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 of Environmental Quality 

1110 W. Washington St. 

Phoenix, Arizona 85007 

Dear Mr. Massey: 

OFFICE OF THE 

REGIONAL ADMINISTRATOR 

This letter responds to Arizona Department of Environmental Quality's (ADEQ) March 14, 2012 

submittal justifying that emissions generated by monsoonal thunderstorm outflow winds caused 

exceedances of the PM10 NAAQS in the Phoenix PM10 nonattainment area at numerous monitoring 

locations from July 3 - July 8, 2011. 

EPA has reviewed the documentation provided by ADEQ to demonstrate that these exceedances on 

July 3 - July 8, 2011 meet the 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 event packages, and additional or alternate evidence may be necessary to make 

an exceptional event determination in other instances or for other types of events. In the submitted 

demonstration for the dates of July 3 - July 8, 2011, EPA concurs based on the weight of the evidence 

that ADEQ has successfully made the demonstrations referred to in 40 CFR §50.14 to EPA's 

satisfaction. In addition, ADEQ has met the schedule and procedural requirements in section 50.14( c) 

with respect to the same data. A more detailed assessment of ADEQ'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 of calculations and 

activities: 

• EPA's Air Quality Data system (AQS) will not count these days as exceedances when 

generating user reports, or include them in design values estimates, unless the AQS user 

specifically indicates that they should be included. 

• EPA will accept the exclusion of these data for the purposes of selecting appropriate 

background concentrations for New Source Review air quality analyses. 1 

• EPA will accept the exclusion of these data for the purposes of selecting appropriate 

background concentrations for transportation conformity hot spot analyses. 2 

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

proposed action, our comments will be consistent with the determinations in this letter. 

2 Applicable only to PM10 and PM2.s· 

Printed on Recycled Paper 
In 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. If EPA is pursuing one 

of these actions for the Phoenix PM1o nonattainment area, EPA will open a new comment period during 

which EPA may receive comments on the exceptional event submission you have made and the 

determinations conveyed in this letter. If so, 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. 

If you have any questions or wish to discuss this matter further, please contact Deborah Jordan, Director 

of the 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, pursuant to the 2005 amendment of Clean Air 

Act (CAA) Section 319. The EER added 40 CFR §50.1(j), (k) and (I); §50.14; and §51.930 to the Code 

of Federal Regulations (CFR). These sections contain definitions, criteria for EPA approval, procedural 

requirements, and requirements for air agency demonstrations, all of which must be met before EPA can 

concur under the EER on the exclusion of air quality data from regulatory decisions. 

Under 40 CFR §50.14(c)(3)(iv), the air agency demonstration to justify exclusion of data must provide 

evidence that: 

A. "The event satisfies the criteria set forth in 40 CFR §50.1 (j)" for the definition of an 

exceptional event; 

• The event "affects air quality." 

• The event "is not reasonably controllable or preventable." 

• The event is "caused by human activity that is unlikely to 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 of normal historical 

fluctuations, including background;" and 

\ 

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

SUMMARY 

Overview 

On March 14, 2011, ADEQ submitted exceptional events demonstrations for 29 exceeedances of the 24-

hour PM10 standard that occurred at several monitoring stations within the Phoenix PM10 nonattainment 

area on the 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 on the 3rd, and southeasterly winds carrying a massive 

dust wall into the Valley on the 5 ,"while the July 4th and 7th events ''were smaller in scale, but were 

still related to thunderstorm activity and thunderstorm outflow boundary winds." Due to the timing of 

the July 7th event, ADEQ explains that, "the impacts in Apache Junction may have occurred around the 

midnight hour, leading to the exceedances there being measured for July gth_, ADEQ provides a 

comprehensive description and discussion of each of these events in Sections I, II, and V of the 

demonstration. 

1

A natural event is further described in 40 CFR SO.l(k) as "an event in which human activity plays little or no direct causal role." 

1 
T a e bl 1 EPAPM 10 E xcee dan ce s 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 04-013-40 I 0-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 04-0 13-9997-4 228 

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 04-0 13-4004-1 360 

West Phoenix 04-0 13-00 19-1 278 

July 7, 2011 Higley 04-0 13-4006-1 266 

West Chandler 04-0 13-4004-1 214 

July 8, 2011 Apache Junction 04-021-3002-1 194 

Not Reasonably Controllable or Preventable (nRCPl 

EPA evaluates whether an event was not reasonably controllable or preventable at the time ofthe event 

by taking into account controls in place and wind speed, along with other factors. 2 For natural sources of 

dust, a high wind dust event can generally be considered to be not reasonably controllable or preventable 

if winds are high enough to cause emissions from natural undisturbed areas. For anthropogenic sources 

of dust, a high wind dust event is also eligible to be considered to be not reasonably controllable or 

preventable if: 

1. The anthropogenic sources of dust have reasonable controls in place, 

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 of required 

controls in the Phoenix PM10 nonattainment area, including information on rule implementation, rule 

effectiveness, compliance and enforcement, real-time monitoring alert systems and public notification 

activities that occurred on the event days. ADEQ concluded, ''the Phoenix area is designated as a serious 

nonattainment area for PM10 and is required to have BACM for all significant sources ofPMJO. BACM2 See e.g., Affirmation of Attainment ofPM-10 NAAQS for the San Joaquin Val1eyNonattainment Area, 73 FR 14691 (March 19, 2008). 

