Appendix B 

Weight-of-Evidence Analysis 

San Joaquin Valley Air Basin: Ozone 
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May 30, 2007 Appendix B 1 

Appendix B 

Weight-of-Evidence Analysis 

San Joaquin Valley Air Basin: Ozone 

Introduction

Under federal law, the San Joaquin Valley Air Basin (San Joaquin Valley, SJV, 

Valley, or Basin) is currently classified as a Serious nonattainment area for the 

federal 8-hour ozone standard. This classification requires attainment by 

June 15, 2013. Given the magnitude of emissions reductions needed to reach 

attainment, including reductions relying on new or the improvement of existing 

control technologies, it is not likely the Valley will meet the 2013 attainment date. 

In such cases, the federal Clean Air Act allows, and U.S. EPA recommends, 

bumping-up to an appropriate higher classification with a later attainment date. 

Based on analyses, an Extreme classification, with an attainment date of 

June 15, 2024, is the Valley’s most realistic option. The following sections 

describe the air quality, emissions, and supplemental analyses, as well as the 

photochemical modeling that support the overall conclusion that the San Joaquin 

Valley can attain the federal 8-hour ozone standard by the 2024 deadline. 

U.S. EPA Attainment Demonstration Requirements

The attainment demonstration portion of a State Implementation Plan or SIP 

consists of the analyses used to determine whether a proposed control strategy 

provides the reductions necessary to meet the federal standard by the attainment 

year. This attainment demonstration includes photochemical modeling which 

predicts that projected controls will result in a high site 8-hour design value for 

the SJV that is below the level of the federal standard by 2024. While 

reclassifying as Extreme extends the attainment date, the Valley should 

nevertheless realize substantial progress over the next decade. Air quality 

modeling predicts that all sites in the Basin will reach attainment prior to 2024, 

with the exception of several sites with the most severe air quality problems. 

However, with the implementation of proposed emissions control measures, 

ozone air quality in these areas is projected to meet the standard by the 2024 

deadline. 

Because of the uncertainties inherent in photochemical modeling, U.S. EPA 

allows states to supplement the modeling results with a “weight of evidence” 

(WOE) demonstration. The WOE assessment provides a set of analyses that 

complement the SIP-required modeling. These analyses can include 

consideration of measured air quality, emissions, and meteorological data, 

evaluation of other air quality indicators, as well as additional air quality 

modeling, if appropriate. Because all analysis methods have inherent strengths 

and weaknesses, examining an air quality problem in a variety of ways helps to 

offset the limitations and uncertainties inherent to air quality modeling. This 
May 30, 2007 Appendix B 2 

approach also provides a better understanding of the overall problem, as well as 

insight about the level and mix of emissions controls needed for attainment. 

The scope of the WOE analysis is different for each nonattainment area, with the 

level of appropriate detail dependent 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. This document summarizes the analyses that 

comprise the WOE assessment for the San Joaquin Valley nonattainment area. 

Historical Context

Over the years, ozone has posed a persistent problem in the San Joaquin Valley. 

Looking at ozone air quality from an historical perspective is challenging because 

of the lack of long-term sites in this area. Between 1975 and 1990, monitoring 

began at a number of sites, but was discontinued after several years. 

Furthermore, these transient monitors did not include sites in the worst areas of 

the central and southern portions of the Basin. For these reasons, 1990 was 

chosen as the start year for long-term trends in the SJV. 1990 is the first year for 

which Arvin, consistently one of the highest sites in the Valley, has complete data 

during the May through October ozone season. In addition, data are available for 

a number of other typically high concentration sites, including Clovis, Edison, 

Parlier, and several Fresno area sites. 

Over the long-term, emissions control programs have improved ozone air quality 

in the SJV, but not to the same degree as seen in other areas of California. Both 

the climate and geography of the Valley present significant challenges to 

progress in the SJV. Figure B-1 shows the 1990 to 2005 basinwide trends for 

several air quality indicators. Because the trend lines for both federal 8-hour 

exceedance days and maximum concentrations reflect values for individual 

years, they show a fair amount of variability, with only a small amount of progress 

over the 15-year period. Overall, the decrease in the number of exceedance 

days was more substantial than the decrease in maximum concentrations. In 

contrast to these two indicators, the other two indicators shown on the graph, the 

design value and the mean of the maximum concentrations on the Top 30 days, 

are less variable because these indicators are more robust. While these two 

indicators show even less change over the 15-year period, the 2005 values are 

lower than the 1990 values. 

Although not shown in Figure B-1, perhaps the greatest indicator of ozone air 

quality improvement in the SJV is the reduction in population-weighted exposure. 

This indicator shows a 50 percent reduction in exposure to concentrations above 

the level of the federal 8-hour standard between 1990 and 2005. Despite the 

gains in improving population-weighted exposure, the overall rate of progress for 

other indicators in the SJV has been slow, and this area will face tremendous 

challenges in reaching attainment. 
May 30, 2007 Appendix B 3 

Figure B-1: San Joaquin Valley Air Basin Ozone Statistics 1990 to 2005 

 

San Joaquin Valley Air Basin Ozone Statistics

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

1990199119921993199419951996199719981999200020012002200320042005

Year

Ozone (ppm)

0

25

50

75

100

125

150

175

200

Maximum Concentration Design Value* Mean Top 30 Concentrations Exceedance Days

 

Assessment of Recent Air Quality Trends

 General Basinwide Perspective 

Over the years, ozone improvement in the SJV has lagged behind other areas of 

California, and the Valley ranks second only to the South Coast Air Basin with 

respect to the nation’s worst ozone air quality. Modest levels of progress have 

occurred in the SJV over the last ten years, with a 15 percent drop in maximum 

concentration, a 5 percent drop in design value, and a 35 percent drop in 

exceedance days between 1995 and 2005 (refer to Figure B-1). However, most 

of this improvement has occurred since 2003. While values for 2006 were up 

slightly from 2005 (maximum concentration of 0.121 ppm and 86 exceedance 

days), they were still among the lowest values over the last 15 years. Ozone 

levels in the SJV are not as high as in the South Coast; however, maximum 

concentrations during 2006 were still more than 40 percent higher than the 

federal standard, with nearly three months of exceedance days each year. 

