State of Arizona 

Exceptional Event Documentation 

for the Event of July 11, 2012, 

for the Phoenix PM10 Nonattainment Area 

Final Report Prepared for 

Arizona Department of Environmental Quality 

Phoenix, AZ 

February 2013 
This PDF document contains blank pages to accommodate two-sided printing. 
State of Arizona 

Exceptional Event Documentation 

for the Event of July 11, 2012, 

for the Phoenix PM10 Nonattainment Area 

Final Report 

STI-912095-5568-FR

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

February 13, 2013 

Cover image illustrates windblown dust and haze with reduced visibility in the Phoenix area at 2245 LST on July 11, 

2012. This photo was taken by an ADEQ visibility camera near downtown Phoenix looking northeast toward 

Camelback Mountain.

Exceptional Event, Phoenix, July 11, 2012 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, July 11, 2012 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 ................................................................................3

5.1.4 Compliance and Enforcement Activities.........................................................4

5.1.5 Review of Source-Permitted Inspections and Public Complaints ..................5

5.2 Forecasts and Warnings ............................................................................................6

5.3 Wind Observations.....................................................................................................6

5.4 Summary....................................................................................................................6

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

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

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

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

v
Exceptional Event, Phoenix, July 11, 2012 Table of Contents 

vi

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

Video Links ..........................................................................................................................1

Media Articles and Images ..................................................................................................2

Appendix A: Air Quality and Meteorological Data...............................................................1

Appendix B: Media Coverage, Videos, and Images ...........................................................1

Appendix C: Historical Fluctuation Time-Series Graphs.....................................................1

Appendix D: ADEQ and NWS Forecast Products ..............................................................1

Appendix E: Affidavit of Public Notice ................................................................................1
Exceptional Event, Phoenix, July 11, 2012 List of Figures 

List of Figures 

Figure Page

2-1. Locations of air quality monitors that recorded exceedances of the 24-hr PM

NAAQS on July 11, 2012, and selected NWS monitors in the Phoenix area....................2

10

2-2. Location of sites monitoring PM10 in Arizona on July 11, 2012. ...........................................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 south-central Arizona moved northnorthwestward and transported dust into the Phoenix area late on July 11, 2012............7

3-1. Hourly PM concentrations and wind speeds at the Greenwood monitor on July 

10−12, 2012 ......................................................................................................................2

10

3-2. Hourly PM concentrations and wind speeds at the Durango Complex monitor on 

July 10−12, 2012...............................................................................................................2

10

3-3. Hourly PM concentrations at the Phoenix-area PM monitors and visibility at 

KPHX.. ..............................................................................................................................3

10 10

3-4. Hourly PM10 concentrations at Pinal County monitors on July 10−12, 2012. .......................3

3-5. Infrared satellite image from 0045 LST on July 12, 2012 (GOES-West) ..............................4

3-6. Phoenix-area air quality and meteorological monitor and general wind direction. ..............5

3-7. Radar base velocity data at 2212 LST on July 11, 2012, from NWS Phoenix Doppler 

radar..................................................................................................................................6

4-1. Twenty-four-hour average PM concentrations at the South Phoenix monitor 

(2007-2012).......................................................................................................................1

10

4-2. Daily maximum hourly average PM concentrations at the South Phoenix monitor 

(2007-2012).......................................................................................................................2

10

5-1. Timeline of Maricopa County fugitive dust rules and ordinances. ........................................4

vii
Exceptional Event, Phoenix, July 11, 2012 List of Tables 

List of Tables 

Table Page

2-1. PM10 measurements on July 11, 2012. ................................................................................8

3-1. Observed wind speeds and wind gusts at Maricopa County air quality and 

meteorological monitors on July 11, 2012.........................................................................5

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

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

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

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

viii
Exceptional Event, Phoenix, July 11, 2012 Introduction

1

1. Introduction 

On July 11, 2012, four 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 these 

exceedances were due to naturally occurring windblown dust, were historically unusual, were 

not reasonably controllable or preventable, and would not have occurred “but for” the windblown 

dust and, therefore, are Exceptional Events 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 July 11, 2012, and provides a background narrative of the exceptional event 

and an overall explanation of how 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 July 11, 2012, and the exceedances of the 24-hr PM10 standard at the four monitoring 

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

quality and was the result of natural events. 

Section 4 of this assessment illustrates that the event of July 11, 2012, 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 

July 11, 2012, was not reasonably controllable or preventable. 

Section 6 of this assessment builds upon the demonstrations made in the previous 

sections, showing a clear causal connection between the natural event and the exceedances, 

and concludes that the exceedances of the 24-hr PM10 standard on July 11, 2012, 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 videos, images, and media reports to supplement Section 3. 

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, July 11, 2012 Introduction

2

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 July 11, 2012. More 

information on ADEQ’s forecasting program can be found in Section 5 of this report. The 

forecast products that were issued pertaining to this 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 (or another agency operating monitors in Arizona) suspects that data may 

be influenced by an exceptional event, ADEQ (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 (or the other 

operating agency) also submits data from continuous monitors into AQS after quality assurance 

is complete. 

If ADEQ (or other operating air quality agencies) have determined that the potential 

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

flag is submitted for those measurement(s) in AQS. The data are not official until they undergo 

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 flags on the July 11, 2012, 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 

2012 that ADEQ intends to analyze under the Exceptional Events Rule. The PM10 exceedances 

that occurred at four monitors on July 11, 2012, in the Phoenix PM10 Nonattainment Area were 

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

these data. 

1.2.4 Document That the Public Comment Process Was Followed for Event 

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

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-day 

public comment period on January 14, 2013. A copy of the public notice certification, along with 
Exceptional Event, Phoenix, July 11, 2012 Introduction 

3

any comments received, will be submitted to EPA, consistent with the requirements of 40 CFR 

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

1.2.5 Submit Demonstration Supporting Exceptional Event Flag (40 CFR 

50.14(a)(1-2)) 

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

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 headquarters 

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

2015. 

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

The EER 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. 

Exceptional Event, Phoenix, July 11, 2012 Conceptual Model

1 

2. Conceptual Model 

This section provides a narrative background and summarizes the meteorological and 

air quality conditions in place on July 11, 2012, in the Phoenix area. Elements described in this 

section include 

 A description and map of the geographic settings 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 July 11, 2012. 

AQS monitors typically 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. The PM10 exceedances on July 11, 2012, were recorded at the 

Durango Complex, West 43rd Avenue, South Phoenix, and Greenwood monitors. The National 

Weather Service (NWS) monitors measure meteorological data only. Phoenix Sky Harbor 

International Airport (KPHX) and Phoenix Deer Valley Airport (KDVT) were the primary NWS 

sites used in this demonstration package because of those sites’ 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 July 11, 2012. 

Please note that NWS data were unavailable for most other Phoenix area airports during the 

time of this dust storm event. 

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 in the air 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. 
Exceptional Event, Phoenix, July 11, 2012 Conceptual Model

2 

Figure 2-1. Locations of air quality monitors (green) that recorded exceedances of the 

24-hr PM10 NAAQS on July 11, 2012, and selected NWS monitors (blue) in the Phoenix 

area. 
Exceptional Event, Phoenix, July 11, 2012 Conceptual Model

3 

Figure 2-2. Location of sites monitoring PM10 in Arizona on July 11, 2012. 
Exceptional Event, Phoenix, July 11, 2012 Conceptual Model

4 

Figure 2-3. Drainage system of Phoenix, Arizona. 
Exceptional Event, Phoenix, July 11, 2012 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 mid- to late-summertime 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. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

20

40

60

80

100

120

Precipitation (inches)

Temperature (°F)

Precipitation

Max Temp

Min Temp

Mean Temp

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 the outflow boundary (also called gust 

fronts), as shown in 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, July 11, 2012 Conceptual Model

6 

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.

