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Review of Exceptional 

Event Requests

Imperial County, CA 

24-hour PM10

• September 2, 2006 

• April 12, 2007 

• June 5, 2007 

US Environmental Protection Agency 

Region 9

December 22, 2009 
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Contents 

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

2.0 Summary of the Events .................................................................................................................. 4

3.0 Requirements of the Exceptional Events Rule .............................................................................. 6

4.0 Criteria Set Forth at 40 CFR 50.1(j) .............................................................................................. 7

4.1 Affects Air Quality ..................................................................................................................... 7

4.2 Not Reasonably Controllable or Preventable ............................................................................. 7

4.2.1 Sources That Need Reasonable Control ............................................................................. 7

4.2.2 Controls for Open Areas ..................................................................................................... 8

4.2.3 Controls for Agricultural Land ........................................................................................... 9

4.3 Natural Event ........................................................................................................................... 10

5.0 Clear Causal Relationship ............................................................................................................ 10

5.1 September 2, 2006 – Calexico Monitors .................................................................................. 11

5.1.1 High Wind ........................................................................................................................ 11

5.1.2 Emission Sources .............................................................................................................. 11

5.1.3 Transport ........................................................................................................................... 12

5.1.4 Comparison to Days with Similar Meteorological Conditions ......................................... 13

5.1.5 Comparison to Neighboring Days .................................................................................... 14

5.1.6 Consideration of Other Causes ......................................................................................... 14

5.1.7 Summary of Clear Causal Relationship for September 2, 2006, Calexico ....................... 15

5.2 September 2, 2006 – Westmorland Monitor ............................................................................ 15

5.2.1 High Wind ........................................................................................................................ 15

5.2.2 Emission Sources .............................................................................................................. 16

5.2.3 Transport ........................................................................................................................... 16

5.2.4 Comparison to Days with Similar Meteorological Conditions ......................................... 18

5.2.5 Comparison to Neighboring Days .................................................................................... 18

5.2.6 Consideration of Other Causes ......................................................................................... 18

5.2.7 Summary of Clear Causal Relationship for September 2, 2006, Westmorland ............... 18

5.3 April 12 and June 5, 2007 Events ............................................................................................ 19

5.3.1 High Wind ........................................................................................................................ 19

5.3.2 Emission Sources .............................................................................................................. 20

5.3.3 Transport ........................................................................................................................... 21

5.3.4 Comparison to Similar and Neighboring Days ................................................................. 22

5.3.5 Consideration of Other Causes ......................................................................................... 23

5.3.6 Summary of Clear Causal Relationship for April 12 and June 5, 2007 ........................... 25

6.0 Concentration in excess of normal historical fluctuations ........................................................... 25

6.1 September 2, 2006 Event – Calexico Monitors ....................................................................... 25

6.2 September 2, 2006 Event – Westmorland Monitor .................................................................. 26

6.3 April 12 and June 5, 2007 Events ............................................................................................ 26

7.0 No exceedance but for the event .................................................................................................. 27

7.1 September 2, 2006 Event – Calexico Monitors ....................................................................... 27

7.2 September 2, 2006 Event – Westmorland Monitor .................................................................. 28

7.3 April 12 and June 5, 2007 Events ............................................................................................ 28

8.0 Procedural requirements .............................................................................................................. 28

9.0 Conclusion ................................................................................................................................... 29

9.1 Summary of EPA’s Evaluation of the September 2, 2006 Event Involving the Calexico Monitors

 29

9.2 Summary for September 2, 2006 Event, Westmorland monitor .................................................. 29
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9.3 Summary for April 12 and June 5, 2007 Events, Multiple Monitors ........................................... 29

10.0 Citation of Exceptional Event Request Documentation .............................................................. 30

Tables 

Table 1. Claimed Exceptional Events .......................................................................................................... 5 

Table 2. September 2, 2006 24 Hour PM10 ................................................................................................................................................. 13 

Table 3. Percentiles of PM10 concentrations .............................................................................................. 28 

Figures 

Figure 1. Imperial County map with PM10 monitors ................................................................................... 5 

Figure 2. HYSPLIT back-trajectories for September 2, 2006 ................................................................... 12 

Figure 3. Imperial County map with PM10 monitors and Sep. 2, 2006 concentrations ............................. 13 

Figure 4. Winds at 6 pm, September 2, 2006 ............................................................................................ 17 

Figure 5. April 11 and 12, 2007 Wind Speed and Direction ..................................................................... 22 

Figure 6. April 11 and 12, 2007 Wind speed and PM10 ............................................................................ 22 

Figure 7. Satellite photograph of Imperial County, with PM10 monitors .................................................. 25 
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1.0 Introduction 

On March 22, 2007, EPA adopted a final rule, Treatment of Data Influenced by Exceptional Events1

(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 rule, EPA may exclude data from use 

in determinations of National Ambient Air Quality Standard (NAAQS) exceedances and violations if a 

state demonstrates that an “exceptional event” caused the exceedances. Before EPA can exclude data 

from these regulatory determinations, the state must flag the data in EPA’s AQS database and, after 

notice and opportunity for public comment, submit a demonstration to justify the exclusion. After 

considering the weight of evidence provided in the demonstration, EPA decides whether or not to 

concur with each flag. 

On May 21, 2009 the California Air Resources Board (ARB) submitted demonstrations for three “high 

wind” events that allegedly caused ten exceedances of the 24-hour PM10 standard at various monitors 

in Imperial County in 2006 and 2007. The demonstrations consist of the following support documents, 

prepared by ARB, the Imperial County Air Pollution Control District (ICAPCD), and ICAPCD’s 

contractor ENVIRON (see the last page for full citations and electronic file names). 

• Natural Event Documentation: Calexico and Westmorland, California - September 2, 2006 

(September NED) [dated January 30, 2009] 

• June 13, 2008 letter to Sean Hogan with enclosures, including: 

o Natural Event Documentation: Brawley and Westmorland, California - April 12, 2007, 

Imperial County Air Pollution Control District (Original April NED) [dated April 15, 

2008] 

• June 13, 2008 letter to Sean Hogan with enclosures, including: 

o Natural Event Documentation: Imperial County, California - June 5, 2007, Imperial 

County Air Pollution Control District (Original June NED) [dated April 15, 2008]) 

• Natural Event Documentation: Brawley and Westmorland, California - April 12, 2007 (April 

NED) [addendum to June 13, 2008 submittal; dated January 30, 2009] 

• Natural Event Documentation: Imperial County, California - June 5, 2007 (June NED) 

[addendum to June 13, 2008 submittal; dated January 30, 2009] 

This document sets forth the basis for EPA’s decision regarding the alleged exceptional events. 

2.0 Summary of the Events

ARB’s requests pertain to ten monitored exceedances of the 24-hour PM10 standard (150 μg/m3

), 

which it generally says were caused by factors such as high winds, extremely high heat, and lack of 

precipitation in the Imperial Valley. Figure 1 shows the monitor locations and Table 1 lists the 

exceedances reviewed in this document and provides an abbreviated description of the events that 

ARB believes caused each exceedance. 

 

1

 72 FR 13560. 
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Figure 1. Imperial County map with PM10 monitors 

Table 1. Claimed Exceptional Events 

Monitor PM10 Concentration 

(μg/m3

) Description of Event Claim 

September 2, 2006

Calexico – 

Grant St. 233 Exceedances caused by high winds from thunderstorm outflows, which entrained emissions from Mexicali and dust 

from unpaved roads and desert areas east and southeast of 

Mexicali. 

Calexico – 

Ethel St. 164 

Westmorland 167 

Exceedances caused by a combination of wind-generated 

PM10 from high winds in the southeast portion of Imperial 

County and Mexico, and from localized high winds from 

thunderstorm outflows north of Imperial County late in the 

day, causing a concentration “spike.” 
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Table 1 (continued) 

Monitor PM10 Concentration 

(μg/m3

) Description of Event Claim 

April 12, 2007 

Brawley 291 Exceedances caused by wind-blown dust from desert areas 

in the west part of Imperial County; associated with an 

increase in wind speed and a shift to a westerly direction. Westmorland 155 

June 5, 2007 

Calexico – 

Ethel St. 282 

Exceedances caused by wind-blown dust from desert areas 

in west part of County; associated with an increase in wind 

speed and a shift to a westerly direction. 

Brawley 281 

El Centro 200 

Westmorland 226 

Niland 162 

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: 

• affects air quality 

• is not reasonably controllable or preventable 

• is caused by human activity that is unlikely to recur at a particular location, or is a natural 

event; 

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

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

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

including background; 

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

The EER also has procedural requirements. 40 CFR §50.14(c)(2)(iii) requires that data claimed to be 

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

event be provided to EPA; both must occur by July 1 of the year following the event. In addition, 40 

CFR §50.14(c)(3)(i) requires that the State: 

• submit a demonstration to EPA within three years of the calendar quarter of the event or 12 

months prior to an EPA regulatory decision; 

• provide notice and opportunity for public comment; 

• submit any public comments along with the demonstration. 

