State of Arizona Exceptional Event Documentation 

for the Event of June 27, 2012, for the 

Phoenix PM10 Nonattainment Area 

Produced by: 

Sierra Research, Inc. 

Arizona Department of Environmental Quality 

Maricopa County Air Quality Department 

Maricopa Association of Governments 

 Final Report 

February 13, 2013 
EXECUTIVE SUMMARY 

In 2005, Congress identified a need to account for events that result in exceedances of the National 

Ambient Air Quality Standards (NAAQS) that are exceptional in nature1 (e.g., not expected to reoccur or 

caused by acts of nature beyond man-made controls). In response, EPA promulgated the Exceptional 

Events Rule (EER) to address exceptional events in 40 CFR Parts 50 and 51 on March 22, 2007 (72 FR 

13560). On May 2, 2011, in an attempt to clarify this rule, EPA released draft guidance documents on the 

implementation of the EER to State, tribal and local air agencies for review. Based on numerous 

comments EPA received on the May 2011 draft, EPA issued a revised draft guidance document in June 

2012. The EER allows for states and tribes to “flag” air quality monitoring data as an exceptional event 

and exclude those data from use in determinations with respect to exceedances or violations of the 

NAAQS, if EPA concurs with the demonstration submitted by the flagging agency. 

Due to the arid nature of the state, Arizona is highly susceptible to windblown dust events. These events 

are often captured by various air quality monitoring equipment throughout the state, sometimes resulting 

in exceedances or violations of the PM10 NAAQS. In the past, the Arizona Department of 

Environmental Quality (ADEQ) has submitted exceptional event documentation for these events. Due in 

part to issues within the EER, obtaining concurrence on these events has been a difficult task. The ADEQ 

is now taking a new approach and focusing exceptional event documentation on events that are believed 

to be clear-cut exceptional events that are not controllable by human intervention, such as the dust events 

that frequently occurred during the monsoon seasons of 2011 and 2012. This new approach was used for 

the exceptional event demonstration submitted in March 2012 by ADEQ for several PM10 events that 

occurred from July 2 – July 8, 2011, in the Phoenix region. In September 2012, EPA concurred that the 

July 2–8, 2011 events were indeed exceptional. As a result, this document is designed to follow the 

format used in the July 2–8, 2011 events submission. 

This demonstration contains detailed information about the windblown dust event that affected the 

Phoenix PM10 nonattainment area on June 27, 2012. On that day, 13 exceedances of the PM10 NAAQS 

occurred at 12 monitoring sites within the nonattainment area. Additional exceedances occurred outside 

of the Phoenix PM10 nonattainment area on this date, and additional documentation may be submitted as 

a separate package at a later time. ADEQ contends that the exceedances that were measured on June 27, 

2012, within the Phoenix PM10 nonattainment area were the result of natural events that were not 

reasonably controllable or preventable. This assessment report of the June 27, 2012, dust event was a 

collaborative effort involving staff from Sierra Research, Inc., the Arizona Department of Environmental 

Quality, Maricopa Association of Governments, Maricopa County Air Quality Department, and Pinal 

County Air Quality Control District. 

 

Section I of this assessment provides a summary of the exceptional event rules and requirements and lays 

out how those rules are met within this specific assessment. 

Section II of this assessment introduces the conceptual model of the thunderstorms and the subsequent 

dust storm that transpired on June 27, 2012, providing general information on the local Phoenix climate, 

monsoons, and the formation of thunderstorm outflows, as well as a background narrative of the 

exceptional event. 

 

1

 Section 319 of the Clear Air Act (CAA), as amended by section 6013 of the Safe Accountable Flexible EfficientTransportation Equity Act: A Legacy for Users (SAFE-TEA-LU of 2005), required EPA to propose the Federal Exceptional 

Events Rule (EER) no later than March 1, 2006. 
Section III of this assessment provides data summaries and time series graphs that help illustrate that the 

event of June 27, 2012, produced PM10 concentrations in excess of normal historical fluctuations. 

Section IV of this assessment details the existing area control measures and demonstrates that despite the 

presence and enforcement of these controls, the event on June 27, 2012, was not reasonably controllable 

or preventable. 

Section V of this assessment establishes a clear causal connection between the natural events of June 27, 

2012, and the exceedances of the 24-hour PM10 standard at the monitoring stations. The evidence in this 

section (and the previous section on historical fluctuations) also confirms that the event in question both 

affected air quality and was the result of natural events. 

Section VI of this assessment builds upon the demonstration showing a clear causal connection between 

the natural events and the exceedances and concludes there would have been no exceedances on June 27, 

2012, but for the presence of the natural events. 

Section VII contains conclusions that summarize the exceptional event that occurred on June 27, 2012, 

and relates the requirements in the EER to the information within this document. 

-ii-
Table of Contents 

I. EXCEPTIONAL EVENT RULE (EER) REQUIREMENTS........................ 1

Procedural Requirements.................................................................................................... 1

Public notification that event was occurring (40 CFR 50.14(c)(1(i)) ................................ 1

Place informational flag on data in AQS (40 CFR 50.14(c)(2)(ii)).................................... 1

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)).................................................... 1

Document that the public comment process was followed for event documentation (40 CFR 

50.14(c)(3)(iv)).................................................................................................................... 2

Submit demonstration supporting exceptional event flag (40 CFR 50.14(a)(1-2))............ 2

Documentation Requirements ............................................................................................ 2

II. CONCEPTUAL MODEL................................................................................. 4

Geographic Setting and Climate......................................................................................... 4

Geographic Setting of Monitors.......................................................................................... 4

Climate ................................................................................................................................ 7

Monsoon Season Description and Event Summary .......................................................... 8

Conclusions......................................................................................................................... 10

III. HISTORICAL FLUCTUATIONS............................................................... 14

Conclusions......................................................................................................................... 17

Regulatory Measures and Control Programs.................................................................. 18

PM10 Rule Effectiveness................................................................................................... 21

Compliance and Enforcement Activities.......................................................................... 22

Conclusions......................................................................................................................... 24

V. CLEAR CAUSAL RELATIONSHIP............................................................ 26

Introduction........................................................................................................................ 26

Event – June 27, 2012 ........................................................................................................ 26

Visibility Cameras.............................................................................................................. 35

Conclusion........................................................................................................................... 35

VI. “BUT FOR” ANALYSIS .............................................................................. 36

VII. CONCLUSIONS .......................................................................................... 37

Affects Air Quality ............................................................................................................. 37

Natural Event ..................................................................................................................... 37

Summary............................................................................................................................. 38

-iii-
 List of Figures 

Figure 2-1. Phoenix Geographic Setting and PM10 Monitor Locations (source: EPA AQS 

DataMart, NASA MODIS Satellite, Google Earth). PM10 monitor locations are indicated by 

the white markers. ..................................................................................................................................5

Figure 2-2. Drainage System of Phoenix, Arizona. ......................................................................................6

Figure 2-3. Phoenix Monthly Precipitation (top) and Maximum Temperature (bottom) 

Climatology (source: National Weather Service) ..................................................................................7

Figure 2-4. Cross-section of a thunderstorm creating an outflow boundary and haboob (Source: 

Desert Meteorology. Thomas T. Warner. 2004.)...................................................................................9

Figure 2-5. U.S. Drought Monitor analysis released during the time period of the exceedances 

described in this report. ........................................................................................................................11

Figure 2-6. Timeline of PM10 concentrations at monitors in the Phoenix area before, during and 

after the June 27, 2012, event...............................................................................................................12

Figure 3-1. Plot of daily maximum hourly average PM10 concentrations (January 1, 2007 – 

September 30, 2012) at the South Phoenix monitoring site.................................................................14

Figure 3-2. Plot of 24-hour average PM10 concentrations (January 1, 2007 – September 30, 2012) 

at the South Phoenix monitoring site. ..................................................................................................15

Figure 3-3. Plot of daily maximum hourly average PM10 concentrations (January 1, 2007 – 

September 30, 2012) at the Higley monitoring site. ............................................................................16

Figure 3-4. Plot of 24-hour average PM10 concentrations (January 1, 2007 – September 30, 2012) 

at the Higley monitoring site................................................................................................................17

Figure 4-1. Timeline of Maricopa County Fugitive Dust Rules and Ordinances. ......................................22

Figure 5-1. Satellite imagery showing strong storms developing and dissipating over southeastern 

Arizona. These storms are believed to have been responsible for the thunderstorm outflow 

boundary that caused PM10 exceedances in Maricopa and Pinal counties on June 27, 2012.............27

Figure 5-2. Phoenix nonattainment area PM10 monitor readings, Sky Harbor wind speed and 

visibility, and Williams Gateway wind speed for June 27, 2012.........................................................28

Figure 5-3. Pinal County PM10 monitor readings and Casa Grande wind speed and visibility for 

June 27, 2012 .......................................................................................................................................29

-iv-
List of Tables 

Table 2-1. Summary of Statewide PM10 Measurements for June 27, 2012...............................................13

Table 4-1. Rules and Ordinances Regulating Particulate Matter Emissions in Maricopa County .............19

Table 4-2. Pinal County Rules Regulating Existing and New Non-point Sources in Pinal County............20

Table 4-3. Pinal County Rules Regulating Fugitive Dust in Pinal County Portion of Maricopa 

County PM10 Nonattainment Area......................................................................................................21

-v-
-viList of Appendices 

Appendix A – 5-year Historical Fluctuation Graphs for the Phoenix Nonattainment Area PM10 Sites 

Appendix B – ADEQ Forecast Products for Phoenix, Maricopa County, and Pinal County 

Appendix C – Notice of Public Comment Period 

Appendix D – National Weather Service Forecasts, Warnings, and Alerts 

Appendix E – Additional National Weather Service Meteorological Data Tables for Maricopa County 

Airports 
I. EXCEPTIONAL EVENT RULE (EER) REQUIREMENTS 

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

be met in order for EPA to concur with the flagged air quality monitoring data. This section of the report 

lays out the requirements of the EER and associated guidance, and discusses how the Arizona Department 

of Environmental Quality (ADEQ) addressed those requirements. 

