State of Arizona Exceptional Event 

Documentation for the Event of 

September 2, 2011, for the Phoenix PM10

Nonattainment Area 

Final Report Prepared for 

Arizona Department of Environmental Quality 

Phoenix, AZ 

January 23, 2013
This PDF document contains blank pages to accommodate two-sided printing. 
State of Arizona Exceptional Event Documentation 

for the Event of September 2, 2011, 

for the Phoenix PM10 Nonattainment Area 

Final Report 

STI-912091-5546-DFR 

Prepared by 

Daniel Alrick 

Angela Ekstrand 

Briana Gordon 

Hilary Hafner 

Clinton MacDonald 

Hilary Minor 

Ashley Russell 

Sonoma Technology, Inc. 

1455 N. McDowell Blvd., Suite D 

Petaluma, CA 94954-6503 

Ph 707.665.9900 | F 707.665.9800 

sonomatech.com 

Prepared for 

Theresa Rigney 

Air Quality Assessment Section Manager 

1110 W. Washington Street 

Phoenix, AZ 85007 

602.771.2274

January 23, 2013 

Cover image depicts suspended dust and haze in the Phoenix area at 0910 LST on September 2, 2011, resulting 

from thunderstorm outflow that transported dust into the region several hours prior. This photo was taken by an 

ADEQ visibility camera in downtown Phoenix, looking northeastward toward Camelback Mountain.

Exceptional Event, Phoenix, September 2, 2011 Acknowledgments 

Acknowledgments 

The authors would like to acknowledge the Arizona Department of Environmental Quality 

for helpful discussion and feedback during the preparation of this document. 

iii

Exceptional Event, Phoenix, September 2, 2011 Table of Contents 

Table of Contents 

Section Page

List of Figures ............................................................................................................................. vii

List of Tables...............................................................................................................................viii

1. Introduction..........................................................................................................................1

1.1 Report Contents .........................................................................................................1

1.2 Exceptional Event Rule Requirements.......................................................................1

1.2.1 Public Notification That the Event Was Occurring (40 CFR 

50.14(c)(1)(i)) .................................................................................................2

1.2.2 Place Informal Flag on Data in AQS (40 CFR 50.14(c)(2)(ii))........................2

1.2.3 Notify EPA of Intent to Flag Through Submission of Initial Event 

Description by July 1 of Calendar Year Following Event (40 CFR 

50.14(c)(2)(iii))................................................................................................2

1.2.4 Document That the Public Comment Process Was Followed for Event 

Documentation (40 CFR 50.14(c)(3)(iv))........................................................2

1.2.5 Submit Demonstration Supporting Exceptional Event Flag (40 CFR 

50.14(a)(1-2)).................................................................................................3

1.2.6 Documentation Requirements (40 CFR 50.14(c)(3)(iii)) ................................3

2. Conceptual Model................................................................................................................1

2.1 Geographic Setting and Monitor Locations ................................................................1

2.2 Climate, Monsoon, and Thunderstorms .....................................................................5

2.3 Event Day Summary ..................................................................................................6

3. Causal Relationship.............................................................................................................1

3.1 Discussion..................................................................................................................1

3.2 Summary....................................................................................................................6

4. Historical Norm ....................................................................................................................1

4.1 Analysis......................................................................................................................1

4.2 Summary....................................................................................................................1

5. Not Reasonably Controllable or Preventable ......................................................................1

5.1 Background................................................................................................................1

5.1.1 Control Measures...........................................................................................1

5.1.2 Additional Measures.......................................................................................3

5.1.3 PM10 Rule Effectiveness ................................................................................4

5.1.4 Compliance and Enforcement Activities.........................................................5

5.1.5 Review of Source-Permitted Inspections and Public Complaints ..................6

5.2 Forecasts and Warnings ............................................................................................7

5.3 Wind Observations.....................................................................................................7

5.4 Summary....................................................................................................................7

6. But-For Analysis ..................................................................................................................1

6.1 Discussion..................................................................................................................1

6.2 Summary....................................................................................................................1

v
Exceptional Event, Phoenix, September 2, 2011 Table of Contents 

vi

Section Page

7. Conclusions .........................................................................................................................1

7.1 Affects Air Quality.......................................................................................................1

7.2 Not Reasonably Controllable or Preventable .............................................................1

7.3 Natural Event .............................................................................................................1

7.4 Clear Causal Relationship..........................................................................................2

7.5 Historical Norm...........................................................................................................2

7.6 Not Reasonably Preventable .....................................Error! Bookmark not defined.

7.7 But For .......................................................................................................................2

Appendix A...........................................................................................................A-1 

Appendix B...........................................................................................................B-1 

Appendix C...........................................................................................................C-1 

Appendix D...........................................................................................................D-1 

Appendix E...........................................................................................................E-1 

 
Exceptional Event, Phoenix, September 2, 2011 List of Figures 

List of Figures 

Figure Page

2-1. Location of air quality and meteorological monitors mentioned in this report and 

relevant geographical features in the Phoenix area..........................................................2

2-2. Location of air quality sites monitoring PM10 statewide on September 2, 2011................3

2-3. Drainage system of Phoenix, Arizona. ..............................................................................4

2-4. Average monthly temperatures and precipitation for Phoenix, 1981–2010.......................5

2-5. Cross-section of a thunderstorm creating an outflow boundary and haboob....................6

2-6. Outflow boundaries from thunderstorms over northwest Mexico and south-central 

Arizona moved northward and transported dust into the Phoenix area on 

September 2, 2011............................................................................................................7

3-1. Infrared satellite image from 0000 LST on September 2, 2011 (GOES-West). ................1

3-2. Phoenix area air quality and meteorological monitors.......................................................2

3-3. Hourly PM concentrations and wind speeds at the West Chandler monitor on 

September 2-3, 2011. .......................................................................................................4

10

3-4. Hourly PM concentrations and wind speeds at the Higley monitor on September 

1-2, 2011. ..........................................................................................................................4

10

3-5. Radar base velocity data at 0150 LST on September 2, 2011, from NWS Phoenix 

Doppler radar. ...................................................................................................................5

3-6. Hourly PM10 concentrations at Phoenix area monitors and visibility at KPHX. .................6

4-1. 24-hr average PM10 concentrations at the Central Phoenix monitor (2007-2011)............1

5-1. Timeline of Maricopa County fugitive dust rules and ordinances......................................5

vii
Exceptional Event, Phoenix, September 2, 2011 List of Tables 

List of Tables 

Table Page

2-1. Statewide monitor information and PM10 measurements on September 2, 2011.............7

3-1. Observed wind speeds and wind gusts at Maricopa County monitors on 

September 2, 2011............................................................................................................3

5-1. Rules and ordinances regulating particulate matter emissions in Maricopa County.........2

5-2. Pinal County rules regulating existing and new non-point sources in Pinal County..........3

5-3. Pinal County rules regulating fugitive dust in Pinal County portion of Maricopa 

County PM10 Nonattainment Area.....................................................................................3

viii
Exceptional Event, Phoenix, September 2, 2011 Introduction

1

ral events. 

1. Introduction 

On September 2, 2011, 11 air quality monitors in the Phoenix PM10 nonattainment area 

recorded 24-hr average PM10 concentrations in excess of the National Ambient Air Quality 

Standard (NAAQS) of 150 μg/m3

. The purpose of this report is to demonstrate that this 

exceedance was due to naturally occurring windblown dust, was not reasonably controllable or 

preventable, was historically unusual, and would not have occurred “but-for” the windblown dust 

and, therefore, is an Exceptional Event as defined by the U.S. Environmental Protection 

Agency’s (EPA) Exceptional Events Rule (EER). 

1.1 Report Contents 

Section 2 of this assessment contains a conceptual model of the wind-blown dust event 

that transpired on September 2, 2011, providing a background narrative of the exceptional event 

and an overall explanation that the event affected air quality. Section 2 also provides evidence 

that the event was a natural event. 

Section 3 of this assessment establishes a clear causal connection between the natural 

event on September 2, 2011, and the exceedances of the 24-hr PM10 standard at the 

monitoring stations. The evidence in this section also confirms that the event in question both 

affected air quality and was the result of natu

Section 4 of this assessment illustrates that the event of September 2, 2011, produced 

PM10 concentrations in excess of normal historical fluctuations. 

Section 5 of this assessment details the existing dust control measures and 

demonstrates that despite the presence and enforcement of these controls, the event of 

September 2, 2011, was not reasonably controllable or preventable. 

Section 6 of this assessment builds upon the demonstration, showing a clear causal 

connection between the natural event and the exceedances, and concludes that the 

exceedances of the 24-hr PM10 standard on September 2, 2011, would not have occurred “but 

for” the event. 

Appendix A contains time-series graphs and data tables to supplement Section 3. 

Appendix B contains links to media coverage pertaining to this dust storm event. Appendix C 

contains time-series graphs to supplement Section 4. Appendix D contains air quality forecasts 

issued by the Arizona Department of Environmental Quality (ADEQ) and weather statements 

and warnings issued by the National Weather Service. Appendix E contains a copy of the 

affidavit of public notice concerning this assessment report. 

1.2 Exceptional Event Rule Requirements 

In addition to the technical requirements that are contained within the EER, procedural 

requirements must also be met in order for the EPA to concur with the flagged air quality 
Exceptional Event, Phoenix, September 2, 2011 Introduction

2

flagging of these data. 

cess Was Followed for Event 

Documentation (40 CFR 50.14(c)(3)(iv)) 

ay 

monitoring data. This section of the report contains the requirements of the EER and 

associated guidance, and discusses how ADEQ addressed those requirements. 

1.2.1 Public Notification That the Event Was Occurring (40 CFR 50.14(c)(1)(i)) 

ADEQ issued Dust Control Action Forecasts and Air Quality Forecasts for Maricopa 

County advising citizens of the potential for high wind dust events on September 2, 2011. More 

information on ADEQ’s forecasting program can be found in Section 5.2 of this report. The 

forecast products that were issued pertaining to the September 2, 2011 event are included in 

Appendix D. 

1.2.2 Place Informal Flag on Data in AQS (40 CFR 50.14(c)(2)(ii)) 

ADEQ and other operating air quality agencies in Arizona submit data into the EPA’s Air 

Quality System (AQS), the official repository of ambient air quality data. This data submittal to 

AQS includes particulate matter (PM) data from both filter-based and continuous monitors 

operated in Arizona. 

When ADEQ and/or another agency operating monitors in Arizona suspect that data 

may be influenced by an exceptional event, ADEQ and/or the other operating agency expedites 

analysis of the filters collected from the potentially-affected filter-based air monitoring 

instruments, quality-assures the results, and submits the data into AQS. ADEQ and/or other 

operating agencies also submit data from continuous monitors into AQS after quality assurance 

is complete. 

If ADEQ and/or other operating air quality agencies have determined that a potential 

exists that a monitor’s reading(s) have been influenced by an exceptional event, a preliminary 

flag is submitted for the measurement in AQS. The data are not official until they undergo more 

thorough quality assurance and quality control, leading to certification by May 1 of the year 

following the calendar year in which the data were collected (40 CFR 58.15(a)(2)). The 

presence of the flag on the September 2, 2011, data can be confirmed in AQS. 

