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 6560-50-P 

ENVIRONMENTAL PROTECTION AGENCY 

40 CFR Part 50 

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

RIN 2060-AS02 

Treatment of Data Influenced by Exceptional Events – Rule Revisions and Notice of 

Availability for Related Draft Guidance 

AGENCY: Environmental Protection Agency (EPA). 

ACTION: Proposed rule; notice of availability. 

SUMMARY: The Environmental Protection Agency (EPA) is proposing revisions to certain 

sections within the March 22, 2007, “Treatment of Data Influenced by Exceptional Events; Final 

Rule” (2007 Exceptional Events Rule) that governs the exclusion of event-affected air quality 

data from regulatory decisions. The EPA is also providing a notice of availability of a draft 

version of the non-binding guidance document titled, Draft Guidance on the Preparation of 

Exceptional Events Demonstrations for Wildfire Events that May Influence Ozone 

Concentrations. 

DATES: Comments. Written comments on this proposal and draft guidance must be received by 

January 19, 2016. 

Public hearing: The EPA will hold a public hearing on this proposal on Tuesday, December 8, 

2015, in Phoenix, Arizona. Please refer to SUPPLEMENTARY INFORMATION for 

additional information on the comment period and public hearing. 

ADDRESSES: Submit your comments on the EPA’s proposed revisions to the 2007 Exceptional 

Events Rule, identified by Docket ID No. EPA-HQ-OAR-2013-0572, at 

http://www.regulations.gov. Follow the online instructions for submitting comments. Once 
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submitted, comments cannot be edited or removed from Regulations.gov. The EPA may publish 

any comment received to its public docket. Do not submit electronically any information you 

consider to be Confidential Business Information (CBI) or other information whose disclosure is 

restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by a 

written comment. The written comment is considered the official comment and should include 

discussion of all points you wish to make. The EPA will generally not consider comments or 

comment contents located outside of the primary submission (i.e., on the web, cloud or other file 

sharing system). For additional submission methods, the full EPA public comment policy, 

information about CBI or multimedia submissions and general guidance on making effective 

comments, please visit http://www2.epa.gov/dockets/commenting-epa-dockets. 

Submit your comments on the EPA’s Draft Guidance on the Preparation of Exceptional 

Events Demonstrations for Wildfire Events that May Influence Ozone Concentrations, identified 

by Docket ID No. EPA-HQ-OAR-2015-0229, at http://www.regulations.gov. Follow the online 

instructions for submitting comments. Once submitted, comments cannot be edited or removed 

from Regulations.gov. The EPA may publish any comment received to its public docket. Do not 

submit electronically any information you consider to be CBI or other information whose 

disclosure is restricted by statute. Multimedia submissions (audio, video, etc.) must be 

accompanied by a written comment. The written comment is considered the official comment 

and should include discussion of all points you wish to make. The EPA will generally not 

consider comments or comment contents located outside of the primary submission (i.e., on the 

web, cloud or other file sharing system). For additional submission methods, the full EPA public 

comment policy, information about CBI or multimedia submissions and general guidance on 

making effective comments, please visit http://www2.epa.gov/dockets/commenting-epa-dockets. 
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Docket. As discussed above, the EPA has established one docket for the proposed 

revisions to the 2007 Exceptional Events Rule and another docket for the draft guidance 

document. All documents in these dockets are listed on the http://www.regulations.gov website 

in the respective docket. The rulemaking docket is Docket ID No. EPA-HQ-OAR-2013-0572. 

The separate docket established for the Draft Guidance on the Preparation of Exceptional Events 

Demonstrations for Wildfire Events that May Influence Ozone Concentrations is Docket No. 

EPA-HQ-OAR-2015-0229. The EPA will not respond to comments relating to the guidance 

document as part of this rulemaking, but will consider these comments in the development of the 

guidance document. If comments on the draft guidance document are submitted to the 

rulemaking docket, the EPA will respond only to the portion of such comments that are relevant 

to the rulemaking. The EPA also relies on the documents in Docket ID No. EPA-HQ-OAR2011-0887, the docket established for the July 2012 notice of availability for the Draft 

Exceptional Events Implementation Guidance, and incorporates this docket into the record for 

this action. However, no new comments may be directed to Docket ID No. EPA-HQ-OAR-2011-

0887 and the EPA will not respond to comments that have already been submitted to this docket 

unless they are resubmitted to Docket ID No. EPA-HQ-OAR-2013-0572. Although listed in the 

indices to the rulemaking docket and the guidance docket associated with this action (i.e., Docket 

ID No. EPA-HQ-OAR-2013-0572 and Docket No. EPA-HQ-OAR-2015-0229), some 

information is not publicly available, (e.g., CBI or other information whose disclosure is

restricted by statute). Certain other material, such as copyrighted material, will not be placed on 

the Internet but may be viewed, with prior arrangement, at the EPA Docket Center. Publicly 

available docket materials are available either electronically in www.regulations.gov or in hard 

copy at the Air and Radiation Docket and Information Center, EPA/D.C., EPA William Jefferson 
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Clinton West Building, Room 3334, 1301 Constitution Avenue, NW, Washington, D.C. The 

Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding 

legal holidays. The telephone number for the Public Reading Room is (202) 566-1744 and the 

telephone number for the Air and Radiation Docket and Information Center is (202) 566-1742. 

For additional information about the EPA’s public docket, visit the EPA Docket Center 

homepage at: http://www.epa.gov/epahome/dockets.htm. 

Public hearing: A public hearing will be held on Tuesday, December 8, 2015, in room 

3175 in the Arizona Department of Environmental Quality main office building located at 1110 

W. Washington Street, Phoenix, Arizona 85007. The public hearing will convene at 10:00 a.m. 

and continue until the earlier of 6:00 p.m. or 1 hour after the last registered speaker has spoken. 

We have scheduled a lunch break from 12:30 p.m. until 2:00 p.m. People interested in presenting 

oral testimony should contact Ms. Pamela Long, Air Quality Planning Division, Office of Air 

Quality Planning and Standards (C504-01), U.S. Environmental Protection Agency, Research 

Triangle Park, NC 27711, telephone (919) 541-0641, fax number (919) 541-5509, email address 

long.pam@epa.gov, at least 2 days in advance of the public hearing (see DATES). People 

interested in attending the public hearing should also call Ms. Long to verify the time, date and 

location of the hearing. The public hearing will provide interested parties the opportunity to 

present data, views or arguments concerning the proposed revisions to the 2007 Exceptional 

Events Rule. The EPA will make every effort to accommodate all speakers who arrive and 

register. Individuals planning to attend the hearing will be required to sign in, and may be 

required to show valid picture identification to the security staff to gain access to the meeting 

room. In addition, no weapons will be allowed in the facility. Any weapons brought to the site 

will be stored in a locker at the facility. No large signs will be allowed in the building, cameras 
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may only be used outside of the building. The EPA may ask clarifying questions during the oral 

presentations but will not respond to the presentations at that time. Written statements and 

supporting information submitted during the comment period will be considered with the same 

weight as oral comments and supporting information presented at the public hearing. 

Commenters must submit written comments on the proposed rule and/or draft guidance by 

January 19, 2016. Commenters should notify Ms. Long if they will need specific equipment, or if 

there are other special needs related to providing comments at the hearing. The EPA will provide 

equipment for commenters to show overhead slides or make computerized slide presentations if 

we receive special requests in advance. Oral testimony will be limited to 5 minutes for each 

commenter. The EPA encourages commenters to provide the EPA with a copy of their oral 

testimony electronically (via email or CD) or in hard copy form. The hearing schedule, including 

the list of speakers, will be posted on the EPA’s website at http://www2.epa.gov/air-qualityanalysis/treatment-data-influenced-exceptional-events. Verbatim transcripts of the hearing and 

written statements will be included in the docket for the rulemaking. The EPA will make every 

effort to follow the schedule as closely as possible on the day of the hearing; however, please 

plan for the hearing to run either ahead of schedule or behind schedule. 

FOR FURTHER INFORMATION CONTACT: For additional information regarding 

this proposed rule, please contact: Beth W. Palma, U.S. EPA, Office of Air Quality Planning and 

Standards, Air Quality Policy Division, Mail Code C539-04, Research Triangle Park, NC 27711, 

telephone (919) 541–5432, email at palma.elizabeth@epa.gov. For additional information 

regarding the Draft Guidance on the Preparation of Exceptional Events Demonstrations for 

Wildfire Events that May Influence Ozone Concentrations, please contact Melinda Beaver, U.S. 

EPA, Office of Air Quality Planning and Standards, Air Quality Policy Division, Mail Code 
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C539-04, Research Triangle Park, NC 27711, telephone (919) 541-1062, email at 

beaver.melinda@epa.gov. For information on the public hearing or to register to speak at the 

hearing, contact Ms. Pamela Long, U.S. Environmental Protection Agency, Office of Air Quality 

Planning and Standards, Air Quality Planning Division, Mail Code C504-01, Research Triangle 

Park, NC 27711, telephone (919) 541-0641, fax number (919) 541-5509, email at 

long.pam@epa.gov (preferred method for registering). 

SUPPLEMENTARY INFORMATION: 

I. General Information 

A. Does this action apply to me? 

Entities potentially directly affected by this proposal and the draft guidance document 

include all state air agencies and any local air quality agency to whom a state has delegated 

relevant responsibilities for air quality management, including air quality monitoring and data 

analysis. Tribal air agencies operating ambient air quality monitors that produce regulatory data 

may also be directly affected. Entities potentially affected indirectly by this proposal and the 

draft guidance document include federal land managers (FLMs) of Class I areas, other federal 

agencies and other entities that operate ambient air quality monitors and submit collected data to 

the EPA’s Air Quality System (AQS) database. 

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

1. Submitting CBI. Do not submit this information to the EPA through 

http://www.regulations.gov or email. Clearly mark the part or all of the information that you 

claim to be CBI. For CBI information in a disk or CD-ROM that you mail to the EPA, mark the 

outside of the disk or CD-ROM as CBI and then identify electronically within the disk or CDROM the specific information that is claimed as CBI. In addition to one complete version of the 
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comment that includes information claimed as CBI, a copy of the comment that does not contain 

the information claimed as CBI must be submitted for inclusion in the public docket. Information 

so marked will not be disclosed except in accordance with procedures set forth in 40 CFR part 2. 

2. Tips for Preparing Your Comments. When submitting comments, remember to:

● Identify the rulemaking and/or draft guidance document by docket number and 

other identifying information (subject heading, Federal Register date, page 

number and guidance document title, if applicable). 

● Follow directions - The agency may ask you to respond to specific questions or 

organize comments by referencing a Code of Federal Regulations (CFR) part or 

section number in the guidance. 

● Explain why you agree or disagree; suggest alternatives and substitute language 

for your requested changes. 

● Describe any assumptions and provide any technical information and/or data that 

you used. 

● If you estimate potential costs or burdens, explain how you arrived at your 

estimate in sufficient detail to allow for it to be reproduced. 

● Provide specific examples to illustrate your concerns, and suggest alternatives. 

● Explain your views as clearly as possible, avoiding the use of profanity or 

personal threats. 

● Make sure to submit your comments by the identified comment period deadline. 

C. Where can I get a copy of these documents and other related information? 
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 In addition to being available in the docket, an electronic copy of this notice and the draft 

guidance will be posted at http://www2.epa.gov/air-quality-analysis/treatment-data-influencedexceptional-events. 

D. What should I know about the public hearing?

The EPA intends to hold a public hearing on Tuesday, December 8, 2015, in room 3175 

in the Arizona Department of Environmental Quality main office building located at 1110 W. 

Washington Street, Phoenix, Arizona 85007. If you would like to attend or speak at the public 

hearing, please contact Ms. Pamela Long, U.S. Environmental Protection Agency, Office of Air 

Quality Planning and Standards, Air Quality Planning Division, Mail Code C504-01, Research 

Triangle Park, NC 27711, telephone (919) 541-0641, fax number (919) 541-5509, email at 

long.pam@epa.gov (preferred method for registering) at least 2 days in advance of the public 

hearing (see DATES). Interested parties may submit oral and/or written comments. Interested 

parties do not need to attend the public hearing to submit written comments. Additional details 

concerning any public hearing for this proposed rule will be posted on the EPA’s website for this 

rulemaking at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptionalevents. 

E. How is this document organized?

The information presented in this document is organized as follows: 

I. General Information 

A. Does this action apply to me? 

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

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

D. What should I know about the public hearing? 

E. How is this document organized? 

II. Glossary of Terms and Acronyms 

III. Executive Summary 

IV. Background for Proposal 

A. Purpose of and Statutory Authority for this Regulatory Action 
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B. The 2007 Exceptional Events Rule 

C. Early Experience in Implementing the 2007 Exceptional Events Rule 

D. The EPA’s Interim Exceptional Events Implementation Guidance 

E. More Recent Implementation Experience Including EPA-Recommended Best 

Practices for the Development of Exceptional Events Demonstrations 

V. Proposed Rule Revisions 

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

 1. Current Situation 

 2. Proposed Changes 

B. What is an exceptional event? 

1. Current Situation 

2. Proposed Changes 

C. What types of ambient concentration data and data uses may be affected by the 

Exceptional Events Rule? 

1. Current Situation 

2. Proposed Changes 

D. What is a natural event? 

1. Current Situation 

2. Proposed Changes 

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

1. Human Activity Unlikely to Recur at a Particular Location or a Natural Event 

2. Not Reasonably Controllable or Preventable 

3. Clear Causal Relationship Supported by a Comparison to Historical 

Concentration Data 

F. Treatment of Certain Events Under the Exceptional Events Rule 

1. Exceedances Due to Transported Pollution 

2. Wildland Fires 

3. Stratospheric Ozone Intrusions 

4. High Wind Dust Events 

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

Submittal and Review 

1. Who may submit a demonstration and request for data exclusion? 

2. Aggregation of Events for NAAQS with Periods Longer Than 24 Hours and 

Demonstrations with Respect to Multiple NAAQS for the Same Pollutant 

3. Exclusion of Entire 24-hour Value Versus Partial Adjustment of the 24-hour 

Value for Particulate Matter 

4. Flagging of Data 

5. Initial Notification of Potential Exceptional Event 

6. Submission of Demonstrations 

7. Timing of the EPA’s Review of Submitted Demonstrations 

8. Dispute Resolution Mechanisms 

VI. Mitigation 

A. Current Situation 

B. Proposed Changes 

1. Defining Historically Documented or Known Seasonal Events 

2. Mitigation Plan Components 
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3. Options for Implementing Mitigation Plans 

VII. Draft Guidance on the Preparation of Exceptional Events Demonstrations for Wildfire 

Events that May Influence Ozone Concentrations 

A. What is this draft guidance about and why is it needed? 

B. What scenarios are addressed in the draft guidance? 

VIII. Environmental Justice Considerations 

 IX. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: 

Improving Regulation and Regulatory Review 

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation and Coordination with Indian Tribal 

Governments 

G. Executive Order 13045: Protection of Children from Environmental Health & Safety 

Risks 

H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect 

Energy Supply, Distribution or Use 

I. National Technology Transfer and Advancement Act 

J. Executive Order 12898: Federal Actions to Address Environmental Justice in 

Minority Populations and Low-Income Populations 

X. Statutory Authority 

II. Glossary of Terms and Acronyms 

The following are abbreviations of terms used in the preamble. 

AQCR Air Quality Control Region 

AQS Air Quality System 

Be Beryllium 

BACM Best available control measures 

BLM Bureau of Land Management 

BMP Best management practice(s) 

BSMP Basic smoke management practices 

CAA Clean Air Act 

CASTNET Clean Air Status and Trends Network 

CBI Confidential business information 

CFR Code of Federal Regulations 

CO Carbon monoxide 

EPA Environmental Protection Agency 
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FLM Federal land manager responsible for management of a federally 

owned area that has been designated a Class I area as codified in 40 

CFR part 81, subpart D 

FR Federal Register 

IPV Isentropic potential vorticity 

Lidar A remote sensing technology that measures distance by 

illuminating a target with a laser and analyzing the reflected light 

μg/m3

 Micrograms per cubic meter 

mph Miles per hour 

NAAQS National ambient air quality standard or standards 

NAM North American Mesoscale Forecast System 

NASA National Aeronautics and Space Administration 

NEPA National Environmental Policy Act 

NO2 Nitrogen dioxide 

NOAA National Oceanic and Atmospheric Administration 

NOV Notice of violation 

NOx Nitrogen oxides 

NPS National Park Service 

NSR New source review 

NRCS Natural Resources Conservation Service 

NRDC Natural Resources Defense Council 

NWCG National Wildfire Coordinating Group 

NWS National Weather Service 

OAQPS Office of Air Quality Planning and Standards, U.S. EPA 

OMB Office of Management and Budget 

Pb Lead 

PM Particulate matter 

PM10 Particulate matter with a nominal mean aerodynamic diameter less 

than or equal to 10 micrometers 

PM2.5 Particulate matter with a nominal mean aerodynamic diameter less 

than or equal to 2.5 micrometers 

ppb Parts per billion 

PSD Prevention of significant deterioration 

PT Potential temperature 

RACM Reasonably available control measures 

RAQMS Real-time Air Quality Modeling System 

RUC Rapid Update Cycle 
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SIP State implementation plan 

SMP Smoke management program 

SO2 Sulfur dioxide 

TAR Tribal Authority Rule 

TIP Tribal implementation plan 

UMRA Unfunded Mandates Reform Act 

USDA U.S. Department of Agriculture 

USFS U.S. Forest Service 

VOC Volatile organic compound or compounds 

III. Executive Summary 

This section summarizes the purpose of this regulatory action, the major provisions of 

this action, and the development of associated guidance. 

Purpose of this Regulatory Action 

Recognizing that it may not be appropriate for the EPA to use certain monitoring data 

collected by the ambient air quality monitoring network and maintained in the air quality data 

system (AQS) in the EPA’s regulatory determinations, in 2005 Congress provided the statutory 

authority for the exclusion of data in specific situations by adding section 319(b) to the CAA. 

The EPA promulgated the 2007 Exceptional Events Rule (March 22, 2007, 72 FR 13560) to 

implement this 2005 amendment of the CAA. The purpose of this action is to propose revisions 

to the 2007 Exceptional Events Rule to address certain substantive issues raised by state, local 

and tribal co-regulators and other stakeholders since promulgation of the rule and to increase the 

administrative efficiency of the Exceptional Events Rule criteria and process. The EPA intends 

to promulgate these rule revisions in advance of the date by which states, and any tribes that wish 

to do so, are required to submit their initial designation recommendations for the revised 2015 

ozone NAAQS (expected in October of 2016). In addition, the EPA intends to address a 2008 
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D.C. Circuit Court decision in which the court found that certain preamble language was “legally 

null” because there was no associated implementing rule language. 

 Interpreting and implementing the 2007 Exceptional Events Rule has been challenging in 

certain respects both for the air agencies developing exceptional events demonstrations and for 

the EPA regional offices reviewing and acting on these demonstrations. Since 2007, air agencies 

have submitted exceptional event demonstrations for a variety of pollutant and event 

combinations ranging from volcanic activity influencing sulfur dioxide and particulate matter 

concentrations to stratospheric ozone intrusions. Air agencies preparing demonstrations have 

expressed specific concerns and identified challenges associated with preparing analyses to 

satisfy the “but for” rule criterion, determining what controls constitute reasonable controls 

particularly for natural sources and for interstate and international transport and identifying how 

much documentation to include in a demonstration. 

As a result of both our experiences and feedback related to implementing the 2007 

Exceptional Events Rule received from state, local and tribal co-regulators and other

stakeholders via letters and numerous conference calls and meetings, the EPA developed and 

released Interim Exceptional Events Implementation Guidance in May of 2013. This guidance 

has addressed some of the concerns and challenges raised by interested parties, has helped 

reduce the burden of preparing demonstrations and has reduced the time needed for review. 

However, the EPA acknowledged that additional changes could only be accomplished through a 

notice-and-comment rulemaking. Therefore, when the EPA released the Interim Exceptional 

Events Implementation Guidance in May of 2013, we simultaneously announced our intent to 

pursue revisions to the Exceptional Events Rule. These changes are reflected in this proposed 

action. 
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Concurrent with preparing this proposed action, the EPA held conference calls with some 

air agencies to discuss more recent implementation experiences and to better understand 

currently employed exceptional events implementation processes and practices. As a result of 

these discussions, the EPA developed a list of best practices for communication and 

collaboration between the EPA and air agencies. Agencies using these approaches have 

developed a common understanding of expectations, terminology and interpretation of the EPA’s 

regulations and policy, which, in turn, helps focus efforts, optimize resources and save time 

during the demonstration development and review process. 

Based on our experiences and the input we have received from our collaborations with 

interested parties (including state, local and tribal air agencies) following the promulgation of the 

2007 Exceptional Events Rule and since the development of the Interim Exceptional Events 

Implementation Guidance and based on the previous legal challenge, we have determined those 

aspects of the 2007 Exceptional Events Rule that most need to be addressed in this proposed 

action. 

Summary of Major Provisions 

For the first time, the EPA proposes to interpret CAA section 319(b) as applying to only 

a specific set of regulatory actions (e.g., designations) because we believe that the criteria and 

process steps specified in the CAA were not clearly intended by Congress to apply to all types of 

regulatory actions and in some cases certain of the criteria and steps are not appropriate. We 

address this concept in this document in general terms, but we also intend to develop a separate 

guidance document to provide guidance on when data can be excluded and when they cannot for 

other specific types of regulatory actions.
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The EPA proposes to return to the core statutory elements and implicit concepts of CAA 

section 319(b): the event affected air quality in such a way that there exists a clear causal 

relationship between the specific event and the monitored exceedance or violation, the event was 

not reasonably controllable or preventable and the event was caused by human activity that is 

unlikely to recur at a particular location or was a natural event. Within each of these elements, 

we are proposing clarifications regarding the desired analyses to include in exceptional events 

demonstrations and we discuss the applicability of these clarifications to certain event types or 

categories. As part of this return to the core statutory elements, we are proposing to remove from 

the Exceptional Events Rule a paragraph containing what is commonly referred to as the “but 

for” criterion. 

The EPA is proposing to incorporate the statutory “affects air quality” criterion and the 

regulatory “historical fluctuations” criterion within the “clear causal relationship” element. We 

believe that if an air agency demonstrates that an event has a clear causal relationship to an 

exceedance or violation of a NAAQS, then the event has certainly affected air quality and that a 

submitting air agency does not need to address “affects air quality” as a distinct component. As 

we indicated in the Interim Exceptional Events Implementation Guidance (see section IV.D), we 

believe that a comparison of the claimed event-influenced concentration(s) to concentrations at 

the same monitoring site at other times is extremely useful evidence in an exceptional events 

demonstration, particularly as part of showing a clear causal relationship, and we propose to 

continue requiring this type of comparison. This proposed action details the minimum set of 

statistical analyses that the EPA expects to see in demonstrations. 

With respect to the “not reasonably controllable or preventable” criterion, many states 

have requested that the EPA automatically consider an event to be reasonably controlled if the 
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EPA has approved a state implementation plan (SIP) that contains controls for anthropogenic 

sources that contribute to the event that are also specific to the pollutant of concern in the 

exceptional events demonstration. In response, the EPA proposes that enforceable control 

measures implemented in accordance with an attainment or maintenance SIP, approved by the 

EPA within 5 years of the date of a demonstration submittal, that address the event-related 

pollutant and all sources necessary to fulfill the requirements of the CAA for the SIP to be 

reasonable controls with respect to all anthropogenic sources that have or may have contributed 

to event-related emissions. Also for this criterion, the EPA clarifies that air agencies generally 

have no obligation to specifically address controls if the event was natural or if it was due to 

emissions originating outside their jurisdictional (i.e., state or tribal) border(s). 

With respect to the “human activity that is unlikely to recur at a particular location or was 

a natural event” criterion, we propose a general approach to determining whether the recurrence 

frequency of an event is “unlikely to recur at a particular location” and an approach applicable to 

prescribed fire on wildland only. We also clarify that natural events can recur, sometimes 

frequently, and reiterate our belief that we generally consider human activity to have played little 

or no direct role in causing emissions if anthropogenic emission sources that contribute to the 

event emissions are reasonably controlled at the time of the event. 

Air agencies must address all of the core statutory elements and implicit concepts of 

CAA section 319(b) within an exceptional events demonstration. In this proposed action, the 

EPA clarifies the content and organization of exceptional events submittals to include the core 

statutory elements, but we also propose that states be required to include a conceptual model, or 

narrative, describing the event(s) causing the exceedance or violation and a discussion of how 

emissions from the event(s) led to the exceedance at the affected monitor(s) and documentation 
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that the air agency conducted a public comment process. We are proposing to require an initial 

notification by the state to the EPA of a potential exceptional event as a preliminary step before 

submitting a demonstration, to ensure the submitting air agency and the reviewing EPA regional 

office share a common understanding regarding the potential event and are in communication 

regarding the timeline for the demonstration to be submitted and to be reviewed by the regional 

office. 

Because affected air agencies have provided feedback regarding the difficulty associated 

with meeting the current regulatory timelines associated with data flagging, initial event 

descriptions and demonstration submittals, the EPA proposes to remove the specific deadlines 

that apply in situations other than initial area designations following promulgation of a new or 

revised NAAQS. Also associated with demonstration timing, the EPA proposes to officially 

terminate review of demonstrations that, due to the passage of time, will have no further 

regulatory significance specifically for the five types of regulatory actions identified in section 

V.C. of this preamble. 

Since promulgation of the 2007 Exceptional Events Rule, stakeholders have raised 

numerous questions about fire-related components that were discussed, but not fully defined or 

clarified, in the preamble to the 2007 Rule. This proposed action addresses fire-related 

definitions, provides more clarity regarding expectations for smoke management programs 

(SMPs) and basic smoke management practices (BSMP), and proposes limited scenarios under 

which federal land managers and other federal agencies may prepare and submit exceptional 

events demonstrations and data exclusion requests directly to the EPA. 

Associated Guidance Documents 

In addition to proposing revisions to the 2007 Exceptional Events Rule, this proposed 
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action simultaneously provides a notice of availability of a draft non-binding guidance document 

titled, Draft Guidance on the Preparation of Exceptional Events Demonstrations for Wildfire 

Events that May Influence Ozone Concentrations, which applies the proposed Exceptional 

Events Rule revisions to wildfire/ozone events. This guidance document is intended to further 

address specific stakeholder questions regarding the Exceptional Events Rule and further 

increase the efficiency of rule implementation. In addition, the EPA is currently developing a 

guidance document titled, Draft Guidance for Excluding Some Ambient Pollutant Concentration 

Data from Certain Calculations and Analyses for Purposes Other than Retrospective 

Determinations of Attainment of the NAAQS, which will apply to the exclusion of certain data for 

certain applications using a process and criteria outside of the Exceptional Events Rule. The EPA 

intends to make this guidance document available shortly after proposing revisions to the 

Exceptional Events Rule. The EPA expects to finalize these guidance documents concurrently 

with promulgating revisions to the Exceptional Events Rule.

IV. Background for Proposal 

A. Purpose of and Statutory Authority for this Regulatory Action 

Part of the EPA’s mission is to preserve and improve, when needed, the quality of our 

nation’s ambient air to protect human health and the environment. As part of accomplishing this, 

the EPA develops the national ambient air quality standards (NAAQS) for criteria pollutants and 

oversees the states’ programs to improve air quality in areas where the current air quality is 

unacceptable and to prevent deterioration in areas where the air quality meets or exceeds the 

NAAQS. The EPA then evaluates the status of the ambient air as compared to these NAAQS by 

using data collected in the national ambient air quality monitoring network established under the 

authority of section 319(a) of the CAA. 
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Recognizing that it may not be appropriate for the EPA to use certain monitoring data 

collected by the ambient air quality monitoring network and maintained in AQS in our regulatory 

determinations, in 2005 Congress provided the statutory authority for the exclusion of data in 

specific situations by adding section 319(b) to the CAA in 2005. The EPA promulgated the 2007 

Exceptional Events Rule (March 22, 2007, 72 FR 13560) to implement this 2005 amendment of 

the CAA. The purpose of this action is to propose revisions to the 2007 Exceptional Events Rule 

to address certain issues raised by stakeholders since promulgation of the rule and to increase the 

administrative efficiency of the Exceptional Events Rule criteria and process. 

In addition to proposing revisions to the 2007 Exceptional Events Rule, we are 

simultaneously providing a notice of availability of a draft non-binding guidance document 

titled, Draft Guidance on the Preparation of Exceptional Events Demonstrations for Wildfire 

Events that May Influence Ozone Concentrations, which applies the proposed Exceptional 

Events Rule revisions to wildfire/ozone events. We seek comment on whether the concepts in 

this guidance document should be finalized as rule text. We are also currently developing a 

second guidance document titled, Draft Guidance for Excluding Some Ambient Pollutant 

Concentration Data from Certain Calculations and Analyses for Purposes Other than 

Retrospective Determinations of Attainment of the NAAQS, which will apply to the exclusion of 

certain data for certain applications using a process and criteria outside of the Exceptional Events 

Rule. Both of these draft guidance documents are intended to further address specific stakeholder 

concerns regarding the Exceptional Events Rule and further increase the efficiency of rule 

implementation. 

B. The 2007 Exceptional Events Rule 
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The 2007 Exceptional Events Rule created a regulatory process codified at 40 CFR parts 

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

procedural requirements, requirements for air agency demonstrations, criteria for the EPA’s 

approval of the exclusion of event-affected air quality data from the data set used for regulatory 

decisions, and requirements for air agencies1

 to take appropriate and reasonable actions to protect 

public health from exceedances or violations of the NAAQS.2

 The 2007 Exceptional Events Rule 

 

 

1

 References to “air agencies” are meant to include state, local and tribal air agencies responsible 

for implementing the Exceptional Events Rule. The regulatory text in the 2007 Exceptional 

Events Rule often uses “State” to apply to “air agencies.” In the context of flagging data and 

preparing and submitting demonstrations, the role of and options available to air agencies would 

also apply to federal land managers of Class I areas and other federal agencies managing federal 

land. 

2

 Per the definition at 40 CFR 50.1(l), an exceedance with respect to a national ambient air 

quality standard means one occurrence of a measured or modeled concentration that exceeds the 

specified concentration level of such standard for the averaging period specified by the standard. 

Violations of a standard are standard-specific and are determined by applying the standardspecific procedures for air quality data handling identified in the appendices to 40 CFR part 50. 

For example, per the requirements in 40 CFR part 50, appendix N, an exceedance of the 2006 

24-hour PM2.5 NAAQS of 35 μg/m3 occurs when the 24-hour concentration is above 35 μg/m3 on 

a single day. A violation of the 2006 24-hour PM2.5 NAAQS occurs when the 3-year average of 

the annual 98th percentile 24-hour concentrations is above 35 μg/m3

. 
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superseded the EPA’s previous natural events guidance and those sections of an earlier guidance 

document that addressed the treatment of data affected by exceptional events.3

In general, the exceptional events regulatory process consists of the following steps. First, 

an air agency identifies a potential event-related exceedance or violation. After noting these data 

in AQS, the air agency prepares a draft demonstration package to support the exclusion of the 

identified event-related data and provides an opportunity for public comment. The air agency 

submits the draft demonstration and any received public comments to its EPA regional office, 

which then reviews the submittal and concurs, nonconcurs or defers a decision related to the air 

agency’s request to exclude data that have been affected by exceptional events. If the EPA agrees 

with the air agency’s request, the data are excluded. If the EPA does not agree with the air 

 

3

 Previous guidance and policy documents that either implied or stated the need for special 

treatment of data affected by an exceptional event include: 

i) Guideline for the Interpretation of Air Quality Standards, U.S. EPA, OAQPS No. 1.2-008, 

Revised February 1977. Available at

http://nepis.epa.gov/Exe/ZyNET.exe/2000YFDB.TXT?ZyActionD=ZyDocument&Client=EPA&I

ndex=1976+Thru+1980&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestric

t=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp

=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5Czyfiles%5CIndex%20Data%5C76thru80%

5CTxt%5C00000007%5C2000YFDB.txt&User=ANONYMOUS&Password=anonymous&SortM

ethod=h%7C-

&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&D

isplay=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Result

s%20page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL.

ii) Guideline on the Identification and Use of Air Quality Data Affected by Exceptional Events

(the Exceptional Events Policy), U.S. EPA, OAQPS, EPA-450/4-86-007, July 1986.

iii) Areas Affected by PM-10 Natural Events (the PM10 Natural Events Policy), memorandum 

from Mary D. Nichols, Assistant Administrator for Air and Radiation, to EPA Regional Offices, 

May 30, 1996. Available at http://www.epa.gov/ttn/caaa/t1/memoranda/nepol.pdf.

iv) Interim Air Quality Policy on Wildland and Prescribed Fires. U.S. EPA. April 23, 1998. 

Available at http://www.epa.gov/ttn/oarpg/t1/memoranda/firefnl.pdf.

v) Guideline on Data Handling Conventions for the PM NAAQS. U.S. EPA, OAQPS, EPA454/R-98-017, December 1998. 
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agency claim, or if the EPA decides to defer a decision on the submittal, the data are used in 

regulatory determinations. 

The 2007 Exceptional Events Rule was challenged in 2008. In NRDC v. EPA, 559 F.3d 

561 (D.C. Cir. 2009), the Natural Resources Defense Council (NRDC) brought a petition for 

review challenging the EPA’s definition of a natural event and seeking to vacate several 

statements in the preamble to the final 2007 Rule concerning the types of events that could 

qualify as being eligible for exclusion under the rule provisions. In particular, NRDC objected to 

treating “events in which human activities play ‘little’ causal role” as natural events. Regarding 

the definition of a natural event, the D.C. Circuit Court determined that NRDC did not identify 

its objection during the rulemaking process and, therefore, did not have standing under CAA 

section 307 to challenge the definition. 

NRDC also challenged the preamble language addressing high wind events. In its 

decision, the D.C. Circuit stated, 

In one section of the preamble, EPA refers to its “final rule concerning high wind 

events,” which “states that ambient particulate concentrations due to dust being raised by 

unusually high winds will be treated as due to uncontrollable natural events” when 

certain conditions apply.... There is no such final rule. The final rule does not mention 

high wind events or anything about “ambient particulate matter concentrations.” EPA 

calls this a drafting error. In light of the error, the high wind events section of the 

preamble is a legal nullity. 

The EPA believes it is clear that the “high wind events section of the preamble” to which the 

court referred is the entire section titled, “B. High Wind Events” beginning at 72 FR 13576. 

Accordingly, since 2007, the EPA has not relied solely on this section of the preamble when 

implementing the 2007 Rule. The EPA maintains that certain of the preamble passages 

determined to be “legally null” are in fact appropriate interpretations of the Exceptional Events 

Rule and are consistent with the CAA. For clarity and regulatory certainty, the EPA is proposing 
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in rule text form some of the interpretative positions originally stated in the High Wind Events 

section of the preamble to the 2007 Exceptional Events Rule. 

Within each topical area of this notice, the EPA has provided more detailed background 

information on specific aspects of the 2007 Exceptional Events Rule and its implementation to 

allow readers to consider the proposed changes in the context of the current situation. 

C. Early Experience in Implementing the 2007 Exceptional Events Rule 

Interpreting and implementing the 2007 Exceptional Events Rule has been a challenging 

process both for the air agencies developing exceptional events demonstrations and for the EPA 

regional offices reviewing and acting on these demonstrations. Shortly after the EPA 

promulgated the rule in 2007, air agencies asked the EPA to clarify key rule provisions and 

expectations for these demonstrations. Air agencies also asked for demonstration templates 

and/or examples of acceptable demonstrations for various event and pollutant combinations. 

Although the EPA provided some of this information via the exceptional events website at 

http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events, air 

agencies noted that, in their view, the information provided was insufficient and sought 

additional guidance to facilitate consistency among the EPA regional offices in interpreting and 

implementing the 2007 Exceptional Events Rule. In the years since rule promulgation, air 

agencies continued to express concern, through various mechanisms including formal letters, 

informal emails, interaction at various meetings and Congressional testimony, about the 

consistent application of the 2007 Exceptional Events Rule and the resources expended to 

prepare exceptional events demonstrations. 

The EPA has also faced challenges in reviewing submitted demonstrations. Because 

exceptional events are fact-specific and thus unique and varied, providing templates or general 
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guidance was, and still is, challenging. The EPA also acknowledges that the final rule and 

preamble language for the 2007 Exceptional Events Rule provided room for interpretation, 

making it difficult for air agencies and the EPA to determine how much evidence or technical 

analysis for demonstrations is needed. We do, however, think that providing additional 

recommendations on appropriate documentation would be helpful. Throughout this proposal, for 

example in section V.E, Technical Criteria for the Exclusion of Data Affected by Events, and in 

section V.F, Treatment of Certain Events Under the Exceptional Events Rule, we provide 

recommendations for language and analyses to include in demonstration packages. Additional 

detail regarding specific recommendations is available in the EPA’s guidance documents and on 

the EPA’s exceptional events website, which the EPA will update to incorporate the finalized 

rule changes concurrently with or shortly after promulgating the final rule. The EPA will also 

continue to maintain and update the exceptional events submissions table on its website with 

examples of approved submissions. These examples may help air agencies develop 

demonstration packages; however, they may not contain the minimum level of data or casespecific analyses necessary for all exceptional events demonstrations of the same event type. In 

addition, commenters on this notice may wish to provide suggestions on the appropriate 

documentation for specific types of exceptional events demonstrations. 

D. The EPA’s Interim Exceptional Events Implementation Guidance 

As a result of stakeholder-identified concerns and the EPA’s own experience related to 

implementing the 2007 Exceptional Events Rule, in 2010 the EPA began developing additional 

implementation guidance. In May of 2011, the EPA released the Draft Exceptional Events 

Implementation Guidance: the Draft Guidance to Implement Requirements for the Treatment of 

Air Quality Monitoring Data Influenced by Exceptional Events, the Draft Exceptional Events 
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Rule Frequently Asked Questions document and the Draft Guidance on the Preparation of 

Demonstrations in Support of Requests to Exclude Ambient Air Quality Data Affected by High 

Winds under the Exceptional Events Rule. The EPA provided these draft guidance documents to 

interested air agencies, FLMs, other federal agencies and other parties upon request, for 

preliminary review to solicit comment and help ensure that the EPA’s final guidance provided an 

efficient and effective process to make determinations regarding air quality data affected by 

exceptional events. The EPA also placed additional examples of approved demonstrations on the 

EPA’s website.

The EPA incorporated the commenters’ feedback, as appropriate, into revised draft 

guidance documents, which were made available for broad public review in a July 6, 2012, 

Federal Register Notice of Availability (77 FR 39959) and in the associated docket (Docket ID 

No. EPA-HQ-OAR-2011-0887).4

 This docket includes a summary of the comment and response 

process from the 2011 preliminary review of the draft guidance documents. In addition to 

identifying specific comments on the draft guidance documents, this summary clearly identifies 

that implementation challenges originated shortly after the EPA promulgated the 2007 

Exceptional Events Rule.5

 In May 2013, after a round of review and comment by the general 

public, the EPA finalized the Interim Exceptional Events Implementation Guidance and made 

 

4

 The EPA established Docket ID No. EPA-HQ-OAR-2011-0887 for the July 2012 notice of 

availability for the Draft Exceptional Events Implementation Guidance and has incorporated this 

docket into the record for this action. 

5 Responses to Significant First-Round Comments on the Draft Guidance Documents on the 

Implementation of the Exceptional Events Rule, U.S. EPA, June 2012. Available in Docket No. 

EPA-HQ-OAR-2011-0887. 
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these documents publicly available on the exceptional events website at 

http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events.

6

 

With the release of the Interim Exceptional Events Implementation Guidance, the EPA 

simultaneously acknowledged the need to consider additional changes that could only be 

accomplished through a notice-and-comment rulemaking to revise the 2007 Exceptional Events 

Rule. To inform the development of proposed rule revisions, the EPA hosted exceptional events 

listening sessions in August and November of 2013 for interested air agencies, FLMs, other 

federal agencies, regional planning organizations, non-governmental organizations and other 

members of the public. The EPA has considered feedback from these listening sessions and the 

previous public comments on the Interim Exceptional Events Implementation Guidance in the 

development of these proposed revisions to the 2007 Exceptional Events Rule. 

E. More Recent Implementation Experience Including EPA-Recommended Best Practices for the 

Development of Exceptional Events Demonstrations

Because of the passage of time since the 2013 exceptional events listening sessions, the 

EPA’s Office of Air Quality Planning and Standards (OAQPS) held conference calls with some 

air agencies and the EPA regional offices between September 2014 and March 2015 to ask 

whether any new implementation concerns had arisen and to better understand currently 

employed exceptional events implementation processes and practices. 

 

6

 The Interim Exceptional Events Implementation Guidance includes: the Interim Guidance to 

Implement Requirements for the Treatment of Air Quality Monitoring Data Influenced by 

Exceptional Events, the Interim Exceptional Events Rule Frequently Asked Questions (the 

Interim Q&A document), and the Interim Guidance on the Preparation of Demonstrations in 

Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds under the 

Exceptional Events Rule (the Interim High Winds Guidance document). 
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As a result of these discussions, the EPA developed a list of best practices for 

communication and collaboration between the EPA and air agencies. These best practices 

include having discussions before, during and after the development and submission of 

exceptional events demonstration packages. Specifically, these best practices recommend that the 

EPA regional offices and their air agencies discuss, on a mutually agreed upon frequency, those 

demonstrations that the agencies have developed and submitted for the EPA’s action. These 

regular discussions should focus on whether the demonstrations have regulatory significance 

(e.g., significance for any of the five types of regulatory actions identified in section V.C.) and, if 

not, whether the EPA can provide general technical or policy feedback that the air agency can 

include in future demonstrations. Prior to an air agency’s development of future demonstrations, 

the air agency and the EPA should identify the relevant days and monitors of focus, the 

regulatory significance of these monitor days, the analyses of particular interest for a specific 

event and pollutant combination and the anticipated timeframe for demonstration submission and 

response. Discussions should continue while the air agency is developing the demonstration and 

after the agency submits the demonstration and while the EPA is reviewing the demonstration, to 

ensure both the air agency and the EPA are aware of status, direction and progress. Finally, after 

the EPA has acted on the demonstration, the reviewing EPA regional office and the air agency 

should discuss elements of the process that should continue and those that should be improved, 

should understand the information in the demonstration that was useful versus the information 

that was extraneous and should discuss the possibility of developing a demonstration template(s) 

for future events of the same type(s). 

Agencies using this communications approach have developed a common understanding 

of expectations, terminology and interpretation of the EPA’s regulations and policy, which, in 
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turn, helps focus efforts, optimize resources and save time during the demonstration development 

and review process. A summary of this “best practices” approach to implementation is available 

at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events. 

