773 San Marin Drive, Suite 2115, Novato, CA 94998 415.899.0700 

Draft Final Report

FINAL 2010 OZONE ATTAINMENT DEMONSTRATION 

MODELING FOR THE DENVER 8-HOUR OZONE 

STATE IMPLEMENTATION PLAN 

Prepared for: 

Regional Air Quality Council 

1445 Market Street, Suite 260 

Denver, Colorado 80202 

Prepared by: 

Ralph Morris 

Tanarit Sakulyanontvittaya 

Edward Tai 

ENVIRON International Corporation 

773 San Marin Dr. Suite 2115 

Novato, CA 94998 

and 

Dennis McNally 

Cyndi Loomis 

Alpine Geophysics, LLC 

7341 Poppy Way 

Arvada, Colorado 80007 

January 12, 2009
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\TOC.doc January 2009 i 

TABLE OF CONTENTS 

Page 

EXECUTIVE SUMMARY .................................................................................................... ES-1 

Overview of Approach.........................................................................................................ES-1 

 Final 2010 Control Strategy.................................................................................................ES-1 

 2010 Ozone Projections .......................................................................................................ES-2 

 2010 Ozone Attainment Demonstration Modeling..............................................................ES-5 

1. INTRODUCTION................................................................................................................ 1-1 

1.1 Background...................................................................................................................... 1-1 

1.2 Approach.......................................................................................................................... 1-1 

1.3 2010 Emissions Modeling Approach............................................................................... 1-4 

2. 2010 OZONE ATTAINMEANT DEMONSTRATION ................................................... 2-1 

2.1 Introduction...................................................................................................................... 2-1 

2.2 Ozone Projection Procedures........................................................................................... 2-1 

2.3 Ozone Attainment Demonstration for the 2010 Base Case and the 

 Final 2010 Control Strategy............................................................................................. 2-3 

2.4 Attainment Demonstration Conclusions........................................................................ 2-12 

3. ALTERNATIVE 2010 OZONE PROJECTIONS 

 AND ADDITIONAL MODEL METRICS ........................................................................ 3-1 

 3.1 Introduction...................................................................................................................... 3-1 

 3.2 Alternative 2010 Ozone Projection Procedures............................................................... 3-1 

 3.3 Additional Modeling Metrics........................................................................................... 3-7 

4. REFERENCES..................................................................................................................... 4-1 

TABLES 

Table ES-1. Projected 2010 8-hour ozone Design Values (DVFs) for the 

 2010 Base Case and Final 2010 Control Strategy .............................................ES-3 

Table 1-1. 2006 Base Case (2006.a3), 2010 Base Case (2010.b1) and 

 Final 2010 Control Strategy (2010.b1-fsip) VOC, NOx and CO 

 emissions (tons per day) across Colorado............................................................ 1-5 

Table 2-1a. Projected 2010 8-hour ozone Design Values (DVFs) for the 

 2010 Base Case and Final 2010 Control Strategy ............................................... 2-4 

Table 2-1b. Relative Response Factors (RRFs) used to project 2010 8-hour 

 ozone Design Values (DVFs) for the 2010 Base Case and the 

 Final 2010 Control Strategy................................................................................. 2-5 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\TOC.doc January 2009 ii 

Table 2-1c. Ozone threshold Cutoff Concentration used to project 2010 8-hour 

 ozone Design Values (DVFs) for the 2010 Base Case and 

 Final 2010 Control Strategy................................................................................. 2-6 

Table 2-1d. Number of days used to project 2010 8-hour ozone Design 

 Values (DVFs) for the 2010 Base Case and Final 2010 

 Control Strategy................................................................................................... 2-7 

Table 3-1 Projected 2010 8-hour ozone Design Values (DVFs) at the 

 Rocky Flats North (RFNO) and Fort Collins West (FTCW) 

 monitoring sites using the EPA guidance default approach and 

 the six alternative projection approaches and the 2010 Base, Control 1 

 and Control 2 emission scenarios ........................................................................ 3-4 

Table 3-2a. Projected 2010 8-hour ozone Design Values (DVFs) at monitoring 

 sites in the DMA using the EPA guidance default approach, the six 

 alternative projection approaches and the 2010 Base Case 

 modeling results................................................................................................... 3-5 

Table 3-2b. Projected 2010 8-hour ozone Design Values (DVFs) at monitoring 

 sites in the DMA using the EPA guidance default approach, the six 

 alternative projection approaches and the Final 2010 Control Strategy 

 modeling results................................................................................................... 3-5 

Table 3-2c. Ozone cut-off threshold concentrations used in the 2010 8-hour 

 ozone Design Value projections at monitoring sites in the DMA 

 using the EPA guidance default and the six alternative projection 

 approaches............................................................................................................ 3-6 

Table 3-2d. Number of modeling days used in the 2010 8-hour ozone Design Value 

projections at monitoring sites in the DMA using the EPA guidance 

 default and the six alternative projection approaches.......................................... 3-7 

FIGURES 

Figure ES-1. Interpolated current year 8-hour ozone Design Values 

 and projected 2010 Base Case 8-hour ozone Design Values.............................ES-4 

Figure ES-2. Projected 2010 8-hour ozone Design Values (DVF) for 

 the Final 2010 Control Strategy and differences in projected 

 2010 DVFs between the 2010 Base Case and Final 2010 Control 

 Strategy (2010 Control – 2010 Base).................................................................ES-5 

Figure 1-1a. Nested 36/12/4 km modeling domains for the Denver 8-hour 

 ozone modeling study. Blue line domains are for CAMx/SMOKE 

 domains that are nested in the MM5 red line domains ........................................ 1-3 

Figure 1-1b. Nested 12/4 km modeling domains for the Denver CAMx air quality 

 and SMOKE emissions modeling........................................................................ 1-3 

Figure 2-1. Interpolated current year observed 8-hour ozone Design Values (DVCs) 

 using the MATS tool (ppb).................................................................................. 2-9 

Figure 2-2. Projected 2010 8-hour ozone Design Values (DVFs) for the 2010 

 Base Case emissions scenario using the MATS tool (ppb) ............................... 2-10 

Figure 2-3. Projected 2010 8-hour ozone Design Values (DVFs) for the Final 2010 

 Control Strategy emissions scenario using the MATS tool (ppb) ..................... 2-11 
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Figure 2-4. Differences in projected 2010 8-hour ozone Design Values (DVFs) 

 between the 2010 Base Case and the Final 2010 Control Strategy 

 emissions scenarios using the MATS tool (2010 Control – 2010 Base). .......... 2-12 

Figure 3-1a. Arrays of 7 x 7 4 km grid cells around monitors in the DMA............................. 3-2 

Figure 3-1b. Arrays of 5 x 5 (left) and 3 x 3 (right) 4 km grid cells a 

 round monitors in the DMA................................................................................. 3-3 

Figure 3-2a. Percent change in Total Ozone greater than 85, 80, 

 75 and 70 ppb between the 2006 Base Case and the 

 2010 Base Case (Base) and Final 2010 Control Strategy 

 (FSIP) emission scenarios.................................................................................... 3-8 

Figure 3-2b. Percent change in Grid Cells greater than 85, 80, 75 and 70 ppb 

 between the 2006 Base Case and the 2010 Base Case (Base) and 

 Final 2010 Control Strategy (FSIP) emission scenarios...................................... 3-9 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Exec_Sum2.doc January 2009 ES-1

EXECUTIVE SUMMARY 

Based on 2005-2007 air quality monitoring data, the Denver Metropolitan Area (DMA) violated 

the 0.08 parts per million (ppm) 8-hour ozone National Ambient Air Quality Standard 

(NAAQS). Thus, in November 2007 the DMA reverted to an 8-hour ozone nonattainment area. 

This requires the DMA to develop an 8-hour ozone State Implementation Plan (SIP) that 

demonstrates the area will achieve the 1997 8-hour ozone NAAQS (0.08 ppm) by 2010. The 

Denver Regional Air Quality Council (RAQC), in consultation with the Colorado Department of 

Health and Environment (CDPHE) Air Pollution Control Division (APCD), contracted with 

ENVIRON International Corporation, and their subcontractor Alpine Geophysics, LLC, to 

develop the photochemical modeling databases necessary to demonstrate that the DMA will 

achieve the 0.08 ppm 8-hour ozone NAAQS by 2010. 

OVERVIEW OF APPROACH 

The Comprehensive Air-quality Model with extensions (CAMx; www.camx.com) was applied to 

the June-July 2006 episode using a 36/12/4 km grid with the 4 km domain focused on Colorado. 

Meteorological inputs were prepared using the MM5 meteorological model whose results and 

evaluation are discussed by McNally and co-workers (2008a). An initial emissions inventory 

was prepared using the SMOKE emissions modeling system and a preliminary 2006 base case 

modeling was performed. A preliminary model performance evaluation was conducted and 

diagnostic sensitivity tests performed to identify an optimal model configuration for simulating 

ozone formation in the DMA (Morris et al., 2008a). A revised final CAMx 2006 base case 

simulation was performed and a comprehensive model performance evaluation was conducted 

(Morris et al., 2008b). Although there were some model performance issues on some of the 

modeling days during the June-July 2006 episode, a vast majority of the modeling days achieved 

EPA’s model performance goals and after the examination of many model performance displays 

and metrics we concluded that the model was simulating the observed ozone sufficiently well for 

use in making future year ozone projections (Morris et al., 2008b). 

