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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.
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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 

(202) 566-1744 and the telephone number for the Air and Radiation Docket Information Center 

is (202) 566-1742. 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 

wallace.larry@epa.gov; 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 

damberg.rich@epa.gov.

SUPPLEMENTARY INFORMATION: 
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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
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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 agencies1to 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 

 

1 The final rule applies to air agencies in all states. The definition of “state” in section 302(d) of 

the Clean Air Act (CAA or Act) means a state, the District of Columbia, the Commonwealth of 

Puerto Rico, the Virgin Islands, Guam, and American Samoa and includes the Commonwealth of 

the Northern Mariana Islands.
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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’s2subsequent 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.

3

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 

 

2 The term Agency is intended to refer to the EPA.

3 The May 2012 White Paper and high-level summaries of stakeholder meetings are available at: 

http://www.epa.gov/oaqps001/sulfurdioxide/implement.html. These documents and written 

comments received from stakeholders are also included in the docket for this rulemaking.
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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, 
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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 
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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,
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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 Stations4(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 

 

4 The SLAMS network is an air quality surveillance system that consists of a network of 

monitoring stations designated as SLAMS which measure ambient concentrations of those 

pollutants for which standards have been established in 40 CFR part 50. 
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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 
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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 
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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
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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
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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 
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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
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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 
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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
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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 
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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
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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.
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(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 
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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, 
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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 
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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 
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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, multipleyear 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.
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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:

- What would be an appropriate emissions metric for identifying sources?

- Should the threshold options require characterization of smaller sources in areas with 

higher populations?

- 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 
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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 
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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 
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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
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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
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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.
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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.
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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
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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.
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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
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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 
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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.

5 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 

 

5 The May 2012 White Paper and high-level summaries of stakeholder meetings are available at: 

http://www.epa.gov/oaqps001/sulfurdioxide/implement.html. These documents and written 

comments received from stakeholders are also included in the docket for this rulemaking.
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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
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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. 
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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 
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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
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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-bycase 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 
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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 
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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 
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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 
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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 
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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.
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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.

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.

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). 
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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 

August 2022 Anticipated due date for state attainment plans for areas 

designated nonattainment in 2020.
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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 
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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 
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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 
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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
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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 
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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 
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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 qualityassured 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
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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 nonthreshold-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
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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 
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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 
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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 
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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
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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 
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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 
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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 
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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
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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 
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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 
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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.

As was the case for the development of the list of sources and characterization 

approaches, the Agency acknowledges that the schedule for state submittal and the EPA review 

of modeling protocols is expeditious. The EPA nevertheless believes that the schedule can be 

achieved with appropriate planning, coordination, and program implementation by air agencies, 

and believes that it is necessary to establish expeditious timelines to ensure timely availability of 

the air quality information. The EPA Regional Office staff will be available to consult with air 

agency officials to refine the modeling protocols for relevant sources. The EPA Regional Offices 

will review the submitted information and consult with the air agency expeditiously to discuss 

any recommended adjustments to the protocol. 
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4. Issues Related to Submittal of Annual Monitoring Network Plans That Include SO2

Monitoring Network Modifications to Satisfy the DRR.

a. Summary of Proposal

In areas where air quality will be characterized through ambient monitoring to satisfy this 

rulemaking, the EPA proposed monitoring requirements in §51.1203(c), including the 

requirement that air agencies submit relevant information about these monitoring sites to the 

EPA Regional Administrator by July 1, 2016, as part of their annual monitoring network plan, in 

accordance with the EPA’s monitoring requirements specified in 40 CFR part 58. In the 

proposal, the EPA encouraged air agencies to work with the EPA Regional Offices in the 

development of an appropriate network plan which would include the rationale for why the 

proposed number of sites and their individual locations are appropriate. The EPA stated in the 

proposal that optional considerations for siting these monitors are discussed in the draft

Monitoring TAD.6See 79 FR 27446, 27458, May 13, 2014 and proposed §51.1203(c).

b. Brief Summary of Comments 

Several state and industry commenters asserted that it is unreasonable for the EPA to 

assume monitoring plans can be submitted by the proposed July 1, 2016, deadline. Some 

commenters stated that it may not be determined that monitoring would be appropriate in certain 

areas until after a lengthy round of initial modeling is complete. Other commenters stated that 

siting monitors is a lengthy process which involves, among other steps, working with the sources 

and the EPA to determine where monitors should be located, obtaining access to sites, 

 

6

The SO2 NAAQS Designations Source-Oriented Monitoring Technical Assistance Document 

can be found at http://www.epa.gov/airquality/sulfurdioxide/pdfs/SO2MonitoringTAD.pdf.
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identifying funding, and procuring and installing equipment. Furthermore, one commenter stated 

that, for sources that choose to operate monitoring equipment, additional time will be needed to

(1) develop documentation between air agencies and sources to ensure that sites are adequately 

maintained and that data are reported in a timely and complete manner, and (2) to put in place a 

quality assurance program consistent with the EPA requirements for the entire monitoring 

network. 

c. EPA Response

The EPA is finalizing the requirement that any plans to conduct monitoring to satisfy

requirements of this rule (by air agencies, industry, or other parties) shall be reflected in the 

state’s Annual Monitoring Network Plan due by July 1, 2016. The Agency believes that 

monitoring resources can be appropriately put in place by the January 1, 2017, deadline to satisfy

this rule, particularly if air agencies begin planning as soon as possible. The EPA has encouraged 

air agencies to begin the monitor planning process early, particularly for the largest sources. As 

stated previously, the EPA believes that while the schedule for meeting the requirements of this 

rule is expeditious, the schedule can be achieved with the appropriate planning, coordination, and 

program implementation by affected air agencies. The EPA strongly encourages air agencies to 

start their investigation of this issue as soon as practicable. The EPA also 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 which would warrant the characterization of nearby air quality. In addition, as 

stated previously, the EPA believes that it is necessary to establish expeditious timelines to 

ensure timely availability of air quality information. With this in mind, and in light of the many 
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logistical concerns raised by commenters and recognized by the EPA, the Agency is encouraging 

air agencies to engage with their respective EPA Regional Offices well in advance of the time by

which the Annual Monitoring Network Plan is due. Notably, there is nothing preventing a state 

from sharing their draft SO2 network design plan for SO2 monitoring intended to satisfy this rule 

with the EPA and the public in advance of the complete Annual Monitoring Network Plan. 

The reality of the sometimes complex process of identifying a location, securing funding 

and installing a new monitoring site, necessitates such an approach. The Agency believes that 

early interaction between air agencies and the EPA Regional Offices and industry will likely

improve the potential for success in installing an appropriate amount of monitors in appropriate 

locations around SO2 emitting facilities identified for characterization in this rulemaking.

5. Issues Related to Deadline for Operation of SO2 Monitors 

a. Summary of Proposal

The EPA proposed in §51.1203(c)(1) that air agencies that have chosen to characterize air 

quality through ambient monitoring must have any relocated and/or new monitors operational by

January 1, 2017. In the preamble, the EPA explained that, under this approach, it is anticipated 

that the first 3 calendar years of data would be collected from 2017 through 2019, allowing the 

first design value for each monitor to be calculated by May 2020. This would allow these new 

monitoring data to be used to inform air agency and the EPA determinations of areas’ attainment 

status in actions that occur in 2020, which could include designations and redesignations. See 79 

FR 27458, May 13, 2014, and proposed §51.1203(c).

b. Brief Summary of Comments 
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One industry commenter stated that the proposed rule reflected a reasonable timeframe 

for air agencies to collect the data, either through monitoring or modeling, that are needed to 

characterize air quality in areas and determine whether the 1-hour SO2 NAAQS is being met. 

One state commenter also asserted that the feasibility of this time period will be dependent upon 

the threshold option selected by the EPA and thus the number of affected sources. 

However, more than 10 state and industry commenters asserted that the short time period 

between the dates when the monitoring plans need to be submitted and the monitors are required 

to be operational is inadequate. One industry commenter stated that it is technically infeasible to 

implement the proposed rule by 2017 and thus the EPA’s proposal is arbitrary and capricious. 

Several state and industry commenters recommended an extension of at least 1 year for

air agencies to begin actual monitoring. One state commenter suggested that the EPA should 

allow monitoring to begin operation between May 1, 2017, and July 1, 2017, which would be 

consistent with its suggested approach allowing air agencies to notify the EPA of selection of the 

monitoring option up to January 1, 2017. This commenter recognized that this approach would 

likely require delaying the attainment date, if designations are not made until after 3 calendar 

years of the new monitoring data are obtained and certified. This commenter also noted that, if 

the EPA’s approval of an SO2 monitoring plan under this proposal does not occur until late 2016, 

air agencies with winter weather concerns would simply not have sufficient time to set up a 

monitoring network by January 1, 2017. Another state commenter noted that other recent rules 

establishing new monitoring requirements (such as NAAQS revisions for NO2, SO2 and PM2.5)

have not required such rapid deployment of monitors, but have each allowed at least 1.5 years 

from submittal of the network plan to operation of the monitor. 
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c. EPA Response

The EPA recognizes that the logistical and financial burdens of installing an ambient air 

monitoring station can vary in difficulty and the resources required. However, as noted earlier 

with regard to the overarching timetables effected by this rule, the Agency believes that, as with 

other parts of the implementation schedule, while the schedule for operating monitors is 

expeditious, it can be achieved with appropriate planning, coordination, and program 

implementation by the air agency which will allow monitoring resources to be in place by the 

deadline. The EPA believes that any further delay in air quality characterization around sources 

identified as a result of this rulemaking will delay implementation of the standard and public 

health protection in areas where there may be a violation of the standard. The Agency believes 

that it is most prudent to maintain the proposed timetable for monitoring network installation 

because of the need for use of these new data in a relatively timely manner for use in making 

attainment status decisions concerning SO2 areas in the country. Therefore, the EPA is finalizing 

the date by which monitors being used to satisfy this rulemaking must be operational to be 

January 1, 2017. 

As noted previously, if a state has chosen to monitor to satisfy the requirements of this 

rule, planning for the installation of new monitors must occur early on, soon after this rule is 

promulgated. With this in mind, and in light of the many logistical concerns raised by

commenters and recognized by the EPA, the Agency is encouraging air agencies to engage their 

respective EPA Regions well in advance of the time by which the Annual Monitoring Network 

Plan and network operations are due. The EPA is encouraging air agencies to engage their 

respective EPA Regional Offices, and possibly the industrial sources needing nearby air quality
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characterization, to plan an adequate network design as early as possible after this rule is 

promulgated. The reality of the sometimes complex process of identifying a location, securing 

funding and installing a new monitoring site, necessitates such an approach. The Agency

believes that early interaction between air agencies and the EPA Regional Office and industry

will likely improve the potential for success in installing an appropriate number of monitors in 

appropriate locations around SO2 emitting facilities identified in this rulemaking as needing 

nearby air quality to be characterized. The EPA also notes that if air agencies conclude that the 

timeline and resource burdens associated with installing and conducting improved monitors are 

less feasible for particular areas, they may instead choose the less resource-demanding and more 

expeditious method of modeling to characterize SO2 emissions impacts in such areas. 

6. Issues Related to Submittal of Modeling Analyses to the EPA

a. Summary of Proposal

The EPA proposed in §51.1203(d)(3) that air agencies that choose modeling to 

characterize ambient air quality be required to submit modeling analyses to the EPA Regional 

Office by January 13, 2017. In the proposal, the EPA recommended that these modeling analyses 

should be conducted in accordance with the recommendations in the EPA’s Modeling TAD or as 

otherwise agreed upon with the EPA Regional Office on a case-by-case basis. The EPA stated 

that the EPA Regional Office and the air agency should engage actively in consultation to 

understand the inputs, assumptions and findings associated with each air quality modeling 

analysis; the air agency should submit thorough documentation of its modeling analysis; and the 

air agency should provide the EPA with supplemental information about the analysis upon 

request. 
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The proposal also indicated that where areas have not already been designated under the 

2010 NAAQS, air agencies could submit updated designation recommendations, if appropriate, 

as informed by their modeling analyses. The proposal noted that in developing any updated 

designation recommendations, the air agency should follow the EPA’s most recent SO2

designation guidance.7See 79 FR 27446, 27458, May 13, 2014 and proposed §51.1203(d).

b. Brief Summary of Comments 

One state commenter disagreed with the requirement that comprehensive modeling 

analyses and related supporting information need to be submitted to the EPA. This commenter 

asserted that the modeling analyses will be conducted by the facility owners and reviewed by the 

state air agency, and the air agency should be able to forward just a summary of the analyses to 

the EPA with sufficient information for the EPA to evaluate. 

c. EPA Response

The EPA is finalizing its proposed approach of requiring that air agencies choosing 

modeling to characterize ambient air quality be required to submit modeling analyses to the EPA 

Regional Office. Irrespective of whether the state or a third party conducts the modeling, it is the 

state’s responsibility under the CAA to submit the information that this rule requires. The EPA 

anticipates that any state submittal of third-party modeling would reflect a review as to whether 

it believes that the modeling satisfies applicable requirements. Moreover, the EPA anticipates 

 

7

The EPA issued a first round of guidance on the SO2 area designations process on March 24, 

2011. See http://www.epa.gov/air/sulfurdioxide/pdfs/20110411SO2designationsguidance.pdf.

Note: The EPA issued an updated SO2 designations guidance. See “Updated Guidance for Area 

Designations for the 2010 Primary Sulfur Dioxide National Ambient Air Quality Standard”, 

March 20, 2015. Available at: 

http://www.epa.gov/airquality/sulfurdioxide/pdfs/20150320SO2designations.pdf.
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that the submittal would provide adequate information for the EPA to review the adequacy of the 

analysis as well.

