# Draft Scope and Methods Plan for Risk/Exposure Assessment: Secondary NAAQS Review for Oxides Of Nitrogen and Oxides of Sulfur[This page intentionally left blank.]EPA-452/D-08-002 March 2008



## Draft Scope and Methods Plan for Risk/Exposure Assessment: Secondary NAAQS Review for Oxides Of Nitrogen and Oxides of Sulfur

Office of Air Quality Planning and Standards US Environmental Protection Agency Research Triangle Park, North Carolina 27711Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur



### Disclaimer
This Draft Scope and Methods Plan for the review of the Secondary National Ambient Air Quality Standards for oxides of nitrogen and oxides of sulfur has been prepared by the Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency (EPA). Any opinions, findings, conclusions , or recommendations are thos e of the authors and do not necessarily reflect the views of EPA. This document is being ci rculated to obtain review and
comment from the Clean Air Scientific Advi sory Committee (CASAC) and the general public.
Comments on this document should be addresse d to Dr. Anne W. Rea, U.S. Environmental Protection Agency, Office of Air Quality Planni ng and Standards, C539-02, Research Triangle Park, North Carolina 27711 (email: rea.anne@epa.gov).

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### Table of Contents

1. Context and Introduction .................................................................................................... ... 1 1.1 Context for this Scope and Methods Plan..................................................................... 1 1.2 Introduction.............................................................................................................. ..... 2 2. Background and Overview .................................................................................................... 5 2.1 Legislative Requirements.............................................................................................. 5 2.2 Background on Previous NO 2 and SO 2 Secondary NAAQS Reviews ......................... 6 2.3 Overview of Ecological Risk Assess ment and the Scope of the Secondary NAAQS Review for NO x and SO x ............................................................................... 7 2.3.1 Overview of Risk Assessment Framework for Deposition-related Ecological Effects ............................................................................................. 9 2.3.2 Overview of Nitrogen Deposition................................................................... 11 2.3.3 Overview of Sulfur Oxides and Sulfur Deposition......................................... 13 2.3.4 Targeted Effects for This Risk/Exposure Assessment.................................... 14 2.4 Key Policy Relevant Questions .................................................................................. 16 2.5 Proposed Schedule...................................................................................................... 19 3. Analysis Plan ....................................................................................................................... 21 3.1 Step 1 – Plan for assessment using documented effects; biological, chemical, and ecological indicators, and potential ecosystem services...................................... 26 3.1.1 Documented Effects and Relevant Indicators of Effects for Each Ecosystem, Including Function and Service................................................... 26 3.1.2 Potential Ecosystem Services ........................................................................ 30 3.2 Step 2 – Define sensitive areas that exhibit effects using research findings and GIS mapping............................................................................................................... 31 3.2.1 Defining Sensitive Ecosystems....................................................................... 31 3.2.2 GIS Mapping................................................................................................... 32 3.3 Step 3 – Select Risk/Exposure Case Study Assessment Area within a Sensitive Area ............................................................................................................................ 35 3.4 Step 4 – Evaluate Current Loads and Effects to Case Study Assessment Area Including Ecosystem Services .................................................................................... 45 3.4.1 Assess Data Availability and Adequacy......................................................... 45 3.4.2 Compute Loading and Exposure fo r Each Ecosystem Effect:........................ 46 3.5 Step 5 – Where Feasible, Scale Up Case Study Assessment Area Findings to Sensitive Areas ........................................................................................................... 51 3.6 Step 6 – Assess the Current Ecological Conditions for Those Sensitive Areas ......... 52 3.6.1 Calculate Response Curves for Each Indicator............................................... 52 3.6.2 Calculate Desired Exposure Endpoint ............................................................ 52 3.6.3 Develop GIS Maps and Overlay Loading and Indicators That Are Representative of Harmful Effects.................................................................. 52 3.6.4 Using GIS Mapping, Compute the Ex tent That Loading Is Greater Than or Less Than the Harmful Effect Level (a.k.a. endpoint)...................... 52 3.6.5 Assess Ecosystem Responses via Ecosystem Services and Valuation ........... 52 3.7 Step 7 – Assess Alternative Levels of Protection Under Different Scenarios of Deposition From Am bient Sources ............................................................................ 55

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4. References............................................................................................................................ 61 Appendix A Identification of Sensitive Ecosystems ................................................................. A-1 Appendix B Other Indicators – Nitrous Oxide .......................................................................... B-1 Appendix C CMAQ Modeling................................................................................................... C-1 Appendix D Multimedia Modeling............................................................................................ D-1 Appendix E Valuation of Ecosystem Services ...........................................................................E-1



### List of Figures
2-1. Risk assessment framework for deposition-related ecological risks. ............................... 10 2-2. Schematic diagram of the cycle of reactive, oxidized nitrogen species in the atmosphere. .................................................................................................................... ... 12 2-3. Percent of Total U.S. Emissions of greenhouse gases in CO 2 equivalents....................... 13 2-4. Transformation of sulfur compounds in the atmosphere. ................................................. 14 2-5. General flow of processes addres sed in the risk/exposure assessment for NO x and SO x secondary standards. ................................................................................................. 16 3-1. Seven step approach to planning a nd implementing risk/exposure assessment. .............. 25 3-2. Ecosystem effects may range from near-field to far-field linkages.................................. 31 3-3. Documented biological, biogeoche mical, and physiographic linkages............................ 33 3-4. Comparison of NAPAP-documented aci d-sensitive ecoregions to CMAQ-moded nitrogen deposition............................................................................................................ 47 3-5. Risk/policy assessment paradigm fo r deposition-related ecological risks........................ 56



### List of Tables
2-1. Proposed Schedule for the Joint NO 2 and SO 2 Secondary NAAQS Review * .................. 19 3-1. Summary of Indicators Categorized by Effect.................................................................. 27 3-2. Key Indicators of Acidification Due to NO x and SO x ...................................................... 28 3-3. Key Indicators of Nutrient Enrichment Due to NO x and SO x .......................................... 29 3-4. Example Datasets Planned for GIS Analysis.................................................................... 34 3-5. Summary of Indicators , Mapping Layers, and Models for Targeted Ecosystems............ 36 3-6. SAB/EES Listing of Potential Assessme nt Areas for Evaluation of Benefits of Reductions in Atmospheric Deposition ............................................................................ 39 3-7. Potential Assessment Areas Identified in the Draft ISA .................................................. 41

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### Key Terms

Acidification: The process of increasing th e acidity of a system (e.g., la ke, stream, forest soil). Atmospheric deposition of aci dic or acidifying compounds can acidify lakes, streams, and forest soils. Adverse Effect: The response or component of an ecosy stem that is deemed harmful in its function. Air Quality Indicator: The substance or set of substances (e.g., PM 2.5 , NO 2 , SO 2 ) occurring in the ambient air for which the National Ambient Air Quality Standards set a standard level and monitoring occurs. Alpine: The biogeographic zone made up of slopes above the tree line, characterized by the presence of rosette-forming herbaceous plants and low, shrubby, slow-growing woody plants. Acid Neutralizing Capacity: A key indicator of the ability of water to neutralize the acid or acidifying inputs it receives. This ability de pends largely on associated biogeophysical characteristics. Arid Region: A land region of low rainfall, where “low ” is widely accepted to be less than 250 mm precipitation per year. Assessment Endpoint: An ecological entity and its attributes impacts to which are considered welfare effects, as defined in Clean Air Act Section 302(h), and that are analyzed in the assessment. Base Cation Saturation: The degree to which soil cation excha nge sites are occupied with base cations (e.g., Ca 2+ , Mg 2+ , K + ) as opposed to Al 3+ and H + . Base cation saturation is a measure of soil acidification, w ith lower values being more acidic. There is a threshold whereby soils with base saturations less than 20% (especiall y between 10–20%) are extremely sensitive to change. Biologically Relevant Indicator: A physical, chemical, or biological entity/feature that demonstrates a consistent degree of respons e to a given level of stressor exposure and that is easily measured/quantified to ma ke it a useful predictor of biological, environmental, or ecological risk. Buffering Capacity: The ability of a body of water and it s watershed to ne utralize introduced acid. Critical Load: A quantitative estimate of the level of e xposure to one or more pollutants, below which significant harmful effects on specific sensitive elements of the environment do not occur according to present knowledge. Denitrification: The anaerobic reduction of oxidized nitroge n (e.g., nitrate or ni trite) to gaseous nitrogen (e.g., N 2 O or N 2 ) by denitrifying bacteria. Dry Deposition: The removal of gases and particles from the atmosphere to surfaces in the absence of precipitation (rain, snow) or occult deposition.

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Ecological Dose: The concentration of a toxicant that inhibits a microbe-mediated ecological process by designated percentage; for example, ED50 inhibits 50%. Ecological Exposure: The exposure of a nonhuman organism to an environmental stressor. Ecological Risk: The likelihood that advers e ecological effects may occur or are occurring as a result of exposure to one or mo re stressors (U.S. EPA, 1992a). Ecological Risk Assessment: A process that evaluates the likelihood that adverse ecological effects may occur or are occurr ing as a result of exposure to one or more stressors (U.S. EPA, 1992a). Ecosystem: The dynamic, complex interaction of plan ts, animals, and microorganisms and the nonliving environment. Ecosystem Benefit: The value, expressed qualitatively, quan titatively, and/or in economic terms, where possible, associated with changes in eco system services that result either directly or indirectly in improved human health and/or welfare. Examples of ecosystem benefits that derive from improved air quality incl ude improvements in habitats for sport fish species, the quality of drinking water and recreational areas, and the visual quality of scenic views. Ecosystem Function: The processes and interactions that operate within an ecosystem. Ecosystem Services: The ecological processes or functions having monetary or nonmonetary value to individuals or soci ety at large. There are (i) supporting services, such as productivity or biodiversity maintenance; (ii) provisioning services, such as food, fiber, or fish; (iii) regulating services, such as climate regulation or carbon sequestration; and (iv) cultural services, such as tourism or spiritual and aesthetic appreciation. Elasticity: The percentage of change in the response variable for a 1% change in the input physical or meteorologi cal characteristic. Eutrophication: The process by which nitrogen additions stimulate the growth of autotrophic biota, usually resulting in the depletion of dissolved oxygen. Greenhouse Gas: Those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of infrared radiation emitted by the earth’s surface, the atmosphere, and clouds. This property causes the greenhouse effect. H 2 O vapor, CO 2 , N 2 O, CH 4 , and O 3 are the primary greenhouse gases in the earth’s atmosphere. As well as CO 2 , N 2 O, and CH 4 , the Kyoto Protocol deals with SF 6 , hydrofluorocarbons, and perfluorocarbons. Nitrogen Enrichment: The process by which a te rrestrial system becomes enriched by nutrient additions to a degree that stimulates the growth of plant or other terre strial biota, usually resulting in an increase in productivity. Nitrogen Saturation: The condition when nitrogen input s from atmospheric deposition and other sources exceed the biological requirements of the ecosystem. Occult Deposition: The removal of gases and particles fr om the atmosphere to surfaces by fog or mist.

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Semiarid Regions: Those regions of moderately low ra infall, typically 25 to 50 centimeters (10 to 20 inches) of rainfall per year, where th e natural vegetation is usually short grasses and shrubs and where the predominan t land use may be as rangelands. Sensitivity: The degree to which a system is affecte d, either adversely or beneficially, by SO x or NO x pollution (e.g., from acidification or nitrog en nutrient enrichment). The impacts to natural environmental systems can be reflected in direct changes in growth or survival rates for individual species or changes at a community level reflected in shifts in measures such as species diversity. Target Load: A policy-based metric that takes into c onsideration such factors as economic costs and time frame for emissions reduction. This can be lower than the crit ical load if a very sensitive area is to be protected in the s hort term, especially if deposition rates exceed critical loads. Total Reactive Nitrogen: This includes all biologically, ch emically, and radiatively active nitrogen compounds in the atmos phere and biosphere, such as NH 3 , NH 4 + , NO, NO 2 , HNO 3 , N 2 O, NO 3 – , and organic compounds (e.g., urea, amines, nucleic acids). Valuation: The economic or noneconomi c process of determining either the value of maintaining a given ecosystem type, state, or condition or the value of a change in an ecosystem, its components, or the services it provides. Vulnerability: The degree to which a system is suscep tible to, and unable to cope with, the adverse effects of NO x and/or SO x air pollution. Vulnerability is a function of the exposed and its sensitivity. Welfare Effects: The effects on soils; water; crops; vegetation; man-ma de materials; animals; wildlife; weather; visibility; and climate as well as damage to, and deterioration of, property; hazards to transpor tation; and the effects on ec onomic values and on personal comfort and well-being, whether caused by tr ansformation, conversion, or combination with other air pollu tants (Clean Air Act Section 302[h]). Wet Deposition: The removal of gases and particles from the atmosphere to surfaces by rain or other precipitation.

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1. CONTEXT AND INTRODUCTION 1.1 Context for This Scope and Methods Plan
The U.S. Environmental Protection Agency (E PA) is currently conduc ting a joint review of the existing secondary (welfare-based) Nation al Ambient Air Quality Standards (NAAQS) for nitrogen dioxide (NO 2 ) and sulfur dioxide (SO 2 ). We recognize that this is the first time that we have conducted a joint, multi-pollutant review of a secondary standard separate from the review of the primary standard. As disc ussed in the Integrated Review Plan (U.S. EPA, 2007a), this was done in recognition of the important linkages be tween ambient nitrogen and sulfur leading to deposition of ambient particles th at can have significant impacts on the environment. We further recognize that a fully comprehensive assessment of such linkages and impacts is very complex and will extend beyond the time available in this review, as constrained by our court-ordered schedule. Thus, this Scope and Methods Plan is more narrowly focused on key aspects of the evolving scientific understanding to provide timely results to meet our court-ordered schedule. Our plan for the current review is to focus on the identification of sensitive ecosystems, the predominant linkages between ambient levels of n itrogen and sulfur, and the levels of deposition that create adverse effects in those ecosystems , building directly from the key findings of our Integrated Science Assessment (ISA). To the de gree possible, our risk and exposure assessment will attempt to evaluate whether ecosystem dama ge is occurring in specific ecosystems under current ambient concentrations, and, if so, what a lternative levels of ambi ent nitrogen and sulfur might be expected to allow vari ous degrees of recovery of impacted systems and prevention of further damage.
In particular, given the data and time constrai nts of our current review, we plan to base
our overall assessment on a small number of local or regional case studies where adequate data are available to enable the quantification of some of the more important linkages, focusing on the impacts of acidification and enrich ment for both terrestrial and aqua tic systems. Along with these case studies, we plan to conduct statistical and spatial characterizations of existing national-scale databases on air quality, nitrogen and sulfur deposition, and ecosystem characteristics to help place the results of these local or regional cas e studies in a broader spatial context. The
combination of these case studies and the nation al characterizations thus forms the body of our planned overall assessment. Additi onal case studies and more comp rehensive investigations of a

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broader set of effects, linkages, and indicators may be identifie d during this process that might reasonably form the basis for further assessment in the next 5-year review cycle, when there will be a more robust database on which to develop a more comprehensive un derstanding of these relationships.

1.2 Introduction
The reviews of the primary NAAQS for NO 2 and SO 2 are addressed in separate plans released during the winter of 2006–2007. The revised, secondary NAAQS review process contains four major components: an integrated review plan, a sc ience assessment, a risk/exposure assessment, and a policy assessment/rulemaking. This Scope and Methods Plan is the first phase of the risk/exposure assessment; it will describe th e scope of the analyses to be performed and
the tools and methods that will be used for the joint review of the secondary NAAQS for these pollutants. In this plan, the terms NO 2 and oxides of nitrogen (NO x ) and SO 2 and oxides of sulfur (SO x ) are not interchangeable. The terms NO x and SO x refer to the listed Criteria Air Pollutants for which EPA has regulatory authority under S ections 108 and 109 of the Clean Air Act (CAA), and for which criteria must be developed and reviewed every 5 years. It is necessary to distinguish between the definition of “nitrogen ox ides” as it appears in the enabling legislation related to the NAAQS and the definition common ly used in the air pollution research and management community. In this document, the te rms “oxides of nitrogen” and “nitrogen oxides” refer to all forms of oxidized nitrogen compounds, including nitric oxide (NO), nitrogen dioxide (NO 2 ), and all other oxidized nitrogen-containi ng compounds transformed from NO and NO 2 . This follows usage in the Clean Air Act Secti on 108(c): “Such criteria [for oxides of nitrogen]
shall include a discussion of n itric and nitrous acids, nitrites, nitrates, nitrosamines, and other carcinogenic and potentially carcinogenic derivatives of oxides of nitrogen.” By contrast, within the air pollution research and c ontrol community, the terms “oxide s of nitrogen” and “nitrogen oxides” are restricted to refer only to the sum of NO and NO 2 , and this sum is commonly
abbreviated as NO x . The category label used by this comm unity for the sum of all forms of oxidized nitrogen compounds including t hose listed in Section 108(c) is NO y . The terms NO 2 and SO 2 refer to the specific air quality indicators (pollutant species) specified by the current standards whose concentr ations are monitored to determine whether the NAAQS are being met in a given location. Th e ecological importance of both oxidized and

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reduced forms of nitrogen has been widely reco gnized by the scientific community. Therefore, this risk/exposure assessment will also evalua te total reactive nitrogen (which includes both
oxidized and reduced forms of nitr ogen) and its impacts on public welf are. It is addressed in this NAAQS review because reduced forms of nitrogen may also cause many of the effects resulting from oxides of nitrogen (e.g., deposition-influenced nitrogen enrichment).

Because NO x , SO x , and their associated transforma tion products are linked from an atmospheric chemistry perspective, as well as fr om an environmental effects perspective, and
because of the National Research Council’s (NRC’s) 2004 recommendations to consider multiple pollutants in forming the scientific ba sis for the NAAQS, EPA has decided, for the first time since NAAQS were established in 1971, to join tly assess the science, risks, and policies relevant to protect the public welfare associated with oxides of nitrogen and oxides of sulfur. Though these interactions have been recognized historically by both the Clean Air Scientific Advisory Committee (CASAC) and EPA, and the science related to these interactions has continued to evolve and grow to the present day, providing ongoing suppor t for considering them together.
This Scope and Methods Plan is organized to provide inform ation consistent with EPA’s
1998 Guidelines for Ecological Risk Assessment (U.S. EPA, 1998) that is representative of the problem formulation phase of the risk assessment. The Scope and Methods Plan is organized to provide the following:

 Background on NAAQS legislati on, previous secondary reviews, an overview of ecological risk assessments, and th e scope of this NAAQS review

 A conceptual model of nitr ogen and sulfur cycling

 Key policy-relevant questions

 EPA’s proposed schedule for the NO x /SO x secondary NAAQS review

 An analysis plan that includes the identific ation of relevant indi cators of effects, a proposed approach to select areas for the risk /exposure assessment, an evaluation of data and models to assess effects, plans for ch aracterization of exposure, and plans for characterization of ecological effects

 An assessment of alternative levels of protection under diffe rent scenarios of deposition
from ambient sources.

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4 DRAFT – March 2008 EPA is consulting with CASAC, an inde pendent scientific advisory committee established under the CAA, on this plan. In part icular, given the context for this NAAQS review and the timing constraints for its completion, we are soliciting the advice from CASAC on how
to best focus our current assessment activities to provide meaningful results to inform the regulatory portion of this review. To aid in this process, we have identified our initial priorities for each step in the process (i.e. case study ar eas, larger assessment areas, endpoints and indicators) for the overall risk/e xposure assessment. As this review proceeds, the plan described here may be modified to reflect information r eceived during the review process and to address
advice and comments received from CASAC and the public.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

2. BACKGROUND AND OVERVIEW 2.1 Legislative Requirements
Two sections of the CAA govern the estab lishment and revision of the NAAQS Section 108 (42 U.S.C. 7408) directs the Administrator to identify and list “air pollutants” that “in his judgment, may reasonably be anticipated to endanger public health and welfare” and whose “presence . . . in the ambient air results from nume rous or diverse mobile or stationary sources” and to issue air quality criteria for those that are listed. Air quality criteria are intended to “accurately reflect the latest sc ientific knowledge useful in in dicating the kind and extent of identifiable effects on public hea lth or welfare which may be e xpected from the presence of [a] pollutant in ambient air. . . .”
Section 109 (42 U.S.C. 7409) directs the Administrator to propose and promulgate “primary” and “secondary” NAAQS for pollutant s listed under Section 108. 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 Admini strator, based on such cr iteria, is required to protect the public welfare from any known or anticipated adverse effects associated with the presence of [the] pollutant in the ambient air.” Welfare effects, as defined in Section 302(h) [42 U.S.C. 7602(h)], include, but ar e not limited to, “effects on soils , water, crops, vegetation, manmade materials, animals, wildlife, weather, visibi lity 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.” The definition of public welfare in Section 302(h) was expanded in the
1990 CAA amendments to state that the welfare effects identified shoul d be protected from
adverse effects associated w ith criteria air pollutants “whe ther caused by transformation, conversion, or combination w ith other air pollutant.”
In setting standards that are “requisite” to protecting public health and welfare, as provided in Section 109(b), EPA’s task is to es tablish standards that are neither more nor less stringent than necessary for th ese purposes. In so doing, EPA may not consider the costs of implementing the standards ( Whitman v. American Trucking Associations , 531 U.S. 457, 465– 472, 475–76 [2001]).

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Section 109(d)(1) requires that “not later than December 31, 1980, and at 5-year intervals thereafter, the Administrator sh all 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 sc ientific review committee “shall complete a
review of the criteria . . . and the nationa l primary and secondary ambient air quality
standards . . . and shall recommend to the Administ rator any new . . . standards and revisions of existing criteria and standards as may be a ppropriate. . . .” Since the early 1980s, this independent review function has been performe d by CASAC of EPA’s Science Advisory Board (SAB).

