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 Federal Equivalent Method (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.

	There were several comments from industry and government organizations supporting the development of a Federal Reference Method (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.

Response:  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.

D.	Collaboration and Stakeholder Participation

Comments:  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.

Response:  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 as described in section IV.D of the final rule. Paralleling this effort, EPA will solicit comment on a draft white paper to enable ongoing review and input from the public. 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. Key topics areas that will be addressed in the draft white paper are described briefly below.

(1) Background, purpose and products. An introductory section will include a statement of program objectives. Since the field pilot program emphasizes atmospheric measurements in 3 to 5 ecoregions, the role and leveraging opportunities of the field pilot program in relation to ongoing water quality modeling, critical load work and atmospheric deposition research will be introduced here and expanded upon in later sections. A brief description of intended data and analysis products from this effort  -  highlighting the types of AAI calculations and variability analyses of NOy and SOx concentrations and deposition rates -- will be introduced here and expanded upon in more depth in the data analysis section. The introductory section will also describe the process for broader public engagement and review. The EPA intends to work closely with two existing venues  -  the NADP critical load and total deposition science committees and the NACAA monitoring subcommittee. Background technical information will include an overview of the AAI equation.

(2) Site selection. Factors to be considered in selecting ecoregions for inclusion in the field pilot program will be discussed and initial thoughts on recommended locations will be presented. Site location considerations include available infrastructure resources (e.g., air monitoring site platforms, nearby water quality monitoring, forest/ecosystem experimental studies), location in acid-sensitive ecoregions, geographic/ecosystem diversity (e.g., east and west locations) and partnership opportunities. To assist in site selection, a series of resource-based maps will be available illustrating infrastructure assets and partnership opportunities. A paring down of the 22 acid-sensitive ecoregions to less than 10 will be accompanied by discussion of the merits of each area, and initial thoughts on groupings of 3, 4 and 5 ecoregions will be provided. This site selection discussion is particularly important as it is initiates broader discussion to enlist interest from potential partners.

(3) Linking atmospheric measurements to the AAI. A suite of atmospheric measurements to be made at the field pilot program sites will be discussed, with a focus on measurements of indicators for oxides of nitrogen and sulfur based on the CFPs and commercial NOy instruments. Complementary measurements such as ammonia gas, precipitation chemistry and other monitoring methods under consideration (e.g., speciated NOy samplers and continuous SO2) will also be discussed, and information on the potential benefits and costs associated with gathering such data will be presented to help inform the program planning effort.

(4) Linking water quality data and critical load calculations to the AAI. The biogeochemical components which support the determination of representative critical loads are an important complement to atmospheric measurements as both biogeochemical and atmospheric processes work together in the calculation of area-specific AAI values. In parallel with the atmospheric measurements, a revised set of critical load estimates will be developed under the umbrella of the NCLDB, which is addressed by the NADP critical loads science committee. In addition to revising critical load estimates, this effort will help inform ongoing critical load model evaluation and development efforts. Topic areas to be considered for inclusion in the program include evaluating the F-factor approach, implications of sulfate retention on steady-state critical load modeling, the role of dynamic models as evaluation tools for generating national critical load data and assessment of Neco characterization approaches. These critical load model evaluation efforts will focus on field pilot program sites contingent on the ability to leverage water quality and soil sampling efforts underway, or new efforts that may be spurred by the pilot program. As part of this effort, attention will be given to characterizing the critical load representativeness at the ecoregion level using existing water quality surveys, which is an important aspect of the AAI equation.

(5) AAI proof-of-concept. Based on newly assembled atmospheric measurements and critical load data, as discussed above, AAI calculations will be performed. As there will be air quality measurements in selected ecoregions for the indicators, SOx and NOy, observation-based AAI values can be calculated for those ecoregions in a manner that would be illustrative of the implementation of a potential future AAI-based standard. Further, based on related national-level efforts to expand the critical load data base and to update CMAQ air quality model simulations, a national set of AAI values will be calculated using approaches similar to those presented in the PA. In addition, the availability of both site-specific measurements and national-scale modeling will allow for assessments that can support monitoring network design.

