vvEPA
2016 Revisions to the
Exceptional Events Rule:
Update to Frequently Asked Questions
United States Environmental Protection Agency
July 2019
Note: This July 23, 2019, document replaces the "Interim Exceptional Events Rule Frequently Asked
Questions" document originally posted on May 14, 2013.

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2016 Revisions to the Exceptional Events Rule: Update to Frequently Asked Questions
July 2019
2016 Revisions to the Exceptional Events Rule:
Update to Frequently Asked Questions
The 2016 revisions to the Exceptional Events Rule1 supersede the EPA's Exceptional Events
Rule of 2007,2 including guidance and policy documents, and revised a regulatory process
codified at 40 CFR parts 50 and 51 (50.1, 50.14 and 51.930). The Exceptional Events Rule
recognizes that each potentially eligible event has unique characteristics that require case-
specific demonstration and evaluation. Therefore, the Exceptional Events Rule continues to rely
on a "weight-of-evidence" approach for evaluating each demonstration to justify excluding air
quality data influenced by an exceptional event. The EPA acknowledges that, due to a variety of
factors including the type and severity of the event, pollutant concentration, spatial extent,
temporal extent, and proximity of the event to the violating monitor, some exceptional events
demonstrations may be limited or may need to be more extensive. The EPA encourages an
iterative demonstration development process informed by frequent communication with the
appropriate EPA Regional office and guided by a principle of "right-sizing" demonstrations.
Air agencies and other stakeholders have raised technical and procedural questions about
implementing the Exceptional Events Rule.3 This document is an update to the 2013 "Interim
Exceptional Events Rule Frequently Asked Questions" document, replacing that document, and
is intended to answer frequently asked air agency questions about the 2016 Exceptional Events
Rule. The EPA recognizes the limited resources of air agencies that prepare exceptional events
demonstrations and EPA Regional offices that review them. One of the EPA's goals in
developing exceptional events implementation guidance is to establish clear expectations to
enable air agencies to better manage resources as they prepare exceptional events
demonstrations. The appropriate level of supporting documentation will vary on a case-by-case
basis under the weight-of-evidence approach. The EPA anticipates that the level of resources
needed to prepare (and review) packages will decrease as we continue to identify ways to
streamline the process and share new demonstrations and analyses for reference.
This document is organized into the following sections:
A. Prior to Developing a Demonstration - Initial Notification and Data Flagging (Page 7)
A. 1. Question: Who can submit an exceptional events demonstration?
A.2. Question: What is the purpose of the exceptional events initial notification
process, and what are the steps involved?
A.3. Question: How does the event data flagging process work in AQS as part of the
initial notification process?
A.4. Question: How long after an event occurs can air agencies flag data and submit
an exceptional events demonstration?
A.5. Question: What is the difference between "R" series flags and "I" series flags
in AQS, and how should they be used?
A.6. Question: Can an air agency flag any data in AQS?
1	"Treatment of Data Influenced by Exceptional Events; Final Rule," 81 FR 68216, October 3, 2016.
2	"Treatment of Data Influenced by Exceptional Events; Final Rule," 72 FR 13563, March 22, 2007.
3	References to "air agencies" are meant to include state, local, and tribal air agencies.
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A.7. Question: What flags does AQS use to describe fires?
A. 8. Question: Some criteria pollutants have multiple forms, averaging periods,
and/or levels of the NAAQS. If the EPA concurs on an exceptional events
demonstration for a measurement that affects one particular form or level of
NAAQS, does the EPA automatically exclude the same measurement for all the
other applicable forms and levels of the NAAQS for that pollutant?
A.9.	Question: When is it appropriate for air agencies to flag concentration values
that are equal to or less than the level of the relevant NAAQS? Under what
circumstances will the EPA concur on such flags?
B. Developing a Demonstration (Page 12)
I. General Topics
B.	1. Question: Is there a template or example for preparing an exceptional events
demonstration?
B.2. Question: What are the required elements for an exceptional events
demonstration?
B.3. Question: Is the Exceptional Events Rule's demonstration requirement for
"analyses comparing the claimed event-influenced concentrations(s) to
concentrations at the same monitoring site at other times" a test that can be
"passed" or "failed" based on the outcome of the statistical comparison? For
example, must the concentration affected by an event exceed a specific
percentile rank in the historical data?
B.4. Question: What types of analyses would most likely be supportive of an
approvable demonstration as part of the comparison of event concentration(s) to
historical concentrations?
B.5. Question: When comparing event concentrations to historical concentrations,
how can air agencies identify and provide evidence to show that specific past
elevated concentrations were associated with known or suspected exceptional
events?
B.6. Question: What other types of evidence can air agencies include in a
demonstration to support that the event affected air quality in such a way that
there exists a clear causal relationship between the specific event and the
monitored exceedance or violation?
II. Fire-Related Events
B.7. Question: What fire-related definitions should air agencies consider as they
develop exceptional events demonstrations for wildfires or prescribed fires on
wildland?
B.8. Question: What types of evidence can air agencies include in a demonstration
to support a clear causal relationship between emissions from a fire event and
ozone exceedances at affected monitor(s)?
B.9. Question: How can air agencies demonstrate that emissions from wildfires are
"not reasonably controllable or preventable"?
B. 10.Question: Can the EPA provide an example exceptional events demonstration
for a wildfire that is claimed to have influenced downwind PM10
concentrations?
B. 11 .Question: Can an air agency submit and the EPA act on a demonstration for a
wildfire or prescribed fire that does not occur on wildland?
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B. 12. Question: Can a prescribed fire be an exceptional event?
B. 13. Question: How can a prescribed fire on wildland satisfy the "human activity
unlikely to recur" criterion?
B. 14. Question: How can a private landowner rely on a land/resource management
plan to satisfy the "human activity unlikely to recur" criterion?
B. 15. Question: How can a prescribed fire satisfy the "not reasonably controllable"
criterion?
B. 16.Question: Is there a deadline by which air agencies must have defined and
implemented BSMPs so that an exceptional events demonstration can rely on
BSMPs to satisfy the "not reasonably controllable" criterion?
B. 17.Question: How can a prescribed fire satisfy the "not reasonably preventable"
criterion?
B. 18. Question: How can air agencies satisfy the "not reasonably controllable or
preventable" criteria for prescribed fires on privately-owned lands?
B. 19. Question: When would a post-burn report be used in an exceptional events
demonstration?
III. High Wind Dust Events
B.20. Question: What is a high wind threshold and how does the EPA apply high
wind thresholds for high wind events?
B.21. Question: Does an air agency need to make an argument or submit evidence
about control measures for air quality impacts from wind-blown dust from
desert land in its natural state?
B.22.	Question: Does the Exceptional Events Rule have special provisions for certain
"extreme" high wind dust events?
C.	Public Comment Process (Page 25)
C.	1. Question: At what point in the exceptional events demonstration development
and review process is public notice and opportunity for comment required?
C.2.	Question: What process or mechanisms should an air agency use to ensure that
an exceptional events demonstration has sufficient opportunity for public
comment?
D.	EPA Review and Response (Page 26)
D.	1. Question: What is meant by a "weight-of-evidence" approach when reviewing
exceptional events demonstrations?
D.2. Question: How can an air agency determine if a demonstration will have
regulatory significance and ensure an EPA action?
D.3. Question: How quickly will the EPA review a demonstration and provide the
air agency with feedback or a decision?
D.4.	Question: How does the EPA publicly communicate its decisions and
supporting rationale on exceptional events demonstrations?
E.	Rule Application and Implementation Issues (Page 27)
E.	1. Question: From which types of calculations will affected data be excluded
when the EPA concurs on an exceptional events demonstration?
E.2. Question: How does the EPA handle data exclusions for exceptional events in
the case of 1-hour values of the 24-hour PM2.5 or PM10 24-hour NAAQS?
E.3. Question: Can droughts be considered exceptional events?
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E.4. Question: It is possible for events to affect more than one state. Would the EPA
accept a jointly developed multi-state/agency demonstration?
E.5. Question: How should air agencies define and support a "particular location" in
the context of a claim that an event was "human activity unlikely to recur at a
particular location?"
E.6. Question: How should an air agency handle a situation in which an exceptional
event influences concentrations at a regulatory monitor that is operated by a
federal land manager or other federal agency?
E.7. Question: Can fireworks displays cause PM10 exceedances/violations?
E.8. Question: In addressing the not reasonably controllable criterion, what kind of
deference is given to control measures in SIPs?
E.9. Question: Can measures either in an approved SIP/TIP or FIP outside of the
five-year period or measures not in an approved SIP/TIP or FIP be considered in
addressing the not reasonably controllable criterion?
E.10. Question: Does a state (or tribe) need to make an argument or submit evidence
about control measures for events that took place in other states or countries, on
federally-owned and managed land, or on state (or tribal) lands not subject to
state (or tribal) regulation?
E.l 1. Question: Does the Exceptional Events Rule contain a dispute resolution
process that air agencies can use to resolve disagreements regarding EPA
decisions on demonstrations?
E.12.	Question: Can air agencies use data from non-regulatory monitors in
exceptional events analyses?
F. Mitigation Plans (Page 32)
F.	1. Question: What is the purpose of mitigation plans under the Exceptional Events
Rule?
F.2. Question: What are the required components of a mitigation plan?
F.3. Question: How will an air agency know if it is required to develop and submit a
mitigation plan?
F.4. Question: Are the mitigation plan components reviewed each time a
demonstration is submitted?
F.5. Question: What if an area already has a plan for public notification or other
components required in a mitigation plan?
F.6. Question: Can the type of flag in AQS impact whether a mitigation plan is
required?
F.7. Question: What happens if an area is subject to the requirements of a mitigation
plan, but submits an exceptional events demonstration without having
developed a complete mitigation plan?