D. Technical Issues Relating to Modeling and Monitoring

1. Technical Assistance Documents (TADs)

This section of the preamble presents a discussion of the threshold-based air quality

characterization approach to implement the SO2 NAAQS in areas that contain sources with 

larger SO2 emissions, in order to address areas where there may be higher potential for NAAQS 

violations that adversely affect public health. This section discusses the different recommended

approaches air agencies may use to provide the necessary air quality information to the EPA for 

areas around those identified sources. 

a. Summary of Proposal

In the preamble of the proposed rulemaking, the EPA noted that the Agency has 

produced draft, non-binding Monitoring and Modeling TADs that discuss options, suggested 

approaches and methods on how monitoring or modeling efforts to characterize air quality

around an identified source might be conducted. The EPA stated that these documents can be 

used to assist air agencies in the implementation of this rulemaking.8See 79 FR 27446, 27460, 

May 13, 2014.

b. Brief Summary of Comments 

One industry commenter stated that the proposed rule references and relies upon 

guidance provided in the Modeling and Monitoring TADs and in Appendix W; therefore, the

 

8

The EPA considered the TADs as being living documents that may be updated on a recurring 

basis as necessary.
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commenter asserted that the documents are subject to review and comment for the proposed rule. 

One state commenter asserted that it is challenging to prepare meaningful comments on the 

proposal since much of it is contingent upon the use of the TADs.

Some state and industry commenters urged the EPA to be clearer in the final preamble 

that the TADs are guidance and therefore are not binding. One state commenter urged the EPA 

to explicitly state in the final rule that air agencies retain the ability to use alternative methods to 

those outlined in the TADs. One industry commenter stated that the EPA's reliance on technical 

guidance documents that have not been subject to public notice and comment undermines 

protections guaranteed by the Administrative Procedure Act. One state commenter stated that 

because the rule “requires” the use of”... separate non-binding draft technical assistance 

documents ...” and creates significant regulatory uncertainty for air agencies, they oppose the 

proposal.

c. EPA Response

The EPA reiterates that the TADs provide recommendations but are not binding or 

enforceable and create no obligations on any person. Although the draft TADs are referenced as 

recommended approaches in the preamble to the proposal and in this final rulemaking, they are 

not required to be adhered to by any air agency required to characterize air quality around an SO2

source identified in this rulemaking. The EPA developed the TADs to aid air agencies seeking 

advice in the air quality characterization process required by this rulemaking. The Agency has 

indicated that the TADs are meant to be used as possible tools to aid air agencies. This 

rulemaking does not codify the TADs, and none of the comments on the proposed rule regarding 

the TADs resulted in changes to the rule itself. The TADs are considered to be living documents 
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that the EPA may update as necessary over time. The Agency believes that a modeling protocol 

or monitoring network design that follows or references the recommended approaches in the 

TADs is likely to be adequate, and will better ensure the success and a timely fulfilment of the 

requirements of this rulemaking. However, air agencies remain free under the final rule to 

suggest alternative approaches to those suggested in the TADs. Whether an agency chooses to 

follow a TAD or suggest an alternative approach does not affect the fact that for every approach

chosen, the air agency will need to submit their rationale and approach to the EPA for review on 

a case-by-case basis. 

The EPA disagrees with the commenters who claimed that the proposal’s reference to the 

TADs violates the rulemaking requirements of the Administrative Procedure Act. The Agency

did not propose, and is not promulgating language that the TADs are required to be followed, 

and is not changing their status as non-binding technical assistance documents. In response to the 

request that the TADs be subjected to notice and comment, in fact the first drafts of the TADs 

were circulated for review and comment by stakeholders, and revised versions of the TADs were 

developed in response to those comments.

2. Monitoring and Network Design Issues

a. Summary of Proposal

The EPA proposed that air agencies that select the monitoring approach to characterize 

air quality in an area would have the option to identify appropriate existing monitoring sites, 

relocate monitors as appropriate or install new monitors, and have them operational by January

1, 2017, in order to provide data for use in the anticipated designations process in calendar year 

2020. The EPA proposed to require that any relocated or new monitors be operated either as 
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SLAMS, or in a manner equivalent to those monitors operated elsewhere in the SLAMS 

network; they do not, however, have to be designated as SLAMS monitors. In the proposal, the 

EPA stated that the monitors should use Federal Reference Methods (FRMs) or Federal 

Equivalent Methods (FEMs) and meet the requirements of 40 CFR part 58, appendices A, C, and 

E. Further, the EPA stated that the resulting data should be reported to the Air Quality System 

(AQS) and would be subject to the same annual data reporting and certification requirements 

listed in 40 CFR 58.15 and 58.16 as required for SLAMS data. See 79 FR 27446, 27461, May

13, 2014.

b. Brief Summary of Comments 

Some commenters suggested that the rule should allow a third party, such as a facility 

owner, to cover the expenses of siting and operating new monitors in coordination with the air 

agency. One public interest group commenter stated that there are numerous considerations that 

make it unlikely that monitors could be sited at ideal modeled locations, including access to the 

location, power hookups, local pollutant effects and safety from vandalism. Several commenters 

expressed concern that the lack of clear criteria for designing an SO2 source-oriented monitoring 

network puts air agencies in the unreasonable position of designing a monitoring network 

without knowing whether it will be approved by the EPA.

Some commenters stated that guidance is needed on the number of monitors required. 

Commenters stated this issue should not be left up to negotiations with the EPA Regional Office;

rather, a procedure should be outlined that will provide consistency for all regional offices and 

air agencies. Some state and industry commenters suggested that one monitor may be sufficient 

and recommended the final rule include a discussion of the adequacy of one monitor in certain 
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situations. One industry commenter stated that, because large gradients in design concentrations 

for SO2 are likely not present to the extent that the EPA may expect, the use of a single monitor 

to demonstrate NAAQS attainment is sufficient in many cases. 

c. EPA Response

The EPA believes that there are no limitations as to who might operate a monitor or 

monitors being used to satisfy the requirements of this rulemaking. It can be a state, local or

tribal government, industry, possibly other third parties or a mix thereof. Whatever the case, the 

monitor or monitors should be included as a part of the state’s monitoring plan. The critical issue 

is that the monitor or monitors must be either a SLAMS monitor or SLAMS-like monitor, where 

the latter might be an industrial or other third party-operated monitor. In either case, the monitor

or monitors must be an FRM or an FEM monitor, and must adhere to requirements in 40 CFR 

part 58, appendices A, C, and E, and adhere to data reporting requirements also contained in 40 

CFR part 58. This does require states to provide oversight to any non-SLAMS sites for which 

they are claiming to satisfy this rulemaking, as the states have the final responsibility to ensure 

the quality of submitted data that satisfies the intent of this rulemaking. 

With respect to concerns over a lack of clear criteria for designing an SO2 source-oriented 

monitoring network, the likelihood to appropriately place one or more monitors, and the issue of 

what number of monitors might be required around a source, there is no one-size-fits-all answer

to this question. The EPA indicated in the preamble to the proposal, and in the draft Monitoring 

TAD, that the relative location and number of monitors that might be sufficient to characterize 

the air quality around a source is a case-by-case determination. In general, the main objective is 

to monitor at, or as near as possible to, the location(s) where ambient SO2 concentration maxima 
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are expected to occur. Site selection for any monitoring network is subject to logistical hurdles 

including site access, identification or installation of appropriate infrastructure, 

telecommunications access, and safety, and state, local, and tribal air agencies are well versed in 

the variety of logistics that can be involved in the installation of an ambient air monitoring 

station. These issues undoubtedly can play into what any ambient air monitoring network 

ultimately looks like. However, as is the case with all required ambient air monitoring, 

responsible air agencies are expected to establish a clear rationale for the number and placement 

of any one or more monitors it is using to satisfy the requirements of the rule. In this process, 

there is flexibility for the state to use professional judgment in determining what is appropriate 

for their individual situations, but they are expected to perform due diligence in attempting to 

locate monitors in the most ideal locations possible. Further, the air agency’s recommended 

number of monitors and preliminary rationale should be discussed with the EPA Regional 

Offices well in advance of the development of an Annual Monitoring Network Plan. As 

discussed in the Monitoring TAD, the development of a network design and its rationale can be 

informed by a number of types of analyses which can include the use of air quality modeling, 

exploratory monitoring, or analysis of existing data. In any scenario, the state would need to have 

a technically credible rationale that supports the monitoring network design approach that has 

been chosen to satisfy requirements in this rulemaking.

As stated previously, the TADs provide recommendations for air agencies, but are not 

binding or enforceable, and they create no obligations on any entity. Although the draft TADs 

are referenced as providing recommended approaches in the preamble to the proposal and in this 

final rulemaking, there is no specific provision in this rule that requires the air agency to adhere
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to the TADs. The TADs have been provided in order to potentially aid air agencies seeking 

advice in the air quality characterization process required by this rulemaking. 

3. Areas Failing to Having New Monitors Operational by January 1, 2017 

a. Summary of Proposal

Where an air agency has chosen the monitoring approach and submitted a list identifying 

the sources near which air quality is to be monitored, the proposed rule addressed the situation 

where it becomes evident that sufficient and appropriate monitoring will not be operational in a 

timely manner. The EPA proposed that the area around the source in question would be

functionally “moved” to the modeling pathway, where air quality data characterized by the state 

under this rule could inform potential future designations that would be intended to occur by

December 2017. The EPA requested comment on this approach, and on any alternative 

approaches that could most effectively address a situation where an air agency is acting in good 

faith to deploy monitors on time but experiences a delay which may be outside of its control, as 

well as a situation where an air agency does not act in good faith to deploy monitors on time. See

79 FR 27446, 27461, May 13, 2014.

b. Brief Summary of Comments 

One public interest group commenter stated that the 2017 modeling pathway discussed in 

the proposal offers a swifter, cheaper, and more accurate way of assessing air quality, and so did 

not believe that states that missed deadlines along the monitoring pathway should be allowed to 

further delay designations. Other commenters stated that the fact that modeling is less expensive

than monitoring is not a substitute for what they believe is the superior accuracy of actual 

monitored data; and that they believe the lower costs of modeling do not offset the regulatory
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costs and other burdens on sources and communities that could result from nonattainment 

designations based on modeling.

One public interest group commenter stated that because the monitoring approach already 

could lead to designations occurring a full decade after the NAAQS was promulgated, it should 

be regarded as an absolute edge-of-the-envelope approach, meaning that failure to meet 

monitoring deadlines should result in areas being treated under the modeling pathway as a 

default. This commenter stated that setting such a policy in any final rule would properly 

incentivize actors to transmit information to the EPA in a timely manner.

A number of state and industry commenters did not agree that a would-be monitored area 

should be automatically designated at the same time as areas for which the modeling option was 

chosen in the event of any delay in monitoring. Commenters also stated that the proposed penalty

for unanticipated monitoring site delays is excessive and there are too many uncertainties which 

argue against such automatic actions; especially in cases where the air agency has exercised all 

due diligence to ensure that the monitors are operational by the deadline in the rule.

c. EPA Response

The EPA is clarifying the relationship between this rule and the schedule for 

promulgating designations under CAA section 107. This rule does not establish any deadlines for 

designations or prescribe the manner in which future designations would occur. Therefore, it has 

never been the role of this rule, even as proposed, to promulgate schedules for designations of 

areas based on whether air agencies timely implement the rule. However, the proposed 

milestones for implementation of the rule were devised in consideration of the Agency’s 

preferred and anticipated schedule for completing area designations. 
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While this rule does not promulgate designation schedules, separate litigation activities 

have affected the schedule. On March 2, 2015, the U.S. District Court for the Northern District 

of California issued an order directing the EPA to complete designations pursuant to the schedule 

discussed earlier in this document. Affected air agencies considering the monitoring option under 

this rule should be aware of this schedule. Under the terms of the consent decree entered by the 

court, in order for the EPA to not be required to designate an area by December 31, 2017, air 

agencies choosing the monitoring option under this rule will need to install and begin operating 

those monitors by January 1, 2017. This is the date that the rule requires. However, while the 

rule does not provide designations schedules, and thus does not address how designation 

schedules would be affected by an air agency missing this deadline, the March 2015 consent 

decree does. If the monitor is not operational by January 1, 2017, the EPA will not be able to use 

the future monitoring information to be generated by those monitors in the initial designation for 

the area, because the court’s order allows those designations to occur as late as 2020 only if the 

monitor is timely installed and operated. Where the January 1, 2017, deadline is not met, the 

designations must occur by December 31, 2017, and will have to depend upon other information 

available at that time. 

The EPA’s proposal addresses circumstances in which an air agency chooses to

characterize through monitoring but fails to have monitors become operational on time. The 

proposal suggests that in these circumstances, the agency (or, for that matter, the EPA) would be 

required to conduct modeling under this rule and be relieved of further obligations to conduct 

monitoring, albeit late. The EPA’s intent in its notice of proposed rulemaking was to explain that 

in these circumstances, where an air agency chooses to characterize air quality with new 
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monitors but failed to have the new monitors operational by the January 1, 2017, deadline, the 

EPA envisioned designating such areas in conjunction with areas being characterized by

modeling. That is, the EPA did not envision delaying the designation for such areas to the 

envisioned 2020 date when the Agency anticipates promulgating designations for areas 

characterizing air quality through a new monitoring network. The EPA must now comply with a 

court ordered designation schedule, in which the court expressly requires that areas that have not 

begun operation of a new monitoring network by January 1, 2017, must be designated by

December 2017. 

Nevertheless, the EPA wishes to clarify that an air agency that chooses monitoring as its 

means to meet the air quality characterization requirements, and commits in its July 2016 Annual 

Monitoring Network Plan to conduct such monitoring, remains obligated to fulfill the original 

requirement to monitor and to provide the resulting air quality characterization around a given 

SO2 source, even if operation of new monitors commences after the January 1, 2017, deadline. If 

a state fails to meet the January 1, 2017, deadline, the state must still meet the monitoring 

requirements for the area pursuant to 40 CFR part 58, or the EPA may disapprove the state’s 

monitoring plan for the following year, unless, of course, the monitoring plan is revised 

accordingly. Although, as discussed previously, the EPA will not be able to rely upon the future 

monitoring data to issue the designation on the court-ordered schedule, the future monitoring 

data may be useful for other purposes such as tracking progress and making later attainment 

status determinations needed for redesignations.

