Technical Support Document 

California 2007 Transport SIP 

Evaluation of Significant Contribution to Nonattainment and 

Interference with Maintenance for the 1997 8-hour Ozone NAAQS 

U.S .EPA, Region 9 

February 25, 2011 

 
EPA has evaluated the potential for California emissions to contribute significantly to 

nonattainment or to interfere with maintenance of the 1997 8-hour Ozone NAAQS at specified 

monitoring sites in Texas. 

To evaluate the potential for ozone transport from California to the “nonattainment” receptors 

that EPA identified in eastern Texas for the 1997 8-hour Ozone NAAQS as part of the Transport 

Rule Proposal (see 75 FR 45210 at 45252, August 2, 2010), we rely on the ozone conceptual 

models prepared by the Texas Commission on Environmental Quality (CEQ) for the Dallas Fort 

Worth area

1

and the Houston-Galveston-Brazoria area,

2

both of which are designated 

nonattainment for the 1997 8-hour Ozone NAAQS. See 40 CFR 81.344. These ozone 

conceptual models discuss formation of ozone in these two nonattainment areas, including 

meteorology and the potential for ozone transport from other states. These analyses include back 

trajectory modeling and are described in Section 1 below (“Evaluation of Significant 

Contribution to Nonattainment for the 1997 8-hour Ozone NAAQS”). To evaluate the potential 

for ozone transport from California to the “maintenance-only” receptors that EPA identified in 

eastern Texas for the 1997 8-hour Ozone NAAQS as part of the Transport Rule Proposal (see 75 

FR 45210 at 45253), we rely on a back trajectory analysis of air parcel pathways to those 

maintenance receptors based on air quality data for the years 2005 - 2006. The back trajectories 

are described in Section 2 below (“Evaluation of Interference with Maintenance for the 1997 8-

hour Ozone NAAQS”). 

Based on these analyses, EPA believes that the potential for emissions from California sources to 

contribute significantly to nonattainment or interfere with maintenance of the 1997 8-hour Ozone 

NAAQS in either the Dallas Fort Worth or Houston-Galveston-Brazoria nonattainment areas in 

Texas is very limited. 

 

1. Evaluation of Significant Contribution to Nonattainment for the 1997 8-hour Ozone NAAQS 

A. Dallas Forth Worth 

Appendix G of the Texas CEQs’ Dallas-Fort Worth Eight-Hour Ozone Nonattainment Area 

Attainment Demonstration includes a conceptual model of ozone formation in Appendix G, 

entitled “DFW Nonattainment Area Ozone Conceptual Model” (“DFW Ozone Model”).3The 

DFW Ozone Model provides a characterization of ozone trends, precursors, formation, and 

transport in the geographic area. It discusses the effect of meteorology on ozone formation in 

Chapter 3, Wind Meteorology and Ozone Levels, and the potential for transport in Chapter 7, 

 

1 Dallas-Fort Worth Eight-Hour Ozone Nonattainment Area Attainment Demonstration, May 23, 2007, at Appendix 

G: “DFW Nonattainment Area Ozone Conceptual Model” (“DFW Ozone Model”), available at 

http://www.tceq.texas.gov/implementation/air/sip/dfw_revisions.html#ad-sip.

2 Draft Houston-Galveston-Brazoria Nonattainment Area Ozone Conceptual Model, May 2009 (“HGB Ozone 

Model”), available at 

http://www.tceq.texas.gov/assets/public/implementation/air/am/modeling/hgb8h2/doc/HGB8H2_Conceptual_Model

_20090519.pdf. 

3

See fn. 1 above.
Ozone Transport. This information creates a comprehensive picture of not only where and when 

ozone forms, but also how and why ozone forms in the geographic area. EPA has reviewed the 

information provided in these chapters to evaluate the potential for California emission sources 

to contribute significantly to nonattainment of the 1997 8-hour Ozone NAAQS in the Dallas 

Forth Worth area. 

Back trajectories are presented in Chapter 3 of the DFW Ozone Model, Wind Meteorology and 

Ozone Levels. These back trajectories are described as follows: 

To determine how the wind patterns effect the ozone concentrations, ozone and 

meteorology data were obtained from the Environmental Protection Agency’s Air Quality 

System (AQS) database. Data from 2001 to 2003 was used for the upper winds analysis, 

and data from 1997 to 2004 was used for the surface winds analysis. Data for long range 

or aloft winds originated from the National Meteorology Center (NMC) using ETA Data 

Assimilations System (EDAS). 

