6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 51

[EPA-HQ-OAR-2013-0711; FRL-9928-18-OAR]

RIN 2060-AR19

Data Requirements Rule for the 2010 1-Hour Sulfur Dioxide (SO2) Primary National Ambient Air Quality Standard (NAAQS)

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.
SUMMARY: The Environmental Protection Agency (EPA) is promulgating a rule directing state and tribal air agencies (air agencies) to provide data to characterize current air quality in areas with large sources of sulfur dioxide (SO2) emissions to identify maximum 1-hour SO2 concentrations in ambient air. The final rule establishes minimum criteria for identifying the emissions sources and associated areas for which air agencies are required to characterize SO2 air quality. Air agencies remain free to also characterize air quality in additional areas beyond those required to be characterized under the rule. The final rule also sets forth a process and timetables by which air agencies must characterize air quality through ambient monitoring and/or air quality modeling techniques and submit such data to the EPA. The EPA has issued separate non-binding draft technical assistance documents recommending how air agencies should conduct such monitoring or modeling. The air quality data developed by air agencies pursuant to this rule may be used by the EPA in future actions to evaluate areas' air quality under the 2010 1-hour SO2 National Ambient Air Quality Standard (NAAQS), including area designations and redesignations, as appropriate.
DATES: This final rule is effective on [INSERT DATE 30 DAYS AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER].
ADDRESSES: The EPA has established a docket for this action under Docket ID
No. EPA-HQ-OAR-2013-0711. All documents in the docket are listed on the http://www.regulations.gov website. Although listed in the index, some information is not publicly available, i.e., confidential business information or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, will be publicly available only in hard copy. Publicly available docket materials are available either electronically in http://www.regulations.gov or in hard copy at the Docket ID No. EPA-HQ-OAR-2013-0711, EPA/DC, William Jefferson Clinton West Building, Room 3334, 1301 Constitution Avenue, NW, Washington, D.C. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal holidays. The telephone number for the Public Reading Room is<PHONE_NUMBER> and the telephone number for the Air and Radiation Docket Information Center is<PHONE_NUMBER>. For additional information about the EPA's public docket, visit the EPA Docket Center homepage at: http://www.epa.gov/epahome/dockets.htm.
FOR FURTHER INFORMATION CONTACT: For further general information on this rulemaking, contact Dr. Larry D. Wallace, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, by phone at (919) 54l-0906, or by email at<EMAIL_ADDRESS>or Mr. Rich Damberg, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, by phone at (919) 54l-5592, or by email at<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:
I. 	General Information

A. 	Does this action apply to me?

      Entities potentially affected directly by this final rulemaking include state, local and tribal governments. Entities potentially affected indirectly by this final rulemaking, depending on how state, local and tribal agencies choose to regulate such entities in the future, include owners and operators of sources of SO2 emissions (such as coal-fired power plants, refineries, smelters, pulp and paper related facilities, waste incinerators, chemical manufacturers and facilities with industrial boilers for power generation) that contribute to ambient SO2 concentrations, as well as people whose air quality is affected by these facilities.
B. 	Where can I get a copy of this document and other related information?
	In addition to being available in the docket, an electronic copy of this notice will be posted at: http://www.epa.gov/air/sulfurdioxide/implement.html. Upon its publication in the Federal Register, only the published version may be considered the final official version of the notice, and will govern in the case of any discrepancies between the Federal Register published version and any other version.
C. 	How is this document organized?

      The information presented in this document is organized as follows:
I.	General Information
	A.	Does this action apply to me?
	B.	Where can I get a copy of this document and other related information?
      C.	How is this document organized?
II.	Background for Final Rulemaking
III.	Summary of the Final Rule Requirements
IV.	Responses to Significant Comments on the Proposed Rule
      A.	The Use of Monitoring and/or Modeling Data
      B.	Source Coverage and Emission Threshold Options
      C.	Data Requirements and Program Implementation Timeline
      D.	Technical Issues Relating to Modeling and Monitoring
      E.	Other Key Issues and Comments
V.	Environmental Justice Considerations
VI. 	Statutory and Executive Order Reviews
      A.	Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review
      B.		Paperwork Reduction Act (PRA)
      C.		Regulatory Flexibility Act (RFA)
      D.		Unfunded Mandates Reform Act (UMRA)
      E.		Executive Order 13132: Federalism
      F.	Executive Order 13175: Consultation and Coordination with Indian Tribal Governments
      G.	Executive Order 13045: Protection of Children from Environmental Health and Safety Risks
      H.	Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution, or Use
      I.		National Technology Transfer and Advancement Act
      J.	Executive Order 12898: Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations
      K.	Congressional Review Act (CRA)
      L.	Judicial Review
Statutory Authority

II. 	Background for Final Rulemaking

	On May 13, 2014, the EPA proposed the Data Requirements Rule (DRR) for the 2010 1-hour SO2 Primary NAAQS, 79 FR 27446, May 13, 2014. The preamble to the proposal provided a discussion of the events that led to the EPA's proposal of a new regulation to direct state, tribal and local agencies to better characterize ambient air SO2 concentrations near large polluting sources. See 79 FR 27446, 27447, May 13, 2014. This discussion addressed the adoption of the 2010 SO2 NAAQS, and the suggested implementation approach described in the preamble of that rulemaking; the area designations process under section 107 of the Clean Air Act (CAA); the history of designations for prior SO2 NAAQS, including the use of air quality modeling information; the Agency's subsequent issuance of an implementation white paper in May 2012 and input received from stakeholder groups; and the EPA's February 2013 SO2 NAAQS implementation and designations strategy paper, developed in response to feedback received through this outreach process. This final rulemaking notice does not repeat all of that discussion, but refers interested readers to the preamble of the proposed rule for this informative background.
	The proposed rule noted that although the current SO2 ambient monitoring network included more than 400 monitors nationwide, the scope of the network had certain limitations, and approximately two-thirds of the monitors are not located to characterize maximum 1-hour SO2 concentration impacts from emissions sources. To more effectively assess potential public health impacts from exposure to high SO2 concentrations, the proposed rule presented options for requiring air agencies to characterize air quality in the vicinity of large sources of SO2 emissions that exceed specified annual emissions thresholds. The EPA's proposed preferred emissions threshold option specified that air agencies would be required to characterize air quality in the vicinity of sources that emit over 1,000 tons of SO2 per year and are located in more highly populated areas (i.e., Core-Based Statistical Areas (CBSA) with population of at least 1 million), and in the vicinity of sources that emit over 2,000 tons of SO2 per year and are located outside metropolitan areas of at least 1 million population. (The EPA also identified two other emission threshold options and requested public comment on these potential emission thresholds values, a CBSA population threshold of 1 million, the combination of emissions and population thresholds as a means of determining how SO2 sources would be identified, and on any possible alternatives.) Under the proposed approach, air agencies, or the EPA, also could require air quality characterization around other sources, if warranted. See 79 FR 27446, 27453, May 13, 2014.
      Under the proposed rule, the air agencies would determine for each emissions source exceeding the threshold whether air quality characterization for that source would be done either through air quality modeling analysis or by conducting ambient monitoring. Apart from the proposed rule, the EPA issued two draft technical assistance documents (TADs) on modeling and monitoring to assist air agencies with this analytical work. The proposed rule also described a process and timetable by which air agencies would be required to identify sources to be characterized, conduct the relevant analyses and submit such data to the EPA. See 79 FR 27446, 27456, May 13, 2014.
      Specific technical considerations regarding air quality monitoring and modeling were also discussed in the proposed rule, along with options for ongoing verification of the air quality characterization in areas that are not otherwise designated as nonattainment. See 79 FR 27446, 27460, May 13, 2014. The proposal also discussed incentives for air agencies and sources to work together to establish federally enforceable limits on emissions expeditiously in order to avoid requirements for air quality characterization altogether. We refer readers to the proposed rule for the technical, policy and legal rationale that were presented in support of the proposal, and for a complete discussion of the issues for which the EPA requested public comment. Several supporting memoranda, analyses and data were included in the docket for the proposed action.
      The 60-day public comment period for the proposed rule closed on July 14, 2014. In section IV of this preamble, we summarize each key issue from the proposal, briefly summarize major comments received and provide a response, and describe the final policy in the rule, including any changes made to the approaches presented in the proposal. A more detailed response to comments document can be found in the docket for this rulemaking, presenting a comprehensive description of comments received and responses to those comments.
III.	Summary of the Final Rule Requirements
      This section provides a brief summary of the requirements of the final rule. Further discussion of the basis for these requirements and responses to significant comments are provided in the next section. The EPA believes that the approach set forth in this rule directing air agencies to gather additional data to characterize ambient air in the vicinity of larger SO2 sources is uniquely suited for implementation of the 1-hour SO2 NAAQS, and the Agency does not anticipate it to be used for other NAAQS pollutants. The final rule establishes minimum requirements for air agencies to characterize 1-hour SO2 air quality concentrations across the country, with an emphasis on doing so in the vicinity of sources that have the largest annual SO2 emissions. Note that there are already minimum SO2 ambient monitoring requirements in place that were established when the 1-hour SO2 NAAQS was adopted. See 75 FR 35520, June 22, 2010. The requirements in the present rule supplement those monitoring requirements, which remain in place. As discussed in more detail in the next section, these requirements are intended to establish a flexible yet effective program for characterizing SO2 air quality in priority areas across the country, given existing funding and resource constraints, and given the particular characteristics of SO2 air pollution in the affected areas. This final rule also reflects the fact that numerous larger sources of SO2 across the country have in recent years installed, and are expected to install in the near future, additional control measures that may substantially reduce SO2 emissions in some cases.
      Under this rule, each air agency is required to submit a list to the EPA by January 15, 2016, that identifies all sources within its jurisdiction that have SO2 emissions that exceeded the 2,000 tons per year (tpy) annual threshold during the most recent year for which emissions data for that source are available, plus any additional sources and their associated areas identified by the air agency or by the EPA as also warranting air quality characterization. (The list does not need to include sources located in areas already designated as nonattainment for the 2010 SO2 NAAQS.) The rule requires air quality characterization of the area associated with each listed source, and provides two options for this characterization, namely the use of monitoring or modeling. The final rule also provides a third option, under which air agencies would establish a limit requiring emissions from a listed source to be below the 2,000 tpy threshold, which, with the concurrence of the EPA Regional Administrator, would result in that source and its associated area not being subject to requirements for air quality characterization. The EPA anticipates discussions with air agencies early in 2016 to resolve any questions as to what areas warrant air quality characterization. These discussions are intended to address whether any additional areas (e.g., areas with clusters of sources) warrant air quality characterization, whether existing monitoring networks might serve to address air quality characterization requirements, and whether any new limits intended by the air agencies negate the need for air quality characterization.
      For each source on the list, the air agency will be required to indicate by July 1, 2016, whether it will characterize air quality through ambient monitoring or through air quality modeling or, alternatively, whether it will be subjecting the pertinent source or sources to emission limit(s) that will keep the source(s) below this rule's 2,000 tpy threshold. The option identified by the air agency for each source and its associated area will determine the submittal and timing requirements for the air agency to provide the required information.
      If the air agency chooses the first option, ambient monitoring for a source, the air agency must include information about the planned new monitor(s) in the annual monitoring plan that the air agency must submit to the EPA by July 1, 2016; and the air agency must also ensure that the new monitor(s) are operational by January 1, 2017. The required monitors shall be sited and operated either as State and Local Air Monitoring Stations (SLAMS) or in a manner equivalent to SLAMS. In either case, monitors shall be subject to reporting and data certification requirements as prescribed in 40 CFR 58.15 and 58.16 (e.g., quarterly reporting of monitoring data to the Air Quality System, and the annual certification of data by May 1 of the following year), and must satisfy applicable criteria in 40 CFR part 58, appendices A, C and E.
      If the air agency chooses the second option, air quality modeling for a source, it must submit a modeling protocol for each such source to the EPA by July 1, 2016, for review and consultation with the EPA Regional Office. The modeling analyses must then be submitted to the EPA by January 13, 2017.
      If the air agency chooses the third option, to provide federally enforceable emissions limitations that limit emissions of an applicable source to less than 2,000 tpy, or to provide documentation that the applicable source has permanently shut down, the air agency must notify the EPA of its decision by July 1, 2016, and provide a description of the planned emission limitation, including identification of the level of the limitation being planned. Especially in areas with multiple sources, the limit(s) should be sufficiently low as to avert the need for air quality characterization that applies for other listed sources. Therefore, the rule requires the concurrence of the EPA as to whether the limit that the air agency intends will suffice in lieu of conducting air quality characterization. By January 13, 2017, the air agency must provide EPA with documentation demonstrating that the emission limits are federally enforceable, adopted, and require compliance by January 13, 2017, in order for areas containing such sources to avoid the need to characterize ambient SO2 emissions under the rule. If EPA approval is required to make a limit federally enforceable, the submittal must be sent to the EPA early enough such that the EPA has enough time to complete a rulemaking to make the limit federally enforceable by the January 13, 2017, date.
      Section IV.D of this preamble provides a discussion of selected technical considerations related to characterizing air quality, but the rule does not prescribe how an ambient monitoring network around an identified SO2 source is to be designed, or how air quality modeling must be specifically done to meet the objectives of this rule. As stated in the proposal, the EPA has developed TADs that provide approaches on ambient monitoring and air quality modeling when planning and executing air quality characterization activities, and recommends that air agencies refer to these documents to support their efforts. For example, the TAD for ambient monitoring suggests potential options and recommendations on different analyses and approaches that could be considered to help the air agency site source-oriented SO2 monitors in locations of expected maximum 1-hour concentrations. The TAD for air quality modeling explains that refined dispersion models are able to characterize SO2 air quality impacts from the modeled sources across the domain of interest on an hourly basis with a high degree of spatial resolution. It suggests that in order to characterize recent air quality levels around a source, it would be acceptable to use actual hourly emissions data, actual meteorological data and actual stack height information as technical inputs to the modeling analysis. However, it is important to note that, except to require that monitoring be sited and operated in a manner equivalent to SLAMS and to provide that modeling may be based on actual or allowable emissions, this rule does not promulgate any specific requirements with regard to these analytical approaches, and air agencies are expected to use their best professional judgment, consulting as appropriate with the EPA, in conducting these analyses. Air agencies should also contact their respective EPA Regional Offices regarding any additional issues beyond those addressed in the TADs.
      The final rule also includes provisions specifying how characterization requirements for listed sources continue into the future (i.e., ongoing data requirements). For areas where air quality is to be characterized through ambient monitoring, the rule requires the monitoring to be conducted for the calendar years of 2017 through 2019, in order to calculate a valid design value for each area. The rule requires that air agencies (or other parties conducting the monitoring) continue the operation of all existing and new monitors used to meet the requirements of this rule. However, it also provides for the possibility that an air agency may obtain EPA approval to terminate operation of a monitor that was established to meet the requirements of this rule if the air quality values at the monitor are low enough to meet specific criteria. Following commencement of operation of a new monitor, the air agency may seek EPA approval to terminate operation of the monitor pursuant to §51.1203(c)(3) of this rule, if the monitored design value for the first 3-year period or second 3-year period is no greater than 50 percent of the 1-hour SO2 NAAQS. After the fourth year following commencement of operation of a new monitor, the air agency may be able to seek approval to shut down the monitor if it meets the criteria specified in existing regulations at 40 CFR 58.14.
      For areas that were characterized using air quality modeling, the ongoing data requirement applies only where the modeling was based on actual emissions and where the area has not subsequently received a nonattainment designation. In such cases, the air agency will be required to submit an annual report to the EPA providing updated emissions information and recommending to the EPA whether further modeling is warranted to assess any expected changes in recent air quality. For example, it may be appropriate for the air agency to conduct updated modeling for an area if there have been increases in short term emissions rates, an increase in annual emissions, or changes in facility operations. Where warranted, the air agency shall conduct updated modeling to characterize air quality in light of the identified emissions changes and present the results in its annual report to the EPA. Analogous to the monitor shutdown provisions noted earlier, the requirement for the annual emissions assessments for an area originally characterized by modeling may be terminated if the air agency provides a modeling analysis demonstrating that actual emissions in the previous year for SO2 sources in the area results in a modeled design value that does not exceed 50 percent of the NAAQS at any receptor within the modeling domain. While the annual assessment requirement under this rule would be terminated in such cases, any other EPA requirements to provide data (e.g., for the Air Emission Reporting Rule (AERR)) would not be affected.
      The EPA received more than 80 comments on the proposed rule. Taking into consideration the range of comments received, the EPA made a number of revisions that are reflected in the final rule, including the following:
         * The source emissions threshold approach was changed to a single 2,000 ton annual SO2 emissions level, so the final rule does not include thresholds that vary depending on the population of the area.
         * Air agencies still need to identify in January 2016 a list of sources in their jurisdiction for which air quality is to be characterized, but they now have until July 2016 to indicate whether, for each source, they plan to use modeling or monitoring to characterize air quality, or to adopt an enforceable emissions limit. (The rule clarifies that this list would not include any source located in an area already designated as nonattainment for the 2010 SO2 NAAQS.) The approach in the proposal would have required the air agency to indicate its planned approach for each source in January 2016.
         * The final rule also includes a set of monitor shutdown provisions that is a hybrid of the options included in the proposed rule and the existing monitor shutdown provisions in 40 CFR part 58. A monitor required under this rule would be eligible for shutdown if it has a design value less than 50 percent of the SO2 standard during one of the first two 3-year periods of operation. After this point in time, any potential shutdown would need to meet the basic shutdown provisions that apply for SLAMS monitors as described in 40 CFR 58.14.
         * The proposal took comment on three potential approaches for ongoing requirements for air agencies to provide modeling or emissions data for areas that were originally characterized with modeling based on actual emissions data. As noted earlier, the approach in the final rule requires the air agency to provide emissions data to the EPA annually for all sources not designated as nonattainment, and to recommend to the EPA whether an emissions change was substantial enough to warrant updated air quality modeling.
         * A number of commenters suggested that an air agency should be able to avoid the air quality characterization requirement for a source if it adopted a federally enforceable requirement limiting annual emissions at the source to less than 2,000 tpy. The final rule now includes such a provision. This type of limit would need to be adopted and in effect by January 2017.
IV. 	Responses to Significant Comments on the Proposed Rule
A.	The Use of Monitoring and/or Modeling Data
1. 	Legal Authority to Require States to Submit Data Pursuant to This Rule