The following sections evaluate ARB’s demonstration for the ten exceedances in question with respect 

to these requirements. 
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4.0 Criteria Set Forth at 40 CFR 50.1(j)

4.1 Affects Air Quality 

As stated in the preamble to the EER, the event in question is considered to have affected air quality if 

it can be shown that there is a clear causal relationship between the monitored exceedance and the 

event, and that the event is associated with a measured concentration in excess of normal historical 

fluctuations.2 These criteria as they apply to the Imperial County events are discussed in detail in 

sections 5 and 6 below. 

4.2 Not Reasonably Controllable or Preventable 

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

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

and other criteria of the EER on a case by case basis. Inherent in the language of this criterion is a 

requirement that the state demonstrate that anthropogenic sources contributing to the exceedance 

caused by the event were reasonably controlled.3

 

4.2.1 Sources That Need Reasonable Control 

In its documentation, ARB concludes that the 2006 Westmorland4 and 2007 events were not 

reasonably controllable or preventable because: 

1. The events were caused largely by windblown dust from uncontrollable sources, i.e., sources in 

Mexico and desert areas in the southeast portion of Imperial County for the 2006 Westmorland 

event, and the Anza Borrego Desert to the west for the 2007 events. 

2. Emissions from agriculture and other anthropogenic sources were small compared to the 

uncontrollable desert and Mexico sources, and were reasonably controlled by the Imperial 

Irrigation District's (IID) fallowing program, ICAPCD Regulation VIII, and other ICAPCD 

regulations. 

While ARB asserts that non-anthropogenic sources are responsible for most of the PM10 emissions 

during the events (e.g., ≈75% for the 2007 events),5

 it acknowledges that there are also anthropogenic 

 

2

 See 72 FR 13569. 

3

 For example, in applying the EER to high wind exceptional event requests for the San Joaquin Valley, EPA determined 

that the requirement for an event to be “not reasonably controllable or preventable” could be met by a demonstration from 

the State that, “despite reasonable and appropriate measures in place, the September 22, 2006 wind event caused the 

exceedance.” See 72 FR 49046, 49051 (August 27, 2007). 

4 On September 2, 2006, three Imperial County air monitoring stations, Calexico Ethel Street, Calexico Grant Street, and 

Westmorland 1st Street, recorded PM10 concentrations in excess of the 24-hour PM10 standard of 150 μg/m3. EPA addresses 

below only sources of PM10 emissions that were associated with the Westmorland exceedance and uses the term “2006 

Westmorland event” as a shorthand reference. For the Calexico monitors, as described below in section 5.1.2 and 

elsewhere, the emission source types and locations contributing to the exceedances are not clear, and therefore it is not 

known whether such sources have been reasonably controlled. Therefore ARB has not shown the event was “not 

reasonably controllable or preventable.”

5

 April NED and June NED for the 2007 events; p. 11 for both these documents. Note that the page and figure numbers are 

the same for the April and June demonstrations. 
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contributions (i.e., the remaining 25% for the 2007 events, and anthropogenic PM10 from the southeast 

quarter of the populated area of Imperial County for the 2006 Westmorland event).6 

Ideally, exceptional event requests would identify all non-de minimis anthropogenic sources that 

contributed to an exceedance and would then describe how each is reasonably controlled. ARB’s 

documentation for the 2006 Westmorland and for the 2007 events, however, does not specify which 

anthropogenic sources need reasonable controls. Rather, there are summary explanations that 

anthropogenic sources are reasonably controlled through Regulation VIII and other local programs.7

The only anthropogenic source discussed in any detail is agriculture. Specifically, the documentation 

for the 2007 events estimates that agricultural lands contribute 4-5% of total emissions during highwind events8

 and states that IID’s fallowing program provides reasonable controls for agricultural land 

not subject to Regulation VIII.9

 In addition to fallow fields for the 2007 events, ARB’s documentation 

implicitly acknowledges that open areas contributed to the 2006 Westmorland and 2007 exceedances, 

and thus need to have been reasonably controlled at the time of the 2006 and 2007 events. These two 

sources are discussed below. 

4.2.2 Controls for Open Areas 

The documentation for the 2007 events concludes that the dominant source of emissions that 

contributed to the exceptional events was natural windblown dust from the Anza Borrego Desert.10 

While the 2007 documentation suggests that all these emissions are non-anthropogenic, it does not 

address the large areas used by recreational off-highway vehicles (OHV) in and around the Anza 

Borrego Desert.11 These areas include, for example, Plaster City, Superstition Mountains, Arroyo 

Salado, Ocotillo Wells and areas around Salton City where OHV use is prohibited but does in fact 

occur.

12 OHV activity indirectly increases PM10 emissions by disturbing vegetation and surface crusts, 

leaving the surface less stable and more vulnerable to emissions during subsequent winds, and directly 

causes PM10 emissions by entraining dust through contact between tires and unpaved surfaces. 

Similarly, the documentation for the 2006 Westmorland event concludes that a dominant source of 

emissions that contributed to the Westmorland exceedance was PM10 from the desert areas to the east 

of Imperial Valley,13 but does not address the large portions of these desert areas that are also used by 

OHVs, including parts of the Imperial Sand Dunes.14

While neither the 2006 nor 2007 event documentation describes OHV contributions to open area PM10

emissions during the events, the 2009 PM10 SIP quantifies a large OHV contribution to the countywide average open area inventory. Table III.B.4 in the 2009 PM10 SIP (Appendix III.B, p. 8, as 

 

6

 September NED for the 2006 event, p. 9. 

7

 April and June NEDs, p. 13, and September NED, p. 18. 

8

 April and June NEDs, p. 11 

9

 The September NED does not discuss any controls in detail.

10 April and June NEDs, p. 6. 

11 April and June NEDs, Table 4, estimate that 1% of open area emissions come from lands disturbed by humans, but it is 

not clear if this refers to OHV activity, particularly since estimates of OHV-related emissions provided in the 2009 PM10

plan (see subsequent footnote and paragraph) represent much more than 1% of the open area inventory. 

12 2009 Imperial County State Implementation Plan for Particulate Matter Less Than 10 Microns in Aerodynamic 

Diameter, Draft Final, Prepared for ICAPCD by ENVIRON Corporation, July 10, 2009 (2009 PM10 SIP), figure III.B.1 

identifies Plaster City, Superstition Mountains, Arroyo Salado and Ocotillo Wells. Additional areas were identified in 

discussions between EPA, ICAPCD and ARB where OHV activity occurs illegally and is reflected in the Erratum Sheet for 

the 2009 Imperial PM10 SIP referenced below. 13 September NED, p. 9. 

14 2009 PM10 SIP, Figure III.B.1 identifies the Imperial Sand Dunes OHV area. 
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amended by the Erratum Sheet15) estimates that up to 22 tons per day16 (tpd) of wind-blown PM10 from 

these areas are emitted from over 250 square miles (164,000 acres) used by OHV. Table III.2 in 

Appendix III, page 6, projects an additional 1.34 tpd of emissions from non-wind emissions from OHV 

activity on U.S. Department of the Interior, Bureau of Land Management (BLM) roads. Additional 

emissions from OHVs directly entraining dust in open areas (i.e., not on established roads/paths) are 

not addressed in existing emission inventories for Imperial County. These estimates should be 

significantly higher during high wind events such as projected in Table 4 of the 2007 event 

documentation. 

The 2007 event documentation references ICAPCD's 2005 BACM Demonstration Report17 as the basis 

for concluding that reasonable measures were in place for fugitive dust sources, presumably including 

OHV activity.18 Because implementation of BACM is required in serious PM10 nonattainment areas 

such as Imperial County under CAA Section 189(b), it is appropriate to consider that level of control in 

evaluating whether reasonable controls are in place for purposes of the Exceptional Events Rule. 

However, neither the documentation for the exceptional event requests nor the 2005 BACM 

Demonstration Report provides any meaningful analysis of BACM or any other level of control for 

OHVs. EPA would expect such an analysis to include as a starting point for example, evaluation of 

EPA’s 1992 potential reasonably available control measures for OHVs,19 as well as OHV controls for 

other areas such as Arizona Revised Statute §49-457.03 which prohibits recreational OHVs in the 

Maricopa PM10 nonattainment area on PM10 high pollution advisory days and Clark County Air 

Quality Regulations, Section 90, which effectively restricts OHV activity on BLM and other open 

areas greater than 5,000 square feet within the Las Vegas PM10 nonattainment area.20

4.2.3 Controls for Agricultural Land 

The 2007 event documentation relies on IID's fallowing program21 as the basis for claiming that 

reasonable measures were in place for fallow fields which are not subject to ICAPCD’s Conservation 

Management Practices (CMP) Rule 806.22 However, there were approximately 32,000 fallow acres in 

Imperial County in 2007 that were not subject to either Rule 806 or IID’s program.23 This fallow land 

represents about 7% of the total area farmable24 in Imperial County, but it is likely to contribute a 

much larger percentage of the overall emissions since there are at least periodic crops or some other 

measure that reduces emissions on all other cropland. Nonetheless, EPA found no meaningful analysis 

 

15 Erratum Sheet for the 2009 Imperial County State Implementation Plan for Particulate Matter Less Than 10 Microns in 

Aerodynamic Diameter Draft Final (Erratum Sheet), distributed by July 31, 2009 email from Emmanuel Sanchez, 

ICAPCD, to ICAPCD PM10 SIP Working Group members. 16 22 tpd = 7.77 + 1.43 + 0.8 + 11.0 (12.3 revised by the Erratum Sheet) + 1.1 (1.5 revised by the Erratum Sheet). 