 

While ozone levels are still unhealthy, modest improvements over the years have 

resulted in a reduction of the extent of the problem, especially in the northern 

portion of the Valley. The maps in Figure B-2 are based on monitoring data and 

estimate the reduction in days exceeding the national 8-hour standard over the 

last decade (1995 to 2005), throughout the San Joaquin Valley, thereby providing 

an estimate of the spatial extent of the ozone problem. Ten years ago (1993 to 

1995 average map), more than half of the SJV experienced between 21 and 

50 federal 8-hour exceedance days, with the worst site experiencing about 

90 days. Areas in the northern SJV were cleaner than areas in the central and 

southern Valley. However, only a relatively small portion of the Basin averaged 

10 or fewer exceedance days. 

* The design value for 1990 is based on 2 years of data, while the design values for all other years are based on 3 years of data.
May 30, 2007 Appendix B 4 

Figure B-2: San Joaquin Valley Air Basin Change in Federal 8-Hour 

 Exceedance Days 1995 to 2005 

Today (2003 to 2005 average map), we see a substantial expansion of areas 

with 10 or fewer exceedance days. Ambient concentrations in most of 

San Joaquin and Stanislaus counties are now below the level of the federal 

8-hour ozone standard. Much of the rest of the Valley experiences an average of 

only 6 to 20 exceedance days per year. Areas with more than 20 exceedance 

days are now generally limited to the eastern portion of the central and southern 

SJV. While the extent of these areas is much smaller than during 1995, the 

areas of poor ozone air quality are also some of the most heavily populated 

(Fresno and Kern counties). Even though these areas still pose a substantial 

challenge, the worst sites show an average reduction in exceedance days of 

approximately 35 percent over the last ten years. 

 

In summary, although there has been some progress in the SJV over the last 

ten years, the rate of progress has been slow in comparison to other areas of the 

State. Overall, the trend lines for various air quality indicators, including 

maximum concentration, exceedance days, design value, and mean of the 

Top 30 concentrations, are relatively flat, with some year-to-year variability 

caused by meteorology (refer to Figure B-1). Most of the progress seen over the 

last 15 years has occurred since 2003. While there has been only a 15 percent 

decrease in maximum concentration since 1995, the decrease in the number of 

exceedance days has been more substantial, at close to 35 percent. In spite of 

the slow rate of progress, the ozone problem is now confined mostly to the 

central and southern portions of the Valley, as continued emissions reductions 

have been successful in shrinking the spatial extent of the problem areas. At the 
May 30, 2007 Appendix B 5 

same time, the “clean” areas have expanded substantially, and nearly all of 

San Joaquin and Stanislaus counties now have air quality that meets the federal 

8-hour standard. However, although these counties are generally clean with 

respect to ozone, emissions from these northern SJV areas can impact ozone air 

quality in other portions of the Valley. 

Regional Analyses 

The basinwide air quality indicators for the SJV show limited progress because 

they are dominated by the high sites, which pose the most severe problems. 

However, when the Basin is subdivided into different regions, different patterns of 

progress emerge. For the following discussion, the Valley is divided into three 

general areas, as shown in Figure B-3: the northern SJV, the central SJV, and 

the southern SJV. For convenience, these regions are divided along county 

boundaries. However, they generally represent three distinct areas with respect 

to geography, meteorology, and air quality. While ozone air quality within each of 

the three subregions tends to be similar, the level of air quality and rates of 

progress from one area to another can vary substantially. 

Figure B-3: San Joaquin Valley Air Basin Subregions 

 

 
May 30, 2007 Appendix B 6 

A third of the Basin population lives in the northern SJV. This lowland area is 

bordered by the Sacramento Valley and Delta lowland to the north, the central 

portion of the SJV to the south, and on the other two sides by mountains. 

Because of the marine influence, which extends into this area through gaps in 

the coastal mountains to the west, the northern SJV experiences a more 

temperate climate than the rest of the Basin. These cooler temperatures and the 

predominant air flow patterns generally favor better ozone air quality. 

In contrast to the northern SJV, most of the Valley population lives in the central 

and southern portions of the Basin, in and around the Fresno and Bakersfield 

urban areas. Sites in the central and southern areas exceed the federal standard 

by the greatest margin, and geography, emissions, and climate pose significant 

challenges to air quality progress. Similar to the northern SJV, the central and 

southern SJV are also low lying areas, flanked by mountains on their west and 

east sides. The southern SJV represents the terminus of the Valley and is 

flanked by mountains on the south, as well. The surrounding mountains in both 

areas act as barriers to air flow, and combined with recirculation patterns and 

stable air, trap emissions and pollutants. The higher temperatures and more 

stagnant conditions in these two regions lead to a build-up of ozone and overall 

poorer air quality. In addition to the urban air quality problems, emissions and 

pollutants from these areas are transported downwind, making for even poorer 

air quality in downwind areas such as Arvin and the Sequoia National Park. 

