1

St. Louis-St. Charles-Farmington, Missouri-Illinois 

Area Designation for the 

2008 Ozone National Ambient Air Quality Standards

Table 1, below, identifies the areas in Illinois and Missouri that EPA is designating as “nonattainment”

for the 2008 ozone NAAQS as part of the St. Louis-St. Charles-Farmington, Missouri-Illinois (MO-IL)

multi-state nonattainment area. In accordance with section 107(d) of the Clean Air Act, EPA must

designate an area (county or part of a county) “nonattainment” if it is violating the 2008 ozone NAAQS

or if it is contributing to a violation of the 2008 ozone NAAQS in a nearby area. The technical analyses 

supporting the boundaries for this nonattainment area are provided below.

Table 1. Areas in Missouri and Illinois Included in the St. Louis-St. Charles-Farmington, MO-IL

Nonattainment Area

State

State Recommended 

Nonattainment Counties/Areas

EPA’s Nonattainment 

Counties/Areas

Missouri Franklin County

Jefferson County

St. Charles County

St Louis County

St Louis City

Franklin County

Jefferson County

St. Charles County

St Louis County

St Louis City

Illinois Madison County

Monroe County

St. Clair County

Madison County

Monroe County

St. Clair County

EPA is designating the remaining counties in Missouri that are not listed in the table above as 

“unclassifiable/attainment” for the 2008 ozone NAAQS. EPA is also designating the remaining counties

in Illinois that are not listed in the table above or being considered for inclusion as part of the ChicagoNaperville, IL-IN-WI ozone nonattainment area as “unclassifiable/attainment” for the 2008 ozone 

NAAQS. EPA will issue no later than May 31, 2012 one or more designations for the areas being 

considered as part of the Chicago-Naperville, IL-IN-WI ozone nonattainment area and will prepare a 

separate technical support document for that action. 

The analysis below provides the basis for nonattainment area boundaries for the St. Louis-St. CharlesFarmington, Missouri-Illinois (MO-IL) multi-state nonattainment area. It relies on our analysis of 

whether and which monitors are violating the 2008 ozone NAAQS, based on certified air quality 

monitoring data from 2008-2010 and an evaluation of whether nearby areas are contributing to such 

violations. EPA has evaluated contributions from nearby areas based on a weight of evidence analysis 

considering the factors identified below. EPA issued guidance on December 4, 2008, that identified

these factors as ones EPA would consider in determining nonattainment area boundaries and 

recommended that states consider these factors in making their designations recommendations to EPA.

1

 

1. Air quality data (including the design value calculated for each Federal Reference Method 

(FRM) or Federal Equivalent Method (FEM) monitor in the area);

2. Emissions and emissions-related data (including location of sources and population, amount of 

emissions and emissions controls, and urban growth patterns);

 1 The December 4, 2008 guidance memorandum “Area Designations for the 2008 Revised Ozone National Ambient Air 

Quality Standards” refers to 9 factors. In this technical support document we have grouped the emissions-related factors 

together under the heading of “Emissions and Emissions-Related Data,” which results in 5 categories of factors.
2

3. Meteorology (weather/transport patterns);

4. Geography and topography (mountain ranges or other basin boundaries);

5. Jurisdictional boundaries (e.g., counties, air districts, existing nonattainment areas, Indian 

country, metropolitan planning organizations (MPOs))

Ground-level ozone is generally not emitted directly into the air, but is created by chemical reactions 

between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. 

Because NOx and VOC emissions from a broad range of sources over a wide area typically contribute to 

violations of the ozone standards, EPA believes it is important to consider whether there are contributing 

emissions from a broad geographic area. Accordingly, EPA chose to examine the 5 factors with respect 

to the larger of the Combined Statistical Area (CSA) or Core Based Statistical Area (CBSA) associated 

with the violating monitor(s).

2 All data and information used by EPA in this evaluation are the latest 

available to EPA and/or provided to EPA by states or tribes.