V. Proposed Rule Revisions 

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

1. Current Situation 

Under the CAA, states are primarily responsible for the administration of air quality 

management programs within their borders, which includes monitoring and analyzing ambient 

air quality, submitting monitoring data to the EPA, which are then stored in the EPA’s AQS 

database, and identifying measurements that may warrant special treatment under the 

Exceptional Events Rule. The 2007 Exceptional Events Rule applies to all state air agencies and 

to local air quality agencies to whom a state has delegated relevant responsibilities for air quality 

management, including air quality monitoring and data analysis. 

Additionally, the 2007 Exceptional Events Rule applies to some tribal air quality agencies 

who have been granted treatment as a state for section 319 of the CAA. Section 301(d) of the 

CAA authorizes the EPA to recognize tribal authority, allowing eligible, federally-recognized 

tribal governments to implement provisions of the CAA. Pursuant to section 301(d)(2), the EPA 

promulgated regulations, known as the Tribal Authority Rule (TAR), on February 12, 1999 (63 

FR 7254, codified at 40 CFR part 49). That rule specifies those provisions of the CAA for which 

it is appropriate to treat tribes in a similar manner as states. Under the TAR, tribes may choose to 

develop and implement their own CAA programs, but are not required to do so. The TAR also 

establishes procedures and criteria by which tribes may request from the EPA a determination of 

eligibility to implement the provisions of the CAA. In cases where a tribal air quality agency is 
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eligible to implement CAA section 319 and has installed and operates an air quality monitoring 

network that produces regulatory data that is affected by emissions from exceptional events, the 

criteria and procedures identified in these proposed rule revisions may be used to exclude data 

for purposes of regulatory decisions. Some tribes may implement only portions of the relevant 

air quality monitoring program and may choose not to address all of the procedures and 

requirements associated with excluding data that have been influenced by exceptional events 

(e.g., a particular tribe may operate a monitoring network for purposes of gathering and 

identifying data appropriate for informational or educational purposes, but may choose not to 

implement relevant programs for the purpose of mitigating the effects of exceptional events). 

Where a tribal air quality agency is not eligible to implement CAA section 319 but operates an 

air quality monitoring network that produces regulatory data that is affected by emissions from 

exceptional events, the tribal air quality agency should consult with the EPA regional office prior 

to addressing the procedures and requirements associated with excluding data that have been 

influenced by exceptional events.7

 In all cases, the EPA will continue to work with tribes in 

implementing any promulgated rule revisions. 

While air agencies are responsible for administering air quality management programs 

within their borders, FLMs of Class I areas, other federal agencies and/or other entities (e.g., 

industrial facilities pursuant to permit conditions) may also operate ambient air quality monitors 

 

 

7

 As of the signature date of this action, only one tribe is eligible to implement all portions of 

CAA section 319 under the TAR. Several other tribes, however, operate air quality monitoring 

networks that produce regulatory data that could be affected by emissions from exceptional 

events. 
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that meet all requirements of 40 CFR parts 50 and 58.8 The FLMs, other federal agencies and 

other entities operating these regulatory monitors may submit collected data to the EPA’s AQS 

database.9 These concentration measurements can be affected by exceptional events. The AQS 

software allows only the entity operating a monitor (and the EPA data system manager) to apply 

exceptional events flags to data from that monitor. Although FLMs and other entities can apply 

exceptional events flags to data from monitors they operate, the Exceptional Events Rule at 40 

CFR 50.14(b)(1) states that the EPA shall exclude data from use in determinations of 

exceedances and NAAQS violations where a state demonstrates to the EPA's satisfaction that an 

exceptional event caused a specific air pollution concentration in excess of one or more NAAQS. 

The language, “where a State demonstrates” has resulted in an interpretation that only states can 

initiate the exceptional events process and submit demonstrations. Some stakeholders have asked 

the EPA to identify the process that the state air agency should follow if the state air agency does 

not have AQS access rights to place exceptional events flags on event-affected data from 

monitoring stations located within the state but not operated by the state. The EPA addressed this 

issue generally in Question 23 of the Interim Q&A document by indicating that air agencies 

should consult with their EPA regional office early in the development of an exceptional event 

demonstration package if they believe that monitors on federally-owned and managed land (e.g., 

 

8

 The Ambient Air Quality Surveillance provisions in 40 CFR part 58 include, among other 

elements, the requirements for monitoring data certification and data submittal and archive in 

AQS. 40 CFR 58.3 provides that these data reporting requirements specifically apply to state air 

pollution control agencies and any local air pollution control agency to which the state has 

delegated authority to operate a portion of the state’s monitoring network. 

9

 For a description of one network of monitoring sites operated by federal agencies, see the 2014 

CASTNET (Clean Air Status and Trends Network) Annual Network Plan, available at 

http://epa.gov/castnet/javaweb/ozone/CASTNET_Plan_2014_Final.pdf, which applies to 

National Park Service (NPS) and Bureau of Land Management (BLM) site managers operating 

CASTNET monitors. 
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national parks within the state) have been affected by an event. In these instances, the EPA has 

assisted in facilitating cross-agency coordination regarding the flagging of data, where needed. 

The 2007 Exceptional Events Rule applies to all criteria pollutant NAAQS.10 This is 

appropriate given the language in CAA section 319(b)(3)(B)(iv), which applies to exceedances 

or violations of “the national ambient air quality standards.” The EPA regulations for the 

interpretation of ambient data with respect to the NAAQS that were in place prior to the 2007 

Exceptional Events Rule and that have not been revised do not contain provisions allowing for 

the special handling of air quality data affected by exceptional events or do so without explicit 

reference to the Exceptional Events Rule as governing such exclusion. One NAAQS without a 

specific provision for handling event-affected data is 40 CFR part 50, appendix K for particulate 

matter (PM) with a nominal mean aerodynamic diameter less than or equal to 10 micrometers 

(PM10). Nevertheless, the EPA has enabled in AQS the capability to flag all criteria pollutant 

data, including the option for the EPA’s concurrence, as the EPA maintains that the monitored 

concentrations of all NAAQS pollutants have the potential to be elevated by one or more event 

types and the Exceptional Events Rule should govern the process of data exclusion for certain 

types of regulatory actions (see section V.C). 

2. Proposed Changes 

As noted above, because FLMs and other federal agencies may operate regulatory 

monitors and submit collected data to the EPA’s AQS database and emissions from exceptional 

 

10 There are NAAQS for carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone, 

particle pollution and sulfur dioxide (SO2). This applicability includes the primary and secondary 

NAAQS. At present, most of the secondary NAAQS are identical to the primary NAAQS for the 

same pollutant, so there is no distinction in how the Exceptional Events Rule applies. To date, 

the EPA has not encountered an exceptional event situation with respect to a non-identical 

secondary NAAQS. 
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events could affect these same monitors, the EPA proposes to allow FLMs and other federal 

agencies to prepare and submit exceptional events demonstrations and data exclusion requests 

directly to the EPA. The EPA believes this is appropriate because, in many cases, the lands 

managed and/or owned by federal entities are not entirely within the jurisdictional boundary of a 

single state or local government. Also, as we discuss in more detail in section V.F.2, federal 

entities may either initiate prescribed fires or fight wildfires on lands managed and/or owned by 

federal entities. The EPA could determine both of types of fires to be exceptional events. The 

EPA expects that allowing FLMs and other federal agencies to submit exclusion requests directly 

will expedite the exceptional events demonstration development and submittal process. The EPA 

solicits comment on this proposed addition to the rule text, which appears at the end of this 

document. Based on comments received, the EPA may retain, modify or not include this 

provision in the final promulgated rule. This provision would apply only to FLMs and other 

federal agencies that either operate a monitor that has been affected by an event or that manage 

land on which an exceptional event originates. The provision would allow such FLMs and other 

federal agencies to provide demonstrations directly to the EPA only after a discussion with the 

state in which the monitor is operated. Alternatively, this discussion might result in an agreement 

that the federal agency flag the data in AQS at the air agency’s request and then provide a draft 

demonstration document to the appropriate state air agency for adoption and submission by the 

air agency to the EPA, as is currently allowed. If the provision for direct submission to the EPA 

is included in the final action, demonstrations prepared by FLMs or other federal agencies would 

be required to meet all provisions in the Exceptional Events Rule, including the requirement for a 

 
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public comment period on a prepared demonstration11 and the requirements related to schedules 

and procedures for demonstration package submittal (see sections V.G.4, V.G.5 and V.G.6) that 

apply to state agencies that operate monitors. 

B. What is an exceptional event? 

1. Current Situation 

The existing definition of an exceptional event at 40 CFR 50.1(j) repeats the CAA 

definition, which provides that an exceptional event is one that affects air quality, is not 

reasonably controllable or preventable, is caused by human activity that is unlikely to recur at a 

particular location or is a natural event, and is determined by the Administrator in accordance 

with 40 CFR 50.14 to be an exceptional event. Also, CAA section 319(b)(3)(B)(ii) requires that a 

clear causal relationship must exist between the measured exceedances of a NAAQS and the 

exceptional event to demonstrate that the exceptional event caused a specific air pollution 

concentration at a particular air quality monitoring location. In addition to these defining 

elements, the 2007 Exceptional Events Rule at 40 CFR 50.14(c)(3)(iv) requires that the 

demonstration provide evidence that “the event is associated with a measured concentration in 

excess of normal historical fluctuations, including background” and evidence that “there would 

have been no exceedance or violation but for the event.” 

Both the statutory and regulatory definitions of an exceptional event include the provision 

that the event affected air quality. Many types of events affect air quality by causing emissions or 

 

11 A public comment opportunity is important prior to submission to the EPA because under the 

Exceptional Events Rule the EPA is not required to provide a public comment opportunity prior 

to concurring with an air agency’s request to exclude data. The EPA generally provides a public 

comment opportunity before we use air quality data, with or without such exclusions, in a final 

regulatory action. States typically provide an opportunity for public comment by posting draft 

demonstrations on a website. Federal agencies could do the same. 
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increasing otherwise occurring emissions. Stratospheric ozone intrusions, one type of event, 

differ from most other event types in that they transport ozone already formed in the stratosphere 

to a surface monitor. High temperatures, air stagnations and meteorological inversions can 

increase the level of air pollution formed from a given amount of emissions. However, both the 

statutory and regulatory definitions of an exceptional event specifically exclude stagnation of air 

masses, meteorological inversions and meteorological events involving high temperatures or lack 

of precipitation, as well as air pollution relating to source noncompliance. 

While the CAA definition of an exceptional event excludes “a meteorological event 

involving high temperatures or lack of precipitation,” high temperatures and drought conditions 

can contribute to exceedances and violations caused by other exceptional events such as high 

wind dust events. If an air agency submits evidence showing that a severe drought that resulted 

in arid conditions (e.g., lower than typical soil moisture content, decreased vegetation, etc.) was 

combined with an event, such as a high wind event, that falls within the CAA definition of an 

exceptional event and has affected air quality data, these data could be considered eligible for 

exclusion under the provisions of the Exceptional Events Rule. Under this scenario, the EPA 

would consider the high wind event as the critical exceptional event. The high wind event would 

need to meet the provisions of the Exceptional Events Rule, including assessing whether the 

event is a natural event or an event due to human activity unlikely to recur at a particular 

location. As another example, if a wildfire exacerbated by drought conditions causes ozone 

exceedances, then the EPA can consider the ozone exceedances for exclusion under the 

Exceptional Events Rule because wildfires, unlike lack of precipitation itself, are not excluded 

from the CAA definition of an exceptional event. However, high temperatures alone that result in 

elevated ozone concentrations would not be eligible for exclusion under the Exceptional Events 
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Rule. Elevated temperatures and inversions can affect ambient air quality apart from any 

interactions with emissions, but such conditions alone are not exceptional events by the very 

clear provisions of the CAA. The EPA believes that Congress intended air agencies to 

compensate for the effects of high temperature and inversions on concentrations formed from 

anthropogenic emissions through the development of SIPs. 

To summarize, the 2007 Exceptional Events Rule specifies six elements that air agencies 

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

determinations: 

 The event affected air quality 

 The event was not reasonably controllable or preventable 

 The event was a human activity that is unlikely to recur at a particular location, or 

was a natural event 

 There exists a clear causal relationship between the specific event and the 

monitored exceedance 

 The event is associated with a measured concentration in excess of normal 

historical fluctuations including background 

 There would have been no exceedance or violation but for the event 

Section 50.14(b)(3) clearly makes the first three of these elements preconditions for the 

EPA to approve an air agency’s request to exclude data. However, the last three of these 

elements are listed only in 50.14(c)(iv), which provides that the state “shall provide evidence” 

that they are true. Since promulgation of the 2007 Exceptional Events Rule, the EPA has treated 

all six elements as conditions that air agencies must address in a demonstration prior to the 

EPA’s concurring with an air agency’s request to exclude data. In the Interim Exceptional Events 
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Implementation Guidance, the EPA stated that for the fifth of these elements (e.g., the “historical 

fluctuations” element), there is no bright line that defines when a concentration is “in excess of 

historical fluctuations.” With respect to the sixth element, referred to as the “but for” criterion, 

although the EPA has, in some cases, expected demonstrations to contain a quantitative estimate 

of the concentration increment caused by the event, more frequently the EPA has considered the 

“but for” criterion to be satisfied by a more qualitative showing that the measured concentration 

was much greater than the non-exceedance concentration that would have normally been 

expected on the day in question. 

In addition to considering whether or not an event is “exceptional” under the Exceptional 

Events Rule, an air agency and the EPA must also decide whether an “event” has occurred. An 

event, or anomaly, is a deviation from normal or expected conditions that contributes to air 

pollution. In some cases, air agencies or other observers can clearly see this “deviation,” for 

example unusually high wind speeds transporting dust, fires generating PM or ozone precursors 

or volcanoes venting plumes of SO2, PM and PM precursors.12 In other cases, such as with 

stratospheric ozone intrusions, the physical effects of the event may not be visible and the 

occurrence of an event can only be inferred from seeing the effect on monitored air quality of 

emissions associated with the event. As described in section V.E.3, comparing the ambient 

pollutant concentrations in question to the historical distribution of concentrations of the same 

pollutant can help an air agency determine whether a deviation from normal concentrations 

occurred. However, such comparisons must consider that multiple factors often contribute to 

 

12 The EPA considers on-going emissions from volcanic activity to be “events” even if they 

occur every day over a long period. The EPA considers this approach to be consistent with 

Congressional intent, but that extending the same treatment to air pollution due to every day 

biological processes or lightning would not be consistent with that intent. 
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high pollutant concentrations. Some events, such as stratospheric ozone intrusions and high wind 

dust events, may last only a few hours at any one location. Still other events, such as volcanic 

activity, may occur and affect pollutant concentrations for a sustained period of time (e.g., 

multiple days). Some events may create pollutant-increasing conditions that persist after the 

original event process has ceased, for example high winds or volcanic eruptions that leave 

deposits of dust on roadways. 

2. Proposed Changes 

The EPA is proposing the following generally applicable changes to the 2007 

Exceptional Events Rule with respect to clarifying what constitutes an exceptional event: 

 Revising the definition of exceptional event by including the concept of 

considering the combined effects of an event and the resulting emissions. 

 Removing the “but for” element. 

 Moving the “clear causal relationship” element into the list of criteria that 

explicitly must be met for data to be excluded. 

 Subsuming the “affects air quality” element into the “clear causal relationship” 

element. 

 Removing the term “historical fluctuations” and replacing it with text referring to 

a comparison to historical concentrations, identifying the types of analyses that 

are necessary in a demonstration to address the comparison of the event-affected 

concentration to historical concentrations and clarifying that an air agency does 

not need to prove a specific “in excess of” fact. 
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Making these changes would result in returning to the following three core statutory 

elements of CAA section 319(b) that air agencies must meet when requesting that the EPA 

exclude event-related concentrations from regulatory determinations: 

 The event affected air quality in such a way that there exists a clear causal 

relationship between the specific event and the monitored exceedance or 

violation. 

 The event was not reasonably controllable or preventable. 

 The event was a human activity that is unlikely to recur at a particular location or 

was a natural event. 

The implicit intent of CAA section 319(b) is that when the above conditions are met, the 

data should be excluded from regulatory decisions so as not to drive SIPs to include 

unreasonable or additional measures to address the effects of certain events. 

a. Definition of an Event 

While an event may have a physical component that is purely natural in origin, for 

example high wind speeds, human activity either prior to or simultaneous with the event may 

influence air quality during the event. In implementing the 2007 Exceptional Events Rule, the 

EPA’s approach in determining whether an exceptional event that affected a monitored 

concentration was natural or due to human activity (an important distinction, as discussed in 

section V.D) has been to consider both whether the initiating physical event was natural or the 

result of human activity and whether human activity had any role in strengthening the emissions 

generation process. In contrast, some parties have argued that only the naturalness of the 

initiating physical event should be considered. To clarify that an event is not a “natural event” 

merely because natural processes initiated the emissions generation process, the EPA proposes to 
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revise the regulatory definition of exceptional event to say that both the naturally occurring 

physical event and its associated resulting emissions are to be considered when applying the 

definitions and criteria for exclusion provided in the Exceptional Events Rule. For example, an 

exceptional event might consist of a high wind and the subsequently entrained PM that is 

transported to a monitoring site or a wildfire that generates ozone or ozone precursors, which are 

transported to a monitoring site. The EPA would not consider the physical event (e.g., in the 

previous example, the high wind or the wildfire) to be an exceptional event unless the resulting 

emissions (e.g., the PM or ozone) reached and elevated the concentration at a monitoring 

location or locations. 

b. “But For” Element 

The EPA proposes to rely more directly upon the statutory requirement at CAA section 

319(b)(3)(B)(ii) by removing the regulatory requirement at 40 CFR 50.14(c)(3)(iv)(D) that 

“there would have been no exceedance or violation but for the event” (i.e., the “but for” 

criterion). In promulgating the 2007 Exceptional Events Rule, the EPA derived the “but for” 

criterion from the language at section 319(b)(3)(B)(ii), which requires “a clear causal 

relationship...between the measured exceedances...and the exceptional event to demonstrate that 

the exceptional event caused a specific air pollution concentration at a particular air quality 

monitoring location.”13 The EPA combined this language with the requirement that there be 

“criteria and procedures for the Governor of a State to petition the Administrator to 

 

13 The EPA believes that the terminology “specific air pollution concentration” refers to the 

identified exceedance or violation rather than a specific increment in the measured concentration, 

which implies quantitative source attribution and a supporting quantitative analysis. 
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exclude...data that is directly due to the exceptional events.”14 Under the EPA’s interpretation of 

CAA section 319(b) at the time, these words suggested that a “but for” causation standard for 

exceptional events was appropriate. 

Air agencies have expressed concern that the EPA has, in many cases, historically 

interpreted the “but for” criterion as implying the need for a strict quantitative analysis to show a 

single value, or at least an explicitly bounded plausible range15, of the estimated air quality 

impact from the event. While a single event can in some cases clearly be shown to be a “but for” 

cause of a NAAQS exceedance or violation in the sense that without the event, the exceedance or 

violation would not have occurred, it is more often the case that the impact of emissions from 

events and other sources cannot be separately quantified and distinguished, and the “but for” role 

of a single source or event is difficult to determine with certainty. Even when the effects of 

events are quantifiable with a sufficient degree of confidence, air agencies have reported 

expending significant resources to quantify them. The EPA was aware of these concerns in 2007 

as a result of public comment on the proposed rule and attempted to alleviate them by stating in 

the preamble to the 2007 Exceptional Events Rule that an air agency’s “but for” analysis does 

 

14 CAA section 319(b)(3)(B)(iv) (emphasis added). The CAA requires regulations that 

encompass the following requirements: (1) “the occurrence of an exceptional event must be 

demonstrated by reliable, accurate data that is promptly produced and provided by the Federal, 

State, or local government agencies”; (2) “a clear causal relationship must exist between the 

measured exceedances of a national ambient air quality standard and the exceptional event to 

demonstrate that the exceptional event caused a specific air pollution concentration at a 

particular air quality monitoring location...”; (3) “there is a public process for determining 

whether an event is exceptional”; and (4) there are procedures for the states to petition the EPA 

to exclude monitoring data directly due to exceptional events. CAA section 319 (b)(3). 

15 The EPA stated in the preamble to the 2007 Exceptional Events Rule that a “weight of 

evidence demonstration can present a range of possible concentrations, which is not as 

technically demanding as justifying a specific adjustment to a measured value.” 72 FR 13570 

(March 22, 2007). 
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not necessarily need to be precise and that the EPA would use a holistic “weight of evidence” 

approach in analyzing submitted demonstration packages.16 Without clear examples of what the 

EPA would accept as satisfying a weight of evidence approach, some air agencies began using 

burdensome approaches to provide quantitative “but for” analyses in their exceptional events 

demonstrations. The reviewing EPA regional offices use similarly resource-intensive approaches 

to validate these quantitative analyses as they review demonstrations. In some cases, the detailed 

quantitative approaches have not produced results any better than what could have been achieved 

with less burdensome measures. Therefore, the EPA is proposing to remove the “but for” 

regulatory language and focus on the “clear causal relationship” statutory criterion applied to the 

specific case, using a weight of evidence approach.17 

In so doing, we propose that in their submittals, air agencies demonstrate by the weight of 

evidence in the record that the event caused the specific air pollution concentration at issue.18

Depending on the event characteristics and the case-by-case nature of the evaluation, an air 

agency may or may not need to provide quantitative analyses or estimates to support the weight 

of evidence approach. The EPA will discuss will discuss with an air agency the appropriate 

approach for a given event demonstration during conversations preceding the submittal of a 

demonstration. For example, when a concentration during an event is higher than any 

concentration previously observed in the same area and time of year, the air agency will 

 

16 72 FR 13570 (March 22, 2007). 

17 The term “weight of evidence” means that the EPA will consider all relevant evidence and 

qualitatively “weigh” this evidence based on its relevance to the Exceptional Events Rule 

criterion being addressed, the degree of certainty, its persuasiveness, and other considerations 

appropriate to the individual pollutant and the nature and type of event. 

18 This approach is consistent with language in the preamble to the 2007 Exceptional Events 

Rule that states, “The final rule permits a case-by-case evaluation, without prescribed threshold 

criteria, to demonstrate that an event affected air quality.” 72 FR 13569 (March 22, 2007). 
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generally not need to quantify the event impact to reach the conclusion that the event “caused” 

the concentration at issue. However, in cases where the concentrations on non-event days during 

the same season come close to or exceed the applicable NAAQS, thus providing evidence that 

non-event pollution sources may produce exceedances of the NAAQS, the EPA would expect an 

air agency’s clear causal relationship showing to include a quantitative estimate (or range of 

estimates) of the specific event’s impact on air pollution concentrations, even if uncertain, as a 

part of a weight of evidence showing alongside other qualitative evidence. Section V.E.3 of this 

proposal clarifies the EPA’s expectations regarding analyses associated with the “clear causal” 

criterion. 

c. Clear Causal Relationship Element 

The EPA is proposing to modify the regulatory language in section 50.14(c)(iv) to more 

clearly indicate, consistent with the CAA directive, the requirement to “demonstrate” versus to 

merely “provide evidence” that a clear causal relationship must exist between the specific event 

and the monitored exceedance. The EPA will evaluate this criterion on a weight of evidence 

basis. 

d. Affects Air Quality Element 

As explained above, the EPA has treated the “affects air quality” element as a distinct 

criterion that air agencies must meet for data to be excluded, and has expected exceptional events 

demonstrations to conclude that the “affects air quality” condition has been satisfied. However, 

after carefully considering Congress’ intent and air agencies’ and the EPA’s experience in 

implementing the 2007 Exceptional Events Rule, we propose to integrate the phrase “affected air 

quality” into the clear causal relationship criterion. We believe that separately requiring an air 

agency to provide evidence to support a conclusion that an event “affects air quality” is 
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unnecessary if we finalize this proposal to require a mandatory clear causal relationship showing. 

If an air agency demonstrates that an event has a clear causal relationship to an exceedance or 

violation of a NAAQS, then the event has certainly affected air quality. This proposed approach 

will reduce the time required to prepare demonstrations, reduce their length, result in more 

understandable demonstrations for the public during the notice-and-comment process, and 

simplify and expedite the EPA’s review process. 

e. Historical Fluctuations Element 

As we indicated in the Interim Exceptional Events Implementation Guidance, we believe 

that a comparison of the claimed event-influenced concentration(s) to concentrations at the same 

monitoring site at other times is extremely useful evidence in an exceptional events 

demonstration. The EPA considers these comparisons as part of the evidence available for 

determining whether an air agency has satisfied the statutory and regulatory “clear causal 

relationship” criterion. Because preparing this type of comparison is within the ability and 

resources of every air agency, the EPA proposes to continue to require this type of comparison in 

every demonstration. However, the EPA is proposing to re-word the requirement to prevent 

misinterpretation that this comparison must show that the concentration in question was “in 

excess of normal historical fluctuations, including background.” This phrase is not clear and has 

caused confusion and regulatory uncertainty. For example, “fluctuations in concentrations” can 

convey either day-to-day or hour-to-hour differences in monitored concentrations. These 

concentration differentials cannot usefully be compared to an absolute concentration (i.e., 

monitored concentration at a given point in time) because many absolute concentrations will be 

larger than the differences between concentrations. The phrase “in excess” might be interpreted 

to mean that the concentration at issue must be higher than all historical concentrations, but the 
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EPA maintains that Congress did not intend this, nor would such an interpretation be reasonable. 

Concentrations that are exceedances of a standard but are not higher than all concentrations 

recorded at a particular monitor may be causally connected to an event of the type that Congress 

clearly identified for treatment as an exceptional event. Finally, the language “including 

background” is confusing. In many cases, the monitor or monitors intended to represent 

“background” concentrations are separated from the event-influenced monitoring site by some 

distance such that the event-influenced monitor and the “background” monitor reflect a different 

mixture of emissions sources, which could lead to misinterpretation. Regardless, the EPA sees no 

clear reason why such “background” concentrations are relevant for analyses associated with 

provisions in the Exceptional Events Rule. 

The change that the EPA is proposing to the text of the 2007 Exceptional Events Rule 

would require demonstrations to include a comparative analysis of the concentration data alleged 

to have been affected by an event and data at other times, and would specify certain aspects of 

the analysis. The change would also make clear that there is no specific “in excess of” 

relationship between the event-affected data and other data that must be proven, for example that 

the event-affected data be above a certain percentile point in the annual distribution of data. 

Section V.E.3 of this proposal contains additional detail regarding the minimum set of statistical 

analyses that the EPA expects to see in demonstrations. 

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

Events Rule? 

The CAA language at 319(b)(3)(B)(iv) requires the Administrator to promulgate 

regulations that provide that there are criteria and procedures for the Governor of a State to 

petition the Administrator to exclude air quality monitoring data that is directly due to 
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exceptional events from use in determinations by the Administrator with respect to exceedances 

or violations of the national ambient air quality standards. The implementing language in the 

2007 Exceptional Events Rule states at 40 CFR part 50.14(a)(1) that air agencies may request 

that the EPA exclude data showing exceedances or violations of the NAAQS that are directly 

due to an exceptional event from use in determinations without naming those determinations in 

that paragraph. The rule at 40 CFR part 50.14(b)(1) states that the EPA shall exclude data from 

use in determinations of exceedances and NAAQS violations where an air agency demonstrates 

to the EPA’s satisfaction that an exceptional event caused a specific air pollution concentration 

in excess of one or more NAAQS.” Thus, both the statutory language and the 2007 Exceptional 

Events Rule use the phrase “in determinations of exceedances and NAAQS violations” with no 

further explanation. 

 In this section, we consider the specific types of determinations by the Administrator that 

should be governed by CAA section 319(b). This issue was not specifically addressed in the 

rulemaking that promulgated the 2007 Exceptional Events Rule and consequently has caused 

some confusion and regulatory uncertainty. 

1. Current Situation 

The EPA believes that Congress clearly intended the CAA language in section 319(b) to 

apply to exclusions of ambient data from determinations of whether a NAAQS exceedance or 

violation occurred at an ambient monitoring site at a particular time in the past. We characterize 

these exceedances or violations as occurring in the “past” because the process of determining 

whether an actual exceedance or violation occurred involves reviewing the ambient air 

monitoring data collected at monitoring sites over some historical timeframe. For example, on 

December 14, 2012, the EPA promulgated a revised primary annual PM2.5 NAAQS of 12.0 
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μg/m3, which is attained when the 3-year average of the annual arithmetic means does not exceed 

12.0 μg/m3.

19 The EPA Administrator made initial area designation decisions for the revised 

NAAQS in December of 2014 based on air quality monitoring data for the most recent period 3-

year period, which was 2011 through 2013.20 Historical, or “past,” data were reviewed and 

assessed to determine whether an exceedance or violation had occurred that would influence a 

current or future regulatory determination. Determinations of “past” exceedances or violations 

are key to the EPA’s actions to designate or redesignate an area, to initially classify an area for a 

NAAQS (where classifications apply), to determine if a nonattainment area has attained the 

NAAQS for which it has previously been designated nonattainment, to determine whether a 

nonattainment area is eligible for an attainment date extension (where applicable) and, in some 

cases, to find that a SIP is inadequate and to issue a SIP call. No affected stakeholders with 

whom the EPA has interacted since 2007 have disputed this interpretation or approach. 

It is not as clear whether CAA section 319(b) also means that data should be excluded 

from determinations of whether a NAAQS exceedance or violation will or is likely to occur in 

the future. Predictions of future NAAQS violation(s) generally involve reviewing the historical 

ambient concentration data that are the evident focus of CAA section 319(b), estimating 

expected future emissions, and then using both of these data sets as inputs to an air quality 

modeling tool or other analytical approach that extrapolates these data to predict a future 

outcome. While science supports and the EPA relies on predictions of future NAAQS violations 

in several parts of the clean air program, such as in the EPA’s approval of attainment 

demonstrations in SIPs, in prevention of significant deterioration (PSD) air permitting programs 

 

19 78 FR 3086 (January 15, 2013). 

20 80 FR 2206 (January 15, 2015). 
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and in actions to reclassify a moderate PM10 or PM2.5 nonattainment area to serious,21the fact that 

these predicted future values rely only in part on historical monitoring data implies that a 

different standard for data exclusion may be appropriate. 

Another interpretation question is whether and under what conditions event-affected data 

should be excluded from determinations that are based wholly or in part on monitoring data but 

formally are not determinations of exceedances or violations of the NAAQS. For example, under 

40 CFR part 51, subpart H, Prevention of Air Pollution Emergency Episodes, the required 

content of a state’s emergency plan depends on whether the state has experienced air pollution 

that exceeds a specified threshold level that is well above any NAAQS. Also, under the EPA’s 

guidance, the eligibility of an area for a simplified maintenance plan for PM10 depends on the 

difference between the better-than-the-NAAQS air quality in an area and the NAAQS. 

To date, the EPA has not issued guidance that explicitly and comprehensively identifies 

the types of data exclusion that are authorized and required by CAA section 319(b) or that may 

be otherwise appropriate and permissible. In the 2013 Interim Q&A document, the EPA 

provided only limited clarification regarding the meaning of “exclude data.”22 Question 14a of 

the Interim Q&A document notes that when the EPA concurs based on the weight of evidence 

that an air agency has successfully made the demonstrations referred to in 40 CFR 50.14(a)(2) 

and (b)(1), then the EPA generally excludes the affected data from the following types of 

calculations and activities: 

 

 

21 Projection of future NAAQS exceedances or violations do not necessarily play a role in 

reclassification of an ozone nonattainment area to a higher classification level. 

22 Interim Exceptional Events Rule Frequently Asked Questions. U.S. EPA. May 2013. Available 

at http://www2.epa.gov/sites/production/files/2015-05/documents/eer_qa_doc_5-10-13_r3.pdf. 
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 The EPA’s AQS does not count these days as exceedances when generating user 

reports, and does not include them in design values estimates,23 unless the AQS 

user specifically indicates that they should be included, which may be appropriate 

for non-regulatory applications of interest to the user.24

 The EPA accepts the exclusion of these data for the purposes of selecting 

appropriate background concentrations for PSD air quality analyses25 and for 

transportation conformity hot spot analyses.26

 The EPA accepts the exclusion of these data for the purposes of selecting 

appropriate ambient data for projecting future year concentrations as part of a 

modeled attainment demonstration. 

 The data continue to be publicly available, but the EPA’s publications and public 

information statements on the status of air quality in the affected area generally do 

 

23 The EPA uses design values in many regulatory decisions, including, but not limited to, when 

designating areas as attainment, nonattainment or unclassifiable for a NAAQS and when 

determining whether a nonattainment area has attained or is still violating a NAAQS. A design 

value is a statistic that describes the air quality status of a given location relative to the level of a 

particular NAAQS. Design values are computed according to the procedures defined in 40 CFR 

part 50 and published annually by EPA's Office of Air Quality Planning and Standards. Design 

values are available at http://www.epa.gov/airtrends/values.html. 24 In some cases where the EPA has revised a NAAQS by strengthening it, the default AQS 

query will exclude data for the more recent, revised NAAQS, but may include concurred data for 

the historical NAAQS. 

25 If the EPA is the permitting authority, the EPA will propose permits on this basis. If the EPA 

is commenting on another permitting authority’s proposed action, the EPA’s comments will be 

consistent with the determinations in this guidance document and any applicable New Source 

Review (NSR) permitting and/or modeling guidance. 

26 Transportation conformity hot spot analysis is applicable only to PM10 and PM2.5. 

“Transportation Conformity Guidance for Quantitative Hot-spot Analyses in PM2.5 and PM10

Nonattainment and Maintenance Areas,” EPA-420-B-10-040, US EPA Office of Transportation 

and Air Quality, December 2010, page 98. 
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not reflect these data in any summary statistic of potential regulatory application, 

unless such inclusion is specifically noted.27

Thus, the EPA has maintained that once data are excluded under the Exceptional Events 

Rule, these same data should be excluded from the above-identified calculations and activities. 

The EPA has not clearly addressed whether approval for exclusion under the Exceptional Events 

Rule means that the data may or must be excluded for the purpose of other types of actions that 

use monitoring data but are not included in the list above. The EPA has also not clearly 

addressed whether data that have not been approved for exclusion under the Exceptional Events 

Rule can nevertheless under some other principle or interpretation be excluded from any of the 

various types of calculations and activities. 

The current situation is further complicated by the fact that the conditions for data 

exclusion in CAA section 319(b) and the Exceptional Events Rule, while logical when applied to 

determinations of NAAQS exceedances or violations occurring in the past, may not be logical 

when applied to predictions of future exceedances or violations. The EPA recognizes, and 

acknowledged in Question 13 of the Interim Q&A document, that an event may have made a 

past air concentration significantly higher than it would have been in the absence of the event 

contribution, and thus elevated an exceedance for a NAAQS pollutant to an even greater degree 

of exceedance. This same event-influenced concentration may not be eligible for exclusion under 

the 2007 Exceptional Events Rule because the “but for” criterion is not satisfied because either 

(1) there would have been a 3-year violation with or without the event or (2) there would not 

 

27 These data may be included in statistics intended to describe current status and trends in actual 

air quality in the area for public information purposes including reporting of the Air Quality 

Index. 
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have been a violation either with or without the event. The 2007 Exceptional Events Rule does 

not explicitly authorize the exclusion of data associated with such an event because the event 

fails to meet the clear causal relationship criterion and “but for” criterion. Retaining the eventinfluenced data could, however, have regulatory implications that seem contrary to the purpose 

of CAA section 319(b). For example, retaining such data in the calculation of the historical 

design value for a nonattainment area can make it seem that the area needs more emissions 

reduction to attain the NAAQS than is actually the case, and could lead to the EPA’s disapproval 

of an attainment demonstration that is in fact adequate, thus requiring the state to adopt 

additional emission controls.28 

As another example, events can make past air concentrations higher without causing an 

actual NAAQS exceedance or violation. However, retaining such data in the calculation of 

background concentrations used in air quality analysis for a PSD permit may suggest that there 

will be a NAAQS violation after construction of a new source and thus could prevent the 

permitting authority from issuing the permit.29

2. Proposed Changes 

To remove the ambiguities described in the preceding section and to provide greater 

regulatory certainty, the EPA proposes in regulatory language to interpret the CAA section 

319(b) phrase “determinations by the Administrator with respect to exceedances or violations of 

 

28 The attainment demonstration would be adequate in the sense that if a similar event does not 

occur during the period on which actual attainment will be based, there would be no monitored 

NAAQS violation, and if a similar event were to occur during that period the event-affected data 

could be excluded and thus there would be no “official” violation. 

29 If a similar event were to occur after completion of construction, the event-affected data could 

be excluded and thus there would be no “official” violation. 
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national ambient air quality standards” to encompass determinations of current30 or historical 

NAAQS exceedances/violations or non-exceedances/non-violations and determinations of the air 

quality “design value” at particular receptor sites when made as part of the basis for any of the 

following five types of regulatory actions: 

 An action to designate or redesignate an area as attainment, unclassifiable/ 

attainment, nonattainment or unclassifiable for a particular NAAQS. Such 

designations rely on a violation at a monitoring site in or near the area being 

designated. 

 The assignment or re-assignment of a classification category (marginal, moderate, 

serious, etc.) to a nonattainment area to the extent this is based on a comparison of 

its “design value” to the established framework for such classifications. 

 A determination regarding whether a nonattainment area has actually attained a 

NAAQS by its CAA deadline. 

 A determination that an area has had only one exceedance in the year prior to its 

deadline and thus qualifies for a 1-year attainment date extension, if applicable. 

 A finding of SIP inadequacy leading to a SIP call to the extent the finding hinges 

on a determination that the area is violating a NAAQS. 

For these types of actions, the EPA proposes to interpret the CAA to require that data be 

excluded only if the requirements of section 319(b) and the Exceptional Events Rule are 

satisfied. In addition, we propose that when one of these determinations is based on a 

combination of monitoring data and air quality modeling, the criterion requiring that there be a 

 

30 The term “current” denotes the determination at issue in the current analysis. In actual 

practice, such determinations are based on historical data and thus reflect a past actual condition. 
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clear causal relationship between the event and a NAAQS exceedance or violation will apply to 

the combined estimate of air pollution levels rather than directly to the monitored background air 

quality data. That is, the event would not be required to have caused an actual exceedance or 

violation at the background ambient monitoring site, but rather to have made the critical 

difference in the combined estimate of air pollution levels (background plus source impact) 

resulting in a NAAQS exceedance or violation, because the event increased the background 

levels that are added to the air quality modeling output. 

When the EPA designates or redesignates areas as attainment or nonattainment for the 

NAAQS; initially classifies ozone nonattainment areas as marginal, moderate, serious, severe or 

extreme; grants a request for a 1-year NAAQS attainment date extension where applicable; or 

determines whether areas designated nonattainment for the NAAQS have attained the respective 

NAAQS by the applicable attainment date, it does so based on monitoring data (where available) 

or modeling of actual air quality, or a combination thereof, as the evidence of the occurrence or 

non-occurrence of a NAAQS exceedance or violation and, in the case of classification actions, 

the degree of violation.31 In the case of reclassifying an ozone nonattainment area to a higher 

classification, the new classification is based on the design value either at the time of the 

determination of attainment by the attainment deadline under section 181(b)(2), or at the time of 

the EPA’s grant of a voluntary request for reclassification from a state under section 181(b)(3).32

This proposal, if finalized, would in effect apply the exceptional events process in the same way 

 

31 The EPA’s initial area designations process also makes use of other information relevant to the 

CAA criteria for designations, such as pollution contributions between nearby areas. 

32 Reclassification of PM10 and PM2.5 nonattainment areas, by contrast, do not exclusively rely 

on area design values (and thus, past monitored violations) but can also result from the 

Administrator’s determination that an area cannot practicably attain a standard by the attainment 

date. See CAA section 188(b)(1). 
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across these related types of determinations and across the NAAQS, which we believe is an 

appropriate interpretation of the CAA 319(b) phrase “determinations by the Administrator with 

respect to exceedances or violations of national ambient air quality standards.” For these types of 

determinations, the EPA proposes to exclude event-affected data only if an air agency satisfies 

the procedural (e.g., event identification, opportunity for public comment, demonstration 

submission) and substantive (i.e., clear causal relationship, not reasonably controllable or 

preventable, and human activity not likely to recur or natural event) requirements of the 

exceptional events process. As indicated previously, the EPA has maintained to this point that 

once data are excluded under the Exceptional Events Rule, these same data also should be 

excluded from (i) design value estimates and AQS user reports (unless the AQS user specifically 

indicates that they should be included), (ii) selecting appropriate background concentrations for 

PSD air quality analyses and transportation conformity hot spot analyses and (iii) selecting 

appropriate ambient data for projecting future year concentrations as part of a modeled 

attainment demonstration. As described below, we intend that EPA approval for exclusion of 

data under the Exceptional Events Rule continue to mean that the same data may be excluded for 

the three applications listed in the previous sentence, but that there should be other pathways for 

exclusion for the second and third of these applications (and others) as well. 

This action proposes to require that data exclusion associated with the five actions in the 

above bulleted list (i.e., initial area designations, classifications, attainment determinations, 

determinations regarding requests for attainment date extensions and findings of SIP inadequacy 

leading to a SIP call) must follow the provisions in the Exceptional Events Rule. It does not, 

however, mean that the EPA would never exclude or agree to exclude event-affected data from 

other types of regulatory determinations. For example, while the EPA would exclude concurred-
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upon event-affected data from the five types of regulatory actions discussed in V.C.1, the EPA 

would not exclude these same data when setting priority classifications for emergency plans 

under 40 CFR 51.150 as the EPA believes that implementing the CAA principle at section 

319(b)(3)(A) that “protection of public health is the highest priority” may necessitate that an air 

agency address in its emergency plan the appropriate planned response for any elevated 

concentration known to be possible because it has already been observed, although the 

appropriate type of response may depend on the cause(s) of the elevated concentration. The 

concept that the EPA does not consider CAA section 319(b) and the revised Exceptional Events 

Rule to be the necessary or sole governing authorities for all data exclusions will be discussed 

further in upcoming, new draft guidance on excluding (or in some cases not excluding) data, 

independent of the Exceptional Events Rule, from several types of determinations and regulatory 

actions. The EPA is currently developing a supplementary guidance document, Draft Guidance 

for Excluding Some Ambient Pollutant Concentration Data from Certain Calculations and 

Analyses for Purposes Other than Retrospective Determinations of Attainment of the NAAQS,

which will describe the appropriate additional pathways that we intend to make available for data 

exclusion for some monitoring data applications (e.g., predicting future attainment that is the 

basis for approval of an attainment demonstration in the SIP for a nonattainment area, preparing 

required air quality analyses in an application for a PSD permit or preparing required air quality 

analysis for the purposes of transportation conformity). The EPA intends to post the draft 

guidance on the exceptional events website at http://www2.epa.gov/air-qualityanalysis/treatment-data-influenced-exceptional-events and expects to finalize the document 

when we finalize these rule revisions. We intend that this guidance will recommend exclusion of 

data for PSD, transportation conformity and certain other applications in any situation in which 
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exclusion has already been approved under the Exceptional Events Rule, as well as in 

applications in which the facts would support exclusion under the criteria of the Exceptional 

Events Rule even if an EPA determination has not yet been made under the Exceptional Events 

Rule and in some other situations that we will describe in the guidance.

