B-1 

APPENDIX B 

WEIGHT OF EVIDENCE 
 

B-2 

SAN JOAQUIN VALLEY PM2.5 SUPPLEMENTAL ANALYSIS 

1. INTRODUCTION 

The State Implementation Plan or SIP serves as a demonstration of attainment 

for the national ambient air quality standards (NAAQS or federal standards). The 

attainment demonstration comprises analyses used to determine the set of 

control measures needed to meet the NAAQS by the attainment year. These 

analyses typically include air quality modeling, which generally guides the 

selection of the most effective pollutants to control and the magnitude of needed 

emissions reductions. The Weight of Evidence (WOE) analysis provides a set of 

complementary analyses that supplement the SIP-required modeling. These 

analyses can include consideration of measured air quality, emissions, and 

meteorological data, evaluation of other air quality indicators, and additional air 

quality modeling. 

A WOE approach looks at the entirety of the information at hand to provide a 

more informed basis for the attainment strategy. Because all methods have 

inherent strengths and weaknesses, examining an air quality problem in a variety 

of ways offsets the limitations and uncertainty that are inherent in air quality 

modeling. This approach also provides a better understanding of the overall 

problem and the level and mix of emissions controls needed for attainment. 

The United States Environmental Protection Agency (U.S. EPA) recognizes the 

importance of a comprehensive assessment of air quality data and modeling and 

encourages this type of broad assessment for all attainment demonstrations. In 

their modeling guidance, they further note that the results of supplementary 

analyses may be used in a WOE determination to show that attainment is likely 

despite modeled results which may be inconclusive (U.S. EPA 2007). Under the 

U.S. EPA guidance, future year modeled annual average design values that fall 

between 14.5 and 15.5 ug/m3

 should be accompanied by a WOE demonstration 

to determine whether attainment will occur. This range in modeled design values 

reflects the uncertainty in predicting absolute PM2.5 concentrations that is 

inherent in air quality modeling, and therefore recognizes that an improved 

assessment of attainment can be derived from examining a broader set of 

analyses. 

U.S. EPA recommends that three basic types of analyses be included to 

supplement the primary modeling analysis in the WOE approach: 

 

1) analyses of trends in ambient air quality and emissions, 

2) observational models and diagnostic analyses, and 

3) additional modeling. 

Each analysis is weighted based on its ability to quantitatively assess the ability 

of the proposed control measures to yield attainment. The scope of the WOE 
 

B-3 

analysis is different for each nonattainment area. The level of detail appropriate 

for each area depends upon the complexity of the air quality problem, how far 

into the future the attainment deadline is, and the amount of data and modeling 

available. For example, less analysis is needed for an area that is projecting 

attainment near-term and by a wide margin, and for which recent air quality 

trends have demonstrated significant progress, than for areas with more severe 

air quality challenges. The following sections present the WOE assessment for 

PM2.5 in the San Joaquin Valley Air Basin for each of the areas outlined in the 

U.S. EPA guidance. 

2. ASSESSMENT OF RECENT AIR QUALITY AND EMISSION TRENDS 

a. Current Air Quality

According to U.S. EPA’s guidance, even though air quality models are regarded 

as the most appropriate tool to assess impacts in emission changes, it is also 

possible to extrapolate future trends in PM2.5 based on measured historical 

trends of air quality and emissions. Strength of the evidence produced by 

emissions and air quality trends is increased if an extensive monitoring network 

exists and if there is good correlation between past emission reductions and 

current trends. 

Along with the South Coast, the San Joaquin Valley Air Basin has one of the 

most severe PM2.5 problems in the nation and represents a considerable 

challenge in attaining the federal PM2.5 standards. These standards consist of 

both a 24-hour standard of 65 ug/m3

, and an annual average standard of 

15 ug/m3. While the majority of monitors with complete data in the San Joaquin 

Valley still exceed the annual PM2.5 standard, PM2.5 air quality has shown 

considerable improvement since 1999 when monitoring data for assessing 

compliance with the federal PM2.5 standard began. When the San Joaquin 

Valley was first designated nonattainment for the federal PM2.5 standards, the 

basin exceeded both the annual and the 24-hour PM2.5 standards. However, 

based on 2004-2006 data, the San Joaquin Valley meets the federal 24-hour 

PM2.5 standard of 65 ug/m3

 throughout the basin. Thus attaining the annual 

standard is the primary challenge in this State Implementation Plan. 

The San Joaquin Valley Air Basin is a lowland area bordered by mountains to the 

east, west, and south. The mountains act as air flow barriers, with the resulting 

stagnant conditions favoring the accumulation of emissions and pollutants. To 

the north, the Valley borders the Sacramento Valley and Delta lowland, which 

allows for some level of pollutant dispersion. As a result, PM2.5 concentrations 

are higher in the southern and central portions of the Valley, where geography, 

emissions, and climate pose significant challenges to air quality progress. 

Chapter 3 and Appendix E of the District Plan provide detailed information on the 

conceptual model of PM2.5 formation in the Valley. 
 

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Currently, eleven sites routinely monitor PM2.5 in the Air Basin. Table 1 provides 

the 2006 annual standard design values and the annual average values for 2004, 

2005, and 2006 for each monitoring site with complete data. The design value is 

a statistic that is used to describe the air quality status of a given area relative to 

the level of the federal standard. The annual design values represent the 

average of the mean annual PM2.5 concentrations measured during the three 

year period. Table 2 provides the 24-hour standard design value for each 

monitoring site with complete data and the yearly 98th percentile value of the 

24-hour concentrations measured in 2004, 2005, and 2006. The 24-hour design 

value represents the average of the yearly 98th percentile of the 24-hour 

concentrations measured during the three year period. Attainment is reached 

when the design value is at or below the corresponding federal standard. 

As shown in Table 1, current 2006 annual average design values (reflecting the 

2004 through 2006 period) range from 12.9 ug/m3 to 18.9 ug/m3. The 

San Joaquin Valley monitoring sites with the highest PM2.5 annual design values 

are located in the southern and central portions of the basin, including 

Bakersfield and Visalia. Monitors located in the northern portion of the Valley, 

including Stockton, Modesto, and Merced have lower annual PM2.5 design 

values and attain the annual PM2.5 standard. As shown in bold in the table, the 

high site for the region is the Bakersfield–Planz monitor with a PM2.5 design 

value of 18.9 ug/m3. 