FINAL 2010 CONTROL STRATEGY 

The Final 2010 Control Strategy was based on a slight modification of the CDPHE’s Ozone 

Action Plan – Alternative Proposal #2 (CDPHE, 2008) presented at the Colorado Air Quality 

Control Commission (AQCC) hearing December 11-12, 2008 (AQCC, 2008). The Denver 8-

hour ozone SIP modeling demonstrates that attainment of the 0.08 ppm ozone NAAQS will be 

achieved in 2010 under the 2010 Base Case emissions scenario. The Final 2010 Control Strategy 

contained two additional federally-enforceable control measures beyond the 2010 Base Case 

(CDPHE, 2008; AQCC, 2008) that provides more certainty that ozone attainment will be 

achieved in 2010: 

• 85% control of VOC emissions on all condensate tanks with annual VOC emissions 

greater than 2 tons per year; and 

• Revisions to condensate tank flaring controls that phases in the auto igniters on flares 

controlling tanks greater than or equal to 50 TPY by May 1, 2009 and on all remaining 

tanks greater than 2 TPY in the Denver NAA by May 1, 2010. 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Exec_Sum2.doc January 2009 ES-2

 

These control measures reduce VOC emissions from oil and gas (O&G) development sources 

that are primarily located in Weld County in the DMA. Across Colorado, the Final 2010 Control 

Strategy reduces anthropogenic VOC emissions by -3.6% from 2010 Base Case conditions. 

2010 OZONE PROJECTIONS 

The procedures given in EPA’s 8-hour ozone modeling guidance were used to project current 

year 8-hour ozone Design Values (DVC) to obtain projected future year 2010 8-hour ozone 

Design Values (DVF) at each of the DMA monitoring sites (EPA, 2007). These procedures use 

the modeling results for the 2006 base case and 2010 emission scenarios in a relative fashion 

where modeled relative response factors (RRFs) are used to scale the current year observed 8-

hour ozone Design Value (DVC) to obtain the projected future year 8-hour ozone Design Value 

(DVF): 

DVF = DVC x RRF 

The 2010 ozone projections were made using EPA’s Modeled Attainment Test Software 

(MATS) tool (http://www.epa.gov/scram001/modelingapps_mats.htm). 

For the Denver 2010 ozone projections, with one exception, the DVCs were based on the 8-hour 

ozone Design Values from the 2005-2007 period (i.e., the three year average of the fourth 

highest daily maximum 8-hour ozone concentration at each monitor). The exception to this was 

for the Fort Collins West (FTCW) monitor that started monitoring in 2006 so a two year average 

of the fourth highest daily maximum 8-hour ozone concentrations was used based on ozone 

observations from the 2006-2007 period. 

Table ES-1 displays the projected future year 2010 8-hour ozone Design Values (DVFs) for the 

2010 Base Case and the Final 2010 Control Strategy emission scenarios. The first set of DVFs 

in Table ES-1 (columns 4 and 5) follow EPA’s guidance approach (EPA, 2007) to truncate the 

final DVFs to the nearest ppb for comparison with the NAAQS. Whereas the last set of DVFs 

for the 2010 Base Case and Final 2010 Control Strategy (columns 6 and 7 in Table ES-1) display 

the DVFs to the nearest tenth of a ppb so that we can distinguish any differences in the ozone 

projections between the 2010 Base Case and Final 2010 Control Strategy. The maximum 

projected 8-hour ozone Design Value for the 2010 Base Case and Final 2010 Control Strategy is 

84 ppb at the Rocky Flats North (RFNO) and Fort Collins West (FTCW) monitoring sites. As all 

projected 2010 8-hour ozone Design Values are below 85 ppb, the 2010 Base Case and Final 

2010 Control Strategy both pass the modeled ozone attainment demonstration test. However, 

since there are four monitoring sites with projected 2010 DVFs of 82 ppb or higher (84 ppb at 

RFNO and FTCW, 83 ppb at Chatfield and 82 ppb at NREL), then additional Weight of 

Evidence (WOE) analysis is required. 

When reporting the DVFs to the nearest tenth of a ppb we see that the maximum projected DVF 

for the 2010 Base Case is 84.9 ppb at both the RFNO and FTCW monitoring sites. The 

implementation of the two additional control measures in the Final 2010 Control Strategy 

reduces the DVF at the FTCW monitoring site by 0.1 ppb (84.8 ppb) and has no effect at the 

RFNO monitoring site (84.9 ppb). Of the 18 sites listed in Table 2-3, the Final 2010 Control 

Strategy is projected to reduce the 2010 DVF by 0.1 ppb at four sites and by 0.2 ppb at one site, 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Exec_Sum2.doc January 2009 ES-3

with the remainder of the monitoring sites having identical 2010 DVFs for the 2010 Base Case 

and Final 2010 Control Strategy. The largest ozone reduction due to the control measures in the 

Final 2010 Control Strategy (0.2 ppb) occurs at the Weld County Tower monitoring site 

(Greely), which is not surprising given that it is the closest monitor to the O&G developments 

that occur primarily in Weld County which is where the largest VOC emission reductions occur 

in the Final 2010 Control Strategy control measures. 

Table ES-1. Projected 2010 8-hour ozone Design Values (DVFs) for the 

2010 Base Case and Final 2010 Control Strategy. 

2010 DVF 

(EPA Guidance) 

2010 DVF 

(nearest 0.1 ppb) 

Name County

DVC 

(2005-

2007) 

Base 

Case 

Final 

Strategy

Base 

Case 

Final 

Strategy 

Welby Adams 70.0 70 70 70.2 70.2 

Highland Arapahoe 78.0 77 77 77.3 77.3 

S. Boulder Creek Boulder 81.0 80 80 80.8 80.7 

Denver - CAMP Denver 56.0 56 56 56.0 56.0 

Carriage Denver 74.0 74 74 74.1 74.1 

Chatfield State Park Douglas 84.0 83 83 83.4 83.4 

USAF Academy El Paso 73.0 72 72 72.0 72.0 

Manitou Springs El Paso 74.0 73 73 73.7 73.7 

Arvada Jefferson 79.0 79 79 79.2 79.1 

Welch Jefferson 75.0 75 75 75.0 75.0 

Rocky Flats North Jefferson 85.0 84 84 84.9 84.9 

NREL Jefferson 82.0 82 82 82.3 82.2 

Fort Collins - West Larimer 86.0* 84 84 84.9 84.8 

Fort Collins Larimer 74.0 73 73 73.0 73.0 

Greeley-WeldTower Weld 78.0 77 77 77.7 77.5 

Gunnison Gunnison 68.0 67 67 67.8 67.8 

Larimer Larimer 76.0 75 75 75.2 75.2 

Larimer Larimer 76.0 75 75 75.2 75.2 

* Fort Collins West DVC based on two-years of measured data. With the 2008 zone season data 

now available the 2006-2008 8-hour ozone Design Value at Fort Collins is now 82 ppb. 

EPA’s 8-hour ozone projection procedure also includes an unmonitored area analysis (EPA, 

2007) that has been codified in MATS. The unmonitored area analysis uses the future-year 8-

hour ozone Design Value projection procedure applied to each grid cell in the modeling domain. 

In this procedure, the current-year Design Values (DVC) are interpolated to each grid cell in the 

modeling domain. This interpolation scheme uses the modeled concentration gradients. RRFs 

are then obtained for each grid cell in the modeling domain using essentially the same approach 

as used for the monitored ozone projections. 

Figure ES-1 displays the interpolated current year 8-hour ozone Design Values (DVC; left) and 

projected 8-hour ozone Design Values (DVFs) for the 2010 Base Case (right) using the MATS 

unmonitored area analysis. Interpolated current year ozone DVCs in excess of 80 ppb are 

estimated to the south, west and northwest of Denver stretching to Fort Collins and then west of 

Fort Collins. In fact, the MATS interpolation procedure estimates 12 grid cells of current-year 

DVCs in excess of the 85 ppb NAAQS west of the Fort Collins (Figure ES-1, left). The 

projected DVFs for the 2010 base case (Figure ES-1, right) have greatly reduced the spatial 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Exec_Sum2.doc January 2009 ES-4

extent of the DVFs in excess of 80 ppb and the 12 cells with DVCs exceeding the 85 ppb 

NAAQS have been reduced by half to 6 grid cells in the 2010 base case emissions scenario. 

Figure ES-2 displays the unmonitored area analysis projected DVFs for the Final 2010 Control 

Strategy (left) and the differences in the 2010 DVFs between the 2010 Base Case and Final 2010 

Control Strategy (right). There are slight reductions in the 2010 DVFs in the Final 2010 Control 

Strategy over the 2010 Base Case, which can be seen more clearly in the difference plot seen in 

Figure ES-2 (right). The 6 remaining grid cells with projected DVFs that are 85 ppb or higher in 

the 2010 Base case are reduced to 5 grid cells in the Final 2010 Control Strategy. 

EPA guidance stresses that the unmonitored area test has more uncertainties than the projections 

at the monitors and it should be treated separately from the monitor based attainment 

demonstration test (EPA, 2007). EPA further notes that while it is expected that additional 

emission controls will likely be needed to eliminate predicted exceedances of the ozone NAAQS 

in the monitor based attainment test, the same requirements may not be appropriate in 

unmonitored areas. In any event, EPA recommends that areas of predicted violations in the 

unmonitored area test be scrutinized and understood to determine whether they are likely to 

really exist in the ambient air, or whether they may be caused by an error or uncertainties in the 

modeling system. It may be appropriate to deploy additional ozone monitors to such areas. In 

the case of the Denver ozone modeling, higher ozone concentrations are estimated west of Fort 

Collins than at the locations of the two monitors in Fort Collins on some days and this does not 

appear to be due to an error in the modeling system. Whether it may be due to uncertainties in 

the modeling system can not be determined. However, it does not seem implausible that higher 

ozone values could exist west of the Fort Collins West monitoring site. 