D. What is a natural event? 

1. Current Situation 

The CAA definition at section 319(b)(1)(iii) specifies that an exceptional event “is an 

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

event.” Thus, the statute limits the expected occurrence frequency of an event caused by human 

activity as “unlikely to recur” but does not limit the occurrence frequency of a natural event. 

Natural events may recur, even frequently.33 Air agencies can request, and the EPA can agree, to 

exclude data affected by a natural event if an air agency’s demonstration meets the other 

requirements of the Exceptional Events Rule. Thus, considering whether an event was a natural 

event or was caused by human activity is important to the content within and to the approval of a 

demonstration. 

As previously discussed, to be considered an exceptional event, an event, whether natural 

or anthropogenic in origin, must affect air quality at the affected monitor. 40 CFR 50.1(k) 

defines a natural event as one in which human activity plays little or no direct causal role in the 

generation of emissions. In some cases, such as stratospheric ozone intrusions or volcanic 

eruptions, the EPA recognizes that human activity plays no role in the magnitude of emissions or 

 

33 See as examples, Hawaii’s exceptional events demonstration for volcanic activity affecting 

PM2.5 concentrations in 2011 – 2012 and California Air Resources Board’s demonstration for 

wildfire events affecting PM2.5 concentrations in 2008, both available at 

http://www2.epa.gov/air-quality-analysis/exceptional-events-submissions-table. 
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level of air pollution that occurs. In other cases, past or current human activity does influence the 

magnitude of emissions and hence the level of air pollution. For example, in high wind dust 

events, the pollution from the event may originate from a mixture of natural lands (e.g., 

undisturbed soil), soil that has been disturbed by human activity and has been made more prone 

to wind-generated dust emissions (e.g., recent construction activity) and materials accumulated 

and stored by human activity (e.g., sand and gravel facilities). 

The EPA generally considers human activity to have played little or no direct role in 

causing emissions if anthropogenic emission sources that contribute to the event emissions are 

reasonably controlled at the time of the event, regardless of the magnitude of emissions 

generated by these reasonably controlled anthropogenic sources and regardless of the relative 

contribution of these emissions and emissions arising from natural sources in which human 

activity has no role.34,35 Thus, the event could be considered a natural event. In such cases, the 

EPA applies the reasonable interpretation that the anthropogenic source had “little” direct causal 

role. If anthropogenic emission sources that contribute to the event emissions can be reasonably 

controllable but reasonable controls were not implemented at the time of the event, then the 

 

34 For example, if an area affected by a high wind dust event has adequate rules or ordinances for 

sources of windblown dust (e.g., rules that establish restrictions for operating vehicles on 

unpaved property, rules that control windblown dust emissions associated with lands disturbed 

by construction, earthwork and land development) and the air agency can provide evidence of 

implementation and enforcement, then the EPA would generally consider human activity to have 

played little or no direct causal role in causing the event-related emissions. 

35 The EPA considers wildfires to be natural events even though some wildfires are initiated by 

human actions and to some degree the frequency and scale of wildfires may be influenced by 

prior land management practices. The EPA believes this interpretation best implements the 

Congressional intent and is a more appropriate approach than expecting air agencies to determine 

the initial cause of each wildfire of interest and classifying it as natural or anthropogenic based 

on that cause. In addition, land owners and managers and government public safety agencies are 

strongly motivated to reduce the frequency and severity of human-caused wildfires and the EPA 

believes they can be presumed to make reasonable efforts to avoid them. 
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event would not be considered a natural event. The EPA explained this concept in the preamble 

to the 2007 Exceptional Events Rule.36 However, the rule text did not reflect the identified 

concept. This has resulted in some regulatory uncertainty as to whether the EPA’s interpretation 

of the CAA and the 2007 Exceptional Events Rule as described here is appropriate. 

2. Proposed Changes 

Based on the discussion above, the EPA proposes to revise the definition of natural event 

to clarify that anthropogenic emission sources that contribute to the event emissions that are 

reasonably controlled do not play a “direct role” in causing emissions. Thus, an event with a mix 

of natural emissions and reasonably controlled human-affected emission sources may be 

considered a natural event. However, an event resulting from only reasonably controlled human 

affected emissions may not be considered a natural event. This proposal is consistent with 

statements made in the preamble to the 2007 Exceptional Events Rule, and including it in the 

rule text provides more regulatory certainty to all parties. 

When addressing the not reasonably controllable or preventable criterion for this same 

event type consisting of a mix of natural emissions and human-affected emission sources (e.g., a 

high wind event affecting both open desert areas and urbanized lands), air agencies must assess 

reasonable controls for both the contributing natural and anthropogenic sources. While air 

agencies must “assess” reasonable controls for most types of contributing natural sources 

because this statutory factor applies to all events, they do not necessarily need to implement 

controls for these same sources. Additionally, because the rule revisions propose a categorical 

presumption of not reasonably controllable for wildfires and large-scale, high-energy and/or 

sudden high wind dust events, “assessing” these events would involve referencing the 

 

36 72 FR 13565-13566 (March 22, 2007). 
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appropriate regulatory citation. As we explain in more detail in section V.E.2, for natural 

sources, we do not think that air agencies need to have implemented any controls for windblown 

dust from never-disturbed, large-scale natural landscapes. Therefore, lack of controls on natural 

sources that contribute to event-related emissions would not disqualify the event from being 

considered as an exceptional event. When assessing the contribution from anthropogenic 

sources, similar to the analyses involved in determining whether these same sources play a 

“direct role” in causing event-related emissions, the air agency should identify the contributing 

anthropogenic sources, explain why the controls specified in rules or ordinances are reasonable, 

and provide evidence of implementation and enforcement. Also as explained in section V.E.2, in 

our view an event is “not reasonably controllable” if an exceedance or violation occurs even 

when reasonable controls were actually in place and any further control would have been beyond 

what was reasonable. The EPA intends to consider these aspects when applying the concept of 

“reasonable controls” on anthropogenic sources to determine whether the event can be 

considered a natural event and to evaluate the not reasonably controllable or preventable 

criterion. 

With respect to determining whether anthropogenic emission sources that contribute to 

the event emissions were reasonably controlled at the time of the event, the EPA also proposes to 

revise the definition of a natural event to indicate that the reasonableness of available controls 

should be assessed as of the date of the event. The EPA does not believe that information related 

to the cost and effectiveness of control measures, or related to the frequency of events, that 

became available to the air agency after the date of the event should affect the assessment of 

whether anthropogenic sources were reasonably controlled and thus the identification of an event 

as natural or caused by human activity. 
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When addressing this criterion as part of an exceptional events demonstration, the EPA 

recommends that the submitting air agency clearly identify whether the event is natural or was a 

human activity that is unlikely to recur at a particular location. If purely natural (e.g., lightningignited wildfire, volcanic or seismic activity, stratospheric ozone intrusion), the EPA 

recommends that the submitting air agency identify the purely natural status in the “human 

activity/natural event” section of its demonstration; provide the type/source of event, the 

resulting emissions, and the documented frequency of the event; and affirmatively state that in 

characterizing the event, the submitting air agency has satisfied the human activity/natural event 

criterion. 

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

 As described in section V.B, the EPA proposes to return to the core statutory elements 

and implicit concepts of CAA section 319(b): that the event affected air quality in such a way 

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

exceedance or violation, the event was not reasonably controllable or preventable and the event 

was caused by human activity that is unlikely to recur at a particular location or was a natural 

event. All exceptional events demonstrations, regardless of event type or relevant NAAQS, must 

address each of these technical criteria. This section describes the EPA’s proposals for rule 

revisions and guidance regarding each of these technical criteria. Section V.G discusses 

additional process-related components of exceptional events demonstration packages. 

1. Human Activity Unlikely to Recur at a Particular Location or a Natural Event 

 The concept of recurrence applies to human activity; the statements in this section are not 

relevant for natural events. Section V.D includes a detailed discussion of a “natural event.” 

a. Current Situation 
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According to both the regulatory and statutory definitions, an exceptional event must be 

“an event caused by human activity that is unlikely to recur at a particular location or a natural 

event” (emphasis added). For clarity, in this section, the EPA focuses on the language “unlikely 

to recur at a particular location.” 

The “unlikely to recur at a particular location” requirement of CAA section 319(b) does 

not define “unlikely to recur.” Thus, this language requires interpretation on a case-by-case or 

event type-by-event type basis. The term “unlikely” implies consideration of the expected future 

frequency of events similar to the event that has already happened, but does not convey any 

particular benchmark for what frequency should be low enough to be considered “unlikely.” 

Also, the term “at a particular location” requires interpretation, as it could refer to the exact area 

or only to the general area of the event, to the location of the ambient monitoring station or 

stations that were affected by the event or to the combination of both. 

The EPA’s 1986 Guideline on the Identification and Use of Air Quality Data Affected by 

Exceptional Events stated that events can be considered exceptional if they are not expected to 

“recur routinely at a given location.” 37 This document did not further define or give specific 

examples of “routinely.” 

The preamble to the 2007 Exceptional Events Rule did not provide specific guidance on 

the unlikely to recur criterion, except to say that recurrence is event-specific and should be 

assessed on a case-by-case basis and that in the particular case of prescribed fires a comparison 

to the natural fire return interval is a relevant consideration for this criterion. 

 

37 Guideline on the Identification and Use of Air Quality Data Affected by Exceptional Events

(the Exceptional Events Policy), U.S. EPA, OAQPS, EPA-450/4-86-007, July 1986. 
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The CAA section 319(b) and the 2007 Exceptional Events Rule do not specifically 

address temporary, but multi-day or multi-year activities such as construction projects. However, 

Question 16 in the Interim Q&A document noted that the 2007 Exceptional Events Rule does not 

explicitly place a limit on the duration of a single event and that a submitting agency could make 

a showing that a prolonged activity (e.g., a multi-year road construction project) is a single event 

that is not likely to recur at the location in question. The Interim High Winds Guidance 

document addressed recurrence for high wind events, as summarized in section V.F.4 of this 

document. Other than this, the Interim Exceptional Events Implementation Guidance did not 

provide any specific guidance on the unlikely to recur criterion. 

b. Proposed Changes 

While we believe that it is appropriate to consider recurrence to be event-specific and for 

the unlikely to recur criterion to be assessed on a case-by-case basis, we also believe that this 

proposed action presents an opportunity to clarify certain points. This section provides general 

clarifications with respect to the meaning of “unlikely to recur.” Section V.F.2 addresses this 

criterion for wildland fires (specifically prescribed fires on wildland) and section V.F.4 

specifically addresses this criterion for high wind dust events. Also, under CAA section 319(b) 

and the 2007 Exceptional Events Rule, air pollution related to source noncompliance is not an 

exceptional event regardless of its frequency. 

The EPA proposes, as guidance, to recommend the following boundaries on the 

interpretation of the unlikely to recur criterion. If an event type has not previously occurred 

within a given air quality control region (AQCR) 38 in the 3 years preceding the submittal of an 

 

38 Air Quality Control Regions are defined in 40 CFR part 81, Subpart B, Designation of Air 

Quality Control Regions. 
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exceptional events demonstration, the EPA will consider this to be a “first” event and will 

generally consider it to be unlikely to recur in the same location. Similarly, a “second” event 

within the 3 years preceding the submittal of an exceptional events demonstration would also 

generally be considered unlikely to recur in the same location. If there have been two prior 

events of a similar type within a 3-year period in an AQCR, that would generally indicate the 

third event, for which the demonstration is being prepared (or would be prepared), does not 

satisfy the “human activity that is unlikely to recur at a particular location” criterion and, thus, 

would not qualify as an exceptional event. The terms “first” and “second” events refer to events 

that affect the same AQCR, even if they have not affected the same monitor.39 This proposed 

guidance is consistent with the approach taken to recurrence in our Interim High Winds 

Guidance document in which we identified non-recurring events as being less than one event per 

year in a given area.40 In the Interim High Winds Guidance, we did not define area other than to 

differentiate areas by attainment status or jurisdiction (i.e., intrastate versus interstate or 

international). 

The EPA solicits comment on this proposed guidance regarding recurrence at a particular 

location, specifically the use of an AQCR to define the bounds for an area subject to event 

recurrence given that some AQCRs may be quite large. The EPA also solicits comments on 

 

39 The EPA will consider previously flagged exceedances within AQS with their associated 

descriptions to be “events” regardless of whether the EPA has received or acted on event 

demonstrations. The EPA also notes that a single event could influence concentrations on 

multiple days. 

40 See footnote 27 in Table 2 of Interim Guidance on the Preparation of Demonstrations in 

Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the 

Exceptional Events Rule. U.S. EPA. May 2013. Available at 

http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_highwinds_guide_130510.pdf. 
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whether this benchmark of three events in 3 years should be incorporated into the rule text, rather 

than being provided only as guidance. 

The EPA proposes, as guidance, that to satisfy the documentation requirements for the 

“human activity that is unlikely to recur at a particular location” criterion, the submitting air 

agency should document and discuss, in a distinct “human activity/natural event” section of the 

demonstration, the type/source of event (e.g., a particular type of chemical spill or other 

industrial accident or a fire in a particular type of structure), the resulting emissions and the 

documented frequency of the event in the prior 3 years. The demonstration should affirmatively 

state that in characterizing the event, the submitting air agency has satisfied the “human activity 

unlikely to recur at a particular location or a natural event” criterion. 

2. Not Reasonably Controllable or Preventable 

The CAA section 319(b) does not restrict the not reasonably controllable or preventable 

criterion to apply only to events caused by human activity. It also applies to natural events. 

Accordingly, the Exceptional Events Rule applies this criterion to all types of events. This 

section discuss the criterion in general terms. We discuss the criterion’s applicability to fire 

events on wildland in section V.F.2 and to high wind dust events in section V.F.4. 

a. Current Situation 

As noted in section V.B of this document, the definition of an exceptional event at 40 

CFR 50.1(j) repeats the CAA definition and includes the requirement at section 319(b)(1)(A)(ii) 

that an exceptional event, whether natural or caused by human activity, is one that “is not 

reasonably controllable or preventable.” Neither the rule text of the 2007 Exceptional Events 

Rule nor the preamble to the final rule provided additional clarification regarding this statutory 

element. Rather, the preamble to the final rule stated, “[w]e are not finalizing more detailed 
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requirements for determining when an event is ‘not reasonably controllable or preventable’ 

because we believe that such determinations will necessarily be dependent on specific facts and 

circumstances that cannot be prescribed by rule.”41 While we maintain that determining whether 

or not an event is not reasonably controllable or preventable is event-specific and necessarily 

requires judgment by the air agency and the EPA, we also believe that some concepts regarding 

this criterion are broadly applicable. 

To begin, the statutory requirement that an exceptional event is one that “is not 

reasonably controllable or preventable” contains two factors: prevention and control. Within the 

context of the Exceptional Events Rule, we intend that “prevent” means to stop or avert the 

event, and “control” means to reduce the magnitude and impact of event-related emissions. We 

interpret CAA section 319(b) to mean that to qualify as an exceptional event, the event cannot be 

reasonably preventable and cannot be reasonably controllable, rather than that only one of the 

two elements must be satisfied. It would be contrary to the emphasis of section 319(b) on 

protection of public health if there were no requirement for reasonable control for an event 

merely because the event could not be reasonably prevented from happening. It is possible for an 

event to not be reasonably preventable, but to be reasonably controllable.42 In this case, if 

emissions were reasonably controlled, then the event could be considered for concurrence as an 

exceptional event. It is also possible that an event be neither preventable nor its air quality 

impacts to be controllable to any degree, such as potential increases in SO2 concentrations 

associated with volcanic eruptions. 

 

41 72 FR 13564 (March 22, 2007). 

42 For example, in section V.F.2, we propose that under certain circumstances a prescribed fire 

may not be reasonably preventable because of the safety or ecosystem benefits that would be 

foregone, but emissions and air quality impacts from the fire may be reasonably controllable 

through the application of basic smoke management practices. 
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The EPA considers the statutory requirement that an exceptional event be “not reasonably 

preventable” to mean that if a set of prevention measures should reasonably have been in place 

for anthropogenically-influenced emission sources that contribute to the event emissions, then 

those measures must have been in place for the event to qualify as an exceptional event under the 

Exceptional Events Rule. Similarly, we consider the statutory requirement that an exceptional 

event be “not reasonably controllable” to mean that if a set of control measures should 

reasonably have been in place for emission sources that contribute to the event emissions, then 

those controls must have been in place for the event to qualify as an exceptional event under the 

Exceptional Events Rule. Satisfying the not reasonably controllable element necessitates a 

showing of reasonable controls. Whether a set of controls constitutes “reasonable controls” is 

event-, time-, and place-dependent, and involves judgment by the air agency when preparing the 

demonstration and by the EPA when reviewing the demonstration.43 We stated in the Interim 

Exceptional Events Implementation Guidance, and we reiterate in this action, that it may not be 

reasonable to apply any prevention or control efforts for some events. 

In the course of implementing the 2007 Exceptional Events Rule, both the EPA and air 

agencies have expressed concern regarding the determination of “reasonable” prevention or 

control efforts for particular events. When an air agency prepares a demonstration, it attempts to 

show that whatever efforts were made were all that were reasonable to make. When the EPA 

reviews a demonstration, we are responsible for determining if the demonstration is credible and 

convincing. The EPA has been unable to make this determination regarding reasonableness for 

some demonstrations because the content regarding the use and implementation of control 

 

43 The EPA has many resources to help states identify appropriate control technologies and 

includes links to some of these sources on the Control Strategies website available at 

http://www3.epa.gov/airquality/aqmportal/management/control_strategies.htm. 
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measures is insufficient. Given the elasticity of the concept of “reasonable,” it is not surprising 

that disagreements have arisen. We have in the past few years, particularly since issuing the 

Interim Exceptional Events Implementation Guidance, worked with states to reach mutual 

understandings of what efforts are reasonable and to have those efforts in place before events 

happen. However, situations will likely occur in the future, as they have in the past, in which an 

assessment of reasonableness must be made retrospectively, when it is too late for the air agency 

to have applied greater efforts. The EPA recognizes that our action on the air agency’s 

demonstration may have important regulatory consequences for the area in question. 

The EPA has stated that for all types of events, we consider reasonableness in light of the 

technical information available to the air agency at the time the event occurred. An air agency 

“caught by surprise” by an event of a given type (or by an unexpected number of such events in a 

period over which NAAQS compliance is evaluated, typically 3 years) should not be expected to 

have implemented the same controls prior to an event as an air agency that has been aware that 

events of a certain type occur with regularity and cause NAAQS exceedances or violations. The 

EPA anticipates that nonattainment (or maintenance) areas have technical information needed to 

understand those measures that constitute reasonable control of anthropogenic sources in their 

jurisdiction for recurring events of the type(s) that cause or contribute to nonattainment (or that 

did previously). In contrast, the EPA generally does not expect areas identified as attainment, 

unclassifiable/attainment or unclassifiable for a NAAQS to have the same understanding or to 

have adopted the same level of event-relevant controls as areas that are nonattainment (or 

maintenance) for the same NAAQS. Also, if an area has been recently designated to 

nonattainment but is still developing its SIP and has not yet reached a deadline to implement 
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controls, the EPA expects the level of controls that is appropriate for that planning stage.44

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

package should address the question of reasonable controls within the not reasonably 

controllable or preventable portion of the demonstration. 

The not reasonably controllable or preventable criterion is a source of particular 

complexity when an event occurs outside the jurisdiction of the state that is requesting that data 

be excluded. The area outside a state’s jurisdiction may be in an area of Indian country, in 

another state, or in a foreign country. For these cases, the air agency requesting data exclusion, 

and other government authorities in the state, generally do not have regulatory authority over 

those who might have been able to prevent or control the event. Therefore, the EPA believes that 

event-related emissions that originate outside of the boundaries of the state within which the 

concentration at issue was monitored are generally “not reasonably controllable or preventable” 

even if no party has made any effort to control or prevent them. To date, we have advised air 

agencies that an exceptional events demonstration for such a case must nevertheless explicitly 

address the question of reasonable efforts towards prevention and control. For these situations, 

we have suggested template language to the effect that satisfying the not reasonably controllable 

or preventable element could consist of an air agency stating that because the event occurred 

outside of its jurisdiction, the not reasonably controllable or preventable criterion is satisfied. 

 

44 The CAA provides different timeframes for developing and implementing SIPs depending on 

the NAAQS and the nonattainment area’s classification (e.g., severity of the nonattainment 

problem). The EPA recognizes that within the SIP development and implementation process, 

some measures may be implemented relatively quickly (e.g., transportation conformity, new 

source review) whereas other programs, such as development or rules for particular source types, 

can take time and involve state legislative processes. 
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Because the reasonableness of controls for event-related emissions is case-specific, the 

EPA has not issued guidance that particular controls are reasonable or are not reasonable. The 

Interim High Winds Guidance document indicates sources of information that identify measures 

that an air agency and the EPA should consider. In that guidance, we said that if the EPA has 

approved a SIP revision to windblown dust controls within the past 3 years of the event, then an 

air agency can rely on the SIP-approved controls to satisfy a portion of a “prospective controls 

analysis.”45 By this, we meant that we would agree with the air agency that for any high wind 

dust events in the next 3 years, implementation of the controls in the SIP would be sufficient to 

establish that those events are not reasonably controllable. In our discussions during the 

development of these proposed revisions of the Exceptional Events Rule, air agencies have urged 

us to give more deference to relevant controls in the EPA-approved SIPs. Some air agencies have 

recommended that we always accept that the controls in the approved SIP are all that should 

have reasonably been in place at the time of the event (and/or that we accept no controls if there 

are no controls in the approved SIP). We understand at least some of those recommending this 

approach to mean it to apply both to nonattainment and maintenance areas that have approved 

attainment or maintenance plans and to areas whose SIPs have been approved only with respect 

to less specific infrastructure SIP requirements. 

b. Proposed Changes 

The EPA generally plans to continue its past interpretations with respect to the “not 

reasonably controllable or preventable” criterion. We propose to codify in regulatory language 

 

45 Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude 

Ambient Air Quality Data Affected by High Winds Under the Exceptional Events Rule. U.S. 

EPA. May 2013. Available at http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_highwinds_guide_130510.pdf.
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key aspects of these past interpretations to reduce uncertainty for air agencies and other parties. 

Specifically, we are proposing changes to the text of the Exceptional Events Rule to indicate 

that: 

 The not reasonably controllable or preventable criterion has two prongs, 

prevention and control. An air agency must demonstrate that an event was both 

not reasonably preventable and not reasonably controllable. 

 An event is not reasonably preventable if reasonable measures to prevent the 

event were applied at the time of the event. 

 An event is not reasonably controllable if reasonable measures to control the 

impact of the event on air quality were applied at the time of the event. 

 The reasonableness of measures is case-specific and is to be evaluated in light of 

information available at the time of the event. 

 No case-specific justification is needed to support the “not reasonably controllable 

or preventable” criterion for emissions-generating activity that occurs outside of 

the boundaries of the state (or tribal lands) within which the concentration at issue 

was monitored.46

With regard to the last of these proposed rule text changes, the EPA maintains that it is 

not reasonable to expect the downwind air agency (i.e., the state or tribe submitting the 

demonstration) to have required or persuaded the upwind foreign country, state or tribe to have 

 

46 Under the CAA, the EPA generally considers a state (not including areas of Indian country) to 

be a single responsible actor. Accordingly, neither the EPA nor the 2007 Exceptional Events 

Rule provides special considerations for intrastate scenarios when an event in one county affects 

air quality in another county in the same state, assuming that the event occurs on land subject to 

state authority (versus tribal government authority). The EPA expects controls appropriate for 

the designation status of the county (or portion of the county) in which the emissions originate. 
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implemented controls on sources sufficient to limit event-related air concentrations in the 

downwind state or tribal lands, nor does the EPA believe that Congress intended to deny the 

downwind state or tribe relief in the form of data exclusion within the context of the Exceptional 

Events Rule. Submitting (downwind) air agencies will, however, need to assess potential 

contribution from local and state-wide sources and submit evidence and statements supporting 

the other exceptional events criteria (i.e., clear causal relationship and human activity unlikely to 

recur or a natural event). 

In addition to proposing to codify the five current interpretations listed above, with regard 

to this criterion, we are proposing and requesting comments on changes from our current 

interpretations and changes in the rule text that are explained below in more detail. 

Natural Events and Natural Sources. The not reasonably controllable or preventable 

criterion applies to natural events, including natural sources and any contributing anthropogenic 

sources and activities.47 The EPA proposes, as guidance, that to satisfy the not reasonably 

controllable or preventable criterion for natural events, air agencies should identify in their 

demonstration the origin and evolution of the natural event, describe any local efforts to prevent 

the event and explain how any efforts to limit the duration, intensity or extent (and thus the 

emissions) from the event were reasonable. 

Large-scale natural landscapes, such as deserts, are one type of natural source from which 

emissions can originate and contribute to event-related emissions. We propose, as guidance for 

these types of natural sources, that air agencies would not need to have implemented any controls 

for windblown dust from never-disturbed, large-scale natural landscapes. If such a landscape is 

 

47 An event with a significant contribution from anthropogenically-influenced emissions sources 

that have not themselves been reasonably controlled cannot be considered a natural event subject 

to this provision. 
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the only source of wind-blown dust, the EPA would consider the event in this scenario to be not 

reasonably controllable or preventable regardless of the past frequency of similar events. Other 

such cases include volcanic releases of SO2 and stratospheric ozone intrusions. In these cases, the 

air agency should affirmatively state that the not reasonably controllable or preventable criterion 

is satisfied by the fact that the natural event was of a character that could not have been 

prevented or controlled and that there were no contributions of event-related emissions from 

anthropogenic sources. 

We also propose, as guidance, for events other than high wind dust events and wildfire on 

wildland (for which the proposed rule revisions take an equivalent approach), to consider the 

direct effects of remote, large-scale, high-energy and/or sudden natural events to generally be not 

reasonable to prevent or control.48 This concept, as it relates specifically to proposed rule 

changes addressing high wind dust events, is discussed in more detail in section V.F.4. Section 

V.F.2.c discusses how the same concept relates to proposed rule changes addressing the “not 

reasonably controllable or preventable” criterion for wildfire on wildland. 

There may, however, be natural events or activities associated with the clean-up 

following a natural event where some type of control effort would be reasonable. For example, 

while an initial volcanic dust event may not be controllable or preventable, it may be reasonable 

 

48 By “remote” events, we mean events that occur in locations where the application of control 

measures is either cost-prohibitive or presents unreasonable risks to worker safety because of the 

distance of the source from logistical staging areas, or absence of roads and/or location on rough 

or steep terrain. By “large-scale” we mean a regional event that involves a significant expanse of 

land and/or affects all/most monitors in an area. “High-energy” means an event involving levels 

of kinetic energy that feasible human efforts cannot absorb or redirect. Example large-scale 

and/or high-energy events might include seismic events, hurricanes, tornadoes and “haboobs” in 

the southwest where sustained wind speeds can exceed 40 mph and generate walls of dust 

several miles wide and more than a mile high. 
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to implement a street cleaning program to control the subsequent re-entrainment of dust 

deposited on roadways after the eruption. Also, air quality impacts during the active period of a 

weather disaster event generally cannot be prevented or controlled and it would be reasonable for 

no effort to have been made to do so. However, air agencies should apply reasonable controls, as 

applicable, in the recovery period after the event (e.g., during the removal or incineration49 of 

debris following a hurricane or tornado). There may also be smaller scale natural sources and 

events for which some control actions would be reasonable. We request comment on additional 

general and event-specific recommendations that would be consistent with the CAA and the 

revised Exceptional Events Rule regarding natural events and sources that the EPA could include 

in guidance to provide more certainty and allow air agencies to efficiently prepare 

demonstrations. 

The Role of Past Occurrences. When assessing the controls that should reasonably have 

been in place in light of information available at the time of the event, both the air agency and 

the EPA should consider the then-known frequency and severity of recurring events of the same 

type as both characteristics should affect decisions regarding those measures that constitute 

reasonable controls. A measure may not be reasonable when the event type and severity was 

known to occur infrequently, but such measures may be reasonable if that event type and severity 

occurs frequently, because there are greater (more frequent) benefits to balance against the cost 

of implementation. If the event was the area’s first experienced event of this type, then the 

submitting air agency would note that fact. The air agency could then rely on measures in the SIP 

 

49 When addressing reasonable controls for the incineration of debris associated with the 

recovery period following a natural disaster, air agencies may want to consider, as appropriate, 

the basic smoke management practices discussed in more detail in section V.F.2.d of this 

proposal. 
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and other controls in place at the time of the event, if any, to satisfy the not reasonably 

controllable or preventable criterion because, at the time of the event, the air agency did not have 

a basis for understanding the possible need for better controls for this type of event. If, however, 

the area has previously experienced events of the type that are the focus of the demonstration, 

then the air agency has a basis for understanding the possible need for better controls.50 

 We note that this consideration of past recurrence when determining what controls would 

have been sufficient to satisfy the not reasonably controllable or preventable criterion is not the 

same as the consideration of the likelihood of future recurrence for the purposes of the unlikely 

to recur criterion. Past experiences are a general guide to future likelihood but the EPA 

recognizes that future recurrence may follow a different pattern and may necessitate new 

measures to prevent events of a given type. 

The Role of the EPA-approved SIP as the Benchmark for Reasonable Measures – In 

General. As already mentioned, some air agencies have urged us to defer to relevant controls in 

EPA-approved SIPs as always sufficient to satisfy the not reasonably controllable or preventable 

criterion. The EPA could conceivably give “deference” to several different types of SIPs. CAA 

section 110(a)(1) and 110(a)(2) requires every state to develop and submit to the EPA an 

“infrastructure SIP” for each NAAQS within 3 years of the promulgation of a new or revised 

NAAQS. Infrastructure SIPs address a number of CAA requirements, including the requirement 

to contain emission limits to ensure attainment and maintenance of a NAAQS. However, under 

the EPA’s interpretation of these CAA sections, infrastructure SIPs are not required to include 

 

50 Because a state is considered a single responsible entity for purposes of SIP development and 

implementation, there may be state governmental authorities whose knowledge of the need for an 

availability of controls at the time of the event is also relevant, particularly for in-state sources 

outside the geographic area covered by the air agency’s regulatory authority. 
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attainment or maintenance demonstrations and are not required to demonstrate that the controls 

on particular sources are “reasonable.” Thus, in general, EPA-approved infrastructure SIPs do 

not necessarily constitute a robust assessment of those controls that are reasonable to have in 

place to address air quality impacts from particular types of events that may become the focus of 

exceptional events demonstrations. 

In contrast, states with areas designated as nonattainment for a NAAQS must prepare 

attainment plan SIPs, which must include an attainment demonstration and reasonably available 

control measures (RACM), among other requirements.51 Attainment plans for serious PM10 or 

PM2.5 areas must also contain best available control measures (BACM). When a nonattainment 

area reaches attainment, it may be redesignated to maintenance area status if it has implemented 

all applicable nonattainment area requirements and obtains the EPA’s approval for a 

maintenance plan for a 10-year period. Thus, in both maintenance and nonattainment areas with 

approved attainment plan SIPs, the air agency and the EPA will have considered what controls 

are necessary and reasonable to provide for attainment, based on information available at the 

time of plan development and approval. 

Taken to its furthest limit, the deference recommended by some air agencies would mean 

that the EPA would always approve a state air agency assertion that the control measures in a 

SIP that has received full approval by the EPA as meeting currently applicable requirements 

related to the event-relevant NAAQS constituted the reasonable set of controls for the event in 

question and thus the event was not reasonably controllable or preventable. We believe that this 

degree of deference could, in some cases, result in the approval for data exclusion contrary to 

 

51 Marginal ozone nonattainment areas are exceptions because they are not required to submit 

attainment demonstrations. 
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CAA requirements. Deference to the measures in an EPA-approved SIP is not always 

appropriate because EPA approval at some time in the past does not necessarily mean that (1) the 

control measures in a current SIP address all event-relevant sources of current importance, (2) 

the control measures that were considered by the air agency and the EPA at the time the EPA last 

approved the SIP are the same measures that were known and available at the time of a more 

recent event, or (3) that conditions in the area have not changed in a way that would affect the 

approvability of the same SIP if it newly needed the EPA’s approval. However, we believe that it 

may be consistent with the CAA to revise the Exceptional Events Rule to identify the conditions 

under which the EPA and air agencies can rely upon measures in an EPA-approved SIP to satisfy 

the not reasonably controllable or preventable criterion. To clarify these scenarios, the EPA is 

proposing, and discusses below, various combinations of rule provisions and guidance for areas 

of different designation status. 

The best time for air agency and federal officials to exchange both technical information 

and views on the balance between costs and benefits related to the sufficiency of reasonable 

controls is before an event happens. To avoid the EPA’s retrospective second guessing of an air 

agency’s consideration of information available to it before an event occurs, we have identified 

and described below several proposals, which would apply when an affected air agency and the 

EPA have not reached a mutual understanding regarding reasonable controls prior to an event. 

The Role of the EPA-approved SIP in Nonattainment and Maintenance Areas. To satisfy 

the not reasonably controllable or preventable criterion for nonattainment or maintenance areas, 

the EPA proposes to establish by rule a non-rebuttable presumption that, during a 5-year window 

(or, alternatively another appropriate timeframe) following approval of an attainment plan or 

maintenance plan SIP during which no subsequent new obligation for the air agency to revise the 
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SIP has arisen, the control measures included in the SIP that are specific to the relevant pollutant, 

sources and event type are sufficient for purposes of the not reasonably controllable or 

preventable criterion.52 The EPA believes that 5 years is an appropriate timeframe upon which to 

rely for SIP deference for several reasons. As noted earlier, deference to the measures in an EPAapproved SIP is not always appropriate because EPA approval at some time in the past does not 

necessarily mean that (1) the control measures in a current SIP address all event-relevant sources 

of current importance, (2) the control measures that were considered by the air agency and the 

EPA at the time the EPA last approved the SIP are the same measures that were known and 

available at the time of a more recent event, or (3) that conditions in the area have not changed in 

a way that would affect the approvability of the same SIP if it newly needed the EPA’s approval. 

A 5-year window provides a reasonable timeframe under which to evaluate the above-identified 

potential changes. Additionally, as we discuss in section V.E.3 of this proposal, we encourage 

the use of 5 years of data when developing analyses to support the clear causal relationship 

criterion because we believe that 5 years of ambient air data represent the range of “normal” air 

quality. 

The EPA would evaluate the not reasonably controllable or preventable criterion on a 

case-by-case basis for those demonstrations involving an event affecting a nonattainment or 

maintenance area with a SIP last approved more than 5 years prior to the submittal of the subject 

demonstration. Because the issue of deference to a SIP is most often applicable for high wind 

events, section V.F.4 further illustrates this proposal. 

 

52 A request for data exclusion must also show that the event was not a result of noncompliance 

with any existing state or local laws or rules that have not been incorporated into the SIP. 
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The Role of the EPA-approved SIP in Attainment, Unclassifiable/Attainment or 

Unclassifiable Areas. Attainment, unclassifiable/attainment and unclassifiable areas should have 

EPA-approved infrastructure SIPs in place that the EPA approved within a few years following 

the promulgation of a new or revised NAAQS. Infrastructure SIPs for the 1987 PM10 NAAQS 

are likely to be many years old, while infrastructure SIPs for ozone, PM2.5, 1-hour SO2 and 1-

hour NO2 have been approved more recently.53 In addition to the EPA-approved infrastructure 

SIPs, these areas may have in place other relevant state or local laws and rules, a natural events 

action plan, a SMP and/or other programs based on voluntary participation. 

 Because the development and the EPA’s review of infrastructure SIPs typically do not 

involve a robust assessment of needed measures to prevent or control the effects of particular 

types of events and because even in the absence of a pending SIP call the SIP may not 

reasonably address events of importance, the EPA does not propose to establish in rule text or in 

guidance any form of general deference to the SIP in attainment, unclassifiable/attainment or 

unclassifiable areas. The EPA will review exceptional events demonstrations on a case-by-case 

basis, applying the Exceptional Events Rule and the EPA’s guidance. A case-by-case review 

may conclude that the measures that were in place under the SIP, a natural events action plan, an 

SMP or other state or local programs were sufficient or insufficient to satisfy the not reasonably 

controllable or preventable criterion. 

If the air agency has historically documented recurring events, then the EPA would 

expect the submitting air agency to identify any anthropogenic emission sources that contribute 

to the event emissions and specifically document the controls that were in place for these sources 

at the time of the event. It is possible that the air agency may not be able to make a sufficient 

 

53 The NAAQS not mentioned here have rarely presented exceptional events issues. 
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showing for the not reasonably controllable or preventable criterion if it has not implemented 

reasonable controls for anthropogenic sources that contribute to recurring events. In this case, the 

EPA regional office may not be able to concur with an air agency’s request for data exclusion. If 

the air agency has no such control plans and has no history of recurring events, then the air 

agency would note this in the not reasonably controllable or preventable portion of its 

demonstration and would rely on the fact that at the time of the event, the air agency did not have 

a basis for understanding the possible need for better reasonable controls. 

Note that in section V.G.7 of this proposed action, “Timing of the EPA’s Review of 

Submitted Demonstrations,” the EPA proposes to work with air agencies to prioritize exceptional 

events determinations that affect near-term regulatory decisions. In an attainment, unclassifiable/ 

attainment or unclassifiable areas, the only likely non-discretionary regulatory action would be 

an initial designation under a new or revised NAAQS. Possible discretionary actions include a 

redesignation under a long-standing NAAQS or a SIP call. Under its planned prioritization 

approach, the EPA would not expect to act on demonstrations for events in an attainment, 

unclassifiable/attainment or unclassifiable areas unless the area could become nonattainment 

under a new or revised NAAQS, the area is the subject of a planned EPA discretionary 

redesignation for a long-standing NAAQS where the approval of a demonstration affects the 

basis for the redesignation, or the area becomes the subject of another EPA discretionary action 

(e.g., a SIP call at the initiative of the EPA or in response to a petition) that hinges on the 

approval of a demonstration.

The Role of Prior Communications with the EPA in Case-Specific Assessments for Not 

Reasonably Controllable or Preventable. As already stated, the EPA believes that an air agency 

must include in its exceptional events demonstration a retrospective assessment of whether an 
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event was not reasonably controllable or preventable. The air agency should base this assessment 

on information available to relevant authorities (e.g., the air agency submitting the demonstration 

and potentially other government authorities in the state, for example an upwind air quality 

control district where the event occurred) that could have implemented measures to prevent or 

control the event and its effects prior to and during the event. We are proposing to adopt the 

following approach as guidance to air agencies submitting demonstrations that will be subject to 

a case-specific assessment (i.e., in situations other than when deferring to a nonattainment or 

maintenance plan SIP). 

To satisfy the not reasonably controllable or preventable criterion in a case-specific 

assessment, the EPA proposes to consider communications between the EPA and the air agency 

when assessing “reasonableness” as part of assessing the technical information available to the 

air agency at the time the event occurred and what should reasonably have been in place at the 

time of the event for anthropogenic emission sources that contribute to the event emissions. It is 

not the EPA’s intent to retroactively apply its current judgments about the reasonableness of 

controls for past events. However, it would also be inappropriate for an air agency to fail to 

respond to the EPA’s recommendations prior to an event and then claim later in an event 

demonstration that it was unaware of a reasonable control issue. 

The EPA recognizes that regulations and an area’s planning status are often evolving and 

changing. The EPA may have recently promulgated new or revised federal rules requiring 

controls on particular sources or promulgated a new or revised nationally applicable standard 

that will ultimately result in an air agency’s adoption of new control measures. The planning 

process to implement these new standards (e.g., the SIP or maintenance plan approval process) 

can be lengthy, sometimes spanning several years and involving multiple rounds of formal and 
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informal communications between the affected air agency and the EPA regarding the 

appropriateness and completeness of planning elements. In some cases, discussion of issues 

regarding appropriate controls, including what controls would constitute “reasonable” controls 

for exceptional events purposes, are part of this iterative communications process. The EPA 

solicits comment on what form of communication (short of a SIP call) would be most effective in 

conveying the EPA’s views to the affected air agency and whether this approach would be most 

appropriately addressed through guidance or regulatory text. 

Prospective Agreement on Assessments of Not Reasonably Controllable or Preventable. 

In the Interim High Winds Guidance, the EPA suggested that an air agency could develop an 

assessment showing that the controls in place for a particular type of event, or a planned 

enhancement of those controls, were sufficient to meet the not reasonably controllable or 

preventable criterion, and then obtain the EPA’s review and concurrence with the assessment 

prior to more events of that type occurring. This prospective approach would reduce 

disagreements that might otherwise occur over later retrospective assessments. To date, most air 

agencies that face recurring event issues have not pursued this option, but the EPA will work 

with any air agency expressing an interest in pursuing this approach. 

Summary of Requests for Comments Regarding Not Reasonably Controllable or 

Preventable. The EPA solicits comment on the following clarifications to the “not reasonably 

controllable or preventable” criterion: 

 The EPA solicits comment on recommending as guidance that 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 event 
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emissions, including the contribution from local sources, (2) identify the relevant 

SIP 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.54 In 

identifying natural and anthropogenic sources, the air agency should assess both 

potentially contributing local/in-state and upwind sources. We also request 

comment on whether we should revise the rule text to require these elements in a 

demonstration. 

 The EPA proposes to codify rule language to specify that no case-specific 

justification is needed to support the “not reasonably controllable or preventable” 

criterion for emissions-generating activity that occurs outside of the boundaries of 

the state (or tribal lands) within which the concentration at issue was monitored. 

 The EPA solicits comment on specific guidance or rule requirements regarding 

what constitutes reasonable control of particular natural events and sources. 

 The EPA proposes to codify in rule language that, provided the air agency is not 

under an obligation to revise the SIP, the EPA would consider (i.e., give 

deference to) enforceable control measures implemented in accordance with a 

state implementation plan, approved by the EPA within 5 years of the date of a 

 

54 The EPA generally expects evidence that the controls determined to be reasonable, if any, 

were effectively implemented and appropriately enforced. This assessment of local sources 

should include a review and description of any known nearby facility upsets or malfunctions that 

could have resulted in emissions of the relevant pollutant(s) that influenced the monitored 

measurements on the day(s) of the claimed events. In the case of a high wind dust event, for 

example, for the identified potentially contributing local and upwind sources, the analysis should 

explain how significant dust emissions occurred despite having reasonable controls in place (e.g., 

that controls were overwhelmed by high wind), if appropriate. 
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demonstration submittal, that address the event-related pollutant and all sources 

necessary to fulfill the requirements of the CAA for the SIP to be reasonable 

controls with respect to all anthropogenic sources that have or may have 

contributed to event-related emissions. 

