[Federal Register: March 22, 2007 (Volume 72, Number 55)]
[Rules and Regulations]
[Page 13559-13581]
[DOCID:fr22mr07-14]

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Part II

Environmental Protection Agency

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40 CFR Parts 50 and 51

Treatment of Data Influenced by Exceptional Events; Final Rule

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50 and 51

[EPA-HQ-OAR-2005-0159; FRL-8289-5]
RIN 2060-AN40

Treatment of Data Influenced by Exceptional Events

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: This action finalizes a rule to govern the review and handling
of air quality monitoring data influenced by exceptional events.
Exceptional events are events for which the normal planning and
regulatory process established by the Clean Air Act (CAA) is not
appropriate. In this rulemaking action, EPA is finalizing the proposal
to: Implement section 319(b)(3)(B) and section 107(d)(3) authority to
exclude air quality monitoring data from regulatory determinations
related to exceedances or violations of the National Ambient Air
Quality Standards (NAAQS) and avoid designating an area as
nonattainment, redesignating an area as nonattainment, or reclassifying
an existing nonattainment area to a higher classification if a State
adequately demonstrates that an exceptional event has caused an
exceedance or violation of a NAAQS. The EPA is also requiring States to
take reasonable measures to mitigate the impacts of an exceptional
event.

DATES: This final rule is effective May 21, 2007.

ADDRESSES: The EPA has established a docket for this action under
Docket ID No. EPA-HQ-OAR-2005-0159. All documents in the docket are
listed on the http://www.regulations.gov Web site. Although listed in

the index, some information is not publicly available, i.e.,
confidential business information (CBI) or other information whose
disclosure is restricted by statute. Certain other material, such as
copyrighted material, is not placed on the Internet and will be
publicly available only in hard copy form. Publicly available docket
materials are available either electronically through http://www.regulations.gov
 or in hard copy at the OAR Docket, EPA/DC, EPA

West, Room 3334, 1301 Constitution Ave., 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<PHONE_NUMBER>.

FOR FURTHER INFORMATION CONTACT: General questions regarding the final
rule should be addressed to Mr. Larry D. Wallace, PhD, Office of Air
Quality Planning and Standards, Air Quality Policy Division, Mail Code
C539-01, Research Triangle Park, NC 27711; telephone<PHONE_NUMBER>,
and e-mail address<EMAIL_ADDRESS>    Questions concerning technical and analytical issues related to
this final rule should be addressed to Mr. Neil Frank, Office of Air
Quality Planning and Standards, Air Quality Assessment Division, Mail
Code C304-01, Research Triangle Park, NC 27711; telephone (919) 541-
5560, and e-mail address<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

I. General Information

A. Does This Action Apply to Me?

    Regulated Entities. This final rule will affect State and local air
quality agencies. This rule may also affect Tribal air quality agencies
that have implemented air quality monitoring networks or have authority
to implement air quality programs.
    This list is not intended to be exhaustive, but rather provides a
guide for readers regarding entities likely to be regulated by this
action. This list gives examples of the types of entities EPA is now
aware could potentially be regulated by this action. Other types of
entities not listed could also be affected. To determine whether your
facility, company, business, organization, etc., is regulated by this
action, you should examine the applicability criteria in section IV of
this preamble. If you have any questions regarding the applicability of
this action to a particular entity, consult the people listed in the
preceding section.

B. How Is This Preamble Organized?

Table of Contents
    The following is an outline of the preamble.

I. General Information
    A. Does This Action Apply to Me?
    B. How Is This Preamble Organized?
II. Preamble Glossary of Terms and Acronyms
III. Background and Purpose of This Rulemaking
    A. Legislative Requirements
    B. Historical Experience Concerning Exceptional and Natural
Events
IV. This Final Action
    A. To Whom and to What Pollutants Does This Rule Apply?
    B. How Does This Rule Relate to Indian Tribes?
    C. Comments Submitted on the Proposed Rule
    D. What Is an Exceptional Event?
    E. Examples of Exceptional Events
    1. Chemical Spills and Industrial Accidents
    2. Structural Fires
    3. Exceedances Due to Transported Pollution
    4. Exceedances Due to a Terrorist Attack
    5. Natural Events
    a. Natural Disasters and Associated Clean-Up Activities
    b. Volcanic and Seismic Activities
    c. High Wind Events
    d. Wildland Fires
    e. Stratospheric Ozone Intrusions
    6. Prescribed Fire
V. The Management of Air Quality Data Affected by Exceptional Events
    A. Flagging of Data in the AQS Database
    1. Background
    2. Final Rule
    3. Comments and Responses
    B. What Does It Mean for an Event to ``Affect Air Quality''?
    1. Background
    2. Final Rule
    3. Comments and Responses
    C. Use of a ``But For'' Test
    1. Background
    2. Final Rule
    3. Comments and Responses
    D. Schedules and Procedures for Flagging and Requesting
Exclusion of Data
    1. Background
    2. Final Rule
    3. Comments and Responses
    E. Exclusion of Entire 24-Hour Value as Opposed to a Partial
Adjustment of the 24-Hour Value
    1. Background
    2. Final Rule
    3. Comments and Responses
    F. What Should States Be Required To Submit in Their Exceptional
Events Demonstrations?
    1. Background
    2. Final Rule
    3. Comments and Responses
    G. Public Availability of Air Quality Data and Demonstrations
Related to Exceptional Events
    1. Background
    2. Final Rule
    3. Comments and Responses
VI. Additional Requirements
    A. Requirements for States To Provide Public Notification,
Public Education, and Appropriate and Reasonable Measures To Protect
Public Health
    1. Background
    2. Final Rule
    3. Comments and Responses
VII. Special Treatment of Certain Exceptional Events Under This
Final Rule
    A. Volcanic and Seismic Activities
    1. Background
    2. Final Rule
    B. High Wind Events
    1. Background
    2. Final Rule
    C. Stratospheric Ozone Intrusion
    1. Background
    2. Final Rule
VIII. Treatment of Fireworks Displays

    A. Background
    B. Final Rule
    C. Comments and Responses
    IX. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and 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 and Safety Risks
    H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution, or Use
    I. National Technology Transfer Advancement Act
    J. Congressional Review Act
    K. Petitions for Judicial Review

II. Preamble Glossary of Terms and Acronyms

    The following are abbreviations of terms used in the preamble.

ARM Approved Regional Methods.
AQS Air Quality System.
BACM Best Available Control Measures.
CAA Clean Air Act.
CAAA Clean Air Act Amendments.
EPA Environmental Protection Agency.
FEM Federal Equivalent Methods.
FIP Federal Implementation Plan.
FR Federal Register.
FRM Federal Reference Methods.
NAAQS National Ambient Air Quality Standards.
NEAP Natural Events Action Plan.
NEPA National Environmental Policy Act.
NTTA National Technology Transfer Advancement Act of 1995.
OAQPS Office of Air Quality Planning and Standards.
OMB Office of Management and Budget.
PM Particulate matter.
PM10 Particles with a nominal mean aerodynamic diameter less
than or equal to 10 micrometers.
PM10-2.5 Particles with a nominal mean aerodynamic diameter
greater than 2.5 micrometers and less than or equal to 10 micrometers.
PM2.5 Particles with a nominal mean aerodynamic diameter
less than or equal to 2.5 micrometers.
RACM Reasonably Available Control Measures.
SIP State Implementation Plan.
SAFE-TEA-LU Safe Accountable Flexible Efficient-Transportation Equity
Act--A Legacy for Users.
SMP Smoke Management Program.
TAR Tribal Authority Rule.
TIP Tribal Implementation Plan.
UMRA Unfunded Mandates Reform Act.
USDA U.S. Department of Agriculture.
VCS Voluntary Consensus Standards.

III. Background and Purpose of This Rulemaking

A. Legislative Requirements

    We \1\ are finalizing a rule to govern the review and handling of
air quality monitoring data influenced by exceptional events. As
discussed below, these are events for which the normal planning and
regulatory process established by the CAA is not appropriate. Section
319 of the CAA, as amended by section 6013 of the Safe Accountable
Flexible Efficient-Transportation Equity Act: A Legacy for Users (SAFE-
TEA-LU) of 2005, required EPA to publish the proposed rule in the
Federal Register no later than March 1, 2006.\2\ Further, EPA must
issue this final rule no later than 1 year from the date of proposal.
The EPA published the proposed rule on March 10, 2006 (See 71 FR
12592).
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    \1\ The U.S. Environmental Protection Agency.
    \2\ All subsequent references to section 319 of the CAA in this
proposal are to section 319 as amended by SAFE-TEA-LU unless
otherwise noted.
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    In this final rule, EPA is establishing procedures and criteria
related to the identification, evaluation, interpretation, and use of
air quality monitoring data related to any NAAQS where States petition
EPA to exclude data that are affected by exceptional events.
    Section 319 defines an event as an exceptional event if the event
affects air quality; is an event that is not reasonably controllable or
preventable; is an event caused by human activity that is unlikely to
recur at a particular location or a natural event; and is determined by
EPA to be an exceptional event. The statutory definition of exceptional
event specifically excludes stagnation of air masses or meteorological
inversions; a meteorological event involving high temperatures or lack
of precipitation; or air pollution relating to source noncompliance.
    Section 319(b)(3)(B)(i) requires a State air quality agency to
demonstrate through ``reliable, accurate data that is promptly
produced'' that an exceptional event occurred.\3\ Section
319(b)(3)(B)(ii) requires that ``a clear causal relationship'' be
established between a measured exceedance of a NAAQS and the
exceptional event demonstrating ``that the exceptional event caused a
specific air pollution concentration at a particular location.'' In
addition, section 319(b)(3)(B)(iii) requires a public process to
determine whether an event is an exceptional event. Finally, section
319(b)(3)(B)(iv) requires criteria and procedures for a Governor to
petition the Administrator to exclude air quality monitoring data that
is directly due to exceptional events from use in determinations with
respect to exceedances or violations of the NAAQS.
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    \3\ While this document refers primarily to States as the entity
responsible for flagging data impacted by exceptional events, other
agencies, such as local or Tribal government agencies, may also have
standing to flag data as being affected by these types of events,
and the criteria and procedures that are discussed in this
rulemaking also apply to these entities.
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    The term exceedance refers to a measured or modeled concentration
greater than the level of one or more for a pollutant. The NAAQS are
also set with particular averaging periods (e.g., 3 years for ozone and
PM2.5) such that a violation of the NAAQS for ozone and
PM2.5 requires an average annual concentration level
specified by appendix I and N to 40 CFR 50 to be greater than the level
of the NAAQS. Public comments favored the consideration of data
contributing to both exceedances and violations for data exclusion
under this Rule. As discussed in section V.C, exceedances of any NAAQS
will be eligible for consideration for data exclusion and any data
contributing to violations of daily or sub-daily standards will also be
eligible for consideration (e.g. 8-hour or 24-hour standards). Data
contributing to annual violations without being exceedances themselves
are considered too close to background air quality levels for exclusion
under this Rule.
    Section 319 also contains a set of five principles for EPA to
follow in developing regulations to implement section 319:

    (i) Protection of public health is the highest priority;
    (ii) Timely information should be provided to the public in any
case in which the air quality is unhealthy;
    (iii) All ambient air quality data should be included in a
timely manner in an appropriate Federal air quality database that is
accessible to the public;
    (iv) Each State must take necessary measures to safeguard public
health regardless of the source of the air pollution; and
    (v) Air quality data should be carefully screened to ensure that
events not likely to recur are represented accurately in all
monitoring data and analyses (42 U.S.C. 7619(b)(3)(A)).

    In adopting revisions to section 319, Congress sought to provide
statutory relief to States to allow them to avoid being designated as
nonattainment or to avoid continuing to be designated

nonattainment as a result of exceptional events in appropriate
circumstances. To accomplish this goal, Congress enumerated certain
minimum requirements for this rulemaking. In addition, Congress
provided certain statutory principles for EPA to follow in promulgating
regulations to exclude data affected by exceptional events.