(6) Coordination with atmospheric deposition research. The field pilot program presents an opportunity to leverage planned EPA dry deposition flux studies and a window of time to more fully explore atmospheric deposition characterization approaches. These research efforts are not part of the field pilot program, but the pilot program provides an opportunity for mutual leveraging between EPA air management and research efforts. The field pilot program offers an extended suite of atmospheric measurements in locations useful for diagnosing model (air quality and deposition focused) behavior. Leveraging refers to locating planned deposition flux studies at field pilot program locations. In addition, the time window afforded by the pilot program allows for continued efforts at improving deposition characterizations in general. Examples of improved characterization approaches include the use of the Parameter-elevation Regressions on Independent Slopes Model (PRISM) gridded precipitation data to improve wet deposition characterization and the merging of modeled and observed data in generating the best available deposition estimates. As part of these research elements, further evaluation of transference ratios can be addressed and the pilot program will include at least one mountainous west ecoregion enabling insights into deposition and transference ratio variability in western regions.

(7) Role of the field pilot program and implementation. As discussed in section IV.A above, the field pilot program provides an opportunity to further explore the issues that would be associated with implementing an AAI-based secondary standard. The field pilot program provides a reasonable time period for EPA and states to become familiar with the AAI approach and develop practical approaches related to implementation issues. For example, such issues are associated with the use of ecoregion boundaries as a planning basis, a focus on rural environments, and the myriad reporting and analysis requirements that are inherent in implementing any such standard. The draft white paper will outline a process for addressing such issues that will involve creating appropriate forums for bringing together EPA and state government experts in technical and air management disciplines.

(8) Data Analysis Products. The atmospheric and biogeochemical measurements to be made as part of the field pilot program, together with updated model simulations that become available during the course of the pilot program, will be used to support analyses that can help inform further development and evaluation of the AAI. Such analyses will be identified and described in terms of how they could advance our understanding and characterization of the degree of protectiveness that would likely be afforded by AAI-based standards that could be considered in the next review of the NAAQS for oxides of nitrogen and sulfur.
	The field pilot program affords 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 this program, 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.

IV. 	RESPONSES TO LEGAL, ADMINISTRATIVE AND PROCEDURAL ISSUES AND MISPLACED COMMENTS
	This section is intended to address those comments received regarding legal, administrative, and procedural issues as well as issues raised in public comments that are not considered when setting a NAAQS.  Many legal comments are presented in previous sections of this document and throughout the rule. Comments included here address issues related to interactions with CASAC, the EPA's authority to define a new indicator of ambient air quality, and responsibility of the EPA under tribal trust agreements. Additionally, a number of comments were submitted related to implementation issues and regulatory impact analyses. As the EPA is not permitted to consider implementation-related issues or the cost associated with meeting a standard when developing NAAQS, these comments are considered misplaced as they are not relevant to determining the appropriate secondary NAAQS for oxides of nitrogen and sulfur. We note that some implementation issues are generally addressed for informational purposes only in section I.C of the preamble to the final rule.
A.	Legal, Administrative and Procedural Issues
(1) Comment:  One commenter (NESCAUM) expressed concern that the proposal was not consistent with CASAC's recommendation to apply an ecologically relevant level and form in this review.
Response:  In considering CASAC's advice and recommendations, the EPA pays close attention to the content of the advice and weighs it carefully, whether it is advice on the science or advice on policy matters related to new or revised NAAQS. The EPA in fact has done this in this review, as seen in the many and varied revisions made to the draft documents CASAC has reviewed and commented on, such as the ISA, the Risk and Exposure Assessment and the Policy Assessment.   It can also be seen in the careful attention paid by the Administrator to CASAC's advice in both the proposed rule and in the final rule. The Administrator has carefully explained where she has accepted CASAC's advice and where she has not, and explained in detail her reasoning.  We acknowledge that in previous comments on the Policy Assessment, CASAC expressed support for setting ecologically relevant standards but also identified areas of concern with regard to uncertainty in modeling and data. The EPA is not, however, required to follow CASAC's recommendations where the Administrator explains the reasons for such differences where she differs in important respects from CASAC's advice (CAA section 307(d) (3), (d) (6)).
(2) Comment:  Some commenters asserted that the consideration of NOy as the indicator for oxides of nitrogen in the AAI is contrary to the EPA's statutory authority. One such commenter asserted that NOy was a broader indicator than had been defined in listing oxides of nitrogen as a criteria pollutant and as such the EPA was prevented from modifying the scope of the criteria pollutant addressed in this review.
Response:  While this issue will be addressed more fully in future reviews, the EPA disagrees that NOy, as considered in this review, is beyond what is permissible in conjunction with the listed criteria pollutant for oxides of nitrogen. In the primary standards, NO2 is used as the indicator of oxides of nitrogen because it is relevant to the exposures of concern to public health. Similarly, in this review NOy is considered as the indicator of oxides of nitrogen in that it is considered to be the most relevant indicator of exposures of concern to aquatic ecosystems with regard to acidifying deposition.

