1

Attachment 1

CALIFORNIA

Area Designations For the 

24-Hour Fine Particle National Ambient Air Quality Standard

Note: As a basic introduction, the following is a summary table listing ALL areas and counties 

proposed for nonattainment in the State. The technical analyses for each individual area then follow. 

The table below identifies the counties in California that EPA has designated as not attaining the 2006 

24-hour fine particle (PM2.5) standard.1 A county (or part thereof) is designated as nonattainment if it 

has an air quality monitor that is violating the standard or if the county is determined to be contributing 

to the violation of the standard. 

1 EPA designated nonattainment areas for the 1997 fine particle standards in 2005. In 2006, the 24-

hour PM2.5 standard was revised from 65 micrograms per cubic meter (average of 98th percentile 

values for 3 consecutive years) to 35 micrograms per cubic meter; the level of the annual standard for 

PM2.5 remained unchanged at 15 micrograms per cubic meter (average of annual averages for 3 

consecutive years). 
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Area 

California Recommended 

Nonattainment Counties

EPA’s Final Designated

Nonattainment Counties

Butte County Butte County - Partial Butte County - Partial

Imperial County Imperial County - Partial Imperial County- Partial

Sacramento County Sacramento Sacramento County

Yolo County - Partial

Placer County – Partial

El Dorado County – Partial

Solano County - Partial

San Francisco Bay Area Sonoma County – Partial

Napa County

Marin County

San Francisco County

Contra Costa County

Alameda County

Santa Clara County

San Mateo County

Solano County - Partial

Sonoma County – Partial

Napa County

Marin County

San Francisco County

Contra Costa County

Alameda County

Santa Clara County

San Mateo County

Solano County - Partial

San Joaquin Valley Air 

Basin 

San Joaquin County

Stanislaus County

Merced County

Madera County

Fresno County

Kings County

Tulare County

Kern County - Partial

San Joaquin County

Stanislaus County

Merced County

Madera County

Fresno County

Kings County

Tulare County

Kern County - Partial

South Coast Air Basin Los Angeles County –

Partial

San Bernardino County

Partial

Riverside County – Partial

Orange County

Los Angeles County –

Partial

San Bernardino County

Partial

Riverside County – Partial

Orange County

Yuba County

Sutter County

Yuba County – Partial

Sutter County - Partial

Yuba County – Partial

Sutter County

EPA has designated the remaining counties in the state as “attainment/unclassifiable.” EPA designated 

a county as “unclassifiable” when: one or more of its monitors recorded a violation in 2004-2006; all 

monitors in the county with complete 2005-2007 data showed attainment; and one or more other 

monitors in the county had 2005-2007 monitoring data that was not complete and could not be used for 

determining compliance with the standard.
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EPA Technical Analysis for Chico (Butte County

Introduction

Pursuant to section 107(d) of the Clean Air Act, EPA must designate as nonattainment those areas that 

violate the NAAQS and those nearby areas that contribute to violations. This technical analysis for 

Butte identifies the counties with monitors that violate the 24-hour PM2.5 standard and evaluates 

nearby counties for contributions to fine particle concentrations in the area. EPA has evaluated these 

counties based on the weight of evidence of the following nine factors recommended in EPA guidance 

and any other relevant information: 

- pollutant emissions

- air quality data

- population density and degree of urbanization

- traffic and commuting patterns

- growth

- meteorology

- geography and topography

- jurisdictional boundaries

- level of control of emissions sources

Figure 1 is a map of the counties in the area and other relevant information such as the locations and 

design values of air quality monitors, the metropolitan area boundary, and counties recommended as 

nonattainment by the State. Figure 2 shows the designated PM 2.5 nonattainment area within Butte 

County.

Figure 1

Figure 1.
4

Figure 2
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The California Air Resources Board (CARB), sent a letter to EPA, dated December 17, 2007 

recommending that the City of Chico in Butte County be designated as “nonattainment” for the 2006 

24-hour PM2.5 standard based on air quality data from 2004-2006. These data are from Federal 

Reference Method (FRM) monitors located in the state.

