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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY ; 1\IfL

REGION IX

75 Hawthorne Street

San Francisco, CA 94105.3901

MAY 06 2013 REGIONAL ADMINISTRATOR

Mr. Eric Massey, Director

Air Quality Division

Arizona Department of Environmental Quality

1110 W. Washington Street

Phoenix, Arizona 85007

Dear Mr. Massey:

This letter responds to Arizona Department of Environmental Quality’s (ADEQ) January 28, 2013

submittal regarding 65 exceeedances ofthe 24-hour PM10 standard that occurred at several monitoring

stations within the Phoenix PM10 nonattainment area on the following days: February 19, July 18,

August 3, August 18, August 25-28, September 2, October 4, November 4, 2011 and February 27,

2012.1

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 ofthe evidence that the exceedances were caused by high

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

40 CFR §50.14 to EPA’s satisfaction. In addition, ADEQ has met the schedule and procedural

requirements in section 50.14(c) with respect to the same data. A more detailed assessment of ADEQ’s

demonstration is enclosed. My 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 exceedances when

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

specifically indicates that they should be included.

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

background concentrations for New Source Review air quality analyses. 2

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

background concentrations for transportation conformity hot spot analyses. 3

‘ADEQ also submitted an exceptional events demonstration for exceedances ofthe 24-hour PM10 standard that occurred on January 21-22,

2012 within the Phoenix PM10 nonattainment area and an exceedance measured in Yuma, AZ on July 3, 2011. At this time, EPA is not

acting on the January 21-22, 2012 or the July 3, 2011 Yuma, AZ demonstrations. 2 If we are the permitting authority, we will propose permits on this basis. If we are commenting on another permitting authority’s

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

Applicable only to PM10 and PM2.5

Printed on Recycled Paper
• The data will continue to be publically 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. 4

In addition, EPA will rely on calculated values that exclude this data in proposed regulatory actions,

such as a proposed designation, classification, attainment demonstration, or finding as to whether the

Phoenix PM10 nonattainment area has met the PM10 NAAQS. These regulatory actions require EPA to

provide an opportunity for public comment prior to taking a final Agency action. If EPA is pursuing one

ofthese 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 ofthe area. Final actions will take place only after EPA completes notice and comment

rulemaking on those determinations. As an additional clarification, the determinations conveyed in this

letter are applicable only to determinations incorporating the submitted data relative to the PM10

NAAQS.

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

Air Division at (415) 947-8715.

Sincerely,

Enclosure

cc: Theresa Rigney, ADEQ

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

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

been adequately demonstrated. 

Natural Event

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

dust is entirely from natural sources or where all significant anthropogenic sources of windblown dust 

have been reasonably controlled.3 This typically 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 January 28, 2013, Arizona Department of Environmental Quality (ADEQ) submitted nine 

exceptional events demonstrations for 65 exceedances of the 24-hour PM10 standard that occurred at 

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

19, July 18, August 3, August 18, August 25-28, September 2, October 4, November 4, 2011 and 

February 27, 2012. Table 1 summarizes these exceedances. 

 

3The EPA 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, the 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 states that the exceedances measured on July 18, August 3, August 18, August 25-28, 

and September 2, 2011 were associated with “monsoonal thunderstorm activity” and “thunderstormdriven high winds,” while the exceedances measured on February 19, October 4, November 4, 2011, and 

February 27, 2012 were associated with the passage of “low pressure systems” or “strong Pacific cold 

fronts.” ADEQ provided a comprehensive description and discussion of each of these events in the 

respective demonstrations.4

Table 1: EPA PM10 Exceedance Summary 

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

) 

February 19, 2011 West Chandler 04-013-4004-1 168 

July 18, 2011 Buckeye 04-013-4011-1 196 

Central Phoenix 04-013-3002-4 210 

Durango Complex 04-013-9812-1 267 

Dysart 04-013-4010-1 163 

Greenwood 04-013-3010-1 208 

South Phoenix 04-013-4003-1 303 

West 43rd 04-013-4009-1 159 

West Phoenix 04-013-0019-1 244 

August 3, 2011 West Chandler 04-013-4004-1 249 

August 18, 2011 Buckeye 04-013-4011-1 296 

Central Phoenix 04-013-3002-4 232 

South Phoenix 04-013-4003-1 179 

West Chandler 04-013-4004-1 186 

August 25, 2011 Buckeye 04-013-4011-1 235 

Central Phoenix 04-013-3002-4 308 

Durango Complex 04-013-9812-1 436 

Dysart 04-013-4010-1 273 

Glendale 04-013-2001-1 240 

Buckeye 04-013-4011-1 388 

JLG Supersite 04-013-9997-3 227 

JLG Supersite 04-013-9997-4 228 

South Phoenix 04-013-4003-1 308 

West 43rd 04-013-4009-1 369 

West Chandler 04-013-4004-1 278 

West Phoenix 04-013-0019-1 212 

Zuni Hills 04-013-4016-1 212 

August 26, 2011 Apache Junction 04-021-3002-1 169 

August 27, 2011 Buckeye 04-013-4011-1 225 

Central Phoenix 04-013-3002-4 233 

Durango Complex 04-013-9812-1 261 

Glendale 04-013-2001-1 219 

Greenwood 04-013-3010-1 207 

South Phoenix 04-013-4003-1 301 

West 43rd 04-013-4009-1 292 

West Chandler 04-013-4004-1 228 

West Phoenix 04-013-0019-1 164 

August 28, 2011 Apache Junction 04-021-3002-1 283 

Higley 04-013-4006-1 175 

 

4ADEQ also submitted an exceptional events demonstration for exceedances of the 24-hour PM10 standard that occurred on January 21-22, 

2012 within the Phoenix PM10 nonattainment area and an exceedance measured in Yuma, AZ on July 3, 2011. At this time, EPA is not 

acting on the January 21-22, 2012 or the July 3, 2011 Yuma, AZ demonstrations. 
4 

Table 1: EPA PM10 Exceedance Summary 

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

) 

September 2, 2011 Apache Junction 04-021-3002-1 217 

Buckeye 04-013-4011-1 169 

Central Phoenix 04-013-3002-4 308 

Durango Complex 04-013-9812-1 225 

Greenwood 04-013-3010-1 198 

Higley 04-013-4006-1 213 

JLG Supersite 04-013-9997-4 208 

South Phoenix 04-013-4003-1 339 

West 43rd 04-013-4009-1 219 

West Chandler 04-013-4004-1 387 

October 4, 2011 Higley 04-013-4006-1 158 

West Chandler 04-013-4004-1 251 

November 4, 2011 Apache Junction 04-021-3002-1 225 

Buckeye 04-013-4011-1 284 

Central Phoenix 04-013-3002-4 223 

Durango Complex 04-013-9812-1 251 

Dysart 04-013-4010-1 224 

Glendale 04-013-2001-1 229 

Greenwood 04-013-3010-1 231 

Higley 04-013-4006-1 258 

JLG Supersite 04-013-9997-3 200 

JLG Supersite 04-013-9997-4 199 

North Phoenix 04-013-1004-1 186 

North Phoenix 04-013-1004-2 186 

South Phoenix 04-013-4003-1 231 

West 43rd 04-013-4009-1 242 

West Chandler 04-013-4004-1 670 

West Phoenix 04-013-0019-1 279 

Zuni Hills 04-013-4016-1 258 

February 27, 2012 West 43rd 04-013-4009-1 167 
5 

FEBRUARY 19, 2011 

Table 2: EPA PM10 Exceedance Summary 

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

) 

February 19, 2011 West Chandler 04-013-4004-1 168 

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 33 mph with gusts of 52 mph, 33 mph with gusts of 43 

mph, and 31 mph with gusts of 44 mph were measured at Chandler Municipal Airport, Williams 

Gateway 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 pre-frontal storm winds 

generated and transported high concentrations of PM10 emissions into, and also overwhelmed controls 

within, the nonattainment area. Sustained winds over 30 mph and gusts over 50 mph easily 

overwhelmed all available efforts to limit PM10 concentrations from the event. The fact that this was a 

natural event involving pre-frontal storm winds that generated and transported PM10 emissions in 

Maricopa County provided strong evidence that the exceedance on February 19, 2011 recorded at the 

West Chandler monitor was not reasonably controllable or preventable.” 

