San Joaquin Valley

1-Hour Ozone

Clean Data Determination

February 8, 2016
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i

Table of Contents

Executive Summary ........................................................................................................ 1

I. INTRODUCTION ...................................................................................................... 3

A. Request for a Clean Data Determination ........................................................... 3

B. History of the Valley’s Progress in Attaining the 1-Hour

Ozone Standard................................................................................................. 3

C. Criteria for U.S. EPA to Approve a Clean Data Determination........................... 4

i. Attainment of the 1-Hour Ozone Standard ......................................................... 4

ii. Network Adequacy ............................................................................................. 4

D. What This Clean Data Determination Request Addresses ................................ 4

II. 1-HOUR OZONE STANDARD ATTAINMENT.......................................................... 5

A. Estimated Expected Number of Exceedance Days for the

2012-2014 Period .............................................................................................. 5

i. Estimated Expected Number of Exceedance Days for the

2012-2014 Period .............................................................................................. 5

ii. Missing Day Analysis for Estimated Expected Number of

Exceedance Days in the 2012-2014 Period....................................................... 6

iii. 2014 Design Values for San Joaquin Valley Sites ............................................. 7

iv. Long-Term Progress in the San Joaquin Valley ................................................. 8

B. Estimated Ozone Levels at Arvin-Bear Mountain ............................................ 16

i. History of the Arvin-Bear Mountain Ozone Site................................................ 16

ii. ARB’s Methods and Results............................................................................. 18

iii. San Joaquin Valley District’s Methods and Results ......................................... 20

iv. Conclusions...................................................................................................... 20

C. Analysis of Ozone Concentration Trends......................................................... 21

i. The Highest Ozone Monitor Has a Tendency to Shift Over Time..................... 21

ii. Topography and Meteorology in the San Joaquin Valley ................................. 27

iii. Ozone Precursor Emissions in the San Joaquin Valley ................................... 28

iv. Limiting Photochemical Conditions in the San Joaquin Valley ......................... 29
ii

Appendices

APPENDIX A: San Joaquin Valley 1-Hour Ozone Estimated Expected

Number of Exceedance Days and Missing Data Analysis .....................................A-1

APPENDIX B: Estimated Design Values and Expected Number

of Exceedance Days at Arvin-Bear Mountain ........................................................B-1

APPENDIX C: Meteorology, Emissions, and Ozone Chemistry

in the San Joaquin Valley .................................................................................... C-1
iii

List of Figures

Figure 1. Arvin-Bear Mountain and the Two Nearest Sites:

Arvin-Di Giorgio (lower middle), and Edison (top left) ................................... 17

Figure 2. Valley National 1-Hour Ozone Design Values for 1990-2014........................ 23

Figure 3. National 1-Hour Ozone Design Values for the Fresno

and Bakersfield MSAs for 1990-2014............................................................ 24

Figure 4. National 1-Hour Ozone Design Values for the Fresno

and Bakersfield MSAs for 1990-2010............................................................ 25

Figure 5. National 1-Hour Ozone Design Values for Sites in the

Fresno MSA from 1990-2014........................................................................ 26

Figure 6. National 1-Hour Ozone Design Values for Sites in the

Bakersfield MSA from 1990-2014 ................................................................. 27
iv

List of Tables

Table 1. Estimated Expected Number of Exceedance Days, 2012-2014...................... 6

Table 2. 2014 1-Hour Ozone Design Values for All Sites in the Valley ......................... 7

Table 3. Three-Year Average Estimated Expected Number of

Exceedance Days for Sites in the San Joaquin Valley,

1990-2014....................................................................................................... 9

Table 3. Three-Year Average Estimated Expected Number of

Exceedance Days for Sites in the San Joaquin Valley,

1990-2014 (cont.).......................................................................................... 10

Table 4. Design Values (in ppm) for Sites in the San Joaquin Valley,

1990-2014..................................................................................................... 12

Table 4. Design Values (in ppm) for Sites in the San Joaquin Valley,

1990-2014 (cont.).......................................................................................... 13

Table 5. Preliminary Estimated Expected Number of Exceedance Days,

2013-2015..................................................................................................... 14

Table 6. Preliminary 2015 1-Hour Ozone Design Values for Sites in the

San Joaquin Valley ....................................................................................... 15

Table 7. Arvin-Bear Mountain 2014 Design Values with Uncertainty .......................... 18

Table 8. Arvin-Bear Mountain Estimated 1-Hour Ozone Design Values and

Three-Year Average Estimated Expected Number of Exceedance Days

for the 2012-2014 Period Based on Measured Data

from Arvin-Di Giorgio..................................................................................... 19

Table 9. National 1-Hour Ozone Design Values for High Sites in the

San Joaquin Valley for 1990-2014 ................................................................ 22

Table 10. The Number of Times Each Site Was the Valley Design Site

from 1990-2014 and 1990-2010 (in parentheses)......................................... 22
1

San Joaquin Valley 1-Hour Ozone Clean Data Determination

Executive Summary

In July 2015, the San Joaquin Valley Air Pollution Control District (District) submitted a 

request to the Air Resources Board (ARB) and the U.S. Environmental Protection 

Agency (U.S. EPA) for a determination that the San Joaquin Valley attained the national 

1-hour ozone standard of 0.12 parts per million (ppm) as of 2014. 

Ozone levels in the Valley have improved significantly over the last 25 years as a result 

of comprehensive emission control programs implemented by ARB and the District. In 

the early 1990s, the 1-hour design value was 0.17 ppm, and the standard was 

exceeded 45 to 50 days each summer. Today, the entire Valley meets the standard, 

with a 1-hour design value of 0.118 ppm measured in 2014. 

To support this determination, ARB and District staff, in consultation with U.S. EPA, 

performed a comprehensive evaluation of currently monitored ozone concentrations, 

frequency of exceedance days, data completeness, and adequacy of the current 

network to characterize 1-hour ozone levels throughout the Valley. This assessment 

demonstrates that the Valley’s 1-hour ozone concentrations and expected number of 

exceedance days in 2012-2014 meet U.S. EPA’s criteria for attainment. 

As part of this assessment, ARB and District staff developed methodologies to estimate 

1-hour ozone concentrations at the former Arvin-Bear Mountain monitoring site. This 

site was shut down after the 2010 ozone season due to circumstances beyond ARB's 

control and was above the 1-hour ozone standard at the time of its closure. ARB 

established a replacement site within two miles of the Arvin-Bear Mountain site at the 

Arvin-Di Giorgio Elementary School. Ozone concentrations at Di Giorgio Elementary 

School were slightly lower than those measured at Arvin-Bear Mountain, however the 

concentrations at both sites were highly correlated. 

The relationship between ozone concentrations at the two sites provided a basis for a 

number of robust statistical methods to estimate 1-hour ozone concentrations and 

expected exceedance days at Arvin-Bear Mountain. The analyses predicted a design 

value of 0.117 ppm for the 2012-2014 period, as well as an expected number of 

exceedance days less than one. These methodologies demonstrate that ozone 

measurements at the Arvin-Di Giorgio Elementary School monitoring site provide an 

adequate basis to assess expected ozone concentrations at the former 

Arvin-Bear Mountain location, and ensure that the region meets the standard even in 

light of the site’s closure. 

Moreover, although the Arvin-Bear Mountain site while in operation periodically had the 

highest 1-hour design value in the Valley, in the five years prior to its closure the highest 

values occurred at other sites in Kern County or the Fresno region. This continued to 

be the case for 2012-2014, with two monitoring sites in Fresno County recording a 

higher design value (0.118 ppm) than the value estimated for Arvin-Bear Mountain. 
2

Taken together, these findings demonstrate that all criteria for attainment have been 

met. Ongoing emission reductions from existing control programs and from efforts to 

attain the 8-hour ozone standards will provide for continuing ozone air quality 

improvement in the San Joaquin Valley.
3

San Joaquin Valley 1-Hour Ozone Clean Data Determination

I. INTRODUCTION

This document is a request for a Clean Data Determination (CDD) for the national

1-hour ozone standard in the San Joaquin Valley (Valley). Its purpose is to 

demonstrate that the Valley now attains the national 1-hour ozone standard of 

0.12 ppm.

A. Request for a Clean Data Determination

On July 13, 2015, the San Joaquin Valley Air Pollution Control District (District) 

submitted a request to the California Air Resources Board (ARB) and the U.S. EPA to 

determine that the Valley has attained the national 1-hour ozone standard. The 

District’s efforts to meet several other national ambient air quality standards, including 

the 1997 and 2008 8-hour ozone standards, as well as the 1997 and 2012 PM2.5

standards, will also continue to reduce ozone precursor emissions in the future.

ARB staff has reviewed the District’s clean data finding report and has performed 

additional analyses, presented in this document. ARB concurs with the District’s finding 

that the Valley meets the national 1-hour ozone standard based on data for the 

2012-2014 period. ARB is therefore submitting a formal request for U.S. EPA to 

approve this CDD for 1-hour ozone with a finding that the Valley is in attainment of that 

standard.

B. History of the Valley’s Progress in Attaining the 1-Hour Ozone Standard

In 1991, U.S. EPA classified the San Joaquin Valley as a serious nonattainment area. 

Due to the severity of the air quality problem in the Valley, U.S. EPA reclassified the 

Valley as extreme nonattainment in 2004 and set 2010 as the attainment deadline. At 

that time, the District set forth a comprehensive emission reduction strategy, as laid out 

in the Extreme Ozone Attainment Demonstration Plan1.

