[Federal Register Volume 80, Number 224 (Friday, November 20, 2015)]
[Proposed Rules]
[Pages 72839-72897]
[FR Doc No: 2015-29350]

Vol. 80

Friday,

No. 224

November 20, 2015

Part III

Environmental Protection Agency

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40 CFR Part 50

Treatment of Data Influenced by Exceptional Events; Proposed Rule

Federal Register / Vol. 80 , No. 224 / Friday, November 20, 2015 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2013-0572, EPA-HQ-OAR-2015-0229; FRL-9935-73-OAR]
RIN 2060-AS02

Treatment of Data Influenced by Exceptional Events

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule; notice of availability of related draft
guidance; notice of hearing.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing
revisions to certain sections within the regulations that govern the
exclusion of event-affected air quality data from regulatory decisions.
The EPA is also providing a notice of availability of a draft version
of the non-binding guidance document titled Draft Guidance on the
Preparation of Exceptional Events Demonstrations for Wildfire Events
that May Influence Ozone Concentrations.

DATES: Comments. Written comments on this proposal and draft guidance
must be received by January 19, 2016.
    Public hearing: The EPA will hold a public hearing on this proposal
on Tuesday, December 8, 2015, in Phoenix, Arizona. Please refer to
SUPPLEMENTARY INFORMATION for additional information on the comment
period and public hearing.

ADDRESSES: Submit your comments on the EPA's proposed revisions to 40
CFR part 50, identified by Docket ID No. EPA-HQ-OAR-2013-0572, at
http://www.regulations.gov. Follow the online instructions for
submitting comments. Once submitted, comments cannot be edited or
removed from Regulations.gov. The EPA may publish any comment received
to its public docket. Do not submit electronically any information you
consider to be Confidential Business Information (CBI) or other
information whose disclosure is restricted by statute. Multimedia
submissions (audio, video, etc.) must be accompanied by a written
comment. The written comment is considered the official comment and
should include discussion of all points you wish to make. The EPA will
generally not consider comments or comment contents located outside of
the primary submission (i.e., on the Web, Cloud or other file sharing
system). For additional submission methods, the full EPA public comment
policy, information about CBI or multimedia submissions and general
guidance on making effective comments, please visit http://www2.epa.gov/dockets/commenting-epa-dockets.
    Submit your comments on the EPA's Draft Guidance on the Preparation
of Exceptional Events Demonstrations for Wildfire Events that May
Influence Ozone Concentrations, identified by Docket ID No. EPA-HQ-OAR-
2015-0229, at http://www.regulations.gov. Follow the online
instructions for submitting comments. Once submitted, comments cannot
be edited or removed from Regulations.gov. The EPA may publish any
comment received to its public docket. Do not submit electronically any
information you consider to be CBI or other information whose
disclosure is restricted by statute. Multimedia submissions (audio,
video, etc.) must be accompanied by a written comment. The written
comment is considered the official comment and should include
discussion of all points you wish to make. The EPA will generally not
consider comments or comment contents located outside of the primary
submission (i.e., on the web, cloud or other file sharing system). For
additional submission methods, the full EPA public comment policy,
information about CBI or multimedia submissions and general guidance on
making effective comments, please visit http://www2.epa.gov/dockets/commenting-epa-dockets.
    Public hearing: A public hearing will be held on Tuesday, December
8, 2015, in room 3175 in the Arizona Department of Environmental
Quality main office building located at 1110 W. Washington Street,
Phoenix, Arizona 85007. The public hearing will convene at 10 a.m. and
continue until the earlier of 6 p.m. or 1 hour after the last
registered speaker has spoken. We have scheduled a lunch break from
12:30 p.m. until 2 p.m. People interested in presenting oral testimony
should contact Ms. Pamela Long, Air Quality Planning Division, Office
of Air Quality Planning and Standards (C504-01), U.S. Environmental
Protection Agency, Research Triangle Park, NC 27711, telephone (919)
541-0641, fax number<PHONE_NUMBER>, email address<EMAIL_ADDRESS>at
least 2 days in advance of the public hearing (see DATES). People
interested in attending the public hearing should also call Ms. Long to
verify the time, date and location of the hearing.

FOR FURTHER INFORMATION CONTACT: For additional information regarding
this proposed rule, please contact: Beth W. Palma, U.S. EPA, Office of
Air Quality Planning and Standards, Air Quality Policy Division, Mail
Code C539-04, Research Triangle Park, NC 27711, telephone (919) 541-
5432, email at<EMAIL_ADDRESS>For additional information
regarding the Draft Guidance on the Preparation of Exceptional Events
Demonstrations for Wildfire Events that May Influence Ozone
Concentrations, please contact Melinda Beaver, U.S. EPA, Office of Air
Quality Planning and Standards, Air Quality Policy Division, Mail Code
C539-04, Research Triangle Park, NC 27711, telephone<PHONE_NUMBER>,
email at<EMAIL_ADDRESS>For information on the public hearing
or to register to speak at the hearing, contact Ms. Pamela Long, U.S.
Environmental Protection Agency, Office of Air Quality Planning and
Standards, Air Quality Planning Division, Mail Code C504-01, Research
Triangle Park, NC 27711, telephone<PHONE_NUMBER>, fax number (919)
541-5509, email at<EMAIL_ADDRESS>(preferred method for registering).

SUPPLEMENTARY INFORMATION:

I. General Information

A. Does this action apply to me?

    Entities potentially directly affected by this proposal and the
draft guidance document include all state air agencies and any local
air quality agency to whom a state has delegated relevant
responsibilities for air quality management, including air quality
monitoring and data analysis. Tribal air agencies operating ambient air
quality monitors that produce regulatory data may also be directly
affected. Entities potentially affected indirectly by this proposal and
the draft guidance document include federal land managers (FLMs) of
Class I areas, other federal agencies and other entities that operate
ambient air quality monitors and submit collected data to the EPA's Air
Quality System (AQS) database.

B. What should I consider as I prepare my comments for the EPA?

    1. Docket. The EPA has established one docket for the proposed
revisions to the 2007 Exceptional Events Rule and another docket for
the draft guidance document. All documents in these dockets are listed
on the http://www.regulations.gov Web site in the respective docket.
The rulemaking docket is Docket ID No. EPA-HQ-OAR-2013-0572. The
separate docket established for the Draft Guidance on the Preparation
of Exceptional Events Demonstrations for Wildfire Events that May
Influence Ozone Concentrations is Docket No. EPA-HQ-OAR-2015-0229. The
EPA will not respond to comments relating to the guidance document as

part of this rulemaking, but will consider these comments in the
development of the final guidance document. If comments on the draft
guidance document are submitted to the rulemaking docket, the EPA will
respond only to the portion of such comments that are relevant to the
rulemaking. The EPA also relies on the documents in Docket ID No. EPA-
HQ-OAR-2011-0887, the docket established for the July 2012 notice of
availability for the Draft Exceptional Events Implementation Guidance,
and incorporates this docket into the record for this action. However,
no new comments may be directed to Docket ID No. EPA-HQ-OAR-2011-0887
and the EPA will not respond to comments that have already been
submitted to this docket unless they are resubmitted to Docket ID No.
EPA-HQ-OAR-2013-0572. Although listed in the indices to the rulemaking
docket and the guidance docket associated with this action (i.e.,
Docket ID No. EPA-HQ-OAR-2013-0572 and Docket No. EPA-HQ-OAR-2015-
0229), some information is not publicly available, (e.g., CBI or other
information whose disclosure is restricted by statute). Certain other
material, such as copyrighted material, will not be placed on the
Internet but may be viewed, with prior arrangement, at the EPA Docket
Center. Publicly available docket materials are available either
electronically in www.regulations.gov or in hard copy at the Air and
Radiation Docket and Information Center, EPA/DC, EPA William Jefferson
Clinton West Building, Room 3334, 1301 Constitution Avenue NW.,
Washington, DC. The Public Reading Room is open from 8:30 a.m. to 4:30
p.m., Monday through Friday, excluding legal holidays. The telephone
number for the Public Reading Room is<PHONE_NUMBER> and the telephone
number for the Air and Radiation Docket and Information Center is (202)
566-1742. For additional information about the EPA's public docket,
visit the EPA Docket Center homepage at: http://www.epa.gov/epahome/dockets.htm.
    2. Submitting CBI. Do not submit this information to the EPA
through http://www.regulations.gov or email. Clearly mark the part or
all of the information that you claim to be CBI. For CBI information in
a disk or CD-ROM that you mail to the EPA, mark the outside of the disk
or CD-ROM as CBI and then identify electronically within the disk or
CD-ROM the specific information that is claimed as CBI. In addition to
one complete version of the comment that includes information claimed
as CBI, a copy of the comment that does not contain the information
claimed as CBI must be submitted for inclusion in the public docket.
Information so marked will not be disclosed except in accordance with
procedures set forth in 40 CFR part 2.
    3. Tips for Preparing Your Comments. When submitting comments,
remember to:
     Identify the rulemaking and/or draft guidance document by
docket number and other identifying information (subject heading,
Federal Register date, page number and guidance document title, if
applicable).
     Follow directions--The agency may ask you to respond to
specific questions or organize comments by referencing a Code of
Federal Regulations (CFR) part or section number in the guidance.
     Explain why you agree or disagree; suggest alternatives
and substitute language for your requested changes.
     Describe any assumptions and provide any technical
information and/or data that you used.
     If you estimate potential costs or burdens, explain how
you arrived at your estimate in sufficient detail to allow for it to be
reproduced.
     Provide specific examples to illustrate your concerns, and
suggest alternatives.
     Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
     Make sure to submit your comments by the identified
comment period deadline.

C. Where can I get a copy of these documents and other related
information?

    In addition to being available in the docket, an electronic copy of
this notice and the draft guidance will be posted at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events.

D. What should I know about the public hearing?

    The EPA intends to hold a public hearing on Tuesday, December 8,
2015, in room 3175 in the Arizona Department of Environmental Quality
main office building located at 1110 W. Washington Street, Phoenix,
Arizona 85007. If you would like to attend or speak at the public
hearing, please contact Ms. Pamela Long, U.S. Environmental Protection
Agency, Office of Air Quality Planning and Standards, Air Quality
Planning Division, Mail Code C504-01, Research Triangle Park, NC 27711,
telephone<PHONE_NUMBER>, fax number<PHONE_NUMBER>, email at
<EMAIL_ADDRESS>(preferred method for registering) at least 2 days in
advance of the public hearing (see DATES). Interested parties may
submit oral and/or written comments. Interested parties do not need to
attend the public hearing to submit written comments. Additional
details concerning any public hearing for this proposed rule will be
posted on the EPA's Web site for this rulemaking at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events.
    The public hearing will provide interested parties the opportunity
to present data, views or arguments concerning the proposed revisions
to the 2007 Exceptional Events Rule. The EPA will make every effort to
accommodate all speakers who arrive and register. Individuals planning
to attend the hearing will be required to sign in, and may be required
to show valid picture identification to the security staff to gain
access to the meeting room. In addition, no weapons will be allowed in
the facility. Any weapons brought to the site will be stored in a
locker at the facility. No large signs will be allowed in the building,
and cameras may only be used outside of the building. The EPA may ask
clarifying questions during the oral presentations but will not respond
to the presentations at that time. Written statements and supporting
information submitted during the comment period will be considered with
the same weight as oral comments and supporting information presented
at the public hearing. Commenters must submit written comments on the
proposed rule and/or draft guidance by January 19, 2016. Commenters
should notify Ms. Long if they will need specific equipment, or if
there are other special needs related to providing comments at the
hearing. The EPA will provide equipment for commenters to show overhead
slides or make computerized slide presentations if we receive special
requests in advance. Oral testimony will be limited to 5 minutes for
each commenter. The EPA encourages commenters to provide the EPA with a
copy of their oral testimony electronically (via email or CD) or in
hard copy form. The hearing schedule, including the list of speakers,
will be posted on the EPA's Web site at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events. Verbatim
transcripts of the hearing and written statements will be included in
the docket for the rulemaking. The EPA will make every effort to follow
the schedule as closely as possible on the day of the hearing; however,
please plan for the hearing to

run either ahead of schedule or behind schedule.