In August 2008, EPA notified California of its intended designations. In this letter, EPA also 

requested that if California wished to provide comments on EPA’s intended designation, it should do 

so by October 20, 2008. EPA stated that it would consider any additional information (e.g., on power 

plants or partial county areas) provided by the state in making final decisions on the designations. 

Air quality monitoring data on the composition of fine particle mass are available from the EPA 

Chemical Speciation Network and the IMPROVE monitoring network, as well as from the Chico 

monitoring site. Analysis of these data indicates that the days with the highest fine particle 

concentrations occur predominantly in the cold season, and the average chemical composition of the 

highest days is characterized by high levels of organic carbon (e.g., 75%). 

Based on EPA's technical analysis described below and currently available information, EPA has 

designated a portion of Butte County as nonattainment for the 24-hour PM2.5 air-quality standard.

Butte County Area 

California Recommended 

Nonattainment Counties

EPA’s Final Designated 

Nonattainment Counties

Butte County Butte County (partial) Butte County (partial)

The following is a technical analysis for Butte County, California.

Factor 1: Emissions data

For this factor, EPA evaluated county level emission data for the following PM2.5 components and 

precursor pollutants: “PM2.5 emissions total,” “PM2.5 emissions carbon,” “PM2.5 emissions other,” 

“SO2,” “NOx,” “VOCs,” and “NH3.” “PM2.5 emissions total” represents direct emissions of PM2.5 and 

includes: “PM2.5 emissions carbon,” “PM2.5 emissions other”, primary sulfate (SO4), and primary

nitrate. (Although primary sulfate and primary nitrate, which are emitted directly from stacks rather 

than forming in atmospheric reactions with SO2 and NOx, are part of “PM2.5 emissions total,” they are 

not shown on the template or data spreadsheet as separate items). “PM2.5 emissions carbon” represents 

the sum of organic carbon (OC) and elemental carbon (EC) emissions, and “PM2.5 emissions other” 

represents other inorganic particles (crustal). Emissions of SO2 and NOx, which are precursors of the 

secondary PM2.5 components sulfate and nitrate, are also considered. VOCs (volatile organic 

compounds) and NH3 (ammonia) are also potential PM2.5 precursors and are included for 

consideration. 

Emissions data were derived from the 2005 National Emissions Inventory (NEI), version 1. See 

http://www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html.

EPA also considered the Contributing Emissions Score (CES) for each county. The CES is a metric 

that takes into consideration emissions data, meteorological data, and air quality monitoring 
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information to provide a relative ranking of counties in and near an area. Note that this metric is not 

the exclusive manner for considering data for these factors. A summary of the CES is included in 

Attachment 2, and a more detailed description can be found at 

http://www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html#C.

Table 1 shows emissions of PM2.5 and precursor pollutants components (given in tons per year) and the 

CES for violating and potentially contributing counties in the Butte County. Counties are listed in 

descending order by CES.

Table 1. PM 2.5 24-hour Component Emissions, and CES.

County State

Reco

mmen

ded 

Nonat

tainm

ent?

CES PM2.5

total

SOx NOx Carbon

PM2.5

PM2.5

other

VOCs NH3

Butte Yes 

(P)

100 2,974 2,115 8,486 1,513 1,461 9,754 1,757

Tehama No 19 1,443 2,087 3,936 823 620 4,150 782

Glenn No 14 1,851 1,347 3,882 833 1,017 4,392 2,139

P = partial

Additional data considered in EPA’s analysis of this factor are summarized in the following table 

derived from the California Air Resources Board Almanac of Emissions and Air Quality Data 

(http://www.arb.ca.gov/Aqd/almanac/almanac.htm). The following table further defines, in tons per 

day, the type of area sources contributing to PM2.5 emissions in Butte County. Area sources include 

residential fuel combustion, farming operations, construction/demolition, paved road dust, unpaved 

road dust, fugitive windblown dust, fires, managed burning and disposal and cooking. Area sources 

represent the dominant emission source category for direct PM2.5 emissions in Butte County, with 

approximately 70% of the total. Based on Table 2, within the area source category, residential wood 

burning is the dominant source of PM2.5. This is consistent with the speciation data discussed below.

