Draft for Public Notice and Comment 

Revision Date: June 2012

Page 1 of 46 

ATTACHMENT 1 

Draft Exceptional Events Rule Frequently Asked Questions 

The Exceptional Events Rule of 20071 supersedes the EPA’s previous Exceptional Events 

guidance and policy documents and creates a regulatory process codified at 40 CFR parts 50 

and 51 (50.1, 50.14 and 51.930). The Exceptional Events Rule (EER) recognizes that each 

potential event can have different or unique characteristics, and thus, requires a case-by-case 

demonstration and evaluation. Therefore, the EER adopts a “weight-of-evidence” approach 

in evaluating each demonstration to justify excluding data affected by an exceptional event. 

The EPA acknowledges that certain extreme2

 exceptional event cases may require more 

limited demonstration packages. 

Technical questions and issues related to implementation have arisen since the EPA 

promulgated the EER. This Question and Answer (Q&A) document is intended to respond to 

some of these frequently asked questions and to provide instruction and clarification to air 

agencies3

 implementing the EER. The EPA recognizes the limited resources of the air 

agencies that prepare and submit exceptional event demonstration packages and of the EPA 

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

developing exceptional event implementation guidance is to establish clear expectations to 

enable affected agencies to better manage resources as they prepare the documentation 

required under the EER. The EPA expects submitters to prepare and submit the appropriate 

level of supporting documentation, which will vary on a case-by-case basis using the weightof-evidence approach. The EPA expects the resources required to prepare (and review) 

packages to decrease as we continue to identify ways to streamline the process and continue 

to build our database of example demonstrations and analyses. In addition, as noted above, 

the EPA acknowledges that certain extreme exceptional event cases may require more 

limited demonstration packages. 

For organizational ease, this document has been divided into the following topical sections: 

A. Historical Fluctuations 

B. “But For” Test 

C. Exceptional Event Data Flagging Schedules 

D. General AQS Procedures 

E. General Exceptional Events Rule Applicability and Implementation Issues 

 

1

 “Treatment of Data Influenced by Exceptional Events; Final Rule,” 72 FR at 13563, March 22, 2007. 

2

 The EPA acknowledges that certain extreme exceptional event cases may require more limited demonstration 

packages. Whether a particular event should be considered “extreme” for this purpose depends on the type and 

severity of the event, pollutant concentration, spatial extent, temporal extent, and proximity of the event to the 

violating monitor. Several meteorological phenomena that could be considered extreme events include 

hurricanes, tornadoes, haboobs, and catastrophic volcanic eruptions. The EPA addresses “extreme” high wind 

dust events in Question 17a in this document, but solicits comment on whether and how specific events of 

various types should be considered to be “extreme.” 

3

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

implementing the EER. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 2 of 46 

F. Exceptional Event Data Flagging for Air Quality Concentrations that Could 

Contribute to an Exceedance or Violation of the National Ambient Air Quality 

Standards 

Each section contains related questions. Readers of this document can find additional 

information at the EPA’s Exceptional Events website located at 

http://www.epa.gov/ttn/analysis/exevents.htm. The EPA’s draft guidance documents and the 

exceptional events website present examples to illustrate specific points. The example 

analyses and level of rigor are not necessarily required for all demonstrations. 

Disclaimer 

The Exceptional Events Rule is the source of the regulatory requirements for exceptional 

events and exceptional event demonstrations. This Q&A document provides guidance and 

interpretation of the Exceptional Events Rule rather than imposing any new requirements and 

shall not be considered binding on any party. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 3 of 46 

A. Historical Fluctuations 

40 CFR 50.14(c)(3)(iv): “The demonstration to justify data exclusion shall provide 

evidence that: 

* * * 

(C) The event is associated with a measured concentration in excess of normal 

historical fluctuations, including background; 

1. Question: Is the Exceptional Events Rule demonstration requirement to provide 

evidence to support “a measured concentration in excess of normal historical fluctuations, 

including background” a test that can be “passed” or “failed” based on the outcome of the 

statistical comparison? For example, must the concentration affected by an event exceed 

a specific percentile point in the historical data? 

Answer: It is a test, but there is no specific percentile point that the EPA will use to 

determine whether the test has been passed. The EPA will use a weight-of-evidence 

approach to review each demonstration on a case-by-case basis. The air agency’s role in 

satisfying this element is to provide appropriate analyses and statistics and conclude that 

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

EPA will review the information provided by the air agency. “Normal historical 

fluctuations” will generally be defined by those days without events for the previous 

years. The EPA acknowledges that natural events can recur and still be eligible for 

exclusion under the EER; therefore, events do not necessarily have to be rare to satisfy 

this element. 

 

The submittal of data showing how the event concentration compared with historical 

concentrations will help the EPA determine whether the “clear causal relationship,” “but 

for,” and “affects air quality” criteria have been satisfied. These EER criteria, as well as 

“not reasonably controllable or preventable,” need to be satisfied for EPA to concur on an 

exceptional event claim. The EPA expects that failure on this element indicates likely 

failure for “clear causal relationship” and/or “but for” as well. However, failure to submit 

a comparison would prevent the EPA from being able to approve exclusion of the data in 

question. 

The EPA recommends that each “historical fluctuation” demonstration submittal contain 

a minimum set of statistical analyses described in more detail in subsequent questions. 

Submission of the identified statistical analyses will be considered to have met the 

requirement to “provide evidence.” 

It is important to note, however, that there is no outcome of the “historical fluctuation” 

statistical comparison that, by itself, can guarantee that the clear causal relationship and 

“but for” elements will also be successfully demonstrated. The EPA will consider in its 

weight-of-evidence approach the comparison of the concentrations during event(s) in 

question with historical concentration data. For example, a uniquely high concentration in 

an area (and season) with no previous exceedances, with a clear causal connection, and 

with no evidence of any other plausible explanation would be a case in which the weight-
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 4 of 46 

of-evidence would indicate that the “but for” criterion has been demonstrated. In contrast, 

if the event-affected concentration does not stand out much from normally occurring 

exceedance concentrations for the same place and season, the statistical comparison will 

not by itself provide much support for “but for” in the weight-of-evidence consideration. 

2. Question: What evidence does the EPA want included in the demonstration as part of a 

comparison of a measured concentration with normal historical fluctuations, including 

background?

Answer: The EPA would prefer an analysis showing how the observed concentration 

compares to the distribution of historical concentrations. To speed the EPA review 

process, avoid the need for the EPA to request additional information, and ensure that the 

EPA understands the position of the submitting agency; this analysis should consist of the 

following types of statistics, graphics, and explanatory text: 

 Comparison of concentrations on the claimed event day with past historical data (see 

Question 3 for additional detail). The historical comparisons can be made on an annual 

and/or seasonal basis, depending on which is more appropriate. For example, if PM or 

ozone data at the location show clear seasonality (i.e., exceedances are nonexistent or 

extremely rare in some seasons but not others, or concentrations vary according to season 

due to meteorological conditions), discussing that information in the demonstration is 

likely appropriate. In contrast, if exceedances can be expected throughout the year, 

analysis of annual data would likely be more appropriate. For seasonal comparisons, the 

EPA recommends using all available seasonal data from 3-5 years (or more, if available). 

The analysis should discuss the seasonal nature of pollution for the location being 

evaluated. Depending on the quantity of data, it may be appropriate to present monthly 

maximums; however, it is not appropriate to present monthly-averaged daily data or any 

other average of the daily data as this masks high values. Regardless of whether seasonal 

or annual data are presented, all data should be provided in the form relevant to the 

standard that is being considered for data exclusion (see Question 30). Specific examples 

of analyses of annual and seasonal data, as well as analyses of historical speciated PM2.5

fluctuations and spatial distribution fluctuations are included in the presentation located 

at http://www.epa.gov/ttn/analysis/docs/IdeasforShowingEEEvidence.ppt. Examples of 

graphics are also included in the response to Question 3. 

Additionally, it may be useful for the comparison of concentrations on the claimed event 

day with past historical data to label appropriate data points as being associated with 

concurred exceptional events, suspected exceptional events, or other unusual occurrences. 

As additional evidence to use in interpreting the data, it may also be useful to include 

comparisons omitting such points. The intent of these comparisons is to present a time 

series of concentration data for the event area, thereby giving a full and accurate portrayal 

of the historical context for the claimed event day. 

 Comparison of concentrations on the claimed event day with a narrower set of similar 

days: Similar days could include neighboring days (e.g., a time series of two weeks) and 

other days with similar meteorological conditions (possibly from other years). The 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 5 of 46 

objective of such a comparison would be to demonstrate that the event caused higher 

concentrations than would be expected for given meteorological and/or local emissions 

conditions. 

 Percentile of concentration relative to annual data. The percentile of the event-day 

concentration should be provided for the event day relative to all measurement days over 

the previous 3-5 years. To ensure statistical robustness, the EPA expects a minimum of 

300 data points to be included in this calculation. The daily statistic should be appropriate 

for the form of the standard being considered for data exclusion (see Question 30). 

 Percentile of concentration relative to seasonal data. The percentile of the event-day 

concentration should be provided for the event day relative to all measurement days for 

the season (or appropriate alternative 3-month period) of the event over the previous 3-5 

years. It is appropriate to use the same time horizon as used for the percentile calculated 

relative to annual data. 

(Note: The use of percentiles is illustrative and should not be seen as a bright line to be 

passed or failed when comparing observed concentrations with historical values.)

3. Question: How will the submitted “historical fluctuations” evidence be considered when 

the EPA assesses whether the “but for” and “clear causal relationship” criteria are met? 

Answer: The EPA will review the submitted analyses showing how the observed 

concentration compares to the distribution of historical concentrations to determine 

whether the event is associated with a measured concentration in excess of normal 

historical fluctuations and will assess the other criteria, in part, based on this historical 

fluctuations comparison. When the observed concentration is higher than all or nearly all 

normal historical concentrations (i.e., concentrations when there was not an event), the 

EPA may need less additional evidence to demonstrate the “but for” finding. When the 

concentration is similar to or lower than a large number of normal historical values, the 

EPA may want additional evidence (e.g., PM or VOC speciation data) to support the “but 

for” and “clear causal relationship” demonstration requirements. The additional evidence 

will help differentiate the concentration increment caused by the event in question from 

other, non-event causes. 

Stated another way, the EPA’s intended use of the data is to review the historical 

fluctuations prong, which may influence how much information of other types is needed 

to successfully meet the other demonstration criteria (i.e., “but for” and “clear causal 

relationship”) of 40 CFR § 50.14 based, in part, on the degree to which the measured 

concentration is in excess of normal historical fluctuations. 

Submitting agencies are encouraged to discuss available historical fluctuation evidence 

with the appropriate EPA regional office prior to submitting the event demonstration 

package to determine if specific information might assist in the review process. 

 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 6 of 46 

Additional Examples and Explanation Concerning “Historical Fluctuations” Evidence

(Note: The discussion and graphics that follow illustrate the type of analyses and 

discussion that are described in this question and in Question 2 and that might be 

included in a submittal showing that an event is associated with a measurement “in 

excess of normal historical fluctuations.”) 

The evidence comparing the event-affected concentration with historical concentrations 

is most helpful to an air agency’s demonstration if it shows that the event-affected 

concentration is high compared to all, or nearly all, historical concentrations generated by 

normal emissions and ambient conditions. This scenario makes it more plausible that the 

event caused the observed excess concentration rather than that some other causal event 

occurred on the same day as the known event. If similar events have been very rare in the 

past, it may be possible to make this point by labeling appropriate data points as being 

associated with concurred exceptional events, suspected exceptional events, or other 

unusual occurrences. To facilitate the EPA’s understanding of the impact of these events, 

air agencies may also include comparisons omitting such points. 

The following figures demonstrate the concept of seasonal emissions fluctuations. The 

first figure shows an exceedance level PM2.5 value in late spring that is outside the range 

of the 3 to 5-year historical data set for non-wintertime PM2.5, while the second figure 

shows a similar data value for a different part of the country where similar exceedance 

concentrations occur throughout the year, suggesting that some non-event process(es) can 

cause high concentrations all during the year. In the first case, a seasonal assessment of 

historical fluctuations would be appropriate, while annualized data analysis might be 

more appropriate for the second case to provide the most robust yet also representative 

historical data set. 

Historical Seasonal Fluctuations in PM2.5, Seasonal Data, 2005-2009

0

10

20

30

40

50

60

70

1 92 183 274 365

Day of Year

24-hour PM2.5, ug/m3

1st Quarter 2nd Quarter 3rd Quarter 4th Quarter

Flagged Value Potentially Outside 

Normal Seasonal Historical Fluctuations
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 7 of 46 

4. Question: The Preamble to the EER states that less documentation or evidence may be 

needed to demonstrate that an event affected air quality for flagged data > 95th percentile 

than for values > 75th percentile. For ozone, PM10 and 24-hour PM2.5, in areas near the 

standard, exceedances are often near or above the 95th percentile of historical data. In 

these cases, will the EPA accept less documentation to demonstrate that an event affected 

air quality simply because an event-affected concentration is above the 95th percentile of 

the historical concentrations? 

Answer: The preamble statement paraphrased in the question above was intended to 

address National Ambient Air Quality Standards (NAAQS) that are based on averaging 

periods of many days, such as annual, quarterly and/or 3-month rolling average NAAQS. 

NAAQS with 1-hour, 8-hour or 24-hour averaging periods only allow a small percentage 

of days to have concentrations above the level of the NAAQS. Flagging and excluding 

data falling at around the 75th percentile point of the historical concentrations are 

extremely unlikely to influence an area’s attainment status with respect to such a shortterm NAAQS. Data around the 75th percentile point can, however, affect compliance with 

NAAQS having a quarterly average, 3-month average, or annual average standard. For 

the annual PM2.5 NAAQS, it is true that showing that the Exceptional Events Rule criteria 

are met will be more difficult for values near the 75th percentile point than for values near 

the 95th percentile point because it is more likely that values near the 75th percentile point 

are related to non-event causes. 

