Denver Metropolitan Area and North Front Range 8-Hour 

Ozone State Implementation Plan 

Weight of Evidence to Support the Modeled Attainment 

Demonstration

DRAFT FINAL 

October 29, 2008 
Preface 

This Weight-of-Evidence report was completed in October 2008 prior to the 

Colorado Air Quality Control Commission’s (AQCC) December 12, 2008 approval 

of the Ozone Action Plan, including revisions to the State Implementation Plan. 

After the hearing and as directed by the AQCC, the photochemical modeling was 

revised in the late December 2008 to early January 2009 timeframe to 

incorporate the final SIP control measures approved by the AQCC, which are 

based on a slight modification of the original Ozone Action Plan’s Alternative 

Proposal #2. 

The emission control scenarios (2010 base case, 2010 control 1, and 2010 

control 2) referenced in this Weight-of Evidence document are summarized in 

Table ES-2 in the following September 2008 report: 

Morris R., T. Sakulyanontvittaya, E. Tai, D. McNally and C. Loomis. 2008. 

2010 Ozone Attainment Demonstration Modeling for the Denver 8-Hour 

Ozone State Implementation Plan Control Strategy. ENVIRON 

International Corporation, Novato, California. Prepared for Denver 

Regional Air Quality Council (RAQC), Denver, Colorado. September 22, 

2008. 

(http://www.colorado.gov/airquality/documents/deno308/Denver_2010Con

trolStrat_Draft_Sep22_2008.pdf) 

The final SIP control measures (referred to as the “Final 2010 Control Strategy”) 

are summarized in the following January 2009 report: 

Morris R., E. Tai, T. Sakulyanontvittaya, D. McNally and C. Loomis. 2009. 

Final 2010 Ozone Attainment Demonstration Modeling for the Denver 8-

Hour Ozone State Implementation Plan. ENVIRON International 

Corporation, Novato, California. Prepared for Denver Regional Air Quality 

Council (RAQC), Denver, Colorado. January 12, 2009. 

(http://www.colorado.gov/airquality/documents/deno308/2010_Denver_Fin

al_Control_Jan12_2009.pdf)

Section 3.2 (‘Alternative 2010 Ozone Projection Procedures’) of the January 

2009 report (Morris, et al, 2009) supplements section 4.3 (‘Alternative 2010 

Ozone Projections’) of this Weight-of-Evidence document by including metrics for 

the “Final 2010 Control Strategy.” Similarly, section 3.3 (‘Additional Modeling 

Metrics’) of the January 2009 report (Morris, et al, 2009) supplements section 4.1 

(‘Additional Modeling Metrics’) of this Weight-of-Evidence document. 

The final ozone modeling and analysis (Morris, et al, 2009) do not contain any 

results that alter the conclusions in this Weight-of-Evidence Report. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

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Table of Contents 

Executive Summary............................................................................................................. I

1 Introduction to the Weight of Evidence (WOE) Analysis.......................................... 1 

2 Air Quality Related Trends......................................................................................... 2 

2.1 Emission Trends.................................................................................................. 2 

2.2 Ozone Monitoring Trends................................................................................... 5 

2.2.1 Time Series of Monitored 4th maximum ozone values............................... 5 

2.2.2 Linear Regression of Monitored 4th maximum ozone values ..................... 6 

2.2.3 Time Series of Monitored Highest Maximum Ozone Concentrations ....... 7 

2.2.4 Trend in Days greater than 75 ppb and 84 ppb........................................... 8 

2.2.5 Trends in Three Year Design Values.......................................................... 9 

2.3 Weather-Corrected Ozone Time Series .............................................................. 9 

2.4 Trends in the Weekend-Weekday Effect .......................................................... 21 

3 Review of the Ozone Conceptual Model for the 8-hour DMA/NFR NAA.............. 29 

3.1 Local and Synoptic Scale Meteorology ............................................................ 29 

3.2 Back Trajectory Analyses for the 2006 Ozone Episodes and Ozone Season... 31 

3.2.1 Trajectory Analysis June 17-19, 2006 ...................................................... 33 

3.2.2 Trajectory Analysis July 13-15, 2006....................................................... 36 

3.2.3 Trajectory Analysis July 27-29, 2006....................................................... 41 

3.2.4 Back Trajectory Climatology for the Summer of 2006 ............................ 45 

3.2.5 Trajectory Analysis Conclusions .............................................................. 48 

4 Review of Modeled Metrics and Modeled Attainment Demonstration.................... 49 

4.1 Additional Modeling Metrics............................................................................ 49 

4.1.1 Relative Change in Grid Cells .................................................................. 50 

4.1.2 Relative Change in Grid Cell-Hours......................................................... 51 

4.1.3 Relative Change in Total 8-Hour Ozone .................................................. 51 

4.2 Review Alternative Attainment Test Methodology.......................................... 53 

4.3 Alternative 2010 Ozone Projections ................................................................. 57 

4.4 Modeled vs. Measured VOC/NOx Ratios ........................................................ 59 

4.5 Model Uncertainty and Limitations.................................................................. 61 

5 Assess the Efficacy of SIP, State-only and Voluntary Control Strategies................ 64 

6 Weight of Evidence – Conclusions........................................................................... 67 

7 References................................................................................................................. 70
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

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List of Tables 

Table 2-1: Linear Regression Statistics for the Relationship between July Mean Daily 

Maximum 8-hour Ozone Concentrations and July Mean 500-millibar Heights....... 13 

Table 2-2: Linear Regression Statistics for the Relationship between Annual 4th

Maximum 8-hour Ozone Concentrations and July Mean 500-millibar Heights....... 15 

Table 2-3: Percent Increase in Weekend Daily Maximum 8-Hour Ozone for June through 

August of 2005 through 2007. .................................................................................. 26 

Table 4-1: 2010 Base Case Design Values for Each Monitoring Site for Modeled Days 

greater than 0.075 ppm ............................................................................................. 54 

Table 4-2: 2010 Base Case Design Values Utilizing EPA’s Recommended DVB 

Calculation Methodology.......................................................................................... 55 

Table 4-3: 2010 “Control 1” Design Values Utilizing EPA’s Recommended DVB 

Calculation Methodology.......................................................................................... 56 

Table 4-4: 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................................................................. 59 

List of Figures 

Figure 2-1: Emissions for 2006 Projected to 2010 Base and 2010 SIP.............................. 3 

Figure 2-2: Emissions for 2006 Projected to 2010 Base and 2010 SIP.............................. 4 

Figure 2-3: Trends in Mobile Source Emissions ................................................................ 4 

Figure 2-4: Time Series of Monitored 4th Maximum 8-Hour Ozone Values (ppm).......... 5 

Figure 2-5: Linear Regression of Monitored 4th maximum ozone values .......................... 6 

Figure 2-6: First Maximum Ozone Concentrations at Rocky Flats-N (RFN), Chatfield 

(CHAT), National Renewable Energy Laboratory (NREL) and Fort Collins West 

(FTCW) [Note: Data for 2008 are preliminary.] ........................................................ 7 

Figure 2-7: Days with 8-hour ozone greater than 75 ppb and greater than or equal to 85 

ppb............................................................................................................................... 8 

Figure 2-8: Design Value Days (Three Year Average of the Fourth High Concentration) 9 

Figure 2-9: Linear regression between July monthly mean daily maximum 8-hour ozone 

and July monthly mean 500-millibar heights at Rocky Flats North. ........................ 11 

Figure 2-10: Linear regression between July monthly mean daily maximum 8-hour 

ozone and July monthly mean 500-millibar heights at NREL.................................. 12 

Figure 2-11: Linear regression between July monthly mean daily maximum 8-hour ozone 

and July monthly mean 500-millibar heights at Chatfield Reservoir. ...................... 12 

Figure 2-12: Linear regression between annual 4th maximum 8-hour ozone and July 

monthly mean 500-millibar heights at Rocky Flats North........................................ 14 

Figure 2-13: Linear regression between annual 4th maximum 8-hour ozone and July 

monthly mean 500-millibar heights at NREL........................................................... 14 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

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Figure 2-14:Linear regression between annual 4th maximum 8-hour ozone and July 

monthly mean 500-millibar heights at Chatfield Reservoir...................................... 15 

Figure 2-15: The trend in weather-corrected July mean daily max 8-hour ozone for Fort 

Collins and Greeley................................................................................................... 17 

Figure 2-16: The trend in weather-corrected July mean daily max 8-hour ozone for Rocky 

Flats, NREL, Chatfield, Carriage, South Boulder Creek, and Arvada. .................... 18 

Figure 2-17: The trend in weather-corrected annual 4th maximum 8-hour ozone for Rocky 

Flats, NREL, Chatfield, Carriage, South Boulder Creek, Arvada, Fort Collins, and 

Greeley...................................................................................................................... 18

Figure 2-18:Trends in the mean strength of the upper level high pressure or 500-millibar 

heights for the Denver-area NCEP Reanalysis data. ................................................ 20 

Figure 2-19: Trends in the mean strength of the upper level high pressure or 500-millibar 

heights for the Denver-area NCEP Reanalysis data, with Lowess trend curve at 40% 

weighting................................................................................................................... 20 

Figure 2-20: Mean hourly ozone concentrations at 5 monitoring sites (the NOAA Erie 

Tower surface and 300-meter levels, NREL, and NOAA’s two Niwot Ridge sites) 

for July 11-31, 2008, showing variations in ozone by time of day and altitude....... 22 

Figure 2-21: Mean hourly ozone concentrations at the NOAA Erie Tower surface and 

300-meter levels showing variations by time of day, by altitude, and for weekends 

and weekdays (for July 1 – August 31, 2008.) ......................................................... 24 

Figure 2-22: Weekend-Weekday differences in mean hourly ozone at the NOAA Erie 

Tower surface and 300-meter levels for July 1 – August 31, 2008, with a Lowess 

curves at 30% weighting........................................................................................... 25 

Figure 2-23: Mean ratio of daily maximum 8-hour ozone on weekends to weekdays for 

June through August of 2005 through 2007. The color-scale legend is not intended 

to define a threshold or cutoff for NOx and VOC limitation, but rather to indicate a 

general tendency towards either state at opposite ends of the scale ......................... 28 

Figure 3-1: HYSPLIT 48-hour back trajectories for 10 (red), 100 (blue), and 1000 (green) 

meter elevations for June 17-19, 2006, at RFN (left) and FTCW (right), 6-hour 

interval markers, 4 PM MDT.................................................................................... 34 

Figure 3-2: June 19, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for 

RFN (left) and FTCW (right). Four trajectories, each starting at 5 AM and arriving 

at 8 AM (short red), 11 AM (green), 2 PM (blue) and 5 PM (long red) with 1-hour 

interval markers. ....................................................................................................... 35 

Figure 3-3: HYSPLIT 48-hour back trajectories for 10 (red), 100 (blue), and 1000 (green) 

meter elevations for July 13-15, 2006, at RFN (left) and FTCW (right), 6-hour 

interval markers, 4 PM MDT.................................................................................... 38 

Figure 3-4: Boulder ozonesonde data for Noon MDT July 14, 2006, showing elevated 

ozone concentrations from the surface to 17,000 feet above sea level..................... 39 

Figure 3-5: July 13, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for 

RFN (left) and FTCW (right). Four trajectories, each starting at 5 AM and arriving 

at 8 AM (short red), 11 AM (green), 2 PM (blue) and 5 PM (long red) with 1-hour 

interval markers. ....................................................................................................... 39 

Figure 3-6: July 14, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for 

RFN (left) and FTCW (right). Four trajectories, each starting at 5 AM and arriving 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

iv

at 8 AM (short red), 11 AM (green), 2 PM (blue) and 5 PM (long red) with 1-hour 

interval markers. ....................................................................................................... 40 

Figure 3-7: July 15, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for 

RFN (left) and FTCW (right). Four trajectories, each starting at 5 AM and arriving 

at 8 AM (short red), 11 AM (green), 2 PM (blue) and 5 PM (long red) with 1-hour 

interval markers. ....................................................................................................... 40 

Figure 3-8: HYSPLIT 48-hour back trajectories for 10 (red), 100 (blue), and 1000 (green) 

meter elevations for July 27-29, 2006, at RFN (left) and FTCW (right), 6-hour 

interval markers, ending 4 PM.................................................................................. 43 

Figure 3-9: July 27, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for 

RFN (left) and FTCW (right). Four trajectories, each starting at 5 AM and arriving 

at 8 AM (short red), 11 AM (green), 2 PM (blue) and 5 PM (long red) with 1-hour 

interval markers. ....................................................................................................... 44 

Figure 3-10: July 28, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for 

RFN (left) and FTCW (right). Four trajectories, each starting at 5 AM and arriving 

at 8 AM (short red), 11 AM (green), 2 PM (blue) and 5 PM (long red) with 1-hour 

interval markers. ....................................................................................................... 44 

Figure 3-11: July 29, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for 

RFN (left) and FTCW (right). Four trajectories, each starting at 5 AM and arriving 

at 8 AM (short red), 11 AM (green), 2 PM (blue) and 5 PM (long red) with 1-hour 

interval markers. ....................................................................................................... 45 

Figure 3-12: NOAA HYSPLIT same-day back trajectory points for May 17 through 

August 15, 2006, for FTCW, RMNP, HLD, and RFN – back trajectories up to 12 

hours in length for each hour that was used to calculate a daily 8-hour maximum 

ozone concentration, each point representing one hour, with average start times of 

6:30 AM MDT. ......................................................................................................... 47 

Figure 3-13: Contours of the mean May 17 – August 15, 2006, Front Range daily 

maximum 8-hour ozone concentrations resulting from transport from given source 

areas, based on a moving spatial average analysis of concentrations associated with 

HYSPLIT back trajectory points for FTCW, RMNP, RFN, and HLD monitors. 

