May 1, 2014

TO: DOCKET EPA-R09-OAR-2014-0185

FROM: Carol Bohnenkamp

SUBJECT: Review of the Modeling for the Attainment Demonstration for the Proposed 

Rulemaking Action on the South Coast 2012 AQMP for the One Hour Ozone Standard

A. CAA and Regulatory Requirements for 1-Hour Ozone Air Quality Modeling and EPA 

Guidance 

For any ozone nonattainment area classified as serious or above, section 182(c)(2)(A) of the 

CAA specifically requires the State to submit a modeled attainment demonstration based on a 

photochemical grid modeling evaluation or any other analytical method determined by the 

Administrator to be at least as effective as photochemical modeling. The following guidance 

documents describe the criteria that an air quality model and its application should meet to 

qualify for use in an ozone attainment demonstration. 

 Appendix W to 40 CFR part 51, as interpreted in EPA guidance. See, e.g., Guideline for 

Regulatory Application of the Urban Airshed Model, EPA-450/4-91-013 (July 1991); 

 “Guidance on Use of Modeled Results to Demonstrate Attainment of the Ozone 

NAAQS,” EPA-454/B-95-007 (June 1996); 

 “Guidance for the 1-hour Ozone Nonattainment Areas that Rely on Weight-of-Evidence 

for Attainment Demonstrations, Mid-Course Review Guidance”(March 28, 2002); 

 “Guidance for Improving Weight-of-Evidence Through Identification of Additional 

Emission Reduction Not Modeled (November 1999);

 “Guidance on the Use of Models and Other Analyses for Air Quality Goals in Attainment 

Demonstrations for Ozone, PM2.5, and Regional Haze,” April 2007. 

These documents describe the components of the attainment demonstration, explain how the 

modeling and other analyses should be conducted, and provide overall guidance on the technical 

analyses for attainment demonstrations.

As with any predictive tool, inherent uncertainties are associated with photochemical grid 

modeling. EPA’s guidance recognizes these limitations and provides recommended approaches 

for considering other analytical evidence to help assess whether attainment of the NAAQS is 

likely. This process is called a weight of evidence (WOE) analysis. EPA’s modeling guidance 

(updated in 1996, 1999, and 2002) discusses various WOE analyses. The guidance recommends 

that all attainment demonstrations include supplemental analyses beyond the recommended 

modeling. These supplemental analyses provide additional information such as data analyses, 
and emissions and air quality trends, which would help strengthen the conclusion based on the 

photochemical grid modeling. 

B. 1-Hour Attainment Demonstration Modeling and Weight of Evidence in the South Coast 

2012 AQMP

1. Modeling Approaches for the SCAQMD Attainment Demonstration

a. Photochemical Grid Model. 

The model selected for the Final 2012 AQMP 1–hour ozone attainment demonstration was 

developed using the U.S. EPA supported Community Multiscale Air Quality (CMAQ) (version 

4.7) air quality modeling platform with Statewide Air Pollution Research Center-99 (SAPRC99) 

chemistry, and the Weather Research and Forecasting Model (WRF) (version 3.3) 

meteorological fields. The modeling system (including the photochemical model, meteorological 

inputs, and chemical mechanism) is consistent with the previous advice of outside peer 

reviewers. CMAQ is a state-of-the-art air quality model that can simulate ozone and PM2.5

concentrations together in a “one-atmosphere” approach for attainment demonstrations. 

Table 1: Summary of Final 2012 AQMP 1-hour Ozone Model Selection and Modeling

Protocol Final 2012 AQMP 1-Hour Ozone Modeling Protocol1

Ozone

Dispersion Platform: CMAQ

Chemistry: SAPRC99

Domain/ Coordinates

Expanded SCOS97

Meteorology, Emissions and Model application: Lambert Conformal

Grid: 4 Km X 4 Km

Ozone: 18 layers

Emissions Inventories

 2008 Base year

 Day-Specific Emissions

 Shipping emissions split into 2 layers

 EMFAC2011

o 3- modules

o Modified DTIM

 Adjustments to fugitive PM2.5 Paved road U.S. EPA with CA modifications

 Day-Specific Biogenic emissions

 Revised Mexican emissions profile

 

1

P.VII-50. Appendix VII, Air Quality Management Plan, 1-Hour Ozone Attainment Demonstration, February 2013, 

South Coast Air Quality Management District. 
Meteorology

 WRF initialized with NCEP data with FDDA

Air Quality Model Performance

 Assess model performance based on both 1-hour statistics: 

Normalized gross bias

Normalized gross error

Peak prediction accuracy

 60 ppb threshold (both indices)

 49 Cell averaging

2008 Base Year Simulations

June – August 2008

92 days of simulations evaluated

Peak Episode 6/18-6/21

Future Year Projections— Deterministic Approach /Tiered RRF Approach

 2022

b. Episode Selection.