Figure ES-1. Interpolated current year 8-hour ozone Design Values (DVC; left) and projected 

2010 Base Case 8-hour ozone Design Values (DVF; right). 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Exec_Sum2.doc January 2009 ES-5

Figure ES-2. Projected 2010 8-hour ozone Design Values (DVF) for the Final 2010 Control 

Strategy (left) and differences in projected 2010 DVFs between the 2010 Base Case and Final 

2010 Control Strategy (2010 Control – 2010 Base) (right). 

2010 OZONE ATTAINMENT DEMONSTRATION MODELING 

The 2010 ozone modeling indicates that the Denver region would achieve attainment of the 1997 

0.08 ppm 8-hour ozone NAAQS by 2010 under both the 2010 Base Case and Final 2010 Control 

Strategy. Both 2010 emission scenarios pass the modeled attainment demonstration test at the 

monitoring sites (EPA, 2007). Examining the unmonitored area analysis, both 2010 emission 

scenarios have several grid cells (5-6) that are projected to still exceed the ozone NAAQS in 

2010. However, these residual 2010 ozone exceedances were due to the assumed 86 ppb current 

year DVC at the Fort Collins West monitoring site that is based on two years (2005-2007) of 

ozone observations. The inclusion of ozone observations from 2008 results in an 8-hour ozone 

Design Value for the Fort Collins West monitoring site of 82 ppb, which if used for the DVC 

would eliminate the residual ozone exceedance areas in the unmonitored area analysis. Although 

both 2010 emissions scenarios demonstrate attainment of the ozone NAAQS in 2010, the 

additional controls in the Final 2010 Control Strategy provide more certainty that ozone 

attainment will be achieved in 2010 than the 2010 Base Case. 

There are numerous uncertainties in the modeling analysis. By definition, models are simplistic 

approximations of complex phenomena. The modeling analysis used to asses whether various 

emission reduction measures will bring the Denver area into attainment of the 8-hour ozone 

NAAQS contain many elements that are uncertain (e.g., emissions inputs and projections, 

meteorological inputs, ozone transport, etc.). There is a lot of year-to-year variability in the 

meteorological for the Denver area that greatly affects the ozone formation potential of the 

region. For example, the most ozone formation conducive year for the DMA in recent record 

was 2003 that was followed by the year with the least ozone formation conducive conditions in 

2004. If the ozone formation conditions in the next few years are much more severe than seen in 

the June-July 2006 modeling period, then that could jeopardize achieving attainment in 2010. 

However, at least for 2008 it appears the opposite is true providing further confidence that the 

DMA will achieve attainment of the 8-hour ozone NAAQS in 2010. 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec1_2010Final_Intro2.doc January 2009 1-1 

1.0 INTRODUCTION 

1.1 BACKGROUND 

Ozone air quality in the Denver Metropolitan Area (DMA) has been near the 8-hour ozone 

National Ambient Air Quality Standard (NAAQS) of 0.08 ppm (exceedance defined by values of 

85 ppb or higher) for several years. In December 2002, the Denver Regional Air Quality 

Council (RAQC) and Colorado Department of Health and Environment (CDPHE) Air Pollution 

Control Division (APCD) and others entered into an 8-hour ozone Early Action Compact (EAC) 

with the U.S. Environmental Protection Agency (EPA). EPA’s EAC allows an area to submit an 

early enforceable 8-hour ozone State Implementation Plan (SIP) by March 2004 that 

demonstrates attainment of the 8-hour ozone NAAQS by 2007. In return, EPA will defer the 

classification of an area as nonattainment until 2007. Based on 2005-2007 measured air quality, 

the DMA violated the 1997 0.08 ppm 8-hour ozone NAAQS, so in November 2007 the DMA 

reverted to an 8-hour ozone nonattainment area and is required to prepare an 8-hour ozone SIP 

that demonstrates attainment by 2010. The contracting team of ENVIRON International 

Corporation, and their subcontractor Alpine Geophysics, LLC, were selected by the RAQC and 

CDPHE to perform the 2010 8-hour ozone attainment demonstration modeling for the new 

Denver 8-hour ozone SIP. 

On March 12, 2008, EPA promulgated a new primary ozone NAAQS that has the same form as 

the 1997 ozone NAAQS, but lowers the threshold from 0.08 ppm (85 ppb) to 0.075 ppm (76 

ppb). Of the ~14 ozone monitors in the greater DMA, half have 2005-2007 8-hour ozone DVs 

that are 0.075 ppm or higher. The current Denver 8-hour ozone SIP modeling effort addresses 

the 0.08 ppm 8-hour ozone NAAQS, the new 0.075 ppm 8-hour ozone NAAQS will be 

addressed in future SIP actions. 

This document presents the ozone attainment demonstration modeling of the 2010 Base Case and 

Final 2010 Control Strategy for the Denver 8-hour ozone SIP that was approved by the Colorado 

Air Quality Control Commissions (AQCC) during their December 11-12, 2008 hearing. 

1.2 APPROACH 

The Denver 8-hour ozone attainment demonstration modeling was performed using the 

following meteorological, emissions and photochemical grid models: the fifth generation 

Mesocale Model (MM5) meteorological model (Anthes and Warner, 1978; Dudhia, 1993); the 

Sparse Matrix Operating Kernel Emissions (SMOKE) modeling system (Coats, 1996); and the 

Comprehensive Air-quality Model with extensions (CAMx) photochemical grid model 

(ENVIRON, 2008). These models were applied to a June-July 2006 modeling period for the 

purposes of demonstrating attainment of the 8-hour ozone standard by 2010. Figure 1-1 displays 

the 36/12/4 km modeling domains used for the MM5 and SMOKE/CAMx modeling. CAMx 

simulations were first performed for the 36 km continental U.S. Inter-RPO modeling domain and 

the results processed to generate boundary conditions (BCs) for the 12 km modeling domain 

(i.e., one-way grid nesting between the 36 km and 12 km CAMx domains). CAMx was then 

used to simulate ozone formation within the 12/4 km modeling domain using two-way 

interactive grid nesting (Figure 1-1b). Once the 12 km BCs were defined from the 2006 and 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec1_2010Final_Intro2.doc January 2009 1-2 

2010 36 km CAMx simulations, sensitivity and control strategy evaluations runs were made on 

the 12/4 km modeling domain. The Denver 8-hour ozone SIP modeling work was performed 

mostly during the 2008 calendar year and produced the following reports: 

• Development of a Denver 8-hour ozone SIP attainment demonstration Modeling Protocol 

(Morris et al., 2007). 

o http://www.ozoneaware.org/documents/DraftFinalProtocolDenver8-

HourOzoneNov282007.pdf

• MM5 meteorological modeling and model performance evaluation (McNally et al., 

2008a). 

o http://www.ozoneaware.org/documents/MM5_Eval_DENSIP_Feb25_2008.pdf

• Development of a preliminary 36/12/4 km photochemical modeling database for the 

June-July 2006 episode, the DMA, and initial model performance evaluation, sensitivity 

test modeling and identification of optimal model configuration for simulating ozone in 

the DMA (Morris et al., 2008a). 

o http://www.ozoneaware.org/documents/Prelim_Ozone_Eval_Denver_SIP_Feb27

_2008.pdf

• Final base case modeling and model performance evaluation for the June-July 2006 

DMA episode (Morris et al., 2008b). 

o http://www.ozoneaware.org/documents/modeling/Denver_2006MPE_DraftFinal_

Aug29_2008.pdf

• 2010 base case modeling, emission sensitivity tests and ozone source apportionment 

modeling (McNally et al., 2008b). 

o http://www.ozoneaware.org/documents/modeling/Exec_Sum1.pdf

• 2010 control strategy and attainment demonstration modeling (Morris et al., 2008c). 

o http://www.ozoneaware.org/documents/modeling/Denver_2010ControlStrat_Draf

t_Sep22_2008.pdf

The last report listed above (Morris et al., 2008c) presented ozone attainment demonstration 

modeling for a 2010 Base Case, a proposed 2010 SIP Control Strategy (Cntl1) and a 2010 

Control Strategy that included the proposed federally-enforceable SIP control measures as well 

as additional state-only control measures (Cntl2). The Colorado Air Quality Control 

Commission (AQCC) held a hearing on the proposed Denver 8-hour ozone 2010 SIP Control 

Strategy and passed a motion for a slightly modified Ozone Action Plan to produce a Final 2010 

Control Strategy for the Denver 8-hour ozone SIP (AQCC, 2008). 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec1_2010Final_Intro2.doc January 2009 1-3 

-2880-2520-2160-1800-1440-1080 -720 -360 0 360 720 1080 1440 1800 2160 2520 2880

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-1800

-1440

-1080

-720

-360

0

360

720

1080

1440

1800

2160

MM5 36 km

CAMx 36 km

MM5 12km

CAMx 12km

MM5 4km

CAMx 4

Figure 1-1a. Nested 36/12/4 km modeling domains for the Denver 8-hour ozone modeling 

study. Blue line domains are for CAMx/SMOKE domains that are nested in the MM5 red line 

domains. 

