APPENDIX B 

SUPPLEMENTAL ANALYSIS: PM2.5 ATTAINMENT 

B-1 

SOUTH COAST PM2.5 SUPPLEMENTAL ANALYSIS 

1. INTRODUCTION 

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

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

demonstration comprises analyses used to determine the set of control measures 

needed to meet the NAAQS by the attainment year. These analyses typically include 

air quality modeling, which generally guides the selection of the most effective pollutants 

to control and the magnitude of needed emissions reductions. Because of the 

uncertainties inherent in photochemical modeling, U.S. Environmental Protection 

Agency (U.S. EPA) modeling guidance recommends that states perform complementary 

analyses. These analyses can include consideration of measured air quality, 

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

additional air quality modeling. 

A supplemental analysis looks at the entirety of the information at hand to provide a 

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

strengths and weaknesses, examining an air quality problem in a variety of ways offsets 

the limitations and uncertainty that are inherent in air quality modeling. This approach 

also provides a better understanding of the overall problem and the level and mix of 

emissions controls needed for attainment. 

In its modeling guidance, U.S. EPA indicates that the results of supplementary analyses 

may be used to demonstrate that attainment is likely despite modeled results, when 

future year modeled annual average design values fall between 14.5 and 15.5 

micrograms per cubic meter (ug/m3). This range in modeled design values reflects the 

uncertainty in predicting absolute PM2.5 concentrations that is inherent in air quality 

modeling, and therefore recognizes that an improved assessment of attainment can be 

derived from examining a broader set of analyses. 

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

the primary modeling analysis: 

 

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

2) observational models and diagnostic analyses, and 

3) additional modeling. 

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

proposed control measures to yield attainment. The scope of the supplemental analysis 

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

depends upon the complexity of the air quality problem, how far into the future the 

attainment deadline is, and the amount of data and modeling available. For example, 

less analysis is needed for an area that is projecting attainment near-term and by a wide 

margin, and for which recent air quality trends have demonstrated significant progress, 
B-2 

than for areas with more severe air quality challenges. The following sections present 

the supplemental assessment for PM2.5 in the South Coast Air Basin for each of the 

areas outlined in the U.S. EPA guidance. 

2. ASSESSMENT OF RECENT AIR QUALITY AND EMISSION TRENDS 

Current Air Quality

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

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

extrapolate future trends in PM2.5 based on measured historical trends of air quality 

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

is increased if an extensive monitoring network exists and if there is a good correlation 

between past emission reductions and current trends in PM2.5. 

The South Coast Air Basin has one of the most severe PM2.5 problems in the nation 

and represents a considerable challenge in attaining the federal PM2.5 standards. 

These standards consist of both a 24-hour standard of 65 ug/m3, and an annual 

average standard of 15 ug/m3. While almost all monitors with complete data in the 

South Coast still exceeded the annual PM2.5 standard as of 2006, PM2.5 air quality has 

shown considerable improvement since 1999 when monitoring data for assessing 

compliance with the federal PM2.5 standard began. When South Coast was first 

designated nonattainment for the federal PM2.5 standard, the basin exceeded both the 

annual and 24-hour PM2.5 standards. However, as of 2005, the South Coast now 

meets the federal 24-hour PM2.5 standard of 65 ug/m3 throughout the basin. Thus, 

attaining the annual standard is the remaining challenge in this State Implementation 

Plan (SIP). 

Currently, South Coast has an extensive monitoring network with seventeen sites 

monitoring for PM2.5 in the air basin. Table B-1 provides the federal 3-year annual 

average values (design values) for each monitoring site for 2006 along with the 2004, 

2005, and 2006 annual averages. Table B-2 provides the 2006 federal 3-year 24-hour 

98th percentile design value along with the 2004, 2005, and 2006 24-hour 98th percentile 

values. The design value is a statistic that is used to describe the air quality status of a 

given area relative to the level of the federal standard. Design values represent the 

yearly annual average or 98th percentile PM2.5 concentrations averaged over a three 

year period. Attainment is reached when the design value is at or below the federal 

standard. As shown in Table B-1, current 2006 design values (reflecting the 2004 

through 2006 period) range from 15.0 ug/m3 to 20.8 ug/m3. 
B-3 

Table B-1: PM2.5 Annual Average Values 

Monitor 2004 Annual 

Average 

(ug/m3) 

2005 Annual 

Average 

(ug/m3) 

2006 Annual 

Average 

(ug/m3) 

2006 

3-year Annual 

Average 

Design Value 

(ug/m3)

Anaheim-Pampas Lane 17.0 14.7 14.0 15.2 

Azusa 18.3 17.0 15.4 16.9 

Burbank-W Palm 

Avenue 19.1 17.8 16.5 17.8 

Fontana-Arrow 

Highway 19.9 18.8 18.4 19.4 

Long Beach East 

Pacific Coast Hwy 16.5 14.7 * * 

Los Angeles-North 

Main Street 18.6 17.8 15.6 17.3 

Lynwood 18.5 17.5 16.7 17.5 

Mission Viejo 26081 Via 

Pera 12.0 10.6 * * 

North Long Beach 17.9 15.9 * * 

Ontario-1408 Francis 

Street 20.9 18.8 18.4 19.4 

Pasadena-S Wilson 

Avenue 16.6 15.1 13.4 15.0 

Pico Rivera 20 * * * 

Reseda 15.7 * * * 

Riverside-Magnolia 20.8 17.9 16.9 18.6 

Riverside-Rubidoux 22.1 21.0 19.2 20.8 

Riverside-5130 

Poinsettia Place * * 20.8 * 

San Bernardino-4th 

Street 21.9 17.3 17.7 19 

*Incomplete data

The South Coast PM2.5 monitoring sites with the highest air quality levels are located in 

the eastern part of the air basin including Fontana, Ontario, Riverside and San 

Bernardino. The eastern part of the basin is most conducive to forming high levels of 

PM2.5 due to the geography and climate. In the South Coast, air typically flows from 

the coast to the eastern part of the basin which is the surrounded by mountains. Along 

with more stagnant conditions, the mountains act as a barrier trapping both emissions 

and pollutants leading to the build-up of high PM2.5 levels. Monitoring sites located 

closer to the coast in Los Angeles and Orange Counties have lower PM2.5 air quality 

levels due to the air flow patterns from the ocean. As shown in bold in the tables, the 

high site for the region is the Rubidoux-Riverside monitor with 2006 design values of 

20.8 ug/m3 for the annual average and 57 ug/m3 for the 24-hour PM2.5 standards. 
B-4 

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

Monitor 2004 

24-hour 98th

Percentile 

(ug/m3) 

2005 

24-hour 98th

Percentile 

(ug/m3) 

2006 

24-hour 

98th 

Percentile 

(ug/m3) 

2006 

3-year 24-hour 98th 

Percentile Design 

Value (ug/m3)

Anaheim-Pampas Lane 48.2 41.8 40.5 44 

Burbank-W Palm 

Avenue 49.3 * 43.4 * 

Fontana-Arrow 

Highway 53.6 * 48.2 * 

Long Beach East 

Pacific Coast Hwy 42.0 37.7 * * 

Los Angeles-North 

Main Street * 53.3 38.3 * 

Lynwood 53.0 48.4 44.4 49 

Mission Viejo 26081 Via 

Pera 38.5 31.4 * * 

Ontario-1408 Francis 

Street * 49.5 * * 

Pasadena-S Wilson 

Avenue 46.5 43.0 32.0 41 

Pico Rivera 52.1 * * * 

Riverside-Magnolia 53.7 * 47.7 * 

Riverside-Rubidoux 59.5 58.3 53.2 57 

Riverside-5130 

Poinsettia Place * * 52.5 * 

San Bernardino-4th 

Street 72.4 43.4 47.7 55 

*Incomplete data 

Recent PM2.5 Mass Trends

Despite the magnitude of the problem, considerable progress has occurred in the South 

Coast over the last seven years due to the ongoing emissions control program. As 

discussed above, 24-hour concentrations, as shown in Table B-2 and Figure B-1, have 

decreased approximately 15 percent and based on the 2004 through 2006 data, now 

attain the federal standard at all sites in the air basin. 
B-5 

Figure B-1: PM2.5 24-hour Design Values 

PM2.5 24-hour Design Value

0

10

20

30

40

50

60

70

80

90

Rubidoux San Bernardino Lynwood

Concentrations (ug/m3)

2001

2006

All South Coast monitors with complete PM2.5 data also show a significant decrease in 

annual average design values (Figure B-2). In 2001, all monitoring sites in the basin 

had annual design values greater than 20 ug/m3, with the Riverside-Rubidoux site at 

twice the level of the standard. By 2006, design values decreased throughout the 

basin, and only one site in the easternmost portion of the basin is still greater than 

20 ug/m3. Riverside-Rubidoux remains the high site, with a design value which is 

39 percent above the standard. However, the greatest rate of progress has also 

occurred in the eastern basin. From 2001 through 2006, the Riverside-Rubidoux site 

design value dropped 30 percent, from 30.1 ug/m3 to 20.8 ug/m3, while the San 

Bernardino site design value dropped 26 percent, from 25.8 ug/m3 to 19.0 ug/m3. In 

contrast, the Los Angeles-North Main site design value dropped 23 percent, from 

22.6 ug/m3 to 17.3 ug/m3. This trend is further illustrated in Figures B-3 and B-4 which 

depict maps of the spatial variations in annual average concentrations in 2001 as 

compared to 2006. A number of sites have annual average concentrations near or 

below 15 ug/m3 in 2005 or 2006, including Anaheim, Mission Viejo, Pasadena, and 

Long Beach. 
B-6 

Figure B-2: Three Year Annual Average PM2.5 Design Values 

Three Year Annual Average PM2.5 Design Values

0

5

10

15

20

25

30

35

2001 2002 2003 2004 2005 2006

Year

Concentrations (ug/m3)

Fontana-Arrow Highw ay

Los Angeles-North Main

Street

Lynw ood

North Long Beach

Ontario-1408 Francis Street

Riverside-Magnolia

Riverside-Rubidoux

San Bernardino-4th Street

Figure B-3: National PM2.5 Annual Average 2001 
B-7 

Figure B-4: National PM2.5 Annual Average 2006 

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

linkage between the two standards. The annual average is comprised of individual 

24-hour samples which vary throughout the year. In the South Coast, PM2.5 

concentrations do not exhibit a pronounced seasonal variation, with monthly average 

concentrations above the level of the annual standard occurring in all parts of the year. 

