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### ATTACHMENT 1

Draft Exceptional Events Rule Frequently Asked Questions

The Exceptional Events Rule of 2007 1 supersedes the EPA’s previous Exceptional Events guidance and policy documents a nd creates a regulatory proce ss codified at 40 CFR parts 50 and 51 (50.1, 50.14 and 51.930). The Exceptional Even ts Rule (EER) recognizes that each potential event can have differe nt or unique characteristics, a nd thus, requires a case-by-case demonstration and evaluation. Therefore, the EER adopts a “weight-of-evidence” approach in evaluating each demonstration to justify excl uding data affected by an exceptional event. The EPA acknowledges that certain extreme 2 exceptional event cases may require more limited demonstration packages.

Technical questions and issues related to implementation have arisen since the EPA promulgated the EER. This Question and Answer (Q&A) document is intended to respond to some of these frequently asked questions and to provide instruction a nd clarification to air agencies 3 implementing the EER. The EPA recognizes the limited resources of the air agencies that prepare and submit exceptional ev ent demonstration packages and of the EPA regional offices that review these demonstr ation packages. One of the EPA’s goals in developing exceptional event implementation guidan ce is to establish cl ear expectations to enable affected agencies to better manage resources as they prepare the documentation required under the EER. The EPA expects submit ters to prepare and submit the appropriate level of supporting documentation, which will va ry on a case-by-case basis using the weightof-evidence approach. The EPA expects the resources required to prepare (and review) packages to decrease as we continue to iden tify ways to streamline the process and continue

to build our database of exam ple demonstrations and analyses . In addition, as noted above, the EPA acknowledges that certain extreme exceptional event cases may require more limited demonstration packages.

For organizational ease, this document has 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 at 13563, March 22, 2007.

2 The EPA acknowledges that certain extreme exceptional event cases may require more limited demonstration packages. Whether a particular event should be considered “extreme” for this purpose depends on the type and severity of the event, pollutant concentration, spatial ex tent, temporal extent, and proximity of the event to the violating monitor. Several meteorological phenomena that could be considered extreme events include hurricanes, tornadoes, haboobs, and catastrophic volca nic eruptions. The EPA addresses “extreme” high wind dust events in Question 17a in this document, but solicits comment on whether and how specific events of various types should be considered to be “extreme.”

3 References to “air agencies” are meant to include st ate, local, and tribal air agencies responsible for implementing the EER.Draft for Public Notice and Comment

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Page 2 of 46 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 draft guidance documents and the exceptional events website present examples to illustrate specific points. The example analyses and level of rigor are not nece ssarily required for all demonstrations.

Disclaimer The Exceptional Events Rule is the source of the regulatory requirements for exceptional events and exceptional event demonstrations . This Q&A document provides guidance and interpretation of the Exceptional Events Rule rather than imposing any new requirements and

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

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Page 3 of 46 A. Historical Fluctuations

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

evidence that:

* * * (C) The event is associated with a measured concentration in excess of normal historical 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 point in the historical data?

Answer: It is a test, but there is no specific percentile point that the EPA will use to determine whether the test has been passe d. The EPA will use a weight-of-evidence approach to review each demonstration on a case-by-case basis. The air agency’s role in satisfying this element is to pr ovide appropriate analyses and statistics and conclude that the provided data show that the event was in excess of normal histor ical fluctuations. The EPA will review the information provided by the air agency. “Normal historical fluctuations” will generally be defined by those days without events for the previous years. The EPA acknowledges that natural even ts can recur and still be eligible for exclusion under the EER; therefore, events do not necessarily have to be rare to satisfy

this element.

The submittal of data showing how the event concentration compared with historical concentrations will help the EPA determine whether the “clear causal relationship,” “but for,” and “affects air quality” criteria have been satisfied. These EER criteria, as well as “not reasonably controllable or preventable,” need to be satisfied for EPA to concur on an exceptional event claim. The EPA expects that failure on this element indicates likely failure for “clear causal relationship” and/or “b ut for” as well. However, failure to submit a comparison would prevent the EPA from being able to approve exclus ion 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 subsequent questions. Submission of the identified statistical analyses will be considered to have met the requirement to “provide evidence.”

It is important to note, however, that there is no outcome of the “h istorical fluctuation” statistical comparison that, by itself, can guarantee that th e clear causal relationship and “but for” elements will also be successfully demonstrated. The EPA will consider in its weight-of-evidence approach the comparison of the concentrations during event(s) in question with historical concen tration data. For example, a uniquely high concentration in an area (and season) with no previous exceed ances, with a clear causal connection, and with no evidence of any other plausible explan ation would be a case in which the weight-Draft for Public Notice and Comment

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Page 4 of 46 of-evidence would indicate that the “but for” criterion has be en demonstrated. In contrast,

if the event-affected concentration does not stand out much from normally occurring exceedance concentrations for the same pl ace and season, the statistical comparison will not by itself provide much support for “but fo r” in the weight-of-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 concentrations. To speed the EPA review process, avoid the need for the EPA to request additional information, and ensure that the EPA understands the position of the submitting agen cy; this analysis should consist of the following types of statistics, gr aphics, 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 exceed ances can be expected throughout the year, analysis of annual data would likely be mo re appropriate. For seasonal comparisons, the EPA recommends using all available seasonal da ta 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 quant ity of data, it may be appropriate to present monthly maximums; however, it is not appropriate to pr esent monthly-averaged daily data or any other average of the daily data as this masks high values. Regardless of whether seasonal or annual data are presented, all data should be provided in the form relevant to the standard that is being considered for data exclusion (see Question 30). Specific examples of analyses of annual and seasonal data, as well as analyses of hi storical speciated PM 2.5 fluctuations and spatial distri bution fluctuations are include d in the presentation located

at http://www.epa.gov/ttn/analysis/doc s/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 area, thereby giving a full and accurate portrayal of the historical context for the claimed event day.

 Comparison of concentrations on the claimed event day with a narrower set of similar days: Similar days could include neighboring days ( e.g., a time series of two weeks) and other days with similar meteorological c onditions (possibly from other years). TheDraft for Public Notice and Comment

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Page 5 of 46 objective of such a comparison would be to demonstrate that the event caused higher concentrations than would be expected for given meteorological and/or local emissions

conditions.

