(i) Definition of a prescribed fire. We are proposing to adopt in
rule language the current NWCG-recommended definition of a prescribed
fire: ``[a]ny fire intentionally ignited by management actions in
accordance with applicable laws, policies and regulations to meet
specific objectives.'' In this definition, ``management'' refers to the
owner or manager of the land area to which prescribed fire is applied,
and ``specific objectives'' refers to specific land or resource
management objectives.
    (ii) Events caused by human activity. We are proposing to say in
rule form that prescribed fires are events caused by human activity.
Thus, to be considered an exceptional event, every prescribed fire
demonstration must address the ``human activity unlikely to recur at a
particular location'' criterion.
    (iii) Unlikely to recur at a particular location. As discussed in
more detail in section V.E.1 of this proposal, this requirement of CAA
section 319(b) is not specific and requires interpretation on a case-
by-case or event type-by-event type basis. The term ``unlikely''
implies consideration of the expected future frequency of events
similar to the event that has already happened, but does not convey any
particular benchmark for what frequency should be low enough to be
considered ``unlikely.'' Also, the term ``at a particular location''
requires interpretation, as it could refer to the exact area or only to
the general area of the event, to the location of the ambient
monitoring station or stations that were affected by the event or to
the combination of both.
    As was our position in 2007, we continue to believe that the
natural fire return interval is a useful and appropriate benchmark for
a satisfactory demonstration that a prescribed fire is unlikely to
recur at a particular location, in the sense that if a planned program
of prescribed fire calls for the application of prescribed fire at a
similar interval to the natural fire return interval at given locations
then each prescribed fire conducted in that program can be considered
not likely to recur at its particular location. However, we now believe
based on experience and further consideration that the natural fire
return interval is not the only appropriate benchmark. It can be
difficult in some cases to determine what fire return interval
prevailed under natural conditions, which may not have existed for
decades or even hundreds of years in a particular area. Also, in some
cases environmental conditions may have changed, for example due to
climate change, such that the original natural ecosystem cannot
realistically be restored and the well-considered land management goal
instead may be the development and maintenance of a sustainable and
resilient ecosystem that is different than what historically existed
and that will likely reduce the risk of catastrophic wildfire. In such
a case, the frequency of prescribed fire needed to establish or restore
such an ecosystem during a transitional period and/or the frequency
needed to sustain the resilient ecosystem may be different than the
natural fire return interval that once prevailed. It is also important
to consider issues of fire personnel and public safety and protection
of nearby property. Land managers may need to apply prescribed fire at
a frequency that maintains the accumulation of fuel loading between
prescribed fires at a level that does not create the risk of a
dangerous wildfire.
    Accordingly, the first proposed change for prescribed fires on
wildland is to include in the rule text two benchmarks for the expected
future frequency of prescribed fires on wildland to meet the not likely
to recur criterion: (1) The natural fire return interval as articulated
in the 2007 preamble and (2) the prescribed fire frequency needed to
establish, restore and/or maintain a sustainable and resilient wildland
ecosystem. If finalized, an air agency could include information
provided by the land manager with respect to the appropriate benchmark
for a prescribed fire on wildland as the basis for satisfying the human
activity unlikely to recur at a particular location criterion in its
exceptional events demonstration.\89\
---------------------------------------------------------------------------

    \89\ The benchmarks for the expected frequency of prescribed
fires not on wildland would be assessed on a case-by-case basis.
---------------------------------------------------------------------------

    Successfully implementing one of these benchmarks for prescribed
fire frequency necessitates that the air agency and the land manager
collaborate to establish and document the appropriate fire return
interval or frequency in a submitted demonstration. Federal agencies
that use prescribed fire to manage lands for which they are responsible
generally prepare multi-year plans for the use of prescribed fire in a
given national park, national forest, armed forces base or other land
area. Many of these plans include an

objective to establish, restore and/or maintain a sustainable and
resilient wildland ecosystem and incorporate the best science available
to determine what prescribed fire cycle will accomplish this.\90\ Some
tribes, private landowners (and federal, state and local agencies
working with private landowners) and state agencies that manage state-
owned lands (e.g., state parks) also prepare multi-year management
plans. While plans developed by public agencies (i.e., state and
federal agencies) often undergo public comment prior to being
finalized, the plans developed by tribes and private landowners may not
follow a public comment process. However, public agencies often work
with tribes and private owners to develop these plans, which are based
on conservation practices and standards that have often undergone
public comment as part of the state or federal agency process.
---------------------------------------------------------------------------

    \90\ An example of a federal land management plan that considers
the need for the use of prescribed fire is the Land and Resource
Management Plan for the Kaibab National Forest, Coconino, Yavapai,
and Mojave Counties, Arizona, USDA Forest Service Southwestern
Region, MB-R3-07-17, February 2014, available at http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprd3791580.pdf.
---------------------------------------------------------------------------

    The EPA understands that multi-year plans incorporate factors
relevant to identifying and selecting the areas and times under which
management will initiate a specific prescribed fire. The EPA also
recognizes that evaluating the behavior and results of prior prescribed
fires aids in determining the frequency and need for future prescribed
fire in a given area. In addition, personnel and equipment must be
available on site, which cannot be specifically planned far in advance.
Thus, it is typical for multi-year plans to identify somewhat general
targets for the frequency of prescribed fire use and for specific burn
plans. Even then, unexpected differences between planned and actual
fire behavior, landscape or ecosystem characteristics, fuel loading
patterns and weather patterns may cause management to deviate from the
general plan and/or the specific burn plan. Therefore, when the EPA
reviews an exceptional events demonstration for a prescribed fire
conducted under a wildland management plan, we intend to compare the
actual time pattern of prescribed fires on the land with the pattern
described in the applicable multi-year plan in a general way, rather
than treating the multi-year plan as containing a specific schedule to
which management must adhere. For example, if the wildland management
plan identified an approximate 5-year burn interval, the EPA would not
disapprove a demonstration if the burn occurred on a 4-year or a 6-year
interval.
    Therefore, we are proposing in rule text that we will consider a
demonstration's referencing of a multi-year land or resource management
plan \91\ (and including either a copy or an internet link to the plan)
with a stated objective to establish, restore and/or maintain a
sustainable and resilient wildland ecosystem and/or to preserve
endangered or threatened species that also identifies the subject area
as a candidate for prescribed fire to be dispositive evidence that a
particular fire conducted in accordance with such a plan satisfies the
``unlikely to recur at a particular location'' criterion. We would also
consider a demonstration's referencing of a fire management plan for
tribal or private lands that has been reviewed and certified by the
appropriate fire and/or resource management professionals and agreed to
and followed by the land owner/manager to be sufficient evidence
satisfying the ``unlikely to recur at a particular location''
criterion.
---------------------------------------------------------------------------

    \91\ These plans could also include fire management plans,
prescribed fire on wildland management plans, landscape management
plans or equivalent public planning documents.
---------------------------------------------------------------------------

    (iv) Not reasonably controllable or preventable. Consistent with
current practice and 2007 preamble/rule language, the EPA considers it
appropriate to allow air agencies to rely on an in-place and
implemented state-certified SMP to satisfy the controllability prong of
the ``not reasonably controllable or preventable'' criterion. The EPA
proposes to incorporate the six elements of SMPs discussed in the 1998
Interim Air Quality Policy on Wildland and Prescribed Fires and
referenced in the preamble to the 2007 Exceptional Events Rule into the
preamble of the final rule for this proposal, where it would serve as
guidance. That is, at a minimum, a state-certified SMP would include
provisions for (i) authorization to burn, (ii) minimizing air pollutant
emissions, (iii) smoke management components of burn plans, (iv) public
education and awareness, (v) surveillance and enforcement, and (vi)
program evaluation. Certification would require that the air agency
certify in a letter to the Administrator of the EPA, or a Regional
Administrator, that it has adopted and is implementing a SMP.\92\
Alternatively, the EPA solicits comment on incorporating these SMP
elements into rule text language. The EPA proposes to accept as
sufficient the testimony of the air agency submitting an exceptional
events demonstration that the SMP is being implemented, provided that
prior to the EPA's acting on a demonstration, the record contains no
clear evidence to the contrary.
---------------------------------------------------------------------------

    \92\ The EPA anticipates that any person within an air agency
responsible for submitting exceptional events demonstrations or SIP
revisions could also be responsible for certifying a Smoke
Management Program.
---------------------------------------------------------------------------

    Consistent with current practice and 2007 preamble and rule
language, the EPA also considers it appropriate to allow air agencies
to rely on a burn manager's use of BSMP that minimize emissions and
control impacts, in lieu of a state-certified SMP, to satisfy the
controllability prong of the ``not reasonably controllable or
preventable'' criterion. To provide clarity and reduce uncertainty for
air agencies and burn managers, the EPA proposes to identify in the
rule text six BSMP practices as being generally applicable for
exceptional events purposes for prescribed fires on wildland as well as
other prescribed fires. The six BSMP, listed and described in more
detail in Table 3, come from guidance on BSMP for prescribed burns
provided by the U.S. Department of Agriculture (USDA) Forest Service
(USFS) and USDA Natural Resources Conservation Service (NRCS).\93\ Land
managers of other federal, state and local agencies and private land
owners generally endorse and follow this BSMP guidance.\94\ While the
listed practices are broadly stated, fire managers use site-specific
considerations to select the exact actions of each type and apply them
to specific burn projects. There may be situations in which one or more
of the six BSMP is clearly not applicable for a particular prescribed
burn--for example, if a prescribed fire is so remote that there are no
neighbors to be notified. The EPA generally does not intend to
challenge a burn manager's selection of the intensity or specific
measure within the BSMP categories when we review a particular
exceptional events demonstration. As part of the on-going assessment of
our

regulatory programs, we intend to generally review those practices
commonly employed by federal agencies and other users of prescribed
fire.
---------------------------------------------------------------------------

    \93\ USDA Forest Service and Natural Resources Conservation
Service, Basic Smoke Management Practices Tech Note, October 2011,
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1046311.pdf.
    \94\ The EPA also addressed how federal agencies may use basic
smoke management practices to establish a presumption of conformity
in the preamble to the EPA's General Conformity Rule at 40 CFR
93.153(g)-(i) (75 FR 17264, April 5, 2010). The six practices
identified in Table 3 are not a presumed to conform action for
purposes of a federal agency satisfying their General Conformity
responsibilities. For basic smoke management practices to provide a
presumption of conformity, the identified basic smoke management
practices must be publicly and state reviewed as part of a presumed
to conform action under 40 CFR 93.153(g) or (f) of the General
Conformity Rule.
---------------------------------------------------------------------------

    As another component of the approach for prescribed fires on
wildland, the EPA is proposing to accept as evidence of the actual use
of BSMP the fire manager's statement that he or she employed applicable
BSMP for a prescribed fire. Documentation of such statement for an
exceptional events demonstration could consist of a copy of the routine
post-burn report or a letter prepared by the fire manager (see example
content of a burn report in Table 4). The EPA and other federal
agencies will work collaboratively to provide access to such post-burn
reports by air agencies that need them. We encourage land managers and
other organizations that employ prescribed fires to work with states
and tribes to develop an efficient process to provide air agencies with
documentation that BSMP were employed for particular prescribed fires.