 The EPA proposes to codify in rule language the time period for such deference to 

be 5 years from the date of the SIP approval measured to the date of an event at 

issue, but is taking comment on whether and what other timeframes might be 

appropriate. To the extent an alternative timeframe might be appropriate, 

comments should explain how it would address the criteria provided above in 

support of the 5-year timeframe. 

 The EPA proposes to consider communications and planning status when 

assessing the status of reasonable controls and proposes to do this through 

guidance. The EPA solicits comment on methods to definitively identify the status 

of communications and planning efforts (e.g., formal correspondence or other 

documentation, timelines for responding) and whether this approach would be 

more appropriately addressed through rule language. 

3. Clear Causal Relationship Supported by a Comparison to Historical Concentration Data 

a. Current Situation 

The CAA at 319(b)(3)(B)(ii) requires that “a clear causal relationship must exist between 

the measured exceedances of a national ambient air quality standard and the exceptional event to 

demonstrate that the exceptional event caused a specific air pollution concentration at a 

particular air quality monitoring location.” The clear causal relationship criterion establishes 

causality between the event and a measured exceedance or violation of a NAAQS. As stated in 
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the preamble to the 2007 Exceptional Events Rule, given the directive in CAA section 

319(b)(3)(B)(ii), it would be unreasonable to exclude data affected by an exceptional event 

simply because of a trivial contribution of an event to air quality.55 The EPA does, however, 

recognize that distinguishing trivial contributions from more significant contributions to an 

exceedance may be difficult. As with the other exceptional events criteria, the EPA has used a 

weight of evidence approach when reviewing analyses to support a causal relationship between 

an event and a monitored exceedance. 

Showing that an event and elevated pollutant concentrations occurred simultaneously 

may not establish causality. The clear causal relationship section of an exceptional events 

demonstration should include analyses showing that the event occurred and that emissions of the 

pollutant of interest resulting from the event were transported to the monitor(s) recording the 

elevated concentration measurement(s). The example analyses to support the clear causal 

relationship criterion, shown in Table 1 and first summarized in the EPA’s Interim Exceptional 

Events Implementation Guidance, are generally appropriate analyses for most event types.56 The 

EPA does not expect air agencies to include all of the evidence and analyses identified in the 

table below, but rather to use available information to build a weight of evidence showing. The 

 

55 72 FR 13569 (March 22, 2007). 

56 For purposes of summarizing example clear causal relationship analyses in one place, the EPA 

has included an entry for the comparison to historical concentrations showing in Table 1. The 

EPA notes that although the Interim High Winds Guidance and the Interim Q&A document 

discussed the comparison to historical concentrations showing, neither of these guidance 

documents presented this showing as part of the clear causal relationship. See specifically 

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

Ambient Air Quality Data Affected by High Winds Under the Exceptional Events Rule. U.S. 

EPA. May 2013. Available at http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_highwinds_guide_130510.pdf and Interim Exceptional Events Rule 

Frequently Asked Questions. U.S. EPA. May 2013. Available at 

http://www2.epa.gov/sites/production/files/2015-05/documents/eer_qa_doc_5-10-13_r3.pdf. 
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EPA may accept limited analyses (e.g., a comparison to historical concentrations in combination 

with one or two additional analyses from Table 1) for areas whose monitored ambient air 

concentrations are generally well below the NAAQS on non-event days. Additional analyses are 

beneficial if they establish a different facet of the event case and/or if they are used in 

combination with other analyses with limited data. For example, the EPA expects that areas 

prone to frequent elevated ozone (or other pollutant) concentrations, such as nonattainment areas, 

to have more sophisticated air quality prediction tools. We would expect these areas could use 

these tools when supporting an exceptional events demonstration and developing analyses to 

support a clear causal relationship. Additionally, photochemical or regression modeling analyses 

may be beneficial in situations where the causality between the event and a measured exceedance 

of a NAAQS is not clearly established with evidence and analyses identified in Table 1. For 

example, if a fire occurs during the normal high ozone season and the ozone level associated 

with the fire is in the range of otherwise-occurring ozone levels and/or only slightly above the 

ozone NAAQS, the causal relationship between the fire and the exceedance or violation may not 

be clear. In such a situation, modeling may produce a specific estimate of the ozone contribution 

from the fire. Air agencies should discuss with their EPA regional office those types of analyses 

that may be adequate to satisfy the weight of evidence requirement in individual exceptional 

events demonstrations. 

Table 1. Example Clear Causal Relationship Evidence and Analyses 

Example of Clear Causal Relationship 

Evidence 

Types of Analyses/Information to Support 

the Evidence 

Comparison to Historical Concentrations Analyses and statistics showing how the 

observed event concentration compares to the 

distribution or time series of historical 

concentrations of the same pollutant 

Occurrence and geographic extent of the 

event 

Special weather statements, advisories, news 

reports, nearby visibility readings, 

measurements from regulatory and non-
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Example of Clear Causal Relationship 

Evidence 

Types of Analyses/Information to Support 

the Evidence 

regulatory (e.g., special purpose, emergency) 

monitoring stations throughout the affected 

area, satellite imagery 

Transport of emissions related to the event in 

the direction of the monitor(s) where the 

measurements were recorded 

Wind direction data showing that emissions 

from sources identified as part of the “not 

reasonably controllable or preventable” 

demonstration were upwind of the monitor(s) 

in question, satellite imagery, monitoring data 

showing elevated concentrations of other 

pollutants expected to be in the event plume 

Spatial relationship between the event, 

sources, transport of emissions and recorded 

concentrations 

Map showing likely source area, wind 

speed/direction and pollutant concentrations 

for affected area during the time of the event, 

trajectory analyses 

Temporal relationship between the event and 

elevated pollutant concentrations at the 

monitor in question 

Hourly time series showing pollutant 

concentrations at the monitor in question in 

combination with wind speed/direction data in 

the area where the pollutant originated/was 

entrained or transported 

Chemical composition and/or size distribution 

(for PM2.5 to PM10) of measured pollution 

that links the pollution at the monitor(s) with 

particular sources or phenomenon 

Chemical speciation data from the monitored 

exceedance(s) and sources, size distribution 

data 

Comparison of event-affected day(s) to 

specific non-event days 

Comparison of concentration and 

meteorology to days preceding and following 

the event, comparison to high concentration 

days in the same season (if any) without 

events, comparison to other event days 

without elevated concentrations (if any), 

comparison of chemical speciation data 

As explained in additional detail in the EPA’s Interim Exceptional Events 

Implementation Guidance, what has previously been called the “historical fluctuations” showing 

(i.e., now referred to as the comparison to historical concentrations) consists of analyses and 

statistics showing how the observed event-affected concentration compares to the distribution or 

time series of historical concentrations of the same pollutant. 
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A demonstration may be less compelling if some evidence is inconsistent with the 

description of how the event caused the exceedance. For example, if an air agency describes an 

event as a regional dust storm or wildfire, then the EPA anticipates that most or all monitors 

within the same regional scale to be similarly affected by the event. That is, the EPA expects 

that the demonstration elements and factors (e.g., clear causal relationship, reasonable controls, 

meteorology, wind speeds) would also support the case for a regional event. Comparison of 

concentrations and conditions at other monitors could thus be very important for the 

demonstration of a clear causal relationship. Alternatively, eliminating plausible non-event 

causes may also support a causal relationship between the event and the elevated concentration. 

The EPA has been recommending that the clear causal relationship section of the 

demonstration should conclude with this type of statement: “On [day/time] an [event type] 

occurred which generated pollutant X or its precursors resulting in elevated concentrations at 

[monitoring location(s)]. The monitored [pollutant] concentrations of [ZZ] were [describe the 

comparison to historical concentrations including the percentile rank over an annual (seasonal) 

basis]. Meteorological conditions were not consistent with historically high concentrations, etc.” 

and “Analyses X, Y and Z support Agency A’s position 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 and thus satisfies the clear causal relationship criterion.” 

b. Proposed Changes 

As previously noted, the EPA is proposing to revise the 2007 Exceptional Events Rule 

text as follows: 

 To move the “clear causal relationship” element into the list of criteria that 

explicitly must be met for data to be excluded 
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 To subsume the “affects air quality” element into the “clear causal relationship” 

element 

 To remove the term “historical fluctuations” and replace it with text referring to a 

comparison to historical concentrations 

 To clarify that the comparison to historical concentrations is not a fact that must 

be proven 

 To clearly identify the types of analyses that are necessary and sufficient in a 

demonstration to address the comparison to historical concentrations 

 To remove the “but for” element (as discussed in section V.B.2) 

Additionally, the EPA proposes to reiterate in guidance the example analyses to support 

the clear causal relationship criterion, shown in Table 1 above, and first summarized in the 

EPA’s Interim Exceptional Events Implementation Guidance. As noted previously, the EPA does 

not expect air agencies to include all of the evidence and analyses identified in Table 2 below, 

but rather to use available information to build a weight of evidence showing. 

The EPA’s rationale for proposing the previously identified changes to the clear causal 

relationship criterion is presented in section V.B. The remainder of this section focuses on the 

types of analyses that an air agency must provide in its demonstration to make the comparison to 

historical concentrations. As noted in the Current Situation section, the EPA has included an 

entry in Table 1 for the comparison to historical concentrations showing. 

The comparison to historical concentrations, referred to as the “historical fluctuations” 

showing in the 2007 Exceptional Events Rule and the Interim Exceptional Events 

Implementation Guidance, is a requirement in the 2007 Rule but it is not a statutory requirement. 

The EPA’s intent with this regulatory element was to require air agencies to present event-
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influenced concentration data along with historical data and to quantify the difference, if any, 

between the event and the non-event concentrations. Comparing event-influenced concentrations 

to historical concentrations bolsters the weight of evidence within the clear causal relationship 

determination. The EPA proposes to re-phrase and incorporate the current regulatory 

requirement at 40 CFR 50.14(c)(3)(iv)(C), which requires that a demonstration to justify data 

exclusion provide evidence that “[t]he event is associated with a measured concentration in 

excess of normal historical fluctuations, including background,” within the “clear causal 

relationship” criterion. In using this approach, we propose to remove from the regulatory text the 

“in excess of normal historical fluctuations, including background” phrase and to subsume the 

concept of historical comparisons into what will effectively be a “completeness” requirement 

within the “clear causal relationship” criterion. As noted above, we specifically propose to 

remove the phrase “in excess of normal historical fluctuations, including background” as we 

believe this language is vague and provides no additional value to historical concentration 

comparisons. 

To aid the EPA’s review, reduce our need to request additional information from air 

agencies and facilitate our understanding of the air agency’s position, we are proposing rule text 

changes to indicate that an air agency submitting a demonstration must provide the following 

types of statistics, graphics and explanatory text regarding comparisons to past data. The rule 

change would also indicate that this information is sufficient to satisfy the rule’s requirement 

regarding the comparison to historical concentration data. Table 2 below identifies appropriate 

analyses and examples for comparing event-related concentrations to historical concentrations 

within the clear causal relationship criterion. 
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Table 2. Evidence and Analyses for the Comparison to Historical Concentrations 

Historical 

Concentration 

Evidence 

Types of Analyses/Supporting Information Required or 

Optional? 

1. Comparison of 

concentrations on the 

claimed event day 

with past historical 

data 

Seasonal (appropriate if exceedances occur 

primarily in one season, but not in others) 

 Use all available seasonal data over the 

previous 5 years (or more, if available) 

 Discuss the seasonal nature of pollution for 

the location being evaluated 

 Present monthly maximums of the NAAQS 

relevant metric (e.g., maximum daily 8-hour 

average ozone or 1-hr SO2) vs monthly or 

other averaged daily data as this masks high 

values 

Annual (appropriate if exceedances are likely 

throughout the year) 

 Use all available data over the previous 5 

years (or more, if available) 

Seasonal and Annual Analyses 

 Provide the data in the form relevant to the 

standard being considered for data exclusion 

 Label “high” data points as being associated 

with concurred exceptional events, 

suspected exceptional events, other unusual 

occurrences, or high pollution days due to 

normal emissions 

 Describe how emission control strategies 

have decreased pollutant concentrations over 

the 5-year window, if applicable 

 Include comparisons omitting known or 

suspected exceptional events points, if 

applicable 

 See examples at 

http://www.epa.gov/ttn/analysis/docs/

IdeasforShowingEEEvidence.ppt and

Question 3 in the Interim Q&A document 

provides additional detaila

Required seasonal 

and/or annual 

analysis (depending 

on which is more 

appropriate)

2. Comparison of 

concentrations on the 

claimed event day 

with a narrower set of 

similar days 

 Include neighboring days at the same 

location (e.g., a time series of two to three 

weeks) and/or other days with similar 

meteorological conditions (possibly from 

other years) at the same or nearby locations 

with similar historical air quality along with 

Optional analysis 
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Historical 

Concentration 

Evidence 

Types of Analyses/Supporting Information Required or 

Optional? 

a discussion of the meteorological 

conditions during the same timeframeb

 Use this comparison to demonstrate that the 

event caused higher concentrations than 

would be expected for given meteorological 

and/or local emissions conditions 

3. Percentile rank of 

concentration when 

compared to annual 

datac

 Provide the percentile rank of the event-day 

concentration relative to all measurement 

days over the previous 5 years to ensure 

statistical robustness and capture non-event 

variability over the appropriate seasons or 

number of yearsd

 Use the daily statistic (e.g., 24-hour average, 

maximum daily 8-hour average, or 

maximum 1-hour) appropriate for the form 

of the standard being considered for data 

exclusion 

Required analysis 

when comparison is 

made on an annual 

basis (see item #1) 

4. Percentile rank of 

concentration relative 

to seasonal datac

 Provide the percentile rank of the event-day 

concentration relative to all measurement 

days for the season (or appropriate 

alternative 3-month period) of the event over 

the previous 5 years 

 Use the same time horizon as used for the 

percentile rank calculated relative to annual 

data, if appropriate 

Required analysis 

when comparison is 

made on a seasonal 

basis (see item #1) 

a

 Interim Exceptional Events Rule Frequently Asked Questions. U.S. EPA. May 2013. Available 

at http://www2.epa.gov/sites/production/files/2015-05/documents/eer_qa_doc_5-10-13_r3.pdf. b

 If an air agency compares the concentration on the claimed event day with days with similar 

meteorological conditions from other years, the agency should also verify and provide evidence 

that the area has not experienced significant changes in wind patterns, and that no significant 

sources in the area have had significant changes in their emissions of the pollutant of concern. c

 The EPA does not intend to identify a particular historical percentile rank point in the seasonal 

or annual historical data that plays a critical role in the analysis or conclusion regarding the clear 

causal relationship. 

d

 Section 8.4.2.e of appendix W (proposed revisions at 80 FR 45374, July 29, 2015) recommends 

using 5 years of adequately representative meteorology data from the National Weather Service 

to ensure that worst-case meteorological conditions are represented. Similarly, for exceptional 

events purposes, the EPA believes that 5 years of ambient air data, whether seasonal or annual, 

better represent the range of “normal” air quality than do shorter periods. 

 

 As with other evidence in an exceptional events demonstration submittal, the EPA will 

use a holistic weight of evidence approach in reviewing submitted demonstration packages and 
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will consider the “clear causal relationship” information, including the comparison to historical 

concentrations showing, along with evidence supporting the other Exceptional Events Rule 

criteria. 

F. Treatment of Certain Events Under the Exceptional Events Rule 

As we stated in the preamble to the 2007 Exceptional Events Rule, we maintain that air 

quality data affected by the following event types are among those that could meet the definition 

of an exceptional event and qualify for data exclusion provided all requirements of the rule are 

met: (1) chemical spills and industrial accidents,57 (2) structural fires, (3) terrorist attacks, (4) 

volcanic and seismic activities, (5) natural disasters and associated cleanup and (6) fireworks.58

We are not proposing any changes to the definition or discussion of these event types. The AQS 

database contains a more detailed list of other similar events that may be identified for special 

consideration. The EPA will consider other types of events on a case-by-case basis. 

 Based on our implementation experience, the following other potential exceptional events 

categories warrant additional discussion: exceedances due to transported pollution, wildland fires 

including wildfires and prescribed fires, stratospheric ozone intrusions and high wind dust 

events. We discuss each of these event categories in the following sections. 

1. Exceedances Due to Transported Pollution 

a. Current Situation 

 

57 A malfunction at an industrial facility could be considered to be an exceptional event if it has 

not resulted in source noncompliance, which is statutorily excluded from consideration as an 

exceptional event, see CAA 319(b)(1)(b)(iii), and if it otherwise meets the requirements of the 

Exceptional Events Rule. 

58 Of these noted event types, only fireworks are currently identified in the regulatory language at 

40 CFR 50.14. We are not proposing any revisions to the exclusion at 40 CFR 50.14(b)(2) for 

fireworks that are demonstrated to be significantly integral to traditional national, ethnic, or other 

cultural events. 
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The 2007 Exceptional Events Rule implements one important CAA provision related to 

transported pollution. Certain events, national or international in origin and from natural or 

anthropogenic sources, may cause exceedances that are eligible for exclusion under the 2007 

Exceptional Events Rule if an air agency satisfies the rule criteria. We discuss in this section our 

position regarding exceedances due to event-related transported pollution. We also clarify in part 

c of this section how the Exceptional Events Rule provisions currently relate to other CAA 

mechanisms that address or involve transported pollution. We are not proposing any changes to 

these relationships. 

The EPA believes that the Exceptional Events Rule will often be the most appropriate 

mechanism to use when addressing transported emissions from out-of-state natural events and/or 

events due to human activity that is unlikely to recur at a particular location, because the 

Exceptional Events Rule may be used during the initial area designations process and may make 

a difference between an attainment versus a nonattainment designation. It is important to note, 

however, that the transported natural emissions must be event-related (e.g., wildfires or 

stratospheric ozone intrusion) versus ongoing on a daily basis. 

b. Proposed Changes 

 If an air agency determines that the Exceptional Events Rule is the most suitable 

approach to address contributions from transported emissions, then the air agency must consider 

the point of origin and the sources contributing to the exceedance or violation to determine how 

to address individual Exceptional Events Rule criteria, specifically the not reasonably 

controllable or preventable criterion and the human activity unlikely to recur or a natural event 

criterion. The analyses to satisfy the clear causal relationship criterion (which would subsume the 

CAA’s affects air quality criterion, if promulgated as proposed and discussed in section V.B) are 
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largely independent of whether the point of origin and contributing sources are within the air 

agency’s jurisdiction. The EPA first addressed these concepts in its Interim Q&A document and 

now proposes to clarify these intrastate and interstate scenarios. 

 Under the CAA, the EPA generally considers a state (not including areas of Indian 

country) to be a single responsible actor. Accordingly, neither the EPA nor the 2007 Exceptional 

Events Rule provides special considerations for intrastate scenarios when an event in one part of 

a state, such as a county or air district, affects air quality in another part of the same state, 

assuming that the event occurs on land subject to state authority (versus tribal government 

authority). For cases involving intrastate transport, the state or local air agency should evaluate 

whether contributing event emissions from all parts of the state were not reasonably controllable 

or preventable. Section V.E.2 discusses the assessment of the not reasonably controllable or 

preventable criterion. Because there may be special considerations regarding air agencies’ 

authority to regulate activity on federally-owned and managed lands (e.g., national parks within 

the state), states and tribes should consult with the appropriate FLM or other federal agency and 

their EPA regional office early in the development of an exceptional events demonstration 

package if they believe that sources on federally-owned and managed land contributed eventrelated emissions to a degree that raises issues of reasonable control. 

Interstate and international transport events are different than intrastate events. As noted 

in section V.E.2, the EPA maintains that it is not reasonable to expect the downwind air agency 

(i.e., the state or tribe submitting the demonstration) to have required or persuaded the upwind 

foreign country, state or tribe to have implemented controls on sources sufficient to limit eventrelated air concentrations in the downwind state nor does the EPA believe that Congress intended 

to deny the downwind state or tribe of relief in the form of data exclusion. As with any 
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demonstration submittal, the submitting (downwind) state should identify all natural and 

anthropogenic contributing sources of emissions (both local/in-state and out-of-state) to show the 

causal connection between an event and the affected air concentration values. A submitting state 

may provide a less detailed characterization of sources in the upwind state or foreign country 

than of sources within its jurisdiction. After completing the source characterization, the 

submitting state should assess whether emissions from sources within its state were not 

reasonably controllable or preventable (see section V.E.2 of this proposal). Although the 

downwind state must still assess potential contribution from in-state sources, we propose that the 

event-related emissions that were transported in the downwind state are “not reasonably 

controllable or preventable” for purposes of data exclusion. The EPA does not expect air 

agencies to submit analyses to satisfy the not reasonably controllable or preventable criterion for 

those upwind, out-of-state sources that contribute to the exceedance as part of a submitted 

demonstration. Rather, with respect to this element for such sources, an air agency would merely 

point to the relevant provision we propose to add to the Exceptional Events Rule. Submitting 

states are, however, still required to assess the contribution and potential controls from local/instate sources and submit evidence/statements supporting the other exceptional events criteria 

(i.e., clear causal relationship, human activity unlikely to recur or a natural event). If the eventrelated emissions are international in origin and affect monitors in multiple states or regions, the 

EPA may assist affected agencies in identifying approaches for evaluating the potential impacts 

of international transport and determining the most appropriate information and analytical 

methods for each area’s unique situation. 

The EPA proposes a similar approach to significant out-of-state anthropogenic sources in 

the case of a mixed natural/anthropogenic event that the submitting state wishes to have treated 
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as a natural event on the grounds that all significant anthropogenic sources were reasonably 

controlled. That is, if a mixture of natural and anthropogenic sources in an upwind state 

contributed to an event, the downwind state is not required to demonstrate that the anthropogenic 

sources in the upwind state were reasonably controlled for those sources to be considered to not 

have directly caused the event. The submitting state could consider the event to be a natural 

event based on the situation within the state requesting the data exclusion (that is, the 

contributing sources within the jurisdiction of the submitting state were either natural or 

reasonably controlled anthropogenic sources). 

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 including any 

information and arguments presented in public comments received by the state in its public 

comment process or by the EPA in a notice-and-comment regulatory action that depends on the 

data exclusion. 

c. Relationship Between Exceptional Events Rule Provisions and Other CAA Transport 

Mechanisms 

Two provisions of the CAA other than section 319(b) also provide regulatory relief for 

transported pollution, for different circumstances than those addressed by the 2007 Exceptional 

Events Rule. These provisions are briefly described here as context for understanding the role of 

the Exceptional Events Rule itself. 

 CAA section 179B, International Transport – CAA section 179B allows states to 

consider in their attainment demonstrations whether a nonattainment area might 

have met the NAAQS by the attainment date “but for” emissions contributing to 

the area originating outside the U.S. This provision addresses sources of 
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emissions originating outside of the U.S. and provides qualifying nonattainment 

areas some regulatory relief from otherwise-applicable additional planning and 

control requirements should the area fail to reach attainment by its deadline. It 

does not provide a pathway for regulatory relief from designation as a 

nonattainment area. 

 CAA section 182(h), Rural Transport Areas – CAA section 182(h) authorizes the 

EPA Administrator to determine that an ozone nonattainment area can be treated 

as a rural transport area, which provides relief from more stringent requirements 

associated with higher nonattainment area classifications (i.e., classifications 

above Marginal). Under CAA section 182(h), a nonattainment area may qualify as 

a Rural Transport Area if it does not contain emissions sources that make a 

significant contribution to monitored ozone concentrations in the area or in other 

areas, and if the area does not include and is not adjacent to a Metropolitan 

Statistical Area. Generally, an area qualifies as a Rural Transport Area because it 

does not contribute to its own or another area’s nonattainment problem; rather, 

ozone exceedances are due to transported emissions, which could be international, 

interstate or intrastate in origin. The Rural Transport Area determination can be 

made during or after the initial area designations and classifications process. 

Two additional provisions of the CAA specifically address and appropriately regulate 

transported pollution that does not qualify for data exclusion under the Exceptional Events Rule 

or for regulatory relief under CAA section 179B or CAA section 182(h). These provisions are 

briefly described here as context for understanding the role of the Exceptional Events Rule itself. 
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 CAA section 110(a)(2)(D)(i)(I), Interstate Transport – CAA section 

110(a)(2)(D)(i)(I) requires states to develop and implement SIPs to address the 

interstate transport of emissions. Specifically, this provision requires each state’s 

SIP to prohibit “any source or other type of emissions activity within the State 

from emitting any air pollutant in amounts which will significantly contribute to 

nonattainment” of any NAAQS in another state, or which will “interfere with 

maintenance” of any NAAQS in another state. When the EPA promulgates or 

revises a NAAQS, each state is required to submit a SIP addressing this interstate 

transport provision as to that NAAQS within 3 years. The EPA interprets this 

interstate transport provision to address anthropogenic sources of emissions from 

other states; we believe that is not intended to address natural sources of 

emissions. 

 CAA section 126, Interstate Transport – CAA section 126 provides states and 

political subdivisions with a mechanism to petition the Administrator for a finding 

that “any major source or group of stationary sources emits or would emit any air 

pollution in violation of the prohibition of CAA 110(a)(2)(D)(i).”59 Where the 

EPA grants such a petition, an existing source may operate beyond a 3-month 

period only if the EPA establishes emissions limitations and compliance 

schedules to bring about compliance with CAA section 110(a)(2)(D)(i) as 

expeditiously as practicable, but no later than 3 years after such finding. Similar 

 

59 The text of section 126 codified in the United States Code cross references section 

110(a)(2)(D)(ii) instead of section 110(a)(2)(D)(i). The courts have confirmed that this is a 

scrivener’s error and the correct cross reference is to section 110(a)(2)(D)(i), See Appalachian 

Power Co. v. EPA, 249 F.3d 1032, 1040–44 (D.C. Cir. 2001). 
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to our interpretation above for CAA section 110(a)(2)(D)(i), the EPA interprets 

the reference to “major source or group of stationary sources” in section 126 to 

refer to anthropogenic sources of emissions from other states. The EPA’s 

interpretation is that this provision is not intended to address natural sources of 

emissions. This mechanism is available to all downwind states, and political 

subdivisions, regardless of area designations, that may be affected by 

anthropogenic sources of emissions from upwind states in violation of the 

prohibition in CAA section 110(a)(2)(D)(i). 

As noted previously, in most cases, the mechanisms in the Exceptional Events Rule often 

provide the most regulatory flexibility in that air agencies can use these provisions to seek relief 

from designation of an area as nonattainment. The CAA section 179B (International Transport) 

and section 182(h) (Rural Transport Areas) apply following, or concurrent with, the initial area 

designations process. 

2. Wildland Fires 

Fires on wildland play an important ecological role across the globe, benefiting those 

plant and animal species that depend upon natural fires for propagation, habitat restoration and 
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reproduction. Wildland can include forestland,60 shrubland,61 grassland62 and wetlands.63 Fires 

on wildland can be of two types: wildfire (unplanned) and prescribed fire (intentionally ignited 

for management purposes). Since promulgation of the 2007 Exceptional Events Rule, the EPA 

has received and acted upon exceptional events demonstrations for both wildfires and prescribed 

fires. The EPA anticipates receiving increasing numbers of fire-related demonstrations in the 

future due to the natural accumulation of fuels in the absence of fire, due to climate change that 

is leading to increased incidence of wildfire,64 which may necessitate land managers employing 

prescribed fire more frequently to manage fuel loads and achieve other benefits as described 

below,65,66 and due to the potential for fire-related demonstrations to affect near-term regulatory 

decisions such as the initial area designations decisions associated with a revised 8-hour ozone 

NAAQS. Consequently, the EPA is proposing revisions to the 2007 Exceptional Events Rule and 

 

60 Forestland is land on which the vegetation is dominated by trees or, if trees are lacking, the 

land shows historic evidence of former forest and has not been converted to other uses. 

Definition available at https://globalrangelands.org/rangelandswest/glossary. 61 Shrubland is land on which the vegetation is dominated by shrubs. Definition available at 

https://globalrangelands.org/rangelandswest/glossary. 62 Grassland is land on which the vegetation is dominated by grasses, grass like plants, and/or 

forbs. Definition available at https://globalrangelands.org/rangelandswest/glossary. 63 Wetlands, as defined in 40 CFR 230.3(t), means those areas that are inundated or saturated by 

surface or ground water at a frequency and duration sufficient to support, and that under normal 

circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil 

conditions. Wetlands generally include swamps, marshes, bogs and similar areas. 

64 The Administrator’s finding on the adverse effects of greenhouse gases included the 

observation that wildfires have increased, and that there are potential serious adverse impacts 

from further wildfire occurrence. 74 FR 66530 (December 15, 2009). 

65 Climate Change in the United States: Benefits of Global Action, U.S. EPA, EPA-430-R-15-

001, June 2015. Available at http://www2.epa.gov/cira.

66 The National Strategy: The Final Phase in the Development of the National Cohesive 

Wildland Fire Management Strategy, Report to Congress developed by the U.S. Department of 

Agriculture and the U.S. Department of the Interior, April 2014. Available at 

http://www.forestsandrangelands.gov/strategy/thestrategy.shtml. 
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developing additional guidance to make the preparation and review of demonstrations for 

wildland fire events more efficient and predictable for all parties. 

Wildfire emissions account for a large portion of direct PM2.5 emissions nationally and 

can contribute to periodic high PM2.5 and PM10 levels. Wildfires also emit volatile organic 

compounds (VOC) and nitrogen oxides (NOx), which are precursors to PM2.5, PM10 and ozone. 

Besides their effect on air quality, wildfires pose a direct threat to public safety. Changes in 

wildfire risk and occurrence are closely associated with the lack of periodic fire in fire-dependent 

ecosystems, demographic changes and associated infrastructure investment at the margins of 

wildland and, as already noted, climate change and climate variability. The threat from wildfires 

can be mitigated through management of wildland vegetation. Attempts to suppress wildfires 

have resulted in unintended consequences, especially the buildup of fuel loads, which can create 

a lingering fire liability that will eventually find resolution, unplanned or planned. Unplanned 

fires in areas with high fuel loads present high risks to both humans and ecosystems.67 Planned 

prescribed fires and letting some wildfires proceed naturally (typically those with lower fire 

intensity and severity) are tools that land managers can use to reduce fuel load, unnatural 

understory and tree density, thus helping to reduce the risk of catastrophic wildfires. Allowing 

some wildfires to continue to burn even though they could be suppressed and the thoughtful use 

of prescribed fire can influence the occurrence, size and severity of catastrophic wildfires, which 

may lead to improved public safety, improved protection of property and an overall reduction in 

fire-induced smoke impacts and subsequent health effects. Thus, appropriate use of prescribed 

 

67 Indeed, “fire policy that focuses on [wildfire] suppression only, delays the inevitable, 

promising more dangerous and destructive future...fires.” Stephens, SL; Agee, JK; Fule, PZ; 

North, MP; Romme, WH; Swetnam, TW. (2013). Managing Forests and Fire in Changing 

Climates. Science 342: 41-42. 
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fire may help manage the contribution of wildfires to both background and peak PM and ozone 

air pollution. However, prescribed fires themselves can affect monitored air quality at some 

times and places affecting public health, and thus give rise to exceptional events issues. This 

action proposes a workable approach to addressing these prescribed fire exceptional events 

issues. 

In addition to reducing wildfire risks to humans and ecosystems and wildfire 

contributions to air pollution, prescribed fires can have benefits to those plant and animal species 

that depend upon natural fires for propagation, habitat restoration and reproduction, as well as 

benefits to a myriad of ecosystem functions (e.g., carbon sequestration, maintenance of water 

supply systems and endangered species habitat maintenance). The EPA understands the 

importance of prescribed fire, which mimics a natural process necessary to manage and maintain 

fire-adapted ecosystems and climate change adaptation, while reducing risk to public safety and 

the risk of uncontrolled emissions and ecosystem damage from catastrophic wildfires. The EPA 

is committed to working with federal land managers, other federal agencies, tribes, states and 

private landowners to effectively manage prescribed fire use to reduce the impact of catastrophic 

wildfire-related emissions on ozone, PM10 and PM2.5. 

a. Current Situation 

When the EPA promulgated the 2007 Exceptional Events Rule, we included definitions 

for fire-related terms (e.g., wildfire, prescribed fire and wildland) in the preamble and attributed 

these definitions to the National Wildland Fire Coordinating Group (now the National Wildfire 

Coordinating Group or NWCG) Glossary of Wildland Fire Terminology, 2003. The EPA did not, 

however, codify these definitions in regulatory text. Since promulgation of the 2007 Exceptional 

Events Rule, the NWCG has modified some of its recommended definitions and the EPA used 
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slightly different definitions in its Interim Exceptional Events Implementation Guidance, 

creating some confusion for air agencies and other entities working with air agencies who have 

tried to use fire-related definitions and concepts when preparing and submitting exceptional 

events demonstrations for fires. 

The preamble to the 2007 Exceptional Events Rule discussed how the EPA expected to 

apply the rule to wildfires and prescribed fires. The EPA stated that wildfires would be 

considered natural events, while prescribed fires would be considered events caused by human 

activity. As events caused by human activity, prescribed fires are subject to the “not likely to 

recur” criterion, and the preamble to the rule discussed the considerations that would apply for 

this criterion. Section V.F.2.d provides a more detailed summary of the current situation and 

planned changes for this criterion. 

Demonstrations for wildfires and those prescribed fires claimed to be exceptional events 

must also address the “not reasonably controllable or preventable” criterion. Neither the 2007 

Exceptional Events Rule nor its preamble addressed this criterion in any depth for wildfires. 

Since promulgating the rule in 2007, the EPA has concluded that short, general statements in 

demonstrations for fire events satisfy this criterion. The EPA has been advising air agencies that 

when documenting the “not reasonably controllable or preventable” criterion in a wildfire 

exceptional events demonstration submittal, air agencies should identify the origin and evolution 

of the wildfire, describe local efforts to prevent fires due to unauthorized activity or accidental 

human-caused actions (if relevant given the origin of the fire) and explain any efforts to limit the 
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duration or extent (and thus the emissions) of the wildfire.68,69,70 We have also advised air 

agencies that if the wildfire originated outside of the jurisdiction of the air agency submitting the 

exceptional events demonstration, then the submitting air agency should identify this fact in its 

demonstration.71

 The 2007 Exceptional Events Rule and its preamble gave more extensive treatment to 

the not reasonably controllable or preventable criterion for prescribed fires. The rule text tied the 

eligibility of prescribed fires as approved exceptional events to the air agency having a 

“certified” SMP in place or, in the alternative, to using BSMP for the prescribed fire(s) in 

question. In the preamble, the EPA did not provide detailed guidance on SMPs or BSMP, but 

committed to defining and developing these concepts when we updated the guidance contained 

in the 1998 Interim Air Quality Policy on Wildland and Prescribed Fires.

72 However, the EPA 

has not revised this guidance document. Although some states have developed demonstrations 

that incorporate BSMP employed for a specific prescribed fire and/or have referenced BSMP in 

their SMPs, the EPA has not in any other more recent guidance clarified what it means for an air 

agency or burner to have a certified SMP in place and/or to implement BSMP. Like the 

inconsistency that has developed since 2007 in fire-related definitions, the absence of further 

 

68 Interim Exceptional Events Rule Frequently Asked Questions. U.S. EPA. May 2013. Available 

at http://www2.epa.gov/sites/production/files/2015-05/documents/eer_qa_doc_5-10-13_r3.pdf. 

See question 20b. 

69 Prevention/control efforts could include posting High Fire Danger signs to make people more 

careful and prevent accidental fires, and/or taking reasonable action to contain a fire once it has 

started. 

70 Example language to limit the duration and extent of the wildfire might include, “During 

wildfires, fire management resources were deployed to the fire event giving first priority to 

protecting life and property.” 

71 Interim Exceptional Events Rule Frequently Asked Questions. U.S. EPA. May 2013. Available 

at http://www2.epa.gov/sites/production/files/2015-05/documents/eer_qa_doc_5-10-13_r3.pdf. 72 Interim Air Quality Policy on Wildland and Prescribed Fires. U.S. EPA. April 23, 1998. 

Available at http://www.epa.gov/ttn/oarpg/t1/memoranda/firefnl.pdf.
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clarifying guidance on SMPs and BSMP has created confusion for air agencies trying to develop 

these plans and/or apply these practices for purposes of developing and submitting exceptional 

events demonstrations for prescribed fires. 

b. Proposed Changes 

In this action, the EPA proposes to codify in regulatory language certain fire-related 

definitions and SMP/BSMP factors necessary for exceptional events demonstration and program 

implementation purposes. Codifying these definitions in 40 CFR 50.1 will promote 

understanding and standardize terminology for the purposes of characterizing an event for 

exceptional events demonstration purposes. 

Finalizing these proposed changes will also decouple implementation of the exceptional 

events process from potential future revisions to the Interim Policy on Wildland and Prescribed 

Fires. Although the EPA solicits comment on the regulatory process associated with developing 

exceptional events demonstrations for fire-related events, the EPA does not intend to take 

comment on any aspects of the 1998 Interim Policy on Wildland and Prescribed Fires as part of 

this effort to revise the Exceptional Events Rule. 

The proposed new definitions, along with other proposed changes, are described in detail 

in the separate sections on wildfires (section V.F.2.c) and prescribed fire (section V.F.2.d) that 

follow immediately below this section. 

c. Wildfires 

Current Situation. The preamble to the 2007 Exceptional Events Rule defined a wildfire 

as “an unplanned, unwanted wildland fire (such as a fire caused by lightning), [to] include 

unauthorized human-caused fires (such as arson or acts of carelessness by humans), escaped 

prescribed fire projects (escaped control due to unforeseen circumstances), where the appropriate 
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management response includes the objective to suppress the fire.” The 2007 Exceptional Events 

Rule preamble also defined a “wildland fire use” as “the application of the appropriate 

management response to a naturally-ignited (e.g., as the result of lightning) wildland fire to 

accomplish specific resource management objectives in predefined and designated areas where 

fire is necessary and outlined in fire management or land management plans.” The 2007 

Exceptional Events Rule preamble further clarified that the EPA believed that both wildfires and 

wildland fire use fires fall within the meaning of “natural events” as that term is used in CAA 

section 319(b). 

In the 2013 Interim Q&A Document, after consulting with other federal agencies that 

manage wildfires and prescribed fires, the EPA defined a wildfire as “[a]ny fire started by an 

unplanned ignition caused by lightning; volcanoes; unauthorized activity; accidental, humancaused actions; and escaped prescribed fires.” 73

Building off of the principles in the February 2009 Guidance for Implementation of 

Federal Wildland Fire Management Policy,

74 the NWCG defines “wildland” as “an area in 

which development is essentially non-existent, except for roads, railroads, power lines, and 

similar transportation facilities. Structures, if any, are widely scattered.” 75

 

73 Interim Exceptional Events Rule Frequently Asked Questions. U.S. EPA. May 2013. Available 

at http://www2.epa.gov/sites/production/files/2015-05/documents/eer_qa_doc_5-10-13_r3.pdf. 

74 Guidance for Implementation of Federal Wildland Fire Management Policy, developed 

cooperatively by the Fire Executive Council, February 13, 2009. Available at 

http://wildfiretoday.com/documents/Federal%20Wildland%20Fire%20Management%20Policy

%2002-13-2009.pdf. 75 National Wildfire Coordinating Group. Glossary of Wildland Fire Terminology, PMS 205. 

October 2014. Available at http://www.nwcg.gov/sites/default/files/products/pms205.pdf. 
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Proposed Changes. Although the EPA has previously approved exceptional events 

demonstrations for wildfires,76 the EPA recognizes air agencies preparing exceptional events 

demonstrations for future wildfires will benefit from additional clarification and guidance related 

to wildfire terminology. This section discusses the EPA’s proposed changes. 

(i) Definition of wildland and wildfire. For purposes of this action, the EPA proposes to 

codify in regulatory language the definition of “wildland” by using the October 2014 NWCG 

Glossary definition that a wildland is “an area in which human activity and development is 

essentially non-existent, except for roads, railroads, power lines, and similar transportation 

facilities. Structures, if any, are widely scattered.” As previously noted, wildland can include 

forestland, shrubland, grassland and wetlands. This proposed definition of wildland would 

include lands that are predominantly wildland, such as land in the wildland-urban interface.77,78 

In proposing this definition for wildland, the EPA considered the types of human 

intervention that could affect whether a land is considered a “wildland.” In our view and the 

view of other federal agencies with which we have consulted in the development of this 

proposed action, the presence of fences to limit the movement of grazing animals, or of 

infrastructure to provide water to grazing animals, does not prevent a land area from being 

 

76 See submitted exceptional events demonstrations available on the EPA’s website at 

http://www2.epa.gov/air-quality-analysis/exceptional-events-submissions-table.

77 The wildland-urban interface is the line, area or zone where structures and other human 

development meet or intermingle with undeveloped wildland or vegetative fuels. The term 

describes an area within or adjacent to private and public property where mitigation actions can 

prevent damage or loss from wildfire. See, Glossary of Wildland Fire Terminology, PMS 205. 

October 2014. Available at http://www.nwcg.gov/sites/default/files/products/pms205.pdf. 78 We would generally treat a large prescribed fire in a wildland-urban interface area as a 

prescribed fire on wildland, subject to the prescribed fire provisions described in this proposal. 

We do not expect a small prescribed fire in an interface area (e.g., a prescribed burn ignited by a 

single landowner on his/her personal property) to generate emissions that would raise 

exceptional events issues. 
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wildland. Cultivated cropland (i.e., a field that is plowed or disked or from which crops are 

removed on an annual or more frequent basis) is not wildland. Land areas on which nursery 

stock is grown to marketable size (e.g., Christmas tree farms) are generally not wildland unless 

they are “wild” in terms of a having only limited human entrance and intervention for 

management or removal purposes thereby resulting in a complex ecosystem. Generally, managed 

timber lands may be considered wildland if they have a complex ecosystem affected by only 

limited human entrance and intervention. The EPA invites comment on whether it would be 

appropriate to incorporate these examples of land use types that can be considered to be (or not 

to be) wildland into the regulatory definition of wildland or whether or it is adequate to discuss 

them in the preamble only. 

Also for purposes of this action, the EPA proposes to codify in regulatory language the 

following definition of “wildfire,” which slightly modifies the definition of “wildfire” with 

respect to prescribed fires that appeared in the Interim Q&A document. 

A wildfire is any fire started by an unplanned ignition caused by lightning; volcanoes; 

other acts of nature; unauthorized activity; or accidental, human-caused actions; or a 

prescribed fire that has been declared to be a wildfire. A wildfire that predominantly 

occurs on wildland is a natural event. 

