In this paper, the results of CAMx modeling analyses as well as monitoring data to explore the general characteristics and patterns for the 8-hour averaging ozone concentrations, as compared to those of the 1-hour averaging ozone concentrations, in an example ozone non-attainment area will be discussed.

1. Modeling Software for EHS Professionals
Utilizing CAMx Modeling Analyses to
Explore 8-hour Ozone Attainment
Strategies
Prepared By:
Weiping Dai
Brian Wulf
BREEZE SOFTWARE
12700 Park Central Drive,
Suite 2100
Dallas, TX 75251
+1 (972) 661-8881
breeze-software.com

2. 1
Utilizing CAMx Modeling Analyses to Explore 8-hour
Ozone Attainment Strategies
Weiping Dai
Trinity Consultants
12770 Merit Drive, Suite 900, Dallas, TX 75251
Email: wdai@trinityconsultants.com
Brian Wulf
Trinity Consultants
174 Clarkson Road, Suite 100, Ellisville, MO 63011
Email: bwulf@trinityconsultants.com
ABSTRACT
On June 15, 2005, the new 8-hour ozone National Ambient Air Quality Standard (NAAQS)
replaced the 1-hour ozone standard. This follows the designation of nonattainment areas for
the 8-hour ozone standard that took effective on June 15, 2004. Each 8-hour ozone
nonattainment area must develop an implementation plan outlining control measures to bring
the area back into attainment. While some of the 8-hour ozone nonattainment counties were
also part of 1-hour ozone nonattainment areas in the past, the necessary attainment strategies
for the 8-hour ozone standard can differ from those for the 1-hour ozone standard,
considering that the photochemical mechanisms for ground-level ozone formation are
complex and the elevated ground-level ozone concentrations typically only occur during
daytime hours in hot summer days.
In this paper, the results of CAMx modeling analyses as well as monitoring data to explore
the general characteristics and patterns for the 8-hour averaging ozone concentrations, as
compared to those of the 1-hour averaging ozone concentrations, in an example ozone non-
attainment area will be discussed. In particular, the CAMx modeling analysis techniques will
be discussed and demonstrated for evaluating the effectiveness of emission control measures.
Furthermore, possible attainment strategies for the 8-hour ozone standard will be discussed.
INTRODUCTION
Ground-level ozone formed through complex photochemistry is one of the criteria pollutants
in the United States for which a National Ambient Air Quality Standard (NAAQS) is
established. Originally, the U.S. Environmental Protection Agency (EPA) established a
1-hour ozone standard at 0.12 parts per million (ppm). An area violates this standard when
1-hour average readings at any one monitor equal or exceed 125 parts per billion (ppb) more
than 3 times during any three-year period. On June 15, 2005, a new 8-hour average ozone

3. 2
NAAQS of 0.08 ppm replaced the 1-hour ozone standard. This follows the designation of
nonattainment areas for the 8-hour ozone standard that took effective on June 15, 2004. An
area violates the 8-hour standard when the three-year average of each year’s fourth highest
daily 8-hour average reading at the controlling monitor equals or exceeds 85 ppb. In essence,
the new 8-hour average ozone standard is different from the old 1-hour average standard on
several aspects: (1) the averaging period is different (8 hours vs. 1 hour); (2) the numerical
value of the standard is different (0.08 ppm vs. 0.12 ppm); and (3) the compliance form is
also different (“three-year average of each year’s fourth highest daily maximum 8-hour
average” vs. “4th
1-hour highest high during any three-year period”). Due to these
differences, as shown in Figure 1 (developed by EPA as of April 15, 2004), there are many
8-hour ozone nonattainment areas that were in compliance with the 1-hour ozone standard.
Note that the current designation of nonattainment areas may be slightly different from those
shown in Figure 1 due to more recent reclassification/redesignation.1
In establishing the new 8-hour ozone NAAQS, the EPA concluded that the 1-hour ozone
standard did not adequately protect the public from adverse health effects. Of particular
concern in revising the standard were numerous scientific studies linking decreased lung
function and increased incidence of respiratory ailments with long-term exposure at ozone
concentrations lower than 0.12 ppm. The three-year average of the fourth-highest daily
maximum 8-hour concentration compliance form was established to provide greater stability
in the designation of areas. Since the previous standard was solely based on the number of
exceedances; all exceedances, regardless of size, were counted equally in attainment
demonstration. The 8-hour ozone considers both the frequency and concentration of peak
ozone values in determining attainment.2

