- 1 - 

## Review of Exceptional Event Requests

Imperial County, CA

24-hour PM 10

• September 2, 2006

• April 12, 2007

• June 5, 2007

US Environmental Protection Agency Region 9

December 22, 2009- 2 - 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 Simila r 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 Relationshi p 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 Simila r 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 Relationshi p for September 2, 2006, Westmorland ............... 18

5.3 April 12 and June 5, 2007 Events ............................................................................................ 1 9

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 Relationshi p for April 12 and June 5, 2007 ........................... 25

6.0 Concentration in excess of norma l 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 ............................................................................................ 2 6

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 ............................................................................................ 2 8

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- 3 - 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 PM 10 ............................................................................................................................

..................... 13
Table 3. Percentiles of PM 10 concentrations .............................................................................................. 28

Figures

Figure 1. Imperial County map with PM 10 monitors ................................................................................... 5 Figure 2. HYSPLIT back-trajector ies for September 2, 2006 ................................................................... 1 2 Figure 3. Imperial County map with PM 10 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 PM 10 ............................................................................ 22 Figure 7. Satellite photograph of Imperial County, with PM 10 monitors .................................................. 25- 4 - 

### 1.0 Introduction

On March 22, 2007, EPA adopted a final rule, Treatment of Data Influen ced by Exceptional Events 1
(EER) to govern the review and handling of certain air quality m onitoring 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) exceedance s and violations if a state demonstrates that an “exceptional event” caus ed 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 justif y the exclusion. After considering the weight of evidence provided in th e 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 PM 10 standard at various monitors in Imperial County in 2006 and 2007. The demonstr ations consist of the following support documents, prepared by ARB, the Imperial County Air Pollu tion Control District (ICAPCD), and ICAPCD’s contractor ENVIRON (see the last page for full citations and electronic file names).

• Natural Event Documentation: Calexico a nd Westmorland, California - September 2, 2006 (September NED) [dated January 30, 2009]

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

o Natural Event Documentation: Brawley and Westmorland, California - April 12, 2007, Imperial County Air Pollution Control Distri ct (Original April NED) [dated April 15, 2008]

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

o Natural Event Documentation: Imperial County, California - June 5, 2007, Imperial County Air Pollution Control District (Ori ginal 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 d ecision regarding the alle ged exceptional events.



### 2.0 Summary of the Events

ARB’s requests pertain to ten monitored exceedances of the 24-hour PM 10 standard (150 μg/m 3 ), 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 sh ows 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.- 5 - Figure 1. Imperial County map with PM 10 monitors

Table 1. Claimed Exceptional Events

Monitor PM 10 Concentration (μg/m 3 ) Description of Event Claim

September 2, 2006
Calexico –

Grant St. 233 Exceedances caused by high winds from thunderstorm outflows, which entrained emi ssions 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

PM 10 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.”- 6 - Table 1 (continued)

Monitor PM 10 Concentration (μg/m 3 ) 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 sh ift to a westerly direction. Westmorland 155

June 5, 2007

Calexico –

Ethel St. 282 Exceedances caused by wind-blow n dust from desert areas in west part of County; associ ated 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 concurren ce 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 lo cation, or is a natural
event;

(B) There is a clear cau sal relationship between the measurem ent under considera tion and the event
that is claimed to have affect ed the air quality in the area;

(C) The event is associated with a measured concen tration 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 th at data claimed to be due to an exceptional event must be flagged in the AQS database, and that an in itial 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) requir es that the State:

• submit a demonstration to EPA within three year s of the calendar quarter of the event or 12 months prior to an EPA regulatory decision;

• provide notice and opportuni ty 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.- 7 - 

### 4.0 Criteria Set Fo rth at 40 CFR 50.1(j)

4.1 Affects Air Quality

As stated in the preamble to the EER, the event in que stion is considered to have affected air quality if
it can be shown that there is a clear causal rela tionship between the monito red exceedance and the event, and that the event is associated with a me asured concentration in ex cess 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 wa s “not reasonably controllable or preventable” depends on the specific facts and circumstances surr ounding 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 demons trate 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 concl udes that the 2006 Westmorland 4 and 2007 events were not reasonably controllable or preventable because:

1. The events were caused largely by windblown dust from uncontrollable sour ces, i.e., sources in Mexico and desert areas in the southeast por tion 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 anth ropogenic sources were small compared to the uncontrollable desert and Mexico sources, and were reasonably contro lled by the Imperial Irrigation District's (IID) fallowing program, ICAPCD Regulation VIII, and other ICAPCD regulations.