Section V 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 and southwest of the nonattainment 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? 

February 19, 2011 Section IV: p. 17-23, Section V: p. 24-41 Sufficient Yes 
6 

Historical Fluctuations (HF)

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

24-hour averages and PM10 daily maximum hourly averages in Figures 3-1 and 3-2 of the 

demonstration, respectively. ADEQ also stated that these figures “indicate that the PM10 concentrations 

seen at the West Chandler monitor on February 19, 2011 were in excess of normal historical 

fluctuations.” ADEQ's analysis sufficiently establishes that the 24-hour PM10 concentrations measured 

on February 19, 2011 were in excess of normal historical fluctuations. 

Table 4: Documentation of HF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 19, 2011 Section III: p. 15-16 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, climate information, and surface 

weather maps for the event for Phoenix area. The conceptual model also included a very detailed 

discussion of the event that occurred on February 19, 2011 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 a detailed and extensive GIS analysis and a number of visibility 

photos that show the spatial and temporal representation of the event as it moves throughout Maricopa 

and Pinal Counties. The analysis included PM10 concentrations, sustained wind speeds, wind gusts, wind 

direction, precipitation, 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. The timing of the event is consistent with the issuance of a National Weather Service (NWS) 

Wind Advisory for the period of 11:00 AM to 2:00 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, while direct links were not included in the final documentation, timelapse videos of the event can be found at the following locations: 

· South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_02192011.mp4

· Superstition Mountains: http://www.phoenixvis.net/videos/mpeg4/SUPM_02192011.mp4

· Camelback Mountains: http://www.phoenixvis.net/videos/mpeg4/CAME_02192011.mp4

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

generated and transported by the pre-frontal storm winds and the exceedance at the West Chandler 

monitor.” ADEQ further stated that “the particular wind magnitudes and wind direction, the proximity 

of the exceeding monitor to open and desert areas of Pinal County, and the delay in the storm 

precipitation reaching the areas around the exceeding monitor provide solid evidence as to why only one 

monitor within the Maricopa County nonattainment area recorded an exceedance.” 

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

measurements, GIS maps, time-lapse video evidence, 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 pre-frontal storm winds and the 

exceedance measured at the West Chandler monitor. Furthermore, while exceedances occurring at only 
7 

one monitor in the network are inherently more complex, the GIS analysis (Figures 5-1 through 5-10) 

indicates that a shift in wind direction from the southwest to west, the spatial extent of precipitation 

throughout the Phoenix PM10 nonattainment area, and stronger wind speeds in the eastern portion of the 

nonattainment area are likely responsible for the isolated exceedance at the West Chandler monitoring 

station. 

Table 5: Documentation of CCR 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 19, 2011 Section II: p.4-14, Section V: p. 24-41, App. B Sufficient Yes

Affects Air Quality (AAQ)

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

requirements, “it is reasonable to 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 6: Documentation of AAQ 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 19, 2011 Section VII: p. 44 Sufficient Yes

Natural Event

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

shown to cause this exceedance was emissions of PM10 caused by pre-frontal storm winds on February 

19, 2011” 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 7: Documentation of Natural Event 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 19, 2011 Section VII: p. 44 Sufficient Yes

No Exceedance or Violation But For the Event (NEBF)

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

requirements and also included a time series graph that included hourly PM10, hourly, wind speeds, and 

wind gusts showing that PM10 concentrations before the event were below the 24-hour PM10 NAAQS 

ADEQ further stated that “the body of evidence presented in this submittal confirms that the exceedance 

on February 19, 2011 was a natural event and that there would have been no exceedance but for the 

presence of the uncontrollable windblown dust from the pre-frontal storm winds.” ADEQ’s summary 

regarding the nRCP and CCR requirements sufficiently establishes that the NEBF criterion has been 

met. 

Table 8: Documentation of NEBF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 19, 2011 Section VI: p. 42-43 Sufficient Yes
8 

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. A 

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) January 28, 

2013 letter5 

Yes

Was the public comment process followed 

and documented? 

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

App. C 

Yes

Conclusion

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

by pre-frontal storm 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 February 19, 2011. EPA has determined that the flagged exceedances at this location on this 

day meet the definition of an exceptional event: the exceedance affected air quality, was not reasonably 

controllable or preventable, and meets 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 transported PM10 from sources outside of the nonattainment area and subsequently overwhelmed 

reasonable controls within the Phoenix PM10 nonattainment area. Also, regardless of transport into the 

area, information pertaining to the controls implemented within the nonattainment area, the spatial 

extent of elevated PM10 concentrations measured in the area, and the wind speeds associated with the 

pre-frontal storm provide sufficient evidence to conclude that the event was not reasonably controllable 

or preventable. Furthermore, EPA has determined that there is 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 is in excess of normal historical fluctuations. 

 

5

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

January 28, 2013. 
9 

JULY 18, 2011 

Table 10: EPA PM10 Exceedance Summary 

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

) 

July 18, 2011 Buckeye 04-013-4011-1 196 

Central Phoenix 04-013-3002-4 210 

Durango Complex 04-013-9812-1 267 

Dysart 04-013-4010-1 163 

Greenwood 04-013-3010-1 208 

South Phoenix 04-013-4003-1 303 

West 43rd 04-013-4009-1 159 

West Phoenix 04-013-0019-1 244 

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. For example, maximum sustained wind speeds of 25 mph with gusts of 37 mph and 29 

mph with gusts of 38 mph were measured at Phoenix Sky Harbor International Airport and Casa Grande 

Municipal Airport, respectively. While not included in the final documentation, it is important to note 

that sustained wind speeds greater than 25 mph were also measured at other locations in the Phoenix 

PM10 nonattainment area, specifically at Williams Gateway Airport, Chandler Municipal Airport, 

Glendale Municipal Airport, and Luke Air Force Base Airport. 

ADEQ further explains 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 typically ranging from 20-30 

mph, and even greater 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 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, provided strong evidence that the events and exceedances of July 18, 

2011, 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 
10 

were affected by PM10 transport from outside the nonattainment area, with the main source areas located 

to the south and southeast of the nonattainment area. In addition to transport, 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 is not reasonably controllable or preventable. 

Table 11: Documentation of nRCP 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

July 18, 2011 Section IV: p.18-24, Section V: p.25-33 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. ADEQ also stated that these figures show 

that “the event that occurred on July 18, 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 July 18, 2011 were in excess of normal historical fluctuations. 

Table 12: Documentation of HF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

July 18, 2011 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 for 

Phoenix area. The conceptual model also included a very detailed discussion of the event that occurred 

on July 18, 2011 and a time series graph for the event that included hourly PM10 concentrations from 

monitors in the Phoenix PM10 nonattainment area. 

Section V of the demonstration included satellite imagery, a time series graph for the event that included 

hourly PM10 concentrations from monitors in the Phoenix PM10 nonattainment area, visibility, and 

sustained wind speed from Phoenix Sky Harbor International Airport, and a time series graph that 

included PM10 concentrations from monitors in Pinal County, visibility, and sustained wind speed from 

Casa Grande Municipal Airport. These data show the spatial and temporal representation of the event as 

it moves throughout Maricopa and Pinal Counties. Also, a time-lapse video of the event was included 

and can be found at the following location: 

· South Mountain: www.phoenixvis.net/videos/mpeg4/SOMT_07182011.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 July 18, 2011 at 3:00 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 Severe Thunderstorm Warning for the period of 3:00 PM to 6:45 

PM, NWS Significant Weather Advisory for the period of 2:38 PM to 7:15 PM, and a NWS Dust Storm 

Warning for the period of 2:30 PM to 7:00 PM, the observed increased PM10 concentrations in the area, 

increased wind speed, reduced visibility, and NWS station reports of thunderstorms (TS), blowing dust 

(BLDU), haze (HZ), and dust storms (DS). 
11 

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 drawing from emission sources outside of 

Maricopa County and were being transported into the Phoenix area.” 

