# State of Arizona Exceptional Event Documentation

for the Event of June 27, 2012, for the

Phoenix PM 10 

# 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 even ts that result in exceedances of the National

Ambient Air Quality Standards (NAAQS ) that are exceptional in nature 1 

# (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 Pa rts 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 On May 2, 2011, in an attempt to clarify this rule, EPA released draft guidance documents on the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 lo cal air agencies for review. Based on numerous

comments EPA received on the May 2011 draft, EPA issu ed a revised draft guidance document in June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 demonstr ation submitted by the flagging agency.

Due to the arid nature of the state, Arizona is high ly 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 exceptiona l event documentation for these events. Due in

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

is now taking a new approach and focusing excepti onal event documentation on ev ents that are believed

to be clear-cut exceptional events that are not controllable by huma n 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 Phoeni x 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 informati on 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. Add itional 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 Depa rtment of Environmental

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

County Air Quality Control District.

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

out how those rules are met with in this specific assessment.

Section II of this assessme nt introduces the conceptual model of the thunderstorms a nd the subsequent

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

monsoons, and the formation of thunderstorm outfl ows, 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 o f this assessment provides data summaries a nd time series graphs that help illustrate that the

event of June 27, 2012, produced PM10 concentrations in excess of normal hist orical 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 esta blishes a clear causal connection betw een 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 hi storical fluctuations) also confir ms that the event in question both

affected air quality and was the result of natural events.

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

the natural events and the exceedances and conclude s 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 even t 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 dat a in AQS (40 CFR 50.14(c)(2)(ii)) .................................... 1

Notify EPA of intent to flag th rough 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 excepti onal 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 Geographi c Setting and PM10 Monitor Locations (source: EPA AQS

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

the white markers.

............................................................................................................................... ... 5

Figure 2-2. Drainage Syst em of Phoenix, Arizona. ......................................................................................6

Figure 2-3. Phoenix Monthly Precipitation (top) and Maxi mum 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 an alysis released during the tim e period of the exceedances

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

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

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

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

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

Figure 3-2. Plot of 24-hour av erage PM10 concentrations (Ja nuary 1, 2007 – September 30, 2012)

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

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

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

Figure 3-4. Plot of 24-hour av erage PM10 concentrations (Ja nuary 1, 2007 – September 30, 2012)

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

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

Figure 5-1. Satellite imagery showing strong stor ms developing and dissipa ting over southeastern

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

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

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

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

Figure 5-3. Pinal County PM10 monito r 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 Existin g 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-

# -vi

# List of Appendices

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

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

Appendix C – Notice of Public Comment Period

Appendix D – National Weather Servic e Forecasts, Warnings, and Alerts

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

Airports

# I. EXCEPTIONAL EVENT RU LE (EER) REQUIREMENTS

In addition to the technical requirements that are c ontained within the EER, proc edural requirements must

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

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

of Environmental Quality (ADEQ) addressed those requirements.

Procedural Requirements

This section presents a review of the procedur al 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 Qua lity System (AQS), notifying EPA of the intent to

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

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

all of these procedural and documentation requirements.

Public notification that event was occurring (40 CFR 50.1 4(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 dus t 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 subm it da ta 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 suspect s that data may be influenced

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

collected from the potentially affected filter-based ai r 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 fl ag is submitted for the measurement in the AQS. The

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

certification by May 1 of the year following the ca lendar 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 in itial eve nt 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 Ru le 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. Th is report serves as the demonstra tion supporting the flagging of these

data.

-1-

# On June 27, 2012, 13 m onitors at 12 monitoring site s within the boundaries of the Phoenix PM10

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

the evening hours. The PM10 monitors impacted were the JLG Supersite TEOM 2 

# and BAM 3 

# operated by

the ADEQ and the Central Phoenix, Durango Comple x, Glendale, Greenwood, Higley, North Phoenix,

South Phoenix, Tempe, West Chandler, West 43 rd 

# , and Zuni Hills monitors operated by Maricopa County

Air Quality Department (MCAQD).

Document that the public comment proce ss was followed for event docu mentation (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 com ments

submitted on this document, ADEQ will submit this document, the comments received, and ADEQ’s

responses to those comments to EPA Region IX headqua rters in San Francisco, California. The deadline

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

Documentation Requirements

Section 50.14(c)(3)(iii) of the EER states that in order to jus tify excluding air quali ty monitoring data,

evidence must be provided for the following elements:

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

(1) the event affected air quality,

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

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

natural event;

b. There is a clear causal relations hip between the measurement under consideration an d the event;

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

and

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

Section II of this assessme nt 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 eviden ce 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 a nd time series graphs that help illustrate that the

event on June 27, 2012, produced PM10 concentrations in excess of normal hist orical 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 esta blishes a clear causal connection betw een 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 hi storical fluctuations) also confir ms that the event in question both

affected air quality and was the result of natural events.

