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# Addendum

Natural Event Documentation

Brawley and Westmorland, California

April 12, 2007



# Imperial County Air Pollution Control District

March 12, 2009

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SUMMARY

This report is an addendum to the April 12, 2007 Na tural Event Documentation (NED) for Brawley and Westmorland monitoring statio ns prepared by the Imperial County Air Pollution Control District (Imperial Cou nty APCD). The Imperial County APCD documentation was submitted to the U.S. EPA by the California Air Resources Board (ARB) on June 13, 2008. This r eport demonstrates that without this regional natural high wind event, ther e would not have been exceedances of the federal PM10 Standard on April 1 2, 2007.

The Code of Federal Regulations (CFR) provides the definition and criteria for determining whether air quality data is impacted by an exceptional event. The 40 CFR 50.1 (j) definition states that “exceptional event means an event that affects air quality, is not reasonably controllable or preventable, is an event caused by human activity that is unlikely to recur at a particular location or a natural event, and is determined by the Administrat or in accordance with 40 CFR 50.14 to be an exceptional event.” The demonstrati on to justify data exclusion as outlined in 40 CFR 50.14 specifies that evidence mu st be provided that:

1. The event meets the definition of an exceptional event; 2. 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;

3. The event is associated with a measured concentr ation in excess of normal historical fluctuations, including backgroun d; and,

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

This report documents that the event meets the abov e criteria and provides analysis to demonstrate that:

I. The dust event was not reasonably controllable o r preventable because the PM10 originated from a non-anthropogenic source;

II. There is a clear-causal connection between the high wind event and the exceedances at Brawley and Westmorland; III. The measured concentrations were beyond normal historical levels; and

IV. The exceedances would not have occurred “but fo r” the high winds.

On April 12, 2007, strong winds from the west with gusts of 30 mph over the Anza Borrego Desert entrained desert soil into the atmosphere and caused two Imperial County Federal Reference Monitors (FRM) to record concentrations in

excess of the federal 24-Hour PM10 standard of 150 μg/m3 (Table 1). According to the Imperial Valley Press, the winds were so hig h on April 12, 2007 that they overturned a big-rig, they toppled trees, and the E nergy Department was put on alert due to wind incidents in Brawley, Imperial, a nd El Centro. In addition, the California Highway Patrol issued a wind advisory wa rning motorists to slow down and drive cautiously since gusts could reach up to 50 mph.

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As specified in their 2005 Natural Event Action Pla n (NEAP), ICAPCD issued warnings of “unhealthy” air quality on April 12th, via their website (NED, p. 30). In addition, the Imperial County APCD published in the local newspaper a notice of petition to exclude April 12, 2007 PM10 monitori ng data affected by the high

wind event.

Table 1. Imperial County APCD NAAQS Exceedances, A pril 12, 2007 Monitoring Site AQS Site Number 24-Hour PM10

Concentration ( μ g/m3) Brawley-Main Street 060250007 291

Westmorland 1 st 

### Street 060254003 155

Figure 1 shows Imperial County in relation to the A nza Borrego Desert and the surrounding counties and states. The Anza Borrego Desert is a broad expanse of land that includes western Imperial County, as w ell as portions of eastern San Diego County and southern Riverside County.

Figure 1. Regional map of Imperial County and the A nza Borrego Desert

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Figure 2 provides a satellite view of the region, a nd includes the PM10 values measured at each monitor on April 12, 2007. The el evated PM10 values measured at multiple sites indicate a large scale r egional event. As seen from the concentrations at the monitors, blowing dust tr ansported from the west was deposited not only at Brawley and Westmorland, but at other areas in the Imperial Valley as well as areas to the north such as Indio and to the west in
Yuma, Arizona.

Figure 2. Imperial County Monitoring Stations and PM10 Concentrations, April 12, 2007

As shown in Table 2, low PM2.5 concentrations on Ap ril 12, 2007 confirm that the majority of particulate matter was in the coars e (PM2.5 to PM10) mass size range, indicating fugitive dust as the primary sour ce of the elevated

concentrations.

Table 2. Imperial County APCD PM2.5 and PM10 conce ntrations, April 12, 2007

Date Site Name PM2.5 PM10

4/12/2007 Brawley 12 291

4/12/2007 Calexico-Ethel 8.2 124

4/12/2007 El Centro 4.8 121

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Anthropogenic emissions in Imperial County were rou ghly constant before, during and after the event, and PM10 concentrations were approximately 50 ug/m3 prior to and after the event, well below t he level of the federal standard. Countywide, windblown dust emissions from urban lan ds are minimal and emissions from agricultural lands are small (6%) co mpared to the desert area emissions in Imperial County. The location of urba n and agricultural lands in relationship to the air quality monitors is shown i n Figure 2. The urban and agricultural lands are located in the center portio n of Imperial County, south of the Salton Sea, and can be identified by the green coloring on the map. The

desert area emissions does not include the extensiv e desert area in San Diego County which also likely contributed to the dust lo ading in Imperial County.

Although windblown dust emissions from agricultural lands are estimated to be only a small contributor to the measured PM10 conce ntrations on April 12, 2007, Imperial County had Best Available Control Measures in place through their conservation management practices rule. This rule was adopted in 2005 after an extensive public process with the general public, U .S. EPA, and local farmers. Dispersed throughout the agricultural lands are fal low farm lands which participate in the Imperial Irrigation District’s F allowing Program. The Fallowing Program also requires farmers to implement United S tates Department of Agriculture Best Management Practices on the land t o control fugitive dust.

In summary, on April 12, 2007, PM10 exceedances at the Westmorland and Brawley monitors were impacted by a natural high wi nd event in which anthropogenic sources played only a small role. Ne vertheless, controls were in place to reduce dust from anthropogenic sources and these control measures were reasonable and appropriate, taking into accoun t the magnitude of the high winds and the small contribution from anthropogenic sources. As such, this high

wind natural event and the associated exceedances w ere not reasonably controllable or preventable and all of the statutor y exceptional events criteria

have been met.

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Analysis

I. Dust Event Not Reasonably Controllable or Preven table

This section will discuss the methods that were use d to estimate the emission inventory for Imperial County and support the concl usion that the dominant

source of dust was natural windblown emissions from the Anza Borrego Desert. Information on the Land Use/Land Cover (LULC) data, the State Soil Geographic Database (STATSGO), and the threshold friction velo city necessary to entrain dust in the Imperial County is presented. This sec tion also discusses the different emission sources in Imperial County along with their controls, and the source activity that occurred on April 12, 2007.

a. Imperial County – Dust Emissive Characteristics.