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, the extremely hot temperatures can 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 evening of July 11, 2012, strong winds generated by thunderstorms over central 

and eastern 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 

four air quality monitors (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 south of the Phoenix area, and wind speeds over 20 mph, 

with gusts over 30 mph, overwhelmed reasonable dust control measures. Monitors across 

Maricopa and Pinal counties recorded high PM10 concentrations during this dust storm, 

illustrating the widespread, regional nature of the event. 
Exceptional Event, Phoenix, July 11, 2012 Conceptual Model

7 

Region of

thunderstorm

development

Windblown dust

transported north‐

northwestward

Figure 2-6. Outflow boundaries from thunderstorms over south-central Arizona moved 

north-northwestward and transported dust into the Phoenix area late on July 11, 2012. 
Exceptional Event, Phoenix, July 11, 2012 Conceptual Model

8 

Table 2-1. PM10 measurements on July 11, 2012. The four Phoenix-area monitors 

discussed in this report are in bold. 

Page 1 of 2 

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 

Apache County 

N/A TEOM WMAT 04-001-1003-81102-1 10 19 1900 

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 23 85 1900 

Maricopa County 

West Phoenix TEOM MC 04-013-0019-81102-1 123 956 2300 

North Phoenix TEOM MC 04-013-1004-81102-1 19 36 2300 

Glendale TEOM MC 04-013-2001-81102-1 89 1433 2300 

Central Phoenix TEOM MC 04-013-3002-81102-4 120 1412 2200 

Greenwood TEOM MC 04-013-3010-81102-1 212 1875 2200 RJ 

South Phoenix TEOM MC 04-013-4003-81102-1 285 3701 2200 RJ 

West Chandler TEOM MC 04-013-4004-81102-1 52 381 2200 

Tempe TEOM MC 04-013-4005-81102-1 96 1413 2300 

Higley TEOM MC 04-013-4006-81102-1 24 76 2300 

West 43rd Ave TEOM MC 04-013-4009-81102-1 172 1644 2200 RJ 

Dysart TEOM MC 04-013-4010-81102-1 84 1126 2300 

Buckeye TEOM MC 04-013-4011-81102-1 150 480 2300 

Zuni Hills TEOM MC 04-013-4016-81102-1 56 912 2300 

Fort 

McDowell/Yuma 

Frank 

TEOM FMIR 04-013-5100-81102-3 25 N/A N/A 

Durango Complex TEOM MC 04-013-9812-81102-1 217 2301 2200 RJ 

JLG Supersite BAM ADEQ 04-013-9997-81102-3 51 581 2200 

JLG Supersite TEOM ADEQ 04-013-9997-81102-4 49 540 2200 

Navajo County 

N/A TEOM WMAT 04-017-1002-81102-1 10 33 1400 

Pima County 

Ajo TEOM ADEQ 04-019-0001-81102-3 33 134 0200 

Rillito TEOM ADEQ 04-019-0020-81102-3 35 182 2200 

Orange Grove FRM PCDEQ 04-019-0011-81102-2 14 N/A N/A 

South Tucson FRM PCDEQ 04-019-1001-81102-1 14 N/A N/A 

Green Valley TEOM PCDEQ 04-019-1030-81102-1 10 21 2000 

Geronimo TEOM PCDEQ 04-019-1113-81102-1 14 73 2100 
Exceptional Event, Phoenix, July 11, 2012 Conceptual Model 

9 

Table 2-1. PM10 measurements on July 11, 2012. The four Phoenix-area monitors 

discussed in this report are in bold. 

Page 2 of 2 

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

Apache Junction 

Fire Station TEOM PCAQCD 04-021-3002-81102-3 N/A N/A N/A 

Casa Grande 

Downtown TEOM PCAQCD 04-021-0001-81102-3 41 326 0000 

Combs School TEOM PCAQCD 04-021-3009-81102-3 28 125 2000 

Cowtown TEOM PCAQCD 04-021-3013-81102-3 273 1688 2200 RJ 

Maricopa TEOM PCAQCD 04-021-3010-81102-3 145 877 0000 RJ 

Pinal County 

Housing TEOM PCAQCD 04-021-3011-81102-3 39 323 0000 

Stanfield TEOM PCAQCD 04-021-3008-81102-3 313 2826 0000 RJ 

Santa Cruz County 

Nogales Post Office BAM ADEQ 04-023-0004-81102-3 17 41 0600 

Yuma County 

Yuma Supersite TEOM ADEQ 04-027-8011-81102-3 79 311 0900 

TEOM: Tapered element oscillating microbalance monitor 

BAM: Beta attenuation monitor 

FRM: Federal reference method 

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

ADEQ: Arizona Department of Environmental Quality 

MC: Maricopa County Air Quality Department 

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 

Exceptional Event, Phoenix, July 11, 2012 Causal Relationship

1 

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 July 11, 2012. Late in the evening on July 10, 2012, thunderstorms developed over the 

higher terrain of central and eastern Arizona, just east of the Phoenix area, generating weak 

outflow boundaries that carried dust westward into the Phoenix area. This outflow caused 

elevated PM10 concentrations at several Maricopa County monitors and high PM10

concentrations (over 2500 g/m3

 at one monitor) in Pinal County very late on July 10, 2012, into 

early July 11, 2012 (Figure 3-1, Figure 3-2, Figure 3-3, and Figure 3-4). 

Late in the evening on July 11, 2012, a second round of thunderstorms developed over 

south-central Arizona, just south of the Phoenix area (Figure 3-5). Outflow from these later 

thunderstorms propagated northwestward across western Pinal County and into Maricopa 

County. This later outflow event had a greater effect on PM10 concentrations and wind speeds 

at Maricopa County monitors compared to the weaker event early on July 11, 2012. As stated 

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

strong winds and blowing dust across Arizona. PM10 concentrations increased at Pinal County 

monitors about one hour before PM10 increased at Maricopa County monitors, which is 

consistent with the location and movement of the outflow boundary and indicates that the likely 

source regions for PM10 during the July 11, 2012, event were the undeveloped, open land areas 

south of Maricopa County. This region largely consists of natural, undisturbed desert. The last 

rainfall in the Phoenix area preceding the July 11, 2012, event was minimal, with 0.14 inches 

recorded at Phoenix-Sky Harbor International Airport (KPHX) on July 4, 2012. The combination 

of geography and lack of rainfall preceding the event resulted in a large source region of soils 

that were particularly vulnerable to particulate suspension. 
Exceptional Event, Phoenix, July 11, 2012 Causal Relationship

Figure 3-1. Hourly PM10 concentrations and wind speeds at the Greenwood monitor on 

July 10−12, 2012. PM10 concentrations and wind gust speeds sharply increased at 

2200 LST on July 11, 2012, indicating the arrival of windblown dust. Wind gusts of over 

50 mph were measured. 

Figure 3-2. Hourly PM10 concentrations and wind speeds at the Durango Complex 

monitor on July 10−12, 2012. PM10 concentrations sharply increased at 2200 LST on 

July 11, 2012, coinciding with wind gusts of nearly 50 mph and indicating the arrival of 

windblown dust. 

2 
Exceptional Event, Phoenix, July 11, 2012 Causal Relationship

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

at KPHX. Visibility was significantly reduced at KPHX between 2200 LST on July 11, 

2012, and 0100 LST on July 12, 2012, coincident with the sharp increase in PM10

concentrations at area PM10 monitors, indicating the arrival of windblown dust. A smaller 

reduction in visibility and increase in PM10 concentrations was evident around 0100 LST 

on July 11, 2012, which also contributed to high daily average PM10 concentrations at the 

exceedance monitors. 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Concentration (ug/m3)

July 10, 2012                                       July 11, 2012                                       July 12, 2012

Time (LST)

Casa Grande Downtown PM10

Stanfield PM10

Combs School PM10

Maricopa PM10

Pinal County Housing PM10

Cowtown PM10

Figure 3-4. Hourly PM10 concentrations at Pinal County monitors on July 10−12, 2012. 