E. How is this document organized?

    The information presented in this document is organized as follows:

I. General Information
    A. Does this action apply to me?
    B. What should I consider as I prepare my comments for the EPA?
    C. Where can I get a copy of these documents and other related
information?
    D. What should I know about the public hearing?
    E. How is this document organized?
II. Glossary of Terms and Acronyms
III. Executive Summary
IV. Background for Proposal
    A. Purpose of and Statutory Authority for This Regulatory Action
    B. The 2007 Exceptional Events Rule
    C. Early Experience in Implementing the 2007 Exceptional Events
Rule
    D. The EPA's Interim Exceptional Events Implementation Guidance
    E. More Recent Implementation Experience Including EPA-
Recommended Best Practices for the Development of Exceptional Events
Demonstrations
V. Proposed Rule Revisions
    A. To whom and to what pollutants does the Exceptional Events
Rule apply?
    1. Current Situation
    2. Proposed Changes
    B. What is an exceptional event?
    1. Current Situation
    2. Proposed Changes
    C. What types of ambient concentration data and data uses may be
affected by the Exceptional Events Rule?
    1. Current Situation
    2. Proposed Changes
    D. What is a natural event?
    1. Current Situation
    2. Proposed Changes
    E. Technical Criteria for the Exclusion of Data Affected by
Events
    1. Human Activity Unlikely To Recur at a Particular Location or
a Natural Event
    2. Not Reasonably Controllable or Preventable
    3. Clear Causal Relationship Supported by a Comparison to
Historical Concentration Data
    F. Treatment of Certain Events Under the Exceptional Events Rule
    1. Exceedances Due to Transported Pollution
    2. Wildland Fires
    3. Stratospheric Ozone Intrusions
    4. High Wind Dust Events
    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?
    2. Aggregation of Events for NAAQS With Periods Longer Than 24
Hours and Demonstrations With Respect to Multiple NAAQS for the Same
Pollutant
    3. Exclusion of Entire 24-Hour Value Versus Partial Adjustment
of the 24-Hour Value for Particulate Matter
    4. Flagging of Data
    5. Initial Notification of Potential Exceptional Event
    6. Submission of Demonstrations
    7. Timing of the EPA's Review of Submitted Demonstrations
    8. Dispute Resolution Mechanisms
VI. Mitigation
    A. Current Situation
    B. Proposed Changes
    1. Defining Historically Documented or Known Seasonal Events
    2. Mitigation Plan Components
    3. Options for Implementing Mitigation Plans
VII. Draft Guidance on the Preparation of Exceptional Events
Demonstrations for Wildfire Events That May Influence Ozone
Concentrations
    A. What is this draft guidance about and why is it needed?
    B. What scenarios are addressed in the draft guidance?
VIII. Environmental Justice Considerations
IX. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From
Environmental Health & Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
X. Statutory Authority

II. Glossary of Terms and Acronyms

    The following are abbreviations of terms used in the preamble.

AQCR Air Quality Control Region
AQS Air Quality System
Be Beryllium
BACM Best available control measures
BLM Bureau of Land Management
BMP Best management practice(s)
BSMP Basic smoke management practices
CAA Clean Air Act
CASTNET Clean Air Status and Trends Network
CBI Confidential business information
CFR Code of Federal Regulations
CO Carbon monoxide
EPA Environmental Protection Agency
FLM Federal land manager responsible for management of a federally
owned area that has been designated a Class I area as codified in 40
CFR part 81, subpart D
FR Federal Register
IPV Isentropic potential vorticity
Lidar A remote sensing technology that measures distance by
illuminating a target with a laser and analyzing the reflected light
[mu]g/m\3\ Micrograms per cubic meter
mph Miles per hour
NAAQS National ambient air quality standard or standards
NAM North American Mesoscale Forecast System
NASA National Aeronautics and Space Administration
NEPA National Environmental Policy Act
NO2 Nitrogen dioxide
NOAA National Oceanic and Atmospheric Administration
NOV Notice of violation
NOx Nitrogen oxides
NPS National Park Service
NSR New source review
NRCS Natural Resources Conservation Service
NRDC Natural Resources Defense Council
NWCG National Wildfire Coordinating Group
NWS National Weather Service
OAQPS Office of Air Quality Planning and Standards, U.S. EPA
OMB Office of Management and Budget
Pb Lead
PM Particulate matter
PM10 Particulate matter with a nominal mean aerodynamic
diameter less than or equal to 10 micrometers
PM2.5 Particulate matter with a nominal mean aerodynamic
diameter less than or equal to 2.5 micrometers
ppb Parts per billion
PSD Prevention of significant deterioration
PT Potential temperature
RACM Reasonably available control measures
RAQMS Real-time Air Quality Modeling System
RUC Rapid Update Cycle
SIP State implementation plan
SMP Smoke management program
SO2 Sulfur dioxide
TAR Tribal Authority Rule
TIP Tribal implementation plan
UMRA Unfunded Mandates Reform Act
USDA U.S. Department of Agriculture
USFS U.S. Forest Service
VOC Volatile organic compound or compounds

III. Executive Summary

    This section summarizes the purpose of this regulatory action, the
major provisions of this action, and the development of associated
guidance.

Purpose of This Regulatory Action

    Recognizing that it may not be appropriate for the EPA to use
certain monitoring data collected by the ambient air quality monitoring
network and maintained in the air quality data system (AQS) in the
EPA's regulatory determinations, in 2005 Congress provided the
statutory authority for the exclusion of data in specific situations by
adding section 319(b) to the Clean Air Act (CAA). The EPA promulgated
the 2007 Exceptional Events Rule (March 22, 2007, 72 FR 13560) to

implement this 2005 amendment of the CAA. The purpose of this action is
to propose revisions to the 2007 Exceptional Events Rule to address
certain substantive issues raised by state, local and tribal co-
regulators and other stakeholders since promulgation of the rule and to
increase the administrative efficiency of the Exceptional Events Rule
criteria and process. The EPA intends to promulgate these rule
revisions in advance of the date by which states, and any tribes that
wish to do so, are required to submit their initial designation
recommendations for the revised 2015 ozone NAAQS (expected in October
of 2016). In addition, the EPA intends to address a 2008 D.C. Circuit
Court decision in which the court found that certain preamble language
was ``legally null'' because there was no associated implementing rule
language.
    Interpreting and implementing the 2007 Exceptional Events Rule has
been challenging in certain respects both for the air agencies
developing exceptional events demonstrations and for the EPA Regional
offices reviewing and acting on these demonstrations. Since 2007, air
agencies have submitted exceptional event demonstrations for a variety
of pollutant and event combinations ranging from volcanic activity
influencing sulfur dioxide (SO2) and particulate matter (PM)
concentrations to stratospheric ozone intrusions. Air agencies
preparing demonstrations have expressed specific concerns and
identified challenges associated with preparing analyses to satisfy the
``but for'' rule criterion, determining what controls constitute
reasonable controls particularly for natural sources and for interstate
and international transport and identifying how much documentation to
include in a demonstration.
    As a result of both our experiences and feedback related to
implementing the 2007 Exceptional Events Rule received from state,
local and tribal co-regulators and other stakeholders via letters and
numerous conference calls and meetings, the EPA developed and released
Interim Exceptional Events Implementation Guidance in May of 2013. This
guidance has addressed some of the concerns and challenges raised by
interested parties, has helped reduce the burden of preparing
demonstrations and has reduced the time needed for review. However, the
EPA acknowledged that additional changes could only be accomplished
through a notice-and-comment rulemaking. Therefore, when the EPA
released the Interim Exceptional Events Implementation Guidance in May
of 2013, we simultaneously announced our intent to pursue revisions to
the Exceptional Events Rule. These changes are reflected in this
proposed action.
    Concurrent with preparing this proposed action, the EPA held
conference calls with some air agencies to discuss more recent
implementation experiences and to better understand currently employed
exceptional events implementation processes and practices. As a result
of these discussions, the EPA developed a list of best practices for
communication and collaboration between the EPA and air agencies.
Agencies using these approaches have developed a common understanding
of expectations, terminology and interpretation of the EPA's
regulations and policy, which, in turn, helps focus efforts, optimize
resources and save time during the demonstration development and review
process.
    Based on our experiences and the input we have received from our
collaborations with interested parties (including state, local and
tribal air agencies) following the promulgation of the 2007 Exceptional
Events Rule and since the development of the Interim Exceptional Events
Implementation Guidance and based on the previous legal challenge, we
have determined those aspects of the 2007 Exceptional Events Rule that
most need to be addressed in this proposed action.

Summary of Major Provisions

    For the first time, the EPA proposes to interpret CAA section
319(b) as applying to only a specific set of regulatory actions (e.g.,
designations) because we believe that the criteria and process steps
specified in the CAA were not clearly intended by Congress to apply to
all types of regulatory actions and in some cases certain of the
criteria and steps are not appropriate. We address this concept in this
document in general terms, but we also intend to develop a separate
guidance document to provide guidance on when data can be excluded and
when they cannot for other specific types of regulatory actions.
    The EPA proposes to return to the core statutory elements and
implicit concepts of CAA section 319(b): The event affected air quality
in such a way that there exists a clear causal relationship between the
specific event and the monitored exceedance or violation, the event was
not reasonably controllable or preventable and the event was caused by
human activity that is unlikely to recur at a particular location or
was a natural event. Within each of these elements, we are proposing
clarifications regarding the desired analyses to include in exceptional
events demonstrations and we discuss the applicability of these
clarifications to certain event types or categories. As part of this
return to the core statutory elements, we are proposing to remove from
the Exceptional Events Rule a paragraph containing what is commonly
referred to as the ``but for'' criterion.
    The EPA is proposing to incorporate the statutory ``affects air
quality'' criterion and the regulatory ``historical fluctuations''
criterion within the ``clear causal relationship'' element. We believe
that if an air agency demonstrates that an event has a clear causal
relationship to an exceedance or violation of a NAAQS, then the event
has certainly affected air quality and that a submitting air agency
does not need to address ``affects air quality'' as a distinct
component. As we indicated in the Interim Exceptional Events
Implementation Guidance (see section IV.D), we believe that a
comparison of the claimed event-influenced concentration(s) to
concentrations at the same monitoring site at other times is extremely
useful evidence in an exceptional events demonstration, particularly as
part of showing a clear causal relationship, and we propose to continue
requiring this type of comparison. This proposed action details the
minimum set of statistical analyses that the EPA expects to see in
demonstrations.
    With respect to the ``not reasonably controllable or preventable''
criterion, many states have requested that the EPA automatically
consider an event to be reasonably controlled if the EPA has approved a
state implementation plan (SIP) that contains controls for
anthropogenic sources that contribute to the event that are also
specific to the pollutant of concern in the exceptional events
demonstration. In response, the EPA proposes that enforceable control
measures implemented in accordance with an attainment or maintenance
SIP, approved by the EPA within 5 years of the date of a demonstration
submittal, that address the event-related pollutant and all sources
necessary to fulfill the requirements of the CAA for the SIP to be
reasonable controls with respect to all anthropogenic sources that have
or may have contributed to event-related emissions. Also for this
criterion, the EPA clarifies that air agencies generally have no
obligation to specifically address controls if the event was natural or
if it was due to emissions originating outside their jurisdictional
(i.e., state or tribal) border(s).
    With respect to the ``human activity that is unlikely to recur at a
particular

