APPENDIX A 

SUPPLEMENTAL ANALYSIS: 8-HOUR OZONE ATTAINMENT 

A-1 

8-HOUR OZONE ATTAINMENT SUPPLEMENTAL ANALYSIS

Introduction

Under federal law, the South Coast Air Basin (South Coast or Basin) is currently 

classified as a Severe-17 nonattainment area for the federal 8-hour ozone standard and 

has a nominal attainment date of June 15, 2021. Given the magnitude of emissions 

reductions needed to reach attainment, including reductions relying on new or improved 

control technologies, the South Coast Air Quality Management District (District) does 

not predict attainment by the 2021 date. Therefore, the District is requesting the Basin 

be reclassified as Extreme, with required attainment by June 15, 2024. The following 

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

photochemical modeling that support the overall conclusion that the South Coast will 

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 South Coast of 

0.083 parts per million (ppm). Because of the uncertainties inherent in photochemical 

modeling, U.S. EPA allows states to show that attainment is likely despite modeled 

results, when the model predicts ozone levels between 0.082 ppm and 0.087 ppm, and 

the weight of evidence (WOE) evaluated supports an alternative attainment 

demonstration. The supplemental 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 offset the limitations and uncertainties that 

are inherent in all air quality modeling. This 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 supplemental analyses is different for each nonattainment area. The level 

of detail appropriate for each area depends upon the complexity of the air quality 

problem, how far into the future the attainment deadline is, and the amount of data and 

modeling available. This document summarizes the ARB’s supplemental assessment 

of the 8-hour ozone NAAQS attainment demonstration for the South Coast 

nonattainment area. 
A-2 

Historical Context

Thirty years ago, high ozone concentrations posed a pervasive problem throughout the 

South Coast. During the mid- to late-1970s, maximum 8-hour concentrations were 

frequently higher than 0.30 ppm, or nearly four times the level of the federal standard. 

During this same time period, the standard was exceeded on more than 200 days each 

year, the federal design value was more than three times the level of the 8-hour 

standard, and Stage 1 episodes (days with a maximum 1-hour ozone concentration of 

at least 0.20 ppm) occurred on roughly one-third of the days during each year (refer to 

Figure A-1). 

 

 

Figure A-1: South Coast Air Basin Ozone Statistics 1975 to 2005 

 

 

South Coast Air Basin Ozone Statistics

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1975

19761977

1978

19791980

1981

1982

1983

1984

1985

1986

1987

19881989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

20002001

2002

20032004

2005

Year

Ozone (ppm)

0

50

100

150

200

250

300

350

400

450

500

Number of Days

Maximum Concentration Design Value* Exceedance Days Stage 1 Episodes

An emissions control strategy based on reductions of both NOx and ROG was 

implemented during the 1970s and began to have an impact on resulting ozone air 

quality. While air quality improvements over the years have been dramatic, progress 

has not been continuous from one year to the next, nor has the rate of progress been 

the same during the entire time period. Although there is often a great deal of 

year-to-year variability, air quality indicators generally showed little progress during the 

mid- to late-1970s, began improving during the early-1980s, and then were relatively flat 

again during the mid- to late-1980s. During the last five to six years, there has again 

been a leveling off in the trends, as shown in Figure A-1, following a period of significant 

improvement throughout the 1990s. Despite the variable rates of progress, there has 

been close to a 60 percent overall reduction in both the maximum concentrations and 

the number of exceedance days since the mid-1970s. 

* Design values for years prior to 1980 may not be based on 3 years of data. 
A-3 

Assessment of Recent Air Quality Trends

 General Basinwide Perspective 

Although there has been significant progress over the last three decades, the South 

Coast Air Basin still faces one of the biggest ozone air quality challenges in the nation, 

whether measured by maximum concentration, design value, or number of exceedance 

days. Furthermore, the South Coast has the distinction of being the only area in the 

nation originally classified as Severe for the federal 8-hour standard. While air quality 

has improved over the years, problems still exist in the northwestern portion of 

Los Angeles County and the South Coast portions of Riverside and San Bernardino 

counties. While exceedances in these areas occasionally occur outside the ozone 

season, most are limited to the May through October timeframe. During 2005, the 

maximum 8-hour ozone concentration was 0.145 ppm, a little less than twice the level of 

the federal standard. The number of exceedance days totaled 83, equivalent to nearly 

three months during the year with unhealthful ozone air quality. 

Despite the magnitude of the problem, air quality indicators show widespread progress 

in improving ozone air quality over the last decade, reflecting the impact of continued 

emissions reductions. Basinwide, maximum concentrations and the number of 

exceedance days both declined by 30 percent, while the design value declined 

25 percent, when comparing 1995 to 2005. Since 1998, Stage 1 Episodes have been 

eliminated, with the exception of a single episode day during 2003, which was a 

meteorologically adverse year with respect to ozone. 

Another way of measuring air quality progress is to estimate the change in the spatial 

extent of the problem using monitoring data. Over the last decade, the spatial extent of 

the “clean” coastal areas has increased substantially, with accompanying improvements 

at sites located inland. Figure A-2 shows contour maps of federal 8-hour exceedance 

days in the South Coast averaged over two three-year periods, ten years apart -- 1995 

and 2005. Ten years ago (1993-1995 average map), the ozone problem was 

widespread throughout the region. About half of the Basin had more than 50 federal 

8-hour exceedance days each year, and nearly all sites had a design value above the 

level of the federal 8-hour ozone standard. Although the coastal areas were cleaner 

than the inland areas, only small, isolated portions of the Basin had ten or fewer 

exceedance days. Today (2003-2005 average map), we see a dramatic expansion of 

clean areas, especially those in the range of 0 to 5 exceedance days. These clean 

areas now include about a third of Orange County and a fourth of Los Angeles County. 

