the Exceptional Events Rule. Also, as explained in this section, events
that include emissions from both natural and anthropogenic sources,
such as high wind dust events, can be considered natural events only if
reasonable controls have been applied to the contributing anthropogenic
sources. Lightning storms occurring close to a regulatory monitor, such
that the particular storm notably affects the monitor close in time to
the storm might qualify as natural events that could also be
exceptional events. However, the ongoing and delayed aggregate impact
of many lightning storms that are not proximate to the monitor is not a
deviation from normal or expected conditions and thus would not be an
exceptional event. Also, routine biological emissions (e.g., including,
but not limited to, emissions from vegetation, microbes and/or animals)
are not deviations from normal or expected conditions. Thus despite
being natural, they are not ``events'' and would not qualify as
exceptional events. As is true for all exceptional events
determinations, the EPA will consider these events, and other event
types not identified here, on a case-by-case basis.

E. Technical Criteria for the Exclusion of Data Affected by Events

    As described in Section IV.B of this preamble, the EPA is
finalizing provisions to return to the core statutory elements and
implicit concepts of CAA section 319(b): That the event affected air
quality in such a way that there exists a clear causal relationship
between the specific event and the monitored exceedance or violation,
the event was not reasonably controllable or preventable, and the event
was caused by human activity that is unlikely to recur at a particular
location or was a natural event. See, e.g., 42 U.S.C. 7619(b)(1). All
exceptional events demonstrations, regardless of event type or relevant
NAAQS, must address each of these technical criteria. The EPA has
posted examples of acceptable demonstrations for various event and
pollutant combinations on its Web site at https://www.epa.gov/air-quality-analysis/exceptional-events-submissions-table. We will update
this Web site as additional examples become available. This section
summarizes the EPA's proposed revisions, final regulatory language and
public comments regarding each of these technical criteria. Section
IV.G of this preamble discusses additional process-related components
of exceptional events demonstrations.
1. Human Activity Unlikely To Recur at a Particular Location or a
Natural Event
    Because Section IV.D of this preamble addresses the definition of a
natural event and those event types that can be considered natural
events under the Exceptional Events Rule, we focus this section of the
preamble on the ``human activity unlikely to recur at a particular
location'' portion of the ``human activity unlikely to recur at a
particular location or a natural event'' technical criterion. In the
final rule description section in this part of the preamble, we provide
example conclusory language that air agencies can use in the portion of
their exceptional events demonstration that addresses this criterion.
This example language applies to both human activity and natural
events.
a. Summary of Proposal
    Our proposal stated that according to both the statutory and
regulatory definitions, an exceptional event must be ``an event caused
by human activity that is unlikely to recur at a particular location or
a natural event'' (emphasis added, see CAA section 319(b)(1)(A)(iii)
and 40 CFR 50.1(j)). As we noted in the discussion of a natural event
in Section IV.D of this preamble, we have come to realize that it may
be helpful to think of an event in terms of the source of its
emissions. If the underlying source is natural and the generated
emissions influence a regulatory monitor, then the ensuing event (i.e.,
event and resulting emissions) could be considered a ``natural event''
under the Exceptional Events Rule. Under this particular criterion, if
the underlying source of emissions is anthropogenic, then the event can
only be ``exceptional'' if the original source is ``unlikely to recur
at a particular location.'' The proposal noted that neither the CAA nor
the 2007 Exceptional Events Rule defined ``unlikely to recur'' or ``at
a particular location.'' Therefore, the proposal sought to clarify both
of these phrases. In addition to proposing a generally applicable
approach for ``unlikely to recur,'' we also proposed specific
approaches for wildland fires, notably prescribed fires on wildland
(which we discuss in Section IV.F.2 of this preamble), and high wind
dust events (which we discuss in Section IV.F.4 of this preamble). The
proposal also clarified that under CAA section 319(b) and a provision
of the 2007 Exceptional Events Rule that we did not propose to change,
air pollution related to source noncompliance is not an exceptional
event regardless of its frequency.
    We proposed, as guidance, to interpret the unlikely to recur
language as follows. If an event type has not previously occurred
within a given air quality control region (AQCR) \31\ in the 3 years
preceding the submittal of an exceptional events demonstration for an
event that has occurred recently, the EPA will consider this recent
event to be a ``first'' event and will generally consider that event
type to be unlikely to recur in the same location.\32\ Similarly, if
there was one prior event (for which a demonstration may or may not
have been submitted) within the 3 years preceding the submittal of an
exceptional events demonstration for the recent event, that event type
would also generally be considered unlikely to recur in the same
location. However, if there have been two prior events of a similar
type within a 3-year period in an AQCR, that would generally indicate
the third event, for which the demonstration is being prepared (or
would be prepared), does not satisfy the ``human activity that is
unlikely to recur at a particular location'' criterion and, thus, would
not qualify as an exceptional event. The terms ``one prior event'' and
``two prior events'' refer to events that affect the same AQCR, even if
they have not affected the same monitor.\33\ This proposed guidance is
consistent with the approach taken to recurrence in our Interim High
Winds Guidance document in which we identified non-recurring events as
being less than one event per year in a given area.\34\ In the Interim
High Winds

Guidance, we did not define ``area'' other than to differentiate areas
by attainment status or jurisdiction (i.e., intrastate versus
interstate or international). The EPA solicited comment on using an
AQCR to define the bounds for an area subject to event recurrence and
on whether to incorporate into rule text the benchmark of three events
in 3 years.
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    \31\ 40 CFR part 81, subpart B, Designation of Air Quality
Control Regions, defines Air Quality Control Regions.
    \32\ While we proposed to define event recurrence as occurring
in the 3 years preceding the submittal of an exceptional events
demonstration, the proposal language should have read in the 3 years
preceding the event that is the subject of an exceptional events
demonstration. We clarify this 3-year timeframe in the final rule
section.
    \33\ The EPA will consider previously flagged exceedances within
AQS with their associated descriptions to be ``events'' regardless
of whether the EPA has received or acted on event demonstrations.
The EPA also notes that a single event could influence
concentrations on multiple days.
    \34\ See footnote 27 in table 2 of Interim Guidance on the
Preparation of Demonstrations in Support of Requests to Exclude
Ambient Air Quality Data Affected by High Winds Under the
Exceptional Events Rule. U.S. EPA. May 2013. Available at http://www2.epa.gov/sites/production/files/2015-05/documents/exceptevents_highwinds_guide_130510.pdf.
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b. Final Rule
    As a result of the feedback from numerous commenters, we are
providing clarifications to the ``unlikely to recur at a particular
location'' language as guidance in this preamble and not regulatory
text. We note here, as guidance, the benchmark of three events in 3
years to define recurrence. We measure the 3-year period backwards from
the date of the most recent event (e.g., for an event occurring on May
1, 2016, the 3-year period would be May 1, 2013, through May 1, 2016).
As described previously, if there have been two prior events of a
similar type (i.e., a similar event type generating emissions of the
same pollutant whether flagged or the subject of a demonstration)
within a 3-year period in ``a particular location,'' the third event,
for which the demonstration is being prepared (or would be prepared),
would generally not satisfy the ``human activity that is unlikely to
recur at a particular location'' criterion and, thus, would not qualify
as an exceptional event. Although under this approach, the third event
essentially confirms that the first two events are ``routine,'' an air
agency would not likely recognize the routine nature of the first two
events until the third occurrence. Also as noted in our proposal, the
EPA will consider previously flagged exceedances within AQS with their
associated descriptions to be ``events'' regardless of whether the
affected air agency has submitted or the EPA has acted on these
``recurring'' event demonstrations. We also note in this final action
that the benchmark of three events in 3 years generally applies
regardless of an area's designation status with respect to the NAAQS
that is the focus of the event demonstration. The EPA could grant
exceptions to the benchmark of three events in 3 years benchmark on a
case-by-case basis. Several commenters supported, and no commenters
opposed, this generally applicable approach.
    With regard to the frequency, several commenters asked the EPA to
clarify how the concept of recurrence applies to a single event
spanning multiple days. First, the EPA notes that for purposes of
exceptional events eligibility, the concept of recurrence only applies
to ``human activity unlikely to recur at a particular location'' and
not to natural events. Natural events can recur. That said, a single
event, natural or caused by human activity, can span multiple days and
result in an air agency flagging multiple monitor-day values in AQS
(i.e., multiple exceedances of a given NAAQS at a single monitor in a
single day or multiple NAAQS exceedances at multiple monitors on
multiple days). The EPA considers a single discrete event to be one
occurrence even if it extends over more than one day. Applying our
benchmark of three events in 3 years, for an area experiencing three
authorized and deliberately set structural fires in 2 years, the EPA
would not consider a third such structural fire in the third year to be
an exceptional event.\35\ Because prescribed fires on wildland eligible
for exceptional events consideration involve igniting and managing the
fire according to the provisions set forth in either a Smoke Management
Program or using basic smoke management practices, we discuss the
unique circumstances associated with the recurrence of prescribed fires
on wildland in IV.F.2.
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    \35\ A deliberately set structural fire that has been authorized
by a responsible government agency is clearly not a natural event.
We are not offering guidance at this time on whether accidentally
set structural fires or arson-set structural fires should be
considered natural or anthropogenic events. We do note, however,
that wildfires on wildland initiated by accident or arson are
considered natural events, and on a case-by-case basis this
treatment for wildfires may bear on the appropriate treatment of
accidental and arson-set structural fires.
---------------------------------------------------------------------------

    While we proposed, as guidance, to use an AQCR to define the bounds
for an area subject to recurrence, in light of the comments received
and issues raised therein, we agree that using AQCRs as the only way in
which to define the bounds for an area subject to recurrence is not
appropriate. Commenters identified the following reasons why an AQCR
may not be suitable: AQCRs can be antiquated and inconsistent with
current jurisdictional boundaries; AQCRs may be too large (particularly
in some areas of the West) for effective analysis of event recurrence;
AQCRs could be subdivided by terrain (e.g., mountains or valleys) that
could affect the transport and/or chemical interactions of pollutants;
pollutant sources and monitors may not fall within the bounds of the
same AQCR. Rather than prescribe an approach to define ``a particular
location,'' commenters suggested that the EPA Regional offices and the
affected air agencies could agree to the bounds of ``a particular
location'' as part of regular, on-going conversations and/or as part of
the Initial Notification of Potential Exceptional Event process.
Commenters suggested that while an AQCR might appropriately define ``a
particular location'' in some areas of the country, other areas may
determine one of the following to be more suitable: Counties or other
political boundaries, core based statistical areas (CBSAs),
nonattainment or unclassifiable area boundaries (if applicable), a
density metric (i.e., number of events per thousand square miles
calculated using the radius around the subject monitor), and/or
distance to the monitor as indicated by a defined radius from the
subject monitor. We agree that some of the commenters' suggestions may
be appropriate in particular cases and we leave it to the EPA Regional
offices and to the affected air agencies to consult on how to
characterize ``a particular location.''
    As stated previously, all exceptional events demonstrations,
regardless of event type or relevant NAAQS, must address each of the
three technical criteria. We proposed conclusory language associated
with the ``human activity that is unlikely to recur at a particular
location or a natural event'' criterion and repeat it here as part of
the preamble to the final Exceptional Events Rule revisions. When
addressing this criterion as part of an exceptional events
demonstration, the EPA recommends that the submitting air agency
document and discuss the following in a distinct ``human activity/
natural event'' section of the demonstration: The type/source of event
(e.g., a particular type of chemical spill or other industrial
accident, fire in a particular type of structure, lightning-ignited
wildfire, etc.), clearly identify whether the event is natural or was a
human activity that is unlikely to recur at a particular location, the
resulting emissions (e.g., characterized in terms of the pollutant and
magnitude, if applicable/available), and the documented frequency of
the event in the prior 3 years (or other appropriate timeframe as
agreed with the reviewing EPA Regional office).\36\

