\O ST41.

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

X\ifL

REGION IX 17 :“ L PRO1 75 Hawthorne Street

San Francisco, CA 94105-3901

OFFICE OF THE JUL u i 2013 REGIONAL ADMINISTRATOR Mr. Eric Massey

Director, Air Division

Arizona Department of Environmental Quality

1110 W. Washington St.

Phoenix. Arizona 85(07

Dear Mr. Massey:

This letter responds to Arizona Department of Environmental Quality’s (ADEQ) February 13. 2013

submittal regarding 37 exceeedanccs of the 24—hour PIV1 10 standard that occurred at several monitoring

stations within the Phoenix PM10 nonattainment area on the following days: September ii and 12, 2011.

June 16, June 27. July 11. August 11, August 14. and September 6, 2012.

ADEQ’s submittal included documentation that these exceedances were caused by high wind

exceptional events. EPA has reviewed the documentation provided by ADEQ to demonstrate that the

exceedances on these days meet the criteria for an exceptional event in the Exceptional Events Rule

(EER). EPA concurs based on the weight of the evidence that the exceedances were caused by high

wind exceptional events and finds that ADEQ has successfully made the demonstrations referred to in

40 CPR 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 staff has 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 exceedanees 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.’

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

o The

background concentrations for transportation conformity hot spot analyses. 2

data will continue to be publicly available, but EPA’s publications and public

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

data in any summary statistic of potential regulatory application, unless such inclusion is

specifically noted.

liwe are the pernlitting 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

These data may be included in statistics intended to describe trends in actual air quality in the area.

Printed on Recycled Paper
In addition. EPA will rely on calculated values that exclude these 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 PM10 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 point 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 PM in

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.

Sincerely,

/ /4

1’kiared Blumenfeld

Enclosure

cc: Ms. Theresa Rigney. ADEQ
1

EXCEPTIONAL EVENTS RULE REQUIREMENTS

EPA promulgated the Exceptional Events Rule (EER) in 2007, pursuant to the 2005 amendment of 

Clean Air Act (CAA) Section 319. The EER added 40 CFR §50.1(j), (k) and (l); §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.”

Not Reasonably Controllable or Preventable (nRCP)

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

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.

 

1A natural event is further described in 40 CFR 50.1(k) as “an event in which human activity plays little or no direct causal role.”

2

See e.g., Affirmation of Attainment of PM-10 NAAQS for the San Joaquin Valley Nonattainment Area, 73 FR 14691 (March 19, 2008).
2

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 typically 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 

elevated concentrations 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.

Affects Air Quality (AAQ)

EPA will generally consider events to have affected air quality if the CCR and HF requirements have 

been adequately demonstrated.

Natural Event

EPA will generally consider a high wind dust event to be a natural event in cases where windblown dust 

is entirely from natural sources or where contributing anthropogenic sources of windblown dust are 

reasonably controlled.3This generally involves adequately demonstrating both the nRCP and CCR 

requirements.

No Exceedance or Violation But For the Event (NEBF)

Generally, for high wind dust events, the NEBF demonstration is similar to and informed by the 

demonstration of the nRCP and CCR requirements, and is expected to show that the measured 

concentration would have been below the applicable NAAQS without the effect of the event. 

OVERVIEW OF EVENTS

On February 13, 2013, Arizona Department of Environmental Quality (ADEQ) submitted seven 

exceptional events demonstrations for 37 exceedances of the 24-hour PM10 standard that occurred at 

several monitoring stations within the Phoenix PM10 nonattainment area on the following days: 

September 11 & 12, 2011, June 16, June 27, July 11, August 11, August 14, and September 6, 2012. 

Table 1 summarized these exceedances.

 

3EPA will generally consider human activity to have played little or no direct role in causing emissions of the dust generated 

by high wind for purposes of the regulatory definition of “natural event” if contributing anthropogenic sources of the dust 

are reasonably controlled, regardless of the amount of dust coming from these reasonably controlled anthropogenic sources, 

and thus the event could be considered a natural event. In such cases, EPA believes that it would generally be a reasonable 

interpretation of its regulations to find that the anthropogenic source had “little” direct causal role. If anthropogenic sources 

of windblown dust that are reasonably controllable but that did not have those reasonable controls applied at the time of the 

high wind event have contributed significantly to a measured concentration, the event would not be considered a natural 

event. See preamble to the EER at 72 FR 13566, f.n.11. 
3

Generally, ADEQ explains that the exceedances measured on September 11 & 12, 2011, June 16, 

June 27, July 11, August 11, August 14, and September 6, 2012, were associated with “monsoonal 

thunderstorm activity” and “thunderstorm-driven high winds.” ADEQ provided a comprehensive 

description and discussion of each of these events in the respective demonstrations. ADEQ's narrative is 

summarized in the following sections.

Table 1: EPA PM10 Exceedance Summary

Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (μg/m3

)

September 11, 2011 JLG Supersite 04-013-9997-3 185

JLG Supersite 04-013-9997-4 178

North Phoenix 04-013-1004-1 184

North Phoenix 04-013-1004-2 183

West Phoenix 04-013-0019-1 168

September 12, 2011 Durango Complex 04-013-9812-1 229

West 43rd 04-013-4009-1 161

West Phoenix 04-013-0019-1 200

June 16, 2012 Buckeye 04-013-4011-1 202

Durango Complex 04-013-9812-1 186

Dysart 04-013-4010-1 167

Higley 04-013-4006-1 194

South Phoenix 04-013-4003-1 165

West 43rd 04-013-4009-1 211

West Phoenix 04-013-0019-1 189

June 27, 2012 Central Phoenix 04-013-3002-4 340

Durango Complex 04-013-9812-1 221

Glendale 04-013-2001-1 337

Greenwood 04-013-3010-1 323

Higley 04-013-4006-1 224

JLG Supersite 04-013-9997-3 344

JLG Supersite 04-013-9997-4 329

North Phoenix 04-013-1004-1 178

South Phoenix 04-013-4003-1 342

Tempe 04-013-4005-1 169

West 43rd 04-013-4009-1 221

West Chandler 04-013-4004-1 220

Zuni Hills 04-013-4016-1 285

July 11, 2012 Durango Complex 04-013-9812-1 217

Greenwood 04-013-3010-1 212

South Phoenix 04-013-4003-1 285

West 43rd 04-013-4009-1 172

August 11, 2012 Higley 04-013-4006-1 159

West Chandler 04-013-4004-1 219

August 14, 2012 Durango Complex 04-013-9812-1 179

West 43rd 04-013-4009-1 254

September 6, 2012 West Chandler 04-013-4004-1 164
4

Event Days: September 11 & 12, 2011

Table 2: EPA PM10 Exceedance Summary

Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (μg/m3

)

September 11, 2011 JLG Supersite 04-013-9997-3 185

JLG Supersite 04-013-9997-4 178

North Phoenix 04-013-1004-1 184

North Phoenix 04-013-1004-2 183

West Phoenix 04-013-0019-1 168

September 12, 2011 Durango Complex 04-013-9812-1 229

West 43rd 04-013-4009-1 161

West Phoenix 04-013-0019-1 200

Not Reasonably Controllable or Preventable (nRCP)

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 stated, “BACM-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 sustained wind speeds associated with these events were 

above 20 mph on September 11, 2011, and above 25 mph on September 12, 2011. For example, 

maximum sustained wind speeds of 20 mph with gusts of 25 mph were measured at Phoenix Sky Harbor 

Airport on September 11, 2011, and maximum sustained wind speeds of 25 mph with gusts of 32 mph, 

25 mph with gusts of 36, and 26 mph with gusts of 33 mph were measured at Phoenix Sky Harbor, Casa 

Grande Municipal Airport, and Williams Gateway Airport on September 12, 2011, respectively. Also, 

while not included in the demonstration, it is important to note that the National Oceanic and 

Atmospheric Administration’s (NOAA) National Climatic Data Center Storm events database included 

reports of strong thunderstorm winds in upwind areas of the exceeding monitoring stations in the area 

north of the city of Maricopa in Pinal County that were observed to be in excess of 46 mph. Also, 50 

mph winds were observed in conjunction with a NOAA report of a dust storm in the greater Phoenix 

Area on September 11, 2011. 

ADEQ further explained that “despite the deployment of comprehensive control measures and 

sophisticated response programs and one localized, low-impact violation of the dust control rules, high 

wind conditions associated with thunderstorms and thunderstorm outflows brought high concentrations 

of PM10 emissions into, and also overwhelmed controls within, the Phoenix PM10 nonattainment area. 

Strong thunderstorm outflows with sustained winds ranging from 20-30 mph, and even greater speeds 

nearest the source regions... were enough to overwhelm all available efforts to limit PM10

concentrations from the events. The fact that these were natural events involving strong thunderstorm 

outflow winds that transported PM10 emissions into the Phoenix PM10 nonattainment area, with a 

majority of the PM10 emissions recorded by Maricopa County area monitors coming from sources to the 

south and southeast of the nonattainment area, provides strong evidence that the events and exceedances 
5

of September 11 & 12, 2011, recorded within the Phoenix PM10 nonattainment area, were not reasonably 

controllable or preventable.”

