Denver Metro Area & North Front Range 

Ozone Action Plan 

Including Revisions to the State Implementation Plan 

Approved by: 

Colorado Air Quality Control Commission 

December 12, 2008

 
 

Contact Information: 

Colorado Air Quality Control Commission 

4300 Cherry Creek Drive South 

Denver, CO 80246 

303-692-3476 

Air Pollution Control Division 

4300 Cherry Creek Drive South 

Denver, CO 80246 

303-692-3100

Regional Air Quality Council 

1445 Market Street, Suite 260 

Denver, CO 80202 

303-629-5450 

North Front Range Transportation and Air Quality Planning Council 

419 Canyon Avenue, Suite 300 

Fort Collins, CO 80521 

970-221-6243
 

Table of Contents 

OVERVIEW: OZONE ACTION PLAN 

8-Hour Ozone Attainment Plan 

CHAPTER I: FEDERAL REQUIREMENTS FOR OZONE ATTAINMENT SIP 

CHAPTER II: OZONE MONITORING INFORMATION 

CHAPTER III: BASE CASE EMISSIONS INVENTORIES 

CHAPTER IV: CONTROL MEASURES 

CHAPTER V: PHOTOCHEMICAL MODELING AND OTHER WEIGHT OF 

EVIDENCE ANALYSES FOR ATTAINMENT 

DETERMINATION 

CHAPTER VI: VOC AND NOx MOTOR VEHICLE EMISSIONS BUDGETS 

ATTACHMENT A: SIP RULE LANGUAGE 

 Revisions to Regulation No. 7 

 Revisions to Regulation No. 3 

 Revisions to Ambient Standards Regulation 

Documents referenced in Attachment A and below are contained in separate 

electronic files. 

State of Colorado Implementation Plan to Meet the 

Requirements of Clean Air Act Section 110 (a)(2)(D)(i)(I) 

Interstate Transport Regarding the 1997 8-Hour Ozone 

Standard 

CHAPTER 1: OZONE-SPECIFIC PROVISIONS TO MEET THE 

REQUIREMENTS OF CAA 110(A)(2)(D)(I)(L) - INTERSTATE 

TRANSPORT 

CHAPTER 2: REFERENCES 
 

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OVERVIEW 

2008 OZONE ACTION PLAN 

This Overview section is provided for information only and shall not be construed to be 

part of the federally-enforceable State Implementation Plan (SIP). 

On November 20, 2007, the United States Environmental Protection Agency (EPA) 

designated the Denver/North Front Range region as nonattainment for the 8-hour ozone 

standard of 0.08 parts per million (ppm) adopted in 1997. The State of Colorado must 

submit an attainment plan (referred to as a revision to the State Implementation Plan, or 

SIP) to EPA by July 1, 2009 that will bring the region back into attainment by November 

2010 (based on data from 2008-2010 ozone seasons). 

BACKGROUND

In 1997 EPA adopted a new, more stringent National Ambient Air Quality Standard 

(NAAQS) for ozone based on the latest ozone health effects information. The standard 

was set as a level of .08 ppm averaged over an 8-hour period. Attainment of the 

standard is based on the 4th maximum 8-hour ozone concentration recorded at each 

monitoring location each year, averaged over a three-year period. 

State and regional agencies in the Denver metropolitan area entered into a voluntary 

agreement with EPA in December 2002 that laid out a process for achieving attainment 

with EPA’s 1997 8-hour ozone standard in an expeditious manner, but no later than 

December 31, 2007. Called the Early Action Compact for Ozone (EAC), the agreement 

sets forth a schedule for the development of technical information and the adoption and 

implementation of the necessary control measures into the State Implementation Plan 

(SIP) to comply with the 8-hour standard by December 31, 2007 and maintain the 

standard beyond that date. The EAC Ozone Action Plan (SIP) was adopted by the 

Colorado Air Quality Control Commission (AQCC) in March 2004 and submitted to EPA 

in the summer 2004. EPA promulgated approval of the Ozone Action Plan in the 

Federal Register (Vol. 70, Number 94, May 17, 2005). A revision to the Ozone Action 

Plan to preserve the reductions estimated in the original plan was approved by the Air 

Quality Control Commission on December 17, 2006 and the Colorado State Legislature 

in spring 2007, and submitted to the EPA by the Governor in August 2007. EPA 

approved this revision in February 2008. 

