1

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 Natural Event Documentation 

(NED) for Brawley and Westmorland monitoring stations prepared by the Imperial 

County Air Pollution Control District (Imperial County 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 report demonstrates that 

without this regional natural high wind event, there would not have been 

exceedances of the federal PM10 Standard on April 12, 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 Administrator in accordance with 40 CFR 

50.14 to be an exceptional event.” The demonstration to justify data exclusion as 

outlined in 40 CFR 50.14 specifies that evidence must 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 concentration in excess of 

normal historical fluctuations, including background; and, 

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

This report documents that the event meets the above criteria and provides 

analysis to demonstrate that: 

I. The dust event was not reasonably controllable or 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 for” 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 high on April 12, 2007 that they 

overturned a big-rig, they toppled trees, and the Energy Department was put on 

alert due to wind incidents in Brawley, Imperial, and El Centro. In addition, the 

California Highway Patrol issued a wind advisory warning 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 Plan (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 monitoring data affected by the high 

wind event. 

Table 1. Imperial County APCD NAAQS Exceedances, April 12, 2007 

Monitoring Site AQS Site Number 24-Hour PM10 

Concentration 

(μg/m3) 

Brawley-Main Street 060250007 291 

Westmorland 1st Street 060254003 155 

Figure 1 shows Imperial County in relation to the Anza Borrego Desert and the 

surrounding counties and states. The Anza Borrego Desert is a broad expanse 

of land that includes western Imperial County, as well as portions of eastern San 

Diego County and southern Riverside County. 

Figure 1. Regional map of Imperial County and the Anza Borrego Desert 
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Figure 2 provides a satellite view of the region, and includes the PM10 values 

measured at each monitor on April 12, 2007. The elevated PM10 values 

measured at multiple sites indicate a large scale regional event. As seen from 

the concentrations at the monitors, blowing dust transported 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 April 12, 2007 confirm that 

the majority of particulate matter was in the coarse (PM2.5 to PM10) mass size 

range, indicating fugitive dust as the primary source of the elevated 

concentrations. 

Table 2. Imperial County APCD PM2.5 and PM10 concentrations, 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 roughly constant before, 

during and after the event, and PM10 concentrations were approximately 

50 ug/m3 prior to and after the event, well below the level of the federal standard. 

Countywide, windblown dust emissions from urban lands are minimal and 

emissions from agricultural lands are small (6%) compared to the desert area 

emissions in Imperial County. The location of urban and agricultural lands in 

relationship to the air quality monitors is shown in Figure 2. The urban and 

agricultural lands are located in the center portion 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 extensive desert area in San Diego 

County which also likely contributed to the dust loading in Imperial County. 

Although windblown dust emissions from agricultural lands are estimated to be 

only a small contributor to the measured PM10 concentrations 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 fallow farm lands which 

participate in the Imperial Irrigation District’s Fallowing Program. The Fallowing 

Program also requires farmers to implement United States Department of 

Agriculture Best Management Practices on the land to control fugitive dust. 

In summary, on April 12, 2007, PM10 exceedances at the Westmorland and 

Brawley monitors were impacted by a natural high wind event in which 

anthropogenic sources played only a small role. Nevertheless, controls were in 

place to reduce dust from anthropogenic sources and these control measures 

were reasonable and appropriate, taking into account the magnitude of the high 

winds and the small contribution from anthropogenic sources. As such, this high 

wind natural event and the associated exceedances were not reasonably 

controllable or preventable and all of the statutory exceptional events criteria 

have been met. 
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Analysis

I. Dust Event Not Reasonably Controllable or Preventable 

This section will discuss the methods that were used to estimate the emission 

inventory for Imperial County and support the conclusion 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 velocity necessary to entrain 

dust in the Imperial County is presented. This section 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 County Air Pollution Control 

District (Imperial County APCD) a report entitled “Development of a Windblown 

Fugitive Dust Model and Inventory for Imperial County” (Windblown Dust Study). 