2.2 Background on Previous NO 2 and SO 2 Secondary NAAQS Reviews
The current secondary NAAQS review for NO x /SO x will examine a number of issues that were of central importance in pr evious NAAQS nitrogen or sulfur reviews. For instance, in the previous review of the NO 2 NAAQS, completed in 1995, welfare effects were assessed primarily with respect to effects of ambient concentrations of NO 2 on vegetation by way of a literature
review. A full risk assessment was not conducted for other welfare effects associated with NO 2 . The decision was made to reta in the current standard for NO 2 as the uncertainty and variability
associated with the science at that time limited any other action. The 1995 NO 2 review did state, however, that “growing evidence does indicate that the impact of nitrogen deposition on sensitive aquatic ecosystems may be significant,” that certain areas of the countr y, including “the Catskills, Northern Appalachians, Valley and Ri dge Province, and Southern Appalachians all show some potential for ch ronic acidification due to NO 3 - ” (U.S. EPA, 1995). Additional topics highlighted within the 1995 NO 2 review also included the im portance of acid neutralizing capacity (ANC) in surface water acidification, the influence of atmospheric nitrogen to eutrophication of the Chesapeake Bay, and the gr owing body of research on developing “critical loads” and “target loads” for n itrogen in various ecosystems.
In the previous review of the SO 2 NAAQS, completed in 1996, vegetation damage (growth, yield, and foliar injury) due to s hort-term and long-term exposures to SO 2 were avoided by maintaining the current sec ondary 3-hour standard (0.053 pp m) (U.S. EPA, 1982). Data on the effects of long-term exposures affecting species richness and species diversity, reduced

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growth, and premature needle drop were cons idered “weak and not developed well enough to provide the principal basis for sel ecting the level of a long-term SO 2 standard” (U.S. EPA, 1982). The 1982 staff paper did note, however, that long-term SO 2 concentrations may be affecting lichen and mosses, and that these should be cons idered in the larger context of regional acid deposition. (Note: the 1986 Addendum (U.S. EPA, 1986) and 1994 Supplement (U.S. EPA, 1994) solely addressed human health effects.)
The planned risk/exposure assessment describe d in this Scope and Methods Plan builds upon the methodology and lessons learned from the previous NAAQS reviews. This plan is based on our current understanding of the NO x and SO x scientific literature and is subject to change as the NO x /SO x ISA undergoes revision. Currently, th e EPA’s Office of Research and Development’s (ORD) National Center for E nvironmental Assessment (NCEA) has compiled and synthesized the most policy-relevant science available to produce a draft of the ISA, which has been used in the development of the approach described here. The appr oach described in this plan may also be modified according to CASA C and public comments following their review of this document as well as any additional informa tion contained in the final version of the ISA.

2.3 Overview of Ecologi cal Risk Assessment and the Scope of the Secondary NAAQS Review for NO x and SO x The conventional framework for ecological risk assessment consists of three phases:

 Problem Formulation,

 Analysis, and

 Risk Characterization.
These phases have been descri bed in more detail in the Guidelines for Ecological Risk
Assessment (U.S. EPA, 1998) and other documents prepared by the Risk Assessment Forum (U.S. EPA, 1991, 1992a, 1992b). Generally, the Problem Formulation Phase describes the goals, breadth, and focus of the assessment including as sessment endpoints, data needs, and anticipated analyses. It results in thre e products: (1) assessment endpoi nts that adequately reflect management goals of the ecosystem they represen t, (2) conceptual mode ls that describe key relationships between stressor and assessment endpoint or between several stressors and assessment endpoints, and (3) an analysis plan. The Scope and Methods Plan for the secondary

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NO 2 /SO 2 NAAQS Review represents the Problem Fo rmulation Phase of the ecological risk assessment framework. The Analysis Phase of an ecological risk assessment might include environmental exposure profiles, the magnitude of spatial and temporal patterns of exposure, and summaries of the data analyses on effects and their association with assessment endpoints. The Risk Characterization Phase of an ecological risk assessment may be a quantitative or qualitative assessment that integrates exposure and effects pr ofiles and estimates risks by categories, such as individuals or populations via m odeling techniques. In the risk /exposure assessment, EPA plans to draw upon the ISA to develop quantitative and qualitative estim ates of the risks of adverse welfare effects occurring as a result of current ambient levels of nitrogen oxides and sulfur oxides, levels that meet the current standards for NO 2 and SO 2 , or levels that meet possible
alternative standards. The issues that ar e addressed by the Analysis Phase and Risk Characterization Phase are part of the Risk/Exposure Assess ment for the Secondary NAAQS
Review for NO x and SO x . Welfare impacts from air pollution to ecosystems are a serious concern. There are many harmful environmental effects of air pollution, including acid rain, ozon e formation, decreased visibility, and effects on climate. In addition to direct adverse impacts to the biological and biogeochemical components of natural ecosystems, these direct ecological impacts can affect the welfare amenities of ecosystems in terms of th eir aesthetic and recreational amenities. Impacts
on climate are also considered welfare effect s, such as those due to nitrous oxide, N 2 O. Welfare effects from the production of greenhouse gases such as N 2 O can also cause significant impairment of ecosystems through climate change processes.
Against this broad background to welfare im pact issues, this risk/exposure assessment will focus on ecological quality and its effects from acidification and enrichment related to nitrogen and sulfur air pollutant s. In previous seconda ry reviews, acidification was evaluated for its damage to materials, including decay of buildi ngs, statues, and sculptures that are part of our national heritage. The current assessment will fo cus on the influence of acid deposition on soil, forests, and waterbodies. Nitrogen deposition ma y also contribute to eutrophication (oxygen depletion) of water bodies, the symptoms of wh ich include algal blooms (some of which may be toxic), fish kills, and loss of plant and animal diversity. These ecological changes impact human populations by changing the availability of seafo od and creating a risk of consuming fish or shellfish contaminated from toxins produced by al gal species that may be at an advantage in

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eutrophic systems as mentioned in the ISA. This reduces our ability to use and enjoy our coastal ecosystems, and causes an economic impact on people who rely on healthy coastal ecosystems,
such as fishermen and those who cater to tourists.
Visibility impairment and ozone formation ar e additional welfare effects that are being addressed in the particulate matter (PM) and ozone NAAQS revi ews. Visibility is being addressed in a separate secondary review effort as part of the PM NAAQS standard review. Ozone pollution’s secondary impact s include damage to plant ve getation. Ozone is a secondary product of precursors NO 2 and volatile organic car bon. A separate rule review effort is in place for ambient ozone concentration, and its secondary impacts will also not be addressed in this
NO x and SO x risk/exposure assessment.
In this current review, the appropriateness of NO 2 as an indicator for NO x species and
SO 2 as an indicator for SO x species will be evaluated. This re view will evaluate new information published in the peer-reviewed literature since the completion of the last NO 2 (1995) and SO 2 (1996) reviews, including assessments of th e adequacy of the current secondary NAAQS, consideration of whether there is a possible need fo r a new single indicator or suite of indicators, as well as changed or retained level(s) and/or averaging time s for the standards, which may include nitrogen and sulfur compounds other than NO 2 and SO 2 . This Scope and Methods Plan is intended to facilitate consultation with the CASAC, as well as the public, and to obtai n advice on the overall scope, a pproaches, and key issues in advance of the completion of such analyses and pr esentation of results in the first draft of the risk/exposure assessment. The risk/exposure assessmen t is intended to be a tool that, together
with other information contained in the NO x /SO x ISA, can aid the Administrator in judging whether the current secondary st andards are requisite to protec t public welfare from any known or anticipated adverse effects, or whether these standards shou ld be retained, revised, revoked and/or replaced with alternativ e standard(s) having different indi cators to provide the required protection.
2.3.1 Overview of Risk Assessment Framework for Deposition-related Ecological Effects The risk/exposure assessment framework is in tended to serve as a conceptual map of the analytical and decision steps necessary to estimate the ecological risks asso ciated with alternative

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forms and levels of standards for NO x and SO x . As noted in the Integrated Review Plan for this review (U.S. EPA, 2007a), the purpose of the ri sk/exposure assessment is to assess the potential adversity of impacts including the effects of th e pollutants on ecosystem goods and services, the degree to which ecosystem functions are impair ed, long-term trends in specific ecosystems (where available), and both monetized and non-m onetized valuation of ecosystem services. The ability of the risk/exposure assessment to charac terize these impacts will depend on a number of factors, including the state of the supporting science, the availability of data on ecosystem baseline conditions and/or respons iveness to changes in nitrogen and sulfur deposition, and the time available under the regulatory development process.
The design of a risk assessment framework is discussed in the 1998 Guidelines for Ecological Risk Assessment (U.S. EPA, 1998). For this NO x /SO x risk/exposure assessment, EPA designed a flow diagram that represents how n itrogen and sulfur compounds move from “source to dose” in the environment. (See Figure 2-1 .) This diagram represen ts the risk assessment framework for deposition-related ecological risks. It consists of two general activities: 1) characterization of exposure and 2) characterization of effect. More specifically, this framework depicts the processes and transf ormations among atmospheric concentrations, deposition, ecosystem impacts, exposure to biol ogically relevant sp ecies, and ecosystem responses via ecosystem services and valuation.

Figure 2-1. Risk assessment framework fo r deposition-related ecological risks.

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The ecosystem response may be expressed as an “endpoint” in the ecological risk assessment. For this review, the level of acidificat ion and nitrogen and/or sulfur enrichment that results in a harmful effect may be deemed an endpoint. This risk assessment framework entails modeling to calculate ecosystem loading and expos ure required to reach the endpoint. This load
is also known as the critical load. A critical lo ad analysis is a form of site-specific risk assessment. It considers exposure to and respons e by various ecosystem receptors to identify the amount of atmospheric deposition (or loading) ab ove which adverse ecological effects occur, serving as the endpoint of the risk assessment. Critical load analysis, within a risk assessment framework, can be a potential in dicator for policy analysis.
Each component of the framework has a numbe r of decisions required in implementing a framework to analyze alternative air quality standards. In an analytical world unconstrained by data and resource limitations, one could envision a nationwide comprehensive risk assessment covering all potentially affected sensitive ecosystems and all sc ientifically supported effects. However, as noted by the Science Advisory Board in their recent review of the analytical plan
for the 2 nd Prospective Analysis of the Costs a nd Benefits of the Clean Air Act, “a comprehensive quantitative national assessmen t of the ecological benefits of the CAA
Amendments is not a realistic expectation.” We recognize this limitation in developing the
framework for the NO x /SO x risk/exposure assessment, and in th e following sections, we describe
a detailed framework for assessi ng ecological risks that follows the principle of obtaining the maximum amount of policy-relevant risk inform ation possible given the data, resources, and
time limitations.
2.3.2 Overview of Nitrogen Deposition The sum of mono-nitrogen oxides, NO 2 and NO, typically are referred to as nitrogen oxides (NO x ) in the atmospheric science community. More formally, the family of nitrogen oxides includes any gaseous combination of nitrogen and oxygen, e.g., NO 2 , NO, N 2 O, N 2 O 3 , N 2 O 4 , and N 2 O 5. Total reduced nitrogen (NO y ) includes all nitrogen oxides as well as gaseous and particulate nitrate species such as HNO 3 , PAN, and aerosol phase ammonium nitrates. Reduced atmospheric nitrogen sp ecies include ammonia gas (NH 3 ) and ammonium ion (NH 4 + ),
the sum of which is referred to as NH x . Atmospheric nitrogen deposition often is delineated

11 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

farther as dry (gas and particulate phases) or as wet (precipitation derived ion phase). (See Figure 2-2 .)

(Source: U.S. EPA, 2007e. ISA for Oxides of Nitrogen and Sulfur Environmental Criteria. December 2007, EPA/600/R-07/145A). Figure 2-2. Schematic diagram of the cycle of reactive, oxidized nitrogen species in the atmosphere. (IN refers to inorganic particul ate species (e.g., Na+,Ca++), MPP to multiphase processes, hv to a solar photon, and R to an organic radical. Particulate phase organic nitrates are also formed from the species on the right side of the figure.)

N 2 O has not been considered in setting previous NO 2 NAAQS standards. In the first NO x review, N 2 O was not considered an air contaminant b ecause there was “no evidence to suggest
N 2 O is involved in photochemical reactions in the lower atmosphere” (U.S. EPA, 1971). N 2 O
was addressed in both the 1982 and 1993 criteria documents. In 1982, it was described as one of the eight nitrogen oxides that may be present in the ambient air, but “not generally considered a pollutant.” The effect of N 2 O on stratospheric ozone was descri bed, and the criteria document
noted that N 2 O may cause a small decrease in stratos pheric ozone (U.S. EPA, 1982). Finally, the
criteria document also concluded that N 2 O significantly contributes to the atmospheric greenhouse effect by trapping outgoing terrestria l radiation, and that the issue was being investigated, but that many years of research were still needed to reliably assess the issue. In 1993, the criteria document again identifies N 2 O as an oxidized nitroge n compound that is not generally considered to be an ai r pollutant, but does have an imp act on stratospheric ozone and is

12 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

considered among the more significant greenhouse gases. These documents clearly consider N 2 O as within the scope of the listed nitrogen oxides criteria pollutant.
The 2007 draft of the ISA (see Sections 2.2, 3.1 and 4.4 in U.S. EPA, 2007b) acknowledges N 2 O as a potent greenhouse gas and discusses N 2 O sources and emissions in the United States, as well as the biogeochemistry of the microbial mediated production via denitrification in natural ecosystems. Based on the current U.S. Greenhouse Gas (GHG) Inventory (U.S. EPA, 2007d), ni trous oxide contributes approxi mately 6.5 % to total greenhouse gas emissions (in CO 2 equivalents) ( Figure 2-3 ).

Source: U.S. EPA, 2007d Figure 2-3. Percent of Total U.S. Emissions of greenhouse gases in CO 2 equivalents.

Since the definition of “welfare effects” includes effects on climate [CAA Section 302(h)], we will include N 2 O within the scope of this review. However, it is most appropriate to analyze the role of N 2 O in anthropogenic climate change in the context of a ll of the greenhouse gases. Since that is outside the scope of this review, it will not be a quantitative part of this

assessment.

2.3.3 Overview of Sulfur Oxides and Sulfur Deposition

SO 2 is one of a group of substances known as SO x , which include multiple gaseous (e.g.,
SO 2 , SO, SO 3 , S 2 O 3 , S 2 O 7 ) and particulate (e.g., amm onium sulfate) species ( Figure 2-4 ).
Acidification can result from th e atmospheric deposition of SO x and NO x ; in acid deposition, these species combine with water in the atmosphere to form sulfuric acid (H 2 SO 4 ) and HNO 3 . Acidification is an environmental effect in which acid precipitation lowers the natural pH of waterbodies and/or damages terres trial ecosystems. Over the past few decades, acidification of

13 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

waterbodies has been recognized as an envi ronmental issue throughout Europe and North America, and steps have been taken to control SO x and NO x emissions and to identify the recovery of the impacted ecosystems. Due to known acute effects on plants, in previous NAAQS reviews, SO 2 served as the chemical indicator for SO x species.

(Source: U.S. EPA, 2007e. ISA for Oxides of Nitrogen and Sulfur Environmental Criteria. December 2007, EPA/600/R-07/145A [adapted from Berresheim et al., 1995]). Figure 2-4. Transformation of sulfur compounds in the atmosphere. 2.3.4 Targeted Effects for This Risk/Exposure Assessment

The two classifications of effects that are targeted for this risk/exposure assessment are acidification and nitrogen and sulf ur enrichment. Both effects occur in response to deposition of
NO x and SO x . Acidification effects can occur from either nitrogen or sulfur deposition, and the relative contribution of each type of deposition depends on the ch aracteristics of the affected ecosystem (see Section 4.2 of the ISA, U.S. EPA, 2007b). Nitrogen and sulfur enrichment represents a continuum of effect s, and it can be characterized as a positive or negative effect, depending on the selected endpoint, location and baseline conditions of an ecosystem. Enrichment effects are caused by nitrogen or sulfur deposition, but are dominated by nitrogen deposition, which will be the focus of the risk /exposure assessment. Nitrogen enrichment in ecosystems may alter the native terrestrial speci es composition (i.e., from wildflower meadows to shrubs), and can result in eutrophication in aquatic systems (s ee Section 4.3 of the ISA, U.S.

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EPA, 2007b). Thus, the framework for this review highlights four main areas that will be evaluated for a risk/exposure assessment in this plan:
1. Risks to terrestrial ecosystems fr om nitrogen enrichment effects
2. Risks to aquatic ecosystems from nitroge n enrichment effects (eutrophication)
3. Risks to terrestrial ecosystems from aci dification effects (nitrogen and sulfur)
4. Risks to aquatic ecosystems from acidi fication effects (nitrogen and sulfur)
In addition to the four targeted effects listed above, we will address, as appropriate, impacts associated with nitrous oxide and the in fluence of sulfur enrichment on methylmercury production. Figure 2-5 provides a conceptual diagram of the processes we will need to model in our risk and exposure assessment. Atmospheric fate and transport is the initial point of departure for the analysis, and is fully integrated in the treatment of NO x , other reactive nitrogen species,
and SO x . The results of the atmospheric fate and tr ansport process are at mospheric loading of sulfur and total reactive nitrogen, as well as concentrations of N 2 O. As mentioned earlier, a quantitative assessment of N 2 O is not currently within the scope of this review.
Atmospheric loadings of total reactive nitrogen and SO x lead to deposition of nitrogen
and sulfur to terrestrial and a quatic ecosystems. In addition to the major ecological effects of
acidification and nutrient enrich ment, sulfur deposition also le ads to enhanced methylmercury (MeHg) production in aquatic systems and, in tu rn, increases the risks for bioaccumulation and biomagnifications of mercury in food chains (see Section 4.4 in U.S. EPA, 2007b). This interaction between sulfur depos ition and methylmercury producti on can exacerbate an already important mercury problem, especially in coasta l and eutrophic waters subject to hypoxic algal blooms. The focus of the quantitative risk/e xposure assessment will be the set of effects associated directly with acidification and ni trogen and sulfur enrichment, but we will also provide a qualitative assessment of potential impacts of sulfur de position on waterbodies that are
vulnerable to increased mercury methylation.

15 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

Atmospheric

Fate and Transport

Total Reactive N and SO x

Deposition Processes

Acidification N 2 O

Acidification

Terrestrial Terrestrial Terrestrial Aquatic Aquatic Aquatic Enrichment MeHg Production Sulfur Nitrogen

Figure 2-5. General flow of processes addressed in the risk/exposure assessment for NO x and SO x secondary standards. 2.4 Key Policy Relevant Questions
The 2007 Integrated Review Plan of the Secondary NAAQS for NO 2 and SO 2 introduced a series of policy-relevant questions to frame th e approach EPA will take in this review (U.S. EPA, 2007a). The Review Plan indicated that is sues of ecosystem susceptibility should be addressed, as well as the issue of whether individual effects or combined effects are more important to a given ecosystem (i.e., is it NO x or SO x acting individually that is important, or is it
the combination of NO x and SO x that needs to be addressed). For example, both NO x and SO x are associated with acidification effects, while nitrogen is associated w ith nutrient enrichment and eutrophication effects, and su lfur is associated with increa sed mercury (Hg) methylation.
Both EPA and CASAC have acknowledged the importance of NO x , SO x , and their associated transformation products with respec t to acidification effects on ecosystems. This review will focus on the ecosystem-related welfare effects that result from the deposition of these pollutants and their transformation products, rather than on the e ffects of aerosol NO x and SO x that remain in the atmosphere.
For this secondary NAAQS review of NO x /SO x , the primary policy-relevant questions
include:

16 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

 What are the known or anticipated welf are effects influenced by ambient NO x and SO x , and for which effects is there sufficient inform ation available to be useful as a basis for considering distinct s econdary standard(s)?

 What is the nature and magnitude of ecosystem responses to NO x and SO x that are understood to have known or anticipated advers e effects, and what is the variability associated with those responses (incl uding ecosystem type, climatic conditions, environmental effects, and interactions with other environmental fact ors and pollutants)?

 What are the biologically relevant indices that adequately capture the relationship between ecosystem exposure and response for the known or anticipat ed welfare effects we are trying to protect?

 To what extent do receptor surfaces influen ce the deposition of gase s and particles (dry deposition), since dry deposition can cont ribute significantly to total deposition?

 What are the appropriate air qua lity indicator(s), averaging tim e(s), form(s), and level(s) of standards that are requisite to avoid those ecosystem responses?

 To what extent do the current standards pr ovide the requisite protection for the public
welfare effects associated with NO x and SO x ?

– Should the current secondary standards for NO 2 (as an indicator of NO x ) and SO 2 (as
an indicator for SO x ) be retained, revised, or revoked and/or replaced with alternative standard(s) having different i ndicators to provide the requ ired protection from known or anticipated adverse pu blic welfare effects?

– Can effects from NO x be distinguished from effects due to total reactive nitrogen?
To the extent that the evidence suggest s revision of the current secondary NO x /SO x NAAQS is appropriate, ranges of st andards will be identified (i ncluding different or alternate indicators, terms of exposure indices, averaging times, levels, and forms) that reflect a range of alternative policy judgments as to the degree of protec tion that is requisite to protect public
welfare from known or an ticipated adverse effects. To acco unt for variability in ecosystem responses and land uses across the nation, ecos ystem characteristics may be an important consideration in evaluating the form(s) of the st andards. The form(s) of the standard(s) may be based on a complex formula that incorporates ecosystem characte ristics, land uses, atmospheric

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transformations, climatic conditions, environmenta l effects and other inte ractions. In so doing, the following questions should be addressed:

 Does the available information provide support for considering different NO x /SO x chemical indicators or exposure indices?