1.2.3 Notify EPA of Intent to Flag Through Submission of Initial Event 

Description by July 1 of Calendar Year Following Event (40 CFR 

50.14(c)(2)(iii)) 

ADEQ submitted notice to EPA on August 29, 2012, listing all days from calendar year 

2011 that ADEQ intends to analyze under the Exceptional Events Rule. The PM10

exceedances that occurred on September 2, 2011, in the Phoenix PM10 nonattainment area 

were included on this list. This assessment report serves as demonstration supporting the 

1.2.4 Document That the Public Comment Pro

ADEQ posted this assessment report on the ADEQ webpage and placed a hard copy of 

the report in the ADEQ Records Management Center for public review. ADEQ opened a 30-d
Exceptional Event, Phoenix, September 2, 2011 Introduction 

3

ements of 40 

CFR 50.14(c)(3)(iv). See Appendix E for a copy of the affidavit of public notice. 

stration Supporting Exceptional Event Flag (40 CFR 

50.14(a)(1-2)) 

to 

s 

Francisco, California. The deadline for the submittal of this package is September 30, 

2014. 

1.2.6 Documentation Requirements (40 CFR 50.14(c)(3)(iii)) 

ion of air quality monitoring data, 

evidence must be provided for the following elements: 

h in 40 CFR 501(j) that 

 activity unlikely to recur in a particular 

nship between the measurement(s) under 

h a measured concentration(s) in excess of normal 

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

public comment period on December 3, 2012. A copy of the public notice certification, along 

with any comments received, will be submitted to EPA, consistent with the requir

1.2.5 Submit Demon

At the close of the public comment period, and after ADEQ has had the opportunity 

consider any comments submitted on this document, ADEQ will submit this document, the 

comments received, and ADEQ’s responses to those comments to EPA Region 9 headquarter

in San 

The EER states that in order to justify the exclus

1. The event satisfies the criteria set fort

a. the event affected air quality, 

b. the event was not reasonably controllable or preventable, and 

c. the event was caused by human

location or was a natural event; 

2. There is a clear causal relatio

consideration and the event; 

3. The event is associated wit

historical fluctuations; and 

4

Exceptional Event, Phoenix, September 2, 2011 Conceptual Model 

1 

2. Conceptual Model 

This section provides a narrative background and summarizes the meteorological and 

air quality conditions in place on September 2, 2011, in the Phoenix area. Elements described 

in this section include 

 A description and map of geographic setting of the air quality and meteorological 

monitors. 

 A description of Phoenix’s climate. 

 An overall description of meteorological and air quality conditions on the event day. 

2.1 Geographic Setting and Monitor Locations 

Phoenix is located in the Salt River Valley in south-central Arizona. It lies at an elevation 

of 1,090 feet above mean sea level (msl) in the northeastern part of the Sonoran Desert. Other 

than the mountains in and around the city, the topography of Phoenix is generally flat. The 

Phoenix area is surrounded by the McDowell Mountains (~4,200 ft msl) to the northeast, the 

foothills of the Bradshaw (~7,900 ft msl) and Mazataal (~7,900 ft msl) ranges to the north, the 

White Tank Mountains (~4,500 ft msl) to the west, the Sierra Estrella (~4,450 ft msl) to the 

southwest, and the Superstition Mountains (~5,000 ft msl) far to the east. Within the City are 

the Phoenix Mountains (~2,600 ft msl) and South Mountain (~2,600 ft msl). Current 

development is pushing north, west, and south into Pinal County. 

A fairly dense network of air quality and meteorological monitors exists throughout the 

Phoenix area, with a much less dense network of monitors throughout the rest of Arizona. 

Figure 2-1 shows the general geographic setting of Phoenix, as well as the locations of 

meteorological monitors and the PM10 monitors that recorded exceedances on September 2, 

2011. It should be noted that some of the monitors shown in Figure 2-1 are filter-based 

monitors; therefore, monitoring data from all locations may only be available for select days (i.e., 

1-in-6 run days). AQS monitors measure both air quality and meteorological data. Some of the 

AQS monitors in the Phoenix area are run by the Maricopa County Air Quality Department 

(MCAQD) while others are run by ADEQ. Arizona Meteorological Network (AZMET) and 

National Weather Service (NWS) monitors measure meteorological data only. Several Arizona 

AZMET and NWS sites are in operation in the Phoenix area. Phoenix Sky Harbor International 

Airport (KPHX) was the primary NWS site used in this demonstration package because of that 

site’s high data quality, data completeness, proximity to AQS sites, and representativeness of 

meteorological conditions in the Phoenix area. Figure 2-2 shows monitors statewide measuring 

PM10 on September 2, 2011. 

Figure 2-3 depicts the drainage systems or watersheds for the State of Arizona. Many 

of the rivers that form Arizona’s drainage system are dry for most of the year and, consequently, 

are sources of silt and fine soils that become suspended and add to regional PM10 loadings 

during high wind events. Much of this alluvial matter and fine soil is deposited in the low lying 

areas of central and southern Arizona, with larger depositional areas focused in and around the 

confluences of dry river channels.
2 

Figure 2-1. Locations of air quality monitors that recorded exceedances of the 24-hr PM10 NAAQS and meteorolo

in the Phoenix area.
Exceptional Event, Phoenix, September 2, 2011 Conceptual Model 

3 

Figure 2-2. Location of sites monitoring PM10 in Arizona on September 2, 2011. 
Exceptional Event, Phoenix, September 2, 2011 Conceptual Model 

4 

Figure 2-3. Drainage system of Phoenix, Arizona. 
Exceptional Event, Phoenix, September 2, 2011 Conceptual Model 

5 

2.2 Climate, Monsoon, and Thunderstorms 

Phoenix has an arid climate, with very hot summers and temperate winters. The 

average summer high temperatures are among the hottest of any populated area in the United 

States (Figure 2-4). Temperatures reach or exceed 100°F an average of 110 days annually, 

and reach or exceed 110°F an average of 18 days annually. Phoenix receives an average of 

7.66 inches of rain per year. The bulk of this rain usually falls during the December through 

March and July through August time periods. During the December through March period, 

winter storms originating from the Pacific Ocean can produce significant rains in southwestern 

Arizona. During the July through early September time period, monsoonal moisture originating 

from the Gulf of California, Gulf of Mexico, and large thunderstorm complexes over the Sierra 

Madre Occidental Mountains in Mexico move northward into Arizona. 

Figure 2-4. Average monthly temperatures and precipitation for Phoenix, 1981–2010. 

The influx of moisture associated with the monsoon, combined with strong solar heating, 

can result in unstable atmospheric conditions that are favorable for the development of 

thunderstorms. Heavy precipitation associated with thunderstorms, and the eventual collapse 

or dissipation of thunderstorms, can generate what are known as downbursts. Downbursts are 

the rapid descent of rain-cooled air in a thunderstorm. Upon reaching the surface, this air 

rapidly disperses horizontally away from the storm as outflow boundaries (also called gust 

fronts; see Figure 2-5). The high winds associated with outflow boundaries can efficiently loft 

dust into the air and transport the dust over long distances, resulting in dust storms (also called 

haboobs) with high PM10 concentrations and low visibilities. 
Exceptional Event, Phoenix, September 2, 2011 Conceptual Model 

6 

Figure 2-5. Cross-section of a thunderstorm creating an outflow boundary and haboob. 

(Source: Desert Meteorology, Thomas T. Warner, 2004.) 

Dust storms associated with these thunderstorms typically occur in the early part of the 

monsoon season (July) before subsequent rains moisten the soil and limit potential lofting of soil 

into the air. However, depending on the amount and frequency of precipitation received during 

the monsoon season, extremely hot temperatures act to dry the surface soils very quickly; thus, 

dust storms can occur at any time during the year. Specific PM10 source regions are difficult to 

determine during thunderstorm-driven dust storms because the thunderstorm outflow can carry 

dust over long distances that encompass many possible sources of dust. Instead, we consider 

general PM10 source regions, which are typically identified based on the locations of the 

thunderstorms that are believed to have generated the dust-laden outflow winds. 

2.3 Event Day Summary 

On the early morning of September 2, 2011, gusty winds generated by thunderstorms 

over northwestern Mexico and south-central Arizona transported dust northward into the 

Phoenix area (Figure 2-6). The windblown dust resulted in 24-hr average PM10 concentrations 

in exceedance of the NAAQS at 11 air quality monitors in the Phoenix area (Table 2-1). The 

PM10 concentrations measured at these monitors were in excess of normal historical 

fluctuations. The dust was naturally occurring and likely originated over undeveloped lands of 

northwestern Mexico and southern Arizona outside the city of Phoenix, and wind gusts in 

excess of 40 mph overwhelmed reasonable dust control measures. Monitors across the 

Phoenix metropolitan area, as well as in Pinal and Pima Counties, recorded high PM10

Cross‐section schematic of a haboob caused by the cool outflow from a

thunderstorm, with the leading edge that is propagating ahead of the storm

called an outflow boundary.  The strong, gusty winds that prevail at the

boundary are defined as a gust front.  The leading edge of the cool air is called

the nose, and the upward‐protruding part of the features is referred to as the

head.  Behind the roll in the windfield at the leading edge is a turbulent wake.  

The rapidly moving cool air and the gustiness at the gust front raise dust

(shaded) high into the atmosphere.
Exceptional Event, Phoenix, September 2, 2011 Conceptual Model 

7 

concentrations as the windblown dust moved through, illustrating the widespread nature of this 

event. 

Region of

thunderstorm

development

Phoenix

Pima

Pinal

Maricopa

Figure 2-6. Outflow boundaries from thunderstorms over northwest Mexico and 

south-central Arizona moved northward and transported dust into the Phoenix area on 

September 2, 2011. 

Table 2-1. Statewide monitor information and PM10 measurements on September 2, 

2011. The exceedance monitors discussed in this report are shown in bold. 

Page 1 of 3 

Monitor Monitor 

Type Operator AQS Monitor ID 

24-hr 

Avg 

PM10

(μg/m3) 

1-hr Max 

PM10

(μg/m3) 

Time of 

Max 1-hr 

PM10

(LST) 

AQS 

Qualifier 

Flag 

Maricopa County 

Buckeye TEOM MC 04-013-4011-81102-1 169 736 200 RJ 

Central Phoenix TEOM MC 04-013-3002-81102-4 308 4392 200 RJ 

Durango 

Complex TEOM MC 04-013-9812-81102-1 255 3217 200 RJ 

Dysart TEOM MC 04-013-4010-81102-1 119 504 300 
Exceptional Event, Phoenix, September 2, 2011 Conceptual Model 

8 

Table 2-1. Statewide monitor information and PM10 measurements on September 2, 

2011. The 11 Phoenix-area monitors discussed in this report are shown in bold font. 

Page 2 of 3 

Monitor Monitor 

Type Operator AQS Monitor ID 

24-hr 

Avg 

PM10

(μg/m3) 

1-hr Max 

PM10

(μg/m3) 

Time of 

Max 1-hr 

PM10

(LST) 