Table 1. PM2.5 Annual Average Values 

 Annual Average 

(ug/m3

) 

Monitor 2004 2005 2006 

2006 

3-year Annual Average 

Design Value (ug/m3) 

Clovis-N Villa Avenue 15.8 16.0 16.8 16.2 

Bakersfield-410 E Planz Road 17.4 19.9 19.3 18.9 

Bakersfield-5558 California 

Avenue 

19.0 17.9 18.7 18.5 

Bakersfield-Golden State 

Highway 

18.1 18.9 18.6 18.5 

Corcoran-Patterson Avenue 17.3 17.6 16.7 17.2 

Fresno-1st Street 16.4 16.9 16.8 16.7 

Fresno-Hamilton and Winery 17.0 16.9 17.6 17.2 

Merced-2334 M Street 15.3 14.1 14.8 14.7 

Modesto-14th Street 13.6 13.9 14.8 14.1 

Stockton-Hazelton Street 13.2 12.5 13.1 12.9 

Visalia-N Church Street 17.0 18.8 18.8 18.2 
 

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Table 2. PM2.5 98th Percentile 24-hour Values 

 Annual 98th Percentile 

(ug/m3

) 

Monitor 2004 2005 2006 

2006 

3-year Average of 

98th Percentile 

Design Value (ug/m3) 

Clovis-N Villa Avenue 42.4 77.0 51.3 57 

Bakersfield-410 E Planz Road 78.6 66.4 50.6 65* 

Bakersfield-5558 California 

Avenue 

59.3 63.6 60.5 62 

Bakersfield-Golden State 

Highway 

53.8 74.9 64.4 64 

Corcoran-Patterson Avenue 49.4 74.5 50.1 58 

Fresno-1st Street 52.0 71.0 51.0 58 

Fresno-Hamilton and Winery 49.4 71.2 55.0 59 

Merced-2334 M Street 43.0 48.3 43.8 45 

Modesto-14th Street 45.0 55.0 52.0 51 

Stockton-Hazelton Street 36.0 44.0 42.0 41 

Visalia-N Church Street 54.0 65.0 50.0 56 

*The 24-hour standard is exceeded when the design value is over 65 ug/m3

 

As previously mentioned, all monitors in the San Joaquin Valley currently attain 

the federal 24-hour PM2.5 standard. As shown on Table 2, 2006 24-hour PM2.5 

design values (reflecting the 2004 through 2006 period) range from 41 ug/m3 to 

65 ug/m3. Monitoring sites with the highest 24-hour PM2.5 design values are 

located in the southern portion of the Valley, around Bakersfield. As shown in 

bold on the table, the high site for the region is Bakersfield-Planz with a 24-hour 

PM2.5 design value of 65 ug/m3

. 

b. Recent PM2.5 Mass Trends

Trends observed in the San Joaquin Valley show that considerable progress has 

occurred in the San Joaquin Valley over the last five years due to the ongoing 

emissions control program. As shown in Figure 1, 24-hour design values have 

decreased approximately 40 percent and, as discussed above, based on the 

2004 through 2006 data, attain the federal standard at all sites in the Basin. 
 

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Figure 1: Comparison of 2006 to 2001 PM2.5 24-hour Design Values 

PM2.5 24-hour Design Value

0

20

40

60

80

100

120

Modesto-14th

Street

Fresno-1st

Street

Visalia BakersfieldGolden

Concentrations (ug/m3)

2001

2006

All San Joaquin Valley monitors with complete data also show a significant 

decrease in annual average design values (Figure 2). In 2001, all monitoring 

sites in the Basin had annual design values greater than 16 ug/m3

, with the 

Visalia site at approximately one and a half times the level of the standard. By 

2006, design values decreased throughout the basin, and only those sites in the 

southern and central portions of the Valley are still greater than 16 ug/m3. 

Bakersfield–Planz is the current high site, with a design value which is 26 percent 

above the standard. The greatest rate of progress has occurred in the northern 

and central basin. From 2001 through 2006, the Modesto site design value 

dropped 28 percent, from 19.7 ug/m3

 to 14.1 ug/m3, while the Visalia site design 

value dropped 26 percent from 24.7 ug/m3

 to 18.2 ug/m3. As a result, all 

monitoring sites in the northern portion of the Valley now attain the annual PM2.5 

standard of 15 ug/m3

. In contrast, the Bakersfield-Golden site dropped 

22 percent from 23.6 ug/m3

 to 18.5 ug/m3 and the Bakersfield-California site 

dropped 21 percent from 23.3 ug/m3 to 18.5 ug/m3. This trend is further 

illustrated in Figure 3 which depicts maps of the spatial variations in annual 

average concentrations in 2001 as compared to 2006. 
 

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Trends in 24-hour and annual average design values were further evaluated 

using the nonparametric statistical analysis test, known as the Theil test, that 

U. S. EPA uses in national air quality trend analysis1

. This method tests for 

whether upward or downward trends are real (significant) or a chance product of 

year-to-year variation (not significant). Theil test results indicate the 2001 to 

2006 downward trends in 24-hour and three-year annual average PM2.5 

concentrations are statistically significant. 

Figure 2: Trends in Annual Average PM2.5 Design Values 

 

1

 U.S. EPA , National Air Quality and Emissions Trends Report, Publication No. 

EPA 454/R-03-005, Office of Air Quality and Standards, Air Quality Strategies and Standards 

Division, Research Triangle Park, North Carolina. 2003. 

http://www.epa.gov/air/airtrends/aqtrnd03/

Three Year Annual Average PM2.5 Design Values

0

5

10

15

20

25

30

2001 2002 2003 2004 2005 2006

Year

Concentrations (ug/m3)

Bakersfield-410 E Planz

Bakersfield-5558 California

Bakersfield-Golden

Modesto-14th Street

Stockton-Hazelton

Visalia-N Church

Fresno-1st Street
 

B-8 

Figure 3: 2001 and 2006 Annual Average Values 

The progress observed in both the annual and the 24-hour PM2.5 standards 

reflects the linkage between the two standards. The annual average is 

comprised of individual 24-hour samples which vary throughout the year. In the 

San Joaquin Valley, PM2.5 concentrations exhibit a pronounced seasonal 

variation, with significantly higher monthly average concentrations during the late 

fall and winter (November through January). PM2.5 concentrations from April 

through September are generally below 15 ug/m3

. Therefore 24-hour PM2.5 

concentrations during the fall and winter seasons strongly drive the annual 

average concentrations. During the late fall and winter, stagnant air, cool 

temperatures, and high humidity can lead to a build-up of PM2.5 over a period of 

several days to weeks. 

Analyses of the changes in the distribution of the 24-hour concentrations 

between 2001-2003 and 2004-2006 indicate that the decrease in both 24-hour 

and annual average concentrations is the result of the downward shift in the 

distribution of daily PM2.5 concentrations during the fall/winter period. This can 

be seen in the decrease in the monthly average concentrations during the 

fall/winter months between 2001 and 2006 at Bakersfield-California and Fresno 

(Figures 4 and 5). 