      a.  	Summary of Proposal
      In the proposed rule, the EPA explained that the requirements for the air agency to submit the SO2 monitoring and modeling data described in §51.1203 of the proposed rule are appropriate steps needed to understand SO2 air quality throughout the country, and are consistent with section 110(a)(2)(B), section 110(a)(2)(K) and section 301(a)(1) of the CAA. See 79 FR 27446, 27457, May 13, 2014.
b. 	Brief Summary of Comments
 Some state commenters asserted that the DRR modifies the CAA and imposes new monitoring and modeling obligations on air agencies. One commenter suggested that requiring states to develop monitoring or modeling data in accordance with this proposal modifies the statutory mandate to designate all areas by June 2013 because the EPA intends to use these data for designations. One industry commenter stated it is not appropriate to replace the CAA's statutory directive for designations with extra-statutory provisions like those proposed in the DRR.
      Several state and industry commenters stated that the proposed requirements and schedules conflict with requirements that apply to the EPA to timely complete designations under section 107 of the CAA. These commenters stated that the CAA required the EPA to make area designations under the new SO2 standard no later than June 3, 2013, and that the EPA failed to comply with that mandatory obligation. Therefore, the commenters claimed, the DRR proposal's discussion of a schedule for issuing designations by December 2020 is beyond the EPA's authority. One state commenter cited EME Homer City Generation LP v. Envt'l Prot. Agency, 696 F. 3d 7, 27 (D.C. Cir. 2012) and stated that the DRR cannot stand as proposed because it fails to follow the mandatory timelines for promulgating area designations, and, therefore, exceeds the EPA's statutory authority.
      c. 	EPA Response
      The comments that assert that the EPA has not designated areas under the 2010 SO2 NAAQS in a timely manner are beyond the scope of this rulemaking, and are not germane to the issue of the EPA's statutory authority to direct air agencies to conduct monitoring or modeling to further characterize ambient air concentrations of SO2. Through this rulemaking, the EPA is not establishing or modifying any area designation requirements provided for in section 107 of the CAA, nor does any aspect of this final rule conflict with any provision of section 107 that directs states and the EPA to take timely action to issue designations. The purpose and effect of this rulemaking is to require air agencies to characterize air quality in priority areas throughout the country where existing ambient monitors may not be adequately characterizing peak 1-hour SO2 ambient air concentrations. The air quality data obtained as a result of this rulemaking then may be used in future analytical actions by the EPA, including designations of any undesignated areas or redesignations of already designated areas. It is true that in the proposed rule preamble we discussed how the timing of the implementation of this rule would fit with our intended schedule for completing area designations, but the proposal did not itself purport to establish a binding schedule for completing designations.
      The EPA notes that litigation was filed against the EPA to compel the Agency to complete designations under CAA section 107, and on March 2, 2015, the court in one of those cases issued a ruling that places the EPA on a binding schedule to complete area designations for the 2010 1-hour SO2 NAAQS. See, Sierra Club, et al. v. McCarthy, Case No. 13-cv-03953-SI (N.D. Cal., March 2, 2015) (Order Granting Joint Motion To Approve And Enter Consent Decree And Denying Other Motions As Moot; and Consent Decree). Copies of the court's order and the March 2015 consent decree setting forth the EPA's schedule for completing designations have been placed in the docket for this this rulemaking. Under the schedule ordered by the court, the EPA is required to complete the designations in no more than three future rounds.
      First, by July 2, 2016 (16 months from the date of the court's order), the EPA must sign a notice for publication in the Federal Register that promulgates designations for remaining undesignated areas that: (a) based on air quality monitoring in the three full calendar years preceding that date have monitored violations of the NAAQS; or (b) contain any stationary source that has not by March 2, 2015, been "announced for retirement" and that, according to data in the EPA's Air Markets Database, either (1) emitted more than 16,000 tons of SO2 in 2012, or (2) emitted more than 2,600 tons of SO2 and had an annual average emission rate of 0.45 lbs. SO2/Mmbtu or higher in 2012. (The March 2015 consent decree defines "announced for retirement" as meaning "any stationary source in the United States with a coal-fired unit that as of January 1, 2010, had a capacity of over five (5) megawatts (MW) and that has announced it will cease burning coal at that unit through a company public announcement, public utilities commission filing, consent decree, public legal settlement, final state or federal permit filing, or other similar means of communication.")
      Second, by December 31, 2017, the EPA must sign such a notice promulgating designations for remaining undesignated areas in which, by January 1, 2017, states have not installed and begun operating a new SO2 monitoring network meeting EPA's specifications referenced in this rulemaking. Finally, by December 31, 2020, the EPA must sign a notice promulgating designations for all remaining undesignated areas.
      The EPA notes that the schedule imposed by the court will allow at least the latter two stages of designations to be informed and benefited by the additional information that is timely obtained pursuant to this final rule, as appropriate. However, we also note that the round of designations that is required to be completed by July 2, 2016, will likely be conducted before state air agencies and the EPA will have been able to implement this final rule, and will instead rely upon data and information that is separately developed or obtained during the designations process. Nevertheless, as explained later in this notice, depending on how those areas become designated in 2016, the rule may still result in additional information that could inform future assessments of attainment status for such areas.
      The EPA continues to believe that the requirements of this rule for air agencies to submit a list of sources where further air quality characterization is needed, and the other data submittal requirements found in §51.1203 of this rule, are appropriate steps needed to better understand SO2 air quality throughout the country, and are consistent with section 110(a)(2)(B), section 110(a)(2)(K), and section 301(a)(1) of the CAA. The commenters did not challenge this view. Section 110(a)(2)(B) indicates that SIPs are to "provide for establishment and operation of appropriate devices, methods, systems, and procedures necessary to (i) monitor, compile and analyze data on ambient air quality and (ii) upon request, make such data available to the Administrator." Section 110(a)(2)(K) states that State Implementation Plans (SIPs) shall "provide for (i) the performance of such air quality modeling as the Administrator may prescribe for the purpose of predicting the effect on ambient air quality of any emissions of any air pollutant for which the Administrator has established a NAAQS, and (ii) the submission, upon request, of data related to such air quality modeling to the Administrator." Section 301(a)(1) provides the EPA with general authority to establish regulations as necessary to carry out the agency's functions, which in this case includes ensuring the attainment of the SO2 NAAQS throughout each state. This section states that "The Administrator is authorized to prescribe such regulations as are necessary to carry out his functions under this chapter."
      The EPA often establishes and revises monitoring requirements for implementing NAAQS. Those requirements will not necessarily always generate new information in time to inform timely area designations under CAA section 107. See, e.g., 75 FR 81126, 81130, December 27, 2010. The validity of such rules does not depend upon whether information generated pursuant to those requirements will be gathered in time to support designations that are timely under section 107. Here, the commenters have raised no objection to the central premise of the rule, which is that additional information that better characterizes air quality near larger sources of SO2 is warranted and is authorized to be required under sections 110 and 301 of the Act. Irrespective of when the EPA uses this information -- for example, irrespective of whether the EPA promulgates initial designations of "unclassifiable" (and then uses the information collected pursuant to this data requirements rule in later redesignations), or whether the EPA promulgates the remaining designations after the information required here becomes available -- the EPA believes that this rule is authorized and is warranted. Therefore, in this final rulemaking, the commenters have provided no basis for the EPA to not require air agencies to submit such SO2 monitoring and modeling data to the EPA, as proposed. The final rule is fully consistent with the Agency's broad authority under section 110 and 301, as well as with the EPA's authority under CAA section 114(a)(1) to direct any person to provide information as is reasonably required to improve characterization of ambient air quality near larger sources of SO2.
2. 	Legal Authority to Base Air Quality Evaluations on Modeling Data

