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[Federal Register Volume 82, Number 187 (Thursday, September 28, 2017)]
[Proposed Rules]
[Pages 45242-45253]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2017-20721]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R01-OAR-2017-0083; FRL-9968-43-Region 1]

Air Plan Approval; New Hampshire; Nonattainment Plan for the 
Central New Hampshire SO2 Nonattainment Area

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing to 
approve the State Implementation Plan (SIP) revision that the State of 
New Hampshire submitted to EPA on January 31, 2017 for attaining the 1-
hour sulfur dioxide (SO2) primary national ambient air 
quality standard (NAAQS) for the Central New Hampshire Nonattainment 
Area. This plan (herein called a ``nonattainment plan'') includes New 
Hampshire's attainment demonstration and other elements required under 
the Clean Air Act (CAA). In addition to an attainment demonstration, 
the nonattainment plan addresses the requirement for meeting reasonable 
further progress (RFP) toward attainment of the NAAQS, reasonably 
available control measures and reasonably available control technology 
(RACM/RACT), base-year and projection-year emission inventories, and 
contingency measures. As a part of approving the attainment 
demonstration, EPA is also proposing to approve SO2 emission 
limits and associated compliance parameters for Merrimack Station into 
the New Hampshire SIP. EPA proposes to conclude that New Hampshire has 
appropriately demonstrated that the nonattainment plan provisions 
provide for attainment of the 2010 1-hour primary SO2 NAAQS 
in the Central New Hampshire Nonattainment Area by the applicable 
attainment date and that the nonattainment plan meets the other 
applicable requirements under the CAA.

DATES: Comments must be received on or before October 30, 2017.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-R01-
OAR-2017-0083 at http://www.regulations.gov, or via email to 
[email protected]. For comments submitted at Regulations.gov, follow 
the online instructions for submitting comments. Once submitted, 
comments cannot be edited or removed from Regulations.gov. For either 
manner of submission, EPA may publish any comment received to its 
public docket. Do not submit electronically any information you 
consider to be Confidential Business Information (CBI) or other 
information whose disclosure is restricted by statute. Multimedia 
submissions (audio, video, etc.) must be accompanied by a written 
comment. The written comment is considered the official comment and 
should include discussion of all points you wish to make. EPA will 
generally not consider comments or comment contents located outside of 
the primary submission (i.e., on the web, cloud, or other file sharing 
system). For additional submission methods, please contact the person 
identified in the FOR FURTHER INFORMATION CONTACT section. For the full 
EPA public comment policy, information about CBI or multimedia 
submissions, and general guidance on making effective comments, please 
visit http://www.epa.gov/dockets/commenting-epa-dockets.

FOR FURTHER INFORMATION CONTACT: Leiran Biton, EPA New England, 5 Post 
Office Square Suite 100, Mail Code OEP05-2, Boston, MA 02109-3912; 
phone: 617-918-1267; fax: 617-918-0267; email: [email protected].

SUPPLEMENTARY INFORMATION:
    Throughout this document whenever ``we,'' ``us,'' or ``our'' is 
used, we mean EPA.

Table of Contents

I. Why was New Hampshire required to submit an SO2 plan 
for the Central New Hampshire Nonattainment area?
II. Requirements for SO2 Nonattainment Area Plans
III. Attainment Demonstration and Longer-Term Averaging
IV. Review of Modeled Attainment Plan
    A. Model Selection and Modeling Components
    B. Area of Analysis
    C. Receptor Grid
    D. Meteorological Data
    E. Source Characterization
    F. Emissions Data
    G. Emission Limits
    1. Enforceability
    2. Longer-Term Average Limits
    H. Background Concentrations
    I. Summary of Results
V. Review of Other Plan Requirements
    A. Emissions Inventory
    B. RACM/RACT
    C. New Source Review (NSR)
    D. Reasonable Further Progress (RFP)
    E. Contingency Measures
VI. Additional Elements of New Hampshire's Submittal
    A. Conformity
    B. Changes in Allowable Emissions
    C. Air Quality Trends
    D. Compliance With Section 110(a)(2) of the CAA
    E. Equivalency Techniques
VII. EPA's Proposed Action
VIII. Incorporation by Reference
IX. Statutory and Executive Order Reviews

I. Why was New Hampshire required to submit an SO2 plan for 
the Central New Hampshire Nonattainment area?

    On June 22, 2010, EPA promulgated a new 1-hour primary 
SO2 NAAQS of 75 parts per billion (ppb), which is met at an 
ambient air quality monitoring site when the 3-year average of the 
annual 99th percentile of 1-hour daily maximum concentrations does not 
exceed 75 ppb, as determined in accordance with appendix T of 40 CFR 
part 50. See 75 FR 35520, codified at 40 CFR 50.17(a)-(b). On August 5, 
2013, EPA designated a first set of 29 areas of the country as 
nonattainment for the 2010 SO2 NAAQS, including the Central 
New Hampshire Nonattainment Area within the State of New Hampshire. See 
78 FR 47191, codified at 40 CFR part 81, subpart C. These area 
designations were effective October 4, 2013. Section 191 of the CAA 
directs states to submit SIPs for areas designated as nonattainment for 
the SO2 NAAQS to EPA within 18 months of the effective date 
of the designation, i.e., by no later than April 4, 2015 in this case. 
These SIPs are required to demonstrate that their respective areas will 
attain the NAAQS as expeditiously as practicable, but no later than 5 
years from the effective date of designation, which is October 4, 2018.
    For a number of areas, including the Central New Hampshire 
Nonattainment Area, EPA published a notice on March 18, 2016 that New 
Hampshire and other pertinent states had failed to submit the required 
SO2 nonattainment plan by the submittal deadline. See 81 FR 
14736. This finding initiated a deadline under CAA section 179(a) for 
the potential imposition of new source and highway funding sanctions, 
and for EPA to promulgate a federal implementation plan (FIP) under 
section 110(c) of the CAA. In response to the requirement for 
SO2 nonattainment plan submittals, New Hampshire submitted a 
nonattainment plan for the Central New Hampshire Nonattainment Area on 
January 31, 2017. Pursuant to New Hampshire's January 31, 2017 
submittal and EPA's subsequent letter dated March 20, 2017 to New 
Hampshire finding the submittal complete and noting the stopping of the 
sanctions deadline, these sanctions under section 179(a) will not be 
imposed. However, to

[[Page 45243]]

stop the deadline for EPA to promulgate a FIP, the state must have made 
the necessary complete submittal and EPA must have approved the 
submittal as meeting applicable requirements no later than two years 
after the prior finding of failure to submit. Therefore, EPA remains 
under a FIP deadline of April 18, 2018. This FIP obligation will not 
apply if EPA issues final approval of New Hampshire's SIP submittal by 
April 18, 2018.
    The remainder of this preamble describes the requirements that 
nonattainment plans must meet in order to obtain EPA approval, provides 
a review of the State's plan with respect to these requirements, and 
describes EPA's proposed action on the plan.

II. Requirements for SO2 Nonattainment Area Plans

    Nonattainment SIPs must meet the applicable requirements of the 
CAA, and specifically CAA sections 110, 172, 191 and 192. EPA's 
regulations governing nonattainment SIPs are set forth at 40 CFR part 
51, with specific procedural requirements and control strategy 
requirements residing at subparts F and G, respectively. Soon after 
Congress enacted the 1990 Amendments to the CAA, EPA issued 
comprehensive guidance on SIPs in a document entitled, ``General 
Preamble for the Implementation of Title I of the Clean Air Act 
Amendments of 1990,'' published at 57 FR 13498 (April 16, 1992) 
(General Preamble). Among other things, the General Preamble addressed 
SO2 SIPs and fundamental principles for SIP control 
strategies. Id., at 13545-49, 13567-68. On April 23, 2014, EPA issued 
recommended guidance for meeting the statutory requirements in 
SO2 SIPs, in a document entitled, ``Guidance for 1-Hour 
SO2 Nonattainment Area SIP Submissions,'' available at 
https://www.epa.gov/sites/production/files/2016-06/documents/20140423guidance_nonattainment_sip.pdf. In this guidance, EPA described 
the statutory requirements for a complete nonattainment area SIP, which 
includes: An accurate emissions inventory of current emissions for all 
sources of SO2 within the nonattainment area, an attainment 
demonstration, demonstration of RFP, implementation of RACM (including 
RACT), an approvable NSR program, enforceable emissions limitations and 
control measures as needed for timely attainment, and adequate 
contingency measures for the affected area.
    In order for EPA to fully approve a SIP as meeting the requirements 
of CAA sections 110, 172, 191, and 192, and EPA's regulations at 40 CFR 
part 51, the SIP for the affected area needs to demonstrate to EPA's 
satisfaction that each of the aforementioned requirements has been met. 
Under CAA sections 110(l) and 193, EPA may not approve a SIP that would 
interfere with any applicable requirement concerning NAAQS attainment 
and RFP, or any other applicable requirement under the CAA. 
Furthermore, no requirement in effect, or required to be adopted by an 
order, settlement, agreement, or plan in effect before November 15, 
1990, in any nonattainment area for any air pollutant, may be modified 
in any manner unless it ensures equivalent or greater emission 
reductions of such air pollutant.

