UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NC 27711 

HAY 1 0 2013 OFFICE OF 

AIR QUALITY PLANNING 

AND STANDARDS 

MEMORANDUM 

SUBJECT: 

FROM: 

TO: 

Interim Guidance to Implement Requirements for the Treatment of Air Quality 

Monitoring Data Influenced by E~cepti na1 7vents 

Stephen D. Page, Director A h . Jv\tut \f-ovltfAJ / 

Office of Air Quality Plann n~ \and Standards ~v 

Regional Air Directors, Regions I-X 

This memorandum and its attachments clarify key provisions of the 2007 Exceptional Events Rule 

(EER) to respond to questions and issues that have arisen since the rule was promulgated. The interim 

guidance in this memorandum and the attachments, along with examples of approved demonstrations on 

the EPA's website1 (collectively the "interim exceptional events guidance materials"), are provided to 

help ensure an efficient and effective process to make determinations regarding air quality data affected 

by exceptional events. Our intent is to streamline processes and reduce costs for air agencies2 preparing 

requests and the EPA offices reviewing these submittals. The EPA is neither setting new policies nor 

raising novel issues through this guidance. 

We first released draft exceptional events implementation guidance documents to air agencies, Federal 

Land Managers, and to other parties as requested, in May of2011. We incorporated some of the 

commenters' feedback into the revised draft guidance documents, which were made available for broad 

public review in a July 6, 2012, Federal Register Notice of Availability (77 FR 39959) and in the 

associated docket (Docket ID No. EPA-HQ-OAR-2011-0887). An accounting ofthe comment and 

response process from the 2011 preliminary review is documented in the docketed response to 

comments document. 3 

One important difference between the interim exceptional events guidance documents released today 

and the draft guidance documents made available to the public via the July 2012 Federal Register 

Notice of Availability is the EPA's clarification that this interim guidance is intended to provide 

recommendations and to indicate the EPA's current thinking on exceptional event issues, rather than 

conveying requirements not already stated in the Clean Air Act (CAA) and the EER. Additionally, the 

EPA revised the interim guidance materials to correct typographical errors, to make editorial changes to 

reflect the December 14, 2012, promulgation of the fine particle (PM2.s) National Ambient Air Quality 

Standards (NAAQS), and to reflect terminology consistent with the ongoing ozone NAAQS review. 

1 Additional information and examples of exceptional event submissions and best practice components can be found at the 

EPA's Exceptional Events website located at http://www.epa.gov/ttnlanalysis/exevents.htm. 2 References to "air agencies" are meant to include state, local, and tribal air agencies responsible for implementing the EER. 3 Response to Comment document titled, "Responses to Significant First-Round Comments on the Draft Guidance 

Documents on the Implementation of the Exceptional Events Rule." 

Internet Address (URL) • http://www.epa.gov 

Recycled/Recyclable • Printed w~h Vegetable Oil Based Inks on Recycled Paper (Minimum 25% Poslconsumer) 
2 

With this memorandum and its attachments, the EPA's Office of Air and Radiation is simultaneously 

announcing its intent to pursue revisions to the 2007 EER. We anticipate proposing these rule revisions 

in late 2013 or early 2014 and finalizing a revised rule in late 2014 or early 2015. As we move forward 

with a notice and comment rulemaking process, there will be an opportunity for all interested parties, 

including those that commented during the 2012 public comment period, to raise any issues or concerns. 

The EPA's regional offices should use the interim guidance as we undertake rule revisions because it is 

consistent with the EER and the guidance already provided in the preamble to the rule. 

The interim guidance materials are based on the following principles: 

1. Air agencies should not be held accountable for exceedances due to exceptional events that were 

beyond their control at the time of the event. 

2. It is desirable to implement reasonable controls to protect public health.4 

3. Clear expectations will enable the EPA and other air agencies to better manage resources related 

to the exceptional events process. 