Windblown Dust Study

In May 2004, ENVIRON prepared for the Imperial Coun ty Air Pollution Control District (Imperial County APCD) a report entitled “ Development of a Windblown Fugitive Dust Model and Inventory for Imperial Coun ty” (Windblown Dust Study). The results of the Windblown Dust Study were revise d in September 2004 as a

result of discussions with ARB and U.S. EPA and we re documented in Appendix A of the 2005 Best Available Control Measu res (BACM) demonstration report (see Attachment A of the NED). The BACM rep ort presents the methodology used in estimating windblown fugitive d ust from agricultural and barren lands in Imperial County. In particular, i t presents information regarding

land use/land cover and soil characteristics for th e desert areas, including the Anza Borrego Desert. The primary source of LULC da ta is based on land use surveys conducted by the California Department of W ater Resources (DWR) and augmented with the USGS National Land Cover Databas e (NLCD). This

information is reiterated here.

Land Use and Land Cover Data

The DWR Land Use Survey databases are developed by the department’s Division of Planning and Local Assistance in order to aid in the efforts of continually monitoring land use for the purpose of tracking the amount of water consumed and changes in its usage. The data is ava ilable at the county level and includes detailed agricultural land use classif ications, as well as less detailed urban and native vegetation land uses. The data wa s compiled from aerial photography alongside extensive field verification and is updated on a rotating basis by the county approximately every seven years . Thus, the data is based on varying years, depending on the county, ranging from the late 1980’s through the late 1990’s. Because the DWR survey data does not explicitly consider certain land use types (i.e., urban lands, desert l ands, forest lands, etc.) it was necessary to augment the DWR land use data with the National Land Cover Data (NCLD).

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The NLCD was developed as part of a cooperative pro ject between the U.S. Geological Survey and U.S. EPA to produce a consistent land cover data layer for the entire conterminous United States, ba sed on 30-meter Landsat thematic mapper (TM) data. The NLCD was developed from TM data acquired from the Multi-Resolution Land Characterization (MR LC) Consortium, a partnership of federal agencies that produce or use land cover data.

A display of the combined DWR and NLCD land use dat abases is shown in Figure 3 for the Imperial Valley domain. This show s the majority of the Anza Borrego Desert covered by sand and shrub land, both of which are characteristic of the desert regions. The Anza Borrego Desert, pa rt of the Sonoran Desert, covers the eastern portion of San Diego County, the western region of Imperial County and the southern section of Riverside County between the eastern slopes of the Peninsular Ranges (east of Highway 79) and t he Salton Sea. Toward the

north it extends eastward across the Santa Rosa Mou ntains from the town of Anza on Highway 371, while toward the south it exte nds up to the International

border between Mexico and the United States.

Imperial County APCD also noted (Table 3 of the NED ) that the 12-month period prior to the event was exceptionally dry, experienc ing only 5% of normal precipitation. This, coupled with extreme high tem peratures experienced in the months preceding the event (Table 4 of the NED), co ntributed to arid conditions in the Anza Borrego Desert area and the availabilit y of material to transport into and around the Imperial Valley.

Figure 3. Land Use Coverage for Imperial Valley Ba sed on

Combined DWR and NLCD Information

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To determine the characteristics or type of soil pr esent in Imperial County, the Wind Blown Dust Study used the STATSGO. The STATSG O database was developed by the Natural Resources Conservation Ser vice of the US Department of Agricultural (USDA), providing detailed informat ion concerning the taxonomy of the soils, including soil texture class, percent age of sand, silt and clay, and the available water capacity of the soil. Figure 4 dis plays the STATSGO data for the Imperial County domain. The data in Figure 4 corro borates the information in Figure 3, which indicates that the soil texture at the Anza Borrego Desert area is composed of sand and sandy loam, making this area u nstable and susceptible for soil erosion even at low wind speeds.

Figure 3 also shows that the center portion of Impe rial County is mostly agricultural land. The main soil texture in this a gricultural area is silty clay loam, as opposed to sand, sand bedrock, and sandy loam to the West. According to the STATSGO soil texture and soil group codes, silt y clay loam has a soil texture code of 8 which is considered stable, whereas sand has a soil texture code of 1 and is very unstable. The assumed soil stability a lso determines the duration of the windblown dust emissions; the Wind Blown Dust S tudy states that unstable soil can emit for up to 10 hours during any single wind event; whereas stable soils emit only for one hour. To account for veget ative canopy cover, the emission rates are reduced according to the assumed percent canopy cover. Crop cover information was based on the crop calend ar for each individual crop. Since the agricultural areas have stable soils with a high percent of canopy
cover, this results in less wind erosion, lower dus t flux rates, and a shorter wind

event duration.

Figure 4. STATSGO Soils Texture Classification for Imperial County and surrounding areas

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Figure 5 shows the monthly PM10 windblown emissions by land use type in 2004 (See Table 3-2 from the Regulation VIII BACM Analys is). Agriculture and Urban open areas emit the least amount of PM10 throughout the year in Imperial County. According to Figure 5, agriculture is resp onsible for only 6% of the total windblown PM10 emissions for the month of April in Imperial County. Sand dunes along with shrubs and grasslands in Imperial County emit 14 times more PM10 than agricultural lands in April.

0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 400.00 450.00 500.00

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

MonthPM10 (Tons/Day) Urban Open Areas

Desert-Other

Desert-Dunes

Shrub/Grassland

Agriculture



### Figure 5. 2004 monthly PM10 emissions by land use type

Table 3 shows the PM10 dust emissions for the major land use categories in 2004 (See Table 3-2 of the Regulation VIII BACM Ana lysis). Urban sources of windblown dust are very minor. In addition, becaus e of its year round crop cover, irrigation, and soil stability, agricultural lands emit very small amounts of PM10.

Table 3 Windblown Emissions from Lands by Month and Annual Average (tons/day) in 2004

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann ual Agriculture 8.59 11.32 19.93 28.48 12.95 16.2 7.37 4.33 9.76 0.38 6.6 4.42 10.81

Urban 0.07 0 0 0.03 0.01 0 0 0 0 0 0 0 0.01

DesertOther 20.55 38.09 52.04 97.31 57.09 71.8 15.7 43.16 29.29 6.36 22.0 13.37 38.74

Forest 0.07 0.06 0.07 0.15 0.06 0.11 0.02 0.07 0.03 0 0.04 0.02 0.06

Shrub/

Grassland 96.15 102.1 120.6 243.2 97.52 189. 40.6 139.9 63.84 7.32 64 25.4 98.75

DesertDunes 21.77 31.88 36.23 62.37 24.77 4.59 16.7 11.98 8.22 0 9.32 11.64 19.85

TOTAL (TPD) 147.2 183.4 228.9 431.5 192.4 282 80.5 199.5 111.1 14.0 102 54.84 168.22

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b. Threshold Dust - Lofting Wind Speed for Imperial County Desert Areas.