-1680-1560-1440-1320-1200-1080 -960 -840 -720 -600 -480 -360 -240 -120 0 120 240

-600

-480

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-240

-120

0

120

240

360

480

CAMx 12km

CAMx 4km

Figure 1-1b. Nested 12/4 km modeling domains for the Denver CAMx air quality and SMOKE 

emissions modeling.
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec1_2010Final_Intro2.doc January 2009 1-4 

1.3 2010 EMISSIONS MODELING APPROACH 

The 2010 base case and control strategy emissions were prepared using the same procedures as 

used to prepare the final 2006 base case emissions scenario (Morris et al., 2008a,b) and the 2010 

Base Case and emissions sensitivity scenarios and are described by McNally and co-workers 

(McNally et al., 2008b). The CDPHE/APCD provided 2010 emissions for all anthropogenic 

emission sources in Colorado except for on-road mobile sources and oil and gas (O&G) 

emissions in the Denver-Julesburg Basin for which 2010 emissions from the WRAP Phase III 

O&G emissions development project were utilized (Bar-Ilan et al., 2008a,b). Outside of 

Colorado, the 2010 anthropogenic emissions were based on the WRAP 2002 and 2018 emissions 

inventories projected to 2010. CAMx-ready emissions were generated using the Sparse Matrix 

Operator Kernel Emissions (SMOKE) emissions modeling system (Coats, 1996) for all 

anthropogenic emissions categories except on-road mobile sources in the DMA, which used the 

Consolidated Community Emissions Processing Tool (ConCEPT) modeling system (Loomis et 

al., 2005) and biogenic emissions. The same biogenic emissions were used for the 2010 base 

case as were used for the final 2006 base case and were based on the Model of Emissions of 

Gases and Aerosols from Nature (MEGAN) biogenic emissions model (Guenther and 

Wiedinmyer, 2004). Emissions from fires were also kept constant between the 2006 base case 

and 2010 emission scenarios. 

Colorado emissions for the 2010 Base Case and Final 2010 Control Strategy were either 

provided by the CDPHE or, in the case of on-road mobile sources, modeled using the CONCEPT 

MV and SMOKE-MOBILE6 on-road mobile source emissions modeling systems. Table 1-1 

summarizes the VOC, NOx and CO emissions within the 4 km Colorado domain (Figure 1-1b) 

for the 2006 Base Case, the 2010 Base Case and the Final 2010 Control Strategy. Between the 

2006 and 2010 Base Case emissions scenarios, VOC, NOx and CO emissions are estimates to be 

reduced by, respectively, 42 tons per day (TPD) (-1%), 50 TPD (-5%) and 386 TPD (-10%). 

Note that the emissions in Table 1-1 are model-ready emissions for the July 27, 2006 

representative day (Thursday) and may differ from the planning inventories and other modeling 

days. In particular, the model-ready emissions have day-specific adjustments for several source 

categories (e.g., on-road mobile sources and biogenic emissions) and the VOC emissions have 

been speciated into the CB05 chemical mechanism used in the CAMx photochemical grid model 

so that any organic compounds that are considered non-reactive in the original VOC inventories 

have been dropped for ozone modeling. 

The Denver 8-hour ozone SIP Final 2010 Control Strategy implemented two additional control 

measures over the 2010 Base Case (AQCC, 2008): 

• 85% control of VOC emissions on all condensate tanks with annual VOC emissions 

greater than 2 tons per year; and 

• Revisions to condensate tank flaring controls that phases in the auto igniters on flares 

controlling tanks greater than or equal to 50 TPY by May 1, 2009 and on all remaining 

tanks greater than 2 TPY in the Denver NAA by May 1, 2010. 

The two control measures in the Final 2010 Control Strategy results mainly in VOC emission 

reductions over the 2010 Base Case. Across Colorado, the 2010 Final Control Strategy 

emissions scenario has 28 TPD less VOC emissions than the 2010 Base Case. This results in a 

-3.6 percent reduction in anthropogenic and a -0.6% reduction in total VOC emissions across 

Colorado over the 2010 Base Case due to the implementation of the two control measures in the 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec1_2010Final_Intro2.doc January 2009 1-5 

Final 2010 Control Strategy. Note that the flaring used in the condensate tank VOC control 

measure in the Final 2010 Control Strategy results in increases in NOx and CO emissions over 

the 2010 Base Case. NOx and CO emissions are estimated to be 0.04 and 0.21 TPD higher in the 

Final 2010 Control Strategy than the 2010 Base Case, respectively. However, these increases in 

NOx and CO emissions are minor representing less than 0.01% of the Colorado NOx and CO 

emissions. 

Table 1-1. 2006 Base Case, 2010 Base Case and Final 2010 Control Strategy VOC, NOx and 

CO emissions (tons per day) across the Colorado 4 km domain. 

Total Colorado Emissions (tons/day) 

July 27, 2006 

Emissions Scenario 

CO 

(tons/day) 

NOx 

(tons/day) 

VOC * 

(tons/day) 

Total Colorado Emissions 

2006 Base Case 4401.1 930.7 4671.4 

2010 Base Case 4015.2 880.7 4629.2 

Final 2010 Control Strategy 4015.4 880.7 4601.6 

Biogenic Emissions for Colorado 4 km Grid 

2006 MEGAN Biogenics 618.4 44.2 3871.5 

Anthropogenic Colorado Emissions 

2006 Base Case 3782.8 886.5 799.9 

2010 Base Case 3396.8 836.5 757.7 

Final 2010 Control Strategy 3397.0 836.5 730.1 

Percent Change in Total Colorado Emissions 

2006 Base Case -- -- -- 

2010 Base Case -8.8% -5.4% -0.9% 

Final 2010 Control Strategy -8.8% -5.4% -0.9% 

Percent Anthropogenic in Total Colorado Emissions 

2006 Base Case -- -- -- 

2010 Base Case -10.2% -5.6% -5.3% 

Final 2010 Control Strategy -10.2% -5.6% -8.7% 

* VOC emissions from CBO5 chemical mechanism. 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-1 

2.0 2010 OZONE ATTAINMEANT DEMONSTRATION 

2.1 INTRODUCTION 

This section presents the 2010 ozone Design Value projections for the 2010 Base Case and the 

Final 2010 Control Strategy approved by the Colorado Air Quality Control Commissions at their 

December 11-12, 2008 hearing (AQCC, 2008). The future year ozone projections demonstrate 

that the Denver area will achieve the 0.08 ppm 8-hour ozone NAAQS by 2010 under the 2010 

Base Case conditions. The 2010 Final Control Strategy includes federally-enforceable control 

measures that will be included in the Denver 8-hour ozone State Implementation Plan (SIP) that 

provide additional certainty that ozone attainment will be achieved in 2010. The 8-hour ozone 

projections are made using the CAMx modeling results for the 2006 Base Case (Morris et al., 

2008a,b) and the 2010 Base Case and Final 2010 Control Strategy emission scenarios. These 

ozone projections are made using EPA’s Modeled Attainment Test Software (MATS) tool that 

can be found at: 

• http://www.epa.gov/scram001/modelingapps_mats.htm

The procedures used by MATS to make ozone projection are described in detail by McNally and 

co-workers (2008b) that presented the 8-hour ozone projections for the Denver 2010 Base Case 

and 2010 sensitivity simulations. Below we provide a brief overview of the ozone projection 

procedures used by MATS, whose results for the 2010 Base Case and Final 2010 Control 

Strategy are presented later in this Chapter. 

2.2 OZONE PROJECTION PROCEDURES 

The Denver 2010 8-hour ozone projections were made using default procedures in EPA’s latest 

modeling guidance (EPA, 2007), with one exception that is described below. These procedures 

use the model in a relative sense to scale the observed current year 8-hour ozone Design Value 

(DVC) to obtain a future year 8-hour ozone Design Value (DVF). The model derived scaling 

factors are referred to as relative response factors (RRF) and are defined as the ratio of daily 

maximum 8-hour ozone concentrations near a monitor averaged over several days of modeling 

results for the 2010 emissions scenario to the 2006 base case: 

RRF = [Σ 2010 scenario] / [Σ 2006 base case] 

DVF = DVC x RRF 

The basic steps in performing the 2010 8-hour ozone projections can be summarized as follows: 

1. Develop an observed current year 8-hour ozone Design Value (DVC) at each monitoring 

site that serves as the starting point for the ozone projections. 

EPA guidance recommends using an average of three years of 8-hour ozone Design 

Values centered on the modeling year, which for the Denver June-July 2006 episode 

modeling would mean averaging 8-hour ozone Design Values from the 2004-2006, 2005-

2007 and 2006-2008 periods. This results in averaging the fourth highest daily maximum 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-2 

8-hour ozone concentration at a monitor across five years of data centered on 2006 using 

weighting factors of 1, 2, 3, 2, and 1 for the years 2004-2008, respectively. However, for 

the Denver 2010 8-hour ozone projections the “five year Design Value” approach 

recommended in EPA’s guidance was not used as the DVCs for the following reasons: 

a. The Denver 2010 ozone projections were initially made before the 2008 ozone 

season was completed so use of the “five year Design Value” approach for the 

DVCs that included 2008 observations was not possible. 

b. Using such a “five year Design Value “ approach would result in the DVCs that 

attain the 0.08 ppm 8-hour ozone NAAQS for all monitors in the Denver region, 

which seemed inconsistent with the designation of the Denver area as 8-hour 

ozone nonattainment in November 2007. 

Thus instead, with one exception, for the DVCs the 8-hour ozone Design Values from the 

2005-2007 three year period that resulted in Denver being classified as nonattainment 

were used. The exception is for the Fort Collins West ozone monitor that started 

monitoring in 2006 so the DVC was based on the average fourth highest measured 8-hour 

ozone for two years (2006-2007). 