 Table 3--Summary of Basic Smoke Management Practices, Benefit Achieved
                  With the BSMP, and When It Is Applied
           [before, during or after ignition of the burn] \a\
------------------------------------------------------------------------
                                                      When the BSMP is
   Basic smoke management       Benefit achieved      applied--before/
          practice                with the BSMP       during/after the
                                                            burn
------------------------------------------------------------------------
Evaluate Smoke Dispersion     Minimize smoke        Before, During,
 Conditions.                   impacts.              After.
Monitor Effects on Air        Be aware of where     Before, During,
 Quality.                      the smoke is going    After.
                               and degree it
                               impacts air quality.
Record-Keeping/Maintain a     Retain information    Before, During,
 Burn/Smoke Journal.           about the weather,    After.
                               burn and smoke. If
                               air quality
                               problems occur,
                               documentation helps
                               analyze and address
                               air regulatory
                               issues.
Communication--Public         Notify neighbors and  Before, During.
 Notification.                 those potentially
                               impacted by smoke,
                               especially
                               sensitive receptors.
Consider Emission Reduction   Reducing emissions    Before, During,
 Techniques.                   through mechanisms    After.
                               such as reducing
                               fuel loading can
                               reduce downwind
                               impacts.
Share the Airshed--           Coordinate multiple   Before, During,
 Coordination of Area          burns in the area     After.
 Burning.                      to manage exposure
                               of the public to
                               smoke.
------------------------------------------------------------------------
\a\ The EPA believes that elements of these BSMP could also be practical
  and beneficial to apply to wildfires for areas likely to experience
  recurring wildfires.

   Table 4--Elements That may be Included in Burn Plans and Post-Burn
      Reports for Prescribed Fires Submitted as Exceptional Events
------------------------------------------------------------------------
           Element                  Burn plan         Post-burn report
------------------------------------------------------------------------
Fire Name \a\...............  X...................  X.
Permit number (if             X...................  X.
 appropriate).
Latitude/longitude and        X...................  X.
 physical description.
Date of burn, ignition time   X...................  X.
 and completion time
 (duration of burn).
AQI status on burn day, if    Predicted...........  Actual.
 available (both in the
 vicinity of the fire and in
 the affected upwind area).
Acres burned................  Planned.............  Actual (blackened).
Description of fuel loading.  Estimated...........  Actual (tons
                                                     consumed).
Meteorological data (weather  Predicted conditions  Actual conditions
 conditions, wind speed and    (including            (including actual
 direction, dispersion).       predicted             dispersion).
                               dispersion).
Smoke Impacts...............  Anticipated smoke     Observed or reported
                               impacts.              smoke impacts
                                                     (include nature,
                                                     duration, spatial
                                                     extent and copies
                                                     of received
                                                     complaints).
BSMP actions to reduce        Expected BSMP         Actual BSMP actions.
 impacts.                      actions.
Recommendations for future    ....................  X.
 burns in similar areas.
Analytics (modeled/actual     ....................  X.
 fire spread, satellite
 imagery and analysis,
 webcam/video, PM/ozone
 concentrations over the
 course of the fire).
------------------------------------------------------------------------
\a\ The ``Fire Name'' should be unique and referenced, to the greatest
  extent possible, in all exceptional events-related documentation,
  including the event name in AQS. The fire name could simply consist of
  the county and state in which the burn occurred (e.g., County X, State
  Y Prescribed Burn on Date Z) if no other name has been assigned.

    States with certified SMPs typically have robust communications
between officials concerned with air quality impacts and officials and
members of the public who use prescribed fire. These groups communicate
during the development of the SMP, during the day-to-day burn
authorization process and in the periodic review and potential revision
of the SMP. States that instead rely on fire managers employing BSMP on
a more individual basis may not have

such regularly occurring communications processes, particularly in
states in which state legislation gives the leadership of fire
management to a forestry or public safety agency rather than to an air
agency. We encourage all agencies and managers/owners involved in land,
air quality and fire management to develop good communications about
both fire use practices in general and plans for specific prescribed
fires with use of BSMP. This will, among other benefits, allow them to
better coordinate on public air quality notice efforts and, if
necessary, public health advisories should smoke enter an inhabited
area. Additionally, the EPA encourages the development of ``prescribed
fire councils,'' comprised of federal, state, tribal, private and other
stakeholders to coordinate activities involving fire planning issues
and, thus, minimize or prevent smoke impacts while using prescribed
fire to accomplish land management objectives.\95\ However, we are not
proposing that notifications between prescribed fire users and specific
types of state agencies be a condition for approval of a prescribed
fire as an exceptional event.
---------------------------------------------------------------------------

    \95\ See additional information on prescribed fire councils on
the Coalition of Prescribed Fire Councils, Inc. Web site at http://www.prescribedfire.net/membership/state-councils.
---------------------------------------------------------------------------

    As previously stated, to date we have considered the existence and
implementation of a SMP or the use of BSMP to be a necessary part of
the supporting evidence needed to satisfy the not reasonably
controllable or preventable criterion, but we have not clearly
addressed what conditions are minimally sufficient to satisfy the
criterion. The remainder of this section focuses on that issue.
    Because prescribed fires are intentionally initiated, clarifying
the minimal conditions for the not reasonably preventable prong is
particularly relevant. The detailed USFS/NRCS guidance on the fifth
listed BSMP, Consider Emission Reduction Techniques, includes the
potential to reduce the fuel loading. It does not suggest that it may
be reasonable to not ignite a particular prescribed fire (i.e., that
the fire be prevented), because this guidance is aimed at those fires
that are already planned to happen. Similarly, SMPs address
coordination of previously planned prescribed fires and typically do
not ask SMP participants to consider whether particular prescribed
fires are reasonably preventable. Therefore, we do not believe that the
existence of an SMP or the use of BSMP is a sufficient basis for
concluding that a prescribed fire is not reasonably preventable. A
prescribed fire should be concluded to be not reasonably preventable on
the basis of the benefits that would be foregone if it were not
conducted, as described below.
    For federal agencies, the planning of prescribed fire programs
typically happens through the development of multi-year plans that
focus on specific land or resource management objectives. This planning
process, and the resulting multi-year plan, typically considers the
importance of a prescribed fire program to achieving land management
goals, which may include an objective to establish, restore and/or
maintain a sustainable and resilient wildland ecosystem, in light of
the availability, cost and effectiveness of other approaches to fuel
management. The final multi-year plans thus generally identify the
level of prescribed fire use necessary to achieve those goals. As noted
previously, some tribes, private landowners (and federal, state and
local agencies working with private landowners) and state agencies that
manage state-owned lands (e.g., state parks) also prepare multi-year
management plans. While plans developed by public agencies (i.e., state
and federal agencies) often undergo public comment prior to being
finalized,\96\ the plans developed by tribes and private landowners may
not follow a public comment process. However, public agencies often
work with tribes and private owners to develop these plans, which are
based on conservation practices and standards that have often undergone
public comment as part of the state or federal agency process. Not
conducting the prescribed fire programs described in such plans could
mean forgoing important ecosystem services and other benefits.
Accordingly, the EPA is proposing in rule text form to consider a
prescribed fire on wildland conducted in accordance with a multi-year
fire management plan that has an objective to establish, restore and/or
maintain a sustainable and resilient wildland ecosystem to be not
reasonably preventable, provided there is no compelling evidence to the
contrary in the record when the EPA approves the associated exceptional
events demonstration.\97\
---------------------------------------------------------------------------

    \96\ Many multi-year plans developed by state and federal
agencies are available electronically online or can be requested
directly from the preparing agency. Interested parties can also
request electronic versions of project level plans, if they are not
available online.
    \97\ On a case-by-case basis, in the absence of a multi-year
plan, the EPA would also consider a prescribed fire on wildland
conducted on a fire return interval established according to
scientific literature to satisfy the not reasonably controllable or
preventable criterion provided the prescribed fire was also
conducted with the objective to establish, restore and/or maintain a
sustainable and resilient wildland ecosystem and conducted in
compliance with either a state-certified SMP or BSMP. This case-by-
case approach is similar to the approach currently used under the
2007 Exceptional Events Rule.
---------------------------------------------------------------------------

    We also propose in rule text that compliance with either a state-
certified SMP or BSMP is sufficient to establish that a prescribed fire
was not reasonably controllable, provided there is no compelling
evidence to the contrary in the record when the EPA concurs with the
associated exceptional events demonstration. This is an appropriate
approach to implementing CAA section 319(b), because SMPs and BSMP aim
to reasonably control the air quality impacts of prescribed fires.\98\
---------------------------------------------------------------------------

    \98\ With respect only to the not reasonably controllable prong,
we also believe that the SMP and BSMP approach each is sufficient
for prescribed fires that are not on wildland.
---------------------------------------------------------------------------

    This two-part categorical approach would reduce the length of
exceptional events demonstrations for prescribed fires on wildland and
make the demonstration preparation and review process more resource
efficient. In summary, to satisfy the not reasonably controllable or
preventable criterion for a prescribed fire on wildland, a
demonstration would need to identify that the prescribed burn was
conducted in accordance with a multi-year plan that has an objective of
the establishment, restoration and/or maintenance of a sustainable and
resilient wildland ecosystem and was conducted in compliance with
either a state-certified SMP or BSMP.
    Finally, we are proposing to remove the phrase ``and must include
consideration of development of a SMP'' from the sentence of the
existing text of 40 CFR 50.14(b)(3) that reads, ``If an exceptional
event occurs using the basic smoke management practices approach, the
State must undertake a review of its approach to ensure public health
is being protected and must include consideration of development of a
SMP.'' While the EPA supports states considering the development of a
SMP in the situation described in this sentence, we believe states have
had ample opportunity to develop such a program since 2007. This rule
language effectively requires an ongoing consideration to develop an
SMP every time a prescribed fire causes a NAAQS exceedance or violation
that merits exclusion as an exceptional event. We do not believe
Congress intended this ongoing consideration to be a requirement
flowing from CAA section 319(b). In addition, we believe that an

SMP is most appropriate when multiple parties wish to employ prescribed
fire at about the same time in the same airshed, which is a more narrow
situation than specified in this sentence. We also do not want our
rules to be open to an inference that development of a SMP should only
be considered following a NAAQS exceedance or violation, because the
impacts from fires may affect public health in areas without NAAQS-
compliance air monitoring stations. Also, we believe that when air
agencies observe NAAQS exceedances or violations attributed to a
prescribed fire, air agencies should consider a wide range of
alternatives including, but not limited to, the development of a SMP.
For example, agencies might also consider the more frequent or
intensive use of BSMP to limit the fuel available to burn in each fire.
    The EPA solicits comment on all aspects of the identified fire-
related approaches.
3. Stratospheric Ozone Intrusions
a. Current Situation
    Stratospheric ozone intrusions are natural events that occur when a
parcel of air originating in the stratosphere is re-entrained into the
troposphere, and in some cases mixes directly to the surface of the
earth. These relatively rare events can create elevated ozone
concentrations that affect areas ranging from a single monitoring site
to a wider area as the air mass with a high ozone concentration moves
across the landscape.
    Normally, the tropopause, the temperature inversion layer of air
that separates the troposphere from the stratosphere, limits the
transport of stratospheric air into the troposphere, the lowest layer
of the Earth's atmosphere.\99\ In some cases, however, parcels or
ribbons of ozone-rich air from the stratosphere can be transported
rapidly to the surface during deep mixing events, such as thunderstorms
or strong frontal passages, by a process known as tropopause folding.
Although this ``folding'' process can occur throughout the year, it is
typically associated with frontal passages and upper level low pressure
systems during the spring season.100 101 The ozone
transported through these ``folds,'' or through other less significant
mechanisms, may disperse within and be destroyed in the upper
troposphere or it may mix down to the surface.\102\ The ``intrusion''
of stratospheric ozone is identified most frequently at high elevation
sites where upper tropospheric air is more likely to reach the surface
than at lower elevation sites where more downward movement would be
needed for a monitoring site to be affected. At these high elevation
sites in particular, stratospheric ozone intrusion has been estimated
to contribute about 20 to 25 percent of the total tropospheric ozone
budget and can cause relatively short-term (i.e., ranging from several
hours to 2-3 days in duration) increases of surface ozone of 10 to 50
ppb above normal background levels.\103\ Stratospheric intrusions with
short-lived surface ozone concentrations of several hundred ppb have
also been observed, although these events are extremely rare.
---------------------------------------------------------------------------