4. Monitor Shut Down

a. Summary of Proposal
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In the preamble, the EPA proposed that a monitor that has been deployed under the 

monitoring option pursuant to this rule, and is located in an area that is subsequently designated 

attainment, may be eligible for shut down provided that the monitor meets certain criteria. The 

EPA proposed in §51.1203(c)(3) that any SO2 monitor identified in an approved state annual 

monitoring network plan to satisfy the rule requirements may be eligible for shut down if the 

following criteria are met: (1) the monitor is not also satisfying other minimum SO2 monitoring 

requirements listed in 40 CFR part 58, appendix D; (2) the monitor is not otherwise required to 

meet requirements in a SIP or permit; and (3) the monitor has recorded a 3-year design value 

(DV) that is no greater than 50 percent of the 1-hour SO2 NAAQS. The EPA also proposed that 

any SO2 monitor eligible for shutting down would need to be approved by the EPA Regional 

Administrator before monitoring operations could cease. As an alternative, the EPA also 

proposed an option in which the same criteria noted earlier would need to be met, except that the 

monitor would be eligible to cease operations if it recorded a DV in the three-year period that is 

no greater than 80 percent of the 1-hour SO2 NAAQS. The EPA requested comment on the two 

proposed options for DV criteria for SO2 monitor shutdown, as well as other potential values 

within the 50-80 percent range. The EPA requested that commenters provide specific technical 

rationale supporting any approach they recommend. See 79 FR 27446, 27462, May 13, 2014.

b. Brief Summary of Comments 

Some state and industry commenters agreed with the proposal that monitors placed 

pursuant to the monitoring option and located in areas that are designated as attainment should 

be eligible for shut down. Commenters also stated that providing state agencies with the 

flexibility to shut down unneeded monitors allows agencies to allocate their limited resources 
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more appropriately. One industry commenter stated that, if the sources are properly controlled 

and/or limited by permit, the risk of significant increases in DVs over time is relatively low 

absent new sources entering the affected area. Several state and industry commenters supported 

the proposal, with one state commenter indicating that the use of the 50 percent threshold would 

be safe to use because the area would require a significant increase in future SO2 emission to 

cause an exceedance of the 1-hour SO2 NAAQS. 

Some state commenters recommended that the threshold of 50 percent be dropped in the 

final rule since 40 CFR 58.14 already contains provisions for shutting down a monitor at 80

percent of the NAAQS. Commenters stated that there does not seem to be a reason to make the 

criteria more stringent than the existing criteria in 40 CFR part 58 and, if the EPA wishes to 

change those criteria, a revision to 40 CFR 58.14(c)(1) should be considered and made available 

for comment. Industry commenters stated that the requirement for annual reporting of changes in 

SO2 emissions with the possibility that further monitoring could be required, argues against the 

more stringent 50 percent option. 

Over 25 commenters supported the use of the 80 percent threshold. Commenters stated 

that 80 percent of the NAAQS is a strong enough criterion for shut down of a SO2 monitor and 

the 80 percent criterion is consistent with criteria for shutting down most regulatory monitors. 

One public interest group commenter stated that new monitors should not be shut down since (1) 

short-term monitor readings may not be consistent with long-term attainment and (2) the SO2

monitor network needs to be rebuilt. In addition, this commenter recommended that monitors not 

be removed if the concentrations they are recording are trending upward, indicative of potential 

future problems.
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c. EPA Response

The EPA is finalizing the rule to allow any SO2 monitor identified by an air agency in its 

approved Annual Monitoring Network Plan as having the purpose of satisfying §51.1203 which 

is not in an SO2 nonattainment area, and is not also being used to satisfy other ambient SO2

minimum monitoring requirements listed in 40 CFR part 58, appendix D, section 4.4, and is not 

otherwise required as part of a SIP, permit, attainment plan or maintenance plan, to be eligible 

for shut down if it produces a DV of no greater than 50 percent of the 1-hour SO2 NAAQS in the 

first or second 3-year periods of its operation. See §51.1203(c)(3). The EPA has chosen to adopt 

this shutdown allowance so that those monitors that record ambient concentrations that are well 

below the NAAQS after 3 or 4 years of operation would no longer be required to operate under 

the unique provisions of this rule, if they are otherwise not required under other requirements. 

This potential ability to shut down monitors would relieve any resource burden under this rule on 

air agencies where NAAQS violations have not and likely will not occur. This particular 

provision will not require estimates of future concentrations as do existing shutdown provisions 

in 40 CFR 58.14.

More specifically, this monitor shutdown provision works by assessing how two design 

values (i.e,, one calculated from monitor data collected in years 1 through 3, and one from years 

2 through 4) would compare to the 50 percent of the NAAQS shutdown criterion. If a monitor 

produces a design value from data collected in years 1 through 3 that is no greater than 50

percent of the NAAQS, it is eligible for shutdown if it is not otherwise required to operate. If the 

design value is above the 50 percent threshold, the monitor must continue operation. If that 

monitor produces a design value no greater than 50 percent of the NAAQS from data in years 2 
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through 4, it is eligible for shutdown if not otherwise required to operate. If, instead, the design 

value is again above the 50 percent threshold, the air agency must continue to operate the 

monitor. From that point forward (i.e., for data collection year 2021 and beyond), the applicable 

monitor shutdown provisions are those that exist in 40 CFR 58.14, which include probabilistic

estimations of future concentrations and other circumstantial situations that might allow for 

monitor shutdown. 

The Agency would like to note language of particular relevance from 40 CFR part 58 

regarding eligibility for shutdown based on recorded data and calculated design values that exists 

in §58.14(c)(1). This particular provision allows monitoring discontinuation with Regional 

Administrator approval for: “Any PM2.5, O3, CO, PM10, SO2, Pb, or NO2 SLAMS monitor which 

has shown attainment during the previous 5 years, that has a probability of less than 10 percent 

of exceeding 80 percent of the applicable NAAQS during the next 3 years based on the levels, 

trends, and variability observed in the past, and which is not specifically required by an 

attainment plan or maintenance plan. In a nonattainment or maintenance area, if the most recent 

attainment or maintenance plan adopted by the state, and approved by the EPA, contains a 

contingency measure to be triggered by an air quality concentration and the monitor to be 

discontinued is the only SLAMS monitor operating in the nonattainment or maintenance area, 

the monitor may not be discontinued.”

In any circumstance regarding monitor shutdown, whether pursuant to this final rule or 

40 CFR part 58, the air agency must receive EPA Regional Administrator approval of a request 

to cease operation of the monitor as part of its action on the annual monitoring plan under 40 

CFR 58.10 prior to the shutdown of any qualifying monitor. Therefore, under the final rule, there 
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are two sequential routes for possibly shutting down a monitor. If a monitor shows concentration 

readings greater than 50 percent of the NAAQS after the first two sets of 3-year periods of its 

operation and cannot be approved for shut down under the first sequential route, the monitoring 

will continue. However, after 5 years of operation it can be considered for shutdown if it meets 

the criteria that the EPA’s rules at 40 CFR 58.14(c)(1) apply, with the EPA Regional 

Administrator’s approval. These monitors might also be subject to shut down eligibility as set 

forth in §58.14 (c)(2), (3), (5), and (6).

5. Annual Reporting Following Monitor Shutdown

a. Summary of Proposal

For any area for which the EPA has approved an air agency’s request for an SO2 monitor 

to cease operations, the EPA proposed that the air agency be required to assess SO2 emissions 

changes annually, beginning in the year after the monitor ceases operation. (The proposal 

contained a similar requirement for modeled areas, discussed later in this section.) For areas 

around these sources in which total SO2 emissions increase over the emissions for the previous 

year, the EPA proposed that the air agency would be required to submit to the EPA an 

assessment of the cause of the increase and provide an initial determination of whether the air 

quality around that source should be further re-assessed. The EPA proposed that the air agency

could choose to reinstate the operation of the air monitor or complete air quality modeling for the 

source area to verify that the area continues to attain the standard. In the proposal, the EPA stated 

that, if modeling or monitoring information required to be submitted by the air agency to the 

EPA pursuant to §51.1205 indicates that an area is not attaining the 2010 SO2 NAAQS, the EPA 

may take appropriate action, including but not limited to disapproving the monitoring plan, 
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requiring adoption of enforceable emission limits to ensure continued attainment of the 2010 SO2

NAAQS, redesignation of the area to nonattainment, or issuance of a SIP Call.

The EPA proposed two options for how the air agency would submit this report and how 

the EPA would review and act on it. Under the first option, the EPA proposed that the air agency

would submit a report to the EPA annually as an appendix to the air agency’s annual monitoring 

plan; the annual monitoring plan is required to be submitted to the EPA Regional Administrator 

by July 1st each year. In the proposal, the EPA stated that the inclusion of this verification report 

as an appendix to the annual monitoring plan would ensure that the report would be subject to 

public review and comments that are to be provided for the monitoring plan pursuant to 

regulations at 40 CFR 58.10. 

Under the second option, the annual report of emissions data for sources for which the 

state ceased the operation of nearby monitors would be submitted to the EPA in the form of a 

separate, independent annual submittal from the state to the EPA Regional Administrator due by

the same July 1st date each year. This independent submittal would follow the general guidelines 

set forth in 40 CFR 58.10 regarding opportunities for public review and comment as described in 

Option 1, but the report would only include the annual assessments associated with sources in 

areas that were designated unclassifiable/attainment and for which the EPA granted approval to 

cease monitoring. The EPA invited comment on any suggested alternatives to these procedural 

options. See 79 FR 27446, 27462, May 13, 2014, and proposed §51.1205(d).

b. Brief Summary of Comments

Several state and industry commenters stated that the proposed annual reporting 

requirement appears to be unduly burdensome. Some industry commenters opposed the annual 
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reporting requirement, stating that SO2 emissions from sources are already available to the EPA 

and the need for ongoing data requirements has not been demonstrated. One state commenter 

suggested that, if the monitors that were removed were providing data under 50 percent of the 

standard, there is no reason to perform such analyses since an increase in emissions that would 

result in such a drastic increase in monitored design values would surely be associated with 

changes to operations that would necessitate air permitting, which evaluates projects for NAAQS 

compliance.

One group of state commenters stated that the EPA’s proposed July 1st submittal date is 

unrealistic because states will not have the required quality-assured emissions monitoring data 

processed by July 1st. Some state and industry commenters recommended a less burdensome 

process in which this verification would take place every 3 to 5 years instead of annually, 

pointing out that the EPA publishes the NEI data every 3 years, the EPA reviews the NAAQS 

every 5 years, and there is a 5-year ambient monitoring assessment plan required by 40 CFR 

58.10. 

Commenters requested clarification regarding the determination of an emissions increase. 

One state commenter stated that it is unclear whether an emission increase should be based on an 

increase greater than the 3 year average of emissions during the initial monitoring analysis, an 

increase above the highest single year of emissions during the initial monitoring analysis, or 

some other metric. Some commenters recommended the comparison be based on some 

compliant level of emissions from the year(s) where the monitor demonstrated attainment with 

the standard, since the "increase" or "decrease" in emissions of SO2 may have resulted in total 
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SO2 emissions levels well below the annual emission rates during the years when monitoring 

data showed compliance. 

One tribal and several state commenters supported the option of including the annual 

emissions analysis with the annual monitoring plan. One commenter stated that the analysis of 

emissions is closely related to network planning, and this procedure would provide a single 

document for public inspection and EPA review and approval. Another commenter stated that 

the annual monitoring plan may not be the best tool or location to place modeled data, emission 

reports, ongoing data requirements, and requests to cease modeling. Other state commenters 

recommended that the monitoring plan verification report be considered a separate element for 

ease of processing and for public review. 

c. EPA Response

The EPA has decided not to finalize the proposed requirement that any state with an area 

for which the EPA has approved the air agency’s request for an SO2 monitor to cease operations

must still assess SO2 emissions changes annually, beginning in the year after the monitor ceases 

operation. The EPA made this decision based upon comments on the proposed rule, and in 

recognition that a cessation of monitoring will not occur unless a monitor has measured SO2

concentrations well below the NAAQS for a given time period and an EPA Regional 

Administrator has allowed the shut-down. The Agency is persuaded by commenters that monitor 

shutdown provisions, along with generally applicable emissions reporting requirements, are of 

sufficient strength that subsequent additional annual observation and reporting of SO2 source 

emissions profiles by states specifically due to this rulemaking is unnecessary. Further, there are 

means by which monitoring can be reinitiated in the future if the unlikely scenario occurs where 
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SO2 emissions rise significantly in an area, or other data indicate possible NAAQS violations in 

an area after a monitor has been shut-down, mainly through the EPA Regional Administrator 

authority granted in 40 CFR part 58, appendix D, section 4.4.3. 

6. Modeling Issues

a. AERMOD

i. Summary of Proposal

In the proposal, the EPA stated that the Agency anticipates that in implementing the rule 

air agencies would likely use AERMOD to conduct modeling, as AERMOD is the EPA’s 

preferred near-field dispersion model and has been demonstrated to be a reliable predictor of SO2

air quality given appropriate input data. The EPA explained in the proposed rule that, as part of 

its development, AERMOD was evaluated using 17 field studies, several of which involved 

short-term measurements of SO2, robust site-specific meteorology and accurate measurements of 

emissions. The EPA stated in the proposal that the Agency is confident that AERMOD can 

provide accurate predictions of actual SO2 concentrations given representative meteorology and 

accurate emissions inputs. See 79 FR 27446, 27463, May 13, 2014.

ii. Brief Summary of Comments 

One industry commenter stated that, for certain conventional SO2 emission scenarios, 

such as tall stacks at coal fired EGUs, AERMOD can be at least reasonably predictive. One 

public interest group commenter stated that AERMOD modeling performs particularly well in 

evaluating emission sources with one or a handful of large emission points. This public interest 

group commenter cited a declaration of Roger W. Brode (EPA) filed in the EPA’s successful 

defense of the 2010 SO2 NAAQS in which he stated that AERMOD is capable of accurately 
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predicting whether the revised primary SO2 NAAQS is attained and whether individual sources 

cause or contribute to a violation of the SO2 NAAQS. This commenter also stated that 

AERMOD has been tested and performs very well during conditions of low wind speeds, citing 

comments of Camille Sears.