In 2005, U.S. EPA revoked the national 1-hour ozone standard based on evidence that 

the 1997 8-hour ozone standard set at 0.08 ppm was more health protective. Although 

the standard was revoked, certain planning obligations associated with the 1-hour

ozone standard still remain, including attainment deadlines. The Valley did not attain the 

1-hour ozone standard by its 2010 deadline. Therefore, in 2014 the District submitted a 

revised plan, which demonstrated attainment by 2017.

The District and ARB have continued development of strategies to meet both the 1-hour 

standard, as well as the more stringent 8-hour ozone standards. These strategies have 

significantly reduced ozone forming precursors and have brought the Valley into 

attainment for the 1-hour ozone standard.

 

1

See http://www.valleyair.org/Air_Quality_Plans/AQ_Final_Adopted_Ozone2004.htm
4

In 2012, U.S. EPA designated the Valley as nonattainment for the 2008 8-hour ozone 

standard. The corresponding attainment plan is due in 2016 with an attainment date

of 2031. On October 1, 2015, U.S. EPA adopted a more stringent 8-hour ozone 

standard of 0.070 ppm. The District and ARB will continue to coordinate future 

emission reduction strategies to attain these air quality standards, providing ongoing 

ozone improvement.

C. Criteria for U.S. EPA to Approve a Clean Data Determination

In order for U.S. EPA to approve a CDD, an area needs to make two necessary 

demonstrations: 1) all available, quality-assured official data for the current or 

applicable three-year period must meet the standard, and 2) the monitoring network 

adequately measures ozone levels in the area. Both of these are discussed in further 

detail below.

i. Attainment of the 1-Hour Ozone Standard

According to the rules set forth in 40 CFR §50.9 and 40 CFR Part 50, Appendix H, an 

area has attained the 1-hour ozone standard if for all monitors the average estimated 

expected number of exceedance days per year for a three-year period (rounded to one 

decimal place) is less than or equal to 1.0. Attainment determinations must also use all 

available, quality-assured official air quality data for the applicable three year period.

ii. Network Adequacy

A finding of attainment also requires a monitoring network that is adequate to 

characterize ozone concentrations in the region. As part of this assessment, federal 

regulations, as stated in 40 CFR Parts 53 and 58, include specifications on the 

minimum monitoring required and other network design considerations, the equipment 

that must be used, and the quality assurance procedures that must be followed.

D. What This Clean Data Determination Request Addresses

Through various analyses and discussions, this CDD request demonstrates that the 

Valley attains the 1-hour ozone standard and that the network is adequate with respect 

to characterizing 1-hour ozone concentrations. This conclusion is based not only on the 

current monitoring network, but also takes into account the closure of the 

Arvin-Bear Mountain site. This site was shut down in 2010 due to circumstances

beyond ARB's control before the site met the 1-hour ozone standard.

A replacement site was established at Arvin-Di Giorgio, which is 2.2 miles 

north-northwest of Arvin-Bear Mountain and has similar adjacent land uses, topography, 

predominant wind direction, and nearby traffic counts as does Arvin-Bear Mountain. 

Parallel monitoring was conducted at the two sites for one year. The parallel monitoring 

showed that while ozone concentrations at the two sites were highly correlated, the 

Arvin-Bear Mountain concentrations read higher (6-8 ppb) on average. The analyses 

presented in this CDD consider this in estimating ozone levels at Arvin-Bear Mountain

since its closure and show that the Valley attains the 1-hour ozone standard.
5

II. 1-HOUR OZONE STANDARD ATTAINMENT

As defined in federal regulation (40 CFR §50.9), the national 1-hour ozone standard is 

0.12 ppm, attained when the average of three consecutive annual estimated expected 

number of exceedance days is one or less. Federal regulation (40 CFR Part 50, 

Appendix H) further defines how the yearly and three-year average estimated expected 

number of exceedance days are to be determined. Using all available quality-assured 

official data from ARB’s Aerometric Data Analysis and Management (ADAM) database, 

which is updated daily with data from U.S. EPA’s Air Quality System (AQS), this section 

demonstrates that all sites in the Valley met this standard during the 2012-2014 period.

Additional analyses in this section show that since 1990, the Valley has made 

substantial progress in improving air quality to achieve the standards. Further 

demonstrations, using preliminary data from ARB’s Air Quality and Meteorological 

Information System (AQMIS) database, will show that these improvements have 

continued through the most recent ozone season of 2015.

This section also describes methods that can be used to estimate ozone concentrations 

and determine attainment at the former Arvin-Bear Mountain site. These methods, 

employed by ARB and the District, rely on the relationships between ozone 

concentrations at Arvin-Bear Mountain and two nearby sites, Arvin-Di Giorgio and 

Edison. The details of the methods are described in Appendix B to this report. 

Application of these methods demonstrate that if the Arvin-Bear Mountain site was still 

in operation today, it would attain the 1-hour ozone standard.

Lastly, this section will show that based on measured data as well as estimated 

concentrations at Arvin-Bear Mountain, the Fresno MSA is now the area in the Valley 

that has the highest 1-hour ozone estimated expected number of exceedance days. 

Evaluations of ozone concentration trends, ozone precursor emissions trends, 

meteorology, and photochemistry explain why the Fresno MSA is expected to continue 

to produce the highest 1-hour estimated expected number of exceedance days in the 

Valley.

A. Estimated Expected Number of Exceedance Days for the 2012-2014 Period

As shown in this section, all monitoring sites currently in operation in the Valley met the 

1-hour ozone standard in the 2012-2014 period, and continued to do so in 2015.

i. Estimated Expected Number of Exceedance Days for the 2012-2014 Period

See Appendix A for more detail.

According to the rules of 40 CFR §50.9 and 40 CFR Part 50, Appendix H, as well as 

Guideline for the Interpretation of Ozone Air Quality Standards, January 1979,

EPA-450/4-79-003, for the 2012-2014 three-year period, all sites in the San Joaquin 

Valley Air Basin met the 1-hour ozone standard. To meet the standard, the three-year
6

average of annual estimated expected number of exceedance days (rounded to one 

decimal place) must be 1.0 or less.

Table 1. Estimated Expected Number of Exceedance Days, 2012-2014

e

Avg e

County Site 2012 2013 2014

Fresno Clovis-N Villa Avenue 0 0 0 0

Fresno Fresno-Drummond Street 1 0 0 0.3

Fresno Fresno-Garland 1 0 0 0.3

Fresno Fresno-Sierra Skypark #2 1 0 1.1 0.7

Fresno Parlier 1 0 0 0.3

Fresno Tranquility-32650 West Adams Avenue 0 0 0 0

Kern Arvin-Di Giorgio 0 0 0 0

Kern Bakersfield-5558 California Avenue 0 0 0 0

Kern Bakersfield-Municipal Airport 0 0 0 0

Kern Edison 0 0 0 0

Kern Maricopa-Stanislaus Street 0 0 0 0

Kern Oildale-3311 Manor Street 0 0 0 0

Kern Shafter-Walker Street 0 0 0 0

Kings Hanford-S Irwin Street 0 0 0 0

Madera Madera-28261 Avenue 14 0 0 0 0

Madera Madera-Pump Yard 0 0 0 0

Merced Merced-S Coffee Avenue 0 0 0 0

San Joaquin Stockton-Hazelton Street 0 0 0 0

San Joaquin Tracy-Airport 0 0 0 0

Stanislaus Modesto-14th Street 0 0 0 0

Stanislaus Turlock-S Minaret Street 0 0 0 0

Tulare Porterville-1839 Newcomb Street 0 0 0 0

Tulare Sequoia and Kings Canyon Natl Park 0 0 0 0

Tulare Sequoia Natl Park-Lower Kaweah 0 0 0 0

Tulare Visalia-N Church Street 0 0 0 0

Table 1 displays "e" values for each ozone monitoring site in the Valley, where "e" is the 

estimated expected number of exceedance days for each year, calculated as required 

by 40 CFR Part 50, Appendix H. "Avg e" is the average of the three "e" values for each 

site, rounded to one decimal place.

Table 1 shows that there were only four sites during the 2012-2014 period that 

experienced exceedances: Fresno-Sierra Skypark #2, Fresno-Garland, 

Fresno-Drummond Street, and Parlier. Of those, only Fresno-Sierra Skypark #2 

experienced more than one day over the standard during the three-year period, with 

one measured exceedance in 2012 and another in 2014. That site's three year average 

"e" was 0.7, the highest 3-year average estimated expected number of exceedance 

days in the Valley during the 2012-2014 period. Therefore all sites meet the 

requirement that the 3-year average estimated expected number of exceedance days is 

1.0 or less.

ii. Missing Day Analysis for Estimated Expected Number of Exceedance Days in

the 2012-2014 Period
7

See Appendix A for more detail.

Staff evaluated the ozone data record for all sites in the Valley during the 2012-2014

period to ensure that data completeness was considered in determining that the Valley 

met the 1-hour ozone standard.

During the 2012-2014 period, only one site in the Valley, Bakersfield-Municipal Airport, 

had insufficient data to be considered complete. The ozone data record for 

Bakersfield-Municipal Airport starts on July 1, 2012. From that date through the end of 

2012, the site experienced no exceedances. Staff used data from three surrounding 

sites (Bakersfield-5558 California Avenue, Oildale-3311 Manor Street, and Edison) to 

determined whether any exceedances of the standard occurred between 

January 1, 2012 and June 30, 2012.

