1 

## Review of Exceptional Event Request

Maricopa County, AZ

24

- Hour PM 10 March 14, 2008 April 30, 2008 May 21, 2008
June 4, 2008

U

. S

. Environmental Protection Agency Region 9

May 12 , 20102



### 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 PM 10

................................ ............................10

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

5.1.3 Review of 24-Hour PM 10 Data ................................................................................11

5.1.4 Review of Hourly PM 10 and Meteorological Data ..................................................12

5.1.5 Review of 5-Min PM 10 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 PM 10

................................ ............................16

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

5.2.3 Review of 24-hour PM 10 Data .................................................................................17

5.2.4 Review of Hourly PM 10 and Meteorological Data ..................................................19

5.2.5 Review of 5-Min PM 10 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 PM 10

................................ ............................21

5.3.2 Visi bility ..................................................................................................................22

5.3.3 Review of 24-Hour PM 10 Data ................................................................................22

5.3.4 Review of Hourly PM 10 and Meteorological Data ..................................................23

5.2.5 Review of 5-Min PM 10 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 C orrelation between Wind Speed and PM 10

................................ ............................26

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

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

5.4.4 Review of 24-Hour PM 10 Data ................................................................................27

5.4.5 Review of Hourly PM 10 and Meteorological Data ..................................................28

5.4.6 Re view of 5-Min PM 10 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 ....................................................................................333

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

9.0 Conclusion ..........................................................................................................................34 Appe ndix A 36 Appe ndix B 40

R eferences 46

T ables

Table 1: Table 1: West 43 rd 2008 PM 10 Exceedances Table 2: March 14, 2008 24-Hour PM 10 Ta ble 3: April 30, 2008 24-Hour PM 10 Ta ble 4: May 21, 2008 24 Hour PM 10 Table 5: June 4, 2008 24-Hour PM 10 Table 6 : Table 6: ADEQ Historical Distribution

Figures

Figure 1: March 1 4, 2008 24-Hour PM 10 Figure 2: March 14, 2008 Hourly PM 10 Figure 3: March 14, 2008 5-Min PM 10 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 PM 10 Figure 8: April 30, 2008 Hourly PM 10 Figure 9: April 30, 2008 5-Min PM 10 and Wind Speed Figure 10: Non-Exceedance Day Comparison Figure 11: May 21, 2008 24-Hour PM 10 Figure 12: May 21, 2008 Maricopa County Hourly PM 10 Figure 13: May 21, 2008 5-Min PM 10 and Wind Speed Figure 14: Non-Exceedance Day Comparison Figure 15: June 4, 2008 24-Hour PM 10 Figure 16: June 4 & 5, 2008 Hourly PM 10 Figure 17: June 4 & 5, 2008 5-Min PM 10 and Wind Speed Figure 18: Non-Exceedance Day Comparison4



### 1.0 Introduction

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

e vidence 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 43 rd monitoring site located in southwestern Phoenix. On November 17, 2009 ADEQ submitted final demonstrations

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

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

exceedances of the 24-hour PM 10 NAAQS in 2008 at the West 43 rd monitoring s ite 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 (PM 10 ) 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 (PM 10 ) 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 (PM 10 ) Concentration Events in the Phoenix and

Yuma Areas on June 4 th , 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 43 rd

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 (PM 10 ) 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 (PM 10 ) Concentration Events in the Phoenix Area on June 4, 2008 (June 4 Assessment); The Impact of Exceptional Events “Unusual Winds” on PM 10 Concentrations in Arizona (Unusual Winds White Paper); High Wind Exceptional Events and Control Measures for PM 10 Ar eas (Controls White Paper); and

DRAFT – Supplemental Report: Assessment of Qualification for Treatment under the Federal Exceptional Events Rule: High Particulate (PM 10 ) 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 PM 10 monitoring sites operating in Maricopa County, ten of which

use continuous PM 10 analyzers that produce hourly data. During 2008, the West 43 rd monitoring site, which measures PM 10 with a continuous analyzer, 5 measured five exceedances of the 24-

hour PM 10 NAAQS , four of which are reviewed in this document. 6 ADEQ has claimed that the

e xceedances at the West 43 rd 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

we re 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 43 rd 2008 PM 10 Excee dances

Date PM 10 (ug/m 3 ) 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 43 rd , 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

e vent 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

a ny 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 p ro cedural requirements. 40 CFR §50.14(c)(2)(iii)

re quires 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 mont hs 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

a ttainment 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 re spect to these requirements.