A number of commenters expressed concern with the use of AERMOD. Some 

commenters stated that AERMOD was intentionally designed to over-predict SO2

concentrations. Several commenters referenced studies that indicate AERMOD over-predicts, 

including studies by the Electric Power Research Institute (EPRI), AECOM and some air 

agencies. Commenters identified a number of issues that they believe need to be addressed 

because they lead to over-predicting SO2 concentrations, including buoyant line sources, 

building downwash, conservative assumptions in terms of model input, modeling of multiple 

sources, periods of low wind speed, steep terrain and lack of representative meteorological data. 

Commenters stated that the individual aspects of AERMOD and the EPA’s guidance that 

contribute to over-prediction of the SO2 concentrations in the context of the 1-hour NAAQS are 

multiplicative.

iii. EPA Response

In this final rule, the EPA is not promulgating a requirement that air agencies use 

AERMOD in all cases, but is retaining the existing flexibility otherwise provided by the EPA’s 

rules for agencies to support the use of the best model for a particular case. The EPA’s latest 

recommendations for making this assessment are contained in the Modeling TAD. In most cases, 

the EPA believes that AERMOD will likely be the model of choice by air agencies to address the 

requirements of this rule, unless the application involves a different recommended model, such 
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as the Buoyant Line and Point Source Dispersion Model (BLP). Models recommended for 

particular applications are listed in appendix A of appendix W. Section 3.2 of appendix W 

outlines the procedures for use of alternative models for those cases where an alternative model 

may be more appropriate than a preferred model. In addition, the Modeling TAD also discusses 

past use of alternative models for particular applications.9The EPA recommends consultation 

with the appropriate reviewing authority or EPA Regional Office to determine if the use of an 

alternative model is valid for that application.

In addition, as stated previously, the TADs are documents that provide recommendations 

but are not binding or enforceable and create no obligations on any person. Although the draft 

TADs are referenced as recommended approaches in the preamble to the proposal and in this 

rulemaking, they are not required to be adhered to by any state who is required to characterize air 

quality around an SO2 source identified in this rulemaking. The TADs have been provided in 

order to potentially aid air agencies seeking advice in the air quality characterization process 

required by this rulemaking. 

With regards to concerns regarding model conservatism, EPA is working on updates to 

AERMOD to address concerns regarding buoyant line sources, building downwash, and low 

wind speed issues. With regards to comments about model inputs that lead to over-estimates, as 

part of its development, AERMOD has been shown to perform well against observed 

concentrations when actual emissions have been used. The modeling of actual emissions for 

 

9

It is the EPA’s intention to update the Modeling and Monitoring TADs as necessary to reflect 

any change in policy or to make clarifications that are necessary. Therefore, any comments on 

the TADs themselves that have been submitted in response to the proposed rule will be 

addressed as a part of any updates made to the TADs in the future, rather than in this final rule.
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multiple sources is not anticipated to cause over-predictions. The modeling TAD also discusses 

that the number of sources explicitly modeled in an application is expected to be low and that in 

many cases, a number of sources in a modeling domain can be represented by background 

concentrations instead of being explicitly modeled, thus reducing potential overestimates in 

modeling.

b. Emissions data

i. Summary of Proposal

The EPA proposed that modeling analyses be based on either actual 1-hour SO2

emissions from the most recent 3 years or federally enforceable allowable emissions. The EPA 

referred readers to the Modeling TAD for a more detailed discussion of a range of recommended 

options for determining actual emissions. While actual emissions would be the preferred choice 

to use for emissions inputs, air agencies have the option of using a more conservative approach 

by inputting a source’s most recent 3 years of allowable, or “potential to emit,” emissions. 

Additional information and recommendations on this approach are discussed in the Modeling 

TAD. See 79 FR 27446, 27464, May 13, 2014.

ii. Brief Summary of Comments 

More than 30 state and industry commenters supported statements in the EPA’s proposal 

that allow the use of actual emissions as an input in the air quality modeling in order to most 

effectively serve as a surrogate for comprehensive ambient monitoring results. Several 

commenters suggested that the use of allowable emissions as an input to air quality modeling 

analyses would result in modeled air quality values that were higher than air quality levels that 

would be expected to be observed by a properly sited ambient monitor. Commenters stated that 
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using actual emissions is even more important when conducting a cumulative impact analysis 

(assessing potential impacts from two or more sources) since the model’s tendency to 

overestimate ambient air impacts is compounded when numerous sources are all modeled at peak 

emissions at all times. 

Several state and industry commenters supported the EPA’s proposal to base the 

modeling analyses on actual emissions over a 3-year period. One commenter noted that, in 

situations where multiple sources are being modeled, the most recent 3 years of actual emissions 

data may not be the same for all sources, particularly if there is a mix of EGUs and non-EGUs. 

One state commenter suggested that, if justification can be provided for an alternative dataset, it 

too may be considered for modeling. One state commenter recommended the rule clarify that 

states must use the most recent 3 years of emissions data that are available at the time that a 

modeling protocol for that area is submitted to the EPA, and that revised modeling should not be 

required if more recent emissions data become available. 

iii. EPA Response

When using actual emissions, the EPA believes the most recent 3 years of time varying 

emissions (e.g., emissions that vary hourly, seasonally, monthly, daily, etc.) should be modeled 

since the air quality modeling is being used as a surrogate for monitoring. The Modeling TAD 

gives recommendations on inputting hourly emissions into AERMOD for those sources with 

hourly continuous emissions monitoring (CEM) data and also gives recommendations on 

inputting time varying emissions (e.g. seasonally, monthly, etc.) when no hourly emissions are 

available and only annual emissions and data such as production logs or fuel usage are available.

However, the final rule does not restrict the ability of air agencies to use more conservative 
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allowable emissions in conducting their modeling. In the event that a particular source does not 

have the most recent 3 years of emissions, it may be possible to use the most recently available 

emissions or develop the most recent 3 years of emissions using recommendations in the TAD. 

The reviewing authority should work with the appropriate EPA Regional Office on the use of 

such emissions. For an application that contains a mix of sources whose emissions data are not 

concurrent with each other, it is possible to model all of these sources together following

recommendations in the TAD. Once a modeling protocol or modeling analyses have been 

submitted, there is no requirement to revise the protocol or modeling respectively if more recent 

emissions have become available since the submission, and in the best professional judgment of 

the reviewing authority, those emission changes do not warrant a revision to the protocol or 

modeling analyses.

c. Accounting for recent emission reductions in modeling analyses

i. Summary of Proposal

In the proposal, the EPA noted that, in some cases, air quality modeling conducted in 

advance of January 2017 may indicate a violation of the 1-hour SO2 standard and, to address 

such situations, the air agency may wish to consult with the source(s) and take action to adopt 

enforceable emissions limitations as necessary prior to January 2017 to potentially avoid a 

nonattainment designation. The EPA proposed that, as long as these controls are implemented 

and enforceable as of January 2017, it would be appropriate for the new lower allowable 

emissions to be used in the modeling analysis in place of the higher actual emissions. The EPA 

proposed that, if the air agency is able to demonstrate attainment with the new controls or 

emission limits, the governor of the state has the opportunity to modify its designation 
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recommendation accordingly, if that designation has not yet been issued. See 79 FR 27446,

27464, May 13, 2014.

ii. Brief Summary of Comments 

A number of commenters supported the inclusion of language providing the option for 

states to model more recent emission rates based on enforceable limits implemented in advance 

of the January 2017 modeling deadline. Commenters stated that this approach is a reasonable 

option which would provide industry with an incentive to achieve timely emission reductions to 

meet the regulatory requirements while potentially relieving air agencies from the requirements 

that a nonattainment designation entails, if such a designation has not yet been issued. One 

industry commenter requested that the method for reducing emissions not be limited to installing 

controls.

Some state commenters requested that the EPA develop methodologies for air agencies to 

work with sources whose 2015 emissions are above the threshold to establish permanent and 

enforceable emission limitations that show attainment with the SO2 standards prior to a 

designation of such sources’ areas. One state commenter stated that there must be a process that 

allows for the air agencies’ discretion under extenuating circumstances in order to account for 

significant changes at a facility that occurred during the most recent 3 years. 

iii. EPA Response

After review of the comments, the EPA continues to believe that it is appropriate for the 

air agency to consult with the affected source(s) and take action to adopt enforceable emissions 

limitations as necessary prior to January 2017. As long as the emissions limitations are in place 
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and enforceable by January 2017, the new allowable emission limit may be input into the model 

instead of the actual emissions of the most recent 3 years.

The EPA expects that a number of emissions sources may be candidates for this optional 

approach. Many EGUs will need to meet compliance deadlines for the Mercury and Air Toxics 

Standards (MATS) in April 2015 (or in some cases, April 2016), and the EPA expects that many

will become subject to Title V permits that require compliance with MATS SO2 emission limits 

as the means of demonstrating compliance with the MATS requirements related to acid gas 

emissions. These EGUs may be able to adopt control technologies and enforceable emission 

limits to reduce emissions of SO2 as well as mercury. Similarly, industrial boiler operators will 

have the incentive to adopt SO2 emission limits as part of their strategy for complying with the 

Industrial Boiler Maximum Achievable Control Technology Standard.

Therefore, the EPA believes that as long as these emissions reductions are implemented 

and enforceable prior to January 2017, it would be appropriate for the new lower allowable 

emissions to be used in a modeling analysis in place of the higher actual emissions. The air 

quality impacts from such a source would be characterized by the new enforceable allowable 

limit (at worst) and could be used as a basis for future determinations regarding areas’ attainment 

status. 

d. Stack height

i. Summary of Proposal

The EPA described its view in the proposed rule that actual stack height is appropriate to 

use in conjunction with actual emissions in a modeling approach to characterize current air 

quality. The EPA also described its view that, if an air agency chooses to use allowable
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emissions, then it should use good engineering practice (GEP) stack height when the actual stack

height exceeds the GEP height because the GEP height is used when calculating the allowable 

emission rates. The EPA noted that additional recommendations on the use of actual stack height 

can be found in the Modeling TAD. See 79 FR 27446, 27464, May 13, 2014.

ii. Brief Summary of Comments 

A number of state and industry commenters supported the EPA’s views on the use of 

actual stack height in conjunction with actual emissions. However, several state and industry

commenters did not agree that GEP stack height should be used if a state chooses to use 

allowable emissions. Commenters stated the EPA should allow sources to model using actual 

stack height regardless of whether they are modeling actual emissions or allowable emissions 

since the purpose of the rule is to estimate, as accurately as possible, conditions that would be 

measured at a monitor. Commenters also stated that GEP stack height is not always a factor in 

establishing the emissions limit, where such limits are not established under an implementation 

plan subject to the restrictions of CAA section 123; for example, in the context of emission limits 

that are established based on emission standards under CAA section 112, such as the MATS rule. 

One commenter stated that the concern about giving inappropriate credit for dispersion 

techniques is irrelevant in the context of this designation modeling as section 123 applies only to 

emission limitation controls. 

iii. EPA Response

After consideration of comments, the EPA continues to recommend the use of actual 

stack heights when using actual emissions and the use of GEP height when modeling with 

allowable emissions where such emissions limits are or would be subject to CAA section 123 
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and to the EPA’s corresponding regulations implementing GEP requirements. This would 

include limits established under any CAA provision that are intended to be credited in an 

implementation plan for attaining and maintaining the NAAQS. The use of GEP for allowable 

emissions modeling in such situations is based on the fact that the modeling conducted to 

determine the emissions limits was or would be based on GEP stack heights. Therefore, if actual 

stack heights (when above GEP) were used in such situations, the behavior of the modeled 

sources would not be consistent with the modeling results used to determine the emissions limits

relied upon to demonstrate attainment of the NAAQS.

e. Meteorological data

i. Summary of Proposal

For purposes of conducting modeling that simulates what might be expected to be 

measured by an ambient monitor, the EPA recommended the use of 3 years of meteorological 

data. The EPA stated that, ideally, air agencies would use the most recent 3 years of 

meteorological data and the same 3 years of actual emissions data when modeling for 

designations. The EPA noted that the Modeling TAD has additional suggestions on these 

meteorological inputs. See 79 FR 27446, 27465, May 13, 2014.

ii. Brief Summary of Comments 

Some commenters recommended the use of 1 year of meteorological data rather than 3 

years and provided several reasons: use of 1 year of on-site meteorological data would yield a 

very robust data set; 3 years does not provide a significant benefit over 1 year; 1 year of 

meteorological data is sufficient for PSD purposes; collection of 3 years of data would delay the 

running of AERMOD; and collection of 3 years of data would be unnecessarily expensive. 
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Commenters stated that, while relatively few meteorological databases with 3 years of on-site 

meteorological data exist, many sources may have previously collected a full year of data and 

should be able to use that data without starting all over again on an expensive 3-year effort. One 

state commenter asked the EPA to clarify what is meant by “the most recent 3 years.”

One state commenter recommended that up to 5 years of meteorological data be used and 

stated that, while a single 3-year period may not provide adequate confidence in the analysis, 5

years will provide more 3-year combinations that can be compared to the NAAQS, and more 

meteorological data improves confidence in the result. Some commenters requested that the EPA 

clarify:

 That air agencies need not use concurrent meteorological data, given that some sites 

simply do not have concurrent meteorological data. 