 Does the available information provide support for considering some joint standard(s) or are separate sta ndards appropriate?

 What range of levels and forms of alternat ive standards are supported by the information,
and what are the uncertainties and li mitations in that information?

 To what extent do specific levels and form s of alternative stan dards reduce adverse impacts attributable to NO x /SO x , and what are the uncertainties in the estimated
reductions?
In order to be able to answer these questions , we believe that the relevant scientific and policy issues that need to be addressed in th e science, risk/exposure, and policy assessment portions of this review include:

 Identifying important chemical species in the atmosphere

 Identifying the atmospheric pathways that gove rn chemical transformation, transport, and deposition of NO x and SO x to the environment

 Identifying the attributes of ecosystem recepto rs that govern their susceptibility to effects from deposition of nitrogen and sulfur compounds

 Identifying the relationships between ambient indicators and biological ly relevant indices of effects, including ecosystem services associ ated with the indicator (but not excluding other non-economic evaluations)

 Evaluating alternative measures to assess the adversity of effects on ecosystem services, including, for example, economic valuation

 Evaluating if current levels may have a long- term impact due to cumulative loadings, and if this is relevant to a NAAQS review

 Evaluating environmental impacts and sensitiv ities to varying mete orological scenarios
and climate conditions

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19 DRAFT – March 2008 2.5 Proposed Schedule
The proposed schedule for the joint NO 2 /SO 2 secondary NAAQS review is shown in
Table 2-1 ; underlined dates indicate the court-orde red schedule. Consultation with CASAC and the public on the first draft of the ISA and this Scope and Methods Plan is planned for April 2008. Based on this consultation, the plan for th e risk/exposure assessment may be revised as needed. The first draft of the risk/exposure asse ssment and the second draft of the ISA will be released to CASAC and the public in Augus t 2008. EPA will receive comments on these draft documents from CASAC and the public at a m eeting in October 2008. A revised risk/exposure
assessment will be released in March 2009 follo wed by a CASAC and public review in May 2009. The final risk/exposure assessment for the secondary NO 2 /SO 2 NAAQS review will be released in July 2009.

Table 2-1. Proposed Schedule for the Joint NO 2 and SO 2 Secondary NAAQS Review *
Stage of Review Major Milestone Draft Target Dates Literature search Ongoing Federal Register call for information December 2005 Prepare the draft NO 2 /SO 2 NAAQS Work Plan December 2005–August 2007 Workshop on science/policy issues July 2007 CASAC consultation October 2007 Planning

Prepare the final integrated NO 2 /SO 2 NAAQS Work Plan December 2007
Prepare first draft of the ISA December 2007 CASAC/public review of the firs t draft of the ISA April 2008 Prepare the second draft of the ISA July/August 2008 CASAC/public review of the second draft of the ISA October 2008 Integrated Science Assessment (ISA)

Prepare the final ISA December 12, 2008 REA methodology released to the CASAC and the public February 2008
CASAC/public consultation on the REA methodology April 2008

First draft of the REA released to the CASAC and the public August 2008
CASAC/public review of the first draft of the REA October 2008 Second draft of the REA released to the CASAC and the public March 2009
CASAC/public review of the second draft of the REA May 2009 Risk/Exposure Assessment (REA)

Final REA released July 2009Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

20 DRAFT – March 2008 Stage of Review Major Milestone Draft Target Dates Publish ANPR August 2009 CASAC review/public comment on ANPR October 2009 Proposed rulemaking February 12, 2010 Policy Assessment/ Rulemaking Final rulemaking October 19, 2010 * Schedule may be modified, as necessary, to re flect actual project requirements and progress.

Underlined dates indicate the court-ordered schedule.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

3. ANALYSIS PLAN
Because ecosystems are diverse in biota, cl imate, geochemistry, and hydrology, response to pollutant exposures can vary significantly. Also, these diverse ecosystems are often neither abundant nor distributed evenly ac ross the United States. To targ et acidification and nitrogen and sulfur enrichment, this Scope and Methods Pl an focuses on four main ecosystem effects on terrestrial and aquatic systems identified in the ISA:

 Terrestrial nitrogen enrichment

 Aquatic nitrogen enrichme nt, including eutrophication

 Terrestrial acidification due to nitrogen and sulfur

 Aquatic acidification due to nitrogen and sulfur
In addition to these four effects, we plan to address, as appropriat e and within our time constraints, impacts associat ed with nitrous oxide (N 2 O)and the influence of sulfur enrichment on methylmercury producti on. Since these ecosystem effects ar e not found evenly distributed across the United States, we plan to perform risk/exposure assessment case studies for specific areas of the U.S. We plan to se lect these case studies from those areas of the United States where ecosystems are identified as sensitive to ni trogen and/or sulfur deposition effects.
Once those sensitive areas are determined, we will decide how best to design and conduct case study analyses that contain th e sensitive ecosystems of interest . These locations may vary in size from a single site to a region containing numerous lakes. Methods of assessments can include cluster analyses of re gions with common ecosystem sens itivities (e.g., lakes and streams of the Adirondack Mountains), site-specific quan titative modeling analyses, qualitative analyses, and review and summary of previous risk/exposure assessments. From these qualitativequantitative analyses of sensitive areas and case studies in different regions of the U.S., we will
discern if the results can be used for a broader characterization of national conditions to represent key components of our nation’s ecology. To be clear, this exercise is not a national-scale ecological risk assessment but, rather, is inte nded to be a qualitative analysis of multiple ecosystems’ quantitative-qualita tive risk/exposure assessments.
The risk/exposure assessment for the Secondary NAAQS for NO x and SO x will build upon the scientific information presented in the 2007 draft ISA (U.S. EPA, 2007b). The ISA

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documents ecological effects of nitrogen and su lfur deposition, biogeoche mical indicators of effects, and areas of the U.S. where ecosyst em effects have been studied. The ISA also
recommends selected indicators and case studies as candidates for risk/exposure assessment and ecosystem services valuation. In this sect ion, we will identify and describe how those
recommendations can be considered in the assess ment in order to provide information for policy decision making.
The risk/exposure assessment will focus on ecosystem welfare effects that result from the deposition of total reactive nitroge n and sulfur. The anticipated sp atial extent and diversity of ecological effects due to depositi on of nitrogen and sulfur do not f acilitate a nationwide analysis. Further, some areas of the United States are more vulnerable to the effects of deposition than others. Because of this diversity, we consider it valuable to form ulate a strategy that is designed to protect sensitive systems, while allowing for flex ibility in areas that are more resilient. As a result, this assessment intends to evaluate potential alternatives to current indices in an attempt to quantify the relationship between ambient concentrations of NO x and SO x and potential welfare effects. To create these indices, we plan to eval uate exposures and impacts in various ecosystem case studies with differing responses relate d to nitrogen and sulf ur inputs and explore relationships between ambient concentrations and depositi on of nitrogen and sulfur.
As previously described, deposition of SO x and NO x compounds affects ecosystems in various adverse ways and at different spatial a nd temporal scales in diverse regions of the country. In its review of th e analytical plan for the 2 nd Prospective Analysis of the Costs and Benefits of the Clean Air Act, the Science Advisory Board recommended that EPA consider including studies of upland as well as coastal sites, because air deposition is often not the primary contributor of nitrogen in coastal sites, while it may be the dominant source in upland locations. The Analysis Plan Phase of a risk asse ssment includes selecting data that will be used, analyzing exposure (including sp atio-temporal conditions), anal yzing effects, and summarizing conclusions about exposure.
In order to address the policy -relevant questions that are gui ding the scope of this review, this risk/exposure assessment intends to evaluate the relationships between atmospheric concentrations, deposition, biologi cally relevant exposures, ecos ystem effects, and ecosystem
services.

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To evaluate the nature and magnitude of eco system responses associated with adverse effects, the risk/exposure assessment plans to ex amine various ways to quantify the relationship between air quality indicators, de position of biologically accessibl e forms of nitrogen and sulfur, biologically-relevant indices rela ting to deposition, exposure and effects on sensitive receptors, and related impacts to ecosystem change and serv ices. To the extent f easible, the risk/exposure
assessment should also evaluate the overall load to the system for nitrogen and sulfur as well as the variability in ecosystem responses to these pollutants. The assessment intends to determine the exposure metrics that incor porate the temporal considerati ons (i.e., biologically relevant timeframes), pathways, and biologically relevant i ndices necessary to main tain the functioning of these ecosystems. In addition, the risk/exposure asse ssment plans to evaluate the contributions of atmospherically deposited nitrogen and sulfur relative to total lo adings in the environment. For the atmospheric contribution to to tal nitrogen, we also plan to evaluate the contribution of NO x to total reactive nitrogen in the atmosphere rela tive to the contributions of reduced forms of nitrogen (e.g., ammonia, ammonium).
The scope of the risk/exposure assessment will depend, in part, on the answers to the following questions:

 What are the appropriate geographic scales a nd/or time frames for the risk assessment?
Information that will be considered in addr essing this question includes mapping datasets, research studies of sensitive ecosystems rangi ng in size from single lakes to stream and lake systems within a large geographic region (e.g., the Southern Appalachian Mountains), identification of representative ecosystem types, air pollution gradient studies, a weight-of-evidence approach incor porating both qualitat ive and quantitative information on risk, or some combination of the above.

 How can regional variation of effects be taken into account? How should the risk/exposure assessment address acidificat ion and nutrient-enrichment effects in different areas and ecosystem types (e.g., mesic, arid, mountain forests, alpine, subalpine)?

 To what degree are assumptions supported by the available scien ce regarding linkages between pollutants in ambient air, deposit ion, and measurable ecosystem effects,

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including effects on ecosystem services? What are the most useful metrics of both ambient pollution and th e resulting effects?

 To what degree should the risk/exposure assess ment take the potential for recovery into account in selecting data for qualit ative and quantitative assessments?

 How can uncertainties be minimized and appropriately characterized?
Because this risk/exposure assessment intends to focus on two basic secondary effects related to sulfur or nitrogen pollutants—acidification and enrichment—and because ecosystems may respond differently to these effects, it will be necessary to first perform risk/exposure assessment case studies unique to the effect and eco system type. We will assess the feasibility to consolidate effects and/or ecosystems in th e risk/exposure assessment and, where feasible, perform a broader characterization. However, so me ecosystems and their effects may be too unique to consolidate into a broad characterization.
Upon completion of all risk/exposure assessm ent case studies, th e results of the assessments performed for unique combinations of effects and ecosystem types will be presented together to facilitate decision making on the to tal effects of nitrogen and sulfur deposition. Ecosystem services that relate to the effects will be identified and valued if possible. Ecosystem services provide an additional way to compare effects across various ecosystems. The following is an overview of the risk/exposur e assessment process, as well as more in-depth discussions on topics addressed in the seven steps (see Figure 3-1 ).
Steps for Characterizing Ecological Effects The seven basic steps guiding the plan for the overall the risk/exposure assessment and the assessments for each case study area of interest are shown in Figure 3-1 . These seven steps capture the components of the risk assessment fr amework by addressing the selection of effects, indicators and ecosystem services measured fo r exposure via atmospheric deposition of total reactive nitrogen and sulfur from ambient air. Th e initial steps of identifying effects, sensitive ecosystems, and potential indicators have been performed and documented in the ISA. In addition, the ISA identifies and reviews candidate multimedia models available for fate and transport analyses of a variety of ecosystems. The science documented in the ISA will play a key role in planning and conducting the risk assessment. It is possible that, for some of the desired case study areas, data may not be abundant enou gh to perform a quantitative assessment for each

24 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

of the steps; in those cases, we may choose to execute some of these steps in a qualitative or semi-quantitative fashion.

Step 1 Plan for assessment using documented effects; biological, chemical, and ecological indicators, and potential ecosystem services

Step 2 Define sensitive areas that exhibit effects using research findings and GIS mapping

Step 3 Select risk/exposure case study assessment area within a sensitive area

Step 4 Evaluate current loads and effects to case study assessment area including ecosystem services

Step 5 Where feasible, scale up case study assessment area findings to sensitive areas

Step 6 Assess the current ecological conditions for those sensitive areas

Step 7 Assess alternative levels of protection under different scenarios of deposition from ambient sources Policy Decision Integrated Science Assessment

Use 2002 CMAQ output to run selected multimed ia models from the integrated Science Assessment

Figure 3-1. Seven step approach to planni ng and implementing risk/exposure assessment.

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3.1 Step 1 – Plan for Assessment Using Documented Effects; Biological, Chemical, and Ecological Indicators , and Potential Ecosystem Services
3.1.1 Documented Effects and Relevant Indicators of Effects for Each Ecosystem, Including Function and Service To assess the impacts of total reactive nitr ogen and sulfur loading, we plan to identify adverse terrestrial and aquatic effects due nitrogen and sulfur deposition. The impacts of acidification, nitrogen-induced eu trophication, and changes in species diversity due to nitrogen
and/or sulfur saturation are documented in EP A’s 2007 ISA (U.S. EPA, 2007b). We plan to use this information to identify those ecosystems and ecosystem services that are considered most
sensitive to acidification and ni trogen and sulfur enrichment and most informative for potential case study analyses in our overa ll risk/exposure assessment.

Environmental indicators are measures that track environmental conditions over time. EPA’s Report on the Environment (ROE) (U.S. EP A, 2007c) defines an indicator as follows:
A numerical value derived from actual measurements of a pressure, state or ambient condition, exposure, or human health or ecological condition over a specified geographic domain, whose trends over time represent or draw attention to underlying trends in the condition of the environment. Indicators of ecosystem response include chemi cal, biological, and habi tat measurements, such as forest extent and type, land cover, lake and stream acidity, nitrogen and phosphorus in streams, and contaminates in fish tissue.
Step 1 entails defining the bi ological and biogeochemical relevance of indicators for acidification and nitrogen and sulfur enrichment in order to select the most appropriate indicators for the risk/exposure assessment. The issue of which soil chemical and physical char acteristics are most appropriate and the spatial variability in these characteristics will be addressed. For sulfur, the adsorption/desorption responses in so ils may be important. In addition, we plan to consider the contribution of any internal sulfur sources (both organic su lfur mineralization and inorganic sulfur mineral weatheri ng) to sulfate fluxes in soil, and the resultant differences in responses to decreases in sulfur deposition in surface and ground waters. For nitrogen, more
attention will be focused on what soil features should be tracked; including how organic matter affects microbial processes. This characterization will also be linked to the role of vegetation, not only with respect to nitrogen cy cling, but also in affecting or ganic matter quality via organic

26 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

matter inputs to the soil. More detail on this appr oach to identifying indica tors and a preliminary list of recommended indicators is provided below.
Preliminary NO x and SO x Indicators: In the ISA, relevant indi cators were divided into one or both of the response categories: acidification or nutrient enrichment. Table 3-1 presents acidification or nutrient en richment indicators.
Table 3-1. Summary of Indicators Categorized by Effect

Acidification Ecosystem Type Nutrient Enrichment Ecosystem Type Acid stress index A Carbon budget (growth, carbon fixation, and respiration) T

Acid neutralizing capacity A, T Concentration of chlorophyll A, T Alkalinity A Concentration of carotenoids T Aluminum, mobilization A Nitrogen, concentrations A Carbon to Nitrogen ratio T Photosynthetic rate A, T Calcium and magnesium concentrations T Phosphorus concentrations T
Community structure A, T Species richness A, T Condition factor A Taxonomic density A, T Dissolved organic carbon A Thallus density T Dissolved organic nitrogen A Transpiration rate T Exchangeable cations T Forest health T Index of biotic integrity A, T Metal mobilization T pH A, T Soil-base saturation T Species composition A, T Taxonomic richness A, T A = Aquatic T = Terrestrial

Key Acidification Indicators: Table 3-2 presents key indicators that play a significant
role in surface water acidificat ion and recovery (summarized fr om Skjelkvale et al., 2005). Specific indicators of acidification ar e discussed in more detail below.

27 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

Table 3-2. Key Indicators of Acidification Due to NO x and SO x Key Indicator Group Examples of Indicators Description Acid anions SO 4 2- , NO 3 - Trends in these concent rations reflect recent trends in atmospheric deposition (especially SO 4 2- ) and in ecosystem responses to long-term deposition (notably NO 3 - and desorbed SO 4 2- ). Base cations Ca 2+ , Mg 2+

, Σ (Ca 2+ +Mg 2+ ) These cations are mobilized by weathering reactions and cation exchange. These respond indirectly to decreases in SO 4 2- and NO 3 - because a reduced input of acids will lead to a reduction of neutralizing processes in the soil, thereby reducing the release of base cations to soil- and runoff waters. Acidity pH, (Gran) alkalinity, ANC These indicators reflect the outcomes of interactions between changing concentration of acid anions and base cations. Organic acidity Dissolved organic carbon (DOC), total organic carbon (TOC) Organic acids are common natural sources of acidity in surface waters.

Metals Al, iron (Fe) These metals are mobilized as a response to the deposition of SO 4 2- and NO 3 -

. Biological Forest health, community structure, species composition, taxonomic richness, Index of Biotic integrity Ecological effects occur at four levels: individual, population, community, and ecosystem. Metrics have been developed for each level to assess the adverse effects of acids.

ANC is an acidification indicat or with relevance to soils, terrestrial ecosystems, and aquatic ecosystems and is a key indicator r ecommended in the ISA. ANC data are widely available for use in a risk/exposure assessment. Other indicators may be used in relation to particular ecosystems or specific sensitive areas. Chemical indicators such as pH or cation exchange capacity (CEC) are more widely avai lable at the present tim e than the biological
indicators.
Key Indicators of Nutrient Enrichment: Major indicators for nutrient enrichment to aquatic and terrestrial systems from air deposi tion of reactive nitrogen involve measurements based on available monitoring stations for wet deposition (NADP/national trend network [NTN]) and limited networks for dry deposition (CAS TNET). Wet-deposition monitoring stations can provide more information on an extensive range of nitrogen species than is possible for drydeposition monitoring stations. Th is creates complications in developing estimates for total nitrogen deposition levels because dry-depositio n data sources will likely be underestimated. Individual studies measuring nitrogen deposit ion to terrestrial ecosystems that involve

28 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

throughfall estimates for forested ecosystems can provide better approximations for total nitrogen deposition levels, but such estimates and related bioassessment data, are not available for the entire country. For terrestrial ecosystems, low calcium to nitrogen ratios in soils are commonly related to increased nitrif ication and potential increases in soil acidity and releases in
NO 3 to receiving waters; however, these measurements are not always widely available.
For aquatic ecosystems, the indicators for “nutrient enrichment” effects reflect a combination of inputs from all media (e.g., air, di scharges to water, di ffuse runoff, groundwater inputs). Major aquatic system indicators invol ve nutrient loadings (Heinz Center, 2006), indicators of excess algal standing crops (U .S. EPA, 2006), or, in larger waterbodies, anoxia/hypoxia in bottom waters. (See Table 3-3 ). For nitrogen, loadings or concentration values related to total nitrogen (a combination of nitrat es, nitrites, organic nitrogen, and total ammonia) are encouraged for inclusion in numeric criteria as part of EPA-approved state water quality standards (U.S. EPA, 2000). Given the nature of the major indicators for atmospheric deposition and indicators for aquatic and terrestrial ecolo gical systems, a data-fusion approach that combines monitoring indicators with modeling input s and outputs is often used (Howarth, 2007).
Table 3-3. Key Indicators of Nutrient Enrichment Due to NO x and SO x Key Indicator Group Examples of Indicators Description Nitrogen deposition Nitrate or ammonia From wet or dry deposition monitoring stations and networks. Nitrogen throughfall deposition Nitrate, ammonia, organic nitrogen Special measurements in terrestrial ecosystem with corrections for nitrogen intercepted by plant canopies. Nitrogen loadings and fluxes to receiving waters Total nitrogen or constituent species combined with flow data from gauged stations Reflects a combination of inputs from all media (air, discharges to water, diffuse runoff, and groundwater inputs). Relative role of air deposition should ideally be compared with air deposition data andalso with available (preferably multimedia) models. Other indicators of aquatic system nutrient enrichment (eutrophication) Algal standing crop (plankton and periphyton); anoxia/hypoxia for estuaries and large rivers Reflects a combination of inputs from all media (air, discharges to water, diffuse runoff, and groundwater inputs). Relative role of air deposition should ideally be compared with air deposition data andalso with available (preferably multimedia) models.

29 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

3.1.2 Potential Ecosystem Services We plan to identify the primary ecosystem serv ice(s) for each of the effect classes (e.g., acidification or enrichment) and for major eco system types and components (e.g., terrestrial ecosystems, soils, aquatic ecosystems). These services may be characterized as: supporting services that are necessary for all other serv ices (for example primary production), provisioning services (food, fuelwood), regulat ing services such as climate regulation or flood control, and
cultural services includin g spiritual or religious values, aesth etic values, recreation values among
others.
Define Change for Each Ecosystem Service: We intend to characterize the type of change, positive or negative, for each ecosystem service. This will be expressed in different ways, relative to the type of environmental system.
Identify the Indicator of Change: We plan to identify indica tors for the major types of change. These indicators may use chemical/physical properties (e.g., ANC), or they may involve biological endpoints (e.g., bioassessmen t metrics, such as a fish or benthic invertebrate Index of Biotic Integrity [IBI]).
Identify Databases of Indicator Conditions: The indicators selected will relate to available compendiums of literature abstracts or actual database systems (as stand alone files or accessed through Web portals) to provide readily available and transparent ways to document the
nature of the indicators and the indicator c onditions used to define the environmental impairments.
Identify and Address Temporal Issues: Different ecoregions or biological provinces may understandably display differing degrees of susceptibility to impairments or differing recovery potential, depending on past land use or pollution histories. Some ecological systems may be capable of fairly rapid recovery res ponses once pollutant load ings are significantly abated; other systems, such as larger estuarine aq uatic systems, may require much longer recovery times. These temporal issues will be documented in the analyses as possible.