    \99\ The height of the tropopause varies with latitude and
season but the average height at mid-latitudes is about 11
kilometers (km) (7 miles or 36,000 feet). The stratosphere, the
second layer of the Earth's atmosphere, is located above the
tropopause at 13 to 50 km (8-31 miles or 43,000 to 160,000 feet)
above the Earth's surface.
    \100\ Pan, L. L., et al. (2014), ``Thunderstorms enhance
tropospheric ozone by wrapping and shedding stratospheric air.''
Geophys. Res. Lett., 41, 7785-7790, doi:10.1002/2014GL061921.
    \101\ A. O. Langford, K. C. Aikin, C. S. Eubank and E. J.
Williams. Stratospheric Contribution to High Surface Ozone in
Colorado During Springtime, Geophysical Research Letters, Vol. 36,
Issue 12, June 2009.
    \102\ Other mechanisms by which stratospheric ozone is
transported into the troposphere include cutoff cyclones, streamers
(long filamentary structures that often roll into vortices), and
clear air turbulence (see http://www.atmos.umd.edu/~dkuhl/documents/
Kuhl_Tropopause_Folding.ppt).
    \103\ Integrated Science Assessment of Ozone and Related
Photochemical Oxidants (Final Report). U.S. EPA.Washington, DC EPA/
600/R-10/076F. Available at: http://www3.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_isa.html.
---------------------------------------------------------------------------

    Because stratospheric intrusion events are relatively infrequent
and because identifying and monitoring these events are challenging,
there have been few direct measurements showing that a given parcel of
surface air contains stratospheric ozone (versus ozone generated by
local natural or anthropogenic sources). Interpreting these direct
measurements is also challenging. One approach to confirm the presence
of stratospheric ozone in surface air is through the use of the
beryllium tracers, beryllium isotopes Be-7 and Be-10, which are
produced primarily in the stratosphere by cosmic ray collisions with
atmospheric gas atoms.\104\ Beryllium isotope measurements are,
however, rare and expensive, and, consequently not normally available.
---------------------------------------------------------------------------

    \104\ Cristofanelli, P. A 6-year analysis of stratospheric
intrusions and their influence on ozone at Mt. Cimone (2165 m above
sea level). Journal of Geophysical Research, Vol. 111, D03306,
doi:10.1029/2005JD006553, 2006.
---------------------------------------------------------------------------

    More common approaches to identify stratospheric ozone in surface
air can include evaluating measurements at the potentially influenced
ozone monitoring site for very low concentrations of CO and/or relative
humidity. Both can be strong indicators of stratospheric air because,
relative to tropospheric air, stratospheric air has very low relative
humidity and very low concentrations of other air pollutants such as
CO, NOX and PM. The concurrent impacts on CO and relative
humidity can be subtle, however, when stratospheric air has mixed with
tropospheric air as the mixing process dilutes the ozone enhancement
and increases CO and water vapor concentrations relative to
stratospheric conditions. Typical CO monitors used for ambient air
monitoring have operational ranges of 500 to 50,000 ppb (0.5 to 50 ppm)
and are not sufficiently sensitive to reliably measure the very low CO
levels found in stratospheric air (50 to 150 ppb). Additionally, few
rural high altitude monitoring sites have both ozone and CO
monitors.\105\ The EPA urges air agencies to provide concurrent
readings of ozone and CO and/or relative humidity in their exceptional
events demonstrations if they have these data. The EPA will evaluate
these data as a part of a weight of evidence showing alongside other
qualitative evidence in a clear causal relationship showing.
---------------------------------------------------------------------------

    \105\ A recent review of AQS data revealed 216 sites in the
United States with collocated ozone and carbon monoxide monitors in
operation after January 1, 2014. Most of these sites are located in
either urban or suburban locations. In these settings, local
emissions would likely hide the stratospheric CO suppression.
---------------------------------------------------------------------------

    A third measurement-based approach to identifying stratospheric
ozone could include measurements of ozone above ground level (i.e.,
measurements in the troposphere). This approach is also uncommon.
Currently, five sites in the U.S. conduct ozone sonde (balloon)
launches two or more times per week and an additional few research
locations operate ozone lidars to vertically measure ozone
profiles.106 107
---------------------------------------------------------------------------

    \106\ Lidar, which stands for Light Detection and Ranging, is a
remote sensing method that uses light in the form of a pulsed laser
to measure ranges (variable distances) to the Earth. These light
pulses, combined with other data recorded by the airborne system,
generate precise, three-dimensional information about the shape of
the Earth and its surface characteristics. See, NOAA definition at
http://oceanservice.noaa.gov/facts/lidar.html.
    \107\ See NOAA data available at ftp://aftp.cmdl.noaa.gov/data/ozwv/Ozonesonde. On April 30, 2015, NOAA's site identified the
following five sites in the U.S. that conduct bi-weekly ozone sonde
launchings: Hilo, HI; Huntsville, AL; Narragansett, RI; Trinidad
Head, CA; and Boulder, CO. Additional launchings occur in Pago Pago,
American Samoa, and outside of the U.S. in Greenland, Antarctica and
Fiji.
---------------------------------------------------------------------------

    In the absence of direct measurements of stratospheric tracers at
ground level,

meteorological models can indicate conditions under which stratospheric
air parcels may reach the surface. Meteorological models such as the
National Oceanic and Atmospheric Administration (NOAA)/National Weather
Service (NWS) North American Mesoscale Forecast System (NAM) or the
NOAA Rapid Update Cycle (RUC) models simulate parameters characteristic
of stratospheric air such as isentropic potential vorticity (IPV) and
potential temperature (PT) that can be used to identify tropopause
folding. Visualization tools using the model output can show spatially
where stratospheric air is located in proximity to or in contact with
the surface. Similarly, atmospheric chemistry models, such as the
National Aeronautics and Space Administration (NASA)/NOAA Real-time Air
Quality Modeling System (RAQMS) \108\ can provide both real time
intrusion forecasting and retrospective analysis of ozone from
intrusions. Finally, satellite observation of atmospheric ozone and CO
can be used to validate predictions based on atmospheric modeling.
---------------------------------------------------------------------------

    \108\ See http://raqms-ops.ssec.wisc.edu/.
---------------------------------------------------------------------------

    Although as of the date of this proposal the EPA has concurred with
only one stratospheric ozone intrusion exceptional events demonstration
prepared under the provisions of the 2007 Exceptional Events Rule (and
disapproved none),\109\ the EPA has been communicating that we consider
it appropriate to use the previously mentioned stratospheric ozone
tools with other event/pollutant exceptional events analyses (e.g.,
seasonal analysis of ozone data, comparison of event days with non-
event days, trajectory analysis, ozone measurement time series and
spatial distribution analysis, meteorological analysis to show the
presence of weather systems associated with typical intrusions and
balloon soundings of the NWS Upper Air Observation Program to detect
parcels of dry air aloft) to successfully demonstrate stratospheric
ozone exceptional events.
---------------------------------------------------------------------------

    \109\ Wyoming Department of Environmental Quality, Air Quality
Division. Big Piney and Boulder, Wyoming Ozone Standard Exceedance,
June 14, 2012. Available at http://www2.epa.gov/air-quality-analysis/exceptional-events-submissions-table.
---------------------------------------------------------------------------

b. Proposed Changes
    As is true for all exceptional events and pollutant combinations,
when submitting a demonstration for stratospheric ozone intrusion
events, air agencies must address all of the Exceptional Events Rule
criteria. As noted in this action, the EPA proposes 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 a human activity that is unlikely to
recur at a particular location or was a natural event. The EPA suggests
the following approach when addressing these technical criteria for an
ozone exceedance or violation caused by a stratospheric intrusion.
    An air agency should begin by showing the geographic extent of
elevated or exceedance-level ozone concentrations associated with the
intrusion event in conjunction with an evaluation of the historical
measured surface ozone levels for the same season (see section V.E.3 of
this proposal for example analyses of how to present the comparison to
historical concentrations within the clear causal relationship
criterion). If the intrusion happened at a time of year when local or
transported photochemical ozone is generally low, evidence that the
intrusion affected ground level air quality may be relatively brief and
still be sufficient, compared to an intrusion occurring at the height
of the historical ozone season.
    If intrusion claims coincide with historically high photochemistry
seasons, then the air agency may need additional evidence to support
the clear causal relationship criterion by showing the relative
contribution estimates to the exceedance from local and transported
anthropogenic pollutants compared to the intrusion contribution. An air
agency can provide additional analyses supporting the clear causal
relationship by showing that an intrusion occurred at or near the
location of an identified monitor by using atmospheric models such at
RAQMS, NAM or RUC, with additional data from satellite observations of
total column ozone and CO.
    The EPA intends to accept a short statement in a demonstration that
because stratospheric ozone intrusions are purely natural events and
are large in scale, they are not reasonably controllable or
preventable.
4. High Wind Dust Events
a. Current Situation
    The 2007 Exceptional Events Rule preamble noted that ``[t]he EPA's
final rule concerning high wind events states that ambient particulate
matter concentrations due to dust being raised by unusually high winds
will be treated as due to uncontrollable natural events where (1) The
dust originated from nonanthropogenic sources, or (2) the dust
originated from anthropogenic sources within the State, that are
determined to have been reasonably well-controlled at the time that the
event occurred, or from anthropogenic sources outside the State.'' As
noted in section IV.B of this document, although this language still
reflects the EPA's interpretation of what might be appropriate under
the Exceptional Events Rule, the D.C. Circuit determined the language
to be a legal nullity because the EPA did not specifically address high
winds or ambient particulate matter concentrations in the promulgated
regulatory language in 40 CFR 50.14.
    The preamble to the 2007 Exceptional Events Rule also noted that
because ``. . . the conditions that cause or contribute to high wind
events vary from area to area with soil type, precipitation, and the
speed of wind gusts, [air agencies] should provide appropriate
documentation which indicates what types of circumstances contributed
to the exceedances or violation at the monitoring site in question.''
The EPA declined to identify a specific high wind threshold to qualify
as being an exceptional event and instead relied on air agencies to
submit appropriate documentation supporting their position.
    Because of the uncertainty associated with these high wind
statements and stakeholder feedback asking the EPA to interpret this
language and provide examples of applying the provisions in the 2007
Exceptional Events Rule to high wind dust events, the EPA clarified
many concepts related to high wind dust events in its May 2013 Interim
Exceptional Events Implementation Guidance, specifically the Interim
High Winds Guidance document.\110\ In this guidance, the EPA defined a
high wind dust event as including the high wind and the dust that the
wind entrains and transports to a monitoring site, clarified our
expectations regarding ``reasonable controls'' for high wind events
with contribution from both natural and anthropogenic sources and
introduced the concept of establishing a value for a high wind
threshold, up to which reasonable windblown dust controls are

expected to be effective in the absence of site specific data or
analyses.
---------------------------------------------------------------------------

    \110\ 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.
---------------------------------------------------------------------------

    As identified in the Interim High Winds Guidance document, dust
phenomena are experienced primarily in the western U.S. where rainfall
is seasonal, creating dry and dusty landscapes.\111\ In high wind dust
events, the meteorological phenomenon (i.e., wind) is purely natural,
but the pollution from the event may be a mixture of natural sources
(e.g., undisturbed soil) and anthropogenic sources (e.g., soil
disturbed by human activity, emissions from sand and gravel facilities,
etc.). The EPA generally classifies high wind dust events as ``natural
events'' in cases where windblown dust is entirely from natural sources
or where all significant anthropogenic sources of windblown dust have
been reasonably controlled.\112\ This long-standing policy was first
established in the PM10 Natural Events Policy, which
provided that:
---------------------------------------------------------------------------

    \111\ We use ``Western U.S.'' to refer to states in the Great
Plains (North Dakota, South Dakota, Nebraska, Kansas, Oklahoma and
Texas) and those farther west including Arizona, California,
Colorado, Nevada, New Mexico, Utah, and Wyoming.
    \112\ As identified in section V.D of this proposal, the EPA
will generally consider human activity to have played little or no
direct role in causing emissions of the dust generated by high wind
for purposes of the regulatory definition of ``natural event'' if
contributing anthropogenic sources of the dust are reasonably
controlled, regardless of the amount of dust coming from these
reasonably controlled anthropogenic sources, and thus the event
could be considered a natural event. In such cases, the EPA believes
that it would generally be a reasonable interpretation to find that
the anthropogenic source had ``little'' direct causal role. If
anthropogenic sources of windblown dust that are reasonably
controllable but that did not have those reasonable controls applied
at the time of the high wind event have contributed significantly to
a measured concentration, the event would not be considered a
natural event.