Each section contains related questions and answers. Readers of this document can find
additional information, including the final rule and example demonstrations, on the EPA's
exceptional events website located at:
https://www.epa.gov/air-qualitv-analvsis/exceptional-events-rule-and-guidance.
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The exceptional events website also contains additional implementation resources, including
Guidance on the Preparation of Exceptional Events Demonstrations for Wildfire Events that
May Influence Ozone Concentrations
Disclaimer
The Exceptional Events Rule is the source of the regulatory requirements for exceptional events
and exceptional events demonstrations. This document provides responses to frequently asked
questions regarding the Exceptional Events Rule and does not impose any new requirements and
shall not be considered binding on any party. Note: If and when the EPA takes a regulatory
action that hinges on a decision to exclude data under the Exceptional Events Rule, the EPA will
consider and respond to any public comments on that regulatory action that address any aspect of
a supporting exceptional events demonstration.
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A. Prior to Developing a Demonstration - Initial Notification and Data Flagging
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Note: "Flag" is the common terminology for a data qualifier code in the EPA's AQS (Air
Quality System). Unless explicitly noted, the process of "flagging" data refers to adding Request
Exclusion ("R ") data qualifier codes to selected data in AQS. "R "flags are the only AQS flags
that satisfy Exceptional Events Rule data flagging as part of the required Initial Notification
Process. The EPA can act/concur only on an "R" flag.
A.l. Question: Who can submit an exceptional events demonstration?
Answer: The EPA may only take action on those demonstrations that meet the requirements of
the Exceptional Events Rule and are submitted by one (or more) of the following authorized
agencies:
•	all state air agencies;
•	local air quality agencies to whom a state has delegated relevant responsibilities for air
quality management;
•	tribal air quality agencies operating ambient air quality monitors that produce regulatory
data;
•	and federal land managers or other federal agencies to whom the relevant state/tribal air
agency(ies) has granted approval to submit a demonstration.
A.2. Question: What is the purpose of the exceptional events initial notification process, and
what are the steps involved?
Answer: The initial notification process is a new requirement in the 2016 Exceptional Events
Rule that is intended to promote early and frequent communication between air agencies and
EPA Regional offices when air agencies first begin to consider developing an exceptional events
demonstration. The initial notification process requires air agencies to signal their intent to
request exclusion of one or more air quality measurements due to an exceptional event by
providing an initial notification to the EPA, creating an initial event description, and flagging the
associated data in the AQS database. The initial notification may be conveyed as an official
letter, electronic mail, or other means of communication from an air agency official with
authority to do so.
Each EPA Regional office can develop its own suggested procedures for the initial notification
process, and air agencies are encouraged to contact their EPA Regional office to discuss options.
Stakeholders are encouraged to review the 2016 Exceptional Events Rule and preamble, which
both contain extensive information about the initial notification process, and can be viewed at the
following link on the Federal Register website:
https ://www.federalregister. gov/documents/2016/10/03/2016-22983/treatment-of-data-
influenced-bv-exceptional-events
A.3. Question: How does the event data flagging process work in AQS as part of the initial
notification process?
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Answer: Flagging event-associated data and creating an initial event description in AQS is part
of the initial notification process, which is intended to promote communication between air
agencies and EPA Regional offices when air agencies begin to consider developing an
exceptional events demonstration. Air agencies should flag event-associated data and create an
initial event description in AQS for data requested for exclusion following initial notification and
discussion with the EPA Regional office that results in submitting a demonstration.
To create a new event within AQS, air agencies should enter an "initial event description" that
contains a qualifier code and qualifier description, (e.g., high wind dust, volcanic eruption,
other); a brief description of the event; optionally may enter the event begin and end date. AQS
allows air agencies to associate affected monitors and specific measurements with a given event.
A.4. Question: How long after an event occurs can air agencies flag data and submit an
exceptional events demonstration?
Answer: The 2016 Exceptional Events Rule revisions removed the "general schedule" deadlines
for data flagging and demonstration submission. Under the 2016 Exceptional Events Rule, there
are only demonstration submission deadlines for data that will influence the initial designation of
areas for any new or revised national ambient air quality standard (NAAQS). These deadlines are
identified in Table 2 of section 50.14(c)(2)(vi) of the Exceptional Events Rule. In all cases, air
agencies should enter "R" flags in AQS for data requested for exclusion, as part of the initial
notification process, following discussion with the EPA Regional office that results in submitting
a demonstration, and before the demonstration is submitted to the EPA. The EPA will not be able
to act on exceptional events demonstrations for event-related data that has not been assigned an
"R" flag in AQS.
As an alternative to "R" flags, air agencies are encouraged to use "I" series flags to initially
identify values they believe may have been affected by an event, but for which they do not yet
know if they will request exclusion or develop a demonstration. Air agencies can later remove
"I" flags or change them to "R" flags in AQS. (see Question A.5. below for additional
information on the difference between "R" flags and "I" flags in AQS).
A.5. Question: What is the difference between "R" series flags and "I" series flags in AQS,
and how should they be used?
Answer: Within AQS, air agencies can use two types of data qualifier codes: Request Exclusion
flags ("R") or Informational Only flags ("I"). Agencies should use the "I" series flags when
identifying informational data and the "R" series flags to identify data points for which the
agency intends to request an exceptional event exclusion and the EPA's concurrence. For
example, air agencies may use an "I" series flag to initially identify values they believe may have
been affected by an event, but for which they do not yet know if they will request exclusion or
develop a demonstration. Air agencies may also use an "I" series flag to identify data for which
they know they will not request exclusion or develop a demonstration, but would like to
document that the values were potentially influenced by an event. The full list of AQS data
qualifier codes can be viewed at:
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https://aqs.epa.gov/aqsweb/documents/codetables/qualifiers.html
Once the air agency collects additional supporting data and determines that it intends to develop
an exceptional events demonstration, it may change the "I" series flag to an "R" series flag. Or,
the air agency may find that additional information does not support flagging the data for
exclusion, and the air agency may remove the flag or retain the "I" series flag. Air agencies may
also want to use "I" flags to identify historical data that may have been influenced by an event in
future exceptional events demonstrations related to other events. Alternatively, air agencies may
also use the "I" series flags simply to note activities or conditions occurring on the data
collection day that are unrelated to exceptional events. The EPA does not intend to review or act
on the "I" series flags.
Air agencies should also be advised that use of "R" (request exclusion) flags may trigger
mitigation plan requirements. The use of "I" (informational) flags will not trigger mitigation plan
requirements. See Section F of this document for additional details on mitigation plan
requirements. If an air agency enters an "R" flag for event-related ambient data in AQS, and later
decides not to develop an exceptional events demonstration for the flagged data, the air agency
can remove the flag or change it from an "R" flag to an "I" flag in AQS.
A.6. Question: Can an air agency flag any data in AQS?
Answer: Yes, but only "R" flags are intended to identify data that might have regulatory
significance, and for which an air agency intends to request exclusion and submit a
demonstration. Air agencies should use the "I" series flags to identify values for informational
purposes (see Question A.5.). AQS only allows the EPA to place concurrence flags on data
identified with an "R" flag. As a general matter, "I" flags do not affect regulatory summary
statistics (e.g., design values, number of exceedances) generated by AQS for the purpose of
informing NAAQS determinations. "R" flags will not affect the regulatory summary statistics
unless or until they are concurred by the EPA.
Further, although the Exceptional Events Rule does not prohibit air agencies from flagging
individual concentration values below the level of the NAAQS, air agencies can generally only
request exclusion for data that contribute to an exceedance or violation of the NAAQS. See
Question A.9. for more information, including clarifications and examples, particularly for PM2.5
and PM10, in which flagging individual concentration values below the level of the NAAQS is
acceptable.
A.7. Question: What flags does AQS use to describe fires?
Answer: Land management agencies modified their fire-related definitions after the EPA
promulgated the 2007 Exceptional Events Rule. The EPA has incorporated the fire-related
terminology into the exceptional events guidance documents to ensure consistency (see also
Question B.7.). These definitional changes resulted in corresponding changes to fire-related flags
in AQS. The EPA eliminated from AQS the Wildland Fire Use Fire - United States ("IU") and
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("RU") flags and the Forest Fire ("E") flag. The EPA continues to use the following flags to
describe fires:
•	IF - Fire - Canadian (Informational Only)
•	IG - Fire - Mexico/Central America (Informational Only)
•	IM - Prescribed Fire (Informational Only)
•	IP - Structural Fire (Informational Only)
•	IT - Wildfire - US (Informational Only)
•	RF - Fire - Canadian (Request Exclusion)
•	RG - Fire - Mexico/Central America (Request Exclusion)
•	RM - Prescribed Fire (Request Exclusion)
•	RP - Structural Fire (Request Exclusion)
•	RT - Wildfire - US (Request Exclusion)
The EPA retained the Fire - Canadian ("IF/RF") and Fire - Mexico/Central America ("IG/RG")
flags because these flags indicate the jurisdictional origin of the fire {i.e., outside of the
submitting state/outside of the United States). Emissions from fires originating outside of the
United States that affect air quality concentrations in the United States may qualify for treatment
under the Exceptional Events Rule.
A.8. Question: Some criteria pollutants have multiple forms, averaging periods, and/or levels
of the NAAQS. If the EPA concurs on an exceptional events demonstration for a
measurement that affects one particular form or level of NAAQS, does the EPA
automatically exclude the same measurement for all the other applicable forms and levels
of the NAAQS for that pollutant?
Answer: No. Air agencies should request (including flagging relevant data in AQS) and provide
support for exclusion of monitored pollutant concentrations separately for each NAAQS that
applies to the pollutant. The EPA will similarly provide separate concurrences.
The EPA did finalize regulatory language to exclude all 24 1-hour values within a given event-
affected day for PM2.5 and PM10 data obtained via monitor instruments that provide 1-hour
measurements. The exclusion of all hours in an event-affected day is consistent with the
approach for filter-based analyzers, and eliminates the calculation of uncertain and potentially
biased daily values for PM2.5 and PM10 NAAQS.