While ARB asserts that non-anthropogenic s ources are responsible for most of the PM 10 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 exception al 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 reas onable and appropriate measures in place, th e September 22, 2006 wind event caused the
exceedance.” See 72 FR 49046, 49051 (Augu st 27, 2007).

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

Westmorland 1 st Street, recorded PM 10 concentrations in excess of the 24-hour PM 10 standard of 150 μ g/m3. EPA addresses below only sources of PM 10 emissions that were associated with the We stmorland 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 contributi ng 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 th e page and figure numbers are the same for the April and June demonstrations.- 8 - contributions (i.e., the remaining 25% fo r the 2007 events, and anthropogenic PM 10 from the southeast quarter of the populated area of Imperial County for the 2006 Westmorland event). 6

Ideally, exceptional event re quests would identify all non- de minimis anthropogenic sources that
contributed to an exceedance and would then descri be how each is reasonably controlled. ARB’s
documentation for the 2006 Westmorland and for th e 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 discusse d in any detail is agriculture. Specifically, the documentation for the 2007 events estimates that agricultural lands contribute 4-5% of to tal emissions during highwind events 8 and states that IID’s fallowing program pr ovides 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 ar eas contributed to the 2006 Westmorland and 2007 exceedances, and thus need to have been reasonably contro lled 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 th at the dominant source of emissions that contributed to the exceptional ev ents 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, Plas ter City, Superstitio n 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 PM 10 emissions by disturbing vege tation and surface crusts, leaving the surface less stable and more vulnerable to emissions during subsequent winds, and directly

causes PM 10 emissions by entraining dust through cont act between tires and unpaved surfaces. Similarly, the documentation for the 2006 Westmorla nd event concludes that a dominant source of
emissions that contributed to the Westmorland exceedance was PM 10 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 PM 10 emissions during the events, the 2009 PM 10 SIP quantifies a large OHV contribution to the countywide average open area inventory. Table III.B.4 in the 2009 PM 10 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 activ ity, particularly since estimates of OHV-related emissions provided in the 2009 PM 10 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 PM 10 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 AR B where OHV activity occurs illegally and is reflected in the Erratum Sheet for the 2009 Imperial PM 10 SIP referenced below.

13 September NED, p. 9.

14 2009 PM 10 SIP, Figure III.B.1 identifies the Imperial Sand Dunes OHV area.- 9 - amended by the Erratum Sheet 15 ) estimates that up to 22 tons per day 16 (tpd) of wind-blown PM 10 from these areas are emitted from over 250 squa re 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 en training dust in open areas (i.e., not on established roads/paths) are not addressed in existing emission inventories fo r 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 IC APCD's 2005 BACM Demonstration Report 17 as the basis for concluding that reasonable measures were in place for fugitive dust sources, presumably including OHV activity. 18 Because implementation of BA CM is required in serious PM 10 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 pl ace for purposes of the Exceptional Events Rule. However, neither the documentation for the exceptional event reque sts nor the 2005 BACM Demonstration Report provides any meaningful analys is of BACM or any other level of control for OHVs. EPA would expect such an an alysis to include as a starting point for example, evaluation of EPA’s 1992 potential reasonably avai lable control measures for OHVs, 19 as well as OHV controls for
other areas such as Arizona Revised Statute §49-457.03 which prohi bits recreational OHVs in the Maricopa PM 10 nonattainment area on PM 10 high pollution advisory days and Clark County Air Quality Regulations, Section 90, which effectiv ely restricts OHV activity on BLM and other open areas greater than 5,000 square feet within the Las Vegas PM 10 nonattainment area. 20