III. Attainment Demonstration and Longer-Term Averaging

    CAA sections 172(c)(1) and (6) direct states with areas designated 
as nonattainment to demonstrate that the submitted plan provides for 
attainment of the NAAQS. Forty CFR part 51, subpart G further 
delineates the control strategy requirements that SIPs must meet, and 
EPA has long required that all SIPs and control strategies reflect four 
fundamental principles of quantification, enforceability, 
replicability, and accountability. See General Preamble, at 13567-68. 
SO2 attainment plans must consist of two components: (1) 
Emission limits and other control measures that assure implementation 
of permanent, enforceable, and necessary emission controls; and (2) a 
modeling analysis that meets the requirements of 40 CFR part 51, 
appendix W (the Guideline on Air Quality Models; ``the Guideline'') and 
demonstrates that these emission limits and control measures provide 
for timely attainment of the primary SO2 NAAQS as 
expeditiously as practicable, but by no later than the attainment date 
for the affected area. In all cases, the emission limits and control 
measures must be accompanied by appropriate methods and conditions to 
determine compliance with the respective emission limits and control 
measures and must be quantifiable (i.e., a specific amount of emission 
reduction can be ascribed to the measures), fully enforceable 
(specifying clear, unambiguous, and measurable requirements for which 
compliance can be practicably determined), replicable (the procedures 
for determining compliance are sufficiently specific and non-subjective 
so that two independent entities applying the procedures would obtain 
the same result), and accountable (source specific limits must be 
permanent and must reflect the assumptions used in the SIP 
demonstrations).
    EPA's April 2014 guidance recommends that the emission limits be 
expressed as short-term average limits (e.g., addressing emissions 
averaged over one or three hours), but also describes the option to 
utilize emission limits with longer averaging times of up to 30 days so 
long as the state meets various suggested criteria. See April 2014 
guidance, pp. 22 to 39. The guidance recommends that--should states and 
sources utilize longer averaging times--the longer-term average limit 
should be set at an adjusted level that reflects a stringency 
comparable to the 1-hour average limit at the critical emission value 
shown to provide for attainment that the plan otherwise would have set.
    The April 2014 guidance provides an extensive discussion of EPA's 
rationale for concluding that appropriately set comparably stringent 
limitations based on averaging times as long as 30 days can be found to 
provide for attainment of the 2010 SO2 NAAQS. In evaluating 
this option, EPA considered the nature of the standard, conducted 
detailed analyses of how 30-day average limits impact attainment of the 
standard, and carefully reviewed how best to achieve an appropriate 
balance among the various factors that warrant consideration in judging 
whether a state's plan provides for attainment. Id. at pp. 22 to 39. 
See also id. at appendices B, C, and D.
    As specified in 40 CFR 50.17(b), the 1-hour primary SO2 
NAAQS is met at an ambient air quality monitoring site when the 3-year 
average of the annual 99th percentile of daily maximum 1-hour 
concentrations is less than or equal to 75 parts per billion. In a year 
with 365 days of valid monitoring data, the 99th percentile would be 
the fourth highest daily maximum 1-hour value. The 2010 SO2 
NAAQS, including this form of determining compliance with the standard, 
was upheld by the U.S. Court of Appeals for the District of Columbia 
Circuit in Nat'l Envt'l Dev. Ass'n's Clean Air Project v. EPA, 686 F.3d 
803 (D.C. Cir. 2012). Because the standard has this form, a single 
exceedance does not create a violation of the standard. Instead, at 
issue is whether a source operating in compliance with a properly set 
longer-term average could cause exceedances, and if so what the 
resulting frequency and magnitude of such exceedances will be, and in 
particular whether EPA can have reasonable confidence that a properly 
set longer-term average limit will provide that the average fourth 
highest daily maximum value will be at or below 75 ppb. A synopsis of 
how EPA judges whether such plans

[[Page 45244]]

``provide for attainment,'' based on modeling of projected allowable 
emissions and in light of the form of the NAAQS for determining 
attainment at monitoring sites, follows.
    For plans for SO2 based on 1-hour emission limits, the 
standard approach is to conduct modeling using fixed emission rates. 
The maximum emission rate that would be modeled to result in attainment 
(i.e., in an ``average year'' \1\ shows three, not four days with 
maximum hourly levels exceeding 75 ppb) is labeled the ``critical 
emission value.'' The modeling process for identifying this critical 
emission value inherently considers the numerous variables that affect 
ambient concentrations of SO2, such as meteorological data, 
background concentrations, and topography. In the standard approach, 
the state would then provide for attainment by setting a continuously 
applicable 1-hour emission limit at this critical emission value.
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    \1\ An ``average year'' is used to mean a year with average air 
quality. While 40 CFR 50 appendix T provides for averaging three 
years of 99th percentile daily maximum values (e.g., the fourth 
highest maximum daily concentration in a year with 365 days with 
valid data), this discussion and an example below uses a single 
``average year'' in order to simplify the illustration of relevant 
principles.
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    EPA recognizes that some sources have highly variable emissions, 
for example due to variations in fuel sulfur content and operating 
rate, that can make it extremely difficult, even with a well-designed 
control strategy, to ensure in practice that emissions for any given 
hour do not exceed the critical emission value. EPA also acknowledges 
the concern that longer-term emission limits can allow short periods 
with emissions above the critical emission value, which, if coincident 
with meteorological conditions conducive to high SO2 
concentrations, could in turn create the possibility of a NAAQS 
exceedance occurring on a day when an exceedance would not have 
occurred if emissions were continuously controlled at the level 
corresponding to the critical emission value. However, for several 
reasons, EPA believes that the approach recommended in our guidance 
document suitably addresses this concern. First, from a practical 
perspective, EPA expects the actual emission profile of a source 
subject to an appropriately set longer-term average limit to be similar 
to the emission profile of a source subject to an analogous 1-hour 
average limit. EPA expects this similarity because it has recommended 
that the longer-term average limit be set at a level that is comparably 
stringent to the otherwise applicable 1-hour limit (reflecting a 
downward adjustment from the critical emission value) and that takes 
the source's emission profile into account. As a result, EPA expects 
either form of emission limit to yield comparable air quality.
    Second, from a more theoretical perspective, EPA has compared the 
likely air quality with a source having maximum allowable emissions 
under an appropriately set longer-term limit, as compared to the likely 
air quality with the source having maximum allowable emissions under 
the comparable 1-hour limit. In this comparison, in the 1-hour average 
limit scenario, the source is presumed at all times to emit at the 
critical emission level, and in the longer-term average limit scenario, 
the source is presumed occasionally to emit more than the critical 
emission value but on average, and presumably at most times, to emit 
well below the critical emission value. In an ``average year,'' 
compliance with the 1-hour limit is expected to result in three 
exceedance days (i.e., three days with hourly values above 75 ppb) and 
a fourth day with a maximum hourly value at 75 ppb. By comparison, with 
the source complying with a longer-term limit, it is possible that 
additional exceedances would occur that would not occur in the 1-hour 
limit scenario (if emissions exceed the critical emission value at 
times when meteorology is conducive to poor air quality). However, this 
comparison must also factor in the likelihood that exceedances that 
would be expected in the 1-hour limit scenario would not occur in the 
longer-term limit scenario. This result arises because the longer-term 
limit requires lower emissions most of the time (because the limit is 
set well below the critical emission value), so a source complying with 
an appropriately set longer-term limit is likely to have lower 
emissions at critical times than would be the case if the source were 
emitting as allowed with a 1-hour limit.
    As a hypothetical example to illustrate these points, suppose a 
source always emits 1,000 pounds of SO2 per hour and results 
in air quality at the level of the NAAQS (i.e., results in a design 
value of 75 ppb). Suppose further that in an ``average year,'' these 
emissions cause the five highest maximum daily average 1-hour 
concentrations to be 100 ppb, 90 ppb, 80 ppb, 75 ppb, and 70 ppb. Then 
suppose that the source becomes subject to a 30-day average emission 
limit of 700 pounds per hour. It is theoretically possible for a source 
meeting this limit to have emissions that occasionally exceed 1,000 
pounds per hour, but with a typical emission profile, emissions would 
much more commonly be between 600 and 800 pounds per hour. In this 
simplified example, assume a zero background concentration, which 
allows one to assume a linear relationship between emissions and air 
quality. (A nonzero background concentration would make the mathematics 
more difficult but would give similar results.) Air quality will depend 
on what emissions occur during critical hours, but suppose that 
emissions at the relevant times on these 5 days are 800 pounds per 
hour, 1,100 pounds per hour, 500 pounds per hour, 900 pounds per hour, 
and 1,200 pounds per hour, respectively. (This is a conservative 
example because the average of these emissions, 900 pounds per hour, is 
well over the 30-day average emission limit.) These emissions would 
result in daily maximum 1-hour concentrations of 80 ppb, 99 ppb, 40 
ppb, 67.5 ppb, and 84 ppb. In this example, the fifth day would have an 
exceedance that would not otherwise have occurred, but the third and 
fourth days would not have exceedances that otherwise would have 
occurred. In this example, the fourth highest maximum daily 
concentration under the 30-day average would be 67.5 ppb.
    This simplified example illustrates the findings of a more 
complicated statistical analysis that EPA conducted using a range of 
scenarios using actual plant data. As described in appendix B of EPA's 
April 2014 SO2 nonattainment planning guidance, EPA found 
that the requirement for lower average emissions is highly likely to 
yield better air quality than is required with a comparably stringent 
1-hour limit. Based on analyses described in appendix B of our April 
2014 guidance, EPA expects that an emission profile with maximum 
allowable emissions under an appropriately set comparably stringent 30-
day average limit is likely to have the net effect of having a lower 
number of exceedances and better air quality than an emission profile 
with maximum allowable emissions under a 1-hour emission limit at the 
critical emission value. This result provides a compelling policy 
rationale for allowing the use of a longer averaging period in 
appropriate circumstances where the facts indicate this result can be 
expected to occur.
    The question then becomes whether this approach--which is likely to 
produce a lower number of overall exceedances even though it may 
produce some unexpected exceedances above the critical emission value--
meets the requirement in section 110(a)(1) and 172(c)(1) and (6) for 
state implementation plans to ``provide for

