### APPENDIX B

SUPPLEMENTAL ANALYSIS: PM2.5 ATTAINMENTB-1 

### SOUTH COAST PM2.5 SUPP LEMENTAL ANALYSIS

1. INTRODUCTION

The State Implementation Plan or SIP serves as a demonstration of attainment for the national ambient air quality st andards (NAAQS or federal st andards). The attainment demonstration comprises analyses used to de termine the set of control measures needed to meet the NAAQS by the attainment year. These analyses typically include air quality modeling, which gene rally guides the selection of the most effective pollutants to control and the magnitude of needed emi ssions reductions. Because of the uncertainties inherent in photochemical mo deling, U.S. Envir onmental Protection Agency (U.S. EPA) modeling guidance recommends that states perform complementary analyses. These analyses can include cons ideration of measured air quality, emissions, and meteorological data, evaluat ion of other air quality indicators, and additional air quality modeling.

A supplemental analysis looks at the entiret y of the information at hand to provide a more informed basis for the attainment stra tegy. Because all methods have inherent strengths and weaknesses, exam ining an air quality problem in a variety of ways offsets the limitations and uncertainty that are inher ent in air quality modeling. This approach also provides a better understanding of t he 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 mo deled results, when future year modeled annual average desi gn 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 conc entrations that is inherent in air quality modeling, and therefore recogni zes that an improved assessment of attainment can be derived from examining a broader set of analyses.

U.S. EPA recommends that three basic ty pes 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 attainm ent. The scope of the supplemental analysis is different for each nonattainment area. T he 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 av ailable. For example, less analysis is needed for an area that is pr ojecting attainment near-term and by a wide margin, and for which recent air quality tr ends 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 t he 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 t hough 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 a ttaining 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. Wh ile almost all monitors with complete data in the South Coast still exceeded the annual PM2.5 st andard as of 2006, PM 2.5 air quality has shown considerable improvement since 1999 when monitoring data for assessing compliance with the federal PM2.5 standar d 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 th roughout 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. T able B-1 provides the federal 3-year annual average values (design values) for each mo nitoring site for 2006 along with the 2004, 2005, and 2006 annual averages. Table B-2 pr ovides the 2006 federal 3-year 24-hour 98 th percentile design value along with the 2004, 2005, and 2006 24-hour 98 th 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 feder al standard. Design values represent the yearly annual average or 98 th percentile PM2.5 concentrations averaged over a three year period. Attainment is reached when t he design value is at or below the federal standard. As shown in Table B-1, curr ent 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 A nnual 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 includi ng 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 clim ate. 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 bu ild-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 pa tterns from the ocean. As show n in bold in the tables, the high site for the region is the Rubidoux-Riverside monito r 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 98 th Percentile 24-hour Values

Monitor 2004 24-hour 98 th

Percentile

(ug/m3) 2005 24-hour 98 th

Percentile

(ug/m3) 2006 24-hour

98 th
Percentile

(ug/m3) 2006 3-year 24-hour 98 th 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 t he problem, considerable progress has occurred in the South Coast over the last seven years due to t he 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 LynwoodConcentrations (ug/m3)

2001

2006

All South Coast monitors with complete PM2. 5 data also show a significant decrease in annual average design values (Fi gure 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 por tion 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 perc ent, 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 furt her illustrated in Figures B-3 and B-4 which depict maps of the spatial variations in annual averag e 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, includi ng 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

YearConcentrations (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 2001B-7 Figure B-4: National PM2.5 Annual Average 2006

The progress observed in both the annual a nd 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 t he year. In the South Coast, PM2.5 concentrations do not exhibit a pronounced s easonal variation, wit h monthly average concentrations above the level of the annual st andard occurring in all parts of the year. This relative uniformity results in part from meteorological conditi ons conducive to the formation of sulfates and nitrat es at different times of the year. Sulfate formation is greater in the warmer months while nitrate fo rmation is greater in the cooler weather. Analyses of the changes in the distribut ion of 24-hour concentrations between 2001-2003 and 2004-2006 indicate that t he decrease in both 24-hour and annual average concentrations is the resu lt of a broad downward shift in the distribution of daily PM2.5 concentrations over time. This can be seen in the decrease in monthly averageconcentrations 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 la rgest 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