B. Historical Experience Concerning Exceptional and Natural Events

    Since 1977, EPA guidance and regulations have either implied or
documented the need for a flagging system for data affected by an
exceptional event. The first EPA guidance related to the exclusion or
discounting of data affected by an exceptional event was an Office of
Air Quality Planning and Standards (OAQPS) guidance document entitled,
``Guideline for the Interpretation of Air Quality Standards,''
Guideline No. 1.2-008 (revised February 1977).\4\
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    \4\ ``Guideline for Interpretation of Air quality Standards,''
U.S. Environmental Protection Agency, Office of Air quality Planning
and Standards, Research Triangle Park, N.C. OAQPS No. 1.2-008
(Revised February 1977). The guidance indicated the need for a data
flagging system which would require the submittal of detailed
information establishing that a violation was due to uncontrollable
natural sources and that the information could be used in decision
making related to the feasibility of modifying control strategies.
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    In July 1986, EPA issued the guidance entitled, ``Guideline On the
Identification and Use of Air Quality Data Affected By Exceptional
Events'' (the Exceptional Events Policy). The Exceptional Events Policy
provided criteria for States to use in making decisions related to
identifying data that have been influenced by an exceptional event.
    In addition to the Exceptional Events Policy, on July 1, 1987, EPA
promulgated the NAAQS for PM10 (particulate matter with an
aerodynamic diameter of 10 micrometers or less), which also addressed
the issue of excluding or discounting data affected by exceptional
events.\5\ Appendix K of that rule allows for special consideration of
data determined to be affected by an exceptional event. Section 2.4 of
appendix K authorizes EPA to discount from consideration in making
attainment or nonattainment determinations air quality data that are
attributable to ``an uncontrollable event caused by natural sources''
of PM10, or ``an event that is not expected to recur at a
given location.'' Section 2.4 of appendix K, together with EPA guidance
contained in the Exceptional Events Policy, describes the steps that
should be taken for flagging PM10 data that a State believes
are affected by an exceptional or natural event.
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    \5\ Federal Register (52 FR 24667), July 1, 1987.
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    In 1990, section 188(f) was added to the CAA. This section of the
CAA provided EPA authority to waive either a specific attainment date
or certain planning requirements for serious PM10
nonattainment areas that are affected by nonanthropogenic sources. In
response to section 188(f), and in consideration of the CAA
consequences for areas affected by elevated concentrations caused by
natural events, in 1996 EPA issued a policy to address data affected by
natural events entitled, ``Areas Affected by PM10 Natural
Events,'' (the PM10 Natural Events Policy).\6\
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    \6\ Memorandum from Mary D. Nicols, Assistant Administrator for
Air and Radiation, to EPA Regional Offices entitled, ``Areas
Affected by PM10 Natural Events,'' May 30, 1996.
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    On July 18, 1997, EPA issued a revised NAAQS for ozone and a new
NAAQS addressing PM2.5. For ozone, the revised NAAQS
provided for an 8-hour averaging period (versus 1 hour for the previous
NAAQS), and the level of the standard was changed from 0.12 ppm to 0.08
ppm (62 FR 38856). For the PM2.5 NAAQS, EPA established both
a new 24-hour standard and a new annual standard. In that Federal
Register, EPA also promulgated appendices I and N to 40 CFR 50.
Appendices I and N provided the methodologies for determining whether
an area is in attainment of the 8-hour ozone and PM2.5 NAAQS
respectively, using ambient air quality data. Section 1.0 of appendix
I, related to the ozone standard, addresses the treatment of data
determined to be influenced natural events, and section 1.0(b) of
appendix N, related to the PM2.5 standard, provides that EPA
may give special consideration to data determined to be affected by an
exceptional or natural event.
    Appendices K, I, and N, which are parts of the NAAQS for the
affected pollutants as described above, provide that, while States must
submit all valid ambient air quality data to EPA's Air Quality System
(AQS) database for use in making regulatory decisions, in some cases it
may be appropriate for EPA to exclude, discount, weight, or make
adjustments to data that have been appropriately flagged from
calculations in determining whether or not an area has attained the
standard. These decisions are to be made on a case-by-case basis using
all available information related to the event in question, and are
required to be made available to the public for review. It should also
be noted that, while it would be desirable to be able to adjust the
daily value to exclude only those portions of the data that are
attributable to the exceptional event, due to technical limitations,
such subtraction has not been possible, and EPA's historical practice
has been to exclude a daily measured value in its entirety when that
value is found to be largely caused by an exceptional event.
    Following the promulgation of the 8-hour ozone and the
PM2.5 NAAQS, EPA provided additional guidance to States on
how to address data affected by exceptional and natural events.\7\ That
guidance directed the States to follow three specific EPA guidance
documents in making determinations related to data influenced by
exceptional and natural events: (1) The Exceptional Events Policy; (2)
The PM10 Natural Events Policy; and (3) The Interim Air
Quality Policy on Wildland and Prescribed Fires, Memorandum from
Richard D. Wilson, Acting Assistant Administrator for Air and
Radiation, to EPA Regional Administrators, May 15, 1998. The Interim
Air Quality Policy on Wildland and Prescribed Fires addressed the
treatment of air quality monitoring data that are affected by wildland
and prescribed fires that are managed for resource benefits.\8\
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    \7\ ``Guideline on Data Handling Conventions for the PM NAAQS,''
United States Environmental Protection Agency, Office of Air Quality
Planning and Standards, Research Triangle Park, NC 27711, EPA-454/R-
98-017, December 1998.
    \8\ Following the promulgation of this rule, it is EPA's
intention to begin the process to revise the ``Interim Air Quality
Policy on Wildland and Prescribed Fires'' in calendar year 2007 to
update the policy and to ensure that the policy is consistent with
this final rulemaking action. In addition, it is EPA's intent that
agricultural prescribed burning will be addressed when this policy
is updated and will also address basic smoke management practices.
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IV. This Final Action

A. To Whom and to What Pollutants Does This Rule Apply?

    Under the statutory scheme established by the CAA, States are
primarily responsible for the administration of air quality management
programs within their borders. This includes the monitoring and
analysis of ambient air quality and submission of monitoring data to
EPA, which are then stored in EPA's AQS database. The EPA retains an
important oversight responsibility for ensuring compliance with CAA
requirements. With respect to the treatment of air quality monitoring
data, States are responsible for ensuring data quality and validity and
for identifying measurements that they believe warrant special
consideration, while EPA is

responsible for reviewing and approving or disapproving any requests
for such consideration. Therefore, this final rule applies to all
States; to local air quality agencies to whom a State has delegated
relevant responsibilities for air quality management, including air
quality monitoring and data analysis; and, as discussed below, to
Tribal air quality agencies where appropriate. This rule governs EPA's
actions in reviewing and approving or disapproving the relevant actions
taken or requested by States. Where EPA implements air quality
management programs on Tribal lands, this rule would govern those
actions as well.
    At present, only the NAAQS for ozone and particulate matter (PM)
contain provisions which allow for the special handling of air quality
data affected by exceptional and natural events (40 CFR part 50,
appendices K, I, and N). The language of section 319 of the CAA is
broad in terms of making its provisions applicable to events that
``affect air quality'' and to exceedances or violations of ``the
national ambient air quality standards'' (42 U.S.C. 7619(b)(1)(A)(i),
(b)(3)(B)(iv)). Thus, its provisions can apply to the NAAQS for any
criteria pollutant. Because the NAAQS established for other criteria
pollutants do not currently contain provisions permitting the
discounting or exclusion of data due to exceptional events, we are only
applying the provisions of this rule initially to ozone and PM.\9\ As
we review and consider the need for revisions to the NAAQS for other
pollutants, we will include provisions to address exceptional events in
those NAAQS in accordance with section 319, as appropriate at that
time. Because issuance of a new or revised NAAQS will necessitate the
initiation of the designation process, EPA believes that the NAAQS
rules are an appropriate place to make provisions for exceptional
events in the evaluation of air quality data. In the interim, where
exceptional events result in exceedances or violations of NAAQS that do
not currently provide for special treatment of the data, we intend to
use our discretion as outlined under section 107(d)(3) not to
redesignate affected areas as nonattainment based on these events. We
also intend to use our discretion under this rule to address
determinations for the ozone standard related to the treatment of data
influenced by both exceptional and natural events. Currently, appendix
I, only addresses the treatment of data determined to be influenced by
a stratospheric ozone intrusion and other natural events, but does not
address the handling of data influenced by other exceptional events.
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    \9\ Section IV.G of the preamble to the Proposed Rule discussed
special considerations relevant to a new NAAQS for
PM10-2.5 proposed by EPA on December 20, 2005. This
proposed standard would have drawn a distinction between coarse
particles of urban versus non-urban origin, which raised new issues
about the handling of exceedances of the coarse particle standard
caused by exceptional events. However, in EPA's final rule on the PM
NAAQS, issued September 21, 2006, EPA retained the existing 24-hour
PM10 standard instead of promulgating the proposed
PM10-2.5 standard. Thus, section IV.G of the preamble to
the Proposed Rule is no longer relevant and has been removed from
this Preamble.
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B. How Does This Rule Relate to Indian Tribes?

    Under the CAA and the Tribal Authority Rule (TAR), eligible Indian
Tribes may develop and submit Tribal Implementation Plans (TIPs) for
EPA approval, to administer requirements under the CAA on their
reservations and other areas under their jurisdiction. However, Tribes
are not required to develop TIPs or otherwise implement relevant
programs under the CAA. The EPA has stated that it will continue to
ensure the protection of air quality throughout the nation, including
in Indian country, and will issue Federal Implementation Plans (FIPs)
as necessary or appropriate to fill gaps in program implementation in
affected areas of Indian country (63 FR 7254, 7265; February 12, 1998).
    In cases where a Tribal air quality agency has implemented an air
quality monitoring network, which is affected by emissions from
exceptional events, the criteria and procedures identified in this
final rule may be used to exclude or discount data for regulatory
purposes. Certain Tribes may implement all relevant components of an
air quality program for purposes of meeting the various requirements of
this rule. In some cases, however, a Tribe may implement only portions
of the relevant program and may not be in a position to address each of
the procedures and requirements associated with excluding or
discounting emissions data (e.g., a particular Tribe may operate a
monitoring network for purposes of gathering and identifying
appropriate data, but may not implement relevant programs for the
purpose of mitigating the effects of exceptional events required under
this rule). The EPA intends to work with Tribes on the implementation
of this rule, which may include appropriate implementation by EPA of
program elements ensuring that any exclusion or discounting of data in
Indian country areas with air quality affected by exceptional events
comports with the procedures and requirements of this rule.

C. Comments Submitted on the Proposed Rule

    The proposed rule on the ``Treatment of Data Influenced by
Exceptional Events'' was issued on March 10, 2006 (71 FR 12592). We
received 98 letters from commenters representing 587 comments from
private citizens, State and local governments, industry, environmental
groups, and Federal agencies. Sections V, VI, VII, and VIII of this
notice describe the primary elements and requirements concerning the
process for the handling of data influenced by exceptional events. Each
section summarizes the relevant issues and options discussed in the
proposed rule and provides the final decisions related to the issues
for each section. In this preamble, we have provided responses to
certain significant comments to elaborate or provide clarification for
EPA's decision on an issue discussed in the relevant section of the
rule. We have developed a response to comments document which addresses
all of the timely comments received on the proposed rule. Following the
promulgation of this rule, the response to comments document will be
placed into the docket of this rulemaking action for public review (See
Docket No. EPA-HQ-OAR-2005-0159).

D. What Is an Exceptional Event?

    In accordance with the language in section 319, EPA is defining the
term ``exceptional event'' to mean an event that:

    (i) Affects air quality;
    (ii) Is not reasonably controllable or preventable;
    (iii) Is an event caused by human activity that is unlikely to
recur at a particular location or a natural event; and
    (iv) Is determined by EPA through the process established in
these regulations to be an exceptional event.