Other questions and answers in this Q&A document address situations involving NAAQS 

with short averaging periods. 

5. Question: Some pollutant demonstrations do not (or poorly) characterize the historical 

fluctuations of the observed concentrations at the monitor affected by the event. How can 

one judge whether the demonstration is adequate in this regard? 

Historical Seasonal Fluctuations in PM2.5, Non-Seasonal Data, 2005-2009

0

10

20

30

40

50

60

70

1 92 183 274 365

Day of Year

24-hour PM2.5, ug/m3

1st Quarter 2nd Quarter 3rd Quarter 4th Quarter

Flagged Value Not Potentially Outside 

Normal Seasonal Historical Fluctuations
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 8 of 46 

Answer: As previously stated in the response to the historical fluctuations question, the 

EPA will review the submitted analyses showing how the observed concentration 

compares to the distribution of historical concentrations to assess whether the event is 

associated with a measured concentration in excess of normal historical fluctuations, and 

when assessing the exceptional event demonstration criteria of “affects air quality,” 

“clear causal relationship,” and “but for” causation. Because the “historical fluctuations” 

showing is not a statistical demonstration with any defined bright line, air agencies need 

only submit (with appropriate descriptions and discussion) the type of statistical analyses 

described in the responses to Questions 2 and 3, and the EPA will review these analyses 

and look at both the relationship between the claimed concentration and historical 

concentrations and the strength of the data set to help inform the evidence needed to 

demonstrate the clear causal relationship and “but for” criteria. 

In the response to Question 2, we identified that 3-5 years of data should be evaluated to 

ensure some degree of statistical validity. We recognize, however, that these data may not 

be available for all monitors and/or all pollutants. If data are not available, please consult 

with the reviewing EPA regional office. 

B. “But For” Test 

 

Section 319 of the Clean Air Act 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...” and that 

[States] can petition [EPA] to “[E]xclude data that is directly due to exceptional 

events from use in determinations...with respect to exceedances or violations.”

The implementing language in the EER at 40 CFR 50.14(c)(3)(iv) states: “The 

demonstration to justify data exclusion shall provide evidence that: 

* * * 

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

6. Question: What types of evidence can be included in a demonstration that ozone 

exceedances would not have occurred but for the effect of a forest fire event? 

Answer: Air Agencies may include any evidence that they consider relevant to the “but 

for” requirement, but they should recognize that the effects of a fire on ozone are 

complex. Fire can generate ozone precursors, but it can also reduce solar radiation needed 

to drive ozone formation. Also, fire plumes containing ozone and ozone precursors can 

pass over a monitoring site without mixing down to ground level and affecting the 

monitored concentration. Additionally, wildfires often occur during the same seasons that 

exhibit high ozone caused by anthropogenic precursor emissions making it difficult to 

separate the wildfire contribution from a high ozone event that would have occurred 

without the fire. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 9 of 46 

Examples of relevant evidence follow. Generally, the more types of evidence the 

demonstration includes, the stronger the case for the exceptional event. 

 Statistical evidence that shows that for the place, time of year, and prevailing 

weather conditions at the time of the event, past ozone data show no history of 

exceedances on days that were not affected by a fire event, or shows that 

exceedances were so infrequent as to make the fire at issue the more likely cause 

of the observed exceedance. 

 Unusual diurnal patterns of hourly or minute-by-minute ozone concentrations, 

such as a spike or peak other than at the normal time of day. This could be 

demonstrated by comparing the event pattern to the range of diurnal patterns 

exhibited on typical high ozone days. 

 Evidence that the normally good correlation between the affected monitor and a 

monitor clearly outside the area of influence of the fire was disrupted on the day 

of the fire event in a manner not seen on non-fire days. 

 Evidence that there were no known unusual emission releases from non-fire 

sources at the time of the fire event, such as from traffic due to a sports or 

entertainment event or source non-compliance. 

 Evidence that the plume from the fire passed over the location of the monitoring 

site, and mixed down to ground level. This can include satellite images, wind data 

including HYSPLIT trajectories, visual smoke observations, and chemical 

analysis of PM filters showing elements and compounds that are markers for 

biomass burning. 

 Altered pollutant amounts, ratios, or patterns that indicate the affect of the event 

rather than non-event sources. This information could include the level, timing 

and patterns of CO and PM; PM size distribution or composition; indicators of 

precursor composition and “age,” such as oxygenated VOCs, radicals, sulfates, 

and timing and pattern of NO2 and NO; and pollutant ratios, such as CO/NOx, 

CO/PM10, Elemental Carbon (EC)/Organic Carbon (OC), O3/NOy and O3/CO. 

 A prediction that the “normal” ozone concentration would have been below the 

level of the NAAQS. “Normal” ozone concentrations can be predicted using 

statistical methods based on previous-day ozone and same-day weather variables 

(like methods used for air quality advisories in some areas) or using air quality 

models. If either type of prediction is included in a demonstration, the EPA will 

likely give it consideration only if the demonstration package also includes 

information on the uncertainty of the prediction methods, i.e., information on its 

past success in predicting normal ozone levels. The demonstration should also 

explain the predictive method in terms that are understandable enough to allow 

informed public comment. 

 A prediction based on air quality/photochemical modeling of the incremental 

ozone concentration due to the emissions from the fire, from comparing modeling 

results with and without the emissions from the fire. A demonstration that 

includes such evidence should address the uncertainties in the emission estimates 

for the fire including the speciation of the VOC and NOx emissions, and the 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 10 of 46 

uncertainties due to other aspects of the modeling platform such as grid cell size, 

etc. 

The EPA intends to prepare a future separate document that provides more guidance for 

preparing a demonstration for wildfire events that are believed to have affected ozone 

concentrations. In addition, the EPA will post on its exceptional events website example 

demonstration packages that illustrate the type and scope of analyses that constitute 

complete submittals for ozone-related exceptional events.4 

C. Exceptional Event Data Flagging Schedules 

Note: “Flag” is the common terminology for a data qualifier code in the EPA’s AQS (Air 

Quality System). Unless explicitly noted, the process of “flagging” data refers to adding 

Request Exclusion (“R”) data qualifier codes to selected data in AQS. “R” flags are the 

only AQS flags that satisfy the EER requirement for initial data flagging. The EPA can 

act/concur only on an “R” flag. 

7. Question: When the EPA revises the National Ambient Air Quality Standards, how will 

it notify air agencies of the schedules and deadlines for flagging and documenting 

exceptional event data for designations purposes? 

Answer: When 40 CFR § 50.14, “Treatment of Air Quality Monitoring Data Influenced 

by Exceptional Events,” was revised in March 2007, the EPA was mindful that 

designations would be occurring under the then-recently revised PM2.5 NAAQS. 

Exceptions to the generic deadline of July 1 of the calendar year following the datum 

year (see 40 CFR § 50.14(c)(2)(iii)) were included for PM2.5 in the rule. The EPA was 

also mindful that similar issues would arise for subsequent new or revised NAAQS. The 

Exceptional Events Rule at section 50.14(c)(2)(vi) indicates “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 data for initial designation of areas for those NAAQS.” See as 

examples, the data flagging schedule identified in the SO2 NAAQS final rule at 75 FR at 

35592 or the data flagging schedule identified in the NO2 NAAQS final rule at 75 FR at 

6531. 

D. General AQS Procedures 

8. Question: What is the difference between the “R” series flags and the “I” series flags, 

and how should they be used? 

Answer: Within AQS, monitoring agencies can use two types of data validation, or data 

qualifier, codes: the Request Exclusion flags (“R”) and the Informational Only flags (“I”). 

Agencies should use the “I” series flags when identifying informational data and the “R” 

series flags to identify data points for which the agency intends to request an exceptional 

event exclusion and the EPA’s concurrence. As an example, air agencies may use an “I” 

series flag to initially identify values they believe were affected by an event. Once the air 

 

4

 http://www.epa.gov/ttn/analysis/exevents.htm 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 11 of 46 

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. Air agencies intending to change “I” flagged data 

to “R” flagged data could approach the EER flagging and initial event description 

deadline of July 1st of the year following the sample measurement. Air agencies must 

change the flag status from “I” to “R” BEFORE the July 1st deadline. Normally, air 

agencies should not modify the flag status after this date and, therefore, will not have met 

the EER initial flagging and event description deadlines. 

The EPA does not intend to review or concur on the “I” series flags. Air agencies must 

submit “R” flags 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 

revisions (see Question 7).

9. Question: May an air agency flag any data in AQS? 

 

Answer: Yes, but the EPA requires air agencies to use the “R” flags to identify data that 

might have a regulatory consequence and for which an air agency intends to request 

exclusion and submit an approvable demonstration. Air agencies should use the “I” series 

flags to identify values for informational purposes (see Question 8). AQS only allows 

EPA to place concurrence flags on data identified with an “R” flag. “I” flags never affect 

regulatory summary statistics (e.g., design values, number of exceedances, 98th percentile 

values) generated by AQS for NAAQS determinations purposes. “R” flags will not affect 

the regulatory summary statistics unless or until they are concurred by the EPA. 

Further, while the EER does not prohibit air agencies from flagging individual 

concentration values below the level of the NAAQS, in general, air agencies can only 

request exclusion for data that contribute to a violation or an exceedance of the NAAQS. 

See Questions 29-31 for more information. 

10. Question: The EPA requires air agencies to provide an initial description for data 

flagged with an “R” data qualifier code. Is it possible for an initial description to be 

inadequate (for example, "fires in surrounding states")? 

Answer: Although the EPA is not specifying pass/fail criteria for the initial description 

associated with “R” flagged data, it is possible for an air agency to enter inadequate 

initial descriptions in AQS. The preamble to the Exceptional Events Rule explains: "At 

the time the [request exclusion] flag is inserted into the AQS database, the State must also 

provide an initial description of the event in the AQS comment field. This initial 

description should include such information as the direction and distance from the event 

to the air quality monitor in question, as well as the direction of the wind on the day in 

question." 72 FR at 13568 (emphasis added). AQS maintains event definitions, including 

their initial descriptions, in fields separate from the raw data flagging fields. As a result, 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 12 of 46 

air agencies can enter more detailed event descriptions either before or after the raw data 

measurements are flagged. Regardless of precise timing, the intent of this initial 

description is to initially explain why the flagged data warrant consideration as 

exceptional events. Although the initial description is not likely to provide enough 

information to assist the EPA with exceptional event planning and prioritization, the act 

of providing the initial description encourages air agencies to review and identify data 

having regulatory consequence and for which they are likely to submit an approvable 

demonstration. To facilitate the EPA’s review of the initial event description, the EPA 

suggests that air agencies notify the appropriate regional office after the air agency 

creates the event description. This allows the air agency and the EPA to discuss and, if 

necessary, develop a mutually agreed-upon description. This initial discussion and the 

optional letter of intent (see Question 27) can assist the EPA and air agencies with 

exceptional event review and prioritization. 

11. Question: The “j” flag was "Construction/Demolition." The new “IE/RE” flag is 

demolition; can it also be used for construction? 

Answer: The “j” flag is obsolete and can no longer be used. The “IE/RE” flag should not

be used for construction. 

Generally, construction activity is not considered to be exceptional. Reasonable and 

appropriate controls capable of preventing localized NAAQS exceedances are expected 

to be available during most construction events. In some cases, however, construction 

activities may involve very high-energy, emissions-generating physical processes, such 

as explosive excavation. Dust control measures may not be adequate to prevent 

exceedances / violations in the vicinity of this type of activity. 

If an agency wishes to “flag” data related to exceedances caused by some construction 

activity, the agency should use the Other (“IL/RL”) exceptional events flag. Air agencies 

should use the “IE/RE” flag only when an exceptional demolition event occurred and the 

air agency wishes to flag the data for exclusion as an exceptional event. Air agencies 

using either the “IE/RE” flag or the “IL/RL” flag to identify an exceptional event would 

be expected to show in a demonstration submittal that all reasonable and appropriate 

controls were in place during the construction / demolition activity, and that those 

controls proved inadequate to prevent NAAQS exceedances. The demonstration would 

also need to meet all other requirements of the Exceptional Events Rule. 

11a.Question: What flags does AQS use to describe fires? 

Answer: Land Management Agencies modified their fire-related definitions after the 

EPA promulgated the Exceptional Events Rule. The EPA has incorporated the firerelated terminology in the exceptional events guidance documents to ensure consistency 

(see also Question 20a). These definitional changes result in corresponding changes to 

fire-related flags in AQS. The EPA eliminated from AQS the Wildland Fire Use Fire – 

United States (“IU”) and (“RU”) flags and the Forest Fire (“E”) flag. The EPA continues 

to use the following flags to describe fires: 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 13 of 46 

 IF – Fire – Canadian (Informational Only) 

 IG – Fire – Mexico/Central America (Informational Only) 

 IM – Prescribed Fire (Informational Only) 

 IP – Structural Fire (Informational Only) 

 IT – Wildfire – US (Informational Only) 

 RF – Fire – Canadian (Request Exclusion) 

 RG – Fire – Mexico/Central America (Request Exclusion) 

 RM – Prescribed Fire (Request Exclusion) 

 RP – Structural Fire (Request Exclusion) 

 RT – Wildfire – US (Request Exclusion) 

The EPA believes it is appropriate to retain the Fire – Canadian (“IF/RF”) and Fire – 

Mexico/Central America (“IG/RG”) flags because these flags indicate the jurisdictional 

origin of the fire (i.e., outside of the submitting state/outside of the US). Emissions from 

fires originating outside of the United States that affect air quality concentrations in the 

United States may qualify for regulatory treatment under the international transport 

provisions of 40 CFR part 179(b) of the Clean Air Act. 