location or was a natural event'' criterion, we propose a general
approach to determining whether the recurrence frequency of an event is
``unlikely to recur at a particular location'' and an approach
applicable to prescribed fire on wildland only. We also clarify that
natural events can recur, sometimes frequently, and reiterate our
belief that we generally consider human activity to have played little
or no direct role in causing emissions if anthropogenic emission
sources that contribute to the event emissions are reasonably
controlled at the time of the event.
    Air agencies must address all of the core statutory elements and
implicit concepts of CAA section 319(b) within an exceptional events
demonstration. In this proposed action, the EPA clarifies the content
and organization of exceptional events submittals to include the core
statutory elements, but we also propose that states be required to
include a conceptual model, or narrative, describing the event(s)
causing the exceedance or violation and a discussion of how emissions
from the event(s) led to the exceedance at the affected monitor(s) and
documentation that the air agency conducted a public comment process.
We are proposing to require an initial notification by the state to the
EPA of a potential exceptional event as a preliminary step before
submitting a demonstration, to ensure the submitting air agency and the
reviewing EPA Regional office share a common understanding regarding
the potential event and are in communication regarding the timeline for
the demonstration to be submitted and to be reviewed by the Regional
office.
    Because affected air agencies have provided feedback regarding the
difficulty associated with meeting the current regulatory timelines
associated with data flagging, initial event descriptions and
demonstration submittals, the EPA proposes to remove the specific
deadlines that apply in situations other than initial area designations
following promulgation of a new or revised NAAQS. Also associated with
demonstration timing, the EPA proposes to officially terminate review
of demonstrations that, due to the passage of time, will have no
further regulatory significance specifically for the five types of
regulatory actions identified in section V.C. of this preamble.
    Since promulgation of the 2007 Exceptional Events Rule,
stakeholders have raised numerous questions about fire-related
components that were discussed, but not fully defined or clarified in
the preamble to the 2007 Exceptional Events Rule. This proposed action
addresses fire-related definitions, provides more clarity regarding
expectations for smoke management programs (SMPs) and basic smoke
management practices (BSMP), and proposes limited scenarios under which
FLMs and other federal agencies may prepare and submit exceptional
events demonstrations and data exclusion requests directly to the EPA.

Associated Guidance Documents

    In addition to proposing revisions to the 2007 Exceptional Events
Rule, this proposed action simultaneously provides a notice of
availability of a draft non-binding guidance document titled, Draft
Guidance on the Preparation of Exceptional Events Demonstrations for
Wildfire Events that May Influence Ozone Concentrations, which applies
the proposed Exceptional Events Rule revisions to wildfire/ozone
events. This guidance document is intended to further address specific
stakeholder questions regarding the Exceptional Events Rule and further
increase the efficiency of rule implementation. In addition, the EPA is
currently developing a guidance document titled, Draft Guidance for
Excluding Some Ambient Pollutant Concentration Data from Certain
Calculations and Analyses for Purposes Other than Retrospective
Determinations of Attainment of the NAAQS, which will apply to the
exclusion of certain data for certain applications using a process and
criteria outside of the Exceptional Events Rule. The EPA intends to
make this guidance document available shortly after proposing revisions
to the Exceptional Events Rule. The EPA expects to finalize these
guidance documents concurrently with promulgating revisions to the
Exceptional Events Rule.

IV. Background for Proposal

A. Purpose of and Statutory Authority for This Regulatory Action

    Part of the EPA's mission is to preserve and improve, when needed,
the quality of our nation's ambient air to protect human health and the
environment. As part of accomplishing this, the EPA develops the
national ambient air quality standards (NAAQS) for criteria pollutants
and oversees the states' programs to improve air quality in areas where
the current air quality is unacceptable and to prevent deterioration in
areas where the air quality meets or exceeds the NAAQS. The EPA then
evaluates the status of the ambient air as compared to these NAAQS by
using data collected in the national ambient air quality monitoring
network established under the authority of section 319(a) of the CAA.
    Recognizing that it may not be appropriate for the EPA to use
certain monitoring data collected by the ambient air quality monitoring
network and maintained in AQS in our regulatory determinations, in 2005
Congress provided the statutory authority for the exclusion of data in
specific situations by adding section 319(b) to the CAA in 2005. The
EPA promulgated the 2007 Exceptional Events Rule (March 22, 2007, 72 FR
13560) to implement this 2005 amendment of the CAA. The purpose of this
action is to propose revisions to the 2007 Exceptional Events Rule to
address certain issues raised by stakeholders since promulgation of the
rule and to increase the administrative efficiency of the Exceptional
Events Rule criteria and process.
    In addition to proposing revisions to the 2007 Exceptional Events
Rule, we are simultaneously providing a notice of availability of a
draft non-binding guidance document titled, Draft Guidance on the
Preparation of Exceptional Events Demonstrations for Wildfire Events
that May Influence Ozone Concentrations, which applies the proposed
Exceptional Events Rule revisions to wildfire/ozone events. We seek
comment on whether the concepts in this guidance document should be
finalized as rule text. We are also currently developing a second
guidance document titled, Draft Guidance for Excluding Some Ambient
Pollutant Concentration Data from Certain Calculations and Analyses for
Purposes Other than Retrospective Determinations of Attainment of the
NAAQS, which will apply to the exclusion of certain data for certain
applications using a process and criteria outside of the Exceptional
Events Rule. Both of these draft guidance documents are intended to
further address specific stakeholder concerns regarding the Exceptional
Events Rule and further increase the efficiency of rule implementation.

B. The 2007 Exceptional Events Rule

    The 2007 Exceptional Events Rule created a regulatory process
codified at 40 CFR parts 50 and 51 (sections 50.1, 50.14 and 51.930).
These regulatory sections contain definitions, procedural requirements,
requirements for air agency demonstrations, criteria for the EPA's
approval of the exclusion of event-affected air quality data from the
data set used for regulatory decisions,

and requirements for air agencies \1\ to take appropriate and
reasonable actions to protect public health from exceedances or
violations of the NAAQS.\2\ The 2007 Exceptional Events Rule superseded
the EPA's previous natural events guidance and those sections of an
earlier guidance document that addressed the treatment of data affected
by exceptional events.\3\
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    \1\ References to ``air agencies'' are meant to include state,
local and tribal air agencies responsible for implementing the
Exceptional Events Rule. The regulatory text in the 2007 Exceptional
Events Rule often uses ``State'' to apply to ``air agencies.'' In
the context of flagging data and preparing and submitting
demonstrations, the role of and options available to air agencies
would also apply to federal land managers of Class I areas and other
federal agencies managing federal land.
    \2\ Per the definition at 40 CFR 50.1(l), an exceedance with
respect to a national ambient air quality standard means one
occurrence of a measured or modeled concentration that exceeds the
specified concentration level of such standard for the averaging
period specified by the standard. Violations of a standard are
standard-specific and are determined by applying the standard-
specific procedures for air quality data handling identified in the
appendices to 40 CFR part 50. For example, per the requirements in
40 CFR part 50, appendix N, an exceedance of the 2006 24-hour
PM2.5 NAAQS of 35 [mu]g/m\3\ occurs when the 24-hour
concentration is above 35 [mu]g/m\3\ on a single day. A violation of
the 2006 24-hour PM2.5 NAAQS occurs when the 3-year
average of the annual 98th percentile 24-hour concentrations is
above 35 [mu]g/m\3\.
    \3\ Previous guidance and policy documents that either implied
or stated the need for special treatment of data affected by an
exceptional event include:
    i) Guideline for the Interpretation of Air Quality Standards,
U.S. EPA, OAQPS No. 1.2-008, Revised February 1977. Available at
http://nepis.epa.gov/Exe/ZyNET.exe/2000YFDB.TXT?ZyActionD=ZyDocument&Client=EPA&Index=1976+Thru+1980&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5Czyfiles%5CIndex%20Data%5C76thru80%5CTxt%5C00000007%5C2000YFDB.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL.
    ii) Guideline on the Identification and Use of Air Quality Data
Affected by Exceptional Events (the Exceptional Events Policy), U.S.
EPA, OAQPS, EPA-450/4-86-007, July 1986.
    iii) 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. Available at http://www.epa.gov/ttn/caaa/t1/memoranda/nepol.pdf.
    iv) Interim Air Quality Policy on Wildland and Prescribed Fires.
U.S. EPA. April 23, 1998. Available at http://www.epa.gov/ttn/oarpg/t1/memoranda/firefnl.pdf.
    v) Guideline on Data Handling Conventions for the PM NAAQS. U.S.
EPA, OAQPS, EPA-454/R-98-017, December 1998.
---------------------------------------------------------------------------

    In general, the exceptional events regulatory process consists of
the following steps. First, an air agency identifies a potential event-
related exceedance or violation. After noting these data in AQS, the
air agency prepares a draft demonstration package to support the
exclusion of the identified event-related data and provides an
opportunity for public comment. The air agency submits the draft
demonstration and any received public comments to its EPA Regional
office, which then reviews the submittal and concurs, nonconcurs or
defers a decision related to the air agency's request to exclude data
that have been affected by exceptional events. If the EPA agrees with
the air agency's request, the data are excluded. If the EPA does not
agree with the air agency claim, or if the EPA decides to defer a
decision on the submittal, the data are used in regulatory
determinations.
    The 2007 Exceptional Events Rule was challenged in 2008. In NRDC v.
EPA, 559 F.3d 561 (D.C. Cir. 2009), the Natural Resources Defense
Council (NRDC) brought a petition for review challenging the EPA's
definition of a natural event and seeking to vacate several statements
in the preamble to the final 2007 Exceptional Events Rule concerning
the types of events that could qualify as being eligible for exclusion
under the rule provisions. In particular, NRDC objected to treating
``events in which human activities play `little' causal role'' as
natural events. Regarding the definition of a natural event, the D.C.
Circuit Court determined that NRDC did not identify its objection
during the rulemaking process and, therefore, did not have standing
under CAA section 307 to challenge the definition.
    NRDC also challenged the preamble language addressing high wind
events. In its decision, the D.C. Circuit stated,

    In one section of the preamble, EPA refers to its ``final rule
concerning high wind events,'' which ``states that ambient
particulate concentrations due to dust being raised by unusually
high winds will be treated as due to uncontrollable natural events''
when certain conditions apply. . . . There is no such final rule.
The final rule does not mention high wind events or anything about
``ambient particulate matter concentrations.'' EPA calls this a
drafting error. In light of the error, the high wind events section
of the preamble is a legal nullity.

    The EPA believes it is clear that the ``high wind events section of
the preamble'' to which the court referred is the entire section
titled, ``B. High Wind Events'' beginning at 72 FR 13576. Accordingly,
since 2007, the EPA has not relied solely on this section of the
preamble when implementing the 2007 Exceptional Events Rule. The EPA
maintains that certain of the preamble passages determined to be
``legally null'' are in fact appropriate interpretations of the
Exceptional Events Rule and are consistent with the CAA. For clarity
and regulatory certainty, the EPA is proposing in rule text form some
of the interpretative positions originally stated in the High Wind
Events section of the preamble to the 2007 Exceptional Events Rule.
    Within each topical area of this notice, the EPA has provided more
detailed background information on specific aspects of the 2007
Exceptional Events Rule and its implementation to allow readers to
consider the proposed changes in the context of the current situation.

