Integrated Review Plan for the 

Secondary National Ambient Air 

Quality Standards for Nitrogen 

Dioxide and Sulfur Dioxide
[This page intentionally left blank.] 
 EPA-452/R-08-006 

December 2007 

Integrated Review Plan for the Secondary National Ambient Air 

Quality Standards for Nitrogen Dioxide and Sulfur Dioxide 

U. S. Environmental Protection Agency 

National Center for Environment Assessment 

Office of Research and Development 

and 

Office of Air Quality Planning and Standards 

Office of Air and Radiation 

Research Triangle Park, North Carolina 27711
December 2007 

DISCLAIMER

This integrated review plan serves as a management tool for the U.S. Environmental Protection 

Agency's National Center for Environmental Assessment and the Office of Air Quality Planning 

and Standards. The approach described in this plan may be modified to reflect information 

developed during this review and to address advice and comments received from the Clean Air 

Scientific Advisory Committee and the public throughout this review. Mention of trade names or 

commercial products does not constitute endorsement or recommendation for use. 
December 2007 

TABLE OF CONTENTS 

KEY TERMS..........................................................................................................................i 

1. INTRODUCTION .......................................................................................................................................... 1-1

1.1 LEGISLATIVE REQUIREMENTS....................................................................................................... 1-2

1.2 REGULATORY HISTORY OF THE SECONDARY NAAQS FOR NO2 AND SO2................................... 1-3

1.2.1 NO2 NAAQS ............................................................................................................................................ 1-3

1.2.2 SO2 NAAQS............................................................................................................................................. 1-4

1.3 OVERVIEW OF THE NAAQS REVIEW PROCESS............................................................................ 1-8

1.4 SCIENCE PRIMER: NOX/SOX EFFECTS ON ECOSYSTEMS.......................................................................... 1-10

1.4.1 Acidification Effects on Freshwater and Terrestrial Ecosystems from Sulfur and Nitrogen Deposition

 ...................................................................................................................................1-12

1.4.2 Excess Nitrogen in Terrestrial and Estuarine Ecosystems .............................................................. 1-13

1.4.3 Reduced Nitrogen ............................................................................................................................ 1-13

2. REVIEW SCHEDULE .................................................................................................................................. 2-1

3. KEY POLICY-RELEVANT ISSUES .......................................................................................................... 3-1

4. SCIENCE ASSESSMENT............................................................................................................................. 4-1

4.1 SCOPE AND ORGANIZATION ........................................................................................................... 4-1

4.2 ASSESSMENT APPROACH......................................................................................................................... 4-2

4.2.1 Literature Search ............................................................................................................................... 4-2

4.2.2 Criteria for Study Selection ............................................................................................................... 4-3

4.2.3 Content and Organization of the ISA................................................................................................. 4-4

4.3 Public and Scientific Review.............................................................................................................. 4-6

4.3.1 Informal Peer Consultation Workshop of the ISA Annexes ............................................................... 4-6

4.3.2 Public Review of External Review Drafts.......................................................................................... 4-6

4.3.3 Review by Clean Air Scientific Advisory Committee ......................................................................... 4-6

5. RISK/EXPOSURE ASSESSMENT.............................................................................................................. 5-1

5.1 ASSESSMENT APPROACH.......................................................................................................................... 5-2

5.2 TOOLS FOR RISK ASSESSMENT ................................................................................................................ 5-4

5.3 KNOWLEDGE GAPS.................................................................................................................................. 5-4

5.3 PUBLIC AND SCIENTIFIC REVIEW.................................................................................................. 5-6

6. POLICY ASSESSMENT............................................................................................................................... 6-1

7. REFERENCES............................................................................................................................................... 7-1

APPENDIX A: CASAC PANEL ........................................................................................................................... A-1

APPENDIX B: ASSESSMENT TOOLS................................................................................................................B-1

B.1 MODELS ..................................................................................................................................................B-1

B.1.1 Community Multi-Scale Air Quality (CMAQ) Model .............................................................................B-1

B.1.2 Response Surface Models (RSM)............................................................................................................B-4

B.1.3 Air Quest.................................................................................................................................................B-5

B.1.4 Total Risk Integrated Methodology (TRIM)............................................................................................B-5

B.1.5 Regional Vulnerability Assessment (ReVA) ............................................................................................B-6

B.2 CASE STUDY MODELING.................................................................................................................................B-7

B.3 ECOSYSTEM SERVICES: IMPACT AND VALUATION ..........................................................................................B-9

B.3.1 Ecosystem services overview and why their consideration is needed................................................B-9

B.3.2 Ecosystem services relating to NOx/SOx secondary standard issues ..............................................B-10

B.3.3 Valuation .........................................................................................................................................B-13
December 2007 i 

 KEY TERMS 

Acidification: The process of increasing the acidity of a system (lake, stream, forest soil). 

Atmospheric deposition of acidic or acidifying compounds can acidify lakes, 

streams and forest soils. 

Air Quality Indicator: The substance or set of substances (e.g., PM2.5, NO2, SO2) occurring 

in the ambient air for which a level is set for a NAAQS standard and monitoring 

occurs. 

ANC (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. 

Biologically Relevant Indicator: A measure that sufficiently characterizes the exposureresponse relationship between a biological receptor and a pollutant stressor so as 

to be useful as a predictor of expected receptor response over a range of pollutant 

exposure levels.

 

Dry Deposition: The removal of gases and particles from the atmosphere to surfaces in the 

absence of precipitation (rain, snow) or occult deposition. 

Ecosystem: A dynamic complex of interacting plants, animals, and microorganisms and the 

non-living environment. 

Ecosystem Benefit: The value, expressed either 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. 

Some examples of ecosystem benefits that derive from improved air quality 

include improvements in habitat for sport fish species, drinking water quality, 

visual quality of scenic views, and quality of recreational areas. 

Ecosystem Services: Conditions and processes through which natural ecosystems, and the 

species that are part of them, currently help sustain and fulfill human life or have 

the potential to do so in the future. Examples include, clean air, clean water, food 

and fiber production, flood protection, water purification, pollination, and pest 

control. Improvements in these services may be valued as an ecosystem benefit. 

Eutrophication: The process by which a body of water becomes enriched in dissolved 

nutrients that stimulate the growth of aquatic plant life usually resulting in the 

depletion of dissolved oxygen 

Occult Deposition: The removal of gases and particles from the atmosphere to surfaces by 

fog or mist. 
December 2007 ii 

Welfare Effects: 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, whether caused by transformation, conversion, 

or combination with other air pollutants (CAA 302(h)) 

Wet Deposition: The removal of gases and particles from the atmosphere to surfaces by rain 

or other precipitation 

Valuation: The economic or non-economic 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. 
December 2007 1-1 

1. INTRODUCTION 

The U.S. Environmental Protection Agency (EPA) is conducting a review of the existing 

primary (health-based) and secondary (welfare-based) National Ambient Air Quality Standards 

(NAAQS) for nitrogen dioxide (NO2) and sulfur dioxide (SO2). The reviews of the primary 

NAAQS for NO2 and for SO2 are addressed in separate plans released during the winter of 

2006/2007. The purpose of this document is to communicate the integrated plan for the joint 

review of the secondary NAAQS for these pollutants. 

In this document, the terms NO2 and NOx and SO2 and SOx are not interchangeable. The 

terms NOx (oxides of nitrogen) and SOx (sulfur oxides) 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. In this review, NOx refers 

to all oxides of nitrogen (conventionally referred to as NOy in the scientific community), not 

simply the sum of NO and NO2 (conventionally referred to as NOx in the scientific community). 

The oxides of nitrogen compounds in the ambient air are nitric oxide (NO), nitrogen dioxide 

(NO2), nitrous oxide (N2O), nitrogen trioxide (NO3), dinitrogen trioxide (N2O3), dinitrogen 

tetroxide (N2O4), dinitrogen pentoxide (N2O5); other ambient oxides of nitrogen include nitric 

acid (HNO3), peroxyacetylnitrate (PAN), and other organic compounds such as nitrites, nitrates, 

nitrogen acids, and N-nitroso compounds (EPA, 1993). 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 is being met in a given location. 

The ecological importance of both oxidized and reduced forms of nitrogen has been widely 

recognized by the scientific community. The science assessment and the risk/exposure 

assessment will also evaluate total reactive nitrogen (which includes both oxidized and reduced 

forms), and its impacts on public welfare. Additionally, we will attempt to evaluate, and quantify 

where possible, the separate contributions of ambient versus other sources of total reactive 

nitrogen in various ecosystems nationwide, as well as the contributions of NOx to ambient 

reactive nitrogen relative to other contributors to ambient reactive nitrogen. 

Section 1.1 below includes additional explanation on the use of these terms within the 

context of the various phases of the new NAAQS review process. This review will evaluate new 
December 2007 1-2 

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, and consider the 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 review plan is organized into six chapters. Chapter 1 presents background 

information on the recently-revised NAAQS review process; on the nature of the NOx/SOx 

problem; on the legislative requirements for the review of the NAAQS; and on the regulatory 

history of past reviews of the NAAQS for NO2 and SO2. Chapter 2 presents the proposed review 

schedule. Chapter 3 presents a set of key policy-relevant questions that will serve to focus the 

NAAQS review process on the critical scientific and policy issues. Chapters 4 through 6 discuss 

the science, risk/exposure, and policy assessment portions of the review. 

We consulted with the Clean Air Scientific Advisory Committee (CASAC, panel 

members are listed in Appendix A), an independent scientific advisory committee established 

under the Clean Air Act, on an earlier draft of this document. 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 the CASAC and from the public throughout this 

review. 

1.1 LEGISLATIVE REQUIREMENTS 

Two sections of the Clean Air Act (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 their “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 
December 2007 1-3 

protect the public welfare from any known or anticipated adverse effects associated with the 

presence of [the] pollutant in the ambient air.”1

In setting standards that are “requisite” to protect public health and welfare, as provided in 

section 109(b), EPA’s task is to establish standards that are neither more nor less stringent than 

necessary for these purposes. In so doing, EPA may not consider the costs of implementing the 

standards. Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001). 

Section 109(d)(1) requires that “not later than December 31, 1980, and at 5-year intervals 

thereafter, the Administrator shall complete a thorough review of the criteria published under 

section 108 and the national ambient air quality standards . . . and shall make such revisions in 

such criteria and standards and promulgate such new standards as may be appropriate . . . .” 

Section 109(d)(2) requires that an independent scientific review committee “shall complete a 

review of the criteria . . . and the national primary and secondary ambient air quality standards . . 

. and shall recommend to the Administrator any new . . . standards and revisions of existing 

criteria and standards as may be appropriate . . . .” Since the early 1980's, this independent 

review function has been performed by the Clean Air Scientific Advisory Committee (CASAC) 

of EPA’s Science Advisory Board. 

1.2 REGULATORY HISTORY OF THE SECONDARY NAAQS FOR NO2 

AND SO2

1.2.1 NO2 NAAQS 

In 1971, the first air quality criteria document for NOx was issued by the National Air 

Pollution Control Association (NAPCA), one of EPA’s predecessor agencies. After reviewing 

the relevant science on the public health and welfare effects associated with oxides of nitrogen, 

EPA promulgated identical primary and secondary NAAQS for NO2 on April 30, 1971. Under 

section 109 of the Act, these standards were set at 0.053 parts per million (ppm) as an annual 

average (36 FR 8186). In 1982, EPA published Air Quality Criteria for Oxides of Nitrogen

(EPA, 1982), which updated the scientific criteria upon which the initial standards were based. 

 

1

 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.” 
December 2007 1-4 

On February 23, 1984, EPA proposed to retain these standards (49 FR 6866). After taking into 

account public comments, EPA published the final decision to retain these standards on June 19, 

1985 (50 FR 25532). 

In November 1991, EPA released an updated draft Air Quality Criteria Document 

(AQCD) for CASAC and public review and comment (56 FR 59285). The draft AQCD 

provided a comprehensive assessment of the available scientific and technical information on 

heath and welfare effects associated with NO2 and other NOx compounds. The CASAC 

reviewed the document at a meeting held on July 1, 1993 and concluded in a closure letter to the 

Administrator that the document “provides a scientifically balanced and defensible summary of 

current knowledge of the effects of this pollutant and provides an adequate basis for EPA to 

make a decision as to the appropriate NAAQS for NO2” (Wolff, 1993). 

The EPA also prepared a draft Staff Paper that summarized and integrated the key studies 

and scientific evidence contained in the revised AQCD and identified the critical elements to be 

considered in the review of the NO2 NAAQS. The Staff Paper received external review at a 

December 12, 1994 CASAC meeting. The CASAC comments and recommendations were 

reviewed by EPA staff and incorporated into the final draft of the Staff Paper, as appropriate. 

The CASAC reviewed the final draft of the Staff Paper in June, 1995 and responded by written 

closure letter (Wolff, 1995). In September, 1995, EPA finalized the Staff Paper, “Review of the 

National Ambient Air Quality Standards for Nitrogen Dioxide: Assessment of Scientific and 

Technical Information,” (EPA, 1995). 

