State of California 

AIR RESOURCES BOARD 

STAFF REPORT 

Analysis of the Imperial County 2009 1997 8-Hour Ozone 

Modified Air Quality Management Plan and 

Reasonably Available Control Technology Plan

Date of Release: 

Scheduled for Consideration: November 18, 2010 

California Environmental Protection Agency

Air Resources Board
This report 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 

mention of trade names or commercial products constitute endorsement or 

recommendation for use. 
Table of Contents 

I. Introduction....................................................................................................1 

II. Background....................................................................................................2 

III. Moderate Area Requirements........................................................................4 

IV. Evaluation of the Imperial County Modified Plan ...........................................6 

A. Attainment of the 1997 8-Hour Ozone Standard ..................................................6

B. Moderate Nonattainment Area Requirements......................................................8

V. 2009 RACT SIP ...........................................................................................11 

VI. Staff Recommendations ..............................................................................11

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I. Introduction 

This staff report reviews plans adopted by the Imperial County Air Pollution 

Control District to address federal Clean Air Act (CAA) nonattainment area plan 

requirements applicable to moderate ozone nonattainment areas. 

With the promulgation of the 1997 8-hour ozone National Ambient Air Quality 

Standard (NAAQS) in April 2004, the United States Environmental Protection 

Agency (U.S. EPA) classified Imperial County as a marginal nonattainment area. 

On February 13, 2008, U.S. EPA found that Imperial County had not attained the 

1997 8-hour ozone NAAQS by the marginal area deadline of June 2007, and 

reclassified the County as a moderate nonattainment area. U.S. EPA’s action 

also required the State to submit a moderate area SIP for the Imperial County 

nonattainment area, by December 31, 2008, that demonstrates attainment of the 

8-hour ozone standard by June 15, 2010. 

On February 19, 2009, ARB requested that U.S. EPA make a “clean data” finding 

for Imperial County. The request demonstrated that the area’s 2008 design 

value for the 8-hour ozone NAAQS, which was based on monitoring data 

collected from 2006 through 2008, showed attainment of the standard. U.S. EPA 

issued its clean data finding on December 3, 2009. 

A clean data finding does not constitute a redesignation to attainment under the 

national CAA, so Imperial County remains a moderate nonattainment area. The 

clean data finding suspended some CAA nonattainment area plan requirements. 

However, the State is still required to submit revisions to the SIP to address 

remaining elements required for moderate ozone nonattainment areas, including 

rule review and adoption relative to the reasonably available control technology 

(RACT) requirements, emission inventories, and new source review 

requirements. 

On July 30, 2010, the Imperial County Air Pollution Control District adopted the 

2009 1997 8-Hour Ozone Modified Air Quality Management Plan (2009 Ozone 

Plan) and 2009 Reasonably Available Control Technology State Implementation 

Plan (2009 RACT SIP) to address the CAA moderate area plan requirements. 

ARB staff reviewed the Imperial County 2009 Ozone Plan and RACT SIP and 

find that both plans meet all applicable CAA requirements for a moderate 

nonattainment area. Staff recommends that the Air Resources Board adopt the 

Imperial County 2009 Ozone Plan and the 2009 RACT SIP and direct the 

Executive Officer to submit the plans to U.S. EPA as proposed revisions to the 

California SIP. 
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II. Background 

Profile of Imperial County 

Imperial County is part of the Salton Sea Air Basin. The Imperial County Ozone 

Nonattainment Area consists of the entire county, and is under the jurisdiction of 

the Imperial County Air Pollution Control District (District). 

Imperial County covers 4,482 square miles of mostly arid lands in the 

southeastern corner of California. It is bordered by Mexico to the south, Arizona 

on the east, San Diego County on the west, and Riverside County on the north. 

