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ENCLOSURE 1

INFORMATION TO SUPPORT 

AREA DESIGNATION BOUNDARY RECOMMENDATIONS 

FOR THE 2008 FEDERAL 8-HOUR OZONE STANDARD:

AMADOR, CALAVERAS, TUOLUMNE, AND MARIPOSA COUNTIES

On December 9, 2011, the United States Environmental Protection Agency 

(U.S. EPA) provided California with notification of its preliminary decision to 

designate 16 areas of California as nonattainment for the revised 2008 federal 

ozone standard. Two of the recommended nonattainment areas differed from 

the State’s initial recommendation: Central Mountain Counties (Amador and 

Calaveras counties) and Southern Mountain Counties (Tuolumne and Mariposa 

counties). In both cases, U.S. EPA’s recommended nonattainment area includes 

counties that attain the standard. 

While the Air Resources Board (ARB) supports U.S. EPA’s proposal to designate 

the Central and Southern Mountain Counties separately from the upwind urban 

areas, ARB reaffirms its recommendation to designate individual counties within 

these areas as separate attainment and nonattainment areas. Additional 

justification for the State recommendation is outlined below, using the nine 

factors U.S. EPA included in its guidance memo (December 4, 2008, Area 

Designations for the 2008 Revised Ozone National Ambient Air Quality 

Standards, Memorandum from Robert J. Meyers, Principal Deputy Assistant 

Administrator, Office of Air and Radiation, to Regional Administrators, 

Regions I-X). These factors include air quality data, emissions data, population 

density and degree of urbanization, traffic and commuting patterns, growth rates 

and patterns, meteorology, geography and topography, jurisdictional boundaries, 

and level of control of emission sources. These nine factors can be combined 

into five general factors, as addressed for each area, below. 

Central Mountain Counties: Amador County and Calaveras County

U.S. EPA recommends that Amador and Calaveras counties be designated 

together as the Central Mountain Counties federal ozone nonattainment area. 

Based on data collected during 2008 through 2010, the design values for both 

areas exceed the federal standard (2010 Amador County design value is 

0.081 parts per million (ppm) and 2010 Calaveras County design value is 

0.083 ppm). However, these design values are higher than what would be 

expected normally, because they include data from 2008, when ozone 

concentrations were adversely impacted by wildfire emissions (refer to air quality 

discussion, below). 

The 2011 design values for both counties (based on complete data for 2009 

through 2011) are much lower. They do not include the 2008 data and 
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therefore, reflect more normal conditions. The 2011 design value for Calaveras 

County is 0.077 ppm, which is close to the level of the standard, but still exceeds 

the standard. In contrast, the 2011 design value for Amador County is 

0.071 ppm, which does not exceed the federal standard. The following 

paragraphs provide a five factor analysis to justify designating Amador County as 

a separate attainment area.

Jurisdictional Boundaries:

Amador and Calaveras counties are located in the Mountain Counties Air Basin

(MCAB). Although both counties are located in the MCAB, they fall under the 

jurisdiction of two separate agencies. Amador County is under the jurisdiction of 

the Amador County Air Pollution Control District (APCD), while Calaveras County 

is under the jurisdiction of the Calaveras County APCD. Air quality in each 

county is managed at the local level through land use and development planning 

practices, and the local APCD is responsible for establishing and enforcing local 

air quality rules and regulations that address the requirements of federal and 

State air quality laws. With respect to nonattainment planning, it is most efficient 

to have the nonattainment boundary coincide with the jurisdictional boundary of 

the area(s) that experience or contribute to violations of the standard. Because 

Amador County attains the standard and does not significantly contribute to 

violations in Calaveras County, it is not appropriate to include Amador County in 

the nonattainment area. 

Geography / Topography:

The MCAB is located in the eastern half of California, comprising a 12,226 

square mile area. Amador and Calaveras counties are located in the central 

portion of the MCAB, with Amador County to the north and Calaveras County to 

the south (refer to Figure 1). 

Amador County covers an area of 605 square miles. Calaveras County is nearly 

twice as large, encompassing an area of approximately 1,037 square miles. The 

Sierra Nevada Mountain range comprises a large portion of both counties, with 

elevations ranging from approximately 100 feet in the western foothill areas to 

over 9,000 feet in the mountainous eastern areas. The physical characteristics

of each county impact local air quality. Because the topography consists largely 

of a succession of east-west canyons and intervening ridges, surface winds are 

generally restricted to flowing in an east-west direction -- uphill from the west 

during the day and downhill from the east at night. 
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FIGURE 1

AMADOR AND CALAVERAS COUNTIES

Meteorology:

ARB staff have conducted extensive studies of meteorological data to better 

understand the impact of transported emissions and pollutants on high ozone 

concentrations in Amador and Calaveras counties. These transport studies 

(ARB 1993 and ARB 1996) concluded that high ozone concentrations in both 

counties are overwhelmingly impacted by the transport of emissions and 

pollutants from the Sacramento Valley, the San Joaquin Valley, a combination of

the San Joaquin Valley and San Francisco Bay Area regions, or a combination of 

the Sacramento Valley, San Joaquin Valley, and San Francisco Bay Area

regions. The analyses showed that the contribution of local emissions generated 

within Amador and Calaveras counties was insignificant on high ozone days. 
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During the summer months, when ozone concentrations are highest, surface 

winds in Amador County are generally upslope (from the west southwest through 

west) during the daytime hours, and winds are generally downslope (from the 

east northeast through east) during the nighttime hours. The upslope airflow is 

the result of daytime heating of the mountain slopes, while the downslope airflow 

is the result of nighttime cooling of the mountain slopes. The airflow patterns 

generally follow the orientation of the mountain valleys and facilitate the transport 

of pollutants and emissions from upwind urban areas into the MCAB, where they 

impact ozone air quality in Amador (and Calaveras) County.

ARB staff reviewed EPA’s Central Mountain Counties Wind Direction Frequency 

diagram. Although the upslope/downslope flow is the primary airflow pattern 

during the summer months, the wind frequency diagram shows that the surface 

winds can blow from Amador County towards Calaveras County (from the west 

northwest and north northwest) about 18% percent of the time. However, 

because ozone concentrations and precursor emissions in Amador County are 

so much lower than concentrations in the upwind areas of Sacramento and the 

San Joaquin Valley, Amador County’s contribution to high concentrations in 

Calaveras County is insignificant (refer to discussion of emissions, below).