2. Select the maximum modeled 8-hour ozone concentrations near a monitor for several 

days from the 2006 base and 2010 emission scenarios and take the ratio of their averages 

to construct the monitor-specific RRFs: 

a. By near a monitor EPA guidance suggests using an array of 7 x 7 grid cells 

centered on the monitoring location for the Denver modeling that uses a 4 km grid 

resolution. 

b. By several days EPA recommends RRFs based on at least 10 modeled days and 

recommends selecting days in which the 2006 base case highest daily maximum 

8-hour ozone concentrations near a monitor are greater than an ozone threshold 

(cut off). Initially, an ozone threshold of 85 ppb is used. If less than 10 days are 

obtained the threshold is reduced by 1 ppb until at least 10 days are obtained for 

the RRF. When the 70 ppb threshold floor is reached and there are at least 5 days 

then the RRF is used. If there are less than 5 modeled days with the 2006 base 

case highest daily maximum 8-hour ozone concentration near the monitor greater 

or equal to 70 ppb, then no RRF and 2010 ozone projection is made for that 

monitoring site. 

3. The RRF is applied to the DVC to obtain the projected DVF for the 2010 emission 

scenarios. The projected DVF is truncated to the nearest ppb. 

4. If the DVFs at all monitoring sites are less than or equal to 84 ppb, then the modeled 

attainment demonstration test is passed. If a DVF at any monitor is 85 ppb or higher, the 

modeled attainment test is not passed. 

5. If there are any DVFs between 82 ppb and 87 ppb then a Weight of Evidence (WOE) 

analysis is required to corroborate the modeled attainment demonstration. 

6. An unmonitored area analysis is also performed that interpolates the DVCs across the 

modeling domain and performs the ozone projections in each grid cell using the 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-3 

procedures given above, except using the modeling results within each grid cell rather 

than near the grid cell. 

a. EPA believes that the unmonitored area analysis is more uncertain than the 

monitor based ozone projections, whereas additional emissions reductions are 

likely required to eliminate any projected monitored ozone exceedances, the same 

is not true in the unmonitored area test. 

b. EPA recommends that the reasons behind any unmonitored area test exceedances 

be understood and explained. 

2.3 OZONE ATTAINMENT DEMONSTRATION FOR THE 2010 BASE CASE AND 

THE FINAL 2010 CONTROL STRATEGY 

The same emissions modeling procedures used for the 2006 Base Case and 2010 sensitivity tests 

and preliminary control strategies were used for the 2010 Base Case and Final 2010 Control 

Strategy (Morris et al, 2008c,d; McNally et al., 2008b). The on-road mobile source emissions 

were modeled using either the CONCEPT MV (area covered by the Denver link-based network) 

or SMOKE-MOBILE6. The two control measures for the Final 2010 Control Strategy were 

included in emissions files provided by the CDPHE/APCD. Table 1-1 presented in Chapter 1 

summarizes the emissions within the Colorado 4 km grid domain for the 2006 Base Case, 2010 

Base Case and Final 2010 Control Strategy. The Final 2010 Control Strategy was based on a 

slight modification to the CDPHE’s Ozone Action Plan – Alternative Proposal #2 (CDPHE, 

2008) presented at the Colorado Air Quality Control Commission hearing December 11-12, 2008 

(AQCC, 2008) and contained two federally-enforceable control measures beyond the 2010 Base 

Case: 85% condensate tank VOC controls on tanks greater than 2 tpy and revised controls on 

condensate flaring. 

2.3.1 Attainment Demonstration at the Monitoring Sites 

Table 2-1 displays the projected future year 2010 8-hour ozone Design Values (DVFs) for the 

2010 Base Case and the Final 2010 Control Strategy emission scenarios. Included in this table 

are the 2010 DVFs (Table 2-1a) and the RRFs (Table 2-1b) the ozone cut-off threshold 

concentrations (Table 2-1c) and the number of modeling days (Table 2-1d) used in the 

construction of the RRFs. The first set of DVFs in Table 2-1a (columns 4 and 5) follow EPA’s 

guidance approach (EPA, 2007) to truncate the final DVFs to the nearest ppb for comparison 

with the NAAQS. Whereas the last set of DVFs for the 2010 Base Case and Final 2010 Control 

Strategy display the DVFs to the nearest tenth of a ppb so that we can distinguish any differences 

in the ozone projections between the 2010 Base Case and Final 2010 Control Strategy. The 

maximum projected 8-hour ozone Design Value for the 2010 Base Case and Final 2010 Control 

Strategy is 84 ppb at the Rocky Flats North (RFNO) and Fort Collins West (FTCW) monitoring 

sites. As all projected 2010 8-hour ozone Design Values are below 85 ppb, the 2010 Base Case 

and Final 2010 Control Strategy both pass the modeled ozone attainment demonstration test. 

However, since there are four monitoring sites with projected 2010 DVFs of 82 ppb or higher 

(84 ppb at RFNO and FTCW, 83 ppb at Chatfield and 82 ppb at NREL), then additional Weight 

of Evidence (WOE) analysis is required. 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-4 

When reporting the DVFs to the nearest tenth of a ppb we see that the maximum projected DVF 

for the 2010 Base Case is 84.9 ppb at both the RFNO and FTCW monitoring sites (Table 2-1a). 

The implementation of the federally-enforceable SIP control measures in the Final 2010 Control 

Strategy reduces the DVF at the FTCW monitoring site by 0.1 ppb (84.8 ppb) and has no effect 

at the RFNO monitoring site (84.9 ppb). Of the 18 sites listed in Table 2-1, the Final 2010 

Control Strategy is projected to reduce the 2010 DVF by 0.1 ppb at four sites and by 0.2 ppb at 

one site, with the remainder of the monitoring sites having identical DVFs for the 2010 Base 

Case and Final 2010 Control Strategy. The largest ozone reduction due to the control measures 

in the Final 2010 Control Strategy (0.2 ppb) occurs at the Weld County Tower monitoring site 

(Greely), which is not surprising given that it is the closest monitor to the O&G developments 

that occur primarily in Weld County, which is where the largest VOC emission reductions occur 

due to the Final 2010 Control Strategy control measures. These results are consistent with the 

2010 sensitivity modeling that found ozone to be more responsive to emission controls at the 

FTCW than RFNO monitoring sites (McNally et al., 2008b). 

Table 2-1a. Projected 2010 8-hour ozone Design Values (DVFs) for the 

2010 Base Case and Final 2010 Control Strategy. 

2010 DVF 

(EPA Guidance) 

2010 DVF 

(nearest 0.1 ppb) 

Name County

DVC 

(2005-

2007) 

Base 

Case 

Final 

Strategy

Base 

Case 

Final 

Strategy 

Welby Adams 70.0 70 70 70.2 70.2 

Highland Arapahoe 78.0 77 77 77.3 77.3 

S. Boulder Creek Boulder 81.0 80 80 80.8 80.7 

Denver - CAMP Denver 56.0 56 56 56.0 56.0 

Carriage Denver 74.0 74 74 74.1 74.1 

Chatfield State Park Douglas 84.0 83 83 83.4 83.4 

USAF Academy El Paso 73.0 72 72 72.0 72.0 

Manitou Springs El Paso 74.0 73 73 73.7 73.7 

Arvada Jefferson 79.0 79 79 79.2 79.1 

Welch Jefferson 75.0 75 75 75.0 75.0 

Rocky Flats North Jefferson 85.0 84 84 84.9 84.9 

NREL Jefferson 82.0 82 82 82.3 82.2 

Fort Collins - West Larimer 86.0* 84 84 84.9 84.8 

Fort Collins Larimer 74.0 73 73 73.0 73.0 

Greeley-WeldTower Weld 78.0 77 77 77.7 77.5 

Gunnison Gunnison 68.0 67 67 67.8 67.8 

Larimer Larimer 76.0 75 75 75.2 75.2 

Larimer Larimer 76.0 75 75 75.2 75.2 

* Fort Collins West DVC based on two-years of measured data. With the 2008 zone season data 

now available the 2006-2008 8-hour ozone Design Value at Fort Collins is now 82 ppb.
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-5 

Table 2-1b. Relative Response Factors (RRFs) used to project 2010 8-hour ozone Design 

Values (DVFs) for the 2010 Base Case and the Final 2010 Control Strategy.

RRF 

Name County DVC 

Base 

Case 

Final 

Strategy

Welby Adams 70.0 1.0042 1.0040 

Highland Arapahoe 78.0 0.9916 0.9915 

S. Boulder Creek Boulder 81.0 0.9976 0.9974 

Denver - CAMP Denver 56.0 1.0017 1.0014 

Carriage Denver 74.0 1.0022 1.0019 

Chatfield State Park Douglas 84.0 0.9934 0.9933 

USAF Academy El Paso 73.0 0.9873 0.9873 

Manitou Springs El Paso 74.0 0.9966 0.9966 

Arvada Jefferson 79.0 1.0026 1.0023 

Welch Jefferson 75.0 1.0004 1.0002 

Rocky Flats North Jefferson 85.0 0.9994 0.9992 

NREL Jefferson 82.0 1.0039 1.0036 

Fort Collins - West Larimer 86.0 0.9874 0.9869 

Fort Collins Larimer 74.0 0.9878 0.9871 

Greeley-WeldTower Weld 78.0 0.9964 0.9944 

Gunnison Gunnison 68.0 0.9984 0.9984 

Larimer Larimer 76.0 0.9903 0.9902 

Larimer Larimer 76.0 0.9903 0.9902 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-6 

Table 2-1c. Ozone threshold Cutoff Concentration used to project 2010 8-hour ozone Design 

Values (DVFs) for the 2010 Base Case and Final 2010 Control Strategy.