These are the average concentrations that result at these four monitors when an air 

mass originates in a given area. ................................................................................ 47 

Figure 4-1: Relative Change in Grid Cells ....................................................................... 51 

Figure 4-2: Relative Change in Grid Cell-hours............................................................... 51 

Figure 4-3: Relative Change in Total 8 hour Ozone .............................................................. 52 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

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Executive Summary

The United States Environmental Protection Agency’s (EPA) 8-hour ozone modeling 

guidance (EPA, 2007) recommends a weight of evidence analysis (WOE) (a set of 

supplemental analyses) to support the attainment determination if the maximummodeled 8-hour ozone design value is between 0.082 ppm and 0.087 ppm at more than 

one monitor. Although all monitoring locations in the Denver/North Front Range SIP 

attainment demonstration indicate modeled attainment of the 8-hour ozone standard, 

four monitors (Rocky Flats North, Fort Collins West, Chatfield and National Renewable 

Energy Laboratory) have modeled concentrations that fall into the 0.082-0.087 ppm 

range. Therefore, a set of supplemental analyses are required to determine if these 

monitors are expected to demonstrate compliance with the ozone standard. 

Overview of Supplemental Analysis and Weight of Evidence

Supplemental analyses used in a weight of evidence will help determine whether 

attainment is likely where modeled attainment test results indicate future air quality 

levels are near the National Ambient Air Quality Standards (NAAQS). A weight of 

evidence determination includes the modeled attainment and screening test results, plus 

results of additional model outputs plus other analyses of air quality, meteorological and 

emissions data. A weight of evidence analysis may be used either to increase or 

decrease emission reductions identified as sufficient to meet the NAAQS by a modeled 

attainment test. 

The final WOE combines and weighs the various supplemental analyses with the results 

of the attainment test resulting in an aggregated, qualitative, and quantitative conclusion 

as to whether the proposed set of control strategies will result in the Denver Metro 

Area/North Front Range reaching attainment in 2010. 

The supplemental analyses used in the weight of evidence for the Denver Ozone SIP 

include: 

• Monitoring and emission inventory trend analysis 

• Review of the conceptual model for ozone formation along the northern Front 

Range 

• Additional modeling metrics 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

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• Alternative attainment test methods 

• Assessment of the efficacy of SIP, state-only, and voluntary control measures 

Monitoring and Emission Inventory Trend Analysis

Trend analyses for ambient measurement of ozone and emission inventory trends are 

presented as one of the supplemental analysis for this weight of evidence. The trends 

analyses indicate several important points: 

• The aggregate trend in weather-corrected 4th maximum time series suggests that 

ozone levels have been flat from 2004 through 2008, although individual 

concentrations have been highly variable. This suggests that without additional 

emission reductions the region will remain at or near the level of the standard. 

• Trends in emissions correlate well with surrogate indicators such as fleet 

turnover. 

• If the emissions trends are correct, then the Relative Response Factors (RRFs) 

are likely to be directionally correct. 

• Meteorological variability is a key component for ozone formation and is reflected 

in the year-to-year variability of peak ozone levels. A key metric for upper level 

high pressure strength has remained steady or trended downward in recent 

years, suggesting a reasonable likelihood for moderate high pressure strength in 

the next few years, which would favor attainment. 

• Analysis of the weekend-weekday effect for the Front Range shows a strong 

effect in Central Denver and weaker effects in outlying areas. This points to the 

possibility for NOx control disbenefits in central Denver due to the role of NOx 

quenching here. The spatial pattern of the weekend effect is consistent with the 

localized NOx disbenefit identified in the photochemical modeling. 

• Reductions in VOC emissions are expected to reduce ozone. 

• Reductions in NOx emissions are expected to reduce ozone, possibly with 

greater efficiency than VOC reductions, at troublesome monitors outside of the 

urban core of metro Denver. Increases in ozone concentrations in the urban core 

of metro Denver due to NOx emissions reductions do not appear to be 

significant. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

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Conceptual Model

A review of the conceptual model for the Front-Range reveals the complexity of the 

meteorological, emission inventory, and photochemical modeling challenges that exist in 

the formation and subsequent control of ozone formation along the northern Front 

Range. Several diagnostic tests with subsequent changes to the science options and 

other input were made to the meteorological model to achieve the best performing 

meteorological model for the Front Range. Trajectory analyses indicate that the base 

case modeling of the June-July 2006 timeframe encompasses various local 

meteorological regimes under which elevated ozone levels have and are expected to 

occur. This assures that the modeled ozone impacts from various 2010 emissions 

scenarios have been examined under a range of meteorological regimes that are 

favorable for ozone formation. 

Additional Modeling Metrics

Additional modeling metrics, such as the reduction in Total Ozone1, the number of gridcells with elevated ozone (Grid Cells2

), and the number of hours of elevated ozone (Grid 

Cell-Hours3

), are used to demonstrate that the existing controls along with potential SIP 

and state-only emissions controls are effective in reducing ozone along the Front Range. 

Additional modeling metrics indicate that: 

• The emission reductions between 2006 and 2010 are effective at reducing 

elevated 8-hour ozone concentrations. 

• 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. This suggests that changes in emissions between 2006 and 2010 

base case emission scenarios reduce ozone levels. 

• The changes in Total Ozone and Grid Cells are greater for the 2010 Control 1 

case (-28% and -17%, respectively) and for the 2010 Control 2 case. 

• Other modeled metrics indicate that there are reductions in Total Ozone, Grid 

Cells, and Grid Cell-Hours of 15% to 30% for thresholds of 85 ppb and 80 ppb 

 

1

 “Total Ozone” is 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. 

2

 “Grid Cells” is the number of grid cell-days with modeled daily maximum 8-hour ozone concentrations 

greater than a given threshold (e.g., 85 ppb) for all grid cells in the NAA and days from the June-July 2006 

episode. 

3

 “Grid Cell-Hours” is the 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. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

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from the 2006 base case through the 2010 base case, the 2010 Control 1 case, 

and the 2010 Control 2 case. 

Alternative Attainment Test Methods

EPA’s modeling guidance for ozone offers several potential approaches for establishing 

base year design values. The guidance recommends the preferred methodology for 

establishing a base year design value as follows: 

“For the modeled attainment tests we recommend using the average of the three 

design value periods which include the baseline inventory year. Based on the 

attributes listed above (in the guidance), the average of the three design value periods 

best represents the baseline concentrations, while taking into account the variability 

of the meteorology and emissions (over a five year period).” 

At the start of the work on the SIP in 2007 and throughout development of the proposed 

plan, the modeling analysis has used the 2005-2007 three-year design value as 

representative of the ozone situation facing the region at the time. In the SIP, modeled 

attainment of the 8-hour ozone standard was demonstrated at all of the monitored 

locations by 2010 as a result of the reductions in ozone precursors expected from 

existing programs and regulations. The use of the three-year design value from 2005-

2007 is a conservative approach with higher base year and future year design values 

when compared to using the EPA preferred method. 

Since the start of the Denver ozone SIP project, additional data from the 2008 ozone 

season (though currently not formally quality assured by the State or EPA) has become 

available. The additional data from 2008 provides the necessary data to estimate the 

average of the three Design Values in the 2004-2008 time period as prescribed in the 

the EPA recommended method. The importance of using 2008 monitoring data in the 

design value estimation is that year 2008 is the first year of the 2008-2010 ambient 

monitoring period used to demonstrates monitored attainment of the ozone standard. 

Using the EPA preferred method of calculating the modeled base year design value 

using years 2004-2008, the current attainment tests show that the Denver region will 

attain the ozone standard by 2010. Table ES-1 presents the results of using EPA’s 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

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recommended methodology for estimating the base year design value with projected 

2010-modeled concentrations. 

Table ES-1: 2010 Control Case Design Values Utilizing EPA’s Recommended DVB Calculation 

Methodology

Site Name 

Current 

(2004-08*) 

Base Case 

Design 

Value 

(ppm) 

Modeled 

Control Case 

Relative 

Reduction 

Factors 

Calculated 

2010 

Control Case 

Design Value 

(ppm) 

Truncated 

2010 

Control Case 

Design Value 

(ppm) 

Welby 0.0707 1.0039 0.0709 0.071 

Arvada 0.0777 1.0022 0.0779 0.077 

NREL 0.0808 1.0027 0.0810 0.081 

Rocky Flats North 0.0840 0.9981 0.0838 0.083 

S. Boulder Creek 0.0791 0.9963 0.0788 0.078 

Fort Collins 0.0728 0.9853 0.0717 0.071 

Fort Collins West** 0.083 0.9852 0.0818 0.081 

Carriage 0.0728 1.0015 0.0729 0.072 

Welch 0.0740 1.0002 0.0740 0.074 

CAMP 0.0560 1.0009 0.0560 0.056 

Weld County Tower 0.0769 0.9925 0.0763 0.076 

Highland 0.0760 0.9900 0.0752 0.075 

Chatfield Res. 0.0829 0.9921 0.0822 0.082 

Rocky Mtn. N.P. 0.0759 0.9892 0.0751 0.075 

* Thru August 31, 2008. 2008 data have not been fully quality assured at this time; 

** FCW only has three years of data and is presented as a Design Value to three places 

Alternative 2010 Ozone Projections

Several alternative 2010 ozone projection procedures for the 2010 base case, Control 1 

and Control 2 scenarios have been examined to estimate the uncertainties in the 

projection procedures and provide confidence that passing the modeled attainment 

demonstration indicates that attainment will likely be achieved in 2010 under the 2010 

base case, Control 1 or Control 2 emission scenarios. Six additional ozone projection 

procedures were analyzed, in addition to the EPA guidance default procedure. 

The six additional tests provide a range of future design values for the 2010 base case 

that is a more robust test than just using the EPA guidance default. In addition, the 

supplemental attainment tests provide a range of concentrations by which the likelihood 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

VI

of reaching attainment can be assessed. As presented in the technical support 

documentation, some of the six alternative projection approaches result in ozone 

concentration increases, whereas others in decreases in the projected 2010 design 

values (DVFs) at the two key sites, Rocky Flats North (RFNO) and Fort Collins West 

(FTCW), relative to the default EPA guidance approach. The future 2010 base year 

design value for the RFNO and the FTCW monitors using the alternative projection 

approach range from 84.6 ppb to 85.2 ppb with an average of 84.9 ppb. Using the 

default EPA guidance approach to show modeled attainment of the ozone standard also 

yielded a 2010 base year design value for RFNO and FTCW of 84.9 ppb, which is 

comparable to the six alternative projection methods. 

When the proposed SIP control strategies are applied, the six alternative projection 

methods indicate that, 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. At the 

FTCW monitoring site the projected DVFs range from 84.4 to 85.0 ppb with an average 

of 84.7 ppb. This indicates the SIP control strategies will provide a slightly better 

‘cushion’ than reliance on only those strategies that are currently on the books. 

The 2010 projected DVFs at RFNO using 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. 

 

Assessment of the Efficacy of SIP, State-only, and Voluntary Control Measures

Along with the control measures contained in this SIP and proposed State-only 

measures contained in the Control 2 scenario, there have been and will continue to be a 

myriad of voluntary measures in the DMA/NFR that are not directly accounted for in the 

current and projected emissions inventories. 

Conclusion

In conclusion, the collective supplemental analyses contained in this weight of evidence 

document support the current photochemical model attainment demonstration for the 

0.08 ppm 8-hour ozone NAAQS using the EPA default approaches for the 2010 base 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

VII

case, 2010 Control 1, and the 2010 Control 2 scenarios. In addition, at this time, the 

photochemical modeling is considered to be the best predictor of future ozone levels. 

The collective supplemental analyses in this weight of evidence analysis support the 

findings using the EPA methods, as specified in the EPA modeling guidance, that the 

2010 base case will likely achieve attainment of the 0.08 ppm 8-hour ozone NAAQS in 

the Denver Metro Area and North Front Range. As demonstrated using alternative 

attainment test methodologies, the same WOE indicators demonstrate 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 emissions scenario. The preponderance of 

evidence suggests that the region will attain the standard in 2010 under the base case, 

Control 1, and Control 2 scenarios, but the safety margin is small. 
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1 Introduction to the Weight of Evidence (WOE) Analysis 

EPA’s 8-hour ozone modeling guidance (EPA, 2007) suggests development of a weight 

of evidence analysis (a set of supplemental analyses) to support the attainment 

determination if the maximum-modeled 8-hour ozone design value is between 0.082 

ppm and 0.087 ppm at more than one monitor. Although all monitoring locations in this 

SIP attainment demonstration indicate modeled attainment of the 8-hour ozone 

standard, four monitors (Rocky Flats North, Fort Collins West, Chatfield and National 

Renewable Energy Laboratory) have modeled concentrations that fall into the 0.082-

0.087 ppm range. Therefore, a set of supplemental analyses are required to determine 

if these monitors are expected to demonstrate compliance with the ozone standard. 

Supplemental analyses used in a weight of evidence will help determine whether 

attainment is likely where modeled attainment test results indicate future air quality 

levels are near the NAAQS. A weight of evidence determination includes the modeled 

attainment and screening test results, plus results of additional model outputs plus other 

analyses of air quality, meteorological and emissions data. A weight of evidence analysis 

may be used either to increase or decrease emission reductions identified as sufficient 

to meet the NAAQS by a modeled attainment test. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

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2 Air Quality Related Trends 

2.1 Emission Trends 

Impacts of federal tailpipe regulations have continued to reduce mobile source 

emissions of VOC and NOx over time. The downward emissions trend in On-Road 

Mobile source emissions between 2006 and 2010 are consistent with the expected 

changes in emissions due to mobile source fleet turnover and federal tailpipe 

regulations. Similarly, downward trends in Non-Road Mobile source emissions are 

consistent with expected changes in emissions from the Tier 2 and Tier 3 non-road 

regulations. The correlation between the expected emission reductions from existing 

regulatory programs and the calculated emissions reductions from the emissions 

modeling systems suggest that the emissions reductions are providing directionally 

correct emissions projections for 2010. 

Reformulation of paints and consumer products are reducing emissions in the area 

source category. Point source growth has been modest. Therefore despite continued 

growth in vehicle miles traveled, population and housing in the Denver/North Front 

Range area, the region has seen declining emissions of VOC and NOx in mobile, nonroad and area sources. 