This relative uniformity results in part from meteorological conditions conducive to the 

formation of sulfates and nitrates at different times of the year. Sulfate formation is 

greater in the warmer months while nitrate formation is greater in the cooler weather. 

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

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

average concentrations is the result of a broad downward shift in the distribution of daily 

PM2.5 concentrations over time. This can be seen in the decrease in monthly average 

concentrations between 2001 and 2006 at Los Angeles-North Main and RiversideRubidoux (Figures B-5 and B-6). Consistent with its greater rate of progress in the 

annual average, the Riverside-Rubidoux site had seen the largest reduction in monthly 

average concentrations. 
B-8 

Figure B-5: Changes in Seasonal Pattern from 2001 to 2006 at Los Angeles-North 

Main Monitoring Site 

Changes in Seasonal Pattern from 2001 to 2006

LA-North Main

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11 12

Month

Concentrations (ug/m3)

2001

2006

Figure B-6: Changes in Seasonal Pattern From 2001 to 2006 at Riverside-Rubidoux 

Monitoring Site 

Changes in Seasonal Pattern from 2001 to 2006

Rubidoux

0

5

10

15

20

25

30

35

40

45

1 2 3 4 5 6 7 8 9 10 11 12

Month

Concentrations (ug/m3)

2001

2006

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

over the last few years provides another means of understanding progress. Data 

collected from 24-hour samples during the 1999–2001 period at the Los Angeles-North 

Main site indicate that 8.2 percent of the samples had concentrations greater than 
B-9 

40 ug/m3 (Figure B-7). In comparison, during the 2004-2006 period the number of 

samples with PM2.5 concentrations greater than 40 ug/m3 fell to 4.2 percent. At the 

Riverside-Rubidoux monitoring site, data collected during the 1999–2001 period 

indicate that 23.3 percent of the samples had concentrations greater than 40 ug/m3 

(Figure B-8), while during the 2004-2006 period, only 8.5 percent of the samples had 

concentrations greater than 40 ug/m3. Significant reductions in the percent of samples 

greater than 20 ug/m3 were also observed at both sites. These analyses illustrate that 

progress in the annual average is linked not just to reductions in peak concentrations at 

certain times of the year, but to decreases across a broad range of conditions. 

Figure B-7: PM2.5 Concentrations at Los Angeles-North Main 1999-2001 versus 

2004-2006 

Decrease in PM2.5 Concentrations at LA-North Main

0

5

10

15

20

25

30

0-10 >10 and

<=15

>15 and

<=20

>20 and

<=30

>30 and

<=40

>40 and

<=50

>50 and

<=65.5

>65.5

% of Samples

1999-2001

2004-2006
B-10 

Figure B-8: PM2.5 Concentrations at Riverside – Rubidoux 1999-2001 versus 

2004-2006 

Decrease in PM2.5 Concentrations at Rubidoux

0

5

10

15

20

25

30

0-10 >10 and

<=15

>15 and

<=20

>20 and

<=30

>30 and

<=40

>40 and

<=50

>50 and

<=65.5

>65.5

% of Samples

1999-2001

2004-2006

Historical PM2.5 Mass Trends

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

the federal standard was first deployed in 1998 and 1999. However, PM2.5 data have 

been routinely collected in the South Coast Air Basin since the late 1980s using 

dichotomous samplers that were operated for research purposes. The PM2.5 

dichotomous data correlate very well with the FRM data (r=0.98), but are biased lower 

than the FRM by approximately 12 percent, therefore an absolute comparison to the 

FRM can not be made. This bias is due to the loss of semi-volatile species on the 

dichotomous sampler which are better captured with the FRM due to improved filter 

handling and temperature controls (Motallebi 2003). However, the relative changes in 

the dichotomous data are still useful by providing a longer-term context for assessing 

trends. Figure B-9 depicts PM2.5 concentrations at three sites. At the highest site, 

Riverside-Rubidoux, the 3-year average concentrations decreased about 30 percent 

from 1990 to 1996. The other two sites shown in the graph, Los Angeles-North Main 

and Azusa, have a longer dichotomous data record, from 1990 to 1998. Between 1990 

and 1998, PM2.5 concentrations decreased 30 percent at Azusa and 40 percent at Los 

Angeles North Main. Therefore, looking at dichotomous data and FRM data trends 

together illustrate a continued long-term improvement in PM2.5 concentrations. 
B-11 

Figure B-9: PM2.5 Dichotomous Three-Year Average Trend 

PM2.5 Dichot Three-Year Average Trend

0

5

10

15

20

25

30

35

40

45

1990 1991 1992 1993 1994 1995 1996 1997 1998

Concentrations (ug/m3)

Azusa

North Long Beach

Riverside-Rubidoux

Meteorology and Air Quality Trends

Although the weather plays a role in the atmospheric formation of PM2.5, the downward 

trends in the South Coast Air Basin do not appear to be an artifact of good 

meteorological conditions, especially given the long-term consistency of the trends. 

Unlike ozone, meteorologically adjusted PM2.5 concentrations have not been 

developed. However, a number of factors which may influence PM2.5 concentrations 

were assessed. Since PM2.5 concentrations in the South Coast Air Basin are high 

throughout the year, we evaluated both annual precipitation and summer ozone forming 

potential to assess the effect of weather on PM2.5 concentrations (Table B-3). Years 

with high precipitation would tend to reduce PM2.5 concentrations during the rainy 

period, while years with high ozone forming potential would tend to be conducive to 

greater secondary particulate matter formation and thus higher PM2.5 concentrations. 

Although there was a range of weather conditions between 2001 and 2005, the yearly 

percent decrease in annual average PM2.5 concentrations remained fairly constant, 

suggesting that weather was not the primary cause of the observed downward trend. 

For example, 2002 was a very dry year, and ozone forming potential was above 

average, suggesting conditions conducive to higher PM2.5 concentrations. However, 

PM2.5 concentrations declined 11 percent in 2002 compared to the previous year. The 

same rate of decline continued in 2003 and 2004, despite the variations in weather. 
B-12 

Table B-3: Impact of Precipitation and Ozone Forming Potential 

on Annual Average PM2.5 Concentrations at Riverside-Rubidoux 

Year 

Precipitation 

% of Average 

Ozone 

Forming 

Potential 

Annual 

Average 

PM2.5 

Concentration 

(ug/m3) 

Change in 

Annual 

Average PM2.5 

(%) 

2001 132% Average 30.95 9%

2002 49% 

Above 

Average 27.44 -11%

2003 89% 

Above 

Average 24.83 -10%

2004 135% 

Below 

Average 22.14 -11%

2005 183% Average 21.04 -5%

On a seasonal basis, Figures B-10 and B-11 show that both the summer and winter 

seasons show improvement since 1999. Between 1999 and 2006, Los Angeles-North 

Main summer and winter season averages decreased by 22 and 40 percent, 

respectively. Between 1999 and 2006, Riverside-Rubidoux summer and winter season 

averages decreased by 35 and 44 percent, respectively. 

Figure B-10: PM2.5 Seasonal Trends at LA-North Main 

Seasonal Trends in PM2.5 Mass at LA-North Main

0

5

10

15

20

25

30

1999 2000 2001 2002 2003 2004 2005 2006

Concentrations (ug/m3)

Summer

Winter
B-13 

Figure B-11: PM2.5 Seasonal Trends at Riverside-Rubidoux 

Seasonal Trends in PM2.5 Mass at Rubidoux

0

5

10

15

20

25

30

35

40

1999 2000 2001 2002 2003 2004 2005 2006

Concentrations (ug/m3)

Summer

Winter

Chemical Composition and Emissions Trends

Current Chemical Composition 

Unlike ozone, particulate matter (PM) is not a single chemical, but instead consists of 

many different chemical components. Investigating these different components and 

how they vary by site and season provides us with a better understanding of their 

complexity and responses to the emission control program. This mix of chemical 

components consists of both directly emitted PM such as geological material and 

elemental carbon (soot), known as primary PM, as well as PM formed in the 

atmosphere from the reactions of precursor gases, known as secondary PM. These 

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

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

ammonium nitrate and sulfate. ROG can form secondary organic carbon, as well as 

participate in the production of secondary ammonium nitrate. 

Chemical components are routinely measured at only two Speciated Trends Network 

sites, Los Angeles-North Main, and Riverside-Rubidoux. These two sites are 

representative of the highly populated western basin (Los Angeles), and the highest 

PM2.5 concentration area in the eastern basin (Riverside). The chemical composition 

of PM2.5, seasonal variability, and trends in these components are discussed below to 

provide further insight into the linkage between observed progress and ongoing 

emission reductions. 

