As discussed throughout the REA (U.S. EPA, 2009 and section II.B above), there are several key areas of risk that are associated with ambient concentrations of oxides of nitrogen and sulfur. As noted earlier, in previous reviews of the secondary standards for oxides of nitrogen and sulfur, the standards were designed to protect against direct exposure of plants to ambient concentrations of the pollutants. A significant shift in understanding of the effects of oxides of nitrogen and sulfur has occurred since the last reviews, reflecting the large amount of research that has been conducted on the effects of deposition of nitrogen and sulfur to ecosystems. The most significant current risks of adverse effects to public welfare are those related to deposition of oxides of nitrogen and sulfur to both terrestrial and aquatic ecosystems. These risks fall into two categories, acidification and nutrient enrichment, which were emphasized in the REA as most relevant to evaluating the adequacy of the existing standards in protecting public welfare from adverse ecological effects.
(a)	Aquatic Acidification
	The focus of the REA case studies was to determine whether deposition of sulfur and oxidized nitrogen in locations where ambient oxides of nitrogen and sulfur were at or below the current standards resulted in acidification and related effects, including episodic acidification and mercury methylation. Based on the case studies conducted for lakes in the Adirondacks and streams in Shenandoah National Park (case studies are discussed more fully in section II.B and U.S. EPA, 2009), there is significant risk to acid sensitive aquatic ecosystems at atmospheric concentrations of oxides of nitrogen and sulfur at or below the current standards. The REA also strongly supports a relationship between atmospheric deposition of oxides of nitrogen and sulfur and loss of ANC in sensitive ecosystems and indicates that ANC is an excellent indicator of aquatic acidification. The REA also concludes that at levels of deposition associated with oxides of nitrogen and sulfur concentrations at or below the current standards, ANC levels are expected to be below benchmark values that are associated with significant losses in fish species richness.
	Significant portions of the United States are acid sensitive, and current deposition levels exceed those that would allow recovery of the most acid sensitive lakes in the Adirondacks (U.S. EPA, 2008, Executive Summary). In addition, because of past loadings, areas of the Shenandoah are sensitive to current deposition levels (U.S. EPA, 2008, Executive Summary). Parts of the West are naturally less sensitive to acidification and subjected to lower deposition (particularly oxides of sulfur) levels relative to the eastern United States, and as such, less focus in the ISA is placed on the adequacy of the existing standards in these areas, with the exception of the mountainous areas of the West, which experience episodic acidification due to deposition.
	In describing the effects of acidification in the two case study areas the REA uses the approach of describing benchmarks in terms of ANC values. Many locations in sensitive areas of the United States have ANC levels below benchmark levels for ANC classified as severe, elevated, or moderate concern (U.S. EPA, 2011, Figure 2-1). The average current ANC levels across 44 lakes in the Adirondack case study area is 62.1 μeq/L (moderate concern). However, 44 percent of lakes had deposition levels exceeding the critical load for an ANC of 50 μeq/L (elevated), and 28 percent of lakes had deposition levels exceeding the (higher) critical load for an ANC of 20 μeq/L (severe) (U.S. EPA, 2009, section <IP_ADDRESS>). This information indicates that almost half of the 44 lakes in the Adirondacks case study area are at an elevated concern level, and almost a third are at a severe concern level. These levels are associated with greatly diminished fish species diversity, and losses in the health and reproductive capacity of remaining populations. Based on assessments of the relationship between number of fish species and ANC level in both the Adirondacks and Shenandoah areas, the number of fish species is decreased by over half at an ANC level of 20 μeq/L relative to an ANC level at 100 μeq/L (U.S. EPA, 2009, Figure 4.2-1). When extrapolated to the full population of lakes in the Adirondacks area using weights based on the EMAP probability survey (U.S. EPA, 2009, section <IP_ADDRESS>), 36 percent of lakes exceeded the critical load for an ANC of 50 μeq/L and 13 percent of lakes exceeded the critical load for an ANC of 20 μeq/L.
	Many streams in the Shenandoah case study area also have levels of deposition that are associated with ANC levels classified as severe, elevated, or moderate concern. The average ANC under recent conditions for the 60 streams evaluated in the Shenandoah case study area is 57.9 μeq/L, indicating moderate concern. However, 85 percent of these streams had recent deposition exceeding the critical load for an ANC of 50 μeq/L, and 72 percent exceeded the critical load for an ANC of 20 μeq/L. As with the Adirondacks area, this information suggests that ANC levels may decline in the future and significant numbers of sensitive streams in the Shenandoah area are at risk of adverse impacts on fish populations if recent conditions persist. Many other streams in the Shenandoah area are also likely to experience conditions of elevated to severe concern based on the prevalence in the area of bedrock geology associated with increased sensitivity to acidification suggesting that effects due to stream acidification could be widespread in the Shenandoah area (U.S. EPA, 2009, section <IP_ADDRESS>).
	In addition to these chronic acidification effects, the ISA notes that "consideration of episodic acidification greatly increases the extent and degree of estimated effects for acidifying deposition on surface waters" (U.S. EPA, 2008, section <IP_ADDRESS>). Some studies show that the number of lakes that could be classified as acid-impacted based on episodic acidification is 2 to 3 times the number of lakes classified as acid-impacted based on chronic ANC. These episodic acidification events can have long-term effects on fish populations (U.S. EPA, 2008, section <IP_ADDRESS>). Under recent conditions, episodic acidification has been observed in locations in the eastern United States and in the mountainous western United States (U.S. EPA, 2008, section <IP_ADDRESS>).
	The ISA, REA and PA all conclude that the current standards are not adequate to protect against the adverse impacts of aquatic acidification on sensitive ecosystems. A recent survey, as reported in the ISA, found sensitive streams in many locations in the United States, including the Appalachian Mountains, the Coastal Plain, and the Mountainous West (U.S. EPA, 2008, section <IP_ADDRESS>). In these sensitive areas, between 1 and 6 percent of stream kilometers are chronically acidified. The REA further concludes that both the Adirondack and Shenandoah case study areas are currently receiving deposition from ambient oxides of nitrogen and sulfur in excess of their ability to neutralize such inputs. In addition, based on the current emission scenarios, forecast modeling out to the year 2020 as well as 2050 indicates a large number of streams in these areas will still be adversely impacted (section II.B). Based on these considerations, the PA concludes that the current secondary NAAQS for oxides of nitrogen and sulfur do not provide adequate protection of sensitive ecosystems with regard to aquatic acidification.
(b)	Terrestrial Acidification
	Based on the terrestrial acidification case studies, Kane Experimental Forest in Pennsylvania and Hubbard Brook Experimental Forest described in section II.B) of sugar maple and red spruce habitat, the REA concludes that there is significant risk to sensitive terrestrial ecosystems from acidification at atmospheric concentrations of NO2 and SO2 at or below the current standards. The ecological indicator selected for terrestrial acidification is the BC/Al, which has been linked to tree health and growth. The results of the REA strongly support a relationship between atmospheric deposition of oxides of nitrogen and sulfur and BC/Al, and that BC/Al is a good indicator of terrestrial acidification. At levels of deposition associated with oxides of nitrogen and sulfur concentrations at or below the current standards, BC/Al levels are expected to be below benchmark values that are associated with significant effects on tree health and growth. Such degradation of terrestrial ecosystems could affect ecosystem services such as habitat provisioning, endangered species, goods production (timber, syrup, etc.) among others.
	Many locations in sensitive areas of the United States have BC/Al levels below benchmark levels classified as providing low to intermediate levels of protection to tree health. At a BC/Al ratio of 1.2 (intermediate level of protection), red spruce growth can be reduced by 20 percent. At a BC/Al ratio of 0.6 (low level of protection), sugar maple growth can be decreased by 20 percent. The REA did not evaluate broad sensitive regions. However, in the sugar maple case study area (Kane Experimental Forest), recent deposition levels are associated with a BC/Al ratio below 1.2, indicating between intermediate and low level of protection, which would indicate the potential for a greater than 20 percent reduction in growth. In the red spruce case study area (Hubbard Brook Experimental Forest), recent deposition levels are associated with a BC/Al ratio slightly above 1.2, indicating slightly better than an intermediate level of protection (U.S. EPA, 2009, section <IP_ADDRESS>).
	Over the full range of sugar maple, 12 percent of evaluated forest plots exceeded the critical loads for a BC/Al ratio of 1.2, and 3 percent exceeded the critical load for a BC/Al ratio of 0.6. However, there was large variability across states. In New Jersey, 67 percent of plots exceeded the critical load for a BC/Al ratio of 1.2, while in several states on the outskirts of the range for sugar maple (e.g. Arkansas, Illinois) no plots exceeded the critical load for a BC/Al ratio of 1.2. For red spruce, overall 5 percent of plots exceeded the critical load for a BC/Al ratio of 1.2, and 3 percent exceeded the critical load for a BC/Al ratio of 0.6. In the major red spruce producing states (Maine, New Hampshire, and Vermont), critical loads for a BC/Al ratio of 1.2 were exceeded in 0.5, 38, and 6 percent of plots, respectively.
	The ISA, REA and PA all conclude that the current standards are not adequate to protect against the adverse impacts of terrestrial acidification on sensitive ecosystems. As stated in the REA and PA, the main drawback, with the understanding of terrestrial acidification lies in the sparseness of available data by which we can predict critical loads and that the data are based on laboratory responses rather than field measurements. Other stressors that are present in the field but that are not present in the laboratory may confound this relationship. The REA does however, conclude that the case study results, when extended to a 27 state region, show that nitrogen and sulfur acidifying deposition in the sugar maple and red spruce forest areas caused the calculated Bc/Al ratio to fall below 1.2 (the intermediate level of protection) in 12 percent of the sugar maple plots and 5 percent of the red spruce plots; however, results from individual states ranged from 0 to 67 percent of the plots for sugar maple and 0 to 100 percent of the plots for red spruce.
(c)	Terrestrial Nutrient Enrichment
	Nutrient enrichment effects are due to nitrogen loadings from both atmospheric and non-atmospheric sources. Evaluation of nutrient enrichment effects requires an understanding that nutrient inputs are essential to ecosystem health and that specific long-term levels of nutrients in a system affect the types of species that occur over long periods of time. Short-term additions of nutrients can affect species competition, and even small additions of nitrogen in areas that are traditionally nutrient poor can have significant impacts on productivity as well as species composition. Most ecosystems in the United States are nitrogen-limited, so regional decreases in emissions and deposition of airborne nitrogen compounds could lead to some decrease in growth of the vegetation that surrounds the targeted aquatic system but as discussed below evidence for this is mixed. Whether these changes in plant growth are seen as beneficial or adverse will depend on the nature of the ecosystem being assessed.
	Information on the effects of changes in nitrogen deposition on forestlands and other terrestrial ecosystems is very limited. The multiplicity of factors affecting forests, including other potential stressors such as ozone, and limiting factors such as moisture and other nutrients, confound assessments of marginal changes in any one stressor or nutrient in forest ecosystems. The ISA notes that only a fraction of the deposited nitrogen is taken up by the forests, most of the nitrogen is retained in the soils (U.S. EPA, 2008, section <IP_ADDRESS>). In addition, the ISA indicates that forest management practices can significantly affect the nitrogen cycling within a forest ecosystem, and as such, the response of managed forests to nitrogen deposition will be variable depending on the forest management practices employed in a given forest ecosystem (U.S. EPA, 2008, Annex C C.6.3). Increases in the availability of nitrogen in nitrogen-limited forests via atmospheric deposition could increase forest production over large non-managed areas, but the evidence is mixed, with some studies showing increased production and other showing little effect on wood production (U.S. EPA, 2008, section 3.3.9). Because leaching of nitrate can promote cation losses, which in some cases create nutrient imbalances, slower growth and lessened disease and freezing tolerances for forest trees, the net effect of increased N on forests in the United States is uncertain (U.S. EPA, 2008, section 3.3.9).
	The scientific literature has many examples of the deleterious effects caused by excessive nitrogen loadings to terrestrial systems. Several studies have set benchmark values for levels of N deposition at which scientifically adverse effects are known to occur. Large areas of the country appear to be experiencing deposition above these benchmarks. The ISA indicates studies that have found that at 3.1 kg N/ha/yr, the community of lichens begins to change from acidophytic to tolerant species; at 5.2 kg N/ha/yr, the typical dominance by acidophytic species no longer occurs; and at 10.2 kg N/ha/yr, acidophytic lichens are totally lost from the community. Additional studies in the Colorado Front Range of the Rocky Mountain National Park support these findings. These three values (3.1, 5.2, and 10.2 kg/ha/yr) are one set of ecologically meaningful benchmarks for the mixed conifer forest (MCF) of the pacific coast regions. Nearly all of the known sensitive communities receive total nitrogen deposition levels above the 3.1 N kg/ha/yr ecological benchmark according to the 12 km, 2002 CMAQ/NADP data, with the exception of the easternmost Sierra Nevadas. The MCFs in the southern portion of the Sierra Nevada forests and nearly all MCF communities in the San Bernardino forests receive total nitrogen deposition levels above the 5.2 N kg/ha/yr ecological benchmark.