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

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

NWS station reports of reduced visibility, thunderstorms, 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? 

July 18, 2011 Section II: p 4-13, Section V: p. 25-33, 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 14: Documentation of AAQ 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

July 18, 2011 Section VII: p.35 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 boundaries on July 18, 2011” 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 15: Documentation of Natural Event 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

July 18, 2011 Section VII: p.35 Sufficient Yes
12 

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 provided no alternative that could tie 

the exceedances of July 18, 2011, 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.” Also, PM10 concentrations before the event were 

below the 24-hour PM10 NAAQS, providing further support of ADEQ’s conclusion. 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? 

July 18, 2011 Section VI: p. 34 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 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) January 28, 

2013 letter6 

Yes

Was the public comment process followed 

and documented? 

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

App. D 

Yes

 

6

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

January 28, 2013. 
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 July 18, 2011. 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 transported PM10 from sources outside of the nonattainment area and subsequently 

overwhelmed reasonable controls within the Phoenix PM10 nonattainment area. Also, regardless of 

transport into the area, information pertaining to the controls implemented within the nonattainment 

area, the spatial extent of elevated PM10 concentrations measured in the area, and the wind speeds 

associated with the thunderstorm outflows provide sufficient evidence to conclude that the event was not 

reasonably controllable or preventable. Furthermore, EPA has determined that there is 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 are in excess of normal historical fluctuations. 
14 

August 3, 2011 

Table 18: EPA PM10 Exceedance Summary 

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

) 

August 3, 2011 West Chandler 04-013-4004-1 249 

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. For example, maximum sustained wind speeds of 22 mph with maximum gusts of 25 

mph were measured at Williams Gateway Airport, while sustained wind speeds of 20 mph were 

measured at other locations in Maricopa and Pinal Counties. For example, sustained wind speeds of 20 

mph with gusts of 32 mph, 20 mph with gusts of 26, and 20 mph with gusts of 29 mph were measured at 

the Pinal County Housing, Higley, and West Chandler monitoring stations, respectively. 

Due to the timing of the event, data from NWS stations that ADEQ typically uses in exceptional event 

demonstrations were not available. Some locations did not begin reporting meteorological measurements 

until after the event had occurred, which was approximately from 1:00 AM to 3:00 AM at the West 

Chandler monitor. For example, the Chandler Municipal Airport station began reporting meteorological 

measurement at 5:47 AM, and the Casa Grande Municipal Airport station began reporting measurements 

at 3:35 AM on August 3, 2011. Despite the lack of data from the early hours of the day, it is nevertheless 

plausible that higher wind speeds occurred in the source area where the thunderstorm outflows were 

strongest. ADEQ explains that “it is very likely the thunderstorm outflow generated dust storm 

developed in the area south of the PCH monitor [in Pinal County], but north of Pima County.” 

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

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

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

nonattainment area. Strong thunderstorm outflows with gusts over 30 mph, were enough to overwhelm 

all available efforts to limit PM10 concentrations from the events. The fact that this was a natural event 

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

from source regions outside of the nonattainment area provided strong evidence that the event and 

exceedance of August 3, 2011 recorded at the West Chandler monitor was not reasonably controllable or 

preventable.” 
15 

Section V 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 and southeast of the nonattainment 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, the timing of the event, 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? 

August 3, 2011 Section IV: p. 16-22, Section V: p. 23-41, App. B Sufficient Yes 

Historical Fluctuations (HF)

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

24-hour averages and PM10 daily maximum hourly averages in Figures 3-1 and 3-2 of the 

demonstration, respectively. ADEQ also stated that these figures “indicate that the PM10 concentrations 

seen at the West Chandler monitor on August 3, 2011 were in excess of normal historical fluctuations.” 

ADEQ's analysis sufficiently establishes that the 24-hour PM10 concentrations measured on August 3, 

2011 were in excess of normal historical fluctuations. 

Table 20: Documentation of HF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 3, 2011 Section III: p. 14-15 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 for 

Phoenix area. The conceptual model also included a very detailed discussion of the event that occurred 

on August 3, 2011 and a time series graph for the event that included hourly PM10 concentration. 

Section V of the demonstration included a detailed and extensive GIS analysis that shows the spatial and 

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

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

base velocity radar 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. The timing of the 

event is consistent with the issuance of a NWS Dust Storm Warning for the period of 1:00 AM to 2:00 

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

visibility, and NWS station reports of haze (HZ). While time-lapse videos of the event are not available, 

ADEQ included a number of visibility images looking northeast towards Camelback Mountain for 12:00 

AM, 3:00 AM and 6:00 AM, and stated that “these images provide additional evidence for a clear causal 

connection between the transported windblown dust from thunderstorm outflow winds with the high 

PM10 concentrations at monitors throughout the nonattainment area.” 
16 

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

between the emissions generated by uncontrollable natural events and the exceedances measured at the 

West Chandler monitor.” ADEQ further stated that “the particular wind magnitudes and wind direction, 

and the proximity of the exceeding monitor to open and desert areas of Pinal County provide solid 

evidence as to why only one monitor within the Maricopa County nonattainment area recorded an 

exceedance. It is clear from these data that thunderstorm outflow winds transported uncontrollable 

windblown PM10 emissions to the West Chandler monitor, demonstrating a clear causal connection 

between the event and the exceedance.” 

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

measurements, GIS maps, visibility images, NWS advisories, and NWS station reports of reduced 

visibility, and haze, 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 5-1 through 5-12) shows that the 

isolated spatial extent of the thunderstorm outflow and stronger wind speeds in the eastern portion of the 

nonattainment area are likely responsible for the isolated exceedance at the West Chandler monitoring 

station. 

Table 21: Documentation of CCR 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 3, 2011 Section V: p. 23-41, Section II: p. 4-13, App. B Sufficient Yes

Affects Air Quality (AAQ)

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

requirements, “it is reasonable to 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? 

August 3, 2011 Section VII: p. 44 Sufficient Yes

Natural Event

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

shown to cause this exceedance was emissions of PM10 driven by high winds caused by thunderstorm 

activity and related outflow boundaries on August 3, 2011” and that “the events therefore qualifies as 

natural event.” 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? 

August 3, 2011 Section VII: p. 44 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 also included a time series graph that included hourly PM10, hourly wind speeds, and wind gusts 

showing that PM10 concentrations before the event were below the 24-hour PM10 NAAQS. ADEQ 

further stated that “the body of evidence presented in this submittal confirms that the exceedance on 

August 3, 2011 was a natural event and that there would have been no exceedance but for the presence 

of the uncontrollable windblown dust from the thunderstorm outflow winds.” 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? 

August 3, 2011 Section VI: p. 42-43 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. A 

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) January 28, 

2013 letter7 

Yes

Was the public comment process followed 

and documented? 