    It is important to note that natural events, which are one form of
exceptional events according to this definition, may recur, sometimes
frequently (e.g., western wildfires). For the purposes of this rule,
EPA is defining ``natural event'' as an event in which human activity
plays little or no direct causal role to the event in question. We
recognize that over time, certain human activities may have had some
impact on the conditions which later give rise to a ``natural'' air
pollution event. However, we do not believe that small historical human
contributions should preclude an event from being deemed ``natural.''
In adopting section 188(f) of part D, subpart 4, of the 1990 amendments
to

the CAA, Congress recognized and provided for distinctions between
these types of events with respect to waiver of applicable requirements
and the extension of otherwise applicable attainment dates for the
PM10 standard. In approving section 188(f) of the CAA, the
House committee of jurisdiction discussed a circumstance in which
recurring emissions from a source should be considered to be
anthropogenic. The House report noted EPA statements that, in the cited
case, high concentrations of dust from a lakebed were due to human
activity, i.e., the long-term diversion of water from a lake. (See Pub.
L. 101-549, CAA Amendments of 1990 House Report No. 101-290(l), May 17,
1990; and discussion of Mono Lake, California therein). Also, EPA
recognized, in recently acting to retain PM10 as a measure
of coarse particulate, that in some instances exceedances of this NAAQS
``may be caused in whole or in part, by exceptional events, including
natural events such as windstorms * * *. (and that) an exceedance may
be treated as an exceptional event even though anthropogenic sources
such as agricultural and mining emissions contribute to the
exceedance.'' (71 FR 61216; October 17, 2006).
    In this final rule, EPA also defines the term ``exceedance'' with
respect to compliance with the NAAQS and establishes criteria for
determining when an event can be said to ``affect air quality.'' We are
not finalizing more detailed requirements for determining when an event
is ``not reasonably controllable or preventable'' because we believe
that such determinations will necessarily be dependent on specific
facts and circumstances that cannot be prescribed by rule.

E. Examples of Exceptional Events

    The EPA believes that the following types of events meet the
definition of exceptional events, as defined above. This means that air
quality data affected by these types of events may qualify for
exclusion under this rule provided that all other requirements of the
rule are met. By providing the examples listed below, EPA is not
determining that such events are the only types of events that may
qualify for exclusion under the rule as exceptional events. Other
events that meet the statutory criteria for an exceptional event as
defined in this rule may also qualify for exclusion. The AQS user
documentation contains a list of other similar events that may be
flagged for special consideration. (http://www.epa.gov/ttn/airs/airsaqs/manuals/qualifiers.htm
).\10\

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    \10\ The EPA will be revising the list of events contained in
the AQS database following the promulgation of this rule to ensure
that the list is consistent with the requirements of the rule.
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    In addition, in the sections below, we have provided responses to
certain significant comments received during the comment period for the
proposed rule regarding the examples of events that may meet the
definition of an exceptional event in order to elaborate upon or
provide clarification about what constitutes an exceptional event.
1. Chemical Spills and Industrial Accidents
    Emissions that result from accidents such as fires, explosions,
power outages, train derailments, vehicular accidents, or combinations
of these may be flagged as an exceptional event.

Comments and Responses

    Comment: Several commenters stated that ``Chemical Spills and
Industrial Accidents'' should generally not be considered exceptional
events. Commenters stated that most industrial accidents and chemical
spills are reasonably controllable and preventable with proper planning
and mitigation efforts. These commenters stated that allowing for
accidents or spills that could have been avoided is inconsistent with
the CAA.
    Response: It is EPA's belief that air quality data that has been
affected by emissions from chemical spills, industrial accidents, or
structural fires may be flagged by a State as an exceptional event and
reviewed by EPA for exclusion on a case-by-case basis to determine
whether it meets the criteria for exceptional events as defined in this
rule. In particular, data influenced by chemical spills or industrial
accidents must be demonstrated to have ``affected air quality'' and
must be demonstrated to be due to circumstances that were not
reasonably controllable or preventable and are events that are unlikely
to recur in a particular location. The EPA agrees with the commenters
that industrial or point source emissions due to malfunctions or non-
compliance would not be considered exceptional events and should be
addressed through the normal State Implementation Planning process.
2. Structural Fires
    Structural fires include any accidental fire involving a manmade
structure.

Comments and Responses

    Comment: Several commenters indicated that ``Structural Fires''
should generally not be considered exceptional events. Commenters
stated that these types of events should be considered as emissions
from anthropogenic sources and handled within the form of the
respective air quality standards where a certain number of exceedances
of the standards are allowed over a 3-year period. Commenters assert
that structural fires, lasting for several hours, are unlikely to cause
an area to reach the level of nonattainment. In cases where structural
fires are determined to be the cause of a monitored violation of the
NAAQS, commenters stated that EPA should adopt a case-by-case review of
these events.
    Response: The definition of structural fires under this rule
pertains to any accidental fire involving a manmade structure. The EPA
believes that structural fires could be an exceptional event under this
rule, provided all other requirements of the rule are met, because they
could ``affect air quality,'' could be an event that is not
``reasonably controllable'' or ``preventable,'' and could be events
that are caused by human activity that are unlikely to recur at the
same location. However, EPA agrees with the commenters that these types
of events, as well as other similar types of events, should be reviewed
on a case-by-case basis to determine whether they meet the criteria for
an exceptional event as defined by this rule.
3. Exceedances Due to Transported Pollution
    Transported pollution, whether national or international in origin,
and whether from natural or anthropogenic sources, may cause
exceedances eligible for exclusion under this rule, as long as all of
the criteria and requirements related to exceptional events are met as
defined in this rule. For example, States may flag, and EPA may
exclude, data associated with fires occurring outside of the borders of
the United States, such as forest fires in Mexico, Central America, and
Canada; or transport events such as African dust and Asian dust which
contribute significantly to ambient concentrations of a pollutant in an
area, leading to exceedances or violations of a NAAQS. An example of
interstate transported emissions which may be flagged as due to an
exceptional event would be emissions due to smoke from wildfires or
wildland fire use fires which cause exceedances or violations of the
NAAQS at monitoring sites in other States. Other examples could include
data affected by emissions from mining and agricultural activities when
such emissions are subjected to long-range transport, and the criteria
and

requirements related to an exceptional event are met as defined in this
rule. In general, events due to transported pollution may be considered
on a case-by-case basis.

Comments and Responses

    Comment: Several commenters expressed concern over EPA allowing the
exceptional events rule to be used to exclude data that has been
affected by emissions emanating from sources outside the borders of the
United States.
    Response: States may flag data that has been affected by sources
emanating from outside the United States that meet the criteria for an
exceptional event as defined under this rule, including requirements
for causation and documentation. In cases where an area is impacted by
emissions from sources outside of the United States which do not meet
the criteria for an exceptional event under this rule, and these
emissions contribute to an area being designated as nonattainment, the
emissions may be addressed under section 179B of the CAA related to
``International Border Areas.'' Section 179B provides that where a
State is required to submit a State Implementation Plan (SIP) to
address issues related to a nonattainment designation, EPA may approve
the SIP for the area provided that the plan (1) meets all the
applicable requirements called for under the CAA, other than the
requirement that the plan demonstrate attainment or maintenance of the
NAAQS, and (2) the SIP must demonstrate that the affected area would be
able to attain the standard by the applicable attainment date ``but
for'' emissions emanating from outside the United States.
4. Exceedances Due to a Terrorist Attack
    Emissions that result from a terrorist attack such as smoke from
fires, dust, explosions, power outages, train derailments, vehicular
accidents, or combinations of these may be flagged as an exceptional
event.

Comments and Responses

    No comments were received on this topic.
5. Natural Events
    The natural events addressed by this final rule are: (1) Natural
disasters and associated cleanup activities; (2) volcanic and seismic
activities; (3) high wind events; (4) wildfires and wildland fire use
fires; and (5) stratospheric ozone intrusions. The EPA will consider
other types of natural events on a case-by-case basis.
a. Natural Disasters and Associated Clean-Up Activities
    For the purpose of flagging, major natural disasters such as
hurricanes and tornadoes for which State, local, or Federal relief has
been granted, and clean-up activities associated with these events, may
be considered exceptional events. The EPA believes that for a major
natural disaster, a timeframe up to 12 months is a reasonable time
period to allow for clean-up activities associated with these types of
activities. In cases where the damage caused by the event is so
substantial that a 12-month period is inadequate to address the clean
up that is necessary, a State may submit a request to EPA for an
extension of the 12-month time period. The EPA will grant requests for
extensions of the time period related to such events on a case-by-case
basis if the States submit adequate supporting information concerning
the reason for the extension as well as the length of time being
requested for the extension.

Comments and Responses

    Comment: Several commenters indicated that EPA should limit the
time period associated with clean-up activities due to a natural
disaster. One commenter indicated that the exceptional events rule as
proposed would allow States to apply the term ``natural disaster'' very
broadly to include circumstances that would circumvent the intent of
the CAA. For example, declaring an episode of high summer temperatures
to be a natural disaster could potentially allow a State to exclude
high ozone levels which commonly occur during hot weather.
    Response: A time period up to 12 months for clean-up activities is
permitted for major natural disasters, such as hurricanes and
tornadoes, for which State, local, or Federal relief has been granted,
may be flagged for exclusion as exceptional events under this rule. The
clean-up activities associated with these types of events may also be
flagged for exclusion as being due to an exceptional event. Given the
nature of a major natural disaster, the 12-month time period allowed
for clean-up activities following such disaster is a reasonable time
period, and is consistent with the time period being allowed for
volcanic and seismic activities under this rule. The period of high
summer temperatures noted in the comment would not represent a major
natural disaster, as described above, subject to the 12-month clean-up
period.
b. Volcanic and Seismic Activities
    Ambient concentrations of particulate matter for which volcanic or
seismic activity caused or significantly contributed to high levels of
particulate matter in an affected area will be treated as natural
events. While generally not occurring frequently, volcanic and seismic
activity can affect air quality data related to the particulate matter
NAAQS for an extended period of time after an event. Volcanic
activities can contribute to ambient concentrations in several ways: it
may influence concentrations of particulate matter due to primary
emissions (e.g., ash), and emissions of precursor pollutants (e.g.,
sulfur dioxide) that contribute to the secondary formation of
particulate matter. Seismic activity (e.g., earthquakes) can also
contribute to ambient particulate matter concentrations by shaking the
ground, causing structures to collapse, and otherwise raising dust
which may lead to exceedances or violations of the NAAQS.

Comments and Responses

    Comment: Several commenters indicated that the rule should provide
sufficient flexibility for data to be excluded where the duration of
the event may last for a long period of time. An example of such an
event is where volcanic activities last for several days.
    Response: The EPA agrees with the commenters and notes that the
rule allows for States to flag data and submit documentation related to
events such as long-term volcanic and seismic activities. States may
also submit requests to EPA to extend the time period up to 12 months
for major natural disasters, for clean-up activities following volcanic
and seismic events. States are encouraged to submit supporting
information related to the reasons for the requested extension and the
length of time being requested for the extension.
c. High Wind Events
    High wind events are events that affect ambient particulate matter
concentrations through the raising of dust or through the re-
entrainment of material that has been deposited. In some locations,
concentrations of coarse particles like PM10 are most likely
affected by these types of events, although PM2.5 standards
may be exceeded under such circumstances as well. Section VII.B. also
includes a discussion of this issue.

