State of California 

AIR RESOURCES BOARD 

Status Report on Imperial County Air Quality and 

Approval of the State Implementation Plan Revision for PM10

Release Date: April 26, 2010 
This report is available for downloading from the Air Resources Board’s Internet site at 

http://www.arb.ca.gov/planning/sip/sip.htm. In addition, written copies may be obtained 

from the Board’s Public Information Office, 1001 I Street, 1st Floor, Environmental 

Services Center, Sacramento, California 95814, (916) 322-2990. 

If you are a person with a disability and desire to obtain this document in an alternative 

format, please contact the Americans with Disabilities Act Coordinator at 

(916) 323-4916, or TDD (916) 324-9531, or (800) 700-8326 for TDD calls from outside 

the Sacramento area. 

This document has been reviewed by the staff of the California Air Resources Board 

and approved for publication. Approval does not signify that the contents necessarily 

reflect the views and policies of the Air Resources Board, nor does the mention of trade 

names or commercial products constitute endorsement or recommendation for use. 
TABLE OF CONTENTS 

I. IMPERIAL COUNTY AIR QUALITY 

 A. Profile of the Imperial County........................................................................1 

 B. Historical Air Quality........................................................................2 

 1. PM10..................................................................................... 2 

 2. PM2.5..................................................................................... 4 

 3. Ozone......................................................................................5 

 

II. IMPERIAL COUNTY PM10 STATE IMPLEMENTATION PLAN 

 A. Planning Requirements....................................................................6 

 1. Moderate Area PM10 Attainment Plan...........................................6 

 2. Serious Area PM10 Attainment Plan.............................................6 

 

III. PLAN EVALUATION 

 A. Air Quality Assessment..........................................................................7 

1. High Wind Natural Events..........................................................7 

2. International Transport Events...................................................8 

3. Specific Attainment Demonstration Provisions..............................8 

 B. Emission Inventory.........................................................................9 

 C. District Control Strategy..................................................................9 

 D. Other Clean Air Act Requirements...................................................11 

1. Transportation Conformity Budgets..........................................11 

 

IV. STAFF RECOMMENDATIONS...............................................................12 

APPENDIX A 

 

1

I. Imperial County Air Quality 

A. Profile of Imperial County 

Imperial County is located in the southeast corner of California. As shown in Figure 1, 

the County extends over 4,597 square miles, bordering Mexico to the south, Riverside 

County to the north, San Diego County to the west, and Arizona to the east. With a 

population of approximately 170,000, the principal industries in Imperial County are 

year-round irrigated farming and retail trade. Few stationary sources are located in the 

region. Most of the population, farming, and retail trade exists in a narrow band of land 

in the central portion of the county. This region comprises on average less than 1/4th

the width of the county and stretches from the south shore of the Salton Sea to the 

Mexican border. The remaining land in Imperial County consists of large expanses of 

open desert, primarily managed by the federal government, including some of the 

largest sand dunes in the State. This arid region receives less than 3 inches of rainfall a 

year. 

Imperial County faces additional air quality challenges due to its proximity to the large 

international city of Mexicali, Mexico. Mexicali has a population of over one million 

people and is located just across the international border from Calexico. As shown in 

Table 1, NOx and VOC emissions in the city of Mexicali are twice those of the entire 

county of Imperial, and SOx emissions are more than 10 times higher. Several major 

border crossings are also located in Calexico. 

Figure 1. Map of Imperial County 
2

Table 1. Imperial County/Mexicali 2005 Emissions in tons per day (tpd) 

 

 

SOx NOx ROG/VOC 

Source 

Type 

City of 

Mexicali 

Imperial 

County 

City of 

Mexicali 

Imperial 

County 

City of 

Mexicali

Imperial 

County 

Stationary 12.7 0.2 39.38 3.56 2.0 1.31 

Area 0.5 0.1 3.3 0.9 41.92 17.08 

Mobile 0.6 0.6 35.77 27.31 26.13 14.98 

Total 13.8 0.9 78.45 31.77 70.05 33.37 

B. Historical Air Quality 

Despite the unique challenges that Imperial County’s geography, climatology, and 

proximity to Mexico pose for air quality, the combined efforts of State and local control 

programs have resulted in improving air quality in the region. The following sections 

characterize the nature of current particulate matter and ozone conditions and provide 

an overview of progress. 