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

the natural events and the exceedances and conclude s 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 even t 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 R ive r Valley in south-central Arizona. It lies at a mean elevation of 1,090

feet above mean sea level (m sl) in the northeastern pa rt of the Sonoran Desert. Other than the mountains

in and around the city, the topography of Phoenix is ge nerally 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 Wh ite Tank Mountains (~4,500 ft msl) to the west, the

Sierra Estrella (~4,450 ft msl) to th e southwest, and the Superstition Mo untains (~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 s hows the general geographic setting of

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

the monitors shown in Figure 2-1 are filter-based mon itors; 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 Monito r Locations (source: EPA AQS DataMart,

NASA MODIS Satellite, Google Eart h). PM10 monitor locations ar e indicated by the white markers.

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

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

soils that become suspended and add to regional PM 10 loadings during high wind events. Much of this

alluvial matter and fine soil is deposited in the lo w lying areas of central a nd southern Arizona, with

larger depositional areas focused in and ar ound 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 Ju ly and lasts until mid-Sept ember, 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, a nd 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 blowi ng dust, large hail, and heavy rain. Dust storms

associated with these thunderstorms typically occur in the ear ly 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 recei ved during the monsoon season, extremely hot

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

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

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

period are typically the wettest part s of the year. Meanwhile, a distin ct dry season occurs from April

through June for the Phoenix area and the rest of Ar izona. While these weathe r patterns describe the

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

Arizona are also prone to a high de gree of variability in these weathe r patterns from year to year.

Figure 2-3. Phoenix Monthly Precipitati on (top) and Maximum Temper ature (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 southeas t Arizona and northern Mexico. The North American Monsoon is a

shift in wind patterns in the summ er 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 particular ly important for low-level moistu re transport into Arizona and

Sonora. Mid- and upper-level moisture is also tran sported into the region, ma inly from the Gulf of

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

western North America, which develops rather quic kly and sometimes dramati cally. Rainfall during the

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

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

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

while other nearby areas experi ence 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 re sult in any precip itation for the Phoenix area, but

the storms were severe in natu re and caused a dust-carry ing 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 burst s 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 th e thunderstorms can kick up large areas of dust,

sometimes called haboobs, and trans port that dust for long distances from the initiat ing thunderstorms

(see Figure 2-4).

The active monsoon occurring on June 27 th 

# led to numerous thunderstorms and a large thunderstorm

outflow that produced blowing dust throughout southeastern and centr al 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

27 th 

# event was a widespread dust event with mostly sout heasterly winds transporti ng dust from the desert

areas of Pima and Pinal Counties into all areas of th e Phoenix region. Further de tails 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 va st 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 lik ely came from areas within Pinal

County to the southeast of Maricopa County. Other potential s ource 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 networ ks in the general source

area.

Another important factor that le d 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 b een 45% of normal, and only 11% of normal since

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

information pertaining to the mons oon 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 th e 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 duri ng the early part of the monsoon season (and which

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

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

counties on June 27, 2012, resulted in very little ra infall throughout south-centr al Arizona, but the high

winds associated with the outfl ow boundary did transport extremel y high levels of particulate

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

6. This figure shows hourly PM10 concentrations for a three-day period ar ound, 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 thunde rstorms produced outflows and moderate to strong

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

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

exceedance of the 24-hour standard. On the event day, June 27 th 

# , the figure shows low PM10

concentrations during the day and then extrememly high PM10 concen trations 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 wit hout the impacts of the thunderstorm outfl ow boundary, PM10 concentrations at all

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

average PM10 exceedance threshold of 150 μg/m 3



# .

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 ev ent day. The table shows exceedances of the national

24-hour PM10 standard at nearly ev ery monitoring site in both Mari copa and Pinal Counties on June 27,

2012, and the timing of the 1-hour maximum values acros s 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 so utheasterly 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 27 th 

# . It should be noted that the 1-hour maximu m PM10 concentration measured at the West

Phoenix monitoring site was one hour later than all other nearby site s 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 dus t event is included in Section V, where a demonstration of the clear

causal connection between the uncontrollable natu ral events and PM10 exceedances is prese nted.Figure 2-5.U.S. Drought Monitor analysisreleased during thetime 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-

# 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/m 3 

# ) 1-hr Max

PM10

(μg/m 3 

# ) 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 – Maric opa 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 Trib e 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 fi ve-plus years. Time se ries plots of the 24-hour

PM10 concentrations for the period January 1, 20 07, through September 30, 2012, were created for all

PM10 monitors in the Phoenix PM10 nonattainment area. Additionally, time seri es 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 contri bution to PM10 exceedances in the Phoenix PM10

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

within this section, while the remaining 24-hour averag e 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 th e June 27 windblown dust even t. 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 27 th 

# event was in the top five most extreme even ts of the last five-plus years for several

sites.

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

30, 2012) at the South Phoenix monitoring site.