Procedural Requirements 

This section presents a review of the procedural requirements of the EER as required by 40 CFR 50.14 

(Treatment of Air Quality Monitoring Data Influenced by Exceptional Events) and explains how ADEQ 

fulfills them. The Federal EER requirements include notifying the public that an event was occurring, 

placing informational flags on data in EPA’s Air Quality System (AQS), notifying EPA of the intent to 

flag through submission of initial event description, documenting that the public comment process was 

followed, and submitting a demonstration supporting the exceptional events flag. ADEQ has addressed 

all of these procedural and documentation requirements. 

Public notification that event was occurring (40 CFR 50.14(c)(1(i)) 

ADEQ issued a Dust Control Action Forecast and an Ensemble Forecast for the Greater Phoenix area on 

June 27, 2012, advising citizens of the general potential for blowing dust due to very dry conditions. 

More information on ADEQ’s forecasting program can be found in Section IV. The forecast products 

that were issued for June 27, 2012, are included in Appendix B. 

Place informational flag on data in AQS (40 CFR 50.14(c)(2)(ii)) 

ADEQ and other operating agencies in Arizona submit data into EPA’s AQS. Data from both filter-based 

and continuous monitors operated in Arizona are submitted to AQS. 

When ADEQ and/or another agency operating monitors in Arizona suspects that data may be influenced 

by an exceptional event, ADEQ and/or the other operating agency expedites analysis of the filters 

collected from the potentially affected filter-based air monitoring instruments, quality assures the results, 

and submits the data into AQS. ADEQ and/or other operating agencies also submit data from continuous 

monitors into AQS after quality assurance is complete. 

If ADEQ and/or the operating agency have determined a potential exists that the monitor reading has been 

influenced by an exceptional event, a preliminary flag is submitted for the measurement in the AQS. The 

data are not official until they undergo more thorough quality assurance and quality control, leading to 

certification by May 1 of the year following the calendar year in which the data were collected 

(40 CFR 58.15(a)(2)). The presence of the flag can be confirmed in AQS. 

Notify EPA of intent to flag through submission of initial event description by July 1 of calendar year 

following event (40 CFR 50.14(c)(2)(iii)) 

ADEQ submitted notice to EPA on August 29, 2012 listing all days from calendar year 2011 that ADEQ 

intends to analyze under the Exceptional Events Rule and notifying the EPA of which PM10 data were 

flagged in AQS. Exceedances that occurred on June 27, 2012, within the Phoenix PM10 nonattainment 

area were included on that list. This report serves as the demonstration supporting the flagging of these 

data. 

-1-
On June 27, 2012, 13 monitors at 12 monitoring sites within the boundaries of the Phoenix PM10 

nonattainment area exceeded the 24-hour PM10 standard during the high wind event that occurred during 

the evening hours. The PM10 monitors impacted were the JLG Supersite TEOM2

 and BAM3 operated by 

the ADEQ and the Central Phoenix, Durango Complex, Glendale, Greenwood, Higley, North Phoenix, 

South Phoenix, Tempe, West Chandler, West 43rd, and Zuni Hills monitors operated by Maricopa County 

Air Quality Department (MCAQD). 

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 hardcopy of the report in the 

ADEQ Records Management Center for public review. ADEQ will open a 30-day public comment period 

on January 14, 2013. A copy of the public notice certification, along with any comments received, will be 

submitted to EPA, consistent with the requirements of 40 CFR 50.14(c)(3)(iv). See Appendix D for a 

copy of the affidavit of public notice. 

Submit demonstration supporting exceptional event flag (40 CFR 50.14(a)(1-2)) 

At the close of the 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 IX headquarters in San Francisco, California. The deadline 

for the submittal of this demonstration package is September 30, 2014. 

Documentation Requirements 

Section 50.14(c)(3)(iii) of the EER states that in order to justify excluding air quality monitoring data, 

evidence must be provided for the following elements: 

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

(1) the event affected air quality, 

(2) the event was not reasonably controllable or preventable, and 

(3) the event was caused by human activity unlikely to recur in a particular location or was a 

natural event; 

b. There is a clear causal relationship between the measurement under consideration and the event; 

c. The event is associated with a measured concentration in excess of normal historical fluctuations; 

and 

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

Section II of this assessment introduces the conceptual model of the high wind event that transpired on 

June 27, 2012, providing a background narrative of the exceptional event and an overall explanation on 

how “the event affected air quality.” Further evidence that “the event affected air quality” is provided in 

Section V. Sections II and V also provide evidence that the event was a natural event. 

Section III of this assessment provides data summaries and time series graphs that help illustrate that the 

event on June 27, 2012, produced PM10 concentrations in excess of normal historical fluctuations. 

 

2

 TEOM is an acronym for a Tapered Element Oscillating Monitoring system. 

3

 BAM is an acronym for a Beta Attenuation Monitoring System. 

-2-
Section IV of this assessment details the existing area control measures and demonstrates that despite the 

presence and enforcement of these controls, the event on June 27, 2012, was not reasonably controllable 

or preventable. 

Section V of this assessment establishes a clear causal connection between the natural events of June 27, 

2012, and the exceedances of the 24-hour PM10 standard at the monitoring stations. The evidence in this 

section (and the previous section on historical fluctuations) also confirms that the event in question both 

affected air quality and was the result of natural events. 

Section VI of this assessment builds upon the demonstration showing a clear causal connection between 

the natural events and the exceedances and concludes there would have been no exceedances on June 27, 

2012, but for the presence of the natural events. 

Section VII contains conclusions that summarize the exceptional event that occurred on June 27, 2012, 

and relates the requirements in the EER to the information within this document. 

-3-
II. CONCEPTUAL MODEL 

Geographic Setting and Climate

 

This section describes the geographic and climatic setting of the monitors. 

Geographic Setting of Monitors 

Phoenix is located in the Salt River Valley in south-central Arizona. It lies at a mean 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. The Phoenix metropolitan area 

contains a fairly dense network of PM10 monitors throughout the area, with a much less dense network of 

monitors located throughout the rest of the state. Figure 2-1 shows the general geographic setting of 

Phoenix, as well as the locations of PM10 monitors throughout the state. It should be noted that some of 

the monitors shown in Figure 2-1 are filter-based monitors; therefore, monitoring data from all locations 

may be available only for select days (i.e., 1-in-6 run days). 

-4-
Figure 2-1. Phoenix Geographic Setting and PM10 Monitor Locations (source: EPA AQS DataMart, 

NASA MODIS Satellite, Google Earth). PM10 monitor locations are indicated by the white markers.

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

form Arizona’s drainage system are dry for most of the year and, consequently, are sources of silt and fine 

soils that become suspended and add to regional PM10 loadings during high wind events. Much of this 

alluvial matter and fine soil is deposited in the low lying areas of central and southern Arizona, with 

larger depositional areas focused in and around the confluences of dry river channels. 

-5-
Figure 2-2. Drainage System of Phoenix, Arizona. 

-6-
Climate 

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

temperature is among the hottest of any populated area in the United States. The temperature reaches or 

exceeds 100oF an average of 110 days during the year and highs top 110oF an average of 18 days during 

the year. Phoenix receives an average of 7.66 inches of rain per year. 

As shown in Figure 2-3, precipitation is sparse during the first part of the summer, but the influx of 

monsoonal moisture, which generally begins in early July and lasts until mid-September, raises humidity 

levels and can cause heavy localized precipitation and flooding. Although thunderstorms are possible at 

any time of the year, they are most common during the monsoon season from July to mid-September as 

humid air is advected from the Gulf of California, Gulf of Mexico, and large thunderstorm complexes that 

form over the Sierra Madre Occidental Mountains in Mexico. This influx of moisture, combined with 

intense solar heating, often creates a very unstable environment that is ripe for thunderstorm development. 

These thunderstorms can produce strong winds and blowing dust, large hail, and heavy rain. Dust storms 

associated with these thunderstorms typically occur in the early part of the monsoon season 

(June/July/August) before soaking rains help keep soil particles bound to one another. However, 

depending on the amount of precipitation received during the monsoon season, extremely hot 

temperatures act to dry out the surface quickly, and dust storms can occur at any time. During the 

December through March period, winter storms moving inland from the Pacific Ocean can produce 

significant rains throughout Arizona. This December–March time period and the July–August time 

period are typically the wettest parts of the year. Meanwhile, a distinct dry season occurs from April 

through June for the Phoenix area and the rest of Arizona. While these weather patterns describe the 

general climatology for the Phoenix area over a long period of time, Phoenix and the entire state of 

Arizona are also prone to a high degree of variability in these weather patterns from year to year. 

Figure 2-3. Phoenix Monthly Precipitation (top) and Maximum Temperature (bottom) Climatology 

(source: National Weather Service)

-7-
Monsoon Season Description and Event Summary

A multi-day period, including the event day of June 27, 2012, was characterized by monsoonal 

thunderstorm activity over southeast Arizona and northern Mexico. The North American Monsoon is a 

shift in wind patterns in the summer that occurs as Mexico and the southwest U.S. warm under intense 

solar heating. As this happens, low level moisture is transported primarily from the Gulf of California 

and eastern Pacific Ocean into the southwestern U.S. The Gulf of California, a narrow body of water 

surrounded by mountains, is particularly important for low-level moisture transport into Arizona and 

Sonora. Mid- and upper-level moisture is also transported into the region, mainly from the Gulf of 

Mexico by easterly winds aloft. This combination causes a distinct rainy season over large portions of 

western North America, which develops rather quickly and sometimes dramatically. Rainfall during the 

monsoon is not continuous and varies considerably, depending on a variety of factors. There are usually 

distinct “burst” periods of heavy rain during the monsoon, and “break” periods with little or no rain. 