Section V of ADEQ’s documentation included a complex Geographic Information System (GIS)

analysis of the event that supports the PM10 transport described above. This analysis indicates that 

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

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

Some of these source areas are located in Pinal County, portions of which were recently designated as a 

moderate nonattainment area (West Pinal) for the 1987 24-hour PM10 NAAQS (77 FR 32024, May 31, 

2012). Currently, the state is undergoing the appropriate process of developing a state implementation 

plan (SIP), due January 2, 2014, that provides for attainment of the PM10 standard as expeditiously as 

practicable but no later than the end of the sixth calendar year after redesignation. The SIP development 

process includes the requirement to identify and implement reasonably available control measures for 

the area. In addition to transport, information pertaining to the controls implemented within the 

nonattainment area, the spatial extent of elevated PM10 concentrations throughout the area, and the wind 

speeds associated with the event sufficiently establishes that the event was not reasonably controllable or 

preventable.

Table 3: Documentation of nRCP

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 11, 2011 Section IV: p. 20-26, Section V: p. 27-42, App. E Sufficient Yes

September 12, 2011 Section IV: p. 20-26, Section V: p. 43-63, App. E Sufficient Yes

Historical Fluctuations (HF)

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 and stated that these figures show that 

“events that occurred on September 11 & 12, 2011, resulted in one of the top ten highest 24-hour 

average PM10 concentrations seen in the last five years.” ADEQ's analysis sufficiently establishes that 

the 24-hour PM10 concentrations measured on September 11 & 12, 2011, were in excess of normal 

historical fluctuations.

Table 4: Documentation of HF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 11, 2011 Section III: p. 16-19, App. A Sufficient Yes

September 12, 2011 Section III: p. 16-19, App. A Sufficient Yes

Clear Causal Relationship (CCR)

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 and climate information. The 

conceptual model also included a very detailed discussion of the events that occurred on September 11 

&12, 2011, and a time series graph for the four day period from September 10, 2011, to September 13, 

2011, that included hourly PM10 concentrations for monitors in the Phoenix PM10 nonattainment area. 

Section V of the demonstration included time-lapse video evidence, satellite imagery, metrological data 

from various National Weather Service (NWS) stations within the Phoenix PM10 nonattainment area and

Pinal County, time series graphs for the events that included hourly PM10 concentrations from monitors 
6

within the Phoenix PM10 nonattainment area, visibility and sustained with speed from Phoenix Sky 

Harbor Airport. ADEQ also included an additional times series graphs for the September 11 & 12, 2011,

events that included 5-minute PM10 concentrations from various PM10 monitors within the Phoenix 

PM10 nonattainment area compared to 5-minute wind speed from the Central Phoenix monitoring 

station, and PM10 concentrations from monitors located in Pinal County compared to wind speed and 

visibility at Casa Grande Municipal Airport, respectively. Finally, ADEQ included a detailed and 

extensive GIS analysis that included PM10 concentrations, sustained wind speeds, wind gusts, wind 

direction, base velocity radar, and visibility to track the transport of PM10 throughout the region. 

Accompanying the analysis, ADEQ provided a discussion for every map that described the conditions at 

that time. These data show the spatial and temporal representation of the event as it moves throughout 

Maricopa and Pinal Counties. The time-lapse videos of the event can be found at the following 

locations:

 September 11, 2011: http://www.phoenixvis.net/videos/mpeg4/SOMT_09112011.mp4

 September 12, 2011: http://www.phoenixvis.net/videos/mpeg4/SOMT_09122011.mp4

While not included in the demonstration, it is important to note that NOAA’s National Climatic Data 

Center Storm events database included dust storm observations on September 11, 2011, at 6:15 PM 

(greater Phoenix area). The timing of these dust storm reports for this event is consistent with the 

issuance of a NWS Dust Storm Warning for the period of 6:00 PM to 7:00 PM, NWS Significant 

Weather Advisory for the period of 5:19 PM to 7:00 PM, the observed increased PM10 concentrations in 

the area, increased wind speed, reduced visibility, and NWS station reports of blowing dust (BLDU), 

haze (HZ), and dust (DU). Also, as previously mentioned in the nRCP section of this document, the 

wind speeds associated with this event reached the 20 mph range at meteorological stations in the area, 

but the NOAA’s National Climatic Data Center Storm events database included observations of strong 

thunderstorm winds in excess of 46 mph just north of the City of Maricopa in Pinal County at 5:30 PM. 

These observations are consistent with the base velocity radar data included in the GIS analysis in 

Figure 5-4 in ADEQ’s demonstration, as well as the location of the collapsing thunderstorm and the area 

of formation of the outflow boundary as described by ADEQ in Section V on p. 27 and p. 32 and of the 

September 11, 2012, event demonstration. The timing of the September 12, 2011, event is consistent 

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

and NWS station reports of thunderstorms (TS) and blowing dust.

ADEQ stated that the evidence presented has “demonstrated a clear causal relationship between the 

emissions generated by uncontrollable natural events and the exceedances measured at the monitors.”

ADEQ further stated that “the satellite images, time series graphs, and meteorological data tables 

provided in this section for September 11 & 12, 2011, show the temporal progression of the dust events 

from the development of the thunderstorms, to the increase in wind speeds, and to the rise in PM10

concentrations. The GIS maps for September 11th also showed how soon after the main outflow 

boundary passed through the Phoenix area, that stagnation allowed PM10 concentrations to remain high 

at a few sites. The GIS maps for September 12th showed how strong winds and high PM10 concentrations 

in Pinal County traveled northward into Maricopa County, leading to a subsequent increase in wind 

speeds and PM10 concentrations at the Phoenix area PM10 monitors. The combination of the PM10

concentrations and wind data from Maricopa County on September 11th, and from Maricopa and Pinal 

Counties on September 12th, supports the conclusion that the events were primarily caused by 

windblown dust from emission sources outside of Maricopa County and the transport of PM10 into the 

Phoenix PM10 nonattainment area.” 
7

The analysis in Sections II and V, specifically, the PM10 time series graph, winds speed and direction 

measurements, the complex GIS maps, time-lapse video evidence, NOAA dust storm and thunderstorm 

wind observations, NWS advisories, and NWS station reports of reduced visibility, blowing dust, haze, 

and dust, sufficiently establishes that there was a clear causal relationship between uncontrollable 

emissions generated from thunderstorm outflow winds and the exceedances measured at monitors 

identified in Table 2 of this document. 

Table 5: Documentation of CCR

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 11, 2011 Section II: p. 4-15, Section V: p. 27-42, App. D, App. E Sufficient Yes

September 12, 2011 Section II: p. 4-15, Section V: p. 43-63, App. D, App. E Sufficient Yes

Affects Air Quality (AAQ)

ADEQ stated that based on the information presented in the demonstrations for both the CCR and HF 

requirements, “we can reasonably conclude that the event[s] in question affected air quality.” ADEQ's 

summary regarding the CCR and HF requirements sufficiently establishes that the event affected air 

quality.

Table 6: Documentation of AAQ

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 11, 2011 Section VII: p. 66 Sufficient Yes

September 12, 2011 Section VII: p. 66 Sufficient Yes

Natural Event

ADEQ stated that based on the documentation for both the nRCP and CCR requirements, “events shown 

to cause these exceedances were emissions of PM10 driven by high winds caused by thunderstorm 

activity and related outflow boundaries on September 11 & 12, 2011” and that “the events therefore 

qualify as a natural events.” ADEQ's summary regarding the CCR and HF requirements sufficiently 

establishes that the event was a natural event.

Table 7: Documentation of Natural Event

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 11, 2011 Section VII: p. 66-67 Sufficient Yes

September 12, 2011 Section VII: p. 66-67 Sufficient Yes

No Exceedance or Violation But For the Event (NEBF)

ADEQ provided a summary of the analysis and information regarding the nRCP and CCR requirements 

and stated that “the body of evidence presented in this submittal provides no alternative that could tie the 

exceedances to any other causal source but transported and re-entrained PM10 generated from 

thunderstorm outflows, confirming that there would have been no exceedances but for the presence of 

these uncontrollable natural events.” ADEQ’s summary regarding the nRCP and CCR requirements 

sufficiently establishes that the NEBF criterion has been met. 
8

Table 8: Documentation of NEBF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 11, 2011 Section VI: p. 65 Sufficient Yes

September 12, 2011 Section VI: p. 65 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 9 outlines EPA’s 

evaluation of these requirements.

Table 9: Schedules and Procedural Criteria

Reference

Demonstration

Citation Criterion Met?

Did the State provide prompt public 

notification of the event?

40 CFR §50.14 (c)(1)(i) Section I: p.1

App. B

Yes

Were flags and initial description placed on 

the data by July 1st of the following year?

40 CFR §50.14 (c)(2)(iii) Section I: p. 1-2 Yes

Was the demonstration submitted within 3 

years of the end of the quarter in which the 

event occurred and 12 months prior to the 

date that any regulatory decision must be 

made by EPA?

40 CFR §50.14 (c)(3)(i) February 13, 

2013 letter4

Yes

Was the public comment process followed 

and documented?

40 CFR §50.14 (c)(3)(v) Section I: p. 2

App. D5

Yes

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 

outlined in Table 2 on September 11 & 12, 2011. EPA has determined that the flagged exceedances at 

these locations 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. In 

addition to 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 events provide sufficient evidence to conclude that the events were not 

reasonably controllable or preventable. Furthermore, EPA has determined that there was a clear causal 

relationship between the events and the measured exceedances, there would have been no exceedances 

but for the events, and the measured exceedances were in excess of normal historical fluctuations.