In EPA’s designations guidance for the 2008 ozone NAAQS EPA recommended examining 

CSA/CBSAs because certain factors used to establish CSAs and CBSAs are similar to the factors EPA is 

using in this technical analysis to determine if a nearby area is contributing to a violation of the 2008

ozone NAAQS. Congress required a similar approach in 1990 for areas classified as serious or above 

for the 1-hour ozone standard and EPA used the same basic approach in the designation process for the 

1997 ozone NAAQS. Where a violating monitor is not located in a CSA or CBSA, EPA’s guidance 

recommended using the boundary of the county containing the violating monitor as the starting point for 

considering the nonattainment area’s boundary. 

Technical Analysis for St. Louis-St. Charles-Farmington, MO-IL

Figure 1 is a map of the St. Louis-St. Charles-Farmington, MO-IL nonattainment area. The map 

provides other relevant information including the locations and design values of air quality monitors, 

county and other jurisdictional boundaries, St. Louis-St. Charles-Farmington, MO-IL CSA boundary, 

existing nonattainment boundary for 1997 ozone NAAQS, and major transportation arteries. 

 2 Lists of CBSAs and CSAs and their geographic components are provided at 

www.census.gov/population/www/metroareas/metrodef.html . The lists are periodically updated by the Office of 

Management and Budget. EPA used the most recent update, based on 2008 population estimates, issued on December 1, 

2009 (OMB Bulletin No. 10-02).
3

Figure 1.

For purposes of the 1997 8-hour ozone NAAQS, portions of this area were designated nonattainment. 

The boundary for the nonattainment area for the 1997 ozone NAAQS included the entire counties of 

Jersey, Madison, Monroe and St. Clair Counties in Illinois and St. Louis City and Franklin, Jefferson, St.

Charles and St. Louis Counties in Missouri.

In March 2009, Missouri submitted its initial designation recommendations based on air quality data 

from 2006-2008. In December 2011, Missouri submitted a revised recommendation to include the entire 

counties of Franklin, Jefferson, St. Charles, and St. Louis and the City of St. Louis as “nonattainment” 

for the 2008 ozone NAAQS based on air quality data from 2008-2010. The State recommended 

designating the remaining areas of the State as attainment/unclassifiable.3 In March 2009, Illinois 

recommended that Madison, Monroe and St. Clair Counties be designated as “nonattainment” for the 

 3 Missouri submitted its designation recommendations in a March 11, 2009, letter from Mark N. Templeton, Director of the 

Missouri Department of Natural Resources, which included enclosures containing an analysis of data supporting the State’s 

recommendations. Missouri revised its recommendations and MDNR requested EPA to act on the revisions in a letter dated 

December 5, 2011.
4

2008 ozone NAAQS based on air quality data from 2006-2008.4 The recommendations from both 

Missouri and Illinois rely on data from FRM monitors or FEM monitors sited and operated in 

accordance with 40 CFR Part 58.

After considering these recommendations and based on EPA's technical analysis described below, EPA 

is designating the areas in Illinois and Missouri listed in Table 1 above as “nonattainment” for the 2008

ozone NAAQS as part of the St. Louis-St. Charles-Farmington, MO-IL multi-state nonattainment area. 

Factor Assessment

Factor 1: Air Quality Data 

For this factor, we considered 8-hour ozone design values (in ppm) for air quality monitors in counties 

in the St. Louis-St. Charles-Farmington, MO-IL CSA based on data for the 2008-2010 period (i.e., the 

2010 design value), which are the most recent years with fully-certified air quality data for both states.

A monitor’s design value is the metric or statistic that indicates whether that monitor attains a specified 

air quality standard. The 2008 ozone NAAQS are met at a monitor when the annual fourth-highest daily 

maximum 8-hour average concentration, averaged over 3 years is 0.075 ppm or less. A design value is 

only valid if minimum data completeness criteria are met. See 40 CFR part 50 Appendix P. Where 

several monitors are located in a county (or a designated nonattainment area or maintenance area), the 

design value for the county or area is determined by the monitor with the highest level.

