1

Mississippi

Area Designations for the 

2008 Ozone National Ambient Air Quality Standards

The table below identifies the areas and associated counties or parts of counties in Mississippi that EPA 

intends to designate as nonattainment for the 2008 ozone national ambient air quality standards (2008 

ozone NAAQS). 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 the individual nonattainment areas are provided below.

Intended Nonattainment Areas in Mississippi

Area 

Mississippi’s Recommended 

Nonattainment Counties

EPA’s Intended Nonattainment 

Counties

Memphis, TN-MS-AR* None DeSoto(partial)

*Memphis, TN-MS-AR is a multi-state nonattainment area. Table 1 below identifies the counties in the 

other states that EPA intends to designate as part of the nonattainment area.

EPA intends to designate the remaining counties in Mississippi that are not listed in the table above as 

“unclassifiable/attainment” for the 2008 ozone NAAQS. 

The analysis below provides the basis for intended nonattainment area boundaries. 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 and other relevant information. 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 

monitors 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);

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 generally is 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 

 

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

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. 

 

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

Technical Analysis for Memphis, TN-MS-AR

Figure 1 is a map of the Memphis, TN-MS-AR intended nonattainment area. The map provides other 

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

jurisdictional boundaries, relevant statistical area boundaries, the nonattainment area boundary for 1997 

ozone NAAQS, and major transportation arteries.

Figure 1. TN-MS-AR Nonattainment Area

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 

Crittenden County, Arkansas, and Shelby County, Tennessee.

In March 2009, Mississippi recommended that DeSoto County, Mississippi be designated as a 

nonattainment area separate from the Memphis nonattainment area for the 2008 ozone NAAQS based 

on air quality data from 2006-2008. Mississippi provided an update to the original recommendation in 

October 2011 based on air quality data from 2008-2010, and preliminary data from 2009-2011. In its 

updated recommendation, Mississippi recommended that all counties in the State be designated 
4

attainment for the 2008 ozone NAAQS. Letter from Haley Barbour, Governor of the State of 

Mississippi to A. Stanley Meiburg, ActingRegional Administrator, US EPA Region 4 (March 3, 2009) 

and Gwendolyn Keyes Fleming, Regional Administrator US EPA Region 4 (October 27, 2011) (on file 

with US EPA Region 4). Also, in March 2009, Tennessee recommended that Shelby County be 

designated “nonattainment” for the 2008 8-hour ozone standard based on air quality data from 2006-

2008. Letter from James H. Fyke, Commissioner, State of Tennessee Department of Environment and 

Conservation to A. Stanley Meiburg, Acting Regional Administrator, US EPA Region 4 (March 10, 

2009) (on file with US EPA Region 4). Tennessee provided an update to its original recommendation in 

November 2011 based on preliminary 2009-2011 air quality data. In Tennessee’s updated 

recommendation, the state did not provide a specific update to its 2009 recommendation for the 

Memphis TN-MS-AR but stated that all other counties (with the exception of those recommended for 

Knoxville) should be designated unclassifiable/attainment. Letter from Robert J. Martineau Jr, 

Commissioner, State of Tennessee Department of Environment and Conservation to Gwendolyn Keyes 

Fleming, Regional Administrator, US EPA Region 4 (November 8, 2011) (on file with US EPA Region 

4).

Additionally, in March 2009, Arkansas recommended that Crittenden County, Arkansas be designated 

nonattainment based on 2006-2008 air quality data. Arkansas did not update its 2009 ozone 

recommendation. These data are from FEM monitors sited and operated in accordance with 40 CFR 

Part 58. Letter from Mike Beebe, Governor of the State of Arkansas to Lawrence E. Starfield, Acting

Regional Administrator, US EPA Region 6 (March 10, 2009) (on file with US EPA Region 6).

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

intends to designate one county in Arkansas, one county (partial) in Mississippi, and one county in 

Tennessee (identified in Table 1 below) as nonattainment for the 2008 ozone NAAQS as part of the 

Memphis, TN-MS-AR multi-state nonattainment area. 

Table 1. State's Recommended and EPA’s Intended Designated Nonattainment Counties for Memphis, 

TN-MS-AR.