The EPA concurrence flags entered into AQS prior to the March 2010 re-engineering of AQS to
accommodate the Exceptional Events Rule did not indicate the specific single NAAQS or the
specific combination of NAAQS for which the exclusion was approved. These "legacy"
concurrence flags have been converted to the new approach as follows:
•	For ozone, all legacy flags were treated as applying to both the 0.08 ppm 8-hour NAAQS
and the 0.12 ppm 1-hour NAAQS. This default was chosen because as of March 2010,
designations under the 2008 NAAQS of 0.075 ppm had been suspended pending reconsideration
of that NAAQS, and AQS staff were not aware of any concurrences already granted with respect
to the 0.075 ppm NAAQS.
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•	For PM2.5, all concurrences on events with dates prior to January 1, 2005 (meaning the
date of the monitored concentration, not the date of the EPA's concurrence), were presumed to
be applicable only to the annual PM2.5 NAAQS. This default was chosen because prior to the
revision of the 24-hour PM2.5 NAAQS in 2006, violations of the 1997 24-hour NAAQS were
extremely rare.
•	For PM2.5, all concurrences on events with dates of January 1, 2005, through March 2010
were presumed to be applicable only to the 24-hour NAAQS because there were no revisions to
the annual PM2.5 NAAQS during this timeframe, so designations of nonattainment for the annual
PM2.5 standard were extremely rare. This 24-hour PM2.5 NAAQS default was chosen because it
was possible for designations under the 2006 24-hour NAAQS to be based on data as early as
2005.
•	For PM10, all concurrences were presumed to apply to the 24-hour NAAQS, as the annual
PM10 NAAQS was revoked in 2006.4
•	For CO, all concurrences were presumed to apply to both the 1-hour and the 8-hour
NAAQS. This default was chosen to ensure that the concurrence applied to whichever NAAQS
had been exceeded and was the basis for the exclusion request.
•	For SO2, all concurrences were presumed to apply to both the 24-hour and the annual
NAAQS. This default was chosen to ensure that the concurrence applied to whichever NAAQS
had been exceeded and was the basis for the exclusion request. No flags were assumed to apply
to the 1-hour NAAQS because the 1-hour SO2 standard was not promulgated until June of 2010,
after the AQS re-engineering.
•	For Pb, all concurrences (if any existed) were presumed to apply to the quarterly average
NAAQS of 1.5 |ig/m3. This default was chosen because March 2010 was prior to the EPA
issuing final designations under the 2008 Pb NAAQS of 0.15 |ig/m3.
•	For NO2, all concurrences were presumed to apply to the annual NAAQS because the 1-
hour NO2 standard was not promulgated until February of 2010.
For concurrences on events with dates after the March 2010 re-engineering of AQS, the EPA
will specify the NAAQS to which the concurrence applies. If this defaulting scheme does not
properly represent the actual concurrence action that was taken by the EPA Regional office, the
Regional office should revise and correct the concurrence flags, if it has not already done so.
Air agencies can find detailed information on the use of events flags in AQS in a tutorial posted
at:
https://www3.epa.gov/ttnairsl/airsaqsORIG/manuals/ExceptionalEventTutorial.pdf.
The tutorial discusses concurrence flags on page 20.
4 The EPA realizes that many of the defaulted EPA concurrences for pre-2006 PM10 concentrations that were below
the level of the 24-hour PM10 NAAQS actually were applicable to the annual PM10 NAAQS, but this approach was
the most practical way to ensure that all other concurrences originally intended to be applicable to the 24-hour
NAAQS were preserved. Because concentrations below the level of the 24-hour NAAQS have no effect on
attainment determinations for the 24-hour NAAQS, no error can come from treating such values as having been
concurred. Nevertheless, the EPA Regional office may choose to update these concurrence flags as time permits.
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A.9.	Question: When is it appropriate for air agencies to flag concentration values that are
equal to or less than the level of the relevant NAAQS? Under what circumstances will the
EPA concur on such flags?
Answer: AQS currently allows an air agency to flag any measured concentration values it
chooses, including values at or below the level of the relevant NAAQS. With respect to the
circumstances when the EPA may concur on a flagged value below the level of a NAAQS, we
offer the following clarifications:
Clean Air Act section 319(b) and the definition of an exceptional event in the 2016 Exceptional
Events Rule state that the event affected air quality in such a way that there exists a clear causal
relationship between the specific event and the monitored exceedance or violation. The 2016
Exceptional Events Rule preamble states that "the concentration values used in calculating a
violating 3-year design value could be considered for exclusion under the Exceptional Events
Rule only if the concentration itself is an exceedance or results in a violating design value. If the
elevated concentration is not itself an exceedance nor does it result in a violating design value,
then the value in question could not be considered as an exceptional event."
The EPA also finalized rule language that will allow an air agency to compare a 24-hour
concentration of any NAAQS pollutant to the NAAQS for the same pollutant with a longer
averaging period as part of a weight-of-evidence showing for the clear causal relationship with
respect to the NAAQS with the longer period. Applying this rule revision to PM2.5 will allow an
air agency to compare a 24-hour averaging period for PM2.5 that is greater than the relevant 12 or
15 [j,g/m3 annual PM2.5 NAAQS to either the 24-hour PM2.5 NAAQS (i.e., 35 (J,g/m3) or the
relevant annual NAAQS (provided the air agency specifically requests exclusion for both
NAAQS and assuming there is regulatory significance for both standards). For example, an
agency could request to exclude a concentration of 32 (J,g/m3, which when excluded from the
calculated 3-year average, could result in a 98th percentile value for the year of 29 [j,g/m3. When
29 [j,g/m3 is averaged with the 98th percentile value for the other two years, the resulting design
value attains the 24-hour standard.
B.	Developing a Demonstration
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B.I. General Topics
B.l. Question: Is there a template or example for preparing an exceptional events
demonstration?
Answer: Air agencies should consult the 2016 Exceptional Events Rule and preamble and the
appropriate EPA Regional office prior to preparing a demonstration. In addition, guidance
documents, example demonstrations, and other rule implementation resources, are available
online at:
https://www.epa.gov/air-qualitv-analvsis/treatment-air-qualitv-data-influenced-exceptional-
events
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B.2. Question: What are the required elements for an exceptional events demonstration?
Answer: An approvable exceptional events demonstration must include the following six
elements:
1)	A narrative conceptual model that describes the event(s) causing the exceedance or violation
and a discussion of how emissions from the event(s) led to the exceedance or violation at the
affected monitor(s);
2)	A demonstration that the event affected air quality in such a way that there exists a clear
causal relationship between the specific event and the monitored exceedance or violation;
3)	Analyses comparing the claimed event-influenced concentration(s) to concentrations at the
same monitoring site at other times. The Administrator shall not require an air agency to prove a
specific percentile point in the distribution of data;
4)	A demonstration that the event was both not reasonably controllable and not reasonably
preventable;
5)	A demonstration that the event was caused by human activity that is unlikely to recur at a
particular location or was a natural event; and
6)	Documentation that the submitting air agency followed the public comment process.
Additional details on what kinds of information could be helpful to include in the sections above
can be found in the 2016 Exceptional Events Rule and preamble, supporting guidance
documents, example demonstrations, and other rule implementation resources, all available
online at:
https://www.epa.gov/air-qualitv-analvsis/treatment-air-qualitv-data-influenced-exceptional-
events
B.3. Question: Is the Exceptional Events Rule's demonstration requirement for "analyses
comparing the claimed event-influenced concentrations(s) to concentrations at the same
monitoring site at other times" a test that can be "passed" or "failed" based on the
outcome of the statistical comparison? For example, must the concentration affected by
an event exceed a specific percentile rank in the historical data?
Answer: There is no specific percentile rank that an air agency must demonstrate in this
monitoring data comparison. The EPA will use a weight-of-evidence approach to review each
demonstration on a case-by-case basis. The air agency's role in satisfying this element is to
provide appropriate analyses and statistics comparing the claimed event-influenced
concentrations(s) to historical concentrations at the same monitoring site.
A comparison of event concentrations with historical concentrations will help the EPA determine
whether the air agency has satisfied the "clear causal relationship" criterion. Although there is no
minimum percentile rank for the event concentration(s) compared to historical concentrations,
the EPA generally anticipates that historical comparisons that show the event-related
concentration(s) to be similar to historical concentrations will likely indicate a less conclusive
clear causal relationship. A comparison of event-related concentrations that are similar to
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historical concentrations would not necessarily preclude concurrence, but a demonstration
without a comparison to historical concentrations would likely not meet the requirements of the
Exceptional Events Rule.
The EPA recommends more than one type of statistical analysis be included when comparing
event-related concentrations to historical concentrations, such as those described in the answer to
Question B.4. It is important to note, however, that no outcome of the statistical comparison, by
itself, can guarantee successful demonstration of a clear causal relationship.
Submitting agencies are encouraged to discuss comparisons of the event-related concentration(s)
to historical concentrations with the appropriate EPA Regional office prior to submitting a
demonstration package to determine what information might be available and appropriate to
support the demonstration.
B.4. Question: What types of analyses would most likely be supportive of an approvable
demonstration as part of the comparison of event concentration(s) to historical
concentrations?
Answer: To aid the review process, reduce requests for additional information, and facilitate the
EPA's understanding of the air agency's position, a submitting air agency can consider providing
some of the following types of statistics, graphics, and explanatory text, which are also described
in Table 2 on page 68242 of the preamble to the Exceptional Events Rule (81 FR 68216, October
3,2016):
• Comparison of concentrations on the claimed event day(s) with past historical data. The
historical comparisons can be made on an annual and/or seasonal basis, depending on which is
more appropriate. For example, if PM or ozone data at the location show clear seasonality {i.e.,
exceedances are nonexistent or extremely rare in some seasons but not others, or concentrations
vary according to season due to meteorological conditions), discussing that information in the
demonstration is likely appropriate. In contrast, if exceedances tend to occur throughout the year,
analysis of annual data may be more appropriate. For seasonal comparisons, an approvable
demonstration will ideally include all available seasonal data from at least 5 years, if available.