On October 2, 1995, the Administrator announced her proposed decision not to revise 

either the primary or secondary NAAQS for NO2 based on the information available to her in 

this review (60 FR 52874; October 11, 1995). After careful evaluation of the comments received 

on the October 1995 proposal, the Administrator made a final determination that revisions to 

neither the primary nor the secondary NAAQS for NO2 were appropriate at that time (61 FR 

52852, October 8, 1996). The level for both the existing primary and secondary NAAQS for 

NO2 remains 0.053 ppm (equivalent to 100 micrograms per cubic meter of air [ug/m3

]) in annual 

arithmetic average, calculated as the arithmetic mean of the 1-hour NO2 concentrations. 

1.2.2 SO2 NAAQS 

 Based on the 1970 sulfur oxides criteria document (DHEW, 1970), EPA promulgated 

primary and secondary NAAQS for SO2, under section 109 of the Act on April 30, 1971 (36 FR 
December 2007 1-5 

8186). The secondary standards included a standard at 0.02 ppm in an annual arithmetic mean 

and a 3-hr average of 0.5 ppm, not to be exceeded more than once per year. These secondary 

standards were established solely on the basis of vegetation effects evidence. In 1973, revisions 

made to Chapter 5 “Effects of Sulfur Oxide in the Atmosphere on Vegetation” of Air Quality 

Criteria for Sulfur Oxides (EPA 1973), indicated that it could not properly be concluded that the 

vegetation injury reported resulted from the average SO2 exposure over the growing season, 

rather than the short-term peak concentrations. EPA, therefore, proposed (38 FR 11355) and 

then finalized a revocation of the annual mean secondary standard (38 FR 25678). At that time, 

EPA was aware that sulfur oxides have other public welfare effects, including effects on 

materials, visibility, soils and water. However, the available data were considered insufficient to 

establish a quantitative relationship between specific sulfur dioxide concentrations and effects 

needed for standard setting (38 FR 25679). 

In 1979, EPA announced that it was revising the AQCD for sulfur oxides concurrently 

with that for particulate matter and would produce a combined particulate matter (PM) and sulfur 

oxides criteria document. Following its review of a draft revised criteria document in August, 

1980, the CASAC concluded that acid deposition was a topic of extreme scientific complexity 

because of the difficulty in establishing firm quantitative relationships among: (1) emissions of 

relevant pollutants (e.g., SO2 and oxides of nitrogen), (2) formation of acidic wet and dry 

deposition products, and (3) effects on terrestrial and aquatic ecosystems. CASAC also noted 

that acid deposition involves, at a minimum, several different criteria pollutants - oxides of 

sulfur, oxides of nitrogen, and the fine particulate fraction of suspended particles. The 

Committee felt that any document on this subject should address both wet and dry deposition, 

since dry deposition was believed to account for at least one half of the total acid deposition 

problem. 

For these reasons, the CASAC recommended that a separate, comprehensive document on 

acid deposition be prepared prior to any consideration of using the NAAQS as a regulatory 

mechanism for the control of acid deposition. CASAC also suggested that a discussion of acid 

deposition be included in the AQCDs for both nitrogen oxides and PM and SOx. In response to 

these recommendations, EPA subsequently prepared the following documents: The Acidic 

Deposition Phenomenon and Its Effects: Critical Assessment Review Papers, Volumes I and II 

(EPA, 1984), and The Acidic Deposition Phenomenon and Its Effects: Critical Assessment 

Document (EPA, 1985) (53 FR 14935 -14936). These documents, though they were not 
December 2007 1-6 

considered criteria documents and did not undergo CASAC review, represented the most 

comprehensive summary of relevant scientific information completed by the EPA at that point. 

 Following CASAC closure on the criteria document for SO2 in December 1981, EPA 

OAQPS published a Staff Paper in November, 1982. The issue of acid deposition was not, 

however, assessed directly in this Staff Paper because EPA followed the guidance given by 

CASAC. 

 On April 26, 1988 (53 FR 14926), EPA proposed not to revise the existing primary and 

secondary standards. This proposal regarding the secondary SO2 NAAQS was due to the 

Administrators conclusions that (1) based upon the then-current scientific understanding of the 

acid deposition problem, it would be premature and unwise to prescribe any regulatory control 

program at that time, and (2) when the fundamental scientific uncertainties had been reduced 

through ongoing research efforts, EPA would draft and support an appropriate set of control 

measures. 

In spite of the complexities and significant remaining uncertainties associated with the 

acid deposition problem, it soon became clear that a program to address acid deposition was 

needed. On November 15, 1990, Amendments to the CAA were passed by Congress and signed 

into law by the President. In Title IV of these Amendments, Congress included a statement of 

findings that had led them to take this action, including that: (1) the presence of acid compounds 

and their precursors in the atmosphere and in deposition from the atmosphere represents a threat 

to natural resources, ecosystems, materials, visibility, and public health; (2) the problem of acid 

deposition is of national and international significance; and (3) current and future generations of 

Americans will be adversely affected by delaying measures to remedy the problem. The goal of 

Title IV was to reduce emissions of SO2 by 10 million tons and NOx emissions by 2 million tons 

from 1980 emission levels in order to achieve reductions over broad geographic regions/areas. 

Congress, however, clearly envisioned that further action might be necessary in the long 

term and reserved judgment on the form it could take, as evidenced by the inclusion of section 

404 of the 1990 Amendments (Clean Air Act Amendments of 1990, Pub. L. 101-549, § 404). 

This section required EPA to conduct a study on the feasibility and effectiveness of an acid 

deposition standard or standards to protect “sensitive and critically sensitive aquatic and 

terrestrial resources” and at the conclusion of the study, submit a report to Congress. Five years 

later EPA submitted to Congress its report titled Acid Deposition Standard Feasibility Study: 

Report to Congress (EPA, 1995) in fulfillment of this requirement. The Acid Deposition 
December 2007 1-7 

Standard Feasibility Study Report to Congress concluded that establishing acid deposition 

standards for sulfur and nitrogen deposition may at some point in the future be technically 

feasible although appropriate deposition loads for these acidifying chemicals could not defined 

with reasonable certainty at that time. 

The 1990 Amendments also added new language to sections of the CAA pertaining to the 

scope or application of the secondary NAAQS designed to protect the public welfare. Section 

108 (g) specified that “the Administrator may assess the risks to ecosystems from exposure to 

Criteria Air Pollutants (as identified by the Administrator in the Administrator’s sole 

discretion).” The definition of public welfare in section 302 (h) was expanded 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 

pollutants.”

In 1999, seven Northeastern States cited this amended language in section 302(h) in a 

petition to EPA to use its authority under the NAAQS program to promulgate secondary 

NAAQS for the criteria pollutants associated with the formation of acid rain. The petition stated 

that this language “clearly references the transformation of pollutants resulting in the inevitable 

formation of sulfate and nitrate aerosols and/or their ultimate environmental impacts as wet and 

dry deposition, clearly signaling Congressional intent that the welfare damage occasioned by 

sulfur and nitrogen oxides be addressed through the secondary standard provisions of Section 

109 of the Act. The petition further stated that “recent federal studies, including the NAPAP 

Biennial Report to Congress: An Integrated Assessment, document the continued-and increasingdamage being inflicted by acid deposition to the lakes and forests of New York, New England 

and other parts of our nation, demonstrating that the Title IV program had proven insufficient.” 

The petition also listed other adverse welfare effects associated with the transformation of these 

criteria pollutants, including visibility impairment, eutrophication of coastal estuaries, global 

warming, tropospheric ozone and stratospheric ozone depletion. 

In a related matter, the Office of the Secretary of the U.S. Department of Interior (DOI) 

requested in 2000 that the EPA initiate a rulemaking proceeding to enhance the air quality in 

national parks and wilderness areas in order to protect resources and values that are being 

adversely affected by air pollution. Included among the effects of concern identified in the 

request were acidification of streams, surface waters and/or soils, eutrophication of coastal 

waters, visibility impairment, and foliar injury from ozone. 
December 2007 1-8 

In a Federal Register notice in 2001, EPA announced receipt of these items and requested 

comment on the issues raised by these requests. EPA stated that it would consider any relevant 

comments and information submitted, along with the information provided by the petitioners and 

DOI, before making any decision concerning a response to these requests for rulemaking. 

 In this review, EPA will again revisit the appropriateness and feasibility of setting a 

secondary NAAQS to address the welfare effects resulting from the deposition of these criteria 

pollutants and their transformation products. This plan describes potential elements of that 

review. 

1.3 OVERVIEW OF THE NAAQS REVIEW PROCESS 

U.S. EPA has recently decided to make a number of changes to the process for reviewing 

the NAAQS (described at www.http://epa.gov/ttn/naaqs/). The revised NAAQS review process 

contains four major components: an integrated review plan, a science assessment, a risk/exposure 

assessment, and a policy assessment/rulemaking. In addition to these procedural modifications, 

for this secondary NAAQS review we have decided to examine two of the six criteria pollutants, 

oxides of nitrogen and sulfur oxides, together, rather than individually, as has been done in the 

past. This decision derives from the fact that NOx, SOx, and their associated transformation 

products are linked from an atmospheric chemistry perspective, as well as from an environmental 

effects perspective (most notably in the case of secondary aerosol formation and acidification in 

ecosystems). These interactions have been recognized historically by both CASAC and EPA; 

the science related to these interactions continues to evolve and grow, emphasizing the 

importance of considering them together. 

The first phase of the revised NAAQS review process is the development of the integrated 

review plan. This document represents the current plan and specifies the schedule of the review, 

the process for conducting the review, and the key policy-relevant science issues that will guide 

the review. This plan will be submitted for review and comment to CASAC, other non-EPA 

scientists, and the public. The final integrated review plan will take into account comments from 

these entities. 

The second phase of the process is the development of the science assessment, which 

consists of the Integrated Science Assessment (ISA) for NOx and SOx and supporting details in 

its annexes. The U.S. EPA’s National Center for Environmental Assessment (NCEA) along with 

contractor support is currently developing the annexes to the ISA. These annexes will contain a 
December 2007 1-9 

comprehensive description and evaluation/assessment of the full breadth of the recent scientific 

literature pertaining to known and anticipated effects on public welfare associated with the 

presence of the NOx and SOx criteria pollutant(s) in the ambient air, emphasizing the 

information that has become available since the last review in order to reflect the current state of 

knowledge. NCEA will then critically evaluate, integrate, and synthesize the most policyrelevant science from the annexes into an ISA. The ISA is intended to provide information useful 

in forming judgments about air quality indicator(s), form(s), averaging time(s) and level(s) for 

the secondary NAAQS. Hence, the ISA and its associated annexes function in the new NAAQS 

review in part as the Air Quality Criteria Document (AQCD) did in previous reviews. The 

schedule includes production of a first and second draft ISA, both of which will undergo CASAC 

and public review prior to completion of the final ISA. Section 4 provides a more detailed 

description of the planned scope, organization and assessment approach for the annexes and ISA, 

as well as EPA’s rationale for conducting a joint review of these criteria pollutants. 

In the third phase of the revised review process, the risk/exposure assessment, OAQPS 

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 NO2 and SO2, or levels that meet possible 

alternative standards. Section 5 of this Plan contains more detail about possible approaches EPA 

could take in conducting the risk/exposure Assessment. Once the draft ISA is complete, EPA 

will release a Scope and Methods Plan/document for CASAC and public review that describes 

the actual scope of the analyses to be performed and the tools/methods that will be employed. 

Once the risk/exposure assessment is complete, a report will be developed that focuses on key 

results, observations, and uncertainties and may include results from specific analyses designed 

to inform the review. This risk/exposure assessment report will also undergo review by CASAC 

and the public. 

The fourth component of the revised process will be a policy assessment/rulemaking. 

Under the revised NAAQS process, a policy assessment reflecting Agency views will be 

published in the Federal Register as an advance notice of proposed rulemaking (ANPR). The 

ANPR will be accompanied by supporting documents, such as air quality analyses and technical 

support documents, as appropriate. The ANPR takes the place of the Staff Paper prepared for 

previous NAAQS reviews. Issuance of a proposed and final rule will then complete the 

rulemaking process. 
December 2007 1-10 

1.4 SCIENCE PRIMER: NOX/SOX EFFECTS ON ECOSYSTEMS 

The following section is intended to provide a brief summary of the generally-accepted 

effects of NOx and SOx pollution on ecosystems. The information presented here is drawn from 

the last NOx (1995) and SOx (1996) NAAQS reviews as well as from more recent peer-reviewed 

scientific literature. Nitrogen and sulfur interactions in the environment are highly complex. Both 

are essential, and sometimes limiting, nutrients needed for growth and productivity. Excesses of 

nitrogen or sulfur can lead to acidification, nutrient enrichment, and eutrophication. Nitrogen 

effects occur along a continuum, from growth-enhancing fertilization to growth-depressing 

conditions associated with varying levels of nitrogen inputs. The excess nitrogen deposition 

described here results when available nitrogen cannot be assimilated by the existing vegetation 

or immobilized by the soil buffering capacity. 