4. 3
Figure 1. Comparison of 1-hour vs. 8-hour Ozone Nonattainment Areas
Ground-level ozone is a secondary pollutant formed by precursors, e.g., nitrogen oxides
(NOx) and volatile organic compounds (VOC), through complex atmospheric photochemical
reactions under certain conditions (e.g., hot summer daytime) in urban areas. The
phenomenon is also called urban smog. The ground-ozone precursors are contributed from
both biogenic sources (e.g., emissions of organic compounds from trees) and anthropogenic
sources (typically grouped into on-road, non-road, point, and area sources). Each 8-hour
ozone nonattainment area must develop an implementation plan outlining control measures
to bring the area back into attainment pursuant to the attainment schedule based on the
nonattainment classification. While some of the 8-hour ozone nonattainment counties were
also part of 1-hour ozone nonattainment areas in the past, the necessary attainment strategies
for the 8-hour ozone standard can differ from those for the 1-hour ozone standard,
considering that the photochemical mechanisms for ground-level ozone formation are
complex and the elevated ground-level ozone concentrations typically only occur during
daytime hours in hot summer days.
Deleted: Page Break

5. 4
Based on the consideration of potential ozone formation due to the emission source
characteristics and other regional/local conditions (e.g., land use and terrain characteristics,
meteorological conditions, and ozone transport outside a nonattainment area), the
effectiveness of potential control measure options with the objective of reducing the ozone
concentrations in a nonattainment area can be developed and evaluated with a photochemical
dispersion model (e.g., CAMx). The following sections will examine the general pattern and
characteristics of 8-hour average ozone in a non-attainment area and explore the potential
strategies to develop effective control measures. The example presented in the paper is based
on available data for the Dallas/Fort Worth (DFW) nonattainment area.3
GENERAL OZONE PATTERNS IN THE DFW AREA
Elevated ground-level ozone concentrations are typically observed in large urban areas
during hot summer daytime (e.g., ozone season from May through September). Formation of
ground-level ozone is a complex atmospheric photochemistry phenomenon illustrated in
Figure 2. Overall, the NOx and VOC are the major chemical precursors of ground-level
ozone.
VOC
CO
Sources
NO
NO2
RONO2 N2O5HNO3
H2O2
O3
NO3
-
NOz
NOx
NOyO3
HO2
RO2
OH
RO2
O3
hv
OH
Removal
VOC
CO
Sources
NO
NO2
RONO2 N2O5HNO3
H2O2
O3
NO3
-
NOz
NOx
NOyO3
HO2
RO2
OH
RO2
O3
hv
OH
Removal
Figure 2. Illustration of Photochemistry for Ground-level Ozone Formation
Due to the variation and availability of precursors in the atmosphere and the conduciveness
of the atmospheric conditions for ozone formation, ground-level ozone concentration in an
urban area typically show a diurnal pattern. For example, in the DFW area, the ozone
episode of August 16-22, 1999 was selected for the 8-hour ozone attainment demonstration.
Figure 3 presents the observed 1-hour and 8-hour average ozone concentrations on August
19, 1999 at the monitor location with the highest 8-hour ozone design value. This figure
demonstrates a general diurnal pattern of both 1-hour and 8-hour average ozone
concentrations in an area. Typically, the hourly ozone concentration starts to increase early
morning (e.g., 8-9am), reaches a peak concentration in the afternoon, and then gradually
decreases to a nighttime level.