These interim guidance materials identify the four independent criteria on which exclusion of eventaffected data depends, describe the administrative process and associated timing for submittal and 

review of demonstrations, provide answers to frequently asked questions, and provide previously 

reviewed demonstrations and best practice components. The EPA recognizes the challenges that air 

agencies face in preparing exceptional event demonstration packages. The EPA also recognizes the 

limited resources of the air agencies that prepare and submit exceptional event demonstration packages 

and of the EPA regional offices that review these demonstration packages. One of the EPA's goals in 

developing exceptional event implementation guidance is to establish clear expectations to enable 

affected air agencies to better manage resources as they prepare the documentation required under the 

EER. These interim guidance documents and the exceptional events website present examples to 

illustrate specific points. The example analyses and level of detail are not necessarily needed for all 

demonstrations. Submitters should prepare and submit the appropriate level of supporting 

documentation, which will vary on a case-by-case basis using the weight-of-evidence approach. The 

EPA anticipates that the resources needed to prepare (and review) packages will decrease as we continue 

to identify ways to streamline the process and continue to build our database of example demonstrations 

and analyses. In addition, extreme exceptional events may justify a more limited demonstration package. 

Exceptional Event Rule Provisions 

On March 22, 2007, the EPA promulgated the "Treatment of Data Influenced by Exceptional Events; 

Final Rule" (72 FR 13560) pursuant to the 2005 amendment ofCAA Section 319. This rule, known as 

the EER, superseded the EPA's previous natural events guidance and those sections ofthe interim fire 

policy document that address exceptional events. 5 The EER created a regulatory process codified at 40 

4 With respect to exceptional events, Section 319 of the CAA states the following guiding principles (among others); 

(i) the principle that protection of public health is the highest priority 

*** 

(iv) the principle that each State must take necessary measures to safeguard public health regardless of the source of the 

air pollution 5

Previous guidance and policy documents that either implied or documented the need for identifying data affected by an 

exceptional event include: 

i) "Guideline for Interpretation of Air Quality Standards," U.S. EPA, OAQPS No. 1.2-008, Revised February 1977. 

ii) "Guideline On the Identification and Use of Air Quality Data Affected by Exceptional Events" (the Exceptional Events 

Policy), U.S. EPA, OAQPS, July 1986. 
3 

CFR parts 50 and 51 (50.1, 50.14 and 51.930). These regulatory sections contain definitions, procedural 

requirements, requirements for air agency demonstrations, and criteria for EPA approval for the 

exclusion of air quality data from regulatory decisions under the EER. 

The definition of an exceptional event at 40 CFR §50.1 (j) repeats the CAA definition, which provides 

that an exceptional event is one that affects air quality, is not reasonably controllable or preventable, and 

is caused by human activity that is unlikely to recur at a particular location or a natural event. 40 CFR 

§50.1 (k) further defines a natural event as one in which human activity plays little or no direct causal 

role. Additional requirements in 40 CFR §50.14( a)(2) and (b )(1) identify that an air agency must 

demonstrate "a clear causal relationship between the measured exceedance or violation of such standard 

and the event" and that "an exceptional event caused a specific air pollution concentration in excess of 

one or more national ambient air quality standards." The rule further requires at 40 CFR §50.14(c)(3)(iv) 

that the demonstration to justify data exclusion shall provide evidence that the event is associated with a 

measured concentration in excess of normal historical fluctuations, including background, and evidence 

that there would have been no exceedance or violation but for the event. 

Treatment of Technical Criteria for Exclusion of Data Affected by Events 

When considered together, the EER provisions summarized above identify the following six elements 

that air agencies must address when requesting that the EPA exclude event-related concentrations from 

regulatory determinations: 

• the event affected air quality 

• the event was not reasonably controllable or preventable 

• the event was caused by human activity that is unlikely to recur at a particular location, or was a 

natural event 

• there exists a clear causal relationship between the specific event and the monitored 

concentration 

• the event is associated with a measured concentration in excess of normal historical fluctuations 

including background 

• there would have been no exceedance or violation but for the event 

In reviewing exceptional events demonstration packages, the EPA has found that the following EER 

elements, along with historical fluctuations, play a significant role in the air agencies' supporting 

documentation: 

1. not reasonably controllable or preventable 

2. ifthe event was caused by human activity, that human activity is unlikely to recur at a particular 

location6 

iii) "Areas Affected by PMlO Natural Events" (the PMlO Natural Events Policy), memorandum from Mary D. Nichols, 

Assistant Administrator for Air and Radiation, to the EPA regional offices, May 30, 1996. 

iv) "The Interim Air Quality Policy on Wildland and Prescribed Fires" (the Interim Fire Policy), memorandum from Richard 

D. Wilson, Acting Assistant Administrator for Air and Radiation, to the EPA regional administrators, May 15, 1998. 

v) "Guideline on Data Handling Conventions for the PM NAAQS," U.S. EPA, OAQPS, EP A-454/R-98-0 17, December 

1998. 

