Technical Support Document

for the

Proposed Rulemaking Action

on the Coachella Valley 

2007 8-Hour Ozone Plan 

Air Division

U.S. EPA Region 9

September 2016
2

1. Introduction

2. Requirements for Air Quality Modeling 

3. Air Quality Modeling and Other Analyses in the 2007 AQMP 

A. Air Quality Modeling Requirements. 

B. Modeled Attainment Test for Ozone -Projected Ozone Air Quality for the Coachella 

Valley Non-attainment Area

4. Weight of Evidence

A. Air Quality Modeling and Other Analyses in the 2012 AQMP

B. Air Quality Monitoring data for the Coachella Valley

5. Evaluation and Conclusion
3

1. Introduction

This document provides supporting information and analysis for EPA’s proposed rulemaking 

action on the attainment demonstration contained in the State Implementation Plan (SIP) for the 

Coachella Valley 1997 8-hour ozone nonattainment area. The attainment demonstration, part of 

the larger SIP submittal, is contained in the South Coast Air Quality Management District‘s 

(SCAQMD) “2007 Air Quality Management Plan,” June 2007 (2007 AQMP) at chapters 5 and 8

and Appendix V. This document also: describes the Clean Air Act (CAA) requirements for an 

attainment demonstration and EPA’s regulations and policies interpreting the CAA requirements; 

summarizes portions of the 2007 AQMP that address CAA requirements, regulations and 

policies; and presents EPA’s evaluation of whether the State’s submittals meet these 

requirements. In preparing this document, EPA reviewed the attainment demonstration in the 

2007 AQMP in support of the final rule,

1

and this document refers to the technical support 

document for that action.

2. Requirements for Air Quality Modeling 

CAA section 172(c)(1) requires states to submit a plan that “shall provide for attainment of the 

national primary ambient air quality standards.” The Act requires SIPs for nonattainment areas to 

demonstrate that the area will attain the 8-hour ozone standard as expeditiously as practicable, 

but no later than the outside dates established by the Act. The Phase 2 Rule provides timing and 

guidance for this requirement for the 1997 8-hour ozone standard and identifies the modeling 

guidance available to make the demonstration.2“Severe-15” 1997 8-hour ozone nonattainment 

areas must attain the standard no later than June 15, 2019.

3 An attainment demonstration SIP 

must include technical analyses to locate and identify sources of emissions that are causing 

violations of the NAAQS within nonattainment areas, adopted measures with schedules for 

implementation and other means and techniques necessary and appropriate for attainment, and 

contingency measures required under section 172(c)(9) of the Act that can be implemented 

without further action by the State or the Administrator to cover failures to meet RFP milestones 

and/or attainment. 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 attainment demonstration requirement is a continuing applicable requirement for the 

Coachella Valley under the EPA’s anti-backsliding rules. See 40 CFR 51.1105(a)(1) and 

51.1100(o)(12). EPA has issued guidance4describing the criteria that an air quality model and 

application should meet to qualify for use in an 8-hour ozone attainment demonstration. 

 

1 Air Quality State Implementation Plans; Approvals and Promulgations: California; South Coast 1-Hour and 8-

Hour Ozone, 79 FR 52526 (September 3, 2014).

2

“Final Rule to Implement the 8-Hour Ozone National Ambient Air Quality Standard – Phase 2,” 70 FR 71612

(November 29, 2005).

3 While the attainment date is not later than June 15, 2019, EPA’s determination of attainment will be based on the

ozone season immediately preceding that date, meaning no later than 2018.

4

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

Ozone, PM2.5, and Regional Haze,” EPA-454/B-07-002, April 2007 (“Modeling Guidance”).
4

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

modeling. EPA’s guidance recognizes these limitations and provides 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 describes the 

importance of conducting a WOE analysis to determine whether an area is likely to achieve 

timely attainment, and strongly recommends that all attainment demonstrations include 

supplemental analyses beyond the recommended modeling.5 These supplemental analyses 

provide additional information such as emissions and air quality trends and monitoring data 

analyses, which strengthen the overall conclusion from the photochemical modeling. A WOE 

analysis is specifically recommended to be included as part of any attainment demonstration SIP 

where the modeling results predict future design values ranging from 82 parts per billion (ppb) to 

less than 88 ppb.6

EPA has reviewed and evaluated the air quality modeling used as a basis for the attainment 

demonstration in the 2007 AQMP. The SCAQMD completed a more recent attainment 

demonstration for EPA’s 1-hour and 1997 8-hour ozone standard in its “Final 2012 Air Quality 