 

4

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

dated February 13, 2013.

5A copy of the affidavit was not included in App. D, as stated in Section I of the final demonstration, but was submitted to EPA as part of 

the February 13, 2013 submission. 
9

Event Day: June 16, 2012

Table 10: EPA PM10 Exceedance Summary

Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (μg/m3

)

June 16, 2012 Buckeye 04-013-4011-1 202

Durango Complex 04-013-9812-1 186

Dysart 04-013-4010-1 167

Higley 04-013-4006-1 194

South Phoenix 04-013-4003-1 165

West 43rd 04-013-4009-1 211

West Phoenix 04-013-0019-1 189

Not Reasonably Controllable or Preventable (nRCP)

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 stated, “BACM-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 sustained wind speeds associated with these events were 

above 25 mph in multiple locations throughout the Phoenix PM10 nonattainment area and Pinal County. 

For example, maximum sustained wind speeds of 29 mph with gusts of 39 mph, and 32 mph with gusts 

of 46 mph, and 30 mph with gusts of 37 were measured at Williams Gateway Airport, Casa Grande 

Municipal Airport, and Falcon Field Airport, respectively. 

ADEQ further explained that “despite the deployment of comprehensive control measures and 

sophisticated response programs and a few localized, low-impact violations of the dust control rules, 

high wind conditions associated with thunderstorms and thunderstorm outflows brought high 

concentrations of PM10 emissions into, and also overwhelmed controls within, the Phoenix PM10

nonattainment area. Strong thunderstorm outflows with sustained winds ranging from 20-30 mph, and 

even greater speeds nearest the source regions, described in Section V, were enough to overwhelm all 

available efforts to limit PM10 concentrations from the event. The fact that these were natural events 

involving strong thunderstorm outflow winds that transported PM10 emissions into Maricopa County, 

with a majority of the PM10 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 June 16, 2012, recorded within the Phoenix PM10 nonattainment area, were not 

reasonably controllable or preventable.”

Section V of ADEQ’s documentation included further analysis of the event that supports the PM10

transport described above. This analysis indicates 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. Some of these source areas are located in Pinal 

County, portions of which were recently designated as a moderate nonattainment area (West Pinal) for 
10

the 1987 24-hour PM10 NAAQS (77 FR 32024, May 31, 2012). Currently, the state is undergoing the 

appropriate process of developing a state implementation plan (SIP), due January 2, 2014, that provides 

for attainment of the PM10 standard as expeditiously as practicable but no later than the end of the sixth 

calendar year after redesignation. The SIP development process includes the requirement to identify and 

implement reasonably available control measures for the area. In addition to transport, information 

pertaining to the controls implemented within the nonattainment area, the spatial extent of elevated PM10

concentrations throughout the area and the wind speeds associated with the event sufficiently establishes 

that the event was not reasonably controllable or preventable. 

Table 11: Documentation of nRCP

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 16, 2012 Section IV: p.18-24, Section V: p. 25-34, App. E Sufficient Yes

Historical Fluctuations (HF)

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 and stated that these figures show that 

“event that occurred on June 16, 2012, resulted in one of the top 12 highest 24-hour average PM10

concentrations seen in the last five-plus years.” ADEQ's analysis sufficiently establishes that the 24-hour 

PM10 concentrations measured on June 16, 2012, were in excess of normal historical fluctuations.

Table 12: Documentation of HF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 16, 2012 Section III: p. 14-17, App. A Sufficient Yes

Clear Causal Relationship (CCR)

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, and climate information. The 

conceptual model also included a very detailed discussion of the event that occurred on June 16, 2012,

and a time series graph for the event that included hourly PM10 concentrations for monitors in the 

Phoenix PM10 nonattainment area. 

Section V of the demonstration included satellite imagery, time-lapse video evidence, a time series 

graph for the event that contained hourly PM10 concentrations from monitors in the Phoenix PM10

nonattainment area, visibility and sustained wind speed from Phoenix Sky Harbor International Airport, 

and sustained wind speed from Williams Gateway Airport. The CCR analysis also included a time series 

graph that contained PM10 concentrations from monitors in Pinal County, visibility, and sustained wind 

speed from Casa Grande Municipal Airport, and the raw data tables for numerous NWS stations in 

Maricopa and Pinal Counties. These data show the spatial and temporal representation of the event as it 

moves throughout Maricopa and Pinal Counties. Time-lapse video of the event was included and can be 

found at the following location:

 South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_06162012.mp4

While not included in the demonstration, it is important to note that NOAA’s National Climatic Data 

Center Storm events database included dust storm observations on June 16, 2012, at 4:30 PM (central 

deserts) and 5:09 PM (greater Phoenix area). The timing of these dust storm reports for this event is 
11

consistent with the issuance of a NWS Significant Weather Advisory for the period of 4:24 PM to 6:00 

PM, NWS Dust Storm Warning for the period of 4:30 PM to 7:00 PM, the observed increased PM10

concentrations in the area, increased wind speed, reduced visibility, and NWS station reports of blowing 

dust (BLDU), haze (HZ), and dust storms (DS).

ADEQ stated that the evidence presented shows a clear causal relationship “between the emissions 

generated by uncontrollable natural events and the exceedances measured at the monitors.” ADEQ 

further stated that “the satellite images, time series graphs, and meteorological data tables provided in 

this section show the temporal progression of the dust events from the development of the 

thunderstorms, to the increase in wind speeds, and to the rise in PM10 concentrations. The combination 

of the PM10 and wind data from Maricopa and Pinal counties shows the transport of particulate matter 

from the south through Pinal County and into the Phoenix PM10 nonattainment area. This information 

supports the conclusion that the events were primarily caused by windblown dust from emission sources 

outside of Maricopa County and the transport of PM10 into the Phoenix PM10 nonattainment area.” 

The analysis in Sections II and V, specifically, the PM10 time series graph, winds speed and direction 

measurements, time-lapse video evidence, NOAA dust storm observations, NWS advisories, and NWS 

station reports of reduced visibility, blowing dust, haze, and dust storms, sufficiently establishes that 

there was a clear causal relationship between uncontrollable emissions generated from thunderstorm 

outflow winds and the exceedances measured at the monitors identified in Table 10 of this document.

Table 13: Documentation of CCR

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 16, 2012 Section V: p. 25-34, App. D, App. E Sufficient Yes

Affects Air Quality (AAQ)

ADEQ stated that based on the information presented in the demonstrations, “we can reasonably 

conclude that the event in question affected air quality.” ADEQ's summary regarding the CCR and HF 

requirements sufficiently establishes that the event affected air quality.

Table 14: Documentation of AAQ

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 16, 2012 Section VII: p. 36 Sufficient Yes

Natural Event

ADEQ stated that based on the documentation for both the nRCP and CCR requirements, “events shown 

to cause these exceedances were emissions of PM10 driven by high winds caused by thunderstorm 

activity and related outflow boundary on June 16, 2012” and that “the events therefore qualify as natural 

events.” ADEQ's summary regarding the CCR and HF requirements sufficiently establishes that the 

event was a natural event.

Table 15: Documentation of Natural Event

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 16, 2012 Section VII: p. 36 Sufficient Yes

No Exceedance or Violation But For the Event (NEBF)
12

ADEQ provided a summary of the analysis and information regarding the nRCP and CCR requirements 

and stated that “the body of evidence presented in this submittal provides no alternative that could tie the 

exceedances of June 16, 2012, to any other causal source but transported and re-entrained PM10

generated from thunderstorm outflows, confirming that there would have been no exceedances but for 

the presence of these uncontrollable natural events.” ADEQ’s summary regarding the nRCP and CCR 

requirements sufficiently establishes that the NEBF criterion has been met. 

Table 16: Documentation of NEBF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 16, 2012 Section VI: p.35 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 17 outlines EPA’s 

evaluation of these requirements.

Table 17: Schedules and Procedural Criteria

Reference

Demonstration

Citation Criterion Met?

Did the State provide prompt public 

notification of the event?

40 CFR §50.14 (c)(1)(i) Section I: p.1, 

App. B

Yes

Were flags and initial description placed on 

the data by July 1st of the following year?

40 CFR §50.14 (c)(2)(iii) Section I: p.1-2 Yes

Was the demonstration submitted within 3 

years of the end of the quarter in which the 

event occurred and 12 months prior to the 

date that any regulatory decision must be 

made by EPA?

40 CFR §50.14 (c)(3)(i) February 13, 

2013 letter6

Yes

Was the public comment process followed 

and documented?

40 CFR §50.14 (c)(3)(v) Section I: p.2, 

App. C

7

Yes

 

6

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

February 13, 2013.