	Coastal Sage Scrub communities are also known to be sensitive to community shifts caused by excess nitrogen loadings. Studies have investigated the amount of nitrogen utilized by healthy and degraded CSS systems. In healthy stands, the authors estimated that 3.3 kg N/ha/yr was used for CSS plant growth. It is assumed that 3.3 kg N/ha/yr is near the point where nitrogen is no longer limiting in the CSS community and above which level community changes occur, including dominance by invasive species and loss of coastal sage scrub. Therefore, this amount can be considered an ecological benchmark for the CSS community. The majority of the known CSS range is currently receiving deposition in excess of this benchmark. Thus, the REA concludes that recent conditions where oxides of nitrogen ambient concentrations are at or below the current oxides of nitrogen secondary standards are not adequate to protect against anticipated adverse impacts from N nutrient enrichment in sensitive ecosystems.
(d)	Aquatic Nutrient Enrichment
	The REA aquatic nutrient enrichment case studies focused on coastal estuaries and revealed that while current ambient loadings of atmospheric oxides of nitrogen are contributing to the overall depositional loading of coastal estuaries, other non-atmospheric sources are contributing in far greater amounts in total, although atmospheric contributions are as large as some other individual source types. The ability of current data and models to characterize the incremental adverse impacts of nitrogen deposition is limited, both by the available ecological indicators, and by the inability to attribute specific effects to atmospheric sources of nitrogen. The REA case studies used ASSETS EI as the ecological indicator for aquatic nutrient enrichment. This index is a six level index characterizing overall eutrophication risk in a water body. This indictor is not sensitive to changes in nitrogen deposition within a single level of the index. In addition, this type of indicator does not reflect the impact of nitrogen deposition in conjunction with other sources of nitrogen.
	Based on the above considerations, the REA concludes that the ASSETS EI is not an appropriate ecological indicator for estuarine aquatic eutrophication and that additional analysis is required to develop an appropriate indicator for determining the appropriate levels of protection from N nutrient enrichment effects in estuaries related to deposition of oxides of nitrogen. As a result, the EPA is unable to make a determination as to the adequacy of the existing secondary oxides of nitrogen standard in protecting public welfare from nitrogen nutrient enrichment effects in estuarine aquatic ecosystems.
	Additionally, nitrogen deposition can alter species composition and cause eutrophication in freshwater systems. In the Rocky Mountains, for example, deposition loads of 1.5 to 2 kg/ha/yr which are well within current ambient levels are known to cause changes in species composition in diatom communities indicating impaired water quality (U.S. EPA, 2008, section <IP_ADDRESS>). This suggests that the existing secondary standard for oxides of nitrogen does not protect such ecosystems and their resulting services from impairment.
(e)	Other Effects
	An important consideration in looking at the effects of deposition of oxides of sulfur in aquatic ecosystems is the potential for production of MeHg, a neurotoxic contaminant. The production of meaningful amounts of MeHg requires the presence of SO4[2-] and mercury, and where mercury is present, increased availability of SO4[2-] results in increased production of MeHg. There is increasing evidence on the relationship between sulfur deposition and increased methylation of mercury in aquatic environments; this effect occurs only where other factors are present at levels within a range to allow methylation. The production of MeHg requires the presence of SO4[2-]  and mercury, but the amount of MeHg produced varies with oxygen content, temperature, pH and supply of labile organic carbon (U.S. EPA, 2008, section 3.4). In watersheds where changes in sulfate deposition did not produce an effect, one or several of those interacting factors were not in the range required for meaningful methylation to occur (U.S. EPA, 2008, section 3.4). Watersheds with conditions known to be conducive to mercury methylation can be found in the northeastern United States and southeastern Canada (U.S. EPA, 2009, section 6).
	With respect to sulfur deposition and mercury methylation, the final ISA determined that "[t]he evidence is sufficient to infer a causal relationship between sulfur deposition and increased mercury methylation in wetlands and aquatic environments."  However, the EPA did not conduct a quantitative assessment of the risks associated with increased mercury methylation under current conditions. As such, the EPA is unable to make a determination as to the adequacy of the existing SO2 secondary standards in protecting against welfare effects associated with increased mercury methylation.
c.	Summary of Adequacy Considerations
	In summary, the PA concludes that currently available scientific evidence and assessments clearly call into question the adequacy of the current standards with regard to deposition-related effects on sensitive aquatic and terrestrial ecosystems, including acidification and nutrient enrichment. Further, the PA recognizes that the elements of the current standards--indicator, averaging time, level and form  -  are not ecologically relevant, and are thus not appropriate for standards designed to provide such protection. Thus, the PA concludes that consideration should be given to establishing a new ecologically relevant multi-pollutant, multimedia standard to provide appropriate protection from deposition-related ecological effects of oxides of nitrogen and sulfur on sensitive ecosystems, with a focus on protecting against adverse effects associated with acidifying deposition in sensitive aquatic ecosystems.
2.	CASAC Views
	In a letter to the Administrator (Russell and Samet 2011a), the CASAC Oxides of Nitrogen and Oxides of Sulfur Panel, with full endorsement of the chartered CASAC, unanimously concluded that:
      "EPA staff has demonstrated through the Integrated Science Assessment (ISA), Risk and Exposure Characterization (REA) and the draft PA that ambient NOx and SOx can have, and are having, adverse environmental impacts. The Panel views that the current NOx and SOx secondary standards should be retained to protect against direct adverse impacts to vegetation from exposure to gas phase exposures of these two families of air pollutants. Further, the ISA, REA and draft PA demonstrate that adverse impacts to aquatic ecosystems are also occurring due to deposition of NOx and SOx. Those impacts include acidification and undesirable levels of nutrient enrichment in some aquatic ecosystems. The levels of the current NOx and SOx secondary NAAQS are not sufficient, nor the forms of those standards appropriate, to protect against adverse depositional effects; thus a revised NAAQS is warranted."

	In addition, with regard to the joint consideration of both oxides of nitrogen and oxides of sulfur as well as the consideration of deposition-related effects, CASAC concluded that the PA had developed a credible methodology for considering such effects. The Panel stated that "the Policy Assessment develops a framework for a multi-pollutant, multimedia standard that is ecologically relevant and reflects the combined impacts of these two pollutants as they deposit to sensitive aquatic ecosystems."
3.	Administrator's Proposed Conclusions
	Based on the above considerations and taking into account CASAC advice, in the proposed rule the Administrator considered the adequacy of the current NO2 and SO2 secondary standards with regard to both direct effects on vegetation, as well as on deposition-related effects on sensitive ecosystems. With regard to direct phytotoxic effects on vegetation, the Administrator concluded that the current secondary standards are adequately protective, and thus proposed to retain the current NO2 and SO2 secondary standards for that purpose.
	With regard to deposition-related effects, the Administrator first considered the appropriateness of the structure of the current standards to address ecological effects of concern. Based on the evidence as well as considering the advice given by CASAC, the Administrator concluded that the elements of the current standards are not ecologically relevant and thus are not appropriate to provide protection of ecosystems. In considering the adequacy of protection with regard to deposition-related effects, the Administrator considered the full nature of ecological effects related to the deposition of ambient oxides of nitrogen and sulfur into sensitive ecosystems across the country. Based on the evidence and information evaluated in the ISA, REA, and PA, and taking into account CASAC advice, the Administrator concluded that current levels of oxides of nitrogen and sulfur are sufficient to cause acidification of both aquatic and terrestrial ecosystems, nutrient enrichment of terrestrial ecosystems and contribute to nutrient enrichment effects in estuaries that could be considered adverse, and that the current secondary standards do not provide adequate protection from such effects.
      Having reached these conclusions, the Administrator determined that it was appropriate to consider alternative standards that are ecologically relevant. These considerations, as discussed below in section III, supported the conclusion that the current secondary standards are neither appropriate nor adequate to protect against deposition-related effects.
C.	Comments on Adequacy of the Current Standards
	The above sections outline the effects evidence and assessments (section II.A) used by the Administrator to inform her proposed judgments about the adequacy of the current secondary NO2 and SO2 standards with regard to both direct effects associated with gas-phase oxides of nitrogen and sulfur (section II.B.1) as well effects associated with deposition of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecosystems (section II.B.2). This section discusses the comments received from the public regarding the adequacy of the current secondary standards with regard to both direct and deposition-related effects. Comments related to the EPA's authority to address deposition-related effects through the NAAQS are discussed above in section I.E. Comments related to the EPA's proposed conclusions regarding alternative secondary standards are discussed below in section III.D.
1.	Adequacy of Current Secondary Standards to Address Direct Effects
	The current secondary NO2 and SO2 secondary standards were set in 1971 to protect against direct effects of gaseous oxides of nitrogen and sulfur. For oxides of nitrogen, the current secondary NO2 standard is an annual standard set to protect against adverse effects on vegetation from direct exposure to ambient oxides of nitrogen. For oxides of sulfur, the current secondary standard is a 3-hour standard intended to provide protection for plants from the direct foliar damage associated with atmospheric concentrations of SO2. As discussed above in section II.B.1, the Administrator proposed to conclude that the current secondary standards are adequate to protect against direct phytotoxic effects on vegetation, and proposed to retain the current standards for that purpose. Many commenters supported the EPA's proposed decision to retain the current secondary standards for various reasons related to their comments on alternative standards (as discussed below in section III.D), a few commenters (Alliance of Automobile Manufacturers (AAM), Pennsylvania Dept. of Environmental Protection) specifically expressed the view that the current standards provide requisite protection from the direct effects on vegetation from exposures to gaseous oxides of nitrogen and sulfur, and no commenters opposed retention of the current secondary standards.
2.	Adequacy of Current Secondary Standards to Address Deposition-related Effects
	As discussed above in section II.B.2, with regard to deposition-related effects, the Administrator proposed to conclude that the elements of the current secondary standards are not ecologically relevant, and thus not appropriate to provide protection of ecosystems, and that they do not provide adequate protection from such acidification and nutrient enrichment effects in both aquatic and terrestrial ecosystems. Having reached these proposed conclusions, she determined that it was appropriate to consider alternative standards that are ecologically relevant.
      One group of commenters that addressed the adequacy of the current standards with regard to deposition-related effects included environmental organizations (Earthjustice, on behalf of the Appalachian Mountain Club, National Parks Conservation Association, Sierra Club, and Clean Air Council; the Center for Biological Diversity; the Nature Conservancy; Adirondack Council; Chesapeake Bay Foundation), the U.S. Department of the Interior, NESCAUM, New York Dept. of Environmental Conservation, and two tribes. These commenters generally expressed the view that the current secondary standards do not provide adequate protection from deposition-related effects. More specifically, some of these commenters stated that there was overwhelming evidence of adversity to sensitive aquatic ecosystems from acidifying deposition. These commenters cited a broad range of scientific evidence that aquatic acidification was ongoing under current conditions allowed by the current secondary standards, and that this acidification represented an adverse effect on public welfare. Several commenters noted that CASAC had agreed that deposition-related effects were ongoing and harmful and that current standards were not adequate to prevent these effects.
	Among these commenters, some also expressed the view that current standards were not adequate to protect against terrestrial acidification or nutrient enrichment. The Department of the Interior as well as Earthjustice noted that the current standards were not sufficient for these additional endpoints and cited ongoing harm under current conditions. Two tribes and the Center for Biological Diversity expressed the view that there was sufficient information to judge that the current standards were not adequate to protect against the adverse welfare effect of mercury methylation, contrary to the EPA's proposed conclusion that the available evidence was not sufficient to reach such a judgment. For example, The Forest County Potawatomi Community provided several citations regarding the relationships between aquatic acidification and mercury methylation and stated that there was sufficient evidence to find that the current standards were not adequate.