17 “Draft Final Technical Memorandum Regulation VIII BACM Analysis,” prepared for ICAPCD by ENVIRON, October 

2005.

18 April and June NEDs, p. 13. 

19 57 FR 13498, 23540 (April 16, 1992); 57 FR 18072 (April 28, 1992) 20 PM10 State Implementation Plan for Clark County, Adopted by Clark County Board of Commissioners, June 19, 2001, 

page 4-81. http://www.accessclarkcounty.com/depts/daqem/aq/planning/Pages/pm10sip2001.aspx. 21 IID’s program, which started in 2003 and will continue until 2017, compensates volunteer participants for fallowing. See 

“Imperial Irrigation District Fallowing Program Status Report,” February 19, 2008, on web page 

http://www.iid.com/Water/FallowingPrograms, direct link: http://www.iid.com/Media/Fallowing-Program-Summary-2003-

2008-Presentation.pdf. 

22 April and June NEDs, p. 13. 

23 Imperial Irrigation District Water Department Monthly Crop Acreage Report, June 13, 2007, pp. 10 and 26. 

24 For a breakdown of what is included in the total area farmable, see Imperial Irrigation District Water Department 

Monthly Crop Acreage Report, June 13, 2007, p. 26.
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of BACM or any other level of control for fallow land referenced or provided in the documentation for 

the exceptional event requests. EPA would expect such analysis to include, for example, evaluation of 

analogous controls in other areas, such as South Coast Air Quality Management District Rule 403, and 

recommendations from the USDA’s National Agronomy Manual to reduce wind erosion based on the 

following principles:25 

• Establishing and maintaining adequate vegetation or other land cover, 

• Reducing unsheltered distance along the wind erosion direction, 

• Producing and maintaining stable clods or aggregates on the land surface, and 

• Roughening the land with ridge and/or random roughness. 

 

4.3 Natural Event 

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

little or no direct causal role.” As discussed elsewhere (section 4.2 on controls, and sections 5.2.2 and 

5.3.2 on emission sources), there are anthropogenic sources which may play a role in these events. 

Nevertheless, as described in the preamble to the EER, high wind events may qualify as exceptional 

events. A high wind event with some anthropogenic contribution can qualify as a natural event if the 

wind speed is “unusual for the affected area during the time of year that the event occurred.”26 In 

addition, where the wind speed results in exceedances or violations, EPA will consider in its weight of 

evidence analysis whether the state has demonstrated that contributing anthropogenic sources “have 

been controlled to the extent possible through use of all reasonably available reasonable and 

appropriate measures.”27 As discussed below in section 5.1.1 and 5.2.1, for the 2006 events ARB 

provides no explicit argument about whether winds were statistically unusually high. For the 2007 

events, as discussed in section 5.3.1 ARB does provide evidence that the winds were statistically 

unusual.28 The other natural event criterion (exceedances despite all reasonable measures) is not 

satisfied because, as stated above, there are anthropogenic contributions to the 2006 Westmorland and 

2007 events for which the State has not demonstrated that all reasonable measures were in place. 

Therefore, ARB has not demonstrated that these events qualify as natural events under the rule. 

5.0 Clear Causal Relationship 

In order for EPA to concur with an exceptional event request, 40 CFR §50.14(c)(3)(iii) 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. The following 

sections evaluate the documentation on this criterion. 

 

25 National Agronomy Manual, USDA NRCS, October 2002, p. 502-17. The National Agronomy Manual also includes the 

following principle which does not generally seem to be applicable to Imperial County sources: “Reshape the land to 

reduce erosion on knolls where converging windflow causes increased velocity and shear stress.” 26 72 FR 13566. 27 72 FR 13566, n. 11. 

28 For the 2006 events, ARB provides a speculative argument that winds were high in an absolute sense upwind of the 

Calexico monitors, and direct evidence that they were moderately high at some distance from the Westmorland monitor. 

For the 2007 events, ARB provides evidence that the winds were high in an absolute sense. 
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5.1 September 2, 2006 – Calexico Monitors 

With regard to the September 2, 2006 Calexico monitors, the “event” is claimed to be high winds 

associated with thunderstorm outflows to the southeast, which generated dust emissions in Mexico. 

ARB claims that these dust emissions were transported to Calexico. ARB describes the weather 

conditions on September 2, 2006 as "monsoonal conditions," characterized by a late summer shift in 

the wind direction from westerly to southeasterly, bringing moisture from the Gulf of Mexico and 

leading to thunderstorms.29 Thunderstorms can have strong outflow winds that generate and entrain 

PM10 from eroded soil. 

ARB’s documentation is reviewed below in terms of six principal factors: 1) high winds in Mexico, 2) 

source areas in Mexico, 3) transport of emissions from Mexico to Calexico, 4) how September 2, 2006 

differs from meteorologically similar days, 5) how this day differs from neighboring days, and 6) the 

likelihood that local emissions caused the exceedances. 

5.1.1 High Wind 

Unusually high speed winds can be evidence that an exceptional event has occurred.30 The 

documentation does not have an explicit argument that the wind speeds were unusually high. For 

September 2, 2006 ARB does not argue that the wind speeds were high in the vicinity of the monitors. 

In fact, hourly wind speeds were generally low (less than 20 mph) in Imperial County that day except 

for an increase around 7 am and a single hour late in the day at the Palo Verde California Irrigation 

Management Information System (CIMIS) meteorological station in the northeast.31 Rather, ARB’s 

argument centers on observed thunderstorm activity that occurred in the general area,32 and the 

premise that thunderstorm cells are known to produce strong outflows. ARB goes on to note that there 

is a general increase in wind speed from west to east, which suggests that wind speeds may have been 

higher at unmeasured locations to the east and southeast of the populated areas in Imperial County and 

Mexicali.33 These potentially higher (but unconfirmed) winds are presumed to have caused the 

exceedances. However, without more direct evidence, ARB’s claim is speculative. Such speculation 

is not adequate to support a demonstration that there is a clear causal relationship between the 

measurement in question and the claimed events. 

5.1.2 Emission Sources 

Based on wind direction data, areas to the southeast are potential sources of the PM10 emissions that 

affected the Calexico monitors. ARB’s documentation notes the presence of desert land directly east 

of Mexicali, and the prevalence of unpaved roads in the broad area southeast of Mexicali.34 While 

EPA agrees these are potential sources, it is difficult to draw any definitive conclusions about their role 

in the September 2, 2006 event in the absence of a more detailed characterization of the potential 

sources than that provided in the documentation. 

 

29 September NED, p. 4. 

30 72 FR 13566 

31 September NED, Tables 1 and 2 at pp. 41-42, tables in Figures 18-20, pp. 33-35 

32 See, for example, September NED text, pp.4-5, satellite infrared imagery (Figure 1, p. 20), and weather forecasts 

(Attachment A, PDF pp. 70-71, Attachment B, PDF p. 73). 

33 September NED, pp. 9 and 15, and Figure 21, p. 36. 

34 September NED, p. 15 and Figure 22, p. 37; see also September NED, Figure 23, p. 38. 
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5.1.3 Transport 

The wind direction on the day of the events was consistently from the southeast.35 A series of 

HYSPLIT back-trajectories provided in Appendix B of ARB’s documentation are also consistent with 

transport from the south and southeast through most of the day. Figure 2 below is an example of one 

of the HYSPLIT model results. Other tables and vector maps provide similar information. It is 

therefore possible that emissions from these areas were transported to Calexico. 

However, this explanation does not account for the significant difference between the PM10

concentrations measured by the Calexico monitors and those directly across the border in Mexicali. 

The distances separating the Calexico monitors from the various monitors in Mexico range from 1 to 

12 km. EPA would generally expect a regional high wind event to have similar impacts on these 

monitors, but this was not observed on the day in question. The 24-hour PM10 concentrations 

measured on September 2, 2006 at the Calexico and surrounding monitors are listed in Table 2 and 

shown graphically on the map in Figure 3. While the Calexico monitors registered concentrations of 

164 μg/m3 and 233 μg/m3, the concentrations measured at the Mexicali monitors ranged from 45 - 67 

μg/m3 (September NED, Figure 13, p. 30), well below the NAAQS. 

One potential explanation for 

this discrepancy is that 

southerly winds in the 

afternoon at the UCSL and 

UCA meteorological stations 

southeast of Mexicali could 

have caused these emissions to 

head north, and then easterly 

winds in unmeasured locations 

could have brought them west 

to Calexico, bypassing 

Mexicali.36 However, this does 

not seem reasonable given the 

relatively low concentrations 

measured at the Mexicali 

Cobach and ITM monitors and 

their very short distance from 

Calexico. This scenario is also 

not supported by the HYSPLIT 

back-trajectories, which travel 

to the Calexico monitor from 

the south-southeast. The 

trajectories neither pass over 

locations to the east-southeast 

nor turn west toward Calexico. 