AQS 

Qualifier 

Flag 

Fort 

McDowell/Yuma 

Frank 

TEOM FMIR 04-013-5100-8112-1 195 N/A N/A 

Glendale TEOM MC 04-013-2001-81102-1 132 621 200 

Greenwood TEOM MC 04-013-3010-81102-1 198 2062 200 RJ 

Higley TEOM MC 04-013-4006-81102-1 213 1289 200 RJ 

JLG Supersite BAM ADEQ 04-013-9997-81102-3 150 985 200 

JLG Supersite TEOM ADEQ 04-013-9997-81102-4 208 2600 200 RJ 

North Phoenix TEOM MC 04-013-1004-81102-1 115* 390 300 

South Phoenix TEOM MC 04-013-4003-81102-1 339 4163 200 RJ 

West Chandler TEOM MC 04-013-4004-81102-1 387 3327 200 RJ 

West 43rd Avenue TEOM MC 04-013-4009-81102-1 219 2523 200 RJ 

West Phoenix TEOM MC 04-013-0019-81102-1 133 294 300 

Zuni Hills TEOM MC 04-013-4016-81102-1 110 484 300 

Apache County 

N/A N/A WMAT 04-001-1003-81102-1 10 24 1500 

Coconino County 

N/A N/A ADEQ 04-005-1237-81102-1 N/A N/A N/A 

Gila County 

Hayden Old Jail TEOM ADEQ 04-007-1001-81102-3 68 180 1300 IJ 

Navajo County 

N/A N/A WMAT 04-017-1002-81102-1 14 42 1400 

Pima County 

Ajo TEOM ADEQ 04-019-0001-81102-3 150 485 200 IJ 

Geronimo BAM PCDEQ 04-019-1113-81102-1 46 137 1400 

Green Valley BAM PCDEQ 04-019-1030-81102-1 27 89 1600 

Orange Grove FRM PCDEQ 04-019-0011-81102-2 N/A N/A N/A 

Rillito TEOM ADEQ 04-019-0020-81102-3 77 222 1300 IJ 

South Tucson FRM PCDEQ 04-019-1001-81102-1 47 N/A N/A 

Pinal County 

Apache Junction 

Fire Station TEOM PCAQCD 04-021-3002-81102-3 217 1441 200 RJ 

Casa Grande 

Downtown TEOM PCAQCD 04-021-0001-81102-3 235 3143 100 RJ 

Combs School TEOM PCAQCD 04-021-3009-81102-3 186 2218 200 RJ 

Cowtown TEOM PCAQCD 04-021-3013-81102-3 588 7909 200 RJ 

Maricopa TEOM PCAQCD 04-021-3010-81102-3 257 2786 200 RJ 
Exceptional Event, Phoenix, September 2, 2011 Conceptual Model 

9 

Table 2-1. Statewide monitor information and PM10 measurements on September 2, 

2011. The 11 Phoenix-area monitors discussed in this report are shown in bold font. 

Page 3 of 3 

Monitor Monitor 

Type Operator AQS Monitor ID 

24-hr 

Avg 

PM10

(μg/m3) 

1-hr Max 

PM10

(μg/m3) 

Time of 

Max 1-hr 

PM10

(LST) 

AQS 

Qualifier 

Flag 

Pinal County 

Housing TEOM PCAQCD 04-021-3011-81102-3 310 4135 100 RJ 

Stanfield TEOM PCAQCD 04-021-3008-81102-3 268 2542 200 RJ 

Santa Cruz County 

Nogales Post 

Office BAM ADEQ 04-023-0004-81102-3 27 65 2100 

Yuma County 

Yuma Supersite TEOM ADEQ 04-027-8011-81102-3 N/A N/A N/A 

* Note: Power failure and/or instrumentation range limitation occurred during expected maximum 

concentration hour(s) likely resulting in an underestimate of 24-hr average. 

TEOM: Tapered element oscillating microbalance monitor 

BAM: Beta attenuation monitor 

FRM: Federal reference method 

WMAT: White Mountain Apache Tribe of Fort Apache Reservation, AZ 

MC: Maricopa County Air Quality Department 

ADEQ: Arizona Department of Environmental Quality 

FMIR: Fort McDowell Indian Reservation 

PCAQCD: Pinal County Air Quality Control District 

PCDEQ: Pima County Department of Environmental Quality 

RJ: qualifier flag for high winds 

IJ: qualifier flag for high winds 

Exceptional Event, Phoenix, September 2, 2011 Causal Relationship

3. Causal Relationship 

3.1 Discussion 

Meteorological and air quality observations indicate that dust carried by thunderstorm 

outflow was directly responsible for the high PM10 concentrations observed in the Phoenix area 

on September 2, 2011. On the evening of September 1, intense thunderstorms developed over 

the higher terrain of northwestern Mexico northward into far southern Arizona (Figure 3-1). 

These thunderstorms expanded northward and weakened, but the thunderstorms generated 

dust-carrying outflow boundaries that propagated into the Phoenix area early on September 2. 

As stated in Section 2.2, thunderstorms associated with the summer monsoon season can 

generate strong winds and blowing dust across Arizona. The likely source regions for PM10

during the September 2, 2011 event were the deserts of far northwestern Mexico and along the 

path of the outflow boundary from those deserts northward through Pima, Pinal, and southern 

Maricopa Counties. This region largely consists of natural, undisturbed desert. In addition, the 

month leading up to the September 2, 2011 event was very dry in the Phoenix area, with the 

last measureable rainfall at KPHX occurring on August 3. This combination of geography and 

lack of rainfall preceding the event resulted in a large fetch of soils that were particularly 

vulnerable to particulate suspension. 

Figure 3-1. Infrared satellite image from midnight LST on September 2, 2011 

(GOES-West). Colder temperatures (blues, purples, and white) indicate tall, convective 

(thunderstorm) clouds. Thunderstorms over northwestern Mexico generated outflow 

boundaries that carried dust northwestward into Arizona. 

1 
Exceptional Event, Phoenix, September 2, 2011 Causal Relationship

Figure 3-2 and Table 3-1 summarize the progression of the thunderstorm outflow and 

windblown dust as it moved into the Phoenix area. The dust-laden outflow arrived in the 

Phoenix area around 0100 LST on September 2. PM10 concentrations at the West Chandler 

(Figure 3-3), Higley (Figure 3-4), and other monitors (Appendix A) increased sharply over this 

time period, with 1-hr PM10 concentrations exceeding 3,000 g/m3

 at some monitors. Several 

wind monitors in the Phoenix area measured sustained winds of over 20 to 25 mph and wind 

gusts in excess of 30 mph coincident with the sharp increase in PM10 concentrations. The 

thunderstorm outflow and associated strong winds were also evident on the NWS Doppler radar 

in Phoenix (Figure 3-5). In addition, visibility at KPHX and other regional airports decreased 

significantly with the arrival of the strong winds and blowing dust (Figure 3-6). Visibility 

cameras in the Phoenix area showed hazy conditions after sunrise on September 2, 2011, likely 

due to suspended dust from the outflow boundary that moved through earlier that morning. 

Links to these videos and other media coverage and images pertaining to this windblown dust 

event are shown in Appendix B. It is also important to note that before the abrupt increase in 

PM10 in the Phoenix area, winds were lighter and PM10 concentrations were much lower, 

illustrating the correlation between the high winds and the dust. 

Figure 3-2. Phoenix area air quality and meteorological monitors. Wind and PM10 data 

and time of observation are shown in Table 3-1. Gusty south-to-southwesterly winds 

transported dust into the Phoenix area early on September 2, 2011. Circular symbols 

and numbers are placed at the location of the monitor site; wind barbs are offset for 

visual clarity. 

2 
Exceptional Event, Phoenix, September 2, 2011 Causal Relationship

Table 3-1. Observed wind speeds and wind gusts at Phoenix area monitors on 

September 2, 2011. Symbols in the first column correspond to monitor locations in 

Figure 3-2. All exceedance monitors reported high PM10 concentrations at 0200 LST on 

September 2, 2011, coincident with high wind gusts. 

Monitor 

1-hr Max 

PM10 

(g/m3) 

Time of 

Observation 

(LST) 

Wind 

Speed 

(mph) 

Wind 

Gust 

(mph) 

Wind 

Direction 

(degrees) 

KIWA - 0105 31 37 180 

Buckeye 735 0200 8 25 219 

Central 

Phoenix 4392 0200 9 28 204 

Durango 

Complex 3217 0200 5 19 204 

Greenwood 2062 0200 5 17 201 

Higley 1289 0200 12 27 205 

JLG Supersite 2600 0200 5 15 197 

South Phoenix 4163 0200 9 26 188 

West 43rd

Avenue 2523 0200 7 19 212 

West Chandler 3327 0200 16 34 198 

Maricopa - 0200 21 32 166 

Paloma - 0200 20 28 202 

Queen Creek - 0200 17 32 189 

KPHX - 0209 28 35 180 

KBXK - 0215 20 25 230 

KLUF - 0229 28 34 240 

KDVT - 0251 21 26 230 

North Phoenix 390 0300 5 13 263 

5 

9 

8 

1 

2 

3 

11 

5 

7 

6 

2 

3 

1 

4 

1 

2 

3 

4 

3 
Exceptional Event, Phoenix, September 2, 2011 Causal Relationship

Figure 3-3. Hourly PM10 concentrations and wind speeds at the West Chandler monitor 

on September 2-3, 2011. PM10 concentrations sharply increased at 0100 LST on 

September 2, 2011, coincident with gusty winds, indicating the arrival of windblown dust. 

Note that PM10 concentration was not available at 800 LST on September 1, 2011 at the 

West Chandler monitor. 

Figure 3-4. Hourly PM10 concentrations and wind speeds at the Higley monitor on 

September 1-2, 2011. PM10 concentrations sharply increased at 0100 LST on 

September 2, 2011, coincident with gusty winds, indicating the arrival of windblown dust. 

4 
Exceptional Event, Phoenix, September 2, 2011 Causal Relationship

Figure 3-5. Radar base velocity data at 0150 LST on September 2, 2011, from NWS 

Phoenix Doppler radar. Green indicates flow toward the radar; orange and red indicate 

flow away from the radar. A well-defined outflow boundary (dashed line) approached the 

Phoenix area from the south. This boundary carried dust northward and westward 

across the Phoenix metropolitan area. 

5 
Exceptional Event, Phoenix, September 2, 2011 Causal Relationship 

Figure 3-6. Hourly PM10 concentrations at Phoenix area monitors and visibility at KPHX. 

Visibility was greatly reduced at 0200 LST on September 2, coincident with the sharp 

increase in PM10 concentrations at Phoenix area monitors, indicating the arrival of 

windblown dust. 

PM10 concentrations decreased after 0400 LST as the dust storm propagated northward 

out of the Phoenix area. Monitors across the Phoenix metropolitan area and areas further south 

in Arizona, including Tucson, reported gusty winds and sharp increases in PM10

concentrations, illustrating the widespread nature of this dust storm event. No measurea

rainfall was reported in Phoenix with the passage of this outflow boundary and the assoc

ble 

iated 

dust. 

3.2 Summary 

n 

x 

 wind speeds 

and wind gusts, and that the strong winds were experienced over a large area. 

The information presented in this section demonstrates a clear causal relationship 

between the windblown dust and the PM10 exceedance measured at Phoenix area monitors o

September 2, 2011. The radar and wind data shown in this section illustrate the spatial and 

temporal representation of the dust storm as it moved through the Phoenix area. In addition, 

the time series plots of air quality and meteorological data found in this section and in Appendi

A show that the sharp increase in PM10 concentrations coincided with the strong

6 
Exceptional Event, Phoenix, September 2, 2011 Historical Norm 

4. Historical Norm 

4.1 Analysis 

PM10 concentrations measured at Phoenix area monitors on September 2, 2011, were 

unusual and in excess of normal historical fluctuations. To establish the severity of this event, 

PM10 concentrations measured on September 2, 2011, were compared to a historical 

2007-2011 five-year annual data set at each monitor (Figure 4-1 and Appendix C). The PM10

concentrations measured at the Central Phoenix monitor (Figure 4-1) and other monitors on 

September 2, 2011, resulted in some of the highest 24-hr averages measured over the five-year 

period. Similar time-series plots for the other Phoenix area monitors with exceedances are 

shown in Appendix C. The graphs also highlight the July to September 2011 season, which had 

many unusually high PM10 24-hr averages, demonstrating the uniqueness of the dust storm 

events during 2011 relative to previous years. 