	With regard to the adequacy of the current secondary standards for NO2 and SO2, the EPA concurs with commenters' assertions that the current standards do not provide adequate protection for ecosystems that are sensitive to aquatic acidification and that effects to these ecosystems are ongoing from ambient deposition of oxides of nitrogen and oxides of sulfur. The EPA also agrees that there is sufficient evidence to conclude that ambient deposition under the current secondary standards is causing or contributing to terrestrial acidification as well as nutrient enrichment in sensitive ecosystems. A complete discussion of considerations with regard to adequacy can be found in section II.B above. In short, the ISA has established that the major effects of concern for this review of the oxides of nitrogen and sulfur standards are associated with deposition of nitrogen and sulfur caused by atmospheric concentrations of oxides of nitrogen and sulfur. The current standards are not directed toward depositional effects, and none of the elements of the current NAAQS  -  indicator, form, averaging time, and level  -  are suited for addressing the effects of nitrogen and sulfur deposition. Additionally, although the proportion of total nitrogen loadings associated with atmospheric deposition of nitrogen varies across locations, the ISA indicates that atmospheric nitrogen deposition is the main source of new anthropogenic nitrogen to most headwater streams, high elevation lakes, and low-order streams. Atmospheric nitrogen deposition contributes to the total nitrogen load in terrestrial, wetland, freshwater and estuarine ecosystems that receive nitrogen through multiple pathways.
	There are expansive data to indicate that the levels of deposition under the current standards are not sufficient to prevent adverse effects in ecosystems. With regard to aquatic acidification, recent data indicate that in the Adirondacks and Shenandoah areas, rates of acidifying deposition of oxides of nitrogen and sulfur are still well above pre-acidification (1860) conditions. Forty-four percent of Adirondack lakes and 85 percent of Shenandoah streams evaluated exceed the critical load for an ANC of 50 μeq/L, and have suffered loss of sensitive fish species. With regard to terrestrial acidification, the REA evaluated a small number of sensitive areas as case studies and showed the potential for reduced growth. When the methodology was extended to a 27-state region, similar results were found to indicate the potential for growth effects in sensitive forests. Nitrogen deposition can alter species composition and cause eutrophication in freshwater systems. In the Rocky Mountains, for example, current deposition levels, which are within the range associated with ambient nitrogen oxide levels meeting the current standard, are known to cause changes in species composition in diatom communities indicating impaired water quality. With regard to terrestrial nutrient enrichment, most terrestrial ecosystems in the United States are nitrogen-limited, and therefore they are sensitive to perturbation caused by nitrogen additions. Under recent conditions, nearly all of the known sensitive mixed conifer forest ecosystems receive total nitrogen deposition levels above the ecological benchmark for changes in lichen species. In addition, in Coastal Sage Scrub ecosystems in California, nitrogen deposition exceeds the benchmark above which nitrogen is no longer a limiting nutrient, leading to potential alterations in ecosystem composition. Therefore, the EPA concludes that the current standards are not adequate for these effects.
	The EPA, however, while agreeing that there is a causal effect between deposition of sulfur and mercury methylation disagrees that there is sufficient evidence to make the quantitative associations that would be necessary to determine that the current standards were not adequate to protect against mercury methylation. The ISA concluded that evidence is sufficient to infer a casual relationship between sulfur deposition and increased mercury methylation in wetlands and aquatic environments. Since the rate of mercury methylation varies according to several spatial and biogeochemical factors whose influence has not been fully quantified, the correlation between sulfur deposition and methylmercury could not be quantified for the purpose of interpolating the association across waterbodies or regions. Therefore, since we are unable to quantify the relationship between atmospherically deposited oxides of sulfur and mercury methylation we cannot we cannot assess adequacy of protection. This subject is discussed more fully in section 6.2 of the REA (EPA 2009).
	Another group of commenters, (e.g. Utility Air Regulatory Group (UARG), Electric Power Research Institute (EPRI), American Petroleum Institute (API), AAM, and American Road and Transportation Builders Association (ARTBA)) generally took the position that the currently available information was not sufficient to make informed judgments about the adequacy of the current standards to address aquatic acidification effects. These commenters generally based this view on the complex nature of the interactions between pollutants and ecosystems and uncertainties in the models and analyses considered in this review. Several commenters asserted that there was not sufficient data available to determine the relationship between acidifying deposition of oxides of nitrogen and sulfur and adverse effects on aquatic ecosystems, such that there was not sufficient information to allow for the assessment of the adequacy of the current standards to provide appropriate protection from this effect. For example, AAM noted the uncertainties in models relating to dry deposition and questioned the linkages between ambient concentrations of oxides of nitrogen and sulfur and the amount of nitrogen and sulfur deposition. In addition to commenting on data limitations, UARG also expressed the view that the ecosystem services analyses included in the proposal were insufficient to make judgments about adversity to aquatic ecosystems resulting from acidifying deposition and that there is a lack of evidence demonstrating that quantifiable changes in public welfare would result from reductions in acidifying deposition. Many commenters within this group did not directly comment on the adequacy of the current standards to protect against aquatic acidification or other deposition-related effects, but instead expressed the view that the EPA did not have the authority to consider deposition-related effects in general or aquatic acidification in particular through the NAAQS. This comment and the EPA's response are discussed above in section I.E.
	With regard to the adequacy of the current standards to protect against aquatic acidification, the EPA disagrees with commenters' assertion that there is insufficient data to make linkages between deposition from the atmosphere and aquatic acidification effects. To the contrary, the EPA is confident that there is sufficient robust science to conclude that aquatic acidification is ongoing in sensitive ecosystems, that ambient deposition of oxides of nitrogen and oxides of sulfur are causative in many ecosystems nationwide and that the current standards are neither appropriate in form nor adequate in level to protect against such effects. The ISA concluded that there was a causal relationship between deposition of oxides of nitrogen and sulfur and NHx and acidification of ecosystems. In addition, the ISA found that effects of acidifying deposition on ecosystems have been well studied over the past several decades, that vulnerable areas have been identified for the United States and that the wealth of available data has led to the development of robust ecological models used for predicting soil and surface water acidification. With regard to the scope of effects, the REA also concluded that the available data are robust and considered high quality. There is high confidence about the use of these data and their value for extrapolating to larger spatial areas.   The EPA TIME/LTM network represents a source of long-term, representative sampling. Data on sulfate concentrations, nitrate concentrations and ANC from 1990 to 2006 used for this analysis as well as the EPA EMAP and Regional Environmental Monitoring and Assessment Program (REMAP) surveys, provide considerable data on surface water trends.
	The EPA also disagrees with commenters' assessment of limitations in wet and dry deposition modeling. Further discussion of characterizing deposition with models can be found in section IV.C. Additionally, while the EPA recognizes that there are limitations associated with modeled deposition values, the linkages between model estimates of deposition and areas exhibiting aquatic acidification effects are consistent and persuasive in considering adequacy of the current standard. Section 2.3 of the PA and sections 2.8 and 2.10 of the ISA provide additional detailed discussions of deposition modeling and spatial resolution for deposition. CASAC concurred with the EPA's conclusion on this matter and encouraged the EPA to move forward in developing a new form of a standard which would address aquatic acidification. Thus, while the EPA is fully mindful of the limitations and uncertainties associated with the data and models, the EPA concludes that the available evidence provides strong scientific support for the view that harm from aquatic acidification is ongoing and attributable in large part to atmospheric deposition of reactive nitrogen and sulfur.
	With regard to the commenters' reliance on ecosystem services analyses included in the proposal to make judgments about adversity and public welfare, the EPA disagrees that comprehensive ecosystems services analyses are necessary to determine adversity. Ecosystem services analyses are used in this review to inform the decisions made with regard to adequacy and as such are used in conjunction with other considerations in the discussion of adversity to public welfare. Section 4 of the PA further refines this discussion of adversity to public welfare. Additionally, the paradigm of adversity to public welfare as deriving from disruptions in ecosystem structure and function has been used broadly by the EPA to categorize effects of pollutants from the cellular to the ecosystem level. An evaluation of adversity to public welfare might consider the likelihood, type, magnitude, and spatial scale of the effect as well as the potential for recovery and any uncertainties relating to these considerations. Within this context, ecosystems services analyses are one of many tools used in this review to help inform the Administrator's decision on adversity. The EPA concludes that the analyses performed as part of this review are sufficient to support the decisions made by the Administrator with regard to the adequacy of the current standards.
D.	Final Decisions on the Adequacy of the Current Standards
	Based on the considerations discussed above, including CASAC advice and public comments, the Administrator believes that the conclusions reached in the proposed rule with regard to the adequacy of the current secondary standards for oxides of nitrogen and sulfur for direct and deposition-related effects continue to be valid. The Administrator recognizes that the purpose of the secondary standard is to protect against "adverse" effects resulting from exposure to oxides of nitrogen and sulfur, discussed above in section II.A. The Administrator also recognizes the need for conclusions as to the adequacy of the current standards for both direct and deposition-related effects as well as conclusions as to the appropriateness and ecological relevance of the current standards.
	In considering what constitutes an ecological effect that is also adverse to the public welfare, the Administrator took into account the ISA conclusions regarding the nature and strength of the effects evidence, the risk and exposure assessment results, the degree to which the associated uncertainties should be considered in interpreting the results, the conclusions presented in the PA, and the views of CASAC and members of the public. On these bases, the Administrator concludes that the current secondary standards are adequate to protect against direct phytotoxic effects on vegetation. Thus, the Administrator has decided to retain the current secondary standards for oxides of nitrogen at 53 ppb, annual average concentration, measured in the ambient air as NO2, and the current secondary standard for oxides of sulfur at 0.5 ppm, 3-hour average concentration, measured in the ambient air as SO2.
	With regard to deposition-related effects, the Administrator first considered the appropriateness of the structure of the current secondary standards to address ecological effects of concern. Based on the evidence as well as considering the advice given by CASAC and public comments on this matter, the Administrator concludes that the elements of the current standards are not ecologically relevant and thus are not appropriate to provide protection of ecosystems. On the subject of adequacy of protection with regard to deposition-related effects, the Administrator considered the full nature of ecological effects related to the deposition of ambient oxides of nitrogen and sulfur into sensitive ecosystems across the country. Her conclusions are based on the evidence presented in the ISA with regard to acidification and nutrient enrichment effects, the findings of the REA with regard to scope and severity of the current and likely future effects of deposition, the synthesis of both the scientific evidence and risk and exposure results in the PA as to the adequacy of the current standards, and the advice of CASAC and public comments. After such consideration, the Administrator concludes that current levels of oxides of nitrogen and sulfur are sufficient to cause acidification of both aquatic and terrestrial ecosystems, nutrient enrichment of terrestrial ecosystems and contribute to nutrient enrichment effects in estuaries that could be considered adverse, and the current secondary standards do not provide adequate protection from such effects
	Having reached these conclusions, the Administrator determined that it was appropriate to consider alternative standards that are ecologically relevant, as discussed below in section III. These considerations further support her conclusion that the current secondary standards for oxides of nitrogen and sulfur are neither appropriate nor adequate to protect against deposition-related effects.
III.	Rationale for Final Decisions on Alternative Secondary Standards
	This section presents the rationale for the Administrator's final decisions regarding alternative secondary standards for oxides of nitrogen and sulfur to address deposition-related effects. Section III.A provides an overview of the aquatic acidification index (AAI) approach presented in the PA to address such effects related to aquatic acidification. Advice from CASAC on such a new approach is presented in section III.B. The Administrator's proposed conclusions on an AAI-based standard are presented in section III.C. Comments on an AAI-based standard are discussed in section III.D as well as in the Response to Comments document. The Administrator's final decisions regarding alternative secondary standards are presented in section III.E
A.	Overview of AAI Approach
	Having reached the conclusion in the proposal that the current NO2 and SO2 secondary standards are not adequate to provide appropriate protection against potentially adverse deposition-related effects associated with oxides of nitrogen and sulfur, the Administrator then considered what new multi-pollutant standard might be appropriate, at this time, to address such effects on public welfare. The Administrator recognizes that the inherently complex and variable linkages between ambient concentrations of nitrogen and sulfur oxides, the related deposited forms of nitrogen and sulfur, and the ecological responses that are associated with public welfare effects call for consideration of a standard with an ecologically relevant design  that reflects these linkages. The Administrator also recognizes that characterization of such complex and variable linkages in this review requires consideration of information and analyses that have important limitations and uncertainties.
