Risk and Exposure Assessment for Review 

of the Secondary National Ambient Air Quality 

Standards for Oxides of Nitrogen and Oxides 

of Sulfur 

First Draft, Chapters 7-8
[This page intentionally left blank.] 
EPA-452/P-08-005b 

August 2008 

RISK AND EXPOSURE ASSESSMENT FOR REVIEW OF THE SECONDARY NATIONAL AMBIENT 

AIR QUALITY STANDARDS FOR OXIDES OF NITROGEN AND OXIDES OF SULFUR 

FIRST DRAFT, CHAPTERS 7-8 

U.S. Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Research Triangle Park, NC 
DISCLAIMER 

This draft document has been prepared by staff from the Health and 

Environmental Impacts and Air Quality Analysis Divisions of the Office of Air 

Quality Planning and Standards, and the Clean Air Markets Division, Office of 

Air Programs, U.S. Environmental Protection Agency. Any opinions, findings, 

conclusions, or recommendations are those of the authors and do not necessarily 

reflect the views of EPA. This document is being circulated to obtain review and 

comment from the Clean Air Scientific Advisory Committee (CASAC) and the 

general public. Comments on this draft document should be addressed to Dr. 

Anne Rea, U.S. Environmental Protection Agency, Office of Air Quality Planning 

and Standards, C539-02, Research Triangle Park, North Carolina 27711 (email: 

rea.anne@epa.gov). 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-1 August 2008 

1 7. SYNTHESIS AND INTEGRATION OF CASE STUDY RESULTS 

2 The final version of this chapter will synthesize the results of the current conditions 

3 assessments for each of the targeted ecosystem effect areas (i.e., aquatic acidification, terrestrial 

4 acidification, aquatic nutrient enrichment, and terrestrial nutrient enrichment). Further, we will 

5 use these summaries of ecological effects and their associated ecological indicators to 

6 characterize the relationship between levels of an ecological indicator and the associated degree 

7 of ecologically adverse effects. This characterization of adversity can then be used to inform 

8 policy judgments on public welfare. Once analyses are completed, the next draft of this chapter 

9 will describe the range of effects associated with the current deposition of nitrogen and sulfur 

10 and demonstrate the links between adverse ecological effects and the level of associated 

11 ecological indicators. These relationships will then be used to inform alternative standards by 

12 translating adverse effects levels into levels of ecological indicators that can be linked through 

13 atmospheric deposition to ambient air concentrations of NOx and SOx. 

14 Conducting a joint risk assessment for two criteria pollutants that impact multiple (and in 

15 some cases overlapping) ecological processes means that disparities exist in the level of 

16 information available for the targeted ecosystem effect areas highlighted in this review. This is 

17 reflected in the nature of the science, data, models, and time available for the case study 

18 analyses. However, a concise synthesis of the information from the case studies, including an 

19 assessment of the similarities and differences in the level of scientific data available to support 

20 the risk assessments for each effect area, will be useful to inform the standard setting process. 

21 The case study synthesis is intended to provide an integrated, weight-of-evidence approach that 

22 will consider the combined information from both evidence-based (i.e., relying primarily on 

23 reference to the ISA review of published literature on the effects of the deposition of NOx and 

24 SOx) and empirical approaches (based on results of applications of data, models, and information 

25 from the published literature), and their associated uncertainties, to explain how the risk and 

26 exposure assessment can inform decision makers about what levels of protection might be 

27 appropriate to protect public welfare from adverse impacts on ecosystems. This synthesis will 

28 look at the ecological indicators selected for each of the targeted effect area and examine the 

29 commonalities and differences among them. This information can then be used to inform a 

30 policy judgment on a known or anticipated adverse public welfare effect. 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-2 August 2008 

1 This chapter summarizes the current progress of the case studies (Section 7.1), begins to 

2 discuss how the results of the case studies can be used to inform potential levels of protection 

3 against ecosystem effects (Section 7.2), and preliminarily points to how this might be used in 

4 designing a meaningful secondary standard or standards for ambient concentrations of NOx and 

5 SOx (Section 7.3). In this draft, we focus on the ecological effects of aquatic acidification, using 

6 surface water ANC as an ecological indicator. As our analyses progress, we will include 

7 additional ecological indicators and explain how they may be used to characterize adverse 

8 ecological effects. 

9 7.1 SUMMARY OF CASE STUDY ANALYSES 

10 Table 7.1-1 summarizes the ecosystem characteristics, ecological indicators, ecological 

11 effects, and ecosystem services evaluated in the case study analyses presented in Chapters 4 and 

12 5. Note: currently these results reflect the present status of the first draft risk and exposure 

13 assessment and will be updated for the second draft as the analyses progress. 

14 7.1.1 Aquatic Acidification 

15 Acidic deposition has altered major biogeochemical processes in the United States by 

increasing the sulfur and nitrogen content of soils. This increase accelerates SO4

2- and NO3! 16 

leaching from soil to drainage water, depletes base cations (namely Ca2+ and Mg2+ 17 ) from soils, 

18 and increases the mobility of toxic aluminum (Al) (see ISA Section 3.2) (U.S. EPA, 2008). Some 

19 ecosystems and some areas of the United States are more sensitive than others to the effects of 

20 acidic nitrogen and sulfur deposition, particularly streams and lakes with naturally low ANC. 

21 The impact of acidic deposition on aquatic systems is controlled by several environmental 

22 factors, such as geology, surface water flow, soil depth, and weathering rates, all of which 

23 contribute to the ability of an ecosystem to neutralize the additional acidic deposition and prevent 

24 the lowering of ANC. The Adirondack Mountains of New York and the Blue Ridge Mountains 

25 in the Shenandoah National Park of Virginia were selected for the evaluation of impacts from 

26 acidic deposition of nitrogen and sulfur. These regions receive relatively high levels of acidic 

27 deposition and are known to be predisposed to acidification, and extensive research, data, and 

28 models exist for these areas. In addition, the results will likely be relevant in characterizing risks 

29 to larger areas of the Appalachian Mountains and the Northeast. 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-3 August 2008 

1 ANC is a key indicator of acidification with relevance to terrestrial and aquatic 

2 ecosystems. ANC is useful because it integrates the overall acid-base status of a lake or stream 

3 and reflects how aquatic ecosystems respond to acidic deposition over time. There is also a 

4 relationship between ANC and the surface water constituents that directly contribute to or 

ameliorate acidity-related stress, in particular, concentrations of hydrogen ion (as pH), Ca2+ 5 , and 

6 aluminum. Moreover, low pH surface waters leach aluminum from soils, which is quite lethal to 

7 fish and other aquatic organisms. In aquatic systems, there is a direct relationship between ANC 

8 and fish and phyto-zooplankton diversity and abundance (see Section 7.3). These relationships 

9 also impact several ecosystem services, such as fishing, recreation, food, natural habitat, and 

10 tourism. 

11 7.1.2 Terrestrial Acidification 

12 Terrestrial acidification occurs where inorganic and mineral acids are added to the soil. 

13 This occurs as a result of both natural biogeochemical processes and acidic deposition of 

14 nitrogen and sulfur. Biological effects of acidification on terrestrial ecosystems are generally 

15 attributable to aluminum toxicity and decreased ability of plant roots to take up base cations. 

16 Acidic deposition increases concentrations of sulfur and nitrogen in soil, which accelerates 

leaching of SO4

2- and NO3- 17 from soil to drainage water. In the absence of anthropogenic sulfur 

18 and nitrogen deposition, the limited mobility of anions in the soil controls the rate of base cation 

19 leaching. Atmospheric deposition of sulfur and nitrogen provides anions that are more mobile in the 

20 soil environment than naturally occurring anions in the soil; these mineral acid anions can accelerate 

natural rates of base-cation leaching, particularly Ca2+ and Mg2+ 21 , leading to mobilization of inorganic 

22 aluminum, which is toxic to tree roots. Acidic deposition can also affect terrestrial ecosystems by 

23 causing direct impacts on plant foliage.

24 The regions considered to be most sensitive to terrestrial acidification effects due to 

25 acidic deposition are forested ecosystems of the Adirondack Mountains of New York, Green 

26 Mountains of Vermont, White Mountains of New Hampshire, Allegheny Plateau of 

27 Pennsylvania, and high-elevation forests in the southern Appalachians. The Kane Forest in the 

28 Allegheny Plateau of Pennsylvania and the Hubbard Brook Experimental Forest in the White 

29 Mountains of New Hampshire were selected to evaluate the effects of acidic deposition on the 

30 sugar maple and red spruce due to known sensitivity of these species in these areas and the 

31 availability of data. The ecosystem services affected include food, natural habitat, and tourism. 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-4 August 2008 

1 For our case study areas, we anticipate that sugar maple and red spruce abundance and growth 

2 (i.e., crown vigor, biomass, and geographic extent) will be quantitatively linked to acidification 

3 symptoms (specifically the calcium to aluminum ratio in the soil) through U.S. Forest Service 

4 Forest Inventory and Analysis (FIA) database analyses and the analysis of estimated sales of 

maple sugar products for the second draft risk and exposure assessment. 5 1

6 7.1.3 Aquatic Nutrient Enrichment 

7 Nitrogen is an essential nutrient for estuarine and marine ecosystem fertility and is often 

8 the limiting nutrient in aquatic ecosystems. Excessive nitrogen contributions can cause habitat 

9 degradation, algal blooms, toxicity, hypoxia (i.e., reduced dissolved oxygen), anoxia (i.e., 

10 absence of dissolved oxygen), fish kills, and decreases in biodiversity. A recent national 

11 assessment of estuary trophic conditions used five biological indicators to assess nutrient 

12 enrichment: chlorophyll a, macroalgae, dissolved oxygen, nuisance/toxic algal blooms, and 

13 SAV. This assessment is discussed in detail in Attachment 5. 

14 The Chesapeake Bay and Pamlico Sound were chosen as case study areas based on the 

15 availability of atmospheric deposition data, water quality modeling data that accounted for 

16 atmospheric deposition, a large main stem river, and scalability. Because the Chesapeake Bay 

17 and Pamlico Sound are fed by multiple river systems, the case study areas were scaled to one 

18 main stem river for each system: the Potomac River Basin for the Chesapeake Bay and the Neuse 

19 River Basin for the Pamlico Sound. A eutrophication index, developed by National Oceanic and 

20 Atmospheric Administration’s (NOAA’s) ) National Estuarine Eutrophication Assessment 

21 (NEEA) was used in this analysis, which estimates the likelihood that the estuary is experiencing 

22 eutrophication or will experience eutrophication in the future. The endpoints of interest are fish 

23 populations; water quality; and habitat quality and their related ecosystem services, including 

24 fisheries (e.g., closings, decreased species richness), recreation, and tourism. 

25 7.1.4 Terrestrial Nutrient Enrichment 

26 Terrestrial ecosystems typically respond to total reactive nitrogen loading and, with few 

27 exceptions, have limited differential responses to the specific chemical species of deposited 

28 nitrogen. Nitrogen compounds that are contained in atmospheric deposition, and that influence 

 

1 The FIA can be found at http://fia.fs.fed.us/. 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-5 August 2008 

1 nutrient dynamics, include multiple oxidized and reduced forms of inorganic nitrogen. Excess 

2 nitrogen in ecosystems causes inadvertent fertilization of trees and grasslands, creating unnatural 

3 growth rates in some species and changing competitive interactions among species, and nutrient 

4 imbalances. These impacts ultimately decrease ecosystem health and biodiversity. Forest growth 

5 enhancement can potentially exacerbate other nutrient deficiencies, such as Ca, Mg, or K, 

6 thereby causing problems with forest health. Enhanced growth generally occurs above ground 

7 level. This can cause changes in the shoot-to-root ratio, causing decreased resistance to 

8 environmental stressors, such as drought. In conifer forests, multiple long-term experiments (see 

9 Attachment 6) have demonstrated transient growth increases (generally at deposition rates lower 

10 than 10 kg N/ha/yr) followed by increased mortality, especially at higher rates of fertilization. In 

11 the western United States, atmospheric nitrogen deposition has been shown to cause increased 

12 litter accumulation and carbon storage in aboveground woody biomass, which, in turn, may lead 

13 to increased susceptibility to more severe fires. Grassland communities that are adapted to low 

14 nutrient supply can exhibit substantial sensitivity to nutrient enrichment effects of nitrogen 

15 deposition.