The results of the Windblown Dust Study were revised in September 2004 as a 

result of discussions with ARB and U.S. EPA and were documented in 

Appendix A of the 2005 Best Available Control Measures (BACM) demonstration 

report (see Attachment A of the NED). The BACM report presents the 

methodology used in estimating windblown fugitive dust from agricultural and 

barren lands in Imperial County. In particular, it presents information regarding 

land use/land cover and soil characteristics for the desert areas, including the 

Anza Borrego Desert. The primary source of LULC data is based on land use 

surveys conducted by the California Department of Water Resources (DWR) and 

augmented with the USGS National Land Cover Database (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 available at the county level 

and includes detailed agricultural land use classifications, as well as less detailed 

urban and native vegetation land uses. The data was 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 lands, 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 project between the 

U.S. Geological Survey and U.S. EPA to produce a consistent land cover data 

layer for the entire conterminous United States, based on 30-meter Landsat 

thematic mapper (TM) data. The NLCD was developed from TM data acquired 

from the Multi-Resolution Land Characterization (MRLC) Consortium, a 

partnership of federal agencies that produce or use land cover data. 

A display of the combined DWR and NLCD land use databases is shown in 

Figure 3 for the Imperial Valley domain. This shows 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, part 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 the Salton Sea. Toward the 

north it extends eastward across the Santa Rosa Mountains from the town of 

Anza on Highway 371, while toward the south it extends 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, experiencing only 5% of normal 

precipitation. This, coupled with extreme high temperatures experienced in the 

months preceding the event (Table 4 of the NED), contributed to arid conditions 

in the Anza Borrego Desert area and the availability of material to transport into 

and around the Imperial Valley. 

Figure 3. Land Use Coverage for Imperial Valley Based on 

Combined DWR and NLCD Information 
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To determine the characteristics or type of soil present in Imperial County, the 

Wind Blown Dust Study used the STATSGO. The STATSGO database was 

developed by the Natural Resources Conservation Service of the US Department 

of Agricultural (USDA), providing detailed information concerning the taxonomy 

of the soils, including soil texture class, percentage of sand, silt and clay, and the 

available water capacity of the soil. Figure 4 displays the STATSGO data for the 

Imperial County domain. The data in Figure 4 corroborates 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 unstable and susceptible 

for soil erosion even at low wind speeds. 

Figure 3 also shows that the center portion of Imperial County is mostly 

agricultural land. The main soil texture in this agricultural 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, silty 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 also determines the duration of 

the windblown dust emissions; the Wind Blown Dust Study 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 vegetative canopy cover, the 

emission rates are reduced according to the assumed percent canopy cover. 

Crop cover information was based on the crop calendar 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 dust 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 Analysis). Agriculture and Urban 

open areas emit the least amount of PM10 throughout the year in Imperial 

County. According to Figure 5, agriculture is responsible 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

Month

PM10 (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 Analysis). Urban sources of 

windblown dust are very minor. In addition, because 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 Annual 

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 methodology 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 erode and entrain fugitive dust 

and is appropriate for unstable areas, such as the Anza Borrego Desert. 

The study follows the methodology of MacDougall, 2002. This methodology was 

applied in the development of the Clark County, Nevada PM10 SIP and 

implemented in the development of the WRAP dust inventory. Following this 

approach, wind events are considered only when the 10-meter wind speeds 

reach or exceed the lowest defined wind speed for which emission factors are 

available. Due to the limited availability of comprehensive wind tunnel studies, 

emission factors were derived only for 10-meter wind speeds of 15 mph or 

higher. The stability of vacant land parcels was determined 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 threshold 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 through 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 wind speeds in April were, 

for the most part, between 5 and 20 mph at both stations. PM10 concentrations 

generally remained below 100 μg/m3 at Brawley and 120 μg/m3 at Westmorland. 

The concentrations seen on April 12, at Westmorland and Brawley were above 

normal historical fluctuations and the winds were unusually high, wind speeds 

reached as high as 12 mph above the entrainment threshold of 15 mph. 