7A copy of the affidavit was not included in App. C, as stated in Section I of the final demonstration, but was submitted to EPA as part of 

the February 13, 2013 submission. 
13

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 

outlined in Table 10 on June 16, 2012. EPA has determined that the flagged exceedances at these 

locations on this day 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. In addition to 

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 event provides sufficient evidence to conclude that the event was not reasonably 

controllable or preventable. Furthermore, EPA has determined that there was a clear causal relationship 

between the event and the measured exceedances, there would have been no exceedances but for the 

event, and the measured exceedances were in excess of normal historical fluctuations.
14

Event Day: June 27, 2012

Table 18: EPA PM10 Exceedance Summary

Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (μg/m3

)

June 27, 2012 Central Phoenix 04-013-3002-4 340

Durango Complex 04-013-9812-1 221

Glendale 04-013-2001-1 337

Greenwood 04-013-3010-1 323

Higley 04-013-4006-1 224

JLG Supersite 04-013-9997-3 344

JLG Supersite 04-013-9997-4 329

North Phoenix 04-013-1004-1 178

South Phoenix 04-013-4003-1 342

Tempe 04-013-4005-1 169

West 43rd 04-013-4009-1 221

West Chandler 04-013-4004-1 220

Zuni Hills 04-013-4016-1 285

Not Reasonably Controllable or Preventable (nRCP)

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 stated, “BACM-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 sustained wind speeds associated with these events were 

above 25 mph in multiple locations throughout the Phoenix PM10 nonattainment area and Pinal County. 

For example, maximum sustained wind speeds of 38 mph with gusts of 45 mph, 34 mph with gusts of 47 

mph, 31 mph with gusts of 39 mph, and 31 mph with gusts of 44 mph were measured at Chandler 

Municipal Airport, Williams Gateway Airport, Casa Grande Municipal Airport, and Phoenix Sky 

Harbor, respectively. 

Section V of ADEQ’s documentation included further analysis of the event that supports the PM10

transport described above. This analysis indicates 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 southeast of the nonattainment area. Some of these source areas are located in Pinal County,

portions of which were recently designated as a moderate nonattainment area (West Pinal) for the 1987 

24-hour PM10 NAAQS (77 FR 32024, May 31, 2012). Currently, the state is undergoing the appropriate 

process of developing a state implementation plan (SIP), due January 2, 2014, that provides for 

attainment of the PM10 standard as expeditiously as practicable but no later than the end of the sixth 

calendar year after redesignation. The SIP development process includes the requirement to identify and 

implement reasonably available control measures for the area. In addition to transport, information 

pertaining to the controls implemented within the nonattainment area, the spatial extent of elevated PM10
15

concentrations throughout the area and the wind speeds associated with the event sufficiently establishes 

that the event was not reasonably controllable or preventable.

Table 19: Documentation of nRCP

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 27, 2012 Section IV: p. 18-25, Section V: p. 26-35, App. E Sufficient Yes

Historical Fluctuations (HF)

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 and stated that these figures show that the 

“event that occurred on June 27, 2012, resulted in one of the top 10 highest 24-hour average PM10

concentrations seen in the last five-plus years.” ADEQ's analysis sufficiently establishes that the 24-hour 

PM10 concentrations measured on July 18, 2011, were in excess of normal historical fluctuations.

Table 20: Documentation of HF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 27, 2012 Section III: p. 14-17. App. A Sufficient Yes

Clear Causal Relationship (CCR)

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, and climate information. The 

conceptual model also included a very detailed discussion of the event that occurred on June 27, 2012,

and a time series graph for the event that included hourly PM10 concentrations for monitors in the 

Phoenix PM10 nonattainment area. 

Section V of the demonstration included satellite imagery, time-lapse video evidence, a time series 

graph for the event that contained hourly PM10 concentrations from monitors in the Phoenix PM10

nonattainment area, visibility and sustained wind speed from Phoenix Sky Harbor International Airport, 

and sustained wind speed from Williams Gateway Airport. The CCR analysis also included a time series 

graph that contained PM10 concentrations from monitors in Pinal County, visibility, and sustained wind 

speed from Casa Grande Municipal Airport, and the raw data tables for numerous NWS stations in 

Maricopa and Pinal Counties. These data show the spatial and temporal representation of the event as it 

moves throughout Maricopa and Pinal Counties. Time-lapse video of the event was included and can be 

found at the following location:

 South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_06272012.mp4

 Superstition Mountains: http://www.phoenixvis.net/videos/mpeg4/SUPM_06272012.mp4

While not included in the demonstration, it is important to note that NOAA’s National Climatic Data 

Center Storm events database included dust storm observations on June 17, 2012, at 6:00 PM (central 

deserts) and 6:40 PM (greater Phoenix area). The timing of these dust storm reports for this event is 

consistent with the issuance of a NWS Blowing Dust Advisory for the period of 5:30 PM to 6:00 PM, 

NWS Severe Thunderstorm Warning for the period of 6:09 PM to 7:30 PM, NWS Significant Weather 

Advisory for the period of 6:58 PM to 8:15 PM, NWS Dust Storm Warning for the period of 6:02 PM to 

9:00 PM, NWS preliminary local storm reports of dust storms, the observed increased PM10
16

concentrations in the area, increased wind speed, reduced visibility, and NWS station reports of blowing 

dust , haze , dust , and dust storms (DS). 

ADEQ stated that the evidence presented shows a clear causal relationship “between the emissions 

generated by uncontrollable natural events and the exceedances measured at the monitors.” ADEQ 

further stated that “the satellite images, time series graphs, and meteorological data tables provided in 

this section show the temporal progression of the dust events from the development of the 

thunderstorms, to the increase in wind speeds, and to the rise in PM10 concentrations. The combination 

of the PM10 and wind data from Maricopa and Pinal counties shows the transport of particulate matter 

from the south through Pinal County and into the Phoenix PM10 nonattainment area. This information 

supports the conclusion that the PM10 exceedances observed on June 27, 2012, were the result of 

emissions entrained into the atmosphere by strong thunderstorm outflow boundary winds from source 

areas to the southeast, outside of the Phoenix PM10 nonattainment area, and transported into the Phoenix 

PM10 nonattainment area.”

The analysis in Sections II and V, specifically, the PM10 time series graph, winds speed and direction 

measurements, time-lapse video evidence, NOAA dust storm observations, NWS local storm reports, 

NWS advisories, and NWS station reports of reduced visibility, blowing dust, haze, dust, and dust 

storms, sufficiently establishes that there was a clear causal relationship between uncontrollable 

emissions generated from thunderstorm outflow winds and the exceedances measured at monitors 

identified in Table 18 of this document. 

Table 21: Documentation of CCR

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 27, 2012 Section V: p. 26-35, App. D, App. E Sufficient Yes

Affects Air Quality (AAQ)

ADEQ stated that based on the information presented in the demonstrations for both the CCR and HF 

requirements, “we can reasonably conclude that the event in question affected air quality.” ADEQ's 

summary regarding the CCR and HF requirements sufficiently establishes that the event affected air 

quality.

Table 22: Documentation of AAQ

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 27, 2012 Section VII: p. 37 Sufficient Yes

Natural Event

ADEQ stated that based on the documentation for both the nRCP and CCR requirements, “the events 

shown to cause these exceedances were emissions of PM10 driven by high winds caused by 

thunderstorm activity and related outflow boundary on June 27, 2012” and that “the events therefore 

qualify as natural events.” ADEQ's summary regarding the CCR and HF requirements sufficiently 

establishes that the event was a natural event.

Table 23: Documentation of Natural Event

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 27, 2012 Section VII: p. 37 Sufficient Yes
17

No Exceedance or Violation But For the Event (NEBF)

ADEQ provided a summary of the analysis and information regarding the nRCP and CCR requirements 

and stated that “the body of evidence presented in this submittal provides no alternative that could tie the 

exceedances of June 27, 2012, to any other causal source but transported and re-entrained PM10

generated from thunderstorm outflows, confirming that there would have been no exceedances but for 

the presence of these uncontrollable natural events.” ADEQ’s summary regarding the nRCP and CCR 

requirements sufficiently establishes that the NEBF criterion has been met.

Table 24: Documentation of NEBF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

June 27, 2012 Section VI: p. 36 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 25 outlines EPA’s 

evaluation of these requirements.

Table 25: Schedules and Procedural Criteria

Reference

Demonstration

Citation Criterion Met?

Did the State provide prompt public 

notification of the event?

40 CFR §50.14 (c)(1)(i) Section I: p. 1

App. B

Yes

Were flags and initial description placed on 

the data by July 1st of the following year?

40 CFR §50.14 (c)(2)(iii) Section I: p 1-2 Yes

Was the demonstration submitted within 3 

years of the end of the quarter in which the 

event occurred and 12 months prior to the 

date that any regulatory decision must be 

made by EPA?

40 CFR §50.14 (c)(3)(i) February 13, 

2013 letter8

Yes

Was the public comment process followed 

and documented?

40 CFR §50.14 (c)(3)(v) Section I: p. 2

App. C

Yes

 

8

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

February 13, 2013.
18

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 

outlined in Table 18 on June 27, 2012. EPA has determined that the flagged exceedances at these 

locations on this day 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. In addition to 

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 event provides sufficient evidence to conclude that the event was not reasonably 

controllable or preventable. Furthermore, EPA has determined that there was a clear causal relationship 

between the event and the measured exceedances, there would have been no exceedances but for the 

event, and the measured exceedances were in excess of normal historical fluctuations.
19

Event Day: July 11, 2012

Table 26: EPA PM10 Exceedance Summary

Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (μg/m3

)

July 11, 2012 Durango Complex 04-013-9812-1 217

Greenwood 04-013-3010-1 212

South Phoenix 04-013-4003-1 285

West 43rd 04-013-4009-1 172

Not Reasonably Controllable or Preventable (nRCP)

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 stated, “BACM 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 sustained wind speeds associated with these events were 

above 20 mph in multiple locations nearby and upwind of the exceeding monitoring stations throughout 

the Phoenix PM10 nonattainment area and above 25 mph in Pinal County. For example, maximum 

sustained wind speeds of 23 mph with gusts of 33 mph, and 25 mph with gusts of 33 mph were 

measured at Phoenix Sky Harbor Airport and Casa Grande Municipal Airport, respectively. 