PM2.5 (ug/m3) 

San Joaquin Valley Air Basin

National PM2.5 Annual Average 

(2001) 

Stockton 

Modesto 

Merced 

Clovis Fresno-1st 

Fresno-Hamilton 

Visalia 

Corcoran 

Bakersfield-Golden 

Bakersfield-California 

Bakersfield-Planz 
 

B-9 

Figure 4: Changes in Seasonal Pattern at the Bakersfield-California 

Monitoring Site 

Figure 5: Changes in Seasonal Pattern at the Fresno Monitoring Site 
 

B-10 

A comparison of the changes in the frequency distribution of 24-hour 

concentrations over the last few years provides another means of understanding 

progress. Data collected from 24-hour samples during 1999-2001 at the 

Bakersfield site indicate that 6 percent of the samples had concentrations greater 

than 65 ug/m3

 (Figure 6). In comparison, during the 2004-2006 period, the 

number of samples with PM2.5 concentrations greater than 65 ug/m3

 fell to 

1 percent. During both periods, 45 percent of the samples had concentrations 

above the level of the annual PM2.5 standard of 15 ug/m3

. 

At the Fresno site data collected during the 1999–2001 period indicate that 

approximately 8 percent of the samples had concentrations greater than 

65 ug/m3 (Figure 7), while during the 2004-2006 period, only 1 percent of the 

samples had concentrations greater than 65 ug/m3. In addition, during 

1999-2001, 46 percent of the samples had concentrations above the level of the 

annual PM2.5 standard of 15 ug/m3, while during the 2004-2006 period, the 

number of samples with concentrations greater than 15 ug/m3

 fell to 34 percent. 

This analysis, together with the analysis of monthly average PM2.5 trends, 

further illustrate that progress to date in the annual average is primarily linked to 

reductions in peak concentrations from October through March. 

Figure 6: PM2.5 Concentrations at Bakersfield-California 1999-2001 versus 

2004-2006 
 

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Figure 7: PM2.5 Concentrations at Fresno 1999-2001 versus 2004-2006 

c. Historical PM2.5 Mass Trends

The PM2.5 Federal Reference Method (FRM) network that is used for 

comparison to the federal standard was deployed in 1998 and 1999. However, 

PM2.5 data have been routinely collected in the San Joaquin Valley Air Basin 

since the late 1980s using dichotomous samplers that were operated for 

research purposes. The PM2.5 dichotomous data correlate very well with the 

FRM data (r=0.98), but are biased lower than the FRM by 12 percent, therefore 

an absolute comparison to the FRM can not be made. However, the relative 

changes in the dichotomous data do provide a longer-term context for assessing 

progress. Figure 8 depicts PM2.5 concentrations at four sites. At all of these 

sites, the 3-year average concentrations decreased until 1998, then increased 

until 2000. At two of the three highest sites, Bakersfield and Visalia, the 3-year 

average concentrations decreased about 40 percent from 1990 to 1998 (the 

Bakersfield site trend has a gap from 1994 to 1996). PM2.5 concentrations at 

Bakersfield then increased 19 percent from 1998 to 2000. The dichotomous data 

record at the third high site, Fresno, started in 1992. From 1992 to 1998, PM2.5 

concentrations decreased 21 percent, followed by a 12 percent increase from 

1998 to 2000. PM2.5 concentrations at the low site, Stockton, decreased 

40 percent from 1991 to 1998, followed by a 20 percent increase from 1998 to 

2000. The period between 1999 and 2001 included a number of adverse winter 

episodes which likely lead to higher annual averages during this period. 

However, overall concentrations declined 20 to 30 percent over the period from 
 

B-12 

1990 through 2001. Looking at dichotomous data and FRM data together 

therefore illustrates an overall long-term improvement in PM2.5. 

Figure 8: Trends in Three-Year Average PM2.5 Concentrations Measured 

with Dichotomous Samplers 

PM2.5 Dichot Three-Year Average Trend

0

5

10

15

20

25

30

35

199019911992199319941995199619971998199920002001

Concentrations (ug/m3)

Bakersfield*

Fresno-1st Street

Stockton

Visalia

d. Chemical Composition and Emissions Trends

Current Chemical Composition 

Particulate matter (PM) consists of many different chemical components. 

Investigating these different components and how they vary by site and season 

provides us with a better understanding of their complexity and responses to the 

emission control program. This mix of chemical components consists of both 

directly emitted PM such as geological material and elemental carbon (soot), 

known as primary PM, as well as PM formed in the atmosphere from the 

reactions of precursor gases, known as secondary PM. These precursor 

pollutants include nitrogen oxides (NOx), sulfur oxides (SOx), reactive organic 

gases (ROG), and ammonia. NOx, SOx, and ammonia combine to form 

secondary ammonium nitrate and sulfate. ROG can form secondary organic 

carbon, as well as participate in the production of secondary ammonium nitrate. 

In the San Joaquin Valley, chemical components are routinely measured at four 

sites, Bakersfield, Fresno, Visalia, and Modesto. These sites represent urban 

areas in different portions in the air basin. The chemical composition of PM2.5, 

seasonal variability, and trends in these components at two of these sites, 
 

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Fresno and Bakersfield are discussed below to provide further insight into the 

linkage between observed progress and ongoing emission reductions. 

The annual average PM2.5 chemical composition for Bakersfield and Fresno is 

shown in Figure 9. Ammonium nitrate and organic carbon are the major 

constituents at both sites. However, at Bakersfield, ammonium nitrate 

constitutes a higher percentage (39 percent) of PM2.5 than organic carbon 

(35 percent), while the reverse (37 percent ammonium nitrate and 41 percent 

organic carbon) occurs in Fresno. Ammonium nitrate is formed in the 

atmosphere from chemical reactions of NOx emitted from motor vehicles and 

stationary combustion sources. Stagnant, cold, and damp conditions in the 

winter promote the formation and accumulation of ammonium nitrate. Burning 

activities, such as residential wood combustion, cooking, and direct tailpipe 

emissions from mobile sources are major sources of organic carbon. 

Ammonium sulfate is also formed in the atmosphere from chemical reactions of 

SOx emitted from combustion sources. Ammonium sulfate constitutes about one 

tenth of ambient PM2.5 at both sites. Elemental carbon resulting from mobile 

and stationary combustion sources, and geological material from roads and other 

dust producing activities also contribute to PM2.5 at both sites, but to a lesser 

extent. 