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

App.C 

Yes

 

7

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

January 28, 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 August 3, 2011. EPA has determined that the flagged exceedance at this location 

on this day meets the definition of an exceptional event: the exceedance affected air quality, was not 

reasonably controllable or preventable, and meets 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 transported PM10 from sources outside of the nonattainment area and subsequently overwhelmed 

reasonable controls within the Phoenix PM10 nonattainment. Also, regardless of transport into the area, 

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

elevated PM10 concentrations measured in the area, the timing of the event, and the wind speeds 

associated with the thunderstorm outflows provide sufficient evidence to conclude that the event was not 

reasonably controllable or preventable. Furthermore, EPA has determined that there is 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 is in excess of normal historical fluctuations. 
19 

August 18, 2011 

Table 26: EPA PM10 Exceedance Summary 

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

) 

August 18, 2011 Buckeye 04-013-4011-1 296 

Central Phoenix 04-013-3002-4 232 

South Phoenix 04-013-4003-1 179 

West Chandler 04-013-4004-1 186 

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 25 mph. For example, maximum sustained wind speeds of 32 mph with gusts of 41 mph and 28 

mph with gusts of 39 mph were measured at Chandler Municipal Airport and Phoenix Sky Harbor 

International Airport, respectively. While not included in the final documentation, it is important to note 

that sustained wind speeds greater than 25 mph were also measured at other locations in the Phoenix 

PM10 nonattainment area and Pinal County, specifically at Williams Gateway Airport, Glendale 

Municipal Airport, Luke Air Force Base Airport, Gila Bend Air Field, and Casa Grande Municipal 

Airport. 

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

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

outflows brought high concentrations 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, provided strong evidence that the exceedances of August 18, 2011 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. 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. 
20 

Table 27: Documentation of nRCP 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 18, 2011 Section V: p. 1-7, Section III: p. 1-6, App. A, App. B 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. ADEQ also stated that these figures show 

that “the PM10 concentrations...on August 18, 2011 were among the highest 24-hr averages measured 

over the five-year period” and “the PM10 levels on August 18, 2011, were outside of normal historical 

fluctuations.” ADEQ's analysis sufficiently establishes that the 24-hour PM10 concentrations measured 

on August 18, 2011 were in excess of normal historical fluctuations 

Table 28: Documentation of HF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 18, 2011 Section IV: p. 1, App. C 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 for Phoenix area. 

The conceptual model also included a brief discussion of the event that occurred on August 18, 2011. 

Section III and Appendix A of the demonstration included satellite imagery, radar base velocity data for 

1806 LST, PM10 and meteorological data for the time period of 6:00 PM to 8:00 PM, and time series 

graphs that included hourly PM10 concentrations, hourly wind speed, and gusts. ADEQ also included a 

time series graph that shows hourly PM10 concentrations from Buckeye, Central Phoenix, South 

Phoenix, and West Chandler monitors, and visibility from Phoenix Sky Harbor International Airport. 

These data show the spatial and temporal representation of the event as it moves throughout Maricopa 

County. Also, time-lapse videos of the event were included in Appendix B of ADEQ’s demonstration 

and can be found at the following locations: 

· South Mountain: www.phoenixvis.net/videos/mpeg4/SOMT_08182011.mp4

· Superstition Mountains: www.phoenixvis.net/videos/mpeg4/SUPM_08182011.mp4

· Camelback Mountains: www.phoenixvis.net/videos/mpeg4/CAME_08182011.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 18, 2011 at 4:00 PM (central 

deserts), and at 5:00 PM hours (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 4:45 PM to 8:00 

PM and a NWS Severe Thunderstorm Warning for the period of 5:00 PM to 7:00 PM, observed 

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

station reports of thunderstorms (TS), blowing dust (BLDU), haze (HZ), and dust storms (DS). 

Again, while not included in the final documentation, it is important to note that hourly PM10

concentrations at the southern monitoring sites (Pinal County Housing, Stanfield, and Casa Grande) in 

Pinal County began also to dramatically increase at 5:00 PM, while PM10 concentrations at more 

northern monitoring sites (Cowtown, Combs School, and Maricopa) in the County began to increase at 
21 

6:00 PM. Sustained wind speeds above 25 mph that were associated with the increase in PM10 in Pinal 

County were measured at Casa Grande Municipal Airport at 4:55, 5:15, 6:15, 6:35, and 6:55 PM. These 

data indicate that PM10 was largely transported from outside of the nonattainment area from 

thunderstorm outflow winds. ADEQ stated that, “the information presented in this section demonstrates 

a clear causal relationship between the windblown dust and the PM10 exceedances measured at four 

Phoenix-area monitors on August 18, 2011.” 

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

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

observations, and NWS station reports of reduced visibility, thunderstorms, 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 exceedance measured at the monitors 

identified in Table 26 of this document. 

Table 29: Documentation of CCR 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 18, 2011 Section III: p. 1-6, Section V: p. 1-7, App. A, App. B Sufficient Yes

Affects Air Quality (AAQ)

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

requirements, “we can reasonably conclude 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? 

August 18, 2011 Section VII: p. 1 Sufficient Yes

Natural Event

ADEQ provided adequate documentation for both the nRCP and CCR requirements and generally stated 

that, “the PM10 exceedances on August 18, 2011, 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.” 

Table 31: Documentation of Natural Event 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 18, 2011 Section VII: p. 1 Sufficient Yes

No Exceedance or Violation But For the Event (NEBF)

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

demonstrate both the nRCP and CCR requirements have been met and stated that “the weight of 

evidence presented in this submittal provided no alternative that could tie the exceedances of August 18, 

2011, to any causal source except PM10 transported by thunderstorm outflow, confirming that there 

would have been no exceedances but for the presence of this uncontrollable natural event.” Also, PM10

concentrations before the event were below the 24-hour PM10 NAAQS, providing further support of 

ADEQ’s conclusion. 
22 

Table 32: Documentation of NEBF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 18, 2011 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 33 outlines EPA’s 

evaluation of these requirements. 

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. 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) January 28, 

2013 letter8 

Yes

Was the public comment process followed 

and documented? 

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

App. E 

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 August 18, 2011. 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 the event was not reasonably controllable and preventable due to high wind 

conditions that transported PM10 from sources outside of the nonattainment area and subsequently 

overwhelmed reasonable controls within the Phoenix PM10 nonattainment. Also, regardless of transport 

into the area, information pertaining to the controls implemented within the nonattainment area, the 

spatial extent of elevated PM10 concentrations measured in the area, and the wind speeds associated with 

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

controllable or preventable. Furthermore, EPA has determined that there is 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 are in excess of normal historical fluctuations. 

 

8

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

January 28, 2013. 
23 

AUGUST 25, 2011 – AUGUST 28, 2011 

Table 34: EPA PM10 Exceedance Summary 

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

) 

August 25, 2011 Buckeye 04-013-4011-1 235 

Central Phoenix 04-013-3002-4 308 

Durango Complex 04-013-9812-1 436 

Dysart 04-013-4010-1 273 

Glendale 04-013-2001-1 240 

Buckeye 04-013-4011-1 388 

JLG Supersite 04-013-9997-3 227 

JLG Supersite 04-013-9997-4 228 

South Phoenix 04-013-4003-1 308 

West 43rd 04-013-4009-1 369 

West Chandler 04-013-4004-1 278 

West Phoenix 04-013-0019-1 212 

Zuni Hills 04-013-4016-1 212 

August 26, 2011 Apache Junction 04-021-3002-1 169 

August 27, 2011 Buckeye 04-013-4011-1 225 

Central Phoenix 04-013-3002-4 233 

Durango Complex 04-013-9812-1 261 

Glendale 04-013-2001-1 219 

Greenwood 04-013-3010-1 207 

South Phoenix 04-013-4003-1 301 

West 43rd 04-013-4009-1 292 

West Chandler 04-013-4004-1 228 

West Phoenix 04-013-0019-1 164 

August 28, 2011 Apache Junction 04-021-3002-1 283 

Higley 04-013-4006-1 175 

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, including maximum sustained wind speeds of 26 mph with gusts of 33 mph at Phoenix 

Sky Harbor International Airport on August 25, 2011, 30 mph at Williams Gateway Airport on August 

26, 2011, and 29 mph at Luke Air Force Base Airport on August 27, 2011. Sustained wind speeds 

greater than 25 mph were also measured at other locations in the Phoenix PM10 nonattainment area 

throughout the three-day period. ADEQ also asserts that due to the timing of the August 27, 2011 late 

evening event, the conditions that led to nine exceedances in the Phoenix PM10 nonattainment area on 
24 

August 27, 2011, were similarly responsible for the exceedances measured at Higley and Apache 

Junction on August 28, 2011. 