Figure 6. Brawley PM10 vs. Maximum Hourly Wind Speed, 

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)
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Figure 7. Westmorland PM10 vs. Maximum Hourly Wind Speed, 

April 2002-2007 

c. Emissions Inventory. 

The PM10 emission inventory compares the type of sources that generate dust 

and could potentially contribute to PM10 NAAQS violations. The 2005 PM10 

emission inventory is summarized in Table 4 and features adjustments made to 

incorporate updated cattle emissions, updated windblown dust model results, 

and updates for entrained and windblown unpaved road dust estimates. The 

middle column of Table 4 portrays annual average emissions. On a day with 

exceptionally high winds such as April 12, 2007, windblown emissions are 

expected to have been significantly higher than their annual average values. 

The Windblown Dust Study showed that spring in Imperial County produces the 

highest PM10 emissions for the entire year (Figure 5). 

A conservative estimate of the emission inventory on a very windy day, derived 

by modifying the annual average inventory to use Windblown 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 right column of Table 4. We note 

that according to both the annual average and the “high wind” inventories, 

windblown emissions from agricultural lands only contribute 5-6% of all 

windblown emissions, or 4-5% of the total inventory. Furthermore, although 

nonanthropogenic sources already account for ~55% of total emissions under 

average conditions, their fractional contribution to the total “high wind” emissions 

is ~75%. Given that the “high wind” inventory of Table 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)
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Table 4. Revised 2005 Annual Average PM10 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 for windblown sources, for which the predictions of the Windblown Dust 

Study for the month of April, 2002 were used. (The month of April had the highest monthly windblown emissions 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, Fugitive Dust Rules, and 

Rule 806, Conservation Management Practices, on November 8, 2005. The 

rules were adopted after an extensive public process with both the general public 

and affected industry. The rules require implementing fugitive dust control 

measures for source categories such as: Bureau of Land Management (BLM) 

controlled areas, construction and earthmoving activities, handling and transport 

of bulk materials, disturbed open areas, paved and unpaved roads, and 

agricultural operations. The rule standards are performance based whereby the 

operators are allowed to determine the control techniques sufficient to limit visible 

dust emissions and, if applicable to that source, to implement requirements for a 

stabilized surface. 

In general, rules can be classified as meeting either Reasonably Available 

Control Technology (RACM), or the more stringent Best Available Control 

Technology (BACM). Imperial County’s rule was adopted to meet BACM 

requirements as outlined in the District’s 2005 BACM Demonstration Report. 

RACM level rules satisfy the Exceptional Events Rule criteria that reasonable 

and appropriate measures for anthropogenic sources were in place during an 

event. As such, the more stringent BACM level rules adopted in Imperial County 

meet this requirement as well. 

Fallow Fields. 

The Imperial Irrigation District (IID) started a “Fallowing Program” in 2003. The 

purpose of the Fallowing Program is to allow willing land owners and/or lessees 

to contract with IID to fallow fields to meet the transfer 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 for voluntary participation to fallow 

a field in return for payment of the conserved water. Fields are then contracted 

based on a random selection to meet the amount of conserved water needed 

each year. Each field’s participation in the fallowing 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 performance, the participants are 

reimbursed for pre-approved dust mitigation Best Management 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 practices are: 

1) Plan ahead to start with plenty of vegetation residue, and maintain as 

much residue on fallowed fields as possible. Residue is more effective for 

wind erosion protection if left standing. 
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2) Avoid any tillage if possible. 

3) Avoid any traffic on the field or tillage when fields 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 rains or soil stabilization 

chemicals may be applied to fallowed lands. 

5) In addition to the above BMPs, in order to satisfy Imperial County dust 

control and mitigation requirements, Fallowing Party will comply with any 

lawful conditions required by the Imperial County Air Pollution Control 

District. 

Farmers who do not participate in IID’s fallowing program will usually continue to 

grow crops on the land and thus would be subject to ICAPCD’s Conservation 

Management Practices rule. Lands will generally only be left fallow if the land is 

converted to a purpose other than agriculture, or the 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 crops. 

e. Source Activity on April 12, 2007 

Agricultural Burning and Fugitive Dust Controls. 

