In addition to the two broad groups of commenters discussed above, a few other commenters offered more general views on an AAI-based standard. For example, some state commenters (NC and PA) expressed support for the concept of developing a multi-pollutant, AAI-based standard, but felt that it would be important to gather additional information before proposing any such standard. One state organization (NESCAUM) expressed concern that the EPA was not following CASAC's recommendation to propose an ecologically relevant level and form for this NAAQS.
      The EPA has carefully considered these comments on whether or not an AAI-based secondary standard for oxides of nitrogen and sulfur is appropriate at this time. The EPA agrees with the second group of commenters and CASAC's advice (outlined in section III.B) that there is a strong scientific basis for development of the structure of such a standard, specifically with regard to a standard that would provide protection from deposition-related aquatic acidification in sensitive ecosystems across the country. As discussed in section II.A and supported by several commenters, the available scientific evidence is sufficient to infer a causal relationship between acidifying deposition of nitrogen and sulfur and potential adverse effects to aquatic ecosystems, and that the deposition of oxides of nitrogen and sulfur both cause such acidification under current conditions that are allowed by the current secondary standards (U.S. EPA, 2008, chapter 3). The EPA agrees with commenters that there are well-established water quality and biological indicators of aquatic acidification as well as well-established models that address deposition, water quality, and effects on ecosystem biota, and that ecosystem sensitivity to acidification varies across the country (U.S. EPA, 2011, chapter 7).
      The EPA also agrees with the second group of commenters and CASAC that ANC would be an appropriate ecological indicator, reflecting the acidifying effects of deposition of nitrogen and sulfur (U.S. EPA, 2011, chapter 7.2 and Russell and Samet, 2011a). Further, the EPA agrees that the structure of an AAI-based standard is well-grounded in science and would address the combined effects of deposition from oxides of nitrogen and sulfur by characterizing the linkages between ambient concentrations, deposition, and aquatic acidification, and that the structure of the standard takes into account relevant variations in these linkages across the country (section III.B. above and U.S. EPA, 2011, chapter 7).
      The EPA disagrees with the first group of commenters that the use of Omernik ecoregions would be inadequate. A full explanation of the EPA's rationale for selecting the Omernik ecoregion scheme for spatial aggregation is found in section 7.2.5 of the PA. Omernik ecoregions include consideration of geology, physiology, vegetation, climate, soils, land use, wildlife, and hydrology. These factors also relate well to sensitivity to acidification. The EPA also evaluated the National Ecological Observatory Network (NEON) and Bailey's ecoregions developed for the US Forest Service and concluded that the Omernik ecoregion classification would be the most appropriate for an AAI-based standard. It offers several levels of spatial delineation, has undergone extensive scientific peer review, and has explicitly been applied to delineating acid sensitive areas of the US.
      Nonetheless, the EPA agrees with the first group of commenters that there are important remaining scientific uncertainties within the derivation of the AAI, with the data used to specify the factors within the AAI equation, and with the models themselves. These uncertainties are more fully discussed in Appendix F and G of the PA and in section III.A.5 above. These uncertainties have been reviewed by CASAC, and the EPA recognizes that further research would help to reduce the uncertainties. In general, the EPA also recognizes that the AAI would depend on atmospheric and ecological modeling, with inherent uncertainties, to specify the terms of an AAI equation that incorporate the linkages between ambient concentrations, deposition, and aquatic acidification.
      The EPA agrees with the first group of commenters that there are several important limitations in the available data upon which elements of the AAI are based (U.S. EPA, 2011, Chapter 7). For example, existing monitors for NOy are generally not located in areas that are representative of sensitive aquatic ecosystems, and there is relatively sparse water quality data coverage in sensitive mountainous western areas. Further, even in areas where relevant data are available, small sample sizes impede efforts to characterize the representativeness of the available data for some ecoregions, which was noted by CASAC as being of particular concern (Russell and Samet, 2011a). Also, measurements of reduced forms of nitrogen are available from only a small number of monitoring sites, and emission inventories for reduced forms of nitrogen used in atmospheric modeling are subject to considerable uncertainty.
      The EPA agrees with the first group of commenters that uncertainties related to the use of ecological and atmospheric models are difficult to evaluate due to a lack of relevant observational data. For example, relatively large uncertainties are introduced by a lack of data with regard to pre-industrial environmental conditions and other parameters that are necessary inputs to critical load models that are the basis for factor F1 in the AAI equation. Also, observational data are not generally available to evaluate the modeled relationships between nitrogen and sulfur in the ambient air and associated deposition, which are the basis for the other factors (i.e., F2, F3, and F4) in the AAI equation. The EPA recognizes that, in contrast, such model-related uncertainties are not relevant in the consideration of other NAAQS since those NAAQS are not defined in terms of factors based on such models.
      The EPA agrees that these data limitations and model uncertainties create a number of inherent uncertainties and complexities in the quantification of the F factors of the AAI and the representativeness of the F factors at an ecoregion scale (U.S. EPA, 2011, Appendix F). These uncertainties and complexities currently lead to large uncertainty in characterizing the degree of protectiveness that would be afforded by an AAI-based standard with quantified F factors derived as discussed above, within the ranges of levels and forms identified in section III.A above.
      The EPA disagrees with the first set of commenters that the selection of sensitive ecoregions and percentile waterbodies would be arbitrary. The EPA fully discussed its rationale and selection of sensitive ecoregions and the range of percentiles used in section 7.2.5 of the PA. The EPA relied on available alkalinity and ANC data to draw distinctions between sensitive and non-sensitive ecoregions. The EPA used its judgment in selecting the range of percentiles for sensitive and non-sensitive ecoregions, attempting to be neither over-protective nor under-protective of the set of waterbodies in each ecoregion.
      In general, the first set of commenters tends to treat all aspects of the AAI as subject to a high to very high degree of uncertainty. The EPA disagrees with this view, and instead views some parts of the AAI as based on more certain scientific information than others. For example, the EPA believes there is a solid scientific basis for the general framework of the AAI and for the relationship between ANC and effects on aquatic life. There is a strong basis for selection of ANC as an ecological indicator, for selection of NOy and SOx as ambient air indicators, for selection of the annual and 3- to 5-year averaging time frame, and for selection of the range of ANC and percentile of water bodies for consideration. Likewise, the EPA believes there is a solid scientific basis for selection of Omernik ecoregions as the geographic basis for development of the AAI F factors. The EPA believes that for many areas there is a strong basis for determining whether an ecoregion is acid sensitive or not acid sensitive, while recognizing there is some uncertainty in some areas as to which category the area should fall in. The EPA's decision not to adopt an AAI-based standard at this time is not driven by uncertainty in these elements of the AAI, but instead in the elements needed to derive the quantified F factors for ecoregions across the country and our ability to evaluate the representativeness of those F factors for an entire ecoregion. The greatest uncertainties concern the F1 and F2 factors, which relate to development of a single critical load to represent a specified percentile of all of the waterbodies in an ecoregion and development of the value for deposition of reduced nitrogen. In addition, there are also important uncertainties related to development of the F3 and F4 factors, which concern the quantified relationship between ambient levels of NOy and SOx and deposition rates of nitrogen and sulfur. The bases for these uncertainties are discussed in more detail in sections III.A.5 above and are considered as well in section III.E below. Thus, while the EPA agrees in part with the first group of commenters, in general they paint with too broad a brush. The EPA's decision is based instead on taking into account the areas where there is less scientific uncertainty as well as the areas where there remain significant scientific uncertainties.
      In general, the second set of commenters does not contest the scientific evidence as discussed by the EPA or the scientific conclusions the EPA draws. They do not contest the existence of scientific uncertainty or the causes of it, and do not present scientific or technical arguments to contest the nature or magnitude of the uncertainty. Instead, they disagree with the conclusions or judgments to draw from the uncertainty. In the view of these commenters, the degree of uncertainty is low enough to warrant setting an AAI standard at this time. They disagree with the Administrator's policy judgment that the nature and magnitude of uncertainty warrants not setting an AAI standard at this time. Their primary disagreement is with this judgment, not with the EPA's underlying views on the science and its uncertainties. As discussed in the proposal and below, however, the Administrator's reasoned judgment is that it is not appropriate to establish an AAI-based secondary standard at this time.  The uncertainties discussed above 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. Therefore, the Administrator is unable to identify an appropriate standard.
      The EPA recognizes that the AAI equation, with factors quantified in the ranges discussed in section III.A above and described more fully in chapter 7 of 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 (PA, chapter 7). However, the various uncertainties discussed above are critical for determining with any degree of confidence 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 with quantified F factors, and thus for determining an appropriate AAI-based standard that meets the requirements of Section 109 of the CAA. The EPA recognizes that 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.
      With regard to comments that the EPA cannot lawfully reject a new AAI-based standard, the EPA disagrees with the second group of commenters that the Administrator is required to set an AAI-based standard at this time. Although the Administrator has concluded that the current secondary standards are neither appropriate nor adequate to protect against potentially adverse deposition-related effects associated with ambient concentrations of oxides of nitrogen and sulfur, such a conclusion does not require the EPA to adopt a new NAAQS where the Administrator cannot reasonably judge that it would meet the criteria for a secondary NAAQS.
      The Administrator judges that the current limitations in relevant data and the uncertainties associated with specifying the elements of a new AAI-based NAAQS defined in terms of modeled factors are of such nature and degree as to prevent her from reaching a reasoned decision 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. As a result, the Administrator has determined that she cannot establish an AAI-based standard that is requisite to protect public welfare. The Administrator has made a similar judgment in deciding not to adopt new secondary NAAQS in the form of 1-hour standards identical to the primary NO2 and SO2 standards, as discussed below. No other NAAQS revisions to address the effects of acid deposition associated with oxides of nitrogen and sulfur in the ambient air have been suggested or considered by the EPA, CASAC, or commenters in this review.  As such, all possible revisions to the secondary NAAQS to address the effects of acid deposition would involve adoption of new secondary standards that are judged by the Administrator to have such a high degree of uncertainty that she cannot make a reasoned decision that a new standard would satisfy the criteria of Section 109(b) of the CAA
      Commenters have pointed to the requirement in Section 109(b)(2) of the CAA that any secondary NAAQS "must specify a level of air quality the attainment and maintenance of which ...is requisite to protect the public welfare from any know or anticipated adverse effects..." in support of the argument that the EPA must adopt a new standard that provides requisite protection, having concluded that the current secondary standards are not sufficient to protect against adverse effects. In considering this comment, the EPA has taken into account the statutory language, as well as the bases for the EPA's conclusion that the current standards for oxides of nitrogen and sulfur are neither appropriate nor adequate to provide protection against potentially adverse deposition-related effects and the data and model uncertainties that limit our efforts to characterize the degree of protectiveness that would be afforded by either an AAI-based standard or a 1-hour standard. We have concluded that Section 109 of the CAA does not require the EPA to adopt a new secondary standard where, as here, in the reasoned judgment of the Administrator, the uncertainties associated with such a standard would prevent her from determining whether or not such a NAAQS is requisite to protect public welfare. Section 109(b) of the CAA does not require the EPA to set a new standard under circumstances where the Administrator cannot reasonably judge that it would meet the criteria for a secondary NAAQS.