PM10 concentrations sharply increased between 2200 LST on July 10, 2012, and 0000 

LST on July 11, 2012, and again between 2100 LST and 2300 LST on July 11, 2012, 

coinciding with the arrival of windblown dust due to thunderstorm outflows. 

3 
Exceptional Event, Phoenix, July 11, 2012 Causal Relationship

Figure 3-5. Infrared satellite image from 0045 LST on July 12, 2012 (GOES-West). 

Colder temperatures (blues, purples, and white) indicate tall, convective (thunderstorm) 

clouds. Thunderstorms over south-central Arizona generated an outflow boundary that 

carried dust northward into Maricopa County. 

Figure 3-6 and Table 3-1 summarize the progression of the thunderstorm outflow 

boundary and windblown dust as it moved through southwestern Arizona on the evening of 

July 11, 2012. The outflow boundary approached the Phoenix area from the southeast, first 

arriving in the Phoenix area around 2200 LST on July 11. These winds carried windblown dust 

to the Durango Complex, West 43rd Ave, South Phoenix, and Greenwood monitors. Sustained 

winds of 23 mph and a peak gust of 33 mph were reported at Phoenix Sky Harbor International 

Airport (KPHX). The outflow boundary continued northwestward and carried windblown dust to 

the Buckeye monitor during the 2300 LST hour. Figure 3-7 shows radar velocity data that 

illustrate the location of the outflow boundary as it moved into the Phoenix metropolitan area. 

Other monitors in the Phoenix area also recorded elevated PM10 concentrations as the outflow 

boundary passed, but the concentrations were not high enough at those monitors to result in 

exceedances of the NAAQS. 

4 
Exceptional Event, Phoenix, July 11, 2012 Causal Relationship

Figure 3-6. Phoenix-area air quality and meteorological monitor and general wind 

direction. Wind and PM10 data for the monitors are shown in Table 3-1, along with the 

names of the stations. South-southeasterly winds associated with thunderstorm outflow 

transported dust across the Phoenix area during the late evening hours of July 11, 2012. 

Table 3-1. Observed wind speeds and wind gusts at Maricopa County air quality and 

meteorological monitors on July 11, 2012. All air quality monitors reported high PM10

concentrations at 2200 and 2300 LST on July 11, 2012, coincident with gusty winds. Filled 

circles identify sites shown in Figure 3-6. 

Site Monitor 

1-hr Max 

PM10

(μg/m3) 

Time of 

Observation 

(LST) 

Wind 

Speed 

(mph) 

Wind Gust 

(mph) 

Wind 

Direction 

(degrees) 

Durango 

Complex 2301 2200 8 26 131 

West 43rd

Ave 1644 2200 9 31 148 

South 

Phoenix 3701 2200 9 28 170 

Greenwood 1875 2200 6 24 108 

KPHX - 2233 23 33 160 

KDVT - 2343 14 29 160 

5 
Exceptional Event, Phoenix, July 11, 2012 Causal Relationship

Tempe

West 43rd St

Greenwood

Durango Complex

Buckeye

Outflow boundary

Radar Site

Figure 3-7. Radar base velocity data at 2212 LST on July 11, 2012, from NWS Phoenix 

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

away from the radar, and yellow indicates little motion away from the radar. A welldefined outflow boundary (dashed line) approached the Phoenix area from the southeast. 

This boundary carried dust northwestward across the Phoenix metropolitan area. 

Monitors in the Phoenix area measured sustained winds of over 20 mph and wind gusts 

in excess of 30 mph, coincident with the sharp increase in PM10 concentrations (Figure 3-1, 

Figure 3-2, and Appendix A). Visibility at KPHX also decreased significantly with the arrival of 

the dust (Figure 3-3). A similar reduction in visibility was reported at KDVT (north of the 

Greenwood PM10 monitor). Weather observations at KPHX also indicated widespread dust 

coincident with the high PM10 concentrations. It is also noteworthy that, immediately before the 

abrupt increase in PM10 concentrations at each monitor, wind gusts were comparatively much 

lighter and PM10 concentrations were much lower, illustrating the correlation between the gusty 

winds and the dust. Visibility cameras in the Phoenix area also clearly depict the rapid arrival of 

dust and the resulting significant reduction in visibility after 2200 LST on July 11, 2012. Links to 

these videos, other media coverage, and images pertaining to this windblown dust event are 

shown in Appendix B. 

3.2 Summary 

The information presented in this section demonstrates a clear causal relationship 

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

6 
Exceptional Event, Phoenix, July 11, 2012 Causal Relationship 

7 

monitors on July 11, 2012. The radar and wind data shown in this section illustrate the spatial 

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

time-series plots of air quality and meteorological data found in this section and in Appendix A 

show that the sharp increase in PM10 concentrations coincided with the strong wind speeds and 

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

Exceptional Event, Phoenix, July 11, 2012 Historical Norm 

1 

4. Historical Norm 

4.1 Analysis 

PM10 concentrations measured at the South Phoenix monitor on July 11, 2012, were 

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

PM10 concentrations measured on July 11, 2012, were compared to a historical 2007–2012 sixyear annual data set at each monitor. The PM10 concentrations measured at the South Phoenix 

monitor on July 11, 2012, were among the highest 24-hr averages (Figure 4-1) and among the 

highest daily maximum hourly average concentrations (Figure 4-2) measured over the five-year 

period. Similar time-series plots for the Durango, Greenwood, and West 43rd monitors are 

shown in Appendix C. 

4.2 Summary 

Given the recorded values and using methods for assessing historical air quality similar 

to those accepted by the EPA, it is clear that the PM10 levels on July 11, 2012, were outside of 

normal historical fluctuations. This analysis provides evidence that the event affected air quality 

on an historic scale. 

0

50

100

150

200

250

300

350

400

450

500

PM10 Concentration (μg/m3)

Date

South Phoenix 5-Year Historical Fluctuation - 24 Hour Averages

July 11, 2012

Figure 4-1. Twenty-four-hour average PM10 concentrations at the South Phoenix 

monitor (2007-2012). The 24-hr average PM10 concentration on July 11, 2012, is 

highlighted in red. 
Exceptional Event, Phoenix, July 11, 2012 Historical Norm 

2 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

PM10 Concentration (μg/m3)

Date

South Phoenix 5-Year Historical Fluctuation - Daily Maximum Hourly Averages

July 11, 2012

Figure 4-2. Daily maximum hourly average PM10 concentrations at the South Phoenix 

monitor (2007-2012). The daily maximum hourly average concentration on July 11, 

2012, is highlighted in red.

Exceptional Event, Phoenix, July 11, 2012 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 Maricopa County Dust Control Forecast of “High Risk”, (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 utilized 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 adopted 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, July 11, 2012 Not Reasonably Controllable or Preventable

2 

Table 5-1. Rules and ordinances regulating PM 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. 

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. 
Exceptional Event, Phoenix, July 11, 2012 Not Reasonably Controllable or Preventable

3 

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. 

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 the 

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. 

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 
Exceptional Event, Phoenix, July 11, 2012 Not Reasonably Controllable or Preventable

4 

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 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. 

March 2008

September 2006

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 and surveillance inspections, conducted both during and after significant 
Exceptional Event, Phoenix, July 11, 2012 Not Reasonably Controllable or Preventable

5 

air quality events. MCAQD routinely inspects dust control-permitted sites and increases 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 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-inspection of sources that had received violations within two business 

days, and an internal MCAQD debriefing of event activities. 

