6560-50-P
                        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: Final rule.
SUMMARY: This final rule is being issued as required by a consent decree governing the schedule for completion of this review of the air quality criteria and the secondary national ambient air quality standards (NAAQS) for oxides of nitrogen and oxides of sulfur. Based on its review, the EPA is retaining the current nitrogen dioxide (NO2) and sulfur dioxide (SO2) secondary standards to address the direct effects on vegetation of exposure to gaseous oxides of nitrogen and sulfur and, for reasons described in detail in this final preamble, is not adding new standards at this time to address effects associated with the deposition of oxides of nitrogen and sulfur on sensitive aquatic and terrestrial ecosystems. In addition, in this rule the EPA describes a field pilot program being developed to enhance our understanding of the degree of protectiveness that would likely be afforded by a multi-pollutant standard to address deposition-related acidification of sensitive aquatic ecosystems.
DATES:  This final rule is effective on [Insert date  -  60 days from date of publication in the Federal Register].
ADDRESSES: The EPA has established a docket for this action under Docket ID No. EPA-HQ-OAR-2007-1145. 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., confidential business information (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: Mrs. Ginger Tennant,  Office of Air Quality Planning and Standards (OAQPS), U.S. Environmental Protection Agency, Mail Code C504-06,  Research Triangle Park, NC 27711; telephone:<PHONE_NUMBER>; fax:<PHONE_NUMBER>; email<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:
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
		1.	Scope Presented in the Proposal
		2.	Comments on the Scope of the Review
II.	Rationale for Final Decisions on the Adequacy of the Current Secondary Standards
    	A.	Introduction
            1.	Overview of Effects
			a.	Effects Associated with Gas-Phase Oxides of Nitrogen and Sulfur
			b.	Effects Associated with Deposition of Oxides of Nitrogen and
				Sulfur
            2.	Overview of Risk and Exposure Assessment
            	a.	Approach to REA Analyses
            	b.	Key Findings
            	c. 	Other Welfare Effects
            3.	Overview of Adversity of Effects to Public Welfare
            	a.	Ecosystem Services
            	b.	Effects on Ecosystem Services
            	c. 	Summary
      B.	Adequacy of the Current Standards
		1.	Adequacy Considerations
			a.	Adequacy of the Current Standards for Direct Effects
			b.	Appropriateness and Adequacy of the Current Standards for
				 Deposition-related Effects
			c.	Summary of Adequacy Considerations
		2.	CASAC Views
		3.	Administrator's Proposed Conclusions
	C.	Comments on Adequacy of the Current Standards
		1.	Adequacy of the Current Standards to Address Direct Effects
		2.	Adequacy of the Current Secondary Standards to Address Deposition-					Related Effects
	D.	Final Decisions on the Adequacy of the Current Standards
III.	Rationale for Final Decisions on Alternative Secondary Standards
	A.	Overview of AAI Approach
		1.	Ambient Air Indicators
			a.	Oxides of Sulfur
			b.	Oxides of Nitrogen
		2.	Form
			a.	Ecological Indicator
			b.	Linking ANC to Deposition
			c.	Linking Deposition to Ambient Air Indicators
			d.	Aquatic Acidification Index
			e. 	Spatial Aggregation
			f.	Summary of the AAI Form
		3.	Averaging Time
		4.	Level
		5.	Characterization of Uncertainties
	B.	CASAC Views
      C.	Proposed Conclusions on Alternative Secondary Standards
      D.	Comments on Alternative Secondary Standards
      	1.	Comments Related to an AAI-based Standard
      		a.	Comments on Consideration of an AAI-based Standard
      		b.	Comments on Specific Aspects of an AAI-based Approach
      	2.	Comments on 1-hour NO2 and SO2 Secondary Standards
      E.	Final Decisions on Alternative Secondary Standards for Oxides of Nitrogen and Sulfur
IV.	Field Pilot Program and Ambient Monitoring
	A.	Overview of Proposed Field Pilot Program
		1. 	Complementary Measurements
		2.	Complementary Areas of Research
		3. 	Implementation Challenges
		4.	Monitoring Plan Development and Stakeholder Participation
	B.	Summary of Proposed Evaluation of Monitoring Methods
	C.	Comments on Field Pilot Program and Monitoring Methods Evaluation
		1. 	Goals, Objectives and Scope of Field Pilot Program
		2. 	Network Design and Role of CASTNET
		3.	Complementary Measurements and Instrumentation
		4.	Collaboration
 V.	Statutory and Executive Order Reviews
      A.	Executive Order 12866:  Regulatory Planning and Review and Executive Order 13563: 	Improving Regulation and Regulatory 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. Section 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. Section 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, the 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, the EPA published Air Quality Criteria Document for Oxides of Nitrogen (U.S. EPA, 1982), which updated the scientific criteria upon which the initial standards were based. In February 1984, the EPA proposed to retain the standards set in 1971 (49 FR 6866). After taking into account public comments, the 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, the 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 the EPA to make a decision as to the appropriate NAAQS for NO2" (Wolff, 1993). The AQCD for Oxides of Nitrogen was then finalized (U.S. EPA, 1995a). The EPA 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. The 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, set at a level of 100 parts per billion (ppb) (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
	The 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 the AQCD for Sulfur Oxides (U.S. 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, the EPA proposed (38 FR 11355) and then finalized (38 FR 25678) a revocation of the annual mean secondary standard. At that time, the 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, the 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. The 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. The 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. The 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, the EPA published a Staff Paper in November 1982, although the paper did not directly assess the issue of acid deposition. Instead, the 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 (U.S. EPA, 1984a, b) and The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Document (U.S. 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 the EPA at that point.
	In April 1988 (53 FR 14926), the 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, the EPA would draft and support an appropriate set of control measures. Although the EPA revised the primary SO2 standard in June 2010 by establishing a new 1-hour standard at a level of 75 ppb and revoking the existing 24-hour and annual standards (75 FR 35520), no further decision on the secondary SO2 standard has 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 CAA 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 (CAA Amendments of 1990, Pub. L. 101-549, Section 404) requiring the EPA to conduct a study on the feasibility and effectiveness of an acid deposition standard or standards to protect "sensitive and critically sensitive aquatic and terrestrial resources."  At the conclusion of the study, the EPA was to submit a report to Congress. Five years later, the EPA submitted its report, entitled Acid Deposition Standard Feasibility Study: Report to Congress (U.S. 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 the 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 the 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 (65 FR 48699), the EPA announced receipt of these requests and asked for comment on the issues raised in them. The 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.
	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 percent 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 Integrated Science Assessment (ISA), Risk and Exposure Assessment (REA), and in other recent reports, such as the NAPAP reports noted above, the 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 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 (U.S. EPA, 2007) for CASAC and public review. The 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 (U.S. EPA, 2008) was released in December 2008.
	Based on the science presented in the ISA, the EPA developed 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 consultation and public review 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 (U.S. EPA, 2009) was released in September 2009. A first draft Policy Assessment (PA) 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 PA (Russell and Samet, 2010a). A second draft PA was released to CASAC and the public in September 2010, and reviewed by CASAC on October 6-7, 2010. The CASAC provided advice and recommendations to the Agency regarding the second draft PA in a December 9, 2010 letter (Russell and Samet 2010b). The CASAC and public comments on the second draft PA were considered by the EPA staff in developing a final PA (U.S. EPA, 2011). CASAC requested an additional meeting to provide additional advice to the Administrator based on the final PA on February 15 - 16, 2011. On January 14, 2011 the EPA released a version of the final PA prior to final document production, to provide sufficient time for CASAC review of the document in advance of this meeting. The final PA, 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 and recommendations to the Administrator with regard to the review of the secondary NAAQS for oxides of nitrogen and oxides of sulfur.
	 In 2005, the Center for Biological Diversity and four other plaintiffs filed a complaint alleging that the 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 agreed to by the parties. 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 the 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 final 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 by the Administrator are noted.