The annual average PM2.5 chemical composition for Riverside-Rubidoux and 

Los Angeles-North Main is shown in Figure B-12. Although ammonium nitrate, 

ammonium sulfate, and organic carbon are the major constituents at both sites, there 

are important differences between the two sites as well. For example, Los 

Angeles-North Main has a greater contribution from ammonium sulfate and elemental 
B-14 

carbon than Riverside-Rubidoux, likely reflecting the greater impact from shipping and 

goods movement related activities in this region. In contrast, Riverside has a greater 

contribution from ammonium nitrate due to its location further downwind in the basin 

and the longer time available for the secondary formation of this constituent. It can be 

important for the attainment strategy to recognize these regional differences. However, 

even though there are regional differences in the composition, the PM2.5 problem is 

widespread throughout the basin and many of the emission sources contributing to the 

problem are regional in nature. 

Figure B-12: 2003 – 2005 Average Composition Los Angeles-North Main Site and 

Riverside-Rubidoux Site 

 

2003-2005 Average Composition

LA-North Main

AmmNitrate

34%

OC

31%

Geological

5%

AmmSulfate

21%

Elements

3%

EC

6%

 

2003-2005 Average Composition

Rubidoux

AmmNitrate

44%

OC

29%

Geological

5%

AmmSulfate

15%

EC

5%

Elements

2%

Figures B-13 and B-14 illustrate the seasonal variability in chemical components that 

make up the annual average for the Los Angeles-North Main and the 

Riverside-Rubidoux sites averaged over 2003 through 2005. At the Los Angeles-North 

Main site (Figure B-13), ammonium nitrate is lowest during the spring months, and 

increases through fall and winter. Sulfate has a photochemical aspect so 

concentrations are greatest in the summer. At the Los Angeles-North Main site, 

ammonium sulfate increases sharply from April to July, and then slowly begins to 

descend to its lowest concentration in December. The Riverside-Rubidoux site follows 

the same trend, with the exception that ammonium sulfate concentrations at the Los 

Angeles-North Main site increase more rapidly and to a higher level than RiversideRubidoux. At the Riverside-Rubidoux site (Figure B-14), ammonium nitrate is highest in 

the summer months from June through October, then decreases sharply from October 

to December. In both locations, organic carbon and the geological components remain 

fairly constant throughout the year. 
B-15 

Figure B-13: PM2.5 Seasonal Pattern in Chemical Components Los Angeles-North 

Main Site 2003-2005 Average 

PM2.5 Seasonal Pattern in Chemical Components

LA-North Main

2003 - 2005 Average

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11 12

Month

Concentrations (ug/m3)

Elements

Geological

Elemental Carbon

Organic Carbon

Ammonium Sulfate

Ammonium Nitrate

Figure B-14: PM2.5 Seasonal Pattern in Chemical Components Riverside-Rubidoux 

Site 2003-2005 Average 

PM2.5 Seasonal Pattern in Chemical Components

Rubidoux

2003 - 2005 Average

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12

Month

Concentrations (ug/m3)

Elements

Geological

Elemental Carbon

Organic Carbon

Ammonium Sulfate

Ammonium Nitrate

Chemical Composition Trends 

As discussed previously, all monitoring sites have shown decreasing PM2.5 

concentrations from 2001 to 2006. Trends in individual chemical components and 

precursor concentrations, as well as emissions inventory trends were evaluated to 
B-16 

understand the primary chemical components responsible for this progress and to 

evaluate the response to ongoing control programs. Riverside-Rubidoux has shown the 

greatest decline in concentrations, with the most significant reductions occurring in 

ammonium nitrate and carbonaceous aerosols components (Figure B-15). Ammonium 

nitrate concentrations have dropped 40 percent, and carbonaceous aerosols have 

dropped 19 percent. Approximately two-thirds of the PM2.5 improvement observed in 

the South Coast can therefore be attributed to reduced ammonium nitrate; a much 

smaller portion of the remaining PM2.5 improvement is due to reductions in 

carbonaceous compounds (19 percent) and ammonium sulfate (~17 percent). Trends 

for Los Angeles-North Main are shown in Figure B-16. 

Figure B-15: Trends in PM2.5 Mass and Chemical Components Riverside-Rubidoux 

Trends in PM2.5 Mass and Chemical Components

Riverside-Rubidoux

0

5

10

15

20

25

30

35

2001* 2002 2003 2004 2005 2006

Concentrations (ug/m3)

PM25 3-year Avg

AmmNitrate

AmmSulfate

CarbonaceousAerosols

Geological

Elements
B-17 

Figure B-16: Trends in PM2.5 Mass and Chemical Components Los Angeles-North 

Main 

Trends in PM2.5 Mass and Chemical Components

LA-North Main

0

5

10

15

20

25

2001 2002 2003 2004 2005 2006

Concentrations (ug/m3)

PM25 3-year Avg

AmmNitrate

AmmSulfate

CarbonaceousAerosols

Geological

Elements

Trends in ammonium nitrate concentrations also correlate well with recent trends in 

basin average NOx, a precursor to ammonium nitrate. Since 2001, basin average NOx 

has decreased by 25 percent while ammonium nitrate concentrations decreased 

40 percent. As our emission control program has reduced the amount of these 

precursors, we have seen a corresponding improvement in air quality. Figure B-17 

illustrates the downward trend of ammonium nitrate concentrations compared with 

similar declines in basin average NOx concentrations at Riverside-Rubidoux. 

Figure B-17: PM2.5 Ammonium Nitrate Compared to NOx Riverside-Rubidoux 

PM2.5 Ammonium Nitrate at Rubidoux

Compared to Basin Average NOx

0

2

4

6

8

10

12

14

16

18

2001 2002 2003 2004 2005 2006

Ammonium Nitrate (ug/m 3

)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

NOx (pphm)

Ammonium Nitrate

NOx
B-18 

Similar relationships between both ammonium nitrate and sulfate and ambient precursor 

concentrations can be seen in the longer-term record. Figures B-18 and B-19 show 

trends in basin average ambient NOx and SO2 concentrations along with corresponding 

ambient nitrate and sulfate measurements from the PM10 network extending back to 

the late 1980s. Both nitrate and sulfate have experienced significant decreases, with 

nitrate decreasing 43 percent, and sulfate 40 percent. During the same period, there 

were concurrent declines in ambient NOx and SO2 concentrations. 

Figure B-18: Three-Year Average Concentrations of PM10 Nitrate and NOx in South 

Coast Air Basin 

Three-year Average Concentrations

of PM10 Nitrate and NOx in South Coast

0

1

2

3

4

5

6

7

8

9

10

19871988198919901991199219931994199519961997199819992000200120022003200420052006

PM10 Nitrate (ug/m 3

)

0

1

2

3

4

5

6

7

8

9

10

NOx (pphm)

NO3-

NOx

Figure B-19: Three-Year Average Concentrations of PM10 Sulfate and SO2 in the 

South Coast Air Basin 

Three-year Average Concentrations of PM10 Sulfate 

and SO2 in the SC

0

1

2

3

4

5

6

7

19871988198919901991199219931994199519961997199819992000200120022003200420052006

PM10 Sulfate (ug/m 3

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

SO2 (pphm)

SO4--

SO2
B-19 

In addition, Figure B-20 compares the PM10 and PM2.5 nitrate at the RiversideRubidoux monitor. Even though PM2.5 monitoring recently began, the data shows that 

the PM2.5 and PM10 nitrate levels track very well. Furthermore, the emission reduction 

strategy in place that reduced PM10 nitrate levels will also reduce PM2.5 nitrate levels. 

As shown in the figure, the PM2.5 nitrate levels are higher than the PM10 nitrate levels. 

This is due to the loss of semi-volatile nitrates with the PM10 monitor configuration, 

which are better captured on the PM2.5 monitor. 

Figure B-20: Three-Year Average Concentrations of PM10 and PM2.5 Nitrate at 

Riverside-Rubidoux 

Three Year Average Nitrate Concentrations at Rubidoux

0

2

4

6

8

10

12

14

16

18

1987198919911993199519971999200120032005

Concentrations (ug/m3)

PM10NO3-

PM25NO3-

Figure B-21: Three-Year Average Concentrations of PM10 and PM2.5 Sulfate at 

Riverside-Rubidoux 

Three-year Average Concentrations of PM10 and 

PM2.5 Sulfate at Rubidoux

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

19871988198919901991199219931994199519961997199819992000200120022003200420052006

Sulfate (ug/m 3

)

PM10SO42-

PM25SO42-
B-20 

Figure B-21 compares the PM10 and PM2.5 sulfate at the Riverside-Rubidoux monitor. 

As with nitrate, the data shows that the PM2.5 and PM10 sulfate levels track very well. 

In contrast with nitrate, the PM10 sulfate is higher than the PM2.5 sulfate. Because 

sulfate is nonvolatile and is retained well by filters, the PM10 monitor is not subject to 

the same sampling artifacts for sulfate as it is for nitrate. Therefore, the difference 

between PM10 and PM2.5 represents sulfate in the coarse particle range. 

Emission Trends 

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

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

decreasing. Figure B-22 illustrates the recent emission trends in the South Coast air 

basin from 2001 through 2006. The greatest decrease in emissions occurred in ROG 

with a decrease of 33 percent, or 339 tpd. NOx also decreased, experiencing a decline 

of 213 tpd, or 18 percent. These reductions reflect the benefits of control programs 

which have provided continuing reductions despite increasing population growth. 

