- s ensitive ecos ys tems. 7 Estimates from the literature of the scope of additional emission reductions that are necessary in 8 order to protect acid

- sensitive ecosy stems range from approximately 40

- 80% beyond full 9 implementation of Title IV.... ” The results of the modeling presented in this Report to Congress 10 indicate that broader recovery is not predicted without additional emission reductions” (NAPAP , 11 2005 ). 12 Given the state of the science as described in the ISA, REA, and in other recent reports, 13 such as the NAPAP reports noted above, EPA has decided, in the context of evaluating the 14 adequacy

of the current NO2 and SO2 secondary standards in this review, to revisit the question 15 of the appropriateness of setting secondary NAAQS to address remaining known or anticipated 16 adverse public welfare effects resulting from the acidic and nutrient deposition of these criteria 17 pollutants. 18 D. History of the Current Review 19 The EPA initiated this current review in December 2005 with a call for information (70 20 FR 73236) for the development of a revised Integrated Science Assessment for Oxides of 21 Nitrogen and Oxides of Sulfur

- Ecological Criteria (henceforth the "ISA"). An Integrated 22 Review Plan (IRP) was developed to provide the framework and schedule as well as the scope of 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

18 the review and to identify policy

- relevant questions to be addressed in the components of the 1 review. The IRP was released in 2007 (US EPA , 2007) for CASAC and public review. EPA 2 held a workshop in July 2007 on the I SA to obtain broad input from the rel evant scientific 3 communities. This workshop helped to inform the preparation of the first draft ISA , which was 4 released for CASAC and public review in December 2007; a CASAC meeting was held on April 5 2

- 3, 2008 to review the first draft ISA. A second draf t ISA was released for CASAC and public 6 review in August 2008, and was discussed at a CASAC meeting held on October 1

- 2, 2008. T he 7 final I SA (US EPA, 2008) was released in December 2008. 8 Based on the science presented in the I SA, EPA developed a Risk and Exposure 9 Assessment for Review of the Secondary National Ambient Air Quality Standards for Oxides of 10 Nitrogen and Oxides of Sulfur (henceforth the “REA”) to further assess the national impact of 11 the effects documented in the I SA. The Draft Scope and Me thods Plan for Risk/Exposure 12 Assessment: Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur outlining 13 the scope and design of the future REA was prepared for CASAC and public in March 2008. A 14 first draft REA was presented to CASAC and the public for review in August 2008 and a second 15 draft was presented for review in June 2009. The final REA (US EPA 2009) was released in 16 September 2009. A first draft Policy Assessment was released in March 2010 and reviewed by 17 CASAC on April 1

- 2, 2010. In a June 22, 2010 letter to the Administrator, CASAC provided 18 advice and recommendations to the Agency concerning the first draft Policy Assessment (Russell 19 and Samet, 2010 a). A second draft Policy Assessment was released to CASAC and the public 20 in Septe mber 2010 and reviewed b y CASAC on October 6

- 7, 2010. CASAC provided advice 21 and recommendations to the Agency regarding the second draft Policy Assessment in a 22 December 9, 2010 letter (Russell and Samet 2010 b ). CASAC and public comments on the 23

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19 second dra ft Policy

Assessment were considered by EPA staff in developing a final Policy 1 Assessment (US EPA, 2011). CASAC requested an additional meeting to provide additional 2 advice to the Administrator based on the final Policy Assessment on February 15

– 16, 2011. 3 EPA released on January 14, 2011, the final Policy Assessment prior to final document 4 production, to provide sufficient time for CASAC review of the document in advance of this 5 meeting. A final Policy Assessment, incorporating final reference checks and document 6 formatting, was released in February 2011. In a May

17, 2011 letter (Russel l and Samet, 2011 a), 7 CASAC offered additional advice to the Administrator with regard to recommendations and 8 revisions to the secondary NAAQS for oxides of nitrogen and ox ides of sulfur. 9 On February 3, 2006, the Center for Biological Diversity and four other plaintiffs filed 10
an amended complaint alleging that EPA had failed to complete the current review within the 11 period provided b y statute. 2 The schedule for completi on of this review is governed b y a consent 12 decree resolving that lawsuit and the subsequent extension agreed to b y the parties . The schedule 13 presented in the original consent decree that governs this review, entered by the court on 14 November 19, 2007, was revised on October 22, 2009 to allow for a 17 month extension of the 15 schedule. The current decree provides that EPA sign for publication notices of proposed and 16 final rulemaking concerning its review of the oxides of nitrogen and oxides of sulfur NAAQS no 17 later than July

12, 2011 and March 20, 2012, respectively

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

oxides of nitrogen and oxides of sulfur standards. Throughout this preamble a number 20 of conclusions, findings, and determinations proposed by the Administrator are noted. While 21 they

identify the reasoning that supports this proposal, they are onl y proposals and are not 22 intended to be final or conclusive in n ature. The EPA invites general, specific, and/or technical 23

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

– 1814 (D.D.C.)

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20 comments on all issues involved with this proposal, including all such proposed judgments, 1 conclusions, findings, and determinations. 2 E. Scope of the Current Review 3 In conducting this periodic re view of the secondary NAAQS for oxides of nitrogen and 4 oxides of sulfur, as discussed in the IRP and REA, EPA decided to assess the scientific 5 information, associated risks, and standards relevant to protecting the public welfare from 6 adverse effects assoc iated jointly with oxides of nitrogen and sulfur. Although EPA has 7 historically

adopted separate secondary standards for oxides of nitrogen and oxides of sulfur, 8 EPA is conducting a joint review of these standards because oxides of nitrogen and sulfur, an d 9 their associated transformation products are linked from an atmospheric chemistry perspective, 10 as well as from an environmental effects perspective. The National Research Council (NRC) has 11 recommended that EPA consider multiple pollutants, as appropriat e, in forming the scientific 12 basis for the NAAQS (NRC, 2004). As discussed in the I SA and REA, there is a strong basis for 13 considering these pollutants together, building upon EPA’s past recognition of the interactions of 14 these pollutants and on the growi ng body of scientific information that is now available related to 15 these interactions and associated ecological effects. 16 In defining the scope of this review, it must be considered that EPA has set secondary 17 standards for two other criteria pollutants rel ated to oxides of nitrogen and sulfur: ozone and 18 particulate matter (PM). Oxides of nitrogen are precursors to the formation of ozone in the 19 atmosphere, and under certain conditions, can combine with atmospheric ammonia to form 20 ammonium nitrate, a compon ent of fine PM. Oxides of sulfur are precursors to the formation of 21 particulate sulfate, which is a significant component of fine PM in many parts of the U.S. There 22 are a number of welfare effects directl y associated with ozone and fine PM, including ozo ne - 23

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21 related damage to vegetation and PM

- related visibility impairment. Protection against those 1 effects is provided b y the ozone and fine PM secondary standards. This review focuses on 2 evaluation of the protection provided b y secondary standards for oxide s of nitrogen and sulfur 3 for two general types of effects: (1) direct effects on vegetation associated with exposure to 4 gaseous oxides of nitrogen and sulfur in the ambient air, which are the effects that the current 5 NO 2 and SO 2 secondary standards protec t against and (2) effects associated with the deposition 6 of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecos ys tems, including 7 deposition in the form of particulate nitrate and particulate sulfate. 8 The I SA focuses on the ecological e ffects associated with deposition of ambient oxides of 9 nitrogen and sulfur to natural sensitive ecos ys tems, as distinguished from commercially managed 10 forests and agricultural lands. This focus reflects the fact that the majority of the scientific 11 evidenc e regarding acidification and nutrient enrichment is based on studies in unmanaged 12
ecos ys tems. Non
- managed terrestrial ecos ys tems tend to have a higher fraction of N deposition 13 resulting from atmospheric nitrogen (US EPA , 2008, section 3.3.2.5). In addition, the I SA notes 14 that agricultural and commercial forest lands are routinely fertilized with amounts of nitrogen 15 that exceed air pollutant inputs even in the most polluted areas (US EPA , 2008, section 3.3.9). 16 This review recognizes that the effects of nitrogen deposition in managed areas are viewed 17 differentl y from a public welfare perspective than are the effects of nitrogen deposition in 18 natural, unmanaged ecosystems, largely due to the more homogeneous, controlled nature of 19 species composition and dev elopment in managed ecos ys tems and the potential for benefits of 20 increased productivity in those ecos ys tems. 21 In focusing on natural sensitive ecosy stems, the Policy Assessment primarily

considers 22 the effects of ambient oxides of nitrogen and sulfur via deposition on multiple ecological 23

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22 receptors. The ISA highlights effects including those associated with acidification and nitrogen 1 nutrient enrichment. With a focus on these deposition

- related effects, EPA’s objective is to 2 develop a framework for oxides o f nitrogen and sulfur standards that incorporates ecologicall y 3 relevant factors and that recognizes the interactions between the two pollutants as they deposit to 4 sensitive ecos ys tems. The overarching policy objective is to develop a secondary standard(s) 5 that is based on the ecological criteria described in the I SA and the results of the assessments in 6 the REA, and is consistent with the requirement of the CAA to set secondary standards that are 7 requisite to protect the public welfare from any known or anticipated adverse effects associated 8 with the presence of these air pollutants in the ambient air. Also consistent with the CAA, this 9 policy

objective necessarily

includes consideration of “variable factors . . . which of themselves 10 or in combination with other factors may alter the effects on public welfare” of the criteria air 11 pollutants included in this review. 12 In addition, we have chosen to focus on the effects of ambient oxides of nitrogen and 13 sulfur on ecological impacts on sensitive aquatic ecosy st ems associated with acidifying 14 deposition of nitrogen and sulfur, which is a transformation product of ambient oxides of 15 nitrogen and sulfur. Based on the information in the I SA, the assessments presented in the REA, 16 and advice from CASAC on earlier draft s of this Policy Assessment (Russell and Samet, 2010a, 17 2010b), and as discussed in detail in the Policy Assessment , we have the greatest confidence in 18 the causal linkages between oxides of nitrogen and sulfur and aquatic acidification effects 19 relative to o ther deposition

- related effects, including terrestrial acidification and aquatic and 20 terrestrial nutrient enrichment. 21 22

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23 **II. Rationale for Proposed Decision on the Adequacy of the Current Secondary**  1 **Standards** 2 D ecisions on retaining or revising the current sec ondary standards for oxides of nitrogen 3 and sulfur are largely public welfare policy

judgments based on the Administrator’s informed 4 assessment of what constitutes requisite protection against adverse effects to public welfare. A 5 public welfare policy dec ision should draw upon scientific information and anal ys es about 6 welfare effects, exposure and risks, as well as judgments about the appropriate response to the 7 range of uncertainties that are inherent in the scientific evidence and analy ses. The ultimate 8 determination as to what level of damage to ecos ystems and the services provided by those 9
ecos ys tems is adverse to public welfare is not wholly a scientific question, although it is 10 informed b y scientific studies linking e cos ys tem damage to losses in ecosy stem services, and 11 information on the value of those losses of ecos ys tem services. In reaching such decisions, the 12 Administrator seeks to establish standards that are neither more nor less stringent than necessary 13 for this purpose. 14 This section presents the rationale for the Administrator’s proposed conclusions with 15 regard to the adequacy of protection and ecological relevance of the current secondary standards 16 for oxides of nitrogen and sulfur. As discussed more full y below, this rationale considered the 17 latest scientific information on ecological effects associated with the presence of oxides of 18 nitrogen and oxides of sulfur in the ambient air. This rationale also takes into account: (1) staff 19 assessments of the most policy

- relevant information in the I SA and staff analy ses of air quality , 20 exposure, and ecological risks, presented more fully in the REA and in the Policy Assessment , 21 upon which staff conclusions on revisions to the secondary oxides of nitrogen and oxides of 22 sulfur standards are based; (2) CASAC advice and recommendations, as reflected in discussions 23

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24 of drafts of the I SA, REA, and Policy Assessment at public meetings, in separate written 1 comments, and in CASAC’s letters to the Administrator; and (3) public comments received 2 during the develo pment of these documents, either in connection with CASAC meetings or 3 separately

. 4 In developing this rationale, EPA has drawn upon an integrative s ynthesis of the entire 5 body of evidence, published through early 2008 , on ecological effects associated wi th the 6 deposition of oxides of nitrogen and oxides of sulfur in the ambient air (US EPA, 2008)

. As 7 discussed below in section II.A, this body of evidence addresses a broad range of ecological 8 endpoints associated with ambient levels of oxides of nitrogen and oxides of sulfur

. In 9 considering this evidence, EPA focuses on those ecological endpoints, such as aquatic 10 acidification, for which the IS A judges associations with oxides of nitrogen and oxides of sulfur 11 to be causal, likel y causal, or for which the e vidence is highl y suggestive that oxides of nitrogen 12 and/or sulfur contribute to the reported effects

. 13 Crucial to this review is the development of a form for an ecologicall y relevant standard 14 that reflects both the geographicall y variable and deposition

- dependent nature of the effects. 15 The atmospheric levels of oxides of nitrogen and sulfur that afford a particular level of 16
ecos ys tem protection are those levels that result in an amount of deposition that is less than the 17 amount of deposition that a giv en ecos ys tem can accept without defined levels of degradation of 18 the ecological indicator for a targeted effect. 19 Drawing from the framework developed in the REA, the framework we used to structure 20 an ecologically

meaningful secondary

standard in the Policy Assessment and to further develop 21 the indicator, form, level, and averaging time of such as standard in section III of this proposal is 22

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25 depicted below and highlights the three key linkages that need to be considered in developing an 1 ecologicall y relevant standard. 2 3

4 **Figure II -**  **1. Sim**  **plified conceptual design of the form of an aquatic acidification standard** 5 **for oxides of nitrogen and sulfur.** 6 7 The following discussion relies heavil y on Chapters 2 and 3 of the Policy Assessment

. 8 The Policy

Assessment include s staff’s evaluation of the policy implications of the scientific 9 assessment of the evidence presented and assessed in the I SA and the results of quantitative 10 assessments based on that information presented and assessed in the REA. Taken together, this 11 inf ormation informs staff conclusions and the development of policy options in the Policy 12 Assessment for consideration in addressing public and welfare effects associated with the 13 presence of oxides of nitrogen and oxides of sulfur in the ambient air. Of par ticular note, chapter 14 2 of the Policy

Assessment presents information not repeated here that characterizes emissions, 15 air quality , deposition and water quality. It includes discussions of the sources of nitrogen and 16 sulfur in the atmosphere as well as current ambient air quality monitoring networks and models. 17 Additional information in this section includes ecological modeling and water quality data 18
sources. 19 Section II.A presents a discussion of the effects associated with oxides of nitrogen and 20 sulfur in the ambient air. The discussion is organized around the ty pes of effects being 21

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26 considered, including direct effects of gaseous oxides of nitrogen and sulfur , deposition

- related 1 effects related to acidification and nutrient enrichment , and other effects s uch as materials 2 damage, climate

- related effects and mercury methylation

. 3 Section II
. B presents a summary and discussion of the risk and exposure assessment 4 performed for each of the four major effects categories. The REA uses case studies representing 5 the broad geographic variability of the impacts from oxides of nitrogen and sulfur to conclude 6 that there are ongoing adverse effects in many ecosy stems from deposition of oxides of nitrogen 7 and sulfur and that under current emissions scenarios these effect s are likely to continue. 8 Section II
. C presents a discussion of adversity linking ecological effects to measures that 9 can be used to characterize the extent to which such effects are reasonabl y considered to be 10 adverse to public welfare. This involves consideration of how to characterize adversity from a 11 public welfare perspective. In so doing, consideration is given to the concept of ecos ys tem 12 services, the evidence of effects on ecos ys tem services, and how ecos ys tem services can be 13 linked to ecological indicators. 14 Section II
. D presents an assessment of the adequacy of the current oxides of nitrogen and 15 oxides of sulfur secondary standards. Consideration is given to the adequacy of protection 16 afforded b y the current standards for both direct and deposition

- related effects, as well as to the 17 appropriateness of the fundamental structure and the basic elements of the current standards for 18 providing protection from deposition

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

- related effects that could reasonably be judged to be adverse to public welfare are 20 occurring under current conditions which are allowed b y the current standards is also considered

. 21 Discussion of the structures and basic elements of the current NO 2 an d SO 2 secondary standards 22 and whether they are adequate to protect against such effects is presented

. 23

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27 A. Ecological Effects 1 This section discusses the known or anticipated ecological effects associated with oxides 2 of nitrogen and sulfur, including the direct effects of gas

- phase exposure to oxides of nitrogen 3 and sulfur (section II.A.1) and effects associated with deposition

- related exposure (sections 4 II.A.2 and 3)

. Section II .A. 2 addresses effects related to acidification of aquatic and terrestrial 5
ecos ys tems and s ection II A.3 addresses effects related to nutrient enrichment of aquatic and 6 terrestrial ecos ys tems
. These sections also address questions about the nature and magnitude of 7
ecos ys tem responses to reactive nitrogen and sulf ur deposition, including responses related to 8 acidification, nutrient depletion, and, in Section II.A 4 the mobilization of toxic metals in 9 sensitive aquatic and terrestrial ecos ys tems . T he uncertainties and limitations associated with the 10 evidence of suc h effects are also discussed throughout this section

. 11 1. Effects Associated with Gas

-Phase Oxides of Nitrogen and Sulfur 12 E cological effects on vegetation as discussed in earlier reviews as well as the I SA can be 13 attributed to gas

- phase oxides of nitrogen and sulfur. Acute and chronic exposures to gaseous 14 pollutants such as sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), nitric oxide (NO), nitric acid 15 (HNO 3 ) and perox yacetyl nitrite (PAN) are associated with negative impacts to vegetation. The 16 current secon dary

NAAQS were set to protect against direct damage to vegetation by exposure to 17 gas - phase oxides of nitrogen and sulfur , such as foliar injury , decreased photosy nthesis, and 18 decreased growth. The following summary is a concise overview of the known or anticipated 19 effects to vegetation caused by gas phase nitrogen and sulfur

. Most phototoxic effects associated 20 with gas phase oxides of nitrogen and sulfur occur at levels well above ambient concentrations 21 observed in the U.S. (US EPA, 2008, section 3.4.2.4 ). 22

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28 a. Nature of ecosystem responses to gas

- phase nitrogen and sulfur 1 The 2008 I SA found that gas phase N and S are associated with direct phy totoxic effects 2 (US EPA, 2008, section 4.4). The evidence is sufficient to infer a causal relationship between 3 expos ure to SO 2 and injury to vegetation (US EPA, 2008, section 4.4.1 and 3.4.2.1). Acute foliar 4 injury

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

- h 5 average of 0.50 ppm). Effects on growth, reduced photosy nthesis and decreased yi eld of 6 vegetation are also associated with increased SO 2 exposure concentration and time of exposure. 7 The evidence is sufficient to infer a causal relationship between exposure to NO, NO 2 8 and PAN and injury

to vegetation (US EPA, 2008, section 4.4 .2 and 3.4.2.2). At sufficient 9 concentrations, NO, NO 2 and PAN can decrease photosy nthesis and induce visible foliar injury 10 to plants. Evidence is also sufficient to infer a causal relationship between exposure to HNO 3 11 and changes to vegetation (US EPA, 2008, section 4.4.3 and 3.4.2.3). Phytotoxic effects of this 12 pollutant include damage to the leaf cuticle in vascular plants and disappearance of some 13 sensitive lichen species. 14 b. Magnitude of ecosystem response to gas

- phase nitrogen and sulfur 15 Vegetation in ecos ys tems near sources of gaseous oxides of nitrogen and sulfur or where 16 SO 2 , NO, NO 2 , PAN and HNO 3 are most concentrated are more likel y to be impacted b y these 17 pollutants. Uptake of these pollutants in a plant canopy is a complex process involving 18 ad sorption to surfaces (leaves, stems and soil) and absorption into leaves (US EPA, 2008, section 19 3.4.2). The functional relationship between ambient concentrations of gas phase oxides of 20 nitrogen and sulfur and specific plant response are impacted b y inter nal factors such as rate of 21 stomatal conductance and plant detoxification mechanisms, and external factors including plant 22

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29 water status, light, temperature, humidity , and pollutant exposure regime (US EPA, 2008, section 1 3.4.2). 2 Entry

of gases into a leaf is dependent upon ph ys ical and chemical processes of gas phase 3 as well as to stomatal aperture. The aperture of the stomata is controlled largely by the 4 prevailing environmental conditions, such as water availability , humidity , temperature, and light 5 inte nsity . When the stomata are closed, resistance to gas uptake is high and the plant has a very 6 low degree of susceptibility to injury . Mosses and lichens do not have a protective cuticle barrier 7 to gaseous pollutants or stomata and are generall y more sensi tive to gaseous sulfur and nitrogen 8 than vascular plants (US EPA, 2008, section 3.4.2). 9 The appearance of foliar injury can vary significantly

across species and growth 10 conditions affecting stomatal conductance in vascular plants (US EPA, 200 9 , section 6.4.1). For 11 example, damage to lichens from SO 2 exposure include s decrease d photosynthesis and 12 respiration, damage to the algal component of the lichen, leakage of electroly tes, inhibition of 13 nitrogen fixation, decreased K+ absorption, and structural chang es. 14 The phy

totoxic effects of gas phase oxides of nitrogen and sulfur are dependent on the 15 exposure concentration and duration and species sensitivity to these pollutants. Effects to 16 vegetation associated with oxides of nitrogen and sulfur are therefore variable across the U.S. 17 and tend to be higher near sources of photochemical smog. For example, SO 2 is considered to be 18 the primary

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

. 19 The I SA states there is very limited new research on phy totoxic effects of NO, NO 2 , PAN 20 and HNO 3 at concentrations currentl y observed in the United States with the exception of some 21 lichen species ( US EPA, 2008, section 4.4). Past and current HNO 3 concentrations may be 22 contributing to the decli ne in lichen s pecies in the Los Angeles basin . Most phototoxic effects 23

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30 associated with gas phase oxides of nitrogen and sulfur occur at levels well above ambient 1 concentrations observed in the U.S. (US EPA, 2008, section 3.4.2.4). 2 2. Acidification Effects A ssociated with Deposition of Oxides of Nitrogen and Sulfur 3 Sulfur oxides and nitrogen oxides in the atmosphere undergo a complex mix of reactions 4 in gaseous, liquid, and solid phases to form various acidic compounds. These acidic compounds 5 are removed fro m the atmosphere through deposition: either wet (e.g., rain, snow), fog or cloud, 6 or dry

(e.g., gases, particles). Deposition of these acidic compounds to ecosy stems can lead to 7 effects on ecos ys tem structure and function. Following deposition, these compo unds can, in 8 some instances unless retained b y soil or biota, leach out of the soils in the form of sulfate (SO 4 2

- 9 ) and nitrate (NO 3 - ), leading to the acidification of surface waters. The effects on ecosy stems 10 depend on the magnitude and rate of deposition , as well as a host of biogeochemical processes 11 occurring in the soils and waterbodies ( US EPA, 200 9 , section 2.1). The chemical forms of 12 nitrogen that may

contribute to acidifying deposition include both oxidized and reduced chemical 13 species , including NH x . 14 When sulfur or nitrogen leaches from soils to surface waters in the form of SO 4 2

- or NO 3 - , 15 an equivalent amount of positive cations, or countercharge, is also transported. This maintains 16 electroneutrality . I f the countercharge is provided by base catio ns, such as calcium (Ca2+ ), 17 magnesium (Mg 2+ ), sodium (Na+ ), or potassium (K+ ), rather than h ydrogen (H + ) and dissolved 18 inorganic aluminum, the acidity of the soil water is neutralized, but the base saturation of the soil 19 decreases. Continued SO 4 2

- or NO 3 - leaching can deplete the available base cation pool in soil. 20 As the base cations are removed, continued deposition and leaching of SO 4 2

- and/or NO 3 - (with 21 H + and Al 3+ ) leads to acidification of soil water, and by connection, surface water. Introduction 22 of strong acid anions such as sulfate and nitrate to an already acidic soil, whether naturally or due 23

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31 to anthropogenic activities, can lead to instantaneous acidification of waterbodies through direct 1 runoff without any significant change in base cation saturation. The ability of a watershed to 2 neutralize acidic deposition is determined by a variety of biogeoph ys ical factors including 3 weathering rates, bedrock composition, vegetation and microbial processes, phy sical and 4 chemical characteristics of soils and hydrologic flowpaths. (US EPA, 200 9 , section 2.1) Some of 5 these factors such as vegetation and soil depth are highl y variable over small spatial scales such 6 as meters, but can be aggregated to evaluate patterns over larger spatial scales. Acidifying 7 depos ition of oxides of nitrogen and sulfur and the chemical and biological responses associated 8 with these inputs vary temporally

. Chronic or long

- term deposition processes in the time scale of 9 ye ars to decades result in increases in inputs of N and S to ecos ys tems and the associated 10 ecological effects. Episodic or short term (i.e., hours or day s) deposition refers to events in which 11 the level of the acid neutralizing capacity (ANC) of a lake or stream is temporaril y lowered. In 12 aquatic ecosy stems, short

- term (i.e., hours or day s) episodic changes in water chemistry can have 13 significant biological effects. Episodic acidification refers to conditions during precipitation or 14 snowmelt events when proportionately more drainage water is routed through upper soil h orizons 15 that tend to provide less acid neutralizing than is passing through deeper soil horizons (US EPA, 16 200 9 , section 4.2). In addition, the accumulated sulfate and nitrate in snow packs can provide a 17 surge of acidic inputs. Some streams and lakes may have chronic or base flow chemistry that is 18 suitable for aquatic biota, but may be subject to occasional acidic episodes with deleterious 19 consequences to sensitive biota. 20 The following summary is a concise overview of the known or anticipated effects caused 21 by acidification to ecos ys tems within the United States. Acidification affects both terrestrial and 22 freshwater aquatic ecosystems. 23

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32 a. Nature of acidification

- related ecosystem responses 1 The I SA concluded that deposition of oxides of nitrogen and sulfur and NH x leads to the 2 varying degrees of acidification of ecos ys tems (US EPA, 2008 ). In the process of acidification, 3 biogeochemical components of terrestrial and freshwater aquatic ecos ys tems are altered in a way 4 that leads to effects on biological o rganisms. Deposition to terrestrial ecosy stems often moves 5 through the soil and eventually leaches into adjacent water bodies. 6 i. Aquatic ecosystems 7 The scientific evidence is sufficient to infer a causal relationship between acidifying 8 deposition and effec ts on biogeochemistry and biota in aquatic ecos ys tems (US EPA, 2008, 9 section 4.2.2). The strongest evidence comes from studies of surface water chemistry in which 10 acidic deposition is observed to alter sulfate and nitrate concentrations in surface waters, the sum 11 of base cations, ANC, diss olved inorganic aluminum and pH (US EPA, 2008, section 3.2.3.2). 12 ANC is a key

indicator of acidification with relevance to both terrestrial and aquatic ecosy stems. 13 ANC is useful because it integrates the overall acid

- base status of a lake or stream and reflects 14 how aquatic ecos ys tems respond to acidic deposition over time. There is also a relationship 15 between ANC and the surface water constituents that directl y contribute to or ameliorate acidity

16 related stress, in particular, concentrations of hydrogen ion (as pH), Ca2+ , and aluminum. 17 Moreover, low pH surface waters leach aluminum from soils, which is quite lethal to fish and 18 other aquatic organisms. In aquatic s ys tems, there is a direct relationship between ANC and fish 19
and phy

to
- zooplankton diversity

and abundance

. 20 Low ANC coincides with effects on aquatic s ys tems (e.g., individual species fitness loss 21 or death, reduced species richness, altered community structure). At the community level, 22 species richness is positive ly correlated with pH and ANC because energy cost in maintaining 23

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33 phy siological homeostasis, growth, and reproduction is high at low ANC levels. For example, 1 there is a logistic relationship between fish species richness and ANC class for Adirondack Case 2 Study

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

. Biota are generally not harmed when ANC values are >100 microequivalents per liter 4 (μeq/L). The number of fish species also peaks at ANC values >100 μeq/L . Below 100 μeq/L 5 ANC , fish fitness and community diversity begin to decline (US EPA, section 4.2) **.** At ANC 6 levels between 100 and 50 μeq/L , the fitness of sensitive species (e.g., brook trout, zooplankton) 7 also begins to decline . When ANC concentrations are <50 μeq/L , they are generall y associated 8 with death or loss of fitness of biota that are sensitive to acidification. 9 Consistent and coherent documentation from multiple studies on various species from all 10 major trophic levels of aquatic sy stems shows that geochemical alteration caused b y acidification 11 can result in the loss of acid

- sensitive biological species (US EPA, 2008, section 3.2.3.3). This is 12 most often discussed with relation to pH. For example, in the Adirondacks, of the 53 fish species 13
reco rded in Adirondack lakes about half (26 species) were absent from lakes with pH below 6.0. 14 Biological effects are linked to changes in water chemistry including decreases in ANC and pH 15 and increases in inorganic Al concentration. The direct biological ef fects are caused b y lowered 16 pH which leads to increased inorganic Al concentrations (US EPA, 20 11 , Figures 3

- 1 and 3

- 2 ). 17 While ANC level does not cause direct biological harm it is a good overall indicator of the risk 18 of acidification (US EPA, 20 11 , sec tion 3.1.3). 19 There are clear associations between ANC, pH, and aquatic species mortality and health 20 which are summarized in section 3.1.1 of the Policy Assessment

. Significant harm to sensitive 21 aquatic species has been observed at pH levels below 6. Nor mal stream pH levels with little to 22 no toxicity

range from 6 to 7 (MacAvo y et al, 1995). Baker et al (1990) observed that “lakes 23

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34 with pH less than approximately 6.0 contain significantly

fewer species than lakes with pH levels 1 above 6.0”. As noted in Ch apter 3, ty picall y at pH <4.5 and an ANC <0 μeq/L , complete to 2
near - complete loss of many taxa of organisms occur, including fish and aquatic insect 3 populations, whereas other taxa are reduced to only acidophilic species. 4 Additional evidence can help re fine the understanding of effects occurring at pH levels 5 between 4.5 and 6. When pH levels are below 5.6, relativel y lower trout survival rates were 6 observed in the Shenandoah National Park. In field observations, when pH levels dropped to 5, 7 mortality

rates went to 100 percent. (Bulger et al, 2000). At pH levels ranging from 5.4 to 5.8, 8 cumulative mortality continues to increase. Several studies have shown that trout exposed to 9 water with varying pH levels and fish larvae showed increasing mortality as pH levels decrease. 10 In one study

almost 100 percent mortality was observed at a pH of 4.5 compared to almost 100 11 percent survival at a pH of 6.5. Intermediate pH values (6.0, 5.5) in all cases showed reduced 12 survival compared with the control (6.5), but not by statisticall y significant amounts (US EPA, 13 2008, section 3.2.3.3). 14 One important indicator of acid stress is increased fish mortality . The response of fish to 15 pH is not uniform across species. A number of s yn optic survey s indicated loss of spe cies 16 diversity

and absence of several fish species in the pH range of 5.0 to 5.5. If pH is lower, there is 17 a greater likelihood that more fish species could be lost without replacement, resulting in 18 decreased richness and diversity . In general, population s of salmonids are not found at pH levels 19 less than 5.0, and smallmouth bass (Micropterus dolomieu) populations are usually not found at 20 pH values less than about 5.2 to 5.5. From Table 3

- 1, only one study

showed significant 21 mortality

effects above a pH o f 6, while a number of studies showed significant mortality when 22 pH levels are at or below 5.5. 23

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35 The highest pH level for any of the studies reported in the I SA, is 6.0, suggesting that pH 1 above 6.0 is protective against mortality effects for most specie s. Most thresholds are in the 2 range of pH of 5.0 to 6.0, which suggests that a target pH should be no lower than 5.0. 3 Protection against mortality in some recreationally important species such as lake trout (pH 4 threshold of 5.6) and crappie (pH threshold of 5.5), combined with the evidence of effects on 5 larval and embryo survival suggests that pH levels greater than 5.5 should be targeted to provide 6 protection against mortality effects throughout the life stages of fish. 7 Non
- lethal effects have been observed at pH levels as high as 6. A study in the 8 Shenandoah National Park found that the condition factor, a measure of fish health expressed as 9 fish weight/length multiplied by a scaling constant, is positively correlated with stream pH 10 levels, and that the condition factor is reduced in streams with a pH of 6.0 ( US EPA, 2008, 11 section 3.2.3.3). 12 Biodiversity is another indicator of aquatic ecos ys tem health. A key study in the 13 Adirondacks found that lakes with a pH of 6.0 had only half the potential specie s of fish (27 of 14 53 potential species). There is often a positive relationship between pH and number of fish 15 species, at least for pH values between about 5.0 and 6.5, or ANC values between about 0 to 16 100 μeq/L . Such observed relationships are complicated, however, b y the tendency for smaller 17 lakes and streams, having smaller watersheds, to also support fewer fish species, irrespective of 18 acid

- base chemistry . This pattern may be due to a decrease in the number of available niches as 19 stream or lake size decreases. Nevertheless, fish species richness is relatively easily determined 20 and is one of the most useful indicators of biological effects of surface water acidification. 21 Changes in stream water pH and ANC also contribute to declines in taxonomic richness 22 of zooplankton, and macroinvertebrates which are often sources of food for fish, birds and other 23

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36 animal species in various ecos ys tems. These fish may also serve as a source of food and 1 recreation for humans. Acidification of ecos ys tems has been shown to disrupt food web 2 dy namics causing alteration to the diet, breeding distribution , and reproduction of certain species 3 of birds (US EPA, 2008, section 4.2.2.2. and Table 3

- 9). For example, breeding distribution of 4 the common goldeneye (Bucephala clangula ) , an insectivorous duck , may be affected b y 5 changes in acidifying deposition. Similarly , decreases in prey diversity and quantity

have been 6 observed to create feeding problems for nesting pairs of loons on low

- pH lakes in the 7 Adirondacks. 8 ii. Terrestrial ecos ystems 9 In terrestrial ecos ys tems, the evidence is sufficient to infer a causal relationship between 10 acidifying deposition and changes in biogeochemistry ( US EPA, 2008, section 4.2.1.1). The 11 strongest evidence comes from studies of forested ecos ys tems, wi th supportive information on 12 other plant taxa, including shrubs and lichens (US EPA, 2008, section 3.2.2.1.). Three useful 13 indicators of chemical changes and acidification effects on terrestrial ecos ystems, showing 14 consistency

and coherence among multiple studies are: soil base saturation, Al concentrations in 15 soil water and soil C:N ratio (US EPA, 2008, section 3.2.2.2). 16 As discussed in the I SA and REA, i n soils with base saturation less than about 15 to 20%, 17 exchange chemistry is dominated by

Al. Unde r these conditions, responses to inputs of sulfuric 18 acid and HNO 3 largel y involve the release and mobilization of dissolved inorganic Al. The 19 effect can be neutralized by weathering from geologic parent material or base cation exchange. 20 The Ca2+ and Al co ncentrations in soil water are strongl y influenced b y soil acidification and 21 both have been shown to have quantitative links to tree health, including Al interference with 22 Ca2+ uptake and Al toxicity to roots. Effects of nitrification and associated acidi fication and 23

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37 cation leaching have been consistently shown to occur onl y in soils with a C:N ratio below about 1 20 to 25. 2 Soil acidification caused by acidic deposition has been shown to cause decreased growth 3 and increased susceptibility to disease and inj ury

in sensitive tree species. Red spruce ( Picea 4 rubens ) dieback or decline has been observed across high elevation areas in the Adirondack, 5 Green and White mountains. The frequency of freezing injury

to red spruce needles has 6 increased over the past 40 ye ars, a period that coincided with increased emissions of S and N 7 oxides and increased acidifying deposition. Acidifying deposition can contribute to dieback in 8 sugar maple (

Acer saccharum ) through depletion of cations from soil with low levels of availa ble 9 Ca. Grasslands are likel y less sensitive to acidification than forests due to grassland soils being 10 generall y rich in base cations. 11 iii. Ecosystem sensitivity 12 The intersection between current deposition loading, historic loading, and sensitivity 13 defines the ecological vulnerability to the effects of acidification. Freshwater aquatic and some 14 terrestrial ecos ys tems, notably forests, are the ecosy stem ty pes which are most sensitive to 15 acidification. The IS A reports that the principal factor governing the s ensitivity of terrestrial and 16 aquatic ecosy stems to acidification from sulfur and nitrogen deposition is geology (particularly 17 surficial geology). Geologic formations having low base cation supply generall y underlie the 18 watersheds of acid

- sensitive lakes and streams. Other factors that contribute to the sensitivity of 19 soils and surface waters to acidifying deposition include topography , soil chemistry , land use, 20 and hydrologic flowpaths. Episodic and chronic acidification tends to occur in areas that have 21 b ase - poor bedrock, high relief, and shallow soils (US EPA, 2008, section 3.2.4.1). 22

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38 b. Magnitude of acidification

- related ecosystem responses 1 Terrestrial and aquatic ecosy stems differ in their response to acidifying deposition. 2 Therefore the magnitude of ecos ys tem response is described separatel y for aquatic and terrestrial 3
ecos ys tems in the following sections. The magnitude of response refers to both the severity of 4 effects and the spatial extent of the U.S. which is affected. 5 i. Aquatic acidification 6 Freshwat er ecos ys tem survey s and monitoring in the eastern United States have been 7 conducted b y man y programs since the mid

- 1980s, including EPA’s Environmental Monitoring 8 and Assessment Program (EMAP), National Surface Water Survey (NSWS), Temporall y 9 Integrated M onitoring of Ecosy stems (TIME), and L ong - term Monitoring (LTM) programs. 10 Based on anal ys es of surface water data from these programs, New England, the Adirondack 11 Mountains, the Appalachian Mountains (northern Appalachian Plateau and Ridge/Blue Ridge 12 region), and the Upper Midwest contain the most sensitive lakes and streams (i.e., ANC less than 13 about 50 μeq/L). Portions of northern Florida also contain many acidic and low

- ANC lakes and 14 streams, although the role of acidifying deposition in this region is less clear. The western U.S. 15 contains many

of the surfac e waters most sensitive to potential acidification effects, but with the 16 exception of the L os Angeles Basin and surrounding areas, the levels of acidifying deposition are 17 low in most areas. Therefore, acidification of surface waters b y acidic deposition i s not as 18 prevalent in the western U.S., and the extent of chronic surface water acidification that has 19 occurred in that region to date has likel y been very limited relative to the Eastern U.S. (US EPA, 20 2008, section 3.2.4.2 and US EPA, 200 9 , section 4.2.2)

. 21 There are a number of species including fish, aquatic insects, other invertebrates and 22 algae that are sensitive to acidification and cannot survive, compete, or reproduce in acidic 23

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39 waters ( US EPA, 2008, section 3.2.3.3). Decreases in ANC and pH have been shown to 1 contribute to declines in species richness and declines in abundance of zooplankton, 2 macroinvertebrates, and fish. Reduced growth rates have been attributed to acid stress in a 3 number of fish species including Atlantic salmon (Salmo salar), Chin ook salmon (Oncorh ynchus 4 tshawy

tscha), lake trout (Salvelinus namaycush), rainbow trout (Oncorhy nchis my kiss), brook 5 trout (Salvelinus Fontinalis), and brown trout (Salmo trutta). In response to small to moderate 6 changes in acidity , acid

- sensitive species are often replaced b y other more acid

- tolerant species, 7 resulting in changes in community composition and richness. The effects of acidification are 8 continuous, with more species being affected at higher degrees of acidification. At a point, 9 ty picall y a pH <4.5 and an ANC <0 μeq/L , complete to near

- complete loss of many taxa of 10 organisms occur, including fish and aquatic insect populations, whereas other taxa are reduced to 11 only

acidophilic species. These changes in taxa composition are associated with the high energy 12 cost in maintaining ph ys iological homeostasis, growth, and reproduction at low ANC levels (US 13 EPA, 2008, section 3.2.3.3) . Decreases in species richness related to acidification have been 14 observed in the Adirondack Mountains and Catskill Mountains of New York, New England and 15 Pennsylvania, and Virginia. From the sensitive areas identified by the ISA, further “case study” 16 anal ys es on aquatic ecosy stems in the Adirondack Mountains and Shenandoah National Park 17 were conducted to better characteri ze ecological risk associated with acidification (US EPA, 18 200 9 , section 4). 19 ANC is the most widely used indicator of acid sensitivity and has been found in various 20 studies to be the best single indicator of the biological response and health of aquatic 21
c ommunities in acid

- sensitive sy stems (Lien et al., 1992; Sullivan et al., 2006; US EPA, 2008). 22 In the REA, surface water trends in SO 4 2

- and NO 3 - concentrations and ANC levels were 23

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40 anal yz ed to affirm the understanding that reductions in deposition could in fluence the risk of 1 acidification. ANC values have been categorized according to their effects on biota, as shown in 2 the table below . Monitoring data from TIME/LTM and EMAP programs were assessed for the 3 ye ars 1990 to 2006, and past, present, and future water quality levels were estimated by both 4 steady

- state and dyn amic biogeochemical models. 5

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41 1 **Table II -**  **1. Ecological effects associated with alternative levels of acid neutralizing capacity** 2 **(ANC). (source: USEPA, Acid Rain Program)** 3 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 repro duce, and fitness. Diversity and distribution of zooplankton communities decline. Moderate Concern 50

– 100

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

lakes). Brook trout populations are sensitive and variable, with possible sublethal effects. Diversity and distribution of zooplankton communities also begin to decline as species that are sensitive to acidifying deposition are affected. Low Concern >100 μeq/L Fish species richness may be unaffected. Reproducing brook trout populations are expected where habitat is suitable. Zooplankton communities are unaffected and exhibit expected diversity and distribution. 4 Studies on fish species richness in the Adirondacks Case Study Area demonstrated the 5 effect of acidificati on. Of the 53 fish species recorded in Adirondack Case Study Area lakes, 6 only 27 species were found in lakes with a pH <6.0. The 26 species missing from lakes with a 7 pH <6.0 include important recreational species, such as Atlantic salmon, tiger trout (Salm o trutta 8

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42 X Salvelinus fontinalis ), redbreast sunfish ( Lepomis auritus ), bluegill ( Lepomis macrochirus ), 1 tiger musky (

Esox masquinongy X lucius ), walleye (Sander vitreus ), alewife ( Alosa 2 pseudoharengus ), and kokanee ( Oncorhynchus nerka ), as well as ecologic ally important 3 minnows that are commonly consumed b y sport fish. A survey of 1,469 lakes in the late 1980s 4 found 346 lakes to be devoid of fish. Among lakes with fish, there was a relationship between 5 the number of fish species and lake pH, ranging from ab out one species per lake for lakes having 6 a pH <4.5 to about six species per lake for lakes having a pH >6.5. In the Adirondacks, a 7 positive relationship exists between the pH and ANC in lakes and the number of fish species 8 present in those lakes ( US EPA, 2008, section 3.2.3.4). 9 Since the mid

- 1990s, streams in the Shenandoah Case Study Area have shown slight 10 declines in NO 3 - and SO 4 2

- concentrations in surface waters. The 2006 concentrations are still 11 above pre

- acidification (1860) conditions. MAGIC mode ling predicts surface water 12 concentrations of NO 3 - and SO 4 2

- are10

- and 32

- fold higher, respectively , in 2006 than in 1860. 13 The estimated average ANC across 60 streams in the Shenandoah Case Study Area is 57.9 μeq/L 14 (± 4.5 μeq/L). 55% of all monitored stre ams in the Shenandoah Case Study Area have a current 15 risk of Elevated, Severe, or Acute. Of the 55%, 18% are chronically acidic today

( US EPA, 16 200 9 , section 4.2.4.3). 17 Based on a deposition scenario for this study area that maintains current emission leve ls 18 from 2020 to 2050, the simulation forecast indicates that a large number of streams still have 19 Elevated to Acute problems with acidity in 2050 . In fact, from 2006 to 2050, the percentage of 20 streams with Acute Concern is predicted to increase by 5%, whil e the percentage of streams in 21 Moderate Concern decreases b y 5%. 22

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43 Biological effects of increased acidification documented in the Shenandoah Case Study 1 Area include a decrease in the condition factor in blacknose dace and a decrease in fish 2 biodiversity

as sociated with decreasing stream ANC. On average, the fish species richness is 3 lower b y one fish species for every 21 μeq/L decrease in ANC in Shenandoah National Park 4 streams (US EPA, 2008, section 3.2.3.4). 5 ii. Terrestrial acidification 6 The I SA identified a variety of indicators that can be used to measure the effects of 7 acidification in soils. Most effects of terrestrial acidification are observed in sensitive forest 8
ecos ys tem in the U.S. Tree health has been linked to the availability of base cations (Bc ) in soil 9 (such as Ca2+ , Mg 2+ and potassium), as well as soil Al content. Tree species show a range of 10 sensitivities to Ca/Al and Bc/Al soil molar ratios, therefore these are good chemical indicators 11 because they

directly relate to the biological effects. Critical Bc/Al molar ratios for a large 12 variety of tree species ranged from 0.2 to 0.8. This range is similar to critical ratios of Ca/Al. 13 Plant toxicity

or nutrient antagonism was reported to occur at Ca/Al molar ratios ranging from 14 0.2 to 2.5 (US EPA, 200 9 ). 15 There has been no s ys tematic national survey of terrestrial ecos ys tems to determine the 16 extent and distribution of terrestrial ecos ys tem sensitivity to the effects of acidifying deposition. 17 However, one preliminary national evaluation estimated th at ~15% of forest ecosy stems in the 18 U.S. exceed the estimated critical load based on soil ANC leaching for S and N deposition by 19 >250 eq ha

- 1 y

r - 1 (McNulty

et al., 2007). Forests of the Adirondack Mountains of New York, 20 Green Mountains of Vermont, White M ountains of New Hampshire, the Allegheny Plateau of 21 Pennsylvania, and high

- elevation forest ecos ys tems in the southern Appalachians are the regions 22 most sensitive to terrestrial acidification effects from acidifying deposition (US EPA, 2008, 23

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44 section 3.2.4. 2). While studies show some recovery of surface waters, there are widespread 1 measurements of ongoing depletion of exchangeable base cations in forest soils in the 2 northeastern U.S. despite recent decreases in acidifying deposition, indicating a slow recovery 3 time. 4 In the REA, a critical load anal ys is was performed for sugar maple and red spruce forests 5 in the eastern United States by using Bc/Al ratio in acidified forest soils as an indicator to assess 6 the impact of nitrogen and sulfur deposition on tree h ealth. These are the two most commonly 7 studied tree species in North America for effects of acidification. At a Bc/Al ratio of 1.2, red 8 spruce growth can be decreased b y 20%. Sugar maple growth can be decreased by 20% at a 9 Bc/Al ratio of 0.6 (U S EPA, 2009 , section 4.4). The REA anal ys is determined the health of at 10 least a portion of the sugar maple and red spruce growing in the United States may have been 11 compromised with acidifyingtotal nitrogen and sulfur deposition. Specifically , total nitrogen and 12 sulf ur deposition levels exceeded three selected critical loads for tree growth in 3% to 75% of all 13 sugar maple plots across 24 states

-- that is, it exceeded the highest (least stringent) of the three 14 critical loads in 3% of plots, and the lowest (most stringent) in 75% of plots . For red spruce, total 15 nitrogen and sulfur deposition levels exceeded three selected critical loads in 3% to 36% of all 16 red spruce plots across eight states ( US EPA, 200 9 , section 4.4). 17

c. Key uncertainties associated with acidification 18 There are different levels of uncertainty associated with relationships between deposition, 19 ecological effects and ecological indicators. In Chapter 7 of the REA, the case study analy ses 20 associated with each targeted effect area were s ynthesized by identi fying the strengths, 21 limitations, and uncertainties associated with the available data, modeling approach, and 22 relationship between the selected ecological indicator and atmospheric deposition as described 23

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45 by the ecological effect function (US EPA, 200 9 , F igure 1