4.2.3 Controls for Agricultural Land

The 2007 event documentation relies on IID's fallowing program 21 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 subj ect to either Rule 806 or IID’s program. 23 This fallow land represents about 7% of the total area farmable 24 in Imperial County, but it is likely to contribute a much larger percentage of the ove rall emissions since there are at least periodic crops or some other measure that reduces emissions on all other cropla nd. 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 PM 10 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 PM 10 State Implementation Plan for Clark County, Adopted by Clark County Board of Commissioners, June 19, 2001, page 4-81. http://www.accessclarkc ounty.com/depts/daqem/aq/plann ing/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 Stat us Report,” February 19, 2008, on web page http://www.iid.com/Water/FallowingProgra ms, direct link: http://www.iid.com/M edia/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 fa rmable, see Imperial Irrigation District Water Department Monthly Crop Acreage Report, June 13, 2007, p. 26.- 10 - of BACM or any other level of control for fallow la nd referenced or provided in the documentation for the exceptional event requests. EPA would expect such analysis to include, fo r example, evaluation of analogous controls in other areas, su ch as South Coast Air Quality Ma nagement District Rule 403, and recommendations from the USDA’s National Agronom y 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 else where (section 4.2 on controls , and sections 5.2.2 and 5.3.2 on emission sources), there are anthropogenic sour ces 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 duri ng 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 demonstrat ed that contributing an thropogenic 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 provi de evidence that the winds were statistically
unusual. 28 The other natural event criterion (exceedan ces despite all reasonable measures) is not satisfied because, as stated above, there are an thropogenic contributions to the 2006 Westmorland and
2007 events for which the State has not demonstrated that all reasonable measur es were in place. Therefore, ARB has not demonstrat ed that these events qualify as natural events under the rule.



### 5.0 Clear Causal Relationship

In order for EPA to concur with an exceptional ev ent request, 40 CFR §50.14(c )(3)(iii) re quires the
state to demonstrate that ther e is a clear causal relationshi p between the measurement under
consideration and the event that is claimed to ha ve affected air quality in the area. The following
sections evaluate the documen tation 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 app licable 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 modera tely high at some distance from the Westmorland monitor. For the 2007 events, ARB provides evidence that the winds were high in an absolute sense.- 11 - 5.1 September 2, 2006 – Calexico Monitors

With regard to the September 2, 2006 Calexico mon itors, the “event” is claimed to be high winds
associated with thunderstorm outflows to the south east, which generated dust emissions in Mexico.
ARB claims that these dust emissions were tran sported to Calexico. AR B 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

PM 10 from eroded soil.

ARB’s documentation is reviewed belo w in terms of six principal factor s: 1) high winds in Mexico, 2) source areas in Mexico, 3) trans port 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 th at the wind speeds were high in th e 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 activ ity that occurred in the general area, 32 and the premise that thunderstorm cells are known to produ ce strong outflows. ARB goe s on to note that there is a general increase in wind speed from west to ea st, 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 PM 10 emissions that
affected the Calexico monitors. ARB’s documentation notes the presence of dese rt land directly east of Mexicali, and the prevalen ce of unpaved roads in the br oad 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 characteri zation of the potential sources than that provide d 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 (Figur e 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.- 12 - 5.1.3 Transport

The wind direction on the day of the events was consiste ntly from the southeast. 35 A series of HYSPLIT back-trajectories provided in Appendix B of ARB’s documenta tion are also consistent with transport from the south and southeast through most of the day. Figure 2 belo w is an example of one
of the HYSPLIT model results. Other tables and v ector maps provide similar information. It is therefore possible that emissions from thes e areas were transpor ted to Calexico.

However, this explanation does not account for the significant difference between the PM 10 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 hi gh wind event to have similar impacts on these monitors, but this was not observed on the day in question. The 24-hour PM 10 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 Ca lexico monitors register ed concentrations of 164 μg/m 3 and 233 μg/m 3 , the concentrations measured at the Mexicali monitors ranged from 45 - 67 μg/m 3 (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-traje ctories 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 PM 10 Country Site Name PM 10 (μg/m 3 )

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 PM 10 monitors and Sep. 2, 2006 concentrations

5.1.4 Comparison to Days with Si milar Meteorological Conditions

Days in 2001-2007 with similar meteorological conditi ons 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 PM 10- 14 - concentrations at various monitors. The fact that these conditions are generally not associated with exceedances suggests that there is something unus ual about September 2, 2006. (September NED, pp. 12-13). Similarly, plots of PM 10 versus wind speed (September NED, Figure 25, p. 40; discussed in text, p.16) also show that September 2, 2006 lies outsi de of the historical relationship, supporting the theory that it is causally different than typi cal 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 th an vice versa, especially since Me xicali has greater emissions than Calexico resulting from less st ringent emission controls.