PM2.5 emissions showed a much smaller decrease of 11 tpd, while SOx emissions 

increased slightly by 5 tpd. Much of the increase in SOx emissions can be attributed to 

growth in the goods movement sector. However, over the longer-term period, both SOx 

and NOx emissions have also dropped substantially as documented by the decrease in 

ambient NOx and SO2 concentrations reflected in Figures B-20 and B-21 above. 

The combined downward trends in PM2.5 components, precursor concentrations, and 

emissions all indicate that over both the long-term and short-term, the ongoing control 

program has had substantial benefits in improving air quality and that similar emission 

reductions in the future should provide continuing progress towards attaining the federal 

PM2.5 standards. 

Figure B-22: South Coast Air Basin Emission Inventory Trends 2001-2006 

South Coast Emission Inventory Trends

2001-2006

0

200

400

600

800

1000

1200

1400

2001 2002 2003 2004 2005 2006

Emissions (tpd)

NOx

ROG

SOx

PM2.5
B-21 

3. OBSERVATIONAL MODELS AND DIAGNOSTIC ANALYSES 

Observational models take advantage of monitored data to draw conclusions about the 

relative importance of different types of emissions and precursors as factors contributing 

to observed PM2.5 concentrations. According to U.S. EPA guidance, observational 

models can be used to corroborate the effects of prior control strategies, as well as 

identify the potential effectiveness of proposed control approaches. U.S. EPA 

recommends using both source apportionment (receptor models) and indicator species 

approaches. The two most widely applied receptor modeling approaches are 

multi-variate statistical models such as positive matrix factorization (PMF) and the 

chemical mass balance model (CMB). Receptor models are particularly useful in 

identifying the source contributions to directly emitted PM. This supplemental analysis 

summarizes results from prior receptor modeling studies as well as new results from 

both PMF and CMB applied to more recent data. In addition, previous work using an 

indicator species approach to assess the limiting precursor in secondary nitrate 

formation in the South Coast Air Basin is discussed. 

Observational Models

Prior Source Apportionment Studies 

Numerous source apportionment studies have been conducted in the South Coast 

reflecting snapshots of different sites and times using various receptor models. While 

the precise contributions vary, these studies have all identified diesel and gasoline 

vehicle exhaust, smoke from wood burning and cooking, and fugitive dust as important 

contributors to directly emitted PM2.5. In 1982, Caltech conducted an annual average 

source apportionment study at four sites in the air basin: West Los Angeles, Downtown 

Los Angeles, Pasadena, and Riverside-Rubidoux (Schauer 1996). This study found 

that directly emitted exhaust from mobile sources (gasoline and diesel) contributed an 

average of 26 percent, dust an average of 13 percent, and wood smoke and cooking an 

average of 7 percent each to annual average PM2.5 concentrations. Figure B-23 

compares the source apportionment results for Downtown Los Angeles and RiversideRubidoux. For example, the largest source in Downtown Los Angeles was diesel 

exhaust, while at Riverside-Rubidoux it was nitrate. Wood smoke and sulfate were 

more predominant at Downtown Los Angeles, while paved road dust was higher at 

Riverside-Rubidoux. 
B-22 

Figure B-23: 1982 Annual Average Source Apportionment of Fine Particulate Matter 

1982 Annual Average Source Apportionment

0

5

10

15

20

25

30

35

40

Riverside-Rubidoux Downtown Los

Angeles

ug/m3

Ammonium Ion

Nitrate Ion

Sulfate Ion

Wood Smoke

Gas Vehicles

Meat Charbroiling

and Frying

Cigarette Smoke

Paved Road Dust

Diesel Exhaust

In a report for ARB, Watson (1993) characterized primary organic carbon sources and 

compared them with PM concentrations in the Los Angeles area during the 1987 

Southern California Air Quality Study. The analysis identified motor vehicles, meat 

cooking, road dust, and wood smoke as contributing to fine particle organic compounds 

in the region. In another study in 1993, annual composite samples from four sites in the 

South Coast found that mobile sources contributed 14 percent of organic carbon, dust 

10 percent, wood smoke 9 percent, and cooking 26 percent (Hannigan 2005). In the 

most recent study conducted in the region, analysis of primary PM10 conducted through 

the Children’s Health Study in 1995 by the University of Wisconsin-Madison and Cal 

Tech (Manchester-Neesvig 2003) demonstrated both a strong spatial and seasonal 

variability in contributions. For example, as shown in Table B-4, in Riverside wood 

smoke contributed an average of 1.83 ug/m3 during November through December (25 

percent of primary PM), dropping to 0.51 ug/m3 during January through April (14 

percent of primary PM). Within the winter season, wood smoke contributions ranged 

from 1.2 ug/m3 at Long Beach (14 percent of primary PM10), up to 3.72 ug/m3 at Mira 

Loma (28 percent of primary PM10). 

Table B-4: 1995 Children’s Health Study CMB analysis of wood smoke contribution to 

PM10 (ug/m3) 

 Riverside Mira Loma Long Beach Upland 

Nov – Dec 1.81 3.72 1.19 2.38 

Jan-April 0.51 0.29 0.68 0.99 

May-Oct 0.30 0.21 0.29 0.40 
B-23 

Recent Studies 

To evaluate the major PM2.5 sources and their contributions in the South Coast Air 

Basin using more recent data, two different source apportionment techniques were 

applied to data collected in 2003 through 2005. The CMB model uses measured source 

profiles and chemical speciation data in combination to determine source contributions. 

A fundamental underlying assumption is that the source profiles used as input are 

appropriate and representative for the region. In contrast, PMF analysis does not 

require the input of source profiles, but rather evaluates the covariance of the chemical 

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

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

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

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

these primary particles. Since each technique has strengths and weaknesses, 

combined the two source apportionment techniques provide complementary results. 

Positive Matrix Factorization 

PMF analysis was conducted for the Los Angeles-North Main and the RiversideRubidoux site data collected between 2003 and 2005. After testing different numbers of 

sources and different modeling parameters, eight and nine major source factors were 

identified at the Los Angeles-North Main and the Riverside-Rubidoux sites, respectively. 

The average source contributions of each source to the PM2.5 mass concentrations are 

provided in Figure B-24. Secondary nitrate particles that accompany secondary 

carbonaceous materials contributed the most at both sites accounting for 30 percent of 

the PM2.5 concentrations at the Los Angeles-North Main site and 49 percent at the 

Riverside-Rubidoux site. In contrast, secondary sulfate particles contributed more at 

the Los Angeles-North Main site (23 percent) than at the Riverside-Rubidoux site 

(9 percent). The possible source directions for the secondary particles and airborne soil 

were estimated by conditional probability functions and are shown as probabilities in 

Figure B-25. Figure B-25 shows that most of the secondary nitrate particles as well as 

the secondary sulfate particles at the Riverside-Rubidoux site were associated with 

westerly winds indicating strong contributions from the western basin. As major 

sources, gasoline vehicles and diesel emissions account for 7 percent and 11 percent, 

respectively, of the PM2.5 concentration at the Los Angeles-North Main site and 

10 percent and 6 percent at the Riverside-Rubidoux site. There appear to be greater 

impacts from diesel emission sources at the Los Angeles-North Main site, potentially 

reflecting the proximity to shipping and goods-movement-related activities. 

Biomass smoke contributed 10 percent of the PM2.5 concentrations at the Los 

Angeles-North Main site and 6 percent at the Riverside-Rubidoux site. The biomass 

smoke category reflects contributions from residential wood burning as well as 

managed and wildland fires; smoke from commercial cooking is also in this category. 

Aged aerosol with sea salt 1 and 2 contributed 9 percent of the PM2.5 concentrations at 

the Los Angeles-North Main site and 11 percent at the Riverside-Rubidoux site. Aged 
B-24 

aerosol with sea salt 1 and 2 reflect particles in which chloride in the fresh sea salt is 

displaced by acidic gases during transport across the air basin. Therefore, aged 

aerosol with sea salt includes other particles such as organic carbon, elemental carbon, 

sulfate, and nitrate in addition to sodium and chloride. Due to transport across the 

basin, there is more aged aerosol with sea salt in Riverside-Rubidoux compared to Los 

Angeles-North Main. Airborne soil contributed 10 percent of the PM2.5 concentrations 

at the Los Angeles-North Main site and 7 percent at the Riverside-Rubidoux site. 

Airborne soil profile contains organic and elemental carbon indicating it also includes resuspended soil by road traffic in addition to wind blown soil dust. Airborne soil identified 

at the Los Angeles-North Main site did not show clear directionality. However, as 

shown in Figure B-24, airborne soil identified at the Riverside-Rubidoux site was 

strongly associated with winds from the northeast. An additional factor contributing 2 

percent of the PM2.5 concentrations was separated at the Riverside-Rubidoux site. 

This source profile includes organic and elemental carbon, zinc, calcium, iron, and lead 

suggesting an industrial emissions source. At the Los Angeles-North Main site, a 

similar signal was not likely strong enough to be separated. 