MonthConcentrations (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

MonthConcentrations (ug/m3)

2001

2006

A comparison of the changes in the frequen cy 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 concentra tions greater thanB-9 40 ug/m3 (Figure B-7). In comparison, during the 2004-2006 per iod 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 duri ng the 1999–2001 period indicate that 23.3 percent of the samples had concentra tions greater than 40 ug/m3 (Figure B-8), while during the 2004-2006 period, only 8.5 per cent of the samples had concentrations greater than 40 ug/m3. Signific ant 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 br oad range of conditions.

Figure B-7: PM2.5 Concentra tions 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

2004-2006B-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 PM 2.5 Mass Trends

The PM2.5 Federal Reference Method (FRM) network t hat is used for comparison to the federal standard was first deployed in 1998 and 1999. Howeve r, PM2.5 data have been routinely collected in the South Coast Air Basin since the late 1980s using dichotomous samplers that were operat ed 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, t herefore 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 capt ured with the FRM due to improved filter handling and temperature controls (Motallebi 2003). However, the relative changes in the dichotomous data ar e 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-y ear average concentrations decreased about 30 percent from 1990 to 1996. The other two sites show n 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. Theref ore, looking at dichotom ous data and FRM data trends together illustrate a conti nued long-term improvement in PM2.5 concentrations.B-11 Figure B-9: PM2.5 Dichotom ous 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 1998Concentrations (ug/m3)

Azusa North Long Beach

Riverside-Rubidoux

Meteorology and Air Quality Trends

Although the weather plays a ro le in the atmospheric forma tion of PM2.5, the downward trends in the South Coast Air Basin do not appear to be an artifact of good meteorological conditions, especially giv en the long-term consistency of the trends. Unlike ozone, meteorologically adjus ted 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 woul d 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 formati on and thus higher PM2.5 concentrations.

Although there was a range of weather c onditions between 2001 an d 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 Precip itation 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, River side-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 2006Concentrations (ug/m3)

Summer

WinterB-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 2006Concentrations (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. In vestigating these different components and how they vary by site and season prov ides us with a better understanding of their complexity and responses to the emission c ontrol program. This mix of chemical components consists of both directly emitte d PM such as geological material and elemental carbon (soot), known as prim ary PM, as well as PM formed in the atmosphere from the reactions of precurso r gases, known as secondary PM. These precursor pollutants include nitr ogen oxides (NOx), sulfur ox ides (SOx), reactive organic gases (ROG), and ammonia. NOx, SOx, and ammonia combine to form secondary ammonium nitrate and sulfate. ROG can form secondary or ganic 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 River side-Rubidoux. These two sites are representative of the highl y populated western basin (L os 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 lin kage between observed progress and ongoing emission reductions.

The annual average PM2.5 chemical comp osition for Riverside-Rubidoux and Los Angeles-North Main is shown in Fi gure B-12. Although ammonium nitrate, ammonium sulfate, and organic carbon are the major constit uents at both sites, there are important differences between the tw o sites as well. For example, Los Angeles-North Main has a greater contribut ion from ammonium sulfate and elementalB-14 carbon than Riverside-Rubidoux, likely reflec ting the greater impact from shipping and goods movement related activities in this r egion. In contrast, Riverside has a greater contribution from ammonium nitrate due to its location fu rther downwind in the basin and the longer time available for the secondar y formation of this c onstituent. It can be important for the attainment strategy to re cognize these regional di fferences. However, even though there are r egional differences in the compos ition, 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 Composit ion 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 th rough 2005. At the Los Angeles-North Main site (Figure B-13), ammonium nitr ate is lowest during the spring months, and increases through fall and winter. Su lfate has a photochemical aspect so concentrations are greatest in the summer. At the Los Angele s-North Main site, ammonium sulfate increases sharply from Ap ril to July, and then slowly begins to descend to its lowest concentration in Decem ber. The Riverside-Rubidoux site follows the same trend, with the exception that am monium sulfate concentrations at the Los Angeles-North Main site increase more rapidly and to a higher level than RiversideRubidoux. At the Riverside-R ubidoux site (Figure B-14), a mmonium nitrate is highest in the summer months from J une 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

