Wyoming

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 Wyoming that EPA is 

designating 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 (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 area 

are provided below.

Table 1. State's Recommended and EPA’s Designated Nonattainment Counties for the Upper Green 

River Basin (UGRB) of Wyoming.

Area

Wyoming’s Recommended 

Nonattainment Counties EPA’s Nonattainment Counties

Upper Green River Basin Area 

Sublette County (all)

Lincoln County (partial)

Sweetwater County (partial)

Sublette County (all)

Lincoln County (partial)

Sweetwater County (partial)

EPA is designating all areas of the State of Wyoming that are not listed in the table above as 

“unclassifiable/attainment” for the 2008 ozone NAAQS. 

The analysis below provides the basis for the 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 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 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 

 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 five categories of factors.
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 five 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. EPA used the same basic approach in the designation process for the 1997 ozone 

NAAQS. For areas such as Wyoming, 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. 

Elevated ozone in the UGRB is associated with distinct meteorological conditions that occur only in 

wintertime, primarily during the first quarter of the year. Wyoming recommended the designation of its 

nonattainment area on a county and partial county basis that reflects the local meteorological conditions 

conducive to wintertime ozone formation, and the likely sources of ozone and ozone precursors. The 

UGRB’s east, north and west county boundaries are natural boundaries of high mountains or elevated 

terrain. The geographical and jurisdictional boundaries also coincide with population and emission 

source boundaries. To the south, the topographical boundaries are less obvious, but there are rivers, 

valleys and buttes that form geographic boundaries near the southern border of Sublette County,

extending slightly into Lincoln and Sweetwater Counties which define the area where emissions 

contribute to nearby violations. As part of its nine-factor analysis, the State evaluated the following 

counties: Sublette, Lincoln and Sweetwater Counties. In tables 4 and 6, Wyoming provided additional 

data on Uinta, Fremont and Teton Counties, which we’ve included in our TSD. Our analysis examines 

the same area evaluated by the State; there is no evidence to support including additional, more distant 

areas as part of our analysis.

 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).
Technical Analysis for the UGRB

Figure 1A is a map of the UGRB nonattainment area. The map provides other relevant information 

including the locations of WDEQ, EPA and National Park Service (NPS) air quality monitors, county 

and other jurisdictional boundaries, including the Wind River Indian Reservation, and the Forest Service 

and NPS Class 1 Areas, but does not show the southern boundary defining the portions of Lincoln and 

Sweetwater counties we are including as part of the designated nonattainment area. Figure 1B was 

provided by Wyoming, and shows that southern boundary, as well as the local topography. 

Figure 1A Upper Green River Basin Nonattainment Area (all of Sublette County, plus parts of Lincoln 

and Sweetwater Counties)
Figure 1B Upper Green River Basin Nonattainment Area (all of Sublette County, plus parts of Lincoln 

and Sweetwater Counties)

In March 2009, the Wyoming Department of Environmental Quality, Air Quality Division (AQD)

recommended that the UGRB, comprising Sublette County and parts of Lincoln and Sweetwater 

Counties, be designated as nonattainment for the 2008 ozone NAAQS based on air quality data from 

2006-2008. Ozone at levels exceeding the standard has been monitored at the Boulder monitor, which is 

located in the center of the UGRB nonattainment area.. Measured ozone levels have not violated the 

standard in the counties adjacent to the UGRB. The highest ozone levels within the UGRB typically 

occur in January, February or March, and VOCs detected in ambient air in the UGRB have a strong oil 

and gas signature. 

Elevated ozone episodes are associated with very light low-level winds, sunshine and snow cover, in 

conjunction with a strong low-level surface-based temperature or “capping” inversion. The longest such 

event (February 19-24, 2008) resulted in a measured ozone level of 122 ppb as an 8-hour average at the 

Boulder station. These data are from monitors sited and operated in accordance with 40 CFR Part 58. 

EPA received certification letters from AQD for 2008, 2009 and 2010 data on March 15, 2011, October 

13, 2011 and July 26, 2011, respectively. These certification letters are available in the docket. 