### 4.0 Criteria S et 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

c oncentration 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 de termination of whether a particular event wa s “not reasonably controllable or preventable” depends on the specific facts and circumstances surrounding the event. Therefore, EPA

a ddress es 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 43 rd monitoring site.. Although the June 4 DSR identifies that the alluvial channels

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

at West 43 rd 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

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

sourc es 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.” A s d escribed 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 t he event is “unusual for the affected area during the time of year that the event occurred, ”

a nd

, 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 discuss ed 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,

c onstruction, 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

c ontribute to elevated PM 10 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 43 rd 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

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

c omplexity 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. ” AD EQ’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 43 rd site makes it difficult to determine the contributing role of human activity to the exceedances at the West 43 rd
site, particularly where it is known that commercial activities such as agriculture, sand and grave l 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 43 rd

, Dur ango Complex, and Higley). The use of a complete year of data in this situation rather than the season during which the events occurred likely bias es the statistical analysis low. The

P hoenix 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 43 rd
monitoring station, this standard would correspond to sustained hourly wind speeds greater than 10 mph and wind gusts 15 greater than 20 mph. ADEQ’s documentation did not provide any

spec ific 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 95 th percentile. While wind

spee ds above the 95 th 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 ha s the ability to create windblown dust.” ADEQ’s definition could be interpreted to treat all

windblown PM 10 a s 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

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

surf ace, 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 2 007 -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 43 rd si te

, 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 43 rd 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

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

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

c ivilian 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

J une 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 43 rd monitoring site, but the specific locations of these facilities were not

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

sourc es 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 §5 0.14(a)(2); 40 CFR §50.14(c)(3)(iii). To address this element for “high wind events,” such as

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

the PM 10 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

tec hnical review also considered additional data regarding wind speed and direction, PM 10 concentration, and visibility. 17

As a preliminary matter relevant to this issue, EPA notes that ADEQ’s limited analysis of the potential sourc es that might have contributed to the exceedances at the West 43 rd site (sections

17 Appendix A contains pollution roses based on % total PM 10 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

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

different sets of PM 10 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

e vent. 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 PM 10

The March 14 Assessment included tabular hourly and maximum wind speed and PM 10 data for five monitoring sites in the Phoenix area: West 43 rd , Durango Complex, West Phoenix, Coyote Lakes, and Central Phoenix. ADEQ also included meteorological data from three National

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

Harbor. 19 EPA notes that ADEQ did not provide hourly PM 10 data from the other four

continuous PM 10 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

spee ds and PM 10 concentrations at the West 43 rd , Dur ango Complex, Greenwood, and South Phoenix monitoring sites. 20 The graphs show that hourly PM 10 concentrations increase with an increase in maximum recorded wind speed at the West 43 rd site, but not at the other three monitoring sites. In fact, the graphs show that the maximum wind speeds at the Durango

C omplex site were higher than those measured at the West 43 rd site, but the Durango Complex site experienced significantly lower PM 10 values during periods of elevated wind speed. These

da ta suggest that the elevated PM 10 concentrations at the West 43 rd 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

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

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

18 Table 1 in Appendix A identifies the PM 10 and meteorological stations ADEQ used in their analysis of the 2008 exceptional events in questio n.