      a.  	Summary of Proposal
      In the proposal, the EPA stated that existing air quality modeling tools are technically sound and historically have been used to characterize SO2 air quality when monitoring data were not available; therefore, the EPA considers these modeling tools appropriate for assessing air quality impacts from SO2 emissions. The EPA stated that historical use of modeling to characterize SO2 air quality concentrations has been affirmed as technically valid and lawful under the CAA by reviewing courts. See 79 FR 27446, 27448, May 13, 2014.
      b. 	Brief Summary of Comments
	Some industry group commenters stated that the DRR provisions allowing for modeling to characterize ambient SO2 concentrations go beyond what is necessary to comply with the CAA, arguing that 40 CFR 50.17 provides that monitoring is the sole basis for determining attainment. Commenters stated that the precise wording of 40 CFR 50.17 establishes ambient air monitoring as the only basis for determining if the SO2 NAAQS is being met because it specifies that:
      (a) The level of the national primary 1-hour annual ambient air quality standard for oxides of sulfur is 75 parts per billion (ppb, which is 1 part in 1,000,000,000), measured in the ambient air as SO2.
      (b) The 1-hour primary standard is met at an ambient air quality monitoring site when the 3-year average of the annual (99th percentile) of the daily maximum 1-hour average concentrations is less than or equal to 75 ppb, as determined in accordance with appendix T of this part.
      (c) The level of the standard shall be measured by a reference method based on appendix A or A-1 of this part, or by a Federal Equivalent Method (FEM) designated in accordance with part 53 of this chapter.
      One public interest group commented that the provisions in the proposed DRR for conducting modeling are consistent with the EPA's historic use of air dispersion modeling for multiple NAAQS implementation purposes. This public interest group stated that dispersion modeling has a lengthy history as an appropriate tool for use in SO2 designations and other actions, and provided several references to the EPA's documents and to court rulings to demonstrate that historic use.
      In contrast, without disputing the fact that the EPA has often relied upon modeling to inform decisions implementing the SO2 NAAQS, several state and industry commenters stated that monitoring, not modeling, has historically been used for designation of areas as attainment or nonattainment under this and other NAAQS. Several industry commenters supported the EPA's use of notice-and-comment rulemaking through the DRR to address certain major issues, including the use of monitoring and/or modeling to characterize air quality and make remaining area designations.
      c. 	EPA Response
      This final rule does not make any decisions or determinations regarding whether any area is in fact meeting or not meeting the NAAQS based on either monitoring or modeling information. Those decisions will be made in separate future actions, or have already been made for some areas in prior actions. See e.g., 78 FR 47191, August 5, 2013. Therefore, this final rule does not take final action on the issue of whether it is permissible to implement the commenter's previous quoted provisions of 40 CFR 50.17(a)-(c) based on a combination of both monitoring and modeling information where both are available, or exclusively on modeling information where appropriate modeling information is available and monitoring is not. The commenters' objections appear to focus on how future-gathered information resulting from the rule may or can be used in subsequent NAAQS implementation actions, but the focus of this rule is on the initial gathering of the information itself. In future designation, redesignation, or other implementation actions, commenters may raise their objections to the validity of information that the EPA relies upon in those specific actions, but such objections are beyond the scope of this final rule.
	The commenters appear to be raising objections that were also raised after the EPA's promulgation of the 2010 SO2 NAAQS, in response to the EPA's final rule preamble discussion explaining the Agency's then-intended implementation approach under the NAAQS. In their petitions for judicial review of the NAAQS, several states claimed that the EPA's discussion of the intended use of modeling in NAAQS implementation contravened the regulatory text of §50.17. However, noting that the petitioners' claims were addressed to potential final implementation actions that had not yet in fact occurred, the U.S. Court of Appeals for the D.C. Circuit dismissed the petitioners' claims without addressing their merits, or lack thereof. See National Environmental Development Association's Clean Air Project v. EPA, 686 F.3d 803 (D.C. Cir. 2012). Likewise here, the EPA is not yet taking any action to apply modeling regarding any decision of whether an area is or is not meeting the NAAQS.
      In any event, we note that although 40 CFR 50.17(a)-(c) very clearly sets forth the criteria for determining whether the NAAQS is met at a monitoring site, it does not by its terms restrict how such decisions may be made more broadly in areas impacted by SO2 sources, including areas where there are no monitoring sites or where monitors are not sited at the point of maximum ambient concentration. Indeed, it is the relative scarcity of such monitors that has caused the EPA to undertake this rulemaking to enable states and the Agency to better understand just what the ambient air impacts are from larger sources of SO2, which may not be captured by the current limited monitoring network. It is true that past area designations processes for most NAAQS (such as for ozone) having violations caused and contributed to by multiple sources over a broad region have relied primarily on air quality monitoring data to identify areas that violate the standard. However, it is important to note, as the EPA explained in the final 2010 SO2 NAAQS preamble, that there is a long history of also using dispersion modeling information to inform area designations for the SO2 NAAQS. See, e.g., 75 FR 35520, 35551, June 22, 2010.
      The EPA and the air quality management community have recognized over many years that peak concentrations of SO2 are commonly caused by one or a few major point sources in an area, and that peak concentrations are typically observed relatively close to the source. Many factors influence the observed SO2 concentrations around emissions sources, including the sulfur content of fuel that is combusted, the sulfur content of material being heated as part of an industrial process, the rate of SO2 emissions per hour, stack height, topography, meteorology, monitor location and source operating schedule. But because ambient SO2 concentrations are not the result of complex atmospheric chemical reactions (unlike ozone or PM2.5), they can be modeled accurately using well-understood air quality modeling tools, especially in areas where one or only a few sources exist. In the 1970's, when the original SO2 NAAQS were established, there were significantly more SO2 monitors in operation nationally than today. Even then, the EPA and air agencies acknowledged the utility of modeling in order to inform area designations under the SO2 NAAQS. See e.g., 43 FR 45993, October 5, 1978.
3. 	The Use of Monitoring and/or Modeling for Making Decisions about Air Quality