Note: Monitors that are eligible for providing design value data generally include State and Local Air 

Monitoring Stations (SLAMS) that are sited in accordance with 40 CFR Part 58, Appendix D (Section 

4.1) and operating with a federal reference method (FRM) or federal equivalent method (FEM) monitor 

that meets the requirements of 40 CFR part 58, appendix A. All data from a special purpose monitor 

(SPM) using an FRM or FEM which has operated for more than 24 months is eligible for comparison to 

the NAAQS unless the monitoring agency demonstrates that the data came from a particular period 

during which the requirements of appendix A (quality assurance requirements) or appendix E (probe and 

monitoring path siting criteria) were not met.

The 2010 design values for the ozone NAAQS for counties in the St. Louis-St. Charles-Farmington, 

MO-IL CSA are shown in Table 2. 

 4 Illinois submitted its designation recommendations in a March 9, 2009, letter from Douglas P. Scott, Director of the Illinois

Environmental Protection Agency. The state also provided a Technical Support Document dated March 9, 2009, which 

included a nine factor analysis to support the State’s recommendations.
5

Table 2. Air Quality Data for Counties in the St. Louis-St. Charles-Farmington, MO-IL CSA.

County State Recommended 

Nonattainment?

2008-2010 Design Value

(ppb)

Missouri:

 Franklin County Yes

 Jefferson County Yes 72

 Lincoln County No 72

 St. Charles County Yes 77

 St. Francois County No

 St. Louis County Yes 71

 St. Louis City Yes 69

 Warren County No

 Washington County No

Illinois:

 Bond County No

 Calhoun County No

 Clinton County No

 Jersey County No 69

 Macoupin County No 66

 Madison County Yes 72

 Monroe County Yes

 St. Clair County Yes 68

St Charles County in Missouri shows a violation of the 2008 ozone NAAQS, therefore this county is

included in the nonattainment area. A county (or partial county) must also be designated nonattainment 

if it contributes to a violation in a nearby area. Each county without a violating monitor that is located 

near a county with a violating monitor has been evaluated, as discussed below, based on the five factors 

to determine whether it contributes to the nearby violation. 

Factor 2: Emissions and Emissions-Related Data

EPA evaluated emissions of ozone precursors (NOx and VOC) and other emissions-related data that 

provide information on areas contributing to violating monitors.

Emissions Data

EPA evaluated county-level emission data for NOx and VOC derived from the 2008 National Emissions 

Inventory (NEI), version 1.5. This is the most recently available NEI. (See

http://www.epa.gov/ttn/chief/net/2008inventory.html) Significant emissions levels in a nearby area 

indicate the potential for the area to contribute to the observed ozone standard violation. 

Table 3 shows 2008 emissions of NOx and VOC (in tons per year) for all counties in the St. Louis-St. 

Charles-Farmington, MO-IL CSA. This table also indicates which of the counties were recommended to 

be nonattainment for the 2008 ozone NAAQS by their respective states.
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Table 3. Total 2008 NOx and VOC Emissions.

County State Recommended

Nonattainment? NOx (tpy) VOC (tpy)

Missouri:

 Franklin County Yes 14,094 4,939

 Jefferson County Yes 11,769 6,729

 Lincoln County No 1,855 2,081

 St. Charles County Yes 15,894 11,652

 St. Francois County No 2,030 2,349

 St. Louis County Yes 36,455 41,894

 St. Louis City Yes 17,576 14,027

 Warren County No 1,749 2,064

 Washington County No 678 862

Missouri total: 102,100 86,597

Illinois:

 Bond County No 1,422 1,181

 Calhoun County No 607 1,057

 Clinton County No 4,409 2,515

 Jersey County No 1,125 1,166

 Macoupin County No 2,286 2,147

 Madison County Yes 23,109 12,351

 Monroe County Yes 2,410 1,551

 St. Clair County Yes 10,804 8,719

Illinois total: 46,172 30,687

Areawide: 148,272 117,284

The emission data in Table 3 indicate that for Missouri, the highest NOx and VOC emissions reside in 

St. Louis County and the City of St. Louis. In 2008 these two areas accounted for 53% and 65% of NOx 

and VOC emissions respectively for the Missouri portion of the area. In addition, Franklin and Jefferson 

Counties also have comparatively high NOx emissions (behind St. Louis and St. Charles Counties and 

the City of St. Louis) and form a contiguous area with the three top emitting areas. Together, the City of 

St. Louis, Franklin, Jefferson, and St. Louis Counties make up 78% of both NOx and VOC emissions in 

the Missouri portion of the area, and 54% of the NOx emissions and 58% of the VOC emissions for the 

entire St. Louis-St. Charles-Farmington, MO-IL CSA. 

