1 

## Addendum

Natural Event Documentation

Imperial County, California
June 5, 2007



## Imperial County Air Pollution Control District

March 12, 20092

SUMMARY

This report is an addendum to the June 5, 2007 Natu ral Event Documentation (NED) for the Niland, El Centro, Westmorland, Brawl ey, and Calexico-Ethel monitoring stations prepared by the Imperial County Air Pollution Control District (Imperial County APCD). The Imperial County APCD d ocumentation was submitted to the U.S. EPA by the California Air Res ources Board (ARB) on June 13, 2008. This report further demonstrates that wit hout this regional natural high wind event, there would not have been exceedances o f 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 Administrat or in accordance with 40 CFR 50.14 to be an exceptional event.” The demonstrati on to justify data exclusion as outlined in 40 CFR 50.14 specifies that evidence mu st be provided that:

1. The event meets the definition of an exceptional event; 2. There is a clear causal relationship between the measurement under

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

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

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

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

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

II. There is a clear-causal connection between the high wind event and the exceedances at Niland, El Centro, Westmorland, Braw ley, and CalexicoEthel; III. The measured concentrations were beyond normal historical levels; and,

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

On June 5, 2007, strong winds from the west with gu sts of 33 mph over the Anza Borrego Desert entrained desert soil into the atmos phere 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 Airpor t. Excessive winds speeds and wind gusts were also recorded at the Blythe, Yu ma and Thermal Airports, all of which are in close proximity to the Imperial Val ley.3 Table 1. Imperial County APCD NAAQS Exceedances, J une 5, 2007 Monitoring Site AQS Site Number 24-Hour PM10 ( μ g/m3) Brawley-Main Street 060250007 281

Westmorland 1 st Street 060254003 226 Niland-English Road 060254004 162

El Centro 9 th Street 060251003 200

Calexico – Ethel Street 060250005 282
*Calexico-Grant site did not sample

The Imperial County APCD published in the local new spaper a notice of petition to exclude June 5, 2007 PM10 monitoring data affect ed by a high wind event.

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

Figure 1. Regional map of Imperial County and the A nza Borrego Desert4

Figure 2 provides a satellite view of the region, a nd includes the PM10 values measured at each monitor on June 5, 2007. The elev ated PM10 values measured at multiple sites indicate a large scale r egional event. As seen from the concentrations at the monitors, blowing dust tr ansported from the west was deposited not only at 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 μ g/m3 and the Mexicali UABC station

recorded a PM10 BAM concentration of 312 μ g/m3.

As shown in Table 2, low PM2.5 concentrations on Ju ne 5, 2007 confirm that the majority of particulate matter was in the coarse (P M2.5 to PM10) mass size range, indicating fugitive dust as the primary sour ce of the elevated

concentrations.

Table 2. Imperial County APCD PM2.5 and PM10 conce ntrations, 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 P M10 Concentrations,
June 5, 20075

Anthropogenic emission in Imperial County were roug hly constant before, during, and after the event, and PM10 concentrations were a pproximately 50 ug/m3 prior to and after the event, well below the federal stan dard. Countywide, windblown

dust emissions from urban lands are minimal and emi ssions from agricultural lands are small (6%) compared to the desert area em issions. 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 loc ated in the center portion of Imperial County, south of the Salton Sea, and can b e identified by the green coloring on the map. The desert area emissions doe s not include the extensive desert area in San Diego County which also likely c ontributed 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 conce ntrations 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, l ocal farmers, and U.S. EPA. Dispersed throughout the agricultural lands are fal low farm lands which participate in the Imperial Irrigation District’s F allowing Program. The Fallowing Program also requires farmers to implement United S tates Department of Agriculture Best Management Practices on the land t o control fugitive dust.

In summary, on June 5, 2007, PM10 exceedances at th e Niland, El Centro, Westmorland, Brawley, and Calexico-Ethel monitors w ere 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 appr opriate taking into account the magnitude of the high winds and the small contr ibution 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.6

Analysis

I. Dust Event Not Reasonably Controllable or Preven table

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

source of dust was natural windblown dust emissions from the Anza Borrego Desert. Information on the Land Use/Land Cover (LU LC) data, the State Soil Geographic Database (STATSGO), and the threshold fr iction velocity necessary to entrain dust in the Imperial County. This secti on also discusses the different emission sources in Imperial County along with thei r 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 Coun ty 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 discussion s with ARB and U.S. EPA, documented in Appendix A of the 2005 BACM demonstra tion report (see Attachment A of the NED). The BACM report presents the methodology used in estimating windblown fugitive dust from agricultura l and barren lands in the Imperial County. In particular, it presents infor mation 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 Dat abase (NLCD). This

information is reiterated here.