Management Plan,” February 2013 (“2012 AQMP”); however, that document states that “the 

2007 AQMP adequately addressed and satisfied the CAA planning requirements for ozone in the 

Coachella Valley, and this chapter [Chapter 7: Current & Future Air Quality – Desert 

Nonattainment Areas] is for information only.” EPA is considering the attainment demonstration 

in the 2012 AQMP as part of a WOE analysis. EPA is also considering air quality monitoring 

data for the Coachella Valley from 2003 through 2015 as part of this analysis.

3. Air Quality Modeling in the 2007 AQMP 

A. Model Performance

EPA guidance lists key elements that should be discussed in an attainment demonstration.

7

EPA’s review of these elements is discussed in detail in the Technical Support Document for the 

Proposed Rulemaking Action on the South Coast 2007 8-Hour Ozone Plan and the South Coast 

Portions of the Revised 2007 State Strategy.8 There, EPA found the model application to be 

acceptable for the South Coast Air Basin portion of the 2007 AQMP. The modeling for the 

Coachella Valley attainment demonstration uses the same modeling approach as was used for the 

South Coast attainment demonstration, which was based on an air quality modeling domain that 

covers the entire South Coast Basin, the Coachella Valley, and much of southern California. The 

air quality model, meteorological model, domain size, spatial resolution, initial and boundary 

conditions selection, which were appropriate for that application, are also be appropriate in the 

adjacent Coachella Valley. The air quality model performance for the plan met EPA goals for the 

South Coast Air Basin. The Coachella Valley portion of the modeling domain is adjacent to the 

South Coast Air Basin portion of the domain, and ozone formation and transport is similar to the 

 

5 Modeling Guidance, page 98.

6 Modeling Guidance, page 17.

7 Modeling Guidance, page, 134.

8

“Technical Support Document for the Proposed Rulemaking Action on the South Coast 2007 8-Hour Ozone Plan 

and the South Coast Portions of the Revised 2007 State Strategy Air Division,” U.S. EPA Region 9, September 6, 

2011.
5

formation in the eastern part of the South Coast Air Basin. Therefore, EPA believes the model 

performance for the domain, including the Coachella Valley, is adequate.

Therefore, EPA believes, based on the review and approval of the modeling and key modeling 

elements from the 2007 AQMP, the modeling is acceptable as a basis for the attainment 

demonstration for the Coachella Valley portion of the domain. 

B. Modeled Attainment Test for Ozone - Projected Ozone Air Quality for the Coachella Valley 

Nonattainment Area

Modeling simulations were conducted for the base year emission scenarios (2002), and future 

year controlled scenarios (2017).9 The results of these scenarios are presented in Table V-4-17 of 

the 2007 AQMP, reproduced below as Table 1. The attainment demonstration shows that both 

Indio and Palm Springs will meet the federal standard in 2017.

Table 1: 2017 Projected Coachella Valley 8-Hour Ozone Design Values10

Site 2002 

Weighted 

Design 

(ppb)

2002

Baseline 

Simulation

(ppb)

2017 

Controlled 

Average 

(ppb)

Number of 

valid days

Relative 

Reduction 

Factor

2024 

Controlled

Design

(ppb)

Palm Springs 105.7 99.9 75.9 9 0.767 81.1

Indio 95.0 84.0 66.2 5 0.791 75.2

4. Weight of Evidence

A. Air Quality Modeling in the 2012 AQMP

The 2012 AQMP was not a required CAA State Implementation Plan submittal for the 1997 

ozone standard. In explaining the reason for the update, the plan explains the difficulty of 

meeting the 1997 standard by 2023 in the South Coast Air Basin, noting that the 2012 plan will 

update emission reduction commitments and control strategies.11 It also states: 

Since the Coachella Valley is not in attainment of the federal 8-hour ozone standards, this 

chapter will address the current status of ozone air quality and provide the latest 

projections of future ozone levels, based on the latest emissions inventories and modeling 

efforts.12

As previously noted, the Coachella Valley portion of the 2012 AQMP was provided for 

informational purposes, not as an update the 2007 AQMP. Therefore, the EPA is considering the 

2012 AQMP as a supplement to the 2007 AQMP in our WOE analysis of whether the area would 

 

9 2007 AQMP, Appendix V, page V-4-52 2007.