Memphis, TN-MS-AR State-Recommended 

Nonattainment Counties

EPA Intended

Nonattainment Counties

Arkansas Crittenden Crittenden

Mississippi None DeSoto (partial)

Tennessee None Shelby

Factor Assessment

Factor 1: Air Quality Data 

For this factor, we considered 8-hour ozone design values (in parts per billion (ppb)) for air quality 

monitors in counties in the Memphis, TN-MS-AR area based on data for the 2008-2010 period (i.e., the 

2010 design value, or DV), which are the most recent years with fully-certified air quality data. A 

monitor’s DV 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 75 ppb or less. A DV 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 DV for the county or 

area is determined by the monitor with the highest level.
5

The 2010 DVs for the ozone NAAQS for counties in the Memphis and nearby surrounding area are 

shown in Table 2. 

Table 2. Air Quality Data3.

County State Recommended 

Nonattainment?

2008-2010 Design Value 

(ppb)

Crittenden, AR Yes 74

DeSoto, MS No 73

Shelby, TN No 76

Shelby County, Tennessee 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 

and other relevant information 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 observed violations. We will also consider any 

additional information we receive on changes to emissions levels that are not reflected in recent 

inventories. These changes include emissions reductions due to permanent and enforceable emissions

controls that will be in place before final designations are issued and emissions increases due to new 

sources. The precursor emission source-category percentages used below and throughout the document 

were derived from emissions data from the 2008 NEI version 1.5 referenced above.

Table 3 shows emissions of NOx and VOC (given in tons per year (tpy)) for violating and nearby

counties that we considered for inclusion in the Memphis, TN-MS-AR area. 

Table 3. Total 2008 NOx and VOC Emissions.

County

State Recommended 

Nonattainment NOx (tpy) VOC (tpy)

Crittenden, AR Yes 4,047 3,805

DeSoto, MS No 5,080 5,222

Fayette, TN No 2,385 1,406

Marshall, MS No 1,769 1,527

 

3 Only counties in the Memphis CBSA that have ozone monitors are included in this table.
6

Shelby, TN No 39,519 27,929

Tate, MS No 3,102 1,392

Tipton, TN No 2,119 2,251

Tunica, MS No 1,598 1,096

Areawide: 59,619 44,628

*Counties that EPA intends to designate as nonattainment are shown in bold.

DeSoto County contributes about 9 percent NOx and 12 percent VOC precursor emissions in the CBSA. 

The County’s 5,080 NOx emissions are mostly comprised of 45 percent area sources, 35 percent mobile 

sources. DeSoto County’s total VOC emissions include 44 percent area sources and 34 percent mobile 

sources.

Shelby County contributes about 66 percent of the NOx and 63 percent of the VOC precursor emissions 

in the CBSA. Shelby makes up 23 percent of the entire CBSA NOx emissions and 22 percent of the 

area’s VOC emissions. Of the county’s 39,519 NOx emissions, 35 percent are from point and mobile 

emissions and 20 percent from area source emissions. The County’s 27, 929 VOC emissions include 36 

percent mobile sources and 32 percent area sources.

Crittenden County contributes less than 10 percent of the precursor CBSA emissions. Of the County’s 

total NOx emissions listed in Table 1, 45 percent are from mobile sources and 34 percent from area 

sources. The County’s total VOC emissions include 35 percent from area sources and 31 percent from 

mobile sources. Only 5 percent of the County’s NOx emissions are from point sources. Both Crittenden 

and DeSoto Counties represent less than 1 percent of the entire area’s NOx and VOC point source 

emissions

Fayette and Tipton Counties in Tennessee and Marshall, Tate, and Tunica counties in Mississippi all 

contribute 5 percent or less NOx and VOC precursor emissions in the CBSA. 

Together, Crittenden, DeSoto and Shelby Counties account for 82 percent of the NOx emissions and 83 

percent of the VOC emissions for the 8-county area. The emissions from Fayette and Tipton Counties in 

Tennessee and Marshall, Tate and Tunica Counties in Mississippi are not thought to contribute to the 

violations of the 2008 ozone NAAQS that have been observed by monitors in Shelby County, Tennessee 

and Crittenden County, Arkansas.