E.	Scope of the Current Review
1.	Scope Presented in the Proposal
	In conducting this periodic review of the secondary NAAQS for oxides of nitrogen and oxides of sulfur, as discussed in the IRP and REA, the 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 the EPA has historically adopted separate secondary standards for oxides of nitrogen and oxides of sulfur, the 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 the 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 the 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 the EPA has set secondary standards for two other criteria pollutants related to oxides of nitrogen and sulfur:  ozone (O3) and 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 United States. 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 nitrogen deposition resulting from atmospheric nitrogen (U.S. 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 (U.S. 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 PA 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 the 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) based on the ecological criteria described in the ISA and the results of the assessments in the REA, and 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. Consistent with the CAA, this policy objective 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 PA (Russell and Samet, 2010a, 2010b), and as discussed in detail in the PA, 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.
2.	Comments on the Scope of the Review
	Comments received regarding the scope of the review were primarily those that questioned the EPA's legal authority under Section 109 of the CAA to set NAAQS that address deposition-related effects, focusing in particular on effects resulting from acidifying deposition to ecosystems.
	While environmental organizations and some other commenters urged the EPA to establish a NAAQS that would protect against the impacts on sensitive ecosystems associated with the acidifying deposition of nitrogen and sulfur, several industry commenters argued that the enactment of Title IV of the CAA in 1990 displaced the EPA's authority to address acidification through the setting of NAAQS. These commenters contend that the existence of a specific regulatory program to address the acidification effects of oxides of nitrogen and sulfur supplants the EPA's general authority under the CAA. According to industry comments, this is demonstrated by a close reading of Section 404 which required the EPA to report to Congress on the feasibility of developing an acid deposition standard and the actions that would be required to integrate such a program into the CAA. The required study described in Section 404, commenters argue, demonstrates that Congress had concluded that the EPA lacked the authority under Section 109 of the CAA to establish a secondary NAAQS to address acid deposition.
	Although the EPA is not adopting a secondary standard designed to protect the public welfare from the effects associated with the acidifying deposition of nitrogen and sulfur, the EPA does not agree that the enactment of Title IV displaced the EPA's authority under Section109 of the CAA to set such a NAAQS. We note that the purpose of Title IV "is to reduce the adverse effects of acid deposition," CAA Section 401(b), while Section 109 directs the Administrator to go beyond this to set a standard that is "requisite to protect public welfare from any known or anticipated adverse effects," CAA Section 109(b)(2). These provisions are not accordingly in conflict, but represent the often typical interlinked approach of Congress to address the frequently complex problems of air pollution.
	Nothing in the text or the legislative history of Title IV of the Act indicates a clear intention by Congress to foreclose the EPA's authority to address acid deposition through the NAAQS process. The requirement in Section 404 of the 1990 CAA Amendments that the EPA send to Congress "a report on the feasibility and effectiveness of an acid deposition standard or standards" does not indicate that Congress had concluded that an amendment to the CAA would be necessary to give the EPA the authority to issue regulations addressing acidification. The significance of the report required by Section 404 cannot be understood clearly in isolation, but should be considered in the overall context of the history of Congress' and the EPA's attempts to understand and to address the causes and effects of acid deposition and the EPA's conclusion in 1988 that the scientific uncertainties associated with acid deposition were too great to allow the Agency to establish a secondary NAAQS at that time. In the proposed rule, we noted that it was clear at the time of the 1990 CAA Amendments that a program to address acid deposition was needed and that the primary and most important of these provisions is Title IV of the Act, establishing the Acid Rain Program. In assessing the import of Section 404 in this overall context, the EPA has noted in the past and in section I.C above that "Congress reserved judgment as to whether further action might be necessary or appropriate in the longer term" to address any problems remaining after implementation of the Title IV program, and "if so, what form it should take" (58 FR 21351, 21356 (April 21, 1993)). Such reservation of judgment does not indicate that Congress viewed the EPA as lacking authority under Section 109 to establish a secondary NAAQS to address acid deposition but a recognition that the uncertainties associated with such a standard may be too significant to allow the Administrator to reach a reasoned conclusion as to the appropriate standard.
	Having carefully considered the public comments, the EPA finds that the conclusions reached in the proposed rule with regard to the scope of the current review continue to be valid. The EPA concludes that the Agency has the authority under Section 109 of the CAA to consider deposition-related to ambient air concentrations of oxides of nitrogen and sulfur and the resulting effects on ecosystems and that the focus of the current review of the NAAQS for oxides of nitrogen and sulfur on aquatic acidification is appropriate. This issue is discussed in more detail in EPA's Response to Comments document.
II.	Rationale for Final Decisions on the Adequacy of the Current Secondary Standards
	This section presents the rationale for the Administrator's final 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 PA, 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 PA 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 as well as comments received on the proposal notice.
	In developing this rationale, the 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 (U.S. EPA, 2008). As discussed below, 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, the EPA focuses on those ecological endpoints, such as aquatic acidification, for which the ISA judges associations with oxides of nitrogen and oxides of sulfur to be causal, likely causal, or for which the evidence is suggestive that oxides of nitrogen and/or sulfur contribute to the reported effects. The categories of causality determinations have been developed in the ISA (U.S. EPA, 2008) and are discussed in section 1.6 of the ISA.
      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.
	Drawing from information in sections II.A-C of the proposal, section II.A below provides overviews of the public welfare effects considered in this review, the risk and exposure assessments, and the adversity of effects on public welfare. Section II.B presents conclusions in the ISA, REA, and PA on the adequacy of the current secondary standards for oxides of nitrogen and oxides of sulfur. Consideration is given 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. The views of CASAC and a summary of the Administrator's proposed conclusions are also included. Section II. C presents a discussion of the comments received on the proposal with regard to the adequacy of the current standards. Section II. D presents the Administrator's final decisions with regard to the adequacy of the current standards for both direct and deposition-related effects on public welfare.
A.	Introduction
	A discussion of the effects associated with oxides of nitrogen and sulfur in the ambient air is presented below in section II.A.1. 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.A.2 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.A.3 presents a discussion of adversity 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.
1.	Overview of 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.a) and effects associated with deposition-related exposure (section II.A.1.b). 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 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.
a.	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 SO2, 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 United States (U.S. EPA, 2008, section <IP_ADDRESS>).
	The 2008 ISA found that gas phase nitrogen and sulfur are associated with direct phytotoxic effects (U.S. EPA, 2008, section 4.4). The evidence is sufficient to infer a causal relationship between exposure to SO2 and injury to vegetation (U.S. 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 (U.S. 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 (U.S. 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.
	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 (U.S. 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 (U.S. 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 (U.S. 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 (U.S. 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 potassium (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 United States 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 (U.S. 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 phytotoxic effects associated with gas phase oxides of nitrogen and sulfur occur at levels well above ambient concentrations observed in the United States (U.S. EPA, 2008, section <IP_ADDRESS>).
b.	Effects Associated with Deposition of Oxides of Nitrogen and Sulfur
	Ecological effects associated with the deposition of oxides of nitrogen and oxides of sulfur can be divided into endpoints related to the type of ecosystem affected and the type of effect. As more fully discussed in section II.A of the proposal and chapter 3 of the PA, this section provides a brief summary of effects on ecosystems related to acidification, nutrient enrichment, and metal toxicity.
i.	Acidification Effects on Aquatic and Terrestrial Ecosystems
	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 aquatic and terrestrial 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 water bodies (U.S. EPA, 2009, section 2.1). The chemical forms of nitrogen that may contribute to acidifying deposition include both oxidized and reduced chemical species, including reduced forms of nitrogen (NHx).