[[Page 45245]]

attainment'' of the NAAQS. For SO2, as for other pollutants, 
it is generally impossible to design a nonattainment plan in the 
present that will guarantee that attainment will occur in the future. A 
variety of factors can cause a well-designed attainment plan to fail 
and unexpectedly not result in attainment, for example if meteorology 
occurs that is more conducive to poor air quality than was anticipated 
in the plan. Therefore, in determining whether a plan meets the 
requirement to provide for attainment, EPA's task is commonly to judge 
not whether the plan provides absolute certainty that attainment will 
in fact occur, but rather whether the plan provides an adequate level 
of confidence of prospective NAAQS attainment. From this perspective, 
in evaluating use of a 30-day average limit, EPA must weigh the likely 
net effect on air quality. Such an evaluation must consider the risk 
that occasions with meteorology conducive to high concentrations will 
have elevated emissions leading to exceedances that would not otherwise 
have occurred, and must also weigh the likelihood that the requirement 
for lower emissions on average will result in days not having 
exceedances that would have been expected with emissions at the 
critical emission value. Additional policy considerations, such as in 
this case the desirability of accommodating real world emissions 
variability without significant risk of violations, are also 
appropriate factors for EPA to weigh in judging whether a plan provides 
a reasonable degree of confidence that the plan will lead to 
attainment. Based on these considerations, especially given the high 
likelihood that a continuously enforceable limit averaged over as long 
as 30 days, determined in accordance with EPA's guidance, will result 
in attainment, EPA believes as a general matter that such limits, if 
appropriately determined, can reasonably be considered to provide for 
attainment of the 2010 SO2 NAAQS.
    The April 2014 guidance offers specific recommendations for 
determining an appropriate longer-term average limit. The recommended 
method starts with determination of the 1-hour emission limit that 
would provide for attainment (i.e., the critical emission value), and 
applies an adjustment factor to determine the (lower) level of the 
longer-term average emission limit that would be estimated to have a 
stringency comparable to the otherwise necessary 1-hour emission limit. 
This method uses a database of continuous emission data reflecting the 
type of control that the source will be using to comply with the SIP 
emission limits, which (if compliance requires new controls) may 
require use of an emission database from another source. The 
recommended method involves using these data to compute a complete set 
of emission averages, computed according to the averaging time and 
averaging procedures of the prospective emission limitation. In this 
recommended method, the ratio of the 99th percentile among these 
longer-term averages to the 99th percentile of the 1-hour values 
represents an adjustment factor that may be multiplied by the candidate 
1-hour emission limit to determine a longer-term average emission limit 
that may be considered comparably stringent.\2\ The guidance also 
addresses a variety of related topics, such as the potential utility of 
setting supplemental emission limits, such as mass-based limits, to 
reduce the likelihood and/or magnitude of elevated emission levels that 
might occur under the longer-term emission rate limit.
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    \2\ For example, if the critical emission value is 1,000 pounds 
of SO2 per hour, and a suitable adjustment factor is 
determined to be 70 percent, the recommended longer-term average 
limit would be 700 pounds per hour.
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    Preferred air quality models for use in regulatory applications are 
described in appendix A of EPA's Guideline on Air Quality Models. In 
2005, EPA promulgated AERMOD as the Agency's preferred near-field 
dispersion modeling for a wide range of regulatory applications 
addressing stationary sources (for example in estimating SO2 
concentrations) in all types of terrain based on extensive 
developmental and performance evaluation. On December 20, 2016, EPA 
revised the Guideline, which provided additional regulatory options and 
updated methods for dispersion modeling with AERMOD; the updates became 
effective on May 22, 2017. Supplemental guidance on modeling for 
purposes of demonstrating attainment of the SO2 standard is 
provided in appendix A to the April 23, 2014 SO2 
nonattainment area SIP guidance document referenced above. Appendix A 
of the guidance provides extensive guidance on the modeling domain, 
source inputs, assorted types of meteorological data, and background 
concentrations. Consistency with the recommendations in this guidance 
is generally necessary for the attainment demonstration to offer 
adequately reliable assurance that the plan provides for attainment.
    As stated previously, attainment demonstrations for the 2010 1-hour 
primary SO2 NAAQS must demonstrate future attainment and 
maintenance of the NAAQS in the entire area designated as nonattainment 
(i.e., not just at the violating monitor) by using air quality 
dispersion modeling to show that the mix of sources and enforceable 
control measures and emission rates in an identified area will not lead 
to a violation of the SO2 NAAQS. For a short-term (e.g., 1-
hour) standard, EPA believes that dispersion modeling using allowable 
emissions and addressing stationary sources in the affected area (and 
in some cases those sources located outside the nonattainment area 
which may affect attainment in the area) is technically appropriate, 
efficient, and effective in demonstrating attainment in nonattainment 
areas because it takes into consideration combinations of 
meteorological and emission source operating conditions that may 
contribute to peak ground-level concentrations of SO2.
    The meteorological data used in the analysis should generally be 
processed with the most recent version of AERMET. Estimated 
concentrations should include ambient background concentrations, should 
follow the form of the standard, and should be calculated as described 
in the August 23, 2010 clarification memo on ``Applicability of 
Appendix W Modeling Guidance for the 1-hr SO2 National 
Ambient Air Quality Standard.''

IV. Review of Modeled Attainment Plan

    The following discussion evaluates various features of the modeling 
that New Hampshire used in its attainment demonstration.

A. Model Selection and Modeling Components

    New Hampshire's attainment demonstration used EPA's preferred model 
AERMOD (version 15181) with default options (e.g., without use of the 
ADJ_U* option) and rural dispersion coefficients for this application. 
The AERMOD modeling system contains the following components:

--AERMOD: The dispersion model
--AERMAP: The terrain processor for AERMOD
--AERMET: The meteorological data processor for AERMOD
--BPIP-PRIME: The building input processor
--AERMINUTE: A pre-processor to AERMET incorporating 1-minute automated 
surface observation system (ASOS) wind data
--AERSURFACE: The surface characteristics processor for AERMET
--AERSCREEN: A screening version of AERMOD

    For any dispersion modeling exercise, the ``urban'' or ``rural'' 
determination of

[[Page 45246]]

a source is important in determining the boundary layer characteristics 
that affect the model's prediction of downwind concentrations. For 
SO2 modeling, the urban/rural determination is important 
because AERMOD invokes a 4-hour half-life for urban SO2 
sources.
    To investigate whether the rural determination was correct, EPA 
examined aerial imagery within 3 km of the facility and classified land 
use within the total area, as described in section 7.2.1.1 of the 
Guideline. Using this approach, EPA found that less than 50 percent of 
the land use in the area reflected urban characteristics, and that 
therefore, consistent with the State's selection, rural dispersion 
characteristics were most appropriate for use in this assessment.
    The State used AERMOD version 15181, the most up-to-date version at 
the time the area was modeled, using all regulatory default options. 
AERMOD version 16216r has since become the regulatory model version. 
There were no updates from 15181 to 16216r that would significantly 
affect the concentrations predicted here.
    The ADJ_U* option, which adjusts the minimum surface roughness 
velocity under stable, low-wind speed conditions, was not invoked by 
the State. Not invoking ADJ_U*, as in the demonstration submitted by 
New Hampshire, may result in higher modeled concentrations; therefore, 
this element of the model option selection is conservative (i.e., 
unlikely to underpredict concentrations).
    EPA finds this selection appropriate because this model version 
using default options is sufficiently up to date, the rural option 
selection is in line with site characteristics, and the selection of 
default surface roughness velocity characteristics (i.e., no ADJ_U*) is 
not expected to underpredict concentrations.