1. Coarse Particulate Matter (PM10) 

Because Imperial County is an arid, desert region, PM10 concentrations are dominated 

by fugitive dust, but are generally quite low. As shown in Figure 2, more than 70% of 

the PM10 concentrations measured between 2005 and 2008 were less than 50 ug/m3, 

and more than 95% were below 100 ug/m3. Concentrations exceed the federal 24-hour 

PM10 standard of 150 ug/m3 on average once or twice a year at any given location. 

These infrequent occurrences are due to two distinct types of conditions – transport of 

emissions from Mexico, or naturally occurring high winds. 

Figure 2. Distribution of PM10 Concentrations in Imperial County (2005-2008) 

0

100

200

300

400

500

600

700

800

<25 26-50 51-75 76-100 101-125 126-150 150<

PM10 Concentration (ug/m3)

Number of measurements
3

The impact of transport from Mexico is seen primarily at monitors in Calexico that are 

near the international border. These transport-related exceedances occur during the 

winter months when conditions are stagnant, and emissions from Mexicali accumulate 

near the border. Research into PM10 concentration differences between Mexicali and 

Calexico showed that average cross border transport of PM10 from Mexico was three 

times higher than from the U.S., and that concentrations in Mexico were almost double 

those at Calexico (Chow. et. al., 2000). Increased residential trash and wood burning 

and charbroiling in Mexicali, often associated with cultural activities during holiday 

periods, are believed to be the primary cause of the transport exceedances at Calexico. 

Figure 3 is an image from the Mexicali newspaper La Cronica that illustrates reduced air 

quality on January 1, 2009. 

 

Figure 3. Photo of the Pollution on January 1, 2009 in Mexicali 

The second type of condition that causes elevated PM10 levels in Imperial County is 

high winds. These high winds events typically occur once or twice a year, usually 

between the months of April and September. Because of the large amount of open 

desert land, coupled with the limited rainfall, elevated wind speeds can loft and disperse 

large amounts of fugitive dust throughout the County. Figure 4 is a photograph taken in 

the western portion of Imperial County on April 15, 2008 that shows the impact of 

wind-generated dust. Evaluation of data between 2004 and 2008 found that after 

identifying exceedances due to international transport, all remaining exceedances 

occurred only once wind speeds exceeded 20 miles per hour (mph), with most occurring 

under conditions when winds reached over 30 mph. Recognizing that these types of 

naturally occurring high wind conditions are not controllable, United States 

Environmental Protection Agency (U.S. EPA) regulations allow areas to identify natural 

events such as high wind days and exclude them from use in determinations of a 

region’s air quality. 
4

Figure 4. Photo of Windblown Dust on April 15, 2008

2. Fine Particulate Matter (PM2.5) 

Unlike PM10, the fine portion of particulate matter (PM2.5) reflects greater contributions 

from combustion related sources, with less influence from fugitive dust. As a result, the 

nature of PM2.5 in Imperial County differs from PM10. In 1997, U.S. EPA established 

new PM2.5 standards to protect against both 24-hour and annual average exposures. 

The 24-hour standard was further tightened in 2006, lowering from 65 ug/m3 to 

35 ug/m3. PM2.5 levels throughout Imperial County have been below the annual 

standard of 15 ug/m3 since 2003. While Imperial County was recently designated as 

nonattainment for the revised 24-hour standard, considerable improvement has 

occurred since 2001. Figure 4 compares 24-hour concentrations at Imperial’s three 

monitoring locations in 2001 and 2008. In 2001, PM2.5 levels throughout Imperial 

County were considered unhealthy. Today, violations of the 24-hour standard are 

limited to the border site of Calexico, and the standard is exceeded by only 1 ug/m3. 