Figure 9: 2004-2006 Average Chemical Composition of PM2.5 at 

Bakersfield and Fresno 

Figures 10 and 11 illustrate the seasonal variability in chemical components that 

make up PM2.5 for the Bakersfield and Fresno sites averaged over 2004 through 

2006. As discussed previously, PM2.5 concentrations are highest during the fall 

and winter. These higher concentrations are driven by increases in ammonium 

2004-2006 Average Composition

Bakersfield

AmmNitrate

39%

AmmSulfate

13%

OC

35%

Geological

6%

EC

5%

Elements

2%

2004-2006 Average Composition

Fresno

AmmNitrate

37%

AmmSulfate

11%

OC

41%

Geological

4% EC

5%

Elements

2%
 

B-14 

nitrate and carbon. At the Bakersfield site (Figure 10), ammonium nitrate is 

highest during the fall and winter months (November through January) and 

lowest from April through September. Ammonium nitrate concentrations at 

Fresno follow the same trend. Cold and humid conditions during the fall and 

winter favor the formation of ammonium nitrate in the atmosphere from chemical 

reactions of nitrogen oxides emitted from mobile and stationary combustion 

sources. At the Fresno site (Figure 11), organic carbon has a similar trend as 

that of ammonium nitrate. At the Bakersfield site, organic carbon is also highest 

during the fall and winter months, but at this site organic carbon levels start high 

in January, decrease until April and then increase almost linearly until reaching a 

maximum in December. Increased activity in residential wood combustion during 

the fall/winter period is one cause of higher organic carbon concentrations. At 

both locations, elemental carbon component remains fairly constant throughout 

the year, while the ammonium sulfate component is more prevalent from May 

through September, and the geological components from April through October. 

Figure 10: Seasonal Variation in PM2.5 Chemical Components at 

Bakersfield 
 

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Figure 11: Seasonal Variation in PM2.5 Chemical Components at Fresno 

Chemical Composition and Precursors Trends 

As discussed previously, monitoring sites in the San Joaquin Valley have shown 

PM2.5 concentrations decreasing from 2001 to 2006. Trends in individual 

chemical components and precursor concentrations, as well as emissions 

inventory trends were evaluated to understand the main chemical components 

responsible for this progress and to evaluate the response to our ongoing control 

program. Figures 12 and 13 illustrate recent trends in PM2.5 concentrations and 

its individual chemical components at Fresno and Bakersfield. Improvements in 

PM2.5 in the San Joaquin Valley can be mostly attributed to reductions in 

carbonaceous aerosols and ammonium nitrate. In Fresno, carbonaceous 

aerosols have dropped 37 percent, and ammonium nitrate concentrations have 

dropped 24 percent. The overall improvement in PM2.5 mass observed in 

Fresno is therefore due in most part to reductions in carbonaceous aerosols 

(63 percent), with a smaller portion of the remaining PM2.5 improvement due to 

reductions in ammonium nitrate (24 percent). In Bakersfield, carbonaceous 

aerosols have dropped by 16 percent and ammonium nitrate concentrations 

dropped by 23 percent since 2002. Therefore 41 percent of the overall reduction 

in PM2.5 mass is due to reductions in carbonaceous aerosols, while 59 percent 

is due to reductions in ammonium nitrate. Understanding the role of 

carbonaceous compounds in PM2.5 pollution, the San Joaquin Valley Air 

Pollution Control District made the Valley’s Residential Wood Combustion Rule 

more stringent in 2003, which may have contributed to the observed decrease in 

carbonaceous aerosols. 
 

B-16 

Figure 12: Trends in PM2.5 Mass and Chemical Components at Fresno 

* The three-year design value is not considered valid. 

Figure 13: Trends in PM2.5 Mass and Chemical Components at 

Bakersfield-California 

Concentrations in ambient NOx, a precursor to nitric acid and ammonium nitrate 

decreased steadily from 2001 through 2006. Figures 3-14 and 3-15 illustrate the 
 

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short-term trends in the Basin average NOx compared to ammonium nitrate 

concentrations in Fresno and Bakersfield. Ambient NOx concentrations in the 

Valley have shown a modest decrease of 15 percent. Ammonium nitrate 

concentrations were highest in 2002 at both sites, with a small decrease between 

2001 and 2006 at Fresno of 7 percent. 

While the short-term trends for ammonium nitrate have been modest, longer-term 

records show concomitant decreases between ambient NOx and ammonium 

nitrate as well as between ambient SO2 and ammonium sulfate. Figure 16 

shows trends in the basin three-year average ambient NOx concentrations and 

the corresponding ambient nitrate measurements from the PM10 network 

extending back to the late 1980s. The hills and valleys in the ammonium nitrate 

concentrations reflect the effects of the varying meteorology on ammonium 

nitrate formation. Since 1987, ambient NOx has decreased 50 percent while 

ammonium nitrate decreased by 35 percent. Figure 17 shows the trends from 

1987 to 1996 in the basin three-year average ambient SO2 concentrations and 

ammonium sulfate measurements from the PM10 network. During this period 

SO2 decreased by 42 percent with a concurrent decrease in PM10 sulfate of 

39 percent. 

Figure 14: PM2.5 Ammonium Nitrate Compared to NOx – Fresno 
 

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Figure 15: PM2.5 Ammonium Nitrate Compared to NOx - Bakersfield 

Figure 16: Long-Term Trends in Three-Year Average Concentrations of 

PM10 Nitrate and NOx in San Joaquin Valley 

Three-year Average Concentrations

of PM10 Nitrate and NOx in San Joaquin Valley

0

1

2

3

4

5

6

7

8

9

10

1987198819891990199119921993199419951996199719981999200020012002200320042005

PM10 Nitrate (ug/m 3

)

0

1

2

3

4

5

NOx (pphm)

NO3-

NOx
 

B-19 

Figure 17: Long-Term Trends in Three-Year Average Concentrations of 

PM10 Sulfate and NOx in the San Joaquin Valley 

Three-year Average Concentrations of PM10 Sulfate 

and SO2 in the SJV

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1987198819891990199119921993199419951996199719981999200020012002200320042005

PM10 Sulfate (ug/m 3

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

SO2 (pphm)

SO4--

SO2

SO2 monitoring discontinued 

in 1997

 Emission Trends 

At the same time that ambient concentrations have been declining, the emissions 

of pollutants that contribute to the different components of PM2.5 have been 

decreasing. Figure 18 illustrates the recent emission trends in the San Joaquin 

Valley air basin from 2001 through 2005. The greatest decrease in emissions 

occurred in ROG with a decrease of 33 tpd or 8 percent. NOx also decreased 

slightly, with a decline of 28 tpd, or 4 percent. Direct PM2.5 emissions showed a 

decrease of 7 tpd, or 7 percent, while SOx emissions did not change. 