Emissions of NOx and SOx compounds into the air react through a complex series of gasphase and heterogeneous reactions to produce additional intermediate compounds and final 

products. These reactions with NOx and SOx often occur and under the same meteorological 

influences as those acting on formation of ozone (O3) and secondary aerosols. These nitrogenand sulfur-containing compounds are removed from the air by deposition – wet (rain, snow), 

cloud and fog, and dry (gases and particles) -- onto surfaces. Prevailing winds can transport 

these compounds hundreds of miles and across state and national borders. 

Deposition of other chemical species derived from NOx and SOx emissions to the 

environment can initiate changes in ecosystem biogeochemistry, structure, and function. Since 

both NOx and SOx emissions can react in the atmosphere to form strong acids, one of the 

possible environmental endpoints is acidification. Acidification results in a cascade of effects 

that alter biogeochemical cycles and harm terrestrial and aquatic ecosystems. These effects 

include slower growth, the injury or death of forest vegetation, and localized extinction of fish 

and other aquatic species. Acid deposition can also cause accelerated erosion of exposed 

materials and structures, such as buildings and statues. 

In addition to acidification, NOx acts with other sources of reactive nitrogen, including 

ammonia-based nitrogen, to increase the total amount of nitrogen available to the organisms in 

terrestrial and aquatic ecosystems. Depending on the ecosystem and the endpoints that are of 

concern, this extra nitrogen can be considered to have both positive and negative effects. For 

example, low-level chronic nitrogen deposition can act as a fertilizer and increase productivity in 
December 2007 1-11 

terrestrial forests, but this extra nitrogen may change the species composition of the understory 

plants and lichens. At high levels of nitrogen deposition there may be negative effects on forest 

productivity. In general, ecosystems adapted to low nitrogen availability are most vulnerable to 

the effects of nitrogen deposition. Excess nitrogen to vulnerable ecosystems can cause “Nitrogen 

pollution,” resulting in a suite of terrestrial and aquatic ecological problems including 

biodiversity losses, community shifts, eutrophication, and harmful algal blooms. Lastly, SOx 

interacts with mercury (Hg) in ecosystems to increase the production of methylmercury, a 

powerful toxin that bioaccumulates, often to the point of causing toxic doses to top members of 

food chains (e.g., river otters and panthers). 

A summary illustration of NOx and SOx effects on the environment is presented in 

Figure 1.1. 
December 2007 1-12 

Figure 1.1 Nitrogen and Sulfur cycling and interactions in the 

environment.

1.4.1 Acidification Effects on Freshwater and Terrestrial Ecosystems from 

Sulfur and Nitrogen Deposition 

The process of acidification affects both freshwater aquatic and terrestrial ecosystems. 

Acid deposition causes acidification of sensitive surface waters. The effects of acid deposition on 

aquatic systems depend largely upon the ability of the ecosystem to neutralize the additional 

acid. As acidity increases, aluminum leached from soils and sediments, flows into lakes and 

streams and can be toxic to both terrestrial and aquatic biota. The lower pH concentrations and 

higher aluminum levels resulting from acidification make it difficult for some fish and other 

aquatic organisms to survive, grow, and reproduce. 

Research on effects of acid deposition on forest ecosystems has come to focus 

increasingly on the biogeochemical processes that affect uptake, retention, and cycling of 

nutrients within these ecosystems. Decreases in available base cations from soils are at least 
December 2007 1-13 

partly attributable to acid deposition. Base cation depletion is a cause for concern because of the 

role these ions play in acid neutralization and, because calcium, magnesium and potassium are 

essential nutrients for plant growth and physiology. Changes in the relative proportions of these 

nutrients, especially in comparison with aluminum concentrations, have been associated with 

declining forest health. 

1.4.2 Excess Nitrogen in Terrestrial and Estuarine Ecosystems 

In addition to acidification effects, NOx contributes to total nitrogen loading of 

ecosystems. In terrestrial systems, too much nitrogen can affect species diversity by favoring 

plant species with high nitrogen requirements over other species which may have evolved 

specialized capabilities to thrive under more nitrogen-limited conditions. Elevated soil nitrogen 

tends to cause changes in the ability of invasive species to colonize, biodiversity, and community 

structure. Animals that depend on specific plants for habitat and food may also be threatened by 

changes in vegetation resulting from increased nitrogen inputs. 

Atmospheric deposition of nitrogen is a significant source of total nitrogen to many 

estuaries in the United States. The amount of nitrogen entering estuaries that is ultimately 

attributable to atmospheric deposition is not well-defined. On an annual basis, atmospheric 

nitrogen deposition may contribute significantly to the total nitrogen load, depending on the size 

and location of the watershed. In addition, episodic nitrogen inputs, which may be ecologically 

important, may play a more important role than indicated by the annual average concentrations. 

Estuaries in the U.S. that suffer from nitrogen enrichment often experience a condition known as 

eutrophication. Symptoms of eutrophication include changes in the dominant species of 

phytoplankton, low levels of oxygen in the water column, fish and shellfish kills, outbreaks of

toxic alga, and other population changes which can cascade throughout the food web. In 

addition, increased phytoplankton growth in the water column and on surfaces can attenuate light 

causing declines in submerged aquatic vegetation, which serves as an important habitat for many 

estuarine fish and shellfish species. 

1.4.3 Reduced Nitrogen 

Although this review must initially address NOx, it appears evident that total reactive 

nitrogen (i.e., both oxidized and reduced forms of nitrogen) is important from an ecosystem 

effects perspective. As such, the interactions between reduced forms of nitrogen (namely 
December 2007 1-14 

ammonia, NH3, and ammonium, NH4

+

) and NOx and SOx in the atmosphere and associated 

deposition processes will be considered. Ammonia is an additional source of nitrogen to the 

environment that contributes to the problems caused by excess nitrogen deposition — nutrient 

enrichment and eutrophication. Ammonia can also contribute to increasing soil acidity via 

nitrification, the microbially-mediated transformation of ammonium to nitrate. The atmospheric 

lifetime of gas-phase ammonia is on the order of a day; however, in the presence of sulfuric or 

nitric acids, ammonia can form aerosols, predominantly ammonium sulfate and ammonium 

nitrate, with lifetimes on the order of 7-10 days. This longer lifetime increases the potential for 

long-range transport of nitrogen and sulfur, and contributes to fine particulate matter pollution 

and regional haze. Ammonium sulfates and nitrates also participate in climate modification 

effects, directly through radiative cooling by light scattering and indirectly through modifications 

of cloud cover and precipitation by acting as cloud condensation nuclei. A detailed discuss of 

these processes is included in the PM review. 
December 2007 2-1 

2. REVIEW SCHEDULE 

EPA’s National Center for Environmental Assessment in Research Triangle Park, NC 

(NCEA-RTP) announced the official initiation of the current periodic review of air quality 

criteria for NOx on December 9, 2005 and for SOx on May 15, 2006. For each of these reviews, 

the Agency began by announcing in the Federal Register (70 FR 73236 and 71 FR 28023) the 

formal commencement of the review and a call for information. With these reviews underway, a 

workshop addressing the separate, joint review of just the secondary standards for these two 

pollutants was announced in the Federal Register on June 20, 2007 (72 FR 11960). The proposed 

schedule for this joint review process is shown in Table 2-1; underlined dates indicate the courtordered schedule. 

Table 2-1. Proposed Schedule for Joint NOx and SOx Secondary Standard Review 1

Stage of Review Major Milestone Target Dates 

Literature Search Ongoing 

Federal Register Call for Information December 2005 

Prepare Draft NO2/SO2 NAAQS Work Plan December 2005-August 2007 

Workshop on science/policy issues July 2007 

CASAC consultation October 2007 

Planning 

Prepare final integrated NO2/SO2 NAAQS Work 

Plan 

December 2007 

Prepare first draft of ISA December 2007 

CASAC/public review first draft ISA April 2008 

Prepare second draft of ISA July/August 2008 

CASAC/public review second draft ISA October 2008 

Integrated Science 

Assessment (ISA) 

Prepare final ISA December 12, 2008

REA methodology released to CASAC and the 

public 

January 2008 

CASAC/public consultation on REA methodology April 2008 

First draft REA released to CASAC and the public August 2008 

CASAC/public review of the first draft of the REA October 2008 

Second draft of REA released to CASAC and the 

public 

March 2009 

CASAC/public review of second draft of REA May 2009 

Risk/Exposure 

Assessment (R/EA) 

Final REA released July 2009 

Publish ANPR August 2009 

CASAC review/public comment on ANPR October 2009 

Proposed rulemaking February 12, 2010

Policy Assessment/ 

Rulemaking 

Final rulemaking October 19, 2010

1 Schedule may be modified from time to time, as necessary, to reflect actual project requirements and progress. 
December 2007 3-1 

3. KEY POLICY-RELEVANT ISSUES 

 In this review of the ecosystem-related effects on public welfare related to NOx and SOx, 

a series of policy-relevant questions will frame our approach and will be addressed in detail in 

the science assessment, risk/exposure assessment, and policy assessment sections of the review. 

For this first time in the NAAQS review process, the secondary standards for NOx and SOx will 

be reviewed together due to the pollutants’ combined effects on atmospheric chemistry, 

deposition processes, and ecosystem-related public welfare effects. As a result, the issue of 

appropriate indicators becomes central to the review of the standards. This review will evaluate 

what appropriate indicators are for NOx and SOx, if these indicators should be combined into 

one standard, if separate standards should be issued, or if a suite of standards should be issued. In 

evaluating environmental responses to these pollutants, the variability present in ecosystems 

across the nation should also be considered. Issues of ecosystem susceptibility should be 

addressed, as should the issue of whether individual effects or combined effects are more 

important to a given ecosystem (i.e., is it NOx or SOx alone or the combination that is 

important). 

 As noted in the introduction, both EPA and CASAC 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. The issues associated with NOx and SOx 

particles; including visibility impairment and climate associated with ambient concentrations of 

particulates and aerosols are being addressed in the secondary PM NAAQS review, which is also 

currently underway. 

The review of the adequacy of the current standard for each effects category involves 

addressing questions such as: 

• To what extent does the available information demonstrate or suggest that NOx/SOxrelated effects are occurring at current ambient conditions or at levels that would meet 

the current standards? 

• To what extent does the available information inform judgments as to whether any 

observed or anticipated effects are adverse to public welfare? 

• To what extent are the current secondary standards likely to be effective in achieving 

protection against any identified adverse effects? 
December 2007 3-2 

To the extent that the evidence suggests that revision of the current secondary NO2/SO2

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 across the nation, ecosystem characteristics may be an important consideration in 

evaluating the form(s) of the standard(s). The form(s) of the standard(s) may be based on a 

complex formula that incorporates ecosystem characteristics, atmospheric 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 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 relevant time scales of ecosystem impacts and matching those time 

scales to relevant time scales for ambient indicators 

• describing 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 
December 2007 3-3 

• 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 
December 2007 4-1 

4. SCIENCE ASSESSMENT 

4.1 SCOPE AND ORGANIZATION 

 The scope of the joint NOX and SOX ISA is limited to welfare topics that do not 

duplicate those addressed by the particulate matter (PM) science assessment. The welfare effects 

of visibility impairment and climate interactions associated with particulate NOX and SOX will 

be addressed within the secondary PM NAAQS review, as these processes occur via NOX and 

SOX residing in the particulate or aerosol phase and interact with other chemical components of 

PM. The issue of NOx as a tropospheric precursor to ozone is discussed in detail in the 

photochemical oxidant NAAQS review (EPA 2006 and 2007). The effects of acidification and 

nitrogen-nutrient deposition on ecosystems are the main focus of this assessment; however, other 

welfare effects are discussed. 

 The science assessment will be organized into the integrated science assessment (ISA) 

and its supporting annexes. The ISA will critically evaluate and integrate the scientific 

information on the exposure and environmental effects associated with atmospheric deposition of 

NOx and SOx to provide a policy-relevant review as discussed in Chapter 32

. The annexes, which 

evaluate and summarize relevant studies, will provide a detailed basis for developing the ISA. 

The annexes will evaluate over a thousand published papers. Key findings will be summarized 

and integrated by discipline, pollutant impact or assessment endpoint pertinent to decisions on 

possible revision of the NO2 and SO2 secondary NAAQSs. Although emphasis will be placed on 

the presentation of environmental effects data, other scientific data will also be presented and 

evaluated in order to provide a better understanding of the nature, sources, distribution, 

measurement, transformations, and concentrations of NOx and SOx in ambient air and in various 

environmental compartments 

The focus of the ISA and its annexes will be literature published since the previous 

reviews of air quality criteria for NOx and SOx. Key findings and conclusions from the previous 

Air Quality Criteria Documents (AQCDs) will be briefly summarized at the beginning of the 

ISA. The results of recent studies will be integrated with previous findings. Important older 

studies will be more specifically discussed if they are open to reinterpretation in light of newer 

 

2

 Note that evidence related to human health effects of NOx and SOx will be considered in two separate 

science assessment documents that will prepared as part of the review of the primary NAAQS. 
December 2007 4-2 

data. Generally, only information that has undergone scientific peer review and that has been 

published (or accepted for publication) in the open literature will be considered. However, 

exceptions may be made depending on the importance of the subject information and its 

relevance to the review of the NO2 and SO2 secondary NAAQS, as determined in consultation 

with CASAC. 