Land Use and Land Cover Data

The DWR Land Use Survey databases are developed by the department’s Division of Planning and Local Assistance in order to aid in the efforts of continually monitoring land use for the purpose of tracking the amount of water consumed and changes in its usage. The data is ava ilable at the county level and includes detailed agricultural land use classif ications, as well as less detailed urban and native vegetation land uses. The data wa s compiled from aerial photography alongside extensive field verification and is updated on a rotating basis by the county approximately every seven years . Thus, the data is based on varying years, depending on the county, ranging from the late 1980’s through the late 1990’s. Because the DWR survey data does not explicitly consider certain land use types (i.e., urban lands, desert l ands, forest lands, etc.) it was necessary to augment the DWR land use data with the National Land Cover Data (NCLD).7

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

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

Sea. Toward the north it extends eastward across th e 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 Uni ted States.

Imperial County APCD noted (Table 3 of the Natural Event documentation) that the 12-month period prior to the event was exceptio nally dry, experiencing only 5% of normal precipitation. This, coupled with extr eme high temperatures experienced in the months preceding the event (Tabl e 4 of the Natural Event Documentation), contributed to arid conditions in t he 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 Ba sed on

Combined DWR and NLCD Information8

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

event duration

To determine the characteristics or type of soil pr esent in Imperial County, the Wind Blown Dust Study used the State Soil Geographi c Database (STATSGO). The STATSGO database was developed by the Natural R esources Conservation Service of the US Department of Agricultural (USDA) , providing detailed information concerning the taxonomy of the soils, i ncluding soil texture class, percentage of sand, silt and clay, and the availabl e 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 areas9

Figure 5 shows the monthly PM10 windblown emissions by land use type in 2004 (See Table 3-2 from the Regulation VIII BACM Analys is). Agriculture and Urban open areas emit the least amount of PM10 throughout the year in Imperial County. According to Figure 5, agriculture is resp onsible for only 6% of the total

windblown PM10 emissions for the month of June in I mperial County. Sand dunes along with shrub and grasslands in Imperial C ounty 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

MonthPM10 (Tons/Day) Urban Open Areas

Desert-Other

Desert-Dunes

Shrub/Grassland

Agriculture

Figure 5. Monthly Windblown Emissions (tons/day) i n 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 Ana lysis). Because of its year round crop cover, irrigation, and soil stability, a gricultural lands emit very small

amounts of PM10.

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

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

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

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

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

Shrub/

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

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

TOTAL (TPD) 147.2 183.4 228.9 431.5 192.4 282 80.5 199.5 111.1 14.0 102 54.84 168.2210

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

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

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

Figures 6 and 7 show the maximum hourly wind speed in relation to PM10

concentrations for the month of June for 2002 throu gh 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 wi nd speeds in June were, for the most part, between 5 and 20 mph at both sta tions. PM10 concentrations generally remain below 110 μg/m3 at Brawley and 150 μg/m3 at Westmorland.

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

normal historical fluctuations and the winds were u nusually high, above the entrainment threshold of 15 mph.

Figure 6. Brawley PM10 vs. Maximum Hourly Wind Spee d,

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

June 2002 through 2007

c. Emissions Inventory.

The PM10 emission inventory compares the type of so urces that generate dust and could potentially contribute to PM10 NAAQS viol ations. The 2005 PM10 emission inventory is summarized in Table 4 and fea tures adjustments made to incorporate updated cattle emissions, updated windb lown dust model results, and updates for entrained and windblown unpaved roa d dust estimates. The middle column of Table 4 portrays annual average em issions. On a day with exceptionally high winds such as June 5, 2007, wind blown emissions are expected to have been significantly higher than the ir annual average values. The Windblown Dust Study showed that summer in Impe rial County produces the second highest PM10 emissions for the entire ye ar; 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 o n a very windy day, derived by modifying the annual average inventory to use Wi ndblown Dust Model results for the month of April (which featured the highest monthly windblown emissions in 2002 according to the model), is given in the ri ght column of Table 4. We note that according to both the annual average and the “ high wind” inventories, windblown emissions from agricultural lands only co ntribute 5-6% of all windblown emissions, or 4-5% of the total inventory . Furthermore, although nonanthropogenic sources already account for ~55% o f total emissions under average conditions, their fractional contribution t o the total “high wind” emissions is ~75%. Given that the “high wind” inventory of T able 4 is expected to