Imperial County’s elevation ranges from more than 2,800 feet on the mountain 

summits to the east to 230 feet below sea level in the Salton Sea. Winters are 

mild and dry with daily average temperature ranges between 65 and 75oF, 

although daily high temperatures of up to 80oF are not uncommon. Summers are 

extremely hot with daily average temperature ranges between 104 and 115oF 

and daily high temperatures of up to 120oF. The County has an annual rainfall of 

approximately 3 inches, occurring mostly from late summer to midwinter. 

Imperial County has a population of approximately 180,000 people. The 

County’s major population centers include the cities of El Centro (25 percent of 

the county population), Calexico (22 percent), and Brawley (15 percent). 

Approximately 22 percent of the population resides in unincorporated areas. 

Mexicali, Mexico – a city of approximately 900,000 people – spreads southward 

from the California/Mexico border at Imperial County. Mexicali’s residential and 

industrial centers begin less than 5 miles from Calexico and 15 and 25 miles, 

respectively, from El Centro and Brawley. Because the New River is the only 

geographical feature separating Imperial County from Mexicali, air pollution 

transport between the two areas is possible. ARB’s June, 1993 transport 

assessment report identified Mexico as the source of emissions that caused or 

contributed to some of the exceedances of the state ozone standard recorded in 

Imperial County from 1989 to 19911

. 

Air Quality History 

Ozone air quality has improved significantly in Imperial County in recent years as 

shown by both the number of days in which ozone values exceed the 8-hour 

ozone NAAQS (exceedance days) and the recorded daily high ozone 

concentrations. In 1997, Imperial County exceeded the 1997 8-hour ozone 

NAAQS of 0.085 parts per million (0.085 ppm), also expressed as 85 parts per 

billion (85 ppb), on 50 days. In 2009, the number of exceedance days declined 

to 2. Table 1 shows the progressive decrease in exceedance days at each 

routine monitoring site in Imperial County. 

 

1

 Air Resources Board, Assessment and Mitigation of the Impacts of Transported Pollutants on Ozone 

Concentrations in California, June 1993 
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Table 1

Imperial County Exceedance Days, 8-Hour Ozone NAAQS

2001 - 2009 

Monitoring Site 2001 2002 2003 2004 2005 2006 2007 2008 2009

Calexico East 7 4 0 1 0 0 2 1 0 

Calexico Ethel 11 1 0 0 1 1 4 4 0 

El Centro 2 9 8 0 5 9 3 0 1 

Niland 1 5 0 0 0 0 0 1 0 

Westmorland 1 0 0 0 10 5 5 2 1 

Figure 1 shows the federal design value trend in Imperial County from 2001 to 

2009 and the respective 1997 8-hour ozone standard. The SIP design value is 

based on the fourth highest value recorded each year, averaged over three 

consecutive years. Imperial County’s 8-hour ozone design value has decreased 

by ten percent since 2001, and now meets the 1997 federal NAAQS. 

When compared to other indicators such as peak concentrations, the design 

value is a more stable indicator for evaluating year to year changes resulting 

from emission control programs, and is the only indicator that can be used to 

determine compliance with the NAAQS. Peak concentrations measure the 

severity of ozone exposure but not the frequency, and can be influenced by year 

to year weather variations. 

The U.S. EPA is in the process of revising the 8-hour ozone NAAQS, and is 

expected to promulgate its revised standard in late 2010. The range of values 

the agency is considering for the new NAAQS – from 0.070 ppm to 0.060 ppm 

(70 ppb to 60 ppb) – is from 29 percent to 17 percent more stringent than the 

current 8-hour ozone NAAQS of 0.085 ppm. Recent monitoring data suggests 

that Imperial County will be designated as nonattainment for the new standard. 
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Figure 1 

Imperial County Design Value 

92

87 87

85 84 85

87

82 83

60

80

100

2000 2002 2004 2006 2008 2010

Year

8-Hour Ozone Design Values

(ppb)

