1

Review of Exceptional 

Event Request 

Maricopa County, AZ

24-Hour PM10

March 14, 2008

April 30, 2008

May 21, 2008

June 4, 2008

U.S. Environmental Protection Agency

Region 9

May 12, 2010
2

Contents 

1.0 Introduction ...........................................................................................................................4

2.0 Summary of the Events .........................................................................................................5

3.0 Requirements of the Exceptional Events Rule ......................................................................5

4.0 Criteria Set Forth in 40 CFR §50.1(j) ...................................................................................6

4.1 Affect Air Quality ..............................................................................................................6

4.2 Reasonably Controllable or Preventable ...........................................................................7

4.3 Human Activity/Natural Event ..........................................................................................7

4.4 Stagnation of Air Masses/Inversions/High Temperature/Lack of Precipitation/Source 

Noncompliance ...................................................................................................................9

5.0 Clear Causal Relationship .....................................................................................................9

5.1 March 14, 2008 ................................................................................................................10

5.1.1 Correlation between Wind Speed and PM10 ............................................................10

5.1.2 Visibility ..................................................................................................................10

5.1.3 Review of 24-Hour PM10 Data ................................................................................11

5.1.4 Review of Hourly PM10 and Meteorological Data ..................................................12

5.1.5 Review of 5-Min PM10 and Wind Speed Data .........................................................13

5.1.6 Days with Similar Meteorological Conditions ........................................................14

5.2 April 30, 2008 ...................................................................................................................16

5.2.1 Correlation between Wind Speed and PM10 ............................................................16

5.2.2 Visibility ..................................................................................................................17

5.2.3 Review of 24-hour PM10 Data .................................................................................17

5.2.4 Review of Hourly PM10 and Meteorological Data ..................................................19

5.2.5 Review of 5-Min PM10 and Wind Speed Data .........................................................19

5.2.6 EPA Review of Days with Similar Meteorological Conditions ..............................20

5.2.7 Summary of Clear Causal Relationship for April 30, 2008 .....................................21

5.3 May 21, 2008 ...................................................................................................................21

5.3.1 Correlation between Wind Speed and PM10 ............................................................21

5.3.2 Visibility ..................................................................................................................22

5.3.3 Review of 24-Hour PM10 Data ................................................................................22

5.3.4 Review of Hourly PM10 and Meteorological Data ..................................................23

5.2.5 Review of 5-Min PM10 and Wind Speed Data .........................................................24

5.3.6 EPA Review of Days with Similar Meteorological Conditions ..............................25

5.3.7 Summary of Clear Causal Relationship for May 21, 2008 ......................................26

5.4 June 4, 2008 Event ...........................................................................................................26

5.4.1 Correlation between Wind Speed and PM10 ............................................................26

5.4.2 High Winds ..............................................................................................................26

5.4.3 Visibility ..................................................................................................................27

5.4.4 Review of 24-Hour PM10 Data ................................................................................27

5.4.5 Review of Hourly PM10 and Meteorological Data ..................................................28

5.4.6 Review of 5-Min PM10 and Wind Speed Data .........................................................30

5.4.7 Review of Days with Similar Meteorological Conditions .......................................31

5.4.8 Summary of Clear Causal Relationship for June 4, 2008 ........................................32

6.0 Concentration in Excess of Normal Historical Fluctuations...............................................32

7.0 No Exceedances But For the Event ....................................................................................33
3

8.0 Procedural Requirements ....................................................................................................33

9.0 Conclusion ..........................................................................................................................34

Appendix A 36

Appendix B 40

References 46

Tables 

Table 1: Table 1: West 43rd 2008 PM10 Exceedances 

Table 2: March 14, 2008 24-Hour PM10

Table 3: April 30, 2008 24-Hour PM10

Table 4: May 21, 2008 24 Hour PM10

Table 5: June 4, 2008 24-Hour PM10

Table 6: Table 6: ADEQ Historical Distribution 

Figures 

Figure 1: March 14, 2008 24-Hour PM10

Figure 2: March 14, 2008 Hourly PM10

Figure 3: March 14, 2008 5-Min PM10 and Wind Speed 

Figure 4: Non-Exceedance Day Comparison 

Figure 5: Non-Exceedance Day Comparison 

Figure 6: Non-Exceedance Day Comparison 

Figure 7: April 30, 2008 24-Hour PM10

Figure 8: April 30, 2008 Hourly PM10

Figure 9: April 30, 2008 5-Min PM10 and Wind Speed 

Figure 10: Non-Exceedance Day Comparison

Figure 11: May 21, 2008 24-Hour PM10

Figure 12: May 21, 2008 Maricopa County Hourly PM10

Figure 13: May 21, 2008 5-Min PM10 and Wind Speed 

Figure 14: Non-Exceedance Day Comparison 

Figure 15: June 4, 2008 24-Hour PM10

Figure 16: June 4 & 5, 2008 Hourly PM10

Figure 17: June 4 & 5, 2008 5-Min PM10 and Wind Speed 

Figure 18: Non-Exceedance Day Comparison 
4

1.0 Introduction 

On March 22, 2007, EPA adopted the Treatment of Data Influenced by Exceptional Events,1also 

known as the Exceptional Events Rule (EER), to govern the review and handling of certain air 

quality monitoring data for which the normal planning and regulatory processes are not 

appropriate. Under the terms of the EER, a state may request EPA to exclude data showing 

exceedances or violations of the National Ambient Air Quality Standard (NAAQS) that are 

directly due to an exceptional event from use in determinations by demonstrating to EPA’s 

satisfaction that such event caused a specific air pollution concentration at a particular air quality 

monitoring location.2 Before EPA will exclude data from these regulatory determinations, the 

state must flag the data in EPA’s AQS database and, after notice and an opportunity for public 

comment, submit a demonstration to justify the exclusion. After considering the weight of 

evidence provided, EPA will determine if the demonstration satisfies all the requirements of the 

EER and either concur or nonconcur with the state’s request. 

On June 30, 2009, the Arizona Department of Environmental Quality (ADEQ) submitted to EPA 

a preliminary demonstration for exceedances that occurred at various monitoring locations 

throughout Arizona on 27 separate days in 2008, including five at the West 43rd monitoring site 

located in southwestern Phoenix. On November 17, 2009 ADEQ submitted final demonstrations 

for twelve of these exceedances, including five at the West 43rd site.3

This document sets forth the legal and factual basis for EPA’s decision regarding four 

exceedances of the 24-hour PM10 NAAQS in 2008 at the West 43rd monitoring site on March 14,

April 30, May 21, and June 4, 2008 that ADEQ has flagged as “high wind” exceptional events.4

EPA has not yet completed its analysis of the remaining dates and is not making a concurrence 

or non-concurrence determination for them at this time. 

The documentation submitted by ADEQ and considered by EPA in support of the exceptional 

events claims includes the following: 

Assessment of Qualification for Treatment under the Arizona Natural and Exceptional 

Events Policy for the High Particulate (PM10) Concentration Events in the Phoenix Area 

on March 14, 2008 (March 14 Assessment); 

Assessment of Qualification for Treatment under the Arizona Natural and Exceptional 

Events Policy for the High Particulate (PM10) Concentration Events in the Phoenix Area 

on April 30, 2008 (April 30 Assessment); 

 

1

 72 FR 13560-13581, March 22, 2007. 

2

 40 CFR §50.14 (a). 

3

 On March 17, 2010 EPA received a draft-supplemental report titled “Assessment of Qualification for Treatment 

Under the Federal Exceptional Events Rule: High Particulate (PM10) Concentration Events in the Phoenix and 

Yuma Areas on June 4th, 2008.” Information presented in this document will be considered in EPA’s 

concurrence/non-concurrence decision for the claimed event that occurred on June 4, 2008. EPA has not received 

additional information concerning the other three events we are reviewing in this document. 

4

 The West 43rd monitor also measured a fifth exceedance on November 9, 2008; EPA is not reviewing this event at 

this time. 
5

Assessment of Qualification for Treatment under the Arizona Natural and Exceptional 

Events Policy for the High Particulate (PM10) Concentration Events in the Phoenix Area 

on May 21, 2008 (May 21 Assessment); 

Assessment of Qualification for Treatment under the Arizona Natural and Exceptional 

Events Policy for the High Particulate (PM10) Concentration Events in the Phoenix Area 

on June 4, 2008 (June 4 Assessment); 

The Impact of Exceptional Events “Unusual Winds” on PM10 Concentrations in Arizona 

(Unusual Winds White Paper); 

High Wind Exceptional Events and Control Measures for PM10 Areas (Controls White 

Paper); and 

DRAFT – Supplemental Report: Assessment of Qualification for Treatment under the 

Federal Exceptional Events Rule: High Particulate (PM10) Concentration Events in the 

Phoenix and Yuma Areas on June 4, 2008 (June 4 DSR). 

2.0 Summary of the Events 

In 2008, there were seventeen PM10 monitoring sites operating in Maricopa County, ten of which 

use continuous PM10 analyzers that produce hourly data. During 2008, the West 43rd monitoring 

site, which measures PM10 with a continuous analyzer,5 measured five exceedances of the 24-

hour PM10 NAAQS, four of which are reviewed in this document.6 ADEQ has claimed that the 

exceedances at the West 43rd site resulted from the transport of dust from soils by high winds, the 

high wind event was a regional phenomenon that affected the entire Phoenix area, and the events 

were the result of the transport of dust and soils from high winds that suspended natural soils and 

soils from areas where BACM was in place.7

Table 1: West 43rd 2008 PM10 Exceedances

Date PM10 (ug/m3) Weather Condition Wind Direction

March 14 251 Low Pressure Trough W

April 30 173 Frontal System Passage WSW

May 21 279 Frontal System Passage W

June 4 194 Frontal System Passage WSW

3.0 Requirements of the Exceptional Events Rule 

Pursuant to 40 CFR §50.14(c)(3)(iii) a request for EPA’s concurrence on an exceptional event 

flag must be accompanied by a demonstration that: 

(A) The event satisfies the criteria set forth in 40 CFR §50.1(j) that it: 

1. affects air quality; 

2. is not reasonably controllable or preventable; 

 

5

 All of the continuous analyzers in Maricopa County, including the analyzer at West 43rd, are Thermo Scientific 

TEOM 1400AB analyzers with EPA FEM designation number EQPM-1090-079.

6

 EPA is not analyzing the exceedance on November 9, 2008 at this time. 