Figure B-24: 2003 – 2005 Average Source Contributions (PMF): Los Angeles-North 

Main Site and Riverside-Rubidoux Site 

Aug. 2003 - Dec. 2005 Average Source 

Contribution in LA-North Main

Secondary 

nitrate

30%

Airborne 

soil

10%

Aged 

aerosol 

with sea 

salt

9%

Gasoline 

vehicle

7%

Diesel

11%

Biomass 

Smoke

10%

Secondary 

Sulfate

23%

 

Aug. 2003 - Dec. 2005 Average Source 

Contribution in Rubidoux

Secondary 

nitrate

49%

Airborne 

soil

7%

Industrial

2%

Aged 

aerosol 

with sea 

salt

11%

Gasoline 

vehicle

10%

Biomass 

Smoke

6%

Diesel

6%

Secondary 

Sulfate

9%

Figure B-25: Possible Source Directions at the Riverside-Rubidoux Site 
B-25 

Source Direction of Secondary Nitrate

Rubidoux

0

0.1

0.2

0.3

0.4

0

15 30

45

60

75

90

105

120

135

150 165

180

195 210

225

240

255

270

285

300

315

330345

Source Direction of Secondary Sulfate

Rubidoux

0

0.1

0.2

0.3

0.4

0

15 30

45

60

75

90

105

120

135

150 165

180

195 210

225

240

255

270

285

300

315

330345

Source Direction of Airborne Soil

Rubidoux

0

0.2

0.4

0.6

0.8

0

15 30

45

60

75

90

105

120

135

150 165

180

195 210

225

240

255

270

285

300

315

330345

The monthly patterns in source contributions are shown in Figures B-26 and B-27. 

Secondary nitrate particles have winter high monthly variations and similar March and 

October/November high patterns at both sites. Secondary sulfate particles show 

monthly variation with higher concentrations in summer at both sites when the 

photochemical activity is highest. At both sites, biomass smoke shows the highest 

contributions during the winter (Figures B-26 and B-27) and on weekends, indicating 

this source is mostly from residential wood burning smoke. Also, slightly elevated 

contributions in July at both sites indicate possible smoke contributions from managed 

burning/wildfires in the summer. Airborne soil has September/October high patterns at 

both sites when the winds from the northeast are most predominant. 

Figure B-26: 2003 – 2005 Monthly Average Source Contributions (PMF): Los 

Angeles-North Main Site 

Monthly Pattern in Source Contributions

LA-North Main

2003 - 2005 Average

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10 11 12

Month

Concentrations 

(μg/m3

)

Aged aerosol w ith sea salt 2

Aged aerosol w ith sea salt 1

Biomass smoke

Airborne soil

Diesel

Gasoline

Secondary sulfate

Secondary nitrate

Figure B-27: 2003 – 2005 Monthly Average Source Contributions (PMF): 

Riverside-Rubidoux Site 
B-26 

Monthly Pattern in Source Contributions

Rubidoux

2003 - 2005 Average

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11 12

Month

Concentrations (

μg/m3

)

Industrial

Aged aerosol w ith sea salt 2

Aged aerosol w ith sea salt 1

Biomass smoke

Airborne soil

Diesel

Gasoline

Secondary sulfate

Secondary nitrate

Chemical Mass Balance Modeling

Additional source apportionment analysis was conducted for the Los Angeles-North 

Main and the Riverside-Rubidoux site data collected between 2003 and 2005 using the 

Chemical Mass Balance (CMB 8.2) model. The annual average source contributions at 

Rubidoux and Los-Angeles-North Main Street were estimated by applying the CMB 

model to 195 and 101, respectively, individual PM2.5 sample concentrations collected 

between 1/1/2004 and 12/31/2005 at Riverside-Rubidoux and Los Angeles-North Main. 

Source profiles for PM2.5 developed during previous studies were applied to the 

measurements to calculate source contributions to the PM2.5 mass. Source 

contribution estimates were calculated based on the 2004-2005 average for the 

individual samples that had acceptable results for the performance measures 

Figure B-28 illustrates the 2004-2005 average source contributions for RiversideRubidoux. Ammonium nitrate was the most significant source contributing 47 percent 

and 34 percent of the PM2.5 mass at Riverside Rubidoux and Los Angeles North Main, 

respectively. Ammonium sulfate accounted for 14 percent at Riverside-Rubidoux and 

20 percent at Los Angeles North Main. Motor vehicle exhaust (diesel and gasoline 

combined) contribution was similar at both sites ranging from 11 percent at RiversideRubidoux to 12 percent at Los Angeles North Main. Vegetative burning, which included 

residential wood combustion and wildfires, contributed 7 percent and 8 percent, 

respectively, of the PM2.5 mass. Sea salt contributed about 1 percent of the PM2.5 

mass at both sites. The sea salt contribution was estimated using a pure sea salt profile 

for Los Angeles North Main and reacted profile for Riverside-Rubidoux, where 25 

percent of chlorine was replaced by nitrate on a molar basis (Chow et al., 1996a). 
B-27 

Figure B-28. Estimated 2004-2005 Average PM2.5 Source Contributions at RiversideRubidoux and Los Angeles North Main 

RUBI Modeled 2004-2005 Avg

AMNIT

47%

AMSUL

14%

Sea Salt

1%

Burning

7%

Cooking

18%

Dust

2%

Motor 

Vehicle

11%

TIRE

0%

Figure B-28 shows that the initial cooking contribution ranged from 19 percent to 

22 percent; however that likely included positive organic carbon sampling artifacts, 

secondary organic aerosols, as well as other combustion sources. In order to test this 

theory, we ran two sets of data; one with organic carbon (OC) as reported in the 

speciation trends network data and another one in which positive sampling artifacts, 

estimated to be 2.77 ug/m3 at Riverside-Rubidoux and 3.17 ug/m3 at Los Angeles 

North Main, were subtracted from the measured OC. The motor vehicle and burning 

contribution were almost the same in both data sets, but the cooking contribution was 

lower for the corrected data set, dropping to approximately 6 percent, which was more 

consistent with estimates of cooking contributions from prior studies (Table B-5). 

Dust and tire contribution were estimated using different profiles at each site. In case of 

Rubidoux, dust was represented by a resuspended road dust profile. A separate tire 

profile was also included in the model, but the estimated concentrations were lower than 

the uncertainty. At LA-North Main, the brake and tire wear were combined into a single 

profile and a separate dust profile was used. 

LANM Modeled 2004-2005 Avg

AMNIT

34%

AMSUL

20%

Sea Salt

1%

Burning

8%

Cooking

22%

Dust

1%

TIRE/BREAK

2%

Motor 

Vehicle

12%
B-28 

Table B-5. Impact of Adjusting Carbon Data on Source Contribution 

(Average of 106 days) 

Source Category Contribution (ug/m3) 

Original Data Blank Corrected 

Data 

Riverside Rubidoux 

Motor Vehicle 2.8 2.8 

Burning 1.8 1.6 

Cooking 5.5 1.6 

Los Angles North Main 

Motor Vehicle 2.8 3.4 

Burning 1.8 1.9 

Cooking 5.0 -1 

Diagnostic Analyses

Indicator Species Approach 

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

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

approach for ensuring further reductions in ammonium nitrate. An indicator species 

approach provides an additional method to investigate which chemical precursor a 

secondary species such as ammonium nitrate is most responsive to control of. 

Blanchard (2000) used a thermodynamic equilibrium model, SCAPE2, to calculate 

indicator species ratios for several locations in California, including the South Coast Air 

Basin. The study looked at nitrate concentration changes in response to the changes in 

nitric acid, ammonia, and sulfuric acid based on the excess of ammonium plus ammonia 

over the sum of nitric acid, nitrate, and sulfate. This approach was applied to the field 

study data measured in the South Coast during the California Acid Deposition 

Monitoring Program and the 1987 Southern California Air Quality Study. The results 

indicated that that nitrate formation was not limited by the availability of ammonia for 

most samples, therefore suggesting that NOx controls are more effective in reducing 

ammonium nitrate concentrations. 

4. AIR QUALITY MODELING 

Rollback Modeling

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

analyses, a rollback modeling analysis was conducted by ARB to estimate the impacts 

of future emission reductions on resulting air quality. Rollback modeling was conducted 

for the two sites with routine speciation data – Riverside-Rubidoux, and Los AngelesNorth Main. Reductions or increases in emissions are assumed to have a linear 

corresponding effect in the ambient PM2.5 concentrations at the monitor, taking into 
B-29 

account background concentrations. The ammonium nitrate component however, was 

assumed to have a non-linear response to reductions in NOx, with a ratio of 1.3:1 (i.e. a 

50% reduction in NOx would provide only a 38% reduction in ammonium nitrate). 

Table B-6 lists the associated emission category and background concentration used 

for each PM2.5 component in the rollback analysis. 

Table B-6: PM2.5 Components, Background Concentrations, and Emission Categories 

use in Rollback Analysis 

PM2.5 Component Background 

Concentration 

Emission Category 

Ammonium Nitrate 0.2 ug/m3 Basin wide NOx 

Ammonium Sulfate 0.6 ug/m3 Basin wide SOx 

Secondary Organic 

Carbon (OC) 

0.2 ug/m3 Basin wide ROG 

Primary Organic Carbon 

(OC) 

1.8 ug/m3 Basin wide PM2.5 

Elemental Carbon (EC) 0.0 ug/m3 Basin wide PM2.5 diesel 

Other 0.5 ug/m3 Basin wide PM2.5 

Rollback modeling of two future emissions scenarios was conducted. In the first 

scenario, the relative reduction or increase of the baseline emission inventories 

between 2005 and 2014 of each PM2.5 component was applied to determine a future 

value for each PM2.5 component. The future PM2.5 design value was determined by 

summing the PM2.5 components and their corresponding background concentration. 