MonthConcentrations (ug/m3)

Elements Geological

Elemental Carbon Organic Carbon

Ammonium Sulfate

Ammonium Nitrate

Figure B-14: PM2.5 Seasonal Pattern in Chemical Component s 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

MonthConcentrations (ug/m3)

Elements Geological

Elemental Carbon Organic Carbon

Ammonium Sulfate

Ammonium Nitrate

Chemical Composition Trends

As discussed previously, all monitori ng sites have shown decreasing PM2.5 concentrations from 2001 to 2006. Trends in individual chemical components and precursor concentrations, as well as emi ssions inventory trends were evaluated toB-16 understand the primary chemic al components responsible for this progress and to evaluate the response to ongoing control pr ograms. Riverside-Ru bidoux 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 per cent, and carbonaceous aerosols have dropped 19 percent. Approximat ely two-thirds of the PM2. 5 improvement observed in the South Coast can therefore be attribut ed to reduced ammonium nitrate; a much smaller portion of the remaining PM2.5 improvement is due to reductions in carbonaceous compounds (19 percent) and amm onium sulfate (~17 percent). Trends for Los Angeles-North Main are shown in Figure B-16.

Figure B-15: Trends in PM2.5 Mass and Ch emical Components Riv erside-Rubidoux

Trends in PM2.5 Mass and Chemical Components Riverside-Rubidoux

0 5 10 15 20 25 30 35

2001* 2002 2003 2004 2005 2006Concentrations (ug/m3) PM25 3-year Avg

AmmNitrate

AmmSulfate

CarbonaceousAerosols Geological

ElementsB-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 2006Concentrations (ug/m3) PM25 3-year Avg

AmmNitrate
AmmSulfate

CarbonaceousAerosols Geological

Elements

Trends in ammonium nitrate concentrations al so 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 amm onium nitrate concentrations decreased 40 percent. As our emission control pr ogram has reduced t he amount of these precursors, we have seen a corresponding im provement in air quality. Figure B-17 illustrates the downward trend of ammoni um nitrate concentra tions compared with similar declines in basin average NOx c oncentrations 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 NO x

0 2 4 6 8 10 12 14 16 18

2001 2002 2003 2004 2005 2006Ammonium Nitrate (ug/m3)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0NOx(pphm)
Ammonium Nitrate
NOxB-18 Similar relationships between both ammonium nitrate and sulfate and ambient precursor concentrations can be seen in the longer-te rm 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 ambi ent NOx and SO2 concentrations.

Figure B-18: Three-Year Average Concentra tions of PM10 Nitrate and NOx in South Coast Air Basin

Three-year Average Concentrations
of PM10 Nitrate and NO x in South Coast

0 1 2 3 4 5 6 7 8 9 1019871988198919901991199219931994199519961997199819992000200120022003200420052006PM10 Nitrate (ug/m3)

0 1 2 3 4 5 6 7 8 9 10NOx(pphm)
NO3-

NOx

Figure B-19: Three-Year Average Concent rations of PM10 Sulfate and SO2 in the South Coast Air Basin

Three-year Average Concentrations of PM10 Sulfate
and SO 2 in the SC

0 1 2 3 4 5 6 719871988198919901991199219931994199519961997199819992000200120022003200420052006PM10 Sulfate (ug/m3)

0.0 0.2 0.4 0.6 0.8 1.0 1.2SO2(pphm)
SO4--

SO2B-19 In addition, Figure B-20 compares the PM 10 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. Furthe rmore, 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 leve ls are higher than the PM10 nitrate levels. This is due to the loss of semi-volatile ni trates with the PM10 m onitor configuration, which are better captured on the PM2.5 monitor.