ADEQ further explained that “despite the deployment of comprehensive control measures and 

sophisticated response programs, high-wind conditions associated with thunderstorms and thunderstorm 

outflows brought high concentrations of PM10 emissions into, and also overwhelmed controls within, the 

Phoenix PM10 Nonattainment Area. Widespread thunderstorm outflows with sustained winds in excess 

of 20 mph with gusts over 30 mph were enough to overwhelm all available efforts to limit PM10

concentrations during the event. The fact that these were natural events involving strong thunderstorm 

outflow winds that transported PM10 emissions into and across the Phoenix area, with a majority of the 

PM10 emissions recorded by Phoenix area monitors coming from sources outside of the Phoenix PM10 

Nonattainment Area, provides strong evidence that the exceedances of July 11, 2012, recorded within 

the Phoenix PM10 Nonattainment Area were not reasonably controllable or preventable.”

While ADEQ states that the majority of the PM10 emissions were generated outside of the Phoenix PM10 

Nonattainment Area, ADEQ’s documentation included information on the event that indicates that 

monitors in the Phoenix PM10 nonattainment area may have been affected by PM10 transport primarily 

from within the nonattainment area, specifically, the areas in the southeastern portion of the 

nonattainment area. Information pertaining to the controls implemented within the nonattainment area, 

the spatial extent of elevated PM10 concentrations throughout the area and the wind speeds associated 

with the event sufficiently establishes that the event was not reasonably controllable or preventable. 

Table 27: Documentation of nRCP

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

July 11, 2012 Section V: p 1-7, Section III: p. 1-7, App. A Sufficient Yes
20

Historical Fluctuations (HF)

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 and stated that these figures show that “the 

PM10 concentrations measured...on July 11, 2012, were among the highest 24-hr averages...measured 

over the five-year period.” ADEQ's analysis sufficiently establishes that the 24-hour PM10

concentrations measured on July 11, 2012, were in excess of normal historical fluctuations.

Table 28: Documentation of HF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

July 11, 2012 Section IV: p. 1-2, App. C Sufficient Yes

Clear Causal Relationship (CCR)

Section II of ADEQ’s demonstration included a conceptual model of the events, including a general 

overview of the geographic setting of the monitors, and climate information. The conceptual model also 

included a discussion of the event that occurred on July 11, 2012, and general map of the region of the 

thunderstorm development in relation to the Phoenix PM10 nonattainment area. 

Section III, Appendix A, and Appendix B of the demonstration included satellite imagery visibility 

photos, time-lapse video evidence, a map of the Phoenix area that displays wind speed and direction at 

the peak hour of PM10 concentrations during the event, a map with radar base velocity data that 

identifies thunderstorm outflow boundaries, a time series of hourly PM10 concentrations from PM10

monitors in Pinal County, a time series graph that shows hourly PM10 concentrations from monitors in 

the Phoenix PM10 nonattainment area and visibility from Phoenix Sky Harbor Airport, and the raw data 

tables for numerous NWS stations in Maricopa and Pinal Counties. These data show the spatial and 

temporal representation of the event as it moves throughout Maricopa and Pinal Counties. ADEQ also 

provided a discussion for every map that described the conditions at that time. Time-lapse videos of the 

event can be found at the following location:

 South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_07112012.mp4 

The timing of the event is consistent with the issuance of a NWS Blowing Dust Advisory for the period 

of 8:30 PM to 12:00 AM, NWS Significant Weather Advisory for the period of 10:35 PM to 11:15 PM,

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

NWS station reports of thunderstorms, haze, and dust. 

ADEQ stated that the evidence presented shows a clear causal relationship “between the windblown dust 

and the PM10 exceedances measured at four Phoenix-area monitors on July 11, 2012. The radar and 

wind data shown in this section illustrate the spatial and temporal extent of the dust storm as it moved 

through the Phoenix area. In addition, the time-series plots of air quality and meteorological data found 

in this section and in Appendix A show that the sharp increase in PM10 concentrations coincided with 

the strong wind speeds and wind gusts.” 

The analysis in Sections II, III, Appendix A, and Appendix B, specifically, the PM10 time series graphs, 

winds speed and direction measurements, maps, time-lapse video evidence, NWS advisories, and NWS 

station reports of reduced visibility, thunderstorms, haze, and dust, sufficiently establishes that there was 

a clear causal relationship between uncontrollable emissions generated from thunderstorm outflow 

winds and the exceedance measured at the monitors identified in Table 26 of this document.
21

Table 29: Documentation of CCR

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

July 11, 2012 Section II: p. 1-9 Section IV: p. 1-2, App. C, B, & D Sufficient Yes

Affects Air Quality (AAQ)

ADEQ stated that based on the information presented in the demonstrations for both the CCR and HF 

requirements, “we can reasonably conclude that the event in question affected air quality.” ADEQ's 

summary regarding the CCR and HF requirements sufficiently establishes that the event affected air 

quality.

Table 30: Documentation of AAQ

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

July 11, 2012 Section VII: p. 1 Sufficient Yes

Natural Event

ADEQ stated that based on the documentation for both the nRCP and CCR requirements, “PM10

exceedances on July 11, 2012, were shown to be caused by PM10 transported into the Phoenix area by 

thunderstorm outflow” and that “the event therefore qualifies as a natural event.” ADEQ's summary 

regarding the CCR and HF requirements sufficiently establishes that the event was a natural event.

Table 31: Documentation of Natural Event

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

July 11, 2012 Section VII: p. 1 Sufficient Yes

No Exceedance or Violation But For the Event (NEBF)

Table 32: Documentation of NEBF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

July 11, 2012 Section VI: p. 1 Sufficient Yes

ADEQ provided a summary of the analysis and information regarding the nRCP and CCR requirements 

and stated that “the weight of evidence presented in this submittal provides no alternative that could tie 

the exceedance of July 11, 2012, to any causal source other than PM10 transported by thunderstorm 

outflow, confirming that there would have been no exceedance but for the presence of this 

uncontrollable natural event.” ADEQ’s summary regarding the nRCP and CCR requirements 

sufficiently establishes that the NEBF criterion has been met.

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 33 outlines EPA’s 

evaluation of these requirements.
22

Table 33: Schedules and Procedural Criteria

Reference

Demonstration

Citation Criterion Met?

Did the State provide prompt public 

notification of the event?

40 CFR §50.14 (c)(1)(i) Section I, p. 1

App. D

Yes

Were flags and initial description placed on 

the data by July 1st of the following year?

40 CFR §50.14 (c)(2)(iii) Section I, p. 1 Yes

Was the demonstration submitted within 3 

years of the end of the quarter in which the 

event occurred and 12 months prior to the 

date that any regulatory decision must be 

made by EPA?

40 CFR §50.14 (c)(3)(i) February 13, 

2013 letter9

Yes

Was the public comment process followed 

and documented?

40 CFR §50.14 (c)(3)(v) Section I, p. 1-2

App. E10

Yes

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 

outlined in Table 26 on July 11, 2012. EPA has determined that the flagged exceedances at these 

locations on this day 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 the event was not reasonably controllable and preventable due to high wind 

conditions that overwhelmed reasonable controls within the Phoenix PM10 nonattainment 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 event 

provide sufficient evidence to conclude that the event was not reasonably controllable or preventable. 

Furthermore, EPA has determined that there was a clear causal relationship between the event and the 

measured exceedances, there would have been no exceedance but for the event, and the measured 

exceedances were in excess of normal historical fluctuations.

 

9

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

February 13, 2013.

10A copy of the affidavit was not included in App. E, as stated in Section I of the final demonstration, but was submitted to EPA as part of 

the February 13, 2013 submission. 
23

Event Day: August 11, 2012

Table 34: EPA PM10 Exceedance Summary

Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (μg/m3

)

August 11, 2012 Higley 04-013-4006-1 159

West Chandler 04-013-4004-1 219

Not Reasonably Controllable or Preventable (nRCP)

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 stated, “BACM on significant anthropogenic sources 

were in place and enforced during the events, and proactive 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 sustained wind speeds associated with these events were 

above 25 mph in multiple locations throughout the Phoenix PM10 nonattainment area and Pinal County. 

For example, maximum sustained wind speeds of 32 mph with gusts of 41 mph, and 25 mph with gusts 

of 36 mph were measured at Chandler Municipal Airport, and Casa Grande Municipal Airport, 

respectively. 

ADEQ further explained that “despite the deployment of comprehensive control measures and 

sophisticated response programs, high wind conditions associated with the thunderstorm outflow 

transported high concentrations of PM10 into, and also overwhelmed controls within, the Phoenix PM10

nonattainment area. Widespread sustained winds in excess of 20 mph with gusts over 30 mph were 

strong enough to overwhelm available efforts to limit PM10 concentrations during the event. The fact 

that these were natural events involving strong winds that transported PM10 emissions into and across 

Maricopa County, with a majority of the PM10 emissions recorded by Maricopa County area monitors 

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

exceedances of August 11, 2012, recorded within the Phoenix PM10 nonattainment area were not 

reasonably controllable or preventable.”