6 Neither the statutory nor regulatory defmition of "exceptional event" requires a demonstration of "unlikely to recur" for 

natural events. 
4 

3. clear causal relationship between specific event and monitored concentration 

4. no exceedance or violation but for the event 7 

As described in the interim guidance materials, the EPA's technical review of a demonstration package 

therefore focuses on these elements. While the EER requires and the EPA anticipates complete 

demonstration packages to contain narrative and evidence supporting all six elements, the EPA's 

position is that these four elements represent distinct facts for air agencies to demonstrate for the EPA to 

concur on an event claim. 8 If an event is natural, then the second element (e.g., human activity unlikely 

to recur) is generally not considered in a demonstration review. In the case of an event that is initiated by 

a natural process, such as a volcano or high wind dust event, the event would be considered a natural 

event if sources are entirely natural or contributing anthropogenic sources are reasonably controlled. 9 

This concept is explained in more detail in Attachment 2, the interim High Winds Guidance document. 

The EPA recognizes the inherent linkages between all six elements and expects that some sections of a 

demonstration package (e.g., affects air quality, natural event) may repeat or refer to other sections of 

the demonstration package (e.g., clear causal relationship, but for). Further, each potential event can 

have varied and differing characteristics, and thus would usually necessitate a case-specific 

demonstration and evaluation. Therefore, the EPA would use a "weight-of-evidence" approach in 

evaluating each element within an exceptional event demonstration package. 

In the interim guidance materials, the requirement that the event was not reasonably controllable or 

preventable, which is part of the definition of an exceptional event in both the CAA and the EER, would 

mean that if a set of control measures should reasonably have been in place for contributing sources at 

the time of the event, then it must have been in place for the event to qualify as an exceptional event 

under the EER. Whether a set of controls should reasonably have been in place is event-, time-, and 

place-dependent, and involves judgement by the air agency when preparing the demonstration and by 

the EPA when reviewing the demonstration. The EER requirement for reasonable control applies to all 

events but is more complicated for high wind dust events because these events typically include both 

natural and anthropogenic sources of dust. In contrast, an event such as a lightning-induced wildfire 

7 Criteria 1, 3, and 4 on this list, along with historical fluctuations, are considered "independent elements" in the interim High 

Winds Guidance document. 8 The EPA generally does not consider the two remaining elements, "affects air quality" and "historical fluctuations," to 

represent "distinct facts." The EPA believes that the "affects air quality'' element is generally satisfied once the air agency 

satisfies the clear causal and historical fluctuations showings. While the "historical fluctuations" element is considered an 

independent element, it also plays an important role in the "clear causal relationship" and "no exceedance but for" 

demonstrations. The EPA will review air agency submissions using a weight-of-evidence approach. The air agency's role in 

satisfying this element is to provide appropriate analyses and statistics comparing the event-affected concentration to normal 

historical fluctuations and conclude that the provided data show that the event was in excess of normal historical fluctuations. 

The EPA will review the information provided by the air agency. ''Normal historical fluctuations" will generally be defined 

by those days without events for the previous years. The EPA acknowledges that natural events can recur and still be eligible 

for exclusion under the EER; therefore, events do not necessarily have to be rare to satisfy this element. However, in most 

cases, the EPA anticipates that less conclusive "historical fluctuations" comparisons will likely indicate less conclusive "clear 

causal relationship" and/or "no exceedance but for" relationships. 9 The EPA will generally consider human activity to have played little or no direct role in causing emissions of the dust 

generated by high wind for purposes of the regulatory defmition of "natural event" if contributing anthropogenic sources of 

dust are reasonably controlled, regardless of the amount of dust coming from these reasonably controlled anthropogenic 

sources, and thus the event could be considered a natural event. In such cases, the EPA believes that it would generally be a 

reasonable interpretation of its regulations to fmd that the anthropogenic source had "little" direct causal role. If 

anthropogenic sources of windblown dust that are reasonably controllable but that did not have those reasonable controls 

applied at the time of the high wind event have contributed significantly to a measured concentration, the event would not be 

considered a natural event. See footnote 11, 72 FR 13566. 
5 

generally does not include an anthropogenic contribution to the event. 10 Among other factors to 

consider, reasonableness would need to be judged in light of the technical information available to the 

air agency at the time the event occurred. The EPA anticipates that nonattainment areas already have the 

technical information needed to reasonably control anthropogenic sources in their jurisdiction. 