The air agency should then affirmatively state that in characterizing
the event, it has satisfied the ``human activity unlikely to recur at a
particular location or a natural event'' criterion.
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    \36\ The frequency of event recurrence is important for both
natural and anthropogenic events. For anthropogenic events,
frequency can determine whether the event satisfies the ``human
activity unlikely to recur at a particular location or a natural
event'' criterion. For a natural event, the frequency can determine
whether a mitigation plan is necessary (see Section V of this
preamble).
---------------------------------------------------------------------------

2. Not Reasonably Controllable or Preventable
    As noted in the proposal, because CAA section 319(b) does not
restrict the applicability of the not reasonably controllable or
preventable criterion to certain types of events, this CAA criterion,
and the implementing Exceptional Events Rule language, applies to both
events caused by human activity and to natural events. This section
discusses the criterion in general terms. We discuss the criterion's
specific applicability to fire events on wildland in Section IV.F.2 of
this preamble and to high wind dust events in Section IV.F.4 of this
preamble.
a. Summary of Proposal
    The EPA proposed to codify in regulatory language key aspects of
the ``not reasonably controllable or preventable'' criterion to reduce
uncertainty for air agencies and other parties. Specifically, we
proposed and solicited comment on the following revisions to the
Exceptional Events Rule to indicate that:
     The not reasonably controllable or preventable criterion
has two prongs, prevention and control. An air agency must demonstrate
that an event was both not reasonably preventable and not reasonably
controllable.
     An event is not reasonably preventable if reasonable
measures to prevent the event were applied at the time of the event.
     An event is not reasonably controllable if reasonable
measures to control the impact of the event on air quality were applied
at the time of the event.
     The reasonableness of measures is case-specific and is to
be evaluated in light of information available at the time of the
event.
     Air agencies do not need to provide case-specific
justification to support the ``not reasonably controllable or
preventable'' criterion for remote, large-scale, high-energy and/or
sudden high wind dust events, such as ``haboobs.''
     Provided the air agency is not under an obligation to
revise the SIP, the EPA would consider (i.e., give deference to)
enforceable control measures implemented in accordance with a state
implementation plan, approved by the EPA within 5 years of the date of
a demonstration submittal, that address the event-related pollutant and
all sources necessary to fulfill the requirements of the CAA for the
SIP to be reasonable controls with respect to all anthropogenic sources
that have or may have contributed to event-related emissions.
     Air agencies do not need to provide case-specific
justification to support the ``not reasonably controllable or
preventable'' criterion for emissions-generating activity that occurs
outside of the boundaries of the state (or tribal lands) within which
the concentration at issue was monitored.

In addition to the identified revisions, the proposal also discussed
and solicited feedback on the role of an EPA-approved SIP in
nonattainment, maintenance, unclassifiable and attainment areas; prior
communications regarding expectations for reasonable controls;
prospective agreements regarding assessments of reasonable controls;
and components of a not reasonably controllable or preventable showing
within a demonstration. We summarize our proposed positions on these
topics in the following paragraphs.
    The proposal stated that while we would defer to the enforceable
control measures in attainment plan SIPs applying to maintenance and
nonattainment areas, we would not give this same deference to
infrastructure SIPs developed for attainment, unclassifiable/attainment
and unclassifiable areas. We differentiated attainment plan SIPs and
infrastructure SIPs by the fact that attainment plan SIPs must include
an attainment demonstration and reasonably available control measures
(RACM), best available control measures (BACM),\37\ and other
requirements,\38\ which together constitute an assessment of reasonable
controls. Infrastructure SIPs typically rely on maintenance and
attainment SIPs to demonstrate compliance with the key infrastructure
elements. Therefore, the EPA proposed that the underlying SIPs, which
would themselves include the control measures, be the relevant SIPs for
exceptional events demonstrations.
---------------------------------------------------------------------------

    \37\ BACM applies to attainment plans for serious
PM10 or PM2.5 areas.
    \38\ Marginal ozone nonattainment areas are exceptions because
they are not required to submit attainment demonstrations.
---------------------------------------------------------------------------

    The proposal also recognized that regulations and an area's
planning status are often evolving and changing, that these changes can
span several years and involve multiple rounds of formal and informal
communications between the affected air agency and the EPA, and that
these changes could ultimately result in an air agency's adoption of
new control measures, which, for exceptional events purposes, could
constitute ``reasonable'' controls. Acknowledging that these
conversations could inform what the air agency knew at the time of the
event and thus could influence a case-specific assessment of the not
reasonably controllable or preventable criterion, the EPA solicited
comment on methods to definitively identify the status of
communications and planning efforts (e.g., formal correspondence or
other documentation, timelines for responding) and whether this
approach would be more appropriately addressed through rule language.
    First appearing in the Interim High Winds Guidance, the proposal
repeated the suggestion that an air agency could prospectively assess
and determine that the controls in place for a particular type of
event, or a planned enhancement of those controls, are sufficient to
meet the not reasonably controllable or preventable criterion, and then
obtain the EPA's review and concurrence of this assessment prior to
more events of that type occurring. The proposal expressed the EPA's
belief that this prospective approach would reduce disagreements that
might otherwise occur over later retrospective assessments.
    The proposal also solicited comment on recommending as either
guidance or rule the following components that an air agency should
include within the not reasonably controllable or preventable showing
in a demonstration: (1) Identify the natural and anthropogenic sources
of emissions causing and contributing to the event emissions, including
the contribution from local sources, (2) identify the relevant SIP or
other enforceable control measures in place for these sources and the
implementation status of these controls, and (3) provide evidence of
effective implementation and enforcement of reasonable controls, if
applicable.\39\ In identifying natural and anthropogenic sources, we
clarified that the air agency should assess both

potentially contributing local/in-state and upwind sources.
---------------------------------------------------------------------------

    \39\ The EPA generally expects evidence that the controls
determined to be reasonable, if any, were effectively implemented
and appropriately enforced. This assessment of local sources should
include a review and description of any known nearby facility upsets
or malfunctions that could have resulted in emissions of the
relevant pollutant(s) that influenced the monitored measurements on
the day(s) of the claimed events. In the case of a high wind dust
event, for example, for the identified potentially contributing
local sources, the analysis should explain how significant dust
emissions occurred despite having reasonable controls in place
(e.g., that controls were overwhelmed by high wind), if appropriate.
---------------------------------------------------------------------------

b. Final Rule
    After considering the public comments we received, we are
finalizing the following not reasonably controllable or preventable
elements, all of which contain associated regulatory language.
     The not reasonably controllable or preventable criterion
has two prongs, prevention and control. An air agency must demonstrate
that an event was both not reasonably preventable and not reasonably
controllable.
     An event is not reasonably preventable if reasonable
measures to prevent the event were applied at the time of the event.
     An event is not reasonably controllable if reasonable
measures to control the impact of the event on air quality were applied
at the time of the event.
     The reasonableness of measures is case-specific and is to
be evaluated in light of information available as of the date of the
event.
     Air agencies do not need to provide case-specific
justification to support the ``not reasonably controllable or
preventable'' criterion for emissions-generating activity that occurs
outside of the boundaries of the state (or tribal lands) within which
the concentration at issue was monitored.\40\
---------------------------------------------------------------------------

    \40\ Under the CAA, the EPA generally considers a state (not
including areas of Indian country) to be a single responsible actor.
Accordingly, neither the EPA nor the 2007 Exceptional Events Rule
provides special considerations for intrastate scenarios when an
event in one county affects air quality in another county in the
same state, assuming that the event occurs on land subject to state
authority (versus tribal government authority). The EPA expects
controls appropriate for the designation status of the county (or
portion of the county) in which the emissions originate.
---------------------------------------------------------------------------

    In addition, as a result of commenter feedback as explained more
fully in subsequent paragraphs, we are promulgating in regulatory text
the following revised versions of elements that we proposed for the not
reasonably controllable or preventable criterion:
     Provided the appropriate federal, state or tribal air
agency is not under an obligation to revise the SIP or FIP or TIP for
an attainment or maintenance area for the event-related pollutant, the
EPA would consider (i.e., give deference to) enforceable control
measures implemented in accordance with such a SIP or FIP or TIP,
approved by the EPA within 5 years of the date of the event, that
address the event-related pollutant and all sources necessary to
fulfill the requirements of the CAA for the SIP or FIP or TIP to be
reasonable controls with respect to all anthropogenic sources that have
or may have contributed to the monitored exceedance or violation.\41\
If the appropriate air agency is under an obligation to revise its
implementation plan with respect to the specific enforceable control
measures applicable to the exceptional events demonstration due to a
SIP call pursuant to CAA section 110(k)(5), the EPA will evaluate on a
case-by-case basis the control measures in place to determine whether
emissions were reasonably controlled at the time of the event.
---------------------------------------------------------------------------

    \41\ Under CAA section 110(c), the EPA is required to issue and
enforce a federal implementation plan if a state fails to develop,
adopt and implement an adequate SIP. States may also choose to adopt
the federal plan as an alternative to developing their own plan. If
a federal plan is implemented in a state, the state may still, at a
later date submit a plan to replace the federal plan either in whole
or in part. States may take over the administrative and enforcement
aspects of a federal plan rather than leaving it to the EPA.
Similarly, under the TAR at 40 CFR 49, tribes can develop their own
plans (i.e., tribal implementation plans) to implement the CAA
provisions. Rather than develop their own TIPs, tribes can request
that the EPA develop a FIP.
---------------------------------------------------------------------------

     When addressing the ``not reasonably controllable or
preventable'' criterion within an exceptional events demonstration, air
agencies should: (1) Identify the natural and anthropogenic sources of
emissions causing and contributing to the monitored exceedance or
violation, including the contribution from local sources,\42\ (2)
identify the relevant SIP, FIP or TIP or other enforceable control
measures in place for these sources and the implementation status of
these controls, and (3) provide evidence of effective implementation
and enforcement of reasonable controls, if applicable.\43\
---------------------------------------------------------------------------

    \42\ In specifying ``local'' sources, we mean those sources that
are both within the jurisdiction of the state or tribe and that are
in the vicinity of or are located upwind of the monitor with the
recorded exceedance or violation. ``Local'' sources could include,
but are not limited to, large point sources (e.g., large industrial
sources, electric power plants, airports, etc), nonpoint sources
(e.g., residential heating, asphalt paving, etc.), mobile sources
(e.g., both on- and off-road vehicles, construction equipment,
trains, and vessels), natural sources or biogenic sources (e.g.,
off-gassing from soil, animals and vegetation).
    \43\ The EPA recognizes that air agencies have various methods
of ensuring source compliance and various methods of permitting and
enforcement. We do not expect nor would all agencies necessarily
need to have enforcement records for all events. However, agencies
should make a general showing that they are enforcing controls to a
reasonable degree (not necessarily on the particular day of the
event). If an air agency identifies several categories of
anthropogenic sources as significant or likely contributors to an
event, the air agency should also describe in the demonstration the
means used to determine compliance with reasonable control
requirements for each category.
---------------------------------------------------------------------------