2 
approved control measures on significant anthropogenic sources were in place and enforced during the 

events, and pro-active tracking and response to the events by regulatory agencies and local governments 

confirmed the uncontrollable nature of the dust emissions; therefore, these pre-existing/prior approved 

required controls are adequate for meeting the requirements of an exceptional event and should be 

considered 'reasonable' for these purposes." 

ADEQ provided documentation showing that, with the exception of the 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 on July 4th, and 25 to 47 mph with 

gusts of 3 5 to 56 mph on July 5th. While sustained wind speeds only reached 18 mph on July 7th, ADEQ 

explains that ''while winds recorded in Pinal and Maricopa County during the early morning 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 of loose dust such that stronger 

winds were not needed to entrain or re-entrain dust into the air." ADEQ also asserts that due to the 

timing of the July 7th late evening event, the conditions that led to exceedances at Higley and West 

Chandler on July 7th were similarly responsible for the exceedance measured at Apache Junction on 

July 8th. 

ADEQ further explains that "despite the deployment of comprehensive 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 PM10 nonattainment area. The events discussed in this document that caused the exceedances in 

this request (see Sections II and V) were caused by thunderstorm driven outflow winds that transported 

dust into Maricopa County from areas largely outside of the 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 of the PM1o emissions recorded by Maricopa County 

area monitors coming from sources outside of the Phoenix PM10 nonattainment area, provides strong 

evidence that the events and exceedances of July 2-8, 2011 recorded within the nonattainment area were 

not reasonably controllable or preventable." 

Section V of ADEQ's documentation includes a complex GIS analysis of each of the events that 

supports the PM10 transport described above. For all of the events, the analysis clearly demonstrates that 

monitors in the Phoenix PM10 nonattainment area were affected by PM10 transport from outside the 

nonattainment area, with the main source areas located to the south and southeast of the nonattainment 

area. In addition to transport, the spatial extent of elevated PM10 concentrations throughout the area and 

the wind speeds associated with the thunderstorm outflows contributes to EPA's evaluation of whether 

these events are not reasonably controllable or preventable. 

Table 2: Documentation ofnRCP 

Exceedance Date Demonstration Citation QualitY of Evidence 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 

3 
Historical Fluctuations (HF) 

EPA evaluates whether a measured exceedance is in excess of historical fluctuation by taking into 

account the level of the exceedance in relation to historical data, which is typically 3 to 5 years. 

To demonstrate that this requirement was met, ADEQ provided 5..:-year time series plots of both PM10 

daily maximum hourly averages and PM10 24-hour averages. ADEQ also explains that PM10 

concentrations measured during the July 2nd_gth period were in the 99.5th percentile range when 

compared to historical data. 

Table 3: Documentation ofHF 

Exceedance Date Demonstration Citation Quality of Evidence 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 of evidence when evaluating whether 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. Demonstrations should include documentation showing that the event in fact 

occurred and that emissions related to the event were transported in the direction of the monitor(s) where 

measurements were recorded; the size of the area affected by the transported emissions; the relationship 

in time between the event, transport of emissions, and recorded concentrations; and, as appropriate, 

pollutant species-specific information supporting a causal relationship between the event and the 

measured concentration. 

Section II of ADEQ's demonstration included a comprehensive conceptual model of the events, 

including a general overview of the geographic setting of the monitors, climate, and drought information 

for Phoenix area. The conceptual model also included a very detailed discussion of each of the events 

that occurred in the July 2nd_gth time period, which included time-lapse videos of the events and time 

series graphs for each event that included hourly PM10 concentration, visibility, and reports of blowing 

dust or haze. The time-lapse videos can be found at the following locations: 

• July 3, 2011: http://www.phoenixvis.net/videos/640x480/SOMT1 07032011.swf 

• July 4, 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 of the demonstration includes a detailed and extensive GIS analysis, that show the spatial and 

temporal representation of the events as they move throughout Maricopa and Pinal Counties. The 

analysis includes PM10 concentrations, sustained wind speeds, wind gusts, wind direction, visibility, and 

base velocity radar to track the transport ofPM10 throughout the region. Accompanying the analysis, 