 Figure 2. HYSPLIT back-trajectories for September 2, 2006 

 

 

35 September NED, table in Figure 20, p. 35, and sequence of wind vector maps in Figures 3-8, pp. 22-27. 

36 September NED, p. 11, and Figure 22, p. 37.
- 13 - 

Table 2. September 2, 2006 24 Hour PM10

Country Site Name PM10 (μg/m3

)

United States 

Calexico – Ethel 164 

Calexico – Grant 233 

El Centro 146 

Brawley 127 

Westmorland 167 

Niland 99 

Mexico 

(all near Mexicali) 

Conalep 67 

Progreso 55 

ITM 67 

UABC 45 

Cobach 55 

Figure 3. Imperial County map with PM10 monitors and Sep. 2, 2006 concentrations 

5.1.4 Comparison to Days with Similar Meteorological Conditions 

Days in 2001-2007 with similar meteorological conditions were examined to assess how September 2, 

2006 differed from other days and to determine the likelihood that emissions from Mexicali impacted 

Calexico. Table 5 of the September NED lists 16 meteorologically similar days and the PM10
- 14 - 

concentrations at various monitors. The fact that these conditions are generally not associated with 

exceedances suggests that there is something unusual about September 2, 2006. (September NED, pp. 

12-13). Similarly, plots of PM10 versus wind speed (September NED, Figure 25, p. 40; discussed in 

text, p.16) also show that September 2, 2006 lies outside of the historical relationship, supporting the 

theory that it is causally different than typical days. Finally, ARB’s documentation also notes 

(September NED, p. 12) that concentrations in Mexicali are generally below average under these 

conditions, whereas those in Calexico are higher than average. This suggests that Mexicali generally 

has a larger impact on Calexico than vice versa, especially since Mexicali has greater emissions than 

Calexico resulting from less stringent emission controls. 

While the foregoing discussion generally supports the theory that September 2, 2006 was in some way 

atypical, it does not provide direct support for the required causal relationship. Indeed, if the 

conditions on September 2, 2006 were sufficient to cause an exceptional event as ARB claims, it is 

unclear why exceedances were not also recorded on the days with similar wind conditions. 

5.1.5 Comparison to Neighboring Days 

ARB compared PM10 concentrations measured on September 2, 2006 with hourly PM2.5 concentrations 

measured on September 1 through September 3, 2006.37 The data show that the neighboring days had 

lower winds and lower PM. These facts are consistent with ARB’s claim that the exceedances were 

caused by higher winds on September 2, even though PM2.5 is not the same as PM10 

5.1.6 Consideration of Other Causes 

ARB considered other potential causes of the exceedances based on the premise that elimination of 

alternative possibilities lends support to the proposed cause. Though the extent of the ICAPCD 

investigation cited by ARB is unclear, ICAPCD did not find any evidence of increased anthropogenic 

activity on September 2, 200638 and the documentation notes the lack of unusual entries in source 

inspection logs for that day.39 ARB also notes its expectation that anthropogenic activity should have 

been lower on September 2 than on the previous day, since September 2 was a Saturday.40 As a result, 

ARB argues that the differences in measured elevated PM10 concentrations on the consecutive days are 

not explained by differences in anthropogenic activity. The documentation also discusses the 

likelihood that the exceedances were caused by local sources as opposed to a large scale wind event. 

The documentation notes that all monitors showed elevated PM10 relative to the norm, which implies 

the cause was regional in nature, since a local cause would affect only certain monitors.41 Still, it is 

unclear why more exceedances were not measured on other days with similar meteorological 

conditions. 

In addition, once surface crusts have been disturbed, emissions can result from OHVs or fallow 

agricultural fields without there being direct anthropogenic activities. As noted in Section 4.2.2, OHV 

activity indirectly increases PM10 emissions by disturbing vegetation on surface crusts, leaving the 

surface less stable and more vulnerable to emissions during subsequent winds. Similarly, a fallow 

agricultural field can also be left in a condition that is vulnerable to wind erosion. Noting the absence 

 

37 September NED text at p. 7-8; hourly PM2.5 concentrations for the three days in figures 18-20, pp. 33-35. 38 September NED, p. 2. 

39 September NED, Attachment H, PDF p. 149.

40 September NED, p. 7 

41 September NED, pp. 12-14. 
- 15 - 

of increased anthropogenic activity on the day of the exceedance does not address previous 

anthropogenic activities that could have left surfaces more vulnerable to emissions during subsequent 

winds. 

5.1.7 Summary of Clear Causal Relationship for September 2, 2006, Calexico 

ARB makes a case that it is theoretically possible that high winds over dusty areas in the southeast 

could have generated PM10 that was transported to Calexico. However, ARB fails to show that high 

winds actually occurred in the area from which the emissions are claimed to have originated. ARB 

also fails to explain why other monitors in the alleged transport path did not register increased 

concentrations. The weight of evidence in this case is not sufficient to demonstrate the clear causal 

relationship required by the exceptional events rule. 

5.2 September 2, 2006 – Westmorland Monitor 

The principal discussion of the Westmorland exceedance and its causes is at pp. 8-11 of the September 

NED. In this case ARB claims that dust was generated by high speed winds from outflows associated 

with thunderstorm activity north of the County. Wind-driven dust emissions from southeastern 

Imperial County and from Mexico were also thought to contribute to the exceedance. Components of 

the demonstration discussed below include 1) high winds north of Imperial County and in Mexico; 2) 

source areas north of Imperial County, in Imperial County, and in Mexico; 3) transport of emissions 

from north of Imperial County and Mexico to Westmorland; 4) how September 2, 2006 differs from 

meteorologically similar days; 5) how this day differs from neighboring days; and 6) the likelihood 

that local emissions caused the exceedances. 

5.2.1 High Wind 

As discussed above for the Calexico exceedances, winds within Imperial County on September 2, 2006 

were below 20 mph, and generally from the southeasterly direction; wind speeds at Westmorland were 

10 mph or less. The documentation does not have an explicit argument that the wind speeds were 

unusually high. Also, no particular wind speed has been established as “high” for Imperial County 

(e.g. based on frequency of occurrence, entrainment speed threshold, or threshold speed for 

“overcoming” controls), For the Westmorland exceedance, ARB’s main argument centers on the 

collapse of thunderstorm cells, which could lead to strong localized downdrafts and outflows capable 

of generating dust. Infrared satellite imagery and weather forecasts establish that there was 

thunderstorm activity in the area. 

Short periods of moderately high and variable wind north of the County at 5 and 6 pm (depending on 

station) are consistent with the occurrence of thunderstorm outflows. At CIMIS meteorological 

stations north of the County, wind speeds reached 23.2 mph at Oasis (NW of the county), 22.9 mph at 

Blythe (NE of the County), and 27 mph at Palo Verde (NE corner of the County). Similar maximum 

speeds were observed at Thermal Airport and Blythe Airport, where gusts reached 29 and 47 mph, 

respectively. However, the 6 pm speed increase at Thermal Airport is not as pronounced as at the 

other stations. This data suggest it was possible that thunderstorm activity led to these moderately high 

winds at locations just outside or within the northern border of Imperial County around 5 or 6 pm. 
- 16 - 

5.2.2 Emission Sources 

As is the case for the Calexico exceedances, areas to the southeast are potential sources of wind-blown 

PM10 emissions that may have affected Westmorland. However, as previously discussed, these source 

areas are not well-characterized in ARB’s documentation. For the areas northwest and northeast of the 

County that are thought to be the emissions source for the 7 pm Westmorland concentration spike, the 

documentation characterizes these as “primarily non-populated areas” (September NED, pp. 3 and 17), 

with little other description. Presumably these are the desert areas north and northwest of Indio and 

Palm Springs, and areas east and west of Blythe. ARB’s documentation states that “anthropogenic 

sources from the southeast quarter of the populated area of Imperial County” contributed to the 

violation (September NED, p. 9), with little other description. The submittal contains little assessment 

of the relative contributions of anthropogenic and non-anthropogenic emissions in the potential source 

areas, which could provide evidence of a causal relationship. 

5.2.3 Transport 

The documentation uses wind speeds and directions at northern meteorological stations to make the 

case that high winds in those areas generated PM10 emissions which were transported to Westmorland. 

In particular the documentation presents the following evidence (September NED text, pp.10-11, and 

Tables 1 and 2, pp. 41-42): 

• As discussed above, wind speeds for some northern stations sharply increased at 5 or 6 pm, 

depending on the station. 

• At 5 pm, the wind at the Palo Verde station (northeast of the County) was 27 mph from the 

east-northeast, i.e., roughly toward Westmorland. Winds at Niland (north of Westmorland) 

were roughly consistent with this flow. Wind speeds were also elevated at Blythe, north of 

Palo Verde (though the wind direction is not toward Westmorland). 

• At 6 pm, the wind speed at Oasis (northwest of the County) suddenly increased to 23 mph. 

• At 6 - 7 pm, winds at both Oasis and the Salton Sea West CIMIS station (between Oasis and 

Westmorland) were from the northwest, i.e., toward Westmorland. 

• At 7 pm, a pronounced concentration "spike" over 600 μg/m3

 is present at Westmorland 

(September NED, Figure 14, p. 31). With winds at 20-25 mph, this spike is consistent with a 

1-2 hour travel time from the Oasis and Palo Verde stations, respectively 45 and 57 miles from 

Westmorland. 