4.2 Summary 

Given the recorded values and using similar methodology to the one accepted by EPA, it 

is clear that the PM10 levels on September 2, 2011, were outside of normal historical 

fluctuations. This analysis provides evidence that the event affected air quality on a historic 

scale. 

 

Figure 4-1. 24-hr average PM10 concentrations at the Central Phoenix monitor 

(2007-2011). The 24-hr average PM10 concentration on September 2, 2011, is 

highlighted in red. The period of July to September 2011 is shown by the vertical box. 

1 
Exceptional Event, Phoenix, September 2, 2011 Not Reasonably Controllable or Preventable

1 

5. Not Reasonably Controllable or Preventable 

5.1 Background 

ADEQ and MCAQD are responsible for implementing regulatory measures to control 

emissions from agricultural sources, stationary sources, fugitive dust sources, and open burning 

within Maricopa County. Three major programs provide or contribute to air pollution control 

measures for the Greater Phoenix area. These programs include: 

1. ADEQ’s Agricultural Best Management Program (AgBMP) 

2. Maricopa County’s Inspection and Compliance Program 

3. ADEQ’s Air Quality Forecasting Program 

Specifically, ADEQ is responsible for compliance assistance and enforcement of 

Agricultural Best Management Practices developed by the Governor’s Agricultural Best 

Management Practices Committee, while MCAQD is responsible for compliance assurance for 

all other significant sources of PM10 emissions. In addition to routine inspections and 

inspections driven by complaints, inspections are often increased when (1) ADEQ forecasters 

issue a High Risk for the Maricopa County Dust Control Forecast, (2) ADEQ forecasters issue a 

High Pollution Advisory, or (3) near real-time monitoring data indicate unique activity via high 

PM concentrations. The forecasting program and inspection/compliance programs work 

together so that resources can be best used during days of greatest risk for elevated PM 

emissions. 

On July 25, 2002, EPA took initial action to finalize approval of the Best Available 

Control Measure (BACM) and the Most Stringent Measure (MSM) demonstrations in the Serious 

Area PM10 plan for the Maricopa County portion of the metropolitan Phoenix PM10

nonattainment area (67 FR 48718). These BACM and MSM demonstrations were again 

approved by EPA on July 14, 2006 (71 FR 43979). The Agricultural Best Management 

Practices General Permit rule and related definitions have been approved into the Arizona 

Administrative Code as R18-2-610 and R18-2-611, pursuant to Arizona Revised Statutes 

§49-4571

. 

5.1.1 Control Measures 

Maricopa County regulations of PM10 emissions are listed in Table 5-1. 

 

1

 Updates to the AgBMP program in December, 2011, clarified BMPs for crops and added BMPs for animal 

operations. Effective 12/29/2011, R18-2-611 was renumbered to R18-2-610.01 Agricultural PM10 General Permit 

for Crop Operations and R18-2-611.01 Animal Operations PM10 General Permit was added. Definitions for Crop 

Operations were revised at R18-2-610 and new definitions for Animal Operations were added at R18-2-611. 
Exceptional Event, Phoenix, September 2, 2011 Not Reasonably Controllable or Preventable

2 

Table 5-1. Rules and ordinances regulating particulate matter emissions in Maricopa County. 

Rule/Ordinance Number & Title Description 

Rule 300: Visible emissions Establishes standards for visible emissions and opacity. 

Rule 310: Fugitive dust from dustgenerating operations 

Establishes limits for the emissions of particulate matter into the 

ambient air from any property, operations, or activity that may serve as 

a fugitive dust source. 

Rule 310.01: Fugitive dust from nontraditional sources of fugitive dust 

Establishes limits for the emissions of particulate matter into the 

ambient air from open areas, vacant lots, unpaved parking lots, and 

unpaved roadways which are not regulated by Rule 310 and which are 

not required to have either a permit or a dust control plan. 

Rule 311: Particulate matter from 

process industries 

Establishes emission rates based on process weight applicable to any 

affected operations not subject to Rule 316. 

Rule 312: Abrasive blasting Establishes limits for particulate emissions from abrasive blasting 

operations. 

Rule 314: Open outdoor fires and 

indoor fireplaces at commercial and 

institutional establishments 

Establishes limits for the emissions of air contaminants produced from 

open burning. 

Rule 316: Nonmetallic mineral 

processing 

Establishes limits for the emissions of particulate matter into the 

ambient air from any nonmetallic mining operation or rock product 

processing plant. 

Rule 317: Hospital/medical/infectious 

waste incinerators 

Establishes limits for the emissions of air pollutants from medical 

waste incinerators. 

Rule 322: Power plant operations 

Establishes limits for the emissions of nitrogen oxides, sulfur oxides, 

carbon monoxide, and particulate matter from existing power plants 

and cogeneration plants. 

Rule 323: Fuel burning equipment 

from industrial/commercial/ 

institutional (ICI) sources 

Establishes limits for the emissions of nitrogen oxides, sulfur oxides, 

carbon monoxide and particulate matter from ICI sources. 

Rule 324: Stationary internal 

combustion (IC) engines 

Establishes limits for the emissions of carbon monoxide, nitrogen 

oxides, sulfur oxides, volatile organic compounds, and particulate 

matter from stationary internal combustion engines, including 

stationary IC engines used in cogeneration. 

Rule 325: Brick and structural clay 

products (BSCP) manufacturing 

Establishes limits for particulate matter emissions from the use of 

tunnel kilns for curing in the BSCP manufacturing processes. 

Ordinance P-25: Leaf blower 

restriction 

Establishes restrictions for leaf blowers in incorporated and 

unincorporated sections of Area A in Maricopa County. 

Ordinance P-26: Residential wood 

burning restriction Establishes restrictions for residential wood burning. 

Ordinance P-27: Vehicle parking 

and use on unstabilized vacant lots 

Establishes restrictions for vehicle parking and use on unstabilized 

vacant lots in unincorporated sections of Area A in Maricopa County. 

Ordinance P-28: Off-road vehicle 

use in unincorporated areas of 

Maricopa County 

Establishes restrictions for operating vehicles on unpaved property in 

unincorporated areas of Maricopa County. 
Exceptional Event, Phoenix, September 2, 2011 Not Reasonably Controllable or Preventable

3 

Rule/Ordinance Number & Title Description 

Arizona Administrative Code R18-

2-611 & 610: Agricultural PM10

general permit 

Establishes a requirement for commercial farmers to implement best 

management practices and maintain a record demonstrating 

compliance. 

5.1.2 Additional Measures 

In addition to the rules and regulations listed in Table 5-1, other PM10-reducing control 

measures (e.g., paving unpaved roads, PM10-certified street sweepers, controlling unpaved 

parking lots, etc.) have been committed to and implemented by local jurisdictions throughout the 

PM10 nonattainment area and incorporated into the Arizona state implementation plan (SIP) 

through PM10 plans such as the Revised Maricopa Association of Governments’ (MAG) 1999 

Serious Area Particulate Plan for PM10 for the Maricopa County Nonattainment Area. The Pinal 

County Air Quality Control District (PCAQCD) also implements regulatory control measures on 

emissions from existing and new non-point sources within Pinal County (see Table 5-2). 

Additionally, the PCAQCD implements specific nonattainment rules for that part of the Phoenix 

PM10 nonattainment area that resides in Pinal County (see Table 5-3). 

Table 5-2. Pinal County rules regulating existing and new non-point sources in Pinal County. 

Article Number & Title Description 

Article 2: Fugitive dust Provides a mechanism to reasonably regulate operations which 

periodically may cause fugitive dust emissions into the atmosphere. 

Article 3: Construction sites – 

fugitive dust 

Improves the control of excessive fugitive dust emissions that have 

been traditionally associated with construction, earthwork, and land 

development, and thereby minimize nuisance impacts. 

Table 5-3. Pinal County rules regulating fugitive dust in Pinal County portion of Phoenix 

PM10 nonattainment area. 

Article Number & Title Description 

Article 4: Nonattainment area rules; 

dustproofing for commercial parking, 

drives, and yards 

Establishes rules to avoid violations of the prevailing PM10 standard 

and additionally minimize nuisance impacts by improving control of 

excessive fugitive dust emissions from unpaved parking lots. 

Article 5: Nonattainment area rules; 

stabilization for residential parking 

and drives 

Establishes rules for stabilizing residential properties. 

Article 6: Restrictions on vehicle 

parking and use on vacant lots Establishes rules for unpaved or unstable vacant lots. 

Article 7: Construction sites in 

nonattainment areas – fugitive dust 

Establishes rules to avoid violations of the prevailing PM10 standard 

and additionally minimize nuisance impacts by improving control of 

excessive fugitive dust emissions from activities associated with 

construction, earthwork, or land development. 
Exceptional Event, Phoenix, September 2, 2011 Not Reasonably Controllable or Preventable

4 

Article Number & Title Description 

Article 8: Nonattainment area rules, 

requirement for stabilization of 

disturbed areas at vacant lots 

Establishes rules for stabilizing disturbed areas at vacant lots. 

5.1.3 PM10 Rule Effectiveness 

MCAQD analyzed the effectiveness of its fugitive dust rules (Rules 310, 310.01 and 316) 

in terms of permit compliance rates. This rule effectiveness (RE) study was designed to assess 

how many sources regulated by MCAQD during the subject time period received no PM10

emissions-related violations. As a basis for comparison, the percentage of permitted sources in 

compliance during calendar year 2007 was 76% for sources subject to Rule 310, 85% for Rule 

310.01 sources, and 40% for Rule 316 sources. In early 2008, Rules 310, 310.01, and 316 

were strengthened, and new ordinances (covering additional source categories such as leaf 

blowers, vacant lots, and off-road vehicles) were adopted. These enhancements resulted from 

MCAQD department’s obligations under such agreements as the 2005 Revised PM10 State 

Implementation Plan for the Salt River Area and the Maricopa Association of Governments 

(MAG) 2007 Five Percent Plan for PM10 for the Maricopa County Nonattainment Area to reduce 

PM10 emissions throughout the county. Three major areas that contributed to increased 

compliance were an increase in departmental staffing (especially inspectors), a robust training 

program, and regulatory changes that broadened and strengthened control measures under 

Rules 310, 310.01, and 316. 

Source compliance rates were re-assessed for FY 2009 (July 2008–June 2009), a 

period that allowed time for the new and revised regulations to take effect. The results showed 

significant increases in compliance compared with the earlier period: to 90% (from 76%) for 

Rule 310 sources, 95% compliance (from 85%) for Rule 310.01 sources, and 65% (from 40%) 

for Rule 316 sources. These improvements continued into calendar year 2010 with compliance 

rates of 94% for Rule 310 sources, 96% for Rule 310.01 sources, and 73% for Rule 316 

sources. The timeline below (Figure 5-1) illustrates the improvements in RE over the last 

several years; it also points out significant revisions to previous rules, as well as newly adopted 

rules and ordinances. 
Exceptional Event, Phoenix, September 2, 2011 Not Reasonably Controllable or Preventable

5 

Figure 5-1. Timeline of Maricopa County fugitive dust rules and ordinances. 

5.1.4 Compliance and Enforcement Activities 

MCAQD is prepared to proactively respond to high wind events and protect human 

health and well-being. MCAQD ‘s approach consists of two primary components: routine 

proactive inspections, as well as surveillance inspections, conducted both during and after 

significant air quality events. MCAQD routinely inspects dust control-permitted sites and 

increase the frequency of inspections for permits covering areas of 10 acres or more. Rule 316 

sources are also regularly inspected multiple times every year. Maricopa County also responds 

to the majority of complaints within 24 hours. 