16 Two of the primary indicators of nitrogen enrichment in forested watersheds are the 

leaching of NO3

- in soil drainage waters and the export of NO3- 17 in stream water, especially during 

18 the growing season. Low C:N ratios in soils are also commonly related to increased nitrification, 

19 potential increases in soil acidity, and releases of nitrate to receiving waters; however, these 

20 measurements are not always widely available. 

21 California’s CSS and mixed conifer forest communities were selected for an initial case 

22 study analysis. These areas were selected based on the availability of atmospheric ambient and 

23 deposition data; the availability of digitized datasets of biotic communities; fire-prone areas; 

24 sensitive, rare species; their representativeness of western United States ecosystems potentially 

25 impacted by nitrogen deposition; and their scalability and generalization potential. 

26 California’s CSS and mixed conifer forests on the slopes of the San Bernardino and 

27 Sierra Nevada Mountains have important recreational value, protect water resources, and provide 

28 habitats for many other species. In the mixed conifer forest, lichen communities and nitrogen 

29 saturation can provide a means to identify the effects of nitrogen loadings. Both habitats provide 

30 a number of ecological services, including biodiversity, water quality, recreation, timber, and fire 

31 hazard mitigation. Linkage methods from endpoint to services may include measurement of 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-6 August 2008 

1 changes in biodiversity and abundance and distribution of threatened and endangered species, 

2 comparison of past and present photography, and measurement of the distribution of soil 

3 moisture with depth and possible nitrate leaching. 

4 7.1.5 Sulfur and Mercury Methylation 

5 The biogeochemical cycle of mercury, a well-known neurotoxin, is closely tied to the 

6 sulfur cycle. Mercury is taken up by living organisms in the methylated form, which is easily 

7 bioaccumulated in the food web. Sulfate-reducing bacteria in wetland and lake sediments play a 

key role in mercury methylation. Changes in sulfate2 8 deposition have resulted in changes in both 

9 the rate of mercury methylation and the corresponding mercury concentrations in fish. 

10 Although sulfur deposition is important to mercury methylation, several other interrelated 

11 factors seem to also be related to mercury uptake, including low lake water pH, dissolved 

12 organic carbon, suspended particulate matter concentrations in the water column, temperature, 

13 and dissolved oxygen. In addition, the proportion of upland to wetland land area within a 

14 watershed, as well as wetland type and annual water yield, appear to be important. 

15 The major ecosystem services potentially impacted by sulfur-mediated mercury 

16 methylation are provisioning and cultural services. Fishing and shellfishing can involve both 

17 commercial operations and sports fishing, which provide food for human populations. For some 

18 socioeconomic groups (especially groups with low incomes), fishing is a subsistence activity that 

19 makes a very significant contribution to household food intake. Sport fishing often involves 

20 important recreational services, and for many groups (e.g., Native Americans, Alaska native 

21 villagers), fishing and consuming local fish or shellfish is of cultural and spiritual significance. 

 

2 EPA notes that in its review of the primary NAAQS for sulfur oxides, it has made the decision to examine all 

sulfate health effects as part of the review of the PM NAAQS (sulfates being one of the chief types of secondary 

PM). EPA believes it appropriate; however, to consider secondary sulfate effects as part of this review, rather than 

in the PM NAAQS review, because the effect at issue is related specifically to chemical composition. The fact 

that sulfate is deposited as a particle is not relevant to its contribution to mercury methylation. 
Chapter 7 – Synthesis and Integration 

DRAFT 7-7 Table 7.1-1. Summary of Characteristics, Ecological Indicators, Effects, and Ecosystem Services Impacted for E

Evaluated in This Review 

Targeted Effect Area 

Characteristics of Sensitivity (Variable Ecological Factors) 

Biological/Chemical Characteristic 

Ecological Indicator 

Ecological Effects Ecosystem 

Services Impacted 

Case Study Locations 

Aquatic Acidification 

Geology, surface water flow, soil depth, weathering rates 

[Al] pH ANC 

Species richness, abundance, composition ANC 

Species losses of fish, phytoplankton, zooplankton; changed community composition, ecosystem structure, and function 

Fisheries, recreation, tourism 

Adirondack Mountains, NY Blue Ridge Mountains, Shenandoah National ParVA 

Terrestrial Acidification 

Geology, surface water flow, soil depth, weathering rates 

Soil base saturation [Al] [Ca] C:N ratio Ca:Al ratio 

Tree health Red spruce, sugar maple ANC Ca:Al ratio 

Decreased tree growth Increased susceptibility to stress, episodic dieback; changed community composition, ecosystem structure, and function 

Food, natural habitat, tourism 

Kane Forest (Allegheny Plateau, PA) Hubbard Brook ExperimentaForest (WhitMountains, NH) 

Aquatic Nutrient Enrichment 

N-limited systems, presence of nitrogen in surface water, eutrophication status, nutrient criteria 

Chlorophyll a, 

macroalgae, dissolved oxygen, nuisance/toxic algal blooms, submerged aquatic vegetation (SAV) 

Eutrophication Index (EI) 

Habitat degradation, algal blooms, toxicity, hypoxia, anoxia, fish kills, decreases in biodiversity 

Fish populations, water quality, habitat quality 

Potomac RivBasin, Chesapeake Bay Neuse River Basin, PamlSound 
Chapter 7 – Synthesis and Integration 

DRAFT 7-8 Targeted Effect Area 

Characteristics of Sensitivity (Variable Ecological Factors) 

Biological/Chemical Characteristic 

Ecological Indicator 

Ecological Effects Ecosystem 

Services Impacted 

Case Study Locations 

Terrestrial Nutrient Enrichment 

Presence of acidophytic lichens, anthropogenic land cover 

Cation Exchange Capacity, C:N ratios, Ca:Al ratios, NO3-

leaching and export 

Species composition 

Species changes, nitrogen enrichment of soil, changes in fire regime, changes in nutrient cycling 

Loss of habitat, loss of biodiversity, recreation, water quality 

Coastal sage scrub and mixed conifeforest (San Bernardino and Sierra Nevada Mountain Ranges, CA) 

Sulfur and Mercury Methylation Potential 

Wetland type, presence of sulfatereducing bacteria, water pH, dissolved organic carbon, suspended particulate matter 

Interaction among dissolved organic carbon, temperature, anoxia, and sulfide Land cover, precipitation response, and limnography 

MeHg concentrations in fish and shellfish 

Neurotoxic effects in fish and throughout food web 

Fishing, shellfishing, sports fishing, food, recreation, biodiversity 

Little Rock Lake, WI (IScase study) 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-9 August 2008 

1 7.2 INTEGRATING CASE STUDY RESULTS 

2 The ISA and case study analyses have shown that, in certain ecosystems, welfare effects 

3 are occurring under current ambient concentrations of NOx and SOx, even though nationwide 

4 (with only a few exceptions) we are in attainment of the current primary and secondary NO2 and 

5 SO2 NAAQS. The next step in the risk and exposure assessment process is to determine what 

6 alternative levels of ambient NOx and SOx might allow (1) various degrees of recovery of 

7 impacted systems, (2) acceleration of recovery of impacted systems, and (3) protection against 

8 further damage, recognizing the variability in ecosystem sensitivities to nitrogen and sulfur 

9 deposition. At this point in the risk and exposure assessment, we have begun to address many of 

10 the policy-relevant questions guiding this review. Our initial case study analyses have begun to 

11 show the variability associated with these responses and the degree to which ecological 

12 indicators are suited to address a particular type of effect (i.e., acidification or nutrient 

13 enrichment in terrestrial or aquatic ecosystems). 

14 There are many factors that determine whether or not an ecological effect occurs in 

15 response to ambient concentrations of NOx and SOx. These “variable factors” may refer to either 

16 ecological factors or atmospheric factors, both of which influence deposition or exposure and the 

17 associated ecological effects (i.e., acidification or nutrient enrichment). The degree to which any 

18 given level of atmospheric deposition of nitrogen and sulfur impacts ecosystems depends in large 

19 part on a suite of variable ecological factors at the point of deposition, including geology, surface 

20 water flow, soil depth, and weathering rates. The levels of these ecological factors in a given 

21 location determine the ecological conditions governing sensitivity to atmospheric deposition of 

22 nitrogen and sulfur. The linkages between a given ecological indicator and deposition, as 

23 influenced by the variable ecological factors, are discussed in Chapter 8. 

24 In the next draft of the risk and exposure assessment, we will look across the results of 

25 the case studies to identify ecological indicators that provide the most comprehensive coverage 

26 across sensitive ecosystems. These indicators will form the basis for assessing links between 

27 adverse effects and potential levels of an ecological indicator to protect against those effects. At 

28 this point, the Aquatic Acidification Case Study analysis is the furthest along and will be used to 

29 illustrate how an ecological indicator of acidification (i.e., ANC) and the levels of ecological 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-10 August 2008 

1 effects associated with changes in that indicator can be used to characterize adversity to public 

2 welfare. 

3 7.3 LINKAGES BETWEEN ECOLOGICAL INDICATORS AND ADVERSE 

4 EFFECTS 

5 Within the limitations of currently available data and models, our case study analyses 

6 indicate the degree, if any, to which adverse ecological effects are already occurring in specific 

7 sensitive ecosystems at current levels of ambient nitrogen and sulfur deposition. Note that 

8 ecological recovery may not immediately result from decreases in atmospheric deposition, but 

9 may lag behind changes in atmospheric deposition. Since current ecological conditions reflect 

10 past deposition and responses to it, it is possible that an ecological indicator such as ANC may 

11 improve over time (years) under current deposition levels. However, the recovery rate may be 

12 affected by current levels of deposition, with lower levels of deposition leading to a more rapid 

13 recovery compared to current levels. 

14 7.3.1 ANC: The Critical Chemical Parameter 

15 Ambient air concentrations are linked to adverse ecological effects via the process of 

16 atmospheric deposition. For our purposes, it is necessary to identify a suitably generalized 

17 ecological indicator for these effects because linking individual ecological endpoints directly to 

18 atmospheric deposition is difficult. The relationship between ecological effects associated with 

19 nitrogen and sulfur deposition and a commonly measured and well understood ecological 

20 indicator, ANC, is described below. In addition, we conceptually describe linking ecological 

21 effects with potential measures of adversity to public welfare by characterizing impacts on 

22 ecosystem services. This information can then help to inform judgments about the extent to 

23 which different levels of a particular ecological indicator might affect public welfare. 

24 The susceptibility of a catchment to terrestrial or aquatic acidification by nitrogen and 

25 sulfur deposition is controlled by the ability of the watershed to neutralize the addition of 

nonmarine acid anion concentrations of SO4

2- or NO3- 26 to the catchment. The neutralizing ability 

27 of a catchment is a function of the acid-base balance; the balance between the sum of base 

cations (i.e., Ca* + Mg* + K* + Na*) inputs minus the sum of acid anions (i.e., SO4

2- or NO3- 28 ) 

29 inputs. In general, catchments with positive acid-base balance have base cations inputs that 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-11 August 2008 

1 exceed acid anions inputs which “buffer” the watershed from becoming acidic. The ability of a 

2 catchment to buffer acid anion deposition is known as the “buffering capacity,” which is 

3 controlled by a host of environmental factors within the watershed. Such factors include base 

4 cation leaching rate, base cation weathering rates, base saturation, uptake by vegetation, rate of 

5 surface water flow, soil depth, bedrock, etc. The buffering capacity of a catchment is controlled 

6 by the flux of base cations, which act to balance acid anion inputs. The effect of acid anion 

7 deposition is to lower this acid-base balance, causing a decline in the ecological system’s 

8 composition, structure, and the biological fitness of its organisms over time. 