By comparing the maximum differences between daily maximum hourly ozone data for 

Bakersfield-Municipal Airport and the three surrounding sites (see Appendix A for 

details about the method used), staff placed an upper bound on the daily maximum 

concentrations that may have occurred during the first six months of 2012 at 

Bakersfield-Municipal Airport. This evaluation demonstrated that the standard may 

have been exceeded on only one day between January 1, 2012 and June 30, 2012. If 

this exceedance did occur, the 3-year average estimated expected number of 

exceedance days at Bakersfield-Municipal Airport for the 2012-2014 period would have 

been 0.3, which does not violate the 1-hour ozone standard.

iii. 2014 Design Values for San Joaquin Valley Sites

Another required metric to measure the severity of an ozone problem in an area and 

determine if it meets the standard is the design value. The design value is used when 

an area is first designated to determine the level of emissions reductions needed and 

how long an area has to attain the standard. For 1-hour ozone, a monitor’s design 

value is the fourth highest daily maximum 1-hour concentration recorded over the 

three-year period (assuming three complete years of data). A nonattainment area’s 

design value is the maximum of the design values for all monitored sites in the area. 

This section demonstrates that the design values for all sites in the Valley are also 

below the national 1-hour ozone standard.

Table 2 shows the 2014 1-hour ozone design values for all sites in the Valley. The 

2014 design value represents the fourth-highest daily maximum 1-hour concentration 

that occurred at each site over the three year period from 2012 to 2014. From Table 2,

the highest 2014 design value for the Valley was 0.118 ppm and occurred at the 

Clovis-North Villa Avenue and Fresno-Drummond Street sites. Fresno-Garland had the 

third highest design value at 0.117 ppm and Parlier came in fourth with 0.116 ppm. The

design values for all sites are well below 0.124 ppm, which is the “effective” level of the 

standard, or the concentration that (when rounded) demonstrates attainment.

Table 2. 2014 1-Hour Ozone Design Values for All Sites in the Valley
8

County Site

Design Value

(ppm)

Fresno Clovis-N Villa Avenue 0.118

Fresno Fresno-Drummond Street 0.118

Fresno Fresno-Garland 0.117

Fresno Fresno-Sierra Skypark #2 0.109

Fresno Parlier 0.116

Fresno Tranquility-32650 West Adams Ave. 0.089

Kern Arvin-Di Giorgio 0.109

Kern Bakersfield-5558 California Ave. 0.101

Kern Bakersfield-Municipal Airport 0.114

Kern Edison 0.108

Kern Maricopa-Stanislaus Street 0.090

Kern Oildale-3311 Manor Street 0.096

Kern Shafter-Walker Street 0.098

Kings Hanford-S Irwin Street 0.104

Madera Madera-28261 Avenue 14 0.102

Madera Madera-Pump Yard 0.100

Merced Merced-S Coffee Avenue 0.098

San Joaquin Stockton-Hazelton Street 0.087

San Joaquin Tracy-Airport 0.098

Stanislaus Modesto-14th Street 0.093

Stanislaus Turlock-S Minaret Street 0.106

Tulare Porterville-1839 Newcomb St. 0.099

Tulare Sequoia & Kings Canyon Natl Park 0.110

Tulare Sequoia Natl Park-Lower Kaweah 0.101

Tulare Visalia-N Church Street 0.101

iv. Long-Term Progress in the San Joaquin Valley

The Valley's attainment of the 1-hour ozone standard in the 2012-2014 period is the 

result of substantial progress that has occurred throughout the last 25 years. From 

1990 to 2014, improvements in ozone reductions occurred over this longer time frame. 

Preliminary data for 2015 also show that this trend continues.
9

Table 3. Three-Year Average Estimated Expected Number of Exceedance Days for Sites in the San Joaquin 

Valley, 1990-2014

County Site Name 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

Fresno Clovis-N Villa 

Avenue 1.4 2.3 7.4 11.3 13.3 9.8 10.7 10.8 17.3 13.7 13.2 7.8 8.5 6.1 3.1 1.4 1.7 1.4 2.4 1.7 2.7 1.7 1.7 0.7 0.0

Fresno Fresno-1st Street 11.0 20.9 18.1 18.2 10.3 10.9 12.2 10.2 10.5 6.7 8.1 4.7 7.1 7.1 5.4 2.7 2.3 2.3 3.7 2.4 3.1 0.7 1.0 0.3 0.3

Fresno FresnoDrummond Street 8.3 7.4 8.0 7.1 4.1 1.7 2.7 3.0 5.8 4.4 4.8 2.4 4.1 4.7 4.4 1.4 0.0 0.0 0.0 0.0 0.0 1.1 1.5 1.5 0.3

Fresno Fresno-Sierra 

Skypark #2 3.8 4.3 4.9 4.0 4.0 3.7 3.0 6.4 5.1 7.5 6.5 11.2 8.8 5.4 1.0 1.0 1.0 1.0 0.7 1.5 0.8 1.1 0.3 0.7

Fresno Parlier 19.5 14.0 10.9 12.5 8.5 7.7 10.4 12.4 13.5 12.7 15.3 15.1 16.9 15.9 11.8 5.1 0.7 0.7 1.0 0.7 1.0 0.7 1.1 0.7 0.3

Fresno Shaver Lake -

Perimeter Road 0.0 3.4 2.9 2.9 1.3 0.7 0.7 2.6

Fresno

Tranquility-32650 

West Adams 

Avenue

0.0 0.0 0.0 0.0 0.0

Kern Arvin-Bear 

Mountain Blvd 25.1 28.2 22.9 19.1 13.5 16.9 24.9 21.3 18.8 9.4 10.2 11.6 13.6 19.3 16.6 13.6 9.0 7.3 10.1 6.9 6.7

Kern Arvin-Di Giorgio 0.0 0.0 0.0 0.0 0.0

Kern Bakersfield-5558 

California Avenue 0.0 1.0 1.7 1.7 1.0 0.0 0.3 0.7 0.7 0.3 0.0 0.0 0.0 0.0 0.3 0.3 0.3 0.0 0.0 0.0 0.0

Kern

BakersfieldGolden State 

Highway

0.0 0.5 1.4 1.4 1.4 0.4 0.4 0.3 0.3 0.3 0.0 0.0 0.0 0.3 0.3

Kern BakersfieldMunicipal Airport 0.0 0.0 0.0

Kern Edison 36.8 24.2 16.0 18.1 21.4 31.9 30.7 21.0 16.8 10.1 12.2 6.8 7.8 5.7 4.0 1.3 3.4 3.4 5.0 2.7 2.7 1.0 0.3 0.0 0.0

Kern MaricopaStanislaus Street 3.5 1.1 0.0 0.0 0.0 0.4 0.4 0.4 2.9 2.9 2.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Kern Oildale-3311 

Manor Street 2.5 1.1 0.7 0.4 0.0 0.3 1.0 1.0 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Kern Shafter-Walker 

Street 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.3 0.3 0.0 0.0 0.0 0.0

Kings CorcoranPatterson Avenue 4.4 2.2

Kings Hanford-S Irwin 

Street 0.0 0.0 2.7 3.4 4.4 2.4 1.7 1.0 0.7 0.7 0.3 0.0 0.4 0.4 2.8 1.4 0.9 0.0 0.0

Madera Madera-28261 

Avenue 14 0.0 0.0 0.0 0.0 0.0

Madera Madera-Pump 

Yard 0.0 0.5 0.3 0.3 0.0 0.7 0.7 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Merced Merced-S Coffee 

Avenue 0.0 0.5 0.3 1.4 1.4 1.4 1.4 1.7 1.7 0.7 0.7 0.7 0.7 0.0 0.0 0.0 1.0 1.0 1.0 0.0 0.0 0.0 0.0

San 

Joaquin

Stockton-E 

Mariposa 2.2 0.4 0.4 0.3 0.3 1.0 0.7 0.7 0.0 0.3 0.3 0.3 0.0

San 

Joaquin

Stockton-Hazelton 

Street 0.4 0.0 0.0 0.0 0.3 0.7 0.7 0.3 0.3 1.0 1.0 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

San 

Joaquin

Tracy-24371 

Patterson Pass 

Road

0.0 1.0 0.7 0.7 0.3 0.3 0.3 0.0 0.0 0.0

San 

Joaquin Tracy-Airport 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
10

Table 3. Three-Year Average Estimated Expected Number of Exceedance Days for Sites in the San Joaquin 

Valley, 1990-2014 (cont.)

County Site Name 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

Stanislaus Modesto-14th 

Street 0.3 0.3 0.3 0.0 0.0 0.7 1.3 1.3 1.7 1.0 1.3 0.3 0.3 0.0 0.0 0.0 0.0 0.0 0.3 0.3 0.3 0.0 0.0 0.0 0.0

Stanislaus Turlock-S Minaret 

Street 0.0 1.1 0.8 1.4 1.0 1.0 1.7 1.3 1.7 0.3 0.7 0.3 0.3 0.0 0.0 0.0 1.0 1.4 1.4 0.3 0.0 0.0 0.0

Tulare Porterville-1839 

Newcomb Street 0.0 0.0 0.0 0.0 0.0

Tulare Sequoia and Kings 

Canyon Natl Park 2.1 1.1 0.7 0.4 1.1 1.1 1.0 0.3 0.3 2.3 2.3 2.3 0.0 0.0 0.0 0.0

Tulare

Sequoia National 

Park-Lookout 

Point

0.0 0.0 0.0 0.0 0.0 0.4 0.8 0.8

Tulare Sequoia Natl ParkLower Kaweah 0.0 0.0 0.0 0.7 1.1 1.1 0.4 0.0 0.4 0.4 0.4 0.0 0.3 0.3 0.3 0.0 0.0 0.0 0.3 0.3 0.3 0.0 0.0 0.0 0.0

Tulare Visalia-N Church 

Street 5.2 4.2 1.4 4.2 7.3 7.2 5.5 2.4 3.7 2.7 2.7 1.4 1.3 1.0 0.7 0.3 0.3 0.0 1.0 1.0 1.0 0.0 0.0 0.0 0.0
11

a. 1990-2014 Estimated Expected Number of Exceedance Days

See Appendix A for more detail.