Maricopa County monitors the ADEQ Five-Day Dust Control Forecast to identify the 

potential for elevated PM10 pollution levels due to high winds or stagnant conditions. When a 

High Pollution Advisory (HPA) is issued for Maricopa County, MCAQD conducts additional 

increased surveillance before, during, and after the forecast event(s). MCAQD also conducts 

event surveillance and post-event activities during exceedance days that had not been forecast 

(i.e., those instances in which an HPA had not been issued). 

Pre-event surveillance consists of surveying high-risk areas for any dust-generating 

activities, educating sources of the impending HPA event, and issuing violations for failure to 

comply with local, state, or federal regulations. During the event, MCAQD inspectors survey 

high-risk areas to confirm that control measures are in place, document any violations, and 

contact other regulatory agencies if necessary. Post-event activities include continued surveys 

of high-risk areas, re-inspecting sources that had received violations within two business days, 

and an internal MCAQD debriefing of event activities. 

During 2011, a total of 14 MCAQD air monitoring sites were upgraded with new 

equipment that allows the monitoring sites to automatically report measured readings at 

5-minute intervals. Previously, only hourly readings were available. The real-time data 

reporting system includes a mechanism to alert MCAQD field staff when PM concentrations are 

elevated. The system allows MCAQD responders to review concentrations at the monitors and 
Exceptional Event, Phoenix, September 2, 2011 Not Reasonably Controllable or Preventable

6 

to consult the National Weather Service website to check for weather event activity. This 

capability allows the MCAQD responder to identify regional events and monitor specific issues. 

If necessary, the MCAQD responders can inform nearby stakeholders and local governments of 

the elevated PM10 concentrations. 

In addition to MCAQD’s efforts in pre-event surveillance and proactive inspections, 

ADEQ’s AgBMP inspector also monitors the ADEQ Five-Day Dust Control Forecast and the 

MCAQD air monitoring sites that include real-time data. The ADEQ AgBMP inspector uses 

specific knowledge of seasonal activities and associations with the local growers and dairymen 

to communicate the importance of limiting dust-generating activities, especially during high wind 

events. Additional outreach is conducted with facility representatives prior to forecasted high 

wind alert days. Should the PM10 readings at a MCAQD air monitoring site show notable 

increase, the ADEQ AgBMP inspector is dispatched to contact the owners and operators of 

agricultural fields in the area to discern if their activities are causing negative impacts. The 

AgBMP inspector is prepared to respond to most agriculture complaints within 24 hours. 

5.1.5 Review of Source-Permitted Inspections and Public Complaints 

ADEQ’s Arizona Unified Repository for Information Tracking of the Environment 

(AZURITE) database was queried to compile a list of inspections for the permitted sources in 

the Phoenix area around the time of the September 2, 2011, PM10 exceedances. An evaluation 

of all inspection reports, air quality complaints, compliance reports, and other documentation do 

not indicate any evidence of unusual anthropogenic-based PM10 emissions. During the time 

period of August 30 through September 5, 2011, MCAQD inspectors conducted a total of 112 

inspections of permitted facilities, of which 89 were at fugitive dust sources. Additionally, 

MCAQD conducted 89 inspections on vacant lots and unpaved parking lots during this period. 

During this seven-day period, nine violations were issued countywide for PM10 and 

non-PM10 related violations. One violation was for PM10 emissions within a four-mile radius of 

an exceeding monitor. 

On September 1, 2011, a violation was issued to a permitted construction site within four 

miles of the Central Phoenix monitor. The violation was issued during a complaint inspection. 

The site was issued a violation for failing to maintain complete dust control records. The site 

only kept records of the amount of water applied for dust control, but failed to maintain records 

of subcontractor registration or maintenance of the trackout control device. The violation was 

corrected on subsequent inspections. 

MCAQD was prepared for any complaints received due to the high wind event. During 

the seven-day period from August 30 through September 2, 2011, MCAQD received 23 

complaints, of which 17 were related to windblown dust. Each complaint was assigned to and 

investigated by a MCAQD inspector. A review of all pertinent records from this period indicates 

that MCAQD inspectors observed only one emission violation, as discussed in the previous 

paragraph, of local, state, or federal PM10 regulations within a four-mile radius of the exceeding 

monitors. 
Exceptional Event, Phoenix, September 2, 2011 Not Reasonably Controllable or Preventable

7 

In addition to MCAQD’s efforts in pre-event surveillance and proactive inspections, 

ADEQ’s Agricultural Best Management Practice Program (Ag BMP) inspector also monitors the 

ADEQ Five-Day Dust Control Forecast and the MCAQD air monitoring sites that include realtime data. The ADEQ Ag BMP inspector uses specific knowledge of seasonal activities and 

associations with the local growers and dairymen to communicate the importance of limiting 

dust-generating activities, especially during high wind events. Additional outreach is conducted 

with facility representatives prior to forecast high wind alert days. Should the PM10 readings at 

a MCAQD air monitoring site show a notable increase, the ADEQ Ag BMP inspector is 

dispatched to contact the owners and operators of agricultural fields in the area to discern 

whether their activities are causing negative impacts. The Ag BMP inspector is prepared to 

respond to most agriculture complaints within 24 hours. 

Based on a review of the inspection reports and site visit documentation, there is no 

evidence to suggest that agricultural activities produced unusual PM10 emissions. On 

September 1, 2011, the ADEQ AgBMP inspector received one complaint and made immediate 

contact with the operation. The subject agriculture field was located in Surprise and was being 

tilled to prepare the field to plant vegetables. Surprise is located in northwestern Maricopa 

County, near the Dysart and Zuni Hills AQS monitors. Since both the Dysart and Zuni Hills AQS 

monitors were not in exceedance on September 2, 2011, this one particular incident is highly 

unlikely to have caused of the widespread regional dust. 

5.2 Forecasts and Warnings 

Dust forecasts were released prior to the event by ADEQ (Appendix D). For September 

2, 2011, the Maricopa County Dust Control Forecast indicated a moderate risk level for 

unhealthy PM10. The Dust Control Forecast also indicated a potential for dense blowing dust 

throughout the area generated by outflow from thunderstorms. 

5.3 Wind Observations 

Sustained wind speeds of over 20 mph with gusts over 30 were reported at monitors 

across the Phoenix area, and sustained winds as high as 28 mph at Sky Harbor and 31 mph at 

Mesa Gateway with gusts as high as 35 mph were reported as the outflow boundary moved 

through (Figure 3-3, Table 3-1 and Appendix A). 

5.4 Summary 

The thunderstorm outflow event of September 2, 2011, produced gusty winds that 

transported dust and PM10 into the Phoenix PM10 nonattainment area. The source region of the 

outflows that caused the exceedances was largely located in areas outside the Phoenix PM10

nonattainment area, primarily the deserts of Pima, Pinal, and southern Maricopa Counties. The 

Phoenix area is designated as a serious nonattainment area for PM10 and is required to have 

BACM for all significant sources of PM10. 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 
Exceptional Event, Phoenix, September 2, 2011 Not Reasonably Controllable or Preventable 

8 

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

be considered “reasonable” for these purposes. 

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, provides strong evidence that the 

exceedances of September 2, 2011, recorded within the Phoenix PM10 nonattainment area 

were not reasonably controllable or preventable. 

Exceptional Event, Phoenix, September 2, 2011 But-For Analysis

1 

6. But-For Analysis 

6.1 Discussion 

Section 50.14(c)(3)(iv)(D) in 40 CFR Part 50 requires that an exceptional event 

demonstration satisfies that “[t]here would have been no exceedance or violation but for the 

event.” The prior sections of this submittal have provided detailed information that in regard to 

the PM10 exceedances in the Phoenix PM10 nonattainment area on September 2, 2011, 

 The exceedances were not reasonably controllable or preventable, and 

 There was a clear causal relationship between PM10 transported from thunderstorm 

outflow originating in desert areas outside the Phoenix metropolitan area and the 

measured PM10 exceedances. 

The weight of evidence in these sections demonstrates that, but for the existence of dust 

emissions generated by thunderstorm outflow and the associated transport of PM10, there would 

have been no exceedances of the NAAQS for 24-hr average PM10. 

As shown in Section 3, radar velocity data and time-series plots of PM10 and wind 

speeds establish a clear causal relationship between the arrival of dust-laden thunderstorm 

outflow and elevated PM10 concentrations at monitors in the Phoenix area. Multiple 

independent measurements of wind speed, wind direction, and visibility all show that 

thunderstorm outflow was the mechanism for transport of PM10 into the Phoenix PM10

nonattainment area. In addition, PM10 concentrations were well below the NAAQS on days 

immediately before and after the windblown dust event. The source regions for the 

thunderstorm outflow and PM10 are clearly identified as desert areas south of Phoenix. The 

weight of evidence presented in this submittal provides 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 these uncontrollable natural events. 

As detailed in Section 5, all reasonable control measures were in place and/or 

implemented on a continual basis. Local regulatory agencies, industry, and the general public 

were alerted to the possibility of dust storms due to thunderstorm activity via daily forecasts and 

media reports. 

6.2 Summary 

The weight of evidence presented in this submittal provides 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 these uncontrollable natural events. 

Exceptional Event, Phoenix, September 2, 2011 Conclusions

1

7. Conclusions 

The PM10 exceedances that occurred on September 2, 2011, satisfy the criteria of the 

EER, which states that in order to justify the exclusion of air quality monitoring data, evidence 

must be provided for the following elements: 

1. The event satisfies the criteria set forth in 40 CFR 501 (j) that 

a. the event affected air quality, 

b. the event was not reasonably controllable or preventable, and 

c. the event was caused by human activity unlikely to recur in a particular location 

or was a natural event; 

2. There is a clear causal relationship between the measurement(s) under 

consideration and the event; 

3. The event is associated with a measured concentration(s) in excess of normal 

historical fluctuations; and 

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

7.1 Affects Air Quality 

As stated in the preamble to the Exceptional Events Rule, the event in question is 

considered to have affected air quality if it can be shown that there is a clear causal relationship 

between the monitored exceedance and the event, and that the event is associated with a 

measured concentration in excess of normal historical fluctuations. Given the information 

presented in Sections 2, 3, 4, and 5, we can reasonably conclude that the event in question 

affected air quality. 

7.2 Not Reasonably Controllable or Preventable 

Section 50.1(j) of 40 CFR Part 50 requires that an event must be “not reasonably 

controllable or preventable” in order to be defined as an exceptional event. This requirement is 

met by demonstrating that despite reasonable control measures in place within Maricopa 

County, high winds overwhelmed all reasonably available controls (Section 5). The PM10

exceedances discussed in this report were caused by naturally occurring thunderstorm outflow 

that transported dust into the Phoenix area from areas largely outside the Phoenix PM10

nonattainment area. These facts provide strong evidence that the PM10 exceedances on 

September 2, 2011, were not reasonably controllable or preventable. 

7.3 Natural Event 

As discussed above, the PM10 exceedances on September 2, 2011, were shown to be 

caused by transport of PM10 into the Phoenix area from thunderstorm outflow. The event 

therefore qualifies as a natural event. 
Exceptional Event, Phoenix, September 2, 2011 Conclusions 

2

7.4 Clear Causal Relationship 

The following points demonstrate that the high PM10 concentrations were caused by 

windblown dust: 

 Time-series of PM10 concentrations show that the timing of high PM10 at Phoenix-area 

monitors was consistent with gusty winds and low visibilities (Section 3). 