Comments and Responses

    Comment: Several commenters suggested that EPA replace the term

``high winds'' with the term ``wind-generated dust'' because (1) it
places the emphasis on the natural mechanism, (2) dust may become
entrained at relatively low wind velocities, and (3) the change will
eliminate confusion between the wind speeds associated with a natural
event and wind speeds needed to qualify for a ``high wind'' exceptional
event under EPA's 1986 guidance.
    Response: The EPA is retaining the term ``high wind'' event because
it accurately connotes the type of natural event that should be
excluded under this rule, as well as the action which caused the
exceedance or violation of the standard. The term also serves as an
indicator concerning the level of wind which caused the exceedance or
violation of the standard and indicates that it was unusually high for
the affected area during the time period that the event occurred.
Therefore, States must provide appropriate documentation to
substantiate why the level of wind speed associated with the event in
question should be considered unusual for the affected area during the
time of year that the event occurred. The EPA will evaluate such
instances on a case-by-case basis, including factors such as
historically typical windspeed levels for the season of the year that
the event is claimed.\11\
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    \11\ As described elsewhere in the preamble, EPA is adopting a
weight of evidence approach to demonstrate that an exceptional event
caused an exceedance or violation. Therefore, in instances where the
level of the wind speed results in exceedances or violations of
particulate matter, for data affected by these events to be
considered for exclusion under the weight of evidence approach, a
clear causal relationship must be demonstrated between the
exceedances measured at the air quality monitoring site and the high
wind event in question. EPA will consider in the weight of evidence
analysis winds that produce emissions contributed to by
anthropogenic activities that have been controlled to the extent
possible through use of all reasonably available reasonable and
appropriate measures.
---------------------------------------------------------------------------

d. Wildland Fires
    Federal land managers have afforded recognition to several
different types of wildland fires (i.e., wildfire, wildland fire use
fire and prescribed fire), depending on their causal circumstances and
the role that such fires play in the affected ecosystems. Prescribed
fire is addressed more fully in the following section.
    The question of what is a natural versus an anthropogenic fire has
particular significance when considering the impacts of wildland fires
(wildfire, wildland fire use fire and prescribed fire) on air quality
and how these impacts should be regarded under this rule. A
``wildfire'' is defined as an unplanned, unwanted wildland fire (such
as a fire caused by lightning), and include unauthorized human-caused
fires (such as arson or acts of carelessness by campers), escaped
prescribed fire projects (escaped control due to unforeseen
circumstances), where the appropriate management response includes the
objective to suppress the fire. In contrast, a ``wildland fire use''
fire is the application of the appropriate management response to a
naturally-ignited (e.g., as the result of lightning) wildland fire to
accomplish specific resource management objectives in predefined and
designated areas where fire is necessary and outlined in fire
management or land management plans.
    Using these definitions, we believe that both wildfires and
wildland fire use fires fall within the meaning of ``natural events''
as that term is used in section 319. Therefore, ambient particulate
matter and ozone concentrations due to smoke from a wildland fire will
be considered for treatment as an exceptional event if the fire is
determined to be either a wildfire or wildland fire use fire.

Comments and Responses

    Comment: In general, commenters strongly supported exempting
wildfires as exceptional events under the rule.
    Response: The EPA acknowledges support for the proposal to classify
wildfires as a potential exceptional event. As noted above, the Agency
states that wildland fires will be excluded as exceptional events if
they meet the criteria and requirements of the exceptional events rule.
    Comment: The Agency received comments both supporting and opposing
the proposal allowing wildland fire use fires to qualify as an
exceptional event.
    Response: After reviewing Congress' revisions to section 319, the
various Agency policies cited in the proposal, and comments received,
the Agency has determined that wildland fire use fires may also qualify
as an exceptional event. However, these types of fires must also meet
certain criteria. For example, these fires must occur on lands that
have been designated in fire management or land management plans as
areas where fires are necessary and desirable to accomplish specific
resource management objectives.
    Comment: Many commenters supported EPA's commitment to update the
1998 Interim Air Quality Policy on Wildland and Prescribed Fires to be
consistent with this rule.
    Response: The Agency plans to begin revising this policy in 2007 as
part of its overall Fire Strategy after promulgation of this rule.

e. Stratospheric Ozone Intrusions

    Stratospheric ozone intrusion is considered to be a natural event.
A stratospheric ozone intrusion occurs when a parcel of air originating
in the stratosphere, which is at an average height of 20 km or 12.4
miles, is transported directly to the surface of the earth.
Stratospheric ozone intrusions are very infrequent, localized events of
short duration. They are typically associated with strong frontal
passages and, thus, may occur primarily during the spring season.

Comments and Responses

    Comment: One commenter stated that EPA should update its approach
to stratospheric events, establish criteria by which such events may be
determined, and credit States for the impact of intrusion events on
non-compliant ozone monitor readings.
    Response: Stratospheric ozone intrusion is identified as a natural
event under 40 CFR part 50, appendix I, for ozone, and will be
considered for treatment as an exceptional event.
6. Prescribed Fire
    A ``prescribed fire'' is defined as any fire ignited by management
actions to meet specific resource management objectives. According to
existing Federal policy, prior to ignition a prescribed fire must have
an approved prescribed fire plan and must meet the National
Environmental Policy Act (NEPA) requirements (where applicable)(see
National Wildland Fire Coordination Group Glossary of Wildland Fire
Terminology, 2003). For purposes of section 319, a prescribed fire
cannot be classified as ``natural,'' given the extent of the direct
human causal connection, however, a prescribed fire may meet the
statutory criteria defined in section 319 of ``affect[ing] air
quality,'' being ``unlikely to recur at a particular location'' and is
``not reasonably controllable or preventable.'' The determination of
whether a prescribed fire can be considered an exceptional event should
be made on a case-by-case basis taking into account the factors
described below.
    A prescribed fire carried out for resource management objectives is
frequently designed to restore essential ecological processes of fire
and mimic fire under natural conditions. As such, a prescribed fire's
expected frequency can vary widely, depending on the natural fire
return interval of a particular landscape or wildland ecosystem. The
natural fire return

interval can range from once every year to less frequently than once in
more than 200 years. Thus, in many, though not all cases, it may be
possible to demonstrate that the likelihood of recurrence is
sufficiently small enough to show that a prescribed fire under these
conditions meets the ``unlikely to recur at a particular location''
requirement of the statutory language.
    A prescribed fire may also meet the condition of ``not reasonably
controllable or preventable'' by examining whether there are reasonable
alternatives to the use of fire in light of the needs and objectives to
be served by it. For instance, there may be a significant build-up of
forest fuels in a particular area that if left unaddressed would pose
an unacceptable risk of catastrophic wildfire, which could result in
adverse impacts of much greater magnitude, duration, and severity than
would result from careful use of prescribed fire. A particular
ecosystem may also be highly dependent on a natural fire return
interval to maintain a sustainable natural species composition.
Alternatively, pest or disease outbreaks in an area may be such that
there are no reasonable alternatives to prescribed fire. In some cases,
other legal requirements may preclude the use of mechanical fuel
reduction methods such as in designated wilderness or National Parks.
Where such ecological conditions exist, or where mechanical or other
treatments are not reasonably feasible for reasons that include, but
are not limited to, a lack of access, or severe topography, we believe
that prescribed fire qualifies as being ``not reasonably controllable
or preventable.'' Thus, we believe that a prescribed fire, conducted by
Federal, State, Tribal or private wildland managers or owners, under
the conditions described above may qualify as an exceptional event.
    In addition, one of the principles contained in SAFE-TEA-LU,
section 6013(b)(3)(A), includes the principle that States must take
necessary measures to safeguard public health regardless of the source
of air pollution. We believe it reasonable to tie the qualifying
criteria for an anthropogenically generated prescribed fire to State
accountability for public health protection. Consistent with historical
practice governed by the guidance contained in the ``Interim Air
Quality Policy on Wildland and Prescribed Fires,'' issued on May 15,
1998, EPA approval of exceedances linked to a prescribed fire used for
resource management purposes is contingent on the State certifying that
it has adopted and is implementing a Smoke Management Program (SMP) as
described in that policy. A State SMP establishes a basic framework of
procedures and requirements for managing smoke from a prescribed fire
managed for resource benefits. A SMP is typically developed by a State
or Tribe with cooperation and participation by wildland managers, both
public and private, and the general public. As reflected in the Interim
Air Quality Policy on Wildland and Prescribed Fires, States are
provided flexibility on the structure of a SMP. Thus, a SMP can be
extensive and detailed, or simply identify the basic smoke management
practices for minimizing emissions, and controlling impacts from a
prescribed fire.\12\ In the proposal to this rule, EPA proposed to
continue the use of that approach. We also proposed to expand the
criteria for contingent approval to a prescribed fire where, in lieu of
a SMP, basic smoke management practices, that minimize emissions and
control impacts, are being employed by burners. In order to protect
public health in areas where a SMP has not been adopted, in the final
rule, the Agency has elected to expand, on a case-by-case basis, the
qualifying criteria by which a prescribed fire may qualify as an
exceptional event. In those cases, the Agency will judge on a case-by-
case basis whether the State has ensured that appropriate basic smoke
management practices have been employed in determining whether the
prescribed fire qualifies as an exceptional event. If an exceptional
event occurs using the basic smoke management practices approach, the
State must undertake a review of their approach to ensure public health
is being protected and must include consideration of development of a
SMP.
---------------------------------------------------------------------------

    \12\ Basic smoke management practices could include, among other
practices, steps that will minimize air pollutant emissions during
and after the burn, evaluate dispersion conditions to minimize
exposure of sensitive populations, actions to notify populations and
authorities at sensitive receptors and contingency actions during
the fire to reduce exposure of people at such receptors, identify
steps taken to monitor the effects of the fire on air quality, and
identify procedures to ensure that burners are using basic smoke
management practices.
---------------------------------------------------------------------------

Comments and Responses

    Comment: Several commenters supported classifying prescribed fire
as qualifying as an exceptional event. However, some commenters
indicated that there should be limitations placed on when this type of
fire should be considered as an exceptional event. A number of
commenters also disapproved of allowing prescribed fire to be
considered as an exceptional event because they believe that this type
of fire is anthropogenic and does not meet the statutory definition of
exceptional event. Some commenters also favored expanding the criteria
for contingent approval to include instances where basic smoke
management practices are used in lieu of a SMP, while other commenters
did not favor this expansion.
    Response: The EPA believes that a prescribed fire may be excluded
as an exceptional event under this rule only in cases where the event
meets the criteria for an exceptional event as defined in this rule, if
documentation is submitted to show that the fire meets the requirement,
as described above, of ``affect[ing] air quality,'' being ``not
reasonably controllable or preventable'' and ``unlikely to recur at
location'' and provided the other requirements of the rule including,
among others, the schedules and procedures for flagging and
demonstration are met. In those instances where a prescribed fire meets
the criteria for an exceptional event, the State must also provide
appropriate documentation to show that a certified SMP was in place or
that the burner employed basic smoke management practices and that the
appropriate practices were being followed at the time that the event
occurred. Because a prescribed fire is an anthropogenic source of
emissions for purposes of section 319, even though it may qualify as an
exceptional event, a State can attempt to limit the health impact of a
prescribed fire through the thoughtful development and implementation
of a SMP or ensuring that basic smoke management practices were
employed that minimize emissions and control impacts from prescribed
fires.

V. The Management of Air Quality Data Affected by Exceptional Events

    The EPA proposed that, in order to exclude air quality data from
consideration for regulatory purposes, States must follow the
procedures, timelines, and other requirements described in the proposed
rule. Under the Final Rule, if an event is determined to be a
qualifying exceptional event according to section IV.D, a State, Tribe,
or designated local agency may petition EPA to classify the event as
exceptional and submit a demonstration to justify data exclusion.\13\
For data exclusion, States must clearly identify, or ``flag,''

data they believe to be influenced by such events. The demonstration to
justify data exclusion shall provide evidence that: (a) The event
qualifies in accordance with section IV.D and with EPA policies and
guidance for certain events as described in section IV.E, (b) there is
a clear causal relationship between the measurement under consideration
and the event that is claimed to have affected the air quality in the
area, (c) the event is associated with an unusual measured
concentration beyond typical fluctuations including background, and (d)
that there would have been no exceedance or violation but for the
event.
---------------------------------------------------------------------------

    \13\ Although a single qualifying exceptional event may affect
air quality for multiple days and at multiple monitors, the
discussions below consider an individual demonstration as justifying
exclusion of a single AQS data point. The EPA encourages State
submittals to package demonstrations about single exceptional events
to expedite the review process.
---------------------------------------------------------------------------

    The SAFE-TEA-LU requirements for exclusion of data from exceptional
events are: (1) The occurrence of the exceptional event must be
demonstrated by reliable and accurate data; (2) the State must show
that there is a ``clear causal relationship'' between the NAAQS
exceedances and the event; (3) there must be a public review process
related to the exceptional event determination; and (4) the rule must
set criteria and procedures for States to petition EPA to exclude data
directly affected by an exceptional event. The sections below describe
how each of these requirements must be met.
    The sections below address the flagging of data as exceptional
events that are determined to have affected air quality, submittal of
demonstrations to request data exclusion, public review, and the
schedule and timing for these processes. After an exceptional event
occurs (judged according to section IV.D) and an agency determines that
the event affected ambient air quality, flagging may occur according to
section V.A. Section V.B describes the evaluation of whether or not the
event affected ambient air quality. Section V.C describes the necessary
``but-for'' test that data would have complied with the applicable
standard but for the occurrence of the exceptional event. Section V.D
explains the schedules and procedures for the flagging and
demonstration submittals, section V.E discusses the applicability to
hourly readings, section V.F states the requirements for determination
submittals if the agency requests EPA to exclude the data from
consideration for regulatory purposes, and section V.G describes the
public review requirements. Some commenters suggested that all data
occurring from exceptional events should be flagged, and EPA will allow
these flags for informational purposes, even if the data do not qualify
for exclusion. If EPA concurs on the exclusion of data from qualifying
exceptional events, the data will be excluded from regulatory
consideration but will still count toward data capture requirements.

