1

Addendum 

Natural Event Documentation 

Imperial County, California 

June 5, 2007 

Imperial County Air Pollution Control District 

March 12, 2009 
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SUMMARY 

This report is an addendum to the June 5, 2007 Natural Event Documentation 

(NED) for the Niland, El Centro, Westmorland, Brawley, and Calexico-Ethel 

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 further demonstrates that without this regional natural high 

wind event, there would not have been exceedances of the federal PM10 

standard on June 5, 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 Niland, El Centro, Westmorland, Brawley, and CalexicoEthel; 

III. The measured concentrations were beyond normal historical levels; and, 

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

On June 5, 2007, strong winds from the west with gusts of 33 mph over the Anza 

Borrego Desert entrained desert soil into the atmosphere and caused all five 

Imperial County Federal Reference Monitors (FRM) to record concentrations in 

excess of the federal 24-Hour PM10 standard of 150 μg/m3 (Table 1). Gusts as 

high as 47 mph were recorded at the Imperial Airport. Excessive winds speeds 

and wind gusts were also recorded at the Blythe, Yuma and Thermal Airports, all 

of which are in close proximity to the Imperial Valley. 

 
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Table 1. Imperial County APCD NAAQS Exceedances, June 5, 2007 

Monitoring Site AQS Site Number 24-Hour PM10 (μg/m3) 

Brawley-Main Street 060250007 281 

Westmorland 1st Street 060254003 226 

Niland-English Road 060254004 162 

El Centro 9th Street 060251003 200 

Calexico – Ethel Street 060250005 282 

 *Calexico-Grant site did not sample 

The Imperial County APCD published in the local newspaper a notice of petition 

to exclude June 5, 2007 PM10 monitoring data affected by a high wind event. 

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 June 5, 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 Imperial County stations, but also to the west in Yuma, 

Arizona and the south in Mexico. On this day, the PM10 FRM monitor in Yuma, 

Arizona recorded a concentration of 224 μg/m3 and the Mexicali UABC station 

recorded a PM10 BAM concentration of 312 μg/m3. 

 

As shown in Table 2, low PM2.5 concentrations on June 5, 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, June 5, 2007 

Date Site Name PM2.5 PM10 

6/5/2007 Brawley 12.3 281 

6/5/2007 Calexico-Ethel 24.9 282 

6/5/2007 El Centro 13.1 200 

Figure 2. Imperial County Monitoring Stations and PM10 Concentrations, 

June 5, 2007
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Anthropogenic emission 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 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. 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 June 5, 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, local farmers, and U.S. EPA. 

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 June 5, 2007, PM10 exceedances at the Niland, El Centro, 

Westmorland, Brawley, and Calexico-Ethel monitors were impacted by a natural 

high-wind event in which anthropogenic sources play 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 dust 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. This section also discusses the different 

emission sources in Imperial County along with their controls, and the source 

activity that occurred on June 5, 2007. 

a. Imperial County – Dust Emissive Characteristics.

Windblown Dust Study

In May 2004, ENVIRON prepared for the 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, 

documented in Appendix A of the 2005 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 the 

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 land use/land cover (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 the 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 noted (Table 3 of the Natural Event documentation) 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 Natural Event 

Documentation), 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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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 

To determine the characteristics or type of soil present in Imperial County, the 

Wind Blown Dust Study used the State Soil Geographic Database (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 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 June in Imperial County. Sand 

dunes along with shrub and grasslands in Imperial County emit 16 times more 

PM10 than agricultural lands in June. 

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. Monthly Windblown Emissions (tons/day) in 2004 

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). 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 June for 2002 through 2007 at the Brawley and 

Westmorland stations. With the exception of June 5, 2007, which is flagged in 

AQS as a high wind natural event, maximum hourly wind speeds in June were, 

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

generally remain below 110 μg/m3 at Brawley and 150 μg/m3 at Westmorland. 

The concentrations seen on June 5th at Westmorland and Brawley were above 

normal historical fluctuations and the winds were unusually high, above the 

entrainment threshold of 15 mph. 

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

 June 2002 through 2007 

June 05 2007

281

0

50

100

150

200

250

300

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

Maximum Wind Speed (mph)

FRM PM10 (microgr/cubic meter)
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Figure 7. Westmorland PM10 vs. Maximum Hourly Wind Speed, 

 June 2002 through 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 June 5, 2007, windblown emissions are 

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

The Windblown Dust Study showed that summer in Imperial County produces 

the second highest PM10 emissions for the entire year; the month of June having 

among the highest emissions in the summer (Figure 5). This is consistent with 

higher winds along with a lack of precipitation. 