Similar to PM10, these elevated levels at Calexico occur during December and January, 

and are due to transport of emissions from the neighboring city of Mexicali. 
5

Figure 5. Imperial County PM2.5 24-hour Design Values (2001-2008) 

0

10

20

30

40

50

60

Calexico Brawley El Centro

Design Value concentration (ug/m3)

2001 2008

3. Ozone 

Ozone air quality has also improved significantly in Imperial County in recent years. 

Both concentrations and the number of exceedance days have decreased between 

1997 and 2009. The number of exceedances days dropped from approximately 50 in 

1997, to less than 2 in 2009. Imperial County attained the 1997 8-hour ozone standard 

of 0.08 ppm in 2008. However, the U.S. EPA is in the process of strengthening the 

8-hour ozone standard. This new standard is expected to be promulgated in 

August 2010, in a range of 0.060 and 0.070 ppm. As a result, we expect that Imperial 

County will be designated as nonattainment for the new standard. 

Meeting this more stringent standard in Imperial County will rely heavily on emission 

reductions in neighboring areas. Ozone levels in Imperial County are affected by 

transport from several other regions. Typically, transport impacts vary from day-to-day, 

depending on variations in wind patterns. However, Mexicali has an overwhelming 

impact on almost all Calexico exceedance days. In the northern area of the County, 

El Centro and Westmorland ozone levels are impacted by transport from both Mexicali 

and the South Coast region, as well as from San Diego County in the west. 
6

II. Imperial County PM10 State Implementation Plan 

Imperial County has been designated as nonattainment for the federal 24-hour PM10

standard. In order to meet Clean Air Act requirements for nonattainment areas, the 

Imperial County Air Pollution Control District (District) developed a State Implementation 

Plan (SIP) in 2009. 

A. Planning Requirements 

In 1987, U.S. EPA replaced its standard for total suspended particulates with standards 

that focused solely on PM10. The 1990 Clean Air Act Amendments (Act) subsequently 

established moderate and serious classifications for PM10 nonattainment areas, with 

planning requirements applicable to each classification. 

 1. Moderate Area PM10 Attainment Plan 

Imperial County was originally classified as a moderate PM10 nonattainment area which 

required reasonably available control measures (RACM), and set an attainment 

deadline of December 31, 1994. The District adopted RACM rules prior to being 

designated nonattainment. In October 9, 2001, U.S. EPA found that Imperial County 

attained the PM10 standard “but for” the emissions from outside the United States. 

However, on December 18, 2001, Earthjustice sued U.S. EPA regarding this 

determination. Subsequently, on August 3, 2004, U.S. EPA reclassified the Imperial 

Valley from a moderate to a "serious" PM10 nonattainment area. On 

December 11, 2007, U.S. EPA determined that the Imperial Valley failed to attain 

the PM10 standard by the required serious area deadline of December 31, 2001. In 

response to this finding, a SIP revision was required that provided for attainment of the 

PM10 standard in the Imperial Valley area as expeditiously as practicable. 

 

 2. Serious Area PM10 Attainment Plan 

Imperial County developed the 2009 PM10 SIP revision with the following required 

elements: 

1) Air Quality Assessment; 

2) Emission Inventory; 

3) Best Available Control Measures (BACM) and Best Available Control 

Technologies (BACT); 

4) Transportation Conformity Budgets; 
7

III. PLAN EVALUATION 

The following sections provide the ARB staff evaluation of the 2009 PM10 SIP with 

respect to meeting the requirements of the Act. 

A. Air Quality Assessment 

The central requirement of an attainment SIP is to demonstrate how a region will meet 

the applicable air quality standard by the federal deadline. Traditionally, the first step in 

developing an attainment plan is the evaluation of recent air quality data and a 

determination of the peak concentration around which the control strategy must be 

designed to reduce concentrations to the level of the standard. This is known as the 

design value. For PM10, it generally represents the highest measured concentration 

over a three year period. As part of the development of the 2009 PM10 SIP, District staff 

evaluated all exceedances that occurred between 2006 and 2008. During this period 

there were five days which exceeded the standard at various locations within the 

District. Based upon extensive technical analysis, the District determined that each of 

the five exceedance days were due to either international transport or high wind natural 

events. 