B. Area of Analysis

    New Hampshire accounted for SO2 impacts in the modeling 
domain, which extends in a 50 km radius around Merrimack Station and 
includes both locations within and outside of the nonattainment area, 
through the inclusion of measured background levels and explicitly 
modeled emission sources. The only source New Hampshire included 
explicitly in the modeling was Merrimack Station. In the narrative of 
the January 31, 2017 SIP submittal, New Hampshire indicated that other 
emitters of SO2 were accounted for in the background levels 
monitored within the nonattainment area. (The approach for developing 
the monitored background levels is described in detail in section IV.H, 
below.) In the submittal, New Hampshire also identified sources with 
annual emissions greater than 100 tons SO2 per year outside 
of the nonattainment area. Specifically, in the submission to EPA, New 
Hampshire identified Schiller Station and Newington Station, which are 
both located in the New Hampshire seacoast area approximately 55 km to 
the east southeast of Merrimack Station, as the principal nearby 
emitters of over 100 tons SO2 annually. Schiller and 
Newington stations are each located about 30 km from the boundary of 
the nonattainment area.
    For the purpose of ensuring that no other sources of SO2 
were inappropriately excluded in New Hampshire's modeling, EPA reviewed 
its 2014 National Emissions Inventory (NEI), version 1 for sources 
within or nearby to the nonattainment area. During this review, EPA 
identified one additional source in the region that has emitted greater 
than 100 tons of SO2 annually, though not within the Central 
New Hampshire Nonattainment Area. The source, Monadnock Paper Mills 
Inc. (Monadnock Paper), a pulp and paper facility located in 
Bennington, New Hampshire approximately 40 km to the southwest of 
Merrimack Station and 24 km from the closest portion of the 
nonattainment area, emitted 148 tons SO2 in 2014 according 
to the 2014 NEI.
    EPA examined whether Monadnock Paper might have an influence on the 
nonattainment area. The main criterion described in section 8.3 of the 
Guideline for establishing whether a secondary source is adequately 
represented by ambient monitoring data is whether that secondary source 
causes a significant concentration gradient in the vicinity of the 
primary source under consideration. In this context, secondary sources 
that do not cause a significant concentration gradient are typically 
considered to be adequately represented in the monitored ambient 
background. Based on the magnitude of emissions and distance relative 
to the nonattainment area, EPA believes it is unlikely that Monadnock 
Paper will cause a significant concentration gradient within the 
nonattainment area and has concluded that Monadnock Paper is adequately 
represented in the monitored ambient background.
    To examine the possible influence of other sources on the 
nonattainment area, EPA considered the most recent modeling assessment 
for Schiller and Newington stations provided by New Hampshire to EPA in 
February 2017 for purposes of SO2 designations. That 
modeling and EPA's evaluation of it are described in detail in the New 
Hampshire technical support document for EPA's intended designations 
for the 2010 SO2 NAAQS, for which EPA sent letters to states 
on August 22, 2017. Based on this information, EPA found no significant 
concentration gradient due to emissions from Schiller Station or 
Newington Station within the nonattainment area and has concluded that 
both stations are adequately represented in the monitored ambient 
background.
    Additionally, EPA believes that the background levels reasonably 
account for other sources influencing air quality within the 
nonattainment area because data used to develop background levels 
include hours during which those sources may have impacted the 
monitors.
    Therefore, based on the reasoning provided in the preceding 
paragraphs, EPA concludes that the State appropriately accounted for 
these other sources through the inclusion of monitored background 
concentrations (see section IV.H below).

C. Receptor Grid

    Within AERMOD, air quality concentration results are calculated at 
discrete locations identified by the user; these locations are called 
receptors. The receptor placement for the area of analysis selected by 
the State is a network of polar grids centered on Merrimack Station to 
a distance of 50 km in all directions. Polar grid radii were spaced at 
10 degree intervals. Receptors were placed every 20 meters along the 
perimeter of and excluded within the facility. Polar receptors along 
the radii were spaced as follows:

--20-meter spacing to 200 meters;
--50-meter spacing from 200 meters to 500 meters;
--100-meter spacing from 500 meters to 2 km;
--250-meter spacing from 2 km to 10 km;
--500-meter spacing from 10 km to 30 km; and
--1,000-meter spacing from 30 km to 50 km.

    In addition to the 4,349 receptors included in the description 
above, the State included 2,308 additional receptors in dense Cartesian 
arrays with 100-meter spatial resolution, over areas of expected 
maximum predicted concentrations based on preliminary modeling. 
Specifically, this was done in areas of complex terrain features at 
distances between 5 and 15 km of Merrimack Station.
    The receptor network contained a total of 6,657 receptors, covering 
a

[[Page 45247]]

circular area of 50 km in radius, including the entirety of the 
nonattainment area. EPA finds that the modeling domain and receptor 
network are sufficient to identify maximum impacts from Merrimack 
Station, and are therefore adequate for characterizing the 
nonattainment area.

D. Meteorological Data

    New Hampshire used AERMOD's meteorological data preprocessor AERMET 
(version 15181) with 2 years of surface and concurrent upper air 
meteorological data. The State relied on site-specific surface 
observations collected at Merrimack Station in Bow, New Hampshire 
during the 23-month period from January 1994 through November 1995 at 
five meteorological tower measurement levels and fifteen SODAR (Sound 
Detection and Ranging) levels. In addition, the State used surface 
observations from the National Weather Service (NWS) station at Concord 
Municipal Airport in Concord, New Hampshire (WBAN Station No. 14745) in 
the following ways: (1) To supplement site-specific surface data with 
additional parameters (sky cover, ceiling height, and surface pressure) 
not available in the site-specific meteorological data, (2) to 
substitute for missing site-specific wind observations (51 hours of the 
16,776 hours of the 23 month period), and (3) to extend the 
meteorological dataset through December 1995 to develop a full 2-year 
analysis period. Concord Municipal Airport is approximately 7 km to the 
north-northwest of Merrimack Station. The State used coincident upper 
air observations from different NWS stations located in Portland, Maine 
(WBAN Station No. 14764) from January 1, 1994 through September 21, 
1994, and Gray, Maine (WBAN Station No. 54762) from September 22, 1994 
through December 31, 1995. (The Portland station ceased its upper air 
observations on September 22, 1994, when the Gray station began its 
upper air observations.) The Portland station is around 110 km to the 
northeast of Merrimack and the Gray station is around 130 km to the 
northeast of Merrimack.
    New Hampshire also considered the use of more recent (2008-2012) 
NWS data collected at Concord Municipal Airport. The State cited two 
potential advantages of using this alternative dataset, mainly that it 
was significantly newer and included data derived from 1-minute 
resolution observations using the AERMINUTE preprocessor to AERMET. New 
Hampshire weighed these considerations against the advantages of using 
the 1994-1995 site-specific data, specifically: (1) The observation 
height for the site-specific data is closer in height to the stacks at 
Merrimack Station than the 8 meter collection height for the NWS data; 
(2) the site-specific wind direction data are more representative of 
the channeling effect within the Merrimack River valley in the location 
of Merrimack Station; and (3) use of the site-specific data would be 
consistent with previous modeling of Merrimack, which relied on the 
site-specific meteorology.
    EPA concurs with the choice of surface and upper air meteorological 
data inputs as being appropriately representative of site-specific 
meteorology. Specifically, EPA has judged the representativeness of the 
measured surface meteorological data according to the following four 
factors, as listed in section 8.4.1(b) to the Guideline: (1) The 
proximity of the meteorological monitoring site to the area under 
consideration, (2) the complexity of the terrain, (3) the exposure of 
the meteorological monitoring site, and (4) the period of time during 
which data are collected. Regarding proximity (factor 1), the site-
specific data is preferred over the more distant NWS data, though both 
data sources are sufficiently close to be appropriately representative 
of the site. Regarding the complexity of terrain (factor 2), both 
Concord and the site-specific location show wind flow patterns with 
predominant northwest flow and secondary southeast flows, but the site-
specific data show a more pronounced valley channeling effect with 
fewer hours with wind flow in other directions. In terms of exposure of 
the site, neither location appears to be exposed in a way that would 
have biased data collection (factor 3). Finally, regarding the data 
collection time period (factor 4), the more recent data at the NWS 
station would allow for use of 1-minute resolution data for more 
accurate wind data inputs, and would be preferred for this factor. 
Notwithstanding the age of the onsite data, current land-use is 
comparable to historical land-use, so that the historic meteorological 
data are sufficiently representative of current conditions. In summary, 
based on the four factors described above, despite the availability of 
recent nearby NWS data, the analysis suggests that the 1994-1995 site-
specific data augmented with NWS data are more representative of 
conditions pertinent to releases at Merrimack Station. The 23 months of 
site-specific data supplemented with 1 additional month of NWS data 
represent an appropriate study period, consistent with EPA guidance 
contained in section 8.4.2(e) of the Guideline, which states that at 
least 1 year of site-specific meteorological data are required to 
ensure that worst-case meteorological conditions are adequately 
represented in the model results. The upper air stations selected for 
the analysis are the closest sites and are suitably representative of 
the upper air in the Central New Hampshire Nonattainment Area, and are 
therefore most appropriate for developing upper air profiles for the 
State's modeling analysis.
    The State used AERSURFACE version 13016 using land cover data from 
the 1992 National Land Cover Dataset (NLCD) for both surface data 
collection locations to estimate the surface characteristics (albedo, 
Bowen ratio, and surface roughness length) of the area of analysis. The 
State estimated surface roughness length values for 12 spatial sectors 
out to the recommended radius of 1 km at a monthly temporal resolution 
for average surface moisture conditions. EPA concurs with New 
Hampshire's approach to developing relevant surface characteristics for 
use in processing meteorological data for this area.

E. Source Characterization

    EPA also reviewed the State's source characterization in its 
modeling assessment, including source types, use of accurate stack 
parameters, and inclusion of building dimensions for building downwash. 
The State's source characterization in its modeling demonstration was 
consistent with the recommendations included in the Guideline. The 
source used actual stack height (445 feet), which EPA determined to be 
good engineering practice (GEP) height using BPIP-PRIME. The State also 
adequately characterized the source's building layout and location, as 
well as the stack parameters, e.g., exit temperature, exit velocity, 
location, and diameter. EPA verified the position of buildings and 
stacks using aerial imagery and relevant stack parameters based on 
permit conditions.