8-Hour Ozone Design Value

8-Hour Ozone Standard

III. Moderate Area Requirements 

On December 3, 2009, U.S. EPA published its finding that Imperial County 

attained the 1997 8-hour ozone standard. U.S. EPA’s “clean data finding” 

suspends certain SIP elements including the attainment demonstration, 

reasonable further progress demonstration, and contingency measures 

requirements. However, some SIP requirements remain, and some were added 

when U.S. EPA reclassified Imperial County from marginal to moderate. Table 2 

shows SIP elements required for marginal and moderate area plans, and identify 

the SIP elements suspended by the clean data finding. 
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Table 2 

Marginal and Moderate Ozone Nonattainment Area SIP Requirements and 

Clean Data Finding Elements 

Ozone Part D SIP 

Requirement 

marginal 

area 

moderate 

area 

requirement suspended with 

clean data finding 

Attainment demonstration 

(182(b)(1)) 

 x x 

Subpart 1 Reasonably 

Available Control Measures 

(172(c)(1)) 

x x x 

Reasonable Further Progress

(172(c)(2), 182(b)(1)) 

x x 

Contingency measures 

(172(c)(9)) 

 x x 

Vehicle 

Inspection/Maintenance 

(182(a)(2)(B), (b)(4)) 

x x Note: N/A in Imperial County 

because of population (<200,000) 

Stage 2 Gasoline Vapor 

Recovery (182(b)(3)) 

Note: Stage 2 GVR no longer required in 

moderate ozone areas per Onboard Refueling 

Vapor Recovery rule. CAA 202(a)(6); 59 FR16262 

(April 6, 1994). 

Emissions Inventory 

(172(c)(3), 182(a)(1) 

x x 

New Source Review 

(172(c)(4) and (5); 

182(a)(2)(C)) 

x x 

Infrastructure SIP (172(c)(7)) x x 

Subpart 2 Reasonably 

Available Control Technology 

for VOCs and NOx (182(b)(2), 

182(f)) 

 x 

Periodic Inventory 

(182(a)(3)(A)) 

x x 

Emissions Statement Rule 

(182(a)(3)(B)) 

x x 

Offset Ratio (182(a)(4); 

182(b)(5)) 

1.1 to 1 1.15 to 1 
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IV. Evaluation of the Imperial County Modified Plan

ARB staff has reviewed the Imperial County 2009 Ozone Plan and the District’s 

supporting technical analysis, and determined that the plan meets CAA SIP 

requirements for moderate ozone nonattainment areas. The following sections 

describe the major elements of the plan. 

 

A. Attainment of the 1997 8-Hour Ozone Standard 

On February 19, 2009, ARB requested that U.S. EPA make a “clean data” finding 

for Imperial County. The request demonstrated that the area’s 2008 design 

value for the 8-hour ozone NAAQS, which was based on monitoring data 

collected from 2006 through 2008, show attainment of the 1997 8-hour ozone 

NAAQS. On December 3, 2009, U.S. EPA published a clean data finding for 

Imperial County. Subsequent monitoring data show that the County continues to 

meet the 1997 8-hour ozone standard. Figure 2 illustrates the Imperial County 

ozone nonattainment area and monitoring locations. 

Figure 2 shows the location of routine ozone monitoring stations in Imperial 

County. The number of routine ozone monitoring sites in Imperial County 

exceeds the monitoring density required in federal regulations for a county of 

approximately 180,000 residents (40 CFR Part 58, Appendix D). 
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Figure 2 

Map of Imperial County and Monitoring Stations 

Table 3 shows the Imperial County air monitoring stations design values for 2006 

through 2009, representing 2004 through 2009 monitoring data. The design 

values for all of the monitoring sites are at or below the mandated 0.085 ppm 

threshold. The El Centro 9th Street and Westmorland started with the highest 

design values; both show a trend of improving air quality. The design values for 

the Niland and Calexico-East sites show slight upward trends, but in both cases 

are still well below the 1997 8-hour ozone standard. On the map above, a dot 

represents an air monitoring station. 
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Table 3 

Imperial Monitoring Station County Design Values 

(ppm)

Monitoring Sites 2006 2007 2008 2009 

El Centro-9th 

Street 0.085 0.086 0.082 0.079 

Westmorland-W 

1st Street 0.085 0.087 0.082 0.079 

Niland 0.073 0.074 0.075 0.076 

Calexico-Ethel 0.074 0.079 0.080 0.083 

Calexico East 0.076 0.079* 0.079* 0.075* 

Calexico Grant 0.069 0.072 Closed Closed 

* Design values impacted by the deletion of June 2007 through October 2007 data due to 

monitoring irregularities. 