 Percentile of concentration relative to a nnual data. The percentile of the event-day concentration should be provided for the even t day relative to all measurement days over the previous 3-5 years. To ensure statisti cal robustness, the EPA expects a minimum of 300 data points to be included in this calculati on. The daily statistic should be appropriate for the form of the standard being consid ered for data exclusion (see Question 30).

 Percentile of concentration relative to seas onal data. The percentile of the event-day concentration should be provided for the even t day relative to all measurement days for the season (or appropriate alte rnative 3-month period) of th e event over the previous 3-5 years. It is appropriate to use the same ti me horizon as used for the percentile calculated

relative to annual data.

(Note: The use of percentiles is illustrative and should not be seen as a bright line to be passed or failed when comparing observed c oncentrations with historical values.)

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

the EPA assesses whether the “but for” and “c lear 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.

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.Draft for Public Notice and Comment

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Page 6 of 46 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 describe d in this question and in Question 2 and that might be included in a submittal showing that an event is associated with a measurement “in excess of normal historical fluctuations.”)

The evidence comparing the event-affected 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 facilita te the EPA’s understanding of the impact 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 would be appropriate, while annuali zed data analysis might be more appropriate for the sec ond case to provide the most r obust yet also representative

historical data set.

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

0 10 20 30 40 50 60 70

1 92 183 274 365

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

Flagged Value Potentially Outside

Normal Seasonal Historical FluctuationsDraft for Public Notice and Comment

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Page 7 of 46 4. Question: The Preamble to the EER states that less documentation or evidence may be

needed to demonstrate that an event affected air quality for flagged data > 95th percentile than for values > 75th percentile. For ozone, 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 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? 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 FluctuationsDraft for Public Notice and Comment

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Page 8 of 46 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 with any defined bright lin e, air agencies need only submit (with appropriate descriptions and di scussion) the type of statistical analyses described in the responses to Questions 2 and 3, and the EPA will review these analyses and look at both the relationship between th e claimed concentration and historical concentrations and the strength of the data se t to help inform the evidence needed to

demonstrate the clear causal relatio nship and “but for” criteria.

In the response to Question 2, we identified that 3-5 years of data s hould be evaluated to ensure some degree of statistic al validity. We recognize, howev er, that these data may not be available for all monitors and/or all polluta nts. If data are not av ailable, please consult with the reviewing EPA regional office.

B. “But For” Test

Section 319 of the Clean Air Act requires that “a clear causal relationship must exist

between the measured exceedances of a national ambient air quality standard and the exceptional event to demonstrate that the exceptional event caused a specific air pollution concentration at a particular air quality monitoring location...” and that [States] can petition [EPA] to “[E]xclude dat a that is directly due to exceptional events from use in determinations...with respect to exceedances or violations.”

The implementing language in the EER at 40 CFR 50.14(c)(3)(iv) states: “The demonstration to 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 be incl uded in a demonstration that ozone

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

Answer: Air Agencies may include any evidence th at they consider relevant to the “but for” requirement, but they should recognize that the effects of a fire on ozone are complex. Fire can generate ozone precursors, but it can also reduce so lar radiation needed to drive ozone formation. Also, fire plumes containing ozone and ozone precursors can pass over a monitoring site without mixi ng down to ground level and affecting the monitored concentration. Additionally, wildfire s often occur during the same seasons that exhibit high ozone caused by anthropogenic pr ecursor emissions making it difficult to separate the wildfire contri bution from a high ozone event that would have occurred

without the fire.Draft for Public Notice and Comment

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Page 9 of 46 Examples of relevant evidence follow. Ge nerally, the more types of evidence the demonstration includes, the stronger the case for the ex ceptional event.

 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.

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

rather than non-event sources. This info rmation could include the level, timing and patterns of CO and PM; PM size di stribution 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. If either type of prediction is included in a demonstration, the EPA will likely give it consideration only if th e demonstration package also includes information on the uncertainty of the pr ediction methods, i.e., information on its past success in predicting normal ozone levels. The demonstration should also explain the predictive method in terms that are understandable enough to allow informed public comment.

 A prediction based on air quality/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 theDraft for Public Notice and Comment

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Page 10 of 46 uncertainties due to other aspects of the modeling platform such as grid cell size,

etc.

The EPA intends to prepare a future separa te document that provides more guidance for preparing a demonstration for w ildfire events that are believed to have affected ozone concentrations. In addition, the EPA will post on its exceptional events website example demonstration packages that illustrate the ty pe 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 40 CFR § 50.14, “Treatment of Ai r Quality Monitoring Data Influenced by Exceptional Events,” was revised in March 2007, the EPA was mindful that designations would be occurring under the then-recently revised PM 2.5 NAAQS. Exceptions to the generic deadline of July 1 of the calendar year following the datum year (see 40 CFR § 50.14(c)(2)( iii)) were included for PM 2.5 in the rule. The EPA was

also mindful that similar issues would aris e for subsequent new or revised NAAQS. The Exceptional Events Rule at section 50.14(c)(2)(v i) indicates “when EPA sets a NAAQS for a new pollutant, or revises the NAAQS for an existing pollutant, it may revise or set a new schedule for flagging data for initial de signation of areas for those NAAQS.” See as examples, the data flagging schedule identified in the SO 2 NAAQS final rule at 75 FR at 35592 or the data flagging sc hedule identified in the NO 2 NAAQS final rule at 75 FR at

6531.

D. General AQS Procedures

8. Question: What is the difference between the “R” series flags and the “I” series flags, and how should they be used?

Answer: Within AQS, monitoring agencies can 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

4 http://www.epa.gov/ttn/analysis/exevents.htmDraft for Public Notice and Comment

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Page 11 of 46 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. Air ag encies intending to ch ange “I” flagged data to “R” flagged data could approach the EER flagging and initia l event description deadline of July 1 st of the year following the sample measurement. Air agencies must change the flag status from “I” to “R” BEFORE the July 1 st deadline. Normally, air agencies should not modify the flag status afte r this date and, therefore, will not have met the EER initial flagging and event description deadlines.