Emphasis will be placed on studies conducted at or near NOx and SOx concentrations 

found in ambient air. Other studies may be included if they contain unique data, such as the 

documentation of a previously unreported effect or of a mechanism for an observed effect; or if 

they were studies that included both higher and lower concentrations designed to determine 

exposure-response relationships. 

4.2 ASSESSMENT APPROACH 

The NCEA-RTP is responsible for preparing the ISA and its annexes for NOx and SOx. 

Expert authors include EPA staff with extensive knowledge in their respective fields and 

extramural scientists contracted to the EPA. 

4.2.1 Literature Search 

The NCEA-RTP uses a systematic approach to identify relevant studies for consideration. 

A Federal Register Notice (FRN) was published on December 9, 2005 for NOx and on May 15, 

2006 for SOx to announce the initiation of this review and request information from the public. 

An initial publication base has been established by searching various databases using as key 

words the terms nitrogen oxides (NOx), nitrogen dioxide (NO2), nitric acid (HNO3), peroxyacytyl 

nitrate (PAN), ammonia (NH3), total reactive nitrogen, atmospheric deposition of nitrogen and 

sulfur, terrestrial ecosystems, aquatic ecosystems, wetlands, etc. This search strategy will 

periodically be reexamined and modified to enhance identification of pertinent published papers. 

Additional papers are identified for inclusion in the publication base in several ways. First, EPA 

staff reviews pre-publication tables of contents for journals in which relevant papers are 

published. Second, expert chapter authors are charged with independently identifying relevant 

literature. Finally, additional publications are identified by both the public and CASAC during 

the external review process. The studies identified will include research published or accepted 

for publication by a date determined to be as inclusive as possible given the relevant target dates 

in the NAAQS review schedule. Some additional studies, published after that date, may also be 
December 2007 4-3 

included if they provide new information that impacts one or more key scientific issues. The 

combination of these approaches should produce a comprehensive collection of pertinent studies 

needed to form the basis of the ISA. 

4.2.2 Criteria for Study Selection 

 Emphasis shall be placed on studies that evaluate effects at realistic ambient levels and 

studies that consider NOX and SOX as components of a complex mixture of air pollutants. 

Studies conducted in any country that contribute significantly to the knowledge base shall be 

included in the assessment, but emphasis may be placed on findings from studies conducted in 

the United States and Canada where differences in emissions and the air pollutant mixture are 

important. In assessing the relative scientific quality of studies reviewed here and to assist in 

interpreting their findings, the following considerations will be taken into account: (1) to what 

extent are the aerometric data/exposure metrics of adequate quality and sufficiently 

representative to serve as credible exposure indicators; (2) were the study populations well 

defined and adequately selected so as to allow for meaningful comparisons between study 

groups; (3) were the ecological assessment endpoints reliable and policy relevant; (4) were the 

statistical analyses used appropriate and properly performed and interpreted; (5) were likely 

important covariates (e.g., potential confounders or effect modifiers) adequately controlled or 

taken into account in the study design and statistical analyses; and (6) were the reported findings 

internally consistent, biologically plausible, and coherent in terms of consistency with other 

known facts. 

These guidelines provide benchmarks for evaluating various studies and for focusing on 

the highest quality studies in assessing the body of environmental effects evidence. Detailed 

critical analysis of all NOX and SOX environmental effects studies, especially in relation to the 

above considerations, is beyond the scope of the ISA and its Annexes. Of most relevance for 

evaluation of studies is whether they provide useful qualitative or quantitative information on 

exposure effect or exposure response relationships for environmental effects associated with 

current ambient air concentrations of NOX and SOX or deposition levels likely to be encountered 

in the United States. 
December 2007 4-4 

4.2.3 Content and Organization of the ISA 

The content of the ISA will be organized and developed to address the overarching 

policy-relevant questions listed in Chapter 3. 

The scientific information in the ISA will be drawn from a series of more comprehensive 

ISA annexes. The annexes will be focused on accomplishing two goals. The first goal will be to 

identify scientific research that is relevant to informing key policy issues. The second goal will 

be to produce a base of evidence containing all of the publications relevant to the review of the 

NOx and SOx secondary standards. Sections of the ISA annexes will include the following, not 

necessarily in this order: 

(1) Atmospheric chemistry of NOx and SOx; emissions, transport, and transformations 

in the atmosphere 

(2) Deposition processes of atmospherically derived NOx and SOx, ambient 

concentrations and the relationship between ambient concentrations and deposition 

(3) Environmental response networks, monitoring networks and models; 

(4) Acidification effects of NOx and SOx on terrestrial and aquatic ecosystems 

(5) Non-acidification effects of NOx on terrestrial and aquatic ecosystems; 

(6) Non-acidification effects of SOx on terrestrial and aquatic ecosystems; 

(7) Impacts of NOx/SOx on ecosystems modified by climate change factors 

(8) Assessing the available information for potentially determining critical loads to 

ecosystems 

(9) Effects of NOx and SOx deposition on acid-driven erosion of man-made materials 

and structures; and 

(10) Valuation of NOx and SOx effects on the environment. 

The ISA will convey the reasoning used to select which scientific studies are most policyrelevant. Detailed discussions of studies considered for the ISA, but not deemed most relevant, 

will be limited to the annexes. The ISA will integrate scientific information from the 10 subject 

areas identified by the annexes into main topic areas. At this point we anticipate these sections 

to be Atmospheric Concentration and Deposition, and Ecosystems. However upon further 

evaluation of the literature Materials and Structures Damage may be a third area of the ISA. 

1. Atmospheric Concentration and Deposition: The ISA will evaluate what we know about 

the factors that influence deposition, the uncertainties associated with extrapolation from 
December 2007 4-5 

ambient air concentrations to ecosystem exposures to NOx and SOx, and spatial patterns 

of NOx and SOx deposition. Specific questions include, but are not limited to: 

a. How are uncertainties described when extrapolating between stationary 

monitoring instruments and ecosystem exposure? 

b. What is the spatial distribution of NOx and SOx on a national scale? What are the 

most effective means of using our observations and predictive models to address 

spatial, temporal and compositional distribution of nitrogen and sulfur? What 

additional information (observations, process formulations) would enhance our 

ability to quantify current exposures and track changes over time? 

c. How does reduced nitrogen (NHx) alter the atmospheric chemistry, transport and 

deposition of NOx or SOx? 

2. Integration of ecological evidence: The integration section will begin with an overview of 

three ecologic concepts that are important to clarify at the onset of the discussion: scale, 

function and structure (organism to region) and ecosystem services. The integration of 

ecologic evidence will be organized according to impacts. These impacts are 

acidification (combined effects of NOx and SOx), nitrogen nutrient effects (NOx) and 

sulfur nutrient effects (SOx). With regard to nitrogen nutrient effects, it is important to 

note that ecosystems are typically sensitive to total reactive nitrogen (Nr) input. 

Therefore NOx is evaluated as a component of total nitrogen input from various 

anthropogenic sources. The following questions will be addressed for each ecological 

impact: 

a. Is the biogeochemistry of ecosystems impacted and what are the chemical 

indicators of impact? 

b. How are organisms impacted? Which biological species are most sensitive? How 

is sensitivity defined? 

c. What ecosystems are most sensitive to NOx and SOx pollution? 

d. How does NOx and SOx pollution impact ecosystem services? 

e. What new information is available on the levels of deposition that can cause harm 

in ecosystems? 

f. What are the most appropriate spatial and temporal scales to evaluate impacts on 

ecosystems? 

g. What is the relationship between ecological vulnerability and NOx and SOx 

pollution variations in current meteorology or gradients in climate? 

h. Are there time lags to ecosystem “recovery”? What are the physical, chemical 

and ecological characteristics of “recovery”? 
December 2007 4-6 

4.3 Public and Scientific Review 

4.3.1 Informal Peer Consultation Workshop of the ISA Annexes 

A combined peer review/policy kickoff workshop was held in Chapel Hill, NC on July 

17-19, 2007. This peer consultation workshop provided a forum for the authors of the ISA and 

its annexes to receive comments from their scientific peers on the first draft of their findings. 

Peer reviewers were given a copy of the chapter they were assigned prior to the workshop and 

were asked to conduct an informal review by providing written comments on its scientific 

content. Each peer consultant was also asked to participate in the workshop. Reviewers 

provided critical comments on the adequacy of the coverage of the literature, the presentation of 

the evidence in both text and figures or tables, the appropriateness of the conclusions from the 

evidence and any new issues or literature that should be considered. At this workshop, expert 

reviewers were also asked to participate in a discussion of the integrative aspects of the evidence, 

to assist the EPA in subsequent preparation of the ISA. 

4.3.2 Public Review of External Review Drafts 

Since the conclusion of the workshop review process, the authors, contributing reviewers, 

and NCEA-RTP Project Team have met to resolve how to address comments received and will 

revise the draft annexes and complete preparation of the First External Review Draft (ERD) NOx 

and SOx ISA. After clearance by the U.S. EPA, the draft document will be released for public 

comment as announced in a Federal Register Notice. The ERD will be made available for 

review during a specified time period, usually of 60 to 90 days; written comments are solicited 

during this time. A similar procedure will be followed for public and CASAC review of a 

Second External Review Draft that EPA expects to be necessary before completion of the Final 

NOx and SOx ISA. 

4.3.3 Review by Clean Air Scientific Advisory Committee 

At the time the First External Review Draft is released to the public, that draft document 

will also be sent to the Clean Air Scientific Advisory Committee (CASAC) of EPA=s Science 
December 2007 4-7 

Advisory Board (SAB). CASAC members and consultants (see Appendix A) will review the 

draft document and discuss their comments in a public meeting announced in the Federal 

Register. At the meeting, the NCEA-RTP Project Team plans to present an overview of the main 

features of the document, a summary of key issues raised by public comments received on the 

document, and the charge to the committee, as well as being prepared to discuss proposed 

revisions, if indicated. Based on CASAC=s past practice, EPA expects that key CASAC advice 

and recommendations for revision of the document will be summarized by the CASAC Chair in 

a letter to the EPA Administrator. EPA will take into account any such recommendations, as 

well as CASAC and public comments at the meeting and any written comments received, in 

revising the draft NOx and SOx ISA. As noted earlier, EPA expects that it will be necessary to 

prepare a Second External Review Draft for further CASAC review and public comment. After 

appropriate revision, the final document will be made available on an EPA website and 

subsequently printed, with its public availability being announced in the Federal Register. 

 
December 2007 5-1 

5. RISK/EXPOSURE ASSESSMENT 

The risk/exposure assessment for the NOx and SOx secondary NAAQS review will build 

upon the scientific information presented in the Integrated Science Assessment. As discussed 

earlier in this document, the risk/exposure assessment will not focus on other welfare effects that 

might be associated with secondary pollutants associated with NOx and SOx. Depending on the 

results of the science assessment, air quality indicators for these criteria pollutants that differ 

from the current indicators may be considered in the risk/exposure assessment. 

The risk/exposure assessment will evaluate potential alternative indices, in an attempt to 

quantify the relationship between ambient concentrations of NOx and SOx and potential welfare 

effects. To create these indices, we will evaluate exposures and impacts in various ecosystems 

with differing responses related to nitrogen and sulfur inputs. One possible approach, 

recommended by both CASAC and the NRC (2004) is considering the use of critical loads when 

evaluating secondary standards. Further, the risk/exposure assessment will use a variety of 

methods 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 economic and non-economic valuation 

of ecosystem services. Our ability to address these issues will enable us to determine the extent 

to which the current standards provide requisite protection from any known or anticipated 

adverse effects associated with ambient levels of NOx and SOx. Given that it is the 

Administrator’s final decision as to whether an effect is significantly adverse to warrant revising 

and/or replacing the current standards, or if the body of scientific evidence and assessment tools 

provide an adequate basis for supporting revised standards, the risk/exposure assessment will be 

designed to provide quantitative results which will best inform that decision. 

To evaluate the nature and magnitude of ecosystem responses associated with adverse 

effects, the risk/exposure assessment will examine various ways to quantify the relationship 

between air quality indicators, deposition of biologically-accessible forms of nitrogen and sulfur, 

biologically-relevant indices relating deposition, exposure and effects on sensitive receptors and 

related impacts to ecosystem service(s). The risk/exposure assessment will evaluate the overall 

load to the system for nitrogen and sulfur as well as the variability in ecosystem responses to 

these pollutants and determining the exposure metric(s) that incorporates the temporal 

considerations (i.e. biologically-relevant timeframes), pathways, and biologically-relevant 
December 2007 5-2 

indices in order to maintain the functioning of these ecosystems. In addition, the risk/exposure 

assessment will attempt to address how sensitive ecosystems and their services affected by 

nitrogen and sulfur deposition may respond to variations in climatic elements such as 

temperature and rainfall. 