- 1). A further discussion of 1 uncertainty in aquatic and terrestrial ecos ys tems is presented below. The key uncertainties were 2 characterized as follows to evaluate the strength of the scientific basis for setting a national 3 standard to protect again st a given effect (US EPA, 200 9 , section 7): 4 (1) Data Availability: high, medium or low quality . This criterion is based on the availability and 5 robustness of data sets, monitoring networks, availability of data that allows for extrapolation 6 to larger assessment areas, and input parameters for modeling and developing the ecological 7 effect function. The scientific basis for the ecological indicator selected is also incorporated 8 into this criterion. 9 (2) Modeling Approach: high, fairl y high, intermediate, or low confi dence. This value is based 10 on the strengths and limitations of the models used in the anal ys is and how accepted they are 11 by

the scientific community for their application in this analy sis. 12 (3) Ecological Effect Function: high, fairl y high, intermediate, or low confidence. This ranking is 13 based on how well the ecological effect function describes the relationship between 14 atmospheric deposition and the ecological indicator of an effect. 15 i. Aquatic acidification 16 The REA concludes that the available data are robust and considered high quality . There 17 is high confidence about the use of these data and their value for extrapolating to a larger 18 regional population of lakes. The EPA TIME/LTM network represents a source of long

- term, 19 representative sampling. Data on sulf ate concentrations, nitrate concentrations and ANC from 20 1990 to 2006 used for this analy sis as well as EPA EMAP and REMAP survey s, provide 21 considerable data on surface water trends. 22

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46 There is fairly

high confidence associated with modeling and input param eters. 1 Uncertainties in water quality estimates (i.e., ANC) from MAGIC were derived from multiple 2 site calibrations. The 95% confidence interval for pre

- acidification of lakes was an average of 15 3 μeq/Ldifference in ANC concentrations or 10% and 8 μeq/L or 5% for streams (US EPA, 200 9 , 4 section 7.1.2). The use of the critical load model to estimate aquatic critical loads is limited by 5 the uncertainties associated with runoff and surface water measurements and in estimating the 6 catchment suppl y of bas e cations from the w eathering of bedrock and soils (McNulty et al., 7 2007). To quantify uncertainty in the model parameters, Monte Carlo methods were employed 8 to develop an inverse function of exceedances

. There is high confidence associated wi th the 9 ecological effect function developed for aquatic acidification

. Incalculating the ANC function, 10 the depositional load for N or S is fixed by the deposition of the other, so deposition for either 11 will never be zero ( US EPA, 200 9 , Figure 7.1

- 6)

. 12 i i. Terrestrial acidification 13 The available data used to quantify the targeted effect of terrestrial acidification are 14 robust and considered high quality. The USFS

- Kane Experimental Forest and significant 15 amounts of research work in the Allegheny Plateau have produced extensive, peer

- reviewed data 16 sets. A meta

- analy sis of laboratory studies showed that tree growth was decreased by 20% 17 relative to controls for BC/Al ratios (US EPA, 2008, section 7.2.1 and Figure 7.2

- 1). Sugar 18 maple and red spruce were the focus of the REA since they are demonstrated to be negativel y 19 affected b y soil available Ca2+ depletion and high concentrations of available Al, and occur in 20 areas that receive high acidifying deposition, There is high confidence about the use of the RE A 21 terrestrial acidification data and their value for extrapolating to a larger regional population of 22 forests. 23

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47 There is high confidence associated with the models, input parameters, and assessment of 1 uncertainty used in the case study for terrestrial ac idification. The Simple Mass Balance (SMB) 2 model, a commonly

used and widel y applied approach for estimating critical loads, was used in 3 the REA anal ys is (US EPA, 2008, section 7.2.2). There is fairly high confidence associated with 4 the ecological effect function developed for terrestrial acidification (US EPA, 200 9 , section 5 7.2.3). 6 3. Nutrient Enrichment Effects Associated with Deposition of Oxides of Nitrogen 7 The following summary is a concise overview of the known or anticipated effects caused 8 by nitroge n nutrient enrichment to ecosy stems within the United States. Nutrient

- enrichment 9 affects terrestrial, freshwater and estuarine ecos ys tems. Nitrogen deposition is a major source of 10 anthropogenic nitrogen. For many terrestrial and freshwater ecos ys tems o ther sources of 11 nitrogen including fertilizer and waste treatment are greater than deposition. Nitrogen deposition 12 often contributes to nitrogen

- enrichment effects in estuaries, but does not drive the effects since 13 other sources of N greatly exceed N depo sition. Both oxides of nitrogen and reduced forms of 14 nitrogen (NH x ) contribute to nitrogen deposition. For the most part, nitrogen effects on 15
ecos ys tems do not depend on whether the nitrogen is in oxidized or reduced form. Thus, this 16 summary

focuses on the effects of nitrogen deposition in total. 17

a. Nature of nutrient enrichment

- related ecosystem responses 18 The I SA found that deposition of nitrogen, including oxides of nitrogen and NH x , leads to 19 the nitrogen enrichment of ecos ys tems (US EPA 2008). In the process of nitrogen enrichment, 20 biogeochemical components of terrestrial and freshwater aquatic ecos ys tems are altered in a way 21 that leads to effects on biological organisms. 22 i. Aquatic ecosystems 23

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48 In freshwater ecos ys tems, the evidence is sufficient to in fer a **causal** relationship between 1 N deposition and the alteration of biogeochemical cycling in freshwater aquatic ecos ys tems (US 2 EPA, 2008, section 3.3.2.3 ). N deposition is the main source of N enrichment to headwater 3 streams, lower order streams and high elevation lakes. The most common chemical indicators 4 that were studied included NO 3 − and dissolved inorganic nitrogen (DIN) concentration in surface 5 waters as well as Chl a:total P ratio. Elevated surface water NO 3 − concentrations occur in both 6 the eastern and western U.S. Studies report a significant correlation between N deposition and 7 lake biogeochemistry by identifying a correlation between wet deposition and [DI N] and Chl a: 8 Total P. Recent evidence provides examples of lakes and streams that are limited by N and show 9 signs of eutrophication in response to N addition. 10 The evidence i s sufficient to infer a **causal** relationship between N deposition and the 11 alteration of species richness, species composition and biodiversity in freshwater aquatic 12
ecos ys tems (US EPA, 2008, section 3.3.5.3 ). Increased N deposition can cause a shift in 13
community

composition and reduce algal biodiversity , especially in sensitive oligotrophic lakes. 14 In the IS A, the evidence is sufficient to infer a **causal** relationship between N deposition 15 and the biogeochemical cycling of N and carbon (C) in estuaries (US EP A, 2008, section 4.3.4.1 16 and 3.3.2.3 ). In general, estuaries tend to be nitrogen

- limited, and many currentl y receive high 17 levels of nitrogen input from human activities (US EPA, 200 9 , section 5.1.1 ). It is unknown if 18 atmospheric deposition alone is suffici ent to cause eutrophication; however, the contribution of 19 atmospheric nitrogen deposition to total nitrogen load is calculated for some estuaries and can be 20 >40% (US EPA, 200 9 , section 5.1.1 ). 21 The evidence is sufficient to infer a **causal** relationship betw een N deposition and the 22 alteration of species richness, species composition and biodiversity in estuarine ecos ys tems (US 23

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49 EPA, 2008, section 4.3.4.2 and 3.3.5.4 ). Atmospheric and non

- atmospheric sources of N 1 contribute to increased phy toplankton and algal productivity , leading to eutrophication. Shifts in 2 community

composition, reduced hypolimnetic DO, decreases in biodiversity , and mortality of 3 submerged aquatic vegetation are associated with increased N deposition in estuarine s ys tems. 4 ii. Terrestrial Ecosy stems 5 The evidence is sufficient to infer a **causal** relationship between N deposition and the 6 alteration of biogeochemical cycling in terrestrial ecos ys tems (US EPA, 2008, section 4.3.1.1 7 and 3.3.2.1 ). This is supported by numerous observational, depositio n gradient and field addition 8 experiments in sensitive ecos ys tems. T he leaching of NO 3 - in soil drainage waters and the export 9 of NO 3 - in stream water w ere identified as two of the primary indictors of N enrichment. Several 10 N
- addition studies indicate that NO 3 - leaching is induced b y chronic additions of N

. Studies 11 identified in the I SA found that surface water NO 3 - concentrations exceeded 1 μeq/Lin 12 watersheds receiving about 9 to 13 kg N/ha/yr of atmospheric N deposition. N deposition 13 disrupts the nutri ent balance of ecos ys tems with numerous biogeochemical effects. The chemical 14 indicators that are typically measured include NO 3 − leaching, soil C:N ratio, rates of N 15 mineralization, nitrification, denitrification, foliar N concentration, and soil water NO 3 − and 16 NH 4 + concentrations. Note that N saturation (N leaching from ecos ys tems) does not need to 17 occur to cause effects. Substantial leaching of NO 3 − from forest soils to stream water can acidify 18 downstream waters, leading to effects described in the previ ous section on aquatic acidification. 19 Due to the complexity of interactions between the N and C cycling, the effects of N on C budgets 20 (quantified input and output of C to the ecosy stem) are variable. Regional trends in net 21
ecos ys tem productivity (NEP) of forests (not managed for silviculture) have been estimated 22 through models based on gradient studies and meta

- anal ys is. Atmospheric N deposition has been 23

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50 shown to cause increased litter accumulation and carbon storage in above

- ground woody 1 biomass. In the West, this has lead to increased susceptibility to more severe fires. Less is 2 known regarding the effects of N deposition on C budgets of non

- forest ecosy stems. 3 The evidence is sufficient to infer a **causal** relationship between N deposition on the 4 alterati on of species richness, species composition and biodiversity in terrestrial ecos ys tems (US 5 EPA, 2008, section 4.3.1.2 ). Some organisms and ecos ys tems are more sensitive to N deposition 6 and effects of N deposition are not observed in all habitats. The most sensitive terrestrial taxa to 7 N deposition are lichens. Empirical evidence indicates that lichens in the U.S. are affected by 8 deposition levels as low as 3 kg N/ha/yr. Alpine ecosystems are also sensitive to N deposition, 9 changes in an individual species ( Carex rupestris ) were estimated to occur at deposition levels 10 near 4 kg N /ha/y

r and modeling indicates that deposition levels near 10 kg N/ha/yralter plant 11 community

assemblages. In several grassland ecosy stems, reduced species diversity and an 12 increase in non

- native, invasive species are associated with N deposition. 13 iii. Ecosystem sensitivity to nutrient enrichment 14 The numerous ecos ys tem ty pes that occur across the U.S. have a broad range of 15 sensitivity

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

- 4 ). Increase d deposition to N

- limited 16
ecos ys tems can lead to production increases that may be either beneficial or adverse depending 17 on the sy stem and management goals. 18 Organisms in their natural environment are commonly adapted to a specific regime of 19 nutrient av ailability . Change in the availability of one important nutrient, such as N, may result 20 in an imbalance in ecological stoichiometry , with effects on ecos ys tem processes, structure and 21 function. In general, N deposition to terrestrial ecosy stems causes acce lerated growth rates in 22 some species deemed desirable in commercial forests but may lead to altered competitive 23

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51 interactions among species and nutrient imbalances, ultimately affecting biodiversity . The onset 1 of these effects occurs with N deposition level s as low as 3 kg N/ha/yr in sensitive terrestrial 2
ecos ys tems to N deposition. In aquatic ecos ys tems, N that is both leached from the soil and 3 directly

deposited to the water surface can pollute the surface water. This causes alteration of the 4 diatom commun ity

at levels as low as 1.5 kg N/ha/y r in sensitive freshwater ecos ys tems. 5 The degree of ecos ys tem effects lies at the intersection of N loading and N

- sensitivity

. 6 N
- sensitivity

is predominately

driven by the degree to which growth is limited by nitroge n 7 availability . Grasslands in the western United States are typically N

- limited ecos ys tems 8 dominated by

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

- analy sis discussed in 9 the I SA (US EPA, 2008, section 3.3.3), indicated that N fertilization i ncreased aboveground 10 growth in all non

- forest ecosy stems except for deserts. In other words, almost all terrestrial 11
ecos ys tems are N
- limited and will be altered b y the addition of anthropogenic nitrogen. 12 Likewise, a freshwater lake or stream must be N

- limi ted to be sensitive to N

- mediated 13 eutrophication. There are many examples of fresh waters that are N

- limited or N and 14 phosphorous (P) co

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

- analy sis 15 discussed in the I SA (US EPA, 2008, section 3.3. 3.2), found that N

- limitation occurred as 16 frequentl y as P
- limitation in freshwater ecos ys tems. Additional factors that govern the 17 sensitivity

of ecos ys tems to nutrient enrichment from N deposition include rates and form of N 18 deposition, elevation, climat e, species composition, plant growth rate, length of growing season, 19 and soil N retention capacity (US EPA, 2008, section 4.3 ). Less is known about the extent and 20 distribution of the terrestrial ecos ys tems in the U.S. that are most sensitive to the effects of 21 nutrient enrichment from atmospheric N deposition compared to acidification. 22

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52 Because the productivity

of estuarine and near shore marine ecos ys tems is generall y 1 limited by

the availability

of N, they

are susceptible to the eutrophication effect of N d eposition 2 (US EPA, 2008, section 4.3.4.1 ). A recent national assessment of eutrophic conditions in 3 estuaries found the most eutrophic estuaries were generall y those that had large watershed

- to
4 estuarine surface area, high human population density, high rai nfall and runoff, low dilution, and 5 low flushing rates. In the REA, the National Oceanic and Atmospheric Administration’s 6 (NOAA) National Estuarine Eutrophication Assessment (NEEA) assessment tool, Assessment of 7 Estuarine Tropic Status (ASSETS) categorical Eutrophication Index (EI) was used to evaluate 8 eutrophication due to atmospheric loading of nitrogen. ASSETS EI is an estimation of the 9 likelihood that an estuary is experiencing eutrophication or will experience eutrophication based 10 on five ecological indicators: chlorophyll a , macroalgae, dissolved oxygen, nuisance/toxic algal 11 blooms and submerged aquatic vegetation (SAV). 12 In the REA, two regions were selected for case study anal ys is using ASSETS EI , the 13 Chesapeake Bay

and Pamlico Sound. Both region s received an ASSETS EIrating of Bad 14 indicating that the estuary had moderate to high pressure due to overall human influence and a 15 moderate high to high eutrophic condition (US EPA, 200 9 , section s 5.2.4.1 and 5.2.4.2 ). These 16 results were then considered with SPAtially Referenced Regression (SPARROW) modeling to 17 develop a response curve to examine the role of atmospheric nitrogen deposition in achieving a 18 desired decrease in load. To change the Neuse River Estuary ’s EI score from Bad to Poor not 19 only

mus t 100% of the total atmospheric nitrogen deposition be eliminated, but considerabl y 20 more nitrogen from other sources as well must be controlled (US EPA, 200 9 , section 5.2.7.2). In 21 the Potomac River estuary, a 78% decrease of total nitrogen could move the EIscore from Bad 22 to Poor (US EPA, 200 9 , section 5.2.7.1 ). The results of this analy sis indicated decreases in 23

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53 atmospheric deposition alone could not eliminate coastal eutrophication problems due to 1 multiple non

- atmospheric nitrogen inputs (US EPA, 200 9 , section 7.3.3 ). However, the 2 somewhat arbitrary

discreteness of the EI scale can mask the benefits of decreases in n itrogen 3 between categories. 4 In general, estuaries tend to be N

- limited, and many currentl y receive high levels of N 5 input from human activities to cause eutrophication. As reported in the IS A (US EPA, 2008, 6 section 3.2.2.2), atmospheric N loads to estuari es in the U.S. are estimated to range from 2

- 8% 7 for Guadalupe Bay , TX on the lowest end to as high as 72% for St Catherines

- Sapelo estuary , 8 GA. The Chesapeake Bay is an example of a large, well

- studied and severely eutrophic estuary 9 that is calculated to receive as much as 30% of its total N load from the atmosphere. 10 b. Magnitude of ecosystem responses 11 i. Aquatic ecosystems 12 The magnitude of ecos ys tem response may be thought of on two time scales, current 13 conditions and how ecosystems have been altered since the onset of anthropogenic N deposition. 14 As noted previousl y, studies found that N

- limitation occurs as frequently as P
- limitation in 15 freshwater ecos ys tems ( US EPA, 2008, section 3.3.3.2 ). Recentl y, a comprehensive study of 16 available data from the northern h emisphere survey s of lakes along gradients of N deposition 17 show increased inorganic N concentration and productivity to be correlated with atmospheric N 18 deposition. The results are unequivocal evidence of N limitation in lakes with low ambient inputs 19 of N, and increased N concentrations in lakes receiving N solel y from atmospheric N deposition. 20 It has been suggested that most lakes in the northern hemisphere may have originall y been N

21 limited, and that atmospheric N deposition has changed the balance of N and P in lakes. 22

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54 Available data suggest that the increases in total N deposition do not have to be large to 1 elicit an ecological effect. For example, a hindcasting exercise determined that the change in 2 Rocky

Mountain National Park lake algae that occurred between 1850 and 1964 was associated 3 with an increase in wet N deposition that was only about 1.5 kg N/ha. Similar changes inferred 4 from lake sediment cores of the Beartooth Mountains of Wyoming also occurred at about 1.5 kg 5 N/ha deposition. Pre

- industrial inorganic N deposition is estimated to have been onl y 0.1 to 0.7 6 kg N/ha based on measurements from remote parts of the world. In the western U.S., pre

7 industrial, or background, inorganic N deposition was estimated b y to range from 0.4 to 0.7 kg 8 N/ha/y

r. 9 Eutrophication effects from N deposition are most likely to be manifested in undisturbed, 10 low nutrient surface waters such as those found in the higher elevation areas of the western U.S. 11 The most severe eutrophication from N deposition effects is expect ed downwind of major urban 12 and agricultural centers. High concentrations of lake or streamwater NO 3 − , indicative of 13
ecos ys tem saturation, have been found at a variety of locations throughout the U.S., including the 14 San Bernardino and San Gabriel Mountains within the L os Angeles Air Basin, the Front Range 15 of Colorado, the Allegheny mountains of West Virg inia, the Catskill Mountains of New York, 16 the Adirondack Mountains of New York, and the Great Smoky Mountains in Tennessee (US 17 EPA, 2008, section 3.3.8 ). 18 In contrast to terrestrial and freshwater s ys tems, atmospheric N load to estuaries 19 contributes to the total load but does not necessarily drive the effects since other combined 20 sources of N often greatly exceed N deposition. In estuaries, N

- loading from multiple 21 anthropogenic and non

- anthropogenic pathway s leads to water quality deterioration, resulting in 22 numerous effects including hypoxic zones, species mortality , changes in community composition 23

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55 and harmful algal blooms that are indicative of eutrophication. The following summary is a 1 concise overview of the known or anticipated effects of nitrogen enrichment on estuaries within 2 the United States. 3 There is a scientific consensus (US EPA, 2008, section 4.3.4) that nitrogen

- driven 4 eutrophication in shallow estuaries has increased over the past several decades and that the 5 environmental degradation of co astal ecos ys tems due to nitrogen, phosphorus, and other inputs is 6 now a widespread occurrence. For example, the frequency of phyt oplankton blooms and the 7 extent and severity

of hypoxia have increased in the Chesapeake Bay and Pamlico estuaries in 8 North Carolina and along the continental shelf adjacent to the Mississippi and Atchafalaya 9 rivers’ discharges to the Gulf of Mexico. 10 A recent national assessment of eutrophic conditions in estuaries found that 65% of the 11 assessed s ys tems had moderate to high ove rall eutrophic conditions and generall y received the 12 greatest N loads from all sources, including atmospheric and land

- based sources . Most eutrophic 13 estuaries occurred in the mid

- Atlantic region and the estuaries with the lowest degree of 14 eutrophication were in the North Atlantic. Other regions had mixtures of low, moderate, and high 15 degrees of eutrophication (US EPA, 2008, section 4.3.4.3 ). 16 The mid

- Atlantic region is the most heavily impacted area in terms of moderate or high 17 loss of submerged aquatic v egetation due to eutrophication (US EPA, 2008, section 4.3.4.2 ). 18 Submerged aquatic vegetation is important to the quality of estuarine ecosy stem habitats because 19 it provides habitat for a variety of aquatic organisms, absorbs excess nutrients, and traps 20
s ediments (US EPA, 2008, section 4.3.4.2). It is partly because many estuaries and near

- coastal 21 marine waters are degraded by nutrient enrichment that they are highl y sensitive to potential 22 negative impacts from nitrogen addition from atmospheric depositio n. 23

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56 ii. Terrestrial ecosystems 1 Little is known about the full extent and distribution of the terrestrial ecos ystems in the 2 U.S. that are most sensitive to impacts caused b y nutrient enrichment from atmospheric N 3 deposition. As previously stated, most terrestri al ecos ys tems are N
- limited, therefore they are 4 sensitive to perturbation caused b y N additions (US EPA, 2008, section 4.3.1) . Effects are most 5 likely

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

- limited 6 plant communiti es. The alpine ecos ys tems of the Colorado Front Range, chaparral watersheds of 7 the Sierra Nevada, lichen and vascular plant communities in the San Bernardino Mountains and 8 the Pacific Northwest, and the southern California coastal sage scrub (CSS) communi ty are 9 among the most sensitive terrestrial ecos ys tems. There is growing evidence (US EPA, 2008, 10 section 4.3.1.2) that existing grassland ecos ys tems in the western United States are being altered 11 by elevated levels of N inputs, including inputs from atmosp heric deposition. 12 In the eastern U.S., the degree of N saturation of the terrestrial ecos ys tem is often 13 assessed in terms of the degree of NO 3 − leaching from watershed soils into ground water or 14 surface water. Studies have estimated the number of surface waters at different stages of 15 saturation across several regions in the eastern U.S. Of the 85 northeastern watersheds examined 16 60% were in Stage 1 or Stage 2 of N saturation on a scale of 0 (background or pretreatment) to 3 17 (visible decline). Of the northe astern sites for which adequate data were available for assessment, 18 those in Stage 1 or 2 were most prevalent in the Adirondack and Catskill Mountains. Effects on 19 individual plant species have not been well studied in the U.S. More is known about the 20 sensi tivity

of particular plant communities. Based largel y on results obtained in more extensive 21 studies conducted in Europe, it is expected that the more sensitive terrestrial ecos ys tems include 22

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57 hardwood forests, alpine meadows, arid and semi

- arid lands, and grassland ecos ys tems (US 1 EPA, 2008, section 3. 3.5)

. 2 The REA used published research results (US EPA, 200 9 , section 5.3.1 and US EPA, 3 2008, Table 4.4) to identify meaningful ecological benchmarks associated with different levels of 4 atmospheric nitrogen depo sition. These are illustrated in Figure 3

- 4 of the Policy Assessment

. 5 The sensitive areas and ecological indicators identified by the IS A were analyzed further in the 6 REA to create a national map that illustrates effects observed from ambient and experimental 7 atmospheric nitrogen deposition loads in relation to Community Multi

- scale Air Quality 8 (CMAQ) 2002 modeling results and NADP monitoring data. This map, reproduced in Figure 3

- 9 5 of the Policy

Assessment , depicts the sites where empirical effects of terrestrial nutrient 10 enrichment have been observed and site proximity to elevated atmospheric N deposition. 11 Based on information in the I SA and initial anal ys is in the REA, further case study 12 anal ys es on terrestrial nutrient enrichment of ecosy stems were developed for the CCS 13 community

and Mixed Conifer Forest (MCF) (US EPA, 2009). Geographic information sy stems 14 (GI S) analy sis supported a qualitative review of past field research to identify ecological 15 benchmarks associated with CSS and mycorrhizal commun ities, as well as MCF nutrient

16 sensitive acidoph yt e lichen communities, fine

- root biomass in Ponderosa pine, and leached 17 nitrate in receiving waters. 18 The ecological benchmarks that were identified for the CSS and the MCF communities 19 are included in the suite of benchmarks identified in the I SA ( US EPA, 2008, section 3.3 ). There 20 are sufficient data to confidently relate the ecological effect to a loading of atmospheric nitrogen. 21 For the CSS community , the following ecological benchmarks were identified: 22

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58 (1) 3.3 kg N/ha/yr
– the amount of nitrogen uptake b y a vigorous stand of CSS; above this level, 1 nitrogen may

no longer be limiting 2 (2) 10 kg N/ha/yr
– mycorrhizal community changes 3 For the MCF community, the following ecological benchmarks were identified: 4 (1) 3.1 kg N/ha/yr
– shift from sensitive to tolerant lichen species 5 (2) 5.2 kg N/ha/yr
– dominance of the tolerant lichen species 6 (3) 10.2 kg N/ha/yr
– loss of sensitive lichen species 7 (4) 17 kg N/ha/yr
– leaching of nitrate into streams. 8 These benchmarks, ranging from 3.1 to 17 kg N/ha/y r, were compared to 2002 9 CMAQ/NADP data to discern an y associations between atmospheric deposition and changing 10 communities. Evidence supports the finding that nitrogen alters CSS and MCF communities

. 11 Key

findings include the following: 2002 C MAQ/NADP nitrogen deposition data show that the 12 3.3 kg N/ha/yrbenchmark has been exceeded in more than 93% of CSS areas (654,048 ha). 13 These deposition levels are a driving force in the degradation of CSS communities. Although 14 CSS decline has been observed in the absence of fire, the contributions of deposition and fire to 15 the CSS decline require further research. CSS is fragmented into many small parcels, and the 16 2002 CMAQ/NADP 12

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

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59 c. Key uncertainties associated with nutrient enrichment 1 There are different levels of uncertainty associated with relationships between deposition, 2 ecological effects and ecological indicators. The criteria used in the REA to evaluate the degree 3 of confidence in the data, modeling and ecological effect function are detailed in Chapter 7 of the 4 REA . Below is a discussion of uncer tainty relating aquatic and terrestrial ecosystems to nutrient 5 enrichment effects. 6 i. Aquatic ecosystems 7 The approach for assessing atmospheric contributions to total nitrogen loading in the 8 REA was to consider the main

- stem river to an estuary (including the estuary) rather than an 9 entire estuary

s ys tem or bay . The biological indicators used in the NOAA ASSETS EIrequired 10 the evaluation of many national databases including the US Geological Survey National Water 11 Quality

Assessment (NAWQA) files, EPA’s STO Rage and RETrieval (STORET) database, 12 NOAA’s Estuarine Drainage Areas data, and EPA’s water quality standards nutrient criteria for 13 rivers and lakes (US EPA, 200 9 , Appendix 6 and Table 1.2.

- 1 ). Both the SPARROW modeling 14 for nitrogen loads and assessment o f estuary conditions under NOAA ASSETS EI , have been 15 applied on a national scale. The REA concludes that the available data are medium quality with 16 intermediate confidence about the use of these data and their values for extrapolating to a larger 17 regional area ( US EPA, 200 9 , section 7.3.1 ). Intermediate confidence is associated with the 18 modeling approach using ASSETS EIand SPARROW. The REA states there is low confidence 19 with the ecological effect function due to the results of the anal ys is which indicat ed that 20 reductions in atmospheric deposition alone could not solve coastal eutrophication problems due 21 to multiple non

- atmospheric nitrogen inputs (US EPA, 200 9 , section 7.3.3 ). 22

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60 ii. Terrestrial ecosystems 1 Ecological thresholds are identified for CSS and MCF areas and these data are considered 2 to be of high quality , however, the ability to extrapolate these data to larger regional areas is 3 limited (U S EPA, 2009 , section 7.4.1 ). No quantitative modeling was conducted or ecological 4 effect function developed for terrestrial nutrient enrichment reflecting the uncertainties 5 associated with these depositional effects. 6 4. Other Ecological Effects 7 It is stated in the ISA ( US EPA, 2008, section 3.4.1 and 4.5 ) that mercury is a highl y 8 neurotoxic contaminant that enter s the food web as a methylated compound, methylmercury

. 9 Mercury

is principally

methylated by

sulfur

- reducing bacteria and can be taken up by 10 microorganisms, zooplankton and macroinvertebrates. The contaminant is concentrated in higher 11 trophic levels, inclu ding fish eaten b y humans. Experimental evidence has established that only 12 inconsequential amounts of methylmercury can be produced in the absence of sulfate. Once 13 methylmercury

is present, other variables influence how much accumulates in fish, but elevat ed 14 mercury

levels in fish can only

occur where substantial amounts of methylmercury are present. 15 Current evidence indicates that in watersheds where mercury is present, increased oxides of 16 sulfur deposition very likely results in additional production of m eth yl mercury which leads to 17 greater accumulation of MeHg concentrations in fish. With respect to sulfur deposition and 18 mercury

methylation, the final I SA determined: The evidence is sufficient to infer a causal 19 relationship between sulfur deposition and in creased mercury methylation in wetlands and 20 aquatic environments. 21 The production of meaningful amounts of methyl mercury (MeHg) requires the presence 22 of SO 4 2

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

- results in 23

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61 increased production of MeHg. There is increasing evidence on the relationship between sulfur 1 deposition and increased methylation of mercury in aquatic environments; this effect occurs onl y 2 where other factors are present at levels within a range to allow methylation. The production of 3 methylmercury

requires the presence of sulfate and mercury , but the amount of methylmercury 4 produced varies with ox ygen content, temperature, pH, and supply of labile organic carbon (US 5 EPA, 2008, section 3.4 ). In watersheds where ch anges in sulfate deposition did not produce an 6 effect, one or several of those interacting factors were not in the range required for meaningful 7 methylation to occur (US EPA, 2008, section 3.4 ). Watersheds with conditions known to be 8 conducive to mercury m ethylation can be found in the northeastern United States and 9 southeastern Canada. 10 While the relationship between sulfur and methylmercury production was concluded to 11 be causal in the IS A, the REA concluded that there was insufficient evidence to quantif y the 12 relationship between sulfur and meth yl mercury . Therefore onl y a qualitative assessment was 13 included in Chapter 6 of the REA

. The Policy Assessment was then unable to make a 14 determination as to the adequacy of the existing SO 2 standards in protecting against welfare 15 effects associated with increased mercury methylation. 16 B. Risk and Exposure Assessment 17 The risk and exposure assessment conducted for the current review was developed to 18 describe potential risk from current and future deposition of oxide s of nitrogen and sulfur to 19 sensitive ecos ys tems. The case study anal ys es in the REA show that there is confidence that 20 known or anticipated adverse ecological effects are occurring under current ambient loadings of 21 nitrogen and sulfur in sensitive ecosy st ems across the United States. An overview of the 22 material covered in the REA , a summary of the key

findings from the air quality analy ses, 23

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62 acidification and nutrient enrichment case studies, and general conclusions from evaluating 1 additional welfare effec ts, are presented below. 2 1. Overview of the Risk and Exposure Assessment 3 The REA evaluates the relationships between atmospheric concentrations, deposition, 4 biologicall y relevant exposures, targeted ecos ys tem effects, and ecos ys tem services. To evaluate 5 the nature and magnitude of adverse effects associated with deposition , the REA also examines 6 various way s to quantify the relationships between air quality indicators, deposition of 7 biologicall y available forms of nitrog en and sulfur, ecologicall y relevant indicators relating to 8 deposition, exposure and effects on sensitive receptors, and related effects resulting in changes in 9
ecos ys tem structure and services. The intent is to determine the exposure metrics that incorpor ate 10 the temporal considerations (i.e., biologically relevant timescales), pathways, and ecologicall y 11 relevant indicators necessary to determine the effects on these ecos ys tems. To the extent feasible, 12 the REA evaluates the overall load to the s ys tem for nitrogen and sulfur, as well as the variability 13 in ecosy stem responses to these pollutants . It also evaluates the contributions of atmosphericall y 14 deposited nitrogen and sulfur individually relative to the c ombined atmospheric loadings of both 15 elements together . Since oxidized nitrogen is the listed criteria pollutant (currentl y measured b y 16 the ambient air quality indicator NO 2 ) for the atmospheric contribution to total nitrogen, the REA 17 examines the contribution of nitrogen oxides to total reactive nitrogen in the atmosphere, relative 18 to the contributions of reduced forms of nitrogen (e.g., ammonia, ammonium), to ultimately 19 assess how a meaningful secondary National Ambient Air Quality Standards (NAAQS) might be 20 structured. 21 The REA focuses on ecosy stem welfare effects that result from the deposition of total 22 reactive nitrogen and sulfur. Because ecos ys tems are diverse in biota, climate, geochemistry , and 23

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63 hydrology , response to pollutant exposures can vary greatl y between ecos ystems. In addition, 1 these diverse ecos ys tems are not distributed evenly across the United States. To target nitrogen 2 and sulfur acidification and nitrogen and sulfur enrichment, the REA addresses four main 3 targeted ecos ys tem effects on t errestrial and aquatic sy stems identified by the IS A (US EPA, 4 2008 ): a quatic acidification due to nitrogen and sulfur ; t errestrial acidification due to nitrogen 5 and sulfur ; a quatic nutrient enrichment, including eutrophication ; and t errestrial nutrient 6
enrichment. 7 In addition to these four targeted ecos ys tem effects, the REA also qualitatively addresses 8 the influence of sulfur oxides deposition on methylmercury production; nitrous oxide (N 2 O) 9 effects on climate; nitrogen effects on primary productivity and biogenic greenhouse gas fluxes; 10 and phy

totoxic effects on plants. 11 Because the targeted ecosy stem effects outlined above are not evenl y distributed across 12 the United States, the REA identified case studies for each targeted effects based on ecos ys tems 13 id entified as sensitive to nitrogen and/or sulfur deposition effects. Eight case study areas and 14 two supplemental study areas (Rocky Mountain National Park and Little Rock Lake, WI) are 15 summarized in the REA based on ecos ys tem characteristics, indicators, and ecos ys tem service 16 information. Case studies selected for aquatic acidification effects were the Adirondack 17 Mountains and Shenandoah National Park. Kane Experimental Forest in Pennsylvania and 18 Hubbard Brook Experimental Forest in New Hampshire were se lected as case studies for 19 terrestrial acidification. Aquatic nutrient enrichment case study locations were selected in the 20 Potomac River Basin upstream of Chesapeake Bay and the Neuse River Basin upstream of the 21 Pamlico Sound in North Carolina. The Coas tal Sage Scrub Communities in southern California 22 and the Mixed Confer Forest Communities in the San Bernardino and Sierra Nevada Mountains 23

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64 of California were selected as case studies for terrestrial nutrient enrichment. Two supplemental 1 areas were also c hosen, one in Rocky Mountain National Park for terrestrial nutrient enrichment 2 and one in L ittle Rock Lake, Wisconsin for aquatic nutrient enrichment. 3 2. Key findings 4 In summary , based on case study anal ys es, the REA concludes that known or anticipated 5
a dverse ecological effects are occurring under current conditions and further concludes that these 6 adverse effects continue into the future

. Key findings from the air quality analyses, acidification 7 and nutrient enrichment case studies, as well as general conclusions from evaluating additional 8 welfare effects, are summarized below. 9

a. Air quality analyses 10 The air quality analyses in the REA encompass the current emissions sources of nitrogen 11 and sulfur, as well as atmospheric concentrations, estimates of dep osition of total nitrogen, 12 policy

- relevant background, and non - ambient loadings of nitrogen and sulfur to ecos ys tems, both 13 nationwide and in the case study areas. Spatial fields of deposition were created using wet 14 deposition measurements from the National Atmospheric Deposition Program (NADP) National 15 Trends Network and dry deposition predictions from the 2002 Community Multi

- Scale Air 16 Quality

(CMAQ) model simulation. Some key conclusions from this analysis are: 17 (1) Total reactive nitrogen deposition and sul fur deposition are much greater in the East 18 compared to most areas of the West. 19 (2) These regional differences in deposition correspond to the regional differences in oxides of 20 nitrogen and SO2 concentrations and emissions, which are also higher in the East. Oxides of 21 nitrogen emissions are much greater and generally more widespread than NH 3 emissions 22 nationwide; high NH 3 emissions tend to be more local (e.g., eastern North Carolina) or sub

- 23

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65 regional (e.g., the upper Midwest and Plains states). The relative amo unts of oxidized versus 1 reduced nitrogen deposition are consistent with the relative amounts of oxides of nitrogen 2 and NH 3 emissions. Oxidized nitrogen deposition exceeds reduced nitrogen deposition in 3 most of the case study areas; the major exception bein g the Neuse River/Neuse River Estuary 4 Case Study

Area. 5 (3) R educed nitrogen deposition exceeds oxidized nitrogen deposition in the vicinity of local 6 sources of NH 3 . 7 (4) There can be relativel y large spatial variations in both total reactive nitrogen deposition and 8 sulfur deposition within a case study area; this occurs particularl y in those areas that contain 9 or are near a high emissions source of oxides of nitrogen , NH 3 , and/or SO 2 . 10 (5) The seasonal patterns in deposition differ between the case study areas. For the c ase study 11 areas in the East, the season with the greatest amounts of total reactive nitrogen deposition 12 correspond to the season with the greatest amounts of sulfur deposition. Deposition peaks in 13 spring in the Adirondack, Hubbard Brook Experimental Forest , and Kane Experimental 14 Forest case study areas, and it peaks in summer in the Potomac River/Potomac Estuary , 15 Shenandoah, and Neuse River/Neuse River Estuary case study

areas. For the case study areas 16 in the West, there is less consistency in the seasons w ith greatest total reactive nitrogen and 17 sulfur deposition in a given area. In general, both nitrogen and/or sulfur deposition peaks in 18 spring or summer. The exception to this is the Sierra Nevada Range portion of the Mixed 19 Conifer Forest Case Study Area, in which sulfur deposition is greatest in winter. 20 b. Deposition

- related aquatic acidification 21 The role of aquatic acidification in two eastern United States areas

— northeastern New 22 York’s Adirondack area and the Shenandoah area in Virginia

— was anal yz ed in the REA to 23

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66 assess surface water trends in SO 4 2

- and NO 3 - concentrations and acid neutralizing capacity (ANC) 1 levels and to affirm the understanding that reductions in deposition could influence the risk of 2 acidification. Monitoring data from the EPA

- administered Temporall y Integrated Monitoring of 3 Ecosy stems (TIME)/Long

- Term Monitoring (LTM) programs and the Environmental Monitoring 4 and Assessment Program (EMAP) were assessed for the years 1990 to 2006, and past, present, 5 and future water quality levels were es timated using both steady

- state and dy namic 6 biogeochemical models. 7 Although wet deposition rates for SO 2 and oxides of nitrogen in the Adirondack Case 8 Study

Area have reduced since the mid

- 1990s, current concentrations are still well above pre

9 acidifi cation (1860) conditions. Model of Acidification of Groundwater in Catchments (MAGIC) 10 modeling predicts NO 3 - and SO 4 2

- are 17

- and 5

- fold higher today , respectively . The estimated 11 average ANC for 44 lakes in the Adirondack Case Study Area is 62.1 microequi valents per liter 12 (μeq/L) (± 15.7 μeq/L); 78 % of all monitored lakes in the Adirondack Case Study Area have a 13 current risk of Elevated, Severe , or Acute . Of the 78%, 31% experience episodic acidification, 14 and 18% are chronicall y acidic today

. 15 (1) Based on the steady- state critical load model for the year 2002, 18%, 28%, 44%, and 58% of 16 169 modeled lakes received combined total sulfur and nitrogen deposition that exceeded 17 critical load s corresponding to ANC limits of 0, 20, 50, and 100 μeq/ L respectivel y. 18 (2) Based on a deposition scenario that maintains current emission levels to 2020 and 2050, the 19 simulation forecast indicates no improvement in water quality in the Adirondack Case Study 20 Area. The percentage of lakes within the Elevated to Acute Concern classes remains the same 21 in 2020 and 2050. 22

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67 (3) Since the mid

- 1990s, streams in the Shenandoah Case Study Area have shown slight declines 1 in NO 3 and SO 4 2

- concentrations in surface waters. ANC levels increased from about 50 2 μeq/Lin the earl y 1990 s to >75 μeq/Luntil 2002, when ANC levels declined back to 1991

– 3 1992 levels. Current concentrations are still above pre

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

- are 10

- and 32

- 5 fold higher today , respectively . The estimated average ANC for 60 streams in the 6 Shenandoah Case Study Area is 57.9 μeq/L (± 4.5 μeq/L). 55% of all monitored streams in 7 the Shenand oah Case Study Area have a current risk of Elevated, Severe , or Acute . Of the 8 55%, 18% experience episodic acidification, and 18% are chronically acidic today

. 9 (4) Based on the steady- state critical load model for the year 2002, 52%, 72%, 85%, and 93% of 10 60 mo deled streams received combined total sulfur and nitrogen deposition that exceeded 11 critical load s corresponding to ANC limits of 0, 20, 50, and 100 μeq/Lrespectivel y. 12 (5) Based on a deposition scenario that maintains current emission levels to 2020 and 2050, the 13 simulation forecast indicates that a large number of streams still have Elevated to Acute 14 problems with acidity. Infact, from 2006 to 2050, the percentage of streams with Acute 15 Concern increases b y 5%, while the percentage of streams in Moderate Concern decreases 16 by

5%. 17
c. Deposition

- related terrestrial acidification 18 The role of terrestrial acidification was examined in the REA using a critical load 19 anal ys is for sugar maple and red spruce forests in the eastern United States by using the base 20 cation to aluminum (Bc/Al) ratio in acidified forest soils as an indicator to assess the impact of 21 nitrogen and sulfur depos ition on tree health. These are the two most commonly studied species 22 in North America for impacts of acidification. At a Bc/Al ratio of 1.2, red spruce growth can be 23

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68 reduced b y 20%. Sugar maple growth can be reduced b y 20% at a Bc/Al ratio of 0.6. Key 1 fi ndings of the case study are summarized below. 2 (1) Case study

results suggest that the health of at least a portion of the sugar maple and red 3 spruce growing in the United States may have been compromised with acidifying total 4 nitrogen and sulfur deposition in 2002

. 2002 CMAQ/NADP total nitrogen and sulfur 5 deposition levels exceeded three selected critical loads in 3% to 75% of all sugar maple plots 6 across 24 states. The three critical loads ranged from 107 to 6,008 eq/ha/y r for the Bc/Al 7 ratios of 0.6, 1.2, and 10.0 (increasing levels of tree protection)

. The 2002 CMAQ/NADP 8 total nitrogen and sulfur deposition levels exceeded three selected critical loads in 3% to 36% 9 of all red spruce plots across 8 states. The three critical loads ranged from 180 to 4,278 10 eq/ha/y

r for the Bc/Al ratios of 0.6, 1 .2, and 10.0 (increasing levels of tree protection). 11 (2) The Simple Mass Balance model assumptions made for base cation weathering (Bcw) and 12 forest soil ANC input parameters are the main sources of uncertainty since these parameters 13 are rarel y measured and require researchers to use default values. Bcw contributed 49% to 14 the total variability in the critical load estimates, and forest soil ANC contributed 46% to the 15 total variability

. 16 (3) The pattern of case study results sug gests that nitrogen and sulfur acidifying deposition in the 17 sugar maple and red spruce forest areas studied were very close to, if not greater than, the 18 critical loads for those areas and both ecos ys tems are likel y to be sensitive to any future 19 changes in the levels of deposition

. 20 d. Deposition

- related aquatic nutrient enrichment 21 The role of nitrogen deposition in two main stem rivers feeding their respective estuaries 22 was anal yz ed in the REA to determine if decreases in deposition could influence the risk of 23

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69 eutrophication as predicted using the Assessment of Estuarine Trophic Status eutrophication 1 index (ASSETS EI) scoring s ys tem in tandem with SPAtially Referenced Regression on 2 Watershed Attributes (SPARROW) modeling. This modeling approach provides a tra nsferrable, 3 intermediate

- level anal ys is of the linkages between atmospheric deposition and receiving waters, 4 while providing results on which conclusions could be drawn . Future application of the methods 5 to case study areas where atmospheric deposition plays a larger role in the nitrogen loading to an 6 estuary

will likely

provide more tangible results. A summary of findings follows: 7 (1) 2002 CMAQ/NADP results showed that an estimated 40,770,000 kg of total nitrogen was 8 deposited in the Potomac River watershed. SPARROW modeling predicted that 7,380,000 kg 9 N/y

r of the deposited nitrogen reached the estuary (20% of the total load to the estuary). The 10 overall ASSETS EIfor the Potomac River and Potomac Estuary was Bad (based on all 11 sources of N)

. 12 (2) To improve the Pot omac River and Potomac Estuary ASSETS EIscore from Bad to Poor , 13 there is a slim chance that a decrease of at least 78% in the 2002 total nitrogen atmospheric 14 deposition load to the watershed would be required. 15 (3) 2002 CMAQ/NADP results showed that an estimated 18,340,000 kg of total nitrogen was 16 deposited in the Neuse River watershed. SPARROW modeling predicted that 1,150,000 kg 17 N/y

r of the deposited nitrogen reached the estuary (26% of the total load to the estuary). The 18 overall ASSETS EIfor the Neuse Rive r/Neuse River Estuary was Bad

. 19 (4) It was found that the Neuse River/Neuse River Estuary ASSETS EIscore could not be 20 improved from Bad to Poor with decreases onl y in the 2002 atmospheric deposition load to 21 the watershed. Additional reductions would be require d from other nitrogen sources within 22 the watershed. 23

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70 The small effect of decreasing atmospheric deposition in the Neuse River watershed is 1 because the other nitrogen sources within the watershed are more influential than atmospheric 2 deposition to the total nitrogen loadings to the Neuse River Estuary as estimated with the 3 SPARROW model . A waterbody’s response to nutrient loading depends on the magnitude (e.g., 4 agricultural sources have a high influence in the Neuse), spatial distribution, and other 5 charact eristics of the sources within the watershed. 6

e. Deposition

- related terrestrial nutrient enrichment 7 California C oastal S age S crub (CSS) and M ixed C onifer F orest (MCF) communities 8 were the focus of the Terrestrial Nutrient Enrichment Case Studies of the REA. Geographic 9 information sy stems (GIS) analy sis supported a qualitative review of past field research to 10 identify

ecological benchmarks associated with CSS and mycorrhizal communities, as well as 11 MCF’s nutrient

- sensitive acidophy te lichen communities, fine

- root biomass in Ponderosa pine, 12 and leached nitrate in receiving waters. These benchmarks, ranging from 3.1 to 17 kg N/ha/yr, 13 were compared to 2002 CMAQ/NADP data to discern any associations between atmospheric 14 deposition and changing communities. Evidence supports the finding that nitrogen alters CSS 15 and MCF. Key

findings include the following: 16 (1) 2002 CMAQ/NADP nitrogen deposition data show that the 3.3 kg N/ha/yrbenchmark has 17 been exceeded in more than 93% of CSS areas (654,048 ha). These deposition levels are a 18 driving force in the degradation of CSS communities. Although CSS decline has been 19 observed in the absence of fire, the contributions of deposition and fire to the CSS decline 20 require further research. CSS is fragmented into many small parcels, and the 2002 21 CMAQ/NADP 12

- km grid data are not fine enough to fully validate the relationship between 22 CSS distribution, nitrogen deposition, and fire. 23

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71 (2) 2002 CMAQ/NADP nitrogen deposition data exceeds the 3.1 kg N/ha/yr benchmark in more 1 than 38% (1,099,133 ha) of MCF areas, and nitrate leaching has been observed in surface 2 waters. Ozone effects confound nitrogen effects on MCF acidophyt e lichen, and the 3 interrelationship between fire and nitrogen cycling requires additional research. 4 f. Additional effects 5 Ecol ogical effects have also been documented across the United States where elevated 6 nitrogen deposition has been observed, including the eastern slope of the Rocky Mountains 7 where shifts in dominant algal species in alpine lakes have occurred where wet nitrog en 8 deposition was only

about 1.5 kg N/ha/yr. High alpine terrestrial communities have a low 9 capacity

to sequester nitrogen deposition, and monitored deposition exceeding 3 to 4 kg N/ha/yr 10 could lead to community

- level changes in plant species, lichens, and mycorrhizae. 11 Additional welfare effects are documented, but examined less extensively , in the REA. 12 These effects include qualitative discussions related to visibility and materials damage, such as 13 corrosion, erosion, and soiling of paint and buildings which are being addressed in the particulate 14 matter (PM) NAAQS review currently underway . A discussion of the causal relationship 15 between sulfur deposition (as sulfate, SO 4 2