The EPA does not intend to review or concur on the “I” series flags. Air agencies must submit “R” flags by July 1 of the calendar 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).

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

Answer: Yes, but the EPA requires air agencies to use the “R” flags to identify data that might have a regulatory consequence and for which an air agency intends to request exclusion and submit an approvable demonstrati on. Air agencies should use the “I” series flags to identify values for informationa l purposes (see Question 8). AQS only allows EPA to place concurrence flags on data identified with an “R” flag. “I” flags never affect regulatory summary statistics (e.g., desi gn values, number of exceedances, 98 th percentile 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.

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 at 13568 (emphasis added). AQS maintains event definitions, including their initial descriptions, in fields separate from the raw data flagging fields. As a result,Draft for Public Notice and Comment

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Page 12 of 46 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 capable of preventing localized NAAQS exceedances are expected to be available during most construction ev ents. In some cases, however, construction activities may involve very high-energy, emissions-generatin g physical processes, such as explosive excavation. Dust control meas ures may 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 would be expected to show in a demonstration submittal that all reasonable and appropriate controls were in place during the construction / demolition activity, and that those controls proved inadequate to prevent NAAQS exceedances. The demonstration would also need to meet all other requirem ents 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:Draft for Public Notice and Comment

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Page 13 of 46  IF – Fire – Canadian (Informational Only)

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

 IM – Prescribed Fire (Informational Only)

 IP – Structural Fire (Informational Only)

 IT – Wildfire – US (Informational Only)

 RF – Fire – Canadian (Request Exclusion)

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

 RM – Prescribed Fire (Request Exclusion)

 RP – Structural Fire (Request Exclusion)

 RT – Wildfire – US (Request Exclusion)

The EPA believes it is appropriate to retain the Fire – Canadian (“IF/RF”) and Fire – Mexico/Central America (“IG/RG”) flags beca use these flags indicate the jurisdictional origin of the fire (i.e., outside of the submitting state/outside of the US). Emissions from fires originating outside of the United States that affect air quality concentr ations in the United States may qualify for regulatory tr eatment under the international transport provisions of 40 CFR 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?

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 EPADraft for Public Notice and Comment

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Page 14 of 46 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 handling associated with events that are not considered “exceptional” under the EER, and does not provide EPA with authority to exclude such data. Yet as the question points out, this event-related concentration could still impact design values. An air agency incorporating the event-related concentration in a design value used for a prospective attainment demonstration might seem to need more 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, for the purpose of making it clear to all parties why the planned guidance on attainment demo nstrations and the SIP approval process,Draft for Public Notice and Comment

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Page 15 of 46 rather than the Exceptional Event Rule and the associated AQS data flagging, demonstration submittal, and review process, will apply to such an event scenario.

To illustrate an attainment 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 at 13574 ("The EPA regional offices will work with the States, Tribes, and local agencies to ensure that proper document ation is submitted to justify data exclusion. EPA will make the response and associated explanation publicly available."). What method does the EPA plan to use to make the explanation "publicly available?"

Answer: The EPA posts example demonstration packages and decisions (consisting of air agency demonstration submittals, the EPA responses, and the EPA technical support documents) on the EPA regional office 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 provide notice and opportunity for public

5 http://www.epa.gov/ttn/analysis/exevents.htmDraft for Public Notice and Comment

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Page 16 of 46 comment prior to taking final Agency action. In addition, an air agency may need to provide an additional opportunity for public co mment if the EPA requests and/or if the air agency provides supplemental information not included in the original documentation made available for public comment. Th e EPA will make a case-by-case decision regarding supplemental opportunities for pub lic comment during the demonstration preparation, submittal, and review process. As part of this decision, the EPA may consider potential impact and/or expressed public interest in the claimed event, data uncertainty, historical application of demonstration approach, etc.

When the EPA concurs based on the weight -of-evidence that the air agency has successfully made the demonstrations referred to in 40 CFR 50.14(a)(2) a nd (b)(1) to the EPA’s satisfaction, the EPA will exclude the a ffected 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 for New Source Review air quality analyses. 7

 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 EPA will exclude these data when a ssessing whether the ambient monitoring requirements of 40 CFR part 58, which are tied to air quality conditions, are being

met.

 The data will continue to be publically available, but the EPA’s publications and public information statements on the status of air quality in the affected area will not reflect these data in any su mmary statistic of potential re gulatory 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) rely on design values th at exclude data that the EPA has determined meet the exceptional event weight-of-evidence requireme nts. These regulatory actions require the EPA to provide an opportunity for public co mment prior to taking a final Agency action.

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.

7 If the EPA is the permitting authority, the EPA will propose permits on this basis. If the EPA is commenting on another permitting authority’s proposed action, the EPA’s comments will be consistent with the determinations in this guidance document.

8 Applicable only to 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.Draft for Public Notice and Comment

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Page 17 of 46 If the EPA pursues one of these actions for a given area, the EPA will open a new comment period during which the public may comment on the exceptional event

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

received comments before 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.

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 impact a previously-sited monitor such that the monitor is no longer representati ve of the area, but rather functions more like a “hot-spot” monitor?

Answer: Except for PM 2.5 , there is no difference in how monitoring data are treated from "area-wide" monitors and hot-spot monito rs. All such data, if meeting applicable CFR regulations, are comparable to the NAAQS. For PM 2.5 a unique microscale or hotspot monitor is only comparable to the 24-hour NAAQS and not to the annual PM 2.5 NAAQS. Air agencies must indicate in their annual monitoring network plan (or an update to that plan) whether PM 2.5 monitors are suitable or not suitable for comparisonDraft for Public Notice and Comment

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Page 18 of 46 against the annual PM 2.5 NAAQS. If the EPA regional offi ce approves a monitor as being unsuitable, this will prevent the data be ing used to compare with the annual PM 2.5 NAAQS (see 40 CFR § 58.30). Note that data co llected at “special purpose” monitors meeting the applicable 40 CFR part 50 and 58 requirements can be compared to the NAAQS when the EPA makes an attainment determination.