 1. High Wind Natural Events 

Imperial County experienced three high wind natural events between 2006 and 2008. 

Table 3 lists the natural event dates along with the affected monitors, the 24-hour 

average PM10 levels and the maximum resultant wind speed. 

Table 3. Imperial County High Wind Exceptional Events (2006-2008) 

Date Location and Concentration Max wind speed 

September 2, 2006 ● Calexico-Ethel 164 μg/m3, FRM 

● Calexico-Grant 233 μg/m3, FRM 

● Westmorland 167 μg/m3, FRM 

23 mph 

April 12, 2007 ● Brawley 291 μg/m3, FRM 

● Westmorland 155 μg/m3, FRM 34 mph 

June 5, 2007 ● Brawley 281 μg/m3, FRM 

● Calexico-Ethel 282 μg/m3, FRM 

● El Centro 200 μg/m3, FRM 

● Niland 162 μg/m3, FRM 

● Westmorland 226 μg/m3

, FRM 

34 mph 

On September 2, 2006, the high PM10 levels were primarily the result of wind-entrained

dust carried up into the atmosphere by high winds associated with a large thunderstorm 

system that impacted the southwestern United States and northwestern Mexico. Strong 

winds were observed on the east, southeast, northeast, and northwest borders of 

Imperial County, with wind gusts up to 47 mph at the Blythe, Yuma, and Thermal 

Airports. Air quality monitoring showed that this natural event also influenced PM10 air 

quality in Yuma, Arizona, and in the Coachella Valley. 

On April 12, 2007, strong winds from the west with gusts of 30 mph over the 
8

Anza Borrego Desert entrained dust into the atmosphere and caused two areas in 

Imperial County to record concentrations in excess of the federal 24-Hour PM10

standard. According to the Imperial Valley Press, winds were so high on April 12, 2007 

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

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

Highway Patrol issued a wind advisory warning motorists to slow down and drive 

cautiously. This natural event also impacted PM10 air quality in the Mojave Desert and 

the South Coast. 

Similarly, on June 5, 2007, strong winds from the west with gusts of 33 mph over the 

Anza Borrego Desert entrained dust into the atmosphere and caused all five 

Imperial County monitors to record concentrations in excess of the federal 24-hour PM10

standard. Gusts as high as 47 mph were recorded at the Imperial Airport. High wind 

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. 

In 2007, U.S. EPA adopted the Exceptional Events Rule (Rule). The Rule recognizes 

that that there are certain naturally occurring, uncontrollable events such as high winds 

and wildfires that can result in exceedances of federal standards for which it is not 

appropriate to apply the normal planning process. The Rule therefore allows 

appropriately documented events to be removed from consideration of a region’s 

attainment status. District staff developed extensive technical documentation for the 

three wind events that occurred in 2006 and 2007. ARB staff concurred with this 

analysis and submitted the documentation to U.S. EPA. 

2. International Transport Events 

Between 2006 and 2008 the remaining two exceedance days at the Calexico monitoring 

sites were due to international transport - December 21, 2006 and December 25, 2006. 

The District provided significant documentation in the 2009 PM10 SIP demonstrating the 

impact of transport and that the PM10 standard would not have been exceeded “but for” 

emissions from Mexico. The Act contains a specific provision (179B) for areas that are 

affected by international transport. While exceedances that occur due to international 

transport are still considered violations of the standard, the Act does not require a State 

to develop an attainment strategy addressing pollution that stems from international 

sources. 

 3. Specific Attainment Demonstration Provisions 

In summary, three of the five exceedances days are considered high wind natural 

events which should be excluded from attainment planning because they are neither 

controllable nor preventable within the scope of the Act’s planning process. The 

remaining two exceedance days would not have occurred in the absence of 

contributions from international transport. As discussed previously, the Act does not 

require an attainment demonstration for these types of events under the international 

transport provisions of 179B. Therefore, Imperial County is considered to have met the 

federal PM10 standard “but-for” these events and no attainment demonstration is 

required. U.S. EPA policy also states that the requirements for reasonable further 
9

progress, a five percent yearly reduction in emissions, and contingency measures are 

not applicable since their sole purpose is to bring an area into attainment of the 

standard. Nevertheless, in the 2009 PM10 SIP, Imperial County addressed contingency 

measures to provide additional assurance that PM10 levels will remain below the 

standard. These contingency measures reflect reductions from adopted regulations 

beyond those required for a serious nonattainment area. 