Upper Level Winds 

Long-range winds tend to have a cyclic variation dependent on the season. Using the 

HYSPLIT back trajectory model, back trajectories were created for multiple years, using 

multiple start times and trajectory elevations. Back trajectories were then grouped by 

season, and trajectory density endpoint plots were created to demonstrate the seasonality 

of the upper winds. Figure 3-1 shows trajectory density plots of daily back trajectories 

over a 3-year period. Each trajectory is 48 hours long, and each day contains three 

trajectories at heights of 100, 500, and 1300 meters. Each day has three start times of 20, 

21, and 22 GMT (2:00 PM, 3:00 PM, and 4:00 PM LST). 

DFW Ozone Model at pg. 3-1. 

The back trajectories shown in Figure 3-1 on page 3-2 of the DFW Ozone Model (reproduced 

below) indicate that there is little potential for transport from California during the summer 

months, when the potential for ozone formation is generally highest. The DFW Ozone Model 

describes these trajectories as follows: 

Figure 3-1 shows that trajectories during the winter months tend to be longer and from 

the northwest, implying that wind speeds are high. This is consistent with cold fronts 

sweeping across the nation from west to east. From June to September, the DFW area 

experiences fewer cases of winds from the west, while from April to June trajectories can 

reach back as far south as the Yucatan peninsula. During the summer, the DFW area 

experiences lower wind speeds and more winds from the Gulf of Mexico. 

DFW Ozone Model at pg. 3-1. 
Figure 3-1: Back Trajectory Densities Demonstrating Seasonal Variation. 
In addition, an analysis of back trajectories for high and low ozone days, presented in Figure 3-2 

on page 3-3 of the DFW Ozone Model (reproduced below), indicates that there is little potential 

for transport from California on high ozone days. The DFW Ozone Model describes these 

trajectories as follows: 

Figure 3-2 shows back trajectory density plots for high and low eight-hour ozone days 

from 2001 to 2004. Each trajectory is 36 hours long, centered at Dallas Hinton St., and 

has a start time of 21 GMT (3:00 PM LST). The top plot shows back trajectory densities 

for all days, and the bottom plot shows the back trajectories for days when the ozone was 

greater than 85 ppb. Figure 3-2 shows that on days when eight-hour ozone concentrations 

are at least 85 ppb, back trajectories tend to have a more easterly and southeasterly 

component. Trajectory lengths also tend to be shorter, implying that wind speeds are 

slower; slow wind speeds are conducive to higher ozone production. Trajectories for all 

days are longer, implying higher wind speeds. Also, wind directions on all days tend to 

be more varied, with only a slight westerly component. 

* * * 

During the summer months, when ozone concentrations are higher, the DFW area’s upper 

winds have a more southerly contribution, a slight easterly contribution, and less of a 

westerly contribution. The wind field analysis showed that both upper level and surface 

winds generally come from the southeastern directions during the ozone season. High 

ozone days tend to have slower winds that come from a southeasterly direction. 

DFW Ozone Model at pp. 3-2, 3-8. 

Chapter 7 of the DFW Ozone Model (Ozone Transport) describes ozone transport as follows: 

Because the DFW area is located far from the Texas borders, it is difficult to determine 

whether transport into the area is from out-of-state sources or in-state sources. In order to 

determine the extent to which the area is effected by transport, the background ozone and 

the upwind ozone were investigated. 

DFW Ozone Model at pg. 7-1. 

The conclusion to Chapter 7 indicates that California is not among the states likely to affect 

ozone concentrations in the Dallas ForthWorth area: 

Conclusions 

These studies on transport in the DFW area show similar results. The background ozone 

analysis and the upwind/downwind analysis both show that background, or upwind, 

ozone concentrations in the DFW area are quite high and that they are even higher on 

eight-hour ozone exceedance days. The high ozone levels could be due to transport from 

other regions, but it cannot be distinguished whether or not the transport emanates from 

in-state sources, out-of-state sources, or the recirculation of the air mass over the DFW 

area. The incremental probability analysis showed that the states likely to affect the ozone 

concentrations in the DFW area are Texas, Oklahoma, Louisiana, Arkansas, Mississippi, 

Georgia, Tennessee, Missouri, Illinois and Kentucky. Although these studies have shown 

that transport affects the DFW area, the upwind and downwind analysis shows that the 

local influences are large enough to drive the eight-hour ozone concentrations above 

exceedance levels. 