A. Flagging of Data in the AQS Database

1. Background
    Air quality data are required, pursuant to 40 CFR 58.16, to be
submitted to EPA by each State on a calendar quarterly basis, with
submissions due not later than 90 days after the end of a quarterly
reporting period. Once air quality data have been submitted to EPA, it
is possible to ``flag'' specific values for various purposes. ``Data
flagging'' refers to the act of making a notation in a designated field
of an electronic data record. The principal purpose of the data
flagging system in the AQS database is to identify those air quality
measurements for which special attention or treatment is warranted.
These include, but are not limited to, those measurements that are
influenced by exceptional events.
    The preamble to the proposed rule stated: ``In the case of
exceptional events, States place the initial flag on the data in the
AQS database. Following an evaluation of the supporting documentation,
EPA will decide whether to concur with the flag; concurrence will be
marked by the placement of a second flag in the AQS database by EPA.
Once EPA has concurred on the flag, the data will be excluded from
regulatory decisions such as determinations of attainment or
nonattainment.''
     ``While the flagging of data by the State is the first
step in an exceptional events demonstration, it is insufficient by
itself to allow for the exclusion of data. In order to have EPA concur
on a flag, States must meet the additional requirements described
below. As stated previously, the State has the responsibility to
document both the occurrence of the event and the causal connection to
the monitoring data under consideration. Because the initial step of
flagging the data is a relatively simple one, States may flag many more
days than the number of days for which they ultimately submit
documentation to support exclusion.''
2. Final Rule
    In the case of exceptional events, States and Tribes place the
initial flag on the data in the AQS database, but EPA determines the
available flags.\14\ States may also delegate authority to local
agencies to submit flags and documentation. In any event, States should
work with their local agencies for the identification and review of
exceptional events and consider requests to flag data from those
agencies. At the time the flag is inserted into the AQS database, the
State must also provide an initial description of the event in the AQS
comment field. This initial description should include such information
as the direction and distance from the event to the air quality monitor
in question, as well as the direction of the wind on the day in
question. The flags, and the initial event description, must be
inserted into the AQS database prior to July 1st following the year in
which the event occurred. Schedules for demonstrations are discussed in
section V.D.
---------------------------------------------------------------------------

    \14\ It is EPA's intention, for purposes of consistency with
this rule, to review the list of exceptional events that are
currently in the AQS database following the promulgation of the
rule.
---------------------------------------------------------------------------

    Following an evaluation of the supporting documentation, EPA will
make a decision concerning whether to concur with the flag; concurrence
will be marked by the placement of a second flag in the AQS database by
EPA. If EPA has concurred on the flag, the data will be excluded from
regulatory determinations such as determinations related to attainment
or nonattainment, or determinations concerning SIP development. The EPA
will use the second flag to indicate the following conditions: EPA
concurrence, EPA non-concurrence, and documentation submitted with EPA
decision pending.
    While flagging of the data in the AQS database by the affected
State, local, or Tribe authority is the first step in an exceptional
events demonstration, it is insufficient in and of itself to allow for
the exclusion of data. In order for EPA to concur on an exceptional
events flag, States, Tribes, and local agencies must meet the
additional requirements described below. As explained, the State,
Tribe, or local agency has the responsibility to document the
occurrence of the event in question, to demonstrate that the event
qualifies as an exceptional event in accordance with section IV.D, is
consistent with EPA policies and guidance for certain events as
described in section IV.E, has provided for public review in accordance
with section V.G, and to document the causal connection between the
measurement under consideration and the event that is claimed to have
affected the air quality in the area. The State, Tribe, or local agency
must also demonstrate that the event is associated with an unusual
measured concentration beyond typical fluctuations including
background, and that there would have been no exceedance or violation
``but for'' the event. Because the initial step of

flagging the data is a relatively simple one, States, Tribes, and local
agencies may flag more days than the number of days for which they
ultimately intend to submit demonstrations to justify data exclusion.
3. Comments and Responses
    Comment: One commenter supported flagging data related to any fire
that caused an exceedance.
    Response: This Rule does not preclude a State, Tribe, or Local
agency from flagging any data allegedly influenced by exceptional
events. However, for the data to qualify as an exceptional event and to
exclude it from regulatory decisions, the data must meet all of the
criteria described in this Rule and all the procedures delineated must
be followed.

B. What Does It Mean for an Event To ``Affect Air Quality''?

1. Background
    It is important to recognize that any emissions-producing event has
the potential to have some influence on downwind air quality. Indeed,
on any given day, measured air quality at any given location will
reflect the influences of a variety of activities, including both
natural and anthropogenic emissions from both local as well as remote
upwind sources. Given the directive in section 319(b)(3)(B)(ii), that a
clear causal connection must exist between the ``measured exceedances''
and the exceptional event, EPA believes that it would be unreasonable
to exclude data affected by an exceptional event simply because of a
trivial contribution of an event to air quality. Furthermore, we
believe that it would be unreasonable to exclude more significant, but
routine background air quality impacts, as this would disregard an
important part of the public's exposure to air pollution upon which
EPA's air quality standards are based. The effect of such exclusion
would be an inappropriate reduction in the stringency of the NAAQS,
rather than providing specific relief under the circumstances provided
in section 319 for which States should not be designated nonattainment
or be required to prepare costly SIP control strategies.
    Neither section 319, nor its legislative history, provides precise
guidance on what should be considered when determining whether an event
``affects air quality'' and thus qualifies to be considered for
exclusion or special treatment. However, section 319(b)(3)(B)(ii) and
(iv) provides that there must be a ``clear causal relationship''
between a measured exceedance of a standard and the event to show that
the event ``caused a specific air pollution concentration;'' and it
must be shown that the data in question are ``directly due'' to an
exceptional event. Moreover, one of the principles provided by section
319(b)(3)(A) indicates that the protection of public health is the
highest priority. For these reasons, we proposed three conditions under
which an event may qualify as ``exceptional'' for purposes of special
regulatory consideration: Its air quality impact must (1) fall both
above the level of the applicable standard (i.e., must be an
``exceedance'' as required by section 319), (2) be significantly beyond
the normal fluctuating range of air quality, including background air
quality concentrations, and (3) should be large enough such that
without it there would have been no exceedance.
    We next provided several alternative approaches to determining
whether and when air quality is ``affected by'' exceptional events and
requested comment on which of these approaches was most suitable for
demonstrating such impacts. These approaches primarily applied to
condition (2) above. Two of the approaches involved statistical
comparisons of existing flagged data. The final rule most closely
reflects the third proposed option with some modifications. This option
considered a case-by-case evaluation of the data against historical,
seasonally adjusted air quality levels. Finally, the proposed rule
provided details regarding what is meant by an exceedance (1) and the
``but-for'' condition (3). These are discussed in detail in section
V.C.
2. Final Rule
    Under the Final Rule, the demonstration to justify data exclusion
must provide a justification that: (a) The event qualifies in
accordance with section IV.D. and if applicable, with EPA policies and
guidance for certain events as described in section IV.E, (b) there is
a clear causal relationship between the measurement under consideration
and the event that is claimed to have affected the air quality in the
area, (c) the event is associated with an unusual measured
concentration beyond typical fluctuations including background, and (d)
there would have been no exceedance or violation but for the event
(discussed in section V.C). The second and third criteria establish
that the event affected air quality.
    The second criterion that the event caused an air quality impact
may be shown through a number of methods including, but not limited to,
modeling and speciation analysis. The third criterion distinguishes
common events from those that are exceptional and may be accomplished
through the presentation of historical evidence.
    The final rule permits a case-by-case evaluation, without
prescribed threshold criteria, to demonstrate that an event affected
air quality. This demonstration would be based on the weight of
available evidence, but must consider the historical frequency of such
measured concentrations. While a State may determine the specific
approach to use for such analysis, it must compare contemporary
concentrations with the distribution of all measured data during the
past several years. The evidence that an event affected air quality may
be presented on a seasonal or other temporal basis to best compare
contemporary concentrations with the distribution of historical values.
For consistency with data reporting and computation of NAAQS
statistics, a calendar quarter basis is suggested. Baseline data may
also be defined differently for each event type (e.g., April and May
data may be the most relevant information for statistical comparison
with certain dust events).
    The general statistical approach of using all measured data during
the past several years is independent of historical flagging practices
and allows States to accurately represent events not likely to recur by
including all monitoring data in analyses.
    In addition, the magnitude of the measured concentration on days
affected by exceptional events relative to historical, temporally
adjusted air quality levels can guide the level of necessary analysis
and documentation to demonstrate that the event affected air quality.
For extremely high concentrations relative to historical values (e.g.,
concentrations greater than the 95th percentile), a lesser amount of
documentation or evidence may be required to demonstrate that the event
affected air quality. The closer the event concentration is to typical
levels (e.g., values less than the historical 75th percentile), the
stronger the necessary evidence would have to be to justify exclusion
of data for regulatory purposes. This weight of evidence approach is
most nearly analogous to our historical treatment of exceptional
events.
3. Comments and Responses
    Comment: One commenter noted that EPA's proposed rule concedes that
the third option would ``provide the least definitive guidance to
assist States in their evaluations,'' and ``may make it

difficult for EPA regions to be consistent when determining whether to
concur on a flag.'' Moreover, ``the case-by-case approach allows for
consideration of days with ambient concentrations which are not
necessarily among the highest concentrations that have been
historically observed. While such days are unlikely to impact short-
term standards, discounting such days can certainly have an impact on
an annual average concentration.'' The commenter asserted that EPA's
description of the proposed case-by-case evaluation makes the case for
rejecting that option because it fails to provide the guidance mandated
by section 319, and is so vague as to be arbitrary.
    Response: The EPA disagrees with the commenter that this option
fails to provide guidance and is so vague as to be arbitrary. The EPA
has explained above the criteria that it will use in making its case-
by-case evaluations. The commenter's concern that the event must
represent concentrations that are not typically observed is addressed
by the third criterion that the event must be associated with an
unusual measured concentration beyond typical fluctuations including
background. Demonstration of the magnitude of the measured
concentrations with respect to historical frequency under similar
conditions will provide a new level of consistency across monitoring
locations.
    Comment: If an area exceeds the NAAQS, one commenter stated that
use of a 95th percentile criterion better ensures that the definition
of an exceptional event is met (i.e., unlikely to recur at a particular
location).
    Response: The EPA recognizes that extreme concentrations (e.g.,
corresponding to values greater than the 95th percentile of historical
values) are more likely associated with exceptional events. With the
final rule, we are not assuming that such values are definitely
exceptional. In fact, some extreme concentrations may be associated
with various emission sources and atmospheric conditions which are
unrelated to a causal connection to the claimed exceptional event.
Instead, the frequency of occurrence relative to historical
concentrations would be used as an important part of the overall weight
of evidence to demonstrate the exceptional nature of the claimed air
quality impact.