In April 2004, EPA designated and classified areas of the country that violated the 8-

hour standard. Based on the 2001-2003 design values, the Denver Metro Area/North 
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Front Range (DMA/NFR) area violated the 8-hour ozone standard at three monitors and 

was included on EPA’s 2004 list of nonattainment areas. However, based on terms in 

the Early Action Compact, EPA deferred the nonattainment area designation pending 

the area continuing to meet the deadlines in the EAC and achieving the 8-hour standard 

by December 31, 2007 (based on data from the 2005-2007 ozone seasons). 

Despite efforts in the EAC Ozone Action Plan (OAP) that reduced ozone-causing 

emissions in the DMA/NFR, the area failed to achieve the standard due to high readings 

in July 2007, resulting in a three-year (2005-2007) design value of 0.085 parts per million 

(ppm) at one monitor (Rocky Flats North) which violated the 8-Hour Ozone NAAQS. 

On November 20, 2007 the EPA did not continue the deferral of the effective date for 

nonattainment in the DMA/NFR 8-hour nonattainment area and the official 

nonattainment designation became effective at that time. 

OZONE HEALTH EFFECTS

Breathing ozone can trigger a variety of health problems including chest pain, coughing, 

throat irritation, and congestion. People with chronic lung and heart diseases, children, 

older adults, and even healthy people who are active outdoors can be affected when 

ozone levels are unhealthy. Ozone can worsen symptoms for those who have preexisting conditions such as bronchitis, emphysema, asthma, chronic obstructive 

pulmonary disease, and heart disease. Ozone can also reduce lung function and 

inflame the linings of the lungs, while repeated exposure may permanently scar lung 

tissue. Ozone exposure can also increase the mortality risk for susceptible individuals, 

including the elderly and those with pre-existing conditions. 

Numerous scientific studies have linked ground-level ozone exposure to a variety of 

problems, including: 

• airway irritation, coughing, and pain when taking a deep breath; 

• wheezing and breathing difficulties during exercise or outdoor activities; 

• inflammation of respiratory tract tissues; 

• aggravation of asthma and increased susceptibility to respiratory illnesses like 

pneumonia and bronchitis; 

• permanent lung damage with repeated exposures; and 

• cardiac impacts. 

The Clean Air Act (CAA) requires EPA to set air quality standards to protect both public 

health and the public welfare (e.g. crops and vegetation) and states and local areas 

must develop plans to achieve these health-based standards as expeditiously as 

practical. 
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HOW OZONE IS FORMED

Ground-level ozone is not emitted directly into the air, but is created by complex 

chemical reactions between nitrogen oxides (NOx) and volatile organic compounds 

(VOC), and to a lesser extent carbon monoxide (CO), in the presence of sunlight. 

Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline 

vapors, and chemical solvents are some of the major sources of NOx and VOC. 

In the Denver/North Front Range area, ozone is principally a summertime problem 

associated with high temperatures, intense sunlight, little cloud cover, little moisture, light 

winds, and persistent high pressure systems. The State of Colorado monitors ambient 

ozone concentrations at 15 sites in the Denver/North Front Range. High ozone levels 

are most likely recorded at monitors along the foothills from Fort Collins south to 

Chatfield Reservoir in Douglas County. Typically, light, easterly winds pick up VOC and 

NOx pollutants throughout the metro area and intense sunlight “bakes” the pollutants, 

resulting in highest concentrations along the foothills during prime ozone meteorological 

conditions. 