	Despite its complexity, an ecologically relevant multi-pollutant standard to address deposition-related effects would still appropriately be defined in terms of the same basic elements that are used to define any NAAQS  -  indicator, form, averaging time, and level. The form would incorporate additional structural elements that reflect relevant multi-pollutant and multimedia attributes. These structural elements include the use of an ecological indicator, tied to the ecological effect we are focused on, and other elements that account for ecologically relevant factors other than ambient air concentrations. All of these elements would be needed to enable a linkage from ambient air indicators to the relevant ecological effect to define an ecologically relevant standard. As a result, such a standard would necessarily be more complex than the NAAQS that have been set historically to address effects associated with ambient concentrations of a single pollutant.
	More specifically, the Administrator considered an ecologically relevant multi-pollutant standard to address effects associated with acidifying deposition-related to ambient concentrations of oxides of nitrogen and sulfur in sensitive aquatic ecosystems. This focus is consistent with the information presented in the ISA, REA, and PA, which highlighted the greater quantity and quality of the available evidence and assessments associated with aquatic acidification relative to the information and assessments available for other deposition-related effects, including terrestrial acidification and aquatic and terrestrial nutrient enrichment. Based on its review of these documents, CASAC agreed that aquatic acidification should be the focus for developing a new multi-pollutant standard in this review. In reaching conclusions about an air quality standard designed to address deposition-related aquatic acidification effects, the Administrator also recognizes that such a standard may also provide some degree of protection against other deposition-related effects.
	As discussed in chapter 7 of the PA, the development of a new multi-pollutant ambient air quality standard to address deposition-related aquatic acidification effects recognizes that it is appropriate to consider a nationally applicable standard for protection against adverse effects of aquatic acidification on public welfare. At the same time, the PA recognizes the complex and heterogeneous interactions between ambient air concentrations of nitrogen and sulfur oxides, the related deposition of nitrogen and sulfur, and associated ecological responses. The development of such a standard also needs to take into account the limitations and uncertainties in the available information and analyses upon which characterization of such interactions are based. The approach used in the PA also recognizes that while such a standard would be national in scope and coverage, the effects to public welfare from aquatic acidification will not occur to the same extent in all locations in the United States, given the inherent variability of the responses of aquatic systems to the effects of acidifying deposition. This contrasts with the relatively more homogeneous relationships between ambient air concentrations of air pollutants and the associated inhalation exposures and related public health responses that are typically considered in setting primary NAAQS.
	As discussed above in section II-A, many locations in the United States are naturally protected against acid deposition due to underlying geological conditions. Likewise, some locations in the United States, including lands managed for commercial agriculture and forestry, are not likely to be negatively impacted by current levels of nitrogen and sulfur deposition. As a result, while a new ecologically relevant secondary standard would apply everywhere, it would be structured to account for differences in the sensitivity of ecosystems across the country. This would allow for appropriate protection of sensitive aquatic ecosystems, which are relatively pristine and wild and generally in rural areas, and the services provided by such sensitive ecosystems, without requiring more protection than is needed elsewhere.
	As discussed below, the multi-pollutant standard developed in the PA would employ (1) total reactive oxidized nitrogen (NOy) and oxides of sulfur (SOx) as the atmospheric ambient air indicators; (2) a form that takes into account variable factors, such as atmospheric and ecosystem conditions that modify the amounts of deposited nitrogen and sulfur; the distinction between oxidized and reduced forms of nitrogen; effects of deposited nitrogen and sulfur on aquatic ecosystems in terms of the ecological indicator ANC; and the representativeness of water bodies within a defined spatial area; (3) a multi-year averaging time, and (4) a standard level defined in terms of a single, national target ANC value that, in the context of the above form, identifies the various levels of concentrations of NOy and SOx in the ambient air that would meet the standard. The form of such a standard has been defined by an index, AAI, which reflects the relationship between ambient concentrations of NOy and SOx and aquatic acidification effects that result from nitrogen and sulfur deposition-related to these ambient concentrations.
	In summarizing the considerations associated with such an air quality standard to address deposition-related aquatic acidification effects, as discussed more fully in sections III.A-F of the proposal and in the PA, the following sections focus on each element of the standard, including ambient air indicators (section III.A.1), form (section III.A.2), averaging time (section III.A.3), and level (section III.A.4). Considerations related to important uncertainties inherent in such an approach are discussed in section III.A.5.
1.	Ambient Air Indicators
	The PA concludes that ambient air indicators other than NO2 and SO2 should be considered as the appropriate indicators of oxides of nitrogen and sulfur in the ambient air for protection against the acidification effects associated with deposition of the associated nitrogen and sulfur. This conclusion is based on the recognition that all forms of nitrogen and sulfur in the ambient air contribute to deposition and resulting acidification, and as such, NO2 and SO2 are incomplete ambient air indicators. In principle, the indicators should represent the species that are associated with oxides of nitrogen and sulfur in the ambient air and can contribute acidifying deposition. This includes both the species of oxides of nitrogen and sulfur that are directly emitted as well as species transformed in the atmosphere from oxides of nitrogen and sulfur that retain the nitrogen and sulfur atoms from directly emitted oxides of nitrogen and sulfur. All of these compounds are associated with oxides of nitrogen and sulfur in the ambient air and can contribute to acidifying deposition.
	The PA focuses in particular on the various compounds with nitrogen or sulfur atoms that are associated with oxides of nitrogen and sulfur, because the acidifying potential is specific to nitrogen and sulfur, and not other atoms (e.g., H, C, O) whether derived from the original source of oxides of nitrogen and sulfur emissions or from atmospheric transformations. For example, the acidifying potential of each molecule of NO2, NO, HNO3 or PAN is identical, as is the potential for each molecule of SO2 or ion of particulate sulfate (p-SO4). Each atom of sulfur affords twice the acidifying potential of each atom of nitrogen.
a.	Oxides of Sulfur
	As discussed in the PA (U.S. EPA, 2011, section 7.1.1), oxides of sulfur include the gases sulfur monoxide (SO), SO2, sulfur trioxide (SO3), disulfur monoxide (S2O), and particulate-phase sulfur compounds (referred to as SO4) that result from gas-phase sulfur oxides interacting with particles. However, the sum of SO2 and SO4 does represent virtually the entire ambient air mass of sulfur that contributes to acidification. In addition to accounting for virtually all the potential for acidification from oxidized sulfur in the ambient air, there are reliable methods to monitor the concentrations of SO2 and particulate SO4. The PA concludes that the sum of SO2 and SO4, referred to as SOx, are appropriate ambient air indicators of oxides of sulfur because they represent virtually all of the acidification potential of ambient air oxides of sulfur and there are reliable methods suitable for measuring SO2 and SO4.
b.	Oxides of Nitrogen
	As discussed in the PA (U.S. EPA, 2011, section 7.1.2), NOy, as defined in chapter 2 of the PA, incorporates basically all of the oxidized nitrogen species that have acidifying potential and as such, NOy should be considered as an appropriate indicator for oxides of nitrogen. Total reactive oxidized nitrogen is an aggregate measure of NO and NO2 and all of the reactive oxidized products of NO and NO2. That is, NOy is a group of nitrogen compounds in which all of the compounds are either an oxide of nitrogen or compounds in which the nitrogen atoms came from oxides of nitrogen. Total reactive oxidized nitrogen is especially relevant as an ambient indicator for acidification in that it both relates to the oxides of nitrogen in the ambient air and also represents the acidification potential of all oxidized nitrogen species in the ambient air, whether an oxide of nitrogen or derived from oxides of nitrogen. The merits of other individual NOy species, particularly total nitrate, are discussed in section 2 of the PA.
2.	Form
	Based on the evidence of the aquatic acidification effects caused by the deposition of NOy and SOx, the PA (U.S. EPA, 2011, section 7.2) presents the development of a new form that is ecologically relevant for addressing such effects. The conceptual design for the form of such a standard includes three main components:  an ecological indicator, deposition metrics that relate to the ecological indicator, and a function that relates ambient air indicators to deposition metrics. Collectively, these three components link the ecological indicator to ambient air indicators, as illustrated below in Fig III-1.

Figure 1II-1. Conceptual design of the form of an aquatic acidification standard for oxides of nitrogen and sulfur
The simplified flow diagram in Figure III-1 compresses the various atmospheric, biological, and geochemical processes associated with acidifying deposition to aquatic ecosystems into a simplified conceptual picture. The ecological indicator (left box) is related to atmospheric deposition through biogeochemical ecosystem models (middle box), which associate a target deposition load to a target ecological indicator. Once a target deposition is established, associated allowable air concentrations are determined (right box) through the relationships between ambient air concentration and deposition that are embodied in air quality models such as CMAQ. The PA describes the development and rationale for each of these components, as well the integration of these components into the full expression of the form of the standard using the concept of a national AAI that represents a target ANC level as a function of ambient air concentrations.
	The AAI was designed to be an ecologically relevant form of the standard that determines the levels of NOy and SOx in the ambient air that would achieve a target ANC limit for the United States. The intent of the AAI is to weight atmospheric concentrations of oxides of nitrogen and sulfur by their propensity to contribute to acidification through deposition, given the fundamental acidifying potential of each pollutant, and to take into account the ecological factors that govern acid sensitivity in different ecosystems. The index also accounts for the contribution of reduced nitrogen to acidification. Thus, the AAI encompasses those attributes of specific relevance to protecting ecosystems from the acidifying potential of ambient air concentrations of NOy and SOx.
a.	Ecological Indicator
 	This section summarizes the rationale in the PA for selecting ANC as the appropriate ecological indicator for consideration. Recognizing that ANC is not itself the causative or toxic agent for adverse aquatic acidification effects, the rationale for using ANC as the relevant ecological indicator is based on the following:
(1) The ANC is directly associated with the causative agents, pH and dissolved Al, both through empirical evidence and mechanistic relationships;
(2) Empirical evidence shows very clear and strong relationships between adverse effects and ANC;
(3) The ANC is a more reliable indicator from a modeling perspective, allowing use of a body of studies and technical analyses related to ANC and acidification to inform the development of the standard; and
(4) The ANC embodies the concept of acidification as posed by the basic principles of acid base chemistry and the measurement method used to estimate ANC and, therefore, serves as a direct index to protect against acidification.
	Because ANC links both to biological effects of aquatic acidification as well as to acidifying inputs of NOy and SOx deposition, the PA concludes that ANC is an appropriate ecological indicator for relating adverse aquatic ecosystem effects to acidifying atmospheric deposition of SOx and NOy, and is preferred to other potential indicators. In reaching this conclusion, the PA notes that in its review of the first draft PA, CASAC concluded that "information on levels of ANC protective to fish and other aquatic biota has been well developed and presents probably the lowest level of uncertainty in the entire methodology" (Russell and Samet, 2010a). In its more recent review of the second draft PA, CASAC agreed "that acid neutralizing capacity is an appropriate ecological measure for reflecting the effects of aquatic acidification" (Russell and Samet, 2010b; p. 4).
b.	Linking ANC to Deposition
	There is evidence to support a quantified relationship between deposition of nitrogen and sulfur and ANC. This relationship was analyzed in the REA for two case study areas, the Adirondack and Shenandoah Mountains, based on time-series modeling and observed trends. In the REA analysis, long-term trends in surface water nitrate, sulfate and ANC were modeled using MAGIC for the two case study areas. These data were used to compare recent surface water conditions in 2006 with preindustrial conditions (i.e. preacidification 1860). The results showed a marked increase in the number of lakes affected by acidifying deposition, characterized as a decrease in ANC levels, since the onset of anthropogenic nitrogen and sulfur deposition, as discussed in chapter 2 of the PA.
	In the REA, the quantified relationship between deposition and ANC was investigated using ecosystem acidification models, also referred to as acid balance models or critical loads models (U.S. EPA, 2011, section 2 and U.S. EPA, 2009, section 4 and Appendix 4). These models quantify the relationship between deposition of nitrogen and sulfur and the resulting ANC in surface waters based on an ecosystem's inherent generation of ANC and ability to neutralize nitrogen deposition through biological and physical processes. A critical load is defined as the amount of acidifying atmospheric deposition of nitrogen and sulfur beyond which a target ANC is not reached. Relatively high critical load values imply that an ecosystem can accommodate greater deposition levels than lower critical loads for a specific target ANC level. Ecosystem models that calculate critical loads form the basis for linking deposition to ANC.
	As discussed in chapter 2 of the PA, both dynamic and steady-state models calculate ANC as a function of ecosystem attributes and atmospheric nitrogen and sulfur deposition, and can be used to calculate critical loads. Steady-state models are time invariant and reflect the long-term consequences associated with an ecosystem reaching equilibrium under a constant level of atmospheric deposition.   Dynamic models are time variant and take into account the time dependencies inherent in ecosystem hydrology, soil and biological processes. Dynamic models like MAGIC can provide the time-series response of ANC to deposition whereas steady-state models provide a single ANC relationship to any fixed deposition level. Dynamic models naturally are more complex than steady-state models as they attempt to capture as much of the fundamental biogeochemical processes as practicable, whereas steady-state models depend on far greater parameterization and generalization of processes that is afforded, to some degree, by not having to account for temporal variability.
