ENVIRONMENTAL PROTECTION AGENCY
                                40 CFR Part 50
                            [EPA-HQ-OAR-2007-1145]
                                RIN: 2060-AO72
Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Sulfur

AGENCY:  Environmental Protection Agency (EPA).
ACTION:  Proposed rule.
SUMMARY:  Based on its review of the air quality criteria and the secondary national ambient air quality standards (NAAQS) for oxides of nitrogen and oxides of sulfur, EPA proposes to retain the current nitrogen dioxide (NO2) and sulfur dioxide (SO2) secondary standards to provide requisite protection for the direct effects on vegetation resulting from exposure to gaseous oxides of nitrogen and sulfur in the ambient air.  With regard to protection from the deposition of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecosystems, including acidification and nutrient enrichment effects, EPA is deferring the setting of a new multi-pollutant secondary standard in this review, and is proposing not to set such a secondary standard at this time.  Instead, EPA has decided to undertake a field pilot program to gather and analyze additional relevant data so as to enhance the Agency's understanding of the degree of protectiveness that a new multi-pollutant standard, defined in terms of an aquatic acidification index (AAI), would afford and to support development of an appropriate monitoring network for such a standard.  The EPA solicits comment on the framework of such a standard and on the design of the field pilot program.
DATES:  Written comments on this proposed rule must be received by [insert date 60 days after date of publication in the Federal Register].
      Public Hearings:  The EPA intends to hold a public hearing around the end of August to early September and will announce in a separate Federal Register notice the date. time, and address of the public hearing on this proposed rule.
ADDRESSES:  Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2007-1145, by one of the following methods:
   * www.regulations.gov:  Follow the on-line instructions for submitting comments.
   * Email<EMAIL_ADDRESS>   * Fax:<PHONE_NUMBER>.
   * Mail:  Docket No. EPA-HQ-OAR-2007-1145, Environmental Protection Agency, Mail code 6102T, 1200 Pennsylvania Ave., NW., Washington, DC  20460.  Please include a total of two copies.
   * Hand Delivery:  Docket No. EPA-HQ-OAR-2007-1145, Environmental Protection Agency, EPA West, Room 3334, 1301 Constitution Ave., NW, Washington, DC.  Such deliveries are only accepted during the Docket's normal hours of operation, and special arrangements should be made for deliveries of boxed information.
      Instructions:  Direct your comments to Docket ID No. EPA-HQ-OAR-2007-1145.  The EPA's policy is that all comments received will be included in the public docket without change and may be made available online at www.regulations.gov, including any personal information provided, unless the comment includes information claimed to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute.  Do not submit information that you consider to be CBI or otherwise protected through www.regulations.gov or email.  The www.regulations.gov website is an "anonymous access" system, which means EPA will not know your identity or contact information unless you provide it in the body of your comment.  If you send an email comment directly to EPA without going through www.regulations.gov, your email address will be automatically captured and included as part of the comment that is placed in the public docket and made available on the Internet.  If you submit an electronic comment, EPA recommends that you include your name and other contact information in the body of your comment and with any disk or CD-ROM you submit.  If EPA cannot read your comment due to technical difficulties and cannot contact you for clarification, EPA may not be able to consider your comment.  Electronic files should avoid the use of special characters, any form of encryption, and be free of any defects or viruses.  For additional information about EPA's public docket, visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm.
	Docket:  All documents in the docket are listed in the www.regulations.gov index.  Although listed in the index, some information is not publicly available, e.g., CBI or other information whose disclosure is restricted by statute.  Certain other material, such as copyrighted material, will be publicly available only in hard copy.  Publicly available docket materials are available either electronically in www.regulations.gov or in hard copy at the Air and Radiation Docket and Information Center, EPA/DC, EPA West, Room 3334, 1301 Constitution Ave., NW, Washington, DC.  The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal holidays.  The telephone number for the Public Reading Room is<PHONE_NUMBER> and the telephone number for the Air and Radiation Docket and Information Center is<PHONE_NUMBER>.
FOR FURTHER INFORMATION CONTACT:  Dr. Richard Scheffe,  Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Mail code C304-02,  Research Triangle Park, NC 27711; telephone:<PHONE_NUMBER>; fax:<PHONE_NUMBER>; email<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:
General Information
What Should I Consider as I Prepare My Comments for EPA?
   1. Submitting CBI.  Do not submit this information to EPA through www.regulations.gov or email.  Clearly mark the part or all of the information that you claim to be CBI.  For CBI information in a disk or CD ROM that you mail to EPA, mark the outside of the disk or CD ROM as CBI and then identify electronically within the disk or CD ROM the specific information that is claimed as CBI.  In addition to one complete version of the comment that includes information claimed as CBI, a copy of the comment that does not contain the information claimed as CBI must be submitted for inclusion in the public docket.  Information so marked will not be disclosed except in accordance with procedures set forth in 40 CFR part 2.
   2. Tips for Preparing Your Comments.  When submitting comments, remember to:
         *    Identify the rulemaking by docket number and other identifying information (subject heading, Federal Register date and page number).
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         *    Make sure to submit your comments by the comment period deadline identified.
Availability of Related Information
	A number of documents relevant to this rulemaking are available on EPA web sites.  The Integrated Science Assessment for Oxides of Nitrogen and Sulfur - Ecological Criteria: FINAL REPORT (ISA) is available on EPAs National Center for Environmental Assessment web site.  To obtain this document, go to http://www.epa.gov/ncea, and click on Air Quality then click on Oxides of Nitrogen and Sulfur.  The Policy Assessment, Risk and Exposure Assessment (REA), and other related technical documents are available on EPA's Office of Air Quality Planning and Standards (OAQPS) Technology Transfer Network (TTN) web site.  The Policy Assessment is available at http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_pa.html, and the exposure and risk assessments and other related technical documents are available at http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_rea.html.  These and other related documents are also available for inspection and copying in the EPA docket identified above.
Table of Contents
The following topics are discussed in this preamble:
I.	Background
	A.	Legislative Requirements
	B.	History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides
		1.	NAAQS for Oxides of Nitrogen
		2.	NAAQS for Oxides of Sulfur
	C.	History of Related Assessments and Agency Actions
	D.	History of the Current Review
	E.	Scope of the Current Review
II.	Rationale for Proposed Decision on the Adequacy of the Current Secondary Standards
   A. Ecological Effects
            1.       Effects Associated with Gas-Phase Oxides of Nitrogen and Sulfur
                  a. Nature of ecosystem responses to gas-phase nitrogen and sulfur
                  b. Magnitude of ecosystem response to gas-phase nitrogen and sulfur
            2.       Acidification Effects Associated with Deposition of Oxides of Nitrogen 			and Sulfur
                  a. Nature of Acidification  - related Ecosystem Responses
                        i. Aquatic ecosystems
                        ii. Terrestrial ecosystems
                        iii. Ecosystem sensitivity
                  b. Magnitude of Acidification-related Ecosystem Responses
                        i. Aquatic acidification
                        ii. Terrestrial acidification
                  c. Key Uncertainties Associated with Acidification
                        i. Aquatic acidification
                        ii. Terrestrial acidification
            3. Nutrient Enrichment Effects Associated with Deposition of Oxides of 	Nitrogen
                  a. Nature of Nutrient Enrichment-related Ecosystem Responses
                  	i.	Aquatic ecosystems
                  	ii.	Terrestrial ecosystems
                  	iii.	Ecosystem sensitivity to nutrient enrichment
                  b. Magnitude of Nutrient Enrichment-related Ecosystem Responses
                  	i.	Aquatic ecosystems
                  	ii.	Terrestrial ecosystems
                  c. Key Uncertainties Associated with Nutrient Enrichment
                        i. Aquatic ecosystems
                        ii. Terrestrial ecosystems
            4.	Other Ecological Effects
   B. Risk and Exposure Assessment
            1.       Overview of Risk and Exposure Assessment
            2.       Key Findings
                  a. Air quality analyses
                  b. Deposition-related aquatic acidification
                  c. Deposition-related terrestrial acidification
                  d. Deposition-related aquatic nutrient enrichment
                  e. Deposition-related terrestrial nutrient enrichment
                  f. Additional effects
            3. Conclusions on Effects
   C. Adversity of Effects to Public Welfare
            1.       Ecosystem Services
            2.       Effects on Ecosystem Services
                  a. Aquatic Acidification
                  b. Terrestrial Acidification
                  c. Nutrient Enrichment
            3. Summary
   D. Adequacy of the Current Standards
      1.       Adequacy of the Current Standards for Direct Effects
      2. Appropriateness and Adequacy of the Current Standards for Deposition-related Effects
                  a. Appropriateness
                  b. Adequacy of Protection
                        i. Aquatic Acidification
                        ii. Terrestrial acidification
                        iii. Terrestrial nutrient enrichment
                        iv. Aquatic nutrient enrichment
                        v. Other effects
      3.       CASAC Views
      4. Administrator's Proposed Conclusions Concerning Adequacy of Current Standard
III. Rationale for Proposed Decision on Alternative Secondary Standards for Aquatic Acidification
      A. Ambient Air Indicators
            1. Oxides of Sulfur
            2. Oxides of Nitrogen
      B. Form
            1. Ecological Indicator
            2. Linking ANC to Deposition
            3. Linking Deposition to Ambient Air Indicators
            4. Aquatic Acidification Index
            5. Spatial Aggregation
                  a. Ecoregion Sensitivity
                  b. Representative Ecoregion-specific Factors
                        i. Factor F1
                              a) Acid-sensitive Ecoregions
                              b) Non-acid sensitive Ecoregions
                        ii. Factor F2
                        iii. Factors F3 and F4
                  c. Factors in Data-limited Ecoregions
                  d. Application to Hawaii, Alaska, and the U.S. Territories
            6. Summary of the AAI Form
      C. Averaging Time
      D. Level
            1. Association Between pH Levels and Target ANC Levels
            2. ANC Levels Related to Effects on Aquatic Ecosystems
            3. Consideration of Episodic Acidity
            4. Consideration of Ecosystem Response Time
            5. Prior Examples of Target ANC Levels
            6. Consideration of Public Welfare Benefits
            7. Summary of Alternative Levels
      E. Combined Alternative Levels and Forms
      F. Characterization of Uncertainties
            1. Overview of Uncertainty
            2. Uncertainties Associated with Data Gaps
            3. Uncertainties in Modeled Processes
      G. CASAC Advice
      H. Administrator's Proposed Conclusions
IV. Field Pilot Program and Ambient Monitoring
      A. Field Pilot Program
               a. Objectives
               b. Overview of Field Pilot Program
               c. Complementary Measurements
               d. Complementary Research Efforts
               e. Implementation Challenges
               f. Final Monitoring Plan Development and Stakeholder Participation
      B. Evaluation of Monitoring Methods
            1. Potential FRMs for SO2 and p-SO4
            2. Potential FRM for NOy
V. Statutory and Executive Order Reviews
      A. Executive Order 12866:  Regulatory Planning and Review
      B. Paperwork Reduction Act
      C. Regulatory Flexibility Act
      D. Unfunded Mandates Reform Act
      E. Executive Order 13132:  Federalism
      F. Executive Order 13175:  Consultation and Coordination with Indian Tribal Governments
      G. Executive Order 13045:  Protection of Children From Environmental Health and Safety Risks
      H. Executive Order 13211:  Actions that Significantly Affect Energy Supply, Distribution, or Use
      I. National Technology Transfer and Advancement Act
      J. Executive Order 12898:  Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations
References

I.	Background
A.	Legislative Requirements
	Two sections of the Clean Air Act (CAA) govern the establishment and revision of the NAAQS.  Section 108 (42 U.S.C. section 7408) directs the Administrator to identify and list certain air pollutants and then to issue air quality criteria for those pollutants.  The Administrator is to list those air pollutants that in her "judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare;" "the presence of which in the ambient air results from numerous or diverse mobile or stationary sources;" and "for which . . . [the Administrator] plans to issue air quality criteria..."  Air quality criteria are intended to "accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of [a] pollutant in the ambient air . . ." 42 U.S.C. § 7408(b).   Section 109 (42 U.S.C. 7409) directs the Administrator to propose and promulgate "primary" and "secondary" NAAQS for pollutants for which air quality criteria are issued.  Section 109(b)(1) defines a primary standard as one "the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect the public health."  A secondary standard, as defined in section 109(b)(2), must "specify a level of air quality the attainment and maintenance of which, in the judgment of the Administrator, based on such criteria, is requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of [the] pollutant in the ambient air." Welfare effects as defined in section 302(h) (42 U.S.C. § 7602(h)) include, but are not limited to, "effects on soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility and climate, damage to and deterioration of property, and hazards to transportation, as well as effects on economic values and on personal comfort and well-being."