Air Quality Data: 

There is one ozone monitor in Amador County (Jackson-Clinton Road), and one 

monitor in Calaveras County (San Andreas-Gold Strike Road; refer to Figure 1). 

Both monitors are located in the more populated western portions of the

counties, where ozone exposure is highest. Although there is only one monitor in 

each county, U.S. EPA has approved both the number and the location of 

monitors as part of California’s network monitoring plan (most recently in the 

2011 Annual Network Report for Small Districts in California). One of the main 

functions identified for both sites is demonstrating compliance with State 

Implementation Plan (SIP) requirements. 

Table 1 shows how the design values for each county compare. It is important to 

note that design values for two different years will be used for designating the 

two counties – 2011 for Amador County and 2010 for Calaveras County. This is 

because using the 2011 design value makes a difference in designation status 

for Amador County, but not for Calaveras County. The 2011 design value for 

Amador County is 0.071 ppm at Jackson. It is based on complete data for 2009 

through 2011 and is 5 percent below the level of the current federal 8-hour 

standard. 
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In contrast, the 2010 design value used for designation purposes in Calaveras 

County is 0.083 ppm at the San Andreas site. This design value is based on 

complete data for 2008 through 2010, and it does exceed the federal standard. 

However, data indicate a 2011 design value of 0.077 ppm. This design value is 

substantially lower than the 2010 design value, but it is still about 3 percent 

above the level of the standard. 

TABLE 1

8-HOUR OZONE DESIGN VALUES FOR 

AMADOR COUNTY AND CALAVERAS COUNTY

COUNTY DESIGN SITE

DESIGN 

VALUE 

(ppm)

PERCENT 

FROM 

STANDARD

Amador County (2008-2010) Jackson 0.081 +8%

Calaveras County (2008-2010) San Andreas 0.083 +11%

Amador County (2009-2011) Jackson 0.071 -5%

Calaveras County (2009-2011) San Andreas 0.077 +3%

Design value trends for Amador (Jackson site) and Calaveras (San Andreas site) 

counties are shown in Figure 2. Although both sites are located at similar 

elevations (Jackson at about 1200 feet and San Andreas at about 1000 feet), the 

design value for San Andreas is generally higher than that for Jackson. 

However, design values for both sites have decreased substantially since 2000. 

Overall, the design value at Jackson decreased 28 percent since 2000. 

The increase in 2008 reflects the substantial impact of wildfire emissions on 

ozone concentrations during 2008. Furthermore, because the design value 

calculation is based on three years of data, the 2008 fire-impacted data also 

impact the design values for 2009 and 2010. Thus, design values for 2008, 

2009, and 2010 are all higher than would normally be expected. If the 2008 

fire-impacted days were removed, there would be a more consistent downward 

trend. Because the Jackson ozone data show a relatively consistent decrease 

over time, despite the influence of the 2008 wildfires, Amador County is expected 

to continue to attain the federal standard. 
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FIGURE 2

The 3-year average trend in days above the federal 8-hour standard

(exceedance days), shown in Figure 3, is similar to the design value trend and

further supports continued attainment in Amador County. Similar to Figure 2, 

Figure 3 shows that the number of exceedance days in Calaveras County is

consistently higher than the number in Amador County. However, both counties 

show significant progress over the years. 

The number of exceedance days at Jackson shows a substantial decrease since 

2000, with an overall 95 percent decrease. Similar to the design value trend, the 

exceedance day trend shows the same increase in 2008 through 2010, because 

the 3-year average exceedance day count for these years includes the 2008 

wildfire-impacted data. If the 2008 wildfire-impacted days were not included, 

there would be a more consistent downward trend. The 2011 value better 

reflects current conditions. 

The form of the federal ozone standard allows several exceedance days each 

year in attainment areas. During the last three years, the number of exceedance 

days at Jackson were very limited, with 1 exceedance day in 2009, 

3 exceedance days in 2010, and 2 exceedance days in 2011. Based on the 

trend, Amador County should continue to attain the federal standard.
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FIGURE 3

Population Density and Degree of Urbanization; Growth Rates and 

Patterns; and Traffic and Commuting Patterns:

Compared with the neighboring upwind regions, Amador and Calaveras counties 

are rural in nature, and the rugged mountain terrain has impacted the way the 

counties have grown. Using ARB 2010 population estimates based on census

data, the population of Amador County was 37,909, and the population of 

Calaveras County was 45,258. Together, the two counties are home to less than 

one percent of the total State population. Table 2 provides some summary 

information, including population density and growth rates over the last decade.

TABLE 2

POPULATION IN AMADOR AND CALAVERAS COUNTIES

COUNTY AREA

(Sq. mi.)

2010 

POPULATION

GROWTH

(2000-2010)

PEOPLE PER

SQUARE MILE

Amador 605 37,909 + 8% 63

Calaveras 1,037 45,258 + 11 % 44
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Although Amador County has more people per square mile, it has a lower overall 

growth rate when compared with Calaveras County. In both counties, most of 

the population is concentrated in just a few towns, including Ione, Jackson, Sutter 

Creek, Buckhorn, and Pine Grove in Amador County and Rancho Calaveras, 

Arnold, Angels Camp, Copperopolis, Valley Springs, and San Andreas in 

Calaveras County. These towns are primarily located in the western lower 

elevation portions of the counties. None of these towns are major population 

centers, when compared with the upwind urban areas in the Sacramento, 

San Joaquin Valley and San Francisco Bay Area regions. Thus, their overall 

contribution to local ozone air quality is minimal. 

California has a number of rural counties that are similar in population to Amador 

and Calaveras counties, including Humboldt, Lake, and Siskiyou counties. 

Although their population numbers are similar, their ozone design values have 

been well below the level of the federal standard for a number of years. This is 

primarily due to the fact that these other rural counties are not located upwind of 

major urban areas. This suggests that the population density and emission 

levels in Amador and Calaveras counties are insufficient on their own to cause 

exceedances of the federal ozone standard.