The scope of the risk/exposure assessment will depend in part on the answers to the 

following questions: 

• What are the appropriate geographic scale(s) and/or time frame(s) for the risk 

assessment? Information that will be considered in addressing this question 

includes: national-scale mapping, case studies of sensitive ecosystems, identification 

of representative ecosystem types, air pollution gradient studies, a weight-ofevidence approach incorporating both qualitative and quantitative information on 

risk, and, perhaps, 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, and 

sub-alpine) at varying ambient concentrations of nitrogen and sulfur compounds? 

Can differences in regional sensitivity to deposition be completely characterized by 

underlying physical or biological attributes? 

• To what degree are assumptions supported by the available science regarding 

linkages between pollutants in ambient air, deposition, and measurable ecosystem 

effects? What are the most useful metrics of both ambient pollution and the resulting 

effect? 

• 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? 

5.1 ASSESSMENT APPROACH 

 There are a variety of ways to address the risk/exposure assessment for the review of the 

secondary NAAQS for NOx and SOx. One of the purposes of this document is to solicit ideas 

and evaluate the most appropriate way to approach this review. In order to assess risks to public 

welfare due to ambient nitrogen and sulfur deposition, the relationships (and associated 

uncertainties) between ambient concentrations, deposition, environmental effects, environmental 

receptors, and ecosystem services and ecosystem valuation should be established. Some of our 

preliminary ideas for the risk/exposure assessment are outlined below. A detailed description of 

the tools mentioned here and others that are currently available are described in detail in 

Appendix B. 
December 2007 5-3 

 Understanding the atmospheric chemistry and deposition of nitrogen and sulfur is central 

to this review. Most likely, we will use the Community Multi-Scale Air Quality (CMAQ) model 

(and derivative models such as the response-surface model) described in Appendix B to estimate 

ambient concentrations and deposition areas. Any assessment of atmospheric chemistry and 

deposition is highly-dependent on the variability associated with baseline inputs. Consideration 

will be given to analyzing five consecutive years of CMAQ outputs (2001-2005) delivered as 

part of EPA’s Interagency Agreement with CDC to improve understanding of baseline 

deposition and the relationships between nitrogen and sulfur concentration and deposition in 

terms of magnitude and spatial relationships. 

Initially we plan to conduct regression and related statistical analyses to generate 

statistical correlations between grid-level ambient concentrations of NOx and SO2 and grid-level 

or appropriately spatially averaged (e.g., at the watershed level) deposition rates of sulfur and 

nitrogen. The regression analysis will use the results from the 2001-2005 CMAQ simulations 

conducted for the joint EPA/CDC project. The analysis will use linear and non-linear regression 

modeling, as well as spatial regression techniques, to develop reduced form equations. To the 

extent that data are available, the statistical modeling may also incorporate relevant 

meteorological and physical attributes, e.g. precipitation, relative humidity, wind speed and 

direction. These statistical relationships are not expected to provide predictive models. Rather, 

the goal is to develop appropriate estimates of the relationship between deposition metrics and 

ambient levels of NOx and SOx. 

 Another central issue in this review is determining which ecosystems are more or less 

sensitive to nitrogen and sulfur deposition. It may be useful to rank ecosystems (where data is 

available) based on a small subset of determinative underlying characteristics. These 

characteristics 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 non-atmospheric sources (such as fertilization, 

sewer leaks, point sources, etc.). Other ecosystem-specific characteristics, which may help assess 

sensitivities, include threatened and endangered species data where available, land-use type 

(including Class I, National Parks and Fish and Wildlife Refuges and National Wilderness 
December 2007 5-4 

areas), and baseline nitrogen and sulfur loading estimates. Where case-study or ecosystemspecific data are available, a subset of maps for the case-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 together, we may 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 if case studies might be appropriate for 

looking at nitrogen and sulfur effects on various ecosystems and geographic regions as a means 

of extrapolating these impacts to the entire country. For those areas where data are available, 

watershed models (e.g., MAGIC, PnET-BGC, and DayCent-Chem) may be useful for evaluating 

the emission-deposition-ecosystem response linkage. (These models are described in more detail 

in Appendix B.) As the integrated science assessment and risk/exposure assessment progress, we 

anticipate identifying additional ways to evaluate biologically-relevant indicators and ecosystem 

valuation techniques that will ultimately help to direct and focus the final Risk Assessment. 

5.2 TOOLS FOR RISK ASSESSMENT 

 In order to develop a risk analysis for a meaningful nationwide or ecosystem-specific 

metric, some of the available analytical tools for conducting a risk assessment are 

described/summarized in Appendix B. The more detailed model evaluation discussion included 

in the annexes and ISA will aid in selecting the appropriate modeling tools. With these tools, the 

risk/exposure assessment should be able to determine the appropriate endpoint(s)/indicator(s), 

geographic level/scale of protection, national or case-study modeling, and which ecosystem 

services are important. 

5.3 KNOWLEDGE GAPS 

 As discussed above and in detail in Appendix B, the risk/exposure assessment will use 

simulation models of atmospheric emissions, transformation, fate, transport, and deposition to 

follow emissions from a source to a receptor, either aquatic or terrestrial. We plan to overlay 

available data sets related to deposition and adverse impacts with GIS-based approaches to aid in 

the evaluation of their spatial correlations. We also have published and evaluated regional or 

area-specific models that will aid in evaluating ecosystem effects in specific watersheds (i.e., for 

the Chesapeake Bay or the Adirondack mountains). Additionally, we are beginning to quantify 
December 2007 5-5 

ecosystem services and the benefits they provide. However, there are knowledge gaps in the 

linkages between these areas. 

 Some of the main questions the risk/exposure assessment will try to answer are, “If we 

can identify appropriate biologically relevant indices, can we establish a link between deposition 

of NOx and SOx and ecosystem response? Additionally, what ecosystem services are associated 

with changes in emissions?” Answering these questions raises other issues regarding the links 

between atmospheric deposition and an ecosystem response, and ecosystem services and 

emissions changes. Some of these questions are listed below. 

Atmospheric Deposition, Fate, and Transport Modeling 

• What are typical uncertainties for observed and regional model estimates of wet and dry 

NOx, NHx, and SOx deposition rates? 

• Can deposition models be used to estimate deposition (wet, dry, cloud, fog) to a landuse specific surface, such as mountainous terrain, coniferous/deciduous forests, etc? 

• If not, are there models that can take this output and estimate deposition to specific 

ecosystems? 

 

Ecological Response Modeling 

• Do we have sufficient data to develop dose-response functions linking changes in 

nitrogen and sulfur deposition to changes in ecosystem functions? 

• Can we adequately characterize the uncertainties in these dose-response functions? 

• Can ecosystem-specific models be used to represent similar ecosystems nationwide? 

What are the uncertainties associated with the transferability between a specific 

watershed model to multiple watersheds? 

 

Ecosystem Services 

• Can the current known ecological responses be translated into an ecosystem service or 

services? 

• Can we value the changes in ecosystem services associated with changes in ecological 

indicators? 

• Can a process-based model serve to link multiple ecosystem services in order to assess 

the ‘true’ impact of air quality on the ‘bundles of services’ provided by that 

ecosystem? 

• How can we show that reducing emissions of NOx/SOx results in a specific ecosystem 

benefit? 
December 2007 5-6 

Valuation 

• What does the current valuation literature indicate about the values of change in 

ecosystems and the services they provide? 

• Are there other methods for developing non-monetized value estimates? 

• How do we link ecosystem responses and ecosystem services and how do we value the 

changes in those services? 

5.3 PUBLIC AND SCIENTIFIC REVIEW 

Drafts of the risk/exposure assessment will be reviewed by CASAC of EPA’s Science 

Advisory Board (SAB). CASAC members and consultants (see Appendix A) will review the 

draft document and discuss their comments in a public meeting announced in the Federal 

Register. Based on CASAC’s past practice, EPA expects that key CASAC advice and 

recommendations for revision of the document will be summarized by the CASAC Chair in a 

letter to the EPA Administrator. In revising the draft risk/exposure assessment for NOx and SOx, 

EPA will take into account any such recommendations. EPA will also consider comments 

received, from CASAC or from the public, at the meeting itself and any written comments 

received. EPA anticipates preparing a second draft of the risk/exposure assessment for CASAC 

review and public comment. After appropriate revision, the final document will be made 

available on an EPA website and subsequently printed, with its public availability being 

announced in the Federal Register. 
December 2007 6-1 

6. POLICY ASSESSMENT 

Based on the information in the ISA and the risk/exposure assessment, the Agency will 

develop an advance notice of proposed rulemaking (ANPR) that reflects EPA’s views regarding 

the need to retain or revise the secondary NAAQS for NO2 and SO2. The ANPR will identify 

conceptual evidence-based and risk/exposure-based approaches for reaching public welfare 

policy judgments. It will discuss the implications of the science and risk/exposure assessments 

for the adequacy of the current standard, and it will present risk/exposure information associated 

with alternative standards. The ANPR will also describe a range of policy options for standard 

setting including a description of the underlying interpretations of the scientific evidence and 

risk/exposure information that might support such alternative standards and that could be 

considered by the Administrator in making NAAQS decisions. 

The use of an ANPR will provide an opportunity for CASAC and the public to evaluate the 

policy options under consideration and to offer comments and recommendations to inform the 

development of a proposed rule. Taking into account CASAC advice and recommendations as well 

as public comment on the ANPR, the Agency will publish a proposed rule. This proposal will be 

followed by a public comment period. Taking into account comments received on the proposed rule, 

the Agency will then issue a final rule to complete the rulemaking process. 

A final decision must draw upon scientific evidence and analyses about effects on public 

welfare and the nature and magnitude of ecosystem responses that are understood to have 

adverse effects, as well as judgments about how to deal with the range of uncertainties that are 

inherent in the scientific evidence and analyses. The Agency’s approach to informing these 

judgments is based on the recognition that the available public welfare effects evidence generally 

reflects a continuum consisting of ambient levels at which scientists generally agree that public 

welfare effects are likely to occur, and that at lower ambient levels the likelihood and magnitude of 

the ecosystem responses become increasingly uncertain. 

This evaluation should recognize that the effects of NOx/SOx compounds on aquatic and 

terrestrial ecosystems are diverse and occur over time as a result of deposition, include 

acidification and fertilization effects, are regionally specific, and occur through the atmospheric 

reactions, transport, and deposition of NOx/SOx compounds emitted from varied and ubiquitous 

sources. The links in the fate, transport, and deposition apply to both acidification and 
December 2007 6-2 

fertilization (which is understood to capture both eutrophication in aquatic systems and 

fertilization in terrestrial systems). 

Other considerations in developing an indicator include using total reactive nitrogen as a 

biologically-relevant index for nutrient enrichment. Total reactive nitrogen deposited (and 

transported) should be viewed in context of baseline nitrogen availability. Ecosystem impacts 

may differ due to form of nitrogen deposited (e.g., oxidized vs reduced nitrogen), and the various 

forms of nitrogen deposited may be important in determining the form of the standard. Nitrogen 

deposition impacts also depend on soil characteristics, species composition and competition, and 

soil composition. In aquatic ecosystems, the range of biologically-relevant indices for nitrogen 

sensitivity may include dissolved oxygen concentrations, and the ratio of algae to diatoms. Acid 

neutralizing capacity (ANC) may be an appropriate biologically-relevant index for acidification 

depending on the time scale being considered. Acidification measured by sulfate and nitrate 

concentrations can change relatively rapidly (over several years), while ANC can take decades to 

change and reach a new equilibrium depending on releases of sulfate and other acidifying 

components due to current emissions/deposition or release of soil constituents including sulfate. 

The short-term effects from acid pulses may point to the need for a seasonal form of the 

standards. Nitrogen impacts through fertilization also have a seasonal component that may 

display a different pattern than that for acidification. The need for seasonal standards will be 

evaluated, noting that impacts may differ by the water type and land use in different areas. 

Ecosystem recovery time may also play an important role in determining the appropriate form 

and averaging time of standards. 

The policy determination of what is an adverse impact may vary, depending on the 

method chosen to assess adversity. The NAAQS provisions in the Act require the Administrator 

to establish secondary standards that are requisite to protect public welfare from any known or 

anticipated adverse effects associated with the presence of the pollutant in the ambient air. In so 

doing, the Administrator seeks to establish standards that are neither more nor less stringent than 

necessary for this purpose. The provisions do not require that secondary standards be set to 

eliminate all welfare effects, but rather at a level that protects public welfare from those effects 

that are judged to be adverse. In the evaluation of ecosystem services and valuation as one 

potential measure of adversity, the assessment should consider the ecosystem benefits that are 

projected to occur as a result of reductions in the current standards. 
December 2007 7-1 

7. REFERENCES 

Aber, J.D., S.V. Ollinger, and C.T. Driscoll, Modeling nitrogen saturation in forest 

ecosystems in response to land use and atmospheric deposition, Ecol. Modell., 101, 61- 

78, 1997. 

