C:\Documents and Settings\krbriggs\Desktop\Exec_Sum1.doc September 2008 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 set up for 

a June-July 2006 episode on 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 

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., 2008b). A revised final CAMx 2006 base case simulation 

was performed and a comprehensive model performance evaluation was conducted (Morris et 

al., 2008c). 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 looking at many model performance displays and metrics we concluded 

that the model was simulating the observed ozone sufficiently well for use in making ozone 

projections. Furthermore, on most days the model reproduced the observed VOC/NOx ratios in 

Denver quite well suggesting that the model is simulating the same chemical regimes as 

observed as well. 

2010 BASE CASE 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 2006 and 2010 base case modeling results in a relative fashion whereby modeled relative 

response factors (RRFs) are used to scale the current year 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). 
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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 from 2006-2007 for 

the FTCW DVC. 

Table ES-1 summarizes the projected 8-hour ozone Design Values (DVF) at the DMA 

monitoring sites for the 2010 base case simulation using the CAMx 2006 and 2010 base case 

modeling results and EPA recommended default ozone projection procedures (EPA, 2007). The 

maximum projected 8-hour ozone Design Value is 84 ppb and occurs at both the Rocky Flats 

North (RFNO) and Fort Collins West (FCTW) monitoring sites (see column 5 in Table ES-1). 

As this value is 84 ppb or lower, then the 2010 base case modeling results pass the model 

attainment demonstration test. EPA’s guidance for making 8-hour ozone projections 

recommends truncating the final projected DVF for comparisons with the 85 ppb NAAQS. In 

column 6 of Table ES-1 the DVFs are presented to the nearest tenth of a ppb before truncation, 

in which case we see that the projected 2010 base case DVFs at RFNO and FTCW are both 84.9 

ppb. Also shown in Table ES-1 are the RRFs and the cut-off thresholds used in selecting days 

and number of days used in calculating the RRF. The EPA desire to use at least 10 modeling 

days and a cutoff threshold of 70 ppb or higher is satisfied using the Denver June-July 2006 

modeling period at all monitoring sites. Modeling days are selected based on whether the 2006 

base case model estimated maximum daily maximum 8-hour ozone concentration near a monitor 

(i.e., within 7 x 7 array of 4 km grid cells centered on the monitor) are above the cut-off 

threshold. In order to achieve at least 10 model days for developing the RRFs, the cut-off 

thresholds of 74 ppb to 78 ppb were used depending on the monitor (the key RFNO and FTCW 

monitors using a 78 and 76 ppb cut-off thresholds, respectively). 

Table ES-1. Current-year (DVC) and projected future-year (DVF) 8-hour ozone Design Values 

using the CAMx 2006 and 2010 base case modeling results. 

2005-07 2010 Base Case 

Site ID Monitor Name County DVC DVF DVF RRF Cutoff #days 

80013001 Welby Adams 70.0 70 70.2 1.0042 77.0 11 

80050002 Highland Arapahoe 78.0 77 77.3 0.9916 78.0 14 

80130011 S. Boulder Creek Boulder 81.0 80 80.8 0.9976 78.0 10 

80310002 Denver - CAMP Denver 56.0 56 56.0 1.0017 78.0 10 

80310014 Carriage Denver 74.0 74 74.1 1.0022 78.0 10 

80350004 Chatfield State Park Douglas 84.0 83 83.4 0.9934 78.0 11 

80410013 USAF Academy El Paso 73.0 72 72.0 0.9873 75.0 10 

80410016 Manitou Springs El Paso 74.0 73 73.7 0.9966 74.0 10 

80590002 Arvada Jefferson 79.0 79 79.2 1.0026 78.0 10 

80590005 Welch Jefferson 75.0 75 75.0 1.0004 78.0 10 

80590006 Rocky Flats North Jefferson 85.0 84 84.9 0.9994 78.0 10 

80590011 NREL Jefferson 82.0 82 82.3 1.0039 78.0 11 

80690011 Fort Collins - West Larimer 86.0 84 84.9 0.9874 76.0 10 

80691004 Fort Collins Larimer 74.0 73 73.0 0.9878 76.0 12 

81230009 Greeley - Weld Tower Weld 78.0 77 77.7 0.9964 75.0 10 

GTH161 Gunnison Gunnison 68.0 67 67.8 0.9984 74.0 10 

ROM206 Larimer Larimer 76.0 75 75.2 0.9903 77.0 10 

ROM406 Larimer Larimer 76.0 75 75.2 0.9903 77.0 10 
C:\Documents and Settings\krbriggs\Desktop\Exec_Sum1.doc September 2008 ES-3