Emissions Data and Level of Emission Controls

Ozone forms when reactive organic gas (ROG) and oxides of nitrogen (NOx) 

precursor emissions react in the presence of sunlight. Figure 4 shows the 

precursor emission trends for Amador and Calaveras counties. Both precursors 

show a relatively consistent decrease over the last two decades. Overall, ROG 

emissions in Amador County decreased about 40 percent, while NOx emissions 

decreased about 30 percent from 1990 to 2010. ROG and NOx emissions also 

decreased in Calaveras County, a more than 30 percent decrease in each

between 1990 and 2010. ROG and NOx emissions are projected to continue 

decreasing in both counties during the next decade. 

Although local emissions continue to decrease, previous ARB staff analyses 

indicate that local precursor emissions are not the primary determinate of high 

ozone concentrations in Amador and Calaveras counties. As discussed 

previously, ARB staff has conducted a comprehensive evaluation of the impact of

emissions and pollutants transported from the Sacramento, San Joaquin, and 

San Francisco Bay Area urban regions (all located to the west and upwind) on 

high ozone concentrations in Amador and Calaveras counties (ARB 1993 and 

ARB 1996). The analyses concluded that high ozone concentrations in Amador 

County are primarily caused by emissions and pollutants from the upwind urban 

areas (the same is true for Calaveras County). Although there is a relatively 

large stationary source located near Jackson that adds to the total Amador 

County emissions, the amount of ozone precursor emissions from this source are 

significantly less than emissions from sources in the upwind regions. 

Furthermore, because of the prevailing local airflow patterns, the impact of 
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emissions from this source on ozone concentrations in Calaveras County is not 

substantial. 

FIGURE 4

The trends in ozone precursor emissions for the three counties located directly 

upwind and adjacent to Amador County are shown in Figure 5. It is clear from 

comparing Figures 4 and 5 that the combined ozone precursor emissions in the 

upwind urban counties are far greater than emissions in Amador County (or in 

Calaveras County). In fact, estimated 2010 ROG emissions in the three upwind 

counties were more than 20 times greater than ROG emissions in Amador 

County, while upwind NOx emissions were nearly 40 times greater. 

Amador County APCD has adopted rules to reduce emissions from sources 

under its jurisdiction. Emission reduction measures adopted by the upwind 

regions as part of the SIP will further improve ozone air quality in Amador 

County. Under California law, upwind areas are responsible for controlling 

transported emissions that contribute to air quality violations in downwind areas. 

ARB has primary responsibility under State law for evaluating the magnitude of 

transport and ensuring the adoption and implementation of appropriate mitigation 

measures. The combination of ARB’s statewide measures, SIP emission 

controls in upwind areas, and local control measures will ensure continued 

attainment in Amador County, as well as future attainment in Calaveras County. 
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FIGURE 5

Summary and Recommendation:

U.S. EPA proposes designating Amador and Calaveras counties together as the 

Central Mountain Counties federal ozone nonattainment area. Based on recent 

air quality data, Amador County attains the federal 8-hour ozone standard of 

0.075 ppm, while Calaveras County does not. Including both Amador County

and Calaveras County in the Central Mountain Counties nonattainment area will 

have no impact on attainment in Calaveras County, where high ozone 

concentrations are dominated by the transport of ozone and ozone precursor 

emissions from the upwind Sacramento, San Joaquin, and San Francisco 

Bay Area regions. Thus, ARB recommends Amador County be designated as a 

separate attainment area for ozone, based on the following information:

• Amador County and Calaveras County fall under the jurisdiction of two 

separate air pollution control agencies. For planning purposes, it is most 

efficient to have the nonattainment boundary coincide with the 

jurisdictional boundary. 

• Air quality data show a consistent decrease in 8-hour ozone 

concentrations in Amador County since 2000. The apparent increase 

during 2008 through 2010 reflects the impact of 2008 wildfire emissions, 

rather than deteriorating air quality.
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• The 2011 design value for Amador County is 0.071 ppm, which meets the 

federal standard. Given the historic trend, the design value is expected to 

continue decreasing, and Amador County will continue to attain the federal 

standard.

• Ozone precursor emissions in Amador County are projected to continue 

decreasing over the next decade.

• The level of ozone precursor emissions generated in Amador County is

dwarfed by the level of emissions in the adjacent upwind counties 

(Sacramento, San Joaquin, and Stanislaus counties). 

• High ozone concentrations in both Amador and Calaveras counties are 

overwhelmed by ozone and ozone precursor emissions transported from 

the Sacramento, San Joaquin, and San Francisco Bay Area regions. The 

contribution of local emissions is insignificant on such days.

• The primary attainment strategy for rural areas such as Amador and 

Calaveras counties relies on statewide controls and control measures 

implemented by upwind districts. However, the local APCDs will continue 

to adopt and enforce rules to reduce emissions from sources under their 

jurisdiction. 
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Southern Mountain Counties: Tuolumne County and Mariposa County

U.S. EPA recommends that Tuolumne and Mariposa counties be designated 

together as the Southern Mountain Counties federal ozone nonattainment area. 

Based on ozone data collected during 2008 through 2010, the design values for 

both areas exceed the federal standard (2010 Tuolumne County design value is 

0.082 ppm and 2010 Mariposa County design value is 0.083 ppm). However, 

these design values are higher than what would be expected normally, because 

they include data from 2008, when ozone concentrations were adversely 

impacted by wildfire emissions (refer to air quality discussion, below).

The 2011 design values for both counties (based on complete data for 2009 

through 2011) are much lower. They do not include the 2008 data, and 

therefore, reflect more normal conditions. The 2011 design value for Mariposa 

County is 0.077 ppm, which is close to the standard, but is still above the level of 

the standard. In contrast, the 2011 design value for Tuolumne County is

0.074 ppm, which does not exceed the federal standard. The following 

paragraphs provide a five factor analysis to justify designating Tuolumne County 

as a separate attainment area.

Jurisdictional Boundaries:

Tuolumne and Mariposa counties are located in the MCAB; however, they fall 

under the jurisdiction of two separate agencies. Tuolumne County is under the 

jurisdiction of the Tuolumne County APCD, while Mariposa County is under the 

jurisdiction of the Mariposa County APCD. Air quality in each county is managed 

at the local level through land use and development planning practices, and the 

local APCD is responsible for establishing and enforcing local air quality rules 

and regulations that address the requirements of federal and State air quality 

laws. With respect to nonattainment planning, it is most efficient to have the 

nonattainment boundary coincide with the jurisdictional boundary of the area(s) 

that experience or contribute to violations of the standard. Because Tuolumne 

County attains the standard and does not significantly contribute to violations in 

Mariposa County, it is not appropriate to include Tuolumne County in the 

nonattainment area. 