ADEQ further explains 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. Numerous strong thunderstorm outflows with sustained winds typically ranging 

from 20-30 mph, and even greater 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 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, provided strong evidence that the events and 

exceedances of August 25-28, 2011, 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. For all events, the 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. In addition to transport, 

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

elevated PM10 concentrations throughout the area, the timing of the August 25, 2011 event, and the wind 

speeds associated with the event sufficiently establishes that these events were not reasonably 

controllable or preventable. 

Table 35: Documentation of nRCP 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 25, 2011 Section IV: p. 22-28, Section V: p. 29-37, p. 63, App. D Sufficient Yes 

August 26, 2011 Section IV: p. 22-28, Section V: p. 38-46, p. 63, App. D Sufficient Yes 

August 27, 2011 Section IV: p. 22-28, Section V: p. 47-62, p. 63, App. D Sufficient Yes 

August 28, 2011 Section IV: p. 22-28, Section V: p. 47-62, p. 63, App. D 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. ADEQ also stated that these figures show 

that for a number of monitors “the highest 24-hour averaged PM10 concentrations measured in the last 

five years occurred on August 25” and that “the other events in the August 25th-28th period were 

generally among the top 12 events in the last five years.” ADEQ's analysis sufficiently establishes that 

the 24-hour PM10 concentrations measured on August 25 – 28, 2011 were in excess of normal historical 

fluctuations. 

Table 36: Documentation of HF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 25, 2011 Section III: p. 18-21, App. A Sufficient Yes 

August 26, 2011 Section III: p. 18-21, App. A Sufficient Yes 

August 27, 2011 Section III: p. 18-21, App. A Sufficient Yes 

August 28, 2011 Section III: p. 18-21, App. A Sufficient Yes 
25 

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 for 

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

occurred in the August 25 – 28, 2011 time period and time series graphs for the events that included 

hourly PM10 concentrations from monitors within the Phoenix PM10 nonattainment area. 

Section V of the demonstration included, for each of the event days, satellite imagery, metrological data 

from various NWS stations within the Phoenix PM10 nonattainment area and Pinal County, time series 

graphs for the events that included hourly PM10 concentrations from monitors in the Phoenix PM10

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

and a time series graph that included PM10 concentrations from monitors in Pinal County, visibility, and 

sustained wind speed from Casa Grande Municipal Airport. These data show the spatial and temporal 

representation of the event as it moves throughout Maricopa and Pinal Counties. Also, while not 

available for the August 25, 2011 event, time-lapse videos of the events on August 26, 2011 and August 

27, 2011 can be found at the following locations: 

· August 26, 2011: http://www.phoenixvis.net/videos/mpeg4/SOMT_08262011.mp4

· August 26, 2011: http://www.phoenixvis.net/videos/mpeg4/SUPM_08262011.mp4

· August 27, 2011: http://www.phoenixvis.net/videos/mpeg4/SOMT_08272011.mp4

The timing of the August 25, 2011 event is consistent with the observed increased PM10 concentrations 

in the area, increased wind speed, reduced visibility, and NWS station reports of haze (HZ). 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 26, 2011 at 4:15 PM (central deserts) and at 

4:30 PM (greater Phoenix area). The timing of these dust storm reports for the August 26, 2011 event is 

consistent with the issuance of a NWS Severe Thunderstorm Warning for the period of 4:00 PM to 8:00 

PM, NWS Dust Storm Warning for the period of 4:00 PM to 6:00 PM, NWS Significant Weather 

Advisory for the period of 12:52 PM to 6:45 PM, the observed increased PM10 concentrations in the 

area, increased wind speed, reduced visibility, and NWS station reports of thunderstorms (TS), blowing 

dust (BLDU), haze (HZ), and dust storms (DS). The timing of the August 27, 2011 – August 28, 2011 

event is consistent with the issuance of a NWS Severe Thunderstorm Warning for the period of 6:00 PM 

to 8:00 PM, NWS Dust Storm Warning for the period of 6:00 PM to 8:00 PM, NWS Significant 

Weather Advisory for the period 6:00 PM to 9:00 PM, the observed increased PM10 concentrations in 

the area, increased wind speed, reduced visibility, and NWS station reports of thunderstorms (TS), 

blowing dust (BLDU), and haze (HZ). 

ADEQ stated that the evidence presented “has adequately 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 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.” 
26 

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

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

NWS station reports sufficiently establishes, for each of the events, 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 34 of this document. 

Furthermore, while exceedances occurring at only one monitor in the network are inherently more 

complex, the time series graphs (Figures 5-5 and 5-6) and NWS meteorological data tables showed that 

stronger wind speeds in the eastern portion of the nonattainment area that were followed a significant 

decrease in wind speed and a shift in wind direction from the west-southwest to south-southeast are 

likely responsible for the isolated exceedance at the Apache Junction monitoring station on August 26, 

2011.

Table 37: Documentation of CCR 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 25, 2011 Section V: p. 29-37, p. 63, App. D Sufficient Yes 

August 26, 2011 Section V: p. 38-46, p. 63, App. D Sufficient Yes 

August 27, 2011 Section V: p. 47-62, p. 63, App. D Sufficient Yes 

August 28, 2011 Section V: p. 47-62, p. 63, 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 the events 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 25, 2011 Section VII: p. 65 Sufficient Yes 

August 26, 2011 Section VII: p. 65 Sufficient Yes 

August 27, 2011 Section VII: p. 65 Sufficient Yes 

August 28, 2011 Section VII: p. 65 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 boundaries during the period of August 25-28, 2011” 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 39: Documentation of Natural Event 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

August 25, 2011 Section VII: p. 65 Sufficient Yes 

August 26, 2011 Section VII: p. 65 Sufficient Yes 

August 27, 2011 Section VII: p. 65 Sufficient Yes 

August 28, 2011 Section VII: p. 65 Sufficient Yes 
27 

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 provided no alternative that could tie 

the exceedances of August 25-28, 2011 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.” Also, PM10 concentrations before the event were 

below the 24-hour PM10 NAAQS. 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 25, 2011 Section VI: p. 64 Sufficient Yes 

August 26, 2011 Section VI: p. 64 Sufficient Yes 

August 27, 2011 Section VI: p. 64 Sufficient Yes 

August 28, 2011 Section VI: p. 64 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. 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 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) January 28, 

2013 letter9 

Yes

Was the public comment process followed 

and documented? 

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

App. D 

Yes

 

9

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

January 28, 2013. 
28 

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 25, August 26, August 27, and August 28, 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. Specifically, EPA has determined that events were not reasonably 

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

the nonattainment area and subsequently overwhelmed reasonable controls within the Phoenix PM10

nonattainment area. Also, regardless of transport into the area, information pertaining to the controls 

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

in the area, the timing of the events, and the wind speeds associated with the thunderstorm outflows 

provide sufficient evidence to conclude that these events were not reasonably controllable or 

preventable. Furthermore, EPA has determined that there is a clear causal relationship between the 

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

measured exceedances are in excess of normal historical fluctuations. 
29 

September 2, 2011 

Table 42: EPA PM10 Exceedance Summary 

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

) 

September 2, 2011 Apache Junction 04-021-3002-1 217 

Buckeye 04-013-4011-1 169 

Central Phoenix 04-013-3002-4 308 

Durango Complex 04-013-9812-1 225 

Greenwood 04-013-3010-1 198 

Higley 04-013-4006-1 213 

JLG Supersite 04-013-9997-4 208 

South Phoenix 04-013-4003-1 339 

West 43rd 04-013-4009-1 219 

West Chandler 04-013-4004-1 387 

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 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. For example, maximum sustained wind speeds of 28 mph with gusts of 34 mph and 31 

mph with gusts of 37 mph were measured at Phoenix Sky Harbor International Airport and Williams 

Gateway Airport, respectively. While not included in the final documentation, it is important to note that 

sustained wind speeds greater than 25 mph were also measured at Casa Grande Municipal Airport in 

Pinal County. 