9 The ANC of surface waters is widely used as a chemical indicator of acidic conditions 

10 because it has been found in many studies to be the best single indicator of the biological 

11 response and health of aquatic communities in acid-sensitive systems (Lien et al., 1992; Sullivan 

12 et al., 2006). Logistic regression of species presence/absence data against ANC provides a 

13 quantitative dose-response function, which indicates the probability of occurrence of an 

14 organism for a given value of ANC. For example, the number of fish species present in a 

15 waterbody has been shown to be positively correlated with the ANC level in the water, with 

16 higher values supporting a greater richness and diversity of fish species (Figure 7.3-1). The 

17 diversity and distribution of phyto-zooplankton communities are also positively correlated with 

18 ANC. 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-12 August 2008 

1 

2 Figure 7.3-1. Number of fish species per lake or stream versus acidity status, expressed 

3 as ANC in lakes in the Adirondacks of New York (Sullivan et al., 2006). Five classes: <0, 

4 0–20, 20–50, 50–100 μeq/L, and >100 μeq/L describe the biological effects at a range of 

5 ANC concentrations. See Table 7.3-1 for more details.

6 For freshwater systems, ANC levels can be grouped into five major classes: <0, 0–20, 

7 20–50, 50–100 μeq/L, and >100 μeq/L with each range representing a probability of ecological 

8 damage to the community (see Attachment 3). ANC values above 100 μeq/L are generally not 

9 harmful (Figure 7.3-1) to biota. Below 100 μeq/L, it has been shown that fish fitness and 

10 community diversity begin to decline, but the overall health of the community remains high as 

11 long as ANC concentrations do not go below 50 μeq/L. ANC concentrations below 50 μeq/L 

12 result in negative effects on sensitive biota. From 50 to 20 μeq/L, it has been shown that fish 

13 diversity and the overall fitness (i.e., health and reproduction) of most aquatic organisms in the 

14 waterbody are reduced. Below 20 μeq/L, all biota exhibit some level of negative effects, 

15 particularly because surface waters at this level are susceptible to episodic acidification and their 

16 associated harmful effects. Fish and plankton diversity and the structure of the communities 

17 continue to decline sharply to levels where acidophilic species begin to outnumber all other 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-13 August 2008 

1 species. Below an ANC of 0 μeq/L, complete loss of fish populations and extremely low 

2 diversity of planktonic communities occur. At these low levels, only acidophilic species are 

3 present, but even their population and community structure are sharply reduced. The five 

4 categories of ANC and expected ecological effects are described in Table 7.3-1 and are 

5 supported by a large body of research completed throughout the eastern United States (Sullivan 

6 et al., 2006). The connection between the ecological effects described in Table 7.3-1 and 

7 ecosystem services that can be indicative of adverse impacts to public welfare will be explored in 

8 the second draft risk and exposure assessment. However, examples of these ecosystem services 

9 include recreational and subsistence fishing, recreational lake use, natural habitat provision, and 

10 tourism. Potential values of these ecosystem services related to alternative levels of ANC will be 

11 assessed in the second draft risk and exposure assessment. 

Table 7.3-1. Aquatic Status Categories 

Category Label ANC Levels Expected Ecological Effects 

Acute 

Concern 

<0 micro 

equivalent 

per Liter 

(μeq/L) 

Complete loss of fish populations is expected. Planktonic 

communities have extremely low diversity and are dominated 

by acidophilic forms. The numbers of individuals in plankton 

species that are present are greatly reduced. 

Severe 

Concern 

 0–20 μeq/L Highly sensitive to episodic acidification. During episodes of 

high acidic deposition, brook trout populations may experience 

lethal effects. Diversity and distribution of zooplankton 

communities decline sharply. 

Elevated 

Concern 

20–50 μeq/L Fish species richness is greatly reduced (i.e., more than half of 

expected species are missing). On average, brook trout 

populations experience sub-lethal effects, including loss of 

health and reproduction (fitness). Diversity and distribution of 

zooplankton communities decline. 

Moderate 

Concern 

50–100 

μeq/L 

Fish species richness begins to decline (i.e., sensitive species 

are lost from lakes). Brook trout populations are sensitive and 

variable, with possible sub-lethal effects. Diversity and 

distribution of zooplankton communities begin to decline as 

species that are sensitive to acidic deposition are affected. 

Low 

Concern 

>100 μeq/L Fish species richness may be unaffected. Reproducing brook 

trout populations are expected where habitat is suitable. 

Zooplankton communities are unaffected and exhibit expected 

diversity and distribution. 

12 Whether the biological community of a waterbody is at risk from current deposition loads 

13 depends on the ability of the watershed to neutralize the incoming acid anion deposition and 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-14 August 2008 

1 maintain an acid-base balance or ANC level that protects the biological community. This linkage 

2 between deposition and biological processes is derived from a series of empirical relationships, 

3 that relate the sustainable buffering capacity (preindustrial) of a catchment to the maximum flux 

4 of acidic deposition which will maintain a particular level of ANC concentration in the surface 

5 water. ANC is operationally defined as the sum of base cations minus the sum of acid anions, 

6 with the proportion of ions deriving from neutral sea-spray removed: 

 ANC = [BC]t

*

 - [AA]t

* 7 (1) 

where [BC]t

* 8 is the current, measured sum of nonmarine base cation equivalent concentrations 

(i.e., Ca* + Mg* + K* + Na*) and [AA]t

* 9 is the sum of nonmarine acid anion equivalent 

10 concentrations (i.e., SO4* + NO3*). It is assumed that all chloride is derived from marine 

11 sources. The * denotes the nonmarine component, whereby the marine contribution of each ion is 

12 subtracted as a proportion of measured chloride concentration from the known ratio of these ions 

13 in seawater. The equation defining ANC forms the basis of the linkage between deposition and 

14 surface water ANC concentrations. This linkage is explored in greater detail in Chapter 8. 
Chapter 7 – Synthesis and Integration of Case Study Results 

DRAFT 7-15 August 2008 

1 7.4 REFERENCES 

2 Lien, L., G.G. Raddum, and A. Fjellheim. 1992. Critical Loads of Acidity to Freshwater: Fish 

3 and Invertebrates. The Environmental Tolerance Levels Programme. Rep. No. 23/1992. 

4 Norwegian Ministry of Environment, Oslo, Norway. 

5 Sullivan, T.J., C.T. Driscoll, B.J. Cosby, I.J. Fernandez, A.T. Herlihy, J. Zhai, R. Stemberger, 

6 K.U. Snyder, J.W. Sutherland, S.A. Nierzwicki-Bauer, C.W. Boylen, T.C. McDonnell, 

7 and N.A. Nowicki. 2006. Assessment of the Extent to Which Intensively-Studied Lakes 

8 are Representative of the Adirondack Mountain Region. Final report. Albany, NY: New 

9 York State Energy Research and Development Authority (NYSERDA). Available at 

10 http://nysl.nysed.gov/uhtbin/cgisirsi/Qcwd6NzFby/NYSL/138650099/8/4298474 

11 (accessed November 1, 2007). 

12 U.S. EPA (Environmental Protection Agency). 2008. Integrated Science Assessment for Oxides 

13 of Nitrogen and Sulfur – Environmental Criteria (Second External Review Draft). 

14 Prepared by U.S. Environmental Protection Agency, National Center for Environmental 

15 Assessment-RTP Division, Office of Research and Development, Research Triangle 

16 Park, NC. EPA/600/R-08-082. August. 
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Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-1 August 2008 

1 8. CONSIDERATIONS IN THE STRUCTURE OF NOX/SOX

2 SECONDARY STANDARD 

3 The previous chapters have provided an understanding of the risks associated with 

4 current deposition of nitrogen and sulfur associated with ambient atmospheric concentrations of 

5 NOx and SOx. Given that information, the next step is to provide a framework for developing 

6 standards for NOx and SOx that provide the appropriate level of protection against those risks. 

7 The purposes of this chapter are 1) to propose one possible framework for a NOx and SOx

8 secondary standard(s), based on the linkages that exist between ambient atmospheric 

9 concentrations of NOx and SOx, atmospheric deposition of N and S, ecological effects and the 

10 related ecological indicators and 2) to illustrate how such a standard might be expressed for 

11 aquatic acidity using data from lakes in the Adirondacks case study area. The framework is 

12 designed to both meet statutory and legal requirements, and to reflect the complex nature of 

13 atmospheric and ecological systems. 

14 In this chapter, we discuss constructing a standard based on achieving a uniform level of 

15 ecosystem protection. We explore a framework by which ambient atmospheric concentrations of 

16 NOx and SOx can be translated into a measure of ecosystem effects, using the transformation 

17 functions described below. To facilitate a more concrete understanding of this framework, we 

18 illustrate how the atmospheric concentration standards can be calculated using the Adirondacks 

19 case study example. 

20 The remainder of this chapter is organized as follows. In Section 8.1, we discuss a 

21 possible structure for a secondary NAAQS for NOx and SOx, building on the conceptual model 

22 introduced in Chapter 1. In section 8.2, we present a discussion of the statutory and legal 

23 considerations that affect the design of an ecologically relevant ambient air quality standard for 

24 NOx and SOx. Section 8.3 discusses the framework for structuring a standard which links 

25 atmospheric concentrations of NOx and SOx to a specific ecological indicator of effects (i.e., 

26 ANC) through the deposition process, taking atmospheric and ecological factors into account. It 

27 also builds upon the current conditions risk and exposure assessment presented in Chapters 3 

28 through 6 as well as the linkages presented in Chapter 7 to discuss the nature of a meaningful 

29 standard in light of the wide range of ecological effects resulting from deposition of NOx and 

30 SOx. In so doing, Section 8.3 expands on the standards framework through which the linkages 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-2 August 2008 

1 between ambient atmospheric concentrations of NOx and SOx, deposition of N and S, and 

2 ecological indicators can be described. Section 8.4, which is being developed for inclusion in the 

3 next draft, will describe an illustrative example of how ambient atmospheric concentrations for 

4 NOx and SOx can be compared to a desired level of ecological protection, as expressed using the 

5 ecological indicator ANC. This example will also show how observed (or modeled) ambient 

6 atmospheric concentrations of NOx and SOx can be transformed into ANC values for 

7 comparison against the level of a standard defined in terms of a value of the ecological indicator. 

8 This example will also provide step-by-step calculations to show how different combinations of 

9 atmospheric concentrations of NOx and SOx can achieve the same level of ANC, based on the 

10 deposition transformation functions and ecological effect functions that are described in Section 

11 8.3. In each case, the example calculations will show clearly how the standards framework can 

12 incorporate 1) the influence of atmospheric factors that affect deposition, 2) the influence of 

13 ecological factors that affect ANC, and 3) the influence of reduced forms of nitrogen (ammonia 

14 and ammonium). 