Even during active monsoon periods, some areas can go without receiving any significant precipitation 

while other nearby areas experience heavy rains and flooding.

The thunderstorm activity in Arizona on June 27, 2012, was active in and around the desert areas of 

southeastern Arizona. These thunderstorms did not result in any precipitation for the Phoenix area, but 

the storms were severe in nature and caused a dust-carrying outflow boundary which propagated into the 

Phoenix area in the evening. Downdrafts associated with heavy precipitation from thunderstorms or from 

collapsing thunderstorms can end up producing what are called downbursts, or sometimes more 

concentrated and severe microbursts, which are rapidly descending bursts of air that come through the 

thunderstorms. These downward bursts of air hit the ground and then disperse away from the storms as 

areas of outflow. These outflow boundaries from the thunderstorms can kick up large areas of dust, 

sometimes called haboobs, and transport that dust for long distances from the initiating thunderstorms 

(see Figure 2-4). 

The active monsoon occurring on June 27th led to numerous thunderstorms and a large thunderstorm 

outflow that produced blowing dust throughout southeastern and central Arizona, including the Phoenix 

metropolitan area. The windblown dust event produced 13 PM10 exceedances at 12 monitoring sites in 

the Phoenix PM10 nonattainment area on June 27, 2012. The event was primarily driven by strong 

outflow winds from severe thunderstorms producing wind gusts from one prominent direction. The June 

27th event was a widespread dust event with mostly southeasterly winds transporting dust from the desert 

areas of Pima and Pinal Counties into all areas of the Phoenix region. Further details on the event can be 

found in Section V. 

-8-
Figure 2-4. Cross-section of a thunderstorm creating an outflow boundary and haboob (Source: Desert 

Meteorology. Thomas T. Warner. 2004.)

The nature of these monsoonal dust events is such that specific source areas are difficult to determine, as 

outflows from thunderstorms can carry dust over vast distances encompassing many source areas. 

Because of this, it is more appropriate to speak of general source regions for these monsoonal dust storms, 

which typically are identified based upon the locations of the thunderstorms that are believed to have 

created the dust generating and carrying outflow winds. The majority of the PM10 impacting the Phoenix 

area from thunderstorm-driven high winds on June 27, 2012, originated outside of the Phoenix PM10 

nonattainment area. The contributing source regions to the dust event were somewhat widespread, but a 

large portion of the PM that was transported into Maricopa County likely came from areas within Pinal 

County to the southeast of Maricopa County. Other potential source contributors to the PM10 events 

were areas in northern Pima County and southeastern Maricopa County. The exact origin of the PM 

sources is often difficult to determine due to the less dense monitoring networks in the general source 

area. 

Another important factor that led to these significant dust storms was the on-going drought across the 

region. The most recent U.S. Drought Monitor placed the area between Tucson and Phoenix, the primary 

dust source region for winds out of the southeast, in D2 (Severe) to D3 (Extreme) drought (see Figure 2-

5). Rainfall in Phoenix since October 1, 2011, had been 45% of normal, and only 11% of normal since 

January 1, 2012 (see the Daily Climate Report for June 27 in Appendix D). For additional general 

information pertaining to the monsoon season in Arizona, see the news media web links provided below. 

2011 Monsoon Season Summary and Review  http://bcove.me/krh3qk29

This video contains information about the 2011 monsoon season and the general formation 

of dust storms in Arizona, which is relevant each year. 

-9-
-10-

2011 Monsoon Season Review 2  http://bcove.me/tc6otk0h

This video contains information about the 2011 monsoon season, focusing on the large 

number of dust storms that occurred, and a discussion of why there were so many dust 

storms in 2011. Since similar meteorological factors were in place in 2012, dust storms 

formed this year for many of the same reasons as they did in 2011. 

The abnormally dry conditions that were in place during the early part of the monsoon season (and which 

persisted throughout much of the monsoon season) resulted in a large fetch of soils that were vulnerable 

to particulate suspension. The thunderstorm outflow boundaries that affected Maricopa and Pinal 

counties on June 27, 2012, resulted in very little rainfall throughout south-central Arizona, but the high 

winds associated with the outflow boundary did transport extremely high levels of particulate 

concentrations into the Phoenix area. A timeline summarizing the June 27 event can be seen in Figure 2-

6. This figure shows hourly PM10 concentrations for a three-day period around, and including, the event 

day. During the 24 hours prior to the event day, there were low PM10 concentrations across the Phoenix 

region for much of the day, but late afternoon thunderstorms produced outflows and moderate to strong 

winds that caused a one hour spike in PM10 concentrations around 8 p.m., but the spike was not high 

enough, nor did PM10 concentrations remain elevated long enough, at any monitoring sites to cause an 

exceedance of the 24-hour standard. On the event day, June 27th, the figure shows low PM10 

concentrations during the day and then extrememly high PM10 concentrations during the evening hours 

when the thunderstorm outflow boundary traveled through the Phoenix area. Lastly, the figure shows that 

PM10 concentrations gradually dropped back down to low levels during the 24 hours after the event, 

indicating that without the impacts of the thunderstorm outflow boundary, PM10 concentrations at all 

sites in the Phoenix PM10 nonattainment area on June 27, 2012, would have been below the 24-hour 

average PM10 exceedance threshold of 150 μg/m3

. 

Following Figure 2-6, Table 2-1 details the PM10 24-hour average concentrations, the daily maximum 

hourly concentrations and the time at which they occurred, and the flag given to the data in EPA’s AQS 

for all PM10 monitoring sites in Arizona for the event day. The table shows exceedances of the national 

24-hour PM10 standard at nearly every monitoring site in both Maricopa and Pinal Counties on June 27, 

2012, and the timing of the 1-hour maximum values across those sites indicates that PM10 was increasing 

sequentially from the southeast to the northwest across central Arizona during the evening hours. This 

timing coincides with a thunderstorm outflow boundary with sustained southeasterly winds over 30 miles 

per hour (mph) that blew through Pinal County and into Maricopa County between 7 p.m. and 9 p.m. on 

June 27th. It should be noted that the 1-hour maximum PM10 concentration measured at the West 

Phoenix monitoring site was one hour later than all other nearby sites because no data were collected 

during the 8 p.m. hour due to an equipment malfunction. 

Conclusions

This Conceptual Model was created to provide a basic description of the general climate and weather 

patterns that led to the active thunderstorm period on June 27, 2012, and produced the dust storm and 13 

PM10 exceedances in the Phoenix PM10 nonattainment area during the one-day episode. A more 

detailed analysis of the windblown dust event is included in Section V, where a demonstration of the clear

causal connection between the uncontrollable natural events and PM10 exceedances is presented. 
Figure 2-5. U.S. Drought Monitor analysis released during the time period of the exceedances described in this r

-11-
Figure 2-6. Timeline of PM10 concentrations at monitors in the Phoenix area before, during and after the June 2

-12-
June 27, 2012 

Table 2-1. Summary of Statewide PM10 Measurements for June 27, 2012. 

Monitor 

Monitor 

Type Operator AQS Monitor ID 

24-hr Avg 

PM10 

(μg/m3) 

1-hr Max 

PM10 

(μg/m3) 

