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

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                                                    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 27711

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                            Secondary 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 those of the authors and do not
necessarily reflect the views of EPA. This document is being circulated to obtain review and
comment from the Clean Air Scientific Advisory Committee (CASAC) and the general public.
Comments on this document should be addressed to Dr. Anne W. Rea, U.S. Environmental
Protection Agency, Office of Air Quality Planning 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 NC>2 and SC>2 Secondary NAAQS Reviews	6
     2.3  Overview of Ecological Risk Assessment and the Scope of the Secondary
          NAAQS Review for NOX  and SOX	7
          2.3.1  Overview of Risk Assessment Framework for Deposition-related
                Ecological Effects	9
          2.3.2  Overview of Nitrogen Deposit on	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 for 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 Extent 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 Ambient Sources	55
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4.    References	61
Appendix A Identification of Sensitive Ecosystems	A-l
Appendix B Other Indicators -Nitrous Oxide	B-l
Appendix C CMAQ Modeling	C-l
Appendix D Multimedia Modeling	D-l
Appendix E Valuation of Ecosystem Services	E-l


                                  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 CO2 equivalents	13
2-4.    Transformation of sulfur compounds in the atmosphere	14
2-5.    General flow of processes addressed in the risk/exposure assessment for NOX and
       SOX secondary standards	16
3-1.    Seven step approach to planning and implementing risk/exposure assessment	25
3-2.    Ecosystem effects may range from near-field to far-field linkages	31
3-3.    Documented biological, biogeochemical, and physiographic linkages	33
3-4.    Comparison of NAPAP-documented acid-sensitive ecoregions to CMAQ-moded
       nitrogen deposition	47
3-5.    Risk/policy assessment paradigm for deposition-related ecological risks	56


                                   List of Tables

2-1.    Proposed Schedule for the Joint NO2 and SO2 Secondary NAAQS Review*	19
3-1.    Summary of Indicators Categorized by Effect	27
3-2.    Key Indicators of Acidification Due to NOX and  SOX	28
3-3.    Key Indicators of Nutrient Enrichment Due to NOX and SOX	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 Assessment 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 the acidity of a system (e.g., lake, stream, forest soil).
       Atmospheric deposition of acidic or acidifying compounds can acidify lakes, streams,
       and forest soils.
Adverse Effect: The response or component of an ecosystem that is deemed harmful in its
       function.
Air Quality Indicator: The substance or set of substances (e.g., PM25, NO2, SO2) 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 depends 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 exchange sites are occupied with base
       cations (e.g., Ca2+, Mg2+, K+) as opposed to A13+ and H+. Base cation saturation is a
       measure of soil acidification, with lower values being more acidic. There is a threshold
       whereby soils with base saturations less than 20% (especially between 10-20%) are
       extremely sensitive to change.
Biologically Relevant Indicator: A physical, chemical, or biological entity/feature that
       demonstrates a consistent degree of response to a given level of stressor exposure and
       that is easily measured/quantified to make it a useful predictor of biological,
       environmental, or ecological risk.
Buffering Capacity: The ability of a body of water and its watershed to neutralize introduced
       acid.
Critical Load:  A quantitative estimate of the level of exposure 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 nitrogen (e.g., nitrate or nitrite) to gaseous
       nitrogen (e.g., N2O or N2) 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 adverse ecological effects may occur or are occurring as a
       result of exposure to one or more stressors (U.S. EPA, 1992a).
Ecological Risk Assessment: A process that evaluates the likelihood that adverse ecological
       effects may occur or are occurring as a result of exposure to one or more stressors (U.S.
       EPA, 1992a).
Ecosystem:  The dynamic, complex interaction of plants, animals, and microorganisms and the
       nonliving environment.
Ecosystem Benefit: The value, expressed qualitatively, quantitatively, and/or in economic terms,
       where possible, associated with changes in ecosystem services that result either directly
       or indirectly in improved human health and/or welfare. Examples of ecosystem benefits
       that derive from improved air quality include 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 society 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 meteorological 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^O vapor, CC>2, N2O, CH/t, and Os are the
       primary greenhouse gases in the earth's atmosphere. As well as CC>2, N2O, and CH4, the
       Kyoto Protocol deals with SF6, hydrofluorocarbons, and perfluorocarbons.
Nitrogen Enrichment: The process by which a terrestrial  system becomes enriched by nutrient
       additions to a degree that stimulates the growth of plant or other terrestrial biota, usually
       resulting  in an increase in productivity.
Nitrogen Saturation: The condition when nitrogen inputs from atmospheric deposition and
       other sources  exceed the biological requirements of the ecosystem.
Occult Deposition: The removal of gases and particles from the atmosphere to surfaces by fog
       or mist.
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Semiarid Regions: Those regions of moderately low rainfall, typically 25 to 50 centimeters
       (10 to 20 inches) of rainfall per year, where the natural vegetation is usually short grasses
       and shrubs and where the predominant land use may be as rangelands.
Sensitivity: The degree to which a system is affected, either adversely or beneficially, by SOX or
       NOX pollution (e.g., from acidification or nitrogen 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 consideration such factors as economic costs
       and time frame for emissions reduction. This can be lower than the  critical load if a very
       sensitive area is to be protected in the short term, especially if deposition rates exceed
       critical loads.
Total Reactive Nitrogen: This includes all biologically, chemically, and radiatively active
       nitrogen compounds in  the atmosphere and biosphere, such as NH3, NH4+, NO,  NO2,
       HNOs, N2O, NOs , and organic compounds (e.g., urea, amines, nucleic acids).
Valuation: The economic or noneconomic 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 susceptible to, and unable to cope with, the
       adverse effects of NOX and/or SOX air pollution. Vulnerability is a function of the
       exposed and its  sensitivity.
Welfare Effects: The effects on soils; water; crops; vegetation; man-made materials; animals;
       wildlife; weather; visibility; and climate as well as damage to, and deterioration of,
       property; hazards to transportation; and the effects on economic values and on personal
       comfort and well-being, whether caused by transformation, conversion, or combination
       with other air pollutants (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 (EPA) is currently conducting a joint review
of the existing secondary (welfare-based) National Ambient Air Quality Standards (NAAQS) for
nitrogen dioxide (NO2) and sulfur dioxide (SO2). 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 discussed in the Integrated Review Plan (U.S. EPA, 2007a), this was
done in recognition of the important linkages between ambient nitrogen and sulfur leading to
deposition of ambient particles that 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 nitrogen 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 degree possible, our risk and exposure assessment
will attempt to evaluate whether ecosystem damage is occurring in specific ecosystems under
current ambient concentrations, and, if so, what alternative levels of ambient nitrogen and sulfur
might be expected to allow various degrees  of recovery of impacted systems and prevention of
further damage.
       In particular, given the data and time constraints 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 enrichment for both terrestrial and aquatic 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 case studies in a broader spatial context.  The
combination of these case studies and the national characterizations thus forms the body of our
planned overall  assessment. Additional case studies and more comprehensive investigations of a

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broader set of effects, linkages, and indicators may be identified 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 understanding of these
relationships.

1.2    Introduction
       The reviews of the primary NAAQS for NO2 and SO2 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 science 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 the 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 NO2 and oxides of nitrogen (NOX) and SO2 and oxides of sulfur
(SOX) are not interchangeable. The terms NOX and SOX refer to the listed Criteria Air Pollutants
for which EPA has regulatory authority under Sections 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 oxides" as it appears in the enabling legislation
related to the NAAQS and the definition commonly used in the air pollution research and
management community. In this document, the terms "oxides of nitrogen" and "nitrogen oxides"
refer to all forms of oxidized nitrogen compounds, including nitric oxide (NO), nitrogen dioxide
(NO2), and all other oxidized nitrogen-containing compounds transformed from NO and NO2.
This follows usage in the Clean Air Act Section 108(c): "Such criteria [for oxides of nitrogen]
shall include a discussion of nitric 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 control community, the terms "oxides of nitrogen" and "nitrogen
oxides" are restricted to refer only to the sum of NO and NO2, and this sum is commonly
abbreviated as NOX. The category label used by this community for the sum of all forms of
oxidized nitrogen compounds including those listed in Section 108(c) is NOy.
       The terms NO2 and SO2 refer to the specific air quality indicators (pollutant species)
specified by the current standards whose concentrations are monitored to determine whether the
NAAQS are being met in a given location. The ecological importance of both oxidized and

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reduced forms of nitrogen has been widely recognized by the scientific community. Therefore,
this risk/exposure assessment will also evaluate total reactive nitrogen (which includes both
oxidized and reduced forms of nitrogen) and its impacts on public welfare. 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 NOX, SOX, and their associated transformation products are linked from an
atmospheric chemistry perspective, as well as from an environmental effects perspective, and
because of the National Research Council's (NRC's) 2004 recommendations to consider
multiple pollutants in forming the scientific basis for the NAAQS, EPA has decided, for the first
time since NAAQS were established in 1971, to jointly 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 support for considering them
together.
       This Scope and Methods Plan is organized to provide information 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 legislation, previous secondary reviews, an overview of
       ecological risk assessments, and the scope of this NAAQS review
    •   A conceptual model of nitrogen and sulfur cycling
    •   Key policy-relevant questions
    •   EPA's proposed schedule for the NOX/SOX secondary NAAQS review
    •   An analysis plan that includes the identification of relevant indicators of effects, a
       proposed approach to select areas for the risk/exposure assessment, an evaluation of data
       and models to assess effects, plans for characterization of exposure, and plans for
       characterization of ecological effects
    •   An assessment of alternative levels of protection under different scenarios of deposition
       from ambient sources.

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       EPA is consulting with CASAC, an independent scientific advisory committee
established under the CAA, on this plan. In particular, 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 areas, larger assessment areas, endpoints and
indicators) for the overall risk/exposure assessment. As this review proceeds, the plan described
here may be modified to reflect information received during the review process and to address
advice and comments received from CASAC and the public.
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2.     BACKGROUND AND OVERVIEW
2.1    Legislative Requirements
       Two sections of the CAA govern the establishment 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 numerous 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 scientific knowledge useful in indicating the kind and extent of
identifiable effects on public health or welfare which may be expected 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 pollutants 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 Administrator, based on  such criteria, 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 are not limited to, "effects on soils, water, crops, vegetation, man-
made materials, animals, wildlife, weather, visibility and  climate, damage to and deterioration of
property, and hazards to transportation,  as well as effects  on economic values and on personal
comfort and well-being." The definition of public welfare in Section 302(h) was expanded in the
1990 CAA amendments to state that the welfare effects identified should be protected from
adverse effects associated with criteria air pollutants "whether caused by transformation,
conversion, or combination with 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 establish standards that are neither more nor less
stringent than necessary for these purposes. In so doing, EPA may not  consider the costs of
implementing the standards (Whitman v. American Trucking Associations., 531 U.S. 457, 465-
472, 475-76 [2001]).
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       Section 109(d)(l) requires that "not later than December 31, 1980, and at 5-year intervals
thereafter, the Administrator shall complete a thorough review of the criteria published under
Section 108 and the national ambient air quality standards . . . and shall make such revisions in
such criteria and standards and promulgate such new standards as may be appropriate. . . ."
Section 109(d)(2) requires that an independent scientific review committee "shall complete a
review of the criteria . .  . and the national primary and secondary ambient air quality
standards . . . and shall recommend to the Administrator any new . . . standards and revisions of
existing criteria and standards as may be appropriate. . .  ." Since the early 1980s, this
independent review function has been performed by CAS AC of EPA's Science Advisory Board
(SAB).

2.2    Background on Previous NO2 and SO2 Secondary NAAQS Reviews
       The current secondary NAAQS review for NOX/SOX will examine a number of issues that
were of central importance in previous NAAQS nitrogen or sulfur reviews. For instance, in the
previous review of the NC>2 NAAQS, completed in 1995, welfare effects were assessed primarily
with respect to effects of ambient concentrations of NC>2 on vegetation by way of a literature
review.  A full risk assessment was not conducted for other welfare effects associated with NC>2.
The decision was made to retain the current standard for NC>2 as the uncertainty and variability
associated with the science at that time limited any other action. The 1995 NC>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 country, including "the
Catskills, Northern Appalachians, Valley and Ridge Province, and Southern Appalachians all
show some potential for chronic acidification due to N(V" (U.S. EPA, 1995). Additional topics
highlighted within the 1995  NC>2 review also included the  importance of acid neutralizing
capacity (ANC) in surface water acidification, the influence of atmospheric nitrogen to
eutrophication of the Chesapeake Bay, and the growing body of research on developing "critical
loads" and "target loads" for nitrogen in various ecosystems.
       In the previous review of the SO2 NAAQS, completed in 1996, vegetation damage
(growth, yield, and foliar injury) due to short-term and long-term exposures to SC>2 were avoided
by maintaining the current secondary 3-hour standard (0.053 ppm) (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 considered "weak and not developed well enough to
provide the principal basis for selecting the level of a long-term SCh standard" (U.S. EPA, 1982).
The 1982 staff paper did note, however, that long-term SO2 concentrations may be affecting
lichen and mosses, and that these should be considered 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 described 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 NOX and SOX scientific literature and is subject to
change as the NOX/SOX ISA undergoes revision. Currently, the EPA's Office of Research and
Development's (ORD) National Center for Environmental 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 approach described in this
plan may also be modified according to CASAC and public comments following their review of
this document as well as any additional information contained in the final version of the ISA.

