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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– ] 

RIN 2060-AS02 

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

Availability for Related Draft Guidance 

******************* 

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. In §50.1: 

a. Revise paragraphs (j) and (k). 

b. Add paragraphs (m), (n), (o), (p), (q) and (r). 

The revisions and additions read as follows: 

§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 
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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 

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 
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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. Amend § 50.14, as amended on October 26, 2015, at 80 FR 65452, effective December 28, 

2015, as follows: 

a. Revise paragraphs (a) and (b); 

b. Revise paragraphs (c)(1), (c)(2)(i) through (v), and (c)(3). 

The revisions 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 

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 
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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 EPA 

Administrator to exclude data showing exceedances or violations of any the

national ambient air quality standard that are directly due to an exceptional event 

from use in determinations by demonstrating to the EPA’s Administrator’s 

satisfaction that such event caused a specific air pollution concentration at a 

particular air quality monitoring location. 

 (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: (1) Either operate a regulatory monitor that has been affected by an 

exceptional event or manage land on which an exceptional event occurred that 
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influenced a monitored concentration at a regulatory monitor; and (2) 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 dDemonstration to justify data exclusion may include any reliable and accurate data, but 

must specifically address the elements in demonstrate a clear causal relationship between 

the measured exceedance or violation of such standard and the event in accordance with 

paragraphs (c)(3)(iv) and (v) of this section.

(b) Determinations by EPAthe Administrator. (1) Generally. EPA The Administrator shall 

exclude data from use in determinations of exceedances and NAAQS violations where a State 

demonstrates to EPA 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. EPA The Administrator shall exclude data from use in determinations of 

exceedances and NAAQS violations where a State demonstrates to EPA 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 be treated in the same manner as exceptional events under this rule, 
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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 which that satisfy the requirements of this section. 

(3) Prescribed fires. (i) EPA The Administrator shall exclude data from use in determinations of 

exceedances and NAAQS violations, where a State demonstrates to EPA’s 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. provided that such emissions are from prescribed 

fires that EPA determines meets the definition in §50.1(j), and. 

(ii) Provided that tIn 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 has certified must either certify to EPA that it 

has adopted and is implementing a SMP smoke management plan or the State has 

ensured that must demonstrate that the burner manager employed the generally 

applicable basic smoke management practices. identified in Table 2 to § 50.14. 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 
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public health is being protected and must include consideration of development of 

a SMP. 

(B) With respect to the requirement that a prescribed fire be not 

reasonably preventable, provided the Administrator determines that there is no 

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. 
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(4)[Reserved] 

Table 2 to § 50.14. Summary of Basic Smoke Management Practices, benefit achieved with 

the BSMP, and when it is applied before, during or after ignition of the burna 

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. 

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 
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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 

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 

paragraph (b)(7) of this section. 

 (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 
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to provide a case-specific justification for a high wind dust event. 

(v) With respect to the not reasonably controllable criterion of paragraph (c)(3)(iv)(D) of 

this section, 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 

with respect to the not reasonable controllable criterion of paragraph (c)(3)(iv)(D) 

of this section. 

(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 
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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. 

 (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 
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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) Flagging of data.Initial notification of potential exceptional event. 

 (i) A State shall notify EPA of its intent to exclude 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: 
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 (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. placing a flag in the 

appropriate field for the data record of concern which has been submitted to the 

AQS database. 

 (ii) Flags placed on data in accordance with this section shall be deemed informational 

only, and tThe 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 the EPA Administrator’s review, EPA the Administrator
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notifies the State of its concurrence by placing a concurrence flag in the 

appropriate field for the data record in the AQS database. 

 (iii) Flags placed on data as being due to an exceptional event together with an initial 

description of the event shall be submitted to EPA not later than July 1st of the 

calendar year following the year in which the flagged measurement occurred, 

except as allowed under paragraph (c)(2)(iv) or (c)(2)(v) (c)(2)(vi) of this section.

[Reserved]

 (iv) [Reserved]For PM2.5 data collected during calendar years 2004-2006, that the State 

identifies as resulting from an exceptional event, the State must notify EPA of the 

flag and submit an initial description of the event no later than October 1, 2007. 

EPA may grant an extension, if a State requests an extension, and permit the State 

to submit the notification of the flag and initial description by no later than 

December 1, 2007. 

 (v) [Reserved]For lead (Pb) data collected during calendar years 2006-2008, that the 

State identifies as resulting from an exceptional event, the State must notify EPA 

of the flag and submit an initial description of the event no later than July 1, 2009. 