underestimate the level of windblown emissions that occur on the windiest days, the predominant source of emissions contributing to the 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)12 Table 4. Revised 2005 Annual Average PM 10 Emission Inventory

For Imperial County (Tons/Day)

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

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

Industrial Processes: 2.8 2.8

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

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

Harvest 0.0 0.0

Cattle 2.8 2.8

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

Canal 30.7 30.7

BLM/USFS 1.4 1.4

Farm 1.4 1.4

Windblown Dust: 200.5 463.7 Open Areas: 157.3 402.9

Grassland*** 98.7 243.2

Dunes*** 19.8 62.4

Other Barren Lands*** 38.7 97.3

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

Pasture 1.8 1.8

Construction 1.9 1.9

Fires 0.0 0.0 Waste Burning 2.8 2.8 Cooking 0.1 0.1

On-Road Mobile 1.1 1.1

Other Mobile 1.0 1.0

Total 284 548 Total Anthropogenic 127 145

* Values in emission inventory table are rounded to the nearest 0.1 ton/day ** Emissions equal to the annual average, except fo r windblown sources, for which the predictions of t he Windblown Dust Study for the month of April, 2002 were used. (The month of April had the highest monthly windblown e missions in 2002 according to the model). *** Natural sources of dust, only 1% of which come from lands disturbed by humans.13

d. Fugitive Dust Control Measures

Fugitive Dust Sources

.

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

stabilized surface.

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

Fallow Fields.

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

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

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

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

1) Plan ahead to start with plenty of vegetation re sidue, and maintain as much residue on fallowed fields as possible. Resid ue is more effective for wind erosion protection if left standing.14 2) Avoid any tillage if possible. 3) Avoid any traffic on the field or tillage when f ields are extremely dry to avoid pulverization. 4) If residues are not adequate, either small grain can be seeded about the first of the year to take advantage of winter r ains or soil stabilization chemicals may be applied to fallowed lands. 5) In addition to the above BMPs, in order to satis fy Imperial County dust control and mitigation requirements, Fallowing Part y will comply with any lawful conditions required by the Imperial County A ir Pollution Control

District.

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

e. Source Activity on 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 gus ts for this day. The Imperial County APCD authorized agricultural burning for a t otal 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 A PCD Daily Weather and Agricultural Burning Information report is attached to this report (Attachment B).

Inspections.

The APCD inspectors performed minimum facility insp ections on June 5, 2007. APCD staff did not observe any unusual emissions ot her 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 th e NED). The results of the investigation indicated that the high winds from th e previous day had resulted in dust deposited onto the property of the complainant s (Table 5).15 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 al ways full of dirt due to an empty lot next to him. Resid ent called in complaint on June 5 th and ICAPCD inspector arrived on

June 6 th to investigate the complaint.

2715 B La Brucherie Rd Imperial, CA

10:40am Occupant called to complain that his property is al ways full of dirt due to an empty lot next to him. Resid ent called in complaint on June 5 th and ICAPCD inspector arrived on

June 6 th to investigate the complaint.

In summary, based on the information presented in t his section, on June 5, 2007

windblown dust emissions from the barren lands to t he west in the Anza Borrego desert were the dominant cause of the suspended dus t measured at the affected monitors. Nevertheless, reasonable and appropriate controls were in place for all anthropogenic sources, and best management practice s 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 includ e: analysis of the PM10 and Beta Attenuation Monitor (BAM) concentrations and t he correlation with wind speeds, wind gusts, and wind direction on June 5, 2 007, as well as running hysplit and Wind2D back trajectories to identify th e source region.