15 

16 8.1 POSSIBLE STRUCTURE OF A SECONDARY NAAQS 

17 Throughout this assessment, we have recognized there are important linkages between 

18 ambient nitrogen and sulfur and their depositional effects on the environment. Our approach to 

19 this joint, multi-pollutant review of two secondary standards has been to evaluate the science 

20 driving welfare effects and consider what is needed to develop a secondary standard or suite of 

21 standards that appropriately protects public welfare. The structure of the current secondary 

22 standards for NOx and SOx is based on the ambient concentration of a single indicator that is 

23 specified as a nitrogen- or sulfur-containing compound (i.e., NO2 and SO2, respectively) where 

24 the level of the indicator does not necessarily correlate well with the ecological effects of 

25 concern (USEPA, 2008). As discussed in Chapter 7, we recognize that the scientific basis that 

26 forms our understanding of the relationship between the criteria air pollutants, NOx and SOx and 

27 their associated public welfare effects supports the use of a more complex structure for the 

28 secondary standard(s). The inherently complex and variable linkages between ambient 

29 concentrations of NOx and SOx, their deposited forms of nitrogen and sulfur, and the ecological 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-3 August 2008 

1 responses that are associated with public welfare effects call for a more complex standard design 

2 that reflects these linkages. 

3 A critical difference between the current risk and exposure assessment and those from 

4 previous reviews is that in the current assessment, exposures and ecological effects are related to 

5 current levels of nitrogen and sulfur deposition in selected case study locations, rather than just 

6 the ambient concentrations of NOx and SOx in those locations. Based on the recognized linkages 

7 between ambient concentrations of NOx and SOx and deposition of nitrogen and sulfur (see 

8 Chapter 3), as well as the linkages between deposition and ecosystem effects, a specified level of 

9 ecosystem effect will require limits on ambient concentrations of NOx and SOx, all other factors 

10 being held constant. A standard that is expressed in terms of atmospheric concentrations of NOx

11 and SOx will require a process to translate ambient concentrations through the deposition process 

12 into ecological effects, to determine if the specified level of ecosystem effect is achieved This 

13 atmospheric concentration-based standard will explicitly identify the role of modifying factors in 

14 affecting the levels of NOx and SOx concentrations that would be necessary to achieve the 

15 standard. 

16 A possible structure for such standards is shown below in Figure 8.1-1, which builds 

17 upon the conceptual model presented in Chapter 1. Figure 8.1-1 represents the pathway from 

18 atmospheric concentrations of NOx and SOx to an ecological effect (aquatic acidification) as 

19 represented by an ecological indicator (ANC), through the processes of deposition, with each of 

20 the steps along the pathway expanded to reflect the example of aquatic acidification using the 

21 ecological indicator ANC. Suites of variable factors reflecting both ecological and atmospheric 

22 complexity modify the pathways. Critical to developing an ecologically appropriate secondary 

23 standard for NOx and SOx is understanding that the different levels of the ecological indicator 

24 can be associated with different levels of protection against adverse public welfare effects. The 

25 ambient standard is then specified as the level(s) of ambient NOx and SOx such that the 

26 calculated ecological indicator meets the level of protection of public welfare set by the 

27 Administrator. Thus, the standard is expressed in terms of the relationship between ambient NOx

28 and SOx and the ecological indicator, linked through the deposition of N and S. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-4 August 2008 

1 

2 Figure 8.1-1. Possible structure of a secondary NAAQS for NOx and SOx based 

3 on the ecological indicator ANC for the effect of acidification. 

4 

5 The variable atmospheric factors which influence atmospheric conditions affecting the 

6 transformation, transport and deposition of ambient concentrations of NOx and SOx include 

7 relative humidity, precipitation, wind conditions, and chemical reactivity. Deposition is also 

8 affected by seasonally dependent landscape factors such as leaf surface area. Since deposition is 

9 the driver for many of the associated ecological effects due to NOx and SOx, such understanding 

10 of the factors that link atmospheric concentrations with deposition is critical. In addition, the 

11 relative contribution of NOx to total deposition of nitrogen is affected by the ambient 

12 concentrations of ammonia and ammonium. The linkages between atmospheric concentrations 

13 and deposition as influenced by the variable atmospheric and landscape factors are discussed in 

14 Section 8.3.1. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-5 August 2008 

1 The degree to which any given level of deposition of nitrogen and sulfur containing species 

2 impacts ecosystems depends in large part on a suite of variable ecological factors at the point of 

3 deposition, including soil base saturation, soil thickness, runoff rate, land use conditions, nutrient 

4 cycling, and non-atmospheric loadings of sulfur and nitrogen. The particular levels of these 

5 factors that occur at any location form the ecological baseline against which ecological effects 

6 from deposition are evaluated. The linkages between deposition and ecological effect as 

7 influenced by the variable ecological factors are discussed in Section 8.3.2. 

8 The relationships illustrated in Figure 8.1-1 in concept can be operated in either direction, 

9 from known or assumed ambient atmospheric concentrations to the resulting ecological effects, 

10 or from desired ecological conditions in an area of interest to ambient concentrations that would 

11 achieve them. In either approach, there may not be a unique combination of ambient NOx and 

12 SOx that achieves a specified level of ecological protection, because of the multiple 

13 combinations of NOx and SOx concentrations that can yield the same level of ecological 

14 indicator. This is particularly true for the case of aquatic acidity, but may not hold true for other 

15 ecological endpoints. Whether precipitation occurs or not also greatly affects the relationship 

16 between ambient atmospheric concentrations of NOx and SOx and deposition of N and S, so 

17 knowledge or an assumption of precipitation is needed to complete the quantitative trail from 

18 ambient concentrations to ecological effects, or the reverse. Finally, concentrations and 

19 deposition of reduced nitrogen species influence ecological conditions, but these species are not 

20 currently encompassed by the listed criteria pollutant NOx. While reduced species of nitrogen do 

21 contribute to the overall loadings of N, as will be demonstrated in a future version of Section 8.4, 

22 it may be possible to focus solely on NOx contributions to loadings while recognizing that there 

23 are impacts from reduced forms of nitrogen that must be taken into account. 

24 8.2 STATUTORY AND LEGAL CONSIDERATIONS IN THE 

25 STRUCTURE OF THE STANDARD 

26 Inherent in the approach described above are various legal considerations as to the 

27 requirements of the CAA and EPA’s ability to implement various types of standards under the 

28 Act. This section will begin to discuss the legal framework for considering appropriate air 

29 quality indicator(s), averaging time(s), form(s), and level(s) of standard(s) to protect against 

30 known or anticipated adverse impacts on public welfare. As noted above, the structure of such a 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-6 August 2008 

1 standard would likely be complex, reflecting the variable factors defining the atmospheric and 

2 environmental conditions that affect deposition of NOx and SOx, and those variable ecological 

3 factors that affect ecosystem responses. 

4 In setting NAAQS to protect public health and welfare, EPA has historically established 

5 standards which require the comparison of locally monitored ambient concentrations of an air 

6 pollutant indicator against a specified numerical level of atmospheric concentration of a 

7 pollutant, using a specified averaging time and statistical form. For example, the current 

8 secondary standard for NOx uses ambient concentrations of NO2 as the indicator, and attainment 

9 is determined by comparing the annual arithmetic mean of measured ambient concentrations, for 

10 a calendar year, against the level of 0.053 ppm. As more is learned about the effects of pollutants 

11 such as NOx and SOx on the environment, such a relatively simple framework may not be the 

12 most appropriate way to protect sensitive ecosystems from effects associated with ambient 

13 concentrations of the air pollutants. EPA is considering in this review of the secondary standard 

14 for NOx and SOx a conceptual framework for evaluating the environmental effect of ambient 

15 concentrations of NOx and SOx that takes into account variable factors, such as atmospheric and 

16 ecosystem conditions that modify the amounts and effects of deposited N and S on ecosystems. 

17 EPA is also considering whether the secondary standard can and should be structured to better 

18 take this framework into account, as a way to appropriately protect the public welfare from the 

19 effects associated with the presence of these pollutants in the ambient air. 

20 As discussed previously, this could involve a standard with an ecological indicator which 

21 can be linked to ambient concentrations of SOx and NOx. The standard could include a nationally 

22 uniform level for a particular ecological indicator, for example a specified level of ANC that 

23 would be determined by the Administrator to represent the appropriate level of protection. This 

24 framework would also take into account other variables, both atmospheric and ecological that 

25 may modify the ecological effects. Compliance would be determined by measuring NOx and SOx

26 concentrations, using these measurements and the various transformation functions to calculate 

27 an ANC level, and comparing the calculated level of ANC to the level of ANC set by the 

28 Administrator, including averaging time and form. Since the effect of a certain ambient 

29 concentration of SOx and NOx on ANC could vary depending on the other variable factors, 

30 whether a certain ambient concentration of SOx and NOx would attain the standard would depend 

31 in part on these other variables, which could vary geographically and temporally. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-7 August 2008 

1 One important issue in evaluating such an approach is EPA’s authority to structure a 

2 NAAQS in such a way. EPA’s review of a NAAQS starts with its review of the air quality 

3 criteria for the pollutant. Under Section 109(d)(1) of the CAA, EPA must undertake a thorough 

4 review of the air quality criteria for the pollutant at issue as part of reviewing a secondary 

5 NAAQS to determine whether a current standard is requisite to protect the public welfare. Under 

6 Section 108, air quality criteria are to “reflect the latest scientific knowledge useful in indicating 

7 the kind and extent of all identifiable effects” associated with the presence of the pollutant in the 

8 ambient air. It is clear from the language of the CAA that where the state of the science provides 

9 a basis for considering such effects as relevant, the review of air quality criteria should 

10 encompass a broad analysis of “any” anticipated adverse effects, as well as the ways in which 

11 variable conditions such as atmospheric conditions or ecological conditions may influence the 

12 effect of a pollutant and the ways in which other air pollutants may interact with the criteria 

13 pollutant to produce adverse effects. Specifically, Section 108(a)(2) of the CAA provides that: 

14 The criteria for an air pollutant to the extent practicable shall include information on the 

15 following: 

16 1. Those variable factors (including atmospheric conditions) which of themselves or in 

17 combination with other factors may alter the effects on public health or welfare of 

18 such air pollutants; 

19 2. The types of air pollutants which, when present in the atmosphere, may interact with 

20 such pollutants to produce an adverse effect on public health or welfare; and 

21 3. Any known or anticipated adverse effects on welfare. 

22 

23 Taking into account this extensive review of the air quality criteria for an air pollutant, 

24 the Administrator is required to review and to revise, as appropriate, the secondary standard. The 

25 secondary standard is to “specify a level of air quality which, in the judgment of the 

26 Administrator, based on such criteria, is requisite to protect public welfare from any known or 

27 anticipated adverse effects associated with the presence of such air pollutant in the ambient air.” 

28 CAA § 109(b) (emphasis added). “Welfare,” in turn, is defined to include a broad array of 

29 effects, including effects on soil, water, crops, vegetation, and manmade materials, “whether 

30 caused by transformation, conversion, or combination with other air pollutants.” CAA § 302(h). 

31 As with the description of the air quality criteria in Section 108, the CAA uses expansive 

32 language in describing the range of effects that EPA should take into account in setting a 

33 standard that is requisite to protect public welfare. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-8 August 2008 

1 Thus, Section 108 and the definition of welfare require that EPA analyze and evaluate 

2 scientific information on the interaction between ambient concentrations of a pollutant and other 

3 variable factors, atmospheric and ecological, that affect the air pollutants impact on the 

4 environment. Section 109(b) makes it clear that EPA must base the standard on this scientific 

5 information but does not specify how EPA is to incorporate this information into the structure of 

6 the standard beyond the requirement that any secondary NAAQS be a level of air quality that is 

7 requisite to protect public welfare. Accordingly, while Sections 108 and 109(b) make it clear that 

8 EPA must take this kind of information into account when setting the NAAQS, specific 

9 directions are not provided on how to accomplish this. 

