Enclosure 2-1 

Enclosure 2 

State of California 

Information to Support Recommendations for 

Federal PM2.5 Nonattainment Area Boundaries 

This document discusses our rationale for the recommended boundaries for each of the 

four preliminary fine particulate matter (PM2.5) nonattainment areas in California. The 

U.S. Environmental Protection Agency’s (U.S. EPA) guidance memorandum 

(April 1, 2003, Designations for the Fine Particle National Ambient Air Quality 

Standards, Memorandum from Jeffrey R. Holmstead, Assistant Administrator, Office of 

Air and Radiation to Regional Administrators, Regions I-X) sets out criteria for 

determining appropriate nonattainment area boundaries. In short, the memorandum 

indicates that nonattainment areas must be large enough to capture the entire area 

violating the standard as well as sources contributing to violations. 

The guidance goes on to describe the association of higher PM2.5 levels with greater 

levels of urban activity and states that the presumptive boundaries for urbanized areas 

violating the PM2.5 standards is the Metropolitan Statistical Area (or Consolidated 

Metropolitan Statistical Area where appropriate.) U.S. EPA’s guidance also suggests 

consideration of eight-hour nonattainment boundaries to support coordination of control 

strategies for ozone and PM2.5. For U.S. EPA to consider an alternative boundary, the 

memorandum calls on states to evaluate the following criteria to support its 

recommendations for the geographic extent of a nonattainment area. 

• Emissions • Air quality 

• Population density • Traffic and commuting patterns 

• Expected growth • Meteorology 

• Geography/topography • Jurisdictional boundaries 

• Level of emission control 

In California, the primary considerations for air quality planning are air basin and air 

district boundaries if the pollution problem is regional in nature. Under State law, air 

basins are based on a rigorous scientific assessment of geography and meteorology, 

with consideration of political jurisdictions. Basin boundaries are formally adopted by 

ARB in regulation. Air districts were established by State statute. ARB typically uses a 

combination of air basin and air district lines to set boundaries for areas that violate 

California air quality standards, with exceptions when a single city or community has a 

unique air pollution problem distinct from the region. 

We evaluated the specific criteria in U.S. EPA guidance, as well as State conventions, 

to identify the most appropriate and effective boundary for each PM2.5 nonattainment 

area. We recommend the air basin boundaries for South Coast, San Joaquin Valley, 

and San Diego to reflect the regional nature of PM2.5 pollution in these areas. We 
 Enclosure 2-2 

 

recommend a focused nonttainment area for the City of Calexico to reflect the localized 

nature of the PM2.5 problem there. 

South Coast Air Basin 

The federal PM2.5 standards are exceeded broadly across the basin. Consideration of 

U.S. EPA’s factors, especially population and emission densities within the basin, which 

indicate broad regional contribution to elevated PM2.5 levels, support use of the air 

basin boundary. 

The recommended South Coast Air Basin PM2.5 nonattainment area includes Western 

Los Angeles (including Catalina and San Clemente Islands), Orange, Southwestern 

San Bernardino, and Western Riverside Counties. This area is under the jurisdiction of 

the South Coast Air Quality Management District. The recommended boundary 

corresponds to the recently modified federal one-hour ozone nonattainment boundary 

and the proposed eight-hour ozone boundary. 

San Joaquin Valley Air Basin

The federal PM2.5 standards are exceeded broadly across the basin. Consideration of 

U.S. EPA’s factors, especially population and emission densities within the basin, which 

indicate broad regional contribution to elevated PM2.5 levels, support use of the air 

basin boundary. 

The recommended San Joaquin Valley PM2.5 nonattainment area consists of 

San Joaquin, Stanislaus, Merced, Madera, Fresno, Kings, Tulare and Western Kern 

Counties. The area is under the jurisdiction of the San Joaquin Valley Unified Air 

Pollution Control District. The recommended boundary for this area coincides with the 

existing federal one-hour ozone nonattainment boundary and ARB’s proposed eighthour ozone boundary. 

San Diego County

The federal annual PM2.5 standard is exceeded broadly across the basin. 

Consideration of U.S. EPA’s factors, especially population and emission densities within 

the basin, which indicate broad regional contribution to elevated PM2.5 levels, support 

use of the County boundary, which is the same as the air basin boundary. 

The proposed PM2.5 nonattainment area would include all of San Diego County, under 

the jurisdiction of the San Diego Air Pollution Control District. This corresponds to the 

existing federal one-hour ozone maintenance area boundary and the proposed eighthour ozone boundary. 
 Enclosure 2-3 

 

City of Calexico

The City of Calexico is on the U.S-Mexico border, at the southern end of Imperial 

County. The County is under the jurisdiction of the Imperial County Air Pollution 

Control District and is part of the Salton Sea Air Basin (which also includes a portion of 

Riverside County). Imperial County is generally rural with relatively small incorporated 

regions -- only El Centro is included in a Metropolitan Statistical Area. 