The one area of significant emissions growth in the region since 2002 has been in the oil 

and gas industry. Controls were first applied to the industry prior to the 2005 peak ozone 

season and tightened prior to the 2007 peak ozone season to regulate previously 

uncontrolled facilities (condensate tanks). Due to continued growth, controls applied 

barely managed to keep pace with the growth in emissions. Additional recommended 

controls in this SIP and currently proposed state-only controls will continue to reduce 

emissions in the oil and gas industry and mobile sources beyond 2010. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

3

 VOC Emissions for 2006 Projected to 2010 Base and 2010 SIP

0.0

100.0

200.0

300.0

400.0

500.0

600.0

Source Category

Tons/day

2006 VOC

2010 VOC

2010 Control VOC

2006 VOC 32.1 187.4 66.3 65.3 129.7 480.8

2010 VOC 37.0 204.6 61.0 61.3 109.2 473.1

2010 Control VOC 37.0 161.2 61.0 61.3 106.0 426.5

Point 

Source

O&G 

Point & 

Area 

Source

NonRoad OnRoad Anthrop

ogenic 

Figure 2-1: Emissions for 2006 Projected to 2010 Base and 2010 SIP 

The total estimate of VOC and NOx emissions from all sources in Figure 2-1 and Figure 

2-2, respectively, demonstrate an overall reduction in emissions between the 2006 base 

case and the 2010 base and control case. In Figure 2-1, the reduction in total 

anthropogenic VOC from 2006 base case to 2010 base case is about 8 tpd. The 2010 

control case will net an additional 46 tpd. The total VOC reduction from 2006 base case 

to 2010 Control Case is around 54 tpd, which is about an 11% reduction. 

In Figure 2-2, the reduction in total anthropogenic NOx from 2006 base case to 2010 

base case is about 36 tpd. The 2010 control case will net an additional 4 tpd. The total 

NOx reduction from 2006 base case to 2010 control case is around 40 tpd, which is 

about a 12% reduction. 

The decrease in emission inventory trends presented in Figure 2-1 and Figure 2-2 are 

directionally consistent with the photochemical modeling results and the sensitivity 

analysis that indicate that a decrease in both VOC and NOx are beneficial for reduction 

in ozone. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

4

NOx Emissions for 2006 Projected to 2010 Base and 2010 SIP

0.0

100.0

200.0

300.0

400.0

500.0

Source category

Tons/day

2006 NOx

2010 NOx

2010 Control 

2006 NOx 81.0 48.9 20.2 70.5 165.5 386.1

2010 NOx 86.4 57.8 22.0 61.0 122.9 350.1

2010 Control 86.4 57.8 22.0 61.0 118.8 346.0

Point 

Sources 

O&G 

Point & 

Area 

Source

NonRoad On-Road 

Mobile

Anthropo

genic 

Figure 2-2: Emissions for 2006 Projected to 2010 Base and 2010 SIP 

Figure 2-3 presents projected trends from on-road mobile sources out to 2035 using the 

current conformity network. The trend indicates that both NOx and VOC emissions will 

continue to decrease through 2025. After 2025, the effectiveness of technology will be 

overtaken by the increase in VMT and VOC emissions will trend upward at that point. 

The reductions in NOx and VOC from on-road mobile sources will likely help reduce 

ozone concentrations into the future. 

On-Road Mobile Source NOx and VOC Emission (tpd) Trends

0.00

50.00

100.00

150.00

200.00

250.00

2002 2005 2006 2010 2015 2025 2035

Year

(Tons/Day)

VOC

NOx

Figure 2-3: Trends in Mobile Source Emissions 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

5

2.2 Ozone Monitoring Trends 

2.2.1 Time Series of Monitored 4th maximum ozone values 

The EAC Ozone Action Plan (OAP) required controls on oil and gas industry condensate 

VOC emissions prior to the 2005 peak ozone season. Due to recorded growth in 

condensate flash emissions, Regulation No. 7 was amended in late 2006 to preserve the 

EAC OAP and additional controls were applied to condensate tanks prior to the 2007 

peak ozone season. The EPA required 7.8 RVP fuel in the DMA 1-hour ozone 

attainment maintenance area prior to the 2004 ozone season. 

Figure 2-4 presents data from 2002 prior to application of controls by the region through 

the end of August 2008. The 4th maximum 8-hour ozone value time series at monitors 

still projecting values between 82 and 87 ppb in the modeling exercise (i.e., NREL, 

Rocky Flats-N, Chatfield and Fort Collins West) are shown in Figure 2-4. 

Figure 2-4: Time Series of Monitored 4th Maximum 8-Hour Ozone Values (ppm) 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

6

2.2.2 Linear Regression of Monitored 4th maximum ozone values 

A linear regression analysis of the 4th high ozone concentration values between 2002 

and 2008 for the Rocky Flats North monitor shown in Figure 2-5 shows what appears to 

be a downward trend. However, the coefficient of determination (R2

) value of the line is 

extremely small, and additional analysis shows that this trend does not pass a standard 

t-test for statistical significance. Therefore, there is not a statistically significant linear 

trend at Rocky Flats North during this period for 4th high values. There is too much interannual variation in the 4th high values at Rocky Flats North to conclude from the trend 

line alone that the 4th high value in future years will be above or below current levels. In 

addition, analyses for the monitors at Chatfield, NREL, and Fort Collins West show that 

there are no statistically significant trends in 4th high values at these sites. Similarly, an 

analysis of all four sites together does not show significant or discernable trends during 

this period. 

50

60

70

80

90

100

2002 2003 2004 2005 2006 2007 2008

Rocky Flats Annual Fourth Max

 8-hr Ozone

Year

Ozone in ppb

R-Squared = 0.02

Figure 2-5: Linear Regression of Monitored 4th maximum ozone values 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

7

2.2.3 Time Series of Monitored Highest Maximum Ozone Concentrations 

Figure 2-6 presents a time series of first maximum ozone concentrations at Rocky FlatsN, Chatfield, National Renewable Energy Laboratory, and Fort Collins West from 2002 

through 2008. As shown in Figure 2-6, there is no discernable trend in the first 

maximum ozone concentration values at these monitoring sites. There is too much interannual variation in the 1st high values to conclude that the 1st high values in future years 

will be above or below current levels. 

8-Hour 1st Maximum Ozone Concentration (ppm)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

200220032004200520062007

2008 *

Year

Ozone Conc. (ppm)

Chatfield State Park 08-

035-0004

Rocky Flats North 08-059-

0006

Nat’l Renew. Energy Lab

08-059-0011

Ft. Collins West 08-069-

0011

Figure 2-6: First Maximum Ozone Concentrations at Rocky Flats-N (RFN), Chatfield (CHAT), 

National Renewable Energy Laboratory (NREL) and Fort Collins West (FTCW) [Note: Data for 2008 

are preliminary.] 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

8

2.2.4 Trend in Days greater than 75 ppb and 84 ppb 

Figure 2-7 presents data from the 2000 ozone season through August 31 of the current 

2008 peak ozone season. The data presents days during the ozone seasons when 

there was a reading at any monitor in the region greater than 75 ppb and greater than or 

equal to 85 ppb. Excluding the year, 2004, the years 2005 through 2008 show a 

reduction in the number of total days of elevated ozone when compared with the years 

2000 through 2003. 

Figure 2-7: Days with 8-hour ozone greater than 75 ppb and greater than or equal to 85 ppb 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

9

2.2.5 Trends in Three Year Design Values 

Figure 2-8 presents a plot of the design value days, which are defined as the 3-year 

average of the fourth high concentration at each monitor. Similar to the analysis of 

fourth high concentrations, there is not a statistically significant linear trend at Rocky 

Flats North, NREL, or Chatfield. There is too much inter-annual variation in the design 

values at the sensitive monitoring sites to conclude from this graph alone that the design 

values in future years will be above or below current levels. 

Average of Three Year Fourth High Design Days

0.072

0.074

0.076

0.078

0.08

0.082

0.084

0.086

0.088

2000-

2002

2001-

2003

2002-

2004

2003-

2005

2004-

2006

2005-

2007

2006-

2008 *

Three-year Period

Average Conc. (ppm)

Rocky Flats North

Chatfield

NREL

Figure 2-8: Design Value Days (Three Year Average of the Fourth High Concentration) 

2.3 Weather-Corrected Ozone Time Series 

A variety of ozone trend-decomposition or filtering methods can be used to remove the 

effects of meteorology from an ozone time series. This filtering or decomposition can 

make it possible to see the effects of changes in emissions in relative isolation from 

many of the meteorological factors that also affect ozone concentrations. For the Early 

Action Compact of 2004 (Colorado Department of Public Health and Environment, 

2004), the Zurbenko-Rao method (Porter et al., 1996; Eskridge et al., 1997; Rao et al., 

1997) was used to cleanly separate ozone time series into distinct short and long-term 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

10

components. Since 2004, the Division has developed a new method of correcting ozone 

time series for weather for monitoring sites along the Front Range. 

For summer months, monthly mean 500-millibar heights are a measure of the mean 

strength of upper level high pressure systems that strongly affect weather in Colorado. 

Monthly mean 500-millibar heights are an excellent predictor of monthly mean daily 

maximum 8-hour ozone concentrations. July monthly mean daily maximum 8-hour 

ozone is more strongly correlated with 500-millibar heights than a host of other logical 

choices for significant predictors of ozone, including mean surface temperatures, mean 

temperatures aloft, winds aloft, cloud cover, solar radiation, and number of days with 

temperatures above 90 degrees. While annual fourth maximum 8-hour ozone 

concentrations can occur in any of the months of summer, it turns out that the mean July 

500-millibar height over Denver is one of the single best predictors for this value at sites 

along the Front Range Urban Corridor. The predictive power of mean August and June 

meteorological variables and meteorological data for shorter averaging times (e.g., daily, 

weekly, etc.) is substantially lower. 

Figure 2-9 through Figure 2-11 show the relationship between July monthly mean daily 

maximum 8-hour ozone concentrations and July monthly mean 500-millibar heights at 

Rocky Flats North, NREL, and Chatfield Reservoir, three of the four key ozone monitors 

identified in the SIP modeling (there are not enough years of data for a similar analysis 

for the fourth key site, Fort Collins West.) The 500-millibar height data are from the 

National Center for Environmental Prediction (NCEP) Reanalysis data set (Kalnay et al., 

1996; and NOAA/OAR/ESRL PSD, 2008). These data are calculated for grid cells with 

dimensions of 2.5 degrees latitude by 2.5 degrees longitude. The grid cell with data 

having the highest predictive power for the Front Range extends from near Colorado 

Springs, north to near Cheyenne Wyoming, and from Denver west to near Glenwood 

Springs (essentially the northern Front Range and north central mountains of Colorado). 

Data from the grid cell immediately to the east, which covers much of the urban corridor 

and the northeast plains, had slightly weaker correlations with mean ozone 

concentrations. 

Figure 2-9 through Figure 2-11 also show the linear regression between July mean daily 

maximum 8-hour ozone and July mean 500-millibar heights, the confidence intervals for 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

11

the regression line, and the coefficient of determination or R-squared value for the 

regression. The R-squared describes the fraction of the variance, or fraction of the yearto-year variation, that can be explained by mean 500-millibar heights. This coefficient 

ranges from 0.66 to 0.88 for these three sites, suggesting that 66% to 88% of the yearto-year variation in July mean daily maximum ozone at these sites can be explained by 

changes in July mean 500-millibar heights. 

Table 2-1 lists regression statistics for 9 Front Range ozone monitors. The trend for 

increasing ozone with increasing high-pressure strength is statistically significant for all 

sites but Welby, which may be disconnected from the influence of heights because of 

the effects of local NOx sources and/or local flow regimes. R-squared values for sites 

with statistical significance range from 0.46 to 0.88. Welch was not included because of 

an apparent local anomaly that may have been caused by facility activities at the 

Colorado Department of Transportation workstation where it is located. Highlands 

Ranch was not included because it was shut down in June of 2008. 

50

60

70

80

5880 5890 5900 5910 5920 5930 5940 5950

1995

1996

1997

1998

1999

20002001

2002

2003

2004

2005

2007 2006

2008

Rocky Flats North

Denver July Mean 500 mb Height in Meters

July Mean Daily Max 8-hr Ozone in ppb

R-Squared = 0.88

Figure 2-9: Linear regression between July monthly mean daily maximum 8-hour ozone and July 

monthly mean 500-millibar heights at Rocky Flats North. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

12

50

60

70

80

5880 5890 5900 5910 5920 5930 5940 5950

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006 2007

2008

NREL

Denver July Mean 500 mb Height in Meters

July Mean Daily Max 8-hr Ozone in ppb

R-Squared = 0.86

Figure 2-10: Linear regression between July monthly mean daily maximum 8-hour ozone and July 

monthly mean 500-millibar heights at NREL. 

50

60

70

80

5880 5890 5900 5910 5920 5930 5940 5950

1995

1996

1997

1998

1999

2000

2001

2002

2003

2005 2006

2008 2007

Chatfield

Denver July Mean 500 mb Height in Meters

July Mean Daily Max 8-hr Ozone in ppb

R-Squared = 0.66

Figure 2-11: Linear regression between July monthly mean daily maximum 8-hour ozone and July 

monthly mean 500-millibar heights at Chatfield Reservoir. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

13

Table 2-1: Linear Regression Statistics for the Relationship between July Mean 

Daily Maximum 8-hour Ozone Concentrations and July Mean 500-millibar Heights 

Monitoring Site Slope 

R-Squared 

Value 

Passed t-test for 

Statistical 

Significance 

Period Used in 

Regression 

Arvada 0.278 0.62 Yes 1995-2008 

Chatfield 0.351 0.66 Yes 1995-2008 

Carriage 0.271 0.84 Yes 1995-2008 

Fort Collins 0.188 0.46 Yes 1988-2008 

Greeley/Weld County 

Tower 

0.227 0.65 Yes 1988-2008 

NREL 0.259 0.86 Yes 1995-2008 

Rocky Flats North 0.279 0.88 Yes 1995-2008 

South Boulder Creek 0.277 0.67 Yes 1995-2008 

Welby 0.049 0.02 No 1995-2008 

Figure 2-12 through Figure 2-14 show the relationship between annual 4th maximum 8-

hour ozone concentrations and July monthly mean 500-millibar heights at Rocky Flats 

North, NREL, and Chatfield Reservoir. R-squared values for these monitors range from 

0.52 to 0.77. This suggests that 52% to 77% of the year-to-year variation in annual 4th

maximum values can be explained by changes in July mean 500-millibar heights. Table 

2-2 shows the linear regression statistics for the same 9 Front Range monitors described 

above. Regression lines show significance for all sites but Welby. Welch and Highlands 

Ranch were excluded from the analysis for reasons already discussed. R-squared 

values for sites with statistical significance range from 0.47 to 0.77. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

14

50

60

70

80

90

100

5880 5890 5900 5910 5920 5930 5940 5950

1995

1996

1997

1998

1999 20002001

2002

2003

2004

2005

2007 206

2008

Rocky Flats Annual Fourth Max

 8-hr Ozone

Denver July Mean 500 mb Height

Ozone in ppb

R-Squared = 0.65

Figure 2-12: Linear regression between annual 4th maximum 8-hour ozone and July monthly mean 

500-millibar heights at Rocky Flats North. 