Furthermore, they cover the region where more than half of the Basin population lives. 

The areas with 6 to 10 and 11 to 20 exceedance days have also grown substantially. 

Although we see substantial amounts of basinwide progress from 1995 to 2005, the rate 

of progress was greatest during the first half of the decade and has leveled off over the 

last five to six years. This holds for a number of different indicators, including 
A-4 

exceedance days, maximum concentration, design value, average concentration on the 

Top 30 days, and population-weighted exposure. Between 1995 and 1999, there was a 

10 percent decrease in design value and a 25 to 30 percent drop in maximum 

concentration. Comparing this with the 2000 to 2005 timeframe, the design value again 

decreased about 10 percent, but the drop in maximum concentration was only a tenth 

and the drop in the number of exceedance days only about half of what it was during 

the earlier five-year period. 

Figure A-2: South Coast Air Basin Change in Federal 8-Hour Exceedance Days 

1995 to 2005

 

 

In summary, basinwide measures of maximum concentration, exceedance days, and 

design value are all down by about 30 percent over the last decade. Although there has 

been a substantial amount of progress since 1995, most of that progress occurred 

during the first half of the time period. In contrast, progress for nearly all indicators has 

been relatively flat since 2000. Despite the slowed progress, the ozone problem is not 

pervasive throughout the Basin. With continuing emissions reductions, the “problem” 

areas are shrinking. At the same time, there has been a substantial expansion of the 

“clean” areas, and a little more than half of the population lives in areas that are now 

clean. 
A-5 

 Regional Analyses 

From a basinwide perspective, there has been substantial improvement in ozone over 

the last decade. However, the basinwide statistics are dominated by the high site(s), 

which pose the most severe and persistent problems. Dividing the Basin into 

sub-regions provides a different perspective, showing an even greater magnitude of 

progress. For the following discussion, the South Coast Air Basin is divided into six 

broad areas, as shown in Figure A-3: the Coastal Plain, the San Fernando Valley, the 

Santa Clarita area, the San Gabriel Valley, the Inland Empire/Desert Region, and the 

Crestline area. These represent six relatively distinct areas, based on geographical, 

meteorological, and air quality characteristics. 

Figure A-3: South Coast Air Basin Sub-regions 

 

 

More than half of the Basin population lives in the highly developed Coastal Plain. The 

bulk of the South Coast emissions are generated in this lowland area which is bordered 

on one side by the coast and on the other sides by mountains. Because of the marine 

influence, the Coastal Plain experiences a more temperate climate than the rest of the 

Basin, and the cooler temperatures and onshore/offshore airflow patterns generally 

favor good air quality. 

As one travels inland, geography and climate pose significant challenges to air quality 

progress. Similar to the Coastal Plain, the San Fernando and San Gabriel valleys also 

represent low lying areas, but are generally surrounded by mountains on all sides. 

These act as barriers to airflow, while also trapping emissions and pollutants. Higher 

temperatures and more stagnant conditions in these valley areas lead to a build-up of 
A-6 

ozone, and overall poorer air quality. The same is true for the Inland Empire/Desert 

Region, as well as the Santa Clarita and Crestline areas, which are even further from 

the coast. 

Previous analyses have shown that air quality progress within each of the sub-regions 

tends to be similar, but between the selected regions, the rates of change often differ 

noticeably. Figure A-4 shows the number of exceedance days during 1995 and 2005 

for each of the sub-regions mapped in Figure A-3. While values for the two highest 

sites in the Basin, Crestline and Santa Clarita, are plotted separately, values for the 

other sub-regions reflect data from multiple sites within each area. 

Figure A-4: South Coast Air Basin Change in Number of Federal 8-Hour Exceedance 

Days by Sub-region 1995 to 2005 

 

Number of Exceedance Days 

During 1995 and 2005

0

10

20

30

40

50

60

70

80

90

100

110

120

Coastal Plain

San Fernando Valley

San Gabriel Valley

Inland Empire/Desert Region

Santa Clarita

Crestline

# of Exceedance Days

1995 2005

 

Based on the bar graph, it is striking how clean the air is in the Coastal Plain – 

Los Angeles County/Orange County – where the bulk of the Basin population lives. 

During the 2005 May through October ozone season, 90 percent of the days were 

below the level of even the more health-protective State 8-hour standard, and no site 

had more than one federal 8-hour exceedance day. Overall, there has been a 

95 percent drop in the number of exceedance days in the Coastal Plain over the last 

decade. 

Although all sites in the Coastal Plain either meet or are close to meeting the federal 

8-hour standard, dominant weather patterns suggest that the Coastal Plain can be an 

important emissions source region for areas located inland. However, growth in the 

inland regions has made them significant sources of ozone precursors, as well. 

The San Fernando Valley, San Gabriel Valley, and Inland Empire/Desert Region, all of 

which are located further inland, pose greater air quality challenges. Even so, these 

areas have shown improvements, including sharp decreases in exceedance days, along 

with increases in the number of “clean” days with concentrations below the State 8-hour 

standard. During the 2005 ozone season, 75 to 80 percent of the days in the 
A-7 

San Fernando and San Gabriel valley areas and 40 percent of the days for the Inland 

Empire/Desert Region met the more health-protective State ozone standard. During 

this same year, the San Gabriel Valley, once the area with the Basin’s worst ozone 

air quality, had a total of 14 federal exceedance days, down from 84 days in 1995 

(an 85 percent decrease). The San Fernando Valley had a 65 percent decrease and 

the Inland Empire/Desert Region a 45 percent decrease in federal exceedance days 

between 1995 and 2005. 