12. Question: The National Park Service operates ozone monitors in some locations that 

meet all requirements of 40 CFR part 58. Can an air agency request exclusion of data 

from such monitors under the EER, and exclusion of other data not collected by the air 

agency itself that may lead to a nonattainment finding? 

Answer: Yes. However, air agencies should take special steps with regard to data 

handling within AQS. To maintain data integrity, AQS is generally designed so that only 

the agency updating a monitoring site may enter or alter data for that site. Under normal 

circumstances, an air agency will not have access rights to apply event flags to data from 

monitors operated by other entities, such as the National Park Service or other state, 

local, or tribal agencies. When an air agency believes that an exceptional event affected 

the concentration recorded by monitors operated by other agencies, the air agency should 

contact the agency operating the monitor and request that the operating agency flag the 

identified data range for exclusion. The affected air agency should also develop and 

forward to the operating agency an initial event description that the operating agency can 

enter in AQS as it enters the appropriate “R” series flags (see Question 10). If an air 

agency is unsuccessful in requesting that another agency apply the appropriate “R” series 

flags and initial event description, the air agency should contact the EPA regional office. 

If the EPA regional office is aware of the request, and if the request was prior to July 1st

of the year following the datum year, the EPA will generally still consider the affected air 

agency’s request. Air agencies should notify the EPA regional office of such an instance 

as soon as possible. 

Regardless of whether the monitor operator flags the data in question or the air agency 

notifies the regional office that a flag is needed, it is the air agency’s responsibility to 

develop an initial event description, prepare the demonstration, and submit it to the EPA 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 14 of 46 

under the applicable schedule. The agency operating the monitor may choose to assist in 

this process. 

13. Question: Events can make an air concentration significantly higher than it would have 

been in the absence of the event contribution, and elevate the 3-year design value for a 

NAAQS pollutant. Depending on the magnitude of the effect and how the “normal” 

concentration compares to the NAAQS, the “but for” test may not be satisfied in that 

there may have been a violation with or without the event. Thus, it appears that data 

associated with the event cannot be handled as an exceptional event. However, retaining 

such data in the calculation of a 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. How will the EPA deal with such a situation when reviewing an attainment 

demonstration? How, if at all, should AQS be used to flag such data? 

Answer: (See also Question 19 for a related question regarding PM10.) The question 

reflects a proper understanding that not every natural or infrequent anthropogenic event 

that affects air quality is a true "exceptional event" under the definition of that term in the 

Exceptional Event Rule. Ambient data affected by an event that does not meet the "but 

for" criterion cannot be excluded under the authority of the Exceptional Events Rule even 

if in all other respects the event meets the definition of an exceptional event. When the 

available evidence indicates that there would have been an exceedance of a NAAQS even 

in the absence of the event, for example when a wildfire makes a summer-time ozone 

exceedance worse than it otherwise would have been, the event is not a true “exceptional 

event” under the EER. The Exceptional Events Rule does not address data handling 

associated with events that are not considered “exceptional” under the EER, and does not 

provide EPA with authority to exclude such data. Yet as the question points out, this 

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

the event-related concentration in a design value used for a prospective attainment 

demonstration might seem to need more emission reductions to attain the NAAQS by its 

attainment deadline than is actually the case. 

However, the EPA intends to achieve much the same effect as if such data were 

excludable under the Exceptional Events Rule, by addressing this topic in future guidance 

on the preparation of attainment demonstrations in required SIPs for areas designated as 

nonattainment. The first pollutant and NAAQS that the EPA will address this way will be 

the 2008 ozone NAAQS. The EPA plans to more formally describe its intention to 

develop such ozone guidance in the preamble of a soon-to-be-proposed rulemaking on 

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

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

consult with their EPA regional office if they face this situation. To avoid confusion, air 

agencies should use AQS informational-only "I" flags on such data, rather than "R" flags. 

In the remainder of this response to the question, the EPA describes in more detail the 

differences between the event scenario described in the question and a true "exceptional 

event" under the Exceptional Events Rule, for the purpose of making it clear to all parties 

why the planned guidance on attainment demonstrations and the SIP approval process, 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 15 of 46 

rather than the Exceptional Event Rule and the associated AQS data flagging, 

demonstration submittal, and review process, will apply to such an event scenario. 

To illustrate an attainment demonstration scenario using the 2006 24-hour PM2.5 NAAQS 

of 35 μg/m3

, assume that the three annual 98th percentile 24-hour PM2.5 concentrations for 

a monitoring site for 2006-2008 are 44, 31, and 37 μg/m3 for each respective year, with a 

resulting 3-year design value of 37 μg/m3

, which is a violation. Also, assume that the next 

highest concentration in 2006 below the 44 μg/m3 was 40 μg/m3. The 44 μg/m3

concentration in 2006 was affected by a one-day wildfire, and the air agency was able to 

show that the concentration would have been 41 μg/m3 without the fire. Because both 44 

μg/m3 and 41 μg/m3 are exceedances, the event on that day does not meet the “but for” 

test when viewed from an “exceedance” perspective. Moreover, from a “violations” 

perspective, the 2006 value also would not meet the “but for” test, because the “no event” 

concentration value of 41 μg/m3

 for the event day in 2006 would still be the 98th

percentile concentration and would still result in a 3-year design value of 36 μg/m3 which 

is a violation. Thus, the 2006 wildfire does not meet the definition of an exceptional 

event. 

E. General Exceptional Events Rule Applicability and Implementation Issues 

14. Question: The Preamble to the Exceptional Events Rule states that the EPA 

headquarters or the EPA regional office will make its decision on demonstrations public. 

See 72 FR at 13574 ("The EPA regional offices will work with the States, Tribes, and 

local agencies to ensure that proper documentation is submitted to justify data exclusion. 

EPA will make the response and associated explanation publicly available."). What 

method does the EPA plan to use to make the explanation "publicly available?" 

Answer: The EPA posts example demonstration packages and decisions (consisting of 

air agency demonstration submittals, the EPA responses, and the EPA technical support 

documents) on the EPA regional office websites and/or the Technology Transfer 

Network website.5

 In certain instances, the EPA’s concurrence or non-concurrence 

determination may be a factor in a rulemaking that includes a public comment period. In 

these cases, the same information that is posted on the EPA websites, and any additional 

supporting correspondence, will also be posted in the relevant rulemaking docket. 

Further, the EPA plans to make the demonstrations and the EPA’s concurrence decisions 

available to interested parties upon request. 

14a. Question: At what point in the exceptional event development and review process is 

public notice and opportunity for comment required? How does the EPA determine the 

need for public comment? 

Answer: The EER requires that air agencies offer notice and opportunity for public 

comment as part of the demonstration development process (see 40 CFR 50.14(c)(3)(i) 

and 40 CFR 50.14(c)(3)(v)). The EPA must provide notice and opportunity for public 

 

5

 http://www.epa.gov/ttn/analysis/exevents.htm 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 16 of 46 

comment prior to taking final Agency action. In addition, an air agency may need to 

provide an additional opportunity for public comment if the EPA requests and/or if the air 

agency provides supplemental information not included in the original documentation 

made available for public comment. The EPA will make a case-by-case decision 

regarding supplemental opportunities for public comment during the demonstration 

preparation, submittal, and review process. As part of this decision, the EPA may 

consider potential impact and/or expressed public interest in the claimed event, data 

uncertainty, historical application of demonstration approach, etc. 

When the EPA concurs based on the weight-of-evidence that the air agency has 

successfully made the demonstrations referred to in 40 CFR 50.14(a)(2) and (b)(1) to the 

EPA’s satisfaction, the EPA will exclude the affected data from the following types of 

calculations and activities: 

 The EPA’s AQS will not count these days as exceedances when generating user 

reports, and will not include them in design values estimates, unless the AQS user 

specifically indicates that they should be included.6

 

 The EPA will accept the exclusion of these data for the purposes of selecting 

appropriate background concentrations for New Source Review air quality analyses.7

 The EPA will accept the exclusion of these data for the purposes of selecting 

appropriate background concentrations for transportation conformity hot spot 

analyses.8

 The EPA will exclude these data when assessing whether the ambient monitoring 

requirements of 40 CFR part 58, which are tied to air quality conditions, are being 

met. 

 The data will continue to be publically available, but the EPA’s publications and 

public information statements on the status of air quality in the affected area will not 

reflect these data in any summary statistic of potential regulatory application, unless 

such inclusion is specifically noted.9

 

In addition, some proposed regulatory actions (e.g., proposed designation, classification, 

attainment demonstration, or finding as to whether the area has met the applicable 

NAAQS) rely on design values that exclude data that the EPA has determined meet the 

exceptional event weight-of-evidence requirements. These regulatory actions require the 

EPA to provide an opportunity for public comment prior to taking a final Agency action. 

 

6

Due to the complexity of the AQS software, inadvertent errors may occur. The EPA asks that agencies provide 

the EPA with information if/when AQS outputs seem inconsistent with the EPA’s intention to exclude 

concurred upon data.

7

 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. 

8

Applicable only to PM10 and PM2.5. See “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. 9

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. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 17 of 46 

If the EPA pursues one of these actions for a given area, the EPA will open a new 

comment period during which the public may comment on the exceptional event 

submission and/or the EPA’s determinations. The EPA must consider and respond to 

received comments before taking final regulatory action. 

15. Question: It is possible for events to affect more than one state. Each state/air agency 

must then submit its own exceptional events demonstration package, which may result in 

redundant work. Could the EPA take on multi-state/agency demonstrations? 

Answer: The primary responsibility for developing demonstrations lies with state, local, 

and tribal air agencies. The EPA encourages states and air agencies to coordinate with 

each other in compiling demonstration packages, and these agencies may submit some of 

the same data and analyses when a single event affects multiple jurisdictions. Each 

NAAQS exceedance, however, will likely have some unique properties (e.g., unique 

monitoring locations, different surrounding and potentially contributing sources with 

varying levels of control, different historical concentration patterns, etc.). States/agencies 

need to address these unique characteristics in individual submittal packages. Similarly, 

where a single event results in exceedances of multiple NAAQS (e.g., annual and 24-hour 

PM), the submitting agency needs to address the unique features of each NAAQS 

exceedance or violation (e.g., potentially different monitoring locations, different 

historical concentration patterns). An air agency could submit a single demonstration 

package for a single event affecting multiple NAAQS provided the air agency clearly 

identifies the unique characteristics of each NAAQS. 

For example, if multiple states or jurisdictions are affected by a Saharan dust plume, they 

could collaborate and submit a common demonstration component (e.g., the same or very 

similar information in multiple submittals) for the “not reasonably controllable or 

preventable” and “human activity unlikely to occur or natural event” elements. Because 

the actual event-related exceedance would have been measured by different monitors 

located in different regions with possibly different contributing factors (e.g., rural 

monitor affected by both dust from feedlots and Saharan dust and urban monitor affected 

by both nearby industrial sources and Saharan dust), the “clear causal relationship,” “but 

for,” and “historical fluctuations” elements are likely to differ from one submittal to 

another. 

16. Question: Does the EER address scenarios in which temporary activities (e.g., multimonth or multi-year road construction / demolition projects) significantly impact a 

previously-sited monitor such that the monitor is no longer representative of the area, but 

rather functions more like a “hot-spot” monitor? 

Answer: Except for PM2.5, there is no difference in how monitoring data are treated 

from "area-wide" monitors and hot-spot monitors. All such data, if meeting applicable 

CFR regulations, are comparable to the NAAQS. For PM2.5 a unique microscale or hotspot monitor is only comparable to the 24-hour NAAQS and not to the annual PM2.5

NAAQS. Air agencies must indicate in their annual monitoring network plan (or an 

update to that plan) whether PM2.5 monitors are suitable or not suitable for comparison 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 18 of 46 

against the annual PM2.5 NAAQS. If the EPA regional office approves a monitor as being 

unsuitable, this will prevent the data being used to compare with the annual PM2.5 

NAAQS (see 40 CFR § 58.30). Note that data collected at “special purpose” monitors 

meeting the applicable 40 CFR part 50 and 58 requirements can be compared to the 

NAAQS when the EPA makes an attainment determination. 

The EER does not specifically address temporary anthropogenic emission sources such as 

construction projects. However, neither does the EER explicitly place a limit on the 

duration of a single event. A submitting agency could make a showing that a claimed 

event (e.g., a multi-year road construction project) is not likely to recur at the location in 

question. If the remaining exceptional event criteria and demonstration criteria are met, 

including the requirement that the event (including the emissions from the project) is not 

reasonably controllable, the activity might qualify as being an exceptional event. 

Air agencies not wishing to develop exceptional event demonstration packages for the 

described scenario can request agreement from the EPA regional office to relocate a 

monitor that no longer meets monitoring objectives. This process is, however, time 

consuming and resource intensive, so air agencies usually "monitor through" the 

disruption or ask their regional offices to support a temporary shut-down. When the EPA 

regional offices approve temporary shut-downs, the operating air agency should assign a 

Null Data Code in AQS for “construction/repairs in area” (AC) to identify and invalidate 

data associated with periods of local construction. 

16a. Question: Are policy relevant background ozone concentrations and exceptional events 

related? 