The combined downward trends in PM2.5 components, precursor 

concentrations, and emissions all indicate that over both in the short- and longterm the ongoing control program has had substantial benefits in improving air 

quality and that similar emission reductions in the future should provide 

continuing progress towards attaining the federal PM2.5 standards. 
 

B-20 

Figure 18: PM2.5 and PM2.5 Precursor Emission Trends in the 

San Joaquin Valley 

Annual Emission Inventory Trends

2001-2005

0

100

200

300

400

500

600

700

2001 2002 2003 2004 2005

Emissions (tpd)

NOx

ROG

SOx

PM2.5

3. OBSERVATIONAL MODELS AND DIAGNOSTIC ANALYSES 

Observational models take advantage of monitored data to draw conclusions 

about the relative importance of different types of emissions and precursors as 

factors contributing to observed PM2.5 concentrations. According to U.S. EPA 

guidance, observational models can be used to corroborate the effects of prior 

control strategies, as well as identify the potential effectiveness of proposed 

control approaches. U.S. EPA recommends using both source apportionment 

(receptor models) and indicator species approaches. The two most widely 

applied receptor modeling approaches are multi-variate statistical models such 

as positive matrix factorization (PMF) and the chemical mass balance model 

(CMB). Receptor models are particularly useful in identifying the source 

contributions to directly emitted PM. This section summarizes results from prior 

source apportionment studies as well as both PMF and CMB applied to recent 

data. In addition, previous work using an indicator species approach to assess 

the limiting precursor in secondary nitrate formation in the San Joaquin Valley is 

discussed. 
 

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a. Observational Models

 Prior Source Apportionment Studies 

The Chemical Mass Balance (CMB), and the Positive Matrix Factorization (PMF) 

and UNMIX (named for its function, which is to "unmix" the concentrations of 

chemical species measured in the ambient air to identify the contributing 

sources) multivariate receptor models have been applied to PM2.5 data collected 

in the San Joaquin Valley. Chow, et al.2 used the CMB source apportionment 

model to estimate the contribution of sources to PM2.5 in Fresno during high 

PM2.5 days occurring from December 15, 2000 through February 3, 2001 

illustrated in Figure 19. Secondary ammonium nitrate is the most significant 

source, contributing 32 percent to the measured PM2.5. Residential wood 

combustion constitutes 30 percent of PM2.5. Vehicle emissions account for 

13 percent of PM2.5, with similar contributions from gasoline (7 percent) and 

diesel (6 percent) fueled vehicles. Cooking accounts for 12 percent of PM2.5. 

Figure 19: Source Contribution to PM2.5 Concentrations at Fresno 

Estimated Using the Chemical Mass Balance (CMB) Method 

 

2

 J .C. Chow, J. G. Watson, D. H. Lowenthal, L.-W. A. Chen, B. Zelinska, L. R. Rinehart, and 

K. L. Magliano: Evaluation of organic markers for chemical mass balance source apportionment 

at the Fresno Supersite, Atmospheric Chemistry and Physics Discussions, 6, 10341 – 10372, 

2006 

PM2.5 Source Contribution (CMB) in Percent 

Fresno - 2000/2001 Winter Average

Wood 

Burning

30%

Ammonium 

Nitrate

32%

Ammonium 

Sulfate

2% Cooking

12%

Vehicle 

Exhaust 

(Gasoline)

7%

Unknown

11% Vehicle 

Exhaust 

(Diesel)

6%
 

B-22 

Chen, et al.3 applied the PMF and UNMIX models to chemically speciated PM2.5 

measurements from 23 sites operated in the in the San Joaquin Valley during the 

2000/2001 Central California PM Air Quality Study (CRPAQS) to estimate source 

contributions. Figure 20 illustrates the resulting source contribution throughout 

the Valley, estimated for the period when high PM2.5 concentrations occurred 

(November through January). PMF and UNMIX source contribution estimates 

are similar. Particles formed through chemical reactions in the atmosphere, 

including ammonium nitrate and secondary organic compounds are the major 

contributors, accounting for 48 percent (PMF) and 51 percent (UNMIX) of PM2.5. 

Residential wood combustion follows, with a contribution of 23 percent (PMF) 

and 24 percent (UNMIX). Particles directly emitted from motor vehicles account 

for 10 percent (PMF) and 15 percent (UNMIX) of PM2.5. In addition, the zinc 

component resulting from the PMF analysis is thought to be related to brake and 

tire wear and contributes 9 percent to PM2.5.

Figure 20: Source Contribution to PM2.5 in the San Joaquin Valley 

Estimated Using the Positive Matrix Factorization (PMF) and the UNMIX 

Models 

 Recent Studies 

To evaluate the major PM2.5 sources and their contributions in the San Joaquin 

Valley using recent data, two different source apportionment techniques were 

applied to data collected in 2003 through 2006. The Chemical Mass Balance 

model (CMB) uses measured source profiles and chemical speciation data in 

combination to determine source contributions. A fundamental underlying 

assumption is that the source profiles used as input are appropriate and 

 

3

 L.-W. A. Chen, J. G. Watson, J. C. Chow, and K. L. Magliano: Quantifying PM2.5 Source 

Contributions for the San Joaquin Valley with Multivariate Receptor Models, submitted for 

publication, 2006. 

Source Contibution (PMF) in Percent

San Joaquin Valley - 2000/2001 Winter Average 

Motor Vehicle

10%

Residential 

Wood 

Combustion

23%

Zinc

9%

Cooking

3%

Fugitive Dust

5% Ag-Dairy

2%

Ammonium 

Nitrate and 

Secondary 

Organic

48%

Source Contribution (UNMIX) in Percent 

San Joaquin Valley - 2000/2001 Winter Average

Motor Vehicle

15%

Ag-Dairy

2%

Fugitive Dust

3%

Residential 

Wood 

Combustion

24% Ammonium 

Nitrate and 

Secondary 

Organic

51%

Cooking

5%
 

B-23 

representative for the region. In contrast, PMF does not require the input of 

source profiles, but rather evaluates the covariance of the chemical species to 

determine a set of factors, which are typically interpreted as source types. 

Source contributions are then calculated for each of the factors. Because the 

factors reflect species which vary in time in a similar manner, the factors may 

reflect the impacts of primary sources, as well the secondary species that have 

condensed on these primary particles. Since each technique has strengths and 

weaknesses, combined the two source apportionment techniques provide 

complementary results. 