Crestline and Santa Clarita continue to stand out as the sites with the worst ozone air 

quality, and these sites will likely pose the most difficulty for attaining the federal 8-hour 

ozone standard. Continuing ROG and NOx emissions reductions have reduced the 

number of federal exceedance days at both sites. However, the number of days during 

2005 was still equal to about one and a half months of the year at Santa Clarita 

(45 days) and two months of the year at Crestline (67 days). While the number of 

exceedance days at Crestline and at Santa Clarita is still substantial, especially when 

compared with other areas of the Basin, there has been improvement over the years. 

For example, Crestline had a 30 percent reduction in the number of exceedance days 

from 1995 to 2005. The problem of attainment at Crestline and Santa Clarita is 

complicated by the distance between the two sites and the fact that less than half of the 

Santa Clarita exceedances (40 percent) occur on the same days as the Crestline 

exceedances. Furthermore, as discussed in the following section on the 

characterization of episodic conditions, the weather conditions favoring high ozone at 

Crestline are different from those for Santa Clarita. 

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

basis in the South Coast. Today, the numbers of exceedance days in all areas are 

more limited than they were ten years ago. However, there have been different rates of 

progress in the different areas. The most progress occurred in the Coastal Plain, where 

the federal standard is exceeded only occasionally now, and all sites are approaching 

attainment of the more health-protective State 8-hour ozone standard. Other areas 

have also made great strides – the San Fernando Valley, San Gabriel Valley, and 

Inland Empire/Desert Region have all seen substantial improvements. As emissions 

continue to decline, ozone air quality in these areas should continue to improve. 

Santa Clarita and Crestline still pose major challenges. Although these two sites have 

shown improvement over the last decade, their rates of progress have been slower than 

in other areas, and these areas may therefore require the greatest emissions reductions 

to attain the standard. 

Meteorology and Air Quality Trends

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

emissions and meteorology. The meteorological and photochemical processes leading 

to ozone formation are complex, 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 
A-8 

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 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 isolate the improvement attributable to 

emissions reductions. One of the goals of these analyses is to determine the role 

meteorology has played in the recent leveling off of ozone air quality. Although the 

analyses indicate that meteorological variability does not account for the flatness of the 

trends, they do show that it now takes more severe meteorological conditions to 

produce ozone exceedances. 

 Characterization and Frequency of Episodic Conditions 

In evaluating the meteorological conditions associated with ozone episodes, three 

well-defined patterns were identified as being associated with the majority of episodes. 

Collectively, these three patterns are associated with over 90 percent of the 8-hour 

exceedances that occurred during May through October of 2000 through 2005. The 

three patterns are characterized by high pressure aloft, generally clear skies, light and 

variable nighttime and morning winds, strong morning inversions, and deep mixing of 

pollutants aloft in the afternoon. The patterns are differentiated from one another 

primarily by small variations in the size of the weather systems influencing each pattern. 

The “typical summer weather pattern” usually brings coastal emissions and ozone from 

the Coastal Plain to the more downwind Inland Empire/Desert Region. It was 

associated with about a third of the 8-hour ozone exceedances recorded during the 

analysis period and tended to result in the highest ozone concentrations in the Basin. 

Under this pattern, high pressure aloft, in combination with calm surface winds during 

the morning, result in stagnant conditions. Maximum temperatures inland are near 

100 degrees, while the Coastal Plain stays relatively cool. In the afternoon, the sea 

breeze moves onshore, in response to increasing and very hot temperatures inland. 

Average afternoon wind speeds range between 5 and 10 miles per hour (mph), with 

only a few hours above 10 mph in the eastern part of the Basin. 

The “eddy weather pattern” brings emissions and ozone northward into the Santa 

Clarita area and San Fernando Valley, as well as eastward into the Crestline area. High 

ozone concentrations under the eddy pattern typically occur at Crestline, at Santa 

Clarita (via Newhall Pass), and as far away as Ventura County. This pattern generated 

the highest number of ozone episodes in the Basin during the study period and is 

characterized by lower pressure aloft. This promotes more wind flow over the upper 

levels of the Basin. At the surface, the Coastal Plain is dominated by a stronger 
A-9 

onshore flow and a counterclockwise eddy circulation centered off the southern 

California coast, near Catalina Island. The sea breeze penetrates much deeper into the 

Basin, causing both temperature and ozone concentrations in the Coastal Plain to be 

much lower than with the other meteorological patterns. 

Finally, the “offshore pattern” typically results in a build-up of ozone near precursor 

sources or only slightly downwind. Upper and surface level winds along the coast are 

light and variable, with weak offshore winds. The influence of the marine layer is felt 

only along the immediate coast, with terrain and slope flows acting as the main 

transport mechanism into the interior valleys. Surface winds remain offshore until the 

afternoon, with average afternoon wind speeds ranging between 5 and 10 mph. 

Maximum temperatures reach near 100 degrees in most eastern areas of the Basin, 

with coastal areas in the 80s. High ozone concentrations typically occur throughout the 

metropolitan area, closer to major source regions. In contrast, areas such as Crestline, 

which are typically dominated by transport, exhibit lower concentrations. This pattern 

was associated with less than a quarter of the 8-hour ozone exceedances during the 

analysis period. 