The Wind Blown Dust Study established the methodolo gy to estimate windblown fugitive dust from agricultural and barren lands in Imperial County. It recognized a wind speed of 15 mph or higher as necessary to er ode and entrain fugitive dust and is appropriate for unstable areas, such as the Anza Borrego Desert.

The study follows the methodology of MacDougall, 20 02. This methodology was applied in the development of the Clark County, Nev ada PM10 SIP and implemented in the development of the WRAP dust inv entory. Following this approach, wind events are considered only when the 10-meter wind speeds reach or exceed the lowest defined wind speed for w hich emission factors are available. Due to the limited availability of comp rehensive wind tunnel studies, emission factors were derived only for 10-meter win d speeds of 15 mph or higher. The stability of vacant land parcels was d etermined based on LULC. As detailed in MacDougall’s methodology, a wind event may be defined as any time period for which the winds reach or exceed the thre shold wind velocity separated by at least 12 hours.

Figures 6 and 7 show the maximum hourly wind speed in relation to PM10 concentrations for the month of April for 2002 thro ugh 2007 at the Brawley and Westmorland stations. With the exception of April 12, 2007, which is flagged in AQS as a high wind natural event, maximum hourly wi nd speeds in April were, for the most part, between 5 and 20 mph at both sta tions. PM10 concentrations generally remained below 100 μg/m3 at Brawley and 1 20 μg/m3 at Westmorland. The concentrations seen on April 12, at Westmorland and Brawley were above

normal historical fluctuations and the winds were u nusually high, wind speeds reached as high as 12 mph above the entrainment thr eshold of 15 mph.

Figure 6. Brawley PM10 vs. Maximum Hourly Wind Sp eed,

April 2002 -2007 April 12 2007

291

0 50 100 150 200 250 300 350

0.0 5.0 10.0 15.0 20.0 25.0 30.0

Maximum Hourly Wind Speed (mph)FRM PM10 (microgr/cubic meter)

### 11 Figure 7. Westmorland PM10 vs. Maximum Hourly Win d Speed, April 2002-2007

c. Emissions Inventory.

The PM10 emission inventory compares the type of so urces that generate dust and could potentially contribute to PM10 NAAQS viol ations. The 2005 PM10 emission inventory is summarized in Table 4 and fea tures adjustments made to incorporate updated cattle emissions, updated windb lown dust model results, and updates for entrained and windblown unpaved roa d dust estimates. The middle column of Table 4 portrays annual average em issions. On a day with exceptionally high winds such as April 12, 2007, wi ndblown emissions are expected to have been significantly higher than the ir annual average values. The Windblown Dust Study showed that spring in Impe rial County produces the highest PM10 emissions for the entire year (Figure 5).

A conservative estimate of the emission inventory o n a very windy day, derived by modifying the annual average inventory to use Wi ndblown Dust Model results for the month of April (which featured the highest monthly windblown emissions in 2002 according to the model), is given in the ri ght column of Table 4. We note that according to both the annual average and the “ high wind” inventories, windblown emissions from agricultural lands only co ntribute 5-6% of all windblown emissions, or 4-5% of the total inventory . Furthermore, although nonanthropogenic sources already account for ~55% o f total emissions under average conditions, their fractional contribution t o the total “high wind” emissions is ~75%. Given that the “high wind” inventory of T able 4 is expected to

underestimate the level of windblown emissions that occur on the windiest days, the predominant source of emissions contributing to the April 12, 2007 Imperial County measurements were from natural desert lands. April 12 2007

155

0 20 40 60 80 100 120 140 160 180

0.0 5.0 10.0 15.0 20.0 25.0 30.0

Maximum Hourly Wind Speed (mph)FRM PM10 (microgr/cubic meter)

### 12 Table 4. Revised 2005 Annual Average PM 10 

### Emission Inventory

for Imperial County (tons/day)

Source Category Annual Average Emissions* (tons/day) “High Wind” Emissions** (tons/day)

Fuel Combustion 0.4 0.4 Waste Disposal 0.0 0.0 Cleaning/Surface Coatings 0.0 0.0 Petroleum Prod/Mktg 0.0 0.0

Industrial Processes: 2.8 2.8

Mineral Processes 2.6 2.6 Food/Agriculture 0.2 0.2 Solvent Evaporation 0.0 0.0

Res Fuel Combustion 0.1 0.1 Farming: 9.9 9.9 Tilling 7.1 7.1

Harvest 0.0 0.0

Cattle 2.8 2.8

Paved Road Dust 4.2 4.2 Entrained Unpaved Road Dust: 59.1 59.1 City/County 25.6 25.6

Canal 30.7 30.7

BLM/USFS 1.4 1.4

Farm 1.4 1.4

Windblown Dust: 200.5 463.7 Open Areas: 157.3 402.9

Grassland*** 98.7 243.2

Dunes*** 19.8 62.4

Other Barren Lands*** 38.7 97.3

Urban 0.0 0.0 Unpaved Roads 30.5 30.5 Ag Lands 12.6 30.3 Non-Pasture Ag Lands 10.8 28.5

Pasture 1.8 1.8

Construction 1.9 1.9

Fires 0.0 0.0 Waste Burning 2.8 2.8 Cooking 0.1 0.1

On-Road Mobile 1.1 1.1

Other Mobile 1.0 1.0

Total 284 548 Total Anthropogenic 127 145

* Values in emission inventory table are rounded to the nearest 0.1 ton/day ** Emissions equal to the annual average, except fo r windblown sources, for which the predictions of t he Windblown Dust Study for the month of April, 2002 were used. (The month of April had the highest monthly windblown e missions in 2002 according to the model). *** Natural sources of dust, only 1% of which come from lands disturbed by humans.

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d. Fugitive Dust Control Measures

Fugitive Dust Sources

.

Imperial County APCD adopted Regulation VIII, Fugit ive Dust Rules, and Rule 806, Conservation Management Practices, on Nov ember 8, 2005. The rules were adopted after an extensive public proces s with both the general public and affected industry. The rules require implement ing fugitive dust control measures for source categories such as: Bureau of L and Management (BLM) controlled areas, construction and earthmoving acti vities, handling and transport of bulk materials, disturbed open areas, paved and unpaved roads, and agricultural operations. The rule standards are pe rformance based whereby the operators are allowed to determine the control tech niques sufficient to limit visible dust emissions and, if applicable to that source, t o implement requirements for a

stabilized surface.