    Ambient PM-10 concentrations due to dust raised by unusually
high winds will be treated as due to uncontrollable natural events
under the following conditions: (1) The dust originated from
nonanthropogenic sources, or (2) the dust originated from
anthropogenic sources controlled with best available control
---------------------------------------------------------------------------
measures (BACM).\113\

    \113\ Areas Affected by PM-10 Natural Events (the
PM10 Natural Events Policy), memorandum from Mary D.
Nichols, Assistant Administrator for Air and Radiation, to EPA
Regional Offices, May 30, 1996.

    Also integral to definition of a high wind dust event is that the
wind speed be ``high,'' or, as indicated in the PM10 Natural
Events Policy, ``unusually high.'' Only ``high wind'' dust events are
exceptional events and ``high'' is area-specific.
    Typically, undisturbed desert landscapes in the western U.S. have a
natural crust that protects the surface and tends to limit emissions of
windblown dust. The wind speed capable of causing emissions from these
natural undisturbed areas varies by location, depending on
characteristics of the local landscape (e.g., soil type and
characteristics, vegetation). Numerous studies have been conducted to
determine the minimum wind speed capable of causing emissions from
natural undisturbed areas and/or overwhelming reasonable controls on
anthropogenic sources.\114\ In the Interim High Winds Guidance, the EPA
called the minimum threshold wind speed capable of causing emissions
from natural undisturbed areas or overwhelming reasonable controls on
anthropogenic sources the ``high wind threshold.''
---------------------------------------------------------------------------

    \114\ See Appendix A1 of the 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.
---------------------------------------------------------------------------

    In the Interim High Winds Guidance, the EPA articulated its
expectations regarding the development and application of high wind
thresholds. In this guidance, the EPA encouraged air agencies to
identify an appropriate high wind threshold for each area experiencing
high wind dust events within their exceptional events submissions for
high wind dust events.\115\ The guidance recommended that these
thresholds should consider local conditions and specify a minimum wind
speed capable of causing emissions from those natural undisturbed areas
or overwhelming reasonable controls on contributing anthropogenic
sources (see section V.E.2 for additional discussion regarding
reasonable controls). This approach was consistent with the
PM10 Natural Events Policy in which the EPA recommended that
air agencies define the conditions in which BACM level controls were
overwhelmed. The area-specific high wind threshold should be
representative of conditions (i.e., sustained wind speeds \116\) that
are capable of overwhelming reasonable controls (whether RACM, BACM or
other) on anthropogenic sources and/or causing emissions from natural
undisturbed areas. The threshold was not intended to represent the
minimum wind speed at which any level of emissions could occur (e.g.,
aerodynamic entrainment), but rather when significant emissions begin
due to reasonable controls or natural undisturbed areas becoming
overwhelmed. We have stated that if an agency is unable to develop an
area-specific high wind threshold, we generally will accept a threshold
of a sustained wind of 25 mph for areas in the western U.S. provided
the agencies support this as the level at which they expect stable
surfaces (i.e., controlled anthropogenic and undisturbed natural
surfaces) to be overwhelmed.\117\ We did not indicate what form such
support could take. We also have said that if we receive specific
information based on relevant studies to choose an alternative high
wind threshold for an identified area, we will notify the affected air
agency.
---------------------------------------------------------------------------

    \115\ See Appendices A2 and A3 in the 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 for additional information
on the development of a high wind threshold.
    \116\ Section <IP_ADDRESS> in the 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 for details on the
calculation of sustained wind speed. Generally, the EPA will accept
that high winds could be the cause of a high 24-hour average
PM10 or PM2.5 concentration if there was at
least one full hour in which the hourly average wind speed was above
the area-specific high wind threshold.
    \117\ The 25 mph threshold is based on studies conducted on
natural surfaces. See additional information relevant to
establishing this threshold in Appendix A1 in the 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.
---------------------------------------------------------------------------

    Also as noted in the Interim High Winds Guidance document, the EPA
has expected air agencies to provide relevant wind data (e.g., wind
speed and direction) as part of an exceptional events submission for
high wind dust events. Wind speed data consist of analyses and
statistics showing how the observed high wind dust event wind speed
compares to the distribution of historical wind speeds and the
established high wind threshold. The EPA has recommended that air
agencies show these historical comparisons on an annual and/or seasonal
basis, depending on which is more appropriate, using a format similar
to the recommended format of the comparison to historical
concentrations showing as part of the clear causal relationship
criterion discussed in section V.E.3 of this proposal. The EPA has
encouraged air agencies to discuss wind direction in the narrative and
to present wind direction information graphically in

maps/plots in the clear causal relationship section of the high wind
dust exceptional events demonstration.
    In considering past high wind dust event demonstrations, the EPA
has found that the ``not reasonably controllable or preventable'' and
the ``clear causal relationship'' (to include the comparison to
historical concentrations showing) criteria play significant roles in
the supporting exceptional events documentation. The EPA has generally
found that for high wind dust events, air agencies can meet the ``human
activity or natural event'' criterion by satisfying the requirements
for not reasonably controllable or preventable and clear causal
relationship as well as addressing the additional components of
exceptional events demonstration packages as discussed in section V.G.
    As is the case with all demonstration packages, air agencies with
agricultural sources that potentially contribute to high wind event-
related emissions should address the question of source contribution
and associated reasonable controls on these sources within the not
reasonably controllable or preventable portion of the demonstration.
The EPA has noted in previous guidance that when considering the
anthropogenic sources that contribute to event-related emissions and
the appropriate ``reasonable controls'' on these sources, air agencies
should be aware of USDA/NRCS-approved Best Management Practices (BMPs)
(also referred to as conservation management practices) that are
designed to effectively reduce fugitive dust air emissions and prevent
soil loss in agricultural applications.\118\ We have stated that these
BMPs could be included in the collection of controls determined to
constitute reasonable controls for wind-blown dust events in areas in
which they have been implemented.
---------------------------------------------------------------------------

    \118\ Reference Guide for Cropping Systems and General Land
Management. USDA, NRCS and U.S. EPA. October 2012. Available at
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1049502.pdf.
---------------------------------------------------------------------------

b. Proposed Changes
    The EPA proposes to include in the preamble to the final rule for
this action a modified version of some of the language that first
appeared in the Interim High Winds Guidance document and to incorporate
into the rule text the revisions proposed in this section. We also
intend to revise the Interim High Winds Guidance to be consistent
following promulgation of final Exceptional Events Rule revisions.
    Definition of an Event. Consistent with the EPA's proposal to
revise the regulatory definition of an exceptional event to include
both the event and its associated resulting emissions, the EPA proposes
to define a high wind dust event as an event that includes the high-
speed wind and the dust that the wind entrains and transports to a
monitoring site. Consistent with the nullified language in the 2007
Exceptional Events Rule preamble, the PM10 Natural Events
Policy and the Interim High Winds Guidance, the EPA proposes to define
high wind dust events in the rule text as ``natural events'' in cases
where windblown dust is entirely from natural sources or where all
significant anthropogenic sources of windblown dust have been
reasonably controlled.
    High Wind Threshold. To facilitate clearer expectations regarding
the evidence needed to demonstrate which controls constitute
``reasonable controls,'' the EPA proposes to codify in rule language
the definition of ``high wind threshold'' as the minimum threshold wind
speed capable of causing particulate matter emissions from natural
undisturbed lands in the area affected by a high wind dust event. The
EPA proposes to accept a threshold of a sustained wind of 25 mph for
areas in the western U.S. provided this value is not contradicted by
evidence in the record when we review a demonstration. If the EPA
receives specific information based on relevant studies that suggest a
different high wind threshold for an identified area, the EPA will
notify the affected air agency so that the agency may consider basing
its demonstration on that threshold value. The EPA would consider such
information as part of the weight of evidence analysis for a submitted
demonstration. In lieu of using the default 25 mph high wind threshold,
air agencies would have the option to identify an area-specific high
wind threshold that is more representative of local/regional
conditions.
    The high wind threshold concept will continue to apply to the
review of demonstrations for events in a nonattainment or maintenance
area for which the dust controls in a recently approved SIP are
generally accepted as sufficient to satisfy the not reasonably
controllable criterion. For such a demonstration, the controls
specified in the SIP should be considered reasonable, while
acknowledging the possibility that the controls are not being complied
with and that uncontrolled anthropogenic sources of PM could be the
contributing to the exceedance. For events with sustained wind speeds
above the high wind threshold, it is very plausible that SIP controls
were being implemented and the high PM concentrations are due to
emissions generated from sources in the area despite implementation of
the SIP measures. Conversely, for events with sustained wind speeds
below the high wind threshold, it becomes more plausible that there may
be noncompliance with control measures or that uncontrolled
anthropogenic sources are contributing to the exceedance. Therefore,
the comparison of sustained wind speeds during an event to the high
wind threshold will help the EPA Regional offices determine what
evidence is required to be included in a demonstration regarding
reasonable controls, the possibility of non-compliance, or non-event
sources.
    Large-Scale or High-Energy High Wind Dust Events. The EPA proposes
to codify in rule language to apply a case-specific approach when
considering reasonableness of controls for remote, large-scale, high-
energy and/or sudden high wind dust events, such as ``haboobs'' in the
southwest where sustained wind speeds can exceed 40 mph and generate
walls of dust several miles wide and more than a mile high. The
proposed rule text provides that in these situations, the event will be
considered not reasonably preventable or controllable. Therefore, a
demonstration limited to such event(s) will not need to substantively
address this criteria. The EPA solicits comment on this proposed, case-
specific approach when considering reasonableness of controls for
remote, large-scale, high-energy and/or sudden high wind dust events.
    Other Types of High Wind Dust Events. Any demonstration for a non-
high-energy event would be evaluated on a case-by-case basis. In doing
so, the EPA would consider what controls are reasonable in light of an
area's attainment status and associated CAA control requirements, the
frequency, and range of non-high energy wind events known (at the time
of the particular event that is the subject of the demonstration) to
occur in the area.
    The Role of the EPA-approved SIP in Nonattainment and Maintenance
Areas. As stated in section V.E.2, the EPA proposes to establish by
rule a non-rebuttable presumption that, during a 5-year window (or,
alternatively another appropriate timeframe) following approval of an
attainment plan or maintenance plan SIP during which no subsequent new
obligation for the air agency to revise the SIP has arisen, the control
measures included in the SIP that are specific to the relevant
pollutant, sources and event type satisfy the not reasonably
controllable or

preventable criterion.\119\ Otherwise, the air agency and the EPA would
evaluate the not reasonably controllable or preventable criterion on a
case-by-case basis.
---------------------------------------------------------------------------

    \119\ A request for data exclusion must also show that the event
was not a result of noncompliance with any existing state or local
laws or rules that have not been incorporated into the SIP.
---------------------------------------------------------------------------

    We describe below one potential scenario in which deference to the
SIP for purposes of ``reasonable controls'' (versus a case-by-case
analysis) satisfies the not reasonably controllable or preventable
criterion. We also provide two other scenarios needing a case-by-case
analysis for purposes of satisfying the not reasonably controllable or
preventable criterion. We identify these scenarios below and then
discuss them in more detail in sequence.
     Nonattainment Area Scenario 1--The EPA approved the SIP
with the enforceable control measures as meeting attainment or
maintenance planning requirements within 5 years of the date of
submittal of the event AND the air agency is not under an obligation to
revise the SIP for the reason listed in Scenario 2 or any other reason.
Additionally, the sustained winds during the event are above the high
wind threshold. The SIP includes enforceable control measures that
address the event-related pollutant and all sources necessary to
fulfill the requirements of the CAA for the SIP that have or may have
contributed to event-related emissions. This indicates that in the
development and approval of the SIP, both the EPA and the state
considered what event-related controls were sufficient to meet the
attainment or maintenance plan requirements of the CAA.
     Nonattainment Area Scenario 2--The air agency is under an
obligation to revise the SIP as a result of a SIP call based on failing
to provide for attainment and maintenance of the relevant NAAQS as
evidenced by current violations.\120\
---------------------------------------------------------------------------