6. 5
0
20
40
60
80
100
120
140
160
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour of 08/17/1999
OzoneConc.(ppb)
1-Hour Ozone
8-hour Ozone
Figure 3. Daily Change of 1-hour and 8-hour Ground-level Ozone
Table 1 documents the statistics developed with the observed data of the 1999 DFW ozone
episode. These statistics show that the 1-hour daily high concentration may occur in a wider
range (as early as 12pm or as late as 5pm) than the 8-hour daily high concentration (whose
last average hour is typically 6pm or 7pm). In other words, the period for the 8-hour daily
high ozone concentration is typically from late morning (e.g., 11am or 12pm) to early
evening (e.g., 6pm or 7pm). The ratio of the 8-hour to 1-hour daily high ozone ranges from
85% to 95%.
Table 1. Statistics of 1-Hour & 8-Hour Average Ozone Concentrations
Parameter 1-Hour Ozone 8-Hour Ozone
Time with Highest Concentration 12pm -5pm
(6 hours span)
6pm-8pm
(3 hours span)
Range of Daily Highest 74-150 ppb 66-126 ppb
8-Hour/1-Hour Daily High Ratio 84-95%
Some other interesting aspects to examine regarding the 1-hour and 8-hour average ozone are
illustrated in Figures 4 and 5.4
Figure 4 shows the change of the 1-hour and 8-hour average
design values in the DFW area over the past 2 decades. The trend lines on the figure imply
that the 8-hour ozone concentration decreases slower than the 1-hour ozone concentration in
responding to the overall change of ozone formation conditions in the area (e.g., emission

7. 6
reductions) over time. Figure 5 presents the number of days in a year that exceed the1-hour
or 8-hour average NAAQS value. Apparently, the number of days exceeding the 8-hour
ozone standard is much greater than that for the 1-hour ozone standard.
80
100
120
140
160
180
200
1977 1982 1987 1992 1997 2002
Year
OzoneDV(ppb)
1-hour DV
8-hour DV
Figure 4. Change of 1-Hour and 8-Hour Ozone Design Values in the DFW Area

8. 7
0
5
10
15
20
25
30
35
40
1996 1997 1998 1999 2000 2001 2002
Year
ExceedanceDays_
1-hour Ozone
8-hour Ozone
Figure 5. Change of 1-Hour and 8-Hour Ozone Exceedances in the DFW Area
CONTROL STRATEGIES WITH PHOTOCHEMICAL MODELING
Performing a photochemical modeling analysis with appropriate techniques is essential for
ozone nonattainment areas to evaluate the effectiveness of potential control measures and
demonstrate attainment. Ozone formation through reactions of various precursors (NOx and
VOC chemicals) is a complex photochemistry phenomenon involving over one hundred
chemical species and reactions. A typical photochemical modeling analysis requires
processing numerous data sets in many steps illustrated below for the selected episode(s):
• Select appropriate ozone episode(s) as the modeling base case for attainment
demonstration;
• Process emission data (including both anthropogenic and biogenic sources) into
proper chemical speciation allocation with temporal and spatial distributions as input
into the photochemical modeling;
• Develop representative meteorological data with selected physics options for the
episode period;

9. 8
• Develop other modeling input data including initial and boundary conditions, surface
characteristics, and other physical/chemical data associated with model domain;
• Validate base-case modeling results via statistical analysis coupling with monitored
data; and
• Develop future-case modeling scenarios considering growth and controls to
demonstrate attainment of ozone standards.
The EPA published a guidance document to recommend procedures for estimating whether a
control strategy to reduce emissions of ozone precursors will lead to attainment of the 8-hour
national ambient air quality standard (NAAQS) for ozone.5
Specifically, the EPA guidance
document discusses: (1) how to interpret whether results of modeling and other analyses
support a conclusion that attainment of NAAQS for 8-hour daily maximum ozone
concentrations will occur by the appropriate attainment date for an area; and (2) how to apply
an air quality model to produce results needed to support an attainment demonstration.
EPA recognizes the uncertainty (i.e., the model estimates will not perfectly predict observed
air quality at any given location, neither at the present time nor in the future) associated with
the modeling predictions due to a variety of reasons (limitations of model formulation and
input data limitation). As such, the guidance recommends several qualitative means for
recognizing model limitations and resulting uncertainties when preparing an attainment
demonstration:
• Use models in a relative sense in concert with observed air quality data (i.e., taking
the ratio of future to present predicted air quality and multiplying it times an
“ambient” design value). This approach should reduce some of the uncertainty
attendant with using absolute model predictions alone;
• Analyze available air quality, meteorological, and emissions data to gain a qualitative
understanding of an area’s nonattainment problem. Such a description should be used
to help guide a model application and may provide a reality check on the model’s
predictions;
• Use several model outputs, as well as other supporting analyses, to provide
corroborative evidence concerning the adequacy of a proposed strategy for meeting
the NAAQS. Modeling results and other supporting analyses can be weighed to
determine whether or not the resulting evidence suggests a proposed control strategy
is adequate to meet the NAAQS; and
• Apply models and corroborative approaches in subsequent reviews and analyses of a
control strategy, such as mid-course reviews.