Geography / Topography:

The MCAB is located in the eastern half of California, comprising a 12,226 

square mile area. Tuolumne and Mariposa counties are located in the southern

portion of the MCAB, with Tuolumne County to the north and Mariposa County to 

the south (refer to Figure 6). 

Tuolumne County covers an area of 2,235 square miles. Mariposa County is 

smaller, encompassing an area of approximately 1,451 square miles. Both 
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Counties include significant portions of Yosemite National Park, and the Sierra 

Nevada Mountain range comprises a substantial portion of both Tuolumne and 

Mariposa counties. Elevations range from around 200 feet in the western foothill 

areas to over 12,000 feet in the mountainous eastern areas. The topography of 

both rural counties is characterized by a series of valleys, generally oriented 

east-northeast to west-southwest, separated by ridges. These topographic 

features channel airflow in a general east-west direction – uphill from the west 

during the daytime and downhill from the east at night. The physical 

characteristics impact local air quality. 

FIGURE 6

TUOLUMNE AND MARIPOSA COUNTIES

Meteorology:

ARB staff have conducted extensive studies of meteorological data to better 

understand the impact of transported emissions and pollutants on high ozone 
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concentrations in Tuolumne and Mariposa counties. These transport studies 

(ARB 1993 and ARB 1996) concluded that high ozone concentrations in both 

counties are overwhelmingly impacted by the transport of emissions and 

pollutants from upwind areas, particularly from the San Joaquin Valley. The 

analyses showed that the contribution of local emissions generated within 

Tuolumne and Mariposa counties was insignificant on high ozone days.

During the summer months, when ozone concentrations are highest, surface 

winds in Tuolumne County are generally upslope during the daytime hours and 

downslope during the nighttime hours. These winds generally follow the 

orientation of the mountain valleys and facilitate the transport of pollutants and 

emissions from the upwind urban areas into Tuolumne (and Mariposa) County,

where they impact ozone air quality. 

ARB staff reviewed EPA’s Southern Mountain Counties Wind Direction 

Frequency diagram. Although the upslope/downslope flow is the primary airflow 

pattern during the summer months, the wind flow frequency diagram shows that 

surface winds can blow from Tuolumne County towards Mariposa County (from 

the north northwest) about 15% of the time. However, because ozone 

concentrations and precursor emissions in Tuolumne County are so much lower 

than concentrations and precursor emissions in the upwind San Joaquin Valley, 

Tuolumne County’s contribution to high concentrations in Mariposa County is 

insignificant (refer to discussion of emissions, below). 

Air Quality Data: 

Currently, there is one ozone monitor in Tuolumne County (Sonora-Barretta 

Street) and two monitors in Mariposa County (Jerseydale-6440 Jerseydale and 

Yosemite-Turtleback Dome; refer to Figure 6). The Sonora monitor is located in

the western portion of Tuolumne County at an elevation of 1,800 feet. Because 

of its location in the town of Sonora, measured ozone concentrations are a good 

indicator of population exposure. Furthermore, the Sonora monitor has been 

identified as appropriate for demonstrating compliance with SIP requirements. 

In contrast to Sonora, the elevation of the Jerseydale (3,723 feet) and Yosemite 

(5,248 feet) monitors is higher. The Jerseydale monitor is operated by ARB staff

as a high concentration site for determining State Designations and assessment 

of transport impacts. In contrast, the Yosemite monitor is operated by the 

National Parks Service and was sited to meet their specific goals. Although the 

number of monitors in Tuolumne and Mariposa counties is limited, U.S. EPA has 

approved both the number and location of the monitors as part of California’s 

network monitoring plan (most recently in the 2011 Annual Network Report for 

Small Districts in California.) 
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Table 3 shows how the design values for sites in Tuolumne and Mariposa County

compare. It is important to note that design values for two different years will be 

used for designating the two counties – 2011 for Tuolumne County and 2010 for 

Mariposa County. This is because using the 2011 design value makes a 

difference in designation status for Tuolumne County, but not for Mariposa 

County. The 2011 design value for Tuolumne County is 0.074 ppm at Sonora. It 

is based on complete data for 2009 through 2011and is 1 percent below the level 

of the current federal 8-hour standard. 

In contrast, the 2010 design value used for designating Mariposa County is 

0.083 ppm at the Yosemite site. This design value is based on complete data for 

2008 through 2010 data, and it does exceed the federal standard. However, 

data indicate a 2011 design value of 0.077 ppm. This design value is 

substantially lower than the 2010 design value, but it is still about 3 percent 

above the level of the standard. The 2011 design value for Jerseydale is 

comparable to the 2011 design value for Yosemite, with a design value of 

0.076 ppm.

TABLE 3

8-HOUR OZONE DESIGN VALUES FOR 

AMADOR COUNTY AND CALAVERAS COUNTY

COUNTY DESIGN SITE

DESIGN 

VALUE 

(ppm)

PERCENT 

FROM 

STANDARD

Tuolumne County (2008-2010) Sonora 0.082 +9%

Mariposa County (2008-2010)

Yosemite 0.083 +11%

Jerseydale 0.080 +7%

Tuolumne County (2009-2011) Sonora 0.074 -1%

Mariposa County (2009-2011)

Yosemite 0.077 +3%

Jerseydale 0.076 +1%

Design value trends for Tuolumne County (Sonora site) and for the design site in 

Mariposa County (Yosemite site) are graphed in Figure 7. It is apparent from the 

graph that the design value for Yosemite is generally about the same or higher 

than for design values for Sonora. Although the trends are variable, design 

values for both counties show an overall decrease close to 15 percent since the 

early 2000s. 
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FIGURE 7

The increase in 2008 reflects the substantial impact of wildfire emissions on 

ozone concentrations during 2008. Furthermore, because the design value 

calculation is based on three years of data, the 2008 fire-impacted data also 

impact the design values for 2009 and 2010. Thus, design values for 2008, 

2009, and 2010 are all higher than would normally be expected. If the 2008 

fire-impacted days were removed, there would be a more consistent downward 

trend. Based on the long-term trend, Tuolumne County is expected to continue 

to attain the federal standard. 