The EER does not specifically address temporary anthropogeni c emission sources such as construction projects. However, neither doe s the EER explicitly place a limit on the duration of a single event. A submitting agen cy could make a showing that a claimed event (e.g., a multi-year road construction project) is not likely to recur at the location in question. If the remaining exceptional event cr iteria and demonstration criteria are met, including the requirement that the event (inclu ding the emissions from the project) is not reasonably controllable, the activity 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 offices approve temporary shut-downs , the operating air agency should assign a Null Data Code in AQS for “construction/repairs in area” (AC) to identify and invalidate data associated with peri ods of local construction.

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

related?

Answer: Policy relevant background ozone con centrations and exceptional events can include partially overlapping c oncepts. The 2007 Staff Paper 10 defines policy relevant background ozone “as the distribu tion of [ozone] concentrations that would be observed in the U.S. in the absence of anthropogenic (man-made) emissions of precursor emissions (e.g., VOC, NOx, and CO) in the U.S., Canada, and Mexico.” An exceptional event is a natural event (excluding stagna tions, inversions, high temperat ures, or precipitation) or an anthropogenic event that is unlikely to recur in the same location. Both exceptional events and policy relevant background include emissions from natural events like forest wildfires or stratospheric ozone intrusi ons. However, exceedances due to natural emissions that occur every day and contribu te to policy relevant background, such as biogenic emissions, do not meet the definition of an exceptional event and are thus not eligible for exclusion under the EER. Rou tine anthropogenic emissions outside of the U.S. contribute to policy relevant background, but are not exceptiona l events. Air agency preparation of a demonstra tion package and the EPA’s subsequent review of the demonstration package is case-by-case base d on a weight-of-evidence approach and does

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

assessment of scientific and technical information. OAQPS staff paper. (U pdated Final) July 2007. Research Triangle Park, NC: Office of Air Qua lity Planning and Standards; EPA report no. EPA-452/R-07-007, available

online at: http://epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_sp.html

.Draft for Public Notice and Comment

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Page 19 of 46 not explicitly consider whether the event type might contribute to policy relevant background. However, if a natural event that contributes to polic y relevant background

causes an observed concentration that meets the statutory definition of an exceptional event and fulfills all of the exceptional even t 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 (see preamble at 75 FR at 35585-35586 and re gulatory text at 75 FR at 35592). The correct flag to use for a volcanic eruption event is “RS.”

17a. Question: The EPA acknowledges that certain extreme exceptional event cases may require more limited demonstration packages . How might the EPA decide whether to consider a particular high wind dust event “extreme” for the purpose of expecting only a very limited demonstration package?

Answer: While many dust storms could qualify as exceptional events, the EPA believes that most events that are conventionally referred to as “dust storms” should not be considered “extreme” events for this 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 caused by severe thunderstorm activity and are typically characterized as “solid walls” of dust that can rise up to 2,000

meters and travel hundreds of miles.

Generally, the EPA would consider sustained wind speed, spatial extent, visibility, and PM levels in determining whether an event is an extreme event. An example of an event that could be considered an exceptional event but not an extreme event would be the Santa Ana winds blowing at 25-30 mph, creating an exceedance at one monitor, with maximum hourly 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 satisfactory The South Coast Air Quality Management District prepared a satisfactory 49-page demonstration package (plus an appendix with additional supporting in formation) for the Santa Ana winds event, parts of which have been used as exam ples in the High Winds guidance document. However, the EPA would expect that much more limited documentation for an event like

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

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Page 20 of 46 meets the several criteria for data exclus ion (clear causal conn ection, not reasonably controllable or preventable, etc.).

18. Question: Carbon monoxide (CO) flags are in AQS for exceedances caused by fires, but the CO NAAQS (40 CFR 50.8) does not 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 gui dance. The EER Preamble (72 FR at 13563) explains the EPA’s position with respect to exceptional event flagging for pollutants for which the statement of the NAAQS in 40 CFR part 50 does not explicitly reference the Exceptional Events Rule: “In th e interim, where exceptional events result in exceedances or violations of NAAQS that do not currently provide for special treatment of the data,

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

affected areas as nonattainment based on these ev ents.” Therefore, air agencies may flag CO data in AQS and the EPA may apply the sa me process and 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 at 54294). Because the EPA made no revisions to the CO standards, it promulgated no related cha nges 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 fo r this regulatory decision. Can these values, which are not exceedances and do not co ntribute to violations, be flagged and

receive the EPA’s concurrence?

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

Division Directors states 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 it mean to use a “weight-of-evidence” approach when the EPA reviews exceptional event de monstration submittals?

Answer: In using the term “weight-of-evidence,” the EPA believes we should consider all relevant evidence and qualitatively “weigh” this evidence based on its relevance to the EER criterion being addressed, the degree of certainty, its persuasiveness, and other considerations appropriate to the individual pollutant and the nature and type of event.Draft for Public Notice and Comment

Revision Date: June 2012

Page 21 of 46 20. Question: Exactly which section(s) of the 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.”

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 at 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. Ra ther, it means that other parts of the preamble and other EPA guidance should be re lied upon instead of statements in these passages of the final rule preamble, whic h 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 will use the following firerelated terminology in the revised draft excep tional 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 EPA recognizes that emissions from wildfires are generally not reasonable to prevent or control. The EPA uses “gener ally” in this statement because the current United States Forest Service (USFS) definitions of “wildfire” and “pre scribed fire” define these events in terms of purpose and delib erateness of ignition (See definitions in response to Question 20a). Based on the USFS definitions, a wildfire may have startedDraft for Public Notice and Comment

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Page 22 of 46 from an unintentional ignition, but the land manager may have allowed the fire to continue to burn although it could have been extinguished. In such a case, some of the explanation of the reasonableness of this decision is needed. Land managers may similarly allow a prescribed fire to bu rn beyond its originally determined bounds. Submitting agencies should also explain the reas ons for extending the bounds of the burn.