      a.  	Summary of Proposal
      In the proposed rule, the EPA explained that the current ambient SO2 monitoring network, on the whole, is not appropriately positioned, or of adequate size, for purposes of the 2010 SO2 standard to characterize the air quality around many of the nation's larger SO2 sources in operation today. The EPA stated that, because ambient SO2 concentrations are not the result of complex chemical reactions (unlike ozone or PM2.5), they can be modeled accurately using well understood air quality modeling tools, especially in areas where one or only a few sources exist. However, the EPA noted that some areas may not be conducive to modeling, and for such areas the EPA encouraged air agencies to consider using enhanced monitoring to characterize air quality. See 79 FR 27446, 27448, May 13, 2014.
      b. 	Brief Summary of Comments
  	Several state and industry commenters supported the provision in the proposed rule allowing air agencies to have the option to use modeling and/or monitoring to characterize SO2 ambient air concentrations, as it provides appropriate flexibility for both the states and affected sources. Several commenters supported the EPA's observation that modeling may not be appropriate for all SO2 evaluation scenarios, and supported the ability of states to choose to evaluate NAAQS attainment through either dispersion modeling or ambient monitoring. However, several state and industry commenters cautioned that monitoring data should be the primary basis for such decisions, especially designating nonattainment areas. Several commenters claimed that, as modeling is frequently affected by factors such as emissions inputs, meteorological data and local geography, it is not as accurate or reliable as real-time, multiple-year monitoring. Other commenters claimed that modeling is advantageous because it characterizes air quality in all directions around a source with appropriate accuracy and can be done with less expense than ambient monitoring, which only characterizes air quality at a single location. Some industry commenters suggested the text of proposed §51.1201 be revised to state that monitoring is the EPA's preferred analytical approach under the rule.
      c. 	EPA Response
	The EPA agrees with commenters who stressed the need to give air agencies the option to characterize SO2 ambient air quality through either enhanced monitoring or modeling, and the EPA is maintaining that approach in this final rule. The EPA believes that the commenters have not presented any persuasive reasons for changing the basic positions previously discussed in the preamble to the final rule of the 2010 SO2 NAAQS rulemaking, the February 2013 Strategy Paper, or in the proposed rule for why both air quality modeling and ambient monitoring are appropriate tools for characterizing ambient air quality for purposes of informing future decisions to implement the SO2 NAAQS. However, as explained earlier, in this final rule the EPA is not taking final action to make any determinations regarding any area's status with respect to attaining or not attaining the NAAQS, but is only prescribing criteria and a process for how and when air agencies are to gather and provide to the EPA additional needed information. How the information is used in subsequent actions evaluating the attainment status of specific areas will depend upon the information that air agencies collect in the future and what it shows about areas' ambient air quality.
B. 	Source Coverage and Emission Threshold Options
1. 	Summary of Proposal
      In the proposal, the EPA recognized that the characterization of air quality in areas around more than 20,000 SO2 sources nationally would not be feasible. The proposal stated that the key objective to be achieved by using SO2 source emission thresholds would be to focus the limited available resources at the state, tribal, local and federal levels toward characterizing air quality in areas having the largest SO2 emitting sources due to the fact that larger sources can be expected to be the most likely potential contributors to violations of the SO2 NAAQS. The EPA stated in the proposed rule that, just as NAAQS ambient monitoring networks are designed to measure air quality in areas meeting specific criteria where the public is likely to be exposed and violations may be likely to occur, the SO2 annual emission threshold options in the rule are designed to meet a similar objective. See 79 FR 27446, 27453, May 13, 2014.
	In considering how to develop effective options for identifying the minimum set of sources around which states would be required to characterize ambient air quality, we considered three important issues and requested comment on each:
   oo What would be an appropriate emissions metric for identifying sources?
   oo Should the threshold options require characterization of smaller sources in areas with higher populations?
   oo What would be an appropriate threshold for identifying sources near which air quality is to be characterized?
      The notice of proposed rulemaking also addressed a number of additional elements of the implementation of these thresholds. In the discussion below, the EPA summarizes these additional proposed features, summarizes comments on these proposed features, and describes the EPA's responses. Note that this section is structured so that all the issues related to emissions thresholds are presented together before proceeding to the comment summaries on these issues, also all presented together, and then to the EPA's responses and final decision.
   a.  Emissions Metric: What would be an appropriate emissions metric for identifying sources?
      The proposal presented a discussion about what emissions-related metric would be most appropriate for this rule. The proposal noted that for the 1-hour SO2 NAAQS, the ideal metric for identifying sources near which air quality is to be characterized would be a 1-hour SO2 emissions rate. However, the EPA observed that while 1-hour SO2 emission rate data are available for most electricity generating units (EGUs) because they operate continuous emission monitors, many non-EGUs do not operate continuous emission monitors on all emission points and produce 1-hour data. For this reason, the proposal stated that the emissions threshold options presented in this rulemaking should be expressed in terms of annual emissions of SO2 because annual emissions information is readily available for all large SO2 sources.
      The EPA requested comment on the use of annual emissions (i.e., tons of SO2 per year) as the metric to be used for an emissions and population-based threshold approach, or, alternatively, for a solely emissions-based threshold approach, to identify SO2 sources around which further ambient air quality characterization with respect to the 1-hour SO2 NAAQS might be required. The EPA also requested comment on any potential alternative factors that should be considered for defining emissions thresholds, along with any information about the availability of data related to any alternative factor for all SO2 sources nationally, the time and resources needed to develop a database for this alternative factor, any associated technical analysis and rationale for using these other factors in defining source thresholds. See 79 FR 27446, 27454, May 13, 2014.
      b. 	Should the threshold options require air quality characterization near smaller sources in areas with higher populations?
      In the proposed rule, the proposed emissions threshold option and the other two options on which the EPA requested comment each had a "two-pronged" form. Each potential option was expressed with a higher emissions threshold for identifying sources located outside of CBSAs with a population equal to or greater than 1 million persons, and a lower emissions threshold for identifying sources located within such CBSAs. The reasoning given for this proposed approach was that a lower threshold for urban sources could help increase public health protection because there are more people in an area that could be impacted by relatively smaller sources. The EPA requested comment on its proposed use of the 1 million person CBSA population threshold for representing the population exposure component of the source threshold options in this rule. The EPA also requested comment on whether to include a population exposure-based threshold at all; and on whether alternative, or additional, criteria would be appropriate to further focus resources on characterizing air quality in areas with a higher likelihood of population exposure. See 79 FR 27446, 27455, May 13, 2014.
      c. 	What is an appropriate threshold level or levels for identifying sources near which air quality is to be characterized?
	 The EPA proposed one preferred option and took comments on two additional options. Option 1 (proposed preferred option) would require ambient air quality characterization around any source with annual emissions greater than 1,000 tpy and which is located within a CBSA having 1,000,000 or more persons, and around sources with emissions greater than 2,000 tpy located outside CBSAs having 1,000,000 or more persons. Option 2 would require ambient air quality characterization around sources with emissions greater than 2,000 tpy that are located within any CBSA having 1,000,000 or more persons, and around sources with emissions greater than 5,000 tpy located outside CBSAs having 1,000,000 or more persons. Option 3 would require ambient air quality characterization around sources with emissions greater than 3,000 tpy that are located within any CBSA having 1,000,000 or more persons, and around sources with emissions greater than 10,000 tpy located outside CBSAs having 1,000,000 or more persons.
      The EPA requested comment on the preferred option and the other two options described in the proposal. The EPA also requested comment on any possible alternatives that might be appropriate for consideration. The EPA requested comment on the scope of sources for which we would require data. In addition, the EPA also requested any information identifying sources that would be identified by these options but that have confirmed documentation to show that they will shut down in the next several years. The EPA noted in the proposed rule that, in addition to meeting the requirements to provide information regarding areas with sources over the future promulgated thresholds, there may still be situations where an air agency would need to characterize air quality for other sources below the thresholds, where the air agency, or the EPA Regional Administrator, determines that they may have the potential to violate the NAAQS. See 79 FR 27446, 27455, May 13, 2014.
      d.	Discretion for air agencies and the EPA to address additional sources
	The EPA proposed to authorize the states and EPA Regional Administrators to apply the DRR requirements to additional areas in addition to those with sources emitting above the applicable threshold. Application of air quality characterization requirements was noted to be possibly warranted where sources emitting below the threshold have the potential to cause or contribute to violations of the NAAQS, for example where multiple smaller sources located in close proximity may collectively exceed the emissions thresholds and/or cause or contribute to NAAQS exceedances.
2. 	Summary of Comments
	This section provides a brief summary of comments received on each of the source threshold issues identified previously, as well as additional features of the EPA's proposed implementation of thresholds.
      a. 	Comments on an appropriate emissions metric
	Most commenters that addressed the emissions metric issue supported using annual SO2 emissions (in tpy) as the appropriate metric for defining source thresholds. Several commenters stated that it is most appropriate to evaluate annual emissions since these data are widely reported to the EPA and are readily available. Some industry commenters stated that using an annual emissions based threshold approach for identifying areas to be evaluated would serve to make more manageable the demands on local, state, tribal and federal resources. Several other commenters stated that the use of additional factors such as stack height, 1-hour SO2 emission rate, proximity to sensitive populations, and topography would make the source identification process unnecessarily difficult and time consuming. On the other hand, a few regulatory agency commenters urged the establishment of supplemental criteria based on short-term spikes in emissions.
      b.	 Comments on whether the options should require characterization near smaller sources in areas with higher populations
 	A number of state and industry commenters supported the application of a lower emission threshold in urban areas. Some commenters stated that population centers represent locations of higher potential public exposure and, therefore, characterization of air quality in these areas would be more representative of the public's SO2 exposure risk. Several state and industry commenters stated that a threshold approach based purely on emissions could inappropriately focus limited resources on areas with limited to no public exposure. Some state commenters noted that, as a precedent, a population threshold has been used to establish the minimum monitoring requirements for the SO2 NAAQS as well as the NAAQS for nitrogen dioxide, carbon monoxide, and particulate matter.
      Some commenters stated that many sources located within an existing CBSA are located on the edge of the boundary in less populated areas and urged the EPA to consider more refined census data based on population density. One industry commenter suggested, for example, that the EPA could use population density data around the effected facilities out to a radius of 10 km and, if average population density from the 2010 census in this area exceeds a certain threshold (e.g. 100 persons/square km), then the lower emissions criteria would be used. Some tribal commenters, some environmental group commenters, and some state commenters recommended against applying different thresholds in less populated areas, in order to assure that all areas are equally protected against violations of the air quality standard.
      c. 	Comments on source threshold options
	One public interest group and several states urged the EPA to adopt the proposed Option 1 level of 1,000 tpy, but apply it uniformly, regardless of population in order to ensure a basic level of health protection to people who live around the sources. Some commenters stated that because modeling has shown that sources with emissions below 2,000 tpy have the potential to cause or contribute to modeled NAAQS violations, an emissions threshold of 1,000 tpy is more appropriate to ensure that air quality characterizations are accurately capturing potential NAAQS violations.
	Several state and industry commenters supported Option 2 stating it balances limited agency resources for the implementation of this rule while still allowing important SO2 emission source areas to be evaluated. Some industry commenters stated Option 2 appears to be the best option because the difference between the number of sources captured by Options 1 and 2 is substantial while the difference in overall emissions covered by the two options is small.
      Numerous state and industry commenters supported Option 3, stating it would apply reasonable thresholds without burdening states with unnecessary modeling or monitoring. One industry commenter stated that this option would allow states to focus their limited resources on the areas with the largest 211 sources of SO2 emissions.
      One industry commenter stated that if the EPA decides that either Option 1 or 2 is preferable, then the source threshold needs to be revised to take into account the following additional factors: the distance a source is located from population centers in general and sensitive populations in particular; stack heights; topography and meteorological factors unique to the source(s); and economic conditions that will affect a source's expected SO2 emissions. This commenter disagreed with the proposal's statement explaining why the Agency does not believe it necessary for air agencies to consider such factors, stating that the lack of a nationwide database with respect to such factors is irrelevant since the modeling is to characterize localized ambient air quality.
      d. 	Comments on discretion to apply data requirements to additional areas