In compliance with the existing Imperial County Smoke Management Program, 

agricultural burning was prohibited in Imperial County 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 Weather and Agricultural 

Burning Information report is attached to this report (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 by the Imperial County 

APCD on April 12, 2007 

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

windblown dust emissions from the barren lands to the west in the Anza Borrego 

desert were the dominant cause of the suspended dust.measured at the Brawley 

and Westmorland monitors. Nevertheless, reasonable and appropriate controls 

were in place for all anthropogenic sources, and best management practices 

were in place for agricultural lands under IID’s Fallowing 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 include: analysis of the PM10 and 

BAM concentrations and the correlation with wind speeds, wind gusts, and wind 

direction on April 12, 2007, as well running hysplit and Wind2D back trajectories 

to identify the source region. 

a. Cause of High PM10 Concentrations on April 12, 2007 

A high wind dust event does not always show an immediate impact on high 

PM10 concentrations at monitoring stations, particularly 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 deposition in a receptor area with 

lower wind speeds. Once suspended upwind of the monitoring stations, dust can 

be carried downwind by winds below the dust entrainment threshold. On 

April 11, 2007, dust was entrained in the Anza Borrego Desert, transported 

toward the east, and deposited at Westmorland and Brawley, and other areas in 

Imperial County, noted by increased PM10 concentrations at all air quality 

monitoring sites. Beginning in the afternoon of April 11, 2007 and continuing 

through April 12, excessive wind speeds and gusts were recorded at Imperial, 

Blythe, Yuma, and Thermal Airports, all bordering or within Imperial County. The 

high PM10 concentration recorded at Westmorland and Brawley during the early 

afternoon hours of April 12, 2007 can, therefore, be 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 and 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 (FRM) monitors in Imperial 

County recorded exceedances of the PM10 National Ambient Air Quality 

Standard (NAAQS) of 150 μg/m3. Above normal concentrations were recorded at

other monitors in and around Imperial County as well as monitors in Riverside 

and San Bernardino Counties, and in Yuma County, Arizona, (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 

(μg/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 FRM sites, elevated 

concentrations were recorded by PM10 continuous Beta Attenuation Monitor 

(BAM) sites operated by Imperial County for forecasting 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 BAM sites 

Monitor AQS No. Concentration (μg/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 afternoon to a more western flow, the 

wind speed also increased significantly. These shifts can be seen at two small 

weather observation sites west of the Imperial monitoring sites. Borrego Springs 

(KCABORRE1) in the northern portion of the Anza Borrego Desert showed wind 

gusts as high as 30 mph, and Cielo Vista Observatory 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 significant wind speed increases earlier 

in the day, allowing time for wind across the desert to entrain and transport dust 

to the Imperial Valley. In a joint effort, the Imperial County APCD and IID provide 

daily weather forecast for Imperial County. On Wednesday 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 Springs, CA and Julian, CA 

[Source: www.wunderground.com] 
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Although no other data is available in the Anza Borrego Desert itself, surface 

analysis maps of wind speeds across the southwest part 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 (14 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 4/11/07 
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In the Imperial Valley, the early afternoon of April 11 saw winds at the 

Westmorland and Niland monitoring stations shifting to an almost uniform 

western flow (Figure 11). At the same time, wind speeds increased and PM10 

concentrations as seen at the BAM sites rose (Figure 12). Concentrations at the 

Brawley PM10 BAM site increased, simultaneously with those at Westmorland 

and Niland. Although the Brawley site does not have meteorology data, the 

corresponding shift in wind direction and increase in wind speed can be 

assumed. Wind speeds remained elevated at Westmorland 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 monitoring sites, California 

Irrigation Management Information System (CIMIS) sites, and the four regional 

airports. 