F. Emissions Data

    New Hampshire included maximum allowable 1-hour emissions from 
Merrimack Station in its modeled attainment demonstration for the 
Central New Hampshire Nonattainment Area. The State indicated that 
SO2 air quality in the area is almost entirely characterized 
by emissions from the two primary boilers at Merrimack Station, and 
this informed the State's decision to only explicitly model 
SO2 emissions from Merrimack Station. Additional

[[Page 45248]]

units (i.e., two peak combustion turbines, an emergency generator, an 
emergency boiler, and a fire pump) at Merrimack Station operate 
infrequently and were treated as intermittent sources; therefore, they 
were excluded from the modeling.\3\ The State provided historical 
(2011-2014) counts of hours of operation for these units to bolster its 
contention that these units do not contribute to the annual 
distribution of daily maximum 1-hour concentrations. Specifically, 
during the 2011-2014 period, the two turbines were operated during an 
average of 40 and 45 hours per year, the emergency generator during an 
average of 17 hours per year, the emergency boiler during an average of 
43 hours per year, and the fire pump during an average of 3 hours per 
year. The maximum annual usage of any of these pieces of equipment 
during that time was 114 hours for combustion turbine 1 in 2014. The 
emergency generator is limited through section Env-A 1311.02(a) of New 
Hampshire's SIP-approved air pollution control regulations, to a 
maximum of 500 hours of operation during any consecutive 12-month 
period. The fire pump is limited to a maximum of 100 hours for 
maintenance and testing during any consecutive 12-month period because 
it is subject to EPA's New Source Performance Standards for stationary 
internal combustion engines, specifically 40 CFR 60.4211(e). These 
utilization levels and patterns are consistent with EPA's assessment of 
intermittent emissions based on the March 1, 2011 EPA guidance. EPA 
believes that this treatment is appropriate for those units in this 
area.
---------------------------------------------------------------------------

    \3\ The March 1, 2011 EPA memorandum from Tyler Fox to EPA 
Regional Air Division Directors entitled ``Additional Clarification 
Regarding Application of Appendix W Modeling Guidance for the 1-hour 
NO2 National Ambient Air Quality Standard,'' which also 
includes information relevant to modeling for SO2, 
addresses treatment of intermittent sources. This guidance indicates 
that air permitting authorities have discretion to exclude certain 
types of intermittent emissions for modeling the 1-hour NAAQS on a 
case-specific basis.
---------------------------------------------------------------------------

    New Hampshire provided attainment modeling used to support its 
establishment of emission rates for Merrimack Station. In establishing 
the emission limits, the State followed EPA's April 2014 guidance by 
using modeling to develop a critical emission value and adjustment 
factor to establish a longer term limit for Merrimack. The State 
modeled three ``normal operating scenarios,'' comprised of one scenario 
with maximum operation of both utility boilers (scenario 1), and two 
other scenarios with maximum operation of each boiler individually 
(scenarios 2 and 3, respectively). In 2011, New Hampshire issued a 
permit (TP-0008) for Merrimack Station that contained, among other 
things, SO2 emission limits associated with a flue gas 
desulfurization (FGD) system. The FGD was required to be installed at 
Merrimack Station by the New Hampshire legislature. See New Hampshire 
Revised Statutes Annotated (RSA) 125-O:11. EPA approved the 
SO2-related source-specific requirements of that permit into 
the New Hampshire SIP as part of the State's regional haze SIP 
submittal. See 77 FR 50602 (August 22, 2012). In September 2016, New 
Hampshire issued a second permit (TP-0189) for Merrimack Station, which 
included SO2 emission limits specifically designed to ensure 
compliance with the SO2 NAAQS. The emission limits included 
in TP-0189, and which New Hampshire has proposed for inclusion in the 
State's SIP, apply at all times. The State's modeling established a 
critical emission value of 2,544 pounds (lb) SO2 per hour 
for scenario 1, which the State concluded is comparably stringent to a 
7-boiler operating day rolling average limit of 0.39 lb SO2 
per million British thermal units (MMBtu). The 7-boiler operating day 
rolling average emissions limits that would be comparably stringent to 
the 1-hour critical emission value under scenarios 2 and 3 would be 
0.92 and 0.47 lb SO2/MMBtu, respectively. Because scenario 1 
was the basis for establishing this limit, and the limit (0.39 lb/
MMBtu) is more stringent than the limits that would have been 
established for either scenario 2 or 3 (0.92 and 0.47 lb/MMBtu, 
respectively), using emissions from scenario 1 as the basis of the 
modeling analysis is appropriate. See section IV.G.2 below for further 
details on the emissions in the State's attainment modeling, including 
discussion of the State's conclusion of comparable stringency with the 
critical emission value.
    In summary, EPA concurs with the State's selection in its 
attainment demonstration modeling of emissions from utility boilers at 
Merrimack Station, and exclusion of additional emission sources at 
Merrimack due to their intermittent operation.

G. Emission Limits

    An important prerequisite for approval of a nonattainment plan is 
that the emission limits that provide for attainment be quantifiable, 
fully enforceable, replicable, and accountable. See General Preamble at 
13567-68. The limits that New Hampshire's plan relies on for Merrimack 
Station are expressed as 7-boiler operating day rolling average limits, 
where a boiler operating day is defined as a 24-hour period that begins 
at midnight and ends the following midnight during which any fuel is 
combusted at any time in the boiler; it is not necessary for the fuel 
to be combusted for the entire 24-hour period. Therefore, part of the 
review of New Hampshire's nonattainment plan must address the use of 
these limits, both with respect to the general suitability of using 
such limits for this purpose and with respect to whether the particular 
limits included in the plan have been suitably demonstrated to provide 
for attainment. The first subsection that follows addresses the 
enforceability of the limits in the plan, and the second subsection 
that follows addresses in particular the 7-boiler operating day average 
limits.
1. Enforceability
    On September 1, 2016, New Hampshire issued a permit, TP-0189, to 
Public Service of New Hampshire d/b/a Eversource Energy for Merrimack 
Station. The permit became effective and enforceable upon issuance, and 
was issued pursuant to RSA 125-C:11. These requirements are more 
stringent than the applicable measures for the facility, which require 
90% reduction for both MK1 and MK2, as incorporated into the SIP by 
reference to Table 4, Items 6 and 8 of TP-0008. EPA considers the 30-
boiler operating day limits included in TP-0189 (specifically, Table 4, 
Item 2) to supersede the conditions specified in Table 4, Items 6 and 8 
of TP-0008.
    Monitoring, testing, and recordkeeping requirements related to all 
of the permit's SO2 emission limits are clearly described in 
the permit and ensure that the limits are quantifiable, fully 
enforceable, and replicable. The accountability of the limits is 
established through the State's inclusion of the permit limits in its 
nonattainment plan, and its modeling demonstration using the 1-hour 
emission levels that are comparably stringent to the permit limits. In 
accordance with EPA policy, the 7-boiler operating day average limit 
for Merrimack Station is set at a lower level than the critical 
emission value used in the attainment demonstration; the relationship 
between these two values is discussed in more detail in the following 
section.
2. Longer-Term Average Limits
    New Hampshire developed a critical emission value for each of the 
three normal operating scenarios (see section IV.F above) using a 
target concentration threshold of 183.2 micrograms per cubic meter 
([mu]g/m\3\) by subtracting a background value of 12.8 [mu]g/m\3\, the

[[Page 45249]]

highest hour-by-season background value (see section IV.H below), from 
196 [mu]g/m\3\, which is equivalent to the level of the NAAQS of 75 
ppb.\4\ The State then divided the target concentration threshold by 
the maximum predicted 99th percentile concentration using a unit 
emission rate (i.e., 1 lb/hr) for each normal operating scenario to 
establish the critical emission value for each scenario (e.g., 2,544 
lb/hr, equivalent to a limit of 0.54 lb/MMBtu at full operating load, 
for scenario 1).
---------------------------------------------------------------------------

    \4\ Using a numerical conversion factor of 2.619 [mu]g/m\3\ per 
ppb, the 2010 SO2 NAAQS of 75 ppb is equivalent to 196.4 
[mu]g/m\3\. The state rounded 196.4 [mu]g/m\3\ down to a more 
protective level of 196 [mu]g/m\3\. EPA is using the lower value in 
this case because it is consistent with the State's analysis and is 
also protective of the NAAQS.
---------------------------------------------------------------------------