B. Emission Inventory

Emission inventories are fundamental elements of any air quality plan, incorporating the 

effects of growth and existing regulations to determine the expected emissions in future 

years. The District selected 2005 as the baseline year for the emission inventory and 

worked closely with ARB staff to improve the emission inventory for Imperial County. 

To determine the estimated emissions beyond 2005, staff prepared projections that 

used the baseline emission inventory, expected growth trends, and reductions from 

rules and regulations. External adjustments and improvements were made to certain 

source categories in the emission inventory. These adjustments and improvements are 

discussed in more detail in the 2009 PM10 SIP. 

Table 2 presents the updated baseline and projected emission inventory for direct PM10

emissions in the County split by main source category for 2006 through 2010. In 2006, 

72 percent (196 tpd) of the total emission inventory was from windblown dust. Area 

wide dust sources include fugitive dust from paved and unpaved roads, construction, 

and farming. The other area wide category includes waste burning, residential fuel 

combustion, and cooking. Stationary sources reflects fuel combustion and other 

industrial processes. Finally, mobile sources include emissions from both on- and 

off-road vehicles and equipment. While emission inventories are a required SIP 

element, it is important to note that understanding the nature of a region’s PM10 

problem is best characterized by what constituents are measured at the monitors. 

Analysis of Imperial County data has shown that fugitive dust is the dominant 

component of PM10. 

Table 2. Imperial County PM10 Annual Average 

Emission inventory in 2006-2010 (tpd) 

Source Category 2006 2007 2008 2009 2010 

Area Wide Dust Sources 70 69 68 68 67 

Other Area Wide Sources 3 3 3 3 3 

Stationary Sources 3 3 3 3 3 

Mobile Sources 2 2 2 2 2 

Subtotal 78 77 76 76 75 

Windblown Dust 196 196 196 196 196

Total emissions including windblown 274 273 272 272 271 

C. District Control Strategy 

The Act requires serious areas PM10 plans to implement Best Available Control 

Measures (BACM) for area sources and Best Available Control Technologies (BACT) 

for major stationary sources. According to the 2005 stationary source emission 
10

inventory, there is only one PM10 major stationary source that operates in Imperial 

County. This source manufactures gypsum wallboard and related products, and is 

located approximately 20 miles west of the nearest PM10 monitoring site in Plaster City. 

This source is currently at BACT levels. 

BACM is required for all area source categories that are considered significant 

contributors to violations of the federal PM10 standard. A source category is considered 

to be significant if its estimated contribution is 5 ug/m3 or higher to the total 

concentration. The significant source analysis outlined in the Imperial County’s 2009 

PM10 SIP determined that there were only two significant source categories in Imperial 

County, agricultural tilling and unpaved road dust. 

Although only two source categories required BACM, in 2005 the District developed a 

comprehensive set of fugitive dust rules collectively known as Regulation VIII, 

addressing all fugitive dust categories in the inventory. The categories covered include: 

• Construction and Earthmoving Activities 

• Bulk Materials 

• Carry-Out and Track-Out 

• Open Areas 

• Paved and Unpaved Roads 

• Agricultural Conservation Management Practices 

The two significant BACM sources are specifically controlled under rules 806 

(Agricultural Conservation Management Practices) and 805 (Paved and Unpaved 

Roads). The selection of control approaches was based upon an assessment of rules 

in other PM10 nonattainment areas and their specific applicability to Imperial County. 

The adopted controls include watering or chemical stabilization of unpaved roads, 

cleaning up dust from paved roads, preventing track-out from construction sites, 

requiring dust control plans for federal lands, and application of dust reducing 

conservation management practices such as providing cover crops or reduced tillage on 

agricultural lands. The fugitive dust rules are expected to reduce PM10 emissions by 

over 16 tpd by 2015. In addition to these reductions, the Bureau of Land Management 

implements additional controls for windblown dust and the Imperial Irrigation District’s 

Fallowing Program reduces dust from fallow fields. 