10 It is also important to keep in mind the meaning of the term “requisite,” as interpreted by 

11 EPA and the Courts. In considering the meaning of the term “requisite” in the context of the 

12 primary standards, the Supreme Court has agreed with EPA that such a standard is one that is 

13 “sufficient, but not more than necessary” to protect public health. Whitman v. American 

14 Trucking, 531 U.S. 457, 473 (2001). In addition, while Section 109(b) requires that EPA 

15 establish “national” standards, it does not define this requirement in any greater detail than 

16 described above. 

17 While EPA has most often made decision based on the results of direct exposure to an air 

18 pollutant in the ambient air in assessing effects on public health and welfare in establishing 

19 NAAQS, such as the health effects on humans when breathing in an air pollutant or the effects 

20 on vegetation through the uptake of air pollutants through leaves, EPA has also considered, 

21 where appropriate, the effects of exposure to air pollutants through more indirect mechanisms. 

22 For example, in 1978, EPA established a NAAQS for lead that included non-respirable particles 

23 based on the conclusion that these particles contribute to blood lead levels once they are 

24 deposited on the ground and then become a source of human lead exposure through the ingestion 

25 of lead-contaminated food or, particularly in children, the placing of hands and other 

26 contaminated objects in the mouth. Lead Industries v. EPA, 647 F.2d 1130 (DC Cir. 1980). In 

27 addition, EPA has previously considered acidification and nutrient enrichment concerns 

28 associated with ambient concentrations of NO2 and other nitrogen compounds, but determined 

29 that the scientific and technical evidence that was available at the time did not provide an 

30 adequate basis to set a separate secondary standard to address such effects. 61 FR 52852, 52855 

31 (October 8, 1996). The deposition of ambient NOx and SOx to terrestrial and aquatic 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-9 August 2008 

1 environments can affect ecosystems through direct and indirect mechanisms, as discussed in 

2 previous chapters. Given Congress’ instruction to set a standard that “is requisite to protect the 

3 public welfare from “any known or anticipated adverse effects associated with the presence of 

4 such air pollutant in the ambient air,” 42 U.S.C. § 109 (b)(2) (emphasis added), this review 

5 appropriately attempts to take into consideration widely acknowledged effects, such as 

6 acidification and nutrient enrichment, which are associated with the presence of ambient SOx and 

7 NOx, as described in previous chapters. 

8 The relationships between the impacts of ambient air concentrations on ecological effects 

9 such as acidification and nutrient enrichment, however, are extremely complex. To take these 

10 effects into account, including the variability in effects resulting from the same ambient 

11 concentrations of SOx and NOx, EPA is developing a biologically relevant framework in this risk 

12 assessment that relates ambient concentrations of an air pollutant to an ecological indicator, 

13 taking into account relevant atmospheric and ecological variables. In that context it is useful to 

14 explore whether a standard structured to take these elements into account is an appropriate way 

15 to provide protection that is sufficient but not more than necessary, as compared to a standard 

16 that treats ambient concentrations the same irrespective of these other biologically relevant 

17 variables. The CAA, which requires EPA to establish “national” standards, does not specifically 

18 address whether EPA can establish standards which could allow for varying levels of SOx and 

19 NOx in different areas. However, such an approach may be necessary if EPA is to establish a 

20 secondary standard that provides sufficient but not more than necessary to protect the public 

21 welfare. Given the lack of specificity in Section 109(b), EPA believes it is appropriate to explore 

22 the approach described in this chapter, with an aim of setting a national standard that provides 

23 sufficient but not more than necessary protection throughout the United States by taking into 

24 account the recognized variation of biologically relevant factors. 

25 8.3 LINKAGES FOR STRUCTURING ECOLOGICALLY RELEVANT 

26 STANDARDS 

27 Consistent with the framework depicted in Figure 8.1-1, the following sections provide a 

28 discussion of both the linkages between ambient air concentrations and resulting deposition 

29 metrics, and between the deposition metric and the ecological indicator of concern. These 

30 linkages are discussed conceptually in Section 8.3.1. Following that discussion, Section 8.3.2 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-10 August 2008 

1 describes the deposition transformation function that relates atmospheric concentrations of NOx 

2 and SOx to nitrogen and sulfur deposition metrics, while Section 8.3.3 describes the ecological 

3 effect function that transforms the deposition metric into the ecological indicator. Modifying 

4 factors that alter the relationship between ambient atmospheric concentrations of NOx and SOx 

5 and depositional loads of nitrogen and sulfur, and those that modify the relationship between 

6 deposition loads and the ecological indicator are also discussed more fully. 

7 8.3.1 A Conceptual Model of the Pathway from Ambient Atmospheric 

8 Concentrations of NOx and SOx to Ecological Effects 

9 For this first draft risk and exposure assessment, we have focused on developing an air 

10 quality indicator for protection against the adverse effects of aquatic acidification. Figure 7.3-1 

11 demonstrates the various levels of protection of fish populations associated with varying ANC 

12 levels. Differing levels of an ecological indicator such as ANC will be associated with differing 

13 levels of protection of ecosystem functions (e.g. fish populations, biological diversity, fish 

14 population health). Different ecological indicators may be necessary for protection against the 

15 adverse effects of terrestrial acidification and aquatic and terrestrial nutrient enrichment. 

16 Indicators for other welfare effects categories of concern will be explored in future drafts of this 

17 risk and exposure assessment. 

18 We have created a system of linked functions that translate an atmospheric indicator 

19 (concentrations of NOx and SOx) into an ecological indicator that expresses the potential for 

20 deposition of nitrogen and sulfur to acidify an ecosystem (Figure 8.1-1). The first part of this 

21 system is the deposition transformation function, which transforms ambient atmospheric 

22 concentrations (the atmospheric indicators) into a deposition metric. In this particular example, 

23 the deposition metric referred to in Figure 8.1-1. is the sum of atmospherically deposited 

24 oxidized nitrogen and sulfur, weighted by their molar ratio. The second part of this system of 

25 functions is an ecological effect function which converts the deposition metric to an ecological 

26 indicator (ANC) by adjusting for levels of other ecological factors that contribute to ANC. 

27 When the amounts of NOx and SOx in the ambient atmosphere can be used to derive a 

28 deposition metric (via the deposition transformation function) that can then be used to derive a 

29 level of ANC (through the ecological effect function) which falls within the range defined as 

30 acceptable by the standard, then those levels of NOx and SOx will be considered to meet that 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-11 August 2008 

1 standard of protection. The atmospheric levels of NOx and SOx that satisfy a particular level of 

2 ecosystem protection are those levels that result in an amount of deposition that is less than the 

3 amount of deposition that a given ecosystem can accept without degradation of ANC. This 

4 latter amount is referred to as the maximum deposition load and is the amount that solves a 

5 mass-balance equation for that particular level of ANC. Mass-balance relationships developed in 

6 the scientific literature provide the basis for established equations that link levels of ANC to 

7 levels of nitrogen and sulfur deposition (given levels of other, non-deposition related parameters 

8 such as weathering rates, base cation uptake rates). Quantitative examples of these relationships 

9 will be provided in Section 8.4 of this chapter in the next draft of this document. 

10 The value of the deposition metric can be calculated in terms of measured (or modeled) 

11 ambient NOx and SOx concentrations by using established relationships between ambient 

12 concentrations of NOx and SOx and nitrogen and sulfur deposition. As will be described in 

13 Section 8.4, these relationships are derived from the physical processes that govern transport and 

14 deposition; they are species dependent; they vary by particle mass, meteorology, compensation 

15 point, terrain and land use type. 

16 It is important to note for this type of air quality indicator that the same level of 

17 protection from acidification can be obtained with different combinations of ambient NOx and 

18 SOx concentrations and their associated levels of nitrogen and sulfur deposition. As a result, the 

19 ambient air quality standard for NOx and SOx may be expressed jointly, and the air quality 

20 standard may be met by various combinations of NOx and SOx. 

21 Determining the level of the air quality indicator that is associated with a particular level 

22 of protection against adverse effects of acidification may include consideration of factors that 

23 could potentially alter the depositional relationship of nitrogen or sulfur to ANC. In addition, the 

24 specific concentrations of ambient NOx and SOx that will result in a particular deposition metric 

25 can vary based on factors that affect the relationship between ambient concentrations and 

26 deposition. These two considerations imply that the levels of ambient NOx and SOx that would 

27 be consistent with a particular level of ANC may differ according to the specific levels of 

28 modifying factors in different locations and whether the ecosystem is nitrogen and/or sulfur 

29 limited or saturated. While the standard is set to achieve the same level of protection (ANC) 

30 nationwide, the atmospheric concentrations of NOx and SOx and the related amounts of 

31 deposition that can occur and still meet the standard may differ. For example, because of the 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-12 August 2008 

1 differences in maximum depositional loads between watersheds with high proportions of 

2 grasslands and those with high proportions of evergreen forest ecosystems, for the same level of 

3 ANC, there may be differences in the ambient concentrations of NOx and SOx that would meet 

4 that ANC level. The response variable associated with these examples will vary across effects 

5 categories, and might be based on soil base saturation or aluminum concentrations in soil water 

6 or biodiversity impacts, rather than ANC. Specific modifying factors for the ambient 

7 concentration to deposition relationship are discussed in Section 8.3.2 of this chapter. Specific 

8 modifying factors for the deposition to acidification relationship will be discussed in Section 

9 8.3.3. The overall transformation function of the standard will be national in scope as will be the 

10 ecological indicator level, although specific modifying factors within the transformation function 

11 and maximum depositional load may vary by location. Therefore, while the Administrator may 

12 choose the level of protection that is desired by specifying a particular limiting value of ANC 

13 (ANClimit), there are a series of transformations that link that level of protection to ambient 

14 concentrations of NOx and SOx. As a conceptual example, to express the previous discussion in 

15 mathematical terms, consider the following: 

16 I= ANClimit= g(·)-DL(S)-DL(N) (1) 

17 where I is an indicator, g(·) is a series of non-atmospheric, non-depositional environmental 

18 factors (e.g., runoff, weathering rates, geology, etc.), ANClimit is a critical limit for ANC 

19 necessary to provide a particular level of protections for biota in an ecosystem, DL(S) is the 

20 implied maximum depositional load of S associated with that ANC limit, and DL(N) is the 

21 implied maximum depositional load of N associated with that ANC limit. Note that DL(S) and 

22 DL(N) are jointly determined by the equation g(·)-ANClimit=DL(S)+DL(N), and that there is no 

23 unique solution for either one unless the other is fixed. Further, we can express the deposition of 

24 NOx and SOx through the following relationships: 

25 

26 SD=fS(NOx, SOx |Z) and ND=fN(NOx, SOx |Z) (2) 

27 

28 where SD is sulfur deposition, ND is nitrogen deposition, Z is a series of parameters affecting 

29 deposition of ambient SOx and NOx (including NH3), fS is a function relating SD to ambient 

30 concentrations of NOx and SOx, given Z, and fN is a function relating ND to ambient 

31 concentrations of NOx and SOx, given Z. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-13 August 2008 

1 It is clear that the indicator of NOx and SOx effects (I) is a function of deposition. 

2 However, by substituting in the ambient to deposition conversion functions in (2), the indicator I 

3 can be made into an ambient indicator: 

4 I= g(·)-SD(fS(NOx, SOx |Z)-ND(fN(NOx, SOx |Z). (3) 

5 Not all values of SD and ND cause adverse welfare effects. The point where the levels of 

6 deposition associated with the ambient NOx and SOx lead to an ANC that does not achieve the 

7 specified ANC limit are when SD and ND are equal to or greater then the DL(S) and DL(N) for a 

8 particular location. 