The AQD’s analysis provided with its recommendation shows that elevated ozone at the Boulder 

monitor is primarily due to local emissions from oil and gas development activities: drilling, 

production, storage, transport and treating of oil and natural gas. The ozone exceedances almost always 

occur when winds are low indicating that there is little to no transport of ozone or precursors from 

distant sources outside the proposed nonattainment area. The ozone exceedances predominantly occur 

in the winter when the following conditions are present: strong temperature inversions, low winds, cold 

temperatures, clear skies and snow cover.

Sublette

Sweetwater

Lincoln
After considering the AQD’s recommendations and based on EPA's technical analysis described below, 

EPA is designating Sublette County and parts of Lincoln and Sweetwater Counties as nonattainment for 

the 2008 ozone NAAQS as together they make up the whole UGRB nonattainment area. 

Factor Assessment

Factor 1: Air Quality Data 

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

the UGRB 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 

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 UGRB and nearby surrounding area are shown 

in Table 2. The WDEQ has flagged a few days in the three-year design set of 2008-2010 as exceptional 

events, but the State has not yet submitted demonstrations for the exceptional event claims at the 

Boulder monitor.

Table 2. Air Quality Data.

County State Recommended 

Nonattainment?

2008-2010 Design Value 

(ppb)

Lincoln, WY (Part) Yes --

Sublette, WY Yes 78

Sweetwater, WY (Part) Yes 64

The Boulder monitor in Sublette County shows a violation of the 2008 ozone NAAQS, therefore 

Sublette 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 based on the 

weight of evidence of the five factors and other relevant information to determine whether it contributes 

to the nearby violation. 

All data are collected by Reference or Equivalent Method monitors and meet EPA’s criteria for quality 

and completeness unless otherwise noted.

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 whether nearby areas are contributing to exceedances of the ozone standard at 

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. 

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

contributing counties in the UGRB area. For the two counties we are designating as partial counties 

(Lincoln and Sweetwater), the table includes all emissions within the counties, not just the emissions 

from the partial county areas. These emissions include some very large point source emissions distant 

from the violating monitor.

Table 3. Total 2008 NOx and VOC Emissions.

County State Recommended 

Nonattainment? NOx (tpy) VOC (tpy)

Lincoln, WY Yes 17,014 10,150 

Sublette, WY Yes 7,802 17,765 

Sweetwater, WY Yes 43,635 30,823 

Areawide: 68,451 58,738 

As part of its nine-factor analysis, the AQD compiled emission estimates for VOCs and NOx for 10 

source categories in the partial county area the state recommended as nonattainment, and counties and 

portions of counties surrounding the nonattainment area. The AQD used emissions information from 

2007, which was the most recently available data for all source sectors. Emissions estimates were 

provided only for the first quarter because elevated ozone in the UGRB occurs primarily during limited 

episodes in the first three months of the calendar year. These data are shown in Table 4.
Table 4 1st Quarter 2007 emissions within UGRB and in surrounding counties outside UGRB

While some surrounding areas (Lincoln and Sweetwater outside of the UGRB) show relatively high 

VOC and NOx emissions when compared to the total emissions within the UGRB, the very large sizes 

of the counties involved and the detailed meteorological and topographical analyses described in later 

sections support the boundary proposed by Wyoming. 

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 the associated area source and mobile 

source emissions may be appropriate to include in the nonattainment area. The area under consideration 

here has no urban core; therefore, these considerations are less relevant than in an urban nonattainment 

area. Table 5 shows the population, population density and population growth information for each 

county in the area.
Table 5. Population and Growth.

County

State 

Recommended 

Nonattainment?

2010 Population

2010 Population

Density

(pop/sq mi)

Absolute Change 

in Population

(2000-2010)

Population % 

Change

(2000-2010)

Lincoln, WY Yes 18,106 4.4 

 

3,492 +24% 

Sublette, WY Yes 10,247 2.1 

 

4,301 +72% 

Sweetwater, WY Yes 43,806 4.2 

 

6,323 +17% 

Areawide: 72,159 3.7 

 

14,116 +24%

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)

Sublette County and the surrounding counties are rural with a low overall population density. There are 

no metropolitan areas with a population of 50,000 or more in this six-county area. The largest 

community in Sublette County is Pinedale, which had a population in 2010 of 2,030. The largest 

communities in the counties surrounding Sublette County are Rock Springs (population 23,036, 

Sweetwater County), Green River (population 12,515, Sweetwater County) and Evanston (population 

12,359, Uinta County). Those communities are classified by the U.S. Census Bureau as Micropolitan 

Statistical Areas.