19 ADEQ also include d 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 PM 10 values

.11

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

sourc es, and PM 10 concentrations at the West 43 rd 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 e vent and the measured exceedance at the West 43 rd site. The visibility at Goodyear Airport before the event ranged from 60 to 20 statute miles, while during the time of the elevated

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

in t he 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 sig nificantly reduced; thus, this information does not significantly contribute to establishing a clear causal relationship. 21

5.1. 3 Review of 24-Hour PM 10 Data

The 24-hour PM 10 concentrations measured on March 14 at the West 43 rd and surrounding sites are listed in Table 2 and shown in Figure 1. On this day, the West 43 rd site was the only site in

the Phoenix area to exceed the 24-hour PM 10 standard. Furthermore, PM 10 concentrations at the

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

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

Table 2 : March 14 , 2008 24

- Hour PM 10 Site Name PM 10 (ug/m3) Site Name P M 10 (ug/m3) Buckeye 23 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 PM 10 data for this site was not included in Arizona’s Assessment.12

Figure 1 : March 1 4, 2008 24

- Hour PM 10

5.1. 4 Review of Hourly PM 10 and Meteorological Data

The hourly PM 10 data are shown in Figure 2. As early at 0500 hrs, the West 43 rd site began to experience an increase in PM 10 concentration that was not characteristic of the other nine monitoring sites in the Phoenix area. From 0500 to 1000 hrs, the hourly PM 10 values increased from 150 μg/m 3 to 360 μg/m 3

. During these hours the hourly wind speeds throughout the Phoenix area remained below 9 mph, which suggests these elevated concentrations were not

dr iven by high wind, but by some other mechanism. Thus, the elevated PM 10 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 (a ccompanied by a 29 mph gust); while an increase in hourly wind speed from 12 to 15.9 mph

wa s recorded at the West 43 rd site. At 1100 hrs, the PM 10 concentration at the West 43 rd site also rose from 355 to 1051 μg/m 3 and continued to increase over the next two hours to a maximum hourly concentration of 1286 μg/m 3

. While the values at some of the other sites in the area increased over the same time period, the values at the West 43 rd 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 43 rd
site, one would expect to see greater consistency in the PM 10 concentrations if a regional high wind event was occurring. It is also worth noting that the West 43 rd 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/m 3 at 1300 hrs,

while West 43 rd ex ceeded this level at 0900 hrs. The inconsistency in the PM 10 concentrations during the period from 1100 to 1400 hrs and the relatively low wind speeds in the morning hours suggest that the West 43 rd 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 PM 10

5.1. 5 Review of 5-Min PM 10 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 43 rd monitor consistently measured much higher PM 10 concentrations than other locations. Figure 3 shows the 5- min PM 10 and wind speed data from West 43 rd and Durango Complex monitoring sites. These monitors are located only 2 miles apart, yet there

se ems to be a considerable difference in the relationship between PM 10 and wind speed on March

14

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

min PM 10 concentrations at the West 43 rd site were more than five times those measured at Durango Complex. These data provide further evidence that the claimed regional high wind

e vent only affected PM 10 concentrations at the West 43 rd site and the elevated PM 10 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 PM 10 and W ind Speed

5.1

. 6 Days with Similar Meteorological Conditions

The following discussion emphasizes that meteorological conditions in upwind locations do not always affect on PM 10 concentrations at the West 43 rd site. The NWS station at Goodyear Airport is located approximately 13 miles to the west of the West 43 rd monitoring site and serves as the closest location with readily available meteorological data for the area directly to the west

of the West 43 rd monitoring site. 24

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

da ys, 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 PM 10 NAAQS

. The following analysis compares hourly PM 10 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 Ma rch 14, the West 43 rd monitor measured elevated PM 10 concentrations of 1051 μg /m 3 and 1270 μg/m 3 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 G oodyear 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 Mar ch 2 correspond ed to an increase in PM 10 from 29 μg/m 3 to 177 μg/m 3 at the West 43 rd
monitoring site. This increase in PM 10 is relat ively minor compared to PM 10 concentrations on measured on March 14, which reached at maximum of 1270 μg/m 3

.

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 speed s of 16 to 17 mph with wind gusts of 29 to 32 mph from the

SSW (2 00°) and the WSW (240°) were recorded at Goodyear Airport for a period of three hours. The corresponding PM 10 concentrations at West 43 rd remained below 130 μg/m 3 for the entire day. On the following day, March 30, wind speeds of 25 to 29 mph from the SW (230°-240°)

we re recorded at Goodyear, which corresponded to a spike in PM 10 concentration at the West

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

a nd 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 PM 10 spike on March 30 was shorter in duration and much smaller in magnitude.