9 I= g(·)-DL(fS(NOx, SOx |Z)-DL(fN(NOx, SOx |Z). (4) 

10 Clearly, the establishment of the functional forms for g(·), fS, and fN, as well as for the 

11 values of the parameter series Z, will be critical. Note that variable factors in g(·) and Z can be in 

12 many forms, some continuous, e.g. precipitation, and some discrete, e.g. type of bedrock. 

13 However, continuous variables can be converted to discrete categories, which may be easier to 

14 implement in the form of a standard. For example, precipitation could be expressed as inches per 

15 year, or precipitation could be categorized as high, medium, and low, possibly based on 

16 percentiles of the observed distributions of precipitation values. 

17 8.3.2 Linkages between Ambient Concentrations and Deposition 

18 Defining the linkages between ambient concentrations of NOx and SOx and deposition of 

19 N and S is a crucial piece of the framework as described in Section 8.1. Atmospheric deposition 

20 of nitrogen from ambient concentrations of NOx (NNOx) and atmospheric deposition of sulfur 

21 from ambient concentrations of SOx (SSOx) to a surface is a complex function of numerous 

22 meteorological parameters and surface properties. A transformation function that incorporates 

23 the nationwide variability of the important meteorological and surface parameters is expected to 

24 be a useful approach to describing the deposition process. The derivation of this transformation 

25 function, which converts ambient concentrations of NOx and SOx to their associated depositions, 

26 is described below. The derivation takes algorithms from the U.S. EPA-developed Community 

27 Multi-scale Air Quality (CMAQ) modeling system (U.S. EPA, 1999) into consideration, and 

28 CMAQ output is used to calculate key parameters in the resulting function. The use of CMAQ 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-14 August 2008 

1 data is intended to be illustrative of the process. Specific models, data sets and methods 

2 appropriate to use in other locations or on different spatial/temporal scales may differ. 

3 8.3.2.1 Data and Tools 

4 8.3.2.1.1 Physics and Chemistry of Atmospheric Deposition 

5 Atmospheric pollutants deposit onto land and water surfaces through at least two major 

6 mechanisms: direct contact with the surface (dry deposition), and transfer into liquid 

7 precipitation (wet deposition). The magnitude of each deposition process is related to the 

8 ambient concentration through the time-, location-, process- and species-specific deposition 

9 velocity (Seinfeld and Pandis, 1998): 

Amb

i

Dry

i

Dry 10 Depi = v ⋅C (19) 

Amb

i

Wet

i

Wet 11 Depi = v ⋅C (20) 

where v

Dry

i and v

Wet

i are the dry and wet deposition velocities, DepDryi and DepWeti 12 are the dry and 

wet deposition fluxes, CAmb

i 13 is the ambient concentration, and the i subscript indicates the 

14 pollutant species under study. The total deposition of each pollutant is 

Wet

i

Dry

i

Tot 15 Depi = Dep + Dep (21) 

16 Substituting Equations 19 and 20 into 21 yields 

Amb

i

Wet

i

Amb

i

Dry

i

Tot 17 Depi = v ⋅C + v ⋅C (22) 

18 The total deposition of sulfur and nitrogen would therefore be: 

= ∑ + ⋅ ⋅ +

i

Amb

i i

Wet

i

Dry

i

Tot 19 DepS N ( (23) v v ) m C

where mi 20 is the molar ratio of the atom (sulfur or nitrogen) of interest to the ith pollutant. 

Ambient sulfur- and nitrogen-containing pollutants include gases such as sulfur dioxide (SO2 21 ), 

ammonia (NH3

), various nitrogen oxides (NO, NO2, N2O5), nitric acid (HNO3 22 ), and peroxyacetyl 

nitrate (PAN); and particulate species such as sulfate (SO2

4), nitrate (NO3), and ammonium (NH+ 23 4

24 ). Oxides of nitrogen and oxides of sulfur are currently regulated by the NAAQS as defined in 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-15 August 2008 

1 the CAA. These compounds include the sulfur-containing species above as well as the above 

2 oxidized forms of nitrogen (referred to here as NOx); ammonia and ammonium, which are 

3 reduced forms of nitrogen, are not currently included as regulated pollutants. 

4 For this section, Equation 5 is termed the deposition transformation function, and it 

5 provides the functional form of the relationship between instantaneous atmospheric pollutant 

6 concentrations and deposition fluxes of sulfur and nitrogen. Equation 5 can also be thought to 

7 represent the relationship between cumulative or average values of the variables over time and/or 

8 space, but doing so raises issues of exactly how integration/averaging over time and/or space is 

9 to be done. To some extent, the degree of temporal and spatial averaging needed will depend on 

10 the ability to measure or model concentrations at particular spatial (e.g. at specific water bodies) 

11 and temporal (e.g. hourly or daily) scales. However, the need for temporal or spatially 

12 aggregated values may also be driven by the form of the ecological indicator, which may be 

13 annual or seasonal, and may reflect local ecological attributes. The remainder of Section 8.3.2 

14 discusses this and other policy-relevant scientific and mathematical issues and proposes methods 

for determining parameter values (i.e. vi

dry and viwet 15 ) for input to Equation 23; results of example 

16 calculations will be shown in Section 8.4. 

17 8.3.2.1.2 Air Quality Simulation Models 

18 Deposition velocities necessary for employing the deposition transformation function can 

19 be obtained either through models or direct measurements, although the logistics of accurate 

20 measurements are currently prohibitive to deploying a measurement network. For purposes of 

21 the current discussion and accompanying future example calculations (Section 8.4), deposition 

22 velocities will be obtained from CMAQ output, and research will be conducted to assess if this is 

23 the optimal source of these parameters for future regulatory analyses. 

24 CMAQ determines the deposition fluxes of the suite of modeled air pollutants at each 

25 time step and location in the user-specified spatial grid. Deposition velocities can be found in 

26 CMAQ output in the following ways: 

1. vdry 27 values of gaseous pollutants are calculated in the CMAQ weather module called 

28 the Meteorology-Chemistry Interface Processor (MCIP) through a complex function 

29 of meteorological parameters (e.g. temperature, relative humidity) and properties of 

30 the geographic surface (e.g. leaf area index, surface wetness) 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-16 August 2008 

2. vdry 1 values for particulate pollutants are calculated in the aerosol module of CMAQ, 

2 which, in addition to the parameters needed for the gaseous calculations, also 

3 accounts for properties of the aerosol size distribution 

4 3. Equation 20 is not directly expressed in CMAQ source code: the cloud processing 

5 module of CMAQ performs simulations of mass transfer into cloud droplets and 

6 aqueous chemistry to incorporate pollutants into rainwater, all of which is 

conceptually contained in the vwet parameter. Explicit vwet 7 values can and will be 

8 calculated in a post-processing step by applying Equation 20 to the wet depositions 

9 and ambient concentrations output by the model. 

10 Due to lack of direct measurements, no performance evaluations of CMAQ’s dry 

11 deposition calculations can be found; however, the current state of MCIP is the product of 

12 development work that has been based on peer-reviewed literature from the past two decades 

13 (U.S. EPA, 1999) and is considered to be EPA’s best estimate of dry deposition velocities. Some 

14 bias has been found between CMAQ’s wet deposition predictions and measured values (Morris 

15 et al., 2005); recent analyses suggest that poor simulation of precipitation could be responsible 

for this (Davis and Swall, 2006), which should be considered if vi

wet 16 values are applied to 

17 ambient measurements (this will be discussed in Section 8.4). Although the model is continually 

18 undergoing improvement, CMAQ is EPA’s state-of-the-science computational framework for 

19 calculating deposition velocities, and was therefore the first choice for exploring inputs to 

Equation 23. Gaseous vdry values are directly output by MCIP, whereas aerosol vdry 20 values and all 

species’ vwet 21 values currently need to be calculated from standard CMAQ output. In all cases, 

22 data aggregation issues need to be carefully considered: while MCIP and CMAQ give hourly 

23 output values, the particular averaging time relevant for ecological processes for NOx and/or SOx

24 is likely to be the same as the time resolution of the ecological indicator of choice (which is 

likely to be seasonal or annual). As such, values of vdry and vwet 25 for each species should either be 

26 representative of the desired averaging period, or aggregation of resulting depositions after 

27 applying hourly deposition velocities to concentrations will need to be performed. It is proposed 

28 that the spatial scale of the deposition velocities used here reflect that of future analyses, e.g. be 

29 relevant at scales that may range from individual water bodies up to regional or national scales, 

30 as discussed in Chapters 4 and 5. Specific methods for performing calculations are discussed 

31 below. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-17 August 2008 

1 8.3.2.2 Analytical Techniques 

2 8.3.2.2.1 Hourly Deposition Velocities 

As stated above, hourly vdry 3 values of gaseous species are complex functions of 

4 meteorology and land surface properties that are directly available in the outputs of MCIP. 

5 Deposition velocities of aerosol species were calculated by applying Equation 19 to CMAQ grids 

of aerosol CAmb

i and DepDryi values to calculate hourly grids of v

Dry

i 6 values for the example in a 

7 future Section 8.4. Plans are underway to obtain the size-specific aerosol deposition velocities 

from future CMAQ model runs. This will provide v

Dry

i 8 for fine as well as coarse mode aerosols. 

9 Unlike dry deposition velocity, the wet deposition velocity is independent of land surface 

10 properties and can be effectively expressed in a simple form (Seinfeld and Pandis, 1998): 

0 v w p r

i

Wet

i 11 = ⋅ (24) 

Amb

i

Aq

r i

i

C

C 12 w = (25) 

where wr

 is the washout ratio, CAq

i is the concentration of species i in the rainwater, and p0 13 is the 

14 precipitation intensity. 

15 If Equation 25 is substituted into 24, and Equation 24 is substituted into Equation 20, the 

16 ambient concentration cancels out and wet deposition can be expressed as a function of only 

17 rainwater concentration and precipitation (both of which are routinely measured by the NADP 

18 network). However, deposition can be expressed in terms of ambient concentrations since the 

19 atmospheric indicators for the framework discussed above are ambient air concentrations of NOx

and SOx. Calculating the transformation function will therefore use values of wr 20 derived from 

CMAQ output for input into Equation 24 (which is then input to Equation 5). Solving for vi

wet 21 in 

22 Equations 2 and 6, equating them and rearranging these equations yields: 

Amb

i

wet

r i

i

p C

Dep

w ⋅ = 0

23 (26) 

24 which expresses the washout ratio in terms of CMAQ outputs. As mentioned above, simulated 

25 precipitation is a source of error in CMAQ’s wet deposition simulations. The washout ratio 

offers a method of dealing with this error: CMAQ output can be used to calculate wi

r 26 values and 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-18 August 2008 

precipitation measurements (e.g. from NADP) can be used to obtain p

0 1 values for input to 

2 Equation 24 to obtain measurement-corrected wet deposition velocities. 

3 8.3.2.2.2 Aggregation Issues 

4 The deposition velocities and washout ratio are dynamic quantities that depend on 

5 numerous conditions that are highly variable with both space and time. However, an ecologically 

6 relevant standard is likely to be based on an averaging time greater than hourly, which might 

7 require temporal aggregation of deposition velocities. Use of deposition velocities that reflect a 

8 spatial scale consistent with this review might also require some level of spatial aggregation of 

deposition velocities. The v

dry, vwet, and wi

r 9 parameters are all expressible as ratios of other 

10 quantities, which allow for multiple possible methods of calculating aggregate values, including 

11 the following: 

12 1. Average ratio: Averaging hourly values of the deposition velocities and the washout 

13 ratio is equivalent to find the average ratios of their component quantities. 

14 2. Weighted Average Ratio: Same as above but weight the hour-specific ratios by either 

15 concentration or deposition during the averaging process. 