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

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

outflows brought high concentrations of PM10 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 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, provided strong evidence that the exceedances of September 2, 2011 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 southwest of the nonattainment area. In addition to transport, information pertaining to 
30 

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

concentrations throughout the area, the timing of the event, 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? 

September 2, 2011 Section V: p. 1-8, Section III: p. 1-6, App. A, App. B 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. ADEQ also explains that these figures 

show that “the PM10 concentrations measured...on September 2, 2011 resulted in some of the highest 

24-hr averages over the five year period” and “PM10 concentrations measured at Phoenix area monitors 

on September 2, 2011 were... in excess of normal historical fluctuations.” ADEQ's analysis sufficiently 

establishes that the 24-hour PM10 concentrations measured on September 2, 2011 were in excess of 

normal historical fluctuations. 

Table 44: Documentation of HF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

September 2, 2011 Section IV: p. 1, App. C 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 for Phoenix area. The 

conceptual model also included a brief discussion of the event that occurred on September 2, 2011. 

Section III and Appendix A of the demonstration included satellite imagery, a map of the Phoenix area 

that displays wind speed and direction at peak hours of PM10 during the event, radar base velocity data 

for 0150 LST, PM10 and meteorological data for the time period of 2:00 AM, and time series graphs that 

included hourly PM10 concentrations, hourly wind speed, and gusts. ADEQ also included a time series 

graph that shows hourly PM10 concentrations from the Buckeye, Central Phoenix, Durango Complex, 

Greenwood, Higley, JLG supersite, North Phoenix, South Phoenix, West 43rd Avenue, and West 

Chandler monitors, and visibility from Phoenix Sky Harbor International Airport. These data show the 

spatial and temporal representation of the event as it moves throughout Maricopa County. 

The timing of the September 2, 2011 event is consistent with the observed increased PM10

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

dust (BLDU), dust (DU), and haze (HZ). Also, while not included in the final documentation, it is 

important to note that hourly PM10 concentrations at the southern monitoring sites (Pinal County 

Housing, Stanfield, and Casa Grande) in Pinal County began also to dramatically increase at 1:00 AM, 

while PM10 concentrations more northern monitoring sites (Cowtown, Combs School, Maricopa, and 

Apache Junction) in the County began to increase at 2:00 AM, and sustained wind speeds above 25 mph 

that were associated with the increase in PM10 in Pinal County were measured at Casa Grande Municipal 

Airport at 1:15 AM. These data indicate that PM10 was largely transported from outside of the 

nonattainment area from thunderstorm outflow winds. 
31 

ADEQ stated that, “the information presented in this section demonstrates a clear causal relationship 

between the windblown dust and the PM10 exceedances measured at four Phoenix area monitors on 

September 2, 2011.” The analysis in Sections II, III and Appendix A, specifically, the PM10 time series 

graph, winds speed and direction measurements, and NWS station reports of reduced visibility, blowing 

dust, dust, and haze, 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 45: Documentation of CCR 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

September 2, 2011 Section III: p. 1-6, Section VII: p. 2, App. A, App. B Sufficient Yes

Affects Air Quality (AAQ)

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

conclude 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? 

September 2, 2011 Section VII: p. 1 Sufficient Yes

Natural Event

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

exceedances on September 2, 2011, were shown to be caused by transport of PM10 into the Phoenix area 

from 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? 

September 2, 2011 Section VII: p. 1 Sufficient Yes

No Exceedance or Violation But For the Event (NEBF)

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

requirements and stated that “the weight of evidence presented in this submittal provided no alternative 

that could tie the exceedances of September 2, 2011, to any causal source except PM10 transported by 

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

this uncontrollable natural event.” Also, PM10 concentrations before the event were below the 24-hour 

PM10 NAAQS. 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? 

September 2, 2011 Section VI: p. 1 Sufficient Yes
32 

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. 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) January 28, 

2013 letter10 

Yes

Was the public comment process followed 

and documented? 

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

App. E 

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 42 on September 2, 2011. 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 the event was not reasonably controllable and preventable due to high wind 

conditions that transported PM10 from sources outside of the nonattainment area and subsequently 

overwhelmed reasonable controls within the Phoenix PM10 nonattainment area. Also, regardless of 

transport into the area, information pertaining to the controls implemented within the nonattainment 

area, the spatial extent of elevated PM10 concentrations measured in the area, the timing of the event, 

and the wind speeds associated with the thunderstorm outflows provide sufficient evidence to conclude 

that the event was not reasonably controllable or preventable. Furthermore, EPA has determined that 

there is 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 are in excess of normal historical 

fluctuations. 

 

10See letter from Eric Massey, Director, Air Quality Division, ADEQ to Deborah Jordan, Director, U.S. EPA Region IX Air Division, dated 

January 28, 2013. 
33 

OCTOBER 4, 2011 

Table 50: EPA PM10 Exceedance Summary 

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

) 

October 4, 2011 Higley 04-013-4006-1 158 

West Chandler 04-013-4004-1 251 

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 34 mph with gusts of 46 mph, 30 mph, and 30 mph 

with gusts of 38 mph were measured at Chandler Municipal Airport, Williams Gateway Airport, and 

Casa Grande Municipal Airport, respectively. 

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

sophisticated response programs, high wind conditions associated with the low pressure system 

generated and transported high concentrations of PM10 emissions into, and also overwhelmed controls 

within, the nonattainment area. Sustained winds over 30 mph and gusts over 45 mph easily 

overwhelmed all available efforts to limit PM10 concentrations from the event. The fact that this was a 

natural event involving a low pressure storm system that generated and transported PM10 emissions in 

Maricopa County provided strong evidence that the exceedances on October 4, 2011 recorded at the 

West Chandler and Higley monitors were not reasonably controllable or preventable.” 

Section V 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 and southwest of the nonattainment 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 51: Documentation of nRCP 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

October 4, 2011 Section IV: p. 19-25, Section V: p. 26-48, App. B Sufficient Yes 
34 

Historical Fluctuations (HF)

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

24-hour averages and PM10 daily maximum hourly averages in Figures 3-1 and 3-2, respectively. ADEQ 

also stated that these figures “indicate that the PM10 concentrations seen at the West Chandler and 

Higley monitors on October 4, 2011 were in excess of normal historical fluctuations.” ADEQ's analysis 

sufficiently establishes that the 24-hour PM10 concentrations measured on October 4, 2011 were in 

excess of normal historical fluctuations. 

Table 52: Documentation of HF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

October 4, 2011 Section III: p. 15-18 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, climate information, surface 

weather maps for the event, and a NOAA Storm Prediction Center mesoscale discussion figure for the 

Phoenix area. The conceptual model also included a very detailed discussion of the event that occurred 

on October 4, 2011 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 a detailed and extensive GIS analysis and a number of visibility 

photos that show the spatial and temporal representation of the event as it moves throughout Maricopa 

and Pinal Counties. The analysis included PM10 concentrations, sustained wind speeds, wind gusts, wind 

direction, 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. 

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 October 4, 2011 at 12:30 PM (south 

central Pinal County) and 2:00 PM (central deserts). 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 1:00 PM to 

7:00 PM, NWS Significant Weather Advisory for the period of 9:56 AM to 2:15 PM, the observed 

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

reports of blowing dust (BLDU). Also, while direct links were not included in the final documentation, 

time-lapse videos of the event can be found at the following locations: 

· South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_10042011.mp4

· Superstition Mountains: http://www.phoenixvis.net/videos/mpeg4/SUPM_10042011.mp4

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

generated and transported by the low pressure system winds and the exceedance at the West Chandler 

and Higley monitors.” ADEQ further stated that “the particular wind magnitudes and wind direction, the 

proximity of the exceeding monitors to open and desert areas of Pinal County, and the delay in the low 

pressure system winds exiting the areas around the exceeding monitors provide solid evidence as to why 

only these monitors within the Maricopa County nonattainment area recorded exceedances.” 
35 

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

measurements, GIS analysis, time-lapse video evidence, NWS advisories, NOAA NCDC dust storm 

observations, and NWS station reports of reduced visibility and blowing dust, sufficiently establishes 

that there was a clear causal relationship between uncontrollable emissions generated from low pressure 

system winds and the exceedances measured at the monitors identified in Table 50 of this document. 