Imperial County is a federal PM10 nonattainment area, with peak PM10 levels occurring 

in Calexico and occasional PM10 exceedances elsewhere in the County (exacerbated 

by transport from Mexico). Within Imperial County, Calexico is just over the annual 

PM2.5 standard for 2000-2002, while monitors in the Brawley and El Centro record 

levels significantly below this standard. None of the Imperial County monitors have 

recorded violations of the 24-hour PM2.5 standards. Data from the portion of Riverside 

County in the basin show attainment of both PM2.5 standards. 

Based on the available information, we believe that violations of the PM2.5 standard are 

localized in Calexico and the much larger adjacent city of Mexicali, Mexico. As a result, 

the federal PM2.5 nonattainment area should be limited to the part of this urban area on 

the U.S. side of the border – the City of Calexico. 

We discuss this recommendation in the context of U.S. EPA’s boundary criteria below, 

specifically the basis for recommending a nonattainment area smaller than the full 

Imperial County. The Calexico/Mexicali urban area is easily distinguished from the rest 

of Imperial County. Since current emission statistics for Mexicali are not readily 

available, we relied on a 1999 report Program to Improve Air Quality in Mexicali 

2000-2005 that was developed by the Mexican government and posted on U.S. EPA’s 

website. We then compared data for the appropriate corresponding year in Calexico 

and Imperial County. 

Population Density From an air quality perspective, Calexico and Mexicali, Mexico 

form one urbanized region divided by an international border. According to 2000 

U.S. Census data, Calexico’s population is approximately 27,000. The official 2000 

Mexican Census placed Mexicali’s population at 760,000, with 3 percent annual growth 

expected. In 2000, the entire Imperial County population was 142,000. Considering the 

geographic size of the two areas as well, the Mexicali population density is two and a 

half times the density for all of Imperial County. 

Air Quality We calculate that the high PM10 site in Mexicali exceeded the level of the 

U.S. federal PM10 standard on 258 days in 2001. During the same year in Imperial 

County, there were 18 calculated exceedances at the Calexico high monitor, zero days 

at Brawley, and 6-7 days at El Centro, Niland, and Westmorland. 

Chemical mass balance speciated data indicate Calexico PM2.5 is dominated by 

organic carbon particles. They are a significant contributor to elevated PM2.5 levels 

throughout the year, peaking during the winter months. On an annual basis, these 
 Enclosure 2-4 

 

carbon particles make up approximately 60 percent of total PM2.5 mass. Organic 

carbon sources in urban areas include burning, cooking, and motor vehicle exhaust. 

Geologic material (dust) is a smaller portion of total PM2.5 in Calexico, making up less 

than 20 percent of total PM2.5 mass on an annual average basis. 

Emissions Calexico (and Mexicali) are distinct from the rest of Imperial County based 

on the distribution and nature of emission sources. Imperial County is largely rural with 

widespread agricultural activity and the associated emissions profile. Urban activities 

and their associated emissions are found only in the few developed areas: Calexico 

(and Mexicali), El Centro, City of Imperial, and Brawley. The Mexicali inventory for 

oxides of nitrogen (NOx) is about 30 percent greater than all of Imperial County’s. The 

Mexicali hydrocarbon (HC) inventory is about 60 percent greater than all of Imperial 

County’s. Finally, the urban Mexicali direct PM10 inventory is nearly the same as 

Imperial County’s. 

Traffic and Commuting Patterns Calexico/Mexicali is home to a busy U.S.-Mexico 

border crossing. In 1996, the border crossing handled almost 7 million vehicles. 

Mexicali has over three times as many motor vehicles as all of Imperial County. 

Level of Control Imperial County has motor vehicle emission controls that are 

consistent with the rest of California. Vehicles must meet California standards; both 

cars and heavy trucks are subject to in-use inspection programs. The Imperial County 

District administers a smoke management program for open burning, consistent with the 

ARB’s statewide regulation. Vehicles in Mexicali are typically older California vehicles 

and there is no in-use inspection program. Finally, Mexicali open burning is widespread 

and uncontrolled. This is particularly significant given the large organic fraction found in 

Calexico PM2.5. 

The table below summarizes how all of Imperial County compares to Mexicali. 

Imperial County/Mexicali Statistical Comparison 

 Imperial County (entire county) Mexicali (city only) 

2000-Population 142,000 760,000 

Population Density* 20 inhabitants/km2 50 inhabitants/km2

2001-days of PM10 over 

150 ug/m3 at high monitor 

18 days @ Calexico Ethel 258 days at Mexicali Progreso 

1996 NOx Emissions* 35 tons per day 51 tons per day 

1996 HC Emissions* 54 tons per day 140 tons per day 

1996 PM10 Emissions* 220 tons per day 195 tons per day 

Vehicle Population* 71,000 241,000 

*Source: Program to Improve Air Quality in Mexicali 2000-2005

Based on all of these factors, ARB staff has concluded that Calexico exceedances of 

the federal PM2.5 standards are the result of urban activity associated with the densely 

populated international Calexico/Mexicali border region. Within Imperial County, the 

level of urban activity is unique to the area and is not representative of the air quality of 

the rest of Imperial County or the Salton Sea Air Basin.