	Several state and tribal commenters requested clarification of criteria the EPA would use to determine additional areas to be characterized beyond those with sources emitting more than the applicable threshold. A few commenters offered specific recommendations, for example to characterize areas of 10 kilometer or 25 kilometer diameters in which total emissions exceed the threshold but those of no single source exceed the threshold. A few commenters recommended that the EPA should not have the discretion to subject additional areas to characterization unless total emissions in the areas exceed the applicable threshold. Some commenters recommended that the rule specify criteria to be used to identify multi-source areas that would need to be characterized. Conversely, some commenters recommended that the EPA not codify any specific criteria, recommending instead that the EPA provide guidance on how it envisions addressing areas with multiple sources and rely on the professional judgment of air agency personnel in consultation with the EPA to identify specific additional areas that warrant being characterized. Also, one state commenter recommended that any area "that, based on the state's knowledge, has the potential to exceed the NAAQS" should become subject to requirements for air quality characterization. Finally, a few industry commenters and a few state commenters urged that the EPA not have the discretion to subject additional areas to DRR requirements.
3. 	EPA Response
	The EPA considered the many and varied comments received on the source threshold options presented in the proposal. After considering the comments received and as explained below, the EPA has decided to establish a requirement for air agencies to identify all sources with annual SO2 emissions that exceed 2,000 tpy (using emissions data from the most recent calendar year for which such data are available) and characterize air quality around such sources according to the timeline in section C of this preamble. The following subsections also address the other comments relating to applicability of the requirements for air quality characterization described previously.
      a.  	Emissions metric
      The EPA agrees with the many commenters who expressed support for using an annual emissions metric because annual emissions data are most readily available for all large SO2 sources, whereas 1-hour emissions rate information is not readily available for all SO2 sources. Since the tpy emissions metric is a common denominator in the emissions inventory and reporting universe, the EPA believes that the use of this metric is most appropriate to be required under a rule that applies broadly to areas with sources that do not already measure 1-hour emissions rates. Using tpy will provide air agencies and the regulated community a common, easily verifiable, straightforward approach for identifying sources around which air agencies are required to characterize air quality. This approach will rely on existing emission inventory collection systems that are already in place. An approach based on tons of emissions per year also should reduce unforeseen or otherwise uneven application of the requirements for air quality characterization that could arise if different metrics are used for different SO2 source sectors to identify areas for which air agencies are required to characterize air quality.
	The EPA acknowledges that some state commenters suggested inclusion of a 1-hour emissions rate-based criterion for identifying certain sources with infrequent, episodic SO2 emissions at atypically high rates that could impact nearby populations. The EPA notes that the emissions threshold included in the final rule establishes only minimum requirements for identifying sources. The EPA agrees with state commenters who recommended that air agencies should also characterize areas that, based on their knowledge of sources and areas, may be at risk of violating the standard. Thus, under this rule air agencies could also require characterization of air quality near sources prone to episodic emissions with relatively high rates or amounts, as appropriate. However, because short-term emissions data are not available for all SO2 sources, the EPA did not include in this rule a minimum requirement for identifying source areas needing air quality characterization based on this metric.
      b. 	No separate threshold for areas with higher population
      The EPA considered the comments received on the issue of whether a lower emissions threshold should be included for areas with more dense populations (e.g., CBSAs greater than 1 million population). A number of commenters appeared to interpret the inclusion of a lower threshold for areas with higher population as being less protective of the public in less populated areas. The EPA wants to clarify that this was not the intention behind the population-inclusive options included in the proposed rule. The SO2 NAAQS, and all NAAQS, provide equal protection for citizens throughout the country. The proposed use of both population and emissions thresholds as a means to require air quality characterization was simply one approach to focus limited federal and state modeling and monitoring resources into characterizing locations where a greater coincidence of people and SO2 emissions occur, and thus a potentially greater potential for exposure is presented. After reviewing the comments on this issue, however, the EPA has decided not to move forward with the proposed preferred approach, and instead to apply requirements for air quality characterization based on emissions levels uniformly across the country for both more urbanized and less urbanized populations so as to focus primarily on the size of the sources.
	It should be noted here that any monitoring that occurs pursuant to this rulemaking is potentially in addition to, or can possibly help to satisfy, required SO2 monitoring stemming from 40 CFR part 58, appendix D, section 4.4. Those monitors required in 40 CFR part 58, appendix D, section 4.4 are determined using a unique metric that accounts for the coincidental occurrences of SO2 emissions and population, namely the Population Weighted Emissions Index (PWEI). This rulemaking does not supplant or otherwise modify those existing requirements.
      c. 	Emissions threshold of 2,000 tpy
      Regarding the comments EPA received expressing preferences on the proposed emission threshold options, the EPA notes the wide range of views. A few commenters recommended alternate thresholds in the range from 1,000 tpy to 10,000 tpy, or recommended pairs of thresholds within this range. Some commenters provided modeling analyses as an indication that sources larger than 12,000 tpy did not cause a violation of the standard, while other commenters recommended a single emissions threshold of 1,000 tpy and provided modeling analyses of different sources as an indication that sources less than 2,000 tpy caused modeled violations. These comments demonstrate that ambient SO2 impacts can be variable, and are dependent on many factors other than annual emissions (such as meteorology, stack height, local topography and plant operations). These factors can only be assessed through analytical approaches, such as ambient monitoring or air quality modeling, which take many of these related factors into account simultaneously. These comments demonstrate why air quality characterization of the area around these sources is needed to protect public health in the first place.
	The EPA believes that, for the purposes of establishing a minimum threshold that prioritizes the resources that will be devoted to characterizing air quality near SO2 sources nationally, the 2,000 tpy source emissions threshold strikes a reasonable balance between the need to characterize air quality near sources that have a higher likelihood of contributing to a NAAQS violation and the analytical burden on air agencies. This threshold is on the lower end of the range of thresholds recommended by commenters because sources on the lower end of the range have the potential to cause or contribute to violations of the NAAQS. As compared to the preferred option in the proposal (i.e., 1,000 tpy sources in CBSAs over 1 million people; 2,000 tpy sources not in CBSAs over 1 million people), the 2,000 tpy threshold would mean that, in the aggregate, air agencies would need to address air quality near about 35 fewer sources (or 7 percent fewer). Nevertheless, the total emissions addressed would still account for 89 percent of the SO2 emissions nationally (based on 2011 emissions), very close to the 90 percent level that has been considered to be reasonable by many stakeholders in the past. National SO2 emissions have declined by a significant amount from 2011 to 2013 (around 1.5 million tons, or more than 20 percent), for various reasons. The EPA assessed the number of sources meeting a 2,000 tpy threshold based on 2013 emissions data now available for EGUs and 2011 emissions data for non-EGUs. Compared to the assessment in the proposal, which assessed the number of sources meeting the proposed threshold (1,000 tpy in urban areas/2,000 tpy elsewhere) based solely on 2011 data, the EPA now estimates that approximately 70 fewer sources (about 15 percent) will need nearby air quality to be characterized than was estimated in the proposal. Based on the updated data, the EPA estimates that already-designated sources plus sources currently exceeding the final threshold in this rule still would account for 86 percent of national emissions. Under this rule, each air agency will be required to identify all sources with annual SO2 emissions that exceed 2,000 tpy (using emissions data from the most recent calendar year for which such data are available) and characterize air quality around such sources according to the timeline in section IV.C of this preamble.
      Of course, if the trend in reduction of SO2 emissions continues at individual sources, there will also be a corresponding reduction in national emissions, and both kinds of reductions are desirable for improving public health, even if that results in fewer source areas becoming subject to the emissions characterization requirements in the final rule. Conversely, if the trend reverses and source emissions increase, more sources and areas will be required to be characterized under the rule. Thus, the exact number of sources and areas that will exceed the promulgated threshold when air agencies begin characterizing areas under the rule cannot be precisely known at this time, nor can their future percentage share of the national inventory be precisely estimated. Nevertheless, the EPA believes that the promulgated threshold strikes a reasonable balance based on the information the Agency currently has regarding recent historical SO2 emissions inventory levels.
      d.	Discretion to require air quality characterization in additional areas
	Section 114(a)(1) of the CAA already provides the EPA authority and discretion to require emissions sources to install, use and maintain monitoring equipment and provide other information as the Agency may reasonably require, even in the absence of this data requirements rule. In addition, the EPA had several reasons for proposing as part of this rule to reinforce state and the EPA discretion to require air quality characterization around additional sources beyond those with emissions above the proposed thresholds. The purpose of proposing the use of emission levels as the criterion for determining applicability of the air quality characterization requirement is that emissions provide a simple means of identifying the sources that are most likely to cause or contribute to violations of the SO2 NAAQS. Nevertheless, the EPA recognizes that a variety of factors other than emission levels can influence the likelihood of NAAQS violations. As one example, source characteristics such as stack height and plume buoyancy can significantly affect source impacts. As another example, clusters of multiple smaller sources that are in close proximity can cause as much impact as a single larger source. Finally, the EPA recognizes that a variety of other reasons may exist that may warrant further characterizing air quality in particular areas, which supports maintaining state and EPA Regional Administrator discretion to require air quality characterization in the area. The EPA continues to believe that states and the EPA should retain this authority and that it would be unreasonable to restrict implicitly, via this rule, the inherent authority that air agencies already have to require sources of air pollution to measure their emissions and characterize their impacts.
      For these purposes, the EPA continues to believe that the rule should make clear that states and the EPA retain the discretion to subject additional areas to the requirements for air quality characterization beyond areas with a single source exceeding the emissions threshold. The use of a simple emission threshold in the rule provides a convenient means of administering the application of the requirements for air quality characterization for the majority of cases. However, the impacts of a given level of emissions vary substantially, such that many areas with a source or sources that do not exceed the emission threshold might be known to have a high risk of contributing to NAAQS violations, potentially resulting in a higher risk of NAAQS violations than other areas exceeding the emission threshold. As a result, a rule that sets forth minimum requirements based on an emissions threshold cannot reasonably be used to support an assumption that no further characterization near smaller sources is warranted, or to preclude authority that air agencies already have to investigate the impacts of such sources. Therefore, while this rule requires the air quality characterization near the above-threshold sources, the EPA and air agencies will also need to consult regarding the need for the characterization of air quality near sources below the threshold as well.
       Among cases in which no single source meets the applicable emission threshold, no simple indicator is available to indicate which of these cases warrants air quality characterization. For areas with a single source, the areas could warrant air quality characterization if the stack height is low, if the plume rise is minimal, if terrain or meteorology is conducive to high impacts, and/or if emissions are just slightly below the threshold. For areas with multiple sources, concentrations are influenced not only by these stack, terrain and meteorological factors but also by the level of emissions at each source and the distances and wind directions between the sources. The EPA appreciates the comments urging the establishment of specific criteria in the rule for identifying additional areas that warrant air quality characterization, but the EPA finds that these areas are better identified on a case-by-case basis reflecting a judgment considering the range of factors that influence the likelihood of NAAQS violations. That is, the EPA agrees with the state commenter urging that the rule provide for discretionary coverage of additional areas, such that additional areas that in the air agency's (and the EPA's) judgment have significant potential for violating the NAAQS can be made subject to requirements for air quality characterization on case-by-case bases.
      Consequently, the EPA is retaining the discretion for air agencies and the EPA to require additional areas to be characterized beyond those with a source exceeding the emission threshold. However, the EPA is not revising the rule to establish specific criteria for identifying such areas; the EPA is instead relying on case-by-case evaluation of the various relevant factors to determine which additional areas warrant air quality characterization.
      For areas with multiple sources, the EPA recognizes that a number of such areas may have no single source that exceeds the threshold discussed earlier and yet may have concentrations similar to other areas with a single source exceeding the threshold. Commonly, such areas would have multiple sources clustered in relatively close proximity and would have total emissions at or above the threshold. The EPA envisions the air agencies and the EPA evaluating multiple source areas on a case-by-case basis to determine whether the areas warrant the same priority as areas where a single source has emissions above the threshold. Generally, the EPA strongly recommends that areas with multiple sources, where the combined impact would be expected to be as much as the impact of a typical single source emitting at least 2,000 tpy, should be carefully considered for air quality characterization, and we expect EPA Regional Administrators to focus on such areas in exercising their discretion. As stated previously, a rule that sets forth minimum requirements based on an emissions threshold cannot also be reasonably used to support an assumption that no further characterization near smaller sources may be required. Therefore, in addition to requiring air quality characterization near sources above the emission threshold, the rule also preserves the discretion of the EPA and air agencies to require air quality characterization in additional areas, which will necessitate consultation on a case-by-case basis regarding the need for characterization of additional areas beyond those containing a source exceeding the threshold in this rule.
      Regarding the comments recommending specific criteria for subjecting multiple source areas to the requirements for air quality characterization, the EPA believes that too many factors influence the combined impact for the EPA to establish a single set of criteria for determining whether each area warrants becoming subject to the requirements for air quality characterization. Nevertheless, for EPA and state agencies considering using their discretion to require characterization of additional areas, the EPA believes that the recommendations of these commenters provide good suggestions for where to begin making such decisions, to be followed by a case-by-case judgment as to the expected degree of combined impacts.
      In numerous cases, areas include multiple operations that previously were all part of a single source that now for business reasons have subdivided their ownership, such that the operations that previously were a single source must now be considered multiple sources. For example, in many cases, where previously the area had a single integrated iron and steel mill, the iron- and steel-making operations now have separate ownership from the coke-making operations, such that the former single source has now become two sources. In these cases, an additional equity concern arises, that otherwise comparable facilities should not be treated differently based on a business decision that has no effect on air quality. If the combined emissions of these now separately-owned operations exceed 2,000 tpy, the impact would commonly be similar to the impacts of single facilities emitting over 2,000 tpy, and such groups of separately owned operations would thus warrant air quality characterization.
      Regarding the commenters who recommended that the EPA stipulate that an area with multiple sources emitting less than the threshold should not be required to characterize air quality under the rule unless the combined emissions exceed the threshold, the EPA does not agree with this approach. Even for single source areas, the EPA is preserving the discretion air agencies and the EPA already have to require air quality characterization where the source emits less than the threshold but where concern about potential NAAQS violations warrants further air quality characterization. By the same logic, the combined impacts of multiple sources may warrant further characterization even if the combined emissions are less than the threshold.
C. 	Data Requirements and Program Implementation Timeline
1. 	Overall Timeline
 	a.	Summary of Proposal
      In the proposed rule, the EPA proposed an implementation timeline addressing feedback and concerns raised in previous stakeholder meetings, which the EPA considered to provide air agencies with sufficient flexibility and time to pursue either improved monitoring or modeling to characterize air quality. The EPA designed the schedule to allow air agencies to account for SO2 reductions that will occur over the next several years as a result of implementation of national and state level programs and facility decisions for complying with such requirements (such as the Mercury and Air Toxics Standards). The EPA solicited comments on the feasibility of the implementation of the proposed timeline. See 79 FR 27446, 27456, May 13, 2014. The notice of proposed rulemaking also included a discussion of when the EPA envisioned the information could potentially be used in designation actions.
      b. 	Brief Summary of Comments
      Several state and industry commenters agreed that the EPA's proposed timeline was reasonable for acquiring data by either modeling or monitoring, and for evaluating the submitted data. Many also agreed that it would be a reasonable schedule for supporting the issuance of designations and submittal of any SIPs, provided future schedules for those actions accommodate the schedule for implementing the rule. However, a larger number of state and industry commenters asserted that the time allotted for installation of monitors was not sufficient. One state commenter stated that the feasibility of the schedule will depend upon the threshold option selected by the EPA. Another state commenter supported the timeline that the EPA proposed as long as the EPA finalizes the rule by late 2014 and added that, if promulgation is delayed, the timeline should be adjusted by as many weeks or months as the delay in finalizing the rule. Some state and industry commenters recommended an extension of at least 1 year on all the proposed actions listed in the implementation timeline. Other commenters felt that the proposed timeline was flawed for multiple reasons and is therefore not achievable.
      c. 	EPA Response