 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 A-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 

emission reductions. Progress is shown when the number of exceedance days 

decreases in relation to the number of high OFP days. 

Looking at the trend lines from 1990 through 1999 provides an illustration of how to 

interpret Figure A-5. From 1990 to 1999, observed exceedance days decreased by 

57 days, from 143 to 86, while high OFP days increased by 1 day, from 76 to 77. 

Therefore, relative to high OFP, the “real” decrease in exceedance days was about 

58 days (the decrease in exceedance days plus the increase in high OFP days). 

 

1

 Above sea level 
A-10 

Figure A-5: South Coast Air Basin Federal 8-Hour Exceedance Days and High OFP 

Days 1990 to 2005 

 

South Coast Air Basin Ozone Trends

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

0

20

40

60

80

100

120

140

160

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

Year

Number of Days (3-yr 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.

The overall analysis suggests that ozone improvements in the South Coast are primarily 

attributable to substantial emissions reductions. The decline in the number of 

exceedance days between 1990 and 1999 was not a result of favorable weather, 

because the number of days with adverse weather conditions did not change 

substantially during this time period, while the number of exceedance days showed a 

steady decrease. Furthermore, even though ozone levels have been relatively flat 

during the last five to six years, the analysis suggests that more adverse weather 

conditions are now required to produce an exceedance of the federal standard. 

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, two groups accounted for the vast 

majority of exceedance days during the ozone season in the South Coast. For each of 

these groups, data from 1990 through 1992 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. 
A-11 

Met-adjusted trends are presented in the following two figures. Both 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. Each line was smoothed 

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

remove meteorological influences perfectly, and other factors also affect year-to-year 

changes. 

In Figure A-6, all three trend lines track together and show significant improvement over 

the time period. Since 2000, the trend has been rather “flat.” A similar period of relative 

flatness is seen from 1996 to 1999, followed by a period of more pronounced progress. 

Figure A-6: South Coast Air Basin Ozone Trends 1990 to 2005 Adjusted for 

 Meteorology 

 

 

South Coast Air Basin -- Basinwide Ozone Trends

(Annual Means for Top 30, Top 20, and Top 10 Met-Adjusted Daily Max-8hour Observations)*

0.100

0.110

0.120

0.130

0.140

0.150

0.160

0.170

0.180

0.190

0.200

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

Year

3-Year Moving 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 A-7 emphasizes the similarity between the trend lines for all three indicators: the 

means of the Top 10, Top 20, and Top 30 basinwide daily maximum 8-hour ozone 

concentrations. When expressed in terms of percent with respect to their values in 

1990, the trends for all three indicators are almost identical. Because of this similarity, it 

is apparent that the top 16 percent2

 of summer ozone concentrations in the South Coast 

Air Basin have responded very similarly to emissions reductions since 1990. 

In summary, a critical question concerning ozone in the South Coast Air Basin is 

whether ozone levels since 2000 have been improving, deteriorating, or unchanged. 

Over short periods, such as 2000 to 2005, it is important to consider the weather 

 

2

 Of the 184 days from May through October, 30 represents 16%. 
A-12 

conditions when studying trends. As described in the above sections, the effects of 

weather on ambient ozone concentrations were considered in two ways: 

(1) OFP/CART trends for exceedance days and (2) met-adjusted trends for average 

ozone on high concentration days. While both analyses confirm that ozone trends have 

been flat, they also suggest that more adverse meteorological conditions are now 

required to produce an exceedance of the federal 8-hour ozone standard. 

Figure A-7: South Coast Air Basin Ozone Trends 1990 to 2005 Adjusted for 

 Meteorology and Expressed as a Percentage of the Base Year 

 

South Coast Air Basin -- Basinwide Ozone Trends

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

60%

70%

80%

90%

100%

110%

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

Year

3-Year Moving Mean Ozone (% 1990-92)

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

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

Emissions and Precursor Trends

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

Emissions controls have significantly reduced the amounts of these precursors in the 

ambient air, resulting in improved air quality. However, similar to ozone, the rate of 

reduction for each precursor has varied over time. This has resulted in a different “mix” 

of precursors over the years. Both the rates of reduction and the mix of precursors can 

impact ozone improvement. The following sections describe the NOx and ROG 

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

the ambient air. 

 Emissions Trends 

Emissions controls have significantly reduced the amounts of both ROG and NOx 

emitted by various sources throughout the South Coast. Figure A-8 shows the 

estimated basinwide trend in 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. 
A-13 

Figure A-8: South Coast Air Basin ROG and NOx Emissions 1994 to 2005 

 

South Coast Air Basin ROG & NOx Emissions

Summer SIP Inventory with Biogenics - Ver 1.06

0

200

400

600

800

1000

1200

1400

1600

1800

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

Year

Tons/Day

South Coast Air Basin ROG South Coast Air Basin NOx

Basinwide, ROG emissions show a relatively steady decline over the entire time period. 

In comparison, NOx emissions also declined, although at a slower overall rate. NOx 

emissions declined between 1994 and 1998, were fairly stable between 1998 and 2001, 

but then showed a small decline again between 2001 and 2005. In terms of total 

reduction, ROG emissions decreased about 45 percent between 1994 and 2005. The 

reduction in NOx emissions was less -- a 25 percent reduction between 1994 and 2005. 

Because the majority of the basinwide emissions (ROG and NOx) occur in the 

Los Angeles County portion of the Basin, most of the emissions reductions have 

occurred in this area (refer to Figure A-9). By comparison, the level of emissions 

reductions in the remaining three counties have been much smaller (refer to Figure A10). 