	In setting standards that are "requisite" to protect public health and welfare, as provided in section 109(b), EPA's task is to establish standards that are neither more nor less stringent than necessary for these purposes.  In so doing, EPA may not consider the costs of implementing the standards.  See generally, Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001).  Likewise, "[a]ttainability and technological feasibility are not relevant considerations in the promulgation of national ambient air quality standards." American Petroleum Institute v. Costle, 665 F. 2d at 1185.  Section 109(d)(1) requires that "not later than December 31, 1980, and at 5-year intervals thereafter, the Administrator shall complete a thorough review of the criteria published under section 108 and the national ambient air quality standards . . . and shall make such revisions in such criteria and standards and promulgate such new standards as may be appropriate . . . ."  Section 109(d)(2) requires that an independent scientific review committee "shall complete a review of the criteria . . . and the national primary and secondary ambient air quality standards . . . and shall recommend to the Administrator any new . . . standards and revisions of existing criteria and standards as may be appropriate . . . ."  Since the early 1980's, this independent review function has been performed by the Clean Air Scientific Advisory Committee (CASAC).
B.	History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides
1.	NAAQS for Oxides of Nitrogen
	After reviewing the relevant science on the public health and welfare effects associated with oxides of nitrogen, EPA promulgated identical primary and secondary NAAQS for NO2 in April 1971.  These standards were set at a level of 0.053 parts per million (ppm) as an annual average (36 FR 8186).  In 1982, EPA published Air Quality Criteria for Oxides of Nitrogen (US EPA, 1982), which updated the scientific criteria upon which the initial standards were based.  In February 1984 EPA proposed to retain these standards (49 FR 6866).  After taking into account public comments, EPA published the final decision to retain these standards in June 1985 (50 FR 25532).
	The EPA began the most recent previous review of the oxides of nitrogen secondary standards in 1987.  In November 1991 EPA released an updated draft air quality criteria document (AQCD) for CASAC and public review and comment (56 FR 59285), which provided a comprehensive assessment of the available scientific and technical information on health and welfare effects associated with NO2 and other oxides of nitrogen.  The CASAC reviewed the draft document at a meeting held on July 1, 1993 and concluded in a closure letter to the Administrator that the document "provides a scientifically balanced and defensible summary of current knowledge of the effects of this pollutant and provides an adequate basis for EPA to make a decision as to the appropriate NAAQS for NO2" (Wolff, 1993).  The Air Quality Criteria for Oxides of Nitrogen was then finalized (US EPA, 1995a).  EPA's OAQPS also prepared a Staff Paper that summarized and integrated the key studies and scientific evidence contained in the revised AQCD for oxides of nitrogen and identified the critical elements to be considered in the review of the NO2 NAAQS.  CASAC reviewed two drafts of the Staff Paper and concluded in a closure letter to the Administrator that the document provided a "scientifically adequate basis for regulatory decisions on nitrogen dioxide" (Wolff, 1995).
	In October 1995 the Administrator announced her proposed decision not to revise either the primary or secondary NAAQS for NO2 (60 FR 52874; October 11, 1995).  A year later, the Administrator made a final determination not to revise the NAAQS for NO2 after careful evaluation of the comments received on the proposal (61 FR 52852; October 8, 1996).  While the primary NO2 standard was revised in January 2010 by supplementing the existing annual standard with the establishment of a new 1-hour standard (75 FR 6474), the secondary NAAQS for NO2 remains 0.053 ppm (100 micrograms per cubic meter [μg/m3] of air), annual arithmetic average, calculated as the arithmetic mean of the 1-hour NO2 concentrations.
2.	NAAQS for Oxides of Sulfur
	EPA promulgated primary and secondary NAAQS for SO2 in April 1971 (36 FR 8186).  The secondary standards included a standard set at 0.02 ppm, annual arithmetic mean, and a 3-hour average standard set at 0.5 ppm, not to be exceeded more than once per year.  These secondary standards were established solely on the basis of evidence of adverse effects on vegetation.  In 1973, revisions made to Chapter 5 ("Effects of Sulfur Oxide in the Atmosphere on Vegetation") of Air Quality Criteria for Sulfur Oxides (US EPA, 1973) indicated that it could not properly be concluded that the vegetation injury reported resulted from the average SO2 exposure over the growing season, rather than from short-term peak concentrations.  Therefore, EPA proposed (38 FR 11355) and then finalized (38 FR 25678) a revocation of the annual mean secondary standard.  At that time, EPA was aware that then-current concentrations of oxides of sulfur in the ambient air had other public welfare effects, including effects on materials, visibility, soils, and water. However, the available data were considered insufficient to establish a quantitative relationship between specific ambient concentrations of oxides of sulfur and such public welfare effects (38 FR 25679).
	In 1979, EPA announced that it was revising the AQCD for oxides of sulfur concurrently with that for particulate matter (PM) and would produce a combined PM and oxides of sulfur criteria document.  Following its review of a draft revised criteria document in August 1980, CASAC concluded that acid deposition was a topic of extreme scientific complexity because of the difficulty in establishing firm quantitative relationships among (1) emissions of relevant pollutants (e.g., SO2 and oxides of nitrogen), (2) formation of acidic wet and dry deposition products, and (3) effects on terrestrial and aquatic ecosystems.  CASAC also noted that acid deposition involves, at a minimum, several different criteria pollutants:  oxides of sulfur, oxides of nitrogen, and the fine particulate fraction of suspended particles.  CASAC felt that any document on this subject should address both wet and dry deposition, since dry deposition was believed to account for a substantial portion of the total acid deposition problem.
	For these reasons, CASAC recommended that a separate, comprehensive document on acid deposition be prepared prior to any consideration of using the NAAQS as a regulatory mechanism for the control of acid deposition.  CASAC also suggested that a discussion of acid deposition be included in the AQCDs for oxides of nitrogen and PM and oxides of sulfur.  Following CASAC closure on the AQCD for oxides of sulfur in December 1981, EPA's OAQPS published a Staff Paper in November 1982, although the paper did not directly assess the issue of acid deposition.  Instead, EPA subsequently prepared the following documents to address acid deposition:  The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Review Papers, Volumes I and II (US EPA, 1984a, b) and The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Document (US EPA, 1985) (53 FR 14935 -14936).  These documents, though they were not considered criteria documents and did not undergo CASAC review, represented the most comprehensive summary of scientific information relevant to acid deposition completed by EPA at that point.
	In April 1988 (53 FR 14926), EPA proposed not to revise the existing primary and secondary standards for SO2.  This proposed decision with regard to the secondary SO2 NAAQS was due to the Administrator's conclusions that (1) based upon the then-current scientific understanding of the acid deposition problem, it would be premature and unwise to prescribe any regulatory control program at that time and (2) when the fundamental scientific uncertainties had been decreased through ongoing research efforts, EPA would draft and support an appropriate set of control measures.  Although EPA revised the primary SO2 standard in June 2010 by establishing a new 1-hour standard and revoking the existing 24-hour and annual standards (75  FR 35520), no further decisions on the secondary SO2 standard have been published.
C.	History of Related Assessments and Agency Actions
	In 1980, the Congress created the National Acid Precipitation Assessment Program (NAPAP) in response to growing concern about acidic deposition.  The NAPAP was given a broad 10-year mandate to examine the causes and effects of acidic deposition and to explore alternative control options to alleviate acidic deposition and its effects.  During the course of the program, the NAPAP issued a series of publicly available interim reports prior to the completion of a final report in 1990 (NAPAP, 1990).
	In spite of the complexities and significant remaining uncertainties associated with the acid deposition problem, it soon became clear that a program to address acid deposition was needed.  The Clean Air Act Amendments of 1990 included numerous separate provisions related to the acid deposition problem.  The primary and most important of the provisions, the amendments to Title IV of the Act, established the Acid Rain Program to reduce emissions of SO2 by 10 million tons and emissions of nitrogen oxides by 2 million tons from 1980 emission levels in order to achieve reductions over broad geographic regions.  In this provision, Congress included a statement of findings that led them to take action, concluding that (1) the presence of acid compounds and their precursors in the atmosphere and in deposition from the atmosphere represents a threat to natural resources, ecosystems, materials, visibility, and public health; (2) the problem of acid deposition is of national and international significance; and (3) current and future generations of Americans will be adversely affected by delaying measures to remedy the problem.
	Second, Congress authorized the continuation of the NAPAP in order to assure that the research and monitoring efforts already undertaken would continue to be coordinated and would provide the basis for an impartial assessment of the effectiveness of the Title IV program.
	Third, Congress considered that further action might be necessary in the long term to address any problems remaining after implementation of the Title IV program and, reserving judgment on the form that action could take, included Section 404 of the 1990 Amendments (Clean Air Act Amendments of 1990, Pub. L. 101-549, § 404) requiring EPA to conduct a study on the feasibility and effectiveness of an acid deposition standard or standards to protect "sensitive and critically sensitive aquatic and terrestrial resources."  At the conclusion of the study, EPA was to submit a report to Congress.  Five years later, EPA submitted its report, entitled Acid Deposition Standard Feasibility Study: Report to Congress (US EPA, 1995b) in fulfillment of this requirement.  That Report concluded that establishing acid deposition standards for sulfur and nitrogen deposition may at some point in the future be technically feasible, although appropriate deposition loads for these acidifying chemicals could not be defined with reasonable certainty at that time.
	Fourth, the 1990 Amendments also added new language to sections of the CAA pertaining to the scope and application of the secondary NAAQS designed to protect the public welfare.  Specifically, the definition of "effects on welfare" in Section 302(h) was expanded to state that the welfare effects include effects "...whether caused by transformation, conversion, or combination with other air pollutants."
	In 1999, seven Northeastern states cited this amended language in Section 302(h) in a petition asking EPA to use its authority under the NAAQS program to promulgate secondary NAAQS for the criteria pollutants associated with the formation of acid rain.  The petition stated that this language "clearly references the transformation of pollutants resulting in the inevitable formation of sulfate and nitrate aerosols and/or their ultimate environmental impacts as wet and dry deposition, clearly signaling Congressional intent that the welfare damage occasioned by sulfur and nitrogen oxides be addressed through the secondary standard provisions of Section 109 of the Act."  The petition further stated that "recent federal studies, including the NAPAP Biennial Report to Congress: An Integrated Assessment, document the continued and increasing damage being inflicted by acid deposition to the lakes and forests of New York, New England and other parts of our nation, demonstrating that the Title IV program had proven insufficient." The petition also listed other adverse welfare effects associated with the transformation of these criteria pollutants, including impaired visibility, eutrophication of coastal estuaries, global warming, and tropospheric ozone and stratospheric ozone depletion.
	In a related matter, the Office of the Secretary of the U.S. Department of Interior (DOI) requested in 2000 that EPA initiate a rulemaking proceeding to enhance the air quality in national parks and wilderness areas in order to protect resources and values that are being adversely affected by air pollution. Included among the effects of concern identified in the request were the acidification of streams, surface waters, and/or soils; eutrophication of coastal waters; visibility impairment; and foliar injury from ozone.
	In a Federal Register notice in 2001, EPA announced receipt of these requests and asked for comment on the issues raised in them.  EPA stated that it would consider any relevant comments and information submitted, along with the information provided by the petitioners and DOI, before making any decision concerning a response to these requests for rulemaking (65 FR 48699).
	The 2005 NAPAP report states that "... scientific studies indicate that the emission reductions achieved by Title IV are not sufficient to allow recovery of acid-sensitive ecosystems.  Estimates from the literature of the scope of additional emission reductions that are necessary in order to protect acid-sensitive ecosystems range from approximately 40-80% beyond full implementation of Title IV...."  The results of the modeling presented in this Report to Congress indicate that broader recovery is not predicted without additional emission reductions" (NAPAP, 2005).
	Given the state of the science as described in the ISA, REA, and in other recent reports, such as the NAPAP reports noted above, EPA has decided, in the context of evaluating the adequacy of the current NO2 and SO2 secondary standards in this review, to revisit the question of the appropriateness of setting secondary NAAQS to address remaining known or anticipated adverse public welfare effects resulting from the acidic and nutrient deposition of these criteria pollutants.
D.	History of the Current Review
	The EPA initiated this current review in December 2005 with a call for information (70 FR 73236) for the development of a revised Integrated Science Assessment for Oxides of Nitrogen and Oxides of Sulfur- Ecological Criteria (henceforth the "ISA").  An Integrated Review Plan (IRP) was developed to provide the framework and schedule as well as the scope of the review and to identify policy-relevant questions to be addressed in the components of the review.  The IRP was released in 2007 (US EPA, 2007) for CASAC and public review.  EPA held a workshop in July 2007 on the ISA to obtain broad input from the relevant scientific communities.  This workshop helped to inform the preparation of the first draft ISA, which was released for CASAC and public review in December 2007; a CASAC meeting was held on April 2-3, 2008 to review the first draft ISA.  A second draft ISA was released for CASAC and public review in August 2008, and was discussed at a CASAC meeting held on October 1-2, 2008.  The final ISA (US EPA, 2008) was released in December 2008.