Answer: Policy relevant background ozone concentrations and exceptional events can 

include partially overlapping concepts. The 2007 Staff Paper10 defines policy relevant 

background ozone “as the distribution of [ozone] concentrations that would be observed 

in the U.S. in the absence of anthropogenic (man-made) emissions of precursor emissions 

(e.g., VOC, NOx, and CO) in the U.S., Canada, and Mexico.” An exceptional event is a 

natural event (excluding stagnations, inversions, high temperatures, or precipitation) or 

an anthropogenic event that is unlikely to recur in the same location. Both exceptional 

events and policy relevant background include emissions from natural events like forest 

wildfires or stratospheric ozone intrusions. However, exceedances due to natural 

emissions that occur every day and contribute to policy relevant background, such as 

biogenic emissions, do not meet the definition of an exceptional event and are thus not 

eligible for exclusion under the EER. Routine anthropogenic emissions outside of the 

U.S. contribute to policy relevant background, but are not exceptional events. Air agency 

preparation of a demonstration package and the EPA’s subsequent review of the 

demonstration package is case-by-case based on a weight-of-evidence approach and does 

 

10 Environmental Protection Agency (2007a) Review of the national ambient air quality standards for ozone: 

assessment of scientific and technical information. OAQPS staff paper. (Updated Final) July 2007. Research 

Triangle Park, NC: Office of Air Quality Planning and Standards; EPA report no. EPA-452/R-07-007, available 

online at: http://epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_sp.html. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 19 of 46 

not explicitly consider whether the event type might contribute to policy relevant 

background. However, if a natural event that contributes to policy relevant background 

causes an observed concentration that meets the statutory definition of an exceptional 

event and fulfills all of the exceptional event criteria, the EPA would consider the event 

to be an exceptional event. 

17. Question: Volcanoes on Hawaii are causing 1-hour and 24-hour SO2 exceedances, 

which are clearly volcanic exceptional events. Section 319 of the Clean Air Act and CFR 

require the EPA to provide air agencies with a method to flag and petition the EPA for 

exclusion of exceptional events data. When will the EPA provide the method for SO2? 

Answer: AQS has been modified to allow flags on all criteria pollutant data. The 

specific schedule for exceptional event flagging and documentation submission for data 

to be used in designations decisions is identified in the final primary SO2 NAAQS rule 

(see preamble at 75 FR at 35585-35586 and regulatory text at 75 FR at 35592). The 

correct flag to use for a volcanic eruption event is “RS.” 

17a. Question: The EPA acknowledges that certain extreme exceptional event cases may 

require more limited demonstration packages. How might the EPA decide whether to 

consider a particular high wind dust event “extreme” for the purpose of expecting only a 

very limited demonstration package? 

Answer: While many dust storms could qualify as exceptional events, the EPA believes 

that most events that are conventionally referred to as “dust storms” should not be 

considered “extreme” events for this purpose. The National Weather Service (NWS) 

defines a “dust storm” as a severe weather condition characterized by strong winds and 

dust-filled air over an extensive area, but does not include any quantified criteria for the 

spatial extent or the concentration of the dust. In contrast, a haboob is of the magnitude 

that could be considered an extreme event. Haboobs are caused by severe thunderstorm 

activity and are typically characterized as “solid walls” of dust that can rise up to 2,000 

meters and travel hundreds of miles. 

Generally, the EPA would consider sustained wind speed, spatial extent, visibility, and 

PM levels in determining whether an event is an extreme event. An example of an event 

that could be considered an exceptional event but not an extreme event would be the 

Santa Ana winds blowing at 25-30 mph, creating an exceedance at one monitor, with 

maximum hourly PM10 levels of less than 800 μg/m3

. In contrast, a haboob that occurred 

in Phoenix in 2011 had downburst winds of 70 mph, with a wall of dust moving at 30-40 

mph for 150 miles; hourly PM10 levels of 50,000 μg/m3

 were monitored during this 

event. Both of these events could be considered for exclusion under the EER. The South 

Coast Air Quality Management District prepared a satisfactory The South Coast Air 

Quality Management District prepared a satisfactory 49-page demonstration package 

(plus an appendix with additional supporting information) for the Santa Ana winds event, 

parts of which have been used as examples in the High Winds guidance document. 

However, the EPA would expect that much more limited documentation for an event like 

the haboob would be sufficient to convince the EPA (and all other parties) that the event 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 20 of 46 

meets the several criteria for data exclusion (clear causal connection, not reasonably 

controllable or preventable, etc.). 

18. Question: Carbon monoxide (CO) flags are in AQS for exceedances caused by fires, but 

the CO NAAQS (40 CFR 50.8) does not reference the Exceptional Event Rule. What is 

the EPA’s approach for the treatment of CO data affected by exceptional events? 

Answer: CO flagging, including the option for the EPA’s concurrence, has been enabled 

in AQS. CO flags from structural fires and wildfires that qualify as exceptional events 

have been allowed in historic EPA guidance. The EER Preamble (72 FR at 13563) 

explains the EPA’s position with respect to exceptional event flagging for pollutants for 

which the statement of the NAAQS in 40 CFR part 50 does not explicitly reference the 

Exceptional Events Rule: “In the interim, where exceptional events result in exceedances 

or violations of NAAQS that do not currently provide for special treatment of the data, 

we intend to use our discretion as outlined under section 107(d)(3) not to redesignate 

affected areas as nonattainment based on these events.” Therefore, air agencies may flag 

CO data in AQS and the EPA may apply the same process and approval criteria as in the 

Exceptional Events Rule. 

On August 12, 2011, the EPA issued a decision to retain the current suite of CO standards 

without revision (see 76 FR at 54294). Because the EPA made no revisions to the CO 

standards, it promulgated no related changes to the Exceptional Events Rule. 

19. Question: The limited maintenance plan requirements for PM10 require a demonstration 

that the area design value is less than or equal to 98 g/m3. Flagging of values between 

98 g/m3

 and the NAAQS are therefore relevant for this regulatory decision. Can these 

values, which are not exceedances and do not contribute to violations, be flagged and 

receive the EPA’s concurrence? 

Answer: Yes. The May 7, 2009, memorandum from William T. Harnett to Regional Air 

Division Directors states the following regarding the PM10 limited maintenance plan 

option: “In determining eligibility for the limited maintenance plan option, the EPA will 

treat 24-hour average air quality data between 98 g/m3 and 155 g/m3 in a manner 

analogous to the treatment of exceedance data under the Exceptional Events Rule, 

provided the impacted data meet the general definition and criteria for exceptional events 

(natural event, or exceptional event that is not reasonable controllable or expected to 

recur).” This memorandum is posted on the EPA website at 

http://www.epa.gov/ttn/oarpg/t1/memoranda/lmp_final_harnett.pdf 

19a. Question: What does it mean to use a “weight-of-evidence” approach when the EPA 

reviews exceptional event demonstration submittals? 

Answer: In using the term “weight-of-evidence,” the EPA believes we should consider 

all relevant evidence and qualitatively “weigh” this evidence based on its relevance to the 

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

considerations appropriate to the individual pollutant and the nature and type of event. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 21 of 46 

 

20. Question: Exactly which section(s) of the preamble to the final Exceptional Event Rule 

has been declared a “legal nullity” by the court, and what does that mean? 

Answer: In NRDC v. EPA, No. 07-1151 (D.C. Cir. 3/20/09), the DC Circuit Court states 

that 

“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 (72 Fed. Reg. 13576). There is no such final rule. The final 

rule [language in 40 CFR 50 and 40 CFR 51.930] 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 considers the “high wind events section of the preamble” to which the court 

referred to be the section titled “B. High Wind Events” beginning on 72 FR at 13576. This 

does not necessarily mean that these passages do not reflect the EPA’s interpretation of 

what might be appropriate under the EER. Rather, it means that other parts of the 

preamble and other EPA guidance should be relied upon instead of statements in these 

passages of the final rule preamble, which should be treated as not having been 

published. 

20a. Question: What fire-related definitions should air agencies use in their exceptional 

event documentation? 

Answer: Land Management Agencies modified their fire-related definitions after the 

EPA promulgated the Exceptional Events Rule. The EPA will use the following firerelated terminology in the revised draft exceptional events guidance documents to ensure 

consistency: 

Prescribed fire - Any fire intentionally ignited by management under an approved plan to 

meet specific objectives. 

Wildfire – Any fire started by an unplanned ignition caused by lightning; volcanoes; 

unauthorized activity; accidental, human-caused actions; and escaped prescribed fires. 

20b. Question: How should air agencies support a claim that emissions from wildfires are 

“not reasonably controllable or preventable”? 

Answer: The EPA recognizes that emissions from wildfires are generally not reasonable 

to prevent or control. The EPA uses “generally” in this statement because the current 

United States Forest Service (USFS) definitions of “wildfire” and “prescribed fire” define 

these events in terms of purpose and deliberateness of ignition (See definitions in 

response to Question 20a). Based on the USFS definitions, a wildfire may have started 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 22 of 46 

from an unintentional ignition, but the land manager may have allowed the fire to 

continue to burn although it could have been extinguished. In such a case, some of the 

explanation of the reasonableness of this decision is needed. Land managers may 

similarly allow a prescribed fire to burn beyond its originally determined bounds. 

Submitting agencies should also explain the reasons for extending the bounds of the burn. 

The EPA’s expectation is that air agencies identify the origin and evolution of the 

wildfire in simple terms when documenting the “not reasonably controllable or 

preventable” criterion in their demonstration submittal. Fires started by lightning, 

volcanoes, unauthorized activity, or accidental human-caused actions are unplanned and 

unwanted fires and therefore, despite efforts to prevent or control such fires11 the 

emissions may not be reasonably controllable or preventable. In these cases, it is often 

sufficient for agencies to provide a statement such as the following to document the “not 

reasonably controllable or preventable” criterion: “Based on the documentation provided 

in [section X] of this submittal, [lightning] caused the unplanned, unwanted wildfire 

event. The responsible agencies did their reasonable best to control the extent of and 

extinguish the fire by taking the following actions [insert list or description of actions 

taken]. Therefore, emissions from this fire were ‘not reasonably controllable or 

preventable.’” Emissions from intentionally ignited fires and escaped prescribed fires that 

burners allow to burn require additional supporting documentation to demonstrate the 

“not reasonably controllable or preventable” criterion. In these cases, the submitting 

agency must explain how the fire escaped or why the land manager allowed the fire to 

burn. The EPA expects to discuss the issue of control/prevention of wildfires in future 

guidance. 

21. Question: The Exceptional Event Rule allows for exclusion of data affected by a 

prescribed fire if the usual requirements of the rule are satisfied and if the air agency has 

adopted and is implementing a Smoke Management Program or if the air agency has 

ensured that the burner employed basic smoke management practices. Are there 

minimum requirements for a Smoke Management Program? What are “basic smoke 

management practices?” 

Answer: The EPA intends to develop separate guidance to address this issue which will 

be issued at a later date following an opportunity for stakeholder input. 

22. Question: Is there a tie between the requirements of 40 CFR 51.930 Mitigation of 

Exceptional Events and the EPA’s approval for exclusion of data affected by an 

exceptional event? 

Answer: While the granting of data exclusion under the EER does not depend on air 

agency actions to meet the requirements of 40 CFR § 51.930, the EPA encourages the 

submittal of a mitigation measures with the demonstration package, particularly for those 

events expected to recur. The Exceptional Events Rule was promulgated pursuant to 

 

11 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. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 23 of 46 

Section 319 of the Clean Air Act which contains a provision that each air agency “must 

take necessary measures to safeguard public health regardless of the source of the air 

pollution...” This provision was the basis for the mitigation requirements in 40 CFR 

§51.930 and the requirement in the EER at 40 CFR §50.14(c)(1)(i) that all air agencies 

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.” The 

language at 40 CFR §51.930 requires that: 

“(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 

events.” 

Although the language at 40 CFR §51.930 does not require the preparation or submittal 

of a mitigation plan, it does require that the air agency develop and implement processes 

and measures that could easily become the elements of a formal, written plan. The 

mitigation criteria focus on specific measures and actions to protect public health, rather 

than on measures that control or prevent emissions associated with a specific event. So, a 

mitigation plan may include measures that apply to emissions sources in general (e.g., 

dust suppression or covering techniques for mineral processing) rather than those 

measures or controls that might be discussed in the “not reasonably controllable or 

preventable” portion of an event demonstration (e.g., controls/measures X, Y, and Z were 

in place on sources A, B, and C during the time of the event). A mitigation plan may also 

include procedures and responsibilities for public alerts and sheltering advisories. 

Because having a mitigation plan in place will help air agencies meet the EER 

requirements at 40 CFR §50.14(c)(1)(i) related to public notification more systematically, 

the EPA encourages the development and submittal of a mitigation plan with the 

demonstration package if one has not already been adopted. 

 

23. Question: Need a state (or tribe) make an argument or submit evidence about control 

measures for events that took place in other states or countries, on federally-owned and 

managed land, or on tribal (or state) lands not subject to state (or tribal) regulation? 

Answer: Under the Clean Air Act, the EPA generally considers a state (not including 

areas of Indian country) to be a single responsible actor. Accordingly, neither the EPA 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 24 of 46 

nor the 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). For cases involving intrastate transport, the state or local air 

agency must evaluate whether emissions from neighboring (or contributing) counties are 

not reasonably controllable or preventable. As discussed in greater detail in the overview 

guidance document and the draft High Winds Guidance document, the assessment of “not 

reasonably controllable or preventable” is based on existing level of required control, 

attainment status, and, for high wind dust events, wind speed and other factors. States and 

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

exceptional event demonstration package if they believe that emissions from sources on 

federally-owned and managed land (e.g., national parks within the state) have been 

affected by an event in a way that raises issues of reasonable control. 