The attainment demonstration modeling focuses on 92 days of ozone air quality observed during 

June through August of the base year 2008. Overall, the 92 day period provides a robust 

description of the 2008 ozone meteorological season. During this period, there were 16 days with 

measured daily 1-hour maximum ozone concentrations of 125 ppb or higher at the Crestline

monitoring site which is the monitoring site with the maximum number of exceedances. Seven 

well defined multi-day ozone episodes occurred in the Basin. When assessed for a normalized 

meteorological ozone episode potential using a regression based weighting covering 30-years of 

data (1998-2010), the June 18-22, 2008 period was ranked in the 99th percentile. This episode 

contained the top four daily Basin ozone maximum concentrations for 2008 and was selected as 

the focus of the attainment demonstration.

c. Model Performance. 

Model performance was evaluated in three zones in the South Coast Basin: the San Fernando 

Valley; the eastern San Gabriel, Riverside and San Bernardino Valleys; and Los Angeles and 

Orange County. 
Figure 1: Model Performance Zones in the South Coast Basin: 

3). the San Fernando Valley; 

4). the eastern San Gabriel, Riverside and San Bernardino Valleys; and

5). Los Angeles and Orange County. 

Normalized Gross Bias, Normalized Gross Error, and Peak Prediction Accuracy were 

determined for each area. 

Normalized Gross Bias < + 15 Paired in space and time 

Normalized Gross Error < 35 Paired in space (+2 grid cells) and time 

Peak Prediction Accuracy < + 20 Unpaired in space and time

Although not a requirement for determining acceptable model performance, the performance 

statistics were compared to the EPA performance goals presented in guidance documents.
Table 2: June 18-22, 2008 Base Year 1-Hour Average Ozone Performance

2

(Bold type indicates meeting statistical performance criteria).

*Normalized bias and normalized error calculated for hours where observations > 60 ppb

 

2

P. VII-57

Date

Zone 3

Observed

(ppb)

Predicted

(ppb)

Unpaired

Peak

Ratio

Normalized

Bias* 

(ppb)

Normalized

Error* 

(ppb)

618 87 93 1.07 -17 25

619 95 109 1.15 4 18

620 111 99 0.89 -10 19

621 122 107 0.87 -19 20

622 123 92 0.75 -29 29

Date

Zone 4

Observed

(ppb)

Predicted

(ppb)

Unpaired

Peak

Ratio

Normalized

Bias* 

(ppb)

Normalized

Error* 

(ppb)

618 162 123 0.76 -17 20

619 152 136 0.90 -1 18

620 176 129 0.73 -12 16

621 156 150 0.96 -1 18

622 123 134 1.09 10 21

Date

Zone 5

Observed

(ppb)

Predicted

(ppb)

Unpaired

Peak

Ratio

Normalized

Bias* 

(ppb)

Normalized

Error* 

(ppb)

618 118 107 0.91 0 22

619 110 111 1.01 11 15

620 114 106 0.93 0 13

621 107 115 1.07 4 12

622 107 121 1.13 13 19
The performance goals for Normalized Gross Error and Peak Prediction Accuracy were met in 

the eastern San Gabriel, Riverside and San Bernardino Valleys. The statistic for bias 

(Normalized Gross Bias) tends to be negative, indicating that the model tends to slightly underpredict ozone. Both June 18th and June 20th failed to meet the model acceptance criteria for the 

unpaired peak analysis. As a result, the attainment demonstration focused on the June 19th and 

21st, days with observed peak concentrations that closely matched the design values. Based on 

their analysis, South Coast concludes and EPA agrees that model performance is acceptable for 

this application.

2. Results of SCAQMD Modeling

Photochemical model simulations were conducted for the base year 2008 emissions, and futureyear 2022 baseline and controlled emissions. The ozone attainment demonstration was based on 

the deterministic approach outlined in the “Guidance on Use of Modeled Results to Demonstrate 

Attainment of the Ozone NAAQS,” EPA-454/B-95-007 (June 1996.) In addition, the weight of 

evidence analysis uses the model in a relative sense, using the relative response factor (RRF) 

technique described in the “Guidance on the Use of Models and Other Analyses for Air Quality

Goals in Attainment Demonstrations for Ozone, PM2.5, and Regional Haze,” April 2007. 

a. Modeled Attainment 

The model predicted a maximum 1-hour ozone concentration for 2022 of 125 ppb (0.125 ppm)

on June 19th at Pasadena. All other predicted concentrations during the five day episode are 

projected to be below a concentration of 0.12 ppm. The results of the attainment demonstration 

for 2022 indicate that, allowing for one day per year above the standard, the federal 1-hour ozone 

standard would be attained by 2022 at all monitors with the controlled emissions inventory. The 

attainment targets (410 tpd VOC and 150 tpd NOx) are based on both short-term and long-term 

control measures. With controls in place, it is expected that all stations in the South Coast ozone 

nonattainment area will meet the federal 1 -hour ozone standard in calendar year 2022.