	Based on the science presented in the ISA, EPA developed a Risk and Exposure Assessment for Review of the Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Oxides of Sulfur (henceforth the "REA") to further assess the national impact of the effects documented in the ISA.  The Draft Scope and Methods Plan for Risk/Exposure Assessment: Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur outlining the scope and design of the future REA was prepared for CASAC and public in March 2008.  A first draft REA was presented to CASAC and the public for review in August 2008 and a second draft was presented for review in June 2009.  The final REA (US EPA 2009) was released in September 2009.  A first draft Policy Assessment was released in March 2010 and reviewed by CASAC on April 1-2, 2010.  In a June 22, 2010 letter to the Administrator, CASAC provided advice and recommendations to the Agency concerning the first draft Policy Assessment (Russell and Samet, 2010a).   A second draft Policy Assessment was released to CASAC and the public in September 2010 and reviewed by CASAC on October 6-7, 2010.  CASAC provided advice and recommendations to the Agency regarding the second draft Policy Assessment in a December 9, 2010 letter (Russell and Samet 2010b).  CASAC and public comments on the second draft Policy Assessment were considered by EPA staff in developing a final Policy Assessment (US EPA, 2011).  CASAC requested an additional meeting provide additional advice to the Administrator based on the final Policy Assessment on February 15 - 16, 2011.  EPA released on January 14, 2011, the final Policy Assessment prior to final document production, to provide sufficient time for CASAC review of the document in advance of this meeting. A final Policy Assessment, incorporating final reference checks and document formatting, was released in February 2011.  In a May 17, 2011 letter (Russell and Samet, 2011a), CASAC offered additional advice to the Administrator with regard to recommendations and revisions to the secondary NAAQS for oxides of nitrogen and oxides of sulfur.
	 On February 3, 2006, the Center for Biological Diversity and four other plaintiffs filed an amended complaint alleging that EPA had failed to complete the current review within the period provided by statute.  The schedule for completion of this review is governed by a consent decree resolving that lawsuit and the subsequent extension granted to the Agency by the plaintiffs. The schedule presented in the original consent decree that governs this review, entered by the court on November 19, 2007, was revised on October 22, 2009 to allow for a 17 month extension of the schedule.  The current decree provides that EPA sign for publication notices of proposed and final rulemaking concerning its review of the oxides of nitrogen and oxides of sulfur NAAQS no later than July 12, 2011 and March 20, 2012, respectively.
	This action presents the Administrator's proposed decisions on the review of the current secondary oxides of nitrogen and oxides of sulfur standards.  Throughout this preamble a number of conclusions, findings, and determinations proposed by the Administrator are noted.  While they identify the reasoning that supports this proposal, they are not intended to be final or conclusive in nature.  The EPA invites general, specific, and/or technical comments on all issues involved with this proposal, including all such proposed judgments, conclusions, findings, and determinations.
E.	Scope of the Current Review
	In conducting this periodic review of the secondary NAAQS for oxides of nitrogen and oxides of sulfur, as discussed in the IRP and REA, EPA decided to assess the scientific information, associated risks, and standards relevant to protecting the public welfare from adverse effects associated jointly with oxides of nitrogen and sulfur.  Although EPA has historically adopted separate secondary standards for oxides of nitrogen and oxides of sulfur, EPA is conducting a joint review of these standards because oxides of nitrogen and sulfur, and their associated transformation products are linked from an atmospheric chemistry perspective, as well as from an environmental effects perspective.  The National Research Council (NRC) has recommended that EPA consider multiple pollutants, as appropriate, in forming the scientific basis for the NAAQS (NRC, 2004).  As discussed in the ISA and REA, there is a strong basis for considering these pollutants together, building upon EPA's past recognition of the interactions of these pollutants and on the growing body of scientific information that is now available related to these interactions and associated ecological effects.
	In defining the scope of this review, it must be considered that EPA has set secondary standards for two other criteria pollutants related to oxides of nitrogen and sulfur:  ozone and particulate matter (PM).  Oxides of nitrogen are precursors to the formation of ozone in the atmosphere, and under certain conditions, can combine with atmospheric ammonia to form ammonium nitrate, a component of fine PM.  Oxides of sulfur are precursors to the formation of particulate sulfate, which is a significant component of fine PM in many parts of the U.S.  There are a number of welfare effects directly associated with ozone and fine PM, including ozone-related damage to vegetation and PM-related visibility impairment.  Protection against those effects is provided by the ozone and fine PM secondary standards.  This review focuses on evaluation of the protection provided by secondary standards for oxides of nitrogen and sulfur for two general types of effects:  (1) direct effects on vegetation associated with exposure to gaseous oxides of nitrogen and sulfur in the ambient air, which are the effects that the current NO2 and SO2 secondary standards protect against and (2) effects associated with the deposition of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecosystems, including deposition in the form of particulate nitrate and particulate sulfate.
	The ISA focuses on the ecological effects associated with deposition of ambient oxides of nitrogen and sulfur to natural sensitive ecosystems, as distinguished from commercially managed forests and agricultural lands.  This focus reflects the fact that the majority of the scientific evidence regarding acidification and nutrient enrichment is based on studies in unmanaged ecosystems.  Non-managed terrestrial ecosystems tend to have a higher fraction of N deposition resulting from atmospheric nitrogen (US EPA, 2008, section <IP_ADDRESS>).  In addition, the ISA notes that agricultural and commercial forest lands are routinely fertilized with amounts of nitrogen that exceed air pollutant inputs even in the most polluted areas (US EPA, 2008, section 3.3.9).  This review recognizes that the effects of nitrogen deposition in managed areas are viewed differently from a public welfare perspective than are the effects of nitrogen deposition in natural, unmanaged ecosystems, largely due to the more homogeneous, controlled nature of species composition and development in managed ecosystems and the potential for benefits of increased productivity in those ecosystems.
	In focusing on natural sensitive ecosystems, the Policy Assessment primarily considers the effects of ambient oxides of nitrogen and sulfur via deposition on multiple ecological receptors.  The ISA highlights effects including those associated with acidification and nitrogen nutrient enrichment.  With a focus on these deposition-related effects, EPA's objective is to develop a framework for oxides of nitrogen and sulfur standards that incorporates ecologically relevant factors and that recognizes the interactions between the two pollutants as they deposit to sensitive ecosystems.  The overarching policy objective is to develop a secondary standard(s) that is based on the ecological criteria described in the ISA and the results of the assessments in the REA, and is consistent with the requirement of the CAA to set secondary standards that are requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of these air pollutants in the ambient air.  Also consistent with the CAA, this policy objective necessarily includes consideration of "variable factors . . . which of themselves or in combination with other factors may alter the effects on public welfare" of the criteria air pollutants included in this review.
	In addition, we have chosen to focus on the effects of ambient oxides of nitrogen and sulfur on ecological impacts on sensitive aquatic ecosystems associated with acidifying deposition of nitrogen and sulfur, which is a transformation product of ambient oxides of nitrogen and sulfur.  Based on the information in the ISA, the assessments presented in the REA, and advice from CASAC on earlier drafts of this Policy Assessment (Russell and Samet, 2010a, 2010b), and as discussed in detail in the Policy Assessment, we have the greatest confidence in the causal linkages between oxides of nitrogen and sulfur and aquatic acidification effects relative to other deposition-related effects, including terrestrial acidification and aquatic and terrestrial nutrient enrichment.

II.	Rationale for Proposed Decision on the Adequacy of the Current Secondary Standards
      Decisions on retaining or revising the current secondary standards for oxides of nitrogen and sulfur are largely public welfare policy judgments based on the Administrator's informed assessment of what constitutes requisite protection against adverse effects to public welfare.  A public welfare policy decision should draw upon scientific information and analyses about welfare effects, exposure and risks, as well as judgments about the appropriate response to the range of uncertainties that are inherent in the scientific evidence and analyses. The ultimate determination as to what level of damage to ecosystems and the services provided by those ecosystems is adverse to public welfare is not wholly a scientific question, although it is informed by scientific studies linking ecosystem damage to losses in ecosystem services, and information on the value of those losses of ecosystem services.  In reaching such decisions, the Administrator seeks to establish standards that are neither more nor less stringent than necessary for this purpose.
      This section presents the rationale for the Administrator's proposed conclusions with regard to the adequacy of protection and ecological relevance of the current secondary standards for oxides of nitrogen and sulfur. As discussed more fully below, this rationale considered the latest scientific information on ecological effects associated with the presence of oxides of nitrogen and oxides of sulfur in the ambient air.  This rationale also takes into account:  (1) staff assessments of the most policy-relevant information in the ISA and staff analyses of air quality, exposure, and ecological risks, presented more fully in the REA and in the Policy Assessment, upon which staff conclusions on revisions to the secondary oxides of nitrogen and oxides of sulfur standards are based; (2) CASAC advice and recommendations, as reflected in discussions of drafts of the ISA, REA, and Policy Assessment at public meetings, in separate written comments, and in CASAC's letters to the Administrator; and (3) public comments received during the development of these documents, either in connection with CASAC meetings or separately.
      In developing this rationale, EPA has drawn upon an integrative synthesis of the entire body of evidence, published through early 2008, on ecological effects associated with the deposition of oxides of nitrogen and oxides of sulfur in the ambient air (US EPA, 2008).  As discussed below in section II.A, this body of evidence addresses a broad range of ecological endpoints associated with ambient levels of oxides of nitrogen and oxides of sulfur. In considering this evidence, EPA focuses on those ecological endpoints, such as aquatic acidification, which the ISA judges associations with oxides of nitrogen and oxides of sulfur to be causal, likely causal, or for which the evidence is highly suggestive that oxides of nitrogen and/or sulfur contribute to the reported effects.
      Crucial to this review is the development of a form for an ecologically relevant standard that reflects both the geographically variable and deposition-dependent nature of the effects.   The atmospheric levels of oxides of nitrogen and sulfur that afford a particular level of ecosystem protection are those levels that result in an amount of deposition that is less than the amount of deposition that a given ecosystem can accept without defined levels of degradation of the ecological indicator for a targeted effect.
      Drawing from the framework developed in the REA, the framework we used to structure an ecologically meaningful secondary standard in the Policy Assessment and to further develop the indicator, form, level, and averaging time of such as standard in section III of this proposal is depicted below and highlights the three key linkages that need to be considered in developing an ecologically relevant standard.

Figure II-1. Simplified conceptual design of the form of an aquatic acidification standard for oxides of nitrogen and sulfur.

      The following discussion relies heavily on Chapters 2 and 3 of the Policy Assessment.  The Policy Assessment includes staff's evaluation of the policy implications of the scientific assessment of the evidence presented and assessed in the ISA and the results of quantitative assessments based on that information presented and assessed in the REA. Taken together, this information informs staff conclusions and the development of policy options in the Policy Assessment for consideration in addressing public and welfare effects associated with the presence of oxides of nitrogen and oxides of sulfur in the ambient air.  Of particular note, chapter 2 of the Policy Assessment presents information not repeated here that characterizes emissions, air quality, deposition and water quality. It includes discussions of the sources of nitrogen and sulfur in the atmosphere as well as current ambient air quality monitoring networks and models. Additional information in this section includes ecological modeling and water quality data sources.
	Section II.A presents a discussion of the effects associated with oxides of nitrogen and sulfur in the ambient air.  The discussion is organized around the types of effects being considered, including direct effects of gaseous oxides of nitrogen and sulfur, deposition-related effects related to acidification and nutrient enrichment, and other effects such as materials damage, climate-related effects and mercury methylation.
	Section II.B presents a summary and discussion of the risk and exposure assessment performed for each of the four major effects categories.  The REA uses case studies representing the broad geographic variability of the impacts from oxides of nitrogen and sulfur to conclude that there are ongoing adverse effects in many ecosystems from deposition of oxides of nitrogen and sulfur and that under current emissions scenarios these effects are likely to continue.
	Section II.C presents a discussion of adversity related to linking ecological effects to measures that can be used to characterize the extent to which such effects are reasonably considered to be adverse to public welfare.  This involves consideration of how to characterize adversity from a public welfare perspective.  In so doing, consideration is given to the concept of ecosystem services, the evidence of effects on ecosystem services, and how ecosystem services can be linked to ecological indicators.
	Section II.D presents an assessment of the adequacy of the current oxides of nitrogen and oxides of sulfur secondary standards.  Consideration is given both to the adequacy of protection afforded by the current standards for both direct and deposition-related effects, as well as to the appropriateness of the fundamental structure and the basic elements of the current standards for providing protection from deposition-related effects.  Considerations as to the extent to which deposition-related effects that could reasonably be judged to be adverse to public welfare are occurring under current conditions which are allowed by the current standards is also considered.  Discussion of the structures and basic elements of the current NO2 and SO2 secondary standards and the degree to which they are inadequate to protect against such effects is presented.
A. Ecological Effects
		This section discusses the known or anticipated ecological effects associated with oxides of nitrogen and sulfur, including the direct effects of gas-phase exposure to oxides of nitrogen and sulfur (section II.A.1) and effects associated with deposition-related exposure (sections II.A.2 and 3).  Section II.A. 2 addresses effects related to acidification of aquatic and terrestrial ecosystems and section II A.3 addresses effects related to nutrient enrichment of aquatic and terrestrial ecosystems.  These sections also address questions about the nature and magnitude of ecosystem responses to reactive nitrogen and sulfur deposition, including responses related to acidification, nutrient depletion, and, in Section II.A 4 the mobilization of toxic metals in sensitive aquatic and terrestrial ecosystems.  The uncertainties and limitations associated with the evidence of such effects are also discussed throughout this section.