2.3    Overview of Ecological Risk  Assessment and the Scope of the Secondary
       NAAQS Review for NOX and SOX
       The conventional framework for ecological risk assessment consists of three phases:
   •   Problem Formulation,
   •   Analysis, and
   •   Risk Characterization.
       These phases have been described 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 assessment endpoints, data needs, and anticipated
analyses. It results in three products: (1) assessment endpoints that adequately reflect
management goals of the ecosystem they  represent, (2) conceptual models 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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NO2/SO2 NAAQS Review represents the Problem Formulation 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 profiles and estimates risks by categories, such as
individuals or populations via modeling techniques. In the risk/exposure assessment, EPA plans
to draw upon the ISA to develop quantitative and qualitative estimates 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 NC>2 and 862, or levels that meet possible
alternative standards. The issues that are addressed by the Analysis Phase and Risk
Characterization Phase are part of the Risk/Exposure Assessment for the Secondary NAAQS
Review for NOX and  SOX.
       Welfare impacts from air pollution to ecosystems are a serious concern. There are many
harmful environmental effects of air pollution, including acid rain, ozone 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 their aesthetic and recreational amenities. Impacts
on climate are also considered welfare effects,  such as those due to nitrous oxide, N2O. Welfare
effects from the production of greenhouse gases  such as N2O can also cause significant
impairment of ecosystems through climate change processes.
       Against this broad background to welfare impact issues, this risk/exposure assessment
will focus on ecological quality and its effects from acidification and enrichment related to
nitrogen and sulfur air pollutants. In previous secondary reviews, acidification was evaluated for
its damage to materials, including  decay of buildings, statues,  and sculptures that are part of our
national heritage. The current assessment will focus on the influence of acid deposition on soil,
forests, and waterbodies. Nitrogen deposition may also contribute to eutrophication (oxygen
depletion) of water bodies, the symptoms of which 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 seafood and creating a risk of consuming fish or
shellfish contaminated from toxins produced by algal species that may be at an advantage in

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

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 are additional welfare effects that are being
addressed in the particulate matter (PM) and ozone NAAQS reviews. Visibility is being
addressed in a separate secondary review effort as part of the PM NAAQS standard review.
Ozone pollution's secondary impacts include damage to plant vegetation. Ozone is a secondary
product of precursors NO2 and volatile organic carbon. A separate rule review effort is in place
for ambient ozone concentration, and its secondary impacts will also not be addressed in this
NOX and SOX risk/exposure assessment.
       In this current review, the appropriateness of NO2 as an indicator for NOX species and
SO2 as an indicator for SOX species will be evaluated. This review will evaluate new information
published in the peer-reviewed literature since the completion of the last NO2 (1995) and SO2
(1996) reviews, including assessments of the adequacy of the current secondary NAAQS,
consideration of whether there is a possible need for a new single indicator or suite of indicators,
as well as changed or retained level(s) and/or averaging times for the standards, which may
include nitrogen and sulfur compounds other than NO2 and SO2.
       This Scope and Methods Plan is intended to facilitate consultation with the CASAC, as
well  as the  public, and to obtain advice on the overall scope, approaches, and key issues in
advance of the completion of such analyses and presentation of results in the first draft of the
risk/exposure assessment. The risk/exposure assessment is  intended to be a tool that, together
with other information contained in the NOX/SOX ISA, can  aid the Administrator in judging
whether the current secondary standards are requisite to protect public welfare from any known
or anticipated adverse effects, or whether these standards should be retained, revised, revoked
and/or replaced with alternative standard(s) having different indicators to provide the required
protection.
2.3.1   Overview of Risk Assessment Framework for Deposition-related Ecological
       Effects
       The risk/exposure assessment framework is intended to  serve as  a conceptual map of the
analytical and decision steps necessary to estimate the ecological risks associated with alternative

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

forms and levels of standards for NOX and SOX. As noted in the Integrated Review Plan for this
review (U.S. EPA, 2007a), the purpose of the risk/exposure assessment is to assess the potential
adversity of impacts including the effects of the pollutants on ecosystem goods and services, the
degree to which ecosystem functions are impaired, long-term trends in specific ecosystems
(where available), and both monetized and non-monetized valuation of ecosystem services. The
ability of the risk/exposure assessment to characterize 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 responsiveness 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 NOX/SOX risk/exposure assessment, EPA
designed a flow diagram that represents how nitrogen and sulfur compounds move from "source
to dose" in the environment. (See Figure 2-1.) This diagram represents 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 transformations among atmospheric concentrations,
deposition, ecosystem impacts, exposure to biologically relevant species, and ecosystem
responses via ecosystem services and valuation.
Characterization of Exposure
Atmospheric
Concentrations
ofNOxand
SOx
j

Deposition of
Sulfur and
Reactive
Nitrogen

—
t


Characterization of Ecological Effects
Ecosystem
Effects

Relevant
Biological
Exposure
Indicators

Ecosystem
Responses,
Setvices and
Valuation
j
n
f
               Climate Change
               (N20)
Nutrient Enrichment
      species alterations
      eutrophication
      mercury methylation
         Figure 2-1. Risk assessment framework for deposition-related ecological risks.
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

       The ecosystem response may be expressed as an "endpoint" in the ecological risk
assessment. For this review, the level of acidification 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 exposure required to reach the endpoint. This load
is also known as the critical load. A critical load analysis is a form of site-specific risk
assessment. It considers exposure to and response by various ecosystem receptors to identify the
amount of atmospheric deposition (or loading) above 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 indicator for policy analysis.
       Each component of the framework has a number 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 scientifically supported effects.
However, as noted by the Science Advisory Board in their recent review of the analytical plan
for the 2nd Prospective Analysis of the Costs and Benefits of the Clean Air Act, "a
comprehensive quantitative national assessment of the ecological benefits of the CAA
Amendments is not a realistic expectation." We recognize this limitation in developing the
framework for the NOX/SOX risk/exposure assessment, and in the following  sections, we describe
a detailed framework for assessing ecological risks that follows the principle of obtaining the
maximum amount of policy-relevant risk information possible given the data, resources, and
time limitations.
2.3.2   Overview of Nitrogen Deposition
       The sum of mono-nitrogen oxides, NC>2 and NO, typically are referred to as nitrogen
oxides (NOX) in the atmospheric science community. More formally, the family of nitrogen
oxides includes any gaseous combination of nitrogen and oxygen, e.g., NO2, NO, N2O, N2Os,
N2O4, and N2O5. Total  reduced nitrogen (NOy) includes all nitrogen oxides as well as gaseous
and particulate nitrate species such as HNOs, PAN, and aerosol phase  ammonium nitrates.
Reduced atmospheric nitrogen species include ammonia gas (NHa) and ammonium ion (NH4+),
the sum of which is referred to as NHX. Atmospheric nitrogen deposition often is delineated
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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

farther as dry (gas and paniculate phases) or as wet (precipitation derived ion phase). (See
Figure 2-2 )
                                                     Long range transport to remote
                                                     regions at low temperatures
Nly    !«y<<
 m\/
  Inorganic
   Nitrates
                      deposition
                                                 ^P
                                                                    nitro-PAHs
                                                                „_  R-C=C-R
                                                                NO-	> RONO,
                                                                 nitrosamines,
                                                                 nitro-phenols, etc.
                                                                    RONO
                                                                deposition
                                                emissions
  (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 particulate 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.)
       N2O has not been considered in setting previous NO2 NAAQS standards.  In the first NOX
review, N2O was not considered an air contaminant because there was "no evidence to suggest
N2O is involved in photochemical reactions in the lower atmosphere" (U.S. EPA, 1971). N2O
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 N2O on stratospheric ozone was described, and the criteria document
noted thatN2O may cause a small decrease in stratospheric ozone (U.S. EPA, 1982). Finally, the
criteria document also concluded thatN2O significantly contributes to the atmospheric
greenhouse effect by trapping outgoing terrestrial 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 N2O as an oxidized nitrogen compound that is not
generally considered to be an air pollutant, but does have an impact on stratospheric ozone and is
                                            12
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

considered among the more significant greenhouse gases. These documents clearly consider N2O
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 N2O as a potent greenhouse gas and discusses N2O 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), nitrous oxide contributes approximately 6.5 % to total greenhouse
gas emissions (in CC>2 equivalents) (Figure 2-3).
                                                        2.2%
                                                    MFCs, PFCs,
                                                         SF6
                                                           6.5%
                                                           N2O
                                                    7.4%
                                                    CH4
                        Source: U.S. EPA, 2007d
               Figure 2-3. Percent of Total U.S. Emissions of greenhouse gases
                                   in CO2 equivalents.
       Since the definition of "welfare effects" includes effects on climate [CAA Section
302(h)], we will include N2O within the scope of this review. However, it is most appropriate to
analyze the role of N2O in anthropogenic climate change in the context of all 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
       SC>2 is one of a group of substances known as SOX, which include multiple gaseous (e.g.,
SC>2, SO, SOs, 8203, 820?) and particulate (e.g., ammonium sulfate) species (Figure 2-4).
Acidification can result from the atmospheric deposition of SOX and NOX; in acid deposition,
these species combine with water in the atmosphere to form sulfuric acid (H2SO4) and HNOs.
Acidification is an environmental effect in which acid precipitation lowers the natural pH of
waterbodies and/or damages terrestrial ecosystems. Over the past few decades, acidification of
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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

waterbodies has been recognized as an environmental issue throughout Europe and North
America, and steps have been taken to control SOX and NOX emissions and to identify the
recovery of the impacted ecosystems. Due to known acute effects on plants, in previous NAAQS
reviews, SC>2 served as the chemical indicator for SOX 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 sulfur enrichment. Both effects occur in response to deposition of
NOX and SOX. Acidification effects can occur from either nitrogen or sulfur deposition, and the
relative contribution of each type of deposition depends on the characteristics of the affected
ecosystem (see Section 4.2  of the ISA, U.S. EPA, 2007b). Nitrogen and sulfur enrichment
represents a continuum of effects, 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 species composition (i.e., from wildflower meadows
to shrubs), and can result in eutrophi cation in aquatic systems (see Section 4.3 of the ISA, U.S.
                                            14
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

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 from nitrogen enrichment effects
       2.  Risks to aquatic ecosystems from nitrogen enrichment effects (eutrophication)
       3.  Risks to terrestrial ecosystems from acidification effects (nitrogen and sulfur)
       4.  Risks to aquatic ecosystems from acidification 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 influence 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 NOX, other reactive nitrogen species,
and SOX. The results of the atmospheric fate and transport process are atmospheric loading of
sulfur and total reactive nitrogen, as well as concentrations of N2O. As mentioned earlier,  a
quantitative assessment of N2O is not currently within the scope of this review.
       Atmospheric loadings of total reactive nitrogen and SOX lead to deposition of nitrogen
and sulfur to terrestrial and aquatic ecosystems. In addition to the major ecological effects of
acidification and nutrient enrichment, sulfur deposition also  leads to enhanced methylmercury
(MeHg) production in aquatic systems and, in turn, 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 deposition and methylmercury production can exacerbate an already
important mercury problem, especially  in coastal and  eutrophic waters subject to hypoxic  algal
blooms. The focus of the  quantitative risk/exposure assessment will be the set of effects
associated directly with acidification and nitrogen and sulfur enrichment, but we  will also
provide a qualitative assessment of potential impacts of sulfur deposition on waterbodies that are
vulnerable to increased mercury methylation.
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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
C\tmospheric
                     Fate and
                     Transport
                    	^
                                              1
                                          Total Reactive \
                                           N and SOX  J
                                           Depositi(
                                           Processes  J
                           Nitrogen  )                      (    Sulfur
                 v       y     V_
                 (  Enrichment  )     ( Acidification  }—,—(  Acidification )     '   MeHg    '
)    \Productiony
           f  Terrestrial  j f   Aquatic  j f  Terrestrial J f   Aquatic  J f  Terrestrial J (   Aquatic
                                                    H	_       I            I
        Figure 2-5. General flow of processes addressed in the risk/exposure assessment
                           for NOX and SOX secondary standards.
2.4    Key Policy Relevant Questions
       The 2007 Integrated Review Plan of the Secondary NAAQS for NO2 and SO2 introduced
a series of policy-relevant questions to frame the approach EPA will take in this review (U.S.
EPA, 2007a). The Review Plan indicated that issues 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 NOX or SOX acting individually that is important, or is it
the combination of NOX and SOX that needs to be addressed). For example, both NOX and SOX
are associated with acidification effects, while nitrogen is associated with nutrient enrichment
and eutrophication effects, and sulfur is associated with increased mercury (Hg) methylation.
       Both EPA and CAS AC have acknowledged the importance of NOX, SOX, and their
associated transformation products with respect 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 effects of aerosol NOX and SOX
that remain in the atmosphere.
       For this secondary NAAQS review of NOX/SOX, the primary policy-relevant questions
include:
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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