30 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

3.2 Step 2 – Define Sensitive Areas That Exhibit Effect s Using Research Findings and GIS Mapping
3.2.1 Defining Sensitive Ecosystems Some ecosystems and areas of the United States are more sensitive than others to the effects of nitrogen and sulfur deposition (i.e., acid ification and nitrogen an d sulfur enrichment). In the risk/exposure assessment, we plan to be gin with those ecosystems and case study areas identified in the ISA and consider pote ntial near-field and far-field linkages.
Identify Biological, Biogeochemical, and Physiographic Linkages in These Ecosystems: Linked systems will be identified (e.g., upl and terrestrial/aqua tic areas linked to downstream estuarine system) where possible. Esp ecially for larger watershed or basin-scale systems, some components of these study areas (e.g., estuaries linked to inland fluvial drainage areas) should show both direct near -field effects from nitrogen or sulfur enrichment as well as linked, far-field effects related to load ings from the inland drainage areas ( Figure 3-2 ).

Figure 3-2. Ecosystem effects may range from near-field to far-field linkages. Assess Significance of Linkages: The spatial extent of linka ges supports different scales of risk assessment. For example, nutrient criteria could serve as the anch or of a broader-scale characterization of U.S. aquatic systems. Inland aci d-sensitive waters in the eastern United States and nitrogen-sensitive ecosystems in the Rocky Mountains and other part s of the western United States may support a large-scale, special-area a ssessment. Ecosystem effects in special areas,
such as the Adirondack Mountains or Class I ar eas of the United States may support area-level assessments. If the linkages are ge ographically significant, we plan to evaluate and determine the maximum scale of the case study area based on th e linkages. The resultant case study area may be a common ecosystem, a subbasin, a riverbasin, or an airshed, a nd may be local, subregional,

31 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

32 DRAFT – March 2008 or regional in scale. If an area is identified as not having importa nt linkages, then that area may be a good candidate for a case study area (e.g., loca l research of MeHg formation in Devil’s Lake, WI).
Of special interest will be the characterization of linkages that reflect conditions for all parts of the country. For instance, national nutrient crite ria for rivers and lakes can provide the
foundation for national-scale characterizations in volving nitrogen-enrichment effects for aquatic systems related to loadings from air deposition and other nitrogen sources. Available information on the nitrogen sensitivit y of estuarine systems can further extend the scope of these analyses to
include estuarine and other near-c oastal waters. Information that deals with special case study areas (e.g., the Adirondack Mountains or special alpine and subalpine eco systems in the West) that reflect impacts affecting sizeable regions may also be of interest. Figure 3-3 provides examples of significant linkages.
3.2.2 GIS Mapping To describe the national picture, we plan to map the locations of those sensitive ecosystems identified in Section 3.2.1 and identi fy the characteristics of the biological and biogeochemical properties that create the sensitivity. Identifying the key properties of sensitive systems may aid in estimating the sensitivity of currently unmapped areas. Sensitive areas can be identified at different spatial sc ales by using different approach es for defining the boundaries of the mapped units. Flexibility in the way sensitive areas are identified will enhance the utility of the highest-quality data. Such flexibility also f acilitates identifying important ecological services or related welfare values of these sensitive natural systems. For instance, primary data collections are often made at the spatial level of small plots or stands. Geographic Information System (GIS)–based spatial analysis tools may be used to gain more insights from the collection
of data. Models are often used to supplement direct measurements for larger spatial units, which may be reasonably localized catchments or la nd-cover patches. Integrated systems using monitoring, modeling, and interpolat ion approaches are often applie d for larger watershed units or physiographical areas related to characteristics of soils, topography, or surficial geology. Sensitive areas may be defined by focusing primarily on terrestrial or aquatic system features. Examples of datasets and GIS maps that can be used to locate ecosystem types are given inSecondary NAAQS Review for Oxides of Nitrogen and Oxides of SulfurFigure 3-3. Documented biological, biogeochemical, and physiographic linkages.33DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

34 DRAFT – March 2008 Table 3-4 . These materials may also be of valu e in identifying a list of nitrogen- and sulfur-sensitive ecosystem services and patterns and trends due to change s in reactive nitrogen and sulfur inputs (Section 3.7 describes this step).
Table 3-4. Example Datasets Planned for GIS Analysis Ecosystem Type Description Effect of Interest Indicator Parameter Internet Link or Other Resource Lakes, rivers, estuaries Nutrient criteria for lakes, rivers, and estuaries NE TN for lakes and rivers www.epa.gov/waterscience/ criteria/nutrient/ecoregions

Lakes, rivers, estuaries Acid sensitive waters of the United States ACID pH, ANC epamap4.epa.gov/cmap/vie wer.htm

All National atlas of Class I areas NE and ACID All www.nationalatlas.gov/mld/f edlanp.html All Ecoregions of the United States NE and ACID All nationalatlas.gov/atlasftp.ht ml#ecoomrp

All Soil characterization data NE and ACID pH, ANC, exchangeable Ca 2+ , dissolved organic content www.ncgc.nrcs.usda.gov/pro ducts/datasets/statsgo/descr iption.html

Lakes, rivers, estuaries Total nitrogen deposition and other analytes NE NH 4 + epamap4.epa.gov/cmap/vie wer.htm

Forests and grasslands U.S. vegetation NE and ACID N and S ivm.cr.usgs.gov/products.ph p Also available from NationalAtlas.gov Rivers, lakes, estuaries Surface water quality Many www.epa.gov/storet

Highelevation– based lakes and rivers U.S. elevation map ACID S ESRI 8.3 data disks

All Land cover NE and ACID N and S www.mrlc.gov/mrlc2k_nlcd.a sp All NADP grid files NE and ACID N and S nadp.sws.uiuc.edu/isopleths/ grids.asp Note: NE = nutrient enrichment, ACID = acidificati on, TN = total nitrogen, S = sulfur, N = nitrogen Recommended Mapping Layers and Models: Table 3-5 summarizes the current plan for GIS mapping layers and models to be appl ied in the risk/exposure assessment of targeted sensitive ecosystems. This plan is prelimin ary and will be reassessed as EPA proceeds in performing the seven steps for char acterizing ecological effects.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

35 DRAFT – March 2008 3.3 Step 3 – Select Risk/Exposure C ase Study Assessment Area Within a Sensitive Area
We intend to use the sensitive areas identified via Step 2 to select/delineate case study assessment areas for the risk/exposure assessment. Where case study or ecosystem-specific data are available, a subset of maps for th e case study assessment area may be created. Complementary to these efforts, we may use a statistical cluster anal ysis to group ecosystem units into similar sets. Clustering ecosystems might reduce the number of locations that need to be modeled to adequately characterize the variability in ecosystem response to changes in nitrogen and sulfur deposition.
In selecting areas to assess ecological effects from air deposition, the SAB Ecological Effects Subcommittee (EES) suggests considerati on of (1) clear quantifiable ecological effects due to air pollution, (2) the degr ee to which a significant compone nt of ecological effects are attributable to air pollution, (3 ) the responsiveness of ecosystem services to changes in air pollution,(4) the cumulative impacts of multiple ai r pollutants, (5) the abundance of ecological
effects and economic benefit cost analysis, and (6) the visibility to the public and value of resources at risk (U.S. EPA, 2005). While these r ecommendations were made in the context of a prospective cost-benefit analysis, many of these recommendations ar e sound in the context of our NAAQS risk analysis. The EES also provided so me specific critiques and recommendations of specific potential cas e study assessment areas. We plan to eval uate their comments as part of our case study assessment area selecti on process. Among other points the EES emphasized are as
follows:

 “Be cautious in any chosen assessment area when using surrogate sources to quantify ecological effects of air pollutants. For exam ple, ‘New’ nitrogen (or mercury) derived from air pollutants is generally more bioava ilable than ‘old’ n itrogen (or mercury). Moreover, ecosystems and associated organi sms respond differently to different species and sources of nitrogen (and mercury).” (p. 6).Secondary NAAQS Review for Oxides of Nitrogen and Oxides of SulfurTable 3-5. Summary of Indicators, Mapping Layers, and Models for Targeted EcosystemsTargetedEcosystemEffectIndicator(s)Mapping LayersModel(s)RemarksTerrestrialNitrogenEnrichmentCECC:N ratiosCa:Al ratiosAir wet/dry deposition (corrected forthroughfall using available data)Forest soils from USFSForest type from USFSStatsgo soilsNLCDCMAQ (N) by HUCMAGIC; PnET-BCGAquatic NitrogenEnrichment andEutrophicationNitrate and ammonia, total nitrogen(major reactive N species)Al toxicity dataChl-a (algal standing crop)anoxia/ hypoxia (primarily estuaries andtidal rivers)N loadings for sub-watersheds or largerbasins and Estuarine Drainage Areas(EDAs)EPA NCCR WQ index and NOAAEstuarine-Coastal Eutrophication IndexDiatom data for N-limited systemsSTORET retrievalsUSGS NAWQA informationUSGS SPARROW informationWQS Nutrient Criteria for riversand lakesEPA NCCR and NOAA estuarineeutrophication indicatorsNOAA EDAsEPA/NOAA airsheds for majorAtlantic and Gulf estuariesCMAQ (N) by HUCUSGS SPARROWPnET-BCGAquatic SulfurEnrichment(MeHg Focus)MeHg (ambient)MeHg (tissue residues)Sulfur (ambient and sediments)Devils Lake, WI, areaLimit to review ofprevious researchExamine studies for Devil’sLake, WI; also examineother literature fromMercury Report toCongress and recent workfor areas in thenortheastern United StatesTerrestrialAcidification Dueto Nitrogen andSulfurSoil ANCSoil pHCECInorganic AlCa:Al ratioSpecial areas (e.g., Class I areas,the Adirondack Mountains)CMAQ (N & S) by HUCForest soils from USFSStatsgo soilsUSFS lichenUSFS forest typesMAGIC; ILWAS;PnET-BGC36DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur37DRAFT – March 2008TargetedEcosystemEffectIndicator(s)Mapping LayersModel(s)RemarksAquaticAcidification Dueto Nitrogen andSulfurANCpHAlNO3and SO4fluxes or loadingsAcid-sensitive watersSelect Class 1 areasEPA STORETUSGS NWISCMAQ (N & S) by HUCMAGIC; PnET-BGC;SPARROWNote: CEC = cation exchange capacity, C:N = carbon:nitrogen, Ca:Al = calcium:aluminum, Chl-a = chlorophyll a, NCCR = National Coastal Condition Reports, WQ= water quality, NOAA = National Oceanic and Atmospheric Association, S = sulfur, USFS = U.S. Forest Service, NLCD = National Land Cover Data, HUC =hydrological unit, STORET = STOrage and RETrieval, USGS = U.S.Geological Survey, NAWQA = National Water Quality Assessment Program, WQS = waterquality standards, NWIS = National Water Information System, ILWAS = Integrated Lake-Watershed Acidification Study.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

 “It is important to choose [assessment areas] where atmospheric deposition itself can be distinguished from other sources contributi ng to ecological effect s of interest. Thus, selection of an [appropriate assessment] ar ea should be based not only on the type of ecosystem and its geographical location, but on the sources and types of air pollutants that impact it.” (p. 7)

 “The EES encourages the EPA to consider sites in different re gions with different resources at risk to help br ing attention to the importance of ecosystem valuation.” EPA could take advantage of the “opportunity to examine the effects of control of multiple pollutants individually and in combination.” (p. 7)
The EES also provided a summary table listi ng potential case study areas for examining ecological benefits of reducing atmospheric deposition. This table is reproduced in Table 3-6

. The ISA also recommended case study areas as candidates for risk/exposure assessments ( Table 3-7 ). Special emphasis was gi ven to the following:

 Adirondacks

 Shenandoah National Park

 Chesapeake Bay

 Alpine and subalpine areas of the Rocky Mountains.
We plan to consider these case study areas a nd any additional sites id entified by the Step 2 mapping exercise in our selecti on of risk/exposure assessment areas. Options for selecting case study assessment areas include s ite-specific quantitative modeling analysis, qua litative analysis, cluster analysis, and review and summary of prev ious risk/exposure assessments. It is likely that since multiple risk/exposure assessments may be needed depending on the number of effects characterized, a combination of a ssessment methods will be used.

38 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

Table 3-6. SAB/EES Listing of Potent ial Assessment Areas for Evaluation of Benefits of Reductions in Atmospheric Deposition

Ecosystem/ Region Main CAA Pollutant(s) Percentage(s) Attributable to Atmospheric

Deposition Quantitative Ecological and Economic Information EES Comments Coastal Waquoit Bay Nitrogen 30% Yes High priority. Higher loading from non-depositional sources may confound analysis. Chesapeake Bay Nitrogen 20–30% Yes High priority. Loading from diverse sources, particularly agricultural, may confound analysis. Long Island Sound Nitrogen; Mercury Nitrogen = 23– 35%; Mercury = ? Yes High priority. High nitrogen loading from wastewater treatment plants may confound analysis. Everglades Mercury 20–85% Ecological = yes; Economic = uncertain Medium priority. Reduction in atmospheric deposition has already resulted in decreased mercury burdens in fish and other biota. Lake Michigan Mercury 87% Ecological = yes; Economic = lacking Medium priority. Lack of quantitative economic data may restrict analysis. Barnegat Bay Nitrogen 50% total Direct deposition 30–39% Yes High priority. Direct linkage of ecological effects with atmospheric deposition; quantitative economic data exist. Tampa Bay Nitrogen; Mercury Nitrogen = 25–30% Yes Medium priority. Examined in previous EPA efforts. Variability in loading data may confound analysis. Gulf of Maine Nitrogen Low ? Low priority. Linkage of nitrogen loadings and ecological impacts is not well established. Major source of nitrogen is open-ocean influx. Casco Bay Nitrogen; Mercury Nitrogen = 30–40% Mercury = 84–92% Yes Medium priority. Good data on ecological and economic impacts are available.

39 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

40 DRAFT – March 2008 Ecosystem/ Region Main CAA Pollutant(s) Percentage(s) Attributable to Atmospheric

Deposition Quantitative Ecological and Economic Information EES Comments Forested Adirondacks Nitrogen; Sulfur; Mercury Nearly 100% Yes High priority. Good quantitative ecological and economic data exist. Previous studies can be augmented readily. Catskills Nitrogen; Sulfur Nearly 100% Yes Medium priority. Economic data may be lacking. Issues similar to the Adirondacks. Southern Appalachian Mountains Nitrogen; Sulfur Nearly 100% Yes Medium priority. Economic data on fisheries are available. Issues similar to the Adirondacks. Rocky Mountains Nitrogen Nearly 100% Yes Medium priority. Levels of nitrogen loading much lower than for northeastern locations. Economic data may be lacking.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of SulfurTable 3-7. Potential Assessment Areas Identified in the Draft ISA (U.S. EPA, 2007b)AreaIndicatorDetailedIndicatorArea StudiesModelsReferences in EPA, 2007bAquatic NutrientEnrichment;TerrestrialNutrientEnrichment;MercuryMethylationPIRLA I and II;AdirondackLakes Survey;EpisodicResponseProject; EMAPMAGIC;PnET-BGCBaker and Laflen, 1983; Baker et al., 1990b;Baker et al., 1990c; Baker et al., 1996; Benoitet al., 2003; Chen and Driscoll, 2004; Conferet al., 1983; Cumming et al., 1992; Driscoll etal., 1987a; Driscoll et al., 1991; Driscoll et al.,1998; Driscoll et al., 2001a; Driscoll et al.,2001b; Driscoll et al., 2003b; Driscoll et al.,2003c; Driscoll et al., 2007a; Driscoll et al.,2007b; Evers et al., 2007; GAO, 2000;Havens et al., 1993; Ito et al., 2002; Johnsonet al., 1994b; Landers et al., 1988; Lawrenceet al., 2007; NAPAP, 1998; Siegfried et al.,1989; U.S. EPA, 2003; Sullivan et al., 1990;Sullivan et al., 2006a; Sullivan et al., 2006b;U.S. Environmental Protection Agency,1995b; Van Sickle et al., 1996; Whittier et al.,2002; Wigington et al., 1996; Zhai et al., 2007ISAAdirondacksAquaticAcidificationILWASGherini et al., 1985Lit.SearchShenandoahNational ParkAquaticAcidification;TerrestrialNutrientEnrichmentShenandoahWatershedStudy; Fish inSensitiveHabitats (FISH)studyMAGICBaker and Christensen, 1991; Baker et al.,1990b; Bulger et al., 1999; Bulger et al., 2000;Cosby et al., 2006; Dennis and Bulger, 1995;Dennis et al., 1995; Deviney et al., 2006;Eshleman and Hyer, 2000; Eshleman et al.,1995; Eshleman et al., 1998; Galloway et al.,1983; Hyer et al., 1995; MacAvoy and Bulger,1995; Molot et al., 1989; Schofield andDriscoll, 1987; Sullivan et al., 2003; Sullivanet al., 2007a; Webb et al., 1995ISA41DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur42DRAFT – March 2008AreaIndicatorDetailedIndicatorArea StudiesModelsReferences in EPA, 2007bChesapeakeBayAquatic NutrientEnrichment;Aquatic NitrogenLimitedEutrophicationBricker et al., 1999; Bricker et al., 2007;Boesch et al., 2001; Boyer et al., 2002; Boyerand Howarth, 2002; Cooper and Brush, 1991;Fisher and Oppenheimer, 1991; Harding andPerry, 1997; Howarth, 2007; Kemp et al.,1983; Malone, 1991, 1992; Officer et al.,1984; Orth and Moore, 1984; Twilley et al.,1985ISAAlpine andSubalpineCommunitiesof the EasternSlope of theRockyMountains inColoradoAquatic NutrientEnrichment;TerrestrialNutrientEnrichmentbiomassproduction; NO3leaching;speciesrichnessBaron et al., 1994; Baron et al., 2000; Baron,2006; Bowman, 2000; Bowman and Steltzer,1998; Bowman et al., 1993; Bowman et al.,1995; Bowman et al., 2006; Burns, 2004;Fenn et al., 2003a; Fisk et al., 1998; Korb andRanker, 2001; Rueth et al., 2003; Seastedtand Vaccaro, 2001; Sherrod and Seastedt,2001; Steltzer and Bowman, 1998; Suding etal., 2006; Williams and Tonnessen, 2000;Williams et al.,1996a; Wolfe et al.,2001ISABeartoothMountain,WyomingAquatic NutrientEnrichmentalgaecompositionswitchSaros et al., 2003ISAFernowExperimentalForest nearParsons,West VirginiaTerrestrialNutrientEnrichmentforest growthAdams et al., 1997, 2000; DeWalle et al.,2006; Edwards and Helvey, 1991; Gilliam etal., 2006; Peterjohn, 1996ISAUintaMountains ofUtah and theBighornMountains ofcentralWyomingAquaticAcidificationlake NO3concentrationsWestern LakesSurveyU.S. EPA, 1987ISAPamlicoestuary inNorth CarolinaAquatic NitrogenLimitedEutrophicationhypoxia;phytoplanktonbloomPaerl et al., 1998ISASecondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur43DRAFT – March 2008AreaIndicatorDetailedIndicatorArea StudiesModelsReferences in EPA, 2007bBear Brook,MaineAquaticAcidification;TerrestrialNutrientEnrichmentsugar maple;red spruceElvir et al., 2003ISAHarvardForestTerrestrialNutrientEnrichmentforest growth—speciesMagill et al., 2004; Magill, 2004ISASouthernCaliforniaTerrestrialNutrientEnrichmentforest growth—species; coastalsage scrubFenn et al., 1996, 2003a; Takemoto et al.,2001ISAJasper RidgeBiologicalPreserve inCaliforniaTerrestrialNutrientEnrichmentgrasslandsZavaleta et al., 2003ISAAlleghenyMountains ofWest VirginiaAquaticAcidificationhighstreamwater orlake NO3concentrationsGilliam et al., 1996ISACatskillMountains ofNew YorkAquaticAcidificationhighstreamwater orlake NO3concentrationsMurdochandStoddard,1992; Stoddard, 1994ISAGreat SmokyMountains inTennesseeAquaticAcidificationhighstreamwater orlake NO3concentrationsCook et al., 1994ISALoch Vale,ColoradoTerrestrialNutrientEnrichmentold-sprucegrowthRueth et al., 2003ISATerrestrialNutrientEnrichmenttundracompositionswitchRockyMountainNational Park,ColoradoAquatic NutrientEnrichmentdiatom shiftsInterlandi and Kilham, 1998ISASecondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur44DRAFT – March 2008AreaIndicatorDetailedIndicatorArea StudiesModelsReferences in EPA, 2007bRockyMountainNational Park,ColoradoAquaticAcidificationsubalpine lakesMAGICSullivan et al., 2005Lit.SearchLake Tahoe,CaliforniaAquatic NutrientEnrichmentprimaryproductivity;chlorophyll aGoldman, 1988; Jassby et al., 1994ISALittle RockLake,WisconsinAquatic SulfurEnhancedMercuryMethylationbioaccumulationof Hg infreshwater fishHrabik and Watras, 2002; Watras and Frost,1989; Watras et al., 2006ISASouthernAppalachiansAquaticAcidificationStreamsMAGICSullivan et al., 2004Lit.SearchHubbardBrook, NewHampshireTerrestrialAcidification;AquaticAcidificationforestecosystem;soils; streamsmany studies fordecadesPnET-BGCGbondo-Tugbawa and Driscoll, 2002;Gbondo-Tugbawa et al., 2002Lit.SearchSecondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

3.4 Step 4 – Evaluate Current Loads and Effects to Case Study Assessment Area Including Ecosystem Services
This step involves evaluating the deposition and terrestrial a nd aquatic fate and transport of nitrogen and sulfur, as well as the ecological effects, and their subsequent effect on ecosystem services, resulting from exposure to certain le vels of nitrogen and sulfur. Depending on the adequacy and abundance of data for areas, th e evaluation may entail computer modeling, statistical analysis, or qualitative analysis.
3.4.1 Assess Data Availability and Adequacy

Determine Which Indicators Have Monitori ng Data and/or Modeling data Available for Analysis: The ISA contains a review of recent (2000 to pres ent) monitoring programs for targeted ecosystem effects and associated indica tors. We propose to use ANC as the first choice
of acidification indicators. Howeve r, we may consider other indicat ors if we determine that data
are insufficient or other factors demonstrate a ne ed to change indicato rs. Nitrogen and sulfur enrichment have both terrestrial and aquatic im pacts, making the selection of indicators more challenging. Currently, we have narrowed the list of indicators for addi tional evaluation to the following:

 Wet and dry nitrate and ammonia deposition

 Nitrate, ammonia, organic nitrogen throughfall deposition for terrestrial ecosystems

 Total nitrogen or constituent species’ loadings and fluxes to receiving waters from runoff, air, discharges, and groundwater inputs

 Algal standing crops and anoxia/ hypoxia for aquatic systems.
Environmental monitoring data and programs will be used to detect long- and short-term effects of nitrogen and sulfur deposition. Theref ore, we will consider the conditions of the monitoring programs supplying data for this as sessment; for example, nationally sponsored, long-term studies versus short-term academic research.
The ISA also reports literature available to assess impacts on ecosystem services that can be used to more fully describe the importance of effects on services that are important to the public.