During 2011 and 2012, a total of 17 MCAQD air monitoring sites were upgraded with 

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

5-min intervals. Previously, 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 to consult 

the NWS 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. 

5.1.5 Review of Source-Permitted Inspections and Public Complaints 

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

(AZURITE) database and Maricopa County’s Environmental Management System were queried 

to compile a list of inspections for the permitted sources in the Phoenix area around the time of 

the July 11, 2012, 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 July 8-14, 2012, 

MCAQD inspectors conducted a total of 188 inspections of permitted facilities, 129 of which 

were sources of fugitive dust. Additionally, MCAQD conducted 7 inspections on vacant lots and 

unpaved parking lots during this period. During this 7-day period, a total of 64 violations were 

issued countywide for PM10 and non-PM10-related violations. No violations were issued for 

PM10 emissions within a 4-mile radius of any exceeding monitor. 

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

the 7- day period from July 8 through July 14, 2012, MCAQD received 18 complaints, of which 

11 were windblown dust-related. Each complaint was assigned to and investigated by a 
Exceptional Event, Phoenix, July 11, 2012 Not Reasonably Controllable or Preventable

6 

MCAQD inspector. A review of all pertinent records from this period indicates that MCAQD 

inspectors did not observe any PM10 emission violations of local, state, or federal regulations 

within a 4-mile radius of an exceeding monitor. 

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 forecast high-wind 

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

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 July 11, 

2012. The ADEQ Ag BMP inspector did not receive any complaints in the three days prior to 

the event. 

5.2 Forecasts and Warnings 

Dust forecasts and statements were released prior to the event by both ADEQ and the 

NWS office in Phoenix (Appendix D). For July 11, 2012, a Maricopa County Dust Control 

Forecast was issued by ADEQ, indicating a low risk level for unhealthy PM10. The Dust Control 

Forecast stated that southeast winds between 5 and 10 mph were expected, and that “during 

the active summer monsoon period, strong outflow winds from even distant thunderstorms can 

generate periods of dense blowing dust.” 

The NWS office in Phoenix issued Blowing Dust Advisories for portions of Pinal and 

Maricopa counties during the period of gusty outflow winds and high PM10 concentrations in the 

Phoenix area. These advisories warned residents of the potential for gusty winds in excess of 

40 mph and visibilities reduced below 1 mile due to blowing dust. 

5.3 Wind Observations 

Wind gusts over 30 mph were measured at several monitors as the outflow boundary 

moved through the Phoenix area (Figure 3-1, Figure 3-2, and Appendix A). 

5.4 Summary 

The thunderstorm outflow event of July 11, 2012, produced strong 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 County. 

The Phoenix area is designated as a serious nonattainment area for PM10 and is required to 
Exceptional Event, Phoenix, July 11, 2012 Not Reasonably Controllable or Preventable 

7 

have BACM for all significant sources of PM10. BACM on significant anthropogenic sources 

were in place and enforced during the events, and pro-active tracking and response to the 

events by regulatory agencies and local governments confirmed the uncontrollable nature o

dust emissions; therefore, these pre-existing prior-approved required controls are adequate

meeting the requirements of an exceptional event and should be considered “reasonable” for 

these purposes. 

f the 

 for 

Despite the deployment of comprehensive control measures and sophisticated response 

programs, high-wind conditions associated with thunderstorms and thunderstorm outflows 

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

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

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

PM10 concentrations during the event. The fact that these were natural events involving strong 

thunderstorm outflow winds that transported PM10 emissions into and across the Phoenix area, 

with a majority of the PM10 emissions recorded by Phoenix area monitors coming from sources 

outside of the Phoenix PM10 Nonattainment Area, provides strong evidence that the 

exceedances of July 11, 2012 recorded within the Phoenix PM10 Nonattainment Area were not 

reasonably controllable or preventable. 

Exceptional Event, Phoenix, July 11, 2012 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 July 11, 2012, 

 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 east and south of Phoenix. 

The weight of evidence presented in this submittal provides no alternative that could tie the 

exceedances of July 11, 2012, to any causal source except PM10 transported by thunderstorm 

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

uncontrollable natural event. 

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

implemented on a continual basis. Air quality-related inspection and compliance data revealed 

no violations or complaints in the vicinity of the exceedance monitors within three days before or 

after the time of the event. 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 July 11, 2012, to any causal source except PM10 transported by 

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

presence of this uncontrollable natural event. 

Exceptional Event, Phoenix, July 11, 2012 Conclusions

1

7. Conclusions 

The PM10 exceedances that occurred on July 11, 2012, 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: 

 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; 

 There is a clear causal relationship between the measurement(s) under consideration 

and the event; 

 The event is associated with a measured concentration(s) in excess of normal historical 

fluctuations; and 

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

7.1 Affects Air Quality 

As stated in the preamble to the EER, 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 

July 11, 2012, were not reasonably controllable or preventable. 

7.3 Natural Event 

As discussed above, the PM10 exceedances on July 11, 2012, were shown to be caused 

by PM10 transported into the Phoenix area by thunderstorm outflow. The event therefore 

qualifies as a natural event. 
Exceptional Event, Phoenix, July 11, 2012 Conclusions 

2

tion 3). 

). 

7.4 Clear Causal Relationship 

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

windblown dust: 

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

Phoenix-area monitors was consistent with gusty winds and low visibilities (Sec

 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 (Section 3). 

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

particulate suspension by high winds (Section 3). 

 Visibility camera imagery shows that large quantities of PM10 were transported into the 

Phoenix area on July 11, 2012. The timing of the dust storm depicted in the visibility 

camera imagery is consistent with the PM10 concentration measurements, elevated 

winds, and reduced visibility. 

7.5 Historical Norm 

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

of normal historical fluctuations, based on a comparison with 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 July 11, 2012, 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 of the Phoenix PM10 Nonattainment Area. 
Exceptional Event, Phoenix, July 11, 2012 Appendix A 

Appendix A: 

Air Quality and Meteorological Data 

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

Phoenix-area monitors on July 10-12, 2012. The data show a region-wide increase in wind 

speeds and wind gusts coincident with the arrival of dust and high PM10. 

Figure A-1. Hourly PM10 concentrations and wind speeds at the South Phoenix monitor 

on July 10−12, 2012. PM10 concentrations and wind gusts sharply increased at 2200 

LST on July 11, 2012, indicating the arrival of windblown dust. 

1
Exceptional Event, Phoenix, July 11, 2012 Appendix A 

Figure A-2. Hourly PM10 concentrations and wind speeds at the West 43rd Avenue 

monitor on July 10−12, 2012. PM10 concentrations and wind gusts sharply increased at 

2200 LST on July 11, 2012, indicating the arrival of windblown dust. 

2
3 

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

HOURLY OBSERVATIONS TABLE 

PHOENIX SKY HARBOR INTL AIRPORT (23183) 

PHOENIX, AZ (07/2012) 

Elevation: 1107 ft. above sea level Latitude: 33.427 Longitude: -112.003 Data Version: VER3

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)

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

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

0051 0151 0251 0351 0451 0551 0651 0751 0851 0951 1051 1151 1251 1351 1451 1551 1651 1751 1851 1951 2051 2151 2233 2237 2251 2256 2315 2347 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

