# Interim Exceptional Events Rule Frequently Asked Questions

United States Environmental Protection Agency

May 2013

Note: This May 14, 2013, document replaces the original document posted on May 13, 2013, to

include the inadvertent omission of footnote 16 on page 28. There are no additional changes and

document pagination is the same in both versions of this document.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 2 of 50 

### ATTACHMENT 1

Interim Exceptional Events Ru le Frequently Asked Questions

The Exceptional Events Rule of 2007 1 superseded the EPA’s previous Exceptional Events guidance and policy documents and created a regu latory process codified at 40 CFR parts 50 and 51 (50.1, 50.14 and 51.930). The Exceptional Even ts Rule (EER) recognizes that each potentially eligible event can have different or unique characteristics, and thus, necessitates a case-by-case demonstration and evaluation. Therefore, the EER adopts a “weight-ofevidence” approach for reviewing each demonstration to justify excluding data affected by an exceptional event. The EPA acknowledges that extreme 2 exceptional events may justify more limited demonstration packages.

Air agencies and other stakeholders have rais ed technical questions and issues related to implementation since the EPA promulgated th e EER. This Question and Answer (Q&A) document is intended to respond to some of thes e frequently asked questions and to provide guidance and clarification to air agencies 3 implementing the EER. The EPA recognizes the limited resources of the air agencies that pr epare 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 even t implementation guidance is to establish clear expectations to enable aff ected air agencies to better manage resources as they prepare the documentation required under the EER. Submitters should prepare and submit the appropriate level of supporting documentation, which will vary on a case-by-case basis under the weight-of-evidence approach. The EPA anticip ates that the resources needed to prepare (and review) packages will decrease as we continue to identify ways to streamline the process and continue to build our database of example demonstrati ons and analyses. In addition, as noted above, the EP A acknowledges that extreme ex ceptional events may justify more limited demonstration packages.

For organizational ease, this document has beendivided into the follo wing topical sections:

A. Historical Fluctuations

B. “But For” Test

C. Exceptional Event Data Flagging Schedules

D. General AQS Procedures

E. General Exceptional Events Rule App licability and Implementation Issues

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

2 Extreme exceptional events may justify a more limite d demonstration package. Wh ether 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 prox imity of the event to the violating monitor. Several meteorological phenomena that could be considered extr eme events include hurricanes , tornadoes, haboobs, and catastrophic volcanic eruptions. The EPA addresses “extrem e” high wind dust events in Question 17a in this

document.

3 References to “air agencies” are meant to include st ate, local, and tribal air agencies responsible for implementing the EER.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 3 of 50 F. Exceptional Event Data Flagging for Air Quality Concentrations that Could

Contribute to an Exceedance or Violati on of the National Ambient Air Quality

Standards

Each section contains related questions. R eaders 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 interim guidance documents and the exceptional events website present examples to illustrate specific points. The example analyses and level of rigor are not nece ssarily needed for all demonstrations.

Disclaimer The Exceptional Events Rule is the source of the regulatory requirements for exceptional events and exceptional event demonstrations. This interim Q&A document provides guidance and interpretation of the Exceptional Events Rule rather than imposing any new requirements and shall not be considered bind ing on any party. Note: If and when the EPA takes a regulatory action that hinges on a d ecision to exclude data under the Exceptional Events Rule, the EPA will consider and appr opriately respond to any public comments on any aspect of a supporting exceptional events demonstration submittal.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 4 of 50 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 fluctuati ons, including background;

1. Question: Is the Exceptional Events Rule de monstration 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 th e concentration affected by an event exceed a specific percentile rank in the historical data?

Answer: The “historical fluctuations” criter ion is a test, but there is no specific percentile rank that the EPA will use to dete rmine whether the test has been passed. The EPA will use a weight-of-evidence approach to review each demonstration on a case-bycase basis. The air agency’s role in satisfy ing this element is to provide appropriate analyses and statistics and conclude that th e 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 year s. The EPA acknowledges that natural events can recur and still be eligible for exclus ion 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 air agency has satisfied the “clear causal relationship,” “but for,” and “aff ects air quality” criteria. Air agencies need to satisfy these EER criteria, as well as “not reasonably controllable or preventable,” for the EPA to concur on an exceptional even t claim. The EPA anticipates that less conclusive historical fluctuation comparisons will likely indicate less conclusive “clear causal relationship” and/or “but for” relati onships. However, a demonstration without a historical fluctuations comparison would pr event the EPA from being able to approve exclusion of the data in question.

The EPA recommends that each “historical fl uctuation” demonstration submittal contain a minimum set of statistical analyses describe d in more detail in Questions 2 and 3. The EPA generally will consider submission of the identified statistical analyses to have met the requirement to “provide evidence.”

It is important to note, however, that there is no outcome of the “h istorical fluctuation” statistical comparison that, by itself, can guarantee successful demonstration of the clear causal relationship and “but for” elements . The EPA will consider in its weight-ofevidence approach the comparison of the concen trations 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 noInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 5 of 50 evidence of any other plausible explanation would be a case in which the weight-ofevidence would generally indicate that the “but for” criterion has been demonstrated. In contrast, if the event-affected concentra tion does not stand out much from normally occurring exceedance concentrations for th e same place and season, the statistical comparison generally will not by itself provide much support for “but for” in the weightof-evidence consideration.

2. Question: What evidence does the EPA want include d in the demonstration as part of a comparison of a measured concentration with normal historical fl uctuations, including background?

Answer: The EPA would prefer an analysis showing how the observed concentration compares to the distribution of historical c oncentrations. To aid the EPA’s review, reduce requests for additional information, and fac ilitate the EPA’s unders tanding of the air agency’s position, a submitting air agency can consider providing some of the following types of statistics, graphi cs, and explanatory text:

 Comparison of concentrations on the claimed event day with past historical data (see Question 3 for additional detail). The histor ical comparisons can be made on an annual and/or seasonal basis, dependi ng on which is more appropriate. For example, if PM or

ozone data at the location show clear seas onality (i.e., exceedances are nonexistent or extremely rare in some seasons but not others , or concentrations va ry according to season due to meteorological conditions), discussing that information in the demonstration is likely appropriate. In contrast, if exceedances are likely throughout th e year, analysis of annual data may 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, generally it is not a ppropriate 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, da ta are most helpful when 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 PM 2.5 fluctuations and spatial distributi on 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 concentr ations on the claimed event day with past historical data to label appr opriate data points as being associated with concurred exceptional events, suspected except ional events, or othe r 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 ar ea, thereby fully and accurately portraying the historical context for the claimed event day.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 6 of 50  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/or other days with similar meteorologi cal conditions (possibl y from other years). This type of comparison could demonstrate th at the event caused higher concentrations than would be expected for given meteor ological and/or local emissions conditions.

 Percentile rank of concentration relative to annual data. The pe rcentile rank of the event-day concentration should be provided fo r the event day relativ e to all measurement days over the previous 3-5 years. To ensure statistical robustne ss, the EPA generally recommends that submitting agencies include a minimum of 300 data points in this calculation. The daily statistic (e.g., 24-hour average, maximum 8-hour average, or maximum 1-hour) should be appropriate for the form of the standard being considered for data exclusion (see Question 30).

 Percentile rank of concentration relative to seasonal data. The percentile rank of the event-day concentration should be provided fo r the event day relativ e to all measurement days for the season (or appropr iate alternative 3-month period) of the event over the previous 3-5 years. It is gene rally appropriate to use the same time horizon as used for the percentile rank cal culated relative to annual data.

(Note: The use of percentile ranks is illustra tive and should not be seen as a bright line to be passed or failed when comparing obser ved concentrations with historical values.)

3. Question: How will the EPA consider the submitted “historical fluctuations” evidence when assessing whether the “but for” and “cl ear 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 m easured concentration in excess of normal historical fluctuations and will assess the other criteria, in part, based on this historical fluctuations comparison. When the observed con centration 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 de monstrate the “but for” finding. When the

concentration is similar to or lower than a la rge number of normal historical values, the EPA may want additional evidence ( e.g., PM or VOC speciation da ta) 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 mu ch 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 nor mal historical fluctuations.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 7 of 50 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 informati on might assist in th e review process.

Additional Examples and Explanation Con cerning “Historical Fluctuations” Evidence (Note: The discussion and graphics that follow illustrat e the type of analyses and

discussion that are described in this question and in Qu estion 2 and that an air agency might include in a submittal showing that an event is associated with a measurement “in excess of normal historical fluctuations.”)

The evidence comparing the event-affected c oncentration with histor ical concentrations is most helpful to an air agency’s demons tration if it shows that the event-affected concentration is high compared to all, or near ly all, historical con centrations 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 be en very rare in the past, it may be possible to ma ke this point by labeling approp riate data points as being associated with concurred exceptional events , suspected exceptional events, or other

unusual occurrences. To facili tate the EPA’s understanding of the influence of these events, air agencies may also include comparisons omitting such points.

The following figures demonstrate the concep t of seasonal emissions fluctuations. The first figure shows an exceedance level PM 2.5 value in late spring that is outside the range of the 3 to 5-year historical data set for non-wintertime PM 2.5 , while the second figure shows a similar data value for a different pa rt of the country where similar exceedance concentrations occur throughout the year, suggesting that so me non-event process(es) can cause high concentrations all during the year. In the first case, a seasonal assessment of historical fluctuations generally would be appropriate, while annualized data analysis might be more appropriate for the second case to provide the most robust yet also representative histor ical data set.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 8 of 50 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, PM 10 and 24-hour PM 2.5 , 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-a ffected concentration is above the 95 th percentile of

the historical concentrations?

Answer: The preamble statement paraphrased in the question above was intended to address National Ambient Air Quality Standa rds (NAAQS) that are based on averaging 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 Year24-hour PM2.5, ug/m3 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter

Flagged Value Potentially Outside

Normal Seasonal Historical Fluctuations

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 Year24-hour PM2.5, ug/m3 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter

Flagged Value Not Potentially Outside

Normal Seasonal Historical FluctuationsInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 9 of 50 periods of many days, such as annual, quart erly and/or 3-month rolling average NAAQS. NAAQS with 1-hour, 8-hour or 24-hour averagin g periods only allow a small percentage of days to have concentrations above th e level of the NAAQS. Flagging and excluding data falling at around the 75 th percentile point of the hist orical concentrations are extremely unlikely to influence an area’s attainment status with respect to such a shortterm NAAQS. Data around the 75 th percentile point can, however , affect compliance with NAAQS having a quarterly average, 3-month average, or annual average standard. For

the annual PM 2.5 NAAQS, it is true that showing that th e Exceptional Events Rule criteria

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

the 95 th percentile point becau se it is more likely that values near the 75 th 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?