    Using hourly emission data provided by EPA's Air Markets Program 
Data database for Merrimack Station for the period between July 4, 2013 
and March 30, 2015 (i.e., since the FGD system became operational), the 
State derived adjustment factors for longer-term averaging periods for 
each scenario. Because the dataset includes only data from Merrimack 
Station using the control technology, it is appropriate for use in 
developing adjustment factors. Prior to deriving the adjustment 
factors, the State removed erroneous data points from the dataset based 
on information provided by the facility. The adjustment factors were 
calculated as the ratio of the 99th percentile of mass emissions for 
the longer-term period to the 99th percentile hourly mass emissions. 
For the rolling 7-day averaging period, the adjustment factor was 0.73 
for each of the three scenarios. That is, the 7-day mass emission rate 
limit would need to be 0.73 times (or 27% lower than) the critical 
emission value to have comparable stringency as a 1-hour rate limit. 
The 7-day adjustment factor of 0.73 for Merrimack Station is similar to 
0.71, EPA's average 30-day adjustment factor for sources with wet 
scrubbers (derived from a database of 210 sources) as listed in 
appendix D of the April 2014 guidance. The State then derived emission 
limits for each scenario on an emission per heat-input basis, and 
selected the lowest level for the 7-day averaging period of 0.39 lb/
MMBtu.
    Based on a review of the State's submittal, EPA believes that the 
7-boiler operating day average limit for Merrimack Station provides a 
suitable alternative to establishing a 1-hour average emission limit 
for this source. The State has used a suitable database in an 
appropriate manner and has thereby applied an appropriate adjustment, 
yielding an emission limit that has comparable stringency to the 1-hour 
average limit that the State determined would otherwise have been 
necessary to provide for attainment. While the 7-boiler operating day 
average limit allows occasions in which emissions may be higher than 
the level that would be allowed with the 1-hour limit, the State's 
limit compensates by requiring average emissions to be lower than the 
level that would otherwise have been required by a 1-hour average 
limit. For the reasons described above and explained in more detail in 
EPA's April 2014 guidance for SO2 nonattainment plans, EPA 
finds that appropriately set longer-term average limits provide a 
reasonable basis by which nonattainment plans may provide for 
attainment. Based on our review of this general information as well as 
the particular information in New Hampshire's plan, EPA finds that the 
7-boiler operating day average limit for Merrimack Station will provide 
for attainment of the SO2 NAAQS.
    In the April 2014 guidance for SO2, EPA also described 
possible supplemental limits on the frequency and/or magnitude of 
elevated emissions to strengthen the justification for the use of 
longer-term average limits to protect against NAAQS violations. One 
option provided in the guidance regarding this topic is the use of 
relatively shorter averaging times, which provide less allowance of 
emission spikes than would longer averaging times, i.e., the 30-day 
averaging time. In this instance, the emission limit for Merrimack 
Station is on a 7-boiler operating day average basis and the limit 
applies at all times. Furthermore, the adjustment factor used to derive 
the limit is similar to 0.71, EPA's average 30-day adjustment factor 
for sources with wet scrubbers as listed in appendix D of the April 
2014 guidance, meaning that the factor used to adjust the emission 
limit downward is more pronounced for a 7-day period than would 
typically be expected. Based on these considerations, EPA believes that 
the 7-boiler operating day limits are sufficiently protective of the 
NAAQS without application of an additional, supplemental limit.

H. Background Concentrations

    To develop background concentrations for the nonattainment area, 
the State of New Hampshire relied on 2012-2014 data from two monitors 
within the nonattainment area: The Pembroke monitor, Air Quality System 
(AQS) number 33-013-1006, and the Concord monitor, AQS number 33-013-
1007. The Pembroke monitor is located on Pleasant Street in Pembroke, 
New Hampshire, about 1.3 km to the southeast of Merrimack Station, and 
the Concord monitor is located at Hazen Drive in Concord, New 
Hampshire, about 9.4 km to the north-northwest of Merrimack Station. 
Each of these monitors was sited to record neighborhood scale exposure 
levels rather than regional background levels; there are currently no 
regional background monitors in the Central New Hampshire Nonattainment 
Area. Per section 8.3.1.a of the Guideline, background air quality 
should not include the ambient impacts of the source under 
consideration. Both the Pembroke and Concord monitors reflect impacts 
attributable to Merrimack Station. One solution to develop background 
concentrations from monitoring data around an isolated source, as 
described in section 8.3.2.c.i of the Guidance, is to exclude monitor 
measurements collected when wind is from a 90[deg] sector centered on 
the source. Due to the low wind speeds and swirling winds 
characteristic of Merrimack Station's river valley location, emissions 
from the source may contribute to the monitors even when the wind 
direction is outside of the 90[deg] sector. Therefore, the State 
determined that the 90[deg] exclusion sector approach was not 
appropriate for this application, and selected an alternative approach 
to develop background levels. Specifically, the State compiled an 
ambient concentration database using the lower observed value for the 
two monitors' hourly values as representing regional background levels. 
This approach accounts for area and mobile sources and more distant 
sources that were not modeled explicitly but affect SO2 
levels in the nonattainment area without also double-counting impacts 
from Merrimack Station, which was modeled explicitly. Using this 
approach, EPA finds the State's treatment of SO2 background 
levels to be suitable for the modeled attainment demonstration.

I. Summary of Results

    The modeling analysis upon which the State relied in establishing a 
critical emission value for setting emission limits for Merrimack 
Station results in concentrations of no greater than 196.0 [micro]g/
m\3\, which is below the level of the 1-hour primary SO2 
NAAQS of 196.4 [micro]g/m\3\. EPA agrees with the State that these 
results indicate that emissions at the critical emission value for 
Merrimack Station provide for attainment of the 1-hour SO2 
NAAQS.

[[Page 45250]]

V. Review of Other Plan Requirements

A. Emissions Inventory

    The emissions inventory and source emission rate data for an area 
serve as the foundation for air quality modeling and other analyses 
that enable states to: (1) Estimate the degree to which different 
sources within a nonattainment area contribute to violations within the 
affected area; and (2) assess the expected improvement in air quality 
within the nonattainment area due to the adoption and implementation of 
control measures. As noted above, the State must develop and submit to 
EPA a comprehensive, accurate, and current inventory of actual 
emissions from all sources of SO2 emissions in each 
nonattainment area, as well as any sources located outside the 
nonattainment area which may affect attainment in the area. See CAA 
section 172(c)(3).
    In its plan, New Hampshire included a current emissions inventory 
for the nonattainment area and also for the three-county area of 
Hillsborough, Merrimack, and Rockingham Counties based on the 2011-2015 
period. The State principally relied on 2014 as the most complete and 
representative record of annual SO2 emissions because it 
coincided with EPA's National Emissions Inventory (NEI), which includes 
a comprehensive inventory of all source types. The State allocated 2014 
NEI version 1 emissions from the portion of each county within the 
nonattainment area using city- and town-level population (for area and 
non-road mobile sources) and vehicle miles traveled (VMT; for on-road 
mobile sources) statistics. The State included emissions from point 
sources (e.g., Merrimack Station) to the area based on location. The 
State calculated emissions for the area from some types of sources 
based on county-level emissions. A summary of the State's emissions 
inventories for 2011, 2014, and 2018 are presented in Table 1. Based on 
the State's inventory, of the 5,471 tons SO2 emitted in 2014 
within the three county area, 1,480 tons were emitted within the 
nonattainment area. Merrimack Station emitted 1,044 tons SO2 
in 2014. These emissions levels are much lower than historical 
emissions levels; for example, in 2011, Merrimack Station emitted 
22,420 tons SO2.

    Table 1--Summary of New Hampshire's Inventory of Actual SO2 Emissions for the Central New Hampshire Area
----------------------------------------------------------------------------------------------------------------
                                                              Hillsborough,      Central New
                                                             Merrimack, and       Hampshire         Merrimack
                           Year                                Rockingham       nonattainment    Station (tons)
                                                            Counties  (tons)    area  (tons)
----------------------------------------------------------------------------------------------------------------
2011......................................................            24,934            22,398            22,420
2014......................................................             5,471             1,480             1,044
2018 (projected)..........................................             6,966             2,473             1,927
----------------------------------------------------------------------------------------------------------------

    New Hampshire also developed a projected emission inventory for the 
2018 attainment year. The emissions projection indicates 1,927 tons of 
SO2 from Merrimack Station and a total of 2,473 tons of 
SO2 within the nonattainment area; however, these 
projections rely on a 90% reduction in SO2 emissions from 
Merrimack Station, which is less stringent than the at least 93.4% 
reduction incorporated into the permit New Hampshire issued for 
Merrimack Station on September 1, 2016, TP-0189.
    EPA agrees that the State's emissions inventories are appropriate 
because they rely on well-established and vetted estimates of emissions 
for the current period and attainment year, respectively.

B. RACM/RACT

    CAA section 172(c)(1) requires that each attainment plan provide 
for the implementation of all reasonably available control measures 
(RACM) as expeditiously as practicable (including such reductions in 
emissions from existing sources in the area as may be obtained through 
the adoption, at a minimum, of reasonably available control technology 
(RACT)) and shall provide for attainment of the NAAQS. EPA interprets 
RACM, including RACT, under section 172, as measures that a state 
determines to be reasonably available and which contribute to 
attainment as expeditiously as practicable for existing sources in the 
area.
    In its January 31, 2017 SIP submittal, New Hampshire identified the 
operational and SO2 emission limits contained in Merrimack 
Station's permit, TP-0189, as meeting RACM/RACT. New Hampshire's plan 
for attaining the 1-hour SO2 NAAQS in the Central New 
Hampshire Nonattainment Area is based on the operational and emission 
limitations contained in Merrimack Station's permit. Specifically, 
Merrimack Station's permit limits SO2 emissions from the MK1 
and MK2 boilers at Merrimack Station to 0.39 lb/MMBtu on a 7-boiler 
operating day rolling average (achieved through operation of the FGD), 
which the State demonstrated was comparably stringent to the critical 
emission value that provides for attainment of the NAAQS, as described 
in section IV.G.2 above. New Hampshire's nonattainment plan includes 
the SO2 control measures required by the permit, which was 
effective immediately upon issuance on September 1, 2016. New Hampshire 
has determined that these measures suffice to provide for timely 
attainment, and plans to incorporate relevant conditions contained in 
TP-0189 into Merrimack's title V operating permit (TV-0055).
    The air modeling analysis submitted to EPA during the development 
of the SO2 limits in TP-0189 confirms that these limits are 
protective of the NAAQS, as described in section IV. Because the 
modeling demonstrates attainment using emission limits contained in 
Merrimack Station's permit, TP-0189, the State determined that controls 
for SO2 emissions at Merrimack Station are appropriate in 
the Central New Hampshire Area for purposes of attaining the 2010 
SO2 NAAQS. Accordingly, New Hampshire only completed a RACM/
RACT analysis for Merrimack Station because the air quality modeling 
showed that the SO2 emission reductions required by TP-0189 
will be sufficient to ensure that the nonattainment area achieves 
attainment with the SO2 NAAQS. EPA believes that New 
Hampshire's approach is consistent with EPA's April 2014 guidance, 
which indicates that ``[a]ir agencies should consider all RACM/RACT 
that can be implemented in light of the attainment needs for the 
affected area(s).''
    The Central New Hampshire Area is currently showing an attaining 
design value for 2014-2016, and has been since