The NOx and VOC emissions are relatively low in Warren, Washington, and Lincoln counties. 

Together in 2008 these counties account for 4% of NOx emissions and 6% of VOC emissions in the 

Missouri portion of the area, and 3% of NOx emissions and 4% of VOC emissions in the entire St. 

Louis-St. Charles-Farmington, MO-IL CSA.

The emissions data in Table 3 show that, for Illinois, comparatively high 2008 NOx and VOC emissions 

originate in Madison and St. Clair Counties. In 2008, these counties account for 73% of the NOx 

emissions and 69% of the VOC emissions for the Illinois portion of the area Taken together, in 2008 the 

three counties that Illinois recommends be included within the nonattainment area - Madison, Monroe 

and St. Claire Counties - account for 79% of the NOx emissions and 74% of the VOC emissions in the 

Illinois portion of the area and 24% of the NOx emissions and 19% of the VOC emissions for the entire 

St. Louis-St. Charles-Farmington, MO-IL CSA. 
7

The VOC and NOx emissions originating in Bond, Calhoun, Clinton, Jersey and Macoupin Counties are 

significantly smaller than those originating in the higher emitting counties elsewhere in the St. Louis-St. 

Charles-Farmington, MO-IL CSA. Taken together, in 2008 these counties account for 21% of the NOx 

emissions and 26% of the VOC emissions for the Illinois portion of the area and only 7% of the NOx

emissions and 7% of the VOC emissions for the entire St. Louis-St. Charles-Farmington, MO-IL CSA. 

Population Density and Degree of Urbanization

EPA evaluated the population and vehicle use characteristics and trends of the area as indicators of the 

probable location and magnitude of non-point source emissions. These include ozone-creating 

emissions from on-road and off-road vehicles and engines, consumer products, residential fuel 

combustion, and consumer services. Areas of dense population or commercial development are an 

indicator of area source and mobile source NOx and VOC emissions. Rapid growth in population or 

Vehicle Miles Traveled (VMT) (see below) in a county on the urban perimeter signifies increasing 

integration with the core urban area, and indicates that it may be appropriate to include the area 

associated with area source and mobile source emissions as part of the nonattainment area. Table 4

shows the population, population density, and population growth information for each county in the 

area.

Table 4. Population and Growth.

County

State 

Recommended 

Nonattainment?

2010 

Population

 2010

 Population

 Density

(1000 pop/sq mi)

 Absolute

 change

in population

(2000-2010)

Population 

% change

(2000-

2010)

Missouri:

 Franklin County Yes 101,492 0.11 7,434 +8%

 Jefferson County Yes 218,733 0.33 19,995 +10%

 Lincoln County No 52,566 0.08 13,310 +34%

 St. Charles County Yes 360,485 0.61 74,322 +26%

 St. Francois County No 65,359 0.14 9,615 +17%

 St. Louis County Yes 998,954 1.91 -17,376 -2%

 St. Louis City Yes 319,294 4.83 -27,570 -8%

 Warren County No 32,513 0.07 7,793 +32%

 Washington County No 25,195 0.03 1,785 +8%

Missouri total: 2,174,591 89,308 +4%

Illinois:

 Bond County No 17,768 0.05 118 +1%

 Calhoun County No 5,089 0.02 -1 0%

 Clinton County No 37,762 0.08 2,233 +6%

 Jersey County No 22,985 0.06 1,330 +6%

 Macoupin County No 47,765 0.06 -1,224 -2%

 Madison County Yes 269,282 0.36 10,165 +4%

 Monroe County Yes 32,957 0.08 5,193 +19%

 St. Clair County Yes 270,056 0.40 13,852 +5%

Illinois total: 703,664 31,666 +5%

Areawide: 2,878,255 120,974 +4%

Sources: U.S. Census Bureau population estimates for 2010 as of August 4, 2011
8

In the Missouri portion of the St. Louis-St. Charles-Farmington, MO-IL CSA, St. Louis and St. Charles 

counties along with the City of St. Louis have comparatively high population and high population 

density. This also correlates to those three counties having the highest amount of NOx and VOC 

emissions in the area. Warren, Washington, and St. Francois Counties have relatively low population, 

population density, and lowest absolute change in population. Lincoln County has relatively low 

population for the area, as well as one of the lowest population densities (behind Warren and 

Washington Counties) among the Missouri Counties in the CSA.

For Illinois, population data show that Madison and St. Clair Counties have comparatively high 

populations and population densities. This implies that the population-related NOx and VOC emissions 

in these counties are comparatively high. Monroe County in Illinois has the highest percentage 

population growth and third highest absolute change in population (behind St. Clair and Madison 

Counties) among the Illinois Counties in the CSA. 

Traffic and Commuting Patterns

EPA evaluated the commuting patterns of residents in the area, as well as the total VMT for each 

county. In combination with the population/population density data and the location of main 

transportation arteries (see Figure 1 above), this information helps identify the probable location of nonpoint source emissions. A county with high VMT and/or a high number of commuters is generally an 

integral part of an urban area and indicates the presence of motor vehicle emissions that may contribute 

to ozone formation. Rapid population or VMT growth in a county on the urban perimeter signifies 

increasing integration with the core urban area, and indicates that it may be appropriate to include this 

county in the ozone nonattainment area, particularly if the VOC and/or NOx emissions in this county are 

a significant portion of the total emissions in the nonattainment area. 

Table 5 shows traffic and commuting pattern data, including total 2008 VMT for each county, number 

of commuters in each county who drive to another county within the area, and the percent of total 

commuters in each county who commute to other counties within the area.

Table 5. Traffic and Commuting Patterns

County

State 

Recommended 

Nonattainment?

2008 VMT*

(million miles)

Number Commuting 

to or within any 

violating counties**

Percent Commuting 

to or within any 

violating counties**

Missouri:

 Franklin County Yes 1,637 776 2%

 Jefferson County Yes 1,885 1,337 1%

 Lincoln County No 495 5,529 30%

 St. Charles County Yes 2,728 70,282 47%

 St. Francois County No 548 92 0%

 St. Louis County Yes 11,925 13,513 3%

 St. Louis City Yes 3,450 1,502 1%

 Warren County No 525 2,967 25%

 Washington County No 220 27 0%

Missouri total: 23,413 96,025 10%

Illinois
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 Bond County No 284 20 0%

 Calhoun County No 40 109 5%

 Clinton County No 389 49 0%

 Jersey County No 192 125 1%

 Macoupin County No 415 36 0%

 Madison County Yes 2,839 1,126 1%

 Monroe County Yes 361 84 1%

 St. Clair County Yes 2,666 729 1%

Illinois total: 7,186 2,278 1%

Areawide: 30,599 98,303 8%

* MOBILE model VMTs are those inputs into the NEI version 1.5. 

** U.S. Census Bureau estimates for 2000 County-to-County Worker Flow 

http://www.census.gov/hhes/commuting/data/commuting.html. 

For Missouri, the VMT data show that VMT levels in St. Louis City, St. Louis County, and St. Charles 

County are significantly higher than those in other Missouri counties in the St. Louis-St. CharlesFarmington, MO-IL CSA. Cumulatively, the VMT in these counties are a significant portion of the total 

VMT for the St. Louis-St. Charles-Farmington, MO-IL CSA.

For Illinois, the VMT data show that VMT levels in Madison and St. Clair Counties are significantly 

higher than those in the other Illinois counties in the St. Louis-St. Charles-Farmington, MO-IL CSA. 

The VMT from these two counties along with the VMT for the Monroe County, which the state also 

recommended as nonattainment, accounts for 82% of the VMT in the Illinois portion of the St. Louis-St. 