Table 3 shows the three-year average estimated expected number of exceedance days 

for all of the operational sites in the Valley from 1990-2014. Table 3 showcases the 

significant air quality progress that has occurred. Twenty-five years ago, the Valley had 

a three-year average estimated expected number of exceedance days of 36.8. By the 

early 2000s this number had been cut in half, and by 2014 that number dropped to less 

than one. As of 2014, all currently operational sites meet the national 1-hour ozone 

standard since their three-year average estimated expected number of exceedance 

days are less than 1.0.

Table 3 also shows that two sites in the Bakersfield region have made the most 

progress over time. Edison and Arvin-Bear Mountain started out as the two highest 

sites in the Valley. The 3-year average estimated expected number of exceedance 

days at Edison decreased by 100 percent, down from 36.8 in 1990 to 0 in 2013 and 

2014. Over a shorter time frame, Arvin-Bear Mountain saw nearly a 75 percent drop in 

the 3-year average estimated expected number of exceedance days from 25.1 in 1990 

to 6.7 in 2010, its last year in operation. As other sites continued to decline from 2010 

through 2014, ARB's analyses demonstrate that a similar downward trend would have 

continued at Arvin-Bear Mountain. ARB staff’s estimates of these values at 

Arvin-Bear Mountain are presented in section II.b.ii., ARB's Methods and Results, 

below.

b. 1990-2014 Design Values

As mentioned earlier, the design value is another important metric when looking at air 

quality progress. Table 4 shows the national 1-hour ozone design values for all sites in 

the Valley from 1990-2014. The numbers below were calculated by finding the 

fourth-highest daily maximum 1-hour ozone concentration at each site in a three-year 

period, while taking into account the national completeness criteria. The design values 

in Table 4 come from ARB’s ADAM database, which is updated daily with official data 

from U.S. EPA’s AQS database.

The Valley has made significant improvements in 1-hour ozone design values. In 1990, 

the basin’s design value was 0.17 ppm, well above the 1-hour standard of 0.12 ppm. 

For the last 25 years, there has been an overall decreasing trend in the design values. 

In the 20 years from 1990 to 2010, the Valley’s design value decreased 18 percent to 

0.140 ppm. Greater progress has been seen in the last five years as the design value 

decreased another 13 percent. For 2014, the basin’s design value was 0.118 ppm, 

which indicates a 31 percent drop in the overall design value for the Valley over the last 

25 years.

This air quality progress has occurred across the Valley. Although there are some 

minor increases throughout the years due to year-to-year variability in weather 

conditions, the predominant trend is a downward one at all of the Valley sites. While
12

Table 4. Design Values (in ppm) for Sites in the San Joaquin Valley, 1990-2014

County Site 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

Fresno Clovis-N Villa Ave 0.13 0.13 0.15 0.14 0.144 0.144 0.146 0.146 0.161 0.161 0.161 0.142 0.137 0.136 0.131 0.126 0.126 0.125 0.140 0.140 0.140 0.130 0.130 0.123 0.118

Fresno Fresno-1st St/

Fresno-Garland 0.15 0.16 0.16 0.16 0.140 0.140 0.140 0.140 0.146 0.146 0.146 0.135 0.135 0.135 0.135 0.131 0.130 0.130 0.133 0.127 0.127 0.124 0.124 0.118 0.117

Fresno Fresno-Drummond 

St 0.15 0.15 0.15 0.15 0.140 0.140 0.131 0.131 0.142 0.137 0.137 0.131 0.131 0.132 0.131 0.128 0.119 0.110 0.114 0.118 0.118 0.127 0.127 0.127 0.118

Fresno Fresno-Fremont 

School 0.119

Fresno Fresno-Mobile 0.125

Fresno Fresno-Olive St 0.15

Fresno Fresno-Sierra 

Skypark #2 0.14 0.14 0.14 0.14 0.130 0.131 0.131 0.131 0.141 0.141 0.141 0.136 0.144 0.144 0.144 0.123 0.124 0.124 0.124 0.119 0.126 0.119 0.118 0.114 0.109

Fresno Parlier 0.15 0.15 0.15 0.15 0.150 0.140 0.144 0.144 0.151 0.145 0.152 0.146 0.151 0.151 0.151 0.135 0.121 0.121 0.122 0.121 0.122 0.121 0.122 0.118 0.116

Fresno Shaver Lake -

Perimeter Rd 0.113 0.135 0.135 0.135 0.129 0.122 0.124 0.128

Fresno Tranquility-West 

Adams Ave 0.093 0.093 0.096 0.091 0.089

Kern Arvin-Bear Mountain

Blvd 0.17 0.16 0.16 0.16 0.150 0.147 0.156 0.156 0.156 0.137 0.141 0.134 0.142 0.150 0.151 0.149 0.134 0.131 0.135 0.135 0.139

Kern Arvin-Di Giorgio 0.120 0.118 0.118 0.112 0.109

Kern Bakersfield-5558 

California 0.123 0.128 0.128 0.126 0.124 0.119 0.119 0.116 0.117 0.116 0.113 0.112 0.117 0.117 0.120 0.117 0.114 0.111 0.104 0.101 0.101

Kern Bakersfield-Chester 

St 0.13 0.13 0.13 0.13 0.130

Kern Bakersfield-Golden 

State 0.122 0.123 0.126 0.125 0.125 0.118 0.118 0.115 0.116 0.116 0.115 0.109 0.105 0.108 0.110

Kern BakersfieldMunicipal Airport 0.120 0.119 0.114

Kern Edison 0.16 0.15 0.15 0.16 0.160 0.165 0.165 0.164 0.158 0.154 0.154 0.138 0.141 0.134 0.134 0.127 0.135 0.135 0.136 0.135 0.134 0.124 0.118 0.113 0.108

Kern Maricopa-Stanislaus 

St 0.13 0.12 0.11 0.11 0.110 0.119 0.121 0.121 0.130 0.130 0.137 0.110 0.114 0.112 0.112 0.102 0.101 0.100 0.097 0.097 0.097 0.102 0.102 0.102 0.090

Kern Oildale-3311 Manor 

St 0.13 0.12 0.12 0.12 0.120 0.116 0.121 0.121 0.122 0.119 0.120 0.113 0.113 0.114 0.112 0.111 0.112 0.112 0.114 0.112 0.110 0.102 0.102 0.099 0.096

Kern Shafter-Walker St 0.12 0.12 0.11 0.11 0.110 0.112 0.118 0.112 0.115 0.112 0.114 0.109 0.109 0.112 0.112 0.111 0.104 0.105 0.106 0.106 0.106 0.102 0.102 0.097 0.098

Kings Corcoran-Patterson 

Ave 0.132 0.132

Kings Hanford 0.13 0.11 0.10 0.11

Kings Hanford-S Irwin St 0.113 0.110 0.138 0.138 0.138 0.128 0.128 0.124 0.124 0.121 0.121 0.113 0.112 0.110 0.131 0.126 0.123 0.109 0.104

Madera Madera-28261 Ave 

14 0.120 0.106 0.106 0.102 0.102

Madera Madera-Health Dept 

#2 0.12 0.12 0.12 0.13 0.130 0.130 0.121

Madera Madera-Pump Yard 0.085 0.123 0.118 0.117 0.104 0.115 0.119 0.119 0.103 0.097 0.095 0.105 0.105 0.110 0.104 0.097 0.096 0.100

Madera Madera-Rd 28

Merced Merced-S Coffee 

Ave 0.13 0.13 0.120 0.125 0.125 0.125 0.131 0.132 0.132 0.120 0.121 0.122 0.122 0.118 0.106 0.102 0.125 0.118 0.118 0.108 0.108 0.098 0.098

San 

Joaquin Stockton-E Mariposa 0.13 0.12 0.12 0.11 0.119 0.124 0.119 0.107 0.105 0.110 0.110 0.109 0.108

San 

Joaquin Stockton-Hazelton St 0.11 0.11 0.11 0.11 0.110 0.120 0.120 0.118 0.104 0.107 0.107 0.107 0.102 0.101 0.101 0.099 0.101 0.101 0.102 0.095 0.105 0.095 0.092 0.086 0.087
13

Table 4. Design Values (in ppm) for Sites in the San Joaquin Valley, 1990-2014 (cont.)