50

60

70

80

90

100

5880 5890 5900 5910 5920 5930 5940 5950

1995

1996

1997

1998

1999

20002001 2002

2003

2004

2005

2006 2007

2008

NREL Annual Fourth Max

 8-hr Ozone

Denver July Mean 500 mb Height

Ozone in ppb

R-Squared = 0.52

Figure 2-13: Linear regression between annual 4th maximum 8-hour ozone and July monthly mean 

500-millibar heights at NREL. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

15

50

60

70

80

90

100

5880 5890 5900 5910 5920 5930 5940 5950

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005 2006

2007 2008

Chatfield Annual Fourth Max

 8-hr Ozone

Denver July Mean 500 mb Height

Ozone in ppb

R-Squared = 0.77

Figure 2-14:Linear regression between annual 4th maximum 8-hour ozone and July monthly mean 500-

millibar heights at Chatfield Reservoir. 

Table 2-2: Linear Regression Statistics for the Relationship between Annual 4th Maximum 8-hour 

Ozone Concentrations and July Mean 500-millibar Heights 

Monitoring Site Slope 

R-Squared 

Value 

Passed t-test for 

Statistical 

Significance 

Period Used in 

Regression 

Arvada 0.269 0.57 Yes 1995-2008 

Chatfield 0.288 0.77 Yes 1995-2008 

Carriage 0.269 0.59 Yes 1995-2008 

Fort Collins 0.212 0.60 Yes 1995-2008 

Greeley/Weld County 

Tower 

0.235 0.67 Yes 1995-2008 

NREL 0.267 0.52 Yes 1995-2008 

Rocky Flats North 0.287 0.65 Yes 1995-2008 

South Boulder Creek 0.232 0.47 Yes 1995-2008 

Welby -0.025 0.01 No 1995-2008 

The correspondence between 500-millibar heights and ozone can be used to correct 

ozone time series for the effects of weather. The differences between the linear 

regression concentrations and the actual concentrations (the differences are the 

residuals) can be plotted by year to show weather-corrected trends in ozone. These 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

16

corrected trends or time series are much more likely to show the effects of changes in 

emissions than the uncorrected time series. 

The trend in weather-corrected July mean daily max 8-hour ozone for Fort Collins and 

Greeley is shown in Figure 2-15 (with a simple cubic spline smoother applied). A 

continuous increase in ozone from the late 1990s through 2005 may be the result of 

local growth and increases in oil and gas emissions. A sudden drop from 2005 through 

2007 may be the result of reductions in area oil and gas emissions. The increase in 

2008 will be discussed below. 

A similar analysis for Rocky Flats, NREL, Chatfield, Carriage, South Boulder Creek, and 

Arvada is shown in Figure 2-16. Gradual decreases through 2004 are replaced by 

apparent step increases in 2005 and 2008. The increases in 2008 in both plots suggest 

that there may have been an increase in background concentrations across all of the 

Front Range, with a magnitude of about 4 ppb. Many factors may have contributed to 

this increase, and it is not possible to complete a thorough analysis of the causes within 

the timeframe of this SIP. It is possible that widespread smoke-enhanced ozone from 

fires on the West Coast, nation-wide reductions in vehicle miles traveled (VMT, Colorado 

VMT values declined about 4% in June of 2008 compared with June of 2007, see 

Federal Highway Administration, 2008) or other factors may have caused this apparent 

increase. VMT reductions may have resulted in reductions in NOx quenching of surface 

ozone, changes in photochemistry, or a redistribution of emissions in time and space. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

17

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

199419951996199719981999200020012002200320042005200620072008

Trend in FTC/GRE July Mean Daily Max 8-hr

Ozone, Effect of Mean 500 mb Heights Removed

Year

Ozone Residuals in ppb

(Residuals are differences between actual and weather

predicted ozone.)

Figure 2-15: The trend in weather-corrected July mean daily max 8-hour ozone for Fort Collins and 

Greeley. 

Correcting all 8 of the Front Range annual fourth max time series for weather leads to 

the pattern shown in Figure 2-17. Here the smoothing algorithm used is a Lowess curve 

with 40% weighting. When smoothed in this way, the trend shows a period of decline 

followed by a rise and ending in a level line from 2004 through 2008. This is consistent 

with the idea that ozone is difficult to control but increases have ceased since 2004. In 

addition, the possible increase in regional background in 2008 seen in earlier plots does 

not appear to have had an impact on these worst-case concentrations. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

18

-10

-8

-6

-4

-2

0

2

4

6

8

10

199419951996199719981999200020012002200320042005200620072008

Trend in DEN Metro July Mean Daily Max 8-hr

Ozone, Effect of Mean 500 mb Heights Removed

Year

Ozone Residuals in ppb

(Residuals are differences between actual and weather

predicted ozone.)

Figure 2-16: The trend in weather-corrected July mean daily max 8-hour ozone for Rocky Flats, 

NREL, Chatfield, Carriage, South Boulder Creek, and Arvada. 

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

14

199419951996199719981999200020012002200320042005200620072008

Trend in Front Range 4th Max Ozone

Effect of Mean 500 mb Heights Removed

Year

Ozone Residuals in ppb

(Residuals are differences between actual and weather

predicted ozone.)

Figure 2-17: The trend in weather-corrected annual 4th maximum 8-hour ozone for Rocky Flats, 

NREL, Chatfield, Carriage, South Boulder Creek, Arvada, Fort Collins, and Greeley. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

19

Why are ozone concentrations along the Front Range so highly correlated with July 

mean 500-millibar heights? Increased heights are associated with strong regional upper 

level high pressure systems. Strong regional upper level high pressure systems lead to 

light winds at the surface and aloft, decreased cloud cover and storms, increased 

temperatures at the surface and aloft, an enhancement of the local thermally driven 

circulations that tend to keep ozone and its precursors in the area, and a greater 

likelihood that ozone and its precursors will accumulate in regional circulation patterns. 

Recent research highlights the role of upper level high pressure systems in the 

accumulation of ozone aloft (Cooper, Owen, et al. 2006, Cooper, Owen, et al. 2007). 

Because of deep vertical mixing during the afternoon over most of the state, this ozone 

aloft affects ground level concentrations, and ozone at ground level is ultimately mixed 

vertically into the upper level high. The Air Pollution Control Division believes that 

persistent upper level high pressure over Colorado in mid summer increases 

background concentrations, providing an increase in both mean and worst-case ozone 

concentrations. 

Since upper level high pressure systems have such a strong effect on ozone, it is 

important to consider recent trends in upper level high pressure strength. Time series 

for 500-millibar heights presented in Figure 2-18 and Figure 2-19 show that a long period 

of increases has recently shifted to a leveling off or perhaps a decline since 2004. A 

Lowess curve with 40% weighting has been applied to the heights data in Figure 2-18 

Climate change or “natural” patterns of drought and heat may alter this trend in the 

future, but there would appear to be a reasonable chance for relatively stable or 

declining heights through 2010. A continued cessation in rapid increases in heights 

would help the area reach attainment by 2010. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

20

5840

5852

5864

5876

5888

5900

5912

5924

5936

5948

5960

198819891990199119921993199419951996199719981999200020012002200320042005200620072008

Mean Strength of July Upper Level High Pressure

Year

500 mb heights in meters

Figure 2-18:Trends in the mean strength of the upper level high pressure or 500-millibar heights for the 

Denver-area NCEP Reanalysis data. 

5840

5852

5864

5876

5888

5900

5912

5924

5936

5948

5960

198819891990199119921993199419951996199719981999200020012002200320042005200620072008

Mean Strength of July Upper Level High Pressure

Year

500 mb heights in meters

Figure 2-19: Trends in the mean strength of the upper level high pressure or 500-millibar heights for 

the Denver-area NCEP Reanalysis data, with Lowess trend curve at 40% weighting. 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

21

2.4 Trends in the Weekend-Weekday Effect 

Lawson (2003) reports that research conducted for the LA Basin and other urban areas 

shows a significant increase in ozone on weekend days compared with weekdays. This 

is known as the weekend effect. For the LA Basin study he reports that “Hypothesis 

testing using empirical observations, data analysis, and different modeling approaches 

suggested that decreased weekend NOx emissions, resulting mostly from fewer trucks 

on the roads on weekends, was the largest single contributor to elevated weekend 

ozone...” Lawson says that the weekend effect offers us a real world analogue of a NOx 

control strategy experiment. Lawson suggests that the existence of the weekend effect 

is evidence that an area is VOC limited. Scientists have identified a number of 

hypotheses for the causes of the weekend effect and what the implications of these are 

for emissions control strategies (Croes et al., 2003). While these include changes in the 

timing of NOx emissions on weekends and changes in VOC to NOx ratios on weekends, 

a recent study suggests that the weekend effect may, in large part, be due a reduction in 

near surface NOx titration on weekends (Murphy et al., 2006). NOx titration can be 

described as follows: Nitric Oxide or NO is emitted by mobile sources and other surface 

sources, and quickly reacts with ozone to reduce concentrations in that portion of the 

atmosphere near the surface. 

The effect of NOx titration and its relative importance in various environments along the 

Front Range is illustrated in Figure 2-20. Mean hourly ozone concentrations for July 11 

through 31, 2008, are plotted for five monitors. Data for four monitor locations have 

been provided by the NOAA Earth Systems Research Laboratory Global Monitoring 

Division (NOAA ESRL GMD) (Oltmans and Levy, 1994). Two of these were sited on the 

NOAA research tower near Erie Colorado, near the surface and at the 300-meter level 

on the tower (984 feet above ground level or 6,180 feet above sea level). The Erie 

Tower is located within the broad Platte Valley northwest of Denver at an elevation of 

about 5,200 feet above sea level. The Erie Tower monitors provide a clear picture of the 

impacts of near-surface NOx titration on hourly ozone. Similarly, NOAA ESRL GMD 

data for two locations at Niwot Ridge near the Continental Divide west of Boulder show 

the temporal behavior of ozone at high altitude locations where the influence of the midatmosphere ozone reservoir is strong and NOx titration is weak. The Tundra monitor is 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

22

above tree line at about 11,500 feet above sea level, and the other Niwot Ridge monitor 

is within a conifer forest at about 9,900 feet above sea level. Data for NREL, which 

shows some of the characteristics of a high altitude site and some of the impacts of NOx 

titration, is included. The NREL monitor is at about 6,000 feet above sea level on top of 

South Table Mountain, which is largely free of NOx sources, and the populated area to 

the south is at about 5,800 feet above sea level. The July 11 through 31, 2008, period 

was chosen because of the availability of data for all five monitors. 

30

40

50

60

70

80

0 2 4 6 8 10 12 14 16 18 20 22 24

July 11-31, 2008

Mean Hourly Ozone

Hour (MDT)

Mean Ozone in ppb

Erie, Niwot Ridge, and NREL 

Temporal Profile: 5 Front Range Altitudes

Variables

Erie 300 meters

Erie Surface

Niwot Ridge

NWR Tundra

NREL

Figure 2-20: Mean hourly ozone concentrations at 5 monitoring sites (the NOAA Erie Tower surface 

and 300-meter levels, NREL, and NOAA’s two Niwot Ridge sites) for July 11-31, 2008, showing 

variations in ozone by time of day and altitude. 

Nighttime and morning surface inversions on the Plains trap NOx from surface sources 

in the atmosphere near the ground. These inversions greatly increase NOx-based 

destruction of ozone. Morning rush hour emissions can lead to an ozone minimum 

within the inversion between 4 AM and 8 AM MDT. This inversion-enhanced NOX 

titration is visible in the hourly data for the Erie Tower surface monitor. By mid-morning 

the inversion begins to transform into a deepening surface mixed layer. The surface 
DRAFT FINAL-Denver Metro Area & North Front Range 8-hr Ozone SIP – Weight of Evidence 

23

mixed layer continues to grow until mid afternoon and can reach to 10,000 to 20,000 feet 

above sea level. As the mixed layer grows, the effects of NOx titration decrease, and 

the photochemical production of ozone leads to an increase in ozone, with peak hourly 

concentrations occurring between 1 PM and 7 PM MDT. The hourly ozone trace for the 

300-meter level of the Erie Tower shows that NOx titration is much less important at this 

altitude above ground level sources. Ozone concentrations at the 300-meter level 

stayed at concentrations of 50 to 60 ppb throughout the nighttime and early morning 

hours. 

Hourly average ozone for the high-altitude Niwot Ridge site within the conifer forest are similar 

to the values measured at 300 meters on the Erie Tower. NOx titration at this remote site is 

probably minimal, but ozone deposition within the forest mimics the nighttime decreases seen 

on the Plains. The Niwot Ridge Tundra site has a narrow diurnal range of ozone 

concentrations, with levels varying between 60 and 67 ppb throughout the day. The daytime 

peak is broad and persists through 11 PM MDT. The absence of significant NOx titration or 

deposition is evident. 