C. Early Experience in Implementing the 2007 Exceptional Events Rule

    Interpreting and implementing the 2007 Exceptional Events Rule has
been a challenging process both for the air agencies developing
exceptional events demonstrations and for the EPA Regional offices
reviewing and acting on these demonstrations. Shortly after the EPA
promulgated the rule in 2007, air agencies asked the EPA to clarify key
rule provisions and expectations for these demonstrations. Air agencies
also asked for demonstration templates and/or examples of acceptable
demonstrations for various event and pollutant combinations. Although
the EPA provided some of this information via the exceptional events
Web site at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events, air agencies noted that, in their view,
the information provided was insufficient and sought additional
guidance to facilitate consistency among the EPA Regional offices in
interpreting and implementing the 2007 Exceptional Events Rule. In the
years since rule promulgation, air agencies continued to express
concern, through various mechanisms including formal letters, informal
emails, interaction at various meetings and Congressional testimony,
about the consistent application of the 2007 Exceptional Events Rule
and the resources expended to prepare exceptional events
demonstrations.
    The EPA has also faced challenges in reviewing submitted
demonstrations. Because exceptional events are fact-specific and thus
unique and varied, providing templates or general guidance was, and
still is, challenging. The EPA also acknowledges that the final rule
and preamble language for the 2007 Exceptional Events Rule provided
room for interpretation, making it difficult for air agencies and the
EPA to determine

how much evidence or technical analysis for demonstrations is needed.
We do, however, think that providing additional recommendations on
appropriate documentation would be helpful. Throughout this proposal,
for example in section V.E, Technical Criteria for the Exclusion of
Data Affected by Events, and in section V.F, Treatment of Certain
Events Under the Exceptional Events Rule, we provide recommendations
for language and analyses to include in demonstration packages.
Additional detail regarding specific recommendations is available in
the EPA's guidance documents and on the EPA's exceptional events Web
site, which the EPA will update to incorporate the finalized rule
changes concurrently with or shortly after promulgating the final rule.
The EPA will also continue to maintain and update the exceptional
events submissions table on its Web site with examples of approved
submissions. These examples may help air agencies develop demonstration
packages; however, they may not contain the minimum level of data or
case-specific analyses necessary for all exceptional events
demonstrations of the same event type. In addition, commenters on this
notice may wish to provide suggestions on the appropriate documentation
for specific types of exceptional events demonstrations.

D. The EPA's Interim Exceptional Events Implementation Guidance

    As a result of stakeholder-identified concerns and the EPA's own
experience related to implementing the 2007 Exceptional Events Rule, in
2010 the EPA began developing additional implementation guidance. In
May of 2011, the EPA released the Draft Exceptional Events
Implementation Guidance: The Draft Guidance to Implement Requirements
for the Treatment of Air Quality Monitoring Data Influenced by
Exceptional Events, the Draft Exceptional Events Rule Frequently Asked
Questions document and the Draft 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. The EPA
provided these draft guidance documents to interested air agencies,
FLMs, other federal agencies and other parties upon request, for
preliminary review to solicit comment and help ensure that the EPA's
final guidance provided an efficient and effective process to make
determinations regarding air quality data affected by exceptional
events. The EPA also placed additional examples of approved
demonstrations on the EPA's Web site.
    The EPA incorporated the commenters' feedback, as appropriate, into
revised draft guidance documents, which were made available for broad
public review in a July 6, 2012, Federal Register Notice of
Availability (77 FR 39959) and in the associated docket (Docket ID No.
EPA-HQ-OAR-2011-0887).\4\ This docket includes a summary of the comment
and response process from the 2011 preliminary review of the draft
guidance documents. In addition to identifying specific comments on the
draft guidance documents, this summary clearly identifies that
implementation challenges originated shortly after the EPA promulgated
the 2007 Exceptional Events Rule.\5\ In May 2013, after a round of
review and comment by the general public, the EPA finalized the Interim
Exceptional Events Implementation Guidance and made these documents
publicly available on the exceptional events Web site at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events.\6\
---------------------------------------------------------------------------

    \4\ The EPA established Docket ID No. EPA-HQ-OAR-2011-0887 for
the July 2012 notice of availability for the Draft Exceptional
Events Implementation Guidance and has incorporated this docket into
the record for this action.
    \5\ 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.
    \6\ The Interim Exceptional Events Implementation Guidance
includes: The Interim Guidance to Implement Requirements for the
Treatment of Air Quality Monitoring Data Influenced by Exceptional
Events, the Interim Exceptional Events Rule Frequently Asked
Questions (the Interim Q&A document), and 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 (the Interim High Winds Guidance document).
---------------------------------------------------------------------------

    With the release of the Interim Exceptional Events Implementation
Guidance, the EPA simultaneously acknowledged the need to consider
additional changes that could only be accomplished through a notice-
and-comment rulemaking to revise the 2007 Exceptional Events Rule. To
inform the development of proposed rule revisions, the EPA hosted
exceptional events listening sessions in August and November of 2013
for interested air agencies, FLMs, other federal agencies, regional
planning organizations, non-governmental organizations and other
members of the public. The EPA has considered feedback from these
listening sessions and the previous public comments on the Interim
Exceptional Events Implementation Guidance in the development of these
proposed revisions to the 2007 Exceptional Events Rule.

E. More Recent Implementation Experience Including EPA-Recommended Best
Practices for the Development of Exceptional Events Demonstrations

    Because of the passage of time since the 2013 exceptional events
listening sessions, the EPA's Office of Air Quality Planning and
Standards (OAQPS) held conference calls with some air agencies and the
EPA Regional offices between September 2014 and March 2015 to ask
whether any new implementation concerns had arisen and to better
understand currently employed exceptional events implementation
processes and practices.
    As a result of these discussions, the EPA developed a list of best
practices for communication and collaboration between the EPA and air
agencies. These best practices include having discussions before,
during and after the development and submission of exceptional events
demonstration packages. Specifically, these best practices recommend
that the EPA Regional offices and their air agencies discuss, on a
mutually agreed upon frequency, those demonstrations that the agencies
have developed and submitted for the EPA's action. These regular
discussions should focus on whether the demonstrations have regulatory
significance (e.g., significance for any of the five types of
regulatory actions identified in section V.C.) and, if not, whether the
EPA can provide general technical or policy feedback that the air
agency can include in future demonstrations. Prior to an air agency's
development of future demonstrations, the air agency and the EPA should
identify the relevant days and monitors of focus, the regulatory
significance of these monitor days, the analyses of particular interest
for a specific event and pollutant combination and the anticipated
timeframe for demonstration submission and response. Discussions should
continue while the air agency is developing the demonstration and after
the agency submits the demonstration and while the EPA is reviewing the
demonstration, to ensure both the air agency and the EPA are aware of
status, direction and progress. Finally, after the EPA has acted on the
demonstration, the reviewing EPA Regional office and the air agency
should discuss elements of the process that should continue and those
that should be improved, should understand the information in the
demonstration that was useful versus the information that was
extraneous and should discuss the possibility of

developing a demonstration template(s) for future events of the same
type(s).
    Agencies using this communications approach have developed a common
understanding of expectations, terminology and interpretation of the
EPA's regulations and policy, which, in turn, helps focus efforts,
optimize resources and save time during the demonstration development
and review process. A summary of this ``best practices'' approach to
implementation is available at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events.

V. Proposed Rule Revisions

A. To whom and to what pollutants does the Exceptional Events Rule
apply?

1. Current Situation
    Under the CAA, states are primarily responsible for the
administration of air quality management programs within their borders,
which includes monitoring and analyzing ambient air quality, submitting
monitoring data to the EPA, which are then stored in the EPA's AQS
database, and identifying measurements that may warrant special
treatment under the Exceptional Events Rule. The 2007 Exceptional
Events Rule applies to all state air agencies and to local air quality
agencies to whom a state has delegated relevant responsibilities for
air quality management, including air quality monitoring and data
analysis.
    Additionally, the 2007 Exceptional Events Rule applies to some
tribal air quality agencies who have been granted treatment as a state
for section 319 of the CAA. Section 301(d) of the CAA authorizes the
EPA to recognize tribal authority, allowing eligible, federally-
recognized tribal governments to implement provisions of the CAA.
Pursuant to section 301(d)(2), the EPA promulgated regulations, known
as the Tribal Authority Rule (TAR), on February 12, 1999 (63 FR 7254,
codified at 40 CFR part 49). That rule specifies those provisions of
the CAA for which it is appropriate to treat tribes in a similar manner
as states. Under the TAR, tribes may choose to develop and implement
their own CAA programs, but are not required to do so. The TAR also
establishes procedures and criteria by which tribes may request from
the EPA a determination of eligibility to implement the provisions of
the CAA. In cases where a tribal air quality agency is eligible to
implement CAA section 319 and has installed and operates an air quality
monitoring network that produces regulatory data that is affected by
emissions from exceptional events, the criteria and procedures
identified in these proposed rule revisions may be used to exclude data
for purposes of regulatory decisions. Some tribes may implement only
portions of the relevant air quality monitoring program and may choose
not to address all of the procedures and requirements associated with
excluding data that have been influenced by exceptional events (e.g., a
particular tribe may operate a monitoring network for purposes of
gathering and identifying data appropriate for informational or
educational purposes, but may choose not to implement relevant programs
for the purpose of mitigating the effects of exceptional events). Where
a tribal air quality agency is not eligible to implement CAA section
319 but operates an air quality monitoring network that produces
regulatory data that is affected by emissions from exceptional events,
the tribal air quality agency should consult with the EPA Regional
office prior to addressing the procedures and requirements associated
with excluding data that have been influenced by exceptional events.\7\
In all cases, the EPA will continue to work with tribes in implementing
any promulgated rule revisions.
---------------------------------------------------------------------------

    \7\ As of the signature date of this action, only one tribe is
eligible to implement all portions of CAA section 319 under the TAR.
Several other tribes, however, operate air quality monitoring
networks that produce regulatory data that could be affected by
emissions from exceptional events.
---------------------------------------------------------------------------

    While air agencies are responsible for administering air quality
management programs within their borders, FLMs of Class I areas, other
federal agencies and/or other entities (e.g., industrial facilities
pursuant to permit conditions) may also operate ambient air quality
monitors that meet all requirements of 40 CFR parts 50 and 58.\8\ The
FLMs, other federal agencies and other entities operating these
regulatory monitors may submit collected data to the EPA's AQS
database.\9\ These concentration measurements can be affected by
exceptional events. The AQS software allows only the entity operating a
monitor (and the EPA data system manager) to apply exceptional events
flags to data from that monitor. Although FLMs and other entities can
apply exceptional events flags to data from monitors they operate, the
Exceptional Events Rule at 40 CFR 50.14(b)(1) states that the EPA shall
exclude data from use in determinations of exceedances and NAAQS
violations where a state demonstrates to the EPA's satisfaction that an
exceptional event caused a specific air pollution concentration in
excess of one or more NAAQS. The language, ``where a State
demonstrates'' has resulted in an interpretation that only states can
initiate the exceptional events process and submit demonstrations. Some
stakeholders have asked the EPA to identify the process that the state
air agency should follow if the state air agency does not have AQS
access rights to place exceptional events flags on event-affected data
from monitoring stations located within the state but not operated by
the state. The EPA addressed this issue generally in Question 23 of the
Interim Q&A document by indicating that air agencies should consult
with their EPA Regional office early in the development of an
exceptional event demonstration package if they believe that monitors
on federally-owned and managed land (e.g., national parks within the
state) have been affected by an event. In these instances, the EPA has
assisted in facilitating cross-agency coordination regarding the
flagging of data, where needed.
---------------------------------------------------------------------------

    \8\ The Ambient Air Quality Surveillance provisions in 40 CFR
part 58 include, among other elements, the requirements for
monitoring data certification and data submittal and archive in AQS.
40 CFR 58.3 provides that these data reporting requirements
specifically apply to state air pollution control agencies and any
local air pollution control agency to which the state has delegated
authority to operate a portion of the state's monitoring network.
    \9\ For a description of one network of monitoring sites
operated by federal agencies, see the 2014 CASTNET (Clean Air Status
and Trends Network) Annual Network Plan, available at http://epa.gov/castnet/javaweb/ozone/CASTNET_Plan_2014_Final.pdf, which
applies to National Park Service (NPS) and Bureau of Land Management
(BLM) site managers operating CASTNET monitors.
---------------------------------------------------------------------------

    The 2007 Exceptional Events Rule applies to all criteria pollutant
NAAQS.\10\ This is appropriate given the language in CAA section
319(b)(3)(B)(iv), which applies to exceedances or violations of ``the
national ambient air quality standards.'' The EPA regulations for the
interpretation of ambient data with respect to the NAAQS that were in
place prior to the 2007 Exceptional Events Rule and that have not been
revised do not contain provisions allowing for the special handling of
air quality data affected by exceptional events or do so without
explicit reference to the Exceptional Events Rule as governing such
exclusion. One NAAQS without a specific provision for handling event-

affected data is 40 CFR part 50, appendix K for PM with a nominal mean
aerodynamic diameter less than or equal to 10 micrometers
(PM10). Nevertheless, the EPA has enabled in AQS the
capability to flag all criteria pollutant data, including the option
for the EPA's concurrence, as the EPA maintains that the monitored
concentrations of all NAAQS pollutants have the potential to be
elevated by one or more event types and the Exceptional Events Rule
should govern the process of data exclusion for certain types of
regulatory actions (see section V.C).
---------------------------------------------------------------------------