	The ISA concluded that deposition of oxides of nitrogen and sulfur and NHx leads to the varying degrees of acidification of ecosystems (U.S. 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. Principal factors governing the sensitivity of terrestrial and aquatic ecosystems to acidification from sulfur and nitrogen deposition include geology, plant uptake of nitrogen, soil depth, and elevation. 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 flowpath.  Chronic as well as episodic acidification tends to occur primarily at relatively high elevations in areas that have base-poor bedrock, high relief, and shallow soils.
	With regard to aquatic acidification, the ISA concluded that the scientific evidence is sufficient to infer a causal relationship between acidifying deposition and effects on biogeochemistry and biota in aquatic ecosystems (U.S. 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, acid neutralizing capacity (ANC), dissolved inorganic aluminum (Al) and pH (U.S. EPA, 2008, section <IP_ADDRESS>). The ANC is a key indicator of acidification with relevance to both terrestrial and aquatic ecosystems. The 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), calcium (Ca[2+]) and Al. 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. Acidification in terrestrial ecosystems has been shown to cause decreased growth and increased susceptibility to disease and injury in sensitive tree species, including red spruce and sugar maple.
	Based on analyses of surface water data from freshwater ecosystem surveys and monitoring, the most sensitive lakes and streams are contained in New England, the Adirondack Mountains, the Appalachian Mountains (northern Appalachian Plateau and Ridge/Blue Ridge region), the mountainous West, and the Upper Midwest. 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. Annual or multi-year average ANC is a good overall indicator of sensitivity, capturing the ability of an ecosystem to withstand chronic acidification as well as episodic events such as spring melting that can lower ANC over shorter time spans.  Biota are generally not harmed when annual average ANC levels are > 100 microequivalents per liter (ueq/L). At annual average ANC levels between 100 and 50 ueq/L, the fitness of sensitive species (e.g., brook trout, zooplankton) begins to decline. When annual average ANC is <50 μeq/L, negative effects on aquatic biota are observed, including large reductions in diversity of fish species, and declines in health of fish populations, affecting reproductive ability and fitness. Annual average ANC levels below 0 ueq/L are generally associated with complete loss of fish species and other biota that are sensitive to acidification. An example of the relationship between ANC level and aquatic effects based on lakes in the Adirondacks is illustrated in the following figure:

	Recent studies indicate that acidification of lakes and streams can result in significant loss in economic value, which is one indicator of adversity associated with loss of ecosystem services. A 2006 study of New York residents found that they are willing to pay between $300 and $800 million annually for the equivalent of improving lakes in the Adirondacks region to an ANC level of 50 ueq/L. Several states have set goals for improving the acid status of lakes and streams, generally targeting ANC in the range of 50 to 60 ueq/L, and have engaged in costly activities to decrease acidification.
	With regard to terrestrial ecosystems, the evidence is sufficient to infer a causal relationship between acidifying deposition and changes in biogeochemistry (U.S. 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 (U.S. EPA, 2008, section <IP_ADDRESS>.). Three useful indicators of chemical changes and acidification effects on terrestrial ecosystems, showing consistency among multiple studies are:  soil base saturation, Al concentrations in soil water, and soil carbon to nitrogen (C:N) ratio (U.S. EPA, 2008, section <IP_ADDRESS>).
	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 and mountainous regions in the West are the regions most sensitive to acidifying deposition. The health of at least a portion of the sugar maple and red spruce growing in the United States may have been compromised by acidifying total nitrogen and sulfur deposition in recent years. 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 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 sulfur and nitrogen oxides and increased acidifying deposition. Acidifying deposition can contribute to dieback in sugar maple through depletion of cations from soil with low levels of available calcium. Grasslands are likely less sensitive to acidification than forests due to grassland soils being generally rich in base cations.
	A commonly used indicator of terrestrial acidification is the base cation-to-aluminum ratio, Bc/Al. Many locations in sensitive areas of the United States have Bc/Al levels below benchmark levels we have classified as providing low to intermediate levels of protection to tree health. At a Bc/Al ratio of 1.2 (intermediate level of protection), red spruce growth can be reduced by 20 percent. At a Bc/Al ratio of 0.6 (low level of protection), sugar maple growth can be reduced by 20 percent. While not defining whether a 20 percent reduction in growth can be considered significant, existing economic studies suggest that avoiding significant declines in the health of spruce and sugar maple forests may be worth billions of dollars to residents of the Eastern United States.
ii.	Nutrient Enrichment Effects in Terrestrial and Aquatic Ecosystems
	The ISA found that deposition of nitrogen, including oxides of nitrogen and NHx, leads to the nitrogen enrichment of terrestrial, freshwater and estuarine ecosystems (U.S. 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. 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 nitrogen greatly exceed nitrogen deposition. Both oxides of nitrogen and 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.
	The numerous ecosystem types that occur across the United States have a broad range of sensitivity to nitrogen deposition. 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 nitrogen, may result in imbalances in ecosystems, with effects on ecosystem processes, structure and function. In certain nitrogen-limited ecosystems, including many ecosystems managed for commercial production, nitrogen deposition can result in beneficial increases in productivity. Nutrient enrichment effects from deposition of oxides of nitrogen are difficult to disentangle from overall effects of nitrogen enrichment. This is caused by two factors:  the inputs of reduced nitrogen from deposition and, in estuarine ecosystems, a large fraction of nitrogen inputs from non-atmospheric sources.
	The numerous ecosystem types that occur across the United States have a broad range of sensitivity to nitrogen deposition (U.S. EPA, 2008, Table 4-4). Increased deposition to nitrogen-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 nitrogen, may result in an imbalance in ecological stoichiometry, with effects on ecosystem processes, structure and function.
	  With regard to terrestrial ecosystems, the ISA concluded that the evidence is sufficient to infer a causal relationship between nitrogen deposition and the alteration of biogeochemical cycling in terrestrial ecosystems (U.S. EPA, 2008, section <IP_ADDRESS> and <IP_ADDRESS>). Due to the complexity of interactions between the nitrogen and carbon cycling, the effects of nitrogen on carbon budgets (quantified input and output of carbon 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 nitrogen 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 nitrogen deposition on carbon budgets of non-forest ecosystems. The ISA also concludes that the evidence is sufficient to infer a causal relationship between nitrogen deposition on the alteration of species richness, species composition and biodiversity in terrestrial ecosystems (U.S. EPA, 2008, section <IP_ADDRESS>).
	Little is known about the full extent and distribution of the terrestrial ecosystems in the United States that are most sensitive to impacts caused by nutrient enrichment from atmospheric nitrogen deposition. Effects are most likely to occur where areas of relatively high atmospheric N deposition intersect with nitrogen-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 (U.S. 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.
	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 (U.S. EPA, 2008, section 3.3.5). The REA used published research results (U.S. EPA, 2009, section 5.3.1 and U.S. 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 PA. 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 National Atmospheric Deposition Program (NADP) monitoring data. This map, reproduced in Figure 3-5 of the PA, depicts the sites where empirical effects of terrestrial nutrient enrichment have been observed and site proximity to elevated atmospheric nitrogen deposition.
	With regard to freshwater ecosystems, the ISA concluded that the evidence is sufficient to infer a causal relationship between nitrogen deposition and the alteration of biogeochemical cycling in freshwater aquatic ecosystems (U.S. EPA, 2008, section <IP_ADDRESS>). Nitrogen deposition is the main source of nitrogen enrichment to headwater streams, lower order streams and high elevation lakes. The ISA also concludes that the evidence is sufficient to infer a causal relationship between nitrogen deposition and the alteration of species richness, species composition and biodiversity in freshwater aquatic ecosystems (U.S. EPA, 2008, section <IP_ADDRESS>).