Concentrations at the NREL monitor fall somewhere between the Erie surface and 300-meter 

monitors for much of the morning and evening periods. NREL is only 200 feet above significant 

local NOx sources, and it may show the effect of a relative lag in photochemical rate increases 

because of its downwind distance from Denver. 

Taken together, the data for these monitors suggests that NOx titration plays a significant role in 

shaping diurnal patterns for ozone concentrations on the Plains and in areas that are more 

directly affected by ground level emissions of NOx. A preliminary analysis of weekend-weekday 

patterns of hourly ozone at the Erie Tower (for July 11 through August 31, 2008) highlights the 

influence of NOx titration on weekend-weekday differences. Figure 2-21 shows mean hourly 

ozone concentrations at the surface and 300 meters on weekdays and weekends. At the 

surface site, morning weekday ozone concentrations are much lower than on weekends 

because of higher rush hour emissions. Similarly, evening concentrations are higher on 

weekends, because of the lack of rush hour emissions. While weekend concentrations at this 

level are generally higher, weekday hourly values are higher during the mid afternoon. 

The hourly patterns for the 300-meter level show that differences are smaller with a tendency for 
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24

higher afternoon and evening concentrations on weekdays and lower morning concentrations 

on weekdays. Since daytime surface ozone is directly affected by the ozone contained in the 

large volume of the atmosphere below the afternoon mixing level, the 300-meter level data 

suggests that there may be no normal weekend effect in this mid-level ozone reservoir. In other 

words, the effect as calculated here may dampen or perhaps reverse with increased altitude. 

20

30

40

50

60

70

80

0 2 4 6 8 10 12 14 16 18 20 22 24

Erie Tower July 11 - August 31, 2008

Mean Hourly Ozone

Hour (MDT)

Mean Ozone in ppb

Weekday/Weekend Patterns

Variables

Weekday: 300m

Weekend: 300m

Weekday: surface

Weekend: surface

Figure 2-21: Mean hourly ozone concentrations at the NOAA Erie Tower surface and 300-meter levels 

showing variations by time of day, by altitude, and for weekends and weekdays (for July 1 – August 

31, 2008.) 

Figure 2-22 shows the differences between hourly weekend and weekday concentrations for the 

two Erie Tower monitors. Lowess curves with 30% weighting have been applied to the data to 

highlight the underlying pattern. Positive differences indicate that weekend concentrations are 

higher. At the surface, the weekend effect is much more pronounced, with higher morning and 

evening concentrations on weekends. At 300 meters, morning concentrations are generally 

higher on weekends, but weekday concentrations are typically higher during the afternoon and 

evening hours. 
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-10

-5

0

5

10

0 2 4 6 8 10 12 14 16 18 20 22 24

Erie Tower July 11 - August 31, 2008

Hour (MDT)

Mean Ozone in ppb

Weekend Hourlies Minus Weekday Hourlies

Variables

300 meters

Surface

Figure 2-22: Weekend-Weekday differences in mean hourly ozone at the NOAA Erie Tower surface 

and 300-meter levels for July 1 – August 31, 2008, with a Lowess curves at 30% weighting. 

Table 2-3 presents weekend effect data that also shows the influence of NOx titration on 

the weekend effect. Based on an analysis of daily maximum 8-hour ozone levels from 

June through August for 2005 through 2007, the mean weekend effect for this three-year 

period ranges from about 1% to 3% at the highest ozone sites (NREL, Chatfield, Fort 

Collins West, and Rocky Flats); 5% to 8% at Arvada, Carriage, Fort Collins, and Welby; 

and 23% at the downtown Denver CAMP monitor. Here the weekend effect is 

expressed as the percentage change in ozone on weekends compared with weekdays. 

The weekend effect clearly increases in those areas with higher densities of mobile NOx 

sources, and peaks in downtown Denver. The spatial distribution of the effect is mapped 

in Figure 2-23, and this shows that central and downtown Denver as well as downtown 

Ft. Collins has the strongest weekend effect. In this map, the contoured values 

represent the ratio of weekend concentrations to weekday concentrations. The colorscale legend is not intended to define a threshold or cutoff for NOx and VOC limitation, 

but rather to indicate a general tendency towards either state at opposite ends of the 

scale. 
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Table 2-3: Percent Increase in Weekend Daily Maximum 8-Hour Ozone for June through August of 

2005 through 2007. 

Monitoring Site 

2005 Percent 

Weekend 

Difference 

2006 Percent 

Weekend 

Difference 

2007 Percent 

Weekend 

Difference 

Mean 2005-

2007 Percent 

Weekend 

Difference 

Welby 2 6 6 5 

Highlands Ranch 0 2 6 2 

S. Boulder Creek -1 2 6 2 

CAMP 22 22 25 23 

Carriage -1 10 7 5 

Chatfield -2 2 3 1 

Academy 1 -1 1 0 

Manitou 1 -3 1 0 

Arvada 3 5 7 5 

Welch 2 6 7 5 

Rocky Flats N. 0 2 6 2 

NREL -2 5 5 3 

RMNP -2 -2 -1 -1 

Ft. Collins West -1 4 1 

Ft. Collins 9 4 12 8 

Mesa Verde -1 -2 -2 -1 

Weld County 

Tower 2 -3 7 2 

Front Range 

Mean 2 4 6 4 

In a recent study of the weekend effect in Southern California, Blanchard et al. (2003) 

concluded, “Nearly all VOC-limited sites exhibited higher weekend peak O3 

concentrations, whereas NOx-limited sites did not show significant differences between 

mean weekday and weekend peak O3 levels. Thus an interpretation of the site’s 

responses as an indication of spatial patterns of VOC or NOx limitation was supported.” 

If we were to assume that this finding also applies in the Denver metro area, it follows 

from the logic presented by Blanchard et al. (2003) that ozone may be VOC-limited at 

central, urban sites in 2005 through 2007. It may be that the metro area as a whole is 

transitional between NOx and VOC limitation or that the high-concentration sites are 
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NOx limited. There is not yet enough information about the relationships between 

changes in weekend emissions and local photochemistry to draw definitive conclusions 

about the implications of this effect for NOx or VOC limitation along the Front Range. 

What is clear, however, is that reductions in NOx are more likely to have a disbenefit in 

central Denver and perhaps downtown Fort Collins, where NOx titration plays a 

significant role in net ozone formation during the day. This phenomenon is supported by 

the photochemical/dispersion modeling for this SIP, which shows a NOx reduction 

disbenefit in downtown Denver. A reduction in morning rush hour NOx emissions, for 

example, would allow ozone concentrations to increase sooner and perhaps more 

rapidly in areas with high densities of mobile source emissions. 

Table 2-3 suggests that the weekend effect increased each year from 2005 through 

2007, with annual mean Front Range differences rising from 2% to 6%. It is possible 

that this increase is a response to reductions in VOC emissions from oil and gas fields 

during the period. Reducing VOC emissions could push the area in the direction of 

VOC-limitation, and in theory this could increase the weekend effect. Other factors 

might be at work, however, including changes in meteorology from year to year. Since 

NOx titration has been shown to be a significant factor and since the magnitude of NOx 

titration is mediated by the strength and duration of the nighttime/morning surface 

inversion and afternoon mixing height, it may be that year-to-year changes in inversion 

and mixing depth climatology are responsible for the increases in the weekend effect. A 

thorough analysis of mixing height and inversion climatology for these summers is 

beyond the scope of this report. 
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Figure 2-23: Mean ratio of daily maximum 8-hour ozone on weekends to weekdays for June through 

August of 2005 through 2007. The color-scale legend is not intended to define a threshold or cutoff 

for NOx and VOC limitation, but rather to indicate a general tendency towards either state at opposite 

ends of the scale 
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3 Review of the Ozone Conceptual Model for the 8-hour 

DMA/NFR NAA 

EPA guidance for the development of a conceptual model defines the meteorological 

conditions associated with high ozone concentrations. A conceptual model of ozone 

formation includes the current understanding of the local meteorological conditions and 

associated large-scale weather patterns typically experienced during periods of elevated 

ozone. Local understanding of ozone formation is not only important for forecasting 

elevated ozone levels to protect public health, but, also to gain an understanding of the 

effectiveness of control strategies. 

As part of the conceptual model, supporting analysis includes a review of available 

ambient air quality data, meteorological data, and photochemical modeling efforts. As 

new meteorological data and emission inventory data becomes available as well as a 

better understanding of the chemical processes that take place in the nonattainment 

area, the opportunity to review the current understanding of local ozone formation in the 

Denver/NFR region presents itself. 

Generally, ozone is formed by a complex series of chemical reactions involving 

photochemical reactive Volatile Organic Compounds (VOCs) and Oxides of Nitrogen 

(NOx) in the presence of sunlight. In the DMA/NFR, ambient concentrations of these 

precursor compounds are sufficient to produce ozone as evident by an occasional 

exceedance of the 8-hour ozone standard of 85 ppb. However, favorable meteorological 

conditions are also required before high ozone concentrations are measured. 

3.1 Local and Synoptic Scale Meteorology 

Meteorology is the single most important factor affecting mid-summer ozone in the 

DMA/NFR. Light winds, a deep layer of thermally-driven upslope flow, local vertical 

recirculation through the actions of a Front Range Mountain-Valley circulation, cloud-free 

skies, and warm temperatures are key ingredients for high ozone at the surface. The 

Mountain Valley circulation consists of thermally-driven surface upslope flow (toward the 

west) to mountain top level during the afternoon, mixing and transport vertically, and 

weak transport to the east at higher altitudes. Vertical mixing over Denver closes this 
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loop, keeping ozone in the area. Nighttime surface drainage along valleys allows 

pooling of morning emissions in lower terrain along the Platte Valley. This phase 

contributes to the accumulation of emissions that are later processed by the sun and the 

daytime mountain-valley circulation during the afternoon. 

Pollutants emitted during the day mix upwards and accumulate in that portion of the 

atmosphere that eventually becomes isolated from the nighttime inversion layer. In 

addition, elevated point sources release pollutants above the inversion layer at night. 

These pollutants are transported aloft by mid-level winds. In the morning, under strong 

insolation, surface temperatures rise rapidly, forming a mixed layer that brings pollutants, 

transported or stored aloft during the night, to the surface. 

High ozone levels along the Front Range are significantly affected by upper air transport 

and the retention of ozone aloft during the nighttime hours. The ozone aloft is 

subsequently incorporated into the surface boundary layer during the day. Research by 

National Oceanic and Atmospheric Administration (NOAA) scientists and APCD staff 

indicate a retention and buildup of ozone in the upper portion of the troposphere (the 

atmosphere below the stratosphere.) Ozone increases in the atmosphere above the 

nighttime boundary layer can be as large as 20 – 40 ppb. 

A key synoptic factor is the multi-day mean 500-millibar height in the area, which is the 

mean strength of the synoptic-scale regional upper level high-pressure system. Since 

the 500-millibar height is directly related to the mean temperature of the column of air 

below about 18,000 feet, it can have a direct effect on the magnitude of regional 

background concentrations. Warm temperatures throughout this layer are a typical 

prerequisite for high ozone concentrations. Higher 500-millibar heights are also 

associated with weaker westerlies and a lower incidence of thunderstorms and can lead 

to the stagnation and re-circulating of ozone and its precursors in the Four Corners 

states. This stagnation and re-circulation, and the retention of ozone in the mid levels 

from one day to the next, can lead to a regional build up an ozone base or background. 

Monthly mean 500-millibar heights are an excellent predictor of monthly mean daily 

maximum 8-hour ozone concentrations. July monthly mean daily maximum 8-hour 

ozone is more strongly correlated with 500-millibar heights than a host of other logical 

choices for significant predictors of ozone, including mean surface temperatures, mean 
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temperatures aloft, winds aloft, cloud cover, solar radiation, and number of days with 

temperatures above 90 degrees. While annual fourth maximum 8-hour ozone 

concentrations can occur in any of the months of summer, it turns out the mean July 

500-millibar height over Denver is one of the single best predictor for this value at sites 

along the Front Range urban corridor.

3.2 Back Trajectory Analyses for the 2006 Ozone Episodes and Ozone 

Season 

Back trajectory calculations for 8-hour ozone exceedance events at worst-case monitors 

are recommended as part of the WOE attainment demonstration and to support the 

understanding of an area’s conceptual model. Back trajectories can also provide 

additional evidence that the behavior of the photochemical dispersion model is 

reasonable and are useful tools for understanding the relative influence of source areas 

within the region. 

Back trajectories were estimated for each episode by using the NOAA Air Resources 

Laboratory (ARL) Hybrid Single Particle Lagrangian Integrated Trajectory Model 

(HYSPLIT, see: http://www.arl.noaa.gov/ss/models/hysplit.html). HYSPLIT uses 

meteorological model data from the National Centers for Environmental Prediction 

(NCEP). Trajectories for the June-July 2006 period were modeled using the threedimensional wind fields provided by the Eta Data Assimilation System (EDAS). The 

EDAS datasets archived by ARL are based on 3-hour data over a 40-km horizontal grid 

(EDAS40). EDAS40 is a reanalysis system based on Eta model simulations heavily 

weighted by a very large collection of surface and upper air observations. HYSPLIT 

further resolves EDAS40 output into a much finer grid space. Although subject to wind 

field estimation errors in complex terrain, HYSPLIT with EDAS40 input has been 

observed to perform well in a variety Air Pollution Control Division analyses. HYSPLIT 

windfields will sometimes miss the thermally-driven upslope flows along the Front 

Range, but catch these flows surprisingly well for many episodes. 

Back trajectories were calculated for the Rocky Flat North (RFN) and Fort Collins West 

(FTCW) sites. RFN and FTCW are both high-concentration monitors. Back trajectories 

for RFN can show the influence of sources within the Denver metro area and the Platte 
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Valley. Back trajectories for FTCW can show the influence of sources within the Platte 

Valley, local emissions, and the Denver metro area. FTCW trajectories can vary 

significantly from those calculated for RFN. Taken together, these sites are fairly 

representative of the temporal and spatial variability of windfields and source region 

influences across the Front Range for a given episode. 