Max 

Time 

AQS 

Qualifier 

Flag 

Apache County 

 N/A TEOM WMAT 04-001-1003-81102-1 21 106 1300 

Cochise County 

 Paul Spur Chemical Lime Plant TEOM ADEQ 04-003-0011-81102-3 18 38 1800 

Coconino County 

 N/A TEOM NN 04-005-1237-81102-1 N/A N/A N/A 

Gila County 

 Hayden Old Jail TEOM ADEQ 04-007-1001-81102-3 47 226 1700 IJ 

Maricopa County 

 Buckeye TEOM MC 04-013-4011-81102-1 111 918 2000 

 Central Phoenix TEOM MC 04-013-3002-81102-4 340 4811 2000 RJ 

 Durango Complex TEOM MC 04-013-9812-81102-1 221 2914 2000 RJ 

 Dysart TEOM MC 04-013-4010-81102-1 123 964 2100 

 Fort McDowell/Yuma Frank TEOM FMIR 04-013-5100-81102-3 N/A N/A N/A 

 Glendale TEOM MC 04-013-2001-81102-1 337 4104 2000 RJ 

 Greenwood TEOM MC 04-013-3010-81102-1 323 4684 2000 RJ 

 Higley TEOM MC 04-013-4006-81102-1 224 3088 1900 RJ 

 JLG Supersite BAM ADEQ 04-013-9997-81102-3 344 4985 2000 RJ, EH 

 JLG Supersite TEOM ADEQ 04-013-9997-81102-4 329 4427 2000 RJ 

 Lehi Air Monitoring Station FRM SRP-MIC 04-013-7022-81102-1 68 N/A N/A 

 North Phoenix BAM MC 04-013-1004-81102-1 178 1952 2000 RJ 

 South Phoenix TEOM MC 04-013-4003-81102-1 342 4496 2000 RJ 

 Tempe TEOM MC 04-013-4005-81102-1 169 1564 2000 RJ 

 West Chandler TEOM MC 04-013-4004-81102-1 221 1383 1900 RJ 

 West Forty Third TEOM MC 04-013-4009-81102-1 220 3034 2000 RJ 

 West Phoenix BAM MC 04-013-0019-81102-1 102 895 2100 

 Zuni Hills TEOM MC 04-013-4016-81102-1 285 2784 2100 RJ 

Navajo County 

 N/A TEOM WMAT 04-017-1002-81102-1 26 75 1900 

Pima County 

 Ajo TEOM ADEQ 04-019-0001-81102-3 37 72 0200 

 Geronimo BAM PCDEQ 04-019-1113-81102-1 27 145 1500 

 Green Valley BAM PCDEQ 04-019-1030-81102-1 15 52 1500 

 Orange Grove FRM PCDEQ 04-019-0011-81102-2 23 N/A N/A 

 Rillito TEOM ADEQ 04-019-0020-81102-3 37 291 1600 IJ 

 South Tucson FRM PCDEQ 04-019-1001-81102-1 26 N/A N/A 

Pinal County 

 Apache Junction Fire Stn. TEOM PCAQCD 04-021-3002-81102-3 N/A 511 2000 

 Casa Grande Downtown TEOM PCAQCD 04-021-0001-81102-3 106 1688 1900 

 Combs School TEOM PCAQCD 04-021-3009-81102-3 179 1829 1900 RJ 

 Cowtown TEOM PCAQCD 04-021-3013-81102-3 504 9639 2000 RJ 

 Maricopa TEOM PCAQCD 04-021-3010-81102-3 235 2086 2000 RJ 

 Pinal County Housing TEOM PCAQCD 04-021-3011-81102-3 44 170 0400 

 Stanfield TEOM PCAQCD 04-021-3008-81102-3 200 1574 1900 RJ 

Santa Cruz County 

 Nogales Post Office BAM ADEQ 04-023-0004-81102-3 27 62 1000 

Yuma County 

 Yuma Supersite TEOM ADEQ 04-027-8011-81102-3 44 89 0700 

Operator Abbreviations: MC – Maricopa County Air Quality Department 

 PCAQCD – Pinal County Air Quality Control District 

 ADEQ – Arizona Department of Environmental Quality 

 FMIR – Fort McDowell Indian Reservation 

 NN – Navajo Nation, AZ, NM, UT 

 PCDEQ – Pima County Department of Environmental Quality 

 SRP-MIC – Salt River Pima-Maricopa Indian Community of Salt River Reservation, AZ 

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

-13-
III. HISTORICAL FLUCTUATIONS 

The PM10 concentrations measured across much of Maricopa County on June 27, 2012, were the highest 

hourly and 24-hour averages measured over the last five-plus years. Time series plots of the 24-hour 

PM10 concentrations for the period January 1, 2007, through September 30, 2012, were created for all 

PM10 monitors in the Phoenix PM10 nonattainment area. Additionally, time series plots of the daily 

maximum hourly average PM10 concentrations were created for two monitors in Maricopa County. 

These two additional plots were created to provide a deeper understanding of the magnitude of short-term 

particulate concentrations and their contribution to PM10 exceedances in the Phoenix PM10 

nonattainment area. Time series plots for two monitoring sites (South Phoenix and Higley) are included 

within this section, while the remaining 24-hour average plots are available in Appendix A. The South 

Phoenix monitor was selected because of its central location in relation to all monitors in Maricopa 

County and it was severely impacted site during the June 27 windblown dust event. The Higley monitor 

was selected because it is representative of eastern Maricopa County, an area of the region frequently 

impacted by dust storms caused by thunderstorm outflow events. The graphs below and in Appendix A 

show that the June 27th event was in the top five most extreme events of the last five-plus years for several 

sites. 

Figure 3-1. Plot of daily maximum hourly average PM10 concentrations (January 1, 2007 – September 

30, 2012) at the South Phoenix monitoring site. 

-14-
Figure 3-1 shows the daily maximum hourly averages from the South Phoenix PM10 monitor. The plot 

shows that the June 27th daily maximum hourly average of 4,496 μg/m3

 was the highest hourly PM10 

concentration recorded in the last five-plus years. The plot also shows that the maximum hourly average 

PM10 concentration on June 27 exceeded that observed during the July 5, 2011, haboob event which 

recently received EPA concurrence. The hourly average concentrations from this event surpassed all of 

the dust storms that occurred during the 2011 monsoon season, which was one of the worst dust storm 

seasons on record. It is also clear that the most significant hourly average PM10 concentrations at the 

South Phoenix monitor typically occur during the June through September time frame, a period during 

which the monsoon season is most active and the Phoenix region is frequently impacted by haboob 

events. 

Figure 3-2 shows the daily 24-hour averages from the South Phoenix PM10 monitor and indicates that the 

June 27th event produced the second highest 24-hour average PM10 concentration measured in the last 

five-plus years. Similar to haboob events that occurred in 2009 and during several months in 2011, the 

June 27th event experienced PM10 concentrations that were well above the 24-hour exceedance threshold. 

Figure 3-2. Plot of 24-hour average PM10 concentrations (January 1, 2007 – September 30, 2012) at the 

South Phoenix monitoring site. 

Figures 3-3 and 3-4 provide an historical look at the daily maximum hourly averages and daily 24-hour 

averaged PM10 concentrations from the Higley monitor. The plots also show that the daily maximum 

hourly PM10 concentration and 24-hour average PM10 exceedance measured at the Higley monitor on 

June 27th were the third and sixth highest, respectively, PM10 concentrations measured over the last five-

-15-
plus years. As was the case for the South Phoenix monitor, the most significant hourly average PM10 

concentrations at the Higley monitor typically occur during the June through September time frame, a 

period during which the monsoon season is most active and the Phoenix region is frequently impacted by 

haboob events. 

Figure 3-3. Plot of daily maximum hourly average PM10 concentrations (January 1, 2007 – September 

30, 2012) at the Higley monitoring site. 

-16-
Figure 3-4. Plot of 24-hour average PM10 concentrations (January 1, 2007 – September 30, 2012) at the 

Higley monitoring site. 

Conclusions

The plots within this section provide an historical perspective of PM10 concentrations since 2007. These 

plots, along with those in Appendix A, show that the haboob event that occurred on June 27, 2012, 

resulted in one of the top 10 highest 24-hour average PM10 concentrations seen in the last five-plus years 

at 10 of the 15 monitoring sites with data over this period. This equates to PM10 concentrations in the 

99.9th percentile when compared to historic data. In the case of the Central Phoenix, Glendale, 

Greenwood, South Phoenix, JLG Supersite, North Phoenix, and Zuni Hills monitoring sites, the 24-hour 

averaged PM10 concentrations measured on June 27 were in the top three highest concentrations observed 

during the last five-plus years, making this one of the most severe dust storms ever measured across the 

Phoenix region. While the PM10 concentrations that were measured on June 27th were extreme in many 

cases, the plots also show that similar events are not uncommon during the monsoon season. As 

documented in several published journals and academic text books, haboobs generated from thunderstorm 

outflow boundaries are a phenomenon that Arizona deals with every year, some years more so than 

others, namely in 2007, 2009, and 2011. 

-17-
IV. NOT REASONABLY CONTROLLABLE OR PREVENTABLE

Section 50.1(j) of Title 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 and the Phoenix PM10 

nonattainment area, high wind conditions overwhelmed all reasonably available controls. The event 

occurring on June 27, 2012, was directly related to strong and gusty winds generated by a thunderstorm 

outflow boundary. The strong winds overwhelmed all reasonably available controls, and were also 

responsible for transporting PM into the Phoenix PM10 nonattainment area from areas outside of the 

nonattainment area. As explained in the conceptual model, downdrafts from collapsing thunderstorms 

produce outflow boundaries that contain gusty, turbulent wakes that raise dust from the desert surfaces 

that can be as wide as 100 miles in severe cases. As shown in Section V, the source region for all of the 

dominant thunderstorm outflows and associated transported dust on June 27, 2012, generally came from 

areas outside of the Phoenix PM10 nonattainment area—primarily from the deserts of Pinal, Pima, and 

southern Maricopa counties. While it is likely that some dust was generated within the PM10

nonattainment area as gusts from the thunderstorm outflows passed through the area, the amount of dust 

generated locally was easily overwhelmed by, and largely unnoticeable as compared to, the dust 

transported in from the source regions of the thunderstorm outflows. Strict controls on local sources of 

fugitive dust were in place and enforced on June 27, 2012, but were not capable of controlling transported 

dust and PM10 raised by the gusty and turbulent thunderstorm outflows on this date. 

The following sections describe the BACM- and MSM-level PM10 control measures in place June 27, 

2012, and the robustness of the programs designed to enforce these measures. Pro-active response and 

analysis of each event as it is occurring verify the uncontrollable nature of each event, while inspections 

of local sources performed before, during, and after the June 27, 2012, event confirmed that no unusual 

anthropogenic PM10-producing activities occurred in Maricopa County, the Phoenix PM10 

nonattainment area, or the local areas surrounding the exceeding monitors. 

Regulatory Measures and Control Programs

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: 

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

2. Maricopa County’s Inspection and Compliance Program; and 

3. ADEQ’s Air Quality Forecasting Program. 

Specifically, ADEQ is responsible for compliance assistance and enforcement of Agricultural Best 

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

while MCAQD is responsible for compliance assurance for all other significant sources of PM10 

emissions. In addition to routine inspections and inspections driven by complaints, inspections are often 

increased when (1) ADEQ forecasters issue a High Risk for the Maricopa County Dust Control Forecast, 

(2) ADEQ forecasters issue a High Pollution Advisory, or (3) near-real-time monitoring data indicate 

unique activity via high PM concentrations. The forecasting program and inspection/compliance 

programs work together so that resources can be best utilized during days that are of greatest risk for 

elevated PM emissions. 

-18-
On July 25, 2002, EPA took initial action to finalize approval of the Best Available Control Measure 

(BACM) and the Most Stringent Measure (MSM) demonstrations in the Serious Area PM10 plan for the 

Maricopa County portion of the metropolitan Phoenix PM10 nonattainment area (67 FR 48718). These 

BACM and MSM demonstrations were again approved by EPA on July 14, 2006 (71 FR 43979). The 

Agricultural Best Management Practices General Permit rule and related definitions have been approved 

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

§ 49-457.4 Maricopa County regulations of PM10 emissions are listed in Table 4-1. 