Positive Matrix Factorization 

The PMF2 model was applied to chemically speciated PM2.5 data collected at 

the Fresno-First St. and Bakersfield-California Ave. Speciation Trends Network 

(STN) monitoring sites from July 2003 through December 2006. The average 

source contribution estimates illustrated in Figure 21 show ammonium nitrate is 

the major contributor to PM2.5 at both sites (41 percent at Fresno and 45 percent 

at Bakersfield). Biomass smoke contributes similar percentages at both sites 

(8 percent at Fresno, 7 percent at Bakersfield) as does the total aged aerosol 

(10 percent at Fresno, 11 percent at Bakersfield). The main differences in 

source contribution between the two sites include particles directly emitted from 

motor vehicles, which at Fresno account for 23 percent of PM2.5, the 2nd major 

component, while at Bakersfield account for 13 percent. Ammonium sulfate 

accounts for 10 percent of PM2.5 at Fresno and 14 percent at Bakersfield. 

Airborne soil is a minor contributor to PM2.5 at Fresno (3 percent), with a 

9 percent contribution at Bakersfield. 

Figure 21. Average Source Contributions Estimated Using PMF2 

(2003-2006) 

July 2003 - Dec. 2006 Average Source 

Contribution in Fresno-1st Street

Airborne 

soil

3%

Secondary 

nitrate

41%

Aged 

aerosol 

with sea 

salt2

5%

Motor 

vehicle

23%

Industrial

5%

Biomass 

smoke

8%

Aged 

aerosol 

with sea 

salt1

5%

Secondary 

Sulfate

10%

July 2003 - Dec. 2006 Average Source 

Contribution in Bakersfield-California Ave.

Secondary 

nitrate

45%

Airborne 

soil

9%

Industrial

1%

Aged 

aerosol 

with sea 

salt2

7%

Motor 

vehicle

13%

Aged 

aerosol 

with sea 

salt1

4%

Biomass 

smoke

7%

Secondary 

Sulfate

14%
 

B-24 

Chemical Mass Balance Modeling

Annual average source contributions at Bakersfield-5558 California Avenue 

(BAC) and Fresno-1st Street (FSF) were also estimated by applying the 

Chemical Mass Balance (CMB 8.2) model to individual PM2.5 sample 

concentrations collected between January 1, 2004 and December 31, 2006 and 

using source profiles for PM2.5 developed during previous studies. Source 

contribution estimates were calculated based on the 2004-2006 annual average 

of the individual samples. Figure 22 shows the calculated contributions to 

ambient fine particulate matter made by sources included in the model. 

Ammonium nitrate was the most significant source contributing 44% and 39% of 

the PM2.5 mass at Bakersfield and Fresno, respectively. Ammonium sulfate 

accounted for 12% at Bakersfield and 10% at Fresno. The motor vehicle exhaust 

(diesel and gasoline combined) contribution was 9% at both sites. Vegetative 

burning, which included residential wood combustion and agricultural and 

prescribed burning, contributed 9% and 14%, respectively, of the PM2.5 mass. 

The sea salt contribution was negligible at both sites. The ‘Other OC’, which 

represents contributions from secondary organic carbon, other unidentified 

primary sources, and the possible positive sampling artifacts of organic carbon, 

accounted for 20% to 23% of the PM2.5 mass. 

Figure 22. CMB Model Calculated 2004-2005 Average PM2.5 Source 

Contributions. 

BAC Modeled 2004-2006 Avg

Ammonium 

Nitrate

45%

Ammonium 

Sulfate

12%

Sea Salt

0%

Biomass 

Burning

9%

Other OC

20%

Airborne 

Soil

4%

Tire/Brake

1%

Motor 

Vehicle

9%

FSF Modeled 2004-2006 Avg

Ammonium 

Nitrate

39%

Ammonium 

Sulfate

10%

Sea Salt

0%

Biomass 

Burning

14%

Other OC

23%

Airborne 

Soil

4%

Motor 

Vehicle

9%

Tire/Brake

1%
 

B-25 

While the specific contributions vary to some extent, taken together these 

sources apportionment studies highlight the importance of secondary 

ammonium nitrate contributions to both the 24-hour and annual average 

concentrations. In addition, biomass burning and mobile sources were found 

to be significant contributors to primary PM2.5. 

Diagnostic Analyses

 Indicator Species Approach 

As discussed in prior sections, trends in ammonium nitrate concentrations have 

tracked well with concurrent reductions in NOx, suggesting that NOx control is an 

effective approach for ensuring further reductions in ammonium nitrate. An 

indicator species approach provides an additional method to investigate which 

chemical precursor a secondary species such as ammonium nitrate is most 

responsive to control of. Ammonium nitrate is formed in the atmosphere through 

the reactions of precursor nitrogen oxides (NOx), reactive organic gases (ROG), 

and ammonia (NH3). The amount of each precursor in the atmosphere relative 

to each other determines how much ammonium nitrate is formed. The chemistry 

is complex, but essentially the precursor in shortest supply will limit how much 

ammonium nitrate is produced. Reducing emissions of this limiting precursor 

provides the best opportunity to cut ammonium nitrate levels. In simple terms, 

photochemical reactions of NOx and ROG form nitric acid (HNO3). Nitric acid 

then reacts with ammonia (NH3) to form ammonium nitrate. Lurmann, et al.4

compared ammonia and nitric acid ambient concentrations measured in the San 

Joaquin Valley during the winter of 2000/2001, as part of CRPAQS. Figures 23 

and 24 show the concentrations of nitric acid and ammonia measured at the rural 

Angiola site and at the urban Fresno site. At both sites ammonia concentrations 

are generally at least an order of magnitude higher than the nitric acid 

concentrations. These ammonia-rich conditions throughout the Valley indicate 

that, during the winter, nitric acid is the limiting precursor. 

 

4

 F. W. Lurmann, S. G. Brown, M. C. McCarthy, and P. T. Roberts: Processes Influencing 

Secondary Aerosol Formation in the San Joaquin Valley during Winter, Journal of Air and Waste 

Management Association, 56, 1679-1693, 2006. 
 