    •   What are the known or anticipated welfare effects influenced by ambient NOX and SOX,
       and for which effects is there sufficient information available to be useful as a basis for
       considering distinct secondary standard(s)?
    •   What is the nature and magnitude of ecosystem responses to NOX and SOX that are
       understood to have known or anticipated adverse effects, and what is the variability
       associated with those responses (including ecosystem type,  climatic  conditions,
       environmental effects, and interactions with other environmental factors and pollutants)?
    •   What are the biologically relevant indices that adequately capture the relationship
       between ecosystem exposure and response for the known or anticipated welfare effects
       we are trying to protect?
    •   To what extent do receptor surfaces influence the deposition of gases and particles (dry
       deposition), since dry deposition can contribute significantly to total deposition?
    •   What are the appropriate air quality indicator(s), averaging time(s), form(s), and level(s)
       of standards that are requisite to avoid those ecosystem responses?
    •   To what extent do the current standards provide the requisite protection for the public
       welfare effects associated with NOX and SOX?
          Should the current secondary standards for NC>2 (as an indicator  of NOX) and SC>2 (as
          an indicator for SOX) be retained, revised, or revoked and/or replaced with alternative
          standard(s) having different indicators to provide the required protection from known
          or anticipated adverse public welfare effects?
       -  Can effects from NOX be distinguished from effects due to total reactive nitrogen?
       To the extent that the evidence suggests revision  of the current secondary NOX/SOX
NAAQS is appropriate, ranges of standards will be identified (including 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 protection that is requisite to protect public
welfare from known or anticipated adverse effects. To account for variability in ecosystem
responses and land uses across the nation, ecosystem characteristics may be an important
consideration in evaluating the form(s) of the standards.  The form(s) of the standard(s) may be
based on a complex formula that incorporates ecosystem characteristics, land uses, atmospheric
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

transformations, climatic conditions, environmental effects and other interactions. In so doing,
the following questions should be addressed:
   •   Does the available information provide support for considering different NOX/SOX
       chemical indicators or exposure indices?
   •   Does the available information provide support for considering some joint standard(s) or
       are separate standards appropriate?
   •   What range of levels and forms of alternative standards are supported by the information,
       and what are the uncertainties and limitations in that information?
   •   To what extent do specific levels and forms of alternative standards reduce adverse
       impacts attributable to NOX/SOX,  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 the science, risk/exposure, and policy assessment
portions of this review include:
   •   Identifying important chemical species in the atmosphere
   •   Identifying the atmospheric pathways that govern chemical transformation, transport, and
       deposition of NOX and SOX to the environment
   •   Identifying the attributes of ecosystem receptors that govern their susceptibility to effects
       from deposition of nitrogen and sulfur compounds
   •   Identifying the relationships between ambient indicators and biologically  relevant indices
       of effects, including ecosystem services associated 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 sensitivities to varying meteorological scenarios
       and climate conditions
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                            Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
2.5    Proposed Schedule
       The proposed schedule for the joint NCVSC^ secondary NAAQS review is shown in
Table 2-1; underlined dates indicate the court-ordered 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 the risk/exposure assessment may be revised as
needed. The first draft of the risk/exposure assessment and the second draft of the ISA will be
released to CASAC and the public in August 2008. EPA will receive comments on these draft
documents from CASAC and the public at a meeting in  October 2008. A revised risk/exposure
assessment will be released in March 2009 followed by  a CASAC and public review in May
2009. The final risk/exposure assessment for the secondary NCVSC^ NAAQS review will be
released in July 2009.
                                                                             *
       Table 2-1. Proposed Schedule for the Joint NO2 and SO2 Secondary NAAQS Review
Stage of
Review
Planning
Integrated
Science
Assessment
(ISA)
Risk/Exposure
Assessment
(REA)
Major Milestone
Literature search
Federal Register call for information
Prepare the draft NO2/SO2 NAAQS Work Plan
Workshop on science/policy issues
CASAC consultation
Prepare the final integrated NO2/SO2 NAAQS
Work Plan
Prepare first draft of the ISA
CASAC/public review of the first draft of the ISA
Prepare the second draft of the ISA
CASAC/public review of the second draft of the
ISA
Prepare the final ISA
REA methodology released to the CASAC and
the public
CASAC/public consultation on the REA
methodology
First draft of the REA released to the CASAC and
the public
CASAC/public review of the first draft of the REA
Second draft of the REA released to the CASAC
and the public
CASAC/public review of the second draft of the
REA
Final REA released
Draft Target Dates
Ongoing
December 2005
December 2005-August 2007
July 2007
October 2007
December 2007
December 2007
April 2008
July/August 2008
October 2008
December 12, 2008
February 2008
April 2008
August 2008
October 2008
March 2009
May 2009
July 2009
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                            Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
Stage of
Review
Policy
Assessment/
Rulemaking
Major Milestone
Publish ANPR
CASAC review/public comment on ANPR
Proposed rulemaking
Final rulemaking
Draft Target Dates
August 2009
October 2009
February 12, 2010
October 19, 2010
Schedule may be modified, as necessary, to reflect actual project requirements and progress.
Underlined dates indicate the court-ordered schedule.
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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

3.     ANALYSIS PLAN
       Because ecosystems are diverse in biota, climate, geochemistry, and hydrology, response
to pollutant exposures can vary significantly. Also, these diverse ecosystems are often neither
abundant nor distributed evenly across the United States. To target acidification and nitrogen and
sulfur enrichment, this Scope and Methods Plan focuses on four main ecosystem effects on
terrestrial and aquatic systems identified in the ISA:
    •   Terrestrial nitrogen enrichment
    •   Aquatic nitrogen enrichment, 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 appropriate and within our time
constraints, impacts associated with nitrous oxide (N2O)and the influence of sulfur enrichment
on methylmercury production.  Since these ecosystem effects are 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 select these case studies from those areas of the United States where
ecosystems are identified as sensitive to nitrogen 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 the 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 regions with common ecosystem sensitivities (e.g., lakes and streams
of the Adirondack Mountains), site-specific quantitative modeling analyses, qualitative analyses,
and review and summary of previous risk/exposure assessments. From these qualitative-
quantitative 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 intended to be a qualitative analysis of multiple
ecosystems' quantitative-qualitative risk/exposure assessments.
       The risk/exposure assessment for the Secondary NAAQS for NOX and SOX will build
upon the scientific information presented in the 2007 draft ISA (U.S. EPA, 2007b). The ISA

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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

documents ecological effects of nitrogen and sulfur deposition, biogeochemical indicators of
effects, and areas of the U.S. where ecosystem 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 section, we will identify and describe how those
recommendations can be considered in the assessment 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 nitrogen and sulfur. The anticipated spatial extent and diversity of
ecological effects due to deposition of nitrogen and sulfur do not facilitate 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 formulate a strategy that is designed
to protect sensitive systems, while allowing for flexibility 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 NOX and SOX and potential welfare
effects. To create these indices, we plan to evaluate exposures and impacts in various ecosystem
case studies with differing responses related to nitrogen and sulfur inputs and explore
relationships between ambient concentrations and deposition of nitrogen and  sulfur.
       As previously described, deposition of SOX and NOX compounds affects ecosystems in
various adverse ways and at different spatial and temporal scales in diverse regions of the
country. In its review of the analytical plan for the 2nd 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 assessment includes selecting data that will be used,
analyzing exposure  (including spatio-temporal conditions), analyzing effects, and summarizing
conclusions about exposure.
       In order to address the policy-relevant questions that are guiding the scope of this review,
this risk/exposure assessment intends to evaluate the relationships between atmospheric
concentrations, deposition, biologically relevant exposures, ecosystem effects, and ecosystem
services.

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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

       To evaluate the nature and magnitude of ecosystem responses associated with adverse
effects, the risk/exposure assessment plans to examine various ways to quantify the relationship
between air quality indicators, deposition of biologically accessible forms of nitrogen and sulfur,
biologically-relevant indices relating to deposition, exposure and effects on sensitive receptors,
and related impacts to ecosystem change and services. To the extent feasible, 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 incorporate the temporal considerations (i.e.,  biologically relevant
timeframes), pathways, and biologically relevant indices necessary to maintain the functioning of
these ecosystems. In addition, the risk/exposure assessment plans to evaluate the contributions of
atmospherically deposited nitrogen and sulfur relative to total loadings in the environment. For
the atmospheric contribution to  total nitrogen, we also plan to evaluate the contribution of NOX to
total reactive nitrogen in the atmosphere relative 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 and/or time frames for the risk assessment?
       Information that will be  considered in addressing this question includes mapping datasets,
       research studies of sensitive ecosystems ranging 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 incorporating both qualitative 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 acidification 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 science regarding linkages
       between pollutants in ambient air, deposition, and measurable ecosystem effects,
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

       including effects on ecosystem services? What are the most useful metrics of both
       ambient pollution and the resulting effects?
    •   To what degree should the risk/exposure assessment take the potential for recovery into
       account in selecting data for qualitative 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 ecosystem type. We will assess the feasibility to
consolidate effects and/or ecosystems in the risk/exposure assessment and, where feasible,
perform a broader characterization. However, some ecosystems and their effects may be too
unique to consolidate into a broad characterization.
       Upon completion of all  risk/exposure assessment case studies, the results of the
assessments performed for unique combinations of effects and ecosystem types will be presented
together to facilitate decision making on the total 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/exposure 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 framework by addressing the selection of effects,
indicators and ecosystem services measured for exposure via atmospheric deposition of total
reactive nitrogen and sulfur from ambient air. The 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 enough to perform a quantitative assessment for each

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                                  Secondary 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.
        Integrated Science Assessment
     Use 2002 CMAQ output to run
     selected multimedia models from
     the integrated Science Assessment
                                               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
    Figure 3-1. Seven step approach to planning and implementing risk/exposure assessment.
                                                   25
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

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 nitrogen and sulfur loading, we plan to identify
adverse terrestrial and aquatic effects due nitrogen and sulfur deposition. The impacts of
acidification, nitrogen-induced  eutrophication, and changes in species diversity due to nitrogen
and/or sulfur saturation are documented in EPA'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 nitrogen and sulfur enrichment and most informative for potential
case study analyses in our overall risk/exposure assessment.
       Environmental indicators are measures that track environmental conditions over time.
EPA's Report on the Environment (ROE) (U.S. EPA, 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 chemical, biological, and habitat 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 biological 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 characteristics
are most appropriate and the spatial variability in these characteristics will be addressed. For
sulfur, the adsorption/desorption responses in soils may be important.  In addition, we plan to
consider the contribution of any internal sulfur sources (both organic sulfur mineralization and
inorganic sulfur mineral weathering) 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 cycling, but also in affecting organic matter quality via organic

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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

matter inputs to the soil. More detail on this approach to identifying indicators and a preliminary
list of recommended indicators is provided below.
       Preliminary NOX and SOX Indicators:  In the ISA, relevant indicators were divided into
one or both of the response categories: acidification or nutrient enrichment. Table 3-1 presents
acidification or nutrient enrichment indicators.
                    Table 3-1. Summary of Indicators Categorized by Effect
Acidification
Acid stress index
Acid neutralizing capacity
Alkalinity
Aluminum, mobilization
Carbon to Nitrogen ratio
Calcium and magnesium
concentrations
Community structure
Condition factor
Dissolved organic carbon
Dissolved organic nitrogen
Exchangeable cations
Forest health
Index of biotic integrity
Metal mobilization
PH
Soil-base saturation
Species composition
Taxonomic richness
Ecosystem
Type
A
A, T
A
A
T
T
A, T
A
A
A
T
T
A, T
T
A, T
T
A, T
A, T
Nutrient Enrichment
Carbon budget (growth, carbon
fixation, and respiration)
Concentration of chlorophyll
Concentration of carotenoids
Nitrogen, concentrations
Photosynthetic rate
Phosphorus concentrations
Species richness
Taxonomic density
Thallus density
Transpiration rate








Ecosystem
Type
T
A, T
T
A
A, T
T
A, T
A, T
T
T








 A = Aquatic
 T = Terrestrial
       Key Acidification Indicators:  Table 3-2 presents key indicators that play a significant
role in surface water acidification and recovery (summarized from Skjelkvale et al., 2005).
Specific indicators of acidification are discussed in more detail below.
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
                 Table 3-2. Key Indicators of Acidification Due to NOX and SOX
Key Indicator
Group
Acid anions
Base cations
Acidity
Organic acidity
Metals
Biological
Examples of Indicators
SO42", NO3"
Ca2+, Mg2+, E(Ca2++Mg2+)
pH, (Gran) alkalinity,
ANC
Dissolved organic carbon
(DOC), total organic
carbon (TOC)
Al, iron (Fe)
Forest health, community
structure, species
composition, taxonomic
richness, Index of Biotic
integrity
Description
Trends in these concentrations reflect recent
trends in atmospheric deposition (especially SO42")
and in ecosystem responses to long-term
deposition (notably NO3" and desorbed SO42").
These cations are mobilized by weathering
reactions and cation exchange. These respond
indirectly to decreases in SO42" and NO3" 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.
These indicators reflect the outcomes of
interactions between changing concentration of
acid anions and base cations.
Organic acids are common natural sources of
acidity in surface waters.
These metals are mobilized as a response to the
deposition of SO42" and NO3".
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 indicator with relevance to soils, terrestrial ecosystems, and
aquatic ecosystems and is a key indicator recommended 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 available at the present time than the biological
indicators.
       Key Indicators of Nutrient Enrichment: Major indicators for nutrient enrichment to
aquatic and terrestrial systems from air deposition of reactive nitrogen involve measurements
based on available monitoring stations for wet deposition (NADP/national trend network [NTN])
and limited networks for dry deposition (CASTNET). Wet-deposition monitoring stations can
provide more information on an extensive range of nitrogen species than is possible for dry-
deposition monitoring stations. This creates complications in developing estimates for total
nitrogen deposition levels because dry-deposition data sources will likely be underestimated.
Individual studies measuring nitrogen deposition to terrestrial ecosystems that involve
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                               Secondary 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 nitrification and potential increases in soil acidity and releases in

NOs 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, discharges to water, diffuse runoff, groundwater

inputs). Major aquatic system indicators involve 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 nitrates,  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 ecological systems, a data-fusion approach that

combines monitoring indicators with modeling inputs and outputs is often used (Howarth, 2007).