In general, rules can be classified as meeting eith er Reasonably Available Control Technology (RACM), or the more stringent Be st Available Control Technology (BACM). Imperial County’s rule was adop ted to meet BACM requirements as outlined in the District’s 2005 BAC M Demonstration Report. RACM level rules satisfy the Exceptional Events Rul e criteria that reasonable and appropriate measures for anthropogenic sources were in place during an event. As such, the more stringent BACM level rule s adopted in Imperial County meet this requirement as well.

Fallow Fields.

The Imperial Irrigation District (IID) started a “F allowing Program” in 2003. The purpose of the Fallowing Program is to allow willin g land owners and/or lessees

to contract with IID to fallow fields to meet the t ransfer and Salton Sea mitigation water needs. Each year the price for the water to be conserved from fallowing is set by IID and solicitations are sent out asking fo r voluntary participation to fallow a field in return for payment of the conserved wate r. Fields are then contracted

based on a random selection to meet the amount of c onserved water needed each year. Each field’s participation in the fallo wing program is limited to two out of every four years. Participants of the fallowing program are responsible for dust control as described below. After proof of pe rformance, the participants are reimbursed for pre-approved dust mitigation Best Ma nagement Practices.

In order to mitigate air quality impacts on Fallow Lands, only the Best Management Practices ("BMPs") recommended by the U. S. Department of Agriculture Natural Resources Conservation Service and listed below will be considered for payment reimbursement. These practi ces are:

1) Plan ahead to start with plenty of vegetation re sidue, and maintain as much residue on fallowed fields as possible. Resid ue is more effective for wind erosion protection if left standing.

### 14 2) Avoid any tillage if possible. 3) Avoid any traffic on the field or tillage when f ields are extremely dry to avoid pulverization. 4) If residues are not adequate, either small grain can be seeded about the first of the year to take advantage of winter r ains or soil stabilization chemicals may be applied to fallowed lands. 5) In addition to the above BMPs, in order to satis fy Imperial County dust control and mitigation requirements, Fallowing Part y will comply with any lawful conditions required by the Imperial County A ir Pollution Control

District.

Farmers who do not participate in IID’s fallowing p rogram will usually continue to grow crops on the land and thus would be subject to ICAPCD’s Conservation Management Practices rule. Lands will generally on ly be left fallow if the land is converted to a purpose other than agriculture, or t he farmer is no longer able to plant on their fields because of economic hardships . Unless a farmer is losing the land because of a bankruptcy or other financial reason farmers will usually rent out their fields to other farmers to grow crop s.

e. Source Activity on April 12, 2007

Agricultural Burning and Fugitive Dust Controls

. In compliance with the existing Imperial County Smo ke Management Program, agricultural burning was prohibited in Imperial Cou nty on April 12, 2007. Weather forecasts had indicated possible high winds with gusts. Fugitive windblown dust controls were also in effect per the requirements of Regulation VIII. A copy of the Imperial County APCD Daily Wea ther and Agricultural Burning Information report is attached to this repo rt (Attachment B).

Inspections

. The Imperial County APCD inspectors did not perform any facility inspections on April 12, 2007. Imperial County APCD staff did not observe any unusual emissions other than the blowing dust on April 12, 2007.

Complaints. No dust or smoke related complaints were received b y the Imperial County APCD on April 12, 2007

In summary, based on the information presented in t his section, on April 12, 2007

windblown dust emissions from the barren lands to t he west in the Anza Borrego desert were the dominant cause of the suspended dus t.measured at the Brawley
and Westmorland monitors. Nevertheless, reasonable and appropriate controls were in place for all anthropogenic sources, and be st management practices were in place for agricultural lands under IID’s Fa llowing Program.

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II. Clear Causal Connection between the Wind Event and the Federal

PM10 Exceedances

.

This section will demonstrate the chronology of the wind events that occurred on April 11 and 12, 2007 causing the PM10 exceedances in Imperial County. The tools used to evaluate the sequence of events inclu de: analysis of the PM10 and BAM concentrations and the correlation with wind sp eeds, wind gusts, and wind direction on April 12, 2007, as well running hyspli t and Wind2D back trajectories to identify the source region.

a. Cause of High PM10 Concentrations on April 12, 2 007

A high wind dust event does not always show an imme diate impact on high PM10 concentrations at monitoring stations, particu larly if transported over some distance. A wind blown dust event may be comprised of entrainment of dust by high winds in the source area, followed by depositi on in a receptor area with lower wind speeds. Once suspended upwind of the mo nitoring stations, dust can be carried downwind by winds below the dust entrain ment threshold. On April 11, 2007, dust was entrained in the Anza Borr ego Desert, transported toward the east, and deposited at Westmorland and B rawley, and other areas in Imperial County, noted by increased PM10 concentrat ions at all air quality monitoring sites. Beginning in the afternoon of Ap ril 11, 2007 and continuing through April 12, excessive wind speeds and gusts w ere recorded at Imperial, Blythe, Yuma, and Thermal Airports, all bordering o r within Imperial County. The high PM10 concentration recorded at Westmorland and Brawley during the early afternoon hours of April 12, 2007 can, therefore, b e explained as an effect of the transport of dust originating from the Anza Borrego Desert and transported by high winds during the evening hours of April 11th a nd the early hours of April 12th.

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b. Chronology of High Wind Exceptional Event

On April 12, 2007, two Federal Reference Method (FR M) monitors in Imperial County recorded exceedances of the PM10 National Am bient Air Quality Standard (NAAQS) of 150 μ g/m3. Above normal concentrations were recorded at other monitors in and around Imperial County as wel l as monitors in Riverside and San Bernardino Counties, and in Yuma County, Ar izona, (Table 5) indicating a regional event.

Table 5. PM10 Concentrations at Federal Reference Monitors on April 12, 2007 (NAAQS exceedances in bold )

Monitor AQS No. Concentration ( μ g/m3) Brawley 60250007 291

Westmorland 60254003 155

Niland 60254004 108

El Centro 60251003 121

Calexico-Ethel 60250005 124

Calexico-Grant 60250004 147

Indio-Jackson 60652002 146

Victorville 60710306 358 Lucerne Valley 60710013 229

Barstow 60710001 202

Perris 60656001 167

Yuma ADEQ 147



### In addition to the concentrations measured at the F RM sites, elevated concentrations were recorded by PM10 continuous Bet a Attenuation Monitor (BAM) sites operated by Imperial County for forecas ting purposes at Brawley, Niland, and Westmorland (Table 6). Although these monitors have shown some irregularities, the concentrations can be used, on a relative basis, to show an hour-by-hour progression of the exceptional event.