Table 4-1. Rules and Ordinances Regulating Particulate Matter Emissions in Maricopa County 

Rule/Ordinance Number 

& Title Description 

Rule 300: Visible 

Emissions 

Establishes standards for visible emissions and opacity. 

Rule 310: Fugitive Dust 

from Dust-Generating 

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 Non-Traditional 

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 that are not regulated by Rule 310 and 

that 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 

 

4

 Updates to the AgBMP program in December, 2011, clarified BMPs for crop 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. 

-19-
Rule/Ordinance Number Description & Title 

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 brick and structural clay product 

(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 Woodburning 

Restriction 

Establishes restrictions for residential woodburning 

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: OffRoad 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 

In addition to the rules and regulations listed in Table 4-1, other PM10 reducing control measures (e.g., 

paving of 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 SIP through PM10 plans such as the Revised 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 4-2). Additionally, the PCAQCD implements 

specific nonattainment rules for that part of the Phoenix PM10 nonattainment area that resides in Pinal 

County (see Table 4-3). 

Table 4-2. Pinal County Rules Regulating Existing and New Non-point Sources in Pinal County 

Article Number & Title Description 

Chapter 4, Article 2: Fugitive 

Dust 

Provides a mechanism to reasonably regulate operations which 

periodically may cause fugitive dust emissions into the 

atmosphere 

Chapter 4, 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 

-20-
Table 4-3. Pinal County Rules Regulating Fugitive Dust in Pinal County Portion of Maricopa 

County PM10 Nonattainment Area 

Article Number & Title Description 

Chapter 4, 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 

Chapter 4, Article 5: 

Nonattainment Area Rules; 

Stabilization for Residential 

Parking and Drives 

Establishes rules for stabilizing residential properties 

Chapter 4, Article 6: Restrictions 

on Vehicle Parking and Use on 

Vacant Lots

Establishes rules for unpaved or unstabilized vacant lots (valid 

in Pinal County portion of Area A) 

Chapter 4, 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 

Chapter 4, Article 8: 

Nonattainment Area Rules, 

Requirement for Stabilization of 

Disturbed Areas at Vacant Lots

Establishes rules for stabilizing disturbed areas at vacant lots 

PM10 Rule Effectiveness

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

permit compliance rates. This rule effectiveness (RE) study was designed to assess how many sources 

regulated by MCAQD during the subject time period received no PM10 emissions-related violations. As 

a basis for comparison, the percentage of permitted sources in compliance during calendar year 2007 was 

76% for sources subject to Rule 310, 85% for Rule 310.01 sources, and 40% for Rule 316 sources. In 

early 2008, Rules 310, 310.01, and 316 were strengthened, and new ordinances (covering additional 

source categories such as leaf blowers, vacant lots, and off-road vehicles) were adopted. These 

enhancements resulted from MCAQD department’s obligations under such agreements as the 2005 

Revised PM10 State Implementation Plan for the Salt River Area and the Maricopa Association of 

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

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

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

and regulatory changes that broadened and strengthened control measures under Rules 310, 310.01, and 

316. 

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

for the new and revised regulations to take effect. The results showed significant increases in compliance 

compared with the earlier period: to 90% (from 76%) for Rule 310 sources, to 95% compliance (from 

85%) for Rule 310.01 sources, and to 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, and 73% for Rule 316 sources. The timeline provided in Figure 4-1 illustrates the improvements 

-21-
in RE over the last several years, and also points out significant revisions to previous rules, as well as 

newly adopted rules and ordinances. 

01/27/2010

Rules 310 & 310.01

Revised

01/12/2011

Ordinances

P‐27 & P‐28 Revised

06/01/2005

06/08/2005

Rule 316 Revised

04/07/2004

Rule 310 Revised

02/17/2005

Rule 310.01 Revised

03/26/2008

Rules 310 & 310.01

Revised

March 2008

September 2006

02/20/2008

Ordinances P‐25, P‐27

& P‐28  Adopted

03/12/2008

Rule 316 Revised

Figure 4-1. Timeline of Maricopa County Fugitive Dust Rules and Ordinances. 

Compliance and Enforcement Activities

MCAQD is prepared to proactively respond to high wind events and to protect human health and wellbeing. MCAQD’s approach consists of two primary components: routine proactive inspections, as well 

as surveillance inspections, conducted both during and after significant 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 also responds to the majority of complaints within 24 hours. 

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

elevated PM10 pollution levels due to high winds or stagnant conditions. When a High Pollution 

Advisory (HPA) is issued for Maricopa County, MCAQD conducts additional 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. Postevent activities include continued surveys of high-risk areas, re-inspecting sources that had received 

violations within two business days, and an internal MCAQD debriefing of event activities. 

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

allow the monitoring sites to automatically report monitored readings at five-minute intervals. 

Previously, hourly readings were only 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 monitor and to consult the National Weather Service 

website to check for weather event activity. This capability allows the MCAQD responder to identify 

-22-
regional events and monitor specific issues. If necessary, the MCAQD responders can inform nearby 

stakeholders and local governments of the elevated PM10 concentrations. 

For June 27, 2012, a Maricopa County Dust Control Forecast was issued indicating a moderate risk level 

for unhealthy PM10. The Dust Control Forecast also indicated mostly light winds were expected, with 

strong gusts possible later in the day. 

An evaluation of all inspection reports, air quality complaints, compliance reports, and other 

documentation indicate no evidence of unusual anthropogenic-based PM10 emissions. During the time 

period of June 24, 2012 through June 30, 2012, MCAQD inspectors conducted a total of 151 inspections 

of permitted facilities, of which 113 were at fugitive dust sources. Additionally, MCAQD conducted one 

inspection on vacant lots and unpaved parking lots during this period. 

During this 7-day period, a total of 27 violations were issued county-wide for PM10 and non-PM10-related 

violations. As described below, on June 28th, eight rule violations were issued for PM10 emissions within 

a 4-mile radius of the following exceeding monitors: Durango Complex, Greenwood, Higley, JLG

Supersite, South Phoenix, and West 43rd. 

On June 28, 2012, a violation was issued to an earthmoving source for failing to have a certified dust 

coordinator onsite, which is required of earthmoving sources with a disturbed area of 5 acres or more. 

The site was stable with soil crust and all required records were on-site and documented compliance with 

the source’s permit conditions. The violation location is approximately 3.05 miles southeast of the South 

Phoenix monitor. The earthmoving site was stable and would not have contributed to the exceedance at 

the South Phoenix monitor on the previous day. 

On June 28, 2012, a violation was issued to an earthmoving source for failing to have the water truck 

driver complete a Basic Dust Control Training Class. The site was stable with visible soil crust and water 

at the time of inspection. The gravel pad was in good condition. All dust control records were 

maintained and document compliance with the site’s earthmoving permit. The water truck driver 

registered for the 310 Basic Dust Control Training class on the day of the violation, June 28, 2012. The 

site was located 2.8 miles east of the Higley monitor. The site was stable and would not have contributed 

to the exceedance at the Higley monitor on the previous day. 

Five violations were issued to a Rule 316 source on June 28, 2012. The violations include exceeding the 

silt loading and silt content limits, failure to implement controls on unpaved haul roads, failure to 

implement control measures on open storage piles, failure to have the water truck driver complete the 

Basic Dust Control Training Class, and failure to provide the required dust control records by the 

requested date. The source notified MCAQD that the stability issues were corrected on the same day of 

the violation, June 28, 2012. The source stabilized and barricaded the haul road to limit access. The 

source did not allow the water truck driver to operate the water truck until he completed a Basic Dust 

Control Training class. Finally, the source provided the required dust control records to the inspector. 

The 316 source had been in compliance with MCAQD rules for the previous 28 months. The violations 

were located 0.67 miles northwest of the West 43rd Ave monitor, 2.1 miles west of the Durango Complex 

monitor, and 3.8 miles southwest of Greenwood. The winds were generally from the south and southwest 

at the West 43rd Ave monitor during the largest exceedances at the West 43rd Ave. monitor from 1745 to 

1820 hours and 1930 to 2330 hours. The source was not within the wind profiles during these times and 

would not have contributed to the exceedance at the West 43rd Ave. monitor. The winds were generally 

from the south, southwest, or east at the Durango Complex monitor, when the monitor registered 

exceedances during the 1800 hours, 2000 hours, and 2100 to 2400 hours. The source was not within the 

wind profile during these times and would not have contributed to the exceedance at the Durango 

-23-
Complex monitor. The winds were generally from the southeast and east at the Greenwood monitor, 

when the monitor registered exceedances from 1810 to 1845 hours and 2000 to 2400 hours. The source 

was not within the wind profile during these times and would not have contributed to the exceedance at 

the Greenwood monitor. Analysis of the wind direction and PM10 concentrations demonstrates the 

violations would not have contributed to the exceedances at the West 43rd Ave, Durango Complex, or 

Greenwood monitors on June 27th. Furthermore, the PM10 concentrations entering the Phoenix 

nonattainment area from the southeast on June 27th exceeded 4,000 μg/m3

, as measured by the South 

Phoenix monitor, vastly overwhelming any potential contribution from PM10 sources within the 

nonattainment area. 

MCAQD was prepared for any complaints received due to the high wind event. During the 7 day period 

from June 24, 2012 through June 30, 2012, MCAQD received 23 complaints, of which 16 were 

windblown dust-related. Each complaint was assigned to and investigated by a MCAQD inspector. A 

review of all pertinent records from this period indicates that MCAQD inspectors observed one PM10

violation of local, state, or federal regulations within a 4-mile radius of the exceeding monitor. 