ARB staff completed an analysis of ozone episodes that occurred in both the 

central and southern SJV during 2004 and 2005. Based on these data, high 

ozone concentrations occurred as multi-day episodes more than 65 percent of 

the time, in both regions. Furthermore, episodes with higher federal 8-hour 

concentrations typically spanned a greater number of days, with the highest 

concentrations occurring in the middle of the episode period. During 2004 and 

2005, more than 75 percent of the central SJV ozone episodes showed their 

highest 8-hour concentration at sites located within the Sequoia National Park. 

During more than 40 percent of the episodes, exceedances were limited only to 

sites located within the Sequoia National Park. While the downwind Sequoia 

sites tend to be the most problematic in the central SJV, is it interesting to note 

that very few central SJV episodes began prior to the start of an episode in the 

southern SJV. In fact, nearly 90 percent of the central SJV episodes started on 

the same day or during an ozone episode in the southern SJV. The most 

problematic site in the southern SJV is Arvin, and during 2004 and 2005, about 

95 percent of the southern SJV ozone episodes showed their highest 8-hour 

concentration at Arvin. 

Figure B-4 shows the average number of exceedance days during 1995 and 

2005 for each of the subregions mapped in Figure B-3. Two sites, Sequoia 

National Park-Lower Kaweah and Arvin are plotted separately, and therefore, 

data for these two sites are not included in the totals for the central and southern 

SJV areas. The Sequoia National Park-Lower Kaweah and Arvin sites are 
May 30, 2007 Appendix B 7 

located downwind of the Fresno and Bakersfield urban areas, respectively, and 

tend to have poorer air quality. 

 

The northern SJVAB continues to be far cleaner than the other areas of the SJV. 

Over the last decade, the number of exceedance days in this area has 

decreased about 70 percent. During 2005, about 80 percent of the days during 

the May through October ozone season were below the more stringent State 

8-hour standard. However, while the number of days in this region has shown 

improvement, Modesto stands out as the high site in the northern SJV. 

Figure B-4: San Joaquin Valley Air Basin Change in Number of Federal 8-Hour 

Exceedance Days by Subregion 1995 and 2005 

 

Number of Exceedance Days 

During 1995 and 2005

0

10

20

30

40

50

60

70

80

90

100

Northern SJV

Central SJV

Southern SJV

Sequoia-Lower Kaweah

Arvin

# of Exceedance Days

1995 2005

 

From north to south, the severity of the ozone problem in the SJV generally 

increases. Between 1995 and 2005, the number of exceedance days at sites in 

the central SJV (excluding the Sequoia area) decreased 55 percent. Although 

the decrease is still relatively high, the number of days in the central SJV during 

2005 was five times higher than in the northern SJV. The number of exceedance 

days in the southern SJV (excluding Arvin) decreased about 65 percent during 

the last decade, and the number of exceedance days during 2005 was just 

slightly higher than the number of days in the central SJV. With respect to days 

below the State 8-hour standard, about 40 percent of the days during the ozone 

season were below this level in both the central SJV and the southern SJV areas 

during 2005. Similar to the basinwide trends, most of the progress in the central 

and southern SJV subregions has occurred since 2003. 

The sites downwind of the Fresno and Bakersfield urban areas continue to pose 

the most severe problems in the SJV, and improvements in these areas have 

been much slower than in other areas. Arvin has always been one of the high 

sites in the Basin. Between 1995 and 2005, federal exceedance days declined 
May 30, 2007 Appendix B 8 

about 30 percent, which is lower than the rate seen at other sites in the southern 

SJV region. In contrast, sites located at higher elevations in the Sequoia 

National Park have shown worsening ozone air quality over the last several 

years. Between 1995 and 2005, the number of federal exceedance days actually 

increased more than 75 percent at the Sequoia-Lower Kaweah site. This 

increase highlights the problem of transported emissions and pollutants from the 

upwind urban area. The Sequoia-Lower Kaweah site was used in this 

comparison because it is a long-term site with data for both 1995 and 2005. 

However, it should be noted that during 2005, the Sequoia-Kings Canyon site 

had even poorer air quality. In fact, during 2005, the Kings Canyon site had the 

same number of exceedance days as Arvin, as well as a similar maximum 

concentration. 

Similar to exceedance days, concentrations have also been decreasing at a 

faster rate in the urban areas than at Arvin or Sequoia. Peak concentrations, as 

measured by the mean of the Top 4 daily concentrations, decreased only 

3 percent over the last five years at Arvin and increased in the Sequoia area. 

However, the same indicator decreased at twice that rate in the Bakersfield and 

Fresno urban areas. Today, the 4th highest 8-hour ozone concentration 

averages 0.095 ppm for sites in both urban areas, compared with 0.105 ppm five 

years ago. Similarly, the mean of the Top 30 concentrations for both urban areas 

is declining and is now close to the level of the federal standard. The mean of 

the Top 30 concentrations is 0.084 ppm for the Fresno/Merced area and 

0.089 ppm for the Bakersfield region. Five years ago, both of these urban areas 

had mean Top 30 concentrations greater than 0.100 ppm. Although the mean of 

the Top 30 concentrations is not directly comparable to the federal standard, it is 

a fairly stable statistic that is less influenced by year-to-year changes in 

meteorology. Therefore, it provides an indication of how concentrations on the 

worst days of the year are changing over time. 