Interstate and international transport events are different than intrastate events. The EPA 

believes it is not reasonable to expect the downwind state (i.e., the state submitting the 

demonstration) to have convinced the upwind country or state to have implemented 

controls on sources sufficient to limit event-related air concentrations in the downwind 

state. As with any demonstration submittal, the submitting (downwind) state should 

sufficiently identify all natural and anthropogenic contributing sources of emissions (both 

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 country than of sources within its jurisdiction. After 

completing the source characterization, the submitting state should assess whether 

emissions from sources within its jurisdiction (i.e., in-state sources) were not reasonably 

controllable or preventable. Although the submitting state should also provide available 

information on the status of control measures for emissions from out-of-state sources, the 

submitting state may determine based on available information that the implemented 

controls on out-of state sources constitute reasonable controls and that the “not 

reasonably controllable or preventable” criterion is satisfied. When assessing emissions 

transported from other states or countries, the submitting state can say that it 

characterized the out of state sources, determined that these sources contributed to the 

noted exceedance or violation, and determined, based on jurisdictional boundaries and 

other available information, that contributing emissions from the upwind state or country 

were not reasonably controllable or preventable. Submitting states are further required to 

submit evidence/statements supporting the other exceptional event criteria (i.e., clear 

causal relationship, but for, human activity unlikely to recur or a natural event, affects air 

quality, and historical fluctuations). 

The EPA recommends 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 consider a natural event of the grounds that all significant anthropogenic 

sources were reasonably controlled. 

As with all exceptional event demonstrations, the EPA will evaluate the information on a 

case-by-case basis based on the facts of a particular exceptional event including any 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 25 of 46 

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. This response is not intended to discourage states from 

working cooperatively to plan and apply controls on both sides of a state boundary for 

their mutual benefit. 

In addition to the provisions in the EER, the Clean Air Act provides mechanisms in 

sections 110(a)(2)(D) and 126 to address interstate transport issues and mechanisms in 

section 179(b) to address international transport issues. 

24. Question: Need an air agency make an argument or submit evidence about control 

measures for air quality impacts from wind-blown dust from desert land in its natural 

state? 

Answer: While the EPA’s position is generally that impacts from wind-blown dust from 

undisturbed natural deserts are inherently not reasonable to control, the air agency would 

need to assert this and provide appropriate supporting documentation in its demonstration 

package. The supporting documentation should include descriptions of the geographic 

area (with maps or available visuals) and a discussion of the historical land use, including 

prior disturbances, water diversions and other historical practices which may have 

occurred on the land, even if the land seems or is considered to be “undisturbed” at 

present. Submitting agencies should also identify all sources contributing to an event and 

identify appropriate control strategies for each anthropogenic source. 

25. Question: Is there a template or example for preparing a demonstration document? 

Answer: The guidance document, “Draft Guidance on the Preparation of 

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

High Winds under the Exceptional Event Rule” (the High Winds guidance document)

provides this type of advice for demonstrations for high wind dust events. While the High 

Winds guidance document speaks specifically to high wind dust events, the EPA expects 

many of the principles discussed therein to extend to all types of exceptional events. The 

EPA has also developed a presentation entitled, “Presenting Evidence to Justify Data 

Exclusion as an Exceptional Event: Ideas based on how the EPA has recently 

documented events to support regulatory decisions.” Interested parties can download this 

presentation from the following site: 

http://www.epa.gov/ttn/analysis/docs/IdeasforShowingEEEvidence.ppt. Additionally, the 

EPA intends to develop a future separate guidance document addressing the preparation 

of demonstrations to support wildfire-related ozone event claims. 

26. Question: Where can an air agency find examples of demonstrations from other air 

agencies that have been approved by the EPA? 

Answer: Examples of approved demonstrations are posted at 

http://www.epa.gov/ttn/analysis/exevents.htm. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 26 of 46 

27. Question: How quickly will the EPA review the demonstration document and provide 

feedback to the air agency on the approval, or on any suggested improvements? 

Answer: The EPA generally intends to conduct its initial review of a submitted 

exceptional event demonstration package within 120 days of receipt. Following this 

initial review, the EPA will generally send a letter to the submitting agency that includes 

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

the supplemental information should be submitted (if applicable)12, and an indicator of 

the timing of the EPA’s final review. The EPA will generally prioritize exceptional event 

determinations that affect near-term regulatory decisions.13 

To promote early communication, the EPA suggests that air agencies provide a letter of 

intent to submit a demonstration package for flagged data in AQS as soon as possible, if 

possible within 12 months from the event occurrence, after the agency identifies the 

event(s) as being significant. A letter of intent is an option for the air agency to use in 

situations where it may help communication and prioritization.14 This initial notification 

can assist both the air agency and the EPA in the planning and prioritization process. The 

EPA intends to respond to such a letter within 60 days of receipt. The EPA response will 

provide the regional office’s best assessment of the priority that can be given to the 

submission once received and any case-specific advice the EPA may have to offer for the 

preparation of the demonstration. 

The EPA intends to make a decision regarding concurrence with an air agency’s flag 

within 18 months of receipt of a complete package for those demonstrations that impact a 

near-term regulatory action, or sooner if necessary to support a regulatory action. The 

EPA intends to communicate with the submitting agency, as needed, during the 

demonstration review period. 

Submitting air agencies that believe their demonstration packages are tied to near-term 

regulatory actions should submit their demonstration packages well in advance of the 

regulatory deadline. Air agencies should also identify the relationship between the 

exceptional event-related flagged data and the anticipated regulatory action in the cover 

letter that accompanies their initial submittal package to the reviewing EPA regional 

office. 

28. Question: Will the EPA ever perform and consider additional data analysis itself before 

deciding whether to approve an air agency-submitted demonstration in support of data 

exclusion? 

 

12 The EPA will generally ask that air agencies provide supplemental information within 60 days from receipt of 

the letter from the EPA. The EPA recognizes that air agencies may need more than 60 days to prepare and 

submit some types of supplemental information. The EPA is willing to work with agencies on supplemental 

timeframes; however, the mandatory timing of EPA actions may limit the response time the EPA allows. 13 “Regulatory decisions” include findings as to whether the area has met the applicable NAAQS, classification 

determinations, attainment demonstrations, the development of Limited Maintenance Plans, clean data findings. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 27 of 46 

Answer: In general, the EPA will not prepare analyses or additional arguments to be 

included as components in a submitted demonstration package. Rather, the EPA will 

recommend demonstration package improvements to the submitting agency. However, if 

a demonstration package is associated with an imminent regulatory action and the public 

interest will be best served by the EPA preparing and/or considering additional analyses, 

the EPA may either assist with or independently prepare supporting analyses that could 

become part of the submission package or an EPA-prepared technical support document. 

Analyses prepared by the EPA could support either approval or disapproval of an air 

agency’s request for concurrence on flagged data. 

28a. Question: Does the Exceptional Events Rule contain a dispute resolution process that 

air agencies can use to resolve disagreements regarding non-concurrence on submittal 

packages? 

Answer: Several mechanisms currently exist that air agencies can use at various points in 

the exceptional events process: 

 Engage in early dialogue with the appropriate EPA regional office. 

 Submit requests for reconsideration to the official who made the determination if a 

request identifies a clear error or if information submitted by the agency was 

overlooked 

 Elevate the concern within the EPA’s chain of command. 

 Participate in the public notice and comment process (see Question 14a). 

 Challenge in an appropriate court the regulatory decision subsequently made that is 

based on the EPA’s exceptional event determination. 

In addition, for complex exceptional events claims or those with significant regulatory or 

other impacts (e.g., those claims that directly influence proposed designation or 

redesignation, classifications, and attainment determinations), the EPA regional office 

staff will generally seek input from other EPA regional offices and/or the EPA 

headquarters staff. 

F. Exceptional Event Data Flagging for Air Quality Concentrations that Could 

Contribute to an Exceedance or Violation of the National Ambient Air Quality 

Standards 

29. Question: Each criteria pollutant except PM10 now has multiple NAAQS in effect that 

differ by averaging period, and/or there is an “original” and a lower “revised” NAAQS 

level each of which has regulatory significance. If a measurement value is approved by 

the EPA for exclusion for one particular NAAQS averaging period and level, is it 

automatically excluded for all the other NAAQS for that pollutant? 

Answer: No. The exclusion of a measured air concentration is to be justified and 

approved separately for each NAAQS that applies to the pollutant. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 28 of 46 

When initially flagging data, an air agency does not need to commit to the specific 

NAAQS for which it seeks to exclude a measured concentration. The EPA’s ambient air 

quality database, AQS, is designed to allow an air agency to apply a single flag to a 

measured concentration value, which merely indicates the agency’s interest in excluding 

that value with respect to one or more of the applicable NAAQS. Later, in the 

justification (i.e., the demonstration) for exclusion, the air agency can indicate the 

specific NAAQS for which it seeks exclusion and for which the demonstration addresses 

the Exceptional Events Rule criteria. When the EPA makes a decision regarding 

concurrence with an air agency’s flag, it will generally identify in its 

approval/disapproval letter (or other official notice) all of the NAAQS for which the EPA 

has concurred on the flag. The EPA will also generally set a flag in AQS indicating 

concurrence with respect to a specific single NAAQS or a specific combination of 

NAAQS for that pollutant (e.g., in the case of PM2.5, the 24-hour NAAQS only, the 

annual NAAQS only, or both the 24-hour and the annual average NAAQS). The EPA 

does this by associating one or more “pollutant standard ID” value with the concurrence. 

The EPA concurrence flags entered into AQS prior to the March 2010 re-engineering of 

AQS to accommodate the Exceptional Events Rule did not indicate the specific single 

NAAQS or the specific combination of NAAQS for which the exclusion was approved. 

These “legacy” concurrence flags have been converted to the new approach using the 

following defaulting scheme: 

 For ozone, all legacy flags were treated as applying to both the 0.08 ppm 8-hour 

NAAQS and the 0.12 ppm 1-hour NAAQS. This default was chosen because as of 

March 2010, designations under the 2008 NAAQS of 0.075 ppm had been 

suspended pending reconsideration of that NAAQS, and AQS staff were not 

aware of any concurrences already granted with respect to the 0.075 ppm 

NAAQS. 

 For PM2.5, all concurrences on events with dates prior to January 1, 2005 

(meaning the date of the concentration, not the date of the EPA’s concurrence) 

were presumed to be applicable only to the annual PM2.5 NAAQS. This default 

was chosen because prior to the revision of the 24-hour PM2.5 NAAQS in 2006, 

violations of the 1997 24-hour NAAQS were extremely rare. 

 For PM2.5, all concurrences on events with dates of January 1, 2005 through 

March 2010 were presumed to be applicable only to the 24-hour NAAQS because 

there were no revisions to the annual PM2.5 NAAQS during this timeframe, so 

designations to nonattainment for the annual PM2.5 standard were extremely rare. 

This 24-hour PM2.5 NAAQS default was chosen because it was possible for 

designations under the 2008 24-hour NAAQS to be based on data as early as 

2005. 

 For PM10, all concurrences were presumed to apply to the 24-hour NAAQS, as 

the annual PM10 NAAQS was revoked in 2006.15 

 

15 The EPA realizes that many of the defaulted EPA concurrences for pre-2006 PM10 concentrations that were 

below the level of the 24-hour PM10 NAAQS actually were applicable to the annual PM10 NAAQS, but this 

approach was the most practical way to ensure that all other concurrences originally intended to be applicable to 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 29 of 46 

 For CO, all concurrences were presumed to apply to both the 1-hour and the 8-

hour NAAQS. This default was chosen to ensure that the concurrence applied to 

whichever NAAQS had been exceeded and was the basis for the exclusion 

request. 

 For SO2, all concurrences were presumed to apply to both the 24-hour and the 

annual NAAQS. This default was chosen to ensure that the concurrence applied to 

whichever NAAQS had been exceeded and was the basis for the exclusion 

request. No flags were assumed to apply to the 1-hour NAAQS because the 1-

hour SO2 standard was not promulgated until June of 2010, after the AQS reengineering. 

 For Pb, all concurrences (if any existed) were presumed to apply to the quarterly 

average NAAQS of 1.5 μg/m3. This default was chosen because March 2010 was 

prior to the EPA issuing final designations under the 2008 Pb NAAQS of 0.15 

μg/m3

. 

 For NO2, all concurrences were presumed to apply to the annual NAAQS because 

the 1-hour NO2 standard was not promulgated until February of 2010. 

For concurrences on events with dates after the March 2010 re-engineering of AQS, the 

EPA will specify the NAAQS to which the concurrence applies. If this defaulting scheme 

does not properly represent the actual concurrence action that was taken by the EPA 

regional office, the regional office should revise and correct the concurrence flags, if they 

have not already done so. 

 

Detailed information on the use of events flags in AQS can be found in a tutorial posted 

at http://www.epa.gov/ttn/airs/airsaqs/manuals/ExceptionalEventTutorial.pdf. 

Concurrence flags are discussed on page 20 of this tutorial. 

30. Question: For a NAAQS that is defined for a multi-hour or multi-day averaging time, 

but for which concentrations are measured, reported, and flagged on the basis of a shorter 

time period, what comparisons between measurements and the NAAQS level should be 

done to satisfy the “but for” test? 

Answer: One requirement for data exclusion under the Exceptional Events Rule is that 

there would have been no exceedance or violation of the NAAQS “but for” the event. In 

AQS, flagging and concurrence are done for each individual reported measurement. 

When the averaging period for the NAAQS is the same as the measurement duration 

period, individual measurements can be compared directly to the level of the NAAQS. 

This is the case for the 1-hour ozone, 1-hour CO, 1-hour SO2, and 1-hour NO2 NAAQS.16

 the 24-hour NAAQS were preserved. Because concentrations below the level of the 24-hour NAAQS have no 

effect on attainment determinations for the 24-hour NAAQS, no error can come from treating such values as 

having been concurred. Nevertheless, the EPA regional office may choose to update these concurrence flags as 

time permits. 

16 Air agencies have for many years reported SO2 concentrations as hourly averages. While some air agencies 

have also voluntarily reported 5-minute average concentrations also, either for each of the 12 5-minute blocks in 

an hour or for the maximum 5-minute average concentrations (block or running) during an hour, it is the hourly 

concentration averages that should be compared to the 1-hour SO2 NAAQS. Under a change in SO2 monitoring 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 30 of 46 

However, a difference exists for the following NAAQS between the time period for 

reporting concentrations and the averaging period to which the level of a NAAQS 

applies. 