- ) and increased mercury methylation in wetlands and 16 aquatic environments is also i ncluded in the REA. On this subject the REA concludes that 17 decreases in sulfate deposition will likely result in decreases in methyl mercury concentration; 18 however, spatial and biogeochemical variations nationally hinder establishing large scale dose

19 respo nse relationships. 20 Several additional issues concerning oxides of nitrogen were addressed in the REA. 21 Consideration was also given to nitrous oxide (N 2 O), a potent GHG. The REA concluded that it 22 is most appropriate to analyze the role of N 2 O in the co ntext of all of the GHGs rather than as 23

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72 part of the REA for this review. The REA considered nitrogen deposition and its correlation 1 with the rate of photos ynthesis and net primary productivity

. Nitrogen addition ranging from 2 15.4 to 300 kg N/ha/yr is documented as increasing wetland N 2 O production by an average of 3 207%. Nitrogen addition ranging from 30 to 240 kg N/ha/y increased CH 4 emissions by 115%, 4 averaged across all ecosystems, and methane uptake was reduced b y 38% when nitrogen addition 5 ranged from 10 to 560 kg N/ha/y r, but reductions were onl y significant for coniferous and 6 deciduous forests. The heterogeneity of ecos ys tems across the United States, however, 7 introduces variations into dose

- response relationships. 8 The p hy

totoxic effects of oxides o f nitrogen and sulfur on vegetation were also briefl y 9 discussed in the REA which concluded that since a unique secondary NAAQS exists for SO 2 , 10 and concentrations of nitric oxide (NO ) , NO 2 , and PAN are rarely high enough to have 11 phy totoxic effects on vegetation , further assessment was not warranted at this time. 12 3. Conclusions on E ffects 13 For aquatic and terrestrial acidification effects, a similar conceptual approach was used 14 (critical loads) to evaluate the impacts of multiple pollutants on an ecologic al e ndpoint, whereas 15 the approaches used for aquatic and terrestrial nutrient enrichment were fundamentally distinct. 16 Although the ecological indicators for aquatic and terrestrial acidification (i.e., ANC and Bc/Al) 17 are very

different, both ecological indicators are well

- correlated with effects such as reduced 18 biodiversity

and growth. While aquatic acidification is clearl y the targeted effect area with the 19 highest level of confidence, the relationship between atmospheric deposition and an ecological 20 indicator is also quite strong for terrestrial acidification. The main drawback with the 21 understanding of terrestrial acidification is that the data are based on laboratory responses rather 22

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73 than field measurements. Other stressors that are present in the field b ut that are not present in 1 the laboratory

may confound this relationship. 2 For nutrient enrichment effects, the REA utilized different types of indicators for aquatic 3 and terrestrial effects to assess both the likelihood of adverse effects to ecosy stems an d the 4 relationship between adverse effects and atmospheric sources of oxides of nitrogen. The 5 ecological indicator chosen for aquatic nutrient enrichment, the ASSETS EI, seems to be 6 inadequate to relate atmospheric deposition to the targeted ecological ef fect, likely

due to the 7 many

other confounding factors. Further, there is far less confidence associated with the 8 understanding of aquatic nutrient enrichment because of the large contributions from non

9 atmospheric sources of nitrogen and the influence of both oxidized and reduced forms of 10 nitrogen, particularl y in large watersheds and coastal areas. However, a strong relationship exists 11 between atmospheric deposition of nitrogen and ecological effects in high alpine lakes in the 12 Rocky

Mountains because atm ospheric deposition is the only source of nitrogen to these 13 sy stems. There is also a strong weight

- of

- evidence regarding the relationships between 14 ecological effects attributable to terrestrial nitrogen nutrient enrichment; however, ozone and 15 climate chang e may be confounding factors. In addition, the response for other species or species 16 in other regions of the United States has not been quantified. 17 C. Adversity of Effects to Public Welfare 18 Characterizing a known or anticipated adverse effect to public welfare is an important 19 component of developing any secondary

NAAQS. According to the Clean Air Act, welfare 20 effects include: “ effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, 21 weather, visibility , and climate, damage to and dete rioration of property, and hazards to 22 transportation, as well as effect on economic values and on personal comfort and well

- being, 23

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74 whether caused b y transformation, conversion, or combination with other air pollutants ” (CAA, 1 Section 302(h)). While the text above lists a number of welfare effects, these effects do not 2 define public welfare in and of themselves. 3 Although there is no specific definition of adversity to public welfare, the paradigm of 4 linking adversity to public welfare to disruptions in ecos ys tem structure and function has been 5 used broadl y b y EPA to categorize effects of pollutants from the cellular to the ecos ys tem level. 6 An evaluation of adversity to public welfare might consider the likelihood, ty pe, magnitude, and 7 spatial scale of the e ffect as well as the potential for recovery and any uncertainties relating to 8 these considerations. 9 Similar concepts were used in past reviews of secondary NAAQS for ozone and PM 10 (relating to visibility ), as well as in initial reviews of effects from l ead deposition. Because 11 oxides of nitrogen and sulfur are deposited from ambient sources into ecosy stems where they 12 affect changes to organisms, populations and ecosystems, the concept of adversity to public 13 welfare as a result of alterations in structure and function of ecos ys tems is an appropriate 14 consideration for this review. 15 Based on information provided in the Policy Assessment , the following section discusses 16 how ecological effects from deposition of oxides o f nitrogen and sulfur relate to adversity to 17 public welfare

. In the Policy Assessment , p ublic welfare was discussed in terms of loss of 18
ecos ys tem services (defined below) , which in some cases can be monetized. Each of the four 19 main effect areas (aquatic and terrestrial acidification and aquatic and terrestrial nutrient over

20 enrichment) are discussed including current ecological effects and associated ecos ys tem services, 21 including potential value of the services. The quantified indicators of value discuss ed below 22

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75 relate to ecos ys tem services generall y that are impacted b y deposition related acidification, not to 1 the specific effects of this action. 2 1. Ecosystem Services 3 The Policy

Assessment defines ecosy stem services as the benefits individuals and 4 organizations obtain from ecosy stems. Ecos ys tem services can be classified as provisioning 5 (food and water), regulating (control of climate and disease), cultural (recreational, existence, 6 spiritual, educational), and supporting (nutrient cycling). Conceptually , changes in ecos ys tem 7 services may

be used to aid in characterizing a known or anticipated adverse effect to public 8 welfare. In the REA and Policy Assessment ecosy stem services are discussed as a method of 9 assessing the magnitude and significance to the public of resource s affected by ambient 10 concentrations of oxides of nitrogen and sulfur and deposition in sensitive ecos ys tems
. 11 EPA has in previous NAAQS reviews defined ecological goods and services for the 12 purposes of a Regulatory Impact Anal ys is as the “outputs of ecological functions or processes 13 that directly

or indirectl y contribute to social welfare or have the potential to do so in the future. 14 Some outputs may

be bought and sold, but most are not marketed.” It is e specially important to 15
a cknowledge that it is difficult to measure and/or monetize the goods and services supplied by 16
ecos ys tems. It can be informative in characterizing adversity to public welfare to attempt to 17 place an economic valuation on the set of goods and services that h ave been identified with 18 respect to a change in policy however it must be noted that this valuation will be incomplete and 19 illustrative only

. 20 Knowledge about the relationships linking ambient concentrations and ecosy stem 21 services is considered in the Pol icy Assessment as one method by which to inform a policy 22 judgment on a known or anticipated adverse public welfare effect. For example, a change in an 23

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76 ecos ys tem structure and process, such as foliar injury , would be classified as an ecological effect, 1 with the associated changes in ecos ys tem services, such as primary productivity , food 2 availability , forest products, and aesthetics (e.g., scenic viewing), classified as public welfare 3 effects. Additionally , changes in biodiversity would be classified as an ecological effect, and the 4 associated changes in ecosy stem services

— productivity , existence (nonuse) value, recreational 5 viewing and aesthetics

—would also be classified as public welfare effects. 6 As described in Chapters 4 and 5 of the REA , case study analy ses were performed that 7 link deposition in sensitive ecos ys tems to changes in a given ecological indicator (e.g., for 8 aquatic acidification, to changes in acid neutralizing capacity [ANC]) and then to changes in 9
ecos ys tems. Appendix 8 of the REA links the changes in ecos ys tems to the services they 10 provide (e.g., fish species richness and its influence on recreational fishing). To the extent 11 possible for each targeted effect area, the REA linked ambient concentrations of nitrogen and 12 sulfur (i.e., ambient air quality indicators) to deposition in sensitive ecosy stems (i.e., exposure 13 pathway s), and then to system response as measured by a given ecological indicator (e.g., lake 14 and stream acidification as measured b y ANC). The ecological effect (e.g., chang es in fish 15 species richness) was then, where possible, associated with changes in ecosy stem services and 16 the corresponding public welfare effects (e.g., recreational fishing). 17 2. Effects on Ecosystem Services 18 The process used to link ecological indicator s to ecosy stem services is discussed 19 extensively

in Appendix 8 of the REA. In brief, for each case study area assessed the ecological 20 indicators are linked to an ecological response that is subsequently linked to associated services 21 to the extent possible . For example in the case study for aquatic acidification the chosen 22 ecological indicator is ANC which can be linked to the ecos ys tem service of recreational fishing. 23

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77 Although recreational fishing losses are the onl y s ervice effects that can be independentl y 1 quantified or monetized at this time, there are numerous other ecos ys tem services that may be 2 related to the ecological effects of acidification. 3 While aquatic acidification is the focus of this proposed standard, the other effect areas 4 were also analy

zed in the REA and these ecos ys tems are being harmed by nitrogen and sulfur 5 deposition and will obtain some measure of protection with any decrease in that deposition 6 regardless of the reason for the decrease. The following summarizes the current levels of specific 7
ecos ys tem services for aquatic and terrestrial acidification, and aquatic and terrestrial nutrient 8
over - enrichment and attempts to quantify and when possible monetize the harm to public 9 welfare, as represented by ecos ys tem services, due to nitrogen and sulfur deposition. 10
a. Aquatic Acidification 11 Acidification of aquatic ecosy stems primarily affects the ecos ys tem services that are 12 derived from the fish and other aquatic life found in surface waters. In the northeastern United 13 States, the surface waters affected by acidification are not a major source of commerciall y raised 14 or caught fish; however, they are a source of food for some recreational and subsistence fishers 15 and for other consumers. Although data and models are available for examining th e effects on 16 recreational fishing, relatively little data are available for measuring the effects on subsistence 17 and other consumers. Inland waters also provide aesthetic and educational services along with 18
non - use services, such as existence value (prote ction and preservation with no expectation of 19 direct use). In general, inland surface waters such as lakes, rivers, and streams also provide a 20 number of regulating services, playing a role in hydrological regimes and climate regulation. 21 There is little ev idence that acidification of freshwaters in the northeastern United States has 22 significantl y degraded these specific services; however, freshwater ecos ys tems also provide 23

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78 biological control services by providing environments that sustain delicate aquatic f ood chains. 1 The toxic effects of acidification on fish and other aquatic life impair these services b y disrupting 2 the trophic structure of surface waters. Although it is difficult to quantify these services and how 3 they are affected by

acidification, it is worth noting that some of these services may be captured 4 through measures of provisioning and cultural services. For example, these biological control 5 services may

serve as “intermediate” inputs that support the production of “final” recreational 6 fishin g and other cultural services. 7 As summarized in Chapter 4 of the Policy Assessment , recent studies indicate that 8 acidification of lakes and streams can result in significant loss in economic value. For example, 9 data indicate that more than 9% of adults in the northeastern part of the country participate 10 annually

in freshwater fishing yielding 140 million freshwater fishing day s. Each fishing day 11 has an estimated average value per day of $35. Therefore, the implied total annual value of 12 freshwater fishing in the northeastern United States was $5 billion in 2006. Embedded in these 13 numbers is a degree of harm to recreational fishing services due to acidification that has occurred 14 over time. These harms have not been quantified on a regional scale; however, a case study was 15 conducted in the Adirondacks area ( US EPA, 20 11 , section 4.4.2). 16 In the Adirondacks case study , estimates of changes in recreational fishing services were 17 determined, as well as changes more broadl y in “cultural” ecos ys tem services (in cluding 18 recreational, aesthetic, and nonuse services). First, the MAGIC model (U S EPA, 2009, 19 Appendix 8 and s ec tion 2.2) was applied to 44 lakes to predict what ANC levels would be under 20 both “business as usual” conditions (i.e., allowing for some declin e in deposition due to existing 21 regulations) and pre

- emission (i.e., background) conditions. Second, to estimate the recreational 22 fishing impacts of aquatic acidification in these lakes, an existing model of recreational fishing 23

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79 demand and site choice was applied. This model predicts how recreational fishing patterns in the 1 Adirondacks would differ and how much higher the average annual value of recreational fishing 2 services would be for New York residents if lake ANC levels corresponded to background 3 (rather than business as usual) conditions. To estimate impacts on a broader category of cultural 4 (and some provisioning) ecos ys tem services, results from the Banzhaf et al (2006) valuation 5 survey

of New York residents were adapted and applied to this conte xt. The survey used a 6 contingent valuation approach to estimate the average annual household WTP for future 7 reductions in the percent of Adirondack lakes impaired b y acidification. The focus of the survey 8 was on impacts on aquatic resources. Pretesting o f the survey indicated that respondents 9 nonetheless tended to assume that benefits would occur in the condition of birds and forests as 10 well as in recreational fishing. 11 By

extrapolating the 44 lake Adiriondack case study to all 3,000 Adirondack lakes and by 12 applying the willingness to pay survey results to all New York residents, the study estimated 13 aggregated benefits between $300 and $800 million annually for the equivalent of improving 14 lakes in the Adirondacks region to an ANC level of 50 μeq/L . The REA estimated 44% of the 15 Adirondack lakes currently fall below an ANC of 50 μeq/L . Several states have set goals for 16 improving the acid status of lakes and streams, generall y targeting ANC in the range of 50 to 60 17 μeq/L , and have engaged in costly activiti es to decrease acidification. 18 These results imply significant value to the public in addition to those derived from 19 recreational fishing services. Note that the results are onl y applicable to improvements in the 20 Adirondacks valued b y residents of New Y ork. If similar benefits exist in other acid

- impacted 21 areas, benefits for the nation as a whole could be substantial. The anal ys is provides results on 22

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80 only a subset of the impacts of acidification on ecosy stem services and suggests that the overall 1 impact on these services is likely to be substantial. 2 b. Terrestrial Acidification 3 Chapter 4.4.3 and 4.4.4 of the Policy Assessment review several economic studies of 4 areas sensitive to terrestrial acidification. Forests in the northeastern United States provide 5 several important and valuable provisioning ecos ystem services, which are reflected in the 6 production and sales of tree products. Sugar maples are a particularly important commercial 7 hardwood tree species in the United States, producing timber and maple s y rup that provide 8 hundreds of millions of dollars in economic value annually . Red spruce is also used in a variety 9 of wood products and provides up to $100 million in economic value annually . Although the 10 data do not exist to directly link acidification damages to economic values of lost recreational 11
ecos ys tem services in forests, these resources are valuable to the public. A recent study, 12 reviewed in the Policy Assessment, suggests that the total annual value of recreational off

- road 13 driving was more th an $9 billion and the value of hunting and wildlife viewing was more than $4 14 billion each in the northeastern United States. EPA is not able to quantify at this time the 15 specific effects on these values of acid deposition, or of any specific reductions i n deposition, 16 relative to the effects of many other factors that may

affect them. 17

c. Nutrient Enrichment 18 Chapter 4.4.5 and 4.4.6 of the Policy Assessment summarize economic studies of east 19 coast estuaries affected by nutrient over

- enrichment or eutrophicatio n. Estuaries in the eastern 20 United States are important for fish and shellfish production. The estuaries are capable of 21 supporting large stocks of resident commercial species, and they serve as the breeding grounds 22 and interim habitat for several migratory species. To provide an indication of the magnitude of 23

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81 provisioning services associated with coastal fisheries, from 2005 to 2007, the average value of 1 total catch was $1.5 billion per year in 15 East Coast states. Estuaries also provide an important 2 and substantial variety of cultural ecos ys tem services, including water

- based recreational and 3 aesthetic services. For example, data indicate that 4.8% of the population in coastal states from 4 North Carolina to Massachusetts participated in saltwater fishing, with a total of 26 million 5 saltwater fishing day s in 2006. Based on estimates in the Policy Assessment , total recreational 6 value from these saltwater fishing days was approximately $1.3 billion. Recreational 7 participation estimates for 1999

–2000 showed almost 6 million individuals participated in 8 motorboating in coastal states from North Carolina to Massachusetts. The aggregate value of 9 these coastal motorboating outings was $2 billion per year. EPA is not able to quantify at this 10 time the specific effe cts on these values of acid deposition, or of any specific reductions in 11 deposition, relative to the effects of many other factors that may affect them. 12 Terrestrial ecos ys tems can also suffer from nutrient over

- enrichment. Each ecos ys tem is 13 different in its composition of species and nutrient requirements. Changes to individual 14
ecos ys tems from changes in nitrogen deposition can be hard to assess economically . Relative 15 recreational values are often determined b y public use information. Chapter 4.4.7 of the Policy 16 Assessment reviewed studies related to park use in California. Data from California State Parks 17 indicate that in 2002, 68.7% of adult residents participated in trail hiking for an average of 24.1 18 day s per year. The anal ys es in the Policy Assess ment indicate that the aggregate annual benefit 19 for California residents from trail hiking in 2007 was $11.59 billion. EPA is not able to quantify 20 at this time the specific effects on these values of acid deposition, or of any specific reductions in 21 depos ition, relative to the effects of many other factors that may affect them. 22

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82 The Policy

Assessment also identified fire regulation as a service that could be affected 1 by nutrient over

- enrichment of the Coastal Sage Scrub and Mixed Conifer Forest ecos ys tems by 2 encouraging growth of more flammable grasses, increasing fuel loads, and altering the fire cycle. 3 Over the 5

- ye ar period from 2004 to 2008, Southern California experienced, on average, over 4 4,000 fires per ye ar, burning, on average, over 400,000 acres p er ye ar. It is not possible at this 5 time to quantify

the contribution of nitrogen deposition, among many other factors, to increased 6 fire risk. 7 3. Summary 8 Adversity

to public welfare can be understood by looking at how deposition of oxides of 9 nitro gen and sulfur affect the ecological functions of an ecos ys tem (see II.A.), and then 10 understanding the ecosy stem services that are degraded. The monetized value of the ecos ys tem 11 services provided b y ecosystems that are sensitive to deposition of oxides of nitrogen and sulfur 12 are in the billions of dollars each ye ar , though it is not possible to quantify or monetize at this 13 time the effects on these values of acid deposition or of any changes in deposition that may result 14 from new secondary stan dards . Man y lakes and streams are known to be degraded b y acidic 15 deposition which affects recreational fishing and tourism. Forest growth is likely suffering from 16 acidic deposition in sensitive areas affecting red spruce and sugar maple timber production , sugar 17 maple sy

rup production, hiking, aesthetic enjoy ment and tourism. Nitrogen deposition 18 contributes significantly to eutrophication in many estuaries affecting fish production, 19 swimming, boating, aesthetic enjo ym ent, and tourism. Ecosy stem services are likel y affected by 20 nutrient enrichment in many natural and scenic terrestrial areas , affecting biodiversity , including 21 habitat for rare and endangered species, fire control, hiking, aesthetic enjoyment, and tourism. 22 D. Adequ acy of the Current Standards 23

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83 An important issue to be addressed in the current review of the secondary standards for 1 oxides and nitrogen and sulfur standard is whether, in view of the scientific evidence reflected in 2 the I SA, additional information on exp osure and risk discussed in the REA, and conclusions 3 drawn from the Policy Assessment , the existing standards provide adequate protection. The 4 Administrator therefore, has considered the extent to which the current standards are adequate 5 for the protecti on of public welfare. Having reached th e general conclusion that aquatic and 6 terrestrial ecos ys tems can be degrade b y deposition of oxides of nitrogen and sulfur , it is then 7 necessary

to evaluate the adequacy of the current secondary standards for oxides of nitrogen and 8 sulfur to provide requisite protection by considering to what degree risks to sensitive ecos ys tems 9 would be expected to occur in areas that meet the current standards. Conclusions regarding the 10 adequacy

of the current standards are based on the available ecological effects, exposure and 11 risk

- based evidence. In evaluating the strength of this information, EPA has taken into account 12 the uncertainties and limitations in the scientific evidence. This section addresses the adequacy 13 of the current standards to protect against direct exposure effects on plants from oxides of 14 nitrogen and sulfur, the appropriateness of the current structure of the standards to address 15 deposition

- related effects of oxides of nitrogen and sulfur on sensitive ecosy stems and finally , 16 the adequacy

of such standards to protect against adverse effects related to the deposition of 17 oxides of nitrogen and sulfur. 18 1. Adequacy of the Current Standards for Direct Effects 19 The current secondary oxides of nitrogen and sulfur standards are intended to protect 20 against adverse effects to public welfare. For oxides of nitrogen, the current secondary standard 21

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84 was set identical to the primary standard 3 , e.g. an annual standard set for NO 2 to protect against 1 adverse effects on vegetation from direct exposure to ambient oxides of nitrogen. For oxides of 2 sulfur, the current secondary standard is a 3

- hour standard intended to provide protection for 3 plants from the direct foliar damage associated with atmospheric concentrations of SO 2 . It is 4 appropriate to consider whether the current standards are adequate to protect against the direct 5 effects on vegetatio n resulting from ambient NO 2 and SO 2 which were the basis for the current 6 secondary

standards. The I SA concluded that there was sufficient evidence to infer a causal 7 relationship between exposure to SO 2 , NO, NO 2 and PAN and injury to vegetation. Addition al 8 research on acute foliar injury has been limited and there is no evidence to suggest foliar injury 9 below the levels of the current secondary standards for oxides of nitrogen and sulfur. There is 10 sufficient evidence to suggest that the levels of the cur rent standards are likely

adequate to 11 protect against direct phytotoxic effects. 12 2. Appropriateness and Adequacy of the Current Standards for Deposition

- related Effects 13 This section addresses two concepts necessary to evaluate the current standards in the 14 context of deposition related effects. First, appropriateness of the current standards is considered 15 with regard to indicator, form, level and averaging time. This discussion centers around the 16 ability

of the current standards to evaluate and provide protection against deposition related 17 effects that vary

spatiall y and temporall y. It includes particular emphasis on the indicators and 18 forms of the current standards and the degree to which they are ecologically

relevant with regard 19 to deposition related effects. Second, this section evaluates the current standards in terms of 20 adequacy

of protection. 21
a. Appropriateness 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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85 The I SA has established that the major effects of concern for this review of the oxides of 1 nitrogen and sulfur standards are a ssociated with deposition of N and S caused by atmospheric 2 concentrations of oxides of nitrogen and sulfur. The current standards are not directed toward 3 depositional effects, and none of the elements of the current NAAQS

– indicator, form, 4 averaging time , and level

– are suited for addressing the effects of N and S deposition. 5 Four issues arise that call into question the ecological relevance of the structure of the 6 current secondary

standards for oxides of nitrogen and sulfur. 7 (1) The current SO 2 seco ndary standard (0.5 ppm SO 2 over a 3

- hour average) does not utilize an 8 exposure period that is relevant for ecos ys tem impacts. The majority of deposition related 9 impacts are associated with depositional loads that occur over periods of months to years. 10 T his differs significantl y from exposures associated with hourly concentrations of SO 2 as 11 measured b y the current secondary standard. By addressing short

- term concentrations, the 12 current SO 2 secondary standard, while protective against direct foliar effect s from gaseous 13 oxides of sulfur, does not take into account the findings of effects in the I SA, which notes the 14 relationship between annual deposition of S and acidification effects which are likel y to be 15 more severe and widespread than phyt otoxic effects under current ambient conditions, and 16 include effects from long term deposition as well as short term. Acidification is a process 17 which occurs over time, as the ability of an aquatic sy stem to counteract acidic inputs is 18 reduced as natural buffers are use d more rapidly than they can be replaced through geologic 19 weathering. The relevant period of exposure for ecosy stems is therefore not the exposures 20 captured in the short averaging time of the current SO 2 secondary standard. The current 21 secondary

standard f

or oxides of nitrogen is an annual standard (0.053 ppm averaged over 1 22 ye ar) and as such is more ecologicall y relevant. Current standards do not utilize appropriate 23

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86 atmospheric indicators. NO 2 and SO 2 are used as the component of oxides of nitrogen and 1
s ulfur that are measured, but they do not provide a complete link to the direct effects on 2
ecos ys tems from deposition of oxides of nitrogen and sulfur as they do not capture all 3 relevant chemical species of oxidized nitrogen and oxidized sulfur that contrib ute to 4 deposition. The I SA provides evidence that deposition related effects are linked with total 5 nitrogen and total sulfur deposition, and thus all forms of oxidized nitrogen and oxidized 6 sulfur that are deposited will contribute to effects on ecos ys tem s. Thus, by using 7 atmospheric NO 2 and SO 2 concentrations as indicators, the current standards address onl y a 8 fraction of total atmospheric oxides of nitrogen and sulfur, and do not take into account the 9 effects from deposition of total atmospheric oxides of nitrogen and sulfur. This suggests that 10 more comprehensive atmospheric indicators should be considered in designing ecologicall y 11 relevant standards. 12 (2) Current standards reflect separate assessments of the two individual pollutants, NO 2 and SO 2 , 13 rather than assessing the joint impacts of deposition to ecos ys tems. Recognizing the role that 14 each pollutant play s in jointly affecting ecos ys tem indicators, functions, and services is vital 15 to developing a meaningful standard. The clearest example of this int eraction is in 16 assessment of the impacts of acidifying deposition on aquatic ecosy stems. Acidification in 17 an aquatic ecos ys tem depends on the total acidifying potential of the deposition of both N 18 and S from both atmospheric deposition of oxides of nitrog en and sulfur as well as the inputs 19 from other sources of N and S such as reduced nitrogen and non

- atmospheric sources. It is 20 the joint impact of the two pollutants that determines the ultimate effect on organisms within 21 the ecos ys tem, and critical ecos ys t em functions such as habitat provision and biodiversity

. 22 Standards that are set independentl y are less able to account for the contribution of the other 23

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87 pollutant. This suggests that interactions between oxides of nitrogen and oxides of sulfur 1 should be a critical element of the conceptual framework for ecologically relevant standards. 2 There are also important interactions between oxides of nitrogen and sulfur and reduced 3 forms of nitrogen, which also contribute to acidification and nutrient enrichment. It is 4 important that the structure of the standards address the role of reduced nitrogen in 5 determin ing the ecological effects resulting from deposition of atmospheric oxides of 6 nitrogen and sulfur. Consideration will also have to be given to total loadings as ecos ys tems 7 respond to all sources of N and S. 8 (3) Current standards do not take into account varia bility in ecos ys tem sensitivity . Ecos ys tems 9 are not uniforml y distributed either spatiall y or temporally in their sensitivity to oxides of 10 nitrogen and sulfur . Therefore, failure to account for the major determinants of variability , 11 includin g geological and soil characteristics related to the sensitivity to acidification or 12 nutrient enrichment as well as atmospheric and landscape characteristics that govern rates of 13 deposition, may lead to standards that do not provide requisite levels of pro tection across 14
ecos ys tems. The current structures of the standards do not address the complexities in the 15 responses of ecos ys tems to deposition of oxides of nitrogen and sulfur. Ecosy stems contain 16 complex groupings of organisms that respond in various ways to the alterations of soil and 17 water that result from deposition of nitrogen and sulfur compounds. Different ecos ys tems 18 therefore respond in different way s depending on a multitude of factors that control how 19 deposition is integrated into the sy stem. For example, the same levels of deposition falling 20 on limestone dominated soils have a very different effect from those falling on shallow 21 glaciated soils underlain with granite. One s ys tem may over time display

no obvious 22 detriment while the other m ay experience a catastrophic loss in fish communities. This 23

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88 degree of sensitivity

is a function of many

atmospheric factors which control rates of 1 deposition as well as ecological factors which control how an ecosy stem responds to that 2 deposition. The current standards do not take into account spatial and seasonal variations not 3 only

in depositional loadings but also in sensitivity of ecos ys tems exposed to those loadings. 4 Based on the discuss ion summarized above, the Policy Assessment concludes that the current 5 secondary

standards for oxides of nitrogen and oxides of sulfur are not ecologically relevant 6 in terms of averaging time, form, level or indicator. 7 b. Adequacy of Protection 8 As described in the Policy Assessment , 2005 ambient conditions indicate that the current 9 SO 2 and NO 2 secondary standards are not exceeded (US EPA, 20 11 , Figures 6

- 1 and 6

- 2) in 10 locations where ecological effects have been observed. In many locations, SO 2 and NO 2 11 concentrations are substantially below the levels of the secondary standards. This pattern 12 suggests that levels of deposition and any effects on ecos ys tems due to deposition of oxides of 13 nitrogen and sulfur under recent conditions are occurring even though areas meet or are below 14 current standards. In addition, these l evels based on conclusions in the REA will not decline in 15 the future to levels below which it is reasonable to anticipate effects. 16 Indetermining the adequacy of the current secondary standards for oxides of nitrogen 17 and sulfur the Policy Assessment cons idered the extent to which ambient deposition contributes 18 to loadings in ecos ys tems. Since the last review of the secondary standard for oxides of nitrogen, 19 a great deal of information on the contribution of atmospheric deposition associated with 20 ambient oxides of nitrogen has become available. The REA presents a thorough assessment of 21 the contribution of oxidized nitrogen to nitrogen deposition throughout the U.S., and the relative 22 contributions of ambient oxidized and reduced forms of nitrogen. The REA concludes that based 23

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89 on that anal ys is, ambient oxides of nitrogen are a significant component of atmospheric nitrogen 1 deposition, even in areas with relatively high rates of deposition of reduced nitrogen. In 2 addition, atmospheric deposition of oxidized nitrogen contributes significantly to total nitrogen 3 loadings in nitrogen sensitive ecosy stems. 4 The I SA summarizes the available studies of relative nitrogen contribution and finds that 5 in much of the U.S., oxides of nitrogen contribute from 50 to 75 percent of total atmospheric 6 deposition relative to total reactive nitrogen that includes oxidized and reduced nitrogen species 7 (US EPA, 2008, section 2.8.4 ). Although the proportion of total nitrogen loadings associated 8 with atmospheric deposition of nitrog en varies across locations (N deposition in the eastern U.S. 9 includes locations with greater than 9 kg N/ha

- yr , and in the central U.S. high deposition 10 locations with values on the order of 6 to 7 kg N/ha

- yr) , the IS A indicates that atmospheric N 11 depositi on is the main source of new anthropogenic N to most headwater streams, high elevation 12 lakes, and low

- order streams. Atmospheric N deposition contributes to the total N load in 13 terrestrial, wetland, freshwater, and estuarine ecosy stems that receive N throu gh multiple 14 pathway s. In several large estuarine s ys tems, including the Chesapeake Bay , atmospheric 15 deposition accounts for between 10 and 40 percent of total nitrogen loadings (U S EPA, 2008 ). 16 Atmospheric concentrations of oxides of sulfur account for nearl y all S deposition in the 17 US. For the period 2004

–2006, mean S deposition in the U.S. was greatest east of the 18 Mississippi River with the highest deposition amount, 21.3 kg S/ha

- yr, in the Ohio River Valley 19 where most recording stations reported 3 ye ar averages >10 kg S/ha

- yr . Numerous other stations 20 in the East reported S deposition >5 kg S/ha

- yr. Total S deposition in the U.S. west of the 100th 21 meridian was relativel y low, with all recording stations reporting <2 kg S/ha

- yr and many 22 reporting <1 kg S/ha

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

− followed in 23

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90 decreasing order b y a smaller proportion of dry SO 2 and a much smaller proportion of deposition 1 as dry

SO 4 2

−

. 2 As discussed throughout the REA (US EPA, 200 9 and s ection IIB above) , there are 3 several key

areas of risk that are associated with ambient concentrations of oxides of nitrogen 4 and sulfur. As noted earlier, in previous reviews of the secondary standards for oxides of 5 nitrogen and sulfur, the standards were designed to prot ect against direct exposure of plants to 6 ambient concentrations of the pollutants. A significant shift in understanding of the effects of 7 oxides of nitrogen and sulfur has occurred since the last reviews, reflecting the large amount of 8 research that has b een conducted on the effects of deposition of nitrogen and sulfur to 9
ecos ys tems. The most significant current risks of adverse effects to public welfare are those 10 related to deposition of oxides of nitrogen and sulfur to both terrestrial and aquatic ecosy stems. 11 These risks fall into two categories, acidification and nutrient enrichment, which were 12 emphasized in the REA as most relevant to evaluating the adequacy of the existing standards in 13 protecting public welfare from adverse ecological effects. 14 i. Aqu atic acidification 15 The focus of the REA case studies was on determining whether deposition of sulfur and 16 oxidized nitrogen in locations where ambient oxides of nitrogen and sulfur were at or below the 17 current standards was resulting in acidification and related effects, including episodic 18 acidification and mercury methyl ation. Based on the case studies conducted for lakes in the 19 Adirondacks and streams in Shenandoah National Park (case studies are discussed more full y in 20
s ection IIB and US EPA, 2009 ), th ere is significant risk to acid sensitive aquatic ecos ys tems at 21 atmospheric concentrations of oxides of nitrogen and sulfur at or below the current standards. 22 The REA also supports strongl y a relationship between atmospheric deposition of oxides of 23

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91 nitrog en and sulfur and loss of ANC in sensitive ecosy stems and indicates that ANC is an 1 excellent indicator of aquatic acidification. The REA also concludes that at levels of deposition 2 associated with oxides of nitrogen and sulfur concentrations at or below the current standards, 3 ANC levels are expected to be below benchmark values that are associated with significant 4 losses in fish species richness. 5 Significant portions of the U.S. are acid sensitive, and current deposition levels exceed 6 those that would all ow recovery of the most acid sensitive lakes in the Adirondacks (US EPA, 7 2008, Executive Summary). In addition, because of past loadings, areas of the Shenandoah are 8 sensitive to current deposition levels (US EPA, 2008, Executive Summary). Parts of the W est 9 are naturall y less sensitive to acidification and subjected to lower deposition (particularl y SO x ) 10 levels relative to the eastern United States, and as such, less focus in the ISA is placed on the 11 adequacy

of the existing standards in these areas, with the exception of the mountainous areas of 12 the West, which experience episodic acidification due to deposition. 13 In describing the effects of acidification in the two case study areas the REA uses the 14 approach of describing benchmarks in terms of ANC valu es. Man y locations in sensitive areas of 15 the U.S. have ANC levels below benchmark levels for ANC classified as severe, elevated, or 16 moderate concern (US EPA, 20 11 , Figure 2

- 1). The average current ANC levels across 44 lakes 17 in the Adirondack case study are a is 62.1 μeq/L (moderate concern). However, 44 percent of 18 lakes had deposition levels exceeding the critical load for an ANC of 50 μeq/L , and 28 percent of 19 lakes had deposition levels exceeding the (higher) critical load for an ANC of 20 μeq/L(US 20 EPA, 20 0 9 , section 4.2.4.2). This information indicates that almost half of the 44 lakes in the 21 Adirondacks case study area are at an elevated concern level , and almost a third are at a severe 22 concern level. These levels are associated with greatly diminished f

ish species diversity , and 23

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92 losses in the health and reproductive capacity of remaining populations. Based on assessments of 1 the relationship between number of fish species and ANC level in both the Adirondacks and 2 Shenandoah areas, the number of fish speci es is decreased b y over half at an ANC level of 20 3 μeq/Lrelative to an ANC level at 100 μeq/L (US EPA, 200 9, Figure 4.2

- 1). When extrapolated 4 to the full population of lakes in the Adirondacks area using weights based on the EMAP 5 probability

survey (US E PA, 200 9 , section 4.2.6.1 **)** , 36 percent of lakes exceeded the critical load 6 for an ANC of 50 μeq/L and 13 percent of lakes exceeded the critical load for an ANC of 20 7 μeq/L

. 8 Many

streams in the Shenandoah case study area also have levels of deposition that are 9 associated with ANC levels classified as severe, elevated, or moderate concern. The average 10 ANC under recent conditions for the 60 streams evaluated in the Shenandoah case study area is 11 57.9 μeq/L , indicating moderate concern. However, 85 percent of these streams had recent 12 deposition exceeding the critical load for an ANC of 50 μeq/L , and 72 percent exceeded the 13 critical load for an ANC of 20 μeq/L

. As with the Adirondacks area, this information suggests 14 that significant numbers of sensitive streams in the Shenandoah area are at risk of adverse 15 impacts on fish populations under recent conditions. Many other streams in the Shenandoah area 16
are also likel y to experience conditions of elevated to severe concern based on the prevalence in 17 the area of bedro ck geology associated with increased sensitivity to acidification suggesting that 18 effects due to stream acidification could be widespread in the Shenandoah area (US EPA, 200 9 , 19 section 4.2.6.2). 20 In addition to these chronic acidification effects, the I SA n otes that “consideration of 21 episodic acidification greatly increases the extent and degree of estimated effects for acidifying 22 deposition on surface waters.” (US EPA, 2008, section 3.2.1.6 ) Some studies show that the 23

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93 number of lakes that could be classifi ed as acid

- impacted based on episodic acidification is 2 to 3 1 times the number of lakes classified as acid

- impacted based on chronic ANC. These episodic 2 acidification events can have long term effects on fish populations (US EPA, 2008, section 3 3.2.1.6 ). Under recent conditions, episodic acidification has been observed in locations in the 4 eastern U.S. and in the mountainous western U.S. (US EPA, 2008, section 3.2.1.6 ). 5 The I SA, REA and Policy Assessment all conclude that the current standards are not 6 ade quate to protect against the adverse impacts of aquatic acidification on sensitive ecos ys tems. 7 In the IS A it is noted that significant portions of the U.S. are acid sensitive, and that current 8 deposition levels exceed those that would allow recovery of th e most acid sensitive lakes in the 9 Adirondacks ( US EPA, 2008, Executive Summary). In addition, because of past loadings, areas 10 of the Shenandoah are sensitive to current deposition levels (US EPA, 2008, Executive 11 Summary). A recent survey , as reported in the ISA, found sensitive streams in many locations in 12 the U.S., including the Appalachian Mountains, the Coastal Plain, and the Mountainous West 13 (US EPA, 2008, section 4.2.2.3). In these sensitive areas, between 1 and 6 percent of stream 14 kilometers are chro nically acidified. The REA further concludes that both the Adirondack and 15 Shenandoah case study areas are currentl y receiving deposition from ambient oxides of nitrogen 16 and sulfur in excess of their ability to neutralize such inputs. In addition based on the current 17 emission scenarios, forecast modeling out to the year 2020 as well as 2050 indicates a large 18 number of streams in these areas will still be adversely impacted ( s ection IIB). Based on these 19 considerations, the Policy Assessment concludes that the current secondary NAAQS for oxides 20 of nitrogen and sulfur do not provide adequate protection of sensitive ecos ystems with regard to 21 aquatic acidification

. 22 ii. Terrestrial acidification 23

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94 Based on the terrestrial acidification case studies (Kane Experimental Forest in 1 Pennsylvania and Hubbard Brook Experimental Forest described in section IIB) on sugar maple 2 and red spruce habitat, the REA concludes that there is significant risk to sensitive terrestrial 3
ecos ys tems from acidification at atmospheric concentrations of NO x and SO X at or below the 4 current standards. The ecological indicator selected for terrestrial acidification is the base 5 cation to aluminum ratio (BC:Al), which has been linked to tree health and growth. The results 6 of the REA strongly support a relationship between atmospheric deposition of oxides of nitrogen 7 and sulfur and BC:Al, and that BC:Al is a good indicator of terrestrial acidification. At levels of 8 deposition associated with oxides of nitrogen and sulfur concentrations at or below the current 9 standards, BC:Al levels are expected to be below benchmark values that are associated with 10 significant effects on tree health and growth. Such degradation of terrestrial ecos ys tems could 11 affect ecos ys tem services such as habitat provisioning, endangered species, goods production 12 (timber, sy

rup, etc.) among others. 13 Many

locations in sensitive areas of the U.S. have BC:Al levels below benchmark levels 14 classified as providing low to intermediate levels of protecti on to tree health. At a BC:Al ratio of 15 1.2 (intermediate level of protection), red spruce growth can be reduced b y 20 percent. At a 16 BC:Al ratio of 0.6 (low level of protection), sugar maple growth can be decreased b y 20 percent. 17 The REA did not evaluat e broad sensitive regions. However, in the sugar maple case study area 18 (Kane Experimental Forest), recent deposition levels are associated with a BC:Al ratio below 19 1.2, indicating between intermediate and low level of protection, which would indicate the 20 potential for a greater than 20 percent reduction in growth. In the red spruce case study area 21 (Hubbard Brook Experimental Forest), recent deposition levels are associated with a BC:Al ratio 22

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95 slightly above 1.2, indicating slightl y better than an intermedi ate level of protection (US EPA, 1 200 9 , section 4.3.5.1 ). 2 Over the full range of sugar maple, 12 percent of evaluated forest plots exceeded the 3 critical loads for a BC:Al ratio of 1.2, and 3 percent exceeded the critical load for a BC:Al ratio 4 of 0.6. However, there was large variability across states. In New Jersey , 67 percent of plots 5 exceeded the critical load for a BC:Al ratio of 1.2, while in several states on the outskirts of the 6 range for sugar maple (e.g. Arkansas, Illinois) no plots exceeded t he critical load for a BC:Al 7 ratio of 1.2. For red spruce, overall 5 percent of plots exceeded the critical load for a BC:Al ratio 8 of 1.2, and 3 percent exceeded the critical load for a BC:Al ratio of 0.6 . In the major red spruce 9 producing states (Maine , New Hampshire, and Vermont), critical loads for a BC:Al ratio of 1.2 10 were exceeded in 0.5, 38, and 6 percent of plots. 11 The I SA, REA and Policy Assessment all conclude that the current standards are not 12 adequate to protect against the adverse impacts of terrestrial acidification on sensitive 13
ecos ys tems. The ISA reported one study that estimated 15 percent of U.S. forest ecos ys tems 14 exceeded the critical loads for acidity for N and S deposition by >250 eq/ha/year under current 15 conditions (US EPA, 2008, sect ion 4.2.1.3 ) . The main drawback, as stated in the REA and 16 Policy

Assessment , with the understanding of terrestrial acidification lies in the availability of 17 data b y which we can predict critical loads and that the data are based on laboratory responses 18

rather than field measurements. Other stressors that are present in the field but that are not 19 present in the laboratory may confound this relationship. The REA does however, conclude that 20 the case study results suggest that nitrogen and sulfur acidifying d eposition in the sugar maple 21 and red spruce forest areas studied were very close to, if not greater than, the critical loads for 22

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96 those areas, and both ecosy stems are likel y to be sensitive to any future changes in the levels of 1 deposition. 2 iii. Terrestrial nu trient enrichment 3 Nutrient enrichment effects are due to nitrogen loadings from both atmospheric and non

4 atmospheric sources. Evaluation of nutrient enrichment effects requires an understanding that 5 nutrient inputs are essential to ecosy stem health and th at specific long term levels of nutrients in 6 a sy stem affect the types of species that occur over long periods of time. Short term additions of 7 nutrients can affect species competition, and even small additions of nitrogen in areas that are 8 traditionally nutrient poor can have significant impacts on productivity as well as species 9 composition. Most ecosy stems in the United States are nitrogen

- limited, so regional decreases 10 in emissions and deposition of airborne nitrogen compounds could lead to some decrease in 11 growth of the vegetation that surrounds the targeted aquatic s ys tem but as discussed below 12 evidence for this is mixed. Whether these changes in plant growth are seen as beneficial or 13 adverse will depend on the nature of the ecosy stem being assesse d. 14 In formation on the effects of changes in nitrogen deposition on forestlands and other 15 terrestrial ecos ys tems is very limited. The multiplicity of factors affecting forests, including other 16 potential stressors such as ozone, and limiting factors such a s moisture and other nutrients, 17 confound assessments of marginal changes in any one stressor or nutrient in forest ecos ys tems. 18 The I SA notes that only a fraction of the deposited nitrogen is taken up by the forests, most of 19 the nitrogen is retained in the soils (US EPA, 2008, section 3.3.2.1 ). In addition, the I SA 20 indicates that forest management practices can significantly affect the nitrogen cycling within a 21 forest ecos ys tem, and as such, the response of managed forests to NO x deposition will be 22 variable depending on the forest management practices employed in a given forest ecos ys tem 23

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97 (US EPA, 2008, Annex C C.6.3 ). Increases in the availability of nitrogen in N

- limited forests 1 via atmospheric deposition could increase forest production over large non

- man aged areas, but 2 the evidence is mixed, with some studies showing increased production and other showing little 3 effect on wood production (US EPA, 2008, section 3.3.9) . Because leaching of nitrate can 4 promote cation losses, which in some cases create nutrie nt imbalances, slower growth and 5 lessened disease and freezing tolerances for forest trees, the net effect of increased N on forests 6 in the U.S. is uncertain ( US EPA, 2008, section 3.3.9). 7 The scientific literature has many examples of the deleterious eff ects caused by

excessive 8 nitrogen loadings to terrestrial sy stems. Several studies have set benchmark values for levels of 9 N deposition at which scientifically adverse effects are known to occur. Large areas of the 10 country

appear to be experiencing depos ition above these benchmarks. The I SA indicates that 11 studies have found that at 3.1 kg N/ha

- yr , the community of lichens begins to change from 12 acidoph yt ic to tolerant species; at 5.2 kg N/ha

- yr, the typical dominance b y acidoph yt ic species 13 no lo nger occurs; and at 10.2 kg N/ha

- yr, acidophytic lichens are totall y lost from the 14 community . Additional studies in the Colorado Front Range of the Rocky Mountain National 15 Park support these findings. These three values (3.1, 5.2, and 10.2 kg/ha

- yr) are on e set of 16 ecologicall y meaningful benchmarks for the mixed conifer forest (MCF) of the pacific coast 17 regions. Nearl y all of the known sensitive communities receive total nitrogen deposition levels 18 above the 3.1 N kg/ha

- yr ecological benchmark according to the12 km, 2002 CMAQ/NADP 19 data, with the exception of the easternmost Sierra Nevadas. MCFs in the southern portion of the 20 Sierra Nevada forests and nearl y all MCF communities in the San Bernardino forests receive 21 total nitrogen deposition levels above the 5. 2 N kg/ha

- yrecological benchmark. 22

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98 Coastal Sage Scrub communities (CSS) are also known to be sensitive to community 1 shifts caused b y excess nitrogen loadings. Studies have investigated the amount of nitrogen 2 utilized by

healthy and degraded CSS sy stems. In healthy stands, the authors estimated that 3.3 3 kg N/ha

- yr was used for CSS plant growth. It is assumed that 3.3 kg N/ha

- yr is near the point 4 where nitrogen is no longer limiting in the CSS community . Therefore, this amount can be 5 considered an ecologic al benchmark for the CSS community . The majority of the known CSS 6 range is currentl y receiving deposition in excess of this benchmark. Thus, the REA concludes 7 that recent conditions where oxides of nitrogen ambient concentrations are at or below the 8
curre nt oxides of nitrogen secondary standards are not adequate to protect against anticipated 9 adverse impacts from N nutrient enrichment in sensitive ecosy stems. 10 iv. Aquatic nutrient enrichment 11 The REA aquatic nutrient enrichment case studies focused on coastal estuaries and 12 revealed that while current ambient loadings of atmospheric oxides of nitrogen are contributing 13 to the overall depositional loading of coastal estuaries, other non

- atmospheric sources are 14 contributing in far greater amounts in total, although atmospheric contributions are as large as 15 some other individual source types. The ability of current data and models to characterize the 16 incremental adverse impacts of nitrogen deposition is limited, both by the available ecological 17 indicators, and b y the inability to attribute specific effects to atmospheric sources of nitrogen. 18 The REA case studies used as the ecological indicator for aquatic nutrient enrichment an index 19 of eutrophication known as the Assessment of Estuarine Trophic Status Eutrophication Index 20 (ASSETS EI). This index is a six level index characterizing overall eutrophication risk in a 21 waterbody. This indictor is not sensitive to changes in nitrogen deposition within a single level 22