The 3-year average trend in days above the federal 8-hour standard 

(exceedance days), shown in Figure 8, further supports continued attainment in 

Tuolumne County. Over the last decade, the number of exceedance days at 

Yosemite is consistently higher than the number of days at Sonora. This is likely 

attributable to the high-elevation transport impact at the Yosemite site. However, 

because of the isloated location of the monitor, a higher number of days does not 

translate into increased exposure. 

As seen in Figure 8, both monitors show significant overall improvement, 

especially since the mid-2000s. The number of exceedance days at Sonora has 

decreased more than 90 percent overall since 2003. Similar to the design value 

trend, the exceedance day trend shows the same increase during 2008 through 

2010, because the 3-year average exceedance day count for these years reflects 
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the impact of wildfire emissions on the 2008 data. If the 2008 wildfire-impacted 

days were not included, there would be a more consistent downward trend. The 

2011 value better reflects current conditions. 

FIGURE 8

The form of the federal ozone standard allows several exceedances per year in 

attainment areas. During the last three years, the number of exceedance days at 

Sonora were limited, with 5 exceedance days in 2009, 3 exceedance days in 

2010, and only 1 exceedance day in 2011. Based on the trend, Tuolumne 

County should continue to attain the federal standard. 
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Population Density and Degree of Urbanization; Growth Rates and 

Patterns; and Traffic and Commuting Patterns:

Compared with the neighboring upwind region, Tuolumne and Mariposa counties 

are rural in nature, and the rugged mountain terrain and National Park area have 

both impacted the way the counties have grown. Using ARB 2010 population 

estimates based on census data, the population of Tuolumne County was 

54,961, and the population of Mariposa County was 18,119. Together, the two 

counties are home to less than 1 percent of the total State population. Table 4

provides some summary information, including population density and growth 

rates over the last decade.

TABLE 4

POPULATION IN AMADOR AND CALAVERAS COUNTIES

COUNTY AREA

(Sq. mi.)

2010 

POPULATION

GROWTH

(2000-2010)

PEOPLE PER 

SQUARE MILE

Tuolumne 2,235 54,961 + 1% 25

Mariposa 1,451 18,119 + 6 % 8

Although Tuolumne County has more people per square mile, it has a much 

lower overall growth rate when compared with Mariposa County. In both 

counties, most of the population is concentrated in just a few towns, including 

Sonora, Phoenix Lake, Jamestown, and Mono Vista in Tuolumne County and 

Mariposa, Midpines, Lake Don Pedro, and Yosemite Valley in Mariposa County. 

None of these towns are major population centers, when compared with the 

upwind urban areas in the San Joaquin Valley. Thus, their overall contribution to 

local ozone air quality is minimal. 

California has a number of rural counties that are similar in population to Amador 

and Calaveras counties, including Humboldt, Lake, and Siskiyou counties. 

Although their population numbers are similar, their ozone design values have 

been well below the level of the federal standard for a number of years. This is 

primarily due to the fact that these other rural counties are not located upwind of 

major urban areas. This suggests that the population density and emission 

levels in Tuolumne and Mariposa counties are insufficient on their own to cause 

exceedances of the federal ozone standard. 

Emissions Data and Level of Emission Controls

As mentioned earlier, ozone forms when reactive organic gas (ROG) and oxides 

of nitrogen (NOx) precursor emissions react in the presence of sunlight. Figure 9

shows the precursor emission trends for Tuolumne and Mariposa counties. Both 
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precursors show a decrease over the last two decades. Overall, ROG emissions 

in Tuolumne County decreased more than 20 percent, while NOx emissions 

decreased about 25 percent from 1990 to 2010. ROG and NOx emissions also 

decreased in Mariposa County, about 30 percent each between 1990 and 2010. 

ROG and NOx emissions are projected to decrease in both counties between 

now and 2020. 

Although local emissions continue to decrease, previous ARB staff analyses 

indicate that local precursor emissions are not the primary determinate of high 

ozone concentrations in Tuolumne and Mariposa counties. As discussed 

previously, ARB staff has conducted a comprehensive evaluation of the impact of 

emissions and pollutants transported from San Joaquin Valley on high ozone 

concentrations in Tuolumne and Mariposa counties (ARB 1993 and ARB 1996). 

The analyses concluded that high ozone concentrations in Tuolumne County are 

primarily caused by emissions and pollutants from the upwind San Joaquin 

Valley. The same is true for Mariposa County. 

FIGURE 9

The trends in ozone precursor emissions for the four San Joaquin Valley 

counties located upwind of Tuolumne and Mariposa counties are shown in 

Figure 10. It is clear from comparing Figures 9 and 10 that the combined ozone 

precursor emissions in the upwind areas are far greater than emissions in 

Tuolumne County (or in Mariposa County). In fact, estimated 2010 ROG 

emissions in the upwind counties are more than eight times greater than ROG 

emissions in Tuolumne County, while upwind NOx emissions are more than 
E1-20

20 times greater. Although the amount of ROG and NOx emissions in Tuolumne 

County are greater than emissions in Mariposa County, Tuolumne County 

emissions are significantly less than emissions from sources in the upwind 

San Joaquin Valley. Furthermore, because of prevailing local air flow patterns, 

Tuolumne County emissions do not substantially impact ozone concentrations in 

Mariposa County.

FIGURE 10

Tuolumne County APCD has adopted rules to reduce emissions from sources 

under its jurisdiction. Emission reduction measures adopted by the upwind 

region as part of the SIP will further improve ozone air quality in Tuolumne

County. Under California law, upwind areas are responsible for controlling 

transported emissions that contribute to air quality violations in downwind areas. 

ARB has primary responsibility under State law for evaluating the magnitude of 

transport and ensuring the adoption and implementation of appropriate mitigation 

measures. The combination of ARB’s statewide measures, SIP emission 

controls in upwind areas, and local control measures will ensure continued 

attainment in Tuolumne County, as well as future attainment in Mariposa County. 