County Site 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

San 

Joaquin

Tracy-24081 

Patterson Pass 0.107 0.107

San 

Joaquin

Tracy-24371 

Patterson Pass 0.124 0.126 0.124 0.116 0.118 0.118 0.118 0.111 0.104 0.104

San 

Joaquin Tracy-Airport 0.120 0.118 0.118 0.104 0.108 0.099 0.106 0.099 0.098

Stanislaus Crows Landing NASFire Station0.110 0.110 0.110

Stanislaus Crows Landing-Davis 0.12 0.12

Stanislaus Modesto-14th St 0.12 0.12 0.11 0.11 0.114 0.123 0.125 0.125 0.131 0.131 0.131 0.109 0.109 0.109 0.106 0.107 0.109 0.109 0.113 0.106 0.106 0.093 0.092 0.091 0.093

Stanislaus Modesto-Rover 14th 

St 0.082

Stanislaus Turlock-Monte Vista 

#1 0.13 0.13 0.12

Stanislaus Turlock-S Minaret St 0.12 0.13 0.120 0.125 0.123 0.123 0.129 0.127 0.130 0.111 0.123 0.119 0.119 0.111 0.106 0.104 0.122 0.125 0.125 0.110 0.112 0.111 0.106

Stanislaus Westley-Truck Stop 0.12 0.12

Tulare Porterville-1839 

Newcomb St 0.111 0.109 0.104 0.104 0.099

Tulare Sequoia and Kings 

Canyon NP 0.127 0.127 0.125 0.124 0.126 0.126 0.126 0.118 0.119 0.132 0.132 0.130 0.112 0.110 0.112 0.110

Tulare Sequoia NP -

Lookout Point 0.120 0.119 0.122 0.122 0.122 0.124 0.124 0.123

Tulare Sequoia NP-Ash 

Mountain #1 0.119 0.119 0.120 0.120 0.126 0.127 0.126

Tulare Sequoia NP-Ash 

Mountain #2 0.12 0.12 0.12 0.12

Tulare Sequoia NP-Lower 

Kaweah 0.112 0.112 0.116 0.121 0.123 0.123 0.122 0.115 0.118 0.112 0.111 0.108 0.118 0.122 0.122 0.115 0.113 0.113 0.115 0.115 0.115 0.095 0.098 0.101 0.101

Tulare Visalia-N Church St 0.14 0.14 0.13 0.14 0.150 0.150 0.140 0.132 0.139 0.127 0.129 0.126 0.126 0.124 0.124 0.117 0.115 0.112 0.121 0.121 0.122 0.117 0.111 0.102 0.101

Tulare Wilsonia-Grant 

Grove 0.12 0.12 0.12 0.12 0.124 0.125
14

sites that had the highest design values made the most progress, sites with lower 

design values, including those below the standard, also declined over that time period. 

For instance, in 1990 Stockton-Hazelton Street had a design value of 0.110 ppm. By 

2014, the design value for that site was 0.087 ppm, well below the national 1-hour 

ozone standard.

c. 2015 Estimated Expected Number of Exceedance Days and Design Values

See Appendix A for more detail.

The most recent data indicate that the declining trend in the estimated expected number 

of exceedance days has continued through the 2015 ozone season. Table 5 calculates 

the preliminary three-year average estimated expected number of exceedance days for 

all sites in the Valley for the 2013-2015 period.

Table 5. Preliminary Estimated Expected Number of Exceedance Days, 2013-2015

e Preliminary

County Site 2013 2014 2015 Avg e

Fresno Clovis-N Villa Avenue 0 0 0 0

Fresno Fresno-Drummond Street 0 0 1.0 0.3

Fresno Fresno-Garland 0 0 0 0

Fresno Fresno-Sierra Skypark #2 0 1.1 0 0.4

Fresno Parlier 0 0 0 0

Fresno Tranquility-32650 West Adams 

Avenue 0 0 0 0

Kern Arvin-Di Giorgio 0 0 0 0

Kern Bakersfield-5558 California Avenue 0 0 0 0

Kern Bakersfield-Municipal Airport 0 0 0 0

Kern Edison 0 0 0 0

Kern Maricopa-Stanislaus Street 0 0 0 0

Kern Oildale-3311 Manor Street 0 0 0 0

Kern Shafter-Walker Street 0 0 0 0

Kings Hanford-S Irwin Street 0 0 0 0

Madera Madera-28261 Avenue 14 0 0 0 0

Madera Madera-Pump Yard 0 0 0 0

Merced Merced-S Coffee Avenue 0 0 0 0

San Joaquin Stockton-Hazelton Street 0 0 0 0

San Joaquin Tracy-Airport 0 0 0 0

Stanislaus Modesto-14th Street 0 0 0 0

Stanislaus Turlock-S Minaret Street 0 0 0 0

Tulare Porterville-1839 Newcomb Street 0 0 0 0

Tulare Sequoia and Kings Canyon Natl Park 0 0 0 0

Tulare Sequoia Natl Park-Lower Kaweah 0 0 0 0

Tulare Visalia-N Church Street 0 0 0 0
15

Only official2data for 2013 and 2014 are used in this analysis. The 2015 data are a 

mixture of official and preliminary data. The extent of the official data record in 2015 

varies from site to site.

Table 6 shows the 2015 1-hour ozone design values for sites in the Valley.

Table 6. Preliminary 2015 1-Hour Ozone Design Values

for Sites in the San Joaquin Valley

County Site

Preliminary

Design Value

(ppm)

Fresno Clovis-N Villa Avenue 0.116

Fresno Fresno-1st St/Fresno-Garland 0.111

Fresno Fresno-Drummond Street 0.107

Fresno Fresno-Sierra Skypark #2 0.109

Fresno Parlier 0.115

Fresno Tranquility-West Adams Ave 0.087

Kern Arvin-Di Giorgio 0.109

Kern Bakersfield-5558 California 0.099

Kern Bakersfield-Municipal Airport 0.111

Kern Edison 0.107

Kern Maricopa-Stanislaus Street 0.090

Kern Oildale-3311 Manor Street 0.093

Kern Shafter-Walker Street 0.098

Kings Hanford-S Irwin Street 0.107

Madera Madera-28261 Avenue 14 0.102

Madera Madera-Pump Yard 0.100

Merced Merced-S Coffee Avenue 0.098

San Joaquin Stockton-Hazelton Street 0.089

San Joaquin Tracy-Airport 0.096

Stanislaus Modesto-14th Street 0.095

Stanislaus Turlock-S Minaret Street 0.102

Tulare Porterville-1839 Newcomb St 0.100

Tulare Sequoia and Kings Canyon NP 0.104

Tulare Sequoia NP-Lower Kaweah 0.101

Tulare Visalia-N Church Street 0.100

 

2

"Official" data are those measurements that been reviewed and verified by the data supplier and that 

have been submitted to ARB via U.S. EPA.
16

The numbers in Table 6 are based on preliminary data that came from ARB’s AQMIS 

database. Design values ranged from 0.087 ppm at Tranquility-West Adams Avenue to 

0.116 ppm at Clovis-N. Villa Avenue. All sites in the Valley have design values below 

the 0.12 ppm standard. The decreasing trend that was evident from 1990-2014, as 

shown in the previous table, has continued through 2015. Tables 5 and 6 show that the 

Valley continues to attain the standard during the most recent ozone season.

d. Conclusions

The Valley has experienced substantial improvements in ozone air quality. As of 2014, 

all currently operating sites in the Valley have attained the national 1-hour ozone 

standard of 0.12 ppm. This is demonstrated by both the three-year average estimated 

expected number of exceedance days and the design values. Both of these metrics 

have also shown significant progress in reducing ozone levels over the last 25 years. 

From 1990 to 2014, the Valley's three-year average estimated expected number of 

exceedance days decreased 98 percent from 36.8 to 0.7, while the Valley’s design 

value decreased 31 percent from 0.17 ppm to 0.118 ppm. Progress continued into the 

most recent ozone season. In 2015, the Valley’s design value dropped 0.002 ppm and 

the three-year average estimated expected number of exceedance days dropped by 

0.3. The District's continued planning efforts for attaining the 8-hour ozone standards

will ensure that ozone levels will remain below the national 1-hour ozone standard.

B. Estimated Ozone Levels at Arvin-Bear Mountain

While in operation, the ozone monitor at the Arvin-Bear Mountain site recorded some of 

the highest 1-hour ozone concentrations in the Valley. However, this site was shut 

down in 2010 due to unavoidable circumstances. Because Arvin-Bear Mountain closed 

before attaining the standard, it is important to consider what ozone concentrations at 

the site would currently be expected to measure. This section presents the history of 

the Arvin-Bear Mountain site and its relationship to the nearest sites; methods based on 

those relationships that can be used to estimate ozone concentrations at 

Arvin-Bear Mountain; the uncertainty in those methods; and how that uncertainty should 

be interpreted. These methods demonstrate that if Arvin-Bear Mountain were still in 

operation today, it would show attainment of the 1-hour ozone standard.

i. History of the Arvin-Bear Mountain Ozone Site

The Arvin-Bear Mountain site was established in 1989 as a maximum concentration site 

in the Bakersfield region. It was the design site, or the site that had the highest design 

value, for the 1-hour ozone standard in the Valley from 1989 through 1993. 

Arvin-Bear Mountain was again the Valley’s design site in 2004 and 2005. The site was 

shut down after the 2010 ozone season. In November 2009, ARB established a new 

site at Arvin-Di Giorgio and conducted parallel monitoring with Arvin-Bear Mountain for 

approximately 13 months, including the 2010 ozone season.
17

Because Arvin-Bear Mountain was periodically a high concentration site during its 

operation and because it was shut down before it was able to show attainment, it is 

necessary to demonstrate in a request for a CDD that it, too, would attain the standard. 

The most robust way to estimate daily ozone data for Arvin-Bear Mountain is to assume 

that prior relationships between measured ozone data at Arvin-Bear Mountain and at 

two other nearby sites (Arvin-Di Giorgio and Edison) would continue as they once did. 