Answer: As previously stated in the response to the historic al fluctuations question, the EPA will review the submitted analyses showing how the observed concentration compares to the distribution of historical c oncentrations to assess whether the event is

associated with a measured concentration in excess of normal historical fluctuations, and when assessing the exceptional event demons tration criteria of “affects air quality,” “clear causal relationship,” and “but for” cau sation. Because the “historical fluctuations” showing is not a statistical demonstration wi th any defined bright line, air agencies should consider submitting (with appropriate de scriptions and discussion) the type of statistical analyses described in the responses to Questions 2 and 3. The EPA will review these analyses and look at both the relations hip 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 a nd “but for” criteria.

In the response to Question 2, we identifie d that air agencies completing historical fluctuation analyses should consider using 3 to 5 years of data to en sure a representative dataset. We recognize, however, that these data may not be available for all monitors and/or all pollutants. If data are not availa ble, please consult w ith 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.”Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 10 of 50 The implementing language in the EER at 40 CFR 50.14(c)(3)(iv) states: “The demonstration to justif y data exclusion shall prov ide evidence that:

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

6. Question: What types of evidence can air agencies include in a demonstration that ozone

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

Answer: Air Agencies may include any evidence th at they consider relevant to the “but for” requirement recognizing th at the effects of a fire on ozone are complex. Fire can generate ozone precursors, but it can also reduce solar radi ation 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, wild fires 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.

Examples of relevant evidence follow. The EPA recognizes that the following example analyses have limitations and may not conclusi vely or quantitatively demonstrate the “but for” criterion. For this reason, the EPA consid ers “but for” evidence using a weight-ofevidence approach on a case-by-case event basis.

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

weather conditions at the time of the even t, 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 patt ern 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, su ch as from traffic due to a sports or entertainment event or source non-compliance.

 Evidence that the plume from the fire passed over the locatio n of the monitoring site and mixed down to ground level. This can include satellite images, wind data including HYSPLIT trajectories, visu al smoke observations, and chemical analysis of PM filters showing elemen ts and compounds that are markers for biomass burning.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 11 of 50  Altered pollutant amounts, ra tios, or patterns that indi cate the influence 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 NO 2 and NO; and pollutant ratios,

such as CO/NO

x , CO/PM 10 , Elemental Carbon (EC)/Organic Carbon (OC),

O 3 /NO y and O 3 /CO.

 A prediction that the “normal” ozone c oncentration would have been below the level of the NAAQS. “Normal” ozone c oncentrations can be predicted using statistical methods based on previous-day ozone and same-day weather variables (like methods used for air quality advisori es in some areas) or using air quality models. The EPA asks that demonstration packages using these predictive techniques also include an easily understandable narrative describing the application of the technique and informa tion on the uncertainty of the prediction methods (i.e., information on its past succe ss in predicting normal ozone levels).

 A prediction based on air quality/photoc hemical 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 NO

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

etc.

The EPA is currently developing a sepa rate guidance document for preparing a

demonstration for wildfire events that are beli eved to have affected ozone concentrations. In addition, the EPA will post on its exceptiona l 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 qual ifier codes to selected dat a in AQS. “R” flags are the only AQS flags that satisfy the EER requirement for init ial 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 th e schedules and deadlines for flagging and documenting exceptional event data for designations purposes?

Answer: When the EPA promulgated 40 CF R § 50.14, “Treatment of Air Quality Monitoring Data Influenced by Excepti onal Events,” in March 2007, the EPA was mindful that designations would be occu rring under the then-recently revised PM 2.5 NAAQS. Exceptions to the generic deadline of July 1 of the calendar year following the

4 http://www.epa.gov/ttn/analysis/exevents.htmInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 12 of 50 datum year (see 40 CFR § 50.14(c )(2)(iii)) were included for PM 2.5 in the rule. The EPA

was also mindful that similar issues woul d 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 th e 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 fl agging schedule identified in the 2012 SO 2 NAAQS final rule at 75 FR 35592, the data fl agging schedule identified in the 2010 NO 2 NAAQS final rule at 75 FR 6531, or the data flagging schedule identified in the 2012

PM 2.5 NAAQS final rule at 78 FR 3086.

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 us e 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 informati onal 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 agency collects additional supporting data, it ma y 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 th e data collection day

that are unrelated to exceptional events.

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 ye ar following the year in which the flagged measurement occurred or by the other dead lines identified with individual NAAQS revisions (see Question 7). Air agencies in tending to change “I” flagged data to “R” flagged data should be aware of the EER fla gging and initial event description deadline of July 1 of the year following the sample m easurement. Air agencies should change the flag status from “I” to “R” BEFORE the July 1 deadline. Normally, air agencies should not modify the flag status after this date a nd, therefore, if they went beyond July 1, they may not be able to meet the EER initia l flagging and event description deadlines.

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

Answer: Yes, but the EPA asks 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 demonstrati on. Air agencies should use the “I” series flags to identify values for informational purposes (see Question 8). AQS only allows the EPA to place concurrence flags on data identified with an “R” flag. “I” flags never affect regulatory summary statistics (e.g., desi gn values, number of exceedances, 98 th percentileInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 13 of 50 values) generated by AQS for NAAQS determinations purposes. “R” flags will not affect the regulatory summary statistics unless or until they are conc urred by the EPA.

Further, while the EER does not prohib it 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, including clarifications and examples, particularly for PM 2.5 and PM 10 , in which flagging individual concentration values below the level of the NAAQS is acceptable.

10. Question: The EPA requires air agencies to pr ovide an initial description for data flagged with an “R” data qualif ier code. Is it possible for an initial description to be inadequate (for example, "fir es in surrounding states")?

Answer: Although the EPA is not specifying pass/f ail 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 insert ed into the AQS database, the State must also provide an initial description of the even t in the AQS comment field. This initial description should include such inform ation as the direction a nd distance from the event to the air quality monitor in question, as we ll as the direction of the wind on the day in question. " 72 FR 13568 (emphasis added). AQS ma intains event defi nitions, including their initial descriptions, in fields separate from the raw data flagging fields. As a result, air agencies can enter more detailed event desc riptions either before or after the raw data measurements are flagged. Regardless of pr ecise 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 exceptiona l event planning and prioritization, the act of providing the initial descri ption 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 even t description, the EPA suggests that air agencies notify the appropr iate regional office af ter the air agency creates the event description. Th is allows the air agency and the EPA to discuss and, if necessary, develop a mutually agreed-upon de scription. This initia l 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/De molition." The new “IE/RE” flag is demolition; can it also be used for construction?

Answer: The “j” flag is obsolete and can no lo nger 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 capab le of preventing localized NAAQS exceedances should be available during most construction events. In some cases, however, construction activitiesInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 14 of 50 may involve very high-energy, emissions-gen erating physical processes, such as explosive excavation. Dust control measures ma y not be adequate to prevent exceedances / violations in the vicinity of this type of activity.

If an agency wishes to “flag” data relate d 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 ex ceptional demolition even t 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 should

show in a demonstration submittal that all re asonable 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 th eir fire-related definitions after the EPA promulgated the Exceptional Events Ru le. 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 corre sponding changes to fire-related flags in AQS. The EPA eliminat ed from AQS the Wildland Fire Use Fire – United States (“IU”) and (“RU”) flags and th e Forest Fire (“E”) flag. The EPA continues to use the following flag s to describe fires:

 IF – Fire – Canadian (Informational Only)

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

 IM – Prescribed Fire (Informational Only)

 IP – Structural Fire (Informational Only)

 IT – Wildfire – US (Informational Only)

 RF – Fire – Canadian (Request Exclusion)

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

 RM – Prescribed Fire (Request Exclusion)

 RP – Structural Fire (Request Exclusion)

 RT – Wildfire – US (Request Exclusion)

The EPA believes it is appropriate to retain the Fire – Canadian (“IF/RF”) and Fire – Mexico/Central America (“IG/RG”) flags beca use these flags indicate the jurisdictional origin of the fire (i.e., outside of the submitting state/outside of the United States). Emissions from fires originating outside of the United States that affect air quality

concentrations in the United States ma y qualify for regulatory treatment under the international transport provisions of 40 CF R part 179(b) of the Clean Air Act.

12. Question: The National Park Servi ce 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 th e EER, and exclusion of other data not collected by the air agency itself that may lead to a nonattainment finding?Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 15 of 50 Answer: Yes. However, air agencies should ta ke 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 ente r or alter data for th at site. Under normal circumstances, an air agency will not have access rights to apply event flags to data from monitors operated by other enti ties, such as the National Pa rk 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 agen cies, the air agency should contact the agency operating th e monitor and request that the operating agency flag the identified data range for exclusion. The a ffected air agency should also develop and forward to the operating agency an initial even t 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 anot her agency apply the appropriate “R” series flags and initial event descri ption, the air agency should c ontact the EPA regional office. If the EPA regional office is aware of the requ est, and if the request was prior to July 1 st
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 flag s the data in question or the air agency notifies the regional office that a flag is need ed, it is the air agency’s responsibility to develop an initial event desc ription, prepare the demonstrat ion, and submit it to the EPA under the applicable schedule. The agency ope rating the monitor may choose to assist in this process.

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

been in the absence of the event contributi on, 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 w ithout 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 va lue for a nonattainment area can make it seem

that the area needs more emissions reducti on 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, s hould AQS be used to flag such data?

Answer: (See also Question 19 for a re lated question regarding PM 10 .) 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 de finition of that term in the Exceptional Event Rule. Ambient data affected by an event that does not meet the "but

for" criterion cannot be exclude d 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 handlingInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 16 of 50 associated with events that are not considered “exceptional” under the EER, and does not provide the 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 emi ssion 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 th at 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 pream ble 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 pollu tant 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 que stion, the EPA describes in more detail the

differences between the event scenario descri bed in the question and a true "exceptional event" under the Exceptional Events Rule, fo r the purpose of clarifying why the planned guidance on attainment demonstrations and th e SIP approval process, rather than the Exceptional Event Rule and the associated AQS data flagging, demonstration submittal, and review process, will appl y to such an event scenario.