 Given the lack of 3 years of on-site data in many areas, the EPA should approve the 

use of prognostic meteorological data. 

iii. EPA Response

The EPA’s recommendation is to use the most recent 3 years of representative sitespecific data or when site-specific data are not readily available, or it is not feasible or costeffective to collect site-specific data, the most recent 3 years of representative National Weather 

Service meteorological data or other representative data. When the most recent 3 years of 

representative meteorological data are not available, the use of older representative 

meteorological data can be used. For such cases, the Modeling TAD offers recommendations on 

synching the older meteorological data with the more recent emissions, especially for those 

sources utilizing hourly emissions. The TAD provides an explanation of the need for 3 years of 
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meteorological data, even if only 1 year of on-site meteorological data are available. With 

regards to the type of meteorological data that are available, i.e. site-specific, NWS data, or 

prognostic data, appendix W should be consulted on the latest acceptable forms of 

meteorological data at the time of the modeling analyses.

f. Modeling protocol, including multiple sources 

i. Summary of Proposal

This rulemaking proposed that each state list the sources that are to be addressed under 

this rule and the approach to be used to meet this rule’s requirements (air quality characterization 

through monitoring, air quality characterization through modeling, or establishment of a 

requirement for a timely source shutdown) for each source. In preparation for conducting 

modeling, the EPA proposed that the state would need to develop a modeling protocol for all the 

sources the state plans to model. Specifically, in §51.1203(d), the EPA proposed that the air 

agency consult with the appropriate EPA Regional Office in developing modeling protocols and 

submit the protocol to the Regional Administrator for review. In §51.1203(d)(1), the EPA 

proposed that the modeling protocol shall include information about the modeling approach to be 

followed, including but not limited to the model to be used, modeling domain, receptor grid, 

emissions dataset, meteorological dataset and how the state will account for background SO2

concentrations. The EPA stated that details on the suggested protocol elements and the 

recommended standard format of this protocol can be found in the Modeling TAD. See 79 FR 

27446, 27465, May 13, 2014.

ii. Brief Summary of Comments 
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Some state and industry commenters requested that the EPA provide more specific 

guidance on conducting multi-source modeling analyses. Commenters stated that leaving these 

topics for negotiation with the EPA Regional Office will lead to inconsistent application of 

guidance among states. Commenters requested guidance on when a source should be modeled by 

itself, when a source should be modeled with other sources in the surrounding area, more detail 

on the size and location of sources that should be included in a multi-source analyses, and who 

would be responsible for conducting analyses when sources are located in multi-state areas. One 

state commenter requested that guidance on modeling facilities across state lines should be 

addressed. 

iii. EPA Response

The determination of whether to include nearby sources in a modeling exercise around a 

source that exceeds the emissions threshold is case specific, and a standardized methodology

cannot be developed to fit all scenarios. Therefore, the final rule does not promulgate 

requirements addressing nearby sources. The EPA has offered technical recommendations in the 

Modeling TAD. The identification of nearby sources for modeling should rely on sound 

technical reasoning and best professional judgment. The EPA emphasizes that not all emissions 

sources near the source of interest need to be explicitly modeled, as in some cases the impacts of 

those sources can be sufficiently represented by a background monitor as discussed in the 

Modeling TAD and section 8.2 of appendix W.

As stated previously, the TADs provide recommendations but are not binding or 

enforceable and create no obligations on any person. Although the draft TADs are referenced as 

recommended approaches in the preamble to the proposal and in this rulemaking, they are not 
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required to be adhered to by any air agency who is required to characterize air quality around an 

SO2 source identified in this rulemaking. The TADs have been provided in order to potentially 

aid air agencies seeking advice in the air quality characterization process required by this 

rulemaking. The Agency has indicated that the TADs are meant to be used as possible tools to 

aid air agencies. The EPA is not codifying changes to the TADs in this rulemaking in response to 

any comments received on the proposed rule. The TADs are living documents which the EPA 

may update as necessary.

g. Ongoing air agency data requirements for areas that were initially modeled

i. Summary of Proposal

The EPA proposed that, for areas with modeled air quality data based on actual emissions 

that did not exceed the standard, air agencies would be required to continue to submit

information to the EPA in subsequent years that provide a reasonable assurance that the area 

continues to have air quality that does not exceed the standard. The EPA proposed 3 options for 

how air agencies that rely on modeling of actual emissions would need to conduct additional 

emissions and/or modeling analyses. In the proposed rule, the EPA believed that such additional 

analyses would only be needed for areas that had been designated as “unclassifiable/attainment” 

based on actual emissions-based modeling. The EPA further noted in the proposed rule that 

modeled source areas would not be subject to these ongoing data requirements if (1) modeling 

for the source was conducted using allowable emissions, or (2) the modeling for the source was 

conducted using actual emissions and the relevant sources then adopted enforceable emission 

limits consistent with the actual emissions rates used in the modeling. 
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In Option 1, the EPA proposed that any air agency that will be subject to an ongoing data

requirement for modeled areas would be required to assess the most recent SO2 emissions data 

annually, beginning in the year after the area is designated unclassifiable/attainment, and to 

conduct updated air quality modeling every 3 years, and in additional years when the air agency

or the EPA determines that such modeling is warranted. Air agencies would be able to request 

that the EPA Regional Administrator approve a suspension of the triennial modeling requirement 

for an area if their most recent modeling design value was less than 50 percent of the NAAQS.

In Option 2, the EPA proposed to require the air agency to provide the EPA with an 

assessment of SO2 emissions changes for each source annually, as in Option 1, but to not have a 

requirement to conduct updated air quality modeling every 3 years. For sources for which the air 

agency determines that emissions have increased, the air agency would be required to submit to 

the EPA an assessment of the cause of the increase, and provide the EPA with an initial 

determination of whether air quality modeling would be needed to verify that the area around 

the source continues to have air quality levels that do not exceed the standard. If the air agency

or the EPA determines that additional air quality modeling is necessary, the air agency would be 

required to submit the results of that assessment in a timely fashion – within 12 months. 

In Option 3, the EPA proposed to require the state to perform periodic screening 

modeling every 3 years for all source areas that had been previously modeled and determined to 

be attaining the standard, and submit such modeling for review to the EPA. Screening modeling 

is commonly performed using a set of default parameters rather than area-specific parameters, 

and it generally simulates air quality levels that are more “conservative” than levels that would 

be estimated using area-specific parameters. In the proposal, the EPA stated that a complete, full-
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scale modeling analysis with updated emissions and meteorological inputs would only be 

required if the state performs screening modeling that indicates a potential violation. Under all 

three options, if the modeling performed indicates that air quality levels in an area exceed the 

SO2 NAAQS, the EPA may take any appropriate action, including, but not limited to, requiring 

adoption of enforceable emission limits to ensure that future air quality levels in the area do not 

exceed the SO2 NAAQS; redesignation of the area to nonattainment; or issuance of a SIP call

requiring action by the state to bring the area into attainment. 

The EPA requested comment on these 3 options for ongoing data requirements for air 

agencies with sources modeled based on actual emissions, and requested that each commenter 

provide a clear rationale for their position. The EPA also invited comments on any alternative 

ideas and asked that the commenter provide a detailed rationale and estimate of any associated 

costs for any such recommendations. See 79 FR 27446, 27465, May 13, 2014.

ii. Brief Summary of Comments 

Several state, environmental, and tribal commenters supported Option 1. These 

commenters stated that an approach that simply assesses SO2 emissions changes at large sources 

would not account for variations in meteorological conditions, increased SO2 emissions from 

interactive sources, or improvements to the actual modeling computer program. One commenter 

stated that annual modeling makes far more sense from the perspective of protecting the public 

health, and suggested that modeling once every 3 years is an extremely periodic and slow way of 

assessing air quality, such that people living in the impacted area could be unaware for years, and 

thus unable to take action to protect themselves or place pressure on their government to correct 

the problem. 
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Several state and industry commenters opposed Option 1 and stated that modeling 

assessments should not be conducted on a 3-year or any other regular basis. Some believed the

requirement to model every 3 years would be an inefficient use of resources and arbitrary since it 

would not take into account information which might show that undergoing a revised modeling 

analysis would be unnecessary. They claimed that as long as conditions have remained the same 

or possibly improved in the intervening timeframe, additional modeling will provide no 

additional useful information. Others opposed Option 1 on the grounds that no other ambient 

standard requires such a detailed on-going analysis. Consistent with their concerns about 

resources, commenters supported the aspect of Option 1 that would enable the air agency to 

terminate certain ongoing data requirements if air quality modeling indicated a design value 

equal to or less than 50 percent of the 1-hour SO2 NAAQS.

A number of state and tribal commenters objected to Option 2. One tribal commenter 

stated that the proposed emissions assessments required in Option 2, which lack a regular air 

quality modeling requirement, are not stringent enough. Some state commenters expressed 

concern that this option could lead to an indeterminate number of future analyses required, and 

that such open-ended requirements have cost implications that could strain states’ alreadylimited resources. On the other hand, more than 20 state and industry commenters supported 

Option 2 because it balances providing air quality protection with level of effort from state 

regulatory authorities. Several commenters noted that with SO2 emissions declining on a national 

level, remodeling would not be expected to be required and a simple analysis of the change in 

emissions would be sufficient to determine the need for additional modeling. A state commenter 

suggested providing clearer guidance regarding what level of emissions increase would trigger 
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further evaluation of sources, rather than having the air agency provide an assessment for each 

source with increased emissions. The commenter suggested (1) if the original modeling level was

equal to or greater than 90 percent of the standard, then new modeling would be required for the 

area in the event there is any increase in emissions in the area; (2) if the original modeling level 

was between 50 percent and 75 percent of the standard, then new modeling would be required 

for the area if area emissions increased by 15 percent or more; and (3) if the original modeling 

level was less than 50 percent of the standard, then the ongoing modeling requirement should not 

apply (similar to the provision in Option 1).

Another state commenter stated that, ideally, under Option 2, agencies would have a 2.5 

year timeframe to complete the entire ongoing data requirement process: the first year would 

consist of preparing and submitting data for the national emissions inventory for the previous 

year; 6 months thereafter agencies would submit a report to the EPA stating whether air quality

modeling is needed; and 12 more months would then be permitted to perform any additional 

modeling deemed necessary.

Regarding Option 3, several state and industry commenters disagreed with having any 

default modeling requirement, even for screening modeling, and opposed this option. Several 

commenters objected to the required use of a screening model for the following reasons: most of 

the facilities will have multiple emission points and the screening tools were not designed to 

evaluate such complex situations; the mandatory use of screening models will result in an overly 

cautious, ineffective approach to verification; and screening modeling is almost as complex and 

time consuming as full-scale modeling and thus this option would not be a good use of state and 

the EPA resources.
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Lastly, some commenters suggested that the air agency should be able to choose which 

ongoing data requirement approach it intends to follow for a particular area. Another commenter 

suggested an approach that would be a combination of all 3 options, where the air agency would 

evaluate emissions changes each year, and then conduct screening modeling or full-scale 

modeling if the magnitude of emission changes warrant.

iii. EPA Response

The EPA recognizes the concerns of commenters about the resource considerations 

associated with Options 1 and 3, which for areas with modeling based on actual emissions and 

designated as attaining would require full-scale modeling or screening modeling every 3 years, 

even if annual emissions in the area were not increasing. We disagree with those commenters 

who oppose any requirement for ongoing data assessment at all; and with those commenters who 

suggest a requirement for annual modeling for all areas. The EPA believes that a reasonable 

requirement for ongoing evaluation of priority areas identified by this rule is important to 

meeting the public health objectives of this NAAQS while balancing resource constraints of air 

agencies in a manageable way. The EPA agrees with commenters that suggest it would be 

reasonable to check emissions changes first, and based on that information, then make a 

determination about whether to conduct additional modeling. The EPA is also mindful of the fact 

that in this rule, modeling is effectively serving as a surrogate for monitoring, and so the EPA 

believes it is reasonable to have similar approaches for terminating the ongoing data 

requirements for both areas where air quality was initially characterized by monitoring, and areas 

where air quality was initially characterized by modeling. 
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After considering the comments received on the proposed rule, the EPA is finalizing a 

combination of elements from Option 1 and Option 2. As outlined in proposed Option 2, the final 

approach requires the air agency to conduct an assessment of emissions changes annually for all 

source areas for which the initial air quality modeling was based on actual emissions and the area 

was designated as attaining the standard. The air agency must provide this assessment to the EPA

in the form of a report, to be submitted by July 1 of the following year. This assessment should 

reflect the most recent quality-assured emissions data available for the relevant sources in the 

area. The report must also describe the reason for emissions increases in the previous year at any

listed sources, and must include a recommendation indicating for which sources and areas the 

emissions increase was substantial enough to warrant updated air quality modeling that would 

help determine air quality levels relative to the standard.

Adapting suggested criteria from a state commenter (with some modification), the EPA 

recommends as a general guideline that the air agency should conduct additional modeling 

(using the most recent actual emissions as inputs) for an area if (1) the original modeling level 

was equal to or greater than 90 percent of the standard, and there is any increase in emissions in 

the area; or (2) if the original modeling level was between 50 percent and 90 percent of the 

standard, and emissions in the area increased by 15 percent or more. However, the EPA is not 

promulgating specific criteria for when additional modeling is required because the EPA believes 

that the need for additional modeling is best judged on a case-by-case basis reflecting casespecific information on emissions changes and prior modeling results. For example, if the 

emissions increase was substantial and the previous modeling had indicated that air quality in the 

area was just under the standard, then air quality modeling would be warranted. In other cases 
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where air quality has been modeled to be well below the standard and annual emissions increase 

only slightly in the following year, the air agency would be able to exercise judgment regarding 

whether additional modeling would be needed. The use of case-specific judgment will be 

especially important in cases involving multiple sources or multiple emission units that may have 

different emissions-air quality relationships.

The modeling analysis for the area would then be due within 12 months of the air agency

recommendation that such modeling is warranted (i.e. by July 1 of the following year). See 

§51.1205(b)(1) of the final rule. In this way, if new modeling is recommended, the whole 

process ideally would take 18 months from the end of the “ongoing data requirement” year to 

when new modeling would be due (not 30 months as suggested by a state commenter). 

The EPA finds that the relatively straightforward approach described in proposed Option 

2 requiring the examination of emissions data annually (rather than conducting updated air 

quality modeling every 3 years for every area) is consistent with the frequency with which 

ambient monitoring data is evaluated. This approach also provides some flexibility to the air 

agency in recommending whether the magnitude of emissions changes in an area would be large 

enough to warrant new modeling. As compared to Options 1 and 3, this approach also would be 

expected to involve less overall workload for air agencies over time. 