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 that may contribute to ozone 

formation. Rapid population or VMT growth (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 the 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.
7

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)

Crittenden, 

AR Yes 50,902 0.08 (75) <1%

DeSoto, MS No 161,252 0.32 52,584 +48%

Fayette, TN No 38,413 0.05 9,313 +32%

Marshall, MS No 37,144 0.05 2,093 +6%

Shelby, TN No 927,644 1.18 29,393 +3%

Tate, MS No 28,886 0.07 3,444 +14%

Tipton, TN No 61,081 0.13 9,545 +19%

Tunica, MS No 10,778 0.02 1,557 +17%

Areawide: 1,316,100 0.28 107,854 +9%

*Counties that EPA intends to designate as nonattainment are shown in bold.

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

(http://factfinder2.census.gov/faces/tableservices/jsf/pages/productview.xhtml?pid=DEC_10_PL_GCTP

L2.STO5&prodType=table)

DeSoto County, Mississippi is moderately populated in the northern portion of the county and mostly 

rural in the remaining portion of the County. DeSoto County contains 12 percent of the CBSA 

population, but experienced 48 percent growth from 2000-2010. Tate, Tunica and Marshall Counties in 

Mississippi all make up 3 percent or less of the CBSA population and are sparsely populated. 

Shelby County, Tennessee is densely populated containing 70 percent of the CBSA population. From 

2000-2010, the County only had 3 percent growth in population. Fayette and Tipton County in 

Tennessee had moderate growth from 2000-2010 but are sparsely populated. 

Crittenden County, Arkansas had less than 1 percent population growth from 2000-2010 and contains 

only 4 percent of the CBSA population. The County is mostly rural with little urbanization. 

The attachment to this document contains Figure 2, Memphis Area Ozone and Ozone Precursor 

Monitoring Network, and Figure 3, Population Density Change Percentage Between 2000 and 2010 

Census for Memphis Ozone and Ozone Precursor Monitoring Network, which present graphical 

information on population density and growth for the Memphis area.
8

Traffic VMT Data and Commuting Patterns

EPA evaluated the total VMT for each county in the Memphis CBSA. In combination with the 

population/population density data and the location of main transportation arteries (see above), this 

information helps identify the probable location of non-point source emissions. A county with high 

VMT is generally an integral part of an urban area and indicates the presence of motor vehicle emissions 

that may contribute to ozone formation that contributes to nonattainment in the area. Rapid population or 

VMT growth in a county on the urban perimeter signifies increasing integration with the core urban 

area, and indicates that the associated area source and mobile source emissions may be appropriate to 

include in the nonattainment area. Table 5 shows total 2008 VMT for each county.

Table 5. Traffic and VMT Data

County

State Recommended 

Nonattainment? 2008 VMT (million miles)

Crittenden, AR Yes 903

DeSoto, MS No 1,629

Fayette, TN No 573

Marshall, MS No 725

Shelby, TN No 8,789

Tate, MS No 376

Tipton, TN No 401

Tunica, MS No 337

Areawide: 13,733

*Counties that EPA intends to designate as nonattainment are shown in bold.

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

DeSoto County has the second highest VMT in the Memphis CBSA (12% of the total Memphis CBSA). 

Additionally, DeSoto County has a 48 percent growth in population from 2000-2010 with approximately 

35 and 34 percent of the County’s NOx and VOC emissions (respectively) deriving from mobile 

sources.

Shelby County is the only county in the Memphis CBSA violating the 2008 ozone NAAQS with 2008-

2010 air quality data and is considered the core CBSA county, with 64 percent of the VMT in the 

Memphis CBSA; Approximately 35 percent of Shelby County’s NOx emissions and 34 percent VOC 

emissions are from mobile sources. 

Crittenden County, has less than 10 percent of the CBSA VMT (third highest in the Memphis CBSA). 

From 2000-2010, Crittenden County had less than 1 percent population growth with 45 percent and 31 

percent of the County’s NOx and VOC emissions(respectively) deriving from mobile sources.