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99 of the index . In addition, this typ e of indicator does not reflect the impact of nitrogen deposition 1 in conjunction with other sources of nitrogen. 2 Based on the above considerations, the REA concludes that the ASSETS EI is not an 3 appropriate ecological indicator for estuarine aquatic eutrophication and that additional analy sis 4 is required to develop an appropriate indicator for determining the appropriate levels of 5 protection from N nutrient enrichment effects in estuaries related to deposition of oxides of 6 nitrogen . As a result, EPA i s unable to make a determination as to the adequacy of the existing 7 secondary oxides of nitrogen standard in protecting public welfare from N nutrient enrichment 8 effects in estuarine aquatic ecos ys tems. 9 Additionally , nitrogen deposition can alter species composition and cause eutrophication 10 in freshwater s ys tems. In the Rocky Mountains, for example, deposition loads of 1.5 to 2 kg/ha

11 yrwhich are well within current ambient levels are known to cause changes in species 12 composition in diatom communities indicating impaired water quality (US EPA, 2008, section 13 3.3.5.3 ). This suggests that the existing secondary standard for oxides of nitrogen may not 14 protect such ecos ys tems and their resulting services from impairment. 15

v. Oth er effects 16 An important consideration in looking at the effects of deposition of oxides of sulfur in 17 aquatic ecosy stems is the potential for production of methylmercury , a neurotoxic contaminant

. 18 The production of meaningful amounts of methyl mercury (MeHg) requires the presence of SO 4 2

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

- results in increased 20 production of MeHg. There is increasing evidence on the relationship between sulfur deposition 21 and increased meth yl ati on of mercury in aquatic environments; this effect occurs onl y where 22 other factors are present at levels within a range to allow methylation. The production of 23

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100 methylmercury

requires the presence of sulfate and mercury , but the amount of methylmercury 1 prod uced varies with ox ygen content, temperature, pH, and supply of labile organic carbon (US 2 EPA, 2008, section 3.4). In watersheds where changes in sulfate deposition did not produce an 3 effect, one or several of those interacting factors were not in the rang e required for meaningful 4 methylation to occur (US EPA, 2008, section 3.4). Watersheds with conditions known to be 5 conducive to mercury methylation can be found in the northeastern United States and 6 southeastern Canada (US EPA, 200 9 , section 6). 7 With res pect to sulfur deposition and mercury methylation, the final ISA determined: The 8 evidence is sufficient to infer a causal relationship between sulfur deposition and increased 9 mercury

methylation in wetlands and aquatic environments. However, EPA did not co nduct a 10 quantitative assessment of the risks associated with increased mercury methylation under current 11 conditions. As such, EPA is unable to make a determination as to the adequacy of the existing 12 SO 2 secondary standards in protecting against welfare eff ects associated with increased mercury 13 methylation. 14 vi. Summary of Adequacy Considerations 15 In summary , the Policy Assessment concludes that currently available scientific evidence 16 and assessments clearl y call into question the adequacy of the current stan dards with regard to 17 deposition

- related effects on sensitive aquatic and terrestrial ecos ys tems, including acidification 18 and nutrient enrichment. Further, the Policy Assessment recognizes that the elements of the 19 current standards

-- indicator, averaging time, level and form

– are not ecologically relevant, and 20 are thus not appropriate for standards designed to provide such protection. Thus, the Policy 21 Assessment concludes that consideration should be given to establishing a new ecologically 22 relevant multi

- pollutant, multimedia standard to provide appropriate protection from deposition

- 23

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101 related ecological effects of oxides of nitrogen and sulfur on sensitive ecosystems, with a focus 1 on protecting against adverse effects associated with acidifying deposition in sensitive aquatic 2
ecos ys tems. 3 3. CASAC Views 4 In a letter to the Administrator (Russell and Samet 2011 a), the CASAC Oxides of 5 Nitrogen and Oxides of Sulfur Panel, with full endorsement of the chartered CASAC, 6 unanimously

concluded that : 7 EPA staff has dem onstrated through the Integrated Science Assessment (I SA), 8 Risk and Exposure Characterization (REA) and the draft PA that ambient NOx 9 and SOx can have, and are having, adverse environmental impacts. The Panel 10 views that the current NOx and SOx secondary st andards should be retained to 11 protect against direct adverse impacts to vegetation from exposure to gas phase 12 exposures of these two families of air pollutants. Further, the ISA, REA and draft 13 PA demonstrate that adverse impacts to aquatic ecosy stems are also occurring due 14 to deposition of NOx and SOx. Those impacts include acidification and 15 undesirable levels of nutrient enrichment in some aquatic ecosy stems. The levels 16 of the current NOx and SOx secondary NAAQS are not sufficient, nor the forms 17 of those standards appropriate, to protect against adverse depositional effects; thus 18 a revised NAAQS is warranted. 19 20 Inaddition, with regard to the joint consideration of both oxides of nitrogen and oxides 21 of sulfur as well as the consideration of deposition rel ated effects, CASAC concluded that the 22 Policy

Assessment had developed a credible methodology for considering such effects. The 23 Panel stated that “the Policy Assessment develops a framework for a multi

- pollutant, multimedia 24 standard that is ecologically relevant and reflects the combined impacts of these two pollutants as 25 they

deposit to sensitive aquatic ecosy stems.” 26 4. Administrator’s Proposed Conclusions Concerning Adequacy of Current Standard 27 Based on the above considerations and taking into account CASA C advice, the 28 Administrator recognizes that the purpose of the secondary standard is to protect against 29 “adverse” effects resulting from exposure to oxides of nitrogen and sulfur, discussed above in 30

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102 section IIA. The Administrator also recognizes the need f or conclusions as to the adequacy of the 1 current standards for both direct and deposition related effects as well as conclusions as to the 2 appropriateness and ecological relevance of the current standards. 3 In considering what constitutes an ecologi cal effect that is also adverse to the public 4 welfare, the Administrator took into account the ISA conclusions regarding the nature and 5 strength of the effects evidence, the risk and exposure assessment results, the degree to which the 6 associated uncertainties should be considered in interpreting the results, the conclusions 7 presented in the Policy Assessment , and the views of CASAC and members of the public. On 8 these bases, the Administrator concludes that the current secondary standard s are adequate to 9 p rotect against direct phytotoxic effects on vegetation. Thus , the Administrator proposes to 10 retain the current secondary standard for oxides of nitrogen at 53 ppb, 4 annual average 11 concentration, measured in the ambient air as NO 2 , and the current secondary standard for oxides 12 of sulfur at 0.5 ppm , 3

- hour average concentration, measured in the ambient air as SO 2 . 13 With regard to deposition

- related effects, the Administrator has first to consider the 14 appropriateness of the structure of the current standards to address ecological effects of concern. 15 Based on the evidence as well as considering the advice given b y CASAC on this matter, the 16 Administrator conclud es that the elements of the current standards are not ecologically relevant 17 and thus are not appro priate to provide protection of ecos ys tems
. On the subject of adequacy of 18 protection with regard to deposition

- related effects, the Administrator considered the full nature 19 of ecological effects related to the deposition of ambient oxides of nitrogen and s ulfur into 20 sensitive ecos ys tems across the country . Her conclusions are based on the evidence presented in 21 the I SA with regard to acidification and nutrient enrichment effects, the findings of the REA with 22

4 The annual secondary standard for oxides of nitrogen is being specified in units of ppb to conform to the current version of the annual primary standard, as specified in the final rule for the most recent review of the NO 2 primary NAAQS (75 FR 6531; February 9, 2010).

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103 regard to scope and severity of the current and l ikely

future effects of deposition, the sy nthesis of 1 both the scientific evidence and risk and exposure results in the Policy Assessment as to the 2 adequacy

of the current standards, and the advice of both CASAC and the public. After such 3 consideration, the Administrator concludes that current levels of oxides of nitrogen and sulfur are 4 sufficient to cause acidification of both aquatic and terrestrial ecos ys tems, nutrient enrichment of 5 terrestrial ecos ys tems and contribute to nutrient enrichment effects in estuaries that could be 6 considered adverse, and the current secondary standards do not provide adequate protection from 7 such effects. 8 Having reached these conclusions, the Administrator determines that it is appropriate to 9 consider alternative standards that are ecologically relevant. These considerations support the 10 conclusion that the current secondary standard s is neither appropriate nor adequate to protect 11 against deposition related effects

. The Administrator’s consideration of such alternative 12
s tandards is discussed below in Section III. 13 **III. Rationale for Proposed Decision on Alternative**  **Multi -**  **pollutant Approach to**  14 **Secondary Standard s**  **for Aquatic Acidification** 15 Having reached the conclusion that the current NO 2 and SO 2 secondary standards are no t 16 adequate to provide appropriate protection against deposition

- related effects associated with 17 oxides of nitrogen and sulfur, the Administrator then considered what new multi

- pollutant 18 standard might be appropriate, at this time, to address such effects o n public welfare. The 19 Administrator recognizes that the inherentl y complex and variable linkages between ambient 20 concentrations of nitrogen and sulfur oxides, the related deposited forms of nitrogen and sulfur, 21 and the ecological responses that are associ ated with public welfare effects call for consideration 22 of an ecologically

relevant design of a standard that reflects these linkages. The Administrator 23

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104 also recognizes that characterization of such complex and variable linkages will necessaril y 1 require c onsideration of information and analy ses that have important limitations and 2 uncertainties. 3 Despite its complexity, an ecologicall y relevant multi

- pollutant standard to address 4 deposition

- related effects could still appropriately be defined in terms of th e same basic elements 5 that are used to define any NAAQS

– indicator, form, averaging time, and level. The form 6 would incorporate additional structural elements that reflect relevant multi

- pollutant and 7 multimedia attributes. These structural elements inc lude the use of an ecological indicator, tied 8 to the ecological effect we are focused on, and other elements that account for ecologically 9 relevant factors other than ambient air concentrations. All of these elements would be needed to 10 enable a linkage from ambient air indicators to the ecological indicator to define an ecologicall y 11 relevant standard. As a result, such a standard would necessaril y be more complex than the 12 NAAQS that have been set historically to address effects associated with ambient con centrations 13 of a single pollutant. 14 More specificall y, the Administrator considered an ecologicall y relevant multi

- pollutant 15 standard to address effects associated with acidifying deposition related to ambient 16 concentrations of oxides of nitrogen and sulfur in sensitive aquatic ecosy stems. This focus is 17 consistent with the information presented in the ISA, REA, and Policy Assessment which 18 highlighted the sufficiency of the quantity and quality

of the available evidence and assessments 19 associated with aquatic acidification relative to the information and assessments available for 20 other deposition

- related effects, including terrestrial acidification and aquatic and terrestrial 21 nutrient enrichment. Based on its review of these documents, CASAC agreed that aqu atic 22 acidification should be the focus for developing a new multi

- pollutant standard in this review. In 23

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105 reaching conclusions about an air quality standard designed to address deposition

- related aquatic 1 acidification effects, the Administrator also recogni zes that such a standard may also provide 2 some degree of protection against other deposition

- related effects. 3 As discussed in chapter 7 of the Policy Assessment, the development of a new multi

4 pollutant standard to address deposition

- related aquatic acidi fication effects recognizes the need 5 for consideration of a nationally applicable standard for protection against adverse effects of 6 aquatic acidification to public welfare, while recognizing the complex and heterogeneous 7 interactions between ambient air c oncentrations of nitrogen and sulfur oxides, the related 8 deposition of nitrogen and sulfur, and associated ecological responses. The development of such 9 a standard also needs to take into account the limitations and uncertainties in the available 10 information and anal ys es upon which characterization of such interactions are based. The 11 approach used in the Policy Assessment also recognizes that while such a standard would be 12 national in scope and coverage, the effects to public welfare from aquatic acidific ation will not 13 occur to the same extent in all locations in the U.S., given the inherent variability of the 14 responses of aquatic s ys tems to the effects of acidifying deposition. 15 As discussed above in section II, many locations in the U.S. are naturall y protected 16 against acid deposition due to underl yi ng geological conditions. Likewise, some locations in the 17 U.S., including lands managed for commercial agriculture and forestry , are not likely to be 18 negativel y impacted b y current levels of nitrogen and sulf ur deposition. As a result, while a new 19 ecologicall y relevant secondary standard would apply everywhere, it would be structured to 20 account for differences in the sensitivity of ecos ystems across the country. This would allow for 21 appropriate protection of sensitive aquatic ecosy stems, which are relativel y pristine and wild and 22

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106 generall y in rural areas, and the services provided by such sensitive ecos ys tems, without 1 requiring more protection than is needed elsewhere. 2 As discussed below, the multi

- pollutant standard developed in the Policy Assessment 3 would employ

(1) NOy and SOx as the atmospheric ambient air indicators; (2) a form that takes 4 into account variable factors, such as atmospheric and ecos ys tem conditions that modify the 5 amounts of deposited ni trogen and sulfur; the distinction between oxidized and reduced forms of 6 nitrogen; effects of deposited nitrogen and sulfur on aquatic ecosy stems in terms of the 7 ecological indicator ANC; and the representativeness of water bodies within a defined spatial 8 area; (3) a multi

- year averaging time, and (4) a standard level defined in terms of a single, 9 national target ANC value that, in the context of the above form, identifies the levels of 10 concentrations of NOy and SOx in the ambient air that would meet the st andard. The form of 11 such a standard has been defined b y an index, termed an aquatic acidification index (AAI), 12 which reflects the relationship between ambient concentrations of NOy and SOx and aquatic 13 acidification effects that result from nitrogen and sulfur deposition related to these ambient 14 concentrations. 15 In presenting the considerations associated with such an air quality standard to address 16 deposition

- related aquatic acidification effects, the following sections focus on each element of 17 the standard, including indicator (section III

. A), form (section III

. B), averaging time (section 18 III
. C), and level (section III

. D). Alternative combinations of levels and forms are discussed in 19 section III.E. Considerations related to important uncertainties inher ent in such an approach are 20 discussed in section III

. F. Advice from CASAC on such a new standard is presented in section 21 III.G. The Administrator’s proposed decisions on such a new standard are presented in section 22 III.H. 23

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107 A. Ambient Air Indicators 1 In co nsidering alternative ambient air indicators, the Policy Assessment primarily

focuses 2 on the important attribute of association. Association in a broad sense refers to how well an 3 ambient air indicator relates to the ecological effects of interest by virtue of both the framework 4 that links the ambient indicator and effects and the empirical evidence that quantifies the 5 linkages. The Policy Assessment also considers how measurable or quantifiable an indicator is 6 to enable its use as an effective indicator of relevant ambient air concentrations. 7 As discussed above in section II.C, the Policy Assessment concludes that indicators other 8 than NO 2 and SO 2 should be considered as the appropriate indicators of oxides of nitrogen and 9 sulfur in the ambient air for p rotection against the acidification effects associated with deposition 10 of the associated nitrogen and sulfur. This conclusion is based on the recognition that all forms 11 of nitrogen and sulfur in the ambient air contribute to deposition and resulting acidi fication, and 12 as such, NO 2 and SO 2 are incomplete indicators. In principle, the ambient indicators should 13 represent the species that are associated with oxides of nitrogen and sulfur in the ambient air and 14 can contribute acidifying deposition. This includes both the species of oxides of nitrogen and 15 sulfur that are directl y emitted as well as species transformed in the atmosphere from oxides of 16 nitrogen and sulfur that retain the nitrogen and sulfur atoms from directly emitted oxides of 17 nitrogen and sulf

ur. All of these compounds are associated with oxides of nitrogen and sulfur in 18 the ambient air and can contribute to acidifying deposition. 19 The Policy

Assessment focuses in particular on the various compounds with nitrogen or 20 sulfur atoms that are as sociated with oxides of nitrogen and sulfur, because the acidifying 21 potential is specific to nitrogen and sulfur, and not other atoms (e.g., H, C, O) whether derived 22 from the original source of oxides of nitrogen and sulfur emissions or from atmospheric 23

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108 transformations. For example, the acidifying potential of each molecule of NO 2 , NO, HNO 3 or 1 PAN is identical, as is the potential for each molecule of SO 2 or ion of particulate sulfate, p - SO 4 . 2 Each atom of sulfur affords twice the acidifying potential of each atom of nitrogen. 3 1. Oxides of Sulfur 4 As discussed in the Policy Assessment (US EPA, 2011, section 7.1.1), oxides of sulfur 5 include the gases sulfur monoxide (SO), sulfur dioxide (SO 2 ), sulfur trioxide (SO 3 ), disulfur 6 monoxide (S 2 O), and particulate

- phase sulfur compounds that result from gas

- phase sulfur 7 oxides interacting with particles. However, the sum of SO 2 and SO 4 does represent virtuall y the 8 entire ambient air mass of sulfur that contributes to acidification. In addition to accounting for 9

v irtually

all the potential for acidification from oxidized sulfur in the ambient air, there are 10 reliable methods to monitor the concentrations of SO 2 and particulate SO 4 . In addition, much of 11 the data used to develop the technical basis for the standard developed in the Policy Assessment 12 is based on monitoring or modeling of these species. 5 The Policy Assessment concludes that the 13 sum of SO 2 and SO 4 , referred to as SOx, are appropriate ambient air indicators of oxides of 14 sulfur because they represent virtually

all of the acidification potential of ambient air oxides of 15 sulfur and there are reliable methods suitable for measuring SO 2 and SO 4 . 16 2. Oxides of Nitrogen 17 As discussed in the Policy Assessment (US EPA, 2011, section 7.1.2), total reactive 18 oxidized n itrogen, NOy , as defined in chapter 2 of the Policy Assessment, incorporates basicall y 19 all of the oxidized nitrogen species that have acidifying potential and as such, NOy should be 20 considered as an appropriate indicator for oxides of nitrogen. NOy is an aggregate measure of 21

5 As discussed in chapter 2 of the P olicy A ssessment , SO 2 and particulate SO 4 are routinely measured in ambient air mo nitoring networks, although only CASTNET filter packs do not intentionally exclude particle size fractions. The CMAQ treatment of SOx is the simple addition of both species, which are treated explicitly in the model formulation. All particle size fractions are included in the CMAQ SOx estimates.

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109 NO and NO 2 and all of the reactive oxidized products of NO and NO 2 . That is, NOy is a group 1 of nitrogen compounds in which all of the compounds are either an oxide of nitrogen or 2 compounds in which the nitrogen atoms came from oxide s of nitrogen. NOy is especiall y 3 relevant as an ambient indicator for acidification in that it both relates to the oxides of nitrogen in 4 the ambient air and also represents the acidification potential of all oxidized nitrogen species in 5 the ambient air, whether an oxide of nitrogen or derived from oxides of nitrogen. 6 There are currently available reliable methods of measuring aggregate NOy . The term 7 “aggregate” measure means that the NO Y , as measured, is not based on measuring each 8 individual species of NOy and calculating an NOy value by summing the individual species. 9 Rather, as described in chapter 2 of the Policy Assessment, current measurement techniques 10 process all of the individual NOy species to produce a single aggregate measure of all of the 11 nitrogen atoms associated with any NOy species. Consequently , the NOy measurement 12 effectivel y provides the sum of all individual species, but the identity of the individual species is 13 lost. As discussed above, the accounting for the individual nitrogen atoms is an accounting of 14 the ambient air acidification potential of oxides of nitrogen and their transformation products and 15 therefore the most relevant ambient indicator for aquatic acidification effects associated with 16 oxides of nitrogen. 17 This loss o f the information on individual species motivated consideration of alternative 18 or more narrowl y defined indicators for oxides of nitrogen in the Policy Assessment. 19 Consideration of a subset of NOy species was based on the following reasoning. First, the actual 20 dry deposition of nitrogen is determined on an individual species basis by multiplying the species 21 concentration times a species

- specific deposition velocity and then summed to develop an 22 estimate of total dry deposition. Consequently , the use of i ndividual ambient species has the 23

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110 potential to be more consistent with the underlying science of deposition and, therefore, has the 1 potential to allow for a more rigorous evaluation of dry deposition with specialized field studies. 2 In addition, there has been a suggestion of focusing onl y on the most quickly depositing NOy 3 species, such as HNO 3 , as contributions from other NOy species such as NO 2 may be negligible. 4 These alternative indicators are discussed below. 5 The Policy

Assessment considers the relative merits of using each individual NOy 6 species as part of a group of indicators. In so doing, it was first noted that dry deposition of NOy 7 is treated as the sum of the deposition of each individual species in advanced process

- based air 8 quality

models l ike CMAQ, as described in chapter 2 of the Policy Assessment. Conceptually 9 one could extend this process

- based approach b y using all NO Y species individually as separate 10 indicators for oxides of nitrogen and requiring, for example, measurements of each of the 11 species, including the dominant species of HNO 3 , particulate nitrate (p

-NO 3 ) , true NO 2 , NO, and 12 PAN. The potential attraction of using individual species would be the reliance on actual 13 deposition velocities. This could have more phys ical meaning i n comparison to a constructed 14 model of aggregate deposition of NOy , which is difficult to evaluate with observations because 15 of the assimilation of many species with disparate deposition behavior. The Policy Assessment 16 notes that the major drawback of using individual NOy species as the indicators is the lack of 17 reliable measurement techniques, especiall y for PAN and NO 2 in rural locations, which renders 18 the use of virtuall y an y i ndividual NOy species, except for NO and perhaps p -NO 3 , as 19 functionally

inade quate from a measurement perspective. 20 The Policy

Assessment next considered the relative merits of using a subset of NOy 21 species as the indicators for oxides of nitrogen, as was discussed above for oxides of sulfur. To 22 the extent that certain species pro vide relativel y minor contributions to total NOy deposition, it 23

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111 may be appropriate to consider excluding them as part of the indicator. As discussed in chapter 2 1 of the Policy

Assessment, each nitrogen species within the array of NOy species has species

2
s pecific dry

deposition velocities. For example, the deposition velocity of HNO 3 is much 3 greater than the velocity for nitrogen dioxide and, consequentl y, for a similar ambient air 4 concentration, HNO 3 contributes more deposition of acidifying nitrogen rel ative to nitrogen 5 dioxide. In transitioning from source

- oriented urban locations to rural environments, the ratio of 6 the concentrations of HNO 3 and PAN to NO 2 increases. 7 Based on the reasoning that a larger fraction of the deposited NO Y is accounted for by 8 total nitrate (the sum of HNO 3 and p -NO 3 ), a surrogate for the more rapidly depositing fraction 9 of NOy , combined with the availability of reliable total nitrate measurements through the Clean 10 Air Status and Trends Network ( CASTNET ) , the Policy Assessment considered using total 11 nitrate as the indicator for oxides of nitrogen ( US EPA, 2011, appendix E ). Nitrate would be 12 expected to correlate well with total reactive oxidized nitrogen deposition relative to NO Y (US 13 EPA, 2011, chapter 2 )
, gi ven the inherent noise associated with variable contributions of low 14 deposition velocity species (e.g., NO 2 ) that may have relatively

high ambient concentrations. 15 However, modeling simulations suggest that NOy may be a more robust indicator, relative to 16 HNO 3 , in terms of relating absolute changes in ambient air concentrations to changes in nitrogen 17 deposition driven by changes in ambient concentrations of oxides of nitrogen (US EPA, 18 2011, Figure 2

- 32 ). 19 Based on the above considerations, the Policy Assessment concludes that total reactive 20 oxidized nitrogen, NOy , should be considered as the appropriate ambient indicator for oxides of 21 nitrogen based on its direct relationship to oxides of nitrogen in the ambient air and its direct 22 relationship to deposition associated with aq uatic acidification. Because NOy represents all of 23

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112 the potentially

acidifying oxidized nitrogen species in the ambient air, it is appropriatel y 1 associated with the deposition of potentially acidifyingcompounds associated with oxides of 2 nitrogen in the ambient air. In addition, there are reliable methods available to measure NOy

. 3 Measurement of each individual species of NOy , or the measurement of only a subset of species 4 of NOy , is less appropriate because there are not reliable measurements methods available to 5 measure all of the individual species of NOy and a subset of species would fail to account for 6 significant portions of the oxidized reactive nitrogen that relates to acidification

. 6 7 B. Form 8 Based on the evidence of the aquatic acidification eff ects caused by

the deposition of 9 NOy and SOx, the Policy

Assessment (US EPA, 2011, section 7.2) presents the development of a 10 new form that is ecologically relevant for addressing such effects. The conceptual design for the 11 form of such a standard includes three main components: an ecological indicator, deposition 12 metrics that relate to the ecological indicator, and a function that relates ambient air indicators to 13 deposition metrics. Collectivel y, these three components link the ecological indicator to a mbient 14 air indicators, as illustrated above in Fig II
- 1. 15 The simplified flow diagram in Figure II
- 1 compresses the various atmospheric, 16 biological, and geochemical processes associated with acidifying deposition to aquatic 17
ecos ys tems into a simplified con ceptual picture. The ecological indicator (left box) is related to 18 atmospheric deposition through biogeochemical ecosy stem models (middle box), which 19 associate a target deposition load to a target ecological indicator. Once a target deposition is 20 establ ished, associated allowable air concentrations are determined (right box) through the 21 relationships between concentration and deposition that are embodied in air quality models such 22

6 The P olicy A ssessment also note s that NO Y is a useful measurement for model evaluation purposes , which is especially im

portant, recognizing the unique role that CMAQ play s in the development of this standard, as described below in section III.B
.

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113 as CMAQ. The following discussion describes the development and rational e for each of these 1 components, as well as the integration of these components into the full expression of the form 2 of the standard using the concept of a national AAI that represents a target ANC level as a 3 function of ambient air concentrations. Spatial aggregation issues associated with defining each 4 of the terms of this index are also addressed below. 5 The AAI is designed to be an ecologically relevant form of the standard that determines 6 the levels of NOy

and SOx in the ambient air that would achieve a target ANC limit for the U.S. 7 The intent of the AAI is to weight atmospheric concentrations of oxides of nitrogen and sulfur b y 8 their propensity

to contribute to acidification through deposition, given the fundamental 9 acidifying potential of each pollutant, and to take into account the ecological factors that govern 10 acid sensitivity

in different ecosy stems. The index also accounts for the contribution of reduced 11 nitrogen to acidification. Thus, the AAI encompasses those attributes of specific relevanc e to 12 protecting ecos ys tems from the acidifying potential of ambient air concentrations of NOy and 13 SOx. 14 1. Ecological Indicator 15 In considering alternative ecological indicators, the Policy Assessment again primarily 16 focuses on the attribute of association . In the case of an ecological indicator for aquatic 17 acidification, association refers to the relationship between the indicator and adverse effects as 18 discussed in section II. Because of the conceptual structure of the form of this standard (Figure 19 III
- 1), this particular ecological indicator must also link up in a meaningful and quantifiable 20 manner with acidifying atmospheric deposition. In effect, the ecological indicator for aquatic 21 acidification is the bridge between biological impairment and deposition of NOy and SOx. 22

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114 This section presents the rationale in the Policy Assessment for selecting acid 1 neutralizing capacity (ANC) as the appropriate ecological indicator for consideration. 2 Recognizing that ANC is not itself the causative or toxic agent for adverse aquatic acidification 3 effects, the rationale for using ANC as the relevant ecological indicator is based on the 4 following: 5 (1) ANC is directly associated with the causative agents, pH and dissolved aluminum, both 6 through empirical evidence and me chanistic relationships; 7 (2) Empirical evidence shows very clear and strong relationships between adverse effects 8 and ANC; 9 (3) ANC is a more reliable indicator from a modeling perspective, allowing use of a body of 10 studies and technical analy ses related to ANC and acidification to inform the 11 development of the standard; and 12 (4) ANC literally embodies the concept of acidification as posed by the basic principles of 13 acid base chemistry and the measurement method used to estimate ANC and, therefore, 14 serves as a direct ind ex to protect against acidification. 15 16 Ecological indicators of acidification in aquatic ecosy stems can be chemical or biological 17 components of the ecosystem that are altered b y the acidifying effects of nitrogen and sulfur 18 deposition. A desirable ecolog ical indicator for aquatic acidification is one that is measurable or 19 estimable, linked causally to deposition of nitrogen and sulfur, and linked causally , either 20 directly

or indirectl y to ecological effects known or anticipated to adversely affect public 21 welfare. 22

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115 As summarized in chapter 2 of the Policy Assessment, atmospheric deposition of NOy 1 and SOx causes aquatic acidification through the input of strong acid anions (e.g., NO 3 - and 2 SO 4 2

- ) that ultimately shifts the water chemistry equilibrium toward i ncreased hydrogen ion 3 levels (or decreased pH). The anions are deposited either directly to the aquatic ecos ys tem or 4 indirectly

via transformation through soil nitrification processes and subsequent drainage from 5 terrestrial ecos ys tems. In other words, when these anions are mobilized in the terrestrial soil, 6 they can leach into adjacent water bodies. Aquatic acidification is indicated by changes in the 7 surface water chemistry of ecos ys tems. In turn, the alteration of surface water chemistry has been 8 link ed to negative effects on the biotic integrity of freshwater ecos ys tems. There is a suite of 9 chemical indicators that could be used to assess the effects of acidifying deposition on lake or 10 stream acid

- base chemistry . These indicators include acid neutrali zing capacity (ANC); 11 alkalinity

(ALK); base neutralizing capacity, commonly referred to as acidity

(ACY); surface 12 water pH; concentrations of trivalent aluminum, Al +3 ; and concentrations of major anions (SO 4 2

- , 13 NO 3 - ), cations (Ca2+ , Mg +2 , K+ ), or sums of c ations or anions. 14 ANC and ALK are very similar quantities and are used interchangeabl y in the literature 15 and for some of the anal yses presented in this document. Both ANC and ALK are defined as the 16 amount of strong acid required to reach a specified equivalence point. For acid

- base solutions, 17 an equivalence point can be thought of as the point at which the addition of strong acids (i.e., 18 titration) is no longer neutralized by the solution. This explains the term acid neutralizing 19 capacity , or ANC, as ANC relates directly to the capacity of a s ys tem to neutralize acids. The 20 differences between ANC and ALK are based on operational definitions and subject to various 21 interpretations. ANC is preferred over ALK as the body of scientific evidence has fo cused on 22 ANC and effects relationships. ALK is more widely associated with more general 23

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116 characterizations of water quality such as the relative hardness of water associated with 1 carbonates. 2 Indictors such as the concentrations of specific anions, cation s, or their groupings, while 3 relevant to acidification processes, are not robust acidification indicators as it is the relative 4 balance of cations and anions that is more directly associated with acidification. That balance is 5 captured b y ANC and ALK. A cidity , ACY, is the coverse of ANC and indicates how much 6 strong base it takes to reach an equivalence point. Because ACY is not used in most ecosy stem 7 assessments, the body of information relating ACY to effects is too l imited to serve as a basis for 8 an appropriate ecological indicator. Aluminum and other metals are causative toxic agents that 9 directly

impair biological functions. However, aluminum, or metals in general, have high 10 variability

in concentrations that can be linked to effects, often at extremely low levels which in 11 some cases approach detectability limits, exhibit rapid transient responses, and are often 12 confounded b y the presence of other toxic metals. These concerns limit the use of metals as 13 reliable and measurable ecological indicators. Hydrogen ion (H + ) concentrations, using their 14 negative logarithmic values, or pH, are well correlated with adverse effects, as discussed above 15 in section II.A, and determine the solubility of metals such as aluminum. Ho wever, pH is not a 16 preferred acidification indicator due to its highl y transient nature and other concerns, as 17 discussed below. 18 Having reasoned that ANC is a preferred indicator to ALK, ACY, individual metals or 19 groupings of ions, the Policy Assessment considers the relative merits of ANC compared to pH, 20 which is a well recognized indicator of acidity and a more direct causative agent with regard to 21 adverse effects. First, the linkage between ANC and pH is considered in recognition of the 22 causative ass ociation between pH and effects. 23

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117 ANC is not the direct causative toxic agent impacting aquatic species diversity . The 1 scientific literature generally emphasizes the links between pH and adverse effects as described 2 above in section II.A. It is important, therefore, to consider the extent to which ANC and pH are 3 well related from a mechanistic perspective as well as through empirical evidence. ANC and 4 pH are co
- dependent on each other based on the requirement that all solutions are electrically 5 neutral, meaning that any solution must satisfy the condition that all negatively charged species 6 must be balanced b y all positively charged species. ANC is defined as the difference between 7 strong anions and cations (US EPA, 2011 , equation 7

- 13). 8 While the c hemistry can be complex, the co

- dependency between ANC and pH is 9 explained by

recognizing that positively charged hydrogen, H + , is incorporated in the charge 10 balance relationships related to the overall solution chemistry which also defines ANC. The 11 posit ive, directional co

- dependency (i.e., ANC and pH increase together) is further explained in 12 concept as ANC reflects how much strong acid (i.e., how much hydrogen ion) it takes to titrate to 13 an equivalence point. Strong observed correlations between pH and ANC as described in the 14 Policy

Assessment support these mechanistic relationships. 15 As discussed above in section II.A, there are well established examples of ANC 16 correlating strongl y with a variety of ecological effects which are summarized in the Poli cy 17 Assessment (US EPA, 2011, Table 3

- 1). Because pH and ANC are well correlated and linearl y 18 dependent over the pH ranges (4.5

- 6) where adverse ecological effects are observed, evidence of 19 clear associations exist between ANC and adverse ecological effec ts as described in the Policy 20 Assessment. In large measure, this dependence between pH and ANC and the relationship of 21 both pH and ANC to effects, speak directly to the appropriateness of ANC with respect to its use 22 as an ecological indicator. 23

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118 Thus, there is a clear association between ANC and ecological effects, although there is a 1 more direct causal relationship between pH and effects. Nonetheless, ANC is preferred as an 2 ecological indicator based on its superior ability to provide a linkage with depos ition in a 3 meaningful and quantifiable manner, a role that is served far more effectively by ANC than b y 4 pH. While both ANC and pH are clearly associated with the effects of concern, ANC is superior 5 in linking these effects to deposition. 6 The Policy

As sessment notes that the basis for this conclusion is that acidifying 7 atmospheric deposition of nitrogen and sulfur is a direct input of potential acidity (ACY), or, in 8 terms of ANC, such deposition is relevant to the major anions that reduce the capacity o f a water 9 body to neutralize acidity. Consequently , there is a well defined linear relationship between 10 potential acidifying deposition and ANC. This ANC

- deposition relationship facilitates the 11 linkage between ecos ys tem models that calculate an ecological indicator and the atmospheric 12 deposition of NOy

and SOx. On the other hand, there is no direct linear relationship between 13 deposition and pH. While acid inputs from deposition lower pH, the relationship can be 14 extremely

nonlinear and there is no direct connection from a modeling or mass balance 15 perspective between the amount of deposition entering a s ys tem and pH. The term “mass 16 balance” underlies the basic formulation of any physical modeling construct, for atmospheric or 17 aquatic s ys tems, and refers t o the accounting of the flow of mass into a s ys tem, the 18 transformation to other forms, and the loss due to flow out of a s ys tem and other removal 19 processes. ANC is a conserved property. This means that ANC in a water body can be 20 accounted for b y knowledg e of how much ANC initially exists, how much flows in and is 21 deposited, and how much flows out. In contrast, hydrogen ion concentration in the water, the 22 basis for pH, is not a conserved property as its concentration is affected b y several factors such 23

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119 as temperature, atmospheric pressure, mixing conditions of a water body, and the levels of 1 several other chemical species in the s ys tem. The disadvantage of pH lacking conservative 2 properties is that there is a very complex connection between changes in ambient air 3 concentrations of NOy and SOx and pH. 4 The discussion of basic water chemistry of natural sy stems in chapter 2 of the Policy 5 Assessment provides further details on why pH is not a conserved quantity and is subject to rapid 6 transient respons e behavior that makes it difficult to use as a reliable and functional ecological 7 indicator. The observed pH

- to
- ANC relationship (US EPA, 2011, figure 7
- 2 ) partiall y explains 8 the concern with pH responding too abruptl y. In the region where pH ranges rou ghly from 4.5 9 to 6 and is of greatest relevance to effects (US EPA, 2011, figure 7

- 4 ), there clearly is more 10 sensitivity

of pH to changes in ANC in the ANC range from approximately 0 to 50 μeq/L . A 11 focus on this part of the ANC

- to
- pH relationship shows that ANC associates well with pH in a 12 fairl y linear manner. However, the pH range from 4.5 to 6 also includes one of the very steepest 13 parts of the slope relating pH as a function of ANC , where ANC ranges down below 0 μeq/L, 14 which is subject to very rapid change in ANC, or deposition inputs. This part of the relationship 15 coincides with reduced levels of ANC and hence with reduced ability to neutralize acids and 16 moderate pH fluctuations. This response behavior can be extended to considering how pH 17 would c hange in response to deposition, or ambient concentrations, of NOy and SOx, which can 18 be viewed as “ANC

- like” inputs. 19 In summary , because ANC clearl y links both to biological effects of aquatic acidification 20 as well as to acidifying inputs of NOy and S Ox deposition, the Policy Assessment concludes that 21 ANC is an appropriate ecological indicator for relating adverse aquatic ecosystem effects to 22 acidifying atmospheric deposition of SOx and NOy , and is preferred to other potential indicators. 23

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120 In reaching this conclusion, the Policy Assessment notes that in its review of the first draft Policy 1 Assessment, CASAC concluded that “information on levels of ANC protective to fish and other 2 aquatic biota has been well developed and presents probably the lowest lev el of uncertainty in 3 the entire methodology” (Russell and Samet, 2010a). In its more recent review of the second 4 draft Policy

Assessment, CASAC agreed “that acid neutralizing capacity is an appropriate 5 ecological measure for reflecting the effects of aquatic acidification” (Russell and Samet, 2010b; 6 p. 4). 7 2. Linking ANC to Deposition 8 There is evidence to support a quantified relationship between deposition of nitrogen and 9 sulfur and ANC. This relationship was anal yz ed in the REA for two case study ar eas, the 10 Adirondack and Shenandoah Mountains, based on time

- series modeling and observed trends. 11 In the REA anal ys is, long

- term trends in surface water nitrate, sulfate and ANC were modeled 12 using Model of Acidification of Groundwater in Catchment (MAGI C ) for the two case study 13 areas. These data were used to compare recent surface water conditions in 2006 with 14 preindustrial conditions (i.e. preacidification 1860). The results showed a marked increase in the 15 number of acid impacted lakes, characterized a s a decrease in ANC levels, since the onset of 16 anthropogenic nitrogen and sulfur deposition, as discussed in chapter 2 of the Policy Assessment. 17 In the REA, more recent trends in ANC, over the period from 1990 to 2006, were assessed using 18 monitoring data c ollected at the two case study areas. In both case study areas, nitrate and 19 sulfate deposition decreased over this time period. In the Adirondack Mountains, this 20 corresponded to a decreased concentration of nitrate and sulfate in the surface waters and an 21 increase in ANC (US EPA, 2009, section 4.2.4.2). In the Shenandoah Mountains, there was a 22 slight decrease in nitrate and sulfate concentration in surface waters corresponding to modest 23

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121 increase in ANC from 50 ueq/Lin 1990 to 67 ueq/L in 2006 (US EPA, 2 009, section 1 4.2.4.3 , Appendix 4, and section 3.4). 2 In the REA, the quantified relationship between deposition and ANC was investigated 3 using ecos ys tem acidification models, also referred to as acid balance models or critical loads 4 models (US EPA, 2011 , s ection 2 and US EPA, 2009, section 4 and Appendix 4). These models 5 quantify

the relationship between deposition of nitrogen and sulfur and the resulting ANC in 6 surface waters based on an ecos ys tem’s inherent generation of ANC and ability to neutralize 7 nitrogen deposition through biological and ph ys ical processes. A critical load is defined as the 8 amount of acidifying atmospheric deposition of nitrogen and sulfur beyond which a target ANC 9 is not reached. Relatively high critical load values imply that an ecos ys tem can accommodate 10 greater deposition levels than lower critical loads for a specific target ANC level. Ecos ys tem 11 models that calculate critical loads form the basis for linking deposition to ANC. 12 As discussed in chapter 2 of the Policy Asses sment, both dy

namic and steady

state 13 models calculate ANC as a function of ecos ys tem attributes and atmospheric nitrogen and sulfur 14 deposition, and can be used to calculate critical loads. Steady state models are time invariant 15 and reflect the long term consequences associated with an ecos ys tem reaching equilibrium under 16 a constant level of atmospheric deposition. Dy namic models are time variant and take into 17 account the time dependencies inherent in ecos ys tem hydrology , soil and biological processes. 18 Dy namic models like MAGI C can provide the time series response of ANC to deposition 19 whereas steady state models provide a single ANC relationship to any fixed deposition level. 20 Dy namic models naturally

are more complex than steady state models as they

attempt to capture 21 as much of the fundamental biogeochemical processes as practicable, whereas steady state 22

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122 models depend on far greater parameterization and generalization of processes that is afforded, 1 somewhat, b y not having to accounting for temporal variability

. 2 The Policy

Assessment notes that steady state models are capable of addressing the 3 question of what does it take to reach and sustain a specific level of ANC

. Dy namic models are 4 also capable of addressing that question, but can also address the question of how long it takes to 5 achieve that result. Dy namic models afford the ability for more comprehensive treatment of a 6 variety of processes throughout the surface, soil and bedrock layers within an ecos ys tem. For 7 example, steady

state models treat sulfate as a mobile anion throughout the sy stem, meaning that 8 the sulfate that is deposited to a watershed enters the water column and is not influenced by soil 9 adsorption or cation exchange. Dy namic models can incorporate these time variant process es. 10 The use of a steady state model treating sulfate as totally mobile does not necessarily conflict 11 with the possibility of sulfate acting as a less than mobile ion at certain times. The steady state 12 assumption is premised on the long term behavior of sulfate which can undergo periods of net 13 adsorption followed b y periods of net desorption which can balance out over time. The Policy 14 Assessment recognizes that as the richness of the available data increases, in terms of parameters 15 and spatial resolution , the incorporation of dy namic modeling approaches in the standard setting 16 process should become more feasible. In determining an appropriate modeling approach for the 17 development of a NAAQS in this review, the Policy Assessment considers both the relevan ce of 18 the question addressed as well as the ability to perform modeling that provides relevant 19 information for geographic areas across the country. 20 Dy

namic models require a large amount of catchment level

- specific data relative to 21 steady

state models. B ecause of the time invariant nature of steady state models, the data 22 requirements that integrate across a broad spectrum of ecos ys tem processes is achievable and 23

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123 available now at the national level. Water quality data to support steady state models currently 1 exist for developing a national data base for modeling nearl y 10,000 catchments in the 2 contiguous U.S. In contrast, the data needs to support dy namic models for national

- scale 3 anal ys es simply

are not available at this time. Further, the information p rovided by steady state 4 modeling would be sufficient to develop and analyze alternative NAAQS and the kind of 5 protection they

would afford. While it would be of interest to also obtain information about how 6 much time it would take for a target ANC level to be achieved, the absence of such information 7 does not preclude developing and evaluating alternative NAAQS using the AAI structure. Based 8 on the above considerations, the Policy Assessment concludes that at this time steady state 9 critical load modeling is an appropriate tool for linking long

- term ANC levels to atmospheric 10 deposition of nitrogen and sulfur for development of an AAI that has national applicability

. 11 A **s** teady state model is used to define the critical load, which is the amount of 12 atmospheric deposition of nitrogen (N) and sulfur (S) beyond which a target ANC is not 13 achieved and sustained. 7 It is expressed as: 14 15 CLANClim (N + S) = ([BC] 0 *

- [ANC lim ]) Q + Neco (III

- 1) 16 17 Where: 18 CLANClim (N + S) is the critical load of deposition, with units o f equivalent charge/(area

- time); 19 [BC] 0 * is the natural contribution of base cations from weathering, soil processes and 20 preindustrial deposition, with units of equivalent charge/volume; 21 [ANC

lim ] is the target ANC value, with units of equivalent charge/volume; 22

7 This section discusses the linkages between deposition of nitrogen and sulfur and ANC. Section III.B.3 then discusses the linkages between atmospheric concentrations of NOy and SOx and deposition of nitrogen and sulfur

.

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124 Q is the catchment level runoff rate governed b y water mass balance and dominated by 1 precipitation, with units of distance/time; and 2 Neco is the amount of nitrogen deposition that is effectivel y neutralized by a variety of biological 3 (e.g., nutrient uptake) and phy sical processes, with units of equivalent charge/ (area

- time). 4 5 Equation III

- 1 is a modified expression that adopts the basic formulation of the steady 6 state models that are described in chapter 2 of the Policy Assessment. More detailed discussion 7 of the rationale, assumptions and derivation of equation III

- 1, as well as all of the equations in 8 this section, are included in Appendix B of the Policy Assessment. The equation simply reflects 9 the amount of deposition of nitrogen and sulfur from the atmosphere, CLANClim (N + S) , that is 10 associated with a sustainable long

- term ANC target, [ANC lim ] , given the capacity of the natural 11 sy stem to generate ANC, [BC] 0 * , and the capacity of the natural s ys tem to neutralize nitrogen 12 deposition, Neco
. This expression of critical load is valid when nitrogen deposition is greater 13 than Neco. 8 The runoff rate, Q , allows for balancing mass in the two environmental mediums

– 14 atmosphere and catchment. This critical load expression can be focused on a single water s ys tem 15 or more broadl y. To extend applicability of the critical load expression (equation III

- 1) from the 16 catchment level to broader spatial areas, the terms Q r and CLr, are used, which are the runoff rate 17 and critical load, respectively , of the reg ion over which all the atmospheric terms in the equation 18 are defined. 19 In considering the contributions of SOx or NOy species to acidification, it is useful to 20 think of every

depositing nitrogen atom as supplying one equivalent charge unit and every sulfur 21 atom as depositing two charge units. The Policy Assessment uses equivalent charge per volume 22

8 Because Neco is only relevant to nitrogen deposition, in rare cases where Neco is greater than the total nitrogen deposition, the critical load would be define d only in terms of acidifying deposition of sulfur and the Neco term in equation III

- 1 wo uld be set to zero.