In summary, there have been changes in the patterns of exceedances on a 

subregional basis in the SJV over the last ten years. Today, the numbers of 

exceedance days in all areas except the Sequoia region are smaller than they 

were ten years ago. The most progress occurred in the northern SJV, and ozone 

concentrations in this area are now below the level of the more stringent State 

8-hour standard 80 percent of the time during the ozone season. Trends in peak 

ozone concentrations reflect similar subregional differences. Based on current 

air quality and past trends, the areas downwind of Bakersfield and Fresno will 

likely pose the most difficulty for attainment. 

Meteorology and Air Quality Trends

Ozone in the ambient air is the result of several factors, two of the most important 

being pollutant emissions and meteorology. The meteorological and 

photochemical processes leading to ozone formation are complex, 
May 30, 2007 Appendix B 9 

involving interactions both at the surface and in the upper air. However, they can 

be characterized in very general terms: strong sunlight and weak dispersion 

generate relatively high ozone levels, while weak sunlight and strong dispersion 

generate relatively low ozone levels. Meteorology, or weather conditions, can 

vary widely, and these day-to-day conditions strongly influence ambient ozone 

concentrations. 

The previous trends discussion looked at air quality as measured at ambient 

monitoring sites, without any consideration of or adjustment for meteorological 

variability. The following discussions characterize the general meteorological 

conditions leading to high ozone concentrations, as well as several different 

methods of accounting for meteorological variability. These analyses are an 

effort to better understand the impact of meteorology on air quality and thereby 

track improvements attributable to emissions reductions. Another goal of these 

analyses is to determine the role meteorology has played in the SJV, where 

ozone improvement has lagged behind other areas of the State. Although ozone 

improvements have been slower to occur in the SJV, the following analyses 

show that modest progress has occurred. 

 Characterization and Frequency of Episodic Conditions 

In evaluating the meteorological conditions associated with ozone episodes, ARB 

staff identified three well-defined patterns as being associated with the majority of 

episodes. Collectively, these three patterns are associated with nearly 

90 percent of the 8-hour exceedances. Summertime meteorology in the Valley is 

dominated by a constant ebb and flow between high atmospheric pressure over 

the Southwest Desert and low pressure over the Gulf of Alaska. When a high 

pressure ridge moves over California, stagnant conditions can persist for several 

days, resulting in widespread violations of the federal 8-hour standard. When a 

low pressure trough pushes inland, the Valley floor may see considerable 

improvements in air quality. However, downwind transport-impacted areas may 

experience higher concentrations. 

In order to categorize the air flow patterns prevalent in the San Joaquin Valley, 

ARB staff performed a cluster analysis involving all days during May through 

October, 1990 through 2005. Subsequently, staff considered the meteorological 

characteristics of a subset of the clusters (2000 through 2005) and the incidence 

of federal 8-hour ozone episodes found in each cluster. These assessments 

show that nearly 90 percent of the SJV federal 8-hour exceedances are 

associated with three meteorological patterns, each distinguishable by the 

relative dominance of a high pressure ridge versus a low pressure trough. These 

patterns are generally characterized by high pressure aloft, mostly clear skies, 

warm temperatures, and strong morning inversions. The differences between 

the patterns are seen through large-scale variations in atmospheric pressure 

along the coast. 
May 30, 2007 Appendix B 10 

Under the “high pressure ridge pattern,” a ridge of high pressure covers all of the 

SJV, leaving the region with clear skies and very warm temperatures. Surface 

winds are terrain driven, with little or no sea breeze moving through the 

Carquinez Strait. Average afternoon wind speeds typically range between 3 and 

7 miles per hour (mph), with only a few hours above 10 mph recorded in the 

northern part of the Basin. Maximum surface temperatures normally reach 

100 degrees in most areas, with occurrences of 105 degrees or greater during 

the peak summer months. The resulting stagnant conditions promote the 

formation of high ozone levels throughout the Basin, with maximum 

concentrations found close to the major source areas. Areas typically dominated 

by transport, such as the higher elevations of Sequoia National Park, will exhibit 

lower concentrations in comparison, as light winds keep pollution movement to a 

minimum. This pattern has led to the highest ozone readings in the Basin, and 

was associated with about half of the 8-hour ozone exceedances recorded during 

May through October of 2000 to 2005. 

The “approaching trough pattern” causes ozone and its precursors to move 

downwind from source locations. Under this pattern, the high pressure ridge 

over central California is weaker, and this increases the sea breeze flowing 

through the Carquinez Strait. Average afternoon wind speeds range between 6 

and 12 mph, with a few hours above 14 mph recorded in the northern parts of the 

Basin. The maximum surface temperatures reach into the upper 90s in most 

southern areas, with northern areas in the upper 80s to lower 90s. High ozone 

levels are observed downwind of the major urban source areas (for example, 

downwind of Stockton, Fresno, and Bakersfield). High ozone levels are also 

observed in the foothill areas, such as the Sequoia National Park area. During 

this weather pattern, the extreme northern sections of the Valley experience 

lower ozone concentrations, as increased wind flow pushes pollutants further 

south and east. During May through October of 2000 to 2005, the approaching 

trough pattern was associated with about 20 percent of the measured 8-hour 

ozone exceedances. 