Therefore, ARB, U.S. EPA, and District staff explored multiple methods to estimate daily 

ozone at Arvin-Bear Mountain based on daily ozone measurements at the two nearest 

ozone monitoring sites, Arvin-Di Giorgio and Edison. All three sites are located in the 

southeast corner of the Valley, as shown in Figure 1. Ozone measurements at 

Arvin-Bear Mountain, Arvin-Di Giorgio, and Edison tend to increase and decrease 

together day-to-day and year-to-year. Methods for estimating daily ozone at 

Arvin-Bear Mountain rely on this relationship.

Figure 1. Arvin-Bear Mountain and the Two Nearest Sites:

Arvin-Di Giorgio (lower middle), and Edison (top left)

The Arvin-Di Giorgio site is slightly over two miles north of Arvin-Bear Mountain, and 

Edison is approximately 10 miles north-northwest of Arvin-Bear Mountain. 

Arvin-Di Giorgio was established as a replacement for Arvin-Bear Mountain, and 
18

measured ozone data in parallel with Arvin-Bear Mountain are available for one ozone 

season (2010). Analysis of the parallel monitoring data showed there was a strong 

correlation between ozone data at these two sites. However, daily maximum 1-hour 

ozone concentrations were lower at Arvin-Di Giorgio compared to Arvin-Bear Mountain. 

Even so, the proximity of the two Arvin sites and their strong correlation support the 

estimation of 1-hour ozone concentrations and design values for Arvin-Bear Mountain

based on measured ozone data that continue to be collected at Arvin-Di Giorgio.

Staff at the ARB and the District used linear regression methods to estimate ozone data

at Arvin-Bear Mountain based on the equivalent data from Arvin-Di Giorgio. ARB also 

considered estimates of daily maximum ozone at Arvin-Bear Mountain based on 

measured ozone at Edison. The methods used by the two agencies are briefly 

identified and the important results are shown in the next two subsections. The 

approaches used by the two agencies varied in terms of the data selected for use, how 

the data were organized, and how uncertainty was evaluated, and all support the CDD. 

ARB and the District used the estimated daily maximum ozone data to calculate design 

values for Arvin-Bear Mountain. Also, ARB used the estimated daily maximum ozone 

data to calculate the estimated expected number of exceedance days for 

Arvin-Bear Mountain.

The estimates from ARB and the District based on measured data at Arvin-Di Giorgio

indicate that Arvin-Bear Mountain would have attained the national 1-hour ozone 

standard in 2014 if ozone had continued to be measured there.

ii. ARB’s Methods and Results

See Appendix B for detail.

ARB staff estimated Arvin-Bear Mountain design values for 2014 based on measured 

data from Arvin-Di Giorgio and from Edison. As shown in Table 7 below, the design 

value based on Edison is substantially lower and subject to greater uncertainty 

compared to the more conservative design value based on Arvin-Di Giorgio.

Table 7. Arvin-Bear Mountain 2014 Design Values with Uncertainty

Method Arvin-Bear Mountain

Design Value Uncertainty *

Based on data from Arvin-Di Giorgio 0.117 ppm 0.0024 ppm

Based on data from Edison 0.110 ppm 0.0039 ppm

* Based on a bootstrap uncertainty assessment

ARB staff concluded that the most conservative and reliable estimates of daily 

maximum 1-hour ozone values at Arvin-Bear Mountain are those based on its 

relationship to Arvin-Di Giorgio. Figures B3 and B4 in Appendix B illustrate why the 

design values based on Arvin-Di Giorgio are more reliable.

Table 8 shows estimated design values and estimated expected number of exceedance 

days, with and without uncertainty added to the estimated daily data, for 

Arvin-Bear Mountain in 2014 based on its relationship to Arvin-Di Giorgio. Consistent 
19

with U.S. EPA criteria for demonstrating attainment, ARB staff applied the estimated 

expected number of exceedance days procedures to the estimated daily maximum 

1-hour ozone values for Arvin-Bear Mountain in the 2012-2014 period. The estimated 

expected number of exceedance days procedures were also applied to the estimated 

values with uncertainty added.

The three-year average estimated expected number of exceedance days at 

Arvin-Bear Mountain for the 2012-2014 period was found to be 0.3 per year, with and 

without adding uncertainty. These results support attainment of the 1-hour ozone 

standard at Arvin-Bear Mountain and a Clean Data Determination for the San Joaquin 

Valley.

In addition, the results in Table 8, compared to results in Tables 1 and 2, indicate that 

Arvin-Bear Mountain, if operating, would not be the site with the highest ozone in the 

Valley. As shown earlier, one monitor in the Fresno region (Fresno-Sierra Skypark #2) 

had a higher three-year average estimated expected number of exceedance days of 0.7 

for the 2012-2014 period. Three other monitors (Fresno-Garland, Fresno-Drummond, 

and Parlier) had the same three-year average estimated expected number of 

exceedance days of 0.3 for the 2012-2014 period.

Table 8. Arvin-Bear Mountain Estimated 1-Hour Ozone Design Values

and Three-Year Average Estimated Expected Number of Exceedance Days

for the 2012-2014 Period Based on Measured Data from Arvin-Di Giorgio

Indicator Estimated Value Estimated Value with 

Uncertainty* Added

Design Value 0.117 ppm 0.120 ppm

Three-Year Average 

Estimated Expected Number 

of Exceedance Days

0.3 0.3

* Uncertainty calculated to be 0.0024 ppm based on a bootstrap assessment

ARB staff also used Arvin-Di Giorgio data (partially preliminary for 2015) to estimate 

design values and the three-year average estimated expected number of exceedance 

days at Arvin-Bear Mountain for the 2013-2015 period. The predicted 2015 design 

value for Arvin-Bear Mountain was 0.117 ppm, and the three-year average estimated 

expected number of exceedance days was 0.3 days. With uncertainty added, the 

results were 0.120 ppm and 0.3 days. These results indicate continued attainment of 

the 1-hour ozone standard for the Valley. From Table 5, Fresno-Sierra Skypark #2 has

a higher (0.4) three-year average estimated expected numbers of exceedance days for 

the 2013-2015 period than the estimated value (0.3) at Arvin-Bear Mountain, and thus 

continues to drive the Valley’s attainment status for the 1-hour ozone standard.
20

iii. San Joaquin Valley District’s Methods and Results

See Appendix B for detail.

In the District’s Clean Data Determination submittal to U.S. EPA and ARB (Attainment 

Determination Request for the Revoked 1-hour Ozone Standard), the District presented 

its methods and results when estimating daily maximum 1-hour ozone for 2012 through 

2014 at Arvin-Bear Mountain. The District developed predictive regression equations 

based on two different sets of daily maximum 1-hour ozone data from 

Arvin-Bear Mountain and Arvin-Di Giorgio. In both cases, the values were matched by 

date, which can lead to biases in the design value results. ARB staff expanded upon 

these analyses to address these potential biases, as described below.

In the first analysis, the District fit a straight line to all of the measured daily maxima 

from November 16, 2009 through October 31, 2010, when both Arvin-Bear Mountain 

and Arvin-Di Giorgio were operating. The slope of the line was adversely affected by 

some ozone data from winter months when ozone is low. These data exerted leverage 

that caused the slope of the line to be inordinately steep, which produced an estimated 

2014 design value of 0.119 ppm for Arvin-Bear Mountain. ARB removed the leverage 

issue by limiting the data to the more appropriate May-October ozone season, which 

produced a more reliable estimated design value of 0.117 ppm.

In the second analysis, the District fit a straight line to those data in 2010 for which daily 

maximum 1-hour ozone concentration at Arvin-Bear Mountain was 0.080 ppm or higher. 

Though this step is reasonable, it retained values above the line at the low end of the 

data while eliminating the counterbalancing values below the line. The unbalanced 

leverage of values above the line caused the slope of the line to be inordinately low, 

which produced an estimated 2014 design value of 0.115 ppm for Arvin-Bear Mountain. 

ARB removed the leverage issue by restoring a balanced selection of data, which 

produced a more reliable estimated design value of 0.116 ppm.

iv. Conclusions

Although the methods used by the ARB and the District differed, the results agree that 

ozone air quality at Arvin-Bear Mountain attained the national 1-hour ozone standard in 

the 2012-2014 period. ARB’s analyses further showed that Arvin-Bear Mountain 

continued to attain the standard for the 2013-2015 three-year period based on the most 

recent preliminary data for 2015.

The analyses also showed that the Fresno region would retain the Valley's high ozone 

site, which shifted there in 2008. This is discussed further in the following section.
21

C. Analysis of Ozone Concentration Trends

The previous sections of this report showed that the three-year average estimated 

expected number of exceedance days for all sites in the Valley are below the 1-hour

ozone standard, as is the three-year average estimated expected number of 

exceedance days for Arvin-Bear Mountain, based on robust statistical methods. This 

section builds on the data provided earlier and examines how ozone behavior in the 

Valley has varied over the last 25 years. Specifically, it discusses how the highest 

ozone levels have shifted over time and space. At times in the past and in recent years, 

the Fresno region has housed the highest ozone concentrations in the Valley. This is 

the case even when considering estimated ozone levels at the Arvin-Bear Mountain 

site.

i. The Highest Ozone Monitor Has a Tendency to Shift Over Time

Design values for a given region, whether it be an air basin or a Metropolitan Statistical 

Area, can often shift from site to site depending on the year. The Valley and two of its 

major MSAs, Fresno and Bakersfield, are no exception to this. The 1-hour ozone 

design site, or the site with the highest 1-hour ozone design value, has shifted 

repeatedly between monitors in each MSA. Table 9 shows the design values for high 

sites in the Valley between 1990 and 2014. Basin-wide design values for each year are 

shaded in gray in the table. The table shows that the design site in each MSA not only 

shifts between sites, it has also shifted repeatedly between the Fresno and Bakersfield 

MSAs.