      The EPA recognizes and agrees with the comment from one environmental group that the EPA is not "foreclosed from setting a standard unless it can identify ...a `perfect' standard level that is free from any noteworthy uncertainty." However, that is not the situation in this rulemaking. The Agency has concluded that it would not be appropriate to promulgate a standard to address the public welfare effects of acidifying deposition where the remaining scientific uncertainties are of such import that they limit the EPA's ability to make a reasoned decision on the degree of protectiveness that would be afforded by such a standard. The EPA recognizes that the result of this decision is that the current secondary standards continue in place and continue to be neither appropriate nor adequate to protect against potentially adverse deposition-related effects associated with ambient concentrations of oxides of nitrogen and sulfur. However, in the Administrator's view the proper response under the current circumstances is to continue to develop the scientific and technical basis for a future revision to the standards, and not to adopt at this time a new secondary standard that she cannot reasonably judge would comply with Section 109 of the CAA.
      Further, the EPA agrees with both groups of commenters and CASAC that collecting further field data would be beneficial. A field pilot program is discussed in detail in section IV below. However, the EPA disagrees with the first group of commenters' assertions that these uncertainties should invalidate or preclude the further development of an AAI-based standard.
b.	Comments on Specific Aspects of an AAI-based Approach
      This section discusses comments on the following four specific aspects of an AAI-based approach to setting a secondary standard for oxides of nitrogen and sulfur:  (1) the inclusion of chemically reduced nitrogen (NHx), in addition to oxides of nitrogen, in the AAI equation; (2)  whether such a standard would be appropriately construed as a national standard versus a regional standard; (3) whether such a standard would be appropriately construed as an ambient air quality standard versus a water quality standard, and (4) whether the EPA has authority under the CAA to set a multi-pollutant NAAQS.
      (1)  As described above in section III.A, the AAI equation contains a separate factor that accounts for the acidifying potential of NHx, in addition to the factor that accounts for the acidifying potential of oxides of nitrogen. Several industry commenters addressed this issue explicitly, with some expressing the view that NHx should be treated the same as NOx in the AAI, while others felt it should not be included at all in the AAI. Several commenters expressed the view that accounting for NHx in the AAI equation represents a de facto regulation of ammonia, which they assert is unlawful since reduced nitrogen is not a listed air pollutant under Section 108 of the CAA.
	Other commenters, including environmental groups and governmental agency commenters, did not explicitly comment on the inclusion of NHx in the AAI equation; however several commenters made note of CASAC's advice on this issue. CASAC advised that it is necessary to include a factor for NHx in the AAI equation, even though it is not a listed pollutant, since aquatic ecosystems respond to inputs of NHx to create acidity just like they do with inputs of NOx and SOx.
	The EPA has included NHx deposition explicitly as part of factor F2 in the AAI expression to account for the acidifying potential afforded by ammonia gas and ammonium ion. Inclusion of NHx deposition, in addition to deposition of oxides of nitrogen, is necessary to account for potential effects of all reactive nitrogen species which, in turn, allows for determining the contributions of oxides of N and S to aquatic acidification. This approach is consistent with the requirement in the CAA that where the state of the science provides a basis for considering such effects, the review of the air quality criteria for a pollutant should encompass the ways in which other air pollutants may interact with the criteria pollutant to produce adverse effects. See CAA Section 108(a)(2). In effect, the inclusion of NHx deposition can be viewed as a necessary component consistent with our scientific understanding that links deposition of all nitrogen species to ecological effects.
	The EPA recognizes that the NAAQS is established to address the pollutants oxides of nitrogen and oxides of sulfur. Consequently, the ambient concentrations of  oxides of  sulfur (as SOx) and nitrogen (as NOy) are accounted for separately from the deposition of NHx in the AAI equation, thus defining the standard specifically in terms of the acidifying potential of levels of oxides of nitrogen and sulfur in the ambient air. More specifically, compliance with an AAI-based standard would be based on using federal reference or equivalent monitoring methods to measure ambient concentrations of NOy and SOx to determine an area's attainment status. Conversely, there would be no requirement to measure concentrations of NHx to determine compliance with an AAI-based standard. Rather, ecoregion-specific values of NHx deposition would be determined by modeling and would be specified by the EPA in conjunction with setting such a standard, and would not be a variable in the AAI equation as would SOx and NOy.  The contribution of reduced forms of nitrogen to total nitrogen deposition would represent an ecosystem-specific environmental factor that plays a necessary background role in characterizing the relationship between the measured, variable levels of the ambient air indicators of oxides of nitrogen and sulfur (NOy and SOx) and the associated degree of aquatic acidification. Section 108 requires the air quality criteria to evaluate to the extent practicable the variable factors such as atmospheric conditions that affect the impact of the ambient air pollutant (in this case oxides of nitrogen and sulfur) on the public welfare. In this review, such variable factors include the deposition of reduced nitrogen in an ecoregion, as well as all of the other elements reflected in the factors F1 to F4, and the designation of an area as acid-sensitive or not acid-sensitive. Section 109 calls for the EPA to base the NAAQS on the air quality criteria, and accounting for the role of reduced nitrogen deposition in the AAI reflects this.
	In considering this aspect of an AAI-based standard, the EPA took into account that in applying the AAI equation, all factors, including NHx deposition, would be updated as appropriate as part of the periodic reviews of the NAAQS, called for at five-year intervals by the CAA, to account for changing environmental conditions and new data. In determining an ecoregion's status with regard to meeting a particular AAI-based standard, NHx deposition reflected in the F2 factor would be treated just as all of the other environmental terms  -  e.g. critical loads and transference ratios -- which influence factors F1, F3 and F4. To the extent that changes in NHx deposition occur from one review to the next, the ecoregion-specific F2 factors would be updated to reflect such changes. To the extent that NHx deposition decreased from one review to the next, an AAI-based standard updated during a periodic review to reflect this change would allow for potentially higher levels of NOy and SOx that would meet a specific AAI-based standard; conversely, increased levels of NHx deposition would allow for potentially lower levels of NOy and SOx. Meeting a specific AAI-based standard would only require that the combined levels of NOy and SOx be such that a calculated AAI value meet or exceed the AAI value set as the level of the standard. Consequently, while the contribution of NHx deposition would be accounted for, NHx emissions would not be regulated through the implementation of an AAI-based standard. NHx deposition would be treated as an ecologically relevant background value that could be updated over time to reflect changes in circumstances, but accounting for such changes would not be required for purposes of determining compliance with an AAI-based standard. Thus, the incorporation of NHx in the AAI equation would not result in de facto regulation of NHx emissions.
      (2)  Some commenters raised the issue of whether an AAI-based standard would be a national standard, as required by Section 109 of the CAA, or whether it is in essence a regional standard. One group of commenters (the Center for Biological Diversity and the National Park Service) generally expressed the view that an AAI-based standard would be a national standard, whereas another group, including industry commenters, asserted that an AAI-based standard would be a regional standard and thus not consistent with the requirements of the CAA.
      The first group of commenters supported the use of a national ANC indicator, recognizing that an AAI approach would account for regional differences in sensitivity and relevant environmental factors while providing a nationally consistent degree of protection across sensitive ecoregions. For example, the National Park Service stated that the AAI approach provides a uniform level of protection to sensitive ecosystems while appropriately taking into account the variability in deposition, meteorology, and other relevant environmental factors across ecoregions.
      The second group of commenters noted that application of the AAI equation in different areas of the country produced different allowable concentrations of NOy and SOx, asserting as a result that an AAI-based standard would be a regional standard. These commenters asserted that the EPA lacks authority under the CAA to set such a regional NAAQS.  For example, UARG states that the AAI is applied differently in different regions of the country (e.g., sensitive vs. non-sensitive ecoregions). The Alliance of Automobile Manufacturers commented that both the EPA and Congress historically have decided that secondary national air quality standards are not an appropriate approach to address regionally variable welfare effects.
      The EPA believes that a secondary NAAQS based on the AAI approach could be a national standard, consistent with the CAA. An AAI-based standard would apply all across the country. It would be defined in part by a single level of the AAI  -  that is, every part of the country would be expected to meet or exceed a specified AAI level. The scientific basis for setting a national AAI level is rooted in the similarity between AAI and acid neutralizing capacity (ANC), which is a widely accepted ecological health indicator for aquatic acidification. The rationale underlying the use of ANC is that the ecosystem health reflected by an ANC value in one part of the country is generally similar to that in another location, irrespective of regional differences in biogeochemistry and atmospheric conditions. The EPA recognizes that allowable concentrations of the ambient air pollutant indicators for oxides of nitrogen and sulfur in the AAI equation can vary from one location to another and result in the same calculated AAI. The difference between an AAI-based standard and the existing primary standards is that the level of the standard is defined directly in terms of the measured ambient air pollutant indicator. That is, the health-based indicator and the measured ambient air indicator are based on the same chemical entity. In an AAI-based standard, the level of the standard, reflecting a nationally consistent degree of protection, would be defined in terms of an ecological indicator, ANC, and compliance would be determined based on concentrations of the ambient air indicators, NOy and SOx. From an ecosystem health perspective, it is most relevant to use the ecological indicator, ANC, to establish a single level that, in the context of an AAI, leads to a similar degree of protection across the country. The allowable levels of NOy and SOx could vary across the country, while the specified AAI level and the corresponding degree of protection, would not. This would facilitate ensuring that such a NAAQS would provide sufficient protection, but not more than was necessary. It should be noted that in the 2006 PM NAAQS decision the EPA set a NAAQS that envisions variation in allowable ambient levels of certain kinds of PM. The EPA set a PM10 standard with a single numerical level, which then allowed varying levels of coarse PM, a subset of PM10. The PM10 standard was designed to allow lower levels of coarse PM in urban areas and higher levels of coarse PM in non-urban, rural areas. The EPA's goal was to target protection at urban areas, where the evidence showed coarse particles presented a greater risk to public health. The single numerical standard for PM10 allowed variable levels of coarse PM, with higher allowable levels where there was less evidence of risk and lower allowable levels where the evidence of risk was greater. This approach was upheld in American Farm Bur. Fed. v. EPA, 559 F.3d 512, 533- 536 (D.C. Cir. 2009).
      In conjunction with consideration of an AAI-based standard, the EPA has recognized that the nation includes some relatively acid-sensitive and some relatively non-acid sensitive ecoregions. This delineation allows for an appropriate application of the AAI equation that increases its relevancy from a national perspective as it avoids creating more than requisite protection in areas that are not acid sensitive. The AAI equation and the selected level of such a standard would be applicable everywhere; however, factors in the AAI equation are appropriately dependent on the sensitive and non-sensitive ecoregion classification. Therefore, the delineation of sensitive and non-sensitive regions allows for a nationally consistent application of the AAI equation as it targets protection on those areas most likely to benefit from reductions in acidifying deposition of oxides of nitrogen and sulfur, and avoids more than requisite protection in areas that would not benefit from such reductions.
 	(3)  Some commenters expressed the view that an AAI-based standard would essentially be a water quality standard, since it would use ANC, a water quality property, as the ecological indicator. For example, UARG expressed this view by noting that an AAI standard would be defined in terms of a single water quality level with multiple allowable air quality concentrations of oxides of nitrogen and sulfur.
      The EPA notes that the AAI relates aquatic acidification to ambient air concentrations of oxides of nitrogen and sulfur. An AAI-based standard would be set at a level such that ambient air concentrations would not cause harmful acidification effects to water quality resources, which is within the scope of welfare effects that secondary NAAQS are to address (i.e., welfare effects include, but are not limited to, "effects on soils, water, ..."). Accordingly, while an AAI-based standard would address effects on water quality, it would do so by defining the allowable ambient air concentrations of oxides of nitrogen and sulfur that would provide appropriate protection against such effects. Compliance with such a standard would be determined by measuring ambient air concentrations of NOy and SOx, not by measuring the water quality property of ANC. The actual water quality of any body of water would not be used to determine compliance with the air quality standard, and no body of water would be considered in "non-compliance" with an AAI air quality standard. Thus, an AAI-based standard is appropriately construed as an air quality standard, not a water quality standard.