     Air agencies do not need to provide case-specific
justification to support the ``not reasonably controllable or
preventable'' criterion for large-scale and high-energy high wind dust
events, such as ``haboobs.'' (We discuss the characteristics of these
events in Section IV.F.4 of this preamble.)
    In addition, we repeat in this final action our suggestion that an
air agency can prospectively assess and determine that the controls in
place for a particular type of event, or a planned enhancement of those
controls, are sufficient to meet the not reasonably controllable or
preventable criterion, and then obtain the EPA's review and concurrence
of this assessment prior to the occurrence of similar events (i.e., a
similar event type generating emissions of the same pollutant). This
prospective approach would reduce disagreements that might otherwise
occur over later retrospective assessments. Although air agencies have
not historically pursued this option, it is our intent going forward to
work with any air agency expressing an interest in pursuing this
approach. Air agencies interested in this process should contact their
reviewing EPA Regional office.
c. Comments and Responses
    While some commenters supported the EPA's stated position in the
proposal that the not reasonably controllable or preventable criterion
consists of two prongs (i.e., control and prevention), other commenters
asserted that the statutory criterion and the implementing language in
the 2007 rule is ``not reasonably controllable or preventable''
(emphasis added). Commenters disagreeing with the EPA's position claim
that the EPA's interpretation is contrary to the CAA and that the EPA
lacks authority to contravene the precise statutory language in the
implementing regulatory language by interpreting the CAA to mean that
an exceptional event must be both not reasonably controllable and not
reasonably preventable.
    As previously noted, we maintain that the criterion consists of two
factors: Prevention and control and that to qualify as an exceptional
event, the event must satisfy both factors. CAA section
319(b)(1)(A)(ii) is ambiguous regarding whether ``not reasonably
controllable or preventable'' requires a demonstration to show both
criteria, or one or the other. In adopting our interpretation, we have
applied a valid rule of inference known as De Morgan's law, which
recognizes that the negation of a disjunction is the conjunction of the
negations. Stated simply, ``not (A or B)'' is the same as ``(not A) and
(not B).'' See, e.g., State v. Nelson, 842 NW.2d 433, at 440-41 (Minn.
2014) (finding it reasonable to apply De Morgan's law to

statutory interpretation); Schane v. Int'l Bhd. Of Teamsters, 760 F.3d
585, 589-92 (7th Cir. 2014) (applying De Morgan's law to address a
pension plan dispute, focusing on the context in which the ``not . . .
or'' phrase was used). Applied to CAA section 319(b)(1)(A)(ii), an
exceptional event means an event that is both not reasonably
controllable and not reasonably preventable. The legislative history
supports this logical reading of the statutory language. Congress
provided the following rationale for promulgating the exceptional
events provisions: ``Events such as forest fires or volcanic eruptions,
should not influence whether a region is meeting its Federal air
quality goals.'' S. Rep. No. 109-53, at Sec. 1618 (2005) and S. Rep.
No. 108-222, at Sec. 1618 (2004). The examples used in the legislative
history--forest fires and volcanic eruptions--are both not reasonably
controllable and not reasonably preventable.
    This interpretation is also supported by the intent of CAA section
319(b), which identifies the limited circumstances in which it is
appropriate to exclude from certain regulatory decisions air monitor
data clearly caused by an exceptional event balanced with the CAA's
goal of protecting human health and the environment. The language ``not
reasonably controllable'' clearly implicates controls, as does
``preventable,'' since an event may be ``preventable'' by mitigating
the conditions under which the event occurs--i.e., by applying
controls. Thus, consideration of the circumstances of the event and
possible application of controls is appropriate in both contexts, and a
separate analysis is required for ``not reasonably controllable'' and
``not reasonably preventable.''
    We note that the commenters who disagree with the EPA's
interpretation failed to identify any scenarios or provide any examples
of why it is problematic for the EPA to require that an exceptional
event must be both not reasonably controllable and not reasonably
preventable. While some air agencies that have submitted demonstrations
have argued that the ``or'' in this criterion allows them to choose
between showing either prevention or control of the event-related
emissions, this type of ``or'' selection is contrary to the emphasis of
CAA section 319(b) on the protection of public health and the exclusion
of data associated with emissions from ``exceptional events.'' The CAA
as a whole, and section 319(b) in particular, is premised on the idea
that states should undertake reasonable actions to control emissions
and protect public health. Exemptions and exceptions apply in addition
to, rather than in place of, reasonable controls. The CAA does not
allow air agencies to avoid applying reasonable controls to address
emissions simply because other factors also contribute to those
emissions. For example, for a high wind event, applying ``or'' might
suggest that because the wind is not preventable, the agency has no
obligation to address reasonable controls (e.g., the application of
water to stockpiles of wood chips) that could reduce emissions in the
case of such an event. For prescribed fire, the use of ``or'' could
allow an air agency to argue that a fire is not reasonably preventable
because of the safety or ecosystem benefits that would be foregone if
the fire were not applied, so the emissions and air quality impacts
from the fire do not need to be reasonably controlled through the
application of basic smoke management practices. Another example of
when applying ``or'' would be problematic is a situation in which a
developer could intentionally set fire to forested land to clear it for
development, as that event would be preventable but possibly not
controllable; such an event should not be considered an exceptional
event. In contrast, elsewhere in the preamble to these final rule
revisions we explain that some events may be neither preventable nor
their air quality impacts to be controllable to any degree, such as
potential increases in SO2 concentrations associated with
volcanic eruptions, and thus would qualify as exceptional events.
    These final rule revisions present that what is ``reasonable'' for
purposes of ``not reasonably controllable or preventable'' should
consider the technical knowledge available to the air agency at the
time of the event. While this concept was supported by some commenters,
others maintain that ``controllable'' is forward looking rather than
backward looking and that air agencies should anticipate future events
and implement controls and measures to account for potential future
impacts.
    We agree with the commenters that a prospective approach to
assessing what might constitute ``reasonable controls'' could be
helpful in some cases, particularly for areas experiencing recurring
events. Therefore, we have modified our proposal as it relates to
mitigation for areas experiencing historically documented or known
seasonal events. We discuss these concepts in Section V of this
preamble. We disagree, however, with the commenters' forward-looking
approach as it applies to other situations. As we noted in the
proposal, an air agency ``caught by surprise'' by an event of a given
type (or by an unexpected number of such events in a period over which
NAAQS compliance is evaluated, typically 3 years) should not be
expected to have implemented the same controls prior to an event as an
air agency that has been aware that events of a certain type occur with
regularity and cause NAAQS exceedances or violations. The EPA
anticipates that nonattainment (or maintenance) areas have technical
information needed to understand those measures that constitute
reasonable control of anthropogenic sources in their jurisdiction for
recurring events of the type(s) that cause or contribute to
nonattainment (or that did previously). In contrast, the EPA generally
does not expect areas identified as attainment, unclassifiable/
attainment or unclassifiable for a NAAQS to have the same understanding
or to have adopted the same level of event-relevant controls as areas
that are nonattainment (or maintenance) for the same NAAQS. Also, if an
area has been recently designated to nonattainment but is still
developing its SIP and has not yet reached a deadline to implement
controls, the EPA expects the level of controls that is appropriate for
that planning stage.\44\
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    \44\ The CAA provides different timeframes for developing and
implementing SIPs depending on the NAAQS and the nonattainment
area's classification (e.g., severity of the nonattainment problem).
The EPA recognizes that within the SIP development and
implementation process, some measures may be implemented relatively
quickly (e.g., transportation conformity, new source review) whereas
other programs, such as development or rules for particular source
types, can take time and involve state legislative processes.
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    As noted previously, the EPA proposed, and is finalizing in rule
language, that an air agency does not need to provide case-specific
justification to support the ``not reasonably controllable or
preventable'' criterion for emissions-generating activity that occurs
outside the boundaries of the state (or tribal lands) within which the
concentration at issue was monitored. While the majority of commenters
supported this provision, other commenters noted that it is
inconsistent with the plain language of CAA section 319, which requires
that an event be not reasonably controllable or preventable and does
not distinguish based on the origin of emissions associated with the
event.
    A review of the legislative history, and the language of section
319, as well as the purpose and intent of the CAA as a whole, reveals
that Congress did not likely intend to deny a downwind state

or tribe relief in the form of data exclusion within the context of the
Exceptional Events Rule for emissions that state or tribe has no
authority to control. See, e.g., H.R. Rep. No. 109-203 (2005) and CAA
section 319(b)(1). As we expressed in the proposal, it is not
reasonable to expect the downwind air agency (i.e., the state or tribe
submitting the demonstration) to have required or persuaded the upwind
state, tribe, or foreign country to have implemented controls on
sources sufficient to limit event-related air concentrations in the
downwind state or tribal lands. In fact, Congress explicitly addressed
interstate pollution transport in CAA sections 110(a)(2)(D)(i) and
(ii), which we discuss in more detail in Section IV.F.1 of this
preamble. There is no evidence that Congress intended for such efforts
to be repeated in the context of exceptional events. We note, however,
that we do expect the submitting (downwind) air agencies to assess
potential contribution from local/in-state sources within their
jurisdiction and submit evidence and statements supporting the other
exceptional events criteria (i.e., clear causal relationship and human
activity unlikely to recur or a natural event) in their demonstrations
for events that originate outside of their jurisdictional bounds.
    Regarding the origin of emissions, several commenters asked that
the EPA clarify how ``outside of jurisdiction'' applies to emissions
from ocean-going vessels (e.g., container ships and large tankers that
are regulated by international treaties) and international natural and
anthropogenic emissions. Although the EPA would consider emissions from
ocean-going vessels regulated by international treaties as well as
other international emissions (regardless of whether they are natural
or anthropogenic in origin) to be emissions originating outside of the
jurisdiction of the affected air agency and these emissions would
therefore satisfy the not reasonably controllable or preventable
criterion, these same emissions would only qualify for treatment under
the Exceptional Events Rule if they also satisfy the clear causal
relationship criterion and the human activity unlikely to recur at a
particular location or a natural event criterion. In these scenarios,
emissions from ships regulated by international treaty and
international emissions from routine anthropogenic activity would not
satisfy the human activity unlikely to recur at a particular location
criterion because they are both routine and occur frequently in the
same area (e.g., the port or coastline). International emissions
originating from a natural, event-based sources (e.g., wildfire,
volcanic activity) or from human activities unlikely to recur at a
particular location (e.g., industrial explosions) are more likely to
qualify as exceptional events. As we have stated multiple times in this
preamble, to qualify for data exclusion under the provisions of the
Exceptional Events Rule, an event must satisfy all of the technical and
administrative requirements under the rule.
    The proposed rule revisions contained regulatory language allowing
air agencies to defer to the control measures included in an attainment
or maintenance SIP, approved by the EPA within 5 years of the date of a
demonstration submittal, that addresses the event-related pollutant and
contributing sources, to satisfy the requirement for reasonable
controls. While the overwhelming majority of commenters, representing
state, local, regional planning organizations and industry, supported
this presumption, a few commenters disagreed with this provision noting
that the EPA should not universally defer to SIP measures, but rather
should assess the not reasonably controllable or preventable criterion
on a case-by-case basis. Commenters supporting deference asked the EPA
to consider the following revisions: (1) Measure the sufficiency of SIP
requirements from the date of the event rather than the date of
demonstration; (2) include reliance on measures in FIPs and/or TIPs in
addition to those in SIPs; (3) include reliance on BACMs in air quality
permits that are designed to control anthropogenic industrial sources;
and (4) expand the reliance to include infrastructure SIPs (with or
without Natural Events Action Plans (NEAP) or other mitigation plans).
    We individually address these general comments and specific
suggestions for revision in the following paragraphs. We maintain, as
supported by many commenters and as opposed by a few, that deference to
enforceable control measures implemented in accordance with an
attainment or maintenance SIP (or FIP or TIP), is appropriate provided
the timeframe for deference is limited and provided the SIP addresses
the pollutant and the sources potentially contributing emissions to the
exceedance or violation that is the subject of the exceptional events
demonstration. SIPs demonstrate that the state has the basic air
quality management program components in place to implement a new or
revised NAAQS by identifying the emission control requirements that
state will rely on to attain/maintain these NAAQS. In developing its
SIP according to the provisions of CAA section 110(a), a state must
identify and assess those sources of emissions that are contributing to
the state's air pollution problem, identify appropriate controls,
identify contingency measures, address provisions for demonstrating
reasonable further progress, identify permitting requirements, and
satisfy other requirements. When a nonattainment area reaches
attainment, it may be redesignated to maintenance area status if it has
implemented all applicable nonattainment area requirements and obtains
the EPA's approval for a maintenance plan for a 10-year period. Thus,
in both maintenance and nonattainment areas with approved attainment
plan SIPs, the air agency and the EPA, with input from the public, will
have considered what controls are necessary and reasonable to provide
for attainment, based on information available at the time of plan
development and approval. Because the attainment/maintenance SIP
development process includes the identification and assessment of those
sources of emissions that are contributing to the state's air pollution
problem, which could include event-related emissions, it is appropriate
to rely on the measures in the SIP as constituting reasonable controls
for purposes of exceptional events demonstrations just as it is
reasonable to rely on the measures in the SIP as constituting
reasonable controls for emissions sources. We do, however, agree with
the commenters that deference to the control measures in an attainment
or maintenance SIP should not be open-ended. We discuss limitations to
this deference in the following paragraphs, including deference for a
limited timeframe (i.e., 5 years).
    As suggested by commenters, we have changed the language in this
provision to be 5 years from the date of the ``event'' rather than the
date of ``demonstration submittal'' as we proposed. We believe that it
is reasonable and appropriate to make this change to ensure that the
exceptional events process is implemented in a manner consistent with
the CAA. We also agree with commenters that ``5 years from the date of
the event'' is the more appropriate time-frame given that we are
promulgating requirements in 50.14(b)(8)(i)-(iv), which also rely on
the date of the event.
    As we noted in this preamble, we also agree with commenter
recommendations that we defer to enforceable control measures