Figure 5 : Non

- Exceedance Day Comparison16

Figure 6 : Non

- Exceedance Day Comparison

These examples illustrate how elevated wind speeds in upwind areas are related to elevated PM 10

c oncentrations on occasion, but the magnitude of PM 10 concentrations measured at the West 43 rd
site seem to be associated with factors in addition to wind speed. Also, March 2, March 29, and

Mar ch 30 were weekend days, which also indicates that elevated wind speeds are not necessarily the primary factor in creating elevated PM 10 concentrations at the West 43 r d 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

e xtent of PM 10 during this day was isolated and not regional in nature. The data also show

diff erences in the measured PM 10 concentrations at the West 43 rd 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 PM 10

The April 30 Assessment included hourly and maximum wind speed and PM 10 data for five sites

in t he Phoenix area: West 43 rd , 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 PM 10 data from the other four continuous PM 10 analyzers in the Phoenix area and did not include wind speed and direction data from numerous other

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

infor mation 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 PM 10 and meteorological data used in the April

30 assessment.

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

spee ds and PM 10 concentrations. The four graphs display data from the West 43 rd , Dur ango Complex, Greenwood, and South Phoenix monitoring sites. While the hourly PM 10

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

there is not a similar correlation between PM 10 and maximum wind speed at the other monitoring sites in the area. These facts suggest that the elevated PM 10 concentrations at West 43 rd 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

a rea. Unfortunately, there is not a significant discernable difference between the conditions preceding and during the event. Therefore, the photographs do not significantly contribute to

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

PM 10 concentrations at the West 43 rd 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 43 rd 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 throug hout the entire day: visibility at Glendale Airport remained at 20 miles and Sky Harbor

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

mi les . 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 PM 10 Data

The 24-hour PM 10 concentrations measured on April 30 at the West 43 rd and surrounding sites are listed in Table 3 and shown in Figure 7. On this day, the West 43 rd monitor was the only site

in the entire Phoenix area to violate the 24-hour PM 10 standard. 27 Furthermore, PM 10 concentrations at the West 43 rd 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 even t.18

Figure 7 : April 30, 2008 24

- Hour PM 10

Table 3 : April 30, 2008 24

- Hour PM 10

Site Name PM 10 (ug/m 3 ) Site Name PM 10 (ug/m 3 )

Buckeye* 28 68 Glendale (FRM) 29 * 43

West 43 rd 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 PM 10 data for these sites were not included in the Assessment.

29 PM 10 FRM samplers operate on a 1 in 6 day schedule.19

5.2. 4 Review of Hourly PM 10 and Meteorological Data

The hourly PM 1 0 data are shown in Figure 8

. As early as 1000 hrs, the West 43 rd site began to experience an increase in PM 10 concentration that was not characteristic of the other nine monitors in the Phoenix area. From 0900 to 1000 hrs the hourly PM 10 values at the West 43 rd
site increased from 85 μg/m 3 to 404 μg/m 3 , while PM 10 values at surrounding sites remained below 120 μg/m 3

. 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 (a ccompanied 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 43 rd site. At 1100 hrs, the PM 10 concentration at the West 43 rd site also rose from 404 μg/m 3 to the day’s maximum value of 1065 μg/m 3

.

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

43 rd site ranged from 5 to 10 times higher than other sites in the Phoenix area. For example, the major ity of the sites measured maximum PM 10 concentrations that were coincident with the

maximum PM 10 concentrations at the West 43 rd site, but all sites in Maricopa County measured

maximum PM 10 concentrations less than 220 μg/m 3 . Given that the Durango Complex, South Phoenix, Greenwood, and West Phoenix sites are located within approximately five miles of the

West 43 rd 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 43 rd 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 PM 10

5. 2

. 5 Review of 5-Min PM 10 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 43 rd monitor consistently measured much higher PM 10 concentrations than other locations. Figure 9 shows the 5- min PM 10 and wind speed data from West 43 rd and Durango Complex. These monitors are located only 2 miles apart, yet there seems to be a

c onsiderable difference in the relationship between PM 10 and wind speed on April 30. Both sites

e xperience similar wind speed levels, but during some periods of the day the 5-min PM 1020

concentrations at West 43 rd site were more than 9 times those measured at Durango Complex. The two highest 5-min PM 10 averages measured at the West 43 rd site were approximately 1920 and 1624 μg/m 3 , while PM 10 concentrations at Durango Complex during the same time period were 178 and 373 μg/m 3 , respectively. These data provide further evidence that the claimed

re gional high wind event only affected PM 10 concentrations at the West 43 rd site and the elevated