The EPA’s expectation is that air agencies identify the origin and evolution of the wildfire in simple terms when documenti ng the “not reasonably controllable or preventable” criterion in their demonstr ation submittal. Fires started by lightning, volcanoes, unauthorized activit y, or accidental human-cause d actions are unplanned and unwanted fires and therefore, despite effo rts to prevent or control such fires 11 the emissions may not be reasonably controllable or preventable. In thes e cases, it is often sufficient for agencies to provide a statement such as the following to document the “not reasonably controllable or preventable” cr iterion: “Based on the documentation provided in [section X] of this submittal, [light ning] caused the unplanned, unwanted wildfire event. The responsible agencies did their r easonable best to contro l the extent of and extinguish the fire by taking the following actio ns [insert list or description of actions taken]. Therefore, emissions from this fire were ‘not reasonab ly controllable or preventable.’” Emissions from intentionally i gnited fires and escaped prescribed fires that burners allow to burn require additional supporting documentation to demonstrate the “not reasonably controllable or preventa ble” criterion. In these cases, the submitting agency must explain how the fire escaped or why the land manager allowed the fire to burn. The EPA expects to discuss the issue of control/prevention of wildfires in future guidance.

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

Answer: The EPA intends to develop separate guidance to address this issue which will be issued at a later date following an opportunity for st akeholder 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?

Answer: While the granting of data exclus ion under the EER does not depend on air agency actions to meet the requirements of 40 CFR § 51.930, the EPA encourages the submittal of a mitigation measures with the demonstration package, particularly for those events expected to recur. The Exceptional Events Rule was promulgated pursuant to

11 Prevention/control efforts could include posting High Fire Danger signs to make people more careful and prevent accidental fires, and/or taking reasonab le action to contain a fire once it has started.Draft for Public Notice and Comment

Revision Date: June 2012

Page 23 of 46 Section 319 of the Clean Air Act which contai ns a provision that each air agency “must take necessary measures to safeguard public health regardless of th e source of the air pollution...” This provision was the basis for the mitigation requirements in 40 CFR §51.930 and the requirement in the EER at 40 CFR §50.14(c)(1)(i) that all air agencies must “notify the public promptly whenever an event occurs or is reasonably anticipated to occur which may result in the exceedance of an applicable air quality standard.” The language at 40 CFR §51.930 requires that:

“(a) A State requesting to exclude air quality data due to exceptional events must take appropriate and reasonable actions to 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 the preparation or submittal of a mitigation plan, it does require that th e air agency develop and implement processes and measures that could easily become th e elements of a formal, written plan. The mitigation criteria focus on specific measures and actions to protect public health, rather than on measures that control or prevent emissi ons associated with a specific event. So, a mitigation plan may include measures that apply to emissions sources in general (e.g., dust suppression or covering techniques for mineral processing) rather than those measures or controls that might be 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?

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 EPADraft for Public Notice and Comment

Revision Date: June 2012

Page 24 of 46 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 must evaluate whether emissions fr om neighboring (or contributing) counties are not reasonably controll able or preventable. As discusse d in greater detail in the overview guidance document and the draft High Winds Guidance document, the assessment of “not reasonably controllable or preventable” is ba sed on existing level of required control, attainment status, and, for high wind dust even ts, wind speed and other factors. States and tribes should consult with their EPA regiona l office early in the development of an exceptional event demonstration package if th ey believe that emissions from sources on federally-owned and managed land (e.g., nationa l parks within the state) have been affected by an event in a way that ra ises issues of r easonable control.

Interstate and international transport events are different than intrastate events. The EPA believes it is not reasonable to expect th e downwind state (i.e., the state submitting the demonstration) to have convinced the upwi nd 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 source s 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 implemented

controls on out-of state sources constitute reasonable controls and that the “not reasonably controllable or preventable” crite rion is satisfied. When assessing emissions transported from other states or count ries, the submitting state can say that it characterized the out of state sources, determ ined that these sources contributed to the noted exceedance or violation, and determin ed, based on jurisdictional boundaries and other available information, that contributing emissions from the upwind state or country were not reasonably controllable or preventable. Submitting states are further required to submit evidence/statements supporting the othe r exceptional event criteria (i.e., clear causal relationship, but for, human activity unlikely to recur or a natural event, affects air quality, and historic al 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.

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 anyDraft for Public Notice and Comment

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Page 25 of 46 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 transp ort 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 assert this and provide appropriate supporting documentation in its demonstration package. The supporting documentation should include descriptions of the geographic area (with maps or available visu als) and a discussion of the hi storical 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, “Draft 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 Ru le” (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 expects many of the principles discussed therein to ex tend 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 intends to develop a future separate guidance document addressing the preparation of demonstrations to support w ildfire-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?

Answer: Examples of approved de monstrations are posted at http://www.epa.gov/ttn/analysis/exevents.htm.Draft for Public Notice and Comment

Revision Date: June 2012

Page 26 of 46 27. Question: How quickly will the EPA review the demonstration document and provide feedback to the air agency on the 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) 12 , and an indicator of the timing of the EPA’s final review. The EPA will generally prioritize exceptional event

determinations that affect near-term regulatory decisions. 13

To promote early communication, the EPA 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. 14 This initial notification can assist both the air agency and the EPA in the planning and prioritization process. The EPA intends to respond to such a letter 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 regardi ng concurrence with an air agency’s flag within 18 months of receipt of a complete p ackage for those demonstrations that impact a near-term regulatory action, or sooner if n ecessary to support a regulatory action. The EPA intends to communicate with the submitting agency, as needed, during the demonstration review period.

Submitting air agencies that believe their 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.

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?

12 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 EPA ac tions may limit the response time the EPA allows.

13 “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.Draft for Public Notice and Comment

Revision Date: June 2012

Page 27 of 46 Answer: In general, the EPA will not prepare analyses or additional arguments to be included as components in a submitted dem onstration package. Rather, the EPA will recommend demonstration package improvement s to the submitting agency. However, if a demonstration package is associated with an imminent regulatory action and the public

interest will be best served by the EPA preparing and/or considering additional analyses, the EPA may either assist with or independe ntly prepare supporting analyses that could become part of the submission package or an EPA-prepared technical support document. Analyses prepared by the EPA could support e ither approval or di sapproval of an air agency’s request for concurrence on flagged data.