Table 2. Area Source Emission (tons per day)

SOURCE PM2.5

Residential Fuel Combustion 2.65

Farming Operations 0.82

Construction/Demolition 0.11

Paved Road Dust 0.53

Unpaved Road Dust 0.76

Fugitive Windblown Dust 0.04

Fires 0.01

Managed Burning & Disposal 1.4

Cooking 0.07

Total Area Wide 6.4

Area Wide percent of total 68%

Total All 9.9

Source: ARB Almanac website (2006) http://www.arb.ca.gov/ei/maps/statemap/cntymap.htm
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Given the significance of NOx emissions in the formation of the PM2.5, EPA also considered emissions 

provided in the CARB Recommendation letter under this factor, along with the NOx data from NEI 

summarized in Table 1. Table 3 summarizes NOx emissions from stationary, area, and mobile source 

categories for 2006, 2010, and 2020. 

Table 3. NOx Winter Emissions for Butte County (tons per day)

Source Category 2006 2010 2020

Stationary Sources 1.4 1.4 1.4

Area Sources 1.7 1.7 1.7

Mobile Sources 23.3 19.9 11.3

Source: California Air Resources Board in their letter of December 17, 2007

The CES shown in Table 1 describes the relative contribution of emissions from surrounding counties 

to the high emission days based on a broad analysis of NOAA HYSPLIT trajectories linking countywide emissions from Butte and the surrounding counties and speciated air monitoring data on high 

days. The CES clearly demonstrates a connection between pollution levels in Chico and sources 

throughout Butte County. The CES shows less of a link between PM2.5 levels in Chico and 

neighboring Tehama and Glenn Counties.

Speciation data from the Chico air monitoring station was considered in evaluating this factor, as a 

way to link emission sources to high PM2.5 levels. As shown in the pie chart below (Figure 3), 

monitored PM2.5 in Chico is 75% organic carbon, for which the major source is residential wood 

burning. Since this source category is associated with population density, this data supports including 

urbanized areas and their surroundings within the nonattainment area.

In addition, 16% of the PM2.5 is ammonium nitrate, formed from precursor NOx emissions. Both 

Table 1 and 3 describe NOx emissions data for Butte County; mobile sources are the dominant source 

of NOx emissions. As discussed below under Factor 4: Traffic and commuting patterns, most of these 

emissions are along the roughly north-south State Route 99 corridor, with substantial additional 

contribution from commuting between Chico and the more eastern cities of Paradise and Oroville.

In light of the commuting patterns discussed under Factor 4 and illustrated in Figure 3, and in light of 

the spatial scale of nitrate formation, mobile source emissions in Butte County are contributing to the 

PM2.5 exceedances measured in Chico.

Emission levels and CES values support designation of Butte County as a 24-hour PM2.5 nonattainment 

area. However, it does not appear that the surrounding counties are significantly contributing to the 

pollution levels in Butte County. With respect to the CES values, Butte County has the highest CES 

score of 100, followed distantly by Glenn and Tehama Counties, at 14 and 19 respectively. Glenn and 

Tehama Counties are located west of Butte County, over 40 miles away. These counties have limited 

populations and no commuting pattern with Butte County. There is no evidence that these counties 

contribute to the PM2.5 air quality problems in Butte County . We have eliminated these counties from 

further consideration given these facts. 
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Factor 2: Air quality data 

This factor considers the 24-hour PM2.5 design values in micrograms per cubic meter (μg/m3) for air 

quality monitors in Butte County based on data for the 2004-2006 and 2005-2007 period. A monitor’s 

design value indicates whether that monitor attains a specified air quality standard. The 24-hour PM2.5

standards are met when the 3-year average of a monitor’s 98th percentile values are 35 μg/m3or less. 

A design value is only valid if minimum data completeness criteria are met. 

The 24-hour PM2.5 design values for Butte County are shown in Table 2. 

Table 4. Air Quality Data

County/ City State 

Recommended 

Nonattainment

24-hour PM2.5

Design Values

2004-06

(μg/m3)

24-hour PM2.5

Design Values

2005-07

(μg/m3)

Butte County, CA

City of Chico 

Yes 56 55

The violating monitor for 2004–2006 and 2005-2007 is located in the City of Chico in Butte County. 