From this table, it can be seen that during the last 25 years, the Fresno MSA housed 

the Valley's design site for 17 years and the Bakersfield MSA for 12 years. Of those 

12 years, four years had design sites in both of the Bakersfield and Fresno MSAs, while 

eight years had design sites exclusively in the Bakersfield region. Of the eight years 

exclusively in the Bakersfield region, the design site was at Edison for six years and 

Arvin-Bear Mountain for only two years. From 2006 through 2010, in the last five years 

of Arvin-Bear Mountain’s operation, the Valley’s design site was located at Edison twice 

(2006-2007) and at Clovis-N. Villa Avenue three times (2008-2010).
22

Table 9. National 1-Hour Ozone Design Values for High Sites

in the San Joaquin Valley for 1990-2014

Year

SJV 

Air 

Basin

Fresno MSA Bakersfield MSA

FresnoSierra 

Skypark 

(FNS)

FresnoDrummond 

Street (FND)

Parlier

(PLR)

Fresno1st 

Street

(FN1)

ClovisN Villa 

Avenue

(CNV)

Edison

(EDI)

Arvin-Bear 

Mountain

Blvd

(ABM)

Arvin-Di

Giorgio

(DIG)

1990 0.17 0.14 0.15 0.15 0.15 0.13 0.16 0.17 --

1991 0.16 0.14 0.15 0.15 0.16 0.13 0.15 0.16 --

1992 0.16 0.14 0.15 0.15 0.16 0.15 0.15 0.16 --

1993 0.16 0.14 0.15 0.15 0.16 0.14 0.16 0.16 --

1994 0.160 0.130 0.140 0.150 0.140 0.144 0.160 0.150 --

1995 0.164 0.131 0.140 0.140 0.140 0.144 0.164 0.147 --

1996 0.165 0.131 0.131 0.144 0.140 0.146 0.165 0.156 --

1997 0.164 0.131 0.131 0.144 0.140 0.146 0.164 0.156 --

1998 0.161 0.141 0.142 0.151 0.146 0.161 0.158 0.156 --

1999 0.161 0.141 0.137 0.145 0.146 0.161 0.154 0.137 --

2000 0.161 0.141 0.137 0.152 0.146 0.161 0.154 0.141 --

2001 0.146 0.136 0.131 0.146 0.135 0.142 0.138 0.134 --

2002 0.151 0.144 0.131 0.151 0.135 0.137 0.141 0.142 --

2003 0.151 0.144 0.132 0.151 0.135 0.136 0.134 0.150 --

2004 0.151 0.144 0.131 0.151 0.135 0.131 0.134 0.151 --

2005 0.149 0.123 0.128 0.135 0.131 0.126 0.127 0.149 --

2006 0.135 0.124 0.119 0.121 0.130 0.126 0.135 0.134 --

2007 0.135 0.124 0.110 0.121 0.130 0.125 0.135 0.131 --

2008 0.140 0.124 0.114 0.122 0.133 0.140 0.136 0.135 --

2009 0.140 0.119 0.118 0.121 0.127 0.140 0.135 0.135 --

2010 0.140 0.126 0.118 0.122 0.127 0.140 0.134 0.139 0.120

2011 0.130 0.119 0.127 0.121 0.124 0.130 0.124 -- 0.118

2012 0.130 0.118 0.127 0.122 0.124 0.130 0.118 -- 0.118

2013 0.127 0.114 0.127 0.118 -- 0.123 0.113 -- 0.112

2014 0.118 0.109 0.118 0.116 -- 0.118 0.107 -- 0.109

Below, Table 10 shows the number of times each site was the exclusive design site (the 

only design site in the basin for the year), the number of times each site was a shared 

design site (the basin had more than one design site for the year), and the total number 

of times each site was a design site (exclusive plus shared) from 1990-2014 and from 

1990-2010 in parentheses.

Table 10. The Number of Times Each Site Was the Valley Design Site

from 1990-2014 and 1990-2010 (in parentheses)

Site Number of Times 

Exclusive Design Site

Number of Times 

Shared Design Site

Total Number of 

Times Design Site

Clovis-N Villa Avenue 8 (6) 1 (0) 9 (6)

Edison 6 (6) 1 (1) 7 (7)

Arvin-Bear Mountain 2 (2) 4 (4) 6 (6)

Parlier 3 (3) 1 (1) 4 (4)

Fresno-1

st Street 0 (0) 3 (3) 3 (3)

Fresno-Drummond Street 1 (0) 1 (0) 2 (0)
23

Clovis-N. Villa Avenue and Edison were each the 1-hour ozone design site more often 

than was Arvin-Bear Mountain between 1990 and 2014. For the period from 1990 to 

2014, Arvin-Bear Mountain ranks third in the total number of times it has been a design 

site (six times) and ranks fourth in the number of times it was the exclusive design site 

for the basin (four times).

ARB staff reviewed data for the period from 1990-2010 to determine whether the results 

through 2014 were an artifact of the 2010 closure of Arvin-Bear Mountain. For the 

1990-2010 period, Edison was most frequently the 1-hour ozone design site (seven 

times total, six exclusively and one shared) for the Valley. Clovis and 

Arvin-Bear Mountain tied for second in the total number of times (six) they have been a 

design site, although Clovis was the exclusive design site all six times, while 

Arvin-Bear Mountain was the exclusive design site only twice. Arvin-Bear Mountain

ranks fourth in the number of times it was the exclusive design site in the Valley, after 

Edison and Clovis with six and Parlier with three.

Figure 2 shows the basin-wide design values for 1990-2014. The colored boxes in the 

plot correspond to a period of three or more consecutive years in which one site was the 

design site. The boxes are labeled and are color coordinated to the sites shown in 

Table 9. All other years are labeled with a three-letter code (also identified in Table 9) 

which signifies the design site for that year.

Figure 2. Valley National 1-Hour Ozone Design Values for 1990-2014

Arvin-Bear Mountain

Arvin-Bear Mountain

EDI EDI

FND

CNV

FND

0.100

0.110

0.120

0.130

0.140

0.150

0.160

0.170

0.180

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011201220132014

National 1-hour Ozone Design Value (ppm)

Year

Arvin-Bear 

Mountain

Fresno1st

Edison

Clovis

Parlier

Clovis
24

Table 9 and Figure 2 also show that the design site has resided in the Fresno MSA 

since 2008. Clovis-N Villa Avenue had the highest design value from 2008-2012, while 

Fresno-Drummond had the highest design value in 2013. Those two sites tied for the 

highest design value in 2014. The estimated design values at Arvin-Bear Mountain for 

2014 was 0.117 ppm, which is also less than the measured design values at the Clovis 

and Fresno sites. Another important observation to be made from Table 9 and Figure 2

is that Arvin-Bear Mountain was not the design site for the last five years it was in 

operation. Furthermore, in 2006 and 2007, the last time the Bakersfield MSA housed 

the Valley’s high site, concentrations were highest at Edison and not at 

Arvin-Bear Mountain.

Figure 3. National 1-Hour Ozone Design Values for the Fresno and Bakersfield 

MSAs for 1990-2014

The design value line shown in Figure 2 can be separated into two more lines showing 

the design values for each MSA. This is shown in Figure 3. The blue squares and lines 

connecting them represent the 1-hour ozone design values for the Fresno MSA, while 

the red circles and lines connecting them represent those for the Bakersfield MSA. The 

straight blue (dashed) and red (solid) lines indicate the trend lines for the Fresno and 

Bakersfield MSAs, respectively. From this, it can be seen that the ozone design values 

for both the Fresno and Bakersfield MSAs have been decreasing at a steady rate over 

the last 25 years. Another noticeable feature of this plot is that the design value for 

Bakersfield has been decreasing at a faster rate than the design value in Fresno.

Figure 4 depicts a comparable analysis that ascertains whether the closure of 

Arvin-Bear Mountain at the end of 2010 affected the trend lines. Figure 4 conveys the 

same findings as Figure 3–declining trends for the Fresno and Bakersfield MSAs, with 

y = -0.001x + 0.162

y = -0.002x + 0.173

0.1

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.18

National 1-Hour Ozone Design Value (ppm)

Year

Fresno MSA Bakersfield MSA Linear (Fresno MSA) Linear (Bakersfield MSA)

National 1-hour Ozone Standard
25

the Bakersfield design value decreasing more steeply than that of Fresno. Additionally, 

Figures 3 and 4 show that the steeper decline in the Bakersfield MSA 1-hour ozone

design values pre-existed the 2010 shutdown of the monitor at Arvin-Bear Mountain.

Figure 4. National 1-Hour Ozone Design Values for the Fresno and Bakersfield 

MSAs for 1990-2010

The lines from Figures 3 and 4 can be broken down further to see the design value 

trends at each of the sites in the Fresno and Bakersfield MSAs. Figures 5 and 6 show 

the 1-hour ozone design values for all operational sites in the Fresno and Bakersfield 

MSAs, respectively, from 1990-2014. There have been significant decreases in 1-hour

design values at all sites in the Fresno and Bakersfield MSAs from 1990-2014, with 

minor ups and downs embedded in the overall downward trend. As of 2014, all 

operational sites in the Fresno and Bakersfield MSAs were below the national 1-hour

ozone standard. In the Bakersfield MSA, no operational site has violated the standard 

since 2011, and the design values for all sites have continued on their downward 

trajectory. Based on the relationships demonstrated in section II.B.ii. ARB's Methods 

and Results, above, if the Arvin-Bear Mountain site were still operating, its design 

values also would have decreased.