    \120\ Other possible reasons for an area to be in a Scenario 2
situation would be if it has been designated nonattainment for a
revised NAAQS for the relevant pollutant or is subject to a SIP call
for the relevant pollutant following the EPA's determination that
the SIP is inadequate for some other reason.
---------------------------------------------------------------------------

     Maintenance Area Scenario 3--The EPA approved the SIP more
than 5 years prior to the date of submittal of a demonstration.
Details for Nonattainment Area Scenario 1
    In this scenario, where the sustained winds during the event are
above the high wind threshold, the EPA would apply a non-rebuttable
presumption that the controls in the existing SIP represent reasonable
measures to prevent or control any event of the given type that occurs
in the 5-year window. To satisfy the not reasonably controllable or
preventable criterion, the EPA would expect the submitting air agency
to identify the emission sources that contribute to the event emissions
and exceedance and identify the associated SIP controls plus any other
enforceable control measures required by state laws or rules. The air
agency would also identify the implementation status of these controls
and provide evidence of effective implementation and enforcement.
    Example: An air agency submits a demonstration for a high wind dust
event in a PM10 nonattainment area that occurred in October
2015. The air agency has an EPA-approved attainment plan SIP for the
affected area that was approved in October 2010 and that SIP includes
enforceable controls implemented in accordance with the SIP that
address the event-related pollutant (i.e., PM) and all sources
necessary to fulfill the requirements of the CAA. The sustained winds
during the event were above the high wind threshold. In the not
reasonably controllable or preventable portion of its high wind dust
demonstration, the air agency would describe the event-related wind
characteristics and identify the natural and anthropogenic emission
sources that contributed to the event emissions and the associated SIP
and other control measures. The air agency would then describe the
implementation status of these controls and provide evidence of
effective implementation and enforcement. The air agency would conclude
the ``not reasonable controllable or preventable'' portion of the
demonstration by affirmatively stating that because the EPA had
approved the SIP within 5 years of the event and because the SIP
measures and other measures specific to the pollutant and at least some
anthropogenic emission sources that contributed to the event emissions
were implemented, the agency has satisfied the not reasonably
controllable or preventable criterion.
    In reviewing the demonstration in this scenario, the EPA would
generally concur that the air agency met the not reasonably
controllable or preventable criterion during the 5-year period provided
the SIP was implemented and the event was not attributable to
noncompliance. Thus, assuming the demonstration also satisfied the
remaining technical and procedural elements in the Exceptional Events
Rule, the EPA would concur with the air agency's request to exclude
data for purposes of regulatory actions within the scope of the final
revised Exceptional Events Rule. If, however, the air agency
experienced an exceedance or violation during the 5-year period for
reasons other than those attributable to the successful exceptional
events demonstration (e.g., industrial source noncompliance or another
type of event), the EPA may take one of these actions. In addition, the
EPA may issue a SIP call because the SIP is inadequate with regard to a
requirement of the CAA that is not tied to the occurrence of NAAQS
violations related to exceptional events. If the EPA issues a SIP call
during the 5-year window, the situation would switch to Scenario 2.
Details for Nonattainment Area Scenario 2
    In this scenario (where the SIP is being revised to respond to a
SIP call involving the PM10 or PM2.5 NAAQS), the
existing SIP controls should not be presumed to satisfy the not
reasonably controllable or preventable criterion regardless of whether
the event-related wind speeds are above or below the high wind
threshold. The EPA recommends that as the first step in preparing an
exceptional events demonstration, the air agency should assess the
case-specific effectiveness of the controls that were in place at the
time of the event and consider potential controls that are more
comprehensive and effective than those in the SIP that the agency could
have implemented before or during the event. This case-specific
assessment should 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 in light of the information in the
record of the EPA action that has created the obligation to revise the
SIP, then those controls must have been in place for the event to
satisfy the not reasonably controllable or preventable criterion.
    The submitting air agency preparing a case-specific assessment
should first identify the natural and anthropogenic emission sources
that significantly contribute to the event emissions and exceedance.
The air agency should categorize sources as those that are addressed
through SIP or other state or local laws or rules and those sources
that are not addressed by SIP measures or other measures. Where the
contributing source has SIP or other controls, the air agency would
identify the implementation status of these controls and provide
evidence of effective implementation and

enforcement. The air agency should also consider whether those SIP
controls should have been made more stringent and effective prior to
the event. For emission sources that contribute to the event emissions
but are not specifically addressed in the SIP or other laws or rules,
the air agency should identify and document why it was reasonable to
have not implemented controls.
    We invite comment on whether there should be a grace or
grandfathering period before a SIP call involving a relevant NAAQS has
the effect of ending the deference that applied prior to the SIP call,
such that for an event occurring during the grace period the SIP would
be given the deference described for the first scenario. We believe
that such a grace period should not extend beyond the due date for the
required SIP revision in response to the SIP call.
    Example: An air agency has an EPA-approved attainment plan SIP for
a PM10 nonattainment area that was approved in 1994 and
includes controls for some of the anthropogenic emission sources that
contribute fugitive dust during high wind events. The nonattainment
area did not include fugitive dust controls for gravel operations in
its 1994 SIP and has not required any controls of these operations in
the years since SIP approval. The area does not have an approved
maintenance plan, in part because it has been experiencing unresolved
exceedances since 1994. The air agency alerts the reviewing EPA
Regional office that it wishes to submit an exceptional events
demonstration for a high wind dust event that occurred in 2015 and
affected several of the area's monitoring sites. This is the second
high wind dust event associated with an exceedance in the past 3 years.
After the first event, the EPA issued a SIP call for the air agency to
revise its PM10 SIP, but the air agency has not yet
submitted a new SIP. Because the EPA issued a SIP call, the air agency
is required to show on a case-specific basis that the not reasonably
controllable or presentable criterion has been met. Applying the
concept that if a set of control measures should reasonably have been
in place for emission sources that contribute to the event emissions to
the information in the record supporting the SIP call would likely
result in a determination that those controls must have been in place
for the event to satisfy the not reasonably controllable or preventable
criterion. Because the gravel operations are not controlled and because
the high wind dust event was the second in 3 years, the air agency had
a basis for understanding the possible need for better controls. Given
the air agency's knowledge of recurring events, the air agency may not
be able to make a sufficient showing for the not reasonably
controllable or preventable criterion and the reviewing EPA Regional
office may not be able to concur with the air agency's request to
exclude data. If, however, the air agency can show that the gravel
operations did not contribute to the event-related emissions, the
reviewing EPA Regional office might be able to concur with the air
agency's request to exclude data.
Details for Maintenance Area Scenario 3
    In this scenario (where the SIP was approved more than 5 years
prior to the date of submittal of a demonstration 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 regardless of whether
the event-related wind speeds are above or below the high wind
threshold. In this case, the air agency should complete a case-specific
assessment of the reasonableness of controls to satisfy the not
reasonably controllable or preventable criterion. The assessment should
consider controls beyond those required by the existing SIP and other
state or local laws and rules. The case-specific assessment should
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. The
submitting air agency should first identify the natural and
anthropogenic emission sources that contribute to the event emissions
and exceedance. The air agency should categorize sources as those that
are addressed through SIP or other state or local laws or rules and
those sources that are not addressed by SIP measures or other measures.
Where the contributing source has SIP or other controls, the air agency
would identify the implementation status of these controls and provide
evidence of effective implementation and enforcement. The air agency
should also consider whether those SIP controls should have been made
more stringent and effective prior to the event. For emission sources
that contribute to the event emissions but are not specifically
addressed in the SIP or other laws or rules, the air agency should
identify and document why it was reasonable to have not implemented
controls.
    Example: An air agency has an EPA-approved attainment plan SIP for
a PM10 former nonattainment area that was approved in 2008
and includes controls for anthropogenic emission sources that
contribute fugitive dust during high wind events. The area has an
approved maintenance plan. Between 2008 and 2014 it has not been
experiencing exceedances related to high winds. In 2014 there is a
single high wind dust event with sustained wind speeds above the high
wind threshold that results in two exceedance days, sufficient to
constitute a 3-year NAAQS violation. The air agency submits a
demonstration covering both exceedances. In the not reasonably
controllable or preventable portion of its demonstration, the air
agency would identify all sources contributing to the event emissions,
including natural sources, sources identified and controlled in the
SIP, and any sources not controlled by the SIP. The air agency would
then identify the applicable controls, the implementation status of
these controls and evidence of enforcement. The air agency should also
consider whether those SIP controls should have been made more
stringent and effective prior to the event. Given the area's past
history of not having events and the fact that the sustained wind speed
during the event was above the high wind threshold, it is likely that
the air agency could make a sufficient showing for the not reasonably
controllable or preventable criterion. In this case, provided the air
agency satisfies the other rule criteria, the EPA Regional office would
likely concur with an air agency's request for data exclusion.
    However, in this maintenance area scenario, another possible
outcome of an event that causes an exceedance or violation is that the
EPA determines that the not reasonably controllable or preventable
criterion is not met and the event-affected data are retained for
regulatory actions within the scope of the Exceptional Events Rule.
This may lead to the EPA taking an action that places the air agency
under an obligation to revise the SIP, in which case the situation
would change into the second scenario for any later events of the same
type.
    Best Management Practices. The EPA solicits comment on whether or
not, as part of the assessment of local sources and reasonable
controls, USDA/NRCS-approved BMPs constitute sufficient reasonable
controls in any or in all high wind event-affected areas and whether
these measures should therefore be specifically and categorically
identified in preamble or rule language as constituting reasonable
controls. As discussed in the ``Current Situation'' section, the EPA
has noted in previous guidance that USDA/NRCS-approved BMPs designed to
effectively reduce

fugitive dust air emissions and prevent soil loss in agricultural
applications could be included in the collection of controls determined
to constitute reasonable controls for wind-blown dust events in areas
in which they have been implemented.\121\ Although the EPA has
addressed the sufficiency of BMPs in decisions on individual
exceptional events demonstrations when the BMPs were part of a SIP-
approved BACM determination, we have not previously addressed whether
or not BMPs individually or in some combination with each other
constitute sufficient reasonable controls nationally or in any
particular types of areas. We recognize that this question may be
difficult to answer because BMPs often describe general types of
practices (e.g., installing wind breaks) rather than specifying the
penetration, scale or intensity of use of these practices by the
landowners who adopt them. Therefore we also solicit comment on the
evidence for degree of penetration, scale and intensity that would be
appropriate in demonstrations to consider BMPs individually or in some
combination with each other to be reasonable controls.
---------------------------------------------------------------------------

    \121\ 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
Guidance to Implement Requirements for the Treatment of Air Quality
Monitoring Data Influenced by Exceptional Events. U.S. EPA. May
2013. Available at http://www2.epa.gov/sites/production/files/2015-05/documents/exceptevents_guidememo_130510.pdf.
---------------------------------------------------------------------------