ADEQ provides a discussion for every map that describes the conditions at that time. While not included 

in the demonstration, it is important to note that NOAA's National Climatic Data Center Storm events 

4 
database 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 of these events is consistent with the 

observed increased PM10 concentrations in the area, increased wind speed, reduced visibility, and NWS 

station reports of thunderstorms (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 by thunderstorm outflow boundaries" that ''were also responsible for transporting 

PM into the Phoenix PM10 nonattainment area from areas outside of the nonattainment area." ADEQ 

further states that "while it is likely that some dust was generated within the PM10 nonattainment area as 

gusts from the thunderstorm outflows passed through the area, the amount of dust generated locally was 

easily overwhelmed by, and largely unnoticeable as compared to the dust transported in from the source 

regions of the thunderstorm outflows." 

Table 4: Documentation of CCR 

Exceedance Date Demonstration Citation Qualitv of Evidence 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 if the 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 of AAQ 

Exceedance Date Demonstration Citation Qualitv of Evidence 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 if both the nRCP and CCR requirements have been 

adequately demonstrated. ADEQ generally states that, ''the events shown to cause these exceedances 

were emissions ofPM10 driven by high winds caused by thunderstorm activity and related outflow 

boundaries during the period of July 2-8, 2011" and that ''the events therefore qualify as natural events." 

Table 6: Documentation ofNatural Event 

Exceedance Date Demonstration Citation QualitY of Evidence Criterion Met? 

July 3, 2011 Section VII: p. 106-107 Sufficient Yes 

July 4, 2011 Section VII: p. I 06-107 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. I 06-107 Sufficient Yes 

5 
· No Exceedance or Violation But For the Event (NEBF) 

Generally, the NEBF demonstration is similar to the demonstration of the nRCP and CCR requirements, 

and should show that the measured concentration would have been below the applicable NAAQS 

without the affect of the event. 

ADEQ provides a summary of the analysis and information presented in the documentation that 

demonstrate both the nRCP and CCR requirements have been met and states that "the body of evidence 

... provides no alternative that could tie the exceedances of July 2-8, 2011 to any other causal source but 

transported andre-entrained PM10 generated from thunderstorm outflows, confirming that there would 

have been no exceedances but for the presence of these uncontrollable natural events." While not 

explicitly stated in the documentation, EPA acknowledges that PM10 concentrations before the periods 

of high winds on the event days were below the 24-hour PM10 NAAQS, providing further support for 

ADEQ's conclusion. 

Table 7· Documentation ofNEBF 

Exceedance Date Demonstration Citation Quality of Evidence 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. I 05 Sufficient Yes 

July 7, 20II Section VI: p. I 05 Sufficient Yes 

July 8, 20II Section VI: p. I 05 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 agency must follow to request data exclusion. Table 8 outlines EPA's 

evaluation of these requirements. 

Table 8: Schedules and Procedural Criteria 

Demonstration 

Reference Citation Criterion Met? 

Did the State provide prompt public 40 CFR §50.I4 (c)(I)(i) Section 1: p.1, Yes 

notification of the event? Appendix 8 

Were flags and initial description placed on 40 CFR §50.I4 (c)(2)(iii) Section 1: p.1 Yes 

the data by July 1st of the following year? 

Was the demonstration submitted within 3 40 CFR §50.14 (c)(3)(i) March 14,2012 Yes 

years ofthe end of the quarter in which the letter3 

event occurred and 12 months prior to 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 

EPA has reviewed documentation provided by ADEQ to support claims that dust emissions generated 

by monsoonal thunderstorm high winds were transported into the Phoenix PM10 nonattainment area 

from areas in Pinal County and caused exceedances of the 24-hour PM10 NAAQS at the locations 

outlinedin Table 1 on 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 of an exceptional event: 

the exceedances affected air quality, were not reasonably controllable or preventable, and meet the 

definition of a natural event. Specifically, EPA has determined that events were not reasonably 

controllable and preventable either due to high wind conditions that transported PM10 from sources 

outside of the nonattainment area and subsequently overwhelmed reasonable controls within the Phoenix 

PM10 nonattainment area (July 3rd, July 4th, and July 5th) or moderate wind speeds re-entrained the large 

amount ofPM10 deEosited by the large July 5th dust storm within and outside of the nonattainment area 

(July 7th and July 8 ). Also, regardless of transport into the area, information pertaining to the controls 

implemented within the nonattainment area, the spatial extent of elevated PM10 concentrations measured 

in the area, and the wind speeds associated with the thunderstorm outflows provide sufficient evidence 

to conclude that these events were not reasonably controllable or preventable. 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 of evidence is sufficient for concurrence on the flagging of the 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 of determining the attainment status of 

the area. Final actions will come only after EPA completes notice and comment rulemaking on those 

determinations. 

7