To illustrate an attainment demonstr ation scenario using the 2006 24-hour PM 2.5 NAAQS

of 35 μ g/m 3 , assume that the three annual 98 th percentile 24-hour PM 2.5 concentrations for a monitoring site for 2006-2008 are 44, 31, and 37 μ g/m 3 for each respective year, with a resulting 3-year design value of 37 μ g/m 3 , which is a violation. Also, assume that the next highest concentration in 2006 below the 44 μ g/m 3 was 40 μ g/m 3 . The 44 μ g/m 3
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 μ g/m 3 without the fire. Because both 44

μ g/m 3 and 41 μ g/m 3 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 μ g/m 3 for the event day in 2006 would still be the 98 th
percentile concentration and would still result in a 3-year design value of 36 μ g/m 3 which is a violation. Thus, the 2006 wildfire does not meet the definition of an exceptional

event.

E. General Exceptional Events Rule A pplicability and Implementation Issues

14. Question: The Preamble to the Exceptional Events Rule states that the EPA headquarters or the EPA regional office will ma ke its decision on demonstrations public. See 72 FR 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. EPAInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 17 of 50 will make the response and associated expl anation 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 webs ites 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 rulema king that includes a public comment period. In these cases, the same information that is pos ted on the EPA websites, and any additional supporting correspondence, will also be poste d in the relevant rulemaking docket. Further, the EPA plans to make the demonstr ations and the EPA’s concurrence decisions

available to interested parties upon request.

14a. Question: At what point in the exceptional even t development and review process is public notice and opportunity for comment re quired? 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 de velopment process (see 40 CFR 50.14(c)(3)(i) and 40 CFR 50.14(c)(3)(v)). The EPA must also provide notice and opportunity for public comment prior to taking a final Agency action, such as acting on an air agency’s request for area redesignation, that may rely upon air quality mon itoring data including exceptional event claims. In addition, an air agency may need to provide an additional opportunity for public comment if the EPA re quests and/or if the air agency provides supplemental information not included in th e original documentation made available for public comment. The EPA will make a case-by-case decision regarding supplemental opportunities for public comment during the de monstration 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, hist orical 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) a nd (b)(1) to the EPA’s satisfaction, the EPA generally will excl ude 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 desi gn values estimates, unless the AQS user specifically indicates that they should be included. 6

 The EPA will accept the exclusion of thes e data for the purposes of selecting appropriate background concentrations fo r New Source Review (NSR) air quality analyses. 7

5 http://www.epa.gov/ttn/analysis/exevents.htm

6 Due to the complexity of the AQS software, inadvertent errors may occur. The EPA as ks that agencies provide the EPA with information if/when AQS outputs seem inconsistent with the EPA’s intention to exclude concurred upon data.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 18 of 50  The EPA will accept the exclusion of thes e data for the purposes of selecting appropriate background concentrations fo r transportation conformity hot spot analyses. 8

 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 generally will not reflect these data in any summary st atistic of potential regulatory application, unless such inclusion is specifically noted. 9

In addition, some proposed regulatory actions (e.g., proposed designa tion, classification, attainment demonstration, or finding as to whether the area has met the applicable NAAQS) will 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. If the EPA pursues one of these acti ons 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 taki ng final regulatory action.

15. Question: It is possible for events to affect mo re than one state. Each state/air agency must then submit its own exceptional events de monstration package, which may result in

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

Answer: The primary responsibility for developing demonstrations lies with state, local, and tribal air agencies. The EPA encourages st ates 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 a nd potentially contri buting sources with varying levels of contro l, different historical concentrati on patterns, etc.). States/agencies need to address these unique characteristic s 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 addr ess the unique features of each NAAQS exceedance or violation (e.g., potentially di fferent monitoring locations, different historical concentration patte rns). 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.

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 and any applicable NSR permitting and/or modeling guidance.

8 Applicable only to PM 10 and PM 2.5 . See “Transportation Conformity Guidance for Quantitative Hot-spot Analyses in PM 2.5 and PM 10 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 descri be current status and trends in actual air quality in the area for public information purposes incl uding reporting of the Air Quality Index.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 19 of 50 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 natu ral event” elements. Because

the actual event-related exceedance would ha ve 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 influence measured concentrations at a long-sited mon itor such that the nature of the monitor changes from “area-wide” to “unique”?

Answer: Generally, all monitori ng data, if meeting appli cable CFR regulations, are comparable to the NAAQS. There are specia l provisions applicable only to the PM 2.5 NAAQS, which provide that monitors must be representative of area-wide air quality to be comparable to the annual NAAQS, and that monitors representative of unique microor middle-scale impact sites are comparable only to the 24-hour PM 2.5 NAAQS. See 40 CFR 58.30. In the provided example, the affected air agency may belie ve that site meets

the criteria for data to be co mparable only to the 24-hour PM 2.5 NAAQS for the period of the construction. The affected air agency c ould request this type of change through updates to its annual monitoring network plan or in a separate request, subject to review and approval by the EPA regional office.

The EER does not specifically address temporary, but multi-day or multi-year, anthropogenic emission sources such as constr uction 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 mu lti-year road construc tion project) is not likely to recur at the location in question. If the remaining exceptio nal event criteria and demonstration criteria are met, including th e requirement that the event (including the emissions from the project) is not reasonabl y controllable, the activ ity might qualify as being an exceptional event.

Air agencies not wishing to develop excepti onal event demonstration packages for the described scenario can request agreement fr om the EPA regional office to relocate a monitor that no longer meets monitoring obj ectives. This process is, however, time consuming and resource intensive, so ai r agencies usually "m onitor through" the disruption or ask their regional offices to support a temporary shut-down. When the EPA regional office approves a temporary shut-dow n, 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 peri ods of local construction.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 20 of 50 16a. Question: Are policy relevant background (PRB) ozone concentrations and exceptional

events related?

Answer: PRB ozone concentrations and excep tional events can include partially overlapping concepts. The 2007 Staff Paper 10 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) emi ssions of precursor emissions (e.g., VOC, NOx, and CO) in the U.S., Canada, and Mexico .” In the current oz one review process, the EPA has more broadly considered b ackground ozone by assessing three separate definitions of background: natural, North American, and U.S. background. 11 As before, each background is defined as the ozone that would be observed in the absence of specific categories of emissi ons. For example, North Amer ican background (NAB) is equivalent to PRB. 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 excep tional events and North American background

can involve emissions from natural events li ke forest wildfires or stratospheric ozone intrusions. However, exceedances due to natural emissions that occur every day and contribute to policy relevant background, su ch as biogenic emissions, do not meet the definition of an exceptional event and are t hus 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. Ai r 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 do es not explicitly consider whether the event type might contribute to North Amer ican background, or any other background definition. However, if a natural event that contributes to back ground ozone causes an

observed concentration that meets the statut ory 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 SO 2 exceedances, which are clearly volcanic ex ceptional 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. Wh en will the EPA provide the method for SO 2 ?

Answer: AQS has been modified to allow flag s on all criteria pollutant data. The specific schedule for exceptional event fla gging and documentation submission for data to be used in designations decisions is identified in the final primary SO 2 NAAQS rule

10 Environmental Protection Agency, Review of the nationa l ambient air quality standards for ozone: assessment of scientific and technical information. OAQPS staff pape r. (Updated Final) July 2 007. Research Triangle Park, NC: Office of Air Quality Planning and Standa rds. EPA-452/R-07-007, available online at: http://epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_sp.html

. 11 Environmental Protection Agency, Integrated Science Assessment for Ozone and Related Photochemical Oxidants. (Third External Review Draft) June 2 012. Research Triangle Park, NC: National Center for Environmental Assessment – RTP Division, Office of Research and Development. EPA-600/R-10-076C, available online at: http://cfpub.epa.gov/nce a/isa/recordisplay.cfm?deid=242490#Download.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 21 of 50 (see preamble at 75 FR 35585-35586 and regulator y text at 75 FR 35592). The correct flag to use for a volcanic eruption event is “RS.”

17a. Question: The EPA acknowledges that extreme exceptional events may justify more limited demonstration packages . How might the EPA decide whether to consider a particular high wind dust event “extreme” wh en reviewing a 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 pur pose. The National Weather Service (NWS)

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

dust-filled air over an extensiv e area, but does not include an y quantified criteria for the spatial extent or the concentr ation of the dust. In contrast , a haboob is of the magnitude

that could be considered an extreme event. Haboobs are often caused by severe weather (e.g., severe thunderstorm activity, cold frontal passages) 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 concentrations in determining whether an event is an extreme event. An example of an event that could be considered an excepti onal event but not an extreme event would be the Santa Ana winds blowing at 25-30 mph, cr eating an exceedance at one monitor, with maximum hourly PM 10 levels of less than 800 μg/m 3 . 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 PM 10 levels of 50,000 μg/m 3 were monitored during this

event. Both of these events could be cons idered for exclusion under the EER. The South Coast Air Quality Management District prep ared a 49-page demonstration package (plus an appendix with additional s upporting information) for the Santa Ana winds event, parts of which have been used as examples in the High Winds guidance document. However, the EPA anticipates that much more limited documentation for an event like the haboob would be sufficient to convince the EPA (and a ll other parties) that the event meets the

several criteria for data exclus ion (clear causal connection, no t 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 refere nce 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 th e 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 guidan ce. The EER Preamble (72 FR 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 sp ecial treatment of the data, we intend to use our discretion as outlined under section 107(d )(3) not to redesignate affected areas asInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 22 of 50 nonattainment based on these events.” Therefor e, air agencies may flag CO data in AQS and the EPA may apply the same process a nd approval criteria as in the Exceptional

Events Rule.

On August 12, 2011, the EPA issued a decision to retain the current su ite of CO standards without revision (see 76 FR 54294). Because the EPA made no revisions to the CO standards, it promulgated no related ch anges to the Exceptional Events Rule.

19. Question: The limited maintenance plan requirements for PM 10 require a demonstration that the area design value is less than or equal to 98  g/m 3 . Flagging of values between

98  g/m 3 and the NAAQS are therefore relevant for this regulatory decision. Can air agencies flag and request/receive the EPA’s concurrence on these values, which are not

exceedances and do not contribute to violations?