Table 6. PM10 Concentrations at Imperial County BA M sites

Monitor AQS No. Concentration ( μ g/m3 ) April 11 April 12 Brawley 60250007 172 196

Niland 60254004 115 131

Westmorland 60254003 108 157

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April 11, 2007

The winds on April 11, 2007 were relatively calm in the morning. When the wind direction shifted in the late morning/early afterno on to a more western flow, the wind speed also increased significantly. These shi fts can be seen at two small weather observation sites west of the Imperial moni toring sites. Borrego Springs (KCABORRE1) in the northern portion of the Anza Bor rego Desert showed wind gusts as high as 30 mph, and Cielo Vista Observator y in Julian (KCAJULIA2), almost directly west of Westmorland on the western edge of the Anza Borrego Desert, noted wind speeds over 35 mph with gusts as high as 45 mph (Figure 8). The site at Julian, in particular, showed significa nt wind speed increases earlier in the day, allowing time for wind across the deser t to entrain and transport dust to the Imperial Valley. In a joint effort, the Imp erial County APCD and IID provide daily weather forecast for Imperial County. On Wed nesday April 11, 2007 weather reports were issued at 6:30 am and 5:30 pm. Both reports predicted strong west winds in the mountain and desert areas causing blowing sand and dust in the region.

Figure 8. Wind speed and direction at Borrego Spri ngs, CA and Julian, CA

[Source: www.wunderground.com ]

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Although no other data is available in the Anza Bor rego Desert itself, surface analysis maps of wind speeds across the southwest p art of the country show wind speeds in the area increasing. Figure 9 shows surface wind speeds across the Desert at 2 p.m. ranged from 12 to 16 knots (1 4 to 18 mph), at and slightly above the dust entrainment threshold of 15 mph. By 6 p.m., wind speeds had increased slightly to upwards of 20 knots (23 mph) (Figure 10).

Figure 9. Surface Wind Speed at 2:00 pm on 4/11/07

Figure 10. Surface Wind Speed at 6:00 pm on 4/11/0 7

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In the Imperial Valley, the early afternoon of Apri l 11 saw winds at the Westmorland and Niland monitoring stations shifting to an almost uniform western flow (Figure 11). At the same time, wind s peeds increased and PM10 concentrations as seen at the BAM sites rose (Figur e 12). Concentrations at the Brawley PM10 BAM site increased, simultaneously wit h those at Westmorland and Niland. Although the Brawley site does not hav e meteorology data, the corresponding shift in wind direction and increase in wind speed can be assumed. Wind speeds remained elevated at Westmorl and and Niland, and all

three BAM sites showed elevated concentrations for the remainder of the day. District monitoring sites do not report wind gusts but maximum hourly wind speeds were as high as 28 mph (25 knots). Table 6 in the District NED noted increased wind speeds at the District air quality m onitoring sites, California Irrigation Management Information System (CIMIS) si tes, and the four regional airports.

Concentrations at the Niland monitoring site on Apr il 12, 2007 were elevated, but

did not exceed the PM10 standard. The Niland monit or is located approximately

3 miles east of the Salton Sea and 2 miles west of a major interstate (Highway 111). Niland is the site furthest east fr om the Anza Borrego desert. In addition, because of its location, the Niland monit oring site is sheltered from much of the windblown dust from the west, the predo minant wind direction at this site. The Salton Sea and agricultural fields are l ocated west of the monitor. The recorded concentration of 108 ug/m3 on April 12th w as the second highest concentration at the Niland site for the year 2007 and the third highest concentration for the years 2005-2007.

April 11

Wind Speed (WS) and Direction (WD)

0 90 180 270 360

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

HourDegree

0 5 10 15 20 25 30knots Niland WS Westmorland WS El Centro WS Calexico-Grant WS

Niland WD Westmorland WD El Centro WD Calexico-Grant WD



### Figure 11. Hourly Average Wind Speed and Direction

,

Imperial County Monitoring Sites

### 20 April 11, 2007
PM10 and Wind Speed

0 200 400 600 800 1000

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

HourPM10 (ug/m3)

0 5 10 15 20 25 30

* - Concentration reached monitor upper detection l imitknots Niland wind Westmorland wind El Centro wind Calexico-Grant wind

Niland BAM Westmorland BAM Brawley BAM

* * * * *



### Figure 12. PM10 BAM Concentrations and Wind Speeds

,

Imperial County Monitoring Sites

### 21

Wind data for the Mexicali-Cobach and Mexicali-ITM stations (which are not BAM sites) corroborate this high wind event (Figure 13) . On the morning of April 11, wind speeds at the Mexicali-Cobach and Mexicali-ITM stations were relatively low with winds coming from the east. At about midd ay, the winds shifted to the west with a parallel increase in wind speeds. Spee ds in the later part of the day ranged from 14 to 21 knots. The BAM site at Mexic ali-UABC, which recorded a 24-hour PM10 concentration of 152 ug/m3, showed PM concentration increases throughout the rest of the day, corresponding with wind speed increases seen at the other Mexicali and Imperial County monitoring s ites. PM10 reached as high as 400 ug/m3 in the early afternoon (Figure 14).

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

0 90 180 270 3600:001:002:003:004:005:006:007:008:009:0010:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

HourDegree

0 5 10 15 20 25knots ITM WS Cobach WS ITM WD Cobach WD



### Figure 13. Hourly Average Wind Speed and Direction

,

Mexicali Monitoring Sites

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

0 50 100 150 200 250 300 350 400 450

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

HourPM10 (ug/m3)

0 5 10 15 20 25 m/s ITM WS Cobach WS UABC PM10 BAM



### Figure 14. PM10 BAM Concentrations and Wind Speeds

,

Mexicali Monitoring Sites

### 22

Figure 15 shows the results of a backward trajector y analysis for the Westmorland monitoring site for the last 12 hours o f April 11. This model, the NOAA HYSPLIT Model, shows impact from the west, ove r the Anza Borrego Desert, during the latter part of the day.