      (4)    Some commenters questioned whether the EPA has the authority to establish a NAAQS that jointly addresses ambient concentrations of oxides of nitrogen and oxides of sulfur. Pointing to language in Section 109(b)(2) that a NAAQS must  address "adverse effects associated with the presence of such air pollutant in the ambient air," these commenters took the position that the EPA may not allow for tradeoffs between two pollutants in setting a NAAQS. See Section109(b)(2)(emphasis added). These commenters suggest the NAAQS must be set for "such air pollutant" only. The EPA disagrees that the phrase "such air pollutant" in Section109(b)(2) would prohibit  the Agency from setting a multi-pollutant NAAQS in the form of an AAI. When the Administrator sets a NAAQS, the standard must be "requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of such air pollutant." CAA Section109(b)(2). Oxides of nitrogen and sulfur, pollutants for which the EPA has issued air quality criteria, both cause acidification of aquatic ecosystems, effects that could be considered adverse to public welfare. As such, acidifying deposition is a "known or anticipated adverse effect[] associated with the presence of [oxides of nitrogen] in the ambient air." This known or anticipated adverse effect is also associated with the presence of oxides of sulfur in the ambient air. Given the scientific links between ambient air concentrations of oxides of nitrogen and sulfur, the related deposition of nitrogen and sulfur, and the associated ecological responses, the EPA appropriately considered a multi-pollutant NAAQS in the form of an AAI to protect against the effects of acidifying deposition to aquatic ecosystems that took into account these linkages. Rather than limiting the EPA's authority, the language cited by the commenters goes to the breadth of the EPA's obligation and authority to set standards to protect against "any known or anticipated adverse effects."  In addition, the NAAQS are to be based on the air quality criteria, which under Section 108 (a)(2) are required to consider the kind of multi-pollutant linkage evident in this review. The EPA does not read the language of Section 109(b) as prohibiting the Administrator from setting a multi-pollutant NAAQS such as the AAI where such an approach would be judged as the appropriate way to satisfy Section 109(b)'s requirements for each of the pollutants involved.
2.	Comments on 1-Hour NO2 and SO2 Secondary Standards
	Comments received on the proposal related to setting new 1-hour NO2 and SO2 secondary standards are addressed in this section. Most generally, there was broad and strong opposition to the EPA's proposed decision to set 1-hour NO2 and SO2 secondary standards identical to the 1-hour NO2 and SO2 primary standards. For example, strong opposition to this proposed decision was expressed by a diverse set of commenters, including some environmental groups (e.g., Environmental Justice, the Adirondack Council) and industry groups (e.g., UARG, AAM, ASARCO, API, Portland Cement Association, Tri-State Generation and Transmission Association, Louisiana Chemical Association, East Kentucky Power Cooperative, FMMI, Rio Tinto), the U.S. Department of the Interior, and some states (e.g., NY, PA, TX). These commenters offered various arguments in support of their views that the proposed decision is unlawful, arbitrary, and not supported by the record of this rulemaking, as outlined below. One commenter (NC) supported setting secondary standards identical to the 1-hour NO2 and SO2 primary standards, while also supporting the EPA's decision to take additional time to develop a multi-pollutant AAI-based secondary standard. Another commenter (SD) simply supported setting secondary standards that are no more stringent than the primary standards.
	In proposing the 1-hour secondary standards, the EPA recognized that such standards would not be ecologically relevant, but concluded that they would nonetheless "directionally provide some degree of additional protection" by reducing deposition to sensitive ecosystems. The EPA also noted that this was 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.
      In arguing that the proposed decision to set 1-hour NO2 and SO2 secondary standards identical to the 1-hour NO2 and SO2 primary standards is unlawful, commenters asserted that the EPA's rationale is not consistent with the requirements of Section 109 of the CAA. Commenters argue that this rationale is not consistent with the CAA requirement that the EPA set secondary NAAQS that are "requisite to protect public welfare;" that is, a standard that is neither more nor less stringent than necessary for this purpose. More specifically, these commenters argue that a standard that is based solely on "directionally" improving the environment, without any evidence or judgment that it would provide "requisite" protection, is not consistent with the requirements of the CAA and is thus unlawful. Some commenters also note that the CAA requires that the EPA revise previously adopted NAAQS as "appropriate" to provide such protection. These commenters assert that since the EPA's proposal concludes that the 1-hour NO2 and SO2 standards are not ecologically relevant to address deposition-related effects on sensitive ecosystems, adding such standards cannot be considered to be an appropriate revision to the NAAQS for the purpose of addressing adverse ecological effects.
	Commenters also raised a number of issues in support of their views that the proposed decision is arbitrary and unsupported by the available information in the record of this rulemaking. Some commenters noted that there is no evidence or analysis in the record that addresses the degree of protection that would likely be afforded by 1-hour NO2 and SO2 standards, and, further, that the EPA does not claim otherwise. In the absence of such information, commenters argue that the EPA cannot make a reasoned judgment as to what levels of such 1-hour NO2 and SO2 standards would be requisite to protect public welfare; in particular, some commenters emphasized that the EPA cannot demonstrate that such standards would not be more stringent than necessary to protect against adverse deposition-related effects to sensitive ecosystems. Thus, in the commenters' view, any such 1-hour standards would be arbitrary.
	One commenter also expressed the view that the EPA's proposed decision to set new 1-hour NO2 and SO2 secondary standards is inconsistent with the reasoning the EPA used as a basis for proposing not to set a new ecologically relevant AAI-based secondary standard at this time. As summarized above, the EPA based its proposed decision not to set an AAI-based standard, which is expressly designed to address important differences in ecosystem sensitivities, in part on uncertainties and limitations in relevant information that were of such nature and degree as to prevent the Administrator from reaching a reasoned decision at this time as to what level and form of such a standard would provide a particular degree of protection. This commenter asserts that the proposed decision to set new 1-hour NO2 and SO2 secondary standards completely ignores such uncertainties inherent in 1-hour standards, which are not even structured to account for differences in ecosystem sensitivities.
	Some commenters asserted not only that the EPA has failed to provide any information on the degree of protection that would likely be afforded by the proposed 1-hour NO2 and SO2 standards, but that such an analysis cannot be done since there is no rational connection between any of the elements of the proposed 1-hour secondary standards  -  including the averaging time and level  -  and the ecological effects the proposed standards are intended to address. In particular,  commenters noted that EPA has not presented any rational basis for concluding that standards designed to reduce human health risks associated with short-term peak concentrations of NO2 and SO2 have any connection whatsoever to addressing long-term deposition of oxides of nitrogen and sulfur and associated impacts on sensitive ecosystems.
	Further, commenters argued that there is no evidence in the record that demonstrates the proposed 1-hour secondary standards would provide any environmental benefit. For example, commenters noted that such standards do not take into account ecosystem sensitivity; they may not result in reductions to long-term deposition that is the relevant time frame for deposition-related effects on sensitive ecosystems; and they would not provide any benefit beyond that which might accrue from the identical primary standards that are already in effect. Some commenters have also noted that many other environmental regulations are already in place that will provide reductions in ambient oxides of nitrogen and sulfur, and that the EPA has not demonstrated that any additional reductions are needed to provide requisite protection.
	The EPA agrees that the Agency has not presented evidence or analysis in the record that addresses the degree of protection that would likely be afforded by secondary standards set identical to the current 1-hour NO2 and SO2 primary standards. The EPA further agrees that such an analysis cannot reasonably be done in the absence of a demonstrable linkage between peak 1-hour average concentrations of NO2 and SO2 in the ambient air and the impact of deposition-related acidification associated with oxides of nitrogen and sulfur on sensitive aquatic ecosystems that the proposed standards were intended to address. As a result, the EPA agrees that there is no factual basis to make a reasoned judgment as to what levels of 1-hour NO2 and SO2 standards would provide a desired degree of protection of the public welfare, such that the EPA cannot demonstrate or judge that the proposed standards would not be more or less stringent than necessary to provide the desired degree of protection against potentially adverse deposition-related effects to sensitive ecosystems.
      As to whether the proposed standards would provide any environmental benefit, it is the EPA's view that it is reasonable to conclude that any standard that would lead to reductions in NO2 and SO2 emissions would likely result in some environmental benefit for some acid-sensitive areas. Nonetheless, the EPA recognizes that any such environmental benefit that would result from reductions in NO2 and SO2 emissions sufficient to attain the 1-hour standards cannot be specifically quantified or linked to reductions in aquatic acidification in specific ecoregions. In addition, unlike an AAI-based standard, the 1-hour standards would tend to provide more protection than is warranted in areas that are not acid-sensitive.
      Further, the EPA recognizes that any such benefits would accrue from the 1-hour NO2 and SO2 primary standards that are in effect, regardless of whether identical secondary standards are adopted. The EPA does not agree, however, that the Agency needs to consider future reductions that may accrue from other environmental regulations in the context of reaching a judgment as to what NAAQS is requisite to protect public welfare.
      The EPA notes that the strongly held view of the commenters with respect to the proposed 1-hour standards is that the EPA should reject and not adopt a standard where there is not an adequate scientific or technical basis for judging the degree of protection which such a standard would provide. The EPA agrees with that general point. According to commenters, the 1-hour standards should be rejected because they do not have such a basis, and, as discussed below, the EPA agrees. This is consistent with the reasoning that the EPA has applied to consideration of an AAI-based standard, as discussed above in response to comments related to an AAI-based standard. As noted above, the limitations and uncertainties in the scientific and technical basis for developing a specific AAI-based standard result in a great degree of uncertainty as to how well the quantified elements of the AAI 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 great uncertainty as to the actual degree of protectiveness that such a standard would provide, especially for acid-sensitive ecoregions. The Administrator judges that the uncertainties are of such a 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.
      The EPA has also considered, in light of the public comments, whether it is necessary or appropriate under Section 109 of the CAA to make any revision to the current secondary standards for oxides of nitrogen and sulfur, having concluded that the current standards are neither adequate nor appropriate.  As discussed above in section III.D.1.a, with regard to comments on the EPA's proposed decision not to set a new multi-pollutant AAI-based standard at this time, some commenters argued that the EPA cannot lawfully use uncertainty as a basis to decline to set an ecologically relevant standard, having concluded that the current secondary standards are neither adequately protective nor appropriate to provide protection to ecosystems. In response, the EPA disagrees, stating that data limitations and uncertainties in key elements of a standard, which are of such nature and degree as to prevent the Administrator from reaching a reasoned decision as to what specific standard would be appropriate to provide requisite protection, are an appropriate basis for deciding not to set such a standard, even one that is of an ecologically relevant form. The EPA concludes that it is appropriate to apply the same reasoning in reaching a decision as to whether to set new 1-hour NO2 and SO2 secondary standards. In this case, the uncertainties are arguably greater than with an AAI-based standard, since as noted above there is no demonstrable linkage between the elements of such standards and impacts on sensitive ecosystems that the standards would be intended to address.
E.	Final Decisions on Alternative Secondary Standards for Oxides of Nitrogen and Sulfur
      In considering the appropriateness of establishing a new multi-pollutant AAI-based standard to provide protection against potentially adverse deposition-related effects associated with oxides of nitrogen and sulfur, or setting new secondary standards identical to the current 1-hour NO2 and SO2 primary standards, the Administrator took into account the information and conclusions in the ISA, REA, and PA, CASAC advice, and the views of public commenters. This consideration follows from her conclusion, discussed above in section II.D, that the existing NO2 and SO2 secondary standards are neither appropriate nor sufficiently protective for this purpose.