-14-

# Figure 3-1 shows the daily m aximum hourly averages from the South Phoenix PM10 monitor. The plot

shows that the June 27 th 

# daily maximum hourly average of 4,496 μ g/m 3 

# was the highest hourly PM10

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

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

recently received EPA concurrence. The hourly averag e 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 PM 10 concentrations at the

South Phoenix monitor typically oc cur during the June through Septem ber 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 27 th 

# event produced the second highest 24-hour aver age PM10 concentration measured in the last

five-plus years. Similar to haboob events that oc curred in 2009 and during se veral months in 2011, the

June 27 th 

# 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 – Septem ber 30, 2012) at the

South Phoenix monitoring site.

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

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

hourly PM10 concentration and 24-hour average PM 10 exceedance measured at the Higley monitor on

June 27 th 

# were the third and sixth highest, respectively, PM10 concentrati ons measured over the last five-

-15-

# plus years. As was the case for the S outh Phoe nix monitor, the most significant hourly average PM10

concentrations at the Higley mon itor 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 m aximum 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 – Septem ber 30, 2012) at the

Higley monitoring site.

Conclusions

The plots within this section provi de 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 PM 10 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

99.9 th 

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

Greenwood, South Phoenix, JLG Supersite, North Phoeni x, and Zuni Hills monito ring 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, maki ng this one of the most severe dust storms ever measured across the

Phoenix region. While the PM10 concentra tions that were measured on June 27 th 

# 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 acad emic text books, haboobs generated from thunderstorm

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

others, namely in 2007, 2009, and 2011.

-17-

# IV. NOT REASONABLY CONT ROLLABLE 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 define d as an exceptional event. This requirement is met by demonstrating

that despite reasonable control measures in pl ace 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 direc tly related to strong and gusty wi nds generated by a thunderstorm

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

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

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

produce outflow boundaries that contai n gusty, turbulent wakes that rais e 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 a nd associated transported dust on June 27, 2012, generally came from

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

southern Maricopa counties. Wh ile it is likely that some dus t 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 ove rwhelmed by, and largely unnoticea ble as compared to, the dust

transported in from the source region s of the thunderstorm outflows. St rict 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 tur bulent thunderstorm out flows on this date.

The following sections describe the BACM- and MS M-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 afte r the June 27, 2012, event confirmed that no unusual

anthropogenic PM10-producing activities occu rred in Maricopa County, the Phoenix PM10

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

Regulatory Measures and Control Programs

ADEQ and MCAQD are responsible for implementing re gulatory measures to control emissions from

agricultural sources, stationary sources, fugitive dus t 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 Mana gement Program (AgBMP);

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

3. ADEQ’s Air Quality Forecasting Program.

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

Management Practices developed by the Governor’s Ag ricultural Best Management Practices Committee,

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

emissions. In addition to routine inspections and in spections 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 Advisor y, or (3) near-real-time monitoring data indicate

unique activity via high PM concentrations. Th e forecasting program a nd inspection/compliance

programs work together so that reso urces can be best utili zed 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 Av ailable Control Measure

(BACM) and the Most S tringent Measure (MSM) demons trations in the Serious Area PM10 plan for the

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

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

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

into the Arizona Administrative Co de as R18-2-610 and R18-2-611 pursu ant 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 Pa rticulate Matter Emissions in Maricopa County

Rule/Ordinance Number

& Title Description

Rule 300: Visible

Emissions Establishes standards for vi sible 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 ei ther 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 emi ssions of air pollutants from

medical waste incinerators

Rule 322: Power Plant

Operations Establishes limits for the emissi ons of nitrogen oxides, sulfur

oxides, carbon monoxide and par ticulate matter from existing

power plants and cogeneration plants

Rule 323: Fuel Burning

Equipment from

Industrial/Commercial/

Institutional (ICI) Sources Establishes limits for the emissi ons of nitrogen oxides, sulfur

oxides, carbon monoxide and pa rticulate matter from ICI

sources

4 

# Updates to the AgBMP program in Decem ber, 2011, clarified BMPs for crop an d added BMPs for animal operations.

Effective 12/29/2011, R18-2-611 was renumbered to R18-2-610.01 Agricultural PM10 Gen eral 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 e ngines 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 fo r vehicle parking and use on

unstabilized vacant lots in unincor porated sections of Area A in

Maricopa County

Ordinance P-28: OffRoad Vehicle Use in

Unincorporated Areas of

Maricopa County Establishes restrictions fo r 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 co mmercial farmers to implement

best management practices and maintain a record demonstrating

compliance

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

paving of unpaved roads, PM10 certified street sweep ers, controlling unpaved parking lots, etc.) have

been committed to, and implement ed by, local jurisdictions throughout the PM10 nonattainment area, and

incorporated into the Arizona SIP through PM10 pl ans 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 regulat ory control measures on emissions from existing and

new non-point sources within Pinal County (see Tabl e 4-2). Additionally, the PCAQCD implements

specific nonattainment rules for that part of the P hoenix PM10 nonattainment ar ea that resides in Pinal

County (see Table 4-3).