2010 CONTROL PLAN EMISSION SCENARIOS 

2010 ozone projections were made for two 2010 emission control plans: (1) 2010 Control 1 that 

consists of the federally-enforceable control measures that are proposed for the Denver 8-hour 

ozone State Implement plan (SIP); and (2) Control 2 that includes the federally-enforceable 

measures of Control 1 plus additional control measures that are adopted as state-only 

enforceable. The 2010 ozone projections for the two control plans were made using the same 

procedures as for the 2010 base case. Table ES-2 summarizes the control measures for the two 

2010 control plans. 

Table ES-2. Summary of control measures in the 2010 Control 1 and Control 2 emission 

scenarios. 

Strategies Under Development for 2008 Proposed Ozone Action Plan 

(All strategies apply to the entire Denver/North Front Range nonattainment area (NAA) unless otherwise 

noted)

Control 1

Recommended Measures 

for Federally-Enforceable 

State Implementation Plan (SIP) 

Potential 

Emission 

Reduction

Control 2

Recommend Measures 

Adopted and Enforced 

as State-only Measures 

Potential 

Emission 

Reduction

3⁄4 More stringent Reg. 11 I/M cutpoints 

 (Denver area) – adopted, effective 

 May 1, 2008 

~ 1 tpd VOC, 

~3 tpd NOx, 

~13 tpd CO

3⁄4 Inspection/maintenance program in 

 North Front Range (structure to be 

 determined) 

~ 1 tpd VOC,

 ~1 tpd NOx, 

~17 tpd CO 

3⁄4 7.8 RVP gasoline regulatory 

requirement in North Front Range 

(consistent with Denver area) 

~ 3 tpd VOC

3⁄4 Mandatory high-emitter pilot

program (Denver area) – began 

January 1, 2008 

unknown at 

this time 

3⁄4 Tighten up collector plate 

requirements for older vehicles 

(statewide) 

< 1 tpd VOC 

~ 7 tpd CO

3⁄4 Increase condensate tank control 

(95%) 

 for all new/modified tanks >2 tpy 

(2009) 

 for all existing tanks >10 tpy (2010) 

VOC 

~ 6-9 tpd 

~19-30 tpd

3⁄4 Increase condensate tank control 

(95%) 

 for all existing tanks >5 tpy (2011) 

 for all existing tanks >2 tpy (2012) 

VOC 

~ 30-35 tpd 

~9-12 tpd

3⁄4 Pneumatic valves controls - require 

 low/no bleed valves on all new and 

 existing valves by 2009 

~ 23 tpd VOC

3⁄4 Statewide Oil & Gas regulations -- 

 Controls on existing reciprocating 

 internal combustion engines 

unknown at 

this time

3⁄4 Expand Reg. 7 (VOC control 

requirements) to entire NAA

unknown at 

this time

3⁄4 Remove current exemptions in Reg. 

3 for selected small sources required 

to file air pollution emission notices 

and obtain permits

unknown at 

this time

3⁄4 Require Reasonably Available 

 Control Technology (RACT) for minor

 sources in NAA (Reg. 3)

unknown at 

this time

TOTAL 

EMISSION 

REDUCTIONS 

VOC 

NOx 

CO 

~52-66 tpd 

~ 3 tpd 

~13 tpd 

VOC 

NOx 

CO 

~41-49 tpd 

NA 

 >24 tpd
C:\Documents and Settings\krbriggs\Desktop\Exec_Sum1.doc September 2008 ES-4

2010 CONTROL PLAN OZONE PROJECTIONS 

Table ES-3 displays the projected 8-hour ozone DVFs for the 2010 base case and the two 2010 

control plans. The maximum projected 8-hour ozone Design Values for the 2010 Base, Control 

1 and Control 2 emissions scenarios are 84 ppb at the Rocky Flats North (RFNO) and Fort 

Collins West (FTCW) monitoring sites. Thus, the 2010 Base Case and two control scenarios 

pass the modeled 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), an additional weight of evidence (WOE) analysis is required. 