While the foregoing discussion generally supports th e theory that September 2, 2006 was in some way atypical, it does not provide di rect 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 PM 10 concentrations measured on September 2, 2006 with hourly PM 2.5 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 consiste nt with ARB’s claim that the exceedances were caused by higher winds on September 2, even though PM 2.5 is not the same as PM 10

5.1.6 Consideration of Other Causes

ARB considered other potential causes of the exc eedances based on the premise that elimination of alternative possibilities lends support to the pr oposed cause. Though the ex tent of the ICAPCD investigation cited by ARB is uncle ar, ICAPCD did not find any eviden ce of increased anthropogenic activity on September 2, 2006 38 and the documentation notes the l ack of unusual entries in source inspection logs for that day. 39 ARB also notes its expectation th at anthropogenic activity should have been lower on September 2 than on the previ ous day, since September 2 was a Saturday. 40 As a result, ARB argues that the differences in measured elevated PM 10 concentrations on the consecutive days are not explained by differences in anthropogenic acti vity. 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 PM 10 relative to the norm, which implies the cause was regional in nature, since a loca l cause would affect only certain monitors. 41 Still, it is unclear why more exceedances were not measur ed on other days with similar meteorological
conditions.

In addition, once surface crusts have been dist urbed, emissions can result from OHVs or fallow agricultural fields without ther e being direct anthropogenic activ ities. As noted in Section 4.2.2, OHV activity indirectly increases PM 10 emissions by disturbing vegetati on on surface crusts, leaving the
surface less stable and more vulnerable to emissi ons 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 PM 2.5 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 surfa ces 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 theo retically possible that high winds over dusty areas in the southeast could have generated PM 10 that was transported to Calexico. However, ARB fa ils 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 ca se is not sufficient to demonstrate the clear causal relationship required by the ex ceptional events rule.

5.2 September 2, 2006 – Westmorland Monitor

The principal discussion of the We stmorland exceedance and its causes is at pp. 8-11 of the September NED. In this case ARB claims that dust was gene rated 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 We stmorland; 4) how September 2, 2006 differs from meteorologically similar days; 5) how this day di ffers 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 southeaste rly 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, entrai nment 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 wi nd 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 Ve rde (NE corner of the County). Similar maximum speeds were observed at Thermal Airport and Blyt he 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 ac tivity led to these moderately high winds at locations just outside or within the nort hern 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 poten tial sources of wind-blown

PM 10 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. Pr esumably these are the desert areas north and northwest of Indio and Palm Springs, and areas east and west of Blythe . ARB’s documentation st ates 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 de scription. The submittal contains little assessment
of the relative contributio ns of anthropogenic and non-anthropogeni c emissions in the potential source areas, which could provide eviden ce 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 PM 10 emissions which were transported to Westmorland. In particular the documentation presents the fo llowing evidence (September NED text, pp.10-11, and Tables 1 and 2, pp. 41-42):

• As discussed above, wind speeds for some norther n 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 to ward Westmorland. Winds at N iland (north of Westmorland) were roughly consistent with this flow. Wind sp eeds were also elevated at Blythe, north of Palo Verde (though the wind directi on 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 Salt on Sea West CIMIS station (between Oasis and Westmorland) were from the north west, i.e., toward Westmorland.

• At 7 pm, a pronounced concentration "spike" over 600 μg/m 3 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. Th e directions can be interpreted as consistent with the theory that dust was tr ansported to Westmorland.

However, the timing of the winds and concentration sp ikes are not entirely cons istent with the distance from the Westmorland monitor. Figure 4 be low shows winds at 6 pm, with predominantly southeasterly flow of moderate sp eed, but high speed flows in the north with varying directions. Most notably, the increased wind at Oa sis toward Westmorland is simultaneous with the Westmorland concentration spike, rather than an hour or tw o before as one would e xpect 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. Fina lly, 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 dire ction at Westmorland itsel f was consistently from
the southeast or east-southeast. HYSPLIT back-tra jectories ending at Westmo rland near the 7 pm high
concentration hour are also inconsis tent with transport from northern stations during the two hours in which high speed winds occurred. 42 For the six-hour period precedi ng the Westmorland concentration spike at 7 pm, the back-trajectories are from the sout h; 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 PM 10 concentration at Indio/Palm Springs and Yuma correspond to the pa ssage of the thunderstorm activity in each area (September NED, Figure 1, p. 20; see also Attachme nt B, PDF p. 73). The 5 pm satellite image does provide evidence of thunde rstorm activity north of Imperial C ounty. 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 ev idence on the transport of emissions from north of the County to the Westmorland monitor, which undermines th e case for a clear causal relationship.