Therefore, Butte County is a candidate for designation as a nonattainment area. Tehama and Glenn 

counties have no PM2.5 monitors , and consequently no data showing violations. Given the air quality

data, including consideration of CES values, and the State’s recommendations, Tehama and Glenn 

Counties were not further considered as nonattainment areas under this factor. 

EPA considered the chemical composition (speciation) of PM2.5 in evaluating this factor. As shown in 

the pie chart below (Figure 3), the chemical makeup of PM2.5 in Chico is dominated by organic carbon, 

at 75% of the total. This reflects emissions from residential wood burning. There is also a large 

ammonium nitrate contribution. The highest concentrations occur during the winter months (i.e., 

November through February). As discussed above, the carbon portion supports nonattainment 

designation of at least the urban areas and their surroundings, while the nitrate portion supports at least 

a large proportion of the county.
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Average Composition on

14 Exceedance Days- Chico

AmmNitrate

16%

AmmSulfate

2%

OC

75%

Other

2%

Geological

1%

Elements

2%

EC

2%

PM2.5 Mass= 52.2 ug/ m3

Sum of Species=51.4 ug/ m3

Figure 3

In summary, the air quality factor supports nonattainment designation of at least large portions of Butte 

County. PM2.5 speciation data support the idea that localized residential wood burning on stagnant 

winter nights is what pushes the monitor into violation, but also support a larger area that includes 

sources contributing to the ammonium nitrate portion of violations.

Eligible monitors for providing design value data generally include State and Local Air Monitoring 

Stations (SLAMS) at population-oriented locations with an FRM. All data from Special Purpose 

Monitors (SPM) using an FRM is eligible for comparison to the relevant NAAQS, subject to the 

requirements given in the October 17, 2006 Revision to Ambient Air Monitoring Regulations (71 FR 

61236). All monitors used to provide data must meet the monitor siting and eligibility requirements 

given in 71 FR 61236 to 61328 in order to be acceptable for comparison to the 24-hr PM2.5 NAAQS 

for designation purposes.

Factor 3: Population density and degree of urbanization (including commercial development)

Population data are relevant in defining the boundaries of the PM2.5 nonattainment area given the 

correlation between population and the emission sources contributing to PM2.5 exceedances (i.e., 

residential wood burning and mobile sources), as well as the population exposed to high PM2.5 levels.

Table 6 shows the 2005 population for each county in the area being evaluated, as well as the 

population density for each county in that area. Population data gives an indication of whether it is 

likely that population-based emissions might contribute to violations of the 24-hour PM2.5 standards.
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Table 6. Population

County State 

Recommended 

Nonattainment

2005 Population 2005 Population 

Density 

(pop/sq mi)

Butte Yes (P) 214,153 128

Tehama No 60,932 21

Glenn No 27,683 21

P= partial

According to Table 3, Butte County has the highest population and population density. Tehama 

County has the next highest population of the adjacent counties, but significantly below Butte (also in 

terms of population density). Population centers in Butte County include Chico (population of 59,444 

per 2000 US Census), Paradise (population of 26408 per 2000 US Census) and Oroville (population of 

13004 per 2000 US Census). Tehama and Glenn County have the same population density of 21 

people per square mile, which is extremely low, compared to Butte County at 128. Both Butte and 

Glenn counties experienced a 5% population growth from 2000-2005, while Tehama County saw 

slightly higher growth at 8%. However, the small populations and moderate growth in Tehama and 

Glenn counties further supports elimination of these counties from consideration as nonattainment 

areas. The presence of population centers outside of Chico supports EPA’s recommendation to include 

these other population centers in the nonattainment area. Relatively little population lives in the 

eastern high elevation portions of the county.