C. Use of a ``But For'' Test

1. Background
    There may be instances in which exceptional events may have a
significant impact on air quality on days when concentrations are
already above the applicable standard in the absence of the influence
of such events. In such cases, it is important to preserve and consider
all valid air quality data influenced by such activities, which
properly fall within the responsibilities of States to manage for
purposes of air quality attainment and maintenance. For this reason, we
proposed to require that air quality data may not be excluded except
where States show that exceedances or violations of applicable
standards would not have occurred ``but for'' the influence of
exceptional events.
    In other words, to the extent that it is possible to determine that
the resulting air quality concentrations and appropriate design values
for an area would be above the level of the standards even without the
influence of the exceptional event, the air quality data for the day(s)
in question should not be excluded. However, consideration of the
impacts of exceptional events on air quality values for control
strategy planning purposes may be appropriate, and States are
encouraged to consult with the appropriate EPA regional office to
further discuss this issue.
2. Final Rule
    The EPA will maintain the proposed ``but-for'' requirement that air
quality data may not be excluded except where States, Tribes, or local
agencies show that exceedances or violations of applicable standards
would not have occurred ``but for'' the influence of exceptional
events. Through analyses, it is possible to demonstrate that an
exceedance or violation would not have occurred but for the event [See
sample ``but-for'' analysis in memo to docket, Husar et al. 2006
(http://www.regulations.gov, EPA-HQ-OAR-2003-0061-0733 thru 0733.5)].

This analysis does not require a precise estimate of the estimated air
quality impact from the event. The 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.
    Because there are two standards for PM2.5, clarification
is needed regarding the measurements that contributed to an exceedance
or a violation that are eligible to be excluded. This rule is limited
to values above the annual standard for PM2.5 because this
simplifies the process for determining which values are eligible for
flagging according to the intent of section 319. The short-term
PM2.5 NAAQS is based on a 3-year average of the annual 98th
percentile of 24-hour values. Therefore, it is possible that one or two
of these annual concentration values may be below the level of the
NAAQS while the 3-year average is above the level of the NAAQS. Because
three annual 98th percentile concentration values are included in the
determination of a short-term PM2.5 NAAQS violation,
individual measurements below the NAAQS may contribute to a violation.
    On the other hand, the annual PM2.5 NAAQS is also a
standard based on a 3-year average. However, violations of the annual
standard that are caused by measurements which are not exceedances of
that standard will be difficult to distinguish from typical air quality
concentrations including background. To accommodate the 3-year form of
the PM2.5 NAAQS, this rule will allow measurements whose
concentrations are greater than the level of the annual NAAQS to be
flagged as being affected by exceptional events for the purposes of
contributing to an exceedance or violation of the PM2.5
NAAQS. Thus, we provide the following clarification that individual
measured values greater than the annual PM2.5 NAAQS will be
considered ``exceedances'' under this rule and therefore eligible to be
considered for exclusion for comparisons to either the annual or 24-
hour NAAQS.
3. Comments and Responses
    Comment: One commenter stated that, while some of those
measurements may not individually be above the NAAQS, taken together
they might be sufficient to put an area in violation of an annual
standard. Any ``but for'' determination must take into account the
aggregate of exceptional events that occurred within the applicable
NAAQS period.
    Response: The rule acknowledges that it is possible that an event
can affect multiple days. The ``but for'' provision allows for data
exclusion if but for the entire event there would have been no
exceedance or violation. Therefore, for those events that can be shown
to affect air quality on multiple consecutive days, measurements for
the entire period are eligible for data exclusion, provided that at
least one measurement day during the episode is an exceedance as
defined by this rule and the air quality impact on each day are
considered exceptional.
    Comment: One commenter cautioned EPA about using the phrase ``to
the extent it is possible to determine'' because a ``bright line''
distinction between the contribution from natural and anthropogenic
sources often does not exist.
    Response: We agree with this comment and for this reason we will
permit a weight of evidence-based

approach to demonstrate that there would not have been an exceedance or
violation but for the event.

D. Schedules and Procedures for Flagging and Requesting Exclusion of
Data

1. Background
    In establishing procedures and time tables for States to request,
and EPA to grant, exclusion of data affected by exceptional events, we
are guided by two competing considerations: Ensuring States have
adequate time and opportunity to compile and evaluate all relevant and
available information in support of such requests; and making
determinations in a timely manner so that all pertinent and valid air
quality data would be appropriately considered in regulatory
determinations. To assist EPA in determining the best approach to
managing the data flagging process and submissions of demonstrations
for the final rule, we proposed three alternatives for public review
and comment. Public comments showed that each option had desirable
aspects, and these are incorporated into the final rule.
2. Final Rule
    A multi-step process will be established for identification of data
and submission of demonstrations. The process is designed to ensure
that States, Tribes, and local agencies have adequate opportunity to
compile and present evidence of exceptional and natural events but also
ensures timely submittals in order to make regulatory decisions and
ensure the protection of human health through NAAQS determinations. The
steps include State flagging, annual State submission of an initial
event description, State submission of a demonstration to justify data
exclusion and EPA review followed by approval or disapproval. Where air
quality in an area is influenced by a relatively small set of emission
sources with well-defined emission profiles and limited pollutant
species, a demonstration that an air quality measurement influenced by
a particular event merits exclusion may be relatively simple to make.
In other cases, such as where the number and types of sources
contributing to measured air quality concentrations are extremely
complex and varied, making it more difficult to distinguish between the
effects of routine activities and unusual ones, more time and effort
will be needed for a State, Tribe, or local agency to provide an
adequate demonstration in support of its request.
    States, Tribes, and local agencies are encouraged to flag the data
that they believe to be affected by exceptional events at the time of
submission of the air quality data to EPA's AQS database, in accordance
with the schedule described in 40 CFR 58.16, which is generally no
later than 90 days after the end of the calendar quarter. This includes
both flagging of data and insertion of the initial event description
into the AQS comment field. This constitutes notification of the
appropriate EPA Regional Office concerning the State's intention to
seek exclusion of data. This approach would ensure that the flagging
process remains consistent with the timeline set forth in rules
governing data submission requirements. The EPA recognizes that
laboratory analyses may delay these submissions and therefore is
extending the required time period for submission to 180 days after the
end of the calendar year (i.e., all flags, along with initial event
descriptions, for a calendar year must be reported by July 1 of the
following year).
    We encourage States, Tribes, and local agencies to submit the
demonstration to justify data exclusion annually for exceedances of
short-term NAAQS by July 1. However, the demonstration to justify data
exclusion must also be submitted no later than 12 months prior to a
regulatory decision. For all flagged events, the demonstration to
justify data exclusion must be submitted within 3 years of the calendar
quarter following an event, but no later than 12 months prior to a
regulatory decision. This period should be used primarily to support
NAAQS compliance with annual averages and violations of the short-term
standard that were not anticipated. For nonattainment designations,
this would occur with the Governor's letter recommending the list of
nonattainment areas. We also recognize that special circumstances could
dictate more expedited data delivery, flagging, and minimal
demonstrations (e.g., PM2.5 designations using 2002-2004
data). The submitted demonstration to justify data exclusion as well as
the EPA responses and the rationale for the EPA decision will be made
publicly available through EPA. The reason for providing the 3-year
timeframe is that for ozone and PM, decisions regarding whether or not
an area is attaining the applicable standard are based on the most
recent 3 years of air quality data. Providing 3 years for submission of
demonstrations would provide States, Tribes, and local agencies with an
opportunity to evaluate whether the influence of one or more
exceptional events will be relevant to determinations of attainment or
nonattainment before undertaking the effort of preparing and submitting
demonstrations.
    Once EPA receives a State's demonstration, EPA generally will
undertake to review the demonstration and provide a concurrence or
nonconcurrence on the flag in the AQS database within 60 days. The EPA
expects that, in most cases, this time period should be enough time to
review and provide a concurrence or non-concurrence related to a
State's request to exclude data affected by an exceptional event.
However, for more complex demonstrations, EPA may require additional
time to make its decision and will notify the State of the additional
time required.
3. Comments and Responses
    Comment: One commenter supported arguments on why the proposed rule
must include a procedure for retrospective flagging that addresses the
full set of the State's needs so that the end result is that the State
can flag any and all events impacted by natural events.
    Response: With the Final Rule, EPA requires annual submittal of
flags. States may, if they so choose, submit them sooner. This schedule
ensures that data are collected and retained shortly after the event
and identification of potential (non-routine) events is done in a
timely fashion to ensure that appropriate corrective actions can be
taken. States would only maintain minimal documentation supporting the
decision to flag the data. The full demonstrations, however, can come
later, in order to allow States time to focus efforts on those events
that are determined to have an impact on attainment. The Agency notes
that the Exceptional Events Rule does not apply to routine natural
events that are part of background air quality.
    Comment: One commenter was concerned that a State may have failed
to flag data impacted by a natural event because the data values were
below the current NAAQS, only to find the State threatened with
nonattainment after NAAQS revisions.
    Response: For data collected before the effective date of this
rule, States may include a demonstration to justify data exclusion with
the Governor's recommendation letter on nonattainment areas, provided
that there was notice and opportunity for public comment. After
considering this and other comments, for PM2.5 data
collected during calendar years 2004-2006, that the State identifies as
resulting from an exceptional event,

EPA is permitting the State to flag and submit an initial description
of the event provided that these are submitted no later than October 1,
2007. In cases where the State is able to show that this time period is
inadequate, a State may submit a request for an extension and EPA will
grant this request for an extension up to but no later than December 1,
2007. This procedure should accommodate States concerned about
potential PM2.5 nonattainment areas using the 2004-2006 data
sets. The EPA may consider a similar exemption of the schedules for
submittal of data for future revision of standards.
    Comment: One commenter stated that EPA should also make allowances
for those situations when a State neglects to flag a value or submit
documentation within the required timeframes. In these cases, the
commenter asserted that EPA should provide some type of petitioning
process.
    Response: If a State fails to meet the schedule for flagging or
document submittal, late petitions will not be considered. Policy
decisions, SIP planning, and dissemination of data should not be
delayed or altered based on a State's failure to submit documentation
or follow the regulatory procedures in a timely manner.