      The EPA recognizes the logistical and financial challenges that were identified by commenters with respect to the timeline. In response, the final rule contains changes to provide additional time for air agencies to determine whether to use modeling or monitoring to characterize air quality near their affected sources, discussed later in this section. However, the final rule retains the proposed deadlines for commencing monitoring or providing modeling. The Agency acknowledges that these deadlines do not provide as much time as some commenters would prefer; however, the EPA believes that these deadlines can be achieved with the appropriate planning, coordination, and program implementation by air agencies. The EPA notes that if air agencies conclude that the timeline and resource burdens associated with installing and conducting improved monitoring are less feasible for particular areas, they may instead choose the modeling approach, which is generally less expensive and can be performed more expeditiously than monitoring, to characterize air quality. Alternatively, in some cases the source owner and the air agency may be able to establish by January 2017 a federally enforceable requirement limiting emissions to less than 2,000 tpy, with the result that further modeling or monitoring in that area would not be required under the rule unless air agencies or EPA Regional Administrators conclude it is otherwise warranted. Because the purpose of this rule is to obtain improved air quality information in an efficient manner in order that these data may be used in future actions (such as area designations, redesignations, or other actions designed to ensure attainment of the 2010 SO2 NAAQS) to protect the public from the short-term health effects associated with exposure to SO2 concentrations that exceed the NAAQS, the EPA believes it would not be appropriate to further extend the timelines for air quality characterization in the rule.
      The EPA believes that any further delay in air quality characterization around sources identified as a result of this rulemaking would delay the implementation of the SO2 NAAQS and therefore would impede public health protection in areas that in the future show violations of the standard based on the data to be gathered under the rule. The EPA also believes that any significant delays in monitors becoming operational past the date of January 1, 2017, will certainly delay the potential for monitoring data to be used to inform actions that depend upon ambient concentration assessments, possibly past calendar year 2021. Finally, the EPA notes that under the terms of the March 2015 consent decree, in order to avoid the EPA being required to designate an area by December 31, 2017, an air agency will need to have installed and begun operating the new SO2 monitoring system no later than January 1, 2017.
      The Agency believes that it is very important to maintain the proposed timetable for conducting modeling and installing monitoring sites because of the need for these new data to be available to support future determinations concerning the attainment status of areas. The EPA encourages each air agency to engage in early dialogue with the appropriate EPA Regional Office and with the identified applicable facilities in order to meet the requirements of the rule. In particular, in light of the reality of the sometimes complex process of identifying potential monitoring locations, securing funding, and installing an appropriate number of new sites, if an air agency is considering the monitoring approach for one or more areas, early coordination should improve the air agency's potential for success in meeting the timing and requirements of the rule.
      The final rule retains the January 15, 2016, date for submittal of a list of sources, because the EPA expects that this information is relatively straightforward to obtain, and it is beneficial for planning purposes to have this list available as soon as possible. However, as mentioned previously, in light of comments, the EPA is promulgating a schedule that provides an additional six months for the air agency to specify how it plans to address the area around each listed source. The EPA is promulgating a schedule in which July 1, 2016, is the deadline for selecting among the monitoring approach, the modeling approach, or establishing source emission requirements. If the air agency selects the monitoring approach for a source area, it must also include in the annual monitoring plan (also due by July 1, 2016) information about any new monitoring sites it will establish by January 1, 2017. If the air agency selects the modeling approach for a source area, it must also submit a modeling protocol at that time. If the air agency chooses the option of establishing an enforceable source limit or limits as an alternative to air quality characterization, it must also at that time provide a description of the planned emission limitation, including such information as emission rate, averaging time, and expected legal mechanism for making the limitation federally enforceable. To suffice as an alternative to the characterization requirement, the emission requirements or limits would need to be adopted by the air agency, made federally enforceable, and require compliance by January 13. 2017. Further discussion of the rationale for these revisions to the timetable is provided in the relevant subsections that follow. Table 1 shows the final rule timetable, including this revision.
                   Table 1: Timeline for DRR Implementation
Date
Action
From promulgation of this rule to January 15, 2016
Air agency and the EPA Regional Office consult on list of SO2 sources; air agency submits its list of sources to EPA by January 15, 2016.
July 1, 2016
Air agency specifies for each source whether it will characterize air quality with modeling, characterize air quality with monitoring, or establish a federally enforceable requirement limiting annual emissions of the source to less than 2,000 tpy. For source areas to be modeled, the air agency submits a modeling protocol. For source areas to be monitored, the air agency submits information about any new monitoring sites it will establish by January 1, 2017. For areas where enforceable emission limits will be established as an alternative to air quality characterization, the air agency submits a description of the planned emission limit.
January 1, 2017
Air agency ensures that SO2 monitors to satisfy the Data Requirements Rule are installed and operational.
January 13, 2017
For any source identified for modeling pursuant to the July 1, 2016, milestone, air agency submits modeling analyses. For any source identified for emission limit approach, air agency submits documentation showing that limits requiring annual emissions to be less than 2,000 tpy are effective and federally enforceable.
May 2020
For any source area identified for monitoring approach, air agency certifies 2019 monitoring data, enabling official design values for the 2017-2019 time period to be calculated.

      In addition, while the proposed rule discussed how the timing of the implementation of this rule would fit with the anticipated schedule for completing area designations, the proposed rule did not itself purport to establish a binding schedule for completing designations. Table 2 provides preliminary information concerning a possible schedule for taking action to designate areas in the future, but is intended for informational purposes only. In this rulemaking we are not addressing comments received on the proposed rule concerning the designation process because those issues would be beyond the intended scope of this rulemaking.
      Table 2: Anticipated Schedule for Future Rounds of SO2 Designations
July 2016
Date by which the EPA must issue final designations for sources meeting specific criteria in the March 2015 consent decree.
August 2017
Expected date by which the EPA would notify states of intended designations based on air quality data obtained pursuant to the first round of the data requirements rule.
December 2017
Date by which the EPA must issue final designations for a majority of the country (pursuant to March 2015 consent decree), except for areas with new monitoring networks commencing operation by January 1, 2017.
August 2019
Anticipated due date for state attainment plans for areas designated nonattainment in 2017.
May 2020
Certification of 2019 monitoring data is required by this date.
August 2020
Expected date by which the EPA would notify states of intended designations for the remainder of the country.
December 2020
Date by which the EPA must issue final designations for the remainder of the country (pursuant to March 2015 consent decree).
August 2022
Anticipated due date for state attainment plans for areas designated nonattainment in 2020.