Generally, the EPA does not expect areas classified as attainment, unclassifiable, or maintenance for a 

NAAQS to have the same level of controls as areas that are nonattainrnent for the same NAAQS. Also, 

if an area has been recently designated to nonattainment but has not yet been required to implement 

controls, the EPA will expect the level of controls that is appropriate for the planning stage. Regardless 

of attainment status or natural/anthropogenic source contribution, each demonstration package should 

address the question of reasonable controls. In general, reasonable controls would not include any 

control on emissions-generating activity outside of the state or tribal boundaries ofthe state (or tribal 

lands) within which the concentration at issue was monitored. As with the other elements, whether an 

event was not reasonably controllable or preventable would be evaluated on a case-by-case basis. If and 

when the EPA takes a regulatory action that hinges on a decision to exclude data under the EER, the 

EPA will consider and appropriately respond to any public comments on whether the event was "not 

reasonably controllable or preventable." 

Timing of EER Demonstration Package Submittal and Review 

The EPA understands that the initial identification of data affected by exceptional events and the 

subsequent preparation, submittal, and review of demonstration packages is a resource intensive process. 

Delays in processing and making decisions on submitted packages create regulatory uncertainty and 

potentially increase the workload for both the submitting air agency and the EPA. In addition, the 

backlog of pending actions makes retrieval of data to support new submittals potentially more difficult. 

Further, air agencies and the EPA often face timelines by which they must make regulatory decisions 

that can be affected by the inclusion or exclusion of event-affected data. 

The EPA will work with air agencies as they prepare complete demonstration packages that meet the 

requirements of the EER. In an effort to streamline this identification~ preparation, submittal, and review 

process, the EPA has developed the following interim guidelines. 

1. Identification of data affected by exceptional events in the EPA's Air Quality System (AQS) -

The EPA is aware that air agencies routinely review their air quality monitoring data, which may 

result in the identification of certain data being affected by an exceptional event. Although air 

agencies may flag any data in AQS that they wish to flag, the EPA encourages air agencies to 

flag only data that might have a regulatory consequence and for which an approvable 

demonstration is likely. 11 Should air agencies wish to flag values for informational purposes, the 

EPA prefers that they use the AQS flags intended for this purpose. 

10 The EPA recognizes that wildfires and emissions from wildfires are generally not reasonable to prevent or control. 

Although the EER requires documentation of this criterion for all event types, the EPA believes that it will generally be 

sufficient for air agencies to provide a brief statement to document the "not reasonably controllable or preventable" criterion 

for wildfires. See Question 20b of the Interim Exceptional Events Rule Frequently Asked Questions document for example 

language. 

11 Air agencies should place flags and an initial event description in AQS either in accordance with the special schedules 

promulgated with new or revised NAAQS or in accordance with the general AQS data submission schedules (i.e., within 90 

days of the end of the previous quarter) but not later than July 1st of the calendar year following the event in which the 

flagged measurement occurred. Note that for data certification purposes, we recommended flagging data prior to submitting 

data certification (May 1 51

). · 
6 

2a. Air agency submittal of letter of intent to submit' a package (optional)- To promote early 

communication, the EPA suggests that air agencies provide a letter of intent to submit a 

demonstration package for flagged data in AQS as soon as possible, if possible within 12 months 

from the event occurrence, after the air agency identifies the event(s) as being significant. 12 This 

initial notification can assist both the air agency and the EPA in the planning and prioritization 

process. 

2b. Air agency notification of intent to submit a package (optional) - Air agencies choosing not to 

submit a letter of intent are still encouraged to contact their EPA regional office more informally 

to alert it of the forthcoming demonstration submittal. 

3. EPA response to air agency letter of intent- The EPA anticipates responding to the air agency's 

letter of intent within 60 days of receipt. The EPA response will provide the regional office's 

best assessment of the priority that can be given to the submission once received and any casespecific advice the EPA may have to · offer for the preparation of the demonstration. 