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

Division Directors states th e following regarding the PM 10 limited maintenance plan option: “In determining eligibility for the li mited maintenance plan option, the EPA will treat 24-hour average air quality data between 98  g/m 3 and 155  g/m 3 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 fo r exceptional events (natural event, or exceptional event that is not reasonable controll able 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 the EPA mean when we say we will review exceptional event demonstration submittals using a “weight-of-evidence” approach?

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.

20. Question: Exactly which section(s) of the preamb le 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 particul ate concentrations due to dust being raised by unusually high winds will be treated as du e 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 concentra tions.’ EPA calls this a drafting error. In light of th e error, the high wind events se ction of the preamble is a legal nullity.”Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 23 of 50 The EPA considers the “high wind events sec tion of the preamble” to which the court

referred to be the section titled “ B. High Wind Events ” beginning on 72 FR 13576. This does not necessarily mean that these passage s do not reflect the EP A’s interpretation of what might be appropriate under the EER. Rather, it means that implementing air agencies and other stakeholders should rely on other parts of the preamble and other EPA guidance instead of statements in these passage s 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 th eir fire-related definitions after theEPA promulgated the Exceptional Events Rule. The EPA is using the following firerelated terminology in the interim exceptiona l 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 i gnition caused by li ghtning; volcanoes; unauthorized activity; accidental , human-caused actions; and escaped prescribed fires.

20b. Question: How should air agencies support a cl aim that emissions from wildfires are “not reasonably controllab le or preventable”?

Answer: The Clean Air Act and the EER appl y the “not reasonably controllable or preventable” requirement to any event that an air agency wishes to be treated as an exceptional event, and thus it applies to w ildfires. The current United States Forest Service (USFS) definitions of “wildfire” and “prescribed fire” define these events in terms of purpose and deliberateness of igniti on (See definitions in response to Question 20a). Based on the USFS definitions, a wildfire is a fire that has started from an unintentional ignition or an unint entional escape of a prescrib ed fire. The initiation of a wildfire is thus by definition unplanned, but the concepts of reas onable prevention and

control should not be over looked in an exceptional ev ent demonstration. The EPA recognizes that wildfires and emissions from wildfires are generally not reasonable to prevent or control.

When documenting the “not reasonably controll able or preventable” criterion in their wildfire exceptional event demonstration submittal, air agencies should identify the origin and evolution of the wildfire, descri be local efforts to prevent fires due to unauthorized activity or accidental human-caused actions (if relevant given the origin of the fire) 12 , and explain how any efforts to limit the duration or extent (and thus the

12 Prevention/control efforts could include posting High Fire Danger signs to make people more careful and prevent accidental fires, and/or taking reasonab le action to contain a fire once it has started.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 24 of 50 emissions) from the wildfire were reasona ble. During wildfires, fire management resources deployed to the fire event give first priority to protecting life and property. Because wildfires are, by definition, unplanned and unwanted, fire management

resources often have limited advance notice of ignition and loca tion, which generally limits preparation time and reasonable efforts to limit the duration or extent of a wildfire. In light of these considerations , the EPA believes that it will generally be sufficient for air agencies to provide a statement such as th e following to document the “not reasonably controllable or preventable” criterion fo r wildfires: “Based on the documentation provided in [section X] of this submittal , [lightning] caused the unplanned, unwanted wildfire event. The responsible agencies did th eir reasonable best to control the extent of and extinguish the fire by taking the following ac tions [insert list or description of actions taken]. Therefore, emissions from this wild fire were ‘not reasonably controllable or preventable.’” For fires that could have b een suppressed or contained but which fire management officials allowed to burn for re source management purposes, air agencies can generally reference or paraphrase a previ ously adopted resource management plan to support the “not reasonabl y controllable or preventable” criterion.

21. Question: The Exceptional Event Rule allows for exclusion of data affected by a prescribed fire if the usual requirements of th e rule are satisfied and if the air agency has adopted and is implementing a Smoke Manageme nt Program (SMP) or if the air agency has ensured that the burner employed basi c smoke management practices. Are there minimum requirements for a Smoke Manage ment Program? What are “basic smoke management practices?”

Answer: The preamble to the Exceptional Even ts Rule at 72 FR 13567 describes an SMP as establishing a basic framework of procedures and requirements for managing smoke from a prescribed fire managed for re source benefits. Further, the EPA’s “Report to Congress on Black Carbon” 13 describes the intent of SMPs as “mitigat[ing] the public health and welfare impacts from prescrib ed fires and promot[i ng] communication and coordination of prescribed burning among la nd owners.” The Report to Congress also states that basic smoke management practi ces could “...include, among other practices, steps to minimize air pollutant emissions dur ing and after the burn, evaluate dispersion conditions to minimize exposure of sensitive populations, and identify procedures to ensure that burners are using basic smoke management practices.” The EPA intends to develop separate guidance to address this i ssue, which will be issued at a later date following an opportunity fo r stakeholder input.

22. Question: Is there a tie between the requi rements of 40 CFR 51.930 Mitigation of Exceptional Events and the EPA’s approval for exclusion of data affected by an exceptional event?

13 Report to Congress on Black Carbon, EPA-450/R-12-001, US EPA, March 2012, page 230. Available at http://www.epa.gov/blackcarbon/

.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 25 of 50 Answer: The EPA encourages the submittal of mitigation measures with the demonstration package, particularly for thos e events likely to recur. The Exceptional Events Rule was promulgated pursuant to 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 §51.93 0 and the requirement in the EER at 40 CFR §50.14(c)(1)(i) that all air agencies must “notify the pub lic 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 pr otect public health from exceedances or violations of the national ambient air quali ty standards. At a minimum, the State

must:

(1) Provide for prompt public notification wh enever air quality concentrations exceed or are expected to exceed an applic able ambient air quality standard;

(2) Provide for public education concerning actions that individuals may take to reduce exposures to unhealt hy levels of air quality during and following an exceptional event; and

(3) Provide for the implementation of appropr iate measures to protect public health

from exceedances or violations of ambien t air quality standards caused by exceptional

events.”

Although the language at 40 CFR §51.930 does not require air agencies to prepare or submit a mitigation plan, it does require that air agencies develop and implement processes and measures that c ould easily become the elements of a formal, written plan. The mitigation criteria focus on specific meas ures and actions to protect public health, rather than on measures that control or prev ent emissions associated with a specific event. So, a mitigation plan may include measures th at apply to emissions sources in general (e.g., dust suppression or covering techniques fo r mineral processing) rather than those measures or controls that might be discu ssed in the “not reason ably controllable or preventable” portion of an event demonstrati on (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 res ponsibilities 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 notifi cation 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 ar gument or submit evidence about control measures for events that took place in othe r states or countries, on federally-owned and managed land, or on tribal (or state) lands not subject to state (or tribal) regulation?Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 26 of 50 Answer: Under the Clean Air Act, the EPA gene rally considers a state (not including areas of Indian country) to be a single responsible actor . Accordingly, neither the EPA nor the Exceptional Events Rule provides special considerations for intra state scenarios when an event in one county affects air quali ty in another county in the same state, assuming that the event occurs on land subj ect to state authority (versus tribal government authority). For cases involving intr astate transport, th e state or local air agency should evaluate whether emissions from neighboring (or c ontributing) counties are not reasonably controllable or preventabl e. As discussed in greater detail in the overview guidance document and the inte rim High Winds Guidance document, the assessment of “not reasonably controllable or preventable” is based on the existing level of required control, attainme nt status, and, for high wind dust events, wind speed and

other factors. States and tribes should consult with their EP A regional office early in the development of an exceptional event dem onstration package if they believe that emissions from sources on federally-owned a nd 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 that generally it is not reasonable to expect the downwind state (i.e., the state submitting the demonstration) to require the upw ind country or state to have implemented

controls on sources sufficient to limit event- related air concentra tions in the downwind state. As with any demonstration submitta l, the submitting (downwind) state should sufficiently identify all natural and anthropoge nic 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 s ubmitting state should also provide available

information on the status of control measures for emissions from out-o f-state sources, the submitting state may determine based on available information that the “not reasonably controllable or preventable” criterion is satisfie d in light of the state’s inability to require controls of the upwind state. 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 availabl e information, that contributing emissions from the upwind state or country were not reas onably controllable or preventable. Submitting states are furthe r required to submit evidence/statements supporting the other exceptiona l event criteria (i.e., clear causal relationship, but for, human activity unlikely to recu r or a natural event, affect s 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/ant hropogenic event that the submitting state

wishes to consider a natural event of th e grounds that all sign ificant anthropogenic

sources were reason ably controlled.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 27 of 50 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 information and arguments presented in publ ic 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 c ontrols 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) an d 126 to address interstate transport issues and mechanisms in section 179(b) to address inte rnational trans port issues.

24. Question: Need an air agency make an ar gument or submit evidence about control measures for air quality impacts from wind-bl own 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 state this and provide appropriat e supporting documentation in its demonstration package. The supporting documentation could in clude descriptions of the geographic area (with maps or available visuals) and a disc ussion of the historical land use, including prior disturbances, water diversions and ot her 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 iden tify all sources contributing to an event and identify appropriate control strate gies for each anthropogenic source.

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

Answer: The guidance document, “Interim Guidance on the Preparation of Demonstrations in Support of Requests to Ex clude Ambient Air Quality Data Affected by High Winds Under the Exceptional Event Rule” (the High Winds guidance document) provides this type of advice for demonstrati ons for high wind dust events. While the High Winds guidance document speaks specifically to high wind dust events, the EPA believes that many of the principles discussed therein to extend to all types of exceptional events. The EPA has also developed a presentation enti tled, “Presenting Evidence to Justify Data Exclusion as an Exceptional Event: Ideas based on how the EPA has recently documented events to support re gulatory decisions.” Interest ed parties can download this presentation from the following site: http://www.epa.gov/ttn/analysis/ docs/IdeasforShowingEEEvidence.ppt . Additionally, the EPA is currently developing separate gui dance to address the preparation of demonstrations to support wildfire-related ozone event claims.