We recognize, however, that these data may not be available for all monitors and/or all
pollutants. If data are not available, please consult with the reviewing EPA Regional office.
The analysis should describe the seasonal nature of the pollutant(s), if appropriate, for the
location being evaluated. Depending on the quantity of data, it may be appropriate to present
monthly maximums; however, generally it is not appropriate to present monthly-averaged daily
data or any other average of the daily data, since this masks high values. Regardless of whether
seasonal or annual data are presented, data are most helpful when provided in the form relevant
to the standard of the pollutant that is being considered for data exclusion. Specific examples of
analyses of annual and seasonal data, as well as analyses of historical PM2.5 speciation data and
spatial distributions, can be found in example demonstrations on the EPA's exceptional events
website at:
https://www.epa.gov/air-qualitv-analvsis/exceptional-events-submissions-table-2016-rule.
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It may also be useful to analyze concentrations of non-NAAQS pollutants which may be
precursors or indicators of the NAAQS pollutants in question, or of a particular type of event.
For example, although PM2.5 is generally not considered to be a precursor or indicator of ozone,
elevated PM2.5, including unusual diurnal patterns, may be an indicator of smoke, and therefore
may provide supporting evidence that elevated ozone in the same area was at least partially
attributable to a wildfire event.
Additionally, it may be useful for the comparison of concentrations on the claimed event day(s)
with past historical data to label appropriate data points as being associated with previously
concurred exceptional events, suspected exceptional events, or other unusual occurrences (see
Question B.5. below).
•	Comparison of concentrations on the claimed event day with a set of similar days:
Similar days could include neighboring days (e.g., a time series of two weeks) and/or other days
with similar meteorological conditions (possibly from other years). Relevant meteorological
conditions may include temperature, wind speed and direction, relative humidity, daily temporal
patterns, and other key factors depending on the pollutant. This type of comparison could
demonstrate that the event caused higher concentrations than would be expected for given
meteorological and/or local emissions conditions.
•	Percentile rank of concentration relative to annual data: The percentile rank of the event-
day concentration should be provided for the event day relative to all measurement days over the
previous 5 years. To ensure statistical robustness, the EPA generally recommends that submitting
agencies include a minimum of 300 data points in this calculation. The daily statistic (e.g., 24-
hour average, maximum 8-hour average, or maximum 1-hour) should be appropriate for the form
of the standard being considered for data exclusion.
•	Percentile rank of concentration relative to seasonal data: The percentile rank of the
event-day concentration should be provided for the event day relative to all monitored
concentrations for the season (or appropriate alternative 3-month period) of the event over the
previous 5 years. It is generally appropriate to use the same time horizon as used for the
percentile rank calculated relative to annual data. The use of percentile ranks should not be seen
as a bright line to be passed or failed when comparing concentrations requested for exclusion
with historical values.
The evidence comparing the event-affected concentration(s) with historical concentrations is
most helpful to an air agency's demonstration if it shows that the event-affected concentration is
high compared to all, or nearly all, historical concentrations generated by normal emissions and
under normal ambient conditions. This scenario makes it more plausible that the event caused the
observed elevated concentration(s) rather than another cause on the same day. If similar events
have been rare in the past, it may be possible to further support that an event caused the observed
elevated concentration by labeling appropriate historical data points as being associated with
concurred exceptional events, suspected exceptional events, or other unusual occurrences. To
illustrate the influence of these events, air agencies may also include comparisons omitting such
points (see Question B.5. below).
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Example Comparisons of Event-Related Concentration^) to Historical Concentrations
The discussion and graphics that follow illustrate the type of analyses that an air agency might
use to compare the claimed event-influenced concentration(s) to historical concentrations at the
same monitoring site at. Additional examples of graphics are also available in the presentation
located at:
https://www.epa.gov/sites/production/files/2015-05/documents/ideasforshowingeeevidence.pdf.
The following figures illustrate seasonal emission variability. The first figure below indicates an
exceedance of the PM2.5 standard in late spring (late 2nd quarter). This concentration is outside
the range of the 5-year historical data set for PM2.5 during the 2nd quarter. The second figure
indicates a similar PM2.5 concentration, but in a different part of the country where similar
exceedance concentrations occur throughout the year. This comparison suggests that, in this
example, some non-event process(es) can cause high concentrations throughout the year. In the
first case, a seasonal comparison of historical concentrations generally would be appropriate,
while annual analysis might be more appropriate for the second case, to provide the most robust
and representative historical data set.
Comparison lo Historical Concentrations forPM2.5, Seasonal Data, 20-16 2017
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B.5. Question: When comparing event concentrations to historical concentrations, how can
air agencies identify and provide evidence to show that specific past elevated
concentrations were associated with known or suspected exceptional events?
Answer: As described in the preamble of the Exceptional Events Rule, the EPA encourages air
agencies to, where applicable in historical concentration plots, label past high data points as
being associated with previously concurred exceptional events, suspected exceptional events,
other unusual occurrences, or high pollution days due to normal emissions. Identifying this
information may be helpful in appropriately characterizing non-event-related historical
concentrations. The EPA recommends providing evidence to support the identification when
possible. For example, air agencies may provide a summary explanation of a past suspected
event along with basic evidence of its occurrence such as news reports, photographs, existing
data and/or analyses (e.g., back trajectories, satellite imagery, record of flagged data in AQS) to
support the claim that elevated concentrations were influenced by an event. Air agencies should
consult with their EPA Regional office to determine what information would be reasonable to
identify potential past event influences on historical data.
Where applicable, air agencies may include historical comparison plots with and without known
or suspected exceptional events points to better illustrate the unusual nature of the exceptional
event(s) in question. Supporting information regarding historical concentrations influenced by
past events will be considered along with all other information and analyses in the demonstration
as part of the weight of evidence for a clear causal relationship. The application of "I" flags in
AQS can help track and identify historical event-related data for this purpose.
B.6. Question: What other types of evidence can air agencies include in a demonstration to
support that the event affected air quality in such a way that there exists a clear causal
relationship between the specific event and the monitored exceedance or violation?
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Answer: The EPA evaluates the clear causal relationship criterion using a weight-of-evidence
approach to evaluate the event and whether relevant emissions were transported from the area of
the event to the monitor(s) recording the exceedance. The air agency's analyses should provide
supporting evidence that emissions were transported from the event onset location to the affected
monitor(s) and that they reached ground level at the location(s) of the affected monitors. Some
examples of evidence of air pollutant transport to the monitor(s) include: speciation data at the
monitor; backward and forward model trajectories; satellite imagery; and spatial extent maps
comparing event days and non-event days. The EPA recommends air agencies consult with their
EPA Regional office to identify which types of analyses may be most useful in supporting the
weight of evidence for a clear causal relationship, and to rule out analyses that may be
unnecessary.
Where a single event results in exceedances or violations of multiple NAAQS (e.g., annual and
24-hour PM2 5), the submitting agency needs to address the unique features of each NAAQS
exceedance or violation (e.g., potentially different monitoring locations, different historical
concentration patterns). An air agency could submit a single demonstration package for a single
event affecting multiple NAAQS provided the air agency clearly identifies all the NAAQS for
which data exclusions are being sought along with the appropriate supporting evidence in the
demonstration relevant to each specific NAAQS.
Examples of supporting analyses can be found in example demonstrations on the EPA's
exceptional events website at:
https://www.epa.gov/air-qualitv-analvsis/exceptional-events-submissions-table-2016-rule.
B.II. Fire-Related Events
B.7. Question: What fire-related definitions should air agencies consider as they develop
exceptional events demonstrations for wildfires or prescribed fires on wildland?
Answer: The EPA uses the following fire-related terminology, consistent with the definitions in
40 C.F.R. § 50.1, in the exceptional events guidance documents to ensure consistency:
Prescribed fire - Any fire intentionally ignited by management actions in accordance with
applicable laws, policies, and regulations to meet specific land or resource management
objectives.
Wildfire - Any fire started by an unplanned ignition caused by lightning; volcanoes; other acts of
nature; unauthorized activity; accidental, human-caused actions, or a prescribed fire that has
developed into a wildfire. A wildfire that predominantly occurs on wildland is a natural event.
Wildland - An area in which human activity and development are essentially non-existent,
except for roads, railroads, power lines, and similar transportation facilities. Structures, if any,
are widely scattered.
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B.8. Question: What types of evidence can air agencies include in a demonstration to support
a clear causal relationship between emissions from a fire event and ozone exceedances at
affected monitor(s)?
Answer: The EPA has developed a separate guidance document to aid air agencies in preparing
a demonstration and supporting a clear causal relationship for wildfire events that are believed to
have influenced ozone concentrations. Air agencies can find the document, Guidance on the
Preparation of Exceptional Events Demonstrations for Wildfire Events that May Influence Ozone
Concentrations, on the EPA's exceptional events website at:
https://www.epa.gov/air-qualitv-analvsis/exceptional-events-rule-and-guidance
Many of the analytical techniques that are useful for showing a clear clausal relationship between
an event and an exceedance or violation in demonstrations for wildfire events that may have
influenced ozone concentrations can also be useful in demonstrations for prescribedfire events
that may have influenced ozone concentrations. The EPA's exceptional events website also
includes example concurred demonstration packages that include different types and scopes of
analyses for ozone-related exceptional events.
B.9. Question: How can air agencies demonstrate that emissions from wildfires are "not
reasonably controllable or preventable"?