E. Exclusion of Entire 24-Hour Value as Opposed to a Partial Adjustment
of the 24-Hour Value

1. Background
    In general, EPA's historical practice has been to exclude a daily
measured value in its entirety when an exceptional event causes that
value, and we retained this approach in the proposed rule. With this
approach, a determination is made that emissions from the event are
largely responsible for the resultant ambient air pollutant
concentration. For example, if the observed concentration is 200 [mu]g/
m3 for PM2.5 and is associated with a nearby
forest fire, then EPA is likely to concur with the claim that the event
was responsible for the ambient concentration. The measured value would
be excluded in its entirety from the data used to judge attainment (as
per 40 CFR 50, appendix N), although the measurement would still count
towards meeting minimum data capture requirements.
    We believe it would be desirable to adjust the daily value to
exclude only those portions of the data that are attributable to the
exceptional event in question, and to retain the remainder of the day's
measurement if appropriate and accurate methods were available to make
such adjustments. For example, if an area affected by a wildfire had a
measured 24-hour PM2.5 concentration of 50 [mu]g/
m3 and the estimated event impact was 30 [mu]g/
m3, then the expected value that would have occurred but for
the event would have been 20 [mu]g/m3. Normal air quality
for this location might be 16 [mu]g/m3 and, therefore, the
``but-for'' concentration of 20 [mu]g/m3 is above average.
Discounting the entire event day could, therefore, inappropriately bias
a determination of nonattainment with the annual PM2.5 NAAQS
(currently set at 15 [mu]g/m3). We are currently seeking to
develop and evaluate new analytical methods that would allow us to
discount only the portion of the daily value attributable to the
exceptional event. However, at present, we are not aware of the
existence of precise and universally applicable techniques that are
administratively and technically feasible and that could support
partial adjustment of air quality data except perhaps in limited cases,
such as where the number and type of pollutant species and contributing
sources are relatively less complex or potentially when sufficient
spatial, temporal, meteorological and chemical data are available [See
memo to docket, Husar et al. 2006, (http://www.regulations.gov, EPA-HQ-

OAR-2003-0061-0733 thru 0733.5)]. When we determine that techniques for
adjustment of air quality data are sufficiently well-demonstrated for
use in exceptional events determinations, we will publish a notice of
proposed rulemaking to seek comment on the appropriateness and scope of
such use and its impact on the requirements set forth in this rule for
determining an exceptional event.
2. Final Rule
    We are retaining in this rule EPA's historical practice to exclude
a daily measured value in its entirety when that value is found to be
caused by a qualifying exceptional event that affected air quality in
accordance with the conditions described in sections V.B and V.D. If
precise and universally applicable techniques that are administratively
and technically feasible and that could support partial adjustment of
air quality data become available in the future, EPA will, through a
rulemaking, propose, and as appropriate, finalize a technique for
partial adjustment of data as well as any other matters in this rule
which may be affected by the availability of this technology.
    One exception may be made to this exclusion of the entire daily
value for monitoring locations with hourly measurements by Federal
Reference Methods (FRM), Federal Equivalent Methods (FEM), and/or
Approved Regional Methods (ARM) where such data are submitted routinely
to AQS. For example, in cases where stratospheric ozone intrusion
occurs, those hourly (but not sub-hourly) measurements affected by the
intrusion may be excluded in order to calculate the ozone measurements
for the day. The individual hours are to be excluded however, if the
resulting calculated NAAQS averaging time value exceeds the level of
the standard, not just if the individual hourly values exceed that
level. Thus, in the case of ozone, the resulting 8-hour average must
exceed 0.08 ppm, and the resulting 24-hour average must exceed 15.0
[mu]g/m3 for PM2.5. Incomplete data substitution
protocols shall also be considered when evaluating the original and
revised NAAQS averaging time value. In other words, an 8-hour ozone
period is considered valid when fewer than six valid hours are present
if one half the minimum detection limit can be substituted for the
missing hours and the resultant 8-hour value still exceeds 0.08 ppm; a
daily (24-hour) PM2.5 value is considered valid when fewer
than eighteen valid hours are present if zeroes can be substituted for
the missing hours and the resultant 24-hour value still exceeds 15.0
[mu]g/m3.
3. Comments and Responses
    Comment: One commenter supported value adjustment rather than
exclusion when, and only when, such adjustment can be accomplished by
the application of various quantitative or semi-quantitative
approaches. When this is not possible, the value in question should be
replaced with a long-term seasonal mean value.
    Response: The EPA will consider such analyses as part of the weight
of evidence to judge ``but-for,'' but will not make quantitative
adjustments to reported measured values because EPA does not believe
sufficient quantitative methods are available at this time.

F. What Should States Be Required To Submit in Their Exceptional Events
Demonstrations?

1. Background
    Section 319 requires that, in order to have a flagged value
excluded from regulatory determinations, a State must make an
affirmative demonstration that an event occurred (as shown by reliable
and accurate data that is promptly produced) and that there is a clear
causal relationship between measured exceedances or violations of a
standard and the exceptional event in question to

``demonstrate that the exceptional event caused a specific air
pollution concentration'' (42 U.S.C. 7619(b)(3)(B)(ii), (iv)). Section
319 also indicates that regulations promulgated under the section
should provide for criteria and procedures to exclude air quality
monitoring data ``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.''
    Therefore, after flagging data in the AQS database, States are
expected to develop appropriate documentation to support each
individual flag. As a general matter, we believe that such
demonstrations should include documentation showing that the event in
fact occurred and that emissions related to the event were transported
in the direction of the monitor(s) where measurements were recorded;
the size of the area affected by the transported emissions; the
relationship in time between the event, transport of emissions, and
recorded concentrations; and, as appropriate, pollutant species-
specific information supporting a causal relationship between the event
and the measured concentration. The latter information could be based
on available data provided by routine speciation, monitoring networks,
or from selective laboratory analysis of archived particulate matter
filters for the day thought to be impacted by specific events. In
certain situations, such data might be useful for evaluation of impacts
from exceptional events, e.g., to distinguish between impacts caused by
natural fires versus impacts caused by industrial sources. States also
need to show that appropriate mitigation actions were taken at the time
that the event occurred, or after an event occurred in order to protect
public health.
    The following examples are intended to further illustrate the kinds
of information that States could consider in preparing their
demonstrations:
     Information demonstrating the occurrence of the event and
its subsequent transport to the affected monitors. This could include,
for instance, documentation from land owners/managers, satellite-
derived pixels (portions of digital images) indicating the presence of
fires; satellite images of the dispersing smoke and smoke plume
transport or trajectory calculations (calculations to determine the
direction of transport of pollutant emissions from their point of
origin) connecting fires with the receptors.
     Identification of the spatial pattern of the affected area
(the size, shape, and area of geographic coverage). This could include,
for instance, the use of satellite or surface measurement data.
     Information about temporal patterns (e.g., the time and
duration of an event in relation to measured downwind concentrations,
air quality trends over time and space). This could include, for
instance, observed sequential concentration spikes at multiple
locations in a downwind direction.
     Identification of the chemical composition of measured
concentrations. This could include, for instance, organic or crustal
material in excess of typically observed quantities to differentiate
from other high concentration events.
     High wind speeds relative to historically typical levels
for the season of the year in which the claimed event occurred.
    This list is not exhaustive and not all of these kinds of
information and/or documentation will need to be provided in every
instance. A particular instance may require more or less documentation,
depending on the particular facts or circumstances in that instance.
The simplest demonstrations could consist of newspaper accounts or
satellite images to demonstrate that an event occurred together with
daily and seasonal average ambient concentrations to demonstrate an
unusually high ambient concentration level, which is clearly indicative
of an exceptional impact. Such is the case with events such as volcanic
eruptions and nearby forest fires. In one instance, we determined that
wildfires upwind of the San Diego area very likely caused high
concentrations of particulate matter measured in October 2003 based on
the actual physical damage caused by fire to the ambient monitor.
Depending on the nature of the event, meteorological conditions,
severity and spatial extent of measured ambient concentrations
(including relevant chemical components when available) relative to
what typically occurs in the area, and on emissions of pollutants from
the exceptional event which have similar characteristics to those of
other sources in the area, additional showings could be required on a
case-by-case basis. In particular, we anticipate that significantly
more effort will be needed to establish that an exceptional event
caused a particular concentration in an urban area in which there are
numerous and diverse sources and complex meteorology and topography,
and where the emissions from the event in question may well be similar
to those from other sources contributing to measured concentrations, as
compared to an area that has relatively few sources, simple terrain and
less complex meteorology, and where emissions associated with the event
are both substantially greater than and different in composition from
those of other nearby sources.
2. Final Rule
    The demonstration to justify data exclusion will address specific
monitor readings reported to the AQS database. As stated in the
previous sections, a complete demonstration shall justify that: (a) The
event qualifies in accordance with section IV.D. and with EPA policies
and guidance for certain events as described in section IV.E, (b) there
is a clear causal relationship between the measurement under
consideration and the event that is claimed to have affected the air
quality in the area, (c) the event is associated with an unusual
measured concentration beyond typical fluctuations including
background, (d) there would have been no exceedance or violation but
for the event, and (e) the State has provided an opportunity for the
public to comment as required under section V.G. The level of
documentation may vary by the type of event and can be guided in part
by the relative magnitude of the observed concentrations. To obtain
concurrence, EPA must determine that the demonstration is complete and
provides a reasonable technical demonstration.
    Because of the variability in the nature of exceptional events and
the resulting demonstration requirements, States should consult with
the appropriate EPA Regional Office early in the process of preparing
their demonstrations. We are not specifying what will be required as a
minimum level of documentation in all cases because facts and
circumstances will vary significantly based on, among other things,
geography, meteorology and the relative complexity of source
contributions to measured concentrations in any particular location. We
believe, however, that at a minimum, the elements of such a
demonstration should include a showing that an event occurred at a time
when meteorological conditions were conducive to transporting emissions
from the event downwind to the monitor recording a high concentration
of one or more criteria pollutants. Acceptable documentation will be
determined through consultation with the EPA regional offices. However,
certain minimum requirements (e.g., ``but for'' test) will be necessary
as discussed in the earlier sections of this rule.

3. Comments and Responses
    Comment: In cases where high wind data cannot be found, one
commenter stated that EPA should use a ``weight of evidence'' approach,
and should recognize that not accepting a demonstration that such
exceedances are exceptional events is equivalent to a determination
that the exceedances were caused by recurring anthropogenic sources.
    Response: The EPA agrees that a weight of evidence approach is the
most appropriate for demonstrations of exceptional impact.
    Comment: One commenter asserted that States should be allowed to
choose not to submit any demonstration, if the flagged value does not
impact a regulatory determination or if more detailed investigation
indicates that the value may not have been caused by an exceptional
event after all. In these cases, the agency should have the option to
remove the flag.
    Response: We agree that the flag can be removed in these
circumstances or left for informational purposes only.
    Comment: One commenter stated that EPA must provide a reasonable
explanation and documentation for their decision to deny any request
for the flagging of data.
    Response: The EPA regional offices will work with the States,
Tribes, and local agencies to ensure that proper documentation is
submitted to justify data exclusion. The EPA will make the response and
associated explanation publicly available.
    Comment: One commenter stated that EPA must establish a
technically-based appellate process for States to follow when Regional
Offices do not concur with a data flag.
    Response: The EPA does not believe that an appellate process is
necessary because we anticipate that the States and Regional Offices
will be working closely through the data and documentation submission
process.

G. Public Availability of Air Quality Data and Demonstrations Related
to Exceptional Events

1. Background
    Section 40 CFR part 58.16 of EPA's air quality monitoring rules
state that all ambient air quality data and associated quality
assurance data, including metadata records and information specified by
the AQS Data Coding Manual epa.gov/ttn/airs/airsaqs/manuals/manuals.htm
must be reported to EPA via AQS. This information includes exceptional
event flags.
2. Final Rule
    We are requiring that all relevant flagged data, along with the
reasons for the data being flagged, and a demonstration that the
flagged data are caused by exceptional events be made available by the
State for 30 days of public review and comment. The State or designated
local agency should consider the public comments prior to the final
demonstration being submitted to EPA for a decision concerning whether
to exclude the data from regulatory consideration. Notice and
availability of such data and demonstrations must be adequate and
consistent with States' administrative procedures governing similar
submissions. The EPA does not require that public hearings be held on
exceptional events demonstrations but leaves this matter to the States'
discretion consistent with their administrative procedures. With the
submission of the demonstration, the State should document that the
public comment process was followed.
3. Comments and Responses
    Comment: One commenter stated that any new rules related to the
flagging of exceptional events should be consistent with prior EPA
policies and provide sufficient time for States to engage the public in
the process prior to data being flagged in the AQS.
    Response: The EPA believes that the data demonstration requirements
of the final rule provide sufficient time to engage the public. Not
only does the final rule require that the public be accorded an
opportunity to comment on the State's findings, but in some instances
there will be further opportunities for public review and comment at
the time that EPA proposes to base specific actions, e.g., approval or
disapproval of SIP revisions. Thus, we do not believe that additional
public review and comment provisions are necessary or appropriate.

VI. Additional Requirements

    Pursuant to section 319, EPA is finalizing this rule to address
data that has been influenced by exceptional events. Also, EPA is
finalizing one of four options put forth in the proposed rule to
address the issue of whether, and to what extent, States are required
to adopt specific mitigation plans or measures to protect the public
from emissions due to exceptional events. Section 319 states that in
promulgating regulations under the section, EPA shall follow certain,
enumerated principles and that regulations must contain certain
requirements. Section 319(b)(3)(A) contains five principles, including
the principle that each State ``must take necessary measures to
safeguard public health regardless of the source of air pollution.'' In
order to address this principle, EPA is finalizing its proposal to
exclude trivial and more routine air quality impacts from qualifying as
an exceptional event and is also finalizing a ``but for'' test as a
precondition to qualification as an exceptional event (See: section V.C
above).