Amar, P., R. Bornstein, H. Feldman, H. Jeffries, D. Steyn, R. Yamartino, and Y. Zhang. 2004. 

Final Report Summary: December 2003 Peer Review of the CMAQ Model. pp. 7. 

Balmford, A., A. Bruner, P.Cooper, R. Costanza, and others. 2002. Economic reasons for 

conserving wild nature. Science 297 (5583): 950-953. 

Banzhaf, S. and J. Boyd. 2005. The architecture and measurement of an ecosystem services 

index. Decision Paper, RFF DP 05-22. Resources for the Future, Washington, D.C. 

Boyd, J. and S. Banzhaf. 2006. What are ecosystem services? The need for standardized 

environmental accounting units. Decision Paper, RFF DP 06-02. Resources for the 

Future, Washington, D.C. 

Byun, D.W., and K.L. Schere. 2006. “Review of the Governing Equations, Computational 

Algorithms, and Other Components of the Models-3 Community Multiscale Air Quality 

(CMAQ) Modeling System.” J. Applied Mechanics Reviews 59(2):51-77. 

The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Document (EPA, 1985) 

(53 FR 14935 -14936) 

The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Review Papers, 

Volumes I and II (EPA, 1984) 

Cosby, B.J., Wright, R.F., Hornberger, G.M., and Galloway, J.N. 1985a. Modelling the Effects 

of Acid Deposition: Assessment of a Lumped Parameter Model of Soil Waterand 

Streamwater Chemistry.” Water Resour. Res. 21:51-63. 

Cosby, B.J., Wright, R.F., Hornberger, G.M. and Galloway, J.N.. 1985b. “Modelling theEffects 

of Acid Deposition: Estimation of Long-Term Water Quality Responses in a Small 

Forested Catchment.” Water Resour. Res. 21:1591-1601. 

Cosby, B.J., Hornberger, G.M., Galloway, J.N., and Wright, R.F. 1985c. Time scales of 

catchment acidification: a quantitative model for estimating freshwater acidification. 

Environ. Sci. and Technol.19, 1145-1149. 

Daily, G.C., S. Alexander; P.R. Ehrlich, L. Goulder, L J. Lubchenco, P.A. Matson, H.A. 

Mooney, S. Postel, S.H. Schneider, D. Tilman, and G.M. Woodwell. 1997. Ecosystem 

Services: Benefits Supplied to Human Societies by Natural Ecosystems. Issues in 

Ecology 1:1-18. 

Dennis, R.L., D.W. Byun, J.H. Novak, K.J. Galluppi, C.J. Coats, and M.A. Vouk. 1996. “The 

next generation of integrated air quality modeling: EPA’s Models-3.” Atmospheric 

Environment 30:1925-1938. 

Department of Health, Education and Welfare (DHEW). 1970. Air Quality 

Criteria For Sulfur Oxides. U.S. Government Printing Office, Washington, D.C., 

AP-50. 

Galloway, J.N., Aber, J.D., Erisman, J.W., Seitzinger, S.P., Howarth, R.W., Cowling, E.B., and 

Cosby, B.J. 2003. The Nitrogen Cascade. BioScience. Vol.53.No.4. 

Gbondo-Tugbawa, S.S., C.T. Driscoll, J.D. Aber, and G.E. Likens, Evaluation of an 

integrated biogeochemical model (PnET-BGC) at a northern hardwood forest ecosystem, 

Water Resour. Res., 37, 1057-1070, 2001. 
December 2007 7-2 

Grell, G., J. Dudhia, and D. Stauffer, 1994: A Description of the Fifth-Generation Penn 

State/NCAR Mesoscale Model (MM5), NCAR/TN-398+STR., 138 pp, National Center 

for Atmospheric Research, Boulder CO. 

Millennium Ecosystem Assessment. 2005. Ecosystems and Human Well-being: Synthesis. 

Island Press, Washington, DC. 

NRC (National Research Council). 2004. Valuing Ecosystem Services: Towards better 

environmental decision-making. Washington, DC: National Academies Press. 277 p 

EPA, 1973. “Effects of Sulfur Oxide in the Atmosphere on Vegetation”. Revised Chapter 

5 of Air Quality Criteria For Sulfur Oxides. U.S. Environmental Protection 

Agency. Research Triangle Park, N.C. EPA-R3-73-030. 

U.S. EPA. Air Quality Criteria for Oxides of Nitrogen (1982). U.S. Environmental Protection 

Agency, Washington, D.C., EPA/600/8-82/026 (NTIS PB83131011), 1982. 

EPA, 1984. The Acidic Deposition Phenomenon and Its Effects: Critical Assessment 

Review Papers. Volume I Atmospheric Sciences. EPA-600/8-83-016AF. Volume 

II Effects Sciences. EPA-600/8-83-016BF. Office of Research and Development, 

Washington, D.C. 

EPA, 1985. The Acidic Deposition Phenomenon and Its Effects: Critical Assessment 

 Document. EPA-600/8-85/001. Office of Research and Development, 

 Washington, D.C. 

EPA, 1993. Air Quality Criteria for Oxides of Nitrogen. Vol. I of III. EPA/600/8-91/048aF. 

Office of Research and Development, Washington, D.C. 

EPA, 1995. Acid Deposition Standard Feasibility Study: Report to Congress. 

U.S. Environmental Protection Agency (1995) Review of the National Ambient Air Quality 

Standards for Nitrogen Dioxide: Assessment of Scientific and Technical Information. 

Office of Air Quality Planning and Standards, Research Triangle Park, N.C., EPA-452/R95-005. 

U.S. Environmental Protection Agency (EPA). 1999. “Science Algorithms of EPA Models-3 

Community Multiscale Air Quality.” (CMAQ Modeling System D.W. Byun and J.K.S. 

Ching, Eds. EPA/600/R-99/030, Office of Research and Development). 

U.S. Environmental Protection Agency. 2006a. Regulatory Impact Analysis for the 2006 

National Ambient Air Quality Standards for Particle Pollution. Office of Air Quality 

Planning and Standards. 

U.S. EPA. 2006b. Ecological Benefits Assessment Strategic Plan. EPA-240-R-06-001. U.S. 

Environmental Protection Agency, Washington D.C. 

U.S. Environmental Protection Agency. 2007. Regulatory Impact Analysis of the Proposed 

Revisions to the National Ambient Air Quality Standards for Ground-Level Ozone. 

Office of Air Quality Planning and Standards. EPA-452/R-07-008, July. 

Wolff, G. T. (1993) CASAC closure letter for the 1993 Criteria Document for Oxides of 

Nitrogen addressed to U.S. EPA Administrator Carol M. Browner dated September 30, 

1993. 

Wolff, G. T. (1995) CASAC closure letter for the 1995 OAQPS Staff Paper addressed to U.S. 

EPA Administrator Carol M. Browner dated August 22, 1995. 

Yantosca, B. 2004. GEOS-CHEMv7-01-02 User’s Guide, Atmospheric Chemistry Modeling 

Group, Harvard University, Cambridge, MA, October 15, 2004. 
December 2007 A-1 

APPENDIX A: CASAC PANEL 

U.S. Environmental Protection Agency 

Science Advisory Board (SAB) Staff Office 

Clean Air Scientific Advisory Committee (CASAC) 

NOX and SOX Secondary NAAQS Review Panel 

 The Clean Air Scientific Advisory Committee (CASAC) was established under section 

109(d)(2) of the Clean Air Act (CAA or Act) (42 U.S.C.7409) as an independent scientific 

advisory committee. CASAC provides advice, information and recommendations on the 

scientific and technical aspects of air quality criteria and national ambient air quality standards 

(NAAQS) under sections 108 and 109 of the Act. The CASAC is a Federal advisory committee 

chartered under the Federal Advisory Committee Act (FACA), as amended, 5 U.S.C., App. The 

Panel will comply with the provisions of FACA and all appropriate SAB Staff Office 

procedural policies. 

 Section 109(d)(1) of the CAA requires that the Agency periodically review and revise, 

as appropriate, the air quality criteria and the NAAQS for the six “criteria”' air pollutants, 

including NOX and SOX. 

CHAIR

Dr. Armistead (Ted) Russell, Georgia Power Distinguished Professor of Environmental 

Engineering, Environmental Engineering Group, School of Civil and Environmental 

Engineering, Georgia Institute of Technology, Atlanta, GA 

MEMBERS

Dr. Praveen Amar, Director, Science and Policy, NESCAUM, Boston, MA 

Dr. Andrzej Bytnerowicz, Senior Scientist, Pacific Southwest Research Station, USDA Forest 

Service, Riverside, CA 

Ms. Lauraine Chestnut, Managing Economist, Stratus Consulting Inc., Boulder, CO 

Dr. Douglas Crawford-Brown, Professor and Director, Department of Environmental Sciences 

and Engineering, Carolina Environmental Program, University of North Carolina at Chapel Hill, 

Chapel Hill, NC 

Dr. Ellis B. Cowling, Emeritus Professor, Colleges of Natural Resources and Agriculture and 

Life Sciences, North Carolina State University, Raleigh, NC 

Dr. Charles T. Driscoll, Jr., Professor, Environmental Systems Engineering, College of 

Engineering and Computer Science, Syracuse University, Syracuse, NY 

Dr. Paul J. Hanson, Distinguished R&D Staff Member, Environmental Sciences Division, Oak 

Ridge National Laboratory, Oak Ridge, TN 
December 2007 A-2 

Dr. Rudolf Husar, Professor and Director, Mechanical Engineering, Engineering and Applied 

Science, Center for Air Pollution Impact & Trend Analysis (CAPITA), Washington University, 

St. Louis, MO 

Dr. Dale Johnson, Professor, Department of Environmental and Resource Sciences, College of 

Agriculture, University of Nevada, Reno, NV 

Dr. Donna Kenski, Director, Lake Michigan Air Directors Consortium, Rosemont, IL 

Dr. Naresh Kumar, Senior Program Manager, Environment Division, Electric Power Research 

Institute, Palo Alto, CA 

Dr. Myron Mitchell, Distinguished Professor and Director, College of Environmental and 

Forestry, Council on Hydrologic Systems Science, State University of New York, Syracuse, NY 

Mr. Richard L. Poirot, Environmental Analyst, Air Pollution Control Division, Department of 

Environmental Conservation, Vermont Agency of Natural Resources, Waterbury, VT 

Mr. David J. Shaw, Director, Division of Air Resources, New York State Department of 

Environmental Conservation, Albany, NY 

Dr. Kathleen Weathers, Senior Scientist, Institute of Ecosystem Studies, Millbrook, NY
December 2007 B-1 

APPENDIX B: ASSESSMENT TOOLS 

B.1 MODELS 

B.1.1 Community Multi-Scale Air Quality (CMAQ) Model 

The CMAQ model is a three-dimensional grid-based Eulerian air quality model designed 

to estimate the formation and fate of ozone and other oxidants and their precursors; primary PM 

and secondary particulate matter precursors and atmospheric concentrations; toxics; and 

deposition of chemical species over scales ranging from continental to regional and urban to 

neighborhood (EPA, 1999; Byun and Schere, 2006; Dennis et al., 1996). The CMAQ model was 

peer-reviewed in 2003 for EPA as reported in “Peer Review of CMAQ Model” (Amar et al., 

2004). 

 CMAQ is most often configured to output spatial fields of gridded concentrations and 

deposition on an hourly basis for the entire modeling domain. Pollutant concentrations are 

output for each vertical layer included the model simulation. Additional information on the 

horizontal and vertical configuration of the model is provided below. The current version of 

CMAQ (v4.6) is capable of one atmosphere modeling in which NOx, SOx, ozone, particulates, 

mercury, and selected toxic pollutants are included in a model simulation. A list of the key 

nitrogen and sulfur containing deposition species output by CMAQ is provided in Table B.1. 

This table also identifies the deposition species derived from CMAQ’s nitrogen and sulfur 

deposition outputs. 

The standard hourly CMAQ model predictions are post-processed to create gridded fields 

of daily average, monthly average, and annual average concentrations for layer 1, which is the 

layer nearest to the ground. The hourly deposition outputs are post-processed to produce gridded 

fields of daily, monthly, and annual total wet, dry and wet plus dry deposition. 