Charles-Farmington, MO-IL CSA and 19 percent of the VMT in the entire CSA.

Factor 3: Meteorology (Weather/Transport Patterns)

EPA evaluated available meteorological data to help determine how meteorological conditions, such as 

weather, transport patterns and stagnation conditions, would affect the fate and transport of precursor 

emissions contributing to ozone formation. EPA examined the frequency distribution of wind directions 

during the summer by averaging National Weather Service direction-sorted wind directions for each 

county for a 30 year period. To apply the results of this data analysis to the St. Louis-St. CharlesFarmington, MO-IL CSA, we have considered the wind direction frequencies during the summer 

months (June-August) for St. Charles County in Missouri, the only county with a recorded violation of 

the 2008 ozone NAAQS (see Table 2). Table 6 shows the summertime 30-year averaged percentages of 

wind directions (winds blowing indo the subject county from the specified wind direction sector) for St. 

Charles County in Missouri.

Table 6. Averaged Summertime Wind Direction Percentages for St. Charles County, MO

Wind Direction Percentage

North-Northeast 9.82%

East-Northeast 7.24%

East-Southeast 13.66%

South-Southeast 14.41%

South-Southwest 20.01%

West-Southwest 14.64%

West-Northwest 12.84%

North-Northwest 7.39%
10

The wind direction percentages show that there is no “preferred” wind direction during the summertime. 

Transport winds can and do blow from all directions into the county with a recorded violation of the 

2008 ozone NAAQS. There is, however an indication that winds from the south-southwest may be 

slightly more prevalent than winds from other directions during the summertime, and there is a southerly 

component 62.72% of the time. 

Illinois provided wind direction data for days from 2006 through 2008 when ozone concentrations were

greater than 75 ppb at any monitor in the Illinois portion of the St. Louis-St. Charles-Farmington, MOIL CSA. Wind data was taken from Edwardsville in Madison County, IL. This data is presented in 

Figure 2.
11

Figure 2. Primary Wind Direction Percentages at Edwardsville in Madison County, IL for Days from 

2006 Through 2008 When Ozone Concentrations in the Illinois Portion of the St. Louis-St. CharlesFarmington, MO-IL CSA Were Greater Than 75ppb 

Data in Figure 2 indicate that when considering wind direction at Edwardsville in Madison County, IL 

for days from 2006 through 2008 when ozone concentrations in the Illinois portion of the St. Louis-St. 

Charles-Farmington, MO-IL CSA were greater than 75ppb, prevailing winds were from the southsouthwest through the southeast approximately 40% of the time. Edwardsville is slightly to the east and 

south of the violating monitor in St. Charles County, Missouri. All of the counties proposed as 

nonattainment lie to the south of or directly east or west of the nonattainment monitor in St. Charles 

County. As shown in Figure 1, Franklin, St. Louis and Jefferson Counties and St. Louis City in 

Missouri and Monroe and St. Clair Counties in Illinois all range from southwest of the violating monitor 

in St. Charles County, to east of the violating monitor. 
12

Factor 4: Geography/Topography (Mountain Ranges or Other Air Basin Boundaries)

The geography/topography analysis evaluates the physical features of the land that might affect the 

airshed and, therefore, the distribution of ozone over the area. The St. Louis-St. Charles-Farmington, 

MO-IL CSA does not have any geographical or topographical barriers significantly limiting air pollution 

transport within its air shed. Therefore, this factor did not play a significant role in this evaluation.

Factor 5: Jurisdictional Boundaries

Once the general areas to be included in the nonattainment area were determined, EPA considered

existing jurisdictional boundaries for the purposes of providing a clearly defined legal boundary and 

carrying out the air quality planning and enforcement functions for nonattainment areas. Examples of 

jurisdictional boundaries include existing/prior nonattainment area boundaries for ozone or other urbanscale pollutants, county lines, air district boundaries, township boundaries, areas covered by a 

metropolitan planning organization, state lines, and Reservation boundaries. 