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125 as a normalizing tool in place of the more familiar metrics such as mass or moles per volume. 1 This allows for a clearer explanation of many of the relationship s between atmospheric and 2
ecos ys tem processes that incorporate mass and volume unit conventions somewhat specific to 3 the environmental media of concern (e.g., m 3 for air and liter for liquid water). Equivalent 4 charge reflects the chemistry equilibrium fun damentals that assume electroneutrality , or 5 balancing charge where the sum of cations alway s equals the sum of anions. 6 As presented above, the terms S and N in the CLANClim (N + S) term broadly represent all 7 species of sulfur or nitrogen that can contrib ute to acidifying deposition. This follows 8 conventions used in the scientific literature that addresses critical loads, and it reflects all 9 possible acidifying contributions from any sulfur or nitrogen species. For all practical purposes, 10 S reflects SOx as described above, the sum of sulfur dioxide gas and particulate sulfate. 11 However, N in equation III

- 1 includes both oxidized forms, consistent with the ambient 12 indicator, NOy , in addition to the reduced nitrogen species, ammonia and ammonium ion, 13 refer

red to as NHx. NHx is included in the critical load formulation because it contributes to 14 potentially

acidifying nitrogen deposition. Consequently , from a mass balance or modeling 15 perspective, the form of the standard needs to account for NHx, as described below. 16 3. Linking Deposition to Ambient Air Indicators 17 The last major component of the form illustrated in Figure III

- 1 addresses the linkage 18 between deposition of nitrogen and sulfur and concentrations of the ambient air indicators, NOy 19 and SOx. T o link ambient air concentrations with deposition, the Policy Assessment defines a 20 transference ratio, T, as the ratio of total wet and dry deposition to ambient concentration, 21 consistent with the area and time period over which the standard is defined. T o express 22 deposition of NOy

and SOx in terms of NOy and SOx ambient concentrations, two transference 23

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126 ratios were defined, where T SOx equals the ratio of the combined dry and wet deposition of SOx 1 to the ambient air concentration of SOx, and TNOy equals the ratio of the combined dry and wet 2 deposition of NOy

to the ambient air concentration of NOy

. 3 As described in Chapter 7 of the Policy Assessment, reduced forms of nitrogen (NH X ) are 4 included in total nitrogen in the critical load equation, III

- 1. Reduce d forms of nitrogen are 5 treated separately , as are NOy and SOx, and the transference ratios are applied. This results in 6 the following critical load expression that is defined explicitly in terms of the indicators NOy and 7 SOx: 8 9 CLANClim (N + S) = ([BC] 0 *

- [ANC lim ]) Q + Neco = [NOy

]T NOy + [SOx

]T SOx + NHx (III

- 2) 10 11 This is the same equation as III

- 1, with the deposition associated with the critical load translated 12 to deposition from ambient air concentrations via transference ratios. In addition, deposition o f 13 reduced nitrogen, oxidize nitrogen and oxidized sulfur are treated separatel y. 14 Transference ratios are a modeled construct, and therefore cannot be compared directl y to 15 measurable quantities. There is an analogy to deposition velocity, as a transfere nce ratio is 16 basically

an aggregated weighted average of the deposition velocities of all contributing species 17 across dry

and wet deposition, and transference ratio units are expressed as distance/time. 18 However, wet deposition commonly is not interpreted as the product of a concentration times a 19 velocity . Direct wet deposition observations are available which integrate all of the processes, 20 regardless of how well they may be understood, related to wet deposition into a measurable 21 quantity . There are reas onable analogies between the processes governing dry and wet 22 deposition, from a fundamental mass transfer perspective. In both cases there is a transfer of 23

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127 mass between the dry

ambient phase and another medium, either a surface or vegetation in the 1
case o f dry

deposition, or a rain droplet or cloud in the case of wet precipitation. The specific 2 thermody

namic properties and chemical/biological reactions that govern the transfer of dry mass 3 to plants or aqueous droplets differ, but either process can be base d on conceptualizing the 4 product of a concentration, or concentration difference, times a mass transfer coefficient which is 5 analogous to the basic dry deposition model: dry deposition = concentration x velocity ( US 6 EPA, 2011, Appendix F ). 7 Transference ratios require estimates of wet deposition of NOy and SOx, dry

deposition 8 of NOy

and SOx, and ambient air concentrations of NOy and SOx. Possible sources of 9 information include model estimates or a combination of model estimates and observations, 10 recogni zing that dry deposition is a modeled quantity that can use observed or modeled estimates 11 of concentration. The limited amount of NOy measurements in acid

- sensitive areas as well as 12 the combination of representative NOy , SO 2 and SO 4 observations generally preclude the use of 13 observations for development of a standard that is applicable nationall y. 14 The Policy

Assessment considers a blending of observations and models to take 15 advantage of their relative strengths; e.g., combining the NADP wet deposition obs ervations, 16 modeled dry

deposition, and a mix of modeled and observed concentrations, using the model for 17 those species not measured or measured with very sparse spatial coverage. A potential 18 disadvantage of mixing and matching observations and model estim ates is to lose consistency 19 afforded b y using just modeling alone. A modeling platform like CMAQ is based on adhering to 20 consistent treatment of mass conservation, by linking emission inputs with air concentrations and 21 concentrations to deposition. Incon sistencies from combining processes from different anal yt ical 22 platforms increase the chance that mass (of nitrogen or sulfur) would unintentionally be 23

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128 increased or decreased as the internal checking that assures mass conservation is lost. 1 Transference ratios incorporate a broad suite of atmospheric processes and consequentl y 2 an anal yt ical approach that instills consistency in the linkage of these processes is preferable to 3 an approach lacking such inherent consistency . This contention does not mean that observations 4 alone, if available, could not be used, but suggests that the inconsistencies in combining models 5 and observations for the purposes of developing transference ratios has the potential for creating 6 unintended artifacts. 7 While there is a re asonable conceptual basis for the concept of an aggregated deposition 8 velocity

referred to in the Policy

Assessment as a transference ratio, there is very limited ability 9 to compare observed and calculated ratios. This is because the deposition velocity i s dependent 10 on individual species, and the mass transfer processes of wet and dry removal, while 11 conceptuall y similar, are different. Consequently, there does not exist a meaningful approach to 12 measure such an aggregated or lumped parameter. Therefore, at this time, the evaluation of 13 transference ratios is based on sensitivity studies, anal ys is of variability, and comparisons with 14 other models, as described in Appendix F of the Policy Assessment. 15 As discussed in Appendix F, the interannual variability, as well as the sensitivity to 16 emission changes of roughly 50%, result s in changes of transference ratios of approximately 5
- 17 10%. Part of the reason for this inherent stability is due to the co

- dependence of concentration 18 and deposition. For example, as concentrations are reduced as a result of emissions reductions, 19 deposition in turn is reduced since deposition is a direct linear function of concentration leading 20 to negligible impact on the deposition

- to
- concentration ratio. L ikewise, an overestimate o f 21 concentration likely does not induce a bias in the transference ratio. While it is important to 22 continue to improve the model’s ability to match ambient concentrations in time and space, the 23

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129 bias of a modeled estimate of concentration relative to observ ations does not necessaril y result in 1 a bias in a calculated transference ratio. In effect, this consideration of bias cancellation reduces 2 the sensitivity

of transference ratios to model uncertainties and affords increased confidence in 3 the stability

of these ratios. Based on the series of sensitivity and variability anal ys es, the Policy 4 Assessment concludes that the transference ratios are relativel y stable and provide a sound 5 metric for linking deposition and concentration in the form of the standard. 6 As discussed in the policy Assessment, transference ratios are dependent on the platform 7 upon which they

are constructed. Comparisons of transference ratios constructed from different 8 modeling platforms do exhibit significant differences. While this div ergence of results may be 9 explained by

a variety of differences in process treatments, input fields and incommensurabilities 10 in species definitions and spatial configurations, it does suggest two very important conclusions. 11 First, the idea of using multiple platforms for different parts of the country may be problematic 12 as there does not exist a reliable approach to judge acceptance which is almost alway s based on 13 comparisons to observations. Second, since transference ratios are based on concentrations and 14 deposition, as the uncertainties in each of those components are reduced, the relative uncertainty 15 in the ratios also is reduced. This means that basic improvements in the model’s ability to 16 reproduce observed wet deposition and ambient concentration fields enhance the relative 17 confidence in the constructed transference ratios. Similarly, as in

- situ dry deposition flux 18 measurements become available that enable a more rigorous evaluation and diagnosis of 19 modeled dry

deposition processes, the expected i mproved treatment of dry deposition also would 20 increase confidence in transference ratios. Finally, deposition is directl y related to ambient air 21 concentrations. Models like CMAQ rel y on the concentration

- to
- deposition linkage to calculate 22 deposition, wh ich is the foundation for broadl y based and robust assessments addressing 23

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130 atmospheric deposition. In principle, the use of a modeled constructed transference ratio is based 1 on the same premise b y which we use models to estimate deposition in the first place. 2 The shortage of widel y available ambient air observations and the fact that estimates of 3 dry deposition requires modeling, collectively suggests that a unified modeling platform is the 4 best approach for constructing transference ratios. The Policy As sessment ( US EPA, 5 2011, section 2) considers CMAQ and other models, such as CAMx and the Canada’s AURAMS 6

- A Unified Regional Air

- quality Modeling S ys tem (Smy

the et al., 2008), and concludes that 7 CMAQ is the preferred modeling platform for constructing tran sference ratios for purposes of 8 developing a new secondary standard for consideration. This conclusion reflects the view that 9 for the purposes of defining transference ratios, a modeling platform should (1) be a multiple 10 pollutant model recognizing the my riad of connections across pollutant categories that directly 11 and indirectl y impact nitrogen and sulfur characterization, (2) include the most comprehensive 12 scientific treatments of atmospheric processes that relate directly and indirectly

to characterizin g 13 concentrations and deposition, (3) have an infrastructure capability that accommodates the 14 inclusion of improved scientific treatments of relevant processes and important input fields, and 15 (4) undergo frequent reviews regarding the adequacy of the underl yi ng science as well as the 16 appropriateness in applications. The CMAQ platform exhibits all these characteristics. It has 17 been (and continues to be) extensively evaluated for several pollutant categories, is supported by 18 a central infrastructure of EPA s cientists, with considerable interfacing with the scientific 19 research communities in academia and industry , whose mission is to improve and evaluate the 20 CMAQ platform. More directly , CMAQ, and its predecessor versions, has a long track record 21 going back to the NAPAP in the 1980’s of specific improvements in deposition processes, which 22 are described in Appendix F of the Policy Assessment. 23

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131 4. Aquatic Acidification Index 1 Having established the various expressions that link atmospheric deposition of nitrogen 2 and sulfur to ANC and the transference ratios that translate atmospheric concentrations to 3 deposition of nitrogen and sulfur, the Policy Assessment derives the following expression of 4 these linkages, which separates reduced forms of nitrogen, NHx, from ox idized forms: 5 6 ANCcalc = {ANClim + CL r /Qr }

– NHx/Qr - T NOy [NOy ]/ Qr - T SOx[SOx ]/ Qr (III

- 3) 7 8 Equation III

- 3 is the basic expression of the form of a standard that translates the 9 conceptual framework into an explicit expression that defines ANC as a function of the ambient 10 air indicators, NOy and SOx. This equation calculates an expected ANC value based on ambient 11 concentrations of NOy and SOx. The calculated ANC will differ from the target ANC (ANClim) 12 depending on how much the nitrogen and sulfur deposition associated with NOy , SOx, and NHx 13 differs from the critical load associated with just achieving the target ANC. If the sum of the 14 three deposition terms is less than or equal to the critical load associated with the target ANC 15 level (ANClim), the standard is achieved. 16 Based on equation III

- 3, the Policy Assessment defines an aquatic acidification index (AAI) that 17 is more simply

stated using terms that highlight the ambient air indicators: 18 19 AAI = F1

– F2

– F3[NOy ]

– F4[SOx

] (III

- 4) 20 21 where the AAI represents the long term (or steady state) ANC level associated with ambient air 22 concentrations of NOy and SOx. The factors F1 through F4 convey three attributes: a relative 23

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132 measure of the ecos ys tem’s ability to neutralize acids (F1), the acidifying potential of reduced 1 nitrogen deposition (F2), and the deposition

- to
- concentration translators for NOy (F3) and SOx 2 (F4). 3 Specificall y: 4 F1 = ANClim + CL r /Qr ; 5 F2 = NHx/ Qr = NHx deposition divided by Q r ; 6 F3 = TNOy/ Qr ; TNOy is the transference ratio that converts ambient air concentrations of NOy to 7 deposition of NOy ; and 8 F4 = T SOx/ Qr ; T SOx is the transference ratio that converts ambient air concentrations of SOx to 9 deposition of SOx. 10 All of these factors include representative Qr to maintain unit (and mass) consistency between 11 the AAI and the terms on the right side of equation III

- 4. 12 The F1 factor is the target ANC level plus the amount of deposition (critical load) the 13
ecos ys tem can receive and still achieve the target level. It incorporates an ecosy stem’s ability to 14 generate acid neutralizing capacity through base cation supply ([BC] * 0 ) and to neutralize 15 acidifying nitrogen deposition through Neco, both of which are incorporated in the CLterm. As 16 noted above, because Neco can onl y neutr alize nitrogen deposition (oxidized or reduced) there 17 may

be rare cases where Neco exceeds the combination of reduced and oxidized nitrogen 18 deposition. Consequently , to ensure that the AAI equation is applicable in all cases that may 19 occur, equation III

- 4 is conditional on total nitrogen deposition, {NHx + F3[NOy]}, being 20 greater than Neco. In rare cases where Neco is greater than {NHx + F3[NOy]}, F2 , F3 , and 21 Neco would be set equal to 0 in the AAI equation. The consequence of setting F2 and F3 to ze ro 22

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133 is simply to constrain the AAI calculation just to SOx, as nitrogen would have no bearing on 1 acidifying contributions in this case. 2 The Policy

Assessment concludes that equation III

- 4 (US EPA, 2011, equation 7

- 12), 3 which defines the AAI, is ecologicall y relevant and appropriate for use as the form of a national 4 standard designed to provide protection for aquatic ecos ys tems from the effects association with 5 acidifying deposition associated with concentrations of oxides of nitrogen and sulfur in the 6 ambie nt air. This AAI equation does not, however, in itself, define the spatial areas over which 7 the terms of the equation would apply . To specify values for factors F1 through F4, it is 8 necessary

to define spatial areas over which these factors are determine d. Thus, it is necessary to 9 identify

an approach for spatially aggregating water bodies into ecologically meaningful regions 10 across the U.S., as discussed below. 11 5. Spatial Aggregation 12 As discussed in the Policy Assessment, one of the unique aspects of this form is the need 13 to consider the spatial areas over which values for the F factors in the AAI equation are 14 quantified. Ecos ys tems across the U.S. exhibit a wide range of geological, hydrological and 15 vegetation characteristics that influence greatl y th e ecos ys tem parameters, Q, BC 0 * and Neco that 16 are incorporated in the AAI. Variations in ecos ys tem attributes naturall y lead to wide variability 17 in the sensitivities of water bodies in the U.S. to acidification, as well as in the responsiveness of 18 water b odies to changes in acidifying deposition. Consequently , variations in ecosy stem 19 sensitivity , and the uncertainties inherent in characterizing these variations, must be taken into 20 account in developing a national standard. In developing a secondary NAAQS to protect public 21 welfare, the focus of the Policy Assessment is on protecting sensitive populations of water 22 bodies, not on each individual water body, which is consistent with the Agency’s approach to 23

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134 protecting public health through primary NAAQS that focus on susceptible populations, not on 1 each individual. 2 The approach used for defining ecologicall y relevant regions across the U.S. in the Policy 3 Assessment (US EPA, 2011, section 7.2.5) is described below, along with approaches to 4 characterizing each region as acid sensitive or relatively non - acid sensitive. This 5 characterization facilitates a more detailed anal ys is and focus on those regions that are relativel y 6 more acid sensitive. This characterization is also used to avoid over

- protection in relati vely non 7 acid sensitive regions, regions that would receive limited benefit from reductions in the 8 deposition of oxides of nitrogen and sulfur with respect to aquatic acidification effects. 9 Approaches to developing representative values for each of the terms in the AAI equation 10 (factors F1 through F4) for each ecologically relevant region for which sufficient data are 11 available are then discussed. These spatial aggregation approaches are generall y applicable to 12 the contiguous United States. The following discussion also addresses the development of 13 factors for data

- limited regions and specifically for Hawaii, Alaska and the U.S. territories. 14 Stated more simply , this section discusses appropriate way s to divide the country into 15 ecologicall y relevant regio ns; to characterize each region as either acid sensitive or relativel y 16
non - acid sensitive; and to determine values of factors F1 through F4 for each region, taking into 17 consideration the acid sensitivity of each region and the availability of relevant data . For each 18 such region, the AAI would be calculated based on the values of factors F1 through F4 specified 19 for that region. 20 In considering approaches to spatial aggregation, the Policy Assessment focuses on 21 methods that have been developed to define ecol ogically relevant regions, referred to as 22 ecoregions, which are meaningfull y related to the factors that are relevant to aquatic 23

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135 acidification. As noted above, the Policy Assessment did not focus on looking at each individual 1 water body, nor did it focus on aggregating over the entire nation, which would preclude taking 2 into account the inherent variability in atmospheric and ecological factors that fundamentall y 3 modify

the relationships that are central to the development of an ecologically relevant AAI. 4 Based on considering available classification schemes, the Policy Assessment concludes 5 that Omernik’s ecoregion classification is the most appropriate method to consider for the 6 purposes of this review. This classification offers several levels of spati al delineation, has 7 undergone an extensive scientific peer review process, and has explicitly been applied to 8 delineating acid sensitive areas within the U.S. Further, the Policy Assessment concludes that 9 ecoregion level III (Figure III
- 1 ) resolution, with 84 defined ecoregions in the contiguous U.S., 9 10 is the most appropriate level to consider for this purpose. The spatial resolution afforded b y 11 level III strikes an appropriate balance relative to the reasoning that supports conclusions on 12 indicators, as discussed above. The Policy Assessment concludes that the most detailed level of 13 resolution (level IV) is not appropriate given the limited data availability to address nearl y 1000 14 subdivisons within that level and the currently evolving nature of level IV regions. Further, level 15 III ecoregions are preferred to level II in that level III ecoregions, but not level II ecoregions, are 16 largel y contiguous in space which allows for a more coherent development of information to 17 quantify

the AAI factors and to ch aracterize the concentrations of NOy and SOx in the ambient 18 air within each ecoregion. 19 Appendix C of the Policy Assessment includes a description of each level III ecoregion. 20 The Policy

Assessment notes that the use of ecoregions is an appropriate spatia l aggregation 21 scheme for this standard focused on deposition

- related aquatic acidification effects, while many 22 of the same ecoregion attributes may be applicable in subsequent NAAQS reviews that may 23

9 We note that an 85 th area within Omernik’s Ecoregion Level III is currently being developed for California

.

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136 address other deposition

- related aquatic and terrestrial ecological effects. Because atmospheric 1 deposition is modified by ecos ys tem attributes, the ty pes of vegetation, soils, bedrock geology , 2 and topographic features that are the basis of this ecoregion classification approach also will 3 likely

be key attributes for other deposition

- related effects (e.g., terrestrial acidification, nutrient 4 enrichment) that link atmospheric concentrations to an aquatic or terrestrial ecological indicator. 5

**Figure III**  **- 1**  **.** Omernik Ecoregion III areas ( **http://www.epa.gov/wed/pages/ecoregions** ). 6 7

a. Ecoregion Sensitivity 8 The Policy

Assessment used Omernik’s original alkalinity data ( US EPA, 2011, as section 9 2 ) and more recent ANC data to delineate two broad groupings of ecoregio ns: acid

- sensitive and 10

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137 relativel y non - acid sensitive ecoregions. This delineation was made to facilitate greater focus on 1 those ecoregions with water bodies that generall y have greater acid sensitivity and to avoid over

2 protection in regions with general ly less sensitive water bodies. The approach used to delineate 3 acid

- sensitive and relatively non - acid sensitive regions included an initial numerical

- based 4 sorting scheme using ANC data, which categorized ecoregions with relatively high ANC values 5 as bein g relativel y non - acid sensitive. This initial delineation resulted in 29 of the 84 Omernik 6 ecoregions being categorized as acid sensitive. Subsequently , land use data were also considered 7 to determine to what extent an ecoregion is of a relatively pristi ne and rural nature b y quantifying 8 the degree to which active management practices related to development and agriculture occur in 9 each ecoregion. 10 The overall objective is to produce a logical and practical grouping of ecoregions that 11 experience adver se conditions with respect to aquatic acidification and are likely to respond to 12 changes in concentrations of NOy and SOx in the ambient air and to the related deposition 13 levels. To achieve this goal, a two

- step process has been applied, first identifying acid sensitive 14 ecoregions based on water quality data alone, and second identifying among those acid

- sensitive 15 ecoregions those with highl y developed and managed areas. These highl y developed and 16 managed ecoregions are placed in a non

- acid sensitive cate gory to avoid over protection beyond 17 what is requisite to protect public welfare. More specificall y, in determining an ecoregion’s acid 18 sensitivity

status in step 1, ANC data across the 84 ecoregions are sorted ( US EPA, 2011, section 19 7 ) to determine the number of water bodies within a region with ANC values suggestive of acid 20 sensitivity , so as to screen out regions with an overabundance of high ANC values. In reviewing 21 the ANC data, the Policy Assessment identified 29 ecoregions that meet two criteria : ( 1) greater 22 than 5% of water bodies with data with ANC values less than 200 μeq/Land (2) greater than 1% 23

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138 of water bodies with ANC values less than 100 μeq/L

. In step 2, land use data were used to 1 identify

those acid sensitive ecoregions with significant managed areas that would not be 2 considered as having a relatively pristine and rural character. The percentage of the 3 combination of developed (residential, transportation, industrial and commercial) and 4 agricultural (croplands, pastures, orchards, vineyards) land use was used as an indicator of 5 managed land use area. Forest cover was used as an indicator of non

- managed land use more 6 directly

reflecting the pristine quality of a region. Based on the 2006 National Land Cover Data 7 base (NLCD, http://www.epa.gov/mrlc/nlcd

- 2006.html ), acid sensitive ecoregions would meet 8 both of the following land use data criteria: percent of developed and agricultural area less than 9 20% combined with forested area greater than 50%. The combination of steps 1 and 2 identify 10 22 relativel y acid sensitive areas (Table III

- 1 and Figure III
- 2 ). 11 Consideration was also given to the use of naturally acidic conditions in defining 12 relativel y non - acid sensitive areas. For examp le, several of the ecoregions located in plains near 13 the coast exhibit elevated dissolved organic carbon (DOC) levels, which is associated with 14 naturally

acidic conditions. DOC in surface waters is derived from a variety of weak organic 15 acid compounds gen erated from the natural availability and decomposition of organic matter 16 from biota. Consequently, high DOC is associated with “natural” acidity , with the implication 17 that a standard intended to protect against atmospheric contributions to acidity is not an area of 18 focus. The evidence suggests that several of the more highl y managed ecoregions in coastal or 19 near coastal transition zones are associated with relatively high DOC values, ty

picall y exceeding 20 on average 5 mg/l, compared to other acid sensitive areas. Although there is sound logic to 21 interpret naturally

acidic areas as relatively non - acid sensitive, natural acidity indicators were not 22 explicitly

included in defining relativel y non - acid sensitive areas as there does not exist a 23

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139 consensus - based quantifiable scientific definition of natural acidity . Approaches to explicitly 1 define natural acidity likely will be pursued in future reviews of the standard. 2 **Table III -**  **1**

. **List of 22 Acid -**  **Sensitive Areas Ecoregion nam**  **e Ecoregion number**  Ridge and Vall ey 8.4.1 Northern Appalachian Plateau and Uplands 8.1.3 Piedmont 8.3.4 Western Allegheny Plateau 8.4.3 Southwestern Appalachians 8.4.9 Boston Mountains 8.4.6 Blue Ridge 8.4.4 Ouachita Mountains 8.4.8 Central Appalachians 8.4.2 Northern Lakes and F orests 5.2.1 Maine/New Brunswick Plains and Hills 8.1.8 North Central Appalachians 5.3.3 Northern Appalachian and Atlantic Maritime Highlands 5.3.1 Columbia Mountains/Northern Rockies 6.2.3 Middle Rockies 6.2.10 Wasatch and Uinta Mountains 6.2.13 No rth Cascades 6.2.5 Cascades 6.2.7 Southern Rockies 6.2.14 Sierra Nevada 6.2.12 Idaho Batholith 6.2.15 Canadian Rockies 6.2.4 3

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140 1 **Figure III**  **- 2**  **.** Acid

- sensitive ecoregions identified in grey fill (22 out of 84 ecoregions). 2 3 b. Representative Ecoregion

-specific Factors 4 Having concluded that the Omernik level III ecoregions are an appropriate approach to 5 spatial aggregation for the purpose of a standard to address deposition

- related aquatic 6 acidification effects, the Policy Assessment uses those ecoregio ns to define each of the factors in 7 the AAI equation. As discussed below, factors F1 through F4 in equation III

- 4 are defined for 8 each ecoregion by

specifyingecoregion

- specific values for each factor based on monitored or 9 modeled data that are representative of each ecoregion. 10

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141 i. Factor F1 1 As discussed above, factor F1 reflects a relative measure of an ecos ys tem’s ability to 2 neutralize acidifying deposition, and is defined as: F1 = ANClim + CL r /Qr . The value of F1 for 3 each ecoregion would be based on a representative critical load for the ecoregion (CLr) 4 associated with a single national target ANC level (ANClim, discussed below in section III.D), 5 as well as on a representative runoff rate (Q r). To specify ecoregion

- specific values for the term 6 Q r , the Policy

Assessment used the median value of the distribution of Q values that are available 7 for water bodies within each ecoregion. To specify ecoregion

- specific representative values for 8 the term CLr in factor F1, a distribution 10 of calculated critical lo ads was created for the water 9 bodies in each ecoregion for which sufficient water quality and h ydrology

data are available. 11 10 The representative critical load was then defined to be a specific percentile of the distribution of 11 critical loads in the ecoregi on. Thus, for example, using the 90 th percentile means that within an 12 ecoregion, 90 percent of the water bodies would be expected to have higher calculated critical 13 loads than the representative critical load. That is, if the representative critical load were to 14 occur across the ecoregion, 90 percent of the water bodies would be expected to achieve the 15 national ANC target or better. 16 The specific percentile selected as part of the definition of F1 is an important parameter 17 that directly

impacts the representative critical load specified for each ecoregion, and therefore 18 the degree of protectiveness of the standard. A higher percentile corresponds to a lower critical 19 load and, therefore, to lower allowable ambient air concentrations of NOy and SOx and relat ed 20

10 T he distribution of critical loads wa s based on CL values calculated with Neco at the lake level. Consideration could also be given to using a distribution of CLs without Neco and adding the ecoregion average Neco value to the nth percentile critical load. This would avoid cases where the lake

- level Neco value potentially could be greater than total nitrogen deposition. The CL at the lake level represents the CL for the lake to achieve the specified national target ANC value. 11 The Policy Assessment judge d the data to be sufficient for this purpose if data are available from more than 10 water bodies in an ecoregion.

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142 deposition to achieve a target AAI level. In conjunction with the other terms in the AAI 1 equation, alternative forms can be appropriatel y characterized in part b y i dentifying a range of 2 alternative percentiles. The choice of an appropriate range of percentiles to consider for acid

3 sensitive and relativel y non - acid sensitive ecoregions, respectivel y, is discussed below. 4 ( a ) Acid

-sensitive Ecoregions 5 In identifying percentiles that are appropriate to consider for the purpose of calculating 6 factor F1 f

or ecoregions characterized as acid sensitive, the Policy Assessment concludes that it 7 is appropriate to focus on the upper part of the distribution of critical loads, so as to ensure that 8 the ecoregion would be represented b y relativel y more acid sensitiv e water bodies within the 9 ecoregion. Specifying factor F1 in this way would help to define a standard that would be 10 protective of the population of acid sensitive water bodies within an ecoregion, recognizing that 11 even ecoregions characterized as acid sen sitive may contain a number of individual water bodies 12 that are not acid sensitive. The Policy Assessment recognizes that there is no basis for 13 independentl y evaluating the degree of protectiveness afforded b y an y specific percentile value, 14 since it is th e combination of form and level, in conjunction with the indicator and averaging 15 time, which determine the degree of protectiveness of a standard. In light of this, the Policy 16 Assessment concludes that it is appropriate to consider a range of percentiles, from well above 17 the 50 th percentile, or median, of the distribution to somewhat below the highest value. More 18 specificall y, the Policy Assessment concludes it is appropriate to consider percentiles in the 19 range of the 70 th to the 90 th percentile . This conclusion is based on the judgment that it would 20 not be appropriate to represent an ecoregion with the lowest or near lowest critical load, so as to 21 avoid potential extreme outliers that can be seen to exist at the extreme end of the data 22 distributions, which would not be representative of the population of acid sensitive water bodies 23

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143 within the ecoregion and could lead to an overly protective standard

. At the same time , in 1 considering ecoregions that are inherentl y acid sensitive, i t is judged to be appropriate to limit 2 the lower end of the range for consideration to the 70 th percentile, a value well above the median 3 of the distribution , so that a substantial majority of acid

- sensitive water bodies are protected

. 4 In considering this conclusion, the CASAC Panel noted that the data bases for calculating 5 critical loads within an ecoregion are not necessarily representative of all water bodies within an 6 ecoregion. That is, in many ecoregions the lake sampling design used in studies that generated 7 the relevant data may have focused on the relatively more sensitive water bodies within an 8 ecoregion (Russell and Samet, 2011 a). Consequently , a given percentile of the distribution of 9 calculated critical loads, based on sampled water bodie s, may not be representative of that 10 percentile of all water bodies across an entire ecoregion. To the extent that the sampling of water 11 bodies within an ecoregion was skewed toward the relativel y more sensitive water bodies, 12 selecting a given percentile from the distribution of available critical loads would result in a 13 somewhat higher percentile of all water bodies within that ecoregion having a higher calculated 14 critical load than the representative critical load value. Thus, the extent to which study sampling 15 designs have resulted in skewed distributions of calculated critical loads is an uncertainty that is 16 appropriate to consider in selecting a percentile for the purpose of defining the factor F1 in the 17 AAI equation. 18 ( b ) Non

- acid sensitive Ecoregion s 19 With regard to identifying percentiles that are appropriate to consider for the purpose of 20 calculating factor F1 for ecoregions characterized as relativel y non - acid sensitive, the Policy 21 Assessment recognizes that while such ecoregions are generall y les s sensitive to acidifying 22 deposition from oxides of nitrogen and sulfur, they may contain a number of water bodies that 23

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144 are acid sensitive This category includes ecoregions that are well protected from acidification 1 effects due to natural production of b ase cations and high ANC levels, as well as naturall y acidic 2 sy stems with limited base cation production and consequently very low critical loads. Therefore, 3 the use of a critical load that would be associated with a highl y sensitive water body in a 4 naturally

acidic s ys tem would impose a high degree of relative protection in terms of allowable 5 ambient air concentrations of oxides of nitrogen and sulfur and related deposition, while 6 potentially

affording little or no public welfare benefit from attempting t o improve a naturally 7 acidic s ys tem. 8 Based on these considerations, the Policy Assessment concludes it is appropriate to 9 consider the use of a range of percentiles that extends lower than the range identified above for 10 acid

- sensitive ecoregions. Consideration of a lower percentile would avoid representing a 11 relativel y non - acid sensitive ecoregion by a critical load associated with relatively more acid

12 sensitive water bodies. In particular, the Policy Assessment concludes it is appropriate to focus 13 on the median or 50 th percentile of the distribution of critical loads so as to avoid over

- protection 14 in such ecoregions. Recognizing that relativel y non - acid sensitive ecoregions generall y are not 15 sampled to the extent that acid

- sensitive ecoregions are, it also is appropriate to consider using 16 the median critical load of all relativel y non - acid sensitive ecoregions for each such ecoregion. 17 ii. Factor F2 18 As discussed above, factor F2 is the amount of reduced nitrogen deposition within an 19 ecoregion, including the deposition of both ammonia gas and ammonium ion, and is defined as: 20 F2 = NHx/ Qr . The Policy Assessment calculated the representative runoff rate, Q r , using a 21 similar approach as noted above for factor F1; i.e., the median value of the distribution o f Q 22 values that are available for water bodies within each ecoregion. In the Policy Assessment, 2005 23

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145 CMAQ model simulations over 12

- km grids are used to calculate an average value of NHx for 1 each ecoregion. The NHx term is based on annual average model out puts for each grid cell, 2 which are spatially

averaged across all the grid cells contained in each ecoregion to calculate a 3 representative annual average value for each ecoregion. The Policy Assessment concludes that 4 this approach of using spatially average d values is appropriate for modeling, largel y due to the 5 relativel y rapid mixing of air masses that typicall y results in relativel y homogeneous air quality 6 patterns for regionall y dispersed pollutants. In addition, there is greater confidence in using 7 spatially

averaged modeled atmospheric fields than in using modeled point

- specific fields. 8 This averaging approach is also used for the air concentration and deposition terms in 9 factors F3 and F4, as discussed below. The Policy Assessment notes that modeled NHx 10 deposition exhibits greater spatial variability than the other modeled terms in factors F3 and F4. 11 Recognizing this greater variability , the Policy Assessment concludes that it would be 12 appropriate to consider alternative approaches to specifying the value of NHx. One such 13 approach might involve the use of more localized and/or contemporaneous modeling in areas 14 where this term is likel y to be particularly variable and important. 15 iii. Factors F3 and F4 16 As discussed above, factors F3 and F4 are the ratios that relate ambient air concentrations 17 of NOy

and SOx to the associated deposition, and are defined as follow: F3 = TNOy / Qr and F4 18 = T SOx/ Qr . TNOy is the transference ratio that converts ambient air concentrations of NOy to 19 deposition of NOy

an d T SOx is the transference ratio that converts ambient air concentrations of 20 SOx to deposition of SOx. The representative runoff rate, Q r , is calculated as for factors F1 and 21 F2. The transference ratios are based on the 2005 CMAQ simulations, using avera ge values for 22 each ecoregion, as noted above for factor F2. More specifically , the transference ratios are 23

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146 calculated as the annual deposition of NOy or SOx spatially

averaged across the ecoregion and 1 divided by

the annual ambient air concentration of NOy or SOx, respectively , spatiall y averaged 2 across the ecoregion. 3

c. Factors in Data

- limited Ecoregions 4 As discussed above in section III.B.5.a, in the Policy Assessment the initial delineation of 5 acid

- sensitive and relatively non - acid sensitive ecoregio ns was based on available ANC and 6 alkalinity

data. Areas not meeting the ANC criteria described above are categorized as 7 relativel y non - acid sensitive. The development of a reasonable distribution of critical loads for 8 water bodies within an ecoregion f or the purpose of identifying the representative critical load 9 requires additional data, including more specific water quality data for major cations and anions. 10 This means that the water bodies that can be used to develop a distribution of critical loads is 11 generall y a subset of those water bodies for which ANC data are available Consequently , there 12 are certain ecoregions with sparse data that are not suitable for developing a distribution of 13 critical loads. 14 As noted above, the Policy Assessment judges that it is not appropriate to develop such 15 distributions based on data from less than ten water bodies within an ecoregion. Twelve such 16 ecoregions, which included only relativel y non - acid sensitive ecoregions, were characterized as 17 being data

- limited. F or these ecoregions, the Policy Assessment considered alternative 18 approaches to specifying values for the terms CLr and Q r for the purpose of determining values 19 for each of the factors in the AAI equation. For these data

- limited ecoregions, the Policy 20 Ass essment judges that it is appropriate to use the median values of CLr and Q r from the 21 distributions of these terms for all other relatively non

- acid sensitive ecoregions, rather than 22 attempting to use severely limited data to develop a value for these term s based solel y on data 23

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147 from such an ecoregion. Further, consideration could be given to using a single national default 1 value for all relativel y non - acid sensitive ecoregions. The Policy Assessment notes that this data 2 limitation is not a concern in spec ifying values for the other terms in the AAI equation for such 3 ecoregions, since those terms are based on data from the 2005 CMAQ model simulation, which 4 covers all ecoregions across the contiguous U.S. 5 d. Application to Hawaii, Alaska, and the U.S. Territories 6 The above methods for specifying ecoregion

- specific values for the factors in the AAI 7 equation appl y to those ecoregions within the contiguous U.S. For areas outside the continental 8 U.S., including Hawaii, Alaska, and the U.S. Territories, there is currentl y a lack of available 9 data to characterize the sensitivity of such areas, as well as a lack of water body

- specific data and 10 CMAQ

- ty

pe modeling to specify

values for the F1 through F4 factors. Thus, the Policy 11 Assessment has considered possible alternative approaches to specifying values for factors F1 12 through F4 in the AAI equation for these areas. 13 One such approach could be to specify area - specific values for the factors based on 14 values derived for ecoregions with similar acid sensitivities, to the extent that relevant 15 information can be obtained to determine such similarities. Such an approach would involve 16 conducting an analy sis to characterize similarities in relevant ecological attributes between 17 ecoregions in the contiguous U.S. and these areas outside the contiguous U.S. so as to determine 18 the appropriateness of utilizing ecoregion

- specific values for the CLr and Q r terms from one or 19 more ecoregions within the contiguous U.S. This approach would also involve conducting 20 additional air qual ity modeling for these area that are outside the geographical scope of the 21 currentl y available CMAQ model simulations, so as to develop the other information necessary 22 to specify

values for factors F2 through F4 for these areas. 23

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148 A second approach could re ly on future data collection efforts to establish relevant 1 ecological data within these areas that, together with additional air quality modeling, could be 2 used to specify

area - specific values for factors F1 through F4. Until such time as relevant data 3 be come available, these areas could be treated the same as data

- limited ecoregions in the 4 contiguous U.S. that are relatively non - acid sensitive. 5 The Policy

Assessment concludes that either approach would introduce substantial 6 uncertainties that arise from attempting to extrapolate values based on similarity assumptions or 7 arbitraril y assigning values for factors in the AAI equation that would be applicable to these 8 areas outside the contiguous U.S. In light of such uncertainties, the Policy Assessment 9
conc ludes that it would also be appropriate to consider rel yi ng on the existing NO 2 and SO 2 10 secondary

standards in these areas for protection of any potential direct or deposition

- related 11 ecological effects that may be associated with the presence of oxides of nitrogen and sulfur in the 12 ambient air. The Policy Assessment concludes that rel yi ng on existing secondary standards in 13 these areas is preferable to using a highl y uncertain approach to allow for the application of a 14 new standard based on the AAI in the absence of relevant area

- specific data. 15 6. Summary of the AAI Form 16 With regard to the form of a multi

- pollutant air quality standard to address deposition

17 related aquatic acidification effects, the Policy Assessment concludes that consideration should 18 be given to an ecologically relevant form that characterizes the relationships between the 19 ambient air indicators for oxides of nitrogen and sulfur, the related deposition of nitrogen and 20 sulfur, and the associated aquatic acidification effects in terms of a relevant ecological indicator. 21 Based on the available information and assessments, consideration should be given to using acid 22 neutralizing capacity (ANC) as the most appropriate ecological indicator for this purpose, in that 23

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149 it provides the most stable m etric that is highl y associated with the water quality properties that 1 are directl y responsible for the principal adverse effects associated with aquatic acidification: 2 fish mortality

and reduced aquatic species diversity

. 3 The Policy

Assessment developed such a form, termed an aquatic acidification index 4 (AAI), using a simple equation to calculate an AAI value in terms of the ambient air indicators of 5 oxides and nitrogen and sulfur and the relevant ecological and atmospheric factors that modify 6 the relati onships between the ambient air indicators and ANC. Recognizing the spatial 7 variability

of such factors across the U.S., the Policy Assessment concludes it is appropriate to 8 divide the country

into ecologicall y relevant regions, characterized as acid

- sens itive or relativel y 9
non - acid

- sensitive, and specify the value of each of the factors in the AAI equation for each such 10 region. Omernik ecoregions, level III, are identified as the appropriate set of regions over which 11 to define the AAI. There are 84 such ecoregions that cover the continental U.S. This set of 12 ecoregions is based on grouping a variety of vegetation, geological, and hydrological attributes 13 that are directly

relevant to aquatic acidification assessments and that allow for a practical 14 application of an aquatic acidification standard on a national scale. 15 The Policy

Assessment defines AAIby the following equation: AAI = F1

– F2

– 16 F3[NO y]

– F4[SO x ]. Factors F1 through F4 would be defined for each ecoregion b y specifying 17 ecoregion

- specific values for each factor based on monitored or modeled data that are 18 representative of each ecoregion. The F1 factor is also defined b y a target ANC value. More 19 specificall y: 20 (1) F1 reflects a relative measure of an ecos ys tem’s ability to neutralize acidifying 21 deposition. The value of F1 for each ecoregion would be based on a representative 22 critical load for the ecoregion associated with a single national target ANC level, as well 23

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150 as on a representative runoff rate. The representative runoff rate, which is also used in 1 specifying values for the other factors, would be the median value of the distributions of 2 runoff rates within the ecoregion. The representative critical load would be derived from 3 a distribution of critical loads calculated for each w ater body in the ecoregion for which 4 sufficient water quality and hydrology data are available. The representative critical load 5 would be defined b y s electing a specific percentile of the distribution. 6 In identifying a range of percentiles that are ap propriate to consider for this 7 purpose, regions categorized as acid sensitive were considered separatel y from regions 8 categorized as relativel y non - acid sensitive. For acid sensitive regions, the Policy 9 Assessment concludes that consideration should be given to selecting a percentile from 10 within the range of the 70 th to the 90 th percentile. The lower end of this range was 11 selected to be appreciably above the median value so as to ensure that the critical load 12 would be representative of the populatio n of relatively more acid sensitive water bodies 13 within the region, while the upper end was selected to avoid the use of a critical load 14 from the extreme tail of the distribution which is subject to a high degree of variability 15 and potential outliers. For relativel y non - acid sensitive regions, the Policy Assessment 16 concludes that consideration should be given to selecting the 50 th percentile to best 17 represent the distribution of water bodies within such a region, or alternatively to using 18 the median critical load of all relativel y non - acid sensitive areas, recognizing that such 19 areas are far less frequently evaluated than acid sensitive areas. Using either of these 20 approaches would avoid characterizing a generally non - acid

- sensitive region with a 21 critical load that is representative of relatively acid sensitive water bodies that may exist 22 within a generall y non - acid sensitive region. 23

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151 (2) F2 reflects the deposition of reduced nitrogen. Consideration should be given to 1 specifying the value of F2 f or each region based on the averaged modeled value across 2 the region, using national CMAQ modeling that has been conducted b y EPA. 3 Consideration could also be given to alternative approaches to specifying this value, such 4 as the use of more localized and/ or contemporaneous modeling in areas where this term is 5 likely

to be particularl y variable and important. 6 (3) F3 and F4 reflect transference ratios that convert ambient air concentrations of NO y 7 and SO x , respectivel y, into related deposition of nitrogen and sulfur. Consideration 8 should be given to specifyingthe values for F3 and F4 for each region based on CMAQ 9 modeling results averaged across the region. We conclude that specifying the values or 10 the transference rations based on CMAQ modeling results alone is preferred to an 11 alternative approach that combines CMAQ model estimates with observational data. 12 (4) The terms [NO y] and [SO x ] reflect ambient air concentrations measured at 13 monitoring sites within each region. 14 Using the equation, a value of AAI can be calculated for any measured values of ambient 15 NOy

and SOx. For such a NAAQS, the Administrator would set a single, national value for the 16 level of the AAI used to determine achievement of the NAAQS, as discussed below in section 17 III.D. The ecoreg ion

- specific values for factors F1 through F4 would be specified by EPA based 18 on the most recent data and CMAQ model simulations, and codified as part of such a standard. 19 These factors would be reviewed and updated as appropriate in the context of each periodic 20 review of the NAAQS. 21 The Policy

Assessment developed specific F factors for each ecoregion based on the 22 approach discussed above, using alternative percentiles and alternative national target ANC 23

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152 levels. The results of this analy sis for ecoregion s characterized as acid sensitive are presented in 1 Table 7

- 1a

- d in the Policy Assessment. 2 C. Averaging Time 3 As discussed in section 7.3 of the Policy Assessment, aquatic acidification can occur over 4 both long

- and short

- term timescales. L ong - term cumulative deposition of nitrogen and sulfur is 5 reflected in the chronic acid

- base balance of surface waters as indicated by measured annual 6 ANC levels. Similarly , the use of steady state critical load modeling, which generates critical 7 loads in terms of annual cumulative deposition of nitrogen and sulfur, means that the focus of 8 ecological effects studies based on critical loads is on the long

- term equilibrium status of water 9 quality

in aquatic ecos ys tems. Much of the evidence of adverse ecological effects associated 10 with aquatic acidification, as discussed above in section II.A, is associated with chronicall y low 11 ANC levels. Protection against a chronic ANC level that is too low is provided by reducing 12 overall annual average deposition levels for nitrogen and sulfur. 13 Reflecting this focus on long

- term acidifying deposition, the Policy Assessment 14 developed the AAI that links ambient air indicators to deposition

- related ecological effects, in 15 terms of several factors, F1 through F4. As discussed above, the se factors are all calculated as 16 annual average values, whether based on water quality and h ydrology data or on CMAQ model 17 simulations. In the context of a standard defined in terms of the AAI, the Policy Assessment 18 concludes that it is appropriate to con sider the same annual averaging time for the ambient air 19 indicators as is used for the factors in the AAI equation. 20 We also recognize that short

- term (i.e., hours or day s) episodic changes in water 21 chemistry , often due to changes in the hydrologic flow paths, can have important biological 22 effects in aquatic ecos ys tems. Such short

- term changes in water chemistry are termed “episodic 23

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153 acidification.” Some streams may have chronic or base flow chemistry that is generall y healthy 1 for aquatic biota, but may be subject to occasional acidic episodes with potentially lethal 2 consequences. Thus, short

- term episodic ecological effects can occur even in the absence of 3 long

- term chronic acidification effects. 4 Episodic declines in pH and ANC are nearl y ubiquitous in drainage waters throughout the 5 eastern U.S. Episodic acidification can result from several mechanisms related to changes in 6 hydrologic flow paths. For example, snow can store nitrogen deposited throughout the winter 7 and snowmelt can then release this stored nitrogen, together with nitrogen derived from 8 nitrification in the soil itself, in a pulse that leads to episodic acidification in the absence of 9 increased deposition during the actual episodic acidification event. The Policy Assessment notes 10 that inputs of nitrogen and sulfur from snowpack and atmospheric deposition largel y c ycl e 11 through soil. As a result, short

- term direct deposition inputs are not necessarily important in 12 episodic acidification. Thus, as noted in chapter 3 of the I SA, protection ag ainst episodic acidity 13 events can be achieved by establishing a higher chronic ANC level. 14 Taken together, the above considerations support the conclusion that it is appropriate to 15 consider the use of a long

- term average for the ambient air indicators NOy and SOx for an 16 aquatic acidification standard defined in terms of the AAI. The use of an annual averaging time 17 for NOy

and SOx concentrations would be appropriate to provide protection against low chronic 18 ANC levels, which in turn would protect against bo th long

- term acidification and acute acidic 19 episodes. 20 The Policy

Assessment has also considered interannual variability in both ambient air 21 quality

and in precipitation, which is directl y related to the deposition of oxides of nitrogen and 22 sulfur from the ambient air. While ambient air concentrations show ye ar - to
- ye ar variability , 23

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154 often the year - to
- ye ar variability in precipitation is considerabl y greater, given the highl y 1 stochastic nature of precipitation. The use of multiple years over which annual av erages are 2 determined would dampen the effects of interannual variability in both air quality

and 3 precipitation. For the ambient air indicators, the use of multiple

- ye ar averages would also add 4 stability

to calculations used to judge whether an area meets a standard defined in terms of the 5 AAI. Consequentl y, the Policy Assessment concludes that an annual averaging time based on 6 the average of each year over a consecutive 3 to 5 year period is appropriate to consider for the 7 ambient air indicators NOy and SOx. In reaching this conclusion, the Policy Assessment notes 8 that in its comments on the second draft Policy Assessment, CASAC agreed that a 3 to 5 year 9 averaging time was appropriate to consider (Russell and Samet, 2010b). 10 D. Level 11 As discussed above, the Policy Assessment concludes that ANC is the ecological 12 indicator best suited to reflect the sensitivity of aquatic ecos ys tems to acidifying deposition from 13 oxides of nitrogen and sulfur in the ambient air. ANC is an indicator of the aquatic acidifica tion 14 expected to occur given the natural buffering capacity of an ecos ys tem and the loadings of 15 nitrogen and sulfur resulting from atmospheric deposition. Thus, the Policy Assessment 16 developed a new standard for aquatic acidification that is based on the use of chronic ANC as the 17 ecological indicator as a component in the AAI. 18 The level of the standard would be defined in terms of a single, national value of the 19 AAI. The standard would be met at a monitoring site when the multi

- ye ar average of the 20 calcul ated annual values of the AAI was equal to or above the specified level of the standard. 12 21 The annual values of the AAI would be calculated based on the AAI equation using the assigned 22

12 Unlike other NAAQS, where the standard is met when the relevant value is at or below the level of the standard since a lower standard level is more protective, in this case a higher standard level is more protective.