1. Effects Associated with Gas-Phase Oxides of Nitrogen and Sulfur
	 Ecological effects on vegetation as discussed in earlier reviews as well as the ISA, can be attributed to gas-phase oxides of nitrogen and sulfur.  Acute and chronic exposures to gaseous pollutants such as sulfur dioxide (SO2), nitrogen dioxide (NO2), nitric oxide (NO), nitric acid (HNO3) and peroxyacetyl nitrite (PAN) are associated with negative impacts to vegetation. The current secondary NAAQS were set to protect against direct damage to vegetation by exposure to gas-phase oxides of nitrogen and sulfur, such as foliar injury, decreased photosynthesis, and decreased growth.  The following summary is a concise overview of the known or anticipated effects to vegetation caused by gas phase nitrogen and sulfur.  Most phototoxic effects associated with gas phase oxides of nitrogen and sulfur occur at levels well above ambient concentrations observed in the U.S. (US EPA, 2008, section <IP_ADDRESS>).
a. Nature of ecosystem responses to gas-phase nitrogen and sulfur
	The 2008 ISA found that gas phase N and S are associated with direct phytotoxic effects (US EPA, 2008, section 4.4).  The evidence is sufficient to infer a causal relationship between exposure to SO2 and injury to vegetation (US EPA, 2008, section 4.4.1 and <IP_ADDRESS>). Acute foliar injury to vegetation from SO2 may occur at levels above the current secondary standard (3-h average of 0.50 ppm). Effects on growth, reduced photosynthesis and decreased yield of vegetation are also associated with increased SO2 exposure concentration and time of exposure.
	The evidence is sufficient to infer a causal relationship between exposure to NO, NO2 and PAN and injury to vegetation (US EPA, 2008, section 4.4.2 and <IP_ADDRESS>).  At sufficient concentrations, NO, NO2 and PAN can decrease photosynthesis and induce visible foliar injury to plants.  Evidence is also sufficient to infer a causal relationship between exposure to HNO3 and changes to vegetation (US EPA, 2008, section 4.4.3 and <IP_ADDRESS>).  Phytotoxic effects of this pollutant include damage to the leaf cuticle in vascular plants and disappearance of some sensitive lichen species.
b. Magnitude of ecosystem response to gas-phase nitrogen and sulfur
	Vegetation in ecosystems near sources of gaseous oxides of nitrogen and sulfur or where SO2, NO, NO2, PAN and HNO3 are most concentrated are more likely to be impacted by these pollutants. Uptake of these pollutants in a plant canopy is a complex process involving adsorption to surfaces (leaves, stems and soil) and absorption into leaves (US EPA, 2008, section 3.4.2).  The functional relationship between ambient concentrations of gas phase oxides of nitrogen and sulfur and specific plant response are impacted by internal factors such as rate of stomatal conductance and plant detoxification mechanisms, and external factors including plant water status, light, temperature, humidity, and pollutant exposure regime (US EPA, 2008, section 3.4.2).
	Entry of gases into a leaf is dependent upon physical and chemical processes of gas phase as well as to stomatal aperture.  The aperture of the stomata is controlled largely by the prevailing environmental conditions, such as water availability, humidity, temperature, and light intensity.  When the stomata are closed, resistance to gas uptake is high and the plant has a very low degree of susceptibility to injury. Mosses and lichens do not have a protective cuticle barrier to gaseous pollutants or stomata and are generally more sensitive to gaseous sulfur and nitrogen than vascular plants (US EPA, 2008, section 3.4.2).
	The appearance of foliar injury can vary significantly across species and growth conditions affecting stomatal conductance in vascular plants (US EPA, 2009, section 6.4.1). For example, damage to lichens from SO2 exposure includes decreased photosynthesis and respiration, damage to the algal component of the lichen, leakage of electrolytes, inhibition of nitrogen fixation, decreased K+ absorption, and structural changes.
	The phytotoxic effects of gas phase oxides of nitrogen and sulfur are dependent on the exposure concentration and duration and species sensitivity to these pollutants.  Effects to vegetation associated with oxides of nitrogen and sulfur are therefore variable across the U.S. and tend to be higher near sources of photochemical smog.  For example, SO2 is considered to be the primary factor contributing to the death of lichens in many urban and industrial areas.
	The ISA states there is very limited new research on phytotoxic effects of NO, NO2, PAN and HNO3 at concentrations currently observed in the United States with the exception of some lichen species (US EPA, 2008, section 4.4).  Past and current HNO3 concentrations may be contributing to the decline in lichen species in the Los Angeles basin.  Most phototoxic effects associated with gas phase oxides of nitrogen and sulfur occur at levels well above ambient concentrations observed in the U.S. (US EPA, 2008, section <IP_ADDRESS>).
2. Acidification Effects Associated with Deposition of Oxides of Nitrogen and Sulfur
	Sulfur oxides and nitrogen oxides in the atmosphere undergo a complex mix of reactions in gaseous, liquid, and solid phases to form various acidic compounds. These acidic compounds are removed from the atmosphere through deposition: either wet (e.g., rain, snow), fog or cloud, or dry (e.g., gases, particles). Deposition of these acidic compounds to ecosystems can lead to effects on ecosystem structure and function. Following deposition, these compounds can, in some instances unless retained by soil or biota, leach out of the soils in the form of sulfate (SO4[2-]) and nitrate (NO3[-]), leading to the acidification of surface waters. The effects on ecosystems depend on the magnitude and rate of deposition, as well as a host of biogeochemical processes occurring in the soils and waterbodies (US EPA, 2009, section 2.1). The chemical forms of nitrogen that may contribute to acidifying deposition include both oxidized and reduced chemical species, including NHx.
	When sulfur or nitrogen leaches from soils to surface waters in the form of SO4[2-] or NO3[-], an equivalent amount of positive cations, or countercharge, is also transported. This maintains electroneutrality. If the countercharge is provided by base cations, such as calcium (Ca[2+]), magnesium (Mg[2+]), sodium (Na[+]), or potassium (K[+]), rather than hydrogen (H[+]) and dissolved inorganic aluminum, the acidity of the soil water is neutralized, but the base saturation of the soil decreases. Continued SO4[2-] or NO3[-] leaching can deplete the available base cation pool in soil. As the base cations are removed, continued deposition and leaching of SO4[2-] and/or NO3[-] (with H[+] and Al[3+]) leads to acidification of soil water, and by connection, surface water. Introduction of strong acid anions such as sulfate and nitrate to an already acidic soil, whether naturally or due to anthropogenic activities, can lead to instantaneous acidification of waterbodies through direct runoff without any significant change in base cation saturation. The ability of a watershed to neutralize acidic deposition is determined by a variety of biogeophysical factors including weathering rates, bedrock composition, vegetation and microbial processes, physical and chemical characteristics of soils and hydrologic flowpaths. (US EPA, 2009, section 2.1)  Some of these factors such as vegetation and soil depth are highly variable over small spatial scales such as meters, but can be aggregated to evaluate patterns over larger spatial scales.  Acidifying deposition of oxides of nitrogen and sulfur and the chemical and biological responses associated with these inputs vary temporally.  Chronic or long-term deposition processes in the time scale of years to decades result in increases in inputs of N and S to ecosystems and the associated ecological effects. Episodic or short term (i.e., hours or days) deposition refers to events in which the level of the acid neutralizing capacity (ANC) of a lake or stream is temporarily lowered.  In aquatic ecosystems, short-term (i.e., hours or days) episodic changes in water chemistry can have significant biological effects.  Episodic acidification refers to conditions during precipitation or snowmelt events when proportionately more drainage water is routed through upper soil horizons that tend to provide less acid neutralizing than was passing through deeper soil horizons (US EPA, 2009, section 4.2).  In addition, the accumulated sulfate and nitrate in snow packs can provide a surge of acidic inputs.  Some streams and lakes may have chronic or base flow chemistry that is suitable for aquatic biota, but may be subject to occasional acidic episodes with deleterious consequences to sensitive biota.
	The following summary is a concise overview of the known or anticipated effects caused by acidification to ecosystems within the United States.  Acidification affects both terrestrial and freshwater aquatic ecosystems.
a. Nature of acidification-related ecosystem responses
	The ISA concluded that deposition of oxides of nitrogen and sulfur and NHx leads to the varying degrees of acidification of ecosystems (US EPA, 2008).  In the process of acidification, biogeochemical components of terrestrial and freshwater aquatic ecosystems are altered in a way that leads to effects on biological organisms.  Deposition to terrestrial ecosystems often moves through the soil and eventually leaches into adjacent water bodies.
i. Aquatic ecosystems
	The scientific evidence is sufficient to infer a causal relationship between acidifying deposition and effects on biogeochemistry and biota in aquatic ecosystems (US EPA, 2008, section 4.2.2). The strongest evidence comes from studies of surface water chemistry in which acidic deposition is observed to alter sulfate and nitrate concentrations in surface waters, the sum of base cations, ANC, dissolved inorganic aluminum and pH (US EPA, 2008, section <IP_ADDRESS>).  ANC is a key indicator of acidification with relevance to both terrestrial and aquatic ecosystems. ANC is useful because it integrates the overall acid-base status of a lake or stream and reflects how aquatic ecosystems respond to acidic deposition over time. There is also a relationship between ANC and the surface water constituents that directly contribute to or ameliorate acidity-related stress, in particular, concentrations of hydrogen ion (as pH), Ca[2+], and aluminum. Moreover, low pH surface waters leach aluminum from soils, which is quite lethal to fish and other aquatic organisms. In aquatic systems, there is a direct relationship between ANC and fish and phyto-zooplankton diversity and abundance.
	Low ANC coincides with effects on aquatic systems (e.g., individual species fitness loss or death, reduced species richness, altered community structure). At the community level, species richness is positively correlated with pH and ANC because energy cost in maintaining physiological homeostasis, growth, and reproduction is high at low ANC levels. For example, there is a logistic relationship between fish species richness and ANC class for Adirondack Case Study Area lakes that indicates the probability of occurrence of an organism for a given value of ANC.  Biota are generally not harmed when ANC values are >100 microequivalents per liter (μeq/L). The number of fish species also peaks at ANC values >100 μeq/L. Below 100 μeq/L, ANC fish fitness and community diversity begin to decline (US EPA, section 4.2). At ANC levels between 100 and 50 μeq/L, the fitness of sensitive species (e.g., brook trout, zooplankton) also begins to decline. When ANC concentrations are <50 μeq/L, they are generally associated with death or loss of fitness of biota that are sensitive to acidification.
      Consistent and coherent documentation from multiple studies on various species from all major trophic levels of aquatic systems shows that geochemical alteration caused by acidification can result in the loss of acid-sensitive biological species (US EPA, 2008, section <IP_ADDRESS>).  This is most often discussed with relation to pH.  For example, in the Adirondacks, of the 53 fish species recorded in Adirondack lakes about half (26 species) were absent from lakes with pH below 6.0.  Biological effects are linked to changes in water chemistry including decreases in ANC and pH and increases in inorganic Al concentration.  The direct biological effects are caused by lowered pH which leads to in increased inorganic Al concentrations (US EPA, 2011, Figures 3-1 and 3-2). While ANC level does not cause direct biological harm it is a good overall indicator of the risk of acidification (US EPA, 2011, section 3.1.3).
	There are clear associations between ANC, pH, and aquatic species mortality and health which are summarized in section 3.1.1 of the Policy Assessment.  Significant harm to sensitive aquatic species has been observed at pH levels below 6.  Normal stream pH levels with little to no toxicity ranges from 6 to 7 (MacAvoy et al, 1995).  Baker et al (1990) observed that "lakes with pH less than approximately 6.0 contain significantly fewer species than lakes with pH levels above 6.0".  As noted in Chapter 3, typically at pH <4.5 and an ANC <0 μeq/L, complete to near-complete loss of many taxa of organisms occur, including fish and aquatic insect populations, whereas other taxa are reduced to only acidophilic species.
	Additional evidence can help refine the understanding of effects occurring at pH levels between 4.5 and 6.  When pH levels are below 5.6, relatively lower trout survival rates were observed in the Shenandoah National Park.  In field observations, when pH levels dropped to 5, mortality rates went to 100 percent. (Bulger et al, 2000).  At pH levels ranging from 5.4 to 5.8, cumulative mortality continues to increase.  Several studies have shown that trout exposed to water with varying pH levels and fish larvae showed increasing mortality as pH levels decrease.  In one study almost 100 percent mortality was observed at a pH of 4.5 compared to almost 100 percent survival at a pH of 6.5.  Intermediate pH values (6.0, 5.5) in all cases showed reduced survival compared with the control (6.5), but not by statistically significant amounts (US EPA, 2008, section <IP_ADDRESS>).
	One important indicator of acid stress is increased fish mortality.   The response of fish to pH is not uniform across species. A number of synoptic surveys indicated loss of species diversity and absence of several fish species in the pH range of 5.0 to 5.5.  If pH is lower, there is a greater likelihood that more fish species could be lost without replacement, resulting in decreased richness and diversity. In general, populations of salmonids are not found at pH levels less than 5.0, and smallmouth bass (Micropterus dolomieu) populations are usually not found at pH values less than about 5.2 to 5.5.  From Table 3-1, only one study showed significant mortality effects above a pH of 6, while a number of studies showed significant mortality when pH levels are at or below 5.5.