Population in Sublette County is expected to increase at a rate of approximately 5% over the next 23 

years. Population in surrounding counties is expected to increase more slowly at rates of 2% or less. 

Sublette County does not have any urbanized areas and the urbanized areas in the surrounding area are 

geographically distant from the Boulder monitor which had the ozone violation in Sublette County. As 

described in Factor 3 Meteorology, meteorological conditions associated with elevated ozone episodes 

greatly limit the possibility of emissions transport.

Table 6 Population Density Data Provided by Wyoming

Population Density

Sublette Sweetwater Lincoln Uinta Fremont Teton

Estimated 2007 Population 7,925 39,305 16,171 20,195 37,479 20,002

Area (square miles) 4,882 10,426 4,069 2,082 9,183 4,008

Population/square mile 2 4 4 10 4 5

Percent in Urbanized Area* 0 89 20 59 48 56

Percent in Rural Area* 100 11 80 41 52 44

* Based on 2000 Census
Traffic and commuting patterns

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

Traveled (VMT) for each county. In combination with the population/population density data (see 

above) and the location of main transportation arteries, 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. 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. In the case of the 

UGRB, with no urbanized areas and a small population, considering those emissions has little relevance 

for this nonattainment area. Table 7 shows traffic and commuting pattern data, including the total 

vehicle miles traveled (VMT) for each county.

Table 7. Traffic and Commuting Patterns.

County State Recommended 

Nonattainment?

2008 Annual VMT*

(million miles)

Lincoln, WY Yes 321 

Sublette, WY Yes 198 

Sweetwater, WY Yes 787 

Areawide: 1,306

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

The number of commuters into or out of Sublette County and the UGRB is small and does not support 

adding other counties or parts of counties into the nonattainment area based on contribution of emissions 

from commuters from those counties. The percent of emissions from on-road mobile sources is small 

within the proposed nonattainment area: 7% of NOx and 0.3% of VOCs. Even if this source of 

emissions increases, it will remain a small percentage of total emissions. 

Approximately 90% of the traffic volume in Sweetwater and Uinta Counties is interstate traffic. 

Interstate 80 is located approximately 80 miles south of the violating Boulder monitor. Five ozone 

monitors located closer to the interstate have operated at times between 2005 and 2010, and none of 

them have shown an ozone violation. Therefore, I-80 traffic is not considered a significant contributor of 

emissions that impact the Boulder monitor during ozone events. 

Consistent with the rural character of the counties in southwest Wyoming, including Sublette County, 

traffic volumes are low. Emissions from this sector are of a much lower significance than is typically 

seen in urban nonattainment areas.
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. The unique meteorology in the UGRB creates conditions 

favorable to wintertime ozone formation. The meteorology within the UGRB during winter ozone 

episodes is much different than on non-high ozone days in the winter, and is also much different than the 

regional meteorology that exists outside of the UGRB during the wintertime high ozone episodes.

There is significant topographic relief in Wyoming which affects climate and daily temperature 

variation. This is a semiarid, dry, cold, mid-continental climate regime. July and August are generally 

the hottest months of the year, while December and January are the coldest. Pinedale’s mean 

temperature in January is 12.5°F with a mean of 60°F in July (Western Regional Climate Center, 2009). 

The high elevation and dry air contribute to a wide variation between daily minimum and maximum 

temperatures. At Pinedale, the total annual average precipitation is about 10.9 inches and an average of 

61 inches of snow falls during the year.

The State’s TSD provides a thorough examination of the meteorological conditions and this portion of 

the State’s TSD is reproduced below. Table numbers have been changed to conform with the structure 

of this TSD:

Strong winds are common in Wyoming, especially in the south. Wind velocity can be attributable, in 

part, to the prevailing westerly winds being funneled through the Rocky Mountains at a low point in the 

Continental Divide. The meteorological conditions conducive to the formation of high ozone levels in 

the UGRB during the winter and early spring are characterized by:

• a stable atmosphere, characterized by light low-level winds

• clear or mostly sunny skies

• low mixing heights or capping inversions

• extensive snow cover

• low temperatures

Because these conditions are common to all of the high ozone episodes in the UGRB, the ozone episode 

of February 19-23, 2008, a five-day period marking the longest consecutive ozone episode observed, is 

considered to be representative of the other ozone episodes. 