    \10\ There are NAAQS for carbon monoxide (CO), lead (Pb),
nitrogen dioxide (NO2), ozone, particle pollution and
sulfur dioxide (SO2). This applicability includes the
primary and secondary NAAQS. At present, most of the secondary NAAQS
are identical to the primary NAAQS for the same pollutant, so there
is no distinction in how the Exceptional Events Rule applies. To
date, the EPA has not encountered an exceptional event situation
with respect to a non-identical secondary NAAQS.
---------------------------------------------------------------------------

2. Proposed Changes
    As noted above, because FLMs and other federal agencies may operate
regulatory monitors and submit collected data to the EPA's AQS database
and emissions from exceptional events could affect these same monitors,
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 believes that the CAA language at
section 319(b)(3)(B)(i), which 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'' provides authority for FLMs to initiate and
submit such demonstration packages and data exclusion requests.
Further, the EPA believes this is appropriate because, in many cases,
the lands managed and/or owned by federal entities are not entirely
within the jurisdictional boundary of a single state or local
government. Also, as we discuss in more detail in section V.F.2,
federal entities may either initiate prescribed fires or fight
wildfires on lands managed and/or owned by federal entities. The EPA
could determine both of types of fires to be exceptional events. The
EPA expects that allowing FLMs and other federal agencies to submit
exclusion requests directly will expedite the exceptional events
demonstration development and submittal process. 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 either operate a monitor that has been affected by an
event or that manage land on which an exceptional event originates. 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. Alternatively, this discussion
might result in an agreement that the federal agency flag the data in
AQS at the air agency's request and then provide a draft demonstration
document to the appropriate state air agency for adoption and
submission by the 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
\11\ 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.
---------------------------------------------------------------------------

    \11\ 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.
---------------------------------------------------------------------------

B. What is an exceptional event?

1. Current Situation
    The existing definition of an exceptional event at 40 CFR 50.1(j)
repeats the CAA definition, which provides that an exceptional event is
one that affects air quality, is not reasonably controllable or
preventable, is caused by human activity that is unlikely to recur at a
particular location or is a natural event, and is determined by the
Administrator in accordance with 40 CFR 50.14 to be an exceptional
event. Also, CAA section 319(b)(3)(B)(ii) requires that a clear causal
relationship must exist between the measured exceedances of a NAAQS and
the exceptional event to demonstrate that the exceptional event caused
a specific air pollution concentration at a particular air quality
monitoring location. In addition to these defining elements, the 2007
Exceptional Events Rule at 40 CFR 50.14(c)(3)(iv) requires that the
demonstration provide evidence that ``the event is associated with a
measured concentration in excess of normal historical fluctuations,
including background'' and evidence that ``there would have been no
exceedance or violation but for the event.''
    Both the statutory and regulatory definitions of an exceptional
event include the provision that the event affected air quality. Many
types of events affect air quality by causing emissions or increasing
otherwise occurring emissions. Stratospheric ozone intrusions, one type
of event, differ from most other event types in that they transport
ozone already formed in the stratosphere to a surface monitor. High
temperatures, air stagnations and meteorological inversions can
increase the level of air pollution formed from a given amount of
emissions. However, both the statutory and regulatory definitions of an
exceptional event specifically exclude stagnation of air masses,
meteorological inversions and meteorological events involving high
temperatures or lack of precipitation, as well as air pollution
relating to source noncompliance.
    While the CAA definition of an exceptional event excludes ``a
meteorological event involving high temperatures or lack of
precipitation,'' high temperatures and drought conditions can
contribute to exceedances and violations caused by other exceptional
events such as high wind dust events. If an air agency submits evidence
showing that a severe drought that resulted in arid conditions (e.g.,
lower than typical soil moisture content, decreased vegetation) was
combined with an event, such as a high wind event, that falls within
the CAA definition of an exceptional event and has affected air quality
data, these data could be considered eligible for exclusion under the
provisions of the Exceptional Events Rule. Under this scenario, the EPA
would consider the high wind event as the critical exceptional event.
The high wind event would need to meet the provisions of the
Exceptional Events Rule, including assessing whether the event is a
natural event or an event due to human activity unlikely to recur at a
particular location. As another example, if a wildfire exacerbated by
drought conditions causes ozone exceedances, then the EPA can consider
the ozone exceedances for exclusion under the Exceptional Events Rule
because wildfires, unlike lack of precipitation itself, are not
excluded from the CAA definition of an exceptional event. However, high
temperatures alone that result in elevated ozone concentrations would
not be eligible for exclusion under the

Exceptional Events Rule. Elevated temperatures and inversions can
affect ambient air quality apart from any interactions with emissions,
but such conditions alone are not exceptional events by the very clear
provisions of the CAA. The EPA believes that Congress intended air
agencies to compensate for the effects of high temperature and
inversions on concentrations formed from anthropogenic emissions
through the development of SIPs.
    To summarize, the 2007 Exceptional Events Rule specifies six
elements that air agencies must address when requesting that the EPA
exclude event-related concentrations from regulatory determinations:
     The event affected air quality.
     The event was not reasonably controllable or preventable.
     The event was a human activity that is unlikely to recur
at a particular location, or was a natural event.
     There exists a clear causal relationship between the
specific event and the monitored exceedance.
     The event is associated with a measured concentration in
excess of normal historical fluctuations including background.
     There would have been no exceedance or violation but for
the event.

Section 50.14(b)(3) clearly makes the first three of these elements
preconditions for the EPA to approve an air agency's request to exclude
data. However, the last three of these elements are listed only in
Sec.  50.14(c)(iv), which provides that the state ``shall provide
evidence'' that they are true. Since promulgation of the 2007
Exceptional Events Rule, the EPA has treated all six elements as
conditions that air agencies must address in a demonstration prior to
the EPA's concurring with an air agency's request to exclude data. In
the Interim Exceptional Events Implementation Guidance, the EPA stated
that for the fifth of these elements (e.g., the ``historical
fluctuations'' element), there is no bright line that defines when a
concentration is ``in excess of historical fluctuations.'' With respect
to the sixth element, referred to as the ``but for'' criterion,
although the EPA has, in some cases, expected demonstrations to contain
a quantitative estimate of the concentration increment caused by the
event, more frequently the EPA has considered the ``but for'' criterion
to be satisfied by a more qualitative showing that the measured
concentration was much greater than the non-exceedance concentration
that would have normally been expected on the day in question.
    In addition to considering whether or not an event is
``exceptional'' under the Exceptional Events Rule, an air agency and
the EPA must also decide whether an ``event'' has occurred. An event,
or anomaly, is a deviation from normal or expected conditions that
contributes to air pollution. In some cases, air agencies or other
observers can clearly see this ``deviation,'' for example unusually
high wind speeds transporting dust, fires generating PM or ozone
precursors or volcanoes venting plumes of SO2, PM and PM
precursors.\12\ In other cases, such as with stratospheric ozone
intrusions, the physical effects of the event may not be visible and
the occurrence of an event can only be inferred from seeing the effect
on monitored air quality of emissions associated with the event. As
described in section V.E.3, comparing the ambient pollutant
concentrations in question to the historical distribution of
concentrations of the same pollutant can help an air agency determine
whether a deviation from normal concentrations occurred. However, such
comparisons must consider that multiple factors often contribute to
high pollutant concentrations. Some events, such as stratospheric ozone
intrusions and high wind dust events, may last only a few hours at any
one location. Still other events, such as volcanic activity, may occur
and affect pollutant concentrations for a sustained period of time
(e.g., multiple days). Some events may create pollutant-increasing
conditions that persist after the original event process has ceased,
for example high winds or volcanic eruptions that leave deposits of
dust on roadways.
---------------------------------------------------------------------------

    \12\ The EPA considers on-going emissions from volcanic activity
to be ``events'' even if they occur every day over a long period.
The EPA considers this approach to be consistent with Congressional
intent, but that extending the same treatment to air pollution due
to every day biological processes or lightning would not be
consistent with that intent.
---------------------------------------------------------------------------

2. Proposed Changes
    The EPA is proposing the following generally applicable changes to
the 2007 Exceptional Events Rule with respect to clarifying what
constitutes an exceptional event:
     Revising the definition of exceptional event by including
the concept of considering the combined effects of an event and the
resulting emissions.
     Removing the ``but for'' element.
     Moving the ``clear causal relationship'' element into the
list of criteria that explicitly must be met for data to be excluded.
     Subsuming the ``affects air quality'' element into the
``clear causal relationship'' element.
     Removing the term ``historical fluctuations'' and
replacing it with text referring to a comparison to historical
concentrations, identifying the types of analyses that are necessary in
a demonstration to address the comparison of the event-affected
concentration to historical concentrations and clarifying that an air
agency does not need to prove a specific ``in excess of'' fact.
    Making these changes would result in returning to the following
three core statutory elements of CAA section 319(b) that air agencies
must meet when requesting that the EPA exclude event-related
concentrations from regulatory determinations:
     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.
     The event was a human activity that is unlikely to recur
at a particular location or was a natural event.
    The implicit intent of CAA section 319(b) is that when the above
conditions are met, the data should be excluded from regulatory
decisions so as not to drive SIPs to include unreasonable or additional
measures to address the effects of certain events.
a. Definition of an Event
    While an event may have a physical component that is purely natural
in origin, for example high wind speeds, human activity either prior to
or simultaneous with the event may influence air quality during the
event. In implementing the 2007 Exceptional Events Rule, the EPA's
approach in determining whether an exceptional event that affected a
monitored concentration was natural or due to human activity (an
important distinction, as discussed in section V.D) has been to
consider both whether the initiating physical event was natural or the
result of human activity and whether human activity had any role in
strengthening the emissions generation process. In contrast, some
parties have argued that only the naturalness of the initiating
physical event should be considered. To clarify that an event is not a
``natural event'' merely because natural processes initiated the
emissions generation process, the EPA proposes to revise the regulatory
definition of exceptional event to say that both the naturally
occurring physical event and its associated resulting emissions are to

be considered when applying the definitions and criteria for exclusion
provided in the Exceptional Events Rule. For example, an exceptional
event might consist of a high wind and the subsequently entrained PM
that is transported to a monitoring site or a wildfire that generates
ozone or ozone precursors, which are transported to a monitoring site.
The EPA would not consider the physical event (e.g., in the previous
example, the high wind or the wildfire) to be an exceptional event
unless the resulting emissions (e.g., the PM or ozone) reached and
elevated the concentration at a monitoring location or locations.
b. ``But For'' Element
    The EPA proposes to rely more directly upon the statutory
requirement at CAA section 319(b)(3)(B)(ii) by removing the regulatory
requirement at 40 CFR 50.14(c)(3)(iv)(D) that ``there would have been
no exceedance or violation but for the event'' (i.e., the ``but for''
criterion). In promulgating the 2007 Exceptional Events Rule, the EPA
derived the ``but for'' criterion from the language at section
319(b)(3)(B)(ii), which requires ``a clear causal relationship. . .
between the measured exceedances . . . and the exceptional event to
demonstrate that the exceptional event caused a specific air pollution
concentration at a particular air quality monitoring location.'' \13\
The EPA combined this language with the requirement that there be
``criteria and procedures for the Governor of a State to petition the
Administrator to exclude . . . data that is directly due to the
exceptional events.'' \14\ Under the EPA's interpretation of CAA
section 319(b) at the time, these words suggested that a ``but for''
causation standard for exceptional events was appropriate.
---------------------------------------------------------------------------