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 June 5, 2007 Imperial 

County measurements were from natural desert lands. 

June 05 2007

226

0

50

100

150

200

250

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

Max Hourly Wind Speed (mph)

FRM PM!) (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 June 5, 2007 

Agricultural Burning and Fugitive Dust Controls. 

Agricultural burning was allowed in Imperial County for the date June 5, 2007. 

However, minimum agricultural burning was conducted on June 5, 2007, due to 

the weather forecast indicating high winds with gusts for this day. The Imperial 

County APCD authorized agricultural burning for a total of 70 acres in the 

Calipatria area during this day. Fugitive windblown dust controls were in affect on 

this high wind day. A copy of the Imperial County APCD Daily Weather and 

Agricultural Burning Information report is attached to this report (Attachment B). 

Inspections.

The APCD inspectors performed minimum facility inspections on June 5, 2007. 

APCD staff did not observe any unusual emissions other than the event of 

blowing dust on June 5, 2007. 

Complaints.

The Imperial county APCD received two dust related complaints which were 

investigated on the following day (Appendix B of the NED). The results of the 

investigation indicated that the high winds from the previous day had resulted in 

dust deposited onto the property of the complainants (Table 5). 
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Table 5. Complaints received on June 5, 2007 

Property address Complaint 

2715 La Brucherie Rd 

Imperial, CA 

10:30am

Occupant called to complain that his property is always 

full of dirt due to an empty lot next to him. Resident called 

in complaint on June 5th and ICAPCD inspector arrived on 

June 6th to investigate the complaint.

2715 B La Brucherie Rd 

Imperial, CA 

10:40am 

Occupant called to complain that his property is always 

full of dirt due to an empty lot next to him. Resident called 

in complaint on June 5th and ICAPCD inspector arrived on 

June 6th to investigate the complaint. 

In summary, based on the information presented in this section, on June 5, 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 affected 

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. 

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 

June 5 and 6, 2007 causing the PM10 exceedances in Imperial County. The 

tools used to analyze the sequence of events include: analysis of the PM10 and 

Beta Attenuation Monitor (BAM) concentrations and the correlation with wind 

speeds, wind gusts, and wind direction on June 5, 2007, as well as running 

hysplit and Wind2D back trajectories to identify the source region. 

a. Cause of High PM10 Concentrations on June 5, 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 

June 5, 2007, dust was entrained in the Anza Borrego Desert, transported toward 

the east, and deposited at areas in Imperial County, noted by increased PM10 

concentrations at all air quality monitoring sites. 

Wind advisories were issued by Imperial County and the Imperial Irrigation 

District on the morning of June 5th and again on June 6th. With the exception of 

Niland, where the wind speeds were higher, winds were originally less than 

5 mph. They began to rise in the late morning of June 5th, particularly at Niland 

and El Centro, and remained fairly high for most of June 6th at most of the air 

quality monitoring sites, the California Irrigation Management Information System 
16

(CIMIS) sites, and the four regional airports. Wind gusts were reported as high 

as 40-50 mph throughout the region. 

b. Chronology of High Wind Exceptional Event 

On June 5, 2007, five Federal Reference Method (FRM) monitors in Imperial 

County recorded exceedances of the PM10 National Ambient Air Quality 

Standard (NAAQS) of 150 μg/m3. The city of Yuma, Arizona also experienced 

increased wind speeds on June 5, 2007 which led to a 24-hour PM10 

concentration of 224 μg/m3 at the Yuma Courthouse monitor, (Table 6) 

substantiating the regional nature of the event. 

Table 6. PM10 Concentrations at Federal Reference Monitors on June 5, 2007 

(NAAQS exceedances in bold) 

Monitor AIRS No. Concentration (ug/m3

) 

Brawley 60250007 281 

Westmorland 60254003 226 

Niland 60254004 162 

El Centro 60251003 200 

Calexico-Ethel 60250005 282 

Yuma ADEQ 224 

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 7). 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 7. PM10 Concentrations at Imperial County BAM sites 

Monitor AIRS No. Concentration (μg/m3 ) 

 June 5 June 6

Brawley 60250007 173 97 

Niland 60254004 245 98 

Westmorland 60254003 216 117 

June 5, 2007

The winds on June 5, 2007 were relatively calm in the morning. When the wind 

direction shifted in the late morning 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 32 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 33 mph with gusts as high as 43 mph (Figure 8). 