7

 March 14, April 30, May 21, and June 4 Assessments at p.4. 
6

3. is caused by human activity that is unlikely to recur at a particular location, or is a 

natural event; 

4. does not include stagnation of air masses or meteorological inversions, a 

meteorological event involving high temperatures or lack of precipitation, or pollution 

relating to source noncompliance; 

(B) There is a clear causal relationship between the measurement under consideration and the 

event that is claimed to have affected the air quality in the area; 

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

fluctuations, including background; and 

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

The demonstrations must fully meet all the above criteria to EPA’s satisfaction; failure to meet 

any one of the criteria will result in the non-concurrence of the event in question.. In addition to 

the technical criteria, the EER also has procedural requirements. 40 CFR §50.14(c)(2)(iii) 

requires that data claimed to be due to an exceptional event must be flagged in the AQS 

database, and that an initial description of the event be provided to EPA; both must occur by July 

1 of the year following the event. In addition, 40 CFR §50.14(c)(3)(i) requires that the State: 

submit a demonstration to EPA within three years of the calendar quarter of the event or 

12 months prior to an EPA regulatory decision; 

provide notice and opportunity for public comment; and

submit any public comments along with the demonstration. 

EPA's concurrence or non-concurrence with a State's flag constitutes its agreement or 

disagreement with the State on whether the data should be excluded from regulatory decisions 

involving a State's compliance with the NAAQS. EPA's determination regarding a State's 

attainment status or action on a state SIP submission will be issued in a rulemaking which is a 

final agency action that is judicially reviewable under CAA section 307(b)(1). 

The following sections evaluate ADEQ’s assessments of March 14, April 30, May 21, and June 

4, 2008 with respect to these requirements. 

4.0 Criteria Set Forth in 40 CFR §50.1(j) 

4.1 Affect Air Quality 

As stated in the preamble to the EER, the event in question shall be considered to have affected 

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

exceedance and the event (section 5.0), and that the event is associated with a measured 

concentration in excess of normal historical fluctuations (section 6.0).8

 

8

 72 FR 13569, 72 FR 49051, and 73 FR 14702. 
7

4.2 Reasonably Controllable or Preventable 

A determination of whether a particular event was “not reasonably controllable or preventable” 

depends on the specific facts and circumstances surrounding the event. Therefore, EPA 

addresses this and the other criteria of the EER on a case by case basis. 

This factor of the analysis should consider whether anthropogenic sources contributing to the 

exceedance caused by the event were reasonably controlled.9 ADEQ’s supporting 

documentation, however, did not specifically identify the type or location of the possible 

contributing sources in the area, other than the Salt and Gila River channels, located upwind of 

the West 43rd monitoring site.. Although the June 4 DSR identifies that the alluvial channels 

located upwind of the West 43rd monitor most likely significantly contributed to the exceedance 

at West 43rd site, ADEQ did not evaluate whether emissions from those sources were reasonably 

controllable or preventable. 

The June 4 DSR included a table titled, “Rules Regulating Particulate Matter Emissions in 

Maricopa County,” which includes the rule number, title, and a brief description of the general 

sources that the rule is designed to control. Without addressing the types, and locations of 

sources in the area, however, it is not possible to evaluate whether sources in the area were 

reasonably controlled. 

4.3 Human Activity/Natural Event 

The term “natural event” is defined at 40 CFR §50.1(k) as “an event in which human activity 

plays little or no direct causal role.” As described in the preamble to the EER, high wind events 

may qualify as exceptional events if the following conditions are met: the wind speed associated 

with the event is “unusual for the affected area during the time of year that the event occurred,” 

and, in instances where wind produces emissions from anthropogenic sources, all reasonable and 

appropriate measures must be in place for all contributing sources.10 An event that was caused by 

human activity, but is unlikely to recur at a given location may be considered an exceptional 

event assuming all other requirements of the rule are met. 

ADEQ’s Assessments briefly discussed the various source categories in the area, including 

industrial sources, construction, area sources (unpaved parking lots and shoulders), roads, track 

out, and windblown dust. According to ADEQ, the windblown dust category includes 

significant contribution from the following sources: agriculture, alluvial channels, vacant lots, 

construction, industrial, disturbed areas, and stockpiles. In addition, EPA has identified, through 

satellite images and visits to the area, numerous anthropogenic sources in the area that could 

contribute to elevated PM10 concentrations. The commercial nature associated with many of 

these activities indicates that some portion of them can be reasonably expected to recur. 

To establish that the exceedances at the West 43rd site may properly be classified as “natural 

events,” the data must support a finding that “human activity plays little or no direct causal

 

9

 EER Preamble, 72 FR 13566, n. 11. 

10 EER Preamble, 72 FR 13566. 
8

role.”11 ADEQ’s Assessments of the four exceedances did not analyze potential contribution 

from anthropogenic sources. The Controls White Paper states that because of “the relative 

complexity of the emitting source mix, parsing out a specific source or source category along 

with the applicable control measures for a determination of relative effectiveness can be difficult 

and may even be counter-productive.” ADEQ’s Assessments also stated that “no specific 

emission allocation is possible based on the data for analysis” and that “the primary source 

appears to be wind-blown dust over central Arizona for which there is not an effective or 

efficient method to estimate the relative contributions from specific sources.”12 

The lack of analysis regarding anthropogenic contribution upwind of the West 43rd site makes it 

difficult to determine the contributing role of human activity to the exceedances at the West 43rd

site, particularly where it is known that commercial activities such as agriculture, sand and gravel 

mining and construction are known to take place. 

EPA notes that the EER did not set a specific threshold to define a “high wind event,”13 but 

suggested the use of a comparison of wind speeds measured on the event day to be compared to 

historical wind speed levels “for the season of the year that the event occurred.”14 The analysis 

that supports ADEQ’s definition of “unusual” wind was based on data from 2005 through 2009 

for the entire year period and was only analyzed for four monitoring sites (Buckeye, West 43rd, 

Durango Complex, and Higley). The use of a complete year of data in this situation rather than 

the season during which the events occurred likely biases the statistical analysis low. The 

Phoenix area experiences more consistent elevated wind speed levels associated with frontal 

passages during the months of March through June. 

Conclusions drawn from this analysis suggest that wind speeds that occur less than 5% of the 

time should be considered “unusual” for exceptional events purposes. For the West 43rd

monitoring station, this standard would correspond to sustained hourly wind speeds greater than 

10 mph and wind gusts15 greater than 20 mph. ADEQ’s documentation did not provide any 

specific analysis pertaining to certain hours of the day and there is no discussion of the wind 

speeds that are associated with the event and their relationship to the 95th percentile. While wind 

speeds above the 95th percentile may seem unusual, the frequency of occurrence of hourly wind 

speeds over 10 mph at this site is approximately 100 days per year.16

 

The Unusual Winds White Paper further stated that “unusual winds can be defined as any wind 

that has the ability to create windblown dust.” ADEQ’s definition could be interpreted to treat all 

windblown PM10 as exceptional as long as the wind speeds are about the threshold friction 

velocity for that area. Threshold wind speeds provide a minimum baseline for wind speeds that 

are capable of producing windblown dust and are based on particle interaction on the ground 

surface, while “high” and “unusual” wind speed definitions should be based on a separate 

analysis. Thus, although this evidence may contribute to the exceptional analysis, it should not 

 

11 40 CFR §50.1(k) 

12 March 14, April 30, May 21, and June 4 Assessments at p.4. 

13 EER Preamble 72 FR 13577.

14 Id. at 13566. 

15 Wind gusts from Maricopa County stations are 1-sec maximum wind speed value for the hour. 

16 Based on data from 2007-2009. 
9

be a major deciding factor when determining whether wind speed associated with an exceptional 

event is “unusual.” 

In summary, considering the limited analysis on the elevated wind speeds associated with the 

event combined with little analysis of possible contributing sources located directly upwind of 

the West 43rd site, EPA has determined that ADEQ’s documentation did not provide sufficient 

evidence to support that the events in question should be considered “natural events” as required 

under the EER. 

4.4 Stagnation of Air Masses/Inversions/High Temperature/Lack of 

Precipitation/Source Noncompliance 

ADEQ did not provide any evidence suggesting that the exceedances at the West 43rd monitoring 

site were the direct result of stagnation of air masses, inversions, high temperature, or lack of 

precipitation. Regarding source noncompliance, ADEQ states that, “no local sources were 

reported as significantly contributing to the air quality episode” for all days except June 4. This 

statement assumes that because there were no observations made (i.e. there were no reported 

civilian complaints or enforcement actions), that all sources in the area were in compliance with 

all applicable fugitive dust control measures. 

The June 4 assessment explained that there were two Notice of Violations (NOV) issued on June 

4 and June 5 for noncompliance with Maricopa County’s (MCAQD) fugitive dust rules. The 

June 4 DSR also states that “one complaint based inspection of a dust control permit on June 4... 

resulted in a Notice of Violation (NOV) for track-out under Rule 310” and on June 5 “an 

inspection of a Rule 316 source resulted in the issuance of a notice of violation for failure to 

install a wheel washer.” Both of the NOVs were issued to sources that are located within a two 

mile radius of the West 43rd monitoring site, but the specific locations of these facilities were not 

identified in the June 4 assessment or DSR. The NOVs provide some evidence that nearby 

sources may not have been reasonably controlled during the time of the event. 

5.0 Clear Causal Relationship 

In order for EPA to concur with an exceptional event request, the EER requires the State to 

demonstrate that there is a clear causal relationship between the measurement under 

consideration and the event that is claimed to have affected air quality in the area. 40 CFR 

§50.14(a)(2); 40 CFR §50.14(c)(3)(iii). To address this element for “high wind events,” such as 

those flagged by Arizona, the state should reasonably consider the relationship between an event, 

the PM10 emissions caused by unusually high winds, and a measured exceedance at a monitoring 

site. Arizona’s Assessments included various data points relevant to this analysis. EPA’s 

technical review also considered additional data regarding wind speed and direction, PM10

concentration, and visibility.17

As a preliminary matter relevant to this issue, EPA notes that ADEQ’s limited analysis of the 

potential sources that might have contributed to the exceedances at the West 43rd site (sections 

 

17 Appendix A contains pollution roses based on % total PM10 mass for all four of the events in question. 
10

4.2 and 4.3) makes it difficult to comprehensively evaluate the causal relationship between the 

event and the exceedance. Another general point concerns the data provided by Arizona for each 

event. EPA notes that, for each of the four events reviewed in this document, Arizona provided 

different sets of PM10 data drawn from among the ten monitoring stations using continuous 

analyzers. EPA also notes that Arizona provided a different set of meteorological data for each 

event. Considering the four events discussed in this document are very similar in nature, it is 

unclear why ADEQ did not provide the same data for each event. In some instances the most 

relevant meteorological data, (those data from the closest or upwind locations) are not included 

in the supporting documentation.18

5.1 March 14, 2008 

5.1.1 Correlation between Wind Speed and PM10

The March 14 Assessment included tabular hourly and maximum wind speed and PM10 data for 

five monitoring sites in the Phoenix area: West 43rd, Durango Complex, West Phoenix, Coyote 

Lakes, and Central Phoenix. ADEQ also included meteorological data from three National 

Weather Service (NWS) stations: Goodyear Airport, Glendale Airport, and Phoenix Sky 

Harbor.19 EPA notes that ADEQ did not provide hourly PM10 data from the other four 

continuous PM10 analyzers in the Phoenix area and did not include wind speed and direction data 

from numerous other meteorological stations in the Phoenix area. 