 High PM10 concentrations and gusty winds were reported throughout the Phoenix 

metropolitan area, illustrating the widespread, regional, and uncontrollable nature of this 

event (Section 3). 

 PM10 concentrations were well below the NAAQS on days immediately before and after 

the windblown dust event (Section3). 

 Dry conditions preceding the event resulted in soils that were particularly susceptible to 

particulate suspension by high winds (Section 3). 

 Visibility camera imagery after sunrise on September 2, 2011, indicated hazy conditions 

in the Phoenix area, likely attributable to suspended dust from the outflow boundary that 

moved through earlier that morning. 

7.5 Historical Norm 

The 24-hr average PM10 values measured at the exceedance monitors were historically 

unusually high compared to a multi-year data set (Section 4). 

7.6 But For 

On the basis of the weight-of-evidence described above and in Section 6, the 

exceedances of the federal 24-hr PM10 standard on September 2, 2011, in the Phoenix PM10

nonattainment area would not have occurred but for the thunderstorm-driven high winds and 

transport of dust from areas largely outside the Phoenix PM10 nonattainment area. 
Exceptional Event, Phoenix, September 2, 2011 Appendix A 

Appendix A: 

Air Quality and Meteorological Data 

This section contains time-series of PM10 concentrations and wind data for Phoenix-area 

monitors on September 1-2, 2011. The data show a region-wide increase in wind gusts 

coincident with the arrival of dust and high PM10. 

Figure A-1. Hourly PM10 concentrations and wind speeds at the Buckeye monitor on 

September 1-2, 2011. PM10 concentrations sharply increased and gusty winds were 

reported at 0200 LST on September 2, 2011, indicating the arrival of windblown dust. 

1
Exceptional Event, Phoenix, September 2, 2011 Appendix A 

2

Figure A-2. Hourly PM10 concentrations and wind speeds at the Central Phoenix 

monitor on September 1-2, 2011. PM10 concentrations and wind increased sharply at 

0200 LST on September 2, 2011, indicating the arrival of windblown dust. Note that wi

speed and gust data were not available between 700 and 1000 LST on September 1, 

2011 at the Central Phoenix m

nd 

onitor. 
Exceptional Event, Phoenix, September 2, 2011 Appendix A 

Figure A-3. Hourly PM10 concentrations and wind speeds at the Durango Complex 

monitor on September 1-2, 2011. PM10 concentrations increased sharply at 0200 LST on 

September 2, 2011, indicating the arrival of windblown dust. 

Figure A-4. Hourly PM10 concentrations and wind speeds at the Greenwood monitor on 

September 1-2, 2011. PM10 concentrations sharply increased at 0200 LST on 

September 2, 2011, coincident with gusty winds, indicating the arrival of windblown dust. 

Note that PM10 concentration was not available at 1000 LST on September 1, 2011 at the 

Greenwood monitor. 

3
Exceptional Event, Phoenix, September 2, 2011 Appendix A 

Figure A-5. Hourly PM10 concentrations and wind speeds at the JLG Supersite monitor 

on September 1-2, 2011. PM10 concentrations sharply increased at 0200 LST on 

September 2, 2011, indicating the arrival of windblown dust. 

Figure A-6. Hourly PM10 concentrations and wind speeds at the North Phoenix monitor 

on September 1-2, 2011. PM10 concentrations sharply increased at 0300 LST on 

September 2, 2011, coincident with gusty winds, indicating the arrival of windblown dust. 

Note that PM10 concentrations were not available at 200 LST on September 2, 2011 at 

the North Phoenix monitor. 

4
Exceptional Event, Phoenix, September 2, 2011 Appendix A 

 Figure A-7. Hourly PM10 concentrations and wind speeds at the South Phoenix monitor 

on September 1-2, 2011. PM10 concentrations sharply increased at 0200 LST on 

September 2, 2011, coincident with gusty winds, indicating the arrival of windblown dust. 

Figure A-8. Hourly PM10 concentrations and wind speeds at the West 43rd Avenue 

monitor on September 1-2, 2011. PM10 concentrations sharply increased at 0200 LST on 

September 2, 2011, coincident with gusty winds, indicating the arrival of windblown dust. 

5
Exceptional Event, Phoenix, September 2, 2011 Appendix A 

Figure A-9. Hourly wind speeds at the Maricopa monitor on September 1-2, 2011. Wind 

speeds and gusts sharply increased at 0200 LST on September 2, 2011. 

Figure A-10. Hourly wind speeds at the Paloma monitor on September 1-2, 2011. Wind 

speeds and gusts sharply increased at 0200 LST on September 2, 2011. 

6
Exceptional Event, Phoenix, September 2, 2011 Appendix A 

7

Figure A-11. Hourly wind speeds at the Queen Creek monitor on September 1-2, 2011. 

Wind speeds and gusts sharply increased at 0200 LST on September 2, 2011. 
8 

U.S. Department of Commerce National Oceanic & Atmospheric Administration

QUALITY CONTROLLED LOCAL CLIMATOLOGICAL DATA 

HOURLY OBSERVATIONS TABLE 

PHOENIX SKY HARBOR INTL AIRPORT (23183) 

PHOENIX, AZ (09/2011) 

Nation

Asheville

 Elevation: 1107 ft. above sea level Latitude: 33.427 Longitude: -112.003 Data Version: VE

Dry 

Bulb 

Temp

Wet 

Bulb 

Temp

Dew 

Point 

Temp Date Time (LST) Station Type Sky Conditions Visibility (SM) Weather Type

(F) (C) (F) (C) (F) (C)

Rel 

Humd 

%

Wind 

Speed 

(MPH)

Wind

Dir

Wind

Gusts

(MPH)

Station

Pressure

(in. hg)

Press

Tend

Net

3-hrChg

(mb)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02

0051 0149 0151 0154 0156 0209 0239 0247 0251 0255 0315 0351 0451 0551 0651 0751 0851 0951 1051 1151 1251 1351 1451 1551 1651 1751 1851 1951 2051 2151 2251 2351

11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11

FEW140 BKN190 FEW120 BKN160 SCT020 BKN140 SCT020 BKN140 VV006 VV008 BKN010 BKN013 BKN110 BKN013 BKN110 BKN019 BKN110 BKN160 SCT019 BKN110 BKN160 FEW110 BKN160 BKN200 FEW110 SCT160 BKN200 FEW110 BKN160 BKN200 SCT130 BKN160 BKN250 FEW110 BKN130 BKN170 FEW110 BKN140 BKN170 FEW110 SCT140 BKN180 FEW110 SCT140 BKN180 FEW140 SCT180 BKN210 FEW140 SCT190 BKN220 FEW140 SCT190 BKN220 FEW140 SCT190 BKN220 FEW140 SCT190 SCT220 FEW140 SCT190 SCT220 FEW140 SCT190 SCT250 FEW140 SCT250 FEW140 SCT250 FEW140 SCT250 CLR CLR FEW170

10.00 2.50 1.50 0.75 0.75 1.00 1.25 2.00 2.50 3.00 3.00 5.00 10.00 10.00 9.00 9.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00

HZ BLDU BLDU BLDU -RA BLDU BLDU BLDU BLDU BLDU BLDU DU -RA -RA 

95979593939191919291919190888788899091939797991011011011019897959393

35.036.035.034.034.033.033.033.033.333.033.032.832.231.130.631.131.732.232.833.936.136.137.238.338.338.338.336.736.135.033.933.9

6466687071737171717071707374747373747373727371707069696968676665

17.518.820.020.921.522.621.721.721.621.221.721.222.823.423.322.822.923.122.922.622.422.921.921.221.020.520.320.719.919.519.018.5

4045525759646161605961596568686666666563606257535250495350504947

4.47.011.114.015.018.016.016.015.615.016.015.018.320.020.018.918.918.918.317.215.616.713.911.711.110.09.411.710.010.09.48.3

15 17 23 30 32 41 37 37 34 34 37 34 44 52 53 48 47 45 42 37 29 31 25 20 19 18 17 22 20 22 22 21

6 26 22 16 18 28 17 14 15 13 11 0 9 7 5 8 7 5 5 3 10 3 8 14 14 13 11 7 10 6 5 6

230 170 180 160 170 180 180 210 220 210 220 000 360 340 VR 320 300 300 290 260 280 290 330 320 290 300 280 250 260 250 250 280

34 34 34 34 34 30 18 18 

28.54 28.60 28.60 28.61 28.61 28.60 28.62 28.62 28.62 28.62 28.63 28.65 28.65 28.66 28.66 28.66 28.68 28.69 28.69 28.66 28.65 28.62 28.60 28.57 28.55 28.54 28.53 28.54 28.55 28.56 28.57 28.57

3 1 1 1 8 6 5 1

024016005007021024003008

Figure A-12. Quality-controlled local climatological data hourly observations table for Phoenix Sky Harbor International Airport

Arizona (09/02/2011). Note in the Weather Type column that BLDU (blowing dust) was reported for several hours. For a more explanation of the weather codes shown in the table above, please see http://www.nws.noaa.gov/oso/oso1/oso12/document/gu

dynamically generated via http://cdo.ncdc.noaa.gov/qclcd/QCLCD.
9 

U.S. Department of Commerce National Oceanic & Atmospheric Administration QUALITY CONTROLLED LOCAL CLIMATOLOGICAL DATA 

(final) 

HOURLY OBSERVATIONS TABLE 

WILLIAMS GATEWAY AIRPORT (23104) 

PHOENIX, AZ 

(09/2011) 

Elevation: 1382 ft. above sea level Latitude: 33.3 Longitude: -111.666 Data Version: VER2

Dry 

Bulb 

Temp

Wet 

Bulb

Temp

Dew

Point

Temp Date Time (LST) Station Type Sky Conditions Visibility (SM) Weather Type

(F) (C) (F) (C) (F) (C)

Rel 

Humd 

%

Wind 

Speed 

(MPH)

Wind

Dir

Wind

Gusts

(MPH)

Station

Pressure

(in. hg)

Press

Tend

Net

3-hr

Chg

(mb)

P

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 

0015 0035 0055 0115 0135 0155 0215 0235 0255 0315 0335 0355 0415 0435 0455 0515 0535 0549 0649 0747 0847 0954 1048 1148 1248 1347 1447 1550 1647 1747 1847 1950 1955 2015 2035 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

CLR CLR CLR BKN002 OVC002 M M SCT001 CLR CLR CLR CLR CLR CLR CLR CLR CLR SCT120 BKN150 OVC200 SCT120 BKN150 OVC200 SCT120 BKN150 OVC200 FEW120 BKN150 BKN200 FEW120 BKN150 BKN200 SCT120 BKN200 SCT120 BKN200 FEW150 SCT200 FEW150 SCT200 FEW150 SCT200 FEW150 SCT200 FEW150 SCT200 FEW150 FEW150 FEW CLR CLR CLR 

10.00 10.00 10.00 1.00 2.00 2.00 2.50 7.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 3.00 5.00 5.00 10.00 10.00 10.00 10.00 15.00s15.00s15.00s15.00s15.00 15.00 10.00 10.00 10.00 

HZ HZ HZ HZ HZ HZ HZ HZ HZ HZ HZ HZ HZ HZ HZ 

979797979393939391888888888888888888909191939799102102102106106102102100999797

36.036.036.036.034.034.034.034.033.031.031.031.031.031.031.031.031.031.032.033.033.034.036.037.039.039.039.041.041.039.039.038.037.036.036.0