4. Air agency submittal of exceptional event demonstration packages13 - Air agencies should 

prepare a technical demonstration package, taking into account the information in the EPA's 

guidance documents, which shows that a particular air quality monitored value(s) was influenced 

by an exceptional event. The EPA acknowledges that extreme exceptional events may justify 

more limited demonstration packages. Air agencies that believe their demonstration packages are 

tied to near-term regulatory actions should 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 in the cover letter 

that accompanies their initial submittal package to the reviewing EPA regional office. 

5. EPA prioritization of submitted demonstration packages- The EPA will generally give priority 

to exceptional event determinations that may affect near-term regulatory decisions, such as state 

implementation plan (SIP) submittal actions, NAAQS designations, and clean data findings. 

6. EPA review of prioritized demonstration packages- The EPA generally intends to conduct its 

initial review of a submitted exceptional event demonstration package within 120 days of receipt. 

Following this initial review, the EPA will generally send a letter to the submitting air agency 

that includes a completeness determination and/or a request for additional information, a date by 

which the supplemental information should be submitted (if applicable), and an indicator of the 

timing of the EPA's final review. The EPA encourages air agencies to provide supplemental 

information for which the EPA asks. The EPA anticipates a 60-day response time for states to 

12 The Letter of Intent is an optional step and the EPA recognizes that air agencies may need additional time to prepare and 

submit demonstration packages particularly where the basis of the exclusion is violating an annual standard or a 3-year 

design value. Similarly, an air agency could consider submitting an annual letter of intent if annual submittal makes sense for 

resource planning or for historically seasonal events. If an air agency decides to submit a letter of intent, the EPA 

recommends that it be submitted as expeditiously as possible after the air agency identifies the event or events as having 

significance. 

13 The general schedule in the EER allows air agencies to submit packages up to 3 years following the end of the calendar 

quarter in which the event occurred, or 12 months prior to the date that a regulatory decision must be made by the EPA. 

When the EPA promulgates a new or revised NAAQS, we may change this schedule to allow air agencies to flag and submit 

documentation for data relevant to the new/revised NAAQS. 
7 

provide additional requested information. 14 The EPA intends to make a decision regarding event 

concurrence as expeditiously as·necessary if required by a near-term regulatory action, but no 

later than 18 months following submittal of a complete package. Determinations on exceptional 

event demonstrations do not constitute final agency action until they are relied upon in a 

regulatory decision such as a finding of attainment or nonattainment which will be conducted 

through notice-and-comment rulemaking procedures. 

Interim Exceptional Events Rule Frequently Asked Questions Document (Attachment 1) 

The "Interim Exceptional Events Rule Frequently Asked Questions" document (the interim Q&A 

document) provides interim responses to questions that have arisen since the EPA promulgated the EER. 

The questions are grouped into six broad areas. The EPA encourages those involved in flagging data and 

preparing demonstration packages to review the questions and answers and to provide input regarding 

their usefulness and appropriateness and regarding additional questions which need answers. The 

following bullets identify key points of interest in the interim Q&A document: 

• Natural events, such as volcanic eruptions, do not have to be infrequent to qualify as exceptional 

events under the EER (see Question 1 of the interim Q&A document). Frequent events with 

natural triggers that have a contribution from anthropogenic activities that are reasonably 

controlled could be eligible "exceptional" events, provided the events meet the demonstration 

requirements for the technical criteria. 

• The EER does not prohibit air agencies from flagging individual concentration values below the 

level of the NAAQS. However, in general, only such data that contribute to a violation of the 

NAAQS are excludable. Questions 29-31 describe the few, limited situations in which 

concentration values below the level of the NAAQS contribute to viol~tions of the NAAQS. 

• An event that an air agency has concluded is associated with a measured concentration "in 

excess of normal historical fluctuations" will be reviewed using a weight-of-evidence approach. 

The comparison of the measured concentration to normal historical concentrations will also 

influence how much information is needed to successfully meet other technical elements. For 

example, when the observed concentration is high compared to historical concentrations, the 

EPA may need less additional evidence to demonstrate the "but for" finding. Questions 1-5 

provide recommendations for showing how the observed concentration compares to the 

distribution ofhistorical concentrations. 

