UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION IX

Mr. Eric Massey Director, Air Division 75 Hawthorne Street San Francisco, CA 94105-3901


## SEP 0 6 2012

Arizona Department ofEnvironmental Quality 1110 W. Washington St. Phoenix, Arizona 85007
DearMr. Massey: OFFICE OFTHE REGIONAL ADMINISTRATOR

This letter responds to Arizona Department ofEnvironmental Quality's (ADEQ) March 14, 2012 submittaljustifying that emissions generatedby monsoonal thunderstorm outflow winds caused exceedances ofthe PM 10 NAAQS in the Phoenix PM 10 nonattainment area at numerous monitoring locations from July 3 - July 8, 2011.
EPA has reviewed the documentation providedbyADEQ to demonstrate that these exceedances on July 3 - July 8, 2011 meetthe criteria for an exceptional event in the Exceptional Events Rule (EER). We note that the information and analyses presented in ADEQ's submittal do not represent all possible evidence for exceptional eventpackages, and additional or alternate evidence maybe necessary to make an exceptional event determination in other instances or for othertypes ofevents. In the submitted demonstration for the dates ofJuly 3 - July 8, 2011, EPA concurs based on the weight ofthe evidence that ADEQ has successfully made the demonstrations referred to in 40 CFR §50.14 to EPA's satisfaction. In addition, ADEQ has metthe schedule andprocedural requirements in section 50.14(c) with respect to the same data. A more detailed assessment ofADEQ's demonstration is enclosed. My staffhas or shortly will enter "concurrence flags" for these data into EPA's AQS data system

. Based on these determinations, EPA will exclude these data from the following types ofcalculations and activities:

• EPA's Air QualityData system (AQS) will not countthese days as exceedances when generating userreports, or include them in design values estimates, unless the AQS user specifically indicates that they should be included. • EPA will accept the exclusion ofthese data for the purposes ofselecting appropriate background concentrations for New Source Review air quality analyses. 1 • EPA will accept the exclusion ofthese data for the purposes ofselecting appropriate background concentrations for transportation conformityhot spot analyses. 2

1 Ifwe are the permitting authority, we will propose permits on this basis. Ifwe are commenting on another permitting authority's

proposed action, our comments will be consistent with the determinations in this letter. 2Applicable only to PM 10 and PM2.s·

Printed on Recycled PaperIn addition, EPA will rely on calculated values that exclude this data in proposed regulatory actions, such as a proposed designation, classification, attainment demonstration, or finding as to whether the Phoenix PM10 nonattainment area has met the PM10 NAAQS. These regulatory actions require EPA to provide an opportunity for public comment prior to taking a final Agency action. IfEPA is pursuing one ofthese actions for the Phoenix PM 1 

### ononattainment area, EPA will open a new comment period during which EPA mayreceive comments on the exceptional event submission you have made and the determinations conveyed in this letter. Ifso, we must consider and respond to those comments before taking final regulatory action. Accordingly, the determinations conveyed in this letter do not constitute final EPA action regarding any matter on which EPA is required to provide an opportunity for public comment. In particular, this applies to determinations regarding the attainment status or classification of the area. Final actions will take place only after EPA completes notice and comment rulemaking on those determinations. As an additional clarification, the determinations conveyed in this letter are applicable only to determinations incorporating the submitted data relative to the PM10 NAAQS

. Ifyou have any questions or wish to discuss this matter further, please contact Deborah Jordan, Director ofthe Air Division at (415) 947-8715.

Enclosure

cc: Theresa Rigney, ADEQ Bryan Paris, ADEQ Sincerely,

### EXCEPTIONAL EVENTS RULE REQUIREMENTS
EPA promulgated the Exceptional Events Rule in 2007, pursuantto the 2005 amendmentofClean Air Act (CAA) Section 319. The EER added 40 CFR §50.1(j), (k) and (I); §50

. 14; and §51.930 to the Code ofFederal Regulations (CFR). These sections contain definitions, criteria for EPA approval, procedural requirements, and requirements for air agency demonstrations, all ofwhich must be met before EPA can concurunderthe EER on the exclusion ofair quality data from regulatory decisions. Under 40 CFR §50.14(c)(3)(iv),the air agency demonstration tojustify exclusion ofdata must provide evidence that: A. "The event satisfies the criteria set forth in 40 CFR §50

. 1 (j)" for the definition ofan exceptional event;

• The event "affects air quality." • The event "is not reasonably controllable orpreventable." • The event is "causedbyhuman activitythat is unlikelyto recur at a particular location or [is] a natural event." 1 B. "There is a clear causal relationship between the measurement under consideration and the event that is claimed to have affected the air quality in the area;" C. "The event is associated with a measured concentration in excess ofnormal historical fluctuations, including background;" and \ D. "There would have been no exceedance or violation but for the event." 