Figure B-20: Three-Year Average Concent rations 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 181987198919911993199519971999200120032005Concentrations (ug/m3)

PM10NO3-

PM25NO3-

Figure B-21: Three-Year Average Concent rations 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 7.019871988198919901991199219931994199519961997199819992000200120022003200420052006Sulfate (ug/m3)

PM10SO42-

PM25SO42-B-20 Figure B-21 compares the PM10 and PM2.5 sulf ate at the Riverside-Rubidoux monitor. As with nitrate, the data show s that the PM2.5 and PM10 sulfat e levels track very well. In contrast with nitrate, t he PM10 sulfate is higher than t he 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. Ther efore, the difference between PM10 and PM2.5 repres ents sulfate in the coarse particle range.

Emission Trends

At the same time that ambient concentra tions have been declining, the emissions of pollutants that contribute to the differ ent components of PM2.5 have also been decreasing. Figure B-22 illustrates the rece nt 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 decr eased, experiencing a decline of 213 tpd, or 18 percent. T hese reductions reflect the benefits of control programs which have provided continuing reductions despite increasing population growth. PM2.5 emissions showed a much smaller dec rease of 11 tpd, wh ile 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 t he longer-term period, both SOx and NOx emissions have also dropped substant ially as documented by the decrease in ambient NOx and SO2 concentrations reflec ted in Figures B-20 and B-21 above.

The combined downward trends in PM2.5 co mponents, precursor concentrations, and emissions all indicate that over both the long-term and s hort-term, the ongoing control program has had substantial benef its in improving air qualit y and that similar emission reductions in the future should provide cont inuing 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 2006Emissions (tpd) NOx
ROG
SOx
PM2.5B-21 3. OBSERVATIONAL MODELS AND DIAGNOSTIC ANALYSES

Observational models take advantage of moni tored data to draw conclusions about the relative importance of different types of em issions and precursors as factors contributing to observed PM2.5 concentrations. Accord ing 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 apportionmen t (receptor models) and indicator species approaches. The two most widely app lied receptor modeling approaches are multi-variate statistical models such as positive matrix factorization (PMF) and the chemical mass balance model (CMB). Rec eptor models are particularly useful in identifying the source contributions to dire ctly emitted PM. This supplemental analysis summarizes results from prior receptor mode ling 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 apportionm ent studies have been conduc ted in the South Coast reflecting snapshots of different sites and time s using various receptor models. While the precise contributions vary, these studi es 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 (Schaue r 1996). This study found that directly emitted exhaus t from mobile sources (gas oline 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 averag e PM2.5 concentrations. Figure B-23 compares the source apportionment results for Downtown Los Angeles and RiversideRubidoux. For example, the largest sour ce in Downtown Los Angeles was diesel exhaust, while at Riverside-R ubidoux 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 A pportionment of Fine Particulate Matter

1982 Annua l Average Source Apportionment

0 5 10 15 20 25 30 35 40

Riverside-Rubidoux Downtown Los Angelesug/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) charac terized primary organic carbon sources and compared them with PM concentrations in the Los Angeles area during the 1987 Southern California Air Quality Study. T he analysis identified motor vehicles, meat cooking, road dust, and wood smoke as contri buting to fine particle organic compounds in the region. In another study in 1993, annual composite samp les from four sites in the South Coast found that mobile sources cont ributed 14 percent of organic carbon, dust 10 percent, wood smoke 9 percent, and cooki ng 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 t he University of Wisconsin-Madison and Cal Tech (Manchester-Neesvig 2003) demonstrat ed 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 0.40B-23 Recent Studies

To evaluate the major PM2.5 sources and thei r contributions in the South Coast Air Basin using more recent data, two differ ent source apportionment techniques were applied to data collected in 2003 through 2005. The CMB model uses measured source profiles and chemical speciation data in comb ination 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 pr ofiles, but rather evaluates t he 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 fact ors may reflect the impacts of primary source s, as well the secondary species that have condensed on these primary particles. Since each te chnique 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 te sting different numbers of sources and different modeling parameters, eight and nine ma jor source factors were identified at the Los Angeles-North Main and the Riverside-Rubidoux sites, respectively.