Finally, the “trough pattern” results in peak ozone further downwind of the major 

emission sources, primarily affecting the eastern and southernmost portions of 

the SJV. Under this pattern, the ridge of high pressure is weakened more 

significantly. This increases wind flow through the Carquinez Strait, causing 

temperature and ozone levels to be much lower over the northern and central 

portions of the Valley. Surface winds are predominantly from the west-northwest 

in the northern portion of the Basin. As the surface winds move southward, 

channeled by the terrain, they veer and become a northwest flow. Average 

afternoon wind speeds range between 8 and 16 mph, with some stronger gusts 

at night. Maximum temperatures still reach near 90 degrees or slightly above in 

most low lying areas of Kern and Tulare counties, with the northern areas in the 

80s. Under this weather pattern, ozone levels peak in the southern region of the 

Basin, downwind of major emission source areas. The highest ozone levels are 
May 30, 2007 Appendix B 11 

found at sites such as Parlier, Arvin, and Sequoia National Park, where transport 

is typically the dominant mechanism for ozone and precursor movement. The 

trough pattern was associated with about 20 percent of the 8-hour ozone 

exceedances recorded during May through October, 2000 to 2005. 

 High Ozone Forming Potential 

As one approach to help understand the types of meteorological conditions 

leading to high ozone concentrations, ARB staff completed an analysis of ozone 

and meteorology using Classification and Regression Tree (CART) techniques. 

The CART analysis determined rules that separated days into 15 groups, based 

on the degree to which weather conditions favor ozone formation. The CART 

rules used daily data for surface air temperature, air temperature at 

1500 meters1, wind speed/direction, atmospheric stability, and other factors in 

relation to daily maximum 8-hour ozone concentrations. From the 15 groups, a 

subset with high average ozone levels and containing on average about one-third 

of the ozone season were considered to represent high ozone forming potential 

(OFP). 

The analysis, presented in Figure B-5, shows the number of days with high OFP 

along with the number of days exceeding the federal 8-hour ozone standard each 

year (three-year moving means). The changes in exceedance days relative to 

the changes in high OFP days helps distinguish changes due to meteorology 

from changes due to emissions reductions. Progress is shown when the number 

of exceedance days decreases in relation to the number of high OFP days. 

The two lines generally track together, indicating that year-to-year changes in 

exceedance days have been largely attributable to year-to-year changes in 

weather, rather than changes in emissions. Relative to the high OFP line, 

however, the number of exceedance days has decreased. During the 1990’s, 

the trend for exceedance days averaged 14 days above the trend for high OFP 

days. Since 1999, however, the trend for exceedance days averaged 4 days 

below the trend for high OFP days, indicating a “real” decrease of about 18 days. 

Furthermore, the unsmoothed trends in Figure B-6 show the 68 exceedance 

days measured in 2005 was a new low for the Basin (note that the 68 

exceedance days reflects only those occurring during the May through October 

ozone season). Three years, 1990, 1997, and 1999, had OFP values similar to 

2005, but exceedance days during these years averaged 13 days above the OFP 

trend. In contrast, the 68 exceedance days measured during 2005 were 

13 fewer than the number of high OFP days. These results indicate that some 

real progress in reducing ozone is now taking place in the SJV, as increasingly 

adverse meteorological conditions are needed to create ozone levels exceeding 

the federal 8-hour standard. 

 

1

 Above sea level 
May 30, 2007 Appendix B 12 

Figure B-5: San Joaquin Valley Air Basin Three-Year Means of Federal 8-Hour 

Exceedance Days and High OFP Days 1990 to 2005 

 

San Joaquin Valley Ozone Trends

(Federal 8-hour Exceedance Days and "High OFP" Days)*

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Number of Days (3-year moving mean)

Fed. 8-hr Exceedance Days Days in High OFP CART Bins

* Based on a CART model relating meteorology to the basinwide daily maximum 8-hour ozone from 1990 - 1992.

Figure B-6: San Joaquin Valley Air Basin Federal 8-Hour Exceedance Days 

and High OFP Days 1990 to 2005

 

San Joaquin Valley Ozone Trends

(Federal 8-hour Exceedance Days and "High OFP" Days)*

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Number of Days

Fed. 8-hr Exceedance Days Days in High OFP CART Bins

* Based on a CART model relating meteorology to the basinwide daily maximum 8-hour ozone from 1990 - 1992.
May 30, 2007 Appendix B 13 

Meteorologically Adjusted Trends 

As discussed above, meteorological parameters such as temperature, pressure, 

and wind speed are systematically correlated with sunlight and dispersion, and 

can be used in formulas that predict daily ozone levels. As a second method to 

address the role of meteorology, a statistical model that predicts daily 

maximum ozone on the basis of daily meteorological data was used to adjust 

daily ozone observations. 

First, days from the May through October ozone season for the years 1990 to 

2005 were assigned to separate groups based on pressure and temperature 

gradients, along with selected wind speeds and directions. Together, three of the 

groups accounted for the vast majority of exceedance days during the ozone 

season in the San Joaquin Valley. For each of these groups, data from 1990 

through 1993 were used to calibrate a within-group model to predict daily 

maximum 8-hour ozone from daily weather data. The limited span of years was 

used for calibration so that when it was applied for all the years (1990 through 

2005), it would provide a level playing field for meteorological effects, apart from 

the influence of changes in emissions. 

Met-adjusted trends are presented in the following three figures. The figures are 

based on data for basinwide daily maximum ozone concentrations after these 

have been reconciled to long-term meteorological norms regarding group 

frequencies and concentrations within each group. The three lines on each 

graph represent the mean of the Top 10, Top 20, and Top 30 met-adjusted 

concentrations. The trends in Figures B-8 and B-9 were smoothed using a 

three-year moving mean, because the process of met-adjustment does not 

remove all meteorological effects perfectly, and other factors also affect the 

year-to-year changes. 