Section III of ADEQ’s documentation included a complex GIS analysis of the event that supports the 

PM10 transport described above. This analysis indicates 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 of the nonattainment area. Some of these source areas are located in 

Pinal County, portions of which were recently designated as a moderate nonattainment area (West Pinal) 

for the 1987 24-hour PM10 NAAQS (77 FR 32024, May 31, 2012). Currently, the state is undergoing the 

appropriate process of developing a state implementation plan (SIP), due January 2, 2014, that provides 

for attainment of the PM10 standard as expeditiously as practicable but no later than the end of the sixth 

calendar year after redesignation. The SIP development process includes the requirement to identify and 

implement reasonably available control measures for the area. In addition to transport, information 

pertaining to the controls implemented within the nonattainment area, the spatial extent of elevated PM10

concentrations throughout the area, and the wind speeds associated with the event sufficiently 

establishes that the event was not reasonably controllable or preventable.
24

Table 35: Documentation of nRCP

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 11, 2012 Section V: p. 1-8, Section III: p. 1-10, App. A Sufficient Yes

Historical Fluctuations (HF)

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 and stated that these figures show that “the 

PM10 concentrations measured...on August 11, 2012, resulted in some of the highest 24-hr

averages...measured over the five-year period.” ADEQ's analysis sufficiently establishes that the 

24-hour PM10 concentrations measured on August 11, 2012, were in excess of normal historical 

fluctuations.

Table 36: Documentation of HF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 11, 2012 Section IV: p. 1-2, App. C Sufficient Yes

Clear Causal Relationship (CCR)

Section II of ADEQ’s demonstration included a conceptual model of the events, including a general 

overview of the geographic setting of the monitors, and climate information. The conceptual model also 

included a discussion of the event that occurred on August 11, 2012, and general map of the region of 

the thunderstorm development in relation to the Phoenix PM10 nonattainment area. 

Section III and Appendix A of the demonstration included satellite imagery visibility photos, time-lapse 

video evidence, the raw data tables for numerous NWS stations in Maricopa and Pinal Counties, and a 

number of time series graphs that include hourly PM10 data from Pinal and Maricopa Counties, wind 

speed, and visibility from Chandler Airport. ADEQ also included a detailed GIS analysis with PM10

concentrations, sustained wind speeds, wind gusts, wind direction, base velocity radar, and visibility to 

track the transport of PM10 throughout the region. These data show the spatial and temporal 

representation of the event as it moves throughout Maricopa and Pinal Counties. ADEQ also provided a 

discussion for every map that described the conditions at that time. Time-lapse videos of the event can 

be found at the following locations:

 South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_08112012.mp4

 Superstition Mountains: http://www.phoenixvis.net/videos/mpeg4/SUPM_08112012.mp4

While not included in the demonstration, it is important to note that NOAA’s National Climatic Data 

Center Storm events database included dust storm observations on August 11, 2012, at 4:30 PM (central 

deserts) and 5:00 PM (greater Phoenix area). The timing of these dust storm reports for this event is 

consistent with the issuance of a NWS Dust Storm Warning for the period of 4:00 PM to 6:00 PM, NWS 

preliminary local storm reports of dust storms, the observed increased PM10 concentrations in the area, 

increased wind speed, reduced visibility, and NWS station reports of blowing dust and dust storms. 

ADEQ stated that the evidence presented shows a clear causal relationship “between the windblown dust 

and the PM10 exceedances measured in the Phoenix PM10 nonattainment area on August 11, 2012.” 

ADEQ further stated that “the wind, visibility, PM10, and radar data shown in this section illustrate the 

spatial and temporal extent of the dust storm as it moved through Maricopa County. In addition, 
25

meteorological data tables found in Appendix A show that the sharp increase in PM10 concentrations 

coincided with gusty winds, low visibilities, and airport observer reports of blowing dust. The fact that 

PM10 concentrations in Pinal County peaked prior to PM10 concentrations peaking in Maricopa County 

illustrates that a vast majority of the dust that impacted the nonattainment area monitors originated 

outside of Maricopa County and was transported into the Phoenix PM10 nonattainment area.” 

The analysis in Sections II, III, and Appendix A specifically, the PM10 time series graphs, winds speed 

and direction measurements, GIS maps, time-lapse video evidence, NOAA dust storm observations, 

NWS advisories, NWS preliminary local storm reports , and NWS station reports of reduced visibility, 

blowing dust and dust storms, sufficiently establishes that there was a clear causal relationship between 

uncontrollable emissions generated from thunderstorm outflow winds and the exceedances measured at 

the West Chandler and Higley monitors. Furthermore, while exceedances occurring at only a few 

monitors in the network are inherently more complex,the GIS analysis (Figures 3-7 through 3-10 in 

ADEQ’s demonstration) indicates higher wind speeds were measured in the eastern portion of the 

nonattainment area influenced the spatial extent of PM10 throughout the Phoenix PM10 nonattainment 

area and was likely responsible for the exceedances at the West Chandler and Higley monitors.

Table 37: Documentation of CCR

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 11, 2012 Section III: p. 1-10, App. A, App. B, App. D Sufficient Yes

Affects Air Quality (AAQ)

ADEQ stated that based on the information presented in the demonstrations for both the CCR and HF 

requirements, “we can reasonably conclude that the event in question affected air quality.” ADEQ's 

summary regarding the CCR and HF requirements sufficiently establishes that the event affected air 

quality.

Table 38: Documentation of AAQ

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 11, 2012 Section VII: p. 1 Sufficient Yes

Natural Event

ADEQ stated that based on the documentation for both the nRCP and CCR requirements, “PM10 

exceedances in the Phoenix area on August 11, 2012, were shown to be caused by transport of PM10 into 

the Phoenix area from gusty winds associated with thunderstorm outflow ” and that “the event therefore 

qualifies as a natural event.” ADEQ's summary regarding the CCR and HF requirements sufficiently 

establishes that the event was a natural event.

Table 39: Documentation of Natural Event

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 11, 2012 Section VII: p. 1 Section VII: p. 1 Yes
26

No Exceedance or Violation But For the Event (NEBF)

ADEQ provided a summary of the analysis and information regarding the nRCP and CCR requirements 

and stated that “the weight of evidence presented in this submittal provides no alternative that could tie 

the exceedance of August 11, 2012, to any causal source other than PM10 transported by gusty winds 

due to thunderstorm outflow, confirming that there would have been no exceedance but for the presence 

of this uncontrollable natural event.” ADEQ’s summary regarding the nRCP and CCR requirements 

sufficiently establishes that the NEBF criterion has been met.

Table 40: Documentation of NEBF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 11, 2012 Section VI: p.1 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 41 outlines EPA’s 

evaluation of these requirements.

Table 41: Schedules and Procedural Criteria

Reference

Demonstration

Citation Criterion Met?

Did the State provide prompt public 

notification of the event?

40 CFR §50.14 (c)(1)(i) Section I: p.2 

App. D

Yes

Were flags and initial description placed on 

the data by July 1st of the following year?

40 CFR §50.14 (c)(2)(iii) Section I: p.2 Yes

Was the demonstration submitted within 3 

years of the end of the quarter in which the 

event occurred and 12 months prior to the 

date that any regulatory decision must be 

made by EPA?

40 CFR §50.14 (c)(3)(i) February 13, 

2013 letter11

Yes

Was the public comment process followed 

and documented?

40 CFR §50.14 (c)(3)(v) Section I: p.2-3

App. E12

Yes

 

11See letter from Eric Massey, Director, Air Quality Division, ADEQ, to Deborah Jordan, Director, U.S. EPA Region IX Air Division, 

dated February 13, 2013.

12A copy of the affidavit was not included in App. E, as stated in Section I of the final demonstration, but was submitted to EPA as part of 

the February 13, 2013 submission. 
27

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 

outlined in Table 34 on August 11, 2012. EPA has determined that the flagged exceedances at these 

locations on this day 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 eventIn addition to

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 event provide sufficient evidence to conclude that the event was not reasonably 

controllable or preventable. Furthermore, EPA has determined that there was a clear causal relationship 

between the event and the measured exceedances, there would have been no exceedances but for the 

event, and the measured exceedances were in excess of normal historical fluctuations.
28

Event Day: August 14, 2012

Table 42: EPA PM10 Exceedance Summary

Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (μg/m3

)

August 14, 2012 Durango Complex 04-013-9812-1 179

West 43rd 04-013-4009-1 254

Not Reasonably Controllable or Preventable (nRCP)

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 stated, “BACM on significant anthropogenic sources 

were in place and enforced during the events, and proactive 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 sustained wind speeds associated with these events were 

above 25 mph in multiple locations throughout the Phoenix PM10 nonattainment area and Pinal County. 

For example, maximum sustained wind speeds of 34 mph with gusts of 41 mph, and 31 mph with gusts 

of 40 mph were measured at Luke Air Force Base, and Casa Grande Municipal Airport, respectively. 

Sustained wind speeds of 23 mph with gusts of 33 mph were also measured at Phoenix Sky Harbor 

Airport. 