10. 9
States/Tribes should estimate the amount of emission reduction needed to demonstrate
attainment by using the modeled attainment test. A modeled attainment test is an exercise in
which an air quality model is used to simulate current and future air quality. If future
estimates of ozone concentrations are < 84 ppb, then this element of the attainment test is
satisfied. The modeled attainment test is linked to the form of the 8-hour NAAQS for ozone
through use of monitored design values. The baseline design values are projected to the
future using relative reduction factors (RRF). The design value is calculated as the 3-year
average of the fourth highest monitored daily 8-hour maximum value at each monitoring site.
The best approach to using models to demonstrate attainment of the 8-hour ozone standard is
to use the model in a relative mode. Model estimates are used in a “relative” rather than
“absolute” sense. RRF are calculated as the ratio of the model’s future to current (baseline)
predictions at ozone monitors. Future ozone concentrations are estimated at existing
monitoring sites by multiplying a modeled relative reduction factor at locations “near” each
monitor by the observation-based, monitor-specific, “baseline” ozone design value. The
resulting predicted “future concentrations” are compared to 84 ppb.
In addition, the results of corroboratory analyses may be used in a weight of evidence
determination to show that attainment is likely despite modeled results that may be
inconclusive. Past modeling analyses have shown that future design value uncertainties of
2-4 ppb can result from use of alternate, yet equally appropriate, emissions inputs, chemical
mechanisms, and meteorological inputs. Because of this uncertainty, EPA believes that
weight of evidence determinations can be used in some cases to demonstrate attainment
conclusions that differ from the conclusions of the model attainment test. There are several
metrics that can be considered as part of this type of additional analysis:
• Percent change in total amount of ozone >= 85 within the nonattainment area;
• Percent change in grid cells >= 85 ppb within the nonattainment area;
• Percent change in grid cell-hours >= 85 ppb within the nonattainment area; and
• Percent change in maximum-modeled 8-hour ozone within the nonattainment area.

11. 10
CAMX MODELING FOR CONTROL MEASURE EVALUATION
Based on the regulatory requirements and EPA modeling guidance, photochemical modeling
analysis can be performed to evaluate the effectiveness of potential control measures for the
purposes of attainment demonstration. The Comprehensive Air Quality Model with
Extensions (CAMx) is one of such models. It is a Eulerian photochemical dispersion model
that implements the “state-of-the-science” in the atmospheric ozone chemistry and allows for
multi-scales ranging from sub-urban to continental. Basically, the control measure
effectiveness can be evaluated through photochemical modeling in the following steps:
• Explore the effectiveness of across-the-board VOC and NOx emission reductions and
distinguish the potential VOC-limited vs. NOx-limited ozone formation.
• Explore the effectiveness of each potential control measure reflecting emission
reductions of various types of emission sources. The effectiveness of a control
measure can be determined by comparing the modeling results against the baseline
results following the EPA modeling guidance.
• Combine the selected potential control measures into a single photochemical
modeling analysis and determine the sufficiency of attainment demonstration.
CAMx provides several techniques that could be utilized in exploring the effectiveness of
potential control measures:
• Source apportionment techniques including Ozone Source Apportionment
Technology (OSAT) or its derivative such as the Anthropogenic Precursor
Culpability Assessment (APCA)
• Decoupled direct method (DDM) for sensitivity analysis
• Brute-force approach (e.g., zero-out) by changing or even eliminating certain
emission sources from the modeling run and compare results to the baseline
CAMx has been selected by the Texas Commission on Environmental Quality (TCEQ) for
the ozone modeling analyses in the course of the State Implementation Plan (SIP)
development and attainment demonstration. The emission data and modeling results
presented in the following discussions are mainly from reports and documents developed by
TCEQ and/or its modeling contractor(s).
VOC-Limited Vs. NOx-Limited Ozone Formation
While both VOC and NOx are the precursors that may contribute to the formation of ground-
level ozone, it is important to distinguish ozone formation under VOC-limited or