Summary and Recommendation:

U.S. EPA proposes designating Tuolumne and Mariposa counties together as 

the Southern Mountain Counties federal ozone nonattainment area. Based on 
E1-21

recent air quality data, Tuolumne County attains the federal 8-hour ozone 

standard of 0.075 ppm, while Mariposa County does not. Including both 

Tuolumne County and Mariposa County in the Southern Mountain Counties 

nonattainment area will have no impact on attainment in Mariposa County, where 

high ozone concentrations are dominated by the transport of ozone and ozone 

precursor emissions from the upwind San Joaquin Valley region. Thus, ARB 

recommends Tuolumne County be designated as a separate attainment area for 

ozone, based on the following information:

• Tuolumne County and Mariposa County fall under the jurisdiction of two 

separate air pollution control agencies. For planning purposes, it is most 

efficient to have the nonattainment boundary coincide with the 

jurisdictional boundary.

• Air quality data show a consistent decrease in 8-hour ozone 

concentrations in Tuolumne County since the early 2000s. The apparent 

increase during 2008 through 2010 reflects the impact of 2008 wildfire 

emissions, rather than deteriorating air quality.

• The 2011 design value for Tuolumne County is 0.074 ppm, which meets 

the federal standard. Given the historic trend, the design value is 

expected to continue decreasing, and Tuolumne County will continue to 

attain the federal standard.

• Ozone precursor emissions in Tuolumne County are projected to 

decrease over the next decade.

• The level of ozone precursor emissions generated in Tuolumne County is

dwarfed by the level of emissions in the upwind counties located in the 

San Joaquin Valley. 

• High ozone concentrations in both Tuolumne and Mariposa counties are 

overwhelmed by ozone and ozone precursors transported from the 

San Joaquin Valley. The contribution of local emissions is insignificant on 

such days.

• The primary attainment strategy for rural areas such as Tuolumne and 

Mariposa counties relies on statewide controls and control measures 

implemented by upwind districts. However, the local APCDs will continue 

to adopt and enforce rules to reduce emissions from sources under their 

jurisdiction. 
E1-22

References: 

ARB, 1993: Assessment and Mitigation of the Impacts on Ozone Concentrations 

in California. Staff Report: Initial Statement of Reasons prepared by Technical 

Support Division, June 1993.

ARB, 1996: Second Triennial Review of the Assessment of the Impacts of 

Transported Pollutants on Ozone Concentrations in California (Revised). Staff 

Report: Initial Statement of Reasons prepared by Technical Support Division, 

November 1996.
E2-1

ENCLOSURE 2

JUSTIFICATION OF SUTTER BUTTES 2011 DESIGN VALUE

Sutter Buttes is a seasonal site in the upper Sacramento Valley that operates during the 

May through October ozone season. Located at an elevation of about 2116 feet, Sutter 

Buttes was sited to monitor the impact of high elevation transport from urban areas. A 

map of the Sutter Buttes monitor location is provided in Figure 1, below. Other locations 

referenced in the following analyses are also included in Figure 1. 

FIGURE 1

MAP OF SUTTER BUTTES AND SURROUNDING AREA
E2-2

Data completeness for Sutter Buttes during 2009 through 2011 is summarized in 

Table 1, below. To be considered complete for regulatory purposes, monitoring data 

must meet the requirements specified in Appendix P to 40 Code of Federal Regulations 

(CFR) Part 50. Under Appendix P, data for each year must be at least 75 percent 

complete, with a three-year average completeness of at least 90 percent. When these 

minimum requirements are not met, meteorological or ambient data may be used to 

demonstrate that conditions on missing days were not conducive to concentrations 

above the standard. Missing days assumed to be less than the standard may be 

counted towards meeting the completeness requirements.

TABLE 1

SUTTER BUTTES DATA COMPLETENESS BY YEAR

YEAR NUMBER OF DAYS 

POSSIBLE

NUMBER OF DAYS 

WITH DATA

PERCENT 

COMPLETE

2009 184 172 93%

2010 184 183 99%

2011 184 140 76%

3-Year Average = 89%

Although the Sutter Buttes data meet the 75 percent completeness requirement for 

each year, the three-year average (89 percent) is just below the overall 90 percent

completeness requirement. A review of the data shows several incomplete days during 

2009 through 2011 that can be assumed to have maximum 8-hour concentrations below 

the standard. Although only one additional day is needed to satisfy the 90 percent 

criteria, the following evaluation focuses on two days, as shown in Table 2. While some 

data are available during each of these days, they are not considered complete 

because more than six hourly measurements are missing, and as a result, there are 

fewer than the required eighteen rolling 8-hour averages. 

TABLE 2

SUTTER BUTTES INCOMPLETE DAYS ASSUMED TO BE BELOW THE STANDARD

DATE MISSING 

HOURS MAX 8-HOUR MAX 8-HOUR DAY BEFORE

MAX 8-HOUR 

DAY AFTER

05/12/2009 00-13 0.057 ppm* Not available 0.041 ppm

06/22/2010 04; 09-12 0.053 ppm 0.050 ppm 0.058 ppm

∗ ppm = parts per million

As summarized in Table 2, the two incomplete candidate days occurred during the 

months of May and June. Typically, these months do not have the highest ozone 
E2-3

concentrations. As shown in Table 3, 90 percent of the high 8-hour ozone 

concentrations relevant to the design value calculation at Sutter Buttes occur during 

July, August, and September, compared with only 10 percent during May and June. 

TABLE 3

FREQUENCY OF THE TOP FOUR 8-HOUR CONCENTRATIONS BY MONTH*

2005 2006 2007 2009 2010 Percent by Month

May 0 0 1 0 0 5%

June 0 0 0 1 0 5%

July 3 2 1 1 0 35%

August 0 0 0 1 2 15%

September 1 2 2 1 2 40%

October 0 0 0 0 0 0%

∗ 2008 data are not included because concentrations were impacted by wildfire emissions; 2011 data are not 

included because August data are not complete.

The following sections provide justification that the maximum 8-hour concentrations on 

May 12, 2009, and June 22, 2010, were below the standard. Our conclusions are

based on the evaluation of several different air quality-related parameters, including the 

following:

• The typical timing of high ozone concentrations at Sutter Buttes, 

• Spatial differences in ozone concentrations on the candidate days,

• Spatial differences in ozone concentrations on the days preceding and following 

the candidate days, and

• The meteorological conditions on the preceding and the candidate days.