Figure A-9: Los Angeles County ROG and NOx Emissions 1994 to 2005 

 

Los Angeles County ROG & NOx Emissions

Summer SIP Inventory - Ver 1.06

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

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

Year

Tons/Day

Los Angeles County ROG Los Angeles County NOx
A-14 

Figure A-10: Orange, Riverside, and San Bernardino County ROG and NOx Emissions 

 1994 to 2005 

 

Orange, Riverside, and San Bernardino County ROG & NOx Emissions

Summer SIP Inventory - Ver 1.06

0

50

100

150

200

250

300

350

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

Year

Tons/Day

Orange County ROG Orange County NOx San Bernardino County ROG

San Bernardino County NOx Riverside County ROG Riverside County NOx

The differing rates of reduction for basinwide ROG and NOx emissions has led to a 

change in the relative amounts of ROG and NOx. Before about 2000, the estimated 

amount of ROG emissions was higher than the estimated amount of NOx emissions. 

However, the levels were about equal in 2000, and the pattern reverses in the following 

years. It is interesting to note that this reversal in the relationship between ROG and 

NOx occurred at about the same time that basinwide ambient ozone concentrations 

leveled off. This provides support for the idea that both NOx and ROG emissions 

reductions are needed to reduce ambient ozone concentrations. Previous emissions 

controls have primarily targeted ROG. However, in the future, there needs to be a 

long-term focus on achieving significant NOx reductions, as well. 

Precursor Trends 

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

inventory, ozone precursor monitoring data confirm the reduction of both ROG and NOx 

in the ambient air. As shown in Figure A-11, there has been an overall reduction in both 

the amount of ROG, as well as the reactivity of the ROG mix measured at the 

Photochemical Assessment Monitoring Stations (PAMS) network in the South Coast. 

Since 1999, both ROG from the PAMS network and NOx from co-located monitors 

show reduction patterns similar to those for the estimated emissions, in that ROG is 

coming down at a faster rate than NOx (about a 60 percent reduction in ROG versus a 

40 percent reduction in NOx). This confirms the effectiveness of the emissions control 

programs, which historically, have focused more heavily on ROG reductions. 
A-15 

Figure A-11: South Coast Air Basin Ambient Summer ROG and NOx Trends at 

 PAMS Stations (5-7 am) 1994 to 2005 

 

July-August Average Ambient PAMS ROG, PAMS Reactivity, and NOx

at South Coast Air Basin PAMS Stations (5-7am) 

0

50

100

150

200

250

300

350

400

450

500

550

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

Year

ppb

PAMS ROG PAMS Reactivity NOx

While the PAMS precursor data show a lower overall decline in NOx, as compared with 

ROG, there have been spatial differences in NOx reductions, as well. Over the 12-year 

time period (1994 to 2005), the average 5 am to 7 am July-August NOx decreased 

about 35 percent at sites located west of Upland. During this same time period, sites 

located east of Upland saw only about a 3 percent reduction in average 5 am to 7 am 

July-August NOx. The slower decline in NOx levels in the eastern part of the Basin is 

consistent with faster growth rates in these downwind portions of the South Coast. This 

growth has partially offset the benefits of emissions control programs in the eastern 

portion of the Basin. 

The ratio of ROG to NOx in the ambient air is an important factor in ozone formation. 

Figure A-12 shows the ambient ROG to NOx ratios for both the total measured ROG 

and the reactive portion of the total measured ROG. Both ratios are quite variable 

during the earlier years, although they track well over the entire timeframe. Over the 

12-year period, the ratios for both have been reduced. The ratio for total ROG to NOx 

was 3.9 in 1994 compared with 2.4 in 2005. Similarly, the ratio for reactive ROG to NOx 

was reduced from 2.8 in 1994 to 1.7 in 2005. This reduction in the ROG to NOx ratio 

reflects the historical emphasis on ROG controls. 

In summary, emissions estimates show a steady decline in ROG over the last 12 years. 

Estimates show a smaller overall decrease in NOx emissions. Prior to 1999, the level of 

estimated NOx emissions was lower than the level of ROG emissions. Both were about 

equal in 2000, and NOx emissions are higher since then. 
A-16 

Figure A-12: South Coast Air Basin Summer Morning PAMS Ratios and Morning 

 ROG and NOx Emissions 1994 to 2005 

 

PAMS Ratio of the July-August Average (5-7 am) at South Coast Air Basin Stations

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

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

Year

PAMS Ratio (ppbC/ppbN)

PAMS ROG/NOx PAMS Reactivity/NOx

 

Similar to the estimated emissions, measured ROG from the PAMS network, as well as 

NOx from co-located samplers also show a relatively steady decline in measurements 

over time. While the measured data show a fairly continuous decrease in ambient 

ROG, ambient NOx concentrations showed the greatest decrease between 1994 and 

2001. Since 2001, there has been little change. This period of relatively stable ambient 

NOx concentrations generally coincides with a leveling off of ambient ozone 

concentrations. In addition to a reduction in the total amounts of precursors, data from 

the PAMS network show a reduction in the overall reactivity of the ROG mix. The ratios 

of ROG to NOx and ROG reactivity to NOx track well over the 12 years, with both 

averaging about 2 or higher during the entire period. 

The ozone control strategy has been effective in reducing precursor emissions, 

especially hydrocarbon emissions and the reactivity of the hydrocarbon mix. However, 

despite these reductions, almost all measures of ozone air quality in the South Coast 

have been relatively flat since 2000. Photochemical modeling suggests that because of 

where the area currently lies with respect to ROG to NOx ratios, slow rates of 

improvement are not unexpected. Furthermore, the decrease in the reactivity of the 

ROG mix means that it now takes longer for photochemical reactions to occur, and this 

is reflected in an eastward movement of the basinwide peak ozone concentrations over 

time. 