Table B.1. Key Predicted and Derived Nitrogen and Sulfur Deposition Species from CMAQ1 

 

Predicted Deposition Species Derived Deposition Species 

Nitrogen Oxide (NO) Oxidized Nitrogen 

Nitrogen Dioxide (NO2) Reduced Nitrogen 
December 2007 B-2 

Nitric Acid (HNO3) Total Nitrogen 

Dinitrogen Pentoxide (N2O5) Total Sulfur 

Peroxyacetyl Nitrate (PAN) 

Ammonia (NH3) 

Sulfur Dioxide (SO2) 

Particulate Sulfate (SO4) 

Particulate Nitrate (NO3) 

Particulate Ammonium (NH4) 1

Model predictions include both wet and dry deposition in units of kg/ha. 

2002-Based Platform CMAQ Modeling 

 EPA/AQAD is developing an air quality modeling platform that includes meteorology 

and emissions3 for 2002 along with projected emissions for 2009, 2014, 2020, and 2030. The 

future year projections reflect the combined effects of emissions growth and reductions 

associated with “national rules” up through the CAIR/CAMR/CAVR. CMAQ will be run for 

each of these five years for a 36 km nationwide domain and for separate 12 km modeling 

domains covering the eastern and western U.S. The area covered by each of these domains is 

shown in Figure B.1. The domain specifications are provided in Table B.2. All three modeling 

domains contain 14 vertical layers with a top at about 16,200 meters, or 100 mb. Note that the 

horizontal and vertical resolution selected for these simulations are not limited by the model. 

Rather, they were selected to balance (1) the desire to have annual nationwide simulations with 

fine scale resolution versus (2) the computational requirements of the simulations which increase 

with horizontal and vertical resolution4

. 

 The CMAQ meteorological input files were derived from simulations of the Pennsylvania 

State University / National Center for Atmospheric Research Mesoscale Model (Grell, Dudhia, 

and Stauffer, 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. The outputs from MM5 were 

processed to create model-ready inputs for CMAQ using the Meteorology-Chemistry Interface 

 

3 Emissions inputs for to CMAQ include anthropogenic and biogenic emissions from areas of the U.S., Canada, and 

Mexico that are within the modeling domain. Anthropogenic emissions for the U.S. were derived from the 2002 

National Emissions Inventory. 
December 2007 B-3 

Processor (MCIP) version 3.2: horizontal wind components (i.e., speed and direction), 

temperature, moisture, vertical diffusion rates, and rainfall rates for each grid cell in each vertical 

layer (EPA, 1999). 

 The lateral boundary and initial species concentrations for the CMAQ 36 km domain 

were obtained from a three-dimensional global atmospheric chemistry model, the GEOS-CHEM 

model (Yantosca, 2004). The global GEOS-CHEM model simulates atmospheric chemical and 

physical processes driven by assimilated meteorological observations from the NASA’s Goddard 

Earth Observing System (GEOS). This model was run for 2002 with a grid resolution of 2 degree 

x 2.5 degree (latitude-longitude) and 20 vertical layers. The predictions were used to provide 

one-way dynamic boundary conditions at 3-hour intervals and the initial concentration field for 

the CMAQ simulations. The outputs from the 36 km domain are utilized to provide the initial 

and boundary condition concentrations for each of the two 12 km domains. 

36km Domain Boundary

12km East Domain Boundary

12km West Domain Boundary

Figure B.1. Map of the CMAQ 36 km and 12 km Modeling Domains. 

Table B.2. Geographic Specifications of Modeling Domains. 

 4 For various special studies, CMAQ has been run at 4 km and 1 km horizontal resolution for urban-scale domains 

and with a vertical resolution of over 30 layers. 
December 2007 B-4 

36 km Domain 

(148 x 112 Grid Cells) 

12 km Eastern Domain 

(279 x 240 Grid Cells) 

12 km Western Domain 

(213 x 192 Grid Cells) 

 Lon lat lon lat lon lat 

SW -121.77 18.17 SW -106.79 24.99 SW -121.65 28.29 

NE -58.54 52.41 NE -65.32 47.63 NE -94.94 51.91 

B.1.2 Response Surface Models (RSM) 

Air quality models can be a powerful regulatory tool for comparing the efficacy of 

various emissions control strategies, regulatory impacts and policy decisions. However, due to 

the often large computational costs and the complication of the required emission inputs and 

processing, using individual simulations from photochemical air quality models to meet policy 

deadlines can be inefficient. A promising tool for addressing this issue, Response Surface 

Modeling (RSM), has been developed by using advanced spatial statistical techniques to 

characterize the relationship between model outputs and input parameters in a highly economical 

manner. The RSM aggregates numerous pre-specified individual air quality modeling 

simulations into a multi-dimensional air quality "response surface." Simply, this metamodeling 

technique is a “model of the model” and can be shown to reproduce the results from an 

individual modeling simulation with little bias or error. The RSM provides a wide breadth of 

model outputs that can be used to rapidly assess air quality impacts of different control measures 

and assist in the development of multi-pollutant control strategies. Specifically, the RSM can be 

used in a variety of ways: (1) strategy design and assessment (e.g. comparison of urban vs. 

regional controls; comparison across sectors; comparison across pollutants); (2) optimization 

(develop optimal combinations of controls to attain standards at minimum cost); (3) model 

sensitivity (systematically evaluate the relative sensitivity of modeled air quality levels to 

changes in emissions inputs. 

Response surface models have been successfully applied in the context of the ozone and 

particulate matter regulatory impact analyses. In those contexts, the RSMs were used to explore 

the sensitivity of ambient ozone and PM concentrations to changes in precursor emissions, for 

the purpose of designing strategies to reduce ambient concentrations. Documentation of these 

applications can be found in the regulatory impact analyses (U.S. EPA, 2006a, 2007). 

Response surface models for nitrogen and sulfur deposition are in development and 

should be available to inform the Risk/Exposure Assessment. These models will allow us to 

explore the sensitivity of nitrogen and sulfur deposition for a number of different deposition 
December 2007 B-5 

species (reduced and oxidized nitrogen, wet and dry sulfur) to emissions of NOx, SO2, and NH3. 

The RSMs will also allow us to explore the chained relationships between emissions of NOx, 

SO2, and NH3 and ambient concentrations of NOx, NHx, and SOx, and the resulting changes in 

deposition. This will help to identify correlations or differences in the spatial patterns of the 

response of ambient concentrations versus the response of deposition. 

B.1.3 Air Quest 

AirQuest is a data management system. It stores air data in a single location and delivers 

it to various tools and applications. The data in AirQuest can be accessed via SAS, Excel, MS 

Access, Google Earth, ArcGIS, and a customized web-based interactive map. 

AIRQuest was designed to serve as a single source for integrated, high quality, readily 

available air quality data, summaries, and statistics from regulatory monitoring networks. In 

addition, AIRQuest represents an opportunity to leverage data processing and storage resources, 

since a fully functional AIRQuest will remove the need for analysts to store large amounts of 

data on their desktops. In addition, by carefully defining and coming to agreement on business 

rules that define “the right way” to calculate air quality metrics, AIRQuest can ensure that every 

analyst has a consistent approach and consistent answers. 

B.1.4 Total Risk Integrated Methodology (TRIM) 

 The TRIM design includes three individual modules which have been the subject of peer 

review and publication. Two of these modules are applicable to ecological risk assessments. The 

Environmental Fate, Transport, and Ecological Exposure module, TRIM.FaTE, accounts for 

movement of a chemical through a comprehensive system of discrete compartments (e.g., media 

and biota) that represent possible locations of the chemical in the physical and biological 

environments of the modeled ecosystem and provides an inventory, over time, of a chemical 

throughout the entire system. 

 TRIM.FaTE is a spatially explicit, compartmental mass balance model that describes the 

movement and transformation of pollutants over time, through a user-defined, bounded system 

that includes both biotic and abiotic compartments. Outputs include pollutant concentrations in 

multiple environmental media and biota, and also biota and pollutant intakes (e.g., mg/kg-day), 

all of which provide exposure estimates for ecological receptors (i.e., plants and animals). 
December 2007 B-6 

Significant features of TRIM.FaTE include: (1) a fully coupled multimedia model; (2) user 

flexibility in defining scenarios, in terms of the links among compartments, and number and 

types of compartments, as appropriate for the application spatial and temporal scale; (3) 

transparent, user-accessible algorithm and input library that allows the user to review and modify 

how environmental transfer and transformation processes are modeled; (4) a full accounting of 

all of the pollutant as it moves among environmental compartments during simulation; (5) an 

embedded procedure to characterize uncertainty and variability; and (6) the capability to provide 

exposure estimates for ecological receptors. 

 In the Risk Characterization module, TRIM.Risk, estimates of human exposures or doses 

are characterized with regard to potential risk using the corresponding exposure- or doseresponse relationships. The TRIM.Risk module is also designed to characterize ecological risks 

from multimedia exposures. The output from TRIM.Risk is intended to include documentation of 

the input data, assumptions in the analysis, and measures of uncertainty, as well as the results of 

risk calculations and exposure analysis. 

 The uncertainty and variability features (e.g., sensitivity & Monte Carlo analysis) 

augment TRIM's capability for performing iterative analyses. For example, the user may perform 

assessments varying from simple deterministic screening analyses using conservative default 

parameters to refined and complex risk assessments where the impacts of parameter uncertainty 

and variability are assessed for critical parameters. 

B.1.5 Regional Vulnerability Assessment (ReVA) 

 The Regional Vulnerability Assessment (ReVA) program conducts research on 

innovative approaches to the evaluation and interpretation of large and complex datasets and 

models to assess the current conditions and likely outcomes of environmental decisions, 

including alternative futures. ReVA works with select client groups to develop research and 

demonstration pilots on how current data and appropriate models can be combined and 

interpreted across a geographic region to set management and ecosystem protection priorities and 

to proactively assess the outcomes of decisions that may impact multiple outcomes or involve 

tradeoffs in a transparent, defensible fashion. 

 To date, ReVA has completed the development of an environmental toolkit for EPA 

Region 3 and is actively engaged in developing tools for decision making that support urban and 
December 2007 B-7 

interstate issues (SEQL), regional multipollutant issues that affect human health and ecosystems 

(Region 4), and multistate partnerships that affect the health of international environmental 

resources (Region 5/ Great Lakes). 

 

B.2 CASE STUDY MODELING 

 Integrated assessment case studies provide results for numerous environmental 

parameters, including emissions, atmospheric concentrations of pollutants, pollutant deposition 

to land and water surfaces, and changes in ecological parameters such as surface water 

chemistry, forest soil chemistry, or fish populations. In some cases, it may also be possible to 

link changes in ecological parameters to changes in ecosystem services, and ultimately to the 

economic value of the changes in ecosystem services (which is one potential measure of 

adversity of effect). By comparing the environmental impacts of existing programs with 

alternative policy scenarios, integrated assessments of this type provide estimates of the 

incremental ecological effects of potential policy changes. The process for assessing the 

ecological outcomes of different potential standards frequently focuses on relationships between 

pollutant emissions, pollutant deposition (the means of exposure for many aquatic and terrestrial 

ecosystems) and ecosystem response. Currently, analytical tools exist to analyze the emissiondeposition-ecosystem response linkage with particular focus on two ecological parameters – 

surface water chemistry and forest soil chemistry – in three case study areas: 

• Northern New England lakes; 

• Adirondack Mountain region lakes; 

• Southern Blue Ridge region streams. 

 The assessment approach involves several steps and employs various analytical and 

modeling tools that, once integrated, provide a means to assess ecological impacts. The National 

Emissions Inventory is used to develop emissions inventories for area, mobile, and point sources 

other than electricity generating utilities (EGU). The Integrated Planning Model (IPM) is used to 

develop emissions inventories for EGUs. The inventories specify profiles of emissions by 

pollutant, place, time, and source type for each policy scenario. Atmospheric models (e.g., 

CMAQ) employ the emissions inventory inputs to project changes in atmospheric concentrations 

and deposition of pollutants to land and water surfaces. Ecological impacts, such as lake and 

stream water chemistry, or changes in forest soils, are assessed by using watershed models (e.g., 

MAGIC, PnET-BGC). These models employ atmospheric modeling output (deposition changes) 
December 2007 B-8 

to project changes in the surface water chemistry of lakes and streams, or changes in forest soil 

chemistry, in regions that are sensitive to atmospheric deposition of sulfur and nitrogen species. 

Ecological impacts of policy scenario implementation are then assessed using algorithms based 

on observed relationships between ecosystem chemical parameters (e.g., acid neutralizing 

capacity, soil base saturation) and biological indicators of ecosystem health (e.g., fish species 

richness). 

 Briefly, MAGIC is a lumped-parameter model of intermediate complexity, developed to 

predict the long-term effects of acidic deposition on surface water chemistry (Cosby et al. 

1985a,b,c, 2001). The model simulates soil solution and surface water chemistry to predict 

average concentrations of the major ions. MAGIC calculates for each time step (in this case year) 

the concentrations of major ions under the assumption of simultaneous reactions involving 

sulphate adsorption, cation exchange, dissolution-precipitation-speciation of aluminium and 

dissolution-speciation of inorganic and organic carbon. MAGIC accounts for the mass balance of 

major ions in the soil by accounting for the fluxes from atmospheric inputs, chemical weathering, 

net uptake in biomass and loss to runoff. 