Figure 15. NOAA Hysplit Model Results, April 11, 2 007

(noon to midnight)

### 23

April 12, 2007

On Thursday April 12, 2007 the Imperial County APCD and IID weather reports were issued at 6:30 am and 5:30 pm. Both reports p redicted high winds in the regions with several areas being affected by blowin g sand and dust. On the morning of April 12, 2007 winds were blowing from t he west at all Imperial County and Mexicali stations. The wind speeds cont inued to be high. In the northern part of the Anza Borrego Desert, winds spe eds, as measured at Borrego Springs (KCABORRE1) were fairly low and fro m the east in the morning (Figure 16). This changed in the late morning when wind speeds increased to 20 mph, gusting up to 40 mph and corresponded to a dir ectional shift back to a primarily western flow. The Julian site (KCAJULIA2 ) also showed continued high winds, decreasing in mid-afternoon. Wind speeds we re peaking at over 35 mph, with some wind gusts as high as 45 mph. The direct ion was almost consistently

from the west.

Figure 16. Wind speed and direction at Borrego Spr ings, CA and Julian, CA

[Source: www.wunderground.com ]

### 24

Wind charts for the southwest portion of the countr y (Figures 17 and 18) show surface wind speeds (in knots) across the Anza Borr ego Desert and Imperial County on April 12. Figure 17, in the late morning , shows a wind speed gradient increasing from 16 to 24 knots (18 to 28 mph) acros s Imperial County. Winds over the Desert were approximately 16 to 20 knots ( 18 to 23 mph), well over the dust entrainment threshold of 15 mph. By early aft ernoon, wind speeds over the Anza Borrego Desert have increased from 20 to 24 kn ots (23 to 28 mph).

Figure 17. Surface Wind Speed at 11:00 am on 4/12/ 07

Figure 18. Surface Wind Speed at 2:00 pm on 4/12/0 7

Wind direction at sites further south and east (Imp erial County and Mexicali) continued to be from the west throughout most of th e day (Figure 19). Wind speeds, which had decreased from the previous day, began to increase again in

### 25

the morning, followed by increased hourly BAM PM10 concentrations at the Imperial County BAM sites (Figure 20); a further wi nd direction shift to northwest/north-northwest (just after 2 p.m.) resul ted in a decrease in both. This directional shift away from the fugitive dust sourc e in the Anza Borrego Desert caused PM10 concentrations at the monitoring sites to fall. An early evening shift back to the west and a slight wind speed incr ease caused BAM concentrations to again rise, but not to the extent seen earlier in the day.

April 12

Wind Speed (WS) and Direction (WD)

0 90 180 270 360

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

HourDegree

0 5 10 15 20 25 30knots Niland WS Westmorland WS El Centro WS Calexico-Grant WS

Niland WD Westmorland WD El Centro WD Calexico-Grant WD



### Figure 19. Hourly Average Wind Speeds and Directio ns,

Imperial County Monitoring Sites

April 12, 2007

PM10 and Wind Speed

0 200 400 600 800 1000

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

HourPM10 (ug/m3)

0 5 10 15 20 25 30

* - Concentration reached monitor upper detection l imitknots Niland wind Westmorland wind El Centro wind Calexico-Grant wind

Niland BAM Westmorland BAM Brawley BAM

*



### Figure 20. PM10 BAM Concentrations and Wind Speeds

Imperial County Monitoring Sites

### 26

Wind data for the Mexicali-Cobach and Mexicali-ITM stations (which are not BAM sites) corroborate this continued high wind event ( Figure 21). Wind speeds at

the Mexicali-Cobach and Mexicali-ITM stations were over 10 knots most of the day, reaching as high as 30 (34 mph) at the ITM sit e in the early afternoon. The BAM site at Mexicali-UABC, which recorded a 24-hour PM10 concentration of 168 ug/m3, showed PM concentrations increased drama tically just after this increase in wind speed (Figure 22) and just behind PM10 increases at the Imperial County BAM sites.

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

0 90 180 270 3600:001:002:003:004:005:006:007:008:009:0010:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

HourDegree

0 5 10 15 20 25 30 35knots ITM WS Cobach WS ITM WD Cobach WD



### Figure 21. Hourly Average Wind Speed and Direction

,

Mexicali Monitoring Sites

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

0 100 200 300 400 500 600 700 800 900

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

HourPM10 (ug/m3)

0 5 10 15 20 25 30 35knots ITM WS Cobach WS UABC PM10 BAM



### Figure 22. PM10 BAM Concentrations and Wind Speeds

Mexicali Monitoring Sites

### 27

Figure 23 shows the results of a backward trajector y analysis for the Westmorland monitoring site for April 12. This mod el, the NOAA HYSPLIT Model, shows impact from the west, over the Anza Bo rrego Desert, throughout the day.

Figure 23. NOAA Hysplit Model Results, April 12, 2 007 (midnight to midnight)

### 28

ARB conducted an additional analysis using a WIND2D back trajectory model (Figure 24). This model traced the air mass 42 hou rs before an arrival time of 6:00 p.m. on April 12. The trajectory (blue line) shows that the air mass moved from the Pacific Ocean into San Diego County, River side County, and finally through Imperial County before impacting the Westmo rland monitor. The trajectory clearly shows the air mass passing throu gh the Anza Borrego Desert before impacting Imperial County.

Figure 24. Westmorland Back trajectory arrival tim e 6:00pm

### 29

c. Additional Spatial and Temporal Analysis of Conc entrations

Analysis of the Imperial County emission inventory (Table 4, Section 1.c) indicates that windblown emissions from agricultura l fields can be expected to

have been 10-20 times lower than emissions from des ert and other natural, unpopulated areas. This subsection evaluates wheth er agricultural-related windblown dust may nevertheless have had a greater impact on air quality at the stations than the emission analysis would indicate, owing to the significantly closer proximity of these sources. To do so, the relationships between surface

wind conditions and PM 10 

### air quality measurements were investigated:

 Very high wind speeds were prevalent on April 12, 2007 throughout the Imperial Valley (Table 7, compare with Figure 19 an d 21): At Calipatria and Westmorland in the central part of the Imperial Valley; at Seeley and Meloland in the south part of the Imperial Valley, and at UC-Andrade in northern Mexicali. Note that the highest 24-hour a verage wind speeds south of the Salton Sea were at the Seeley and Melo land station, directly to the west and east of El Centro;

 Winds were from the West throughout the Valley sta rting in the early afternoon on April 11 until the end of the day on A pril 12; and

 The range of agricultural lands to the west of Imp erial County stations is greater for the El Centro station than for the Braw ley or Westmorland stations (by a factor of approximately 2 and 3, res pectively).