	In the PA, a steady-state model is used to define the relevant critical load, which is the amount of atmospheric deposition of nitrogen (N) and sulfur (S) beyond which a target ANC is not achieved and sustained.  It is expressed as:
CLANClim(N + S) = ([BC]0[*] - [ANClim])Q + Neco					(III-1)
Where:
 CLANClim(N + S) is the critical load of deposition, with units of equivalent charge/(area-time);
 [BC]0[*] is the natural contribution of base cations from weathering, soil processes and preindustrial deposition, with units of equivalent charge/volume;
[ANClim] is the target ANC value, with units of equivalent charge/volume;
Q is the catchment level runoff rate governed by water mass balance and dominated by precipitation, with units of distance/time; and
Neco is the amount of nitrogen deposition that is effectively neutralized by a variety of biological (e.g., nutrient uptake) and physical processes, with units of equivalent charge/(area-time).
	Equation III-1 is a modified expression that adopts the basic formulation of the steady-state models that are described in chapter 2 of the PA. More detailed discussion of the rationale, assumptions and derivation of equation III-1, as well as all of the equations in this section, are included in Appendix B of the PA. The equation simply reflects the amount of deposition of nitrogen and sulfur from the atmosphere, CLANClim(N + S), that is  associated with a sustainable long-term ANC target, [ANClim], given the capacity of the natural system to generate ANC, [BC]0[*], and the capacity of the natural system to neutralize nitrogen deposition, Neco. This expression of critical load is valid when nitrogen deposition is greater than Neco.  The runoff rate, Q, allows for balancing mass in the two environmental mediums  -  atmosphere and catchment. This critical load expression can be focused on a single water system or more broadly. To extend applicability of the critical load expression (equation III-1) from the catchment level to broader spatial areas, the terms Qr and CLr, are used, which are the runoff rate and critical load, respectively, of the region over which all the atmospheric terms in the equation are defined.
	As presented above, the terms S and N in the CLANClim (N + S) term broadly represent all species of sulfur or nitrogen that can contribute to acidifying deposition. This follows conventions used in the scientific literature that addresses critical loads, and it reflects all possible acidifying contributions from any sulfur or nitrogen species. For all practical purposes, S reflects SOx as described above, the sum of sulfur dioxide gas and particulate sulfate. However, N in equation III-1 includes both oxidized forms, consistent with the ambient indicator, NOy, in addition to the reduced nitrogen species, ammonia and ammonium ion, referred to as NHx. The NHx is included in the critical load formulation because it contributes to potentially acidifying nitrogen deposition. Consequently, from a mass balance or modeling perspective, the form of the standard needs to account for NHx, as described below.
c.	Linking Deposition to Ambient Air Indicators
	The last major component of the form illustrated in Figure III-1 addresses the linkage between deposition of nitrogen and sulfur and concentrations of the ambient air indicators, NOy and SOx. To link ambient air concentrations with deposition, the PA defines a transference ratio, T, as the ratio of total wet and dry deposition to ambient concentration, consistent with the area and time period over which the standard is defined. To express deposition of NOy and SOx in terms of NOy and SOx ambient concentrations, two transference ratios were defined, where TSOx equals the ratio of the combined dry and wet deposition of SOx to the ambient air concentration of SOx, and TNOy equals the ratio of the combined dry and wet deposition of NOy to the ambient air concentration of NOy.
	As described in chapter 7 of the PA, reduced forms of nitrogen (NHx) are included in total nitrogen in the critical load equation, III-1. Reduced forms of nitrogen are treated separately, as are NOy and SOx, and the transference ratios are applied. This results in the following critical load expression that is defined explicitly in terms of the indicators NOy and SOx:
CLANClim(N + S) = ([BC]0[*] - [ANClim])Q + Neco = [NOy]TNOy + [SOx]TSOx + NHx	(III-2)
This is the same equation as III-1, with the deposition associated with the critical load translated to deposition from ambient air concentrations via transference ratios. In addition, deposition of reduced nitrogen, oxidized nitrogen and oxidized sulfur are treated separately.
	Transference ratios are a modeled construct, and therefore cannot be compared directly to measurable quantities. Section III.B.3 of the proposal discusses approaches to quantifying these ratios that consider blending observational data and models. The PA more fully discusses the rationale underlying transference ratios, as well as analyses illustrating the relative stability and variability of these ratios
d.	Aquatic Acidification Index
	Having established the transference ratios that translate atmospheric concentrations to deposition of nitrogen and sulfur and the various expressions that link atmospheric deposition of nitrogen and sulfur to ANC, the PA derived the following expression of these linkages, which separates reduced forms of nitrogen, NHx, from oxidized forms:
ANCcalc = {ANClim + CLr/Qr }  -  NHx/Qr - TNOy [NOy]/Qr - TSOx[SOx]/Qr 		(III-3)
	Equation III-3 is the basic expression of the form of a standard that translates the conceptual framework into an explicit expression that defines ANC as a function of the ambient air indicators, NOy and SOx, reduced nitrogen deposition, and the critical load necessary to achieve a target ANC level. This equation calculates an expected ANC value based on ambient concentrations of NOy and SOx. The calculated ANC will differ from the target ANC (ANClim) depending on how much the nitrogen and sulfur deposition associated with NOy, SOx, and NHx differs from the critical load associated with just achieving the target ANC.
	Based on equation III-3, the PA defines an AAI that is more simply stated using terms that highlight the ambient air indicators:
AAI  =  F1  -  F2  -  F3[NOy]  -  F4[SOx]						(III- 4)
where the AAI represents the long-term (or steady-state) ANC level associated with ambient air concentrations of NOy and SOx. The factors F1 through F4 convey three attributes:  a relative measure of the ecosystem's ability to neutralize acids (F1), the acidifying potential of reduced nitrogen deposition (F2), and the deposition-to-concentration translators for NOy (F3) and SOx (F4).
Specifically:
F1 = ANClim + CLr/Qr ;
F2 =  NHx/Qr  = NHx deposition divided by Qr;
F3 =  TNOy/ Qr ; TNOy is the transference ratio that converts ambient air concentrations of NOy to deposition of NOy; and
F4 =  TSOx/ Qr ; TSOx is the transference ratio that converts ambient air concentrations of SOx to deposition of SOx.
All of these factors include representative Qr to maintain unit (and mass) consistency between the AAI and the terms on the right side of equation III-4.
	The F1 factor is the target ANC level plus the amount of deposition (critical load) the ecosystem can receive and still achieve the target level. It incorporates an ecosystem's ability to generate acid neutralizing capacity through base cation supply ([BC]*0) and to neutralize acidifying nitrogen deposition through Neco, both of which are incorporated in the CL term. As noted above, because Neco can only neutralize nitrogen deposition (oxidized or reduced) there may be rare cases where Neco exceeds the combination of reduced and oxidized nitrogen deposition. Consequently, to ensure that the AAI equation is applicable in all cases that may occur, equation III-4 is conditional on total nitrogen deposition, {NHx + F3[NOy]}, being greater than Neco. In rare cases where Neco is greater than {NHx + F3[NOy]}, F2, F3, and Neco would be set equal to 0 in the AAI equation. The consequence of setting F2 and F3 to zero is simply to constrain the AAI calculation just to SOx, as nitrogen would have no bearing on acidifying contributions in this case.
	The PA concludes that equation III-4 (U.S. EPA, 2011,equation 7-12), which defines the AAI, is ecologically relevant and appropriate for use as the form of a national standard designed to provide protection for aquatic ecosystems from the effects of acidifying deposition associated with concentrations of oxides of nitrogen and sulfur in the ambient air. This AAI equation does not, however, in itself, define the spatial areas over which the terms of the equation would apply. To specify values for factors F1 through F4, it is necessary to define spatial areas over which these factors are determined. Thus, it is necessary to identify an approach for spatially aggregating water bodies into ecologically meaningful regions across the United States, as discussed below.
e.	Spatial Aggregation
	As discussed in the PA, one of the unique aspects of this form is the need to consider the spatial areas over which values for the F factors in the AAI equation are quantified. Ecosystems across the United States exhibit a wide range of geological, hydrological and vegetation characteristics that influence greatly the ecosystem parameters, Q, BC0[*] and Neco that are incorporated in the AAI. Variations in ecosystem attributes naturally lead to wide variability in the sensitivities of water bodies in the United States to acidification, as well as in the responsiveness of water bodies to changes in acidifying deposition. Consequently, variations in ecosystem sensitivity, and the uncertainties inherent in characterizing these variations, must be taken into account in developing a national standard. In developing a secondary NAAQS to protect public welfare, the focus of the PA is on protecting sensitive populations of water bodies, not on each individual water body, which is consistent with the Agency's approach to protecting public health through primary NAAQS that focus on susceptible populations, not on each individual.
	The approach used for defining ecologically relevant regions across the United States, along with approaches to characterizing each region as acid sensitive or relatively non-acid sensitive is discussed in detail in the PA (U.S. EPA, 2011, section 7.2.5). This characterization facilitates a more detailed analysis and focus on those regions that are relatively more acid sensitive, as well as avoiding over-protection in relatively non-acid sensitive regions that would receive limited benefit from reductions in the deposition of oxides of nitrogen and sulfur with respect to aquatic acidification effects.
	Based on considering available classification schemes for spatial aggregation, the PA concludes that Omernik's ecoregion classification (as described at http://www.epa.gov/wed/pages/ecoregions) is the most appropriate method to consider for the purposes of this review. The PA concludes that ecoregion level III (Figure IV-1) resolution, with 84 defined ecoregions in the contiguous United States, is the most appropriate level to consider for this purpose. The PA notes that the use of ecoregions is an appropriate spatial aggregation scheme for an AAI-based standard focused on deposition-related aquatic acidification effects, while many of the same ecoregion attributes may be applicable in subsequent NAAQS reviews that may address other deposition-related aquatic and terrestrial ecological effects. Because atmospheric deposition is modified by ecosystem attributes, the types of vegetation, soils, bedrock geology, and topographic features that are the basis of this ecoregion classification approach also will likely be key attributes for other deposition-related effects (e.g., terrestrial acidification, nutrient enrichment) that link atmospheric concentrations to an aquatic or terrestrial ecological indicator.
	The PA used Omernik's original alkalinity data (U.S. EPA, 2011, section 2) and more recent ANC data to delineate two broad groupings of ecoregions:  acid-sensitive and relatively non-acid sensitive ecoregions. This delineation was made to facilitate greater focus on those ecoregions with water bodies that generally have greater acid sensitivity and to avoid over-protection in regions with generally less sensitive water bodies. The approach used to delineate acid-sensitive and relatively non-acid sensitive regions included an initial numerical-based sorting scheme using ANC data, which categorized ecoregions with relatively high ANC values as being relatively non-acid sensitive. This initial delineation resulted in 29 of the 84 Omernik ecoregions being categorized as acid sensitive. Subsequently, land use data based on the 2006 National Land Cover Data base (NLCD, http://www.epa.gov/mrlc/nlcd-2006.html) were also considered to determine to what extent an ecoregion is of a relatively pristine and rural nature by quantifying the degree to which active management practices related to development and agriculture occur in each ecoregion, resulting in 22 relatively acid-sensitive ecoregions (Table III-1).