5.2.4 Comparison to Days with Si milar Meteorological Conditions

ARB’s documentation contains li ttle discussion of meteorological ly similar days, but Table 8 (September NED, p. 48) shows that on days in 2001-2007 when Westmorland experienced similar wind speeds and directions, PM 10 concentrations at the various Impe rial stations are well below what was observed on September 2, 2006. As previously di scussed, while this s uggests that September 2 was in some way atypical, it does not provide di rect support for the require d causal relationship.

5.2.5 Comparison to Neighboring Days

As discussed above with respect to the Calexi co monitors in secti on 5.1.5, wind speeds and PM 2.5 concentrations were lower on September 1 and 3 than on September 2, 2006. These facts are
consistent with the ARB’s claim that the exceed ances were caused by higher winds on September 2, even though PM 2.5 is not the same as PM 10

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, a nd the lack of unusual entrie s in source inspection logs. 43 This evidence is
consistent with ARB’s conclusion th at the cause of the exceedance wa s not local; however, the extent of ICAPCD’s investigation is unclear and th is evidence does not dir ectly 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 PM 10 from high winds in the southeast portion of Imperial County and Mexico, and fro m localized high winds from thunderstorm outflows north of Imperial County late in the day, causing a concentration “spike.” While EPA does not dispute that thu nderstorm activity occurred, the ev idence 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 Westmorl and is simultaneous with the Westmorland concentration spike, rather than an hour or two before as one w ould 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 contributio ns of possible source areas in th e northwest, northeast, east, and southeast are little examined. The weight of evid ence 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

PM 10 concentrations at multiple monitors are well correlated with high speed winds from the west, where there are large areas of natural desert. Th e correlation is supplemente d by back-trajectories.
The demonstrations assume that the source is dese rt areas to the west, since they are the dominant

PM 10 emission sources in the county- wide inventory. However, as discussed below, there are deficiencies in the inventory reli ed upon with respect to determini ng 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 ha s been established as “high” fo r Imperial County (e.g. based on frequency of occurrence, entrainment speed threshol d, or threshold speed for “overcoming” controls), ARB’s demonstration provides evidence that wi nds 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-tra iler 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 Bo rrego 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 ma ximum wind speeds at Westmorland on April 12, 2007 to those in the months of April between 2004 an d 2007, and finds that those speeds are 3 standard deviations above the average daily maximum for the month. 49 While the suggested PM 10 source area is not at Westmorland itself, these data are consis tent 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 numb er 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 stati ons, 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 Or iginal 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, Bo rrego 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 fo r Julian. This is consistent with a wind-driven

PM 10 source area in the western part of Imperial County.

The demonstration compares maximum wind speeds e xperienced 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 approxi mately 4 standard deviations above the average daily maximum values for the time period (J une NED, Table 8, p. 29). Graphs of historical days in June show June 5, 2007 to have the firs t 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 PM 10 source area is not at the monitors themselves, the comparison table and the graph are eviden ce 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 moni tors, including Anza Borrego De sert (ABD). This is certainly plausible given the wi nd direction from the west, the high winds around ABD at Borrego Springs and Julian (measurements within ABD itsel f are not available), an d the relatively high
emissions ascribed to desert land in the inventory. Howeve r, off-highway vehicle activity occurs in the Superstition Mountain and Plaste r City BLM Open Area and the Ocotillo Wells State Vehicle Recreation Area (SVRA), to the west of the Brawley and Westmorla nd 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 emi ssion contributions in “Other caus es” 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 decidi ng which sources need reas onable measures (section 4.2 above), and also for establishing th e required clear cau sal relationship.

A county-wide monthly or annual inventory may not be a sufficien t basis for the demonstrations'
conclusion that the contribution of windblown dust emi ssions from agricultural lands is small, or for source attribution in general. Su ch inventories lack temporal and spatial resolution, and do not account
for the distance of emissions from the monitor.