In addition, as provided in Option 1, the final rule also includes a provision in 

§51.1205(b) enabling the air agency to terminate the ongoing data requirement for a modeled 

area if it meets certain criteria. The provision is analogous to §51.1205(a), which allows for the 

air agency to obtain EPA approval to cease operation of a new ambient monitor if the most 

recent design value is low enough to meet certain criteria (e.g. less than or equal to 50 percent of 
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the level of the NAAQS, or meeting the criteria of 40 CFR 58.14). Thus, for areas that were 

originally modeled based on actual emissions, §51.1205(b) of the rule allows termination of the 

air agency’s annual emission reporting requirement if the air agency submits an air quality

modeling analysis, using updated actual emissions data from the most recent 3 years, that 

demonstrates that air quality design values at all receptors in the analysis are less than or equal to 

50 percent of the 1-hour SO2 NAAQS, and such demonstration is approved by the EPA Regional 

Administrator. Likewise, if the initial modeling of a source area demonstrates that air quality 

design values at all receptors in the analysis are less than or equal to 50 percent of the 1-hour 

SO2 NAAQS, and such demonstration is approved by the EPA Regional Administrator, the area 

would not be subject to ongoing data requirements as well. The EPA believes that including this 

type of provision in the final rule structures the rule in a balanced way for both modeled and 

monitored areas in order to meet the objectives of ensuring that such areas continue to not exceed 

the standard and continue to protect public health, while recognizing the resource constraints of 

air agencies.

h. Procedural approach for post-attainment annual reporting

i. Summary of Proposal

The EPA also proposed two options regarding the procedures by which air agencies 

would submit ongoing data reports to the EPA for source areas characterized through modeling,

and by which the EPA would review and act on them. Under procedural Option 1, the EPA

proposed that the air agency would submit a report to the EPA annually as an appendix to its 

annual monitoring plan. The annual monitoring plan is required to be submitted to the EPA 

Regional Administrator by July 1 each year. The inclusion of this report as an appendix to the 
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annual monitoring plan would ensure that the report would be subject to the same opportunities 

for public review and comment that are to be provided for the monitoring plan pursuant to 

regulations at 40 CFR 58.10. Those regulations specify that if the air agency modifies the 

monitoring plan from the previous year, then prior to taking final action to approve or disapprove 

the plan, the EPA would be required to provide an opportunity for public comment on its 

proposed action. The regulations also indicate that if the air agency has already provided a public 

comment opportunity in developing its revised monitoring plan and has made no further changes 

to the plan after reviewing the public comments that were received, then it could submit the 

public comments along with the revised plan to the EPA, and the EPA Regional Administrator 

would not need to provide a separate opportunity for comment before approving or disapproving 

the plan. 

Under procedural Option 2, the ongoing report would not be submitted to the EPA as an 

appendix to the annual monitoring network plan, but it would take the form of a separate, 

independent submittal from the state to the EPA Regional Administrator. The EPA proposed that 

this report would be due by the same July 1st date each year and that this independent submittal 

would follow the general guidelines set forth in 40 CFR 58.10 regarding opportunities for public 

review as described in procedural Option 1, but the report would only include the annual 

assessments associated with sources in areas that were designated unclassifiable/attainment 

based on modeling of actual emissions. 

In the proposed rule, the EPA requested comment on the two procedural options 

presented earlier as well as any alternative ideas suggested by commenters. For any such 
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recommendations, the EPA requested that the commenter provide a detailed rationale and 

estimate of any associated costs. See 79 FR 27446, 27467, May 13, 2014.

ii. Brief Summary of Comments 

Some state, tribal and industry commenters recommended that this information be 

included as an appendix to the annual monitoring plan, rather than as a stand-alone document. 

One commenter stated that, since both options have a deadline of July 1st each year, a separate 

document would only add more time and resource use. Several state commenters recommended 

that the assessment be submitted separately from the annual monitoring plan. These commenters 

provided the following rationale: since these documents are not related, they should be kept 

separate; since the annual report refers to modeling, it will cause less confusion for the general 

public if it is a separate document from the annual monitoring plan; and because the annual 

monitoring plan and the emissions inventory submittals are performed by separate work units on 

different timelines, it would be better to deliver the products separately rather than delay one or 

the other to deliver them together.

iii. EPA Response

After considering the comments received related to both of the proposed options, the 

EPA believes that the best approach for the final rule is to allow the affected air agencies the 

discretion to either include the required annual data requirements report for modeled areas either 

as an appendix to the state’s monitoring plan, or as a stand-alone document. The air agency will 

have the flexibility under the final rule to select the approach that best meets the agency’s 

workload, schedule, and particular needs. The EPA believes that either of the procedural 

approaches will be sufficient to implement the ongoing data requirements. Regardless of which 
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approach is chosen by the air agency, the report must be submitted to the respective EPA 

Regional Office by July 1st annually and made available for public review and comment. The 

first report is due on July 1st of the year after the effective date of the area’s initial designation 

and additional reports are due July 1st of each subsequent year. 

E. Other Key Issues and Comments

Comments on the proposed rule also raised several other issues not already addressed in 

this document. This section identifies and addresses the key issues raised by those comments. 

1. March 2015 Consent Decree

The proposed rule did not contain any regulatory deadlines for the EPA to complete area 

designations under the 2010 SO2 NAAQS. However, at the same time that the EPA was 

developing the proposed rule and the final rule, the agency was also engaged in district court 

litigation from public interest groups and some states and state agencies seeking to have the EPA 

placed on a binding schedule to complete the designations. The parties in these cases filed 

complete briefs in one of these cases, resulting first in the court finding that the EPA was liable 

for having failed to meet the statutory deadline to complete all area designations. Subsequently, 

the EPA and the other parties conducted extensive settlement discussions over the remedy, i.e., 

the schedule by which the EPA would complete its duties. This resulted in a settlement between 

the EPA and the public interest group plaintiffs, which the plaintiff-interveners did not join. 

On June 2, 2014, the EPA published notice of a proposed consent decree (CD) reflecting 

this settlement (Sierra Club et al v. McCarthy, Civil Action No. 3:13-cv-3953-SI (N.D. Cal.)). 79 

FR 31325. This proposed CD included deadlines for the EPA to complete designations in three 

phases, the latter two of which were due on the same dates that the EPA discussed as its intended 
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designations dates in the preamble to the proposed DRR. The EPA received several comments 

on the notice informing the public of the proposed CD itself, and in response to this proposed 

rule. 

The EPA is not promulgating deadlines for its completion of area designations in this 

final rule. Therefore, any comments directed to the merits of the CD itself are outside the scope 

of this rulemaking, and we will not respond to them here. Instead, as discussed earlier in this 

notice, on March 2, 2015, the court issued an order entering the CD and establishing its deadlines 

as binding on the EPA. As also explained earlier, the 2017 and 2020 deadlines for the latter two 

stages of designations established by the CD will allow the EPA and states to use the new data 

and information that is timely generated by the implementation of this rule to inform the 

designations required to be completed by those dates, but it is not likely that full implementation 

of the rule can occur quickly enough to support the next round of designations required by the 

court’s order to be completed by July 2, 2016.

2. Recommendations for the EPA to Designate Areas as Unclassifiable

Several commenters recommended that the EPA take prompt action to designate areas 

with inadequate data for air quality characterization as unclassifiable. A number of commenters

asserted that the EPA cannot use the rule to supersede the statutory schedule under which the 

EPA is required to make area designations, including statutorily-appropriate “unclassifiable”

designations. One industry group commented that the CAA does not authorize the EPA to 

conduct designations according to the schedule anticipated by the proposed rule preamble, 

commenting that the EPA must instead complete designations in accordance with the schedule 

under CAA section 107(d)(1) (designating areas unclassifiable where appropriate), and then 
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redesignating unclassifiable areas as either attainment or nonattainment later. Similarly, a state 

commenter expressed the view that further data are not necessary to meet the CAA. Several 

commenters also stated that the proposed rule effectively nullifies the “unclassifiable”

designation, use of which would have allowed the EPA to meet its statutory deadline. One 

commenter also stated that the EPA should continue to use the "unclassifiable" designation 

where appropriate, and should not seek to designate all areas attainment or nonattainment. 

Several commenters also addressed the interrelationship between the proposed rule and 

the proposed consent decree for settling the lawsuit regarding the EPA’s failure to promulgate 

designations for areas without monitored violations. One state commenter urged that the EPA 

codify the proposed consent decree into the rule. Another state commenter objected to this 

suggestion, stating that the proposed consent decree specifies a designations schedule that 

conflicts with the proposed schedule and compromises a commenter’s ability to comment on the 

impact of that consent decree on the rule. An industrial commenter found the consent decree to 

undermine the proposed rule. These commenters urged that the EPA re-propose the relationship 

between the consent decree and the rule. An industry group stated that the issuance of the 

proposed consent decree undermines the rule because it would require an early round of 

designations that would be based on modeling, in contravention of the process under the 

proposed rule that offers the option of basing designations on monitoring data.

As stated previously, the EPA is not establishing or modifying any area designation 

requirements provided for in section 107 of the CAA through this rulemaking. The purpose of 

this rulemaking is to require states to characterize air quality in priority areas throughout the 

country where existing ambient monitors may not be adequately characterizing peak 1-hour SO2
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concentrations. The air quality data obtained as a result of this rulemaking then may be used in 

future designations or redesignations, as appropriate. While the notice of proposed rulemaking 

described the EPA’s anticipated designations schedule, for purposes of explaining the timeline 

by which the EPA anticipates that the data the EPA was proposing to require will be used, the 

timeline for possible future use of these data does not dictate the schedule or the substantive 

features of the requirements for obtaining data for air quality characterization purposes, and the 

Agency believes it will be highly valuable to obtain these data even if that occurs after initial

designations occur. 

While the notice of proposed rulemaking described the EPA’s expectations that 

designations for areas not already completed in August 2013 would be completed either in 2017

or in 2020, the timetables for obtaining additional data are as prompt as the EPA considers 

reasonable whether or not such data can be used to inform the remaining designations, and thus

alternate approaches and timetables for designations would not result in a different timetable for 

implementation of the rule’s requirements. In particular, whether designations proceed according 

to the approach described in the EPA’s notice of proposed rulemaking, or whether areas are first 

designated unclassifiable and subsequently redesignated to attainment or nonattainment, the 

same timetable, and substance of requirements for data to support more properly informed future 

judgments regarding areas’ attainment status is warranted. Because this rulemaking is not 

intended to define the designations process and did not propose regulatory deadlines for issuing 

designations, it would be inappropriate in this final rulemaking to codify any particular schedule 

for designations action.
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The proposed consent decree referenced by the comments concerns separate legal

proceedings that are addressing the EPA’s obligations to designate areas under CAA section 107. 

The commenters have not identified why any potential outcome of those proceedings warrants 

any particular revision to the rule, nor have they explained why the validity of the DRR is 

contingent on use of any particular designations approach. While the court’s decision 

establishing timing requirements for the EPA’s designations obligations will of course affect the 

EPA’s approach to designations, including affecting the extent to which the EPA will be able to 

use the data required under the rule at various times in the designations or redesignations 

processes, these effects do not determine the validity of the data collection requirements of the 

rule. For these reasons, the EPA believes that the ability of commenters to address issues 

relevant to the rule was not compromised by the proposed consent decree and other actions or 

statements in the proceedings regarding the EPA’s timetable for designations, and the EPA finds 

that re-proposal of the rule is not justified.

3. The Cost of Monitoring or Modeling Under this Rule.

Several state and industry commenters stated that, because of funding limitations at the 

state level, any monitoring or modeling done to meet the requirements of the rule would likely 

need to be done by the affected sources. Commenters also stated that the rule will present yet 

another burden on the regulated community when facilities are already spending resources on 

emissions reductions projects that are required as the result of other EPA air quality rules. 

Commenters also stated that even if sources voluntarily set up and operate their own 

monitors, state and local agencies will nevertheless still need to dedicate resources to administer 

the program, provide technical assistance, conduct performance audits, ensure data quality and 
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submit the data to the EPA’s Air Quality System (AQS) database each year. Commenters also 

stated that the initial state funding should be provided by the EPA through CAA section 103 or 

105 grant funds in order to establish the monitoring sites required to meet the requirements of the 

rule. 

The EPA recognizes the fact that there will be costs and resources required to satisfy the 

requirements of this rulemaking. As suggested by both state and industry stakeholders who 

attended the May-June 2012 stakeholder meetings, in the absence of increased grant funding it 

may be necessary for air agencies to rebalance their existing grant funds for this purpose, or to 

consider alternative funding approaches such as working closely with affected sources to assist 

in funding either the modeling or monitoring required to meet the requirements of the rule. Early 

planning may be helpful to address these funding needs.

Because the CAA assigns to states much of the responsibility for developing air quality

characterization data, the EPA describes the requirements of this rule in a consistent manner: air 

agencies are the entities with principal responsibility to establish and operate monitors, and

conduct modeling, and to provide air quality data to the EPA. However, the EPA recognizes that 

other parties (such as facility owners) also may perform significant portions of the work that this 

rule requires. The EPA would consider monitoring or modeling conducted by a third party to be

an appropriate means for air agencies to obtain the data necessary to meet the requirements of 

this rule, provided that the state provides oversight to assure that (1) any monitoring is conducted 

in a manner that is equivalent to SLAMs and quality-assured in accordance with applicable 

requirements, and (2) any modeling analysis that the state submits, even if it was initially
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provided to the state by a third party, is done in a reasonable manner and follows the 

recommendations in the Modeling TAD.

4. How Should the DRR Address SO2 Sources in Areas that Are Already Designated?

The intent of this SO2 data requirements rulemaking is to direct state and tribal air 

agencies to characterize air quality in areas around the largest sources of SO2 emissions, through 

the use of either air quality modeling or ambient monitoring, and to provide such data to the 

EPA. The additional information required by this rule will be able to inform future action by the 

EPA or the state (e.g., future designation decisions). 

The proposed rule did not specifically address whether the requirement to characterize a 

sources’ SO2 emission impacts would apply differently based on whether areas containing 

sources were still undesignated, or whether they had already been designated as nonattainment, 

attainment, or unclassifiable. However, much of the discussion in the proposed rule preamble 

concerned how implementation of the rule might inform future area designations, thus implying 

that the air quality characterization requirement might apply only to areas that remained 

undesignated at the time of the rule’s implementation. The EPA believes it is necessary to clarify

how the rule applies to areas that have already been designated in some manner, either during the 

initial round of designations in August 2013 or in subsequent rounds of designations pursuant to 

the March 2015 consent decree or this rule.