The site at Julian, in particular, showed significant wind speed increases earlier 
17

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 Tuesday June 5th, 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 for the Anza Borrego Desert, 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 16 to 20 knots (18 to 23 mph), above the dust entrainment 

threshold of 15 mph. By 5 p.m., wind speeds had increased slightly to upwards 

of 24 knots (28 mph) (Figure 10). 

Figure 9. Surface Wind Speed at 2:00 pm on 6/5/07 

Figure 10. Surface Wind Speed at 5:00 pm on 6/5/07 
19

Hourly data from District monitoring sites shows wind speeds throughout the 

region increased beginning in the late morning on June 5th, at the same time that 

the wind direction shifted 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). Hourly averages reached as high as 38 mph at the Niland 

monitoring site. These high winds continued into June 6th, decreasing slightly in 

the late morning of the 6th before rising again and remaining relatively high 

throughout the rest of the day, peaking around 5pm.

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 and all three BAM 

sites showed elevated concentrations for the remainder of the day. 

June 5

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

35

40

mph

N WS W WS EC WS CG WS

N WD W WD EC WD CG WD

Figure 11. Hourly Average Wind Speed and Direction, 

Imperial County Monitoring Sites 

June 5, 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

35

40

wind (mph)

Brawley Westmorland Niland

Niland wind Westmorland wind Calexico-Grant wind

El Centro wind

* BAM concentrations exceeded monitor measurement range

 * * * * * * * ** ** ** * * *** * * * *

Figure 12. PM10 BAM Concentrations and Wind Speeds, 

Imperial County Monitoring sites 
20

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 June 5th

, 

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 10 to 23 knots. The BAM site at Mexicali-UABC, which recorded a 

24-hour PM10 concentration of 286 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 857 ug/m3 in the early afternoon (Figure 14). 

June 5, 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

Cobach WS ITM WS Cobach WD ITM WD

Figure 13. Hourly Average Wind Speed and Direction, 

Mexicali Monitoring Sites 

June 5, 2007

Mexicali PM10 and Wind Speed (WS) 

0

100

200

300

400

500

600

700

800

900

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

PM10 (ug/m3)

0

5

10

15

20

25

Knots

ITM WS Cobach WS UABC PM10 BAM

Figure 14. PM10 BAM Concentrations and Wind Speeds, 

Mexicali Monitoring Sites 
21

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

Imperial County PM10 BAM sites. Figure 15 shows impact from the west at 

three levels in the atmosphere (10, 100, and 500 meters) at the time of the peak 

PM10 concentrations. The back trajectory shows that the air parcels that affected 

the monitors at 3:00 p.m. on June 5, 2007 came from the west and passed 

through the Anza Borrego Desert. 

Figure 15. NOAA Hysplit Model Results, June 5, 2007 (3pm) 
22

ARB conducted an additional analysis using a WIND2D back trajectory model 

(Figure 16). This model traced the air mass 42 hours before an arrival time of 

6:00 p.m. on June 5th. The trajectory (blue line) shows that the air mass moved 

from the Pacific Ocean into San Diego County and 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 16. Westmorland Back trajectory arrival time 6:00pm 
23

June 6, 2007

On the morning of June 6, 2007 winds were blowing predominantly 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, wind speeds, as 

measured as Borrego Springs (Figure 17) were fairly low and from the southwest 

in the morning. This changed in the late morning when wind speeds increased to 

15 mph, gusting up to 40 mph. This corresponded to a directional shift back to a 

primarily western flow. 

Figure 17. Wind speed and direction at Borrego Springs, CA and Julian, CA 

[Source: www.wunderground.com] 

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

continued to be from the west throughout most of the day (Figure 18). Wind 

speeds, began to increase again in the morning, followed by increased hourly 

BAM PM10 concentrations at the Imperial County BAM sites (Figure 19); a 

further wind direction shift to northwest (just after 9 a.m.) resulted in a decrease 
24

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 afternoon shift back to the west and a slight wind speed increase caused 

BAM concentrations to again rise. 