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

did not exceed the PM10 standard. The Niland monitor 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 from the Anza Borrego desert. In 

addition, because of its location, the Niland monitoring site is sheltered from 

much of the windblown dust from the west, the predominant wind direction at this 

site. The Salton Sea and agricultural fields are located west of the monitor. The 

recorded concentration of 108 ug/m3 on April 12th was 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

Hour

Degree

0

5

10

15

20

25

30

knots

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

Hour

PM10 (ug/m3)

0

5

10

15

20

25

30

* - Concentration reached monitor upper detection limit

knots

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 midday, the winds shifted to the 

west with a parallel increase in wind speeds. Speeds in the later part of the day 

ranged from 14 to 21 knots. The BAM site at Mexicali-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 sites. 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

360

0:001:002:003:004:005:006:007:008:009:00

10:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

Hour

Degree

0

5

10

15

20

25

knots

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

Hour

PM10 (ug/m3)

0

5

10

15

20

25

ITM WS Cobach WS m/s UABC PM10 BAM

Figure 14. PM10 BAM Concentrations and Wind Speeds, 

Mexicali Monitoring Sites 
22

Figure 15 shows the results of a backward trajectory analysis for the 

Westmorland monitoring site for the last 12 hours of April 11. This model, the 

NOAA HYSPLIT Model, shows impact from the west, over the Anza Borrego 

Desert, during the latter part of the day. 

Figure 15. NOAA Hysplit Model Results, April 11, 2007 

(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 predicted high winds in the 

regions with several areas being affected by blowing sand and dust. On the 

morning of April 12, 2007 winds were blowing from the west at all Imperial 

County and Mexicali stations. The wind speeds continued to be high. In the 

northern part of the Anza Borrego Desert, winds speeds, as measured at 

Borrego Springs (KCABORRE1) were fairly low and from 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 directional shift back to a 

primarily western flow. The Julian site (KCAJULIA2) also showed continued high 

winds, decreasing in mid-afternoon. Wind speeds were peaking at over 35 mph, 

with some wind gusts as high as 45 mph. The direction was almost consistently 

from the west. 

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

[Source: www.wunderground.com] 
24

Wind charts for the southwest portion of the country (Figures 17 and 18) show 

surface wind speeds (in knots) across the Anza Borrego 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) across 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 afternoon, wind speeds over the 

Anza Borrego Desert have increased from 20 to 24 knots (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 4/12/07 

Wind direction at sites further south and east (Imperial County and Mexicali) 

continued to be from the west throughout most of the 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 wind direction shift to 

northwest/north-northwest (just after 2 p.m.) resulted in a decrease in both. This 

directional shift away from the fugitive dust source 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 increase 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

Hour

Degree

0

5

10

15

20

25

30

knots

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 Directions, 

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

Hour

PM10 (ug/m3)

0

5

10

15

20

25

30

* - Concentration reached monitor upper detection limit

knots

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 site 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 dramatically 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

360

0:001:002:003:004:005:006:007:008:009:00

10:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

Hour

Degree

0

5

10

15

20

25

30

35

knots

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

Hour

PM10 (ug/m3)

0

5

10

15

20

25

30

35

knots

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 trajectory analysis for the 

Westmorland monitoring site for April 12. This model, the NOAA HYSPLIT 

Model, shows impact from the west, over the Anza Borrego Desert, throughout 

the day. 

Figure 23. NOAA Hysplit Model Results, April 12, 2007 

(midnight to midnight) 
28

ARB conducted an additional analysis using a WIND2D back trajectory model 

(Figure 24). This model traced the air mass 42 hours 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, Riverside County, and finally 

through Imperial County before impacting the Westmorland monitor. The 

trajectory clearly shows the air mass passing through the Anza Borrego Desert 

before impacting Imperial County. 

Figure 24. Westmorland Back trajectory arrival time 6:00pm 
29

c. Additional Spatial and Temporal Analysis of Concentrations 

Analysis of the Imperial County emission inventory (Table 4, Section 1.c) 

indicates that windblown emissions from agricultural fields can be expected to 

have been 10-20 times lower than emissions from desert and other natural, 

unpopulated areas. This subsection evaluates whether 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 PM10 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 and 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 average wind speeds 

south of the Salton Sea were at the Seeley and Meloland station, directly 

to the west and east of El Centro; 

 Winds were from the West throughout the Valley starting in the early 

afternoon on April 11 until the end of the day on April 12; and 

 The range of agricultural lands to the west of Imperial County stations is 

greater for the El Centro station than for the Brawley or Westmorland 

stations (by a factor of approximately 2 and 3, respectively). 