Answer: According to the Clean Air Act and the Exceptional Events Rule, an exceptional event
must be "not reasonably controllable or preventable." The preamble to the Exceptional Events
Rule clarifies that the EPA interprets this requirement to contain two factors: the event must be
both not reasonably controllable and not reasonably preventable at the time the event occurred.
This requirement applies to both natural events and events caused by human activities. However,
it is presumptively assumed that if evidence supports that a wildfire occurred on wildland, such a
wildfire event will satisfy both factors of the "not reasonably controllable or preventable"
criterion, provided the Administrator determines that there is no compelling evidence to the
contrary in the record.
In showing that a prescribed fire has developed into a wildfire, the demonstration should include
the following documentation: (1) news reports or notifications to the public characterizing the
nature of the fire; and (2) the demonstration submitters' explanation and other evidence
documenting the origin and evolution of the fire. If a prescribed fire has developed into a
wildfire, some of the basic smoke management practices that were planned for use for the
prescribed fire may continue to be reasonable to apply during the wildfire period.
The Exceptional Events Rule accepts that wildfire events on wildland are not generally
reasonable to control or prevent. Therefore, a statement that the wildfire event was caused by one
of the causes identified in the definition of wildfire (such as lightning) and occurred
predominantly on wildland, and thus by the terms of the Exceptional Events Rule, was not
reasonably controllable or preventable, should satisfy this rule element. A report based on
information from other agencies or from news reports, possibly supplemented by maps of fire
locations, may potentially be sufficient for this statement. The air agencies should work with
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their EPA Regional office to ensure that their statements about the causes of the wildfire events
are sufficient.
B.10. Question: Can the EPA provide an example exceptional events demonstration for a
wildfire that is claimed to have influenced downwind PMio concentrations?
Answer: PMio generally tends to "fall" to ground level relatively quickly in the vicinity of the
event and, in our experience, is not usually subject to long-range transport. However, all
demonstrations are evaluated case-by-case based on the weight of evidence and the EPA is open
to evidence on this subject. Air agencies should contact their EPA Regional office to discuss
scientific evidence to support such a claim.
B.ll. Question: Can an air agency submit and the EPA act on a demonstration for a wildfire or
prescribed fire that does not occur on wildland?
Answer: Air agencies contemplating preparing fire-related exceptional events demonstrations
for fires not on wildland should consult with the reviewing EPA Regional office. The EPA may
review such a demonstration on a case-by-case basis considering the specific merits of each
event.
B.12. Question: Can a prescribed fire be an exceptional event?
Answer: The 2016 Exceptional Events Rule states that prescribed fire on wildland can be a
human-caused event eligible for treatment as an exceptional event. As with any other type of
event, a supporting demonstration for a prescribed fire event must include all required elements
(see Question B.2. for information on the six required elements).
Questions B.13.-B.19. provide additional information specific to prescribed fires about how to
satisfy the "not reasonably controllable or preventable" and "human activity unlikely to recur at a
particular location or a natural event" criteria.
B.13. Question: How can a prescribed fire on wildland satisfy the "human activity unlikely to
recur" criterion?
Answer: The Exceptional Events Rule recognizes that prescribed fires are intentionally ignited
by land management actions. Since they are caused by human activity, to be considered an
exceptional event, every prescribed fire must address the "human activity unlikely to recur"
criterion. To satisfy this criterion for demonstrations involving prescribed fires on wildland, the
demonstration must describe the actual frequency with which a burn was conducted, and may
rely upon an assessment of either the natural fire return interval or the prescribed fire frequency
needed to establish, restore and/or maintain a sustainable and resilient wildland ecosystem (as
documented in a land or resource management plan).
Multi-year land or resource management plans prepared by the land management agency or any
private property owner generally include documentation of natural fire return intervals, and can
be used to identify the natural fire return interval in an exceptional events demonstration. While
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the EPA will generally defer to the interval described in these plans, the actual burn frequency is
not required to match the described interval exactly to satisfy human activity unlikely to recur
criterion. Based on the complexity of prescribed fire for sustainable land management, the EPA
acknowledges that portions of natural ecosystems may have different natural fire return intervals,
which may influence how a burn manager applies the prescribed fire. The EPA will compare
these intervals to determine whether the actual burn frequency mimics the natural fire return
interval. On a case-by-case basis, in the absence of a multi-year land or resource management
plan, the natural fire return interval may be established according to scientific literature.
Fire recurrence can also be addressed by identifying the prescribed fire frequency needed to
establish, restore and/or maintain a sustainable and resilient wildland ecosystem and/or to
preserve endangered or threatened species. The EPA acknowledges that a multi-year plan with
such a stated objective may include general targets for the frequency of prescribed fire use and
that management may deviate from the general plan due to unexpected differences between
planned and actual fire behavior, landscape or ecosystem characteristics, fuel loading patterns,
and weather patterns.
B.14. Question: How can a private landowner rely on a land/resource management plan to
satisfy the "human activity unlikely to recur" criterion?
Answer: If private landowners want to rely on a land/resource management plan, and burns on
private land contribute to an exceedance or violation addressed in an exceptional events
demonstration, then the multi-year land or resource management plan should address the private
land to be burned and include information about the fire-return interval (that either mimics the
natural fire-return interval or the interval needed to create or maintain a sustainable ecosystem).
B.15. Question: How can a prescribed fire satisfy the "not reasonably controllable" criterion?
Answer: The Exceptional Events Rule allows for exclusion of data affected by a prescribed fire
if the general requirements of the rule are satisfied and if the air agency has adopted and is
implementing a Smoke Management Program (SMP) or if the air agency has ensured that the
burn manager employed basic smoke management practices (BSMPs).
The preamble to the 2016 Exceptional Events Rule identifies the following six recommended
SMP components: authorization to burn, minimizing air pollutant emissions, smoke management
components of burn plans, public education and awareness, surveillance and enforcement, and
program evaluation.
SMPs must be state-certified, which means that a responsible state or delegated local agency has
certified in a letter to the air agency's EPA Regional office that it has adopted and is
implementing an SMP. Past certifications provided to the EPA through this process may qualify
as "certified" for purposes of the 2016 Exceptional Events Rule. An air agency with a current
SMP that has not been certified according to this process could pursue certification of its existing
SMP. SMPs that have been incorporated into a SIP are "certified."
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Table 1 in the 2016 Exceptional Events Rule lists suggested BSMPs, but this listing is not
intended to be all-inclusive, and not all BSMP are appropriate for all burns. Goals for
applicability should retain flexibility to allow for onsite variation and site-specific conditions that
can be variable on the day of the burn. Burn managers can consider other appropriate BSMP as
they become available due to technological advancement or programmatic refinement.
B.16. Question: Is there a deadline by which air agencies must have defined and implemented
BSMPs so that an exceptional events demonstration can rely on BSMPs to satisfy the
"not reasonably controllable" criterion?
Answer: The Exceptional Events Rule does not require an air agency to define or implement
BSMPs by a fixed date. However, if an air agency intends to rely on BSMPs to satisfy the "not
reasonably controllable" criterion in an exceptional events demonstration, the Exceptional
Events Rule requires the air agency to provide documentation in its demonstration that the burn
manager for the prescribed fire event(s) employed appropriate BSMPs as part of the event. The
demonstration may rely on a statement or other documentation provided by the burn manager
regarding what BSMPs were employed for the prescribed fire. Appropriate BSMPs may be fire-
specific, and what is reasonably considered to be appropriate may change over time. The
Exceptional Events Rule does not require an air agency to define appropriate BSMPs for each
individual prescribed fire event.
The Exceptional Events Rule also requires that, after September 30, 2018, for any demonstration
for a prescribed fire event that relies on BSMPs, the air agency, federal land managers, and other
appropriate entities (e.g., state forestry agencies, state prescribed fire councils, burn associations)
must have collaborated with burn managers operating within the jurisdiction of the State to
discuss and document the process by which air agencies and land managers will work together to
protect public health and manage air quality impacts during the conduct of prescribed fires on
wildland. Such discussions must include outreach and education aimed at burn managers
regarding general expectations for the selection and application of appropriate basic smoke
management practices and goals for advancing strategies and increasing adoption and
communication of the benefits of appropriate basic smoke management practices. Under the
Exceptional Events Rule, the EPA cannot concur with a demonstration for prescribed fire that
relies on BSMP after September 30, 2018, until this collaboration has been conducted and
documented. The EPA encourages all states to engage in these collaborative discussions
irrespective of interest in pursuing an exceptional events demonstration.
B.17. Question: How can a prescribed fire satisfy the "not reasonably preventable" criterion?
Answer: For prescribed fires, the not reasonably preventable criterion can be satisfied by an
assessment of benefits that would have been foregone had the prescribed fire not been
conducted.
To assess the benefits of a prescribed fire, an air agency may rely on and reference a multi-year
land or resource management plan for a wildland area with a stated objective to establish, restore
and/or maintain a sustainable and resilient wildland ecosystem and/or to preserve endangered or
threatened species through a program of prescribed fire provided that the Administrator
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determines that there is no compelling evidence to the contrary in the record and the use of
prescribed fire in the area has not exceeded the frequency indicated in that plan.
B.18. Question: How can air agencies satisfy the "not reasonably controllable or preventable"
criteria for prescribed fires on privately-owned lands?
Answer: For the not reasonably controllable criterion, an air agency submitting a demonstration
would normally rely on BSMPs to address fires on privately-owned lands. The air agency would
include evidence of the use of BSMP, such as the burn manager's affirmation of use supported
by a copy of the post-burn report or a letter from the burn manager.
For the not reasonably preventable criterion, the air agency submitting the demonstration should
explain how the prescribed fire is "not reasonably preventable" based on the benefits that would
be foregone if the fire were not conducted. "Forgone benefits" would be those objectives in a
multi-year fire/land management plan that establish, restore and/or maintain a sustainable and
resilient wildland ecosystem. If a prescribed fire on private land contributes to the exceedance,
then the comprehensive land management plan must either include the private land, or the
private landowners must have a land management plan that addresses the above objectives.