45 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

Evaluate the Adequacy of Spatio-temporal Data and the Statistical Adequacy of
Available Data: We plan to evaluate the spatial ad equacy of available monitoring data including GIS mapping of documented data to id entify any meaningful spatial gaps. For each ecosystem effect, we plan to dete rmine if there is a temporal dime nsion to exposure. That is, if effects “lag” behind exposure at different scales of time, we plan to develop an approach to address those time-steps (e.g., measur e effects on a daily or annual scale), and we plan to seek
and assess data available for candidate indicato rs from both monitoring and modeling data that address the temporal dimensions required.

We intend to review and determine the best indicators for acidifica tion and nitrogen and sulfur enrichment for each targeted ecosystem effect. To accomplish this, we plan to identify spatial and temporal gaps, document uncertainti es and limitations raised in the research literature, and raise any additional points about un certainty identifiable th rough either statistical analysis or qualitative evaluation (dependi ng on the form of information available).
Resolve Gaps and Disadvantages (e.g., integrate datasets and/or models; interpolate): The location and type of ecosystem ef fects resulting from nitrogen and sulfur deposition do not lend themselves to a traditional large-scale ri sk/exposure assessment. This is due to both the isolated or regional effects obs erved and the potential lag in time for observed effects. Therefore, it will likely be necessary to resolve spatial a nd temporal gaps in data needed to perform a risk/exposure assessment. We propose to resolve gaps through a combination of integrating datasets, modeling, and interpolati on/extrapolation. We pl an to use CMAQ output data at 12 km grids for regions of interest, and an appropriate multimedia model to address spatial data needs for terrestrial and aquatic e xposure assessments for the targeted ecosystem effect. Additional information on CMAQ and multim edia models is presented in Appendices C and D, respectively.
3.4.2 Compute Loading and Exposure for Each Ecosystem Effect: We plan to use current data and models to analyze reactive nitrogen and sulfur loads and exposures. Major categories of lo ading data include the following:

 Atmospheric deposition across the landscap e (available from CMAQ modeling for 2002 for 8-digit hydrological units [HUCs] and a 12-km grid) ( Figure 3-4 presents past

46 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

applications of CMAQ modeli ng to support the evaluation of National Acid Precipitation Assessment Program [NAPAP]–monitored acidi fication effects in the United States.)

 Atmospheric deposition monitoring data (N ational Atmospheric Deposition Program [NADP] and Clean Air Status a nd Trends Network [CASTNET])

 National Pollutant Discharge Elimination Syst em (NPDES) point source discharge data

 Agricultural runoff modeling

 Urban non-point source runoff modeling

 Other urban or rural load ing sources (e.g., onsite septic system modeling).

Figure 3-4. Comparison of NAPAP-documented acid-sensitive ecoregions to CMAQ-modeled nitrogen deposition. CMAQ Deposition Modeling: It is necessary to understand the role of re ceptor surfaces (i.e., land use/land cover) influencing atmosphe ric deposition. Receptor surfaces affect the dry deposition of gases and particles. Dry deposition can contribute significantly to total deposition (in many locations and for many chemical speci es >50% of total de position). CMAQ modeling should account for the role of deposition due to bot h atmospheric conditions and land cover. We plan to use land cover datasets, such as the Nation al Atlas, to support our an alysis of the role of land cover in targeted ecosystem effects. Appe ndix C provides a more extensive description of CMAQ modeling plans for this risk/exposure assessment.
Multimedia Modeling: The ISA (U.S. EPA, 2007e) provides a survey of multimedia models, and a summary of the ISA review is in Appendix D. The recommended models for the risk/exposure assessment are presented here. The two distinct environmental effects of nitrogen and sulfur deposition for which indicators can be defined and models identified based on past

47 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

48 DRAFT – March 2008 applications, geographic applic ability, and use of atmospheri c inputs are acidification and nitrogen and sulfur enrichment.
To provide more detail for the models, thos e that readily accept atmospheric deposition inputs of the same nature as CMAQ output (flux in mass per area per time) are presented in bold. Those that accept atmospheric c oncentration data (mass per volum e) are presented in italics. Although these classifications note whether a model accepts atmo spheric deposition input, they do not specify whether temporal and spatial reso lutions will match between the atmospheric and ecosystem models.
Acidification As explained in Appendix A, acidification is an environmental effect in which acid precipitation lowers the natural pH of waterbodies and/or dama ges terrestrial ecosystems. The key indicators of acidification lik ely to be considered in any modeling efforts to determine the effects of acidification on an ecosystem include both chemical measurements and ecological/biological indices and fa ctors. The biological indicators of acidificat ion can be found in both terrestrial and aquatic systems; however , models are much less developed for simulating or estimating measures of the bi ological indicators on land. The ch emical indicators of acidity are much more likely to be included in a model.

Both MAGIC and WARMF 1 account for several of the chemical indicators of
acidification for both terrest rial and aquatic systems. Either of these models could be used for an acidification analysis, although WARMF contains a more robust analysis with higher-level processes, including biologi cal processes, and a GUI.
The ILWAS and WARMS models both provide for acidification indicators but have limited applications outside of their development areas of Illinois and Ontario, respectively. ILWAS is a highly parameterized model.

The TMDB/IBIS model does not include the mi nerals as its state variables but does simulate nitrogen and carbon, as well as vegetatio n, which could be considered a measure of
forest health. This model can be used on a global scale.

1 WARMF currently accepts atmospheri c deposition in concentration fo rm; however, EPRI and Systech Engineering, Inc., the developers of WARMF, have rece ntly undertaken a project to modify WARMF to accept depositional fluxes such as those output by CMAQ.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

49 DRAFT – March 2008 Several of the terrestrial/watershed models could be used to provi de input loads to a receiving-water model for acidification studies. The choice of model would depend on the timestep needed and the geographic location of analysis. Applicable terrestrial/watershed models
include PnET-BGC , DayCent, Century 4/5, NuCM , and SAFE

. Our current intention is to run MAGIC for a case study area to l ook at changes in ANC. The other models may be considered if further analysis would prove useful.
Nitrogen Enrichment Indicators of nitrogen enrichment are addr essed in detail in Section 3.1.1. Atmospheric deposition has been shown to increase nitrogen enri chment in Atlantic Coast estuaries and is now of concern in high-altitude, alpine lakes. Becau se the key indicators for nitrogen enrichment depend on measurements of nitrog en, there are many models availa ble to estimate the ecosystem effects, but special attention should be paid to how these models simulate the nitrogen cycle and whether biological processes, whic h are vital to the cycle, are in cluded. Nitrogen, either total or speciated, is probably the most highly modele d chemical parameter in ecosystems. Both receiving-water and terrestrial/watershed models may be used in these analyses. Additionally, it is possible to use a combination of models so th at a watershed model provides input loadings to the receiving water model.
The following receiving-water models all es timate speciated nitrogen (e.g., ammonia, nitrate, organic, or total) and can be used in most geographic locations: AQUATOX , QUAL2K
, WASP , and the CE-QUAL family of models.

The THMB/IBIS model simulates the nitr ogen cycle, but has limited geographic applicability for small sites. It is more applicable for large-scale simulations.

HSPF , SWAT 2 , and WARMF simulate the nitrogen cy cle across land and water; however, SWAT contains a more simplified appro ach to the nitrogen cycle in reservoirs and
lakes. HSPF accepts the atmospheric flux of nitrogen that is output by CMAQ. While SWAT currently only considers a state nitrate concentration in wet deposition, WARMF currently

2 SWAT currently accepts only a static input nitrate concentratio n in precipitation. An un tested concept to utilize SWAT’s fertilizer management function to simulate atmospheric deposition may provide a work-around function.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

accepts atmospheric concentrations of NO x , ammonia, and nitrate in both wet and dry atmospheric conditions.
Nitrogen enrichment, more than acidificati on, may be measured across the terrestrial landscape, in addition to waterbodies. Many of the terrestrial/watershed models may be used for nitrogen enrichment analyses, with the choice depending on time-step, geographic location, nitrogen species desired, and simulation of th e nitrogen cycle within the model. For daily simulations, possible models include: DayCent, DNDC, DRAINMOD-N II , EPIC/APEX , GLEAMS, INCA
, RHESSys , and GT/MEL . For longer-term simulations (monthly, annual), the following models may be used: Century, MAGIC
, MERLIN , PnET-BGC , ReNuMa, and
SPARROW

. Approach for Selecting Geographic Regions to Model: Selection of geographic regions of the United States to model will depend on a number of factors:

 Observed data that are indicative of the eco system and response of interest or observed characteristics of an ecosystem that implies it has the potential to respond

 Availability of a model capable of analyzing the region of interest

 Availability of model input data for the region.
It may be possible to explore adapting an ex isting model to an untested geographic area or using synthetic or surrogate data if there is a geographic region of pa rticular interest with insufficient model input data. In contrast, if there are multiple geographic regions suitable for modeling, we intend to examine the feasibility of clustering the regions, increasing the scale of the modeling to capture the regions , or selecting the one region with the best potential to provide valid and representative data on ecosystem response.
Assess Uncertainty in Loading and Exposure Computations: The risk/exposure
assessment will need to account for the fact that current runoff models vary in resolution. Runoff and other fate and transport models should be selected to adequately address the differing complexities of the identified sensitive ecosystems. To the extent possible, the analyses should try to include all reac tive nitrogen species, r ecognizing that inventories may need improving. Most likely, the assessments will initially be a snapshot of the loadings and exposures with more dynamic assessments to follow in future reviews. Where possible, the focus will be to develop

50 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

the best available numeric measures of the loading contribution from all major sources, so that the contributions from air deposition sources can be analyzed relativ e to all other major anthropogenic or natural sources for se nsitive ecosystems across the country.

3.5 Step 5 – Where Feasible, Scale- Up Case Study Assessment Area Findings to Sensitive Areas
Several approaches can be applied to take th e results of analyses for specific case study assessment areas and relate these findings to more spatially extensive sensitive areas. For instance, analyses using MAGIC taking advant age of good quality Temporally Integrated Monitoring of Ecosystems (TIME) and Long-Term Monitoring (LTM) data sets may be scaled up to several extensive ecological and physiological provinces such as the Adirondacks, the Appalachian Plateau (primarily in Pennsylvania), and the Ridge & Blue Ridge Provinces (largely in the Shenandoah National Park and along th e Shenandoah Parkway). Where the TIME/LTM datasets have previously been a pplied to define the extent of sensitive areas, scaling up the new analyses to the same previously used larger se nsitive areas may be readily justified. Where the case study assessment areas are defined using cluste rs of sampling sites, more sophisticated GISbased spatial interpolation techniques may be applied where the case study assessment area sites are related to bounding polygons reflecting standa rd ecoregion delineation systems (e.g., the EPA Omernik Level II ecoregions or the aggreg ated Omernik ecoregi ons developed for EPA
criteria for numeric nutrient crite ria for lakes and streams). Statis tical cluster anal ysis techniques may be applied to provide a means for objec tively grouping localized case study assessment areas into larger sensitive area s. For larger case study assessment areas such as the Chesapeake Bay system, available national indicator produc ts such as the USGS SPARROW models for HUC8 (sub-basin) watersheds, or EPA and NOA A estuary eutrophication indicators may be
considered as tools to document similarities and differences for major estuarine and near coastal aquatic systems, taking into account the rela tive importance of atmo spheric deposition of pollutants such as nitrogen to s ources within estuarine drainage areas related to point source, urban, or rural nutrient loadings.
A qualitative or semi-quantitative characterization of these larger sensitive areas may be developed, depending on data availability and time constraints. Where feasible, we plan to also discuss the effect on related ecosystem services of these sensitive areas.

51 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

3.6 Step 6 – Assess the Current Ecol ogical Conditions for Those Sensitive Areas
3.6.1 Calculate Response Curves for Each Indicator If feasible, we intend to generate indicator response curves from the multimedia model runs. The response curves can supply useful info rmation about the degree of impairment and sensitivity of the system.
3.6.2 Calculate Desired Exposure Endpoint Once the sensitive ecosystems response curves are generated, it may be possible to calculate a desired exposure endpoint . This requires similar information and steps required for a critical load analysis. Critical load analyses ma y be considered, where ap propriate, in identifying ways to select endpoints in terms of ecosys tem responses relative to atmospheric levels (concentrations or loadings) of n itrogen and sulfur. A detailed desc ription of critical loads and its use as an analysis tool are presen ted in the ISA (U.S. EPA, 2007b).
3.6.3 Develop GIS Maps and Overlay Loading and Indicators That Are Representative of Harmful Effects For each targeted ecosystem effect, we plan to create GIS maps from the modeling and monitoring data generated/compiled for selected indicators. Then, a second data layer can be created to represent the level of exposure at which harmful eff ects based on the indicator, and these data layers can be combined to facilitate mapping.
3.6.4 Using GIS Mapping, Compute the Extent That Loading Is Greater Than or Less Than the Harmful Effect Level (a.k.a. endpoint) We plan to use GIS to comput e the difference in exposure re lative to the response-based harmful effect level. We recognize that the loadin g and exposure may be less than or greater than the findings of the exposure response research. Ba sed on our findings, we plan to determine if a particular geographic region requires additional analysis or if we ar e able to proceed to Step 7.
3.6.5 Assess Ecosystem Responses via Ecosystem Services and Valuation One component of an integrated assessment of risks to the pub lic welfare involves determining what level of ecosystem response transl ates into an effect that could reasonably be considered adverse to the public welfare. There are a number of ways to do this, including the following:

52 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

 Direct measures of quantities that are of known value to the public (e.g., numbers of endangered species)

 Translation of ecosystem attributes into meas ures of ecosystem services, which can then be quantified

 Direct economic valuation of ecosystem func tions and services, including use and nonuse values (values that do not re quire an individual’s direct use of an ecosystem—for example, the value of preserving an enda ngered species habitat, even though that individual will ever see that species in the wild)

 Direct nonmonetary valuation of ecosystem functions based on enumeration of preferences using nonmonetized indices of preferences.
The specific methods used to evaluate advers ity will depend on the av ailability of data
and methods for the indicators of interest related to acidificati on and nutrient enrichment and on an assessment of the appropriatene ss of each type of qua ntification in comparing different levels
and forms of the standards. In our initial assessment of the available data, given the timeframe for this NAAQS review, we have determined that the most useful approaches will be those focusing on quantifying the link between change s in ecosystem indicators and ecosystem services. Linking these changes in ecosystem serv ices to changes in economic values will likely be beyond the scope of the current assessment. A brief discussion is provided in Appendix E on how valuation approaches might be used to co mbine and/or compare changes across ecosystem
services.
The EPA SAB Committee on Valuing the Protec tion of Ecological Systems and Services (SAB CVPESS) has recently dr afted major recommendations for improving ecological benefits assessment to facilitate consideration of ecosystem benefits in the decision-making process. This SAB panel recommends taking “a more comprehe nsive approach to assessing, valuing, and reporting on the ecological benefits of its act ions” (SAB CVPESS, 2007). The conceptual model
described in Figure 2-5 that links atmospheric concentra tions and deposition to ecosystem effects and biologically relevant indicators, and tying these to eco system services and valuation is supported by the SAB panel. One of the cr itical gaps identified by the SAB panel is “identifying how the biophysical effects of an action on an ecosystem w ill in turn impact the ecosystem services of importance to the public” (SAB CVPESS, 2007).

53 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

In broad terms, the most important informa tion gap in this review is an incomplete understanding of how hydrol ogical and biogeochemical pr ocesses (e.g., cycling of H 2 O, N, C, P) interact to control the respons e of ecosystem services to NO x and SO x and other forcing variables (e.g., climate). A major consequence of these intera ctions is that ecosystem services tend to be linked or “bundled,” so that ac tions taken to improve one may result in an improvement or deterioration of other services. Thus, when ecosy stem services are quantified and their ecological response functions to NO x and SO x are modeled, it is imperativ e that the entire bundle of services be evaluated, and that the linkages and tradeoffs among ecosystem services be included in the quantification (i.e., ecol ogical tradeoff functions [ETFs]). The key feature distinguishing the services from the underlying ecosystem functi on or processes is the explicit involvement of beneficiaries, so consideration of which ecosyst em services to target in assessment activities
involves consideration of the re lative demand for the service, its spatial distribution, and its magnitude.
Ecological Response Functions (ERFs) and Ec ological Tradeoff Functions (ETFs) can be developed to quantify the response of a service to changes in NO x /SO x concentrations and the tradeoffs between different services given these am bient air quality drivers. Many of the services of importance in assessing risk of NO x and SO x have not been quantified (i.e., developed ERFs and ETFs) or adequately scaled for a comprehensiv e evaluation of the benef its of these services. Such research is being planned, and will, no doubt, contribute to future NO x /SO x assessments. Therefore, at this time, data mining will be central to developing at least a preliminary assessment of potential impacts of NO x /SO x deposition and acidity on ecosystem services. In the current plan, process-based models are being cons idered to be used to (1) synthe size/link the suite of ERFs and ETFs and (2) generate maps and summaries of ecosystem services and tradeoffs in response to current and future ambient air indicators for NO x and SO x . The collection of response and tradeoff functions wi ll aid in the valuation of the serv ices at risk to these criteria pollutants where possible.
A risk/exposure assessment approach using the most relevant and best available data on deposition, acidity, and measured effects will be nefit from spatial and temporal mapping. In preparation for the NO x /SO x risk/exposure assessment, a suggested series of mapping exercises would proceed in the following order:

54 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

1. Identify areas/regions of the country receiving high levels of NO x /SO x deposition and acidity impacts.
2. In those regions, identify ecosystems sensit ive to elevated levels of nitrogen and sulfur, using some comm on selection criteria.
3. In those sensitive ecosystems, ask what ecosystem services are expected to be prevalent and “valued” (i.e., some s ubset of all potential services).
4. In those areas, identify what data are avai lable to develop ERFs and ETFs, at least to a qualitative degree that would enable produc tion of spatial and temporal maps to identify different degrees of protection th at would exist under alternative secondary NAAQS. Chan et al., (2006) produced such maps for several ecosystem services in the Central Coast ecoregion of California. The linkage and comparison of multiple ecosystem services in the region would pr ovide information for consideration of
tradeoff value of one service versus another.
Both economic and biophysical valuation can be considered in determining adverse effects and can be used in determining benefits of protection. As noted above, valuation is a useful way to compare disparate ecosystem impact s, and it is a potential ly important component in the risk characterization phase of the risk a ssessment. As a result, an additional step in the above process is to identify potentially relevant economic va luation studies for ecosystem services and map the location of these studies relative to the availa ble data on ecosystem
services. Note that while we are consideri ng all potential methods for obtaining ecosystem valuation estimates, the current focus of this plan is to use literature-based estimates of ecosystem service values previously identified in the ISA that can be applied in a valuation transfer approach. Realization of concepts like the conjoint analys is approach would require time and resources, which are not avai lable for the current analysis, but which should be considered
for future reviews.

3.7 Step 7 – Assess Alternative Levels of Protection Under Different Scenarios of Deposition From Ambient Sources
A secondary NAAQS standard, while national in scope, might have a form that allows for consideration of regional heterogeneity in ecosystem sensitivity and heterogeneity in

55 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

56 DRAFT – March 2008 atmospheric composition of nitrogen and sulfur deposition either alone or combined. Such a form should be designed to provide for the adequate protection of sensitive ecosystems and to allow flexibility for those ecosystems with mo re ecological resilience. Assessing alternative levels of protection with differe nt loadings scenarios under different forms of the standards will be an important step in this process.
We intend the risk/exposure assessment to characterize exposure and ecological effects associated with current levels of nitrogen and sulfur deposition as described in this Scope and Methods Plan. However, the NAAQS are ba sed on ambient concentrations of NO x and SO x in the atmosphere, and, therefore, additional anal yses are required to move from deposition-based risk estimates to policy-relevant ambient i ndicators. The policy asse ssment will build upon the current conditions risk/exposure as sessment to develop these policy -relevant ambient indicators ( Figure 3-5 ). This requires synthesizing the ecosyst em responses, biological indicators, and ecosystem effects related to deposition loadings and translating those loadings back to their corresponding ambient air conditions.