    \13\ The EPA believes that the terminology ``specific air
pollution concentration'' refers to the identified exceedance or
violation rather than a specific increment in the measured
concentration, which implies quantitative source attribution and a
supporting quantitative analysis.
    \14\ CAA section 319(b)(3)(B)(iv) (emphasis added).
---------------------------------------------------------------------------

    Air agencies have expressed concern that the EPA has, in many
cases, historically interpreted the ``but for'' criterion as implying
the need for a strict quantitative analysis to show a single value, or
at least an explicitly bounded plausible range,\15\ of the estimated
air quality impact from the event. While a single event can in some
cases clearly be shown to be a ``but for'' cause of a NAAQS exceedance
or violation in the sense that without the event, the exceedance or
violation would not have occurred, it is more often the case that the
impact of emissions from events and other sources cannot be separately
quantified and distinguished, and the ``but for'' role of a single
source or event is difficult to determine with certainty. Even when the
effects of events are quantifiable with a sufficient degree of
confidence, air agencies have reported expending significant resources
to quantify them. The EPA was aware of these concerns in 2007 as a
result of public comment on the proposed rule and attempted to
alleviate them by stating in the preamble to the 2007 Exceptional
Events Rule that an air agency's ``but for'' analysis does not
necessarily need to be precise and that the EPA would use a holistic
``weight of evidence'' approach in analyzing submitted demonstration
packages.\16\ Without clear examples of what the EPA would accept as
satisfying a weight of evidence approach, some air agencies began using
burdensome approaches to provide quantitative ``but for'' analyses in
their exceptional events demonstrations. The reviewing EPA Regional
offices use similarly resource-intensive approaches to validate these
quantitative analyses as they review demonstrations. In some cases, the
detailed quantitative approaches have not produced results any better
than what could have been achieved with less burdensome measures.
Therefore, the EPA is proposing to remove the ``but for'' regulatory
language and focus on the ``clear causal relationship'' statutory
criterion applied to the specific case, using a weight of evidence
approach.\17\
---------------------------------------------------------------------------

    \15\ The EPA stated in the preamble to the 2007 Exceptional
Events Rule that a ``weight of evidence demonstration can present a
range of possible concentrations, which is not as technically
demanding as justifying a specific adjustment to a measured value.''
72 FR 13570 (March 22, 2007).
    \16\ 72 FR 13570 (March 22, 2007).
    \17\ The term ``weight of evidence'' means that the EPA will
consider all relevant evidence and qualitatively ``weigh'' this
evidence based on its relevance to the Exceptional Events Rule
criterion being addressed, the degree of certainty, its
persuasiveness, and other considerations appropriate to the
individual pollutant and the nature and type of event.
---------------------------------------------------------------------------

    In so doing, we propose that in their submittals, air agencies
demonstrate by the weight of evidence in the record that the event
caused the specific air pollution concentration at issue.\18\ Depending
on the event characteristics and the case-by-case nature of the
evaluation, an air agency may or may not need to provide quantitative
analyses or estimates to support the weight of evidence approach. The
EPA will discuss with an air agency the appropriate approach for a
given event demonstration during conversations preceding the submittal
of a demonstration. For example, when a concentration during an event
is higher than any concentration previously observed in the same area
and time of year, the air agency will generally not need to quantify
the event impact to reach the conclusion that the event ``caused'' the
concentration at issue. However, in cases where the concentrations on
non-event days during the same season come close to or exceed the
applicable NAAQS, thus providing evidence that non-event pollution
sources may produce exceedances of the NAAQS, the EPA would expect an
air agency's clear causal relationship showing to include a
quantitative estimate (or range of estimates) of the specific event's
impact on air pollution concentrations, even if uncertain, as a part of
a weight of evidence showing alongside other qualitative evidence.
Section V.E.3 of this proposal clarifies the EPA's expectations
regarding analyses associated with the ``clear causal'' criterion.
---------------------------------------------------------------------------

    \18\ This approach is consistent with language in the preamble
to the 2007 Exceptional Events Rule that states, ``The final rule
permits a case-by-case evaluation, without prescribed threshold
criteria, to demonstrate that an event affected air quality.'' 72 FR
13569 (March 22, 2007).
---------------------------------------------------------------------------

c. Clear Causal Relationship Element
    The EPA is proposing to modify the regulatory language in section
50.14(c)(iv) to more clearly indicate, consistent with the CAA
directive, the requirement to ``demonstrate'' versus to merely
``provide evidence'' that a clear causal relationship must exist
between the specific event and the monitored exceedance. The EPA will
evaluate this criterion on a weight of evidence basis.
d. Affects Air Quality Element
    As explained above, the EPA has treated the ``affects air quality''
element as a distinct criterion that air agencies must meet for data to
be excluded, and has expected exceptional events demonstrations to
conclude that the ``affects air quality'' condition has been satisfied.
However, after carefully considering Congress' intent and air agencies'
and the EPA's experience in implementing the 2007 Exceptional Events
Rule, we propose to integrate the phrase ``affected air quality'' into
the clear causal relationship criterion. We believe that separately
requiring an air agency to provide evidence to support a conclusion
that an event ``affects air quality'' is unnecessary if we finalize
this proposal to require a mandatory clear causal relationship showing.
If an air agency demonstrates that an event has a clear causal
relationship to an exceedance or violation of a NAAQS,

then the event has certainly affected air quality. This proposed
approach will reduce the time required to prepare demonstrations,
reduce their length, result in more understandable demonstrations for
the public during the notice-and-comment process, and simplify and
expedite the EPA's review process.
e. Historical Fluctuations Element
    As we indicated in the Interim Exceptional Events Implementation
Guidance, we believe that a comparison of the claimed event-influenced
concentration(s) to concentrations at the same monitoring site at other
times is extremely useful evidence in an exceptional events
demonstration. The EPA considers these comparisons as part of the
evidence available for determining whether an air agency has satisfied
the statutory and regulatory ``clear causal relationship'' criterion.
Because preparing this type of comparison is within the ability and
resources of every air agency, the EPA proposes to continue to require
this type of comparison in every demonstration. However, the EPA is
proposing to re-word the requirement to prevent misinterpretation that
this comparison must show that the concentration in question was ``in
excess of normal historical fluctuations, including background.'' This
phrase is not clear and has caused confusion and regulatory
uncertainty. For example, ``fluctuations in concentrations'' can convey
either day-to-day or hour-to-hour differences in monitored
concentrations. These concentration differentials cannot usefully be
compared to an absolute concentration (i.e., monitored concentration at
a given point in time) because many absolute concentrations will be
larger than the differences between concentrations. The phrase ``in
excess'' might be interpreted to mean that the concentration at issue
must be higher than all historical concentrations, but the EPA
maintains that Congress did not intend this, nor would such an
interpretation be reasonable. Concentrations that are exceedances of a
standard but are not higher than all concentrations recorded at a
particular monitor may be causally connected to an event of the type
that Congress clearly identified for treatment as an exceptional event.
Finally, the language ``including background'' is confusing. In many
cases, the monitor or monitors intended to represent ``background''
concentrations are separated from the event-influenced monitoring site
by some distance such that the event-influenced monitor and the
``background'' monitor reflect a different mixture of emissions
sources, which could lead to misinterpretation. Regardless, the EPA
sees no clear reason why such ``background'' concentrations are
relevant for analyses associated with provisions in the Exceptional
Events Rule.
    The change that the EPA is proposing to the text of the 2007
Exceptional Events Rule would require demonstrations to include a
comparative analysis of the concentration data alleged to have been
affected by an event and data at other times, and would specify certain
aspects of the analysis. The change would also make clear that there is
no specific ``in excess of'' relationship between the event-affected
data and other data that must be proven, for example that the event-
affected data be above a certain percentile point in the annual
distribution of data. Section V.E.3 of this proposal contains
additional detail regarding the minimum set of statistical analyses
that the EPA expects to see in demonstrations.

C. What types of ambient concentration data and data uses may be
affected by the Exceptional Events Rule?

    The CAA language at section 319(b)(3)(B)(iv) requires the
Administrator to promulgate regulations that provide that there are
criteria and procedures for the governor of a state to petition the
Administrator to exclude air quality monitoring data that is directly
due to exceptional events from use in determinations by the
Administrator with respect to exceedances or violations of the national
ambient air quality standards. The implementing language in the 2007
Exceptional Events Rule states at 40 CFR 50.14(a)(1) that air agencies
may request that the EPA exclude data showing exceedances or violations
of the NAAQS that are directly due to an exceptional event from use in
determinations without naming those determinations in that paragraph.
The rule at 40 CFR 50.14(b)(1) states that the EPA shall exclude data
from use in determinations of exceedances and NAAQS violations where an
air agency demonstrates to the EPA's satisfaction that an exceptional
event caused a specific air pollution concentration in excess of one or
more NAAQS. Thus, both the statutory language and the 2007 Exceptional
Events Rule use the phrase ``in determinations of exceedances and NAAQS
violations'' with no further explanation.
    In this section, we consider the specific types of determinations
by the Administrator that should be governed by CAA section 319(b).
This issue was not specifically addressed in the rulemaking that
promulgated the 2007 Exceptional Events Rule and consequently has
caused some confusion and regulatory uncertainty.
1. Current Situation
    The EPA believes that Congress clearly intended the CAA language in
section 319(b) to apply to exclusions of ambient data from
determinations of whether a NAAQS exceedance or violation occurred at
an ambient monitoring site at a particular time in the past. We
characterize these exceedances or violations as occurring in the
``past'' because the process of determining whether an actual
exceedance or violation occurred involves reviewing the ambient air
monitoring data collected at monitoring sites over some historical
timeframe. For example, on December 14, 2012, the EPA promulgated a
revised primary annual PM2.5 NAAQS of 12.0 [mu]g/m\3\, which
is attained when the 3-year average of the annual arithmetic means does
not exceed 12.0 [mu]g/m\3\.\19\ The EPA Administrator made initial area
designation decisions for the revised NAAQS in December 2014 based on
air quality monitoring data for the most recent period 3-year period,
which was 2011 through 2013.\20\ Historical, or ``past,'' data were
reviewed and assessed to determine whether an exceedance or violation
had occurred that would influence a current or future regulatory
determination. Determinations of ``past'' exceedances or violations are
key to the EPA's actions to designate or redesignate an area, to
initially classify an area for a NAAQS (where classifications apply),
to determine if a nonattainment area has attained the NAAQS for which
it has previously been designated nonattainment, to determine whether a
nonattainment area is eligible for an attainment date extension (where
applicable) and, in some cases, to find that a SIP is inadequate and to
issue a SIP call. No affected stakeholders with whom the EPA has
interacted since 2007 have disputed this interpretation or approach.
---------------------------------------------------------------------------

    \19\ 78 FR 3086 (January 15, 2013).
    \20\ 80 FR 2206 (January 15, 2015).
---------------------------------------------------------------------------

    It is not as clear whether CAA section 319(b) also means that data
should be excluded from determinations of whether a NAAQS exceedance or
violation will or is likely to occur in the future. Predictions of
future NAAQS violation(s) generally involve reviewing the historical
ambient concentration data that are the evident focus of CAA section
319(b), estimating expected

future emissions, and then using both of these data sets as inputs to
an air quality modeling tool or other analytical approach that
extrapolates these data to predict a future outcome. While science
supports and the EPA relies on predictions of future NAAQS violations
in several parts of the clean air program, such as in the EPA's
approval of attainment demonstrations in SIPs, in prevention of
significant deterioration (PSD) air permitting programs and in actions
to reclassify a moderate PM10 or PM2.5
nonattainment area to serious,\21\ the fact that these predicted future
values rely only in part on historical monitoring data implies that a
different standard for data exclusion may be appropriate.
---------------------------------------------------------------------------

    \21\ Projection of future NAAQS exceedances or violations do not
necessarily play a role in reclassification of an ozone
nonattainment area to a higher classification level.
---------------------------------------------------------------------------