The same technique was then used to evaluate the impact of the April 24, 2007 ARB 

staff proposed State Strategy for California’s 2007 State Implementation Plan together 

with local South Coast measures (an additional reduction of 142 tpd of NOx, 23 tpd of 

SOx, 56 tpd of ROG, and 11 tpd of directly emitted PM2.5). The results of this modeling 

are shown in Tables B-7 and B-8. The rollback technique estimated that the substantial 

additional emission reductions contained in the ARB State Strategy as proposed in the 

April 24, 2007 document would be sufficient to provide for attainment at RiversideRubidoux as well Los Angeles-North Main. 
B-30 

Table B-7: Rollback Modeling Predicted PM2.5 Concentrations at Riverside-Rubidoux 

 PM2.5 

(ug/m3) 

Ammonium 

Nitrate 

(ug/m3)

Ammonium 

Sulfate 

(ug/m3)

Secondary 

OC 

(ug/m3)

Primary 

OC 

(ug/m3) 

EC 

(ug/m3)

Other 

(ug/m3)

2005 22.7 10.1 3.4 1.9 4.5 1.1 1.6 

2014 

baseline 

18.1 7.3 2.7 1.4 4.4 0.6 1.6 

2014 

control 

14.8 6.3 1.6 1.3 4.2 0.2 1.5 

*Numbers may not add due to rounding 

Table B-8: Rollback Modeling Predicted PM2.5 Concentrations at Los Angeles-North 

Main 

 PM2.5 

(ug/m3) 

Ammonium 

Nitrate 

(ug/m3)

Ammonium 

Sulfate 

(ug/m3)

Secondary 

OC 

(ug/m3)

Primary 

OC 

(ug/m3) 

EC 

(ug/m3)

Other 

(ug/m3)

2005 19.6 6.6 4.2 1.8 4.3 1.2 1.5 

2014 

baseline 

15.3 4.9 3.3 1.4 4.2 0.7 0.9 

2014 

control 

13.0 4.2 2.1 1.3 3.9 0.2 1.3 

*Numbers may not add due to rounding 

Grid-Based Modeling

Annual average grid-based aerosol modeling was conducted by the South Coast 

District. The PM modeling for the South Coast Air Basin was based on the 1997 

Southern California Ozone Study domain which covers San Luis Obispo, Santa 

Barbara, Kern, Ventura, San Bernardino, Los Angeles, Orange, Riverside, San Diego, 

and Imperial Counties and extends into Northwestern Mexico. During 2005, the MATES 

III field study was conducted in the South Coast Air Basin during which speciated PM2.5 

concentrations were measured at 9 monitoring sites throughout the year. The year 

2005 was selected for the annual PM2.5 simulation since the resulting MATES III data 

set was robust and useful for model evaluation. The PM modeling was run using the 

Comprehensive Air Quality Model with extensions (CAMx) air quality model. The model 

was run for emissions inventories representing the year 2005 and the year 2014. 

To demonstrate attainment of the federal PM2.5 standard, U.S. EPA guidelines 

recommend using a model-based relative reduction or response factor (RRF) approach 

for projecting future design values. The RRF approach projects design values from a 

specific baseline year (in this case 2005) to a future year (2014). For PM2.5, this is 

done on a species basis (known as the Speciated Model Attainment Test), which are 
B-31 

then summed to determine the future year PM2.5 concentrations. The modeled future 

year design values representing ARB’s control strategy along with local South Coast 

District measures are listed in Table B-9. As shown in the table, the proposed control 

strategy significantly reduces PM2.5 concentrations in the South Coast Air Basin 

regionwide. However, a few areas are near, but do not meet the 15.0 ug/m3 federal 

PM2.5 standard, including the high site at Riverside-Rubidoux, with a predicted 2014 

concentration of 15.7 ug/m3. 

Table B-9: South Coast 2014 Grid-Based Modeling Predicted PM2.5 Concentrations 

with ARB Control Measures 

 

PM2.5 

Mass 

(ug/m3) 

Ammonium 

(ug/m3) 

 

Nitrate 

(ug/m3) 

Sulfate 

(ug/m3)

OC 

(ug/m3)

EC 

(ug/m3)

Others/ 

Metals 

(ug/m3) 

Water 

(ug/m3)

Blank 

(ug/m3)

Anaheim 12.6 1.2 2.9 2 1.7 1.2 1.9 1.3 0.5

Burbank 15.5 1.6 3.7 1.9 2.9 1.6 2 1.4 0.5

Compton 14.7 1.5 3.4 2.3 2.2 1.5 1.9 1.5 0.5

Fontana 15.2 1.6 4 2 1.4 1.8 2.5 1.4 0.5

Los 

Angeles 

15.4 

1.8 4.4 2.3 0.9 1.7 2.1 1.8 0.5

Long 

Beach 

12.9 

1.4 3.1 2.3 1.3 1.2 1.7 1.4 0.5

Rubidoux 15.7 1.7 4.5 1.9 1.9 1.6 2.1 1.5 0.5

Wilmington 12.1 1.2 2.5 2.2 1.4 1.4 1.7 1.3 0.5

The grid-based modeling indicates that PM2.5 progress will slow down and be less 

responsive to controls than has been previously observed. For example, between 2001 

and 2006 observed ammonium nitrate concentrations dropped by 40 percent. The gridbased modeling predicts a similar decline will occur between 2006 and 2014. However, 

emissions, specifically NOx emissions, are projected to drop by about 6 percent per 

year from 2006 to 2014 – approximately twice as fast as they fell between 2001 and 

2006. Figure B-29 below illustrates the actual progress from 2001 to 2006 as compared 

to the progress predicted by the grid-based modeling from 2006 through 2014. 

Therefore the grid-based model appears to be somewhat “stiff” or less responsive to 

NOx controls than expected. 
B-32 

Figure B-29: Comparison of Measured Progress in PM2.5 Annual Average Design 

Values Compared to Predicted Progress 

South Coast PM2.5 Air Quality and Modeling 

(Rubidoux)

0

5

10

15

20

25

30

35

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

Concentrations (ug/m3)

PM2.5 Target

Measured

Measured

Modeled

0.7ug/m3 gap

Retrospective Modeling

While chemical transport models may represent the best tool to assess and address a 

region’s PM problem, these models are not without uncertainties. Uncertainties can be 

introduced at any step of the model formulation and application and lie primarily in (1) 

the simplifications needed to efficiently simulate complex atmospheric processes and 

(2) the model’s meteorological and emissions inputs. In order to test the ability of a 

model to effectively capture observed phenomena, model performance tests are 

necessary. Diagnostic tests to determine how well a model simulates concentration 

changes due to changes in emissions are particularly useful, because this is how 

chemical transport models are applied in the U.S. EPA recommended attainment 

demonstration. A “retrospective analysis” is one such diagnostic test to determine how 

well a model responds to historical emissions changes. In a retrospective analysis the 

model is run for a number of prior years so that observed and modeled trends can be 

compared. 

ARB ran the CAMx model using 2005 meteorology and (1) 1995, (2) 1999, and (3) 2005 

emissions for two domains over the South Coast Air Basin (SCAB). The predicted 

species concentrations were then compared to particulate observations at western (e.g., 

Los Angeles) and eastern (e.g., Rubidoux) sites in the SCAB in order to evaluate how 

well the modeled concentration trends track those observed. 

It is important to note here that the meteorology was held constant while emissions 

were backcasted for 1995 and 1999. This was due to a lack of resources to exercise 

the prognostic meteorology model for those two years. Since the meteorology was not 

identical for all three years, the modeled trend is only an approximation to the more 
B-33 

appropriate modeled trend that would have been obtained using actual meteorology for 

1995 and 1999. Thus, the comparison of the modeled and observed trends presented 

here is not expected to be quantitative, as the model only captures the trend due to 

emissions changes. In reality the trends are due to both emissions changes and 

meteorological variability. 

A description of the model and simulation configuration is given below and is followed 

by a graphical comparison of modeled and observed annual PM concentrations at sites 

throughout the South Coast Air Basin. 

Model Inputs and Domain Specifications

In order to capture the long-term spatiotemporal variation of PM2.5 and its response to 

emissions changes for the South Coast Air Basin, the CAMx model was run for 1995, 

1999, and 2005 over two domains (Figure B-30) using a Lambert Conformal coordinate 

system. The first domain is a 65x40 5-km2 cell domain including Orange, Los Angeles, 

Riverside, San Bernardino and Ventura counties (SCAQS). The second domain 

referenced here is the 116x80 5-km2 cell “SCOS” domain which covers San Luis 

Obispo, Santa Barbara, Kern, Ventura, San Bernardino, Los Angeles, Orange, 

Riverside, San Diego, and Imperial Counties and extends into Northwestern Mexico. 

The larger domain, which is nearly 4 times larger than the smaller domain, was 

originally employed to reduce the impact of the shipping lanes on the western and 

southern boundaries. 

Both domains have a similar vertical structure, extending to approximately 5 kilometers 

above ground level and divided into 8 vertical layers, with the CAMx layer structure 

largely dependent upon the layer structure of the meteorological model. Meteorological 

inputs were generated with the fifth generation Penn State/NCAR Mesoscale Model 

(MM5). MM5 was run for 2005 in 5-day segments without Four Dimensional Data 

Assimilation (FDDA). 