Composite 48-hour back trajectories were generated for each day of three ozone 

episodes during the photochemical model base year of 2006. These episodes coincide 

with periods modeled with the CAMx photochemical model. These three episodes 

represent three distinct meteorological regimes when high ozone concentrations were 

modeled in the DMA//NFR. The EPA recommends that various meteorological 

conditions be modeled for the attainment demonstration in order to estimate the benefit 

of the various control strategies. The back trajectory analysis demonstrates that the twomonth period of June-July 2006 contains a variety of meteorological regimes with 

elevated ozone ensuring a more representative analysis of the control strategy packages 

in this SIP. 

Back trajectories were developed for three ozone episodes in 2006: 

• June 17-19, 2006 

• July 13-15, 2006 

• July 27-29, 2006 

Substantial transport of regional background ozone and precursor compounds 

contributes to elevated ozone levels along the northern Front Range during most high 

ozone episodes. The potential effects of long-range transport and local sources were 

investigated by calculating 48-hour HYSPLIT back-trajectories for each of the days in the 

three high ozone episodes in 2006. Trajectories arrived at each monitor location at 4:00 

PM MDT, a time that approximates the midpoint for periods of elevated concentrations 

on most episode days. Composite back trajectories were generated and these include 

three arrival heights: 10 meters, 100 meters and 1000 meters. Analysis for a variety of 

arrival height levels makes it possible to assess the transport of low-level air parcels into 

the area as well as air parcels aloft. It also provides an indication of the level of wind 

shear in the atmosphere as well as the presence of absence of decoupled air masses. 
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Day-specific trajectories for each episode were also estimated for the 5:00 AM through 

5:00 PM MDT period. Daily back trajectories were calculated using 10 meters as the 

arrival height. The day specific trajectory analyses consist of four trajectories, each 

starting at 5 AM and arriving at 8 AM, 11 AM, 2 PM and 5 PM with 1-hour interval 

markers. 

3.2.1 Trajectory Analysis June 17-19, 2006 

Figure 3-1 shows that, during the 48-hours proceeding June 17 and 18, the air mass 

source was generally from the west and northwest. Transport distances were also 

substantial, suggesting that there was limited potential for stagnation and an increased 

potential for good dispersion. On June 19 when the highest monitored concentrations 

were measured, the air mass source was in the eastern plains of Colorado. Transport 

distances were minimal, suggesting an increased potential for stagnation and limited 

dispersion. On June 19, a thermally driven upslope condition occurred over the Platte 

Valley and the Front Range, and this is consistent with the conceptual model for highozone events. Upslope occurred at each of the three arrival heights for both sites, and 

the 6 PM MDT Denver sounding showed that the top of the mixed layer was at about 

19,000 feet above sea level. This would indicate that there was deep mixing of the 

atmosphere sufficient to mix any aged air mass aloft down to the surface. This day was 

representative of two processes described in the conceptual model where upslope 

involves local sources and deep vertical mixing is important in ozone formation and 

accumulation in the non-attainment area. 

High ozone concentrations were monitored at RFN (94 ppb), FTCW (87 ppb) and SBC 

(87 ppb) on June 19. The 94 ppb ozone recorded at RFN was the highest monitored 

concentration at that site in 2006. On June 17 and 18, area-wide ozone concentrations 

were about 60-80 ppb over the entire network. 

Back trajectories in Figure 3-2 show that the local scale transport was generally from the 

eastern plains of Colorado towards the RFN and FTCW monitors. Thermally driven 

upslope flow would have transported precursors from the Platte Valley in Weld County to 

FTCW and precursors from the Denver area to RFN. Local control strategies would be 

expected to have a more significant impact under conditions found on this day. 
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Figure 3-1: HYSPLIT 48-hour back trajectories for 10 (red), 100 (blue), and 1000 (green) meter 

elevations for June 17-19, 2006, at RFN (left) and FTCW (right), 6-hour interval markers, 4 PM MDT. 
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Figure 3-2: June 19, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for RFN (left) and 

FTCW (right). Four trajectories, each starting at 5 AM and arriving at 8 AM (short red), 11 AM 

(green), 2 PM (blue) and 5 PM (long red) with 1-hour interval markers. 
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3.2.2 Trajectory Analysis July 13-15, 2006 

Figure 3-3 presents backward trajectories for the July 13-15, 2006, episode. The 

composite backward trajectories suggest that at both the upper and lower levels, the air 

mass originated west of the Front Range on July 13 and 14 with some additional 

influence from plains sources on July 14. On July 15, the transport was generally from 

the south. A large upper level high pressure system moved into the region during this 

period. Data from a single Boulder ozonesonde is available for this episode (from the 

NOAA Earth Systems Research Laboratory Global Monitoring Division (NOAA ESRL 

GMD), see: http://www.esrl.noaa.gov/gmd/ozwv/). Results from the July 14 

ozonesonde, launched at noon MDT, are presented in Figure 3-4 and show that ozone 

concentrations ranged from about 74 to 91 ppb in the atmosphere below 17,000 feet 

above sea level, with the highest concentrations at the top of this layer. This is evidence 

of a large reservoir of ozone aloft within the residual layer and suggests that this episode 

is representative of those strongly affected by transport and storage aloft within the core 

of an upper level high pressure system. Denver mixing heights at 6 PM MDT on July 13, 

14, and 15 were approximately 21,000, 17,500, and 19,000 feet above sea level 

respectively; and these mixing levels also support the hypothesis that ozone retained 

within a deep residual layer within the upper level high pressure had an impact on 

ground level concentrations each day. 

High eight-hour ozone concentrations were monitored at HLD (85 ppb), CHAT (92 ppb), 

WCH (87 ppb), and the Weld County Tower (90 ppb) on July 13. Notably, ozone 

concentration at RFN (81 ppb) and FTCW (82 ppb) were below the standard. Ozone 

levels were elevated at most locations in the network on June 13. On July 14, ozone 

concentrations were high at SBC (86 ppb), RFN (91 ppb), and RMNP (91 ppb). The 97 

ppb ozone concentration recorded at FTCW on July 14 was the highest monitored 

concentration at that site in 2006. Elevated concentrations continued on July 15 at SBC 

(86 ppb), RFN (90 ppb), and FTCW (87 ppb). 

Figure 3-5 through Figure 3-7 show same-day back trajectories for four arrival times 

during the daylight hours for July13 through 15. Each shows some influence from 

transport from the west. Transport distances are highest on the 13th but are quite short 

on the 14th and 15th. Transport from the west was greatest on the 13th. Local source 

areas had more of a same-day influence on the 14th and 15th. On the 13th, surface 
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ozone concentrations may have been more heavily influenced by regional transport and 

ozone in the residual layer. On the 14th and 15th, surface ozone may have been 

influenced by both retention of ozone within the residual layer and local sources. Local 

controls would be more likely to have an impact during conditions similar to those on the 

14th and 15th than the 13th. 
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Figure 3-3: HYSPLIT 48-hour back trajectories for 10 (red), 100 (blue), and 1000 (green) meter 

elevations for July 13-15, 2006, at RFN (left) and FTCW (right), 6-hour interval markers, 4 PM MDT. 
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0

5000

10000

15000

20000

25000

30000

35000

0 10 20 30 40 50 60 70 80 90 100

Boulder Noon Ozonesonde Data

July 14, 2006

Ozone in ppb

Altitude Above Sea Level in Feet

Denver Mean Summer Afternoon Mixing Height

Horizontal White Line

Variables

O3ppb

Figure 3-4: Boulder ozonesonde data for Noon MDT July 14, 2006, showing elevated ozone 

concentrations from the surface to 17,000 feet above sea level. 

Figure 3-5: July 13, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for RFN (left) and 

FTCW (right). Four trajectories, each starting at 5 AM and arriving at 8 AM (short red), 11 AM 

(green), 2 PM (blue) and 5 PM (long red) with 1-hour interval markers. 
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Figure 3-6: July 14, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for RFN (left) and 

FTCW (right). Four trajectories, each starting at 5 AM and arriving at 8 AM (short red), 11 AM 

(green), 2 PM (blue) and 5 PM (long red) with 1-hour interval markers. 

Figure 3-7: July 15, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for RFN (left) and 

FTCW (right). Four trajectories, each starting at 5 AM and arriving at 8 AM (short red), 11 AM 

(green), 2 PM (blue) and 5 PM (long red) with 1-hour interval markers. 
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3.2.3 Trajectory Analysis July 27-29, 2006 

The composite trajectory analysis for July 27-29, 2006, (Figure 3-8) shows a complex 

pattern with generally favorable conditions for high ozone concentrations. A broad upper 

level high pressure system west of the state on the 27th moved over Colorado on the 28th

and 29th. Transport to RFN for July 27 shows that source regions were within Colorado, 

while the back trajectories for FTCW indicate that the air mass had origins as far away 

as Idaho and Montana. On the 28th, sources for air arriving at near-surface levels for 

both sites were within Colorado, while parcels arriving at the 1500-meter level came 

from northern Wyoming. On the 29th, transport source regions for both monitors were 

largely within eastern Colorado and Western Kansas. Transport distances for the 10-

meter and 100-meter arrival levels were relatively short on both the 28th and 29th, 

suggesting poor dispersion conditions. Denver mixing heights at 6 PM MDT were 

roughly 17,500, 13,000, and 14,000 feet above sea level on the 27th, 28th, and 29th, 

respectively. 

Elevated ozone concentrations were monitored at CHAT (86 ppb) on July 27 and at RFN 

(88 ppb) on July 28. Widespread excedances of the ozone standard occurred on July 

29 especially at CARRIAGE (92 ppb), CHAT (88 ppb), WCH (96 ppb), RFN (93 ppb), 

NREL (94 ppb) and FTCW (95 ppb) and Weld (87 ppb) on July 27. 

The composite trajectory analysis for July 27-29, 2006 indicates that stagnant and 

recirculation conditions existed over the Front Range both at the surface and aloft as 

illustrated in Figure 3-8. The vertical plot for the composite trajectory indicate that there 

was turbulence in the atmosphere where upper air and surface air was mixed. Both the 

stagnant conditions and the interaction between the upper level and surface conditions 

represent conditions for the conceptual model. This is also one of the best periods for 

analyzing the effects of local controls on ozone levels along the Front Range. 

Generally, the daily trajectory plots for July 27-29 indicate that the surface air over the 

Front Range was recirculated or stagnant. 

Figure 3-9 through Figure 3-11 show same-day back trajectories for four arrival times 

during the daylight hours for the July 27-29, 2006 episode. The same-day trajectories 

generally show upslope transport from key source areas such as the Platte Valley, Weld 
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County, and the Denver metro area. The trajectories for FTCW on July 27 are the 

exception here, showing transport from the west and north. Transport distances are also 

typically short, and this is consistent with limited dispersion conditions. This episode is 

another good example of one in which thermally-driven upslope affects transport of local 

emissions and vertical mixing taps into ozone and its precursors that reside in the 

residual layer aloft. This episode should be quite well suited to the evaluation of local 

control strategy evaluations. 
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Figure 3-8: HYSPLIT 48-hour back trajectories for 10 (red), 100 (blue), and 1000 (green) meter 

elevations for July 27-29, 2006, at RFN (left) and FTCW (right), 6-hour interval markers, ending 4 PM 
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Figure 3-9: July 27, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for RFN (left) and 

FTCW (right). Four trajectories, each starting at 5 AM and arriving at 8 AM (short red), 11 AM 

(green), 2 PM (blue) and 5 PM (long red) with 1-hour interval markers. 

Figure 3-10: July 28, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for RFN (left) and 

FTCW (right). Four trajectories, each starting at 5 AM and arriving at 8 AM (short red), 11 AM 

(green), 2 PM (blue) and 5 PM (long red) with 1-hour interval markers. 
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Figure 3-11: July 29, 2006, 10-meter back trajectories from 5 AM to 5 PM (MDT) for RFN (left) and 

FTCW (right). Four trajectories, each starting at 5 AM and arriving at 8 AM (short red), 11 AM 

(green), 2 PM (blue) and 5 PM (long red) with 1-hour interval markers. 

3.2.4 Back Trajectory Climatology for the Summer of 2006 

The NOAA ARL HYSPLIT model has also been used to calculate same-day back 

trajectories for each day during the May 17 through August 15 period of 2006. Figure 

3-12 shows back trajectory points for this entire period. Back trajectories were 

calculated for surface arrival for each of the eight hours associated the daily maximum 8-

hour ozone concentrations at FTCW, RFN, Highlands Ranch, and Rocky Mountain 

National Park. Back trajectories were up to 12 hours in length with an average start time 

of about 6:30 AM MDT. Each point represents one hour of transport. The EDAS40 data 

set was used to drive HYSPLIT. This spread of points provides a comprehensive picture 

of the possible air mass source areas for these sites during the summer of 2006. 

This map shows that the plains, Platte Valley and metropolitan areas of the Front Range 

are typical source areas for these four monitors, and this is consistent with the 

conceptual model that has already been discussed. Transport from the west also 

occurred, especially at Rocky Mountain National Park. 
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It is possible to estimate the average air quality contributions of any area covered by 

back trajectory points using a moving spatial average method. The results of such an 

analysis are presented in Figure 3-13. For this analysis, the concentration values 

associated with a given sample were assigned to each of the calculated back trajectory 

points for that sample. A moving spatial average was applied to all of the point 

concentrations for the entire year. A computer routine placed an oval window with a 

height of 0.2 degrees latitude and width of 0.2 degrees longitude over each grid node 

(the grid node spacing was 0.07 degrees latitude in the Y direction and 0.07 degrees 

longitude in the X direction). If the number of points in the oval was 90 or higher, the 

routine calculated the mean concentration for that node. The distribution of mean values 

on the map was then contoured using the Kriging method. Limiting the averaging to 

nodes with a high number of points reduces the chances that a few points associated 

with a high or low sample concentration bias the estimates of source contributions from 

that locale. The resulting map provides an estimate of the mean contribution for any 

area covered by the analysis. 