At the heart of MAGIC is the size of the pool of exchangeable base cations in the soil. As 

the fluxes to and from this pool change over time owing to changes in atmospheric deposition, 

the chemical equilibria between soil and soil solution shift to give changes in surface water 

chemistry. The degree and rate of change of surface water acidity thus depend both on flux 

factors and the inherent characteristics of the affected soils. Data inputs required for calibration 

of MAGIC comprise lake and catchment characteristics, soil chemical and physical 

characteristics, input and output fluxes for water and major ions, and net uptake of base cations 

by vegetation. 

PnET-BGC is a comprehensive forest-soil-water model developed by linking a monthly 

carbon, nitrogen, and water balance model PnET (Aber et al., 1997) with a soil model BGC to 

allow for comprehensive simulations of element cycling within forest and the interconnected 

aquatic ecosystems (Gbondo-Tugbawa et al., 2001). The model is able to simulate both abiotic 

processes and biotic processes. Especially the representation of biomass accumulation and the 

associated element cycling enable the evaluation of land disturbance and climatic events on soil 

and water chemistry (Gbondo-Tugbawa et al., 2001). The model uses relatively simple 

formulations and requires a moderate number of inputs to quantify the acid-base status of soil 
December 2007 B-9 

and surface waters under various levels of atmospheric deposition; therefore it can serve both as 

a research and a management tool. Its simplicity also makes it a good candidate for regional 

applications. 

B.3 ECOSYSTEM SERVICES: IMPACT AND VALUATION 

The final step 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 

· direct measures of quantities that are of known value to the public, e.g. numbers of 

endangered species, 

· direct economic valuation of ecosystem functions, 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), 

· translation of ecosystem attributes into measures of ecosystem services which can then 

be valued using economic valuation methods, and 

· direct non-economic valuation of ecosystem functions based on enumeration of 

preferences using non-monetized indices of preferences. 

Similar to other elements of the Risk/Exposure Assessment, we are seeking input on the 

range of valuation methods that should be considered in helping to inform the discussion of 

adversity of ecosystem impacts. 

B.3.1 Ecosystem services overview and why their consideration is needed 

One way to assess adverse effects on welfare is through valuation 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. The 

EPA defines ecosystem services as the outputs of healthy, intact ecosystems and the underlying 

ecosystem processes and functions that contribute to human well-being (USEPA 2006b). As 

articulated by the Millenium Ecosystem Assessment (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, etc.), supporting services (e.g. nutrient cycling, soil 
December 2007 B-10 

formation), and cultural services (e.g. recreation, spiritual) (Figure B.2). Regulating services are 

of key importance to the EPA because they directly impact air and water quality, and they have 

strong links to human heath and well-being. As such, valuation of ecosystem services may be 

one means of assessing whether an effect is adverse. Valuation provides a means for evaluating 

different services. The valuation may be monetized or non-monetized. Non-monetized measures 

of valuation include such things as quality of critical habitat, biodiversity, species composition, 

control/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. 

Figure B.2. Ecosystem services as defined by the Millennium Assessment and their linkage to 

human well being (From Millennium Assessment, 2005). 

B.3.2 Ecosystem services relating to NOx/SOx secondary standard issues 

Ecosystems provide services through a complex web of ecosystem processes that create 

and sustain the services. For example, if we are interested in how terrestrial and riparian 
December 2007 B-11 

ecosystems can help regulate nitrogen inputs to aquatic systems, we should consider all the 

nitrogen transformations and flow paths within these ecosystems (Figure B.3). Ecosystem 

processes can help mitigate adverse effects from environmental stressors, but processes can also 

be negatively affected by environmental stressors, decreasing the amount of ecosystem services 

they provide. For example, plant nutrient uptake and microbial denitrification and 

immobilization decrease the nutrient content in soil water, decreasing the potential for leaching 

into aquatic systems. Environmental stressors that decrease plant growth can increase nutrient 

flows to aquatic systems. 

The web of ecosystem processes that influence the regulation of nitrogen transport to 

aquatic systems will also affect other ecosystem services such as carbon sequestration, so that 

management activities that alter these processes will affect a bundle of services, not just one. 

The impact of a land-use change will not have the same impact on all of the ecosystem services; 

while one ecosystem service may be improved by a land-use change, it may come at a cost of 

decreasing another ecosystem service. Food production is a classic example of tradeoffs 

between ecosystem services. Adding fertilizer to crops will increase food production but may 

decrease water quality as some fertilizer ends up in the waterways. Thus, when ecosystems 

services are quantified, it is imperative that the entire bundle of services is evaluated, and that the 

tradeoffs between ecosystem services be included in the quantification. 
December 2007 B-12 

Figure B.3. The Nitrogen Cascade illustrates the complexity of pools and processes at the 

watershed scale that are interconnected with the addition of nitrogen to the system, and with the 

output of the system in terms of ecological services to benefit man. Adapted from, Galloway et 

al. 2003 (Ecological Research Program Strategic Directions, ORD, April 2007, Preliminary 

Draft) 

Energy production

NOx

People

(food; fiber)

Ozone

effects

NHx

Terrestrial

Ecosystems

Norganic

Groundwater

effects

Particulate

Matter 

effects

Stratospheric

effects

N2O Atmosphere 

Food

production

Surface water

effects

Ocean

effects

NH3

Aquatic Ecosystems

NO3

Soil

Agroecosystem

effects

Soil

Crop Animal

Forests &

Grasslands

effects

Indicates denitrification potential

Coastal

effects

NHx

NOx NOy

Greenhouse

effects

N2O

N2O

(terrestrial)

Galloway et al., 2003 
December 2007 B-13 

B.3.3 Valuation 

While not the sole means of assessing adversity of effect, ecological improvements can 

benefit people indirectly by increasing the delivery of “ecosystem services,” which are the end 

products of ecological functions important to humans (Dailey 1997, Balmford et al. 2002, NRC 

2004, Banzhaf and Boyd 2005, Boyd and Banzhaf 2006). Such valuable ecological functions 

include the maintenance of stable climate conditions, the regulation of water availability and 

quality, and nutrient retention (Dailey 1997). Through their effect on ecosystem services, 

ecological improvements may lead to improved agricultural yields, recreational opportunities, 

human health or other types of benefits. For example, protecting wetlands and the natural flow 

regulation and water purification services they provide may lead to enhanced recreational fishing 

or swimming opportunities in connected water bodies, reduced flooding in downstream 

residential areas, or reduced incidences of illness from contaminated drinking water. Ecological 

improvements may also benefit people directly through aesthetic improvements or through 

increases in “nonuse” values (e.g., NRC 2004), which can arise from a variety of motivations 

including an intrinsic concern for the existence of species populations or ecosystems in a 

relatively undisturbed state or a desire to preserve healthy ecosystems for future generations. 

Valuation is a tool for integrating information on the effects of NOx and SOx on the 

multiple ecosystem processes that form the basis for the ecosystem service. It provides a 

common currency to compare impacts. The National Research Council (2004) noted that the 

translation from ecosystem structure and function to ecosystem goods and services is described 

by an ecological production function (goods and services that can be produced from inputs of 

natural and human capital, labor, and other resources). Many ecological relationships can be 

quantified using existing data and models, at least for particular settings where data are 

quantified and models have been calibrated. A very simple example of some ecological 

processes or functions that would need to be quantified in order to determined the impact of 

nitrogen deposition on an ecological service is shown in Figure B.4. 
December 2007 B-14 

Figure B.4. Ecological Relationships Potentially disrupted by nitrogen deposition and the 

Ecosystem Service potentially affected (prepared by Dr. Bob McKane, ORD) 

A number of different approaches have been suggested for valuing ecosystem services: 

1) Provisioning services commodity values. There are methods and data for obtaining 

monetary values for commodities under scenarios of change. These break down into ‘natural 

resource values’ for extracted goods (e.g. cut and sell the tree in the market) and ‘environmental 

quality values’ to leave the tree in the forest and obtain the benefits of it in its role in the 

ecosystem. 

2) Other kinds of monetization. Economic methods that have been used to value 

ecological improvements include production or cost function approaches, travel cost models, 

hedonic property models, and stated preference surveys. Bioeconomic modeling, which involves 

combining models of species population or ecosystem dynamics with economic models of 

human behavior, is another approach that can potentially be used to value ecological 

improvements. Most bioeconomic models have been applied to fishery and forestry 

management problems, but in many cases these models could be adapted to estimate willingness 

N Deposition to Watershed 

Ecological 

Process/Function 

Alter microbial communities – biomass, 

fungal:bacterial, ectomycorrhizal 

Alter soil chemistry, organic matter 

dynamics 

Changing C:N Ratios 

Altering Plant available Nutrients (e.g. N, Ca) 

Alteration of Production 

Altered Plant Communities 

Ecosystem Service 

Habitat 

Changes in timing and amount of N leaching, 

acidity 

Food and Fiber 

Carbon sequestration 

Habitat 

Water Quality 

Alteration of lake and stream biota Water Quality 

Habitat

Surface water

effects

Ocean

effects

Surface water

effects

Ocean

effects

Surface water

effects

Ocean

effects
December 2007 B-15 

to pay for environmental improvements that may affect the growth rates or carrying capacities of 

the focal species. 

3) When direct valuation is infeasible, benefits or value transfer methods may be 

employed. This method takes direct values for ecosystem services from a primary study for a 

particular location and transfers those values to a different location using either a point estimate 

or value transfer function. Direct transfer of point value estimates has the potential to lead to 

large uncertainties and in some cases biases in estimated values for ecosystem services, because 

of the high dependency of most ecosystem values on the specific characteristics of the location in 

which they were collected. Value transfer functions which allow the values to vary with 

ecosystem characteristics can provide for more accurate estimation, but still have large 

uncertainties. 

4) Non-monetary values for individual and bundled services. These are developed from 

experimental field studies, literature and modeling activities. There are two types of nonmonetary valuation methods, one based on direct physical or biological tradeoffs, and the other 

based on tradeoffs defined by human preferences for the ecosystem functions defined by 

different bundles of physical and biological attributes. Figure B.5 illustrates a conceptual 

approach for developing non-monetary values using physical and/or biological tradeoff 

functions. 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. ERFs and ETFs will 

have to be, for the most part, defined by data mining from existing published literature. In the 

future, however, these quantifying functions will be the focus of research to broaden the scope of 

the science assessment and risk/exposure assessment. Process-based models will 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. 

Ecological response functions are developed to show the response of a service to changes in an 

ecosystem driver and ecological tradeoff functions combining response functions to show the 

tradeoffs between different services given a driver. 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 
December 2007 B-16 

preferences for different levels of ecosystem functions as expressed through ecosystem service 

levels. Figure B.6 shows an example of how ecosystem services information might be presented 

to a survey respondent in a conjoint analysis framework. 

Figure B.5. Conceptual approach for quantifying Ecological Response Functions (ERF) and 

Ecological Tradeoff Functions (ETF) for ecosystem services in relation to different levels of 

NOx or SOx (forcing variables) 

N Fertilization

Crop Yield

ERF1

N Fertilization

N Export

ERF2

Socioeconomics

Valuation & Trading of 

Ecosystem Services

ERF X-axis: Forcing Variables

• N Fertilization (rate, timing, form)

• Harvest (interval, intensity, residues)

• Climate (Temp, Precip, Light, CO2)

• Cover type (% landscape coverage) 

• Riparian buffers (width, age, species)

ERF Y-axis: Ecosystem Services

• Food/fiber yield

• Water quality

• Water quantity

• C sequestration 

• N2O, NOx, CH4

Ecosystem Service vs. Forcing Variable = ERF

N Export

ETF

Another way 

to show this 

ETF

(ES vs. ES)

Crop Yield

ERF1 + ERF2

N Fertilization

N Export

Yield

N Exp

ETF

Crop Yield
December 2007 B-17 

Figure B.6. Example Survey Design for Preference Based Ecosystem Tradeoffs 

Water Quality Habitat

Food and Fiber Production

Carbon Sequestration

Scenario 1 Drinkable Low diversity High HighScenario 2 Drinkable High diversity Low LowScenario 3 Swimmable Medium diversity Medium MediumScenario 4 Boatable Low diversity High MediumScenario 5 Swimmable Low diversity Low Low

Highly 

Desirable Quite Desirable Desirable

Slightly 

Desirable

Neither 

Desirable 

nor 

Undesirable

Slightly 

Undesirable Unde

Scenario 1 1 2 3 4 5 6 Scenario 2 1 2 3 4 5 6 Scenario 3 1 2 3 4 5 6 Scenario 4 1 2 3 4 5 6 Scenario 5 1 2 3 4 5 6 

Ecosystem Services Scenario Descriptions

Scenario Rating

Please rate how desirable each scenario is overall by circling one of the numbers in each row of the 

Example Conjoint Scenario Rating Form
United States 

Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Health and Environmental Impacts Division 

Research Triangle Park, NC 

Publication N