Table 53: Documentation of CCR 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

October 4, 2011 Section V: p. 26-48, App. B Sufficient Yes

Affects Air Quality (AAQ)

ADEQ stated that based on the information presented in the demonstrations, “it is reasonable to 

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? 

October 4, 2011 Section VII: p. 51 Sufficient Yes

Natural Event

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

shown to cause these exceedances were emissions of PM10 caused by low pressure system winds on 

October 4, 2011” 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? 

October 4, 2011 Section VII: p. 51 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 also included a time series graph included presenting hourly PM10, hourly wind speeds, and wind 

gusts that show that PM10 concentrations before the event were below the 24-hour PM10 NAAQS. 

ADEQ further stated that “the body of evidence presented in this submittal confirms that the 

exceedances on October 4, 2011 were a natural event and that there would have been no exceedance but 

for the presence of the uncontrollable windblown dust from the low pressure system winds.” 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? 

October 4, 2011 Section VI: p. 49-50 Sufficient Yes
36 

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 57 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. A 

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) January 28, 

2013 letter11 

Yes

Was the public comment process followed 

and documented? 

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

App. C 

Yes

Conclusion

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

by low pressure system 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 50 on October 4, 2011. 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 event was not reasonably controllable and preventable due to high wind conditions that 

transported PM10 from sources outside of the nonattainment area and subsequently overwhelmed 

reasonable controls within the Phoenix PM10 nonattainment area. Also, regardless of transport into the 

area, information pertaining to the controls implemented within the nonattainment area, the spatial 

extent of elevated PM10 concentrations measured in the area, and the wind speeds associated with the 

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

controllable or preventable. Furthermore, EPA has determined that there is 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 are in excess of normal historical fluctuations. 

 

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

January 28, 2013. 
37 

NOVEMBER 4, 2011 

Table 58: EPA PM10 Exceedance Summary 

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

) 

November 4, 2011 Apache Junction 04-021-3002-1 225 

Buckeye 04-013-4011-1 284 

Central Phoenix 04-013-3002-4 223 

Durango Complex 04-013-9812-1 251 

Dysart 04-013-4010-1 224 

Glendale 04-013-2001-1 229 

Greenwood 04-013-3010-1 231 

Higley 04-013-4006-1 258 

JLG Supersite 04-013-9997-3 200 

JLG Supersite 04-013-9997-4 199 

North Phoenix 04-013-1004-1 186 

North Phoenix 04-013-1004-2 186 

South Phoenix 04-013-4003-1 231 

West 43rd 04-013-4009-1 242 

West Chandler 04-013-4004-1 670 

West Phoenix 04-013-0019-1 279 

Zuni Hills 04-013-4016-1 258 

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 states, “BACM-approved control measures 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 previously 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. For example, maximum sustained wind speeds of 34 mph with gusts of 40 mph and 31 

mph with gusts of 37 mph were measured at Chandler Municipal Airport and Luke Air Force Base 

Airport, respectively. While not included in the final documentation, it is important to note that 

sustained wind speeds greater than 25 mph were also measured at other locations in the Maricopa and 

Pinal Counties, specifically at the Gila Bend Air Field, and Casa Grande Municipal Airport. 

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

sophisticated response programs, high wind conditions associated with the approaching cold front 

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 some sustained winds as high 

as 32 mph and gusts frequently over 35 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
38 

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

PM10 nonattainment area, provided strong evidence that the exceedances of November 4, 2011, 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 southwest of the nonattainment 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 59: Documentation of nRCP 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

November 4, 2011 Section V: p. 1-9, Section III: p. 1-5, App. A, B, and D 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. ADEQ also explains that these figures 

show that “PM10 concentrations measured at Phoenix area monitors on November 4, 2011 were... in 

excess of normal historical fluctuations.” ADEQ's analysis sufficiently establishes that the 24-hour PM10

concentrations measured on November 4, 2011 were in excess of normal historical fluctuations. 

Table 60: Documentation of HF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

November 4, 2011 Section IV: p. 1, App. C 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 for Phoenix area. 

The conceptual model also included a brief discussion of the event that occurred on November 4, 2011. 

Section III and Appendix A of the demonstration included satellite imagery, a map of the Phoenix area 

that displays wind speed and direction at peak hours of PM10 during the event, PM10 and meteorological 

data for the time period of 3:00 PM to 6:00 PM, and time series graphs that include hourly PM10

concentrations, hourly wind speed, and gusts. ADEQ also included a time series graph that shows hourly 

PM10 concentrations from monitors in the Phoenix PM10 nonattainment area and visibility from Phoenix 

Sky Harbor International Airport. These data show the spatial and temporal representation of the event 

as it moves throughout Maricopa County. Also, time-lapse videos of the event were included in 

Appendix B and can be found at the following locations: 

· South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_11042011.mp4

· Superstition Mountains: http://www.phoenixvis.net/videos/mpeg4/SUPM_11042011.mp4
39 

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 November 4, 2011 at 1:15 PM 

(central deserts). The timing of the dust storm report for this event is consistent with the issuance of a 

NWS Wind Advisory for the period of 11:00 AM to 11:00 PM, NWS Blowing Dust Advisory for the 

period of 11:00 AM to 8:00 PM, NWS Dust Storm Warning for the period of 2:00 PM to 8:00 PM, 

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

and NWS station reports of blowing dust (BLDU) and haze (HZ). Again, while not included in the final 

documentation, it is important to note that hourly PM10 concentrations at the southern monitoring sites 

(Stanfield and Casa Grande) in Pinal County began to dramatically increase at 12:00 PM, while PM10

concentrations monitoring sites located further north and east (Cowtown, Combs School, Maricopa, and 

Apache Junction) in the County began to increase at 1:00 PM and measured peak PM10 concentrations at 

3:00 PM. Also, sustained wind speeds above 25 mph that were associated with the increase in PM10 in 

Pinal County were measured at Casa Grande Municipal Airport at 1:35, 2:15, 2:35, 2:55, and 3:35 PM. 

These data indicate that PM10 was largely transported from outside of the nonattainment area from 

strong winds associated with an approaching cold front. 

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

windblown dust and the PM10 exceedances measured in the Phoenix PM10 nonattainment area on 

November 4, 2011.” The analysis in Sections II, II, and V, specifically, the PM10 time series graph, 

winds speed and direction measurements, the GIS map, time-lapse video evidence, NWS advisories, 

NOAA NCDC dust storm observations, and NWS station reports of reduced visibility, blowing dust and 

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

generated from winds associated with an approaching cold front and the exceedances measured at the 

monitors identified in Table 58 of this document. 

Table 61: Documentation of CCR 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

November 4, 2011 Section III: p. 1-5, App. A, B, and 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 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 62: Documentation of AAQ 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

November 4, 2011 Section VII: p. 1 Sufficient Yes

Natural Event

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

exceedances in the Phoenix area on November 4, 2011, were shown to be caused by transport of PM10

into the Phoenix area from widespread strong winds associated with an approaching cold front” 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. 
40 

Table 63: Documentation of Natural Event 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

November 4, 2011 Section VII: p. 1 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 weight of evidence presented in this submittal provided no alternative that could tie 

the exceedance of November 4, 2011, to any causal source other than PM10 transported by strong winds 

associated with an approaching cold front, confirming that there would have been no exceedance but for 

the presence of this uncontrollable natural event.” Also, PM10 concentrations before the event were 

below the 24-hour PM10 NAAQS. ADEQ’s summary regarding the nRCP and CCR requirements 

sufficiently establishes that the NEBF criterion has been met. 