26. Question: Where can an air agency find exampl es of demonstrations from other air agencies that have been approved by the EPA?Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 28 of 50 Answer: The EPA has posted examples of approved demonstrations at http://www.epa.gov/ttn/analysis/exevents.htm.

27. Question: How quickly will the EPA review the demonstration document and provide feedback to the air agency on the appr oval, or on any suggested improvements?

Answer: The EPA generally intends to conduc t its initial review of a submitted exceptional event demonstration package w ithin 120 days of receipt. Following this initial review, the EPA will generally send a le tter 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) 14 , 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. 15

To promote early communication, the EPA suggest s that air agencies provide a letter of intent to submit a demonstration package for fl agged data in AQS as soon as possible, if possible within 12 months from the event o ccurrence, after the ag ency 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. 16 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 with in 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-specifi c 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 as expeditiously as necessary if required by a near-term regulatory act ion, but no later than 18 months following submittal of a complete package. The EPA intends to communicate with the submitting agency, as needed, dur ing the demonstration review period.

Submitting air agencies that believe their demo nstration packages are tied to near-term regulatory actions should submit their demons tration packages well in advance of the regulatory deadline. Air agencies should also identify the rela tionship between the exceptional event-related flagge d data and the anticipated regulatory action in the cover letter that accompanies their initial subm ittal package to the reviewing EPA regional

office.

14 The EPA will generally ask that air agencies provide su pplemental information within 60 days from receipt of the letter from the EPA. The EPA recognizes that air ag encies may need more than 60 days to prepare and submit some types of supplemental information. The EP A is willing to work with agencies on supplemental timeframes; however, the mandatory timing of the EPA’s actions may limit the response time the EPA allows.

15 “Regulatory decisions” include findin gs as to whether the area has met th e applicable NAAQS, classification determinations, attainment demonstrations, the developmen t of Limited Maintenance Plans, clean data findings.

16 The Letter of Intent is an optional step and the EPA recognizes that air agencies may need additional time to prepare and submit demonstration packages particularly where the basis of the exclusion is violating an annual standard or a 3-year design value. Similarly, an air agency could consider submitting an annual letter of intent if annual submittal makes sense for resource planning or for historically seasonal events.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 29 of 50 28. Question: Will the EPA ever perform and consider additional data analysis itself before deciding whether to approve an air agency-s ubmitted demonstration in support of data

exclusion?

Answer: In general, the EPA does not prepare analyses or additional arguments for inclusion in a submitted demonstration packag e or to support the EPA’s concurrence on a demonstration package. Rather, the EP A 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’s preparing and/or considering additional analyses, the EPA may either assist with or independently prepare supporting an alyses that could become part of the submission package or an EPA-prepared tec hnical 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 cont ain a dispute resolution process that air agencies can use to resolve disagreements rega rding non-concurrence on submittal packages?

Answer: Several mechanisms currently exist that ai r agencies can use at various points in the exceptional events process:

 Engage in early dialogue with th e 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 th e EPA’s chain of command.

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

 Challenge in an appropriate court the regul atory decision subsequently made that is based on the EPA’s exceptional event determination.

In addition, for complex exceptional events clai ms or those with signi ficant regulatory or other impacts (e.g., those claims that di rectly influence proposed designation or redesignation, classifications, and attainment determinations), the EPA regional office staff will generally seek input from ot her EPA regional offices and/or the EPA headquarters staff.

28b. Question: Can air agencies use data from non-re gulatory monitors in exceptional events analyses ?

Answer: Yes, air agencies can use data from non-regulatory monito rs to support their exceptional event demonstrations. Generally, monitoring data used for NAAQS regulatory purposes are collected from Federal Reference Method (FRM), Federal Equivalent Method (FEM), a nd/or Approved Regional Method (ARM) monitors that are sited and operated in accordance with 40 CFR Part 58. Exceedances or violationsInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 30 of 50 identified as exceptional events originate from these same data from FRM/FEM/ARM monitors. The AQS, the EPA's repository of ambient air quality data, stores data from more than 10,000 monitors, about 5,000 of whic h are currently active. Although not all of these monitors are FRM/FEM/ARM-approved, data from non-FRM/FEM/ARM monitors can be used in exceptional event analyses. Fo r example, air quality data summaries from non-FRM/ FEM/ARM monitors may be helpfu l in defining the duration and geographic extent of the event, including the area of exceedance/violation and the area containing

sources that contribute to the exceedances/vio lations. Similarly, chemical speciation data from monitor samples can help characterize the nature of the violation and identify contributing emissions sources.

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 PM 10 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 signifi cance. If the EPA approves a measurement value for exclusion for one particular NAAQS averaging period and level, does the EPA automatically exclude the same value fo r all the other NAAQS for that pollutant?

Answer: No. Air agencies should request and su pport the exclusion of a measured air concentration separately for each NAAQS that applies to the pollutant. The EPA will similarly provide sepa rate concurrences.

When initially flagging data, an air agency does not need to iden tify the specific NAAQS

for which it seeks to exclude a measured c oncentration. The EPA’s ambient air quality database, AQS, is designed to allow an air ag ency to apply a single flag to a measured concentration value, which merely indicates th e agency’s interest in excluding that value with respect to one or more of the appli cable NAAQS. Later, in the request for data exclusion (i.e., the demonstration), the air agency can indicate the specific NAAQS for

which it seeks exclusion and for which th e demonstration addr esses 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 le tter (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 combina tion of NAAQS for that pollu tant (e.g., in the case of

PM 2.5 , 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.

Air agencies preparing demonstrations to support requests to excl ude 24-hour average

values for PM 2.5 and PM 10 should flag all 24 1-hour values within a given day. If concurred upon, flagging all 1-hour values wi ll ultimately result in the same available remaining data for regulatory analysis and calculation regardless of whether the 24-hour

PM 2.5 or PM 10 measurement data are collected from filter-based or continuous monitoringInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 31 of 50 instruments. 17 The EPA believes flagging all 24 hourly values is appropriate because flagging only peak or selected hours could result in the remaining hourly values still meeting the data completeness requirement s. Exclusion of only the high hourly concentrations could result in AQS calculati ng a valid low (or, pot entially high) biased

24-hour concentration under the rules for data interpretation. 18

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 combin ation of NAAQS for which th e exclusion was approved. These “legacy” concurrence flags have been converted to the ne w 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 PM 2.5 , all concurrences on events w ith 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 PM 2.5 NAAQS. This default was chosen because prior to the revision of the 24-hour PM 2.5 NAAQS in 2006, violations of the 1997 24-hour NAAQS were extremely rare.

 For PM 2.5 , all concurrences on events with dates of January 1, 2005 through March 2010 were presumed to be appli cable only to the 24-hour NAAQS because

there were no revisions to the annual PM 2.5 NAAQS during this timeframe, so designations to nonattainment for the annual PM 2.5 standard were extremely rare.

This 24-hour PM 2.5 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 PM 10 , all concurrences were presumed to apply to the 24-hour NAAQS, as

the annual PM 10 NAAQS was revoked in 2006. 19

17 Filter based instruments typically record a single value within a 24-hour period while continuous monitors typically collect 24 1-hour measurements. Because AQ S can calculate a valid 24-hour average concentration with as few as 18 hours, it may be necessary to exclude hours not actually affected by the event to ensure the

same data exclusion outcome as if the measurement had been made with a 24-hour filter.

18 The form of the 24-hour PM 2.5 NAAQS of 35 μ g/m 3 is 98 th percentile averaged over 3 years. The form of the primary annual PM 2.5 NAAQS of 12 μ g/m 3 is an annual mean averaged over 3 years. The form of the 24-hour

PM 10 NAAQS of 150 μ g/m 3 is not to be exceeded more than once per year on average over 3 years. Biased concentrations can potentially sk ew the determination of the 98 th percentile and/or the annual mean for PM 2.5 and the averages for PM 2.5 or PM 10 calculated to determine compliance with the relevant NAAQS.

19 The EPA realizes that many of the defaulted EPA concurrences for pre-2006 PM 10 concentrations that were

below the level of the 24-hour PM 10 NAAQS actually were applicable to the annual PM 10 NAAQS, but this approach was the most practical way to ensure that all ot her concurrences originally intended to be applicable to the 24-hour NAAQS were preserved. B ecause concentrations below the leve l of the 24-hour NAAQS have no

effect on attainment determinations for the 24-hour NAAQS, no error can come from treating such values asInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 32 of 50  For CO, all concurrences were presumed to apply to both the 1-hour and the 8- hour NAAQS. This default was chosen to en sure that the concurrence applied to whichever NAAQS had been exceeded and was the basis for the exclusion

request.

 For SO 2 , all concurrences were presumed to apply to both the 24-hour and the annual NAAQS. This default was chosen to en sure that the conc urrence applied to whichever NAAQS had been exceeded and was the basis for the exclusion request. No flags were assumed to a pply to the 1-hour NAAQS because the 1-

hour SO 2 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/m 3 . This default was chosen because March 2010 was prior to the EPA issuing final design ations under the 2008 Pb NAAQS of 0.15 μg/m 3

.

 For NO 2 , all concurrences were presumed to apply to the annual NAAQS because the 1-hour NO2 standard was not pr omulgated until February of 2010.

For concurrences on events with dates afte r the March 2010 re-engineering of AQS, the EPA will specify the NAAQS to which the concu rrence applies. If this defaulting scheme does not properly represent the actual conc urrence action that was taken by the EPA regional office, the regional office should revise and correct the concur rence flags, if it has not already done so.

Air agencies can find detailed information on the use of events flags in AQS in a tutorial posted at http://www.epa.gov/ttn/airs/airsaqs/m anuals/ExceptionalEventTutorial.pdf . The tutorial discusses concurrence flags on page 20.

30. Question: For a NAAQS that is defined for a multi-hour or multi-day averaging time, but for which concentrations are measured, re ported, and flagged on the basis of a shorter time period, what comparisons between meas urements and the NAAQS level should air agencies prepare to satisfy the “but for” test?

Answer: One requirement for data exclusion unde r the Exceptional Events Rule is that

there would have been no exceedance or violat ion 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 that have even t flags attached can be compared directly to the level of the NAAQS. This is the case for the 1-hour ozone, 1-hour CO, 1-hour SO 2 , and 1-hour NO 2 NAAQS. This is also the case when 24-hour filter-based PM 10 or PM 2.5 concentrations are compared to the respective 24-hour NAAQS. 20 However, a difference

having been concurred. Nevertheless, the EPA regional office may choose to update these concurrence flags as time permits.