G. Other Aspects of Flagging Exceptional Events-Influenced Data and
Demonstration Submittal and Review

1. Who may submit a demonstration and request for data exclusion?
a. Current Situation
    Before addressing the schedule and mechanics of flagging event-
influenced data and preparing demonstrations, the EPA believes it is
necessary to first clarify which parties can submit an exceptional
events demonstration package to the EPA. The CAA language at section
319(b)(3)(B)(i) states that ``the occurrence of an exceptional event
must be demonstrated by reliable, accurate data that is promptly
produced and provided by Federal, State, or local government
agencies.'' As noted in section V.A of this proposal, state, local and
some tribal agencies administer air quality management programs within
their jurisdiction, which includes monitoring and analyzing ambient air
quality and submitting monitoring data to the EPA, which are then
stored in the EPA's AQS database. Also, FLMs and other federal agencies
operate air quality monitoring stations on some lands they manage, and
some of these monitors meet the technical specifications and quality
assurance requirements for their data to be used in regulatory
determinations. As operators of regulatory monitors, each of these
agencies can flag their own data within AQS for consideration as an
exceptional event.
    As discussed in section V.F.1 of this proposed action, however, the
EPA generally considers a state, exclusive of tribal lands, to be a
single responsible actor, and, as the state is the entity primarily
responsible for administering air quality planning and management
activities, the state has been ultimately responsible for submitting
exceptional events demonstrations for exceedances that occur at all
regulatory monitoring sites within the boundary of the state, including
exceedances occurring at monitoring sites operated by local air quality
agencies to whom a state has delegated relevant responsibilities or at
regulatory monitoring sites operated by any other entity within the
state, such as FLMs of Class I areas, other federal agencies and/or
industrial facilities. Although the state is responsible, a local
agency, an FLM, another federal agency or another entity operating a
regulatory monitor with an event-influenced exceedance can develop a
demonstration for submittal by the state. If a state disagrees with the
local agency's, FLM's, other federal agency's or other entity's
exceptional events claim, the state can decide not to act on or forward
that submittal to the EPA. A state can request that operators of other
regulatory monitors experiencing event-influenced exceedances prepare
or assist in the preparation of demonstration analyses for ultimate
submittal by the state.
    Because some tribal air quality agencies also operate regulatory
ambient air quality monitoring sites and submit these data to the EPA's
AQS database, as appropriate, these tribal agencies may also submit
exceptional events demonstrations for exceedances that occur at their
monitoring sites.
b. Proposed Changes
    As indicated in section V.A of this proposal, because FLMs and
other federal agencies may operate regulatory monitors and submit
collected data to the EPA's AQS database and these same monitors could
be affected by emissions from exceptional events, the EPA proposes to
allow FLMs and other federal agencies to prepare and submit exceptional
events demonstrations and data exclusion requests directly to the EPA.
The EPA solicits comment on this proposed addition to the rule text,
which appears at the end of this document. Based on comments received,
the EPA may retain, modify or not include this provision in the final
promulgated rule. This provision would apply only to FLMs and other
federal agencies that manage land on which an exceptional event
originates or that operate a monitor that has been affected by an
event. The provision would allow such FLMs and other federal agencies
to provide demonstrations directly to the EPA only after a discussion
with the state in which the monitor is operated. This discussion might
instead result in an agreement that the federal agency (or another
party) will provide a draft demonstration document to the appropriate
state air agency for adoption and submission by the state air agency to
the EPA, as is currently allowed. Regardless of who ultimately submits
the demonstration, the EPA encourages collaboration between the FLMs
and other federal agencies and the appropriate state air agency during
the event identification and demonstration development process. If the
provision for direct submission to the EPA is included in the final
action, demonstrations prepared by FLMs or other federal agencies would
be required to meet all provisions in the Exceptional Events Rule,
including the requirement for a public comment period on a prepared
demonstration \122\ and the requirements related to schedules and
procedures for demonstration package submittal (see sections V.G.4,
V.G.5 and V.G.6) that apply to state agencies that operate monitors.
---------------------------------------------------------------------------

    \122\ A public comment opportunity is important prior to
submission to the EPA because under the Exceptional Events Rule the
EPA is not required to provide a public comment opportunity prior to
concurring with an air agency's request to exclude data. The EPA
generally provides a public comment opportunity before we use air
quality data, with or without such exclusions, in a final regulatory
action. States typically provide an opportunity for public comment
by posting draft demonstrations on a Web site. Federal agencies
could do the same.
---------------------------------------------------------------------------

2. Aggregation of Events for NAAQS With Periods Longer Than 24 Hours
and Demonstrations With Respect to Multiple NAAQS for the Same
Pollutant
a. Current Situation
    The EPA's AQS database houses ambient air quality monitoring and
related data. The data in AQS are maintained as individual reported
measurements, which can range from 5-minute maximum concentrations per
hour for SO2, to hourly data for ozone,

CO, NO2, SO2 and some PM measurements to 24-hour
measurements for lead and other particulate matter measurements. Air
agencies identify and the EPA concurs with exceptional event-related
data in AQS that are reported as individual measurements.
    Some NAAQS have long averaging periods, such that multiple
independent events may affect the period-average concentration of the
NAAQS pollutant. In the aggregate, a clear causal relationship may
exist between the events and an exceedance or violation, but no single
event satisfies the clear causal relationship criterion because each
event has too small of an effect on the longer-period metric to do so
by itself. CAA section 319(b) and the 2007 Exceptional Events Rule do
not clearly allow the aggregation of events for purposes of the clear
causal relationship criterion, yet aggregation seems consistent with
the intent of section 319(b). The EPA has not to date indicated that
actual aggregation of events is permitted. However, Question 30 in the
Interim Q&A document provided guidance that can be of some help in this
situation. This guidance was that 24-hour concentrations of Pb,
NO2, or SO2 can be individually compared to the
NAAQS level defined for a longer period, for purposes of meeting ``but
for'' with respect to both the 24-hour NAAQS, if applicable, and for
purposes of meeting ``but for'' with respect to the NAAQS with the
longer averaging period. This guidance focused on the intention of a
passage in the preamble to the 2007 Exceptional Events Rule addressing
the PM2.5 NAAQS in particular, and extended the approach of
the 2007 preamble to other cases of NAAQS for the same pollutant that
have different averaging periods. The practical effect of this approach
is that several events that individually have effects too small to have
a causal connection to a longer-period exceedance or violation might be
excluded one-by-one, and the net effect of the exclusions may make a
difference to compliance with the longer-period NAAQS. As guidance,
however, the Interim Q&A document does not provide full certainty that
an air agency may rely on the recommended approach.
    As noted in section IV.B of this proposal, the 2007 Exceptional
Events Rule requires that for data exclusion, among other requirements,
an air agency must demonstrate that there would have been no exceedance
or violation of the NAAQS ``but for'' the event. The ``but for''
criterion necessarily requires comparing the individual measurements in
AQS to the averaging period of the relevant NAAQS to determine whether
an exceedance or violation occurred. When the averaging period for the
NAAQS is the same as the measurement duration period, this comparison
is relatively straightforward. For example, air agencies and the EPA
can directly compare 1-hour ozone, 1-hour CO, 1-hour SO2,
and 1-hour NO2 measurements to the respective 1-hour NAAQS.
This comparison becomes more complicated, however, when there is a
difference between the pollutant measurement duration and the averaging
time of the NAAQS, which is the case when comparing a 1-hour
measurement to an 8-hour, 24-hour, 3-month or annual NAAQS (or in the
case of 1-hour ozone the previously existing NAAQS). In fact, the EPA
devoted Questions 29-31 in the Interim Q&A document to explaining how
to make these complicated comparisons.\123\ The Interim Q&A document
also explained that because these comparisons are NAAQS-specific, air
agencies should request and support the exclusion of a measured air
concentration separately for each NAAQS that applies to the pollutant
and the EPA will similarly provide separate concurrences. Under the
2007 Exceptional Events Rule provisions, this means, for example, that
an air agency with several 24-hour measurements of event-influenced
PM2.5 data measuring 75 micrograms per cubic meter ([mu]g/
m\3\) would need to separately flag the data within AQS on a NAAQS-
specific basis, and submit separate requests, analyses and
demonstration components to support exclusion of the identified event-
influenced data for the 1997 annual secondary PM2.5 NAAQS of
15 [mu]g/m\3\, the 2012 annual primary PM2.5 NAAQS of 12
[mu]g/m\3\ and the 2006 primary and secondary 24-hour PM2.5
NAAQS of 35 [mu]g/m\3\. Depending on the outcome of the ``but for''
criterion with respect to each PM2.5 NAAQS, it could be that
the data would be excluded for purposes of determinations with respect
to only some of these NAAQS.
---------------------------------------------------------------------------

    \123\ 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.
---------------------------------------------------------------------------

    This current situation can result in complicated demonstrations for
air agencies seeking data exclusion from determinations with respect to
multiple NAAQS for the same pollutant. This complexity may make it more
difficult for the public to comment, and requires time for the EPA to
review such a demonstration.
b. Proposed Changes
    The EPA is taking comment on proposed rule text allowing 24-hour
concentrations of any NAAQS pollutant to be compared to a NAAQS level
defined for a longer period as part of a weight of evidence showing for
the clear causal relationship with respect to the NAAQS with the longer
period. This approach would be more amenable to less quantified weight
of evidence demonstrations, since only one day would be examined at a
time.
    The EPA is also proposing that for NAAQS with averaging or
cumulative periods longer than 24 hours, events occurring on different
days may be aggregated for the purpose of determining whether their
collective effect has caused an exceedance or violation, without regard
to whether the events are of the same type (e.g., stratospheric ozone
intrusion followed by a wildfire). The EPA notes that such aggregation
may be very difficult if the effects of the individual events on their
individual days are not fully quantified. Proposed rule text for this
change is provided for comment.
    Finally, to simplify some demonstrations, the EPA is also taking
comment on whether a successful demonstration with respect to any NAAQS
for a given pollutant would suffice to qualify the data in question for
exclusion with respect to all NAAQS for that pollutant. The EPA
believes it is useful to invite public comment on this ``approved for
one NAAQS approved for all NAAQS for the same pollutant'' concept.
    The EPA will carefully consider the comments it receives on these
concepts and may finalize all, some or none of the three proposals
described in this section.
3. Exclusion of Entire 24-Hour Value Versus Partial Adjustment of the
24-Hour Value for Particulate Matter
a. Current Situation
    As indicated in Question 29 of the Interim Q&A document, we have
advised air agencies preparing demonstrations to support requests to
exclude PM2.5 and PM10 data obtained via monitor
instruments that provide 1-hour measurements that they should flag all
24 1-hour values within a given event-affected day, even if the event
did not last all these hours. If concurred upon, flagging all 1-hour
values will ultimately result in the same available remaining data for
regulatory analysis and calculation as would be the case had the 24-
hour PM2.5 or PM10 measurement data been
collected from filter-based (24-hour) monitoring

instruments.\124\ Another reason we have taken the position that
flagging all 24 1-hour values is appropriate is because flagging only
peak or selected hours could result in the remaining 1-hour values
still meeting the data completeness requirements because flagged and
excluded data do not count against completeness even though they cannot
be used in calculating an average concentration for a 24-hour period.
Under the rules for data interpretation, exclusion of only the event-
affected 1-hour concentrations could result in AQS calculating a
seemingly valid 24-hour concentration that is actually highly uncertain
because it is based on only a few hours and thus may be biased.\125\
---------------------------------------------------------------------------