              Table 3-3. Key Indicators of Nutrient  Enrichment Due to NOX and SOX
   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 and
also with available (preferably multi-
media) 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 and
also with available (preferably multi-
media) models.
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

3.1.2   Potential Ecosystem Services
       We plan to identify the primary ecosystem service(s) for each of the effect classes (e.g.,
acidification or enrichment) and for major ecosystem types and components (e.g., terrestrial
ecosystems, soils, aquatic ecosystems). These services may be characterized as: supporting
services that are necessary for all other services (for example primary production), provisioning
services (food, fuelwood), regulating services such as climate regulation or flood control, and
cultural services including spiritual or religious values, aesthetic 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 indicators for the major types of
change. These indicators may use chemical/physical properties (e.g., ANC), or they may involve
biological endpoints (e.g., bioassessment 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 conditions 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 responses once pollutant loadings are significantly
abated;  other  systems, such as larger estuarine aquatic systems, may require much longer
recovery times. These temporal issues will be documented in the analyses as possible.
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

3.2    Step 2 - Define Sensitive Areas That Exhibit Effects 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., acidification and nitrogen and sulfur enrichment).
In the risk/exposure assessment, we plan to begin with those ecosystems and case study areas
identified in the ISA and consider potential near-field and far-field linkages.
       Identify Biological, Biogeochemical, and Physiographic Linkages in These
Ecosystems: Linked systems will be identified (e.g., upland terrestrial/aquatic areas linked to
downstream estuarine system) where possible. Especially 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 loadings from the inland drainage areas (Figure 3-2).
       Plats, Stands               Hills lopes. Catchments           Basin, Region
          Figure 3-2. Ecosystem effects may range from near-field to far-field linkages.
       Assess Significance of Linkages: The spatial extent of linkages supports different scales
of risk assessment. For example, nutrient criteria could serve as the anchor of a broader-scale
characterization of U.S. aquatic  systems. Inland acid-sensitive waters in the eastern United States
and nitrogen-sensitive ecosystems in the Rocky Mountains and other parts of the western United
States may support a large-scale, special-area assessment. Ecosystem effects in special areas,
such as the Adirondack Mountains or Class I areas of the United States may support area-level
assessments. If the linkages are geographically significant, we plan to evaluate and determine the
maximum scale of the case study area based on the linkages. The resultant case study area may
be a common ecosystem, a subbasin, a riverbasin, or an airshed, and may be local, subregional,
                                           31
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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

or regional in scale. If an area is identified as not having important linkages, then that area may
be a good candidate for a case study area (e.g., local 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 criteria for rivers and lakes can provide the
foundation for national-scale characterizations involving nitrogen-enrichment effects for aquatic
systems related to loadings from air deposition and other nitrogen sources. Available information
on the nitrogen sensitivity of estuarine systems can further extend the scope of these analyses to
include estuarine and other near-coastal waters. Information that deals with special case study
areas (e.g., the Adirondack Mountains or special alpine and subalpine ecosystems 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 identify 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 scales by using different approaches 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 facilitates 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 (GlS)-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 land-cover patches. Integrated systems using
monitoring, modeling, and interpolation approaches are often applied 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 in
                                            32                       DRAFT - March 2008

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                                                                                           Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
                                                            Altered plant
                                                             communities
                                                           (plants, lichens,
                                                           Increased Invowv* 1J^
                                                             Species end
                                                             Altered Fire
                                                                Regime     .
                Draft Aggregations of Level III Ecoregions
                for the National Nutrient Strai
 I I. HiHaau-ltr anil Cramtl Valiiy
            JMaataiaam
  HI. XrnrllW
  JV. Grant Phuti* Ojiu fl/irf Sij-uWitnc/s
  ',. LSnij^n i .'nirjii i 'ui'i.-v.ii.'ij -;i
  V/. foo) HcJ( amiSarthfrn
II 1 \ N A/Ml*)- Clnrutrd Daii

  J.t SbutJmutwn T*
  .V. 7'r.vns-i.rcursinn.n ('oairtjij nn
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

       Table 3-4. These materials may also be of value in identifying a list of nitrogen- and
sulfur-sensitive ecosystem services and patterns and trends due to changes in reactive nitrogen
and sulfur inputs (Section 3.7 describes this step).
                    Table 3-4. Example Datasets Planned for GIS Analysis
Ecosystem
Type
Lakes, rivers,
estuaries
Lakes, rivers,
estuaries
All
All
All
Lakes, rivers,
estuaries
Forests and
grasslands
Rivers, lakes,
estuaries
High-
elevation-
based lakes
and rivers
All
All
Description
Nutrient criteria
for lakes, rivers,
and estuaries
Acid sensitive
waters of the
United States
National atlas of
Class 1 areas
Ecoregions of
the United
States
Soil
characterization
data
Total nitrogen
deposition and
other analytes
U.S. vegetation
Surface water
quality
U.S. elevation
map
Land cover
NADP grid files
Effect of
Interest
NE
ACID
NE and ACID
NE and ACID
NE and ACID
NE
NE and ACID

ACID
NE and ACID
NE and ACID
Indicator
Parameter
TN for lakes
and rivers
pH, ANC
All
All
pH,ANC,
exchangeable
Ca2+, dissolved
organic content
NH4+
NandS
Many
S
NandS
NandS
Internet Link or Other
Resource
www.epa.gov/waterscience/
criteria/nutrient/ecoregions
epamap4.epa.gov/cmap/vie
wer.htm
www.nationalatlas.gov/mld/f
edlanp.html
nationalatlas.gov/atlasftp.ht
ml#ecoomrp
www.ncgc.nrcs.usda.gov/pro
ducts/datasets/statsgo/descr
iption.html
epamap4.epa.gov/cmap/vie
wer.htm
ivm.cr.usgs.gov/products.ph
p Also available from
NationalAtlas.gov
www . e pa . g o v/sto ret
ESRI 8.3 data disks
www.mrlc.gov/mrlc2k_nlcd.a
sp
nadp.sws.uiuc.edu/isopleths/
grids. asp
Note: NE = nutrient enrichment, ACID = acidification, 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 applied in the risk/exposure assessment of targeted
sensitive ecosystems. This plan is preliminary and will be reassessed as EPA proceeds in
performing the seven steps for characterizing ecological effects.
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

3.3    Step 3 - Select Risk/Exposure Case 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 the case study assessment area may be created.
Complementary to these efforts, we may use a statistical cluster analysis 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 consideration of (1) clear quantifiable ecological effects
due to air pollution, (2) the degree to which a significant component 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 air 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 recommendations were made in the context of a
prospective cost-benefit analysis, many of these recommendations are sound in the context of our
NAAQS risk analysis. The EES also provided  some specific critiques and recommendations of
specific potential case study assessment areas.  We plan to evaluate their comments as part of our
case study assessment area selection 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 example, 'New' nitrogen (or mercury) derived
       from air pollutants is generally more bioavailable than 'old' nitrogen (or mercury).
       Moreover, ecosystems and associated organisms respond differently to different species
       and sources of nitrogen (and mercury)." (p. 6).
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                                                                 Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
                     Table 3-5. Summary of Indicators, Mapping Layers, and Models for Targeted Ecosystems
   Targeted
  Ecosystem
     Effect
            Indicators)
        Mapping Layers
     Model(s)
       Remarks
Terrestrial
Nitrogen
Enrichment
CEC
C:N ratios
Ca:AI ratios
Air wet/dry deposition (corrected for
throughfall using available data)
Forest soils from USFS
Forest type from USFS
Statsgo soils
NLCD
CMAQ (N) by HUC
MAGIC; PnET-BCG
Aquatic Nitrogen
Enrichment and
Eutrophication
Nitrate and ammonia, total nitrogen
(major reactive N species)
Al toxicity data
Chl-a (algal standing crop)
anoxia/ hypoxia (primarily estuaries and
tidal rivers)
N loadings for sub-watersheds or larger
basins and Estuarine Drainage Areas
(EDAs)
EPA NCCR WQ index and NOAA
Estuarine-Coastal Eutrophication Index
Diatom data for N-limited systems
STORET retrievals
USGS NAWQA information
USGS SPARROW information
WQS Nutrient Criteria for rivers
and lakes
EPA NCCR and NOAA estuarine
eutrophication indicators
NOAA EDAs
EPA/NOAA airsheds for major
Atlantic and Gulf estuaries
CMAQ (N) by HUC
USGS SPARROW
PnET-BCG
Aquatic Sulfur
Enrichment
(MeHg Focus)
MeHg (ambient)
MeHg (tissue residues)
Sulfur (ambient and sediments)
Devils Lake, Wl, area
Limit to review of
previous research
Examine studies for Devil's
Lake, Wl; also examine
other literature from
Mercury Report to
Congress and recent work
for areas in the
northeastern United States
Terrestrial
Acidification Due
to Nitrogen and
Sulfur
Soil ANC
Soil pH
CEC
Inorganic Al
Ca:AI ratio
Special areas (e.g., Class I areas,
the Adirondack Mountains)
CMAQ (N & S) by HUC
Forest soils from USFS
Statsgo soils
USFS lichen
USFS forest types
MAGIC; ILWAS;
PnET-BGC
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                                                                      Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
Targeted
Ecosystem
Effect
Aquatic
Acidification Due
to Nitrogen and
Sulfur
Indicators)
ANC
PH
Al
NO3 and SO4 fluxes or loadings
Mapping Layers
Acid-sensitive waters
Select Class 1 areas
EPA STORE!
USGS NWIS
CMAQ (N & S) by HUC
Model(s)
MAGIC; PnET-BGC;
SPARROW
Remarks

Note: CEC = cation exchange capacity, C:N = carbon:nitrogen, Ca:AI = 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, STORE! = STOrage and RETrieval, USGS = U.S. Geological Survey, NAWQA = National Water Quality Assessment Program, WQS = water
quality standards, NWIS = National Water Information System, ILWAS = Integrated Lake-Watershed Acidification Study.
                                                                   37
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                             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 contributing to ecological effects of interest. Thus,
       selection of an [appropriate assessment] area 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 regions with different
       resources at risk to help bring 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 listing 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 given 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  and any additional sites identified by the Step 2
mapping exercise in our  selection of risk/exposure assessment areas. Options for selecting case
study assessment areas include site-specific quantitative modeling analysis, qualitative analysis,
cluster analysis,  and review and summary of previous 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 assessment methods will be used.
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                  Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
Table 3-6. SAB/EES Listing of Potential 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
Chesapeake
Bay
Long Island
Sound
Everglades
Lake
Michigan
Barnegat Bay
Tampa Bay
Gulf of Maine
Casco Bay
Nitrogen
Nitrogen
Nitrogen;
Mercury
Mercury
Mercury
Nitrogen
Nitrogen;
Mercury
Nitrogen
Nitrogen;
Mercury
30%
20-30%
Nitrogen = 23-
35%; Mercury = ?
20-85%
87%
50% total
Direct deposition
30-39%
Nitrogen = 25-30%
Low
Nitrogen = 30-40%
Mercury = 84-92%
Yes
Yes
Yes
Ecological = yes;
Economic =
uncertain
Ecological = yes;
Economic = lacking
Yes
Yes
?
Yes
High priority. Higher loading
from non-depositional
sources may confound
analysis.
High priority. Loading from
diverse sources, particularly
agricultural, may confound
analysis.
High priority. High nitrogen
loading from wastewater
treatment plants may
confound analysis.
Medium priority. Reduction in
atmospheric deposition has
already resulted in decreased
mercury burdens in fish and
other biota.
Medium priority. Lack of
quantitative economic data
may restrict analysis.
High priority. Direct linkage of
ecological effects with
atmospheric deposition;
quantitative economic data
exist.
Medium priority. Examined in
previous EPA efforts.
Variability in loading data may
confound analysis.
Low priority. Linkage of
nitrogen loadings and
ecological impacts is not well
established. Major source of
nitrogen is open-ocean influx.
Medium priority. Good data
on ecological and economic
impacts are available.
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Ecosystem/
Region
Main CAA
Pollutant(s)
Percentage(s)
Attributable to
Atmospheric
Deposition
Quantitative
Ecological and
Economic
Information
EES Comments
Forested
Adirondacks
Catskills
Southern
Appalachian
Mountains
Rocky
Mountains
Nitrogen;
Sulfur;
Mercury
Nitrogen;
Sulfur
Nitrogen;
Sulfur
Nitrogen
Nearly 100%
Nearly 100%
Nearly 100%
Nearly 100%
Yes
Yes
Yes
Yes
High priority. Good
quantitative ecological and
economic data exist. Previous
studies can be augmented
readily.
Medium priority. Economic
data may be lacking. Issues
similar to the Adirondacks.
Medium priority. Economic
data on fisheries are
available. Issues similar to the
Adirondacks.
Medium priority. Levels of
nitrogen loading much lower
than for northeastern
locations. Economic data may
be lacking.
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                                       Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
Table 3-7. Potential Assessment Areas Identified in the Draft ISA (U.S. EPA, 2007b)