3. Ratio of averages: Instead of working with hourly deposition velocities and wi

r 16 

17 values, concentrations and depositions can be aggregated (concentrations are 

18 averaged, depositions and precipitation are summed) over space and/or time and the 

19 ratios of the resulting aggregate values can be computed. This method removes the 

20 outlier effect of unrealistically large ratios resulting from small denominators (this 

seems most likely to be an issue in calculations of wi

r 21 when precipitation is very small 

22 but non-zero). 

23 4. Conservatism: Single hourly values from the pool of available data can be chosen that 

24 provide a more conservative calculation of depositions, such as predicting the largest 

25 possible depositions from a given concentration (i.e., choose the annual and spatial 

maximum values of vi

dry and/or wir 26 for each species and region). 

27 Temporal aggregation is known to produce substantial biases in dry depositions when 

correlation exists between vdry and CAmb

i 28 (Clarke et al., 1997), and it is presumed that this bias 

29 needs consideration when performing spatial aggregation as well. Some exploration of the 

conservative method of finding vdry 30 values has been performed, and it was found that this 

31 produced unrealistically high annually representative values. For purposes of the example 

32 calculations that will be presented in Section 8.4, the Ratio of Averages method will be 

33 employed to obtain all annual deposition velocities, but the best method of obtaining such values 

34 remains an open research question. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-19 August 2008 

1 8.3.2.3 Results and Findings 

2 Potential applications of the transformation function to the standard setting and 

3 implementation processes will be presented in Section 8.4. In this section, the variability of the 

4 deposition velocities resulting from CMAQ output is explored to 1) highlight the complexity of 

5 the process of converting between ambient concentrations and depositions and 2) illustrate some 

6 possible visualization techniques 

7 Fields of dry deposition velocities of SO2 output by MCIP are shown in Figure 8.3-2. 

8 The figure shows maps of mean and standard deviation of hourly dry deposition velocities over 

9 24 hours during four seasonally distinct days across the eastern U.S. output by MCIP. The plots 

show that vdry 10 values vary by at least two orders of magnitude over the spatial domain, and that 

there appears to be some spatial correlation between the magnitude of vdry 11 values (mean plots) 

12 and the hourly variability (standard deviation plots). These maps highlight the difficulty of 

choosing a single nationally representative vdry 13 value. 

Therefore, vdry 14 values could also be studied at smaller spatial scales. Figures 8.3-3 and 

8.3-4 show distributions of vdry 15 values from a Case Study Area (Pamlico Sound, NC) of the same 

four days used for Figure 8.3-2. vdry 16 values in Figure 8.3-3 show an increasing trend in the 

17 January 1 plot, a decreasing trend on April 1, and relatively stable trends on July 1 and October 

18 1, but with much greater spatial variability in the July 1 plot. Figure 8.3-4 shows maps of the 

19 diurnal distributions of the same data, and reveals that a substantial portion of the spatial 

20 variability might be explained by differences in the dominant land use of the grid cells (see 

21 Appendix A for the full list of land use categories in the model), especially when distinguishing 

between water and land surfaces. This suggests that vdry 22 values specific to at least the land use 

23 type, if not the grid cell, should be considered when using the transformation function. 
Chapter 8 – Considerations in the Structure of the NOx/SO

DRAFT 8-20 

1 

2 

Figure 8.3-2: Mean and standard deviations of SO2 vdry 3 values across the CMAQ eastern United States mo

4 first day of four seasonally distinct months. Note that the high value of the legend represents the maximum o5 data. 
Chapter 8 – Considerations in the Structure of the NOx/SO

DRAFT 8-21 

1 2 

Figure 8.3-3: Hourly distributions of SO2 vdry 3 values for one HUC (#3020105: Pamlico Sound, NC) over th

4 seasonally distinct months.
Chapter 8 – Considerations in the Structure of the NOx/SO

DRAFT 8-22 

1 

2 

Figure 8.3-4: Distributions of SO2 vdry 3 in each gridcell of one HUC (#3020105: Pamlico Sound, NC) over 

4 seasonally distinct months. Colors indicate the dominant land use category (DLUSE) of the gridcell; see Ap5 description of each categorical value. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-23 August 2008 

1 

Figure 8.3-5 shows domain-wide SO2 2 washout ratios values for the same four seasonally 

3 distinct days as above, calculated using the Average Ratio and the Ratio of Averages methods. 

4 These plots illustrate that the potential for unrealistically extreme values is far greater using the 

5 Average Ratio method. Exploration of the numbers in these calculations showed that, despite 

excluding times when p

0 6 was 0, very low ambient concentration values could also cause extreme 

7 values such as those shown in Figure 8.3-5. These results thus suggest that the Ratio of Averages 

might be a better choice for calculations of wr

i 8 rather than the Average ratio method to help 

9 mitigate the use of unrealistically large outlier values. 

10 8.3.2.4 Uncertainties 

11 The most prominent uncertainty in the process of converting between concentrations and 

12 depositions is the known lack of accuracy of the deposition velocities that are input to Equation 

13 23. The CASTNET and NADP networks measure concurrent weekly values of wet deposition 

14 and ambient concentrations of select species; these data might be used to estimate the bias 

present in calculations of v

Wet

i 15 . Such data explorations are also expected to aid in understanding 

the stability of wr

i 16 values, which might be low due to the dependence of cloud chemistry 

17 dynamics on meteorology of the upper atmosphere. Estimates of deposition velocities might be 

18 improved over CMAQ predictions by packaging the relevant portions of the MCIP and CMAQ 

19 source code into a stand-alone tool that generates values using local meteorological 

20 measurements (as opposed to the simulated values that are used during model runs). Further 

21 discussions of uncertainty will be included in future drafts of this document.
Chapter 8 – Considerations in the Structure of the NOx/S

DRAFT 8-24 August 2008 

1 

2 

Figure 8.3-5: Daily average washout ratios of SO2 3 across the CMAQ eastern United States modeling dom

4 four seasonally distinct months (calculated using two different aggregation methods).
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-25 August 2008 

1 8.3.3 Linkages between Deposition and Ecological Indicators 

2 The equation defining ANC forms the basis of the linkage between deposition and 

3 surface water acidic condition and the modeling approach used. In the discussion below, the 

4 functions are framed in terms of a maximum depositional load given a specified ANC limit, for 

5 ease of discussion. However the maximum depositional load is not the deposition metric 

6 employed in Fig 8.1. The deposition metric is the deposition associated with a certain measured 

7 or assumed level of atmospheric concentration of NOx and SOx, and under Fig 8.1 the 

8 deposition metric is used to calculate an ANC level associated with that atmospheric level. The 

9 next draft will also frame the functions in terms of calculating an ANC level from a specific 

10 deposition metric, whether it is above or below the depositional load. 

11 Given an ANC concentration [ANClevel]), the corresponding depositional load (DL) is 

12 simply the input flux of acid anions from atmospheric deposition, which gives the [ANClevel] 

13 when subtracted from the sustainable acidity or charge balance, defined as the uptake (Nupt), 

14 immobilization (Nimm), and denitrification (Nden) of NOx deposition, the in-lake retention of NOx

15 deposition (Nre) and SOx deposition (Sret), and the acid anion leaching from the catchment as a 

function of the pre-industrial flux of base cations [BC]0

* 16 from weathering, a described in 

17 Equation 5: 

18 

DL(NNOx) + DL(SSOx) ={fNupt + (1 - r)(Nimm + Nden) + r(Nret + Sret)} + ( [BC]0

* 19 - [ANClevel])Q (5) 

20 

21 where f and r are dimensionless parameters that define the fraction of forest cover in the 

22 catchment and the lake/catchment ratio, respectively. The right hand side of equation (5) is 

23 equivalent to the g(·) function discussed in Section 8.3.1. To convert into fluxes, concentrations 

24 ([x] = X/Q) are simply multiplied by runoff (Q) (in m/yr) from the site. The charge balance 

25 equates the deposition inputs of acid anions with the sum of processes which control their long 

26 term storage, removal and leaching exports. Several major assumptions are made: 1) steady-state 

27 conditions, 2) the effect of nutrient cycling between plants and soil is ignored 3) there are no 

28 significant N inputs from sources other than atmospheric deposition, 4) ammonium leaching is 

29 negligible because any inputs are either taken up by biota, adsorbed onto soils or nitrate 

30 compounds, and 5) long-term sinks of sulfate in the catchment soils are negligible. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-26 August 2008 

1 Using equation 5 to derive the terms for internal catchment processes, the charge balance 

2 provides those combinations of NNOx and SSOx deposition that defines the DL function given by: 

3 

 DLmax(SSOx ) = (( [BC]0

* 4 - [ANClevel])Q)/ (1- ps) (6) 

5 DLmin(SSOx ) = 0 (7) 

 DLmax(NNOx ) = fNupt + (1-r)( Nimm + Nden) + [( [BC]0

* 6 - [ANClevel])Q]/ (1-pn) (8) 

7 DLmin(NNOx ) = fNupt + (1-r)( Nimm + Nden) (9) 

8 where ps and pn define the fraction of in-lake retention of Nret and Sret, which is similarly 

9 proportional to the net input of NNOx and SSOx to the lake. It is not possible to define a single 

10 value of maximal loading of total acidity, since the acid anions sulfate and nitrate behave 

11 differently in the way they are transported with hydrogen ions; one unit of deposition of S will 

12 not have the same net effect on surface water ANC as an equivalent unit of N deposition. 

13 Equations 2 and 4 define the deposition loads for NNOx and SSOx individually. DLmax(SSOx) 

14 defines the loading for sulfur when total NNOx deposition is less then DLmin(NNOx). 

15 DLmin(NNOx) defines the deposition of NNOx at which terrestrial catchment processes effectively 

16 remove NNOx, so that deposition loads lower then DLmin(NNOx) result in no leaching of nitrate. 

17 DLmax(NNOx) defines the load for total NNOx deposition when S deposition is zero. In reality, 

18 neither NNOx and SSOx deposition will ever be zero, so the DL for the deposition of one is fixed 

19 by the deposition of the other, according to the line defining in Figure 8.3-1. It is our intention to 

20 construct a similar line for N deposition originating from NOx only for the second draft of this 

21 risk assessment. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-27 August 2008 

NNOx Deposition 

DLmax(NNOx)

SSOx Deposition 

DLmin(NNOx)

DLmax(SSOx)

NNOx Deposition 

DLmax(NNOx)

SSOx Deposition 

DLmin(NNOx)

DLmax(SSOx)

1 

2 Figure 8.3-1. The deposition load function defined by the model. 

3 The thick lines indicate all possible pairs of DL of NNOx and SSOx acidity. Note that in the 

4 above formulation individual DL of NNOx and SSOx are not specified; each pair of depositions 

5 (NNOx and SSOx) fulfilling equations 2-5. 

6 Given a [ANClevel] for a desired level of protection, the modeling has to provide the 

7 calculations of the NNOx and SSOx sinks (Nupt, Nimm, Nden, Nret, Sret), landscape characteristics (f 

and r) and critical leaching of acid anions (( [BC]0

* 8 - [ANClevel])Q), which are described below. 

9 

10 Given a maximum deposition load, ANC concentration at a site can be provided by the 

11 difference between future base cation and acid anion concentrations, which is equal to the 

12 predicted, steady-state sulfate (Sleach) and nitrate (Nleach) leaching fluxes: 

 [ANC] = [BC]0

* 13 - (Sleach + Nleach)/Q (10) 

14 where Sleach and Nleach are: 

15 Sleach = (1 – ps)Sdep (11)

16 Nleach = (1- pn)(Ndep – fNupt – (1 – r)(Nimm + Nden)) (12)

17 

18 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-28 August 2008 

1 8.3.3.1 Parameters 

2 Net growth uptake of NNOx in forest (Nupt) 

3 At steady-state or over long timescales, the only permanent sink for N (from any source) 

4 in the terrestrial system occurs when there is removal of biomass from the catchment. Short term, 

5 seasonal cycling of N does not affect the mean annual budget over these timescales. The main 

6 removal of N from the catchment is through the harvesting of the forest or grazing. There are 

7 various ways for estimating this N loss from the catchment, which take into account the N 

8 content of various tree species and the time periods of harvesting cycle. For forests without 

9 harvesting, like the Adirondack region of New York, Nupt is zero. 