The St. Louis, MO-IL area has previously established nonattainment boundaries associated with the 1-

hour and 1997 8-hour ozone NAAQS. The portion of the St. Louis-St. Charles-Farmington, MO-IL 

CSA that we are designating as nonattainment for the 2008 ozone NAAQS is the same area that we 

designated as nonattainment under the 1-hour NAAQS. Under the 1997 8-hour ozone NAAQS, Jersey 

County, IL was also included in the nonattainment area due to a monitored violation of the standard in 

that county. The state of Illinois has recommended designating Jersey County, IL as attainment for the 

2008 ozone NAAQS. The monitor in Jersey County, IL is currently monitoring attainment and, we do 

not believe Jersey County is contributing to nonattainment at the violating monitors in the area. 

Conclusion

Based on the assessment of factors described above, EPA has concluded that the following 

counties/areas meet the CAA criteria for inclusion in the St. Louis-St. Charles-Farmington, MO-IL

nonattainment area: Franklin County, Jefferson County, St. Charles County, St. Louis City, and St. 

Louis County in Missouri and Madison County, Monroe County, and St. Clair County in Illinois. These 

are the same counties that were included in the St. Louis, MO-IL nonattainment area for the 1-hour 

ozone NAAQS. Under the 1997 ozone NAAQS, Jersey County, IL was also included in the

nonattainment area due to a monitored violation of the NAAQS in that county. The monitor in Jersey 

County, IL is currently well below the standard and the other factors do not support inclusion of the 

county within the St. Louis-St. Charles-Farmington, MO-IL nonattainment area. Jersey county has low 

emissions, low population, low VMT and is located north of, and therefore generally downwind of, the 

violating monitor in St. Charles County. One air quality monitor, located in St. Charles County,

Missouri, indicates a violation of the 2008 ozone NAAQS based on 2010 design values, therefore this 

county is included in the nonattainment area. There are no air quality monitors in Illinois counties

indicating violations of the 2008 ozone NAAQS based on the 2010 design values. Franklin County, 

Jefferson County, St. Louis City, and St. Louis County in Missouri and Madison County, Monroe 

County, and St. Clair County in Illinois are nearby counties that do not have violating monitors based on 

2010 design values, but EPA has concluded that these areas contribute to the ozone concentrations in 

violation of the 2008 ozone NAAQS in St. Charles County. Based on 2008 data, Franklin County, 

Jefferson County, St. Charles County, St. Louis City, and St. Louis County in Missouri and Madison 

County and St. Clair County in Illinois together represent the counties with the seven highest emissions 

of NOx and VOC. 
13

While emissions are low in Monroe County, we note that the State has recommended inclusion of the 

county as part of the nonattainment area. Monroe County is an integral part of the core area of the St. 

Louis-St. Charles-Farmington, MO-IL area and borders Jefferson County, St. Louis County and the City 

of St. Louis in Missouri and St. Clair County in Illinois, all of which are included in the nonattainment 

area and all of which have high ozone precursor emissions and high VMT. These high emissions areas 

surround Monroe County on three sides. Furthermore, prevailing wind direction on days where ozone 

levels exceeded 75 parts per billion (ppb) were from the southeast through south-southwest. Given that 

the violating ozone monitor in St. Charles County, Missouri is located directly north of Monroe County, 

Illinois and that the monitor is within 30 miles of Monroe County, we can conclude that any emissions 

originating in Monroe County are likely to contribute to elevated ozone levels at the St. Charles ozone 

monitor. Taken together Franklin County, Jefferson County, St. Charles County, St. Louis City, and St. 

Louis County in Missouri and Madison County, Monroe County, and St. Clair County in Illinois 

represent 89 percent of the total NOx emissions and 87 percent of the total VOC emissions in the St. 

Louis-St. Charles-Farmington, MO-IL CSA. Comparatively low emissions, population, and VMT in the 

remaining counties in the St. Louis-St. Charles-Farmington, MO-IL CSA and the fact that these counties 

are largely downwind of the violating monitor on days when it has exceeded the standard support the 

exclusion of these counties from the ozone nonattainment area for the 2008 8-hour ozone NAAQS.