	The highest pH level for any of the studies reported in the ISA, is 6.0, suggesting that pH above 6.0 is protective against mortality effects for most species.  Most thresholds are in the range of pH of 5.0 to 6.0, which suggests that a target pH should be no lower than 5.0.  Protection against mortality in some recreationally important species such as lake trout (pH threshold of 5.6) and crappie (pH threshold of 5.5), combined with the evidence of effects on larval and embryo survival suggests that pH levels greater than 5.5 should be targeted to provide protection against mortality effects throughout the life stages of fish.
	Non-lethal effects have been observed at pH levels as high as 6.  A study in the Shenandoah National Park found that the condition factor, a measure of fish health expressed as fish weight/length3 multiplied by a scaling constant, is positively correlated with stream pH levels, and that the condition factor is reduced in streams with a pH of 6.0 (US EPA, 2008, section <IP_ADDRESS>).
	Biodiversity is another indicator of aquatic ecosystem health.  A key study in the Adirondacks found that lakes with a pH of 6.0 had only half the potential species of fish (27 of 53 potential species). There is often a positive relationship between pH and number of fish species, at least for pH values between about 5.0 and 6.5, or ANC values between about 0 to 100 ueq/L. Such observed relationships are complicated, however, by the tendency for smaller lakes and streams, having smaller watersheds, to also support fewer fish species, irrespective of acid-base chemistry. This pattern may be due to a decrease in the number of available niches as stream or lake size decreases. Nevertheless, fish species richness is relatively easily determined and is one of the most useful indicators of biological effects of surface water acidification.
	Changes in stream water pH and ANC also contribute to declines in taxonomic richness of zooplankton, and macroinvertebrates which are often sources of food for fish, birds and other animal species in various ecosystems.  These fish may also serve as a source of food and recreation for humans. Acidification of ecosystems has been shown to disrupt food web dynamics causing alteration to the diet, breeding distribution and reproduction of certain species of birds (US EPA, 2008, section <IP_ADDRESS>. and Table 3-9).  For example, breeding distribution of the common goldeneye (Bucephala clangula) an insectivorous duck may be affected by changes in acidifying deposition.  Similarly, decreases in prey diversity and quantity have been observed to create feeding problems for nesting pairs of loons on low-pH lakes in the Adirondacks.
ii. Terrestrial ecosystems
	In terrestrial ecosystems, the evidence is sufficient to infer a causal relationship between acidifying deposition and changes in biogeochemistry (US EPA, 2008, section <IP_ADDRESS>).  The strongest evidence comes from studies of forested ecosystems, with supportive information on other plant taxa, including shrubs and lichens (US EPA, 2008, section <IP_ADDRESS>.).  Three useful indicators of chemical changes and acidification effects on terrestrial ecosystems, showing consistency and coherence among multiple studies are: soil base saturation, Al concentrations in soil water and soil C:N ratio (US EPA, 2008, section <IP_ADDRESS>).
	As discussed in the ISA and REA, in soils with base saturation less than about 15 to 20%, exchange chemistry is dominated by Al.  Under these conditions, responses to inputs of sulfuric acid and HNO3 largely involve the release and mobilization of dissolved inorganic Al.  The effect can be neutralized by weathering from geologic parent material or base cation exchange. The Ca[2+] and Al concentrations in soil water are strongly influenced by soil acidification and both have been shown to have quantitative links to tree health, including Al interference with Ca[2+] uptake and Al toxicity to roots.  Effects of nitrification and associated acidification and cation leaching have been consistently shown to occur only in soils with a C:N ratio below about 20 to 25.
	Soil acidification caused by acidic deposition has been shown to cause decreased growth and increased susceptibility to disease and injury in sensitive tree species.  Red spruce (Picea rubens) dieback or decline has been observed across high elevation areas in the Adirondack, Green and White mountains.  The frequency of freezing injury to red spruce needles has increased over the past 40 years, a period that coincided with increased emissions of S and N oxides and increased acidifying deposition.  Acidifying deposition can contribute to dieback in sugar maple (Acer saccharum) through depletion of cations from soil with low levels of available Ca. Grasslands are likely less sensitive to acidification than forests due to grassland soils being generally rich in base cations.
iii.  Ecosystem sensitivity
	The intersection between current deposition loading, historic loading, and sensitivity defines the ecological vulnerability to the effects of acidification. Freshwater aquatic and some terrestrial ecosystems, notably forests, are the ecosystem types which are most sensitive to acidification.  The ISA reports that the principal factor governing the sensitivity of terrestrial and aquatic ecosystems to acidification from sulfur and nitrogen deposition is geology (particularly surficial geology). Geologic formations having low base cation supply generally underlie the watersheds of acid-sensitive lakes and streams. Other factors that contribute to the sensitivity of soils and surface waters to acidifying deposition include topography, soil chemistry, land use, and hydrologic flowpaths. Episodic and chronic acidification tends to occur in areas that have base-poor bedrock, high relief, and shallow soils (US EPA, 2008, section <IP_ADDRESS>).
a. Magnitude of acidification-related ecosystem responses
	Terrestrial and aquatic ecosystems differ in their response to acidifying deposition.  Therefore the magnitude of ecosystem response is described separately for aquatic and terrestrial ecosystems in the following sections.  The magnitude of response refers to both the severity of effects and the spatial extent of the U.S. which is affected.
i. Aquatic acidification
	Freshwater ecosystem surveys and monitoring in the eastern United States have been conducted by many programs since the mid-1980s, including EPA's Environmental Monitoring and Assessment Program (EMAP), National Surface Water Survey (NSWS), Temporally Integrated Monitoring of Ecosystems (TIME), and Long-term Monitoring (LTM) programs. Based on analyses of surface water data from these programs, New England, the Adirondack Mountains, the Appalachian Mountains (northern Appalachian Plateau and Ridge/Blue Ridge region), and the Upper Midwest contain the most sensitive lakes and streams (i.e., ANC less than about 50 μeq/L). Portions of northern Florida also contain many acidic and low-ANC lakes and streams, although the role of acidifying deposition in this region is less clear. The western U.S. contains many of the surface waters most sensitive to potential acidification effects, but with the exception of the Los Angeles Basin and surrounding areas, the levels of acidifying deposition are low in most areas.  Therefore, acidification of surface waters by acidic deposition is not as prevalent in the western U.S., and the extent of chronic surface water acidification that has occurred in that region to date has likely been very limited relative to the Eastern U.S. (US EPA, 2008, section <IP_ADDRESS> and US EPA, 2009, section 4.2.2).
	There are a number of species including fish, aquatic insects, other invertebrates and algae that are sensitive to acidification and cannot survive, compete, or reproduce in acidic waters (US EPA, 2008, section <IP_ADDRESS>). Decreases in ANC and pH have been shown to contribute to declines in species richness and declines in abundance of zooplankton, macroinvertebrates, and fish. Reduced growth rates have been attributed to acid stress in a number of fish species including Atlantic salmon (Salmo salar), Chinook salmon (Oncorhynchus tshawytscha), lake trout (Salvelinus namaycush), rainbow trout (Oncorhynchis mykiss), brook trout (Salvelinus Fontinalis), and brown trout (Salmo trutta).  In response to small to moderate changes in acidity, acid-sensitive species are often replaced by other more acid-tolerant species, resulting in changes in community composition and richness. The effects of acidification are continuous, with more species being affected at higher degrees of acidification.   At a point, typically a pH <4.5 and an ANC <0 μeq/L, complete to near-complete loss of many taxa of organisms occur, including fish and aquatic insect populations, whereas other taxa are reduced to only acidophilic species. These changes in taxa composition are associated with the high energy cost in maintaining physiological homeostasis, growth, and reproduction at low ANC levels (US EPA, 2008, section <IP_ADDRESS>). Decreases in species richness related to acidification have been observed in the Adirondack Mountains and Catskill Mountains of New York, New England and Pennsylvania, and Virginia. From the sensitive areas identified by the ISA, further "case study" analyses on aquatic ecosystems in the Adirondack Mountains and Shenandoah National Park were conducted to better characterize ecological risk associated with acidification (US EPA, 2009, section 4).
 	ANC is the most widely used indicator of acid sensitivity and has been found in various studies to be the best single indicator of the biological response and health of aquatic communities in acid-sensitive systems (Lien et al., 1992; Sullivan et al., 2006; US EPA, 2008). In the REA, surface water trends in SO4[2-] and NO3[-] concentrations and ANC levels were analyzed to affirm the understanding that reductions in deposition could influence the risk of acidification. ANC values have been categorized according to their effects on biota, as shown in the table below. Monitoring data from TIME/LTM and EMAP programs were assessed for the years 1990 to 2006, and past, present, and future water quality levels were estimated by both steady-state and dynamic biogeochemical models.
Table II-1.  Ecological effects associated with alternative levels of acid neutralizing capacity (ANC). (source: USEPA, Acid Rain Program)
Category Label ANC Levels and Expected Ecological Effects
Acute Concern
<0 μeq/L
Complete loss of fish populations is expected. Planktonic communities have extremely low diversity and are dominated by acidophilic taxa. The numbers of individuals in plankton species that are present are greatly reduced.
Severe
Concern
0 - 20 μeq/L
Highly sensitive to episodic acidification. During episodes of high acidifying deposition, brook trout populations may experience lethal effects. The diversity and distribution of zooplankton communities decline sharply.
Elevated Concern
20 - 50 μeq/L
Fish species richness is greatly reduced (i.e., more than half of expected species can be missing). On average, brook trout populations experience sublethal effects, including loss of health, ability to reproduce, and fitness. Diversity and distribution of zooplankton communities decline.
Moderate
Concern
50 - 100 μeq/L
Fish species richness begins to decline (i.e., sensitive species are lost from lakes). Brook trout populations are sensitive and variable, with possible sublethal effects. Diversity and distribution of zooplankton communities also begin to decline as species that are sensitive to acidifying deposition are affected.
Low Concern
>100 μeq/L
Fish species richness may be unaffected. Reproducing brook trout populations are expected where habitat is suitable. Zooplankton communities are unaffected and exhibit expected diversity and distribution.

	Studies on fish species richness in the Adirondacks Case Study Area demonstrated the effect of acidification. Of the 53 fish species recorded in Adirondack Case Study Area lakes, only 27 species were found in lakes with a pH <6.0. The 26 species missing from lakes with a pH <6.0 include important recreational species, such as Atlantic salmon, tiger trout (Salmo trutta X Salvelinus fontinalis), redbreast sunfish (Lepomis auritus), bluegill (Lepomis macrochirus), tiger musky (Esox masquinongy X lucius), walleye (Sander vitreus), alewife (Alosa pseudoharengus), and kokanee (Oncorhynchus nerka), as well as ecologically important minnows that are commonly consumed by sport fish. A survey of 1,469 lakes in the late 1980s found 346 lakes to be devoid of fish. Among lakes with fish, there was a relationship between the number of fish species and lake pH, ranging from about one species per lake for lakes having a pH <4.5 to about six species per lake for lakes having a pH >6.5.  In the Adirondacks, a positive relationship exists between the pH and ANC in lakes and the number of fish species present in those lakes (US EPA, 2008, section <IP_ADDRESS>).
	Since the mid-1990s, streams in the Shenandoah Case Study Area have shown slight declines in NO3[-] and SO4 [2-] concentrations in surface waters. The 2006 concentrations are still above pre-acidification (1860) conditions. MAGIC modeling predicts surface water concentrations of NO3[-] and SO4[2-] are10- and 32-fold higher, respectively, in 2006 than in 1860. The estimated average ANC across 60 streams in the Shenandoah Case Study Area is 57.9 μeq/L (+- 4.5 μeq/L). 55% of all monitored streams in the Shenandoah Case Study Area have a current risk of Elevated, Severe, or Acute.  Of the 55%, 18% are chronically acidic today (US EPA, 2009, section <IP_ADDRESS>).
	Based on a deposition scenario for this study area that maintains current emission levels from 2020 to 2050, the simulation forecast indicates that a large number of streams still have Elevated to Acute problems with acidity. In fact, from 2006 to 2050, the percentage of streams with Acute Concern is predicted to increase by 5%, while the percentage of streams in Moderate Concern decreases by 5%.
	Biological effects of increased acidification documented in the Shenandoah Case Study Area include a decrease in the condition factor in blacknose dace and a decrease in fish biodiversity associated with decreasing stream ANC.  On average, the fish species richness is lower by one fish species for every 21 μeq/L decrease in ANC in Shenandoah National Park streams (US EPA, 2008, section <IP_ADDRESS>).
ii.  Terrestrial acidification
	The ISA identified a variety of indicators that can be used to measure the effects of acidification in soils.  Most effects of terrestrial acidification are observed in sensitive forest ecosystem in the U.S. Tree health has been linked to the availability of base cations (Bc) in soil (such as Ca[2+], Mg[2+] and potassium), as well as soil Al content. Tree species show a range of sensitivities to Ca/Al and Bc/Al soil molar ratios, therefore these are good chemical indicators because they directly relate to the biological effects. Critical Bc/Al molar ratios for a large variety of tree species ranged from 0.2 to 0.8. This range is similar to critical ratios of  Ca/Al. Plant toxicity or nutrient antagonism was reported to occur at Ca/Al molar ratios ranging from 0.2 to 2.5  (US EPA, 2009).