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155 ecoregion

- specific values for factors F1 through F4 and monitored annu al average NOy and SOx 1 concentrations. Since the AAI equation is based on chronic ANC as the ecological indicator, the 2 level chosen for the standard would reflect a target chronic ANC value. As noted above, the 3 assigned F factors for each ecoregion would be determined b y EPA in the rulemaking to set the 4 NAAQS, based on water quality and h ydrology data, CMAQ modeling, the selected percentile 5 that is used to identify a representative critical load within the ecoregion, and the selected level 6 of the standard . The combination of the form of the standard, discussed above in section III.B, 7 defined b y the AAI equation and the assigned values of the F factors in the equation, other 8 elements of the standard including the ambient air indicators (section III.A) and their averaging 9 time (section III.C), and the level of the standard determines the allowable levels of NOy and 10 SOx in the ambient air within each ecoregion. All of the elements of the standard together 11 determine the degree of protection from adverse aquat ic acidification effects associated with 12 oxides of nitrogen and sulfur in the ambient air. The level of the standard play s a central role in 13 determining the degree of protection provided and is discussed below. 14 The Policy

Assessment focuses primaril y on information that relates degrees of 15 biological impairment associated with adverse ecological effects to aquatic ecos ys tems to 16 alternative levels of ANC in reaching conclusions regarding the range of target ANC levels that 17 is appropriate to consider for the level of the standard. The Policy Assessment develops the 18 rationale for identifying a range of target ANC levels that is appropriate to consider by 19 addressing questions related to the following areas: (1) associations between ANC and pH levels 20 to provid e an initial bounding for the range of ANC values to be considered; (2) evidence that 21 allows for the delineation of specific ANC ranges associated with varying degrees of severity of 22 biological impairment ecological effects; (3) the role of ANC in affording protection against 23

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156 episodic acidity ; (4) implications of the time lag response of ANC to changes in deposition; (5) 1 past and current examples of target ANC values applied in environmental management 2 practices; and (6) data linking public welfare benef its and ANC. 3 1. Association Between pH Levels and Target ANC Levels 4 As discussed above in section II.A and more full y in chapter 3 of the Policy Assessment, 5 specific levels of ANC are associated with differing levels of risk of biological impairment in 6 aquatic ecosy stems, with higher levels of ANC resulting in lower risk of ecosy stem impacts, and 7 lower ANC levels resulting in risk of both higher intensity of impacts and a broader set of 8 impacts. While ANC is not the causal agent determining biological e ffects in aquatic ecos ys tem, 9 it is a useful metric for determining the level at which a water body is protected against risks of 10 acidification. There is a direct correlation between ANC and pH levels which, along with 11 dissolved aluminum, are more closel y linked to the biological causes of ecosy stem response to 12 acidification. 13 Because there is a direct correlation between ANC and pH levels, the selection of target 14 ANC levels is informed in part through information on effects of pH as well as direct studi es of 15 effects related to ANC. Levels of pH are closely associated with ANC in the pH range of 16 approximately

4.5 to 7. Within this range, higher ANC levels are associated with higher pH 17 levels. At a pH level of approximately 4.5, further reductions in AN C generally do not correlate 18 with pH, as pH levels remain at approximately 4.5 while ANC values fall substantially

. 19 Similarly , at a pH value of approximately 7, ANC values can continue to increase with no 20 corresponding increase in pH. As pH is the primary causal indicator of effects related to aquatic 21 acidification, this suggests that ANC values below approximately

- 50 μeq/L (the apparent point 22 in the relationship between pH and ANC where pH reaches a minimum) are not likely to result in 23

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157 further damage. In addition, ANC values around and above approximately 100 μeq/L(the 1 apparent region in the relationship where pH reaches a maximum) are not likely to confer 2 additional protection. As a result, the initial focus in the Policy Assessment was on target ANC 3 values in the range of

- 50 to 100 μeq/L

. 4 2. ANC Levels Related to Effects on Aquatic Ecosystems 5 As discussed above in section II.A, the number of fish species present in a water body has 6 been shown to be positively correlated with the ANC level in the water, with higher values 7 supporting a greater richness and diversity of fish species. The diversity and distribution of 8 phy to
- zooplankton communities also are positively correlated with ANC. 9 A summary

of effects related to ANC ranges is shown above in Table II
- 1. Within the 10 ANC range from approximate ly

- 50 to 100 μeq/L, linear and sigmoidal relationships are 11 observed between ANC and ecos ys tem effects. On average, fish species richness is lower b y one 12 fish species for every 21 μeq/Ldecrease in ANC in Shenandoah National Park streams (I SA, 13 section 3.2 .3.4). As shown in Table II

- 1, ANC levels have been grouped into five categories 14 related to expected ecological effects, including categories of acute concern ( <0 μeq/L), severe 15 concern (0

- 20 μeq/L), elevated concern (20

- 50 μeq/L), moderate concern (50

- 1 00 μeq/L), and 16 low concern (>100 μeq/L). This categorization is supported by a large body

of research 17 completed throughout the eastern U.S. (Sullivan et al., 2006). 18 Water bodies with ANC values less than or equal to 0 μeq/Lat based flow are chronicall y 19 acidic. Such ANC levels can lead to complete loss of species and major changes in the ability of 20 water bodies to support diverse biota, especiall y in water bodies that are highly sensitive to 21 episodic acidification. Based on the above considerations, the Policy Assessment has focused on 22 target ANC levels no lower than 0 μeq/L. 23

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158 As discussed in the Policy Assessment, biota generally are not harmed when ANC values 1 are >100 μeq/L , due to the low probability that pH levels will be below 7. In the Adirondacks, 2 the number of fish species also peaks at ANC values >100 μeq/L. This suggests that at ANC 3 levels greater than 100 μeq/L , little risk from acidification exists in many aquatic ecos ys tems. At 4 ANC levels below 100 μeq/L , overall health of aquatic communiti es can be maintained, although 5 fish fitness and community diversity begin to decline **.** At ANC levels ranging from 100 down to 6 50 μeq/L , there is increasing likelihood that the fitness of sensitive species (e.g., brook trout, 7 zooplankton) will begin to decl ine. When ANC concentrations are below 50 μeq/L , the 8 probability

of acidification increases substantially , and negative effects on aquatic biota are 9 observed, including large reductions in diversity of fish species and changes in the health of fish 10 populations, affecting reproductive ability and fitness, especiall y in water bodies that are affected 11 by episodic acidification. While there is evidence that ANC levels above 50 can confer 12 additional protection from adverse ecological effects associated with aq uatic acidification in 13 some sensitive ecos ys tems, the expectation that such incremental protection from adverse effects 14 will continue up to an ANC level of 100 is substantially reduced. The Policy

Assessment 15 concludes that the above considerations support a focus on target ANC levels up to a level 16 greater than 50 μeq/L but below 100 μeq/L , such as up to a level of 75 μeq/L. 17 In considering the available scientific evidence, as summarized here and discussed in 18 more detail in the ISA and REA, in its review o f the second draft Policy Assessment, CASAC 19 expressed the following views about the range of biological responses that corresponds to this 20 range of ANC levels (i.e., 0

- 100 μeq/L): 21 There will likel y be biological effects of acidification at higher ANC value s 22 within this range, and there are relatively insensitive organisms that are not 23 impacted at ANC values at the low end of this range. Adverse effects of 24 acidification on aquatic biota are fairl y certain at the low end of this range of 25

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159 ANC and incremental benefits of shifting waters to higher ANC become 1 more uncertain at higher ANC levels. There is substantial confidence that 2 there are adverse eff ects at ANC levels below 20 μeq/L , and reasonable 3 confidence that there are adverse effects below 50 μeq/L . Levels of 50 4 μeq/Land higher would provide additional protection, but the Panel has less 5 confidence in the significance of the incremental benefit s as the level 6 increases above 50 μeq/L. (Russell and Samet, 2010b) 7 8 The Policy

Assessment concludes that the above considerations, including the views of 9 CASAC, provide support for focusing on target ANC levels in the range of 20 to 75 μeq/L

. 10 3. Conside ration of Episodic Acidity 11 As discussed in the Policy Assessment, across the broad range of ANC values from 0 to 12 100 μeq/L , ANC affords protection against the likelihood of decreased pH (and associated 13 increases in Al) during long or short periods. In general, the higher the ANC within this range, 14 the lower the probability of reaching low pH levels where direct effects such as increased fish 15 mortality

occur, as shown in Table 3

- 1 of the Policy Assessment. Accordingl y, greater 16 protection would be achiev ed b y target chronic ANC values set high enough to avoid pH 17 depression to levels associated with elevated risk. 18 The specific relationship between ANC and the probability of reaching pH levels of 19 elevated risk varies b y water body and fish species. ANC levels below 20 μeq/Lare generall y 20 associated with high probability of low pH, leading to death or loss of fitness of biota that are 21 sensitive to acidification (US EPA, 2008 , section 5.2.2.1; US EPA, 2009 , section 5.2.1.2). At 22 these levels, during episo des of high acidifying deposition, brook trout populations may 23 experience lethal effects. In addition, the diversity and distribution of zooplankton communities 24 decline sharpl y at ANC levels below 20 μeq/L. Overall, there is little uncertainty that signi ficant 25 effects on aquatic biota are occurring at ANC levels below 20 μeq/L

. 26

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160 It is clear that at ANC levels approaching 0 μeq/L (Table II

- 1), there is significant 1 impairment of sensitive aquatic ecos ys tems with almost complete loss of fish species. Avoi ding 2 ANC levels approaching 0 μeq/Lis particularl y relevant to episodic spikes in acidity that occur 3 during periods of rapid snow melt and during and after major precipitation events. Since the 4 ANC range considered in the Policy Assessment reflects avera ge, long

- term base flow values, it 5 is appropriate to consider protecting against episodic drops in ANC values to a level as low as 0 6 μeq/L . Staddard et al. (2003) noted on average a 30 μeq/Ldepression of ANC between spring 7 and summer time values, indicating the need to maintain higher base flow ANC levels to protect 8 against ANC levels below 0 μeq/L . The above considerations do not provide support for a target 9 chronic ANC level as low as 0 μeq/Lfor a standard that would protect against significant harm t o 10 aquatic ecosy stems, including harm from episodic acidification. The Policy Assessment 11 concludes that these considerations also support a lower end of the range for consideration no 12 lower than 20 μeq/L **.** 13 The CASAC agreed with this conclusion in its comments on the second draft Policy 14 Assessment (Russell and Samet, 2010b). CASAC noted that “there are clear and marked 15 biological effects at ANC values near 0 μeq/L, so this is probably not an appropriate target 16 value” for the AAI. With regard to the likelih ood of impairment of aquatic ecos ys tems due to 17 episodic acidification, in terms of specific target levels for chronic ANC, CASAC expressed the 18 following view: 19 Based on surface waters studied in the Northeast, decreases in ANC 20 associated with snowmelt [are ] approximately 50 μeq/L . Thus, based on these 21 studies, a long term ANC target level of 75 μeq/L would generall y guard against 22 effects from episodic acidification down to a level of about 25 μeq/L . (Russell and 23 Samet, 2010b) 24 25 26 27

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161 4. Consideration of Ecosystem Response Time 1 The Policy

Assessment notes that when considering a standard level to protect against 2 aquatic acidification, it is appropriate to take into account both the time period to recovery as 3 well as the potential for recovery in acid

- sensitive ecoregions. Ecos ys tems become adversel y 4 impacted b y acidif

yi ng deposition over long periods of time and have variable time frames and 5 abilities to recover from such perturbations. Modeling presented in the REA (US EPA, 2009 , 6 section 4.2.4) shows the esti mated ANC values for Adirondack lakes and Shenandoah streams 7 under pre

- acidification conditions and indicates that for a small percentage of lakes and streams, 8 natural ANC levels would have been below 50 μeq/L . Therefore, for these water bodies, 9 reduction s in acidifying deposition are not likely to achieve an ANC of 50 μeq/Lor greater. 10 Conversel y, for some lakes and streams the level of perturbation from long periods of acidifying 11 deposition has resulted in very low ANC values compared to estimated natural conditions. For 12 such water bodies, the time to recovery would be largel y dependent on future inputs of acidifying 13 deposition. 14 Setting a standard level in terms of a target chronic ANC level is based on the long

- term 15 response of aquatic ecosy stems. The time required for a water body to achieve the target ANC 16 level

-- given a decrease in ambient air concentrations of NOy and SOx and related acidifying 17 deposition such that the critical load for a target ANC is not exceeded

– is often decades if not 18 centuries. In recognition of the potential public welfare benefits of achieving the target ANC in 19 a shorter time frame, the concept of target loads had been developed. Target loads represent the 20 depositional loading that is expected to achieve a particular lev el of the ecological indicator by a 21 given time. For example, to achieve an ANC level of 20 μeq/Lby 2030, it might be necessary to 22 specify

a higher target ANC level of, for example, 50 μeq/L , such that the depositional loading 23

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162 would be reduced more quickl y than would occur if the depositional loading was based on 1 achieving a target ANC level of 20 μeq/L as a long

- term equilibrium level. In this example, the 2 target ANC of 50 μeq/L would ultimately be realized many

years later. 3 The above considerations hav e implications for selecting an appropriate standard level, in 4 that the standard level affects not only the ultimate degree of protection that would be afforded 5 by the standard, but also the time frame in which such protection would be realized. However, 6 the Policy

Assessment recognizes that there is a great deal of heterogeneity in response times 7 among water bodies and that there is only very limited information from dy namic modeling that 8 would help to quantify recovery time frames in areas across the country . As a consequence, 9 quantification of a general relationship between critical loads associated with a specific long

10 term target ANC level and target loads associated with achieving the target ANC level within a 11 specific time frame is not currentl y pos sible. Thus, while the time frame for recovery is an 12 important consideration in selecting an appropriate range of levels to consider, the Policy 13 Assessment concludes that it can only be considered in a qualitative sense at this time. 14 5. Prior Examples of Target ANC Levels 15 A number of regional organizations, states, and international organizations have 16 developed critical load frameworks to protect against acidification of sensitive aquatic 17
ecos ys tems. In considering the appropriate range of target ANC levels for consideration in this 18 review, it is informative to evaluate the target ANC levels selected b y these different 19 organizations, as well as the rationale provided in support of the selected levels. Chapter 4 of the 20 Policy

Assessment provides a detail ed discussion of how critical loads have been developed and 21 used in other contexts. Specific target values and their rationales are summarized below. 22

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163 The UNECE has developed critical loads in support of international emissions reduction 1 agreements. As n oted in chapter 4 of the Policy Assessment, critical loads were established to 2 protect 95 percent of surface waters in Europe from an ANC less than 20 μeq/L based on 3 protection of brown trout. Individual countries have set alternative ANC targets; for example, 4 Norway

targets an ANC of 30 μeq/Lbased on protection of Atlantic salmon. 5 Several states have established target ANC or pH values related to protection of lakes and 6 streams from acidification. While recognizing that some lakes in the Adirondacks wil l have a 7 naturally

low pH, the state of New York has established a target pH value of 6.5 for lakes that 8 are not naturally

below 6.5. As noted above, this level is associated with an ANC value that is 9 likely

to be between 20 and 50 μeq/L or possibl y higher. New Hampshire and Vermont have set 10 ANC targets of 60 μeq/L and 50 μeq/L , respectively . Tennessee has established site

- specific 11 target ANC values based on assessments of natural acidity , with a default value of 50 μeq/L 12 when specific data are not avail able. 13 Taken together, these policy responses to concerns about ecological effects associated 14 with aquatic acidification indicate that target ANC values between 20 and 60 μeq/Lhave been 15 selected b y states and other nations to provide protection of lakes and streams in some of the 16 more sensitive aquatic ecosy stems. 17 6. Consideration of Public Welfare Benefits 18 The point at which effect s on public welfare become adverse is not defined in the CAA. 19 Characterizing a known or anticipated adverse effect to public welfare is an important 20 component of developing any secondary

NAAQS. According to the CAA, welfare effects 21 include: 22 ...effects on s oils, water, crops, vegetation, manmade materials, animals, 23 wildlife, weather, visibility , and climate, damage to and deterioration of 24

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164 property, and hazards to transportation, as well as effect on economic values and 1 on personal comfort and well

- being, whe ther caused by transformation, 2 conversion, or combination with other air pollutants. (CAA, section 302(h)). 3 4 Consideration of adversity to public welfare in the context of the secondary NAAQS for oxides 5 of nitrogen and sulfur can be informed b y information about losses in ecosystem services 6 associated with acidifying deposition and the potential economic value of those losses, as 7 summarized above in section II. C and discussed more full y in chapter 4 of the Policy 8 Assessment. 9 Ecosy stem service losses at alternative ANC levels are difficult to enumerate. However, 10 in general there are categories of ecos ys tem services, discussed i n chapter 4 of the Policy 11 Assessment, that are related to the specific ecos ys tem damages expected to occur at alternative 12 ANC levels. Losses in fish populations due to very low ANC (below 20 μeq/L) are likel y 13 associated with significant losses in value fo r recreational and subsistence fishers. Many acid 14 sensitive lakes are located in areas with high levels of recreational fishing activity . For example, 15 in the northeastern U.S., where nearl y 8 percent of lakes are considered acidic, more than 9 16 percent of adults participate in freshwater fishing, with an estimated value of approximately $5 17 billion in 2006. This suggests that improvements in lake fish populations may be associated with 18 significant recreational fishing value. 19 As discussed in th e Policy Assessment, inland surface waters also provide cultural 20 services such as aesthetic and existence value and educational services. To the extent that 21 piscivorous birds and other wildlife are harmed b y the absence of fish in these waters, hunting 22
an d birdwatching activities are likel y to be adversely affected. A case study of the value to New 23 York residents of improving the health of lakes in the Adirondacks found significant willingness 24 to pay

for those improvements. When scaled to evaluate the improvement in lake health from 25

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165 achieving ANC values of 20 to 50 μeq/L , the study implies benefits to the New York population 1 roughl y on the order of $600 million per year (in constant 2007$). The survey administered in 2 this study

recognized that particip ants were thinking about the full range of services provided by 3 the lakes in question

– not just the recreational fishing services. Therefore the estimates of 4 willingness to pay

include resident’s benefits for potential hunting and birdwatching activities 5 and other ancillary

services. These results are just for New York populations. The Policy 6 Assessment concludes that if similar benefits exist for improvements in other acid sensitive 7 lakes, the economic value to U.S. populations could be very substantial , suggesting that, at least 8 by one measure of impact on public welfare, impacts associated with ANC less than 50 μeq/L 9 may be adverse to public welfare. 10 7. Summary of Alternative Levels 11 Based on all the above considerations, the Policy Assessment conclude s that 12 consideration should be given to a range of standard levels from 20 to 75 μeq/L . The available 13 evidence indicates that target ANC levels below 20 μeq/L would be inadequate to protect against 14 substantial ecological effects and potential catastrophic loss of ecos ys tem function in some 15 sensitive aquatic ecosy stems. While ecological effects occur at ANC levels below 50 μeq/Lin 16 some sensitive ecos ys tems, the degree and nature of those effects are less significant than at 17 levels below 20 μeq/L. Levels at and above 50 μeq/Lwould be expected to provide additional 18 protection, although uncertainties regarding the potential for additional protection from adverse 19 ecological effects are much larger for target ANC levels above about 75 μeq/L , as effects are 20 g enerall y appreciably less sensitive to changes in ANC at such higher levels. 21 In reaching this conclusion in the Policy Assessment, consideration was given to the 22 extent to which a target ANC level within this range would protect against episodic as well a s 23

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166 long

- term ecological effects. Levels in the mid

- to upper part of this range would be expected to 1 provide greater protection against short

- term, episodic peaks in aquatic acidification, while lower 2 levels within this range would give more weight to prot ection from long

- term rather than 3 episodic acidification. Similarly , levels in the mid

- to upper part of this range would be expected 4 to result in shorter time periods for recovery given the lag in ecos ys tem response in some 5 sensitive ecos ys tems relative to levels in the lower part of this range. The Policy Assessment 6 also note that this range encompasses target ANC values that have been established by various 7 States and regional and international organizations to protect against acidification of aquatic 8
ecos ys tems. 9 The Policy

Assessment recognizes that the level of the standard together with the other 10 elements of the standard, including the ambient air indicators, averaging time, and form, 11 determine the overall protectiveness of the standard. Thus, cons ideration of a standard level 12 should reflect the strengths and limitations of the evidence and assessments as well as the 13 inherent uncertainties in the development of each of the elements of the standard. The 14 implications of considering alternative standa rds, defined in terms of alternative combinations of 15 levels and percentile values that are a critical component of factor F1 in the form of the standard, 16 are discussed below in section III.E. Key uncertainties in the various components of the standard 17
are summarized and considered below in section III.F. 18 E. Combined Alternative Levels and Forms 19 To provide some perspective on the implications of various alternative multi

- pollutant, 20 AAI

- based standards, the Policy Assessment presented the number of acid

- se nsitive ecoregions 21 that would likely

not meet various sets of alternative standards. The alternative standards 22 considered were based on combinations of alternative target ANC levels, within the range of 20 23

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167 to 75 μeq/L , and alternative forms, characterized by alternative representative percentiles within 1 the range of the 70 th to 90 th percentile. These alternative standards are also defined in terms of 2 the other elements of the standard: ambient air indicators NOy and SOx, discussed above in 3 section III.A; other elements of the form of the standard, including ecoregion

- specific values for 4 factors F1 through F4 in the AAI equation, discussed above in section III.B.5; and an annual 5 averaging time for NOy and SOx, discussed above in section III.C. With regard to the averaging 6 time, the assessment did not consider multi

- ye ar averaging of the calculated annual AAI values 7 due to data limitations, including, for example, the lack of CMAQ modeling for multiple 8 consecutive ye ars. In this assessment, we characterize an ecoregion as likely not meeting a given 9 alternative standard if the calculated AAI value is less than the target ANC level of the standard, 10 recognizing that higher AAI values are more protective than lower values. 11 The results of this assessment are pr esented in Table 7

- 1a

- d in the Policy Assessment for 12 a subset of ecoregions including those characterized as acid sensitive. Calculated annual AAI 13 values at the ecoregion level are shown for each alternative standard considered. Based on these 14 AAI values , Table 7

- 2 in the Policy Assessment summarizes the number of acid

- sensitive 15 ecoregions that would likely not meet each of the alternative standards considered. 13 Calculated 16 AAI values for all ecoregions categorized as relatively non - acid sensitive are sh own in Table D

- 5 17 in Appendix D of the Policy Assessment. In all cases, these relatively non

- acid sensitive 18 ecoregions were estimated to meet all of the alternative standards considered in this assessment. 19 As described above, the AAI values presented in Table 7

- 1a

- d of the Policy Assessment 20 are based in part on data from 2005 CMAQ model simulations, which was used to generate 21

13 Ta

bles 7

- 1a

- d and 7

- 2 in the Policy Assessment present assessment results for 29 ecoregions that had been initially characterized as acid sensitive. Subsequently, based on a broader set of criteria used to characterize ecoregions as acid sensitive, as discussed above in section III.B.5.a, the set of ecoregions characterized as acid sensitive was narrowed to include 22 ecoregions.

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168 values for F2 through F4 in the AAI equation, as well as to estimate annual average ambient air 1 concentrations of NOy and SOx that reflect recent air quality in the absence of currentl y 2 available monitored concentrations in sensitive ecoregions across the country . Water quality and 3 hydrology

data from water bodies within each ecoregion were also used in calculating the AAI 4 values. S uch data were initially used to calculate critical loads for each water body with 5 sufficient data within an ecoregion so as to identify the nth percentile critical load representative 6 of the ecoregion used in calculating the F1 factor for the ecoregion. A s expected, the number of 7 ecoregions that likel y would not meet alternative standards increases with increasing percentile 8 values and target ANC levels (US EPA, 2011, Table 7

- 2 ). Out of 22 acid

- sensitive ecoregions, 9 the number of ecoregions that would lik ely not meet the alternative standards ranges from 22 for 10 the most protective alternative standard considered (75 μeq/L , 90 th percentile) to 4 for the least 11 protective alternative standard (20 μeq/L, 70 th percentile). It is apparent that both the percentile 12 and the level chosen have a strong influence, over the ranges considered, in determining the 13 number of areas that would likely not meet this set of alternative standards. 14 The Policy

Assessment observes that there is one grouping of these acid

- sensiti ve 15 ecoregions that would likely not meet almost all combinations of level and form under 16 consideration (US EPA, 2011, Table 7

- 2 and Appendix D ). This group is made up of southern 17 Appalachian mountain areas, including North Central Appalachians, 5.3.3; Ridge and Valley , 18 8.4.1; Central Appalachians, 8.4.2; Blue Ridge, 8.4.4; and Southwestern Appalachians, 8.4.9. In 19 addition, these ecoregions exhibit the highest amounts of exceedance relative to alternative 20 standards. 21 The Northern Appalachian and Atlantic Maritime Highlands (5.3.1), which includes the 22 Adirondacks, and the Northern Lakes and Forests (5.2.1) of the upper midwest exhibit similar 23

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169 patterns with respect to in the role of level and percentile in identifying regions not likely to meet 1 alternative s tandards, although there are considerably fewer cases compared to the regions in the 2 Appalachians. 3 In the mountainous west, the Sierra Nevada (6.2.12), Idaho Batholith (6.2.15) and the 4 Cascades (6.2.7) ecoregions likely would not meet alternative standard s in fewer cases relative to 5 eastern regions, with the Sierra Nevada ecoregion exhibiting relatively greater sensitivity 6 compared to all western regions. Onl y in the upper part of the ranges of level and percentile do 7 regions in the northern and central Rockies likel y not meet alternative standards. 8 In considering these findings, the Policy Assessment observes that the standard as 9 defined b y the AAI behaves in an intuitively logical manner. That is, an increase in ecoregions 10 likely

not to meet the standard is associated with higher alternative levels and percentiles, both of 11 which contribute to a lower regionally representative critical load. Moreover, the areas of known 12 adverse aquatic acidification effects are identified, mostly in high elevation regio ns or in the 13 northern latitudes

-- the Adirondacks, Shenandoahs, northern midwest lakes and the mountainous 14 west. These results reflect the first application of a nationwide model that integrates water 15 quality

and atmospheric processes at a national scale and provides findings that are consistent 16 with our basic understanding of the extent of aquatic acidification across the U.S. What is 17 particularl y noteworthy is that this model is not initialized with a starting ANC based on water 18 quality

data, which lik ely would result in a reproduction of water quality observations. Rather, 19 this standard reflects the potential of the changes in atmospheric concentrations of NOy and SOx 20 to induce long

- term sustained changes in surface water s ys tems. The Policy Assessme nt notes 21 that the fact that the patterns of adversity based on applying this standard are commensurate with 22

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170 what is observed in surface water s ys tems provides confidence in the basic underlying 1 formulation of the standard. 2 The Policy

Assessment notes that the Appalachian mountain regions merit further 3 inspection as they

stand out as areas with the largest relative exceedances from a national 4 perspective. Water quality data from these regions as well as an emissions sensitivity CMAQ 5 simulation were considered to better understand the simulated behavior of these regions. The 6 maps and tables in appendix D of the Policy Assessment include paired comparisons of the 7 CMAQ 2005 and emissions sensitivity simulations. The emissions sensitivity simulation reflects 8 domain

- wide reductions in NOy and SOx emissions of 48% and 42%, respectively , relative to 9 2005 base year emissions. The Policy Assessment assumes that this emissions sensitivity 10 simulation is indicative of future conditions. 11 The emissions sensitivity results project that many of the regions that likel y would not 12 meet the alternative standards based on recent air quality , especiall y at alternative levels of 20 13 and 35 μeq/L , would likely meet such standards in the future year scenario for the Appalachian 14 mountain regions. It is apparent that the AAI calculations are especiall y sensitive to changes in 15 SOx emissions as the Appalachian regions have the highest SOx concentrations and deposition 16 rates (US EPA, 2011, section 2), and the AAI equation responds a s expected to modeled 17 reductions in SOx. The emissions sensitivity scenario is a prospective application of the 18 standard, in the sense that rules derived from the air quality management process result in 19 reductions of NOy

and SOx emissions. Expected emi ssion changes over the next two decades 20 should be far greater than the 42 and 48% SOx and NOy reductions used in this analy sis, with a 21 consequent further reduction in areas that would likely not meet alternative standards. 22

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171 The Appalachian mountain regions generally have low DOC levels, average runoff rates, 1 moderatel y low base cation supply and highl y elevated sulfate concentrations. Collectively, 2 those attributes do not suggest naturall y acidic conditions as the availability of anthropogenic 3 contribution s of mineral acids is likely responsible for observed low ANC values in those 4 regions. 5 The Policy

Assessment notes the Sierra Nevada region as an interesting case study , as it 6 has some of the lowest critical load values nationally (US EPA, 2011, Table D

- 3 ). Water quality 7 data indicate extremel y l ow sulfate, as expected given the relativel y low SO 2 emissions in the 8 western U.S. Extremely low base cation suppl y and low Neco, which mitigate the effect of 9 nitrogen deposition, explain the low critical load v alues. Low Neco values appear to associate 10 well with high elevation western U.S. regions, perhaps reflecting the more arid and reduced 11 vegetation density

relative to eastern U.S. regions. The proximity to high level nitrogen 12 emissions combined with very l ow base cation supply

explains the cases where the Sierra region 13 likely

does not meet alternative standards. Because Neco values are low in the Sierras, the 14 sy stem responds effectively to reductions of NOx emissions, as illustrated in the maps and tables 15 of Appendix D of the Policy Assessment. Although Neco affords protection from the acidifying 16 effects of nitrogen deposition, the availability of excessive nitrogen neutralization capacity also 17 means that reductions in nitrogen are not as effective as redu ctions in SOx in reducing the 18 calculated AAI. 19 In reviewing these results, the Policy Assessment observes that the anal ys is of the 20 alternative combinations of level and form presented provide context for considering the impact 21 of different standards. Sin ce the AAI equation has been newl y developed in the Policy 22 Assessment, these examples of estimated exceedances help to address the question of whether 23

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172 the AAI equation responds in a reasonable manner with regard to identifying areas of concern 1 and to prosp ective changes in atmospheric conditions likely to result from future emissions 2 reduction strategies. The Policy Assessment concludes that the behavior of the AAI calculations 3 is both reasonable and explainable, which the Policy Assessment concludes serve s to increase 4 confidence in considering a standard defined in terms of the AAI. 5 F. Characterization of Uncertainties 6 This section summarizes discussions of the results of anal ys es and assessments, presented 7 more fully

in the Policy

Assessment (US EPA, 201 1 , section 7.6 and Appendices F and G), 8 intended to address the relative confidence associated with the linked atmospheric

- ecological 9 effects sy stem described above. An overview of uncertainties is presented in the context of the 10 major structural components underl yi ng the standard, as well as with regard to areas of relativel y 11 high uncertainty. The section closes with a discussion of data gaps and uncertainties associated 12 with the use of ecological and atmospheric modeling to specify the factors in the AA I equation, 13 which can be used to guide future field programs and longer

- term research efforts. 14 1. Overview of Uncertainty 15 As discussed in the Policy Assessment (US EPA, 2011, Table 7
- 3), there is relativel y low 16 uncertainty with regard to the conceptual f ormulation of the overall structure of the AAI

- based 17 standard that incorporat es the major associations linking biological effects to air concentrations. 18 Based on the strength of the evidence that links species richness and mortality to water quality , 19 the associations are strongl y causal and without any obvious confounding influence. The strong 20 association between the ecosy stem indicator (ANC) and the causative water chemistry species 21 (dissolved aluminum and hydrogen ion) reinforces the confidence in the l inkage between 22 deposition of nitrogen and sulfur and effects . This strong association between ANC and effects 23

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173 is supported by

a sound mechanistic foundation between deposition and ANC. The same 1 mechanistic strength holds true for the relationship between ambient air levels of nitrogen and 2 sulfur and deposition, which completes the linkage from ambient air indicators through 3 deposition to ecological effects. 4 There are relativel y higher uncertainties, however, in considering specific elements 5 within the structure of an AAI

- based standard, including the deposition of SOx, NOy , and NHx 6 as well as the critical load

- related component, each of which can vary within and across 7 ecoregions. Overall sy stem uncertainty relates not just to the uncertainty in eac h such element, 8 but also to the combined uncertainties that result from linking these elements together within the 9 AAI

- based structure. Some of these elements

– including , for example, dry deposition, pre

10 industrial base cation production , and reduced nitrogen deposition

– are estimated with less 11 confidence than other elements (US EPA, 2011, Table 7.3 ). The uncertainties associated with all 12 of these elements, and the combination of these elements through the AAI equation, are 13 discussed below and in the fo llowing sections related to measured data gaps and modeled 14 processes for both air quality and water quality

. 15 The lack of observed dry deposition data is constrained b y resources and the lack of 16 efficient measurement technologies. Progress in reducing unc ertainties in dry deposition will 17 depend on improved atmospheric concentration data and direct deposition flux measurements of 18 the relevant suite of NOy and SOx species

. 19 Pre
- industrial base cation productivity by definition is not observable. Contemporary 20 observations and inter

- model comparisons are useful tools that would help reduce the uncertainty 21 in estimates of preindustrial base cation productivity used in the AAI equation

. In characterizing 22 contemporary

base cation flux using basic water quality m easurements (i.e., major anioni and 23

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174 cation species as defined in equation 2.11 in the Policy Assessment) , it is reasonable to assume 1 that a major component of contemporary base cation flux is associated with pre

- industrial 2 weathering rates. To the extent that multiple models converge on similar solutions, greater 3 confidence in estimating pre

- industrial base cation production would be achieved. 4 Characterization of reduced nitrogen (NHx) deposition has been evolving over the last 5 decade. The relativel y high uncertainty in characterizing NHx deposition is due to both the lack 6 of field measurements and the inherent complexity of characterizing NHx with respect to source 7 emissions and dry

deposition. Because ammonia emissions are generated through a combinati on 8 of man

- made and biological activities, and ammonia is semi

- volatile, the ability to characterize 9 spatial and temporal distributions of NHx concentration s and deposition patterns is challenging. 10 While direct measurement of NHx deposition is resource int ensive because of the diffuse nature 11 of sources (i.e., area

- wide and not point sources), there have been more frequent deposition flux 12 studies, relative to other nitrogen species, that enable the estimation of both emissions and dry 13 deposition. Also, while ammonia has a relativel y high deposition velocity and traditionally was 14 thought to deposit close to the emissions release areas, the semi

- volatile nature of ammonia 15 results in re

- entrainment back into the lower boundary layer resulting in a more dis persed 16 concentration pattern exhibiting transport ty pe characteristics similar to longer lived atmospheric 17 species. These inherent complexities in source characterization and ambient concentration 18 patterns raise the uncertainty level of NHx in general. H owever, the Policy Assessment notes 19 that progress is being made in measuring ammonia with cost efficient samplers and anticipates 20 the gradual evolution of a spatially robust ammonia sampling network that would help support 21 anal ys es to reduce underlyingunc ertainties in NHx deposition. Also, from an aquatic 22 acidification perspective, NHx is not as important a driver as NOy and SOx in the mountainous 23

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175 areas in the eastern U
. S. However, the relative importance of NHx is likely to increase over 1 tim e, in light of air quality rules in place designed to reduce emissions of NOy and SOx. 2 2. Uncertainties Associated with Data Gaps 3 In summarizing uncertainties with respect to available measurement data and the use of 4 ecological and atmospheric models, the Policy Assessment indentified data gaps and model 5 uncertainties in relative terms by comparing, for example, the relative richness of data between 6 geographic areas or environmental media. With regard to relevant air quality measurements, the 7 Policy

Asses sment notes that such measurements are relativel y sparse in the western U.S. While 8 the spatial extent of CASTNET coverage has gradually incorporated western U.S. locations with 9 support from the National Parks Service (NPS), the relative density of monitoring sites is much 10 less than that in the eastern U.S. This relative disparity in spatial density

of monitors is 11 exacerbated as air quality patterns in the mountainous west generally exhibit greater spatial 12 heterogeneity

due to dramatic elevation gradients that impact meteorology and air mass flow 13 patterns. Similarly , water quality data coverage is far more comprehensive in the eastern U.S. 14 relative to the west 15 Measurements of NOy notably are lacking in both eastern and western acid

- sensitive 16 ecoregions. This adds uncertainty to the use of the AAI equation as the lack of NOy data limits 17 efforts to evaluate air quality model ing of NOy that is the basis for quantifying factor F3 in the 18 AAI equation . The lack of NOy measurements also limits efforts to charac terize the variability 19 and representativeness of modeled NOy concentrations within and across ecoregions

. Currently , 20 the Agency ’s ability to define the protection likely to be afforded by

alternative standards (in 21 terms of alternative levels and percentil es) is compromised by the lack of a full set of ambient air 22

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176 quality indicator measurements , notably including NOy , throughout sensitive ecoregions across 1 the U.S

. 2 Further, obtaining measurement of the dominant species that comprise NOy ( HNO 3 , true 3 NO 2 , NO , p
–

NO 3 , and PAN) would be useful to evaluate performance of NOy samplers. 4 Be yond the more well known dominant components of NOy , research efforts would be needed to 5 characterize total reactive nitrogen that may include significant amounts of organical ly

- b ound 6 nitrogen (beyond PAN) which is poorly understood with regard to emission sources and 7 concentration levels. 8 Field measurements of NHx have been extremel y limited, but have begun to be enhanced 9 through the National Atmospheric Deposition Program’s (NADP) passive ammonia network 10 (AMoN). AMoN measures ammonia at over 50 sites, with more than 35 at CASTNET locations. 11 Enhanced spatial coverage of reduced nitrogen measurements , particularl y t o understand within 12 and across ecoregion variability , and the inclusion of some continuous observations would 13 provide a better understanding of the uncertainty in the F2 factor in the AAI equation and of the 14 representativeness of modeled NHx deposition within and across ecoregions. 15 With regard to water quality dat a, the Policy

Assessment notes that such data are 16 ty picall y limited relative to air quality data sets, and are also relativel y sparse in the western U.S. 17 The TIME/LTM water quality sampling program in the eastern U.S. (as described in chapter 2 18 of the Po licy

Assessment) is an appropriate complement to national air monitoring programs as it 19 affords consistency across water bodies in terms of sampling frequency and anal ys is protocols. 20 Consideration should be given to extending the TIME/L TM design to all ac id sensitive 21 ecoregions, with priority for areas in the western mountains that are data limited and showing 22 initial signs of adversity particularl y with respect to aquatic acidification. The lack of a 23

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177 regulatory

requirement for TIME/LTM often jeopardizes funding support of this resource that is 1 especiall y valuable and cost effective. While there are several state and local agency water 2 quality

data bases, it is unclear the extent to which differences in sampling, chemical analy sis 3 and reporting protocols would impact the use of such data for the purpose of better 4 understanding the degree of protectiveness that would be afforded b y an AAI

- based standard 5 within sensitive ecoregions across the country . In addition, our understanding of water quality in 6 Alask a and Hawaii and the acid sensitiv ity of their ecoregions is particularly limited

. 7 Water quality data and modeling support the standard setting process. As more water 8 bodies are sampled, the critical load data bases would expand, enabling clearer delin eation of 9 ecoregion representative critical loads in terms of the nth percentile. This w ould provide more 10 refined characterization of the degree of protection afforded by a given standard. Longer term, 11 the availability

of water quality

trend data (annual to monthly sampled) would support 12 accountability

assessments that examine if an ecoregion’s response to air management efforts is 13 as predicted b y earlier model forecasting. The most obvious example is the long

- term response 14 of water quality

ANC change to changes in calculated AAI, deposition, ambient NOy and SOx 15 concentrations, and emissions. In addition, water quality trends data provide a basis for 16 evaluating and improving the parameterizations of processes in critical load models applied at 17 the ecoreg ion scale related to nitrogen retention and base cation suppl y. A better understanding 18 of soil processes , especially in the southern Appalachians, would enhance efforts to examin e the 19 variability

within ecoregions of the soil

- based adsorption and exchange processes which 20 moderate the suppl y of major cations and anions to surface waters and strongl y influence the 21 response of surface water ANC to changes in deposition of nitrogen and sulfur. 22

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178 3. Uncertainties in Modeled Processes 1 As discussed in the Policy A ssessment, from an uncertainty perspective, gaps in field 2 measurement data are related to uncertainties in modeled processes and in the specific 3 application of such models

. As noted above, processes that are embodied in an AAI

- based 4 standard are modeled u sing the CMAQ atmospheric model and steady state ecological models. 5 These models are characterized in the Integrated Science Assessment as being well established 6 and they

have undergone extensive peer review. Nonetheless, the application of these models for 7 purposes of specifying the factors in the AAI equation, on an ecoregion scale, is a new 8 application that introduces uncertainties, as noted below, especially in areas with limited 9 observational data that can be used to evaluate this specific applicatio n. Understanding 10 uncertainties in relevant modeled process thus involves consideration of the uncertainties 11 associated with appl yi ng each model as well as the combination of these uncertainties as the 12 models are applied in combination within the AAI frame work. 13 With regard to the application of CMAQ for purposes of use in an AAI

- based standard, 14 the modeling of dry deposition has been identified as having a relativel y high degree of 15 uncertainty. Due to a combination of sy stem complexity and resource constraints, there is no 16 routine observational basis for directl y comparing modeled dry deposition and measurements. 17 Periodic dry

deposition flux experiments covering a variety of vegetation, surfaces and 18 meteorology

across seasons would enable a more robust evaluation of modeled deposition of 19 nitrogen and sulfur. Given the difficulty in acquiring dry deposition observations, it becomes 20 especiall y important to evaluate the model’s ability to capture temporal and spatial ambient air 21 patterns of individual nitrogen and sulfur species which are used to drive dry deposition 22 calculations in models. For example, reducing a generall y acknowledged positive bias in model

- 23

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179 predicted SO 2 relative to observations is especiall y relevant to the AAI

- based standard, as SO 2 1 depos ition is a dominant contributor to total acidifyingdeposition in the eastern U.S. With 2 respect to oxidized nitrogen, observations of individual NOy species are important as air quality 3 models calculate the individual deposition of each species. The mode led transference ratios, 4 TNOy and T SOx used in factors F3 and F4 rel y on CMAQ’s ability to characterize both deposition 5 and concentration. Consequently , a better understanding of the variability of these factors 6 within and across ecoregions could be achi eved by improved availability of measured ambient 7 concentration s and deposition observations. 8 Steady

state biogeochemical ecos ys tem modeling is used to develop critical load 9 estimates that are incorporated in the AAI equation through factor F1. Consequently , the Policy 10 Assessment notes that an estimate of the temporal response of surface water ANC to deposition 11 and air concentration changes is not directly available. Lacking a predicted temporal response 12 impairs the ability

to conduct accountability asse ssments down to the effects level. 13 Accountability

assessments would examine the response of each step in the emissions source 14 through air concentration

– deposition

-- surface water quality
– biota continuum. The steady 15 state assumption at the ecosy stem level does not impair accountability assessments through the 16 air concentration/deposition range of that continuum. However, i n using steady state ecos ys tem 17 modeling, several assumptions are made relative to the long

- term importance of processes 18 related to soil adsorption of major ions and ecos ys tem nitrogen dyn amics. Because these models 19 often were developed and applied in glaciated areas with relativel y thin and organicall y rich 20 soils, their applicability is relativel y more uncertain in areas such as thos e in the non

- glaciated 21 clay

- based soil regions of the central Appalachians. Consequently , it is desirable to develop the 22 information bases to drive simple dy namic ecos ys tem models that incorporate more detailed 23

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180 treatment of subsurface processes, such as adsorption and exchange processes and sulfate 1 absorption. 2 4. Applying Knowledge of Uncertainties 3 An understanding of the relative uncertainties in a sy stem assists in setting priorities for 4 data collection efforts and research, with the expectation that su ch efforts would reduce 5 uncertainties over time and afford greater confidence in applications of an AAI

- based standard. 6 Because of the uniquel y wide breadth of pollutants and environmental media addressed b y an 7 AAI

- based multi

- pollutant standard, there are a wide range of uncertainties that are important to 8 consider relative to single pollutant standards that ty picall y address onl y direct effects of ambient 9 air exposures. For an AAI

- b ased standard, a reduction of the uncertaintie s across the various 10 modeled processes at the ecoregion scale would lead to greater confidence in the degree of 11 protection afforded b y the standard. 12 The Policy

Assessment notes that there is general ly low uncertainty with regard to the 13 conceptual devel opment and related major components of this standard. In recognizing the 14 scientific soundness of the basic structure of this standard, the Policy Assessment notes that 15 future efforts would be appropriatel y directed at expanding the availability of relevant data for 16 ecoregion

- specific e valuati on and application of the relevant modeling of ecological and 17 atmospheric processes, as identified above . Such efforts would further support consideration of 18 an AAI

- based standard and would guide field studies and analy ses designed to improve the 19 longer

- term confidence in such a standard. 20 G. CASAC Advice 21 The CASAC has advised EPA concerning the Integrated Science Assessment, the Risk 22 and Exposure Assessment, and the Policy Assessment. CASAC has endorsed EPA’s 23

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181 interpretation of the science embodied in the Integrated Science Assessment and the assessment 1 approaches and conclusions incorporated in the Risk and Exposure Assessment

. 2 Most recentl y, CASAC has considered the information in the final Policy Assessment in 3 prov iding its recommendations on the review of the new multi

- pollutant standard developed in 4 that document and discussed above (Russell and Samet, 2011 a). In so doing, CASAC has 5 expressed general support for the conceptual framework of the standard based on the underl yi ng 6 scientific information, as well as for the conclusions in the Policy Assessment with regard to 7 indicators, form, averaging time, and level of the standard that are appropriate for consideration 8 by the Agency

in reaching decisions on the revie w of the secondary NAAQS for oxides of 9 nitrogen and sulfur: 10 The final Policy Assessment clearl y s ets out the basis for the recommended ranges 11 for each of the four elements (indicator, averaging time, level and form) of a 12 potential NAAQS that uses ambient air indicators to address the combined effects 13 of oxides of nitrogen and oxides of sulfur on aquatic ecos ys tems, primaril y 14 streams and lakes. As requested in our previous letters, the Policy Assessment also 15 describes the implications of choosing specific co mbinations of elements and 16 provides numerous maps and tabular estimates of the spatial extent and degree of 17 severity of NAAQS exceedances expected to result from possible combinations of 18 the elements of the standard. 19 20 We believe this final PA is appropriat e for use in determining a secondary 21 standard to help protect aquatic ecos ys tems from acidifying deposition of oxides 22 of sulfur and nitrogen. EPA staff has done a commendable job developing the 23 innovative Aquatic Acidification Index (AAI), which provides a framework for a 24 national standard based on ambient concentrations that also takes into account 25 regional differences in sensitivities of ecosy stems across the country to effects of 26 acidifying deposition. (Russell and Samet, 2011 a) 27 28 CASAC also recommended that as EPA moves forward in the regulatory process “some 29 attention should be given to our residual concern that the available data may reflect the more 30 sensitive water bodies and thus, the selection of percentiles of waterbodies to be protected could 31

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182 be c onservativel y biased” (Russell and Samet, 2011 a). In additional, CASAC found some 1 improvements could be made to the uncertainty analy sis, as noted below. 2 With respect to indicators, CASAC supports the use of SOx and NOy as ambient air indicators 3 (discusse d above in section III.A) and ANC as the ecological indicator (discussed above in 4 section III.B.1): 5 The use of total reactive oxidized nitrogen (NOy) and sulfur oxides (SOx) as atmospheric 6 indicators of oxidized nitrogen (N) and sulfur (S) atmospheric co ncentrations is well justified. 7 The use in the Aquatic Acidification Index (AAI) of NOy and SOx as atmospheric indicators of 8 N and S concentrations is useful and corresponds with other efforts by EPA. As we have stated 9 previously , CASAC also agrees that a cid neutralizing capacity (ANC) is the most appropriate 10 ecological indicator of aquatic ecos ys tem response and resiliency to acidification. (Russell and 11 Samet, 2011 a) 12 With respect to the form of the standard (discussed above in section III.B), CASAC 13 state d the following: 14 EPA has developed the AAI, an innovative “form” of the NAAQS itself that 15 incorporates the multi

- pollutant, multi

- media, environmentally modified, 16 geographically variable nature of SOx/NOy deposition

- related aquatic 17 acidification effects. W ith the caveats noted below, CASAC believes that this 18 form of the NAAQS as described in the final Policy Assessment is consistent with 19 and directl y reflective of current scientific understanding of effects of acidifying 20 deposition on aquatic ecosy stems. ( Russell and Samet, 2011 a) 21 22 CASAC agrees that the spatial components of the form in the Policy Assessment 23 are reasonable and that use of Omernick’s ecoregions (Level III) is appropriate for 24 a secondary NAAQs intended to protect the aquatic environment from 25 acidification . . . (Russell and Samet, 2011 a) 26 27 The “caveats” noted b y CASAC include a recognition of the importance of continuing to 28 evaluate the performance of the CMAQ and ecological models to account for model 29