Area
Adirondacks


















Shenandoah
National Park









Indicator
Aquatic Nutrient
Enrichment;
Terrestrial
Nutrient
Enrichment;
Mercury
Methylation










Aquatic
Acidification
Aquatic
Acidification;
Terrestrial
Nutrient
Enrichment





Detailed
Indicator






























Area Studies
PIRLA 1 and II;
Adirondack
Lakes Survey;
Episodic
Response
Project; EMAP













Shenandoah
Watershed
Study; Fish in
Sensitive
Habitats (FISH)
study





Models
MAGIC;
PnET-BGC















ILWAS

MAGIC










References in EPA, 2007b
Baker and Laflen, 1983; Baker etal., 1990b;
Baker etal., 1990c; Baker etal., 1996; Benoit
et al., 2003; Chen and Driscoll, 2004; Confer
et al., 1 983; Gumming et al., 1 992; Driscoll et
al., 1987a; Driscoll et al., 1991; Driscoll et al.,
1998; Driscoll et al., 2001 a; Driscoll et al.,
2001 b; 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; Johnson
et al., 1994b; Landers et al., 1988; Lawrence
et al., 2007; NAPAP, 1998; Siegfried et al.,
1 989; U.S. EPA, 2003; Sullivan et al., 1 990;
Sullivan et al., 2006a; Sullivan et al., 2006b;
U.S. Environmental Protection Agency,
1 995b; Van Sickle et al., 1 996; Whittier et al.,
2002; Wigington et al., 1996; Zhai et al., 2007
Gherini etal., 1985

Baker and Christensen, 1991; Baker etal.,
1990b; Bulger et al., 1999; Bulger et al., 2000;
Cosby et al., 2006; Dennis and Bulger, 1995;
Dennis et al., 1 995; Deviney et al., 2006;
Eshleman and Hyer, 2000; Eshleman et al.,
1995; Eshleman et al., 1998; Galloway et al.,
1983; Hyeret al., 1995; MacAvoy and Bulger,
1 995; Molot et al., 1 989; Schofield and
Driscoll, 1987; Sullivan et al., 2003; Sullivan
etal.,2007a; Webb etal., 1995


ISA
















Lit.
Search
ISA









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   Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
Area
Chesapeake
Bay
Alpine and
Subalpine
Communities
of the Eastern
Slope of the
Rocky
Mountains in
Colorado
Beartooth
Mountain,
Wyoming
Fernow
Experimental
Forest near
Parsons,
West Virginia
Uinta
Mountains of
Utah and the
Bighorn
Mountains of
central
Wyoming
Pamlico
estuary in
North Carolina
Indicator
Aquatic Nutrient
Enrichment;
Aquatic Nitrogen
Limited
Eutrophication
Aquatic Nutrient
Enrichment;
Terrestrial
Nutrient
Enrichment
Aquatic Nutrient
Enrichment
Terrestrial
Nutrient
Enrichment
Aquatic
Acidification
Aquatic Nitrogen
Limited
Eutrophication
Detailed
Indicator

biomass
production; NO3
leaching;
species
richness
algae
composition
switch
forest growth
lake N03
concentrations
hypoxia;
phytoplankton
bloom
Area Studies




Western Lakes
Survey

Models






References in EPA, 2007b
Brickeretal., 1999; Bricker et al., 2007;
Boesch et al., 2001 ; Boyer et al., 2002; Boyer
and Howarth,2002; Cooper and Brush, 1991;
Fisher and Oppenheimer, 1991; Harding and
Perry, 1997; Howarth, 2007; Kemp et al.,
1983; Malone, 1991, 1992; Officer et al.,
1 984; Orth and Moore, 1 984; Twilley et al.,
1985
Baron 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 and
Ranker, 2001; Rueth et al., 2003; Seastedt
and Vaccaro, 2001 ; Sherrod and Seastedt,
2001; Steltzer and Bowman, 1998; Suding et
al., 2006; Williams and Tonnessen, 2000;
Williams et al.,1996a; Wolfe et al.,2001
Sarosetal., 2003
Adams et al., 1997, 2000; DeWalle et al.,
2006; Edwards and Helvey, 1991 ; Gilliam et
al., 2006; Peterjohn, 1996
U.S. EPA, 1987
Paerletal., 1998

ISA
ISA
ISA
ISA
ISA
ISA
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   Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
Area
Bear Brook,
Maine
Harvard
Forest
Southern
California
Jasper Ridge
Biological
Preserve in
California
Allegheny
Mountains of
West Virginia
Catskill
Mountains of
New York
Great Smoky
Mountains in
Tennessee
Loch Vale,
Colorado
Rocky
Mountain
National Park,
Colorado
Indicator
Aquatic
Acidification;
Terrestrial
Nutrient
Enrichment
Terrestrial
Nutrient
Enrichment
Terrestrial
Nutrient
Enrichment
Terrestrial
Nutrient
Enrichment
Aquatic
Acidification
Aquatic
Acidification
Aquatic
Acidification
Terrestrial
Nutrient
Enrichment
Terrestrial
Nutrient
Enrichment
Aquatic Nutrient
Enrichment
Detailed
Indicator
sugar maple;
red spruce
forest growth —
species
forest growth —
species; coastal
sage scrub
grasslands
high
streamwater or
lake NO3
concentrations
high
streamwater or
lake NO3
concentrations
high
streamwater or
lake NO3
concentrations
old-spruce
growth
tundra
composition
switch
diatom shifts
Area Studies










Models










References in EPA, 2007b
Elviretal.,2003
Magilletal., 2004; Magill, 2004
Fenn et al., 1996, 2003a; Takemoto et al.,
2001
Zavaleta et al., 2003
Gilliametal., 1996
Murdoch and Stoddard, 1992; Stoddard, 1994
Cooketal., 1994
Rueth et al., 2003

Interlandi and Kilham, 1998

ISA
ISA
ISA
ISA
ISA
ISA
ISA
ISA

ISA
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   Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
Area
Rocky
Mountain
National Park,
Colorado
Lake Tahoe,
California
Little Rock
Lake,
Wisconsin
Southern
Appalachians
Hubbard
Brook, New
Hampshire
Indicator
Aquatic
Acidification
Aquatic Nutrient
Enrichment
Aquatic Sulfur-
Enhanced
Mercury
Methylation
Aquatic
Acidification
Terrestrial
Acidification;
Aquatic
Acidification
Detailed
Indicator
subalpine lakes
primary
productivity;
chlorophyll a
bioaccumulation
of Hg in
freshwater fish
Streams
forest
ecosystem;
soils; streams
Area Studies




many studies for
decades
Models
MAGIC


MAGIC
PnET-BGC
References in EPA, 2007b
Sullivan etal., 2005
Goldman, 1988; Jassby et al., 1994
Hrabik and Watras, 2002; Watras and Frost,
1989; Watras etal. ,2006
Sullivan etal., 2004
Gbondo-Tugbawa and Driscoll, 2002;
Gbondo-Tugbawa et al., 2002

Lit.
Search
ISA
ISA
Lit.
Search
Lit.
Search
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                             Secondary 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 and 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 levels of nitrogen and sulfur. Depending on the
adequacy and abundance of data for areas, the evaluation may entail computer modeling,
statistical analysis, or qualitative analysis.
3.4.1   Assess  Data Availability and Adequacy
       Determine Which Indicators Have Monitoring Data and/or Modeling data Available
for Analysis: The ISA contains a review of recent (2000 to present) monitoring programs for
targeted ecosystem effects and associated indicators. We propose to use ANC as the first choice
of acidification indicators. However, we may consider other indicators if we determine that data
are insufficient or other factors demonstrate a need to change indicators.  Nitrogen and sulfur
enrichment have both terrestrial and aquatic impacts, making the selection of indicators more
challenging. Currently, we have narrowed the list of indicators for additional 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.  Therefore, we will consider the conditions of the
monitoring programs supplying data for this assessment; 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.
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       Evaluate the Adequacy of Spatio-temporal Data and the Statistical Adequacy of
Available Data: We plan to evaluate the spatial adequacy of available monitoring data
including GIS mapping of documented data to identify any meaningful spatial gaps. For each
ecosystem effect, we plan to determine if there is a temporal dimension 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., measure effects on a daily or annual scale), and we plan to seek
and assess data available for candidate indicators from both monitoring and modeling data that
address the temporal dimensions required.
       We intend to review and determine the best indicators for acidification and nitrogen and
sulfur enrichment for each targeted ecosystem effect. To accomplish this, we plan to identify
spatial and temporal gaps, document uncertainties and limitations raised in the research
literature, and raise any additional points about uncertainty identifiable through either statistical
analysis or qualitative evaluation (depending on the form of information available).
       Resolve Gaps and Disadvantages (e.g., integrate datasets and/or models;
interpolate): The location and type of ecosystem effects resulting from nitrogen and sulfur
deposition do not lend themselves to a traditional large-scale risk/exposure assessment. This is
due to both the isolated or regional effects observed and the potential lag in time for observed
effects. Therefore, it will likely be necessary to resolve spatial and temporal gaps in data needed
to perform a risk/exposure assessment. We propose to resolve gaps through a combination of
integrating datasets, modeling, and interpolation/extrapolation. We plan 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 exposure assessments for the targeted ecosystem
effect. Additional information on CMAQ and multimedia 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 loading data include the following:
    •   Atmospheric deposition across the landscape (available from CMAQ modeling for 2002
       for 8-digit hydrological units [HUCs] and a 12-km grid) (Figure 3-4 presents past
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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

       applications of CMAQ modeling to support the evaluation of National Acid Precipitation
       Assessment Program [NAPAP]-monitored acidification effects in the United States.)
       Atmospheric deposition monitoring data (National Atmospheric Deposition Program
       [NADP] and Clean Air Status and Trends Network [CASTNET])
       National Pollutant Discharge Elimination System (NPDES) point source discharge data
       Agricultural runoff modeling
       Urban non-point source runoff modeling
       Other urban or rural loading sources (e.g., onsite septic system modeling).
        Sensitive ecological receptors in the United
  Annual Total Nitrogen Deposition (Wet + Dry)
       CMAQ 36-km Grid (kg-N/ha)
16.0011? |
                                              0.00  1
                                            kg/hectare  1                             148
           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 receptor surfaces
(i.e., land use/land cover) influencing atmospheric 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 species >50% of total deposition). CMAQ modeling
should account for the role of deposition due to both atmospheric conditions and land cover. We
plan to use land cover datasets, such as the National Atlas, to support our analysis of the role of
land cover in targeted ecosystem effects. Appendix 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
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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

applications, geographic applicability, and use of atmospheric inputs are acidification and
nitrogen and sulfur enrichment.
       To provide more detail for the models, those 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 concentration data (mass per volume) are presented in italics.
Although these classifications note whether a model accepts atmospheric deposition input, they
do not specify whether temporal and spatial resolutions 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 damages terrestrial ecosystems. The
key indicators of acidification likely 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 factors. The biological indicators of acidification can be found
in both terrestrial and aquatic systems; however, models are much less developed for simulating
or estimating measures of the biological indicators on land. The chemical indicators of acidity
are much  more likely to be included in a model.
       Both MAGIC and WARMF1 account for several of the chemical indicators of
acidification for both terrestrial 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 biological 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 minerals as its state variables but does
simulate nitrogen and carbon, as well as vegetation, which could be considered a measure of
forest health. This model can be used on a global scale.
1 WARMF currently accepts atmospheric deposition in concentration form; however, EPRI and Systech
Engineering, Inc., the developers of WARMF, have recently undertaken a project to modify WARMF to accept
depositional fluxes such as those output by CMAQ.
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       Several of the terrestrial/watershed models could be used to provide input loads to a
receiving-water model for acidification studies. The choice of model would depend on the time-
step 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 look at changes in ANC.
The other models may be considered if further analysis would prove useful.
Nitrogen Enrichment
       Indicators of nitrogen enrichment are addressed in detail in Section 3.1.1. Atmospheric
deposition has been shown to increase nitrogen enrichment in Atlantic Coast estuaries and is now
of concern  in high-altitude, alpine lakes. Because the key indicators for nitrogen enrichment
depend on measurements of nitrogen, there are many models available to estimate the ecosystem
effects, but special attention should be paid to how these models simulate the nitrogen cycle and
whether biological processes, which are vital to the cycle, are included. Nitrogen, either total or
speciated, is probably the most highly modeled 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 that a watershed model provides input loadings to
the receiving water model.
       The following receiving-water models all estimate 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 nitrogen cycle, but has limited geographic
applicability for small sites. It is more applicable for large-scale simulations.
       HSPF, SWAT2, and  WARMF simulate the nitrogen cycle across land and water;
however, SWAT contains a more simplified approach 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 concentration in precipitation. An untested concept to utilize
SWAT's fertilizer management function to simulate atmospheric deposition may provide a work-around function.