	There has been no systematic national survey of terrestrial ecosystems to determine the extent and distribution of terrestrial ecosystem sensitivity to the effects of acidifying deposition. However, one preliminary national evaluation estimated that ~15% of forest ecosystems in the U.S. exceed the estimated critical load based on soil ANC leaching for S and N deposition by >250 eq ha-1 yr-1 (McNulty et al., 2007). Forests of the Adirondack Mountains of New York, Green Mountains of Vermont, White Mountains of New Hampshire, the Allegheny Plateau of Pennsylvania, and high-elevation forest ecosystems in the southern Appalachians are the regions most sensitive to terrestrial acidification effects from acidifying deposition (US EPA, 2008, section <IP_ADDRESS>). While studies show some recovery of surface waters, there are widespread measurements of ongoing depletion of exchangeable base cations in forest soils in the northeastern U.S. despite recent decreases in acidifying deposition, indicating a slow recovery time.
	In the REA, a critical load analysis was performed for sugar maple and red spruce forests in the eastern United States by using Bc/Al ratio in acidified forest soils as an indicator to assess the impact of nitrogen and sulfur deposition on tree health. These are the two most commonly studied tree species in North America for effects of acidification. At a Bc/Al ratio of 1.2, red spruce growth can be decreased by 20%. Sugar maple growth can be decreased by 20% at a Bc/Al ratio of 0.6 (US EPA, 2009, section 4.4). The REA analysis determined the health of at least a portion of the sugar maple and red spruce growing in the United States may have been compromised with acidifying total nitrogen and sulfur deposition. Specifically, total nitrogen and sulfur deposition levels exceeded three selected critical loads for tree growth in 3% to 75% of all sugar maple plots across 24 states. For red spruce, total nitrogen and sulfur deposition levels exceeded three selected critical loads in 3% to 36% of all red spruce plots across eight states (US EPA, 2009, section 4.4).
a. Key uncertainties associated with acidification
	There are different levels of uncertainty associated with relationships between deposition, ecological effects and ecological indicators.  In Chapter 7 of the REA, the case study analyses associated with each targeted effect area were synthesized by identifying the strengths, limitations, and uncertainties associated with the available data, modeling approach, and relationship between the selected ecological indicator and atmospheric deposition as described by the ecological effect function (US EPA, 2009, Figure  1-1). A further discussion of uncertainty in aquatic and terrestrial ecosystems is presented below. The key uncertainties were characterized as follows to evaluate the strength of the scientific basis for setting a national standard to protect against a given effect (US EPA, 2009, section 7):
(1) Data Availability: high, medium or low quality. This criterion is based on the availability and robustness of data sets, monitoring networks, availability of data that allows for extrapolation to larger assessment areas, and input parameters for modeling and developing the ecological effect function. The scientific basis for the ecological indicator selected is also incorporated into this criterion.
(2) Modeling Approach: high, fairly high, intermediate, or low confidence. This value is based on the strengths and limitations of the models used in the analysis and how accepted they are by the scientific community for their application in this analysis.
(3) Ecological Effect Function: high, fairly high, intermediate, or low confidence. This ranking is based on how well the ecological effect function describes the relationship between atmospheric deposition and the ecological indicator of an effect.
i. Aquatic acidification
	The REA concludes that the available data are robust and considered high quality.  There is high confidence about the use of these data and their value for extrapolating to a larger regional population of lakes.  The EPA TIME/LTM network represents a source of long-term, representative sampling.  Data on sulfate concentrations, nitrate concentrations and ANC from 1990 to 2006 used for this analysis as well as EPA EMAP and REMAP surveys, provide considerable data on surface water trends.
	There is fairly high confidence associated with modeling and input parameters. Uncertainties in water quality estimates (i.e., ANC) from MAGIC were derived from multiple site calibrations.  The 95% confidence interval for pre-acidification of lakes was an average of 15 ueq/L difference in ANC concentrations or 10% and 8 ueq/L or 5% for streams (US EPA, 2009, section 7.1.2). The use of the critical load model used to estimate aquatic critical loads is limited by the uncertainties associated with runoff and surface water measurements and in estimating the catchment supply of base cations from the weathering of bedrock and soils (McNulty et al., 2007).  To propagate uncertainty in the model parameters, Monte Carlo methods were employed to develop an inverse function of exceedances.  There is high confidence associated with the ecological effect function developed for aquatic acidification.  In calculating the ANC function, the depositional load for N or S is fixed by the deposition of the other, so deposition for either will never be zero (US EPA, 2009, Figure 7.1-6).
ii.	Terrestrial acidification
	The available data used to quantify the targeted effect of terrestrial acidification are robust and considered high quality.  The USFS-Kane Experimental Forest and significant amounts of research work in the Allegheny Plateau have produced extensive, peer-reviewed data sets.  A meta-analysis of laboratory studies showed that tree growth was decreased by 20% relative to controls for BC/Al ratios (US EPA, 2008, section 7.2.1 and Figure 7.2-1).  Sugar maple and red spruce were the focus of the REA since they are demonstrated to be negatively affected by soil available Ca[2+] depletion and high concentrations of available Al, and occur in areas that receive high acidifying deposition, There is high confidence about the use of the REA terrestrial acidification data and their value for extrapolating to a larger regional population of forests.
	There is high confidence associated with the models, input parameters, and assessment of uncertainty used in the case study for terrestrial acidification. The Simple Mass Balance (SMB) model, a commonly used and widely applied approach for estimating critical loads, was used in the REA analysis (US EPA, 2008, section 7.2.2).  There is fairly high confidence associated with the ecological effect function developed for terrestrial acidification (US EPA, 2009, section 7.2.3).
3. Nutrient Enrichment Effects Associated with Deposition of Oxides of Nitrogen
	The following summary is a concise overview of the known or anticipated effects caused by nitrogen nutrient enrichment to ecosystems within the United States.  Nutrient-enrichment affects terrestrial, freshwater and estuarine ecosystems.  Nitrogen deposition is a major source of anthropogenic nitrogen.  For many terrestrial and freshwater ecosystems other sources of nitrogen including fertilizer and waste treatment are greater than deposition.  Nitrogen deposition often contributes to nitrogen-enrichment effects in estuaries, but does not drive the effects since other sources of N greatly exceed N deposition.  Both oxides of nitrogen and reduced forms of nitrogen (NHx) contribute to nitrogen deposition.  For the most part, nitrogen effects on ecosystems do not depend on whether the nitrogen is in oxidized or reduced form.  Thus, this summary focuses on the effects of nitrogen deposition in total.
a. Nature of nutrient enrichment-related ecosystem responses
	The ISA found that deposition of nitrogen, including oxides of nitrogen and NHx, leads to the nitrogen enrichment of ecosystems (US EPA 2008).  In the process of nitrogen enrichment, biogeochemical components of terrestrial and freshwater aquatic ecosystems are altered in a way that leads to effects on biological organisms.
i. Aquatic ecosystems
	In freshwater ecosystems, the evidence is sufficient to infer a causal relationship between N deposition and the alteration of biogeochemical cycling in freshwater aquatic ecosystems (US EPA, 2008, section <IP_ADDRESS>). N deposition is the main source of N enrichment to headwater streams, lower order streams and high elevation lakes. The most common chemical indicators that were studied included NO3[−] and dissolved inorganic nitrogen (DIN) concentration in surface waters as well as Chl a:total P ratio. Elevated surface water NO3[−] concentrations occur in both the eastern and western U.S. Studies report a significant correlation between N deposition and lake biogeochemistry by identifying a correlation between wet deposition and [DIN] and Chl a: Total P. Recent evidence provides examples of lakes and streams that are limited by N and show signs of eutrophication in response to N addition.
	The evidence is sufficient to infer a causal relationship between N deposition and the alteration of species richness, species composition and biodiversity in freshwater aquatic ecosystems (US EPA, 2008, section <IP_ADDRESS>). Increased N deposition can cause a shift in community composition and reduce algal biodiversity, especially in sensitive oligotrophic lakes.
	In the ISA, the evidence is sufficient to infer a causal relationship between Nr deposition and the biogeochemical cycling of N and carbon (C) in estuaries (US EPA, 2008, section <IP_ADDRESS> and <IP_ADDRESS>). In general, estuaries tend to be nitrogen-limited, and many currently receive high levels of nitrogen input from human activities (US EPA, 2009, section 5.1.1). It is unknown if atmospheric deposition alone is sufficient to cause eutrophication; however, the contribution of atmospheric nitrogen deposition to total nitrogen load is calculated for some estuaries and can be >40% (US EPA, 2009, section 5.1.1).
	The evidence is sufficient to infer a causal relationship between N deposition and the alteration of species richness, species composition and biodiversity in estuarine ecosystems (US EPA, 2008, section <IP_ADDRESS> and <IP_ADDRESS>).  Atmospheric and non-atmospheric sources of N contribute to increased phytoplankton and algal productivity, leading to eutrophication. Shifts in community composition, reduced hypolimnetic DO, decreases in biodiversity, and mortality of submerged aquatic vegetation are associated with increased N deposition in estuarine systems.
ii. Terrestrial Ecosystems
	The evidence is sufficient to infer a causal relationship between N deposition and the alteration of biogeochemical cycling in terrestrial ecosystems (US EPA, 2008, section <IP_ADDRESS> and <IP_ADDRESS>). This is supported by numerous observational, deposition gradient and field addition experiments in sensitive ecosystems. The leaching of NO3- in soil drainage waters and the export of NO3- in stream water were identified as two of the primary indictors of N enrichment.  Several N-addition studies indicate that NO3- leaching is induced by chronic additions of N. Studies identified in the ISA found that surface water NO3- concentrations exceeded 1 ueq/L in watersheds receiving about 9 to 13 kg N/ha/yr of atmospheric N deposition.  N deposition disrupts the nutrient balance of ecosystems with numerous biogeochemical effects. The chemical indicators that are typically measured include NO3[−] leaching, soil C:N ratio, rates of N mineralization, nitrification, denitrification, foliar N concentration, and soil water NO3 − and NH4[+] concentrations. Note that N saturation (N leaching from ecosystems) does not need to occur to cause effects. Substantial leaching of NO3− from forest soils to stream water can acidify downstream waters, leading to effects described in the previous section on aquatic acidification. Due to the complexity of interactions between the N and C cycling, the effects of N on C budgets (quantified input and output of C to the ecosystem) are variable. Regional trends in net ecosystem productivity (NEP) of forests (not managed for silviculture) have been estimated through models based on gradient studies and meta-analysis. Atmospheric N deposition has been shown to cause increased litter accumulation and carbon storage in above-ground woody biomass.  In the West, this has lead to increased susceptibility to more severe fires. Less is known regarding the effects of N deposition on C budgets of non-forest ecosystems.
	The evidence is sufficient to infer a causal relationship between N deposition on the alteration of species richness, species composition and biodiversity in terrestrial ecosystems (US EPA, 2008, section <IP_ADDRESS>). Some organisms and ecosystems are more sensitive to N deposition and effects of N deposition are not observed in all habitats.  The most sensitive terrestrial taxa to N deposition are lichens. Empirical evidence indicates that lichens in the U.S. are affected by deposition levels as low as 3 kg N/ha/yr. Alpine ecosystems are also sensitive to N deposition, changes in an individual species (Carex rupestris) were estimated to occur at deposition levels near 4 kg N /ha/yr and modeling indicates that deposition levels near 10 kg N/ha/yr alter plant community assemblages. In several grassland ecosystems, reduced species diversity and an increase in non-native, invasive species are associated with N deposition.
iii. Ecosystem sensitivity to nutrient enrichment
	The numerous ecosystem types that occur across the U.S. have a broad range of sensitivity to N deposition (US EPA, 2008, Table 4-4).  Increased deposition to N-limited ecosystems can lead to production increases that may be either beneficial or adverse depending on the system and management goals.
	Organisms in their natural environment are commonly adapted to a specific regime of nutrient availability. Change in the availability of one important nutrient, such as N, may result in an imbalance in ecological stoichiometry, with effects on ecosystem processes, structure and function. In general, N deposition to terrestrial ecosystems causes accelerated growth rates in some species deemed desirable in commercial forests but may lead to altered competitive interactions among species and nutrient imbalances, ultimately affecting biodiversity. The onset of these effects occurs with N deposition levels as low as 3 kg N/ha/yr in sensitive terrestrial ecosystems to N deposition. In aquatic ecosystems, N that is both leached from the soil and directly deposited to the water surface can pollute the surface water. This causes alteration of the diatom community at levels as low as 1.5 kg N/ha/yr in sensitive freshwater ecosystems.