Table III-1.   List of 22 Acid-Sensitive Areas
Ecoregion name
Ecoregion number
Ridge and Valley
8.4.1
Northern Appalachian Plateau and Uplands
8.1.3
Piedmont
8.3.4
Western Allegheny Plateau
8.4.3
Southwestern Appalachians
8.4.9
Boston Mountains
8.4.6
Blue Ridge
8.4.4
Ouachita Mountains
8.4.8
Central Appalachians
8.4.2
Northern Lakes and Forests
5.2.1
Maine/New Brunswick Plains and Hills
8.1.8
North Central Appalachians
5.3.3
Northern Appalachian and Atlantic Maritime Highlands
5.3.1
Columbia Mountains/Northern Rockies
6.2.3
Middle Rockies
6.2.10
Wasatch and Uinta Mountains
6.2.13
North Cascades
6.2.5
Cascades
6.2.7
Southern Rockies
6.2.14
Sierra Nevada
6.2.12
Idaho Batholith
6.2.15
Canadian Rockies
6.2.4

	Consideration was also given to the use of naturally acidic conditions in defining relatively non-acid sensitive areas. For example, several of the ecoregions located in plains near the coast exhibit elevated dissolved organic carbon (DOC) levels, which is associated with naturally acidic conditions. The DOC in surface waters is derived from a variety of weak organic acid compounds generated from the natural availability and decomposition of organic matter from biota. Consequently, high DOC is associated with "natural" acidity, with the implication that a standard intended to protect against atmospheric contributions to acidity is not an area of focus. The evidence suggests that several of the more highly managed ecoregions in coastal or near coastal transition zones are associated with relatively high DOC values, typically exceeding on average 5 milligrams per liter, compared to other acid sensitive areas. Although there is sound logic to interpret naturally acidic areas as relatively non-acid sensitive, natural acidity indicators were not explicitly included in defining relatively non-acid sensitive areas as there does not exist a generally accepted quantifiable scientific definition of natural acidity. Approaches to explicitly define natural acidity likely will be pursued in future reviews of the standard.
	Having concluded that the Omernik level III ecoregions are an appropriate approach to spatial aggregation for the purpose of a standard to address deposition-related aquatic acidification effects, the PA uses those ecoregions to define each of the factors in the AAI equation. As discussed below, factors F1 through F4 in equation III-4 are defined for each ecoregion by specifying ecoregion-specific values for each factor based on measured and modeled data.
i.	Factor F1
      As discussed above, factor F1 reflects a relative measure of an ecosystem's ability to neutralize acidifying deposition, and is defined as:  F1 = ANClim + CLr/Qr. The value of F1 for each ecoregion would be based on a calculated critical load used to represent  the ecoregion (CLr) associated with a single national target ANC level (ANClim, discussed below in section III.D), as well as on a runoff rate (Qr) to represent the region. To specify ecoregion-specific values for the term Qr, the PA used the median value of the distribution of Q values that are available for water bodies within each ecoregion. To specify ecoregion-specific values for the term CLr in factor F1, a distribution of calculated critical loads was created for the water bodies in each ecoregion for which sufficient water quality and hydrology data are available.  The specified critical load was then defined to be a specific percentile of the distribution of critical loads in the ecoregion. Thus, for example, using the 90[th] percentile means that within an ecoregion, the goal would be for 90 percent of the water bodies to have higher calculated critical loads than the specified critical load. That is, if the specified critical load were to occur across the ecoregion, the goal would be for 90 percent of the water bodies to achieve the national ANC target or better.
      The specific percentile selected as part of the definition of F1 is an important parameter that directly impacts the critical load specified to represent each ecoregion, and therefore the degree of protectiveness of the standard. A higher percentile corresponds to a lower critical load and, therefore, to lower allowable ambient air concentrations of NOy and SOx and related deposition to achieve a target AAI level. In conjunction with the other terms in the AAI equation, alternative forms can be appropriately characterized in part by identifying a range of alternative percentiles. The choice of an appropriate range of percentiles to consider for acid-sensitive and relatively non-acid sensitive ecoregions, respectively, is discussed below.
      For relatively acid-sensitive ecoregions, the PA concludes it is appropriate to consider percentiles in the range of the 70[th] to the 90[th] percentile (of sensitivity). This conclusion is based on the judgment that it would not be appropriate to represent an ecoregion with the lowest or near lowest critical load, so as to avoid potential extreme outliers that can be seen to exist at the extreme end of the data distributions, which would not be representative of the population of acid sensitive water bodies within the ecoregion and could lead to an overly protective standard. At the same time, in considering ecoregions that are inherently acid sensitive, it is judged to be appropriate to limit the lower end of the range for consideration to the 70[th] percentile, a value well above the median of the distribution, so that a substantial majority of acid-sensitive water bodies are protected. Since the percentile value influences the relative degree of protectiveness afforded by the AAI approach, the degree of confidence in characterizing the representativeness of sampled water bodies relative to all water bodies within an ecoregion is a critical issue, and it is important to continually improve this confidence.
      For relatively non-acid sensitive ecoregions, the PA concludes it is appropriate to consider the use of a range of percentiles that extends lower than the range identified above for acid-sensitive ecoregions. Consideration of a lower percentile would avoid representing a relatively non-acid sensitive ecoregion by a critical load associated with relatively more acid-sensitive water bodies. In particular, the PA concludes it is appropriate to focus on the median or 50[th] percentile of the distribution of critical loads so as to avoid over-protection in such ecoregions.
ii.	Factor F2, F3 and F4
	As discussed above, factor F2 is the amount of reduced nitrogen deposition within an ecoregion, including the deposition of both ammonia gas and ammonium ion, and is defined as:  F2 = NHx/Qr. The PA calculated the representative runoff rate, Qr, using a similar approach as noted above for factor F1; i.e., the median value of the distribution of Q values that are available for water bodies within each ecoregion. In the PA, 2005 CMAQ model simulations over 12-km grids are used to calculate an average value of NHx for each ecoregion. The NHx term is based on annual average model outputs for each grid cell, which are spatially averaged across all the grid cells contained in each ecoregion to calculate a representative annual average value for each ecoregion. The PA concludes that this approach of using spatially averaged values is appropriate for modeling, largely due to the relatively rapid mixing of air masses that typically results in relatively homogeneous air quality patterns for regionally dispersed pollutants. In addition, there is greater confidence in using spatially averaged modeled atmospheric fields than in using modeled point-specific fields.
	This averaging approach is also used for the air concentration and deposition terms in factors F3 and F4, which are the ratios that relate ambient air concentrations of NOy and SOx to the associated deposition, and are defined as follows:  F3 =  TNOy/ Qr  and F4 =  TSOx/ Qr.  TNOy is the transference ratio that converts ambient air concentrations of NOy to deposition of NOy and TSOx is the transference ratio that converts ambient air concentrations of SOx to deposition of SOx. The transference ratios are based on the 2005 CMAQ simulations, using average values for each ecoregion, as noted above for factor F2. More specifically, the transference ratios are calculated as the annual deposition of NOy or SOx spatially averaged across the ecoregion and divided by the annual ambient air concentration of NOy or SOx, respectively, spatially averaged across the ecoregion.
f.	Summary of the AAI form
	The PA developed an ecologically relevant form of an ambient air quality standard to address deposition-related aquatic acidification effects using an equation to calculate an AAI value in terms of the ambient air indicators of oxides of nitrogen and sulfur and the relevant ecological and atmospheric factors that modify the relationships between the ambient air indicators and ANC. Recognizing the spatial variability of such factors across the United States, the PA concludes it is appropriate to divide the country into ecologically relevant regions, characterized as acid-sensitive or relatively non-acid sensitive, and specify the value of each of the factors in the AAI equation for each such region.
	Using the equation, a value of AAI can be calculated for any measured values of ambient NOy and SOx. For such a NAAQS, the Administrator would set a single, national value for the level of the AAI used to determine achievement of the NAAQS, as summarized below in section III.A.4. The ecoregion-specific values for factors F1 through F4 would be specified by the EPA based on the most recent data and CMAQ model simulations, and codified as part of such a standard. These factors would be reviewed and updated as appropriate in the context of each periodic review of the NAAQS.
3.	Averaging Time
	Reflecting a focus on long-term effects of acidifying deposition, the PA developed the AAI that links ambient air indicators to deposition-related ecological effects, in terms of several factors, F1 through F4. As discussed above, these factors are all calculated as annual average values, whether based on water quality and hydrology data or on CMAQ model simulations. In the context of a standard defined in terms of the AAI, the PA concludes that it is appropriate to consider the same annual averaging time for the ambient air indicators as is used for the factors in the AAI equation. As noted in chapter 3 of the ISA, protection against episodic acidity events can be achieved by establishing a higher chronic ANC level.
	The PA also considered interannual variability in both ambient air quality and in precipitation, which is directly related to the deposition of oxides of nitrogen and sulfur from the ambient air. While ambient air concentrations show year-to-year variability, often the year-to-year variability in precipitation is considerably greater, given the highly stochastic nature of precipitation. The use of multiple years over which annual averages are determined would dampen the effects of interannual variability in both air quality and precipitation. Consequently, the PA concludes that an annual averaging time based on the average of each year over a consecutive 3- to 5-year period is appropriate to consider for the ambient air indicators NOy and SOx.
4.	Level
	The PA concludes that the level of a standard for aquatic acidification based on the AAI would be defined in terms of a single, national value of the AAI. Such a standard would be met at a monitoring site when the multi-year average of the calculated annual values of the AAI was equal to or above the specified level of the standard.   The annual values of the AAI would be calculated based on the AAI equation using the assigned ecoregion-specific values for factors F1 through F4 and monitored annual average NOy and SOx concentrations. Since the AAI equation is based on chronic ANC as the ecological indicator, the level chosen for the standard would reflect a target chronic ANC value. The combination of the form of the standard, discussed above in section III.A.2, defined by the AAI equation and the assigned values of the F factors in the equation, other elements of the standard including the ambient air indicators (section III.A.1) and their averaging time (section III.A.3), and the level of the standard determines the allowable levels of NOy and SOx in the ambient air within each ecoregion. All of the elements of the standard together determine the degree of protection from adverse aquatic acidification effects associated with oxides of nitrogen and sulfur in the ambient air. The level of the standard plays a central role in determining the degree of protection provided and is discussed below.
      Based on associations between pH levels and target ANC levels and between ANC levels and aquatic ecosystem effects, as well as consideration of episodic acidity, ecosystem response time, precedent uses of target ANC levels, and public welfare benefits, the PA concludes that consideration should be given to a range of standard AAI levels from 20 to 75 μeq/L. The available evidence indicates that target ANC levels below 20 μeq/L would be inadequate to protect against substantial ecological effects and potential catastrophic loss of ecosystem function in some sensitive aquatic ecosystems. While ecological effects occur at ANC levels below 50 μeq/L in some sensitive ecosystems, the degree and nature of those effects are less significant than at levels below 20 μeq/L. Levels at and above 50 μeq/L would be expected to provide additional protection, although uncertainties regarding the potential for additional protection from adverse ecological effects are much larger for target ANC levels above about 75 μeq/L, as effects are generally appreciably less sensitive to changes in ANC at such higher levels.
	The PA recognizes that the level of the standard together with the other elements of the standard, including the ambient air indicators, averaging time, and form, determine the overall protectiveness of the standard. Thus, consideration of a standard level should reflect the strengths and limitations of the evidence and assessments as well as the inherent uncertainties in the development of each of the elements of the standard. The implications of considering alternative standards, defined in terms of alternative combinations of levels and percentile values that are a critical component of factor F1 in the form of the standard, are discussed in section III.E of the proposal and more fully in the PA.
5.	Characterization of Uncertainties
	The characterization of uncertainties is intended to address the relative confidence associated with the linked atmospheric-ecological effects system described above, and is described in detail in the PA (U.S. EPA, 2011, section 7.6 and Appendices F and G) and summarized in section III.F of the proposal. A brief overview of uncertainties is presented here in the context of the major structural components underlying the standard, as well as with regard to areas of relatively high uncertainty.
	As discussed in the PA (U.S. EPA, 2011,Table 7-3), there is relatively low uncertainty with regard to the conceptual formulation of the overall structure of the AAI-based standard that incorporates the major associations linking biological effects to air concentrations. Based on the strength of the evidence that links species richness and mortality to water quality, the associations are strongly causal and without any obvious confounding influence. The strong association between the ecosystem indicator (ANC) and the causative water chemistry species (dissolved aluminum and hydrogen ion) reinforces the confidence in the linkage between deposition of nitrogen and sulfur and effects. This strong association between ANC and effects is supported by a sound mechanistic foundation between deposition and ANC. The same mechanistic strength holds true for the relationship between ambient air levels of nitrogen and sulfur and deposition, which completes the linkage from ambient air indicators through deposition to ecological effects.
	There are higher uncertainties, however, in considering and quantifying the specific elements within the structure of an AAI-based standard, including the deposition of SOx, NOy, and NHx as well as the critical load-related component, each of which can vary within and across ecoregions. Overall system uncertainty with an AAI approach relates not just to the uncertainty in each element, but also to the combined uncertainties that result from linking these elements together within the AAI-based structure and over the defined spatial scale (i.e., ecoregions). Some of these elements  -  including, for example, dry deposition, pre-industrial base cation production, and reduced nitrogen deposition  -  are estimated with less confidence than other elements (U.S. EPA, 2011, Table 7.3). The uncertainties associated with all of these elements, and the combination of these elements through the AAI equation and over the ecoregion spatial scale, are summarized below.