implemented in accordance with an attainment or maintenance SIP, FIP or
TIP. We have included these implementation plans in the regulatory
text. We agree that FIPs and TIPs provide the same level of assessment
of control measures during the development and approval process as
attainment/maintenance SIP process previously described and that the
only difference between these plans lies in the agency developing the
plan and the agency to whom the plan applies, neither of which impact
whether the measures contained in the plans constitute reasonable
controls for purposes of exceptional events demonstrations. For several
reasons, however, we do not agree that we should universally extend
this same deference to BACM or fugitive dust control plans contained in
air quality permits. First, control measures in air quality permits may
or may not be EPA-approved and evaluated using the same rigor as
controls in a SIP, FIP or TIP. Second, the best available control
measures in an air quality permit apply to the permit holder and not to
all sources potentially contributing emissions to a monitored
exceedance or violation. While we are not deferring to BACM controls in
air quality permits, we encourage air agencies to identify these
measures in the collection of controls that they determine constitute
``reasonable'' controls for purposes of addressing the not reasonably
controllable or preventable criterion.
    The EPA disagrees with the suggestion from a few other commenters
to defer to provisions in infrastructure SIPs to satisfy the not
reasonably controllable or preventable criterion. CAA sections
110(a)(1) and 110(a)(2) require every state to develop and submit to
the EPA an ``infrastructure SIP'' for each NAAQS within 3 years of the
promulgation of a new or revised NAAQS. While infrastructure SIPs
address a number of CAA requirements, including the requirement to
identify emission limits for specific pollutants, infrastructure SIPs
are not required to include attainment or maintenance demonstrations
and are not required to demonstrate that the controls on particular
sources are ``reasonable.'' Thus, the EPA-approved infrastructure SIPs
do not necessarily constitute an assessment of those controls that are
reasonable to have in place to address air quality impacts from
particular types of events that may become the focus of exceptional
events demonstrations. As with measures in air quality permits, while
we are not deferring to measures identified in infrastructure SIPs to
universally satisfy the not reasonably controllable or preventable
criterion, we encourage air agencies to identify measures in
infrastructure SIPs, NEAPs, mitigation plans, SMP and prospective
assessments of reasonable controls in the collection of controls that
they determine constitute ``reasonable'' controls for purposes of
addressing the not reasonably controllable or preventable criterion. We
note that provisions in these plans could, on a case-by-case basis with
the proper showing, satisfy the not reasonably controllable or
preventable criterion.
    We are promulgating rule language that the timeframe for
attainment/maintenance SIP deference is 5 years from the date of the
SIP approval measured to the date of an event at issue. We solicited
comment on whether and what other timeframes might be appropriate for
this deference. In responding to this specific solicitation for
feedback, commenters provided a range of options for SIP deference
including 3 years, 5 years, 10 years, reliance on the SIP until a new
NAAQS is adopted or until the EPA disapproves or calls the SIP, and, as
previously noted, no reliance on the SIP because any such deference is
inappropriate. One commenter noted that a deference timeframe of 3
years is more consistent with design value averaging and the timeframe.
We previously suggested in the 2013 Interim Exceptional Events
Implementation Guidance, and other commenters argued, that 10 years is
consistent with the timeframe for maintenance plan updates. The EPA
considered this information and is now promulgating, as proposed, a
deference timeframe of 5 years. After reviewing feedback received
during the comment period, we retain our proposed language that 5 years
represents a reasonable timeframe during which (1) the control measures
in a current SIP (or FIP or TIP) address all event-relevant sources of
current importance, (2) the control measures that were considered by
the air agency and the EPA at the time the EPA last approved the SIP
(or FIP or TIP) are the same measures that are known and available at
the time of a more recent event, and (3) the conditions in the area
have not changed in a way that would affect the approvability of the
same SIP (or FIP or TIP) if it newly needed the EPA's approval.
Additionally, as we discuss in Section IV.E.3 of this preamble, we
encourage the use of 5 years of data when developing analyses to
support the clear causal relationship criterion because we believe that
5 years of ambient air data represent the range of ``normal'' air
quality. Using a 3-year period of deference might mask (or accentuate)
the range of ``normal'' air quality, while using a 10-year deference
timeframe could overlook new emissions sources, relevant control
measures and control measure technologies, and other changes in the
affected area that could influence the approvability of a SIP (or FIP
or TIP).
    We also note that in establishing a period of deference of 5 years,
we are not implying that in periods longer than 5 years, the controls
in a SIP automatically become inappropriate or insufficient. Rather, we
are saying that in cases where the SIP was approved more than 5 years
prior to the date of the event (and the air agency is not under an
obligation to revise the SIP), because of the passage of time, the SIP
controls should not be presumed to satisfy the not reasonably
controllable or preventable criterion. In such a case, the air agency
should complete a case-specific assessment of the reasonableness of
controls to satisfy the not reasonably controllable or preventable
criterion. This case-by-case assessment would include the following
components, which we are promulgating as rule text: (1) Identify the
natural and anthropogenic sources of emissions causing and contributing
to the monitored exceedance or violation, including the contribution
from local sources, (2) identify the relevant SIP or other enforceable
control measures in place for these sources and the implementation
status of these controls, and (3) provide evidence of effective
implementation and enforcement of reasonable controls, if applicable.
As we identified earlier in this preamble, when we specify ``local''
sources, we mean those sources that are both within the jurisdiction of
the state or tribe and that are also in the vicinity of or are located
upwind of the monitor with the recorded exceedance or violation.
``Local'' sources could include, but are not limited to, large point
sources (e.g., large industrial sources, electric power plants,
airports, etc), nonpoint sources (e.g., residential heating, asphalt
paving, etc.), mobile sources (e.g., both on- and off-road vehicles,
construction equipment, trains, and vessels), natural or biogenic
sources (e.g., off-gassing from soil, animals and vegetation).
    We identified in the proposal these three components of a case-by-
case assessment of the not reasonably controllable or preventable
criterion and solicited comment on including these components as
regulatory language. One commenter supported this suggestion, and, as a
result, we are promulgating associated rule text. Although no

commenters opposed including the components as rule text, a number of
commenters asked that we clarify our expectations with respect to these
components. We do so here.
    When identifying the sources of emissions causing and contributing
to the monitored exceedance or violation, the air agency should first
discuss the scope of the analysis with the reviewing EPA Regional
office. This scope will be determined on a case-by-case basis
considering the specifics of the individual event. For example, if an
air agency claims that an event was regional in nature, then the area
of focus for the not reasonably controllable or preventable criterion
would likely be the county or counties involved in the ``region.'' If
an affected air agency claims that an exceedance or violation was
caused by an event originating in a nearby state, then the air agency
would include in its assessment the area and the potentially
contributing sources located between the subject upwind source and the
affected monitor. Once the air agency and the EPA determine the
appropriate area of analysis, the air agency should identify, within
the area of analysis, those stationary, mobile (if applicable) and area
sources and any other natural sources that emit the pollutant or
precursors that are the subject of the demonstration.\45\ In doing
this, the air agency should include, for ``major'' point sources,\46\
the facility name, the distance of the facility to the affected
monitor, and emissions in terms of tons per year (tpy) of the pollutant
in question. Air agencies may identify other point sources and area
sources by category.
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    \45\ A recent emissions inventory could serve as a starting
point when identifying sources of emissions within a given area of
analysis. Air agencies should also consider other sources that
potentially contribute to event-related emissions that may not be
the focus of routine annual inventories, which are often required by
federal, state or local rules for only a specific set of sources or
pollutants.
    \46\ The term ``major'' can vary by pollutant and NAAQS and
affected air agencies should discuss the expectation during the
initial notification of a potential exceptional event process.
Generally, however, we would consider ``major'' to be the thresholds
used in the initial area designations process for the NAAQS in
question. For example, for PM2.5, major point sources are
those whose sum of PM precursor emissions (PM2.5 +
NOX + SO2 + VOC + NH3) are greater
than 500 tpy based on the most recent National Emissions Inventory
(NEI) or SIP inventory.
---------------------------------------------------------------------------

    For each source category and/or individual source, if appropriate,
the air agency should identify applicable control measures in the SIP
or in other state rules or ordinances and provide a statement as to why
these controls are reasonable.\47\ In addition to the SIP, state rules
or local ordinances, air agencies could also identify control measures
in individual permits, NEAPs, SMP, other mitigation plans, or USDA/
Natural Resources Conservation Service (NRCS)-approved Best Management
Practices (BMPs) (discussed in more detail in Section IV.F.2.b of this
preamble). The air agency may also consider recent Reasonably Available
Control Technology (RACT)/Best Available Control Technology (BACT)/
Lowest Achievable Emission Rate (LAER) determinations in the affected
area or in another area with similar sources or other appropriate
measures. This assessment should include a review and description of
any known instances of source noncompliance (e.g., nearby facility
upsets or malfunctions, failure to comply with applicable rules such as
vacant lot stabilization or moisture requirements for area sources)
that could have resulted in emissions of the relevant pollutant(s) that
influenced the monitored measurements on the day(s) of the claimed
events. The air agency would then identify the implementation status of
these controls and provide evidence of enforcement. As we indicated
earlier, the EPA generally expects evidence that the controls
determined to be reasonable, if any, were effectively implemented and
appropriately enforced.
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    \47\ To clarify, the EPA does not need to formally approve an
air agency's rules and SIP before reasonable controls are officially
in place for an exceptional events determination. These final rule
revisions and final rule preamble indicate that we will defer to
controls in a SIP/FIP/TIP approved by the EPA within 5 years of the
date of the event provided the controls are specific to the
pollutant and contributing anthropogenic sources. Thus, a SIP/FIP/
TIP approved within 5 years of an event satisfies reasonable
controls, but an area could also satisfy the not reasonably
controllable or preventable criterion a number of other ways as
discussed in this preamble. We also note that if an air agency has a
record of other controls that are not yet part of a SIP/FIP/TIP (as
could be the case for an attainment, unclassifiable/attainment or
unclassifiable area or for a nonattainment or maintenance area
undergoing SIP planning or revision process) but that are
implemented and enforced and not just contemplated, that we would
consider these controls to be SIP/FIP/TIP controls.
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    After addressing these components and in concluding that they have
shown that reasonable measures to control the impact of the event on
air quality were applied at the time of the event and that the event
was therefore not reasonably controllable, the air agency should then
apply the concept that if a set of control measures should reasonably
have been in place for emission sources that contribute to the event
emissions, then those controls must have been in place for the event to
satisfy the not reasonably controllable or preventable criterion. To do
this, the air agency should ask the following questions: (1) Do the
control measures in the current SIP (or other programs) address all
event-relevant sources of current importance? (2) Are the control
measures that were considered by the air agency and the EPA at the time
the EPA last approved the SIP the same measures that are known and
available at the time of the more recent event? and (3) Have the
conditions in the area changed in a way that would affect the
approvability of the same SIP if it newly needed the EPA's approval? In
our view an event is ``not reasonably controllable'' if an exceedance
or violation occurs even when reasonable controls were actually in
place and any further control would have been beyond what was
reasonable. As indicated in these rule revisions, the EPA intends to
consider these aspects when applying the concept of ``reasonable
controls'' on anthropogenic sources.
    The EPA notes that there are several instances in which this step-
wise approach to addressing the not reasonably controllable or
preventable criterion is not necessary. This analysis is not required
when an air agency can rely on deference to control measures contained
in a SIP (or FIP or TIP). It is also not required for exceedances or
violations caused by events whose emissions are solely from natural
sources (e.g., wildfire; stratospheric ozone intrusions; windblown dust
from natural, undisturbed landscapes; large-scale and high-energy high
wind dust events, volcanic activity) as demonstrated by satisfying the
clear causal relationship (discussed in more detail in Section IV.E.3
of this preamble). In these cases, after addressing the clear causal
relationship criterion, the air agency should affirmatively state that
the not reasonably controllable or preventable criterion is satisfied
by the fact that the natural event was of a character that could not
have been prevented and could not have been controlled and that there
were no contributions of event-related emissions from anthropogenic
sources as demonstrated in the clear causal relationship showing. To
clarify, once an air agency has satisfied the clear causal relationship
criterion and has shown that the subject exceedance or violation was
caused by an event whose emissions are solely from natural sources,
then the not reasonably controllable criterion applies only to
emissions from natural sources/event and not to local sources. And, for
natural sources, air agencies can satisfy