[[Page 45251]]

the 2012-2014 period, which means that attainment of the NAAQS is as 
expeditious as practicable.
    Based on New Hampshire's modeling demonstration, which accounted 
for the SO2 emission limits contained in Merrimack Station's 
permit, TP-0189, the Central New Hampshire Area is projected to attain 
the 2010 SO2 NAAQS by the 2018 attainment date. Because the 
area is currently attaining the 2010 SO2 NAAQS, EPA proposes 
to find that the control strategy will ensure attainment of the NAAQS 
by the required attainment date.
    The State's plan also includes a broader discussion of the 
SO2 control strategy beyond Merrimack Station's permit, TP-
0189. Merrimack Station is also subject to requirements of the Mercury 
and Air Toxics Standards (MATS), which promotes reductions at subject 
facilities of certain hazardous air pollutants, including hydrochloric 
acid; such reductions are achieved at Merrimack Station through the 
operation of the FGD system, which concurrently reduces emissions of 
SO2. New Hampshire also notes in its nonattainment plan the 
anticipated 73% reduction in SO2 emissions among upwind 
states subject to EPA's Cross State Air Pollution Rule (CSAPR), which 
will lessen the contribution of sources from other states into the 
nonattainment area in future years. New Hampshire also described 
emissions reductions at Schiller Station as part of statewide efforts 
to reduce SO2, as well as other state rules.
    EPA concurs with New Hampshire's approach and analysis, and 
proposes to conclude that the State has satisfied the requirement in 
section 172(c)(1) to adopt and submit all RACM as needed to attain the 
SO2 NAAQS as expeditiously as practicable.

C. New Source Review (NSR)

    EPA last approved New Hampshire's Env-A 618 nonattainment new 
source review rules on May 25, 2017 (82 FR 24057). These rules provide 
for appropriate new source review for SO2 sources undergoing 
construction or major modification in the Central New Hampshire 
Nonattainment Area without need for modification of the approved rules. 
Therefore, EPA concludes that this requirement has already been met for 
this area.

D. Reasonable Further Progress (RFP)

    New Hampshire concluded that the appropriate control measures were 
implemented as expeditiously as practicable in order to ensure 
attainment of the standard by the applicable attainment date. 
Specifically, the State implemented its main control strategy, i.e., 
establishment of federally enforceable SO2 emissions limits 
and operational conditions in TP-0189 for Merrimack Station in 
September 2016. New Hampshire concluded that this plan therefore 
provides for RFP in accordance with the approach to RFP described in 
EPA's guidance. EPA concurs and proposes to conclude that the plan 
provides for RFP.

E. Contingency Measures

    As discussed in our guidance, Section 172(c)(9) of the CAA defines 
contingency measures as such measures in a SIP that are to be 
implemented in the event that an area fails to make RFP, or fails to 
attain the NAAQS, by the applicable attainment date. Contingency 
measures are to become effective without further action by the state or 
EPA, where the area has failed to (1) achieve RFP or (2) attain the 
NAAQS by the statutory attainment date for the affected area. These 
control measures are to consist of other available control measures 
that are not included in the control strategy for the nonattainment 
area SIP. EPA guidance describes special features of SO2 
planning that influence the suitability of alternative means of 
addressing the requirement in section 172(c)(9) for contingency 
measures for SO2. Because SO2 control measures 
are by definition based on what is directly and quantifiably necessary 
emissions controls, any violations of the NAAQS are likely related to 
source violations of a source's permit terms. Therefore, an appropriate 
means of satisfying this requirement for SO2 is for the 
state to have a comprehensive enforcement program that identifies 
sources of violations of the SO2 NAAQS and to undertake an 
aggressive follow-up for compliance and enforcement.
    For its contingency program, New Hampshire proposed to continue to 
operate a comprehensive program to identify sources of violations of 
the SO2 NAAQS and undertake aggressive compliance and 
enforcement actions, including expedited procedures for establishing 
consent agreements pending the adoption of the revised SIP. New 
Hampshire's program for enforcement of SIP measures for the 2010 
SO2 NAAQS was approved by EPA on June 15, 2016. See 81 FR 
44542. As EPA stated in its April 2014 guidance, EPA believes that this 
approach continues to be a valid approach for the implementation of 
contingency measures to address the 2010 SO2 NAAQS.
    Based on the contingency measures identified by the State in its 
plan submittal, EPA believes that New Hampshire's plan provides for 
satisfying the contingency measure requirement. EPA concurs and 
proposes to approve New Hampshire's plan for meeting the contingency 
measure requirement in this manner.

VI. Additional Elements of New Hampshire's Submittal

A. Conformity

    The State addresses general conformity and transportation 
conformity requirements as they apply to the nonattainment area. 
Generally, as set forth in section 176(c) of the Clean Air Act, 
conformity requires that actions by federal agencies do not cause new 
air quality violations, worsen existing violations, or delay timely 
attainment of the relevant NAAQS. General conformity applies to federal 
actions, other than certain highway and transportation projects, if the 
action takes place in a nonattainment area or maintenance area (i.e., 
an area which submitted a maintenance plan that meets the requirements 
of section 175A of the CAA and has been redesignated to attainment) for 
ozone, particulate matter, nitrogen dioxide, carbon monoxide, lead, or 
SO2. EPA's General Conformity Rule (40 CFR 93.150 to 93.165) 
establishes the criteria and procedures for determining if a federal 
action conforms to the SIP. With respect to the 2010 SO2 
NAAQS, federal agencies are expected to continue to estimate emissions 
for conformity analyses in the same manner as they estimated emissions 
for conformity analyses under the previous NAAQS for SO2. 
EPA's General Conformity Rule includes the basic requirement that a 
federal agency's general conformity analysis be based on the latest and 
most accurate emission estimation techniques available (40 CFR 
93.159(b)). When updated and improved emissions estimation techniques 
become available, EPA expects the federal agency to use these 
techniques. New Hampshire addresses general conformity under SIP-
approved state rule Env-A 1500.
    Federal Highway and Federal Transit Administration projects are 
subject to transportation conformity rather than general conformity 
requirements, with some exceptions. New Hampshire asserts in its plan 
that due to minimal impact on SO2 from combustion of 
gasoline and diesel fuels, transportation conformity rules do not 
generally apply to SO2 unless the EPA Regional Administrator 
or the state air director finds that its transportation-related 
SO2 emissions are a significant contributor to fine 
particulate matter as a precursor.

[[Page 45252]]

This reasoning is consistent with EPA's April 2014 guidance and EPA 
proposes to conclude that New Hampshire's plan meets our guidance and 
rule requirements with regard to general and transportation conformity.

B. Changes in Allowable Emissions

    The State quantified the changes in allowable emissions expected to 
result from implementation of its nonattainment area plan. To do so, 
the State compared allowable annual emissions at Merrimack Station 
prior to installation of the FGD control system with those after the 
system was operational and with those with the conditions of TP-0189 in 
place (i.e., allowable emissions under the plan). Prior to the 
effective date of TP-0189, under the conditions of TP-0008 (see 77 FR 
50602), Merrimack Station was permitted to operate the MK1 boiler 
through the bypass stack (i.e., now the emergency stack) for no more 
than 840 hours during any consecutive 12-month period and thereby 
bypass SO2 controls; the MK2 boiler is unable to operate 
through the bypass stack. The State quantified emissions from these 
boilers which were allowed prior to installation of the FGD and the 
effective date of TP-0008. Then, the State quantified emissions from 
the MK1 and MK2 boilers under the provisions of TP-0008 (i.e., using a 
90% emissions reduction). Finally, the State quantified emissions for 
MK1 and MK2 allowed under the provisions of TP-0189, i.e., assuming an 
average of 0.39 lb/MMBtu. A summary of these allowable emissions is 
presented in Table 2. According to the plan, allowable annual 
SO2 emissions prior to the FGD installation (and the 
conditions of TP-0008) were 82,537 tons, compared to 8,254 tons under 
the permit conditions of TP-0008, and 8,047 tons under the 
nonattainment plan (namely the SO2 emissions limit for NAAQS 
compliance included in TP-0189). That is, the State expects 
implementation of the plan to allow 207 tons fewer than prior to plan 
implementation, and 74,490 tons fewer than prior to installation and 
operation of the FGD.