Despite the slow rate of basinwide improvement, ROG emissions reductions over the 

last several years have resulted in improved ozone in the coastal urban areas of the 

South Coast. However, the NOx reductions have not been sufficient to lower ozone 

concentrations in the eastern (downwind) portions of the Basin. Over the long-term, 

emissions controls must focus on NOx reductions. Modeling results support this, 
A-17 

showing that substantial reductions in NOx emissions (a 75 percent reduction), coupled 

with a lesser reduction in ROG emissions (a 20 percent reduction) are needed to reach 

attainment throughout the Basin, including downwind, transport-impacted areas. As 

additional controls are implemented, ozone should once again begin to improve at a 

more rapid rate. 

Modeling Results

The ozone modeling domain for the South Coast was based on the domain defined for 

the 1997 Southern California Ozone Study (SCOS) and includes not only the South 

Coast Air Basin, but the surrounding coastal, desert, and mountain areas, as well. The 

domain horizontal grid is 116 by 80 cells, with a resolution of five kilometers. The ozone 

episode periods selected for modeling comprised a combination of five episodes from 

the years 2004 and 2005, chosen to reflect recent ozone design values, along with a 

1997 episode to maintain continuity with the 2003 South Coast Air Quality Management 

District (District) SIP. 

The required meteorological fields were generated using the MM5 prognostic 

meteorological model, and the required emissions inventories were developed by 

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

Model with Extensions (CAMx) model, with initial and boundary conditions based on 

estimates of clean-air concentrations. 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 

sub-region within the domain. 

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

used in projecting future 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 (in this case 2002). A reference or base year design value is then multiplied 

by the RRF to project a future year design value. The District’s modeling satisfies the 

minimum five episode requirement for use in developing a site-specific RRF for most 

sites, as recommended by the U.S. EPA’s guidance for modeling 8-hour ozone design 

values. 

Results of the modeling analyses show that all sites in the South Coast Air Basin will 

attain the federal 8-hour ozone standard by June 15, 2024, as required for Extreme 

ozone nonattainment areas. The amounts of emissions reductions needed to reach 

attainment vary throughout the Basin, because the magnitude of the ozone problem 

varies from place to place. Sites located in the eastern portions of the Basin, in 

particular Crestline, Fontana, and Redlands, are projected to have the highest future 

year design values, at 0.083 ppm, 0.081 ppm, and 0.081 ppm, respectively. These 

future year values are just below the level of a federal 8-hour exceedance. The future 

year ozone projection for Santa Clarita, another high site in the Basin, is 0.074 ppm, 

which is a little more than 10 percent below the level of a federal 8-hour exceedance. 
A-18 

 

In addition to projecting attainment by 2024, results of the modeling analyses confirm 

the recent flatness of the ozone trends in the South Coast Air Basin. As described 

previously, maximum concentrations measured at the Crestline site showed little 

progress between 2002 and 2005. Given the emissions reductions that occurred during 

this timeframe, the modeling analyses indicate that little progress should have been 

expected. However, despite the recent plateau, the modeling analyses show that in 

future years, a dual pollutant control strategy will be effective in reducing ambient ozone 

concentrations, eventually bringing the South Coast Air Basin into attainment for the 

federal 8-hour ozone standard. However, because of where the Basin is, with respect 

to the current mix of ROG and NOx emissions, it may be several years before 

substantial progress occurs. Therefore, it is important to recognize that progress must 

be evaluated in the long-term, and that short-term plateaus are not unexpected. 

Modeled Versus Ambient Changes in Ozone on Meteorologically Similar Days 

As part of the supplemental analysis, ARB staff compared changes in modeled ozone to 

changes in measured ambient ozone from 1996 to 2005. The purpose of this analysis 

was to corroborate the modeled results and to assess the model’s response to a 

particular pattern of emissions reductions. The analysis is intended to help in 

interpreting the modeling results. 

Methods 

The key step in comparing changes in modeled versus ambient ozone was selecting 

days in each year for which meteorological conditions were similar to conditions that 

prevailed on the modeled days. For the South Coast, a total of six 2004 and 2005 

episode days with satisfactory model performance were available for comparison. Each 

selected day was part of a multi-day episode that was successfully modeled as a “base 

case,” using day-specific inventories. Each multi-day episode was subsequently re-run 

using summer planning inventories for 1996 and 2005. The six episode days that were 

modeled and selected for comparison were August 6-8, 2004, and July 17-19, 2005. 

ARB staff compared the relative (percent) change in modeled ozone from 1996 to 2005 

for the modeled days to the relative change in ambient ozone on days with similar 

meteorological conditions for the same period. Meteorological similarity was 

determined using surface temperature, surface wind speed and direction, temperature 

aloft (about 5000 feet above sea level), atmospheric stability, and integrated wind speed 

and direction aloft (surface to about 5000 feet). The similarity scale for each 

meteorological factor was standardized before determining a composite similarity for 

meteorology. Composite similarity was a weighted sum reflecting the relative 

importance of each meteorological characteristic: surface temperature (weight = 1), 

surface winds (weight = 0.25), temperature aloft (weight = 1), atmospheric stability 

(weight = 0.25), and winds aloft (weight = 0.5). For this analysis, the sum of the weights 

did not need to equal 1. 
A-19 

For each modeled day, ARB staff identified the four most similar days in each year. If 

each selected day was similar to only one of the modeled days, a total of 24 days in 

each year would have been selected. However, because some of the modeled days 

were similar to each other, the number of days selected to reflect ambient conditions 

ranged from 11 to 19 for the years 1996 to 2005. The average number of days selected 

per year was slightly more than 15. 