	The lack of observed dry deposition data, which affects confidence in the AAI on an ecoregion scale, is constrained in part by the lack of efficient measurement technologies. Progress in reducing uncertainties in dry deposition will depend on improved atmospheric concentration data and direct deposition flux measurements of the relevant suite of NOy and SOx species.
      Pre-industrial base cation productivity by definition is not observable. Contemporary observations and inter-model comparisons are useful tools that help reduce the uncertainty in estimates of pre-industrial base cation productivity used in the AAI equation. In characterizing contemporary base cation flux using basic water quality measurements (i.e., major anion and cation species as defined in equation 2.11 in the PA), it is reasonable to assume that a major component of contemporary base cation flux is associated with pre-industrial weathering rates. To the extent that multiple models converge on similar solutions within and across ecoregions, greater confidence in estimating pre-industrial base cation production within the AAI and ecoregion frameworks would be achieved.
	While characterization of NHx deposition has been evolving over the last decade, the relatively high uncertainty in characterizing NHx deposition is due to both the lack of field measurements and the inherent complexity of characterizing NHx with respect to source emissions and dry deposition.  Because ammonia emissions are generated through a combination of man-made and biological activities, and ammonia is semi-volatile, the ability to characterize spatial and temporal distributions of NHx concentrations and deposition patterns is challenging. While direct measurement of NHx deposition is resource intensive because of the diffuse nature of sources (i.e., area-wide and non-point sources), there have been more frequent deposition flux studies, relative to other nitrogen species, that enable the estimation of both emissions and dry deposition. Also, while ammonia has a relatively high deposition velocity and traditionally was thought to deposit close to the emissions release areas, the semi-volatile nature of ammonia results in re-entrainment back into the lower boundary layer of the atmosphere resulting in a more dispersed concentration pattern exhibiting transport characteristics similar to longer lived atmospheric species. These inherent complexities in source characterization and ambient concentration patterns increase the degree of uncertainty in NHx deposition in general, and in the AAI equation applied on an ecoregion scale in particular. However, the PA notes that progress is being made in measuring ammonia with cost efficient samplers and anticipates the gradual evolution of a spatially robust ammonia sampling network that would help support analyses to reduce underlying uncertainties in NHx deposition.
	In characterizing uncertainties with respect to available measurement data and the use of ecological and atmospheric models, as summarized in sections III.F.2-3 of the proposal, the PA identified data gaps and model uncertainties in relative terms by comparing, for example, the relative richness of data between geographic areas or environmental media. As discussed in the proposal and more fully in the PA, from an uncertainty perspective, gaps in field measurement data increase uncertainties in modeled processes and in the specific application of such models. As noted above, processes that are embodied in an AAI-based standard are modeled using the CMAQ atmospheric model and steady-state ecological models. These models are characterized in the ISA as being well-established and have undergone extensive peer review. Nonetheless, the application of these models for purposes of specifying the factors in the AAI equation, on an ecoregion scale, is a new application that introduces uncertainties, especially in areas with limited observational data that can be used to evaluate this specific application. Understanding uncertainties in relevant modeled processes thus involves consideration of the uncertainties associated with applying each model as well as the combination of these uncertainties as the models are applied in combination within the AAI framework applied on an ecoregion scale.
      Our confidence in improving critical load estimates can be increased by expanding water quality data bases used as inputs and evaluation metrics for critical load models. With regard to water quality data, the PA notes that such data are typically limited relative to air quality data sets, and are also relatively sparse in the western United States. While there are several state and local agency water quality data bases, it is unclear the extent to which differences in sampling, chemical analysis and reporting protocols would impact the use of such data for the purpose of better understanding the degree of protectiveness that would be afforded by an AAI-based standard within sensitive ecoregions across the country. In addition, our understanding of water quality in Alaska and Hawaii and the acid sensitivity of their ecoregions is particularly limited. Expanding the water quality data bases would enable clearer delineation of ecoregion representative critical loads in terms of the n[th] percentile. This would provide more refined characterization of the degree of protection afforded by a given standard. Longer term, the availability of water quality trend data (annual to monthly sampled) would support accountability assessments that examine if an ecoregion's response to air management efforts is as predicted by earlier model forecasting. The most obvious example is the long-term response of water quality ANC change to changes in calculated AAI, deposition, ambient NOy and SOx concentrations, and emissions. In addition, water quality trends data provide a basis for evaluating and improving the parameterizations of processes in critical load models applied at the ecoregion scale related to nitrogen retention and base cation supply. A better understanding of soil processes, especially in the southern Appalachians, would enhance efforts to examine the variability within ecoregions of the soil-based adsorption and exchange processes which moderate the supply of major cations and anions to surface waters and strongly influence the response of surface water ANC to changes in deposition of nitrogen and sulfur.
      Steady-state biogeochemical ecosystem modeling is used to develop critical load estimates that are incorporated in the AAI equation through factor F1. Consequently, the PA notes that an estimate of the temporal response of surface water ANC to deposition and air concentration changes is not directly available. Lacking a predicted temporal response impairs the ability to conduct accountability assessments down to the effects level. Accountability assessments would examine the response of each step in the emissions source through air concentration  -  deposition -- surface water quality  -  biota continuum. The steady-state assumption at the ecosystem level does not impair accountability assessments through the air concentration/deposition range of that continuum. However, in using steady- state ecosystem modeling, several assumptions are made relative to the long-term importance of processes related to soil adsorption of major ions and ecosystem nitrogen dynamics. Because these models often were developed and applied in glaciated areas with relatively thin and organically rich soils, their applicability is relatively more uncertain in areas such as those in the non-glaciated clay-based soil regions of the central Appalachians. Consequently, it is desirable to develop the information bases to drive simple dynamic ecosystem models that incorporate more detailed treatment of subsurface processes, such as adsorption and exchange processes and sulfate absorption.
B.	CASAC Views
	The CASAC has advised the EPA concerning the ISA, the REA, and the PA. The CASAC supported the EPA's interpretation of the science embodied in the ISA and the assessment approaches and conclusions incorporated in the REA.
	Most recently, CASAC considered the information in the final PA in providing its recommendations on the review of the new multi-pollutant standard developed in that document and discussed above (Russell and Samet, 2011a). In so doing, CASAC expressed general support for the conceptual framework of the standard based on the underlying scientific information, as well as for the conclusions in the PA with regard to indicators, averaging time, form and level of the standard that are appropriate for consideration by the Agency in reaching decisions on the review of the secondary NAAQS for oxides of nitrogen and sulfur:
      The final Policy Assessment clearly sets out the basis for the recommended ranges for each of the four elements (indicator, averaging time, level and form) of a potential NAAQS that uses ambient air indicators to address the combined effects of oxides of nitrogen and oxides of sulfur on aquatic ecosystems, primarily streams and lakes. As requested in our previous letters, the Policy Assessment also describes the implications of choosing specific combinations of elements and provides numerous maps and tabular estimates of the spatial extent and degree of severity of NAAQS exceedances expected to result from possible combinations of the elements of the standard.

      We believe this final PA is appropriate for use in determining a secondary standard to help protect aquatic ecosystems from acidifying deposition of oxides of sulfur and nitrogen. The EPA staff has done a commendable job developing the innovative Aquatic Acidification Index (AAI), which provides a framework for a national standard based on ambient concentrations that also takes into account regional differences in sensitivities of ecosystems across the country to effects of acidifying deposition. (Russell and Samet, 2011a)

	With respect to indicators, CASAC supported the use of SOx and NOy as ambient air indicators (discussed above in section III.A) and ANC as the ecological indicator (discussed above in section III.B.1). With respect to averaging time (discussed above in section III.C), CASAC agreed with the conclusions in the PA that "an averaging time of three to five years for the AAI parameters is appropriate."  CASAC noted that "a longer averaging time would mask possible trends of AAI, while a shorter averaging time would make the AAI being more influenced by the conditions of the particular years selected" (Russell and Samet, 2011a).
      With respect to the form of the standard (discussed above in section III.B), CASAC stated the following:
      EPA has developed the AAI, an innovative "form" of the NAAQS itself that incorporates the multi-pollutant, multi-media, environmentally modified, geographically variable nature of SOx/ NOy deposition-related aquatic acidification effects. With the caveats noted below, CASAC believes that this form of the NAAQS as described in the final Policy Assessment is consistent with and directly reflective of current scientific understanding of effects of acidifying deposition on aquatic  ecosystems." (Russell and Samet, 2011a)

      CASAC agrees that the spatial components of the form in the Policy Assessment are reasonable and that use of Omernik's ecoregions (Level III) is appropriate for a secondary NAAQs intended to protect the aquatic environment from acidification..." (Russell and Samet, 2011a)

	The caveats noted by CASAC include a recognition of the importance of continuing to evaluate the performance of the CMAQ and ecological models to account for model uncertainties and to make the model-dependent factors in the AAI more transparent. In addition, CASAC noted that the role of DOC and its effects on ANC would benefit from further refinement and clarification (Russell and Samet, 2011a). While CASAC expressed the view that the "division of ecoregions into `sensitive' and `non-sensitive' subsets, with a more protective percentile applied to the sensitive areas, also seems reasonable" (Russell and Samet, 2011a), CASAC also noted that there was the need for greater clarity in specifying how appropriate screening criteria would be applied in assigning ecoregions to these categories. Further, CASAC identified potential biases in critical load calculations and in the regional representativeness of available water chemistry data, leading to the observation that a given percentile of the distribution of estimated critical loads may be protective of a higher percentage of surface waters in some regions (Russell and Samet, 2011a). Such potential biases led CASAC to recommend that "some attention be given to our residual concern that the available data may reflect the more sensitive water bodies and thus, the selection of the percentiles of waterbodies to be protected could be conservatively biased" (Russell and Samet, 2011a).
	With respect to level as well as the combination of level and form as they are presented as alternative standards (discussed above in sections III.D-E), CASAC agreed with the PA conclusions that consideration should be given to standard levels within the range of 20 and 75 μeq/L. CASAC also recognized that the level and the form of any AAI-based standard are so closely linked that these two elements should be considered together:
      When considered in isolation, it is difficult to evaluate the logic or implications of selecting from percentiles (70th to 90th) of the distribution of estimated critical loads for lakes in sensitive ecoregions to determine an acceptable amount of deposition for a given ecoregion. However, when these percentile ranges are combined with alternative levels within the staff-recommended ANC range of 20 to 75 microequivalents per liter (μeq/L), the results using the AAI point to the ecoregions across the country that would be expected to require additional protection from acidifying deposition. Reasonable choices were made in developing the form. The number of acid sensitive regions not likely to meet the standard will be affected both by choice of ANC level and the percentile of the distribution of critical loads for lakes to meet alternative ANC levels in each region. These combined recommendations provide the Administrator with a broad but reasonable range of minimally to substantially protective options for the standard. (Russell and Samet, 2011a)

	CASAC also commented on the EPA's uncertainty analysis, and provided advice on areas requiring further clarification in the proposed rule and future research. The CASAC found it "difficult to judge the adequacy of the uncertainty analysis performed by the EPA because of lack of details on data inputs and the methodology used, and lack of clarity in presentation" (Russell and Samet, 2011a). In particular, CASAC identified the need for more thorough model evaluations of critical load and atmospheric modeling, recognizing the important role of models as they are incorporated in the form of the standard. In light of the innovative nature of the standard developed in the PA, CASAC identified "a number of areas that should be the focus of further research" (Russell and Samet, 2011a). While CASAC recognized that the EPA staff was able to address some of the issues in the PA, they also noted areas "that would benefit from further study or consideration in potential revisions or modifications to the form of the standard."  Such research areas include "sulfur retention and mobilization in the soils, aluminum availability, soil versus water acidification and ecosystem recovery times."  Further, CASAC encouraged future efforts to monitor individual ambient nitrogen species, which would help inform further CMAQ evaluations and the specification of model-derived elements in the AAI equation (Russell and Samet, 2011a).