Figure 3-5. Risk/policy assessment paradigm for deposition-related ecological risks. R I S K / E X P O S U R E A S S E S S M E N T

P O L I C Y A S S E S S M E N TSecondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

After the current conditions risk/exposure assessment develops an understanding of baseline conditions, it will be necessary to defi ne the amounts of risk under varying levels and
forms of the standards. The risk /exposure assessment intends to establish whether current levels are causing effects, which ecosystems are most sens itive to those effects, and the magnitude of the risks existing at current levels. After this ba seline is established, mode ling techniques can be used to change the ambient level and exam ine how the corresponding exposures and risks change. In this way, dose-response curv es can be generated that es tablish the range of impacts and effects associated with nitrogen and sulfur in puts. For example, suppose at a particular level of loading (X), a negative effect is observed (Y). Th is analysis can be used to establish what X is equivalent to in terms of ambient concentra tions, so that Y can be reduced or avoided. Several iterations of this process may be needed to examine different forms and levels of the standard(s) and determine if the form(s) of the standard(s) ha s an impact on the risk associated with that
form.

Some of the issues associated with this type of an alysis include:

 What adverse effects are we trying to protect against?

 If total nitrogen is the relevant biological indicator, what is the relative contribution of oxidized versus reduced forms of nitroge n, and what is the ambient contribution compared to other sources?

 Do these effects occur due to different am bient levels and/or forms of nitrogen and
sulfur?

 How should alternative levels be selected (i.e., via a dose response curve, based on threshold events, or is anot her method more appropriate)?

 What are the correct temporal and spatial scal es for the level of the ambient air indicator and how do they relate to the temporal and spatial scales of the deposition indicator?

 What are the associated uncertainties?
In determining the appropriate level and fo rm of a standard, we plan to evaluate
alternative levels and forms of the standard and evaluate the ecological risks associated with those levels and forms. Due to limits in data, modeling, and time, we are not able to conduct a national assessment. Instead, we plan to extend the assessment area analysis approach used in the

57 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

current conditions analysis. Our initial thoughts on evaluating the appropriate level and form of the standard using the assessment area an alyses approach in clude the following:
1. Identify the relative contri bution of loadings associated with atmospheric deposition of nitrogen and sulfur.
2. Identify the most critical impacts from nitrogen and/or sulfur loadings (i.e., acidification, nutrient enrich ment, or eutrophication).
3. Identify the contribution to atmospheric loadings from total reactive nitrogen, NO x , and SO x. 4. Identify the biogeochemical indicators/res ources of concern in the assessment area and the ecosystem services associated with those indicators.
a. Determine the ecosystem service effects a ssociated with the most-critical impacts.
b. Bundle ecosystem services to find common metrics for comparison across
locations.
5. Define the exposure-response (loading-respons e) functions (ERFs) for the ecological
indicators of concern.
6. Estimate the loadings/exposures associated with current and alternative levels of the
NO x and SO x standards (using CMAQ modeling).
a. Analyze the relationships between NO x , SO x , and other reactive forms of nitrogen.
b. Assess the impacts of meteorological variability on these relationships.
7. Estimate the ecosystem impacts associated with estimated loadings.

8. Convert estimates of individual eco system risks to common units using

a. economic valuation based on benefits tran sfer from existing literature estimates
b. biogeochemical equivalents using eco logical tradeoff f unctions (ETFs).

9. Combine individual risk estimates to produce overall impact estimates.

Results from individual assessments can be evalua ted relative to the national maps of ecosystem types and sensitivities. This analysis may char acterize how well the assessment areas represent

58 DRAFT – March 2008Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

overall national ecosystem type s and sensitivities, and how similar impacts of alternative standards might be across ecosystems throughout the country. The output of the quantitative assessment and the science assessment feed the po licy assessment, so that it can examine a range of alternative standards applicable for the agen cy to consider. The following are questions to address in identifying specifi c elements of standards:

 How do alternative levels and forms of the standards relate to a given exposure metric?

 What are the appropriate averaging times for alternative levels and forms of the
standards?

 What alternative leve ls of the standards should be considered?

 Should there be alternative levels of the standards (i.e., individual NO x and SO x standards
or a combined NO x /SO x standard)?

 Do the ambient air indicator forms allow fo r site-specific protection while maintaining national consistency?

 Does the ambient air indicator adequately account for the effect s of total reactive nitrogen?

 Does the form of the standard have an impact on the risk?

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4. REFERENCES
Berresheim, H., P.H. Wine, and D.D. Davi s. 1995. Sulfur in the atmosphere. Pp. 251-302 in Composition, Chemistry and Climate of the Atmosphere . Edited by H.B. Singh. New York: Van Nostrand Reinhold. Chan, K.M.A., M.R. Shaw, D.R. Cameron, E.C. Underwood, and G.C. Daily. 2006. Conservation planning for ecosystem services. PLoS Biology 4 (11):e379. Heinz Center. 2006. Filling the Gaps, Priority Data N eeds and Key Management Challenges for National Reporting on Ecosystem Condition . A Report of the Heinz Center’s State of the Nation’s Ecosystems Project. The H. John Hein z Center for Science, Economics and the Environment, Washington, DC. Howarth, R.W. 2007. Chapter 6: Atmospheric de position and nitrogen pollution in coastal marine ecosystems. In Acid in the Environment: Less ons Learned and Future Prospects

. United States: Springer. NRC (National Research Council). 2004. Air Quality Management in the United States . 401 pp. Washington, DC: National Academies Press.
SAB CVPESS. 2007. Valuing the Protection of Ec ological Systems and Services . Science Advisory Board Committee on Valuing the Protection of Ecological Systems and Services Draft Report, September 24. Skjelkvale, B.L., J.L. Stoddard, D.S. Jeffries, K. Torseth, T. Hogasen, J. Bowman, J. Mannio, D.T. Monteith, R. Mosello, M. Rogora, D. Rz ychon, J. Vesely, J. Wieting, A. Wilander, and A. Worsztynowicz. 2005. Regional scale ev idence for improvements in surface water chemistry 1990–2001. Environmental Pollution 137 (1):165–176. U.S. EPA (Environmental Protection Agency). 2007a. Plan for Review of the Secondary National Ambient Air Quality Standards fo r Nitrogen Dioxide and Sulfur Dioxide

. Available at http://www.epa.gov/ttn/ naaqs/standards/no2so2sec/index.html. U.S. EPA (Environmental Protection Agency). 2007b. Preliminary Draft Excerpts of the Integrated Science Assessment for the NO x and SO x Secondary Draft document. Presented at the PM Secondary NAAQS Kickoff Meeting and NO x /SO x Secondary NAAQS Workshop, Chapel Hill, NC, sponsored by the U.S. Environmental Protection Agency, July 16–19. U.S. EPA (Environmental Protection Agency). 2007c. EPA’s Report on the Environment, SAB Review Draft . Office of Research and Devel opment, Washington, DC. EPA 841-B-06- 002. December. U.S. EPA (Environmental Protection Agency). 2007d. The U.S. Inventory of Greenhouse Gas Emissions and Sinks: Fast Facts . Office of Atmospheric Programs. EPA 430-F-07-004. April. U.S. EPA (Environmental Protection Agency). 2007e. Integrated Science Assessment for Oxides of Nitrogen and Sulfur Environmental Criteria. Office of Research and Development, Research Triangle Park, NC. EPA/600/R-07/145A, December.

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62 DRAFT – March 2008 U.S. EPA (Environmental Protection Agency). 2006. Wadeable Streams Assessment . Office of Research and Development, Wa shington, DC. EPA/600/R-07/045. May. U.S. EPA (Environmental Protection Agency). 2005. EPA-COUNCIL-ADV-05-001 Advisory on Plans for Ecological Effects Analysis in the Analytical Plan for EPA’s Second Prospective Analysis—Benefits and Co sts of the Clean Air Act, 1990–2020 . June 23. U.S. EPA (Environmental Protection Agency). 2003. Response of Surface Water Chemistry to the Clean Air Act Amendments of 1990 . U.S. EPA Office of Research and Development, National Health and Environmental Effect s Research Laboratory, Research Triangle Park, NC. EPA/620/R-03/001. U.S. EPA (Environmental Protection Agency). 2000. Nutrient Criteria Technical Guidance Manual: Rivers and Streams . Office of Water, Wash ington, DC. EPA-822-B-00-002. July. U.S. EPA (Environmental Protection Agency). 1998. Guidelines for Ecological Risk Assessment

. Risk Assessment Forum. EPA/630/R-95/002F. April. U.S. EPA (Environmental Protection Agency). 1995. Review of the National Ambient Air Quality Standards for Nitrogen Dioxide: Assessment of Scientific and T echnical Information

. OAQPS Staff Paper. EPA- 452/R-95-005. September. U.S. EPA (Environmental Protection Agency). 1994. Review of the Natio nal Ambient Air Quality Standards for Sulfur Oxides: Assessment of Scientific and Tec hnical Information

. Supplement to the 1986 OAQPS Staff Pape r Addendum. EPA 452/R-94-013. September. U.S. EPA (Environmental Protection Agency). 1992a. Framework for Ecological Risk Assessment . Risk Assessment Forum. EPA/630/R-92/001. February. U.S. EPA (Environmental Protection Agency). 1992b. Report on the Ecological Risk Assessment Guidelines Strategic Planning Workshop . Risk Assessment Forum. EPA/630/R-92/002. February. U.S. EPA (Environmental Protection Agency). 1991. Summary Report on Issues in Ecological Risk Assessment . Risk Assessment Forum. EPA/625/3-91/018. February. U.S. EPA (Environmental Protection Agency). 1986. Review of the National Ambient Air Quality Standards for Sulfur Oxides: Updated Assessment of Scientific and Technical Information . Addendum to the 1982 OAQPS Staff Paper. EPA 450/05/86-013. December. U.S. EPA (Environmental Protection Agency). 1982. Review of the National Ambient Air Quality Standards for Sulfur Dioxide: Assessment of Scientific and Te chnical Information

. OAQPS Staff Paper. EP A-450/5-82-007. November. U.S. EPA (Environmental Protection Agency). 1971. Air Quality Criteria for Nitrogen Oxides

. Air Pollution Control Office. No. AP-84. January.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

APPENDIX A

IDENTIFICATION OF SENSITIVE ECOSYSTEMS A.1 Targeted Ecosystem Effects

One of the central issues in this seco ndary NAAQS review is determining which ecosystems are sensitive to nitrogen and sulfur deposition and their degree of sensitivity compared to one another. Characteristics to determine sensitivity may include, but are not limited to, (1) potential nitrogen and sulfur rete ntion rates; (2) potenti al nitrogen and sulfur uptake rates, which might include vegetative uptake, potential denitr ification, and potential mobilization of nitrogen and su lfur; (3) potential residence time based on local hydrology (precipitation rates, conductivity) and geology (b edrock type, pervious surfaces, soil properties); and (4) total supply of nitrogen and sulfur, in cluding current and historical atmospheric deposition and other nonatmosphe ric sources (e.g., applied fer tilizers, sewer leaks, point sources). Other ecosystem-specific characterist ics that may help assess sensitivities include threatened and endangered species data where av ailable, land cover, land-use type (including Class I, National Park, and Fish and Wildlife Re fuges and National Wilderness areas), species of community shifts (or invasive species), and ba seline nitrogen and sulf ur loading estimates. Where ecosystem-specific data are available, a subset of maps for the study region may be created. Complementary to these efforts, we may use statistical clus ter analysis to group ecosystem units into similar sets. By cluste ring ecosystems, we might reduce the number of locations that need to be modeled to adequa tely characterize the va riability in ecosystem response to changes in nitrogen and sulfur deposition.
These types of analyses may aid in dete rmining whether area-based risk/exposure assessments are appropriate for looking at nitr ogen and sulfur effects on various ecosystems and geographic regions as a means of extrapolating th ese impacts to character ize the entire country. For those areas where data are available, watershed models (e.g., MAGIC, PnET-BGC, DayCent-chem) may be useful for evaluati ng the emission-deposition-ecosystem response linkage.
These are the main questions the risk/exposure assessment will address:

 If we can identify appropriate biologically relevant indicators, can we establish a link between deposition of NO x (and/or reactive nitrogen), SO x , and ecosystem response?

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 What ecosystem services are associated with changes in emissions?
Ecosystem response to nitrogen and sulf ur deposition varies across the landscape, depending on the local physical, chemical, a nd biological characteri stics. Atmospheric deposition effects range from sp ecies alterations (due to nutri ent enrichment) to anthropogenic eutrophication of estuaries to ac idification of forests, lakes, and streams. This Scope and Methods Plan focuses on five ecosystem eff ects on terrestrial and aquatic systems.
A.1.1 Terrestrial Nitrogen Enrichment Deposited nitrogen compounds can act as a fertili zer to increase the pr oductivity of plants and algae. However, too much nitrogen can lead to a surplus of nutrients, resulting in overnutrient enrichment. Table A-1 presents examples of nitrogen enrichment effects, which can impact species diversity by favor ing nitrogen-tolerant species ove r other species that are more sensitive to nitrogen limits. Certain ecosyste ms may be dominated by species that have a competitive advantage in low nitrogen environmen ts. When nitrogen increases, species that are normally kept in check by low nitrogen levels fl ourish and out-compete the other species in the community; thereby, potentially altering specie s’ composition and diversity, nutrient cycling, and other ecosystem properties and functions. New plants may also move into ecosystems that are enriched in nitrogen, further challenging the native species. Animals th at depend on specific plants for habitat and food may th en be threatened by the changes to the plant communities that result from nitrogen inputs.
Table A-1. Examples of Nitrogen Enrichment Effects Nitrogen Load (kg N ha-1• yr-1) Nitrogen Enrichment Effects Altered algal communities in high-elev ation freshwater lakes. (Colorado) >1.5 Elevated N in tree leaf tissue high-elevation forests. (Colorado) 3 to 8 Mortality of sensitive lichen species. (West Coast) 5 to 35 A transect of 68 acid grasslands; specie s richness declines as a linear function of the rate of inorganic nitrogen deposition, with a reduction of one species per 4-m 2 quadrat for every 2.5kg N -1 yr -1 . (United Kingdom) 7 Community-level shifts in ombrotrophic bogs. (North Central and Eastern United States) <8 to 10 N retained or denitrified (i.e., limited leaching). (many U.S. forests) 10 to 15 Change in plant species’ competitive interactions lead to community-level shift in native grassland. (California)

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Increased nitrogen deposition in western grass/sh rub lands is also implicated in increased fire frequency in some areas because the nitr ogen over-enrichment has favored the growth and production of fire-prone grass species. Emergi ng research has also li nked increased nitrogen deposition to habitat alteration for threatened species.
Nitrogen enrichment, in combination with oz one exposure, causes major changes in tree health by reducing fine-root biomass and car bon allocation below ground and by greatly decreasing the lifespan of pine foliage. Nitrogen en richment results in greater leaf growth, while

ozone causes premature leaf loss at the end of the growing season. The net result of these pollutants is significant litter accumulation on the fo rest floor. Nitrogen cycling rates in soil are also stimulated by high nitrogen inputs, resulting in large leachate losses of nitrate from these
watersheds and elevated fluxes of nitric oxide gas from soil. In coastal sage ecosystems that
occur in low-elevation sites in this region, greenhouse and fiel d studies indicate that nitrogen deposition may be one factor that enhances the invasion of exotic annual grasses.
A.1.2 Aquatic Nitrogen Enrichment and Eutrophication Atmospheric deposition of nitrogen is a si gnificant source of to tal nitrogen to many
estuaries in the United States. National O ceanic and Atmospheric Association (NOAA) has
calculated the amount of nitrogen entering estuaries ultimate ly attributable to atmospheric deposition for many of the East Coast estuaries. The amount of nitrogen entering estuaries due to atmospheric deposition varies widely, depending on the size and location of the estuarine watershed and other sources of nitrogen in the watershed. A number of uncertainties may result in a greater relative contribution of atmospheri c deposition in some places. In addition, episodic inputs, which may be ecologically significant, ma y be higher than the annual average. Studies have shown that atmospherically deposited nitrogen (AD-N) contri butes from 20 to >40% of the new nitrogen flux to estuaries and waters along the East Coast (Whitall and Paerl, 2001). The areas with the highest deposition rates stretch from Massachusetts to the Chesapeake Bay, and along the Central Gulf Coast.
The supply of nitrogen tends to limit the pr oductivity of coastal ecosystems. However, nitrogen over-enrichment can alte r a series of complex biogeoc hemical cycles that affect community processes (e.g., competition, community structure) and ecosystem processes (e.g., ecosystem efficiency, decomposition). Approximate ly 60% of estuaries in the United States

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(65% of the estuarine surface area) suffer from over-enrichm ent of nitrogen, resulting in excessive algal growth, the outcome of whic h is eutrophication—the depletion of oxygen concentrations as the algae die and decompose. Symptoms of eutrophica tion include changes in the dominant species of phytoplankton (the primar y food source for many kinds of marine life), low levels of oxygen in the water column, fish and shellfish kills, outbreaks of the toxic dinoflagellate, such as Pfiesteria piscicida , and cascading population changes up the food chain. In addition, encrustation and incr eased levels of turbidity in th e water due to large amounts of algae can kill off submerged aqua tic vegetation, which is an impor tant habitat for many estuarine fish and shellfish species.

Often the most severe result of eutrophication is the depl etion of dissolved oxygen (DO) in the water column by the decomposition of orga nic matter, produced within the ecosystem as a result of the nitrogen saturation. Anoxia (lack of oxygen) or hypoxia ( DO concentrations lower than required by indigenous organisms) is a part icular concern in coastal estuaries that exhibit density stratification (the divisi on of water into layers with different temperatures and oxygen content), which occurs mostly during the summ er months. Organic matter produced in lighted surface waters either sinks to the bottom waters where it decomposes, consuming oxygen inventories that are not replen ished by photosynthesis, or it mixes with oxygen-rich surface waters. These low-oxygen zones may cause massi ve deaths of aquatic life and reduce the population densities of many important commerci al fish and shellfish. Hypoxic bottom waters have expanded during the latter 20th century in many coastal eco systems, including large areas
of the continental shelf of the northern Gulf of Mexico near the mouth of the Mississippi River. Many of the highly eutrophic es tuaries are along the gulf and mid-Atlantic coasts, overlapping many of the areas with the highest nitrogen deposition, but there are eutrop hic estuaries in every region of the conterminous U.S. coastline.
Emerging ecological studies in the western Unit ed States demonstrate that some aquatic and terrestrial communities are significantly al tered by increased nitrogen deposition. The major concerns are for ecological systems that are natu rally adapted to low-nitrogen inputs, because increases related to anthropogeni c atmospheric deposition can lead to nitrogen saturation. Where the inputs of nitrogen exceed the ecological syst em’s need for them, excess nitrogen is leached into surrounding waterways. Althoug h much of the western United St ates is exposed to relatively low deposition of nitrogen, hotspots of n itrogen deposition occu r downwind of large

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metropolitan areas or large agricultural operations. Some of the most sensitive systems affected by the elevated nitrogen deposition include high-e levation catchments in Colorado and chaparral
catchments in the southwestern Sierra Nevada Mountains. The primary c oncerns in the West have been for the critical load of nitrogen de position, which affects both terrestrial and aquatic resources through eutrophicati on and/or nitrogen enrichment, thereby altering community

structure. The over-enrichment of nitrogen deposition in high-elevation lakes has led to increased biomass of phytoplankton, resulting in eutr ophication and shifts in diatom community composition. In the western United States, episodic acidification is also an important issue for surface waters throughout high-elevation areas. Where soils are sparse, as in alpine regions, most snowpack nitrogen is flushed to surface waters early in the snowmelt pe riod. In addition to highelevation lakes, snowpack nitrogen has also been reported to cause temporary acidification of alpine streams. Snowmelt-rela ted temporary acidification of alpine lakes and streams and associated effects have been reported in numer ous studies, largely for CAA Class 1 areas in the southern and central Rocky Mountains.
Increased nitrogen deposition in western arid and semiarid grass/shrub lands is also implicated in increased fire frequency in so me areas because the nitrogen over-enrichment has favored the growth and production of fire-prone grass species. Em erging research has also linked increased nitrogen deposition to habitat alteration for threatened species. Nitrogen fertilization experiments in arid and semiarid plant communities have shown that changes in plant biomass
associated with increased nitr ogen deposition tend to alter sp ecies’ composition, with negative impacts on biodiversity. Such plant community cha nges resulting from experimental fertilization have been reported in Joshua Tree National Park in California, coastal sage shrub communities of southern California, areas in the Chihuahuan Desert and Mojave Desert, and for arid grassland
areas on the Colorado Plateau.
A.1.3 Aquatic Sulfur Enrichment Atmospheric deposition of sulfur to pr imarily aquatic environments with high concentrations of organic matter leads to an increase in production of MeHg. The increased availability of sulfur as sulfate in la kes and streams due to the deposition of SO x accelerates the conversion of elemental mercury and mercuric salts to highly toxic, bioaccumulative, and

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persistent MeHg compounds that build up in living ti ssue and increase in concentration up the food chain. Sulfate-dependant bacteria, al ong with methanogenic (methane-producing) microorganisms, are involve d in the conversion of Hg 2+ to MeHg under the anaerobic conditions found in wetlands, river sediments, and in certain soils. The presence of sulfates stimulates the growth of these methylating microbes. Acid rain is thought to increase biomethylation as more MeHg is formed under acidic conditions (less than a pH of 6).
The “cause-and-effect relationship” between sulfur and mercury deposition from the atmosphere has been demonstrated in the lab and in small-scale field experiments. Using sulfate levels equivalent to hi storical levels of sulfate deposition from acid rain in the northeastern United States, researchers in Minnesota have been able to the confirm a surge in MeHg levels with increasing levels of sulfates in a large-sc ale, fresh water wetland ecosystem (Jeremiason et al., 2006). While an in-depth assessment of mercury methylation relative to NO x and SO x inputs
is not feasible at this time, reducing NO x and SO x inputs may also reduce the mercury methylation potential of some systems.
A.1.4 Terrestrial Acidification Due to Nitrogen and Sulfur The current understanding of th e effects of acidifying deposition on forest ecosystems has focused increasingly on the biogeochemical proce sses that affect plant uptake, retention, and cycling of nutrients within fore sted ecosystems. Research results from the 1990s indicate that decreases in base cations (e.g., ca lcium, magnesium, potassium) from soils are at least partially attributable to acid deposition in the northeastern and southeaste rn United States. Base cation depletion is a cause for concern because of the role these ions play in acid neutralization, as
discussed above: In the case of calcium, magnesium, and potassium , these are essential nutrients for plant growth and physiology (e.g., nutrient upta ke). As mobile aluminum increases due to soil acidification, the calcium/al uminum ratios change, which is partly related to the higher affinity of aluminum during passive uptake by ro ots. The change in these relative nutrient proportions has been correlated w ith declining forest health. Recen t research indicates that the loss of cations also leads to aluminum leaching from the soil to stream waters, which can have
harmful effects on fish.