	As an initial matter, the Administrator has again considered whether it is appropriate at this time to set a new multi-pollutant standard to provide protection against potentially adverse deposition-related effects associated with oxides of nitrogen and sulfur, with a structure that would better reflect the available science regarding acidifying deposition. In considering this, she recognizes 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 has focused on the new AAI-based standard developed in the PA and reviewed by CASAC, as discussed above in section III.A. In so doing, the Administrator has again considered the extent to which there is a scientific basis for development of such a standard, specifically with regard to a standard that would provide protection from deposition-related aquatic acidification in sensitive aquatic ecosystems in areas across the country.
	The Administrator notes that the ISA concludes that the available scientific evidence is sufficient to infer a causal relationship between acidifying deposition of nitrogen and sulfur in aquatic ecosystems, and that the deposition of oxides of nitrogen and sulfur both cause such acidification under current conditions in the United States. Further, the ISA concludes that there are well-established water quality and biological indicators of aquatic acidification as well as well-established models that address deposition, water quality, and effects on ecosystem biota, and that ecosystem sensitivity to acidification varies across the country according to present and historic nitrogen and sulfur deposition as well as geologic, soil, vegetative, and hydrologic factors. In considering public comments on the relevant scientific evidence, the Administrator notes that some commenters agree with these conclusions in the ISA, whereas other commenters question the extent to which the scientific information provides evidence of well-established water quality and biological indicators of aquatic acidification and the extent to which relevant models appropriately account for important factors or have been adequately evaluated. The Administrator has carefully considered these comments and the Agency's responses to these comments, as discussed above in section III.D. The Administrator also has considered the views of CASAC, including its general support for the conceptual framework of the AAI-based standard developed in the PA based on the assessments of the underlying scientific information in the ISA and REA.
	Based on these considerations, the Administrator again concludes that the general structure of  an AAI-based standard addresses the combined effects of deposition from oxides of nitrogen and sulfur by characterizing the linkages between ambient concentrations, deposition, and aquatic acidification, and that it takes into account relevant variations in these linkages across the country. She recognizes that while such a standard clearly would be quite innovative and unique, the general structure of such a standard is nonetheless 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. Based on these considerations, the Administrator continues to agree with the conclusion in the PA, and supported by CASAC, that there is a strong scientific basis for continued development of a standard with the general structure presented in the PA.
	Nonetheless, while the Administrator recognizes the strong scientific foundation for the general structure of an AAI-based standard, she also recognizes that a specific AAI-based standard would depend to a great degree on atmospheric and ecological modeling, in combination with appropriate data, to specify the quantified terms of an equation that incorporates the linkages between ambient concentrations, deposition, and aquatic acidification. This equation, which defines an aquatic acidification index (AAI), has the effect of translating spatially variable ambient concentrations and ecological effects into a potential national standard. With respect to establishing the specific terms of this equation, there are a number of important uncertainties and complexities that are critical to the question of whether it is appropriate under Section 109 of the CAA to set a specific AAI-based standard at this time, recognizing that such a standard must be one that in the judgment of the Administrator is requisite to protect public welfare without being either more or less stringent than necessary for this purpose. As discussed above in section III.A, these uncertainties and complexities generally relate not to the structure of the standard, but to the quantification of the various elements of the standard, i.e., the F factors, and their representativeness at an ecoregion scale. These uncertainties and complexities, which are unique to this NAAQS review, currently limit efforts to characterize the degree of protectiveness that would be afforded by such a standard, within the ranges of levels and forms identified in the PA, and the representativeness of F factors in the AAI equation described above and in the PA. These important uncertainties have been generally categorized as limitations in available field data as well as uncertainties that are related to reliance on the application of ecological and atmospheric modeling at the ecoregion scale to specify the various elements of the AAI. In addition, while ambient measurements of NOy are made as part of a national monitoring network, the monitors are not located in locations that have been determined to be representative of sensitive aquatic ecosystems or individual ecoregions.
	With regard to data limitations, the Administrator observes that there are several important limitations in the available data upon which elements of the AAI are based. For example, while air and water quality data are generally available in areas in the eastern United States, there is relatively sparse coverage in mountainous western areas where a number of sensitive aquatic ecosystems are located. Further, even in areas where relevant data are available, small sample sizes impede efforts to characterize the representativeness of the available data, which was noted by CASAC and some commenters as being of particular concern. Also, measurements of reduced forms of nitrogen are available from only a small number of monitoring sites, and emission inventories for reduced forms of nitrogen used in atmospheric modeling are subject to considerable uncertainty.
	With regard to uncertainties related to the use of ecological and atmospheric modeling, the Administrator notes in particular that model results are difficult to evaluate due to a lack of relevant observational data. For example, relatively large uncertainties are introduced by a lack of data to inform the necessary inputs to critical load models that are the basis for factor F1 in the AAI equation. Also, observational data are not generally available to evaluate the modeled relationships between nitrogen and sulfur in the ambient air and associated deposition, which are the basis for the other factors (i.e., F2, F3, and F4) in the AAI equation.
	In combination, these limitations and uncertainties result in a large 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 large uncertainty as to the actual degree of protectiveness that such a standard would provide, especially for acid-sensitive ecoregions. The Administrator recognizes 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 directionally 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 of the CAA.
	In considering these uncertainties in light of CASAC's advice, the Administrator notes 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. She also notes that CASAC encouraged the Agency to engage in future monitoring and model evaluation efforts to help inform the specification of model-derived elements in the AAI equation. CASAC supported the view in the PA that there was a scientific basis for consideration of an AAI, and that is what the Administrator has done in that she has fully considered an AAI-based standard. However, CASAC did not indicate that there was such a degree of scientific support for quantifying the terms of the AAI equation and setting a specific AAI-based standard at this time that it would be inappropriate to consider not setting an AAI-based standard in this review in light of the uncertainties that CASAC itself recognized.
	Further, in considering these uncertainties in light of the public comments discussed above, the Administrator notes that these uncertainties and limitations have been highlighted by a number of public commenters in support of their view that it would be inappropriate to establish an AAI-based standard at this time. Other commenters, however, noted that NAAQS decisions are always made in the face of uncertainties, and expressed the view that the uncertainties in this NAAQS review are not so great as to preclude establishing such a standard at this time.
	The Administrator agrees with the commenters that note that NAAQS decisions are always made in the face of uncertainties, since the latest available scientific information upon which NAAQS are to be based is often at the leading edge of research.  Thus, the EPA Administrator must always consider uncertainties in scientific and other information in reaching decisions on whether to retain or revise an existing NAAQS or to adopt a new NAAQS.  As a result, it is clear that the existence of scientific uncertainty does not preclude adoption of a new or revised NAAQS.  The issue here, however, is not whether uncertainty exists, but whether it is of such a nature and magnitude that it warrants not adopting an AAI-based standard at this time.  In that context, the Administrator recognizes that the AAI-based standard considered in this review is by far the most complex form of a NAAQS standard that EPA has considered, to date, and that this is the first review in which the scientific and technical details of an AAI-based standard have been developed for consideration.  This review has served to bring into focus for the first time the nature and degree of the uncertainties associated with quantifying the specific factors in the equation that defines the AAI.  Thus, in this review, the Administrator must newly consider not only the scientific basis for the conceptual framework of such a standard, but also the extent to which the available data, models, and analyses provide a reasoned basis to choose a specific AAI-based standard consistent with the requirements of Section 109 of the CAA.
	The nature of the uncertainties present in this review, and the implications of those uncertainties for reaching a reasoned decision as to whether an AAI-based NAAQS could be set consistent with the requirements of section 109(b), are in sharp contrast to the nature of uncertainties present in other NAAQS reviews.  In other NAAQS reviews, studies are generally available directly linking ambient air concentrations of the pollutant to evidence of effects on public health or welfare.  For example, in reviewing a health-based primary NAAQS EPA typically considers a wide range of clinical, epidemiologic, toxicologic, and other studies that evaluate the relationship between direct exposure to an ambient air pollutant and human health.  EPA also often considers laboratory or field studies or surveys that evaluate and characterize the relationship between ambient levels of an air pollutant and welfare effects, such as effects of the ambient air pollutant on the growth of plants or on injury to plants.  These kinds of scientific studies have provided a reasoned basis in other reviews for the selection of an appropriate level and form of a standard, with EPA taking into account the nature and degree of uncertainties, for example, in the relationships between varying ambient air concentrations and the impact on human health or the environment.
      Further, the uncertainties present in the evidence available for other NAAQS reviews have not been of a nature that have precluded a reasoned assessment of the degree of protectiveness that would likely be afforded by specific alternative standards under consideration. In this case, however, unlike in other NAAQS reviews, multi-pollutant and multi-media pathways of exposure must be considered, and characterized in terms of an equation with several factors, where the values of those factors vary from ecoregion to ecoregion.  The quantification of these factors must be based on the use of ecological and atmospheric modeling at an ecoregion scale. Further, the appropriateness of these factors depends upon analyses that could be used to determine the representativeness of the data at an ecoregion level. These circumstances, which are unique to this review, result in large uncertainties at this time that prevent the development of a reasoned assessment of the degree of protectiveness that specific alternative AAI-based standards would provide.
	Based on the above considerations, the Administrator has determined that at this time it is not appropriate under Section 109 of the CAA to set a new multi-pollutant standard to address deposition-related effects of oxides of nitrogen and sulfur on aquatic acidification. As the Administrator noted in the proposal, setting a NAAQS properly 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. As noted above, 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, in the Administrator's judgment, the unique uncertainties present in this review prevent a reasoned 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, together with an AAI defined in terms of ecoregion-specific F factors. The Administrator has considered whether these uncertainties could be appropriately accounted for by choosing either a more or less protective target ANC level and percentile of water bodies than would otherwise be chosen if the uncertainties did not substantially limit the confidence that can appropriately be ascribed to the quantification of the AAI factors. However, in the Administrator's judgment, the uncertainties are of such nature and magnitude that there is no reasoned way to choose such 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.
	Based on the above considerations, the Administrator judges that the current limitations in relevant data and the uncertainties associated with specifying the elements of the AAI are of such nature and degree as to prevent her from reaching a reasoned decision as to what level and form (in terms of a selected percentile) of an AAI-based standard would provide the degree of protection that the Administrator determined was requisite.  While acknowledging that CASAC supported consideration of moving forward to establish the standard developed in the PA at this time, the Administrator also observes that CASAC supported conducting further field studies that would better inform the continued development or modification of such a standard. Given the current large uncertainties and complexities inherent in quantifying the elements of such a standard, largely deriving from the nature of the standard under consideration for the first time in this review, and having fully considered CASAC's advice and public comments, the Administrator concludes that it would be premature and not appropriate to set a new, multi-pollutant AAI-based secondary standard for oxides of nitrogen and sulfur at this time.
	While the Administrator has concluded that it is not appropriate to set such a multi-pollutant standard at this time, she has determined that the Agency should undertake a field pilot program to gather additional data, and that it is appropriate that such a program be undertaken before, rather than after, reaching a decision to set such a standard. As described below in section IV, the purpose of the program is to collect and analyze data so as to enhance our understanding of the degree of protectiveness that would likely be afforded by a standard based on the AAI as developed in the PA. This will provide additional information to aid the Agency in considering an appropriate multi-pollutant standard in future reviews, specifically with respect to the acidifying effects of deposition of oxides of nitrogen and sulfur. Data generated by this field program will also support development of an appropriate monitoring network that would work in concert with such a standard to result in the intended degree of protection. The information generated during the field program can also be used to help state agencies and the EPA better understand how an AAI-based standard would work in terms of the implementation of such a standard.
