1

ARKANSAS 

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 Arkansas that EPA 

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

NAAQS). In accordance with section 107(d) of the Clean Air Act, EPA must designate an area 

“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 Arkansas

Area 

Arkansas’ Recommended 

Nonattainment Counties 

EPA’s Intended Nonattainment 

Counties 

Memphis, TN-MS-AR* Crittenden Crittenden 

Little Rock, AR Pulaski None 

*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 Arkansas 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. 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 FRM or FEM2

 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); 

 

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

 FRM refers to Federal Reference Method, and FEM refers to Federal Equivalent Method. FRM monitors utilize a chemiluminescent technique to measure ozone, while many FEM monitors use a technique involving ultraviolet photometry. FEM 

methods began to be developed in the late 1970’s and early1980’s and are now the most widely utilized methods for 

monitoring ozone levels. Refer to 40 CFR Part 53 for a more detailed description of FEM and FRM methods. 

http://www.epa.gov/ttnamti1/files/ambient/criteria/reference-equivalent-methods-list.pdf 
2

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 

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

with the violating monitor(s).3

 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 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. 

 

3

 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. Intended Memphis, 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 20094

, Arkansas recommended that Crittenden County be designated as nonattainment for the 

2008 ozone NAAQS based on validated air quality data from 2006 – 2008. These data are from Federal 

Equivalent Method (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 

intends to designate two full counties, one each in Arkansas and Tennessee, and one partial county in 

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

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

 

4

 2008 ozone NAAQS designation recommendation letter from Governor Beebe to Acting Regional Administrator Starfield, 

dated March 10, 2009. 
4

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

TN-AR-MS

Memphis, TN-MS-AR State-Recommended 

Nonattainment Counties 

EPA Intended 

Nonattainment Counties 

Arkansas Crittenden Crittenden 

Mississippi None DeSoto (partial) 

Tennessee None* Shelby 

*Recommendation based on preliminary air quality data from 2009 – 2011. 

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 Memphis, TN-MS-AR CBSA 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 0.075 ppm 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. 

The 2010 DVs for the ozone NAAQS for counties in the Memphis, TN-MS-AR CBSA area are shown 

in Table 2. 

Table 2. Air Quality Data.

County State Recommended 

Nonattainment? 

2008-2010 Design Value 

(ppb) 

Crittenden, AR Yes 74 

DeSoto, MS No 73 

Fayette, TN No -- 

Marshall, MS No -- 

Shelby, TN No* 76 

Tate, MS No -- 

Tipton, TN No -- 

Tunica, MS No -- 

*Recommendation based on preliminary air quality data from 2009 – 2011; Tennessee intends to certify 

data early for EPA to consider for final designation decisions. 

Air quality monitoring in Shelby County, Tennessee indicates 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. 
5

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. 

Table 3 shows emissions of NOx and VOC (given in tons per year) 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 

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 

Area-wide: 59,619 44,628 

*Recommendation based on preliminary air quality data from 2009 – 2011; Tennessee intends to certify 

data early for EPA to consider for final designation decisions. 

Of the eight counties evaluated for inclusion in the nonattainment area, the emissions from Shelby 

County, Tennessee eclipse those of the surrounding counties and account for about 66 percent of the 

NOx and 63 percent of the VOC precursor emissions in the CBSA. Of the county’s 39,519 tons per year 

of NOx emissions, 35 percent are from point and mobile emissions and 20 percent from area source 

emissions. Of the county’s 27, 929 tons per year of VOC emissions, 36 percent are from mobile sources 

and 32 percent are from area sources. 

DeSoto County contributes about 9 percent NOx and 12 percent VOCprecursor 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. 

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 mobile sources and 34 percent from area sources. 
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The county’s total VOC emissions include 35 percent area sources and 31 percent mobile sources. Only 

5 percent of the County’s NOx emissions are from point sources. Both Crittenden and DeSoto Counties 

have 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 

significantly 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 that 

contributes to nonattainment in the area. 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 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) 

Crittenden, AR Yes 50,902 0.08 -75 0 

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 

Area-wide: 1,316,100 0.28 107,854 9 

*Recommendation based on preliminary air quality data from 2009 – 2011; Tennessee intends to certify 

data early for EPA to consider for final designation decisions. 

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_GCTPL2.STO5&

prodType=table)

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. 

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 
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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. 

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. 

Traffic and commuting patterns

EPA evaluated the commuting patterns of residents in the area, as well as the total Vehicle Miles 

Traveled (VMT) for each county. 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 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 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 traffic and commuting pattern data, including total 

2008 VMT and 10-year VMT growth for each county. 

Table 5. Traffic and Commuting Patterns.

County State Recommended 

Nonattainment? 

2008 VMT* 

(million miles) 

% Change in VMT 

(2002 – 2008) 

Crittenden, AR Yes 903 8.0 

DeSoto, MS No 1,629 20.0 

Fayette, TN No 573 6.0 

Marshall, MS No 725 20.0 

Shelby, TN No** 8,789 1.0 

Tate, MS No 376 20.0 

Tipton, TN No 401 0 

Tunica, MS No 337 20.0 

Area-wide: 13,733 -- 

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

**Recommendation based on preliminary air quality data from 2009 – 2011; Tennessee intends to 

certify data early for EPA to consider for final designation decisions. 

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. The County accounts for 64 percent of 

the VMT in the CBSA; and approximately 35 percent of Shelby County’s NOx emissions and 34 

percent VOC emissions are from mobile sources. 