If emissions from nearby agricultural lands had mad e a dominant contribution to

PM 10 

### concentrations at Imperial County stations on Apri l 12, 2007, then PM 10 

### impacts at El Centro would have been larger than th ose at Brawley and
Westmorland. However, the observed concentrations of 121 μ g/m 3 

### at El Centro,

155 μ g/m 3 

### at Westmorland, and 291 μ g/m 3 

### at Brawley indicate that this is not the case. Thus, this observation confirms the results of the emission analysis that windblown dust from the desert is the overwhelmingl y dominant cause of the

exceedences.

### 30

Table 7. Wind Speed (WS)* and Wind Direction (WD ) at Select Imperial County CIMIS Stations** on April 11 and 12, 2007

Westmorland (181) Calipatria

(41) Seeley

(68) Meloland

(87) UC Andrade

(201) Date and

Time

WS WD WS WD WS WD WS WD WS WD

4

- 11

- 07: 0 8.9 NW 5.4 ESE 11.1 W 4.9 SSW 6.2 SSE

1 5.8 NNW 5.7 ESE 10.3 W 7.1 SE 2.6 SSE

2 4.8 E 6.7 SE 8.1 WSW 6.8 SE 3.3 SE

3 4.2 SE 7.4 ESE 4.9 SE 7.2 SE 5.2 SE

4 5.2 SE 7.6 SE 5.4 ESE 7.1 SE 5.7 SE

5 6.6 SE 6.4 ESE 3.7 SE 7.3 SE 4.3 E

6 7.3 SS E 9.7 SE 5.2 SE 9.7 SE 8.1 ESE

7 8.2 S 13.7 SE 6.4 SE 12.2 SE 10.5 SSE

8 5.7 S 12.0 SE 5.8 ESE 11.7 SE 10.8 SSE

9 3.3 SE 10.8 SE 6.8 ENE 9.1 SE 10.3 SSE

10 2.6 SSE 6.3 SSE 7.7 NE 6.6 SE 9.7 S

11 3.9 S 6.6 SSW 14.4 WSW 5.8 S 10.3 S

12 3.9 SE 6.2 SW 23 .3 WSW 5.9 S 10.2 SSW

13 10.8 NNW 6.3 W 25.3 WSW 22.1 WSW 10.8 WSW

14 19.9 WNW 15.1 NW 26.2 W 25.8 WSW 25.7 WNW

15 22.4 WNW 27.6 WNW 24.2 WSW 21.2 WSW 26.0 W

16 24.4 WNW 29.0 WNW 23.7 WSW 17.4 WSW 23.1 W

17 28.4 WNW 25.6 WNW 19.0 WSW 19.5 WSW 26.0 W

18 26.8 WNW 24.4 WNW 17.4 WSW 14.0 WSW 32.5 W

19 18.9 NW 23.4 WNW 23.2 WSW 17.8 WSW 31.0 W

20 22.4 WNW 20.0 WNW 25.1 WSW 21.8 WSW 26.2 WNW

21 21.0 WNW 14.5 W 25.7 WSW 21.4 WSW 23.3 WNW

22 16.9 NW 19.5 WNW 31.5 W 21.9 W 26.7 WNW

23 12.9 WNW 16.4 WNW 31 .5 W 26.6 W 21.4 NW

4

- 12

- 07: 0 18.4 WNW 9.1 WSW 24.1 W 28.2 W 20.7 NW

1 16.6 WNW 9.1 W 24.1 W 24.3 W 18.8 NW

2 16.9 NW 6.9 W 28.7 W 23.7 WNW 21.0 WNW

3 15.2 WNW 8.1 W 23.9 W 21.3 W 22.7 NW

4 16.0 WNW 8.7 W 20.3 W 21.3 W 20.4 WNW

5 17.1 WNW 6.6 W 18.6 W 17.3 W 21.2 WNW

6 19.4 NW 9.5 W 16.9 W 14.5 W 17.0 WNW

7 20.7 NW 15.2 WNW 20.9 W 15.8 W 18.4 WNW

8 23.6 WNW 23.7 WNW 25.3 W 22.3 W 23.1 WNW

9 23.6 WNW 16.8 W 29.9 WSW 26.8 W 23.6 WNW

10 17.1 WNW 23.1 W 25.5 WSW 26.8 W 28.0 W

11 19.3 NW 22.1 W 25.7 WSW 23.9 WSW 21.7 W

12 26.3 NW 24.9 NW 32.0 WSW 27.7 W 30.1 WNW

13 28.6 WNW 33.6 NW 28.5 WSW 29.2 W 30.9 NW

14 26.3 N 31.1 NNW 28.5 W 28.1 WNW 33.4 NW

15 20.5 N 17.5 N 17.9 WNW 34.5 NNW 32.1 NNW

16 17.5 NW 15.9 WNW 19.4 WNW 18.5 NNW 23.9 NNW

17 12.7 WNW 12.1 WNW 19.4 W 19.0 WNW 19.8 WNW

18 11.3 WNW 5.8 W 19.0 W 18.5 W 17.3 WNW

19 14.0 W 5.7 WSW 21.3 W 17.3 W 15.4 WNW

20 15.9 WNW 4.9 W 20.4 W 20.9 W 17.4 WNW

21 17.3 WNW 3.9 W 18.4 W 18.0 W 16.6 WNW

22 14.9 NW 6.6 NW 17.4 W 14.9 W 15.0 NW

23 13.1 NW 3.0 NNW 10.5 W 6.8 WNW 9.2 NW

* Wind speed 10 m above the ground (in miles/hour), estimated as 1.26 x the measured wind speed 2 m ab ove the ground. Wind speeds in the 15-20 mph and 20-25 mph ranges are in blue and pink, respectively, while w ind speeds > 25 mph are in red. ** Refer to Figure 12 and Table 7 of the initial d ocumentation of this event (ICAPCD, April 15, 2008, submitted with this report) for information about the location of the s tations.

### 31

III. Concentrations Beyond Normal Historical Fluctu ations

This section discusses the average wind speeds in I mperial County as well as

the historical PM10 concentrations to demonstrate t hat both the wind speeds and the resulting PM10 concentrations were above normal levels.

a. Wind Speed Averaging and Historical Analysis.

Wind Speed Averaging

.