a. Cause of High PM10 Concentrations on June 5, 20 07

A high wind dust event does not always show an imme diate impact on high PM10 concentrations at monitoring stations, particu larly if transported over some distance. A wind blown dust event may be comprised of entrainment of dust by high winds in the source area, followed by depositi on in a receptor area with lower wind speeds. Once suspended upwind of the mo nitoring stations, dust can be carried downwind by winds below the dust entrain ment threshold. On June 5, 2007, dust was entrained in the Anza Borreg o 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 5 th and again on June 6 th . With the exception of Niland, where the wind speeds were higher, winds we re originally less than 5 mph. They began to rise in the late morning of J une 5 th , particularly at Niland and El Centro, and remained fairly high for most of June 6 th at most of the air quality monitoring sites, the California Irrigation Management Information System16

(CIMIS) sites, and the four regional airports. Win d 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 Am bient Air Quality Standard (NAAQS) of 150 μg/m3. The city of Yuma, A rizona 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 m onitor, (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/m 3 ) 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 F RM sites, elevated concentrations were recorded by PM10 continuous Bet a Attenuation Monitor (BAM) sites operated by Imperial County for forecas ting purposes at Brawley, Niland, and Westmorland (Table 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 BA M sites

Monitor AIRS No. Concentration ( μ g/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 t he morning. When the wind direction shifted in the late morning to a more wes tern flow, the wind speed also increased significantly. These shifts can be seen a t two small weather observation sites west of the Imperial monitoring s ites. Borrego Springs (KCABORRE1) in the northern portion of the Anza Bor rego Desert showed wind gusts as high as 32 mph, and Cielo Vista Observator y in Julian (KCAJULIA2), almost directly west of Westmorland on the western edge of the Anza Borrego Desert, noted wind speeds over 33 mph with gusts as high as 43 mph (Figure 8). The site at Julian, in particular, showed significa nt wind speed increases earlier17

in the day, allowing time for wind across the deser t to entrain and transport dust to the Imperial Valley. In a joint effort, the Imp erial County APCD and IID provide daily weather forecast for Imperial County. On Tue sday June 5th, 2007 weather reports were issued at 6:30 am and 5:30 pm. Both r eports predicted strong west winds in the mountain and desert areas causing blow ing sand and dust in the region.

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

[Source: www.wunderground.com ]18

Although no other data is available for the Anza Bo rrego Desert, surface analysis maps of wind speeds across the southwest part of th e country show wind speeds in the area increasing. Figure 9 shows surface win d speeds across the Desert at 2 p.m. ranged from 16 to 20 knots (18 to 23 mph), a bove the dust entrainment threshold of 15 mph. By 5 p.m., wind speeds had in creased 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 6/5/0719

Hourly data from District monitoring sites shows wi nd speeds throughout the region increased beginning in the late morning on J une 5 th , at the same time that

the wind direction shifted to an almost uniform wes tern flow (Figure 11). At the same time, wind speeds increased and PM10 concentra tions 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 J une 6 th , decreasing slightly in the late morning of the 6 th before rising again and remaining relatively high throughout the rest of the day, peaking around 5pm.

Concentrations at the Brawley PM10 BAM site increas ed simultaneously with those at Westmorland and Niland. Although the Braw ley site does not have meteorology data, the corresponding shift in wind d irection and increase in wind speed can be assumed. Wind speeds remained elevate d and all three BAM

sites showed elevated concentrations for the remain der 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

HourDegree

0 5 10 15 20 25 30 35 40mph 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

HourPM10 (ug/m3)

0 5 10 15 20 25 30 35 40wind (mph) Brawley Westmorland Niland Niland wind Westmorland wind Calexico-Grant wind El Centro wind

* BAM concentrations exceeded monitor measurement r ange * * * * * * * ** ** ** * * *** * * * *

Figure 12. PM10 BAM Concentrations and Wind Speeds

,

Imperial County Monitoring sites20

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 5 th

, wind speeds at the Mexicali-Cobach and Mexicali-ITM stations were relatively low with winds coming from the east. At about midd ay, the winds shifted to the west with a parallel increase in wind speeds. Spee ds in the later part of the day ranged from 10 to 23 knots. The BAM site at Mexica li-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 s ites. 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 3600:001:002:003:004:005:006:007:008:009:0010:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

HourDegree

0 5 10 15 20 25knots 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 9000:001:002:003:004:005:006:007:008:009:0010:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