The wind directions for 5 pm at Palo Verde and 6 pm at Oasis described above differ from the 

surrounding hours, but may be consistent with short-lived high wind with a direction different from 

the underlying flow, such as might be caused by thunderstorm outflow. The directions can be 

interpreted as consistent with the theory that dust was transported to Westmorland. 

However, the timing of the winds and concentration spikes are not entirely consistent with the distance 

from the Westmorland monitor. Figure 4 below shows winds at 6 pm, with predominantly 

southeasterly flow of moderate speed, but high speed flows in the north with varying directions. Most 

notably, the increased wind at Oasis toward Westmorland is simultaneous with the Westmorland 

concentration spike, rather than an hour or two before as one would expect based on the distance 

between the two locations. Further, in order for dust generated at Oasis to reach Westmorland one 

must assume the wind followed a straight line path over the 50 mile distance for two hours, despite the 

observed variability in speed and direction. Finally, Palo Verde experienced increased wind speed 
- 17 - 

before Oasis, which is inconsistent with the path of the storm from west to east. These discrepancies 

undermine the transport scenario proposed by ARB. 

There is additional evidence which contradicts ARB’s claim that dust was transported to Westmorland 

from the northeast or northwest. First, the wind direction at Westmorland itself was consistently from 

the southeast or east-southeast. HYSPLIT back-trajectories ending at Westmorland near the 7 pm high 

concentration hour are also inconsistent with transport from northern stations during the two hours in 

which high speed winds occurred.42 For the six-hour period preceding the Westmorland concentration 

spike at 7 pm, the back-trajectories are from the south; all but one are from the south even twelve hours 

before the concentration spike. 

Figure 4. Winds at 6 pm, September 2, 2006 

Finally, ARB presents satellite imagery to show that the times of elevated PM10 concentration at 

Indio/Palm Springs and Yuma correspond to the passage of the thunderstorm activity in each area 

(September NED, Figure 1, p. 20; see also Attachment B, PDF p. 73). The 5 pm satellite image does 

provide evidence of thunderstorm activity north of Imperial County. However, it does not provide 

 

42 September NED, Figure B.6b, p. B-6; also see Figure 3 above. 
- 18 - 

clear evidence of a causal relationship because the images are not taken frequently enough to compare 

them with the timing of the concentration spike. 

In summary, there is conflicting evidence on the transport of emissions from north of the County to the 

Westmorland monitor, which undermines the case for a clear causal relationship. 

5.2.4 Comparison to Days with Similar Meteorological Conditions 

ARB’s documentation contains little discussion of meteorologically similar days, but Table 8 

(September NED, p. 48) shows that on days in 2001-2007 when Westmorland experienced similar 

wind speeds and directions, PM10 concentrations at the various Imperial stations are well below what 

was observed on September 2, 2006. As previously discussed, while this suggests that September 2 

was in some way atypical, it does not provide direct support for the required causal relationship. 

5.2.5 Comparison to Neighboring Days 

As discussed above with respect to the Calexico monitors in section 5.1.5, wind speeds and PM2.5

concentrations were lower on September 1 and 3 than on September 2, 2006. These facts are 

consistent with the ARB’s claim that the exceedances were caused by higher winds on September 2, 

even though PM2.5 is not the same as PM10 

5.2.6 Consideration of Other Causes

As discussed above in the Calexico discussion, 5.1.6., ARB notes an absence of unusual activity that 

would lead to increased anthropogenic emissions on this day. This is supported by ICAPCD's 

investigation of the period, and the lack of unusual entries in source inspection logs.43 This evidence is 

consistent with ARB’s conclusion that the cause of the exceedance was not local; however, the extent 

of ICAPCD’s investigation is unclear and this evidence does not directly support the causal 

relationship. 

5.2.7 Summary of Clear Causal Relationship for September 2, 2006, Westmorland

In summary, ARB claims the exceedance was caused by a combination of wind-generated PM10 from 

high winds in the southeast portion of Imperial County and Mexico, and from localized high winds 

from thunderstorm outflows north of Imperial County late in the day, causing a concentration “spike.” 

While EPA does not dispute that thunderstorm activity occurred, the evidence provided is not entirely 

consistent with ARB’s claim that this thunderstorm activity caused the exceedance measured at the 

Westmorland monitor. In particular: 

• the increased wind at Oasis toward Westmorland is simultaneous with the Westmorland 

concentration spike, rather than an hour or two before as one would expect based on the 

distance between the two locations; and 

• HYSPLIT back-trajectories are inconsistent with the claim that wind-blown dust was 

transported from northern areas. 

In addition, the relative contributions of possible source areas in the northwest, northeast, east, and 

southeast are little examined. The weight of evidence does not demonstrate a clear causal relationship 

as required by the EER. 

 

43 September NED, p.2; see also September NED, Attachment H, PDF p. 149. 
- 19 - 

5.3 April 12 and June 5, 2007 Events 

ARB’s principal argument regarding a clear causal relationship for the 2007 events is that high hourly 

PM10 concentrations at multiple monitors are well correlated with high speed winds from the west, 

where there are large areas of natural desert. The correlation is supplemented by back-trajectories. 

The demonstrations assume that the source is desert areas to the west, since they are the dominant 

PM10 emission sources in the county-wide inventory. However, as discussed below, there are 

deficiencies in the inventory relied upon with respect to determining relative source contributions. 

These deficiencies are particularly significant in this case given the discussion above regarding the lack 

of controls on OHV activity and agricultural land. 

5.3.1 High Wind 

April 12, 2007

While no particular wind speed has been established as “high” for Imperial County (e.g. based on 

frequency of occurrence, entrainment speed threshold, or threshold speed for “overcoming” controls), 

ARB’s demonstration provides evidence that winds were indeed high during April 11-12, 2007. 

ICAPCD and the IID issued wind advisories for April 11 and 12, warning of 35 mph winds and gusts 

over 50 mph.44 A newspaper article noted a tractor-trailer was overturned and some trees were 

knocked down.45 Wind speeds reached 28 mph at Niland and 20 mph Westmorland.46 The CIMIS 

station at Calipatria/Mulberry east of Westmorland, and Meloland east of El Centro, as well as other 

stations, had similar high values.47 Maximum wind speeds at Borrego Springs (KCABORRE1) and 

Julian (KCAJULIA2) stations, just north and just west of the Anza Borrego Desert, were 30 and 35 

mph, respectively, with gusts to 45 mph at both.48

In addition, the demonstration compares the maximum wind speeds at Westmorland on April 12, 2007 

to those in the months of April between 2004 and 2007, and finds that those speeds are 3 standard 

deviations above the average daily maximum for the month.49 While the suggested PM10 source area 

is not at Westmorland itself, these data are consistent with the conclusion that there were unusually 

high winds on April 12. A graph of historical April days shows April 12, 2007 to have the highest 

daily maximum wind speed in the 2004-2007 time period.50 A longer time period would have been 

useful, and the daily maximum is only a single number for the day, but both the comparison table and 

the graph are evidence that April 12, 2007 had unusually high wind. 

June 5, 2007

ARB’s demonstration provides evidence that winds were also high on June 5, 2007. ICAPCD and the 

IID issued wind advisories for the day.51 Wind speeds at Niland reached 29 mph (with a later hour at 

34 mph), and at Westmorland reached 17 mph.52 The nearby CIMIS stations, such as Calipatria/ 

 

44 Original April NED, pp. 10-11 and Attachments 6 and 7, PDF pp. 51-54. 

45 Original April NED, p. 12 and Attachment 8, PDF p. 55. 

46 April NED, Figures 11 and 19; wind data tables in Original April NED, Attachments 12 and 13, PDF pp. 70-71. 47 Original April NED, Attachment 14, PDF pp. 72-83. 

48 April NED, Figure 8 and text, p. 17. 

49 April NED, Table 8, p. 32. 

50 April NED, Figure 25, p. 32.

51 Original June NED, Attachment 6, PDF p. 53. 52 June NED, Figure 11, p. 19; Original June NED, Figure 9 p. 17; wind data tables in Original June NED, Attachment 11, 

PDF pp. 63-66. 
- 20 - 

Mulberry, Meloland, and Seeley, reached mph wind speeds in the high teens.53 These speeds seem 

only moderately high, except for Niland. However, Borrego Springs and Julian stations, just north and 

just west of the Anza Borrego Desert, had wind gusts up to 32 and 43 mph, respectively; these gust 

speeds remained moderately high for extended periods.54 Sustained mph wind speeds were in the high 

teens for Borrego Springs, and in the 20s to low 30s for Julian. This is consistent with a wind-driven 

PM10 source area in the western part of Imperial County. 

The demonstration compares maximum wind speeds experienced on June 5, 2007 at Calexico-Ethel, El 

Centro, Niland, and Westmorland to the wind speeds measured during the months of June between 

2004 and 2007; the June 5, 2007 wind speeds are approximately 4 standard deviations above the 

average daily maximum values for the time period (June NED, Table 8, p. 29). Graphs of historical 

days in June show June 5, 2007 to have the first or second highest wind speed, except for CalexicoEthel's 5th high (June NED, Figures, 24-27, pp.31-32). On all the graphs, the June 5, 2007 wind 

speeds are outliers. Although the suggested PM10 source area is not at the monitors themselves, the 

comparison table and the graph are evidence that June 12, 2007 had unusually high wind. 