PM 10 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 PM 10 and Wind Speed

5

. 2

. 6 Review of Days with Similar Meteorological Conditions

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

those da ys, wind gusts exceeded 25 mph. The following analysis compares the hourly PM 10 da ta, wind speed, and wind gusts recorded at Goodyear Airport on April 30 with the same data

fr om a similar day in April.

On April 30, the West 43 rd monitor experienced elevated PM 10 concentrations of 404 μg/m 3 and 1065 μg/m 3 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

μ g/m 3 was observed during this period, but it is considerably lower than the PM 10 concentrations measured on the day the exceptional event is claimed to have occurred. This example illustrates

how e levated wind speeds in upwind areas are related to elevated PM 10 concentrations on occasion, but the magnitude of PM 10 concentrations measured at the West 43 rd 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

AD EQ’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

e xtent of PM 10 during this day was isolated and not regional in nature. The data also show

diff erences in the measured PM 10 concentrations at the West 43 rd 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

e vidence 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 PM 10

The May 21 Assessment included tabular hourly and maximum wind speed and PM 10 data for

five sites in the Phoenix area: West 43 rd , 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 PM 10 and meteorological data from the remaining five continuous PM 10

a nalyzers 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

PM 10 and meteorological data used in the May 21assessment.

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

spee ds and PM 10 concentrations. The three graphs display data from the West 43 rd , Dur ango Complex, and South Phoenix monitoring sites. While the hourly PM 10 concentrations significantly increase with an increase in maximum recorded wind speeds at the West 43 rd 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 43 rd site . W e 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 PM 10 and maximum wind speed at the other monitoring sites in the area. These facts suggest that the elevated PM 10 concentrations at West 43 rd 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.

P hotographs taken at 1330 hrs show evidence of reduced visibility and a potential regional event; however, PM 10 concentrations at the West 43 rd site began to increase at 0800 hrs. Photographs

we re provided for 0930, 1330, 1430, and 1530 hrs. Photographs were not submitted for the hours preceding the elevated PM 10 concentrations measured at the West 43 rd site. Therefore, the photog raphs do not significantly contribute to establishing a causal relationship between wind speed, potential contributing sources, and PM 10 concentrations at the West 43 rd 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

be fore the event ranged from 20 to 7 statute miles; visibilities of 7 miles were recorded at 1047, 1647, a nd 1747 hrs. Chandler Airport recorded observations of blowing dust (BLDU) at 1347

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

statio ns 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 PM 10 Data

The 24-hour PM 10 concentrations measured on May 21 at the West 43 rd and surrounding sites are listed in Table 4 and shown geographically in Figure 11. On this day, the West 43 rd monitor was the only site in the entire Phoenix area to violate the 24-hour PM 10 standard. Furthermore, PM 10 concentrations at West 43 rd 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 PM 10

Table 4 : May 21 , 2008 24

- Hour PM 10 Site Name PM 10 (ug/m3) Site Name PM 10 (ug/m3) Buckeye * 32 123 West PHX * 83

West 43 rd 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 PM 10 and Meteorological Data

The hourly PM 10 data for Maricopa County are shown in Figure 12

. The peak PM 10 concentration of 1207 μg/m 3 at 09 00 hrs measured at the West 43 rd site coincides with an

incr ease in wind speed from 11 to 22 mph, and a recorded wind gust of 2 8 mph at the Goodyear

32 24 -hour PM 10 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 43 rd monitoring

site. Similar to previously discussed events, the measured PM 10 concentrations at West 43 rd were more than 3.9 times the PM 10 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 PM 10 concentrations suggest that the West 43 rd 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 PM 10