SCT160 BKN180 BKN240 SCT160 BKN180 BKN240 SCT180 BKN240 SCT180 BKN240 FEW140 SCT180 BKN240 FEW140 SCT200 BKN240 FEW140 SCT180 BKN240 FEW180 BKN240 FEW180 BKN240 FEW090 SCT180 BKN240 FEW095 BKN240 FEW110 BKN240 FEW100 BKN240 FEW110 BKN240 FEW100 SCT160 BKN250 FEW100 SCT130 SCT200 FEW090 SCT130 SCT200 FEW090 SCT110 SCT200 FEW090 SCT110 SCT200 FEW110 SCT170 SCT200 FEW110 SCT170 SCT200 FEW090 BKN130 BKN200 SCT014 BKN120 BKN200 BKN012 BKN120 BKN200 BKN012 BKN075 BKN110 BKN012CB BKN075CB BKN120BKN005CB BKN020 BKN070 BKN018CB BKN070CB BKN120BKN018CB BKN070CB BKN120

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 2.00 1.75 2.00 2.00 3.00 6.00 7.00

DU DU DU -TSRA TSDU TSDU 

93939291919091939810010310610510910810811010810710510510397979493939394

33.933.933.332.832.832.232.833.936.737.839.441.140.642.842.242.243.342.241.740.640.639.436.036.034.434.034.034.034.4

7171707171707171737373737374737373737373737372727272727373

21.821.521.321.721.721.321.421.822.522.822.722.922.523.122.722.523.022.722.622.822.522.522.122.121.922.022.022.622.5

6059596161606060606058575656555455555557565759596061616362

15.615.015.016.116.115.615.615.615.615.614.413.913.3

13.312.812.212.812.812.813.913.313.915.015.015.616.016.017.016.7

33 32 33 37 37 37 35 33 28 27 23 20 20 18 17 17 16 17 18 21 20 22 28 28 32 34 34 37 35

11 11 8 6 9 10 6 0 6 7 13 9 3 7 5 0 15 9 6 13 10 3 23 20 17 14 14 14 14

130100120100070120180000VR 120140170VR 140VR 000310290VR 330330010160160150170320290280

18 20 18 33 32 24 23 25 

28.63 28.64 28.65 28.66 28.66 28.68 28.69 28.70 28.71 28.69 28.69 28.66 28.64 28.60 28.56 28.53 28.52 28.51 28.52 28.54 28.54 28.57 28.61 28.61 28.60 28.61 28.63 28.62 28.62

1 1 1 8 8 6 3 3 

022007016008029025005021

222222222222222222222222

Figure A-4. Quality-controlled local climatological data hourly observations table for Phoenix Sky Harbor International Airport, 

(07/11/2012). Note that visibilities were reduced with gusty winds and DU (widespread dust) reported after 2200 LST coinciden

concentrations in the Phoenix area. Dynamically generated via http://cdo.ncdc.noaa.gov/qclcd/QCLCD.
4 

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 (07/2012) 

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)

P

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

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

0053 0153 0253 0353 0453 0553 0653 0753 0853 0953 1053 1153 1253 1353 1453 1553 1653 1753 1853 1953 2053 2153 2253 2343 2346 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

CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR SCT120 FEW046 BKN095 OVC110 FEW044 SCT070 BKN110 FEW042 SCT070 BKN100

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 4.00 2.50 3.00 7.00

HZ +RA +RA RA

92919190898991939698101102103103104104106103102100999998908885

33.332.832.832.231.731.732.833.935.636.738.338.939.439.440.040.041.139.438.937.837.237.236.732.031.029.4

7070697070707171727273737272727273737272717171707072

21.321.120.621.021.121.121.421.722.222.222.722.522.422.222.322.322.822.722.322.021.621.621.421.021.222.3

5959575960606060605959585756565657585757565656596166

15.015.013.915.015.615.615.615.615.615.015.014.413.913.313.313.313.914.413.913.913.313.313.315.016.018.9

33 34 32 35 38 38 35 33 30 27 25 23 22 21 20 20 20 23 23 24 24 24 25 35 40 53

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

160140100100090080000120VR 150140180VR 000240VR 340310330330330010120160150180

16 29 29 21

28.27 28.28 28.28 28.29 28.30 28.32 28.33 28.35 28.35 28.34 28.32 28.30 28.27 28.23 28.20 28.18 28.17 28.17 28.17 28.18 28.20 28.21 28.22 28.25 28.25 28.25

1 3 1 8 8 6 3 3 

027006014008028022006012

22222222222222222222222M M 2

Figure A-5. Quality-controlled local climatological data hourly observations table for Phoenix Deer Valley Airport, Phoenix, AZ Note in the Weather Type column that HZ (haze) was reported with reduced visibilities and gusty winds coincident with the arriv

concentrations in the Phoenix area. Dynamically generated via http://cdo.ncdc.noaa.gov/qclcd/QCLCD.
5 

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

(final) 

HOURLY OBSERVATIONS TABLE 

CASA GRANDE MUNICIPAL ARPT (03914) 

CASA GRANDE, AZ 

(07/2012) 

Elevation: 1462 ft. above sea level Latitude: 32.95 Longitude: -111.766 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 

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 11 11 

0015 0035 0055 0115 0135 0155 0215 0235 0255 0315 0335 0355 0415 0435 0455 0515 0535 0555 0615 0635 0655 0715 0735 0755 0815 0835 0855 0915 0935 0955 1015 1035 1055 1115 

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 CLR CLR CLR FEW065 FEW080 SCT090 CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR 

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 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 

88868488888886848484848482848484828484868888888888889091939797979799

31.030.029.031.031.031.030.029.029.029.029.029.028.029.029.029.028.029.029.030.031.031.031.031.031.031.032.033.034.036.036.036.036.037.0

71707071707171717270707070707070707070717171717272717273737474747473

21.820.921.221.821.221.821.821.522.121.221.221.220.821.221.221.220.821.221.221.521.821.821.822.122.121.822.422.622.923.523.523.223.222.9

63616363616364646663636363636363636363636363636464636464646464636361

17.0

16.017.017.016.017.018.018.019.017.017.017.017.017.017.017.017.017.017.017.017.017.017.018.018.017.018.018.018.018.018.017.017.016.0

43 43 49 43 40 43 48 51 55 49 49 49 53 49 49 49 53 49 49 46 43 43 43 45 45 43 42 41 38 34 34 33 33 29 

22 20 7 0 6 8 9 5 5 8 10 9 13 9 7 8 5 5 0 6 5 6 6 0 0 5 0 5 5 5 0 5 7 8 

160170200000140090070040020080090090080110110090100100000080060140160000000090000180160170000170150100

30 31 

28.28 28.27 28.28 28.28 28.30 28.31 28.31 28.32 28.32 28.32 28.32 28.32 28.33 28.33 28.34 28.34 28.34 28.35 28.35 28.35 28.36 28.36 28.38 28.38 28.39 28.38 28.38 28.38 28.37 28.37 28.36 28.36 28.36 28.35 

2 2 2 7 

003010014007

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 
6 

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 11 11 11 11 11 11

1135 1155 1215 1235 1255 1315 1335 1355 1415 1435 1455 1515 1535 1555 1615 1635 1655 1715 1735 1755 1815 1835 1855 1915 1935 1955 2015 2035 2055 2115 2135 2155 2215 2235 2255 2315 2335 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 0 0 0 0 0 0 0 0 0 0

CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR SCT120 BKN120 BKN120 FEW095 BKN120 FEW025 SCT055 BKN070 FEW024 SCT055 BKN070 FEW015 SCT021 BKN037 CLR CLR FEW110 CLR CLR

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 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 8.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00

VCTS TS TS VCTS TS VCTS 

100100100102102104104104102102102104106106106106104106104104104104102102102102100100100998882757979818181

38.038.038.039.039.040.040.040.039.039.039.040.041.041.041.041.040.041.040.040.040.040.039.039.039.039.038.038.038.037.031.028.024.026.026.027.027.027.0