ADEQ also provided four graphs that show the potential correlation between maximum wind 

speeds and PM10 concentrations at the West 43rd, Durango Complex, Greenwood, and South 

Phoenix monitoring sites. 20 The graphs show that hourly PM10 concentrations increase with an 

increase in maximum recorded wind speed at the West 43rd site, but not at the other three 

monitoring sites. In fact, the graphs show that the maximum wind speeds at the Durango 

Complex site were higher than those measured at the West 43rd site, but the Durango Complex 

site experienced significantly lower PM10 values during periods of elevated wind speed. These 

data suggest that the elevated PM10 concentrations at the West 43rd site may have been caused by 

local upwind sources and were not due to a high wind event that was regional in nature. 

5.1.2 Visibility 

The March 14, Assessment included photographs from numerous locations throughout the 

Phoenix area. Unfortunately, there is not a significant discernable difference between the 

conditions preceding and during the event. Therefore, the photographs do not significantly 

 

18 Table 1 in Appendix A identifies the PM10 and meteorological stations ADEQ used in their analysis of the 2008 

exceptional events in question.

19 ADEQ also included meteorological data from two AZMET stations. These data are collected at 3 meters, while 

NWS and Maricopa County data are collected at 10 meters. There does not seem to be any correction or 

adjustment for the difference in the heights of these stations. 

20 The max wind speed values used in this comparison are the instantaneous max wind speed values recorded by 

onsite data loggers, which have the capability of recording these instantaneous values in a fraction of a second. 

ADEQ does not explain why the use of the maximum 1-sec value for an hour is the appropriate measure for 

comparison to hourly average PM10 values. 
11

contribute to establishing a causal relationship between wind speed, potential contributing 

sources, and PM10 concentrations at the West 43rd site. 

The March 14 Assessement also stated that reduced visibility during the event throughout 

portions of the Phoenix provides further evidence of a clear causal relationship between the high 

wind event and the measured exceedance at the West 43rd site. The visibility at Goodyear 

Airport before the event ranged from 60 to 20 statute miles, while during the time of the elevated 

PM10 concentrations at West 43rd the visibility ranged from 15 to 10 miles. Other NWS stations 

in the area did not record any decrease in visibility throughout the entire day: visibility at 

Glendale Airport remained at 20 miles, Sky Harbor remained at 10 miles, and Luke Air Force 

Base remained at 10 miles. Visibility throughout the day in the Phoenix area was never 

significantly reduced; thus, this information does not significantly contribute to establishing a 

clear causal relationship.21

5.1.3 Review of 24-Hour PM10 Data

The 24-hour PM10 concentrations measured on March 14 at the West 43rd and surrounding sites 

are listed in Table 2 and shown in Figure 1. On this day, the West 43rd site was the only site in 

the Phoenix area to exceed the 24-hour PM10 standard. Furthermore, PM10 concentrations at the 

West 43rd site were 2-3 times higher than those measured at other sites, which is generally 

inconsistent with the notion that a regional high wind event caused the exceedance.22

Table 2: March 14, 2008 24-Hour PM10

Site Name PM10 (ug/m3) Site Name PM10 (ug/m3)

Buckeye23 80 West PHX 57

West 43rd 251 Central PHX 69

Durango Complex 92 JLG Supersite 41

South PHX 120 Higley 54

Greenwood 71 Coyote Lakes 48

 

21 Appendix B contains information pertaining to reduced visibility and dust storms in Arizona. 

22 The only other exceedance recorded in Arizona on March 14, 2008, was the Cowtown monitoring site in Pinal 

County, which was not flagged as an exceptional event. 

23 24-hour PM10 data for this site was not included in Arizona’s Assessment. 
12

Figure 1: March 14, 2008 24-Hour PM10

5.1.4 Review of Hourly PM10 and Meteorological Data 

The hourly PM10 data are shown in Figure 2. As early at 0500 hrs, the West 43rd site began to 

experience an increase in PM10 concentration that was not characteristic of the other nine 

monitoring sites in the Phoenix area. From 0500 to 1000 hrs, the hourly PM10 values increased 

from 150 μg/m3 to 360 μg/m3. During these hours the hourly wind speeds throughout the 

Phoenix area remained below 9 mph, which suggests these elevated concentrations were not 

driven by high wind, but by some other mechanism. Thus, the elevated PM10 during these hours 

do not appear to have been caused by elevated wind conditions. 

The first sign of any elevated winds occurred at the majority of the stations around 1100 hrs. 

NWS data for Goodyear Airport showed an increase in wind speed from 6 to 14 mph 

(accompanied by a 29 mph gust); while an increase in hourly wind speed from 12 to 15.9 mph 

was recorded at the West 43rd site. At 1100 hrs, the PM10 concentration at the West 43rd site also 

rose from 355 to 1051 μg/m3 and continued to increase over the next two hours to a maximum 

hourly concentration of 1286 μg/m3. While the values at some of the other sites in the area 

increased over the same time period, the values at the West 43rd site ranged from 3-20 times 

higher than other sites in the Phoenix area. Given that the Durango Complex, South Phoenix, 
13

Greenwood, and West Phoenix sites are located within approximately five miles of the West 43rd

site, one would expect to see greater consistency in the PM10 concentrations if a regional high 

wind event was occurring. It is also worth noting that the West 43rd site came close to reaching 

the peak concentration seen by other nearby sites well before the arrival of elevated wind speeds. 

The closest site, Durango Complex, reached a maximum concentration of 310 μg/m3 at 1300 hrs, 

while West 43rd exceeded this level at 0900 hrs. The inconsistency in the PM10 concentrations 

during the period from 1100 to 1400 hrs and the relatively low wind speeds in the morning hours 

suggest that the West 43rd site was most likely significantly influenced by local upwind sources 

and the claimed exceptional event was not regional in nature. 

Figure 2: March 14, 2008 Hourly PM10

5.1.5 Review of 5-Min PM10 and Wind Speed Data 

The 5-min data reinforce the fact that even though elevated wind speeds were measured at other 

nearby locations, the West 43rd monitor consistently measured much higher PM10 concentrations 

than other locations. Figure 3 shows the 5- min PM10 and wind speed data from West 43rd and 

Durango Complex monitoring sites. These monitors are located only 2 miles apart, yet there 

seems to be a considerable difference in the relationship between PM10 and wind speed on March 

14. Both sites experience similar wind speed levels, but during some periods of the day the 5-

min PM10 concentrations at the West 43rd site were more than five times those measured at 

Durango Complex. These data provide further evidence that the claimed regional high wind 

event only affected PM10 concentrations at the West 43rd site and the elevated PM10

concentrations measured at this site were most likely significantly influenced by local sources 

and the claimed exceptional event was not regional in nature. 
14

Figure 3: March 14, 2008 5-Min PM10 and Wind Speed

5.1.6 Days with Similar Meteorological Conditions 

The following discussion emphasizes that meteorological conditions in upwind locations do not 

always affect on PM10 concentrations at the West 43rd site. The NWS station at Goodyear 

Airport is located approximately 13 miles to the west of the West 43rd monitoring site and serves 

as the closest location with readily available meteorological data for the area directly to the west 

of the West 43rd monitoring site.24

Wind speeds at Goodyear Airport exceeded 15 mph on ten days in March 2008. On six of those 

days, wind gusts exceeded 25 mph. Despite these facts, March 14 was the only day in the month 

of March that measured an exceedance of the 24-hour PM10 NAAQS. The following analysis 

compares hourly PM10 data, wind speed, and wind gusts recorded at Goodyear Airport on March 

14 with the same data for three days in March with similar meteorological conditions. 

On March 14, the West 43rd monitor measured elevated PM10 concentrations of 1051 μg/m3 and 

1270 μg/m3 at 1100 and 1200 hrs, respectively. Wind speeds at Goodyear Airport during this 

period were from the west (260°) at 14 and 18 mph with gusts of 29 and 34 mph. On March 2, 

the Goodyear station measured wind speeds and gusts of equal or higher magnitude: 23 mph 

with 34 mph gusts from the NW (310°-320°) for two consecutive hours. Elevated wind speeds 

on March 2 corresponded to an increase in PM10 from 29 μg/m3 to 177 μg/m3 at the West 43rd

monitoring site. This increase in PM10 is relatively minor compared to PM10 concentrations on 

measured on March 14, which reached at maximum of 1270 μg/m3.

 

24 NWS stations report meteorological data differently than meteorological stations operated by Maricopa County. 

NWS service stations report wind speeds as a 2-min average and wind gusts are defined as “a rapid fluctuation of 

wind speed with variations of 10 knots or more between peaks and lulls,” which are reported as a 5-sec average. 

Maricopa County meteorological stations have the capability of reporting wind speeds as a 5-min average, an 

hourly average, or a maximum wind speed, which is recorded as an instantaneous reading that can be less than 

one second in duration. 
15

Figure 4: Non-Exceedance Day Comparison

Similarly, on March 29, wind speeds of 16 to 17 mph with wind gusts of 29 to 32 mph from the 

SSW (200°) and the WSW (240°) were recorded at Goodyear Airport for a period of three hours. 

The corresponding PM10 concentrations at West 43rd remained below 130 μg/m3 for the entire 

day. On the following day, March 30, wind speeds of 25 to 29 mph from the SW (230°-240°) 

were recorded at Goodyear, which corresponded to a spike in PM10 concentration at the West 

43rd site. There are, however, significant differences between the spike measured on March 30 

and the one measured on March 14 and flagged as an exceptional event. First, the spike on 

March 30 clearly follows a period of elevated wind speed while the spike measured on March 14 

was coincident with or even precedes the elevated wind. In addition, the PM10 spike on March 

30 was shorter in duration and much smaller in magnitude. 

Figure 5: Non-Exceedance Day Comparison
16

Figure 6: Non-Exceedance Day Comparison

These examples illustrate how elevated wind speeds in upwind areas are related to elevated PM10

concentrations on occasion, but the magnitude of PM10 concentrations measured at the West 43rd

site seem to be associated with factors in addition to wind speed. Also, March 2, March 29, and 

March 30 were weekend days, which also indicates that elevated wind speeds are not necessarily 

the primary factor in creating elevated PM10 concentrations at the West 43rd site. 

5.1.7 Summary of Clear Causal Relationship for March 14, 2008 

ADEQ’s conclusions that the recorded exceedance was caused by a regional high wind event are 

not substantiated by relevant monitoring and meteorological data. The data show that the spatial 

extent of PM10 during this day was isolated and not regional in nature. The data also show 

differences in the measured PM10 concentrations at the West 43rd site and the remaining sites in 

the Phoenix area. In addition, as explained above, ADEQ provided only limited analysis of 

possible contribution from human activity, making it difficult to determine the relationship 

between the claimed event and the exceedance. Therefore, EPA has determined that the weight 

of evidence presented in the March 14 Assessment does not demonstrate a clear causal 

relationship as required by the EER. 