### SUMMARY
Overview
OnMarch 14, 2011, ADEQ submitted exceptional events demonstrations for 29 exceeedances ofthe 24- hour PM 10 standard that occurred at several monitoring stations within the Phoenix PM 10 nonattainment area onthe following days: July 3, July 4, July 5, July 7, and July 8, 2011. Table 1 summarizes these exceedances. ADEQ describes the July 3rd and 5th events as "large-scale and widespread dust events with mostly south-southeasterly winds**carryin~** in the dust onthe 3rd, and southeasterly winds carrying a massive dust wall into the Valley onthe 5 ,"whilethe July 4th and 7th events ''were smaller in scale, but were still related to thunderstorm activity and thunderstorm outflowboundarywinds." Due to the timing of the July 7th event, ADEQ explains that, "the impacts in Apache Junction may have occurred around the midnighthour, leading to the exceedances there being measured for July gth

_ , ADEQ provides a comprehensive description and discussion ofeach ofthese events in Sections I, II, and V ofthe demonstration.

1A natural event is further described in 40 CFR SO

.l(k) as "an event in which human activity plays little orno direct causal role

." 1T bl 1 EPAPM E a e 10 xcee

### dan 

# s ce ummary Exceedance Date Monitor/Site Name AQSID 24-hour Avg. (pg!m"') July 3, 2011 Buckeye 04-013-4011-1 385 Central Phoenix 04-013-3002-4 279 Durango Complex 04-013-9812-1 277 Dysart 04-013-40I0-I 239 Glendale 04-013-2001-1 242 Greenwood 04-013-3010-1 254 Higley 04-0 13-4006-1 196 JLG Supersite**04~013-9997-1** 227 JLG Supersite 04-013-9997-4 228 South Phoenix 04-013-4003-1 280 West Chandler 04-0 13-4004-1 198

. West43ro 04-013-4009-1 250 West Phoenix 04-013-0019-1 243 Zuni Hills 04-013-4016-1 260 July 4, 2011 Higley 04-013-4006-1 198 July 5, 2011 Buckeye 04-013-4011-1 163 Central Phoenix 04-013-3002-4 277 Durango Complex 04-013-9812-1 156 D_ysart 04-013-4010-1 219 Glendale 04-013-2001-1 167 Greenwood 04-013-3010-1 155 Higley 04-013-4006-1 362 JLG Supersite 04-013-9997-4 331 South Phoenix 04-013-4003-1 206 West Chandler 04-013-4004-1 360 West Phoenix 04-013-0019-1 278 July 7, 2011 Higley 04-013-4006-1 266 West Chandler 04-013-4004-1 214 July 8, 2011 Apache Junction 04-021-3002-1 194 Not Reasonably Controllable or Preventable (nRCPl
EPA evaluates whether an event was notreasonably controllable orpreventable at the time ofthe event bytaking into account controls inplace and wind speed, along with otherfactors. 2 Fornatural sources of dust, a high wind dust event can generally be consideredto be notreasonably controllable orpreventable ifwinds are high enough to cause emissions from natural undisturbed areas. Foranthropogenic sources ofdust, a high wind dust event is also eligible to be consideredtobenotreasonably controllable or preventable if: 1. The anthropogenic sources ofdust have reasonable controls inplace, 2. The reasonable controls have been effectively implemented and enforced, and 3. The wind speed was high enough to overwhelm the reasonable controls.
In addressing reasonable controls, ADEQ provided detailed information on the current set ofrequired controls inthe Phoenix PM 10 nonattainment area, including information onrule implementation, rule effectiveness, compliance and enforcement, real-time monitoring alert systems and public notification activities that occurred onthe event days. ADEQ concluded, ''the Phoenix area is designated as a serious nonattainment area for PM 10 and is required to have BACM for all significant sources ofPMJO. BACM