Cutoff 

Concentration 

Name County DVC 

Base 

Case 

Final 

Strategy

Welby Adams 70.0 77.0 77.0 

Highland Arapahoe 78.0 78.0 78.0 

S. Boulder Creek Boulder 81.0 78.0 78.0 

Denver - CAMP Denver 56.0 78.0 78.0 

Carriage Denver 74.0 78.0 78.0 

Chatfield State Park Douglas 84.0 78.0 78.0 

USAF Academy El Paso 73.0 75.0 75.0 

Manitou Springs El Paso 74.0 74.0 74.0 

Arvada Jefferson 79.0 78.0 78.0 

Welch Jefferson 75.0 78.0 78.0 

Rocky Flats North Jefferson 85.0 78.0 78.0 

NREL Jefferson 82.0 78.0 78.0 

Fort Collins - West Larimer 86.0 76.0 76.0 

Fort Collins Larimer 74.0 76.0 76.0 

Greeley-WeldTower Weld 78.0 75.0 75.0 

Gunnison Gunnison 68.0 74.0 74.0 

Larimer Larimer 76.0 77.0 77.0 

Larimer Larimer 76.0 77.0 77.0 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-7 

Table 2-1d. Number of days used to project 2010 8-hour ozone Design Values (DVFs) for the 

2010 Base Case and Final 2010 Control Strategy.

Number of Day 

Name County DVC 

Base 

Case 

Final 

Strategy

Welby Adams 70.0 11 11 

Highland Arapahoe 78.0 14 14 

S. Boulder Creek Boulder 81.0 10 10 

Denver - CAMP Denver 56.0 10 10 

Carriage Denver 74.0 10 10 

Chatfield State Park Douglas 84.0 11 11 

USAF Academy El Paso 73.0 10 10 

Manitou Springs El Paso 74.0 10 10 

Arvada Jefferson 79.0 10 10 

Welch Jefferson 75.0 10 10 

Rocky Flats North Jefferson 85.0 10 10 

NREL Jefferson 82.0 11 11 

Fort Collins - West Larimer 86.0 10 10 

Fort Collins Larimer 74.0 12 12 

Greeley-WeldTower Weld 78.0 10 10 

Gunnison Gunnison 68.0 10 10 

Larimer Larimer 76.0 10 10 

Larimer Larimer 76.0 10 10 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-8 

2.3.2 Unmonitored Area Analysis 

EPA’s 8-hour ozone projection procedure also includes an unmonitored area analysis (EPA, 

2007) that has been codified in MATS. The unmonitored area analysis uses the future-year 8-

hour ozone Design Value projection procedure applied to each grid cell in the modeling domain. 

In this procedure, the current-year Design Values (DVC) are interpolated to each grid cell in the 

modeling domain. This interpolation scheme uses the modeled concentration gradients so that 

the gridded DVCs may have some locations that are higher than the observed DVCs at the 

monitoring sites. RRFs are then obtained for each grid cell in the modeling domain using 

essentially the same approach as used for the monitored ozone projections, only RRFs are based 

on the model estimates within each grid cell rather than near a grid cell as done for the 

projections at the monitor. 

Figure 2-1 displays the interpolated current year 8-hour ozone Design Values (DVC) using the 

MATS unmonitored area analysis. Interpolated current year ozone DVCs in excess of 80 ppb 

are estimated to the south, west and northwest of Denver stretching to Fort Collins and then west 

of Fort Collins. In fact, the MATS interpolation procedure estimates 12 grid cells of current-year 

DVCs in excess of the 0.08 ppm NAAQS (i.e., 85 ppb or higher) that occur west of Fort Collins 

(Figure 2-1). 

The projected DVFs for the 2010 Base Case (Figure 2-2) have greatly reduced the spatial extent 

of the DVFs in excess of 80 ppb and the 12 cells with DVCs exceeding the 0.08 ppm NAAQS 

have been reduced by half to 6 grid cells in the 2010 Base Case emissions scenario. Note that 

the unshaded areas in the unmonitored area analysis 2010 ozone projections are areas in which 

there were less than 5 days with 2006 Base Case daily maximum 8-hour ozone estimates of 70 

ppb or greater so no 2010 ozone projection were made for those grid cells. 

Figure 2-3 displays the unmonitored area analysis projected DVFs for the Final 2010 Control 

Strategy emissions scenario. There are slight reductions in the 2010 DVFs over the 2010 Base 

Case, which can be seen more clearly in the 2010 DVF difference plot shown in Figure 2-4. The 

6 remaining exceedance grid cells with projected DVFs that are 85 ppb or higher in the 2010 

Base case are reduced to 5 grid cells under the Final 2010 Control Strategy. 

EPA guidance stresses that the unmonitored area test has more uncertainties than the projections 

at the monitors and it should be treated separately from the monitor based attainment 

demonstration test (EPA, 2007). EPA further notes that while it is expected that additional 

emission controls will likely be needed to eliminate predicted exceedances of the ozone NAAQS 

in the monitor based attainment test, the same requirements may not be appropriate in 

unmonitored areas. In any event, EPA recommends that areas of predicted violations in the 

unmonitored area test be scrutinized and understood to determine whether they are likely to 

really exist in the ambient air, or whether they may be caused by an error or uncertainties in the 

modeling system. It may be appropriate to deploy additional ozone monitors to such areas. In 

the case of the Denver ozone modeling, higher ozone concentrations are estimated west of Fort 

Collins than at the locations of the two monitors in Fort Collins on some days and this does not 

appear to be due to an error in the modeling system. Whether it may be due to uncertainties in 

the modeling system can not be determined. However, it does not seem implausible that higher 

ozone values could exist west of the Fort Collins West monitoring site. These high 2010 

projected ozone DVFs west of Fort Collins are driven by the high DVC of 86 ppb at the FTCW 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-9 

monitor that is based on two-years (2006-2007) of monitoring data. When a third year of 

monitoring data is included (2008) the 8-hour ozone Design Value at FTCW is reduced from 86 

ppb to 82 ppb. If the 82 ppb lower actual 8-hour ozone Design Value was used for the DVC at 

the FTCW monitor in the unmonitored area analysis instead of 86 ppb, there would be no grid 

cells exceeding the 8-hour ozone NAAQS in the unmonitored area analysis for both the 2010 

Base Case and Final 2010 Control Strategy emission scenarios. 

Figure 2-1. Interpolated current year observed 8-hour ozone Design Values (DVCs) using the 

MATS tool (ppb). 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-10 

Figure 2-2. Projected 2010 8-hour ozone Design Values (DVFs) for the 2010 Base Case 

emissions scenario using the MATS tool (ppb).
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-11 

Figure 2-3. Projected 2010 8-hour ozone Design Values (DVFs) for the Final 2010 Control 

Strategy emissions scenario using the MATS tool (ppb).
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec2_2010Final_AttainDemo2.doc January 2009 2-12 

Figure 2-4. Differences in projected 2010 8-hour ozone Design Values (DVFs) between the 

2010 Base Case and the Final 2010 Control Strategy emissions scenarios using the MATS tool 

(2010 Control – 2010 Base).

2.4 ATTAINMENT DEMONSTRATION CONCLUSIONS 

The photochemical modeling of the Denver region demonstrates that the 2010 Base Case 

emissions scenario is sufficient to achieve attainment of the 0.08 ppm ozone NAAQS by 2010. 

The additional control measures in the Final 2010 Control Strategy provide additional assurances 

that ozone attainment will be achieved in the Denver area by 2010. 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec3_Control_ModelMetrics2.doc January 2009 3-1 

3.0 ALTERNATIVE 2010 OZONE PROJECTIONS 

AND ADDITIONAL MODEL METRICS 

3.1 INTRODUCTION 

As noted in Chapter 2, the maximum projected 2010 8-hour ozone Design Value (DVF) at any 

monitor for the 2010 Base Case and Final 2010 Control Strategy is 84 ppb at the Rocky Flats 

North (RFNO) and Fort Collins West (FTCW) monitoring sites, so both 2010 emission scenarios 

pass the modeled attainment demonstration test. As these projected 2010 DVFs are 82 ppb or 

higher, then a Weight of Evidence (WOE) analysis is required to corroborate the modeled 

attainment demonstration test. The WOE analysis examines observed emissions and air quality 

data and their trends, assesses the conceptual model of ozone formation in the region, examines 

additional modeling metrics and performs additional analysis. All of the elements of the WOE 

analysis are examined together to determine whether the preponderance of evidence suggests that 

Denver area will in fact achieve attainment of the 0.08 ppm 8-hour ozone National Ambient Air 

Quality Standard (NAAQS) by 2010. Below we provide additional modeling metrics and 

alternative ozone projection procedures that are one component of a WOE analysis. 

3.2 ALTERNATIVE 2010 OZONE PROJECTION PROCEDURES 

Several alternative 2010 ozone projection procedures were analyzed for the 2010 Base Case, and 

Final 2010 Control Strategy emission scenarios to estimate the uncertainties in the projection 

procedures and provide confidence that passing the modeled attainment demonstration test does 

indicate attainment will likely be achieved in 2010. These alternative ozone projection 

procedures differ in the days used and how modeled ozone near the monitor is selected to 

construct the RRFs. Six additional ozone projection procedures were analyzed, in addition to the 

EPA guidance default approach discussed in Chapter 2: 

Minimum 5 Days to Develop RRF using 85-70 ppb Sliding Threshold (5dth): In the EPA 

default approach, modeling days are selected for use in constructing RRFs based on 

whether the maximum daily maximum 8-hour ozone concentration near the monitor 

(with 7 x 7 array of grid cells) in the 2006 Base Case is greater than a threshold, with the 

threshold determined when at least 10 days are obtained for the RRF. In this alternative 

projection approach, we require a minimum of 5 modeled days to construct the RRFs. 