      While not a basis for this decision, the Administrator also recognizes, as she did at the time of the proposal, that a new, innovative AAI-based standard would raise significant implementation issues that would need to be addressed consistent with the CAA requirements for implementation-related actions following the setting of a new NAAQS. It will take time to address these issues, during which the Agency will be conducting a field pilot program to gather relevant data and the environment will benefit from reductions in oxides of nitrogen and sulfur resulting from the new NO2 and SO2 primary standards, as noted above, as well as reductions expected to be achieved from the EPA's Cross-State Air Pollution Rule and Mercury and Air Toxics standards. These implementation-related issues are discussed in more detail below in section IV.A.5.
	The Administrator has also reconsidered whether it is appropriate at this time to set new secondary standards identical to the current 1-hour NO2 and SO2 primary standards. In the proposal, the Administrator recognized that the new NO2 and SO2 primary 1-hour standards set in 2010 were not ecologically relevant for a secondary standard to address deposition-related effects associated with oxides of nitrogen and sulfur. Nonetheless, the Administrator proposed to set new secondary standards identical to the 1-hour NO2 and SO2 primary standards on the basis that they would directionally provide some degree of additional protection. At that time, the Administrator reasoned that setting such standards would be consistent with her conclusions that the current NO2 and SO2 secondary standards are neither sufficiently protective nor appropriate in form, and that it is not appropriate to set a new, ecologically relevant multi-pollutant secondary standard at this time.
	In reconsidering this proposal, the Administrator first notes that although the ISA, REA, and PA did not directly consider secondary standards set identical to the 1-hour NO2 and SO2 primary standards, the information and conclusions in those documents provide strong support for the judgment that such short-term, peak standards are not ecologically relevant to address deposition-related effects associated with long-term deposition from ambient concentrations of oxides of nitrogen and sulfur. The Administrator notes that commenters on this aspect of the proposal broadly and strongly supported this view. The Administrator also recognizes that the Agency has not presented in these documents or elsewhere any analysis of the degree of protectiveness that would likely be afforded by such standards with regard to deposition-related effects in general or aquatic acidification effects in particular. She also recognizes, as discussed above in response to comments on this issue, that such an analysis cannot be done since there is no demonstrable linkage between 1-hour average concentrations of NO2 and SO2 in the ambient air and the impact of longer-term deposition-related acidification associated with oxides of nitrogen and sulfur on sensitive aquatic ecosystems that the proposed standards were intended to address. As a result, as in the case of an AAI-based standard as discussed above, the Administrator concludes that there is no basis to make a reasoned judgment as to what levels of 1-hour NO2 and SO2 standards would be requisite to protect public welfare, such that the EPA cannot demonstrate a reasoned basis for judging that the proposed standards would be sufficient but not more stringent than necessary to protect against adverse deposition-related effects to sensitive ecosystems.
	With regard to considering the views of CASAC, the Administrator notes that the PA did not discuss the alternative of setting secondary standards that are identical to the 1-hour NO2 and SO2 primary standards. As a consequence, this alternative was not presented for consideration by CASAC and therefore CASAC has not expressed its views on this alternative set of standards.
      In light of the above considerations, and taking into consideration public comments, the Administrator has further considered whether it is necessary or appropriate under Section 109 of the CAA to set such 1-hour NO2 and SO2 secondary standards, having concluded that the current NO2 and SO2 secondary standards are neither adequate nor appropriate to address potentially adverse deposition-related effects on sensitive ecosystems associated with oxides of nitrogen and sulfur. In reaching this decision, the Administrator concludes that it is appropriate to apply the same reasoning as she did in reaching the decision that it is premature and not appropriate under Section 109(b) to set a new AAI-based standard at this time. In considering such 1-hour standards, the Administrator judges that the uncertainties are likely greater than with an AAI-based standard, since as noted above there is no demonstrable linkage between the elements of such standards and impacts on sensitive ecosystems that the standards would be intended to address. In addition, with respect to areas that are not acid sensitive, and unlike an AAI standard, it is likely that the proposed 1-hour standards directionally would provide more protection than is warranted. Therefore, the Administrator now concludes that it is neither necessary nor appropriate to set 1-hour NO2 and SO2 secondary standards, since in her judgment setting such standards cannot reasonably be judged to provide requisite protection of public welfare.
	In summary, for the reasons discussed above, and taking into account information and assessments presented in the ISA, REA, and PA, the advice and recommendations of CASAC, and the public comments on the proposal, the Administrator has decided that it is not appropriate under Section 109(b) to set any new secondary standards at this time to address potentially adverse deposition-related effects associated with oxides of nitrogen and sulfur. Further, as discussed above in section II.D, she has also decided to retain the current NO2 and SO2 secondary standards to address direct effects of gaseous NO2 and SO2 on vegetation. Thus, taken together, the Administrator has decided to retain and not revise the current NO2 and SO2 secondary standards. Specifically these secondary standards include an NO2 standard set at a level of 0.053 ppm, annual arithmetic average, and an SO2 standard set at a level of 0.5 ppm, 3-hour average, not to be exceeded more than once per year.
IV.	Field Pilot Program and Ambient Monitoring
      This section discusses elements of a field pilot program and the evaluation of monitoring methods for ambient air indicators of NOy and SOx that could be conducted to implement the Administrator's decision to undertake such a field monitoring program in conjunction with her decision not to set a new multi-pollutant secondary standard in this review, as discussed above in section III.E. The PA included considerations related to monitoring methods and network design that could support an AAI-based standard, which were reviewed by the CASAC Ambient Monitoring Methods Subcommittee (AMMS) (Russell and Samet, 2011b). As discussed below, the CASAC AMMS supported the approach of basing a potential future air monitoring network on the existing Clean Air Status and Trends Network (CASTNET) program. In addition, the CASAC AMMS supported the use of the CASTNET filter packs (CFPs) as appropriate methods to measure the oxides of sulfur indictor, SOx, and the use of commercially available NOy instruments to measure the oxides of nitrogen indicator, NOy. CASAC AMMS also supported the inclusion of complementary measurements in any future field monitoring program that would support the evaluation of the monitoring methods and air quality models upon which the AAI developed in the PA was based.
      Section IV.A below outlines the objectives, scope, and key elements of the field pilot program as presented in the proposal and section IV.B summarizes EPA's proposed approach to evaluating monitoring methods. These approaches reflect consideration of the advice of the CASAC AMMS. Public comments on the field pilot program and evaluation of monitor methods are discussed below in section IV.C. These comments have been helpful in shaping the process that EPA is now undertaking to develop the field pilot program and monitoring methods evaluation.
      The following sections provide insight into EPA's current ideas about what could be incorporated into the pilot program, but EPA has not made any final decisions on what will be included. These ideas will be discussed further in a draft white paper to be made available later this year for public comment. The draft white paper will present more detailed plans for the field pilot program and monitoring methods evaluation. The draft white paper is intended to serve as both a draft work plan and a vehicle for continued input from outside interests. Taking into consideration comments received on the draft white paper, EPA will prepare a final white paper that will serve as a program management and communication document to facilitate engagement with interested stakeholders and convey EPA's final plans.
      A. Overview of Proposed Field Pilot Program
As discussed in the proposal, the primary goal of this field pilot program, and the related monitoring methods evaluation summarized below in section IV.B, is to enhance our understanding of the degree of protectiveness that would likely be afforded by a standard based on the AAI, as described above in section III.A. This program is intended to aid the Agency in considering in future reviews an appropriate multi-pollutant standard that would be requisite to protect public welfare consistent with Section 109 of the CAA, through the following objectives:
(1) evaluate measurement methods for the ambient air indicators of NOy and SOx and consider designation of such methods as Federal Reference Methods (FRMs);
(2) examine the variability and improve characterization of concentration and deposition patterns of NOy and SOx, as well as reduced forms of nitrogen, within and across a number of sensitive ecoregions across the country;
(3) develop updated ecoregion-specific factors (i.e., F1 through F4) for the AAI equation based in part on new observed air quality data within the sample ecoregions as well as on updated nationwide air quality model results and expanded critical load data bases, and explore alternative approaches for developing such representative factors;
(4) calculate ecoregion-specific AAI values using observed NOy and SOx data and updated ecoregion-specific factors to examine the extent to which the sample ecoregions would meet a set of alternative AAI-based standards;
(5) develop air monitoring network design criteria for an AAI-based standard;
(6) assess the use of total nitrate measurements as a potential alternative indicator for NOy;
(7) support related longer-term research efforts, including enhancements to and evaluation of modeled dry deposition algorithms; and
(8) facilitate stakeholder engagement in addressing implementation issues associated with possible future adoption of an AAI-based standard.
      The EPA proposed to use CASTNET sites (Figure IV-1) in selected acid-sensitive ecoregions to serve as the platform for this pilot program, potentially starting in late 2012 and extending through 2018. The CASTNET sites in three to five acid-sensitive ecoregions would collect NOy and SOx (i.e., SO2 and  p-SO4) measurements over a 5-year period. The initial step in developing a data base of observed ambient air indicators for oxides of nitrogen and sulfur requires the addition of NOy samplers at the pilot study sites so that a full complement of indicator measurements are available to calculate AAI values. These CASTNET sites would also be used to make supplemental observations useful for evaluation of CMAQ's characterization of factors F2  - F4 in the AAI equation.
      The selected ecoregions would account for geographic variability by including regions from across the United States, including the east, upper midwest, and west. Each selected ecoregion would have at least two existing CASTNET sites.

Figure IV-1. Location of CASTNET sites in relation to acid sensitive ecoregions.
       Over the course of this 5-year pilot program, the most current national air quality modeling, based on the most current national emissions inventory, would be used to develop an updated set of F2  -  F4 factors. A parallel multi-agency national critical load data base development effort would be used as the basis for calculating updated F1 factors. As discussed above in section III.A, these factors would be based on average parameter values across an ecoregion. Using this new set of F factors, observations of NOy and SOx derived from the field pilot program, averaged across each ecoregion, would be used to calculate AAI values in the sample ecoregions. The data from the field pilot program would also be used to examine alternative approaches to generating representative air quality values, such as examining the appropriateness of spatial averaging in areas of high spatial variability.
      Beyond this basic overview of the field pilot program, the following sections highlight complementary measurements that may be performed as part of the program (section IV.A.1), complementary areas of related research (section IV.A.2), a discussion of implementation challenges that would be addressed during the course of the field pilot program (section IV.A.3), and plans for program development and stakeholder participation (section IV.A.4).
1.	Complementary Measurements
      Complementary measurements may be performed at some sites in the pilot network to reduce uncertainties in the recommended methods for measuring ambient oxides of nitrogen and sulfur and to better characterize model performance and application to the AAI. The CASAC AMMS advised EPA that such supplemental measurements were of critical importance in a field measurement program related to an AAI-based standard (Russell and Samet, 2011b).
      Candidate complementary measurements to address sulfur, in addition to those provided by CFPs, include trace gas continuous SO2 and speciated PM2.5 measurements. The co-located deployment of a continuous SO2 analyzer with the CFP for SO2 will provide test data for determining suitability of continuous SO2 measurements as a Federal Equivalent Method (FEM) for an AAI-based standard, as well as producing valuable time-series data for model evaluation purposes. The weekly averaging time provided by the CFP adequately addresses the annual-average basis of an AAI-based secondary standard, but would not be applicable to short-term (i.e., 1-hour) averages associated with the primary SO2 standard. Conversely, because of the relatively low SO2 concentrations associated with many acid-sensitive ecoregions, existing SO2 FRMs designated for use in determining compliance with the primary standard, which typically are used in higher concentration environments, would not necessarily be appropriate for use in conjunction with an AAI-based secondary standard.