the criterion with a statement similar to that in the following
example.
    Consider, as an example, a stratospheric ozone intrusion event.
Stratospheric intrusions are by nature not reasonably controllable or
preventable. If an air agency has shown in the clear causal portion of
its demonstration that ozone transported from the stratospheric ozone
intrusion overwhelmingly caused each of the identified exceedances,
then it has shown these are natural, intrusion events and controls on
anthropogenic sources are irrelevant. The air agency can include the
following statements in its demonstration:

    The analysis shows that ozone transported via a stratospheric
ozone intrusion caused each of the identified exceedances in
[Section A] of this demonstration. We conclude that the event
identified should be considered a natural, stratospheric ozone
intrusion event. (An air agency may include this type of conclusory
language in the natural events section of the demonstration.)
    The analysis shows that ozone transported via a stratospheric
ozone intrusion caused each of the identified exceedances in
[Section A] of this demonstration. We conclude that the event in
question was a stratospheric ozone intrusion event and thereby an
unpreventable and uncontrollable natural event, and therefore not
reasonably controllable or preventable. (An air agency may include
this type of conclusory language in the not reasonably controllable
or preventable portion of the demonstration.)

    The proposal also discussed and solicited feedback on the role of
prior communications regarding expectations for reasonable controls.
The proposal indicated that the EPA would consider communications
between the EPA and the air agency when assessing ``reasonableness'' as
part of assessing the technical information available to the air agency
at the time the event occurred and what should reasonably have been in
place at the time of the event for anthropogenic emission sources that
contribute to the event emissions. We noted that because regulations
and an area's planning status are often evolving and changing and
because these changes and iterative discussions often include issues
regarding appropriate controls, including what controls would
constitute ``reasonable'' controls for exceptional events purposes, we
solicited comment on what form of communication would be most effective
in conveying the EPA's views to the affected air agency and whether
this approach would be most appropriately addressed through guidance or
regulatory text. Although one commenter responding to this specific
solicitation for comment indicated that our decision should be
promulgated in rule text, the majority of commenters indicated that
expectations in guidance were appropriate. These commenters suggested
that any formal communication notifying an air agency of specific
expectations regarding reasonable controls that should be, but are not
yet, included in the SIP (or FIP or TIP) would be sufficient to
override the deference to existing SIP (or FIP or TIP) controls.
Commenters noted that such communications, either electronic or in hard
copy, come from an authorized person within the EPA and be transparent
and publicly available. One commenter suggested that the ``authorized''
person be the Regional Administrator. The EPA agrees with commenters
that we would consider as sufficient any formal communication notifying
the affected air agency of SIP (or FIP or TIP) deficiencies with
respect to those controls that constitute reasonable controls for the
sources and pollutants that are contained within the SIP (or FIP or
TIP) and are the subject of an exceptional events demonstrations.\48\
These communications can be conveyed electronically or in hard copy and
come from any person within the EPA who is authorized to make such
decisions. Generally, these authorized persons could be branch chiefs,
air program managers, air division directors or the equivalent highest
manager who exclusively oversees air programs, or regional
administrators.
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    \48\ The EPA acknowledges that not all SIP (or FIP or TIP)-
related communications would negatively impact deference to the
control measures contained within the SIP (or FIP or TIP). For
example, if the EPA issued a letter notifying an air agency that its
existing SIP (or FIP or TIP)-approved controls appear to meet a new
SIP (or FIP or TIP) requirement (i.e., BACM for the 2008 Ozone NAAQS
would also be BACM for 2015 Ozone NAAQS), this same correspondence
could support continued use of those controls as ``reasonable'' for
exceptional events purposes.
---------------------------------------------------------------------------

    Related to these communications regarding expectations for
reasonable controls, the proposal invited comment on whether there
should be a grace or grandfathering period before a SIP (or FIP or TIP)
call involving a relevant NAAQS that would effectively end the
deference that applied prior to the SIP (or FIP or TIP) call. If an
event were to occur during such a grace period, the existing SIP (or
FIP or TIP) controls would still be given the deference. Several
commenters supported, and no commenters opposed, incorporating this
concept into regulatory language, noting that agencies should be given
time to enact appropriate control measures after the EPA has identified
this need. Commenters also noted that the timeframe for enacting these
measures often depends on the widely-varying state/area-specific
administrative requirements. In many cases, state and local agencies
are prohibited by state law from enacting ``stricter than federal''
controls unless required by a federal action such as a nonattainment
designation or SIP call. Therefore, in most circumstances, when a SIP
(or FIP or TIP) revision is required, such as when new regulations must
be incorporated or when an area receives a new designation, we think it
is reasonable that agencies be given time to enact appropriate control
measures after the need to do so has been identified and justification
is in place to satisfy state laws. However, in some circumstances, the
requirement to revise particular emission control measures in an
implementation plan might be pursuant to a SIP call under CAA section
110(k)(5), which represents a determination by the EPA that the control
measures in the existing implementation plan are substantially
inadequate. In the proposal, the EPA acknowledged that such SIP calls
might necessitate different treatment and took comment on that issue
(see 80 FR 81878). After fully considering the issue, including
comments received, we have determined that in such cases involving a
SIP call, we do not think it would be reasonable for an air agency to
continue to rely on those deficient measures in an exceptional events
demonstration. Accordingly, we are including regulatory text that
extends the deference to emission control measures contained in a SIP
that is subject to a revision requirement to the due date for a
required SIP revision. However, the regulatory text also explains that
when the control measures applicable to the exceptional events
demonstration are subject to a SIP call under CAA section 110(k)(5),
the EPA will evaluate on a case-by-case basis the control measures in
place to determine whether emissions were reasonably controlled at the
time of the event.
3. Clear Causal Relationship Supported by a Comparison to Historical
Concentration Data
a. Summary of Proposal
    The EPA proposed to revise the 2007 Exceptional Events Rule
language related to the clear causal relationship criterion as follows:

 To move the ``clear causal relationship'' element into the
list of criteria that explicitly must be met for data to be excluded

 To subsume the ``affects air quality'' element into the
``clear causal relationship'' element
 To remove the ``but for'' element
 To remove the term ``historical fluctuations'' and replace it
with text referring to a comparison to historical concentrations,
 To clarify that the comparison to historical concentrations is
not a fact that must be proven
 To clearly identify in regulatory language the types of
analyses that are necessary and sufficient in a demonstration to
address the comparison to historical concentrations

    As noted in the proposal, CAA section 319(b)(3)(B)(ii) requires
that ``a clear causal relationship must exist between the measured
exceedances of a national ambient air quality standard and the
exceptional event to demonstrate that the exceptional event caused a
specific air pollution concentration at a particular air quality
monitoring location.'' The clear causal relationship criterion
establishes causality between the event and a measured exceedance or
violation of a NAAQS. If the actual effect of the event were small, it
may be very difficult to distinguish the effect of the event with
sufficient confidence because many other factors could have produced
similar effects. As with the other exceptional events criteria, the EPA
has used a weight of evidence approach when reviewing analyses to
support a causal relationship between an event and a monitored
exceedance or violation.
    Showing that an event and elevated pollutant concentrations
occurred simultaneously may not establish causality. The clear causal
relationship section of an exceptional events demonstration should
include analyses showing that the event occurred and that emissions of
the pollutant of interest resulting from the event were transported to
the monitor(s) recording the elevated concentration measurement(s). The
last three of the bullets, summarized here, relate to analyses
associated with demonstrating that a clear causal relationship exists
between the event-related emissions and the monitored exceedance or
violation (i.e., they relate to the technical treatment of data, which
is the subject of this section of the preamble). We discussed our
proposed rationale for the first three bullets in Section V.B.1 of this
preamble, Definition and Scope of an Exceptional Event.
    The EPA proposed to remove the regulatory language in the 2007
Exceptional Events Rule that ``[t]he event is associated with a
measured concentration in excess of normal historical fluctuations,
including background'' and replace it with text referring to a
comparison to historical concentrations. Our intent with the original
language in the 2007 rule was to require air agencies to present event-
influenced concentration data along with historical data and to
quantify the difference, if any, between the event and the non-event
concentrations thus supporting the weight of evidence within the clear
causal relationship determination. We indicated in our November 2015
proposal that the phrase ``in excess of normal historical fluctuations,
including background'' is vague and provides no additional value to
historical concentration comparisons. Rather than use this language, we
proposed that every exceptional events submittal must include a
demonstration of a clear causal relationship between the event-related
emissions and the monitored exceedance or violation as supported by a
comparison to historical concentration data.
    To support the clear causal relationship generally, we proposed
example analyses and guidance, shown in Table 1, as being appropriate
for most event types.\49\
---------------------------------------------------------------------------

    \49\ For purposes of summarizing example clear causal
relationship analyses in one place, the EPA has included an entry
for the comparison to historical concentrations showing in Table 1.
The EPA notes that although the Interim High Winds Guidance and the
Interim Q&A document discussed the comparison to historical
concentrations showing, neither of these guidance documents
presented this showing as part of the clear causal relationship. See
specifically Interim Guidance on the Preparation of Demonstrations
in Support of Requests to Exclude Ambient Air Quality Data Affected
by High Winds Under the Exceptional Events Rule. U.S. EPA. May 2013.
Available at http://www2.epa.gov/sites/production/files/2015-05/documents/exceptevents_highwinds_guide_130510.pdf and Interim
Exceptional Events Rule Frequently Asked Questions. U.S. EPA. May
2013. Available at http://www2.epa.gov/sites/production/files/2015-05/documents/eer_qa_doc_5-10-13_r3.pdf.