20 Air agencies have for many years reported SO 2 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 av erage concentrations (block or running) during an hour, it is the hourly concentration averages that should be compared to the 1-hour SO 2 NAAQS. Under a change in SO 2 monitoringInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 33 of 50 exists for the following NAAQS between the time period for reporting concentrations and the averaging period to which th e level of a NAAQS applies.

 Ozone, CO, NO 2 , and SO 2 are reported to AQS as 1-hour measurements, but all three have NAAQS defined for longer av eraging periods (3-hours, 8-hours, 24- hours, and/or annual). The longer-period c oncentration values that are compared to these NAAQS are calculated from the s ubmitted hourly values within AQS and cannot have event flag s attached to them.

 Pb is reported as 24-hour measurements , but the old and new NAAQS are both for three-month averages (quarterly av erages and 3-month rolling averages, respectively). The quarterly and 3-month c oncentration values that are compared to these NAAQS are calculated from the submitted 24-hour measurements within AQS and cannot have event fl ags attached to them.

 When using automated/continuous monitoring equipment, PM 2.5 and PM 10 data are reported as 1-hour measurements but there are PM 2.5 and PM 10 NAAQS with 24-hour averaging periods and a PM 2.5 NAAQS with an annual averaging period. The 24-hour and annual values compared to the NAAQS are calculated within AQS and cannot have event flags attached to them. As described in more detail in the response to Question 29, to ensure the same data exclusion outcome regardless of whether PM 2.5 and PM 10 measurements are made with filter-based or continuous monitoring equipment, the EP A intends to exclude all 24 1-hour measurements in a given day whenever the “but for” cr iterion (and other exceptional event criterion) are satisfied for that day even if an event only affected discrete hours of the day. The EPA will be able to do this only if the air agency has applied “R” fl ags to each of those hours. 21

 When using filter-based monitoring equipment, PM 2.5 and PM 10 are reported as

24-hour measurements but there is a PM 2.5 NAAQS with an annual averaging period. The annual values used in compar isons the NAAQS are calculated within AQS and cannot have event flags attached to them.

requirements that accompanied th e promulgation of the 1-hour SO 2 NAAQS, the EPA now requires that air agencies report the maximum 5-minute block average concen tration, 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; AQ S does not use 5-minute data to replace the submitted hourly concentration. While 5-minute concentrations ma y play a role in evaluating whether Exceptional Event criteria are satisfied for a given hour and event, for exam ple to establish a clear causa l connection, they are not to be compared to the level of the 1-hour (or any other) NAAQS for SO 2 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 SO 2 data.

21 Because AQS can calculate a valid 24-hour average concentr ation with as few as 18 hours, it may be necessary to exclude hours not actually affected by the ev ent to ensure the same data exclusion outcome as if

the measurement had been made with a 24-hour filter. Ex clusion of only the high hourly concentrations could result in AQS calculating a valid low (or, potentially high) biased 24-hour concentration under the rules for data interpretation.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 34 of 50 The mismatches of time periods make this a question with a complex answer. The following paragraphs, summarized in Table Q 30-1, explain the genera l rationale behind the pollutant and NAAQS-specifi c 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 overl apping with the event and its effects on air quality, and which would not have occurred “but for” the effects of the event. 22 By definition, an exceedance necessarily involves a comparison between an air concentration, averaged over a time period equa l 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 me t, because the outcome of the comparison for a single hour does not indicate whether an ex ceedance or violation of the 8-hour NAAQS occurred, or whether it would not have occurred “but for” the event. Instead, air agencies

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, rather than the individual c oncentrations comprising the average, to the identified NAAQS. 23 Air agencies should, however, id entify in their exceptional event submission those particular measurements that caused 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 PM 2.5 , the direct comparison of a single 24-hour average con centration (determined from a single filterbased measurement or by averaging 24 1-hour measurements from a continuous equivalent instrument) to the level of the annual NAAQS can be the basis for meeting the

22 The EPA interprets the Exceptional Ev ent Rule and its preamble to mean “exceedance or violation” each time that “exceedance” or “violation” occurs in the text, cons istent with the obvious in tent of the Clean Air Act amendment requiring the EPA to prom ulgate the Rule. An “exceedance” occu rs each time the concentration in the air for the averaging period appli cable to the NAAQS is higher than the level of the NAAQS. Most NAAQS allow some such occurrences in a 1-y ear 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.

23 A scenario could exist in which the effect of an event on one or more 24-hour PM 2.5 concentration creates a

“but for” difference on the annual concentration even t hough the actual 24-hour concentration(s) on the day(s) of the event was below the level of the annual NAAQS. This implies that the EPA could concur with the exclusion request for the 24-hour concentration value. However, the Exceptional Events Rule preamble makes clear that only 24-hour PM 2.5 concentrations that are above the level of the annual NAAQS maybe excluded. Similarly, the EPA generally does not intend to concur with respect to any NAAQS on a flag for a 1-hour NO 2 and SO 2 concentration that is below the level of the resp ective annual NAAQS, regardless of the outcome of “but for” tests based on comparison of 24-hour or annu al average concentrations to their same-period NAAQS. Also, the EPA generally does not 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 (r olling 3-month average) Pb NAAQS. The EPA believes that it is generally appropriate to use the similar restriction for PM 2.5 stated and explained in the preamble to the Exceptional Event Rule. Moreover, it is highly unlikely th at even several hourly concentrations below the level

of the annual NO 2 NAAQS of 53 ppb could include an event contribution that when summed with all other hourly concentrations and then divided by 8760 (24 hours times 365 days), could result in the annual average

NO 2 concentration crossing from below the level of the annual NAAQS to above the level of the annual NAAQS.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 35 of 50 “but for” criterion for exceedances or violations of the annual NAAQS. 24 In context, it is clear that based on this comp arison, a 24-hour concentration can be excluded from the

calculation of the annual PM 2.5 NAAQS design value, if other rule criteria are also met. It is therefore not necessary to show that the annual average PM 2.5 concentration was above

12 or 15 μ g/m 3 with the event and would have been below 12 or 15 μ g/m 3 “but for” the single event at issue. Such a concentration ca n also be excluded from the calculation of the design value for the 24-hour PM 2.5 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 μ g/m 3 . This special case is re flected in Table Q30-2.

In light of this departure in the preamble from a formal definitional approach in the case

of a 24-hour PM 2.5 measurement and the annual PM 2.5 NAAQS, Table Q30-2 also provides a parallel special approach for similar comparisons involving Pb, NO 2 and SO 2 that the EPA generally intends to apply. The EPA believes ap plying this interpretation for Pb, NO 2 , and SO 2 is consistent with the interpretation in the preamble for PM 2.5 and is consistent with the EPA’s intent in drafting the Exceptional Events Rule. That is, a 24- hour average concentration of Pb, NO 2 , or SO 2 can be compared to the NAAQS level defined for a longer period, for purposes of meet ing “but for” with respect to both the 24- hour NAAQS, if applicable, and the NAAQS with the longer averaging period.

Table Q30-1. Principles for General Approac h to Satisfying the “But For” Test Note: The principles identified in this table ar e 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, SO 2 , NO 2 , and ozone.

 24-hour filter-based PM 2.5 or PM 10 measurements vs. 24-hour NAAQS.

24 When the EPA promulgated the Exceptional Events Rule in 2007, the level of the annual PM 2.5 NAAQS was

15 μ g/m 3 . On December 14, 2012, the EPA promulgated a revised annual PM 2.5 NAAQS of 12 μ g/m 3 (78 FR 3086). Because both standards apply, an air agency can choose the appropri ate level of the annual NAAQS (i.e.,

either 12 μ g/m 3 or 15 μ g/m 3 ) as the basis for meeting the “but for” criterion. For example, an air agency developing an exceptional events demonstration package that may influence an attainment demonstration for

the annual PM 2.5 NAAQS of 15 μ g/m 3 would likely use 15 μ g/m 3 as the basis for meeting the “but for” criterion while an air agency preparing a demonstration package that may influence initial area designation status for the

2012 annual PM 2.5 NAAQS of 12 μ g/m 3 would likely use 12 μ g/m 3 as the basis for meeting the “but for”

criterion.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 36 of 50 Principle Application to Specific NAAQS Exceptions

2 When the measurement time is shorter than the averaging time of the NAAQS (e.g., 1-hour O 3 measurements and the 8-hour O 3 NAAQS), air agencies can compare the average of the 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 particular measurements that caused the elevated average.  1-hour ozone measurements vs. 8- hour NAAQS.

 1-hour CO measurements vs. 8- hour NAAQS.

 1-hour SO 2 measurements vs. 3- hour, 24-hour, and annual NAAQS.

 1-hour NO 2 measurements vs. annual average .NAAQS.

 1-hour PM 2.5 measurements vs. 24- hour and annual average NAAQS.

 1-hour PM 10 measurements vs. 24- hour average NAAQS.

 24-hour PM 2.5 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 generally will not be

considered for exclusion regardless of

the outcome of comparing the longer period average to the NAAQS level.

3 When the PM 2.5 or Pb measurement time is 24 hours (and when hourly

PM 2.5 measurements are used to calculate a 24-hour concentration), it is also permitted to compare the

24-hour concentration to the annual average PM 2.5 NAAQS or the quarterly or rolling 3-month Pb NAAQS.  24-hour PM 2.5 concentrations 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 a consistent

with the intent of the PM 2.5 provision in the preamble).

4 1-hour SO 2 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 with respect to the 24-hour NAAQS for those individual 1-hour

measurements in the 24-hour averaging period that were affected by the event.  A comparison of 1-hour SO 2 measurements vs. the annual average NAAQS (where the 30 ppb annual SO 2 NAAQS still applies) is recommended in this guidance to create a reasonable benchmark for judging the excludability of 1-hour SO 2 measurements for the purpose of the annual NAAQS, for cases

when the event did not affect the annual average enough to make a

“but for” difference relative to the annual average NAAQS. 5 When there is no NAAQS for the 24-hour averaging period, 1-hour measurements may be compared directly to the annual NAAQS.  A comparison of 1-hour NO 2 measurements vs. annual average NAAQS is recommended in this guidance to create a reasonableInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 37 of 50 Principle Application to Specific NAAQS Exceptions
benchmark for judging the excludability of 1-hour NO 2 measurements for the purpose of the annual NAAQS, for cases

when the event did not affect the annual average enough to make a

“but for” difference relative to the annual average NAAQS. 6 Otherwise, single 1-hour measurements generally may not be compared to the level of the annual average NAAQS.  Single 1-hour SO 2 measurements generally may not be compared the annual average NAAQS (because there is a 24-hour NAAQS for SO 2 with a defined averaging methodology).