      Co-locating the PM2.5 sampler used in the EPA Chemical Speciation Network and the Interagency Monitoring of Protected Visual Environments (IMPROVE) network at pilot network sites would allow for characterizing the relationship between the CFP-derived p-SO4 and the speciation samplers used throughout the state and local air quality networks. EPA notes that CASTNET already has several co-located IMPROVE chemical speciation samplers. Because the AAI equation is based in part on the concentration of p-SO4, the original motivation for capturing all particle size fractions is not as important relative to simply capturing the concentration of total p-SO4.
      Candidate measurements to complement oxidized nitrogen measurements, in addition to the CFP, include a mix of continuous and periodic sampling for the dominant NOy species, namely NO, directly measured NO2, PAN, HNO3, and particulate nitrate, p-NO3. The CASAC AMMS (Russell and Samet, 2011b) recommended that EPA consider the use of total nitrate (t-NO3) obtained from CASTNET sampling as an indicator for NOy, reasoning that t-NO3 is typically a significant fraction of deposited oxidized nitrogen in rural environments and CASTNET measurements are widely available. Collection of these data would support further consideration of using the CFP for t-NO3 as the indicator of oxides of nitrogen for use in an AAI-based secondary standard.
      The CASAC AMMS also recommended that total NHx (NH3 and particulate ammonium (p-NH4)) be considered as a proxy for reduced nitrogen species, reasoning that the subsequent partitioning to NH3 and p-NH4 may be estimated using equilibrium chemistry calculations. Reduced nitrogen measurements are used to evaluate air quality modeling that is used in generating factor F2. Additional studies are needed to determine the applicability of NHx measurements and calculated values of NH3 and ammonium (NH4) to the AAI.
      The additional supplemental measurements of speciated NOy, continuous SO2 and NHx will be used in future air quality modeling evaluation efforts. Because there often is significant lag in the availability of contemporary emissions data to drive air quality modeling, the complete use of these data sets will extend beyond the 5-year collection period of the pilot program. Consequently, the immediate application of those data will address instrument performance comparisons that explore the feasibility of using continuous SO2 instruments in rural environments, and using the speciated NOy data to assess NOy instrument performance. Although contemporary air quality modeling will lag behind measurement data availability, the observations can be used in deposition models to compare observed transference ratios with the previously calculated transference ratios to test temporal stability of the ratios.
      An extended water quality sampling effort that would parallel the air quality measurement program would help to address some of the uncertainties related to factor F1 and the representativeness of the n[th] percentile critical load, as discussed in section III.B.5.b.i of the proposal. The objective of the water quality sampling would be to develop a larger data base of critical loads in each of the pilot ecoregions such that the n[th] percentile can adequately be characterized in terms of representing all water bodies. Opportunities to leverage and perhaps enhance existing ecosystem modeling efforts enabling more advanced critical load modeling and improved methods to estimate base cation production could be pursued. For example, areas with ongoing research studies producing data for dynamic critical load modeling could be considered when selecting the pilot ecoregions.
2.	Complementary Areas of Research
      The EPA recognizes that a source of uncertainty in an AAI-based secondary standard that would not be directly addressed in the pilot program stems from the uncertainty in the model used to link atmospheric concentrations to dry deposition fluxes. Currently, there are no ongoing direct dry deposition measurement studies at CASTNET sites that can be used to evaluate modeled results. It was strongly recommended by CASAC AMMS that a comprehensive sampling-intensive study be conducted in at least one, preferably two sites in different ecoregions to assess characterization of dry deposition of sulfur and nitrogen. These sites would be the same as those for the complementary measurements described above, but they would afford an opportunity to also complement dry deposition process research that benefits from the ambient air measurements collected in the pilot program. The concerns regarding uncertainties underlying an AAI-based secondary standard suggest that research that includes dry deposition measurements and evaluation of dry deposition models would be a high priority.
      Similar leveraging could be pursued with respect to ecosystem research activities. For example, studies that capture a suite of soil, vegetation, hydrological, and water quality properties that can help evaluate more advanced critical load models would complement the atmospheric-based pilot program. In concept, such studies could provide the infrastructure for true multi-pollutant, multimedia "super" sites assuming the planning, coordination, and resource facets can be aligned. While this discussion emphasizes the opportunity of leveraging ongoing research efforts, consideration could be given to explicitly including related research components directly in the pilot program.
3.	Implementation Challenges
	The CAA requires that once a NAAQS is established, designation and implementation must move forward. With a standard as innovative as the AAI-based standard considered in this review, the Administrator believes that should such a standard be adopted in the future, its success would be greatly improved if, while additional data are being collected to reduce the uncertainties discussed above, the implementing agencies and other stakeholders have an opportunity to discuss and thoroughly understand how such a standard would work. And since, as noted above, emissions reductions that are directionally correct to reduce aquatic acidification will be occurring as a result of other CAA programs, the Administrator believes that this period of further discussion will enable agencies to implement a multi-pollutant standard to address aquatic acidification if one is adopted in a future review.
      Consideration of an AAI-based secondary standard for oxides of nitrogen and sulfur would present significant implementation challenges because it involves multiple, regionally-dispersed pollutants and relatively complex compliance determinations based on regionally variable levels of NOy and SOx concentrations that would be necessary to achieve a national ANC target. The anticipated implementation challenges fall into three main categories:  monitoring and compliance determinations for area designations, pre-construction permit application analyses of individual source impacts, and State Implementation Plan (SIP) development. Several overarching implementation questions that we anticipate will be addressed in parallel with the field pilot program's five-year data collection period include:
(1) What are the appropriate monitoring network density and siting requirements to support a compliance system based on ecoregions?
(2) Given the unique spatial nature of the secondary standard (e.g., ecoregions), what are the appropriate parameters for establishing nonattainment areas?
(3) How can new or modified major sources of oxides of nitrogen and oxides of sulfur emissions assess their ambient impacts on the standard and demonstrate that they are not causing or contributing to a violation of the NAAQS for preconstruction permitting? To what extent does the fact that a single source may be impacting multiple areas, with different acid sensitivities and variable levels of NOy and SOx concentrations that would be necessary to achieve a national ANC target, complicate this assessment and how can these additional complexities best be addressed?
(4) What additional tools, information, and planning structures are needed to assist states with SIP development, including the assessment of interstate pollutant transport and deposition?
(5) Would transportation conformity apply in nonattainment and maintenance areas for this secondary standard, and, if it does, would satisfying requirements that apply for related primary standards (e.g., ozone, PM2.5, and NO2) be demonstrated to satisfy requirements for this secondary standard?
4.	Monitoring Plan Development and Stakeholder Participation.
      The existing CASTNET sampling site infrastructure provides an effective means of quickly and efficiently deploying a monitoring program to support potential implementation of an AAI-based secondary standard, and also provides an additional opportunity for federally managed networks to collaborate and support the states, local agencies and tribes (SLT) in determining compliance with a secondary standard. A collaborative effort would help to optimize limited federal and SLT monitoring funds and would be beneficial to all involved. The CASTNET is already a stakeholder-based program with over 20 participants and contributors, including federal, state and tribal partners.
      The CASAC AMMS generally endorsed the technical approaches used in CASTNET, but concerns were raised by individual representatives of state agencies concerning the perception of EPA-controlled management aspects of CASTNET and data ownership. Potential approaches to resolve these issues will be developed and evaluated in existing National Association of Clean Air Agencies (NACAA)/EPA ambient air monitoring and National Atmospheric Deposition Program (NADP) science committees. The EPA Office of Air and Radiation (which includes the Office of Air Quality Planning Standards, OAQPS; and the Office of Atmospheric Program's Clean Air Markets Division, OAP-CAMD), and their partners on the NACAA monitor steering committee will work to develop a prioritized plan that identifies three to five ecoregions and specific instrumentation to be deployed. Although this pilot program is focused on data collection, the plan will also include data analysis approaches as well as a process to facilitate engagement by those within EPA and the SLTs to foster progress on the implementation questions noted above.
B.	Summary of Proposed Evaluation of Monitoring Methods
      This section provides a brief overview of EPA's plans for evaluating monitoring methods of NOy and SOx, as discussed in section IV.B of the proposal. The EPA generally relies on monitoring methods that have been designated as FRMs or FEMs for the purpose of determining the attainment status of areas with regard to existing NAAQS. Such FRMs or FEMs are generally required to measure the air quality indicators that are compared to the level of a standard to assess compliance with a NAAQS. Prior to their designation by EPA as FRM/FEMs through a rulemaking process, these methods must be determined to be applicable for routine field use and need to have been experimentally validated by meeting or exceeding specific accuracy, reproducibility, and reliability criteria established by EPA for this purpose. As discussed above in section III.A, the ambient air indicators being considered for use in an AAI-based standard include SO2, p-SO4, and NOy.
      The CASTNET provides a well-established infrastructure that would meet the basic location and measurement requirements of an AAI-based secondary standard given the rural placement of sites in acid sensitive areas. In addition, CFPs currently provide very economical weekly, integrated average concentration measurements of SO2, p-SO4, NH4 and t-NO3, the sum of HNO3 and p-NO - 3.
      While routinely operated instruments that measure SO2, p-SO4, NOy and/or t-NO3 exist, instruments that measure p-SO4, NOy, t-NO3, or the CFP for SO2 have not been designated by EPA as FRMs or FEMs. The EPA's Office of Research and Development has initiated work that will support future FRM designations by EPA for SO2 and p-SO4 measurements based on the CFP. Such a designation by EPA could be done for the purpose of facilitating consistent research related to an AAI-based standard and/or in conjunction with setting and supporting an AAI-based secondary standard.
      Based on extensive review of literature and available data, the EPA has identified potential methods that appear suitable for measuring each of the three components of the indicators. As discussed more fully in section IV.B of the proposal, these methods are being considered as new FRMs to be used for measuring the ambient concentrations of the three components (SO2, p-SO4 and NOy) that would be needed to determine compliance with an AAI-based secondary standard.
      For the SO2 and p-SO4 measurements, EPA is considering the CFP method, which provides weekly average concentration measurements for SO2 and p-SO4. This method has been used in the EPA's CASTNET monitoring network for 15 years, and experience with this method strongly indicates that it will meet the requirements for use as an FRM for the SO2 and p-SO4 concentrations for an AAI-based secondary standard.
      Although the CFP method would provide measurements of both the SO2 and p-SO4 components in a unified sampling and analysis procedure, individual FRMs will be considered for each. The EPA recognizes that an existing FRM to measure SO2 concentrations using ultra-violet fluorescence (UVF) exists (40 CFR Part 50, Appendix A-1) for the purpose of monitoring compliance for the primary SO2 NAAQS. However, several factors suggest that the CFP method would be superior to the UVF FRM for monitoring compliance with an AAI-based secondary standard.
	For monitoring the NOy component, a continuous analyzer for measuring NOy is commercially available and is considered by EPA to be likely suitable for use as an FRM. This method is similar in design to the existing NO2 FRM (described in 40 CFR Part 50, Appendix F), which is based on the ozone chemiluminescence measurement technique. The method is adapted to and further optimized to measure all NOy. However, this NOy method requires further evaluation before it can be fully confirmed as a suitable FRM. The EPA is currently completing a full scientific assessment of the NOy method to determine whether it would be appropriate to consider for designation by EPA as an FRM.