Use of 80 ppb Cutoff Threshold and Minimum of 1 Day (1dth80): The second 

alternative ozone projection approach uses an 80 ppb cutoff threshold and RRFs are 

allowed to be calculated with as few as one modeling day. 

Use of 75 ppb (1dth75) and 70 ppb (1dth70) Cutoff Thresholds: These two alternative 

ozone projection approaches use cutoff thresholds of 75 and 70 ppb. 

Use of 5 x 5 and 3 x 3 Array of Grid Cells: Select the maximum daily maximum 8-hour 

ozone concentration from a 5 x 5 or 3 x 3 array of grid cells centered on the monitor, 

instead of using a 7 x 7 array as used in the EPA default procedure. 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec3_Control_ModelMetrics2.doc January 2009 3-2 

In regards to these last two alternative 2010 ozone projection methods, Figure 3-1 displays the 

sizes of the arrays of 7 x 7, 5 x 5 and 3 x 3 4 km grid cells around the monitors in the DMA. 

With the EPA default 7 x 7 array of grid cells around each monitor, there is a lot of overlap of 

the areas searched to obtain the maximum daily maximum 8-hour ozone concentrations near a 

monitor used in the RRFs. This can potentially result in selecting the same maximum modeled 

concentrations from nearby grid cells to develop the RRFs for different monitors. Using the 

tighter 5 x 5 and 3 x 3 array of grid cells centered on each monitor (Figure 3-1b) reduces the 

overlap among nearby monitors and potentially retains the different characteristics of the 

monitoring sites, if such differences were captured by the model. For example, the CAMP 

monitor, and other more urban Denver monitoring sites, is clearly affected by the high NOx 

concentrations in metropolitan Denver that inhibit ozone formation. Use of the 7 x 7 array of 

grid cells results in selecting maximum modeled concentrations that are potentially outside of the 

influence of the high NOx concentration region for use in the RRFs thereby not capturing the 

NOx inhibition effect of these monitoring sites in the metropolitan Denver region (Figure 3-1). 

Figure 3-1a. Arrays of 7 x 7 4 km grid cells around monitors in the DMA. 

 

-800 -760 -720 -680 -640 -600

-120

-80

-40

0

40

80

120

SBC

ACAD

 ARVA

NREL 

RMNP

FTCW

 GREE

CHAT CHAT

MANI

FTCO

WCTO

 CARR

WELB

HIGH

CAMP

 RFNO

WELC

LOOK

Denver Ozone Monitoring Sites 

with surrounding 7x7 cells
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec3_Control_ModelMetrics2.doc January 2009 3-3 

 Figure 3-1b. Arrays of 5 x 5 (left) and 3 x 3 (right) 4 km grid cells around monitors in the DMA. 

Table 3-1 lists the projected 2010 DVFs at the RFNO and FTCW monitoring sites for the 2010 

Base Case and Final 2010 Control Strategy using the EPA guidance default and the six 

alternative ozone projection procedures discussed above. Also shown in Table 3-1 are the ozone 

cutoff thresholds and number of days used in calculating the RRFs for each alternative 2010 

ozone project method and the RFNO and FTCW monitoring sites. Results for all monitoring 

sites in the DMA are shown in Table 3-2. It should be noted that there is really no one “correct” 

method for projecting future year ozone concentrations that has been proven the most reliable. 

Methods based on just a few number of days have been shown to be less robust than ones based 

on more days. And it is logical that methods based on modeled concentrations closer to the 

observed 8-hour ozone Design Values would be more representative of the conditions that 

produced those Design Values than methods based on days with much higher or lower ozone 

concentrations than the Design Values. 

2010 Base Case: For the 2010 Base Case, the projected 2010 DVF using the EPA guidance 

default approach was 84.9 ppb at both the RFNO and FTCW monitoring sites. Some of the six 

alternative projection approaches result in increases, whereas others in decreases in the projected 

DVF relative to the EPA default approach at these two sites. The projected DVFs at RFNO for 

the 2010 Base Case range from 84.5 to 85.2 ppb with an average value of 84.950 ppb. A similar 

range for the FTCW monitor is 84.6 to 85.2 ppb with an average of 85.013 ppb. At the RFNO 

monitoring site, 3 of the 7 projection methods pass the modeled attainment demonstration test 

(43%), while at the FTCW 5 of the 7 methods pass the test (71%). 

Final 2010 Control Strategy: The 2010 ozone projections at the RFNO monitoring site for the 

Final 2010 Control Strategy are the same as for the 2010 Base Case using all the alternative 

projections procedures (e.g., average 84.950 ppb). At the at the FTCW monitoring site, the 

projected DVFs for the Final 2010 Control Strategy range from 84.6 to 85.1 ppb with an average 

of 84.963 ppb. 

-800 -760 -720 -680 -640 -600

-120

-80

-40

0

40

80

120

SBC

ACAD

 ARVA

NREL 

RMNP

FTCW

 GREE

CHAT CHAT

MANI

FTCO

WCTO

 CARR

WELB

HIGH

CAMP

 RFNO

WELC

LOOK

Denver Ozone Monitoring Sites

with surrounding 3x3 cells

-800 -760 -720 -680 -640 -600

-120

-80

-40

0

40

80

120

SBC

ACAD

 ARVA

NREL 

RMNP

FTCW

 GREE

CHAT CHAT

MANI

FTCO

WCTO

 CARR

WELB

HIGH

CAMP

 RFNO

WELC

LOOK

Denver Ozone Monitoring Sites

with surrounding 5x5 cells
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec3_Control_ModelMetrics2.doc January 2009 3-4 

An examination of the different 2010 ozone projection methods across monitoring sites shows no 

method is trending toward estimating higher or lower DVFs than the EPA default method across 

all monitoring sites. This is clearly shown in Table 3-1 for the RFNO and FTCW monitoring 

sites where, in most cases, a method in which the projected DVF at RFNO is greater than the 

EPA default method is below the EPA default method at FTCW and vice versa. The possible 

exception to this is at the downtown ozone monitors where as the array of grid cells becomes 

smaller, the projected DVF goes up. For example, at the CAMP monitor and the 2010 Base Case 

the projected DVF using the 7 x 7, 5 x 5 and 3 x 3 array of grid cells are, respectively, 56.0, 56.8 

and 57.3 ppb (Table 3-2a). This reflects the selection of modeling results closer to the urban 

core where the model is less responsive and where ozone increases from the 2006 to 2010 due to 

NOx emissions reductions (NOx disbenefits). It is encouraging that the EPA default 2010 ozone 

projection approach falls in between the alternative approaches and, in most cases, the average of 

the seven ozone projection approaches is close to the EPA default method approach. 

In conclusion, the alternative ozone projection approaches support the findings using the EPA 

default approach that the 2010 Base Case will likely achieve attainment in the Denver region of 

the 0.08 ppm 8-hour ozone NAAQS. The ozone projection methods indicate that there will be 

more certainty that the Denver region will achieve 8-hour ozone attainment in 2010 under the 

Final 2010 Control Strategy. 

Table 3-1. Projected 2010 8-hour ozone Design Values (DVFs) at the Rocky Flats North 

(RFNO) and Fort Collins West (FTCW) monitoring sites using the EPA guidance default 

approach and the six alternative projection approaches and the 2010 Base, Control 1 and 

Control 2 emission scenarios. 

Alternative 2010 Ozone Projection Procedures 

Name DVC EPA 5dth 1dth80 1dth75 1dth70 5x5 3x3 Avg 

2010 Base Case (Base) DVFs (ppb) 

Rocky Flats North 85.0 84.9 85.2 85.1 84.9 85.0 85.0 84.5 84.950 

Fort Collins - West 86.0 84.9 84.6 84.6 84.9 85.1 84.8 85.2 85.013 

Final 2010 Control Strategy DVFs (ppb) 

Rocky Flats North 85.0 84.9 85.2 85.1 84.9 85.0 85.0 84.5 84.950 

Fort Collins - West 86.0 84.8 84.6 84.6 84.8 85.0 84.8 85.1 84.963 

Cut-Off Concentration (ppb) 

Rocky Flats North 78 81 80 75 70 76 75 

Fort Collins - West 76 81 80 75 70 75 73 

Number of Days Used 

Rocky Flats North 10 6 7 19 27 11 10 

Fort Collins - West 10 5 5 13 22 10 10 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec3_Control_ModelMetrics2.doc January 2009 3-5 

Table 3-2a. Projected 2010 8-hour ozone Design Values (DVFs) at monitoring sites in the DMA 

using the EPA guidance default approach, the six alternative projection approaches and the 

2010 Base Case modeling results.

2010 Base Case 

Name DVC base 5dth 1dth80 1dth75 1dth70 5x5 3x3 

Welby 70.0 70.2 69.4 69.4 70.3 71.1 70.4 71.4 

Highland 78.0 77.3 76.2 77.4 77.5 78.0 77.6 78.0 

S. Boulder Creek 81.0 80.8 80.5 80.5 80.5 80.5 80.8 80.5 

Denver - CAMP 56.0 56.0 55.8 55.8 56.2 56.7 56.8 57.3 

Carriage 74.0 74.1 73.8 73.8 74.3 75.0 75.0 74.8 

Chatfield State Park 84.0 83.4 83.0 83.3 83.6 84.0 83.2 82.9 

USAF Academy 73.0 72.0 72.4 72.2 72.0 72.2 72.1 72.3 

Manitou Springs 74.0 73.7 73.7 73.6 73.8 73.5 73.6 73.7 

Arvada 79.0 79.2 78.9 78.9 79.4 79.7 79.5 79.4 

Welch 75.0 75.0 75.0 75.0 74.8 74.9 74.5 74.6 

Rocky Flats North 85.0 84.9 85.2 85.1 84.9 85.0 85.0 84.5 

NREL 82.0 82.3 82.6 82.5 82.2 82.0 82.0 81.8 

Fort Collins - West 86.0 84.9 84.6 84.6 84.9 85.1 84.8 85.2 

Fort Collins 74.0 73.0 72.8 72.8 73.0 73.3 73.3 73.6 

Greeley - Weld 78.0 77.7 78.0 77.5 77.7 77.7 77.7 77.8 

Gunnison 68.0 67.8 67.9 68.0 67.9 67.8 67.8 67.8 

Larimer 76.0 75.2 75.0 75.0 75.0 75.2 75.2 75.2 

Larimer 76.0 75.2 75.0 75.0 75.0 75.2 75.2 75.2 

Table 3-2b. Projected 2010 8-hour ozone Design Values (DVFs) at monitoring sites in the DMA 

using the EPA guidance default approach, the six alternative projection approaches and the 

Final 2010 Control Strategy modeling results.