The remaining counties in the Memphis CBSA all have low total population and population growth with 

little urbanization and low precursor emission contribution suggesting negligible contribution of 

population-based emissions.
9

Factor 3: Meteorology (weather/transport patterns)

For this factor, EPA analyzed 30-years of National Weather Service (NWS) wind speed and wind 

direction data collected at the Memphis International Airport (NWS Station 13893) to help determine 

transport patterns and source contributions. EPA assessed wind direction and speed for the 2008-2010 

“ozone season” (March through October) in the Memphis CBSA as well as on days when area ozone 

monitors exceeded the 2008 ozone NAAQS. Additionally, EPA evaluated wind back trajectories (which 

are an analysis of meteorological patterns) specifically on days when the current ozone design value 

monitor in Shelby County (Frayser monitor) exceeded the 2008 NAAQS. These analyses were 

conducted to better understand the fate and transport of precursor emissions contributing to ozone 

formation. 

EPA’s analysis of the NWS data indicate predominate south and south-southwest component for the 

Memphis CBSA. However, an examination on days when monitors in DeSoto County (Hernando) 

exceeded the 2008 ozone NAAQS suggested a northerly component. Additionally, on days when 

monitors in Shelby County exceeded the 2008 NAAQS, the data indicated a southerly wind component. 

Figure 2, Memphis Area Ozone and Ozone Precursor Monitoring Network, and Figure 4 present 

graphical information on 24-hour back trajectories for exceedances in 2008-2010 at the Frayser monitor, 

locations of major stationary sources, and locations of ambient monitors with their design values. An 

examination of the meteorological data indicates that, for the 2008-2010 days with ozone concentrations 

above 75 ppb at the Memphis 2008-2010 Design Value site (Frayser monitor), the wind back trajectories 

primarily go back through Shelby County, TN (on 10 out of 10 days) and DeSoto County, MS (on 7 out 

of 10 days), with back trajectories going back through Crittenden County, AR on only 1 out of 10 days. 

As mentioned in Factor 1, the Shelby County monitor is the only monitor in the Memphis CBSA with a 

2008-2010 violation of the 2008 ozone NAAQS.

Since the 2008-2010 data is only for three years and has only 10 exceedance days, we evaluated more 

years to better understand the meteorological transport conditions that exist during ozone exceedances. 

Normally when we are developing a conceptual model understanding of what yields ozone exceedances 

in an area we will evaluate 5 to 10 years worth of meteorological data. Therefore we decided to evaluate 

all days that had ozone exceedances at the Design Value monitor (Frayser) for the 2006-2010 period. 

The 2006 and 2007 years had more meteorology that was conducive for ozone formation than the years 

of 2008, 2009, and 2010. Figure 5 in the attachment to this document includes 72-hour back trajectories 

for 2006-2010 ozone exceedances at the Frayser monitor using the National Oceanic and Atmospheric 

Administration Hybrid Single Particle Lagrangian Integrated Trajectory Model (NOAA HYSPLIT). To 

further understand the meteorological transport conditions within the regional area around Memphis, we 

also evaluated24-hour back trajectories for the 2006-2010 time-periods using the NOAA HYSPLIT 

model. The results of these back trajectories are included in the attachment to this document as Figure 6 

with a further zoom in view in Figure 7.

Evaluation of Figures 6 and 7 further supports our previous conclusions based on the 2008-2010 back 

trajectories when the Memphis area Frayser monitor has ozone exceedances. The 2006-2010 data further 

supports that most of the centerlines of the back trajectories passes through Shelby County TN, and 

many of the back trajectory centerlines pass through DeSoto county in northern Mississippi with smaller 

percentage passing through Crittenden County, Arkansas. 

EPA’s meteorological assessment of the area monitors ozone exceedances and specifically the wind 

back trajectory analysis at the Frayser monitor indicate that Shelby County is likely an emission 
10

contributor to exceedances of the 2008 NAAQS at the Frayser monitor. Furthermore, the assessment 

also suggests that DeSoto and Crittenden Counties should be considered for potential inclusion in the 

intended Memphis nonattainment area. 

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 Memphis area does not have any geographical or topographical barriers 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 we identified the general areas we anticipated we would recommend for nonattainment, we then 

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

boundary and to help identify the areas appropriate for 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 urban-scale pollutants, county lines, air 

district boundaries, township boundaries, area covered by an MPO, state lines, Reservation boundaries,

and urban growth boundaries. Where existing jurisdictional boundaries were not adequate or 

appropriate to describe the nonattainment area, other clearly defined and permanent landmarks or 

geographic coordinates were considered.