Table 64: Documentation of NEBF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

November 4, 2011 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 65 outlines EPA’s 

evaluation of these requirements. 

Table 65: 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) January 28, 

2013 letter12 

Yes

Was the public comment process followed 

and documented? 

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

App. E 

Yes

 

12See letter from Eric Massey, Director, Air Quality Division, ADEQ to Deborah Jordan, Director, U.S. EPA Region IX Air Division, dated 

January 28, 2013. 
41 

Conclusion

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

by high winds associated with the passage of a cold front 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 58 on November 4, 2011. 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 transported PM10 from sources outside of the nonattainment area and 

subsequently overwhelmed reasonable controls within the Phoenix PM10 nonattainment area. Also, 

regardless of transport into the area, information pertaining to the controls implemented within the 

nonattainment area, the spatial extent of elevated PM10 concentrations measured in the area, and the 

wind speeds associated with the passing cold front provide sufficient evidence to conclude that the event 

was not reasonably controllable or preventable. Furthermore, EPA has determined that there is 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 are in excess of normal historical 

fluctuations. 
42 

FEBRUARY 27, 2012 

Table 66: EPA PM10 Exceedance Summary 

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

) 

February 27, 2012 West 43rd 04-013-4009-1 167 

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 30 mph with gusts of 36 mph and 31 mph with gusts 

of 37 mph were measured at Luke Air Force Base Airport and Casa Grande Municipal Airport, 

respectively. Sustained wind speeds greater than 25 mph were also measured at other locations, 

specifically at Williams Gateway Airport, Chandler Municipal Airport, and Gila Bend Air Field. 

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

sophisticated response programs, high wind conditions associated with the low pressure system 

generated high concentrations of PM10 within the nonattainment area. Sustained winds up to 30 mph and 

gusts up to 43 mph easily overwhelmed all available efforts to limit PM10 concentrations from the event. 

The fact that this was a natural event involving a low pressure storm system that generated PM10

emissions in the nonattainment area provided strong evidence that the exceedance on February 27, 2012 

recorded at the West 43rd Avenue monitor was not reasonably controllable or preventable.” 

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

statements described above. This analysis clearly demonstrates the “spatial and temporal representation 

of the low pressure system winds and associated windblown dust as they move throughout Maricopa and 

Pinal counties.” 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 67: Documentation of nRCP 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 27, 2012 Section IV: p. 17-23, Section V: p. 25-51, App. B Sufficient Yes
43 

Historical Fluctuations (HF)

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

24-hour averages and PM10 daily maximum hourly averages in Figures 3-1 and 3-2, respectively. ADEQ 

also stated that these figures “indicate that the PM10 concentrations seen at the West 43rd Avenue 

monitor on February 27, 2012 were in excess of normal historical fluctuations.” ADEQ's analysis 

sufficiently establishes that the 24-hour PM10 concentrations measured on February 27, 2012 were in 

excess of normal historical fluctuations. 

Table 68: Documentation of HF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 27, 2012 Section III: p. 15-16 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, climate, surface weather maps, 

and NOAA 500-Milibar wind fields for the Phoenix area. The conceptual model also included a very 

detailed discussion of the event that occurred on February 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 a detailed and extensive GIS analysis and a number of visibility 

photos that show the spatial and temporal representation of the event as it moves throughout Maricopa 

and Pinal Counties. The analysis included PM10 concentrations, sustained wind speeds, wind gusts, wind 

direction, 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. 

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

of 12:30 PM to 11:00 PM, NWS Wind Advisory for the period of 4:00 PM to 11:00 PM, NWS Dust 

Storm Warning for the period of 1:00 PM to 3:00 PM, the observed increased PM10 concentrations in the 

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

while direct links were not included in the final documentation, time-lapse videos of the event can be 

found at the following locations: 

· South Mountain: http://www.phoenixvis.net/videos/mpeg4/SOMT_02272012.mp4

· Superstition Mountains: http://www.phoenixvis.net/videos/mpeg4/SUPM_02272012.mp4

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

windblown dust emissions generated by uncontrollable natural events and the exceedances measured at 

the monitors.” ADEQ further stated that “It is clear from these data that sustained wind speeds of 30 

mph and gusts of 43 mph were strong enough to generate uncontrollable windblown PM10 emissions to 

the West 43rd Avenue monitor and demonstrates the clear causal relationship between the low pressure 

system winds and the recorded exceedance.” 
44 

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

measurements, GIS analysis, time-lapse video evidence, NWS advisories, and NWS station reports of 

reduced visibility and blowing dust, sufficiently establishes that there was a clear causal relationship 

between uncontrollable emissions generated from low pressure system winds and the exceedance 

measured at the West 43rd Avenue monitor. Furthermore, while exceedances occurring at only one 

monitor in the network are inherently more complex, the GIS analysis (Figures 5-1 through -5-21) 

shows stronger wind speeds in the western portion of the nonattainment area are likely responsible for 

the isolated exceedance at the West 43rd Avenue monitoring station. 

Table 69: Documentation of CCR 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 27, 2012 Section V: p. 25-51, App. B Sufficient Yes

Affects Air Quality (AAQ)

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

requirements, “it is reasonable to 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 70: Documentation of AAQ 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 27, 2012 Section VII: p. 54 Sufficient Yes

Natural Event

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

shown to cause the exceedance was emissions of PM10 caused by low pressure system winds on 

February 27, 2012” 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 71: Documentation of Natural Event 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 27, 2012 Section VII: p. 54 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 also included a time series graph included that presents hourly PM10, hourly, wind speeds, and wind 

gusts showing that PM10 concentrations before the event were below the 24-hour PM10 NAAQS.ADEQ 

further also stated that “the body of evidence presented in this submittal confirms that the exceedance on 

February 27, 2012 was a natural event and that there would have been no exceedance but for the 

presence of the uncontrollable windblown dust from the low pressure system winds.” ADEQ’s summary 

regarding the nRCP and CCR requirements sufficiently establishes that the NEBF criterion has been 

met. 

Table 72: Documentation of NEBF 

Exceedance Date Demonstration Citation Quality of Evidence Criterion Met? 

February 27, 2012 Section VI: p. 52-53 Sufficient Yes
45 

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 73 outlines EPA’s 

evaluation of these requirements. 

Table 73: 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. A 

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) January 28, 

2013 letter13 

Yes

Was the public comment process followed 

and documented? 

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

App. C 

Yes

Conclusion

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

by low pressure system high winds caused exceedances of the 24-hour PM10 NAAQS at the locations 

outlined in Table 66 on February 27, 2012. EPA has determined that the flagged exceedances at this 

location on this day meet the definition of an exceptional event: the exceedance affected air quality, was 

not reasonably controllable or preventable, and meets the definition of a natural event. Specifically, EPA 

has determined that event was not reasonably controllable and preventable due to high wind conditions 

that overwhelmed reasonable controls within the Phoenix PM10 nonattainment area. In reviewing the 

GIS analysis included in Section V of the demonstration and time series of PM10 for the Phoenix PM10

nonattainment area in conjunction with the time-lapse video evidence for the event, it is plausible that 

the elevated PM10 from the hours of 11:00 AM to 2:00 PM was caused by transported PM10 from 

sources outside of the nonattainment area, which indicates that the emissions causing the exceedance at 

West 43rd Avenue monitor were not reasonably controllable or preventable. Also, 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 were not reasonably controllable or preventable. 

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

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

exceedance is in excess of normal historical fluctuations. 

 

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

January 28, 2013. 
46 

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 February 19, July 18, August 3, August 18, August 25-28, September 

2, October 4, November 4, 2011 and February 27, 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.