The first question is whether air agencies are required under this rule to characterize air 

quality near sources in areas that were designated as nonattainment in August 2013. See 78 FR 

47191, August 5, 2013. In general, we expect nonattainment plans to provide adequate 

characterization of the impacts of sources within those nonattainment areas. Therefore, we have 
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concluded that an air agency will not be required under this rule to characterize air quality

around SO2 sources located in a nonattainment area. Specifically, we have clarified the definition 

of “applicable source” in §51.1200 of the rule to be “a stationary source that is (1) not located in 

a designated nonattainment area, and (2) has annual actual SO2 emissions of 2,000 tons or more, 

or has been identified by an air agency or by the EPA Regional Administrator as requiring 

further air quality characterization.” Thus, as a general matter, this rule does not require the 

state’s January 2016 list of sources triggering the requirements of this rule to include sources 

located within areas already designated nonattainment.

However, it may be possible that in some cases an SO2 source or group of sources within 

the boundary of an existing nonattainment area can have significant impacts outside the 

nonattainment area, potentially raising concerns that these impacts might not be adequately

evaluated in a nonattainment plan. The EPA notes that for such cases, the air agency and the 

Regional Administrator retain the authority under this rule to require additional characterization 

of air quality around specific sources located in an existing nonattainment area, in the same 

manner that they retain the authority, as warranted, to require characterization of air quality

around sources that are below the emissions threshold identified in this rule. 

Related questions also arise for sources in areas that will be subject to evaluation and

designation by July 2016 under the March 2015 consent decree regarding SO2 designations.

Because all sources that meet the March 2015 consent decree criteria for designation by July

2016 will also exceed the 2,000 ton threshold under this data requirements rule, these sources

will need to be included on the January 2016 list of sources subject to requirements for air 

quality characterization under this rule. Subsequent designations do not alter this list; the list is a 
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permanent list of prioritized sources that excludes sources in areas designated nonattainment 

before January 2016 but is not altered by designations promulgated after January 2016. In 

particular, the list of sources would not be altered by promulgation of nonattainment designations 

in July 2016. Nevertheless, the EPA expects that if the area around a “consent decree” source is 

designated as nonattainment by July 2016, pursuant to the consent decree, then the information 

that was adequate to inform this designation would also satisfy the air agency’s obligation under 

this rule to submit modeling information in January 2017 characterizing air quality around that 

source. 

The next question is how this rule applies to sources in areas that have been designated as 

“unclassifiable” or as “unclassifiable/attainment.”10 The EPA did not apply these designations to 

any areas in August 2013, but the EPA may apply these designations to some areas in the 

designations required to be completed by July 2016. This rule requires air quality

characterization for areas previously designated as unclassifiable, just as it requires air quality

characterization for undesignated areas. If the EPA has previously determined through a 

designation action that sufficient information has not yet been identified to support an attainment 

or nonattainment designation (i.e., the area was initially designated as unclassifiable), then the 

additional information required by this rule will be used to inform possible future actions by the 

 

10 While states have and may continue to submit designations recommendations identifying areas 

as “attainment,” the EPA expects to continue its traditional approach, where appropriate, of using 

a designation category of "unclassifiable/attainment” for areas that the EPA determines meet the 

NAAQS. The EPA expects to reserve the category “unclassifiable” for areas where the EPA 

cannot determine based on available information whether the area is meeting or not meeting the 

NAAQS or where the EPA cannot determine whether the area contributes to a violation in a 

nearby area. See SO2 designations guidance issued by Stephen D. Page on March 20, 2015, 

available at http://www.epa.gov/airquality/sulfurdioxide/pdfs/20150320SO2designations.pdf.
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EPA or the state (e.g., to determine whether the area is attaining or not attaining the standard, 

and change designation status). 

With regard to “unclassifiable/attainment” areas, no areas were given this designation in 

the August 2013 designations. However, it is possible that some areas may be given this 

designation in the July 2016 designations based on relevant air quality characterization 

information (such as air quality modeling) that has been provided by the air agency or other 

parties in the designations process. The applicable sources in any such areas designated pursuant 

to the March 2015 consent decree would have also been included in the list of sources that air 

agencies would be required to submit to the EPA in January 2016 according to this rule. If an 

area has already been designated by the EPA as “unclassifiable/attainment” by July 2016 

pursuant to the consent decree, then the EPA expects that, as was the case for areas designated 

nonattainment, the information that was adequate to inform an unclassifiable/attainment 

designation would also satisfy the air agency’s obligation under this rule to submit modeling 

information in January 2017 characterizing air quality around that source. As a result, under this 

rule, the air agency would not be required to provide additional air quality characterization 

information to the EPA by January 2017. 

However, these already-designated “unclassifiable/attainment” areas would nevertheless 

be subject to the ongoing data requirements included in §51.1205 of this rule. While modeling 

for purposes of informing designations promulgated by July 2016 would also be considered 

modeling to address the requirements of this rule, the EPA is promulgating revised rule language 

that clarifies that the ongoing data requirements apply to areas modeled based on actual 

emissions whether that modeling was conducted for purposes of informing designations by July
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2016 or conducted only for satisfying the requirements of this rule. Accordingly, §51.1205(b) 

has been modified to apply to any unclassifiable/attainment area “where modeling of actual 

emissions was conducted to characterize air quality pursuant to §51.1203 of this subpart or 

pursuant to the March 2015 consent decree.”

5. How Air Agencies Should Address Modeling and Monitoring in Multi-state Areas to 

Meet the Requirements of the Rule.

As with the previous issue, a review of the comments and questions received from states 

has made the EPA aware of the need to clarify how the rule applies to situations where an 

applicable source that is located in one state or tribal jurisdiction has an impact on SO2

concentrations in one or more other jurisdictions. While the final rule preserves the option of the 

air agency of the jurisdiction in which the source is located to choose how to satisfy the air 

quality characterization requirements of the rule (i.e., through either monitoring or modeling),

the EPA urges all air agencies involved to consult and coordinate in order to make appropriate 

decisions concerning whether modeling or monitoring would be the most effective method to 

characterize the peak 1-hour SO2 concentrations in the ambient air affected by such sources.

If the jurisdiction in which the source is located prefers to employ ambient monitoring to 

characterize air quality, the EPA believes it would be appropriate to use ambient monitoring only

if: (1) the air agency coordinates with the other jurisdiction in identifying appropriate ambient 

monitoring sites; and (2) there is an agreement established with the other jurisdiction (in which 

peak 1-hour SO2 impacts are being experienced), and possibly with the facility owner, regarding 

logistical, financial and operational responsibilities associated with the purchase, installation and 

operation of the monitor or monitors that is acceptable to all parties. However, if one or both 
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jurisdictions do not wish to employ ambient monitoring, and a monitoring agreement cannot be 

reached, the EPA believes that the obligation to characterize air quality rests with the jurisdiction 

in which the source is located. Without an adequate multi-jurisdiction monitoring plan, the air 

agency would need to use modeling analyses to characterize air quality in the multi-jurisdiction 

area. Consultation among all involved jurisdictions will be important for planning and 

conducting technically appropriate modeling. The EPA expects that early and active coordination 

among all involved parties can lead to beneficial agreements for characterizing air quality in 

multi-jurisdiction areas, and the EPA will work with air agencies to help facilitate such 

agreements.

V. Environmental Justice Considerations

The EPA believes the human health or environmental risk addressed by this action will 

not have disproportionately high and adverse human health or environmental effects on minority, 

low-income, or indigenous populations because it does not affect the level of protection provided 

to human health or the environment under the SO2 NAAQS. When promulgated, these 

regulations will require that air agencies characterize air quality around certain large emissions 

sources, or secure emission limits on sources to reduce annual emissions below 2,000 tpy. It is 

intended that the actions resulting from this rule would lead to greater protection for U.S. 

citizens, including minority, low-income, or indigenous populations, by reducing exposure to 

high ambient concentrations of SO2. In addition, this rule will help communities by informing 

residents about ambient air quality around the largest sources of SO2. 

VI. Statutory and Executive Order Reviews
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A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: 

Improving Regulation and Regulatory Review

This action is not an economically significant action, but raises novel policy issues and 

was submitted to the Office of Management and Budget (OMB) for review. Any changes made 

in response to OMB recommendations have been documented in the docket.

B. Paperwork Reduction Act (PRA)

The information collection activities in this rule have been submitted for approval to the 

Office of Management and Budget (OMB) under the PRA. The information Collection Request 

(ICR) document that the EPA prepared has been assigned EPA ICR number 2495.01. A copy of 

the ICR is available in the docket for this rule, and is briefly summarized here. The information 

collection requirements are not enforceable until OMB approves them.

The EPA is requiring air agencies to more extensively characterize ambient SO2 air 

quality concentrations, pursuant to sections 110(a)(2)(B), 110(a)(2)(K), 301(a) and 114 of the 

CAA. For purposes of analysis of the estimated paperwork burden, the EPA assumed that 43

states and tribes would take actions to characterize air quality through either air quality modeling 

or ambient monitoring in 412 areas around SO2 sources emitting 2,000 tpy of more across the 

country, and such states would submit the results of these analyses to the EPA. Under this rule, 

the air agency will have the ability to choose, on an area-by-area basis, the analytical approach to 

follow for characterizing air quality around each qualifying source. For this reason, there is no 

way of determining exactly how many areas may be characterized through ambient monitoring 

versus air quality modeling approaches. Therefore, this section presents two sets of estimated 
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costs, one that assumes all source areas would be characterized through ambient monitoring, and 

the other that assumes that all source areas would be characterized through air quality modeling.

Potential ambient air monitoring costs are estimated based on the assumption that air 

quality for each of the 412 SO2 sources exceeding the 2,000 ton per year threshold would be 

characterized through a single newly deployed air monitor. (Note, however, that the Monitoring 

TAD discusses situations where more than one monitor may be appropriate or necessary to 

properly characterize peak 1-hour SO2 concentrations in certain areas, which would increase 

costs proportionally.) Estimates are provided for a 3-year period and include a calculation for 

equipment amortization over 7 years (as is typically done in monitoring-related ICRs). For the 

period of 2016, 2017, and 2018 (monitoring related expenditures would begin in 2016), the total 

approximate average annual monitoring cost, including a calculation for equipment amortization 

is $8,662,110 (total capital, and labor and non-labor operation and maintenance) with a total 

burden of 102,869 hours. The annual labor costs associated with these hours is $7,080,572. 

Included in the $8,662,110 total are other annual costs of non-labor operations and maintenance 

of $706,827 and equipment and contract costs of $874,711. For reference purposes, an estimate 

for initial establishment of a new SO2 monitoring station is $92,614 (does not include equipment 

amortization). In addition to the costs that would be incurred by the state and local air agencies, 

there would be an estimated burden to the EPA related to salary cost and equipment cost, etc, of 

a total of 52,717 hours and $776,005. 

Potential air quality modeling costs are estimated based on the assumption that air quality

for each of the 412 SO2 sources exceeding the 2,000 ton per year threshold would be 

characterized through air quality modeling analyses. Based on market research, stakeholder 
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feedback and assumptions about the procedures to follow when conducting modeling for 

designations purposes,11 an estimate of modeling costs for a single modeling run centered on an 

identified source would be approximately $30,000. If air agencies choose to characterize air 

quality through modeling analyses around all 412 sources expected to be identified as exceeding

the source threshold, then total national costs for modeling analyses would be estimated at 

$12,360,000. If these costs were incurred over the course of 3 years, then the approximate annual 

cost for each year over that period would be $4,120,000.

An agency may not conduct or sponsor, and a person is not required to respond to, a 

collection of information unless it displays a currently valid OMB control number. The OMB 

control numbers for the EPA's regulations in 40 CFR are listed in 40 CFR part 9. When OMB 

approves this ICR, the agency will announce that approval in the Federal Register and publish a 

technical amendment to 40 CFR part 9 to display the OMB control Number for the approved 

information collection activities contained in this final rule.

C. Regulatory Flexibility Act (RFA)

I certify that this action will not have a significant economic impact on a substantial 

number of small entities under the RFA. In making this determination, the impact of concern is 

any significant adverse economic impact on small entities. An agency may certify that a rule will 

not have a significant economic impact on a substantial number of small entities if a rule relieves 

regulatory burden, has no net burden or otherwise has a positive economic effect on the small 

entities subject to the rule. This final rule will not impose any requirements directly on small 

 

11 The Draft SO2 NAAQS Designations Modeling Technical Assistance Document can be found 

at: http://www.epa.gov/airquality/sulfurdioxide/pdfs/SO2ModelingTAD.pdf.
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entities. Entities potentially affected directly by this final rule include state, local and tribal 

governments and none of these governments are small entities. Other types of small entities are 

also not directly subject to the requirements of this rule. 

D. Unfunded Mandates Reform Act (UMRA)

This action does not contain any unfunded mandate of $100 million or more as described 

in UMRA, 2 U.S.C. 1531, and does not significantly or uniquely affect small governments. 

E. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial direct 

effects on the states, on the relationship between the national government and the states, or on 

the distribution of power and responsibilities among the various levels of government. The 

requirement to characterize air quality to inform the area designation process for the revised 

NAAQS is imposed by the CAA. This rule is intended to interpret those requirements as they

apply to the 2010 1 hour SO2 NAAQS.

F. Executive Order 13175: Consultation and Coordination with Indian Tribal Governments

This action does not have tribal implications, as specified in Executive Order 13175. It 

would not have a substantial direct effect on one or more Indian tribes. Furthermore, this

regulation does not affect the relationship or distribution of power and responsibilities between 

the federal government and Indian tribes. The CAA and the Tribal Air Rule establish the 

relationship of the federal government and tribes in characterizing air quality and developing 

plans to attain the NAAQS, and this regulation does nothing to modify that relationship. Thus, 

Executive Order 13175 does not apply to this action. 
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Consistent with the EPA Policy on Consultation and Coordination with Indian tribes, the 

EPA held several meetings with tribal environmental professionals to discuss issues associated 

with this rule, including discussions at the National Tribal Forum on May 1, 2013, and on 

National Tribal Air Association policy calls. These meetings discussed the SO2 implementation 

White Paper. The EPA provided an opportunity for tribes and stakeholders to provide written 

comments on the concepts discussed in the White Paper. Summaries of these meetings are 

included in the docket for this rule. The EPA also provided information on the proposed rule and 

conducted consultation with the National Tribal Air Association in the form of a briefing on 

April 24, 2014, and a webinar on May 21, 2014.