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183 uncertainties and to make the model

- depen dent factors in the AAI more transparent. In addition, 1 CASAC noted that the role of DOC and its effects on ANC would benefit from further 2 refinement and clarification (Russell and Samet, 2011 a). While CASAC expressed the view that 3 the “division of ecoreg ions into ‘sensitive’ and ‘non

- sensitive’ subsets, with a more protective 4 percentile applied to the sensitive areas, also seems reasonable” (Russell and Samet, 2011 a), 5 CASAC also noted that there was the need for greater clarity in specifying how appropriate 6 screening criteria would be applied in assigning ecoregions to these categories. Further, CASAC 7 identified potential biases in critical load calculations and in the regional representativeness of 8 available water chemistry data, leading to the observatio n that a given percentile of the 9 distribution of estimated critical loads may be protective of a higher percentage of surface waters 10 in some regions (Russell and Samet, 2011 a). 11 With respect to averaging time (discussed above in section III.C), CASAC stat ed the 12 following: 13 Considering the cumulative nature of the long

- term adverse ecological effects and 14 the year - to
- year variability of atmospheric conditions (mainly in the amount of 15 precipitation), CASAC concurs with EPA that an averaging time of three to fi ve 16 ye ars for the AAI parameters is appropriate. A longer averaging time would mask 17 possible trends of AAI, while a shorter averaging time would make the AAI being 18 more influenced b y the conditions of the particular years selected. (Russell and 19 Samet, 2011 a) 20 21 With respect to level as well as the combination of level and form as they are presented 22 as alternative standards (discussed above in sections III.D

- E), CASAC stated the following: 23 CASAC agrees with EPA staff’s recommendation that the “level” of the 24 alternative AAI standards should be within the range of 20 and 75 μeq/L. We also 25 recognize that both the “level” and the form of any AAIstandard are so closely 26 linked in their effectiveness that these two elements should be considered 27 together. (Russell an d Samet, 2011 a) 28 29

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184 When considered in isolation, it is difficult to evaluate the logic or implications 1 of selecting from percentiles (70th to 90th) of the distribution of estimated critical 2 loads for lakes in sensitive ecoregions to determine an acceptable amount of 3 deposition for a given ecoregion. However, when these percentile ranges are 4 combined with alternative levels within the staff

- recommended ANC range of 20 5 to 75 microequivalents per liter (μeq/L), the results using the AAI point to the 6 ecoregions across the country that would be expected to require additional 7 protection from acidifying deposition. Reasonable choices were made in 8 developing the form. The number of acid sensitive regions not likely to meet the 9 standard will be affected both by choi ce of ANC level and the percentile of the 10 distribution of critical loads for lakes to meet alternative ANC levels in each 11 region. These combined recommendations provide the Administrator with a 12 broad but reasonable range of minimally to substantially

prot ective options for the 13 standard. (Russell and Samet, 2011 a) 14 15 CASAC also commented on EPA’s uncertainty analysis, and provided advice on areas 16 requiring further clarification in the proposed rule and future research. CASAC found it 17 “difficult to judge the adequacy of the uncertainty anal ys is performed by EPA because of lack of 18 details on data inputs and the methodology used, and lack of clarity

in presentation” (Russell and 19 Samet, 2011 a). In particular, CASAC identified the need for more thorough model ev aluations 20 of critical load and atmospheric modeling, recognizing the important role of models as they are 21 incorporated in the form of the standard. In light of the innovative nature of the standard 22 developed in the Policy Assessment, CASAC identified “a number of areas that should be the 23 focus of further research” (Russell and Samet, 2011 a). While CASAC recognized that EPA staff 24 was able to address some of the issues in the Policy Assessment, they

also noted areas “that 25 would benefit from further study or consideration in potential revisions or modifications to the 26 form of the standard.” Such research areas include “sulfur retention and mobilization in the 27 soils, aluminum availability , soil versus water acidification and ecos ys tem recovery times.” 28 Furthe r, CASAC encouraged future efforts to monitor individual ambient nitrogen species, 29

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1 85 which would help inform further CMAQ evaluations and the specification of model

- derived 1 elements in the AAI equation (Russell and Samet, 2011 a). 2 H. Administrator’s Proposed Conclusions 3 Having concluded that the existing NO 2 and SO 2 secondary standards are neither 4 sufficiently

protective nor appropriate to address deposition

- related effects associated with 5 oxides of nitrogen and sulfur (section II.D above), the Administrator has considered whether it is 6 appropriate at this time to set a new multi

- pollutant standard for that purpose , with a structure 7 that would better reflect the available science regarding acid deposition . In particular, she has 8 focused on the new standard de veloped in the Policy Assessment and reviewed by CASAC, as 9 discussed above. In so doing, the Administrator first considered the extent to which there is a 10 scientific basis for development of a standard that wou ld be requisite to protect public welfare, as 11 provided in section 109(b) of the CAA, specifically with respect to deposition

- related aquatic 12 acidification in sensitive aquatic ecosy stems , that would be ne ither more nor less stringent than 13 necessary

for this purpose

. As discussed above, the Administrator notes that the Integrated 14 Science Assessment concludes that the available scientific evidence is sufficient to infer a causal 15 relationship between acidif yi ng deposition of nitrogen and sulfur in aquatic ecos ys tems, and that 16 the deposition of oxides of nitrogen and sulfur both cause such acidification under current 17 conditions in the U.S. Further, the Integrated Science Assessment concludes that there are well

18 established water quality and biological indicators of aquatic acidification as well as well

19 established models that address deposition, water quality , and effects on ecosy stem biota, and 20 that ecos ys tem sensitivity to acidification varies across the c ountry according to present and 21 historic nitrogen and sulfur deposition as well as geologic, soil, vegetative, and hydrologic 22 factors. Based on these considerations, the Administrator agrees with the conclusion in the 23

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186 Policy Assessment, and supported by C ASAC, that there is a strong scientific basis for 1 development of a standard with the general structure presented in the Policy Assessment

. 2 The Administrator also recognizes that the conceptual framework for an ecologically 3 relevant, multi

- pollutant standard, which was initially explored in the Risk and Exposure 4 Assessment and further developed in the Policy Assessment, builds on the information in the 5 Integrated Science Assessment. She notes that the structure of the standard addresses the 6 combined effect s of deposition from oxides of nitrogen and sulfur by characterizing the linkages 7 between ambient concentrations, deposition, and aquatic acidification, and that the structure of 8 the standard takes into account relevant variations in these linkages across the country . She 9 recognizes that while the standard is innovative and unique, the structure of the standard is well 10 grounded in the science underlying the relationships between ambient concentrations of oxides 11 of nitrogen and sulfur and the aquatic acidif ication related to deposition of nitrogen and sulfur 12 associated with such ambient concentrations. 13 While the Administrator recognizes the strong scientific foundation for the structure of an 14 AAI

- based standard, she also recognizes that the standard depends on atmospheric and 15 ecological modeling , based on appropriate data, to specify the terms of an equation that 16 incorporates the linkages between ambient concentrations, deposition, and aquatic acidification. 17 This equation, which defines an aquatic acidification index (AAI), has the effect of translating 18 spatially

variable ecological effects into a potential national standard. With respect to 19 establishing the specific terms of this equation, there are a number of inherent uncertainties and 20 complexities that are relevant to the question of whether it is feasible to set a specific AAI

- based 21 standard at this time that is requisite to protect public welfare without being either more or less 22 stringent than necessary for this purpose

. As discussed abo ve, these uncertainties and 23

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187 complexities generally

relate not to the structure of the standard, but to the quantification of the 1 various elements of the standard, such as the F factors discussed earlier in this section and their 2 representativeness at an ec oregion scale

. These uncertainties and complexities currentl y limit 3 efforts to determine an appropriate form an level for such a standard, within the ranges of levels 4 and forms identified in the Policy Assessment, and the representativeness of F factors in the AAI 5 equation described above and in the Policy Assessment. These important uncertainties have 6 been generall y categorized as limitations in available field data as well as uncertainties that are 7 related to reliance on the application of ecological and atmospheric modeling at the ecoregion 8 scale to specify

the various elements of the AAI. 9 With regard to data limitations, the Administrator observes that there are several 10 important limi tations in the available data upon which elements of the AAI are based. For 11 example, while ambient measurements of NOy are made as part of a national monitoring 12 network, the monitors are not located in locations that are representative of sensitive aquati c 13
ecos ys tems. While air and water quality data are generall y available in areas in the eastern U.S., 14 there is relativel y sparse coverage in mountainous western areas where a number of sensitive 15 aquatic ecosy stems are located. Further, even in areas where relevant data are available, small 16 sample sizes impede efforts to characterize the representativeness of the available data, which 17 was noted b y CASAC as being of particular concern. Also, measurements of reduced forms of 18 nitrogen are available from only a small number of monitoring sites, and emission inventories for 19 reduced forms of nitrogen used in atmospheric modeling are subject to considerable uncertainty. 20 With regard to uncertainties related to the use of ecological and atmospheric modeling, 21 the Administrator notes in particular that model results are difficult to evaluate due to a lack of 22 relevant observational data. For example, relatively large uncertainties are introduced by a lack 23

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188 of data with regard to pre

- industrial environmental conditions and other parameters that are 1 necessary

inputs to critical load models that are the basis for factor F1 in the AAI equation. 2 Also, observational data are not generall y available to evaluate the modeled relationships 3 between nitrogen and sulfur in the ambi ent air and associated deposition, which are the basis for 4 the other factors (i.e., F2, F3, and F4) in the AAI equation. 5 In combination, these limitations and uncertainties result in a considerable degree of 6 uncertainty as to how well the quantified elements of the AAI standard would predict the actual 7 relationship between varying ambient concentrations of oxides of nitrogen and sulfur and steady 8 state ANC levels across the distribution of water bodies within the various ecoregions in the U.S. 9 Because of this, there is considerable uncertainty as to the actual degree of protectiveness that 10 such a standard would provide, especiall y for acid

- sensitive ecoregions. The Administrator 11 recognizes that the AAI equation, with factors quantified in the ranges discu ssed above and 12 described more full y in the Policy Assessment, generall y performs well in identifying areas of 13 the country

that are sensitive to such acidifying deposition and indicates, as expected, that lower 14 ambient levels of oxides of nitrogen and sulfur would lead to higher calculated AAI values. 15 However, the uncertainties discussed here are critical for determining the actual degree of 16 protection that would be afforded such areas b y any specific target ANC level and percentile of 17 water bodies that wou ld be chosen in setting a new AAI

- based standard , and thus for determining 18 an appropriate AAI

- based standard that meets the requirements of the CAA

. 19 In considering these uncertainties, the Administrator notes that CASAC acknowledged 20 that important uncertainties remain that would benefit from further study and data collection 21 efforts, which might lead to potential revisions or modifications to the form of the standard 22 developed in the Policy Assessment. She also notes that CASAC encouraged the Agency to 23

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189 en gage in future monitoring and model evaluation efforts to help inform the specification of 1 model

- derived elements in the AAI equation. 2 Based on the above considerations, the Administrator has determined that it is not 3 appropriate to set a new multi

- pol lutant standard to address deposition

- related effects of oxides 4 of nitrogen and sulfur on aquatic acidification at this time. Setting a NAAQS generall y involves 5 consideration of the degree of uncertainties in the science and other information, such as gap s in 6 the relevant data and, in this case, limitations in the evaluation of the application of relevant 7 ecological and atmospheric models at an ecoregion scale

. The issue here is identifying a specific 8 form and level for the standard that would provide an appropriate level of public welfare 9 protection, consistent with the requirements of the CAA, in light of the specific uncertainties and 10 limitations that are discussed in detail throughout this preamble . As noted above, it is not a 11 question of uncertainties about the scientific soundness of the structure of the AAI, but instead 12 uncertainties in the quantification and representativeness of the elements of the AAI as they vary 13 in ecoregions across the country . At present, these unc ertainties prevent an understanding of the 14 degree of protectiveness that would be afforded to various ecoregions across the country by a 15 new standard defined in terms of a specific nationwide target ANC level and a specific percentile 16 of water bodies for a cid

- sensitive ecoregions , and thus prevent identification of an appropriate 17 form and level for the standard . The Administrator has considered whether these uncertainties 18 could be appropriatel y accounted for b y choosing either a more or less protective target ANC 19 level and percentile of water bodies than would otherwise be chosen if the uncertainties did not 20 substantially

limit the confidence that can appropriately be ascribed to the quantification of the 21 AAI elements. However, in the Administrator’s judgm ent, the uncertainties are of such nature 22 and magnitude that there is no reasoned way to choose such a specific nationwide target ANC 23

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190 level or percentile of water bodies that would appropriately account for the uncertainties, since 1 neither the direction no r the magnitude of change from the target level and percentile that would 2 otherwise be chosen can reasonabl y be ascertained at this time. 3 Based on the above considerations, the Administrator judges that the current limitations 4 in relevant data and the un certainties associated with specifying the elements of the AAI based 5 on modeled factors are of such nature and degree as to prevent her from reaching a reasoned 6 decision such that she is adequatel y confident as to what level and form (in terms of a selecte d 7 percentile) of such a standard would provide an y particular intended degree of protection of 8 public welfare that the Administrator determined satisfied the requirements of the CAA

. While 9 acknowledging that CASAC supported moving forward to establis h the standard developed in 10 the Policy

Assessment, the Administrator also observes that CASAC supported conducting 11 further field studies that would better inform the continued development or modification of such 12
a standard. Given the large uncertainties and complexities inherent in quantifying the elements 13 of such a standard, largely deriving from the unprecedented nature of the standard under 14 consideration in this review, and having fully considered CASAC’s advice, the Administrator 15 provisionally conclud es that it is premature to set a new, multi

- pollutant secondary standard for 16 oxides of nitrogen and sulfur at this time, and as such she is proposing not to set such a new 17 secondary

standard. 18 While it is premature to set such a multi

- pollutant standard at this time , the Administrator 19 determines that the Agency should undertake a field pilot program to gather additional data, and 20 that it is appropriate that such a program be undertaken before, rather than after, reaching a 21 decision to set such a standard. As described below in section IV, the purpose of the program is 22 to collect and anal yz e data so as to enhance our understanding of the degree of protectiveness 23

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191 that would likely

be afforded by

a standard based on the AAI as developed in the Policy 1 Assessment. This will provide a better basis for the Administrator to identify a form and level of 2 the standard that is requisite to protect the public welfare consistent with the CAA, specifically 3 with respect to the acidifyingeffects of deposition of oxides of nitrogen and sulfur. Data 4 generated b y this field program will also support development of an appropriate monitoring 5 network that would work in concert with such a standard to result in the intended degree of 6 protection. The data and anal ys es generated a s a result of this program will serve to inform the 7 next review of the NAAQS for oxides of nitrogen and sulfur. The information generated during 8 the field program can also be used to help state agencies and EPA better understand how an 9 AAI

- based standard w ould work in terms of the implementatio n of such a standard. 10 Based on the above considerations, the Administrator is proposing not to set a new multi

11 pollutant AAI

- based secondary standard for oxides of nitrogen and sulfur in this review. In 12 reaching th is decision, the Administrator recognizes that the new NO 2 and SO 2 primary 1

- hour 13 standards set in 2010, while not ecologically relevant for a secondary

standard , will nonetheless 14 result in reductions in oxides of nitrogen and sulfur that will directionall y benefit the 15 environment by

reducing NOy

and SOx deposition to sensitive ecosy stems. EPA is proposing to 16 revise the secondary standards by

adding secondary standards identical to the NO 2 and SO 2 17 primary

1

- hour standards set in 2010. While this will not add secondary standards of the 18 ecologicall y relevant form to address acid deposition , it will directionally provide some degree of 19 additional protection. This is consis tent with the view that the current secondary standards are 20 neither sufficiently protective nor appropriate in form, but that it is not appropriate to propose to 21 set a new, ecologicall y relevant multi

- pollutant secondary standard at this time, for all of the 22 reasons discussed above

. 23

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192 While not a basis for this decision, the Administrator also recognizes that a new, 1 innovative AAI

- based standard would raise significant implementation issues that would need to 2 be addressed consistent with the CAA requirements for implementation

- related actions following 3 the setting of a new NAAQS. It will take time to address these issues, during which the Agency 4 will be conducting a field pilot program to gather necessary data and the environment will 5 benefit from r eductions in oxides of nitrogen and sulfur resulting from the new NO 2 and SO 2 6 primary

standards, as noted above , as well as reductions expected to be achieved from EPA’s 7 proposed Transport Rule and Mercury and Air Toxics standards. These implementation

- re lated 8 issues are discussed in more detail below in section IV.A.5. 9 The Administrator solicits comment on all aspects of this proposed decision, including 10 the framework and elements of a multi

- pollutant standard for oxides of nitrogen and sulfur to 11 address deposition

- related effects on sensitive ecosy stems, with a focus on aquatic acidification, 12 and the uncertainties and complexities associated with the development of such a standard at this 13 time. The Administrator also solicits comment on the field pilot program and related monitoring 14 methods as discussed below in section IV. 15 **IV. Field Pilot Program and Ambient Monitoring**  16 This section describes EPA’s plans for a field pilot program and the evaluation of 17 monitoring methods for ambient air indicators of oxi des of nitrogen (NOy) and oxides of sulfur 18 (SOx) to implement the Administrator’s decision to undertake such a field monitoring program 19 in conjunction with her decision to propose not to set a new multi

- pollutant secondary standard 20 in this review, as discu ssed above in section III.H. As noted above and discussed below in 21 section IV.A, the field pilot program is intended to collect and analyze data so as to enhance our 22 understanding of the degree of protectiveness that would likely be afforded by a standard based on 23 the AAI as developed in the Policy Assessment. Data generated by this field program would also 24

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193 support development of an appropriate monitoring network that would work in concert with such a 1 standard to result in the intended degree of protection. As discussed below in section IV.B, the 2 evaluation of monitoring methods focuses on the development of Federal Reference 3 Methods/Federal Equivalent Methods (FRM/FEM) for NOy and SOx. 4 A. Field Pilot Program 5 This section presents the objectives of a field pilot program (section IV.A.1) that would 6 gather relevant field data over a five

- ye ar period in a sample of 3 to 5 sensitive ecoregions across 7 the country . An overview of the scope and structure of the field program, with a focus on 8 measurements of ambient air indicators of oxides of nitrogen and sulfur , is presented in section 9 IV .A.2. Section IV.A.3 explai ns the role of additional complementary measurements beyond the 10 ambient air indicators that would be included in the program, and section IV.A.4 discusses a 11 parallel longer

- term research agenda, both of which are guided b y the uncertainties discussed 12 above in section III. Section IV.A.5 identifies implementation challenges presented b y an AAI

13 based standard that could be addressed in parallel with a field pilot program. Section IV.A.6 14 discusses engagement with stakeholder groups as part of the planned pil ot program. 15 1. Objectives 16 Consideration of a new multi

- pollutant standard to address deposition

- related effects on 17 sensitive aquatic ecoregions raises unique challenges relative to those typically raised in reviews 18 of existing NAAQS for which an establishe d network of FRM/FEM monitor s, designed to 19
measure the indicator pollutant , is generall y available. The primary goal of this field pilot 20 program, and the related monitoring program discussed in section IV.B, is to enhance our 21 understanding of the degree o f protectiveness that would likely be afforded by

a standard based 22 on the AAI, as described above in section III, and to support the identification of a specific form 23

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194 and level for such a standard that would be requisite to protect public welfare consistent with the 1 requirements of the CAA, through the following objectives: 2 (1) evaluate measurement methods for the ambient air indicator s of NOy and SOx and 3 consider designation of such methods as FRMs; 4 (2) examine the variability and improve characterizatio n of concentration and 5 deposition patterns of oxides of nitrogen and sulfur, as well as reduced forms of 6 nitrogen, within and across a number of sensitive ecoregions across the country ; 7 (3) develop updated ecoregion

- specific factors (i.e., F1 through F4) for the AAI 8 equation based in part on new observed air quality data within the sample ecoregions 9 as well as on updated nationwide air quality model results and expanded critical load 10 data bases , and explore alternative approaches for developing such repres entative 11 factors; 12 (4) calculate ecoregion

- specific AAI values using observed NOy and SOx data and 13 updated ecoregion

- specific factors to examine the extent to which the sample 14 ecoregions would meet a set of alternative AAI

- based standards; 15 (5) develop air monitoring network design criteria for an AAI

- based standard; 16 (6) assess the use of total nitrate measurements as a potential alternative indicator for 17 oxides of nitrogen; and 18 (7) support related longer

- term research efforts, including enhancements to and 19 evaluation of modeled dry deposition algorithms. 20 2. Overview of Field Pilot Program 21 The CASTNET program (Figure IV
- 1) affords an available infrastructure relevant to an 22 AAI

- based standard, given the location of sites in some acid

- sensitive ecoregions and various 23

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195 measurements of sulfur and nitrogen species. The EPA plans to use CASTNET sites in selected 1 acid

- sensitive ecoregions to serve as the platform for this pilot program, potentially starting in 2 late 2012 and extending through 2018. CASTNET sites in three to five ecoregions in acid

3 sensitive areas would collect NOy and SOx (i.e., SO 2 and p

- SO 4 ) measurements over a five

- ye ar 4 period . The initial step in developing a data base of observed ambient air indicators for oxides of 5 nitrogen and sulfur requi res the addition of NOy samplers at the pilot study sites so that a full 6 complement of indicator measurements are available to calculate AAIvalues . These CASTNET 7 sites would also be used to make supplemental observations useful for evaluation of CMAQ’s 8
c haracterization of factors F2

–F4 in the AAI equation. 9 The selected ecoregions would account for geographic variability b y including regions 10 from across the U.S., including the east, upper midwest and west. Each selected region would 11 have at least two exi sting CASTNET sites. Each of the pilot CASTNET sites would be used to 12 verify and enhance the performance of the established methods, data retrieval and reporting 13 procedures . This would in turn inform the future selection of specific parameters for the AAI 14 equation. 15 16

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196 1 **Figure IV -**  **1. Location of CASTNET sites in relation to acid sensitive ecoregions.** 2
3 Over the course of this five

- year pilot program, the most current national air quality 4 modeling , based on the mo st current national emissions inventory , would be used to develop an 5 updated set of F 2

– F4 factors. A parallel multi

- agency national critical load data base 6 development effort would be used as the basis for calculating updated F1 factors. As discussed 7
a bove in section III.B, these factors would be based on average parameter values across an 8 ecoregion. Using this new set of F factors, observations of NOy and SOx derived from the pilot 9 program , averaged across each ecoregion, would be used to calculate AA I values in the sample 10 ecoregions

. The data from the pilot program would also be used to examine alternative 11

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197 approaches to generating representative air quality values, such as examining the appropriateness 1 of spatial averaging in areas of high spatial v ariability

. 2 3 . Complementary Measurements 3 Complementary measurements may

be performed at some sites in the pilot network to 4 reduce uncertainties in the recommended methods and better characterize model performance 5 and application to the AAI. The CASAC A ir Monitoring and Methods Subcommittee (AMMS) 6 advised EPA that such supplemental measurements were of critical importance in a field 7 measurement program related to an AAI

- based standard (Russell and Samet, 2011 b ). 8 Candidate complementary measurements to address sulfur, in addition to those provided 9 by the CASTNET filter pack (CFP), include trace gas continuous SO 2 and speciated PM 2.5 10 measurements. The co

- located deploy ment of a continuous SO 2 analy zer with the CFP for SO 2 11 will provide test data for determining suitability of continuous SO 2 measurements as an FEM, as 12 well as producing valuable time series data for model evaluation purposes. The weekl y 13 averaging time provided by the CFP adequatel y address es the annual

- average basis of an AAI

14 based secondary s tandard, but would not be applicable to short

- term (i.e., 1

- hour) average s 15 associated with the primary SO 2 standard. Conversely , because of the low concentrations 16 associated with many acid

- sensitive ecoregions, existing SO 2 FRMs designated for use in 17 determining compliance with the primary standard would not necessarily be appropriate for use 18 in conjunction with an AAI

- based secondary standard

. 19 Co

- locating the PM 2.5 sampler used in the EPA Chemical Speciation Network and the 20 IMPROVE network at pilot network sites would allow for characterizing the relationship 21 between the CFP

- derived p - SO 4 and the speciation samplers used throughout the state and local 22 air quality

networks. Note that CASTNET already has several co

- located IMPROVE chemical 23

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198 speciation samplers. Because the AAI equation is based on concentration of p - SO 4 , the original 1 motivation for capturing all particle size fractions is not as important relative to simply capturing 2 the concentration of total p - SO 4 . 3 Candidate measurements to complement oxidized nitrogen measurements, in addition to 4 the CFP, include a mix of continuous and periodic sampling for the dominant NOy species, 5 namely

NO, true NO 2 , PAN, HNO 3 , and p -NO 3 . While there are several approaches to acquiring 6 these measurements, perhap s the most efficient strategy would take advantage of the available 7 CFP for total nitrate, and add a three

- channel chemiluminescence instrument that will cycle 8 between NOy , true NO 2 and NO b y adding photolytic detection for true NO 2 . Other options for 9
mea suring true NO 2 would include adding either a stand

- alone photol yt ic or cavity ring

- down 10 spectroscop y instrument. Measurements of PAN may be acquired either on a periodic basis 11 through canister sampling and subsequent laboratory anal ys is or through emergi ng in

- situ 12 sampling and anal ys is methods. Although the CFP yields a reliable measurement of total nitrate, 13 the t
-NO 3 (i.e., the sum of HNO 3 and p

-NH 4 ) value, strong consideration may be given to direct 14 measurement of HNO 3 , which has the highest deposition velocity of all the dominant NOy 15 species. Similar to the use of continuous SO 2 data, these speciated NOy data serve two purposes : 16 evaluating total NOy instrument behavior , and air quality model evaluation. The measurement 17 of individual NOy

species can be used to generate site

- specific NOy values for comparison to 18 modeled NOy , and will likely provide insight into and improvement of modeled dry deposition. 19 The CASAC AMMS (Russell and Samet, 2011 b ) recommended that EPA consider the 20 use of t
-NO 3 obtained from CASTNET sampling as an indicator for NOy , reasoning that t
-NO 3 is 21 ty picall y a significant fraction of deposited oxidized nitrogen in rural environments and 22 CASTNET measurements are widel y available. Collection of this data would support further 23

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199 consi deration of using the CFP for t
-NO 3 as the indicator of oxides of nitrogen for use in an AAI

1 based secondary

standard. 2 The CASAC AMMS also recommended that total NHx (NH 3 and p

-NH 4 ) be considered 3 as a prox y for reduced nitrogen species, reasoning that the subsequent partitioning to NH 3 and p

4 NH 4 may

be estimated using equilibrium chemistry calculations. Reduced nitrogen 5 measurements are used to evaluate air quality modeling which is used in generating factor F2. 6 Additional studies are needed to determine the applicability of NHx measurements and 7 calculated values of NH 3 and NH 4 to the AAI. 8 The additional supplemental measurements of speciated NOy , continuous SO 2 and NHx 9 will be used in future air quality modeling evaluation efforts. Because there often i s significant 10 lag in the availability of contemporary

emissions data to drive air quality modeling, the complete 11 use of these data sets will extend beyond the five

- year collection period of the pilot program. 12 Consequently , the immediate application of tho se data will address instrument performance 13 comparisons that explore the feasibility of using continuous SO 2 instruments in rural 14 environments, and using the speciated NOy data to assess NOy

instrument performance. 15 Although contemporary air quality

model ing will lag behind measurement data availability , the 16 observations can be used in deposition models to compare observed transference ratios with the 17 previously

calculated transference ratios to test temporal stability of the ratios. 18 An extended water q uality sampling effort should parallel the air quality measurement 19 program to address some of the uncertainties related to factor F1 and the representativeness of 20 the nth percentile critical load as discussed in section III. B.5.b.i. The objective of the w ater 21 quality

sampling would be to develop a larger data base of critical loads in each of the pilot 22 ecoregions such that the nth percentile can adequately be characterized in terms of representing 23

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200 all water bodies. Opportunities to leverage and perhaps enhance existing ecosy stem modeling 1 efforts enabling more advanced critical load modeling and improved methods to estimate base 2 cation production would be pursued. For example, areas with ongoing research studies 3 producing data for dynamic critical load mod eling would be considered when selecting the pilot 4 ecoregions. 5 4. Complementary areas of research 6 The EPA recognizes that a source of uncertainty in an AAI

- based secondary standard that 7 would not be directl y addressed in the pilot program stems from the uncertainty in the model 8 used to link atmospheric concentrations to dry deposition fluxes. Currently, there are no ongoing 9 direct dry

deposition measurement studies at CASTNET sites that can be used to evaluate 10 modeled results. It was strongl y recommended by CASAC AMMS that a comprehensive 11 sampling

- intensive study be conducted in at least one, preferabl y two sites in different 12 ecoregions to assess characterization of dry deposition of sulfur and nitrogen. These sites would 13 be the same as those for the complementary measurements described above, but they would 14 afford an opportunity to also complement dry

deposition process research that benefits from the 15 ambient air measurements collected in the pilot program. The concerns regarding uncertainties 16 underlyin g an AAI

- based secondary standard suggest that research that includes dry deposition 17 measurements and evaluation of dry deposition models should be a high priority

. 18 Similar leveraging should be pursued with respect to ecosy stem research activities. For 19
e xample, studies that capture a suite of soil, vegetation, hydrological, and water quality 20 properties that can help evaluate more advanced critical load models would complement the 21 atmospheric

- based pilot program. In concept, such studies could provide the infrastructure for 22 true multi

- pollutant , multi

- media “super” sites assuming the planning, coordination , and resource 23

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201 facets can be aligned. While this discussion emphasizes the opportunity of leveraging ongoing 1 research efforts, consideration could be gi ven to explicitly including related research components 2 directly

in the pilot program. 3 5. Implementation challenges 4 The CAA requires that once a NAAQS is established, designation and implementation 5 must move forward. With a standard as innovative as the AAI

- based standard considered in this 6 review, the Administrator believes that its success will be greatl y improved if, while additional 7 data are being collected to reduce the uncertainties discussed above, the implementing agencies 8 and other stakeholders h ave an opportunity to discuss and thoroughl y understand how such a 9 standard would work. And since, as noted above, emissions reductions that are directionally 10 correct to reduce aquatic acidification will be occurring as a result of other CAA programs, th e 11 Administrator believes that this period of further discussion will not delay progress but w i ll 12 ensure that once implementation is triggered, agencies will be prepared to implement it 13 successfull y. 14 Consideration of an AAI

- based secondary standard for oxid es of nitrogen and sulfur 15 would present significant implementation challenges because it involves multiple, regionall y16 dispersed pollutants and relativel y complex compliance determinations. The anticipated 17 implementation challenges fall into three main categories: monitoring and compliance 18 determinations for area designations, pre

- construction permit application analy ses of individual 19 source impacts, and State Implementation Plan (SIP) development. Several overarching 20 implementation questions that we anticipate will be addressed in parallel with the field pilot 21 program’s five

- ye ar data collection period include: 22 (1) What are the appropriate monitoring network density and siting requirements to 23

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202 support a compliance s ys tem based on ecoregions? 1 (2) Given the unique spatial nature of the secondary standard (e.g., ecoregions), what are 2 the appropriate parameters for est ablishing nonattainment areas? 3 (3) How can new or modified major sources of oxides of nitrogen and oxides of sulfur 4 emissions assess their ambient impacts on the standard and demonstrate that they are not 5 causing or contributing to a violation of the NAAQS f or preconstruction permitting? 6 (4) What additional tools, information, and planning structures are needed to assist states 7 with SIP dev elopment, including the assessment of interstate pollutant transport and 8 deposition? 9 (5) Would transportation conformity appl y in nonattainment and maintenance areas for 10 this secondary standard, and, if it does, would satisfying requirements that apply fo r 11 related primary standards ( e.g., ozone, PM 2.5 , and NO 2 ) be demonstrated to satisfy 12 requirements for this secondary standard? 13 6. Final monitoring plan development and stakeholder participation 14 The existing CASTNET sampling site infrastructure provides an effective means of 15 quickly

and efficiently

deploying a monitoring program to support potential implementation of 16 an AAI

- based secondary standard, and also provides an additional opportunity for federall y 17 managed networks to collaborate and support the stat es, local agencies and Tribes (SLT) in 18 determining compliance with a secondary standard. A collaborative effort would help to 19 optimize limited federal and SL T monitoring funds and would be beneficial to all involved. 20 CASTNET is already a stakeholder

- base d program with over 20 participants and contributors, 21 including federal, state and tribal partners. CASTNET partners are willing to adapt the network 22

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203 to meet the monitoring requirements of an AAI

- based secondary standard, provided the current 1 requirements for program assessment continue to be met. 2 The CASAC AMMS generall y endorsed the technical approaches used in CASTNET, but 3 concerns were raised b y i ndividual representatives of State agencies concerning the perception of 4 EPA

- controlled manage ment aspects of CASTNET and data ownership. Potential approaches to 5 resolve these issues will be developed and evaluated in existing NACAA/EPA ambient air 6 monitoring workgroups. The EPA Office of Air and Radiation (which includes the Office of Air 7 Quality Planning Standards, OAQPS; and the Office of Atmospheric Program’s Clean Air 8 Markets Division, OAP

- CAMD), and their partners on the NACAA monitor steering committee 9 will develop a prioritized specific plan that identifies the 3

- 5 ecoregions and the instrumentation 10 to be deployed. EPA anticipates that a cost estimate of the plan with priorities and options will 11 be developed b y J anuary , 2012. Although this pilot program is focused on data collection, the 12 plan will include details of the data anal ys is approaches as well as a vehicle that incorporates 13 engagement from those within EPA and SL Ts to foster progress on the implementation questions 14 noted above in section IV.A. 5. 15 If an AAI

- based secondary standard were to be set in the future, deploy ment of a full 16 national network would follow the pilot monitoring program. The number of sites deployed in 17 the network will lead to increased confidence in capturing spatial patterns of air quality. 18 Recognizing that this section presents the general elements of the fie ld pilot programs, EPA 19 intends to develop a more detailed field pilot program plan through a process that will engage the 20 air quality

management and research (atmospheric and ecos ys tem) communities , as well as other 21 federal agencies, state and local agenci es , and non - government based centers of expertise. The 22 EPA is seeking comment and input on all aspects of this field pilot program. 23

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204 . B. Evalu ation of Monitoring Methods 1 The EPA generally relies on monitoring methods that have been designated as FRMs or 2 FE Ms for the purpose of determining the attainment status of areas with regard to existing 3 NAAQS. Such FRMs or FEMs are generall y required to measure the air quality indicators that 4 are compared to the level of a standard to assess compliance with a NAAQS. Prior to their 5 designation b y EPA as FRM/FEMs through a rulemaking process, these methods must be 6 determined to be applicable for routine field use and need to have been experimentally validated 7 by meeting or exceeding specific accuracy , reproducibility, and reliability criteria established b y 8 EPA for this purpose. As discussed above in section III .A, the ambient air indicators being 9 considered for use in an AAI

- based standard include SO 2 , particulate sulfate (p

- SO 4 ), and total 10 reactive oxides of nitrogen (NO y). 11 The CASTNET provides a well established infrastructure that would meet the basic 12 location and measurement requirements of an AAI

- based secondary standard given the rural 13 placement of sites in acid sensitive areas. In addition, CFPs currentl y provi de very economical 14 weekl y, integrated average concentration measurements of SO 2 , p - SO 4 , ammonium ion (NH 4 ) 15 and t
-NO 3 , the sum of HNO 3 and p -NO 3 . 16 While routinely operated instruments that measure SO 2 , p - SO 4 , NOy and/or t
-NO 3 exist, 17 instruments that measure p - SO 4 , NOy , t
-NO 3 , or the CFP for SO 2 have not been designated b y 18 EPA as FRMs or FEMs The EPA’s ORD has initiated work that will support future FRM 19 designations b y EPA for SO 2 and p

- SO 4 measurements based on the CFP . Such a designation by 20 EPA could b e done for the purpose of facilitating consistent research related to an AAI

- based 21 standard and/or in conjunction with setting and supporting an AAI

- based secondary standard. 22

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205 Based on extensive review of literature and available data, the EPA has identif ied 1 potential methods that appear suitable for measuring each of the three components of the 2 indicators. These three methods are being considered as new FRMs to be used for measuring the 3 ambient concentrations of the three components that would be needed to determine compliance 4 with an AAI

- based secondary standard. 5 For the SO 2 and p

- SO 4 measurements, EPA is considering the CFP method, which 6 provides weekl y average concentration measurements for SO 2 and p

- SO 4 . This method has been 7 used in the EPA’s CASTNET monitoring network for 15 years, and strongly indicates that it will 8 meet the requirements for use as an FRM for the SO 2 and p

- SO 4 concentrations for an AAI

- based 9 secondary standard. 10 Although the CFP method would provide measurements of both the SO 2 and p - SO 4 11 components in a unified sampling and anal ys is procedure, individual FRMs will be considered 12 for each. The EPA recognizes that an existing FRM to measure SO 2 concentrations using ultra

13 violet fluorescence (UVF) exists (40 CFR Part 50, Appendix A

- 1) f or the purpose of monitoring 14 compliance for the primary SO 2 NAAQS. However, several factors suggest that the CFP method 15 would be superior to that UVF FRM for monitoring compliance with an AAI

- based secondary 16 standard and will be discussed in more detail b elow. 17 For monitoring the NOy component, a continuous anal yz er for measuring NOy is 18 commerciall y available and is considered to be suitable for use as an FRM. This method is 19 similar in design to the existing NO 2 FRM (described in 40 CFR Part 50, Appendix F), which is 20 based on the ozone chemiluminescence measurement technique. The method is adapted to and 21 further optimized to measure all oxides of nitrogen (NOy). However, this NOy method requires 22 further evaluation before it can be full y confirmed as a suitable FRM. The EPA is currentl y 23

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206 completing a full scientific assessment of the NOy method to determine whether it would be 1 appropriate to consider for designation by EPA as an FRM. Specific details on these three 2 methods are given below. 3 On February 16, 2011, EPA presented this set of potential FRMs to the CASAC AMMS 4 for their consideration and comment. In response, the CASAC AMMS stated that, overall, it 5 believes that EPA’s planned evaluation of methods for measuring NOy , SO 2 and p

- SO 4 as 6 ambient air indicators are suitable approaches in concept. On supporting the CFP method as a 7 potential FRM for SO 2 , CASAC stated that they felt that the CFP is adequate for measuring 8 long

- term average SO 2 gas concentrations in rural areas with low l evels (less than 5 ppbv) and is 9 therefore suitable for consideration as an FRM. For p

- SO 4 , CASAC generally supports the use 10 of the CFP as a potential FRM for measuring p

- SO 4 for an AAI

- based secondary standard. The 11 method has been relatively well

- charact erized and evaluated, and it has a documented, long

- term 12 track record of successful use in a field network designed to assess spatial patterns and long

- term 13 trends. 14 On supporting the photometric NOy method as a potential FRM, CASAC concluded that 15 the existing NOy method is generall y an appropriate approach for the indicator. However, 16 CASAC agrees that additional characterization and research is needed to fully understand the 17 method in order to designate it as a FRM. The EPA is now soliciting public comment on these 18 methods as to their adequacy , suitability , and relative merits as FRMs for purposes of monitoring 19 to determine compliance with an AAI

- based secondary standard. 20 1. Potential FRMs for SO2 and p

- SO 4 21 The CFP is a combined, integrated sampling and analy sis method based on the well

22 established measurement technology that has been used extensively in EPA’s CASTNET 23

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207 monitoring network (see http://www.epa.gov/castnet). This method is in current use at over 80 1 moni toring sites and has been in use at not less than 40 sites for over 15 ye ars. This method 2 employ s a relativel y simple and inexpensive sampler and uses four 47

- mm filters placed in an 3 open - faced filter pack to simultaneously collect integrated filter sampl es for the SO 2 and p

- SO 4 4 components. In addition, the CFP is also capable of the collection of t

-NO 3 , the sum of HNO 3 5 and p -NO 3 . 6 The first stage of the filter pack assembly contains a Teflon® filter that collects p - SO 4 2

- 7 and p -NO 3 , the second stage contai ns a n yl on filter that collects SO 2 (as SO 4 2

- ) and HNO 3 , and 8 the third stage contains two cellulose fiber filters impregnated with potassium carbonate (K 2 CO 3 ) 9 that collect any remaining SO 2 (as SO 4 2

- ). The sampler collects one

- week integrated samples at a 10 very

low, controlled flow rate (1.5 or 3 L/min) in an attempt simulate actual deposition. Weekly 11 averaged SO 2 and p

- SO 4 concentrations could then be averaged over a 1

- year period to calculate 12 annual average values. 13 Upon sample completion, the species

- sp ecific filters are extracted, with subsequent 14 anal ys is by

the well

- established and documented ion chromatographic (IC) anal yt ical technique. 15 During the IC anal ys is, an aliquot of a filter extract is injected into a stream of eluent (ion 16 chromatograph y mobi le phase, generall y a millimolar

- strength solution of carbonate

17 bicarbonate) and passed through a series of ion exchangers. The anions of interest are separated 18 on the basis of their relative affinities for a low capacity and the strongl y basic anion exch anger 19 (guard and separator column). The separated anions are directed onto a cation exchanger 20 (suppressor column) where they are converted to their highl y conductive acid form, and the 21 eluent is converted to a weakl y conductive form. The now

- separated an ions, each in their acid 22 form, are measured b y conductivity . They are identified on the basis of retention time compared 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

208 to that of standards and quantified by measurement of peak area compared to the peak areas of 1 calibration standards. 2 Calibration and quality assurance for the method are applied to the sample filters, the 3 anal yt ical processes, and the flow rate measurement and control aspects of the sampler. Overall 4 method performance is ty pically assessed with collocated samplers. These quality as surance 5 techniques are routinel y used and have proved adequate for other types of FRMs and equivalent 6 methods in air monitoring network service. 7 The measurement and analy tical procedures and past performance data associated with 8 the C FP method are well documented and available through Quality Assurance Performance 9 Plans (QAPPs), Standard Operating Procedures ( SOPs ) and annual reports (US EPA, 2010a and 10 2010b). The accumulated database on the CFP method is substantial and indicates that the 11 method is sound, stable and has good reliability in routine, field operation. Data quality 12 assessment results show the method to have good reproducibility , with collocated and anal yt ical 13 precision values in the range of 2%

– 10% (excluding very low concentration measurem ents near 14 the method detection limits; US EPA 2010b). 15 Data quality objectives (DQOs) for a new FRM would be based upon current DQOs 16 being used for this method by EPA’s OAP/CAMD and the National Park Service (NPS), the 17 federal managers of CASTNET (US EPA, 2 010a). In its current state, the CFP method is 18 expected to meet or exceed (as past CASTNET data have indicated; US EPA, 2010b) the 19 expected FRM DQOs, even when deplo ye d in new monitoring networks outside of CASTNET. 20 In addition, CASTNET samples have agre ed favorably with other measures of SO 2 and p

- SO 4 in 21 comparison studies. For example, in direct comparison with an annular denuder sampler (ADS) 22 method, CASTNET/ADS ratios for SO 2 and p

- SO 4 were generall y on the order of 0.9

- 1.1 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

209 (Lavery et al, 2009; Sickl es et al, 1999; Sickles et al, 2008) , thus illustrating the accuracy of the 1 CFP method in the determination of long

- term average SO 2 and p

- SO 4 concentrations. The EPA 2 believes that the CFP method would be fully adequate as an FRM in determining yearl y ave rage 3 SO 2 and p

- SO 4 concentrations for compliance determination purposes. 4 The EPA recognizes that an existing FRM for SO 2 has proven adequate for the purposes 5 of monitoring compliance for the primary SO 2 NAAQS, specifically the newly

- promulgated 1

- 6 hour stan dard. However, this FRM is better suited to the shorter

- term, higher concentration 7 primary

and secondary

SO 2 NAAQS, and there is substantial uncertainty as to the adequacy of 8 this SO 2 FRM for monitoring the lower concentrations relevant to determining com pliance with 9 an AAI

- based secondary standard. The performance specifications for SO 2 FRM anal yz ers (40 10 CFR Part 53, Table B

- 1) require a lower detectable limit (LDL) of 0.002 ppm for the standard 11 measurement range and 0.001 ppm for the lower measurement r ange. These requirements 12 correspond to mass per unit volume concentrations of 5.24 and 2.62 μg/m 3 , respectivel y. 13 Analy sis of 2009 CASTNET data shows that of the 84 CASTNET sampling sites, 63 measured 14 annual average SO 2 concentrations below even the lower of these LDL requirements of 2.62 15 μg/m 3 for the lower range SO 2 FRM (US EPA, 2010a). In addition, 11 of the 84 sites measured 16 annual (2009) average SO 2 concentrations very near or below the manufacturers’ reported 17 detection limits for trace level UVF SO 2 monitors. Further, it is likely that the number of sites 18 with annual average SO 2 concentration below both the SO 2 FRM LDL and the manufacturers 19 reported detection limits will increase due to expected declines in mean SO 2 concentrations (US 20 EPA, 2010b). For these reasons, EPA is considering the CFP method for use as the FRM for 21 monitoring the SO 2 component of an ambient air indicator for oxides of sulfur, with a 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

210 recommendation for additional study and data collection to further evaluate the possible 1 appli cability

of the continuous UVF SO 2 FRM for this purpose. 2 2. Potential FRM for NOy 3 Atmospheric concentrations of NOy are measured continuously by an analyzer that 4 photometrically

measures the light intensity , at wavelengths greater than 600 nanometers (nm), 5 resulting from the chemiluminescent reaction of ozone (O 3 ) with NO in sampled air. This 6 method is very

similar to the chemiluminescence NO/NO 2 anal yz ers widely used to collect NO 2 7 monitoring data for determining compliance with the NO 2 NAAQS. The vari ous oxides of 8 nitrogen species, excluding NO, are first quantitatively reduced to NO by

means of a catal yt ic 9 converter. These species include NO 2 , HNO 2 , PANs, HNO 3 and p -NO 3 . The NO, which 10 commonly

exists in ambient air, passes through the converter unch anged, and, when combined 11 with the NO resulting from the catal yt ic conversion of the other oxides of nitrogen, a 12 measurement of the total NOy concentration results. To maximize the conversion of the more 13 chemicall y active oxides of nitrogen species, the c onverter is located externally , at or near the air 14 sample inlet probe. This location minimizes losses of these active species that could otherwise 15 occur from chemical reactions and wall losses in the sample inlet line. 16 The NOy

anal yz er is a suitable, comm erciall y produced continuous chemiluminescence 17 anal yz er that includes an ozone generator, a reaction cell, a photometric detector, wavelength 18 filters as necessary

to reduce sensitivity

to wavelengths below 600 nm, a pump and flow control 19 sy stem to draw atm ospheric air through the converter and into the reaction cell, a suitable 20 converter, a s ys tem to control the operation of the anal yz er, and appropriate electronics to 21 process and quantitatively scale the photometric signals. The converter contains a catal ys t such 22 as moly

bdenum and is heated to an optimum temperature designed to optimize the conversion of 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

211 the various oxides of nitrogen to NO. It is connected to the anal yz er via suitable lengths of 1 Teflon® tubing. Hourly NOy measurements obtained by the an al yz er would be averaged over 2 the same 7

- day

period used by

the CFP method to measure the SO 2 and p

- SO 4 components, with 3 further averaging over a 1

- ye ar period

. 4 Commercial NOy analyzers are currentl y available, and the anal yz ers have been used for 5 a variety of monitoring applications. During the 2006 TexAQS Radical and Aerosol 6 Measurement Project (TRAMP), Luke et. al. , 2010, compared measured NOy concentrations 7 obtained with an NOy instrument based upon the above mentioned methodology with the sum of 8
measured individual NOy species (i.e., NOyi = NO+NO 2 +HNO 3 +PANs+HNO 2 +p
-NO 3 ). This 9 comparison yi elded excellent overall agreement during both day ([NOy](ppb) = [NOy i ](ppb) × 10 1.03