HourPM10 (ug/m3)

0 5 10 15 20 25Knots ITM WS Cobach WS UABC PM10 BAM

Figure 14. PM10 BAM Concentrations and Wind Speeds

,

Mexicali Monitoring Sites21

Figure 15 shows the results of a backward trajector y analysis for the three Imperial County PM10 BAM sites. Figure 15 shows im pact from the west at three levels in the atmosphere (10, 100, and 500 me ters) 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, 200 7 (3pm)22

ARB conducted an additional analysis using a WIND2D back trajectory model (Figure 16). This model traced the air mass 42 hou rs before an arrival time of 6:00 p.m. on June 5 th . The trajectory (blue line) shows that the air ma ss moved from the Pacific Ocean into San Diego County and th rough Imperial County before impacting the Westmorland monitor. The traj ectory clearly shows the air mass passing through the Anza Borrego Desert before impacting Imperial County.

Figure 16. Westmorland Back trajectory arrival tim e 6:00pm23

June 6, 2007

On the morning of June 6, 2007 winds were blowing p redominantly 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 D esert, 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 wh en 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 Spr ings, CA and Julian, CA

[Source: www.wunderground.com ]

Wind direction at sites further south and east (Imp erial County and Mexicali) continued to be from the west throughout most of th e day (Figure 18). Wind speeds, began to increase again in the morning, fol lowed by increased hourly BAM PM10 concentrations at the Imperial County BAM sites (Figure 19); a further wind direction shift to northwest (just aft er 9 a.m.) resulted in a decrease24

in both. This directional shift away from the fugi tive dust source in the Anza Borrego Desert caused PM10 concentrations at the mo nitoring 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

HourDegree

0 5 10 15 20 25 30mph 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

HourPM10 (ug/m3)

0 5 10 15 20 25 30wind (mph) Brawley Westmorland Niland Niland wind Westmorland wind Calexico-Grant wind El Centro wind

* BAM concentrations exceeded monitor measurement r ange * * * * ** ** ** * ** **

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 the25

day, reaching as high as 21 knots (24 mph) at the I TM 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 wi th 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 3600:001:002:003:004:005:006:007:008:009:0010:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

HourDegree

0 5 10 15 20 25knots 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 6000:001:002:003:004:005:006:007:008:009:0010:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

HourPM10 (ug/m3)

0 5 10 15 20 25Knots ITM WS Cobach WS UABC PM10 BAM

Figure 21. PM10 BAM Concentrations and Wind Speeds

,

Mexicali Monitoring Sites26

Figure 22 shows the results of a backward trajector y analysis for the Westmorland monitoring site for June 6. This model , the NOAA HYSPLIT Model, shows impact from the west, over the Anza Borrego D esert, throughout the day.

Figure 22. NOAA Hysplit Model Results, June 6, 200 7 (midnight to midnight)27

ARB conducted an additional analysis using a WIND2D back trajectory model (Figure 23). This model traced the air mass 42 hou rs before an arrival time of 6:00 p.m. on June 6 th . The trajectory (blue line) shows that the air ma ss 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 C ounty.

Figure 23. Westmorland Back trajectory arrival tim e 6:00pm28

c. Additional Spatial and Temporal Analysis of Conc entrations

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

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

. To do so, the relationships between surface

wind conditions and PM 10 air quality measurements are investigated:

 Although very high wind speeds were prevalent on J une 5, 2007 throughout the Imperial Valley (Table 8, compare wi th Figure 11 and 13), the highest winds south of the Salton Sea were reco rded at Seeley and Meloland, directly to the west and east of El Centr o, respectively. Note also that wind speeds at the El Centro ARB station were significantly greater than those at either the Westmorland or Cal exico 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 5 th and throughout the following day), winds were from the west throughout the valley; and,

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

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

PM 10 concentrations at Imperial County stations on June 5, 2007, then PM 10 impacts at El Centro would have been larger than th ose at Brawley, Westmorland, and Calexico. However, the observed c oncentrations of

200 μ g/m 3 at El Centro, 226 μ g/m 3 at Westmorland, 281 μ g/m 3 at Brawley, and

282 μ g/m 3 at Calexico Ethel indicate that this is not the ca se. Instead, the meteorology-air quality relationship confirms the e mission inventory analysis that it is not the emissions from agricultural field dus t (i.e., the nearby sources), but rather the greater emissions from the desert and ba rren areas, that are the dominant contributor to the June 5, 2007 exceedence s 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 ab ove the ground. Wind speeds in the 15-20 mph and 20-25 mph ranges are in blue and pink, respectively, while w ind speeds > 25 mph are in red. ** Refer to Figure 12 and Table 7 of the initial d ocumentation of this event (ICAPCD, April 15, 2008, submitted with this report) for information about the location of the s tations.