B.19. Question: When would a post-burn report be used in an exceptional events
demonstration?
Answer: If an exceedance or violation of a NAAQS occurs when a prescribed fire is employing
an appropriate BSMP approach, the air agency and the burn manager should undertake a review
of the subject fire, including a review of the BSMPs applied during the subject fire to ensure the
protection of air quality and public health and progress towards restoring and/or maintaining a
sustainable and resilient wildland ecosystem. If the prescribed fire becomes the subject of an
exceptional events demonstration, documentation of the post-burn review must accompany the
demonstration. Recommended elements that may be included in post-burn reports are listed in
Table 4 of the 2016 Exceptional Events Rule preamble.
B.III. High Wind Dust Events
B.20. Question: What is a high wind threshold and how does the EPA apply high wind
thresholds for high wind events?
Answer: For purposes of the Exceptional Events Rule, a high wind threshold is defined as the
minimum threshold wind speed capable of causing particulate matter emissions from natural
undisturbed lands in the area affected by a high wind dust event.
As indicated in the 2016 Exceptional Events Rule at 40 CFR 50.14(b)(5)(iii), "The Administrator
will accept a high wind threshold of a sustained wind of 25 mph for areas in the States of
Arizona, California, Colorado, Kansas, Nebraska, Nevada, New Mexico, North Dakota,
Oklahoma, South Dakota, Texas, Utah, and Wyoming provided this value is not contradicted by
evidence in the record at the time the State submits a demonstration." States can also identify and
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use an Administrator-approved alternate area-specific high wind threshold that is more
representative of local or regional conditions, if appropriate.
B.21. Question: Does an air agency need to make an argument or submit evidence about
control measures for air quality impacts from wind-blown dust from desert land in its
natural state?
Answer: Although impacts from wind-blown dust from undisturbed natural deserts are often
inherently not reasonable to control, the air agency would need to state that the wind-blown dust
was from undisturbed natural deserts and provide appropriate supporting documentation in its
demonstration. The supporting documentation could include descriptions of the geographic area
(with maps or available visuals) and a discussion of historical land use, including prior
disturbances, water diversions and other historical practices that may have occurred on the land,
even if the land seems or is generally considered to be "undisturbed" at present.
Submitting agencies should also identify and evaluate controls for significant anthropogenic
sources of event-related windblown dust. When addressing the "not reasonably controllable or
preventable" criterion within an exceptional events demonstration for a high wind dust event, air
agencies should: (1) identify the event-related natural and anthropogenic sources of emissions
causing and contributing to the monitored exceedance or violation, including the contribution
from local sources; (2) identify the relevant state implementation plan (SIP), federal
implementation plan (FIP) or tribal implementation plan (TIP) or other enforceable control
measures in place for these sources and the implementation status of these controls; and (3)
provide evidence of effective implementation and enforcement of reasonable controls, if
applicable. Special provisions for satisfying this criterion apply to large-scale and high-energy
high wind dust events, as described in Question B.22.
B.22. Question: Does the Exceptional Events Rule have special provisions for certain
"extreme" high wind dust events?
Answer: Yes, the 2016 Exceptional Events Rule added provisions for large-scale and high-
energy high wind dust events, in some cases referred to as "haboobs", that allow for more
streamlined documentation requirements to support the "not reasonably controllable" criterion
than other high wind events.
Air agencies do not need to provide the case-specific justification described in Question B.21 to
satisfy the "not reasonably controllable or preventable" criterion for large-scale and high-energy
high wind dust events, such as "haboobs."
Instead, to satisfy the "not reasonably controllable or preventable" criterion for large-scale and
high-energy high wind dust events, the demonstration must document that the event: (1) Was
associated with a dust storm and is the focus of a Dust Storm Warning; (2) Had sustained winds
that were greater than or equal to 40 mph; and (3) Had reduced visibility equal to or less than 0.5
miles. In addition, as stated in the Exceptional Events Rule preamble (p. 68259), a large-scale
and high-energy high wind dust event would be associated with measured exceedances occurring
at multiple monitoring sites over a large geographic area unless the area has only a single PM
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monitor or if the area has monitors operating on a sampling frequency that does not coincide
with the timing of the event.
The National Weather Service definition of a dust storm referenced in the Exceptional Events
Rule preamble (p. 68259) and found at http://wl.weather.gov/glossarv/is: "a severe weather
condition characterized by strong winds and dust-filled air over an extensive area." Evidence of a
Dust Storm Warning, either from the National Weather Service or a similar scientifically based
government entity, may be used to satisfy the Dust Storm Warning criterion above. Although
events that are the subject of other types of blowing dust advisories and alerts (not a "Dust Storm
Warning") generally would not qualify for the more streamlined documentation requirements to
satisfy the "not reasonably controllable" criterion for large-scale and high energy high wind dust
events, such events may still satisfy this criterion in accordance with the Exceptional Events
Rule's provisions for less "extreme" high wind dust events.
EPA will generally consider documentation of the nature and extent of the large-scale and high-
energy high wind dust event in a demonstration to be sufficient with respect to the "not
reasonably controllable criterion." Examples of documentation may include dust storm warnings,
as well as visibility data. Storm data, including date, estimated damage, as well as meteorological
descriptions may be found via National Climatic Data Center (NCDC) Storm data at the
following link: https://www.ncdc.noaa.gov/IPS/sd/sd.html. Meteorological data, including hourly
visibility measurements, may be found at https://www.ncdc.noaa.gov/cdo-web/datatools/lcd.
To satisfy the visibility criterion, a demonstration for a large-scale high-energy high wind dust
event should endeavor to include the best evidence available. Less precise photographic data or
video documentation of visibility reduction could be considered on a case-by-case basis for this
criterion in circumstances where precise visibility measurements are not available.
C. Public Comment Process
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C.l. Question: At what point in the exceptional events demonstration development and
review process is public notice and opportunity for comment required?
Answer: The Exceptional Events Rule requires that air agencies offer notice and opportunity for
public comment as part of the demonstration development process (see 40 CFR 50.14(c)(3)(i)
and 40 CFR 50.14(c)(3)(v)). The EPA encourages air agencies to work closely with their EPA
Regional office throughout the development and submittal of the demonstration, including
ensuring that air agencies provide sufficient opportunity for public comment.
C.2. Question: What process or mechanisms should an air agency use to ensure that an
exceptional events demonstration has sufficient opportunity for public comment?
Answer: Providing sufficient opportunity for public comment for a demonstration is case-by-
case and depends on the circumstances and intended audience. In particular, the EPA encourages
air agencies to provide electronic notice, which provides an effective, convenient, and cost-
efficient way to communicate information to the majority of the public (see, for example, the
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EPA's October 18, 2016, action related to electronic notice for air permitting, available at
https://www.epa.gov/nsr/clean-air-act-permitting-electronic-notice-final-rule).
In general, it may be sufficient for an air agency to post a draft demonstration on its website,
provided the interested stakeholders are given a timely notification of the opportunity to review
and comment on the demonstration. For example, notification of the availability of a
demonstration for review can be accomplished through an email from the air agency to the air
agency's public electronic distribution list, announcing the posting of the draft demonstration for
review and comment along with the web link to the draft demonstration document(s) and
instructions for submitting comments to the air agency. The air agency should seek to ensure that
interested stakeholders are aware of the posting, and should allow public comment for a period
of at least 30 days.
D. EPA Review and Response
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D.l. Question: What is meant by a "weight-of-evidence" approach when reviewing
exceptional events demonstrations?
Answer: In using the term "weight of evidence," the EPA and relevant air agency will consider
all relevant evidence and qualitatively "weigh" this evidence based on its relevance to the
Exceptional Events Rule criterion being addressed, the degree of certainty, its persuasiveness,
and other considerations appropriate to the individual pollutant, as well as the nature and type of
event.
D.2. Question: How can an air agency determine if a demonstration will have regulatory
significance and ensure an EPA action?
Answer: The 2016 Exceptional Events Rule applies to the following types of regulatory
determinations by the Administrator, as specified in 50.14(a)(l)(i).
•	An action to designate an area, pursuant to Clean Air Act section 107(d)(1), or
redesignate an area, pursuant to Clean Air Act section 107(d)(3), for a particular
NAAQS;
•	The assignment or re-assignment of a classification category to a nonattainment area
where such classification is based on a comparison of pollutant design values,
calculated according to the specific data handling procedures in 40 CFR part 50 for
each NAAQS, to the level of the relevant NAAQS;
•	A determination regarding whether a nonattainment area has attained the level of the
appropriate NAAQS by its specified deadline;
•	A determination that an area has data for the specific NAAQS, which qualify the area
for an attainment date extension under the Clean Air Act provisions for the applicable
pollutant;
•	A determination under Clean Air Act section 110(k)(5), if based on an area violating
a NAAQS, that the state implementation plan (SIP) is inadequate under the
requirements of Clean Air Act section 110; and
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• Other actions on a case-by-case basis as determined by the Administrator.
In general, if a demonstration does not affect one these specific regulatory determinations, it is
considered to not have "regulatory significance," and EPA Regional offices may defer action on
the demonstration or recommend another existing mechanism for addressing the air agency's
concern.
The 2016 Exceptional Events Rule's initial notification requirement is intended to begin a
process of communication and consultation between air agencies and the EPA Regional offices
to determine the regulatory implications of a potential demonstration for a specific event. Please
consult with your EPA Regional office if you have additional questions about regulatory
significance.
D.3. Question: How quickly will the EPA review a demonstration and provide the air agency
with feedback or a decision?
Answer: The Exceptional Events Rule preamble identifies the following intended response
timelines for the EPA: a formal response to the initial notification within 60 days; initial review
of an exceptional events demonstration with regulatory significance within 120 days of receipt; a
decision regarding event concurrence or nonconcurrence within 12 months of receipt of a
complete demonstration; and a "deferral letter" within 60 days of receipt of a demonstration that
the EPA determined during the initial notification process to not to have regulatory significance.