The map in Figure 3-13 shows the resulting summer mean 8-hour maximum ozone for 

that occurred when an air mass was from a given source area. The Denver metro area 

and the Platte Valley are responsible for the highest concentrations. More specifically, 

the cool pool area of the Platte Valley in Weld County (where drainage flow creates a 

deeper decoupled air mass that is typically the last location to se inversion burn off) 

results in a mean daily maximum 8-hour ozone concentration of 71 ppb at the four Front 

Range monitors considered. This is the average concentration based on all four 

monitors. This plot clearly supports the conceptual model and shows that local plains 

sources and pooling of emissions within the Platte Valley contribute to the highest 

concentrations at monitors subject to thermally driven upslope flows during the 

afternoon. Flow from the west is typically responsible for lower mean concentrations 

over the course of this three-month period. 
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Figure 3-12: NOAA HYSPLIT same-day back trajectory points for May 17 through August 15, 2006, 

for FTCW, RMNP, HLD, and RFN – back trajectories up to 12 hours in length for each hour that was 

used to calculate a daily 8-hour maximum ozone concentration, each point representing one hour, with 

average start times of 6:30 AM MDT. 

Figure 3-13: Contours of the mean May 17 – August 15, 2006, Front Range daily maximum 8-hour 

ozone concentrations resulting from transport from given source areas, based on a moving spatial 

average analysis of concentrations associated with HYSPLIT back trajectory points for FTCW, 

RMNP, RFN, and HLD monitors. These are the average concentrations that result at these four 

monitors when an air mass originates in a given area. 
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3.2.5 Trajectory Analysis Conclusions 

The trajectory analyses for three key episodes show that they represent a variety of 

meteorological and transport conditions. In particular they show gradations between 

long-distance transport from the west and more localized upslope flows within eastern 

Colorado. In the conceptual model, the roles of upper level transport and storage and 

surface upslope transport from local sources have been highlighted. While June 17 and 

18 and July 13 were dominated by long-range transport from the west and the impacts of 

ozone aloft, the remaining episode days show a combination of influences. In particular, 

the meteorology of these days favored contributions from ozone within the residual layer 

and short-range transport from local sources on the eastern plains. The range of 

conditions supports the conceptual models and demonstrates that the modeling period 

includes a representative variety of meteorological conditions. 

In addition, the climatological analysis of back trajectories for FTCW, RMNP, RFN, and 

HLD and the moving spatial analysis of source areas based on these trajectories 

demonstrate that local sources along the Platte Valley, in the Denver metro area, and 

within Weld County play a key role in ozone formation during thermally-driven upslope 

conditions. This is consistent with the conceptual model. While MM5 does not always 

reproduce these thermally-driven upslope flows, it does so often enough to insure some 

confidence in the overall performance of the photochemical modeling. 
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4 Review of Modeled Metrics and Modeled Attainment 

Demonstration 

4.1 Additional Modeling Metrics 

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

metrics from the current year base case to future year emissions scenarios to assure 

that 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. 

To support the attainment demonstration in the Denver Ozone SIP, additional modeling 

metrics are used to assess the changes in ozone levels at grid cells in the NAA from 

2006 base case to 2010 base case to 2010control cases. In the figures that follow, 

“Base” represents the 2010 base case, “Cntrl1” represents the Control 1 scenario,, and 

“Cntrl2” represents the Control 2 scenario (i.e., proposed state-only strategies that are 

not included in the SIP but will provide additional reductions in the DMA/NFR area). All 

three of the metrics presented below (grid cells, grid cell hours and total ozone) show 

decreases in peak elevated ozone ≥ 85 ppb from emissions reductions due to existing 

controls and regulations and continued decreases due to the Control 1 and Control 2 

scenarios. 

ENVIRON/Alpine Geophysics (Morris, et al 2008a) calculated the change in daily 

maximum 8-hour ozone concentrations between the 2006 base case and 2010 emission 

scenarios across grid cells in the Denver NAA and across all days in the June-July 2006 

modeling episode. The changes 8-hour ozone concentrations were calculated for values 

above four separate threshold concentrations: 85, 80, 75 and 70 ppb. The additional 

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. 
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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

• 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. 

4.1.1 Relative Change in Grid Cells 

As can be seen in Figure 4-1 the emissions reductions from the 2006 base case to the 

2010 base case achieve a 14% reduction in grid cells ≥ 85 ppb and an additional 3.5% 

reduction in grid cells due to the Control 1 strategies. Continued reduction of emissions 

due to Control 2 strategies (through state-only or voluntary measures) continues to 

demonstrate reductions of cells ≥ 85 ppb. Grid cells ≥80 ppb, ≥ 75 ppb and ≥70 ppb 

show an initial reduction due to existing controls and regulation from the 2006 base case 

to the 2010 base case and show ≥1% reduction due to the Control 1 and Control 2 

scenarios. 
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Percent Change in Grid Cell Hours for 

2010 Base, Cntrl1, and Cntrl2

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

Base CNTL1 CNTL2

% Change

Grid Cell

Hours 85

ppb

Grid Cell

Hours 80

ppb

Grid Cell

Hours 75

ppb

Grid Cell

Hours 70

ppb

Figure 4-1: Relative Change in Grid Cells 

4.1.2 Relative Change in Grid Cell-Hours 

As can be seen in Figure 4-2 the emissions reductions from the 2006 base case to the 

2010 base case achieve a 22% reduction in grid cells-hours ≥ 85 ppb and an additional 

3% reduction in grid cells due to the Control 1 strategies. Continued reduction of 

emissions (through state-only or voluntary measures) continues to demonstrate 

reduction of cells-hours ≥85 ppb. Grid cell-hours ≥80 ppb, ≥75 ppb and ≥70 ppb show an 

initial reduction due to existing controls and regulation from the 2006 base case to the 

2010 base case and show ≥1% increased reduction in cell-hours due to the SIP and 

state-only controls. 

Figure 4-2: Relative Change in Grid Cell-hours 

4.1.3 Relative Change in Total 8-Hour Ozone 
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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

As can be seen in Figure 4-3 below, the emissions reductions from the 2006 base case 

to the 2010 base case achieve a 21% reduction in total 8-hour ozone ≥ 85 ppb and an 

additional 7% reduction in total 8-hour ozone due to the Control 1 strategies. Continued 

reduction of emissions (through state-only or voluntary measures) continues to 

demonstrate reduction of total 8-hour ozone ≥85 ppb. Total 8-hour ozone ≥80 ppb, ≥75 

ppb and ≥70 ppb show an initial reduction due to existing controls and regulation from 

the 2006 base case to the 2010 base case and show ≥1% increased reduction in cellhours due to the Control 1 and Control 2 strategies. 

Figure 4-3: Relative Change in Total 8 hour Ozone 

In summary, the additional model metrics presented in Figure 4-1 through Figure 4-3 

demonstrate that the emission reductions between 2006 and 2010 are having their 

intended effect of being effective at reducing the elevated 8-hour ozone concentrations. 

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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4.2 Review Alternative Attainment Test Methodology 

While EPA’s modeling guidance offers several potential approaches for establishing 

base year design values, the guidance recommends the preferred methodology for 

establishing a base year design value as follows: 

“For the modeled attainment tests we recommend using the average of the three 

design value periods which include the baseline inventory year. Based on the 

attributes listed above (in the guidance), the average of the three design value periods 

best represents the baseline concentrations, while taking into account the variability 

of the meteorology and emissions (over a five year period).” 

At the start of the work on the SIP in 2007 and throughout development of the proposed 

plan, the modeling analysis has used the 2005-2007 three-year design value as 

representative of the ozone situation facing the region at the time. In the SIP, attainment 

of the 8-hour ozone standard was demonstrated at all of the monitored locations by 2010 

for the 8-hour ozone nonattainment as a result of the reductions expected from existing 

programs and regulations. Table 4-1 presents as summary of the current attainment 

demonstration in the Denver Ozone SIP. Data from the 2008 ozone season (though 

currently not formally quality assured by the State or EPA) is now available and the 

average of the three Design Values in the 2004-2008 periods can be calculated, as the 

recommended method by EPA. 
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Table 4-1: 2010 Base Case Design Values for Each Monitoring Site for Modeled Days greater than 

0.075 ppm 

Site Name 

8-Hour Ozone 

Current 

(2005-2007) 

Base Case 

Design Values 

(ppm) 

Modeled 

Base Case 

Relative 

Response 

Factors 

Calculated 

8-Hour Ozone 

Future (2010) 

Base Case 

Design Values 

(ppm) 

Truncated 

8-Hour Ozone 

Future (2010) 

Base Case 

Design Values 

(ppm) 

Welby 0.070 1.0042 0.0702 0.070 

Arvada 0.079 1.0026 0.0792 0.079 

NREL 0.082 1.0039 0.0823 0.082 

Rocky Flats North 0.085 0.9994 0.0849 0.084 

S. Boulder Creek 0.081 0.9976 0.0808 0.080 

Fort Collins 0.074 0.9878 0.0730 0.073 

Fort Collins West* 0.086 0.9874 0.0849 0.084 

Carriage 0.074 1.0022 0.0741 0.074 

Welch 0.075 1.0004 0.0750 0.075 

CAMP 0.056 1.0017 0.0560 0.056 

Weld County Tower 0.078 0.9964 0.0777 0.077 

Highland 0.078 0.9916 0.0773 0.077 

Chatfield Res. 0.084 0.9934 0.0834 0.083 

Rocky Mtn. N.P. 0.076 0.9903 0.0752 0.075 

* FCW has only 2 years of complete data available, 2006 and 2007 

In this supplemental analysis, the EPA’s recommended methodology is applied to 

establish the base year Design Values and project the 2010 base case Design Value. 

This methodology requires that the average of the base year Design Value over 2004-

2008 be rounded to the 4th place and presented to the 4th place in ppm. After application 

of the Relative Response Factors, the future year (2010) Design Values are rounded to 

the 4th place and then truncated for comparison with the NAAQS. Table 4-1 presents 

the results of applying EPA’s recommended base year Design Value calculation 

approach. As shown in Table 4-2, by applying EPA’s recommended design value 

methodology, the ozone standard is achieved at all the monitoring sites with more of a 

safety margin than the results presented in Table 4-2 using the three-year average 

(2005-07) base case design values for the 2010 base case. 
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Table 4-2: 2010 Base Case Design Values Utilizing EPA’s Recommended DVB Calculation 

Methodology 

Site Name 

Current 

(2004-08*) 

Base Case 

Design Value 

(ppm) 

Modeled 

Control Case 

Relative 

Response 

Factors 

Calculated 

2010 

Base Case 

Design 

Value 

(ppm) 

Truncated 

2010 

Base Case 

Design Value 

(ppm) 

Welby 0.0707 1.0042 0.0710 0.071 

Arvada 0.0777 1.0026 0.0779 0.077 

NREL 0.0808 1.0039 0.0811 0.081 

Rocky Flats North 0.0840 0.9994 0.0839 0.083 

S. Boulder Creek 0.0791 0.9976 0.0789 0.078 

Fort Collins 0.0728 0.9878 0.0719 0.071 

Fort Collins West** 0.083 0.9874 0.0820 0.082 

Carriage 0.0728 1.0022 0.0730 0.073 

Welch 0.0740 1.0004 0.0740 0.074 

CAMP 0.0560 1.0017 0.0561 0.056 

Weld County Tower 0.0769 0.9964 0.0766 0.076 

Highland 0.0760 0.9916 0.0754 0.075 

Chatfield Res. 0.0829 0.9934 0.0824 0.082 

Rocky Mtn. N.P. 0.0759 0.9903 0.0752 0.075 

* Thru August 31, 2008. 2008 data have not been fully quality assured at this time; 

** FCW only has three years of data and is presented as a Design Value to three places 
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Table 4-3 presents the future year Design Values from the 2010 Control 1 scenario 

modeling using EPA’s recommended DVB calculation approach. Table 4-3 also 

demonstrates that the ozone standard will be achieved at all the monitor sites using the 

2010 Control 1 scenario. 

Table 4-3: 2010 “Control 1” Design Values Utilizing EPA’s Recommended DVB Calculation 

Methodology

Site Name 

Current 

(2004-08*) 

Base Case 

Design Value 

(ppm) 

Modeled 

Control Case 

Relative 

Reduction 

Factors 

Calculated 

2010 

Control Case 

Design Value 

(ppm) 

Truncated 

2010 

Control Case 

Design Value 

(ppm) 

Welby 0.0707 1.0039 0.0709 0.071 

Arvada 0.0777 1.0022 0.0779 0.077 

NREL 0.0808 1.0027 0.0810 0.081 

Rocky Flats North 0.0840 0.9981 0.0838 0.083 

S. Boulder Creek 0.0791 0.9963 0.0788 0.078 

Fort Collins 0.0728 0.9853 0.0717 0.071 

Fort Collins West** 0.083 0.9852 0.0818 0.081 

Carriage 0.0728 1.0015 0.0729 0.072 

Welch 0.0740 1.0002 0.0740 0.074 

CAMP 0.0560 1.0009 0.0560 0.056 

Weld County Tower 0.0769 0.9925 0.0763 0.076 

Highland 0.0760 0.9900 0.0752 0.075 

Chatfield Res. 0.0829 0.9921 0.0822 0.082 

Rocky Mtn. N.P. 0.0759 0.9892 0.0751 0.075 

* Thru August 31, 2008. 2008 data have not been fully quality assured at this time; 

** FCW only has three years of data and is presented as a Design Value to three places 
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4.3 Alternative 2010 Ozone Projections 

ENVIRON/Alpine Geophysics (Morris, et al, 2008a) analyzed several alternative 2010 

ozone projection procedures for the 2010 base case, Control 1 and Control 2 scenarios 

to estimate the uncertainties in the projection procedures and provide confidence that 

passing the modeled attainment demonstration indicates attainment will likely be 

achieved in 2010 under 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 Relative Response Factors (RRFs). 

Six additional ozone projection procedures were analyzed, in addition to the EPA 

guidance default. 

• 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 4-4 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. Table 4-4 shows the ozone cutoff thresholds and number of days used in 

calculating the RRFs for each alternative 2010 ozone project methods, and, at the 

RFNO and FTCW monitoring sites. 
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• 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%). 