      On February 16, 2011, EPA presented this set of potential FRMs to the CASAC AMMS for their consideration and comment. In response, the CASAC AMMS stated that, overall, it believes that EPA's planned evaluation of methods for measuring NOy, SO2 and p-SO4 as ambient air indicators is a suitable approach in concept. On supporting the CFP method as a potential FRM for SO2, CASAC stated that they felt that the CFP is adequate for measuring long-term average SO2 gas concentrations in rural areas with low levels (less than 5 parts per billion by volume (ppbv)) and is therefore suitable for consideration as an FRM. For p-SO4, CASAC AMMS generally supports the use of the CFP as a potential FRM for measuring p-SO4 for an AAI-based secondary standard. The method has been relatively well-characterized and evaluated, and it has a documented, long-term track record of successful use in a field network designed to assess spatial patterns and long-term trends. On supporting the photometric NOy method as a potential FRM, CASAC AMMS concluded that the existing NOy method is generally an appropriate approach for the indicator of an AAI-based standard. However, CASAC AMMS agreed that additional characterization and research is needed to fully understand the method in order to designate it as a FRM.
C.	Comments on Field Pilot Program and Monitoring Methods Evaluation
Public comments on EPA's proposed plans for a field pilot program and related evaluation of monitoring methods generally fell into the following four topic areas:  (1) goals, objectives, and scope; (2) monitoring network and site selection; (3) complementary measurements and instrumentation; and (4) collaboration and stakeholder participation. An overview of these comments and EPA's responses are discussed below. In addition, many commenters generally requested that EPA provide clarification of its plans regarding the field pilot program.
1.	Goals, Objectives, and Scope of Field Pilot Program
	There was a mix of comments regarding the need for and the overall purpose and scope of the field pilot program. In general commenters that supported the AAI approach ((e.g., DOI/National Park Service (NPS), Nature Conservancy, Adirondack Council, NESCAUM, NY, PA, NC) also supported the concept of deploying a field pilot program as well the proposed goals and objectives, while offering specific comments on the scope of the proposed monitoring effort. Other commenters supporting the AAI approach, including Earthjustice and the Center for Biological Diversity, expressed the view that a field pilot program was not needed to support adoption of such a standard in this review. A variety of commenters expressed the view that a field pilot program in 3 to 5 ecoregions was too limited to adequately capture differences in concentrations and deposition patterns across the nation.
	Commenters that did not support the adoption or future development of an AAI-based secondary NAAQS (e.g., EPRI, UARG, AAM, NCBA, Aluminum Association, and TX) expressed the view that a field pilot program was therefore not needed. However, these commenters nonetheless expressed the view that if EPA intended to consider such a standard in future reviews, the field pilot program would need to expand in coverage and incorporate a much more comprehensive research program to address data gaps and uncertainties inherent in such an approach. These commenters suggested that the field pilot program should be more responsive to the issues raised by the members of the CASAC review panel. One commenter (API) expressed the view that even if EPA intended to consider such a standard in the future, a field study was not appropriate at this time on the basis that the AAI-based approach was still only very preliminary in nature.
	These commenters not supporting the AAI and the field pilot program as proposed contended that the proposed program fails to address key scientific uncertainties and data needs with regard to a methodology based on the AAI, and cannot meaningfully reduce the uncertainties that would be associated with such a standard. Some of these commenters offered specific recommendations for areas of research, noted below, that in their view would be necessary to support any further consideration of such a standard. For example, these commenters contended that it was necessary to conduct research in the following areas before further consideration of an AAI-based standard:  (1) the effect of other sources, including wastewater pollution from permitted or unpermitted sources and fertilization of farm lands, on aquatic acidification; (2) relationships between measured air quality and deposition rates and related model performance evaluations; (3) improved methods for measuring dry deposition; and (4) characterization of NHx concentrations that are representative of specific ecoregions for all ecoregions based on a model performance evaluation.
	Additional views were expressed by various commenters in regard to implementation, site selection and data availability. Many commenters from State agencies and industry agreed with EPA that implementation challenges should be addressed during the course of the field pilot program. For example, commenters expressed the view that guidance should emerge for monitoring network design accounting for the influence of variability of air concentration and deposition patterns within specific ecoregions. Some commenters also noted that much of the underlying information for the AAI was based on the Adirondacks and Shenandoah regions which are relatively rich data sources and the field pilot program should consider under-sampled areas in other parts of country such as the mountainous West. Also, some commenters requested that relatively non-acid sensitive areas be included in the field pilot program in the interest of broader national applicability or, as one state agency suggested, the availability of a rich data base in the Chesapeake Bay region. Some commenters also expressed the view that results from the field pilot program would not be available for the next periodic review of the secondary standards for oxides of nitrogen and sulfur.
	Having considered these comments contending that the scope of the field pilot program is too limited spatially and not sufficiently comprehensive, EPA maintains that the purpose and scope of the pilot studies program as presented in the proposal remain appropriate. As summarized above in section IV.A, the primary goal of the field pilot program is to collect and analyze data so as to enhance the Agency's understanding of the degree of protectiveness that would likely be afforded by an AAI-based standard. EPA also intends that data generated by this program would support development of an appropriate monitoring network for such a standard. This field pilot program is not intended to be a research program, but rather to be a more targeted data collection and analysis effort, which will be done in conjunction with ongoing research efforts that are better suited to address some of the issues raised by commenters on the breadth of the field pilot program.
	EPA largely agrees that the scope of the field pilot program is not adequate to address many of the issues raised by the commenters regarding either the ability to adequately capture air quality and deposition patterns in all ecoregions or fully addressing scientific uncertainties related to numerous investigations into measurement development methods and biogeochemical and atmospheric deposition processes. However, as noted earlier, a field pilot program by definition is limited in scope and intended to guide future broader applications. Toward that end, the field pilot program is intended to provide an intermediate link between initial conceptual design and potential future development and adoption of a standard, where the breadth and depth of spatial coverage would explicitly be addressed through monitoring network rules and implementation guidance.
	The relevant ongoing programs addressing underlying atmospheric deposition uncertainties and development of critical load models include EPA's atmospheric deposition research program and the multi-agency National Critical Load Data Base (NCLDB) program, respectively. In addition, the NAAQS review process of iterative science review and assessment provides a framework for evaluating newly available information that may address current data gaps and scientific uncertainties. These research programs are appropriate venues for addressing comments, including relevant CASAC recommendations, regarding desired improvements in the science underlying an AAI-based standard. In light of these ongoing research programs, it is not appropriate to duplicate these efforts through an expanded scope of the field pilot program. Rather, the most efficient approach is to increase the coordination between the field pilot program and these existing efforts. For example, EPA plans to explore co-locating planned dry deposition studies at field pilot program sites that would result in mutually beneficial data enhancements that support both pilot program and research program objectives.
	With regard to views regarding the importance of water quality monitoring, the EPA agrees with comments recommending increased coordination with water quality sampling and critical load modeling programs. In addition to working closely with the NCLDB, the EPA plans to factor in availability of water quality monitoring data in selecting field pilot program sites. The field pilot program has the potential to spur increased water quality monitoring in under-sampled areas which would improve confidence in generating ecoregion representative critical loads, as well as enhancing longer-term assessment of progress.
	In addressing the last group of comments concerning implementation, site selection and data availability, the EPA offers the following views. The field pilot program does provide an opportunity to assist in answering a number of implementation challenges, including the design of a future network that could support an AAI-based secondary standard. Toward that end, EPA plans to work closely with its state and local agency partners in utilizing the field pilot program as a test case for implementation-based issues. In optimizing the design of a field pilot program, emphasis will be placed on relatively acid-sensitive areas given that those are areas an AAI-based standard would be intended to protect. Nevertheless, EPA will consider ecoregions that may offer advantages in having multiple deposition-based effects beyond aquatic acidification that potentially could support future reviews that consider multiple ecological effects. In addition, nearly all ecoregions have a mix of acid-sensitive and non-acid sensitive water bodies which will allow for assessing some of the AAI applicability to different aquatic systems. EPA also notes that the field pilot program will provide data and analyses that will help inform consideration of an AAI-based standard in the next review. For example, data and analyses generated as part of the field pilot program will be incorporated into EPA's characterization of environmental factors and evaluations of alternative approaches to specifying the terms of an AAI that would be included in the exposure/risk assessment and policy assessment prepared as part of the next review.
2.	Network Design and Role of CASTNET
	Most commenters expressed the view that CASTNET was an appropriate program to support the field pilot program and a potential AAI-based standard. While government agencies generally supported the use of CASTNET, some State organizations suggested that the NCore monitoring network may be more efficient given that the costs of adding CASTNET filter packs (CFPs) to NCore locations is less than that of adding NOy instruments, which exist at NCore locations, to CASTNET locations. Support also was expressed by New York State and NESCAUM for the use of rural NCore monitoring stations, where appropriate, in combination with CASTNET sites. Some states requested that access to the sampling methods and laboratory analyses used in the program and all data results be made through a national contract for States and local agencies, a concern related to CASTNET operations being managed by EPA. Environmental groups also supported the use of CASTNET and encouraged EPA to adopt the multiple stakeholder process of the NCLDB program and to align CASTNET sites with the Temporally Integrated Monitoring of Ecosystems and Long-Term Monitoring (TIME/LTM) water sampling programs. These water sampling programs should also be extended to other under-sampled areas of the country that are acid sensitive. Some industry commenters raised concerns regarding the CFPs as they have measurement artifacts associated with both mass loss and gain.
	Some state agencies commented that states should not be required to fund or implement the pilot monitoring studies, and funding should arise from sources other than State and Territorial Air Grant (STAG) funds. Relatedly, the NPS and environmental groups encouraged EPA to make this effort a priority for funding.
 	The EPA has considered all available monitoring networks in the interest of locating the most suitable sites for a pilot study and to effectively leverage resources. The CASTNET monitoring program offers substantially more available platforms in acid-sensitive ecoregions relative to rural NCore sites and CASTNET sites already include the CFP method for measurements of key atmospheric species. Consequently, the financial burden on states, tribes and local air monitoring agencies would be less using this existing infrastructure instead of expanding measurements at or relocating rural NCore sites. The CASTNET siting design originally was intended to discern contributions of acidifying deposition of NOx and SOx to sensitive ecosystems, which is especially relevant for the AAI applications. NCore was designed as a more generalized network to collect measurements in a variety of geographical areas, with no specific focus on acid-sensitive ecosystems. Moreover, CASTNET has established a track record over the last two decades of providing quality measurements, whereas NCore is a relatively new network that has been fully deployed for less than two years and therefore not been subjected to review and analysis commensurate with the CASTNET program. Nevertheless, as some states suggested, this pilot program should afford an opportunity to explore the use of existing rural NCore sites in acid-sensitive ecoregions. EPA welcomes the inclusion of rural NCore sites into the pilot study in cases where there are clear advantages of using such sites, and especially where such sites provide additional information likely resulting in more conclusive data findings. The development of site selection criteria and site selection will be conducted in partnership with other federal, state and local agencies. Although CASTNET is managed by EPA, the agency has aggressively supported the user community management approach adopted in the NADP and views the field pilot program as an opportunity to expand ownership of CASTNET analysis and data products, which currently can be accessed by the public.
	While the field pilot program resources are focused on atmospheric measurements, as noted above EPA will try to leverage existing water quality monitoring programs such as TIME/LTM in selecting field pilot program site locations. EPA would rely heavily on the NCLDB critical load work for generating AAI values at monitoring locations as part of the field pilot program. In regard to issues raised by commenters regarding artifacts in the CFP, which would be the basis for SOx data in the field pilot program, EPA notes that these methods have been extensively deployed and evaluated and have exhibited generally excellent performance. As part of the CASAC review on measurement methods, CASAC pointed out that the CFPs are preferred methods for measuring SOx in rural, low concentration environments due to the sensitivity of the CFP method.
3.	Complementary Measurements and Instrumentation
	In general, commenters across government agencies, environmental groups and industry supported the use of complementary measurements that would be deployed in addition to the CFP and NOy instruments used to measure the indicators, NOy and SOx. Comments regarding these measurements were provided in different contexts. For example, industry views reflected a position that complementary measurements were necessary to address information gaps, whereas state agencies and environmental groups expressed more general support in the interest of adding additional useful data, but not as a required component of the field pilot program.