    Table 1--Example Clear Causal Relationship Evidence and Analyses
------------------------------------------------------------------------
   Example of clear causal      Types of analyses/information to support
    relationship evidence                     the evidence
------------------------------------------------------------------------
Comparison to Historical       Analyses and statistics showing how the
 Concentrations.                observed event concentration compares to
                                the distribution or time series of
                                historical concentrations of the same
                                pollutant.
Occurrence and geographic      Special weather statements, advisories,
 extent of the event.           news reports, nearby visibility
                                readings, measurements from regulatory
                                and non-regulatory (e.g., special
                                purpose, emergency) monitoring stations
                                throughout the affected area, satellite
                                imagery.
Transport of emissions         Wind direction data showing that
 related to the event in the    emissions from sources identified as
 direction of the monitor(s)    part of the ``not reasonably
 where the measurements were    controllable or preventable''
 recorded.                      demonstration were upwind of the
                                monitor(s) in question, satellite
                                imagery, monitoring data showing
                                elevated concentrations of other
                                pollutants expected to be in the event
                                plume.
Spatial relationship between   Map showing likely source area, wind
 the event, sources,            speed/direction and pollutant
 transport of emissions and     concentrations for affected area during
 recorded concentrations.       the time of the event, trajectory
                                analyses.
Temporal relationship between  Hourly time series showing pollutant
 the event and elevated         concentrations at the monitor in
 pollutant concentrations at    question in combination with wind speed/
 the monitor in question.       direction data in the area where the
                                pollutant originated/was entrained or
                                transported.
Chemical composition and/or    Chemical speciation data from the
 size distribution (for PM2.5   monitored exceedance(s) and sources,
 to PM10) of measured           size distribution data.
 pollution that links the
 pollution at the monitor(s)
 with particular sources or
 phenomenon.
Comparison of event-affected   Comparison of concentration and
 day(s) to specific non-event   meteorology to days preceding and
 days.                          following the event, comparison to high
                                concentration days in the same season
                                (if any) without events, comparison to
                                other event days without elevated
                                concentrations (if any), comparison of
                                chemical speciation data.
------------------------------------------------------------------------

    We noted that we do not expect nor would all air agencies
necessarily need to include all of the evidence and analyses identified
in Table 1, but rather to use available information to build a weight
of evidence showing. The proposal also noted that the EPA expects
nonattainment areas to have more sophisticated air quality prediction
tools, in some cases these tools include photochemical or regression
models and modeling experience. Depending on the case-by-case nature of
the event, these tools may be beneficial, particularly in situations
where the causality between the event and a measured exceedance of a
NAAQS is not clearly established with evidence and analyses identified
in Table 1.
    As we have noted previously, the EPA's mission includes preserving
and improving, when needed, the quality of our nation's ambient air to
protect human health and the environment. The EPA accomplishes this by
developing the NAAQS for criteria pollutants, evaluating the status of
the ambient air as compared to these NAAQS using data collected in the
national ambient air quality monitoring network established under the
authority of section 319(a) of the CAA, and by overseeing the states'
programs to improve air quality, as needed. Thus, ambient air quality
data are fundamental to the CAA and the protection of public health.
Data exclusions must also be consistent with the CAA. The ``comparison
to historical concentration'' portion of the clear causal relationship
criterion shows how the event-influenced data compare to other non-
event related air quality data.
    To clarify our expectations for the ``comparison to historical
concentrations'' portion of the clear causal relationship showing, we
proposed the evidence and analyses shown in Table 2 as rule text to
indicate types of statistics, graphics and explanatory text regarding
comparisons to past data. The proposed rule language also indicated
that the analyses described in Table 2 are sufficient to satisfy the
rule's requirement regarding the comparison to historical concentration
data and that the submitting air agency does not need to prove any
specific threshold or ``in excess of'' fact.
    As with other evidence in an exceptional events demonstration
submittal, the EPA will use a weight of evidence approach in reviewing
submitted demonstrations and will consider the ``clear causal
relationship'' information, including the comparison to historical
concentrations showing, along with evidence supporting the other
Exceptional Events Rule criteria.
b. Final Rule
    After considering the public comments as described in the following
text, many of which supported our proposed approach, we are finalizing
as proposed and revising the regulatory requirement that the
demonstration to justify data exclusion must include a demonstration
that the event affected air quality in such a way that there exists a
clear causal relationship between the specific event and the monitored
exceedance or violation. We are also finalizing a modified version of
our proposal that the demonstration include analyses comparing the
claimed event-influenced concentration(s) to concentrations at the same
monitoring site at other times to support the clear causal relationship
criterion. The modification to the language within 40 CFR
50.14(c)(3)(iv)(C) retains the statement that the Administrator shall
not require an air agency to prove a specific percentile point in the
distribution of data. We note, in response to comments, that
``proving'' a specific percentile point is different than
``determining'' a specific percentile point. Also in response to
commenter feedback, we have removed the regulatory table identifying
the specific analyses associated with the comparison to historical
concentrations and included a revised version of the proposed table
(see Table 2) in this preamble as guidance. Although the table includes
several changes and clarifications suggested by commenters, we have
retained the proposed analysis that involves ``determining'' the
percentile ranking of the concentration in question because this
assessment provides perspective for the clear causal showing.

     Table 2--Evidence and Analyses for the Comparison to Historical
                             Concentrations
------------------------------------------------------------------------
   Historical concentration     Types of analyses/supporting information
           evidence                               \a\
------------------------------------------------------------------------
1. Compare the concentrations   Provide the data in the form
 on the claimed event day       relevant to the standard being
 with past historical data.     considered for data exclusion.
                                Present monthly maximums of the
                                NAAQS relevant metric (e.g., maximum
                                daily 8-hour average ozone or 1-hr SO2)
                                vs presenting monthly or other averaged
                                daily data as this masks high values for
                                the most recent 5-year period that
                                includes the event(s).\b\
                                Alternatively, if informative,
                                include separate plots for each year (or
                                season).\c\
                                See examples at https://www.epa.gov/sites/production/files/2015-05/documents/ideasforshowingeeevidence.pdf and
                                Question 3 in the Interim Q&A document
                                provides additional detail.\d\
2. Demonstrate spatial and/or   Prepare one or more time series
 temporal variability of the    plots showing the concentrations of the
 pollutant of interest in the   pollutant of interest at the affected
 area.                          monitor and nearby monitors.
                                Compare concentrations on the
                                claimed event day with a narrower set of
                                similar days by including neighboring
                                days at the same location (e.g., a time
                                series of two to three weeks) and/or
                                other days with similar meteorological
                                conditions (possibly from other years)
                                at the same or nearby locations with
                                similar historical air quality along
                                with a discussion of the meteorological
                                conditions during the same timeframe.\e\
3. Determine percentile         Determine 5-year percentile of
 ranking.                       the data requested for exclusion on a
                                per monitor basis.
                                Determine the annual ranking of
                                the data requested for exclusion. This
                                assessment may be potentially helpful to
                                show when the non-event concentrations
                                during the year with the exclusion
                                request were lower than surrounding
                                years.
4. Plot annual time series to   Prepare a time series plot
 show the range of ``normal''   covering 12 months (or all months in
 values (i.e., Display          which the data were collected)
 Interannual Variability) \f\.  overlaying at least 5 years of
                                monitoring data from the event-
                                influenced monitor to show how monitored
                                concentrations compare at a given time
                                of year and/or coincide with the subject
                                event. This plot will display the non-
                                event variability over the appropriate
                                seasons or number of years.
                                For annual comparisons, use the
                                daily statistic (e.g., maximum daily 8-
                                hour average, or maximum 1-hour)
                                appropriate for the form of the standard
                                being considered for data exclusion.

5. Identify all ``high''        Label ``high'' data points as
 values in all plots.           being associated with concurred
                                exceptional events, suspected
                                exceptional events, other unusual
                                occurrences, or high pollution days due
                                to normal emissions (provide evidence to
                                support the identification when
                                possible).
                                Include comparisons omitting
                                known or suspected exceptional events
                                points, if applicable.
6. Identify historical trends   Describe how pollutant
 (optional if this trends       concentrations have decreased over the 5-
 analysis provides no           year window, if applicable.
 additional ``weight'').        Identify and discuss trends due
                                to emission reductions from planning
                                efforts and/or implementing emission
                                control strategies.
                                Identify and discuss trends or
                                other variability due to meteorology or
                                economics of an area.
                                If appropriate, create a plot to
                                show how a downward trend in pollutant
                                concentrations over the 5-year
                                historical data record obscures the
                                uniqueness of the event-related
                                concentration.
7. Identify diurnal or          Show how the diurnal or seasonal
 seasonal patterns.             pattern differs due to the event, if the
                                event causes a change from typical
                                diurnal/seasonal patterns.
------------------------------------------------------------------------
\a\ While the EPA recommends using 5 years of data in analyses to
  support the comparison to historical concentrations, we recognize that
  there may be exceptions to using 5 years of data such as when 5 years
  of data are not available for a given monitor or in case-by-case
  analyses such as those for prescribed fire on wildlands.
\b\ Section 8.4.2.e of appendix W (proposed revisions at 80 FR 45374,
  July 29, 2015) recommends using 5 years of adequately representative
  meteorology data from the National Weather Service (NWS) to ensure
  that worst-case meteorological conditions are represented. Similarly,
  for exceptional events purposes, the EPA believes that 5 years of
  ambient air data, whether seasonal or annual, better represent the
  range of ``normal'' air quality than do data from shorter periods.
\c\ ``Season'' can be pollutant and area specific. For example, the EPA
  defines ozone monitoring seasons in Table D-3 to Appendix D of Part
  58: ``Ozone Monitoring Season by State.'' These seasons include, but
  may be longer than, an area's typical photochemical ozone season. For
  exceptional events purposes, an area may want to include both the
  typical photochemical ozone season and the ``season'' in which the
  event happened (if they are different). Similarly, the ``season'' for
  PM may be in the winter (for areas influenced by wood smoke). The
  general concept behind ``seasonal'' analyses is to compare the season
  of anthropogenic pollutant generation to the season in which the event
  occurred.
\d\ Interim Exceptional Events Rule Frequently Asked Questions. U.S.
  EPA. May 2013. Available at http://www2.epa.gov/sites/production/files/2015-05/documents/eer_qa_doc_5-10-13_r3.pdf.
\e\ If an air agency compares the concentration on the claimed event day
  with days with similar meteorological conditions from other years, the
  agency should provide information regarding any changes in wind
  patterns or sources of emissions of the pollutant(s) of concern in the
  area, including increases or reductions in the emissions inventory, or
  other known source of emissions information, that could affect the
  concentration of the pollutant(s) of concern during the exceptional
  event. If an air agency compares the concentration on the claimed
  event day to days immediately preceding and following the event day,
  the air agency should discuss and compare the meteorology on those
  days.
\f\ The EPA does not intend to identify a particular historical
  percentile rank point in the seasonal or annual historical data that
  plays a critical role in the analysis or conclusion regarding the
  clear causal relationship.