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

Article Number & Title Description

Chapter 4, Article 2: Fugitive

Dust Provides a mechanism to reasona bly 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 viol ations of the prevailing PM10

standard and addi tionally 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 viol ations of the prevailing PM10

standard and addi tionally 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 effectivenes s of its fugitive dust rules (Rul es 310, 310.01 and 316) in terms of

permit compliance rates. This ru le effectiveness (RE) study was de signed to assess how many sources

regulated by MCAQD during the subj ect time period received no PM10 emi ssions-related violations. As

a basis for comparison, the percentage of permitted s ources 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 strengthene d, 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 Sa lt 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. Thr ee major areas that contributed to increased

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

and regulatory changes that broa dened 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% fo r Rule 310 sources, 96% for Rule

310.01, and 73% for Rule 316 sources. The timeline pr ovided in Figure 4-1 illustrates the improvements

-21-

# in RE over the last several years, and also points out signifi cant revisions to previ ous rules, as well as

newly adopted rules and ordinances.

0 1 / 27

/ 2 01

0

Rules 3 1 0 &

3 10.

0 1

R ev i s ed 0 1 / 1 2/

2 011

O rdinances

P ‐ 27

&

P

‐ 28

Revised 06/01/2005

06/08/20 0 5

Rule 3 1 6 Rev

is ed

04/

0 7/2004

R u le

3 10

Revised

0 2 /1

7 / 2 005

Rul

e 31

0 .01

Revised

0 3/26/200 8

R u les

31

0 &

3 10

.0 1 Revised

M arc h 20 08 S e p tem b er 200 6



# 0 2/

2 0/20 08

O rdina n ces

P ‐ 25,

P

‐ 27

&

P ‐ 2 8

Adopted

0 3/12/20 0 8

R u le

3 16

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 co mponents: 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 sour ces are also regularly inspected mu ltiple 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 iden tify the potential for

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

Advisory (HPA) is issued for Maricopa County, MC AQD conducts additional surveillance before, during,

and after the forecast event(s). MCAQD also conducts event surveillance and pos t-event activities during

exceedance days that had not been forecast (i.e., thos e 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 viola tions for failure to comply with local, state, or

federal regulations. During the event, MCAQD inspector s survey high-risk areas to confirm that control

measures are in place, document any violations, and contact other regulatory agen cies if necessary. Postevent activities include continued su rveys of high-risk areas, re-inspe cting 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 re porting system includes a

mechanism to alert MCAQD field staff when PM con centrations are elevated. The system allows

MCAQD responders to review concentr ations at the monitor and to c onsult the National Weather Service

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

-22-

# regional events and m onitor specifi c issues. If necessary, the MC AQD responders can inform nearby

stakeholders and local governments of the elevated PM10 concentrations.

For June 27, 2012, a Maricopa County Dust Control Fore cast was issued indicati ng a moderate risk level

for unhealthy PM 10 

# . 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 qua lity complaints, compliance reports, and other

documentation indicate no evidence of unusual anthropogenic-based PM 10 

# emissions. During the time

period of June 24, 2012 through June 30, 2012, MCAQD in spectors conducted a to tal of 151 inspections

of permitted facilities, of which 113 were at fugi tive 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 vi olations were issued county-wide for PM 10 

# and non-PM 10 

# -related

violations. As described below, on June 28 th 

# , eight rule violations were issued for PM 10 

# emissions within

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

Supersite, South Phoenix, and West 43 rd



# .

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

coordinator onsite, which is required of earthmoving sources with a dist urbed 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 approximatel y 3.05 miles southeast of the South

Phoenix monitor. The earthmoving site was stable a nd would not have contribute d to the exceedance at

the South Phoenix monitor on the previous day.

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

driver complete a Basic Dust Contro l Training Class. The site was stab le with visible soil crust and water

at the time of inspection. The gravel pad was in good condition. All dus t 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 vi olation, June 28, 2012. The

site was located 2.8 miles east of the Higley monitor. The site was stab le 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 im plement controls on unpaved haul roads, failure to

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

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

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

the violation, June 28, 2012. The source stabilized a nd 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 co ntrol 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 43 rd 

# Ave monitor, 2.1 miles west of the Durango Complex

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

at the West 43 rd 

# Ave monitor during the larges t exceedances at the West 43 rd 

# 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 43 rd 

# 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-

# Com plex monitor. The winds were generally from the southeast and east at the Greenwood monitor,

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

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

the Greenwood monitor. Analysis of the wind direction and PM 10 

# concentrations demonstrates the

violations would not have contributed to the exceedances at the West 43 rd 

# Ave, Durango Complex, or

Greenwood monitors on June 27 th 

# . Furthermore, the PM10 concen trations entering the Phoenix

nonattainment area from the southeast on June 27 th 

# exceeded 4,000 μg/m 3 

# , as measured by the South

Phoenix monitor, vastly overwhelming any poten tial 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 assi gned to and investigated by a MCAQD inspector. A

review of all pertinent records from this peri od indicates that MCAQD inspectors observed one PM 10 

# violation of local, state, or fede ral 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 violati on for unstable soil on an abandoned golf course. The site was stable by

soil crust and vegetation growth on s ubsequent inspections. The viola tion 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 27 th 

# exceedance.

In addition to MCAQD’s efforts in pre-event surveillance and proact ive 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 associa tions 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 representati ves prior to for ecasted high-wind

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

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

area to discern if their activities ar e 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 activ ities 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 tur bulent wakes that transpor ted 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 seriou s nonattainment area for

PM10 and is required to have BACM for all si gnificant 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 th e events by regulatory agencies and local governments confirmed the

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

-24-

# are adequ ate for meeting the requirements of an ex ceptional event and should be considered “reasonable”

for these purposes.