ADEQ further explained that “despite the deployment of comprehensive control measures and 

sophisticated response programs, high wind conditions associated with the thunderstorm outflow 

transported high concentrations of PM10 into, and also overwhelmed controls within, the Phoenix PM10

nonattainment area. Widespread sustained winds in excess of 20 mph with gusts over 40 mph were 

strong enough to overwhelm available efforts to limit PM10 concentrations during the event. The fact 

that these were natural events involving strong winds that transported PM10 emissions into and across 

Maricopa County, with a majority of the PM10 emissions recorded by Maricopa County area monitors 

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

exceedances of August 14, 2012, recorded within the Phoenix PM10 nonattainment area were not 

reasonably controllable or preventable.”

Section III of ADEQ’s documentation included a complex GIS analysis of the event that supports the 

PM10 transport described above. This analysis indicates 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 of the nonattainment area. Some of these source areas are located in 

Pinal County, portions of which were recently designated as a moderate nonattainment area (West Pinal) 

for the 1987 24-hour PM10 NAAQS (77 FR 32024, May 31, 2012). Currently, the state is undergoing the 

appropriate process of developing a state implementation plan (SIP), due January 2, 2014, that provides

for attainment of the PM10 standard as expeditiously as practicable but no later than the end of the sixth 

calendar year after redesignation. The SIP development process includes the requirement to identify and 

implement reasonably available control measures for the area. In addition to transport, information 

pertaining to the controls implemented within the nonattainment area, the spatial extent of elevated PM10
29

concentrations throughout the area, and the wind speeds associated with the event sufficiently

establishes that the event was not reasonably controllable or preventable.

Table 43: Documentation of nRCP

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 14, 2012 Section V: p. 1-7, Section III: p. 1-12, App A Sufficient Yes

Historical Fluctuations (HF)

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 and stated that these figures show that “the 

PM10 concentrations measured...on August 14, 2012, resulted in one of the highest 24-hr 

averages...measured over the five-year period.” ADEQ's analysis sufficiently establishes that the 

24-hour PM10 concentrations measured on August 14, 2012, were in excess of normal historical 

fluctuations.

Table 44: Documentation of HF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 14, 2012 Section IV: p. 1-2, App. C Sufficient Yes

Clear Causal Relationship (CCR)

Section II of ADEQ’s demonstration included a conceptual model of the events, including a general 

overview of the geographic setting of the monitors, and climate information. The conceptual model also 

included a discussion of the event that occurred on August 14, 2012, and a general map of the region of 

the thunderstorm development in relation to the Phoenix PM10 nonattainment area. 

Section III of the demonstration included satellite imagery visibility photos, time-lapse video evidence, 

the raw data tables for numerous NWS stations in Maricopa and Pinal Counties, and a number of time 

series graphs that include hourly PM10 data from Pinal and Maricopa Counties, wind speed, and 

visibility from Phoenix Sky Harbor Airport. ADEQ also included a detailed GIS analysis with PM10

concentrations, sustained wind speeds, wind gusts, wind direction, base velocity radar, and visibility to 

track the transport of PM10 throughout the region. These data show the spatial and temporal 

representation of the event as it moves throughout Maricopa and Pinal Counties. ADEQ also provided a 

discussion for every map that described the conditions at that time. Time-lapse videos of the event can 

be found at the following locations:

 South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_08142012.mp4

The timing of the August 14, 2012, event is consistent with the issuance of a NWS Blowing Dust 

Advisory for the period of 8:00 PM to 11:00 PM, the observed increased PM10 concentrations in the 

area, increased wind speed, reduced visibility, and NWS station reports of thunderstorms, blowing dust, 

haze, and dust storms. 

ADEQ stated that the evidence presented shows a clear causal relationship “between the windblown dust 

and the PM10 exceedances measured in the Phoenix PM10 nonattainment area on August 14, 2012.” 

ADEQ further stated that “The wind, visibility, PM10, and radar data shown in this section illustrate the 

spatial and temporal extent of the dust storm as it moved through Maricopa County. In addition, 
30

meteorological data tables found in Appendix A show that the sharp increase in PM10 concentrations 

coincided with the gusty winds, low visibilities, and airport reports of blowing dust. The fact that PM10

concentrations in Pinal County spiked before PM10 concentrations increased in Maricopa County shows 

that a vast majority of the dust that impacted the nonattainment area monitors originated outside of 

Maricopa County and was transported into the Phoenix PM10 nonattainment area.”

The analysis in Sections II, III, and Appendix A specifically, the PM10 time series graphs, winds speed 

and direction measurements, GIS maps, time-lapse video evidence, NWS advisories, and NWS station 

reports of reduced visibility, blowing dust and dust storms, sufficiently establishes that there was a clear 

causal relationship between uncontrollable emissions generated from thunderstorm outflow winds and 

the exceedances measured at the West 43rd Avenue and Durango Complex monitors. Furthermore, while 

exceedances occurring at only a few monitors in the network are inherently more complex, the GIS 

analysis (Figures 3-7 through 3-10 in ADEQ’s demonstration) indicates higher wind speeds were 

measured in the western portion of the nonattainment area and variation in wind direction at nearby 

monitors influenced the spatial extent of PM10 throughout the Phoenix PM10 nonattainment area and was 

likely responsible for the exceedances at the West 43rd Avenue and Durango Complex monitors.

Table 45: Documentation of CCR

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 14, 2012 Section III: p. 1-12, App. A, App. B, App. D Sufficient Yes

Affects Air Quality (AAQ)

ADEQ stated that based on the information presented in the demonstrations for both the CCR and HF 

requirements, “we can reasonably conclude that the event in question affected air quality.” ADEQ's 

summary regarding the CCR and HF requirements sufficiently establishes that the event affected air 

quality.

Table 46: Documentation of AAQ

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 14, 2012 Section VII: p. 1 Sufficient Yes

Natural Event

ADEQ stated that based on the documentation for both the nRCP and CCR requirements, “PM10 

exceedances in the Phoenix area on August 14, 2012, were shown to be caused by transport of PM10 into 

the Phoenix area from gusty winds associated with thunderstorm outflow ” and that “the event therefore 

qualifies as a natural event.” ADEQ's summary regarding the CCR and HF requirements sufficiently 

establishes that the event was a natural event.

Table 47: Documentation of Natural Event

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 14, 2012 Section VII: p. 1 Sufficient Yes
31

No Exceedance or Violation But For the Event (NEBF)

ADEQ provided a summary of the analysis and information regarding the nRCP and CCR requirements 

and stated that “the weight of evidence presented in this submittal provides no alternative that could tie 

the exceedance of August 14, 2012, to any causal source other than PM10 transported by gusty winds 

due to thunderstorm outflow, confirming that there would have been no exceedance but for the presence 

of this uncontrollable natural event.” ADEQ’s summary regarding the nRCP and CCR requirements 

sufficiently establishes that the NEBF criterion has been met.

Table 48: Documentation of NEBF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

August 14, 2012 Section VI: p. 1 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 49 outlines EPA’s 

evaluation of these requirements.

Table 49: Schedules and Procedural Criteria

Reference

Demonstration

Citation Criterion Met?

Did the State provide prompt public 

notification of the event?

40 CFR §50.14 (c)(1)(i) Section I: p. 1

App. D

Yes

Were flags and initial description placed on 

the data by July 1st of the following year?

40 CFR §50.14 (c)(2)(iii) Section I: p. 2 Yes

Was the demonstration submitted within 3 

years of the end of the quarter in which the 

event occurred and 12 months prior to the 

date that any regulatory decision must be 

made by EPA?

40 CFR §50.14 (c)(3)(i) February 13, 

2013 letter13

Yes

Was the public comment process followed 

and documented?

40 CFR §50.14 (c)(3)(v) Section I: p. 2-3

App. E14

Yes

 

13See letter from Eric Massey, Director, Air Quality Division, ADEQ, to Deborah Jordan, Director, U.S. EPA Region IX Air Division, 

dated February 13, 2013.

14A copy of the affidavit was not included in App. E, as stated in Section I of the final demonstration, but was submitted to EPA as part of 

the February 13, 2013 submission. 
32

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 

outlined in Table 42 on August 14, 2012. EPA has determined that the flagged exceedances at these 

locations on this day 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 eventIn addition to

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 event provide sufficient evidence to conclude that the event was not reasonably 

controllable or preventable. Furthermore, EPA has determined that there was a clear causal relationship 

between the event and the measured exceedances, there would have been no exceedances but for the 

event, and the measured exceedances were in excess of normal historical fluctuations.
33

Event Day: September 6, 2012

Table 50: EPA PM10 Exceedance Summary

Exceedance Date Monitor/Site Name AQS ID 24-hour Avg. (μg/m3

)

September 6, 2012 West Chandler 04-013-4004-1 164

Not Reasonably Controllable or Preventable (nRCP)

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 stated, “BACM on significant anthropogenic sources 

were in place and enforced during the events, and proactive 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 sustained wind speeds associated with these events were 

above 20 mph in multiple locations nearby and upwind of the exceeding monitoring stations throughout 

the Phoenix PM10 nonattainment area and Pinal County. For example, maximum sustained wind speeds 

of 23 mph with gusts of 40 mph, 24 mph with gusts of 29, and 24 mph with gusts of 33 mph were 

measured at Chandler Municipal Airport, Phoenix Sky Harbor Airport, and Casa Grande Municipal 

Airport, respectively. 