(2) Comment:  Two tribes, the Forest County Potawatomi Community and the Fond du Lac Reservation, commented that failure of the EPA to implement standards and determine adequacy with regard to mercury methylation and deposition of oxides of sulfur represents a violation of tribal trust responsibilities. The commenters explained that methylation of mercury from sulfur deposition continues to be a significant adverse effect on tribal lands and that the EPA has the responsibility to prevent such effects.
Response:  The adequacy of the current standards to address mercury methylation is discussed in section II.B.2. above and in greater detail in the final rule section II.B. With regard to tribal trust, the EPA agrees with the commenters that the definition of "tribal implications" can also include rule impacts to tribal resources, whether occurring on reservations or on lands managed by federal agencies under the federal Trust Responsibility. These resources include tribes' lands, waters, and natural, religious and cultural resources. NAAQS are intended to provide additional protection, as warranted, to adversely impacted systems and as such are expected to also provide additional protection to such resources that fall on tribal lands or lands held in trust by federal agencies. On August 3, 2011, the EPA sent letters to all Tribal leaders offering to consult with the tribes on the proposed rule. On October 6, 2011 the EPA held a call consultation call with the Forest County Potawatomi Community and 5 other tribes participated for informational purposes.
	With respect to EO 13175, the EPA has determined however, that this rule does not have tribal implications. The rule concerns the establishment of a secondary Oxides of Nitrogen and Oxides of Sulfur NAAQS. The Tribal Authority Rule gives Tribes the opportunity to develop and implement CAA programs such as the PM NAAQS, but it leaves to the discretion of the Tribe whether to develop these programs and which programs, or appropriate elements of a program, they will adopt. Thus, the rule does not have substantial direct effects on Tribes, which means it does not have tribal implications and is not subject to EO 13175. Nevertheless, the Agency has, as explained in section x, worked to obtain timely and meaningful input from Indian Tribes and their leaders.
B.	Misplaced Comments
Comment:  Several commenters stated that additional implementation language should have been included in the proposal both with regard how the EPA would implement an AAI-based standard in the future and with regard to the proposed decision to set the secondary NAAQS for oxides of nitrogen and sulfur equal to the primary NAAQS for NO2 and SO2. Some commenters also noted that further regulatory analyses such as regulatory impact analyses and regulatory flexibility analyses should have been conducted.
Response:  Comments concerning implementation of a NAAQS are legally irrelevant in determining which standards are requisite to protect public welfare. (API v. Costle, 665 F. 2d at 1185-86). The Clean Air Act also specifically prohibits the EPA from considering costs associated with implementation or setting of NAAQS. The EPA therefore did not consider these comments in its decision making process on the NAAQS standard.

V.	REFERENCES
Russell, A and J. M. Samet, 2011a. Review of the Policy Assessment for the Review of the Secondary National Ambient Air Quality Standard for NOx and SOx: FINAL. EPA-CASAC-11-005.
U.S. EPA 2008. Integrated Science Assessment (ISA) for Oxides of Nitrogen and Sulfur Ecological Criteria (Final Report). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/082F, 2008.
U.S. EPA 2009. Risk and Exposure Assessment for Review of the Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Oxides of Sulfur-Main Content - Final Report. U.S. Environmental Protection Agency, Washington, D.C., EPA-452/R-09-008a.
U.S. EPA 2011. Policy Assessment for the Review of the Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Oxides of Sulfur. U.S. Environmental Protection Agency, Washington, D.C., EPA-452/R-11-005a.