    In summarizing the clear causal relationship section of its
demonstration, the air agency should conclude with this type of
statement: ``On [day/time] an [event type] occurred which generated
pollutant X or its precursors resulting in elevated concentrations at
[monitoring location(s)]. The monitored [pollutant] concentrations of
[ZZ] were [describe the comparison to historical concentrations
including the percentile rank over an annual (seasonal) basis].
Meteorological conditions were not consistent with historically high
concentrations, etc.'' and ``In addition to the comparison to
historical concentrations showing, analyses X, Y and Z support Agency
A's position that the event affected air quality in such a way that
there exists a clear causal relationship between the specific event and
the monitored exceedance or violation and thus satisfies the clear
causal relationship criterion.''
c. Comments and Responses
    As indicated previously, numerous commenters supported revising the
regulatory language from ``event is associated with a measured
concentration in excess of normal historical fluctuations, including
background'' to ``a comparison to historical concentrations.''
Commenters supportive of the proposal agreed with the EPA's position
that the phrase ``in excess of normal historical fluctuations,
including background'' is vague and provides no additional value to
historical concentration comparisons. Commenters representing the
environmental community urged the EPA to maintain the ``in excess of
normal historical fluctuations, including background'' language
included in the 2007 rule, arguing that removing this language simply
because it is unclear effectively weakens clean air protections. The
EPA does not see this change to the rule text as weakening the CAA
protections. An analysis of measured concentrations, which inherently
includes background, and evidence that supports a comparison to
historical concentrations is still required to support the
demonstration of the clear causal criterion for the data exclusion
request to qualify as an exceptional event. Thus, the ``comparison to
historical concentrations'' showing is not less stringent than the ``in
excess of normal historical fluctuations, including background''
showing because the technical analysis remains robust.
    Commenters generally supported requiring a historical
concentrations showing as part of the clear causal relationship
criterion. Several of these commenters suggested that the EPA include
the proposed regulatory table identifying these historical
concentrations analyses as guidance in the preamble rather than in
regulatory text. Commenters offering this suggestion stated that
because some of the identified analyses are required and others are
optional, they are not universally applicable and are therefore best
presented as guidance. As indicated in the final rule discussion, the
EPA agrees with this approach and is removing the table from the final
rule language and retaining it as guidance, with changes, in this
preamble.
    A number of other commenters provided feedback regarding the
details of the clear causal relationship criterion, particularly asking
that we lessen or remove certain analyses. Although we address these
comments here and/or in the Response to Comments document that
accompanies this final rule, we note that CAA section 319(b)(3)(B)
requires the EPA to promulgate regulations, which ``at a minimum''
provide that exceptional events must be ``demonstrated by reliable,
accurate

data.'' The requirement for a ``demonstration'' necessarily imposes
data-driven analyses.
    One commenter requested that the EPA eliminate what is now Table 2
in this preamble from both rule and guidance because the EPA did not
provide an acceptable range of percentiles or a process/methodology to
determine whether the historical concentrations showing had been
satisfied. In response to this commenter, the EPA notes that
comparisons to historical concentrations help build a weight of
evidence showing for the clear causal relationship criterion and add
perspective to other analyses that air agencies may use in their clear
causal showing. A demonstration may be less compelling if some evidence
is inconsistent with the description of how the event caused the
exceedance. For example, if an air agency describes an event as a
regional dust storm or wildfire, then the EPA anticipates that most or
all monitors within the same regional scale would be similarly affected
by the event. That is, the EPA expects that the demonstration elements
and factors (e.g., clear causal relationship, reasonable controls,
meteorology, wind speeds) would also support the case for a regional
event. Comparison of concentrations and conditions at other monitors
could thus be very important for the demonstration of a clear causal
relationship. Alternatively, eliminating plausible non-event causes may
also support a causal relationship between the event and the elevated
concentration. In response to the commenter's request to eliminate the
showing based on a lack of information about an acceptable range of
percentiles or a process/methodology to determine whether the criterion
has been satisfied, the EPA points to language in this section of the
preamble and rule text that provides such criteria by indicating that
the analyses described in Table 2 are sufficient to satisfy the rule's
requirement regarding the comparison to historical concentration data
and that the submitting air agency does not need to prove any specific
threshold or ``in excess of'' fact (see 40 CFR 50.14(c)(3)(iv)(C)).
    In response to other specific comments regarding the analyses in
Table 2, two commenters noted that a comparison involving 5 years of
data is an inappropriate time for the comparison to historical
concentrations. As we note in footnote ``a'' to Table 2, we believe
that 5 years of ambient air data, whether seasonal or annual, better
represent the range of ``normal'' air quality than do data from shorter
periods. We recognize, however, that some monitors do not have 5 years
of data and/or may have periods of invalid data. The EPA recognizes
that there may be exceptions to using 5 years of data. One commenter
suggested that an appropriate comparison to historical concentrations
for prescribed fires may involve ``visual observations and/or modeled
impacts based on biomass consumption or other ecological parameters''
rather than comparisons using 5 years of monitoring data. The commenter
explains that while we were not measuring air quality impacts 100 years
ago, current fuel models may be used to estimate the area's fire
history and, thus, historical concentrations influenced by smoke. The
EPA agrees that the commenter's comparative analysis for prescribed
fire on wildland could supplement the comparison to historical
concentrations using monitoring data as part of the clear causal
relationship showing. The EPA acknowledges that current fuel models
could incorporate a timeframe for comparison that is longer than 5
years and could incorporate contributions from both prescribed fire and
wildfire. We further note that such modeling could support the clear
causal relationship by showing that a given observed ambient
concentration is similar to concentrations associated with past
fires.\50\ Such modeling, however, is not a substitute for the
comparison to historical concentrations using monitoring data. The
title of CAA section 319(b) is ``Air quality monitoring data influenced
by exceptional events.'' The language at section 319(b)(3)(B)(ii)
requires that ``a clear causal relationship must exist between the
measured exceedances of a national ambient air quality standard and the
exceptional event to demonstrate that the exceptional event caused a
specific air pollution concentration at a particular air quality
monitoring location.'' Monitoring data are at the core of the rule.
Generally, the form of most primary NAAQS (carbon monoxide and lead
excluded) relies on 3 years of data. Regulatory determinations
associated with these NAAQS employ data from regulatory monitors.
Therefore, if an exceptional event influences a regulatory monitor that
produces data, which will be used for a regulatory decision, 3 years of
data will be available. Comparisons of monitored event-influenced data
to modeled data, which are inherently predicted or estimated, do not
carry the same weight under a weight of evidence approach.
Additionally, because these monitoring data are readily available and
accessible, these analyses are also relatively easy to produce.
---------------------------------------------------------------------------

    \50\ While this comparison contributes to plausibility, it does
not necessarily mean that in the subject case, the exceedance or
violation was not caused by some other source or factor. The
comparison to actual historical concentrations on days not affected
by fire can make this point.
---------------------------------------------------------------------------

    In the same table, commenters asked for clarification regarding
``seasonal'' analyses. In response to this comment, the EPA has added a
new footnote clarifying that ``season'' can be pollutant and area
specific. For example, the EPA defines ozone monitoring ``seasons'' in
40 CFR part 58, appendix D, Table D-3, ``Ozone Monitoring Season by
State.'' These seasons include, but may be longer than, an area's
typical photochemical ozone season. For exceptional events purposes, an
area may want to include both the typical photochemical ozone season
and the ``season'' in which the event happened (if they are different).
Similarly, the ``season'' for PM may be in the winter (for areas
influenced by wood smoke). The general concept behind ``seasonal''
analyses is to compare the season of anthropogenic pollutant generation
to the season in which the event occurred.
    Continuing with additional requested clarifications regarding Table
2, another commenter asked that we clarify the language ``time
horizon.'' As a result of the modifications to this table, we no longer
use this term. Another commenter asks that we revise the language in
footnote ``e'' to Table 2, which reads ``. . . the agency should also
verify and provide evidence that the area has not experienced
significant changes in wind patterns, and that no significant sources
in the area have had significant changes in their emissions of the
pollutant of concern'' to ``. . . the agency should provide information
regarding any changes in wind patterns or sources of emissions of the
pollutant(s) of concern in the area, including increases or reductions
in the emissions inventory that could affect the pollutant
concentration during the exceptional event.'' The EPA agrees that the
suggested language better conveys our intent to require details of any
changes rather than evidence of lack of changes. We have incorporated
the commenter's suggested language with the following revision into the
footnote in Table 2 of this preamble: ``. . . the agency should provide
information regarding any changes in wind patterns or sources of
emissions of the pollutant(s) of concern in the area, including
increases or reductions in the emissions inventory, or other known
source of emissions information, that could affect the concentration of
the

pollutant(s) of concern during the exceptional event.''
    In response to a commenter's request to clarify that the burden on
the air agency does not change with moving the ``clear causal
relationship'' element into the list of criteria that explicitly must
be met for data to be excluded, we affirm that the burden does not
increase. In our rule revisions, we have clarified that air agencies
must address all three of the core statutory elements and implicit
concepts of CAA section 319(b) (i.e., the event affected air quality in
such a way that there exists a clear causal relationship between the
specific event and the monitored exceedance or violation, the event was
not reasonably controllable or preventable and the event was caused by
human activity that is unlikely to recur at a particular location or
was a natural event) in an exceptional events demonstration. Prior to
these rule revisions, the elements ``affects air quality,'' ``not
reasonably controllable or preventable,'' and ``human activity unlikely
to recur at a particular location or a natural event'' were included in
the definition of an exceptional event, while the requirement at 40 CFR
50.14(c)(3)(iv) that a ``demonstration to justify data exclusion shall
provide evidence'' included addressing the exceptional events
definitional requirements, ``clear causal relationship,'' ``historical
fluctuations'' and ``but for.'' Based on our experience implementing
the rule, it is more clear to explicitly include all of the elements in
a single place in the regulatory language.

F. Treatment of Certain Events Under the Exceptional Events Rule

    The preamble of the November 2015 proposal stated that air quality
data affected by the following event types are among those that could
meet the definition of an exceptional event and qualify for data
exclusion provided all requirements of the rule are met: (1) Chemical
spills and industrial accidents, (2) structural fires, (3) terrorist
attacks, (4) volcanic and seismic activities, (5) natural disasters and
associated cleanup, and (6) fireworks.\51\ We did not propose any
changes to the definition of exceptional event to address these event
types nor did we intend to imply that these are the only event types
that could be considered for data exclusion under the Exceptional
Events Rule. We simply repeated these event categories because they
were specifically identified and discussed in the preamble to the 2007
Exceptional Events Rule and we wanted to acknowledge our continued
belief that the identified events could be considered ``exceptional.''
The AQS database contains a more detailed list of other events that may
also be identified for consideration. The EPA will consider other types
of events on a case-by-case basis.
---------------------------------------------------------------------------

    \51\ Of these noted event types, the regulatory language at 40
CFR 50.14 only specifically addresses fireworks. We did not propose
any revisions to the exclusion at 40 CFR 50.14(b)(2) for fireworks
that are demonstrated to be significantly integral to traditional
national, ethnic or other cultural events.
---------------------------------------------------------------------------

    Based on our implementation experience, our proposal, and commenter
feedback, the following sections clarify details for other potential
exceptional events categories: Transported pollution, wildland fires
(including wildfires and prescribed fires), stratospheric ozone
intrusions, and high wind dust events. We discuss each of these event
categories in the following sections of this preamble.
    Several commenters provided feedback on the EPA's list of
identified, but not discussed, potential exceptional events. One
commenter noted that fireworks cannot be an exceptional event. This
comment is beyond the scope of this rulemaking because we did not
propose to change our consideration of fireworks under the Exceptional
Events Rule and did not open this issue for comment (see additional
explanation in footnote 51).
    Another commenter asked why the EPA added as an explanation for the
``chemical spills and industrial accidents'' event type the following
footnote: ``A malfunction at an industrial facility could be considered
to be an exceptional event if it has not resulted in source
noncompliance, which is statutorily excluded from consideration as an
exceptional event, see CAA 319(b)(1)(b)(iii), and if it otherwise meets
the requirements of the Exceptional Events Rule.'' While we are
deleting the footnote in this final action, we note that we added the
footnote to the proposal to clarify the position stated in previous EPA
guidance \52\ that limited noncompliance of local sources can be
expected from time to time as a result of process upsets or
malfunctioning control equipment. These events are usually classified
as ``upsets'' or ``malfunctions'' as defined by the applicable State or
local agency regulations, or they may be considered a violation of
applicable emission or opacity limits. If these events are caused by
upsets or malfunctions, they should be so noted and reported to the
appropriate control agency. If they constitute a violation, legal
remedies are available to relevant parties. In summary, if a
malfunction is caused by or results in source noncompliance, then the
resulting emissions cannot be considered for exclusion under the
Exceptional Events Rule in light of the plain language of CAA section
319(b)(1)(B)(iii). However, if the malfunction was not caused by nor
did it result from source noncompliance (e.g., it resulted from an act
of nature, such as a lightning strike) AND if the resulting emissions
caused a NAAQS exceedance or violation AND if it otherwise meets the
requirements of the Exceptional Events Rule, then the emissions from
the malfunction could be considered for exclusion under the provisions
of 40 CFR 50.14.
---------------------------------------------------------------------------

    \52\ Guideline on the Identification and Use of Air Quality Data
Affected by Exceptional Events (the Exceptional Events Policy), U.S.
EPA, OAQPS, EPA-450/4-86-007, July 1986.
---------------------------------------------------------------------------