Attachment 3 has a chart that shows the area, population, car traffic and truck traffic for Butte County 

for the county as a whole, as well as just for the nonattainment area. The numbers clearly reflect that 

the population and traffic numbers are very high, and that most of the population and traffic is captured 

within the nonattainment area. 
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Figure 4
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Factor 4: Traffic and commuting patterns

This factor considers the number of commuters in each county who drive to Butte 

County, the percent of total commuters in each county who commute to Butte County, as 

well as the total Vehicle Miles Traveled (VMT) for each county in millions of miles (see 

Table 5). A county with numerous commuters is generally an integral part of an urban 

area and is likely contributing to fine particle concentrations in the area. Such an area 

could be an appropriate county for implementing mobile source emission control 

strategies, thus warranting inclusion in the nonattainment area.

Table 5. Traffic and Commuting Patterns

County State 

Rec. NA

2005 

VMT

(millions 

Number 

Commutin

g to any 

violating 

counties 

Percent

Commuting 

to any 

violating 

counties 

Number 

Commut

ing into 

statistic

al area 

Percent

Commuting 

into 

statistical 

area 

Butte 

County 

Yes (P) 2,078 74,510 91% 73,000 91%

Tehama 

County

No 599 1,170 6%] 1,140 6%

Glenn 

County 

No 330 1,770 17% 1,770 17%

P = partial

According to the data in Table 8, Butte County has a significantly larger number of 

commuters commuting into the violating area, 75,510 or 92%. Butte County has a large 

number of commuters traveling to and from Chico, the location of the violating monitor. 

There is also significant traffic into and out of Chico from the Cities of Paradise (on 

Highway 91) and to Oroville (on Highway 149).

In addition to the contribution of Butte County to traffic levels in the City of Chico, 

average daily truck traffic on Highway 162 is in the range of 5001 to 10,000. This 

highway travels from Sutter County to Butte County beyond the city limits of Chico. The 

daily car and truck traffic from Chico to Paradise, and from Chico to Oroville is much 

lower, in the range of 0 to 2000, but shows a daily traffic pattern.

Factor 4 further supports eliminating Tehama, and Glenn Counties from consideration as 

a nonattainment area. However, Butte County has significant commuter and truck traffic 

which argues for including most of Butte County as a nonattainment area. Figure 4 shows 

the traffic patterns in and around Chico. There is relatively little traffic in the eastern 

high elevation portions of the county.

The 2005 VMT data used for tables 5 and 6 of the technical analysis have been derived 

using methodology such as that described in "Documentation for the 2005 Mobile 
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National Emissions Inventory, Version 2," December 2008, prepared for the Emission 

Inventory Group, U.S. EPA. This document may be found at: 

ftp://ftp.epa.gov/EmisInventory/2005_nei/mobile_sector/documentation/2005_mobile_ne

i_version_2_report.pdf

Factor 5: Growth rates and patterns 

This factor considers population growth for 2000-2005 and growth in vehicle miles 

traveled for 1996-2005 for Butte County and the surrounding counties, as well as patterns 

of population and VMT growth. A county with rapid population or VMT growth is 

generally an integral part of an urban area and likely to be contributing to fine particle 

concentrations in the area. 

Table 8 below shows population, population growth, VMT and VMT growth for counties 

that are in the area adjacent to Butte County. Counties are listed in descending order 

based on VMT growth between 2000 and 2005.

Table 6. Population and VMT Growth and Percent Change

County Population 

(2005)

Population 

Density

Population 

% change 

(2000 -

2005)

2005 VMT

( millions)

% VMT

change

(% 2000-

2005

Butte 214,153 128 5% 2,078 61%

Tehama 60,932 21 8% 485 (41)%

Glenn 27,683 21 5% 253 (40)%

According to Table 6, Butte County has the highest population and population density. 

Tehama County has the next highest population of the adjacent counties, but significantly 

below Butte (also in terms of population density). Tehama and Glenn County have the 

same population density of 21 pop/sq mi, compared to Butte County at 128. Both Butte 

and Glenn counties experienced a 5% population growth from 2000-2005, while Plumas 

and Tehama counties also saw slightly higher growth at 8%. Glenn and Tehama 

Counties, while having a relatively small increase in population from 2000 to 2005, also 

experienced a decline in VMT growth from 2000 to 2005. 

Based on the analysis under Factor 5, Tehama and Glenn Counties, while experiencing 

modest growth in population, also had significant decreases in VMT which further 

supports elimination of these counties from consideration as nonattainment areas. 