The average source contributions of each s ource to the PM2.5 ma ss concentrations are provided in Figure B-24. Secondary nitrat e 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, secon dary sulfate particles contributed more at the Los Angeles-North Main site (23 perc ent) than at the River side-Rubidoux site (9 percent). The possible source directions for the secondary particles and airborne soil were estimated by conditional probability f unctions and are shown as probabilities in Figure B-25. Figure B-25 shows that most of the secondary ni trate particles as well as the secondary sulfate particles at the Riverside-Rubidoux site were associated with westerly winds indicating st rong contributions from the western basin. As major sources, gasoline vehicles and diesel emi ssions 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 t he Riverside-Rubidoux site. There appear to be greater impacts from diesel emission sources at th e 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 concentra tions at the Los Angeles-North Main site and 6 percent at the Riverside-Rubidoux site. The biomass smoke category reflects contributions fr om residential wood bur ning as well as managed and wildland fires; smoke from commerci al 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 perc ent at the Riverside-R ubidoux site. AgedB-24 aerosol with sea salt 1 and 2 reflect particles in which chloride in the fresh sea salt is displaced by acidic gases during transpor t 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 sa lt in Riverside-Rubidoux compared to Los Angeles-North Main. Airbor ne 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 co ntains 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 di rectionality. However, as shown in Figure B-24, airbor ne soil identified at the Riv erside-Rubidoux site was strongly associated with winds from the nort heast. An additional factor contributing 2 percent of the PM2.5 concentrations was s eparated at the Riverside-Rubidoux site. This source profile includes organic and elem ental carbon, zinc, calcium, iron, and lead suggesting an industrial emissions source. At the Los Angeles-North Main site, a similar signal was not likel y strong enough to be separated.

Figure B-24: 2003 – 2005 Average Source C ontributions (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% A ug. 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 Directi ons at the Riverside-Rubidoux SiteB-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

330 345 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

330 345 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

330 345

The monthly patterns in source contributio ns 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 site s. Secondary sulfate particles show monthly variation with higher concentrati ons 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 indica te possible smoke contributions from managed burning/wildfires in the summer. Airborne soil has September/October high patterns at both sites when the winds from t he northeast are most predominant.

Figure B-26: 2003 – 2005 Mont hly 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

MonthConcentrations(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 Averag e Source Contributions (PMF): Riverside-Rubidoux SiteB-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

MonthConcentrations (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 apporti onment 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 Str eet 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 contri butions to the PM2.5 mass. Source contribution estimates were calculat ed based on the 2004-2005 average for the individual samples that had acceptable re sults for the perf ormance measures

Figure B-28 illustrates the 2004-2005 averag e 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 per cent 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 repl aced by nitrate on a molar basis (Chow et al., 1996a).B-27 Figure B-28. Estimated 2004-2005 Average PM 2.5 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 cook ing contribution ranged from 19 percent to 22 percent; however that likely included posit ive organic carbon sampling artifacts, secondary organic aerosols, as well as other com bustion 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 m easured 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 cont ributions 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 resus pended 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 brak e and tire wear were co mbined into a single profile and a separate dus t 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 Ca rbon Data on Source Contribution (Average of 106 days)

Contribution (ug/m3) Source Category 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 a mmonium nitrate concentrations have tracked well with concurrent reductions in NOx, suggesting that NOx control is an effective approach for ensuring further reductions in a mmonium nitrate. An indicator species approach provides an additional method to in vestigate which chemical precursor a secondary species such as ammonium nitrat e is most responsive to control of. Blanchard (2000) used a thermodynamic equ ilibrium 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 bas ed on the excess of ammonium plus ammonia over the sum of nitric acid, nitrate, and sulfate. This a pproach was applied to the field study data measured in the South Coast during the California Acid Deposition Monitoring Program and the 1987 Southern Calif ornia Air Quality Study. The results indicated that that nitrate fo rmation was not limited by the availability of ammonia for most samples, therefore suggesting that NO x controls are more effective in reducing ammonium nitrate concentrations.