 Single 1-hour PM 2.5 measurements generally may not be compared to the annual average NAAQS (because there is a 24-hour NAAQS for PM 2.5 with a defined averaging methodology).

Table Q30-2 identifies the comparisons and c onclusions that generally would help satisfy the “no exceedance 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 ev ent might cause an exceedance of the annual average NO 2 NAAQS. Also, note that Table Q30- 2 addresses only the “no exceedance but for” question. As indicated in the answer to Question 31, even if an event cannot be

demonstrated to make a “but for” differ ence in whether an exceedance occurred, it is possible that it makes a “but for” difference in whether a 3-year vi olation of the NAAQS occurred, for the NAAQS that are defined based on a 3-year average design value concentration. The logic behind Table Q30-2 app lies to a “no violation but for” test also. Air agencies may request assistance from th e EPA regional office on applying this logic when performing a “no violation but for” test.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 38 of 50 Table Q30-2. General Approaches for Satisfying th e “No Exceedance But For” Test

Pollutant Specific Case: NAAQS level NAAQS averaging period Measurement period General 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 fo r 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 pu rposes 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.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 39 of 50 Table Q30-2. General Approaches for Satisfying th e “No Exceedance But For” Test

Pollutant Specific Case: NAAQS level NAAQS averaging period Measurement period General Approach

4 PM 2.5 35 μ g/m 3
24-hour averaging period

1-hour measurement

(Note: Air agencies can use either

15.0 μ g/m 3 or 12.0 μ g/m 3 as a basis for comparison.)  If the 24-hour average concentration based on 1-

hour measurements was above 35.4 μ g/m 3 (after truncating after the first decimal digit, per 40 CFR 50 Appendix N section 3.0(c)) but would

have been 35.4 μ g/m 3 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 35

μ g/m 3 24-hour PM 2.5 NAAQS.

 Also, if the 24-hour average concentration based

on 1-hour measurements was above 12.0 / 15.0

μ g/m 3 (after truncation after the first decimal digit) but would have been 12.0 / 15.0 μ g/m 3 or less in the absence of the event, those 1-hour concentration values that were affected by the single event are eligible to be considered for exclusion for purposes of comparison to the 35

μ g/m 3 24-hour PM 2.5 NAAQS.

5 PM 2.5 12.0 μ g/m 3
Annual averaging period

1-hour measurement

(Note: Air agencies preparing demonstrations involving PM concentrations for comparison against the 1997 annual PM 2.5 standard of 15.0 μ g/m 3 should

substitute 12.0 μ g/m 3 with 15.0

μ g/m 3 in the “General Approach” steps in the next column.)  If the annual average PM 2.5 concentration was

above 12.0 μ g/m 3 but would have been equal to

or less than 12.0 μ g/m 3 (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 12 μ g/m 3 annual PM 2.5 NAAQS.

 Also, if the 24-hour average concentration based

on 1-hour measurements was above 12.0 μ g/m 3
(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 12.0 μ g/m 3 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 12 μ g/m 3 annual PM 2.5 NAAQS.

However, an hourly value must be part of a 24-hour average concentration that is above 12 μ g/m 3 (after rounding to one decimal digit) to be excluded from an annual NAAQS calculation.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 40 of 50 Table Q30-2. General Approaches for Satisfying th e “No Exceedance But For” Test

Pollutant Specific Case: NAAQS level NAAQS averaging period Measurement period General Approach

6 PM 2.5 35 μ g/m 3
24-hour averaging period

24-hour measurement

(Note: Air agencies can use either

15.0 μ g/m 3 or 12.0 μ g/m 3 as a basis for comparison.)  If the 24-hour average concentration was above

35.4 μ g/m 3 (after truncating after the first decimal digit, per 40 CFR 50 Appendix N section 3.0(b)) but would have been 35.4

μ g/m 3 or less in the absence of the event, the 24-

hr concentration value meets the “but for” test for purposes of comparison to 35 μ g/m 3 24-hour

PM 2.5 NAAQS.

 Also, if the 24-hour average concentration was

above 12.0 / 15.0 μ g/m 3 (after truncating after the first decimal digit, per 40 CFR 50 Appendix N section 3.0(b)) but would have been 12.0 / 15.0

μ g/m 3 or less in the absence of the event, the 24 average concentration meets the “but for” test for purposes of comparison to 35 μ g/m 3 24-hour

PM 2.5 NAAQS.

7 PM 2.5 12 μ g/m 3
Annual averaging period

24-hour measurement

(Note: Air agencies preparing demonstrations involving PM concentrations for comparison against the 1997 annual PM 2.5 standard of 15.0 μ g/m 3 should

substitute 12.0 μ g/m 3 with 15.0

μ g/m 3 in the “General Approach” steps in the next column.)  If the annual average PM 2.5 concentration was

above 12.0 μ g/m 3 (after rounding to one decimal digit per 40 CFR 50 Appendix N section 4.2(a)) but would have been equal to or less than 12.0

μ g/m 3 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 12 μ g/m 3 annual PM 2.5 NAAQS.

 Also, if the 24-hour average concentration from the filter-based sampler was above 12.0 μ g/m 3
(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 12.0

μ g/m 3 in the absence of the event, the 24-hour value meets the “but for” test for purposes of comparison to 12 μ g/m 3 annual PM 2.5 NAAQS.

8 PM 10 150 μ g/m 3
24-hour averaging period

1-hour measurement  If the 24-hour average concentration based on 1-

hour measurements was above 150 μ g/m 3 (after rounding to the nearest 10 μ g/m 3 , per 40 CFR 50 Appendix K section 1.0(b)) but would have been equal to or less than 150 μ g/m 3 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

μ g/m 3 24-hour PM 10 NAAQS.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 41 of 50 Table Q30-2. General Approaches for Satisfying th e “No Exceedance But For” Test

Pollutant Specific Case: NAAQS level NAAQS averaging period Measurement period General Approach

9 PM 10 150 μ g/m 3
24-hour averaging period

24-hour measurement  If the 24-hour average concentration from the filter-based sampler was above 150 μ g/m 3 (after rounding to the nearest 10 μ g/m 3 , per 40 CFR 50 Appendix K section 1.0(b)) but would have been equal to or less than 150 μ g/m 3 in the absence of the event, the 24-hour value meets the “but for” test for purposes of comparison to the 150 μ g/m 3

24-hour PM 10 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 va lue 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-hourperiod 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.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 42 of 50 Table Q30-2. General Approaches for Satisfying th e “No Exceedance But For” Test

Pollutant Specific Case: NAAQS level NAAQS averaging period Measurement period General Approach

12 Pb 1.5 μ g/m 3
Quarterly averaging period

24-hour measurement  If the quarterly mean was above 1.5 μ g/m 3 (after rounding to one decimal digit) but would have been equal to or less than 1.5 μ g/m 3 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 μ g/m 3 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 μ g/m 3
(after rounding to one decimal digit) but would have been equal to or less than 1.5 μ g/m 3 in the absence of the event, th e 24-hour value meets the “but for” test for purposes of comparison to 1.5

μ g/m 3 quarterly average Pb NAAQS.

A 24-hour Pb concentration that is equal to or less

than 1.5 μ g/m 3 will generally not be considered for

exclusion.

13 Pb 0.15 μ g/m 3
Rolling 3-month averaging period

24-hour measurement  If a 3-month mean was above 0.15 μ g/m 3 (after rounding to two decimal digits) but would have been equal to or less than 0.15 μ g/m 3 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 μ g/m 3 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 μ g/m 3
(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 μ g/m 3 in the absence of the event, th e 24-hour value meets the “but for” test for purpos es of comparison to the

0.15 μ g/m 3 quarterly average Pb NAAQS.

A 24-hour Pb concentration that is equal to or less

than 0.15 μ g/m 3 will generally not be considered for

exclusion.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 43 of 50 Table Q30-2. General Approaches for Satisfying th e “No Exceedance But For” Test

Pollutant Specific Case: NAAQS level NAAQS averaging period Measurement period General Approach

14 NO 2 100 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 ev ent, the 1-hour NO 2 concentration value meets the “but for” test for purposes of comparison to the 1-hour NAAQS.

15 NO 2 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 NO 2 NAAQS.

 If the 1-hour concentration was above 53 ppb (after truncating to a wh ole 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 NO 2 concentration that is below 53 ppb (after rounding to a whole number) will generally

not be considered for exclusion.

16 SO 2 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 ev ent, the 1-hour SO 2 concentration value meets the “but for” test for purposes of comparison to the 1-hour SO 2 NAAQS.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 44 of 50 Table Q30-2. General Approaches for Satisfying th e “No Exceedance But For” Test

Pollutant Specific Case: NAAQS level NAAQS averaging period Measurement period General Approach

17 SO 2 140 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 SO 2 NAAQS.

 Also, if the 24-hour average concentration based on 1-hour measurements was above 30 ppb (after rounding to the nearest 10 ppb per 40 CFR 50.4(b)) but would have been equal to or less than 30 ppb in the absenc e 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 140 ppb 24-hour SO 2 NAAQS.

18 SO 2 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 SO 2 NAAQS.

If the 30 ppb annual SO 2 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)) will generally not be considered for

exclusion.

19 SO 2 (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 SO 2 NAAQS.

31. Question: When is it appropriate fo r air agencies to flag con centration values that are less than the level of the relevant NAAQS ? Under what circumstances will the EPA concur on such flags?Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 45 of 50 Answer: ( Please read Q30 before re ading 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 Exceptional Events Rule does not 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 ev idence/analysis package 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 viol ation of the NAAQS), for purpos es of being able to later identify historical data that have not been a ffected so that “normal” concentration patterns can be presented as part of meeting the “in ex cess 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 establ ish that the concentrations were in fact affected by an event and should be ex cluded from the “normal” baseline.