In the South Coast, comparisons were based on the average of model results at 

selected sites in the Riverside and San Bernardino County portions of the Basin, which 

comprise the Inland Empire/Desert Region. Also included in the analysis area was 

Crestline, the current design site for the federal 8-hour ozone standard. Crestline 

routinely shows the highest ozone concentrations in the Basin, both on a daily and an 

annual basis. Long-term sites in the analysis area include Crestline, Riverside, San 

Bernardino, Banning, and Perris. In addition to a regional comparison, model and 

ambient results were compared at the first three sites. Banning and Perris were not 

considered individually because of problematic model performance in these areas. 

The San Fernando Valley/Santa Clarita sub-regions were not considered because of 

problems with the ambient data. Specifically, Santa Clarita, the key monitoring site, was 

relocated from a fire station, where diesel fire engines produced NOx that scavenged 

ozone locally, to a more representative location with lower NOx emissions. Ozone 

readings at the new location were systematically higher, and an appropriate adjustment 

of the ambient trend was problematic. 

The Coastal Plain sub-region also was not considered because of spotty model 

performance. While model performance was satisfactory at some locations, it was not 

at others. In addition, the Coastal Plain generally has relatively low ozone 

concentrations, making comparisons for the Inland Empire/Desert Region and Crestline 

area relatively more important. 

Results 

The comparisons of modeled ozone to ambient ozone do not constitute proof that the 

model results are either good or bad, since assumptions implicit in the methodology 

could lead to differences. For example, the criteria used to identify days with similar 

meteorology could not ensure that the selected days were identical to the modeled 

episodes. Also, the day-specific inventories used to validate the models’ base cases 

differ to some degree from the summer planning inventories used to represent 1996 and 

2005. Nevertheless, this analysis is a reasonable approach for examining model 

behavior and real-world behavior under similar atmospheric conditions. The key 

conclusions of the ARB analysis include the following: 

• Model performance was strongest in the Inland Empire/Desert 

Region and Crestline area which comprise the Riverside and 

San Bernardino County portions of the Basin. Sites in these 

areas regularly record the highest 8-hour ozone concentrations 

in the Basin. 
A-20 

• For the Inland Empire/Desert Region and Crestline area, the 

modeled improvement in daily maximum 8-hour ozone from 

1996 to 2005 was 17 percent, while the ambient improvement 

was 14 percent. 

• Modeled and ambient improvements at multiple sites in the 

Inland Empire/Desert Region and Crestline area were 

reasonably consistent. 

As shown in Table A-1, the agreement between the ambient and modeled trends was 

quite good for the Inland Empire/Desert Region and Crestline area as a whole and for 

the Crestline and San Bernardino sites specifically. This provides some indication that 

the modeling system as a whole is able to reasonably reproduce the ambient trends on 

a relative basis. That is, while the model may not be able to replicate the exact air 

quality measurements that occurred in 1996 and 2005, it is able to reasonably capture 

the relative change in ozone. This performance is consistent with the use of relative 

reduction factors in the application of air quality models for federal 8-hour ozone SIPs. 

Results for the Riverside site were not as good as for the other two sites. Nevertheless, 

the modeled and ambient trends still compare relatively well. Additional analysis is 

needed to better understand the cause or causes of the larger difference at Riverside. 

Table A-1: Modeled versus Ambient Ozone Progress from 1996 to 2005 in the 

 South Coast Air Basin. 

Basin Sub-region 

or Site 

Modeled 

Progress 

Ambient 

Progress 

Inland Empire / 

Desert Region / 

Crestline area 

17 percent 14 percent 

Crestline 16 percent 15 percent 

San Bernardino 18 percent 18 percent 

South Coast 

Riverside 18 percent 12 percent 

Figures A-13 through A-16 give a graphical representation of the results in Table A-1. 

The graphs show the modeled values for 1996 and 2005. In contrast, the ambient 

trends are shown as annual values from 1996 through 2005. 

In Figures 13, 14, and 16, the ambient ozone level for 1998 is notable. This analysis 

emphasized meteorological similarity among the days selected for the ambient trend. 

Accordingly, most of the weather effects related to ozone formation and accumulation 

should have little impact on the trend. The persistence of anomalous values in 1998 is 
A-21 

consistent with other analyses conducted by ARB staff. Efforts to remove the effects of 

meteorology from the ozone trends in the Inland Empire/Desert Region, and at Crestline 

in particular, have been unsuccessful in coaxing the 1998 data into reasonable 

agreement with the rest of the trend. Several alternative explanations are under 

investigation: (1) perhaps some unusual meteorological pattern affected the study area 

in 1998, and this pattern is not reflected in the factors used to adjust the ozone data, or 

(2) it is possible that a factor unrelated to meteorology, such as a temporary effect of 

monitor recalibration or unusual local activity, may have strongly affected the ozone 

data for 1998. The trend for the Inland Empire/Desert Region and Crestline area 

reflects the regional daily maximum 8-hour ozone data, which often is dominated by the 

Crestline site. Therefore, the Inland Empire/Desert Region trend is interconnected with 

the Crestline trend. Furthermore, Crestline is located high in the mountains, in contrast 

to San Bernardino and Riverside, which are not elevated sites and represent large 

suburban populations. 