65646470727373737272737272727272717272717374747372737172706868M676669

18.417.618.021.022.022.622.622.622.322.122.822.122.122.122.122.121.822.122.121.722.623.523.522.922.322.821.622.120.820.219.8M 19.319.020.3

4339415561636363636466646464646463646361646664615759545448484648464652

6.04.05.013.0

16.017.017.017.017.018.019.018.018.018.018.018.017.018.017.016.018.019.018.016.014.015.012.012.09.09.08.09.08.08.011.0

16 13 14 24 34 37 37 37 39 45 48 45 45 45 45 45 43 45 41 37 41 41 34 29 23 24 20 18 14 16 15 M 16 17 22 

0 6 8 31 20 20 20 10 8 10 7 7 5 3 3 0 0 3 7 7 7 8 7 8 9 9 13 14 11 14 11 3 0 0 3 

000120130180200210210170120110120110120100090000000VR 350350330270300300290280280290300310300VR 000000230

37 29 26 25 17 17 18 18 

28.30 28.31 28.33 28.35 28.36 28.36 28.37 28.39 28.40 28.41 28.41 28.41 28.41 28.41 28.41 28.41 28.41 28.40 28.40 28.42 28.43 28.44 28.43 28.42 28.39 28.37 28.34 28.32 28.30 28.29 28.29 M 28.31 28.31 28.31 

M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M 
10 

02 02 02 02 02 02 02 02 02 02 02

2047 2055 2115 2135 2155 2215 2235 2255 2315 2335 2355

0 0 0 0 0 0 0 0 0 0 0

BKN CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR

20.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00

9797979090909088888888

36.036.036.032.032.032.032.031.031.031.031.0

M69696666666667676666

M 20.320.318.619.118.618.619.319.318.818.8

5252525052505054545252

11.011.011.010.011.010.010.012.012.011.011.0

M 22 22 25 27 25 25 31 31 29 29

3 5 0 3 0 5 7 8 7 6 3

VR 250000110000110120130130110110

M 28.32 28.32 28.32 28.32 28.32 28.33 28.34 28.33 28.33 28.33

M M M M M M M M M M M

Figure A-13. Quality-controlled local climatological data hourly observations table for Williams Gateway Airport, Phoenix, Arizo

Note in the Weather Type column that HZ (haze) was reported for several hours. Dynamically generated via http://cdo.ncdc.noaa.gov/qclcd/QCLC. 
11 

U.S. Department of Commerce National Oceanic & Atmospheric Administration QUALITY CONTROLLED LOCAL CLIMATOLOGICAL DATA (final) 

HOURLY OBSERVATIONS TABLE 

LUKE AFB AIRPORT (23111) 

GLENDALE, AZ (09/2011)

Elevation: 1085 ft. above sea level Latitude: 33.55 Longitude: -112.366 Data Version: VER2

Dry 

Bulb 

Temp

Wet 

Bulb 

Temp

Dew 

Point

Temp Date Time (LST) Station Type Sky Conditions Visibility (SM) Weather Type

(F) (C) (F) (C) (F) (C)

Rel

Humd

%

Wind 

Speed 

(MPH)

Wind

Dir

Wind

Gusts

(MPH)

Station

Pressure

(in. hg)

Press

Tend

Net

3-hr

Chg

(mb)

L

Pr

(i

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02

0055 0155 0229 0246 0249 0255 0306 0355 0455 0555 0655 0755 0855 0955 1055 1155 1255 1355 1455 1555 1655 1755 1855 1955 2055 2155 2255 2355

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

FEW140 BKN210 FEW140 BKN170 FEW010 SCT140 BKN140 SCT140 SCT140 FEW140 SCT180 BKN200 BKN160 CLR FEW190 CLR CLR FEW180 FEW180 FEW130 SCT200 FEW160 FEW170 FEW190 CLR CLR CLR CLR CLR CLR CLR CLR CLR

10.00 10.00 6.00 8.00 8.00 9.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00

-RA -DZ -DZ 

93969390908988888686878788909192929396981021031019594918987

33.835.734.032.032.031.431.030.930.130.230.430.531.332.032.833.133.233.835.436.639.039.738.235.134.533.031.430.4

64667272737575757575757575747375757474747167676967666665

17.918.822.022.222.523.624.123.723.823.823.623.923.823.422.924.023.723.223.423.121.619.619.520.719.118.818.718.4

44466163646870697070697069676568676564625444455549505151

6.67.816.017.018.020.021.020.821.221.020.620.920.319.618.419.819.318.517.816.512.06.87.312.89.710.210.610.7

18 18 34 41 42 50 55 53 59 59 55 57 53 47 42 45 44 40 35 31 20 13 15 26 22 25 27 29

6 20 28 18 18 14 17 22 9 5 0 3 0 0 3 8 7 3 5 0 6 6 6 3 5 7 6 6

180 250 240 170 170 190 190 220 240 200 000 360 000 000 VR 190 200 VR 210 000 320 300 230 230 230 170 210 200

34 23 28 

28.58 28.60 28.62 28.65 28.66 28.66 28.66 28.66 28.67 28.68 28.69 28.70 28.71 28.71 28.71 28.70 28.68 28.66 28.63 28.60 28.58 28.56 28.56 28.57 28.58 28.58 28.59 28.60

3 1 2 1 8 6 5 2

013025009005019026003008

29292929292929292929292929292929292929292929292929292929

Figure A-14. Quality-controlled local climatological data hourly observations table for Luke Air Force Base, Glendale, Arizona (

Dynamically generated via http://cdo.ncdc.noaa.gov/qclcd/QCLCD.
12 

U.S. Department of Commerce National Oceanic & Atmospheric Administration QUALITY CONTROLLED LOCAL CLIMATOLOGICAL DATA (final) 

HOURLY OBSERVATIONS TABLE 

PHOENIX DEER VALLEY ARPT (03184) 

PHOENIX, AZ (09/2011)

Elevation: 1455 ft. above sea level Latitude: 33.688 Longitude: -112.081 Data Version: VER2

Dry 

Bulb 

Temp

Wet 

Bulb 

Temp

Dew 

Point

Temp Date Time (LST) Station Type Sky Conditions Visibility (SM) Weather Type

(F) (C) (F) (C) (F) (C)

Rel

Humd

%

Wind 

Speed 

(MPH)

Wind

Dir

Wind

Gusts

(MPH)

Station

Pressure

(in. hg)

Press

Tend

Net

3-hr

Chg

(mb)

L

Pr

(i

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02

0053 0153 0234 0240 0251 0253 0256 0307 0351 0353 0453 0553 0653 0753 0853 0953 1053 1153 1253 1353 1453 1553 1653 1753 1853 1953 2053 2153 2253 2353

12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12

CLR CLR BKN013 VV012 BKN013 BKN013 BKN013 BKN017 BKN031 BKN033 OVC032 FEW032 FEW028 CLR SCT100 SCT120 FEW050 FEW110 CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR

10.00 10.00 2.00 1.00 1.50 2.00 4.00 5.00 9.00 9.00 9.00 9.00 8.00 8.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00

HZ HZ -RA -RA -RA HZ 

9192939391929190868685848585878788899092959799100999692898685

32.833.334.034.033.033.333.032.030.030.029.428.929.429.430.630.631.131.732.233.335.036.137.237.837.235.633.331.730.029.4

636266676565646771717272737272717171717071716966646463626262

17.316.718.719.118.318.317.919.621.821.822.322.122.622.322.021.621.821.721.521.321.721.820.619.018.017.617.016.916.516.8

433948504847465464646666676664636362615959585244394041434446

6.13.99.010.09.08.38.012.018.017.818.918.919.418.917.817.217.216.716.115.015.014.411.16.73.94.45.06.16.77.8

19 16 21 23 23 21 21 29 48 48 53 55 55 53 46 45 43 41 38 33 30 27 21 15 12 14 17 20 23 26

0 8 13 16 21 16 15 14 10 10 10 6 0 7 9 6 5 3 0 3 9 9 11 11 6 3 0 0 5 0

000 220 170 180 230 230 230 230 240 240 240 250 000 360 330 300 260 250 000 290 270 260 270 270 280 220 000 000 110 000

25 26 26 26 

28.18 28.20 28.24 28.25 28.26 28.26 28.26 28.27 28.29 28.29 28.30 28.30 28.30 28.31 28.31 28.32 28.32 28.31 28.29 28.26 28.24 28.21 28.20 28.17 28.17 28.18 28.20 28.20 28.20 28.21

3 1 1 3 8 7 5 3

009031004004020024003008

2929M M M 29M M M 292929292929292929292929292929292929292929

Figure A-15. Quality-controlled local climatological data hourly observations table for Phoenix Deer Valley Airport, Phoenix, Ar

(09/02/2011). Note in the Weather Type column that HZ (haze) was reported for several hours. Dynamically generated via http://cdo.ncdc.noaa.gov/qclcd/QCLCD.
Exceptional Event, Phoenix, September 2, 2011 Appendix B 

Appendix B: Media Coverage, Videos, and Images 

ADEQ visibility cameras in the Phoenix area: 

South Mountain: www.phoenixvis.net/videos/mpeg4/SOMT_09022011.mp4

Estrella Mountains: www.phoenixvis.net/videos/mpeg4/ESMO_09022011.mp4

Camelback Mountains: www.phoenixvis.net/videos/mpeg4/CAME_09022011.mp4

Superstition Mountains: www.phoenixvis.net/videos/mpeg4/SUPM_09022011.mp4

Other media sources: 

http://www.youtube.com/watch?v=0ckZh57Lsx4

http://www.azfamily.com/younews/129081558.html?img=1&mg=t

Figure B-1. Dust storm in Phoenix at approximately 0200 LST on September 2, 2011. 

This photo was taken on 20th Street and Indian School Road. Image source: 

http://www.azfamily.com/younews/129081558.html?img=1&mg=t.

1

Exceptional Event, Phoenix, September 2, 2011 Appendix C 

Appendix C: Historical Fluctuation Time Series Graphs 

Figure C-1. 24-hr average PM10 concentrations at the Greenwood monitor (2007–2011). 

The 24-hr average PM10 concentration on September 2, 2011, is shown in red and 

highlighted by the arrow. 

1
Exceptional Event, Phoenix, September 2, 2011 Appendix C 

Figure C-2. 24-hr average PM10 concentrations at the South Phoenix monitor (2007–

2011). The 24-hr average PM10 concentration on September 2, 2011, is shown in red 

and highlighted by the arrow. 

Figure C-3. 24-hr average PM10 concentrations at the West Chandler monitor (2007–

2011). The 24-hr average PM10 concentration on September 2, 2011, is shown in red 

and highlighted by the arrow. 

2
Exceptional Event, Phoenix, September 2, 2011 Appendix C 

Figure C-4. 24-hr average PM10 concentrations at the Higley monitor (2007–2011). The 

24-hr average PM10 concentration on September 2, 2011, is highlighted in red. The 

period of July to September 2011, is shown in red and highlighted by the arrow. 

Figure C-5. 24-hr average PM10 concentrations at the West 43rd monitor (2007–2011). 

The 24-hr average PM10 concentration on September 2, 2011, is shown in red and 

highlighted by the arrow. 

3
Exceptional Event, Phoenix, September 2, 2011 Appendix C 

Figure C-6. 24-hr average PM10 concentrations at the Buckeye monitor (2007–2011). 

The 24-hr average PM10 concentration on September 2, 2011, is shown in red and 

highlighted by the arrow. 

Figure C-7. 24-hr average PM10 concentrations at the Durango monitor (2007–2011). 