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

accepts atmospheric concentrations of NOX, ammonia, and nitrate in both wet and dry
atmospheric conditions.
       Nitrogen enrichment, more than acidification, 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 the nitrogen cycle within the model. For daily
simulations, possible models include: DayCent, DNDC, DRAINMOD-NII, 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 ecosystem 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 existing model to an untested geographic area
or using synthetic or surrogate data if there is a geographic region of particular 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 reactive nitrogen species, recognizing 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


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                             Secondary 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 relative to all other major
anthropogenic or natural sources for sensitive 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 the 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 advantage 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 the  Shenandoah Parkway).  Where the TIME/LTM
datasets have previously been applied to define  the extent of sensitive areas, scaling up the new
analyses to the same previously used larger sensitive areas may be readily justified. Where the
case study assessment areas are defined using clusters of sampling sites, more sophisticated GIS-
based spatial interpolation techniques may be applied where the case study assessment area sites
are related to bounding polygons reflecting standard ecoregion delineation systems (e.g., the
EPA Omernik Level II ecoregions or the aggregated Omernik ecoregions developed for EPA
criteria for numeric nutrient criteria for lakes and streams). Statistical cluster analysis techniques
may be applied to provide a means for objectively grouping localized case study assessment
areas into larger sensitive areas. For larger case  study assessment areas such as the Chesapeake
Bay system, available national indicator products such as the USGS SPARROW models for
HUC8 (sub-basin) watersheds, or EPA and NOAA 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 relative importance of atmospheric deposition of
pollutants such as nitrogen to sources 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.
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3.6    Step 6 - Assess the Current Ecological 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 information 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 may be considered, where appropriate, in identifying
ways to select endpoints in terms of ecosystem responses relative to atmospheric levels
(concentrations or loadings) of nitrogen and sulfur. A detailed description of critical loads and its
use as an analysis tool are presented 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 effects 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 compute the difference in exposure relative to the response-based
harmful effect level. We recognize that the loading and exposure may be less than or greater than
the findings of the exposure response research. Based on our findings, we plan to  determine if a
particular geographic region requires additional analysis or if we are 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 public welfare involves
determining what level of ecosystem response translates 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:

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                             Secondary 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 measures of ecosystem services, which can then
       be quantified
    •   Direct economic valuation of ecosystem functions and services, including use and nonuse
       values (values that do not require an individual's direct use of an ecosystem—for
       example, the value of preserving an endangered 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 adversity will depend on the availability of data
and methods for the indicators of interest related to acidification and nutrient enrichment and on
an assessment of the appropriateness of each type of quantification 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 changes in ecosystem indicators and ecosystem
services. Linking these changes in ecosystem services 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 combine and/or compare changes across ecosystem
services.
       The EPA SAB Committee on Valuing the Protection of Ecological Systems and Services
(SAB CVPESS) has recently drafted 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 comprehensive approach to assessing, valuing, and
reporting on the ecological benefits of its actions"  (SAB CVPESS, 2007).  The conceptual model
described in Figure 2-5 that links atmospheric concentrations and deposition to ecosystem
effects and biologically relevant indicators, and tying these to ecosystem services and valuation
is supported by the SAB panel. One of the critical gaps identified by the SAB panel is
"identifying how the biophysical effects of an action on an ecosystem will in turn impact the
ecosystem services of importance to the public" (SAB CVPESS, 2007).

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       In broad terms, the most important information gap in this review is an incomplete
understanding of how hydrological and biogeochemical processes (e.g., cycling of H2O, N, C, P)
interact to control the response of ecosystem services to NOX and SOX and other forcing variables
(e.g., climate). A major consequence of these interactions is that ecosystem services tend to be
linked or "bundled,"  so that actions taken to improve one may result in an improvement or
deterioration of other services. Thus, when ecosystem services are quantified and their ecological
response functions to NOX and SOX are modeled, it is imperative that the entire bundle of
services be evaluated, and that the linkages and tradeoffs among ecosystem services be included
in the quantification (i.e., ecological tradeoff functions [ETFs]). The key feature  distinguishing
the services from the underlying ecosystem function or processes is the explicit involvement of
beneficiaries, so consideration of which ecosystem services to target in assessment activities
involves  consideration of the relative demand for the service, its spatial distribution, and its
magnitude.
       Ecological Response Functions (ERFs) and Ecological Tradeoff Functions (ETFs) can be
developed to quantify the response of a service to changes in NOX/SOX concentrations and the
tradeoffs between different services given these ambient air quality drivers. Many of the services
of importance in assessing risk of NOX and SOX have not been quantified (i.e., developed ERFs
and ETFs) or adequately scaled for a comprehensive evaluation of the benefits of these services.
Such research is being planned, and will, no doubt, contribute to future NOX/SOX assessments.
Therefore, at this time, data mining will be central to developing at least a preliminary
assessment of potential impacts of NOX/SOX deposition and acidity on ecosystem services. In the
current plan, process-based models are being considered to be used to (1) synthesize/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 NOX and SOX. The collection
of response and tradeoff functions will aid in the valuation of the services 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 benefit from spatial and temporal mapping. In
preparation for the NOX/SOX risk/exposure assessment, a suggested series of mapping exercises
would proceed in the following order:
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       1.  Identify areas/regions of the country receiving high levels of NOX/SOX deposition and
          acidity impacts.
       2.  In those regions, identify ecosystems sensitive to elevated levels of nitrogen and
          sulfur, using some common selection criteria.
       3.  In those sensitive ecosystems, ask what ecosystem services are expected to be
          prevalent and "valued" (i.e., some subset of all potential services).
       4.  In those areas, identify what data are available to develop ERFs and ETFs, at least to
          a qualitative degree that would enable production of spatial and temporal maps to
          identify different degrees of protection that 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 provide 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 impacts, and it is a potentially important component
in the risk characterization phase of the risk assessment. As a result, an additional step in the
above process is to identify  potentially relevant economic valuation studies for ecosystem
services and map the location of these studies relative to the available data on ecosystem
services. Note that while we are considering 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 analysis approach would require time
and resources, which are not available 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
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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 more ecological resilience. Assessing alternative
levels of protection with different 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 based on ambient concentrations of NOX and SOX in
the atmosphere, and, therefore, additional analyses are required to move from deposition-based
risk estimates to policy-relevant ambient indicators. The policy assessment will build upon the
current conditions risk/exposure assessment to develop these policy-relevant ambient indicators
(Figure 3-5). This requires synthesizing the ecosystem responses, biological indicators, and
ecosystem effects related to deposition loadings and translating those loadings back to  their
corresponding ambient air conditions.
                               ISK/EXPOSURE ASSESSMENT
Characterization of Exposure
Atmospheric
Concentrations
ofNOxand
SOx
	 	 	 1 	

Deposition of
Sulfur and
Reactive
Nitrogen


^-^

Characterization of Ecological Effects
Ecosystem
Effects

Relevant
Biological
Exposure
ndicators

Ecosystem
Responses,
Services and
Valuation


1
                          Climate Change
                          (N20)
Acidification
Nutrient Enrichment
    species alterations
    eutrophication
    mercury methylation
                                                 ASSESSMENT
       Figure 3-5. Risk/policy assessment paradigm for deposition-related ecological risks.
                                           56
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                             Secondary 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 define 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 sensitive to those effects, and the magnitude of
the risks existing at current levels. After this baseline is established, modeling techniques can be
used to change the ambient level and examine how the corresponding exposures and risks
change. In this way, dose-response curves can be generated that establish the range of impacts
and effects associated with nitrogen and sulfur inputs. For example, suppose at a particular level
of loading (X), a negative effect is observed (Y). This analysis can be used to establish what X is
equivalent to in terms of ambient concentrations, 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) has an impact on the risk associated with that
form.
       Some of the issues associated with this type of analysis 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 nitrogen, and what is the ambient contribution
       compared to other sources?
   •   Do these effects occur due to different ambient 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 another method more appropriate)?
   •   What are the correct temporal and spatial scales 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 form 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
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current conditions analysis. Our initial thoughts on evaluating the appropriate level and form of
the standard using the assessment area analyses approach include the following:
       1.  Identify the relative contribution 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 enrichment, or eutrophication).
       3.  Identify the contribution to atmospheric loadings from total reactive nitrogen, NOX,
          and SOX.
       4.  Identify the biogeochemical indicators/resources of concern in the assessment area
          and the ecosystem services associated with those indicators.
          a.  Determine the ecosystem service effects associated with the most-critical impacts.
          b.  Bundle ecosystem services to find common metrics for comparison across
              locations.
       5.  Define the exposure-response (loading-response) functions (ERFs) for the ecological
          indicators of concern.
       6.  Estimate the loadings/exposures associated with current and alternative levels of the
          NOX and SOX standards  (using CMAQ modeling).
          a.  Analyze the relationships between NOX, SOX, 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 ecosystem risks to common units using
          a.  economic valuation  based on benefits transfer from existing literature estimates
          b.  biogeochemical equivalents using  ecological tradeoff functions (ETFs).
       9.  Combine individual risk estimates to produce overall impact estimates.
Results from individual assessments can be evaluated relative to the national maps of ecosystem
types and sensitivities. This analysis may characterize how well the assessment areas represent

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overall national ecosystem types 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 policy assessment, so that it can examine a range
of alternative standards applicable for the agency to consider. The following are questions to
address in identifying specific 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 levels of the standards should be considered?
   •   Should there be alternative levels of the standards (i.e., individual NOX and SOX standards
       or a combined NOX/SOX standard)?
   •   Do the ambient air indicator forms allow for site-specific protection while maintaining
       national consistency?
   •   Does the ambient air indicator adequately account for the effects 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. Davis. 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, B.C. Underwood, and G.C. Daily. 2006.
       Conservation planning for ecosystem services. PLoSBiology 4(11):e379.
Heinz Center. 2006. Filling the Gaps, Priority Data Needs 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 Heinz Center for Science, Economics and the
       Environment, Washington, DC.
Howarth, R.W. 2007. Chapter 6: Atmospheric deposition and nitrogen pollution in coastal
       marine ecosystems. In Acid in the Environment: Lessons 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 Ecological 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. Rzychon, J. Vesely, J. Wieting, A. Wilander,
       and A. Worsztynowicz. 2005. Regional scale evidence for improvements in surface water
       chemistry 1990-2001. Environmental Pollution 737(1): 165-176.
U.S. EPA (Environmental  Protection Agency). 2007a. 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). 2007b. Preliminary Draft Excerpts of the
       Integrated Science  Assessment for the NOX and SOX Secondary Draft document.
       Presented at the PM Secondary NAAQS Kickoff Meeting and NOX/SOX 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 Development, 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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U.S. EPA (Environmental Protection Agency). 2006. Wadeable Streams Assessment. Office of
      Research and Development, Washington, 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 Costs 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 Effects 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, Washington, 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 Technical Information.
      OAQPS Staff Paper. EPA-452/R-95-005. September.
U.S. EPA (Environmental Protection Agency). 1994. Review of the National Ambient Air Quality
      Standards for Sulfur Oxides: Assessment of Scientific and Technical Information.
      Supplement to the 1986 OAQPS Staff Paper Addendum. EPA452/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 Technical Information.
      OAQPS Staff Paper. EPA-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.
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                                     APPENDIX A
                   IDENTIFICATION OF SENSITIVE ECOSYSTEMS
A.1    Targeted Ecosystem Effects
       One of the central issues in this secondary 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 retention rates; (2) potential nitrogen and sulfur
uptake rates, which might include vegetative uptake, potential denitrification, and potential
mobilization of nitrogen and sulfur; (3) potential residence time based on local hydrology
(precipitation rates, conductivity) and geology (bedrock type, pervious surfaces, soil properties);
and (4) total supply of nitrogen and sulfur, including current and historical atmospheric
deposition and other nonatmospheric sources (e.g., applied fertilizers, sewer leaks, point
sources). Other ecosystem-specific characteristics that may help assess sensitivities include
threatened and endangered species data where available, land cover, land-use type (including
Class I, National Park, and Fish and Wildlife Refuges and National Wilderness areas), species of
community shifts (or invasive species), and baseline nitrogen and sulfur 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 cluster analysis to group
ecosystem units into similar sets. By clustering ecosystems, we 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.
       These types of analyses may aid in determining  whether area-based risk/exposure
assessments are appropriate for looking at nitrogen and sulfur effects on various ecosystems and
geographic regions as a means of extrapolating these  impacts to characterize the entire country.
For those areas where data are available, watershed models (e.g., MAGIC, PnET-BGC,
DayCent-chem) may be useful for evaluating 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 NOX (and/or reactive nitrogen), SOX, and ecosystem response?