This proposed definition of wildfire does not require that the objective be to put out a fire 

for the fire to be categorized as a wildfire. When an unplanned fire on wildland does not threaten 

catastrophic consequences, it may be very appropriate to allow it to continue. The proposed 

definition therefore encompasses the type of activity previously referred to as a wildland fire use 

(i.e., a situation in which a fire manager deliberately allows a wildfire to continue to burn over a 

certain land area rather than immediately extinguish it or block its progress into that area). This 

inclusion is consistent with the approach taken in 2007 that all types of wildfire were considered 

to be natural. We note here, as guidance, that the part of the proposed definition referring to a 
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prescribed fire that has been declared to be a wildfire refers to specific instances in which the 

conditions of a particular prescribed fire have developed in a way that leads the fire manager to 

decide that the fire should be treated as a wildfire, for example if it has escaped secure 

containment lines and requires suppression along all or part of its boundary or no longer meets 

the resource objectives (e.g., smoke impact, flame height). It is not the intention that land 

managers may categorically re-define some types of prescribed fire to be wildfires. 

Because the EPA is proposing in rule text that all wildfires on wildland are always 

considered natural events, the proposed definition of wildland will in turn determine which fires 

are considered to be natural events. This is consistent with the approach in the 2007 Exceptional 

Events Rule. The EPA realizes that some wildfires are initiated by human actions (e.g., careless 

use of campfires or leaf and brush pile fires). The EPA also realizes that past human activity in 

the form of decades of suppressing wildfires has influenced the size and emissions of wildfires 

that do occur. However, wildfire is mostly dominated by natural factors, and the EPA believes 

that treating all wildfires on wildland as natural events is in keeping with Congressional intent 

and not contradictory to any plain meaning of CAA section 319(b). Therefore, because a wildfire 

on wildland is a natural event and because natural events can recur, an air agency would not need 

to address event recurrence in the “human activity unlikely to recur at a particular location or a 

natural event” portion of its exceptional events demonstration. 

(ii) Not reasonably controllable or preventable. Although a wildfire is unplanned, the 

“not reasonably controllable or preventable” criterion still applies. Another function of the 

definition of wildland is that the EPA is proposing that the treatments of wildfires and prescribed 

fires on wildland with regard to the not reasonably controllable or preventable criterion have a 

number of common aspects, as described here and below in the section on prescribed fires, 
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because a wildland situation presents particular considerations applicable to both fire types with 

respect to what prevention or control measures may be reasonable. The EPA is not proposing any 

general approach for wildfires or prescribed fires that are not on wildland.79

Because wildfires on wildland are unplanned, fire management agencies generally have 

either no advanced notice or limited and uncertain notice, of wildfire ignition and location. In 

addition, many areas of wildland are very remote and rugged, and thus not easily reached and 

traversed. These factors generally limit preparation time and on-site resources to prevent or 

control the initiation, duration or extent of a wildfire. Also, by their nature, catastrophic wildfires 

typically present some risk of property damage, ecosystem damage and/or loss of life (of the 

public or firefighters), which is a strong motivation for appropriate suppression and control 

efforts. The EPA believes that land managers and other fire management entities have the 

motivation and the best information for taking action to reasonably prevent and limit the extent 

of wildfires on wildland, thus also controlling the resulting emissions. Therefore, the EPA 

believes that it is not useful to require air agencies to include in their individual wildfire 

exceptional events demonstrations descriptions of prevention and control efforts employed by 

burn managers to support a position that such efforts were reasonable. To increase the efficiency 

of the exceptional events process, the EPA proposes a new approach for wildfires on wildland, 

under which there would be a rebuttable presumption that every wildfire on wildland satisfies the 

“not reasonably controllable or preventable” criterion, unless evidence in the record 

 

79 While we are proposing special provisions only for fires that occur predominantly on wildland, 

we do not intend to restrict fires on other types of land from receiving similar treatment. In 

addressing the not reasonably controllable or preventable criterion in a demonstration for a 

wildfire that is not on wildland, air agencies should state that available resources were 

reasonably aimed at suppression and avoidance of loss of life and property and that no further 

efforts to control air emissions from the fire would have been reasonable.
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demonstrates otherwise. Applying this categorical presumption of not reasonably controllable for 

wildfires would involve referencing the appropriate regulatory citation in the demonstration. 

As previously stated, there will be situations in which a fire manager could have 

suppressed or contained a wildfire but has allowed the fire to continue burning through an area 

with a current, in-place land management plan calling for restoration through natural fire or 

mimicking the natural role of fire. The EPA recognizes that this scenario could occur when a fire 

manager has a plan for acquiring personnel and equipment to address the wildfire (either 

coincidentally or because the wildfire was originally a prescribed fire) but the manager 

determines that allowing the wildfire to continue burning is safe and will conserve overall fire 

management resources compared to suppressing or containing the current wildfire and then 

conducting a separate prescribed burn at a later time. In such a scenario, even though the fire 

would meet the definition of a wildfire and even though we are proposing that in general 

wildfires will not need to be reviewed individually against the not reasonably controllable or 

preventable criterion, we would expect the fire manager to employ appropriate BSMP as 

described in section V.F.2.d when possible. 

(iii) Coordinated communications. Regardless of the above considerations for wildfires, 

the EPA urges land managers and air agencies to coordinate, as appropriate, in developing plans 

and appropriate public communications regarding public safety and reducing exposure in 

instances where wildfires are potential exceptional events and contribute to exceedances of the 

NAAQS. Coordinated efforts can help air agencies satisfy the Exceptional Events Rule 

obligation at 40 CFR 51.930 that air agencies must provide public notice and public education 

and must provide for implementation of reasonable measures to protect public health when an 
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event occurs.80 Also, when wildfire impacts are frequent and significant in a particular area, land 

managers, land owners, air agencies and communities may be able to lessen the impacts of 

wildfires by working collaboratively to take steps to minimize fuel loading in areas vulnerable to 

fire.81 Fuel load minimization steps can consist of both prescribed fire and mechanical 

treatments, such as using mechanical equipment to reduce accumulated understory. 

d. Prescribed Fires 

As noted previously, the EPA recognizes and acknowledges the potential significant 

impact on air quality posed by catastrophic wildfires. The use of prescribed fire on wildland can 

influence the occurrence, severity, behavior and effects of catastrophic wildfires, which may help 

manage the contribution of wildfires to measured ambient pollutant levels (particularly ozone 

and PM concentrations). Additionally, prescribed fires can benefit the plant and animal species 

that depend upon natural fires for propagation, habitat restoration and reproduction, as well as a 

myriad of ecosystem functions (e.g., carbon sequestration, maintenance of water supply systems 

and endangered species habitat maintenance). The EPA formally recognized in the 1998 Interim 

Air Quality Policy on Wildland and Prescribed Fires that federal, tribal and state owners and 

land managers use prescribed fire on wildland to achieve some of these resource benefits, to 

correct the undesirable conditions created by past wildfire suppression management strategies 

and to reduce the risk of wildfires to the public. Although the 1998 Interim Air Quality Policy on 

 

80 72 FR 13575 (March 22, 2007). 

81 One example of this collaborative approach is the evolving interagency Wildland Fire Air 

Quality Response Program, which has developed resources to help address and predict smoke 

impacts from wildfires to reduce public exposure to wildfire smoke. Additional information is 

available at 

http://www.westar.org/Docs/Business%20Meetings/Spring14/Boise/12.3%20Lahm_WFAQRP_5

_28_14.pdf.
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Wildland and Prescribed Fires focused on the role of federal, tribal and state owners/land 

managers, it also recognized that prescribed fires on private lands achieve some of the same 

goals. These concepts, also noted in the preamble to the 2007 Exceptional Events Rule, are 

summarized in more detail immediately below. In recent regulatory actions,82 the EPA has 

continued to express an understanding of the importance of prescribed fire, noting that it can be 

used to mimic the natural process necessary to manage and maintain existing fire-adapted 

ecosystems and/or return an area to its historical ecosystem (or another natural ecosystem if the 

historical ecosystem is no longer attainable) while reducing the risk to public safety and the risk 

of uncontrolled emissions from catastrophic wildfires. 

Current Situation. The 2007 Exceptional Events Rule recognized the benefits of 

prescribed fire as summarized earlier in this section and included provisions for these event types 

in both the preamble to the final rule and in regulatory language. The preamble to the 2007 

Exceptional Events Rule defined a prescribed fire as “a fire ignited by management objectives to 

meet specific resource management needs.” This was consistent with the definition of prescribed 

fire in general use by the fire management community at the time.83 Also in the preamble 

language, the EPA explained that prescribed fire cannot be classified as natural given the extent 

of the direct human causal connection. However, the preamble explained that a prescribed fire 

that causes or contributes to an exceedance or violation of an ambient air quality standard could 

 

82 See, for example, National Ambient Air Quality Standards for Ozone; Proposed Rule (79 FR 

75234, December 17, 2014) and Implementation of the 2008 National Ambient Air Quality 

Standards for Ozone: State Implementation Plan Requirements; Final Rule (80 FR 12264, March 

6, 2015). 

83 The October 2014 NWCG definition of “prescribed fire” is similar but includes the concept 

that the fire is not illegal: “[a]ny fire intentionally ignited by management actions in accordance 

with applicable laws, policies, and regulations to meet specific objectives.” 
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still be considered an exceptional event if it satisfies all core statutory elements of CAA section 

319(b), including the “human activity that is unlikely to recur at a particular location” and the 

“not reasonably controllable or preventable” criteria. 

The 2007 Exceptional Events Rule preamble further explained that air agencies should 

take into account the natural fire return interval84 as part of the basis for establishing that the 

human activity (i.e., the prescribed fire) is “unlikely to recur at a particular location.” The 

preamble acknowledged that the natural fire return interval can vary widely and range from once 

every year to less frequently than once every 200 years. 

When addressing the “not reasonably controllable or preventable” criterion, the 2007 

Exceptional Events Rule preamble instructed agencies to examine whether there are “reasonable 

alternatives,” such as mechanical or other (e.g., chemical) treatments to the use of prescribed 

fire. The preamble language recognized that, although case- and area-specific, any number of 

conditions could exist that would favor the use of prescribed fire rather than alternate treatments. 

Such scenarios identified in the preamble included: significant build-up of forest fuels in a 

particular area that if left unaddressed would pose an unacceptable risk of catastrophic wildfire; 

pest or disease outbreak; natural species composition dependent on a specific fire return interval; 

and legal requirements precluding the use of mechanical fuel reduction methods.

The 2007 Exceptional Events Rule also indicated, in both preamble discussion and rule 

text, that to further satisfy the “not reasonably controllable or preventable” criterion for 

prescribed fires and to address the principle at section 319(b)(3)(A) of the CAA that the 

 

84 The natural fire return interval is the typical number of years between two successive 

naturally-occurring fires in a specified area or ecosystem. The historical rate of return of these 

fires resulted in plant communities that evolved with recurring fire and therefore became 

dependent on fire for maintenance.
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protection of public health is the highest priority, a prescribed fire would be considered to be an 

exceptional event only if the state has certified to the EPA that it has adopted and is 

implementing a SMP or the state has ensured that the burner has employed BSMP. While the 

EPA did not identify in the 2007 Rule the necessary components of an SMP or what SMP 

certification entails, the preamble cited the 1998 Interim Air Quality Policy on Wildland and 

Prescribed Fires.85 This policy identified the following basic components of a certifiable SMP:86 

 Authorization to Burn - includes a process for authorizing or granting approval to 

manage fires for resource benefits within a region, state or on Indian lands and 

identify a central authority responsible for implementing the program. The 

authorization process could include burn plans that consider air quality and the 

ability of the airshed to disperse emissions from all burning activities on the day 

of the burn. 

 Minimizing Air Pollutant Emissions – encourages wildland owners/managers to 

consider and evaluate alternative treatments to fire, but if fire is the selected 

approach to follow emission reduction techniques. 

 Smoke Management Components of Burn Plans – identifies the following 

components if the SMP requires burn plans: actions to minimize fire emissions, 

evaluate smoke dispersion, public notification and exposure reduction procedures 

 

85 The discussion of the 1998 Interim Air Quality Policy on Wildland and Prescribed Fires

recommendations regarding SMPs appears in the preamble to the 2007 Exceptional Events Rule 

at 72 FR 13567 (March 22, 2007). 

86 The language associated with the six basic components of a certifiable SMP was taken directly 

from the 1998 Interim Air Quality Policy on Wildland and Prescribed Fires. For context, the 

EPA notes that the identified components of a certifiable SMP apply to managing smoke from 

prescribed fires managed for resource benefits. The EPA would expect burn managers to 

consider actions and approaches where applicable or where appropriate rather than in all 

prescribed fire scenarios. 
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and air quality monitoring. 

 Public Education and Awareness – establishes the criteria for issuing health 

advisories when necessary and procedures for notifying potentially affected 

populations. 

 Surveillance and Enforcement – includes procedures to ensure that wildland 

owners/managers comply with the SMP. 

 Program Evaluation – provides for periodic review by all stakeholders of the SMP 

effectiveness and program revision as necessary. 

The 1998 Interim Air Quality Policy on Wildland and Prescribed Fires also noted, 

regarding the certification process for SMPs, that to receive special consideration for air quality 

data whose concentrations were influenced by prescribed fires, “[t]he State/tribal air quality 

manager must certify in a letter to the Administrator of EPA that at least a basic [smoke 

management] program has been adopted and implemented....” The 1998 Interim Air Quality 

Policy on Wildland and Prescribed Fires further identified that federal agencies intending to use 

prescribed fire should operate under an approved prescribed fire plan and meet the National 

Environmental Policy Act (NEPA) requirements, where applicable, prior to ignition.87

 

87 In specifying the basic components of certifiable SMPs that would include the requirement for 

agency approval of prescribed fire plans, the 1998 Interim Air Quality Policy on Wildland and 

Prescribed Fires noted that SMPs can mitigate the nuisance and public safety hazards associated 

with smoke from prescribed fires intruding into populated areas, prevent deterioration of air 

quality and NAAQS violations and address visibility impacts in mandatory Class I Federal areas. 

Since the EPA issued the Interim Policy in 1998, some federal agencies have reported to us their 

assessment that some states and/or local air agencies have managed their SMP (or other 

regulatory programs) in such a way as to exclude the use of prescribed fires in areas to such an 

extent that fuels have continued to accumulate to levels that increase the likelihood of 

catastrophic wildfires. 
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The EPA did not use the preamble to the 2007 Exceptional Events Rule to expand on the 

concept of using BSMP in lieu of an SMP. Rather, the EPA only noted that burners could use 

BSMP to minimize emissions and control the impacts of fire. Although the EPA identified 

several example BSMP in a footnote in the preamble (footnote 12 at 72 FR 13567), we also 

committed to developing the concept when we updated the guidance in the 1998 Interim Air 

Quality Policy on Wildland and Prescribed Fires. The EPA has not revised this guidance and 

does not currently plan to do so. Although some states have developed demonstrations that 

incorporate BSMP employed for a specific prescribed fire and/or referenced BSMP in their 

SMPs, the EPA has not in any other more recent guidance clarified what it means for an air 

agency or burner to have a certified SMP in place and/or to implement BSMP. 

In addition to conditioning the approval of a prescribed fire as an exceptional event on 

the existence and implementation of a certified SMP or the actual use of BSMP, as described 

above, the 2007 Exceptional Events Rule also requires that “[i]f an exceptional event occurs 

using the basic smoke management practices approach, the State must undertake a review of its 

approach to ensure public health is being protected and must include consideration of 

development of a SMP.” To date, air agencies have submitted few exceptional events 

demonstrations for prescribed fires. One recent submission came from Kansas, a state already 

operating an SMP.88

The 2007 Exceptional Events Rule at 40 CFR part 50.14(b)(3) allows for the exclusion of 

data where a state demonstrates to the EPA’s satisfaction that emissions from prescribed fires 

 

88 Treatment of Data Influenced by Exceptional Events: Exceptional Events Submissions Table. 

U.S. Environmental Protection Agency, Research Triangle Park, NC. See links associated with 

“April 2011 Fires” in the ozone section of the table available at http://www2.epa.gov/air-qualityanalysis/exceptional-events-submissions-table. 
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caused a specific air pollution concentration in excess of one or more NAAQS at a particular air 

quality monitoring location and otherwise satisfies the requirements in the Exceptional Event 

Rule. The regulatory language also requires that the subject prescribed fire meets the definition 

in 40 CFR part 50.1(j) and requires that the state has certified to EPA that it has adopted and is 

implementing a Smoke Management Program or the state has ensured that the burner employed 

basic smoke management practices. The definition of an exceptional event at 40 CFR 50.1(j) 

includes the requirement that an event “is not reasonably controllable or preventable.” Thus, the 

EPA has interpreted that a demonstration for a prescribed fire independently address both the 

SMP/BSMP element and the not reasonably controllable or preventable criterion. We have not 

indicated that meeting the SMP/BSMP condition is sufficient to satisfy the not reasonably 

controllable or preventable criterion. 

Proposed Changes. As previously noted, the EPA has not to date clarified fire-related 

definitions or its expectations regarding SMPs or BSMP in rule or preamble form. This 

uncertainty has created confusion for air and fire management agencies trying to develop firerelated plans and/or apply fire management practices for prescribed fires. It has also created 

confusion for air agencies when developing and submitting exceptional events demonstrations 

for both wildfires and prescribed fires. 

 To assist air agencies in documenting an exceptional events package for a prescribed fire 

on wildland, the EPA proposes to clarify its expectations for a satisfactory demonstration, as 

follows. 

(i) Definition of a prescribed fire. We are proposing to adopt in rule language the current 

NWCG-recommended definition of a prescribed fire: “[a]ny fire intentionally ignited by 

management actions in accordance with applicable laws, policies and regulations to meet 
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specific objectives.” In this definition, “management” refers to the owner or manager of the land 

area to which prescribed fire is applied, and “specific objectives” refers to specific land or 

resource management objectives. 

(ii) Events caused by human activity. We are proposing to say in rule form that prescribed 

fires are events caused by human activity. Thus, to be considered an exceptional event, every 

prescribed fire demonstration must address the “human activity unlikely to recur at a particular 

location” criterion. 

(iii) Unlikely to recur at a particular location. As discussed in more detail in section 

V.E.1 of this proposal, this requirement of CAA section 319(b) is not specific and requires 

interpretation on a case-by-case or event type-by-event type basis. The term “unlikely” implies 

consideration of the expected future frequency of events similar to the event that has already 

happened, but does not convey any particular benchmark for what frequency should be low 

enough to be considered “unlikely.” Also, the term “at a particular location” requires 

interpretation, as it could refer to the exact area or only to the general area of the event, to the 

location of the ambient monitoring station or stations that were affected by the event or to the 

combination of both. 

As was our position in 2007, we continue to believe that the natural fire return interval is 

a useful and appropriate benchmark for a satisfactory demonstration that a prescribed fire is 

unlikely to recur at a particular location, in the sense that if a planned program of prescribed fire 

calls for the application of prescribed fire at a similar interval to the natural fire return interval at 

given locations then each prescribed fire conducted in that program can be considered not likely 

to recur at its particular location. However, we now believe based on experience and further 

consideration that the natural fire return interval is not the only appropriate benchmark. It can be 
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difficult in some cases to determine what fire return interval prevailed under natural conditions, 

which may not have existed for decades or even hundreds of years in a particular area. Also, in 

some cases environmental conditions may have changed, for example due to climate change, 

such that the original natural ecosystem cannot realistically be restored and the well-considered 

land management goal instead may be the development and maintenance of a sustainable and 

resilient ecosystem that is different than what historically existed and that will likely reduce the 

risk of catastrophic wildfire. In such a case, the frequency of prescribed fire needed to establish 

or restore such an ecosystem during a transitional period and/or the frequency needed to sustain 

the resilient ecosystem may be different than the natural fire return interval that once prevailed. It 

is also important to consider issues of fire personnel and public safety and protection of nearby 

property. Land managers may need to apply prescribed fire at a frequency that maintains the 

accumulation of fuel loading between prescribed fires at a level that does not create the risk of a 

dangerous wildfire. 

Accordingly, the first proposed change for prescribed fires on wildland is to include in 

the rule text two benchmarks for the expected future frequency of prescribed fires on wildland to 

meet the not likely to recur criterion: (1) the natural fire return interval as articulated in the 2007 

preamble and (2) the prescribed fire frequency needed to establish, restore and/or maintain a 

sustainable and resilient wildland ecosystem. If finalized, an air agency could include 

information provided by the land manager with respect to the appropriate benchmark for a 

prescribed fire on wildland as the basis for satisfying the human activity unlikely to recur at a 

particular location criterion in its exceptional events demonstration.89

 

89 The benchmarks for the expected frequency of prescribed fires not on wildland would be 

assessed on a case-by-case basis. 
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Successfully implementing one of these benchmarks for prescribed fire frequency 

necessitates that the air agency and the land manager collaborate to establish and document the 

appropriate fire return interval or frequency in a submitted demonstration. Federal agencies that 

use prescribed fire to manage lands for which they are responsible generally prepare multi-year 

plans for the use of prescribed fire in a given national park, national forest, armed forces base or 

other land area. Many of these plans include an objective to establish, restore and/or maintain a 

sustainable and resilient wildland ecosystem and incorporate the best science available to 

determine what prescribed fire cycle will accomplish this.90 Some tribes, private landowners (and 

federal, state and local agencies working with private landowners) and state agencies that 

manage state-owned lands (e.g., state parks) also prepare multi-year management plans. While 

plans developed by public agencies (i.e., state and federal agencies) often undergo public 

comment prior to being finalized, the plans developed by tribes and private landowners may not 

follow a public comment process. However, public agencies often work with tribes and private 

owners to develop these plans, which are based on conservation practices and standards that have 

often undergone public comment as part of the state or federal agency process. 

The EPA understands that multi-year plans incorporate factors relevant to identifying and 

selecting the areas and times under which management will initiate a specific prescribed fire. 

The EPA also recognizes that evaluating the behavior and results of prior prescribed fires aids in 

determining the frequency and need for future prescribed fire in a given area. In addition, 

personnel and equipment must be available on site, which cannot be specifically planned far in 

 

90 An example of a federal land management plan that considers the need for the use of 

prescribed fire is the Land and Resource Management Plan for the Kaibab National Forest, 

Coconino, Yavapai, and Mojave Counties, Arizona, USDA Forest Service Southwestern Region, 

MB-R3-07-17, February 2014, available at 

http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprd3791580.pdf. 
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advance. Thus, it is typical for multi-year plans to identify somewhat general targets for the 

frequency of prescribed fire use and for specific burn plans. Even then, unexpected differences 

between planned and actual fire behavior, landscape or ecosystem characteristics, fuel loading 

patterns and weather patterns may cause management to deviate from the general plan and/or the 

specific burn plan. Therefore, when the EPA reviews an exceptional events demonstration for a 

prescribed fire conducted under a wildland management plan, we intend to compare the actual 

time pattern of prescribed fires on the land with the pattern described in the applicable multi-year 

plan in a general way, rather than treating the multi-year plan as containing a specific schedule to 

which management must adhere. For example, if the wildland management plan identified an 

approximate 5-year burn interval, the EPA would not disapprove a demonstration if the burn 

occurred on a 4-year or a 6-year interval. 

Therefore, we are proposing in rule text that we will consider a demonstration’s 

referencing of a multi-year land or resource management plan91 (and including either a copy or 

an internet link to the plan) with a stated objective to establish, restore and/or maintain a 

sustainable and resilient wildland ecosystem and/or to preserve endangered or threatened species 

that also identifies the subject area as a candidate for prescribed fire to be dispositive evidence 

that a particular fire conducted in accordance with such a plan satisfies the “unlikely to recur at a 

particular location” criterion. We would also consider a demonstration’s referencing of a fire 

management plan for tribal or private lands that has been reviewed and certified by the 

appropriate fire and/or resource management professionals and agreed to and followed by the 

 

91 These plans could also include fire management plans, prescribed fire on wildland 

management plans, landscape management plans or equivalent public planning documents. 
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land owner/manager to be sufficient evidence satisfying the “unlikely to recur at a particular 

location” criterion. 

(iv) Not reasonably controllable or preventable. Consistent with current practice and 

2007 preamble/rule language, the EPA considers it appropriate to allow air agencies to rely on an 

in-place and implemented state-certified SMP to satisfy the controllability prong of the “not 

reasonably controllable or preventable” criterion. The EPA proposes to incorporate the six 

elements of SMPs discussed in the 1998 Interim Air Quality Policy on Wildland and Prescribed 

Fires and referenced in the preamble to the 2007 Exceptional Events Rule into the preamble of 

the final rule for this proposal, where it would serve as guidance. That is, at a minimum, a statecertified SMP would include provisions for (i) authorization to burn, (ii) minimizing air pollutant 

emissions, (iii) smoke management components of burn plans, (iv) public education and 

awareness, (v) surveillance and enforcement, and (vi) program evaluation. Certification would 

require that the air agency certify in a letter to the Administrator of the EPA, or a Regional 

Administrator, that it has adopted and is implementing a SMP.92 Alternatively, the EPA solicits 

comment on incorporating these SMP elements into rule text language. The EPA proposes to 

accept as sufficient the testimony of the air agency submitting an exceptional events 

demonstration that the SMP is being implemented, provided that prior to the EPA’s acting on a 

demonstration, the record contains no clear evidence to the contrary. 

Consistent with current practice and 2007 preamble and rule language, the EPA also considers it 

appropriate to allow air agencies to rely on a burn manager’s use of BSMP that minimize 

emissions and control impacts, in lieu of a state-certified SMP, to satisfy the controllability prong 

 

92 The EPA anticipates that any person within an air agency responsible for submitting 

exceptional events demonstrations or SIP revisions could also be responsible for certifying a 

Smoke Management Program. 
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of the “not reasonably controllable or preventable” criterion. To provide clarity and reduce 

uncertainty for air agencies and burn managers, the EPA proposes to identify in the rule text six 

BSMP practices as being generally applicable for exceptional events purposes for prescribed 

fires on wildland as well as other prescribed fires. The six BSMP, listed and described in more 

detail in Table 3, come from guidance on BSMP for prescribed burns provided by the U.S. 

Department of Agriculture (USDA) Forest Service (USFS) and USDA Natural Resources 

Conservation Service (NRCS).93 Land managers of other federal, state and local agencies and 

private land owners generally endorse and follow this BSMP guidance.94 While the listed 

practices are broadly stated, fire managers use site-specific considerations to select the exact 

actions of each type and apply them to specific burn projects. There may be situations in which 

one or more of the six BSMP is clearly not applicable for a particular prescribed burn--for 

example, if a prescribed fire is so remote that there are no neighbors to be notified. The EPA 

generally does not intend to challenge a burn manager’s selection of the intensity or specific 

measure within the BSMP categories when we review a particular exceptional events 

demonstration. As part of the on-going assessment of our regulatory programs, we intend to 

generally review those practices commonly employed by federal agencies and other users of 

prescribed fire. 

 

93 USDA Forest Service and Natural Resources Conservation Service, Basic Smoke Management 

Practices Tech Note, October 2011, 

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1046311.pdf. 94 The EPA also addressed how federal agencies may use basic smoke management practices to 

establish a presumption of conformity in the preamble to the EPA’s General Conformity Rule at 

40 CFR 93.153(g)-(i) (75 FR 17264, April 5, 2010). The six practices identified in Table 3 are 

not a presumed to conform action for purposes of a Federal agency satisfying their General 

Conformity responsibilities. For basic smoke management practices to provide a presumption of 

conformity, the identified basic smoke management practices must be publicly and state 

reviewed as part of a presumed to conform action under 40 CFR 93.153(g) or (f) of the General 

Conformity Rule. 
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As another component of the approach for prescribed fires on wildland, the EPA is 

proposing to accept as evidence of the actual use of BSMP the fire manager’s statement that he 

or she employed applicable BSMP for a prescribed fire. Documentation of such statement for an 

exceptional events demonstration could consist of a copy of the routine post-burn report or a 

letter prepared by the fire manager (see example content of a burn report in Table 4). The EPA 

and other federal agencies will work collaboratively to provide access to such post-burn reports 

by air agencies that need them. We encourage land managers and other organizations that 

employ prescribed fires to work with states and tribes to develop an efficient process to provide 

air agencies with documentation that BSMP were employed for particular prescribed fires. 

Table 3. Summary of Basic Smoke Management Practices, benefit achieved with the 

BSMP, and when it is applied (before, during or after ignition of the burn).a 

Basic Smoke Management 

Practice

Benefit achieved with the 

BSMP

When the BSMP is Applied 

– Before/During/After the 

Burn

Evaluate Smoke Dispersion 

Conditions 

Minimize smoke impacts Before, During, After 

Monitor Effects on Air 

Quality 

Be aware of where the smoke 

is going and degree it impacts 

air quality 

Before, During, After 

Record-Keeping/Maintain a 

Burn/Smoke Journal 

Retain information about the 

weather, burn and smoke. If 

air quality problems occur, 

documentation helps analyze 

and address air regulatory 

issues 

Before, During, After 

Communication – Public 

Notification 

Notify neighbors and those 

potentially impacted by 

smoke, especially sensitive 

receptors 

Before, During 

Consider Emission Reduction 

Techniques 

Reducing emissions through 

mechanisms such as reducing 

fuel loading can reduce 

downwind impacts 

Before, During, After 

Share the Airshed – 

Coordination of Area Burning 

Coordinate multiple burns in 

the area to manage exposure 

of the public to smoke 

Before, During, After 
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a

 The EPA believes that elements of these BSMP could also be practical and beneficial to apply 

to wildfires for areas likely to experience recurring wildfires. 

Table 4. Elements that May Be Included in Burn Plans and Post-Burn Reports for 

Prescribed Fires Submitted as Exceptional Events. 

Element Burn Plan Post-Burn Report

Fire Namea X X 

Permit number (if 

appropriate) X X 

Latitude/longitude and 

physical description X X 

Date of burn, ignition time 

and completion time 

(duration of burn) 

X X 

AQI status on burn day, if 

available (both in the vicinity 

of the fire and in the affected 

upwind area) 

Predicted Actual 

Acres burned Planned Actual (blackened) 

Description of fuel loading Estimated Actual (tons consumed) 

Meteorological data (weather 

conditions, wind speed and 

direction, dispersion) 

Predicted conditions 

(including predicted 

dispersion) 

Actual conditions (including 

actual dispersion) 

Smoke Impacts Anticipated smoke impacts 

Observed or reported smoke 

impacts (include nature, 

duration, spatial extent and 

copies of received 

complaints) 

BSMP actions to reduce 

impacts Expected BSMP actions Actual BSMP actions 

Recommendations for future 

burns in similar areas X 

Analytics (modeled/actual 

fire spread, satellite imagery 

and analysis, webcam/video, 

PM/ozone concentrations 

over the course of the fire) 

 X 

a

 The “Fire Name” should be unique and referenced, to the greatest extent possible, in all 

exceptional events-related documentation, including the event name in AQS. The fire name 

could simply consist of the county and state in which the burn occurred (e.g., County X, State Y 

Prescribed Burn on Date Z) if no other name has been assigned. 

States with certified SMPs typically have robust communications between officials 

concerned with air quality impacts and officials and members of the public who use prescribed 
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fire. These groups communicate during the development of the SMP, during the day-to-day burn 

authorization process and in the periodic review and potential revision of the SMP. States that 

instead rely on fire managers employing BSMP on a more individual basis may not have such 

regularly occurring communications processes, particularly in states in which state legislation 

gives the leadership of fire management to a forestry or public safety agency rather than to an air 

agency. We encourage all agencies and managers/owners involved in land, air quality and fire 

management to develop good communications about both fire use practices in general and plans 

for specific prescribed fires with use of BSMP. This will, among other benefits, allow them to 

better coordinate on public air quality notice efforts and, if necessary, public health advisories 

should smoke enter an inhabited area. Additionally, the EPA encourages the development of 

“prescribed fire councils,” comprised of federal, state, tribal, private and other stakeholders to 

coordinate activities involving fire planning issues and, thus, minimize or prevent smoke impacts 

while using prescribed fire to accomplish land management objectives.95 However, we are not 

proposing that notifications between prescribed fire users and specific types of state agencies be 

a condition for approval of a prescribed fire as an exceptional event. 

As previously stated, to date we have considered the existence and implementation of a 

SMP or the use of BSMP to be a necessary part of the supporting evidence needed to satisfy the 

not reasonably controllable or preventable criterion, but we have not clearly addressed what 

conditions are minimally sufficient to satisfy the criterion. The remainder of this section focuses 

on that issue. 

 

95 See additional information on prescribed fire councils on the Coalition of Prescribed Fire 

Councils, Inc. website at http://www.prescribedfire.net/membership/state-councils. 
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Because prescribed fires are intentionally initiated, clarifying the minimal conditions for 

the not reasonably preventable prong is particularly relevant. The detailed USFS/NRCS guidance 

on the fifth listed BSMP, Consider Emission Reduction Techniques, includes the potential to 

reduce the fuel loading. It does not suggest that it may be reasonable to not ignite a particular 

prescribed fire (i.e., that the fire be prevented), because this guidance is aimed at those fires that 

are already planned to happen. Similarly, SMPs address coordination of previously planned 

prescribed fires and typically do not ask SMP participants to consider whether particular 

prescribed fires are reasonably preventable. Therefore, we do not believe that the existence of an 

SMP or the use of BSMP is a sufficient basis for concluding that a prescribed fire is not 

reasonably preventable. A prescribed fire should be concluded to be not reasonably preventable 

on the basis of the benefits that would be foregone if it were not conducted, as described below. 

For federal agencies, the planning of prescribed fire programs typically happens through 

the development of multi-year plans that focus on specific land or resource management 

objectives. This planning process, and the resulting multi-year plan, typically considers the 

importance of a prescribed fire program to achieving land management goals, which may include 

an objective to establish, restore and/or maintain a sustainable and resilient wildland ecosystem, 

in light of the availability, cost and effectiveness of other approaches to fuel management. The 

final multi-year plans thus generally identify the level of prescribed fire use necessary to achieve 

those goals. As noted previously, some tribes, private landowners (and federal, state and local 

agencies working with private landowners) and state agencies that manage state-owned lands 

(e.g., state parks) also prepare multi-year management plans. While plans developed by public 

agencies (i.e., state and federal agencies) often undergo public comment prior to being 
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finalized,96 the plans developed by tribes and private landowners may not follow a public 

comment process. However, public agencies often work with tribes and private owners to 

develop these plans, which are based on conservation practices and standards that have often 

undergone public comment as part of the state or federal agency process. Not conducting the 

prescribed fire programs described in such plans could mean forgoing important ecosystem 

services and other benefits. Accordingly, the EPA is proposing in rule text form to consider a 

prescribed fire on wildland conducted in accordance with a multi-year fire management plan that 

has an objective to establish, restore and/or maintain a sustainable and resilient wildland 

ecosystem to be not reasonably preventable, provided there is no compelling evidence to the 

contrary in the record when the EPA approves the associated exceptional events demonstration.97 

We also propose in rule text that compliance with either a state-certified SMP or BSMP 

is sufficient to establish that a prescribed fire was not reasonably controllable, provided there is 

no compelling evidence to the contrary in the record when the EPA concurs with the associated 

exceptional events demonstration. This is an appropriate approach to implementing CAA section 

319(b), because SMPs and BSMP aim to reasonably control the air quality impacts of prescribed 

fires.98 

 

96 Many multi-year plans developed by state and federal agencies are available electronically 

online or can be requested directly from the preparing agency. Interested parties can also request 

electronic versions of project level plans, if they are not available online. 

97 On a case-by-case basis, in the absence of a multi-year plan, the EPA would also consider a 

prescribed fire on wildland conducted on a fire return interval established according to scientific 

literature to satisfy the not reasonably controllable or preventable criterion provided the 

prescribed fire was also conducted with the objective to establish, restore and/or maintain a 

sustainable and resilient wildland ecosystem and conducted in compliance with either a statecertified SMP or BSMP. This case-by-case approach is similar to the approach currently used 

under the 2007 Exceptional Events Rule. 

98 With respect only to the not reasonably controllable prong, we also believe that the SMP and 

BSMP approach each is sufficient for prescribed fires that are not on wildland. 
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This two-part categorical approach would reduce the length of exceptional events 

demonstrations for prescribed fires on wildland and make the demonstration preparation and 

review process more resource efficient. In summary, to satisfy the not reasonably controllable or 

preventable criterion for a prescribed fire on wildland, a demonstration would need to identify 

that the prescribed burn was conducted in accordance with a multi-year plan that has an objective 

of the establishment, restoration and/or maintenance of a sustainable and resilient wildland 

ecosystem and was conducted in compliance with either a state-certified SMP or BSMP. 

Finally, we are proposing to remove the phrase “and must include consideration of 

development of a SMP” from the sentence of the existing text of 40 CFR 50.14(b)(3) that reads, 

“If an exceptional event occurs using the basic smoke management practices approach, the State 

must undertake a review of its approach to ensure public health is being protected and must 

include consideration of development of a SMP.” While the EPA supports states considering the 

development of a SMP in the situation described in this sentence, we believe states have had 

ample opportunity to develop such a program since 2007. This rule language effectively requires 

an ongoing consideration to develop an SMP every time a prescribed fire causes a NAAQS 

exceedance or violation that merits exclusion as an exceptional event. We do not believe 

Congress intended this ongoing consideration to be a requirement flowing from CAA section 

319(b). In addition, we believe that an SMP is most appropriate when multiple parties wish to 

employ prescribed fire at about the same time in the same airshed, which is a more narrow 

situation than specified in this sentence. We also do not want our rules to be open to an inference 

that development of a SMP should only be considered following a NAAQS exceedance or 

violation, because the impacts from fires may affect public health in areas without NAAQScompliance air monitoring stations. Also, we believe that when air agencies observe NAAQS 
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exceedances or violations attributed to a prescribed fire, air agencies should consider a wide 

range of alternatives including, but not limited to, the development of a SMP. For example, 

agencies might also consider the more frequent or intensive use of BSMP to limit the fuel 

available to burn in each fire. 

The EPA solicits comment on all aspects of the identified fire-related approaches. 

3. Stratospheric Ozone Intrusions 

a. Current Situation 

 Stratospheric ozone intrusions are natural events that occur when a parcel of air 

originating in the stratosphere is re-entrained into the troposphere, and in some cases mixes 

directly to the surface of the earth. These relatively rare events can create elevated ozone 

concentrations that affect areas ranging from a single monitoring site to a wider area as the air 

mass with a high ozone concentration moves across the landscape. 

 Normally, the tropopause, the temperature inversion layer of air that separates the 

troposphere from the stratosphere, limits the transport of stratospheric air into the troposphere, 

the lowest layer of the Earth’s atmosphere.99 In some cases, however, parcels or ribbons of 

ozone-rich air from the stratosphere can be transported rapidly to the surface during deep mixing 

events, such as thunderstorms or strong frontal passages, by a process known as tropopause 

folding. Although this “folding” process can occur throughout the year, it is typically associated 

 

99 The height of the tropopause varies with latitude and season but the average height at midlatitudes is about 11 kilometers (km) (7 miles or 36,000 feet). The stratosphere, the second layer 

of the Earth’s atmosphere, is located above the tropopause at 13 to 50 km (8-31 miles or 43,000 

to 160,000 feet) above the Earth’s surface. 
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with frontal passages and upper level low pressure systems during the spring season.100,101 The 

ozone transported through these “folds,” or through other less significant mechanisms, may 

disperse within and be destroyed in the upper troposphere or it may mix down to the surface.102

The “intrusion” of stratospheric ozone is identified most frequently at high elevation sites where 

upper tropospheric air is more likely to reach the surface than at lower elevation sites where 

more downward movement would be needed for a monitoring site to be affected. At these high 

elevation sites in particular, stratospheric ozone intrusion has been estimated to contribute about 

20 to 25 percent of the total tropospheric ozone budget and can cause relatively short-term (i.e., 

ranging from several hours to 2-3 days in duration) increases of surface ozone of 10 to 50 ppb 

above normal background levels.103 Stratospheric intrusions with short-lived surface ozone 

concentrations of several hundred ppb have also been observed, although these events are 

extremely rare. 

 Because stratospheric intrusion events are relatively infrequent and because identifying 

and monitoring these events are challenging, there have been few direct measurements showing 

that a given parcel of surface air contains stratospheric ozone (versus ozone generated by local 

natural or anthropogenic sources). Interpreting these direct measurements is also challenging. 

 

100 Pan, L. L., et al. (2014), “Thunderstorms enhance tropospheric ozone by wrapping and 

shedding stratospheric air.” 

Geophys. Res. Lett., 41, 7785–7790, doi:10.1002/2014GL061921. 

101 A. O. Langford, K. C. Aikin, C. S. Eubank and E. J. Williams. Stratospheric Contribution to 

High Surface Ozone in Colorado During Springtime, Geophysical Research Letters, Vol. 36, 

Issue 12, June 2009. 

102 Other mechanisms by which stratospheric ozone is transported into the troposphere include 

cutoff cyclones, streamers (long filamentary structures that often roll into vortices), and clear air 

turbulence (see http://www.atmos.umd.edu/~dkuhl/documents/Kuhl_Tropopause_Folding.ppt). 103 Integrated Science Assessment of Ozone and Related Photochemical Oxidants (Final Report). 

U.S. EPA.Washington, D.C. EPA/600/R-10/076F. Available at: 

http://www3.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_isa.html. 
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One approach to confirm the presence of stratospheric ozone in surface air is through the use of 

the beryllium tracers, beryllium isotopes Be-7 and Be-10, which are produced primarily in the 

stratosphere by cosmic ray collisions with atmospheric gas atoms.104 Beryllium isotope 

measurements are, however, rare and expensive, and, consequently not normally available. 

 More common approaches to identify stratospheric ozone in surface air can include 

evaluating measurements at the potentially influenced ozone monitoring site for very low 

concentrations of CO and/or relative humidity. Both can be strong indicators of stratospheric air 

because, relative to tropospheric air, stratospheric air has very low relative humidity and very 

low concentrations of other air pollutants such as CO, NOx and PM. The concurrent impacts on 

CO and relative humidity can be subtle, however, when stratospheric air has mixed with 

tropospheric air as the mixing process dilutes the ozone enhancement and increases CO and 

water vapor concentrations relative to stratospheric conditions. Typical CO monitors used for 

ambient air monitoring have operational ranges of 500 to 50,000 ppb (0.5 to 50 ppm) and are not 

sufficiently sensitive to reliably measure the very low CO levels found in stratospheric air (50 to 

150 ppb). Additionally, few rural high altitude monitoring sites have both ozone and CO 

monitors.105 The EPA urges air agencies to provide concurrent readings of ozone and CO and/or 

relative humidity in their exceptional events demonstrations if they have these data. The EPA 

will evaluate these data as a part of a weight of evidence showing alongside other qualitative 

evidence in a clear causal relationship showing. 