On June 29, 2012, MCAQD received a windblown dust complaint on a vacant lot. A MCAQD inspector 

investigated and issued a violation for unstable soil on an abandoned golf course. The site was stable by 

soil crust and vegetation growth on subsequent inspections. The violation location was 3.5 miles north of 

the JLG Supersite monitor, which was not within the wind profile of the predominant southeasterly winds 

during the June 27th exceedance. 

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

Agricultural Best Management Practice Program (Ag BMP) inspector monitors the ADEQ Five-Day Dust 

Control Forecast and the MCAQD air monitoring sites that include real-time data. The ADEQ Ag BMP 

inspector uses specific knowledge of seasonal activities and associations with the local growers and 

dairymen to communicate the importance of limiting dust-generating activities, especially during highwind events. Additional outreach is conducted with facility representatives prior to forecasted high-wind 

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

ADEQ Ag BMP inspector is dispatched to contact the owners and operators of agriculture fields in the 

area to discern if their activities are causing negative impacts. The Ag BMP inspector is prepared to 

respond to most agriculture complaints within 24 hours. 

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

that agricultural activities produced unusual PM10 emissions. On June 27, 2012, the ADEQ Ag BMP 

inspector observed activity in the Higley area and noted no harvesting or tilling activities. No complaints 

were received in the 2 days prior to the event. 

Conclusions

The thunderstorm outflow event of June 27, 2012, first described in Section II and discussed in more 

detail in Section V, produced strong gusts and turbulent wakes 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 Pinal, 

Pima, and southern Maricopa County. The Phoenix area is designated as a serious nonattainment area for 

PM10 and is required to have BACM for all significant sources of PM10. BACM-approved control 

measures on significant anthropogenic sources were in place and enforced during the events, and proactive tracking and response to the events by regulatory agencies and local governments confirmed the 

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

-24-
are adequate for meeting the requirements of an exceptional event and should be considered “reasonable” 

for these purposes. 

Despite the deployment of comprehensive control measures and sophisticated response programs and a 

few localized, low-impact violations of the dust control rules, high wind conditions associated with 

thunderstorms and thunderstorm outflows brought high concentrations of PM10 emissions into, and also 

overwhelmed controls within, the Phoenix PM10 nonattainment area. Strong thunderstorm outflows with 

sustained winds exceeding 30 mph, and even greater speeds nearest the source regions, as discussed in 

Section V, were enough to overwhelm all available efforts to limit PM10 concentrations during the 

extreme windblown dust event. The fact that these were natural events involving strong thunderstorm 

outflow winds that transported PM10 emissions into Maricopa County, with a majority of the PM10 

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

nonattainment area, provides strong evidence that the events and exceedances of June 27, 2012, recorded 

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

-25-
V. CLEAR CAUSAL RELATIONSHIP 

Introduction

A demonstration of the clear causal connection between uncontrollable natural events or uncontrollable 

emissions associated with the thunderstorm outflow boundaries and PM10 exceedances on June 27, 2012, 

is presented in the following time series graphs, infrared satellite imagery, and National Weather Service 

(NWS) data tables for selected, representative airports. These various pieces of information contain data 

on sustained wind speed, wind speed gusts, PM10 concentrations, visibility, weather observations, and 

thunderstorm development areas and provide the causal connection between the uncontrollable emissions 

associated with thunderstorm outflows and elevated PM10 concentrations. For reference, Appendix D 

provides additional NWS text products issued preceding and on June 27 containing forecasts and 

warnings of blowing dust due to thunderstorms outflows. A summary of the event day is provided below. 

Event – June 27, 2012

On June 27, 2012, the Phoenix region experienced typical summertime weather conditions with mostly 

clear skies, a maximum temperature of 110°F, relative humidity of 15-25%, winds <6 miles per hour 

(mph) from various directions for much of the day, and visibility at 10 miles through 7 p.m., as reported at 

the Phoenix Sky Harbor Airport. Nearly identical surface weather conditions in the Phoenix area on the 

preceding day, June 26th, resulted in generally low PM10 concentrations (20-50 μg/m3

 range) through the 

early evening hours, but general early-season monsoon conditions allowed thunderstorms to develop over 

the eastern half of Arizona on the June 26th, producing a thunderstorm outflow boundary with moderately 

strong winds that caused a spike in PM10 concentrations during the 8 p.m. hour. However, due to the 

short duration of the elevated PM10 concentrations, no exceedances were observed on June 26th and 

PM10 concentrations dropped back down to low levels for June 27th. Similar to the 26th, though, between 

6 p.m. Mountain Standard Time (MST) and 8 p.m. MST on June 27th, severe thunderstorms developed 

over a large area southeast of the Phoenix region (as shown in Figure 5-1) and by 7-8 p.m., outflows from 

the thunderstorms entrained dust from desert areas of Pinal County and generated a haboob that 

progressed northwestward into Maricopa County, transporting extremely large amounts of PM10 into the 

Phoenix PM10 nonattainment area. 

As shown in Figure 5-2, the haboob had the following impacts in the Phoenix area based on readings at 

the Phoenix Sky Harbor Airport, the Gateway Williams Airport, and local PM10 monitoring stations: 

southeasterly winds increased to 31-34 mph, with gusts up to 44-47 mph; visibility dropped from 10 miles 

to as low as 0.25 miles; weather conditions were reported as “blowing dust” and “dust storm”; and hourly 

PM10 concentrations increased to nearly 5,000 μg/m3 at the JLG Supersite monitoring site in the central 

Phoenix area. Additionally, PM10 concentrations exceeded 4,000 μg/m3

 at five other monitoring sites. 

Once the winds began to subside during the 8 p.m. hour, PM10 concentrations remained high for several 

hours, resulting in 24-hour average concentrations above 300 μg/m3

, more than double the 24-hour PM10 

standard, at five monitoring sites on June 27th. In total, 13 monitors at 12 monitoring sites in the region 

exceeded the PM10 NAAQS and, as was mentioned in the Historical Fluctuation section, seven of the 

PM10 monitoring sites saw one of their three highest 24-hour average PM10 concentrations measured in 

the past five-plus years. Without the existence of the emissions generated by the thunderstorm outflows, 

there would not have been any exceedances of the 24-hour PM10 standard in the Phoenix PM10 

nonattainment area. 

-26-
12:00 p.m. 2:00 p.m. 4:00 p.m. 6:00 p.m. 

1900z Jun 27 2100z Jun 27 2300z Jun 27 0100z Jun 28 

8:00 p.m. 10:00 p.m. 12:00 a.m. 2:00 a.m. 

0300z Jun 28 0500z Jun 28 0700z Jun 28 0900z Jun 28 

Figure 5-1. Satellite imagery showing strong storms developing and dissipating over southeastern 

Arizona. These storms are believed to have been responsible for the thunderstorm outflow boundary that 

caused PM10 exceedances in Maricopa and Pinal counties on June 27, 2012. 

Figure 5-2 depicts the dust event that occurred in Maricopa County impacting the Phoenix area on June 

27, 2012. Hourly averaged PM10 concentrations at monitors throughout Maricopa County are shown 

along with visibility readings (blue diamonds) and wind speeds reported at the Phoenix Sky Harbor 

Airport and the Williams Gateway Airport. Measurements at Sky Harbor were used due to its central 

location amongst most of the Phoenix-area monitors that experienced exceedances on June 27 and wind 

speeds for the Williams Gateway Airport represented conditions on the eastern-side of the region. As can 

be seen in the graph, visibility was reduced down to 1 mile at the Sky Harbor Airport during the 

windblown dust event, and this visibility reduction coincided with rapidly increasing PM10

concentrations and sustained wind speeds up to 31-34 mph in the Phoenix area. The NWS data tables 

after Figure 5-3 also show reports of “blowing dust” and “dust storm” during the 7 p.m. and 8 p.m. hours, 

when PM10 concentrations were the highest at various sites across the region. As noted earlier, data 

during the 8 p.m. hour, when PM10 concentrations were highest at most of the Phoenix-area monitoring 

sites, no data were collected by the West Phoenix monitor due to an equipment malfunction. 

Additionally, the NWS issued dust storm warnings (see Appendix D) for many areas in Pinal and 

Maricopa Counties due to the large haboob that was moving across southeastern and central Arizona for 

upwards of 100 miles. The daily climatology report June 27th also listed under “Weather Conditions” for 

the day: “Sandstorm”, providing another clear indication of an exceptional natural event. 

-27-
Figure 5-2. Phoenix nonattainment area PM10 monitor readings, Sky Harbor wind speed and visibility, 

and Williams Gateway wind speed for June 27, 2012.

As shown in Figure 5-3, several monitoring sites in the upwind, dust source region of Pinal County 

measured 24-hour average PM10 exceedances, with the peak site (Cowtown) having a 24-hour average of 

504 μg/m3

 and an extremely high maximum 1-hour average during the 5 p.m. hour of 9,639 μg/m3, when 

the southeasterly winds were strongest and the visibility was lowest at the Casa Grande Airport (see the 

NWS data tables below). Four other monitoring sites also measured hourly concentrations over 1,500 

μg/m3

. These spikes at the Pinal County monitors preceded the PM10 concentration spikes at the 

monitoring sites in Maricopa by one to two hours, which is consistent with wind speeds and directions 

observed at the Phoenix Sky Harbor Airport in Maricopa County and the Casa Grande Airport in Pinal 

County for the propagation of the haboob across the region. Only monitoring sites in Maricopa and Pinal 

counties exceeded the NAAQS due to thunderstorm outflows on this day, but the broad range of sites and 

the extreme hourly- and daily-averaged PM10 concentrations reflect the magnitude of the haboob. 

Additionally, the NWS airport meteorological data tables for the Phoenix Sky Harbor and Casa Grande 

airports show that the strongest winds associated with the haboob were out of the southeast. These winds, 

combined with the high PM10 concentrations in Pinal County, indicate that PM10 was already entrained 

in the atmosphere outside of the Phoenix PM10 nonattainment area and then transported into the region 

by the strong winds. 