Of the flagged cases that appear in both AQS and in demonstration packages, the EPA may find it appropriate to concur with flags for concentrations that are below the NAAQS only in five very narrow condi tions described below. If the EPA determines that a flag on

a value less than the level of the NAAQS cannot meet the “but fo r” 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 PM 10 limited maintenance plans 25 , the EPA intends to prioritize

events that result in a violation or ex ceedance 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 indi vidual 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 O 3 measurement of 100 ppb) is not above the level of that averaging time NAAQS (e.g., 1-hour O 3 NAAQS of 120 ppb), but is above a longer averaging time NAAQS (e.g., 8-hour O 3 NAAQS of 80 ppb) a nd contributes to a violation of the longer averaging time NAAQ S. In such cases, although the individual measurement may not exceed the level of the shorter-term NAAQS, it may be possible for air agencies to present sufficient eviden ce to satisfy the “but-for” criterion for a longer-term NAAQS.

(See Questions 8, 9, 13, and 19 for additional information.)

25 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 PM 10 values between 98 and 154

μ g/m 3 (inclusive) to be flagged, concurred, and excluded for purposes of qualifying an area for reliance on only a limited maintenance plan.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 46 of 50 First, PM 10 values between 98 and 154 μ g/m 3 (inclusive) may be flagged, concurred, and excluded for purposes of qualifying an area fo r reliance on only a limited maintenance plan (see footnote 24). Because of th e expected exceedance form of the PM 10 NAAQS, concentrations in this range cannot possibly affect whether a site actually meets the NAAQS, so there is no reason for flagging th em except when the acceptability of a limited maintenance plan is an issue. The normal AQS flagging and concurrence procedures may be used in this situation. 26

A second scenario in which the EPA may find it appropriate to concur with flags for concentrations that are below the NAAQS is indicated at 72 FR 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 ai r 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 th e context of this provision, “exceptional” encompasses all the requirements of the Excepti onal Events Rule other than the “but for” test (e.g., clear causal connection, “in excess of normal historical fluctuations, including background,” not reasonably c ontrollable or preventable).

Scenarios in which the measur ed 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 PM 2.5 measurement that is greater than the 12 or 15 μ g/m 3 annual PM 2.5 NAAQS but not greater than the 35 μ g/m 3

24-hour PM 2.5 NAAQS for exclusion for the purposes of the 24-hour PM 2.5 NAAQS.

This is the result if the actual 24-hour concentration was between 12 or 15 and 35 μ g/m 3

but would have been below 12 or 15 μ g/m 3 but for the effect of the event. It should be

noted that an exclusion made under this very specific provision for the 24-hour PM 2.5 NAAQS will only affect the outcome of an attainment determination for the 24-hour NAAQS if the concentration value in que stion 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 leve l of the 24-hour NAAQS does affect the 3- year design value, the applica tion of the guidance for the four th situation (below), which is applicable to all four NAAQS pollutants with multi-year design values, would get to the same result as applica tion of this paragraph.

Fourth, assuming that all other Exceptional Ev ents Rule requirements and conditions are met, the EPA may concur with flags for ozone, PM 2.5 , 1-hour NO 2 , and 1-hour SO 2 that are “less than the level of the NAAQS” if adjusting the flagged concentrations for the

estimated contribution from the event would ch ange the 3-year design value from being

26 Values in this range can potentially affect the design value for PM 10 , but these design values are primarily informational and are not likely to influence designations or regulatory determinations of attainment. The procedure for determining a PM 10 design value in units of μ g/m 3 is given in section 6.3 of the EPA guidance

document “PM 10 SIP Development Guideline,” June 1987, posted at http://www.epa.gov/ttn/oarpg/t1/memoranda/pm10sip_dev_guide.pdf

.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 47 of 50 above the NAAQS to being equal to or be low the NAAQS. However, as indicated in footnote 21, concentrations below certain values generally will not be excluded.

Fifth, a 1-hour measurement of a pollutant th at 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 acc ount the event’s effect on all the hours in the longer period the effect of the event on the longer averag ing period’s concentrations

satisfies the “but for” criter ion. These situations are descri bed in Table Q30-2 (rows 3, 4, 8, 11, 12, 13, 17, and 19). However, as indicate d in Table Q30-2, concentrations below certain values generally will not be excluded.

The following NAAQS-specific discussions prov ide further explanati ons regarding some

of the situations in which a concentrati on less than the level of the NAAQS may qualify

for exclusion. These discussions are not exhaus tive and do not obviate the need to refer to Table Q30-2.

24-hour PM 2.5

Assume for illustration th at the three annual 98 th percentile 24-hour PM 2.5 concentrations for a monitoring site for 2006-2008 are 41, 31, and 37 μ g/m 3 for each respective year with a resulting 3-year design value of 36 μ g/m 3 which is a violation of the 24-hour PM 2.5 NAAQS of 35 μ g/m 3 . Also, assume that the next highe st concentration in 2007 below the

31 μ g/m 3 was only 20 μ g/m 3 . The 31 μ g/m 3 concentration in 2007 was affected by a oneday wildfire. The air agency has been able to show that the concentration would have

been 17 μ g/m 3 without the fire. Because neither 20 μ g/m 3 nor 31 μ g/m 3 exceed the NAAQS, the event on that day does not meet th e “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 th e 24-hour NAAQS. Had there been no fire, the

98 th percentile concentration in 2007 would have been 20 μ g/m 3 which would result in a 3-year design value of 33 μ g/m 3 (i.e., less than the 24-hour PM 2.5 NAAQS of 35 μ g/m 3 ). Therefore, the 2007 value of 31 μ g/m 3 meets the “but for” test when the focus in on NAAQS violations rather th an individual exceedances. A ssuming other requirements are met, the 31 μ g/m 3 concentration would be approved by the EPA for exclusion from the 2006-2008 design value. Note that in doing a “v iolations-based” “but for” analysis, one does not simply substitute the “no even t” concentration for the original 98 th percentile day into the design value calculation. Rather, one must re-select the 98 th percentile day, which sometimes will result in a different day’s actual measured value being used in the design value calculation. 27

It is conceivable that the e ffect 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

27 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 12 or 15 to above 12 or 15 μ g/m 3 the event does not me et the first condition specified in row 7 of Table Q30-2. It is also very improb able that an event affecting a single day would meet the second condition in row 7 of Table Q30-2.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 48 of 50 paragraph for one three-year period, but that it do es satisfy it for an ea rlier or later 3-year period when it is combined with one or two di fferent concentrations to calculate a 3-year design values, since the outcome of the “vio lations” analysis may change. After the EPA has approved the exclusion of a concentrati on 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.

For the 24-hour PM 2.5 NAAQS, it is possible that multip le days with concentrations below the NAAQS within one year are flagged. Excluding just one of these concentrations may not change the annual 98 th percentile concentr ation enough to cause the 3-year design value to change from “vio lating” 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 concen trations that are above the level of the NAAQS. In such cases, the exclusion decisi ons 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 c onditions 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 fla gged concentrations below the level of the NAAQS would cause a switch in whether th e 3-year design value violates the NAAQS

then if the EPA determines that value is to be excluded then th ere 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 PM 2.5 concentrations below the level of the a nnual NAAQS cannot be excluded for purposes of comparisons to the annual NAAQS. (72 FR 13570, bottom of middle column) Even if the conditions described in the preceding para graph are met, values below 12 or 15 μ g/m 3

cannot be excluded.

Annual PM 2.5

The preamble to the Exceptional Events Rule explicitly states that PM 2.5 concentrations below the level of the annual NAAQS cannot be excluded for purposes of comparisons to the annual NAAQS. (72 FR 13570, bottom of middle column)

Ozone (0.075 ppm 8-hour NAAQS)

Assume for illustration that the three annual 4 th highest daily 8-hour ozone values in 2006-2008 are 0.077, 0.076, and 0.075 ppm respec tively. The 0.075 ppm value in 2008 was affected by an exceptional event. Th e 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 4 th 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 circ umstances even though it is not itself an exceedance. Furthermore, the exclusion also applies to the use of this value whenInterim Exceptional Events Rule Frequently Asked Questions May 2013

Page 49 of 50 calculating the 2007-2009 and 2008-2010 design va lues, regardless of whether such exclusion causes those design values to sw itch from violating to complying with the NAAQS.

For ozone, as for 24-hour PM 2.5 , it is possible that an air ag ency could flag multiple days within one year with con centrations below the NAAQS. Excluding just one of these concentrations may not change the annual 4 th highest concentrati on enough to cause the 3-year design value to change from “viola ting” to “complying,” but excluding several of them may. Also, the outcome for the desi gn value may depend, in part, on whether exclusion is granted for some other concen trations that are above the level of the NAAQS. In such cases, the exclusion decisi ons 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 c onditions 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 fla gged concentrations below the level of the NAAQS would cause a switch in whether th e 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.

PM 10

The only current PM 10 NAAQS is the 24-hour NAAQS based on the expected number of exceedances over a 3-year period. Since a conc entration below the level of the NAAQS

would not be an exceedance and cannot aff ect 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 PM 10 values as low as 98 μ g/m 3 can be concurred for exclusion when determining whether an area is eligible fo r 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 μ g/m 3 would appear to have little regulatory significance, the EPA discourages the flagging of such data.

Pb

The legacy 1.5 μ g/m 3 and current 0.15 μ g/m 3 NAAQS for lead are both based on a maximum three-month average concentration. The 1.5 μ g/m 3 standard is based on the maximum quarterly average, while the 0.15 μ g/m 3 NAAQS is based on the highest rolling 3-month average during a 3-year period. As previously explained, the EPA is not likely to concur on the exclusi on of a 24-hour concentration va lue that is below the level of the NAAQS, and we discourage air ag encies from flagging such values.Interim Exceptional Events Rule Frequently Asked Questions May 2013

Page 50 of 50 NO 2

As previously explained, the EPA is not lik ely to concur on the exclusion of a 1-hour

NO 2 concentration that is belo w the level of the annual NO 2 NAAQS, and we discourage air agencies from flagging such values.

SO 2

As previously explained, the EPA is not lik ely to concur on the exclusion of a 1-hour

SO2 concentration that is be low the level of the annual SO 2 NAAQS, and we discourage air agencies from flagging such values.