12. 11
NOx-limited conditions, even though such a distinction is subject to local conditions and
chemical species availability. In general, the ozone formation depends on the initial amount
of VOC-to-NOx concentration ratio in the atmosphere. Under a VOC-limited condition,
reducing VOC emissions may be more effective in reducing ground-level ozone. On the
other hand, under a NOx-limited condition, reducing NOx emissions could be more effective.
Due to the complex dynamics of the ozone formation process, it is possible that a VOC-
limited condition may become NOx-limited during the course of the day because NOx can be
depleted more quickly than VOC. Evaluating the overall NOx-limited or VOC-limited
condition present in an area can be conducted with photochemical modeling. Figures 6 and 7
show the anthropogenic NOx and VOC emission rates (in tons per day) for the DFW area in
2002 (which is the baseline year for the attainment demonstration). Such information
provides us the overall conditions of the NOx and VOC emissions in the area in both the
absolute and relative senses. In the DFW area, the anthropogenic NOx emissions are
dominantly contributed by the on-road and non-road mobile sources and the anthropogenic
VOC emissions are mainly from the on-road mobile and area sources in the area. Note that
biogenic sources (e.g., trees) also contribute significant VOC emissions. However, for the
purposes of attainment demonstration, potential control measures should be focused on the
anthropogenic sources.
2002 - NOx Emissions by Source Type for DFW Area
Onroad Mobile -
349.87 tpd
58%
Nonroad Mobile -
136.24 tpd
23%
Point - 79.31 tpd
13%
Area - 38.03 tpd
6%
Point - 79.31 tpd
Area - 38.03 tpd
Onroad Mobile - 349.87 tpd
Nonroad Mobile - 136.24 tpd
Figure 6. NOx Emission Rates by Source

13. 12
2002 - VOC Emissions by Source Type for DFW Area
Area - 204.42 tpd
44%
Point - 28.31 tpd
6%
Nonroad Mobile -
70.08 tpd
15%
Onroad Mobile -
161.42 tpd
35% Point - 28.31 tpd
Area - 204.42 tpd
Onroad Mobile - 161.42 tpd
Nonroad Mobile - 70.08 tpd
Figure 7. VOC Emission Rates by Source
Effectiveness of reducing 8-hour ozone concentrations by reducing overall anthropogenic
NOx and/or VOC emissions can be evaluated with a series of model runs reflecting certain
level of reductions on NOx and/or VOC emissions. The modeling results can be presented in
absolute ozone concentrations in ppb or relative reduction factors. Figure 8 shows the
predicted future (2010) ozone concentrations at the monitor location where the highest
8-hour ozone design value was observed in the DFW area.6
Overall, the analysis shows that
reducing NOx emissions in the area will be more effective in reducing the 8-hour ozone
concentrations than reducing VOC emissions in the DFW area. Combining both NOx and
VOC emission reductions provides small incremental benefits in reducing 8-hour ozone
concentrations. Therefore, performing a series of model runs with the CAMx photochemical
model can provide insights on the effectiveness of overall NOx and/or VOC emission
reductions.