III. Concentrations Beyond Normal Historical Fluctu ations

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

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

a. Wind Speed Averaging and Historical Analysis.

Wind Speed Averaging

.

The Imperial County APCD and ARB monitoring sites c ollect hourly averages of wind speed and wind direction. Each hourly average is comprised of 60 data points (each minute of the hour). Because of limit ed monitoring data capacity, the Imperial County APCD is not able to store these minute data points and only retains the hourly averages. Federal Aviation Admi nistration/National Weather Service (FAA/NWS) stations located in Imperial Coun ty report wind speed and direction following the standard METAR observation periods, which are30

approximately 10 minutes prior to the observation t ime. In other words, the Imperial County APCD’s wind speed data recorded is an hourly average for the complete hour and the FAA/NWS data is a 10 minute a verage at the end of each hour. In addition, airport monitors have wind gust reporting capabilities not available to the APCD’s monitoring sites. Because of the different averaging periods between the APCD’s monitors and the airport monitors, the wind speeds and directions may not always be comparable. The F AA/NWS stations typically record hourly wind averages higher than those calcu lated at the APCD and ARB monitoring sites. However, the Imperial County APC D utilizes both data sets to further substantiate the high wind event.

Historical Wind Speed Analysis.

Resultant maximum wind speeds at the Imperial Count y monitoring sites (Table 8) showed June mean daily maximum wind speed s 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 affect ed Imperial County monitoring stations was four standard deviations ab ove the June historical average, making it clearly an exceptional wind even t.

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 3.331 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

Daymph 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

Daymph 2004 2005 2006 2007 2004-2007 Mean

June 5, 2007

Figure 25. Maximum Resultant Wind Speeds, El Centro , June 2004-200732 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

Daymph 2004 2005 2006 2007 2004-2007 Mean

June 5, 2007

Figure 26. Maximum Resultant Wind Speeds, Niland, J une 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

Daymph 2004 2005 2006 2007 2004-2007 Mean

June 5, 2007

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

b. Historical PM10 Concentration Analysis

PM Concentrations at the Brawley, Westmorland, Nila nd, El Centro, and Calexico-Ethel monitoring stations are generally we ll below the NAAQS. As shown in Figures 28-32, exceedances like the ones o n June 5 th occur less than 2% of the time at all of the affected sites, with t he exception of the Calexico-Ethel

monitor. The June 5 th concentrations were far above historical fluctuati ons as well as above background levels.33 Brawley

0 5 10 15 20 25 3020455565758595105115125135145155170190250More

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 1020455565758595105115125135145155170190250More

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 3020455565758595105115125135145155170190250More

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 Centro34 Niland

0 5 10 15 2020455565758595105115125135145155170190250More

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 2520455565758595105115125135145155170190250More

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 exa mined for the weeks prior to

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

have been an exceedance.

There were no unusual local emissions reported befo re, during, and after the event. Figure 33 shows that PM10 BAM data at the t hree sites (Niland, Brawley, and Westmorland) was well below the federal PM10 st andard 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 fo llowing two weeks were also low, rising only during the event on June 5 th . Without the shift in wind direction, bringing dust from the Anza-Borrego Dese rt, 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 anthropogen ic emissions were approximately constant before, during and after the event, indicating that the significant increase was not due to anthropogenic e missions activity.

PM10 BAM Daily Averages

May 29-June 12, 2007

0 50 100 150 200 250 300 3505/29/20075/30/20075/31/20076/1/20076/2/20076/3/20076/4/20076/5/20076/6/20076/7/20076/8/20076/9/20076/10/20076/11/20076/12/2007

DatePM10 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 6/17/2007PM10 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 exceeda nce 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 t o 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 BA CM Analysis. October

2005.

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

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

Plan. Adopted August 9, 2005.

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