Regarding temporal resolution, th e monthly average inventory does not account for the particular conditions of the exceedance days. The demonstrati ons do argue that such an episodic inventory would have a higher relative contribution fr om wind-blown dust compared to other PM 10 sources than does the monthly inventory relied upon, strengthen ing 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 agricultura l lands relative to natural desert emissions would change if an episodic inventory were used. If OHV and agricultu ral emissions increase more quickly with wind speed than natural desert emissions, thei r 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 in ventory relied upon does not distingui sh portions of the County that are upwind of a monitor from those that are downwind; all locations ar e 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 importa nt for the April 12, 2007 Westmorland value of 156 μg/m 3 , and the June 5, 2007 Niland value of 162 μg/m 3 . For these moderate exceedances, a relatively
small reduction in the local emissi ons contribution could have a rela tively 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., dist ance 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. (“Devel opment 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 invent ory is a substantial improvement over previous efforts, it makes generi c assumptions about the de gree 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 measurem ents and the use of more spatially resolved parameters, a re-run of the existi ng model with episode-specific winds would be an improvement over the monthly inventory.

A spatially resolved episodic inventory that addresse s 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 plot s (April NED, Figures 11 and 19, pp. 19 and 25; see Figure 5 below), after about noon on April 11, wind dire ctions 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, a nd 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 corr elation between the shift in wind di rection from east-south east at noon on April 11, and shifts to high wind speed and high PM 10 at around the same time (April NED, Figures 12 and 20, pp. 20 and 25; see Fi gure 6 below). The winds and PM 10 declined after midnight on April 11, increasing again after around 7 am on Apr il 12. Mexicali stations show a similar pattern (April NED, Figures. 13, 14, 21 and 22, pp. 21 and 26). The increase in PM 10 tends to lag behind the increase in wind speed by an hour or so, which is consistent with tran sport from some distance to the

west.

April 12 HYSPLIT trajectories ending at Westmorl and (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

HourMiles per hour

0 100 200 300 400Degrees 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 PM 10

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

HourMiles per hour

0 200 400 600 800 1000μg/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 PM 10 is not quite as striking as for the April event, but is still c onvincing evidence of causality.

HYSPLIT and WIND2D for June 5 c onfirm flow from the west, incl uding 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 PM 10 concentrations throughout the area, including the monitored NAAQS exceedances. When wind is high and from the west, PM 10 is high; when it is not,

PM 10 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 PM 10 and maximum wind speeds on Apr il 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 PM 10 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 PM 10 from Beta Attenuated Monitors (BAM) on Ap ril 5-19, 2007, and shows that concentrations on April 11 and 12 at Brawley, Westmorland, and Niland were approximat ely double those of neighboring days. Westmorland concen trations were also a bit elevat ed two days later. The same applies to the PM 10 Federal Reference Monitor (FRM) data (every 6th day sche dule) from preceding and succeeding weeks (April NED, Figure 29, p.34).

June 5, 2007 The June demonstration compares PM 10 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 PM 10 and wind speed values are clear outliers. (As noted previously, the page and figure num bers are the same for the April and June demonstrations).

At the Westmorland, Brawley, and Niland BAMs, the J une 5 monitored values were more than double
the PM 10 values for neighboring days. 59 The same was true for FRM PM 10 values for all the exceeding monitors for neighboring weeks. 60

5.3.5 Consideration of Other Causes

ARB’s demonstrations addr ess the issues of whether the exce edances 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 demonstrati on points out that PM 10 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 re gional 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 regiona l meteorological occurrence.

The demonstration also argues th at if agricultural lands were a dominant contributor to PM 10 on April 12, then the El Centro monitor would have had hi gher concentrations than Brawley or Westmorland, since it has roughly 2-3 times as much ag ricultural land upwind toward the west. 63 However, examination of all the monitors and their upw ind agricultural fetch doe s not show any clear relationship between concentr ation and upwind agriculture. 64 Other monitors do not fit the relationship cited in the documentation: Braw ley had nearly double the concentration that Westmorland had (291 vs. 155 μg/m 3 ), 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/m 3 ) and the lowest fetch. These figures would seem to indicate a direct
correlation between the amount of upwind agricultural land and PM 10 concentrations. Without knowing the particulars of the land uses upwind of each site, it is di fficult to draw conclusions for either of these possible relationships. The demons tration’s conclusion that agriculture was not the

dominant PM 10 source may be correct, but even a part ial contribution could be important at Westmorland with its 155 μg/m 3 concentration.