• Question 6 describes types of evidence that could be submitted as part of a demonstration 

showing that an ozone exceedance would not have occurred but for the effect of a fire event. In 

particular, statistical or photochemical dispersion model predictions of the ozone concentration 

that would have occurred in the absence of the fire would be a relevant type of evidence, 

provided the demonstration package is transparent about the technical basis for the model and its 

uncertainties. Also, as noted below, the EPA intends to .develop a separate draft document to 

provide guidance in preparing demonstrations for wildfire-related events that influence ambient 

ozone concentrations. 

14The EPA recognizes that air agencies may need more than 60 days to prepare and submit some types of supplemental 

information. The EPA will work with air agencies on supplemental timeframes; however, the mandatory timing of the EPA 

actions may limit the response time the EPA allows. 
8 

• Not every natural or infrequent anthropogenic event that affects air quality is a true "exceptional 

event" under the definition of that term in the EER. Ambient data affected by an event that does 

not meet the "but for" criterion cannot be excluded under the authority of the EER even if in all 

other respects the event meets the definition of an exceptional event. The EER does not address 

data handling associated with events that are not considered "exceptional" under the EER, and 

does not provide the EPA with authority to exclude such data. Yet, the event-related 

concentration could still influence design values. An air agency incorporating the event-related 

concentration in a design value used for a prospective attainment demonstration might seem to 

need more emission reductions to attain the NAAQS by its attainment deadline than is actually 

the case. The EPA plans to more formally address this topic on a pollutant/NAAQS basis, the 

first of which will be ozone guidance in the preamble of a soon-to-be-proposed rulemaking on 

SIP requirements for areas designated nonattainment for the 2008 ozone NAAQS. Until the 

planned guidance for a pollutant and NAAQS of interest is issued, air agencies should consult 

with their EPA regional office if they face this situation. The EPA further discusses this issue in 

Question 13. 

• To remove any possible confusion, the passages of the preamble that were declared to be a legal 

nullity by the court that reviewed the EER are specifically identified in Question 20. 15 While air 

agencies canriot rely on these passages as the EPA guidance on interpretation of the EER, this 

interim guidance overview document and its attachments are consistent with those sections. 

• The EPA identifies in Question 28a currently existing mechanisms that air agencies can use at 

various points in the exceptional events process to resolve disagreements regarding nonconcurrence on submittal packages. 

Interim High Winds Guidance Document (Attachment 2) 

The attached "Interim Guidance on the Preparation of Demonstrations in Support of Requests to 

Exclude Ambient Air Quality Data Affected by High Winds Under the Exceptional Events Rule" (the 

High Winds Guidance document) is a resource for air agencies when flagging data and preparing 

demonstrations packages for high wind dust events that have affected particulate matter concentrations -

both particles less than or equal to 10 micrometers (J.Lm) in diameter (PM10) and PM2.5. The interim 

document applies the provisions of the EER, the general guidance conveyed in this memorandum, and 

the general guidance in the interim Q&A document to the particular situation of a high wind dust event. 

While the document is specific to high wind dust events, it outlines how the EPA generally intends to 

implement the preparation and review process for exceptional events and, therefore, may have relevance 

for other types of exceptional events. The following are some of the highlights of the interim High 

Winds Guidance document: 

• In nonattainment areas, a reference point for considering what constitutes reasonable control of 

wind-blown dust during high wind events would be the set of measures that are identified as 

Reasonably Available Control Measures (RACM) or Best Available Control Measures (BACM) 

in the approved SIPs of other areas with similar wind-blown dust conditions, depending on area 

classification. U.S. Department of Agriculture (USDA) I Natural Resources Conservation 

Service (NRCS)-approved conservation management practices designed to effectively reduce 

fugitive dust air emissions and prevent loss of soil during high winds could also be considered. 

15 SeeNRDCv. EPA, No. 07-1151 (D.C. Cir. 3/20/09). 
9 

All dust-related control measures and/or dust suppression measures in an area's own approved 

SIP should be considered part of the set of controls that would have been reasonable to have 

been in place at the time of the event. The assessment of whether an event was not reasonably 

controllable will be made on a case-by-case basis. Like other elements included in an exceptional 

events demonstration, when the EPA takes a regulatory action that hinges on a decision to 

exclude data under the EER, the EPA will consider and appropriately respond to any public 

comments on whether the event was "not reasonably controllable or preventable." 