7474747474757574737373737473737374747473737372727272727272747371696970717171

23.023.023.023.323.323.623.623.122.822.822.822.623.322.822.822.823.123.323.122.622.622.622.322.322.322.322.022.022.023.522.721.820.620.621.321.621.621.6

6161616161616159595959575957575759595957575757575757575757636666666466666666

16.016.016.016.016.016.016.015.015.015.015.014.015.014.014.014.015.015.015.014.014.014.014.014.014.014.014.014.014.017.019.019.019.018.019.019.019.019.0

28 28 28 26 26 24 24 23 24 24 24 21 21 20 20 20 23 21 23 21 21 21 23 23 23 23 24 24 24 31 48 58 74 60 65 60 60 60

9 5 8 11 8 11 7 10 5 8 8 6 6 3 7 11 13 11 13 11 14 13 10 9 7 5 5 3 0 13 25 20 11 6 9 11 8 7

080180090150160160130070320160100040110200240270280270270260260250240240240250250260000170060020320230220210230200

17 17 18 21 21 16 16 18 18 20 33 29 

28.34 28.33 28.32 28.31 28.30 28.29 28.28 28.27 28.27 28.25 28.25 28.23 28.21 28.21 28.20 28.21 28.20 28.20 28.20 28.20 28.19 28.20 28.20 28.20 28.21 28.22 28.22 28.22 28.24 28.27 28.32 28.36 28.30 28.29 28.27 28.28 28.29 28.30

7 7 2 2 

030027007020

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 M M M

Figure A-6. Quality-controlled local climatological data hourly observations table for Casa Grande Municipal Airport, Casa Gra(07/11/2012). Note that reduced visibilities with gusty winds occurred after 2100 LST coincident with the arrival of high PM10 cothe Casa Grande area. Dynamically generated via http://cdo.ncdc.noaa.gov/qclcd/QCLCD.
Exceptional Event, Phoenix, July 11, 2012 Appendix B 

Appendix B: Media Coverage, Videos, and Images 

Video Links 

ADEQ visibility cameras in the Phoenix area: 

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

Estrella Mountains: www.phoenixvis.net/videos/mpeg4/ESMO_07112012.mp4

Camelback Mountains: www.phoenixvis.net/videos/mpeg4/CAME_07112012.mp4

Superstition Mountains: www.phoenixvis.net/videos/mpeg4/SUPM_07112012.mp4

Local citizens often create videos during storms, documenting their observations. Here are 

some links to these videos. 

Lightning Over Phoenix, AZ – July 11, 2012 (clip 1) 

http://www.youtube.com/watch?v=zz0Lc9hlKVA

AZ monsoon video – July 11, 2012 

http://www.youtube.com/watch?v=20sdQEZka-Q

1
Exceptional Event, Phoenix, July 11, 2012 Appendix B 

Media Articles and Images 

http://www.accuweather.com/en/weather-blogs/clark/another-scorcherthunderstorms-toincrease/67876

Another Scorcher-Thunderstorms to Increase 

Ken Clark 

July 11, 2012; 12:52 PM 

Today has been another sizzler in the Southwest. Many places inland from the cooling effects of 

the Pacific were at, or well above, 100. An increase in low-level moisture in the southeast 

deserts of California have kept temperatures lower than yesterday but, of course, the added 

moisture is making up for that. The AccuWeather RealFeel temperatures are pretty much the 

same as yesterday in a lot of these areas. I am not sure how hot the tables and slots are in 

Vegas today in giving up money but I can guarantee you those moving between casinos are hot 

and giving up a lot of sweat as temperatures on the Strip have risen to between 111 and 114 

degrees. 

It will stay hot the next couple of days but it probably will not be as hot in all the deserts as the 

last two days because of increase moisture causing an increase in cloud cover and also the 

development of thunderstorms. A look at the water vapor satellite picture from early afternoon 

Wednesday shows moisture moving in from the southeast. 

2
Exceptional Event, Phoenix, July 11, 2012 Appendix B 

A big increase in available moisture from today to tomorrow will take place over all of Southern 

California north into southern Nevada and into portions of the Sierra. This will help the 

development of showers and thunderstorms in all the deserts and mountains. The most active 

time will be in the afternoon and evening hours. With a lot more water vapor in the air tomorrow 

expect thunderstorms that develop to bring locally heavy rain that could cause some temporary 

flash flooding. Friday looks active too then over the weekend moisture will be on the decrease 

as a trough drops down from the north. 

As an interesting side not. The so-called low temperature this morning in Death Valley was a 

toasty 103 degrees. 

http://thetandd.com/news/more-thunderstorms-may-rumble-through/article_66eaea26-cb1c11e1-80a1-001a4bcf887a.html

More thunderstorms may rumble through

By Gene Zaleski 

July 11, 2012 5:30 am 

Mother Nature was not so nurturing Tuesday afternoon as downed trees, power outages and 

nickel-size hail were reported primarily in the southern portion of The Times and Democrat 

Region. 

And there could be more on the way. 

“We are expecting a good chance of showers and thunderstorms through the end of this week,” 

National Weather Service meteorologist Dan Miller said. 

Miller said the rain is welcome not only because conditions have been dry, but it has helped 

reduce the temperatures which have hovered near the century mark. 

On Tuesday, the National Weather Service reported trees down from Bamberg to Denmark. Hail 

was reported near Cope, along with windy conditions. 

In Orangeburg County, trees were down from Bowman’s Winter Creek Road to Interstate 26. 

Trees were also reported down on Vance Road in the Providence area as well as on Bass 

Drive. 

In Bamberg, a couple thousand Edisto Electric Cooperative Inc. customers were without power 

for about one hour Tuesday afternoon as storms rolled through. 

“It has been kind of crazy,” Edisto Electric Manager of Member Services Frank Furtick said. 

“The storm took a transmission line down that had most of our system around Bamberg and the 

town of Bamberg.” 

Furtick said the transmission line was rerouted and customers had power restored. 

3
Exceptional Event, Phoenix, July 11, 2012 Appendix B 

4

As of late Tuesday afternoon, Furtick said there were still scattered outages in the Bamberg, 

Ehrhardt, Cope and St. George areas. 

Sharon Hammond, Bamberg County Emergency Services director, says trees were down 

throughout the county. 

Calhoun County Emergency Services Director Bill Minikiewicz said there were no reports of 

significant weather-related damage on Tuesday afternoon. 

“We are the blessed people,” he said. 

The T&D Region is under a hazardous weather outlook, with the forecast calling for more 

thunderstorms today and through the end of the week. 

The chance of rain is 60 percent for today and 50 percent for Thursday. The chance of rainfall 

over the weekend is about 30 percent. 
Exceptional Event, Phoenix, July 11, 2012 Appendix C 

Appendix C: Historical Fluctuation Time-Series Graphs 

0

50

100

150

200

250

300

350

400

450

500

PM10 Concentration (μg/m3)

Date

Durango 5-Year Historical Fluctuation - 24 Hour Averages

July 11, 2012

Figure C-1. Twenty-four-hour average PM10 concentrations at the Durango monitor 

(2007–2012). The 24-hr average PM10 concentration on July 11, 2012, is shown in red. 

1 
Exceptional Event, Phoenix, July 11, 2012 Appendix C 

0

50

100

150

200

250

300

350

400

450

500

PM10 Concentration (μg/m3)

Date

Greenwood 5-Year Historical Fluctuation - 24 Hour Averages

July 11, 2012

Figure C-2. Twenty-four-hour average PM10 concentrations at the Greenwood monitor 

(2007–2012). The 24-hr average PM10 concentration on July 11, 2012, is shown in red. 