The loss of calcium from forest soils and fo rested watersheds has now been documented as a sensitive, early indicator of the soil response to acidifying deposition for a wide range of

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forest soils in the northeastern United States (Likens, Driscoll , and Buso, 1996). There is a strong relationship between acid deposition and leaching of base cations from soils in hardwood forests (e.g., maple, oak), as indicated by long-term data on watershed mass balances, plot- and watershed-scale acidification experiments in the Adirondack Mountains and in Maine, and
studies of soil solution chemistr y along an acid-deposition gradient from Minnesota to Ohio.
A.1.5 Aquatic Acidification Due to Nitrogen and Sulfur Acidifying deposition causes acidification of sensitive surface waters. The effect of acidifying deposition on aquatic systems depends largely upon the ability of the ecosystem to
neutralize the additional aci d. This is referred to as an ecosystem’s ANC.
ANC levels depend largely on a watershed’s physical charact eristics: geology, soils, and size. Water systems that are sensitive to acidifica tion tend to be located in small watersheds that have few alkaline minerals and shallow soils. Larg e, forested watersheds have been shown to acidify during large rainfall and snowmelt episod es. As acidity increases, aluminum leached from soils and sediments flows into lakes and streams and can be toxic to aquatic species. The lower pH levels and higher aluminum levels that result from acidificati on make it difficult for some fish and other aquatic species to survive, grow, and reproduce. In some waters, the number of fish species able to survive has been directly correlated to water acidity. Acidification can also decrease fish population density and individual fish size.
In western regions, some high-el evation lakes, particularly in the Rocky Mountains, have become acidic, especially during snowmelt. Howe ver, while many western lakes and streams are sensitive to acidification, they are not subject to con tinuously high levels of acid deposition, and, therefore, have not become chronically acid ified. During the 1980s and 1990s, an integrated study of atmospheric deposition, terrestrial ec osystems, and aquatic ecosystems was conducted
in several watersheds in the Sierra Nevada M ountains to determine if acidifying deposition was affecting these areas and to infer the implications of acidification on surface waters in the region. Chronic acidification of high-elevation surface wa ters in the Sierra Nevada Mountains was not found, but episodic changes in stream water chem istry did occur. In many of the watersheds studied, for example, the pH decreased as the wa ters became more acidic with increasing runoff, reaching a minimum during peak snowmelt.

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Anthropogenic atmospheric nitrogen and sulfur deposition impacts the surface seawater chemistry by increasing acidificati on and reducing alkalinity. The impacts of these changes are
more substantial in coastal waters where the eco systems are more vulnerable due to other human impacts, such as nutrient enrichment and pollutio n. Ocean acidification is a significant threat to
coral reefs and coastal benthic and planktonic foodwebs which, in turn, impacts fish populations.

A.2 Additional Effects Related to Acidifi cation and/or Nutrient Enrichment on Sensitive Ecosystems
It important to note that several additional indicators are worthy of investigation:

 Nitrogen saturation

 Maple decline

 Ammonia air deposition and toxicity to native mussels

 Relationship between acidity/nutrient en richment and mercury methylation

 Sensitive areas for acidity/nutrient enrichment impacts.
A concise summary of information on each topi c is introduced in the following sections.
A.2.1 Nitrogen Saturation Terrestrial and aquatic ecosystems in the eas tern and northeastern United States receive nitrogen deposition loadings rela ted to air pollution that are fa r in excess of natural background
levels. When combined with othe r acid rain impacts to soil systems and receiving waters, the terrestrial ecosystem, in particular, can become nitrogen saturated. This leads to the increased “leakage” of nitrogen into groundw ater and surface water. Studi es are now documenting similar
concerns for sensitive areas that often involve national parks in the Appalachians (e.g., Smoky Mountains National Park), the Rocky Mountains, the Cascades, and other areas in the West. Several forests throughout the Unit ed States are beginning to show signs of nitrogen saturation, a condition where the inputs of nitrogen exceed the forest’s need for them and excess nitrogen is leached into surrounding waterways. Significant st udies include those identified by Driscoll and colleagues (2001); Fenn and coll eagues (2003); and NAPAP (2005).
A.2.2 Maple Decline Maple decline is a generalized term for a se t of symptoms that may be applied to any species of tree suffering a wide range of different stressors, resulting in a loss of vigor or habitat.

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These symptoms have been studied for the northea stern United States in recent decades, with a focus on native maples, such as the sugar maple. In rural areas, maple dec line is often attributed to soil acidification caused by acid rain. Soils that have devel oped from nutrient-poor parent materials, such as sandstone, qua rtzite, and granite are most sens itive to acidification. Some of
the best-documented work has been conducted at the Hubbard Brook Experimental Forest (HBEF), a 3,160-hectare reserve near North Woodstock, NH, where scientists have measured soil composition for the past 50 years. A recent study (Juice et al., 2006) documents how scientists added nutrients in a test plot to repli cate soil conditions that exis ted prior to the loss of sugar maples over the past 25 years. This reprod uced the favorable soil conditions that existed prior to 20th-century industrial pollution, with the result that sugar maples on the test plot rebounded dramatically. Nitric and sulphuric acid in acid rain leaches calcium from the soil. Calcium is the second most abundant plant nutr ient after nitrogen. In addition, the loss of calcium leads directly to acidic so ils. When soils become too acidi c, trees such as sugar maples
become stressed and have a harder time growing or producing seeds and seedlings.
A.2.3 Ammonia Air Deposition and Toxicity to Native Mussels Across North America, populations of freshwat er mussels have fallen drastically to the point where more than 70% of native unioni d mussel species are considered endangered, threatened, or of special concern. Ammonia toxicity is of concern for the survival of juvenile mussels (the most sensitive life stage), and EPA’ s Office of Water (OW) is pursuing studies to
consider revisions to its total ammonia criterion guidance to provi de protections against acute or chronic toxicity (U.S. EPA, 2005).
A.2.4 Relationship between Acidity/Nutrient Enrichment and Mercury Methylation Research recently summarized for the northeaste rn United States suggests a relationship between sulfur deposition and processes promo ting mercury methylation, which increases the risks for bioaccumulation and biomagnification in food chains. Ongoing research in the Gulf of Mexico is also addressing the following three research goals:

 Research Goal 1 – Test the hypothesis that ra tes of MeHg production in coastal sediments are in part controlled by temporal and spat ial hypoxia patterns that result from coastal eutrophication, and that maximum MeHg pr oduction occurs in regions adjacent to hypoxic zones.

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 Research Goal 2 – Test the hypothesis that coastal eutrophication and hypoxia can result in elevated MeHg accumulation and biomagnifi cation in red snapper and gray snapper, both commercially and recrea tionally important fish sp ecies in this region.

 Research Goal 3 – Test the hypothesis that anglers who proportiona tely consume fish from areas of higher MeHg production relate d to hypoxia will have higher rates of mercury exposure (as measured by the concentr ation of mercury in hair) than anglers consuming similar amounts of fish from ot her coastal Louisiana locations where hypoxia
does not occur.
The EPA ORD has also done some prelim inary work on developing large-basin multimedia modeling systems that would handle impacts from nutrients (especially nitrogen species) and the complicated processes related to mercury. Significant sour ces identified include Chesney and colleagues (2006), Driscoll and co lleagues (2007), and Evers and colleagues (2007).

A.3 Sensitive Areas for Acidity/Nutrient Enrichment Impacts
Research accomplished through the NAPAP (2005) has identified spa tial areas in terms of both terrestrial and aquatic ecosystems that show sensitivity to impacts from acid rain–related air pollution ( Figure A-1 ). The NAPAP has been able to deve lop GIS layers of such sensitive areas as ecoregions or hydrologic basins.
The degree to which other types of sensitive areas (related to such themes as nitrogen saturation, maple decline, or the locations of waterbodies showi ng nitrogen limitations) can be at least robustly defined in terms of their geographical extents could be examined. Where such geospatial data stratification materials can be identified, this would be helpful in both the analysis of indicators and in the development and interpretation of results from models. An
examination of these materials would also be useful in identifying critical loads for acidification and nutrient enrichment stressors, which in most cases will vary for different ecoregions and waterbody types.

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Figure A-1. Sensitive ecological receptors in the United States.
References
Chesney, E., R. Lincoln, M.S. Bank, D.B. Senn, D.J. Vorhees, P. Grandjean, and N.N. Rabalais. 2006. Exploring Links Between Mercury, the Co astal Environment, and Human Health in Coastal Louisiana . Available at http://www.sdafs .org/laafs/MeetingProgram200616.pdf. Driscoll, C.T., Y.-J. Han, C.Y. Chen, D.C. Evers, K.F. Lambert, T.M. Holsen, N.C. Kamman, and R.K. Munson. 2007. Mercury contamination in forest and freshwater ecosystems in the northeastern United States. BioScience 57 (1):17–28. Driscoll, C.T., G.B. Lawrence, A.J. Bulger, T.J. Butler, C.S. Cronan, C. Eager, K.F. Lambert, G.E. Likens, J.L. Stoddard, and K.C. W eathers. 2001. Acidic deposition in the northeastern United States: s ources and inputs, ecosystem effects, and management strategies. BioScience 51 :180–198. Evers, D.C., Y.-J. Han, C.T. Driscoll, N.C. Kamman, M.W. Goodale, K.F. Lambert, T.M. Holsen, C.Y. Chen, T.A. Clair, and T. Bu tler. 2007. Biological mercury hotspots in the northeastern United States and southeastern Canada. BioScience 57 (1):29–43. Fenn, M.E., R. Haeuber, G.S. Tonnesen, J.S. Baron, S. Grossman-Clarke, D. Hope, D. Jaffe, S. Copeland, L. Geiser, H.M. Rueth, and J.O. Sickman. 2003. Nitrogen emissions, deposition, and monitoring in the western United States. Bioscience 53 (4):391–403.

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A- 12 DRAFT – March 2008 Jeremiason, J.D., D.R. Engstrom, E.B. Swain, E. A. Natur, B.M. Johnson, J.E. Almendinger, B.A. Monson, and R.K. Kolka. 2006. Sulfate addition increases methylmercury production in an experimental wetland. Environ. Sci. Technol

. 40 (12)3800–3806. (DOI: 10.1021/es0524144). Juice, S.M., T.J. Fahey, T.G. Siccama, C.T. Driscoll, E.G. Denny, C. Eagar, N.L. Cleavitt, R. Minocha, and A.D. Richardson. 2006. Response of sugar maple to calcium addition to northern hardwood forest at Hubbard Brook, NH. Ecology 87 (5):1267–1280. Likens, G.E., C.T. Driscoll, and D.C. Buso. 1996. Long-term effects of acid rain response and recovery of a forest ecosystem. Science 272 :244–246. NAPAP (National Acid Precip itation Assessment Program) 2005. National Acid Precipitation Assessment Program Report to Cong ress: An Integrated Assessment . National Acid Precipitation Assessment Program. National O ceanic and Atmospheric Administration, Silver Spring, MD (NAPAP coordination now under the U.S. Geological Survey). General information on NAPA is available at: http://ny.cf.er.usgs.gov/napap/index.html

. The 2005 report can be downloaded from the following url: http://ny.cf.er.usgs.gov/napap/index.html. U.S. EPA (Environmental Protection Agency). 2005. Mussel Toxicity Testing Workshop

. Conducted by U.S. EPA HQ and EPA Region 5, Chicago, IL. August 23–24. Whitall, D.R. and H.W. Paerl. 2001. Spatiotempor al variability of wet atmospheric nitrogen deposition to the Neuse River Estuary, North Carolina. Journal of Environmental Quality 30 :1508–1515.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

APPENDIX B

OTHER INDICATORS – NITROUS OXIDE
NAAQS’ purpose to protect welf are entails not only ecological welfare, but also other forms of welfare that are potenti ally impacted, such as materials, climate change, and visibility.
N 2 O is a naturally occurring greenhouse gas; however, human activit ies have increased atmospheric concentrations by 18% since th e preindustrial era (IPCC, 2001). The global warming potential of CO 2 is 1, while the global warming potential of N 2 O is 310, indicating that
a molecule of N 2 O is 310 times more effective in tra pping heat in the atmosphere than CO 2 over a 100-year period. Based on the current U.S. Greenhouse Gas (GHG) Inventory (U.S. EPA, 2007), nitrous oxide contributes approximately 6.5% to total greenhouse gas emissions (in CO 2 equivalents)
Elevated nitrogen loading to ecosystems can significantly enhance the production of N 2 O. For example, the nitrogen nutrient lo ading in water bodies can enhance N 2 O emissions from the bacterial breakdown of nitrogen from these source s. In another example, numerous studies have
shown that N 2 O emissions from soils increase upon ar tificial nitrogen additions (Brumme and Beese, 1992; Matson et al., 1992; Klemedtsson et al., 1997; Papen et al., 2001). Regions with elevated atmospheric nitrogen deposition due to anthropogenic activity also show increased N 2 O emissions (Butterbach-Bahl et al., 1998, 2002). N itrous oxide emissions from soils are also influenced by precipitation and temperature. The Photosynthesis-Evapotranspiration-Model– Nitrogen–Denitrification-Decom position (PnET-N-DNDC) model is designed to simulate and predict soil carbon and nitrogen bi ogeochemistry in temperate forest ecosystems and to simulate
the emissions of N 2 O and NO from forest soils. The model couples the PnET model, the DNDC model, and a nitrogen module that are further described in Li and colleagues (1992, 1996, 2000), Li (2000), and Stange and colleagues (2000). The capacity of this model to simulate nitrogen trace gas emissions from forest soils was tested by comparing model results with results from
field measurements at 19 different field sites acro ss Europe and 1 site in the United States (Kesik et al., 2005). Denitrification is de scribed in the model as a series of sequential reductions driven by a microorganism using nitrogen oxides as electron acceptors under an aerobic conditions. As intermediates of the processes, NO and N 2 O are tightly controlled by the kinetics of each step in the sequential reactions. Interacti ons between temperature, precip itation, and forest soil NO and

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N 2 O emissions in Europe were investigated us ing PnET-N-DNDC (Kesik et al., 2006) abiotic parameters that are included in the model and are summarized in Table B-1

.
Table B-1. Parameters Included in the PnET-N-DNDC Model as Modeled for European Forest (Kesik et al.

, 2006)

Forest Properties Soil Properties Daily Climate Input Parameters Tree Species/Genera Forest type Age Above and below ground biomass Plant physiology parameters Texture Clay content pH Soil organic carbon content Stone content Humus type Precipitation Min and max air temps Inorganic [N] in Precip Pine Spruce Hemlock Fir Oak Birch Beech Slash Pine Larch Cypress Evergreen Oak

References
Brumme, R., and F. Beese. 1992. Effects of limi ng and nitrogen fertilization on emissions of CO 2 and N 2 O from a temporate forest. Journal of Geophysical Research 97 :12851– 12858. Butterbach-Bahl, K., L. Breuer, R. Gasche, G. Willibald, and H. Papen. 2002. Exchange of trace gases between soils and the atmosphere in Scots pine forest ecosystems of the northeastern German lo wlands 1. Fluxes of N 2 O, NO/NO 2 and CH 4 at forest sites with different N-Deposition. Forest Ecology and Management 167 :123–134. Butterbach-Bahl, K., R. Gasche, C. Huber, K. Kreutzer, and H. Papen. 1998. Impact of nitrogen input by wet deposition on N-trace gas fluxes and CH 4 -oxidation in spruce forest ecosystems of the temperate zone in Europe. Atmos. Environ

. 32 :559–564. IPCC, 2001: Climate Change 2001: Synthesis Report. A Contribution of Working Groups I, II, and III to the Third Assessment Report of the Intergovernmental Panel on Climate Change . 398 pp. Edited by R.T. Watson and the Core Writing Team., Cambridge, United Kingdom, and New York, NY: Camb ridge University Press. Kesik, M., P. Ambus, R. Baritz, N. Brüggemann, K. Butterbach-Bahl, M. Damm, J. Duyzer, L. Horváth, R. Kiese, B. Kitzler, A. Leip, C. Li , M. Pihlatie, K. Pilegaard, S. Seufert, D. Simpson, U. Skiba, G. Smiatek, T. Vesa la, and S. Zechmeister-Boltenstern. 2005. Inventories of N 2 O and NO emissions from European forest soils . Copernicus GmbH on behalf of the European Geosciences Union (EGU). Sweden. Kesik, M., S. Blagodatsky, H. Papen, and K. Bu tterbach-Bahl. 2006. Effect of pH, temperature and substrate on N 2 O, NO, and CO 2 production by Alcaligenes faecalis p. Journal of Applied Microbiology 10 (3)655–667.

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Klemedtsson, A., L. Klemedtsson, K. Burglund, P. Martikainen, J. Silvola, and O. Oenema. 1997. Greenhouse gas emissions from fa rmed organic soils: a review. Soil Use of Management 13 :245–250. Li, C. 2000. Modeling trace gas emissions from agricultural ecosystems. Nutr. Cycl. Agroecosys

. 58 :259–276. Li, C., J. Aber, F. Stange, K. Butterbach-Bahl, and H. Papen. 2000. A process-oriented model of N 2 O and NO emissions from forest soils: 1. Model development. Journal of. Geophysical Research 105 :4369–4384. Li, C., S. Frolking, and T.A. Fr olking. 1992. A model of nitrous oxide evolution from soil driven by rainfall events: 1. Model structure and sensitivity. Journal of Geophysical Research 97 :9759–9776. Li, C., V. Narayanan, and R. Harriss. 1996. Model estimates of nitrous oxide emissions from agricultural lands in the United States. Global Biogeochemical Cycles 10 :297–306. Matson, P.A., S.T. Gower, C. Volkmann, C. Billo w, and C.C. Grier. 1992. Soil nitrogen cycling and nitrous oxide fluxes in fertilized Rocky Mountain Douglas-fir forests. Biogeochemistry 18 :101–117. Papen, H., M. Daum, R. Steinkamp, and K. Butterbach-Bahl. 2001. N 2 O and CH 4 -fluxes from soils of a N-limited and N-fertilized spruce forest ecosystem of the temperate zone. J. Appl. Bot. Angew. Bot

. 75 :159–163. Stange, F., K. Butterbach-Bahl, and H. Papen. 2000.A process-oriented model of N 2 O and NO emissions from forest soils, 2. Se nsitivity analysis and validation. Journal of Geophysical Research 105 (15)4385–4398. U.S. EPA (Environmental Protection Agency). 2007. The U.S. Inventory of Greenhouse Gas Emissions and Sinks: Fast Facts . Office of Atmospheric Programs, EPA 430-F-07-004. April.

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APPENDIX C
CMAQ MODELING

The 1998 Guidelines for Ecologi cal Risk Assessment (U.S. EPA, 1998) state that “At the beginning of the analysis phase, the [risk] assessor critically examines the data and models to ensure that they can be used to evaluate the conceptual model developed in problem formulation.” The assessor evaluates the strengths and limitations of different types of data and evaluated measurement or modeling studies to be used, including acc ounting for uncertainty. This section presents our curre nt thoughts for using existing am bient air quality and deposition monitoring data, as well as the application of atmospheric and multimedia models to predict ecological risk and exposure.

C.1 Atmospheric Modeling
The atmospheric processes that transform and transport ambient NO x and SO x to deposition species are simila r. (NOTE: In addition, NO x and SO x interact photochemically in the PM formation. Further, since NO x participates in both the ozone and PM photochemistry, those pollutants are linked and need to be simulated to gether in a single model.) Therefore, we intend to use the same process model for atmospheric fate , transport, and deposition. We plan to use the CMAQ model, a peer-reviewed, state-of-the-a rt model of the atmosphere. The 2002-based CMAQ modeling platform will likely be used as the tool for the air quality modeling.

As shown in Figure C-1, the CMAQ modeling domain covers the continental United States and portions of Canada and Mexico. There are two 12 x 12 km horizontal-grid resolution modeling domains, an eastern United States domain (outlined in red), and a western United States domain (outlined in blue). The modeling do main contains 14 vertical layers, with the top of the modeling domain at about 16,200 meters.