2.  	Issues Related to Submittal of List of SO2 Sources Where Air Quality is to Be Characterized, and Election of Modeling or Monitoring
      a.	Air agency submits its list of sources where air quality is to be characterized
      i  	Summary of Proposal
      In §51.1203(a), the EPA proposed to require each air agency to submit to its respective EPA Regional Administrator by January 15, 2016, a list identifying the specific sources in the state around which SO2 air quality is to be characterized. The EPA stated that this proposed requirement for the air agency to submit a list of source areas identified for further air quality characterization, and the other data submittal requirements found in §51.1203 of the proposed rule, are appropriate steps necessary to understand SO2 air quality throughout the country, and are consistent with sections 110(a)(2)(B), 110(a)(2)(K) and 301(a)(1) of the CAA. In the docket, the EPA provided a preliminary list of sources that appeared to meet the EPA's proposed thresholds (based on 2011 emissions data), and the EPA solicited comments on this list. See 79 FR 27446, 27461, May 13, 2014.
      ii.   	Brief Summary of Comments
      Some state and industry commenters opposed the requirement that, by January 15, 2016, air agencies must submit a list of sources. Some commenters also stated that submitting a list of sources is unnecessary for various reasons such as data is already made publicly available on an annual basis through the national emissions inventory; that it does not make sense to establish a list that is expected to change; and that air agencies and the EPA can work cooperatively without a binding requirement. Commenters also recommended that any listing of sources, and any identification of the selected air quality characterization approach for specific source areas, should wait until the January 2017 analysis for individual sources or areas is to be completed. One state commenter indicated that they did not find merit in the citations that the EPA provided in the proposal regarding the authority for requiring this list submittal. This commenter stated that the section 110(a)(2) citations address the requirements for SIP submittals by states for implementation, maintenance and enforcement of the standard. Several state commenters also suggested updates or revisions to the EPA's preliminary list of sources potentially subject to this rule.
      iii.  	 EPA Response
      The EPA does not agree with commenters who claim that submittal of an initial list of sources near which air quality is to be characterized is not needed in January 2016. The EPA believes that it is important to receive the list of source areas to be characterized under the rule by January 15, 2016, because it will provide timely clarity for both EPA and the air agency about which sources and associated areas are to be characterized for air quality under this rule. See section 51.1203(a).  In EPA's judgment, such timely clarity is essential to the success of the characterization efforts that follow the source identification step. The list will identify the sources in the state that exceed the 2,000 ton emissions threshold based on the most recently available emissions data, as well as any other sources or groups of sources identified by the air agency or the EPA Regional Administrator as warranting air quality characterization . Development of this initial list will be important for air agencies as they prepare to generate timely air quality information that may be used to inform future designation, redesignation, or other decisions concerning whether sources are affecting the attainment of areas.
      Retaining this deadline will provide the early opportunity for the air agency and the EPA to discuss and resolve questions about whether air quality characterization should be required for a particular area if, for example, emissions are low in some years and high in others, if an area has a cluster of smaller sources, or if source-specific factors may warrant air quality characterization. As a further example, there may also be situations for which the state and the EPA need to reach agreement on what constitutes the most recent year of emissions data for specific EGU and non-EGU sources. The list requirement and deadline will ensure resolution of such questions in time to enable further characterization requirements to be met.
      Thus, the EPA is retaining the January 2016 deadline, as proposed, for submittal of the list of sources in order to initiate an orderly process to obtain additional information on ambient SO2 concentrations, and ensure these data are available to support actions taken for the implementation of the 2010 SO2 NAAQS. While the Agency has previously acknowledged that some of the deadlines in this rule do not provide as much time as some commenters would prefer, the EPA believes that the schedule for providing the list of sources is a relatively straightforward exercise that can be accomplished within the required time frame.
      The EPA strongly encourages each air agency to consult with its respective EPA Regional Office to identify sources exceeding the emission threshold in the final rule, and to identify any other areas near sources that do not exceed the emission threshold but which would be appropriate for further air quality characterization. It will be important for air agencies and the EPA to carry out this consultation process as early as possible and to reach agreement on the list of sources to characterize under the rule as quickly and efficiently as possible. It is also important to note that, due to the overlap between the criteria for inclusion of sources in this final rule and those in the March 2015 consent decree, all of the sources identified in the March 2015 consent decree should also be included on the January 2016 list of sources required for characterization under this rule. The consent decree requires the designation in July 2016 of areas associated with an initial list of sources meeting specific criteria. Depending on the specifics of those designation actions, information developed to support those actions may serve to meet some or all of the requirements of this data requirements rule. (See section IV.E, Other Key Issues and Comments, for more discussion of these issues.)
      Regarding comments about EPA's authority to require submittal of a source list, the EPA believes that the requirements of this rule for air agencies to submit a list of source areas identified for further air quality characterization, and the other data submittal requirements found in §51.1203 of this rule are appropriate steps needed to better understand SO2 air quality throughout the country, and that including such requirements is consistent with sections 110(a)(2)(B), 110(a)(2)(K), and 301(a)(1) of the CAA.
      Section 110(a)(2)(B) indicates that state SIPs are to "provide for establishment and operation of appropriate devices, methods, systems, and procedures necessary to (i) monitor, compile and analyze data on ambient air quality and (ii) upon request, make such data available to the Administrator." Section 110(a)(2)(K) states that SIPs shall "provide for (i) the performance of such air quality modeling as the Administrator may prescribe for the purpose of predicting the effect on ambient air quality of any emissions of any air pollutant for which the Administrator has established a NAAQS and (ii) the submission, upon request, of data related to such air quality modeling to the Administrator." Although both of these provisions direct what air agencies are required to include in SIPs, they clearly support the authority of the EPA to prescribe requirements that the information that SIPs are to ensure can be provided is collected in the first instance.
       In addition, section 301(a)(1) provides the EPA with general authority to establish regulations as necessary to carry out the agency's functions, which in this case includes ensuring that additional information is collected and provided so that air agencies and the EPA can ensure attainment and maintenance of the SO2 NAAQS throughout each state. Finally, the EPA notes that CAA section 114(a)(1) also provides broad authority for the EPA, for the purposes of developing any implementation plan under section 110 or carrying out any provision of the CAA, to require monitoring and provision of other information the Agency may reasonably require (such as modeling information).
      The EPA appreciates the comments on the preliminary list of sources that appeared likely to be subject to this rule as proposed. The EPA acknowledges that, for various reasons, such a list of sources could change up until the time that the list is required to be submitted. Accordingly, such a list is not being promulgated as part of this rule. The EPA plans on continuing consultations with air agencies regarding the source areas that the final rule will require to be characterized.
      b. 	 Choose monitoring or modeling
      i.  	Summary of Proposal
      In §51.1203(b), the EPA proposed to require each air agency to state whether  it will characterize air quality through improved ambient air quality monitoring or through air quality modeling techniques by January 15, 2016. The EPA also proposed in §51.1203(b) that in an area with multiple subject sources, the air agency (or air agencies if a multi-state area) shall use the same technique (monitoring or modeling) to characterize air quality for all sources in the area. For situations where multiple sources are located in proximity across state boundaries, the EPA recommended that the relevant air agencies work together to determine a common analytical approach for assessing air quality in that area. See 79 FR 27446, 27460, May 13, 2014.
      ii.  	Brief Summary of Comments
      Several state and industry commenters stated that the EPA should provide a more reasonable schedule for air agencies to elect the monitoring option under the proposed rule. Some commenters suggested that air agencies should have until January 1, 2017, to make this determination because they could benefit from using initial modeling results to inform this decision, such flexibility would reduce burdens on state regulators, and it could lead to more accurate determinations, while not impacting the EPA's expected attainment dates for such areas should the areas become designated nonattainment.
      iii.  	EPA Response
      In response to these comments, the EPA is providing additional time for making the election of modeling or monitoring (or, as discussed later, for making the election of an alternative approach that enforceably limits an applicable source's emissions). Accordingly, the deadline for this election will be July 1, 2016. The EPA recognizes that evaluating the relative merits of modeling and monitoring for any particular area, including identification of funding sources for any new monitoring that might be under consideration, warrants more time than was provided under the proposed rule. Consistent with this revision, the EPA is also revising the deadline for air agencies using modeling to submit modeling protocols for the applicable areas. Thus, under the final rule, by July 1, 2016, the air agency must submit its selection of whether each area will be characterized through modeling or monitoring and, depending on that selection, either must submit a modeling protocol or must include information in the Annual Monitoring Network Plan that specifies the monitoring to be conducted to address the requirements of this rule. The EPA believes that this revised deadline still provides for timely planning for air quality characterization to occur (through modeling) or begin (through monitoring) at the beginning of 2017. Conversely, the EPA does not agree that any later deadline for selecting the means of addressing air quality characterization requirements would provide the time and flexibility to address in a timely way any issues that arise after the selection is made. The result would be that a later deadline for this selection could jeopardize timely receipt of information characterizing air quality.
      Notwithstanding this revision, the Agency encourages air agencies to start their investigation of this issue as soon as practicable. The EPA strongly encourages each air agency to consult with its respective EPA Regional Office to identify sources exceeding the emission threshold in the final rule and any other sources that do not exceed the emission threshold but near which further air quality characterization would be warranted. Similarly, the EPA strongly encourages air agencies to hold early discussions regarding the manner in which modeling or monitoring might be used. As one example, if the air agency believes that the existing monitoring network suffices to characterize air quality, early discussions with the EPA would be essential for assuring that the intended selection of monitoring is based on appropriate assumptions regarding the network's ability to characterize air quality near the applicable source(s) without further network adjustments.
      c. 	Use most recent publicly available data
      i.  	Summary of Proposal
      In §51.1202, the EPA proposed that the air agency should identify applicable sources of SO2 based on the most recent publicly available annual SO2 emissions data for such sources. The EPA specified in proposed §51.1200, that "annual SO2 emissions data" means the quality-assured annual SO2 emissions data for a stationary source as reported to the EPA in accordance with any existing regulatory requirement (such as requirements to report continuous emissions monitoring data for EGUs subject to the acid rain program). The EPA stated that, by January 15, 2016, data for 2014 would be available for EGU sources and 2013 data would be available for non-EGU sources. By considering the most recent emissions data, the EPA noted that air agencies and the EPA will be able to take into account any recent emissions increases or decreases that would cause a source to be subject to the requirements in this proposed rule. The EPA included in the docket to the proposed rule a preliminary list of sources that appeared to meet the criteria described in the EPA's proposed source threshold approach and requested that air agencies provide in their comments on this proposed rule any relevant updated information that would support the addition or removal of a source from that preliminary list. See, 79 FR 27446, 27457, May 13, 2014.
      ii.  	Brief Summary of Comments
 	 Several state and industry commenters generally supported the approach that the basis for the emissions to be compared to the threshold would be the latest available 1-year of SO2 emissions data. One industry commenter stated that using the most recent year of data ensures that any recent emissions reductions that have occurred will be properly taken into consideration.
      One public interest group commenter stated that using the most recent year as a snapshot may fail to capture sources that simply have a low year, but normally emit at higher levels, and recommended that the EPA require that facilities only be excluded under the threshold if, in prior years, the facilities had similar low total emissions below the limit. A number of states provided information suggesting specific modifications to the EPA's preliminary list of sources.
      One commenter stated that the rule should not take an "all in" or "all out" approach based on a simple analysis of 1 year's emissions or even a 3-year average of emissions alone. The commenter stated that the EPA seems to allow, or consider, the potential addition of non-threshold-meeting sources but does not appear to recognize that there may be instances where the air agencies knowledge and judgment warrants exclusion of threshold triggering sources. They suggested that air agencies should be able to take into consideration operational changes during the 3-year period to determine if a different methodology is appropriate for determining if a source should be a part of the analysis.
 	iii. 	EPA Response
      The EPA continues to believe that the most appropriate generally applicable basis for determining applicability of the air quality characterization requirements is the most recent available year of emissions data for a stationary source as reported to the EPA in accordance with any existing regulatory requirement. As we have previously explained, SO2 emissions are trending downward, due to numerous national and regional requirements that have recently been adopted and are taking effect. The Agency believes it is reasonable to account for this trend by basing applicability for this data requirements rule on the most recent available year of emissions.
      By January 15, 2016, the EPA would expect that 2014 data will be available for all EGU sources, and 2015 data may be available for many EGUs in accordance with the requirements of the acid rain program and other emission trading programs that require data certification soon after the end of the year. These sources report hourly emissions data to the EPA on a quarterly basis. Emissions data for large SO2 sources also would be available from annual reporting required for the AERR. Every 3 years (i.e., 2011, 2014, 2017 and so on), air agencies must submit to EPA emissions data for SO2 sources with the potential to emit more than 100 tpy. In other years, the AERR requires states to report emissions data for SO2 sources with the potential to emit more than 2,500 tpy. These annual reports under the AERR are due 12 months after the end of the emissions year. Thus, the EPA would expect that in January 2016, states would have emissions data for calendar year 2014 available for non-EGU sources over 100 tons potential to emit. Emissions reporting requirements for the acid rain and AERR programs would be expected to cover the vast majority, if not all, of the sources subject to the SO2 data requirements rule.
      By considering the most recent emissions data, the air agency and the EPA will be able to take into account any recent emissions increases or decreases that would cause a source to be subject to the requirements in this rule or not. Although identifying sources based on the most recent year of emissions is a reasonable basis for prioritizing limited modeling and monitoring resources for characterizing current air quality, the EPA recognizes the concern of some commenters that there may be sources that in the most recent year have emissions that are lower than normal and are not representative of normal operations. In these cases, i.e., where recent emissions are below 2,000 tpy but no controls have been installed and past representative emission levels are typically above 2,000 tpy, the state and the EPA should consider using their discretion to require additional air quality characterization near such sources.
      The EPA also recognizes the concern about sources for which the most recent year's emissions are unrepresentatively high, i.e., that some sources may have recent year emissions above 2,000 tpy but normally emit below that level. Given the trends in emissions, the EPA believes that situation will be relatively rare. Moreover, the existence of such sources does not negate the general conclusion that recent emissions data are an appropriate means for targeting limited modeling and monitoring resources for characterizing current air quality.
      The EPA believes that a rule that prioritizes resources based on the most recent year's data is more appropriate for a broader range of circumstances. The EPA notes, however, that after a source is initially identified, the air quality characterization requirements require air agencies to provide at least 3 years of monitoring or modeling data. The availability of such data will provide the opportunity to give appropriate consideration to representative emissions when using such data, as appropriate to the specific use.
      d.	Shutdowns and limitations on emissions levels by January 13, 2017
      i.  	Summary of Proposal
      The EPA noted in the proposed rule that there may be sources in the power industry and other sectors that are in operation as of January 15, 2016, but may be scheduled to shut down (e.g., due to a consent decree or other legal requirement), or may choose to shut down, prior to January 2017 (when the air agency should have ambient monitors operational and air quality modeling completed). The EPA proposed that any applicable source that intends to shut down but is still in operation on January 15, 2016, should be included on the air agency's list for SO2 air quality characterization. However, if by January 13, 2017, the air agency can provide the EPA with a legal agreement confirming that the listed source has permanently and enforceably shut down, then under the proposal the air agency would have no further obligation regarding air quality characterization for this source pursuant to this rulemaking. See 79 FR 27446, 27458, May 13, 2014.
      ii.  	Brief Summary of Comments
      One state commenter recommended that the EPA revise the rule to exempt from the list those sources that take an enforceable emission limitation below the 2,000 tpy emissions threshold before January 13, 2017, even if reductions and applicability of the limitation are only realized within a reasonable time after January 13, 2017. Several commenters stated that there is no basis to distinguish between shutdowns and enforceable limits because in each case the source would no longer meet the criteria for characterization under the rule. Another commenter stated that sources should be able to take federally enforceable limits on a tpy basis prior to the January 13, 2017 date for air agencies to submit their modeling analysis to avoid characterization under the rule. Another state commenter stated that requiring sources to implement controls prior to submittal of future required SIPs would encourage sources to make emission reductions while allowing sufficient time to implement these actions.
      Some state and industry commenters recommended that sources should have until the applicable attainment date for a designated nonattainment area to complete any enforceable actions that achieve attainment, provided those actions are committed to by January 13, 2017. Commenters stated that there is insufficient time for sources to take all the actions needed to implement these controls (including conducting modeling, determining the required reductions and control strategies, procuring capital funds, obtaining permits and installing equipment) under the proposed rule. Commenters stated that allowing sources to implement controls after
January 13, 2017, but before future attainment dates supports the EPA's desired outcome of achieving emission reductions as quickly as possible; in contrast, under the EPA's proposal, sources unable to have enforceable limits in place by the January 13, 2017, deadline have little incentive to take any action prior to the anticipated designation deadline of 2020.
       iii.  	EPA Response
	The EPA is finalizing the proposed approach to allow a state with a source that is in operation as of January 15, 2016, but that will shut down permanently prior to January 13, 2017 pursuant to a federally enforceable mechanism (e.g., source-specific SIP revision or minor NSR permit revision submitted to the EPA by January 13, 2017), to avoid being subject to the requirement to characterize air quality in the vicinity of the source.
      As a result of comments received on the proposed rule, the EPA is clarifying how this exclusion would work relative to the requirement for development and submittal in January 2016 of the list of sources near which air quality is to be characterized. The EPA appreciates that there might be a source whose most recent year of actual emissions exceeds the threshold for inclusion on the list, but for which the state has already adopted, or will soon adopt, enforceable requirements to shut down by January 2017. Such a source may have significant emissions during the most recent available year, or may even still be in operation on January 15, 2016. The EPA has determined that the clearest way to implement the exclusion from the air quality characterization requirement is to require that the air agency initially identify such a source on its list for SO2 air quality characterization because emissions in the previous year, which serve as the basis for listing under this rule, exceeded the emissions threshold. However, the final rule now includes language in §51.1203(b) allowing the air agency to indicate by July 1, 2016 that it will provide the EPA with a federally enforceable requirement confirming that the source will be permanently and enforceably shut down by January 13, 2017. For a source with a federally enforceable requirement to shut down, the air agency will have no further obligation regarding air quality characterization pursuant to this rulemaking. This approach accomplishes the intent of the proposal by implementing the approach in a more clear and straightforward manner.
      Commenters on the proposed rule also suggested that, in a similar manner, an air agency should not be subject to the air quality characterization obligation for any source that is initially on the list of sources due in January 15, 2016 (based on most recent actual emissions), but that becomes subject to a federally enforceable requirement to limit annual SO2 emissions to below the 2,000 tpy emissions threshold. The EPA finds merit in those comments that suggest that the rule allow for similar treatment for sources that become subject to a federally enforceable emission limit as is allowed for sources that will be shut down. The EPA has revised the final rule accordingly, and provides further discussion below. However, EPA does not agree with commenters who suggest that sources should have until the applicable designation date, or attainment date for an area that is designated nonattainment, to implement controls that were committed to prior to January 13, 2017. Relying on commitments for emission reductions to occur after 2017 would not be consistent with the main focus of this rule, which is to provide current, updated information on priority SO2 sources to the EPA beginning in early 2017 that will inform future area designations (now required in December 2017 and December 2020 per the March 2015 consent decree).
      As indicated above, a source would be listed for air quality characterization if its most recent emissions were above the 2,000 tpy threshold. However, the final rule also allows the air agency to meet the requirements of this rule by submitting a federally enforceable emissions limitation (e.g., source-specific SIP revision or minor NSR permit revision) to the EPA prior to January 13, 2017 that requires the affected source to reduce allowable emissions at the source to an annual rate below the 2,000 tpy threshold level prior to January 13, 2017. By July 1, 2016, the air agency would be required to identify the sources on the list for which it would be using such an approach as an alternative to modeling or monitoring. For such a source identified on the list, if the affected air agency has adopted and the source has become subject to effective federally enforceable control measures lowering emissions below 2,000 tpy by January 13, 2017, the air agency will generally not be required to further characterize the impacts from the source's emissions solely due to its size as of January 15, 2016.
      Although air agencies may follow this option as an alternative to characterizing areas with sources that limit their emissions to below the 2,000 tpy size threshold, the EPA believes that air agencies and the EPA must apply judgment as to whether there are still reasons to characterize these areas due to other factors. As discussed above, some areas where all sources emit less than 2,000 tpy may nevertheless warrant air quality characterization, for example because the area has a cluster of sources with intermediate emission levels or because the characteristics of a source or the area warrant it. Thus, some areas with all sources limited to below 2,000 tpy may still warrant air quality characterization. Therefore, the EPA urges air agencies to consult early with the EPA regarding areas that are under consideration for being addressed in this manner, in order to develop a common understanding as to whether emission limits under consideration would suffice as an alternative to air quality characterization for the area.
      The EPA believes that allowance for this alternative emission limit approach is not only consistent with the intent of this rule to prioritize resources to focus on the largest sources of SO2, but it also has the additional benefit of providing an incentive for early emission reductions to occur which will improve air quality in these areas in an expeditious manner. However, we do acknowledge the distinction between a formerly large source with no future emissions and a source with reduced but continuing emissions. The Agency does not believe it would be appropriate to provide that the latter source can be excluded from evaluation in all cases. It may be that a source with emissions newly limited to below the applicability threshold  -  particularly one with limits established just below the threshold  -  may warrant further characterization, just as a source with actual emissions below the threshold may warrant characterization in some instances. For example, air quality characterization would continue to be warranted in areas with other sources over the applicability threshold, and in areas where no single source has emissions over the threshold but the combined emissions of multiple sources warrant air quality characterization. In evaluating such cases, the air agency should account for all source emissions contributing to ambient concentrations in the area, including those remaining emissions from the source that has just reduced its levels to below the applicability threshold. For this reason, the rule does not automatically exempt sources with emissions limited to less than 2,000 tpy from air quality characterization requirements; the rule instead provides that the air agency or the EPA may judge that the area should continue to be required to characterize air quality notwithstanding the new emission limits. Air agencies are thus advised to consult with their EPA Regional Office before pursuing this alternative to air quality characterization for a particular source area.
3.	Issues Related to Submittal of Modeling Protocols.