B-26 

Figure 23: Comparison of Ammonia and Nitric Acid Concentrations 

Measured at Angiola during the Winter of 2000/2001 as Part of CRPAQS 

Figure 24: Comparison of Ammonia and Nitric Acid Concentrations 

Measured at Fresno during the Winter of 2000/2001 as Part of CRPAQS 

Comparison of Ammonia and Nitric Acid Concentrations 

Fresno - Winter 2000/2001

-4

0

4

8

12

16

20

-2 0 2 4 6 8

HNO3 Concentration (ug-N/m3

)

NH3 Concentration (ug-N/m3)

1:1 Line

Comparison of Ammonia and Nitric Acid Concentrations 

Angiola - Winter 2000/2001

-10

0

10

20

30

40

50

60

-2 0 2 4 6 8 10 12 14 16 18

HNO3 Concentration (ug-N/m3)

NH3 Concentration (ug-N/m3)

1:1 Line
 

B-27 

4. AIR QUALITY MODELING 

a. Rollback Modeling

In addition to the evaluation of air quality trends and the observational and 

diagnostic analyses, a rollback modeling analysis was conducted by the District 

to estimate the impacts of future emission reductions on resulting air quality. An 

extensive discussion of the rollback methodology and the results are provided in 

Chapter 3 and associated spreadsheets of the District Plan. The rollback 

modeling showed that Fresno and Corcoran would attain the annual PM2.5 

standard with baseline emission reductions in 2014. With the addition of the 

ARB and District proposed control strategy, both Visalia and Bakersfield-Planz 

would attain in 2014 as well. 

b. Grid-Based Modeling

As stipulated in the EPA Modeling Guidance, a grid-based photochemical model 

is necessary to perform the modeled attainment test for PM2.5 (EPA, 20075

). 

Such models offer the best available representation of important atmospheric 

processes and are an essential tool in analyzing the impacts of proposed 

emissions controls on pollutant concentrations. The EPA recommends 

guidelines for choosing a model for use in the attainment test. For example, the 

model source code should be free or low cost, modeling elements should have 

undergone rigorous scientific peer-review, and it should have been shown to 

perform well in the past for similar applications. 

The Community Multiscale Air Quality Modeling System (CMAQ) has been 

selected for use in the PM2.5 modeled attainment demonstration for the San 

Joaquin Valley Air Pollution Control District. CMAQ is a state-of-the-science 

“one-atmosphere” system that treats major atmospheric and land processes 

(e.g., advection, diffusion, gas phase chemistry, gas-particle mass transfer, 

nucleation, coagulation, wet and dry deposition, aqueous phase chemistry, etc.) 

and a range of species (e.g., anthropogenic and biogenic, primary and 

secondary, gaseous and particulate) in a comprehensive framework (EPA, 

19996

; CMAS, 20077). 

 

5 U.S. Environmental Protection Agency. (2007) Guidance on the Use of Models and Other 

Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional 

Haze. EPA-454/B-07-002 

6

 U.S. Environmental Protection Agency (1999) Science Algorithms of the EPA Models-3 

Community Multiscale Air Quality (CMAQ) Modeling System. EPA/600/R-99/030. 

http://www.epa.gov/asmdnerl/CMAQ/CMAQscienceDoc.html

7 Community Modeling and Analysis System (2007) CMAQ v4.6 Operational Guidance 

Document. http://www.cmaq-model.org/op_guidance_4.6/manual.pdf
 

B-28 

CMAQ was run for the year 2000 to provide the basis for the model performance 

evaluation. It was during 2000 that the California Regional PM10/PM2.5 Air 

Quality Study (CRPAQS) took place. The study resulted in a wealth of data with 

which to evaluate model performance. As it is necessary to execute simulations 

for a model reference year and a future year to perform the recommended 

modeled attainment demonstration, 2005 and 2014 were also simulated. 

Simulations for all years were driven by the meteorological inputs for 2000, while 

emissions varied from year to year. 

Regional air quality modeling only represents a portion of the attainment test. In 

order to perform the EPA-recommended Speciated Modeled Attainment Test, or 

SMAT, the relative response between the modeled reference and future years 

must be considered in conjunction with observations. This approach minimizes 

the uncertainties in predicting future year attainment that result from potential 

model bias in predicting absolute species concentrations. 

Federal Reference Method (FRM) PM2.5 mass measurements provide the basis 

for nonattainment designations. For this reason it is recommended that the FRM 

data also be used to project future air quality and progress towards attainment of 

the health-based National Ambient Air Quality Standard (NAAQS) for PM2.5. 

However, given the complex physicochemical nature of PM2.5, it is necessary to 

consider individual species as well. While the FRM measurements give the 

mass of the bulk sample, a method for apportioning this bulk mass to individual 

PM2.5 components is a first step towards determining the best targets for 

emissions controls in order to reach NAAQS levels in a timely manner. Given 

that (1) attainment status is currently dependent upon FRM measurements and 

(2) concentrations of individual PM2.5 species need to be considered in order to 

understand the nature of and efficient ways to ameliorate the PM2.5 problem in a 

given region, a method has been developed to speciate bulk FRM PM2.5 mass 

with known FRM limitations in mind. This method is referred to as the measured 

Sulfate, Adjusted Nitrate, Derived Water, Inferred Carbonaceous material and 

estimated acidity (H+) approach or “SANDWICH”. SANDWICH is based on 

speciated measurements from other (often co-located) samplers, such as those 

from the Speciated Trends Network (STN), and the known sampling environment 

of the FRM. The approach serves to provide mass closure, reconciliation 

between speciated and bulk mass concentration measurements, and the basis 

for a connection between observations, modeled PM2.5 concentrations, and the 

air quality standard. 

The SMAT procedure was applied to FRM monitors operating in the San Joaquin 

Valley with minimal deviations from U.S. EPA’s recommended procedure. The 

2006 design value was used as a basis from which to project forward to 

estimated future year design values for the year 2014. Speciation data for four 

STN (speciation) sites was used to speciate the FRM mass for all FRM sites. 

For those sites not collocated with STN monitors, “surrogate” speciation sites 

were determined based on analysis of CRPAQS data to determine which sites 
 

B-29 

had similar speciation profiles. The composition was assumed to be the same at 

all three Bakersfield sites (BAC, BGS, and BEP). Similarly, the percent 

composition at the two Fresno sites (FSF and FSH) was assumed to be the 

same. In addition, Stockton (SOH), Clovis (CLO), Corcoran (COP), and Modesto 

(MRM), were assumed to have the same speciation as one of the four speciation 

sites based on CRPAQS data analysis. 

Quarterly average species concentrations were calculated at each STN site 

using the SANDWICH procedure. Modeled concentrations for the reference year 

(2005) and future year (2014) for each component were extracted for the FRM 

sites as a nine-cell average. The relative response factors were calculated for 

each component for each quarter. These calculations were performed using all 

modeled days, as we assumed that the selected FRM measurements provided a 

stable quarterly average value. 

These quarterly species percentages were then multiplied against the base year 

design value for 2006 (the average FRM PM2.5 concentrations for 2004, 2005, 

and 2006). The quarterly observed species concentrations were then multiplied 

by the RRFs and summed and averaged to get a future year PM2.5 design value 

at each FRM site. See Table 3 for the predicted baseline and controlled 2014 

PM2.5 design values. For the “controlled” 2014 emissions sensitivity scenario 

described above, future annual PM2.5 concentrations at all FRM sites are below 

the annual PM2.5 NAAQS of 15.0 μg/m3

, and, therefore, the San Joaquin Valley 

has passed the speciated modeled attainment test for the annual PM2.5 NAAQS. 