10 2007 AQMP, Appendix V, Table V-4-17.

11 2012 AQMP, page ES-2.

12 2012 AQMP, page 7-1.
6

attain the standard by the attainment date. The 2012 AQMP, which has the benefit of more recent 

emissions budgets and projected emission estimates over a shorter time span, confirmed that the 

air quality in the Coachella Valley continues to make progress toward attainment by the area’s 

2019 attainment date. Appendix V of the 2012 AQMP projects attainment by the June 15, 2019. 

The 2012 AQMP no longer projects attainment in 2017, but in 2018, stating:

The results of the ... 8-hour ozone simulations conducted for 2014 and 2019 also indicate 

that the two Coachella sites, Palm Springs and Indio will meet the federal standard by the 

2019 attainment date. The projected 2018 8-hour ozone design for the Coachella Valley 

portion of the Salton Sea Air Basin will be 84 ppb.13

B. Air Quality Monitoring data for the Coachella Valley

EPA has reviewed the most recent air quality monitoring data for the two air quality monitoring

sites in the Coachella Valley.14 The data, presented in Figure 1 and listed in Table 2, indicate that 

the 8-hour ozone design value for 2013-2015 for the Indio site is 82 ppb, which is below the 8-

hour ozone NAAQS. The 8-hour ozone design value for 2013-2015 for the Palm Springs site is 

88 ppb, which is slightly above the 8-hour ozone NAAQS. Because the design value is close to

the standard, and there has been a consistent downward trend of the ozone design values at this 

site, EPA believes the data for this site is consistent with modeled results which show that the 

area will attain the standard by the attainment date. EPA believes the most recent ambient data

support the modeling which shows that the area will achieve attainment of the 8-hour standard 

by the attainment date.

 

13 2012 AQMP, Appendix V, page V-7-27.

14 Air Quality Subsystem (AQS) Preliminary Design Value Report, dated September 7, 2016, for the Coachella 

Valley and Western Mojave Desert 
7

Figure 1: 8- Hour ozone Design Values for the Coachella Valley (ppm)

Table 2: 8- Hour ozone Design Values for the Coachella Valley (ppm)

Site

2003-

2005

2004-

2006 

2005-

2007 

2006-

2008 

2007-

2009 

2008-

2010 

2009-

2011 

2010-

2012 

2011-

2013 

2012-

2014 

2013-

2015

Indio 0.095 0.090 0.088 0.086 0.086 0.085 0.084 0.084 0.085 0.084 0.082

Palm 

Springs 0.104 0.102 0.101 0.97 0.096 0.095 0.093 0.092 0.092 0.091 0.088

5. Evaluation and Conclusion

As part of the 2007 AQMP, the modeling used for the attainment demonstration was made 

available to the public for comment, and was subject to public hearing.15 The air quality 

modeling documentation in Appendix V of the 2007 AQMP contains all the elements required 

by EPA’s modeling guidance, and is based on the best and most current information available at 

the time the 2007 AQMP was developed. EPA believes that the available modeling, evidence, 

analyses, monitoring data, and additional information, support, when taken together, support the 

conclusion that the Coachella area will reach attainment of the 1997 8-hour ozone standard by its

attainment date of June 15, 2019. The modeling shows significant reductions in ozone from the 

base period. The modeling predicts values below the 1997 8-hour ozone standard at the two

ozone monitoring stations. The more recent modeling, included as weight of evidence, is 

consistent with attainment. With the attainment date so near, EPA has also heavily weighed the 

most recent ambient data indicating that the area is on a track to achieve attainment of the 8-hour 

standard by June 15, 2019. Taking these factors together, we believe the modeling and WOE are

consistent with attainment. 

Based on our evaluation, discussed above, we propose to find that the modeling analysis used for 

the attainment demonstration in the 2007 AQMP meets the CAA section 172(c)(1) requirement 

that the plan provide for attainment of the national primary ambient air quality standards at the 

time of its submittal in 2007, and that the modeling in the 2007 AQMP provides an adequate 

basis for the reasonably available control measure, reasonable further progress, and attainment 

demonstrations in the 2007 8-Hour Ozone Plan.

 

15 The docket for EPA’s action includes proof of publication for the start of the public notice for the 2007 AQMP on 

Mach 2, 2007 and a public hearing on June 1, 2007.