28a. Question: Does the Exceptional Events Rule contain a dispute resolution process that air agencies can use to resolve disagreements 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.

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 significan ce. If a measurement value is approved by the EPA for exclusion for one particular NAAQS averaging period and level, is it automatically excluded for all the other NAAQS for that pollutant?

Answer: No. The exclusion of a measured air co ncentration is to be justified and approved separately for each NAAQS th at applies to the pollutant.Draft for Public Notice and Comment

Revision Date: June 2012

Page 28 of 46 When initially flagging data, an air agency does not need to commit to the specific NAAQS for which it seeks to exclude a meas ured concentration. The EPA’s ambient air quality database, AQS, is designed to allow an air agency to apply a single flag to a measured concentration value, which merely in dicates the agency’s interest in excluding that value with respect to one or more of the applicable NAAQS. Later, in the justification (i.e., the demons tration) for exclusion, the air agency can indicate the specific NAAQS for which it seeks exclusion and for which the demonstration addresses the Exceptional Events Rule criteria. When the EPA makes a decision regarding concurrence with an air agency’s fla g, it will generally identify in its approval/disapproval letter (or ot her official notice) all of the NAAQS for which the EPA has concurred on the flag. The EPA will also generally set a flag in AQS indicating concurrence with respect to a specific single NAAQS or a specific combination of NAAQS for that pollutant (e.g., in the case of 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 w ith the concurrence.

The EPA concurrence flags entered into AQS prior to the March 2010 re-engineering of AQS to accommodate the Exceptional Events Rule did not indicate the specific single NAAQS or the specific 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. 15

15 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 toDraft for Public Notice and Comment

Revision Date: June 2012

Page 29 of 46  For CO, all concurrences were presumed to apply to both the 1-hour and the 8- hour NAAQS. This default was chosen to 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 concurrence flags, if they have not already done so.

Detailed information on the use of events fl ags in AQS can be found in a tutorial posted

at http://www.epa.gov/ttn/airs/airsaqs/m anuals/ExceptionalEventTutorial.pdf

. Concurrence flags are discussed on page 20 of this tutorial.

30. Question: For a NAAQS that is defined for a multi-hour or multi-day averaging time, but for which concentrations are measured, re ported, and flagged on the basis of a shorter time period, what comparisons between meas urements and the NAAQS level should be done 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 can be compar ed 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. 16

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

16 Air agencies have for many years reported 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 monitoringDraft for Public Notice and Comment

Revision Date: June 2012

Page 30 of 46 However, a difference exists for the following NAAQS between the time period for reporting concentrations and the averaging period to which the level of a NAAQS applies.

 Ozone, CO, 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).

 Pb is reported as 24-hour measurements , but the old and new NAAQS are both for three-month averages (quarterly aver ages and three-month rolling averages, respectively).

 When using automated/continuous monitoring equipment, PM 2.5 and PM 10 are sometimes 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.

 When using filter-based monitoring equipment, PM 2.5 and PM 10 are sometimes reported as 24-hour measurem ents but there is a PM 2.5 NAAQS with an annual averaging period.

The mismatches of time periods make this a question with a complex answer. The following paragraphs, summarized in Table 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 ove rlapping with the even t and its effects on air quality, and which would not have occurr ed “but for” the effects of the event. 17
By definition, an exceedance necessarily involves a comparison between an air concentration, averaged over a time period eq ual 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 concen tration to the level of the 8-hour NAAQS as part of a test of whether the “but for” criterion is met, because the outcome of the

requirements that accompanied 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.

17 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.Draft for Public Notice and Comment

Revision Date: June 2012

Page 31 of 46 comparison for a single hour does not indicate whether an exceedance or violation of the 8-hour NAAQS occurred, or whether it w ould not have occurred “but for” the event. Instead, one should consider whether the event made a “but for” difference in the average concentration over the period that is the same as the averaging period for the NAAQS. That is, air agencies making a “but for” argument should compare the average concentration to the identifie d NAAQS rather than the individual concentrations that comprise the average. Air agencies should, however, identify in their exceptional event submission those cas es in which a single measurement or several, but not all, measurements cause the elevated average.

The preamble to the Exceptional Events Rule provides one exception from this formal definitional approach. The preamble states that in the particular case of PM 2.5 , the direct comparison of a single 24-hour av erage concentration (determined from a single filter-based measurement or by averaging 24 1-hour measurements from a continuous equivalent instrument) to the level of the annual NAAQS (currently 15

μ g/m 3 ) can be the basis for meeting the “but for” criterion for exceedances or violations of the annual NAAQS. In context, it is clear that ba sed on this comparison,

a 24-hour concentration can be excluded from the calculation of the annual PM 2.5 NAAQS design value, if other rule criteria ar e also met. It is therefore not necessary to show that the annual average PM 2.5 concentration was above 15 μ g/m 3 with the

event and would have been below 15 μ g/m 3 “but for” the single event at issue. Such a concentration can also be excluded from the calculation of the design value for the

24-hour PM 2.5 NAAQS, although this is likely to make a difference to meeting the NAAQS only if the actual measured con centration were close to or above 35 μ g/m 3

. This special case is reflected in Table Q30-2.

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

case of a 24-hour 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 . The EPA believes applying th is 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 should be applicable to all pollutants. That is, a 24-hour av erage concentration of Pb, NO 2 , or SO 2 can be compared to the NAAQS level defined for a longer period, for purposes of meeting “but for” with respect to the NAAQS with the longer averaging period. However, the EPA does not intend to concur on flags for a 1-hour NO 2 and SO 2 concentration that is below the level of the annual NAAQS, re gardless of the outcome of “but for” comparisons based on 24-hour or annual averaging periods. 18 Also, the EPA does not

18 This restriction is intended to parallel the similar restriction for PM 2.5 stated in the preamble to the Exceptional Event Rule. It likely has no practical eff ect. It is highly unlikely that even several hourly

concentrations below the level of the annual NO 2 NAAQS (53 ppb) could include an event contribution that would, when divided by 8760 (24 hours times 365 days), result in the annual average NO 2 concentration crossing from below to above the level of the annual NAAQ S. Similarly, it is highly unlikely that even several hourly concentrations below the level of annual SO 2 NAAQS (30 ppb) could include an event contribution that would, when divided by 24, result in the 24-hour average SO 2 concentration crossing fr om below to above the

level of the 24-hour SO 2 NAAQS (140 ppb).Draft for Public Notice and Comment

Revision Date: June 2012

Page 32 of 46 intend to concur on flags for a 24-hour Pb measurement below the level of the old (fixed quarterly average) Pb NAAQS or the new (rolling 3-month average) Pb NAAQS.