    Another interpretation question is whether and under what
conditions event-affected data should be excluded from determinations
that are based wholly or in part on monitoring data but formally are
not determinations of exceedances or violations of the NAAQS. For
example, under 40 CFR part 51, subpart H, Prevention of Air Pollution
Emergency Episodes, the required content of a state's emergency plan
depends on whether the state has experienced air pollution that exceeds
a specified threshold level that is well above any NAAQS. Also, under
the EPA's guidance, the eligibility of an area for a simplified
maintenance plan for PM10 depends on the difference between
the better-than-the-NAAQS air quality in an area and the NAAQS.
    To date, the EPA has not issued guidance that explicitly and
comprehensively identifies the types of data exclusion that are
authorized and required by CAA section 319(b) or that may be otherwise
appropriate and permissible. In the 2013 Interim Q&A document, the EPA
provided only limited clarification regarding the meaning of ``exclude
data.'' \22\ Question 14a of the Interim Q&A document notes that when
the EPA concurs based on the weight of evidence that an air agency has
successfully made the demonstrations referred to in 40 CFR 50.14(a)(2)
and (b)(1), then the EPA generally excludes the affected data from the
following types of calculations and activities:
---------------------------------------------------------------------------

    \22\ 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.
---------------------------------------------------------------------------

     The EPA's AQS does not count these days as exceedances
when generating user reports, and does not include them in design
values estimates,\23\ unless the AQS user specifically indicates that
they should be included, which may be appropriate for non-regulatory
applications of interest to the user.\24\
---------------------------------------------------------------------------

    \23\ The EPA uses design values in many regulatory decisions,
including, but not limited to, when designating areas as attainment,
nonattainment or unclassifiable for a NAAQS and when determining
whether a nonattainment area has attained or is still violating a
NAAQS. A design value is a statistic that describes the air quality
status of a given location relative to the level of a particular
NAAQS. Design values are computed according to the procedures
defined in 40 CFR part 50 and published annually by EPA's Office of
Air Quality Planning and Standards. Design values are available at
http://www.epa.gov/airtrends/values.html.
    \24\ In some cases where the EPA has revised a NAAQS by
strengthening it, the default AQS query will exclude data for the
more recent, revised NAAQS, but may include concurred data for the
historical NAAQS.
---------------------------------------------------------------------------

     The EPA accepts the exclusion of these data for the
purposes of selecting appropriate background concentrations for PSD air
quality analyses \25\ and for transportation conformity hot spot
analyses.\26\
---------------------------------------------------------------------------

    \25\ If the EPA is the permitting authority, the EPA will
propose permits on this basis. If the EPA is commenting on another
permitting authority's proposed action, the EPA's comments will be
consistent with the determinations in this guidance document and any
applicable New Source Review (NSR) permitting and/or modeling
guidance.
    \26\ Transportation conformity hot spot analysis is applicable
only to PM10 and PM2.5. ``Transportation
Conformity Guidance for Quantitative Hot-spot Analyses in
PM2.5 and PM10 Nonattainment and Maintenance
Areas,'' EPA-420-B-10-040, U.S. EPA Office of Transportation and Air
Quality, December 2010, page 98.
---------------------------------------------------------------------------

     The EPA accepts the exclusion of these data for the
purposes of selecting appropriate ambient data for projecting future
year concentrations as part of a modeled attainment demonstration.
     The data continue to be publicly available, but the EPA's
publications and public information statements on the status of air
quality in the affected area generally do not reflect these data in any
summary statistic of potential regulatory application, unless such
inclusion is specifically noted.\27\
---------------------------------------------------------------------------

    \27\ These data may be included in statistics intended to
describe current status and trends in actual air quality in the area
for public information purposes including reporting of the Air
Quality Index.
---------------------------------------------------------------------------

    Thus, the EPA has maintained that once data are excluded under the
Exceptional Events Rule, these same data should be excluded from the
above-identified calculations and activities. The EPA has not clearly
addressed whether approval for exclusion under the Exceptional Events
Rule means that the data may or must be excluded for the purpose of
other types of actions that use monitoring data but are not included in
the list above. The EPA has also not clearly addressed whether data
that have not been approved for exclusion under the Exceptional Events
Rule can nevertheless under some other principle or interpretation be
excluded from any of the various types of calculations and activities.
    The current situation is further complicated by the fact that the
conditions for data exclusion in CAA section 319(b) and the Exceptional
Events Rule, while logical when applied to determinations of NAAQS
exceedances or violations occurring in the past, may not be logical
when applied to predictions of future exceedances or violations. The
EPA recognizes, and acknowledged in Question 13 of the Interim Q&A
document, that an event may have made a past air concentration
significantly higher than it would have been in the absence of the
event contribution, and thus elevated an exceedance for a NAAQS
pollutant to an even greater degree of exceedance. This same event-
influenced concentration may not be eligible for exclusion under the
2007 Exceptional Events Rule because the ``but for'' criterion is not
satisfied because either (1) there would have been a 3-year violation
with or without the event or (2) there would not have been a violation
either with or without the event. The 2007 Exceptional Events Rule does
not explicitly authorize the exclusion of data associated with such an
event because the event fails to meet the clear causal relationship
criterion and ``but for'' criterion. Retaining the event-influenced
data could, however, have regulatory implications that seem contrary to
the purpose of CAA section 319(b). For example, retaining such data in
the calculation of the historical design value for a nonattainment area
can make it seem that the area needs more emissions reduction to attain
the NAAQS than is actually the case, and could lead to the EPA's
disapproval of an attainment demonstration that is in fact adequate,
and thus require the state to adopt additional emission controls.\28\
---------------------------------------------------------------------------

    \28\ The attainment demonstration would be adequate in the sense
that if a similar event does not occur during the period on which
actual attainment will be based, there would be no monitored NAAQS
violation, and if a similar event were to occur during that period
the event-affected data could be excluded and thus there would be no
``official'' violation.
---------------------------------------------------------------------------

    As another example, events can make past air concentrations higher
without causing an actual NAAQS exceedance or violation. However,
retaining such data in the calculation of background concentrations
used in air quality analysis for a PSD permit may suggest that there
will be a NAAQS violation after construction of a new source and

thus could prevent the permitting authority from issuing the
permit.\29\
---------------------------------------------------------------------------

    \29\ If a similar event were to occur after completion of
construction, the event-affected data could be excluded and thus
there would be no ``official'' violation.
---------------------------------------------------------------------------

2. Proposed Changes
    To remove the ambiguities described in the preceding section and to
provide greater regulatory certainty, the EPA proposes in regulatory
language to interpret the CAA section 319(b) phrase ``determinations by
the Administrator with respect to exceedances or violations of national
ambient air quality standards'' to encompass determinations of current
\30\ or historical NAAQS exceedances/violations or non-exceedances/non-
violations and determinations of the air quality ``design value'' at
particular receptor sites when made as part of the basis for any of the
following five types of regulatory actions:
---------------------------------------------------------------------------

    \30\ The term ``current'' denotes the determination at issue in
the current analysis. In actual practice, such determinations are
based on historical data and thus reflect a past actual condition.
---------------------------------------------------------------------------

     An action to designate or redesignate an area as
attainment, unclassifiable/attainment, nonattainment or unclassifiable
for a particular NAAQS. Such designations rely on a violation at a
monitoring site in or near the area being designated.
     The assignment or re-assignment of a classification
category (marginal, moderate, serious, etc.) to a nonattainment area to
the extent this is based on a comparison of its ``design value'' to the
established framework for such classifications.
     A determination regarding whether a nonattainment area has
actually attained a NAAQS by its CAA deadline.
     A determination that an area has had only one exceedance
in the year prior to its deadline and thus qualifies for a 1-year
attainment date extension, if applicable.
     A finding of SIP inadequacy leading to a SIP call to the
extent the finding hinges on a determination that the area is violating
a NAAQS.

For these types of actions, the EPA proposes to interpret the CAA to
require that data be excluded only if the requirements of section
319(b) and the Exceptional Events Rule are satisfied. In addition, we
propose that when one of these determinations is based on a combination
of monitoring data and air quality modeling, the criterion requiring
that there be a clear causal relationship between the event and a NAAQS
exceedance or violation will apply to the combined estimate of air
pollution levels rather than directly to the monitored background air
quality data. That is, the event would not be required to have caused
an actual exceedance or violation at the background ambient monitoring
site, but rather to have made the critical difference in the combined
estimate of air pollution levels (background plus source impact)
resulting in a NAAQS exceedance or violation, because the event
increased the background levels that are added to the air quality
modeling output.
    When the EPA designates or redesignates areas as attainment or
nonattainment for the NAAQS; initially classifies ozone nonattainment
areas as marginal, moderate, serious, severe or extreme; grants a
request for a 1-year NAAQS attainment date extension where applicable;
or determines whether areas designated nonattainment for the NAAQS have
attained the respective NAAQS by the applicable attainment date, it
does so based on monitoring data (where available) or modeling of
actual air quality, or a combination thereof, as the evidence of the
occurrence or non-occurrence of a NAAQS exceedance or violation and, in
the case of classification actions, the degree of violation.\31\ In the
case of reclassifying an ozone nonattainment area to a higher
classification, the new classification is based on the design value
either at the time of the determination of attainment by the attainment
deadline under CAA section 181(b)(2), or at the time of the EPA's grant
of a voluntary request for reclassification from a state under CAA
section 181(b)(3).\32\ This proposal, if finalized, would in effect
apply the exceptional events process in the same way across these
related types of determinations and across the NAAQS, which we believe
is an appropriate interpretation of the CAA 319(b) phrase
``determinations by the Administrator with respect to exceedances or
violations of national ambient air quality standards.'' For these types
of determinations, the EPA proposes to exclude event-affected data only
if an air agency satisfies the procedural (e.g., event identification,
opportunity for public comment, demonstration submission) and
substantive (i.e., clear causal relationship, not reasonably
controllable or preventable, and human activity not likely to recur or
natural event) requirements of the exceptional events process. As
indicated previously, the EPA has maintained to this point that once
data are excluded under the Exceptional Events Rule, these same data
also should be excluded from (i) design value estimates and AQS user
reports (unless the AQS user specifically indicates that they should be
included), (ii) selecting appropriate background concentrations for PSD
air quality analyses and transportation conformity hot spot analyses,
and (iii) selecting appropriate ambient data for projecting future year
concentrations as part of a modeled attainment demonstration. As
described below, we intend that EPA approval for exclusion of data
under the Exceptional Events Rule continue to mean that the same data
may be excluded for the three applications listed in the previous
sentence, but that there should be other pathways for exclusion for the
second and third of these applications (and others) as well.
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    \31\ The EPA's initial area designations process also makes use
of other information relevant to the CAA criteria for designations,
such as pollution contributions between nearby areas.
    \32\ Reclassification of PM10 and PM2.5
nonattainment areas, by contrast, do not exclusively rely on area
design values (and thus, past monitored violations) but can also
result from the Administrator's determination that an area cannot
practicably attain a standard by the attainment date. See CAA
section 188(b)(1).
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    This action proposes to require that data exclusion associated with
the five actions in the above bulleted list (i.e., initial area
designations, classifications, attainment determinations,
determinations regarding requests for attainment date extensions and
findings of SIP inadequacy leading to a SIP call) must follow the
provisions in the Exceptional Events Rule. It does not, however, mean
that the EPA would never exclude or agree to exclude event-affected
data from other types of regulatory determinations. For example, while
the EPA would exclude concurred-upon event-affected data from the five
types of regulatory actions discussed in V.C.1, the EPA would not
exclude these same data when setting priority classifications for
emergency plans under 40 CFR 51.150 as the EPA believes that
implementing the CAA principle at section 319(b)(3)(A) that
``protection of public health is the highest priority'' may necessitate
that an air agency address in its emergency plan the appropriate
planned response for any elevated concentration known to be possible
because it has already been observed, although the appropriate type of
response may depend on the cause(s) of the elevated concentration. The
concept that the EPA does not consider CAA section 319(b) and the
revised Exceptional Events Rule to be the necessary or sole governing
authorities for all data exclusions will be discussed further in
upcoming, new draft guidance on excluding (or in some cases not
excluding) data, independent of the Exceptional Events Rule, from
several

types of determinations and regulatory actions. The EPA is currently
developing a supplementary guidance document, Draft Guidance for
Excluding Some Ambient Pollutant Concentration Data from Certain
Calculations and Analyses for Purposes Other than Retrospective
Determinations of Attainment of the NAAQS, which will describe the
appropriate additional pathways that we intend to make available for
data exclusion for some monitoring data applications (e.g., predicting
future attainment that is the basis for approval of an attainment
demonstration in the SIP for a nonattainment area, preparing required
air quality analyses in an application for a PSD permit or preparing
required air quality analysis for the purposes of transportation
conformity). The EPA intends to post the draft guidance on the
exceptional events Web site at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events and expects to
finalize the document when we finalize these rule revisions. We intend
that this guidance will recommend exclusion of data for PSD,
transportation conformity and certain other applications in any
situation in which exclusion has already been approved under the
Exceptional Events Rule, as well as in applications in which the facts
would support exclusion under the criteria of the Exceptional Events
Rule even if an EPA determination has not yet been made under the
Exceptional Events Rule and in some other situations that we will
describe in the guidance.