– 0.42; r2 = 0.9933) and night time ([NOy](ppb) = [NOy i ](ppb) × 1.01

– 0.18; r2 = 0 .9975) 11 periods ( Luke et al , 2010 ). The results of this study show that this NOy method is capable of the 12 accurate determination of all the atmosphericall y relevant NOy components, resulting in an 13 accurate determination of total NOy concentrations. NOy

in struments have been routinel y 14 operated in networks such as SEARCH, dating back several years. In addition, state monitoring 15 agencies across the U.S. have begun, starting in 2009, the routine operation of commercially 16 available NOy

instrumentation in antic ipation of EPA’s NCore network transitioning to full 17 operation in 2011. 18 These initial assessments described above are promising and indicate that the photometric 19 NOy

method appears to be accurate, reliable, and capable of routine network operation. As a 20 result, the method is most likely capable for use as an FRM for determining atmospheric NOy 21 concentrations as a component in determining compliance with an AAI

- based secondary 22 standard. However, as described below, this continuous method for NOy requires additional 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

212 time for further evaluation before it can be full y confirmed for adoption as a FRM. The EPA has 1 identified measurement uncertainties and some remaining science questions associated with this 2 method. Among these are (a) the ability of the meth od to capture all components of NOy 3 relevant to nitrogen deposition, (b) the efficiency of the moly

bdenum converter in converting all 4 oxides of nitrogen to NO for detection (excluding NO 2 , as this conversion is already well 5 documented), (c) appropriate inl et height specifications to minimize any bias associated with 6 vertical concentration gradients of key NOy components, (d) identification and quantification of 7 potential measurement interferences in the NOy determination, and (e) development and 8 demonstrati on of effective calibration/challenge procedures to best represent the various 9 mixtures of NOy

components that are expected to be present in the different air sheds across the 10 U.S. 11 To address these NOy method uncertainties and to fully assess this method f

or use as the 12 NOy

FRM, EPA has developed a detailed research plan ( Russell and Samet, 2011b) which was 13 presented to the CASAC AMMS on February 16, 2011. In response, CASAC recognized the 14 need for, and supported the general outline of EPA’s research plan to evaluate the NOy method 15 for potential designation as an FRM (US EPA, 2011). In addition, the CASAC AMMS 16 suggested additional areas of research associated with the photometric NOy method that warrant 17 further assessment prior to final designation of the m ethod as the NOy FRM. These include 18 operation of the method during extremel y low temperature conditions to investigate possible 19 condensation in sample lines, method detection limits relative to low levels expected in remote 20 areas, and ambient

- based method evaluations in various air sheds across the U.S. In response to 21 these CASAC AMMS suggestions, EPA is carrying out studies, in addition to the tasks outlined 22 in the research plan, for the NOy method. The results of these studies will likely take a year o r 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

213 more to become available. As noted previousl y, EPA anticipates that these results will be 1 favorable and will confirm the adequacy of the NOy

method as a suitable FRM for determining 2 compliance with an AAI

- based secondary standard. 3 **V. Statutory and Execu**  **tive Order Reviews** 4 A. Executive Order 12866: Regulatory Planning and Review and Review and Executive 5 Order 13563: Improving Regulation and Regulatory Review 6 Under Executive Order 12866 (58 FR 51735, October 4, 1993), this action is a 7

“ significant regulatory action

. ” Accordingl y, EPA submitted this action to the Office of 8 Management and Budget (OMB) for review under Executive Orders 12866 and 13563 (76 FR 9 3821, January

21, 2011), and any chan ges made in response to OMB recommendations have 10 been documented in the docket for this action. 11 B. Paperwork Reduction Act 12 This action does not impose an information collection burden under the provisions of the 13 Paperwork Reduction Act, 44 U.S.C. 3501 et seq. Burden is defined at 5 CFR 1320.3(b). There 14 are no information collection requirements directly associated with the establishment of a 15 NAAQS under section 109 of the CAA. 16 C. Regulatory Flexibility Act 17 For purposes of assessing the impacts of today ’s rule on small entities, small entity is 18 defined as: (1) a small business that is a small industrial entity as defined by

the Small Business 19 Administration’s (SBA) regulations at 13 CFR 121.201; (2) a small governmental jurisdiction 20 that is a government of a city , county, town, school district or special district with a population of 21 less than 50,000; and (3) a small organization that is any not
- for

- profit enterprise which is 22 independentl y owned and operated and is not dominant in its field. 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

214 After consid ering the economic impacts of today's proposed rule on small entities, I 1 certify

that this action will not have a significant economic impact on a substantial number of 2 small entities. This proposed rule will not impose any requirements on small entities. Rather, 3 this rule establishes national standards for allowable concentrations of oxides of nitrogen and 4 sulfur in ambient air as required b y section 109 of the CAA. See also American Trucking 5 Associations v. EPA. 175 F. 3d at 1044

- 45 (NAAQS do not have significant impacts upon small 6 entities because NAAQS themselves impose no regulations upon small entities). We continue to 7 be interested in the potential impacts of the proposed rule on small entities and welcome 8 comments on issues related to such impact s 9 D. Unfunded Mandates Reform Act 10 Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public Law 104

- 4, 11 establishes requirements for Federal agencies to assess the effects of their regulatory actions on 12 State, local, and Tribal governments and the private sector. Under section 202 of the UMRA, 13 EPA generall y must prepare a written statement, including a cost

- benefit analy sis, for proposed 14 and final rules with “Federal mandates” that may result in expenditures to State, local, and Tribal 15 government s, in the aggregate, or to the private sector, of $100 million or more in any 1 year. 16 Before promulgating an EPA rule for which a written statement is needed, section 205 of the 17 UMRA generally

requires EPA to identify and consider a reasonable number of r egulatory 18 alternatives and to adopt the least costly , most cost
- effective or least burdensome alternative that 19 achieves the objectives of the rule. The provisions of section 205 do not apply when they

are 20 inconsistent with applicable law. Moreover, sectio n 205 allows EPA to adopt an alternative 21 other than the least costly, most cost

- effective or least burdensome alternative if the 22 Administrator publishes with the final rule an explanation why that alternative was not adopted. 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

215 Before EPA establishes any regulatory

requirements that may

significantl y or uniquely affect 1 small governments, including Tribal governments, it must have developed under section 203 of 2 the UMRA a small government agency plan. The plan must provide for notifying potentiall y 3 affected sm all governments, enabling officials of affected small governments to have meaningful 4 and timely

input in the development of EPA regulatory proposals with significant Federal 5 intergovernmental mandates, and informing, educating, and advising small governments on 6 compliance with the regulatory requirements. 7 This action contains no Federal mandates under the provisions of Title II of the 8 Unfundated Mandates Reform Act of 1995 (UMRA), 2 U.S.C. 1531

- 1538 for State, local, or 9 tribal governments or the private s ector. Therefore, this action is not subject to the requirements 10 of sections 202 or 205. Furthermore, as indicated previously , in setting a NAAQS EPA cannot 11 consider the economic or technological feasibility of attaining ambient air quality standards; 12 al though such factors may be considered to a degree in the development of State plans to 13 implement the standards. See also American Trucking Associations v. EPA, 175 F. 3d at 1043 14 (noting that because EPA is precluded from considering costs of implementatio n in establishing 15 NAAQS, preparation of a Regulatory Impact Analy sis pursuant to the Unfunded Mandates 16 Reform Act would not furnish any information which the court could consider in reviewing the 17 NAAQS). Accordingl y, EPA has determined that the provisions of sections 202, 203, and 205 of 18 the UMRA do not apply to this proposed decision. The EPA acknowledges, however, that any 19 corresponding revisions to associated SIP requirements and air quality surveillance requirements, 20 40 CFR part 51 and 40 CFR part 58, respectivel y, might result in such effects. Accordingl y, 21 EPA will address, as appropriate, unfunded mandates if and when it proposes any revisions to 40 22 CFR parts 51 or 58. 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

216 E. Executive Order 13132: Federalism 1 This proposed rule does not have federalism implications. It will not have substantial 2 direct effects on the States, on the relationship between the national government and the States, 3 or on the distribution of power and responsibilities among the various levels of government, as 4 specified in Executive Order 13132 because it does not contain legally binding requirements. 5 Thus, the requirements of Executive Order 13132 do not apply to this rule. 6 EPA believes, however, that this proposed rule may be of significant interest to State 7 governments. A s also noted in section E (above) on UMRA, EPA recognizes that States will 8 have a substantial interest in this rule and any corresponding revisions to associated SIP 9 requirements and air quality surveillance requirements, 40 CFR part 51 and 40 CFR part 58, 10 respectivel y. Therefore, in the spirit of Executive Order 13132 and consistent with EPA policy 11 to promote communications between EPA and State and local governments, EPA specificall y 12 solicits comment on this proposed rule from State and local off icials. 13 F. Executive Order 13175: Consultation and Coordination with Indian Tribal Governments 14 Executive Order 13175, entitled “Consultation and Coordination with Indian Tribal 15 Governments” (65 FR 67249, November 9, 2000), requires EPA to develop an accountable 16 process to ensure “meaningful and timely input by tribal officials in the development of 17 regulatory

policies that have tribal implications.” This rule concerns the establishment of 18 national standards to address the public welfare effects of o xides of n itrogen and s ulfur

. 19 This action does not have Tribal implications, as specified in Executive Order 13175 (65 20 FR 67249, November 9, 2000). It does not have a substantial direct effect on one or more Indian 21 tribes, since tribes are not obligated to adopt or implement any NAAQS. Thus, Executive Order 22 13175 does not apply to this rule. 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

217 G. Executive Order 13045: Protection of Children from Environmental Health &Safety 1 Risks 2 This action is not subject to EO 13045 because it is not an economicall y significant r ule 3 as defined in EO 12866. 4 H. Executive Order 13211: Actions that Significantly Affect Energy Supply, Distribution or 5 Use 6 This action is not a “significant energy action” as defined in Executive Order 13211 (66 7 FR 28355, May

22, 2001), because it is not likely to have a significant adverse effect on the 8 supply , distribution, or use of energ y. This action concerns the establishment of national 9 standards to address the public welfare effects of oxides of nitrogen and sulfur. This action does 10 not prescribe specific pollution control strategies by which these ambient standards will be met. 11 S uch strategies will be developed b y States on a case

- by

- case basis, and EPA cannot predict 12 whether the control options selected b y States will include regulations on energy suppliers, 13 distributors, or users. 14 I. National Technology Transfer and Advancement Act 15 Section 12(d) of the National Technology Transfer and Advancement Act of 1995 16 (NTTAA), Public Law No. 104

- 113, 12(d) (15 U.S.C. 272 note) directs EPA to use voluntary 17 consensus standards in its regulatory activities unless to do so would be inconsistent with 18 applicable law or otherwise impractical. Voluntary consensus standards are technical standards 19 (e.g., materials specifications, test methods, sampling procedures, and business practices) that are 20 developed or adopted b y voluntary consensus standards bodies. The NTTAA directs EPA to 21 provide Congress, through OMB, explanations when the Agency decides not to use available and 22 applicable voluntary consensus standards. 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

218 EPA is not aware of any

voluntary

consensus standards that are relevant to the 1 provisions of this proposed rule. EPA welcomes any feedback on such standards that may be 2 applicable. J. Executive Order 12898: Federal Actions to Address Environmental Justice in 3 Minority Populations and Low

-Income Populations 4 Executive Order 12898 (59 FR 7629 (Feb. 16, 1994)) establishes federal executive policy 5 on environmental justice. Its main provision directs federal agencies, to the greatest extent 6 practicable and permitted by law, to make environmental justice part of their mission by 7 identifying and addressing, as appropriate, disproportionately high and adverse human health or 8 environmental effects of thei r programs, policies, and activities on minority populations and low

9 income populations in the United States. 10 EPA has determined that this proposed rule will not have disproportionatel y high and 11 adverse human health or environmental effects on minority or low

- income populations because it 12 retain s the level of environmental protection for all affected populations without having any 13 disproportionately

high and adverse human health or environmental effects on any population, 14 including any minority or low

- in come population. 15 16 **References** 17 Baker JP; Bernard DP; Christensen SW; Sale MJ. 1990. Biological effects of changes in surface water acid

- base 18 chemistry. (State of science / technology report #13).Washington DC: National Acid Precipitation 19 Assessment Program (NAPAP). 20 Bulger AJ; Cosby BJ; Webb JR. 2000. Current, reconstructed past, and projected future status of brook trout 21 ( Salvelinus fontinalis ) streams in Virginia. Can J Fish Aquat Sci, 57, 1515

- 1523. 22 Banzhaf, S., D. Burtraw , D. Evans, and A. Krupnick. 2006. “Valuation of Natural Resource Improvements in the 23 Adirondacks.” Land Economics 82:445

- 464. 24 Lavery, T.F. C.M. Rogers, R. Baumgardner, and K.P. Mishoe. 2009. Intercomparison of Clean Air Status and 25 Trends Network Nitrate and Nitric Acid Measurements with Data from Other Monitoring Programs **.** J . Air 26 & Waste Manage. Assoc. , 59, 214

- 226. 27

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

219 Lien L; Raddum GG; Fjellheim A. 1992. Critical loads for surface waters: invertebrates and fish. (Acid rain research 1 report no 21). Oslo, Norway : Norwegian Institute for Water Research. 2 Luke, W.T. , P.K. Barry, L. Lefer, J. Flynn, B. Rappenglück, M. Leuchner, J.E. Dibb, L.D. Ziemba, C.H. Anderson, 3 and M. Buhr. 2010. Measurements of primary trace gases and NO Y com position in Houston, Texas. 4 Atmospheric Environment , 44, 4068

- 4080. 5 MacAvoy SW; Bulger AJ. 1995. Survival of brook trout (Salvelinus fontinalis) embryos and fry in streams of 6 different acid sensitivity in Shenandoah National Park, USA. Water Air Soil Pollut, 85, 445

- 450. 7 McNulty SG; Cohen EC; Myers JAM; Sullivan TJ; Li H. 2007. Estimates of critical acid loads and exceedances for 8 forest soils across the conterminous United States. Environ Pollut, 149, 281

- 292. 9 NAP AP. 1990. Acid Deposition: State of Science and Technology. National Acid Precipitation Assessment 10 Program. Offi ce of the Director, Washington, DC. 11 NAP AP. (2005). National acid precipitation assessment program report to Congress: An integrated assessment. 12 http://www.esrl.noaa.gov/csd/aqrs/reports/napapreport05.pdf. Silver Spring, MD: National Acid 13 Precipitation Ass essment Program (NAPAP); Committee on Environment and Natural Resources (CENR) 14 of the National Science and Technology Council (NSTC). 15 NRC (National Research Council). 2004 . Air quality management in the United States. Washington, DC: National 16 Research Coun cil (NRC); The National Academies Press. 17 Russell, A and J. M. Samet, 2010a. Review of the Policy Assessment for the Review of the Secondary National 18 Ambient Air Quality Standard for NOx and SOx: First Draft. EPA

- CASAC

- 10

- 014. 19 Russell, A and J. M. Samet, 20 10b. Review of the Policy Assessment for the Review of the Secondary National 20 Ambient Air Quality Standard for NOx and SOx: Second Draft. EPA

- CASAC

- 11

- 003. 21 Russell, A and J. M. Samet, 2011. Review of the Policy Assessment for the Revie w of the Secondary National 22 Ambient Air Quality Standard for NOx and SOx: FINAL. EPA

- CASAC

- 11

- 005. 23 Russell and Samet, 2011b Review of EPA Draft Documents on Monitoring and Methods for Oxides of Nitrogen 24 (NOx) and Sulfur (SOx) http://yosemite.epa.gov/sab/sabpeople.nsf/WebCommittees/CASAC 25 Sickles II, J.E. , L. L. Hodson, and L. M. Vorburger. 1999

. Evaluation of the filter pack for long

- duration sampling 26 of ambient air

. Atmospheric Environment , 33, 2187

- 220 2. 27 Sickles II, J.E. and D.S. Shadwick. 2008. Comparison of particulate sulfate and nitrate at collocated CASTNET and 28 IMPROVE sites in the eastern US

. Atmospheric Environment , 42, 2062

- 2073. 29 Smyth SC, W. Jiang, and H. Roth. 2008. A comparative performance evaluation of the AURAMS and CMAQ air 30 quality modeling systems. Atmos Envir 43:1059

- 1070. 31 Stoddard J; Kahl JS; Deviney FA; DeWalle DR; Driscoll CT; Herlihy AT; Kellogg JH; Murdoch PS; Webb JR; 32 Webster KE. (2003). Response of surface water chemistry to the Clean Air Act Amendments of 1990 (No. 33 EPA 620/R

- 03/001). Research Triangle Park, NC; National Health and Environmental Effects Research 34 Laboratory; Office of Research and Development; U.S. Environmental Protection Agency. 35 Sullivan TJ; Driscoll CT; Cosby BJ ; Fernandez IJ; Herlihy AT; Zhai J; Stemberger R; Snyder KU; Sutherland JW; 36 Nierzwicki

- Bauer SA; Boylen CW; McDonnell TC; Nowicki NA. 2006. Assessment of the extent to which 37 intensively studied lakes are representative of the Adirondack Mountain region. (F inal Report no 06

- 38 17).Corvallis, OR; prepared by Environmental Chemistry, Inc. for: Albany, NY; Environmental Monitoring 39

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

220 Evaluation and Protection Program of the New York State Energy Research and Development Authority 1 (NYSERDA). 2 US EPA, 1973. “Effects of Sulfur Oxide in the Atmosphere on Vegetation”. Revised Chapter 5 of Air Quality 3 Criteria For Sulfur Oxides. U.S. Environmental Protection Agency. Research Triangle Park, N.C. EPA

- R3

- 4 73

- 030. 5 US EPA. 1982. Review of the National Ambient Air Quality Standards for Sulfur Oxides: Assessment of Scientific 6 and Technical Information . OAQPS Staff Paper. EPA

- 450/5

- 82

- 007. U.S. Environmental Protection 7 Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC. 8 US EPA, 1984a. The Acidic Depositio n Phenomenon and Its Effects: Critical Assessment Review Papers. Volume I 9 Atmospheric Sciences. EPA

- 600/8

- 83

- 016AF. Office of Research and Development, Washington, DC 10 US EPA, 1984b. The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Revi ew Papers. Volume 11 II Effects Sciences. EPA

- 600/8

- 83

- 016BF. Office of Research and Development, Washington, DC 12 US EPA, 1985. The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Document. EPA

- 600/8

- 13 85/001. Office of Research and Development , Washington, DC 14 US EPA. 1995a. Review of the National Ambient Air Quality Standards for Nitrogen Dioxide: Assessment of 15 Scientific and Technical Information . OAQPS Staff Paper. EPA

- 452/R

- 95

- 005. U.S. Environmental 16 Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC. September. 17 US EPA. 1995b. Acid Deposition Standard Feasibility Study Report to Congress. U.S. Environmental Protection 18 Agency, Washington, DC. EPA

- 430/R

- 95

- 001a. 19 US EPA 2007. Integrated Review Plan for th e Secondary National Ambient Air Quality Standards for Nitrogen 20 Dioxide and Sulfur Dioxide . U.S. Environmental Protection Agency, Research Triangle Park, NC, EPA

21 452/R

- 08

- 006. 22 US EPA 2008. Integrated Science Assessment (ISA) for Oxides of Nitrogen and Sulf ur Ecological Criteria (Final 23 Report). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R

- 08/082F, 2008. 24 US EPA 2009. Risk and Exposure Assessment for Revie w of the Secondary National Ambient Air Quality 25 Standards for Oxides of Nitrogen and Oxides of Sulfur

- Main Content

- Final Report. U.S. Environmental 26 Protection Agency, Washington, D.C., EPA

- 452/R

- 09

- 008a 27 US EPA, 2010a. CASTNET Quality Assurance Project Plan, Revision 7.0, October 2010, 28 http://java .epa.gov/castnet/ 29 US EPA, 2010b. CASTNET Annual Reports, 2004

- 2009, http://java.epa.gov/castnet/ 30 US EPA 2011. Policy Assessment for the Review of the Secondary National Ambient Air Quality Standards for 31 Oxides of Nitrogen and Oxides of Sulfur. U.S. Environmental Protection Agency, Washington, D.C., EPA

32 452/R

- 11

- 005a. 33 US EPA, 2011b. Federal Reference Methods for NO y and p

- SO 4 for the New Combined NO x and SO x Secondary 34 NAAQS Research Plan, EPA/600/1

- 11/002 January 20, 2011. 35 Wolff, G. T. 1993. CASAC closure letter for the 1993 Criteria Document for Oxides of Nitrogen addressed to U.S. 36 EPA Administrator Carol M. Browner dated September 30, 1993. 37 Wolff, G. T. 1995. CASAC closure letter for the 1995 OAQPS Staff Paper addressed to U.S. EPA Administrator 38 Carol M. Browner dated August 22, 1995. 39

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

221 1 **List of Subjects in 40 CFR Part 50** 2 Environmental protection, Air pollution control, Carbon monoxide, Lead, Nitrogen 3 dioxide, Ozone, Particulater matter, Sulfur oxides. 4 5

________

________________________ 6 Dated: July

12, 2011 7 8 9 Lisa P. Jackson, 10 Administrator 11 12 For the reasons set forth in the preamble, part 50 of chapter 1 of title 40 of the code of 13 Federal regulations is proposed to be amended as follows: 14
15
**P ART**  **50 -**  **NATIONAL PRIMARY**  **AND SECONDARY AMBIENT AIR QUALITY**  16 **STANDARDS** 17
1. The authority citation for part 50 continues to read as follows: 18 Authority : 42 U.S.C. 7401, et seq. 19
20

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

222 2. Section 50.5 is amended by revising paragraphs (b) and (c) and b y adding paragraphs (d) and 1 (e) to read as follows: 2 **§50. 5**  **National secondary**  **am bient air quality standards for sulfur oxides (sulfur dioxide).**  3 * * * * * 4 (b) The level of the national secondary 1

- hour ambient air quality standard for oxides of sulfur is 5 75 parts per billion (ppb, which is 1 part in 1,000,000,000), measured in the ambient air as sulfur 6 dioxide (SO 2 )

. 7 (c) The levels of the standards shall be measured by a reference method based on Appendix A

- 1 8 or A

- 2 of this part, or b y a Federal Equivalent Method (FEM) designated in accordance with part 9 53 of this chapter. 10 (d) To demonstrate attainment with the 3

- hour secondary standard, the second

- highest 3

- hour 11 average must be based upon hourly data that are at least 75 percent complete in each calendar 12 quarter. A 3

- hour block average shall be considered valid only if all three hourly

averages for 13 the 3

- hour period are available. If onl y one or two hourly averages are available, but the 3

- hour 14 average would exceed the level of the standard when zeros are substituted for the missing values, 15 subject to the rounding rule of paragraph (a) of this section, then this shall be considered a valid 16 3

- hour average. In all cases, the 3

- hour block average shall be computed as the sum of the 17 hourly

averages divided by

3. 18 (e) The 1

- hour secondary standard is met at an ambient air quality monitoring site when the 19 three

- ye ar average of the annual 99 th percentile of the daily maximum 1

- hour average 20 concentrations is less than or equal to 75 ppb, as determined in accordance with Appendix T of 21 this part. 22 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

223 3. Section 50.11 is amended by deleting paragraphs (f) and (g) and revis ing paragraphs (a) 1 through (e) to read as follows: 2 **§50.11 National primary and secondary ambient air quality standards for oxides of**  3 **nitrogen (with nitrogen dioxide as the indicator).**  4 (a) The level of the national primary and secondary annual ambient air quality standard s for 5 oxides of nitrogen is 53 parts per billion (ppb, which is 1 part in 1,000,000,000), annual average 6 concentration, measured in the ambient air as nitrogen dioxide. 7 (b) The level of the national primary and secondary 1

- hour ambient air quality standard s for 8 oxides of nitrogen is 100 ppb, 1

- hour average concentration, measured in the ambient air as 9 nitrogen dioxide. 10 (c) The levels of the standard s shall be measured by : 11 (1) a reference method b ased on appendix F to this part; or 12 (2) a Federal equivalent method (FEM) designated in accordance with part 53 of this chapter. 13 (d) The annual primary and secondary standard s are met when the annual average concentration 14 in a calendar ye ar is less than or equal to 53 ppb, as determined in accordance with Appendix S 15 of this part for the annual standard. 16 ( e ) The 1

- hour primary and secondary standard s are met when the three

- ye ar average of the 17 annual 98 th perce ntile of the daily maximum 1

- hour average concentration is less than or equal to 18 100 ppb, as determined in accordance with Appendix S of this part for the 1

- hour standard. 19
20

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224 4. Appendix S is amended as follows: 1 a. by revising paragraph 1.(a), 2 b. by revising the definition of “Design values” under paragraph 1.(c), 3 c. by revising paragraph 2.(b), 4
d. by revising paragraphs 3.1(a) through (d), 5 e. by revising paragraphs 3.2(a) through (e), 6 f. by revising paragraph 4.1(b), 7 g. b y revising paragraph 4.2 (c), 8 h. by revising paragraph 5.1(b), and 9 i. by revising paragraph 5.2(b) to read as follows: 10 **Appendix S to Part 50**

**— Interpretation of the Primary**  **and Secondary National Ambient**  11 **Air Quality Standards for Oxides of Nitrogen (Nitrogen Dioxide)** 12
1. General. 13 (a) This app endix explains the data handling conventions and computations necessary for 14 determining when the primary and secondary national ambient air quality standards for oxides of 15 nitrogen as measured b y nitrogen dioxide (“NO 2 NAAQS”) specified in § 50.11 are met. 16 Nitrogen dioxide (NO 2 ) is measured in the ambient air by a Federal reference method (FRM) 17 based on appendix F to this part or b y a Federal equivalent method (FEM) designated in 18 accordance with part 53 of this chapter. Data handling and computation procedu res to be used in 19

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

225 making comparisons between reported NO 2 concentrations and the levels of the NO 2 NAAQS 1 are specified in the following sections. 2
* * * * * 3 (c) The terms used in this appendix are defined as follows: 4
* * * * * 5 Design values are the metrics ( i.e. , statistics) that are compared to the NAAQS levels to 6 determine compliance, calculated as specified in section 5 of this appendix. The design values 7 for the primary and secondary NAAQS are: 8 (1) The annual mean v alue for a monitoring site for one year (referred to as the “ annual primary 9 or secondary standard design value ”). 10 (2) The 3

- ye ar average of annual 98 th percentile daily maximum 1

- hour values for a monitoring 11 site (referred to as the “ 1

- hour primary or sec ondary standard design value ”). 12 * * * * * 13 2. Requirements for Data Used for Comparisons with the NO 2 NAAQS and Data 14 Reporting Considerations. 15
* * * * * 16 (b) When two or more NO 2 monitors are operated at a site, the state m ay in advance designate 17 one of them as the primary monitor. If the state has not made this designation, the Administrator 18 will make the designation, either in advance or retrospectivel y. Design values will be developed 19 using onl y the data from the primary monitor, if this results in a valid design value. If data from 20 the primary

monitor do not allow the development of a valid design value , data solely from the 21 other monitor(s) will be used in turn to develop a valid design value , if this results in a val id 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

226 design value . If there are three or more monitors, the order for such comparison of the other 1 monitors will be determined by the Administrator. The Administrator may combine data from 2 different monitors in different ye ars for the purpose of developing a valid 1

- hour primary or 3 secondary

standard design value, if a valid design value cannot be developed solely with the data 4 from a single monitor. However, data from two or more monitors in the same year at the same 5 site will not be combined in an attemp t to meet data completeness requirements, except if one 6 monitor has phy sicall y replaced another instrument permanentl y , in which case the two 7 instruments will be considered to be the same monitor, or if the state has switched the 8 designation of the primary monitor from one instrument to another during the year . 9
* * * * * 10 3. Comparisons with the NO 2 NAAQS. 11 3.1 The Annual Primary and Secondary NO 2 NAAQS. 12 (a) The annual primary and secondary NO 2 NAAQS are met at a site when the valid annual 13 primary

standard design value is less than or equal to 53 parts per billion (ppb). 14 (b) An annual primary or secondary standard design value is valid when at least 75 percent of the 15 hours in the year are reported. 16 (c) An annual primary or secondary

standard design value based on data that do not meet the 17 completeness criteria stated in section 3.1(b) may also be considered valid with the approval of, 18 or at the initiative of, the Administrator, who may consider factors such as monitoring site 19 closures/moves, m onitoring diligence, the consistency and levels of the valid concentration 20 measurements that are available, and nearb y concentrations in determining whether to use such 21 data. 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

227 (d) The procedures for calculating the annual primary and secondary standard des ign values are 1 given in section 5.1 of this appendix. 2 3.2 The 1

- hour Primary and Secondary NO 2 NAAQS. 3 (a) The 1

- hour primary or secondary NO 2 NAAQS is met at a site when the valid 1

- hour primary 4 or secondary standard design value is less than or equal to 1 00 parts per billion (ppb). 5 (b) An NO 2 1

- hour primary or secondary standard design value is valid if it encompasses three 6 consecutive calendar years of complete data. A year meets data completeness requirements 7 when all 4 quarters are complete. A quarter is complete when at least 75 percent of the sampling 8 day s for each quarter have complete data. A sampling day has complete data if 75 percent of the 9 hourly

concentration values, including state

- flagged data affected b y exceptional events which 10 have been a

pproved for exclusion by the Administrator, are reported

. 11 (c) In the case of one, two, or three ye ars that do not meet the completeness requirements of 12 section 3.2(b) of this appendix and thus would normally not be useable for the calculation of a 13 valid 3

- ye ar 1

- hour primary or secondary standard design value, the 3

- ye ar 1

- hour primary or 14 secondary standard design value shall nevertheless be considered valid if one of the following 15 conditions is true. 16 (i) At least 75 percent of the days in each quarter o f each of three consecutive years have at least 17 one reported hourl y value, and the design value calculated according to the procedures specified 18 in section 5.2 is above the level of the primary or secondary

1

- hour standard. 19 (ii) (A) A 1

- hour primary or sec ondary standard design value that is below the level of the 20 NAAQS can be validated if the substitution test in section 3 .2 (c)(i i ) (B) results in a “test design 21 value” that is below the level of the NAAQS. The test substitut es actual ‘‘ high ’’ reported dail y 22 maximum 1

- hour values from the same site at about the same time of the year ( specifically , in 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

228 the same calendar quarter) for unknown values that were not successfully measured . Note that 1 th e test is merely

diagnostic in nature, intended to confirm that there is a very high likelihood 2 that the original design value (the one with less than 75 percent data capture of hours b y day and 3 of day s by

quarter) reflects the true under

- NAAQS

- level status for that 3

- year period; the result 4 of th is data substitution te st (the ‘‘test design value”, as defined in section 3 .2 (c)(i i ) (B)) is not 5 considered the actual design value. For this test , substitution is permitted only if there are at least 6 200 day s across the three matching quarters of the three years under consideration (which is 7 about 75 percent of all possible daily values in those three quarters) for which 75 percent of the 8 hours in the day , including state

- flagged data affected by exceptional events which have been 9 approved for exclusion by the Administrator, hav e reported concentrations. However, maximum 10 1

- hour values from day s with less than 75 percent of the hours reported shall also be considered 11 in identifying the high value to be used for substitution. 12 (B) The substitution test is as follows: Data substituti on will be performed in all quarter periods 13 that have less than 75 percent data capture but at least 50 percent data capture , including state

14 flagged data affected b y exceptional events which have been approved for exclusion by the 15 Administrator ; if any qu arter has less than 50 percent data capture then this substitution test 16 cannot be used

. Identify for each quarter (e.g., January

- March) the highest reported daily 17 maximum 1

- hour value for that quarter , excluding state
- flagged data affected by exceptional 18 events which have been approved for exclusion by the Administrator, looking across those three 19 months of all three years under consideration

. All daily maximum 1

- hour values from all day s in 20 the quarter period shall be considered when identifying this highest value, including day s with 21 less than 75 percent data capture. If after substituting the highest non - excluded reported dail y 22 maximum 1

- hour value for a quarter for as much of the missing dail y data in the matching 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

229 deficient quarter ( s ) as is needed to make them 100 percent complete, the procedure in section 5.2 1 yi elds a recalculated 3

- ye ar 1

- hour standard “test design value” below the level of the standard, 2 then the 1

- hour primary or secondary standard design value is deemed to have passed the 3 diagnosti c test and is valid, and the level of the standard is deemed to have been met in that 3

- 4 ye ar period. As noted in section 3 .2 (c)(i), in such a case, the 3

- ye ar design value based on the 5 data actually

reported, not the ‘‘test design value’’, shall be used as the valid design value. 6 (iii) (A) A 1

- hour primary or secondary standard design value that is above the level of the 7 NAAQS can be validated if the substitution test in section 3 .2 (c)(i ii ) (B) results in a “test design 8 value” that is above the level of the NAAQS. The test substitut es actual ‘‘ low” reported dail y 9 maximum 1

- hour values from the same site at about the same time of the year ( specifically , in 10 the same three months of the calendar) for unknown values that were not successfull y measured

. 11 Note th at th e test is merely diagnostic in nature, intended to confirm that there is a very high 12 likelihood that the original design value (the one with less than 75 percent data capture of hours 13 by day and of days by

quarter) reflects the true above

-NAAQS

- level status for that 3

- ye ar period; 14 the result of th is data substitution test (the ‘‘test design value”, as defined in section 15 3 .2 (c)(i ii ) (B)) is not considered the actual design value. For this test , substitution is permitted 16 only

if there are a minimum number of available daily data points from which to identify the low 17 quarter
- specific daily maximum 1

- hour values, specificall y if there are at least 200 day s across 18 the three matching quarters of the three years under consideration (which is about 75 percent of 19 all possible daily

values in those three quarters) for which 75 percent of the hours in the day have 20 reported concentrations. Only day s with at least 75 percent of the hours reported shall be 21 considered in identifying the low value to be used for substitution. 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

230 (B) The substitution test is as follows: Data substitution will be performed in all quarter periods 1 that have less than 75 percent data capture. Identify for each quarter (e.g., January

- March) the 2 lowest reported daily maximum 1

- hour value for that quarter , looking across those three months 3 of all three ye ars under consideration

. All daily maximum 1

- hour values from all day s with at 4 least 75 percent capture in the quarter period shall be considered when identifying this lowest 5 value. If after substi tuting the lowest reported daily maximum 1

- hour value for a quarter for as 6 much of the missing daily data in the matching deficient quarter ( s ) as is needed to make them 75 7 percent complete, the procedure in section 5.2 yi elds a recalculated 3

- year 1

- hour s tandard “test 8 design value” above the level of the standard, then the 1

- hour primary or secondary standard 9 design value is deemed to have passed the diagnostic test and is valid, and the level of the 10 standard is deemed to have been exceeded in that 3

- ye ar period. As noted in section 3 .2 (c)(i), in 11 such a case, the 3

- ye ar design value based on the data actually reported, not the ‘‘test design 12 value’’, shall be used as the valid design value. 13 (d) A 1

- hour primary or secondary

standard design value based on d ata that do not meet the 14 completeness criteria stated in 3.2(b ) and also do not satisfy section 3.2(c), may

also be 15 considered valid with the approval of, or at the initiative of, the Administrator, who may consider 16 factors such as monitoring site closures /moves, monitoring diligence, the consistency and levels 17 of the valid concentration measurements that are available, and nearb y concentrations in 18 determining whether to use such data. 19 (e ) The procedures for calculating the 1

- hour primary and secondary stan dard design values are 20 given in section 5.2 of this appendix. 21 4. Rounding Conventions. 22 4.1 Rounding Conventions for the Annual Primary and Secondary NO 2 NAAQS. 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

231 * * * * * 1 (b) The annual primary or secondary standard design value is calculated pursuant to section 5.1 2 and then rounded to the nearest whole number or 1 ppb (decimals 0.5 and greater are rounded up 3 to the nearest whole number, and any decimal lower than 0.5 is rounded down to the nearest 4 whole number). 5 4.2 Rounding Conventions fo r the 1

- hour Primary and Secondary NO 2 NAAQS. 6 * * * * * 7 (c) The 1

- hour primary or secondary standard design value is calculated pursuant to section 5.2 8 and then rounded to the nearest whole number or 1 ppb (decimals 0.5 and greater are rounded up 9 to the nearest whole number, and any decimal lower than 0.5 is rounded down to the nearest 10 whole number). 11 5. Calculation Procedures f or the Primary and Secondary NO 2 NAAQS. 12 5.1 Procedures for the Annual Primary and Secondary NO 2 NAAQS 13 * * * * * 14 (b) The annual primary or secondary standard design value for a site is the valid annual mean 15 rounded according to the conventions in section 4.1. 16 5.2 Calculation Procedures f or the 1

- hour Primary and Secondary NO 2 NAAQS

. 17 * * * * * 18 (b) The 1

- hour primary or secondary standard design value for a site is the mean of the three 19 annual 98 th percentile values, rounded according to the conventions in section 4. 20
* * * * * 21
22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

232 5. Appendix T is amended as follows: 1 a. by rev ising paragraph 1.(a), 2 b. by revising the definition of “Design values” under paragraph 1.(c), 3 c. by revising paragraph 2.(b), 4 d. by revising paragraphs 3.(a) through (e), 5 e. by revising paragraph 4.(c), and 6 f. by revising paragraph 5.(b) to read as follo ws: 7 **Appendix T to Part 50** 

**— Interpretation of the Primary**  **and Secondary National Ambient**  8 **Air Quality Standards for Oxides of Sulfur**  **( Sulfur Dioxide**  **)** 9
1. General. 10 (a) This appendix explains the data handling conventions and computations necessary for 11 determin ing when the primary and secondary national ambient air quality standards for Oxides 12 of Sulfur as measured b y Sulfur Dioxide (“ SO 2 NAAQS”) specified in § 50.17 and §50.5 (b), 13 respectivel y, are met at an ambient air quality monitoring site

. Sulfur Dioxide ( SO 2 ) is measured 14 in the ambient air b y a Federal reference method (FRM) based on appendix A

- 1 or A

- 2 to this 15 part or b y a Federal equivalent method (FEM) designated in accordance with part 53 of this 16 chapter. Data handling and computation procedures to be used in making comparisons between 17 reported SO 2 concentrations and the levels of the SO 2 NAAQS are specified in the following 18 sections. 19

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

233 * * * * * 1 (c) The terms used in this appendix are defined as follows: 2 * * * * * 3 Design values are the metrics ( i.e. , statistics) that are compared to the NAAQS levels to 4 determine compliance, calculated as specified in section 5 of this appendix. The design value for 5 the primary and secondary 1

- hour NAAQS is the 3

- ye ar average of annual 99 th percentile daily 6 maximum 1

- hour values for a monitoring site (referred to as the “ 1

- hour primary standard design 7 value ”). 8
* * * * * 9 2. Requirements for Data Used for Comparisons with the SO 2 NAAQS and Data 10 Reporting Considerations. 11
* * * * * 12 (b) Data from two or more monitors from the same year at the same site reported to EPA under 13 distinct Pollutant Occurrence Codes shall not be combined in an attempt to meet data 14 completeness requirements

. The Administrator will combin e annual 99 th percentile dail y 15 maximum concentration values from different monitors in different ye ars, selected as described 16 here, for the purpose of developing a valid 1

- hour primary or secondary

standard design value. 17 If more than one of the monitors m eets the completeness requirement for all four quarters of a 18 ye ar, the steps specified in section 5(a) of this appendix shall be applied to the data from the 19 monitor with the highest average of the four quarterly completeness values to derive a valid 20
annual 99 th percentile daily maximum concentration. If no monitor is complete for all four 21 quarters in a ye ar, the steps specified in section 3(c) and 5(a) of this appendix shall be applied to 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

234 the data from the monitor with the highest average of the four quar terl y completeness values in 1 an attempt to derive a valid annual 99 th percentile daily maximum concentration. This paragraph 2 does not prohibit a monitoring agency from making a local designation of one phy sical monitor 3 as the primary

monitor for a Pollutant Occurrence Code and substituting the 1

- hour data from a 4 second ph ys ical monitor whenever a valid concentration value is not obtained from the primary 5 monitor; if a monitoring agency substitutes data in this manner, each substituted value must be 6 accompanied by

an AQS qualifier code indicating that substitution with a value from a second 7 phy sical monitor has taken place. 8
* * * * * 9 3. Comparisons with the 1

- hour Primary and Secondary SO 2 NAAQS. 10 (a) The 1

- hour primary or secondary SO 2 NAAQS is met at an ambient air quality monitoring 11 site when the valid 1

- hour primary or secondary standard design value is less than or equal to 75 12 parts per billion (ppb). 13 (b) An SO 2 1

- hour primary or secondary standard design value is valid if it encompasses three 14 consecutive calendar years of complete data. A year meets data completeness requirements 15 when all 4 quarters are complete. A quarter is complete when at least 75 percent of the sampling 16 day s for each quarter have complete data. A sampling day has c omplete data if 75 percent of the 17 hourly

concentration values, including State

- flagged data affected b y exceptional events which 18 have been approved for exclusion by the Administrator, are reported. 19 (c) In the case of one, two, or three ye ars that do not m eet the completeness requirements of 20 section 3(b) of this appendix and thus would normally not be useable for the calculation of a 21 valid 3

- ye ar 1

- hour primary or secondary standard design value, the 3

- ye ar 1

- hour primary or 22

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

235 secondary standard design value shall nevertheless be considered valid if one of the following 1 conditions is true. 2 (i) At least 75 percent of the days in each quarter of each of three consecutive years have at least 3 one reported hourl y value, and the design value calculated according t o the procedures specified 4 in section 5 is above the level of the primary or secondary

1

- hour standard. 5 (ii) (A) A 1

- hour primary or secondary standard design value that is equal to or below the level 6 of the NAAQS can be validated if the substitution test in section 3(c)(i i ) (B) results in a “test 7 design value” that is below the level of the NAAQS. The test substitut es actual ‘‘ high ’’ reported 8 daily maximum 1

- hour values from the same site at about the same time of the year ( specifically , 9 in the same calend ar quarter) for unknown values that were not successfully measured . Note that 10 th e test is merely

diagnostic in nature, intended to confirm that there is a very high likelihood 11 that the original design value (the one with less than 75 percent data capture of hours b y day and 12 of day s by

quarter) reflects the true under

- NAAQS

- level status for that 3

- year period; the result 13 of th is data substitution test (the ‘‘test design value”, as defined in section 3(c)(i i ) (B)) is not 14 considered the actual design value. Fo r this test , substitution is permitted only if there are at least 15 200 day s across the three matching quarters of the three years under consideration (which is 16 about 75 percent of all possible daily values in those three quarters) for which 75 percent of th e 17 hours in the day , including State

- flagged data affected b y exceptional events which have been 18 approved for exclusion by the Administrator, have reported concentrations. However, maximum 19 1

- hour values from day s with less than 75 percent of the hours reported shall also be considered 20 in identifying the high value to be used for substitution. 21 (B) The substitution test is as follows: Data substitution will be performed in all quarter periods 22 that have less than 75 percent data capture but at least 50 percent data capture, including State

- 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

236 flagged data affected b y exceptional events which have been approved for exclusion by the 1 Administrator ; if any quarter has less than 50 percent data capture then this substitution test 2 cannot be used

. Identify for each quart er (e.g., January

- March) the highest reported daily 3 maximum 1

- hour value for that quarter, excluding State

- flagged data affected by exceptional 4 events which have been approved for exclusion by the Administrator, looking across those three 5 months of all three years under consideration

. All daily maximum 1

- hour values from all day s in 6 the quarter period shall be considered when identifying this highest value, including day s with 7 less than 75 percent data capture. If after substituting the highest reported daily maximum 1

- hour 8 value for a quarter for as much of the missing daily data in the matching deficient quarter ( s ) as is 9 needed to make them 100 percent complete, the procedure in section 5 yi elds a recalculated 3

- 10 ye ar 1

- hour standard “test design value” le ss than or equal to the level of the standard, then the 1

- 11 hour primary or secondary standard design value is deemed to have passed the diagnostic test 12 and is valid, and the level of the standard is deemed to have been met in that 3

- ye ar period. As 13 noted in section 3(c)(i), in such a case, the 3

- ye ar design value based on the data actually 14 reported, not the ‘‘test design value’’, shall be used as the valid design value. 15 (iii) (A) A 1

- hour primary or secondary standard design value that is above the level of the 16 NAAQS can be validated if the substitution test in section 3(c)(i ii ) (B) results in a “test design 17 value” that is above the level of the NAAQS. The test substitut es actual ‘‘ low” reported dail y 18 maximum 1

- hour values from the same site at about the same time of the year ( specifically , in 19 the same three months of the calendar) for unknown hourly values that were not successfully 20 measured . Note that th e test is merely diagnostic in nature, intended to confirm that there is a 21 very

high likelihood that the original design value (the one with less than 75 percent data capture 22 of hours b y day and of days b y quarter) reflects the true above

-NAAQS

- level status for that 3

- 23

### Interagency Working Comments on Draft Language under EO12866 and 1356 3 Interagency Review. Subject to Further Policy Review.

237 ye ar period; the result of th is data substitution test (the ‘‘test design value”, as defined in section 1 3(c)(i ii ) (B)) is not considered the actual design value. For this test , substitution is permitted only 2 if there are a minimum number of available dail y data points from which to identify the low 3 quarter
- specific daily maximum 1

- hour values, spe cificall y if there are at least 200 day s across 4 the three matching quarters of the three years under consideration (which is about 75 percent of 5 all possible daily

values in those three quarters) for which 75 percent of the hours in the day have 6 reported c oncentrations. Only day s with at least 75 percent of the hours reported shall be 7 considered in identifying the low value to be used for substitution. 8 (B) The substitution test is as follows: Data substitution will be performed in all quarter periods 9 that have less than 75 percent data capture. Identify for each quarter (e.g., January

- March) the 10 lowest reported daily maximum 1

- hour value for that quarter , looking across th ose three months 11 of all three ye ars under consideration

. All daily maximum 1

- hour values from all day s with at 12 least 75 percent capture in the quarter period shall be considered when identifying this lowest 13 value. If after substituting the lowest reported daily maximum 1

- hour value for a quarter for as 14 much of the missing daily data in the matching deficient quarter ( s ) as is needed to make them 75 15 percent complete, the procedure in section 5 yi elds a recalculated 3

- year 1

- hour standard “test 16 design value” above the level of the standard, then the 1

- hour primary or secondary standard 17 design value is deemed to have passed the diagnostic test and is valid, and the level of the 18 standard is deemed to have been exceeded in that 3

- ye ar period. As noted in section 3(c)(i), in 19 such a case, the 3

- ye ar design value based on the data actually reported, not the ‘‘test design 20 value’’, shall be used as the valid design value. 21 (d) A 1

- hour primary or secondary

standard design value based on data that do not meet the 22 comple teness criteria stated in 3(b ) and also do not satisfy section 3(c), may

also be considered 23

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238 valid with the approval of, or at the initiative of, the Administrator, who may consider factors 1 such as monitoring site closures/moves, monitoring diligence, the c onsistency and levels of the 2 valid concentration measurements that are available, and nearb y concentrations in determining 3 whether to use such data. 4 (e ) The procedures for calculating the 1

- hour primary or secondary standard design values are 5 given in section 5 of this appendix. 6 4. Rounding Conventions for the 1

- hour Primary and Secondary SO 2 NAAQS. 7 * * * * * 8 (c) The 1

- hour primary or secondary standard design value is calculated pursuant to section 5 9 and then rounded to the nearest whole number or 1 ppb (decimals 0.5 and greater are rounded up 10 to the nearest whole number, and any decimal lower than 0.5 is rounded down to the nearest 11 whole number). 12 5. Calculation Procedures f or the 1

- hour Primary and Secondary SO 2 NAAQS. 13 * * * * * 14 (b) The 1

- hour primary or secondary standard design value for an ambient air quality monitoring 15 site is the mean of the three annual 99 th percentile values, rounded according to the conventions 16 in section 4. 17