	There are many examples of fresh waters that are nitrogen-limited or nitrogen and phosphorous (P) co-limited (U.S. EPA, 2008, section <IP_ADDRESS>). Less is known about the extent and distribution of the terrestrial ecosystems in the United States that are most sensitive to the effects of nutrient enrichment from atmospheric nitrogen deposition compared to acidification. Grasslands in the western United States are typically nitrogen-limited ecosystems dominated by a diverse mix of perennial forbs and grass species. A meta-analysis discussed in the ISA (U.S. EPA, 2008, section 3.3.3), indicated that nitrogen fertilization increased aboveground growth in all non-forest ecosystems except for deserts. Because the productivity of estuarine and near shore marine ecosystems is generally limited by the availability of nitrogen, they are also susceptible to the eutrophication effect of nitrogen deposition (U.S. EPA, 2008, section <IP_ADDRESS>).
	The magnitude of ecosystem response to nutrient enrichment may be thought of on two time scales, current conditions and how ecosystems have been altered since the onset of anthropogenic nitrogen deposition. As noted previously, studies found that nitrogen-limitation occurs as frequently as phosphorous-limitation in freshwater ecosystems (U.S. EPA, 2008, section <IP_ADDRESS>). Recently, a comprehensive study of available data from the northern hemisphere surveys of lakes along gradients of nitrogen deposition show increased inorganic nitrogen concentration and productivity to be correlated with atmospheric nitrogen deposition. The results are unequivocal evidence of nitrogen limitation in lakes with low ambient inputs of nitrogen, and increased nitrogen concentrations in lakes receiving nitrogen solely from atmospheric nitrogen deposition. It has been suggested that most lakes in the northern hemisphere may have originally been nitrogen-limited, and that atmospheric nitrogen deposition has changed the balance of nitrogen and phosphorous in lakes.
	Eutrophication effects from nitrogen 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 United States. The most severe eutrophication from nitrogen 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 United States, 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 (U.S. EPA, 2008, section 3.3.8).
	With regard to estuaries, the ISA concludes that the evidence is sufficient to infer a causal relationship between nitrogen deposition and the biogeochemical cycling of nitrogen and carbon in estuaries (U.S. 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 (U.S. 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 percent (U.S. EPA, 2009, section 5.1.1). The evidence is also sufficient to infer a causal relationship between nitrogen deposition and the alteration of species richness, species composition and biodiversity in estuarine ecosystems (U.S. EPA, 2008, section <IP_ADDRESS> and <IP_ADDRESS>). Atmospheric and non-atmospheric sources of nitrogen contribute to increased phytoplankton and algal productivity, leading to eutrophication. Shifts in community composition, reduced hypolimnetic dissolved oxygen (DO), decreases in biodiversity, and mortality of submerged aquatic vegetation are associated with increased N deposition in estuarine systems.
	 In contrast to terrestrial and freshwater systems, atmospheric nitrogen load to estuaries contributes to the total load but does not necessarily drive the effects since other combined sources of nitrogen often greatly exceed nitrogen deposition. In estuaries, nitrogen-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.
	A recent national assessment of eutrophic conditions in estuaries found that 65 percent of the assessed systems had moderate to high overall eutrophic conditions. 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 (U.S. 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 (U.S. 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 (U.S. 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.
iii.	Effects on Metal Toxicity
	As discussed in the ISA (U.S. EPA, 2008, section 3.4.1 and 4.5), mercury is a highly neurotoxic contaminant that enters the food web as a methylated compound, methylmercury (MeHg). 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 MeHg can be produced in the absence of sulfate. Once MeHg is present, other variables influence how much accumulates in fish, but elevated mercury levels in fish can only occur where substantial amounts of MeHg are present. Current evidence indicates that in watersheds where mercury is present, increased oxides of sulfur deposition very likely results in additional production of MeHg which leads to greater accumulation of MeHg concentrations in fish. With respect to sulfur deposition and mercury methylation, the final ISA determined that "[t]he 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 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 MeHg requires the presence of SO4[2-] and mercury, but the amount of MeHg produced varies with oxygen content, temperature, pH, and supply of labile organic carbon (U.S. 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 (U.S. 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 ISA concluded that the evidence was sufficient to infer a causal relationship between sulfur deposition and increased MeHg production in wetlands and aquatic ecosystems, the REA concluded that there was insufficient evidence to quantify the relationship between sulfur deposition and MeHg production. Therefore, only a qualitative assessment was included in chapter 6 of the REA. As a result, the PA could not reach a conclusion as to the adequacy of the existing SO2 standards in protecting against welfare effects associated with increased mercury methylation.
2.	Overview of 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 analytic approaches used in the REA, a summary of the key findings from the air quality analyses and acidification and nutrient enrichment case studies, and general conclusions regarding other welfare effects are presented below.
a.	Approach to REA Analyses
	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 adverse effects associated with deposition, 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 determine the effects on 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. It also evaluates the contributions of atmospherically deposited nitrogen and sulfur individually relative to the combined atmospheric loadings of both elements together. 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 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 (U.S. 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 MeHg production; nitrous oxide (N2O) effects on climate; nitrogen effects on primary productivity and biogenic greenhouse gas (GHG) 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, Wisconsin) 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 CSS communities in southern California and the mixed conifer forest (MCF) 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.
      For aquatic and terrestrial acidification effects, a similar conceptual approach was used (critical loads) to evaluate the impacts of multiple pollutants on an ecological endpoint, whereas the approaches used for aquatic and terrestrial nutrient enrichment were fundamentally distinct. Although the ecological indicators for aquatic and terrestrial acidification (i.e., ANC and BC/Al) are very different, both ecological indicators are well-correlated with effects such as reduced biodiversity and growth. While aquatic acidification is clearly the targeted effect area with the highest level of confidence, the relationship between atmospheric deposition and an ecological indicator is also quite strong for terrestrial acidification. The main drawback with the understanding of terrestrial acidification is that the data are based on laboratory responses rather than field measurements. Other stressors that are present in the field but that are not present in the laboratory may confound this relationship.
	For nutrient enrichment effects, the REA utilized different types of indicators for aquatic and terrestrial effects to assess both the likelihood of adverse effects to ecosystems and the relationship between adverse effects and atmospheric sources of oxides of nitrogen. The ecological indicator chosen for aquatic nutrient enrichment, the Assessment of Estuarine Trophic Status Eutrophication Index (ASSETS EI), seems to be inadequate to relate atmospheric deposition to the targeted ecological effect, likely due to the many other confounding factors. Further, there is far less confidence associated with the understanding of aquatic nutrient enrichment because of the large contributions from non-atmospheric sources of nitrogen and the influence of both oxidized and reduced forms of nitrogen, particularly in large watersheds and coastal areas. However, a strong relationship exists between atmospheric deposition of nitrogen and ecological effects in high alpine lakes in the Rocky Mountains because atmospheric deposition is the only source of nitrogen to these systems. There is also a strong weight-of-evidence regarding the relationships between ecological effects attributable to terrestrial nitrogen nutrient enrichment; however, ozone and climate change may be confounding factors. In addition, the response for other species or species in other regions of the United States has not been quantified.
b.	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.
i.	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-atmospheric 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 NADP National Trends Network and dry deposition predictions from the 2002 CMAQ model simulation. Some key conclusions from this analysis are:
(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 ammonia (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 MCF Case Study Area, in which sulfur deposition is greatest in winter.
ii.	Aquatic Acidification Case Studies
	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 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/Long-Term Monitoring (TIME/ 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 are still well above pre-acidification (1860) conditions. The 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 μeq/L (+- 15.7 μeq/L); 78 percent of all monitored lakes in the Adirondack Case Study Area have a current risk of Elevated, Severe, or Acute. Of the 78 percent, 31 percent experience episodic acidification, and 18 percent are chronically acidic today.
(1) Based on the steady-state critical load model for the year 2002, 18 percent, 28 percent, 44 percent, and 58 percent of 169 modeled lakes received combined total sulfur and nitrogen deposition that exceeded critical loads corresponding to 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. The ANC levels increased from about 50 μeq/L in the early 1990s to >75 μeq/L until 2002, when ANC levels declined back to 1991 - 1992 levels. Current concentrations are still above pre-acidification (1860) conditions. The 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). Fifty-five percent of all monitored streams in the Shenandoah Case Study Area have a current risk of Elevated, Severe, or Acute. Of the 55 percent, 18 percent experience episodic acidification, and 18 percent are chronically acidic today.