5.2. 5 Review of 5-Min PM 10 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 43 rd monitor consistently measured much higher PM 10 concentrations than other locations. Figure 13 shows the 5- min PM 10 and wind speed data from the West 43 rd
and Durango Complex sites. These monitors are located only 2 miles apart, yet there seems to

be a considerable difference in the relationship between PM 10 and wind speed on May 21. Both sites experience similar wind speed levels, but during some periods of the day the 5-min PM 10 concentrations at West 43 rd site ranged from 3 -6 times higher than those measured at Durango Complex. The two highest 5-min PM 10 averages measured at the West 43 rd site were approximately 1837 and 1769 μg/m 3 , while PM 10 concentrations at Durango Complex during the same time period were 290 and 362 μg/m 3 , respectively. These data provide further evidence

that the c laimed regional high wind event only affected PM 10 concentrations at West 43 rd and the

elevated PM 10 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 1 3 : May 21 , 2008 5

- Min PM 10 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 PM 10 da ta, wind speed, and wind gusts on May 21 with the same data from a similar day in May.

On May 21, the West 43 rd monitor experienced elevated PM 10 concentrations of 518 μg/m 3 and

1207 μg/m 3 at 08 00 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

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

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

c orrespond to moderate hourly PM 10 values at the West 43 rd site. Hourly PM 10 concentrations on

Ma y 12 were considerably lower than the PM 10 concentrations measured on the day the exceptional event is claimed to have occurred; maximum PM 10 values on May 12 only reached 500 μg/m 3

. This example illustrates how elevated wind speeds in upwind areas are related to

elevated PM 10 concentrations on occasion, but the magnitude of PM 10 concentrations measured

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

Figure 1 4 : Non

- Exceedance Day Comparison26

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

e xtent of PM 10 during this day was isolated and not regional in nature. The data also show

diff erences in the measured PM 10 concentrations at the West 43 rd 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

c omprehensively 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

sit es in both the Phoenix area (Buckeye, Coyote Lakes, and West 43 rd 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

se ction, it is clear that the PM 10 concentrations at these sites are not correlated to those measured

at the West 43 rd 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 PM 10

The assessment included tabular wind speed and PM 10 data for five sites in the Phoenix area:

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

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

pr ovide tabular hourly PM 10 data from the remaining five continuous PM 10 an alyzers in the Phoenix area and did not include wind speed and direction data from numerous other

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

mete orological data used in the June 4 supporting documentation.

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

spee ds and PM 10 concentrations. The graphs show that, at the West 43 rd site, the hourly PM 10 concentrations increase with an increase in maximum recorded wind speeds at the West 43 rd site; however, there does not seem to be a similar correlation between PM 10 and maximum wind speed for the other monitoring sites in the area until later in the evening. These data suggest that

the e levated PM 10 concentrations in the morning and early afternoon hours at the West 43 rd 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 with27

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

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

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

0356 hr s 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 43 rd 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 dur ing 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

discusse d below, the West 43 rd monitoring site began measuring elevated PM 10 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.

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

event, 33 but it is important to note that PM 10 con centrations at the West 43 rd monitoring site began to increase at 1200 hrs. No photographs were submitted for this time period or for hours

pr eceding the elevated PM 10 concentrations. Therefore, the photographs do not significantly contribute to establishing a causal relationship between observed wind speeds, potential contributing sources, and PM 10 concentrations at the West 43 rd 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

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

obser ved at numerous NWS after 1800 hrs suggests a regional event may have occurred, it is important to note that PM 10 concentrations at the West 43 rd site began to increase at 1200 hrs: a time when visibility was between 10 to 20 miles.

5

. 4

. 4 Review of 24-Hour PM 10 Data

The 24-hour PM 10 concentrations measured on June 4 at the West 43 rd and surrounding sites are shown in Figure 15

. On this day, the West 43 rd monitor measured PM 10 concentrations that were

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

a nd Coyote Lakes sites, which recorded similar concentrations.