When reporting the DVFs to the nearest tenth of a ppb we see that the projected 2010 DVF for 

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

of the federally enforceable SIP control measures in the 2010 Control 1 emissions scenario 

reduces the DVFs at the RFNO and FTCW by, respectively, 0.1 and 0.2 ppb (to 84.8 and 84.7 

ppb, respectively). The addition of the state-enforceable control measures in the 2010 Control 2 

scenario reduces the DVFs at RFNO and FRTCW by an additional 0.1 and 0.2 ppb, respectively 

(to 84.7 and 84.5 ppb, respectively). 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 ES-3. Projected 2010 8-hour ozone Design Values (DVFs) for the 

2010 Base Case and 2010 Control 1 (Cntl1) and Control 2 (Cntl2) control strategies. 

DVF (EPA Recommended) DVF (to nearest 0.1 ppb) 

Name County DVC Base Cntl1 Cntl2 Base Cntl1 Cntl2 

Welby Adams 70.0 70 70 70 70.2 70.2 70.2 

Highland Arapahoe 78.0 77 77 77 77.3 77.2 77.1 

S. Boulder Creek Boulder 81.0 80 80 80 80.8 80.7 80.6 

Denver - CAMP Denver 56.0 56 56 55 56.0 56.0 55.9 

Carriage Denver 74.0 74 74 74 74.1 74.1 74.0 

Chatfield State Park Douglas 84.0 83 83 83 83.4 83.3 83.3 

USAF Academy El Paso 73.0 72 71 71 72.0 71.9 71.9 

Manitou Springs El Paso 74.0 73 73 73 73.7 73.7 73.7 

Arvada Jefferson 79.0 79 79 79 79.2 79.1 79.1 

Welch Jefferson 75.0 75 75 74 75.0 75.0 74.9 

Rocky Flats North Jefferson 85.0 84 84 84 84.9 84.8 84.7 

NREL Jefferson 82.0 82 82 82 82.3 82.2 82.1 

Fort Collins - West Larimer 86.0 84 84 84 84.9 84.7 84.5 

Fort Collins Larimer 74.0 73 72 72 73.0 72.9 72.7 

Greeley-WeldTower Weld 78.0 77 77 77 77.7 77.4 77.0 

Gunnison Gunnison 68.0 67 67 67 67.8 67.8 67.9 

Larimer Larimer 76.0 75 75 75 75.2 75.1 75.1 

Larimer Larimer 76.0 75 75 75 75.2 75.1 75.1 
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2010 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. 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 using the modeling data within each grid cell 

rather than near a grid cell as done for the projections at the monitor. 

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

projected 8-hour ozone Design Values (DVFs) for the 2010 Base Case 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 occur 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 

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 2010 Control 1 (left) 

and 2010 Control 2 (right) emission scenarios. There are slight reductions in the 2010 DVFs 

over the 2010 Base Case, which can be seen more clearly in the difference plots seen in Figure 

ES-3. The 6 remaining grid cells with projected DVFs that are 85 ppb or higher in the 2010 Base 

case are reduced to 4 and 3 grid cells in the, respectively, 2010 Control 1 and Control 2 emission 

scenarios. 

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. At a minimum, 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. 
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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). 

Figure ES-2. Projected 2010 8-hour ozone Design Values (DVF) for the 2010 Control 1 (left) 

and Control 2 (right) emission scenarios. 

Figure ES-3. Differences in projected 2010 8-hour ozone Design Values (DVF) between the 

2010 Control 1 (left) and 2010 Control 2 (right) and the 2010 Base Case. 

 
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ALTERNATIVE 2010 OZONE PROJECTION RESULTS 

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

Control 1 and Control 2 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 under either the 2010 Base Case, Control 1 or 

Control 2 emission scenarios. 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 previously: 

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

default approach, days are selected for use 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: In those two alternative 

projection approaches the cutoff threshold is reduced to 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 

Table ES-4 lists the projected 2010 DVFs at the key RFNO and FTCW monitoring sites for the 

EPA guidance default and the six alternative ozone projection procedures discussed above. Also 

shown in Table ES-4 are the ozone cutoff thresholds and number of days used in calculating the 

RRFs for each alternative 2010 ozone project methods and the RFNO and FTCW monitoring 

sites. 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 much higher or lower 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 

2010 DVFs at these two sites relative to the EPA guidance default approach. The projected 

DVFs at RFNO for the 2010 Base Case range from 84.5 to 85.2 with an average value of 84.9 

ppb. A similar range for the FTCW monitor is 84.6 to 85.2 ppb with an average of 84.9 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%). 
C:\Documents and Settings\krbriggs\Desktop\Exec_Sum1.doc September 2008 ES-8

2010 Control 1 Case: A majority of the 2010 ozone projection procedures pass the modeled 

attainment demonstration test at both the RFNO (4 out of 7, 57%) and FTCW (6 out of 7, 86%) 

monitoring sites. At the RFNO monitoring site, the projected DVFs for the 2010 Control 1 

scenario range from 84.3 to 85.1 ppb with an average of 84.8 ppb. And at the FTCW monitoring 

site the projected DVFs range from 84.4 to 85.0 ppb with an average of 84.7 ppb. 