(4) Based on the steady-state critical load model for the year 2002, 52 percent, 72 percent, 85 percent and 93 percent of 60 modeled streams received combined total sulfur and nitrogen deposition that exceeded critical loads corresponding to ANC limits of 0, 20, 50, and 100 μeq/L respectively.
(5) Based on a deposition scenario that maintains current emission levels to 2020 and 2050, the simulation forecast indicates that a large number of streams would still have Elevated to Acute problems with acidity.
iii.	Terrestrial Acidification Case Studies
	The role of terrestrial acidification was examined in the REA using a critical load analysis for sugar maple and red spruce forests in the eastern United States by using the 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 species in North America for impacts of acidification. At a BC/Al ratio of 1.2, red spruce growth can be reduced by 20 percent. Sugar maple growth can be reduced by 20 percent at a BC/Al ratio of 0.6. Key findings of the case study are summarized below.
(1) Case study results suggest that 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 in 2002. The 2002 CMAQ/NADP total nitrogen and sulfur deposition levels exceeded three selected critical loads in 3 percent to 75 percent of all sugar maple plots across 24 states. The three critical loads ranged from 6,008 to 107 eq/ha/yr for the BC/Al ratios of 0.6, 1.2, and 10.0 (increasing levels of tree protection). The 2002 CMAQ/NADP total nitrogen and sulfur deposition levels exceeded three selected critical loads in 3 percent to 36 percent of all red spruce plots across eight states. The three critical loads ranged from 4,278 to 180 eq/ha/yr for the Bc/Al ratios of 0.6, 1.2, and 10.0 (increasing levels of tree protection).
(2) The SMB model assumptions made for base cation weathering (Bcw) and forest soil ANC input parameters are the main sources of uncertainty since these parameters are rarely measured and require researchers to use default values.
(3) The pattern of case study results suggests that nitrogen and sulfur acidifying deposition in the sugar maple and red spruce forest areas studied were similar in magnitude to the critical loads for those areas and both ecosystems are likely to be sensitive to any future changes in the levels of deposition.
iv.	Aquatic Nutrient Enrichment Case Studies
	The role of nitrogen deposition in two main stem rivers feeding their respective estuaries was analyzed in the REA to determine if decreases in deposition could influence the risk of eutrophication as predicted using the ASSETS EI scoring system in tandem with SPARROW (SPAtially Referenced Regression on Watershed Attributes) modeling. This modeling approach provides a transferrable, intermediate-level analysis of the linkages between atmospheric deposition and receiving waters, while providing results on which conclusions could be drawn. A summary of findings follows:
(1) The 2002 CMAQ/NADP results showed that an estimated 40,770,000 kilograms (kg) of total nitrogen was deposited in the Potomac River watershed. The SPARROW modeling predicted that 7,380,000 kg N/yr of the deposited nitrogen reached the estuary (20 percent of the total load to the estuary). The overall ASSETS EI for the Potomac River and Potomac Estuary was Bad (based on all sources of N).
(2) To improve the Potomac River and Potomac Estuary ASSETS EI score from Bad to Poor, a decrease of at least 78 percent in the 2002 total nitrogen atmospheric deposition load to the watershed would be required.
(3) The 2002 CMAQ/NADP results showed that an estimated 18,340,000 kg of total nitrogen was deposited in the Neuse River watershed. The SPARROW modeling predicted that 1,150,000 kg N/yr of the deposited nitrogen reached the estuary (26 percent of the total load to the estuary). The overall ASSETS EI for the Neuse River/Neuse River Estuary was Bad.
(4) It was found that the Neuse River/Neuse River Estuary ASSETS EI score could not be improved from Bad to Poor with decreases only in the 2002 atmospheric deposition load to the watershed. Additional reductions would be required from other nitrogen sources within the watershed.
	The small effect of decreasing atmospheric deposition in the Neuse River watershed is because the other nitrogen sources within the watershed are more influential than atmospheric deposition in affecting the total nitrogen loadings to the Neuse River Estuary, as estimated with the SPARROW model. A water body's response to nutrient loading depends on the magnitude (e.g., agricultural sources have a higher influence in the Neuse than in the Potomac), spatial distribution, and other characteristics of the sources within the watershed; therefore a reduction in nitrogen deposition does not always produce a linear response in reduced load to the estuary, as demonstrated by these two case studies.
v.	Terrestrial Nutrient Enrichment Case Studies
	California CSS and MCF communities were the focus of the Terrestrial Nutrient Enrichment Case Studies of the REA. Geographic information systems  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. 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:
(1) The 2002 CMAQ/NADP nitrogen deposition data show that the 3.3 kg N/ha/yr benchmark has been exceeded in more than 93 percent  of CSS areas (654,048 ha). This suggests that such deposition is a driving force in the degradation of CSS communities. One potentially confounding factor is the role of fire. Although CSS decline has been observed in the absence of fire, the contributions of deposition and fire to the CSS decline require further research. The 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.
(2) The 2002 CMAQ/NADP nitrogen deposition data exceeds the 3.1 kg N/ha/yr benchmark in more than 38percent (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.
c.	Other Welfare Effects
	Ecological effects have also been documented across the United States where elevated nitrogen deposition has been observed, including the eastern slope of the Rocky Mountains where shifts in dominant algal species in alpine lakes have occurred where wet nitrogen deposition was only about 1.5 kg N/ha/yr. High alpine terrestrial communities have a low capacity to sequester nitrogen deposition, and monitored deposition exceeding 3 to 4 kg N/ha/yr could lead to community-level changes in plant species, lichens and mycorrhizae.
	Additional welfare effects are documented, but examined less extensively, in the REA. These effects include qualitative discussions related to visibility and materials damage, such as corrosion, erosion, and soiling of paint and buildings which are being addressed in the PM NAAQS review currently underway. A discussion of the causal relationship between sulfur deposition (as sulfate, SO4[2-]) and increased mercury methylation in wetlands and aquatic environments is also included in the REA. On this subject the REA concludes that decreases in SO4[2-] deposition will likely result in decreases in MeHg concentration; however, spatial and biogeochemical variations nationally hinder establishing large scale dose-response relationships.
	Several additional issues concerning oxides of nitrogen were addressed in the REA. Consideration was also given to N2O, a potent GHG. The REA concluded that it is most appropriate to analyze the role of N2O in the context of all of the GHGs rather than as part of the REA for this review. The REA considered nitrogen deposition and its correlation with the rate of photosynthesis and net primary productivity. Nitrogen addition ranging from 15.4 to 300 kg N/ha/yr is documented as increasing wetland N2O production by an average of 207 percent across all ecosystems. Nitrogen addition ranging from 30 to 240 kg N/ha/yr increased methane (CH4) emissions by 115 percent, averaged across all ecosystems, and methane uptake was reduced by 38 percent averaged across all ecosystems when nitrogen addition ranged from 10 to 560 kg N/ha/yr, but reductions were only significant for coniferous and deciduous forests. The heterogeneity of ecosystems across the United States, however, introduces variations into dose-response relationships.
	The phytotoxic effects of oxides of nitrogen and sulfur on vegetation were also briefly discussed in the REA which concluded that since a unique secondary NAAQS exists for SO2, and concentrations of nitric oxide (NO), NO2 and PAN are rarely high enough to have phytotoxic effects on vegetation, further assessment was not warranted at this time.