The Imperial County APCD and ARB monitoring sites c ollect hourly averages of wind speed and wind direction. Each hourly average is comprised of 60 data points (each minute of the hour). Because of limit ed monitoring data capacity, the Imperial County APCD is not able to store these minute data points and only retains the hourly averages. Federal Aviation Admi nistration/National Weather Service (FAA/NWS) stations located in Imperial Coun ty report wind speed and direction following the standard METAR observation periods, which are approximately 10 minutes prior to the observation t ime. In other words, the Imperial County APCD’s wind speed data recorded is an hourly average for the complete hour and the FAA/NWS data is a 10 minute a verage at the end of each hour. In addition, airport monitors have wind gust reporting capabilities not available to the APCD’s monitoring sites. Because of the different averaging periods between the APCD’s monitors and the airport monitors, the wind speeds and directions may not always be comparable. The F AA/NWS stations typically record hourly wind averages higher than those calcu lated at the APCD and ARB monitoring sites. However, the Imperial County APC D utilizes both data sets to further substantiate the high wind event.

### 32

Historical Wind Speed Analysis. Resultant maximum wind speeds at the Westmorland monitoring site (Table 8) showed April mean daily maximum wind speeds of approximately 10 mph. A standard deviati on of the combined data was 5.14. As noted in Figure 25, the maximum wind speed at Westmorland was three standard deviations above the April historica l average, making it clearly an exceptional wind event.

Table 8. April Daily Maximum Wind Speed Statistics for Westmorland

2004 2005 2006 2007 April

TOTAL

Mean 9.566667 10.42 9.846667 9.796667 9.9075

Standard Error 0.785828 1.018105 0.916411 1.051714 0.469208

Median 8.65 8.1 7.5 7.5 8.1

Mode 5.8 5.8 5.8 5.8 5.8

Standard

Deviation 4.304155 5.576391 5.019392 5.760477 5.139911



### Westmorland Wind - April Max Values

0 5 10 15 20 25 30

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Daymph 2004 2005 2006 2007 2004-2007 Mean

April 12, 2007



### Figure 25. Maximum Resultant Wind Speeds, Westmorl and, April 2004-2007

PM Concentrations at the Brawley and Westmorland si tes are generally well below the NAAQS. As shown in Figures 26 and 27, ex ceedances like the ones on April 12 occur less than 2% of the time at Brawl ey and Westmorland. The April 12 concentrations were far above historical f luctuations as well as above background levels.

### 33 Westmorland
April data - 1995 to 2007

0 5 10 15 20 25 30 35 40 45

2 0 4 5 55 65 75 85 9 5 105
1

15
1 25
1 3 5
1 4 5 155 170
1

90 2 50 More

PM10 Concentration (ug/m3)Frequency

0.00% 20.00% 40.00% 60.00% 80.00% 100.00% Frequency Cumulative %

90% 85%

April 12, 2007



### Figure 26. Historical April PM10 concentrations at Westmorland

Brawley (combined sites)
April data - 1987 to 2008

0 10 20 30 40 50 60

20 45 5

5 6 5 75 85 9

5 105
1

1 5
1 2 5
1 3 5 145 155
1

7 0
1 9 0 250 Mo

r e

PM10 Concentration (ug/m3)Frequency

0.00% 20.00% 40.00% 60.00% 80.00% 100.00% Frequency Cumulative %

90% 85%

April 12, 2007



### Figure 27. Historical April PM10 concentrations at Brawley

(combined Brawley-Main and Brawley-220 Main sites)



### IV. No Exceedance “But For” the Event

In this section, BAM and FRM concentrations are exa mined for the weeks prior to

and after this event to demonstrate that but for th e high winds there would not

have been an exceedance.

There were no unusual local anthropogenic emissions reported before, during, and after the event. Figure 28 shows that PM10 con centrations at the three BAM sites (Niland, Brawley, and Westmorland) were well below the federal 24-hour standard of 150 ug/m3 for one week before and after the event. FRM data from

### 34

all sites in Imperial County (Figure 29) shows that concentrations during the two weeks prior to and following the event were low. W ithout the shift in wind direction, bringing dust from the Anza-Borrego Dese rt, there would have been no

exceedance of the federal 24-hour PM10 standard.

Based on the fact that dust and smoke controls were in place, no complaints were received, and the BAM concentrations prior to and after the event were low, the Imperial County APCD estimates that anthropogen ic emissions were approximately constant before, during and after the event, indicating that the significant increase was not due to anthropogenic e missions activity.

PM10 BAM Daily Averages
April 5-19, 2007

0 50 100 150 200 2504/5/20074/6/20074/7/20074/8/20074/9/20074/10/20074/11/20074/12/20074/13/20074/14/20074/15/20074/16/20074/17/20074/18/20074/19/2007

DatePM10 BAM Concentrations (ug/m3) Westmorland Niland Brawley



### Figure 28. PM10 BAM Concentrations, April 5-19, 20 07

April 2007 Exceptional Event

0 50 100 150 200 250 300 350

3/31/2007 4/6/2007 4/12/2007 4/18/2007 4/24/2007PM10 Conc (ug/m3) Niland Westmorland Brawley El Centro Calexico-Ethel Calexico-Grant



### Figure 29. PM10 FRM Data March 31 to April 24, 200 7

### 35

Finally, a satellite view of the Imperial Valley (F igure 30) on April 12, gives additional evidence of a regional dust event. Dust storms south of Mexicali and in northwest Imperial County are visible inside of the red circles on the image

below.

Figure 30. Satellite Image of dust storms on April 12, 2007 at noon

All of the PM10 monitors in Imperial County, Rivers ide County, Mexicali, and Yuma, Arizona were impacted by this exceptional eve nt. High winds were reported on a regional basis.

Based on the analysis of wind speed and direction, PM10 concentrations, and a review of satellite images and modeling data, there would not have been an

exceedance of the federal PM10 standard at the West morland and Brawley monitoring stations on April 12, 2007 if not for th e high winds entraining dust from the Anza Borrego Desert which was subsequently tran sported to the two

monitors.

### 36

References

ENVIRON. Development of a Wind Blown Fugitive Dust Model and Inventory for Imperial County, California. May 12, 2004.

ENVIRON. Draft Final Technical Memorandum Regulation VIII BA CM Analysis.

October 2005.

Imperial County Air Pollution Control District. Natural Event Documentation. Brawley and Westmorland, California April 12, 2007 . April 15, 2008.

Imperial County Air Pollution Control District. Imperial County Natural Events Action

Plan. Adopted August 9, 2005.

Imperial County Air Pollution Control District. Smoke Management Program. Procedures for Operating the District Agricultural Burning Program. Adopted February

2001.

Imperial County Air Pollution Control District. Regulation VIII Fugitive Dust Rules. Adopted 10/10/1994-11/08/2005.

Imperial County Air Pollution Control District. Rule 806: Conservation Management

Practices. Adopted 11/08/2005