	The degree of ecosystem effects lies at the intersection of N loading and N-sensitivity.  N-sensitivity is predominately driven by the degree to which growth is limited by nitrogen availability. Grasslands in the western United States are typically N-limited ecosystems dominated by a diverse mix of perennial forbs and grass species. A meta-analysis discussed in the ISA (US EPA, 2008, section 3.3.3), indicated that N fertilization increased aboveground growth in all non-forest ecosystems except for deserts. In other words, almost all terrestrial ecosystems are N-limited and will be altered by the addition of anthropogenic nitrogen. Likewise, a freshwater lake or stream must be N-limited to be sensitive to N-mediated eutrophication. There are many examples of fresh waters that are N-limited or N and phosphorous (P) co-limited (US EPA, 2008, section <IP_ADDRESS>). A large dataset meta-analysis discussed in the ISA (US EPA, 2008, section <IP_ADDRESS>), found that N-limitation occurred as frequently as P-limitation in freshwater ecosystems.   Additional factors that govern the sensitivity of ecosystems to nutrient enrichment from N deposition include rates and form of N deposition, elevation, climate, species composition, plant growth rate, length of growing season, and soil N retention capacity (US EPA, 2008, section 4.3). Less is known about the extent and distribution of the terrestrial ecosystems in the U.S. that are most sensitive to the effects of nutrient enrichment from atmospheric N deposition compared to acidification.
	Because the productivity of estuarine and near shore marine ecosystems is generally limited by the availability of N, they are susceptible to the eutrophication effect of N deposition (US EPA, 2008, section <IP_ADDRESS>). A recent national assessment of eutrophic conditions in estuaries found the most eutrophic estuaries were generally those that had large watershed-to-estuarine surface area, high human population density, high rainfall and runoff, low dilution, and low flushing rates.  In the REA, the National Oceanic and Atmospheric Administration's (NOAA) National Estuarine Eutrophication Assessment (NEEA) assessment tool, Assessment of Estuarine Tropic Status (ASSETS) categorical Eutrophication Index (EI) was used to evaluate eutrophication due to atmospheric loading of nitrogen.  ASSETS EI is an estimation of the likelihood that an estuary is experiencing eutrophication or will experience eutrophication based on five ecological indicators: chlorophyll a, macroalgae, dissolved oxygen, nuisance/toxic algal blooms and submerged aquatic vegetation (SAV).
	In the REA, two regions were selected for case study analysis using ASSETS EI, the Chesapeake Bay and Pamlico Sound.  Both regions received an ASSETS EI rating of Bad indicating that the estuary had moderate to high pressure due to overall human influence and a moderate high to high eutrophic condition (US EPA, 2009, sections <IP_ADDRESS> and <IP_ADDRESS>).  These results were then considered with SPAtially Referenced Regression (SPARROW) modeling to develop a response curve to examine the role of atmospheric nitrogen deposition in achieving a desired decrease in load.  To change the Neuse River Estuary's EI score from Bad to Poor not only must 100% of the total atmospheric nitrogen deposition be eliminated, but considerably more nitrogen from other sources as well must be controlled (US EPA, 2009, section <IP_ADDRESS>).  In the Potomac River estuary, a 78% decrease of total nitrogen could move the EI score from Bad to Poor (US EPA, 2009, section <IP_ADDRESS>).  The results of this analysis indicated decreases in atmospheric deposition alone could not eliminate coastal eutrophication problems due to multiple non-atmospheric nitrogen inputs (US EPA, 2009, section 7.3.3). However, by decreasing atmospheric contributions, it may help avoid the need for more costly controls on nitrogen from other sources.  In addition, the somewhat arbitrary discreteness of the EI scale can mask the benefits of decreases in nitrogen between categories.
	In general, estuaries tend to be N-limited, and many currently receive high levels of N input from human activities to cause eutrophication. As reported in the ISA (US EPA, 2008, section <IP_ADDRESS>), atmospheric N loads to estuaries in the U.S. are estimated to range from 2-8% for Guadalupe Bay, TX on the lowest end to as high as 72% for St Catherines-Sapelo estuary, GA. The Chesapeake Bay is an example of a large, well-studied and severely eutrophic estuary that is calculated to receive as much as 30% of its total N load from the atmosphere.
a. Magnitude of ecosystem responses
i. Aquatic ecosystems
 	The magnitude of ecosystem response may be thought of on two time scales, current conditions and how ecosystems have been altered since the onset of anthropogenic N deposition.  As noted previously, studies found that N-limitation occurs as frequently as P-limitation in freshwater ecosystems (US EPA, 2008, section <IP_ADDRESS>). Recently, a comprehensive study of available data from the northern hemisphere surveys of lakes along gradients of N deposition show increased inorganic N concentration and productivity to be correlated with atmospheric N deposition. The results are unequivocal evidence of N limitation in lakes with low ambient inputs of N, and increased N concentrations in lakes receiving N solely from atmospheric N deposition. It has been suggested that most lakes in the northern hemisphere may have originally been N-limited, and that atmospheric N deposition has changed the balance of N and P in lakes.
	Available data suggest that the increases in total N deposition do not have to be large to elicit an ecological effect. For example, a hindcasting exercise determined that the change in Rocky Mountain National Park lake algae that occurred between 1850 and 1964 was associated with an increase in wet N deposition that was only about 1.5 kg N/ha. Similar changes inferred from lake sediment cores of the Beartooth Mountains of Wyoming also occurred at about 1.5 kg N/ha deposition. Pre-industrial inorganic N deposition is estimated to have been only 0.1 to 0.7 kg N/ha based on measurements from remote parts of the world. In the western U.S., pre-industrial, or background, inorganic N deposition was estimated by to range from 0.4 to 0.7 kg N/ha/yr.
	Eutrophication effects from N deposition are most likely to be manifested in undisturbed, low nutrient surface waters such as those found in the higher elevation areas of the western U.S. The most severe eutrophication from N deposition effects is expected downwind of major urban and agricultural centers. High concentrations of lake or streamwater NO3[−], indicative of ecosystem saturation, have been found at a variety of locations throughout the U.S., including the San Bernardino and San Gabriel Mountains within the Los Angeles Air Basin, the Front Range of Colorado, the Allegheny mountains of West Virginia, the Catskill Mountains of New York, the Adirondack Mountains of New York, and the Great Smoky Mountains in Tennessee (US EPA, 2008, section 3.3.8).
	In contrast to terrestrial and freshwater systems, atmospheric N load to estuaries contributes to the total load but does not necessarily drive the effects since other combined sources of N often greatly exceed N deposition.  In estuaries, N-loading from multiple anthropogenic and non-anthropogenic pathways leads to water quality deterioration, resulting in numerous effects including hypoxic zones, species mortality, changes in community composition and harmful algal blooms that are indicative of eutrophication.  The following summary is a concise overview of the known or anticipated effects of nitrogen enrichment on estuaries within the United States.
	There is a scientific consensus (US EPA, 2008, section 4.3.4) that nitrogen-driven eutrophication in shallow estuaries has increased over the past several decades and that the environmental degradation of coastal ecosystems due to nitrogen, phosphorus, and other inputs is now a widespread occurrence.  For example, the frequency of phytoplankton blooms and the extent and severity of hypoxia have increased in the Chesapeake Bay and Pamlico estuaries in North Carolina and along the continental shelf adjacent to the Mississippi and Atchafalaya rivers' discharges to the Gulf of Mexico.
	A recent national assessment of eutrophic conditions in estuaries found that 65% of the assessed systems had moderate to high overall eutrophic conditions and generally received the greatest N loads from all sources, including atmospheric and land-based sources.  Most eutrophic estuaries occurred in the mid-Atlantic region and the estuaries with the lowest degree of eutrophication were in the North Atlantic. Other regions had mixtures of low, moderate, and high degrees of eutrophication (US EPA, 2008, section <IP_ADDRESS>).
	The mid-Atlantic region is the most heavily impacted area in terms of moderate or high loss of submerged aquatic vegetation due to eutrophication (US EPA, 2008, section <IP_ADDRESS>).  Submerged aquatic vegetation is important to the quality of estuarine ecosystem habitats because it provides habitat for a variety of aquatic organisms, absorbs excess nutrients, and traps sediments (US EPA, 2008, section <IP_ADDRESS>).  It is partly because many estuaries and near-coastal marine waters are degraded by nutrient enrichment that they are highly sensitive to potential negative impacts from nitrogen addition from atmospheric deposition.
ii. Terrestrial ecosystems
	Little is known about the full extent and distribution of the terrestrial ecosystems in the U.S. that are most sensitive to impacts caused by nutrient enrichment from atmospheric N deposition. As previously stated, most terrestrial ecosystems are N-limited, therefore they are sensitive to perturbation caused by N additions (US EPA, 2008, section 4.3.1). Effects are most likely to occur where areas of relatively high atmospheric N deposition intersect with N-limited plant communities.  The alpine ecosystems of the Colorado Front Range, chaparral watersheds of the Sierra Nevada, lichen and vascular plant communities in the San Bernardino Mountains and the Pacific Northwest, and the southern California coastal sage scrub (CSS) community are among the most sensitive terrestrial ecosystems. There is growing evidence (US EPA, 2008, section <IP_ADDRESS>) that existing grassland ecosystems in the western United States are being altered by elevated levels of N inputs, including inputs from atmospheric deposition.
	In the eastern U.S., the degree of N saturation of the terrestrial ecosystem is often assessed in terms of the degree of NO3[−] leaching from watershed soils into ground water or surface water. Studies have estimated the number of surface waters at different stages of saturation across several regions in the eastern U.S. Of the 85 northeastern watersheds examined 60% were in Stage 1 or Stage 2 of N saturation on a scale of 0 (background or pretreatment) to 3 (visible decline). Of the northeastern sites for which adequate data were available for assessment, those in Stage 1 or 2 were most prevalent in the Adirondack and Catskill Mountains. Effects on individual plant species have not been well studied in the U.S. More is known about the sensitivity of particular plant communities. Based largely on results obtained in more extensive studies conducted in Europe, it is expected that the more sensitive terrestrial ecosystems include hardwood forests, alpine meadows, arid and semi-arid lands, and grassland ecosystems (US EPA, 2008, section 3.3.5).
	The REA used published research results (US EPA, 2009, section 5.3.1 and US EPA, 2008, Table 4.4) to identify meaningful ecological benchmarks associated with different levels of atmospheric nitrogen deposition. These are illustrated in Figure 3-4 of the Policy Assessment.  The sensitive areas and ecological indicators identified by the ISA were analyzed further in the REA to create a national map that illustrates effects observed from ambient and experimental atmospheric nitrogen deposition loads in relation to Community Multi-scale Air Quality (CMAQ) 2002 modeling results and NADP monitoring data.  This map, reproduced in Figure 3-5of the Policy Assessment, depicts the sites where empirical effects of terrestrial nutrient enrichment have been observed and site proximity to elevated atmospheric N deposition.
	Based on information in the ISA and initial analysis in the REA, further case study analyses on terrestrial nutrient enrichment of ecosystems were developed for the CCS community and Mixed Conifer Forest (MCF) (US EPA, 2009).  Geographic information systems (GIS) analysis supported a qualitative review of past field research to identify ecological benchmarks associated with CSS and mycorrhizal communities, as well as MCF's nutrient-sensitive acidophyte lichen communities, fine-root biomass in Ponderosa pine, and leached nitrate in receiving waters.
	The ecological benchmarks that were identified for the CSS and the MCF are included in the suite of benchmarks identified in the ISA (US EPA, 2008, section 3.3). There are sufficient data to confidently relate the ecological effect to a loading of atmospheric nitrogen. For the CSS community, the following ecological benchmarks were identified:
(1) 3.3 kg N/ha/yr  -  the amount of nitrogen uptake by a vigorous stand of CSS; above this level, nitrogen may no longer be limiting
(2) 10 kg N/ha/yr  -  mycorrhizal community changes
For the MCF community, the following ecological benchmarks were identified:
(1) 3.1 kg N/ha/yr  -  shift from sensitive to tolerant lichen species
(2) 5.2 kg N/ha/yr  -  dominance of the tolerant lichen species
(3) 10.2 kg N/ha/yr  -  loss of sensitive lichen species
(4) 17 kg N/ha/yr  -  leaching of nitrate into streams.