Plumas County also had slight growth in population, but saw increased VMT. However, 

the total numbers for Plumas are still very low further supporting its elimination from 

consideration as a nonattainment area. Butte County has the largest population, by far, 

and also the most significant growth in VMT. 
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Factor 6: Meteorology (weather/transport patterns)

For this factor, EPA considered data from National Weather Service instruments in the 

area. Wind direction and wind speed data for 2004-2006 were analyzed, with an 

emphasis on “high PM2.5 days” for each of two seasons (an October-April “cold” season 

and a May-September “warm” season). These high days are defined as days where any 

FRM or Federal Equivalent Method (FEM) air quality monitors had 24-hour PM2.5

concentrations above 95% on a frequency distribution curve of PM2.5 24-hour values, or 

were 24-hr values exceeded 35.1 μg/m3.

For each air quality monitoring site, EPA developed a “pollution rose” to understand the 

prevailing wind direction and wind speed on the days with highest fine particle 

concentrations. Figure 4 identifies 24-hour PM2.5 values by color; days exceeding 35 

ug/m3 are denoted with a red or black icon. A dot indicates the day occurred in the warm 

season; a triangle indicates the day occurred in the cool season. The center of the figure 

indicates the location of the air quality monitoring site, and the location of the icon in 

relation to the center indicates the direction from which the wind was blowing on that 

day. An icon that is close to the center indicates a low average wind speed on that day. 

Higher wind speeds are indicated when the icon is further away from the center.

The pollution rose for Butte County, shown below, indicates that the elevated levels of 

the PM2.5 24-hour values for the Chico monitoring site occur primarily when the wind is 

from the south, and occasionally when the wind is from the north. The pollutant rose for 

Butte County also indicates that elevated PM2.5 24-hour values occur during the cool 

season, during time periods of low wind speeds.
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Figure 5

These data are consistent with the analysis provided by California, and may also support 

the CARB position that the organic carbon portion of the particulate matter problem is 

localized. However, as discussed in Factor 2: Air Quality, above, based on the nitrate 

portion, emissions from a larger area contribute to PM2.5 violations. This factor, together 

with Factor 2, supports the inclusion of at least major portions of Butte County in the

nonattainment area.

Note: the meteorology factor is also considered in each county’s Contributing Emissions 

Score because the method for deriving this metric included an analysis of trajectories of 

air masses for high PM2.5 days.

Factor 7: Geography/topography (mountain ranges or other air basin boundaries)

The geography/topography analysis looks at physical features of the land that might have 

an effect on the airshed and, therefore, on the distribution of PM2.5 within Butte County.

S

W E

2 4 6 8 10 12+

Wind Speed (mph)

Site 060070002

Chico, CA [Butte County, CA]

Pollution Rose, 2005-2007

Year

2004

2005

2006

98th %-ile

54.0

59.0

53.0

# days > 35

4

5

4

Design

Value 55-NA

7 exceedance(s) not plotted 

(due to missing or variable wind data) Chico, CA

Meteorological data from 34.7 miles away

RED_BLUFF_MUNICIPAL_ARPT (ID=24216)

2005

2006

2007

Concentration:

n > 40 μg/m3

n

35 - 40 μg/m3

n

30 - 35 μg/m3

n

< 30 μg/m3

Season:

cool (Oct-Apr)

warm (May-Sep)
16

Butte County is part of the larger Northern Sacramento Valley Air Basin (NSVAB), 

which includes the counties of Butte, Colusa, Glenn, Shasta, and Tehama. The NSVAB is 

bounded on north and west by the Coastal Mountain Range and on the east by the 

southern portion of the Cascade Mountain Range and the northern portion of the Sierra 

Nevada Mountains. These mountain ranges reach heights in excess of 6,000 feet with 

peaks rising much higher. This provides a substantial physical barrier to locally created 

pollution. 