5.2 April 30, 2008 

5.2.1 Correlation between Wind Speed and PM10

The April 30 Assessment included hourly and maximum wind speed and PM10 data for five sites 

in the Phoenix area: West 43rd, Durango Complex, South Phoenix, Central Phoenix, and Higley. 

ADEQ also included meteorological data from the NWS Sky Harbor and Deer Valley stations.25

ADEQ did not provide tabular hourly PM10 data from the other four continuous PM10 analyzers 

in the Phoenix area and did not include wind speed and direction data from numerous other 

meteorological stations in the Phoenix area. The assessment also did not include any 

information discussing the 7 filter-based monitoring sites that collected samples on this day. 

 

25 ADEQ also includes meteorological data from two AZMET stations. These data are collected at 3 m while NWS 

and Maricopa County data are collected at 10 m. There does not seem to be any correction or adjustment for the 

collection heights of these stations, and therefore should not be used in the exceptional events analysis. 
17

Refer to Appendix A for more information on the PM10 and meteorological data used in the April 

30 assessment. 

ADEQ also provided four graphs that show the potential correlation between maximum wind 

speeds and PM10 concentrations. The four graphs display data from the West 43rd, Durango 

Complex, Greenwood, and South Phoenix monitoring sites. While the hourly PM10

concentrations increase with an increase in maximum recorded wind speeds at the West 43rd site, 

there is not a similar correlation between PM10 and maximum wind speed at the other monitoring 

sites in the area. These facts suggest that the elevated PM10 concentrations at West 43rd may 

have been caused by local upwind sources and were not regional in nature. 

5.2.2 Visibility 

The April 30 assessment included photographs from numerous locations throughout the Phoenix 

area. Unfortunately, there is not a significant discernable difference between the conditions 

preceding and during the event. Therefore, the photographs do not significantly contribute to 

establishing a clear causal relationship between wind speed, potential contributing sources, and 

PM10 concentrations at the West 43rd monitoring site. 

ADEQ also stated that reduced visibility during the event at Goodyear Airport provides further 

evidence of a causal relationship between the high wind event and the measured exceedance at 

the West 43rd site. The visibility at Goodyear Airport before and during the event ranged from 

20 to 7 statute miles. Other NWS stations in the area did not record any decrease in visibility 

throughout the entire day: visibility at Glendale Airport remained at 20 miles and Sky Harbor 

remained at 10 miles. At the Goodyear Airport, the minimum recorded visibility was 7 statute 

miles. The visibility throughout the day in the Phoenix area was never significantly reduced, and 

thus this information does not significantly contribute to establishing a clear causal 

relationship.26

5.2.3 Review of 24-hour PM10 Data

The 24-hour PM10 concentrations measured on April 30 at the West 43rd and surrounding sites 

are listed in Table 3 and shown in Figure 7. On this day, the West 43rd monitor was the only site 

in the entire Phoenix area to violate the 24-hour PM10 standard.27 Furthermore, PM10

concentrations at the West 43rd site were more than double those recorded at other local sites, 

which is generally inconsistent with the notion that a regional high wind event caused the 

exceedance. 

 

26 See Appendix B for information regarding reduced visibility and dust storms in Arizona. 

27 Similar to the data for March 14, 2008, the only other exceedance recorded in Arizona on this day was the 

Cowtown monitoring site in Pinal County, which was not flagged as an exceptional event.
18

Figure 7: April 30, 2008 24-Hour PM10

Table 3: April 30, 2008 24-Hour PM10

Site Name PM10 (ug/m3) Site Name PM10 (ug/m3)

Buckeye*28 68 Glendale(FRM) 29* 43

West 43rd 172 Mesa (FRM)* 38

Durango Complex 69 North PHX (FRM)* 39

South PHX 88 South Scottsdale (FRM)* 40

Greenwood 63 West Chandler (FRM)* 49

West PHX 55 Bethune Elementary (FRM)* 48

Central PHX 51 Dysart (FRM)* 43

JLG Supersite 46 Coyote Lakes 53

Higley 63

 

28 24-hour PM10 data for these sites were not included in the Assessment. 

29 PM10 FRM samplers operate on a 1 in 6 day schedule. 
19

5.2.4 Review of Hourly PM10 and Meteorological Data 

The hourly PM10 data are shown in Figure 8. As early as 1000 hrs, the West 43rd site began to 

experience an increase in PM10 concentration that was not characteristic of the other nine 

monitors in the Phoenix area. From 0900 to 1000 hrs the hourly PM10 values at the West 43rd

site increased from 85 μg/m3 to 404 μg/m3, while PM10 values at surrounding sites remained 

below 120 μg/m3. The first sign of any elevated winds occurred at the majority of the stations 

around 1100 hrs. NWS data for Goodyear Airport showed an increase in wind speed from 16 to 

17 mph (accompanied by a 29 mph gust); while an increase in hourly wind speed from 12.7 to 

the day’s maximum value of 16 mph was recorded at the West 43rd site. At 1100 hrs, the PM10

concentration at the West 43rd site also rose from 404 μg/m3 to the day’s maximum value of 1065 

μg/m3. 

While values at other sites in the area increased over the same time period, the values at the West 

43rd site ranged from 5 to 10 times higher than other sites in the Phoenix area. For example, the 

majority of the sites measured maximum PM10 concentrations that were coincident with the 

maximum PM10 concentrations at the West 43rd site, but all sites in Maricopa County measured 

maximum PM10 concentrations less than 220 μg/m3. Given that the Durango Complex, South 

Phoenix, Greenwood, and West Phoenix sites are located within approximately five miles of the 

West 43rd site, one would expect to see greater consistency in the concentrations if a regional 

high wind event was occurring. The data suggest that the West 43rd site was most likely 

significantly influenced by local upwind sources and the claimed exceptional event was not 

regional in nature. 

Figure 8: April 30, 2008 Hourly PM10

5.2.5 Review of 5-Min PM10 and Wind Speed Data 

The 5-min data reinforce the fact that even though elevated wind speeds were measured at other 

nearby locations, the West 43rd monitor consistently measured much higher PM10 concentrations 

than other locations. Figure 9 shows the 5- min PM10 and wind speed data from West 43rd and 

Durango Complex. These monitors are located only 2 miles apart, yet there seems to be a 

considerable difference in the relationship between PM10 and wind speed on April 30. Both sites 

experience similar wind speed levels, but during some periods of the day the 5-min PM10
20

concentrations at West 43rd site were more than 9 times those measured at Durango Complex. 

The two highest 5-min PM10 averages measured at the West 43rd site were approximately 1920 

and 1624 μg/m3, while PM10 concentrations at Durango Complex during the same time period 

were 178 and 373 μg/m3, respectively. These data provide further evidence that the claimed 

regional high wind event only affected PM10 concentrations at the West 43rd site and the elevated 

PM10 concentrations measured at this site were most likely significantly influenced by local 

sources and the claimed exceptional event was not regional in nature. 

Figure 9: April 30, 2008 5-Min PM10 and Wind Speed

5.2.6 Review of Days with Similar Meteorological Conditions 

On twenty days in April 2008, the wind speeds at Goodyear Airport exceeded 15 mph. On six of 

those days, wind gusts exceeded 25 mph. The following analysis compares the hourly PM10

data, wind speed, and wind gusts recorded at Goodyear Airport on April 30 with the same data 

from a similar day in April. 

On April 30, the West 43rd monitor experienced elevated PM10 concentrations of 404 μg/m3 and 

1065 μg/m3 at 1000 and 1100 hrs, respectively. Wind speeds at Goodyear Airport during this 

period were from the WSW (240°-260°) at 17 mph with gusts of 29 mph. On April 29, the 

Goodyear station measured wind speeds and gusts of equal magnitude; 17 mph winds and 29 

mph gusts from the SW (230°) for three consecutive hours. A maximum concentration of 177 

μg/m3 was observed during this period, but it is considerably lower than the PM10 concentrations 

measured on the day the exceptional event is claimed to have occurred. This example illustrates 

how elevated wind speeds in upwind areas are related to elevated PM10 concentrations on 

occasion, but the magnitude of PM10 concentrations measured at the West 43rd site seem to be 

dependent on a number of different factors. 
21

Figure 10: Non-Exceedance Day Comparison

5.2.7 Summary of Clear Causal Relationship for April 30, 2008 

ADEQ’s conclusions that the recorded exceedance was caused by a regional high wind event are 

not substantiated by relevant monitoring and meteorological data. The data show that the spatial 

extent of PM10 during this day was isolated and not regional in nature. The data also show 

differences in the measured PM10 concentrations at the West 43rd site and the remaining sites in 

the Phoenix area. In addition, ADEQ provided only limited analysis of possible contribution 

from human activity, making it difficult to comprehensively evaluate the relationship between 

the claimed event and the exceedance. Therefore, EPA has determined that the weight of 

evidence presented in the April 30 Assessment does not demonstrate a clear causal relationship 

as required by the EER. 

5.3 May 21, 2008 

5.3.1 Correlation between Wind Speed and PM10

The May 21 Assessment included tabular hourly and maximum wind speed and PM10 data for 

five sites in the Phoenix area: West 43rd, Durango Complex, South Phoenix, and Buckeye.30

ADEQ also included meteorological data from the NWS Luke Air Force Base station. ADEQ 

did not provide hourly PM10 and meteorological data from the remaining five continuous PM10

analyzers in the Phoenix area and did not include wind speed and direction data from numerous 

other meteorological stations in the Phoenix area. Appendix A contains more information on the 

PM10 and meteorological data used in the May 21assessment. 

ADEQ also provided three graphs that show the potential correlation between maximum wind 

speeds and PM10 concentrations. The three graphs display data from the West 43rd, Durango 

Complex, and South Phoenix monitoring sites. While the hourly PM10 concentrations 

significantly increase with an increase in maximum recorded wind speeds at the West 43rd site, 

 

30 ADEQ’s supporting documentation for this event also contained information pertaining to measured exceedances 

at monitoring sites in Yuma County (Yuma Courthouse site). The Yuma monitor is more than 150 miles from the 

West 43rd site. We expect the circumstances that caused the exceedance at the Yuma MCAS site to be different 

that those affecting the Phoenix area; therefore we are giving this data relatively little weight in our evaluation. 
22

there is not a similar correlation between PM10 and maximum wind speed at the other monitoring 

sites in the area. These facts suggest that the elevated PM10 concentrations at West 43rd may 

have been caused by local upwind sources and were not regional in nature. 

5.3.2 Visibility 

The assessment included photographs from numerous locations throughout the Phoenix area. 