CAMx has been set up to run in a two-section fine-coarse PM configuration with gasphase chemistry represented by the Carbon Bond IV chemical mechanism. Some 

changes to the default partitioning of secondary particulates were made in accordance 

with the AERO-LT model developed by the South Coast Air Quality Management 

District (SCAQMD). 
B-34 

Figure B-30: SCOS Modeling Domain with SCAQS Modeling Domain Inset 

Emissions were prepared for 1995, 1999, and 2005 using CEFS version 1.06. CAMx 

was run for those years over both domains, while keeping boundary concentrations and 

meteorology constant. 

Results 

Figures B-31 and B-32 show modeled and observed PM2.5 total carbon, ammonium, 

nitrate, and sulfate concentrations at Los Angeles and Rubidoux. The modeled results 

that follow are for the simulations with the larger SCOS domain. However, the modeled 

trends for the two domains are remarkably similar (Figures B-33 and B-34). While the 

remaining figures show results for the “SCOS” simulations, it should be understood that 

the same conclusions can be drawn for simulations over both the large and small 

domains. 

In Figures B-31 through B-34, the modeled concentrations are compared with PM2.5 

data collected during the 1995 and 1999 PTEP/TEP studies and, during later years, with 

data collected at the Speciated Trends Network (STN) monitors at Los Angeles and 

Rubidoux. The main goal of a retrospective analysis such as this one is to show that 

the model behaves “predictably”. This is, therefore, a somewhat qualitative test of 

predicted and observed trends. Due to the fact that meteorology is not changed 

between years, the model results can only capture the concentration changes due to 

emissions variations. Observed concentrations vary from year to year and reflect 

concentration fluctuations that are a result of emissions changes and meteorological 

variability. 
B-35 

For both Los Angeles and Rubidoux, the predicted trends for total PM2.5 carbon 

concentrations match the observed trends very well. This may be due to the mostly 

primary nature of the elemental and organic carbon and their relative insensitivity to 

meteorological changes. For ease of comparison of the trends, modeled and observed 

concentrations are “normalized” by the 2005 concentrations. For the largely secondary 

species of nitrate, ammonium, and sulfate, however, the observed and modeled trends 

do not agree as well, and further analysis is required to determine whether the model 

behaves as is “expected”. It is expected that meteorological changes would play a 

significant role in the formation of secondary compounds such as nitrate and sulfate. 

However, constructing prognostic meteorology fields for the entire years of 1995 and 

1999 is a major undertaking that is beyond the scope of this initial study. 

Domainwide NOx emissions decrease on the order of 25% between 1995 and 2005, 

and while this is not a uniform decrease throughout the entire domain, most inland cells 

see a decrease in NOx emissions. This ~25% decrease in domainwide emissions is 

large enough that the model should clearly predict a downward trend in nitrate 

concentrations. Indeed, the model does predict a downward trend. Between 1995 and 

2005 predicted PM2.5 nitrate concentrations decrease by 10% and 16% at Los Angeles 

and Rubidoux, respectively. For PM2.5 sulfate, the emissions changes are more 

complex, and the modeled trends are expected to be less distinct. While domainwide 

SO2 emissions are expected to increase (due mainly to increased shipping activity), 

inland SO2 emissions are expected to decrease. In some areas, however, primary 

particulate sulfate emissions are expected to increase. In Figures B-35 and B-36, a 

spatial plot of the absolute difference of SO2 emissions between 1995 and 2005 and 

primary PM2.5 SO4 emissions between 1995 and 2005 are shown for a randomly 

selected day (February 25th). While SO2 emissions are decreasing in the immediate 

vicinity of Los Angeles and Rubidoux, primary PM2.5 sulfate emissions look to be 

increasing near Rubidoux. This is reflected in the predicted sulfate concentrations 

where there is a 12% decrease in PM2.5 sulfate at Los Angeles and a 3% increase at 

Rubidoux from 1995 to 2005. The model behaves predictably given the emissions 

changes, but the relation of modeled and observed trends is not as clear. 

Due to the limited number of speciated PM2.5 data and the possible disconnect 

between data taken from the PTEP (PM10 Technical Enhancement Program) and STN 

samplers, it is difficult to make a stronger statement about the model’s retrospective 

performance in comparison to observed trends. While there is a relative scarcity of 

PM2.5 data for previous years in the SCAB, PM10 nitrate and sulfate measurements 

have been taken at various sites for many years. Contrary to the PM2.5 PTEP and STN 

data where differences in sampling protocols may skew the observed trends, the 

available PM10 measurements present an opportunity to examine the observed nitrate 

and sufate trends without uncertainties associated with varying measurement 

techniques. Figures B-37 and B-38 show the measured and modeled trends for PM10 

nitrate and sulfate at 5 sites, Burbank, Fontana, Los Angeles, North Long Beach, and 

Rubidoux. This larger dataset indicates a pronounced variability in annual average 

nitrate and sulfate concentrations that was not clear with the PM2.5 observations. For 

sulfate (Figure B-38), the modeled and observed trends compare very well for most 
B-36 

sites. The observed flattening of the sulfate trends in later years is also predicted by the 

model. While PM10 nitrate predictions fall in line with the observed trends at Burbank, 

at the other sites, the predicted trends are less steep than those observed. Since the 

simulation is only capturing emissions changes while the observations include 

meteorological and background variability as well, it is difficult to determine if the 

agreement for nitrate is unsatisfactory. 

Conclusion 

CAMx was run for three years (1995, 1999, and 2005) with 2005 meteorology and 

boundary conditions held constant for all simulations in an effort to compare model and 

observed response to historical emissions changes. While the three years were not 

modeled as rigorously as possible (meteorology was held constant), the model 

responds as expected, predicting lower 2005 concentrations for species that see a 

decrease in emissions over the 10-year period. Comparisons with PM10 and PM2.5 

species data at multiple sites throughout the South Coast Air Basin allow a qualitative 

comparison between the model and observations. Observed and total PM2.5 carbon 

trends correspond well for eastern and western sites, as do PM10 sulfate concentrations. 

While predicted and observed PM10 nitrate concentrations both show a downward 

trend over the ten-year period, modeled trends are less steep than those observed. 

Since the observed concentrations include meteorological and background 

concentration variability while the model does not, it is difficult to determine if the 

agreement between the observed and predicted trends, especially for nitrate, is 

unsatisfactory. 
B-37 

Figure B-31: Observed and Predicted PM2.5 Total Carbon, Ammonium, Nitrate, and 

Sulfate at Los Angeles (SCOS Domain) 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[TC (year)] / [TC (2005)]

TCmod

TCobs

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[NH4

+ (year)] / [NH4

+ (2005)]

NH4mod

NH4obs

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[NO3

- (year)] / [NO3

- (2005)]

NO3mod

NO3obs

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1990 1995 2000 2005 2010

year

[SO42- (year)] / [SO42- (2005)]

SO4mod

SO4obs
B-38 

Figure B-32: Observed and Predicted PM2.5 Total Carbon, Ammonium, Nitrate, and 

Sulfate at Rubidoux (SCOS Domain) 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[TC (year)] / [TC (2005)]

TCmod

TCobs

0

0.5

1

1.5

2

2.5

3

1990 1995 2000 2005 2010

year

[NH4

+ (year)] / [NH4

+ (2005)]

NH4mod

NH4obs

0

0.5

1

1.5

2

2.5

1990 1995 2000 2005 2010

year

[NO3

- (year)] / [NO3

- (2005)]

NO3mod

NO3obs

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[SO42- (year)] / [SO42- (2005)]

SO4mod

SO4obs
B-39 

Figure B-33: Observed and Predicted PM2.5 Total Carbon, Ammonium, Nitrate, and 

Sulfate at Los Angeles (SCAQS Domain) 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[TC(year)] / [TC(2005)]

TCmod

TCobs

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[NH4

+(year)] / [NH4

+(2005)]

NH4mod

NH4obs

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[NO3

-(year)] / [NO3

-(2005)]

NO3mod

NO3obs

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1990 1995 2000 2005 2010

year

[SO42-(year)] / [SO42-(2005)]

SO4mod

SO4obs
B-40 

Figure B-34: Observed and Predicted PM2.5 Total Carbon, Ammonium, Nitrate, and 

Sulfate at Rubidoux (SCAQS Domain) 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[TC(year)] / [TC(2005)]

TCmod

TCobs

0

0.5

1

1.5

2

2.5

3

1990 1995 2000 2005 2010

year

[NH4

+(year)] / [NH4

+(2005)]

NH4mod

NH4obs

0

0.5

1

1.5

2

2.5

1990 1995 2000 2005 2010

year

[NO3

-(year)] / [NO3

-(2005)]

NO3mod

NO3obs

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1990 1995 2000 2005 2010

year

[SO42-(year)] / [SO42-(2005)]

SO4mod

SO4obs
B-41 

Figure B-35: Absolute SO2 Emissions Change Between 1995 and 2005 at Noon for a 

Day in February (February 25th). A negative value indicates an increase in emissions 

between 1995 and 2005. Los Angeles (west) and Rubidoux (east) are marked by stars. 