      a. Summary of Proposal

	For source areas that an air agency identifies are to be evaluated through air quality modeling, the EPA proposed in §51.1203(d) that an air agency must also provide a modeling protocol to the EPA Regional Administrator for review by January 15, 2016. In the proposal, the EPA stated that the EPA Regional Offices would review the submitted information and consult with the air agency as expeditiously as practicable, either approving the submitted information in a similar manner to approval of annual monitoring plan updates, or having further discussion with the air agency if adjustments to modeling protocols are warranted. See 79 FR 27446, 27458, May 13, 2014.
      b. 	Brief Summary of Comments
      Several commenters stated that 1 year is not enough time to complete modeling demonstrations. These commenters stated that depending on the scope of the modeling required, it would take 2 to 4 years to complete the entire process. The modeling time estimate will increase if refined modeling is required to site monitors and if the EPA expects the states to submit modeling protocols and not conduct any refined modeling to support monitor placement decisions until the EPA approves the protocols.
      Several state and industry commenters objected to the EPA oversight of the modeling protocols. Commenters were concerned that the EPA could not review the plans in a timely manner and could cause delays in the process. One state commenter stated that, if this oversight and approval is finalized in this rule, they have serious concerns about whether 2 years from promulgation of the final rule is a reasonable amount of time for air agencies to prepare the necessary data inputs and conduct such modeling for all subject sources. One state commenter suggested that the EPA should clarify that air agencies could provide to the EPA a modeling protocol framework for review and approval, and that source-specific review of protocols should be left up to the respective state agency, consistent with past practices in PSD SIP approved states as well as past practices supporting Best Available Retrofit Technology (BART).
      Some state and industry commenters recommended that the EPA oversight/approval of model protocols should be eliminated and air agencies should be able to determine the best approach, using the normal course of discussion and cooperation with their respective EPA Regional modeling contacts, and document that approach with the final submittal to the EPA. One industry commenter stated that it is arbitrary and capricious to require EPA approval of state monitoring and modeling plans when the EPA's technical resources are too stretched to provide this oversight in a timely manner.
      c. 	EPA Response
      The EPA recognizes the concerns of the commenters about the time and resources needed to develop effective modeling protocols. p clarify, the final rule does not require EPA approval of modeling protocols before air agencies may begin conducting modeling, but does direct air agencies to submit to the EPA modeling protocols by July 1, 2016. As with the modeling itself, directing submission of protocols is within the EPA's authority to prescribe modeling for the purpose of predicting the effect on ambient air quality of emissions under CAA section 110(a)(2)(K), and to prescribe such regulations as are necessary for the EPA to carry out its functions under CAA section 301(a)(1).  It is reasonable for the EPA to establish a process that provides an opportunity for preliminary EPA assistance to air agencies to ensure that their subsequent modeling is conducted in a manner that results in information that can reliably inform subsequent EPA actions determining air quality status under the SO2 NAAQS. As explained below, the submission of modeling protocols will reduce the likelihood that subsequent air agency modeling is insufficient for this purpose, and thus will clearly assist the EPA in carrying out its functions of determining air quality status.
      As noted above, the EPA is allowing air agencies approximately six additional months to determine whether to characterize air quality through modeling or monitoring in order to accommodate the concerns about time needed to make this determination, without delaying the date by which information for characterizing air quality becomes available. Consistent with this revision, the EPA is delaying the deadline for states to submit modeling protocols for sources for which they choose to characterize air quality through modeling, to match the July 1, 2016, deadline for selecting an air quality characterization approach. The EPA believes that it is important and valuable for the EPA Regional Offices to work closely with air agencies to ensure that modeling protocols are adequate to ensure that the modeling for sources accurately characterizes air quality near sources. Requiring modeling protocols will help to keep air agencies from getting too far into the modeling process in a manner that may not be appropriate, which could occur absent such preliminary consultation with EPA and, if it occurred, could result in the air agency needing to re-conduct modeling after submission to the EPA. Although EPA does not intend to formally approve these protocols, EPA believes that a process designed to reach agreement on a modeling protocol up front also would help to avoid a later situation in which the EPA would not be able to rely upon the air agency's modeling in subsequent actions determining air quality status. Review of modeling protocols by the EPA will help ensure that the air agency's modeling will be appropriate for use in making future determinations regarding areas' attainment status, such as designations or redesignations. If an air agency's modeling protocol is not submitted in advance of the subsequent modeling, the chances are greater that the EPA may not have critical air quality information when it is needed (for example, when the EPA intends to make area designations). Therefore, the EPA believes that a requirement for the air agency to provide modeling protocols for relevant sources to the EPA Regional Administrator by July 1, 2016 is a reasonable requirement. The modeling protocol should include information about issues such as emissions input data, modeling domain, receptor grid, meteorological data and how to account for background concentrations.