1. Transported Pollution
    We did not propose any new guidance or specific regulatory language
addressing the transported pollution that could be considered for
exclusion under the Exceptional Events Rule. Rather, the proposal
discussed the provisions within the CAA that provide regulatory relief
for, or otherwise regulate, transported pollution and identified the
circumstances under which air agencies can use these provisions. While
our focus in this action is the Exceptional Events Rule (CAA section
319(b)), we also discuss transport under other CAA sections for context
(i.e., 179B (International Transport), 182(h) (Rural Transport Areas),
110(a)(2)(D)(i)(I) (Interstate Transport) and 126 (Interstate
Transport)). We are finalizing the language from our proposal with
additional clarifications resulting from commenter feedback as guidance
in this preamble.
a. Transported Pollution Within the Exceptional Events Rule
    To be considered for data exclusion, transported pollution must
meet all of the Exceptional Events Rule criteria. Specifically,
transported pollution must be event-related AND be either natural or
caused by a human activity unlikely to recur at a particular location
(see 40 CFR 50.14(c)(3)(iv)(E)). Human activities unlikely to recur at
a particular location could include some of the event types mentioned
in the introduction to this section of this preamble, such as chemical
spills, industrial accidents, or terrorist activities. Routine
emissions generated by and transported from anthropogenic sources are
not

exceptional events.\53\ Additionally, transported emissions from
natural sources must be event-related (e.g., wildfires, stratospheric
ozone intrusion, Saharan dust) versus ongoing on a daily basis to
qualify for data exclusion under the Exceptional Events Rule. Natural
emissions that occur every day and contribute to background levels,
such as routine biogenic emissions of ozone precursors from vegetation
and soils, do not meet the definition of an exceptional event because
they are not deviations from normal or expected conditions. Despite
being natural, they are not ``events.''
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    \53\ An example of routine emissions generated by and
transported from anthropogenic sources might include emissions of
ozone precursors or directly emitted particulate matter (or PM
precursors) from one state or foreign country's power plants
transported into another state or the U.S. The CAA provides other
mechanisms like 179B (for international transport) or 110(a)(2)(D)
and/or 126 (for interstate transport) to address these types of
emissions.
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    In most cases, of the previously identified CAA sections, the
mechanisms in the Exceptional Events Rule provide the most regulatory
flexibility in that air agencies can use these provisions to seek
relief from designation as a nonattainment area.\54\ Because the
Exceptional Events Rule may be used during the initial area
designations process and may make a difference in an attainment versus
a nonattainment decision, the EPA believes that the Exceptional Events
Rule will often be the most appropriate mechanism to use when
addressing transported emissions from out-of-state natural events or
events due to human activity that is unlikely to recur at a particular
location.
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    \54\ The CAA section 179B (International Transport) and CAA
section 182(h) (Rural Transport Areas) apply following, or
concurrent with, the initial area designations process.
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    If an air agency determines that the Exceptional Events Rule is the
most suitable approach to address contributions from event-related
transported emissions, then the air agency must consider the source(s)
of emissions contributing to the exceedance or violation to determine
how to address individual Exceptional Events Rule criteria,
specifically the not reasonably controllable or preventable criterion
and the human activity unlikely to recur or a natural event criterion.
    Under the CAA, the EPA generally considers a state (not including
areas of Indian country) to be a single responsible actor. Accordingly,
neither the EPA nor the 2007 Exceptional Events Rule provides special
considerations for intrastate scenarios when an event in one part of a
state, such as a county or air district, affects air quality in another
part of the same state, assuming that the event occurs on land subject
to state authority (versus tribal government authority). For cases
involving intrastate transport, the state or local air agency should
evaluate whether contributing event emissions from those parts of the
state located between the subject upwind source and the affected
monitor were not reasonably controllable or preventable. Section IV.E.2
of this preamble discusses the not reasonably controllable or
preventable criterion in more detail. Because there may be special
considerations regarding air agencies' authority to regulate activity
on federally-owned and managed lands (e.g., national parks within the
state), states and tribes should discuss with the appropriate FLM or
other federal agency and their EPA Regional office early in the
development of an exceptional events demonstration if they believe that
sources on federally-owned and managed land contributed event-related
emissions to a degree that raises issues of reasonable control.
    Interstate and international transport events are different than
intrastate events. As noted in Section IV.E.2 of this preamble and in
the final regulatory language at 40 CFR 50.14(b)(8)(vii), the EPA
maintains that it is not reasonable to expect the downwind air agency
(i.e., the state or tribe submitting the demonstration) to have
required or persuaded the upwind foreign country, state or tribe to
have implemented controls on sources sufficient to limit event-related
emissions in the downwind state. As with any demonstration, the
submitting (downwind) state should identify all natural and
anthropogenic contributing sources of emissions (both local/in-state
and out-of-state) to show the causal connection between an event and
the monitored exceedance or violation. Although the downwind state must
still assess potential contribution from in-state sources as discussed
in Section IV.E.2 of this preamble, we are finalizing regulatory
language at 40 CFR 50.14(b)(8)(vii) that the event-related emissions
that were transported in the downwind state are ``not reasonably
controllable or preventable'' for purposes of data exclusion. If the
event-related emissions are international in origin and affect monitors
in multiple states or regions, the EPA may assist affected agencies in
identifying approaches for evaluating the potential impacts of
international transport and determining the most appropriate
information and analytical methods for each area's unique situation.
    As with all exceptional events demonstrations, the EPA will
evaluate the information on a case-by-case basis based on the facts of
a particular exceptional event including any information and arguments
presented in public comments received by the state in its public
comment process or by the EPA in a notice-and-comment regulatory action
that depends on the data exclusion.
b. Other Transport Mechanisms Within the CAA
    In the following paragraphs, we discuss other provisions within the
CAA that provide regulatory relief for, or otherwise regulate,
transported pollution and identify the circumstances under which air
agencies can use these provisions.
     CAA section 179B, International Transport--CAA section
179B allows states to consider in their attainment demonstrations
whether a nonattainment area might have met the NAAQS by the attainment
date ``but for'' emissions contributing to the area originating outside
the U.S. This provision addresses sources of emissions originating
outside of the U.S. and provides qualifying nonattainment areas some
regulatory relief from otherwise-applicable additional planning and
control requirements should the area fail to reach attainment by its
deadline. It does not provide a pathway for regulatory relief from
designation as a nonattainment area; rather, CAA section 179B applies
following the initial area designations process.
     CAA section 182(h), Rural Transport Areas--CAA section
182(h) authorizes the EPA Administrator to determine that certain ozone
nonattainment areas can be treated as rural transport areas, which
provides relief from more stringent requirements associated with higher
nonattainment area classifications (i.e., ozone classifications above
Marginal). Under CAA section 182(h), a nonattainment area may qualify
as a Rural Transport Area if it does not contain emissions sources that
make a significant contribution to monitored ozone concentrations in
the area or in other areas, and if the area does not include and is not
adjacent to a Metropolitan Statistical Area. Generally, an area
qualifies as a Rural Transport Area because it does not contribute to
its own or another area's nonattainment problem; rather, ozone
exceedances are due to transported emissions, which could be
international, interstate or

intrastate in origin. The Rural Transport Area determination can be
made during or after the initial area designations and classifications
process.
     CAA section 110(a)(2)(D)(i)(I), Interstate Transport--CAA
section 110(a)(2)(D)(i)(I) requires states to develop and implement
SIPs to address the interstate transport of emissions from sources
within their jurisdiction. Specifically, this provision requires each
state's SIP to prohibit ``any source or other type of emissions
activity within the State from emitting any air pollutant in amounts
which will significantly contribute to nonattainment'' of any NAAQS in
another state, or which will ``interfere with maintenance'' of any
NAAQS in another state. When the EPA promulgates or revises a NAAQS,
each state is required to submit a SIP addressing this interstate
transport provision as to that NAAQS within 3 years. The EPA interprets
this interstate transport provision to address anthropogenic sources of
emissions from other states, and not to address natural sources of
emissions.
     CAA section 126, Interstate Transport--CAA section 126
provides states \55\ and political subdivisions with a mechanism to
petition the Administrator for a finding that ``any major source or
group of stationary sources emits or would emit any air pollution in
violation of the prohibition of CAA 110(a)(2)(D)(i).'' \56\ Where the
EPA grants such a petition, an existing source may operate beyond a 3-
month period only if the EPA establishes emissions limitations and
compliance schedules to bring about compliance with CAA section
110(a)(2)(D)(i) as expeditiously as practicable, but no later than 3
years after such finding. Similar to our interpretation for CAA section
110(a)(2)(D)(i), the EPA interprets the reference to ``major source or
group of stationary sources'' in CAA section 126 to refer to
anthropogenic sources of emissions from other states. The EPA's
interpretation is that this provision is not intended to address
natural sources of emissions.
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    \55\ Tribes with treatment as a state authority (under the TAR)
for CAA section 126 could also use this CAA provision.
    \56\ The text of CAA section 126 codified in the United States
Code cross references CAA section 110(a)(2)(D)(ii) instead of
section 110(a)(2)(D)(i). The courts have confirmed that this is a
scrivener's error and the correct cross reference is to section
110(a)(2)(D)(i), See Appalachian Power Co. v. EPA, 249 F.3d 1032,
1040-44 (D.C. Cir. 2001).
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c. Comments and Responses
    Several commenters asked that the EPA clarify how the provisions in
the Exceptional Events Rule apply to background ozone concentrations
and longer duration emissions sources such as biogenics, lightning and
international transport. We provide some clarification in this section
of the preamble, but also refer to the discussion in Section IV.B.3,
which discusses rule applicability to background ozone.
    Commenters also asked for clarification regarding assessing
``event-related emissions that originate outside of the boundaries of
the state within which the concentration at issue was monitored'' for
purposes of the not reasonably controllable or preventable criterion.
As discussed in Section IV.E.2 of this preamble, the state or local air
agency should evaluate whether contributing event emissions from those
parts of the state located between the subject upwind source and the
affected monitor were not reasonably controllable or preventable.
    Another commenter suggests that where meteorological conditions
play a pronounced role in transporting extra-jurisdictional emissions,
those emissions would not prevent classification as a natural event.
The commenter notes that because recurring natural events may qualify
as exceptional events under the Exceptional Events Rule, international
event-related emissions, because they are transported by recurring
natural meteorological mechanisms, could also be exceptional events
even if the source of emissions in another country is anthropogenic.
The commenter continued that if the EPA does not consider all
international emissions to be ``natural events,'' then the data
associated with international emissions could still qualify for
exclusion under the Exceptional Events Rule in those instances in which
the magnitude of transported emissions or the resulting concentrations
are ``unusual.'' As we have noted, over the course of implementing the
Exceptional Events Rule, we have come to realize that an event needs to
be defined by the source of the emissions. If the underlying source is
a natural event (e.g., wildfire) and the emissions influence a
regulatory monitor, then it can be considered for exclusion under the
Exceptional Events Rule. If the underlying source is anthropogenic then
it can only be considered under the Exceptional Events Rule if the
emissions from the original source is unlikely to recur at a particular
location. The meteorological processes that result in pollutant
transport are ongoing and thus not an event, even though their
influence on ambient concentrations at a particular time and location
may be observed only occasionally and thus seem ``event-like.''
2. Wildland Fires
    The proposal noted that fires on wildland can play an important
ecological role across the nation, benefiting those plant and animal
species that depend upon natural fires for propagation, habitat
restoration and reproduction. The proposed rule also noted the large
contribution that wildfire can make to air pollution (including
periodic high PM2.5 and PM10, and VOC and
NOX, which are precursors to PM2.5,
PM10 and ozone) and wildfire's potential threat to public
safety. The proposal further recognized that these adverse effects can
be mitigated through management of wildland vegetation, including
planned prescribed fires and letting some wildfires proceed naturally
(typically those with lower fire intensity and severity).
    The proposal also recognized, consistent with the EPA's past
practice, that both wildfires and prescribed fires, under certain