Despite the deployment of comprehensive control m easures and sophisticated response programs and a

few localized, low-impact violati ons of the dust control rules, hi gh 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 sp eeds nearest the source regions, as discussed in

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

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

outflow winds that transported PM 10 emissions into Maricopa Count y, 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 th e events and exceedances of June 27, 2012, recorded

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

-25-

# V. CLEAR CAUSAL RELATIONSHIP

Introduction

A demonstration of the clear causa l connection between uncontrollable natural events or uncontrollable

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

is presented in the following time se ries graphs, infrared satellite im agery, and National Weather Service

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

on sustained wind speed, wind speed gus ts, PM10 concentrations, visibi lity, weather observations, and

thunderstorm development areas and provide the causal connection betw een the uncontrollable emissions

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

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

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

Event – June 27, 2012

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

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

(mph) from various directions for much of the day, a nd visibility at 10 miles th rough 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 26 th 

# , resulted in generally low PM10 concentrations (20-50 μg/m 3 

# range) through the

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

the eastern half of Arizona on the June 26 th 

# , 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 concentr ations, no exceedances were observed on June 26 th 

# and

PM10 concentrations dropped back down to low levels for June 27 th 

# . Similar to the 26 th 

# , though, between

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

# , 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 ar eas of Pinal County and generated a haboob that

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

Phoenix PM10 nonattainment area.

As shown in Figure 5-2, the haboob had the following im pacts 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 co nditions were reported as “blowing dust” and “dust storm”; and hourly

PM10 concentrations increased to nearly 5,000 μg/m 3 

# at the JLG Supersite monitoring site in the central

Phoenix area. Additionally, PM10 concentrations exceeded 4,000 μg/m 3 

# 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 aver age concentrations above 300 μg/m 3 

# , more than double the 24-hour PM10

standard, at five mon itoring sites on June 27 th 

# . In total, 13 monitors at 12 monitoring sites in the region

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

PM10 monitoring sites saw one of their three highes t 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 de veloping and dissipating over southeastern

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

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

Figure 5-2 depicts the dust event th at occurred in Maricopa County im pacting the Phoenix area on June

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

along with visibility readings (b lue diamonds) and wind speeds repor ted at the Phoenix Sky Harbor

Airport and the Williams Gateway Airport. Measuremen ts 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 c onditions 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 coincide d 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 th e 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 Phoe nix 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 27 th 

# also listed under “Weather Conditions” for

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

-27-

# Figure 5-2. Phoenix nonattainm ent area PM10 monitor readi ngs, 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) ha ving a 24-hour average of

504 μg/m 3 

# and an extremely high maximum 1-hour average during the 5 p.m. hour of 9,639 μg/m 3 

# , when

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

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

μg/m 3 

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

monitoring sites in Ma ricopa by one to two hours, which is cons istent with wind speeds and directions

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

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

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

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

Additionally, the NWS airport meteor ological data tables for the Phoe nix Sky Harbor and Casa Grande

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

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

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

by the strong winds.

-28-

# Figure 5-3. Pinal County PM10 m onitor readin gs and Casa Grande wind speed and visibility for June 27,

2012

A visualization of this dust event wa s 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/ m peg4/SOMT_06272012.mp4