 

 

Figure 13: Modeled versus Ambient Trends from 1996 to 2005 in the Inland Empire / 

Desert Region / Crestline Area 

 

SoCAB: Inland Empire Trend for Daily Max. 8-hr Ozone

(Avg. on 11-19 days selected for meteorology similar to 6 modeled episodes)

70%

75%

80%

85%

90%

95%

100%

105%

110%

115%

120%

125%

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Average Ozone (% of 1996 value)

Ambient Modeled
A-22 

Figure 14: Modeled versus Ambient Trends from 1996 to 2005 at Crestline 

 

 

Crestline Trend for Daily Maximum 8-hour Ozone

(Avg. on 11-19 days selected for meteorology similar to 6 modeled episodes)

60%

65%

70%

75%

80%

85%

90%

95%

100%

105%

110%

115%

120%

125%

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Average Ozone (% of 1996 value)

Ambient Modeled

Figure 15: Modeled versus Ambient Trends from 1996 to 2005 at San Bernardino 

 

 

San Bernardino Trend for Daily Maximum 8-hour Ozone

(Avg. on 11-19 days selected for meteorology similar to 6 modeled episodes)

60%

65%

70%

75%

80%

85%

90%

95%

100%

105%

110%

115%

120%

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Average Ozone (% of 1996 value)

Ambient Modeled
A-23 

Figure 16: Modeled versus Ambient Trends from 1996 to 2005 at Riverside 

 

Riverside Trend for Daily Maximum 8-hour Ozone

(Avg. on 11-19 days selected for meteorology similar to 6 modeled episodes)

60%

65%

70%

75%

80%

85%

90%

95%

100%

105%

110%

115%

120%

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Average Ozone (% of 1996 value)

Ambient Modeled

Summary

The supplemental analysis supports the modeled attainment demonstration. Currently, 

the South Coast is classified as Severe-17 with respect to the federal 8-hour ozone 

standard, with attainment required by 2021. Because of the magnitude of emissions 

reductions required, the District is requesting reclassification as Extreme, with a 

June 15, 2024 attainment deadline. 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. 

Photochemical modeling completed by the District indicates that all sites in the South 

Coast Air Basin will reach attainment by the 2024 deadline with a 76 percent reduction 

in NOx emissions and a 22 percent reduction in ROG emissions. Based on these 

reductions, Crestline, the high site, will have a future year design value of 0.083 ppm, 

which is below the level of a federal 8-hour ozone exceedance. Based on the modeling 

results and supplemental analyses, attainment by 2024 can be projected because of the 

following factors: 
A-24 

• Over the last decade, basinwide measures of maximum concentration, design 

value, and number of exceedance days all declined by about 30 percent. 

Because these are basinwide numbers, they reflect the “worst case” sites. 

On a sub-regional basis, the amount of improvement during the last ten years 

is even greater. 

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

South Coast Air Basin. Today, the spatial extent of the relatively clean areas 

has expanded substantially. Furthermore, more than half of the Basin 

population now lives in areas with design values that meet the federal 8-hour 

ozone standard. 

• Progress has occurred at different rates in different parts of the Basin. The 

greatest amount of progress occurred in the Coastal Plain, where all sites are 

approaching attainment of the more health-protective State 8-hour ozone 

standard. Other areas have also improved significantly. Even the two high 

sites, Crestline and Santa Clarita, have made substantial gains, although at 

slower overall rates. 

• Emissions estimates and ambient precursor data show that both ROG and 

NOx have declined, although ROG has declined at the faster rate. Prior to 

2000, the level of estimated NOx emissions was greater than ROG 

emissions. However, beginning in 2000, NOx emissions have been about 

equal to or higher than ROG emissions. This crossover in the emissions 

trend lines occurred at about the same time that the ozone trends began to 

level off, lending support to the idea than both ROG and NOx reductions are 

important for ozone improvement. 

• Emissions estimates indicate a continuing decline in emissions for both 

precursors, as the State and District continue to pursue an aggressive dual 

pollutant emissions control program. However, 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 being proposed as 

providing the most efficient path to attainment. 

• Because emissions from the South Coast can be transported to areas outside 

the Basin, emissions reductions in the South Coast area should result in 

ozone improvements not only there, but also in other downwind, 

transport-impacted areas such as Ventura County, the Coachella Valley, and 

the Mojave Desert Air Basin. 

• Analyses suggest that the ozone improvements that have been achieved are 

primarily attributable to substantial emission reductions. The decline in the 

number of exceedance days between 1990 and 2000 was not a result of 

favorable weather, because the number of days with adverse weather 

conditions actually increased or remained level during this time period, while 
A-25 

the number of exceedance days decreased. Furthermore, even though 

ozone levels have been relatively flat during the last five to six years, 

analyses suggest that more adverse weather conditions are now required to 

produce an exceedance of the federal standard. 

• Photochemical modeling shows a design value of 0.083 ppm at Crestline by 

the end of 2023. The design values at other sites in the Basin are predicted 

at or below 0.081 ppm. All of these values are below the level of a federal 

8-hour ozone exceedance. 

• A comparison of modeled to measured changes on meteorologically similar 

days showed model performance was strongest in the Inland Empire/Desert 

Region and Crestline area, where the highest 8-hour ozone concentrations 

regularly occur. Results showed a 17 percent improvement from 1996 to 

2005 based on modeling, compared with a 14 percent improvement based on 

changes in the measured data. 

Taken together, the results of these analyses indicate that all sites in the South Coast 

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

deadline for an Extreme ozone nonattainment area.