D. What is a natural event?

1. Current Situation
    The CAA definition at section 319(b)(1)(iii) specifies that an
exceptional event ``is an event caused by human activity that is
unlikely to recur at a particular location or a natural event.'' Thus,
the statute limits the expected occurrence frequency of an event caused
by human activity as ``unlikely to recur'' but does not limit the
occurrence frequency of a natural event. Natural events may recur, even
frequently.\33\ Air agencies can request, and the EPA can agree, to
exclude data affected by a natural event if an air agency's
demonstration meets the other requirements of the Exceptional Events
Rule. Thus, considering whether an event was a natural event or was
caused by human activity is important to the content within and to the
approval of a demonstration.
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    \33\ See as examples, Hawaii's exceptional events demonstration
for volcanic activity affecting PM2.5 concentrations in
2011-2012 and California Air Resources Board's demonstration for
wildfire events affecting PM2.5 concentrations in 2008,
both available at http://www2.epa.gov/air-quality-analysis/exceptional-events-submissions-table.
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    As previously discussed, to be considered an exceptional event, an
event, whether natural or anthropogenic in origin, must affect air
quality at the affected monitor. 40 CFR 50.1(k) defines a natural event
as one in which human activity plays little or no direct causal role in
the generation of emissions. In some cases, such as stratospheric ozone
intrusions or volcanic eruptions, the EPA recognizes that human
activity plays no role in the magnitude of emissions or level of air
pollution that occurs. In other cases, past or current human activity
does influence the magnitude of emissions and hence the level of air
pollution. For example, in high wind dust events, the pollution from
the event may originate from a mixture of natural lands (e.g.,
undisturbed soil), soil that has been disturbed by human activity and
has been made more prone to wind-generated dust emissions (e.g., recent
construction activity), and materials accumulated and stored by human
activity (e.g., sand and gravel facilities).
    The EPA generally considers human activity to have played little or
no direct role in causing emissions if anthropogenic emission sources
that contribute to the event emissions are reasonably controlled at the
time of the event, regardless of the magnitude of emissions generated
by these reasonably controlled anthropogenic sources and regardless of
the relative contribution of these emissions and emissions arising from
natural sources in which human activity has no role.34 35
Thus, the event could be considered a natural event. In such cases, the
EPA applies the reasonable interpretation that the anthropogenic source
had ``little'' direct causal role. If anthropogenic emission sources
that contribute to the event emissions can be reasonably controllable
but reasonable controls were not implemented at the time of the event,
then the event would not be considered a natural event. The EPA
explained this concept in the preamble to the 2007 Exceptional Events
Rule.\36\ However, the rule text did not reflect the identified
concept. This has resulted in some regulatory uncertainty as to whether
the EPA's interpretation of the CAA and the 2007 Exceptional Events
Rule as described here is appropriate.
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    \34\ For example, if an area affected by a high wind dust event
has adequate rules or ordinances for sources of windblown dust
(e.g., rules that establish restrictions for operating vehicles on
unpaved property, rules that control windblown dust emissions
associated with lands disturbed by construction, earthwork and land
development) and the air agency can provide evidence of
implementation and enforcement, then the EPA would generally
consider human activity to have played little or no direct causal
role in causing the event-related emissions.
    \35\ The EPA considers wildfires to be natural events even
though some wildfires are initiated by human actions and to some
degree the frequency and scale of wildfires may be influenced by
prior land management practices. The EPA believes this
interpretation best implements the Congressional intent and is a
more appropriate approach than expecting air agencies to determine
the initial cause of each wildfire of interest and classifying it as
natural or anthropogenic based on that cause. In addition, land
owners and managers and government public safety agencies are
strongly motivated to reduce the frequency and severity of human-
caused wildfires and the EPA believes they can be presumed to make
reasonable efforts to avoid them.
    \36\ 72 FR 13565-13566 (March 22, 2007).
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2. Proposed Changes
    Based on the discussion above, the EPA proposes to revise the
definition of natural event to clarify that anthropogenic emission
sources that contribute to the event emissions that are reasonably
controlled do not play a ``direct role'' in causing emissions. Thus, an
event with a mix of natural emissions and reasonably controlled human-
affected emission sources may be considered a natural event. However,
an event resulting from only reasonably controlled human affected
emissions may not be considered a natural event. This proposal is
consistent with statements made in the preamble to the 2007 Exceptional
Events Rule, and including it in the rule text provides more regulatory
certainty to all parties.
    When addressing the not reasonably controllable or preventable
criterion for this same event type consisting of a mix of natural
emissions and human-affected emission sources (e.g., a high wind event
affecting both open desert areas and urbanized lands), air agencies
must assess reasonable controls for both the contributing natural and
anthropogenic sources. While air agencies must ``assess'' reasonable
controls for most types of contributing natural sources because this
statutory factor applies to all events, they do not necessarily need to
implement controls for these same sources. Additionally, because the
rule revisions propose a categorical presumption of not reasonably
controllable for wildfires and large-scale, high-energy and/or sudden
high wind dust events, ``assessing'' these events would involve
referencing the appropriate regulatory citation. As we explain in more
detail in section V.E.2, for natural sources, we do not think that air
agencies need to have implemented any controls for windblown dust from
never-disturbed,

large-scale natural landscapes. Therefore, lack of controls on natural
sources that contribute to event-related emissions would not disqualify
the event from being considered as an exceptional event. When assessing
the contribution from anthropogenic sources, similar to the analyses
involved in determining whether these same sources play a ``direct
role'' in causing event-related emissions, the air agency should
identify the contributing anthropogenic sources, explain why the
controls specified in rules or ordinances are reasonable, and provide
evidence of implementation and enforcement. Also as explained in
section V.E.2, in our view an event is ``not reasonably controllable''
if an exceedance or violation occurs even when reasonable controls were
actually in place and any further control would have been beyond what
was reasonable. The EPA intends to consider these aspects when applying
the concept of ``reasonable controls'' on anthropogenic sources to
determine whether the event can be considered a natural event and to
evaluate the not reasonably controllable or preventable criterion.
    With respect to determining whether anthropogenic emission sources
that contribute to the event emissions were reasonably controlled at
the time of the event, the EPA also proposes to revise the definition
of a natural event to indicate that the reasonableness of available
controls should be assessed as of the date of the event. The EPA does
not believe that information related to the cost and effectiveness of
control measures, or related to the frequency of events, that became
available to the air agency after the date of the event should affect
the assessment of whether anthropogenic sources were reasonably
controlled and thus the identification of an event as natural or caused
by human activity.
    When addressing this criterion as part of an exceptional events
demonstration, the EPA recommends that the submitting air agency
clearly identify whether the event is natural or was a human activity
that is unlikely to recur at a particular location. If purely natural
(e.g., lightning-ignited wildfire, volcanic or seismic activity,
stratospheric ozone intrusion), the EPA recommends that the submitting
air agency identify the purely natural status in the ``human activity/
natural event'' section of its demonstration; provide the type/source
of event, the resulting emissions, and the documented frequency of the
event; and affirmatively state that in characterizing the event, the
submitting air agency has satisfied the human activity/natural event
criterion.

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

    As described in section V.B, 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 caused by human activity that is
unlikely to recur at a particular location or was a natural event. All
exceptional events demonstrations, regardless of event type or relevant
NAAQS, must address each of these technical criteria. This section
describes the EPA's proposals for rule revisions and guidance regarding
each of these technical criteria. Section V.G discusses additional
process-related components of exceptional events demonstration
packages.
1. Human Activity Unlikely To Recur at a Particular Location or a
Natural Event
    The concept of recurrence applies to human activity; the statements
in this section are not relevant for natural events. Section V.D
includes a detailed discussion of a ``natural event.''
a. Current Situation
    According to both the regulatory and statutory definitions, an
exceptional event must be ``an event caused by human activity that is
unlikely to recur at a particular location or a natural event''
(emphasis added). For clarity, in this section, the EPA focuses on the
language ``unlikely to recur at a particular location.''
    The ``unlikely to recur at a particular location'' requirement of
CAA section 319(b) does not define ``unlikely to recur.'' Thus, this
language 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.
    The EPA's 1986 Guideline on the Identification and Use of Air
Quality Data Affected by Exceptional Events stated that events can be
considered exceptional if they are not expected to ``recur routinely at
a given location.'' \37\ This document did not further define or give
specific examples of ``routinely.''
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    \37\ Guideline on the Identification and Use of Air Quality Data
Affected by Exceptional Events (the Exceptional Events Policy), U.S.
EPA, OAQPS, EPA-450/4-86-007, July 1986.
---------------------------------------------------------------------------

    The preamble to the 2007 Exceptional Events Rule did not provide
specific guidance on the unlikely to recur criterion, except to say
that recurrence is event-specific and should be assessed on a case-by-
case basis and that in the particular case of prescribed fires a
comparison to the natural fire return interval is a relevant
consideration for this criterion.
    The CAA section 319(b) and the 2007 Exceptional Events Rule do not
specifically address temporary, but multi-day or multi-year activities,
such as construction projects. However, Question 16 in the Interim Q&A
document noted that the 2007 Exceptional Events Rule does not
explicitly place a limit on the duration of a single event and that a
submitting agency could make a showing that a prolonged activity (e.g.,
a multi-year road construction project) is a single event that is not
likely to recur at the location in question. The Interim High Winds
Guidance document addressed recurrence for high wind events, as
summarized in section V.F.4 of this document. Other than this, the
Interim Exceptional Events Implementation Guidance did not provide any
specific guidance on the unlikely to recur criterion.
b. Proposed Changes
    While we believe that it is appropriate to consider recurrence to
be event-specific and for the unlikely to recur criterion to be
assessed on a case-by-case basis, we also believe that this proposed
action presents an opportunity to clarify certain points. This section
provides general clarifications with respect to the meaning of
``unlikely to recur.'' Section V.F.2 addresses this criterion for
wildland fires (specifically prescribed fires on wildland) and section
V.F.4 specifically addresses this criterion for high wind dust events.
Also, under CAA section 319(b) and the 2007 Exceptional Events Rule,
air pollution related to source noncompliance is not an exceptional
event regardless of its frequency.
    The EPA proposes, as guidance, to recommend the following
boundaries on the interpretation of the unlikely to recur criterion. If
an event type has not previously occurred within a given air

quality control region (AQCR) \38\ in the 3 years preceding the
submittal of an exceptional events demonstration, the EPA will consider
this to be a ``first'' event and will generally consider it to be
unlikely to recur in the same location. Similarly, a ``second'' event
within the 3 years preceding the submittal of an exceptional events
demonstration would also generally be considered unlikely to recur in
the same location. If there have been two prior events of a similar