Based on these analyses, conditions on the candidate days were not conducive to high 

ozone concentrations. Therefore, it can be assumed that the maximum 8-hour average 

ozone concentration at Sutter Buttes on May 12, 2009, and on June 22, 2010, did not 

exceed the federal standard.

May 12, 2009

The maximum 8-hour concentration at Sutter Buttes on May 12, 2009, was 0.057 parts 

per million (ppm), which is below the level of the federal standard. Data for the day are 

not complete because hourly measurements are missing during the first half of the day 

(hours 00 through 13; refer to Table 2). Based on the available hourly data, twelve valid 

8-hour averages can be calculated, which is below the eighteen averages required for a 

complete day. The missing 8-hour averages have start hours of 00 through 11.

1

 1 Although hourly data are missing for hours 12 and 13 on May 12, 2009, the 8-hour averages for these 

start hours are valid because hourly measurements are available for at least six other hours during the 

8-hour period.
E2-4

Despite the missing data, it is highly unlikely the federal 8-hour ozone standard was 

exceeded at Sutter Buttes on May 12, 2009. A review of recent Sutter Buttes data 

shows the missing hours are not critical for determining the design value. Figure 2 

provides a summary of the frequency of high 8-hour ozone concentrations at Sutter 

Buttes by start hour. Based on data for the five years, 8-hour averages with start hours 

00 through 11 are not critical to the design value calculation, as the four highest ozone 

concentrations during a year rarely occur during these hours. 

FIGURE 2

FREQUENCY OF TOP FOUR 8-HOUR AVERAGES BY START HOUR

(based on data from 2005 - 2010*)

∗ 2008 data are not included because concentrations were impacted by wildfire emissions; 2011 data are not included 

because August data are incomplete.

An evaluation of the 1-hour measurements for May 12 indicates that despite the missing 

hours, the remaining data (hours 14 through 23) likely capture the peak 1-hour ozone 

concentration at Sutter Buttes. The hourly measurements show ozone concentrations 

slowly increasing from 0.057 ppm at hour 14, to a peak concentration of 0.060 ppm at 

hour 19. Following hour 19, concentrations decline. Thus, the available data appear to 

capture the highest hourly concentration at Sutter Buttes, and even this concentration is 

below the level of the 8-hour standard. Similarly, peak 1-hour ozone concentrations at 

0

1

2

3

4

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Frequency of Occurrence

Start Hour of Day
E2-5

all other northern California sites on May 12 were below the level of the 8-hour 

standard, reaching a high of only 0.067 ppm. 2

Since all 1-hour ozone concentrations were below the level of the 8-hour standard on 

May 12, 2009, it follows that all 8-hour concentrations were below the standard, as well. 

Indeed, the maximum 8-hour concentration in northern California on May 12 was 

0.064 ppm. This concentration was measured at three sites: Folsom-Natoma in the 

Sacramento Metro Area, Grass Valley-Litton Building in Western Nevada County, and 

Echo Summit in the Mountain Counties Air Basin. 

In addition to regional 8-hour concentrations below the standard on May 12, 

concentrations throughout northern California were below the 8-hour standard during

the three preceding days (May 9 through May 11, 2009), as well as the three following 

days (May 13 through May 15, 2009). At Sutter Buttes, there are no ozone data for the 

days prior to May 12, as monitoring had not yet started for the 2009 ozone season. 

However, the maximum 8-hour concentrations on the three following days were 

0.041 ppm (May 13), 0.038 ppm (May 14), and 0.055 ppm (May 15). These 

concentrations are all well below the standard.

In addition to air quality data, some meteorological data are available for May 12, 2009. 

Temperature data for Sutter Buttes show a maximum of 69° Fahrenheit on May 12, and 

daily maximum temperatures throughout northern California reached only

85° Fahrenheit. These maximums are below temperatures normally associated with 

federal ozone exceedance days. 

No wind data are available for Sutter Buttes on May 12, 2009. However, surface wind 

data are available for Tuscan Butte, which is located north of Sutter Buttes. Tuscan 

Butte is another rural high elevation (1876 feet) site that was established to study 

transport impacts. Wind data are also available for Elk Grove-Bruceville Road, a site in 

the Sacramento urban area, south of Sutter Buttes. The Elk Grove data comprise 

measurements at the surface, as well as aloft (2000 feet). 

Data for both Tuscan Butte and Elk Grove (2000 foot level) show strong northerly and 

westerly components during all hours on both May 11 and May 12. In addition, surface 

winds at Elk Grove were from the southwest during the late afternoon hours of May 11,

through the early morning hours of May 12. Around 8 a.m., these surface winds shifted, 

blowing from the northwest through the afternoon of May 12. The wind patterns, both at 

the surface and aloft, prevented emissions from the Sacramento urban area from 

impacting ozone concentrations at Sutter Buttes, thus keeping concentrations low 

during the missing hours. Furthermore, the southwesterly component of the surface 

winds at Elk Grove during May 11 and 12 supports the movement of emissions from the 

Sacramento urban area eastward, contributing to the higher ozone concentrations at 

Folsom, Grass Valley, and Echo Summit on May 12. 

 2 Northern California sites include all ozone monitoring sites from Merced County in the south to the Oregon 

border in the north.
E2-6

In summary, it can be assumed that the maximum 8-hour average ozone concentration 

at Sutter Buttes on May 12, 2009, was below the level of the federal standard, based on 

the following information:

• 8-hour ozone concentrations at Sutter Buttes during the missing hours 

on May 12, 2009, are not critical to the design value calculation.

• The increase in 1-hour concentrations at Sutter Buttes during the 

afternoon hours of May 12, 2009, suggests the available data capture 

the peak 1-hour concentration, and even this concentration is below 

the level of the 8-hour standard.

• No site in northern California had a maximum 8-hour average 

concentration exceeding the standard on May 12, 2009, and even the 

1-hour concentrations at the northern California sites were below the 

level of the federal 8-hour standard.

• The federal 8-hour standard was not exceeded at any site in northern 

California during the three days prior to May 12, 2009, or during the 

three days after May 12, 2009.

