Modeling Guidance and Examples for
Commonly Asked Questions (Part II, Ozone)
Advanced Air Permitting Seminar October 16, 2014
I.
Purpose of Presentation
To provide guidance to Texas Commission on Environmental Quality (TCEQ) customers
on when and how to conduct an ozone impacts analysis. Examples of a quantitative and
qualitative demonstration are provided.
II.
Background Information on Types of Ozone
Ozone is a gas formed in the atmosphere when three atoms of oxygen combine. Ozone is
found high in the Earth’s upper atmosphere and at ground level. Ozone has the same
chemical structure wherever it occurs.
Stratospheric ozone occurs naturally in the Earth’s upper atmosphere where it forms a
protective layer that shields us from the sun’s harmful ultraviolet rays. This is good
ozone.
Ground-level ozone is not emitted directly into the air but is created by chemical
reactions between nitrogen oxides (NOX) and volatile organic compounds (VOCs) in the
presence of sunlight. This is bad ozone.
III.
Background Information on Ground-level Ozone
Ground-level ozone is the main component of smog. The major man-made sources of
NOX and VOCs that form ozone are emissions from industrial facilities, electric utilities,
motor vehicle exhaust, gasoline vapors, and chemical solvents.
Ground-level ozone occurs most frequently during the summertime because sunlight
and hot weather accelerates its formation.
Ozone is likely to reach unhealthy levels on hot sunny days in urban environments.
Ozone can also be transported long distances by wind. For this reason, even rural areas
can experience high ozone levels.
IV.
Ozone Standards
The Clean Air Act requires the Environmental Protection Agency (EPA) to set National
Ambient Air Quality Standards (NAAQS) for ozone and five other pollutants considered
harmful to public health and the environment. Ozone standards have been revised over
the years. The EPA by law is required to review the standards and their scientific basis
every five years to determine whether revisions are appropriate.
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The current primary and secondary 8-hour ozone standards are 75 parts per billion
(ppb). The 75 ppb design value is based on the 3-year average of the annual fourth
highest daily maximum 8-hour concentration.
V.
When an Ozone Impacts Analysis Is Required
When is an ozone impacts analysis required? That depends on the type of review
required once an application is submitted. If the proposed project is major, will the
application go through a non-attainment review or a Prevention of Significant
Deterioration (PSD) review?
VI.
What Areas in Texas are in Non-attainment for Ozone
First, it must be determined if the proposed project is located in an area that is in
attainment or non-attainment for ozone. Currently, there are 18 counties in Texas that
are in non-attainment for ozone. These counties are located in the Dallas-Fort Worth
area and the Houston-Galveston-Brazoria area.
VII.
Non-attainment Reviews
If it is determined that the proposed project will go through an ozone non-attainment
review, then an ozone impacts analysis is not required. This is because emissions of
NOX and/or VOCs are offset in order to improve the air quality in the airshed. In other
words, the emissions of NOX and/or VOCs are reduced at a larger amount than what the
project is proposing to emit.
VIII.
PSD Reviews
If the proposed project is located in an area that is in attainment for ozone and is major
for NOX and/or VOCs, then an ozone impacts analysis may or may not be required. If
the NOX and/or VOCs emissions for the proposed project are less than 100 tons per year
(tpy), then an ozone impacts analysis is not required. If the NOX and/or VOCs
emissions for the proposed project are greater than 100 tpy, then an ozone impacts
analysis is required.
IX.
What Criteria Are Included in an Ozone Impacts Analysis
The ozone impacts analysis should include a monitor that is representative of the
proposed project site. Details on how to determine if a monitor is representative will
not be discussed since the earlier presentations went through those criteria.
The ozone impacts analysis should make clear if the area around the proposed project
site is NOX-limited or VOC-limited. Being a NOX-limited area means that VOC
concentrations in the area are greater than NOX concentrations, and ozone formation is
more effectively reduced by lowering current and future NOX emissions rather than
lowering emissions of VOCs. Being a VOC-limited area means that NOX concentrations
in the area are greater than VOC concentrations, and ozone formation is more effectively
reduced by lowering current and future VOC emissions rather than lowering NOX
emissions. In areas where TCEQ has conducted photochemical modeling for SIPs, most
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urban and rural areas of Texas are NOX-limited. To demonstrate if the proposed project
site is NOX-limited or VOC-limited , it is recommended that the applicant obtain
photochemical modeling representative of the proposed project site or obtain
representative VOC-to-NOX ratios from a representative monitor to support that
determination. Photochemical modeling from the TCEQ can be obtained from the
TCEQ’s Air Quality Division Air Modeling Team (contact at 512-239-1459).
Photochemical modeling from other sources may be used as well.
The next aspect of the ozone impacts analysis will be for the applicant to demonstrate
compliance by either a quantitative or qualitative demonstration.
X.
Quantitative Demonstrations
The EPA has not developed and/or recommended a near-field model that includes the
necessary chemistry algorithms to estimate ozone formation from the reaction of NOX
and VOCs in the presence of sunlight. However, there are two methods for conducting a
quantitative demonstration for ozone. The first method is for the applicant to conduct
photochemical modeling. This is typically done using the Comprehensive Air Quaility
Model with extensions (CAMx) model. The CAMx model simulates air quality over
many geographic scales. The model treats a wide variety of inert and chemically active
pollutants, such as ozone. Photochemical modeling is not required by regulation, but it
can be performed in support of the ozone impacts analysis to determine the ozone
impacts from a proposed project and to quantify its potential impact on ozone
concentrations within the surrounding area. If an applicant chooses this approach, the
applicant should follow EPA guidance for conducting photochemical modeling.
The second method is a demonstration based on comments by the EPA utilizing the
refined air dispersion model, AERMOD. The screening approach is only appropriate for
NOX-limited areas, which as previously mentioned, is the case for most areas in Texas.
Therefore, in most cases, this approach could be used.
XI.
Criteria for Screening Approach Using AERMOD
The screening approach using AERMOD is a conservative analysis based on the
proposed project’s NOX modeling. The steps that are involved in the demonstration
consist of:

Determining if the project is NOX-limited or VOC-limited;

If VOC-limited, determining the GLCmax at a distance of 10-11 kilometers (km)
downwind from the proposed source since it takes time for the NOX emissions
to react to generate ozone;

Conservatively assuming a 90 percent conversion of NOX to nitrogen dioxide
(NO2);

Assuming the emission source would have an average ozone yield of three
ozone molecules per NOX molecule (Luria, Menachem, Imhoff, Robert E.,
Valente, Ralph J., and Tanner, Roger L., 2003, Ozone yields and production
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efficiencies in a large power plant plume, Atmospheric Environment 37
3593-3603);
XII.

Adding the result of the analysis to a representative monitored concentration;
and

Finally, comparing the result to the standard.
Example of Screening Approach Using AERMOD
The screening approach using AERMOD begins by obtaining a representative monitor
concentration. The bluish-green object highlighted in Slide 12 represents the property
line of the project site.
XIII.
Example of Screening Approach Using AERMOD (Cont.)
The bluish-green object highlighted on Slide 13 represents the representative monitor
that was selected for this particular project. As you can tell from this slide and the
previous slide, the areas surrounding the two objects are very similar. However, the
applicant provided additional information to justify the use of this monitor, such as a
quantitative assessment of the NOX and VOC emissions within 10 km of the project site
and the monitor, county emission comparisons, and population comparisons.
The representative ozone concentration based on the 3-year average of the 4th highest
daily maximum 8-hour concentration from the most recent consecutive three years was
69 ppb.
XIV.
Example of Screening Approach Using AERMOD (Cont.)
Next, receptors were placed 10-11 km from the proposed project to determine the
GLCmax using AERMOD.
XV.
Example of Screening Approach Using AERMOD (Cont.)
The result from the AERMOD run of the project NOX emissions was 2.96 micrograms
per cubic meter (µg/m3), and when converted to parts per billion, the GLCmax is 1.57
ppb.
XVI.
Example of Screening Approach Using AERMOD (Cont.)
Assuming 90 percent conversion of NOX to NO2 and three molecules of ozone for every
molecule of NOX, the predicted concentration is conservatively estimated to be 4.24 ppb.
Now adding the predicted concentration to the representative monitored concentration,
the result is 73.24 ppb, which is less than the 8-hour ozone standard of 75 ppb.
XVII.
Qualitative Demonstrations
A qualitative demonstration should consist of an assessment of the current air quality
where the proposed project will be located and an analysis of the potential ozone impact
from the proposed project.
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When assessing the current air quality, the applicant may want to show what the trends
are like for ozone, NOX, and VOC in the area where the proposed project will be located.
This can be accomplished by looking at historical and current monitoring data as well as
emissions inventory data.
To analyze the potential ozone impact from the proposed project, the applicant should
review existing photochemical modeling analyses that represents the area of the
proposed project. Some things to consider when selecting an analysis are as follows:
XVIII.