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

Figure 1 5 : June 4, 2008 24

- Hour PM 10

Table 5 : June 4 , 2008 24

- Hour PM 10 Site Name PM 10 (ug/m3) Site Name PM 10 (ug/m3) Buckeye 204 Greenwood * 97

West 43rd 194 West P HX * 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 PM 10 and Meteorological Data

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

c laimed 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 PM 10 data for this station was not included in the Assessment.29

discussed events, the West 43 rd site measured elevated PM 10 concentrations earlier and of a higher magnitude than other monitoring sites located nearby. For example, on the early afternoon of June 4, the West 43 rd monitor began measuring PM 10 concentrations ranging from 165 μg/m 3 to 645 μg/m 3 between 1200 and 1400 hrs, while all other monitors in the Phoenix area

re mained below 200 μg/m 3 for the same time period. The inconsistencies in these concentrations

sugg est that the West 43 rd site was most likely significantly influenced by local upwind sources and the claimed exceptional event was not regional in nature. ADEQ acknowledged that the

c oncurrent 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

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

short distance to the West 43 rd 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 PM 10 concentrations at 1600 hrs, followed by an increase in PM 10 at the Coyote Lakes

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

show elevated PM 10 from the regional event until 2200 hrs on June 4. While the West 43 rd

, B uckeye, 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

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

at West 43 rd 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, PM 10 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 PM 10 concentration that can be attributed to this evening event, the West 43 rd monitor began measuring high PM 10 concentrations well before the arrival of the “dust plume” described in the June 4 DSR. Furthermore, the arrival of the dust plum e began at around 2100 hrs and only contributed to approximately 11.3 percent of the total

PM 10 mass concentration for June 4. With such a small total contribution, if all PM 10

c oncentrations measured after 2100 hrs were completely removed from the data set, the PM 10 24

- hour average for June 4 would still be above the PM 10 24 -hour NAAQS. The majority of the

PM 10 mass was measured well before the arrival of the evening event described above and the high PM 10 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 1 6 : June 4 & 5, 2008 Hourly PM 10

5.4.6 Review of 5-Min PM 10 and Wind Speed Data

The 5 -min data also show that even though elevated wind speeds were measured at other nearby locations, the West 43 rd monitor consistently measured much higher PM 10 co ncentrations than other locations. Figure 18 shows the 5- min PM 10 and wind speed data from West 43 rd and Durango Complex:. these monitors are located only 2 miles apart, yet there seems to be a

c onsiderable difference in the relationship between PM 10 an d 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 PM 10 concentrations at West 43 rd site ranged from four

to ni ne times higher than those measured at Durango Complex. The two highest 5-min PM 10

a verages measured at the West 43 rd site were approximately 1475 and 975 μg/m 3 , while PM 10 concentrations at Durango Complex during the same time period were 153 and 264 μg/m 3

, respectively.

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

PM 10 concentrations at West 43 rd and the elevated PM 10 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 PM 10 unrelated to the
evening regional event Regional event31

Figure 1 7 : June 4 & 5 , 2008 5

- Min PM 10 and Wind Speed

5

. 4. 7 Review of Days with Similar Meteorological Conditions

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

those da ys, wind gusts exceeded 25 mph. The following analysis compares the hourly PM 10 da ta, wind speed, and wind gusts on June 4 with the same data from a similar day in May.

On June 4, the West 43 rd monitor experienced elevated PM 10 concentrations of 307 μg/m 3 and 644.9 μg/m 3 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

J une 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,

onl y correspond to a slight increase in hourly PM 10 values at the West 43 rd site. This example illustrates how elevated wind speeds in upwind areas are related to elevated PM 10 concentrations on occasion, but the magnitude of PM 10 concentrations measured at the West 43 rd site seem to be dependent on a number of different factors.

Figure 18 : Non

- Exceedance Day Comparison32

5 .4. 8 Summary of Clear Causal Relationship for June 4, 2008

The data show that the spatial extent of PM 10 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 43 rd monitor, which makes a causal relationship

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

PM 10 c oncentrations at the West 43 rd site and the remaining sites in the Phoenix area experiencing similar wind conditions.

AD EQ 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,

AD EQ explained that the exceedance at West 43 rd “was due to generally elevated PM 10 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

43 rd 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.

W hile there appears to be some component of the PM 10 that could be attributed to a regional dust storm event, the time series (Figures 1 6 and 1 7 ) indicate that the regional event did not influence

the measured PM 10 at the West 43 rd 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

de monstrate 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 PM 10 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 c omplete five year data set. Table 6 summarizes these data.