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

27, 2012. The snapshot on the cover of this report wa s 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 mon itoring 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 CommerceNational Oceanic & Atmospheric AdministrationQUALITY CONTROLLED LOCALCLIMATOLOGICAL DATA(final)HOURLY OBSERVATIONS TABLEPHOENIX SKY HARBOR INTL AIRPORT (23183)PHOENIX, AZ(06/2012)National Climatic Data CenterFederal Building151 Patton AvenueAsheville, North Carolina 28801Elevation: 1107 ft. above sea levelLatitude: 33.427Longitude: -112.003Data Version: VER3DryBulbTempWetBulbTempDewPointTempDateTime(LST)StationTypeSkyConditionsVisibility(SM)WeatherType(F)(C)(F)(C)(F)(C)RelHumd%WindSpeed(MPH)WindDirWindGusts(MPH)StationPressure(in. hg)PressTendNet3-hrChg(mb)SeaLevelPressure(in. hg)ReportTypePrecip.Total(in)Altimeter(in. hg)123456789101112131415161718192021222327272727272727272727272727272727272727272727272727272727272727005101510251035104510551065107510851095110511151125113511451155116511751185119512003202420342051210021162134215121582251235111111111111111111111111111111111111111111111111111111111111111FEW250FEW250FEW250FEW250FEW200FEW150 SCT200FEW150FEW150FEW150FEW150FEW100 SCT150FEW110 SCT150FEW110 SCT150FEW120FEW120FEW120FEW120FEW120 SCT170 SCT210FEW120 SCT180 SCT250BKN028 BKN180 BKN250BKN011 BKN180 BKN250BKN012 BKN180 BKN250BKN008 BKN180 BKN250BKN008 BKN180 BKN250BKN008 BKN180 BKN250BKN008 BKN180 BKN250BKN013 BKN180FEW013 SCT180 SCT250FEW013 SCT180 SCT250FEW015 SCT140 SCT180SCT140 SCT19010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.001.001.002.001.501.752.503.003.006.006.006.008.00BLDUBLDUBLDUBLDUBLDUBLDUBLDUBLDUBLDUBLDUBLDU96949392908989939710010110410610710810911010910699939393939393939493929235.634.433.933.332.231.731.733.936.137.838.340.041.141.742.242.843.342.841.137.234.034.034.033.934.034.034.034.434.033.333.37069696969696970707171717271717172727271727372727272737271717120.920.620.720.520.420.520.620.921.321.821.421.722.021.921.621.822.322.422.221.922.022.622.022.322.022.022.621.921.521.621.65555565657585857565654535352505052535457616361626161636059606012.812.813.313.313.914.414.413.913.313.312.211.711.711.110.010.011.111.712.213.916.017.016.016.716.016.017.015.615.015.615.6252729303335353025232118171614141516182534373436343437323234343653691053333651131111731141113910898988130110130130110110130110270230VRVRVR33033033033024023015015015016016016007011009011010007018181844332328.5228.5228.5328.5428.5628.5928.6128.6128.6128.6328.6328.6128.6028.5828.5628.5428.5328.5328.5528.6228.6228.6328.6428.6528.6528.6528.6628.6628.6628.6628.652312863000201201800401501703101329.6229.6229.6329.6429.6629.6829.7129.7129.7229.7229.7229.7129.7029.6829.6629.6429.6329.6329.6529.72MMM29.75MMM29.77M29.7629.75AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAASPSPSPAASPSPSPAASPAAAA29.6929.6929.7029.7129.7329.7629.7829.7829.7829.8029.8029.7829.7729.7529.7329.7129.7029.7029.7229.7929.7929.8029.8129.8229.8229.8229.8329.8429.8429.8329.82-31-U.S. Department of CommerceNational Oceanic & Atmospheric AdministrationQUALITY CONTROLLED LOCALCLIMATOLOGICAL DATA(final)HOURLY OBSERVATIONS TABLEWILLIAMS GATEWAY AIRPORT (23104)PHOENIX, AZ(06/2012)National Climatic Data CenterFederal Building151 Patton AvenueAsheville, North Carolina 28801Elevation: 1382 ft. above sea levelLatitude: 33.3Longitude: -111.666Data Version: VER2DryBulbTempWetBulbTempDewPointTempDateTime(LST)StationTypeSkyConditionsVisibility(SM)WeatherType(F)(C)(F)(C)(F)(C)RelHumd%WindSpeed(MPH)WindDirWindGusts(MPH)StationPressure(in. hg)PressTendNet3-hrChg(mb)SeaLevelPressure(in. hg)ReportTypePrecip.Total(in)Altimeter(in. hg)1234567891011121314151617181920212223272727272727272727272727272727272727272727272727272727272727272727001500350055011501350155021502350255031503350355041504350455051505470647074708470947104711471247134714471547164718181835190019081947000000000000000000000000000000000CLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRBKN100SCT100SCT150FEW200FEW200SCT200FEW200SCT120SCT120SCT120SCT120SCT120VV000BKN120BKN120BKN120BKN10.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0045.0045.0045.0035.0035.0035.0035.0035.0035.0035.0035.0035.000.50s0.752.000.25s2.00DSsDSBLDUDSs8888888888888888888686868686868684889797102104106108108109109109109108106979131.031.031.031.031.031.031.031.031.030.030.030.030.030.030.030.029.031.036.036.039.040.041.042.042.043.043.043.043.042.041.036.033.06968696967676767676867676969696969717271727272727270707070727273M20.620.120.620.619.519.519.519.519.519.719.219.220.320.320.320.320.521.822.121.522.322.122.222.422.021.321.321.320.922.422.222.6M59575959555555555557555559595959616359575755545452484848465454616115.014.015.015.013.013.013.013.013.014.013.013.015.015.015.015.016.017.015.014.014.013.012.012.011.09.09.09.08.012.012.016.016.03835383833333333333735354040404046432826232018171613131312171830M75665357655665581110698386111010581020342814014013012012012012012013013012012013012010012012012012013014014026025028028027031030026014014015036473328.2928.2828.2828.2828.2828.2828.2828.2928.2928.2928.3028.3028.3128.3128.3228.3428.3628.3628.3628.3728.3828.3928.3728.3628.3428.3228.3028.2828.3028.3228.3628.37MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA29.7529.7429.7429.7429.7429.7429.7429.7529.7529.7529.7629.7629.7729.7729.7829.8029.8229.8229.8229.8329.8429.8529.8329.8229.8029.7829.7629.7429.7629.7829.8229.8329.86-32-27272727272727272727272727271955200820152035205521152135215522152235225523152335235500000000000000CLRBKN120CLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLR7.005.004.0010.0010.0010.0010.007.0010.0010.0010.0010.0010.0010.00-RA