2 See e.g., Affirmation ofAttainment ofPM-10 NAAQS for the San Joaquin Val1eyNonattainment Area, 73 FR 14691 (March 19, 2008). 2approved control measures on significant anthropogenic sources were in place and enforced during the events, and pro-active tracking and response to the events byregulatory agencies and local governments confirmed the uncontrollable nature ofthe dust emissions; therefore, these pre-existing/prior approved required controls are adequate for meeting the requirements ofan exceptional event and should be considered 'reasonable' for these purposes." ADEQ provided documentation showing that, with the exception ofthe July 7th-July gth event, sustained wind speeds associated with these events were above 25 mph. For example, maximum sustained wind speeds of26 to 31 mph were measured on July 3rd, 28 to 34 mph onJuly4th, and 25 to 47 mph with gusts of3 5 to 56 mph on July 5th. While sustained wind speeds onlyreached 18 mph on July 7th, ADEQ explains that ''while winds recorded in Pinal and Maricopa County during the earlymorning hours of July 7th were only somewhat moderate, it is possible that the large-scale windblown dust event that occurred on July 5th had conditioned soils and deposited large amounts ofloose dust such that stronger winds were not needed to entrain orre-entrain dust into the air." ADEQ also asserts that due to the timing ofthe July 7th late evening event, the conditions that led to exceedances at Higley and West Chandler on July 7th were similarlyresponsible for the exceedance measured at Apache Junction on July 8th. ADEQ further explains that "despite the deployment ofcomprehensive control measures and sophisticated response programs, high wind conditions associated with thunderstorms and thunderstorm outflows brought high concentrations ofPM10 emissions into, and also overwhelmed controls within, the Phoenix PM 10 nonattainment area. The events discussed in this documentthat caused the exceedances in this request (see Sections II and V) were causedbythunderstorm driven outflow winds that transported dust into Maricopa County from areas largely outside ofthe Phoenix PM10 nonattainment area. The fact that these were natural events involving strong thunderstorm outflow winds that transported PM10 emissions into Maricopa County, with a majority ofthe PM1o emissions recorded byMaricopa County area monitors coming from sources outside ofthe Phoenix PM10 nonattainment area, provides strong evidence that the events and exceedances ofJuly 2-8, 2011 recorded within the nonattainment area were not reasonably controllable orpreventable." Section V ofADEQ's documentation includes a complex GIS analysis ofeach ofthe events that supports the PM10 transport described above. For all ofthe events, the analysis clearly demonstrates that monitors in the Phoenix PM10 nonattainment area were affectedbyPM10 transport from outside the nonattainment area, with the main source areas locatedto the south and southeast ofthe nonattainment area. In addition to transport, the spatial extent ofelevated PM10 concentrations throughout the area and the wind speeds associated with the thunderstorm outflows contributes to EPA's evaluation ofwhether these events are notreasonably controllable orpreventable.
Table 2: Documentation ofnRCP

Exceedance Date Demonstration Citation QualitYofEvidence Criterion Met? July 3, 2011 Section IV: p.39-45, Section V: p.48-62 Sufficient Yes July 4, 2011 Section IV: p.39-45, Section V: p.63-7J Sufficient Yes July 5, 2011 Section IV: p.39-45, Section V: p.74-86 Sufficient Yes July 7, 2011 Section IV: p.39-45, Section V: p. 87-101 Sufficient Yes July 8, 2011 Section IV: p.39-45, Section V: p. 87-101 Sufficient Yes

3Historical Fluctuations (HF)
EPA evaluates whether a measured exceedance is in excess ofhistorical fluctuation bytaking into account the level ofthe exceedance inrelation to historical data, which is typically 3 to 5 years. To demonstrate thatthis requirement was met, ADEQ provided 5..:-year time series plots ofboth PM10 daily maximum hourly averages andPM10 24-hour averages. ADEQ also explains thatPM10 concentrations measured during the July 2nd_gth period were inthe 99.5th percentile range when compared to historical data.
Table 3: Documentation ofHF

Exceedance Date Demonstration Citation Quality ofEvidence Criterion Met? July 3, 2011 Section III: p.35-38, App. A Sufficient Yes July 4, 2011 Section III: p.35-38 App

. A Sufficient Yes July 5, 20ll Section III: p.35-38, App. A Sufficient Yes July 7, 2011 Section ill: p.35-38, App. A Sufficient Yes July 8, 2011 Section Ill: p.35-38, App. A Sufficient Yes Clear Causal Relationship (CCR)
EPA considers a variety ofevidence when evaluating whetherthere is a clear causal relationship between the measurement under consideration and the event that is claimed to have affected the air quality in the area. Demonstrations should include documentation showing that the event in fact occurred and that emissions related to the event were transported in the direction ofthe monitor(s) where measurements were recorded; the size ofthe area affected bythe transported emissions; the relationship in time between the event, transport ofemissions, and recorded concentrations; and, as appropriate, pollutant species-specific information supporting a causal relationship betweenthe event and the measured concentration. Section II ofADEQ's demonstration included a comprehensive conceptual model ofthe events, including a general overview ofthe geographic setting ofthe monitors, climate, and drought information for Phoenix area. The conceptual model also included a very detailed discussion ofeach ofthe events that occurred in the July2nd_gth time period, which included time-lapse videos ofthe events and time series graphs for each event that included hourlyPM 10 concentration, visibility, and reports ofblowing dust orhaze. The time-lapse videos canbe found at the following locations:

• July 3, 2011: http://www.phoenixvis.net/videos/640x480/SOMT1 07032011.swf • July4, 2011: http://www.phoenixvis.net/videos/640x480/SUPM1 07042011.swf • July 5, 2011:**http:/ www~phoenixvis.net/videos/640x480/SOMT1**  07052011.swf • July 7, 2011: http://www.phoenixvis

.net/videos/640x480/SUPM1 0707201l.swf • July 8, 2011: http://www.phoenixvis.net/videos/640x480/SUPM1

_0708201 l.swf Section V ofthe demonstration includes a detailed and extensive GIS analysis, that show the spatial and temporal representation ofthe events as they move throughout Maricopa and Pinal Counties. The analysis includes PM 10 concentrations, sustained wind speeds, wind gusts, wind direction, visibility, and base velocityradarto trackthe transport ofPM 10 throughout the region. Accompanying the analysis, ADEQ provides a discussion for everymap that describes the conditions at that time. While not included in the demonstration, it is important to note thatNOAA'sNational Climatic Data Center Storm events 4database includes dust storm observations on July 

### 2nd at 

### 1815 hours (central deserts), July 

### 3rd at 

### 1743 hours (greater Phoenix area), July

### 4th at 

### 1830 hours (central deserts), and July 

### 5th at 

### 1920 hours (greater Phoenix area). The timing ofthese dust storm reports for each ofthese events is consistent with the observed increased PM10 concentrations in the area, increased wind speed, reduced visibility, and NWS station reports ofthunderstorms (TS), blowing dust (BLDU), haze (HZ), and dust storms (DS). ADEQ generally summarizes that ''the events occurring from July 

### 2-8 were directly related to strong and gusty winds generated bythunderstorm outflow boundaries" that ''were also responsible for transporting PM into the Phoenix PM10nonattainment area from areas outside ofthe nonattainment area." ADEQ further states that "while itis likelythat some dust was generated within the PM10 nonattainment area as gusts from the thunderstorm outflows passedthrough the area, the amount ofdust generated locally was easily overwhelmed by, and largelyunnoticeable as compared to the dust transported in from the source regions ofthe thunderstorm outflows." Table 4: Documentation ofCCR

Exceedance Date Demonstration Citation Qualitv ofEvidence Criterion Met? July 3, 2011 Section II: o.4-12, o.l3-19, Section V: p.48-62, App. C&E Sufficient Yes July 4, 2011 Section II: p.4-12, p.l3-19, Section V: p.48-62, App. C&E Sufficient Yes July 5, 2011 Section II: o.4-12, o.13-19, Section V: p.48-62, App. C&E Sufficient Yes July 7, 2011 Section II: o.4-12, o.l3-19, Section V: p.48-62, App. C&E Sufficient Yes July 8, 2011 Section II: o.4-12, o.13-19, Section V: p.48-62, App. C&E Sufficient Yes
Affects Air Quality (AAQ)
EPA will consider events to have affected air quality ifthe CCR and HF requirements have been adequately demonstrated. ADEQ states that due to the information presented in the demonstrations, ''we can reasonably conclude the events in question affected air quality."
Table 5: Documentation ofAAQ

Exceedance Date Demonstration Citation Qualitv ofEvidence Criterion Met? July 3, 2011 Section VII: p. 106 Sufficient Yes July 4, 2011 Section VII: p. 106 Sufficient Yes July 5, 2011 Section VII: p. 106 Sufficient Yes July 7, 2011 Section VII: o. 106 Sufficient Yes July 8, 2011 Section VII: p. 106 Sufficient Yes
Natural Event
EPA will consider an event to be a natural event ifboth the nRCP and CCRrequirements have been adequately demonstrated. ADEQ generally states that, ''the events shown to cause these exceedances were emissions ofPM10 driven byhigh winds causedbythunderstorm activity and related outflow boundaries during the period ofJuly