The 24-hr average PM10 concentration on September 2, 2011, is shown in red and 

highlighted by the arrow. 

4
Exceptional Event, Phoenix, September 2, 2011 Appendix C 

Figure C-8. 24-hr average PM10 concentrations at the JLG Supersite monitor (2007–

2011). The 24-hr average PM10 concentration on September 2, 2011, is shown in red 

and highlighted by the arrow. 

Figure C-9. 24-hr average PM10 concentrations at the North Phoenix monitor (2007–

2011). The 24-hr average PM10 concentration on September 2, 2011, is shown in red 

and highlighted by the arrow. 

5
Exceptional Event, Phoenix, September 2, 2011 Appendix C 

Figure C-10. 24-hr average PM10 concentrations at the Fort McDowell/Yuma Frank 

monitor (2007–2011). The 24-hr average PM10 concentration on September 2, 2011, is 

shown in red and highlighted by the arrow. 

Figure C-11. 24-hr average PM10 concentrations at the Apache Junction monitor (2007–

2011). The 24-hr average PM10 concentration on September 2, 2011, is shown in red 

and highlighted by the arrow. 

6
Exceptional Event, Phoenix, September 2, 2011 Appendix D 

Appendix D: 

ADEQ and NWS Forecast Products 

MARICOPA COUNTY DUST CONTROL FORECAST

ISSUED THURSDAY, SEPTEMBER 01, 2011

Five-day weather outlook:

DURING ACTIVE SUMMER MONSOON EPISODES STRONG GUSTY WINDS AND DENSE BLOWING DUST ARE 

POSSIBLE EVEN FROM DISTANT THUNDERSTORMS

Although the prospects for thunderstorms and significant rainfall in the Valley are remote, an increase in summer monsoon moisture 

and thunderstorm activity east and southeast of the metro area the next few days will mean a higher risk for unhealthy levels of PM10 due to dense blowing dust generated by outflow boundaries. Otherwise, low-level gradient winds and upslope flow will yield 

breezy to gusty conditions at times each afternoon thru Sunday with a light easterly wind regime possible by Monday. 

R I S K F A C T O R S

WINDS STAGNATION

UNHEALTHY PM-10

RISK LEVEL

D-1

Day 1: Fri 09/02/2011

Southwest to westerly 

10-20 mph during the 

afternoon except strong 

and gusty due to 

outflow from 

thunderstorms. 

No significant 

+ stagnation expected. = MODERATE

Day 2: Sat 09/03/2011

Southwest to westerly 

10-20 mph during the 

afternoon except strong 

and gusty due to 

outflow from 

thunderstorms. 

No significant 

+ stagnation expected. = MODERATE

Day 3: Sun 09/04/2011

Southwest to westerly 

10-20 mph during the 

afternoon except strong 

and gusty due to 

outflow from 

thunderstorms. 

No significant 

+ stagnation expected. = MODERATE

EXTENDED OUTLOOK

Day 4: Mon 09/05/2011

Easterly 5-10 mph 

except strong and gusty 

due to outflow from 

thunderstorms. 

No significant 

+ stagnation expected. = MODERATE

Day 5: Tue 09/06/2011

Easterly 5-10 mph 

except strong and gusty 

due to outflow from 

thunderstorms. 

No significant 

+ stagnation expected. = MODERATE
Exceptional Event, Phoenix, September 2, 2011 Appendix D 

D-2

The Maricopa County Dust Control Action Forecast is issued to assist in the planning of work activities to help 

reduce dust pollution. A recorded message of this forecast can be accessed at 602-771-2368. To review the 

complete air quality forecast for the Phoenix metropolitan area, as well as the health impacts and reduction 

methods for different air pollutants, call 602-771-2367 for recorded forecast information or click on ADEQ's 

Air Quality Forecast at http://www.azdeq.gov/environ/air/ozone/ensemble.pdf.

CKR 04/28/2011
Exceptional Event, Phoenix, September 2, 2011 Appendix D 

D-3

VERY UNHEALTHY (201-300) 

UNHEALTHY (151-200)

UNHEALTHY FOR SENSITIVE GROUPS (101-150)

MODERATE (51-100)

GOOD (0-50)

For more information visit:

http://www.airnow.gov/index.cfm?action=aqibasics.aqi

NEW!!! CLICK HERE FOR UPDATED OZONE SEASON STATS NEW!!!

AIR QUALITY FORECAST FOR FRIDAY, SEPTEMBER 02, 2011

This report is updated by 1:00 p.m. Sunday thru Friday and is valid 

for areas within and bordering Maricopa County in Arizona

FORECAST 

DATE

NOTICES 

(*SEE BELOW 

FOR DETAILS)

AIR POLLUTANT 

YESTERDAY

WED 08/31/2011

DUST

NWS EXCESSIVE 

HEAT WARNING

Highest AQI Reading/Site

(Preliminary data only)

TODAY

THU 09/01/2011

OZONE HEALTH 

WATCH

DUST

NWS EXCESSIVE 

HEAT WARNING

TOMORROW

FRI 09/02/2011

OZONE HEALTH 

WATCH

DUST

NWS EXCESSIVE 

HEAT WARNING

EXTENDED

SAT 09/03/2011

OZONE HEALTH 

WATCH 

POSSIBLE

DUST

O3* 93

QUEEN VALLEY

97

MODERATE

93

MODERATE

90

MODERATE

CO* 07

GREENWOOD

06

GOOD

05

GOOD

06

GOOD

PM-10* 58

WEST FORTY THIRD

75

MODERATE

75

MODERATE

75

MODERATE

PM-2.5* 41

PHOENIX SUPERSITE

50

GOOD

49

GOOD

47

GOOD

* O3 = Ozone CO = Carbon Monoxide PM-10 = Particles 10 microns & smaller PM-2.5 = Particles smaller than 2.5 microns 

*“Ozone Health Watch” means that the highest concentration of OZONE may approach the federal health standard.
Exceptional Event, Phoenix, September 2, 2011 Appendix D 

D-4

“PM-10 or PM-2.5 Health Watch” means that the highest concentration of PM-10 or PM-2.5 may approach the federal health standard. 

“High Pollution Advisory” means that the highest concentration of OZONE, PM-10, or PM-2.5 may exceed the federal health standard.

“DUST” means that short periods of high PM-10 concentrations caused by outflow from thunderstorms are possible.
Exceptional Event, Phoenix, September 2, 2011 Appendix D 

Health message for Thursday September 01: Unusually sensitive people should consider 

reducing prolonged or heavy exertion outdoors.

Health message for Friday September 02: Unusually sensitive people should consider 

reducing prolonged or heavy exertion outdoors.

Synopsis and Discussion

OZONE: AN OZONE HEALTH WATCH HAS BEEN ISSUED FOR TODAY AND FRIDAY SEPT 02

It appears that last evening’s thunderstorm outflow boundary has significantly altered the local ozone chemistry due to 

a suspected increase in ozone precursors. Around 9:00 p.m. Wednesday hourly ozone time series’ from nearly all 

Valley ozone monitoring sites showed a subtle increase in ozone concentrations following east to southeasterly wind 

gusts of up to 25 mph. Ozone readings at some metro locations subsequently remained elevated thru the overnight 

period and hourly concentrations Valley-wide were ramping up rapidly as of this writing. As of 10:00 a.m. hourly 

concentrations were between 20 and 30 parts per billion higher than those of 24 hours ago at sites such South Phoenix, 

West Phoenix, and South Scottsdale. Since a westerly wind regime remains in place, highest 8-hour average ozone 

levels are still expected to occur over the eastern portion of the metro area but readings may now approach unhealthy 

concentrations over some central city locations as well. As a result, an Ozone Health Watch has been issued for the 

remainder of today. A similar situation looks possible for Friday so the Watch will continue in effect. 

PARTICLES: A localized blowing dust event occurred Wednesday evening with the highest PM-10 concentration of 

428.1ug/m3 measured at the Higley monitoring site at 8:00 p.m. Visibilities as low as five miles and wind gusts up to 26 mph were 

recorded. Although the prospects for thunderstorms and significant rainfall in the Valley are remote, an increase in 

summer monsoon moisture and thunderstorm activity east and southeast of the metro area the next few days will mean 

a higher risk for unhealthy levels of PM-10 due to dense blowing dust generated by outflow boundaries. Otherwise, 

low-level gradient winds and upslope flow will yield breezy to gusty conditions at times each afternoon thru Saturday. 

MONITORING SITE MAPS: STATIC MAP - http://www.azdeq.gov/environ/air/monitoring/images/map.jpg

D-5

INTERACTIVE MAPS - http://aqwww.maricopa.gov/AirMonitoring/SitePollutionMap.aspx

http://www.airnow.gov/

POLLUTION MONITOR READINGS FOR WEDNESDAY, AUGUST 31, 2011

O3 (OZONE)

Info on current 8-hour ozone standard: http://www.epa.gov/air/ozonepollution/pdfs/2008_03_aqi_changes.pdf

For archived AQI maps go to: http://www.airnow.gov/index.cfm?action=airnow.maps

SITE NAME MAX 8-HR VALUE (PPB) MAX AQI AQI COLOR CODE

Alamo Lake (La Paz County) 55 47

Apache Junction (Pinal County) 63 61

Blue Point 62 58

Buckeye 42 36

Casa Grande (Pinal County) 50 42

Cave Creek 57 48

Central Phoenix 54 46

Dysart 47 40

Falcon Field 61 54

Fountain Hills 63 61

Glendale 52 44

Humboldt Mountain 57 48

North Phoenix 58 49

Phoenix Supersite 57 48

Pinal Air Park (Pinal County) 52 44

Pinnacle Peak 55 47

Queen Valley (Pinal County) 73 93

Rio Verde 62 58

South Phoenix 53 45

South Scottsdale 57 48

Tempe 50 42

Tonto Nat’l Mon. (Gila County) 63 61

West Chandler 63 61

West Phoenix 55 47

Yuma (Yuma County) 47 40
Exceptional Event, Phoenix, September 2, 2011 Appendix D 

D-6

CO (CARBON MONOXIDE)

SITE NAME MAX 8-HR VALUE (PPM) MAX AQI AQI COLOR CODE

Central Phoenix 0.5 06

Greenwood 0.6 07

Phoenix Supersite NOT AVBL NOT AVBL NOT AVBL

West Phoenix 0.4 05

PM-10 (PARTICLES)

SITE NAME MAX 24-HR VALUE (ug/m3) MAX AQI AQI COLOR CODE

Buckeye 62 54

Central Phoenix 48 44

Combs School(Pinal County 77 59

Durango 63 55

Dysart 38 35

Glendale 41 38

Greenwood 42 39

Higley 62 54

Maricopa (Pinal County) 79 63

Phoenix Supersite 38 35

South Phoenix 62 54

West Chandler 47 44

West Forty Third 69 58

West Phoenix 52 48

Zuni Hills 38 35

PM-2.5 (PARTICLES)

(Some data derived from light-scattering equipment)

For maps go to: http://www.airnow.gov/

SITE NAME MAX 24-HR VALUE (ug/m3) MAX AQI AQI COLOR CODE

Durango 11.4 37

Dysart 8.4 27

Estrella Mountain Park 4.4 14

Glendale 9.5 31

Phoenix Supersite 12.6 41

South Phoenix 8.6 28

Vehicle Emissions Lab 9.1 30

West Phoenix 9.9 32

Exceptional Event, Phoenix, September 2, 2011 Appendix E 

Appendix E: 

Affidavit of Public Notice 

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