-28-
Figure 5-3. Pinal County PM10 monitor readings and Casa Grande wind speed and visibility for June 27, 

2012

A visualization of this dust event was created using Phoenix visibility camera images that can be viewed 

as an animation here: 

Looking south toward Phoenix, with South Mountain in the distance: 

www.phoenixvis.net/videos/mpeg4/SOMT_06272012.mp4

The video clearly shows the wall of dust blowing across the Phoenix region in the early evening of June 

27, 2012. The snapshot on the cover of this report was taken from the 8:05 p.m. mark of the video and 

coincides with wind speed increases and visibility reductions at the airports and PM10 concentration 

spikes seen in the hourly data from the PM10 monitoring sites in the central Phoenix area. Videos 

showing other views across the Valley on this date are also available (see Visibility Cameras section).

-29-
-30-

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 

(06/2012) 

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

Ne

3-h

Ch

(m

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

27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27

0051 0151 0251 0351 0451 0551 0651 0751 0851 0951 1051 1151 1251 1351 1451 1551 1651 1751 1851 1951 2003 2024 2034 2051 2100 2116 2134 2151 2158 2251 2351

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

FEW250 FEW250 FEW250 FEW250 FEW200 FEW150 SCT200 FEW150 FEW150 FEW150 FEW150 FEW100 SCT150 FEW110 SCT150 FEW110 SCT150 FEW120 FEW120 FEW120 FEW120 FEW120 SCT170 SCT210 FEW120 SCT180 SCT250 BKN028 BKN180 BKN250 BKN011 BKN180 BKN250 BKN012 BKN180 BKN250 BKN008 BKN180 BKN250 BKN008 BKN180 BKN250 BKN008 BKN180 BKN250 BKN008 BKN180 BKN250 BKN013 BKN180 FEW013 SCT180 SCT250 FEW013 SCT180 SCT250 FEW015 SCT140 SCT180 SCT140 SCT190

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 1.00 1.00 2.00 1.50 1.75 2.50 3.00 3.00 6.00 6.00 6.00 8.00

BLDU BLDU BLDU BLDU BLDU BLDU BLDU BLDU BLDU BLDU BLDU

969493929089899397100101104106107108109110109106999393939393939394939292

35.634.433.933.332.231.731.733.936.137.838.340.041.141.742.242.843.342.841.137.234.034.034.033.934.034.034.034.434.033.333.3

70696969696969707071717172717171727272717273727272727372717171

20.920.620.720.520.420.520.620.921.321.821.421.722.021.921.621.822.322.422.221.922.022.622.022.322.022.022.621.921.521.621.6

55555656575858575656545353525050525354576163616261616360596060

12.812.813.313.313.914.414.413.913.313.312.211.711.711.110.010.011.111.712.213.916.017.016.016.716.016.017.015.615.015.615.6

25 27 29 30 33 35 35 30 25 23 21 18 17 16 14 14 15 16 18 25 34 37 34 36 34 34 37 32 32 34 34

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

130110130130110110130110270230VR VR VR 330330330330240230150150150160160160070110090110100070

18 18 18 44 33 23 

28.52 28.52 28.53 28.54 28.56 28.59 28.61 28.61 28.61 28.63 28.63 28.61 28.60 28.58 28.56 28.54 28.53 28.53 28.55 28.62 28.62 28.63 28.64 28.65 28.65 28.65 28.66 28.66 28.66 28.66 28.65

2 3 1 2 8 6 3 0

002012018004015017031013
-31-

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

(final) 

HOURLY OBSERVATIONS TABLE 

WILLIAMS GATEWAY AIRPORT (23104) 

PHOENIX, AZ 

(06/2012) 

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

Dry 

Bulb 

Temp

Wet 

Bulb 

Temp

Dew 

Point 

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

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

Rel 

Humd

%

Wind 

Speed (MPH)

Wind 

Dir

Wind

Gusts

(MPH)

Station

Pressure

(in. hg)

Press

Tend

Net

3-hr

Chg

(mb)

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

27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 

0015 0035 0055 0115 0135 0155 0215 0235 0255 0315 0335 0355 0415 0435 0455 0515 0547 0647 0747 0847 0947 1047 1147 1247 1347 1447 1547 1647 1818 1835 1900 1908 1947 

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 BKN100 SCT100 SCT150 FEW200 FEW200 SCT200 FEW200 SCT120 SCT120 SCT120 SCT120 SCT120 VV000 BKN120 BKN120 BKN120 BKN 

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 45.00 45.00 45.00 35.00 35.00 35.00 35.00 35.00 35.00 35.00 35.00 35.00 0.50s 0.75 2.00 0.25s 2.00 

DSs

DS BLDU DSs 

88 88 88 88 88 88 88 88 88 86 86 86 86 86 86 86 84 88 97 97 10210410610810810910910910910810697 91 

31.031.031.031.031.031.031.031.031.030.030.030.030.030.030.030.029.031.036.036.039.040.041.042.042.043.043.043.043.042.041.036.033.0

6968696967676767676867676969696969717271727272727270707070727273M

20.620.120.620.619.519.519.519.519.519.719.219.220.320.320.320.320.521.822.121.522.322.122.222.422.021.321.321.320.922.422.222.6M 

595759595555555555575555595959596163595757555454524848484654546161

15.014.015.015.013.0

13.013.013.013.014.013.013.015.015.015.015.016.017.015.014.014.013.012.012.011.09.0 9.0 9.0 8.0 12.012.016.016.0

38 35 38 38 33 33 33 33 33 37 35 35 40 40 40 40 46 43 28 26 23 20 18 17 16 13 13 13 12 17 18 30 M 

7 5 6 6 5 3 5 7 6 5 5 6 6 5 5 8 11 10 6 9 8 3 8 6 11 10 10 5 8 10 20 34 28 

140 140 130 120 120 120 120 120 130 130 120 120 130 120 100 120 120 120 120 130 140 140 260 250 280 280 270 310 300 260 140 140 150 

36 47 33 

28.29 28.28 28.28 28.28 28.28 28.28 28.28 28.29 28.29 28.29 28.30 28.30 28.31 28.31 28.32 28.34 28.36 28.36 28.36 28.37 28.38 28.39 28.37 28.36 28.34 

28.32 28.30 28.28 28.30 28.32 28.36 28.37 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 
-32-

27 27 27 27 27 27 27 27 27 27 27 27 27 27

1955 2008 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

CLR BKN120 CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR

7.00 5.00 4.00 10.00 10.00 10.00 10.00 7.00 10.00 10.00 10.00 10.00 10.00 10.00

-RA BLDU HZ 

90 97 97 93 91 93 93 91 93 93 93 93 93 93

32.036.036.034.033.034.034.033.034.034.034.034.034.034.0

7173727270717169706969696968

21.522.622.122.021.121.521.520.620.920.420.420.420.420.1

6161596159595957575555555554

16.016.015.016.015.015.015.014.014.013.013.013.013.012.0

38 30 28 34 34 32 32 32 30 28 28 28 28 27

24 23 17 15 15 15 18 13 10 8 8 6 6 3

170 170 150 100 100 100 100 090 100 100 080 080 090 060

31 40 28 21 24 

28.41 28.40 28.40 28.41 28.40 28.40 28.41 28.41 28.41 28.41 28.40 28.41 28.41 28.40

M M M M M M M M M M M M M M

Dynamically generated Wed Dec 26 22:27:59 EST 2012 via http://cdo.ncdc.noaa.gov/qclcd/QCLCD
-33-

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 

(06/2012) 

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

Ne

3-h

Ch

(m

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

27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 

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 

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 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 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 

8888888484848284848281828282818282828282848488909193939597979999100

31.031.031.029.029.029.028.029.029.028.027.028.028.028.027.028.028.028.028.028.029.029.031.032.033.034.034.035.036.036.037.037.038.0

696868666666656666656565656565656666666768686869696970707070707071

20.620.120.118.818.818.818.218.818.818.518.318.518.518.518.318.519.019.019.019.619.919.920.120.420.620.420.921.221.021.021.321.321.5

595757555555545555555555555555555757575959595757575557575555555555

15.014.014.013.013.0

13.012.013.013.013.013.013.013.013.013.013.014.014.014.015.015.015.014.014.014.013.014.014.013.013.013.013.013.0

38 35 35 37 37 37 38 37 37 40 41 40 40 40 41 40 43 43 43 46 43 43 35 33 32 28 30 28 24 24 23 23 22 

5 0 0 0 0 0 6 3 3 0 0 0 3 0 3 11 6 0 3 3 6 5 0 0 6 6 6 3 0 0 0 5 0 

090000000000000000050060110000000000250000130120130000090070090060000000160100120160000000000160000

28.20 28.20 28.20 28.20 28.20 28.21 28.21 28.21 28.21 28.22 28.22 28.23 28.23 28.24 28.25 28.27 28.27 28.27 28.27 28.27 28.27 28.27 28.27 28.27 28.28 

28.29 28.29 28.29 28.29 28.30 28.30 28.30 28.29 

4 2 2 2 

000014010007
-34-

27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27

1115 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 0

CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR CLR SCT120 FEW022 SCT050 BKN110 FEW021 SCT035 BKN050 SCT025 BKN036 BKN060 FEW022 BKN120 SCT120 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 7.00 10.00 7.00 6.00 10.00 10.00 10.00 9.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00

TS TS TS TS TS VCTS VCTS 

10010210210210610210610610810810810810810810910810910910810810410410497847975798488909190908886888482

38.039.039.039.041.039.041.041.042.042.042.042.042.042.043.042.043.043.042.042.040.040.040.036.029.026.024.026.029.031.032.033.032.032.031.030.031.029.028.0