2 
Exceptional Event, Phoenix, July 11, 2012 Appendix C 

0

50

100

150

200

250

300

350

400

450

500

PM10 Concentration (μg/m3)

Date

West 43rd 5-Year Historical Fluctuation - 24 Hour Averages

July 11, 2012

Figure C-3. Twenty-four-hour average PM10 concentrations at the West 43rd monitor 

(2007–2012). The 24-hr average PM10 concentration on July 11, 2012, is shown in red. 

3 
Exceptional Event, Phoenix, July 11, 2012 Appendix C 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

PM10 Concentration (μg/m3)

Date

Durango 5-Year Historical Fluctuation - Daily Maximum Hourly Averages

July 11, 2012

Figure C-4. Daily maximum hourly average PM10 concentrations at the Durango monitor 

(2007–2012). The daily maximum hourly average concentration on July 11, 2012, is 

shown in red. 

4 
Exceptional Event, Phoenix, July 11, 2012 Appendix C 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

PM10 Concentration (μg/m3)

Date

Greenwood 5-Year Historical Fluctuation - Daily Maximum Hourly Averages

July 11, 2012

Figure C-5. Daily maximum hourly average PM10 concentrations at the Greenwood 

monitor (2007–2012). The daily maximum hourly average concentration on July 11, 

2012, is shown in red. 

5 
Exceptional Event, Phoenix, July 11, 2012 Appendix C 

6 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

PM10 Concentration (μg/m3)

Date

West 43rd 5-Year Historical Fluctuation - Daily Maximum Hourly Averages

July 11, 2012

Figure C-6. Daily maximum hourly average PM10 concentrations at the West 43rd 

monitor (2007–2012). The daily maximum hourly average concentration on July 11, 

2012, is shown in red.
Exceptional Event, Phoenix, July 11, 2012 Appendix D 

1 

Appendix D: ADEQ and NWS Forecast Products 

MARICOPA COUNTY DUST CONTROL FORECAST

ISSUED TUESDAY, JULY 10, 2012

Three-day weather outlook:

NOTE: DURING THE ACTIVE SUMMER MONSOON PERIOD, STRONG OUTFLOW WINDS FROM EVEN DISTANT

THUNDERSTORMS CAN GENERATE PERIODS OF DENSE BLOWING DUST

The Phoenix forecast area is still looking for its first valley-wide rain event of the monsoon season. Spotty storm across 

the deserts to southeast have lowered the chances for blowing dust impacting the area. With high pressure now over 

the Four Corners, the steering flow is out of the northeast. These storms tend to be more electric rather than dust 

producing. Thus, the risk of exceeding the 24-hr PM10 health standard in Phoenix has been lowered to Low through 

Sunday. 

R I S K F A C T O R S

RISK LEVEL SURFACE WINDS STAGNATION UNHEALTHY PM-10

Day 1: Wed 07/11/2012

Southeast winds 

between 5 and 10 

mph are expected 

(20% chance of rain). 

No significant 

+ stagnation is 

expected. 

= LOW

Day 2: Thu 07/12/2012

Southeast winds 

between 5 and 10 

mph are expected 

(20% chance of rain). 

No significant 

+ stagnation is 

expected. 

= LOW

Day 3: Fri 07/13/2012

Southeast winds 

between 5 and 10 

mph are expected 

(30% chance of rain). 

No significant 

+ stagnation is 

expected. 

= LOW

EXTENDED OUTLOOK

Day 4: Sat 07/14/2012 = LOW

Southeast winds 

between 5 and 10 

mph are expected 

(30% chance of rain). 

+ No significant 

stagnation is likely. 

Day 5: Sun 07/15/2012 = LOW

Southeast winds 

between 5 and 10 

mph are expected 

(30% chance of rain). 

+ No significant 

stagnation is likely. 
Exceptional Event, Phoenix, July 11, 2012 Appendix 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.

JRP 04/28/2011 
Exceptional Event, Phoenix, July 11, 2012 Appendix 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

*LINK TO 2012 AIR POLLUTION EXCEEDANCE GRAPH*

AIR QUALITY FORECAST FOR WEDNESDAY, JULY 11, 2012

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

MON 07/09/2012

Ozone Health

Watch

DUST

Highest AQI Reading/Site 

(*Preliminary data only*)

TODAY

TUE 07/10/2012

Ozone Health

Watch

DUST

NWS Excessive

Heat Warning

TOMORROW

WED 07/11/2012

NWS Excessive

Heat Warning

EXTENDED

THU 07/12/2012

O3* 90

BLUE POINT

97

MODERATE

54

MODERATE

57

MODERATE

CO* 5

CENTRAL PHOENIX

6

GOOD

8

GOOD

6

GOOD

PM-10* 36

DURANGO

65

MODERATE

45

GOOD

42

GOOD

PM-2.5* 27

WEST PHOENIX

38

GOOD

43

GOOD

31

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.

“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, July 11, 2012 Appendix D 

Health Statements

Tuesday, 07/10/12 Unusually sensitive people should consider limiting prolonged or heavy 

exertion outdoors.

Wednesday, 07/11/12 Unusually sensitive people should consider limiting prolonged or heavy 

exertion outdoors.

SYNOPSIS AND DISCUSSION

...An Ozone Health Watch remains in effect for today, Tuesday, July 10, 2012...

High pressure is in a favorable position to drive thunderstorms off of the Mogollon Rim and 

towards the Valley. Such storms tend to be more electrical in nature due to the topography, 

additional vertical movement and associated friction of the air molecules within the developing 

storms. Such storms also tend to be less apt to generate wide-spread blowing dust coming off of 

the more vegetated mountainous areas. Thus, it’s less likely that we will see significant PM10 

levels over the next few days. 

The subtle “shift” we’ve been waiting for has been underway. Ozone levels reached the 

upper Moderate range under the east-northeast flow. Today, day 2 of the “shift” reveals lower 

concentrations than 24-hr earlier. The forecast calls for a continuation of the northeast-tosouthwest flow. Thus, ozone concentrations should drop a bit more with highest levels towards 

the southwest part of the forecast area (Buckeye) on Wednesday. 

Check back tomorrow for more. Until then, have a good day. -J.Paul

MONITORING SITE MAPS

STATIC MAP http://www.azdeq.gov/environ/air/monitoring/images/map.jpg

4 

http://aqwww.maricopa.gov/AirMonitoring/SitePollutionMap.aspx

INTERACTIVE MAPS http://156.42.96.39/alert/Google/air.html

http://www.airnow.gov/

POLLUTION MONITOR READINGS FOR MONDAY, JULY 9, 2012

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 53 45

Apache Junction 51 43

Blue Point 72 90

Buckeye 49 42

Casa Grande 56 47

Cave Creek 65 67

Central Phoenix 59 50

Dysart 52 44

Falcon Field 61 54

Fountain Hills 64 64

Glendale 56 47

Humboldt Mountain 63 61

North Phoenix 63 61

Phoenix Supersite 60 51

Pinal Air Park 60 51

Pinnacle Peak 71 87

Queen Valley 65 67

Rio Verde 69 80

South Phoenix 58 49
Exceptional Event, Phoenix, July 11, 2012 Appendix D 

5 

South Scottsdale 67 74

Tempe 60 51

Tonto Nat’l Mon. 61 54

West Chandler 59 50

West Phoenix 56 47

Yuma 48 41

CO (CARBON MONOXIDE)

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

Central Phoenix 0.4 5

Greenwood 0.4 5

West Phoenix 0.3 3

PM-10 (PARTICLES)