Photographs taken at 1330 hrs show evidence of reduced visibility and a potential regional event; 

however, PM10 concentrations at the West 43rd site began to increase at 0800 hrs. Photographs 

were provided for 0930, 1330, 1430, and 1530 hrs. Photographs were not submitted for the 

hours preceding the elevated PM10 concentrations measured at the West 43rd site. Therefore, the 

photographs do not significantly contribute to establishing a causal relationship between wind 

speed, potential contributing sources, and PM10 concentrations at the West 43rd monitoring site 

during the morning hours. 

ADEQ also stated that reduced visibility during the event throughout portions of Phoenix 

provides further evidence of a clear causal relationship. The visibility at Goodyear Airport 

before the event ranged from 20 to 7 statute miles; visibilities of 7 miles were recorded at 1047, 

1647, and 1747 hrs. Chandler Airport recorded observations of blowing dust (BLDU) at 1347 

hrs, which was followed by a recorded visibility of 7 miles at 1447 hrs. Visibility at other NWS 

stations in the area remained above 10 miles for the entire day: Glendale Airport ranged from 10 

to 20 miles, Sky Harbor remained at 10 miles, and Luke Air Force Base remained at 10 miles. 

The visibility throughout the day in the Phoenix area was never significantly reduced, and thus 

this information does not significantly contribute to establishing a clear causal relationship.31

5.3.3 Review of 24-Hour PM10 Data

The 24-hour PM10 concentrations measured on May 21 at the West 43rd and surrounding sites are 

listed in Table 4 and shown geographically in Figure 11. On this day, the West 43rd monitor was 

the only site in the entire Phoenix area to violate the 24-hour PM10 standard. Furthermore, PM10

concentrations at West 43rd were more than double those recorded at other local sites, which is 

generally inconsistent with the notion that a regional high wind event caused the exceedance. 

 

31 See Appendix B for information regarding reduced visibility and dust storms in Arizona. 
23

Figure 11: May 21, 2008 24-Hour PM10

Table 4: May 21, 2008 24-Hour PM10

Site Name PM10 (ug/m3) Site Name PM10 (ug/m3)

Buckeye*

32 123 West PHX* 83

West 43rd 278 Central PHX* 92

Durango Complex 110 JLG Supersite* 62

South PHX 122 Higley* 111

Greenwood 89 Coyote Lakes* 71

Bethune (FRM)* 111

5.3.4 Review of Hourly PM10 and Meteorological Data 

The hourly PM10 data for Maricopa County are shown in Figure 12. The peak PM10 

concentration of 1207 μg/m3at 0900 hrs measured at the West 43rd site coincides with an 

increase in wind speed from 11 to 22 mph, and a recorded wind gust of 28 mph at the Goodyear 

 

32 24-hour PM10 data from these sites were not included in the Assessment. 
24

station and an increase in hourly wind speed from 15.3 to 18.1 mph at the West 43rd monitoring 

site. Similar to previously discussed events, the measured PM10 concentrations at West 43rd were 

more than 3.9 times the PM10 values measured at the Durango Complex station just 2 miles to 

the northeast and 3.6 times the values measured at the South Phoenix station 4 miles to the east. 

The inconsistencies in PM10 concentrations suggest that the West 43rd site most likely was 

influenced by local upwind sources and the claimed exceptional event was not regional in nature. 

Figure 12: May 21, 2008 Maricopa County Hourly PM10

5.2.5 Review of 5-Min PM10 and Wind Speed Data 

The 5-min data reinforce the fact that even though elevated wind speeds were measured at other 

nearby locations, the West 43rd monitor consistently measured much higher PM10 concentrations 

than other locations. Figure 13 shows the 5- min PM10 and wind speed data from the West 43rd

and Durango Complex sites. These monitors are located only 2 miles apart, yet there seems to 

be a considerable difference in the relationship between PM10 and wind speed on May 21. Both 

sites experience similar wind speed levels, but during some periods of the day the 5-min PM10

concentrations at West 43rd site ranged from 3-6 times higher than those measured at Durango 

Complex. The two highest 5-min PM10 averages measured at the West 43rd site were 

approximately 1837 and 1769 μg/m3, while PM10 concentrations at Durango Complex during the 

same time period were 290 and 362 μg/m3, respectively. These data provide further evidence 

that the claimed regional high wind event only affected PM10 concentrations at West 43rd and the 

elevated PM10 concentrations measured at this site were most likely significantly influenced by 

local sources and the claimed exceptional event was not regional in nature. 
25

Figure 13: May 21, 2008 5-Min PM10 and Wind Speed

5.3.6 EPA Review of Days with Similar Meteorological Conditions 

On fourteen days in May 2008, the wind speed at Goodyear Airport exceeded 15 mph. On three 

of those days, wind gusts exceeded 25 mph. The following analysis compares the hourly PM10

data, wind speed, and wind gusts on May 21 with the same data from a similar day in May. 

On May 21, the West 43rd monitor experienced elevated PM10 concentrations of 518 μg/m3 and 

1207 μg/m3 at 0800 and 0900 hrs, respectively. Wind speeds at Goodyear Airport during this 

period were from the WSW (240°) at 22 and 21 mph with gusts of 28 and 30 mph. Similarly, on 

May 12, the Goodyear station measured wind speeds and gusts of equal magnitude; 21 mph wind 

speeds and 30 mph gusts from the SW (230°). These elevated wind speeds, however, only 

correspond to moderate hourly PM10 values at the West 43rd site. Hourly PM10 concentrations on 

May 12 were considerably lower than the PM10 concentrations measured on the day the 

exceptional event is claimed to have occurred; maximum PM10 values on May 12 only reached 

500 μg/m3. This example illustrates how elevated wind speeds in upwind areas are related to 

elevated PM10 concentrations on occasion, but the magnitude of PM10 concentrations measured 

at the West 43rd site seem to be dependent on a number of different factors. 

Figure 14: Non-Exceedance Day Comparison
26

5.3.7 Summary of Clear Causal Relationship for May 21, 2008

ADEQ’s conclusions that the recorded exceedance was caused by a regional high wind event are 

not substantiated by relevant monitoring and meteorological data. The data show that the spatial 

extent of PM10 during this day was isolated and not regional in nature. The data also show 

differences in the measured PM10 concentrations at the West 43rd site and the remaining sites in 

the Phoenix area. In addition, as explained above, ADEQ provided only limited analysis of 

possible contribution from human activity (sections 4.2 and 4.3 above), making it difficult to 

comprehensively evaluate the relationship between the claimed event and the exceedance. 

Therefore, EPA has determined that the weight of evidence presented in the May 21 Assessment 

does not demonstrate a clear causal relationship as required by the EER. 

5.4 June 4, 2008 Event 

The June 4 assessment contained information pertaining to measured exceedances at monitoring 

sites in both the Phoenix area (Buckeye, Coyote Lakes, and West 43rd site) and Yuma County 

(Yuma MCAS site). These two locations are over 150 miles apart and the data concerning the 

Yuma area has limited value in determining whether or not exceptional events occurred in the 

Phoenix area. It is also important to note that EPA is not evaluating the exceedances measured at 

the Buckeye and Coyote Lakes monitoring sites in this document. As discussed in the next 

section, it is clear that the PM10 concentrations at these sites are not correlated to those measured 

at the West 43rd site for the majority of the day on June 4 and were most likely influenced by a 

different set of sources and meteorological conditions (Figure 16).

5.4.1 Correlation between Wind Speed and PM10

The assessment included tabular wind speed and PM10 data for five sites in the Phoenix area: 

West 43rd, Durango Complex, Central Phoenix, Coyote Lakes, and Buckeye. ADEQ also 

included meteorological data from the NWS Luke Air Force Base station. ADEQ did not 

provide tabular hourly PM10 data from the remaining five continuous PM10 analyzers in the 

Phoenix area and did not include wind speed and direction data from numerous other 

meteorological stations in the Phoenix area. Appendix A contains more information on the 

meteorological data used in the June 4 supporting documentation. 

ADEQ also provided seven graphs that show the potential correlation between maximum wind 

speeds and PM10 concentrations. The graphs show that, at the West 43rd site, the hourly PM10

concentrations increase with an increase in maximum recorded wind speeds at the West 43rd site; 

however, there does not seem to be a similar correlation between PM10 and maximum wind 

speed for the other monitoring sites in the area until later in the evening. These data suggest that 

the elevated PM10 concentrations in the morning and early afternoon hours at the West 43rd site 

were most likely caused by local upwind sources and are not regional in nature. 

5.4.2 High Winds 

While Section 4.3 contains a general discussion of ADEQ’s high wind analysis, ADEQ’s DSR 

provided a more detailed discussion of the meteorological conditions that were associated with 
27

the claimed exceptional event occurring on June 4. Unlike the previously discussed events, the 

DSR contained copies of NWS advisories concerning the meteorological conditions in the 

Phoenix area. These reports provide additional evidence of the nature of the wind speeds 

associated with the claimed exceptional event. Specifically, NWS issued a wind advisory at 

0356 hrs on June 4 that was set to be in effect from 1500 to 2100 hrs. The advisory states that 

“wind speeds of 25 to 30 mph with gusts up to 40 mph can be expected” and warned that “strong 

winds over desert areas could result in briefly lowered visibilities to well under a mile at times in 

blowing dust or blowing sand...especially near empty farm fields and construction areas.” 

While these advisories continued to be in effect during the afternoon hours of June 4, the average 

hourly wind speeds observed at the West 43rd monitoring site never exceeded 17.1 mph for the 

entire day, while wind gusts reached a maximum of 36 mph at 1600 hrs. ADEQ’s DSR states 

that during the afternoon hours the Phoenix area experienced “unusually high gusts of 35-40 mph 

which would likely overwhelm BACM in place for PM in the Phoenix” area, but as discussed in 

section 4.2.2, ADEQ has not determined at which wind speeds this may be occurring. As 

discussed below, the West 43rd monitoring site began measuring elevated PM10 concentrations at 

1200 hrs, well before the NWS advisories were put into effect. 

5.4.3 Visibility 

The assessment included photographs from numerous locations throughout the Phoenix area. 

Photographs taken at 1830 hrs show evidence of reduced visibility and a potential regional 

event,33 but it is important to note that PM10 concentrations at the West 43rd monitoring site 

began to increase at 1200 hrs. No photographs were submitted for this time period or for hours 

preceding the elevated PM10 concentrations. Therefore, the photographs do not significantly 

contribute to establishing a causal relationship between observed wind speeds, potential 

contributing sources, and PM10 concentrations at the West 43rd monitoring site during the late 

morning, early afternoon hours. 

ADEQ also stated that reduced visibility during the event at Goodyear Airport provides further 

evidence of a clear causal relationship. The visibility at Goodyear Airport during the morning 

and early afternoon hours ranged from 20 to 10 statute miles. While the reduced visibility 

observed at numerous NWS after 1800 hrs suggests a regional event may have occurred, it is 

important to note that PM10 concentrations at the West 43rd site began to increase at 1200 hrs: a 

time when visibility was between 10 to 20 miles.