14. 13
74
76
78
80
82
84
86
88
90
92
8-HrOzone(ppb)
VOC Reduction Only
NOx Reduction Only
Both NOx and VOC Reductions
Figure 8. Effectiveness of NOx and/or VOC Emission Reductions
Sensitivity Analysis on Potential Control Measures
Besides understanding the overall effectiveness of reducing NOx and/or VOC emissions on
the 8-hour ozone level in the area, it is also critical to evaluate how effective specific control
measures are in bringing the nonattainment area back to attainment. Such an analysis can
provide sights on the most effective control measures for the area. For example, for the
DFW area, CAMx modeling analysis were performed on the following scenarios (each
scenario represents a 40 tons per day NOx or VOC emission reductions from specific source
category):
• NOx reduction from on-road mobile sources
• NOx reduction from off-road and area sources
• NOx reduction from point sources
• VOC reduction from on-road mobile sources
• VOC reduction from off-road and area sources
0 25 50 75VOC Reduc%
0 20 40 60NOx Reduc%

15. 14
Figure 9 shows the daily highest 8-hour ozone concentrations at the monitor locations
with highest design value corresponding to various control measures. The results
demonstrate that reducing NOx emissions from on-road and off-road mobile sources will
be more effective in reducing the 8-hour ozone in the area than reducing NOx from point
sources or VOC from onroad or off-road mobile sources. This is consistent with the
finding for the overall emission reductions discussed above.
When applicable, more sensitivity analysis can be done to evaluate the potential benefits
of different control measures or strategies. Overall, sensitivity analysis through CAMx
modeling can help to determine the effectiveness of different control measures and
establish control strategies for the attainment demonstration required for an 8-hour ozone
non-attainment area.
89.5
90
90.5
91
91.5
92
92.5
Baseline
O
nroad
N
O
x
O
ffroad/A
rea
N
O
x
PointN
O
x
O
nroad
V
O
C
O
ffroad/A
rea
V
O
C
8-HourOzoneConc.(ppb)
Figure 9. Effectiveness of Control Measures By Source Category
Combined Run with Selected Control Measures
Once a set of selected control measures is identified through the above steps, a combined
CAMx run incorporating all the control measures can be performed in order to demonstrate
the overall effects of the selected control measures. If the predicted design values for an area

16. 15
are less than the standard, the attainment demonstration is achieved. Otherwise, additional
sensitivity analyses may be necessary for additional control measures.
Furthermore, emission controls and their effectiveness should also be evaluated with the
consideration of (1) short-term emission rate variations (e.g., traffic hours for mobile sources
and short-term emission fluctuations for stack sources); (2) emission dispersion and
transport; (3) local wind patterns; and (4) impacts on 1-hour vs. 8-hour ozone concentrations.
In addition, the analyses should also consider potential persistent ozone conducive
meteorological conditions in the area, including high insolation, high temperature, high
stability (as often reflected by low mixing heights), low winds, and low midday relative
humidity.
SUMMARY
Ozone formation in the lower atmosphere is a complex photochemistry phenomenon. The
patterns and characteristics of the 8-hour average ozone concentrations can be different from
those of the 1-hour average ozone in an area. The case study presented in this paper shows
that the daily high 8-hour average ozone concentration typically occurs as the average of the
ozone observed during the summer daytime hours (e.g., from noon to early evening).
Therefore, reducing the ozone level during these daytime hours will be critical. Strategies for
demonstrating attainment with the new 8-hour ozone standard can be explored with the
CAMx photochemical modeling analysis. Specifically, the effectiveness of various control
measures on 8-hour ground-level ozone in an urban area can be evaluated with the CAMx
modeling system.
REFERENCES
1. The most recent ozone nonattainment area designation can be found at the EPA
website: http://www.epa.gov/ozonedesignations/index.htm
2. U.S. EPA, “National Ambient Air Quality Standards for Ozone”, 62 FR 38856,
July 18, 1997.
3. More technical information for the DFW ozone nonattainment area can be found at
the website: http://www.tceq.state.tx.us/implementation/air/airmod/data/dfw1.html
4. Texas Commission on Environmental Quality, “Conceptual Model of Ozone
Formation in the Dallas/Fort Worth Ozone Non-attainment Area”, October 16, 2002,
Prepared by ENVIRON.

17. 16
5. U.S. EPA, “Guidance on the Use of Models and Other Analyses in Attainment
Demonstrations for the 8-Hour Ozone NAAQS”, EPA-454/R-05-002, October 2005.
6. Texas Commission on Environmental Quality, “Final Report – Dallas/Fort Worth
Future Case Control Requirement Assessment”, October 11, 2005, Prepared by
ENVIRON.