Pollution transport from Mexico contributed to high readings in some ozone 

exceedance days in Calexico, and possibly in some other Imperial County sites. 

The CAA allows the exclusion of concentrations caused by transport from 

another country in the development of attainment demonstration plans. This plan 

does not utilize international transport exclusion since the clean data finding does 

not rely on the transport argument.

B. Moderate Nonattainment Area Requirements 

As discussed earlier, U.S. EPA’s clean data finding suspends certain SIP 

requirements. This section reviews the elements that the District submitted to 

fulfill the remaining moderate area nonattainment requirements. 

New Source Review and Offset Ratio 

Federal New Source Review (NSR) regulations are applied to the siting and 

modification of sources that are located in areas designated as federal 

nonattainment for NAAQS. The purpose of federal NSR is to ensure that 

ambient air quality does not deteriorate any further in nonattainment areas. 

Imperial’s SIP-approved NSR rule, Rule 207, was adopted by the Imperial 

County Air Pollution Control Board on November 10, 1980. The CAA requires 

moderate nonattainment areas to have offsets ratios of 1.15 to 1. Rule 207 has a 

1.15 to 1 offset ratio for ozone precursors. This rule and the offset ratio fulfill the 

requirements of the CAA. 
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RACT Analysis 

The District completed its RACT analysis in a separate document (please refer to 

Section V). The Imperial County Air Pollution Control Board adopted the 2009 

RACT SIP at the July 13, 2010, Board meeting. 

Emissions Inventory 

The precursors of ozone are oxides of nitrogen (NOx) and reactive organic gases 

(ROG). Mobile sources contribute the bulk of Imperial County’s NOx emissions 

with heavy duty trucks clearly leading locomotives, and passenger vehicles as 

the top source (Table 4). Livestock waste, passenger vehicles, and pesticides 

constitute the top sources for ROG emissions (Table 5). ROG and NOx 

emissions decreased significantly in these categories between 2002 and 2009, 

as did total emissions, as shown in Tables 4 and 5. 

The Emission Statement Rule requires the owner or operator of each stationary 

source that emits NOx or ROG to provide a statement for classes or categories 

of sources showing the actual emissions. The Imperial County Air Pollution 

Control Board adopted an Emissions Statement and Certification Rule on 

February 23, 2010. 

Air Quality Planning 

In developing this plan, the District held regularly scheduled SIP working group 

meetings that included various community representatives, health, 

environmental, and industry and military representatives, local and county 

governments, and state and federal agencies. The meetings were initiated in 

March 2008 and ended in June 2009. The District also posted drafts of the 2009 

Ozone Plan on their website throughout the process. 
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Table 4 

Imperial County NOx Sources 

Imperial County - Top Sources of NOx (tons per day)

Rank (2002) Source Category 2002 2009

1 HEAVY DUTY DIESEL TRUCKS 13 11 

2 LOCOMOTIVES 7 5 

3 PASSENGER VEHICLES 6 5 

4 FARM EQUIPMENT (COMBINES AND TRACTORS) 3 2 

5 OFF-ROAD EQUIPMENT 2 1 

6 ELECTRIC UTILITIES 1 1 

7 MANUFACTURING AND INDUSTRIAL (BOILERS, IC ENGINES) 1 1 

8 MANAGED BURNING AND DISPOSAL 1 1 

9 AIRCRAFT 1 2 

10 AGRICULTURAL IRRIGATION PUMPS 1 1 

TOP 10 CATEGORIES TOTAL 36 31 

TOTAL EMISSIONS 37 32 

TOP 10 CATEGORIES PERCENT OF TOTAL 96% 95% 

Numbers may not add due to rounding 

“Top 10” categories are prioritized by 2002 emissions

Table 5 

Imperial County ROG Sources 

Imperial County - Top Sources of ROG (tons per day)