Figures B-7 and B-9 show that ozone declined approximately five percent from 

1990 to 2005. An upswing in the trend from 2001 to 2004 may be attributable to 

meteorological effects for which the process of met-adjustment is incomplete. 

Following the upswing, the met-adjusted values for 2005 reached a new low for 

all three indicators, indicating that modest improvement (5 percent) in ozone 

occurred in the San Joaquin Valley in the 2000s, compared to the 1990s. It is 

also noteworthy that this progress has been similar for all three indicators: mean 

of the Top10, Top20, and Top30 ozone concentrations. This shows that the 

Top 30 (top 16 percent2

) summer ozone concentrations have responded very 

similarly to emissions reductions in the SJV since 1990. 

 

2

 The May – October ozone season has 184 days, of which 30 is 16%. 
May 30, 2007 Appendix B 14 

Figure B-7: San Joaquin Valley Air Basin Ozone Trends 1990 to 2005 Adjusted 

 for Meteorology 

 

San Joaquin Valley Air Basin -- Basinwide Ozone Trends

(Annual Means for Top 10, Top 20, and Top 30 Met-Adjusted Daily Max 8-Hour Obs.)*

0.095

0.100

0.105

0.110

0.115

0.120

0.125

0.130

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Mean Ozone (ppm)

Mean of Top 10 Mean of Top 20 Mean of Top 30

* Relationship between ozone and meteorology was calibrated using data from 1990-1992.

Figure B-8: San Joaquin Valley Air Basin Three-Year Mean Ozone Trends 

 1990 to 2005 Adjusted for Meteorology 

 

San Joaquin Valley Air Basin -- Basinwide Ozone Trends

(Annual Means for Top 10, Top 20, and Top 30 Met-Adjusted Daily Max 8-Hour Obs.)*

0.095

0.100

0.105

0.110

0.115

0.120

0.125

0.130

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Moving Avg. (3-year) Ozone (ppm)

Mean of Top 10 Mean of Top 20 Mean of Top 30

*Relationship between ozone and meteorology was calibrated using data from 1990-1992.
May 30, 2007 Appendix B 15 

Figure B-9: San Joaquin Valley Air Basin Ozone Trends 1990 to 2005 Adjusted 

 for Meteorology and Expressed as a Percentage of the Base Year 

 

San Joaquin Valley Air Basin -- Basinwide Ozone Trends

(Annual Means for Top 10, Top 20, and Top 30 Met-Adjusted Daily Max 8-Hour Obs.)*

85%

90%

95%

100%

105%

110%

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Moving 3-Year Mean Ozone (% 1990-1992)

Mean of Top 10 Mean of Top 20 Mean of Top 30

*Relationship between ozone and meteorology was calibrated using data from 1990-1992.

The above analyses use different methods to account for the variable impacts of 

meteorology on ozone air quality. Results of these analyses confirm that 

progress has occurred in the San Joaquin Valley Air Basin, especially during the 

last several years. Although adjusting ozone air quality for meteorology does not 

change the overall flatness of the trend in the SJV throughout most of the 

analysis period, it does show some measure of real progress during the most 

recent years. 

Emissions and Precursor Trends

Oxides of nitrogen (NOx) and reactive organic gases (ROG) are precursors to 

ozone. Emissions controls have reduced the amounts of these precursors 

throughout the Basin, resulting in the modest improvement in ozone air quality 

observed in the SJV. The following sections describe the NOx and ROG 

emissions trends in the SJV since 1994, as well as the amounts of these 

precursors measured in the ambient air. 

 Emissions Trends 

Emissions controls have substantially reduced the amounts of both ROG and 

NOx emitted by various sources throughout the San Joaquin Valley. Figure B-10

shows the estimated basinwide trend in these precursor emissions from 1994 to 

2005. The totals reflect estimates for the summer season in tons per day and 

include emissions from natural biogenic sources. 
May 30, 2007 Appendix B 16 

Figure B-10: San Joaquin Valley Air Basin Estimated NOx and ROG Emissions 

 1994 to 2005 

 

San Joaquin Valley Air Basin NOx & ROG Emissions

Summer SIP Inventory with Biogenics - Ver 1.06

0

100

200

300

400

500

600

700

800

900

1000

1100

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Tons/Day

San Joaquin Valley Air Basin ROG San Joaquin Valley Air Basin NOx

 

Since 1994, there has been a fairly steady decrease, basinwide, in both ROG 

and NOx emissions. While the relative amounts of the two precursors have 

remained fairly constant over the 12-year period, the overall reduction in ROG 

emissions (about 15 percent) has been slightly smaller than the overall reduction 

in NOx emissions (about 20 percent). However, it is important to note that a 

substantial portion of the ROG emissions (from 45 and 50 percent of the total 

ROG between 1994 and 2005) comes from biogenic sources. Because the 

biogenic portion is constant over time, it masks the reductions attributable to 

emissions control programs. When looking only at the anthropogenic portion of 

the ROG inventory, ROG emissions impacted by control programs decreased 

nearly 25 percent between 1994 and 2005. 

Figures B-11 and B-12 show the estimated emissions trends for the three SJV 

subregions. In all three areas, both ROG and NOx emissions are decreasing. 

Similar to the basinwide trend, NOx emissions are decreasing at a faster rate 

than ROG in both the central and southern areas, but not in the northern SJV. 