	  Commenters expressed support for including trace gas continuous SO2 and speciated PM2.5 measurements in the field pilot program to provide test data for determining the suitability of continuous SO2 measurements as a FEM for secondary standards and to characterize the relationship between CFP-based particulate sulfate and the national network of speciation samplers used throughout the state and local air quality networks. Industry commenters suggested that dry deposition flux measurements be conducted at the field pilot program sites, while also indicating that having sites in only 3 to 5 ecoregions would be inadequate. Industry commenters also suggested deploying multiple co-located methods measuring the same species as a quality assurance step and advocated measuring individual NOy species. Several commenters suggested adding NADP wet deposition samplers.
	Several commenters supported the development of a FRM for NOy and CFP-based SO2 and sulfate measurements. Greater attention was addressed to NOy measurements as the technology has only recently been used in routine monitoring applications. Some commenters supported EPA's approach of using EPA's research office to conduct instrument evaluation as a related but separate program from the field pilot program. Some commenters also recommended testing NOy at locations with extreme temperature and relative humidity regimes.
	The EPA appreciates the support expressed by commenters regarding the use of complementary measurements. While EPA agrees with views expressing the importance of additional measurements, complementary measurements will not have the same funding priority as indictor measurements for NOy and SOx. Nevertheless, it is reasonable to expect that all field pilot program sites will also include NADP precipitation samplers and NADP passive ammonia samplers, both of which are located in roughly half of all CASTNET sites. EPA agrees that the formal NOy FRM development should be decoupled from the pilot studies, while recognizing that separate NOy measurements are an important component of the pilot study. Although NOy measurement technology is relatively mature, the effort to develop FRM certification will promote more confidence in the data due to standardized operational and quality assurance protocols.
4.	Collaboration
	Most commenters agreed with EPA's intention to broaden review and participation in the field pilot program, given that the AAI approach cuts across multiple organizations and technical disciplines. Both industry and state governments suggested that some level of initial and ongoing external peer review is needed for evaluating design of the field pilot program and subsequent data analyses, with one state suggesting using NACAA's Monitoring Steering Committee. Some state commenters also reasoned that an agency's participation in the pilot program should be optional, because some states cannot support additional monitoring even if it were to be fully funded. The NPS in particular indicated a desire to participate with EPA in the field pilot program. Clearly, many of the comments described above suggesting added emphasis on water quality monitoring and research collectively emphasize strengthening the collaborative aspects of this field pilot program.
	EPA is encouraged by commenters' interest in the field pilot program. While EPA's Office of Air and Radiation (OAR) will assume primary leadership of this program, EPA OAR will take several actions to promote collaboration across internal EPA research programs and other government agencies. Paralleling this effort, EPA will solicit comment on a draft white paper to enable ongoing review and input from the public.
	These pilot studies afford an excellent opportunity to coordinate air quality monitoring and related critical load and water quality assessment activities (modeling and measurements). As part of the planning effort for these pilot studies, EPA will engage other federal agencies (U.S. Geological Survey, NPS, U.S. Forest Service) and state and local agencies primarily through existing NADP and NACAA committee structures.
V.	Statutory and Executive Order Reviews
A.	Executive Order 12866: Regulatory Planning and Review and Executive Order 13563:  Improving Regulation and Regulatory Review
	 Under Executive Order 12866 (58 FR 51735, October 4, 1993), this action is a "significant regulatory action."   Accordingly, the EPA submitted this action to the Office of Management and Budget (OMB) for review under Executive Orders 12866 and 13563 (76 FR 3821, January 21, 2011), and any changes made in response to Office of Management and Budget (OMB) recommendations have been documented in the docket for this action.
B.	Paperwork Reduction Act
	This action does not impose an information collection burden under the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq.  Burden is defined at 5 CFR 1320.3(b). There are no information collection requirements directly associated with the establishment of a NAAQS under Section 109 of the CAA and this rulemaking will retain current standards and will not establish any new standards.
C.	Regulatory Flexibility Act
	For purposes of assessing the impacts of today's rule on small entities, small entity is defined as:  (1) a small business that is a small industrial entity as defined by the Small Business Administration's (SBA) regulations at 13 CFR 121.201;  (2) a small governmental jurisdiction that is a government of a city, county, town, school district or special district with a population of less than 50,000; and (3) a small organization that is any not-for-profit enterprise which is independently owned and operated and is not dominant in its field.
	After considering the economic impacts of today's final rule on small entities, I certify that this action will not have a significant economic impact on a substantial number of small entities.  This final rule will not impose any requirements on small entities.  Rather, this rule will retain the current secondary standards and does not establish any new national standards.  See also American Trucking Associations v. EPA. 175 F. 3d at 1044-45 (NAAQS do not have significant impacts upon small entities because NAAQS themselves impose no regulations upon small entities).
D.	Unfunded Mandates Reform Act
 	Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public Law 104-4, establishes requirements for Federal agencies to assess the effects of their regulatory actions on State, local, and Tribal governments and the private sector. Under Section 202 of the UMRA, the EPA generally must prepare a written statement, including a cost-benefit analysis, for proposed and final rules with "Federal mandates" that may result in expenditures to state, local, and tribal governments, in the aggregate, or to the private sector, of $100 million or more in any 1 year.  Before promulgating an EPA rule for which a written statement is needed, Section 205 of the UMRA generally requires the EPA to identify and consider a reasonable number of regulatory alternatives and to adopt the least costly, most cost-effective or least burdensome alternative that achieves the objectives of the rule. The provisions of Section 205 do not apply when they are inconsistent with applicable law.  Moreover, Section 205 allows the EPA to adopt an alternative other than the least costly, most cost-effective or least burdensome alternative if the Administrator publishes with the final rule an explanation why that alternative was not adopted. Before the EPA establishes any regulatory requirements that may significantly or uniquely affect small governments, including tribal governments, it must have developed under Section 203 of the UMRA a small government agency plan. The plan must provide for notifying potentially affected small governments, enabling officials of affected small governments to have meaningful and timely input in the development of the EPA regulatory proposals with significant Federal intergovernmental mandates, and informing, educating, and advising small governments on compliance with the regulatory requirements.
	This action contains no Federal mandates under the provisions of Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), 2 U.S.C. 1531-1538 for state, local, or tribal governments or the private sector.  Therefore, this action is not subject to the requirements of Sections 202 or 205.  Furthermore, as indicated previously, in setting a NAAQS the EPA cannot consider the economic or technological feasibility of attaining ambient air quality standards; although such factors may be considered to a degree in the development of state plans to implement the standards.  See also American Trucking Associations v. EPA, 175 F. 3d at 1043 (noting that because the EPA is precluded from considering costs of implementation in establishing NAAQS, preparation of a Regulatory Impact Analysis pursuant to the Unfunded Mandates Reform Act would not furnish any information which the court could consider in reviewing the NAAQS).  Accordingly, the EPA has determined that the provisions of Sections 202, 203, and 205 of the UMRA do not apply to this final decision not to establish new standards.
E.	Executive Order 13132: Federalism
	This final rule does not have federalism implications.  It will not have substantial direct effects on the states, on the relationship between the national government and the states, or on the distribution of power and responsibilities among the various levels of government, as specified in Executive Order 13132 because it does not contain legally binding requirements. Thus, the requirements of Executive Order 13132 do not apply to this rule.
F.	Executive Order 13175: Consultation and Coordination with Indian Tribal Governments
	Executive Order 13175, entitled ``Consultation and Coordination with Indian Tribal Governments'' (65 FR 67249, November 9, 2000), requires EPA to develop an accountable process to ensure ``meaningful and timely input by tribal officials in the development of regulatory policies that have tribal implications.'' This rule concerns the establishment of national standards to address the public welfare effects of oxides of nitrogen and sulfur.
	This action does not have tribal implications, as specified in Executive Order 13175 (65 FR 67249, November 9, 2000) as tribes are not obligated to adopt or implement any NAAQS. We recognize, however, that this rule does concern resources of special interest to the tribes. Accordingly, on August 3, 2011, EPA sent letters to all tribal leaders offering to consult with the tribes on the proposed rule.  On October 6, 2011 EPA held a consultation call with the Forest County Potawatomi Community; five other tribes participated in the call for informational purposes.  EPA also received public comment from two tribes on this rule. EPA has responded to the tribal comments in its Response to Comments Document.
G.	Executive Order 13045: Protection of Children from Environmental Health & Safety Risks
This action is not subject to EO 13045 because it is not an economically significant rule as defined in EO 12866.
H.	Executive Order 13211: Actions that Significantly Affect Energy Supply, Distribution or Use
	This action is not a "significant energy action" as defined in Executive Order 13211 (66 FR 28355, May 22, 2001), because it will not have a significant adverse effect on the supply, distribution, or use of energy.  This action does not establish new national standards to address the public welfare effects of oxides of nitrogen and sulfur.
I.	National Technology Transfer and Advancement Act
	Section 12(d) of the National Technology Transfer and Advancement Act of 1995 (NTTAA), Public Law No. 104-113, 12(d) (15 U.S.C. 272 note) directs the EPA to use voluntary consensus standards in its regulatory activities unless to do so would be inconsistent with applicable law or otherwise impractical.  Voluntary consensus standards are technical standards (e.g., materials specifications, test methods, sampling procedures, and business practices) that are developed or adopted by voluntary consensus standards bodies.  The NTTAA directs the EPA to provide Congress, through OMB, explanations when the Agency decides not to use available and applicable voluntary consensus standards.
	The EPA is not aware of any voluntary consensus standards that are relevant to the provisions of this final rule.
J.	Executive Order 12898: Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations
	Executive Order 12898 (59 FR 7629 (Feb. 16, 1994)) establishes federal executive policy on environmental justice.  Its main provision directs federal agencies, to the greatest extent practicable and permitted by law, to make environmental justice part of their mission by identifying and addressing, as appropriate, disproportionately high and adverse human health or environmental effects of their programs, policies, and activities on minority populations, low-income populations, or indigenous populations in the United States.
	The EPA has determined that this final rule will not have disproportionately high and adverse human health or environmental effects on minority, low-income populations, or indigenous populations because it retains the level of environmental protection for all affected populations without having any disproportionately high and adverse human health or environmental effects on any population, including any minority, low-income population, or indigenous population.
References
Banzhaf, S., D. Burtraw, D. Evans, and A. Krupnick. 2006. "Valuation of Natural Resource Improvements in the Adirondacks." Land Economics 82:445-464.
NAPAP. 1990. Acid Deposition: State of Science and Technology. National Acid Precipitation Assessment Program. Office of the Director, Washington, DC.
NAPAP. 2005. National acid precipitation assessment program report to Congress: An integrated assessment.  http://www.esrl.noaa.gov/csd/aqrs/reports/napapreport05.pdf. Silver Spring, MD: National Acid Precipitation Assessment Program (NAPAP); Committee on Environment and Natural Resources (CENR) of the National Science and Technology Council (NSTC).
NRC (National Research Council). 2004. Air quality management in the United States. Washington, DC: National Research Council (NRC); The National Academies Press.
Russell, A and J. M. Samet, 2010a. Review of the Policy Assessment for the Review of the Secondary National Ambient Air Quality Standard for NOx and SOx: First Draft. EPA-CASAC-10-014.
Russell, A and J. M. Samet, 2010b. Review of the Policy Assessment for the Review of the Secondary National Ambient Air Quality Standard for NOx and SOx: Second Draft. EPA-CASAC-11-003.