Figure B-36: Absolute PM2.5 Sulfate Emissions Change Between 1995 and 2005 at 

Noon for a Day in February (February 25th). A negative value indicates an increase in 

emissions between 1995 and 2005. 
B-42 

Figure B-37: Observed and Modeled Trends for PM10 NO3 at Burbank, Fontana, Los 

Angeles, North Long Beach, and Rubidoux 

Burbank

0.00

0.50

1.00

1.50

2.00

2.50

1980 1990 2000 2010

year

[NO3

- (year)] / [NO3

- (2005)]

obs-no3

mod-no3

Fontana

0.00

0.50

1.00

1.50

2.00

2.50

3.00

1980 1990 2000 2010

year

[NO3

- (year)] / [NO3

- (2005)]

obs-no3

mod-no3

Los Angeles

0.00

0.50

1.00

1.50

2.00

2.50

1980 1990 2000 2010

year

[NO3

- (year)] / [NO3

- (2005)]

obs-no3

mod-no3

North Long Beach

0.00

0.50

1.00

1.50

2.00

2.50

1980 1990 2000 2010

year

[NO3

- (year)] / [NO3

- (2005)]

obs-no3

mod-no3

Rubidoux

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

1980 1990 2000 2010

year

[NO3

- (year)] / [NO3

- (2005)]

obs-no3

mod-no3
B-43 

Figure B-38: Observed and Modeled Trends for PM10 SO4 at Burbank, Fontana, Los 

Angeles, North Long Beach, and Rubidoux 

Burbank

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1980 1990 2000 2010

year

[SO42- (year)] / [SO42- (2005)]

obs-so4

mod-so4

Fontana

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1980 1990 2000 2010

year

[SO42- (year)] / [SO42- (2005)]

obs-so4

mod-so4

Los Angeles

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

1980 1990 2000 2010

year

[SO42- (year)] / [SO42- (2005)]

obs-so4

mod-so4

North Long Beach

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1980 1990 2000 2010

year

[SO42- (year)] / [SO42- (2005)]

obs-so4

mod-so4

Rubidoux

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

1980 1990 2000 2010

year

[SO42- (year)] / [SO42- (2005)]

obs-so4

mod-so4
B-44 

Assessment of Alternative Attainment Scenarios

Sensitivity testing conducted by the South Coast District indicated that reductions in 

SOx have the biggest impact on PM2.5 concentrations, followed by directly emitted 

PM2.5 and then NOx. Reductions in VOCs had the least impact on PM2.5 

concentrations. The relative effectiveness of emission reductions in terms of 

micrograms of PM2.5 reduced per ton of emissions reduction for each PM precursor is 

shown in Table B-10. 

Table B-10: Relative Effectiveness of PM2.5 Emission Reductions 

Emissions 

Category 

Relative Effectiveness 

in Reducing PM2.5 

SOx Factor of 10 

PM2.5 Factor of 5 

NOx Factor of 3 

ROG Factor of 1 

Given the relatively greater effectiveness of reductions in directly emitted PM2.5 as 

compared to NOx, ARB staff conducted a grid-based modeling sensitivity analysis to 

determine the potential impact of further residential wood combustion and cooking 

controls, categories that contribute significantly to the primary PM levels in the South 

Coast as indicated by source apportionment studies. ARB staff evaluated two 

scenarios; a 100 percent reduction in residential wood combustion emissions for 

November through February, and a 50 percent reduction in residential wood combustion 

emissions for November through February, both coupled with a 20 percent reduction in 

cooking emissions for the entire year. In addition, the South Coast District evaluated 

the impact of curtailing wood burning on days when PM2.5 exceeds 35 ug/m3, coupled 

with control of cooking emissions. The results of these three scenarios at the RiversideRubidoux site are shown in Table B-11. This grid-based modeling analysis indicates 

that further controls on these sources of directly emitted PM2.5 can have significant 

benefits in further reducing PM2.5 concentrations. 
B-45 

Table B-11: Predicted Reduction in Annual Average 2014 PM2.5 Concentrations for 

Residential Wood Burning and Cooking Control Scenarios 

 Scenario Description Annual Average 

PM2.5 Reduction 

(ug/m3) 

Scenario 1 100% reduction in woodsmoke from November – 

February + 20% reduction in cooking year-round 

0.89 

Scenario 2 50% reduction in woodsmoke from NovemberFebruary + 20% reduction in cooking year-round 

0.45 

Scenario 3 Curtailment of woodsmoke on days where 

PM2.5 > 35 ug/m3 and control of cooking 

0.10 

Based upon this analysis, the South Coast District added an additional control measure 

that reflects scenario 3 in Table B-11, reducing the predicted annual average design 

value at Riverside-Rubidoux to 15.6 ug/m3. Analysis by ARB staff also identified an 

additional 14 tons of mobile source NOx reductions. Taken together, the attainment 

demonstration design value drops to just below 15.5 ug/m3 at the high site of Riverside 

Rubidoux, and therefore a predicted design value within the uncertainty bounds 

identified by U.S. EPA as qualifying for a weight of evidence attainment demonstration. 

5. SUMMARY 

In summary, the South Coast has experienced tremendous progress in reducing PM2.5 

concentrations since initial monitoring began in 1990. Between 1990 and 1998, PM2.5 

annual average concentrations dropped by 30 to 40 percent throughout the basin. 

Since 1999, when the official regulatory monitoring program for comparison to the 

federal PM2.5 standard began, PM2.5 annual average concentrations have dropped a 

further 23 to 30 percent. While the peak annual average design value in the basin was 

30 ug/m3 in 2000, twice the level of the standard, it dropped to 22.7 ug/m3 in 2005, with 

a continued decline in 2006 to 20.8 ug/m3. In addition, the South Coast now attains the 

federal 24-hour standard of 65 ug/m3. Although concentrations remain the highest in 

the eastern basin, the rate of decrease in PM2.5 and the response to ongoing control 

programs has also been the greatest in this region. 

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

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

ammonium nitrate have had the greatest contribution to declining PM2.5 mass trends, 

with ammonium nitrate concentrations at Riverside-Rubidoux dropping by 40 percent. 

During this same time period, ambient NOx concentrations, a precursor to ammonium 

nitrate, as well as NOx emissions have also decreased by 25 and 18 percent 

respectively. Looking back to the late 1980s, both ammonium nitrate and ammonium 

sulfate have declined significantly, along with concurrent declines in ambient NOx and 

SO2 concentrations and NOx and SOx emissions. These significant trends in declining 

PM2.5 concentrations, ambient precursor concentrations, and associated emissions all 
B-46 

suggest that continuing reductions of both NOx and SOx emissions of the same order of 

magnitude as seen in the past will provide significant further progress in attaining the 

annual PM2.5 standard. 

 

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

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

PM2.5 annual average concentrations in 2014 based on emission reductions achieved 

from both the ongoing control program, as well as the new measures specified in the 

Statewide Strategy, were estimated with both methods. Substantial further emission 

reductions of approximately 40 to 50 percent for all key pollutants will occur between 

2005 and 2014. 

Based on the linear rollback modeling approach, all sites in the basin would be 

expected to attain the annual standard by 2014, including the high site at RiversideRubidoux. Although the linear rollback approach does not incorporate the potentially 

non-linear and inter-pollutant interactions of controls that can be addressed through 

grid-based modeling, the predicted future year concentrations from the rollback 

modeling are consistent with observed air quality trends and the response to past 

emission reductions. In contrast, the grid-based modeling shows a significantly lower 

response to emission controls, with higher predicted concentrations in 2014 as 

compared to the rollback modeling. This modeling suggests that a much greater rate of 

emission reductions will be needed over the next ten years in order to sustain the same 

amount of progress observed in recent years. As a result, while the grid-based 

modeling indicates that the coastal sites would still attain the annual standard, portions 

of the western and eastern basin are predicted to remain above the standard in 2014, 

with a maximum predicted concentration design value of just below 15.5 ug/m3 at 

Riverside-Rubidoux. This is within the range for which a weight-of-evidence attainment 

demonstration is appropriate. 

Considering all of the information available, on balance, ARB staff believes that the 

evidence provided by the supplemental analysis suggests that the South Coast will 

attain the annual average PM2.5 standard throughout the basin by 2014. This is due to 

the following considerations: 

 The significant progress that has already occurred, a 9.2 ug/m3 drop in 

annual average design value between 2001 and 2006, which represents twothirds of the progress needed to attain the annual standard by 2014; 

 Attainment of the 24-hour standard has already occurred; 

 The positive response of ammonium nitrate and ammonium sulfate 

concentrations to past reductions in NOx and SOx emissions; 

 The pace of future emission reductions for NOx and SOx, reflecting a 

50 percent decrease in NOx and a 45 percent decrease in SOx, which is 

significantly greater than during the 2001-2006 period; 
B-47 

 Speciated rollback modeling which indicates attainment in 2014; and, 

 The small degree by which the attainment demonstration modeling design 

value is above the annual standard, which is within the range of the 

uncertainty of this type of modeling. This, coupled with the relatively 

conservative response of the model to NOx reductions, would suggest that 

attainment is likely. 

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Identify which Precursor Species Limit Aerosol Nitrate Formation, J. Air & Waste 

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Chow, J.C., Watson, J. G., Lowenthal, D. H., and Countess, R. J.: Sources and 

chemistry of PM10 aerosol in Santa Barbara County, CA, Atmospheric Environment, 

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Hannigan, M.P.; Busby, W.F.; Cass J.R. Source Contributions to the Mutagenicity of 

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Manchester-Neesvig, J.B.; Schauer, J.J.; Cass, G.R. The Distribution of Particle-Phase 

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Air & Waste Management Association. 2003, 53, 1509-1516. 

Schauer, J.J.; Rogge, W.F; Hildemann, L.M.; Mazukek, M.A.; Cass, G.R. Source 

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Atmospheric Environment. 1996, Vol 30, No. 22, 3837-3855. 

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Watson, J. G.; Chow, J.C.; Lu, Z.; Gertler, A.W. Particulate and Gaseous Organic 

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