3.	Overview of Adversity of Effects to Public Welfare
	Characterizing a known or anticipated adverse effect to public welfare is an important component of developing any secondary NAAQS. According to the CAA, welfare effects include: "effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, weather, visibility, and climate, damage to and deterioration of property, and hazards to transportation, as well as effect on economic values and on personal comfort and well-being, whether caused by transformation, conversion, or combination with other air pollutants" (CAA, Section 302(h)). While the text above lists a number of welfare effects, these effects do not define public welfare in and of themselves.
	Although there is no specific definition of adversity to public welfare, the paradigm of linking adversity to public welfare to disruptions in ecosystem structure and function has been used broadly by the EPA to categorize effects of pollutants from the cellular to the ecosystem level. An evaluation of adversity to public welfare might consider the likelihood, type, magnitude, and spatial scale of the effect as well as the potential for recovery and any uncertainties relating to these considerations.
	Similar concepts were used in past reviews of secondary NAAQS for ozone and PM (relating to visibility), as well as in initial reviews of effects from lead deposition. Because oxides of nitrogen and sulfur are deposited from ambient sources into ecosystems where they affect changes to organisms, populations and ecosystems, the concept of adversity to public welfare as a result of alterations in structure and function of ecosystems is an appropriate consideration for this review.
	Based on information provided in the PA, the following section discusses how ecological effects from deposition of oxides of nitrogen and sulfur relate to adversity to public welfare. In the PA, public welfare was discussed in terms of loss of ecosystem services (defined below), which in some cases can be monetized. Each of the four main effect areas (aquatic and terrestrial acidification and aquatic and terrestrial nutrient over-enrichment) are discussed including current ecological effects and associated ecosystem services.
a.	Ecosystem Services
	The PA defines ecosystem services as the benefits individuals and organizations obtain from ecosystems. Ecosystem services can be classified as provisioning (food and water), regulating (control of climate and disease), cultural (recreational, existence, spiritual, educational), and supporting (nutrient cycling). Conceptually, changes in ecosystem services may be used to aid in characterizing a known or anticipated adverse effect to public welfare. In the REA and PA ecosystem services are discussed as a method of assessing the magnitude and significance to the public of resources affected by ambient concentrations of oxides of nitrogen and sulfur and deposition in sensitive ecosystems.
	The EPA has in previous NAAQS reviews defined ecological goods and services for the purposes of a Regulatory Impact Analysis (RIA) as the "outputs of ecological functions or processes that directly or indirectly contribute to social welfare or have the potential to do so in the future. Some outputs may be bought and sold, but most are not marketed."   It is especially important to acknowledge that it is difficult to measure and/or monetize the goods and services supplied by ecosystems. It can be informative in characterizing adversity to public welfare to attempt to place an economic valuation on the set of goods and services that have been identified with respect to a change in policy however it must be noted that this valuation will be incomplete and illustrative only.
	Knowledge about the relationships linking ambient concentrations and ecosystem services is considered in the PA as one method by which to inform a policy judgment on a known or anticipated adverse public welfare effect. For example, a change in an ecosystem structure and process, such as foliar injury, would be classified as an ecological effect, with the associated changes in ecosystem services, such as primary productivity, food availability, forest products, and aesthetics (e.g., scenic viewing), classified as public welfare effects. Additionally, changes in biodiversity would be classified as an ecological effect, and the associated changes in ecosystem services -- productivity, existence (nonuse) value, recreational viewing and aesthetics -- would also be classified as public welfare effects.
	As described in chapters 4 and 5 of the REA, case study analyses were performed that link deposition in sensitive ecosystems to changes in a given ecological indicator (e.g., for aquatic acidification, to changes in ANC) and then to changes in ecosystems. Appendix 8 of the REA links the changes in ecosystems to the services they provide (e.g., fish species richness and its influence on recreational fishing). To the extent possible for each targeted effect area, the REA linked ambient concentrations of nitrogen and sulfur (i.e., ambient air quality indicators) to deposition in sensitive ecosystems (i.e., exposure pathways), and then to system response as measured by a given ecological indicator (e.g., lake and stream acidification as measured by ANC). The ecological effect (e.g., changes in fish species richness) was then, where possible, associated with changes in ecosystem services and the corresponding public welfare effects (e.g., recreational fishing).
b.	Effects on Ecosystem Services
	The process used to link ecological indicators to ecosystem services is discussed extensively in appendix 8 of the REA. In brief, for each case study area assessed, the ecological indicators are linked to an ecological response that is subsequently linked to associated services to the extent possible. For example, in the case study for aquatic acidification the chosen ecological indicator is ANC which can be linked to the ecosystem service of recreational fishing. Although recreational fishing losses are the only service effects that can be independently quantified or monetized at this time, there are numerous other ecosystem services that may be related to the ecological effects of acidification.
	While aquatic acidification is the focus of this proposed standard, the other effect areas were also analyzed in the REA and these ecosystems are being harmed by nitrogen and sulfur deposition and will obtain some measure of protection with any decrease in that deposition regardless of the reason for the decrease. The following summarizes the current levels of specific ecosystem services for aquatic and terrestrial acidification and aquatic and terrestrial nutrient over-enrichment and attempts to quantify and when possible monetize the harm to public welfare, as represented by ecosystem services, due to nitrogen and sulfur deposition.
i.	Aquatic Acidification
	Acidification of aquatic ecosystems primarily affects the ecosystem services that are derived from the fish and other aquatic life found in surface waters. In the northeastern United States, the surface waters affected by acidification are not a major source of commercially raised or caught fish; however, they are a source of food for some recreational and subsistence fishers and for other consumers. Although data and models are available for examining the effects on recreational fishing, relatively little data are available for measuring the effects on subsistence and other consumers. Inland waters also provide aesthetic and educational services along with non-use services, such as existence value (protection and preservation with no expectation of direct use). In general, inland surface waters such as lakes, rivers, and streams also provide a number of regulating services, playing a role in hydrological regimes and climate regulation. There is little evidence that acidification of freshwaters in the northeastern United States has significantly degraded these specific services; however, freshwater ecosystems also provide biological control services by providing environments that sustain delicate aquatic food chains. The toxic effects of acidification on fish and other aquatic life impair these services by disrupting the trophic structure of surface waters. Although it is difficult to quantify these services and how they are affected by acidification, it is worth noting that some of these services may be captured through measures of provisioning and cultural services. For example, these biological control services may serve as "intermediate" inputs that support the production of "final" recreational fishing and other cultural services.
	As summarized in Chapter 4 of the PA, recent studies indicate that acidification of lakes and streams can result in significant loss in economic value. Embedded in these numbers is a degree of harm to recreational fishing services due to acidification that has occurred over time. These harms have not been quantified on a regional scale; however, a case study was conducted in the Adirondacks area (U.S. EPA, 2011, section 4.4.2).
	In the Adirondacks case study, estimates of changes in recreational fishing services were determined, as well as changes more broadly in "cultural" ecosystem services (including recreational, aesthetic, and nonuse services). First, the MAGIC model (U.S. EPA, 2009, Appendix 8 and section 2.2)  was applied to 44 lakes to predict what ANC levels would be under both "business as usual" conditions (i.e., allowing for some decline in deposition due to existing regulations) and pre-emission (i.e., background) conditions. Second, to estimate the recreational fishing impacts of aquatic acidification in these lakes, an existing model of recreational fishing demand and site choice was applied. This model predicts how recreational fishing patterns in the Adirondacks would differ and how much higher the average annual value of recreational fishing services would be for New York residents if lake ANC levels corresponded to background (rather than business as usual) conditions. To estimate impacts on a broader category of cultural (and some provisioning)  ecosystem services, results from the Banzhaf et al (2006) valuation survey of New York residents were adapted and applied to this context. The survey used a contingent valuation approach to estimate the average annual household willingness to pay (WTP) for future reductions in the percent of Adirondack lakes impaired by acidification. The focus of the survey was on impacts on aquatic resources. Pretesting of the survey indicated that respondents nonetheless tended to assume that benefits would occur in the condition of birds and forests as well as in recreational fishing.