### 2-8, 2011" and that ''the events therefore qualify as natural events." Table 6: Documentation ofNatural Event

Exceedance Date Demonstration Citation QualitYofEvidence Criterion Met? July 3, 2011 Section VII: p. 106-107 Sufficient Yes July 4, 2011 Section VII: p. I06-107 Sufficient Yes July 5, 2011 Section VII: p. 106-107 Sufficient Yes July 7, 2011 Section VII: p. 106-107 Sufficient Yes July 8, 2011 Section VII: p. I06-107 Sufficient Yes 5· No Exceedance or Violation But For the Event (NEBF) Generally, the NEBF demonstration is similarto the demonstration ofthe nRCP and CCRrequirements, and should show that the measured concentration would have been below the applicable NAAQS without the affect ofthe event. ADEQ provides a summary ofthe analysis and information presented in the documentation that demonstrate both the nRCP and CCRrequirements have been met and states that "the bodyofevidence

...provides no alternative that could tie the exceedances ofJuly 2-8, 2011 to any other causal source but transported andre-entrained PM 10 generated from thunderstorm outflows, confirming that there would have been no exceedances but for the presence ofthese uncontrollable natural events." While not explicitly stated in the documentation, EPA acknowledges that PM10 concentrations before the periods ofhigh winds on the event days were belowthe 24-hour PM 10 NAAQS, providing further support for ADEQ's conclusion. Table 7· Documentation ofNEBF

Exceedance Date Demonstration Citation Quality ofEvidence Criterion Met? July 3, 20II Section VI: p. I05 Sufficient Yes July 4, 20II Section VI: p. I05 Sufficient Yes July 5, 2011 Section VI: p. I05 Sufficient Yes July 7, 20II Section VI: p. I05 Sufficient Yes July 8, 20II Section VI: p. I05 Sufficient Yes
Schedule and Procedural Requirements
In addition to technical demonstration requirements, 40 CFR §50.14 (c) specifies the schedule and procedural requirements an air agencymust follow to request data exclusion. Table 8 outlines EPA's evaluation ofthese requirements. Table 8: Schedules and Procedural Criteria Demonstration Reference Citation Criterion Met? Didthe State provide prompt public 40 CFR §50.I4 (c)(I)(i) Section 1: p.1, Yes notification ofthe event? Appendix 8 Were flags and initial description placed on 40 CFR §50.I4 (c)(2)(iii) Section 1: p.1 Yes the databy July 1st ofthe following year? Was the demonstration submitted within 3 40 CFR §50.14 (c)(3)(i) March 14,2012 Yes years ofthe end ofthe quarter in which the letter3 event occurred and 12 months priorto the date that any regulatory decision must be made by EPA? Was the public comment process followed 40 CFR §50.I4 (c)(3)(v) Section 1: p.2, Yes and documented? Appendix D

3 See letter from Eric Massey, Director, Air Quality Division, ADEQ to Deborah Jordan, Director, U.S. EPA Region IX Air Division, dated March 14, 2012

. 6

### CONCLUSION
EPAhas reviewed documentation provided byADEQ to support claims that dust emissions generated bymonsoonal thunderstorm high winds were transported into the Phoenix PM10 nonattainment area from areas in Pinal County and caused exceedances ofthe 24-hour PM10 NAAQS atthe locations outlinedin Table 

### 1on July 3, July 4, July 5, July 7, and July 8, 

### 2011. EPA has determined that the flagged exceedances at these locations and on these days meet the definition ofan exceptional event: the exceedances affected air quality, were not reasonably controllable orpreventable, and meet the definition of

# a natural event. Specifically, EPA has determined that events were not reasonably controllable andpreventable either due to high wind conditions that transported PM10 from sources outside ofthe nonattainment area and subsequently overwhelmed reasonable controls within the Phoenix PM10 nonattainment area (July 

### 3rd, July4th, and July 5th) ormoderate wind speeds re-entrained the large amount ofPM10 deEosited bythe large July 5th dust storm within and outside ofthe nonattainment area (July 7th and July 8 ). Also, regardless oftransport into the area, information pertaining to the controls implemented within the nonattainment area, the spatial extent ofelevated PM10 concentrations measured in the area, and the wind speeds associated with the thunderstorm outflows provide sufficient evidence to conclude that these events were notreasonably controllable orpreventable. Furthermore, EPA has determined that there is a clear causal relationship between the events and the measured exceedances, there would have been no exceedance but for the events, and the measured exceedances are in excess of normal historical fluctuations.
EPA fmds that the weight ofevidence is sufficient for concurrence on the flagging ofthe data for these monitors on July 3, July 4, July 5, July 7, and July 8, 2011. These concurrences do not constitute final EPA action to exclude these data from consideration for purposes ofdetermining the attainment status of the area. Final actions will come only after EPA completes notice and commentrulemaking onthose determinations.

7