2010 Control 2 Case: The 2010 projected DVFs at RFNO for the 2010 Control 2 case are similar 

to the 2010 Control 1 case ranging from 84.3 to 85.1 ppb, with an average of 84.8 ppb. More 

benefits are seen at FTCW where the 2010 projected DVFs range from 84.3 to 84.8 ppb with an 

average of 84.5 ppb. 

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

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

all monitoring sites. This is clearly shown in Table ES-4 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. 

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 

2010 Control 1 and Control 2 emission scenarios. 

Table ES-4. 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, the six alternative projection approaches and the 2010 Base, Control 1 and Control 2 

modeling results. 

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 84.9 

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

2010 Control Strategy No. 1 (Cntl1) DVFs (ppb) 

Rocky Flats North 85.0 84.8 85.1 85.0 84.8 85.0 84.9 84.3 84.8 

Fort Collins - West 86.0 84.7 84.4 84.4 84.7 84.9 84.6 85.0 84.7 

2010 Control Strategy No. 2 (Cntl2) DVFs (ppb) 

Rocky Flats North 85.0 84.7 85.1 84.9 84.8 84.9 84.8 84.3 84.8 

Fort Collins - West 86.0 84.5 84.3 84.3 84.5 84.7 84.5 84.8 84.5 

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 
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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 scenario 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 was 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 NAA and days from the 

June-July 2006 episode. 

Grid Cell-Hours: Number of grid cell-hours with modeled running 8-hour ozone 

concentrations greater than the threshold for all grid cells in the NAA and hours during 

the June-July 2006 episode. 

Figure ES-4 displays the percent change in the Total Ozone and Grid Cells between the 2006 

Base Case and the 2010 emission scenarios (the change in Grid-Cell Hours is similar). There are 

small reductions between 2006 and 2010 in the Total Ozone (~-5%) and Grid Cell (~-3%) 

modeling metrics greater than the 70 ppb threshold. However, the emission reductions between 

2006 and 2010 are having their intended effect in being more effective at reducing the elevated 

8-hour ozone concentrations. For example, the changes in Total Ozone and Grid Cells greater 

than 85 ppb modeling metrics between the 2006 and 2010 Base Cases are -21% and -14% , 

respectively. These reductions are even greater for the 2010 Control 1 case (-28% and -17%) 

and even greater still for the 2010 Control 2 scenario. 
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Percent Change in Total Ozone for 2010 Base, Cntrl1, and 

Cntrl2

-35.0

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

Base CNTL1 CNTL2

% Change

Total Ozone

85 ppb

Total Ozone

80 ppb

Total Ozone

75 ppb

Total Ozone

70 ppb

Percent Change in Grid Cells for 2010 Base, Cntrl1, and 

Cntrl2

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

Base CNTL1 CNTL2

% Change

Grid Cells

85 ppb

Grid Cells

80 ppb

Grid Cells

75 ppb

Grid Cells

70 ppb

Figure ES-4. Percent change in Total Ozone (left) and Grid Cells (right) greater than 85, 80, 75 

and 70 ppb between the 2006 Base Case and the 2010 Base Case (Base), Control 1 (CNTL1) 

and Control 2 (CNTL2) emission scenarios.

2010 CONTROL PLAN MODELING CONCLUSIONS 

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

8-hour ozone NAAQS (0.08 ppm) by 2010 under any of the three 2010 emission scenarios 

studied. All three 2010 emission scenarios pass the modeled attainment demonstration test. 

Examining the unmonitored area test, the alternative ozone projection procedures and additional 

modeling metrics we conclude that the two 2010 control strategies 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 are a lot of year-to-year meteorological 

variations in the Denver area that greatly affect 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 JuneJuly 2006 modeling period, then that could jeopardize achieving attainment in 2010. However, 

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.