If emissions from nearby agricultural lands had made a dominant contribution to 

PM10 concentrations at Imperial County stations on April 12, 2007, then PM10

impacts at El Centro would have been larger than those at Brawley and 

Westmorland. However, the observed concentrations of 121 μg/m3 at El Centro, 

155 μg/m3 at Westmorland, and 291 μg/m3 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 overwhelmingly 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 SSE 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 above the 

ground. Wind speeds in the 15-20 mph and 20-25 mph ranges are in blue and pink, respectively, while wind speeds > 25 

mph are in red. 

 ** Refer to Figure 12 and Table 7 of the initial documentation of this event (ICAPCD, April 15, 2008, submitted with this 

report) for information about the location of the stations. 
31

III. Concentrations Beyond Normal Historical Fluctuations 

This section discusses the average wind speeds in Imperial County as well as 

the historical PM10 concentrations to demonstrate that 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 collect hourly averages of 

wind speed and wind direction. Each hourly average is comprised of 60 data 

points (each minute of the hour). Because of limited monitoring data capacity, 

the Imperial County APCD is not able to store these minute data points and only 

retains the hourly averages. Federal Aviation Administration/National Weather 

Service (FAA/NWS) stations located in Imperial County report wind speed and 

direction following the standard METAR observation periods, which are 

approximately 10 minutes prior to the observation time. 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 average 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 FAA/NWS stations typically 

record hourly wind averages higher than those calculated at the APCD and ARB 

monitoring sites. However, the Imperial County APCD 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 deviation 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 historical 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

Day

mph

2004 2005 2006 2007 2004-2007 Mean

April 12, 2007

Figure 25. Maximum Resultant Wind Speeds, Westmorland, April 2004-2007 

PM Concentrations at the Brawley and Westmorland sites are generally well 

below the NAAQS. As shown in Figures 26 and 27, exceedances like the ones 

on April 12 occur less than 2% of the time at Brawley and Westmorland. The 

April 12 concentrations were far above historical fluctuations as well as above 

background levels. 
33

Westmorland

April data - 1995 to 2007

0

5

10

15

20

25

30

35

40

45

204555

65

75

8595

105115

125135

145155

170

190250

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

2045

55

65

75

8595

105

115

125135

145155

170190

250

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 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 examined for the weeks prior to 

and after this event to demonstrate that but for the 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 concentrations 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. Without the shift in wind 

direction, bringing dust from the Anza-Borrego Desert, 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 anthropogenic emissions were 

approximately constant before, during and after the event, indicating that the 

significant increase was not due to anthropogenic emissions activity. 

PM10 BAM Daily Averages

April 5-19, 2007

0

50

100

150

200

250

4/5/20074/6/20074/7/20074/8/20074/9/2007

4/10/20074/11/20074/12/20074/13/20074/14/20074/15/20074/16/20074/17/20074/18/20074/19/2007

Date

PM10 BAM Concentrations (ug/m3)

Westmorland Niland Brawley

Figure 28. PM10 BAM Concentrations, April 5-19, 2007 

April 2007 Exceptional Event

0

50

100

150

200

250

300

350

3/31/2007 4/6/2007 4/12/2007 4/18/2007 4/24/2007

PM10 Conc (ug/m3)

Niland Westmorland Brawley El Centro Calexico-Ethel Calexico-Grant

Figure 29. PM10 FRM Data March 31 to April 24, 2007 
35

Finally, a satellite view of the Imperial Valley (Figure 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, Riverside County, Mexicali, and 

Yuma, Arizona were impacted by this exceptional event. 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 Westmorland and Brawley 

monitoring stations on April 12, 2007 if not for the high winds entraining dust from 

the Anza Borrego Desert which was subsequently transported 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 BACM 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