	L. The REA estimated 44 percent of the Adirondack lakes currently fall below an ANC of 50 ueq/L. Several states have set goals for improving the acid status of lakes and streams, generally targeting ANC in the range of 50 to 60 ueq/L, and have engaged in costly activities to decrease acidification.
 	These results imply significant value to the public in addition to those derived from recreational fishing services. Note that the results are only applicable to improvements in the Adirondacks valued by residents of New York. If similar benefits exist in other acid-impacted areas, benefits for the nation as a whole could be substantial. The analysis provides results on only a subset of the impacts of acidification on ecosystem services and suggests that the overall impact on these services is likely to be substantial.
ii.	Terrestrial Acidification
	Chapters 4.4.3 and 4.4.4 of the PA review several economic studies of areas sensitive to terrestrial acidification. Forests in the northeastern United States provide several important and valuable provisioning ecosystem services, which are reflected in the production and sales of tree products. Sugar maples are a particularly important commercial hardwood tree species in the United States, producing timber and maple syrup that provide hundreds of millions of dollars in economic value annually. Red spruce is also used in a variety of wood products and provides up to $100 million in economic value annually. Although the data do not exist to directly link acidification damages to economic values of lost recreational ecosystem services in forests, these resources are valuable to the public. The EPA is not able to quantify at this time the specific effects on these values of acid deposition, or of any specific reductions in deposition, relative to the effects of many other factors that may affect them.
iii.	Nutrient Enrichment
	Chapters 4.4.5 and 4.4.6 of the PA summarize economic studies of east coast estuaries affected by nutrient over-enrichment or eutrophication. Estuaries in the eastern United States are important for fish and shellfish production. The estuaries are capable of supporting large stocks of resident commercial species, and they serve as the breeding grounds and interim habitat for several migratory species. To provide an indication of the magnitude of provisioning services associated with coastal fisheries, from 2005 to 2007, the average value of total catch was $1.5 billion per year in 15 East Coast states. Estuaries also provide an important and substantial variety of cultural ecosystem services, including water-based recreational and aesthetic services. For example, data indicate that 4.8 percent of the population in coastal states from North Carolina to Massachusetts participated in saltwater fishing, with a total of 26 million saltwater fishing days in 2006. Recreational participation estimates for 1999 - 2000 showed almost 6 million individuals participated in motor boating in coastal states from North Carolina to Massachusetts. The EPA is not able to quantify at this time the specific effects on these values of nitrogen deposition, or of any specific reductions in deposition, relative to the effects of many other factors that may affect them.
	Terrestrial ecosystems can also suffer from nutrient over-enrichment. Each ecosystem is different in its composition of species and nutrient requirements. Changes to individual ecosystems from changes in nitrogen deposition can be hard to assess economically. Relative recreational values are often determined by public use information. Chapter 4.4.7 of the PA reviewed studies related to park use in California. Data from California State Parks indicate that in 2002, 68.7 percent of adult residents participated in trail hiking for an average of 24.1 days per year. The analyses in the PA indicate that the aggregate annual benefit for California residents from trail hiking in 2007 was $11.59 billion. The EPA is not able to quantify at this time the specific effects on these values of nitrogen deposition, or of any specific reductions in deposition, relative to the effects of many other factors that may affect them.
	The PA also identified fire regulation as a service that could be affected by nutrient over-enrichment of the CSS and MCF ecosystems by encouraging growth of more flammable grasses, increasing fuel loads, and altering the fire cycle. Over the 5-year period from 2004 to 2008, Southern California experienced, on average, over 4,000 fires per year, burning, on average, over 400,000 acres per year. It is not possible at this time to quantify the contribution of nitrogen deposition, among many other factors, to increased fire risk.
c.	Summary
	Adversity to public welfare can be understood by looking at how deposition of oxides of nitrogen and sulfur affect the ecological functions of an ecosystem (see II.A.), and then understanding the ecosystem services that are degraded. The monetized value of the ecosystem services provided by ecosystems that are sensitive to deposition of oxides of nitrogen and sulfur are in the billions of dollars each year, though it is not possible to quantify or monetize at this time the effects on these values of nitrogen and sulfur deposition or of any changes in deposition that may result from new secondary standards. Many lakes and streams are known to be degraded by acidic deposition which affects recreational fishing and tourism. Forest growth is likely suffering from acidic deposition in sensitive areas affecting red spruce and sugar maple timber production, sugar maple syrup production, hiking, aesthetic enjoyment and tourism. Nitrogen deposition contributes significantly to eutrophication in many estuaries affecting fish production, swimming, boating, aesthetic enjoyment and tourism. Ecosystem services are likely affected by nutrient enrichment in many natural and scenic terrestrial areas, affecting biodiversity, including habitat for rare and endangered species, fire control, hiking, aesthetic enjoyment and tourism.
B.	Adequacy of the Current Standards
	An important issue to be addressed in this review of the secondary standards for oxides of nitrogen and sulfur is whether, in view of the scientific evidence reflected in the ISA, additional information on exposure and risk discussed in the REA, and conclusions drawn from the PA, the current standards provide adequate protection of public welfare. In this review, consideration is given to the adequacy of the current standards with regard to both the direct effects of exposure to gaseous oxides of nitrogen and sulfur on vegetation and on potentially adverse deposition-related effects on sensitive aquatic and terrestrial ecosystems. This section is drawn from section II.D of the proposal. The following discussion summarizes the considerations related to the adequacy of the standards as discussed in the PA (section II.B.1), CASAC's views on adequacy (section II.B.2), and the Administrator's proposed conclusions on the adequacy of the current standards
1.	Adequacy Considerations
	This discussion is based on the information presented in the PA and includes considerations related to the adequacy of the current NO2 and SO2 secondary standards with regard to direct effects (section II.B.1.a), as well as considerations related to both the appropriateness and the adequacy of protection of the current standards with regard to deposition-related effects (section II.B.1.b).
a.	Adequacy of the Current Standards for Direct Effects
      For oxides of nitrogen, the current secondary standard was set identical to the primary standard, i.e., an annual standard set for NO2 to protect against adverse effects on vegetation from direct exposure to ambient oxides of nitrogen. For oxides of sulfur, the current secondary standard is a 3-hour standard intended to provide protection for plants from the direct foliar damage associated with atmospheric concentrations of SO2. In considering the adequacy of these standards, it is appropriate to consider whether they are adequate to protect against the direct effects on vegetation resulting from exposure to ambient oxides of nitrogen and sulfur, which was the basis for initially setting the standards in 1971. The ISA concludes that there was sufficient evidence to infer a causal relationship between exposure to SO2, NO, NO2 and PAN and injury to vegetation. Additional research on acute foliar injury has been limited and there is no evidence to suggest foliar injury below the levels of the current secondary standards. Based on information in the ISA, the PA concludes that there is sufficient evidence to suggest that the levels of the current standards are likely adequate to protect against phytotoxic effects caused by direct gas-phase exposure.
b.	Appropriateness and Adequacy of the Current Standards for Deposition-related Effects
	This section addresses two concepts necessary to evaluate the current standards in the context of deposition-related effects. First, appropriateness of the current standards is considered with regard to indicator, form, level and averaging time. This discussion includes particular emphasis on the indicators and forms of the current standards and the degree to which they are ecologically relevant with regard to deposition-related effects that vary spatially and temporally. Second, this section considers the current standards in terms of adequacy of protection.
i.	Appropriateness
	The ISA has established that the major effects of concern for this review are associated with deposition of nitrogen and sulfur caused by atmospheric concentrations of oxides of nitrogen and sulfur. As discussed below, the current standards are not directed toward depositional effects, and none of the elements of the current NAAQS  -  indicator, form, averaging time, and level  -  are suited for addressing the effects of nitrogen and sulfur deposition.
	Four issues arise that call into question the ecological relevance of the structure of the current secondary standards for oxides of nitrogen and sulfur.