5.4.4 Review of 24-Hour PM10 Data

The 24-hour PM10 concentrations measured on June 4 at the West 43rd and surrounding sites are 

shown in Figure 15. On this day, the West 43rd monitor measured PM10 concentrations that were 

more than double those measured at other monitoring sites in the area, except for the Buckeye 

and Coyote Lakes sites, which recorded similar concentrations. 

 

33 See Appendix B regarding visibility and dust storms in Arizona. 
28

Figure 15: June 4, 2008 24-Hour PM10

Table 5: June 4, 2008 24-Hour PM10

Site Name PM10 (ug/m3) Site Name PM10 (ug/m3)

Buckeye 204 Greenwood* 97

West 43rd 194 West PHX* 83

Coyote Lakes 187 Central PHX* 93

Durango Complex*

34 94 JLG Supersite* 65

South PHX* 82 *Higley* 84

5.4.5 Review of Hourly PM10 and Meteorological Data 

The patterns observed through the morning hours and mid-day on June 4 are similar to the 

claimed exceptional event days discussed in previous sections and the data from this time period 

does not indicate an influence from a regional high wind event. Also, similar to the previously 

 

34 24-hour PM10 data for this station was not included in the Assessment. 
29

discussed events, the West 43rd site measured elevated PM10 concentrations earlier and of a 

higher magnitude than other monitoring sites located nearby. For example, on the early 

afternoon of June 4, the West 43rd monitor began measuring PM10 concentrations ranging from 

165 μg/m3to 645 μg/m3 between 1200 and 1400 hrs, while all other monitors in the Phoenix area 

remained below 200 μg/m3for the same time period. The inconsistencies in these concentrations 

suggest that the West 43rd site was most likely significantly influenced by local upwind sources 

and the claimed exceptional event was not regional in nature. ADEQ acknowledged that the 

concurrent timing of elevated wind speeds “may indicate that PM sources in close proximity to 

the monitor contributed significantly to the dust event” and “it is likely that the loose particulates 

deposited in the dry river bed to the west and south-west of the monitor were transported the 

short distance to the West 43rd monitor by the high winds.”35

It appears that a regional weather event began on the evening of June 4 and lasted into the 

morning of June 5. Figure 16 shows that the Buckeye site begins to measure significantly 

elevated PM10 concentrations at 1600 hrs, followed by an increase in PM10 at the Coyote Lakes 

site a few hours later. The West 43rd site lagged behind Buckeye and Coyote Lakes and did not 

show elevated PM10 from the regional event until 2200 hrs on June 4. While the West 43rd

, 

Buckeye, and Coyote Lakes sites all exceeded on June 4, the cause of the exceedances seems to 

be different. The exceedances at Buckeye and Coyote Lakes were most likely due to a regional 

event that began in the evening and did not reach West 43rd until 2200 hrs, while the exceedance 

at West 43rd was most likely caused by a different set of circumstances (Figure 16). Also, 

beginning around 2200 hrs and extending into the early morning hours of June 5, PM10

concentrations at all sites in the Phoenix area were elevated and uniformly consistent, illustrating 

a potential regional event. In the DSR, ADEQ acknowledged that “a more homogeneous dust 

plume affected the area just after midnight on the following day.” 

While there was some contribution to the 24-hour PM10 concentration that can be attributed to 

this evening event, the West 43rd monitor began measuring high PM10 concentrations well before 

the arrival of the “dust plume” described in the June 4 DSR. Furthermore, the arrival of the dust 

plume began at around 2100 hrs and only contributed to approximately 11.3 percent of the total 

PM10 mass concentration for June 4. With such a small total contribution, if all PM10

concentrations measured after 2100 hrs were completely removed from the data set, the PM10 24-

hour average for June 4 would still be above the PM10 24-hour NAAQS. The majority of the 

PM10 mass was measured well before the arrival of the evening event described above and the 

high PM10 concentrations measured in the late morning and early afternoon hours have been 

determined to be independent from the regional event that took place on the evening of June 4. 

 

35 June 4 DSR at p. 24. 
30

Figure 16: June 4 & 5, 2008 Hourly PM10

5.4.6 Review of 5-Min PM10 and Wind Speed Data 

The 5-min data also show that even though elevated wind speeds were measured at other nearby 

locations, the West 43rd monitor consistently measured much higher PM10 concentrations than 

other locations. Figure 18 shows the 5- min PM10 and wind speed data from West 43rd and 

Durango Complex:. these monitors are located only 2 miles apart, yet there seems to be a 

considerable difference in the relationship between PM10 and wind speed on June 4 during the 

late morning and early afternoon hours. Both sites experience similar wind speed levels, but 

during some periods of the day the 5-min PM10 concentrations at West 43rd site ranged from four 

to nine times higher than those measured at Durango Complex. The two highest 5-min PM10

averages measured at the West 43rd site were approximately 1475 and 975 μg/m3

, while PM10

concentrations at Durango Complex during the same time period were 153 and 264 μg/m3, 

respectively. 

These data provide further evidence that the claimed regional high wind event only affected 

PM10 concentrations at West 43rd and the elevated PM10 concentrations measured at this site in 

the morning and early afternoon hours were most likely significantly influenced by local sources 

and the claimed exceptional event was not regional in nature. 

High values of PM10

unrelated to the 

evening regional event

Regional event
31

Figure 17: June 4 & 5, 2008 5-Min PM10 and Wind Speed

5.4.7 Review of Days with Similar Meteorological Conditions 

On twelve days in June 2008, the wind speed at Goodyear Airport exceeded 15 mph. On two of 

those days, wind gusts exceeded 25 mph. The following analysis compares the hourly PM10

data, wind speed, and wind gusts on June 4 with the same data from a similar day in May. 

On June 4, the West 43rd monitor experienced elevated PM10 concentrations of 307 μg/m3and 

644.9 μg/m3 at 1300 and 1400 hrs, respectively. Wind speeds at Goodyear Airport during this 

period were from the WSW (242°) at 17 and 18 mph with gusts of 23 and 29 mph. Similarly, on 

June 10, the Goodyear station measured wind speeds and gusts of similar magnitude; 17 mph 

wind speeds and 23 mph gusts from the WSW (240°). These elevated wind speeds, however, 

only correspond to a slight increase in hourly PM10 values at the West 43rd site. This example 

illustrates how elevated wind speeds in upwind areas are related to elevated PM10 concentrations 

on occasion, but the magnitude of PM10 concentrations measured at the West 43rd site seem to be 

dependent on a number of different factors.

 

Figure 18: Non-Exceedance Day Comparison
32

5.4.8 Summary of Clear Causal Relationship for June 4, 2008 

The data show that the spatial extent of PM10 during the early portion of the day was isolated and 

not regional in nature. In addition, ADEQ did not adequately address the possible contributing 

sources in the area directly upwind of the West 43rd monitor, which makes a causal relationship 

difficult to evaluate. ADEQ has also failed to adequately explain the differences in the measured 

PM10 concentrations at the West 43rd site and the remaining sites in the Phoenix area 

experiencing similar wind conditions. 

ADEQ asserted that while elevated wind speeds occurred throughout the Phoenix area, “the 

blowing dust that was generated from these high winds occurred at sporadic locations;” the “high 

concentrations of blowing dust only occurred where dust sources were located;” and “these dust 

sources are typically located in depositional areas where fine and coarse particles are deposited 

during times of precipitation.” ADEQ further concluded that “cause of the exceedances for the 

Maricopa County monitors was alluvial dust generated by high winds in the river channels, 

coupled with the generally elevated dust from the region-wide dust storm.” Even more explicitly, 

ADEQ explained that the exceedance at West 43rd “was due to generally elevated PM10 from the 

dust storm coupled with contributions from dust generated in the alluvial plain of the Salt and 

Gila Rivers due to high, gusty winds.” While ADEQ has concluded that the exceedance at West 

43rd was caused by emissions originating in the Salt and Gila River channels, there little 

technical justification supporting this conclusion and there is no discussion explaining how 

emissions from these sources are not reasonably controllable or preventable. 

While there appears to be some component of the PM10 that could be attributed to a regional dust 

storm event, the time series (Figures 16 and 17) indicate that the regional event did not influence 

the measured PM10 at the West 43rd site until very late on June 4 and the principal cause of the 

exceedance were emissions from local sources. Therefore, the weight of evidence does not 

demonstrate a clear causal relationship as required by the EER. 

6.0 Concentration in Excess of Normal Historical Fluctuations 

Pursuant to 40 CFR 50.14(c)(3)(iii)(C), the demonstration must show that “the event is 

associated with a measured concentration in excess of normal historical fluctuations.” ADEQ 

provided tables for each event that ranked the PM10 exceedances using data from the past five 

years (2003-2008). A comparison was made to five years of data from the “spring season” and 

the complete five year data set. Table 6 summarizes these data. 

Table 6: ADEQ Historical Distribution 

Exceedance Date PM10 Concentration Seasonal Percentile Yearly Percentile

3.14.08 251 μg/m3< 99.5 < 99.5

4.30.08 172 μg/m3< 97.5 < 99.7

5.21.08 279 μg/m3< 99.5 < 99.5

6.4.08 194 μg/m3< 97.5 < 99.0
33

There is no specific threshold test for this requirement, but concentrations in the high percentiles 

can provide supporting evidence and informs EPA’s weight of evidence analysis of the 

exceptional events in question. As stated in the EER preamble, “For extremely high 

concentrations relative to historical values, a lesser amount of documentation or evidence may be 

required.” 36 While the relative comparison to the historical fluctuations informs the amount of 

evidence required, for an event to be considered an exceptional event, all criteria listed under 

section 3.0 must be met. 

7.0 No Exceedances But For the Event 

Pursuant to 40 CFR §50.14(c)(3)(iii)(D), the demonstration must show that “there would have 

been no exceedance or violation but for the event.” The weight of evidence in a demonstration 

does not require a precise estimate of the air quality impact from the event,37 though such 

information could be useful. 

Assessments for all events include an “event contribution analysis” to support the notion that 

there would have been no exceedance but for the event. This analysis consists of a table that 

calculates the 24-hour PM10 concentration excluding the hours of the day that the event was 

assumed to have occurred. There is no explanation of how to interpret this analysis, and it is 

unclear how these hours are chosen for exclusion. Also, from the documentation alone, it is 

unclear how this calculation is performed. After conversations with staff members of ADEQ, it 

was determined that the hours that have been chosen for exclusion are replaced by the average 

PM10 concentration calculated with remaining hours of the day. This is equivalent to assuming 

there is no normal increase during those hours. If there is a typical rise during that period, then 

the average used may not be representative of typical conditions. Considering the weight of 

evidence, the assessments for all four events do not provide sufficient evidence to establish that 

there would not have been an exceedance but for the event. 