4. AIR QUALITY MODELING

Rollback Modeling

In addition to the evaluation of air qualit y trends and the observational and diagnostic analyses, a rollback modeling analysis was c onducted 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 c oncentrations at the monitor, taking intoB-29 account background concentrations. The a mmonium nitrate com ponent however, was assumed to have a non-linear response to reducti ons in NOx, with a ra tio 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 associ ated emission category and background concentration used for each PM2.5 component in the rollback analysis.

Table B-6: PM2.5 Component s, 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 increas e 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 futu re PM2.5 design value was determined by summing the PM2.5 components and their co rresponding background concentration. The same technique was then used to evaluat e the impact of t he April 24, 2007 ARB staff proposed State Strategy for California’s 2007 State Im plementation 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 rollba ck technique estimated that the substantial additional emission reductions contained in t he ARB State Strategy as proposed in the April 24, 2007 document would be sufficient to provide for attainment at RiversideRubidoux as well Los Angele s-North Main.B-30 Table B-7: Rollback Modelin g 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 Modelin g 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 modelin g was conducted by the South Coast District. The PM modeling for the Sout h Coast Air Basin was based on the 1997 Southern California Ozone Study domain wh ich covers San Luis Obispo, Santa Barbara, Kern, Ventura, San Bernardino, Los Angeles, Orange, Riverside, San Diego, and Imperial Counties and extends into Northw estern Mexico. During 2005, the MATES III field study was conducted in the South Co ast Air Basin during which speciated PM2.5 concentrations were measured at 9 monito ring sites throughout the year. The year 2005 was selected for the annual PM2.5 simula tion since the resulting MATES III data set was robust and useful for model evaluati on. The PM modeling was run using the Comprehensive Air Quality Mode l with extensions (CAMx) air quality model. The model was run for emissions inv entories representin g the year 2005 and the year 2014.

To demonstrate attainment of the federal PM 2.5 standard, U.S. EPA guidelines recommend using a model-based relative reducti on 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 Specia ted Model Attainment Test), which areB-31 then summed to determine the future year PM2.5 concentrations. The modeled future year design values representing ARB’s cont rol strategy along with local South Coast District measures are listed in Table B-9. As shown in t he table, the proposed control strategy significantly reduces PM2.5 conc entrations 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 PM 2.5 progress will slow down and be less responsive to controls than has been previous ly observed. For example, between 2001 and 2006 observed ammonium nitrate concentra tions dropped by 40 percent. The gridbased modeling predicts a similar decline wi ll 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 twic e as fast as they fell between 2001 and 2006. Figure B-29 below illustrates the ac tual 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 “sti ff” or less responsive to NOx controls than expected.B-32 Figure B-29: Comparison of Measured Pr ogress 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 2015Concentrations (ug/m3)

PM2.5 Target Measured

Measured

Modeled 0.7ug/m3 gap

Retrospective Modeling

While chemical transport models may repres ent 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 t he model formulation and applicatio n and lie primarily in (1) the simplifications needed to efficiently si mulate complex atmospheric processes and (2) the model’s meteorological and emissions inputs. In order to test the ability of a model to effectively capture observ ed 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 emissi ons changes. In a retrospective analysis the model is run for a number of prior year s so that observed and modeled trends can be compared.