 Ozone, CO, NO2, and SO2 are reported to AQS as 1-hour measurements, but all 

three have NAAQS defined for longer averaging periods (3-hours, 8-hours, 24-

hours, and/or annual). 

 Pb is reported as 24-hour measurements, but the old and new NAAQS are both 

for three-month averages (quarterly averages and three-month rolling averages, 

respectively). 

 When using automated/continuous monitoring equipment, PM2.5 and PM10 are 

sometimes reported as 1-hour measurements but there are PM2.5 and PM10

NAAQS with 24-hour averaging periods and a PM2.5 NAAQS with an annual 

averaging period. 

 When using filter-based monitoring equipment, PM2.5 and PM10 are sometimes 

reported as 24-hour measurements but there is a PM2.5 NAAQS with an annual 

averaging period. 

The mismatches of time periods make this a question with a complex answer. The 

following paragraphs, summarized in Table Q30-1, explain the general rationale behind 

the pollutant and NAAQS-specific entries in Table Q30-2. 

To satisfy the “but for” criterion, there must have actually been an exceedance or 

violation of the NAAQS in a time period overlapping with the event and its effects on 

air quality, and which would not have occurred “but for” the effects of the event.17 

By definition, an exceedance necessarily involves a comparison between an air 

concentration, averaged over a time period equal in length to the averaging time of 

the NAAQS, and the level of the NAAQS. For example, it does not make sense to 

compare an individual 1-hour ozone concentration to the level of the 8-hour NAAQS 

as part of a test of whether the “but for” criterion is met, because the outcome of the 

 requirements that accompanied the promulgation of the 1-hour SO2 NAAQS, the EPA now requires that air 

agencies report the maximum 5-minute block average concentration, as well as the hourly concentration (see 40 

CFR § 58.12(g)). Air agencies may satisfy the 5-minute reporting requirement by submitting all twelve 5-

minute block averages or by reporting only the maximum 5-minute block average concentration. The EPA’s 

AQS retains the hourly concentration as submitted; AQS does not use 5-minute data to replace the submitted 

hourly concentration. While 5-minute concentrations may play a role in evaluating whether Exceptional Event 

criteria are satisfied for a given hour and event, for example to establish a clear causal connection, they are not 

to be compared to the level of the 1-hour (or any other) NAAQS for SO2 as part of a “but for” demonstration 

and should not be flagged for exclusion under the EER. Air agencies may, however, use “I” series flags 

(Information only) with 5-minute SO2 data. 17 The EPA interprets the Exceptional Event Rule and its preamble to mean “exceedance or violation” each time 

that “exceedance” or “violation” occurs in the text, consistent with the obvious intent of the Clean Air Act 

amendment requiring the EPA to promulgate the Rule. An “exceedance” occurs each time the concentration in 

the air for the averaging period applicable to the NAAQS is higher than the level of the NAAQS. Most NAAQS 

allow some such occurrences in a 1-year or 3-year time period (depending on the NAAQS). A “violation” of the 

NAAQS occurs when there have been enough high-concentration episodes that the statistical form of the 

particular NAAQS indicates a failure to meet the NAAQS. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 31 of 46 

comparison for a single hour does not indicate whether an exceedance or violation of 

the 8-hour NAAQS occurred, or whether it would not have occurred “but for” the 

event. Instead, one should consider whether the event made a “but for” difference in 

the average concentration over the period that is the same as the averaging period for 

the NAAQS. That is, air agencies making a “but for” argument should compare the 

average concentration to the identified NAAQS rather than the individual 

concentrations that comprise the average. Air agencies should, however, identify in 

their exceptional event submission those cases in which a single measurement or 

several, but not all, measurements cause the elevated average. 

The preamble to the Exceptional Events Rule provides one exception from this formal 

definitional approach. The preamble states that in the particular case of PM2.5, the 

direct comparison of a single 24-hour average concentration (determined from a 

single filter-based measurement or by averaging 24 1-hour measurements from a 

continuous equivalent instrument) to the level of the annual NAAQS (currently 15 

μg/m3

) can be the basis for meeting the “but for” criterion for exceedances or 

violations of the annual NAAQS. In context, it is clear that based on this comparison, 

a 24-hour concentration can be excluded from the calculation of the annual PM2.5

NAAQS design value, if other rule criteria are also met. It is therefore not necessary 

to show that the annual average PM2.5 concentration was above 15 μg/m3

 with the 

event and would have been below 15 μg/m3

 “but for” the single event at issue. Such a 

concentration can also be excluded from the calculation of the design value for the 

24-hour PM2.5 NAAQS, although this is likely to make a difference to meeting the 

NAAQS only if the actual measured concentration were close to or above 35 μg/m3. 

This special case is reflected in Table Q30-2. 

In light of this departure in the preamble from a formal definitional approach in the 

case of a 24-hour PM2.5 measurement and the annual PM2.5 NAAQS, Table Q30-2 

also provides a parallel special approach for similar comparisons involving Pb, NO2

and SO2. The EPA believes applying this interpretation for Pb, NO2, and SO2 is 

consistent with the interpretation in the preamble for PM2.5 and is consistent with the 

EPA’s intent in drafting the Exceptional Events Rule that should be applicable to all 

pollutants. That is, a 24-hour average concentration of Pb, NO2, or SO2 can be 

compared to the NAAQS level defined for a longer period, for purposes of meeting 

“but for” with respect to the NAAQS with the longer averaging period. However, the 

EPA does not intend to concur on flags for a 1-hour NO2 and SO2 concentration that 

is below the level of the annual NAAQS, regardless of the outcome of “but for” 

comparisons based on 24-hour or annual averaging periods.18 Also, the EPA does not 

 

18 This restriction is intended to parallel the similar restriction for PM2.5 stated in the preamble to the 

Exceptional Event Rule. It likely has no practical effect. It is highly unlikely that even several hourly 

concentrations below the level of the annual NO2 NAAQS (53 ppb) could include an event contribution that 

would, when divided by 8760 (24 hours times 365 days), result in the annual average NO2 concentration 

crossing from below to above the level of the annual NAAQS. Similarly, it is highly unlikely that even several 

hourly concentrations below the level of annual SO2 NAAQS (30 ppb) could include an event contribution that 

would, when divided by 24, result in the 24-hour average SO2 concentration crossing from below to above the 

level of the 24-hour SO2 NAAQS (140 ppb). 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 32 of 46 

intend to concur on flags for a 24-hour Pb measurement below the level of the old 

(fixed quarterly average) Pb NAAQS or the new (rolling 3-month average) Pb 

NAAQS. 

Table Q30-1. Principles for Correct Approaches for Helping to Show That the “But 

For” Test Is Met 

Note: The principles identified in this table are presented from the more general and/or selfevident to the more specialized and/or derivative. 

 Principle Application to Specific 

NAAQS 

Exceptions 

1 A single measurement may be 

compared directly to the level of 

the NAAQS if the averaging times 

are the same. 

 1-hour NAAQS for CO, SO2, 

NO2, and ozone. 

 24-hour filter-based PM2.5 or 

PM10 measurements vs. 24-

hour NAAQS. 

2 When the measurement time is 

shorter than the averaging time of 

the NAAQS (e.g., 1-hour O3

measurements and the 8-hour O3

NAAQS), air agencies can compare 

the average of multiple 

measurements within the averaging 

period of the NAAQS to the level 

of the NAAQS (e.g., compare the 

average of eight 1-hour 

measurements to the 8-hour 

NAAQS). If this comparison shows 

that the average is more than the 

NAAQS but would have been 

below the NAAQS in the absence 

of the event, then the “but for” test 

will have been met for those 

individual measurements in the 

longer averaging period that were 

affected by the event. Air agencies 

should, however, identify in their 

exceptional event submission those 

cases in which a single 

measurement or several, but not all, 

measurements cause the elevated 

average. 

 1-hour ozone measurements 

vs. 8-hour NAAQS. 

 1-hour CO measurements vs. 

8-hour NAAQS. 

 1-hour SO2 measurements vs. 

3-hour, 24-hour, and annual 

NAAQS. 

 1-hour NO2 measurements vs. 

annual average .NAAQS. 

 1-hour PM2.5 measurements 

vs. 24-hour and annual 

average NAAQS. 

 1-hour PM10 measurements 

vs. 24-hour average NAAQS. 

 24-hour PM2.5 measurements 

vs. annual average NAAQS. 

 24-hour Pb measurements vs. 

quarterly average NAAQS. 

 24-hour Pb measurements vs. 

rolling 3-month average 

NAAQS. 

If a measurement value is 

below the level of the 

quarterly, rolling 3-month, or 

annual average NAAQS, it 

cannot be excluded, 

regardless of the outcome of 

comparing the longer period 

average to the NAAQS level. 

3 When the PM2.5 or Pb measurement 

time is 24 hours, it is also permitted 

to compare the 24-hour 

measurement to the annual average 

PM2.5 NAAQS or the quarterly or 

rolling 3-month Pb NAAQS. 

 24-hour PM2.5 filter 

measurements vs. the annual 

average NAAQS (expressly 

permitted in the preamble to 

the Exceptional Events Rule). 

 24-hour Pb filter 

measurements vs. the 

quarterly average and rolling 

3-month average NAAQS 

(suggested by this guidance as 

If a measurement value is 

below the level of the 

quarterly, rolling 3-month, or 

annual average NAAQS, it 

cannot be excluded. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 33 of 46 

 Principle Application to Specific 

NAAQS 

Exceptions 

a consistent with the intent of 

the PM2.5 provision in the 

preamble). 

4 1-hour PM2.5 and SO2

measurements may be averaged to 

24-hour periods and then compared 

to the annual average NAAQS. If 

the “but for” test is supported by 

this comparison, the showing 

supports a “but for” finding for 

those individual 1-hour 

measurements in the 24-hour 

averaging period that were affected 

by the event. 

 1-hour PM2.5 measurements 

vs. annual average NAAQS 

(suggested by this guidance to 

create a level playing field 

between filter-based and 

continuous PM2.5

measurements). 

 1-hour SO2 measurements vs. 

annual average NAAQS 

(where the 30 ppb annual SO2

NAAQS still applies) 

If the average of the 24 1-

hour measurements is below 

the level of the annual 

average NAAQS, it cannot 

be excluded. 

5 When there is no NAAQS for the 

24-hour averaging period, 1-hour 

measurements may be compared 

directly to the annual NAAQS. 

 1-hour NO2 measurements vs. 

annual average NAAQS 

(suggested by this guidance to 

create a benchmark for 

judging the excludability of 1-

hour NO2 measurements, 

other than whether the event 

affected the annual average 

enough to make a “but for” 

difference relative to the 

annual average NAAQS). 

If a measurement value is 

below the level of the annual 

average NAAQS, it cannot 

be excluded. 

6 Otherwise, single 1-hour 

measurements may not be 

compared to the level of the annual 

average NAAQS. 

 Single 1-hour SO2

measurements may not be 

compared the annual average 

NAAQS (because there is a 

24-hour NAAQS for SO2 with 

a defined averaging 

methodology). 

 Single 1-hour PM2.5

measurements may not be 

compared to the annual 

average NAAQS (because 

there is a 24-hour NAAQS for 

PM2.5 with a defined 

averaging methodology). 

Table Q30-2 identifies the comparisons and conclusions that would help satisfy the “but 

for” test for each pollutant, for each current NAAQS. Note that for completeness Table 

Q30-2 addresses some situations that may be very unlikely to actually occur – for 

example, that a single event might cause an exceedance of the annual average NO2

NAAQS. 

 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 34 of 46 

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

Correct Approach

1 Ozone 0.12 ppm 

1-hour averaging period 

1-hour measurement 

If a 1-hour measured concentration was above 0.124 

ppm but would have been 0.124 ppm or less in the 

absence of the event, the 1-hour ozone concentration 

value meets the “but for” test for purposes of 

comparison to the 1-hour NAAQS. If other criteria are 

also met for that hour (e.g., there was a clear causal 

relationship between the event and that hour’s ozone 

level, among other criteria), then the hour can be 

flagged and concurred for exclusion. 

2 Ozone 0.08 ppm 

8-hour averaging period 

1-hour measurement 

 If the daily maximum 8-hour average of 

measured concentrations was above 0.084 ppm 

but would have been 0.084 ppm or less in the 

absence of the event, those 1-hour concentration 

values that were affected by the single event meet 

the “but for” test for purposes of comparison to 

the 0.08 ppm 8-hour ozone NAAQS. 

The exclusion of some or all hours of the 8-hour 

period that was originally the daily maximum 8-hour 

period may cause another 8-hour period to become the 

daily maximum. The “but for” comparison can be 

repeated for this new 8-hour period, which may result 

in flagging and concurrence for more 1-hour values. It 

is also possible for additional hourly concentrations 

that were not included in the original 8-hour block to 

be excluded as part of a second 8-hour block. 

3 Ozone 0.075 ppm 

8-hour averaging period 

1-hour measurement 

 If the daily maximum 8-hour average of 

measured concentrations was above 0.075 ppm 

but would have been 0.075 ppm or less in the 

absence of the event, those 1-hour concentration 

values that were affected by the single event 

meet the “but for” test for purposes of comparison 

to the 0.075 ppm 8-hour ozone NAAQS. 