    \124\ Filter based instruments typically record a single value
within a 24-hour period while continuous monitors typically collect
24 1-hour measurements. Because AQS can calculate a valid 24-hour
average concentration with as few as 18 hours, it may be necessary
to exclude hours not actually affected by the event to ensure the
same data exclusion outcome as if the measurement had been made with
a 24-hour filter.
    \125\ The form of the 24-hour PM2.5 NAAQS of 35
[mu]g/m\3\ is 98th percentile averaged over 3 years. The form of the
primary annual PM2.5 NAAQS of 12 [mu]g/m\3\ is an annual
mean averaged over 3 years. The form of the 24-hour PM10
NAAQS of 150 [mu]g/m\3\ is not to be exceeded more than once per
year on average over 3 years. Biased concentrations can potentially
skew the determination of the 98th percentile and/or the annual mean
for PM2.5 and the averages for PM2.5 or
PM10 calculated to determine compliance with the relevant
NAAQS.
---------------------------------------------------------------------------

b. Proposed Changes
    The EPA solicits comment on codifying its current approach in the
rule text to eliminate any regulatory uncertainty. If finalized, this
modification to the data handling procedures will be made to occur
automatically within AQS.
4. Flagging of Data
    While neither the preamble to nor the rule text contained within
the 2007 Exceptional Events Rule discuss data handling within AQS,
explaining certain AQS processes and functions will be useful to an
understanding of the data flagging situation that has developed in
implementing the requirements in 40 CFR 50.14(c)(2) and why we are
proposing changes.
a. Current Situation
    Within AQS, monitoring agencies can use two types of data
validation, or data qualifier, codes related to exceptional events: The
Request Exclusion flags (R) and the Informational Only flags (I).\126\
The EPA has advised air agencies to use the I series flags when
identifying informational data and the R series flags to identify data
points for which the agency intends to request an exceptional events
exclusion and the EPA's concurrence. As an example, air agencies may
currently use an I series flag to initially identify values they
believe were affected by an event. Once the air agency collects
additional supporting data, it may change the flag to an R series flag
and submit an initial event description. Or, the air agency may find
that additional information does not support flagging the data as an
exceptional event, and the air agency may, therefore, delete the flag
or retain the I series flag. Air agencies may also use the I series
flags simply to note activities or conditions occurring on the data
collection day that are unrelated to exceptional events or that do not
result in an exceedance or violation of a NAAQS. Air agencies have
previously indicated that they generally see little value in the use of
I series flags.\127\
---------------------------------------------------------------------------

    \126\ ``Flag'' is the common terminology for a data qualifier
code in the EPA's AQS. Unless explicitly noted, the process of
``flagging'' data refers to adding Request Exclusion (R) data
qualifier codes to selected data in AQS. R flags are the only AQS
flags that satisfy the 2007 Exceptional Events Rule requirement for
initial data flagging. The current design of the AQS software is
such that EPA can act/concur only on an R flag.
    \127\ Responses to Significant First-Round Comments on the Draft
Guidance Documents on the Implementation of the Exceptional Events
Rule, U.S. EPA, June 2012. Available in Docket No. EPA-HQ-OAR-2011-
0887.
---------------------------------------------------------------------------

    Flagging of event-influenced data has traditionally also involved
associating a one- or two-character code with a monitored value within
AQS indicating that the data have potentially been influenced by a
particular type of exceptional event (e.g., ``RT'' is the character
code used to request exclusion for data that have been influenced by
wildfires in the U.S.). The 2007 Exceptional Events rule added a
requirement to include a more detailed initial description of the
particular event associated with such a character code. This
description consists of text of variable length.
    The EPA does not review or concur on the I series flags. Rather, an
air agency must use an R flag to request data exclusion. The language
at 40 CFR 50.14(c)(2), Flagging of Data, requires that an air agency
notify the EPA of its intent to exclude one or more measured
exceedances of an applicable NAAQS as being due to an exceptional event
by placing a flag and an initial event description in the appropriate
fields in AQS for the data record(s) of concern no later than July 1 of
the calendar year following the year in which the flagged measurement
occurred. Only R flags fulfill this requirement. This ``general''
schedule date of July 1 applies unless the data are associated with the
initial area designations process for a new or revised NAAQS in which
case the specific schedule in Sec.  50.14(c)(2)(vi) applies.
    Air agencies have previously expressed concern that the timelines
for event flagging and demonstration submittal are not always
appropriate.\128\ While the EPA has historically promulgated revised
flagging and demonstration submittal schedules in the regulatory
actions for new and revised NAAQS for those data years that might be
used in the initial area designations process for those NAAQS, the EPA
does not promulgate revised schedules for other regulatory actions such
as clean data or attainment determinations. Rather, the EPA has relied
upon the ``general'' flagging and demonstration submittal schedules in
40 CFR 50.14(c)(2) and (c)(3)(i). Meeting the requirement at 40 CFR
50.14(c)(2)(iii) to submit R flags and an initial description of the
event ``not later than July 1st of the calendar year following the year
in which the flagged measurement occurred'' can be difficult in the
case of an annual standard where an air agency needs all 12 months of
data to calculate an annual average and then needs 3 years of annual
averages to identify whether or not the event-influenced data results
in a violation of a 3-year design values. An air agency may not know
that data influenced by an exceptional event caused the design value to
become a NAAQS violation until 3 years after the event occurred.
---------------------------------------------------------------------------

    \128\ Responses to Significant First-Round Comments on the Draft
Guidance Documents on the Implementation of the Exceptional Events
Rule, U.S. EPA, June 2012. Available in Docket No. EPA-HQ-OAR-2011-
0887.
---------------------------------------------------------------------------

    Some air agencies have used and applied I and R flags in AQS
inconsistently with this intended scheme, by including applying
numerous R flags in AQS with no real intention to submit an exceptional
events demonstration. Also, R flags may be set immediately before a
demonstration is submitted or even as late as when the EPA needs to
indicate in AQS our approval of a request for data exclusion. As a
result, neither the presence nor the absence of these flags provide the
EPA with an indication of anticipated exceptional events
demonstrations.
b. Proposed Changes
    As part of this action, the EPA proposes to revise the ``general''
schedule language contained within 40 CFR 50.14(c)(2) by removing the
timelines associated with initial event flagging. The EPA also proposes
to modify the associated data flagging process within AQS to correspond
with

these proposed regulatory changes. These proposed changes would include
eliminating the use of the current exceptional events data validation/
data qualifier codes: The Request Exclusion flags (R) and the
Informational Only flags (I). The one- or two-character event type
codes would be retained. The EPA solicits comment on the approach that
is discussed below in additional detail.
    The EPA is proposing to change the definition and process for
flagging exceptional event data. Flagging would in effect become the
application of the one- or two-character event type and event
description text as described below, along with a concurrent or
subsequent request for data exclusion communicated to the EPA through
other channels.
    Because the flagging of data necessarily begins with the
identification of an event, the EPA proposes to retain, with
modifications, the AQS free-form text field for an initial event
description. As is currently the practice, we request that air agencies
use the ``initial event description'' to identify a unique, real-world
event. We propose that this ``initial event description'' be expanded
to contain a unique event name; the type of the event (e.g., high wind
dust, volcanic eruption, other); a brief description of the event; and,
to the extent known, the scope of the event in terms of geography and
time (e.g., likely affected area using latitude and longitude and a
radius of influence and beginning day/time and ending day/time).\129\
AQS would also be modified to allow the air agency to associate
specific AQS sites and potentially affected monitors and specific data
points with a given event as so described. This will enable air
agencies and the EPA to ``flag'' or add qualifier codes to selected
data in a single step rather than adding this information or the
necessary codes on a per entry basis. Historically, when events have
influenced the concentrations at multiple monitors for multiple days,
the air agency has added initial event descriptions and set flags on
each monitored concentration, sometimes resulting in hundreds of
identical individual entries. ``Associating'' monitors with an event
defined in time and space will save resources.
---------------------------------------------------------------------------

    \129\ The EPA is proposing that air agencies select the ``type
of event'' from a pre-set list of event types, which would likely
consist of those event types currently identified by existing
Informational and Request Exclusion flags within AQS.
---------------------------------------------------------------------------

    Once an air agency has identified an event and created the initial
event description within AQS, the agency should begin the process of
requesting exclusion for identified data, which will consist of two
discrete operations: (1) Indicating in a separate communication to the
EPA that specific ambient air quality measurements are affected by a
defined event (see section V.G.5 related to Initial Notification of
Potential Exceptional Event), and (2) requesting that these identified
ambient air quality measurements be excluded from regulatory actions
according to the terms of the revised Exceptional Events Rule and EPA
guidance for other applications of air quality data. AQS would retain a
field to allow the EPA to concur or not concur with a given request for
exclusion for one or more of the data points associated with a
described event, once review of the air agency's request and
demonstration is completed.
    In addition to the proposed AQS modifications described above, the
EPA is proposing to remove the ``general'' flagging schedule in 40 CFR
50.14(c)(2)(iii). This regulatory language currently requires that air
agencies submit [R] flags and an initial description of the event by
July 1 of the calendar year following the year in which the flagged
measurement occurred or by the other deadlines identified with
individual NAAQS. As noted earlier in this section, an air agency may
not know that data influenced by an exceptional event caused a
violation of a NAAQS until after the initial event flagging deadline
has passed. The EPA proposes to remove and reserve the current language
at 40 CFR 50.14(c)(2)(iii). Additional changes to the regulatory
language in 40 CFR 50.14(c)(2) will be discussed in the next section.
    The EPA notes that the recent ozone NAAQS action \130\ also removed
and reserved the subsequent sections at 40 CFR 50.14(c)(2)(iv) and (v),
which addressed the submittal of exceptional events demonstrations that
could affect regulatory determinations associated with initial area
designations for the 2006 24-hour PM2.5 NAAQS and the 2010
Lead NAAQS and were made obsolete by the passage of time. The EPA will
retain these removed and reserved sections as promulgated in the ozone
NAAQS and proposes no additional changes to these sections.
---------------------------------------------------------------------------

    \130\ 80 FR 65292 (October 26, 2015).
---------------------------------------------------------------------------

5. Initial Notification of Potential Exceptional Event
a. Current Situation
    As the EPA acknowledged in the Interim Exceptional Events
Implementation Guidance and in discussions with stakeholders, the EPA
understands that the initial identification of data affected by
exceptional events and the subsequent preparation, submittal and review
of demonstration packages is a resource intensive process both for the
preparing air agency and the reviewing EPA Regional office.\131\ Delays
in processing and making decisions on submitted packages create
regulatory uncertainty and potentially increase the workload for both
the submitting air agency and the EPA. In addition, the backlog of
pending actions makes selection of the best information to support new
submittals potentially more uncertain. Further, air agencies and the
EPA often face timelines by which they must make regulatory decisions
that can be affected by the inclusion or exclusion of event-affected
data. In the Interim Exceptional Events Implementation Guidance and
through the EPA's best practices discussions identified in section
IV.E, the EPA committed to work with air agencies as they prepare
complete demonstration packages and we developed some guidelines to
increase the efficiency of the process.
---------------------------------------------------------------------------

    \131\ Interim Guidance to Implement Requirements for the
Treatment of Air Quality Monitoring Data Influenced by Exceptional
Events. Memorandum from Stephen D. Page, U.S. EPA Office of Air
Quality Planning and Standards, to Regional Air Directors, Regions
I-X. May 10, 2013. Available at http://www2.epa.gov/sites/production/files/2015-05/documents/exceptevents_guidememo_130510.pdf.
---------------------------------------------------------------------------