These rules were developed through a public process that included representatives of 

the agricultural community, private industry, Coalition of Labor and Business, Farm 

Bureau, Bureau of Land Management, Border Patrol, Imperial Irrigation District, County 

Public Works Department, ARB, and U.S. EPA. The District held six public workshops 

to solicit comments. ARB staff reviewed the rules at that time and supported the 

District’s adoption of these rules as BACM. In addition, at the Imperial County Board 

hearing, U.S. EPA staff testified that they believed that the Imperial County fugitive dust 

rules represented BACM level controls. Subsequent to their adoption, ARB staff 

submitted the rules to U.S. EPA in 2006 for adoption into the SIP. No action was taken 

by U.S. EPA until February 2010, at which time U.S. EPA proposed a partial approval of 

the rules, but also identified several rule components which they believed required 
11

additional analysis in order to demonstrate BACM level equivalence. When evaluating 

BACM, U.S. EPA does not have specific defined criteria, but rather the analysis is done 

on a case by case basis, reflecting the nature of the sources in the region, and 

considering cost-effectiveness. Because of this, ARB staff believes that given the 

specific nature of sources in Imperial County, the rules continue to reflect an appropriate 

BACM level of control. 

However, the District continues to look for cost-effective rule improvements that would 

further improve air quality. As discussed in the letter contained in Appendix A, the 

District has committed to additional rule improvements that would represent a 

strengthening of the SIP for Imperial County. These changes include more specific 

controls for off-highway vehicles, revising the conservation management practices 

application forms to be more specific on the required controls and frequency, narrowing 

the exemption and specifying dust control measures required for Border Patrol unpaved 

roads, and adding windblown dust controls for fallowed land. In a separate effort 

Imperial County is also in the process of updating their Smoke Management Plan to 

enhance public outreach and include a “good neighbor policy” to alert nearby residents 

of an upcoming burn. ARB staff support these efforts for the purpose of further 

improving public health in the region. 

D. Other Clean Air Act Requirements 

1. Transportation Conformity Budgets 

The 2009 PM10 SIP establishes on-road motor vehicle emissions budgets for the years 

2010, 2020, 2030, and 2035. The new emissions budgets for PM10 are shown in 

Table 4. The budgets are derived with EMFAC2007 projections and matched to activity 

data reported by the South Coast Association of Governments (SCAG). The new 

budgets will become applicable when U.S. EPA finds the budgets adequate. The 

conformity budgets are based on the average annual daily emissions for the Imperial 

County nonattainment area. The emissions budgets established in this Plan fulfill the 

requirements of the Act and U.S. EPA regulations to ensure that transportation activities 

support progress and attainment of the PM10 standards. 

Table 4. Motor Vehicle Emission Budgets for PM10* 

Imperial County, Annual Average, Tons per Day 

2010 2020 2030 2035 

EMFAC Output* 0.8 0.7 0.8 0.9

Paved Road Dust 3.9 6.5 7.9 8.5

Unpaved Road Dust 24.5 24.5 24.5 24.5

Road Construction Dust 0.5 0.4 1.4 1.9

Reductions from District Rules ** 4.5 8.8 9.1 9.2

Total 25.1 23.3 25.5 26.5

Motor Vehicle Emission Budgets 26 24 26 27

(rounded up to the nearest ton) 

* EMFAC 2007 with Imperial County activity provided by SCAG April 2009.

** Reductions from Imperial County APCD rules 801, 803 and 805.
12

IV. STAFF RECOMMENDATIONS 

ARB staff has reviewed Imperial County’s 2009 PM10 SIP and finds that the SIP meets 

all applicable CAA requirements. Therefore, we recommend that the Board adopt the 

Imperial County 2009 PM10 SIP, including updated transportation conformity budgets, 

and emission inventory, as a revision to the California SIP for submittal to U.S. EPA. 
13

APPENDIX A