G. Executive Order 13045: Protection of Children from Environmental Health and Safety 

Risks

The EPA interprets Executive Order 13045 as applying only to those regulatory actions 

that concern environmental health or safety risks that the EPA has reason to believe may 

disproportionately affect children, per the definition of “covered regulatory action” in section 2-

202 of the Executive Order. This action is not subject to Executive Order 13045 because it does 

not directly involve an environmental health risk or safety risk.

H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect 

Energy Supply, Distribution, or Use

This action is not a “significant energy action” because it is not likely to have a 

significant adverse effect on the supply, distribution or use of energy. The EPA is finalizing this 

SO2 DRR to require air agencies to more extensively characterize ambient SO2 air quality

concentrations, pursuant to sections 110(a)(2)(B), 110(a)(2)(K), 301(a) and 114 of the CAA. The 
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rule does not prescribe specific control strategies by which the SO2 NAAQS will be met. Such 

strategies will be developed by states on a case-by-case basis only if the information generated 

by this rule results in an area being designated nonattainment, thereby triggering the need for the 

state to develop an attainment plan for the area. The EPA cannot predict whether the attainment 

plan prepared by the state will include regulations on energy suppliers, distributors, or users. 

Thus, the EPA concludes that this rule is not likely to have any adverse energy effects.

I. National Technology Transfer and Advancement Act

This action does not involve technical standards. 

J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations

The EPA believes the human health or environmental risk addressed by this action will 

not have potential disproportionately high and adverse human health or environmental effects on 

any population, including any minority, low-income or indigenous populations, because it does 

not affect the level of protection provided to human health or the environment. That level of 

protection is established by the NAAQS itself. The results of the evaluation of environmental 

justice considerations is contained in section V of this preamble titled, “Environmental Justice 

Considerations.”

K. Congressional Review Act (CRA)

This action is subject to the CRA, and the EPA will submit a rule report to each House of 

the Congress and to the Comptroller General of the United States. This action is not a “major 

rule” as defined by 5 U.S.C. 804(2). 

L. Judicial Review
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Under section 307(b)(1) of the CAA, petitions for judicial review of this final action 

must be filed in the United States Court of Appeals for the District of Columbia Circuit by

[INSERT DATE 60 DAYS AFTER DATE OF PUBLICATION IN THE FEDERAL

REGISTER]. Filing a petition for reconsideration by the Administrator of this final action does 

not affect the finality of this action for the purposes of judicial review nor does it extend the time 

within which a petition for judicial review must be filed, and shall not postpone the effectiveness 

of this action.

Page 135 of 143 - Data Requirements Rule for the 2010 1-Hour Sulfur Dioxide (SO2) 

Primary National Ambient Air Quality Standard (NAAQS)

Statutory Authority

The statutory authority for this action is provided by 42 U.S.C. sections 7401 et seq., and 

particularly sections 7403, 7407, 7410, 7414 and 7601.

List of Subjects in 40 CFR Part 51

Environmental protection, Air pollution control, Intergovernmental relations, Sulfur oxides.

______________________________________

Dated: 

______________________________________

Gina McCarthy,

Administrator.
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For the reasons stated in the preamble, title 40, chapter I, part 51 of the Code of Federal 

Regulations is amended as follows:

PART 51–REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL 

OF IMPLEMENTATION PLANS

1. The authority citation for part 51 continues to read as follows:

Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q. 

2. Subpart BB is added to read as follows:

Subpart BB--Data Requirements for Characterizing Air Quality for the Primary SO2

NAAQS 

Sec.

51.1200 Definitions.

51.1201 Purpose.

51.1202 Applicability.

51.1203 Air agency requirements.

51.1204 Enforceable emission limits providing for attainment. 

51.1205 Ongoing data requirements.

Subpart BB--Data Requirements for Characterizing Air Quality for the Primary SO2

NAAQS 

§51.1200 Definitions.

The following definitions apply for the purposes of this subpart. All terms not defined herein will 

have the meaning given them in §51.100 of this part or in the CAA.

Air agency means the agency or organization responsible for air quality management within a 

state, local governmental jurisdiction, territory or area subject to tribal government. 

Annual SO2 emissions data means the quality-assured annual SO2 emissions data for a stationary

source. Such data may have been required to be reported to the EPA in accordance with an 
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existing regulatory requirement (such as the Air Emissions Reporting Rule or the Acid Rain 

Program); however, annual SO2 emissions data may be obtained or determined through other 

reliable means as well. 

Applicable source means a stationary source that is (1) not located in a designated nonattainment 

area, and (2) has actual annual SO2 emissions data of 2,000 tons or more, or has been identified 

by an air agency or by the EPA Regional Administrator as requiring further air quality

characterization.

2010 SO2 NAAQS means the primary National Ambient Air Quality Standard for sulfur oxides 

(sulfur dioxide) as codified at 40 CFR 50.17, as effective August 23, 2010. 

§51.1201 Purpose.

The purpose of this subpart is to require air agencies to develop and submit air quality data 

characterizing maximum 1-hour ambient concentrations of SO2 across the United States through 

either ambient air quality monitoring or air quality modeling analysis at the air agency’s election. 

These monitoring and modeling data may be used in future determinations by the EPA regarding 

areas’ SO2 NAAQS attainment status, or for other actions designed to ensure attainment of the 

2010 SO2 NAAQS and provide protection of the public from the short-term health effects 

associated with exposure to SO2 concentrations that exceed the NAAQS.

§51.1202 Applicability.

This subpart applies to any air agency in whose jurisdiction is located one or more applicable 

sources of SO2 emissions that have annual actual SO2 emissions of 2,000 tons or more; or in 

whose jurisdiction is located one or more sources of SO2 emissions that have been identified by 

the air agency or by the EPA Regional Administrator as requiring further air quality
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characterization. For the purposes of this subpart, the subject air agency shall identify applicable 

sources of SO2 based on the most recently available annual SO2 emissions data for such sources.

§51.1203 Air agency requirements.

(a) The air agency shall submit a list of applicable SO2 sources located in its jurisdiction 

to the EPA by January 15, 2016. This list may be revised by the Regional Administrator after 

review based on available SO2 emissions data. 

(b) For each source area subject to requirements for air quality characterization, the air 

agency shall notify the EPA by July 1, 2016, whether it has chosen to (1) characterize peak 1-

hour SO2 concentrations in such area through ambient air quality monitoring; (2) characterize 

peak 1-hour SO2 concentrations in such area through air quality modeling techniques; or (3) 

provide federally enforceable emission limitations by January 13, 2017 that limit emissions of 

applicable sources to less than 2,000 tpy, in accordance with §51.1203(e), or provide 

documentation that the applicable source has permanently shut down. Emission limits in 

accordance with §51.1203(e) may be established in lieu of conducting monitoring or modeling 

unless, in the judgment of the air agency or the EPA Regional Administrator, the area warrants 

further air quality characterization even with the establishment of any new emission limit(s). If 

the air agency has chosen to establish requirements to limit emissions for applicable sources in 

an area, the notification from the air agency shall describe the requirements and emission limits

the air agency intends to apply. For any area with multiple applicable sources, the air agency (or 

air agencies if a multi-state area) shall use the same technique (monitoring, modeling, or 

emissions limitation) for all applicable sources in the area. If multiple air agencies have 
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applicable sources in an area, the air agencies must consult with each other to employ a common 

technique for the area.

(c) Monitoring. For each area identified in the notification submitted pursuant to 

§51.1203(b) of this subpart as an area for which SO2 concentrations will be characterized through 

ambient monitoring, the required monitors shall be sited and operated either as SLAMS or in a 

manner equivalent to SLAMS. In either case, monitors shall meet applicable criteria in 40 CFR 

part 58, appendices A, C, and E and their data shall be subject to data certification and reporting 

requirements as prescribed in 40 CFR 58.15 and 58.16. These requirements include quarterly

reporting of monitoring data to the Air Quality System, and the annual certification of data by

May 1 of the following year.

(1) The air agency shall include relevant information about monitors used to meet the 

requirements of paragraph (c) in the air agency’s Annual Monitoring Network Plan required by 

40 CFR 58.10 due July 1, 2016. The air agency shall consult with the appropriate EPA Regional 

Office in the development of plans to install, supplement, or maintain an appropriate ambient 

SO2 monitoring network pursuant to the requirements of 40 CFR part 58 and of this subpart. 

(2) All existing, new, or relocated ambient monitors intended to meet the requirements of 

paragraph (c) must be operational by January 1, 2017 and must be operated continually until 

approved for shut down by EPA.

(3) Any SO2 monitor identified by an air agency in its approved Annual Monitoring 

Network Plan as having the purpose of meeting the requirements of paragraph (c) that: is not 

located in an SO2 nonattainment area; is not also being used to satisfy other ambient SO2

minimum monitoring requirements listed in 40 CFR part 58, appendix D, section 4.4; and is not 
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otherwise required as part of a SIP, permit, attainment plan or maintenance plan, may be eligible 

for shut down upon EPA approval if it produces a design value no greater than 50 percent of the 

2010 SO2 NAAQS from data collected in either its first or second 3-year period of operation. 

The air agency must receive EPA Regional Administrator approval of a request to cease 

operation of the monitor as part of the EPA’s action on the Annual Monitoring Network Plan 

under 40 CFR 58.10 prior to shutting down any qualifying monitor under this paragraph.

(d) Modeling. For each area identified in the notification submitted pursuant to 

§51.1203(b) of this subpart as an area for which SO2 concentrations will be characterized 

through air quality modeling, the air agency shall submit by July 1, 2016, a technical protocol for 

conducting such modeling to the Regional Administrator for review. The air agency shall consult 

with the appropriate EPA Regional Office in developing these modeling protocols. 

(1) The modeling protocol shall include information about the modeling approach to be 

followed, including but not limited to the model to be used, modeling domain, receptor grid, 

emissions dataset, meteorological dataset and how the air agency will account for background 

SO2 concentrations. 

(2) Modeling analyses shall characterize air quality based on either actual SO2 emissions 

from the most recent 3 years, or on any federally enforceable allowable emission limit or limits 

established by the air agency or the EPA and that are effective and require compliance by

January 13, 2017. 

(3) Except as provided by §51.1204 of this subpart, the air agency shall conduct the 

modeling analysis for any applicable source identified by the air agency pursuant to §51.1203(a)
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of this subpart, and for its associated area and any nearby area, as applicable, and submit the 

modeling analysis to the EPA Regional Office by January 13, 2017. 

(e) Federally enforceable requirement to limit SO2 emissions to under 2,000 tons per 

year. For each area identified in the notification submitted pursuant to §51.1203(b) of this 

subpart as an area for which the air agency will adopt federally enforceable requirements in lieu 

of characterizing air quality through monitoring or modeling, the air agency shall submit 

documentation to the EPA by January 13, 2017, showing that such requirements have been 

adopted and been made federally enforceable by January 13, 2017, through an appropriate legal 

mechanism, and the provisions either: (1) require the applicable sources in the area to emit less 

than 2,000 tons of SO2 per year for calendar year 2017 and thereafter; or (2) document that the 

applicable sources in the area have permanently shut down by January 13, 2017. 

§51.1204 Enforceable emission limits providing for attainment. 

At any time prior to January 13, 2017, the air agency may submit to the EPA effective federally

enforceable SO2 emissions limits for one or more applicable sources that provide for attainment 

of the 2010 SO2 NAAQS in the area affected by such emissions. The submittal shall include

associated air quality modeling and other analyses that demonstrate that all modeling receptors in 

the area will not violate the 2010 SO2 NAAQS, taking into account the updated allowable 

emission limits on applicable sources as well as emissions limits that apply to any other sources 

in the area. The air agency shall not be subject to the ongoing data requirements of §51.1205 for 

such area if the air quality modeling and other analyses demonstrate that the area will not violate 

the 2010 SO2 NAAQS.

§51.1205 Ongoing data requirements. 
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(a) Monitored areas. For any area where SO2 monitoring was conducted to characterize 

air quality pursuant to §51.1203 of this subpart, the air agency shall continue to operate the 

monitor(s) used to meet those requirements and shall continue to report ambient data pursuant to 

existing ambient monitoring regulations, unless the monitor(s) have been approved for shut 

down by the EPA Regional Administrator pursuant to §51.1203(c)(3) of this subpart or pursuant 

to 40 CFR 58.14. 

(b) Modeled areas. For any area where modeling of actual emissions was conducted to 

characterize air quality pursuant to §51.1203 of this subpart or pursuant to the March 2015 

consent decree and such area was designated as attaining the 2010 SO2 NAAQS, the air agency

shall submit an annual report to the EPA Regional Administrator by July 1 of each year, either as 

a stand-alone document made available for public inspection, or as an appendix to its Annual 

Monitoring Network Plan (also due on July 1 each year under 40 CFR 58.10), that documents the 

annual SO2 emissions of each applicable source in each such area and provides an assessment of 

the cause of any emissions increase from the previous year. The first report for each such area is 

due by July 1 of the calendar year after the effective date of the area’s initial designation.

(1) The air agency shall include in such report a recommendation regarding whether 

additional modeling is needed to characterize air quality in any area to determine whether the 

area meets or does not meet the 2010 SO2 NAAQS. The EPA Regional Administrator will 

consider the emissions report and air agency recommendation, and may require that the air 

agency conduct updated air quality modeling for the area and submit it to the EPA within 12 

months.
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(2) An air agency will no longer be subject to the requirements of §51.1205(b) for a 

particular area if it provides air quality modeling demonstrating that air quality values at all 

receptors in the analysis are no greater than 50 percent of the 1-hour SO2 NAAQS, and such 

demonstration is approved by the EPA Regional Administrator.

(c) Any air agency that demonstrates that an area would meet the 2010 SO2 NAAQS 

with allowable emissions pursuant to §51.1205(b) to submit future annual reports for the area. 

(d) If modeling or monitoring information required to be submitted by the air agency to 

the EPA pursuant to this subpart indicates that an area is not attaining the 2010 SO2 NAAQS, the 

EPA may take appropriate action, including but not limited to requiring adoption of enforceable 

emission limits to ensure continued attainment of the 2010 SO2 NAAQS, designation or 

redesignation of the area to nonattainment, or issuance of a SIP Call.