Another interesting observation is that similar to the basinwide trend, ROG 

emissions are at a higher level than NOx emissions only in the central SJV. In 

the other two areas, NOx emissions are higher. Overall, the greatest reductions 

in NOx emissions (with respect to both percentages and tons per day) occurred 

in the southern SJV. This area has also seen substantial reductions in the 

number of 8-hour exceedance days in both the upwind and downwind portions of 

the region. 
May 30, 2007 Appendix B 17 

Figure B-11: Northern and Central SJV ROG and NOx Emissions 1994 to 2005 

 

 

San Joaquin Valley Air Basin NOx & ROG Emissions

Northern & Central SJV - Summer SIP Inventory w/ Biogenics - Ver 1.06

0

50

100

150

200

250

300

350

400

450

500

550

600

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Tons/Day

Central SJV ROG Central SJV NOx Northern SJV NOx Northern SJV ROG

Figure B-12: Southern SJV ROG and NOx Emissions 1994 to 2005 

 

San Joaquin Valley Air Basin NOx & ROG Emissions

Southern SJV - Summer SIP Inventory with Biogenics - Ver 1.06

0

50

100

150

200

250

300

350

400

450

500

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Tons/Day

Southern SJV NOx Southern SJV ROG

 

 Precursor Trends 

In addition to the reduction in overall emissions as estimated by the emissions 

inventory, measured data show a reduction in the amounts of ROG and NOx in 

the ambient air. Ambient monitoring data from the Photochemical Assessment 

Monitoring Stations (PAMS) network are plotted in Figure B-13 and show 

reductions in both precursors. Since 1994, ROG, as measured by the PAMS 

network in the SJV, shows an overall reduction of about 75 percent. Coupled 

with the reduction in ROG is a similar reduction in the reactivity of the 

hydrocarbon mix (about 70 percent between 1994 and 2005). During this same 

timeframe, ambient NOx concentrations decreased approximately 50 percent, 

with the greatest decrease occurring between 1994 and 1995. Between 1995 

and 2005, ambient NOx concentrations were flatter, with only a modest overall 
May 30, 2007 Appendix B 18 

reduction. The overall trends from the ambient monitoring network are generally 

consistent with the trends in estimated emissions in that they both show both 

precursors decreasing. 

Figure B-13: San Joaquin Valley Air Basin Summer Morning Average ROG, 

 Reactivity, and NOx at PAMS Stations 

 

San Joaquin Valley Air Basin July-August Average at PAMS Stations (5-7 am/6-8 am)* 

for PAMS ROG, PAMS Reactivity, and NOx

0

50

100

150

200

250

300

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

ppb(C)

0

10

20

30

40

50

60

 NOx ppb(N)

PAMS NMOC PAMS Reactivity NOx

* 3-hour ROG/PAMS samples were from either 5-7 am or 6-8 am. The same 55 compounds were used for 1994-2005 (carbonyls were not included). Several 

 samples with very high mixing ratios (orders of magnitude higher than the rest) for one or more compounds were identified/excluded .

 

Based on the PAMS ambient data, the reductions in precursor and reactivity 

levels in the Valley suggest there should have been improvements in peak ozone 

levels. However, SJV ozone levels have changed very little, except during the 

last three years. This implies that there are other factors at work that affect the 

production of ozone. Perhaps emissions in the SJV remain in the Valley for 

longer periods of time, providing additional opportunities for lower reactivity 

precursors to contribute to peak ozone levels. In addition, because ambient 

ROG levels have changed faster than NOx levels, the ratio of ROG to NOx has 

decreased from 5.0 in 1994 to 2.8 in 2005. This change in the precursor ratio 

has likely changed the responsiveness of the Valley to predominately ROG 

emissions reductions. As a result, NOx emissions reductions may become more 

important for future emission control strategies. The greater decrease in 

estimated NOx emissions, coupled with the substantial drop in exceedance days 

in the southern SJV, appear to support this conclusion. Finally, meteorology is 

an important factor. Changes in temperature and vertical mixing are known to 

impact ozone levels. Therefore, accounting for meteorology is one way to 

provide a better understanding of the available data (refer to previous section on 

Meteorology and Air Quality Trends). 
May 30, 2007 Appendix B 19 

Modeling Results

The ozone modeling domain for the SJV was based on the domain defined for 

the year 2000 Central California Ozone Study (CCOS) and includes not only the 

Central Valley, but the San Francisco Bay area, as well. The domain comprises 

a grid measuring 185 by 185 cells with a horizontal resolution of four kilometers. 

The required meteorological fields were generated using the MM5 prognostic 

meteorological model, and the required emissions inventories were developed by 

ARB staff. The ozone air quality modeling utilized the Comprehensive Air Quality 

Model with Extensions (CAMx) air quality model, with initial and boundary 

conditions based on estimates of clean-air concentrations. 

ARB staff completed ozone modeling for two episode periods – one during 

July 1999 and the other during July/August 2000. During these episodes, 8-hour 

ozone concentrations exceeding the federal standard occurred throughout the 

region. Analysis of the model outputs included the estimation of 1-hour and 

8-hour ozone concentrations for each ozone monitoring site within the domain, 

as well as statistical measures comparing observed and simulated ozone 

concentrations. These analyses were used to evaluate model performance by 

subregion within the domain. 

As required by U.S. EPA, a relative reduction factor (RRF) approach was used in 

projecting future year design values. The RRF reflects the ratio between the 

future year model prediction (in this case the end of 2023) and the reference year 

model prediction. A reference year design value is then multiplied by the RRF to 

project a future year design value. 