• Maximum ambient temperatures throughout northern California on 

May 12, 2009, including temperatures at Sutter Buttes, were lower 

than those generally associated with ozone exceedances. 

• Winds on May 12, 2009, both at the surface and aloft, were not 

consistent with the transport of ozone and ozone precursor emissions 

from the urban area to Sutter Buttes, where they could impact ozone 

concentrations.

June 22, 2010

The maximum 8-hour concentration at Sutter Buttes on June 22, 2010, was 0.053 ppm, 

which is below the level of the federal standard. Data for the day are not complete 

because five hourly measurements are missing during the first half of the day (refer to 

Table 2). Based on the hourly data that are available, sixteen valid 8-hour averages 

can be calculated, which is just below the eighteen averages required for a complete 

day. The missing averages have start hours of 03 through 10.3

 A review of recent 

Sutter Buttes data shows these hours are not critical for determining design value 

because the four highest 8-hour averages during a year do not occur during these hours 

(refer to Figure 2). 

 3 Although hourly data are missing for hour 04 and hours 09 through 12, the 8-hour averages for some start hours 

within this range are valid because hourly measurements are available for at least six other hours during the 

8-hour period.
E2-7

Despite the missing hours, it is highly unlikely that ozone concentrations at Sutter Buttes 

exceeded the federal 8-hour standard on June 22, 2010. In fact, an evaluation of daily 

maximum 8-hour concentrations shows the 8-hour standard was not exceeded 

anywhere in northern California on June 22. The maximum 8-hour average was 

0.074 ppm, measured at the Folsom-Natoma site in the Sacramento Metro Area. 

Ozone concentrations at Folsom-Natoma are typically higher than concentrations at 

Sutter Buttes. 

In addition to regional 8-hour concentrations below the standard on June 22, 

concentrations throughout northern California (including Sutter Buttes) were below the 

8-hour standard during the three preceding days (June 19 through June 21, 2010), 

ranging from less than 0.010 ppm to 0.062 ppm. During the three following days 

(June 23 through June 25, 2010), a single 8-hour exceedance was measured at the 

White Cloud Mountain site in Nevada County on June 23. At a level of 0.076 ppm, this 

maximum was just above the federal 8-hour standard. Concentrations at all other 

northern California sites ranged from less than 0.010 ppm to 0.075 ppm. The 8-hour 

averages at Sutter Buttes during these days were all well below the standard: 

0.058 ppm (June 23), 0.042 ppm (June 24), and 0.045 ppm (June 25). 

In addition to air quality data, wind data are available for both Tuscan Butte and 

Elk Grove. Surface winds at Elk Grove, in the Sacramento area, were from the south 

and southwest during the evening hours on June 21 and into the early morning hours on 

June 22. Later in the morning, winds were very light and variable, changing to a 

stronger, westerly wind in the afternoon. In contrast, winds measured at Tuscan Butte, 

located north of Sutter Buttes and at a similar elevation, were from the north to north 

northwest on both days. Wind speeds at Tuscan Butte were about 8 miles per hour on 

June 21, strengthening to 15 miles per hour on June 22. At Elk Grove, winds aloft 

(2000 foot level) were westerly during the afternoon of June 21, becoming light and 

variable during the early morning through afternoon of June 22. Given the relatively 

light southwest to westerly winds both at the surface and aloft at Elk Grove, combined 

with the stronger northerly component at Tuscan Butte, it is unlikely that emissions from 

the Sacramento urban area had an impact on ozone concentrations at Sutter Buttes on 

June 22. Therefore, it is unlikely that concentrations during the missing hours of 

June 22 would have been high enough to cause an exceedance of the federal 8-hour 

standard. 

In summary, it can be assumed that the maximum 8-hour average ozone concentration 

at Sutter Buttes on June 22, 2010, was below the level of the federal standard, based 

on the following information:

• Concentrations during the missing hours on June 22, 2010, are not 

critical to the design value calculation.

• No site in northern California had a maximum 8-hour average 

concentration exceeding the standard on June 22, 2010.
E2-8

• The federal 8-hour standard was not exceeded at any site in northern 

California during the three days prior to June 22, 2010, and only one 

exceedance was recorded during the three days after June 22, 2010.

• Winds on June 22, 2010, both at the surface and aloft, were not 

consistent with the transport of ozone and ozone precursor emissions 

from the urban area to Sutter Buttes, where they could impact ozone 

concentrations.

2011 Sutter Buttes 8-Hour Ozone Design Value

As shown in Table 4, assuming the maximum 8-hour average ozone concentrations on 

May 12, 2009, and June 22, 2010, were below the level of the federal standard brings 

the average completeness for the 2009 through 2011 three-year period to 90 percent. 

This meets the completeness requirements specified in Appendix P to 40 CFR Part 50.

TABLE 4

SUTTER BUTTES DATA COMPLETENESS BY YEAR WITH ADDITIONAL DAYS

YEAR NUMBER OF DAYS 

POSSIBLE

NUMBER OF DAYS 

WITH DATA

PERCENT 

COMPLETE

2009 184 173 94%

2010 184 183 100%

2011 184 140 76%

3-Year Average = 90%

The fourth high values used in the design value calculation are shown in Table 5. The 

three-year average of these gives Sutter Buttes a 2011 design value of 0.071 ppm, 

which is below the level of the federal 8-hour ozone standard. 

TABLE 5

FOURTH HIGHEST 8-HOUR AVERAGE OZONE CONCENTRATIONS

AND 2011 DESIGN VALUE FOR SUTTER BUTTES

YEAR 4th HIGH 8-HOUR CONCENTRATION

2009 0.072 ppm

2010 0.068 ppm

2011 0.075 ppm

2011 Design Value: 0.071 ppm

Because concentrations at Sutter Buttes during May and June generally are not as high 

as those in July, August, and September, ozone concentrations on May 12, 2009, and 
E2-9

June 22, 2010, do not impact the design value calculation. Furthermore, for either 

day to make a difference in the 2011 design value, May 12, 2009, would need a 

maximum 8-hour concentration of at least 0.075 ppm, and June 22, 2010, would 

need a maximum 8-hour concentration of at least 0.073 ppm. Even if both days had 

8-hour concentrations above these threshold levels, the 2011 design value would be 

0.074 ppm, which would still be below the level of the federal 8-hour ozone standard.