Air agencies that believe their demonstrations are tied to near-term regulatory actions should
discuss this with their EPA Regional office and work collaboratively on identifying an
appropriate and timely review schedule. In addition, air agencies may wish to submit their
demonstration packages well in advance of the regulatory deadline. Air agencies should also
identify the relationship between the exceptional event-related flagged data and the anticipated
regulatory action as part of their initial notification to the reviewing EPA Regional office.
D.4.	Question: How does the EPA publicly communicate its decisions and supporting
rationale on exceptional events demonstrations?
Answer: In most instances, the EPA's concurrence or nonconcurrence determination may be a
factor in a rulemaking that includes a public comment period. In these cases, documents related
to the relevant exceptional events demonstration and any official supporting correspondence will
also be posted in the associated rulemaking docket. As a general matter, and in consultation with
relevant air agencies, the EPA also posts demonstration packages and decisions (consisting of air
agency demonstration submittals, EPA responses, and EPA technical support documents) on the
EPA's exceptional events webpage once the demonstration review process has been completed.5
E.	Rule Application and Implementation Issues
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5 https://www.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events
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E.l. Question: From which types of calculations will affected data be excluded when the
EPA concurs on an exceptional events demonstration?
Answer: When the EPA concurs based on the weight of evidence that the air agency has
successfully made the demonstrations referred to in 40 CFR 50.14(a)(2) and (b)(1), the EPA
generally will exclude the affected data from the following types of calculations and activities:
•	The EPA's AQS will not include these days in user reports or in design value estimates if
the "exclude events" report option is chosen when the report is run.6
•	The data will continue to be publicly available, but the EPA's publications and public
information statements on the status of air quality in the affected area generally will not
reflect these data in any summary statistic of potential regulatory application, unless such
inclusion is specifically noted.7
In addition, some proposed regulatory actions (e.g., proposed designation, classification,
attainment demonstration, or finding as to whether the area has met the applicable NAAQS) will
rely on design values that exclude data that was approved for exclusion in a concurred
demonstration. These regulatory actions require the EPA to provide an opportunity for public
comment prior to taking final action. If the EPA pursues one of these actions for a given area, the
EPA will open a new comment period during which the public may comment on the exceptional
event submission and/or the EPA's determinations. The EPA must consider and respond to
received comments before taking final regulatory action.
E.2. Question: How does the EPA handle data exclusions for exceptional events in the case of
1-hour values of the 24-hour PM2.5 or PM10 24-hour NAAQS?
Answer: When the EPA concurs on an exceptional events demonstration affecting the 24-hour
PM2.5 or PM10 24-hour NAAQS, the EPA intends to exclude all 24 1-hour values within a given
event-affected day for PM2.5 and PM10 data obtained via monitor instruments that provide 1-hour
measurements.
E.3. Question: Can droughts be considered exceptional events?
Answer: Meteorological events involving high temperatures or drought conditions do not
directly cause pollutant emissions and are therefore not considered exceptional events. However,
events involving high temperatures or lack of precipitation may promote occurrences of certain
types of exceptional events, such as wildfires or high wind events, which do directly cause
emissions.
E.4. Question: It is possible for events to affect more than one state. Would the EPA accept a
jointly developed multi-state/agency demonstration?
6Due to the complexity of the AQS software, inadvertent errors may occur. The EPA asks that agencies provide the
EPA with information if/when AQS outputs seem inconsistent with the EPA's intention to exclude concurred upon
data.
7These data may be included in statistics intended to describe current status and trends in actual air quality in the
area for public information purposes, including reporting of the Air Quality Index.
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Answer: The primary responsibility for developing demonstrations lies with state, local, and
tribal air agencies. The EPA encourages air agencies to coordinate and cooperate with each other
in compiling demonstration packages when a single event affects multiple jurisdictions. Each
NAAQS exceedance or violation, however, will likely have some unique properties (e.g., unique
monitoring locations, different surrounding and potentially contributing sources with varying
levels of control, different historical concentration patterns, etc.). States/agencies will need to
address these unique characteristics individually, but may coordinate with other air agencies
similarly affected by the same event. Air agency collaboration can range from informal
information-sharing to a closely coordinated joint demonstration to be submitted separately by
each participating air agency. Additional information on coordinating preparation of
demonstrations for events that affect more than one air agency jurisdiction can be found in the
EPA document, Best Practices for Preparation of Multi-State/Local Air Agency Exceptional
Events Demonstrations, located at:
https://www.epa.gov/air-qualitv-analvsis/exceptional-events-implementation-tools-templates-
and-links
E.5. Question: How should air agencies define and support a "particular location" in the
context of a claim that an event was "human activity unlikely to recur at a particular
location?"
Answer: How a "particular location" is characterized in this context can vary depending on the
specifics of the area. Air agencies and EPA Regional offices should proactively discuss what
"particular location" means during the initial notification process before a demonstration is
prepared. A "particular location" will most commonly be defined by jurisdictional boundaries
(e.g., a county) or an air quality control region, but definitions may vary based on local
topography, jurisdiction sizes, air movement patterns, and other considerations.
Regarding the concept of recurrence, and as stated in the preamble of the Exceptional Events
Rule, we generally measure the 3-year period backwards from the date of the most recent event
(e.g., for an event occurring on May 1, 2016, the 3-year period would be May 1, 2013, through
May 1, 2016). As described previously, if there have been two prior events caused by human
activity of a similar type (i.e., a similar event type generating emissions of the same pollutant
whether flagged and/or the subject of a demonstration) within a 3-year period in "a particular
location," the third event, for which the demonstration is being prepared (or would be prepared),
would generally not satisfy the "human activity that is unlikely to recur at a particular location"
criterion and, thus, would not qualify as an exceptional event.
E.6. Question: How should an air agency handle a situation in which an exceptional event
influences concentrations at a regulatory monitor that is operated by a federal land
manager or other federal agency?
Answer: Air agencies may submit demonstrations for any regulatory monitor within its
jurisdictional bounds, including those operated by federal land managers, other federal agencies,
and delegated local agencies. To maintain data integrity, AQS is generally designed so that only
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the agency updating a monitoring site may enter or alter data for that site. Under normal
circumstances, an air agency will not have access rights to apply event flags to data from
monitors operated by other entities, such as a federal land manager. When an air agency believes
that an exceptional event affected the concentration recorded by monitors operated by other
agencies, the air agency should contact the agency operating the monitor and request that the
operating agency flag the identified data range for exclusion. The affected air agency should also
develop and forward to the operating agency an initial event description that the operating
agency can enter in AQS along with the associated "R" series flags (see Question A.3.). If an air
agency is unsuccessful in requesting that another agency apply the appropriate "R" series flags
and initial event description, the air agency should contact its EPA Regional office.
A federal land manager or other federal agency may initiate a request for data exclusion, but only
after the air agency in which the affected monitor is located concurs with the federal land
manager's or other federal agency's submittal.
E.7. Question: Can fireworks displays cause PMio exceedances/violations?
Answer: All demonstrations are evaluated case-by case based on the weight of evidence.
Although the EPA has not seen definitive scientific evidence that fireworks are likely to cause
PMio exceedances at a monitor, the EPA is open to reviewing new evidence on this subject.
E.8. Question: In addressing the not reasonably controllable criterion, what kind of deference
is given to control measures in SIPs?
Answer: Deference is given to measures in a nonattainment or maintenance SIP, TIP, or FIP
approved within 5 years of the date of the event. The measures must address the event-related
pollutant for all appropriate sources to qualify for consideration that the requirements of the SIP,
TIP, or FIP are reasonable controls (with respect to all anthropogenic sources that have or may
have contributed to the exceedance or violation.) This deference does not apply if the air agency
is under obligation to revise the relevant SIP/TIP.
When addressing the "not reasonably controllable or preventable" criterion within an exceptional
events demonstration, air agencies should: (1) identify the natural and anthropogenic sources of
emissions causing and contributing to the monitored exceedance or violation, including the
contribution from local sources; (2) identify the relevant SIP, FIP or TIP or other enforceable
control measures in place for these sources and the implementation status of these controls; and
(3) provide evidence of effective implementation and enforcement of reasonable controls, if
applicable.
E.9. Question: Can measures either in an approved SIP/TIP or FIP outside of the five-year
period or measures not in an approved SIP/TIP or FIP be considered in addressing the not
reasonably controllable criterion?
Answer: Yes. Such measures may be presented in a demonstration and evaluated as part of the
weight of evidence for reasonable controls, but such measures are not given the deference
described above in question E.8.
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E.10. Question: Does a state (or tribe) need to make an argument or submit evidence about
control measures for events that took place in other states or countries, on federally-
owned and managed land, or on state (or tribal) lands not subject to state (or tribal)
regulation?
Answer: Under the Exceptional Events Rule, air agencies generally have no obligation to
specifically address controls if the event was due to emissions originating outside their
jurisdictional {i.e., state or tribal) border. It is generally not reasonable to expect the downwind
state {i.e., the state submitting the demonstration) to require the upwind country or state to have
implemented controls on sources sufficient to limit event-related air concentrations in the
downwind state. However, a demonstration based on emissions originating outside of the
submitting air agency's jurisdictional borders must demonstrate that the event also meets the
other exceptional events criteria.
In the case of a federal land manager or other federal agency submitting a demonstration under
the requirements of this section, the jurisdictional boundaries that apply are those of the state or
the tribe depending on which has jurisdiction over the area where the event has occurred. States
and tribes should consult with their EPA Regional office early in the development of an
exceptional events demonstration package if they believe that emissions from sources on
federally-owned and managed land {e.g., national parks within the state) have been affected by
an event in a way that raises issues about reasonable control.