June 6

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

mph

N WS W WS EC WS CG WS

N WD W WD EC WD CG WD

Figure 18. Hourly Average Wind Speed and Direction, 

Imperial County Monitoring Sites 

June 6, 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

wind (mph)

Brawley Westmorland Niland

Niland wind Westmorland wind Calexico-Grant wind

El Centro wind

* BAM concentrations exceeded monitor measurement range

* * * * ** ** ** * ** ** 

Figure 19. PM10 BAM Concentrations and Wind Speeds, 

Imperial County Monitoring Sites 

Wind data for the Mexicali-Cobach and Mexicali-ITM stations (which are not BAM 

sites) corroborate this continued high wind event (Figure 20). Wind speeds at 

the Mexicali-Cobach and Mexicali-ITM stations were over 10 knots most of the 
25

day, reaching as high as 21 knots (24 mph) at the ITM site in the early evening. 

The BAM site at Mexicali-UABC, which recorded a 24-hour PM10 concentration 

of 97 ug/m3, showed PM concentration increased with an increase in wind speed 

(Figure 21). The Mexicali PM10 BAM’s corroborate with the PM10 increases 

seen at the Imperial County BAM sites. 

June 6, 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

Cobach WS ITM WS Cobach WD ITM WD

Figure 20. Hourly Average Wind Speed and Direction, 

Mexicali Monitoring Sites

June 6, 2007

Mexicali PM10 and Wind Speed (WS) 

0

100

200

300

400

500

600

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

PM10 (ug/m3)

0

5

10

15

20

25

Knots

ITM WS Cobach WS UABC PM10 BAM

Figure 21. PM10 BAM Concentrations and Wind Speeds, 

Mexicali Monitoring Sites
26

Figure 22 shows the results of a backward trajectory analysis for the 

Westmorland monitoring site for June 6. This model, the NOAA HYSPLIT Model, 

shows impact from the west, over the Anza Borrego Desert, throughout the day. 

Figure 22. NOAA Hysplit Model Results, June 6, 2007 (midnight to midnight) 
27

ARB conducted an additional analysis using a WIND2D back trajectory model 

(Figure 23). This model traced the air mass 42 hours before an arrival time of 

6:00 p.m. on June 6th. The trajectory (blue line) shows that the air mass moved 

from the Pacific Ocean into San Diego and 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 23. Westmorland Back trajectory arrival time 6:00pm 
28

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 are investigated: 

 Although very high wind speeds were prevalent on June 5, 2007 

throughout the Imperial Valley (Table 8, compare with Figure 11 and 13), 

the highest winds south of the Salton Sea were recorded at Seeley and 

Meloland, directly to the west and east of El Centro, respectively. Note 

also that wind speeds at the El Centro ARB station were significantly 

greater than those at either the Westmorland or Calexico ARB stations 

(and that maximum wind speeds at all these stations were significantly 

lower than those at Seeley); 

 During the time of elevated wind speeds (from noon on June 5th and 

throughout the following day), winds were from the west throughout the 

valley; 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 2 and 3, respectively). 

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

PM10 concentrations at Imperial County stations on June 5, 2007, then PM10

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

Westmorland, and Calexico. However, the observed concentrations of 

200 μg/m3 at El Centro, 226 μg/m3 at Westmorland, 281 μg/m3 at Brawley, and 

282 μg/m3 at Calexico Ethel indicate that this is not the case. Instead, the 

meteorology-air quality relationship confirms the emission inventory analysis that 

it is not the emissions from agricultural field dust (i.e., the nearby sources), but 

rather the greater emissions from the desert and barren areas, that are the 

dominant contributor to the June 5, 2007 exceedences at the affected monitors. 
29

Table 8. Wind Speed (WS)* and Wind Direction (WD) at Select Imperial County 

CIMIS Stations** on June 5, 2007 

Westmorland 

(181) 

Calipatria 

(41) 

Seeley 

(68) 

Meloland 

(87) 

UC Andrade 

(201) 