Figure 5 also shows that all sites in the Fresno MSA show similar rates of decline in 

design value concentrations over the past 25 years. Figure 6 shows that all sites in the 

Bakersfield MSA show a decreasing trend, but that the two sites with the highest ozone 

(Arvin-Bear Mountain and Edison) show a steeper decline than do the remaining sites 

(i.e., those with lower ozone concentrations) over the same period. The design values 

for Arvin-Bear Mountain and Edison also decrease faster over the 25 years than do 

those for the sites in the Fresno MSA. If just the most recent years (the 2012-2014 

y = -0.001x + 0.160

y = -0.002x + 0.170

0.1

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.18

National 1-Hour Ozone Design Value (ppm)

Year

Fresno MSA Bakersfield MSA Linear (Fresno MSA) Linear (Bakersfield MSA)

National 1-hour Ozone Standard
26

period) are considered, design values at all the sites in the Bakersfield region indicate a 

similar rate of decline. Figures 5 and 6, in conjunction with Figures 3 and 4, show that 

ozone levels have been decreasing at a faster rate in the Bakersfield region than in the 

Fresno region.

Figure 5. National 1-Hour Ozone Design Values for Sites in the Fresno MSA

from 1990-2014

0.08

0.09

0.1

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.18

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011201220132014

National 1-Hour Ozone Design Value (ppm)

Year

Fresno-Drummond St. Parlier Fresno-Sierra Skypark #2

Fresno-1st St. Clovis-N Villa Ave. Fresno-Garland

National 1-hour Ozone Standard
27

Figure 6. National 1-Hour Ozone Design Values for Sites in the Bakersfield MSA

from 1990-2014

The Valley ozone trends discussed above showed progress in all subregions with the 

fastest progress in the southern subregion. These trends are the result of the Valley’s 

topography and meteorology, general and regional changes in ozone-precursor 

emissions, and the Valley’s NOx-limited photochemical conditions. These factors are 

discussed below in relation to both valley-wide effects and varying subregional effects.

ii. Topography and Meteorology in the San Joaquin Valley

See Appendix C for more detail.

The San Joaquin Valley’s topography, weather, and transport patterns strongly 

influence the geographic distribution of ozone, resulting in lower levels in the north, with 

higher levels in the central and southern portions of the Valley.

Topographically, the valley is broad, flat, and bounded by mountain ranges that impede 

the free flow of air on three sides: the Coast Range to the west, the Sierra Nevada to 

the east, and the Tehachapi Mountains to the south. Marine air from the Pacific Ocean 

frequently flows through the Golden Gate, across the San Francisco Bay, through the 

Carquinez Strait, and into the northern end of the San Joaquin Valley. The marine air 

often ventilates the northern subregion, while it exerts less ventilating influence on the 

central and southern subregions as the topography impedes its inflow.

During the May through October ozone season, the Valley usually experiences hot and 

dry conditions associated with prevailing high-pressure weather systems. These 

0.08

0.09

0.1

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.18

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011201220132014

National 1-hour Ozone Design Value (ppm)

Year

Edison Oildale-3311 Manor St.

Maricopa-Stanislaus St. Arvin-Bear Mountain Blvd.

Shafter-Walker St. Arvin-Di Giorgio

Bakersfield-5558 California Ave.

National 1-hour 

Ozone Standard
28

systems tend to trap pollutants and retain them over multiple days. The persistent 

stagnation promotes high ozone concentrations. Under cooler and breezier 

low-pressure weather systems, the Valley may become well-ventilated with low ozone 

concentrations, as free-flowing air dilutes airborne materials and removes them from the 

region.

Wind flow patterns play a significant role in the distribution of air pollutants in the San 

Joaquin Valley. The dominant wind flow pattern3during the daytime is upvalley, from 

the northwest to the southeast, with airflow generally moving from Stockton toward 

Bakersfield. At night, the general northwest to southeast surface wind flow pattern 

continues but slows under the influence of surface cooling, downslope flows, and 

eddies. Transport tends to carry ozone and its precursors southward, mixing them via 

circular eddies, and accumulating ozone and other photochemical products in the 

central and southern subregions.

From year to year, annual differences in meteorology and the pace of emission 

reductions may cause the location of the highest site(s) to fluctuate between the central 

and southern subregions of the San Joaquin Valley. While ozone concentrations have 

decreased throughout the Valley, since 2008, the highest ozone design values have 

remained in the central subregion, shifting between sites in the Fresno area.

Through 2020, generally decreasing ozone levels are expected to continue, with the 

highest levels remaining in the central subregion. This expectation is due to parallel 

patterns of emission reductions in the central and southern subregions, in which 

substantial NOx reductions should reduce ozone concentrations, especially so in the 

southern subregion on high-temperature days. The following two sections address 

these points.

iii. Ozone Precursor Emissions in the San Joaquin Valley

See Appendix C for more detail.

ARB's emissions inventories show that anthropogenic emissions of nitrogen oxides 

(NOx) and reactive organic gases (ROG) have decreased substantially throughout the 

Valley from 2000 to 2014, with some small differences in the relative rates between 

subregions of the Valley. Though small, the differences are directionally consistent with 

somewhat greater impact of NOx reductions in the southern subregion compared to the 

central subregion of the Valley.

In the southern subregion (defined as Tulare County and the valley portion of Kern 

County), NOx decreased by 56 percent and ROG decreased by 40 percent. In the 

central subregion (defined as Madera, Fresno, and Kings Counties), NOx decreased by 

53 percent and ROG decreased by 36 percent. And in the northern subregion (defined 

as Merced, Stanislaus, and San Joaquin Counties), NOx decreased by 51 percent and 

ROG decreased by 32 percent. The fastest rates of decrease were in the southern 

subregion and coincide with the fastest improvements in ozone.

 

3 California Surface Wind Climatology, California Air Resources Board, 1992.
29

From 2014 to 2020, anthropogenic NOx is expected to decrease by almost 30 percent 

in the central and southern subregions. These changes are consistent with the view 

that the Valley’s design value is likely to continue to occur in the central subregion.

iv. Limiting Photochemical Conditions in the San Joaquin Valley

See Appendix C for more detail.

The atmospheric chemistry by which NOx and ROG produce ozone is complex and can 

produce nonlinear results. The ratio of ROG to NOx (ROG/NOx ratio) is a key indicator 

of the likely effects of NOx and ROG reductions. When ozone production is limited by 

the available NOx, the ROG/NOx ratio is relatively high and the system is called 

NOx-limited. In a NOx-limited system, reducing NOx tends to cause ozone production 

to decrease. Sophisticated modeling and air quality studies are needed to identify and 

quantify the effects of NOx and ROG reduction.

ARB staff provided the results of modeling studies and air quality analyses as 

weight-of-evidence (WOE) support for the San Joaquin Valley District’s 2013 plan to 

attain the national 1-hour ozone standard.4 The WOE document includes the following 

statements from ARB staff:

 The trend towards higher ROG/NOx ratios in the SJV indicates that the area will 

become more NOx limited, thus NOx controls will become increasingly more 

effective for lowering ozone concentrations.

 The results of modeling carrying capacity diagrams also indicate that ozone in 

the SJV is NOx-limited, and thus the continuing NOx reductions from ARB and 

District control programs will be the most important contributor to achieving the 

1-hour ozone standard in the SJV.

Two recently published air quality studies, Pusede (2012)5and Pusede (2014)6, indicate

that photochemical production of ozone in the Valley in 2010 was or soon would be 

NOx-limited (NOx-sensitive) on high-temperatrue days that favor high ozone. 

Therefore, further NOx reductions are likely to be effective in reducing the levels of 

ambient ozone in the San Joaquin Valley. This is especially true of the southern

subregion, which showed NOx-limited characteristics, including higher ROG/NOx ratios, 

in advance of the central subregion ozone. Therefore while both regions will continue to 

see ozone improvements going forward due to ongoing emission reductions, it is 

 

4

Appendix G Weight of Evidence at: 

http://www.valleyair.org/Air_Quality_Plans/Ozone-OneHourPlan-2013.htm

5

S. E. Pusede and R. C. Cohen, On the observed response of ozone to NOx and VOC reactivity 

reductions in San Joaquin Valley California 1995–present[2010], Atmos. Chem. Phys., 12, 8323-8339, 

2012.

6

S. E. Pusede, et al., On the temperature dependence of organic reactivity, nitrogen oxides, ozone 

production, and the impact of emission controls in the San Joaquin Valley, California, Atmos. Chem. 

Phys., 14, 3373-3395, 2014.
30

expected that ozone levels will remain highest in the central subregion and continue to 

drive the Valley’s attainment status.

Rapid improvements in measured ozone air quality since 2010 provide confirmation that

ozone production the San Joaquin Valley is now NOx-limited. Although Valley-wide 

emissions of anthropogenic NOx and ROG are projected to decrease substantially 

between 2010 and 2020, emissions of biogenic ROG are substantial and unlikely to 

decrease. As a result, ROG/NOx ratios are projected to continue to increase, moving 

the Valley further into NOx-limited conditions.