As with all exceptional events demonstrations, the EPA will evaluate the information on a case-
by-case basis based on the facts of a particular exceptional event. The EPA encourages air
agencies to work cooperatively to plan and apply controls on both sides of jurisdictional
boundaries for their mutual benefit.
E.ll. Question: Does the Exceptional Events Rule contain a dispute resolution process that air
agencies can use to resolve disagreements regarding EPA decisions on demonstrations?
Answer: There are several mechanisms for preventing or resolving disagreements that air
agencies can use at various points in the exceptional events or subsequent regulatory
determination process:
•	Engage in early dialogue with the appropriate EPA Regional office.
•	Participate in the public notice and comment process for the regulatory determination
affected by EPA nonconcurrence on the demonstration.
•	Challenge the regulatory decision made based on the EPA's exceptional event
disapproval in an appropriate court.
For complex exceptional events claims or those with immediate regulatory or other impacts {e.g.,
those claims that directly influence proposed designation or redesignation, classification, and
attainment determinations), EPA Regional office staff may also seek input from other EPA
Regional offices and/or EPA headquarters staff.
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E.12.	Question: Can air agencies use data from non-regulatory monitors in exceptional events
analyses?
Answer: Yes, air agencies can use data from non-regulatory monitors to support their
exceptional events demonstrations. Generally, monitoring data used for NAAQS regulatory
purposes are collected from Federal Reference Method (FRM), Federal Equivalent Method
(FEM), and/or Approved Regional Method (ARM) monitors that are sited and operated in
accordance with 40 CFR Part 58. Exceedances or violations identified as exceptional events
originate from these same data from FRM, FEM, and/or ARM monitors. The EPA's repository of
ambient air quality data, AQS, stores data from more than 10,000 monitors, about 5,000 of which
are currently active. Although not all of these monitors are FRM/FEM/ARM-approved, data
from non-FRM/FEM/ARM monitors can be used in exceptional events analyses. For example,
air quality data summaries from non-FRM/FEM/ARM monitors may be helpful in defining the
duration and geographic extent of the event, including the area of exceedance/violation and the
area containing sources that contribute to exceedances/violations. Similarly, chemical speciation
data from monitor samples can help characterize the nature of the violation and identify
contributing emissions sources.
F.	Mitigation Plans
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F.l. Question: What is the purpose of mitigation plans under the Exceptional Events Rule?
Answer: Section 319(b)(3)(A) of the Clean Air Act identifies five principles that the EPA must
follow in developing implementing regulations for exceptional events:
(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.
The EPA finalized revisions to the Exceptional Events Rule, which governs the exclusion of
event-influenced air quality data, on September 30, 2016 (codified at 40 CFR 50.1, 50.14, and
51.930). Pursuant to the rule revisions, areas with historically documented or known seasonal
events are subject to the development of mitigation plans. With the overarching goal of
protecting public health, mitigation plan requirements (40 CFR 51.930) contain elements of
principles (i), (ii), and (iv) above that meet statutory requirements in section 319(b)(3)(A) of the
Clean Air Act. More generally, the purpose of mitigation plans under the Exceptional Events
Rule (40 CFR 51.930) is to ensure that air agencies take appropriate and reasonable actions to
protect public health from exceedances or violations of the NAAQS.
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The EPA also recognizes air agencies will generally be unable to prevent or "mitigate" the
occurrence of a natural event, which is why the primary objective of mitigation plans under the
Exceptional Events Rule is to minimize negative event-associated impacts on public health. The
six required components for mitigation plans (see Question F.2.) were developed with that
objective in mind. The EPA will be reviewing mitigation plans only to ensure the requisite
elements are included in the plan, and that an appropriate public comment process was followed
as part of its development process.
Additional information on mitigation plan components and the submission process can be found
in 40 CFR §51.930 of the Exceptional Events Rule.
F.2. Question: What are the required components of a mitigation plan?
Answer: At a minimum, each mitigation plan developed shall contain the following
components:
a.	Public notification and education programs for affected or potentially affected
communities.
b.	Steps to identify, study, and implement mitigating measures, including:
i.	measures to abate or minimize contributing controllable sources;
ii.	methods to minimize public exposure to high concentrations of identified
pollutants;
iii.	processes to collect and maintain data pertinent to the event; and
iv.	mechanisms to consult with other air quality managers in the affected area
with regards to appropriate responses to abate and minimize impacts.
c.	Provisions for periodic review and evaluation of the mitigation plan and its
implementation and effectiveness by the State and all interested stakeholders.
EPA Regional offices will review mitigation plans only for completeness {i.e., inclusion of the
required components above) rather than detailed review and approval. The EPA encourages air
agencies to leverage and incorporate by reference the applicable content of existing documents
when developing mitigation plans for exceptional events (see Question F.5.).
If possible, air agencies should notify the public of the actual or anticipated event at least 48
hours in advance. Outreach mechanisms include web site alerts, National Weather Service alerts,
telephone or text bulletins, and television or radio campaigns or other messaging campaigns.
Public notification and education may include adoption of methods for forecasting/detection,
consultation with appropriate health department personnel regarding health advisories and
recommended actions to reduce exposure. More information on the required components for an
exceptional events mitigation plan can be found in 40 CFR 51.930(b)(2), and a Mitigation Plan
Checklist designed to facilitate the development of a mitigation plan can be found - along with
other implementation resources - on EPA's exceptional events website at:
https://www.epa.gov/air-qualitv-analvsis/exceptional-events-implementation-tools-templates-
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F.3. Question: How will an air agency know if it is required to develop and submit a
mitigation plan?
Answer: In general, if an area is impacted by the same type of event for the same pollutant over
three consecutive years or for three seasons over a three-year period, development and
submission of a mitigation plan is required. Table 6 in the preamble to the 2016 Exceptional
Events Rule provided the initial list of areas that met the aforementioned criteria for event
recurrence, and for which mitigation plans are required. Mitigation plans are specific to area,
pollutant, and event type. The EPA will employ concepts articulated in the 2016 Exceptional
Events Rule and preamble to identify which areas may be subject to mitigation plan requirements
in the future. The EPA will solicit feedback from air agencies regarding this process, and future
notifications based on the established process for required mitigation plans will come from the
EPA Regional offices when the EPA identifies an air agency's obligation to develop such plans.
F.4. Question: Are the mitigation plan components reviewed each time a demonstration is
submitted?
Answer: No. The appropriate EPA Regional office will review each mitigation plan developed
once it is submitted, and notify the submitting air agency upon completion of such review and
completeness determination, not a review of the plan content. The EPA seeks to avoid redundant
effort on the part of air agencies to develop mitigation plans by allowing them to rely upon and
reference existing mitigation-related documents to satisfy mitigation plan requirements for
exceptional event demonstrations (see Question F.5.).
F.5. Question: What if an area already has a plan for public notification or other components
required in a mitigation plan?
Answer: The EPA recognizes that required mitigation plan components may be satisfied by
preexisting air agency documents with potentially applicable provisions such as High Wind
Action Plans, Emergency Episode Action Plans, and Smoke Management Programs. The EPA
therefore encourages air agencies to leverage and incorporate by reference the applicable content
of existing relevant documents when developing mitigation plans for exceptional events.
F.6. Question: Can the type of flag in AQS impact whether a mitigation plan is required?
Answer: Yes. EPA views event request data exclusion flags, or "R" flags, as documentation
from an air agency that, in its view, an event and its associated emissions contributed to an
exceedance or violation at the monitor for the relevant pollutant and area. Flagging data with an
"R" flag is part of the initial notification of intent to submit a demonstration and request
exclusions of measured exceedances of aNAAQS for a potential exceptional event (40 CFR
50.14(c)(2)(i)).
In general, if an area is impacted by the same type of event for the same pollutant over three
consecutive years or three event seasons over a three-year period, a mitigation plan must be
developed and submitted. The EPA would not count an "R" flag toward the aforementioned
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three-year event totals if the air agency submitted a demonstration for that area and pollutant and
the EPA nonconcurred on that demonstration.
The EPA suggests air agencies use informational "I" flags in cases where the relevant data may
have been influenced by an event, but has not been determined by an air agency to merit an
exceptional events demonstration. Informational flags, or "I" flags, will not count toward the
determination of mitigation plan requirements. Furthermore, the 2016 Exceptional Events Rule
revisions eliminated the general schedule deadlines for flagging events for data exclusion, so air
agencies no longer need to submit "R" flags soon after an event occurs to ensure the ability to
submit a future exceptional events demonstration.
F.7. Question: What happens if an area is subject to the requirements of a mitigation plan, but
submits an exceptional events demonstration without having developed a complete
mitigation plan?
Answer: Air agencies with identified areas shall submit mitigation plans to the applicable EPA
Regional Administrator within 2 years of notification of this requirement. After this initial 2-year
timeframe, even if a demonstration otherwise satisfies the Exceptional Events Rule criteria, the
EPA will not concur with an air agency's request to exclude data that have been influenced by an
event of the type that is the subject of a required mitigation plan until a mitigation plan for the
relevant area has been determined by the EPA to be complete. The EPA could, however, either
nonconcur or defer action on a demonstration for such event-influenced data.
In accordance with the five principles of Clean Air Act Section 319(b)(3)(A), the purpose of
mitigation plans under the Exceptional Events Rule is to protect public health through public
notification, minimization of exposure, and relevant source mitigation measures. If areas are
subject to the requirements of a mitigation plan, but do not have a mitigation plan that the EPA
has determined to be complete, then public health is at risk of not being adequately protected,
and data influenced by exceptional events shall not be concurred upon for regulatory purposes.
Once an agency has submitted a mitigation plan, the EPA will review each submitted plan and
verify that it includes the required elements. Within 60 days of receipt of such a plan, the EPA
intends to notify the submitting air agency that it has reviewed the mitigation plan and either
verified that it contains the required elements or identified any missing elements.
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