Time

WS WD WS WD WS WD WS WD WS WD

0 9.6 WNW 5.3 SE 11.6 W 6.4 SSE 2.9 ESE

1 6.0 W 5.8 SE 8.8 WSW 7.2 SE 2.9 ENE

2 5.2 SE 6.3 SE 5.8 SW 6.9 SE 3.5 SE

3 5.9 SSW 5.9 SE 4.0 E 6.4 SE 3.5 E 

4 5.2 SE 7.2 SE 2.9 SE 6.6 SE 3.5 ENE

5 7.6 SSE 6.8 SE 4.8 WSW 6.8 SSE 2.0 SE

6 4.5 SE 6.9 SE 7.2 W 7.6 SE 6.8 SSE

7 5.0 SE 7.6 SSE 14.1 W 7.3 SE 8.6 SSE

8 2.3 ESE 7.3 S 8.1 SW 5.7 SE 9.1 SSE

9 4.5 WSW 6.9 WSW 15.2 WSW 6.9 SSE 5.5 S

10 5.8 WSW 11.7 WSW 17.8 WSW 7.9 SSE 5.3 S

11 8.1 NNW 15.8 WSW 21.4 W 13.7 WSW 9.2 S

12 16.1 NW 18.4 WSW 22.9 W 15.0 W 10.5 SSW

13 18.8 NW 13.1 W 26.0 W 21.5 WNW 8.8 SW

14 20.2 WNW 13.2 W 25.8 W 25.1 W 18.4 WNW

15 22.3 NW 16.9 WNW 23.4 W 28.7 W 24.6 WNW

16 20.7 NW 20.0 NW 20.9 W 29.6 W 27.0 WNW

17 21.2 NW 30.1 NW 21.4 W 24.6 W 24.3 NW

18 18.0 WNW 25.7 NW 32.5 W 23.9 WNW 24.7 NW

19 23.7 WNW 16.1 NW 26.7 W 28.6 WNW 25.3 NW

20 18.5 WNW 18.8 WNW 13.6 W 33.1 WNW 25.2 NW

21 21.3 WNW 13.5 NW 28.1 W 29.7 WNW 22.6 NW

22 21.7 NW 12.3 W 33.4 W 35.2 W 19.2 NNW

23 24.3 NW 13.1 WSW 27.1 W 26.0 W 17.1 NNW

* 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. 

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 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 
30

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. 

Historical Wind Speed Analysis. 

Resultant maximum wind speeds at the Imperial County monitoring sites 

(Table 8) showed June mean daily maximum wind speeds of approximately 

11 mph. A standard deviation of the combined data was 4.36. As noted in 

Figures 24-27, the maximum wind speed at the affected Imperial County 

monitoring stations was four standard deviations above the June historical 

average, making it clearly an exceptional wind event. 