Results of the modeling analyses indicate that NOx reductions will be relatively 

more effective than ROG reductions in reducing ozone concentrations in the 

San Joaquin Valley. Therefore, the attainment strategy relies on a 75 percent 

reduction in NOx, in combination with a 25 percent reduction in ROG, to bring the 

worst-case sites into attainment. 

Because ozone concentrations vary throughout the Valley, the modeling 

indicates different overall reductions are needed in different areas to reach 

attainment. However, as emissions controls are implemented, ozone 

concentrations will decrease in all areas of the Valley. San Joaquin Valley 

Unified Air Pollution Control District ( District) staff estimate that by 2015, over 

50 percent of the Valley’s population will live in areas that meet the federal 

8-hour ozone standard. By 2020, the portion of the population living in clean 

areas will increase to 90 percent. Although ozone concentrations will drop 

measurably throughout the entire Valley over time, downwind areas such as 

Arvin will require the greatest level of emissions reductions to meet the standard. 

Some of these reductions rely on technological advancements and are expected 

after 2020, but no later than 2023. These reductions are expected to bring the 

last remaining nonattainment areas into compliance with the federal 8-hour 

ozone standard. 
May 30, 2007 Appendix B 20 

Summary

The Weight of Evidence (WOE) package comprises a set of complementary 

analyses that supplement the SIP-required modeling, thereby providing 

additional support for the attainment demonstration. Currently, the San Joaquin 

Valley is classified as a Serious nonattainment area with an attainment date of 

June 15, 2013. Because of the magnitude of emissions reductions required, the 

District is requesting reclassification as Extreme, with a required attainment date 

of June 15, 2024. The Extreme classification allows the attainment 

demonstration to rely on emissions reductions from measures that anticipate the 

development of new technologies or improvement of existing technologies. 

These measures are often referred to as “black box” measures and go beyond 

the short-term measures that are based on known and demonstrated 

technologies. 

U.S. EPA guidance allows the use of a Weight of Evidence (WOE) analysis to 

supplement photochemical modeling in demonstrating attainment. The ARB 

staff’s modeling results indicate substantial NOx and ROG emissions reductions 

will be needed to bring the Valley into attainment. However, with reclassification 

as an Extreme nonattainment area, the SJV will be able to reach attainment by 

the 2024 deadline with a 75 percent reduction in NOx emissions, in combination 

with a 25 percent reduction in ROG emissions. Based on these reductions, even 

the worst-case sites will attain the federal 8-hour ozone standard. Based on 

modeling, as well as supporting analyses completed as part of this WOE 

evaluation, attainment by 2024 is anticipated because of the following factors: 

 Over the last decade, the number of basinwide exceedance 

days decreased 35 percent. The maximum concentration and 

design value show more modest reductions, with decreases of 

15 and 5 percent, respectively. Because these are basinwide 

numbers, they reflect the “worst case” sites. On a subregional 

basis, the amount of improvement during the last ten years is 

greater. While values for 2006 were up slightly from 2005, they 

are still among the lowest values over the last 15 years. 

 

 During the mid-1990s, the ozone problem was widespread 

throughout the San Joaquin Valley. Today, the spatial extent of 

the relatively clean areas has expanded substantially. The 

greatest amounts of improvement have occurred in the 

northern SJV, and ambient concentrations in most of

San Joaquin and Stanislaus counties now meet the federal 

standard. Over the last decade, the number of exceedance 

days in the northern SJV decreased 70 percent, and during 

2005, about 80 percent of the days during the ozone season 

were below the more stringent State 8-hour ozone standard. 
May 30, 2007 Appendix B 21 

 The ozone problem is now confined primarily to the central and 

southern portions of the Valley. Sites located downwind of the 

Fresno and Bakersfield urban areas continue to pose the most 

severe problems. While the number of exceedance days has 

declined substantially at most sites in the central and southern 

SJV (55 to 65 percent between 1995 and 2005), exceedance 

days have declined more slowly at Arvin, while increasing at 

sites located at higher elevations in the Sequoia National Park. 

 Estimated ROG and NOx emissions trends, as well as

measured data from the PAMS monitoring network in the 

San Joaquin Valley indicate reductions in both precursors since 

1994. These reductions have resulted in modest ozone 

improvements over the last decade. 

 Analyses suggest that recent ozone improvements are linked to 

emissions reductions. The decline in the number of

exceedance days relative to the number of days with a high 

potential for ozone formation indicates that the modest 

improvements in ozone over the last few years were related to 

emissions reductions rather than favorable meteorological 

conditions. Results of these analyses also indicate that 

increasingly adverse meteorological conditions are now needed 

to create ozone levels exceeding the federal 8-hour standard. 

 Emissions estimates indicate a continuing decline in ROG and 

NOx emissions as the State and District pursue the aggressive 

dual pollutant strategy outlined in the SIP. Based on the 

magnitude of emissions reductions needed for ozone 

attainment, as well as the readiness of NOx control

technologies, a NOx-heavy strategy is proposed and will provide 

the most efficient path to attainment (75 percent NOx reduction, 

coupled with 25 percent ROG reduction). 

 Photochemical modeling shows all sites in the San Joaquin 

Valley Air Basin will attain the federal 8-hour ozone standard by 

the end of 2023, as required for Extreme nonattainment areas. 

Many areas will reach attainment before this date. The District 

estimates that by 2015, 50 percent of the Basin population will 

live in areas attaining the standard, and by 2020, the number 

will increase to 90 percent. 

Taken together, all of these factors indicate that all sites in the San Joaquin 

Valley can expect to attain the federal 8-hour ozone standard by June 15, 2024, 

the required attainment date for an Extreme ozone nonattainment area.