The Memphis Area MPO is comprised of two study areas; the Memphis Urban Area MPO and the West 

Memphis MPO. Both organizations are considered multi-jurisdictional agencies responsible for the 

implementation and coordination of urban transportation planning and establishing transportation 

conformity infrastructure within their respective boundaries. The Memphis Urban jurisdiction is 

comprised of all of Shelby County, Tennessee, the western four miles of Fayette County, Tennessee and 

the northern twelve miles of DeSoto County. The portion of the Memphis Urban MPO in DeSoto 

County captures the more urbanized portion of the county that has experience continuous growth as well 

as the ozone air quality monitor. The West Memphis jurisdiction is comprised of the current and 

potential future urbanized portion of Crittenden County (including the ozone air quality monitor) with 

the following legal description:

That area west from the Mississippi River along the southern right of way line of County Road 

18 (Miller Road and Caldwell Road) to the western right of way line of County Road 205 

(Hinkley Road); then north along said right of way line and continuing north to the intersection 

of the southern right of way line of the St. Louis-Southwestern Railroad; then in a southwesterly 

direction along said right of way line to the intersection of eastern right of way line of State 

Highway 147; then north along said right of way to the intersection of the southern right of way 

line of State Highway 131; then west along said right of way line to the western right of way line 

of County Road 51(Eubank Road); then north along said right of way line to U.S. 70; then 

continuing north along the western right of way line of County Road 25 (Katie Goodhope) to the 

northern right of way line of County Road 12 (Buck Lake Road); then east along said northern 

right of way line to State Road 306; then continuing east along the northern right of way line of 

State Road 306 to the western right of way line of County Road 165; then north along said right 

of way line to the northern right of way line of County Road 168; then northeasterly along said 
11

right of way line to the intersection of the northern right of way of County Road 172; then east 

along said right of way line to the intersection of the western right of way line of County Road 5; 

then north along said right of way line to the intersection of the northern right of way line of 

James Mill Road; then east along said northern right of way line to the Mississippi River being 

the eastern boundary of the study area.

Memphis, TN-MS-AR Area has previously established nonattainment boundaries associated with both 

the 1-hour ozone and 1997 8-hour ozone NAAQS. The Memphis nonattainment boundary for the 1-

hour ozone NAAQS included Shelby County, Tennessee in its entirety. Whereas the Memphis 

nonattainment boundary for the 1997 8-hour ozone NAAQS included Crittenden County, Arkansas and 

Shelby County, Tennessee in their entireties. Tennessee has recommended a different boundary for the 

2008 ozone NAAQS for their portion of this Area. Arkansas recommended the same as the previous 

boundary for their portion of this Area. In addition, there is current infrastructure for meeting the 

transportation conformity requirements in Shelby County and the urbanized portions of DeSoto County 

and Crittenden County since both the Memphis Urban area and West Memphis MPO are currently 

implementing these requirements for the 1997 8-hour ozone standard.

Even though, DeSoto and Crittenden Counties do not have violating monitors for the 2008 ozone 

NAAQS based on air quality data from 2008-2010, our analysis suggest that both are likely contributing 

to the violation in Shelby County due to potential population-based emissions from mobile sources 

(VMT) and area source, meteorology and population growth.

Conclusion

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

following counties should be included as part of the intended Memphis nonattainment area because they 

are either violating the 2008 ozone NAAQS or contributing to a violation in a nearby area: Crittenden 

County, Arkansas, and Shelby County, Tennessee in their entireties, and the portion of DeSoto County 

that is included in the Memphis MPO boundary. Two of these counties (i.e., Crittenden County,

Arkansas and Shelby County, Tennessee) are included in the Memphis nonattainment area for the 1997 

ozone NAAQS. One of the air quality monitors in Shelby County indicates violation of the 2008 ozone

NAAQS based on 2010 DVs, therefore this county is preliminarily included in the nonattainment area. 

Crittenden County, Arkansas, and DeSoto County, Mississippi are nearby counties that do not have

monitors indicating a violation of the standard based on 2010 DVs. However, EPA has preliminarily 

concluded that these counties (or portions thereof) contribute to the ozone concentrations in violation of 

the 2008 ozone NAAQS through population-based emissions from mobile and area sources (e.g., 

vehicles and other small area sources) and county VMT. 