After elevated ozone levels were monitored in the winter of 2005 and 2006, the AQD initiated intensive 

field studies to collect meteorological and ambient data in the first quarter of 2007, 2008 and 2009 

throughout the Green River Basin to better understand the relationships between winter meteorological 

conditions and high ozone levels versus low ozone levels. The winter of 2008 included several high 

ozone episodes which provided a significant amount of data on ozone formation. A map showing the 

monitoring sites employed in the 2008 field study and regional terrain features in the 2008 study area is 

shown in Figure 2.
Figure 2. Surface and upper air monitoring sites employed in the 2008 field study.

With the addition of the temporary mesonet monitoring sites to the existing permanent meteorological 

monitoring stations in the 2007 and 2008 field studies, a fairly detailed picture of wind flow patterns 

within the UGRB was obtained, revealing that the wind flow patterns were distinctly different 

throughout the northern and southern portions of southwest Wyoming. A composite map of wind rose 

plots generated from meteorological data collected throughout southwest Wyoming during the time 

period 18 – 22, February 2008 is provided in Figure 3.
Figure 3. Composite wind rose map for February 18 – 22, 2008 at monitoring sites located throughout 

southwest Wyoming

CalDESK Trajectory Analysis 

AQD developed a high resolution (spatial and temporal) 3-dimensional wind field that uses the National 

Center for Environmental Prediction (NCEP) Rapid Update Cycle (RUC) model at 20 kilometer 

resolution, coupled with the high resolution observational database of surface and upper air 

meteorological data measurements obtained during the 2008 field study. It should be noted that the 

terrain elevation data used in this wind field is based on much higher terrain resolution than is currently 

used in the HYSPLIT model. The RUC and field meteorological data were processed through the 

CALMET diagnostic wind model to generate a 1 kilometer gridded wind field, using high resolution 

terrain and land use/land cover data, and actual observations of daily snow cover to account for actual 

snow cover (and albedo effects) within the CALMET domain. The complexity of the terrain, as 

represented in this 3-dimensional (3-D) CALMET wind field in shown in Figure 4.
Figure 4. Terrain features in CALMET modeling domain (464 km x 400 km)

Figure 5 provides a snapshot of the wind field based on the winds at 4:00 am (MST) on February 20, 

2008, and shows the complexity of the terrain surrounding the UGRB is very well represented in the 

CALMET wind field. The wind field captures the strong terrain-dominated down slope winds during the 

early morning hours, and the strong channeling and drainage effects which are exhibited throughout the 

UGRB – CALMET “sees” the influence of the terrain.
Figure 5. CALMET wind field at 4:00 am (MST) on February 20, 2008. The 2008 field study 

meteorological monitoring sites are shown for reference

The trajectory analyses using this wind field lead to the conclusion that regional transport is 

insignificant, and local-scale precursor emissions transport is the dominant means of precursor 

transport during the high ozone periods. The trajectory analyses that follow were a key factor in 

selection of an appropriate southern boundary of the nonattainment area. The trajectory analyses 

demonstrate that the proposed southern boundary of the nonattainment area is reasonable, and that 

there is no significant contribution of ozone or ozone precursors from areas or sources outside the 

proposed nonattainment area during elevated ozone events.

Based on this wind field, AQD used the CalDESK visualization software to run forward trajectory 

analyses to evaluate air parcel transport into and out of the UGRB, specifically with respect to air 

parcels from large stationary sources (power plants and Trona plants) located to the south of the 

UGRB, and to evaluate the southern extent of air parcel inflow into the UGRB. 

An extensive series of CalDESK forward trajectory analyses were conducted for high ozone days 

throughout the winter days of 2008. The analyses consistently showed that emissions from large point 

sources south of the state-recommended nonattainment area boundary did not reach the central portion 

of the nonattainment area with ozone violations. The boundary was therefore defined to include those 
point and area emissions sources consistently constrained and transported within the ozone impacted 

area by local winds, and to exclude more remote sources which were never seen to transport into the 

ozone impacted area.

Representative forward trajectories for source regions within and outside the nonattainment area are 

shown below for February 20, 2008. Very similar trajectories for the other high ozone days, consistent 

with the trajectories shown are included within the Wyoming recommendation supporting materials 

included in the docket but omitted here for brevity. 