b. Weight of Evidence Analysis

Relative reduction factor

The weight of evidence analysis for the ozone attainment demonstration relies on the use of sitespecific relative response factors (RRFs) being applied to the 2008 weighted design values. The 

RRFs are determined from the future year controlled and the 2008 base year simulations. The 

results of the relative response factor analysis supports the deterministic attainment 

demonstration and the level of emission reductions needed for attainment. 

The second element of the weight of evidence discussion utilizes the tiered RRF approach to 

determine station specific future year design concentrations based on base year 2008 emissions 
and 2022 controlled emissions. 3

The proposed methodology tiers the concentration threshold for 

accepting a simulation station day based on three criteria for evaluation: (1) the base year daily

maximum concentration absolute prediction error (calculated for a station per episode day) must 

be 20 percent or less; (2) the observed station concentration must be within 25 percent of the 

design value; and (3) a minimum of four station specific days simulated must meet the error at 

the set concentration threshold for the RRF to be calculated. The 20 percent error criteria is the 

same level used in the 8-hour ozone analysis and the four day minimum was iteratively

determined to provide a measure of robustness to the RRF calculation. Basically, the four day

criteria represented a balance between an analysis based on a higher error criteria with 

potentially more days included at a higher concentration threshold vs. a limited set of better 

simulated station days with lower prediction error. Table VII-5-9 lists the impacted stations and 

the threshold concentration used for the RRF calculation. Also listed in Table VII-5-9 are the 

base year average percentage prediction bias and error for those days included in the future year 

projection. Overall, the base year tendency is towards under prediction. 

 

3

P. VII-75
Air Quality Data Trends

The most recent ambient air quality data that we have reviewed indicate that the area is on track 

to attain the 1-hour ozone standard by 2022. The 1-hour ozone design value has decreased from 

23.4 expected exceedence days 2000-2002 to 5.5 expected exceedence days in 2010-2012. The 

peak 1-hour concentration has decreased from 0.169 ppb in 2002 to 0.147 ppb in 2012. The peak 

levels have decreased at all monitors throughout the South Coast Basin. 

Figure 3: South Coast Air Basin One hour Ozone Trends from 1990 to 2013 – at 

Maximum Monitoring Site.

0

20

40

60

80

100

120

140

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

3-Year Average Expected Exceedances

1-Hour Ozone Concentration (ppm)

South Coast Air Basin: 1-Hour Ozone 

 Max 1-Hour Ozone Concentration 1-Hour NAAQS (Level)

1-Hour Design Value (Expected Exceedance) 1-hour NAAQS (Form)
Figure 4: South Coast Air Basin One hour Ozone Trends from 1990 to 2013 – at All 

Monitors.
c. EPA’s Evaluation and Proposed Conclusion on the Modeling Demonstration

Our evaluation of the air quality modeling analyses and supporting information provided in the 

South Coast 2012 AQMP addressing 1-hour ozone indicate that the South Coast area will attain 

the 1-hour ozone standard by its attainment year of 2022. The appropriate episode days that 

represent days conducive to ozone formation in the South Coast Air Basin were selected. The 

modeling platform, including photochemical model, domain, number of vertical layers, and 

initial and boundary conditions most appropriate for the South Coast Air Basin, and other factors 

are appropriate. Finally, the performance of the selected modeling platform on each of the 

modeled episode days meets the performance criteria specified in EPA guidance. The modeling 

predicts attainment at all of the twenty ozone monitoring stations.

In addition to the attainment demonstration provided in the South Coast 2012AQMP, we have 

considered supplemental technical information, including ambient air quality monitoring data, 

which was not available at the time the attainment modeling was performed by SCAQMD. The 

modeling shows significant reductions in ozone from the base period. 
References:

Appendix VII, Air Quality Management Plan, 1-Hour Ozone Attainment Demonstration, 

February 2013, South Coast Air Quality Management District. 

Guideline for Regulatory Application of the Urban Airshed Model, EPA-450/4-91-013 (July

1991).

Guidance on Use of Modeled Results to Demonstrate Attainment of the Ozone NAAQS, EPA454/B-95-007 (June 1996).

Guidance for the 1-hour Ozone Nonattainment Areas that Rely on Weight-of-Evidence for 

Attainment Demonstrations, Mid-Course Review Guidance (March 28, 2002).

Guidance for Improving Weight-of-Evidence Through Identification of Additional Emission 

Reduction Not Modeled (November 99).