Was EPA guidance followed when the modeling was conducted?

Are the sources used in the modeling similar to the proposed project?

Are the meteorological conditions representing the modeling similar to the
proposed project?
Example of a Qualitative Demonstration
For this example, the qualitative demonstration is for a liquefaction facility to liquefy
natural gas for export from an existing terminal and was made to show that the
proposed project would not cause or contribute to a violation of the ozone NAAQS. The
location of the proposed project was the Beaumont/Port Arthur (BPA) area. The BPA
area includes Hardin, Jefferson, and Orange Counties. The BPA area is currently in
attainment for all criteria pollutants, including ozone.
This analysis begins by showing the ozone trend for the area. The graph on Slide 18
shows the ozone design values for all the ozone monitoring sites in the BPA area from
1992-2013. The downward trajectory shows that the air quality for ozone has improved
over the years.
XIX.
Example of a Qualitative Demonstration (Cont.)
The chart on Slide 19 shows the summary of NOX emissions in the BPA area. The first
column represents the NOX emissions in tons per day from the 2005 National Emissions
Inventory (NEI). The column breaks down the categories of NOX emissions, which
include non-road mobile, on-road mobile, area, and point sources. The second and
third columns similarly represent the NOX emissions from the 2008 and 2011 NEI data,
respectively. The chart shows that NOX emissions have declined significantly since
2005.
XX.
Example of a Qualitative Demonstration (Cont.)
For the chart on Slide 20, the maximum NOX concentration across all NOX monitors was
obtained to evaluate the trend in NOX concentrations for the BPA area from 1998-2013.
The chart shows that regional NOX levels in the BPA area have decreased. Reductions in
NOX emissions can be attributed to air quality regulations and control strategies that
have targeted both stationary and mobile sources over the last three decades.
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XXI.
Example of a Qualitative Demonstration (Cont.)
Similar to the evaluation of the NOX trend, the chart on Slide 21 shows the summary of
VOC emissions in the BPA area. The three columns represent the VOC emissions in
tons per day from the 2005, 2008, and 2011 NEI. The chart shows that VOC emissions
have declined since 2005 with the greatest decline coming in 2011.
XXII.
Example of a Qualitative Demonstration (Cont.)
In the next two slides, the applicant reviewed ethylene and propylene emissions in the
BPA area to assess trends in VOC emissions. For this analysis, the applicant noted that
neither ethylene nor propylene are assumed to be surrogates for total VOC; however,
these two chemicals are the most reactive and abundantly available species of VOC
emitted by man-made sources in the BPA area. This is based on a 2012 study for ozone
formation and accumulation in the BPA area. Since these two chemicals are primarily
emitted from point sources, the applicant made the case that the trend in ethylene and
propylene are good indicators of trends in VOC emissions released to the atmosphere
from point sources. The graph on Slide 22 shows the trend in ambient ethylene levels
from 1997-2013.
XXIII.
Example of a Qualitative Demonstration (Cont.)
The graph on Slide 23 shows the trend in ambient propylene levels from 1996-2013. As
was the case with the decline in NOX emissions in the BPA area, reduction in VOC
emissions can be attributed to air quality regulations and control strategies
implemented over the last three decades.
XXIV.
Example of a Qualitative Demonstration (Cont.)
The previous slides of the qualitative demonstration example dealt with assessing the air
quality of the BPA area and illustrating the declining trends of ozone and precursor
emissions. The applicant then supplemented the demonstration with review of
photochemical modeling that included the BPA area. The photochemical modeling was
conducted in support of an air permit near the BPA area to address concerns regarding
ozone formation and was subsequently accepted by the EPA (i.e. the photochemical
modeling followed EPA guidance and was deemed appropriate). For the example on
Slide 24, the photochemical modeling project was conducted for a site that is a few
kilometers away from the proposed project.
The photochemical modeling project was conducted to construct a liquefaction facility
to liquefy natural gas for export from the existing terminal. As stated earlier, the
qualitative demonstration for the proposed project was for a liquefaction facility to
liquefy natural gas for export from the existing terminal. Therefore, using the existing
photochemical modeling to represent the proposed project is an appropriate approach.
Obviously, the fact that photochemical modeling was done for the same type of project
and was located nearby is atypical for most projects, but the methodology and concepts
used to conduct the evaluation can generally be applied to other projects.
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The applicant went on to compare the types of sources from the two projects. The
photochemical modeling project consisted of the following sources:

24 natural gas-fired refrigeration compressor turbines;

4 acid gas vents;

1 marine flare;

2 wet gas flares;

2 dry gas flares;

2 natural gas-fired generator turbines; and

2 emergency generators.
The proposed project consisted of the following sources:

6 natural gas-fired refrigeration compressor turbines;

1 liquefied natural gas storage LP flare;

1 wet/dry gas ground flare;

1 auxiliary boiler;

4 thermal oxidizers;

7 diesel generators;

1 natural gas-fired essential generator; and

1 blowdown vent.
Both projects have similar sources, which is expected since both projects represent the
same types of facilities.
XXV.
Example of a Qualitative Demonstration (Cont.)
Slide 25 shows how the applicant went on to compare the potential NOX emissions from
each project. The photochemical modeling project had approximately four times the
amount of NOX than the proposed project.
XXVI.
Example of a Qualitative Demonstration (Cont.)
To support the conclusions that the photochemical modeling project is representative of
the meteorological parameters and regional transport criteria, the applicant analyzed
various meteorological parameters based on the North American Meso (NAM) 12-km
gridded data assimilation accessed from the National Oceanic and Atmospheric
Administration (NOAA) Air Resources Laboratory READY website
(http://ready.arl.noaa.gov/index.php). Slide 26 shows the comparison of the two
projects for surface pressure and relative humidity. Although hard to tell from the slide,
the black star in the map represents the proposed project and the black diamond
represents the photochemical modeling project.
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XXVII.
Example of a Qualitative Demonstration (Cont.)
Slide 27 shows the comparison of the two projects for surface roughness and
temperature.
XXVIII.
Example of a Qualitative Demonstration (Cont.)
Slide 28 shows the comparison of the two projects for wind vectors and wind velocity.
XXIX.
Example of a Qualitative Demonstration (Cont.)
The applicant went on to discuss the results of the photochemical modeling. The
predicted impacts at monitors in the BPA area ranged from 0.1-0.5 ppb for the project
allowable case, which represents the base case with the estimated allowable emissions
and sources from the photochemical modeling project. At areas removed from
monitors, the model predicts impacts to be less than one ppb either on land or offshore.
Therefore, the applicant concluded that the proposed project is expected to be
insignificant for ozone based on the current air quality and the photochemical modeling,
which demonstrates that the same type of project with approximately four times as
much NOX is insignificant.
XXX.
Questions
Any questions regarding particulate matter with a diameter less than or equal to 2.5
microns (PM2.5) or ozone?
XXXI.
Contact Information
Justin Cherry’s and Reece Parker’s contact information:

Justin Cherry – 512-239-0955; justin.cherry@tceq.texas.gov

Reece Parker – 512-239-1348; reece.parker@tceq.texas.gov
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