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    •   What ecosystem services are associated with changes in emissions?
       Ecosystem response to nitrogen and sulfur deposition varies across the landscape,
depending on the local physical, chemical, and biological characteristics. Atmospheric
deposition effects range from species alterations (due to nutrient enrichment) to anthropogenic
eutrophication of estuaries to acidification of forests, lakes, and streams. This Scope and
Methods Plan focuses on five ecosystem effects on terrestrial and aquatic  systems.
A.1.1  Terrestrial Nitrogen Enrichment
       Deposited nitrogen compounds can act as a fertilizer to increase the productivity of plants
and algae. However, too much nitrogen can lead to a surplus of nutrients,  resulting in over-
nutrient enrichment. Table A-l presents  examples of nitrogen enrichment effects, which can
impact species diversity by favoring nitrogen-tolerant species over other species that are more
sensitive to nitrogen limits. Certain ecosystems may be dominated by species that have a
competitive advantage in low nitrogen environments. When nitrogen increases, species that are
normally kept in check by low nitrogen levels flourish and out-compete the other species in the
community; thereby, potentially altering  species' 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 that depend on specific
plants for habitat and food may then be threatened by the changes to the plant communities that
result from nitrogen inputs.
                     Table A-1. Examples of Nitrogen Enrichment Effects
Nitrogen Load
(kgNha-1'yr-1)
>1.5
3 to 8
5 to 35
7
<8 to 1 0
10to15
Nitrogen Enrichment Effects
Altered algal communities in high-elevation freshwater lakes. (Colorado)
Elevated N in tree leaf tissue high-elevation forests. (Colorado)
Mortality of sensitive lichen species. (West Coast)
A transect of 68 acid grasslands; species richness declines as a linear function of
the rate of inorganic nitrogen deposition, with a reduction of one species per4-m2
quadrat for every 2.5kg N"1 yr"1. (United Kingdom)
Community-level shifts in ombrotrophic bogs. (North Central and Eastern United
States)
N retained or denitrified (i.e., limited leaching), (many U.S. forests)
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/shrub lands is also implicated in increased
fire frequency in some areas because the nitrogen over-enrichment has favored the growth and
production of fire-prone grass species. Emerging research has also linked increased nitrogen
deposition to habitat alteration for threatened species.
       Nitrogen enrichment, in combination with ozone exposure, causes major changes in tree
health by reducing fine-root biomass and carbon allocation below ground and by greatly
decreasing the lifespan of pine foliage. Nitrogen enrichment 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 forest 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 field 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 significant source of total nitrogen to many
estuaries in the United States. National Oceanic and Atmospheric Association (NOAA) has
calculated the amount of nitrogen entering estuaries ultimately 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 atmospheric deposition in  some places. In addition, episodic
inputs, which may be ecologically significant, may be higher than the annual  average. Studies
have shown that atmospherically deposited nitrogen (AD-N) contributes 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 productivity of coastal ecosystems. However,
nitrogen over-enrichment can alter a series of complex biogeochemical cycles that affect
community processes (e.g., competition, community structure) and ecosystem processes (e.g.,
ecosystem efficiency, decomposition). Approximately 60% of estuaries in the United States

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(65% of the estuarine surface area) suffer from over-enrichment of nitrogen, resulting in
excessive algal growth, the outcome of which is eutrophication—the depletion of oxygen
concentrations as the algae die and decompose. Symptoms of eutrophication include changes in
the dominant species of phytoplankton (the primary 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 Pfiesteriapiscicida, and cascading population changes up the food chain.
In addition, encrustation and increased levels of turbidity in the water due to large amounts of
algae can kill off submerged aquatic vegetation, which is an important habitat for many estuarine
fish and shellfish species.
       Often the most severe result of eutrophication is the depletion of dissolved oxygen (DO)
in the water column by the decomposition of organic 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 particular concern in coastal estuaries that exhibit
density stratification (the division of water into layers with different temperatures and oxygen
content), which occurs mostly during the summer months.  Organic matter produced in lighted
surface waters either sinks to the bottom waters where it decomposes, consuming oxygen
inventories that are not replenished by  photosynthesis, or it mixes with oxygen-rich surface
waters. These low-oxygen zones may cause massive deaths of aquatic life and reduce the
population densities of many important commercial fish and  shellfish. Hypoxic bottom waters
have expanded during the latter 20th century in many coastal ecosystems, 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 estuaries are along the gulf and mid-Atlantic coasts, overlapping
many of the areas with the highest nitrogen deposition, but there are eutrophic estuaries in every
region of the  conterminous U.S. coastline.
       Emerging ecological  studies in  the western  United States demonstrate that some aquatic
and terrestrial communities are significantly altered by increased nitrogen deposition. The major
concerns are for ecological systems that are naturally adapted to low-nitrogen inputs, because
increases related to anthropogenic atmospheric deposition can lead to nitrogen saturation. Where
the inputs of nitrogen exceed the ecological system's need for them, excess nitrogen is leached
into surrounding waterways. Although much of the western United States is exposed  to relatively
low deposition of nitrogen, hotspots of nitrogen deposition occur 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-elevation catchments in Colorado and chaparral
catchments in the southwestern Sierra Nevada Mountains. The primary concerns in the West
have been for the critical load of nitrogen deposition, which affects both terrestrial and aquatic
resources through eutrophication 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 eutrophication 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 period. In addition to high-
elevation lakes, snowpack nitrogen has also been reported to cause temporary acidification of
alpine streams. Snowmelt-related temporary acidification of alpine lakes and streams and
associated effects have been reported in numerous 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 some areas because the nitrogen over-enrichment has
favored the growth and production of fire-prone grass species. Emerging 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 nitrogen deposition tend to alter species' composition, with negative
impacts on biodiversity. Such plant community changes 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 primarily aquatic  environments with high
concentrations of organic matter leads to an increase in production of MeHg. The increased
availability of sulfur as sulfate in lakes and streams due to the deposition of SOX 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 tissue and increase in concentration up the
food chain. Sulfate-dependant bacteria, along with methanogenic (methane-producing)
microorganisms, are involved in the conversion of Hg2+ 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 historical 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-scale, fresh water wetland ecosystem (Jeremiason et
al., 2006). While an in-depth assessment of mercury methylation relative to NOX and SOX inputs
is not feasible at this time, reducing NOX and SOX 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 the effects of acidifying deposition on forest ecosystems has
focused increasingly on the biogeochemical processes that affect plant uptake, retention,  and
cycling  of nutrients within forested ecosystems. Research results from the 1990s indicate that
decreases in base cations  (e.g., calcium, magnesium, potassium) from soils are at least partially
attributable to acid deposition in the northeastern and southeastern 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 uptake). As mobile aluminum increases due to
soil acidification, the calcium/aluminum ratios change, which is partly related to the higher
affinity  of aluminum during passive uptake by roots. The change in these relative nutrient
proportions has been correlated with declining forest health. Recent 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 forested 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 chemistry 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 acid. This is referred to as an ecosystem's ANC.
       ANC levels depend largely on a watershed's physical characteristics: geology, soils, and
size. Water systems that are sensitive to acidification tend to be located in small watersheds that
have few alkaline minerals and shallow soils. Large, forested watersheds have been shown to
acidify during large rainfall and snowmelt episodes. 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 acidification make it difficult for
some fish and other aquatic species to survive, grow, and reproduce. In some waters, the number
offish 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-elevation lakes, particularly in the Rocky Mountains, have
become acidic, especially during snowmelt. However, while many western lakes and streams are
sensitive to acidification, they are not subject to continuously high levels of acid deposition, and,
therefore, have not become chronically acidified. During the 1980s and  1990s, an integrated
study of atmospheric deposition, terrestrial  ecosystems, and aquatic ecosystems was conducted
in several watersheds in the Sierra Nevada Mountains 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 waters in the Sierra Nevada Mountains was not
found, but episodic changes in stream water chemistry did occur. In many of the watersheds
studied, for example, the pH decreased as the waters 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 acidification and reducing alkalinity. The impacts of these changes are
more substantial in coastal waters where the ecosystems are more vulnerable due to other human
impacts, such as nutrient enrichment and pollution. 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 Acidification 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 enrichment and mercury methylation
   •   Sensitive areas for acidity/nutrient enrichment impacts.
       A concise summary of information on each topic is introduced in the following sections.
A.2.1  Nitrogen Saturation
       Terrestrial and aquatic ecosystems in the eastern and northeastern United  States receive
nitrogen deposition loadings related to air pollution that are far  in excess of natural background
levels.  When combined with other 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 groundwater and surface water. Studies 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 United 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 studies include those identified by Driscoll and
colleagues (2001); Fenn and colleagues (2003); and NAPAP (2005).
A.2.2  Maple Decline
       Maple decline is a generalized term for a set 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 northeastern United States in recent decades, with a
focus on native maples, such as the sugar maple. In rural areas, maple decline is often attributed
to soil acidification caused by acid rain. Soils that have developed from nutrient-poor parent
materials, such as sandstone, quartzite, and granite are most sensitive 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 replicate soil conditions that existed prior to the loss of
sugar maples over the past 25 years. This reproduced 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 nutrient after nitrogen. In addition, the loss of
calcium leads directly to acidic soils. When soils become too acidic, 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 freshwater mussels have fallen drastically to the
point where more than 70% of native unionid 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 provide 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 northeastern United States suggests a relationship
between sulfur deposition and processes promoting 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 rates of MeHg production in coastal sediments
       are in part controlled by temporal and spatial hypoxia patterns that result from coastal
       eutrophication, and that maximum MeHg production 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 biomagnification in red snapper and gray snapper,
       both commercially and recreationally important fish species in this region.
    •   Research Goal 3 - Test the hypothesis that anglers who proportionately consume fish
       from areas of higher MeHg production related to hypoxia will have higher rates of
       mercury exposure (as measured by the concentration of mercury in hair) than anglers
       consuming similar amounts offish from other coastal Louisiana locations where hypoxia
       does not occur.
       The EPA ORD has also done some preliminary 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 sources identified include
Chesney and colleagues (2006), Driscoll and colleagues (2007), and Evers and colleagues
(2007).

A.3   Sensitive Areas for Acidity/Nutrient Enrichment Impacts
       Research accomplished through the NAPAP (2005) has identified spatial areas in terms
of both terrestrial and aquatic ecosystems that show sensitivity to impacts from acid rain-related
air pollution (Figure A-l). The NAPAP has been able to develop 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 showing 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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            Sensitive ecological receptors in the United State?
                                                                         Sensitive
                                                                         Ecological Areas
                                                                         Acidic Surface
                                                                         Waters
                                                                         Class I Areas
                                                                       C3 Estuanne
                                                                         Drainage Areas
             Source EPA
                Figure A-1. Sensitive ecological receptors in the United States.

References

Chesney, E., R. Lincoln, M.S. Bank, D.B. Senn, DJ. Vorhees, P. Grandjean, andN.N. Rabalais.
       2006. Exploring Links Between Mercury, the Coastal 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, AJ. Bulger, TJ. Butler, C.S. Cronan, C. Eager, K.F. Lambert,
       G.E.  Likens, J.L. Stoddard, and K.C. Weathers. 2001. Acidic deposition in the
       northeastern United States: sources and inputs, ecosystem effects, and management
       strategies. BioScience 57:180-198.
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. Butler. 2007. Biological mercury hotspots in the
       northeastern United States and southeastern Canada. BioScience 57(l):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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Jeremiason, J.D., D.R. Engstrom, E.B. Swain, E.A. Natur, B.M. Johnson, I.E. Almendinger,
       B.A. Monson, and R.K. Kolka. 2006.  Sulfate addition increases methyl mercury
       production in an experimental wetland. Environ. Sci. Technol. ₯0(12)3800-3806. (DOT:
       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 57(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 Precipitation Assessment Program) 2005. National Acid Precipitation
       Assessment Program Report to Congress: An Integrated Assessment. National Acid
       Precipitation Assessment Program. National Oceanic 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. Spatiotemporal variability of wet atmospheric nitrogen
       deposition to the Neuse River Estuary, North Carolina. Journal of Environmental Quality
       30:1508-1515.
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                                     APPENDIX B
                      OTHER INDICATORS - NITROUS OXIDE
       NAAQS' purpose to protect welfare entails not only ecological welfare, but also other
forms of welfare that are potentially impacted, such as materials, climate change, and visibility.
N2O is a naturally occurring greenhouse gas; however, human activities have increased
atmospheric concentrations by 18% since the preindustrial era (IPCC, 2001). The global
warming potential of CC>2 is 1, while the global warming potential of N2O is 310, indicating that
a molecule of N2O is 310 times more effective in trapping heat in the atmosphere than CC>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 CC>2
equivalents)
       Elevated nitrogen loading to ecosystems can significantly enhance the production of N2O.
For example, the nitrogen nutrient loading in water bodies can enhance N2O emissions from the
bacterial breakdown of nitrogen from these  sources. In another example, numerous studies have
shown thatN2O emissions from soils increase upon artificial 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 N2O
emissions  (Butterbach-Bahl et al., 1998, 2002). Nitrous oxide emissions from soils are also
influenced by precipitation and temperature. The Photosynthesis-Evapotranspiration-Model-
Nitrogen-Denitrification-Decomposition (PnET-N-DNDC) model is designed to simulate and
predict soil carbon and nitrogen biogeochemistry in temperate forest ecosystems and to simulate
the emissions of N2O 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 across Europe and 1 site in the United States (Kesik
et al., 2005). Denitrification is described in the model as a series of sequential reductions driven
by a microorganism using nitrogen oxides as electron acceptors under anaerobic conditions.  As
intermediates of the processes, NO and N2O are tightly controlled by the kinetics of each step in
the sequential reactions. Interactions between temperature, precipitation, and forest soil NO  and


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N2O emissions in Europe were investigated using PnET-N-DNDC (Kesik et al., 2006) abiotic

parameters that are included in the model and are summarized in Table B-l.