717171717271727273737373737373737373727372727272716968706969696869717171697070

21.521.821.821.822.121.522.122.122.722.722.722.722.722.622.822.622.822.622.422.622.122.122.122.121.520.619.921.320.520.620.420.020.421.521.821.520.621.221.1

555555555454545455555555555555555554545555555559646464666159575557616363596364

13.013.013.013.012.012.012.012.013.013.013.013.013.013.013.013.013.012.012.013.013.013.013.015.018.018.018.019.016.015.014.013.014.016.017.017.015.017.018.0

22 21 21 21 18 20 18 18 17 17 17 17 17 17 17 17 17 16 17 17 20 20 20 28 51 60 69 65 46 38 33 30 33 38 43 46 38 49 55

5 0 0 0 9 0 7 6 7 11 8 10 9 7 7 7 8 13 15 20 15 16 10 16 20 31 24 22 20 20 22 16 9 7 7 8 14 10 7

230000000000260000280280320240280280270280200280310270310290280310310120140120120150130130120120160310320310010340010

17 18 16 24 24 20 17 33 31 39 41 25 37 31 28 22 28 

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

7 7 2 7 

017017047017

Dynamically generated Wed Dec 26 22:01:20 EST 2012 via http://cdo.ncdc.noaa.gov/qclcd/QCLCD

A description of the fields and data codes/abbreviations used in these tables can be found here: http://cdo.ncdc.noaa.gov/qclcd/qclcddocumentation.pdf
Visibility Cameras

Visibility camera images were compiled into time lapse photography videos in order to visually 

show the dust impacts on portions of Maricopa County on June 27. The animations are available 

online for each event and for each of the visibility camera sites in Maricopa County. These 

views can be accessed using the camera site codes SOMT (South Mountain), CAME (Camelback 

Mountains), SUPM (Superstition Mountains), ESMO (Estrella Mountains), and WHTM (White 

Tank Mountains). It should be noted that these time lapse photography videos may require the 

installation of a mp4 viewer. 

 

The video files can be accessed as follows: 

 

http://www.phoenixvis.net/videos/mpeg4/xxxx_mmddyyyy.mp4

 

xxxx = 4-character site code (ie. SOMT) 

mm = 2-character month (ie. 06) 

dd = 2-character day (ie. 27) 

yyyy = 4-character year (ie. 2012) 

Conclusion

The information presented within this section has adequately demonstrated a clear causal 

relationship between the emissions generated by uncontrollable natural events and the 

exceedances measured at the monitors. The satellite images, time series graphs, and 

meteorological data tables provided in this section show the temporal progression of the dust 

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

in PM10 concentrations. The combination of the PM10 and wind data from Maricopa and Pinal 

counties shows the transport of particulate matter from the south through Pinal County and into 

the Phoenix PM10 nonattainment area. This information supports the conclusion that the PM10 

exceedances observed on June 27, 2012, were the result of emissions entrained into the 

atmosphere by strong thunderstorm outflow boundary winds from source areas to the southeast, 

outside of the Phoenix PM10 nonattainment area, and transported into the Phoenix PM10 

nonattainment area. 

-35-
VI. “BUT FOR” ANALYSIS 

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

must satisfy 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 the exceedances on June 

27, 2012, were not reasonably controllable or preventable and there is a clear causal relationship 

between transported PM10 from thunderstorm outflows originating in the desert areas outside of 

the Phoenix PM10 nonattainment area and the measured exceedances within the nonattainment 

area. The weight of evidence in these sections demonstrates that but for the existence of 

emissions generated by these thunderstorm outflows and associated transported PM10, there 

would have been no exceedances of the 24-hour PM10 standard on June 27, 2012. 

As detailed in Section IV, all reasonable control measures were in place and actively enforced 

before, during, and after the exceedances of June 27, 2012. Inspection and compliance data of 

local fugitive dust sources during this time period revealed that PM10 from anthropogenic 

activities was well controlled and constant. Local regulatory agencies, industry, and the general 

public were alerted to the possibility of dust storms due to thunderstorm activity through daily 

forecasts and media reports. Heightened, real-time surveillance of PM10 monitoring stations 

during the events established a clear link between rapidly rising PM10 concentrations and the 

arrival of the thunderstorm outflows. On-the-ground observations recorded during the events 

consistently identify transported or re-entrained PM10 as the cause of the elevated concentrations 

near the exceeding monitors. 

As shown in Section V, detailed, time series data establish a clear causal relationship between 

the arrival of emissions generated by thunderstorm outflows and elevated PM10 concentrations 

at the monitors. Multiple, independent measurements of wind speed, wind direction, and 

visibility all point to the presence of thunderstorm outflows as the delivery vehicle for 

transported PM10 into the nonattainment area. Any local PM10 emissions generated during the 

arrival of the thunderstorm outflows are linked to the turbulent and gusty outflow front of the 

storm and pale in comparison to the amount of transported PM10 originating outside the 

nonattainment area. The source region for the primary thunderstorm outflow boundary and 

transported PM10 are clearly identified as the desert areas to the southeast of the Phoenix PM10 

nonattainment area. 

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

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

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

but for the presence of these uncontrollable natural events. 

-36-
VII. CONCLUSIONS 

The exceedances that occurred on June 27, 2012, satisfy the following criteria of 40 CFR 50.1(j) 

and meet the definition of an exceptional event: 

 The event affects air quality; 

 The event is not reasonably controllable or preventable; and 

 The event is unlikely to reoccur at a particular location or [is] a natural event. 

Affects Air Quality

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

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

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

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

Sections II, III, IV, and V, we can reasonably conclude that the event in question affected air 

quality. 

Not Reasonably Controllable or Preventable

Section 50.1(j) of Title 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 

and the Phoenix PM10 nonattainment area, high wind conditions overwhelmed all reasonably 

available controls. 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. The event discussed in this document that caused 

the exceedances in this request (see Sections II and V) were caused by thunderstorm-driven 

outflow winds that transported dust into Maricopa County from areas largely outside of the 

Phoenix PM10 nonattainment area. The fact that these were natural events involving strong 

thunderstorm outflow winds that transported PM10 emissions into Maricopa County, with a 

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

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

and exceedances of June 27, 2012, recorded within the nonattainment area were not reasonably 

controllable or preventable. 

Natural Event

As discussed above, the events shown to cause these exceedances were emissions of PM10 

driven by high winds caused by thunderstorm activity and related outflow boundary on June 27, 

2012. The events therefore qualify as natural events. 

-37-
-38-

Summary

In summary, the exceedances of the federal 24-hour PM10 standard on June 27, 2012, would not 

have occurred but for the monsoonal thunderstorm-driven high winds and windblown dust 

transport from areas largely outside the Phoenix PM10 nonattainment area, based on the weight 

of evidence summarized below. 

 Graphs of PM10 at various Phoenix area monitors in Section V and the tables containing 

meteorological data for representative NWS airport sites show that the timing of elevated 

PM10 events was consistent with decreased visibility and high winds at the NWS sites. 

 Visibility camera imagery discussed in Section V indicates that large quantities of PM10

were transported into the Phoenix Metro area during the June 27th event. The timing of 

the dust storms depicted in the visibility camera imagery is consistent with the PM10

concentration measurements, elevated winds, and reduced visibility reported during the 

event. 

 Historical Fluctuation analyses and graphs in Section III and Appendix A showing fiveplus years of 24-hour average data for each of the Phoenix PM10 nonattainment area 

monitors depict numerous atypically high PM10 concentrations on June 27, 2012. The 

extreme PM10 concentrations measured across a broad range of monitoring sites prove 

that the exceedances of June 27, 2012, were exceptional from an historical perspective. 

 Each of the exceedances of the PM10 standard recorded on June 27 were a direct result 

of thunderstorm activity and thunderstorm generated outflow winds, as can be seen in 

data and analyses in Section V. 

 Figures and tables in Section V show that the timing of the thunderstorm-generated 

outflow boundary passage, and increases in wind speeds at the monitoring locations and 

National Weather Service stations during this event, is consistent with the timing of 

elevated PM10 concentrations recorded at the monitoring locations in the Phoenix PM10 

nonattainment area. 

 Wind directions, thunderstorm-generated outflow boundary propagation, and 

concentration patterns showing elevated levels of PM10 in Pinal County prior to levels 

increasing in Maricopa County, all depicted in Section V, help show that the majority of 

the dust which impacted Phoenix PM10 nonattainment area monitors originated in Pinal 

County and other areas located generally to the south-southeast of the Phoenix PM10

nonattainment area. 

 Section IV discusses rules that are in place in the Phoenix PM10 nonattainment area as 

well as inspections that were conducted in the area to verify compliance with those rules 

in order to show that the events are not reasonably controllable or preventable. 

Additionally, the visibility camera time lapse photography videos referenced in Section V 

help illustrate the magnitude and scale of this event, which supports the claim that the 

exceedances recorded during this period were not reasonably controllable or preventable. 
0

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JLG Supersite 5-Year Historical Fluctuation - 24 Hour Averages
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West 43rd 5-Year Historical Fluctuation - 24 Hour Averages
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South Phoenix 5-Year Historical Fluctuation - 24 Hour Averages
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Durango 5-Year Historical Fluctuation - 24 Hour Averages
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West Chandler 5-Year Historical Fluctuation - 24 Hour Averages
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Higley 5-Year Historical Fluctuation - 24 Hour Averages
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Glendale 5-Year Historical Fluctuation - 24 Hour Averages
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North Phoenix 5-Year Historical Fluctuation - 24 Hour Averages
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Dysart 5-Year Historical Fluctuation - 24 Hour Averages
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Zuni Hills 5-Year Historical Fluctuation - 24 Hour Averages
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Apache Junction 5-Year Historical Fluctuation - 24 Hour Averages