BLDUHZ909797939193939193939393939332.036.036.034.033.034.034.033.034.034.034.034.034.034.0717372727071716970696969696821.522.622.122.021.121.521.520.620.920.420.420.420.420.1616159615959595757555555555416.016.015.016.015.015.015.014.014.013.013.013.013.012.0383028343432323230282828282724231715151518131088663170170150100100100100090100100080080090060314028212428.4128.4028.4028.4128.4028.4028.4128.4128.4128.4128.4028.4128.4128.40MMMMMMMMMMMMMMAAAAAAAAAAAAAAAAAAAAAAAAAAAA29.8729.8629.8629.8729.8629.8629.8729.8729.8729.8729.8629.8729.8729.86Dynamically generated Wed Dec 26 22:27:59 EST 2012 viahttp://cdo.ncdc.noaa.gov/qclcd/QCLCD-33-U.S. Department of CommerceNational Oceanic & Atmospheric AdministrationQUALITY CONTROLLED LOCALCLIMATOLOGICAL DATA(final)HOURLY OBSERVATIONS TABLECASA GRANDE MUNICIPAL ARPT (03914)CASA GRANDE, AZ(06/2012)National Climatic Data CenterFederal Building151 Patton AvenueAsheville, North Carolina 28801Elevation: 1462 ft. above sea levelLatitude: 32.95Longitude: -111.766Data Version: VER2DryBulbTempWetBulbTempDewPointTempDateTime(LST)StationTypeSkyConditionsVisibility(SM)WeatherType(F)(C)(F)(C)(F)(C)RelHumd%WindSpeed(MPH)WindDirWindGusts(MPH)StationPressure(in. hg)PressTendNet3-hrChg(mb)SeaLevelPressure(in. hg)ReportTypePrecip.Total(in)Altimeter(in. hg)1234567891011121314151617181920212223272727272727272727272727272727272727272727272727272727272727272727001500350055011501350155021502350255031503350355041504350455051505350555061506350655071507350755081508350855091509350955101510351055000000000000000000000000000000000CLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLR10.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.009.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.00888888848484828484828182828281828282828284848890919393959797999910031.031.031.029.029.029.028.029.029.028.027.028.028.028.027.028.028.028.028.028.029.029.031.032.033.034.034.035.036.036.037.037.038.069686866666665666665656565656565666666676868686969697070707070707120.620.120.118.818.818.818.218.818.818.518.318.518.518.518.318.519.019.019.019.619.919.920.120.420.620.420.921.221.021.021.321.321.559575755555554555555555555555555575757595959575757555757555555555515.014.014.013.013.013.012.013.013.013.013.013.013.013.013.013.014.014.014.015.015.015.014.014.014.013.014.014.013.013.013.013.013.0383535373737383737404140404041404343434643433533322830282424232322500000633000303116033650066630005009000000000000000005006011000000000025000013012013000009007009006000000016010012016000000000016000028.2028.2028.2028.2028.2028.2128.2128.2128.2128.2228.2228.2328.2328.2428.2528.2728.2728.2728.2728.2728.2728.2728.2728.2728.2828.2928.2928.2928.2928.3028.3028.3028.294222000014010007MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA29.7429.7429.7429.7429.7429.7529.7529.7529.7529.7629.7629.7729.7729.7829.7929.8229.8229.8229.8229.8229.8229.8229.8229.8229.8329.8429.8429.8429.8429.8529.8529.8529.84-34-272727272727272727272727272727272727272727272727272727272727272727272727272727111511351155121512351255131513351355141514351455151515351555161516351655171517351755181518351855191519351955201520352055211521352155221522352255231523352355000000000000000000000000000000000000000CLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRCLRSCT120FEW022 SCT050 BKN110FEW021 SCT035 BKN050SCT025 BKN036 BKN060FEW022 BKN120SCT120CLRCLRCLRCLRCLRCLRCLRCLRCLRCLR10.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.007.0010.007.006.0010.0010.0010.009.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.00TSTSTSTSTSVCTSVCTS1001021021021061021061061081081081081081081091081091091081081041041049784797579848890919090888688848238.039.039.039.041.039.041.041.042.042.042.042.042.042.043.042.043.043.042.042.040.040.040.036.029.026.024.026.029.031.032.033.032.032.031.030.031.029.028.071717171727172727373737373737373737372737272727271696870696969686971717169707021.521.821.821.822.121.522.122.122.722.722.722.722.722.622.822.622.822.622.422.622.122.122.122.121.520.619.921.320.520.620.420.020.421.521.821.520.621.221.155555555545454545555555555555555555454555555555964646466615957555761636359636413.013.013.013.012.012.012.012.013.013.013.013.013.013.013.013.013.012.012.013.013.013.013.015.018.018.018.019.016.015.014.013.014.016.017.017.015.017.018.02221212118201818171717171717171717161717202020285160696546383330333843463849555000907671181097778131520151610162031242220202216977814107230000000000260000280280320240280280270280200280310270310290280310310120140120120150130130120120160310320310010340010171816242420173331394125373128222828.2928.2928.2928.2828.2728.2728.2728.2528.2528.2528.2428.2428.2328.2228.2228.2128.2028.2028.2028.2128.2228.2228.2328.2928.3428.3428.3328.3328.3328.3228.3128.3028.3028.3128.3028.2828.3028.3028.327727017017047017MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA0.250.250.2529.8429.8429.8429.8329.8229.8229.8129.8029.7929.7929.7829.7829.7729.7629.7629.7529.7429.7429.7429.7529.7629.7629.7729.8429.8929.8929.8829.8829.8829.8729.8629.8529.8529.8629.8529.8329.8529.8529.87Dynamically generated Wed Dec 26 22:01:20 EST 2012 viahttp://cdo.ncdc.noaa.gov/qclcd/QCLCDA description of the fields anddata codes/abbreviations used inthese tables can be found here:http://cdo.ncdc.noaa.gov/qclcd/qclcddocumentation.pdf