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

Apache Junction 22.3 21

Buckeye 35.1 32

Central Phoenix 24.9 23

Combs School (Pinal County) 45.6 42

Durango 39.8 36

Dysart 24.0 22

Glendale 24.4 22

Greenwood 30.8 28

Higley 32.2 29

Maricopa (Pinal County) 35.5 33

North Phoenix 21.1 19

Phoenix Supersite 21.6 20

South Phoenix 32.6 30

Tempe 22.4 20

West Chandler 22.9 21

West Forty Third NOT AVBL NOT AVBL NOT AVBL 

West Phoenix 27.7 25

Zuni Hills 21.2 19

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 (μg/m3) MAX AQI AQI COLOR CODE

Durango 6.1 20

Dysart 2.8 9

Estrella Mountain Park 5.0 16

Glendale 6.6 21

Phoenix Supersite 3.4 11

North Phoenix 6.3 20

South Phoenix 4.9 16

Tempe 6.4 21

Vehicle Emissions Lab 3.0 10

West Phoenix 8.2 27
Exceptional Event, Phoenix, July 11, 2012 Appendix D 

6 

Statements, Advisories, and Warnings issued by the National Weather Service office in 

Phoenix, AZ, pertaining to the July 11, 2012, dust storm event:

264 

WWUS75 KPSR 120335 

NPWPSR 

URGENT - WEATHER MESSAGE 

NATIONAL WEATHER SERVICE PHOENIX AZ 

835 PM MST WED JUL 11 2012 

AZZ028-120500- 

/O.NEW.KPSR.DU.Y.0011.120712T0335Z-120712T0500Z/ 

NORTHWEST AND NORTH CENTRAL PINAL COUNTYINCLUDING THE CITIES OF...APACHE JUNCTION...CASA GRANDE... 

FLORENCE 

835 PM MST WED JUL 11 2012 

...BLOWING DUST ADVISORY IN EFFECT UNTIL 10 PM MST THIS 

EVENING... 

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A BLOWING DUST 

ADVISORY...WHICH IS IN EFFECT UNTIL 10 PM MST THIS EVENING. 

* AFFECTED AREA...NORTHERN PINAL COUNTY...INCLUDING FLORENCE... 

 COOLIDGE...HIGHWAY 79...INTERSTATE 10...AND CASA GRANDE. 

* TIMING...VALID UNTIL 1000 PM MST. 

* WINDS...WIDESPREAD WINDS OF 25 MPH...GUSTS TO 40 MPH. 

* VISIBILITY...WIDESPREAD VISIBILITIES LESS THAN 3 MILES...LOCALLY BELOW 1 

 MILE. 

* IMPACTS...RAPID CHANGES IN VISIBILITY ON AREA ROADS. 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 

BE READY FOR A SUDDEN DROP IN VISIBILITY. IF YOU ENCOUNTER 

BLOWING DUST OR BLOWING SAND ON THE ROADWAY OR SEE IT 

APPROACHING...PULL OFF THE ROAD AS FAR AS POSSIBLE AND PUT YOUR 

VEHICLE IN PARK. TURN THE LIGHTS ALL THE WAY OFF AND KEEP YOUR 

FOOT OFF THE BRAKE PEDAL. 

REMEMBER...PULL ASIDE...STAY ALIVE. 

&& 
Exceptional Event, Phoenix, July 11, 2012 Appendix D 

URGENT - WEATHER MESSAGE 

NATIONAL WEATHER SERVICE PHOENIX AZ 

1027 PM MST WED JUL 11 2012 

AZZ023-027-120700- 

/O.NEW.KPSR.DU.Y.0012.120712T0527Z-120712T0700Z/ 

GREATER PHOENIX AREA-SOUTHWEST MARICOPA COUNTYINCLUDING THE CITIES OF...MESA...PHOENIX...GILA BEND 

1027 PM MST WED JUL 11 2012 

...BLOWING DUST ADVISORY IN EFFECT UNTIL MIDNIGHT MST TONIGHT... 

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A BLOWING DUST 

ADVISORY...WHICH IS IN EFFECT UNTIL MIDNIGHT MST TONIGHT. 

* AFFECTED AREA...SOUTHERN AND SOUTHWESTERN PORTIONS OF THE PHOENIX 

 METROPOLITAN AREA...AND SOUTHWESTERN MARICOPA COUNTY. 

* TIMING...THROUGH MIDNIGHT MST. 

* WINDS...SUSTAINED 20-30 MPH...GUSTS TO 45 MPH. 

* VISIBILITY...WIDESPREAD 2-3 MILES...LOCALLY BELOW 1 MILE. 

* IMPACTS...RAPID CHANGES IN VISIBILITY...ESPECIALLY ALONG 

 INTERSTATES 8 AND 10...AND HIGHWAY 85. 

PRECAUTIONARY/PREPAREDNESS ACTIONS... 

BE READY FOR A SUDDEN DROP IN VISIBILITY. IF YOU ENCOUNTER 

BLOWING DUST OR BLOWING SAND ON THE ROADWAY OR SEE IT 

APPROACHING...PULL OFF THE ROAD AS FAR AS POSSIBLE AND PUT YOUR 

VEHICLE IN PARK. TURN THE LIGHTS ALL THE WAY OFF AND KEEP YOUR 

FOOT OFF THE BRAKE PEDAL. 

REMEMBER...PULL ASIDE...STAY ALIVE. 

SPECIAL WEATHER STATEMENT 

NATIONAL WEATHER SERVICE PHOENIX AZ 

1035 PM MST WED JUL 11 2012 

AZZ027-028-120615- 

PINAL AZ-MARICOPA AZ1035 PM MST WED JUL 11 2012 

...SIGNIFICANT WEATHER ADVISORY... 

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A 

SIGNIFICANT WEATHER ADVISORY FOR... 

 SOUTH CENTRAL MARICOPA COUNTY IN SOUTH CENTRAL ARIZONA 

 EXTREME WEST CENTRAL PINAL COUNTY IN SOUTH CENTRAL ARIZONA 

7 
Exceptional Event, Phoenix, July 11, 2012 Appendix D 

8 

UNTIL 1115 PM MST 

AT 1032 PM MST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A 

STRONG THUNDERSTORM 8 MILES SOUTHWEST OF ESTRELLA SAILPORT...MOVING 

WEST AT 20 MPH. 

WIND GUSTS UP TO 50 MPH ARE EXPECTED WITH THIS STORM...ALONG WITH 

HEAVY RAIN. GUSTY WINDS WILL SPREAD AHEAD OF THE STORM...AND WILL 

ARRIVE WELL BEFORE THE RAIN. 

LOCATIONS IMPACTED INCLUDE... 

 GILA BEND... 

 BIG HORN... 

 ESTRELLA... 

 SONORAN NATIONAL MONUMENT... 

 BOSQUE... 

IF THIS STORM INTENSIFIES...A WARNING MAY BE NEEDED. STAY TUNED TO 

NOAA WEATHER RADIO...LOCAL RADIO OR TV FOR THE LATEST UPDATES. 

SOME UNSECURED OBJECTS WILL BE BLOWN AROUND...TREE LIMBS COULD BE 

SNAPPED OFF...AND ISOLATED POWER OUTAGES WILL BE POSSIBLE. SEEK 

SHELTER INDOORS UNTIL THE STORM PASSES. 

BLOWING DUST IS POSSIBLE....ESPECIALLY ALONG INTERSTATE 8 AND 

HIGHWAY 85. IF YOU ENCOUNTER BLOWING DUST WHILE DRIVING...PULL OVER 

AS FAR OFF THE ROADWAY AS POSSIBLE AND PARK. TURN OFF YOUR 

HEADLIGHTS AND KEEP YOUR FOOT OFF THE BRAKE. REMEMBER...PULL 

ASIDE...AND STAY ALIVE. 
Exceptional Event, Phoenix, July 11, 2012 Appendix E 

Appendix E: Affidavit of Public Notice 

1