• Reasonable controls generally would not need to be implemented for wind-blown dust from 

undisturbed natural landscapes or previously disturbed landscapes that are being allowed to 

return to natural conditions. 

• For purposes of qualifying for the exclusion of data affected by wind events with sustained wind 

speeds above 25 miles per hour (or above another threshold determined to be appropriate for a 

particular area), the demonstration of reasonable controls appliedto disturbed landscapes and 

other anthropogenic sources of dust could be less rigorous because: (1) the contribution from 

natural undisturbed lands is likely to be high and, (2) at such high wind speeds many available 

controls would have been ineffective in significantly reducing wind-generated dust emissions. 

• In response to commenter feedback, the EPA has added the optional prospective controls 

analysis, which air agencies can prepare to document existing controls and facilitate the EPA's 

review and evaluation of the not reasonably controllable and preventable criterion. In the 

prospective controls analysis, the air agency would provide information on attainment status, 

identify natural and anthropogenic windblown dust sources and emissions, provide the status of 

SIP submittals and their implementation (if applicable), and identify the wind speed up to which 

the collective windblown dust controls are expected to be effective. Air agencies would submit 

their prospective controls analysis in advance of an air agency submittal and the EPA review of 

any specific demonstration submittal, with a letter of intent, or with their demonstration package 

submittal. The EPA review and approval of controls and an appropriate high wind threshold 

would typically be effective for a minimum of three years. 

• If the EPA has approved a SIP containing wind-blown dust controls within the past three years, 

then the submitting air agency has the option of using their current, implemented SIP-approved 

controls and specifying a high wind threshold to which the controls are expected to be effective 

to constitute the set of controls that would have been reasonable to implement. 

• Air agencies are encouraged to work with the EPA regional offices to develop High Wind Action 

Plans, which need not be incorporated into the SIP, as a way to develop a mutual understanding 

ofwhat additional controls are reasonable to implement in light of foreseeable high wind 

conditions. Similar to a Natural Events Action Plan16, the optional High Wind Action Plari is a 

mechanism to implement necessary controls on newly-identified sources needing reasonable 

controls such that the EPA could consider future high wind events not reasonably controllable or 

preventable. 

160n May 30, 1996, Mary D. Nichols, Assistant Administrator for Air and Radiation issued a memorandum to the EPA 

regional offices titled, "Areas Affected by PM10 Natural Events." The policy, known as the PM10 Natural Events Policy, or 

sirnply the Natural Events Policy, set forth procedures for protecting public health through the development of a Natural 

Events Action Plan, which implements Best Available Control Measures for human-generated particulate emissions in areas 

that could violate the PM10 NAAQS due to natural events. Promulgation of the EER superseded the Natural Events Policy. 
10 

On-line Availability of Exceptional Event Packages and Best Practice Components 

To assist air agencies in deciding what type and how much evidence/technical analysis to include in 

their demonstration packages, the EPA has developed a public website at 

http://www. epa.gov/ttnlanalysis/exevents. htm that contains EPA-approved demonstration packages and 

links to best practice components. The EPA developed this website to provide examples to illustrate 

specific points; the example analyses and level of rigor are not necessarily needed for all 

demonstrations. The website will continue. to evolve as air agencies submit, and the EPA reviews, 

additional demonstration packages. 

Draft Guidance Under Development 

The EPA is currently developing a separate draft guidance document addressing the preparation of 

demonstrations to support data exclusion requests for wildfire-related events that may have affected 

ozone concentrations. We anticipate preparing this guidance within the same timeframe as the EER 

revisions with draft guidance available in late 2013/early 2014 and final guidance available in late 

2014/early 2015. We will provide an opportunity for stakeholder input on this guidance. 

Conclusion 

Regional offices should use the interim guidance provided in this overview document and its 

attachments as we undertake rule revisions, because it is consistent with the EER and the guidance 

already provided in the preamble to the rule. 

Staff in the EPA's Office of Air Quality Planning and Standards are available for assistance and 

consultation. For interim guidance-related questions, please contact Beth W. Palma at (919) 541-5432 or 

palma. elizabeth@epa.gov. 

Attachments: 

1. Interim Exceptional Events Rule Frequently Asked Questions 

2. Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude Ambient 

Air Quality Data Affected by High Winds Under the Exceptional Events Rule