5.3.2 Emission Sources

ARB asserts that the principal emission source for both the April 5 and June 12, 2007 events was the 

desert area some 30 miles to the west of the monitors, including Anza Borrego Desert (ABD). This is 

certainly plausible given the wind direction from the west, the high winds around ABD at Borrego 

Springs and Julian (measurements within ABD itself are not available), and the relatively high 

emissions ascribed to desert land in the inventory. However, off-highway vehicle activity occurs in the 

Superstition Mountain and Plaster City BLM Open Area and the Ocotillo Wells State Vehicle 

Recreation Area (SVRA), to the west of the Brawley and Westmorland monitors, and in the Plaster 

City BLM Open Area west of El Centro. Their emissions and contribution to exceedances are not 

discussed in the demonstrations for the 2007 events. Further, there is agricultural land relatively close 

to the monitors, which could have had a relatively larger contribution than indicated by the inventory 

totals (but see discussion of emission contributions in “Other causes” subsection, 5.3.5 below. It may 

be true that native desert is the largest source of emissions contributing to the exceedances, but there 

should be fuller source attribution, both for deciding which sources need reasonable measures (section 

4.2 above), and also for establishing the required clear causal relationship. 

A county-wide monthly or annual inventory may not be a sufficient basis for the demonstrations' 

conclusion that the contribution of windblown dust emissions from agricultural lands is small, or for 

source attribution in general. Such inventories lack temporal and spatial resolution, and do not account 

for the distance of emissions from the monitor. 

Regarding temporal resolution, the monthly average inventory does not account for the particular 

conditions of the exceedance days. The demonstrations do argue that such an episodic inventory 

would have a higher relative contribution from wind-blown dust compared to other PM10 sources than 

does the monthly inventory relied upon, strengthening the conclusion that wind-blown dust is the 

dominant source (April NED and June NED, p.11). However, this argument does not address how the 

contribution of OHVs and agricultural lands relative to natural desert emissions would change if an 

episodic inventory were used. If OHV and agricultural emissions increase more quickly with wind 

speed than natural desert emissions, their relative contribution will also increase. 

 

53 Original June NED, Attachment 13, PDF pp. 71-76.

54 June NED, Figure 8, p. 17, and text pp. 16-17. 
- 21 - 

Regarding spatial resolution, the inventory relied upon does not distinguish portions of the County that 

are upwind of a monitor from those that are downwind; all locations are assumed to contribute equally 

to monitored concentrations. The inventory also does not account for the relatively small distance 

between anthropogenic sources and the monitors, as opposed to the greater distance to some natural 

desert areas. This may be especially important for the April 12, 2007 Westmorland value of 156 

μg/m3

, and the June 5, 2007 Niland value of 162 μg/m3. For these moderate exceedances, a relatively 

small reduction in the local emissions contribution could have a relatively large effect, possibly even 

preventing the exceedances. The lower value at Niland relative to other monitors is consistent with 

shielding by the Salton Sea from the effect of western sources, but it is also consistent with a smaller 

proportion of agricultural lands in the upwind “fetch” (i.e., distance from the western desert). 

The emission inventory relied upon is partly based on results from the ENVIRON Wind Blown 

Fugitive Dust Model (WBFDM) from 2005. (“Development of a Wind Blown Fugitive Dust Model 

and Inventory for Imperial County, California,” ENVIRON and Eastern Research Group, May 12, 

2004; April Original NED, Attachment 26, PDF pp. 114-208). While this inventory is a substantial 

improvement over previous efforts, it makes generic assumptions about the degree of land disturbance 

(stability), uses a threshold emissions wind speed that is uniform instead of varying with surface 

roughness, is not based on Imperial County-specific measurements of threshold friction velocities or 

available dust reservoirs, and has various components that have not been validated against ambient 

measurements, such as agricultural emissions that decline exponentially with crop canopy cover. Even 

without new Imperial-specific emissions measurements and the use of more spatially resolved 

parameters, a re-run of the existing model with episode-specific winds would be an improvement over 

the monthly inventory. 

A spatially resolved episodic inventory that addresses the above concerns would help in demonstrating 

that the clear causal relationship criterion is met.

5.3.3 Transport

April 12, 2007

As shown in wind speed and wind direction plots (April NED, Figures 11 and 19, pp. 19 and 25; see 

Figure 5 below), after about noon on April 11, wind directions were almost uniformly from the west 

(270 degrees) at the two stations near ABD (April NED, Figures 8 and 16, pp. 17 and 23), as well as 

for the monitoring sites having meteorological data, and for most of the CIMIS stations (April Original 

NED, Attachments 13 and 14, PDF pp. 72-83). (See also April NED, Table 7, p. 30.) These data are 

consistent with the claim that the source area was in the west. 

Additional strong evidence is the correlation between the shift in wind direction from east-south east at 

noon on April 11, and shifts to high wind speed and high PM10 at around the same time (April NED, 

Figures 12 and 20, pp. 20 and 25; see Figure 6 below). The winds and PM10 declined after midnight 

on April 11, increasing again after around 7 am on April 12. Mexicali stations show a similar pattern 

(April NED, Figures. 13, 14, 21 and 22, pp. 21 and 26). The increase in PM10 tends to lag behind the 

increase in wind speed by an hour or so, which is consistent with transport from some distance to the 

west. 

April 12 HYSPLIT trajectories ending at Westmorland (April NED, Figures. 15 and 23, pp. 22 and 27) 

confirm the flow from the west. An additional WIND2D back-trajectory (April NED, Figure 24, p. 28) 

is consistent with this, including flow over ABD. 
- 22 - 

Figure 5. April 11 and 12, 2007 Wind Speed and Direction 

April 11 and 12, 2007 Wind Speed (WS) and Wind Direction (WD)

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Hour

Miles per hour

0

100

200

300

400

Degrees from north

Niland WS Westmorland WS El Centro WS Calexico-Grant WS

Niland WD Westmorland WD El Centro WD Calexico-Grant WD

Figure 6. April 11 and 12, 2007 Wind speed and PM10

April 11 and 12, 2007 Wind Speed (WS) and PM10

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Hour

Miles per hour

0

200

400

600

800

1000

μg/m3

Niland WS Westmorland WS El Centro WS Calexico-Grant WS

Niland BAM Westmorland BAM Braw ley BAM

June 5, 2007

ARB presents evidence similar to April 12, 2007 for June 5, 2007 (June NED, Figures 11-14, pp.19-

20). The correlation between the shift in the wind and the increase in PM10 is not quite as striking as 

for the April event, but is still convincing evidence of causality. 

HYSPLIT and WIND2D for June 5 confirm flow from the west, including over potential source areas 

in the ABD.55

Taken together, this evidence makes a compelling case of a causal relationship between some winddriven dust source in the west, and elevated PM10 concentrations throughout the area, including the 

monitored NAAQS exceedances. When wind is high and from the west, PM10 is high; when it is not, 

PM10 is low. 

5.3.4 Comparison to Similar and Neighboring Days 

April 12, 2007

The April demonstration does not include an explicit comparison to similar days. However, the 

demonstration does compare PM10 and maximum wind speeds on April 12, 2007 to historical Aprils.56 

 

55 June NED, Figures 15 and 22, pp. 21 and 26 for HYSPLIT; Figure 23, p. 27 for WIND2D. 

56 April NED, Figures 6 and 7, pp. 10 and 11. 
- 23 - 

For both the Brawley and Westmorland monitors, the April 12, 2007 PM10 and wind speed values are 

clear outliers, indicating something unusual occurred on this day. 

The April demonstration also compares April 12, 2007 to neighboring days and weeks.57 Figure 28 

has PM10 from Beta Attenuated Monitors (BAM) on April 5-19, 2007, and shows that concentrations 

on April 11 and 12 at Brawley, Westmorland, and Niland were approximately double those of 

neighboring days. Westmorland concentrations were also a bit elevated two days later. The same 

applies to the PM10 Federal Reference Monitor (FRM) data (every 6th day schedule) from preceding 

and succeeding weeks (April NED, Figure 29, p.34). 

June 5, 2007

The June demonstration compares PM10 and maximum wind speed on June 5, 2007 to historical 

conditions during the months of June between 2002 and 2007.58 The data show that for both the 

Brawley and Westmorland monitors, the June 5, 2007 PM10 and wind speed values are clear outliers. 

(As noted previously, the page and figure numbers are the same for the April and June 

demonstrations). 

At the Westmorland, Brawley, and Niland BAMs, the June 5 monitored values were more than double 

the PM10 values for neighboring days.59 The same was true for FRM PM10 values for all the exceeding 

monitors for neighboring weeks.60 

5.3.5 Consideration of Other Causes 

ARB’s demonstrations address the issues of whether the exceedances were caused by sources other 

than high wind-driven ones, and also whether they were caused by wind-driven sources other than 

natural desert. 

April 12, 2007

The April demonstration points out that PM10 was elevated over a large area on this date, including at 

monitors in northern Mexico and southwestern Arizona;61 wind speeds were also elevated over a large 

area.62 This information supports claims regarding the regional nature of the event, as opposed to local 

causes specific to the exceeding monitors. It is also evidence that the event was caused by wind-driven 

emissions stemming from a regional meteorological occurrence. 