 

104 Cristofanelli, P. A 6-year analysis of stratospheric intrusions and their influence on ozone at 

Mt. Cimone (2165 m above sea level). Journal of Geophysical Research, Vol. 111, D03306, 

doi:10.1029/2005JD006553, 2006. 

105 A recent review of AQS data revealed 216 sites in the United States with collocated ozone 

and carbon monoxide monitors in operation after January 1, 2014. Most of these sites are located 

in either urban or suburban locations. In these settings, local emissions would likely hide the 

stratospheric CO suppression. 
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 A third measurement-based approach to identifying stratospheric ozone could include 

measurements of ozone above ground level (i.e., measurements in the troposphere). This 

approach is also uncommon. Currently, five sites in the U.S. conduct ozone sonde (balloon) 

launches two or more times per week and an additional few research locations operate ozone 

lidars to vertically measure ozone profiles.106,107

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

meteorological models can indicate conditions under which stratospheric air parcels may reach 

the surface. Meteorological models such as the National Oceanic and Atmospheric 

Administration (NOAA)/National Weather Service (NWS) North American Mesoscale Forecast 

System (NAM) or the NOAA Rapid Update Cycle (RUC) models simulate parameters 

characteristic of stratospheric air such as isentropic potential vorticity (IPV) and potential 

temperature (PT) that can be used to identify tropopause folding. Visualization tools using the 

model output can show spatially where stratospheric air is located in proximity to or in contact 

with the surface. Similarly, atmospheric chemistry models, such as the National Aeronautics and 

Space Administration (NASA)/NOAA Real-time Air Quality Modeling System (RAQMS)108 can 

provide both real time intrusion forecasting and retrospective analysis of ozone from intrusions. 

 

106 Lidar, which stands for Light Detection and Ranging, is a remote sensing method that uses 

light in the form of a pulsed laser to measure ranges (variable distances) to the Earth. These light 

pulses, combined with other data recorded by the airborne system, generate precise, threedimensional information about the shape of the Earth and its surface characteristics. See, NOAA 

definition at http://oceanservice.noaa.gov/facts/lidar.html. 107 See NOAA data available at ftp://aftp.cmdl.noaa.gov/data/ozwv/Ozonesonde/. On April 30, 

2015, NOAA’s site identified the following five sites in the U.S. that conduct bi-weekly ozone 

sonde launchings: Hilo, HI; Huntsville, AL; Narragansett, RI; Trinidad Head, CA; and Boulder, 

CO. Additional launchings occur in Pago Pago, American Samoa, and outside of the U.S. in 

Greenland, Antarctica and Fiji. 

108 See http://raqms-ops.ssec.wisc.edu/. 
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Finally, satellite observation of atmospheric ozone and CO can be used to validate predictions 

based on atmospheric modeling. 

 Although as of the date of this proposal the EPA has concurred with only one 

stratospheric ozone intrusion exceptional events demonstration prepared under the provisions of 

the 2007 Exceptional Events Rule (and disapproved none),109 the EPA has been communicating 

that we consider it appropriate to use the previously mentioned stratospheric ozone tools with 

other event/pollutant exceptional events analyses (e.g., seasonal analysis of ozone data, 

comparison of event days with non-event days, trajectory analysis, ozone measurement time 

series and spatial distribution analysis, meteorological analysis to show the presence of weather 

systems associated with typical intrusions and balloon soundings of the NWS Upper Air 

Observation Program to detect parcels of dry air aloft) to successfully demonstrate stratospheric 

ozone exceptional events. 

b. Proposed Changes 

As is true for all exceptional events and pollutant combinations, when submitting a 

demonstration for stratospheric ozone intrusion events, air agencies must address all of the 

Exceptional Events Rule criteria. As noted in this action, the EPA proposes to return to the core 

statutory elements and implicit concepts of CAA section 319(b): that the event affected air 

quality in such a way that there exists a clear causal relationship between the specific event and 

the monitored exceedance or violation, the event was not reasonably controllable or preventable 

and the event was a human activity that is unlikely to recur at a particular location or was a 

 

109 Wyoming Department of Environmental Quality, Air Quality Division. Big Piney and 

Boulder, Wyoming Ozone Standard Exceedance, June 14, 2012. Available at 

http://www2.epa.gov/air-quality-analysis/exceptional-events-submissions-table. 
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natural event. The EPA suggests the following approach when addressing these technical criteria 

for an ozone exceedance or violation caused by a stratospheric intrusion. 

An air agency should begin by showing the geographic extent of elevated or exceedancelevel ozone concentrations associated with the intrusion event in conjunction with an evaluation 

of the historical measured surface ozone levels for the same season (see section V.E.3 of this 

proposal for example analyses of how to present the comparison to historical concentrations 

within the clear causal relationship criterion). If the intrusion happened at a time of year when 

local or transported photochemical ozone is generally low, evidence that the intrusion affected 

ground level air quality may be relatively brief and still be sufficient, compared to an intrusion 

occurring at the height of the historical ozone season. 

If intrusion claims coincide with historically high photochemistry seasons, then the air 

agency may need additional evidence to support the clear causal relationship criterion by 

showing the relative contribution estimates to the exceedance from local and transported 

anthropogenic pollutants compared to the intrusion contribution. An air agency can provide 

additional analyses supporting the clear causal relationship by showing that an intrusion occurred 

at or near the location of an identified monitor by using atmospheric models such at RAQMS, 

NAM or RUC, with additional data from satellite observations of total column ozone and CO. 

The EPA intends to accept a short statement in a demonstration that because stratospheric 

ozone intrusions are purely natural events and are large in scale, they are not reasonably 

controllable or preventable. 

4. High Wind Dust Events 

a. Current Situation 
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The 2007 Exceptional Events Rule preamble noted that “[t]he EPA’s final rule 

concerning high wind events states that ambient particulate matter concentrations due to dust 

being raised by unusually high winds will be treated as due to uncontrollable natural events 

where (1) the dust originated from nonanthropogenic sources, or (2) the dust originated from 

anthropogenic sources within the State, that are determined to have been reasonably wellcontrolled at the time that the event occurred, or from anthropogenic sources outside the State.” 

As noted in section IV.B of this document, although this language still reflects the EPA’s 

interpretation of what might be appropriate under the Exceptional Events Rule, the D.C. Circuit 

determined the language to be a legal nullity because the EPA did not specifically address high 

winds or ambient particulate matter concentrations in the promulgated regulatory language in 40 

CFR 50.14. 

The preamble to the 2007 Exceptional Events Rule also noted that because “...the 

conditions that cause or contribute to high wind events vary from area to area with soil type, 

precipitation, and the speed of wind gusts, [air agencies] should provide appropriate 

documentation which indicates what types of circumstances contributed to the exceedances or 

violation at the monitoring site in question.” The EPA declined to identify a specific high wind 

threshold to qualify as being an exceptional event and instead relied on air agencies to submit 

appropriate documentation supporting their position. 

Because of the uncertainty associated with these high wind statements and stakeholder 

feedback asking the EPA to interpret this language and provide examples of applying the 

provisions in the 2007 Exceptional Events Rule to high wind dust events, the EPA clarified many 

concepts related to high wind dust events in its May 2013 Interim Exceptional Events 
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Implementation Guidance, specifically the Interim High Winds Guidance document.110 In this 

guidance, the EPA defined a high wind dust event as including the high wind and the dust that 

the wind entrains and transports to a monitoring site, clarified our expectations regarding 

“reasonable controls” for high wind events with contribution from both natural and 

anthropogenic sources and introduced the concept of establishing a value for a high wind 

threshold, up to which reasonable windblown dust controls are expected to be effective in the 

absence of site specific data or analyses. 

As identified in the Interim High Winds Guidance document, dust phenomena are 

experienced primarily in the western U.S. where rainfall is seasonal, creating dry and dusty 

landscapes.111 In high wind dust events, the meteorological phenomenon (i.e., wind) is purely 

natural, but the pollution from the event may be a mixture of natural sources (e.g., undisturbed 

soil) and anthropogenic sources (e.g., soil disturbed by human activity, emissions from sand and 

gravel facilities, etc.). The EPA generally classifies high wind dust events as “natural events” in 

cases where windblown dust is entirely from natural sources or where all significant 

 

 

110 Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude 

Ambient Air Quality Data Affected by High Winds Under the Exceptional Events Rule. U.S. 

EPA. May 2013. Available at http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_highwinds_guide_130510.pdf.

111 We use “Western U.S.” to refer to states in the Great Plains (North Dakota, South Dakota, 

Nebraska, Kansas, Oklahoma and Texas) and those farther west including Arizona, California, 

Colorado, Nevada, New Mexico, Utah, and Wyoming. 
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anthropogenic sources of windblown dust have been reasonably controlled.112 This long-standing 

policy was first established in the PM10 Natural Events Policy, which provided that: 

Ambient PM-10 concentrations due to dust raised by unusually high winds will be treated 

as due to uncontrollable natural events under the following conditions: (1) the dust 

originated from nonanthropogenic sources, or (2) the dust originated from anthropogenic 

sources controlled with best available control measures (BACM).113 

Also integral to definition of a high wind dust event is that the wind speed be “high,” or, 

as indicated in the PM10 Natural Events Policy, “unusually high.” Only “high wind” dust events 

are exceptional events and “high” is area-specific. 

Typically, undisturbed desert landscapes in the western U.S. have a natural crust that 

protects the surface and tends to limit emissions of windblown dust. The wind speed capable of 

causing emissions from these natural undisturbed areas varies by location, depending on 

characteristics of the local landscape (e.g., soil type and characteristics, vegetation). Numerous 

studies have been conducted to determine the minimum wind speed capable of causing emissions 

from natural undisturbed areas and/or overwhelming reasonable controls on anthropogenic 

 

112 As identified in section V.D of this proposal, the EPA will generally consider human activity 

to have played little or no direct role in causing emissions of the dust generated by high wind for 

purposes of the regulatory definition of “natural event” if contributing anthropogenic sources of 

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

reasonably controlled anthropogenic sources, and thus the event could be considered a natural 

event. In such cases, the EPA believes that it would generally be a reasonable interpretation to 

find that the anthropogenic source had “little” direct causal role. If anthropogenic sources of 

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

applied at the time of the high wind event have contributed significantly to a measured 

concentration, the event would not be considered a natural event.

113 Areas Affected by PM-10 Natural Events (the PM10 Natural Events Policy), memorandum 

from Mary D. Nichols, Assistant Administrator for Air and Radiation, to EPA Regional Offices, 

May 30, 1996. 
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sources.114 In the Interim High Winds Guidance, the EPA called the minimum threshold wind 

speed capable of causing emissions from natural undisturbed areas or overwhelming reasonable 

controls on anthropogenic sources the “high wind threshold.” 

In the Interim High Winds Guidance, the EPA articulated its expectations regarding the 

development and application of high wind thresholds. In this guidance, the EPA encouraged air 

agencies to identify an appropriate high wind threshold for each area experiencing high wind 

dust events within their exceptional events submissions for high wind dust events.115 The 

guidance recommended that these thresholds should consider local conditions and specify a 

minimum wind speed capable of causing emissions from those natural undisturbed areas or 

overwhelming reasonable controls on contributing anthropogenic sources (see section V.E.2 for 

additional discussion regarding reasonable controls). This approach was consistent with the PM10

Natural Events Policy in which the EPA recommended that air agencies define the conditions in 

which BACM level controls were overwhelmed. The area-specific high wind threshold should be 

 

 

114 See Appendix A1 of the Interim Guidance on the Preparation of Demonstrations in Support 

of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the Exceptional 

Events Rule. U.S. EPA. May 2013. Available at http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_highwinds_guide_130510.pdf. 

115 See Appendices A2 and A3 in the Interim Guidance on the Preparation of Demonstrations in 

Support of Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the 

Exceptional Events Rule. U.S. EPA. May 2013. Available at 

http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_highwinds_guide_130510.pdf for additional information on the 

development of a high wind threshold. 
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representative of conditions (i.e., sustained wind speeds116) that are capable of overwhelming 

reasonable controls (whether RACM, BACM or other) on anthropogenic sources and/or causing 

emissions from natural undisturbed areas. The threshold was not intended to represent the 

minimum wind speed at which any level of emissions could occur (e.g., aerodynamic 

entrainment), but rather when significant emissions begin due to reasonable controls or natural 

undisturbed areas becoming overwhelmed. We have stated that if an agency is unable to develop 

an area-specific high wind threshold, we generally will accept a threshold of a sustained wind of 

25 mph for areas in the western U.S. provided the agencies support this as the level at which they 

expect stable surfaces (i.e., controlled anthropogenic and undisturbed natural surfaces) to be 

overwhelmed.117 We did not indicate what form such support could take. We also have said that 

if we receive specific information based on relevant studies to choose an alternative high wind 

threshold for an identified area, we will notify the affected air agency. 

Also as noted in the Interim High Winds Guidance document, the EPA has expected air 

agencies to provide relevant wind data (e.g., wind speed and direction) as part of an exceptional 

events submission for high wind dust events. Wind speed data consist of analyses and statistics 

showing how the observed high wind dust event wind speed compares to the distribution of 

 

116 Section 6.3.2.2 in the Interim Guidance on the Preparation of Demonstrations in Support of 

Requests to Exclude Ambient Air Quality Data Affected by High Winds Under the Exceptional 

Events Rule. U.S. EPA. May 2013. Available at http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_highwinds_guide_130510.pdf for details on the calculation of 

sustained wind speed. Generally, the EPA will accept that high winds could be the cause of a 

high 24-hour average PM10 or PM2.5 concentration if there was at least one full hour in which the 

hourly average wind speed was above the area-specific high wind threshold. 

117 The 25 mph threshold is based on studies conducted on natural surfaces. See additional 

information relevant to establishing this threshold in Appendix A1 in the Interim Guidance on 

the Preparation of Demonstrations in Support of Requests to Exclude Ambient Air Quality Data 

Affected by High Winds Under the Exceptional Events Rule. U.S. EPA. May 2013. Available at 

http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_highwinds_guide_130510.pdf.
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historical wind speeds and the established high wind threshold. The EPA has recommended that 

air agencies show these historical comparisons on an annual and/or seasonal basis, depending on 

which is more appropriate, using a format similar to the recommended format of the comparison 

to historical concentrations showing as part of the clear causal relationship criterion discussed in 

section V.E.3 of this proposal. The EPA has encouraged air agencies to discuss wind direction in 

the narrative and to present wind direction information graphically in maps/plots in the clear 

causal relationship section of the high wind dust exceptional events demonstration. 

In considering past high wind dust event demonstrations, the EPA has found that the “not 

reasonably controllable or preventable” and the “clear causal relationship” (to include the 

comparison to historical concentrations showing) criteria play significant roles in the supporting 

exceptional events documentation. The EPA has generally found that for high wind dust events, 

air agencies can meet the “human activity or natural event” criterion by satisfying the 

requirements for not reasonably controllable or preventable and clear causal relationship as well 

as addressing the additional components of exceptional events demonstration packages as 

discussed in section V.G. 

As is the case with all demonstration packages, air agencies with agricultural sources that 

potentially contribute to high wind event-related emissions should address the question of source 

contribution and associated reasonable controls on these sources within the not reasonably 

controllable or preventable portion of the demonstration. The EPA has noted in previous 

guidance that when considering the anthropogenic sources that contribute to event-related 

emissions and the appropriate “reasonable controls” on these sources, air agencies should be 

aware of USDA/NRCS-approved Best Management Practices (BMPs) (also referred to as 

conservation management practices) that are designed to effectively reduce fugitive dust air 
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emissions and prevent soil loss in agricultural applications.118 We have stated that these BMPs 

could be included in the collection of controls determined to constitute reasonable controls for 

wind-blown dust events in areas in which they have been implemented. 

b. Proposed Changes 

The EPA proposes to include in the preamble to the final rule for this action a modified 

version of some of the language that first appeared in the Interim High Winds Guidance 

document and to incorporate into the rule text the revisions proposed in this section. We also 

intend to revise the Interim High Winds Guidance to be consistent following promulgation of 

final Exceptional Events Rule revisions.

Definition of an Event. Consistent with the EPA’s proposal to revise the regulatory 

definition of an exceptional event to include both the event and its associated resulting 

emissions, the EPA proposes to define a high wind dust event as an event that includes the highspeed wind and the dust that the wind entrains and transports to a monitoring site. Consistent 

with the nullified language in the 2007 Exceptional Events Rule preamble, the PM10 Natural 

Events Policy and the Interim High Winds Guidance, the EPA proposes to define high wind dust 

events in the rule text as “natural events” in cases where windblown dust is entirely from natural 

sources or where all significant anthropogenic sources of windblown dust have been reasonably 

controlled. 

High Wind Threshold. To facilitate clearer expectations regarding the evidence needed to 

demonstrate which controls constitute “reasonable controls,” the EPA proposes to codify in rule 

language the definition of “high wind threshold” as the minimum threshold wind speed capable 

 

118 Reference Guide for Cropping Systems and General Land Management. USDA, NRCS and 

U.S. EPA. October 2012. Available at 

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1049502.pdf.
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of causing particulate matter emissions from natural undisturbed lands in the area affected by a 

high wind dust event. The EPA proposes to accept a threshold of a sustained wind of 25 mph for 

areas in the western U.S. provided this value is not contradicted by evidence in the record when 

we review a demonstration. If the EPA receives specific information based on relevant studies 

that suggest a different high wind threshold for an identified area, the EPA will notify the 

affected air agency so that the agency may consider basing its demonstration on that threshold 

value. The EPA would consider such information as part of the weight of evidence analysis for a 

submitted demonstration. In lieu of using the default 25 mph high wind threshold, air agencies 

would have the option to identify an area-specific high wind threshold that is more representative 

of local/regional conditions. 

The high wind threshold concept will continue to apply to the review of demonstrations 

for events in a nonattainment or maintenance area for which the dust controls in a recently 

approved SIP are generally accepted as sufficient to satisfy the not reasonably controllable 

criterion. For such a demonstration, the controls specified in the SIP should be considered 

reasonable, while acknowledging the possibility that the controls are not being complied with 

and that uncontrolled anthropogenic sources of PM could be the contributing to the exceedance. 

For events with sustained wind speeds above the high wind threshold, it is very plausible that 

SIP controls were being implemented and the high PM concentrations are due to emissions 

generated from sources in the area despite implementation of the SIP measures. Conversely, for 

events with sustained wind speeds below the high wind threshold, it becomes more plausible that 

there may be noncompliance with control measures or that uncontrolled anthropogenic sources 

are contributing to the exceedance. Therefore, the comparison of sustained wind speeds during 

an event to the high wind threshold will help the EPA regional offices determine what evidence 
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is required to be included in a demonstration regarding reasonable controls, the possibility of 

non-compliance, or non-event sources. 

Large-Scale or High-Energy High Wind Dust Events. The EPA proposes to codify in rule 

language to apply a case-specific approach when considering reasonableness of controls for 

remote, large-scale, high-energy and/or sudden high wind dust events, such as “haboobs” in the 

southwest where sustained wind speeds can exceed 40 mph and generate walls of dust several 

miles wide and more than a mile high. The proposed rule text provides that in these situations, 

the event will be considered not reasonably preventable or controllable. Therefore, a 

demonstration limited to such event(s) will not need to substantively address this criteria. The 

EPA solicits comment on this proposed, case-specific approach when considering reasonableness 

of controls for remote, large-scale, high-energy and/or sudden high wind dust events. 

Other Types of High Wind Dust Events. Any demonstration for a non-high-energy event 

would be evaluated on a case-by-case basis. In doing so, the EPA would consider what controls 

are reasonable in light of an area’s attainment status and associated CAA control requirements, 

the frequency, and range of non-high energy wind events known (at the time of the particular 

event that is the subject of the demonstration) to occur in the area. 

The Role of the EPA-approved SIP in Nonattainment and Maintenance Areas. As stated 

in section V.E.2, the EPA proposes to establish by rule a non-rebuttable presumption that, during 

a 5-year window (or, alternatively another appropriate timeframe) following approval of an 

attainment plan or maintenance plan SIP during which no subsequent new obligation for the air 

agency to revise the SIP has arisen, the control measures included in the SIP that are specific to 

the relevant pollutant, sources and event type satisfy the not reasonably controllable or 
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preventable criterion.119 Otherwise, the air agency and the EPA would evaluate the not 

reasonably controllable or preventable criterion on a case-by-case basis. 

We describe below one potential scenario in which deference to the SIP for purposes of 

“reasonable controls” (versus a case-by-case analysis) satisfies the not reasonably controllable or 

preventable criterion. We also provide two other scenarios needing a case-by-case analysis for 

purposes of satisfying the not reasonably controllable or preventable criterion. We identify these 

scenarios below and then discuss them in more detail in sequence. 

 Nonattainment Area Scenario 1 – The EPA approved the SIP with the enforceable 

control measures as meeting attainment or maintenance planning requirements 

within 5 years of the date of submittal of the event AND the air agency is not 

under an obligation to revise the SIP for the reason listed in Scenario 2 or any 

other reason. Additionally, the sustained winds during the event are above the 

high wind threshold. The SIP includes enforceable control measures that address 

the event-related pollutant and all sources necessary to fulfill the requirements of 

the CAA for the SIP that have or may have contributed to event-related emissions. 

This indicates that in the development and approval of the SIP, both the EPA and 

the state considered what event-related controls were sufficient to meet the 

attainment or maintenance plan requirements of the CAA. 

 

119 A request for data exclusion must also show that the event was not a result of noncompliance 

with any existing state or local laws or rules that have not been incorporated into the SIP. 
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 Nonattainment Area Scenario 2 – The air agency is under an obligation to revise 

the SIP as a result of a SIP call based on failing to provide for attainment and 

maintenance of the relevant NAAQS as evidenced by current violations.120

 Maintenance Area Scenario 3 – The EPA approved the SIP more than 5 years 

prior to the date of submittal of a demonstration. 

Details for Nonattainment Area Scenario 1 

In this scenario, where the sustained winds during the event are above the high wind 

threshold, the EPA would apply a non-rebuttable presumption that the controls in the existing 

SIP represent reasonable measures to prevent or control any event of the given type that occurs 

in the 5-year window. To satisfy the not reasonably controllable or preventable criterion, the 

EPA would expect the submitting air agency to identify the emission sources that contribute to 

the event emissions and exceedance and identify the associated SIP controls plus any other 

enforceable control measures required by state laws or rules. The air agency would also identify 

the implementation status of these controls and provide evidence of effective implementation and 

enforcement. 

Example: An air agency submits a demonstration for a high wind dust event in a PM10

nonattainment area that occurred in October 2015. The air agency has an EPA-approved 

attainment plan SIP for the affected area that was approved in October 2010 and that SIP 

includes enforceable controls implemented in accordance with the SIP that address the eventrelated pollutant (i.e., PM) and all sources necessary to fulfill the requirements of the CAA. The 

 

120 Other possible reasons for an area to be in a Scenario 2 situation would be if it has been 

designated nonattainment for a revised NAAQS for the relevant pollutant or is subject to a SIP 

call for the relevant pollutant following the EPA’s determination that the SIP is inadequate for 

some other reason. 
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sustained winds during the event were above the high wind threshold. In the not reasonably 

controllable or preventable portion of its high wind dust demonstration, the air agency would 

describe the event-related wind characteristics and identify the natural and anthropogenic 

emission sources that contributed to the event emissions and the associated SIP and other control 

measures. The air agency would then describe the implementation status of these controls and 

provide evidence of effective implementation and enforcement. The air agency would conclude 

the “not reasonable controllable or preventable” portion of the demonstration by affirmatively 

stating that because the EPA had approved the SIP within 5 years of the event and because the 

SIP measures and other measures specific to the pollutant and at least some anthropogenic 

emission sources that contributed to the event emissions were implemented, the agency has 

satisfied the not reasonably controllable or preventable criterion. 

In reviewing the demonstration in this scenario, the EPA would generally concur that the 

air agency met the not reasonably controllable or preventable criterion during the 5-year period 

provided the SIP was implemented and the event was not attributable to noncompliance. Thus, 

assuming the demonstration also satisfied the remaining technical and procedural elements in the 

Exceptional Events Rule, the EPA would concur with the air agency’s request to exclude data for 

purposes of regulatory actions within the scope of the final revised Exceptional Events Rule. If, 

however, the air agency experienced an exceedance or violation during the 5-year period for 

reasons other than those attributable to the successful exceptional events demonstration (e.g., 

industrial source noncompliance or another type of event), the EPA may take one of these 

actions. In addition, the EPA may issue a SIP call because the SIP is inadequate with regard to a 

requirement of the CAA that is not tied to the occurrence of NAAQS violations related to 
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exceptional events. If the EPA issues a SIP call during the 5-year window, the situation would 

switch to Scenario 2. 

Details for Nonattainment Area Scenario 2 

 In this scenario (where the SIP is being revised to respond to a SIP call involving the 

PM10 or PM2.5 NAAQS), the existing SIP controls should not be presumed to satisfy the not 

reasonably controllable or preventable criterion regardless of whether the event-related wind 

speeds are above or below the high wind threshold. The EPA recommends that as the first step in 

preparing an exceptional events demonstration, the air agency should assess the case-specific 

effectiveness of the controls that were in place at the time of the event and consider potential 

controls that are more comprehensive and effective than those in the SIP that the agency could 

have implemented before or during the event. This case-specific assessment should apply the 

concept that if a set of control measures should reasonably have been in place for emission 

sources that contribute to the event emissions in light of the information in the record of the EPA 

action that has created the obligation to revise the SIP, then those controls must have been in 

place for the event to satisfy the not reasonably controllable or preventable criterion. 

 The submitting air agency preparing a case-specific assessment should first identify the 

natural and anthropogenic emission sources that significantly contribute to the event emissions 

and exceedance. The air agency should categorize sources as those that are addressed through 

SIP or other state or local laws or rules and those sources that are not addressed by SIP measures 

or other measures. Where the contributing source has SIP or other controls, the air agency would 

identify the implementation status of these controls and provide evidence of effective 

implementation and enforcement. The air agency should also consider whether those SIP 

controls should have been made more stringent and effective prior to the event. For emission 
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sources that contribute to the event emissions but are not specifically addressed in the SIP or 

other laws or rules, the air agency should identify and document why it was reasonable to have 

not implemented controls. 

 We invite comment on whether there should be a grace or grandfathering period before a 

SIP call involving a relevant NAAQS has the effect of ending the deference that applied prior to 

the SIP call, such that for an event occurring during the grace period the SIP would be given the 

deference described for the first scenario. We believe that such a grace period should not extend 

beyond the due date for the required SIP revision in response to the SIP call. 

 Example: An air agency has an EPA-approved attainment plan SIP for a PM10

nonattainment area that was approved in 1994 and includes controls for some of the 

anthropogenic emission sources that contribute fugitive dust during high wind events. The 

nonattainment area did not include fugitive dust controls for gravel operations in its 1994 SIP 

and has not required any controls of these operations in the years since SIP approval. The area 

does not have an approved maintenance plan, in part because it has been experiencing 

unresolved exceedances since 1994. The air agency alerts the reviewing EPA regional office that 

it wishes to submit an exceptional events demonstration for a high wind dust event that occurred 

in 2015 and affected several of the area’s monitoring sites. This is the second high wind dust 

event associated with an exceedance in the past 3 years. After the first event, the EPA issued a 

SIP call for the air agency to revise its PM10 SIP, but the air agency has not yet submitted a new 

SIP. Because the EPA issued a SIP call, the air agency is required to show on a case-specific 

basis that the not reasonably controllable or presentable criterion has been met. Applying the 

concept that if a set of control measures should reasonably have been in place for emission 

sources that contribute to the event emissions to the information in the record supporting the SIP 
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call would likely result in a determination that those controls must have been in place for the 

event to satisfy the not reasonably controllable or preventable criterion. Because the gravel 

operations are not controlled and because the high wind dust event was the second in 3 years, the 

air agency had a basis for understanding the possible need for better controls. Given the air 

agency’s knowledge of recurring events, the air agency may not be able to make a sufficient 

showing for the not reasonably controllable or preventable criterion and the reviewing EPA 

regional office may not be able to concur with the air agency’s request to exclude data. If, 

however, the air agency can show that the gravel operations did not contribute to the eventrelated emissions, the reviewing EPA regional office might be able to concur with the air 

agency’s request to exclude data. 

 Details for Maintenance Area Scenario 3 

 In this scenario (where the SIP was approved more than 5 years prior to the date of 

submittal of a demonstration and the air agency is not under an obligation to revise the SIP), 

because of the passage of time the SIP controls should not be presumed to satisfy the not 

reasonably controllable or preventable criterion regardless of whether the event-related wind 

speeds are above or below the high wind threshold. In this case, the air agency should complete a 

case-specific assessment of the reasonableness of controls to satisfy the not reasonably 

controllable or preventable criterion. The assessment should consider controls beyond those 

required by the existing SIP and other state or local laws and rules. The case-specific assessment 

should apply the concept that if a set of control measures should reasonably have been in place

for emission sources that contribute to the event emissions, then those controls must have been in 

place for the event to satisfy the not reasonably controllable or preventable criterion. The 

submitting air agency should first identify the natural and anthropogenic emission sources that 
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contribute to the event emissions and exceedance. The air agency should categorize sources as 

those that are addressed through SIP or other state or local laws or rules and those sources that 

are not addressed by SIP measures or other measures. Where the contributing source has SIP or 

other controls, the air agency would identify the implementation status of these controls and 

provide evidence of effective implementation and enforcement. The air agency should also 

consider whether those SIP controls should have been made more stringent and effective prior to 

the event. For emission sources that contribute to the event emissions but are not specifically 

addressed in the SIP or other laws or rules, the air agency should identify and document why it 

was reasonable to have not implemented controls. 

 Example: An air agency has an EPA-approved attainment plan SIP for a PM10 former 

nonattainment area that was approved in 2008 and includes controls for anthropogenic emission 

sources that contribute fugitive dust during high wind events. The area has an approved 

maintenance plan. Between 2008 and 2014 it has not been experiencing exceedances related to 

high winds. In 2014 there is a single high wind dust event with sustained wind speeds above the 

high wind threshold that results in two exceedance days, sufficient to constitute a 3-year NAAQS 

violation. The air agency submits a demonstration covering both exceedances. In the not 

reasonably controllable or preventable portion of its demonstration, the air agency would identify 

all sources contributing to the event emissions, including natural sources, sources identified and 

controlled in the SIP, and any sources not controlled by the SIP. The air agency would then 

identify the applicable controls, the implementation status of these controls and evidence of 

enforcement. The air agency should also consider whether those SIP controls should have been 

made more stringent and effective prior to the event. Given the area’s past history of not having 

events and the fact that the sustained wind speed during the event was above the high wind 
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threshold, it is likely that the air agency could make a sufficient showing for the not reasonably 

controllable or preventable criterion. In this case, provided the air agency satisfies the other rule 

criteria, the EPA regional office would likely concur with an air agency’s request for data 

exclusion. 

 However, in this maintenance area scenario, another possible outcome of an event that 

causes an exceedance or violation is that the EPA determines that the not reasonably controllable 

or preventable criterion is not met and the event-affected data are retained for regulatory actions 

within the scope of the Exceptional Events Rule. This may lead to the EPA taking an action that 

places the air agency under an obligation to revise the SIP, in which case the situation would 

change into the second scenario for any later events of the same type. 

Best Management Practices. The EPA solicits comment on whether or not, as part of the 

assessment of local sources and reasonable controls, USDA/NRCS-approved BMPs constitute 

sufficient reasonable controls in any or in all high wind event-affected areas and whether these 

measures should therefore be specifically and categorically identified in preamble or rule 

language as constituting reasonable controls. As discussed in the “Current Situation” section, the 

EPA has noted in previous guidance that USDA/NRCS-approved BMPs designed to effectively 

reduce fugitive dust air emissions and prevent soil loss in agricultural applications could be 

included in the collection of controls determined to constitute reasonable controls for windblown dust events in areas in which they have been implemented.121 Although the EPA has 

 

121 Interim Guidance on the Preparation of Demonstrations in Support of Requests to Exclude 

Ambient Air Quality Data Affected by High Winds Under the Exceptional Events Rule. U.S. 

EPA. May 2013. Available at http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_highwinds_guide_130510.pdf and Interim Guidance to Implement 

Requirements for the Treatment of Air Quality Monitoring Data Influenced by Exceptional 

Events. U.S. EPA. May 2013. Available at http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_guidememo_130510.pdf. 
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addressed the sufficiency of BMPs in decisions on individual exceptional events demonstrations 

when the BMPs were part of a SIP-approved BACM determination, we have not previously 

addressed whether or not BMPs individually or in some combination with each other constitute 

sufficient reasonable controls nationally or in any particular types of areas. We recognize that 

this question may be difficult to answer because BMPs often describe general types of practices 

(e.g., installing wind breaks) rather than specifying the penetration, scale or intensity of use of 

these practices by the landowners who adopt them. Therefore we also solicit comment on the 

evidence for degree of penetration, scale and intensity that would be appropriate in 

demonstrations to consider BMPs individually or in some combination with each other to be 

reasonable controls. 

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

and Review 

1. Who may submit a demonstration and request for data exclusion? 

a. Current Situation 

Before addressing the schedule and mechanics of flagging event-influenced data and 

preparing demonstrations, the EPA believes it is necessary to first clarify which parties can 

submit an exceptional events demonstration package to the EPA. The CAA language at section 

319(b)(3)(B)(i) states that “the occurrence of an exceptional event must be demonstrated by 

reliable, accurate data that is promptly produced and provided by Federal, State, or local 

government agencies.” As noted in section V.A of this proposal, state, local and some tribal 

agencies administer air quality management programs within their jurisdiction, which includes 

monitoring and analyzing ambient air quality and submitting monitoring data to the EPA, which 

are then stored in the EPA’s AQS database. Also, FLMs and other federal agencies operate air 
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quality monitoring stations on some lands they manage, and some of these monitors meet the 

technical specifications and quality assurance requirements for their data to be used in regulatory 

determinations. As operators of regulatory monitors, each of these agencies can flag their own 

data within AQS for consideration as an exceptional event. 

As discussed in section V.F.1 of this proposed action, however, the EPA generally 

considers a state, exclusive of tribal lands, to be a single responsible actor, and, as the state is the 

entity primarily responsible for administering air quality planning and management activities, the 

state has been ultimately responsible for submitting exceptional events demonstrations for 

exceedances that occur at all regulatory monitoring sites within the boundary of the state, 

including exceedances occurring at monitoring sites operated by local air quality agencies to 

whom a state has delegated relevant responsibilities or at regulatory monitoring sites operated by 

any other entity within the state, such as FLMs of Class I areas, other federal agencies and/or 

industrial facilities. Although the state is responsible, a local agency, an FLM, another federal 

agency or another entity operating a regulatory monitor with an event-influenced exceedance can 

develop a demonstration for submittal by the state. If a state disagrees with the local agency’s, 

FLM’s, other federal agency’s or other entity’s exceptional events claim, the state can decide not 

to act on or forward that submittal to the EPA. A state can request that operators of other 

regulatory monitors experiencing event-influenced exceedances prepare or assist in the 

preparation of demonstration analyses for ultimate submittal by the state. 

Because some tribal air quality agencies also operate regulatory ambient air quality 

monitoring sites and submit these data to the EPA’s AQS database, as appropriate, these tribal 

agencies may also submit exceptional events demonstrations for exceedances that occur at their 

monitoring sites. 
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b. Proposed Changes 

As indicated in section V.A of this proposal, because FLMs and other federal agencies 

may operate regulatory monitors and submit collected data to the EPA’s AQS database and these 

same monitors could be affected by emissions from exceptional events, the EPA proposes to 

allow FLMs and other federal agencies to prepare and submit exceptional events demonstrations 

and data exclusion requests directly to the EPA. The EPA solicits comment on this proposed 

addition to the rule text, which appears at the end of this document. Based on comments 

received, the EPA may retain, modify or not include this provision in the final promulgated rule. 

This provision would apply only to FLMs and other federal agencies that manage land on which 

an exceptional event originates or that operate a monitor that has been affected by an event. The 

provision would allow such FLMs and other federal agencies to provide demonstrations directly 

to the EPA only after a discussion with the state in which the monitor is operated. This 

discussion might instead result in an agreement that the federal agency (or another party) will 

provide a draft demonstration document to the appropriate state air agency for adoption and 

submission by the air agency to the EPA, as is currently allowed. If the provision for direct 

submission to the EPA is included in the final action, demonstrations prepared by FLMs or other 

federal agencies would be required to meet all provisions in the Exceptional Events Rule, 

including the requirement for a public comment period on a prepared demonstration122 and the 

 

 

122 A public comment opportunity is important prior to submission to the EPA because under the 

Exceptional Events Rule the EPA is not required to provide a public comment opportunity prior 

to concurring with an air agency’s request to exclude data. The EPA generally provides a public 

comment opportunity before we use air quality data, with or without such exclusions, in a final 

regulatory action. States typically provide an opportunity for public comment by posting draft 

demonstrations on a website. Federal agencies could do the same. 
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requirements related to schedules and procedures for demonstration package submittal (see

sections V.G.4, V.G.5 and V.G.6) that apply to state agencies that operate monitors. 

2. Aggregation of Events for NAAQS with Periods Longer Than 24 Hours and Demonstrations 

with Respect to Multiple NAAQS for the Same Pollutant 

a. Current Situation 

The EPA’s AQS database houses ambient air quality monitoring and related data. The 

data in AQS are maintained as individual reported measurements, which can range from 5-

minute maximum concentrations per hour for SO2, to hourly data for ozone, CO, NO2, SO2 and 

some PM measurements to 24-hour measurements for lead and other particulate matter 

measurements. Air agencies identify and the EPA concurs with exceptional event-related data in 

AQS that are reported as individual measurements. 

Some NAAQS have long averaging periods, such that multiple independent events may 

affect the period-average concentration of the NAAQS pollutant. In the aggregate, a clear causal 

relationship may exist between the events and an exceedance or violation, but no single event 

satisfies the clear causal relationship criterion because each event has too small of an effect on 

the longer-period metric to do so by itself. CAA section 319(b) and the 2007 Exceptional Events 

Rule do not clearly allow the aggregation of events for purposes of the clear causal relationship 

criterion, yet aggregation seems consistent with the intent of section 319(b). The EPA has not to 

date indicated that actual aggregation of events is permitted. However, Question 30 in the 

Interim Q&A document provided guidance that can be of some help in this situation. This 

guidance was that 24-hour concentrations of Pb, NO2, or SO2 can be individually compared to 

the NAAQS level defined for a longer period, for purposes of meeting “but for” with respect to 

both the 24-hour NAAQS, if applicable, and for purposes of meeting “but for” with respect to the 
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NAAQS with the longer averaging period. This guidance focused on the intention of a passage in 

the preamble to the 2007 Exceptional Events Rule addressing the PM2.5 NAAQS in particular, 

and extended the approach of the 2007 preamble to other cases of NAAQS for the same pollutant 

that have different averaging periods. The practical effect of this approach is that several events 

that individually have effects too small to have a causal connection to a longer-period 

exceedance or violation might be excluded one-by-one, and the net effect of the exclusions may 

make a difference to compliance with the longer-period NAAQS. As guidance, however, the 

Interim Q&A document does not provide full certainty that an air agency may rely on the 

recommended approach. 

As noted in section IV.B of this proposal, the 2007 Exceptional Events Rule requires that 

for data exclusion, among other requirements, an air agency must demonstrate that there would 

have been no exceedance or violation of the NAAQS “but for” the event. The “but for” criterion 

necessarily requires comparing the individual measurements in AQS to the averaging period of 

the relevant NAAQS to determine whether an exceedance or violation occurred. When the 

averaging period for the NAAQS is the same as the measurement duration period, this 

comparison is relatively straightforward. For example, air agencies and the EPA can directly 

compare 1-hour ozone, 1-hour CO, 1-hour SO2, and 1-hour NO2 measurements to the respective 

1-hour NAAQS. This comparison becomes more complicated, however, when there is a 

difference between the pollutant measurement duration and the averaging time of the NAAQS, 

which is the case when comparing a 1-hour measurement to an 8-hour, 24-hour, 3-month or 

annual NAAQS (or in the case of 1-hour ozone the previously existing NAAQS). In fact, the 

EPA devoted Questions 29-31 in the Interim Q&A document to explaining how to make these 
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complicated comparisons.123 The Interim Q&A document also explained that because these 

comparisons are NAAQS-specific, air agencies should request and support the exclusion of a 

measured air concentration separately for each NAAQS that applies to the pollutant and the EPA 

will similarly provide separate concurrences. Under the 2007 Exceptional Events Rule 

provisions, this means, for example, that an air agency with several 24-hour measurements of 

event-influenced PM2.5 data measuring 75 micrograms per cubic meter (μg/m3

) would need to 

separately flag the data within AQS on a NAAQS-specific basis, and submit separate requests, 

analyses and demonstration components to support exclusion of the identified event-influenced 

data for the 1997 annual secondary PM2.5 NAAQS of 15 μg/m3

, the 2012 annual primary PM2.5

NAAQS of 12 μg/m3 and the 2006 primary and secondary 24-hour PM2.5 NAAQS of 35 μg/m3. 

Depending on the outcome of the “but for” criterion with respect to each PM2.5 NAAQS, it could 

be that the data would be excluded for purposes of determinations with respect to only some of 

these NAAQS. 

This current situation can result in complicated demonstrations for air agencies seeking 

data exclusion from determinations with respect to multiple NAAQS for the same pollutant. This 

complexity may make it more difficult for the public to comment, and requires time for the EPA 

to review such a demonstration. 

b. Proposed Changes 

The EPA is taking comment on proposed rule text allowing 24-hour concentrations of 

any NAAQS pollutant to be compared to a NAAQS level defined for a longer period as part of a 

weight of evidence showing for the clear causal relationship with respect to the NAAQS with the 

 

123 Interim Exceptional Events Rule Frequently Asked Questions. U.S. EPA. May 2013. 

Available at http://www2.epa.gov/sites/production/files/2015-05/documents/eer_qa_doc_5-10-

13_r3.pdf. 
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longer period. This approach would be more amenable to less quantified weight of evidence 

demonstrations, since only one day would be examined at a time. 

The EPA is also proposing that for NAAQS with averaging or cumulative periods longer 

than 24 hours, events occurring on different days may be aggregated for the purpose of 

determining whether their collective effect has caused an exceedance or violation, without regard 

to whether the events are of the same type (e.g., stratospheric ozone intrusion followed by a 

wildfire). The EPA notes that such aggregation may be very difficult if the effects of the 

individual events on their individual days are not fully quantified. Proposed rule text for this 

change is provided for comment. 

Finally, to simplify some demonstrations, the EPA is also taking comment on whether a 

successful demonstration with respect to any NAAQS for a given pollutant would suffice to 

qualify the data in question for exclusion with respect to all NAAQS for that pollutant. The EPA 

believes it is useful to invite public comment on this “approved for one NAAQS approved for all 

NAAQS for the same pollutant” concept. 