    One of the efficiency-increasing measures we suggested in the
Interim Exceptional Events Implementation Guidance was the Letter of
Intent. The guidance explained that the Letter of Intent was a
voluntary process by which the submitting air agency notifies the
reviewing EPA Regional office of the air agency's intent to submit a
demonstration for an identified exceptional event. The purpose of the
letter was to promote early communication between the submitting air
agency and the reviewing EPA Regional office. In the time since issuing
the Interim Exceptional Events Implementation Guidance, several air
agencies and the EPA Regional offices have successfully used this
voluntary process to discuss expectations regarding specific
exceptional events demonstrations.
b. Proposed Changes
    As part of the best practices for communications during the
exceptional events process and to aid all agencies in resource planning
and prioritization, the EPA proposes that air agencies and the EPA
engage in regular communications

to identify those data that have been potentially influenced by an
exceptional event, to determine whether the identified data affect a
regulatory determination, and to discuss whether an air agency should
develop and submit an exceptional events demonstration. In most
instances, these discussions will be between individual air agencies
and the reviewing EPA Regional office. In other cases, the EPA regional
office, or an individual air agency within the purview of the EPA
Regional office, may initiate and/or host a general discussion with all
air agencies in the region followed by individual discussions, as
needed. In still other cases, such as where large events cross state
lines and when two or more states are pursuing exclusion for the same
event(s), the EPA region or regions may initiate discussions will all
potentially affected states/agencies to assist in coordinating states
affected by regional events.
    For purposes of this proposed action, the EPA is referring to these
communications as the ``Initial Notification of Potential Exceptional
Event'' (Initial Notification) process. The EPA has changed the name of
this process from the Letter of Intent in recognition of the fact that
effective communication may have multiple formats and does not
necessarily consist of a formal, written letter to convey important
information. As with the voluntary Letter of Intent, the ultimate
purpose of the Initial Notification process is to initiate
conversations between an air agency and the EPA if not already on-
going, or engage in more detailed discussions if a process is currently
in place, regarding specific data and whether the identified data are
ripe for submittal as exceptional events. As stakeholders have
repeatedly expressed and as the EPA acknowledges, the identification of
data affected by exceptional events and the subsequent preparation,
submittal and review of demonstration packages is a resource intensive
process both for the preparing air agency and the reviewing EPA
Regional office.
    However, in considering the exceptional events process, it is
important to note that if these data do not have regulatory
significance, then engaging in the development and review of an
exceptional events demonstration is generally not an efficient use of
an air agency's or the EPA's limited resources. The Initial
Notification process will focus efforts on the relevant data and
provide the EPA with the opportunity to convey to the affected air
agency our initial thoughts regarding the identified event and analyses
that may or may not be appropriate for inclusion in a demonstration,
and, with respect to regulatory significance, which demonstrations the
EPA will consider for review. We believe that this approach will help
air agencies make the best use of their available resources.
    As noted earlier, the Initial Notification could include any form
of communication (e.g., letter, email, in-person meeting with an
attendees' list and discussion summary or phone conversation with
follow-up email) that ultimately identifies the potential need to
develop an exceptional events demonstration and communicates key
information related to the data identified for potential exclusion.
Where an air agency independently identifies event-affected data and
the need to submit an exceptional events demonstration outside of its
regular, on-going communications with the EPA Regional office, the air
agency could prepare a letter or email communicating its Initial
Notification. Generally, the EPA anticipates that air agencies would
develop and provide an Initial Notification as soon as the agency
identifies event-influenced data that potentially influence a
regulatory decision or when an agency wants the EPA's input on whether
or not to prepare a demonstration.\132\ The EPA further proposes that
each Initial Notification would include the following components:
---------------------------------------------------------------------------

    \132\ The EPA recognizes that air agencies can immediately
identify those events that result in an exceedance of a NAAQS with a
short averaging time (e.g., 1-hour, 8-hour or 24-hour standards) but
may need additional time for an annual average standard. An air
agency could also submit an annual Initial Notification if annual
submittal makes sense for resource planning or for recurring
seasonal events.
---------------------------------------------------------------------------

     Unique event name (field in AQS)--facilitates future
communication and understanding between the submitting air agency and
the reviewing EPA Regional office, particularly if an air agency has
submitted multiple exceptional events demonstration packages.
     Initial event description (field in AQS)--provides a brief
narrative of the event that could also include maps or graphs similar
to what an air agency might include in the proposed conceptual model
discussed in section V.G.6 of this proposed action; the event
description would include a qualitative description of the event and,
at a minimum, briefly describe the agency's current understanding of
interaction of emissions with the event, transport and meteorology
(e.g., wind patterns such as strength, convergence, subsidence,
recirculation) and pollutant formation in the area.
     Affected regulatory decision--provides a description of
the regulatory action or actions potentially affected by the claimed
event-influenced data and the anticipated timing of this action.
     Proposed target date for demonstration submittal--
identifies the proposed target date by which the air agency would
submit a demonstration package to the reviewing EPA Regional office.
     Most recent design value including and excluding the
event-affected data (optional)--the air agency's assessment of the most
recent design value both with and without the identified event(s) is
helpful when assessing regulatory significance. The EPA cannot
calculate this value (and therefore may not be able to determine
significance) if the air agency has flagged more data than it intends
to include in an exceptional events demonstration.
     Information specific to each monitored day--see Table 5,
which would be developed by the submitting air agency and generated
from the initial event description in AQS (see discussion in section
V.G.4).

                                        Table 5--Initial Notification Information Specific to Each Monitored Day
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Monitor AQS
      Agency/planning area            State        County      Event name      Type of        NAAQS      ID and site   Date(s) of    Monitor exceedance
                                                                 in AQS         event                       name          event         concentration
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                  ............  ............  ............  ............  ............  ............  ............  ....................
                                                                                -----------------------------------
                                  ............  ............  ............  ............  ............  ............  ............  ....................
                                                                                -----------------------------------
                                  ............  ............  ............  ............  ............  ............  ............  ....................
--------------------------------------------------------------------------------------------------------------------------------------------------------

    The EPA anticipates promptly acknowledging an air agency's Initial
Notification and then formally responding within 90 days of receipt via
letter, email or in-person meeting with an attendees' list and
discussion summary. We also anticipate having informal phone
conversations with the air agency prior to this formal response. As
previously discussed, the EPA will generally prioritize exceptional
events determinations that affect near-term regulatory decisions.\133\
Where the data are to be used in initial area designations, the EPA
proposes to rely on the promulgated documentation submission schedule
in Table 1 at Sec.  50.14(c)(2)(vi). Where the data will influence
another near-term regulatory decision, the EPA proposes to rely on the
case-by-case timelines by which the air agency should submit the
demonstration. For case-by-case demonstrations, the EPA's recommended
date for demonstration submittal would consider the nature of the
event, the anticipated timing of the regulatory decision, and would
allow time for both an air agency's preparation of the demonstration
and the EPA's review. The EPA may not be able to review and act on
demonstrations submitted after the recommended submittal date.
Additionally, the EPA will request in its response that, if the
submitting air agency has not already identified the affected data
within AQS, that it undertake this effort according to the process
described in section V.G.4. If the data identified in the Initial
Notification do not have regulatory significance (and there is no
compelling reason for excluding data), then the EPA will indicate this
in its correspondence back to the air agency and will discourage the
air agency from devoting resources to developing a demonstration
because the EPA will likely not review or act upon the submittal.
---------------------------------------------------------------------------

    \133\ ``Regulatory decisions'' include findings as to whether
the area has met the applicable NAAQS, classification
determinations, attainment demonstrations, the development of
Limited Maintenance Plans and clean data findings.
---------------------------------------------------------------------------

    If after discussing the content of a submitted Initial Notification
and/or receiving the EPA's response to the Initial Notification, the
EPA acknowledges that identified data have regulatory significance (or
some other compelling reason for excluding data), then the air agency
should proceed with the development of a technical demonstration
package that satisfies the requirements in 40 CFR 50.14 and accounts
for any case-specific advice from the EPA and additional information in
the EPA's guidance documents.\134\ Although air agencies can submit
demonstrations for events that do not affect a regulatory action, the
EPA will likely not review or act on such submittals.
---------------------------------------------------------------------------

    \134\ Interim Guidance to Implement Requirements for the
Treatment of Air Quality Monitoring Data Influenced by Exceptional
Events. Memorandum from Stephen D. Page, U.S. EPA Office of Air
Quality Planning and Standards, to Regional Air Directors, Regions
I-X. May 10, 2013. Available at http://www2.epa.gov/sites/production/files/2015-05/documents/exceptevents_guidememo_130510.pdf.
---------------------------------------------------------------------------

    For these reasons described in this section and in section V.G.4,
the EPA proposes to revise the language in 40 CFR 50.14(c)(2)(i) as
follows: ``A State shall notify EPA of its intent to request exclusion
of one or more measured exceedances of an applicable ambient air
quality standard as being due to an exceptional event by creating an
initial event description and flagging the associated data that have
been submitted to the AQS database and by engaging in the Initial
Notification of Potential Exceptional Event process.'' Specific steps
in the Initial Notification process are identified in rule text at the
end of this document. The EPA solicits comment on the proposed rule
text revision (in 40 CFR 50.14(c)(2)) to require an Initial
Notification of Potential Exceptional Event, with a provision that the
EPA can waive the Initial Notification requirement on a case-by-case
basis. Alternatively, the EPA solicits comment on making the Initial
Notification of Potential Exceptional Event a voluntary process.
    Additional proposed revisions would continue at (ii): ``The data
shall not be excluded from determinations with respect to exceedances
or violations of the national ambient air quality standards unless and
until, following the State's submittal of its demonstration pursuant to
paragraph (c)(3) of this section and EPA review, EPA notifies the State
of its concurrence by placing a concurrence flag in the appropriate
field for the data record in the AQS database.''
    As noted in section V.G.4, the EPA is proposing to remove the
``general'' flagging schedule in 40 CFR 50.14(c)(2)(iii). The EPA seeks
comments on these proposed changes to the language at 40 CFR
50.14(c)(2), which more clearly identify the process for flagging data
in AQS and requesting exclusion of one or more measured exceedances of
an applicable ambient air quality standard.
    The EPA notes that the recent final rule to revise the ozone NAAQS
also removed and reserved the subsequent sections at 40 CFR
50.14(c)(2)(iv) and (v), which addressed the submittal of exceptional
events demonstrations that could affect regulatory determinations
associated with initial area designations for the 2006 24-hour
PM2.5 NAAQS and the 2010 Lead NAAQS and were made obsolete
by the passage of time. The EPA will retain these removed and reserved
sections as promulgated in the ozone NAAQS and proposes no additional
changes to these sections.
6. Submission of Demonstrations
a. Current Situation
    With the recent ozone NAAQS, the EPA proposed and promulgated
changes to the current exceptional events regulatory language at 40 CFR
50.14(c)(2) and (3) to include finalizing exceptional events flagging
and demonstration submittal schedules related to implementing the
revised ozone standards and future revised NAAQS and removing obsolete
regulatory language for expired exceptional events deadlines. Sections
V.G.4 and V.G.5 discuss the current situation and additional proposed
changes to 40 CFR 50.14(c)(2). This section discusses the current
situation and proposed revisions to 40 CFR 50.14(c)(3).
    As part of the recent final rule to revise the ozone NAAQS, the
regulatory language at 40 CFR 50.14(c)(3)(i) now refers to a revised
exceptional events flagging and demonstration submittal schedule for
data that could be used in initial area designation decisions following
promulgation of any future revised NAAQS. However, the language at 40
CFR 50.14(c)(3)(i) still requires air agencies to ``. . . submit a
demonstration to justify data exclusion to EPA not later than the
lesser of, 3 years following the end of the calendar quarter in which
the flagged concentration was recorded or, 12 months prior to the date
that a regulatory decision must be made by EPA.''
    As identified in section V.G.4 of this proposal, air agencies have
previously expressed concern that the timelines for event flagging and
demonstration submittal are not always appropriate because an air
agency may not know that data influenced by an exceptional event caused
the design value exceedance until 3 years after the event
occurred.\135\ The EPA acknowledges that this scenario can occur.
---------------------------------------------------------------------------

    \135\ Responses to Significant First-Round Comments on the Draft
Guidance Documents on the Implementation of the Exceptional Events
Rule, U.S. EPA, June 2012. Available in Docket No. EPA-HQ-OAR-2011-
0887.

---------------------------------------------------------------------------

    In addition to establishing a general schedule for demonstration
submittal, the regulatory language at 40 CFR 50.14(c)(3)(i) requires
that ``A State must submit the public comments it received along with
its demonstration to EPA.'' Although this language is included in 40
CFR 50.14(c)(3)(i), it refers to the regulatory language at 40 CFR
50.14(c)(3)(v), which requires the air agency to document, and submit
with its demonstration, evidence that it followed the public comment
process. Regarding this requirement to ``document that the public
comment process was followed,'' neither the Exceptional Events Rule
language in 40 CFR 50.14 nor the preamble to the promulgated 2007
Exceptional Events Rule specifies a minimum timeframe for public
comment. Many air agencies have been posting draft demonstrations for
public review on their Web sites. The EPA has reviewed several of these