Table 6: ADEQ Historical Distrib ution

Exceedance Date PM 10 Concentration Seasonal Percentile Yearly Percentile

3.14.08 251 μg/m 3 < 99.5 < 99.5

4.30.08 172 μg/m 3 < 97.5 < 99.7

5.21.08 279 μg/m 3 < 99.5 < 99.5

6.4.08 194 μg/m 3 < 97.5 < 99.033

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

c oncentrations 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 N o 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

doe s 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

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

c alculates the 24-hour PM 10 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

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

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

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

PM 10 concentration calculated with remaining hours of the day. This is equivalent to assuming

ther e 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

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

ther e 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

Nove mber 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

doc umentation 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

c omment 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 43 rd 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

spee ds 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

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

those conditions affected the elevated PM 10 concentrations at the West 43 rd monitoring site. The major ity 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

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

elevated PM 10 concentrations resulted from controllable anthropogenic sources or natural desert

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

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

unc ontrolled anthropogenic sources. With little discussion of the meteorological conditions on

the e vent 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

e levated PM 10 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 43 rd

. 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 43 rd 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 PM 10 within Region 9 due to its proximity to a large cattle feedlot. Also, the

diff erences between the hourly PM 10 concentrations at West 43 rd 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 me asured exceedances at the West 43 rd site, the assessments did not explain these differences

in P M 10 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 PM 10 concentrations at the West 43 rd monitoring site. The additional documentation asserted that the

exceedance measured at the West 43 rd 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 technical35

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 r egional dust storm during the late night hours of June 4 was not significant and the exceedance was most likely driven by the elevated PM 10 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 PM 10 values seen at the Durango Complex and South Phoenix monitoring sites. The

doc umentation 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 PM 10 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

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

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

the w eight of available evidence, EPA does not concur that the March 14, April 30, May 21 and June 4, 2008 exceedances at the West 43 rd monitoring site should be treated as exceptional

even ts.36



### Appendix A

Table 1 : Meteo rological Data Used in ADEQ ’s Assessments 39

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

H IGLEY 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 GATEW AY 29 ESE

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

West 43 rd monitoring site.37

Table 2 : PM 10 Data Used in ADEQ ’s Assessments 40

Site Name Distance to

West 43rd Direction 24 Hour PM 10 Data Hourly PM 10 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

COYO TE 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 43 rd 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 NNE39

Figure 1: West 43 rd Pollution Roses % Total PM 10 Mass40



### 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

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

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

c onclude 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

a nthropogenic disturbances)”. For the years 1965 -1980, 80% of all intense dust storms 41 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 a re caused by powerful downdrafts from thunderstorms and have the potential to create solid

wa lls 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

be low 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

re duced 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

NW S 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

a ccompanying 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

0.25 44 N N N N

1 45 N N N N

7 46 N N N N

Figure 1 further illustrates how PM 10 concen trations can be affected during these events. The

S eptember 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

re ported 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

eas t movement of the dust storm and its effect on the PM 10 monitoring stations in the Phoenix

area. PM 10 concentrations spike first at the Buckeye monitor at 1700 hrs and the rest of the central Phoenix area experiences elevated PM 10 concentrations at 1800 h rs

.

Figure 1 : PM 10 concentrations 9/ 11 /08

43 Visibility during periods of elevated wind speed and elevated PM 10 at West 43 rd

44 NWS Warning

45 NWS Advisory & Nickling and Brazel

46 Orgill and Sehmel42

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 BL DU

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 BLD U

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

– Squall 47

Table 3 : NWS Storm Data and Unusual Weather Phenomena Reports 2008

D ate L ocation 48 E vent T ime D escription

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 G ateway 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 Areas43

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 airp ort 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 m easured 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 Thun derstorm

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 Mari copa

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 P ower 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 equiv alent 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 r eached 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 Seve re 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

. Win ds 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 b lowing 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.

AZZ02 3

- 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 Th understorm

Wind 1815

-

1840 According to A rizona 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 Statio n tower.

9.26.08 NONE

9.27.08 NONE

11.9.08 NONE

12.13.08 NONE45

Figure 2 : NWS Forecast Areas46



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