The exclusion of some or all hours of the 8-hour 

period that was originally the daily maximum 8-hour 

period may cause another 8-hour period to become the 

daily maximum. The “but for” comparison can be 

repeated for this new 8-hour period, which may result 

in flagging and concurrence for more 1-hour values. It 

is also possible for additional hourly concentrations 

that were not included in the original 8-hour block to 

be excluded as part of a second 8-hour block. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 35 of 46 

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

Correct Approach

4 PM2.5 35 μg/m3 

24-hour averaging period 

1-hour measurement 

 If the 24-hour average concentration based on 1-

hour measurements was above 35.4 μg/m3 (after 

truncating after the first decimal digit, per 40 

CFR 50 Appendix N section 3.0(c)) but would 

have been 35.4 μg/m3 or less in the absence of 

the event, those 1-hour concentration values that 

were affected by the single event meet the “but 

for” test for purposes of comparison to 35 μg/m3 

24-hour PM2.5 NAAQS. 

 Also, if the 24-hour average concentration based 

on 1-hour measurements was above 15.0 μg/m3 

(after truncation after the first decimal digit) but 

would have been 15.0 μg/m3 or less in the 

absence of the event, those 1-hour concentration 

values that were affected by the single event 

meet the “but for” test for purposes of 

comparison to 35 μg/m3 24-hour PM2.5 NAAQS. 

5 PM2.5 15.0 μg/m3 

Annual averaging period 

1-hour measurement 

 If the annual average PM2.5 concentration was 

above 15.0 μg/m3 but would have been equal to 

or less than 15.0 μg/m3

 (after rounding to one 

decimal digit) in the absence of the single 

event’s effect on one or more hours, those 1-hour 

concentration values that were affected by the 

single event meet the “but for” test for purposes 

of comparison to 15 μg/m3 annual PM2.5

NAAQS. 

 Also, if the 24-hour average concentration based 

on 1-hour measurements was above 15.0 μg/m3 

(after rounding to one decimal digit, per 40 CFR 

50 Appendix N section 4.3(a)) but would have 

been equal to or less than 15.0 μg/m3in the 

absence of the event, those 1-hour concentration 

values that were affected by the single event 

meet the “but for” test for purposes of 

comparison to 15 μg/m3 annual PM2.5 NAAQS. 

However, an hourly value must be part of a 24-hour 

average concentration that is above 15 μg/m3

 (after 

rounding to one decimal digit) to be excluded from an 

annual NAAQS calculation. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 36 of 46 

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

Correct Approach

6 PM2.5 35 μg/m3 

24-hour averaging period

24-hour measurement 

 If the 24-hour average concentration was above 

35.4 μg/m3 (after truncating after the first 

decimal digit, per 40 CFR 50 Appendix N 

section 3.0(b)) but would have been 35.4 

μg/m3

or less in the absence of the event, the 24-

hr concentration value meets the “but for” test 

for purposes of comparison to 35 μg/m3 24-hour 

PM2.5 NAAQS. 

 Also, if the 24-hour average concentration was 

above 15.0 μg/m3 (after truncating after the first 

decimal digit, per 40 CFR 50 Appendix N 

section 3.0(b)) but would have been 15.0 

μg/m3

or less in the absence of the event, the 24 

average concentration meets the “but for” test for 

purposes of comparison to 35 μg/m3 24-hour 

PM2.5 NAAQS. 

7 PM2.5 15 μg/m3 

Annual averaging period 

24-hour measurement 

 If the annual average PM2.5 concentration was 

above 15.0 μg/m3 (after rounding to one decimal 

digit per 40 CVFR 50 Appendix N section 

4.2(a)) but would have been equal to or less than 

15.0 μg/m3

 in the absence of the single event’s 

effect on one or more days, those 24-hour 

concentration values that were affected by the 

single event meet the “but for” test for purposes 

of comparison to 15 μg/m3 annual PM2.5

NAAQS. 

 Also, if the 24-hour average concentration from 

the filter-based sampler was above 15.0 μg/m3 

(after truncating after the first decimal digit, per 

40 CFR 50 Appendix N section 3.0(b)) but 

would have been equal to or less than 15.0 

μg/m3

in the absence of the event, the 24-hour 

value meets the “but for” test for purposes of 

comparison to 15 μg/m3 annual PM2.5 NAAQS. 

Note that a 24-hour concentration that is equal to or 

less than 15.0 μg/m3

 (after truncation to one decimal 

digit) cannot be approved for exclusion, regardless of 

the outcome of the comparison just described. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 37 of 46 

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

Correct Approach

8 PM10 150 μg/m3 

24-hour averaging period 

1-hour measurement 

 If the 24-hour average concentration based on 1-

hour measurements was above 150 μg/m3 (after 

rounding to the nearest 10 μg/m3

, per 40 CFR 50 

Appendix K section 1.0(b)) but would have been 

equal to or less than 150 μg/m3

 in the absence of 

the event, those 1-hour concentration values that 

were affected by the single event meet the “but 

for” test for purposes of comparison to the 150 

μg/m3 24-hour PM10 NAAQS. 

9 PM10 150 μg/m3 

24-hour averaging period 

24-hour measurement 

 If the 24-hour average concentration from the 

filter-based sampler was above 150 μg/m3 (after 

rounding to the nearest 10 μg/m3

, per 40 CFR 50 

Appendix K section 1.0(b)) but would have been 

equal to or less than 150 μg/m3

 in the absence of 

the event, the 24-hour value meets the “but for” 

test for purposes of comparison to the 150 μg/m3 

24-hour PM10 NAAQS. 

10 CO 35 ppm 

1-hour averaging period 

1-hour measurement 

 If a 1-hour measured concentration was above 

35.0 ppm (after rounding to one decimal digit 

per 40 CFR 50.8(d)) but would have been 35.0 

ppm or less in the absence of the event, the 1-

hour CO concentration value meets the “but for” 

test for purposes of comparison to the 1-hour 

NAAQS. 

11 CO 9 ppm 

8-hour averaging period 

1-hour measurement 

 If an 8-hour average of measured concentrations 

is one of the two highest non-overlapping 8-hour 

periods of the year and was above 9.0 ppm (after 

rounding to one decimal digit per 40 CFR 

50.8(d)) but would have been equal to or less 

than 9.0 ppm in the absence of the event, those 

1-hour concentration values that were affected 

by the single event meet the “but for” test for 

purposes of comparison to the 9 ppm 8-hour CO 

NAAQS. 

The exclusion of some or all hours of the 8-hour 

period that was originally one of the two highest nonoverlapping 8-hour periods of the year may cause 

another 8-hour period to become one of two highest 

non-overlapping 8-hour periods of the year. The “but 

for” comparison can be repeated for this new 8-hour 

period, which may result in flagging and concurrence 

for more 1-hour values. It is also possible for 

additional hourly concentrations that were not 

included in the original 8-hour block to be excluded as 

part of a second 8-hour block. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 38 of 46 

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

Correct Approach

12 Pb 1.5 μg/m3 

Quarterly averaging period

24-hour measurement 

 If the quarterly mean was above 1.5 μg/m3 (after 

rounding to one decimal digit) but would have 

been equal to or less than 1.5 μg/m3 in the 

absence of the single event’s effect on some 

day(s), the 24-hour value(s) affected by the 

single event meets the “but for” test for purposes 

of comparison to the 1.5 μg/m3 quarterly average 

Pb NAAQS. (Note that given the 1-in-6 

sampling schedule for Pb, it will be unusual for a 

single event to affect multiple sampling days.) 

 Also, if the 24-hour average concentration from 

the filter-based sampler was above 1.5 μg/m3 

(after rounding to one decimal digit) but would 

have been equal to or less than 1.5 μg/m3

in the 

absence of the event, the 24-hour value meets the 

“but for” test for purposes of comparison to 1.5 

μg/m3 quarterly average Pb NAAQS. 

 

A 24-hour Pb concentration that is equal to or less 

than 1.5 μg/m3 can never be excluded, regardless of 

the outcome of the comparison just described. 

13 Pb 0.15 μg/m3 

Rolling 3-month averaging period

24-hour measurement 

 If a 3-month mean was above 0.15 μg/m3 (after 

rounding to two decimal digits) but would have 

been equal to or less than 0.15 μg/m3

 in the 

absence of the single event’s effect on some 

day(s), the 24-hour value affected by the single 

event meets the “but for” test for purposes of 

comparison to the 0.15 μg/m3 quarterly average 

Pb NAAQS. (Note that given the 1-in-6 

sampling schedule for Pb, it will be unusual for a 

single event to affect multiple sampling days.) 

 Also, if the 24-hour average concentration from 

the filter-based sampler was above 0.15 μg/m3 

(after rounding to two decimal digits per 40 CFR 

50 Appendix R section 5(b)) but would have 

been equal to or less than 0.15 μg/m3

 in the 

absence of the event, the 24-hour value meets the 

“but for” test for purposes of comparison to the 

0.15 μg/m3 quarterly average Pb NAAQS. 

A 24-hour Pb concentration that is equal to or less 

than 0.15 μg/m3 can never be excluded, regardless of 

the outcome of the comparison just described. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 39 of 46 

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

Correct Approach

14 NO2 100 ppb 

1-hour averaging period 

1-hour measurement 

 If a 1-hour measured concentration was above 

100 ppb (after truncating to a whole number per 

40 CFR 50 Appendix S section 4.2(c)) but would 

have been equal to or less than 100 ppb in the 

absence of the event, the 1-hour NO2

concentration value meets the “but for” test for 

purposes of comparison to the 1-hour NAAQS. 

15 NO2 53 ppb 

Annual averaging period 

1-hour measurement 

 If the annual average of all the measured 1-hour 

concentrations in a year was above 53 ppb (after 

rounding to a whole number per 40 CFR 50 

Appendix S section 4.1(b)) but would have been 

53 ppb or less in the absence of the event, those 

1-hour values that were affected by the single 

event meet the “but for” test for purposes of 

comparison to the 53 ppb annual average NO2

NAAQS. 

 Provided there is an exceedance of the annual 

standard, if the 1-hour concentration was above 

53 ppb (after truncating to a whole number per 

40 CFR 50 Appendix S section 4.2(c)) but would 

have been equal to or less than 53 ppb in the 

absence of the event meets the “but for” test for 

purposes of comparison to annual NAAQS. 

However, a 1-hour NO2 concentration that is below 53 

ppb (after rounding to a whole number) can never be 

excluded, regardless of the outcome of the comparison 

just described. 

16 SO2 75 ppb 

1-hour averaging period 

1-hour measurement 

If a 1-hour measured concentration was above 75 ppb 

(after rounding to a whole number per 40 CFR 50 

Appendix T section 4(c)) but would have been equal 

to or less than 75 ppb in the absence of the event, the 

1-hour SO2 concentration value meets the “but for” 

test for purposes of comparison to the 1-hour SO2

NAAQS. 

17 SO2 140 ppb 

24-hour averaging period 

1-hour measurement 

If the 24-hour average concentration based on 1-hour 

measurements was above 140 ppb (after rounding to 

the nearest 10 ppb per 40 CFR 50.4(b)) but would 

have been equal to or less than 140 ppb in the absence 

of the event, those 1-hour concentration values that 

were affected by the single event meet the “but for” 

test for purposes of comparison to 140 ppb 24-hour 

SO2 NAAQS. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 40 of 46 

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

Correct Approach

18 SO2 30 ppb 

Annual averaging period 

1-hour measurement 

 If the annual average of measured 1-hour 

concentrations was above 30 ppb (after rounding 

to a whole number per 40 CFR 50.4(a))) but 

would have been 30 ppb or less in the absence of 

the event, those 1-hour values that were affected 

by the single event meet the “but for” test for 

purposes of comparison to the 30 ppb annual 

average SO2 NAAQS. 

 Also, if the 24-hour average concentration based 

on 1-hour measurements was above 140 ppb

(after rounding to the nearest 10 ppb per 40 CFR 

50.4(b)) but would have been equal to or less 

than 140 ppb in the absence of the event, those 1-

hour concentration values that were affected by 

the event meet the “but for” test for purposes of 

comparison to the 30 ppb annual SO2 NAAQS. 

If the 30 ppb annual SO2 NAAQS still applies in the 

affected area, a 1-hour concentration equal to or below 

30 ppb (after rounding to a whole number per 40 CFR 

50.4(a)) may never be excluded, regardless of the 

outcome of the comparison just described. 

19 SO2

(secondary) 

500 ppb 

3-hour averaging period 

1-hour measurement 

 If the 3-hour average of measured 1-hour 

concentrations was above 500 ppb (rounded to 

the nearest 100 ppb per 40 CFR 50.5(a)) but 

would have been equal to or less than 500 ppb in 

the absence of the event, those 1-hour values that 

were affected by the single event meet the “but 

for” test for purposes of comparison to the 3-

hour average secondary SO2 NAAQS. 

31. Question: When is it appropriate for air agencies to flag concentration values that are 

less than the level of the relevant NAAQS? Under what circumstances will the EPA 

concur on such flags? 

Answer: (Please read Q30 before reading this response.) 

AQS currently allows an air agency to flag any measured concentration values it chooses, 

including values below the level of the relevant NAAQS. The EPA does not plan to 

implement any new technical restrictions through the AQS software. Also, the EPA does 

not consider the Exceptional Events Rule to prohibit air agencies from flagging values 

below the level of the NAAQS. However, the EPA does not intend to review data flags in 

AQS for concurrence until the air agency submits its evidence/analysis package 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 41 of 46 

demonstrating that exclusion of the flagged values is consistent with the criteria in the 

Exceptional Events Rule, including the “but for” analysis at 40 CFR 50.14(c)(3)(iv)(D). 

Air agencies wishing to flag values for informational purposes should use the “I” series 

flags in AQS. 

Air agencies may see an advantage in flagging all values they believe were affected by an 

event (and contribute to a violation of the NAAQS), for purposes of being able to later 

identify historical data that have not been affected so that “normal” concentration patterns 

can be presented as part of meeting the “in excess of historical fluctuations” prong of the 

exclusion criteria. AQS does not prevent such flagging, but air agencies should be aware 

that agency flagging by itself does not establish that the concentrations were in fact 

affected by an event and should be excluded from the “normal” baseline. 