DFW Ozone Model at pg. 7-11. 

In sum, the analysis of the effects of meteorology on ozone formation and the potential for ozone 

transport provided in chapters 3 and 7 of the DFW Ozone Model support a conclusion that 

California emission sources do not contribute significantly to nonattainment of the 1997 8-hour 

Ozone NAAQS in the Dallas Forth Worth area. 

B. Houston-Galveston-Brazoria Nonattainment Area 

EPA evaluated the Texas CEQ’s May 2009 Draft Houston-Galveston-Brazoria Nonattainment 

Area Ozone Conceptual Model (“HGB Ozone Model”),4which includes detailed analyses of 

emissions patterns across time and space, and extensive examination of background and 

transport phenomena. Chapter 4 of the HGB Ozone Model (Recent Findings in Background 

Ozone and Ozone Transport Into the Houston-Galveston-Brazoria Area) discusses long range 

transport into the Houston-Galveston-Brazoria area. In particular, Section 4.2, Long Range 

Transport Trends, discusses the back trajectory approach that was used to investigate the patterns 

of long range transport of ozone into Texas as follows: 

 

4

 See fn. 2 above.
This section investigates patterns in continental transport of ozone into Texas. The EPA 

AIRNow website reports daily national ozone levels. Eight-hour daily ozone 

concentrations from AIRNow, for May through September, 2004 through 2008, were 

used to identify days on which transport of ozone into Texas was likely. Candidate days 

were selected by examining AIRNow ozone plots for days when ozone over 85 ppb 

occurred in multiple states upwind of Texas, generally as far east as the Midwestern U.S. 

For these days, back trajectories were created using the National Oceanic and 

Atmospheric Administration (NOAA) HYSPLIT (Hybrid Single Particle Lagrangian 

Integrated Trajectory) (Draxler, et. al, 2003) model and EDAS (Eta Data Assimilation 

System) (Rolph, 2003) data for each identified day, to verify whether transport was 

indeed possible from the Midwest. 

HGB Ozone Model at pg. 4-7. 

Chapter 4 presents an analysis of long range transport through a back trajectory analysis during 

the ozone season (May through October) for the eight years from 2000-2007. Section 4.4 of the 

HGB Ozone Model (Transport into the Houston-Galveston-Brazoria Area) describes this 

analysis as follows: 

Previous sections have identified that, while transport plays a role in elevated ozone 

levels in the HGB area, the level of transport may not significantly change from year to 

year. It is important to be able to estimate the levels of ozone transported into the HGB 

area so that the amount of ozone than can be controlled (locally produced ozone) is 

identified. Studies done for the TCEQ (Sullivan, 2009) used HYSPLIT back trajectories 

along with EDAS to estimate the amount of ozone entering into the HGB area. 

Numerous 72-hour back trajectories were run starting at 12:00 CST in central Houston, at 

300 meters above ground level, during the peak of ozone season in the HGB area (May 1 

to October 31, 2000 to 2007). Use of multiple trajectories can distinguish between source 

areas, or paths, that lead to higher than average, or lower than average ozone 

concentrations (Sullivan, 2009). A clustering algorithm was used to group multiple 

trajectories based on trajectory size and shape. Six identified clusters appear in Figure 4-

16. Forty back trajectories that did not end up in any of the six clusters, illustrated in 

Figure 4-17, were grouped and assigned a cluster number of zero Note that all but one of 

the back trajectories in cluster zero originate from the south and that these trajectories 

appear to extend beyond the EDAS modeling domain, which is the only difference 

between cluster zero and cluster two. 

Figure 4-16 shows that cluster one and cluster two represent days in which winds 

originated from the Gulf of Mexico; these trajectories typically represent cleaner air 

coming from the Gulf and hence lower observed ozone concentrations in the HGB area. 

Cluster three represents long trajectories which come into the HGB area from the north as 

far away as Canada; this type of trajectory can also bring clean air from northern states 

and Canada into the HGB area. Cluster four represents stagnant air conditions, a frequent 

cause of ozone accumulation. Cluster five shows trajectories that originate from the 

northeast, in the Ohio River Valley, that bring polluted air from more populated areas 
into the HGB area. Cluster six illustrates trajectories from the east, some of which cross 

over Louisiana and the Beaumont-Port Author (BPA) area, and some of which cross over 

the Gulf. 