Final 2010 Control Strategy 

Name DVC EPA 5dth 1dth80 1dth75 1dth70 5x5 3x3 

Welby 70.0 70.2 69.4 69.4 70.3 71.1 70.3 71.3 

Highland 78.0 77.3 76.2 77.4 77.5 78.0 77.6 78.0 

S. Boulder Creek 81.0 80.7 80.5 80.5 80.5 80.5 80.8 80.4 

Denver - CAMP 56.0 56.0 55.8 55.8 56.2 56.7 56.8 57.3 

Carriage 74.0 74.1 73.8 73.8 74.3 75.0 75.0 74.8 

Chatfield 84.0 83.4 82.9 83.3 83.6 83.9 83.2 82.9 

USAF Academy 73.0 72.0 72.4 72.2 72.0 72.2 72.1 72.3 

Manitou Springs 74.0 73.7 73.7 73.6 73.8 73.5 73.6 73.7 

Arvada 79.0 79.1 78.9 78.9 79.3 79.7 79.4 79.3 

Welch 75.0 75.0 75.0 75.0 74.7 74.9 74.5 74.5 

Rocky Flats North 85.0 84.9 85.2 85.1 84.9 85.0 85.0 84.5 

NREL 82.0 82.2 82.5 82.5 82.2 82.0 82.0 81.8 

Fort Collins - West 86.0 84.8 84.6 84.6 84.8 85.0 84.8 85.1 

Fort Collins 74.0 73.0 72.8 72.8 73.0 73.2 73.2 73.6 

Greeley - Weld 78.0 77.5 77.8 77.3 77.5 77.5 77.6 77.7 

Gunnison 68.0 67.8 67.9 68.0 67.9 67.8 67.8 67.8 

Larimer 76.0 75.2 75.0 75.0 75.0 75.2 75.2 75.2 

Larimer 76.0 75.2 75.0 75.0 75.0 75.2 75.2 75.2 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec3_Control_ModelMetrics2.doc January 2009 3-6 

Table 3-2c. Ozone cut-off threshold concentrations used in the 2010 8-hour ozone Design 

Value projections at monitoring sites in the DMA using the EPA guidance default and the six 

alternative projection approaches.

Cut-Off Concentration (ppb) 

Name EPA 5dth 1dth80 1dth75 1dth70 5x5 3x3 

Welby 77.0 80.0 80.0 75.0 70.0 76.0 74.0 

Highland 78.0 83.0 80.0 75.0 70.0 78.0 76.0 

S. Boulder Creek 78.0 80.0 80.0 75.0 70.0 76.0 75.0 

Denver - CAMP 78.0 80.0 80.0 75.0 70.0 74.0 72.0 

Carriage 78.0 80.0 80.0 75.0 70.0 76.0 73.0 

Chatfield State Park 78.0 81.0 80.0 75.0 70.0 78.0 77.0 

USAF Academy 75.0 78.0 80.0 75.0 70.0 75.0 73.0 

Manitou Springs 74.0 78.0 80.0 75.0 70.0 73.0 72.0 

Arvada 78.0 80.0 80.0 75.0 70.0 76.0 75.0 

Welch 78.0 81.0 80.0 75.0 70.0 77.0 76.0 

Rocky Flats North 78.0 81.0 80.0 75.0 70.0 76.0 75.0 

NREL 78.0 81.0 80.0 75.0 70.0 77.0 75.0 

Fort Collins - West 76.0 81.0 80.0 75.0 70.0 75.0 73.0 

Fort Collins 76.0 80.0 80.0 75.0 70.0 75.0 73.0 

Greeley - Weld 75.0 77.0 80.0 75.0 70.0 74.0 73.0 

Gunnison 74.0 78.0 80.0 75.0 70.0 74.0 73.0 

Larimer 77.0 80.0 80.0 75.0 70.0 77.0 76.0 

Larimer 77.0 80.0 80.0 75.0 70.0 77.0 76.0 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec3_Control_ModelMetrics2.doc January 2009 3-7 

Table 3-2d. Number of modeling days used in the 2010 8-hour ozone Design Value projections 

at monitoring sites in the DMA using the EPA guidance default and the six alternative projection 

approaches.

Number of Days Used 

Name EPA 5dth 1dth80 1dth75 1dth70 5x5 3x3 

Welby 11 6 6 13 29 10 10 

Highland 14 5 9 17 37 10 11 

S. Boulder Creek 10 6 6 18 28 12 11 

Denver - CAMP 10 7 7 14 31 12 10 

Carriage 10 6 6 15 32 10 12 

Chatfield State Park 11 5 6 18 33 11 10 

USAF Academy 10 6 3 10 30 10 10 

Manitou Springs 10 5 3 9 25 12 12 

Arvada 10 7 7 16 25 12 11 

Welch 10 5 5 17 31 12 11 

Rocky Flats North 10 6 7 19 27 11 10 

NREL 11 5 6 16 30 13 10 

Fort Collins - West 10 5 5 13 22 10 10 

Fort Collins 12 5 5 14 24 10 10 

Greeley - Weld 10 5 2 10 22 11 11 

Gunnison 10 7 3 9 14 10 10 

Larimer 10 5 5 13 21 10 10 

Larimer 10 5 5 13 21 10 10 

3.3 ADDITIONAL MODELING METRICS 

EPA’s 8-hour ozone modeling guidance recommends calculating additional modeling metrics 

from the current year base case to future year control scenarios to assure that they indicate the 

modeled ozone concentrations are going down. These additional modeling metrics examine the 

ozone differences between the current year base case and future year emission scenarios in the 

modeling domain to assure that ozone is going down, on average, across the entire nonattainment 

area (NAA) rather than just limited to a few key monitoring sites. 

The changes in daily maximum 8-hour ozone concentrations between the 2006 Base Case and 

2010 emission scenarios were calculated across grid cells in the Denver NAA and across all days 

in the June-July 2006 modeling episode. The changes 8-hour ozone concentrations are 

calculated for values above four separate threshold concentrations: 85, 80, 75 and 70 ppb. These 

modeling metrics consist of the following: 

Total Ozone: Defined as the difference between the modeled daily maximum 8-hour 

ozone concentrations and the threshold concentration, for modeled values above the 

threshold, summed across all grid cells in the Denver NAA and modeling days during 

June-July 2006. 

Grid Cells: Number of grid cell-days with modeled daily maximum 8-hour ozone 

concentrations greater than the threshold for all grid cells in the Denver NAA and days 

from the June-July 2006 episode. 

Figure 3-2 displays the percent change in the Total Ozone and Grid Cells between the 2006 Base 

Case and the 2010 Base Case and Final 2010 Control Strategy emission scenarios. Using the 70 

ppb threshold, there are small reductions between 2006 and 2010 in the Total Ozone 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec3_Control_ModelMetrics2.doc January 2009 3-8 

(approximately -5%) and Grid Cell (approximately -3.5%) modeling metrics. However, the 

emission reductions between 2006 and 2010 are having their intended effect by having more 

reductions in the higher 8-hour ozone concentrations in the Denver region. For example, the 

changes in Total Ozone and Grid Cells with ozone greater than the 85 ppb threshold are reduced 

by approximately -21% and -14%, respectively. These reductions are even greater for the Final 

2010 Control Strategy scenario than the 2010 Base Case. For example, the Total Ozone above 

85 ppb is reduced by -21.9% in the 2010 Base Case and reduced event more (-23.4%) in the 

Final 2010 Control Strategy. 

Percent Change in Total Ozone for 2010 Base and 2010 

FSIP

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

Base FSIP

% Change

Total Ozone

85 ppb

Total Ozone

80 ppb

Total Ozone

75 ppb

Total Ozone

70 ppb

Figure 3-2a. Percent change in Total Ozone greater than 85, 80, 75 and 70 ppb between the 

2006 Base Case and the 2010 Base Case (Base) and Final 2010 Control Strategy (FSIP) 

emission scenarios. 
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec3_Control_ModelMetrics2.doc January 2009 3-9 

Percent Change in Grid Cells for 2010 Base and 2010 FSIP

-16.0

-14.0

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

Base FSIP

% Change

Grid Cells

85 ppb

Grid Cells

80 ppb

Grid Cells

75 ppb

Grid Cells

70 ppb

Figure 3-2b. Percent change in Grid Cells greater than 85, 80, 75 and 70 ppb between the 

2006 Base Case and the 2010 Base Case (Base) and Final 2010 Control Strategy (FSIP) 

emission scenarios.
F:\Denver_O3_2008\Reports\2010_Final_Control_Strategy\Draft_Final\Sec4_Refs2.doc January 2009 4-1 

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