• 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 shown clearly in Table 4-4 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. 
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Table 4-4: 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 

4.4 Modeled vs. Measured VOC/NOx Ratios 

The CDPHE/APCD collected 3-hour VOC samples at several sites during a few days of 

the June-July 2006 modeling period. As part of the model performance evaluation 

ENVIRON/Alpine Geophysics compared these ambient VOC samples with modeled 

data. In the comparisons, the observed VOC species were converted to the Carbon 

Bond V (CB05) lumped VOC species, which was the chemical mechanism used in 

CAMx for the Denver SIP. The modeled and observed CB05 species were summed to 

get the total predicted and observed VOC concentrations, respectively. Ethane (ETHA) 

was not included in the total VOC species when summing the CB05 lumped species. 

Also note that since the VOC sampling did not collected measurements for methanol 

and ethanol, those species were also not accounted for when summing the CB05 

species. 

The comparison that ENVIRON/AG (Morris, et al, 2008b) conducted indicated that the 

model systematically under predicts the observed VOC concentrations at the CAMP 

monitoring site, whereas for NOx and CO there are days with under predictions and 

days with over predictions. On average NOx is under predicted as well. Given the 

location of the CAMP monitor in downtown Denver, VOC, NOx and CO emissions are 

dominated by on-road mobile sources. The comparisons of monitored and modeled data 
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provide an opportunity to evaluate the on-road mobile source emissions with the caveat 

that the ambient measurement occurs at a specific point and time and the modeled 

concentrations are averaged over a 4-km grid space. Since a direct comparison of 

ambient data and modeled data cannot be made, better indications of the accuracy of 

the on-road mobile source emissions are the VOC/NOx and CO/NOx key indicator ratio 

comparisons. Comparisons of the predicted and observed VOC/NOx ratios provide an 

indication of whether the model is reproducing the correct chemical regime in the DMA. 

Of the 15 days with morning VOC/NOx comparisons, very good comparisons are seen 

on 11 (73%) of the days. One of the days where the model under predicted the 

VOC/NOx ratio one was July 4th that has atypical traffic patterns which is not simulated 

well in the CONCEPT modeling. Two of the days where the VOC/NOx ratio was over 

predicted were weekend days. However on two other weekend days, good performance 

was demonstrated. 

When ENVIRON/AG compared individual VOC species, the modeled versus ambient 

measurement data indicated that Ethane (ETHA) is under predicted by approximately a 

factor of 10 at the CAMP monitor with observed values of ~25 ppbC and predicted 

values of ~2.5 ppbC. The under prediction of ethane was even greater at the two Weld 

County monitoring sites where the CB05 paraffin species was also under predicted. As 

ethane is primarily associated with natural gas, the under prediction of ethane indicates 

that organic emissions from natural gas related and oil and gas development sources 

are understated in the inventory. 

In addition to the underestimation of observed ethane and paraffin VOC species noted 

above, the comparison also revealed that carbonyl VOC species were also 

systematically under predicted by the model. The under prediction of acetaldehyde may 

be due in part to the SMOKE VOC speciation profile for on-road mobile sources not 

accounting for ethanol blended gasoline whose combustion produces higher 

acetaldehyde than conventional gasoline. However, the reasons for the large under 

prediction of formaldehyde were less clear. Formaldehyde is an important VOC species 

that initiates the radical cycle. The under prediction of formaldehyde may help explain 

why the model tends to form ozone too slowly and does not obtain as high ozone peaks 
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as observed. The consequence of under estimating VOC emissions is that the 

effectiveness of controls is under estimated on mass basis. 

The CO/NOx ratios were over predicted on most days, which may indicate that 

MOBILE6 is overestimating the on-road mobile source CO emissions. These are similar 

to other studies where the CO/NOx ratio is over predicted and may not be unique to the 

Denver emission inventory (Pollack, et. al., 2004). 

4.5 Model Uncertainty and Limitations 

A common thread throughout the modeling and WOE analyses is the uncertainty in the 

modeling process. While modeling is by far the best tool for evaluating proposed control 

strategies, it is imperative to recognize its limitations and the uncertainty in the model 

predictions. 

The photochemical modeling process is composed of a suite of mathematical models 

that are used to evaluate current and future air quality processes. The suite of 

mathematical models have inherent limitations owing to the necessary simplifications 

and approximations made in formulating the governing equations, implementing them for 

numerical solution on fast computers, and in supplying them with input data sets and 

parameters that are themselves approximations of the full state of the atmosphere and 

emission processes. Each component adds its own uncertainty to the process, so that 

the overall uncertainty is a composite of hundreds of individual uncertainties. To put the 

air quality model results in full perspective, the technical support document contains a 

full analysis of the model performance evaluation for the meteorological and 

photochemical model. 

. 

Fortunately, photochemical grid models have proven to be fairly robust in hundreds of 

applications, and provide reasonable answers under most circumstances. Nonetheless, 

the policy maker must be aware that the model can only provide general guidance for 

control strategy development, and cannot be expected to predict future ozone 

concentrations with high precision. 

The uncertainties in the modeling process are inevitably reduced over time, but will 

never be entirely eliminated. Thus, controls must be implemented before it is possible to 
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judge their impact with as much precision as we would like. The WOE process allows for 

a middle ground, where a reasonable control package is sufficient to demonstrate 

probable attainment. 

A brief summary of photochemical model performance used for the Denver Ozone SIP 

and Plan are summarized as follows: 

• The model has a tendency to under predict the observed peaks in ozone 

concentration that is believed to be due in part to: 

o The inability of the meteorology model to push the ozone and its 

precursors far enough into the foothills; 

o The inability to replicate retention of ozone aloft for a sustained period of 

days; and 

o On some days, the estimation of the contributions of transport and the 

amount of ozone generated in the Denver urban plume. 

• The model meets EPA’s peak, bias and error ozone performance goals for ozone 

modeling on a vast majority of the modeled days. The model meets EPA 

guidance performance on a vast majority of the modeled days with the goal that 

most of the matched pairs near the monitor are within ± 20% of the observed 

value. A summary of the model’s performance for the matched pairs is as 

follows: 

o Maximum modeled daily maximum is 76% within ± 20% of the observed 

value; 

o Closest modeled daily maximum is 91% within ± 20% observed value; 

and 

o Spatial paired modeled daily maximum is 82% within ± 20% observed 

value. 

o There is agreement between the modeled and measured volatile organic 

compounds/nitrogen oxides (VOC/NOx) ratios in Denver on most days 

suggesting that the mobile sources inventory is representative and the 

model is simulating the correct chemical regimes. 

The model performance evaluations included in the Technical Support Appendices 

indicate that the photochemical grid modeling is the best available tool to assess 

progress in reducing ozone concentrations. Further, to mitigate the limitations of the 
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modeling platform, the results are not used in an absolute sense, but rather are used in 

relative sense. 
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5 Assess the Efficacy of SIP, State-only and Voluntary Control 

Strategies 

The reduction in emissions from the 2006 base case to the 2010 Control 1 case, which 

includes reductions from current state and federal regulations and newly proposed state 

regulations for inclusion in the SIP, reduces VOC and NOx emissions by 11% from the 

2006 base case. Photochemical grid modeling has shown that these reductions reduce 

ozone concentrations in the nonattainment area. 

The proposed state-only regulations controlling mobile source emissions and oil and gas 

facilities in the nonattainment area and statewide, plus a request of EPA for a change in 

RVP in the NFR area, anticipate an approximate additional 50-60 tpd of VOC reduction 

and 20-21 tpd of NOx reduction state-wide and in the nonattainment area. 

Photochemical grid modeling has shown that these reductions will provide additional 

reduction in ozone concentration levels in the nonattainment area. 

In addition in the DMA/NFR there have been and will continue to be a myriad of 

voluntary measures that are not directly accounted for in the current and projected 

emissions inventories. Such programs include: 

• The summertime Ozone Alert Program where citizens are alerted when elevated 

ozone levels are predicted and are encouraged to reduce their ozone-causing 

activities. 

• The Regional Air Quality Council’s “Let’s Take Care of Our Summer Air” public 

awareness program that includes media advertising and community outreach to 

encourage citizen action to reduce ozone-causing activities. 

• Lawn mower exchange programs in the Denver area and the North Front Range 

that offers discounts for citizens to replace and recycle old gasoline-powered 

mowers with electric mowers and lawn equipment. 

• Replacement of faulty gas caps on cars and trucks through employer-sponsored 

activities and fleet testing programs. 

• Marketing efforts with Colorado Wyoming Petroleum Marketers Association and 

other gasoline retailers to educate motorists at their stores to “Stop at the Click,” 
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65

refuel in the evening, and maintain their vehicles to reduce ozone-forming 

emissions. 

• Efforts by the Regional Air Quality Council and the Colorado Department of 

Public Health and Environment repair or salvage high-emitting vehicles that are 

identified on the road by remote-sensing technology. 

• Pollution Prevention programs implemented by local business and industry to 

reduce their loss of product and prevent emissions of ozone-causing emissions. 

• Employer-based travel reduction programs that are implemented by the Denver 

Regional Council of Governments, area transportation management 

associations, the Regional Transportation District, local governments, and local 

businesses that encourage reduced automobile travel and increased use of 

alternative transportation and workplace options. 

• Efforts by the Regional Air Quality Council, local school districts, and government 

and private fleets to reduce emissions from diesel vehicles through education 

and application emission control and anti-idling equipment. 

• Car Care Fairs where area motorists can have their cars and trucks evaluated to 

improve vehicle performance and increase gas mileage. 

• Implementation of land use and design policies by local governments to 

encourage sustainable development practices and mixed-use, transit-oriented 

development. 

• Efforts by the State of Colorado to improve energy efficiency in state government 

and promote energy efficient practices throughout the state. 

• Household chemical recycling events conducted by local governments and local 

health departments through the Denver area and North Front Range. 

• Greenprint Denver, an initiative of the Denver Mayor’s Office, promotes energy 

efficient practices, sustainable development, increased use of alternative fuels 

and low-emission vehicles, recycling programs, and increased tree planting. 

• The Colorado Department of Transportation’s (CDOT) Air Quality Programmatic 

Agreement is being crafted to identify and commit to a number of proactive 

measures that will reduce mobile source air toxics and greenhouse gas 

emissions throughout Colorado, in addition to criteria air pollutants. Due to the 

inherent nature of air quality, it is more efficient to mitigate these impacts utilizing 

a programmatic approach rather than negotiating individual mitigation for new 

environmental documents. It is hoped that this agreement will be signed by the 
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66

participating parties (between CDOT, EPA, Colorado Department of Health and 

Environment (CDPHE), Federal Highway Authority (FHWA), Federal Transit 

Administration (FTA), Regional Air Quality Council (RAQC), and Regional 

Transportation District (RTD)) by the end of 2008. 
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6 Weight of Evidence – Conclusions 

The final WOE combines and weighs the various supplemental analyses with the results 

of the attainment test resulting in an aggregated, qualitative, and quantitative conclusion 

as to whether the proposed set of control strategies will result in the Denver Metro Area 

and North Front Range reaching attainment in 2010. A number of conclusions can be 

drawn from the weight of evidence: 

• Trends in emissions correlate well with surrogate indicators such as fleet 

turnover. 

• Meteorological variability is a key component for ozone formation and is reflected 

in the year-to-year variability of peak ozone levels. A key metric for upper level 

high pressure strength has remained steady or trended downward in recent 

years, suggesting a reasonable likelihood for moderate high pressure strength in 

the next few years, which would favor attainment. 

• Analysis of the weekend-weekday effect for the Front Range shows a strong 

effect in Central Denver and weaker effects in outlying areas. This points to the 

possibility for NOx control disbenefits in central Denver due to the role of NOx 

quenching here. The spatial pattern of the weekend effect is consistent with the 

localized NOx disbenefit identified in the photochemical modeling. 

• If the emissions trends are correct, then the Relative Response Factors (RRFs) 

are likely to be directionally correct. 

• Reductions in VOC emissions are expected to reduce ozone. 

• Reductions in NOx emissions are expected to reduce ozone, possibly with 

greater efficiency than VOC reductions, at troublesome monitors outside of the 

urban core of metro Denver. Increases in ozone concentrations in the urban core 

of metro Denver due to NOx emissions reductions do not appear to be 

significant. 

• The aggregate trend in weather-corrected 4th maximum time series suggests that 

ozone levels have been flat from 2004 through 2008, although individual 

concentrations have been highly variable. This suggests that without additional 

emission reductions the region will remain at or near the level of the standard. 
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• The base case modeling of the June-July 2006 timeframe encompasses the 

various local meteorological regimes under which elevated ozone levels have 

and are expected to occur. 

• Other modeled metrics indicate that there are reductions in Total Ozone, Grid 

Cells, and Grid Cell-Hours of 15% to 30% for thresholds of 85 ppb and 80 ppb 

from the 2006 base case through the 2010 base case, the 2010 Control 1 case 

and the 2010 Control 2 case. 

• A comparison of ambient and modeled data indicates that VOC emissions from 

the oil and gas sector may be under estimated. The consequence of under 

estimating VOC emissions is that the effectiveness of controls is under estimated 

on mass basis. 

• At this time, the photochemical modeling is considered to be the best predictor of 

future ozone levels. 

In conclusion, the collective supplemental analyses contained in this weight of evidence 

document support the current photochemical model attainment demonstration for the 

0.08 ppm 8-hour ozone NAAQS using the EPA default approaches for the 2010 base 

case, 2010 Control 1, and the 2010 Control 2 scenarios. In addition, at this time, the 

photochemical modeling is considered to be the best predictor of future ozone levels. 

The collective supplemental analyses in this weight of evidence analysis support the 

findings using the EPA methods, as specified in the EPA modeling guidance, that the 

2010 base case will likely achieve attainment of the 0.08 ppm 8-hour ozone NAAQS in 

the Denver Metro Area and North Front Range. As demonstrated using alternative 

attainment test methodologies, the same WOE indicators demonstrate 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 emissions scenario. The preponderance of 

evidence suggests that the region will attain the standard in 2010 under the base case, 

Control 1, and Control 2 scenarios, but the safety margin is small. 
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