Of the flagged cases that appear in both AQS and in demonstration packages, the EPA 

can concur with flags for concentrations that are below the NAAQS only in five very 

narrow conditions described below. If the EPA determines that a flag on a value less than 

the level of the NAAQS cannot meet the “but for” test, it is likely the EPA would 

nonconcur or leave the default/null value of the AQS concurrence flag (indicating no 

EPA action) in place. 

Except in cases involving PM10 limited maintenance plans19, the EPA intends to prioritize 

events that result in a violation or exceedance of a NAAQS or those that otherwise 

impact a regulatory decision. As described below and in the response to Question 30, 

there may be specific instances where individual measurements fall below a NAAQS but 

still contribute to a violating design value. There may also be instances where a shorter 

averaging time measurement (e.g., 1-hour O3 measurement of 100 ppb) is not above the 

level of that averaging time NAAQS (e.g., 1-hour O3 NAAQS of 120 ppb), but is above a 

longer averaging time NAAQS (e.g., 8-hour O3 NAAQS of 80 ppb) and contributes to a 

violation of the longer averaging time NAAQS. In such cases, although the individual 

measurement may not exceed the level of the (short-term) NAAQS, it may be possible 

for air agencies to present sufficient evidence to satisfy the “but-for” criterion. 

(See Questions 8, 9, 13, and 19 for additional information.) 

First, PM10 values between 98 and 154 μg/m3 (inclusive) may be flagged, concurred, and 

excluded for purposes of qualifying an area for reliance on only a limited maintenance 

plan.20 Because of the expected exceedance form of the PM10 NAAQS, concentrations in 

this range cannot possibly affect whether a site actually meets the NAAQS, so there is no 

reason for flagging them except when the acceptability of a limited maintenance plan is 

 

19 See May 7, 2009 policy memorandum from William T. Harnett to Regional Air Division Directors at 

http://www.epa.gov/ttn/oarpg/t1/memoranda/lmp_final_harnett.pdf that allows PM10 values between 98 and 154 

μg/m3 (inclusive) to be flagged, concurred, and excluded for purposes of qualifying an area for reliance on only 

a limited maintenance plan. 

20 See May 7, 2009 policy memorandum from William T. Harnett to Regional Air Division Directors at 

http://www.epa.gov/ttn/oarpg/t1/memoranda/lmp_final_harnett.pdf. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 42 of 46 

an issue. The normal AQS flagging and concurrence procedures may be used in this 

situation.21

A second scenario in which the EPA can concur with flags for concentrations that are 

below the NAAQS is indicated at 72 FR at 13570. If (i) an event has affected air quality 

on multiple consecutive days, (ii) at least one measured concentration during the episode 

can be found to meet the “but for” test using the relevant comparison specified in Table 

Q30-2, and (iii) the air quality impact on each day is “exceptional,” measurements for the 

entire period are eligible for data exclusion regardless of how they compare to the level 

of the NAAQS. In the context of this provision, “exceptional” encompasses all the 

requirements of the Exceptional Events Rule other than the “but for” test (e.g., clear 

causal connection, “in excess of normal historical fluctuations, including background,” 

not reasonably controllable or preventable). 

Scenarios in which the measured concentration is greater than a NAAQS with a 

longer averaging time but less than the level of a NAAQS with a shorter averaging 

time

Third, applying Table Q30-2 may result in qualifying a 24-hour PM2.5 measurement that 

is greater than the 15 μg/m3 annual PM2.5 NAAQS but not greater than the 35 μg/m3 24-

hour PM2.5 NAAQS for exclusion for the purposes of the 24-hour PM2.5 NAAQS. This is 

the result if the actual 24-hour concentration was between 15 and 35 μg/m3 but would 

have been below 15 μg/m3 but for the effect of the event. It should be noted that an 

exclusion made under this very specific provision for the 24-hour PM2.5 NAAQS will 

only affect the outcome of an attainment determination for the 24-hour NAAQS if the 

concentration value in question is one of the few highest daily concentrations during the 

year, because only then could it have affected the 3-year design value. When a 24-hour 

value below the level of the 24-hour NAAQS does affect the 3-year design value, the 

application of the guidance for the fourth situation (below), which is applicable to all four 

NAAQS pollutants with multi-year design values, would get to the same result as 

application of this paragraph. 

Fourth, assuming that all other Exceptional Events Rule requirements and conditions are 

met, the EPA may concur with flags for ozone, PM2.5, 1-hour NO2, and 1-hour SO2 that 

are “less than the level of the NAAQS” if adjusting the flagged concentrations for the 

estimated contribution from the event would change the 3-year design value from being 

above the NAAQS to being equal to or below the NAAQS. However, as indicated in 

Table Q30-2, concentrations below certain values may never be excluded. 

Fifth, a 1-hour measurement of a pollutant that is below the level of the 8-hour, 3-hour, 

24-hour, or quarterly NAAQS for that pollutant can be excluded if (1) the event affected 

the 1-hour measurement, and (2) taking into account the event’s effect on all the hours in 

 

21 The procedure for determining a PM10 design value in units of μg/m3 is given in section 6.3 of the EPA 

guidance document “PM10 SIP Development Guideline,” June 1987, posted at 

http://www.epa.gov/ttn/oarpg/t1/memoranda/pm10sip_dev_guide.pdf . 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 43 of 46 

the longer period the effect of the event on the longer averaging period’s concentrations 

satisfies the “but for” criterion. These situations are described in Table Q30-2 (rows 3, 4, 

8, 11, 12, 13, 17, and 19). However, as indicated in Table Q30-2, concentrations below 

certain values may never be excluded. 

The following NAAQS-specific discussions provide further explanations regarding some 

of the situations in which a concentration less than the level of the NAAQS may qualify 

for exclusion. These discussions are not exhaustive and do not obviate the need to refer to 

Table Q30-2. 

24-hour PM2.5 

Assume for illustration that the three annual 98th percentile 24-hour PM2.5 concentrations 

for a monitoring site for 2006-2008 are 41, 31, and 37 μg/m3 for each respective year with 

a resulting 3-year design value of 36 μg/m3 which is a violation of the 24-hour PM2.5 

NAAQS of 35 μg/m3. Also, assume that the next highest concentration in 2007 below the 

31 μg/m3 was only 20 μg/m3. The 31 μg/m3 concentration in 2007 was affected by a oneday wildfire. The air agency has been able to show that the concentration would have 

been 17 μg/m3 without the fire. Because neither 20 μg/m3 nor 31 μg/m3 exceed the 

NAAQS, the event on that day does not meet the “but for” test when viewed from an 

“exceedance” perspective. However, the effect of the fire on the 2007 value determines 

whether the 3-year design value passes the 24-hour NAAQS. Had there been no fire, the 

98th percentile concentration in 2007 would have been 20 μg/m3 which would result in a 

3-year design value of 33 μg/m3 (i.e., less than the 24-hour PM2.5 NAAQS of 35 μg/m3). 

Therefore, the 2007 value of 31 μg/m3 meets the “but for” test when the focus in on 

NAAQS violations rather than individual exceedances. Assuming other requirements are 

met, the 31 μg/m3 concentration would be approved by the EPA for exclusion from the 

2006-2008 design value. Note that in doing a “violations-based” “but for” analysis, one 

does not simply substitute the “no event” concentration for the original 98th percentile 

day into the design value calculation. Rather, one must re-select the 98th percentile day, 

which sometimes will result in a different day’s actual measured value being used in the 

design value calculation.22 

It is conceivable that the effect of an event on a given day is not enough to satisfy the 

“but for” test with regard to the “violation” perspective explained in the preceding 

paragraph for one three-year period, but that it does satisfy it for an earlier or later 3-year 

period when it is combined with one or two different concentrations to calculate a 3-year 

design values, since the outcome of the “violations” analysis may change. After the EPA 

has approved the exclusion of a concentration based on a “violations” analysis for one 3-

year period, the EPA will also exclude that concentration when calculating design values 

and attainment for the other two 3-year periods that include that same year. 

 

22 Note that exclusion of this 24-hour value from design values for the annual average NAAQS is a separate 

question, the likely answer to which is that the value is not excludable. If the event did not make the 24-hour 

concentration change from below 15 to above 15 μg/m3

 the event does not meet the first condition specified in 

row 7 of Table Q30-2. It is also very improbable that an event affecting a single day would meet the second 

condition in row 7 of Table Q30-2. 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 44 of 46 

For the 24-hour PM2.5 NAAQS, it is possible that multiple days with concentrations 

below the NAAQS within one year are flagged. Excluding just one of these 

concentrations may not change the annual 98th percentile concentration enough to cause 

the 3-year design value to change from “violating” to “complying,” but excluding several 

of them may. The outcome for the design value may also depend in part on whether 

exclusion is granted for some other concentrations that are above the level of the 

NAAQS. In such cases, the exclusion decisions should first be made for each of the 

flagged concentrations that are above the NAAQS. All remaining flagged concentrations 

(those meeting all other requirements and conditions of the Exceptional Events Rule) 

should then be considered in progressively larger groups ranked by concentration. That 

is, if excluding the highest one of the flagged concentrations below the level of the 

NAAQS would cause a switch in whether the 3-year design value violates the NAAQS 

then if the EPA determines that value is to be excluded then there is no impact to 

retaining all others and, thus, no need to make determinations for those others. If 

excluding the two highest such concentrations causes a switch, then there is no impact to 

determining whether others beyond those two should be retained. 

However, the preamble to the Exceptional Events Rule explicitly states that PM2.5

concentrations below the level of the annual NAAQS cannot be excluded for purposes of 

comparisons to the annual NAAQS. (72 FR at 13570, bottom of middle column) Even if 

the conditions described in the preceding paragraph are met, values below 15 μg/m3 

cannot be excluded. 

Annual PM2.5

The preamble to the Exceptional Events Rule explicitly states that PM2.5 concentrations 

below the level of the annual NAAQS cannot be excluded for purposes of comparisons to 

the annual NAAQS. (72 FR at 13570, bottom of middle column) 

Ozone (0.075 ppm 8-hour NAAQS)

Assume for illustration that the three annual 4th highest daily 8-hour ozone values in 

2006-2008 are 0.077, 0.076, and 0.075 ppm respectively. The 0.075 ppm value in 2008 

was affected by an exceptional event. The 3-year average would be 0.076 ppm, a 

NAAQS violation. If the 0.075 ppm value for 2008 were to be excluded and if, as a 

result, 2008’s new 4th highest value was 0.074 ppm or less, the 3-year average (after 

Appendix P truncation) would be 0.075 ppm, which is not a NAAQS violation. The 0.075 

ppm value may be excluded under these circumstances even though it is not itself an 

exceedance. Furthermore, the exclusion also applies to the use of this value when 

calculating the 2007-2009 and 2008-2010 design values, regardless of whether such 

exclusion causes those design values to switch from violating to complying with the 

NAAQS. 

For ozone, as for 24-hour PM2.5, it is possible that an air agency could flag multiple days 

within one year with concentrations below the NAAQS. Excluding just one of these 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 45 of 46 

concentrations may not change the annual 4th highest concentration enough to cause the 

3-year design value to change from “violating” to “complying,” but excluding several of 

them may. Also, the outcome for the design value may depend, in part, on whether 

exclusion is granted for some other concentrations that are above the level of the 

NAAQS. In such cases, the exclusion decisions should first be made for each of the 

flagged concentrations that are above the NAAQS. All remaining flagged concentrations 

(those meeting all other requirements and conditions of the Exceptional Events Rule) 

should then be considered in progressively larger groups ranked by concentration. That 

is, if excluding the highest one of the flagged concentrations below the level of the 

NAAQS would cause a switch in whether the 3-year design value violates the NAAQS 

then if the EPA determines that value is to be excluded, all others can be retained without 

impact. If exclusion of the two highest such concentrations causes a switch, then the EPA 

may focus first on whether only those are to be excluded. 

PM10

The only current PM10 NAAQS is the 24-hour NAAQS based on the expected number of 

exceedances over a 3-year period. Since a concentration below the level of the NAAQS 

would not be an exceedance and cannot affect compliance with the NAAQS in any way, 

a concentration below the level of the NAAQS usually cannot be excluded. However, 

under an EPA policy memo, for the purpose of the EPA approval of a limited 

maintenance plan PM10 values as low as 98 μg/m3 can be concurred for exclusion when 

determining whether an area is eligible for a limited maintenance plan. (See May 7, 2009 

memorandum from William T. Harnett to Regional Air Division Directors, 

http://www.epa.gov/ttn/oarpg/t1/memoranda/lmp_final_harnett.pdf). Because 

concentrations less than 98 μg/m3 would appear to have little regulatory significance, the 

EPA discourages the flagging of such data. 

Pb 

The legacy 1.5 μg/m3

 and current 0.15 μg/m3 NAAQS for lead are both based on a 

maximum three-month average concentration. The 1.5 μg/m3

 standard is based on the 

maximum quarterly average, while the 0.15 μg/m3

 NAAQS is based on the highest 

rolling 3-month average during a 3-year period. The EPA will not concur on the 

exclusion of a 24-hour concentration value that is below the level of the NAAQS, and we 

discourage air agencies from flagging such values. 

NO2 

The EPA will not concur on the exclusion of a 1-hour NO2 concentration that is below 

the level of the annual NO2 NAAQS, and we discourage air agencies from flagging such 

values. 

 
Draft for Public Notice and Comment 

Revision Date: June 2012

Page 46 of 46 

SO2

The EPA will not concur on the exclusion of a 1-hour SO2 concentration that is below 

the level of the annual SO2 NAAQS, and we discourage air agencies from flagging such 

values.