A similar procedure to the attainment demonstration for the annual PM2.5 

standard was followed for the 24-hour PM2.5 standard attainment demonstration. 

The exception was that only the top 25% of the measured and modeled days for 

each quarter were used instead of all available days. The top 25% of the days 

are expected to be more representative of the 24-hour design value than would 

all available days for a given quarter. 

Table 4 shows the predicted 2014, 24-hour PM2.5 design values for the top five 

2006 design value sites for the controlled emissions case. As shown, all sites in 

the SJV attained the 24-hour standard in 2006, and further emissions controls do 

not cause any monitors to become non-attainment. 
 

B-30 

Table 3. Reference and future year annual design values for SJV FRM sites 

Site Code Speciation 2006 

DV 

2014 

Baseline 

DV 

2014 

"Controlled" 

DV 

Bakersfield - 5558 California BAC BAC 18.51 15.86 14.28 

Bakersfield - 410 E Planz 

Road BEP BAC 18.86 16.26 14.70 

Bakersfield - Golden State BGS BAC 18.64 15.98 14.39 

Clovis - N Villa Avenue CLO FSF 16.39 14.10 12.72 

Corcoran - Patterson 

Avenue COP VCS 17.24 14.75 13.27 

Fresno - 1st Street FSF FSF 16.68 14.43 13.01 

Fresno - Hamilton and 

Winery FSH FSF 17.16 14.93 13.47 

Merced - 2334 M Street MRM M14 14.69 12.85 11.76 

Modesto - 14th Street M14 M14 14.10 12.52 11.44 

Stockton - Hazelton Street SOH M14 12.93 11.77 10.87 

Visalia - N. Church Street VCS VCS 18.20 16.05 14.47 

Table 4. Reference and future year 24-hour design values for SJV FRM sites 

Site Code Speciation 2006 

DV 

2014 

"Controlled" 

DV 

Bakersfield - 5558 California BAC BAC 62.4 46.2 

Bakersfield - 410 E Planz 

Road BEP BAC 65.2 45.9 

Bakersfield - Golden State BGS BAC 64.4 45.3 

Fresno - 1st Street FSF FSF 58.0 41.2 

Fresno - Hamilton and 

Winery FSH FSF 58.5 41.7 

5. SUMMARY 

In summary, the San Joaquin Valley has experienced tremendous progress in 

reducing PM2.5 concentrations since initial monitoring began in 1990. Between 

1990 and 1998, PM2.5 annual average concentrations dropped by approximately 

40 percent throughout the Basin. Since 1999, when the official regulatory 

monitoring program for comparison to the federal PM2.5 standard began, PM2.5 

annual average concentrations have dropped a further 19 to 29 percent. While 

the peak annual average design value in the basin was 24.7 ug/m3 in 2001, 

approximately one and a half times the level of the standard, it has dropped to 

18.9 ug/m3

 in 2006. Currently, annual design values in the northern portion of 

the Valley are below the standard. In addition, based on 2004-2006 data, the 

San Joaquin Valley attains the federal 24-hour standard of 65 ug/m3. Although 

concentrations remain the highest in the central and southern portions of the 
 

B-31 

Valley, the rate of decrease in PM2.5 in response to ongoing control programs 

has been significant. 

Unlike ozone, PM2.5 consists of many different components. Analysis of the 

trends in different components of PM2.5 has shown that over the last six years, 

decreases in carbonaceous aerosols and ammonium nitrate have had the 

greatest contribution to declining PM2.5 mass trends, with carbonaceous aerosol 

concentrations dropping by 16 to 37 percent and ammonium nitrate 

concentrations dropping by 23 to 24 percent. During this same time period, 

ambient NOx concentrations, a precursor to ammonium nitrate, decreased by 

15 percent, while NOx emissions dropped by 4 percent. Looking back to the late 

1980s, both ammonium nitrate and ammonium sulfate have declined 

significantly, along with concurrent declines in ambient NO2 and SO2

concentrations and NOx and SOx emissions. These significant trends in 

declining PM2.5 concentrations, ambient precursor concentrations, and 

associated emissions all suggest that a continuing focus on further NOx 

reductions, as well as sources which contribute to carbonaceous aerosols will 

provide significant further progress in attaining the annual PM2.5 standard. 

In order to estimate the impact of further emission reductions on future PM2.5 

concentrations, both linear rollback and grid-based aerosol modeling were 

conducted. PM2.5 annual average concentrations in 2014 based on emission 

reductions achieved from both the ongoing control program, as well as new State 

and local measures, were estimated with both methods. Substantial further 

emission reductions for all key pollutants will occur between 2005 and 2014. 

Based on the grid-based modeling, several sites in the District would be 

expected to attain the annual standard in 2014 with baseline emission 

reductions. With the addition of the new State and local measures focusing on 

NOx, carbon, and directly-emitted combustion emissions, the sites with the most 

severe problems – Visalia and Bakersfield – would also attain in 2014 with a 

maximum predicted design value of 14.7 in Bakersfield. The linear rollback 

modeling showed similar results, providing a consistent assessment of 

attainment prospects. It also appears that 2014 is the earliest feasible attainment 

date. This is due to consideration of the following factors: 1) baseline emission 

reductions that occur during the period up to 2014 are not sufficient alone to 

provide for attainment at all sites, 2) the additional emission reductions that will 

result from the State Strategy will not phase in prior to 2014 due to the 

extraordinarily complex and innovative nature of developing and implementing 

these new measures, and 3) the San Joaquin Valley has experienced periods of 

slower air quality progress due to the influence of meteorology, suggesting that 

the full timeframe up to 2014 will be needed to ensure attainment. 

Therefore, considering all of the information available, the weight-of-evidence 

analyses provide a consistent assessment that the San Joaquin Valley will attain 
 

B-32 

the annual average PM2.5 standard throughout the District in 2014. This is due 

to the following considerations: 

 The significant progress that has already occurred, a 6 ug/m3 drop in 

annual average design value between 2001 and 2006, which 

represents two-thirds of the progress needed to attain the annual 

standard by 2014; 

 The positive response of ammonium nitrate, ammonium sulfate, and 

carbon concentrations to past reductions in NOx, SOx, and primary 

PM2.5 emissions; 

 The substantial future emission reductions that will occur due to both 

baseline commitments as well as new measures from NOx, and 

primary PM2.5 emissions that have worked in the past; 

 Linear rollback modeling which indicates attainment in 2014; and, 

 Grid-based aerosol modeling which also indicates attainment in 2014 

at all sites in the District.