Figure 7. Satellite photograph of Imperial County, with PM 10 monitors

The demonstration states that “no unusual local anthropogenic emissions [were] reported before, during, and after the event” and th at “Imperial County APCD estimates that anthropogenic emissions were approximately constant befo re, during, and after the event.” 65 However, the extent of the investigation underlying these statements is not cl ear. Also, presumably “anthropogenic emissions”
refers to direct emissions from a subset of anthropogenic activities, si nce wind would increase emissions from land disturbed by anthropogenic activity before or during the ev ent (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 PM 10 NAAQS exceedances.

65 April NED, p. 33.- 25 - June 5, 2007 Like the April demonstration, the June demonstration points out that both PM 10 concentrations and wind speeds were elevated over a large area, indica ting the regional nature of the event (September NED, pp. 4 and 16). This evidence suggests the cause was no t specific to a singl e monitor (regionally elevated winds could generate dust locally).

Again, the June demonstration argue s that if agricultural lands were a dominant contributor to PM 10 on June 5, the El Centro monitor (200 μg/m 3 ) would have had higher concen trations than Brawley (281 μg/m 3 ), Westmorland (226 μg/m 3 ), and Calexico-Ethel (282 μg/m 3 ), 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 othe r locations, so locally-generated agricultural dust would be exp ected to be greater at El Centro, whereas the observed PM 10 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 r easonable, 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 dem onstrates that the April 12 and June 5, 2007 PM 10 exceedances were probably caused by wind-driven PM 10 emissions from some sources west of the monitors. However, without better and more comp lete information regardi ng source attribution, the cause of the exceedances remain unclear, as do the ex tent 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 histori cal fluctuations.” Ther e is no specific threshold test for this requirement, but c oncentrations in the hi gh percentiles can prov ide 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 PM 10 vs. wind speed graph in Fi gure 25 of the September NED shows only meteorologically similar days, not the full data set, but consid ering just the vertical spread of concentrations in the graph, it is clea r that the September 2, 2006 concentrations are among the highest values. Thus, the concentrations ar e 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 PM 10 observations against time. While not am ong the very highest values recorded, the Westmorland exceedance of 167 μg/m 3 is clearly beyond the range of normal historical fluctuations, which vary seasonally around 50 μg/m 3

.

6.3 April 12 and June 5, 2007 Events

April 12, 2007

PM 10 data from the months of April in 1995-2007 for Westmorland and 1987-2008 for Brawley show that the April 12, 2007 PM 10 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/m 3 was the highest during 2001-2007; the Westmorland value of 155 μg/m 3 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 hi storical fluctuations.

As mentioned above in section 5.3.1, th e demonstration’s graphs comparing PM 10 and maximum wind speed on April 12, 2007 to historical months of Apr il show that for both the Brawley and Westmorland monitors, the April 12, 2007 PM 10 and wind speed values are clear outliers. 70

EPA agrees that the evidence presented demonstrates that the April 12, 2007 PM 10 concentrations at Brawley and Westmorland are in excess of normal histori cal fluctuations.

June 5, 2007
Historical PM 10 data for the months of June (during an uns pecified time period) for Brawley, CalexicoEthel, El Centro, Niland, Westmo rland show that the June 5, 2007 PM 10 concentration is at the high
end of the concentration distri butions 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 consid ering all data, and the 100th percentile considering just the months of June ; all were in the top five concentr ations recorded. These facts are
evidence that the measured concentrations are in excess of normal hist orical fluctuations.

As mentioned above in section 5.3.1, the graph in the demonstration comparing PM 10 and maximum wind speed on June 5, 2007 to histori cal data for the months of June show that for both the Brawley and Westmorland monitors, the June 5, 2007 PM 10 and wind speed values are clear outliers.