The demonstration also argues that if agricultural lands were a dominant contributor to PM10 on April 

12, then the El Centro monitor would have had higher concentrations than Brawley or Westmorland, 

since it has roughly 2-3 times as much agricultural land upwind toward the west.63 However, 

examination of all the monitors and their upwind agricultural fetch does not show any clear 

relationship between concentration and upwind agriculture.64 Other monitors do not fit the 

relationship cited in the documentation: Brawley had nearly double the concentration that 

Westmorland had (291 vs. 155 μg/m3

), and about double the agricultural fetch; Niland has the lowest 

 

57 April NED, pp. 33-34. 

58 June NED, Figures 6 and 7, pp. 10 and 11. 

59 June NED, Figure 33, p. 35. 

60 June NED, Figure 34, p. 35. 

61 April NED, p. 29. 

62 April NED, p. 16. 

63 April NED, p. 29. 

64 April NED, Figure 2, p. 4; similar satellite photograph below, Figure 7.
- 24 - 

concentration (108 μg/m3

) and the lowest fetch. These figures would seem to indicate a direct 

correlation between the amount of upwind agricultural land and PM10 concentrations. Without 

knowing the particulars of the land uses upwind of each site, it is difficult to draw conclusions for 

either of these possible relationships. The demonstration’s conclusion that agriculture was not the 

dominant PM10 source may be correct, but even a partial contribution could be important at 

Westmorland with its 155 μg/m3 concentration. 

Figure 7. Satellite photograph of Imperial County, with PM10 monitors 

The demonstration states that “no unusual local anthropogenic emissions [were] reported before, 

during, and after the event” and that “Imperial County APCD estimates that anthropogenic emissions 

were approximately constant before, during, and after the event.”65 However, the extent of the 

investigation underlying these statements is not clear. Also, presumably “anthropogenic emissions” 

refers to direct emissions from a subset of anthropogenic activities, since wind would increase 

emissions from land disturbed by anthropogenic activity before or during the event (any such increases 

may have been assumed not to count, due to the ARB claim of reasonable measures in place). The 

lack of other known causes supports the theory that high wind on April 11-12 was responsible for the 

April 12 PM10 NAAQS exceedances. 

 

65 April NED, p. 33. 
- 25 - 

June 5, 2007

Like the April demonstration, the June demonstration points out that both PM10 concentrations and 

wind speeds were elevated over a large area, indicating the regional nature of the event (September 

NED, pp. 4 and 16). This evidence suggests the cause was not specific to a single monitor (regionally 

elevated winds could generate dust locally). 

Again, the June demonstration argues that if agricultural lands were a dominant contributor to PM10 on 

June 5, the El Centro monitor (200 μg/m3

) would have had higher concentrations than Brawley (281 

μg/m3), Westmorland (226 μg/m3), and Calexico-Ethel (282 μg/m3), since El Centro has much more 

such lands upwind toward the west (June NED, p. 28). Further, wind speeds at El Centro and at 

nearby Seeley and Meloland on either side were higher than at the other locations, so locally-generated 

agricultural dust would be expected to be greater at El Centro, whereas the observed PM10 is lower. 

The demonstration also argues that anthropogenic emissions were approximately constant before, 

during, and after the event, so the exceedances are not likely to be due to them, but rather to the high 

wind (June NED in “but for” section, p. 35). 

As was the case for the April demonstration, both of these arguments are reasonable, and support the 

conclusion of a non-local cause, despite lacking some of the particulars that would make the argument 

more conclusive. 

5.3.6 Summary of Clear Causal Relationship for April 12 and June 5, 2007 

EPA agrees that the evidence presented demonstrates that the April 12 and June 5, 2007 PM10

exceedances were probably caused by wind-driven PM10 emissions from some sources west of the 

monitors. However, without better and more complete information regarding source attribution, the 

cause of the exceedances remain unclear, as do the extent of the anthropogenic contribution, and which 

anthropogenic sources should have reasonable controls applied. 

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.” There is no specific threshold 

test for this requirement, but concentrations in the high percentiles can provide supporting evidence.66

6.1 September 2, 2006 Event – Calexico Monitors 

The Calexico concentrations are stated to be in 99th percentile at Grant St. and in the 98th percentile at 

Ethel St. in the 2001-2007 period.67 The PM10 vs. wind speed graph in Figure 25 of the September 

NED shows only meteorologically similar days, not the full data set, but considering just the vertical 

spread of concentrations in the graph, it is clear that the September 2, 2006 concentrations are among 

the highest values. Thus, the concentrations are outside the normal range expected for these 

conditions. 

 

66 EER Preamble, 72 FR 13569

67 September NED, p. 16.
- 26 - 

6.2 September 2, 2006 Event – Westmorland Monitor 

The Westmorland concentration is in the 98th percentile.68 Figure 24 of the September NED plots the 

last 7 years of PM10 observations against time. While not among the very highest values recorded, the 

Westmorland exceedance of 167 μg/m3

 is clearly beyond the range of normal historical fluctuations, 

which vary seasonally around 50 μg/m3. 

6.3 April 12 and June 5, 2007 Events

April 12, 2007

PM10 data from the months of April in 1995-2007 for Westmorland and 1987-2008 for Brawley show 

that the April 12, 2007 PM10 concentration is at the high end of the concentration distributions for both 

monitors.69 The April NED states that they are in the 98th percentile of April concentrations. EPA 

also examined data from 2001-2007. As shown in Table 3 below, the Brawley value of 291 μg/m3 was 

the highest during 2001-2007; the Westmorland value of 155 μg/m3

 was at the 98th percentile, the 

tenth highest observed. These values are evidence that the concentrations measured on April 12 , 2007 

are in excess of normal historical fluctuations. 

As mentioned above in section 5.3.1, the demonstration’s graphs comparing PM10 and maximum wind 

speed on April 12, 2007 to historical months of April show that for both the Brawley and Westmorland 

monitors, the April 12, 2007 PM10 and wind speed values are clear outliers.70

EPA agrees that the evidence presented demonstrates that the April 12, 2007 PM10 concentrations at 

Brawley and Westmorland are in excess of normal historical fluctuations. 

June 5, 2007

Historical PM10 data for the months of June (during an unspecified time period) for Brawley, CalexicoEthel, El Centro, Niland, Westmorland show that the June 5, 2007 PM10 concentration is at the high 

end of the concentration distributions for all the monitors.71 The June NED states that they are in the 

98th percentile of June concentrations. EPA also examined data from 2001-2007. As shown in Table 

3, the exceedances were at the 99th percentile considering all data, and the 100th percentile 

considering just the months of June; all were in the top five concentrations recorded. These facts are 

evidence that the measured concentrations are in excess of normal historical fluctuations. 

As mentioned above in section 5.3.1, the graph in the demonstration comparing PM10 and maximum 

wind speed on June 5, 2007 to historical data for the months of June show that for both the Brawley 

and Westmorland monitors, the June 5, 2007 PM10 and wind speed values are clear outliers. 

EPA agrees that the evidence presented demonstrates that the June 5, 2007 PM10 concentrations at 

Brawley, Calexico-Ethel, El Centro, Niland, Westmorland are in excess of normal historical 

fluctuations. 

 

68 September NED, p. 16. 69 April NED, Figures 26 and 27, p. 33. 

70 April NED, Figures 6 and 7, pp. 10 and 11. 

71 June NED, Figures 28-32, pp. 33-34. 
- 27 - 

Table 3. Percentiles of PM10 concentrations for Imperial County exceptional event claims, 

using all 2001-2007 data and using only data from corresponding month

Brawley Calexico -

Ethel 

Calexico -

Grant El Centro Niland Westmorland 

 April 12th, 2007 

(μg/m3) 291 124 147 121 108 155 

rank 1 27 20 8 11 10

percentile, all data 100% 93% 95% 98% 97% 98%

Aprils only 100% 97% 100% 100% 96% 100%

 June 5th, 2007 

(μg/m3) 281 282 #N/A 200 162 226 

rank 2 4 #N/A 3 4 6

percentile, all data 100% 99% #N/A 99% 99% 99%

Junes only 100% 100% #N/A 100% 100% 100%

 Sept. 2nd, 2006 

(μg/m3) 127 164 233 146 99 167 

rank 5 11 3 5 13 8

percentile, all data 98% 98% 99% 99% 97% 98%

Sept. only 100% 100% 100% 100% 97% 100%

# obs, 2001-2007 226 423 365 391 393 398

Note: percentiles rounded to whole percents; 100% is not necessarily the overall maximum. 

7.0 No exceedance but for the event 

Pursuant to 40 CFR §50.14(c)(3)(iii)(D), the demonstration must show that “there would have been no 

exceedance or violation but for the event.” The weight of evidence in a demonstration does not require 

a precise estimate of the air quality impact from the event,72 though such information could be useful. 

Concentrations on days with similar emissions but without the influence of the event are rough 

evidence of what the concentration on the event day would have been but for the event.