C.	Proposed Conclusions on Alternative Secondary Standards
	As discussed in section III.H of the proposal, the Administrator considered whether it is appropriate at this time to set a new multi-pollutant standard to address deposition-related effects associated with oxides of nitrogen and sulfur, with a structure that would better reflect the available science regarding acidifying deposition to sensitive aquatic ecosystems. In so doing, she recognized that such a standard, for purposes of Section 109(b) and (d) of the CAA, must in her judgment be requisite to protect public welfare, such that it would be neither more nor less stringent that necessary for that purpose. In particular, she focused on the AAI-based standard developed in the PA and reviewed by CASAC, as discussed above. Based on consideration of the scientific basis for such a standard and the conclusions reached in the ISA, the Administrator agreed with the conclusion in the PA, and supported by CASAC, that there is a strong scientific basis for development of a standard with the general structure presented in the PA. She recognized that while the standard is innovative and unique, the structure of the standard is well-grounded in the science underlying the relationships between ambient concentrations of oxides of nitrogen and sulfur and the aquatic acidification related to deposition of nitrogen and sulfur associated with such ambient concentrations.
	Nonetheless, the Administrator also recognized that such a standard would depend on atmospheric and ecological modeling, based on appropriate data, to specify the terms of an equation that incorporates the linkages between ambient concentrations, deposition, and aquatic acidification, for each separate ecoregion, and that there are a number of inherent uncertainties and complexities that are relevant to the question of whether it is appropriate under Section 109 of the CAA to set a specific AAI-based standard at this time. Based on her consideration of these important uncertainties and limitations, the Administrator recognized that in combination, these limitations and uncertainties result in a considerable degree of uncertainty as to how well the quantified elements of the AAI standard would predict the actual relationship between varying ambient concentrations of oxides of nitrogen and sulfur and steady- state ANC levels across the distribution of water bodies within the various ecoregions in the United States. Because of this, there is considerable uncertainty as to the actual degree of protectiveness that such a standard would provide, especially for acid-sensitive ecoregions. The Administrator recognized that the AAI equation, with factors quantified in the ranges discussed above and described more fully in the PA, generally performs well in identifying areas of the country that are sensitive to such acidifying deposition and indicates, as expected, that lower ambient levels of oxides of nitrogen and sulfur would lead to higher calculated AAI values. However, the uncertainties discussed here are critical for determining the actual degree of protection that would be afforded such areas by any specific target ANC level and percentile of water bodies that would be chosen in setting a new AAI-based standard, and thus for determining an appropriate AAI-based standard that meets the requirements of Section 109.
      The Administrator noted that setting a NAAQS generally involves consideration of the degree of uncertainties in the science and other information, such as gaps in the relevant data and, in this case, limitations in the evaluation of the application of relevant ecological and atmospheric models at an ecoregion scale. She noted that the issue here is not a question of uncertainties about the scientific soundness of the structure of the AAI, but instead uncertainties in the quantification and representativeness of the elements of the AAI as they vary in ecoregions across the country. At present, these uncertainties prevent an understanding of the degree of protectiveness that would be afforded to various ecoregions across the country by a new standard defined in terms of a specific nationwide target ANC level and a specific percentile of water bodies for acid-sensitive ecoregions and thus prevent identification of an appropriate standard.
	The Administrator judged that the uncertainties are of such nature and magnitude that there is no reasoned way to choose a specific AAI-based standard, in terms of a specific nationwide target ANC level or percentile of water bodies that would appropriately account for the uncertainties, since neither the direction nor the magnitude of change from the target level and percentile that would otherwise be chosen can reasonably be ascertained at this time. Further, she noted that CASAC acknowledged that important uncertainties remain that would benefit from further study and data collection efforts, which might lead to potential revisions or modifications to the form of the standard developed in the PA, and that CASAC encouraged the Agency to engage in future monitoring and model evaluation efforts to help inform further development of the elements of an AAI-based standard. Based on these considerations the Administrator judged that the current limitations in relevant data and the uncertainties associated with specifying the elements of the AAI-based on modeled factors are of such nature and degree as to prevent her from reaching a reasoned decision such that she is adequately confident as to what level and form (in terms of a selected percentile) of such a standard would provide any particular intended degree of protection of public welfare that the Administrator determined satisfied the requirements to set an appropriate standard under Section 109 of the CAA.
	Based on the above considerations, the Administrator provisionally concluded that it is premature to set a new, multi-pollutant secondary standard for oxides of nitrogen and sulfur at this time, and as such she proposed not to set such a new secondary standard. Nonetheless, while the Administrator concluded that it is premature to set such a multi-pollutant standard at this time, she determined that the Agency should undertake a field pilot program to gather additional data (discussed below in section IV). She concluded that it is appropriate that such a program be undertaken before, rather than after, reaching a decision to set such a standard.
	In reaching her proposed decision not to set a new AAI-based standard at this time, the Administrator recognized that the new NO2 and SO2 primary 1-hour standards set in 2010, while not ecologically relevant for a secondary standard, will nonetheless result in reductions in oxides of nitrogen and sulfur that will directionally benefit the environment by reducing NOy and SOx deposition to sensitive ecosystems. The Administrator proposed to revise the secondary standards by adding secondary standards identical to the NO2 and SO2 primary 1-hour standards set in 2010, including a 1-hour secondary NO2 standard set at a level of 100 ppb and a 1-hour secondary SO2 standard set at a level of 75 ppb. The EPA noted that while this will not add secondary standards of an ecologically relevant form to address deposition-related effects, it will provide additional protection for sensitive areas. The EPA further noted that this proposed decision is consistent with the view that the current secondary standards are neither sufficiently protective nor appropriate in form, but that it is not appropriate to propose to set a new, ecologically relevant multi-pollutant secondary standard at this time, for the reasons summarized above.
	The EPA solicited comment on all aspects of this proposed decision, as discussed in the following section.
D.	Comments on Alternative Secondary Standards
      In this section, comments received on the proposal related to an AAI-based standard are discussed in section III.D.1 and comments related to the proposed decision to set 1-hour NO2 and SO2 secondary standards are discussed in section III.D.2.
1.	Comments Related to an AAI-based Standard
      General comments that either supported or opposed the proposed decision not to set an AAI-based standard in this review are addressed in this section. Two groups of commenters offered sharply divergent views on whether it is appropriate for the EPA to set or even consider an AAI-based standard to protect against the effects in aquatic ecosystems from acidifying deposition associated with ambient concentrations of oxides of nitrogen and sulfur. These groups provided strongly contrasting views on the strength and limitations in the underlying scientific information upon which such a standard could be based, as well as on the legal authority and requirements in the CAA for the EPA to set such a standard. These comments are discussed below in section III.D.1.a, and build in part on the overarching issue raised by some commenters as to the EPA's authority under the CAA to include deposition-related effects within the scope of a NAAQS review, which is discussed above in section I.E. Some commenters also expressed views about specific aspects of an AAI-based approach, as discussed below in section III.D.1.b. More technical comments on specific elements and factors of the AAI are discussed in the Response to Comments document. General comments based on implementation-related factors that are not a permissible basis for considering an alternative standard are noted in the Response to Comments document.
a.	Comments on Consideration of an AAI-based Standard
      The first group of commenters, including several industry groups (e.g., EPRI, UARG, and API), individual companies (e.g., East Kentucky Power Cooperative), and two states (TX, SD), strongly supported the EPA's proposed decision not to set an AAI-based standard in this review. These commenters generally focused on the limitations and uncertainties in the scientific evidence used by the EPA as a basis for its consideration of an AAI-based standard, expressing the view that these limitations and uncertainties were so great as to preclude setting such a standard at this time. Several industry commenters felt the uncertainties were of sufficient magnitude as to invalidate the AAI approach for use in the NAAQS, while others agreed with the EPA's finding that further information and analysis is needed, and further noted that this work should be completed before EPA could propose a new multi-pollutant standard. More fundamentally, some commenters in this group expressed the view that any consideration of such a standard is inconsistent with various provisions of the CAA and thus unlawful.
      With regard to their views on the underlying scientific information, many of these commenters focused on what they asserted were areas of substantial uncertainty in the AAI approach including uncertainties in the individual F factors of the AAI, air deposition modeling, critical loads modeling, and available water quality and watershed data. Several commenters felt a more rigorous uncertainty and variability analysis of the AAI, beyond the analyses that the EPA presented in the PA, would be needed if the EPA were to consider such a standard in the future.
      Some commenters expressed concerns with specific aspects of the AAI, such as the adequacy of the Omernik ecoregion approach as a method of waterbody aggregation for critical load calculations and whether ANC was an appropriate ecological indicator. The commenters asserted that the EPA needed to explore different methods for calculating critical loads, collect essential data, and employ mechanistic water chemistry models. The commenters also felt that the EPA was arbitrary in choosing its criteria for sensitive ecoregions and percent waterbodies, and that there was a bias in the field data toward sensitive areas. Several commenters felt a more comprehensive research program was needed to improve characterization of the biogeochemical and deposition processes incorporated into the AAI.
      Some industry groups commented on uncertainties in the CMAQ modeling, including high levels of uncertainty surrounding measurement and modeling of chemically reduced forms of nitrogen (NHx). Other commenters were also critical of the reliance of the AAI on modeling, and expressed the view that CMAQ would require intensive deposition-focused evaluation.
      A second group of commenters, including several environmental groups (e.g., Center for Biological Diversity, Earthjustice, and Adirondack Council), the U.S. Department of Interior and the National Park Service, the New York Department of Environmental Conservation, and two tribes (Fond du Lac Band and Potawatomi) strongly disagreed with the EPA's proposed decision not to set an AAI-based standard in this review. These commenters generally focused on the strengths of the evidence of deposition-related effects, the extent to which analyses presented in the PA addressed uncertainties and limitations in the evidence, and on information regarding the adversity of such effects as a basis for their views that such a standard was warranted at this time. Many of these commenters pointed to CASAC's review of the underlying scientific evidence and its support for moving forward with an AAI-based standard at this time as support for their views.
      In general, the environmental group commenters expressed the view that the current standards are clearly not adequate and that a combined NOx/SOx standard that links ambient air quality to an ecosystem indicator is appropriate, founded in science, and necessary for protection of public welfare. The commenters stated the current standards are neither sufficiently protective nor appropriate to address deposition-related effects. They also noted that the EPA has worked for decades to solve the acid deposition problem and that in their view the AAI represents an elegant solution to that problem.
      With regard to their views on the underlying scientific information, these commenters generally agreed with the EPA's proposed conclusions that there are well-established water quality and biological indicators of aquatic deposition and well-established models that address air deposition, water quality impacts, and effects on biota. Many of these commenters expressed the view that the uncertainties and limitations in the scientific evidence were adequately addressed in the PA, which was reviewed by CASAC. Many of these commenters pointed to CASAC's support for adopting an AAI-based standard in this review while concurrently conducting additional field monitoring and longer-term research that might reduce uncertainties in future reviews of secondary NAAQS for oxides of nitrogen and sulfur.
      Some governmental agency commenters were strongly supportive of an AAI-based standard and clearly felt such a standard should be adopted now. They also noted that the current ambient concentrations of NOx and SOx are causing adverse ecological impacts and they believe that ongoing damage due to acidic deposition and the risks to ecosystems far outweigh the risk of setting an AAI-based standard while some uncertainties remain. They assert that NOx and SOx deposition is causing adversity to public welfare and that the scientific uncertainties do not preclude setting an AAI-based standard, and point to CASAC as generally supporting this view. The commenters believe that the EPA has ample evidence to support a new ecologically based standard and that the AAI is reasonable and scientifically defensible. NY specifically recommended an AAI of 50 with some flexibility built into the F factors.
      Some of these agency and environmental group commenters also referenced CASAC's support for specific elements of the AAI-based standard developed in the PA, including 1) the use of ANC as an appropriate ecological indicator for such a standard, 2) the use of NOy and SOx as well-justified indicators of atmospheric concentrations of oxides of nitrogen and sulfur, 3) the use of Omernik Level III ecoregions, 4) the division of ecoregions into sensitive and non-sensitive categories, 5) the use of a 3 to 5 year averaging time, and 6) the appropriateness of an AAI level between 20 to 75 ueq/L.
      With regard to their views on the requirements of the CAA, several environmental group commenters stated that given the large body of evidence supporting significant ongoing harm to the public welfare and the EPA's finding the current standards are neither sufficiently protective nor appropriate to address deposition-related effects, the EPA's reliance on uncertainty as grounds for failing to propose protective standards is irrational, arbitrary, and legally flawed. They believe that the EPA cannot lawfully reject a new AAI-based standard while continuing to rely solely on a form of the standard that is inadequate and allows serious harms to the public welfare to continue. When confronted with scientific uncertainties and incomplete data, they feel the EPA must act in a precautionary manner that errs toward stronger protections. Further, they believe that the EPA's reliance on scientific uncertainty as a basis for its inaction is unsupportable in light of CASAC's advice and the EPA staff's conclusions in the ISA, REA and PA.