The EPA will carefully consider the comments it receives on these concepts and may 

finalize all, some or none of the three proposals described in this section. 

3. Exclusion of Entire 24-hour Value Versus Partial Adjustment of the 24-hour Value for 

Particulate Matter

a. Current Situation 

As indicated in Question 29 of the Interim Q&A document, we have advised air agencies 

preparing demonstrations to support requests to exclude PM2.5 and PM10 data obtained via 

monitor instruments that provide 1-hour measurements that they should flag all 24 1-hour values 

within a given event-affected day, even if the event did not last all these hours. If concurred 
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upon, flagging all 1-hour values will ultimately result in the same available remaining data for 

regulatory analysis and calculation as would be the case had the 24-hour PM2.5 or PM10

measurement data been collected from filter-based (24-hour) monitoring instruments.124Another 

reason we have taken the position that flagging all 24 1-hour values is appropriate is because 

flagging only peak or selected hours could result in the remaining 1-hour values still meeting the 

data completeness requirements because flagged and excluded data do not count against 

completeness even though they cannot be used in calculating an average concentration for a 24-

hour period. Under the rules for data interpretation, exclusion of only the event-affected 1-hour 

concentrations could result in AQS calculating a seemingly valid 24-hour concentration that is 

actually highly uncertain because it is based on only a few hours and thus may be biased.125

b. Proposed Changes 

The EPA solicits comment on codifying its current approach in the rule text to eliminate 

any regulatory uncertainty. If finalized, this modification to the data handling procedures will be 

made to occur automatically within AQS.

4. Flagging of Data 

While neither the preamble to nor the rule text contained within the 2007 Exceptional 

Events Rule discuss data handling within AQS, explaining certain AQS processes and functions 

 

124 Filter based instruments typically record a single value within a 24-hour period while 

continuous monitors typically collect 24 1-hour measurements. Because AQS can calculate a 

valid 24-hour average concentration with as few as 18 hours, it may be necessary to exclude 

hours not actually affected by the event to ensure the same data exclusion outcome as if the 

measurement had been made with a 24-hour filter. 

125 The form of the 24-hour PM2.5 NAAQS of 35 μg/m3 is 98th percentile averaged over 3 years.

The form of the primary annual PM2.5 NAAQS of 12 μg/m3 is an annual mean averaged over 3 

years. The form of the 24-hour PM10 NAAQS of 150 μg/m3 is not to be exceeded more than once 

per year on average over 3 years. Biased concentrations can potentially skew the determination 

of the 98th percentile and/or the annual mean for PM2.5 and the averages for PM2.5 or PM10

calculated to determine compliance with the relevant NAAQS. 
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will be useful to an understanding of the data flagging situation that has developed in 

implementing the requirements in 40 CFR 50.14(c)(2) and why we are proposing changes. 

a. Current Situation 

Within AQS, monitoring agencies can use two types of data validation, or data qualifier, 

codes related to exceptional events: the Request Exclusion flags (R) and the Informational Only 

flags (I).126 The EPA has advised air agencies to use the I series flags when identifying 

informational data and the R series flags to identify data points for which the agency intends to 

request an exceptional events exclusion and the EPA’s concurrence. As an example, air 

agencies may currently use an I series flag to initially identify values they believe were affected 

by an event. Once the air agency collects additional supporting data, it may change the flag to 

an R series flag and submit an initial event description. Or, the air agency may find that 

additional information does not support flagging the data as an exceptional event, and the air 

agency may, therefore, delete the flag or retain the I series flag. Air agencies may also use the I

series flags simply to note activities or conditions occurring on the data collection day that are 

unrelated to exceptional events or that do not result in an exceedance or violation of a NAAQS. 

Air agencies have previously indicated that they generally see little value in the use of I series 

flags.127

 

126 “Flag” is the common terminology for a data qualifier code in the EPA’s AQS. Unless 

explicitly noted, the process of “flagging” data refers to adding Request Exclusion (R) data 

qualifier codes to selected data in AQS. R flags are the only AQS flags that satisfy the 2007 

Exceptional Events Rule requirement for initial data flagging. The current design of the AQS 

software is such that EPA can act/concur only on an R flag. 

127 Responses to Significant First-Round Comments on the Draft Guidance Documents on the 

Implementation of the Exceptional Events Rule, U.S. EPA, June 2012. Available in Docket No. 

EPA-HQ-OAR-2011-0887. 
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Flagging of event-influenced data has traditionally also involved associating a one- or 

two-character code with a monitored value within AQS indicating that the data have potentially 

been influenced by a particular type of exceptional event (e.g., “RT” is the character code used 

to request exclusion for data that have been influenced by wildfires in the US). The 2007 

Exceptional Events rule added a requirement to include a more detailed initial description of the 

particular event associated with such a character code. This description consists of text of 

variable length. 

The EPA does not review or concur on the I series flags. Rather, an air agency must use 

an R flag to request data exclusion. The language at 40 CFR 50.14(c)(2), Flagging of Data, 

requires that an air agency notify the EPA of its intent to exclude one or more measured 

exceedances of an applicable NAAQS as being due to an exceptional event by placing a flag 

and an initial event description in the appropriate fields in AQS for the data record(s) of concern 

no later than July 1 of the calendar year following the year in which the flagged measurement 

occurred. Only R flags fulfill this requirement. This “general” schedule date of July 1 applies 

unless the data are associated with the initial area designations process for a new or revised 

NAAQS in which case the specific schedule in 50.14(c)(2)(vi) applies. 

Air agencies have previously expressed concern that the timelines for event flagging and 

demonstration submittal are not always appropriate.128 While the EPA has historically 

promulgated revised flagging and demonstration submittal schedules in the regulatory actions 

for new and revised NAAQS for those data years that might be used in the initial area 

designations process for those NAAQS, the EPA does not promulgate revised schedules for 

 

128 Responses to Significant First-Round Comments on the Draft Guidance Documents on the 

Implementation of the Exceptional Events Rule, U.S. EPA, June 2012. Available in Docket No. 

EPA-HQ-OAR-2011-0887. 
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other regulatory actions such as clean data or attainment determinations. Rather, the EPA has 

relied upon the “general” flagging and demonstration submittal schedules in 40 CFR 

50.14(c)(2) and 40 CFR 50.14(c)(3)(i). Meeting the requirement at 40 CFR 50.14(c)(2)(iii) to 

submit R flags and an initial description of the event “not later than July 1st of the calendar year 

following the year in which the flagged measurement occurred” can be difficult in the case of an 

annual standard where an air agency needs all 12 months of data to calculate an annual average 

and then needs 3 years of annual averages to identify whether or not the event-influenced data 

results in a violation of a 3-year design values. An air agency may not know that data influenced 

by an exceptional event caused the design value to become a NAAQS violation until 3 years 

after the event occurred. 

Some air agencies have used and applied I and R flags in AQS inconsistently with this 

intended scheme, by including applying numerous R flags in AQS with no real intention to 

submit an exceptional events demonstration. Also, R flags may be set immediately before a 

demonstration is submitted or even as late as when the EPA needs to indicate in AQS our 

approval of a request for data exclusion. As a result, neither the presence nor the absence of 

these flags provide the EPA with an indication of anticipated exceptional events 

demonstrations. 

b. Proposed Changes 

As part of this action, the EPA proposes to revise the “general” schedule language 

contained within 40 CFR 50.14(c)(2) by removing the timelines associated with initial event 

flagging. The EPA also proposes to modify the associated data flagging process within AQS to 

correspond with these proposed regulatory changes. These proposed changes would include 

eliminating the use of the current exceptional events data validation/data qualifier codes: the 
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Request Exclusion flags (R) and the Informational Only flags (I). The one- or two-character 

event type codes would be retained. The EPA solicits comment on the approach that is discussed 

below in additional detail. 

The EPA is proposing to change the definition and process for flagging exceptional event 

data. Flagging would in effect become the application of the one- or two-character event type 

and event description text as described below, along with a concurrent or subsequent request for 

data exclusion communicated to the EPA through other channels. 

Because the flagging of data necessarily begins with the identification of an event, the 

EPA proposes to retain, with modifications, the AQS free-form text field for an initial event 

description. As is currently the practice, we request that air agencies use the “initial event 

description” to identify a unique, real-world event. We propose that this “initial event 

description” be expanded to contain a unique event name; the type of the event (e.g., high wind 

dust, volcanic eruption, other); a brief description of the event; and, to the extent known, the 

scope of the event in terms of geography and time (e.g., likely affected area using latitude and 

longitude and a radius of influence and beginning day/time and ending day/time).129 AQS would 

also be modified to allow the air agency to associate specific AQS sites and potentially affected 

monitors and specific data points with a given event as so described. This will enable air 

agencies and the EPA to “flag” or add qualifier codes to selected data in a single step rather than 

adding this information or the necessary codes on a per entry basis. Historically, when events 

have influenced the concentrations at multiple monitors for multiple days, the air agency has 

added initial event descriptions and set flags on each monitored concentration, sometimes 

 

129 The EPA is proposing that air agencies select the “type of event” from a pre-set list of event 

types, which would likely consist of those event types currently identified by existing 

Informational and Request Exclusion flags within AQS. 
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resulting in hundreds of identical individual entries. “Associating” monitors with an event 

defined in time and space will save resources. 

Once an air agency has identified an event and created the initial event description within 

AQS, the agency should begin the process of requesting exclusion for identified data, which will 

consist of two discrete operations: (1) indicating in a separate communication to the EPA that 

specific ambient air quality measurements are affected by a defined event (see section V.G.5 

related to Initial Notification of Potential Exceptional Event), and (2) requesting that these 

identified ambient air quality measurements be excluded from regulatory actions according to the 

terms of the revised Exceptional Events Rule and EPA guidance for other applications of air 

quality data. AQS would retain a field to allow the EPA to concur or not concur with a given 

request for exclusion for one or more of the data points associated with a described event, once 

review of the air agency’s request and demonstration is completed. 

In addition to the proposed AQS modifications described above, the EPA is proposing to 

remove the “general” flagging schedule in 40 CFR 50.14(c)(2)(iii). This regulatory language 

currently requires that air agencies submit [R] flags and an initial description of the event by July 

1 of the calendar year following the year in which the flagged measurement occurred or by the 

other deadlines identified with individual NAAQS. As noted earlier in this section, an air agency 

may not know that data influenced by an exceptional event caused a violation of a NAAQS until 

after the initial event flagging deadline has passed. The EPA proposes to remove and reserve the 

current language at 40 CFR 50.14(c)(2)(iii). Additional changes to the regulatory language in 40 

CFR 50.14(c)(2) will be discussed in the next section. 
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The EPA notes that the recent ozone NAAQS action130 also removed and reserved the 

subsequent sections at 40 CFR 50.14(c)(2)(iv) and (c)(2)(v), which addressed the submittal of 

exceptional events demonstrations that could affect regulatory determinations associated with 

initial area designations for the 2006 24-hour PM2.5 NAAQS and the 2010 Lead NAAQS and 

were made obsolete by the passage of time. The EPA will retain these removed and reserved 

sections as promulgated in the ozone NAAQS and proposes no additional changes to these 

sections. 

5. Initial Notification of Potential Exceptional Event 

a. Current Situation 

As the EPA acknowledged in the Interim Exceptional Events Implementation Guidance 

and in discussions with stakeholders, the EPA understands that the initial identification of data 

affected by exceptional events and the subsequent preparation, submittal and review of 

demonstration packages is a resource intensive process both for the preparing air agency and the 

reviewing EPA regional office.131 Delays in processing and making decisions on submitted 

packages create regulatory uncertainty and potentially increase the workload for both the 

submitting air agency and the EPA. In addition, the backlog of pending actions makes selection 

of the best information to support new submittals potentially more uncertain. Further, air 

 

130 On October 1, 2015, the EPA Administrator signed a final action strengthening the groundlevel ozone NAAQS to 70 ppb. The EPA posted a pre-publication version of this notice on its 

website at http://www3.epa.gov/ozonepollution/pdfs/20151001fr.pdf. Once the official version of 

the notice is published in the Federal Register, the EPA will remove the posted pre-publication 

version and replace it with a link to the official version of the action. 

131 Interim Guidance to Implement Requirements for the Treatment of Air Quality Monitoring 

Data Influenced by Exceptional Events. Memorandum from Stephen D. Page, U.S. EPA Office 

of Air Quality Planning and Standards, to Regional Air Directors, Regions I-X. May 10, 2013. 

Available at http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_guidememo_130510.pdf. 
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agencies and the EPA often face timelines by which they must make regulatory decisions that 

can be affected by the inclusion or exclusion of event-affected data. In the Interim Exceptional 

Events Implementation Guidance and through the EPA’s best practices discussions identified in 

section IV.E, the EPA committed to work with air agencies as they prepare complete 

demonstration packages and we developed some guidelines to increase the efficiency of the 

process. 

One of the efficiency-increasing measures we suggested in the Interim Exceptional 

Events Implementation Guidance was the Letter of Intent. The guidance explained that the Letter 

of Intent was a voluntary process by which the submitting air agency notifies the reviewing EPA 

regional office of the air agency’s intent to submit a demonstration for an identified exceptional 

event. The purpose of the letter was to promote early communication between the submitting air 

agency and the reviewing EPA regional office. In the time since issuing the Interim Exceptional 

Events Implementation Guidance, several air agencies and the EPA regional offices have 

successfully used this voluntary process to discuss expectations regarding specific exceptional 

events demonstrations. 

b. Proposed Changes 

As part of the best practices for communications during the exceptional events process 

and to aid all agencies in resource planning and prioritization, the EPA proposes that air agencies 

and the EPA engage in regular communications to identify those data that have been potentially 

influenced by an exceptional event, to determine whether the identified data affect a regulatory 

determination, and to discuss whether an air agency should develop and submit an exceptional 

events demonstration. In most instances, these discussions will be between individual air 

agencies and the reviewing EPA regional office. In other cases, the EPA regional office, or an 
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individual air agency within the purview of the EPA regional office, may initiate and/or host a 

general discussion with all air agencies in the region followed by individual discussions, as 

needed. In still other cases, such as where large events cross state lines and when two or more 

states are pursuing exclusion for the same event(s), the EPA region or regions may initiate 

discussions will all potentially affected states/agencies to assist in coordinating states affected by 

regional events. 

For purposes of this proposed action, the EPA is referring to these communications as the 

“Initial Notification of Potential Exceptional Event” (Initial Notification) process. The EPA has 

changed the name of this process from the Letter of Intent in recognition of the fact that effective 

communication may have multiple formats and does not necessarily consist of a formal, written 

letter to convey important information. As with the voluntary Letter of Intent, the ultimate 

purpose of the Initial Notification process is to initiate conversations between an air agency and 

the EPA if not already on-going, or engage in more detailed discussions if a process is currently 

in place, regarding specific data and whether the identified data are ripe for submittal as 

exceptional events. As stakeholders have repeatedly expressed and as the EPA acknowledges, 

the identification of data affected by exceptional events and the subsequent preparation, 

submittal and review of demonstration packages is a resource intensive process both for the 

preparing air agency and the reviewing EPA regional office. 

However, in considering the exceptional events process, it is important to note that if 

these data do not have regulatory significance, then engaging in the development and review of 

an exceptional events demonstration is generally not an efficient use of an air agency’s or the 

EPA’s limited resources. The Initial Notification process will focus efforts on the relevant data 

and provide the EPA with the opportunity to convey to the affected air agency our initial 
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thoughts regarding the identified event and analyses that may or may not be appropriate for 

inclusion in a demonstration, and, with respect to regulatory significance, which demonstrations 

the EPA will consider for review. We believe that this approach will help air agencies make the 

best use of their available resources. 

As noted earlier, the Initial Notification could include any form of communication (e.g., 

letter, email, in-person meeting with an attendees’ list and discussion summary or phone 

conversation with follow-up email) that ultimately identifies the potential need to develop an 

exceptional events demonstration and communicates key information related to the data 

identified for potential exclusion. Where an air agency independently identifies event-affected 

data and the need to submit an exceptional events demonstration outside of its regular, on-going 

communications with the EPA regional office, the air agency could prepare a letter or email 

communicating its Initial Notification. Generally, the EPA anticipates that air agencies would 

develop and provide an Initial Notification as soon as the agency identifies event-influenced data 

that potentially influence a regulatory decision or when an agency wants the EPA’s input on 

whether or not to prepare a demonstration.132 The EPA further proposes that each Initial 

Notification would include the following components: 

 Unique event name (field in AQS) - facilitates future communication and 

understanding between the submitting air agency and the reviewing EPA regional 

office, particularly if an air agency has submitted multiple exceptional events 

demonstration packages. 

 

132 The EPA recognizes that air agencies can immediately identify those events that result in an 

exceedance of a NAAQS with a short averaging time (e.g., 1-hour, 8-hour or 24-hour standards) 

but may need additional time for an annual average standard. An air agency could also submit an 

annual Initial Notification if annual submittal makes sense for resource planning or for recurring 

seasonal events. 
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 Initial event description (field in AQS) – provides a brief narrative of the event 

that could also include maps or graphs similar to what an air agency might include 

in the proposed conceptual model discussed in section V.G.6 of this proposed 

action; the event description would include a qualitative description of the event 

and, at a minimum, briefly describe the agency’s current understanding of 

interaction of emissions with the event, transport and meteorology (e.g., wind 

patterns such as strength, convergence, subsidence, recirculation) and pollutant 

formation in the area. 

 Affected regulatory decision – provides a description of the regulatory action or 

actions potentially affected by the claimed event-influenced data and the 

anticipated timing of this action. 

 Proposed target date for demonstration submittal – identifies the proposed target 

date by which the air agency would submit a demonstration package to the 

reviewing EPA regional office. 

 Most recent design value including and excluding the event-affected data

(optional) – the air agency’s assessment of the most recent design value both with 

and without the identified event(s) is helpful when assessing regulatory 

significance. The EPA cannot calculate this value (and therefore may not be able 

to determine significance) if the air agency has flagged more data than it intends 

to include in an exceptional events demonstration. 

 Information specific to each monitored day – see Table 5, which would be 

developed by the submitting air agency and generated from the initial event 

description in AQS (see discussion in section V.G.4). 
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Table 5. Initial Notification Information Specific to Each Monitored Day 

Agency/ 

Planning 

Area 

State County Event 

Name 

in 

AQS 

Type 

of 

Event 

NAAQS Monitor 

AQS ID 

and Site 

Name 

Date(s) 

of 

Event 

Monitor 

Exceedance 

Concentration

 

 

 

 The EPA anticipates promptly acknowledging an air agency’s Initial Notification and 

then formally responding within 90 days of receipt via letter, email or in-person meeting with an 

attendees’ list and discussion summary. We also anticipate having informal phone conversations 

with the air agency prior to this formal response. As previously discussed, the EPA will 

generally prioritize exceptional events determinations that affect near-term regulatory 

decisions.133 Where the data are to be used in initial area designations, the EPA proposes to rely 

on the promulgated documentation submission schedule in Table 1 at 50.14(c)(2)(vi). Where the 

data will influence another near-term regulatory decision, the EPA proposes to rely on the caseby-case timelines by which the air agency should submit the demonstration. For case-by-case 

demonstrations, the EPA’s recommended date for demonstration submittal would consider the 

nature of the event, the anticipated timing of the regulatory decision, and would allow time for 

both an air agency’s preparation of the demonstration and the EPA’s review. The EPA may not 

be able to review and act on demonstrations submitted after the recommended submittal date. 

Additionally, the EPA will request in its response that, if the submitting air agency has not 

 

133 “Regulatory decisions” include findings as to whether the area has met the applicable 

NAAQS, classification determinations, attainment demonstrations, the development of Limited 

Maintenance Plans and clean data findings. 
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already identified the affected data within AQS, that it undertake this effort according to the 

process described in section V.G.4. If the data identified in the Initial Notification do not have 

regulatory significance (and there is no compelling reason for excluding data), then the EPA will 

indicate this in its correspondence back to the air agency and will discourage the air agency from 

devoting resources to developing a demonstration because the EPA will likely not review or act 

upon the submittal. 

If after discussing the content of a submitted Initial Notification and/or receiving the 

EPA’s response to the Initial Notification, the EPA acknowledges that identified data have 

regulatory significance (or some other compelling reason for excluding data), then the air agency 

should proceed with the development of a technical demonstration package that satisfies the 

requirements in 40 CFR 50.14 and accounts for any case-specific advice from the EPA and 

additional information in the EPA’s guidance documents.134 Although air agencies can submit 

demonstrations for events that do not affect a regulatory action, the EPA will likely not review or 

act on such submittals. 

For these reasons described in this section and in section V.G.4, the EPA proposes to 

revise the language in 40 CFR 50.14(c)(2)(i) as follows: “A State shall notify EPA of its intent to 

request exclusion of one or more measured exceedances of an applicable ambient air quality 

standard as being due to an exceptional event by creating an initial event description and 

flagging the associated data that have been submitted to the AQS database and by engaging in 

the Initial Notification of Potential Exceptional Event process.” Specific steps in the Initial 

 

134 Interim Guidance to Implement Requirements for the Treatment of Air Quality Monitoring 

Data Influenced by Exceptional Events. Memorandum from Stephen D. Page, U.S. EPA Office 

of Air Quality Planning and Standards, to Regional Air Directors, Regions I-X. May 10, 2013. 

Available at http://www2.epa.gov/sites/production/files/2015-

05/documents/exceptevents_guidememo_130510.pdf.
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Notification process are identified in rule text at the end of this document. The EPA solicits 

comment on the proposed rule text revision (in 40 CFR 50.14(c)(2)) to require an Initial 

Notification of Potential Exceptional Event, with a provision that the EPA can waive the Initial 

Notification requirement on a case-by-case basis. Alternatively, the EPA solicits comment on 

making the Initial Notification of Potential Exceptional Event a voluntary process. 

Additional proposed revisions would continue at (ii): “The data shall not be excluded 

from determinations with respect to exceedances or violations of the national ambient air quality 

standards unless and until, following the State’s submittal of its demonstration pursuant to 

paragraph (c)(3) of this section and EPA review, EPA notifies the State of its concurrence by 

placing a concurrence flag in the appropriate field for the data record in the AQS database.” 

As noted in section V.G.4, the EPA is proposing to remove the “general” flagging 

schedule in 40 CFR 50.14(c)(2)(iii). The EPA seeks comments on these proposed changes to the 

language at 40 CFR 50.14(c)(2), which more clearly identify the process for flagging data in 

AQS and requesting exclusion of one or more measured exceedances of an applicable ambient 

air quality standard. 

The EPA notes that the recent final rule to revise the ozone NAAQS also removed and 

reserved the subsequent sections at 40 CFR 50.14(c)(2)(iv) and (c)(2)(v), which addressed the 

submittal of exceptional events demonstrations that could affect regulatory determinations 

associated with initial area designations for the 2006 24-hour PM2.5 NAAQS and the 2010 Lead 

NAAQS and were made obsolete by the passage of time. The EPA will retain these removed and 

reserved sections as promulgated in the ozone NAAQS and proposes no additional changes to 

these sections. 

6. Submission of Demonstrations 
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a. Current Situation 

With the recent ozone NAAQS, the EPA proposed and promulgated changes to the 

current exceptional events regulatory language at 40 CFR 50.14(c)(2) and 40 CFR 50.14(c)(3) to 

include finalizing exceptional events flagging and demonstration submittal schedules related to 

implementing the revised ozone standards and future revised NAAQS and removing obsolete 

regulatory language for expired exceptional events deadlines. Sections V.G.4 and V.G.5 discuss 

the current situation and additional proposed changes to 40 CFR 50.14(c)(2). This section 

discusses the current situation and proposed revisions to 40 CFR 50.14(c)(3). 

As part of the recent final rule to revise the ozone NAAQS, the regulatory language at 40 

CFR 50.14(c)(3)(i) now refers to a revised exceptional events flagging and demonstration 

submittal schedule for data that could be used in initial area designation decisions following 

promulgation of any future revised NAAQS. However, the language at 40 CFR 50.14(c)(3)(i) 

still requires air agencies to “...submit a demonstration to justify data exclusion to EPA not later 

than the lesser of, 3 years following the end of the calendar quarter in which the flagged 

concentration was recorded or, 12 months prior to the date that a regulatory decision must be 

made by EPA.” 

As identified in section V.G.4 of this proposal, air agencies have previously expressed 

concern that the timelines for event flagging and demonstration submittal are not always 

appropriate because an air agency may not know that data influenced by an exceptional event 

caused the design value exceedance until 3 years after the event occurred.135 The EPA 

acknowledges that this scenario can occur. 

 

135 Responses to Significant First-Round Comments on the Draft Guidance Documents on the 

Implementation of the Exceptional Events Rule, U.S. EPA, June 2012. Available in Docket No. 

EPA-HQ-OAR-2011-0887. 
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 In addition to establishing a general schedule for demonstration submittal, the regulatory 

language at 40 CFR 50.14(c)(3)(i) requires that “A State must submit the public comments it 

received along with its demonstration to EPA.” Although this language is included in 40 CFR 

50.14(c)(3)(i), it refers to the regulatory language at 40 CFR 50.14(c)(3)(v), which requires the 

air agency to document, and submit with its demonstration, evidence that it followed the public 

comment process. Regarding this requirement to “document that the public comment process 

was followed,” neither the Exceptional Events Rule language in 40 CFR 50.14 nor the preamble 

to the promulgated 2007 Exceptional Events Rule specifies a minimum timeframe for public 

comment. Many air agencies have been posting draft demonstrations for public review on their 

websites. The EPA has reviewed several of these postings and identified 30-days as an often 

cited timeframe for public comment on a draft exceptional events demonstration submittal. 

 The current rule also provides at 40 CFR 50.14(c)(3)(iv) that the demonstration to justify 

data exclusion shall provide evidence that the event satisfies the definition of an exceptional 

event provided at 40 CFR 50.1(j); that there is a clear causal relationship between the monitored 

exceedance and the event that is claimed to have affected the air quality in the area; that the 

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

including background; and that there would have been no exceedance or violation but for the 

event. Air agencies have found this section of the 2007 Exceptional Events Rule to be confusing 

because it contains a mix of statutory requirements and regulatory language without clearly 

identifying the components that the EPA expects to see in an exceptional events demonstration. 

As the EPA expressed in the Interim Exceptional Events Implementation Guidance, all parties 

would benefit from clear expectations regarding demonstration components. 
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The EPA further believes, and recommended in the Interim High Winds Guidance 

document, that each demonstration begin with a conceptual model, or narrative, describing the 

event(s) causing the exceedance or violation and a discussion of how emissions from the event(s) 

led to the exceedance at the affected monitor(s). As described in the Interim High Winds 

Guidance document, the narrative conceptual model could include varying levels of detail 

depending on the event complexity, but in all cases would provide a qualitative description of the 

event, interaction of the event-generated emissions with transport and meteorology (e.g., wind 

patterns such as strength, convergence, subsidence, recirculation) and pollutant formation in the 

area with the exceeding monitor. Because, in some cases, monitored data or technical analyses 

may seem to contradict the event claim, particularly the clear causal relationship, an air agency 

can use the conceptual model to explain, with a weight of evidence approach, why the majority 

of the data or analyses are consistent with the event’s impact on a measured exceedance or 

violation (for example, for a wildfire, why most of the meteorology would have indicated a 

lower ozone day without the fire emissions, even if the temperature were high). A useful 

conceptual model also includes (1) a description of the regulatory decision impacted by the 

exceptional event, (2) a summary table of the data requested for exclusion and (3) maps and/or 

summary tables of event-related information including location; size and extent; point and 

explanation of origin. A conceptual model can additionally include examples of media coverage 

of the event.136 Since releasing the Interim High Winds Guidance document in 2013, the EPA 

has received several demonstrations that included a conceptual model. The EPA has found it 

very helpful to understand the event formation and the event’s influence on monitored pollutant 

 

136 The EPA expects that air agencies could use some of the same information and tables in both 

the conceptual model and the Initial Notification of Potential Exceptional Event, which is 

discussed in section V.G.5 of this proposal. 
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concentrations before beginning to review the individual technical evidence to support the 

requested data exclusion. 

b. Proposed Changes 

For the previously mentioned reasons, the EPA is proposing and soliciting comment on 

the following changes to the regulatory language in 40 CFR 50.14(c)(3) regarding the 

submission of demonstrations: 

 Removing the general schedule provisions in 40 CFR 50.14(c)(3)(i) for 

submitting demonstrations. 

 Moving the language requiring a state to include the comments it received during 

the public comment period for the subject demonstration from 40 CFR 

50.14(c)(3)(i) to 40 CFR 50.14(c)(3)(v). 

 Modifying the language at 40 CFR 50.14(c)(3)(iv) to more clearly identify the 

required elements of an exceptional events demonstration to include (1) a 

narrative conceptual model and (2) demonstrations and analysis that address the 

core statutory technical criteria [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 (as indicated by the comparison to historical 

concentrations showing and other analyses), the event was a human activity that is 

unlikely to recur at a particular location or was a natural event, the event was not 

reasonably controllable or preventable]. 

 Modifying the language at 40 CFR 50.14(c)(3)(v) to identify that a demonstration 

submittal must include (1) documentation that the air agency conducted a public 

comment process on its draft exceptional events demonstration that was a 
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minimum of 30 days, which could be concurrent with the EPA’s review, (2) any 

public comments received during the public comment period and (3) an 

explanation of how the air agency addressed the public comments. 

To elaborate on removing the general schedule provisions in 40 CFR 50.14(c)(3)(i), the 

EPA proposes to remove the provision in 40 CFR 50.14(c)(3)(i) that requires air agencies to 

submit a demonstration “not later than the lesser of 3 years following the end of the calendar 

quarter in which the flagged concentration was recorded or 12 months prior to the date that a 

regulatory decision must be made by EPA.” In place of this language, the EPA proposes to rely 

on the promulgated documentation submission schedule in Table 1 at 50.14(c)(2)(vi) in those 

cases where the data are to be used in initial area designations. If the data could influence a 

regulatory determination other than initial area designations, the EPA proposes to rely on the 

case-by-case timelines established by the reviewing EPA regional office as part of the Initial 

Notification of Potential Exceptional Event process. 

With respect to the public comment provisions for a developed demonstration, for the 

reasons stated previously, the EPA proposes to move the language requiring an air agency to 

include the comments it received during the public comment period for the subject 

demonstration from 40 CFR 50.14(c)(3)(i) to 40 CFR 50.14(c)(3)(v) to consolidate the required 

elements of the public comment process for exceptional events demonstrations within a single 

regulatory provision. The EPA also proposes to specify a minimum 30-day public comment 

process, which provides sufficient time for exchange between the reviewing public and the air 

agency. Shorter comment periods may not provide necessary time for the public to research the 

identified event and associated supporting data while longer timeframes may not be possible 

where a near-term regulatory decision relies on an exceptional events decision. The EPA notes 
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that in very limited cases where the air agency is relying on exceptional events claims as part of a 

near-term regulatory action, such as a demonstration for events in the third year of a 3-year 

design value that will be used in initial area designations for a new or revised NAAQS under a 2-

year designation schedule, the public comment period could be concurrent with the EPA’s 

review provided the submitting air agency sends any public comments and responses to the EPA 

by a specified date should comments be submitted. If an air agency receives public comment 

disputing the technical elements of a demonstration during a comment period that runs 

concurrent with the EPA’s review and these comments result in the air agency’s need to 

reanalyze or reassess the validity of a claimed event, a second public comment period may be 

necessary. 

The EPA also proposes to revise the language at 40 CFR 50.14(c)(3)(iv) so that it more 

clearly identifies the required elements of an exceptional events demonstration. As previously 

described, the EPA proposes that each demonstration begin with a narrative conceptual model, 

which summarizes the event in question and provides context for required statutory technical 

criteria analyses. The EPA further proposes, consistent with other proposed changes in this 

action, that an air agency include in its demonstration to justify data exclusion evidence that the 

following statutory technical criteria are satisfied: 

(A) The event was a human activity that is unlikely to recur at a particular location or 

was a natural event. 

(B) The event was not reasonably controllable or preventable. 

(C) 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 

(supported in part by the comparison to historical concentrations and other 

analyses). 
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The EPA seeks comments on the identified proposed changes to the language at 40 CFR 

50.14(c)(3)(i), (iv) and (v), which more clearly identify the required elements of an exceptional 

events demonstration. 

The EPA notes that the recent final rule to revise the ozone NAAQS also removed and 

reserved the subsequent sections at 40 CFR 50.14(c)(3)(ii) and (c)(3)(iii), which addressed the 

submittal of exceptional events demonstrations that could affect regulatory determinations 

associated with initial area designations for the 2006 24-hour PM2.5 NAAQS and the 2010 Lead 

NAAQS and were made obsolete by the passage of time. The EPA will retain these removed and 

reserved sections as promulgated in the ozone NAAQS and proposes no additional changes to 

this language. 

7. Timing of the EPA’s Review of Submitted Demonstrations 

a. Current Situation 

Since promulgation of the Exceptional Events Rule in 2007, stakeholders have 

questioned the process by which the EPA reviews submitted demonstrations.137 Specifically, 

stakeholders have expressed concern that the EPA has a backlog of submittals but acts only on 

EPA-prioritized packages. Stakeholders have stated that because the EPA has not acted on all 

submissions, the air quality values used for planning and regulatory purposes are higher than 

they would be if the effects of non-controllable emissions were removed from the data set. Air 

agencies have also noted that without feedback, they do not know the EPA’s expectations 

regarding future submittals. 

 

137 See comment letters in Docket ID No. EPA-HQ-OAR-2011-0887 for the July 2012 notice of 

availability for the Draft Exceptional Events Implementation Guidance, which the EPA has 

incorporated into the record for this action. 
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The EPA addressed these questions and comments in the Interim Exceptional Events 

Implementation Guidance. In Question 27 of the Interim Q&A document, the EPA identified the 

general process and timing for demonstration reviews. In this document, the EPA clarified the 

process by which it prioritizes submittals and indicated that we may not act on submittals with no 

regulatory significance. The guidance also presented the voluntary Letter of Intent concept as a 

mechanism to aid in planning and prioritization. Additionally, we stated that we intend to make a 

decision regarding concurrence with an air agency’s request to exclude data as expeditiously as 

necessary following submittal of a complete package if required by a near-term regulatory 

action. We also indicated our intent to communicate with the submitting agency, as needed, 

during the demonstration review period. 

b. Proposed Changes 

In this proposal, the EPA is clarifying some of our previous statements regarding the 

prioritization and submittal of demonstrations. As noted in several subsections within section 

V.G of this proposal, we also propose to codify in regulatory language approaches to increase the 

efficiency of preparing, submitting and reviewing exceptional events demonstrations. Although 

the EPA is not proposing to codify in regulatory language any changes pertaining to the timing 

of the EPA review process, the EPA offers the following discussion to clarify expectations and 

facilitate communications, which are at the center of timing-related issues. 

As noted in the Interim Exceptional Events Implementation Guidance and in the EPA’s 

best practices discussions described in section IV.E, the EPA is committed to working with air 

agencies as they prepare complete demonstration packages. The EPA encourages ongoing 

discussions between the reviewing EPA regional office and the submitting air agency from the 

onset of the Initial Notification of Potential Exceptional Event process through official package 
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submittal. Since renewing our focus on improved communications, the EPA has received 

positive feedback from engaged agencies that have used this approach. Additionally, these 

communications have resulted in decreased instances of submissions containing insufficient or 

unnecessary information. 

In reviewing submitted demonstration packages, the EPA will generally give priority to 

exceptional events determinations that may affect near-term regulatory decisions, such as EPA 

action on SIP submittals, NAAQS designations and clean data determinations. The EPA intends 

to make a decision regarding event status expeditiously following submittal of a complete 

package if required by a near-term regulatory action. If during the review process the EPA 

identifies the need for additional information to determine whether the exceptional events criteria 

are met, the EPA will notify the submitting air agency and encourage the agency to provide the 

supplemental information. If the information needed is minor and a natural outgrowth of what 

was previously submitted, the EPA will not require the air agency to undergo an additional 

public notice and comment process. However, if the needed information is significant, the EPA 

may request that the air agency re-notice the demonstration before resubmitting it to the EPA, 

thus requiring an additional EPA review following resubmittal. 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. The EPA proposes to include as rule text a requirement 

for the air agency to submit additional information within 12 months. If additional information is 

not received in 12 months, then the EPA will consider the submitted demonstration inactive, and 

will not continue the review or take action. In effect, an air agency’s lack of response within a 

12-month period will “void” the submittal. In these cases, the EPA does not intend to issue a 

formal notice of deferral. If the air agency later decides to pursue the exceptional events claim 
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after a 12-month period of inactivity, it may re-initiate the exceptional events process by 

submitting a new Initial Notification of Potential Exceptional Event followed by a new 

demonstration, which could simply be revising the original submittal to include the additional 

information previously requested by the EPA. 

At the conclusion of the EPA’s review, the EPA will make a determination regarding the 

status of a submitted exceptional events demonstration. The EPA’s decision could result in 

concurrence, nonconcurrence or deferral.138 In acting on a submitted demonstration covering 

multiple event days and/or multiple flags, the EPA could concur with part of a demonstration and 

nonconcur or defer other flagged values. If the EPA determines that the events addressed in an 

exceptional events demonstration are not anticipated to affect any future regulatory decision, the 

EPA could defer review of these events and notify the submitting agency if a subsequent review 

results in a determination that the events do affect a regulatory decision.139 Formal mechanisms 

for deferral could include the EPA’s indicating this decision by letter, by email to a responsible 

official or during a high-level meeting with an attendees’ list and discussion summary. 

8. Dispute Resolution Mechanisms 

Since promulgation of the 2007 Exceptional Events Rule and through the development of 

the Interim Exceptional Events Implementation Guidance, some interested parties have asked the 

EPA to identify a process by which submitting air agencies can formally dispute the EPA’s 

decision regarding requests for additional information to support submitted demonstration 

 

138 The EPA anticipates a reduced number of deferrals and/or nonconcurrences for 

demonstrations associated with the Initial Notification of Potential Exceptional Event process as 

discussed in section V.G.5 because the EPA and the affected air agency would have discussed 

issues/concerns prior to the EPA’s decision on a submitted demonstration. 

139 Routine status calls between the reviewing EPA regional office and air agencies could include 

an agenda item to review the status of all submitted demonstrations, including those that the EPA 

has deferred. 
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packages and/or decisions regarding concurrence, nonconcurrence or deferral of submitted 

demonstration packages. While the EPA acknowledges the expressed concerns and desire for a 

formally identified dispute resolution process, the EPA also believes that several mechanisms 

currently exist that air agencies can use at various points in the exceptional events process. These 

mechanisms include engaging in early dialogue with the reviewing EPA regional office, 

submitting requests for reconsideration to the official who made the determination if a request 

identifies a clear error or if the reviewing EPA regional office overlooked information submitted 

by the affected air agency, and/or elevating the concern within the EPA’s chain of command. 

Additionally, air agencies can raise any unresolved event-related issues during the regulatory 

process that relies upon the claimed event-influenced data by participating in related public 

notice and comment processes and/or challenging in an appropriate court the regulatory decision 

subsequently made based in part on the EPA’s exceptional events determination. These currently 

available dispute resolution approaches to address exceptional events decisions are consistent 

with the mechanisms available for other EPA actions. With exceptional events decisions, 

however, the air agency has opportunities to elevate concerns during two processes: the 

exceptional events determination and the subsequent regulatory action that relies on the 

exceptional events decision. 

The EPA believes that the existing mechanisms identified above combined with the 

EPA’s commitment to focus on communication and collaboration with the submitting air agency 

through the exceptional events demonstration process, and the clarifications that would be in 

effect with these proposed revisions to the 2007 Exceptional Events Rule and associated 

guidance, will avoid the need for a formal dispute resolution mechanism for exceptional events. 
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Therefore, the EPA does not intend to address dispute resolution within these proposed rule 

revisions and does not intend to respond to comments on this issue.

VI. Mitigation

A. Current Situation 

Section 319(b)(3)(A) of the CAA identifies five principles for the EPA to 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 regulatory requirements implementing (iii) and (v) of this part of the statute are 

found only in 40 CFR 50.14 while the regulatory requirements implementing (i) and (iv) are 

found only in 40 CFR 51.930, Mitigation of Exceptional Events. Both section 50.14(c)(1) and 

section 51.930(a)(1) require states to provide notice of events to the public (the second of the 

five principles). 

The language at 40 CFR 51.930 requires air agencies requesting data exclusion to “take 

appropriate and reasonable actions to protect public health from exceedances or violations of the 

NAAQS” and at a minimum do each of the following: 
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 Provide for prompt public notification whenever air quality concentrations exceed 

or are expected to exceed the NAAQS. 

 Provide for public education concerning actions that individuals may take to 

reduce exposures to unhealthy levels of air quality during and following an 

exceptional event. 

 Provide for the implementation of appropriate measures to protect public health 

from exceedances or violations of ambient air quality standards caused by 

exceptional events. 

The EPA promulgated the existing requirements in 2007 after considering and proposing 

several approaches to implementing CAA section 319(b)(3)(A). Some of the proposed 

approaches would have established a more formal structure by which air agencies prepared and 

submitted to the EPA mitigation plans to protect public health during events. These plans would 

have been subject to the EPA’s approval and/or the approval of the exclusion of event-affected 

data would have been contingent on the approval of such a plan. Comments on these proposed 

options varied widely.140,141

In the final 2007 rule, “mitigation” measures142 became part of the 2007 Exceptional 

Events Rule, but they were not incorporated into the criteria and processes by which data are 

excluded from use in regulatory determinations. There is no requirement to submit such 

 

140 The Treatment of Data Influenced by Exceptional Events; Proposed Rule, 71 FR 12592 

(March 10, 2006). 

141 See Comments and Responses related to “Requirements for States To Provide Public 

Notification, Public Education, and Appropriate and Reasonable Measures To Protect Public 

Health” in Treatment of Data influenced by Exceptional Events; Final Rule, 72 FR 13574-13576 

(March 22, 2007). 

142 The term “mitigation” does not appear in CAA section 319(b). It appears in the title but not 

the text of 40 CFR 51.930. 
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measures to the EPA for either prospective or retrospective review and approval as a condition 

for approval for exclusion of event-affected data. Neither are air agencies required to notify the 

EPA of the measures an air agency plans to take or has taken. In the preamble to the 2007 

Exceptional Events Rule, we stated that states should take “reasonable and appropriate 

measures” to protect public health related to the occurrence of an event and that states should 

determine what measures constitute those that are “reasonable and appropriate.”143 We did not 

clarify how measures should be determined to be “appropriate” measures. 