For the areas under consideration, high PM2.5 concentrations mostly occur during 

stagnant conditions during winter, with radiation inversions. The cooling of the ground,

as heat is radiated away, creates an inversion, since air near the ground is cooler than that 

above. This inhibits mixing and confines pollutants to a relatively shallow layer near the 

ground. EPA considered radar wind profiler data collected at Chico during CRPAQS

study, and analyzed by CARB to derive mixing heights for the area. A typical value for 

maximum mixing height during high PM2.5 conditions is 300 m (984 ft) AGL (above 

ground level). EPA recognizes that an inversion height is not a rigid boundary extending 

through a fixed elevation. In reality the inversion would be partly terrain-following, and 

the degree of stagnation would be subject to additional influences at the foothill edges, 

such as strong diurnal slope flows. In any case, mixing heights vary by site and date, so 

any single height can provide only a scale for comparison, not a definitive value. 

Nevertheless, the inversion height provides an indicator of the area over which inversions 

may be enhancing pollution concentrations, and of the extent of the area that may be 

contributing to NAAQS violations.

Because the Butte area has topographical features higher the typical daytime height of the 

inversion layer, to help determine an appropriate eastern boundary EPA considered the 

inversion height to estimate the size of the area likely to have similar pollution conditions 

and to contribute to NAAQS violations. The eastern portion of Butte County extends into 

the foothills of the Sierra Nevada Mountains. For Chico, the 300 m AGL (984 ft)

inversion layer thickness translates to an elevation contour of 375 m (1221 ft) MSL

(above Mean Sea Level). Much of eastern portion Butte County is above this elevation, 

as shown in Figure 6 below. The urbanized area of Paradise is right about at this line.

In summary, topography is an important factor for Butte given that the inversion layer, 

which can lead to winter PM2.5 exceedances in the Sacramento Valley, is blocked by the 

Sierra foothills. In addition to affecting the City of Chico, similar pollution conditions 

are expected throughout Butte County, and areas below or near the inversion height could 

contribute to PM2.5 violations.
17

Figure 6
18

Factor 8: Jurisdictional boundaries (e.g., existing PM and ozone areas) 

EPA believes consideration also should be given to existing boundaries and organizations 

as they may facilitate air quality planning and the implementation of control measures to 

attain the standard. Areas already designated as nonattainment represent important 

boundaries for state air quality planning. To the degree appropriate based upon violations 

and contribution to violations of the respective NAAQS, EPA believes it can be helpful 

for air planning purposes and for attainment of both NAAQS for ozone and PM2.5 

nonattainment area boundaries to be consistent.

As compared to EPA’s original intention, the final designation places little emphasis on 

the ozone nonattainment area boundary, especially in setting the boundary within Butte 

County. Instead, more weight was placed on the areas generating pollution likely to 

contribute to NAAQS violations and on the topographic barrier of the Sierras. The lower 

elevation portions of Butte County is being recommended for a nonattainment 

designation for PM2.5 and is under the jurisdiction of the Butte County Air Quality 

Management District (AQMD). 

Factor 9: Level of control of emission sources 

This factor considers emission controls currently implemented for major sources in Butte 

County. There are no large contributing sources that have been excluded from the 

Sacramento nonattainment area. Figure 6 identifies both NOx and PM 2.5 sources within 

Butte County, as well as their emissions levels.

The emission estimates in Table 1 (under Factor 1) include any control strategies 

implemented by Butte County area before 2005 that may influence emissions of any 

component of PM2.5 emissions (i.e., total carbon, SO2, NOx, and crustal PM2.5). 

Conclusion

EPA is designating the lower elevation portions of Butte County as nonattainment for the 

PM2.5 NAAQS. The chosen area boundaries are supported by most of the factors EPA 

considered. Butter County contains a violating PM2.5 monitor, and the cities of Paradise 

and Oroville are connected by traffic distributions. These cities have substantial 

residential wood smoke emissions of carbon, by far the largest component of PM2.5

violations. Despite evidence that the effect of these sources is relatively localized, these 

sources are not limited to the city boundaries, nor is their influence. Also, ammonium 

nitrate, mainly from mobile source NOx emissions, is an important PM2.5 component; its 

larger spatial scale justifies including relatively large portions of the counties within the 

nonattainment area. The winter inversions that lead to PM2.5 NAAQS exceedances limit 

the vertical extent of pollution, as reflected in additional data provided by CARB. The 