            Table B-1. Parameters Included in the PnET-N-DNDC Model as Modeled
                          for European Forest (Kesik et al., 2006)
Forest Properties
Forest type
Age
Above and below
ground biomass
Plant physiology
parameters





Soil Properties
Texture
Clay content
PH
Soil organic carbon
content
Stone content
Humus type




Daily Climate Input
Parameters
Precipitation
Min and max air temps
Inorganic [N] in Precip








Tree Species/Genera
Pine
Spruce
Hemlock
Fir
Oak
Birch
Beech
Slash Pine
Larch
Cypress
Evergreen Oak
References

Brumme, R., and F. Beese. 1992. Effects of liming and nitrogen fertilization on emissions of
       CC>2 and N2O from a temperate 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 lowlands 1. Fluxes of N2O, NO/NO2 and CFLt 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 CH4-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: Cambridge University Press.
Kesik, M., P. Ambus, R. Baritz, N. Briiggemann, K. Butterbach-Bahl, M. Damm, J. Duyzer, L.
       Horvath, R. Kiese, B. Kitzler, A. Leip, C. Li, M. Pihlatie, K. Pilegaard, S. Seufert, D.
       Simpson, U. Skiba, G. Smiatek, T.  Vesala, and S. Zechmeister-Boltenstern. 2005.
       Inventories ofN2O 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. Butterbach-Bahl. 2006. Effect of pH, temperature
       and substrate on N2O, NO, and CC>2 production by Alcaligenesfaecalisp. 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 farmed organic soils: a review. Soil Use of
       Management 73:245-250.
Li, C. 2000. Modeling trace gas emissions from agricultural ecosystems. Nutr. Cycl. Agroecosys.
       55:259-276.
Li, C., J. Aber, F. Stange, K. Butterbach-Bahl, and H. Papen. 2000. A process-oriented model of
       N2O and NO emissions from forest soils: 1. Model development. Journal of. Geophysical
       Research 705:4369-4384.
Li, C., S. Frolking,  and T.A. Frolking. 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. GlobalBiogeochemical Cycles 70:297-306.
Matson, P.A., S.T. Gower,  C. Volkmann, C. Billow, and C.C. Grier.  1992. Soil nitrogen cycling
       and nitrous oxide fluxes in fertilized Rocky Mountain Douglas-fir forests.
       Biogeochemistry  75:101-117.
Papen, H., M. Daum, R.  Steinkamp, and K. Butterbach-Bahl. 2001. N2O and CH4-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 N2O and NO
       emissions from forest soils, 2. Sensitivity analysis and validation. Journal of Geophysical
       Research 705(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 Ecological 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 accounting for uncertainty.
This section presents our current thoughts for using existing ambient 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 NOX and SOX to
deposition species are similar. (NOTE: In addition, NOX and SOX interact photochemically in the
PM formation. Further,  since  NOX participates in both the ozone and PM photochemistry, those
pollutants are linked and need to be simulated together 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-art 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-l, 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 domain contains 14 vertical layers, with the top
of the modeling domain at about 16,200 meters.
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         12km Western Domain (WRAP)
         origin: -2412000. -972000
         col 213 row 192

                                                                      12km Eastern Domain
         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 and boundary conditions. The CMAQ meteorological
input files were derived for the entire year of 2002 from a simulation of the Pennsylvania State
University/National Center for Atmospheric Research Mesoscale Model (Grell et al., 1994). This
model, commonly referred to as MM5, is a limited-area, nonhydrostatic, terrain-following
system that solves for the full set of physical and thermodynamic equations, which govern
atmospheric motions. Anthropogenic and biogenic emissions from the 2002 CMAQ modeling
platform were obtained from the 2002 National Emissions Inventory (NEI).  These data were then
processed using the Sparse Matrix Operator Kernel Emissions (SMOKE) processing system.
       In addition to understanding the relationship between ambient concentrations and
deposition, we also need to understand how the existing patterns of deposition modeled using
CMAQ may vary with variability in meteorology (e.g., temperature, precipitation). By making
use of the 5 years of CMAQ  simulations conducted 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 analyses designed to compare the magnitude and
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spatial patterns of deposition for sulfur and nitrogen containing species predicted by CMAQ
across the 5 years modeled. The analysis intends to highlight the similarities and differences in
the distribution of monthly, seasonal, and annual wet, dry, and total deposition for grid cells
covering selected watersheds, forested ecosystems, and croplands. Specifically, for each
geographic area we plan to calculate (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 individual years compares to the 5-year average. We
may also examine the spatial variation in deposition 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 and range of values for the different areas. The
feasibility of using statistical techniques to  quantify  year-to-year variability in the spatial patterns
of predicted deposition within a geographic 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 NOX and SOX standards, the risk assessment will need to characterize both
the effects associated with the deposition of nitrogen and sulfur and the relationship between
ambient concentrations at a specific geographic 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 NOX and SOX and estimates of nitrogen 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 Quality System (AQS). There are also databases of
deposition monitoring data, including the NADP, the CASTNet, and the Park Research and
Intensive Monitoring of Ecosystems NET work (PRIMENet).
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                Figure C-2. Development of ambient NOX and SOX and nitrogen
                          and sulfur deposition characterizations.
       We plan to assess the feasibility of combining data from modeling and monitoring to
produce ambient air quality and deposition surfaces that are grounded in observational data but
make use of the models' ability to capture the impacts of emissions and meteorology on the
geographic gradients of air quality and deposition. To assess the feasibility of combining data,
we intend to implement one or more methods on data associated with a region 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 alternative standards.

References
Grell, G., J. Dudhia, and D. Stauffer.  1994. A Description of the Fifth-Generation Penn
       State/NCARMesoscale Model (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., models for terrestrial systems or aquatic systems)
to determine the environmental effects of atmospheric deposition of NOX and SOX. 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 multimedia models, there are numerous technical
formulations, parameterizations, and geographic considerations to consider in choosing the
correct model for the task at hand. This decision depends on desired outcomes and simulated
parameters. Outcomes may range from quantified, speciated sulfur or nitrogen 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-l  represent a wide diversity of types of ecosystems;
history, location,  and spatial/temporal scale of application; scientific acceptance and
organizational and agency support; complexity and requirements; state variables 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/watershed models,  although these distinctions are
somewhat arbitrary. In recognition of some models having enough common features or history of
integration to warrant merging of the discussion, the table also presented several models
together; however, although presented collectively, these models may not share the same history,
theory, development, and application.
                                         D-1                       DRAFT - March 2008

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
                  Table D-1. Information Detailed for Each Model of Interest
Model Information
Name
Description
Supporting organization(s)/agency endorsements
Scientific acceptance
Type of ecosystems modeled
Where has it been applied?
Spatial scale
Transferability
Temporal features
Model inputs related to deposition
Model type
Indicators
Resource requirements
Data requirements
Model performance/evaluation
Model technological integration
Key environmental/ecological processes
Management use
Citations/URLs
Notes (miscellaneous)
       Table D-2 presents a more concise overview of the models identified; information is
presented that summarizes both the type of ecosystem that the model has been designed to
represent and the relative level of complexity 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. Complexity 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 degree of agency support. Further discernment
regarding "quality" or applicability of the different models to support elements of the secondary
NC>2 and SC>2 NAAQS should carefully consider a variety of contextual drivers when using an
integrated modeling approach.
                                         D-2
DRAFT - March 2008

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                              Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
            Table D-2. Supported Model Components And Level Of Representation



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                                           D-3
DRAFT - March 2008

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

D.2    Multimedia Model Selection
       There are four basic steps necessary 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 area, data availability,  and necessary
          uncertainty/sensitivity analyses for the models.
       3.  Determine and set up any processes/algorithms necessary to match atmospheric
          modeling output (assumed to be from CMAQ) to the chosen receiving water or
          terrestrial/watershed model.
       4.  Obtain the data needed for model parameterization.
       The model details presented in the summary 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 utilize 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, spatial 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 inputs of the ecosystem model.
       Most applications for the determination of acidification effects 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 are 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

                                         EMDRAFT - March 2008

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

alpine regions of the United States. The remaining 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 that 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 country, 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 processes involving nitrogen
    •   Mass balance - may be applied anywhere data are available
    •   MERLIN - no applications uncovered for U. S. sites but has been used 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 WARMF to accept atmospheric deposition fluxes
       to make it more compatible with CMAQ).


                                         I>5DRAFT - March 2008

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur


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 Agency, Research Triangle Park, NC.
                                         D-6                      DRAFT - March 2008

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                             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 quantification of ecosystem services. The adverse
effect would be the loss or reduction of those services through the effects of NOX and SOX on the
underlying ecological processes and functions that constitute the service. EPA defines ecosystem
services as the outputs of healthy, intact ecosystems and the underlying ecosystem processes and
functions that contribute to human well-being (U.S. EPA, 2006). As articulated by the
Millennium Ecosystem Assessment (United Nations, 2005) from the United Nations, these
include provisioning services (e.g., clean water, food, wood, fiber, fuel), regulating services (e.g.,
water purification, climate regulation),  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, assessing 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 potential indicators of ecosystem services include the quality of a
critical habitat, biodiversity, species composition, controlling/limiting invasive species, and pest
outbreaks. Determining the exposure-response  relationships of NOX and SOX on the ecosystem
service and the underlying ecological process and 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 Secondary 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 Assessment Strategic Plan (EBASP) (U.S. EPA, 2006): "Economists
generally attempt to estimate the value  of ecological goods and services based on what people
are willing to pay (WTP) to increase 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 express the value of  ecological goods and services. The dollar is the

                                         E^l                       DRAFT - March 2008

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

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. Three 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 further explaining these types of economic
monetary valuation methods.
       For market-based valuation, for many regions of the country, there are diverse crop
assemblages, including timber for building materials, timber for pulp and paper, biofuel  crops,
grain and soybean feed crops, grass seed, orchard 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 economists 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  approach will first require mapping biophysical
measurements of services as discussed in Section 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/english/default.htm] and other sources of
information).
       Biophysical Valuation. Because monetization of many ecosystems 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 modeling data is what has been called relative value
indicators or relative  benefit indicators (Wainger et al., 2001; Boyd and Wainger 2003).  In this
approach,  a set of indicators is defined that reflect site and landscape 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 services, and (4) the reliability of the flows  of
ecosystem services. These indicators are then aggregated into a combined site or place-based
index using standardized scoring (Wainger et al., 2001).  This is an explicit approach to a
                                          E-2                       DRAFT - March 2008

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur

"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 economic method that develops "productivity
indexes" and now calls these "environmental performance indexes" (Fare et al., 2004). This
approach attempts to combine positive and negative outcomes from a system (e.g., factory,
region, county) into a single index of performance. To the degree that this approach is science-
based, it can be developed with available data and offers potential in a qualitative risk
assessment.
       A second approach to biophysical valuation 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, 2006) and other reviews of valuation.  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 elaborating on these approaches.
       By describing the differing bundles of ecosystem services under varying levels of NOX
and SOX; and offering choices between those bundles, tradeoff or indifference curves  can also be
generated reflecting individual- and population-level preferences for different ecosystem
functions. These tradeoffs can be presented to survey respondents using methods such as
conjoint analysis to provide measures of preferences for different levels of ecosystem functions
as expressed through ecosystem service levels. Table E-l shows an example of how  ecosystem
services information might be presented to a survey respondent in a conjoint analysis framework.
       The results of a conjoint analysis can be used to convert all ecosystem services to a
common unit based on preference weightings for different types of services. This may allow
ecosystem services to be combined for comparing impacts across alternative forms or levels of
NOX and SOX standards.
                                          E-3                       DRAFT - March 2008

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                            Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur
         Table E-1. Example Survey Design for Preference-Based Ecosystem Tradeoffs
Example Conjoint Scenario Rating Form

Ecosystem Services Scenario Descriptions

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


Scenario Rating
Please rate how desirable each scenario is overall by circling one of the num bers in each row of the following table:

Scenario 1
Scenario 2
Scenario 3
Scenario 4
Scenario 5
Highly
Desirable
1
1
1
1
1
Quite Desirable
2
2
2
2
2
Desirable
3
3
3
3
3
Slightly
Desirable
4
4
4
4
4
Neither
Desirable
nor
Undesirable
5
5
5
5
5
Slightly
Undesirable
6
6
6
6
6
Undesirable
7
7
7
7
7
Quite
Undesirable
8
8
8
8
8
Highly
Undesirable
9
9
9
9
9


References

Boyd, J., and L. Wainger. 2003. Measuring Ecosystem Service Benefits: The Use of Landscape
      Analysis to Evaluate Environmental Trades and Compensation Resources for the Future.
      Washington, D.C. Discussion Paper 02-63.
Fare, R., S. Grosskopf, and F. Hernadez-Sancho. 2004. Environmental performance: an index
      number approach. Resources and Energy Economics, 26: 343-352.
Hau, J.L., and B.R. Bakshi. 2004a. Expanding exergy analysis to account for ecosystem products
      and services. Environmental Science and Technology 35:3768-3777.
Hau, J.L. and B.R. Bakshi. 2004b. Promise and problems of emergy analysis. Ecological
      Modeling 17'5:215-225.
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 ecosystem 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 implications 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 application to 1992 U.S. economy. Environmental Science and
      Technology 35:4810-4827.
United Nations. 2005. Millennium Ecosystem Assessment Reports. Available at
      http://www.millenniumassessment.org/en/index.aspx.
                                         E-4
DRAFT - March 2008

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                             Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur


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.
                                         E-5                       DRAFT - March 2008

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United States                             Office of Air Quality Planning and Standards              Publication No. EPA-452/D-08-002
Environmental Protection                   Health and Environmental Impacts Division                                   March 2008
Agency                                         Research Triangle Park, NC

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