# Visibility Cameras

Visibility camera images were compiled into tim e 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 visibi lity camera sites in Maricopa County. These

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

Mountains), SUPM (Superstiti on Mountains), ESMO (Estrella Mountains), and WHTM (White

Tank Mountains). It should be noted that thes e 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 generate d by uncontrollable natu ral events and the

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

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

events from the development of th e 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 pa rticulate matter from the south through Pinal County and into

the Phoenix PM10 nonattainment area. This info rmation 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 ar ea, and transported in to the Phoenix PM10

nonattainment area.

-35-

# VI. “BUT FOR” ANALYSIS

Section 50.14(c)(3)(iv)(D) in 40 CFR part 50 re quires 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 contro llable or preventable and there is a clear causal relationship

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

the Phoenix PM10 nonattainment area and the m easured exceedances within the nonattainment

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

emissions generated by these thunderstorm outfl ows 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 J une 27, 2012. Inspection and compliance data of

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

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

public were alerted to the possi bility of dust storms due to thunderstorm activ ity through daily

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

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

arrival of the thunderstorm outfl ows. 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 thundersto rm outflows and elevated PM10 concentrations

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

visibility all point to the pr esence 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 outfl ows 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 th e primary thunderstorm outflow boundary and

transported PM10 are clearly identified as the de sert areas to the south east of the Phoenix PM10

nonattainment area.

The body of evidence presented in this submitta l provides no alternativ e 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 thes e 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 cont rollable or preventable; and

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

Affects Air Quality

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

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

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

concentration in excess of normal historical fl uctuations. Given the information presented in

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

quality.

Not Reasonably Controllable or Preventable

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

controllable or preventable” in orde r to be defined as an exceptiona l event. This requirement is

met by demonstrating that, despite reasonable cont rol measures in place w ithin Maricopa County

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

available controls. Despite the deployment of comprehensive cont rol measures and sophisticated

response programs, high wind conditions associ ated with thunderstorms and thunderstorm

outflows brought high concentrations of PM10 emi ssions 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 ac tivity 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 Phoe nix 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 air port sites show that the timing of elevated

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

 Visibility camera imagery discussed in Sec tion V indicates that la rge quantities of PM10

were transported into the Phoe nix Metro area during the June 27 th 

# 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 acro ss a broad range of monitoring sites prove

that the exceedances of June 27, 2012, were exce ptional 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 gene rated 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 th understorm-generated

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

National Weather Service stati ons during this event, is c onsistent with the timing of

elevated PM10 concentrations recorded at the mon itoring locations in the Phoenix PM10

nonattainment area.

 Wind directions, thunderstorm-generat ed outflow boundary propagation, and

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

increasing in Maricopa County, a ll 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 th e area to verify compliance with those rules

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

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

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

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

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date Buckeye 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date Central Phoenix 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date West Phoenix 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date Greenwood 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 

# 450 500 550

PM 10 Concentration ( μg/m 3 )



# Date JLG Supersite 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date West 43rd 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date South Phoenix 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date Durango 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date West Chandler 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date Higley 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date Glendale 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date North Phoenix 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date Dysart 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration ( μg/m 3 )



# Date Zuni Hills 5

- Year Historical Fluctuation

- 24 Hour Averages

# 0

50 100 150 200 250 300 350 400 450 

# 500

PM 10 Concentration (μg/m 3 )



# Date Apache Junction 5

- Year Historical Fluctuation

- 24 Hour Averages