       Table 2--Summary of Annual Allowable SO2 Emissions for the MK1 and MK2 Boilers at Merrimack Station
----------------------------------------------------------------------------------------------------------------
                                                                                   Difference in   Difference in
                                                                                     allowable       allowable
                                                                       Total      emissions from  emissions from
                                                                     allowable     prior to TP-    prior to TP-
                                                                     emissions     0008  (tons)    0189  (tons)
 
----------------------------------------------------------------------------------------------------------------
Prior to TP-0008................................................          82,537
With TP-0008....................................................           8,254      \a\-74,283
Nonattainment Area Plan (With TP-0189)..........................           8,047      \a\-74,489        a b-206
----------------------------------------------------------------------------------------------------------------
\a\ Reported negative emissions values for differences indicate emission reductions.
\b\ New Hampshire reported a difference of 206 tons compared with the numerical difference of 207 tons between
  the reported total allowable emissions. This slight difference can be attributed to rounding.

C. Air Quality Trends

    New Hampshire also included trends in ambient monitoring data for 
the nonattainment area. In its nonattainment plan, the State shows that 
ambient concentrations in the area have dropped markedly since 2011, 
when Merrimack Station began operation of its FGD system under the SIP-
approved conditions of TP-0008, and are now below 75 ppb, the level of 
the NAAQS. The monitored design value for the Pembroke monitor (AQS 
number 33-013-1006), consistently the highest in the area, was 23 ppb 
for 2012 to 2014, and 20 ppb for both 2013 to 2015 and 2014 to 2016.

D. Compliance With Section 110(a)(2) of the CAA

    Section 172(c)(7) of the CAA requires nonattainment SIPs to meet 
the applicable provisions of section 110(a)(2) of the CAA. While the 
provisions of 110(a)(2) address various topics, EPA's past 
determinations suggest that only the section 110(a)(2) criteria linked 
with a particular area's designation and classification are relevant to 
section 172(c)(7). This nonattainment SIP submittal satisfies all 
applicable criteria of section 110(a)(2) of the CAA, as evidenced by 
the State's nonattainment new source review program which addresses 
110(a)(2)(I), the included control strategy, and the associated 
emissions limits which are relevant to 110(a)(2)(A). In addition, EPA 
approved the State's SO2 infrastructure SIP on May 25, 2017 
(82 FR 24057). EPA will take action in a separate rulemaking on the 
remaining portion of the State's infrastructure SIP, the so-called 
SO2 ``good neighbor'' or ``interstate transport'' SIP to 
satisfy section 110(a)(2)(D)(i)(I) of the CAA. EPA is proposing to 
conclude that the State has meet the requirements of 172(c)(7) of the 
CAA.

E. Equivalency Techniques

    Section 172(c)(8) of the CAA states that upon application by any 
state, the Administrator may allow the use of equivalent modeling, 
emission inventory, and planning procedures, unless the Administrator 
determines that the proposed techniques are, in the aggregate, less 
effective than the methods specified by the Administrator.
    The State's nonattainment SIP indicates that it followed existing 
regulations, guidance, and standard practices when conducting modeling, 
preparing the emissions inventories, and implementing its planning 
procedures. Therefore, the State did not use or request approval of 
alternative or equivalent techniques as allowed under of the CAA and 
EPA is proposing to conclude that the State's nonattainment SIP meets 
the requirements of section 172(c)(8) of the CAA.

VII. EPA's Proposed Action

    EPA has determined that New Hampshire's SO2 
nonattainment plan meets the applicable requirements of sections 110, 
172, 191, and 192 of the CAA. EPA is proposing to approve New 
Hampshire's January 31, 2017 SIP submission for attaining the 2010 1-
hour SO2 NAAQS for the Central New Hampshire Nonattainment 
Area and for meeting other nonattainment area planning requirements. 
This SO2 nonattainment plan includes New Hampshire's 
attainment demonstration for the SO2 nonattainment area. The 
nonattainment area plan also addresses requirements for RFP, RACT/RACM, 
enforceable emission limits and control measures, base-year and 
projection-year emission inventories, and contingency measures.
    In the January 31, 2017 submittal to EPA, New Hampshire included 
the applicable monitoring, testing, recordkeeping, and reporting

[[Page 45253]]

requirements contained in Merrimack Station's permit, TP-0189, to 
demonstrate how compliance with Merrimack Station's SO2 
emission limit will be achieved and determined. EPA is proposing to 
approve into the New Hampshire SIP the provisions of Merrimack 
Station's permit, TP-0189, that constitute the SO2 operating 
and emission limits and their associated monitoring, testing, 
recordkeeping, and reporting requirements. EPA is proposing to approve 
these provisions into the State's SIP through incorporation by 
reference, as described in section VIII, below. EPA's analysis is 
discussed in this proposed rulemaking.
    EPA is not proposing to remove from the existing New Hampshire SIP, 
Table 4, items 6, 8, and 10 contained in Merrimack Station's July 2011 
permit, TP-0008, because EPA has not received a request from the State 
to do so. See 52.1520(d) EPA-approved State Source specific 
requirements. However, EPA considers those provisions to be superseded 
by the conditions of TP-0189, which are more stringent, and which are 
to be incorporated into the SIP in this proposed action. Specifically, 
two of the provisions, items 6 and 8 from Table 4, relate to 
SO2 emissions limits that have been superseded by Merrimack 
Station's September 2016 permit, TP-0189. Item 10 from Table 4 has also 
been superseded by Merrimack Station's September 2016 permit, TP-0189, 
in that the existing SIP provision allowed operation of one of 
Merrimack Station's two boilers, MK1, for up to 840 hours in any 
consecutive 12-month period through the emergency bypass stack, i.e., 
not through the FGD. Each of the corresponding provisions of Merrimack 
Station's September 2016 permit, TP-0189, are more stringent than those 
existing SIP provisions. EPA is taking public comments for thirty days 
following the publication of this proposed action in the Federal 
Register. We will take all comments into consideration in our final 
action.

VIII. Incorporation by Reference

    In this rule, EPA is proposing to include in a final EPA rule 
regulatory text that includes incorporation by reference. In accordance 
with requirements of 1 CFR 51.5, EPA is proposing to incorporate by 
reference certain federally enforceable provisions of Merrimack 
Station's permit, TP-0189, effective on September 1, 2016. 
Specifically, the following provisions of that permit are proposed to 
be incorporated by reference: Items 1, 2, and 3 in Table 4 (``Operating 
and Emission Limits''); items 1 and 2 in Table 5 (``Monitoring and 
Testing Requirements''); items 1 and 2 in Table 6 (``Recordkeeping 
Requirements''); and items 1 and 2 in Table 7 (``Reporting 
Requirements'').
    EPA has made, and will continue to make, these materials generally 
available through www.regulations.gov and/or at the EPA Region 1 Office 
(please contact the person identified in the For Further Information 
Contact section of this preamble for more information).

IX. Statutory and Executive Order Reviews

    Under the CAA, the Administrator is required to approve a SIP 
submission that complies with the provisions of the Act and applicable 
Federal regulations. 42 U.S.C. 7410(k); 40 CFR 52.02(a). Thus, in 
reviewing SIP submissions, EPA's role is to approve state choices, 
provided that they meet the criteria of the CAA. Accordingly, this 
proposed action merely approves state law as meeting Federal 
requirements and does not impose additional requirements beyond those 
imposed by state law. For that reason, this proposed action:
     Is not a ``significant regulatory action'' subject to 
review by the Office of Management and Budget under Executive Order 
12866 58 FR 51735, October 4, 1993) and 13563 (76 FR 3821, January 21, 
2011);
     does not impose an information collection burden under the 
provisions of the Paperwork Reduction Act (44 U.S.C. 3501 et seq.);
     is certified as not having a significant economic impact 
on a substantial number of small entities under the Regulatory 
Flexibility Act (5 U.S.C. 601 et seq.);
     does not contain any unfunded mandate or significantly or 
uniquely affect small governments, as described in the Unfunded 
Mandates Reform Act of 1995 (Pub. L. 104-4);
     does not have Federalism implications as specified in 
Executive Order 13132 (64 FR 43255, August 10, 1999);
     is not an economically significant regulatory action based 
on health or safety risks subject to Executive Order 13045 (62 FR 
19885, April 23, 1997);
     is not a significant regulatory action subject to 
Executive Order 13211 (66 FR 28355, May 22, 2001);
     is not subject to requirements of Section 12(d) of the 
National Technology Transfer and Advancement Act of 1995 (15 U.S.C. 272 
note) because application of those requirements would be inconsistent 
with the CAA; and
     does not provide EPA with the discretionary authority to 
address, as appropriate, disproportionate human health or environmental 
effects, using practicable and legally permissible methods, under 
Executive Order 12898 (59 FR 7629, February 16, 1994).
    In addition, the SIP is not approved to apply on any Indian 
reservation land or in any other area where EPA or an Indian tribe has 
demonstrated that a tribe has jurisdiction. In those areas of Indian 
country, the rule does not have tribal implications and will not impose 
substantial direct costs on tribal governments or preempt tribal law as 
specified by Executive Order 13175 (65 FR 67249, November 9, 2000).

List of Subjects in 40 CFR Part 52

    Environmental protection, Air pollution control, Incorporation by 
Reference, Intergovernmental relations, Reporting and recordkeeping 
requirements, Sulfur oxides.

    Authority: 42 U.S.C. 7401 et seq.

    Dated: September 15, 2017.
Ken Moraff,
Acting Regional Administrator, EPA New England.
[FR Doc. 2017-20721 Filed 9-27-17; 8:45 am]
BILLING CODE 6560-50-P
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