Table 8. June Daily Maximum Wind Speed Statistics for 

Imperial County Monitors 

Calexico-Ethel 2004 2005 2006 2007 TOTAL 

Mean 18.2 11.2 10.7 10.2 12.6 

Standard Error 1.0 0.7 0.6 0.8 0.5 

Median 17.3 10.4 10.4 8.6 11.5 

Mode 16.1 12.7 8.1 8.1 8.1 

Standard Deviation 5.6 4.0 3.5 4.4 5.5 

El Centro 

Mean 8.2 10.0 8.3 8.3 8.7 

Standard Error 0.5 0.6 0.5 0.8 0.3 

Median 8.1 9.2 8.1 7.5 8.1 

Mode 8.1 8.1 6.9 4.6 8.1 

Standard Deviation 2.9 3.5 2.8 4.2 3.4 

Niland 

Mean 12.9 15.0 13.3 14.0 13.8 

Standard Error 0.9 1.1 0.6 1.1 0.5 

Median 11.5 14.4 12.7 12.1 12.7 

Mode 10.4 6.9 10.4 9.2 10.4 

Standard Deviation 4.9 5.9 3.5 6.2 5.2 

Westmorland 

Mean 7.0 9.6 8.2 7.1 8.0 

Standard Error 0.4 0.6 0.6 0.7 0.3 

Median 6.3 9.2 6.9 5.8 6.9 

Mode 5.8 6.9 6.9 4.6 6.9 

Standard Deviation 2.4 3.5 3.0 3.8 3.3 
31

Calexico-Ethel Wind - June Max Values

0

5

10

15

20

25

30

35

40

45

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

Day

mph

2004 2005 2006 2007 2004-2007 Mean

June 5, 2007

Figure 24. Maximum Resultant Wind Speeds, Calexico-Ethel, June 2004-2007 

El Centro Wind - June Max Values

0

5

10

15

20

25

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

Day

mph

2004 2005 2006 2007 2004-2007 Mean

June 5, 2007

Figure 25. Maximum Resultant Wind Speeds, El Centro, June 2004-2007 
32

Niland Wind - June Max Values

0

5

10

15

20

25

30

35

40

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

Day

mph

2004 2005 2006 2007 2004-2007 Mean

June 5, 2007

Figure 26. Maximum Resultant Wind Speeds, Niland, June 2004-2007 

Westmorland Wind - June Max Values

0

5

10

15

20

25

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

Day

mph

2004 2005 2006 2007 2004-2007 Mean

June 5, 2007

Figure 27. Maximum Resultant Wind Speeds, Westmorland, June 2004-2007 

b. Historical PM10 Concentration Analysis 

PM Concentrations at the Brawley, Westmorland, Niland, El Centro, and 

Calexico-Ethel monitoring stations are generally well below the NAAQS. As 

shown in Figures 28-32, exceedances like the ones on June 5th occur less than 

2% of the time at all of the affected sites, with the exception of the Calexico-Ethel 

monitor. The June 5th concentrations were far above historical fluctuations as 

well as above background levels. 
33

Brawley

0

5

10

15

20

25

30

20455565758595

105115125135145155170190250

More

PM10 Concentrations (ug/m3)

Frequency

0.00%

25.00%

50.00%

75.00%

100.00%

Frequency Cumulative %

June 5, 2007

85% 90%

Figure 28. Historical June PM10 concentrations at Brawley 

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

Calexico-Ethel

0

2

4

6

8

10

20455565758595

105115125135145155170190250

More

PM10 Concentrations (ug/m3)

Frequency

0.00%

25.00%

50.00%

75.00%

100.00%

Frequency Cumulative %

June 5, 2007

85% 90%

Figure 29. Historical June PM10 concentrations at Calexico-Ethel 

El Centro

0

5

10

15

20

25

30

20455565758595

105115125135145155170190250

More

PM10 Concentrations (ug/m3)

Frequency

0.00%

25.00%

50.00%

75.00%

100.00%

Frequency Cumulative %

June 5, 2007

85%

90%

Figure 30. Historical June PM10 concentrations at El Centro 
34

Niland

0

5

10

15

20

20455565758595

105115125135145155170190250

More

PM10 Concentrations (ug/m3)

Frequency

0.00%

25.00%

50.00%

75.00%

100.00%

Frequency Cumulative %

June 5, 2007

85% 90%

Figure 31. Historical June PM10 concentrations at Niland 

Westmorland

0

5

10

15

20

25

20455565758595

105115125135145155170190250

More

PM10 Concentrations (ug/m3)

Frequency

0.00%

25.00%

50.00%

75.00%

100.00%

Frequency Cumulative %

June 5, 2007

85% 90%

Figure 32. Historical June PM10 concentrations at Westmorland 

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 emissions reported before, during, and after the 

event. Figure 33 shows that PM10 BAM data at the three sites (Niland, Brawley, 

and Westmorland) was well below the federal PM10 standard for one week 

before and after the event. In Figure 34, FRM data from all sites in Imperial 

County shows that the previous two weeks and the following two weeks were 

also low, rising only during the event on June 5th. Without the shift in wind 

direction, bringing dust from the Anza-Borrego Desert, there would have been no 

exceedance of the federal PM10 standard. 
35

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 

May 29-June 12, 2007

0

50

100

150

200

250

300

350

5/29/20075/30/20075/31/2007

6/1/20076/2/20076/3/20076/4/20076/5/20076/6/20076/7/20076/8/20076/9/2007

6/10/20076/11/20076/12/2007

Date

PM10 BAM Concentrations (ug/m3)

Westmorland Brawley Niland

Figure 33. PM10 BAM concentrations, May 29-June 12, 2007 

June 2007 Exceptional Event

0

50

100

150

200

250

300

5/24/2007 5/30/2007 6/5/2007 6/11/2007 6/17/2007

PM10 Conc (ug/m3)

Niland Westmorland Brawley El Centro Calexico-Ethel Calexico-Grant

Figure 34. PM10 FRM Data May 24 to June 17, 2007 

Based on the analysis of wind speed and direction, PM10 concentrations, and 

modeling data, there would not have been an exceedance of the federal PM10 

standard at the affected if not for the high winds entraining dust from the Anza 

Borrego Desert which was subsequently transported to the affected 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