For Pb data collected during calendar year 2009, that the State identifies as 

resulting from an exceptional event, the State must notify EPA of the flag and 

submit an initial description of the event no later than July 1, 2010. For Pb data 

collected during calendar year 2010, that the State identifies as resulting from an 

exceptional event, the State must notify EPA of the flag and submit an initial 
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description of the event no later than May 1, 2011. 

 (vi) When EPA sets a NAAQS for a new pollutant or revises the NAAQS for an existing 

pollutant, it may revise or set a new schedule for flagging exceptional event data, 

providing initial data descriptions and providing detailed data documentation in 

AQS for the initial designations of areas for those NAAQS. Table 1 provides the 

schedule for submission of flags with initial descriptions in AQS and detailed 

documentation. These schedules shall apply for those data which will or may 

influence the initial designation of areas for those NAAQS. EPA anticipates 

revising Table 1 as necessary to accommodate revised data submission schedules 

for new or revised NAAQS. Table 1 identifies the data submission process for 

new or revised NAAQS. This process shall apply to those data that will or may 

influence the initial designation of areas for any new or revised NAAQS. 

 
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Table 1. 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 

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. 
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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. 

[Note: the above table does not appear in the Federal Register. The promulgated 2015 Ozone 

NAAQS at 80 FR 65292 replaced Table 1 in its entirety. For ease of reading, we are not 

including the previous Table 1 in this redlined/strikeout document.] 

 (3) Submission of demonstrations. (i) Except as allowed under paragraph (c)(2)(vi) of this 

section, aA 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 EPA the Administrator according to the schedule established under paragraph 

(c)(2)(i)(B). 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. A State must submit the 

public comments it received along with its demonstration to EPA. 

 (ii) [Reserved] A State that flags data collected during calendar years 2004-2006, 

pursuant to paragraph (c)(2)(iv) of this section, must adopt the procedures and 

requirements specified in paragraph (c)(3)(i) of this section and must include a 

demonstration to justify the exclusion of the data not later than the submittal of 

the Governor’s recommendation letter on nonattainment areas. 

 (iii) [Reserved] A State that flags Pb data collected during calendar years 2006-2009, 

pursuant to paragraph (c)(2)(v) of this section shall, after notice and opportunity 
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for public comment, submit to EPA a demonstration to justify the exclusion of the 

data not later than October 15, 2010. A State that flags Pb data collected during 

calendar year 2010 shall, after notice and opportunity for public comment, submit 

to EPA a demonstration to justify the exclusion of the data not later than May 1, 

2011. A state must submit the public comments it received along with its 

demonstration to EPA. 

 (iv) The demonstration to justify data exclusion shall provide evidence thatmust include: 

 (A) The event satisfies the criteria set forth in 40 CFR 50.1(j)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); 

 (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 violationThere is a clear causal relationship between the 

measurement under consideration and the event that is claimed to have affected 

the air quality in the area; 

 (C) Analyses identified in Table 3 to § 50.14 comparing the claimed 

event-influenced concentration(s) to concentrations at the same monitoring site at 

other times consistent with Table 3 to § 50.14 to support the requirement at 

paragraph (c)(3)(iv)(B) of this section. The Administrator shall not require a State 

to prove a specific percentile point in the distribution of data.The event is 
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associated with a measured concentration in excess of normal historical 

fluctuations, including background; and; 

 (D) There would have been no exceedance or violation but for the event. 

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 paragraph 

(c)(3)(iv) of this section, 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 

section and the Administrator has reviewed such demonstration and requested 

additional evidence to support one of the elements in paragraph (c)(3)(iv) of this 

section, the State shall have 12 months from the date of the Administrator’s 
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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 in paragraph (c)(2)(i) of this section.

Table 3 to § 50.14. 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 timeframe.a

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 

data.b

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 data.b

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 
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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. 

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. 

PART 51—REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL 

OF IMPLEMENTATION PLANS 

4. The authority citation for part 51 continues to read as follows: 

Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q. 

§51.930 Mitigation of Exceptional Events. 

(a) A State requesting to exclude air quality data due to exceptional events must take appropriate 

and reasonable actions to protect public health from exceedances or violations of the 

national ambient air quality standards. At a minimum, the State must: 

 (1) Provide for prompt public notification whenever air quality concentrations exceed or 

are expected to exceed an applicable ambient air quality standard; 

 (2) 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; and 

 (3) Provide for the implementation of appropriate measures to protect public health from 

exceedances or violations of ambient air quality standards caused by exceptional 
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events. 

 (b) [Reserved]