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Figure C-1. CMAQ 12-km eastern and western United States modeling domains. The key inputs to the CMAQ model include emissions from anthropogenic and biogenic sources, meteorological data, and initial a nd boundary conditions. The CMAQ meteorological input files were derived for the entire year of 2002 from a simu lation of the Pennsylvania State University/National Center for Atmospheric Res earch Mesoscale Model (Grell et al., 1994). This model, commonly referred to as MM5, is a limited-area, nonhydrosta tic, terrain-following system that solves for the full set of phys ical and thermodynamic equations, which govern atmospheric motions. Anthropogenic and bioge nic emissions from the 2002 CMAQ modeling platform were obtained from the 2002 National Emi ssions Inventory (NEI). These data were then processed using the Sparse Matrix Operator Kernel Emissions (SMOKE) processing system.
In addition to understanding the relations hip between ambient concentrations and deposition, we also need to understand how the existing patterns of deposition modeled using CMAQ may vary with variability in meteorol ogy (e.g., temperature, precipitation). By making use of the 5 years of CMAQ simulations conduc ted for the joint EPA/Centers for Disease Control and Prevention (CDC) project, we may be able to provide a quantitative assessment of the variability in deposition patterns induced by the meteorological variability between years. This assessment plans to include statistical an alyses designed to compare the magnitude and

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spatial patterns of deposition fo r sulfur and nitrogen contai ning species predicted by CMAQ across the 5 years modeled. The anal ysis intends to highlight the similarities and differences in the distribution of monthly, seasonal, and annua l wet, dry, and total deposition for grid cells covering selected watersheds, forested ecosy stems, and croplands. Specifically, for each geographic area we plan to calcu late (1) the 5-year average in monthly, seasonal, and annual nitrogen and sulfur deposition and (2) the range in the deposition for each of these time periods within the 5 years modeled. This information may be presented in tabular and graphical forms to reveal (1) how deposition varies by year, season, and month for the 5 years, on average, and (2) how monthly and seasonal deposition in the individua l years compares to the 5-year average. We may also examine the spatial variation in deposit ion within and between the selected geographic areas. For this we plan to prepare histograms showing the distribution of deposition by month and season across the multiple grid cells within each of the selected geographic areas. The results may be analyzed to compare the distribution a nd range of values for the different areas. The feasibility of using statistical t echniques to quantify year-to-year variability in the spatial patterns of predicted deposition within a ge ographic area may be explored.

C.2 Ambient Air Concentration Data and Deposition Data
Because the risk/exposure assessment will ultimately be used to assess current and
alternative ambient NO x and SO x standards, the risk assessment will need to characterize both
the effects associated with the deposition of nitrogen and sulf ur and the relationship between ambient concentrations at a specific geogra phic resolution and deposition at “downstream” receptors. Figure C-2 shows the flow of information and modeling we propose to use to
characterize the ambient levels of NO x and SO x and estimates of nitroge n and sulfur deposition.

As shown in Figure C-2 , in addition to CMAQ modeling, we plan to evaluate the
available observational data on deposition for use in characterizing ambient and deposition surfaces across the United States. There are a number of databases of ambient monitoring data, most of which are collected in EPA’s Air Qualit y System (AQS). There are also databases of deposition monitoring data, incl uding the NADP, the CASTNet, and the Park Research and Intensive Monitoring of Ecosystems NETwork (PRIMENet).

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Ambient and Deposition Surfaces Monitoring Emissions Meteorology

CMAQ Modeling

Figure C-2. Development of ambient NO x and SO x and nitrogen and sulfur deposition characterizations. We plan to assess the feasibility of comb ining data from modeling and monitoring to produce ambient air quality and de position surfaces that are gr ounded in observational data but make use of the models’ ability to capture the impacts of emissions and meteorology on the geographic gradients of air quali ty and deposition. To assess the feasibility of combining data, we intend to implement one or more methods on data associated with a regi on of interest to this project. The estimated air quality and deposition surfaces will be helpful in assessing baseline risks and serve as baselines for assessing the impacts of attaining a lternative standards.

References
Grell, G., J. Dudhia, and D. Stauffer. 1994. A Description of the Fifth-Generation Penn State/NCAR Mesoscale Mode l (MM5), NCAR/TN-398+STR . 138 pp. National Center for Atmospheric Research, Boulder CO. U.S. EPA (Environmental Protection Agency). 1998. Guidelines for Ecological Risk Assessment

. Risk Assessment Forum. EPA/630/R-95/002F. April.

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APPENDIX D

MULTIMEDIA MODELING
Atmospheric modeling for this quantitative risk/exposure assessment intends to use, where possible, the CMAQ modeling system. The outputs from the CMAQ model must be linked to one or more multimedia models (e.g., mode ls for terrestrial systems or aquatic systems) to determine the environmental effects of atmospheric deposition of NO x and SO x . For this analysis, there is a need to consider models that can potentially handle all types of total reactive nitrogen (e.g., consideration of such species as ammonia and nitrous oxide) and models that can combine analyses of sulfur with mercury methylation processes.

The choice of multimedia models to determine and quantify these effects is not as clear as in the atmospheric modeling. With multim edia models, there are numerous technical formulations, parameterizations, and geographic considerations to consider in choosing the
correct model for the task at hand. This deci sion depends on desired outcomes and simulated parameters. Outcomes may range from quantified, speciated sulfur or nitr ogen concentrations in the soil and surface water to measures of species’ richness in a forest landscape. Not all effects can be estimated with multimedia models, so a decision on the models to use must balance the pros and cons of the models that are available.

D.1 Model Characterization

The 35 models listed in Table D-1 represent a wide diversity of types of ecosystems; history, location, and sp atial/temporal scale of application; scien tific acceptance and organizational and agency support; complexity and requirements; state vari ables and processes; and management uses. Each model was examined to elucidate the necessary information for choosing the appropriate multimedia model. Of the 35 models, 9 are classified as receiving-water models, and 26 are classified as terrestrial/wat ershed models, although th ese distinctions are somewhat arbitrary. In recogniti on of some models having enough common features or history of integration to warrant merging of the discus sion, the table also presented several models together; however, although presented collectively, these models may not share the same history, theory, development, and application.

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Table D-1. Information Detailed for Each Model of Interest
Model Information Name Model type Description Indicators Supporting organization(s)/agency end orsements Resource requirements Scientific acceptance Data requirements Type of ecosystems modeled Model performance/evaluation Where has it been applied? Model technological integration Spatial scale Key environmental/ecological processes Transferability Management use Temporal features Citations/URLs Model inputs related to deposition Notes (miscellaneous)

Table D-2 presents a more concise overview of the models identified; information is presented that summarizes both the type of eco system that the model has been designed to represent and the relative level of complex ity with which the model considers system components and processes. Model complexity was measured by the underlying theory and the effort necessary to parameterize a model. For instance, an empirical model that requires only basic nutrient information and relies on many assumptions would constitute a low level of complexity, whereas a mechanistic model with a large number of variables to parameterize would constitute a highly complex model. Comple xity does not necessarily suggest desirability, utility, or credibility, as these concepts are highly context-specific. All of the models presented have arguably demonstrated substantial technical acceptance based on their historical use, presence in peer-reviewed publications, and de gree of agency support. Further discernment regarding “quality” or applicability of the different models to support elements of the secondary
NO 2 and SO 2 NAAQS should carefully consider a variety of contextual drivers when using an integrated modeling approach.

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Table D-2. Supported Model Components And Level Of Representation

ModelUrbanRuralAgricultureForestRiverLakeReservoirEstuaryCoastalGroundwater

AQUATOX — — — — ■ ■ ■ — — — QUAL2K — — — — ◙ — — — — — WASP7 — — — — ■ ■ ■ ■ ■ — CE-MODELS — — — — ■ ■ ■ ■ ■ — ILWAS — □ — ◙ ◙ ◙ ◙ — — □ LWWM ■ — — — ■ ■ ■ ■ ■ ◙ RCA — — — — ■ ■ ■ ■ ■ — THMB/IBIS — ◙ ◙ ◙ ◙ — — — — ◙WATER MODELS

WARMS — — — — — ◙ ◙ — — — HSPF/LSPC ■ ■ ■ ■ □ □ □ — — □ PLOAD □ □ □ □ — — — — — — SWAT ◙ ■ ■ ■ ■ ◙ ◙ □ — ■ WARMF ■ ■ ◙ ■ ■ ■ ■ — — ◙ BIOME-BGC — ■ ◙ ■ — — — — — — PnET-BGC — ◙ — ◙ ◙ ◙ ◙ — — — CENTURY 4/ CENTURY 5 — ■ ■ ■ — — — — — □ DayCent — ■ ■ ■ — — — — — □ DNDC — □ ◙ □ — — — — — — DRAINMOD — □ ■ — □ — — — — ◙ EPIC/APEX — ◙ ■ □ — — — — — □ GLEAMS — ◙ ■ □ — — — — — ◙ Hole-in-the-pipe — ■ — ■ — — — — — — INCA — ◙ ◙ ◙ □ — — — — — MAGIC — ◙ — ◙ ◙ ◙ ◙ — — ◙ MERLIN — ■ □ ■ — — — — — □ NLOAD/NLM/ELM □ □ □ □ — — — □ — □ NuCM — ◙ — ■ — — — — — — ReNuMa ◙ ◙ ◙ ◙ — — — — — □ NWPCAM □ □ □ □ □ — — — — — RHESSys — ■ ■ ■ □ — — — — — SAFE — ■ — ■ — — — — — ◙ Simple Mass Balance □ □ □ □ □ □ □ □ □ — SPARROW □ □ □ □ □ — □ — — — TOPMODEL or GT/MEL — ◙ ◙ ◙ ◙ — — — — —WATERSHED/TERRESTRIAL MODELS

WATERSN □ □ □ □ — — — — — — — Not supported

□ Relatively simple representation of features and processes

◙ Moderate level of representation of features and processes

■ Most complex representation of features and processes

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D.2 Multimedia Model Selection
There are four basic steps nece ssary to undertake a modeling effort to examine the effects of nitrogen and sulfur deposition (RTI 2007):
1. Choose the specific question/problem to address.

2. Choose the best models based on model formulation (e.g., are biological processes considered?), desired output, study ar ea, data availability, and necessary uncertainty/sensitivity analyses for the models.
3. Determine and set up any processes/algor ithms necessary to match atmospheric modeling output (assumed to be from CM AQ) to the chosen receiving water or
terrestrial/watershed model.
4. Obtain the data needed for model parameterization.
The model details presented in the summa ry table and concluding discussion should assist the reader in making an informed decision on the best model for a task. The difficulty with
this area of work lies with the desire to uti lize atmospheric modeling in combination with the receiving-water and terrestrial/watershed models. The multi-media approach to modeling is still in development, so, at this time, not many models are set up to immediately accept the output from an atmospheric model such as CMAQ. Several of the models examined accept atmospheric concentration or flux data, but the time-step, spa tial resolution, and exact species required might all differ from the atmospheric model output. For those models that accept atmospheric inputs in a form other than that output by CMAQ, efforts can be made to reconcile the outputs of the atmospheric model and the input s of the ecosystem model.
Most applications for the de termination of acidification e ffects have taken place in the northeastern United States because of the interest in quantification of acid rain effects. A specific
effort was made to determine which models of the 35 could apply to other regions of the United
States.
Review of past applications and technical documentation for each of the 9 receiving
water models reveals that 4 of the 9 models ar e accepting of parameters for all regions of the country. These models are: AQUATOX, the CE-QUAL family of models, QUAL2K, and
WASP. The WARMS model has been used in Canada ; therefore, it may be applicable in more

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alpine regions of the United States. The remaini ng 4 models have either been developed for a specific area of the country or show limitations in their extension beyond the regions in which they have already been applied.
A listing of the terrestrial/watershed models th at show promise in applications across the United States, specifically in western states, follows with a short explanation of support.

 BIOME-BGC – validated with western locations , including arid, cold, western climates

 CENTURY 4/5 – has been used in the Great Plains and Rocky Mountain areas of the
United States

 DayCent (DayCent-Chem) – validated in an alpine/subalpine watershed of Colorado and
used at other sites across the United States

 EPIC/APEX – used across the countr y, but only for agricultural lands

 GT/MEL – a relatively new model that has been tested in the western Oregon Cascades

 HSPF – used in applications across the country

 MAGIC – used worldwide and on many types of systems but does not include biological processes directly and lacks features to simulate proce sses involving nitrogen

 Mass balance – may be applied anywhere data are available

 MERLIN – no applications uncovered for U.S. sites but has been us ed in locations in Europe

 PLOAD – a screening-level model that may be applied anywhere data are available

 PnET-BGC – many applications in the northeastern United States; developed for forest landscapes

 RHESSys – used in applications in the mountains of Montana

 SAFE – wide applications in forest systems

 SWAT – used throughout the country, but does not readily accept the atmospheric deposition inputs required for this study, although a workaround may be possible

 WARMF – used across the country (currently utilizes atmospheric concentration data; a newly undertaken project will modify WARM F to accept atmospheric deposition fluxes to make it more compatible with CMAQ).

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D- 6 DRAFT – March 2008 References

RTI International. 2007. Review of Candidate Fate and Transport and Ecological Models . Final Report. Report for the Office of Air Quality, Planning, and Standards, U.S. Environmental Protection Agenc y, Research Triangle Park, NC.Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

APPENDIX E

VALUATION OF ECOSYSTEM SERVICES E.1 Introduction to Ecosystem Services

The definition of adverse is “unfavorable or antagonistic in purpose or effect.” One way to assess adverse effects on welfare is through qu antification of ecosystem services. The adverse
effect would be the loss or reduction of those services through the effects of NO x and SO x on the underlying ecological processes and functions that constitute the service. EPA defines ecosystem services as the outputs of hea lthy, intact ecosystems and the underlying ecosystem processes and
functions that contri bute to human well-being (U.S. EP A, 2006). As articulated by the Millennium Ecosystem Assessment (United Na tions, 2005) from the United Nations, these include provisioning services (e .g., clean water, food, wood, fiber, fuel), regulating services (e.g., water purification, climate re gulation), supporting services (e.g., nutrient cycling, soil formation), and cultural services (e.g., recreation, spiritual). Regulating services are of key importance to EPA because they directly impact air and water quality, and they have strong links to human heath and well-being. Therefore, assess ing changes in ecosystem services may be one means of assessing whether an effect is adverse. Key issues are how to aggregate across different ecosystem services or how to select a representative ecosystem service that is most sensitive to deposition effects. Some potenti al indicators of ecosystem serv ices include the quality of a critical habitat, biodiversity, species compositi on, controlling/limiting invasive species, and pest outbreaks. Determining the exposure-response relationships of NO x and SO x on the ecosystem service and the underlying ecological process a nd function will provide a broader focus in determination of adverse impacts. For more background information on ecosystem services, see The Integrated Review Plan for the Secondar y National Ambient Air Quality Standards for Nitrogen Dioxide and Sulfur Dioxide (U.S. EPA, 2007).

Economic Valuation. A succinct statement of the economic approach to valuation is in the Ecological Benefits Asse ssment Strategic Plan (EBASP) (U.S. EPA, 2006): “Economists generally attempt to estimate the value of eco logical goods and services based on what people are willing to pay (WTP) to incr ease ecological services or by what people are willing to accept (WTA) in compensation for reductions in them. To enable a comparison of policy options, a common unit is needed to expre ss the value of ecologi cal goods and services . The dollar is the

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preferred unit for valuation, because there is an extensive body of literature addressing its application and interpretation and it is easily compared with costs for considering the net effects of alternative policy choices. Th ree primary approaches for estimating these values exist: market-based methods, revealed preference methods , and stated preference methods (U.S. EPA, 2006).” The EBASP document continues by furt her explaining these types of economic monetary valuation methods.
For market-based valuation, for many regi ons of the country, th ere are diverse crop assemblages, including timber for building materi als, timber for pulp and paper, biofuel crops, grain and soybean feed crops, grass seed, or chard crops, row crops, Christmas trees, and horticultural crops. There are numerous methods and data for valuation of these market
commodities.
A useful approach to pursue with economist s would be mapping the monetary values of ecosystem services by land use across the ecosystem service district, using the approach described in Troy and Wilson (2006). This appr oach will first require mapping biophysical
measurements of services as discussed in Sect ion 3.2. Then, economics expertise will be needed to apply monetary values to these services as well as to other services, for which there are
transferable methods and values from studies elsewhere, using the “Environmental Valuation Reference Inventory” ([http://www.evri.ca/en glish/default.htm] and other sources of information).
Biophysical Valuation. Because monetization of many eco systems services is either very difficult or problematic for a number of reasons, nonmonetary valuation using biophysical measurements and concepts will be pursued in addition to economic valuation. One approach that fits well with the empirical and modeli ng data is what has b een called relative value indicators or relative benefit indicators (Wainge r et al., 2001; Boyd and Wa inger 2003). In this approach, a set of indicators is de fined that reflect site and landscap e features that affect (1) the quality and quantity of ecosystem functions or services, (2) the availability of complementary goods and services, (3) the scarcity of goods and se rvices, and (4) th e reliability of the flows of ecosystem services. These indicators are then aggr egated into a combined site or place-based index using standardized scori ng (Wainger et al., 2001). This is an explicit approach to a

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“bundling” of services. The development of ERFs and ETFs in Section 3.7 may be adapted to such an approach.
A related approach derives from an ec onomic method that develops “productivity
indexes” and now calls these “en vironmental performance indexe s” (Färe et al., 2004). This approach attempts to combine positive and negative outcomes from a system (e.g., factory, region, county) into a single inde x of performance. To the degree that this approach is sciencebased, it can be developed with available data and offers potential in a qualitative risk

assessment. A second approach to biophysical valuati on assigns values to ecosystem goods and services through the use of the common currency of energy. This approach has been supported in part by the EBASP (U.S. EPA, 200 6) and other reviews of valuat ion. A comprehensive, holistic method of energy-based valuation is the energy systems analysis of environmental accounting developed by H. T. Odum (1996). A similar approach has been developed by Bakshi and colleagues (Hau and Bakshi, 2004a, 2004b; Ukidwe and Bakshi, 2004, 2005). There is extensive literature supporting and elabor ating on these approaches.
By describing the differing bundles of ecosyst em services under va rying levels of NO x and SO x, and offering choices between those bundles, tr adeoff or indifference curves can also be generated reflecting individual- and population-level preferen ces for different ecosystem functions. These tradeoffs can be presented to survey respondents using methods such as conjoint analysis to provide measures of prefer ences for different levels of ecosystem functions as expressed through ecosystem service levels . Table E-1 shows an example of how ecosystem services information might be pr esented to a survey respondent in a conjoint analysis framework.
The results of a conjoint anal ysis can be used to convert all ecosystem services to a common unit based on preference weightings for di fferent types of services. This may allow ecosystem services to be combined for compari ng impacts across alternative forms or levels of
NO x and SO x standards.

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Table E-1. Example Survey Design for Preference-Based Ecosystem Tradeoffs

Water Quality Habitat Food and Fiber Production Carbon Sequestration Scenario 1 Drinkable Low diversity High High Scenario 2 Drinkable High diversity Low Low Scenario 3 Swimmable Medium diversity Medium Medium Scenario 4 Boatable Low diversity High Medium Scenario 5 Swimmable Low diversity Low Low

Highly Desirable Quite Desirable Desirable Slightly Desirable Neither Desirable

nor Undesirable Slightly Undesirable Undesirable Quite Undesirable Highly Undesirable Scenario 1 1 2 3 4 5 6 7 8 9 Scenario 2 1 2 3 4 5 6 7 8 9 Scenario 3 1 2 3 4 5 6 7 8 9 Scenario 4 1 2 3 4 5 6 7 8 9 Scenario 5 1 2 3 4 5 6 7 8 9 Ecosystem Services Scenario Descriptions

Scenario Rating Please rate how desirable each scenario is overall by circ ling one of the numbers in each row of the following table: Example Conjoint Scenario Rating Form

References
Boyd, J., and L. Wainger. 2003. Measuring Ecosystem Service Benefits: The Use of Landscape Analysis to Evaluate Environmental Trade s and Compensation Resources for the Future

. Washington, D.C. Discussion Paper 02-63. Fare, R., S. Grosskopf, and F. Hernadez-Sanc ho. 2004. Environmental performance: an index number approach. Resources and Energy Economics, 26: 343–352. Hau, J.L., and B.R. Bakshi. 2004a. Expanding exer gy analysis to account for ecosystem products and services. Environmental Science and Technology 38 :3768–3777. Hau, J.L. and B.R. Bakshi. 2004b. Promise and problems of emergy analysis. Ecological Modeling 178 :215–225. Odum, H.T. 1996. Environmental Accounting: Energy and Environmental Decision Making. 370 p.p. New York: John Wiley & Sons, Inc. Troy, A., and M.A. Wilson. 2006. Mapping ecosyst em services: practical challenges and opportunities in linking GIS and value transfer. Ecological Economics 60 :435–449. Ukidwe, N.U., and B.R. Bakshi. 2005. Flow of natural versus economic capital in industrial supply networks and its impli cations to sustainability. Environmental Science and Technology 39 :9759–9769. Ukidwe, N.U., and B.R. Bakshi . 2004. Thermodynamic accounting of ecosystem contribution to economic sectors with app lication to 1992 U.S. economy. Environmental Science and Technology 38 :4810–4827. United Nations. 2005. Millennium Ecosystem Assessment Reports . Available at http://www.millenniumassessment.org/en/index.aspx

.

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E- 5 DRAFT – March 2008 U.S. EPA (Environmental Protection Agency). 2007. Plan for Review of the Secondary National Ambient Air Quality Standards for Nitrogen Dioxide and Sulfur Dioxide . Available at http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html. U.S. EPA (Environmental Protection Agency). 2006. Ecological Benefits Assessment Strategic Plan. P. A-4. Washington, DC. EPA 240-R-06-001. October. Wainger, L.A., D. King, J. Salzman, and J. Boyd. 2001. Wetland value indicators for scoring mitigation trades. Stanford Environmental Law Journal 20 :413–478.United StatesEnvironmental ProtectionAgencyOffice of Air Quality Planning and StandardsHealth and Environmental Impacts DivisionResearch Triangle Park, NCPublication No. EPA-452/D-08-002March 2008