Table Q30-1. Principles for Correct Approac hes for Helping to Show That the “But

For” Test Is Met 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.

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 multiple measurements within the averaging period of the NAAQS to the level of the NAAQS (e.g ., compare the average of eight 1-hour

measurements to the 8-hour NAAQS). If this comparison shows that the average is more than the NAAQS but would have been below the NAAQS in the absence of the event, then the “but for” test

will have been met for those

individual measurements in the longer averaging period that were affected by the event. Air agencies should, however, identify in their exceptional event submission those cases in which a single measurement or several, but not all,

measurements cause the elevated

average.  1-hour ozone measurements vs. 8-hour NAAQS.

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

 1-hour 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 cannot be excluded, 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, it is also permitted to compare the 24-hour

measurement to th e annual average

PM 2.5 NAAQS or the quarterly or rolling 3-month Pb NAAQS.  24-hour PM 2.5 filter

measurements vs. the annual average NAAQS (expressly permitted in the preamble to the Exceptional Events Rule).

 24-hour Pb filter

measurements vs. the quarterly average and rolling 3-month average NAAQS (suggested by this guidance as If a measurement value is

below the level of the quarterly, rolling 3-month, or annual average NAAQS, it

cannot be excluded.Draft for Public Notice and Comment

Revision Date: June 2012

Page 33 of 46 Principle Application to Specific NAAQS Exceptions

a consistent with the intent of

the PM 2.5 provision in the preamble).

4 1-hour PM 2.5 and 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 for

those individual 1-hour

measurements in the 24-hour averaging period that were affected by the event.  1-hour PM 2.5 measurements vs. annual average NAAQS (suggested by this guidance to create a level playing field

between filter-based and

continuous PM 2.5 measurements).

 1-hour SO 2 measurements vs. annual average NAAQS (where the 30 ppb annual SO 2 NAAQS still applies) If the average of the 24 1-

hour measurements is below

the level of the annual average NAAQS, it cannot

be excluded.

5 When there is no NAAQS for the 24-hour averaging period, 1-hour measurements may be compared directly to the annual NAAQS.  1-hour NO 2 measurements vs. annual average NAAQS (suggested by this guidance to

create a benchmark for judging the excludability of 1-

hour NO 2 measurements,
other than whether the event affected the annual average enough to make a “but for”

difference relative to the annual average NAAQS). If a measurement value is

below the level of the annual average NAAQS, it cannot

be excluded.

6 Otherwise, single 1-hour measurements may not be compared to the level of the annual average NAAQS.  Single 1-hour SO 2 measurements 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 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 would help satisfy the “but for” test for each pollutant, for each curre nt NAAQS. Note that for completeness Table Q30-2 addresses some situations that may be very unlikely to actually occur – for example, that a single event might cause an exceedance of the annual average NO 2 NAAQS.Draft for Public Notice and Comment

Revision Date: June 2012

Page 34 of 46

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met

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

1 Ozone 0.12 ppm 1-hour averaging period

1-hour measurement If a 1-hour measured concentration was above 0.124 ppm but would have been 0.124 ppm or less in the absence of the event, the 1-hour ozone concentration value meets the “but for” test for purposes of comparison to the 1-hour NAAQS. If other criteria are also met for that hour (e.g., there was a clear causal relationship between the even t 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.Draft for Public Notice and Comment

Revision Date: June 2012

Page 35 of 46

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met

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

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

1-hour measurement  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 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 15.0 μ g/m 3
(after truncation after the first decimal digit) but

would have been 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 meet the “but for” test for purposes of comparison to 35 μ g/m 3 24-hour PM 2.5 NAAQS.

5 PM 2.5 15.0 μ g/m 3
Annual averaging period

1-hour measurement  If the annual average PM 2.5 concentration was

above 15.0 μ g/m 3 but would have been equal to

or less than 15.0 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 15 μ g/m 3 annual PM 2.5 NAAQS.

 Also, if the 24-hour average concentration based

on 1-hour measurements was above 15.0 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 15.0 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 15 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 15 μ g/m 3 (after rounding to one decimal digit) to be excluded from an annual NAAQS calculation.Draft for Public Notice and Comment

Revision Date: June 2012

Page 36 of 46

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met

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

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

24-hour measurement  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 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 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 15 μ g/m 3
Annual averaging period

24-hour measurement  If the annual average PM 2.5 concentration was

above 15.0 μ g/m 3 (after rounding to one decimal digit per 40 CVFR 50 Appendix N section 4.2(a)) but would have been equal to or less than

15.0 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 15 μ g/m 3 annual PM 2.5 NAAQS.

 Also, if the 24-hour average concentration from the filter-based sampler was above 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 equal to or less than 15.0

μ g/m 3 in the absence of the event, the 24-hour value meets the “but for” test for purposes of comparison to 15 μ g/m 3 annual PM 2.5 NAAQS.

Note that a 24-hour concentration that is equal to or

less than 15.0 μ g/m 3 (after truncation to one decimal digit) cannot be approved for exclusion, regardless of the outcome of the comparison just described.Draft for Public Notice and Comment

Revision Date: June 2012

Page 37 of 46

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met

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

8 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.

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.Draft for Public Notice and Comment

Revision Date: June 2012

Page 38 of 46

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met

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

12 Pb 1.5 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 can never be excluded, regardless of the outcome of the comparison just described.

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 can never be excluded, regardless of the outcome of the comparison just described.Draft for Public Notice and Comment

Revision Date: June 2012

Page 39 of 46

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met

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

14 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.