Rank (2002) Source Category 2002 2009 

1 LIVESTOCK WASTE 9 6 

2 PASSENGER VEHICLES 7 5 

3 PESTICIDES 5 3 

4 OFF-ROAD RECREATIONAL VEHICLES 3 4 

5 MANAGED BURNING AND DISPOSAL 2 2 

6 ASPHALT PAVING / ROOFING 2 2 

7 CONSUMER PRODUCTS 1 1 

8 AIRCRAFT 1 2 

9 HEAVY DUTY DIESEL TRUCKS 1 1 

10 RECREATIONAL BOATS 1 1 

TOP 10 CATEGORIES TOTAL 33 27 

TOTAL EMISSIONS 37 32 

TOP 10 CATEGORIES PERCENT OF TOTAL 87% 86% 

Numbers may not add due to rounding 

“Top 10” categories are prioritized by 2002 emissions

The District released the draft 2009 Ozone Plan and the 2009 RACT SIP on June 

13, 2010 following several months of coordinated efforts between District staff 

and the SIP working group. The District released a notice of public hearing for 

the adoption of the 2009 Ozone Plan and associated negative declaration 30 

days prior to its Board meeting, and held two public workshops to discuss both 

the 2009 Ozone Plan and 2009 RACT SIP. 
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Air Pollution Transport 

Since Imperial County in effect shares an air basin with the Baja California city of 

Mexicali, ozone levels in southern part of the county are likely impacted by 

emissions from Mexico. The CAA allows states to demonstrate that an area 

would have attained the NAAQS by the applicable attainment date but for 

emissions emanating from outside of the United States (CAA section 179B(b)). 

Imperial County’s 2009 Ozone Plan does not utilize the international transport 

provision since it was not needed to demonstrate attainment of the 1997 8-hour 

ozone NAAQS. However, ARB has previously assessed the impacts of 

transported pollutants on violations of the State Ambient Air Quality Standard 

(SAAQS) for ozone, and determined that transport from Mexico can cause or 

contribute to high ozone levels in Imperial County. ARB, the local District, and 

U.S. EPA continue to coordinate on efforts to improve air quality and reduce 

transported pollutants throughout the border region. 

V. 2009 RACT SIP 

Ozone non-attainment areas classified as moderate or above are required by 

CAA sections 182(b)(2) and 182(f) to implement RACT for sources subject to 

Control Techniques Guideline (CTG) documents and for all major sources of 

VOC and NOx that are not subject to a CTG. The 2009 RACT SIP fulfills CAA 

requirements for a moderate area, and ARB staff recommends that it be 

submitted as a SIP revision simultaneously with the 2009 Ozone Plan. The 

District worked closely with U.S. EPA and stakeholders as it prepared its RACT 

SIP. The District also released a public notice 30 days prior to its Board hearing, 

and held two workshops to discuss the draft 2009 RACT SIP. On July 13, 2010, 

the Imperial County Air Pollution Control Board adopted the 2009 RACT SIP. 

VI. Staff Recommendations 

ARB staff has reviewed the Imperial County 2009 Ozone Plan for the Imperial 

County ozone nonattainment area, including the 2009 RACT SIP, and consulted 

with District staff during its review. ARB staff finds that the Imperial County 2009 

Ozone Plan and 2009 RACT SIP meet all applicable CAA requirements for a 

moderate nonattainment area. Staff recommends that the Board adopt the 

Imperial County 2009 1997 8-Hour Ozone Modified Air Quality Management Plan 

and 2009 Reasonably Available Control Technology State Implementation Plan, 

and direct the Executive Officer to submit them to U.S. EPA as proposed 

revisions to California’s SIP.