A. Requirements for States To Provide Public Notification, Public
Education, and Appropriate and Reasonable Measures To Protect Public
Health

1. Background
    The EPA proposed one approach and took comments on three
alternative options concerning what actions a State should take in
anticipation of, or in response to, the occurrence of an exceptional
event. The options that were proposed ranged from being very detailed
and prescriptive to being very flexible and less prescriptive in terms
of the actions that States should take to mitigate the impact of an
exceptional event on the public. While EPA does not believe that
section 319(b)(3)(A) explicitly requires, in and of itself, that States
must develop mitigating measures or plans, EPA solicited comment in the
proposed rule on whether this subparagraph supports the use of other
legal authority to require mitigating actions or plans when an
exceptional event occurs, and solicited comment on issues regarding its
legal authority to require mitigation measures and plans, and the legal
basis for not requiring mitigation measures or plans.
    Option 1 in the proposed rule provided that in cases where
exceedances of a NAAQS are caused by an exceptional event, once a State
becomes aware that an exceptional event is occurring, is predicted to
occur, or has occurred, the State must take reasonable and appropriate
actions to:
     Provide notice to the public of the event. This may
include, but is not limited to, using the media to alert the public of
the event.
     Provide public education concerning the potential health
risks associated with being exposed to high ambient concentrations of
pollutant(s) related to the event. This may include, but is not limited
to, providing information to sensitive populations related to the
health risks associated with the event.
     Take appropriate and reasonable measures to abate or
minimize the exposure of the public to high

concentrations of air pollution associated with the exceptional event.
This may include, but is not limited to, taking reasonable and
appropriate actions to implement control measures on significant
contributing anthropogenic sources to reduce potential exposure of the
public to emissions associated with natural events. States must review
the need to implement controls on contributing anthropogenic sources on
a case-by-case basis. For example, in the case of volcanic or seismic
activity, this may include, but is not limited to, providing for prompt
clean-up of the ash deposits related to the event to prevent re-
entrainment.
    Under option 1, EPA also proposed that, where a State is requesting
that air quality data be excluded as an exceptional event, the State
must submit, as a part of its demonstration, appropriate documentation
to show that the State provided public notice and public education
concerning the event in question, and that the State took reasonable
and appropriate measures to abate or minimize the exposure of the
public to the emissions from the event, where appropriate.
    Option 2 in the proposed rule provided that, States are required to
adopt a general mitigation plan to address exceptional events before
the occurrence of an event as a part of the State's SIP required under
section 110(a)(1) of the CAA. Section 110(a)(1) requires States to
adopt and submit to EPA, within 3 years following the promulgation of a
new or revised NAAQS, a plan which provides for the implementation,
maintenance, and enforcement of the standard in each air quality region
within the State. Under this option, States would be required to
develop and adopt the general requirements and procedures necessary for
the implementation of a mitigation plan to address exceptional events
as a part of its section 110(a)(1) SIP to address a new or revised
NAAQS. The general plan related to exceptional events would include
provisions providing for public notice, public education related to an
event, and provide a requirement for a State to take reasonable and
appropriate measures to mitigate the public health impacts of an
exceptional event. Under this option, in cases where control measures
are required to address the impacts associated with an exceptional
event, the State would be required to implement appropriate measures on
an episodic basis, meaning in response to a specific event that affects
the air quality of a particular area.
    Option 3 in the proposed rule required that, where appropriate, EPA
would require a State to develop and implement a mitigation plan for an
area following the occurrence of an exceptional event. This is in
contrast to option 2 above, which would require each State to adopt a
plan under section 110(a)(1) of the CAA which would contain the general
provisions of a mitigation plan in advance of the occurrence of any
exceptional event. Under option 3, the mitigation plan would only be
developed by the State following the occurrence of an exceptional event
for which the State requested exclusion of the air quality data, and
would not be submitted as a part of the SIP. The mitigation plan would
be required to address the actions that would be taken by the State
related to future similar events. The mitigation plan under this option
would have the same provisions as required of plans developed under
Option 2 above, including the requirements to notify the public that an
event is expected to occur, or is occurring, or has occurred, to
provide for public education related to the health effects associated
with the event, and to identify the actions that would be taken by the
State to mitigate the impact of any recurrence of the event on public
health.
    Option 4 provided that EPA would not require a State to develop and
implement a mitigation plan for exceptional events, or to take specific
mitigation measures as described in options 1-3 in order for EPA to
exclude data from regulatory consideration. This approach proposed to
allow States to have the maximum degree of flexibility in determining
what actions should be taken to mitigate the impacts of exceptional
events, e.g., public notification, public education, efforts to reduce
exposures, or other necessary measures to safeguard public health.
Thus, under this proposed option States would not be obligated to take
any particular actions to mitigate exposures such as those contained in
Option 1, to develop and implement a formal mitigation plan as part of
the SIP such as those contained in Option 2, or to develop a more
formal plan with requirements not a part of the SIP such as those
contained in Option 3.
2. Final Rule
    The EPA is adopting a modified version of Option 1 from the
proposed rule, as described above. This option does not require States
to submit formal mitigation plans; however, States must provide public
notice, public education, and must provide for implementation of
reasonable measures to protect public health when an event occurs.
3. Comments and Responses
    Comment: Several commenters supported option 1 because they stated
that it provides more flexibility for States to determine the
appropriate measures to be implemented related to the occurrence of an
exceptional event. Other commenters supported option 1 for well
defined, well understood events that are non-recurring or unlikely to
recur. The majority of the commenters who commented on option 2
strongly opposed that option. The commenters indicated that option 2
would waste scarce local resources in developing a mitigation plan.
Other commenters stated that issues concerning exceptional events
should be dealt with outside the SIP process and section 110 of the
CAA. With regard to Option 3, one commenter indicated that a preemptive
plan similar to a Natural Events Action Plan (NEAP) (which includes
Reasonably Available Control Measures (RACM)/Best Available Control
Measures (BACM) is necessary to mitigate the poor air quality impacts
associated with exceptional events. The commenter stated that BACM, not
RACM, must be implemented on all contributing anthropogenic sources
related to an exceptional event. Several commenters supported option 3
for addressing public health impacts related to recurring natural
events. The commenters stated that mitigation plans should include BACM
for contributing anthropogenic sources, not RACM. The majority of
commenters who commented on option 4 stated that they supported the
implementation of option 4 because it allows States the most
flexibility for developing and tailoring programs for public
notification of exceptional events, the implementation of education
programs on exceptional events, and implementation of reasonable
measures to protect public health.
    Response: States have an inherent responsibility to protect its
citizens and as such to provide appropriate and reasonable actions to
mitigate the impact of exceptional events on the public health. This
includes alerting the public when such events occur, providing public
education concerning the health effects of such events, and
implementing reasonable measures to mitigate the impact of such events
on public health. Consistent with this inherent responsibility, it is
EPA's belief that States are in a better position to make decisions
concerning what actions should be taken to protect the public when an
exceptional event occurs. This

being the case, States should have the necessary flexibility to take
appropriate actions when exceptional events occur. The EPA is adopting
a modified version of its proposed preferred option 1, which requires
States to provide public notification, public education, and provides
that States should take ``reasonable and appropriate measures'' to
protect public health related to the occurrence of an event. Because
States are inherently responsible for the public health of its
citizens, and are capable of making the determinations of what actions
should be taken to mitigate the impact of such events on the public
when they occur. The EPA has modified option 1 from the proposed rule
and will not be requiring States to submit documentation concerning the
actions that it took to mitigate the impact of exceptional events, in
order for EPA to exclude data from regulatory consideration. As
proposed in option 1, States may still make determinations regarding
reasonable measures in a particular instance, which may or may not
include the implementation of control measures on contributing
anthropogenic sources related to an event, and are not limited to any
particular measure. Therefore, under this option the implementation of
RACM or BACM is not required, but a State has the necessary flexibility
to determine if, and what, controls should be implemented following an
event, as well as the level of control that is required. The EPA
believes that this modified option 1 provides suitable flexibility to
allow States to take those actions that it deems necessary and
appropriate to protect public health. While section 319, as revised by
SAFE-TEA-LU, does not specifically provide that States must implement
mitigation plans, in developing the exceptional events rule, EPA is
required to consider the enumerated principles including the principle
that States must take necessary measures to protect public health
regardless of the source of air pollution. Therefore, under the
modified version of option 1 adopted in this final rule, States must
take reasonable and appropriate actions to protect public health.
    Comment: Several commenters stated that the exceptional events rule
should be consistent with the current requirements under existing
policies with respect to the need for a NEAP to address recurring
natural events such as high wind events.
    Response: The EPA believes that it is advantageous for States to
keep NEAPs in place that are currently being implemented in order to
address the public health impacts associated with recurring natural
events such as high wind events. However, following the promulgation of
this rule, States will no longer be required to keep NEAPs in place
that were not approved as a part of a SIP for an area. Where a NEAP, as
well as BACM, has been approved as a part of a nonattainment SIP for an
area, the NEAP, as well as the associated BACM, must remain in place.
States may, however, submit a request to EPA to remove the NEAP and
BACM from the SIP. The request must contain an approvable
demonstration, as required by section 110(l), which shows that the
removal of the NEAP and BACM will not interfere with any applicable
requirement concerning attainment or maintenance of the NAAQS for an
area, reasonable further progress, or any other applicable requirement
for the area.

VII. Special Treatment of Certain Exceptional Events Under This Final
Rule

    As stated in section IV.D above, this final rule applies to data
affected by natural events (which are a subset of exceptional events)
at air quality monitoring sites where it has been determined that
concentrations due to these events have caused, or substantially
contributed to, exceedances of the NAAQS in an affected area. This
final rule applies to several types of natural events, including, but
not limited to, volcanic and seismic activities, natural disasters,
high wind events, certain fires, and stratospheric ozone intrusions. It
also applies to transported pollution originating from national and
international sources that otherwise meets the criteria and
requirements for exceptional events. Some types of exceptional events
have unusual characteristics that require special consideration in the
context of this final rulemaking. We discuss each of these special
issues, and the necessary accommodations, below.

A. Volcanic and Seismic Activities

1. Background
    Volcanic and seismic activities may affect air quality for an
extended period of time after the initial occurrence of the event in
question. Therefore, EPA believes that it is appropriate to consider an
extended timeframe for flagging and exclusion of data associated with
such events. Specifically, EPA believes that emissions attributed to
anthropogenic activities associated with clean-up that re-entrain
volcanic ash and dust from seismic activity during the first year (12
months) following an event will be treated as due to the natural event.
2. Final Rule
    The EPA is finalizing its proposal with regards to volcanic and
seismic activities. The EPA will allow up to 12 months for the clean-up
of ash deposits due to volcanic/seismic events. During that time
period, emissions of re-entrained dust due to anthropogenic activities
associated with cleanup may be treated as exceptional events. In cases
where the damage caused by the event is so substantial that a 12-month
period is inadequate to address the clean-up that is necessary, a State
may submit a request for an extension of the 12-month time period to
EPA. As stated elsewhere in this rule, EPA will grant requests for
extensions of the time period related to such events on a case-by-case
basis. States are encouraged to submit supporting information
concerning the reason for the extension and the length of time being
requested for the extension.

B. High Wind Events

1. Background
    Where high wind events result in exceedances or violations of the
particulate matter standards, EPA proposed that they be treated as
natural events if there is a clear causal relationship demonstrated
between the exceedances measured at the air quality monitoring site and
the high wind event in question, and if anthropogenic activities which
contribute to particulate matter emissions in conjunction with the high
wind event are reasonably well-controlled.
2. Final Rule
    The EPA's final rule concerning high wind events states that
ambient particulate matter concentrations due to dust being raised by
unusually high winds will be treated as due to uncontrollable natural
events where (1) the dust originated from nonanthropogenic sources, or
(2) the dust originated from anthropogenic sources within the State,
that are determined to have been reasonably well-controlled at the time