ARB ran the CAMx model using 2005 meteorol ogy and (1) 1995, (2) 1999, and (3) 2005 emissions for two domains over the Sout h Coast Air Basin (SCAB). The predicted species concentrations were then compared to par ticulate observations at western (e.g., Los Angeles) and eastern (e.g., Rubidoux) si tes in the SCAB in order to evaluate how well the modeled concentration tr ends track those observed.

It is important to note here t hat 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 tw o years. Since the meteorology was not identical for all three years, the modeled trend is only an approximation to the moreB-33 appropriate modeled trend that would have been obtained using actual meteorology for 1995 and 1999. Thus, the comparison of the modeled and obser ved trends presented here is not expected to be quantit ative, 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 conf iguration 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-km 2 cell domain including Orange, Los Angeles, Riverside, San Bernardino and Ventura c ounties (SCAQS). The second domain referenced here is the 116x80 5-km 2 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 ti mes 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 structur e, extending to approximately 5 kilometers above ground level and divided into 8 vertical layers, with the CAMx layer structure largely dependent upon the layer st ructure of the meteorologic al model. Meteorological inputs were generated with the fifth generat ion Penn State/NCAR Mesoscale Model (MM5). MM5 was run for 2005 in 5-day s egments without Four Dimensional Data Assimilation (FDDA).

CAMx has been set up to run in a two-secti on fine-coarse PM configuration with gasphase chemistry represented by the Carbon Bond IV chemical mechanism. Some changes to the default partitioning of sec ondary 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 obs erved PM2.5 total carbon, ammonium, nitrate, and sulfate concentrations at Los Angeles and Rubidoux. The modeled results that follow are for the simula tions with the larger SCOS domain. However, the modeled trends for the two domains are remarkably si milar (Figures B-33 and B-34). While the remaining figures show result s for the “SCOS” simulations, it should be understood that the same conclusions can be drawn for si mulations 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 Netw ork (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, th erefore, a somewhat qualitative test of predicted and observed trends. Due to the fact that meteorology is not changed between years, the model re sults can only capture the concentration changes due to emissions variations. Observed concentrati ons vary from year to year and reflect concentration fluctuations that are a resu lt 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 co mparison of the trends, modeled and observed concentrations are “normalized” by the 2005 c oncentrations. For the largely secondary species of nitrate, ammonium, and sulfat e, however, the obser ved 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 secondar y 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 thro ughout the entire domain, most inland cells see a decrease in NOx emissions. This ~2 5% 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 co ncentrations decrease by 10% and 16% at Los Angeles and Rubidoux, respectively. For PM2.5 su lfate, the emissions changes are more complex, and the modeled trends are expected to be less distinct. While domainwide SO 2 emissions are expected to increase (due ma inly to increased shipping activity), inland SO 2 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 abs olute difference of SO 2 emissions betw een 1995 and 2005 and primary PM2.5 SO 4 emissions between 1995 and 200 5 are shown for a randomly selected day (February 25 th ). While SO 2 emissions are decreasing in the immediate vicinity of Los Angeles and Rubidoux, prim ary PM2.5 sulfate emissions look to be increasing near Rubidoux. This is reflec ted in the predicted su lfate concentrations where there is a 12% decrease in PM2.5 sulfate at Los Angel es and a 3% increase at Rubidoux from 1995 to 2005. The model beh aves predictably given the emissions changes, but the relation of modeled and observed trends is not as clear.

Due to the limited number of speciat ed PM2.5 data and the possible disconnect between data taken from the PTEP (PM 10 Technical Enhancement Program) and STN samplers, it is difficult to make a str onger 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 SC AB, 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 prot ocols may skew the observed trends, the available PM10 measurements present an opportunity to ex amine 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 indica tes 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 obser ved trends compare very well for mostB-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 predi cted trends are less steep than those observed. Since the simulation is only capturing emissions changes while the observations include meteorological and background vari ability as well, it is diffi cult 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 simula tions in an effort to compare model and observed response to historical emissions changes. While the three years were not modeled as rigorously as possible (meteor ology was held cons tant), the model responds as expected, predicting lower 2005 concentrations for species that see a decrease in emissions over the 10-year period. Compar isons with PM10 and PM2.5 species data at multiple sites throughout the South Coast Air Basin allow a qualitative comparison between the model and observati ons. Observed and total PM2.5 carbon trends correspond well for eastern and western sites, as do PM 10 sulfate concentrations. While predicted and observed PM10 nitrate concentrations both show a downward trend over the ten-year period, modeled tr ends are less steep than those observed. Since the observed concentrations include meteorological and background concentration variability while the model does no t, 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