Figure 6. 24-hour forward trajectory analysis at Labarge, Wyoming (oil and gas production area) on 

February 20, 2008.
A point at the northernmost location in the Moxa Arch oil and gas field (located in Lincoln County, 

south of the nonattainment area boundary), the point nearest the violating monitor, was selected for 

evaluation of the potential for Moxa Arch emissions to travel to the area violating the ozone standard. 

No trajectories were observed which suggested emissions from this gas field contribute to the measured 

violations. In its response to EPA’s proposed boundary, Wyoming noted that a new monitor just east of 

the Moxa Arch field recorded no ozone exceedances in 2011 at a time the monitors within the UGRB 

nonattainment area were recording very high exceedances (based on preliminary, uncertified data). In 

addition, Wyoming provided wind roses for each of the high ozone days in 2011 for each monitor in the 

area. The wind roses showed patterns of flow in 2011 consistent with the earlier trajectory analyses for 

2008 (see Feb. 29, 2012 letter from John V. Corra, Director, Wyoming DEQ, in the docket).

Figure 7. 24-hour forward trajectory analysis at Moxa Arch (oil and gas production area) on February 

20, 2008.
Figure 8. 24-hour forward trajectory analysis at Naughton Power Plant on February 20, 2008.
Figure 9. 24-hour forward trajectory analysis at OCI Trona Plant on February 20, 2008.
Figure 10. 24-hour forward trajectory analysis at Bridger Power Plant on February 20, 2008.

In summary, the high fidelity trajectory analysis conducted by Wyoming in support of its 

recommendation for the southern boundary of the Upper Green River nonattainment area shows some 

minor transport of emissions from areas just north of the boundary toward the most impacted ozone 

monitors, but also showed that emissions from sources south of the boundary were consistently 

transported east and out of the region without entering the area with the violating monitor.
Factor 4: Geography/topography (mountain ranges or other air basin boundaries)

The State’s recommendation included a detailed description of the UGRB terrain, some of which is 

reproduced here:

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. Southwest Wyoming and the UGRB are 

within the Wyoming Basin Physiographic Province. Topography in the UGRB is characterized by low, 

gently rolling hills interspersed with buttes. Elevations range from approximately 7,000 to 7,400 feet 

above mean sea level (AMSL) in the lowest portion of the UGRB. The Wind River Range, with peaks up 

to 13,800 feet, bounds the UGRB to the east and north. The Wyoming Range, with peaks up to 11,300 

feet, bounds the UGRB to the west. There are also important low terrain features such as the Green 

River Basin and the Great Divide Basin. Other terrain features such as river and stream valleys also 

influence local wind patterns. Mountain-valley weather patterns in the UGRB tend to produce limited 

atmospheric mixing during periods when a high pressure system is in place, setting up conditions for

temperature inversions, which are enhanced by the effect of snow cover.

Mountain elevations decrease moving south along both the Wyoming and Wind River ranges. Along the 

western boundary of the Green River Basin, in the southern part of the Wyoming Range, the elevation 

decreases to about 6,900 feet AMSL with some peaks in the 7,500 to 8,000-foot range. Moving south 

along the Wind River Range, the elevation decreases to 7,800 feet at South Pass.

The surrounding significant terrain features effectively create a bowl-like basin in the northern portion 

of the Green River Basin, which greatly influences localized meteorological and climatological patterns 

relative to geographical areas located outside of the UGRB. Although difficult to quantify over the 

entire UGRB valley, the UGRB is roughly 3,000 to 4,300 feet lower than the terrain features bounding 

the UGRB to the east and west. 

The southern boundary of the area is defined by river and stream channels. To the east the Big Sandy, 

Little Sandy and Pacific Creek drainages define the boundary and to the west the Green River and 

Fontenelle Creek drainages define the boundary.

Significant terrain in the UGRB has an impact on the local meteorology (wind speed, wind direction and 

atmospheric stability). In the mountain-valley areas such as the UGRB, during the night cold air will 

accelerate down the valley sides (downslope winds), while during the day warmer air will flow up the 

valley sides (upslope winds). At night, this can create a cold pool of air within the UGRB that stratifies 

the atmosphere (inhibits mixing) since colder, denser air exists at the surface with warmer air above. 