EPA agrees that the evidence presente d demonstrates that the June 5, 2007 PM 10 concentrations at Brawley, Calexico-Ethel, El Centro, Niland, West morland 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 PM 10 concentrations for Imperial County exceptional event claims, using all 2001-2007 data and using onl y data from corresponding month

Brawley Calexico -

Ethel Calexico -
Grant El Centro Niland Westmorland

April 12th, 2007 (μg/m 3 ) 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/m 3 ) 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/m 3 ) 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; 10 0% is not necessarily the overall maximum.



### 7.0 No exceedan ce 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 wei ght 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 influen ce of the event are rough
evidence of what the concentration on the ev ent day would have b een 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 Im perial 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 exceedan ces 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 a nd 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 Mexi co, it is not clear that th e 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/m 3 concentration "spike" at 7 pm, the 24-hour average 169 μg/m 3 BAM concentration at Westmorland would not have been an exceedance 74 - without that value, the 24-hour average would have been 149 μg/m 3

. 75 Even though ARB states the BAM monitor in this case is used only for forecasting 76 rather than directly in assess ing NAAQS exceedances, this is good evidence that the regular Size-Sel ective 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 conflic ting 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/m 3
concentration spike. Therefore the “but fo r” regulatory criteri on has not been met.

7.3 April 12 and June 5, 2007 Events

The clear correlation between elevated PM 10 concentrations and the sh ift at around noon on April 11 and on June 5 to high wind speed and westerly di rection (see “transport ” section 5.3.3, above) is
evidence that PM 10 concentrations may have continued at the non-exceeding level of about 50 μg/m 3
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) 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 by July 1 of the year following the event. For the 2007 even ts, ICAPCD sent a letter to EPA dated September 17, 2007 describing the events as likely due to hi gh 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 demonstr ation to EPA within three years of the event. For the September 2, 2006 event, draf t 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 we re 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 re quires notice and opportunity for public comment. ARB’s
documentation shows evidence of news paper publication for a ll three of the event days. In addition,
EPA found that the demonstrations were available to the public via the IAPC D web site starting in March, 2009.

Finally, 40 CFR §50.14(c)(3)(i) requ ires 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 Sept ember 2, 2006 Event Involving the Calexico
Monitors

ARB states that the Calexico exceedances resulte d from a combination of typical emissions from Mexicali, and high wind-driven emissions from unpa ved 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 measur ed 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 fo r 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.

PM 10 emissions were allegedly transported to West morland causing a single hour “spike” of very high concentration. However, wind direct ion data and HYSPLIT trajectories are not consistent with ARB’s
claim. PM 10 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. Furthe r, due to inconsistencies in the timing of winds and concentrations, and due to th e conflicting evidence rega rding the wind direction,
the “but for” criterion has not been met. Base d on the weight of evidence provided, EPA does not
concur that the Westmorland exceedance shoul d 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 loca tions consistently exceeded 15 mph, approached 25 mph during several hours, and gusted above 25 mph on a few occasions. During these periods, PM 10 concentrations sharply incr eased. The correlation of wind speed and direction, PM 10 concentrations, and back-trajectories passing over desert areas to the west are evidence of a high wind event that influenced the monitors. However, agricultura l and off-highway vehicl e activity are potential significant sources of emissions. A RB neither provided source attributio n 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 regardi ng the causal relationship
between the exceedances and the wind-driven PM 10 emissions and the extent to which the proffered events meet the requirement at 40 CFR 50.1(j) that they were not reason ably controllable or preventable. ARB’s failure to prov ide 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, Se ptember 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, reque sting exclusion of September 2, 2006, April 12, 2007, and June 5, 2007 Imperial County PM 10 Exceedances SS_051409_EPA_ltr.pdf

“Proof of Publication”, st atement of the principal clerk for the printer of the Imperial Valley Press, dated April 1, 2009, including photocopy of newspape r announcement, “Notice of Petition for
Exclusion of PM 10 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 West morland, 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 excl usion 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, Brawle y and Westmorland, California, April 12, 2007”, Imperial County Air Pollution Control Di strict 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 excl usion of June 5, 2007 exceedances, with enclosures: June ARB staff analysis; June original NED : ”Natural Event Documentation, Imperial County, California, June 5, 2007”, Impe rial 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_Ex ceptional_Event_Report.pdf

Electronic versions available at: http://www.imperialcounty.net/AirPollution/We b%20Pages/2009%20March%20Natural%20Events.ht

m