SO4obsB-38 Figure B-32: Observed and Predicted PM2. 5 Total Carbon, Ammonium, Nitrate, and Sulfate at Rubidou x (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

SO4obsB-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

SO4obsB-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

SO4obsB-41 Figure B-35: Absolute SO 2 Emissions Change Between 1995 and 2005 at Noon for a Day in February (February 25th). A negative va lue 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 betw een 1995 and 2005.B-42 Figure B-37: Observed and M odeled Trends for PM10 NO 3 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-no3B-43 Figure B-38: Observed and M odeled Trends for PM10 SO 4 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-so4B-44 Assessment of Alternati ve Attainment Scenarios

Sensitivity testing conducted by the South Coast District i ndicated that reductions in SOx have the biggest impact on PM2.5 concent rations, followed by directly emitted PM2.5 and then NOx. Reductions in VOCs had the least impact on PM2.5 concentrations. The relative effectivene ss of emission reductions in terms of micrograms of PM2.5 r educed per ton of emissions reduct ion for each PM precursor is shown in Table B-10.

Table B-10: Relative Effectivene ss 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 staf f conducted a grid-based model ing sensitivity analysis to determine the potential impact of further re sidential wood combustion and cooking controls, categories that contribute significant ly to the primary PM levels in the South Coast as indicated by source apporti onment studies. ARB staff evaluated two scenarios; a 100 percent reduction in resi dential wood combus tion emissions for November through February, and a 50 percent r eduction in residential wood combustion emissions for November through February, bot h 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 day s when PM2.5 exceeds 35 ug/m3, coupled with control of cooking emissions. The result s of these three scenari os at the RiversideRubidoux site are shown in Table B-11. Th is grid-based modeli ng analysis indicates that further controls on thes e sources of directly emitt ed PM2.5 can have significant benefits in further reduci ng PM2.5 concentrations.B-45 Table B-11: Predicted Reduction in Annua l 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 woodsm oke 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 pr edicted design value within the uncertainty bounds identified by U.S. EPA as qualifying for a wei ght of evidence attainment demonstration.

5. SUMMARY

In summary, the South Coast has experienc ed 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 an nual average design value in the basin was 30 ug/m3 in 2000, twice the level of the st andard, 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 ha s shown that over the last six years, decreases in ammonium nitrate have had the greatest cont ribution to declining PM2.5 mass trends, with ammonium nitrate concentrations at Riv erside-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 co ncentrations, and associated emissions allB-46 suggest that continuing reductions of both NO x 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 pr ogram, as well as the new measures specified in the Statewide Strategy, were estimated with bot h 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 in teractions of controls t hat can be addressed through grid-based modeling, the predicted future year c oncentrations from the rollback modeling are consistent wit h observed air quality trends and the response to past emission reductions. In cont rast, 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 t hat 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 w ould still attain the a nnual standard, portions of the western and eastern basin are predi cted 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 informa tion available, on balance, ARB staff believes that the evidence provided by the supplemental ana lysis 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 al ready occurred, a 9.2 ug/m3 drop in annual average design value between 20 01 and 2006, which represents twothirds of the progress needed to a ttain the annual standard by 2014;

 Attainment of the 24-hour standard has already occurred;

 The positive response of ammoni um 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 per cent decrease in SOx, which is significantly greater than during the 2001-2006 period;B-47  Speciated rollback modeling which i ndicates attainment in 2014; and,

 The small degree by which the attain ment demonstrati on 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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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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