HGB Ozone Model at pg. 4-14. 

Figure 4-16 of the HGB Ozone Model (Six Clusters Found Using HYSPLIT Back Trajectory 

Clustering Algorithm), reproduced below, illustrates six sets of back trajectories starting at 18:00 

UTC that were created for each day from May 1 to October 31, 2000 to 2007. The number of 

back trajectories in each cluster is found at the top of each map. 



In addition, Figure 4-17 of the HGB Ozone Model (Cluster 0 Back Trajectories), reproduced 

below, illustrates additional back trajectories that did not end up in any of the previous six 

clusters. 

 

None of the back trajectories in Figures 4-16 or 4-17 indicate a potential for ozone transport from 

California to the Houston-Galveston-Brazoria area. 

Chapter 6 of the HGB Ozone Model, Conclusions and Future Research, states as follows: 

Investigation of HYSPLIT back trajectories identified likely transport of ozone into 

Texas on days following elevated ozone episodes in eastern U.S. states. Further 

examination of state-wide design values for Texas, and eastern states where high ozone 

likely originates, revealed clear downward trends from 2000 to 2008. This may lead to 

lower background levels of ozone being transported into Texas from eastern states, but 

there was insufficient evidence to support this conclusion based on the lack of trend in 

background ozone values. 
HGB Ozone Model at pg. 6-7. 

In sum, although the analyses provided in the HGB Ozone Model indicate a downward trend in 

ozone transport from likely sources in eastern states, none of the several hundred back trajectory 

analyses for the Houston-Galveston-Brazoria nonattainment area provided in the HGB Ozone 

Model identifies a potential for ozone transport from California. As such, this analysis supports 

a conclusion that California emission sources do not contribute significantly to nonattainment of 

the 1997 8-hour Ozone NAAQS in the Houston-Galveston-Brazoria area. 

2. Evaluation of Interference with Maintenance for the 1997 8-hour Ozone NAAQS 

To evaluate the impact of wind direction on ozone transport from California to the 

“maintenance-only” receptors that EPA identified in eastern Texas for the 1997 8-hour Ozone 

NAAQS as part of the Transport Rule Proposal (see 75 FR 45210 at 45253, August 2, 2010), we 

rely on the results of an EPA Region 8 back trajectory study, including a set of trajectories with 

end points at those maintenance receptors. EPA generated these trajectories using the HYSPLIT 

4.9 online computer application, selecting the archived Eta Data Assimilation System (EDAS) 

meteorological data sets with the highest degree of resolution (40 km). Back trajectories were 

run for the days in 2005 and 2006 when ozone concentrations at these receptors exceeded the 

1997 8-hour NAAQS--i.e., when monitored ozone concentrations were 85 ppb or above. 

Exceedance days were identified using the Air Quality System (AQS), EPA's repository of 

monitored ambient air quality data. At each monitor, trajectories were started at 22 Coordinated 

Universal Time (UTC), equivalent to 4 p.m. CST, and were run backwards in time for 72 hours 

(three days). The trajectory height at the starting point is 1500 meters above ground level. These 

maps indicate that air parcel pathways from California and ending at nonattainment receptors do 

not exist during the three days preceding ozone exceedances at these receptors. 

The back trajectory analysis for each of these days is presented in Appendix A: Back 

Trajectories, Texas and California Maintenance-Only Monitors, 2005-2006, 1997 Ozone 

NAAQS Exceedance Days.5 Because back trajectory analysis results map pathways of air 

parcels that may or may not transport pollutants, they cannot be considered determinative as to 

the transport of ozone and its precursors, or of the absence of such transport. However, the fact 

that air parcel trajectories to the Texas maintenance receptors do not originate in California 

strongly supports a conclusion that California sources of ozone precursor emissions do not 

interfere with maintenance of the 1997 8-hour ozone NAAQS at these locations. 

 

5 Appendix A: Back Trajectories, Texas and California Maintenance-Only Monitors, 2005-2006, 1997 Ozone 

NAAQS Exceedance Days, U.S. EPA Region 8, available at http://www.regulations.gov, Docket ID # EPA-R08-

OAR-2007-1035.