ADEQ further explained that “despite the deployment of comprehensive control measures and 

sophisticated response programs, high wind conditions associated with the thunderstorm outflow 

transported high concentrations of PM10 into, and also overwhelmed controls within, the Phoenix PM10

nonattainment area. Widespread wind gusts over 30 mph were strong enough to overwhelm available 

efforts to limit PM10 concentrations during the event. The fact that these were natural events involving 

strong winds that transported PM10 emissions into and across Maricopa County, with a majority of the 

PM10 emissions recorded by Maricopa County area monitors coming from sources outside of the 

Phoenix PM10 nonattainment area, provides strong evidence that the exceedances of September 6, 2012, 

recorded within the Phoenix PM10 nonattainment area were not reasonably controllable or preventable.”

Section III of ADEQ’s documentation included a complex GIS analysis of the event that supports the 

PM10 transport described above. This analysis indicates 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 of the nonattainment area. Some of these source areas are located in 

Pinal County, portions of which were recently designated as a moderate nonattainment area (West Pinal) 

for the 1987 24-hour PM10 NAAQS (77 FR 32024, May 31, 2012). Currently, the state is undergoing the 

appropriate process of developing a state implementation plan (SIP), due January 2, 2014, that provides 

for attainment of the PM10 standard as expeditiously as practicable but no later than the end of the sixth 

calendar year after redesignation. The SIP development process includes the requirement to identify and 

implement reasonably available control measures for the area. In addition to transport, information 

pertaining to the controls implemented within the nonattainment area, the spatial extent of elevated PM10

concentrations throughout the area, and the wind speeds associated with the event sufficiently 

establishes that the event was not reasonably controllable or preventable.
34

Table 51: Documentation of nRCP

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 6, 2012 Section V: p. 1-8, Section III: p. 1-10, App. A Sufficient Yes

Historical Fluctuations (HF)

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 and stated that these figures show that “the 

PM10 concentrations measured at the West Chandler monitor on September 6, 2012, resulted in some of 

the highest 24-hr averages...measured over the five-year period.” ADEQ's analysis sufficiently 

establishes that the 24-hour PM10 concentrations measured on September 6, 2012, were in excess of 

normal historical fluctuations.

Table 52: Documentation of HF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 6, 2012 Section IV: p. 1-2 Sufficient Yes

Clear Causal Relationship (CCR)

Section II of ADEQ’s demonstration included a conceptual model of the events, including a general 

overview of the geographic setting of the monitors, and climate information. The conceptual model also 

included a discussion of the event that occurred on September 6, 2012.

Section III and Appendix A of the demonstration included satellite imagery visibility photos, time-lapse 

video evidence, the raw data tables for numerous NWS stations in Maricopa County, and a number of 

time series graphs that include hourly PM10 data from Pinal and Maricopa Counties, wind speed, and 

visibility from Chandler Airport. ADEQ also included a detailed GIS analysis with PM10 concentrations, 

sustained wind speeds, wind gusts, wind direction, base velocity radar, and visibility to track the 

transport of PM10 throughout the region. These data show the spatial and temporal representation of the 

event as it moves throughout Maricopa and Pinal Counties. ADEQ also provided a discussion for every 

map that described the conditions at that time. Time-lapse videos of the event can be found at the 

following locations:

 South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_09062012.mp4

 Superstition Mountains: www.phoenixvis.net/videos/mpeg4/SUPM_09062012.mp4

While not included in the demonstration, it is important to note that NOAA’s National Climatic Data 

Center Storm events database included dust storm observations on September 6, 2012, at 4:15 PM 

(central deserts) and 4:50 PM (greater Phoenix area). The timing of these dust storm reports for this 

event is consistent with the issuance of a NWS Dust Storm Warning for the period of 4:16 PM to 7:00 

PM, NWS Blowing Dust Advisory for the period of 4:41 PM to 5:00 PM, NWS Severe Thunderstorm 

Warning for the period of 4:19 PM to 5:00 PM, NWS preliminary local storm reports of dust storms, the 

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

station reports of thunderstorms, blowing dust, haze, and dust storms. 

ADEQ stated that the evidence presented shows a clear causal relationship “between the windblown dust 

and the PM10 exceedances measured in the Phoenix PM10 nonattainment area on September 6, 2012.” 

ADEQ further stated that “The wind, visibility, PM10, and radar data shown in this section illustrate the 
35

spatial and temporal extent of the dust storm as it moved through Maricopa County. In addition, 

meteorological data tables found in Appendix A show that the sharp increase in PM10 concentrations 

coincided with the gusty winds, low visibilities, and airport observer reports of blowing dust. The fact 

that PM10 concentrations in Pinal County spiked prior to PM10 concentrations increasing in Maricopa 

County illustrate that a vast majority of the dust that impacted the nonattainment area monitors 

originated outside of Maricopa County and was transported into the Phoenix PM10 nonattainment area.” 

The analysis in Sections II, III, and Appendix A specifically, the PM10 time series graphs, winds speed 

and direction measurements, GIS maps, time-lapse video evidence, NOAA dust storm observations, 

NWS advisories, NWS preliminary local storm reports, and NWS station reports of reduced visibility, 

blowing dust and dust storms, sufficiently establishes that there was a clear causal relationship between 

uncontrollable emissions generated from thunderstorm outflow winds and the exceedance measured at 

the West Chandler monitor. Furthermore, while exceedances occurring at only one monitor in the 

network are inherently more complex, the GIS analysis (Figures 3-6 through 3-9 in ADEQ’s 

demonstration) indicates that a shift in wind direction from the southwest to west influenced the spatial 

extent of PM10 throughout the Phoenix PM10 nonattainment area and was likely responsible for the 

isolated exceedance at the West Chandler monitor.

Table 53: Documentation of CCR

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 6, 2012 Section III: p. 1-10, App. A, App. B, App. C Sufficient Yes

Affects Air Quality (AAQ)

ADEQ stated that based on the information presented in the demonstrations for both the CCR and HF 

requirements, “we can reasonably conclude that the event in question affected air quality.” ADEQ's 

summary regarding the CCR and HF requirements sufficiently establishes that the event affected air 

quality.

Table 54: Documentation of AAQ

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 6, 2012 Section VII: p. 1 Sufficient Yes

Natural Event

ADEQ stated that based on the documentation for both the nRCP and CCR requirements, “ PM10 

exceedances in the Phoenix area on September 6, 2012, were shown to be caused by transport of PM10

into the Phoenix area from gusty winds associated with thunderstorm outflow” and that “the event 

therefore qualifies as a natural event.” ADEQ's summary regarding the CCR and HF requirements 

sufficiently establishes that the event was a natural event.

Table 55: Documentation of Natural Event

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 6, 2012 Section VII: p. 1 Sufficient Yes
36

No Exceedance or Violation But For the Event (NEBF)

ADEQ provided a summary of the analysis and information regarding the nRCP and CCR requirements 

and stated that “the weight of evidence presented in this submittal provides no alternative that could tie 

the exceedance of September 6, 2012, to any causal source other than PM10 transported by gusty winds 

due to thunderstorm outflow, confirming that there would have been no exceedance but for the presence 

of this uncontrollable natural event.” ADEQ’s summary regarding the nRCP and CCR requirements 

sufficiently establishes that the NEBF criterion has been met.

Table 56: Documentation of NEBF

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met?

September 6, 2012 Section VI: p. 1 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 49 outlines EPA’s 

evaluation of these requirements.

Table 57: Schedules and Procedural Criteria

Reference

Demonstration

Citation Criterion Met?

Did the State provide prompt public 

notification of the event?

40 CFR §50.14 (c)(1)(i) Section I: p. 1

App. C

Yes

Were flags and initial description placed on 

the data by July 1st of the following year?

40 CFR §50.14 (c)(2)(iii) Section I: p. 1 Yes

Was the demonstration submitted within 3 

years of the end of the quarter in which the 

event occurred and 12 months prior to the 

date that any regulatory decision must be 

made by EPA?

40 CFR §50.14 (c)(3)(i) February 13, 

2013 letter15

Yes

Was the public comment process followed 

and documented?

40 CFR §50.14 (c)(3)(v) Section I: p. 1

App. D16

Yes

 

15See letter from Eric Massey, Director, Air Quality Division, ADEQ, to Deborah Jordan, Director, U.S. EPA Region IX Air Division, 

dated February 13, 2013.

16A copy of the affidavit was not included in App. D, as stated in Section I of the final demonstration, but was submitted to EPA as part of 

the February 13, 2013 submission. 
37

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 an exceedance of the 24-hour PM10 NAAQS at the location 

outlined in Table 50 on September 6, 2012. EPA has determined that the flagged exceedance at this 

location on this day meets the definition of an exceptional event: the exceedances affected air quality, 

was not reasonably controllable or preventable, and met the definition of a natural event. In addition to

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 low pressure system provide sufficient evidence to conclude that the event was not 

reasonably controllable or preventable. Furthermore, EPA has determined that there was a clear causal 

relationship between the event and the measured exceedance, there would have been no exceedance but 

for the event, and the measured exceedance was in excess of normal historical fluctuations.
38

CONCLUSION

EPA finds that the weight of evidence is sufficient for concurrence on the flagging of the data for the 

monitors identified in Table 1 on September 11 & 12, 2011, June 16, June 27, July 11, August 11, 

August 14, and September 6, 2012. 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 any such determinations.