 Provided there is an ex ceedance of the annual standard, if the 1-hour concentration was above 53 ppb (after truncating to a whole number per 40 CFR 50 Appendix S section 4.2(c)) but would have been equal to or less than 53 ppb in the

absence of the event meets the “but for” test for purposes of comparison to annual NAAQS.

However, a 1-hour NO 2 concentration that is below 53 ppb (after rounding to a whole number) can never be excluded, regardless of the outcome of the comparison just described.

16 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 event, the

1-hour SO 2 concentration value meets the “but for” test for purposes of comparison to the 1-hour SO 2 NAAQS.

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.Draft for Public Notice and Comment

Revision Date: June 2012

Page 40 of 46

Table Q30-2. Correct Approaches for Helping to Show That the “But For” Test Is Met

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

18 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.

 Also, if the 24-hour average concentration based on 1-hour measurements was above 140 ppb (after rounding to the nearest 10 ppb per 40 CFR 50.4(b)) but would have been equal to or less than 140 ppb in the absence of the event, those 1- hour concentration values that were affected by the event meet the “but for” test for purposes of comparison to the 30 ppb annual 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)) may never be excluded, regardless of the outcome of the comparison just described.

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?

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 th rough the AQS software. Also, the EPA does not consider the Exceptional Events Rule to prohibit air agencies from flagging values below the level of the NAAQS. However, the EP A does not intend to review data flags in AQS for concurrence until the air agency submits its evidence/analysis packageDraft for Public Notice and Comment

Revision Date: June 2012

Page 41 of 46 demonstrating that exclusion of the flagged values is consistent with the criteria in the Exceptional Events Rule, including the “but for” analysis at 40 CFR 50.14(c)(3)(iv)(D). Air agencies wishing to flag values for in formational 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 can concur with flags for concentrations that are below the NAAQS only in five very

narrow conditions described below. If the EPA de termines that a flag on a value less than the level of the NAAQS cannot meet the “but for” test, it is likely the EPA would

nonconcur or leave the default/null value of the AQS concurrence flag (indicating no EPA action) in place.

Except in cases involving PM 10 limited maintenance plans 19 , 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 th e (short-term) NAAQS, it may be possible for air agencies to present sufficient evid ence to satisfy the “b ut-for” criterion.

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

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. 20 Because of the expected exceedance form of the PM 10 NAAQS, concentrations in this range cannot possibly affect whether a si te actually meets the NAAQS, so there is no reason for flagging them except when the accepta bility of a limited maintenance plan is

19 See May 7, 2009 policy memorandum from William T. Harnett to Regional Air Division Directors at http://www.epa.gov/ttn/oarpg/t1/memoranda/lmp_final_harnett.pdf that allows 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.

20 See May 7, 2009 policy memorandum from William T. Harnett to Regional Air Division Directors at http://www.epa.gov/ttn/oarpg/t1/memoranda/lmp_final_harnett.pdf.Draft for Public Notice and Comment

Revision Date: June 2012

Page 42 of 46 an issue. The normal AQS flagging and concu rrence procedures may be used in this

situation. 21

A second scenario in which the EPA can concur with flags for concentrations that are below the NAAQS is indicated at 72 FR at 13570. If (i) an event has affected air quality on multiple consecutive days, (ii) at least one measured concentration during the episode

can be found to meet the “but for” test usi ng the relevant comparis on specified in Table Q30-2, and (iii) the air quality impact on each day is “exceptional,” measurements for the entire period are eligible for data exclusi on regardless of how they compare to the level of the NAAQS. In the context of this provision, “exceptional” encompasses all the requirements of the Exceptional Events Rule other than the “but for” test (e.g., clear causal connection, “in excess of normal historical fluctua tions, including background,” not reasonably controll able or preventable).

Scenarios in which the measured concentr ation is greater than a NAAQS with a longer averaging time but less than the le vel 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 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 th e purposes of the 24-hour PM 2.5 NAAQS. This is

the result if the actua l 24-hour concentration was between 15 and 35 μ g/m 3 but would

have been below 15 μ g/m 3 but for the effect of the event. It should be noted that an exclusion made under this very sp ecific 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 question is one of the few highest da ily concentrations during the year, because only then could it have affect ed the 3-year design value. When a 24-hour value below the level of the 24-hour NAAQS do es affect the 3-year design value, the application of the guidance for th e fourth situation (below), whic h is applicable to all four NAAQS pollutants with multi-year design va lues, would get to the same result as application 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 above the NAAQS to being equal to or be low the NAAQS. However, as indicated in Table Q30-2, concentrations below cert ain values may never 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

21 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

.Draft for Public Notice and Comment

Revision Date: June 2012

Page 43 of 46 the longer period the effect of the event on the longer 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 may never 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. 22

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 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.

22 Note that exclusion of this 24-hour value from design values for the annual average NAAQS is a separate question, the likely answer to which is that the value is not excludable. If the event did not make the 24-hour concentration change from below 15 to above 15 μ g/m 3 the event does not meet the first condition specified in row 7 of Table Q30-2. It is also very improbable that an event affecting a single day would meet the second condition in row 7 of Table Q30-2.Draft for Public Notice and Comment

Revision Date: June 2012

Page 44 of 46 For the 24-hour 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 at 13570, bottom of middle column) Even if the conditions described in the precedi ng paragraph are met, values below 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 at 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 when 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 theseDraft for Public Notice and Comment

Revision Date: June 2012

Page 45 of 46 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. The EPA will not concur on the

exclusion of a 24-hour concentration value thatis below the level of the NAAQS, and we discourage air agencies from flagging such values.

NO 2

The EPA will not concur on the exclusion of a 1-hour NO 2 concentration that is below

the level of the annual NO 2 NAAQS, and we discourage air agencies from flagging such

values.Draft for Public Notice and Comment

Revision Date: June 2012

Page 46 of 46 SO 2

The EPA will not concur on the exclusion of a 1-hour SO2 concentration that is below

the level of the annual SO 2 NAAQS, and we discourage air agencies from flagging such

values.