(1) The current SO2 secondary standard (0.5 ppm SO2 over a 3-hour average) does not utilize an averaging time that relates to an exposure period that is relevant for ecosystem impacts. The majority of deposition-related impacts are associated with depositional loads that occur over periods of months to years. This differs significantly from exposures associated with hourly concentrations of SO2 as measured by the current secondary standard. By addressing short-term concentrations, the current SO2 secondary standard, while protective against direct foliar effects from gaseous oxides of sulfur, does not take into account the findings of effects in the ISA, which notes the relationship between annual deposition of sulfur and acidification effects which are likely to be more severe and widespread than phytotoxic effects under current ambient conditions, and include effects from long-term and short-term deposition. Acidification is a process that occurs over time because the ability of an aquatic system to counteract acidic inputs is reduced as natural buffers are used more rapidly than they can be replaced through geologic weathering. The relevant period of exposure for ecosystems is, therefore, not the exposures captured in the short averaging time of the current SO2 secondary standard. The current secondary standard for oxides of nitrogen is an annual standard (0.053 ppm averaged over 1 year) and as such the averaging time of the standard is more ecologically relevant.
(2)  Current standards do not utilize appropriate atmospheric indicators. Nitrogen dioxide and SO2 are used as the species of oxides of nitrogen and sulfur that are measured to determine compliance with the standards, but they do not capture all relevant chemical species of oxides of nitrogen and sulfur that contribute to deposition-related effects. The ISA provides evidence that deposition-related effects are associated with total nitrogen and total sulfur deposition, and thus all chemical species of oxidized nitrogen and oxidized sulfur that are deposited will contribute to effects on ecosystems. Thus, by using atmospheric NO2 and SO2 concentrations as indicators, the current standards address only a fraction of total atmospheric oxides of nitrogen and sulfur, and do not take into account the effects from deposition of total atmospheric oxides of nitrogen and sulfur. This suggests that more comprehensive atmospheric indicators should be considered in designing ecologically relevant standards.
(3) Current standards reflect separate assessments of the two individual pollutants, NO2 and SO2, rather than assessing the joint impacts of deposition of nitrogen and sulfur to ecosystems. Recognizing the role that each pollutant plays in jointly affecting ecosystem indicators, functions, and services is vital to developing a meaningful standard. The clearest example of this interaction is in assessment of the impacts of acidifying deposition on aquatic ecosystems. Acidification in an aquatic ecosystem depends on the total acidifying potential of the nitrogen and sulfur deposition resulting from oxides of nitrogen and sulfur as well as the inputs from other sources of nitrogen and sulfur such as reduced nitrogen and non-atmospheric sources. It is the joint impact of the two pollutants that determines the ultimate effect on organisms within the ecosystem, and critical ecosystem functions such as habitat provision and biodiversity. Standards that are set independently are less able to account for the contribution of the other pollutant. This suggests that interactions between oxides of nitrogen and oxides of sulfur should be a critical element of the conceptual framework for ecologically relevant standards. There are also important interactions between oxides of nitrogen and sulfur and reduced forms of nitrogen, which also contribute to acidification and nutrient enrichment. It is important that the structure of the standards address the role of reduced nitrogen in determining the ecological effects resulting from deposition of atmospheric oxides of nitrogen and sulfur. Consideration will also have to be given to total loadings as ecosystems respond to all sources of nitrogen and sulfur.
(4) Current standards do not take into account variability in ecosystem sensitivity. Ecosystems are not uniformly distributed either spatially or temporally in their sensitivity to oxides of nitrogen and sulfur. Therefore, failure to account for the major determinants of variability, including geological and soil characteristics related to the sensitivity to acidification or nutrient enrichment, as well as atmospheric and landscape characteristics that govern rates of deposition, may lead to standards that do not provide requisite levels of protection across ecosystems. The current structures of the standards do not address the complexities in the responses of ecosystems to deposition of oxides of nitrogen and sulfur. Ecosystems contain complex groupings of organisms that respond in various ways to the alterations of soil and water that result from deposition of nitrogen and sulfur compounds. Different ecosystems therefore respond depending on a multitude of factors that control how deposition is integrated into the system. For example, the same levels of deposition falling on limestone dominated soils have a very different effect from those falling on shallow glaciated soils underlain with granite. One system may over time display no obvious detriment while the other may experience a catastrophic loss in fish communities. This degree of sensitivity is a function of many atmospheric factors that control rates of deposition as well as ecological factors that control how an ecosystem responds to that deposition. The current standards do not take into account spatial and seasonal variations, not only in depositional loadings, but also in sensitivity of ecosystems exposed to those loadings. Based on the discussion summarized above, the PA concludes that the current secondary standards for oxides of nitrogen and oxides of sulfur are not ecologically relevant in terms of averaging time, form, level or indicator.
ii.	Adequacy of Protection
	As described in the PA, ambient conditions in 2005 indicate that the current SO2 and NO2 secondary standards were not exceeded at that time (U.S. EPA, 2011, Figures 6-1 and 6-2) in locations where negative ecological effects have been observed. In many locations, SO2 and NO2 concentrations are substantially below the levels of the secondary standards. This pattern suggests that levels of deposition and any negative effects on ecosystems due to deposition of oxides of nitrogen and sulfur under recent conditions are occurring even though areas meet or are below current standards. In addition, based on conclusions in the REA, these levels will not decline in the future to levels below which it is reasonable to anticipate effects.
	 In determining the adequacy of the current secondary standards for oxides of nitrogen and sulfur the PA considered the extent to which ambient deposition contributes to loadings in ecosystems. Since the last review of the secondary standard for oxides of nitrogen, a great deal of information on the contribution of atmospheric deposition associated with ambient oxides of nitrogen has become available. The REA presents a thorough assessment of the contribution of oxidized nitrogen relative to total nitrogen deposition throughout the United States, and the relative contributions of ambient oxidized and reduced forms of nitrogen. The REA concludes that based on that analysis, ambient oxides of nitrogen are a significant component of atmospheric nitrogen deposition, even in areas with relatively high rates of reduced nitrogen deposition. In addition, atmospheric deposition of oxidized nitrogen contributes significantly to total nitrogen loadings in nitrogen sensitive ecosystems.
	The ISA summarizes the available studies of relative nitrogen contribution and finds that in much of the United States, oxides of nitrogen contribute from 50 to 75 percent of total atmospheric deposition relative to total reactive nitrogen, which includes oxidized and reduced nitrogen species (U.S. EPA, 2008, section 2.8.4). Although the proportion of total nitrogen loadings associated with atmospheric deposition of nitrogen varies across locations, the ISA indicates that atmospheric nitrogen deposition is the main source of new anthropogenic nitrogen to most headwater streams, high elevation lakes, and low-order streams. Atmospheric nitrogen deposition contributes to the total nitrogen load in terrestrial, wetland, freshwater and estuarine ecosystems that receive nitrogen through multiple pathways. In several large estuarine systems, including the Chesapeake Bay, atmospheric deposition accounts for between 10 and 40 percent of total nitrogen loadings (U.S. EPA, 2008).
	Atmospheric concentrations of oxides of sulfur account for nearly all sulfur deposition in the US. For the period 2004-2006, mean sulfur deposition in the United States was greatest east of the Mississippi River with the highest deposition amount, 21.3 kg S/ha-yr, in the Ohio River Valley where most recording stations reported 3-year averages >10 kg S/ha-yr. Numerous other stations in the East reported S deposition >5 kg S/ha-yr. Total sulfur deposition in the United States west of the 100th meridian was relatively low, with all recording stations reporting <2 kg S/ha-yr and many reporting <1 kg S/ha-yr. Sulfur was primarily deposited in the form of wet SO4 [2−] followed in decreasing order by a smaller proportion of dry SO2 and a much smaller proportion of deposition as dry SO4[2−].