Sierra foothills to the east provide a corresponding topographic barrier. This and the 

relatively small population and emissions in eastern Butte County justify excluding it 

from the nonattainment area.
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Additional information regarding responses to specific State comments can be found in 

EPA's Response to Comments document at 

http://www.epa.gov/pmdesignations/2006standards/tech.htm. 
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Attachment 2

Description of the Contributing Emissions Score

The CES is a metric that takes into consideration emissions data, meteorological data, 

and air quality monitoring information to provide a relative ranking of counties in and 

near an area. Using this methodology, scores were developed for each county in and 

around the relevant metro area. The county with the highest contribution potential was 

assigned a score of 100, and other county scores were adjusted in relation to the highest 

county. The CES represents the relative maximum influence that emissions in that 

county have on a violating county. The CES, which reflects consideration of multiple 

factors, should be considered in evaluating the weight of evidence supporting designation 

decisions for each area.

The CES for each county was derived by incorporating the following significant 

information and variables that impact PM2.5 transport:

· Major PM2.5 components: total carbon (organic carbon (OC) and elemental 

carbon (EC)), SO2, NOx, and inorganic particles (crustal).

· PM2.5 emissions for the highest (generally top 5%) PM2.5 emission days (herein 

called “high days”) for each of two seasons, cold (Oct-Apr) and warm (May-Sept)

· Meteorology on high days using the NOAA HYSPLIT model for determining 

trajectories of air masses for specified days

· The “urban increment” of a violating monitor, which is the urban PM2.5 

concentration that is in addition to a regional background PM2.5 concentration, 

determined for each PM2.5 component

· Distance from each potentially contributing county to a violating county or 

counties

A more detailed description of the CES can be found at 

http://www.epa.gov/ttn/naaqs/pm/pm25_2006_techinfo.html#C
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Attachment 3: Population and Traffic Data

County County Area (sq. miles) Nonattainment Area (sq. miles)

Butte 1,677.50 1,158.33

El Dorado 1,787.99 620.92

Imperial 4,481.67 690.54

Placer 1,501.86 432.38

Sacramento 992.04 992.04

Sutter 608.39 608.39

Yolo 1,023.36 698.67

Yuba 643.57 482.77

Solano (SV) 470.55 470.55

Solano (SF) 419.01 419.01

County County Population Nonattainment Area Population

Butte 203,171.00 196,300.00

El Dorado 156,299.00 110,528.00

Imperial 142,361.00 122,775.00

Placer 248,399.00 207,156.00

Sacramento 1,223,499.00 1,223,499.00

Sutter 78,930.00 78,930.00

Yolo 168,660.00 163,193.00

Yuba 60,219.00 56,293.00

Solano (SV) 120,697.00 120,697.00

Solano (SF) 273,845.00 273,845.00

County County Annual Non-truck Traffic Nonattainment Area Annual Non-truck Traffic

Butte 2,237,170.00 2,225,913.00

El Dorado 1,573,738.00 1,547,018.00

Imperial 1,677,333.00 1,229,867.00

Placer 4,777,401.00 4,548,701.00

Sacramento 33,510,398.00 33,510,398.00

Sutter 361,416.00 361,416.00

Yolo 4,677,258.00 4,671,958.00

Yuba 426,377.00 422,677.00

Solano (SV) 3,988,735.00 3,988,735.00

Solano (SF) 12,301,618.00 12,301,618.00

County County Annual Truck Traffic Nonattainment Area Annual Truck Traffic

Butte 169,932.00 169,280.00

El Dorado 86,212.00 84,588.00

Imperial 229,338.00 139,186.00

Placer 422,876.00 400,706.00

Sacramento 2,515,749.00 2,515,749.00

Sutter 33,714.00 33,714.00

Yolo 614,874.00 614,512.00

Yuba 23,724.00 23,036.00

Solano (SV) 369,035.00 369,035.00

Solano (SF) 933,605.00 933,605.00

Sources: U.S. Census Department (2000), Federal Highway Administration (2002), EPA (2008)
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