	These benchmarks, ranging from 3.1 to 17 kg N/ha/yr, were compared to 2002 CMAQ/NADP data to discern any associations between atmospheric deposition and changing communities. Evidence supports the finding that nitrogen alters CSS and MCF. Key findings include the following: 2002 CMAQ/NADP nitrogen deposition data show that the 3.3 kg N/ha/yr benchmark has been exceeded in more than 93% of CSS areas (654,048 ha). These deposition levels are a driving force in the degradation of CSS communities. Although CSS decline has been observed in the absence of fire, the contributions of deposition and fire to the CSS decline require further research. CSS is fragmented into many small parcels, and the 2002 CMAQ/NADP 12-km grid data are not fine enough to fully validate the relationship between CSS distribution, nitrogen deposition, and fire. 2002 CMAQ/NADP nitrogen deposition data exceeds the 3.1 kg N/ha/yr benchmark in more than 38% (1,099,133 ha) of MCF areas, and nitrate leaching has been observed in surface waters. Ozone effects confound nitrogen effects on MCF acidophyte lichen, and the interrelationship between fire and nitrogen cycling requires additional research.
a. Key uncertainties associated with nutrient enrichment
	There are different levels of uncertainty associated with relationships between deposition, ecological effects and ecological indicators.  The criteria used in the REA to evaluate the degree of confidence in the data, modeling and ecological effect function are detailed in Chapter 7 of the REA.  Below is a discussion of uncertainty relating aquatic and terrestrial ecosystems to nutrient enrichment effects.
i. Aquatic ecosystems
	The approach for assessing atmospheric contributions to total nitrogen loading in the REA was to consider the main-stem river to an estuary (including the estuary) rather than an entire estuary system or bay.  The biological indicators used in the NOAA ASSETS EI required the evaluation of many national databases including the US Geological Survey National Water Quality Assessment (NAWQA) files, EPA's STORage and RETrieval (STORET) database, NOAA's Estuarine Drainage Areas data, and EPA's water quality standards nutrient criteria for rivers and lakes (US EPA, 2009, Appendix 6 and Table 1.2.-1).  Both the SPARROW modeling for nitrogen loads and assessment of estuary conditions under NOAA ASSETS EI, have been applied on a national scale.  The REA concludes that the available data are medium quality with intermediate confidence about the use of these data and their values for extrapolating to a larger regional area (US EPA, 2009, section 7.3.1).  Intermediate confidence is associated with the modeling approach using ASSETS EI and SPARROW.  The REA states there is low confidence with the ecological effect function due to the results of the analysis which indicated that reductions in atmospheric deposition alone could not solve coastal eutrophication problems due to multiple non-atmospheric nitrogen inputs (US EPA, 2009, section 7.3.3).
ii. Terrestrial ecosystems
	Ecological thresholds are identified for CSS and MCF and these data are considered to be of high quality, however, the ability to extrapolate these data to larger regional areas is limited (US EPA, 2009, section 7.4.1).  No quantitative modeling was conducted or ecological effect function developed for terrestrial nutrient enrichment reflecting the uncertainties associated with these depositional effects.
4.	Other Ecological Effects
	It is stated in the ISA (US EPA, 2008, section 3.4.1 and 4.5) that mercury is a highly neurotoxic contaminant that enters the food web as a methylated compound, methylmercury. Mercury is principally methylated by sulfur-reducing bacteria and can be taken up by microorganisms, zooplankton and macroinvertebrates. The contaminant is concentrated in higher trophic levels, including fish eaten by humans. Experimental evidence has established that only inconsequential amounts of methylmercury can be produced in the absence of sulfate. Once methylmercury is present, other variables influence how much accumulates in fish, but elevated mercury levels in fish can only occur where substantial amounts of methylmercury are present. Current evidence indicates that in watersheds where mercury is present, increased oxides of sulfur deposition very likely results in additional production of methylmercury which leads to greater accumulation of MeHg concentrations in fish. With respect to sulfur deposition and mercury methylation, the final ISA determined: The evidence is sufficient to infer a causal relationship between sulfur deposition and increased mercury methylation in wetlands and aquatic environments.
	The production of meaningful amounts of methylmercury (MeHg) requires the presence of SO4[2-] and mercury, and where mercury is present, increased availability of SO4[2-] results in increased production of MeHg. There is increasing evidence on the relationship between sulfur deposition and increased methylation of mercury in aquatic environments; this effect occurs only where other factors are present at levels within a range to allow methylation. The production of methylmercury requires the presence of sulfate and mercury, but the amount of methylmercury produced varies with oxygen content, temperature, pH, and supply of labile organic carbon (US EPA, 2008, section 3.4). In watersheds where changes in sulfate deposition did not produce an effect, one or several of those interacting factors were not in the range required for meaningful methylation to occur (US EPA, 2008, section 3.4). Watersheds with conditions known to be conducive to mercury methylation can be found in the northeastern United States and southeastern Canada.
	While the relationship between sulfur and methylmercury production was concluded to be causal in the ISA, the REA concluded that there was insufficient evidence to quantify the relationship between sulfur and methylmercury.  Therefore only a qualitative assessment was included in Chapter 6 of the REA. The Policy Assessment was then unable to make a determination as to the adequacy of the existing SO2 standards in protecting against welfare effects associated with increased mercury methylation.
B.	Risk and Exposure Assessment
	The risk and exposure assessment conducted for the current review was developed to describe potential risk from current and future deposition of oxides of nitrogen and sulfur to sensitive ecosystems. The case study analyses in the REA show that there is confidence that known or anticipated adverse ecological effects are occurring under current ambient loadings of nitrogen and sulfur in sensitive ecosystems across the United States.  An overview of the material covered in the REA, a summary of the key findings from the air quality analyses, acidification and nutrient enrichment case studies, and general conclusions from evaluating additional welfare effects, are presented below.
1.	Overview of the Risk and Exposure Assessment
	The REA evaluates the relationships between atmospheric concentrations, deposition, biologically relevant exposures, targeted ecosystem effects, and ecosystem services. To evaluate the nature and magnitude of ecosystem responses associated with adverse effects, the REA also examines various ways to quantify the relationships between air quality indicators, deposition of biologically available forms of nitrogen and sulfur, ecologically relevant indicators relating to deposition, exposure and effects on sensitive receptors, and related effects resulting in changes in ecosystem structure and services. The intent is to determine the exposure metrics that incorporate the temporal considerations (i.e., biologically relevant timescales), pathways, and ecologically relevant indicators necessary to maintain the functioning of these ecosystems. To the extent feasible, the REA evaluates the overall load to the system for nitrogen and sulfur, as well as the variability in ecosystem responses to these pollutants as well as evaluating the contributions of atmospherically deposited nitrogen and sulfur relative to the combined atmospheric loadings of both elements. Since oxidized nitrogen is the listed criteria pollutant (currently measured by the ambient air quality indicator NO2) for the atmospheric contribution to total nitrogen, the REA examines the contribution of nitrogen oxides to total reactive nitrogen in the atmosphere, relative to the contributions of reduced forms of nitrogen (e.g., ammonia, ammonium), to ultimately assess how a meaningful secondary National Ambient Air Quality Standards (NAAQS) might be structured.
	The REA focuses on ecosystem welfare effects that result from the deposition of total reactive nitrogen and sulfur. Because ecosystems are diverse in biota, climate, geochemistry, and hydrology, response to pollutant exposures can vary greatly between ecosystems. In addition, these diverse ecosystems are not distributed evenly across the United States. To target nitrogen and sulfur acidification and nitrogen and sulfur enrichment, the REA addresses four main targeted ecosystem effects on terrestrial and aquatic systems identified by the ISA (US EPA, 2008): aquatic acidification due to nitrogen and sulfur; terrestrial acidification due to nitrogen and sulfur; aquatic nutrient enrichment, including eutrophication; and terrestrial nutrient enrichment.
	In addition to these four targeted ecosystem effects, the REA also qualitatively addresses the influence of sulfur oxides deposition on methylmercury production; nitrous oxide (N2O) effects on climate; nitrogen effects on primary productivity and biogenic greenhouse gas fluxes; and phytotoxic effects on plants.
	Because the targeted ecosystem effects outlined above are not evenly distributed across the United States, the REA identified case studies for each targeted effects based on ecosystems identified as sensitive to nitrogen and/or sulfur deposition effects.  Eight case study areas and two supplemental study areas (Rocky Mountain National Park and Little Rock Lake, WI) are summarized in the REA based on ecosystem characteristics, indicators, and ecosystem service information.  Case studies selected for aquatic acidification effects were the Adirondack Mountains and Shenandoah National Park.   Kane Experimental Forest in Pennsylvania and Hubbard Brook Experimental Forest in New Hampshire were selected as case studies for terrestrial acidification.  Aquatic nutrient enrichment case study locations were selected in the Potomac River Basin upstream of Chesapeake Bay and the Neuse River Basin upstream of the Pamlico Sound in North Carolina.  The Coastal Sage Scrub Communities in southern California and the Mixed Confer Forest Communities in the San Bernardino and Sierra Nevada Mountains of California were selected as case studies for terrestrial nutrient enrichment.  Two supplemental areas were also chosen, one in Rocky Mountain National Park for terrestrial nutrient enrichment and one in Little Rock Lake, Wisconsin for aquatic nutrient enrichment.
2.	Key findings
	In summary, based on case study analyses, the REA concludes that known or anticipated adverse ecological effects are occurring under current conditions and further concludes that these adverse effects continue into the future.  Key findings from the air quality analyses, acidification and nutrient enrichment case studies, as well as general conclusions from evaluating additional welfare effects, are summarized below.
a. Air quality analyses
	The air quality analyses in the REA encompass the current emissions sources of nitrogen and sulfur, as well as atmospheric concentrations, estimates of deposition of total nitrogen, policy-relevant background, and non-ambient loadings of nitrogen and sulfur to ecosystems, both nationwide and in the case study areas. Spatial fields of deposition were created using wet deposition measurements from the National Atmospheric Deposition Program (NADP) National Trends Network and dry deposition predictions from the 2002 Community Multi-Scale Air Quality (CMAQ) model simulation.
(1) Total reactive nitrogen deposition and sulfur deposition are much greater in the East compared to most areas of the West.
(2) These regional differences in deposition correspond to the regional differences in oxides of nitrogen and SO2 concentrations and emissions, which are also higher in the East. Oxides of nitrogen emissions are much greater and generally more widespread than NH3 emissions nationwide; high NH3 emissions tend to be more local (e.g., eastern North Carolina) or sub-regional (e.g., the upper Midwest and Plains states). The relative amounts of oxidized versus reduced nitrogen deposition are consistent with the relative amounts of oxides of nitrogen and NH3 emissions. Oxidized nitrogen deposition exceeds reduced nitrogen deposition in most of the case study areas; the major exception being the Neuse River/Neuse River Estuary Case Study Area.
(3) Reduced nitrogen deposition exceeds oxidized nitrogen deposition in the vicinity of local sources of NH3.
(4) There can be relatively large spatial variations in both total reactive nitrogen deposition and sulfur deposition within a case study area; this occurs particularly in those areas that contain or are near a high emissions source of oxides of nitrogen, NH3, and/or SO2.
(5) The seasonal patterns in deposition differ between the case study areas. For the case study areas in the East, the season with the greatest amounts of total reactive nitrogen deposition correspond to the season with the greatest amounts of sulfur deposition. Deposition peaks in spring in the Adirondack, Hubbard Brook Experimental Forest, and Kane Experimental Forest case study areas, and it peaks in summer in the Potomac River/Potomac Estuary, Shenandoah, and Neuse River/Neuse River Estuary case study areas. For the case study areas in the West, there is less consistency in the seasons with greatest total reactive nitrogen and sulfur deposition in a given area. In general, both nitrogen and/or sulfur deposition peaks in spring or summer. The exception to this is the Sierra Nevada Range portion of the Mixed Conifer Forest Case Study Area, in which sulfur deposition is greatest in winter.
a. Deposition-related aquatic acidification
	The role of aquatic acidification in two eastern United States areas -- northeastern New York's Adirondack area and the Shenandoah area in Virginia -- was analyzed in the REA to assess surface water trends in SO4[2-]and NO3[-]concentrations and acid neutralizing capacity (ANC) levels and to affirm the understanding that reductions in deposition could influence the risk of acidification.  Monitoring data from the EPA-administered Temporally Integrated Monitoring of Ecosystems (TIME)/Long-Term Monitoring (LTM) programs and the Environmental Monitoring and Assessment Program (EMAP) were assessed for the years 1990 to 2006, and past, present, and future water quality levels were estimated using both steady-state and dynamic biogeochemical models.
	Although wet deposition rates for SO2 and oxides of nitrogen in the Adirondack Case Study Area have reduced since the mid-1990s, current concentrations in are still well above pre-acidification (1860) conditions. Model of Acidification of Groundwater in Catchments (MAGIC) modeling predicts NO3[-] and SO4[2-] are 17- and 5-fold higher today, respectively. The estimated average ANC for 44 lakes in the Adirondack Case Study Area is 62.1 microequivalents per liter (μeq/L) (+- 15.7 μeq/L); 78 % of all monitored lakes in the Adirondack Case Study Area have a current risk of Elevated, Severe, or Acute. Of the 78%, 31% experience episodic acidification, and 18% are chronically acidic today.
(1) Based on the steady-state critical load model for the year 2002, 18%, 28%, 44%, and 58% of 169 modeled lakes received combined total sulfur and nitrogen deposition that exceeded their critical load, with critical ANC limits of 0, 20, 50, and 100 μeq/L respectively.
(2) Based on a deposition scenario that maintains current emission levels to 2020 and 2050, the simulation forecast indicates no improvement in water quality in the Adirondack Case Study Area. The percentage of lakes within the Elevated to Acute Concern classes remains the same in 2020 and 2050.
(3) Since the mid-1990s, streams in the Shenandoah Case Study Area have shown slight declines in NO3 and SO4[2-] concentrations in surface waters. ANC levels increased from about 50 μeq/L in the early 1990 to >75 μeq/L until 2002, when ANC levels declined back to 1991 - 1992 levels. Current concentrations are still above pre-acidification (1860) conditions. MAGIC modeling predicts surface water concentrations of NO3 and SO4[2-] are 10- and 32-fold higher today, respectively. The estimated average ANC for 60 streams in the Shenandoah Case Study Area is 57.9 μeq/L (+- 4.5 μeq/L). 55% of all monitored streams in the Shenandoah Case Study Area have a current risk of Elevated, Severe, or Acute. Of the 55%, 18% experience episodic acidification, and 18% are chronically acidic today.