8.0 Procedural Requirements 

The EER at 40 CFR §50.14(c)(2)(iii) requires that data claimed to be due to an exceptional event 

must be flagged in the AQS database, and that an initial description of the event be provided to 

EPA by July 1 of the year following the event. 

Pursuant to 40 CFR §50.14(c)(3)(i) the State must submit a demonstration to EPA within three 

years of the event. EPA received the final demonstrations for the 2008 events in question on 

November 17, 2009, which satisfies the three year submission requirement. 

40 CFR §50.14(c)(3)(i) also requires notice and opportunity for public comment. ADEQ’s 

documentation was available on the ADEQ web-site and the ADEQ Library in Phoenix 

beginning on October 15, 2009. No comments were received from the public during the 

comment period.38 Information included in the draft supplemental report, received by EPA on 

March 17, 2010, has not yet gone through the public comment process. 

 

36 EER Preamble, 72 FR 13569. 

37 Id. at 13570. 

38 Letter from Nancy Wrona, ADEQ, to Deborah Jordan, USEPA Region 9 received on November 17, 2009. 
34

9.0 Conclusion 

ADEQ stated that the measured exceedances at the West 43rd monitoring site, during these days, 

were a result of the transport of dust from soils by high winds that were associated with 

approaching low pressure systems. For all of the events, there appears to be elevated wind 

speeds in various locations throughout the Phoenix area, but as discussed in section 4.3, ADEQ’s 

approach to defining “unusual” winds relies on complete yearly data, rather than the season 

during which the events occurred. In addition, ADEQ’s approach would find that “unusual” 

winds occur on approximately 100 days a year. Also, there is little discussion or explanation 

concerning the meteorological conditions that were occurring on the days in question and how 

those conditions affected the elevated PM10 concentrations at the West 43rd monitoring site. The 

majority of the data concerning these relationships are presented in tables and a small number of 

graphs with no explanation of the interpretation of the information that has been presented. 

Moreover, the Assessments did not adequately address the sources that may have been 

contributing to the event. Without this information, it is difficult to determine whether the 

elevated PM10 concentrations resulted from controllable anthropogenic sources or natural desert 

sources. Since there are numerous anthropogenic sources located in upwind areas, this 

information is critical to assessing whether an exceedance is the result of an exceptional event or 

uncontrolled anthropogenic sources. With little discussion of the meteorological conditions on 

the event days combined with a very limited discussion on possible sources, the Assessments did 

not adequately establish a clear source-receptor relationship or make a convincing demonstration 

that the events in question should be considered natural events under the EER. 

Furthermore, the information in the Assessments did not support the broad conclusion that the 

elevated PM10 concentrations were caused by transport of dust from soils by high winds. Again, 

without acknowledging the sources that may be contributing to the event, it is difficult to 

determine where the dust originated from and how it was transported to West 43rd. Also, the 

monitoring data is inconsistent with the notion of transport. If transport was occurring on these 

days, one would expect to see similar concentrations at nearby monitoring locations. One of the 

most interesting aspects of these events is that on March 14 and April 30 the West 43rd monitor is 

the only monitor to violate the 24-hour standard, not only in the Phoenix area, but the entire state 

of Arizona except for the Cowtown monitor in Pinal County, which consistently measures the 

highest levels of PM10 within Region 9 due to its proximity to a large cattle feedlot. Also, the 

differences between the hourly PM10 concentrations at West 43rd and other sites that are located 

just a few miles away are striking. Although it is very clear that there is something unique about 

the measured exceedances at the West 43rdsite, the assessments did not explain these differences 

in PM10 concentrations and how they are inconsistent with a regional high wind event. 

The June 4 DSR included a more detailed discussion of the meteorological conditions during the 

event and provided some discussion on the sources that may be influencing the elevated PM10

concentrations at the West 43rd monitoring site. The additional documentation asserted that the 

exceedance measured at the West 43rd monitoring site can be attributed to emissions from the dry 

Gila and Salt River channels that were coupled with contributions from a regional dust storm. 

As previously discussed, the documentation Assessments did not provide sufficient technical 
35

justification of this conclusion and did not explain how emissions from these sources were not 

reasonably controllable or preventable. Furthermore, the data show that the contribution from 

the regional dust storm during the late night hours of June 4 was not significant and the 

exceedance was most likely driven by the elevated PM10 concentrations measured in the late 

morning and early afternoon hours. 

The June 4 DSR provided some new information regarding the significant differences in the 

hourly PM10 values seen at the Durango Complex and South Phoenix monitoring sites. The 

documentation stated that “it is also entirely possible that the urbanized core of the Phoenix 

metro area acted to reduce the amount of blowing dust compared to the western periphery due to 

increased surface roughness.” While this might be relevant, it does not account for the nearly 

identical PM10 concentrations measured throughout the entire Phoenix area in the evening hours 

of June 4 and the morning hours of June 5. The June 4 DSR did not provide sufficient technical 

analysis to support a clear source receptor relationship or provide new evidence to support the 

notion that the June 4 event should be considered a natural event under the EER. Considering 

the weight of available evidence, EPA does not concur that the March 14, April 30, May 21 and 

June 4, 2008 exceedances at the West 43rd monitoring site should be treated as exceptional 

events. 
36

Appendix A 

Table 1: Meteorological Data Used in ADEQ’s Assessments39

Available Met Data

Distance to 

West 43rd Direction 3.14.08 4.30.08 5.21.08 6.4.08

WEST PHOENIX 5 N x x

MESA 16 E x

NORTH PHOENIX 12 NNE

GLENDALE 12 N

PINNACLE PEAK 27 NE

CENTRAL PHOENIX 7 ENE x x x

SOUTH SCOTTSDALE 14 ENE x

GREENWOOD 4 NNE

SOUTH PHOENIX 4 E x x

COYOTE LAKES* 21 NNW x x

WEST CHANDLER 17 ESE x

TEMPE 12 E

HIGLEY 25 ESE x

WEST 43RD n/a n/a x x x x

DYSART 20 NNE

BUCKEYE 28 W x x

DURANGO COMPLEX 2 NE x x x x

JLG SUPERSITE 7 NNE

WEST INDIAN ROAD 6 N

FALCON FIELD 24 E

CAVE CREEK 29 NNE

BLUE POINT 32 ENE

FOUNTAIN HILLS 28 ENE

GOODYEAR 13 W x x

LUKE AFB 16 NW x

GLENDALE 13 NW x

SKY HARBOR 9 ENE x x

DEER VALLEY 20 N x

SCOTTSDALE 20 NE

FALCON FIELD 24 E

CHANDLER 21 ESE

WILLIAMS GATEWAY 29 ESE

 

39 The highlighted areas in Table 1 correspond to either the closest meteorological station or stations upwind of the 

West 43rd monitoring site. 
37

Table 2: PM10 Data Used in ADEQ’s Assessments40

Site Name

Distance to 

West 43rd Direction 

24 Hour PM10

Data Hourly PM10 Data

3.14.08

WEST PHOENIX 5 N x x

CENTRAL PHOENIX 7 ENE x x

GREENWOOD 4 NNE x x

SOUTH PHOENIX 4 E x

COYOTE LAKES 21 NNW x x

HIGLEY 25 ESE x

WEST 43RD n/a n/a x x

BUCKEYE 28 W

DURANGO COMPLEX 2 NE x x

JLG SUPERSITE 7 NNE x

4.30.08

WEST PHOENIX 5 N x

MESA 16 E n/a

NORTH PHOENIX 12 NNE n/a

GLENDALE 12 N n/a

CENTRAL PHOENIX 7 ENE x x

SOUTH SCOTTSDALE 14 ENE n/a

GREENWOOD 4 NNE x x

SOUTH PHOENIX 4 E x x

COYOTE LAKES 21 NNW x

WEST CHANDLER 17 ESE n/a

HIGLEY 25 ESE x x

WEST 43RD n/a n/a x x

DYSART 20 NNE n/a

BUCKEYE 28 W

BETHUNE 4 NE n/a

DURANGO COMPLEX 2 NE x x

JLG SUPERSITE 7 NNE x

5.21.08

WEST PHOENIX 5 N x

CENTRAL PHOENIX 7 ENE

GREENWOOD 4 NNE x

SOUTH PHOENIX 4 E x x

COYOTE LAKES 21 NNW

HIGLEY 25 ESE

WEST 43RD n/a n/a x x

BUCKEYE 28 W x

BETHUNE 4 NE n/a

DURANGO COMPLEX 2 NE x x

JLG SUPERSITE 7 NNE

 

40 The highlighted areas in Table 1 correspond to either the closest meteorological station or stations upwind of the 

West 43rd monitoring site. 
38

6.4.08

WEST PHOENIX 5 N x

CENTRAL PHOENIX 7 ENE x

GREENWOOD 4 NNE x

SOUTH PHOENIX 4 E

COYOTE LAKES 21 NNW x x

HIGLEY 25 ESE

WEST 43RD n/a n/a x x

BUCKEYE 28 W x x

BETHUNE 4 NE

DURANGO COMPLEX 2 NE x

JLG SUPERSITE 7 NNE
39

Figure 1: West 43rd Pollution Roses % Total PM10 Mass
40

Appendix B 

EPA acknowledges that massive dust storms do occur in the Southwestern United States and that 

these events could quality as exceptional events if all requirements of the EER were satisfied. 

The following information could be used as evidence in an exceptional events demonstration if 

the conditions of the event were consistent with those observed during a dust storm. 

The relationship between weather types, wind speed, and dust storm generation has been 

researched and examined for many years. Generally, there are generally four different weather 

types that are capable of producing dust storms. These conditions were examined in further 

detail by Brazel and Nickling in two separate research papers during the 1980’s. Both studies 

conclude that the frequency of dust storms can be directly linked to specific weather conditions 

which are accompanied by elevated wind speeds, but also note that dust events are “strongly 

affected by antecedent conditions (i.e. surface moisture, vegetation cover, surface crusting, and 

anthropogenic disturbances)”. For the years 1965 -1980, 80% of all intense dust storms41 in the 

Phoenix area were related to thunderstorm activity in the region. The mean wind speed for dust 

storms during this time period in the Phoenix area were 12.4 m/s or 27.7 mph, while the mean 

peak gusts were 17.8 m/s or 39.8 mph (Nickling W.G., Brazel A.J., 1984). Some of these intense 

dust storms that occur in the Phoenix area could potentially be classified as “haboobs”: events 

that are caused by powerful downdrafts from thunderstorms and have the potential to create solid 

walls of advancing dust (Idso, 1972). 

There are a number of different definitions of “dust storms” based on different levels of reduced 

visibility. The National Weather Service issues a dust storm advisory when visibility drops 

below 1 mile and a dust storm warning when visibility is less than 1⁄4 mile. NWS further states 

that “typically, Blowing Dust Advisories are issued for widespread winds that may produce 

localized areas of blowing dust” and “dust storms can occur with widespread winds, or may be 

associated with thunderstorm outflow.” Table 2 lists all days in 2008 that had reports of blowing 

dust or dust storms at Phoenix NWS stations. 