7.1 September 2, 2006 Event – Calexico Monitors 

If it is granted that there was a high wind event in areas southeast of Mexicali that created dust 

transported to Calexico, that Imperial winds were too low on their own to cause increased dust 

emissions, and that anthropogenic activities were the same on September 2 as on neighboring days 

without exceedances, then it follows that the cause of the exceedances was the high wind event, and 

that there would have been no exceedances but for the event. The documentation has some discussion 

about the likelihood that there were no causes for concentrations to be above normal other than higher 

wind speeds: the absence of citizen complaints and the absence of unusual activity that would have 

 

72 72 FR 23570. 
- 28 - 

been noted during inspections in Attachment H.73 However, as discussed above in section 5.5.1, due to 

the lack of actual measurements of high wind in Mexico, it is not clear that the wind generating dust in 

Mexico qualifies as a high wind event. As a result, the “but for” regulatory criterion has not been 

clearly met. 

 

7.2 September 2, 2006 Event – Westmorland Monitor 

ARB points out that absent the 619 μg/m3 concentration "spike" at 7 pm, the 24-hour average 169 

μg/m3 BAM concentration at Westmorland would not have been an exceedance74 - without that value, 

the 24-hour average would have been 149 μg/m3

.

75 Even though ARB states the BAM monitor in this 

case is used only for forecasting76 rather than directly in assessing NAAQS exceedances, this is good 

evidence that the regular Size-Selective Inlet (SSI) Westmorland monitor likewise would not have 

exceeded but for the short period around 7 pm. If it is accepted that high winds to the north caused this 

concentration spike, then it follows that there would have been no exceedance but for the event. 

However, as discussed above in section 5.2.3, due to inconsistencies in the timing of winds and 

concentrations, and due to conflicting evidence regarding the wind direction, the causal relationship 

has not been demonstrated. As a result, it is not clearly appropriate to disregard the 619 μg/m3

concentration spike. Therefore the “but for” regulatory criterion has not been met. 

7.3 April 12 and June 5, 2007 Events

The clear correlation between elevated PM10 concentrations and the shift at around noon on April 11 

and on June 5 to high wind speed and westerly direction (see “transport” section 5.3.3, above) is 

evidence that PM10 concentrations may have continued at the non-exceeding level of about 50 μg/m3

without that shift. Still, as discussed above, uncertain source apportionment means that ARB has not 

fully characterized the causality of the event (i.e. the generation of dust from some unspecified sources 

by the wind). As a result, EPA cannot conclude that there would have been no exceedance or violation 

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. For the 2007 events, ICAPCD sent a letter to EPA dated September 

17, 2007 describing the events as likely due to high wind under dry conditions. There was no such 

letter for the 2006 events; however, that event predated adoption of the EER, and there was informal 

communication with EPA about 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. For the September 2, 2006 event, draft demonstrations were provided in 2008 and an 

official submittal was dated May 19, 2009; this is within three years of the event. For the 2007 events, 

ARB prepared an official demonstration dated June 18th, 2008, and an official re-submittal dated May 

21, 2009; using either date, the demonstrations were submitted within three years of the events. 

 

73 September NED, PDF p. 149. 

74 September NED, p. 17 

75 24 hours at 169, less the effect of the 619: (24×169 - 619)÷(24-1) = 149 76 September NED, p.6 
- 29 - 

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

documentation shows evidence of newspaper publication for all three of the event days. In addition, 

EPA found that the demonstrations were available to the public via the IAPCD web site starting in 

March, 2009. 

Finally, 40 CFR §50.14(c)(3)(i) requires that any public comments be submitted along with the 

demonstrations. The documentation stated that no public comments had been received, and ARB 

committed to submitting any received thereafter. 

9.0 Conclusion 

9.1 Summary of EPA’s Evaluation of the September 2, 2006 Event Involving the Calexico 

Monitors 

ARB states that the Calexico exceedances resulted from a combination of typical emissions from 

Mexicali, and high wind-driven emissions from unpaved roads and desert areas east and southeast of 

Mexicali. However, ARB failed to provide wind speed data showing that high winds actually occurred 

in the area in question. Moreover, neighboring monitors south of the US-Mexico border measured 

concentrations significantly lower than those measured by the Calexico monitors. This evidence is 

inconsistent with ARB’s claim that a regional high-wind event southeast of the monitors caused the 

exceedances. Because a clear causal relationship has not been established, EPA also can not conclude 

that there would have been no exceedance but for the event. Based on the weight of evidence 

provided, EPA does not concur that the exceedances at the Calexico monitors in 2006 should be 

treated as exceptional events. 

9.2 Summary for September 2, 2006 Event, Westmorland monitor 

Although wind speeds throughout Imperial County were low, ARB states that the Westmorland 

exceedance was caused by localized high winds from thunderstorm outflows north of the County. 

PM10 emissions were allegedly transported to Westmorland causing a single hour “spike” of very high 

concentration. However, wind direction data and HYSPLIT trajectories are not consistent with ARB’s 

claim. PM10 was elevated through much of the day, with winds blowing from the direction of Mexico 

and the southeast portion of Imperial County. Neither the latter contribution nor potential source areas 

in the north have been demonstrated to be reasonably controlled. Further, due to inconsistencies in the 

timing of winds and concentrations, and due to the conflicting evidence regarding the wind direction, 

the “but for” criterion has not been met. Based on the weight of evidence provided, EPA does not 

concur that the Westmorland exceedance should be treated as an exceptional event. 

9.3 Summary for April 12 and June 5, 2007 Events, Multiple Monitors 

Starting at noon on April 11 and at noon on June 5, the wind direction shifted to blow from the west. 

At the same time, the wind speed at various locations consistently exceeded 15 mph, approached 25 

mph during several hours, and gusted above 25 mph on a few occasions. During these periods, PM10

concentrations sharply increased. The correlation of wind speed and direction, PM10 concentrations, 

and back-trajectories passing over desert areas to the west are evidence of a high wind event that 

influenced the monitors. However, agricultural and off-highway vehicle activity are potential 

significant sources of emissions. ARB neither provided source attribution data adequate to show that 

these sources did not contribute to the exceedance nor showed that these anthropogenic sources were 
- 30 - 

reasonably controlled at the time of the events. This raises questions regarding the causal relationship 

between the exceedances and the wind-driven PM10 emissions and the extent to which the proffered 

events meet the requirement at 40 CFR 50.1(j) that they were not reasonably controllable or 

preventable. ARB’s failure to provide adequate source attribution data also calls into question whether 

there would have been no exceedance or violation but for the event. In light of all of the available 

evidence, EPA does not concur that the April 12 and June 5, 2007 exceedances should be treated as 

exceptional events. 

10.0 Citation of Exceptional Event Request Documentation 

Letter from Emmanuel Sanchez, Imperial County Air Pollution Control District, to Sean Hogan, U.S. 

Environmental Protection Agency Region 9, September 17, 2007, with initial description of 

April 12 and June 5, 2007 events ImpCo_initial_description_2007_events.pdf 

Letter from James N. Goldstene, Air Resources Board to Deborah Jordan, U.S. Environmental 

Protection Agency Region 9, May 19, 2009, requesting exclusion of September 2, 2006, April 

12, 2007, and June 5, 2007 Imperial County PM10 Exceedances SS_051409_EPA_ltr.pdf 

“Proof of Publication”, statement of the principal clerk for the printer of the Imperial Valley Press, 

dated April 1, 2009, including photocopy of newspaper announcement, “Notice of Petition for 

Exclusion of PM10 Monitoring Data Affected by an Exceptional Event on September 2,2006, 

April 12, 2007 and June 5,2007” ImpCo_public_notice.pdf 

September NED September 2nd, 2006 Natural Event Documentation 

“Natural Event Documentation, Calexico and Westmorland, California, September 2, 2006”, Imperial 

County Air Pollution Control District, January 30, 2009, with attachments 

September_2_2006_event_documentation_ENVIRON_031309.pdf

Letter from Karen Magliano, Air Resources Board, to Sean Hogan, U.S. Environmental Protection 

Agency Region 9, June 13, 2008 requesting exclusion of April 12, 2007 exceedances, with 

enclosures: April ARB staff analysis; April original NED: “Natural Event Documentation, 

Brawley and Westmorland, California, April 12, 2007”, Imperial County Air Pollution Control 

District, dated April 15, 2008; and additional attachments

April_12_Report_and_Attachments.pdf

April NED April 12, 2007 Natural Event Documentation 

“Addendum, Natural Event Documentation, Brawley and Westmorland, California, April 12, 2007”, 

Imperial County Air Pollution Control District March 12, 2009, with attachments 

Final_Addendum_for_April_12_Event.pdf

Letter from Karen Magliano, Air Resources Board, to Sean Hogan, U.S. Environmental Protection 

Agency Region 9, June 13, 2008 requesting exclusion of June 5, 2007 exceedances, with 

enclosures: June ARB staff analysis; June original NED: ”Natural Event Documentation, 

Imperial County, California, June 5, 2007”, Imperial County Air Pollution Control District, 

dated April 15, 2008; and additional attachments 

June_5_Report_and_Attachments.pdf
- 31 - 

June NED June 5, 2007 Natural Event Documentation 

“Addendum, Natural Event Documentation, Imperial County, California, June 5, 2007”, Imperial 

County Air Pollution Control District, March 12, 2009, with attachments; 

Final_Addendum_for_June_5th_Exceptional_Event_Report.pdf

Electronic versions available at: 

http://www.imperialcounty.net/AirPollution/Web%20Pages/2009%20March%20Natural%20Events.ht

m