10 

11 Immobilization (Nimm) and denitrification (Nden)of NNOx in catchment soils 

12 For long-term immobilization of N is forest soils, a constant value of 2 kg N / ha/ yr is 

13 used. This value represents the lower end of the range suggested from studies (Downing et al 

14 1993). Thus, the amount of N removed for a given NNOx depositional load is simply 

15 propositional to its total Ntot load (NNOx/Ntot) 

16 It is assumed that the denitrification is related to the net input of NNOx loading into the soil 

17 system after accounting for the removal of NNOx by immobilization and uptake as follows: 

18 Nden = fde(Nnox – Nimm – Nupt) (13) 

19 where fde is the denitrification faction (value between 0 and 1), ascribed to the catchment soil. 

20 Denitrification is an anaerobic process and therefore a function of soil moisture content, with 

21 higher rates in peat soils. A typical upland soils denitrify N at a rate of 10% of N input while 

22 peat soils work at a rate of 70 to 80 %. Therefore, the average denitrification fraction for a 

23 catchment is approximated by the following linear relationship: 

24 fde = 0.1 + 0.7fpeat (14) 

25 where fpeat is the proportion of peat soils within the catchment. 

26 

27 In-lake retention of NNOx and SSOx

28 The in-lake retention of acid anions is assumed to be a linear function of net input of acidity. It 

29 is sequentially the final sink of acidity encountered by inputs that move through the catchment. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-29 August 2008 

1 The in-lake retention fractions for NNOx (pn) and SSOx (ps) are calculated from a kinetic equation 

2 accounting for water retention time (Kelly et al 1987; Posch et al 1997). 

3 

4 pn = (sn / sn + Q/r) and Ps = (ss / ss + Q/r) (15) 

5 

6 where Q is the runoff and sn and ss are the net mass transfer coefficient for NNOx and SSOx. Due to 

7 a lack of data, the net mass transfer coefficient for sulfate (ss) was taken from a retention model 

8 calibration to mass balance data of 11 lakes located in North America and northern Europe 

9 (Baker and Brezonik 1988). The mean value of ss for these lakes, 0.5 m/yr, was used for the 

10 maximum depositional load calculations. The value for sn, 5 m/yr, was obtained from a similar 

11 study of 12, mostly Canadian, lakes (Dillon and Molot 1990). 

12 Pre-Industrial Base Cation Concentration 

The pre-industrial concentration of base cations [BC]0

* 13 effectively set the long term 

14 capacity of the catchment to neutralize acidic deposition, because it represents the only source of 

15 base cation input that is sustainable over the long-term. Input of cations from weathering is 

16 assumed to be a relatively constant process driven largely by the reaction of CO2 with primary 

17 minerals in the soils and bedrock. Base cations are removed by leaching from the soil solution 

18 through surface water runoff. At a steady-state, the leaching rate of base cation occurs at lesser or 

19 greater rates than the weathering supply. However, base cation leaching is not at steady-state 

20 today because anthropogenic acid deposition actually increases the leaching of base cations 

21 through ion-exchange within catchment soils. Soils contain a store of adsorbed base cations, as 

22 measured as base saturation, which are derived from weathering, but have accumulated in the 

23 soil over millennia, until eventually a steady-state is achieved, whereby the supply of base 

24 cations from weathering was in approximate equilibrium with the removal of base cations by 

rainwater, itself in equilibrium with the atmosphere. For this reason, [BC]0

* 25 cannot be derived 

26 from measured data in runoff, but derived from a empirical relationships (i.e., pre-industrial base 

27 cation concentration). 

The pre-industrial base cation concentration is the sum of weathering ([BC* 28 w]) supply 

plus base cation deposition ([BC* 29 dep]), if it is assumed that base cation deposition has not 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-30 August 2008 

1 significantly changed since pre-industrial times, minus long-term average uptake of base cations 

2 in the biomass (i.e., the annual average removal of base cations due to harvesting): 

 [BC]0

* = [BCw] + [BC* 3 dep] – [BCu]. (16) 

4 The first step is to quantify the proportion of measured base cation leaching which is 

5 derived from transient ion-exchange processes (BCex) and is proportional to the load of acid 

6 anions. This proposition is represented by the term ‘F’, calculated according to the methodology 

7 of Brakke and colleagues (1990): 

 F = sin (π/2 . [BC* 8 ]t/[S] (17) 

where [BC* 9 ]t is the measured non-marine base cation concentration and S is a constant which 

10 varies regionally according to geology, but from empirical studies is taken as 400 μeq/L 

11 (Harriman and Christie, 1995). This constant determines the measured non-marine base cation 

concentration which represents a catchment likely to be unaffected by acid deposition; [BC* 12 ]t = 

13 S, F =1 and base cation leaching is increased by exactly the value of the acid anion load, 

resulting in no change in the ANC of runoff. For values of [BC* 14 ]t greater then S, F is 1. 

15 This F value is then used to calculate the pre-industrial base cation concentration 

16 according to the following equation: 

 [BC*

]0

* = [BC*

]t – F([AA]t

*

 - [AA]0

* 17 ) (18) 

where [AA]0

* 18 is the pre-acidification concentration of non-marine acid anions from weathering 

19 and natural atmospheric sources and the measured leaching rate of non-marine base cations 

[BC* 20 ]t represent the sum of weathering, non-marine deposition and leaching sources. We 

assumed the steady-state concentration of nitrate ([AA]0) was zero ([AA]0

*

= 0). Since [BC*]0

* 21 is 

22 now known, the maximum depositional load can be defined by Equation 9. 

23 Catchment Parameters (r, f, and Q) 

24 Lake and catchment areas can be estimated from topographical maps (1:50,000) and from 

25 the EMAP lake survey. The annual runoff can be obtained by interpolation from a national 

26 runoff map (Krug et al., 1990) for water years 1951-80. The fraction of forest in the catchment 

27 can be estimated from 2001 National Land Cover Data (NLCD). 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-31 August 2008 

1 8.4 EXAMPLE APPLICATIONS OF SECONDARY NAAQS 

2 STRUCTURE 

3 This section is intended to provide an illustrative example applying actual data sets for 

4 the Adirondacks to the above derivations and linkage calculations. It is our intent that this 

5 section will be developed in the second draft of this document. 

6 8.5 REFERENCES 

7 Baker, L. A., and P. L. Brezonik. 1988. Dynamic model of in-lake alkalinity generation. Water 

8 Resources Research 24: 65–74. 

9 Brakke, D.F., A. Henriksen, and S.A. Norton. 1990. A variable F-factor to explain changes in 

10 base cation concentrations as a function of strong acid deposition. Verh. Internat. Verein. 

11 Limnol. 24:146–149. 

12 Clarke, J., Edgerton, E., Martin, B., 1997. Dry deposition calculations for the Clean Air Status 

13 and Trends Network. Atmospheric Environment 31 (21), 3667–3678. 

14 Davis, J. M., Swall, J. L., 2006. An examination of the CMAQ simulations of the wet deposition 

15 of ammonium from a Bayesian perspective. Atmospheric Environment 40, 4562–4573. 

16 Dillon, P. J., and L. A. Molot. 1990. The role of ammonium and nitrate in the acidification of 

17 lakes and forested catchments. Biogeochemistry 11:23–43. 

18 Downing, R. J., J.-P. Hettelingh, and P. A. M. de Smet (eds.). 1993. Calculation and mapping of 

19 critical loads in Europe, status report 1993. Coordination Center for Effects, National 

20 Institute of Public Health and Environmental Protection (RIVM), Bilthoven, The 

21 Netherlands. 

22 Harriman R, Christie AEG. 1995 Estimating critical loads for biota: the steady-state water 

23 chemistry Henriksen.model. Critical Loads of Acid Deposition for United Kingdom 

24 Freshwaters:7_8. 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-32 August 2008 

1 Kelly, C. A., J. W. M. Rudd, R. H. Hesslin, D. W. Schindler, P. J. Dillon, C. T. Driscoll, S. A. 

2 Gherini, and R. H. Heskey. 1987. Prediction of biological acid neutralization in acid 

3 sensitive lakes. Biogeochemistry 3:129–140. 

4 Krug, W. R., W. A. Gebert, D. J. Graczyk, D. L. Stevens, B. P. Rochelle, and M. R. Church, 

5 1990. Map of Mean Annual Runoff for the Northeastern, Southeastern, and Mid-Atlantic 

6 United States, Water Years 1951-1980. U.S. Geological Survey Water- Resources 

7 Investigations Report 88-4904, Madison, Wisconsin. 

8 Morris, R. E., McNally, D. E., Tesche, T. W., Tonnesen, G., Boylan, J. W., Brewer, P., 2005. 

9 Preliminary evaluation of the community multiscale air quality model for 2002 over the 

10 Southeastern United States. Journal of the Air and Waste Management Association 55, 

11 1694–1708. 

12 Posch, M, J.K.MA, R. Forsius, A. Henriksen, A. Wilander 1997 Exceedance of Critical Loads 

13 for Lakes in Finland, Norway, and Sweden: Reduction Requirements for Acidifying 

14 Nitrogen and Sulfur Deposition. Environmental Management Vol. 21, No. 2, pp. 291–

15 304. 

16 Seinfeld, J., Pandis, S., 1998. Atmospheric Chemistry and Physics. John Wiley and Sons, Inc., 

17 New York. 

18 U.S. EPA. 2008. Integrated Science Assessment for Oxides of Nitrogen and Sulfur – 

19 Environmental Criteria (Second External Review Draft). Prepared by U.S. Environmental 

20 Protection Agency, National Center for Environmental Assessment-RTP Division, Office 

21 of Research and Development, Research Triangle Park, NC. EPA/600/R-08-082. August. 

22 U.S. EPA, 1999. Science Algorithms of the EPA Models-3 Community Multiscale Air Quality 

23 (CMAQ) Modeling System. Tech. Rep. EPA/600/R-99/030, U.S. Environmental 

24 Protection Agency, Washington DC. 

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28 
Chapter 8 – Considerations in the Structure of the NOx/SOx Secondary Standard(s) 

DRAFT 8-33 August 2008 

1 APPENDIX A 

2 Land Use Categories in MCIP (from USGS). 

3 1. Urban Land 

4 2. Dryland Cropland and Pasture 

5 3. Irrigated Cropland and Pasture 

6 4. Mixed Dryland and Irrigated Cropland 

7 5. Cropland/Grassland Mosaic 

8 6. Cropland/Woodland Mosaic 

9 7. Grassland 

10 8. Shrubland 

11 9. Mixed Shrubland/Grassland 

12 10. Savannah 

13 11. Deciduous Broadleaf Forest 

14 12. Deciduous Needleleaf Forest 

15 13. Evergreen Broadleaf Forest 

16 14. Evergreen Needleleaf Forest 

17 15. Mixed Forest 

18 16. Water 

19 17. Herbaceous Wetland 

20 18. Wooded Wetland 

21 19. Barren or Sparsely Vegetated 

22 20. Herbaceous Tundra 

23 21. Wooded Tundra 

24 22. Mixed Tundra 

25 23. Bare Ground Tundra 

26 24. Snow or Ice 

27 
United States 

Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Health and Environmental Impacts Division 

Research Triangle Park, NC 

Publication No