The mitigation measures that the EPA sees states most commonly practicing are ones 

related to the requirement that air agencies “provide for prompt public notification whenever air 

quality concentrations exceed or are expected to exceed the NAAQS.” Often, these public 

notifications include public health alerts for high wind dust events or wildfires. We believe that 

other aspects of mitigation, including implementing appropriate measures to protect public health 

beyond notification, are also important in implementing the CAA guiding principle that “each 

State must take necessary measures to safeguard public health regardless of the source of the air 

pollution.” 

B. Proposed Changes 

Given the EPA’s and the states’ experience implementing the 2007 Exceptional Events 

Rule as indicated above, we consider it appropriate to consider possible changes to the 

mitigation-related rule components with the benefit of additional public input. We are seeking 

comment on approaches ranging from retaining the existing rule requirements at 40 CFR 51.930 

to the various possible new components described in this section. We invite comment on these 

alternatives and on other concepts. We may make no change; we may adopt all of the described 

 

143 72 FR 13574 (March 22, 2007). 
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new components; or we may adopt only some features or variations of the described options. 

Note that we are not considering requiring all states to develop formal mitigation plans. We are 

seeking comment on the concept of only some states being required to develop mitigation plans 

for their particular “historically documented” or “known seasonal” exceptional events, defined 

below in section VI.B.1; on recommended elements for such mitigation plans described below in 

section VI.B.2; and on options for implementing mitigation plans described in section VI.B.3. 

Section VI.B.4 summarizes the EPA’s potential options for mitigation elements for exceptional 

events purposes. 

1. Defining Historically Documented or Known Seasonal Events 

The EPA seeks comment on whether an air agency should develop a mitigation plan for 

its particular type of “historically documented” or “known seasonal” exceptional events, if any. 

The EPA would consider “historically documented” or “known seasonal” exceptional events to 

include events of the same type and pollutant (e.g., high wind dust/PM or wildfire/ozone) that 

meet any of the following criteria: an event for which an air agency has previously submitted 

exceptional events demonstrations; an event that an air agency has previously flagged for 

concurrence in AQS (regardless of whether the air agency submitted a demonstration); or an 

event that has been the subject of local news articles, public health alerts or published scientific 

journal articles. The EPA would not require an air agency to develop a mitigation plan for the 

first event of a given type (e.g., if an area is prone to wildfires but has never experienced a high 

wind dust event, then it would not be expected to develop a mitigation plan for its first high wind 

dust event, but it would be expected to develop a mitigation plan for wildfires). A second event 

of a given type within a 3-year period would subject the area to “having a history” and, therefore, 
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needing a mitigation plan.144 This option avoids plan development for a one-of-a-kind 

occurrence.145 In defining “first” and “second” events, the EPA could consider events that affect 

the same AQCR, but not necessarily the same monitor.146 For example, high wind dust events 

occur seasonally in the Phoenix, Arizona metropolitan area, which is part of the Maricopa 

Intrastate Air Quality Control Region (see 40 CFR 81.36). These events have influenced 

particulate matter concentrations at multiple monitors within the Maricopa Intrastate AQCR. 

Under this proposal, high wind dust events in Phoenix (i.e., the Maricopa Intrastate AQCR) are 

known events requiring a mitigation plan. On the other hand, a high wind dust event in Sedona, 

Arizona, part of the Northern Arizona Intrastate Air Quality Control Region (see 40 CFR 

81.270), would be a first event and not subject to the development of a mitigation plan. As a 

variation of this concept on which we also seek comment, the EPA could consider a first season 

of events as one of three required seasons of events, so that a mitigation plan would be required 

only when an event type persists across several years. For example, an area may not have 

previously experienced wildfires in the past 10 years, but then experiences multiple wildfires and 

multiple exceedances in a single wildfire season. If these multiple wildfires affect the same 

general geographic area and monitors in a relatively short period of time (e.g. 2-3 months), then 

they could be considered a single event for purposes of developing a mitigation plan and would 

not trigger the requirement for a mitigation plan. 

 

144 A 3-year period is determined based on the submittal date of an exceptional events 

demonstration. 

145 Because the form of the NAAQS varies by pollutant, it is possible that multiple events in a 3-

year period may not cause a NAAQS violation. An air agency that identifies multiple events of 

the same type (e.g., wildfire/ozone) in AQS, but prepares and submits a demonstration for only 

one of these events, would trigger the proposed requirement to develop a mitigation plan. 

146 Air Quality Control Regions are defined in 40 CFR part 81, subpart B, Designation of Air 

Quality Control Regions. 
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2. Mitigation Plan Components 

The EPA solicits comment on the following three plan components that could be 

recommended or required in order to implement the mitigation principles found in section 

319(b)(3)(A) of the CAA: public notification and education; steps to identify, study and 

implement mitigating measures and provision for periodic revision of the mitigation plan (to 

include public review of plan elements). This section discusses these elements in more detail. A 

mitigation plan should address actions that would be taken within a state’s own territory for 

events that happen within its own territory or that of another jurisdiction. 

a. Public notification to and education programs for affected or potentially affected 

communities. Air agencies could be required or encouraged to include in their mitigation 

plans steps to activate public notification and education systems whenever air quality 

concentrations exceed or are expected to exceed an applicable national ambient air 

quality standard. If possible, air agencies would notify the public of the actual or 

anticipated event at least 48 hours in advance of the event using methods appropriate to 

the community being served. Outreach mechanisms could include website alerts, 

National Weather Service alerts, telephone or text bulletins, television or radio campaigns 

or other messaging campaigns. Public notification and education programs could be 

encouraged or required to include some or all of the following actions to support the 

outreach system: adoption of methods for forecasting/detection, consultation with 

appropriate health department personnel regarding issuing health advisories and 

suggested actions for exposure minimization for sensitive populations (e.g., remain 

indoors, avoid vigorous outdoor activity, avoid exposure to tobacco smoke and other 

respiratory irritants and, in extreme cases, evacuation or public sheltering procedures). 
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b. Steps to identify, study and implement mitigating measures, including approaches to 

address each of the following: 

(i) Mandatory or voluntary measures to abate or minimize contributing controllable 

sources of identified pollutants. A state could be required to include or 

encouraged to consider full-time or contingent controls on event-related sources 

as well as non-event related sources. For example, these measures might include 

continuously operating control measures during an extreme event for identified 

sources that normally operate these same controls on an intermittent basis. It 

could also involve including work practices (e.g., water spray for dust 

suppression) or contingent limits during extreme events on emissions from nonevent related sources that, under non-event periods, have no or less stringent 

emissions limits or work practices. 

(ii) Methods to minimize public exposure to high concentrations of identified 

pollutants. 

(iii) Processes to collect and maintain data pertinent to the event (e.g., to identify the 

data to be collected, the party responsible for collecting and maintaining the data 

and when, how and to whom the data will be reported). 

(iv) Mechanisms to consult with other air quality managers in the affected area 

regarding the appropriate responses to abate and minimize impacts. Consultation 

could include collaboration between potentially affected local, state, tribal and 

federal air quality managers and/or emergency response personnel. 

c. Provision for periodic review and evaluation of the mitigation plan and its 

implementation and effectiveness by the air agency and all interested stakeholders (e.g., 
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public and private land owners/managers, air quality, agriculture and forestry agencies, 

the public). For example, air agencies could be required to use this review process and to 

revise, if appropriate, and certify the mitigation plan every 3 years or every three events, 

whichever is longer. The air agency could be required to submit a summary and response 

to the comments received during the public plan review process to the EPA along with 

the recertification statement and/or revised mitigation plan. If the historically documented 

or known seasonal exceptional events continue to result in elevated pollutant 

concentrations above the relevant NAAQS, thus showing that the combination of the 

existing SIP and the existing mitigation plan does not effectively safeguard public health, 

the mitigation plan might need to be strengthened during this review. 

If the EPA adopts requirements like those described above, it would not necessarily mean 

that all affected air agencies would have to prepare new plans. If an air agency has developed 

and implemented a contingency plan under 40 CFR part 51, subpart H, Prevention of Air 

Pollution Emergency Episodes, that meets the requirements of 40 CFR 51.152, and that includes 

provisions for events that could be considered “exceptional events” under the provisions in 40 

CFR 50.14, then the subpart H contingency plan would likely satisfy the mitigation requirements 

described above. If the identified basic elements are included and addressed, including the 

element for public comment, then other types of existing mitigation or contingency plans may 

satisfy the possible mitigation plan requirement described above. For example, if an area has 

developed a natural events action plan or a high wind action plan covering high wind dust 

events, this plan likely would satisfy mitigation elements for high wind dust events. Smoke 

management plans and/or forest management plans might also satisfy the mitigation elements for 

prescribed fires and wildfires. Most air agencies generally have sufficient, established processes 
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that meet the public notification and education element, which can be easily adapted or modified 

to meet the mitigation elements proposed in this action. The EPA is requesting comment on how 

much time air agencies should be allowed to develop a plan. 

3. Options for Implementing Mitigation Plans 

The EPA is seeking comment on two options for tying the proposed mitigation plan 

components discussed in section VI.B.2 to the EPA review of exceptional events demonstrations. 

Option 1 includes the EPA’s review for completeness but not substantive approval or 

disapproval, while Option 2 includes the EPA’s approval of the substance of the mitigation plan. 

These options are discussed below in more detail, but neither option would require a mitigation 

plan to be included in a SIP or to be otherwise federally-enforceable. 

Under both options, air agencies with historically documented or known seasonal 

exceptional events could submit the mitigation plan to the EPA in advance of an event, or submit 

a mitigation plan along with an exceptional events demonstration. The EPA would concur with a 

demonstration for the relevant event type only if a mitigation plan has passed the type of EPA 

review described in the option. Given that the air agency would have advance notification of the 

need to develop a plan, the air agency could develop and submit the mitigation plan in advance 

of any exceptional events demonstration so that the EPA could pre-review the mitigation plan 

and take faster action on an exceptional events submittal once one is submitted. 

Option 1: Under this option, the EPA would review for inclusion of required elements as 

described above and to ensure that the development of the mitigation plan included a public 

comment process. We would not formally review the substance of the plan in the sense of 

approving the details of the specific measures and commitments in the plan. 
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Option 2: Under this option, EPA approval of the substance of the mitigation plan would 

be a precondition for EPA concurrence on an exceptional events demonstration. Because the 

EPA would approve the content, completeness and sufficiency of a mitigation plan, the EPA’s 

disapproval of the plan could result in the EPA’s nonconcurrence on a current or future 

exceptional events demonstration. 

VII. Draft Guidance on the Preparation of Exceptional Events Demonstrations for Wildfire 

Events that May Influence Ozone Concentrations 

A. What is this draft guidance about and why is it needed? 

The Exceptional Events Rule contains the regulatory requirements and criteria necessary 

for the EPA’s approval of the exclusion of air quality data from regulatory determinations related 

to NAAQS exceedances or violations. During the implementation of the 2007 Exceptional 

Events Rule, the EPA and stakeholders have identified a need for implementation guidance that 

provides an interpretation of and examples for addressing the regulatory requirements specific to 

the most common event types. One event type that has been identified by the EPA and 

stakeholders is wildfire influence on ozone concentrations. In 2013, the EPA finalized the 

Interim Exceptional Events Implementation Guidance documents (see section IV.D), which 

included the Interim High Winds Guidance document and an Interim Overview document that 

also committed to the preparation of a Draft Wildfire Ozone Guidance document. The EPA 

intends to address this need and commitment via the Draft Guidance on the Preparation of 

Exceptional Events Demonstrations for Wildfire Events that May Influence Ozone 

Concentrations (“Draft Wildfire Ozone Guidance document”), which accompanies this proposed 

rule and is also available for comment. 
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This Draft Wildfire Ozone Guidance document includes example analyses, conclusion 

statements and technical tools that air agencies can use to provide evidence that the wildfire 

event influenced the monitored ozone concentration. The Draft Wildfire Ozone Guidance 

document also identifies fire and monitor-based characteristics that might allow for a simpler and 

less resource-consuming demonstration package. The EPA has developed the Draft Wildfire 

Ozone Guidance document concurrently with the proposed Exceptional Events Rule revisions so 

that the Draft Wildfire Ozone Guidance reflects the proposed rule changes. Once finalized, this 

guidance will provide the EPA regional offices and air agencies with guidance on how to prepare 

and submit evidence to meet the Exceptional Events Rule requirements for monitored ozone 

exceedances caused by wildfires. The guidance, when finalized, will not be an EPA rule, and in 

specific cases the EPA may depart from the guidance for reasons that the EPA will explain at the 

time of the action. 

B. What scenarios are addressed in the draft guidance? 

The EPA has prepared the Draft Wildfire Ozone Guidance document to provide 

assistance and example analyses for wildfire events that may influence ozone concentrations. 

Though many of the technical analyses included in the draft document may also be applied to 

prescribed fire events, the draft guidance document available for comment at this time does not 

provide guidance specific on how prescribed fire events can address all proposed rule 

requirements. Limiting the scope to wildfire events is intended to make the document easier to 

use for wildfire events. With this notice, the EPA invites comment on the content of this 

guidance document and whether it is appropriate to expand the scope of the guidance to include 

prescribed fire events. If commenters believe it is necessary to expand the scope of the EPA’s 

final new guidance beyond the scope of the Draft Wildfire Ozone Guidance document, the EPA 
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seeks comment on whether wildfire and prescribed fire events should be addressed in a single 

fire ozone guidance document or in separate guidance documents. 

VIII. Environmental Justice Considerations 

The Exceptional Events Rule provides the criteria by which state, local and tribal air 

agencies identify air quality data they believe have been influenced by exceptional events, which 

by statutory definition are not reasonably controllable or preventable. Because these events are 

not reasonable to prevent or control, they can affect all downwind populations including minority 

and low-income populations. For this reason, in adding section 319(b) to the CAA, Congress 

identified as a guiding principle in developing regulations, “the principle that protection of public 

health is the highest priority.” The 2007 Exceptional Events Rule at 40 CFR 50.14 requires air 

agencies to seek public comment on prepared exceptional events demonstrations prior to 

submitting them to the reviewing EPA regional office. The public can also comment on 

rulemakings that include decisions related to the exclusion of event-influenced data. The 

mitigation of exceptional events language at 40 CFR 51.930 also requires that air agencies 

provide public notification and education programs related to events. To protect all people and 

communities, notably minority and low-income populations, air agencies should ensure that 

notifications and education programs are communicated using the language (e.g., English and 

Spanish) and media (e.g., radio and postings in local community centers) best suited to the target 

audience(s). Additionally, these proposed revisions are part of a public notice-and-comment 

rulemaking effort, which will include a public hearing. These opportunities for public input and 

education ensure that all those residing, working, attending school or otherwise present in areas 

affected by exceptional events, regardless of minority and economic status, are protected.

IX. Statutory and Executive Order Reviews
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A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: 

Improving Regulation and Regulatory Review 

This action is a significant regulatory action that was submitted to the Office of 

Management and Budget (OMB) for review because it raises novel policy issues. Any changes 

made in response to OMB recommendations have been documented in the docket.

B. Paperwork Reduction Act 

This action does not impose any new information collection burden under the PRA. The 

information being requested under these proposed rule revisions is consistent with current 

requirements related to information needed to verify the authenticity of monitoring data 

submitted to the EPA’s AQS database, and to justify exclusion of data that have been flagged as 

being affected by exceptional events. OMB has previously approved the information collection 

activities for ambient air monitoring data and other supporting measurements reporting and 

recordkeeping activities associated with the 40 CFR part 58 Ambient Air Quality Surveillance 

rule and has assigned OMB control number 2060-0084. 

C. Regulatory Flexibility Act 

 I certify that this action will not have a significant economic impact on a substantial 

number of small entities under the RFA. This action will not impose any requirements on small 

entities. Instead, the proposed rule revisions provide the criteria and increase the efficiency of the 

process by which state, local and tribal air agencies identify air quality data they believe have 

been influenced by an exceptional event. The proposed rule revisions also clarify those actions 

that state, local and tribal air agencies should take to protect public health during and following 

an exceptional event. Because affected air agencies would have discretion to implement controls 

on sources that may need to be regulated due to anthropogenic contribution in the area 
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determined to be influenced by an exceptional event, the EPA cannot predict the indirect effect 

of the rule on sources that may be small entities. 

D. Unfunded Mandates Reform Act 

 This action does not contain an unfunded mandate of $100 million or more as described in 

UMRA, 2 U.S.C. 1531–1538, and does not significantly or uniquely affect small governments. The 

action imposes no enforceable duty on any state, local or tribal governments or the private sector. 

E. Executive Order 13132: Federalism

 This action does not have federalism implications. The EPA believes, however, that this 

action may be of significant interest to states and to local air quality agencies to whom a state has 

delegated relevant responsibilities for air quality management. Consistent with the EPA’s policy 

to promote communications between the EPA and state and local governments, the EPA 

consulted with representatives of state and local governments early in the process of developing 

this action to permit them to have meaningful and timely input into its development. A summary 

of the concerns raised during that consultation is provided in section IV of this preamble. 

F. Executive Order 13175: Consultation and Coordination with Indian Tribal Governments 

This action does not have tribal implications as specified in Executive Order 13175. It 

would not have a substantial direct effect on one or more Indian tribes. Furthermore, these 

proposed regulation revisions do not affect the relationship or distribution of power and 

responsibilities between the federal government and Indian tribes. The CAA and the Tribal Air 

Rule establish the relationship of the federal government and tribes in characterizing air quality 

and developing plans to attain the NAAQS, and these revisions to the regulations do nothing to 

modify that relationship. Thus, Executive Order 13175 does not apply to this action. 
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Although Executive Order 13175 does not apply to this action, the EPA held public 

meetings attended by tribal representatives and separate meetings with tribal representatives to 

discuss the revisions proposed in this action. The EPA also provided an opportunity for all 

interested parties to provide oral or written comments on potential concepts for the EPA to 

address during the rule revision process. Summaries of these meetings are included in the docket 

for this proposed rule. The EPA specifically solicits additional comment on this proposed action 

from tribal officials. 

G. Executive Order 13045: Protection of Children from Environmental Health & Safety Risks 

The EPA interprets Executive Order 13045 as applying only to those regulatory actions 

that concern environmental health or safety risks that the EPA has reason to believe may 

disproportionately affect children, per the definition of “covered regulatory action” in section 2-

202 of the Executive Order. This action is not subject to Executive Order 13045 because it does 

not concern an environmental health risk or safety risk. 

H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy 

Supply, Distribution or Use 

This action is not a “significant energy action” because it is not likely to have a 

significant adverse effect on the supply, distribution or use of energy. The purpose of this 

proposed rule is to provide the criteria, and increase the efficiency of the process, by which state, 

local and tribal air agencies may identify air quality data they believe have been influenced by an 

exceptional event. The EPA does not expect these activities to affect energy suppliers, 

distributors or users. 

I. National Technology Transfer and Advancement Act 

This rulemaking does not involve technical standards. 
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J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations 

The EPA believes the human health or environmental risk addressed by this action will 

not have potential disproportionately high and adverse human health or environmental effects on 

minority, low-income or indigenous populations. The results of this evaluation are contained in 

the section of the preamble titled “Environmental Justice.” This proposed action provides the 

criteria and increases the efficiency of the process by which state, local and tribal air agencies 

identify air quality data they believe have been influenced by exceptional events, which by 

statutory definition, are not reasonably controllable or preventable. These proposed regulatory 

provisions do, however, provide information concerning actions that state, local or tribal air 

agencies might take to uniformly protect public health once the EPA has concurred with an air 

agency’s request to exclude data influenced by an exceptional event. The mitigation component 

of the proposed rule could ultimately provide additional protection for minority, low income and 

other populations located in areas affected by exceptional events. Therefore, the EPA finds that 

this proposed action would not adversely affect the health or safety of minority or low-income 

populations, and that it is designed to protect and enhance the health and safety of these and 

other populations. 

X. Statutory Authority

The statutory authority for this action is provided by 42 U.S.C. 7401, et seq. 

 
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Page 201 of 218 - Treatment of Data Influenced by Exceptional Events – Rule Revisions 

and Notice of Availability for Related Draft Guidance 

List of Subjects in 40 CFR Part 50 

Environmental protection, Air pollution control, National parks, Wilderness areas. 

__________________________________ 

Dated: 

__________________________________ 

Gina McCarthy, 

Administrator. 
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For the reasons set forth in the preamble, it is proposed that 40 CFR part 50 be amended as follows: 

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

1. The authority citation for part 50 continues to read as follows: 

Authority: 42 U.S.C. 7401, et seq. 

2. Amend §50.1 by: 

a. Revising paragraphs (j) and (k). 

b. Adding paragraphs (m), (n), (o), (p), (q) and (r). 

§50.1 Definitions. 

* * * * * 

(j) Exceptional event means an event and its resulting emissions that affect air quality in such a 

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

exceedance or violation, is not reasonably controllable or preventable, is an event caused by 

human activity that is unlikely to recur at a particular location or a natural event, and is 

determined by the Administrator in accordance with 40 CFR 50.14 to be an exceptional event. It 

does not include stagnation of air masses or meteorological inversions, a meteorological event 

involving high temperatures or lack of precipitation, or air pollution relating to source 

noncompliance. 

(k) Natural event means an event and its resulting emissions, which may recur, in which human 

activity plays little or no direct causal role. Anthropogenic sources that are reasonably controlled 

shall be considered to not play a direct role in causing emissions. 

***** 

(m) Prescribed fire is any fire intentionally ignited by management actions in accordance with 
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applicable laws, policies, and regulations to meet specific land or resource management 

objectives. 

(n) Wildfire is any fire started by an unplanned ignition caused by lightning; volcanoes; other 

acts of nature; unauthorized activity; or accidental, human-caused actions, or a prescribed fire 

that has been declared to be a wildfire. A wildfire that predominantly occurs on wildland is a 

natural event. 

(o) Wildland means an area in which human activity and development is essentially non-existent, 

except for roads, railroads, power lines, and similar transportation facilities. Structures, if any, 

are widely scattered. 

(p) High wind dust event is an event that includes the high-speed wind and the dust that the wind 

entrains and transports to a monitoring site. 

(q) High wind threshold is the minimum wind speed capable of causing particulate matter 

emissions from natural undisturbed lands in the area affected by a high wind dust event. 

(r) Federal land manager means, consistent with the definition in 40 CFR 51.301, the Secretary 

of the department with authority over the Federal Class I area (or the Secretary’s designee) or, 

with respect to Roosevelt-Campobello International Park, the Chairman of the RooseveltCampobello International Park Commission. 

3. Revise §50.14 to read as follows: 

§50.14 Treatment of air quality monitoring data influenced by exceptional events. 

(a) Requirements. (1) Scope. (i) This section applies to the treatment of data showing 

exceedances or violations of any national ambient air quality standard for purposes of the 

following types of regulatory determinations by the Administrator: 

(A) An action to designate an area, pursuant to Clean Air Act section 
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107(d)(1), or redesignate an area, pursuant to Clean Air Act section 107(d)(3), for 

a particular national ambient air quality standard; 

(B) 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 national ambient air quality standard, to 

the level of the relevant national ambient air quality standard; 

(C) A determination regarding whether a nonattainment area has attained 

the level of the appropriate national ambient air quality standard by its specified 

deadline; 

(D) A determination that an area has had only one exceedance in the year 

prior to its attainment deadline and thus qualifies for a 1-year attainment date 

extension, if applicable; and 

(E) A determination under Clean Air Act section 110(k)(5), if based on an 

area violating a national ambient air quality standard, that the state 

implementation plan is inadequate to the requirements of Clean Air Act section 

110. 

(ii) A State, federal land manager or other federal agency may request the Administrator 

to exclude data showing exceedances or violations of any national ambient air 

quality standard that are directly due to an exceptional event from use in 

determinations by demonstrating to the Administrator’s satisfaction that such 

event caused a specific air pollution concentration at a particular air quality 

monitoring location. 
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 (A) For a federal land manager or other federal agency to be eligible to 

initiate such a request for data exclusion, the federal land manager or other federal 

agency must (i) either operate a regulatory monitor that has been affected by an 

exceptional event or manage land on which an exceptional event occurred that 

influenced a monitored concentration at a regulatory monitor, and (ii) initiate such 

a request only after discussing such submittal with the State in which the affected 

monitor is located, and 

 (B) When initiating such a request, all provisions in this section that are 

expressed as requirements applying to a State shall, except as noted, be 

requirements applying to the federal land manager or other federal agency. 

(2) A demonstration to justify data exclusion may include any reliable and accurate data, but 

must specifically address the elements in paragraphs (c)(3)(iv) and (c)(3)(v) of this 

section. 

(b) Determinations by the Administrator. (1) Generally. The Administrator shall exclude data 

from use in determinations of exceedances and violations where a State demonstrates to the 

Administrator’s satisfaction that an exceptional event caused a specific air pollution 

concentration in excess of one or more national ambient air quality standards at a particular air 

quality monitoring location and otherwise satisfies the requirements of this section. 

(2) Fireworks displays. The Administrator shall exclude data from use in determinations of 

exceedances and violations where a State demonstrates to the Administrator’s satisfaction 

that emissions from fireworks displays caused a specific air pollution concentration in 

excess of one or more national ambient air quality standards at a particular air quality 

monitoring location and otherwise satisfies the requirements of this section. Such data will 
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be treated in the same manner as exceptional events under this rule, provided a State 

demonstrates that such use of fireworks is significantly integral to traditional national, 

ethnic, or other cultural events including, but not limited to, July Fourth celebrations that 

satisfy the requirements of this section. 

(3) Prescribed fires. (i) The Administrator shall exclude data from use in determinations of 

exceedances and violations, where a State demonstrates to the Administrator’s 

satisfaction that emissions from prescribed fires caused a specific air pollution 

concentration in excess of one or more national ambient air quality standards at a 

particular air quality monitoring location and otherwise satisfies the requirements 

of this section. 

(ii) In addressing the requirements set forth in paragraph (c)(3)(iv)(D) of this section 

regarding the not reasonably controllable or preventable criterion: 

(A) With respect to the requirement that a prescribed fire be not 

reasonably controllable, the State must either certify to the Administrator that it 

has adopted and is implementing a smoke management plan or the State must 

demonstrate that the burn manager employed the generally applicable basic 

smoke management practices identified in Table 1. To make the latter 

demonstration, the State may rely on a statement or other documentation provided 

by the burn manager that he or she employed those practices. If an exceptional 

event occurs using the basic smoke management practices approach, the State 

must undertake a review of its approach to ensure public health is being protected. 

(B) With respect to the requirement that a prescribed fire be not 

reasonably preventable, provided the Administrator determines that there is no 
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compelling evidence to the contrary in the record, the State may rely upon 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, but also provided that the use of prescribed fire in the 

area has not exceeded the frequency indicated in that plan. 

(iii) Provided the Administrator determines that there is no compelling evidence to the 

contrary in the record, in addressing the requirements set forth in paragraph 

(c)(3)(iv)(E) of this section regarding the human activity unlikely to recur at a 

particular location criterion for demonstrations involving prescribed fires on 

wildland, the State must describe the actual frequency with which a burn was 

conducted, but may rely upon and reference an assessment of the natural fire 

return interval or the prescribed fire frequency needed to establish, restore, and/or 

maintain a sustainable and resilient wildland ecosystem contained in a multi-year 

land or resource management plan with a stated objective to establish, restore, 

and/or maintain a sustainable and resilient wildland ecosystem and/or to preserve 

endangered or threatened species through a program of prescribed fire. 

Table 1. Summary of Basic Smoke Management Practices, benefit achieved with the 

BSMP, and when it is applied (before, during or after ignition of the burn).a

 

Basic Smoke 

Management Practice

Benefit achieved with the 

BSMP

When the BSMP is Applied 

– Before/During/After the 

Burn

Evaluate Smoke Dispersion 

Conditions 

Minimize smoke impacts Before, During, After 

Monitor Effects on Air 

Quality 

Be aware of where the smoke is 

going and degree it impacts air 

quality 

Before, During, After 

Record-Keeping/Maintain 

a Burn/Smoke Journal 

Retain information about the 

weather, burn and smoke. If air 

Before, During, After 
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Basic Smoke 

Management Practice

Benefit achieved with the 

BSMP

When the BSMP is Applied 

– Before/During/After the 

Burn

quality problems occur, 

documentation helps analyze and 

address air regulatory issues 

Communication – Public 

Notification 

Notify neighbors and those 

potentially impacted by smoke, 

especially sensitive receptors 

Before, During 

Consider Emission 

Reduction Techniques 

Reducing emissions through 

mechanisms such as reducing 

fuel loading can reduce 

downwind impacts 

Before, During, After 

Share the Airshed – 

Coordination of Area 

Burning 

Coordinate multiple burns in the 

area to manage exposure of the 

public to smoke 

Before, During, After 

a

 Elements of these BSMP could also be practical and beneficial to apply to wildfires for areas likely to 

experience recurring wildfires. 

(4) Wildfires. The Administrator shall exclude data from use in determinations of exceedances 

and violations where a State demonstrates to the Administrator’s satisfaction that 

emissions from wildfires caused a specific air pollution concentration in excess of one or 

more national ambient air quality standard at a particular air quality monitoring location 

and otherwise satisfies the requirements of this section. Provided the Administrator 

determines that there is no compelling evidence to the contrary in the record, the 

Administrator will determine every wildfire occurring predominantly on wildland to have 

met the requirements identified in paragraph (c)(3)(iv)(D) of this section regarding the 

not reasonably controllable or preventable criterion. 

(5) High wind dust events. (i) The Administrator shall exclude data from use in determinations of 

exceedances and violations, where a State demonstrates to the Administrator’s 

satisfaction that emissions from a high wind dust event caused a specific air 

pollution concentration in excess of one or more national ambient air quality 
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standards at a particular air quality monitoring location and otherwise satisfies the 

requirements of this section provided that such emissions are from high wind dust 

events. 

 (ii) The Administrator will consider high wind dust events to be natural events in cases 

where windblown dust is entirely from undisturbed natural lands or where all 

anthropogenic sources are reasonably controlled as determined in accordance with 

§50.14(b)(7). 

 (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. 

(iv) In addressing the requirements set forth in paragraph (c)(3)(iv)(D) of this section 

regarding the not reasonably preventable criterion, the State shall not be required 

to provide a case-specific justification for a high wind dust event. 

(v) With respect to the not reasonably controllable criterion of (c)(3)(iv)(D), dust controls 

on an anthropogenic source shall be considered reasonable in any case in which 

the controls render the anthropogenic source as resistant to high winds as a natural 

undisturbed land area. The Administrator may determine lesser controls 

reasonable on a case-by-case basis. 

(vi) For remote, large-scale, high-energy and/or sudden high wind dust events, such as 

“haboobs” in the southwest, the Administrator will generally consider a 

demonstration documenting the nature and extent of the event to be sufficient 
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with respect to the not reasonable controllable criterion of (c)(3)(iv)(D). 

(6) Determinations with respect to event aggregation and multiple national ambient air quality 

standards for the same pollutant. (i) Where a State demonstrates to the 

Administrator’s satisfaction that for national ambient air quality standards with 

averaging or cumulative periods longer than 24-hours the aggregate effect of 

events occurring on different days has caused an exceedance or violation, the 

Administrator shall determine such collective data to satisfy the requirements in 

paragraph (c)(3)(iv)(B) of this section regarding the clear causal relationship 

criterion and otherwise satisfies the requirements of this section. 

(ii) The Administrator shall accept as part of a demonstration for the clear causal 

relationship in paragraph (c)(3)(iv)(B) of this section, a State’s comparison of a 

24-hour concentration of any national ambient air quality standard pollutant to the 

level of a national ambient air quality standard for the same pollutant with a 

longer averaging period. 

(7) Determinations with respect to the not reasonably controllable or preventable criterion. (i) 

The Administrator shall determine that an event is not reasonably preventable if 

the State shows that reasonable measures to prevent the event were applied at the 

time of the event. 

 (ii) The Administrator shall determine that an event is not reasonably controllable if the 

State shows that reasonable measures to control the impact of the event on air 

quality were applied at the time of the event. 

 (iii) The Administrator shall assess the reasonableness of available controls for 

anthropogenic sources based on information available as of the date of the event. 
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 (iv) Except where a State is obligated to revise its state implementation plan, the 

Administrator shall consider enforceable control measures implemented in 

accordance with a state implementation plan, approved by the EPA within 5 years 

of the date of a demonstration submittal, that address the event-related pollutant 

and all sources necessary to fulfill the requirements of the Clean Air Act for the 

state implementation plan to be reasonable controls with respect to all 

anthropogenic sources that have or may have contributed to event-related 

emissions. 

 (v) The Administrator shall not require a State to provide case-specific justification to 

support the not reasonably controllable or preventable criterion for emissionsgenerating activity that occurs outside of the State’s jurisdictional boundaries 

within which the concentration at issue was monitored. In the case of a tribe with 

treatment as a state status with respect to exceptional events requirements, the 

tribe’s jurisdictional boundaries for purposes of requiring or directly 

implementing emission controls apply. 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. 

(c) Schedules and procedures. (1) Public notification. (i) All States and, where applicable, their 

political subdivisions must notify the public promptly whenever an event occurs or is reasonably 

anticipated to occur which may result in the exceedance of an applicable air quality standard. 

 (ii) [Reserved] 

(2) Initial Notification of Potential Exceptional Event. 
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 (i) A State shall notify the Administrator of its intent to request exclusion of one or more 

measured exceedances of an applicable national ambient air quality standard as 

being due to an exceptional event by creating an initial event description and 

flagging the associated data that have been submitted to the AQS database and by 

engaging in the Initial Notification of Potential Exceptional Event process as 

follows: 

 (A) The State and the appropriate EPA regional office shall engage in 

regular communications to identify those data that have been potentially 

influenced by an exceptional event, to determine whether the identified data may 

affect a regulatory determination and to discuss whether the State should develop 

and submit an exceptional events demonstration according to the requirements in 

this section; 

 (B) For data that may affect an anticipated regulatory determination or 

where circumstances otherwise compel the Administrator to prioritize the 

resulting demonstration, the Administrator shall respond to a State’s Initial 

Notification of Potential Exceptional Event with a due date for demonstration 

submittal that considers the nature of the event and the anticipated timing of the 

associated regulatory decision; 

 (C) The Administrator may waive the Initial Notification of Potential 

Exceptional Event process on a case-by-case basis. 

 (ii) The data shall not be excluded from determinations with respect to exceedances or 

violations of the national ambient air quality standards unless and until, following 

the State’s submittal of its demonstration pursuant to paragraph (c)(3) of this 
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section and the Administrator’s review, the Administrator notifies the State of its 

concurrence by placing a concurrence flag in the appropriate field for the data 

record in the AQS database. 

 (iii) [Reserved] 

 (iv) [Reserved] 

 (v) [Reserved] 

 (vi) Table 2 identifies the data submission process for a new or revised national ambient 

air quality standard. This process shall apply to those data that will or may 

influence the initial designation of areas for any new or revised national ambient 

air quality standard. 

Table 2. Schedule for Flagging and Documentation Submission for Data Influenced by 

Exceptional Events for Use in Initial Area Designations 

Exceptional Events / Regulatory 

Action Exceptional Events Deadline Scheduled

Flagging and initial event description 

deadline for data years 1, 2 and 3.a

 

If state and tribal initial designation 

recommendations for a new/revised NAAQS 

are due August through January, then the 

flagging and initial event description 

deadline will be the July 1 prior to the 

recommendation deadline. If state and tribal 

recommendations for a new/revised NAAQS 

are due February through July, then the 

flagging and initial event description 

deadline will be the January 1 prior to the 

recommendation deadline. 

Exceptional events demonstration 

submittal deadline for data years 1, 2 

and 3.a

 

No later than the date that state and tribal 

recommendations are due to EPA. 

Flagging, initial event description and 

exceptional events demonstration 

submittal deadline for data year 4b

 and, 

where applicable, data year 5.c

By the last day of the month that is 1 year 

and 7 months after promulgation of a 

new/revised NAAQS, unless either option a 

or b applies. 

a. If the EPA follows a 3-year designation 

schedule, the deadline is 2 years and 7 
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months after promulgation of a 

new/revised NAAQS. 

b. If the EPA notifies the state/tribe that it 

intends to complete the initial area 

designations process according to a 

schedule between 2 and 3 years, the 

deadline is 5 months prior to the date 

specified for final designations decisions 

in such EPA notification. a Where data years 1, 2, and 3 are those years expected to be considered in state and tribal 

recommendations. 

b Where data year 4 is the additional year of data that the EPA may consider when it makes final 

area designations for a new/revised NAAQS under the standard designations schedule. c Where data year 5 is the additional year of data that the EPA may consider when it makes final 

area designations for a new/revised NAAQS under an extended designations schedule. 

d The date by which air agencies must certify their ambient air quality monitoring data in AQS 

is annually on May 1 of the year following the year of data collection as specified in 40 CFR 

58.15(a)(2). In some cases, however, air agencies may choose to certify a prior year's data in 

advance of May 1 of the following year, particularly if the EPA has indicated its intent to 

promulgate final designations in the first 8 months of the calendar year. Data flagging, initial 

event description and exceptional events demonstration deadlines for “early certified” data 

will follow the deadlines for “year 4” and “year 5” data. 

(3) Submission of demonstrations. (i) Except as allowed under paragraph (c)(2)(vi) of this 

section, a State that has flagged data as being due to an exceptional event and is 

requesting exclusion of the affected measurement data shall, after notice and 

opportunity for public comment, submit a demonstration to justify data exclusion

to the Administrator according to the schedule established under paragraph 

(c)(2)(i)(B). 

 (ii) [Reserved] 

 (iii) [Reserved] 

 (iv) The demonstration to justify data exclusion must include: 

 (A) 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 at the affected monitor(s); 
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 (B) 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; 

 (C) Analyses identified in Table 3 comparing the claimed eventinfluenced concentration(s) to concentrations at the same monitoring site at other 

times consistent with Table 3 to support the requirement at (3)(iv)(B) of this 

paragraph. The Administrator shall not require a State to prove a specific 

percentile point in the distribution of data; 

 (D) A demonstration that the event was both not reasonably controllable 

and not reasonably preventable; and 

 (E) A demonstration that the event was a human activity that is unlikely to 

recur at a particular location or was a natural event. 

 (v) With the submission of the demonstration containing the elements in (c)(3)(iv), the 

State must: 

 (A) Document that the public comment process was followed and that the 

comment period was open for a minimum of 30 days, which could be concurrent 

with the Administrator’s review of the associated demonstration provided the 

State can meet all requirements in this paragraph; 

 (B) Submit the public comments it received along with its demonstration 

to the Administrator; and 

 (C) Address in the submission to the Administrator those comments 

disputing or contradicting factual evidence provided in the demonstration. 

 (vi) Where the State has submitted a demonstration according to the requirements of this 
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section and the Administrator has reviewed such demonstration and requested 

additional evidence to support one of the elements in (c)(3)(iv), the State shall 

have 12 months from the date of the Administrator’s request to submit such 

evidence. At the conclusion of this time, if the State has not submitted the 

requested additional evidence, the Administrator will consider the demonstration 

to be inactive and will not pursue additional review of the demonstration. After a 

12-month period of inactivity, if a State desires to pursue the inactive 

demonstration, it must reinitiate its request to exclude associated data by 

following the process beginning with paragraph (c)(2)(i). 

Table 3. Evidence and Analyses for the Comparison to Historical Concentrations 

Historical 

Concentration 

Evidence 

Types of Analyses/Supporting Information Required or 

Optional? 

1. Comparison of 

concentrations on the 

claimed event day with 

past historical data 

Seasonal (appropriate if exceedances occur 

primarily in one season, but not in others) 

 Use all available seasonal data over the 

previous 5 years (or more, if available) 

 Discuss the seasonal nature of pollution for 

the location being evaluated 

 Present monthly maximums of the NAAQS 

relevant metric (e.g., maximum daily 8-

hour average ozone or 1-hr SO2) vs 

monthly or other averaged daily data as this 

masks high values 

Annual (appropriate if exceedances are likely 

throughout the year) 

 Use all available data over the previous 5 

years (or more, if available) 

Seasonal and Annual Analyses 

 Provide the data in the form relevant to the 

standard being considered for data 

exclusion 

 Label “high” data points as being 

associated with concurred exceptional 

events, suspected exceptional events, other 

unusual occurrences, or high pollution days 

due to normal emissions 

Required seasonal 

and/or annual 

analysis (depending 

on which is more 

appropriate)
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Historical 

Concentration 

Evidence 

Types of Analyses/Supporting Information Required or 

Optional? 

 Describe how emission control strategies 

have decreased pollutant concentrations 

over the 5-year window, if applicable 

 Include comparisons omitting known or 

suspected exceptional events points, if 

applicable 

2. Comparison of 

concentrations on the 

claimed event day with 

a narrower set of 

similar days 

 Include neighboring days at the same 

location (e.g., a time series of two to three 

weeks) and/or other days with similar 

meteorological conditions (possibly from 

other years) at the same or nearby locations 

with similar historical air quality along with 

a discussion of the meteorological 

conditions during the same timeframea

 Use this comparison to demonstrate that the 

event caused higher concentrations than 

would be expected for given meteorological 

and/or local emissions conditions 

Optional analysis 

3. Percentile rank of 

concentration when 

compared to annual 

datab

 Provide the percentile rank of the event-day 

concentration relative to all measurement 

days over the previous 5 years to ensure 

statistical robustness and capture non-event 

variability over the appropriate seasons or 

number of years.c

 Use the daily statistic (e.g., 24-hour 

average, maximum daily 8-hour average, or 

maximum 1-hour) appropriate for the form 

of the standard being considered for data 

exclusion 

Required analysis 

when comparison is 

made on an annual 

basis (see item #1) 

4. Percentile rank of 

concentration relative 

to seasonal datab

 Provide the percentile rank of the event-day 

concentration relative to all measurement 

days for the season (or appropriate 

alternative 3-month period) of the event 

over the previous 5 years 

 Use the same time horizon as used for the 

percentile rank calculated relative to annual 

data, if appropriate 

Required analysis 

when comparison is 

made on a seasonal 

basis (see item #1) 

a

 If an air agency compares the concentration on the claimed event day with days with similar 

meteorological conditions from other years, the agency should also verify and provide evidence 

that the area has not experienced significant changes in wind patterns, and that no significant 

sources in the area have had significant changes in their emissions of the pollutant of concern. 
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b

 The EPA does not intend to identify a particular historical percentile rank point in the seasonal 

or annual historical data that plays a critical role in the analysis or conclusion regarding the clear 

causal relationship. 

c

 Section 8.4.2.e of appendix W (proposed revisions at 80 FR 45374, July 29, 2015) recommends 

using 5 years of adequately representative meteorology data from the National Weather Service 

to ensure that worst-case meteorological conditions are represented. Similarly, for exceptional 

events purposes, the EPA believes that 5 years of ambient air data, whether seasonal or annual, 

better represent the range of “normal” air quality than do shorter periods.