Further, at the valley floor, the wind speed is likely to be lower than in the open plain as the roughness 

of the surrounding terrain tends to decrease wind speeds at the surface. The terrain obstacles 

surrounding the UGRB also tend to cut-off, block or redirect air that might normally flow through the 

valley. This effect is exacerbated by during times of calm weather, such as the passage of a high 

pressure system that tends to set up conditions for strong surface-based temperature inversions.
The Wind River Range on the east and the Wyoming Range on the west provide significant barriers to 

movement of ozone and ozone precursors into the nonattainment area. Although the southern boundary 

is not bordered by a mountain range, the southern boundary lies along two significant drainages: the 

Fontenelle/Green River to the southwest and the Pacific/Big Sandy River to the southeast. These 

geographic features influence air flow, although they do not provide an absolute barrier to migration. 

The influence of these geographic features on wind flows, especially during periods of low winds which 

are needed for ozone formation is illustrated by figure 5. This figure shows winds generally conforming 

to the drainages which establish the southern boundary of the nonattainment area. The nonattainment 

area is outlined in blue below against an aerial view of the topography in the UGRB in Figure 11.

Figure 11: Nonattainment area shown (blue outline) against an aerial view of the topography in the 

UGRB and adjacent areas 
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 areas for ozone or other urban-scale 

pollutants, counties, air districts, townships, metropolitan planning organizations, state lines, areas of 

Indian country, urban growth boundary, etc. Where existing jurisdictional boundaries are not adequate 

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

coordinates were considered.
Figure 12: Nonattainment Boundary and Jurisdictions

The Sublette County jurisdictional boundary forms the northern and most of the western and eastern 

boundaries of the nonattainment area. The remainder of the boundary is not jurisdictional but is based on 

topographical and meteorological considerations. There is no existing local authority that transcends county 

boundaries, so the nonattainment area has no single local administrative authority other than the state of 

Wyoming. 

The nonattainment area includes all of Sublette County; the portion of Lincoln County northeast of the 

waterways of Aspen, Fontenelle and Roney Creeks, and northeast of Fontenelle Reservior and the Green 

River; and the portion of Sweetwater County northwest of the waterways of the Green River, the Big Sandy 
River, Little Sandy Creek, Pacific Creek, and Whitehorse Creek. This area includes the town of LaBarge in 

Lincoln County. The southern boundary of the nonattainment area is defined based on topographical and 

meteorological considerations rather than jurisdictional boundaries. 

The Sublette County borders to the north, east and west follow topographic features (mountain ranges) and 

are appropriate boundaries for the nonattainment area. The five counties in Southwest Wyoming which were 

also included in the analysis are: Teton, Lincoln, Uinta, Sweetwater and Fremont. Two Indian Tribal Nations 

are also located near the nonattainment area, the Northern Arapahoe and Eastern Shoshone, at the Wind 

River Reservation in Fremont County. The reservation and the counties are shown in Figure 12. We did not 

include the Wind River Reservation as part of the nonattainment area.

Conclusion

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

meet the CAA criteria for inclusion in the UGRB nonattainment area: Sublette County and portions of 

Lincoln and Sweetwater Counties. Based on monitoring results from 2006 through 2010, the state of 

Wyoming is in compliance with the 2008 0.075 ppm ozone standard with the exception of the Boulder 

monitor in Sublette County. The 2010 DV for the ozone NAAQs at the Boulder monitor was 0.078 

ppm. The AQD performed a nine-factor analysis that formed the basis of the state’s recommendation 

that this area be designated for nonattainment and EPA is adopting the AQD’s recommendation. 

Because this is a low-population, rural area, EPA agrees with the state’s conclusion that elevated ozone 

at the Boulder monitor is primarily due to local emissions from oil and gas development activities: 

drilling, production, storage, transport and treatment of oil and natural gas. 

The boundary recommendations are based on meteorological conditions that occur in the first quarter of 

the year, when elevated ozone events occur. The nonattainment area boundary includes the violating 

monitor and the sources which are most likely to contribute ozone and ozone precursors to the 

monitored area. While the east, north and west boundaries are natural geographic boundaries of high 

mountains, they also coincide with population, jurisdictions, growth and emission source boundaries. 

Meteorology provided the strongest basis for setting the southern boundary, which includes portions of 

Lincoln and Sweetwater Counties.