Similarly, Nickling and Brazel (1984) also use a reduced visibility of 1 mile as a cut-off point for 

dust storm classification. This criterion was chosen to be the most representative of the 

conditions that can be attributed to dust storms in Arizona. Earlier research suggests that 

reduced visibility less than 7 miles constitutes dust storm classification (Orgill, Sehmel, 1976). 

Table 1 shows the visibility recorded at Goodyear Airport during the event days in question 

compared to the various dust storm definitions discussed above. 

In 2008, the Phoenix area experienced numerous occurrences of thunderstorm activity and 

elevated winds. A detailed account of these events is displayed in table 3 and is available in the 

NWS report “Storm Data and Unusual Weather Phenomena”. Four of the events are described 

as dust storms.42 For example, an event occurring on September 11 was described as “a 

 

41 Intense dust storms (IDS) correspond with visibility ≤ 1 mile, while moderate-to-weak dust storms (MWDS) 

correspond to visibility >1 mile but ≤ 7 miles. 

42 The events described by the NWS as dust storms occurred on May 15, July 1, July 10, and September 9. The 

meteorological events that occurred on the days of concern for the present analysis (March 14, April 30, May 21, 

and June 4) were not characterized as such. 
41

spectacular dust storm moved across west-central and central Maricopa County, including the 

Greater Phoenix area. Dust moved southwest to northeast, with winds typically 30-50 mph 

accompanying the blowing dust. A 3-mile stretch of power poles was blown down along old 

U.S. highway 80 south of Buckeye and north of Gila Bend (615 PM)”. 

Table 1: Dust Storm Comparison

Event Date

3.14.08 4.30.08 5.21.08 6.4.08

Event Visibility 

(miles)43

10 7 7 20

0.2544 N N N N

1

45 N N N N

7

46 N N N N

Figure 1 further illustrates how PM10 concentrations can be affected during these events. The 

September 9 dust storm originated in the southwest and moved through Phoenix, heading 

northeast. Wind speeds throughout the Phoenix area reached 30 mph, with 40 mph wind gusts 

reported at the NWS Luke Air Force Base station. Wind direction during the event was 

predominately from the west/southwest. The visibility during the event dropped below 1⁄4 mile at 

one station and remained below 5 miles for other stations in the area. Figure 1 shows the west to 

east movement of the dust storm and its effect on the PM10 monitoring stations in the Phoenix 

area. PM10 concentrations spike first at the Buckeye monitor at 1700 hrs and the rest of the 

central Phoenix area experiences elevated PM10 concentrations at 1800 hrs. 

Figure 1: PM10 concentrations 9/11/08

 

43 Visibility during periods of elevated wind speed and elevated PM10 at West 43rd

44 NWS Warning 

45 NWS Advisory & Nickling and Brazel 

46 Orgill and Sehmel 
42

Table 2: National Weather Service Significant Weather Types 2008

Date Goodyear

Luke 

AFB Glendale

Sky 

Harbor Chandler

Williams 

Gateway

Falcon 

Field Scottsdale

Deer 

Valley

3.14.08

4.16.08 BLDU

4.30.08

5.15.08 BLDU SQ BLDU

5.21.08 BLDU

6.4.08 BLDU

BLDU

DU

6.5.08 HZ

BLDU

HZ BLDU HZ HZ HZ

7.1.08 BLDU BLDU

HZ

SQ

7.3.08 BLDU

7.4.08 BLDU HZ

7.10.08 BLDU

7.13.08 BLDU

7.26.08 BLDU

8.7.08 BLDU BLDU BLDU

8.14.08 BLDU BLDU BLDU BLDU BLDU

8.25.08 BLDU

BLDU

SQ BLDU BLDU

8.30.08 BLDU BLDU

9.8.08 BLDU

9.11.08 DS

BLDU 

HZ BLDU

BLDU

HZ

SQ BLDU DS HZ BLDU HZ SQ HZ

9.26.08 BLDU

9.27.08 BLDU

11.9.08 BLDU HZ BLDU BLDU BLDU HZ

12.13.08 BLDU

Notes: BLDU - Blowing Dust DS - Dust Storm DU - Dust HZ - Haze SQ – Squall47

Table 3: NWS Storm Data and Unusual Weather Phenomena Reports 2008

Date Location48 Event Time Description

3.14.08 NONE

4.16.08 NONE

4.30.08 NONE

5.15.08

AZZ028-

Central 

Deserts 

Dust Storm 1515-

1640

Strong and gusty winds uprooted trees in Eastern 

Chandler. Dense blowing dust with low visibility 

was reported at Gateway airport

Maricopa 

County

Thunderstorm 

Wind

1535-

1610

Portions of eastern Maricopa county and Pinal county 

received gusty winds from a line of thunderstorms 

that moved rapidly toward the south.

5.21.08 NONE

 

47 NWS definition: sudden onset of a strong wind with increase of at least 16 knots and sustained at 22 knots or 

more for at least one minute 

48 See Figure 2 for NWS Forecast Areas 
43

6.4.08 NONE

6.5.08 NONE

7.1.08

AZZ028-

Central 

Deserts 

Dust Storm 1740-

1800

Low visibility due to blowing dust resulted from 

strong winds from nearby thunderstorms. Winds 

were generally in the 30 to 40 mph range with reports 

of blowing dust in the Phoenix East Valley Late in

the afternoon and early afternoon.

7.3.08

Maricopa 

County

Funnel Cloud 2140 Pilot reported sighting a brief funnel cloud. 

Thunderstorms were triggered by an old outflow 

boundary. The associated peak wind gusts were 28 

knots at Sky Harbor and 39 knots at Scottsdale airport

Pinal County Thunderstorm 

Wind

1630-

1631

Several trees were uprooted at Saddlebrook

7.4.08

Maricopa 

County

Thunderstorm 

Wind

1900-

2000

Scottsdale airport recorded peak winds of 53 mph 

during thunderstorms. Winds at Sky Harbor airport 

reached as high as 39 mph and some tents at the 

fireworks display were blown down

7.10.08

AZZ026 -

Southwestern 

Deserts

Dust Storm 1540-

1700

Strong winds from nearby thunderstorms resulted in 

dense blowing dust.

Maricopa 

County

Thunderstorm 

Wind

1830-

2045

Winds caused power outages and property damage 

due to microburst winds as high as 65 mph. Winds 

blew down a tree near 78th Street and McDonald 

which damaged a covered parking structure.

Maricopa 

County

Thunderstorm 

Wind

1915-

1925

Winds speed measured at 68 mph at Bush Highway 

and Usery Pass Road. According to radar, these 

storms were moving west at about 35 mph.

Pinal County Thunderstorm 

Wind

1927-

1940

Spotters in two locations in Apache Junction had 

gusts to 67 and 89 mph

7.13.08

Maricopa 

County

Thunderstorm 

Wind

1600-

1630

Winds from a microburst blew down about 25 trees 

and damaged light poles at Mesa Community 

College. A security officer was slightly injured when 

the strong winds blew him from his golf cart.

7.26.08

Maricopa 

County

Thunderstorm 

Wind

1830 Power poles and trees were reported down at 

Chandler Heights and Greenfield roads, as well as 

Ocotillo and Higley and at Ocotillo and Power roads. 

Brief strong winds caused isolated damage to parts of 

the Southeast Valley

8.7.08

Maricopa 

County

Thunderstorm 

Wind

1940-

1950

Power poles and lines reported blown down. As 

many as 70 poles were down in the Buckeye area 

alone. Note: the estimated wind gust of 60 knots is 

equivalent to 69 mph.

Maricopa 

County

Thunderstorm 

Wind

2017-

2020

Power poles down in Central Phoenix. Note: the 

estimated wind gust of 60 knots is equivalent to 69 

mph.

Maricopa 

County

Thunderstorm 

Wind

2020-

2025

Large branches blown from trees. Note: the 

estimated wind gust of 55 knots is equivalent to 63 

mph.

8.14.08 Pinal County Thunderstorm 

Wind

1810-

1850

Strong winds reported by spotter. Note: the estimated 

wind gust of 52 knots is equivalent to 60 mph.
44

Maricopa 

County

Thunderstorm 

Wind

1812-

1852

Several crashes on the Loop 202 were blamed on 

strong winds and rain. Power outages were reported 

after winds and rain moved through the East Valley. 

SRP reported about 3,000 customers were left 

without electricity...and APS reported 2,000 

customers without power. Note: the estimated wind 

gust of 52 knots is equivalent to 60 mph.

Maricopa 

County

Thunderstorm 

Wind

1838 Strong winds reported at Brown and Mesa. Trees 

were damaged. Note: the estimated wind gust of 50 

knots is equivalent to 58 mph.

Maricopa 

County

Thunderstorm 

Wind

1905 Winds at Chandler Airport reached 50 knots as severe 

thunderstorms moved toward the west. The southern 

and central portions of Arizona were very moist and 

unstable. Storms developed and moved toward the 

southwest and strong winds kicked up widespread 

areas of blowing dust. A Severe Thunderstorm 

Watch was in effect for much of the evening. Note: 

the measured wind gust of 50 knots is equivalent to 

58 mph.

8.25.08

Maricopa 

County

Thunderstorm 

Wind

1510-

1526

Microburst winds hit Chandler airport and flipped at 

least two planes. Winds also damaged a fence and 

other property. Northeast winds peaked at 67 mph at 

3:25 pm. Thunderstorm winds over 80 mph damaged 

planes at Chandler Municipal Airport. Strong winds 

also blew down tress and damaged some homes in the 

Chandler area. Dense blowing dust was also 

reported. Note: the measured wind gust of 58 knots 

is equivalent to 67 mph.

8.30.08 NONE

9.8.08 NONE

9.11.08

Maricopa 

County

Thunderstorm 

Wind

1710-

1720

Shingles were blown off homes, and a few trees were 

uprooted.

Maricopa 

County

Thunderstorm 

Wind

1734-

1742

Winds estimated to reach as high as 60 mph along 

with visibility to less than a 1/4 mile in blowing dust.

Maricopa 

County

Thunderstorm 

Wind

1740 Flood control district sensor measured a gust to 60 

mph.

AZZ023-

Greater 

Phoenix 

Area

Dust Storm 1745-

1815

Thunderstorms moved steadily toward the northeast 

during afternoon hours. As a result, locally heavy 

rain, strong winds and very low visibility due to dust 

and sand moved across the deserts

Maricopa 

County

Thunderstorm 

Wind

1815-

1840

According to Arizona Public Service, 48 power poles 

across a distance of three miles were blown down 

along Old Highway 80 between Buckeye and Gila 

Bend. Winds were measured up to 56 mph on the 

Palo Verde Nuclear Generating Station tower.

9.26.08 NONE

9.27.08 NONE

11.9.08 NONE

12.13.08 NONE
45

Figure 2: NWS Forecast Areas
46

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