In DeSoto County accounts for 12 percent of the CBSA VMT (which is the second highest in the 

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. 
8

In Crittenden County accounts for less than 10 percent of the CBSA VMT. 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. 

Of the 8-county area, Crittenden County, Arkansas, DeSoto County, Mississippi, and Shelby County, 

Tennessee are all characterized by comparatively high VMT that exceed 900 million miles. Collectively, 

these counties account for 82 percent of the total VMT in the eight-county area. 

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 a predominant south and south-southwest component for the 

Memphis CBSA, which means the winds blow most often from the south and south-southwest. 

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. The attachment to 

this document contains Figure 4, Memphis Area Ozone and Ozone Precursor Monitoring Network, 

which presents 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. 

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 evaluated 24-hour back trajectories for the 2006-2010 time period 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. 
9

Evaluation of Figures 6 and 7 further support 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 a 

comparatively smaller percentage passing through Crittenden County, Arkansas. 

EPA’s meteorological assessment of the areas’ emissions, the area monitors’ ozone exceedances, and 

specifically the wind back trajectory analysis at the Frayser monitor indicate that these areas should be 

considered for potential inclusion in the Memphis, TN-MS-AR 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, TN-MS-AR area 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 we identified the general areas we anticipated we would recommend to be included in the 

nonattainment area, 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, areas covered by a metropolitan planning 

organization, 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 to 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 Area MPO 

jurisdiction is comprised of all of Shelby County, TN, the western four miles of Fayette County, TN and 

the northern twelve miles of DeSoto County. The portion of the Memphis Urban Area 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 
10

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 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. With the exception of those counties that comprise the 1997 8-

hour ozone boundary for this Area and the portion of Desoto County, Mississippi discussed in this TSD 

for inclusion, we believe that the remainder of the counties in the CBSA do not contribute to the 

violations at the monitors in this Area and therefore are not necessary be considered as part of the 

nonattainment area. There is current infrastructure for meeting the transportation conformity 

requirements in Shelby County and the urbanized portions for 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, our analysis suggest that both are likely contributing to the violation in Shelby County due to 

potential population-based emissions from mobile sources (commuting patterns) and area source, 

meteorology and population growth. 

The remaining Tennessee and Mississippi counties all have relatively low population and urbanization, 

and precursor emission contribution and transport suggesting negligible contribution to the violating 

county. 

Conclusion

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

following counties, or parts of counties, should be included as part of the Memphis, TN-MS-AR 

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 Urban Area MPO 

boundary. Two of these counties, Shelby and Crittenden, are included in the Memphis, TN-MS-AR 

nonattainment area for the 1997 ozone NAAQS. One of the air quality monitors in Shelby County

indicates a violation of the 2008 ozone NAAQS based on the 2010 DVs; therefore this county is 
11

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 

2008 – 2010 data. 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 non-point sources (e.g., vehicles and other small area sources). 

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 

significant total population or population growth and precursor emission transport would indicate a 

county with contribution in the Memphis Area. 

Shelby County, TN dominates the CBSA in terms of urbanization, precursor emission contribution and 

transport (through commuting patterns) which indicate significant 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. More than 30 percent of the County’s NOx and VOC emission derives from mobile 

sources and point sources. 

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

portion) with a 48 percent increase, even though it only makes up 12 percent of the population. The 

County has the area’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 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. 

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, analysis of the meteorological data indicates 

that emissions from Crittenden County do occasionally contribute to violations of the 2008 ozone 

NAAQS in Shelby County. EPA is proposing to include all of Crittenden County in the 2008 ozone 

Memphis, TN-MS-AR nonattainment area because the county was included in its entirety in the 1997 

ozone Memphis, TN-MS-AR nonattainment area and because Arkansas recommended inclusion of the 

county in its entirety. 

Technical Analysis for Little Rock, AR 

In its March 2009 initial 2008 ozone NAAQS designation recommendation letter to EPA, Arkansas 

recommended that Pulaski County be designated as nonattainment. This recommendation was based on 

ambient air quality monitoring data for the period from 2006 – 2008, which indicated a design value of 

0.080 ppm. More recent validated ambient air quality monitoring data indicates that the Little Rock area 

is meeting the 2008 ozone NAAQS, and there are currently no monitors in the Little Rock area that 

indicate a violation of the standard. 

Although Arkansas has not submitted an updated 2008 ozone recommendation letter to rescind its initial 

recommendation of nonattainment for Pulaski County, EPA is basing its designation regarding Pulaski 

County and other counties in the Little Rock area on the most recently available validated ambient air 
12

quality monitoring data. The 2008-2010 8-hour ozone design value for Little Rock is 0.070 ppm (0.070 

ppm at two sites and 0.067 ppm at one site). 

Conclusion

Based on the most recently available validated ambient air quality monitoring data, EPA has 

preliminarily concluded that no counties or portions of counties comprising the Little Rock, Arkansas 

metropolitan statistical area should be designated as nonattainment for the 2008 ozone NAAQS. 
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. 
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Figure 2. Memphis Ozone and Ozone Precursor Monitoring Network, with Population De
15

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

for Memphis Ozone and Ozone Precursor Monitoring Network 
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Figure 4. Overlay of 24-hour HYSPLIT back trajectories of all 75 ppb exceedances at the Frayser monitor fo
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Figure 5. NOAA HYSPLIT MODEL 72-Hour Back Trajectory Frayser Exceedances (2006-10) 
18

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