Source category emissions data indicate that mobile sources and area sources are the primary 

contributors to ozone formation in the Memphis CBSA. Thus, population-based emissions such as total 

population or population growth, and precursor emission transport would indicate a county with 

contribution in the Memphis Area.

The population in DeSoto County, Mississippi has grown steadily from 2000-2010 (particularly the 

northern portion) with a 48 percent increase, even though it only makes up 12 percent of the total 

population in the CBSA. The County also has the CBSA’s second highest VMT. More than 30 percent 

of the County’s NOx and VOC emissions are from mobile sources and over 40 percent from area 
12

sources. In addition, meteorology suggests that DeSoto County is likely contributing to the violation in 

Shelby County due to potential southerly transport of mobile and area emissions.

Shelby County, Tennessee dominates the CBSA in terms of urbanization, precursor emission 

contribution and transport which indicate population-based emission (mobile and area sources) 

contribution to its own violating monitor. Although the County population growth was less than 5 

percent from 2000-2010, it is densely populated with 70 percent of the CBSA population and five times 

DeSoto County’s population. Shelby County makes up over 60 percent of the Area’s NOx and VOC 

emissions. The County’s has over 30 percent of the County’s NOx and VOC emission coming from 

mobile sources and point sources. Meteorological analysis also indicates that Shelby County is 

contributing to its own violation as well as other monitors in the Memphis CBSA.

Crittenden County, Arkansas makes up less than 5 percent of the CBSA population with less than a 1

percent population growth from 2000-2010. Crittenden County is mostly rural with the least 

urbanization compared to Shelby and DeSoto Counties. The County contributes less than 10 percent of 

the CBSA NOx and VOC precursor emissions. However, Crittenden County has over 40 percent of its 

NOx emission deriving from area sources which is considered a primary contributor to the formation of 

ozone in the Memphis area. EPA is proposing to include all of Crittenden County in the 2008 ozone 

Memphis nonattainment area because the county was included in its entirety in the 1997 ozone Memphis 

nonattainment area and because Arkansas recommended inclusion of the county in its entirety.

The remaining Tennessee (Tipton, Fayette) and Mississippi (Marshal, Tate, and Tunica) counties all 

have low population and urbanization, and precursor emission contribution and transport suggesting 

negligible contribution to the violating county. With the exception of those counties that comprise the 

Memphis, TN-MS-AR 1997 8-hour ozone boundary and the portion of DeSoto County, Mississippi 

discussed in this TSD for inclusion, EPA preliminarily concludes that the remainder of the counties in 

the CBSA do not contribute to the violations at the monitors in the CBSA and therefore are not being

considered as part of the nonattainment area. 
13

ATTACHMENTS

Figure 2. Memphis Ozone and Ozone Precursor Monitoring Network, with Population Density.

Figure 3. Population Density Change Percentage Between 2000 and 2010 Census for Memphis Ozone 

and Ozone Precursor Monitoring Network.

Figure 4. Overlay of 24-hour HYSPLIT back trajectories of all 75 ppb exceedances at the Frayser 

monitor for the 2008-2010 period.

Figure 5. NOAA HYSPLIT MODEL 72-Hour Back Trajectory Frayser Exceedances (2006-10).

Figure 6. NOAA HYSPLIT MODEL 24-Hour Back Trajectory Frayser Exceedances (2006-10).

Figure 7. NOAA HYSPLIT MODEL 24-Hour Back Trajectory Frayser Exceedances (2006-10) - Zoom 

View.
14

Figure 2. Memphis Ozone and Ozone Precursor Monitoring Network, with Population Density
15

Figure 3. Population Density Change Percentage Between 2000 and 2010 Census

for Memphis Ozone and Ozone Precursor Monitoring Network
16

Figure 4 - Overlay of 24-hour HYSPLIT back trajectories of all 75 ppb exceedances 

at the Frayser monitor for the 2008-2010 period.
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Figure 5. NOAA HYSPLIT MODEL 72-Hour Back Trajectory Frayser Exceedances (2006-10)
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Figure 6. NOAA HYSPLIT MODEL 24-Hour Back Trajectory Frayser Exceedances (2006-10)
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Figure 7. NOAA HYSPLIT MODEL 24-Hour Back Trajectory Frayser 

Exceedances (2006-10) - Zoom View