AIR QUALITY AGENCIES IN COLORADO

Regional Air Quality Council 

The Regional Air Quality Council (RAQC) is designated by Governor Ritter as the lead 

air quality planning agency for the Denver metropolitan area. In this capacity, the 

mission of the RAQC is to develop effective and cost-efficient air quality initiatives with 

input from state and local government, the private sector, stakeholder groups, and 

private citizens. The RAQC’s primary task is to prepare state implementation plans 

(SIPs) for compliance with federal air quality standards. The RAQC consists of an 11-

member board appointed by the Governor. 

In July 2007, when it was clear that the region was in violation of the 8-hour ozone 

standard, Governor Bill Ritter directed the RAQC to develop an effective plan (SIP) to 

reduce ozone in the Denver/North Front Range area by September 2008. The Governor 

also urged the RAQC to propose measures that would further reduce ozone 

concentrations during the 2008 summer season and set as its immediate goal the 

reduction or elimination of ozone levels measured above 0.08 ppm. In addition, the 

Governor directed the RAQC to begin the process for considering additional measures 

that may be necessary to meet an anticipated lower federal standard for ozone. 
 iv 

North Front Range Transportation and Air Quality Planning Council 

The North Front Range Transportation and Air Quality Planning Council (NFRTAQCPC) 

is designated by the Governor as the lead air quality planning organization for the North 

Front Range region. The North Front Range Transportation and Air Quality Planning 

Council is a nonprofit public organization of 15 local and county governments in Larimer 

and Weld counties and is funded through federal and state grants, and local funds. The 

goal of the North Front Range Transportation and Air Quality Planning Council is to 

enhance air quality and mobility among northern Colorado communities and between the 

North Front Range and the Denver Metro area by developing cooperative working 

relationships and financial partnerships among its member governments, the Colorado 

Department of Transportation (CDOT), Federal Highway Administration (FHA), the 

Federal Transit Administration (FTA), and the private sector. 

The North Front Range Transportation and Air Quality Planning Council is responsible 

for proposing air quality measures affecting the North Front Range and performing 

conformity determinations to ensure its transportation plans and programs comply with 

the state implementation plan. 

Colorado Air Quality Control Commission 

The Colorado Air Quality Control Commission (AQCC) is the regulatory body with 

responsibility for adopting air quality regulations consistent with state statute including 

the responsibility and the authority to adopt state implementation plans (SIPs) and 

implementing regulations. The AQCC takes action on SIPs and regulations through a 

public rule-making process. The AQCC has nine members who are appointed by the 

Governor and confirmed by the State Senate. 

DENVER METRO AREA/NORTH FRONT RANGE NONATTAINMENT AREA

The boundary of the DMA/NFR 8-hour ozone nonattainment area was established in 

EPA’s April 2004 designation of nonattainment areas, as follows: 

All of Adams, Arapahoe, Boulder, Broomfield, Denver, Douglas, Jefferson 

Counties and portions of Larimer and Weld Counties. 

A map describing the current nonattainment area boundaries is included in the Figure at 

the end of this section. 
 v 

NEW 8-HOUR OZONE STANDARD

In March 2008 EPA established a new, more stringent standard for ozone based on a 

review of the most recent health effects information. The new 8-hour standard is set at a 

level of 0.075 ppm (or 75 parts per billion (ppb)) averaged over an eight-hour period. As 

with the 1997 standard, a violation of the standard occurs when the three-year average 

of the fourth maximum values at a monitor exceeds the federal standard. Due to 

rounding of monitoring values, a violation occurs when the three-year average is equal 

to or greater than 0.076 ppm (or 76 ppb). 

Under EPA’s rule establishing the new standard, the Governor is required to make 

recommendations for areas of nonattainment by March 2009. EPA will review the 

Governor’s recommendations and make final nonattainment determinations in March 

2010. States will have to submit revised state implementation plans for the new ozone 

standard by March 2013. EPA will later establish attainment dates for areas, which will 

range between 2013 and 2030 depending on the severity and classification of the area. 

In the meantime, the 1997 8-hour ozone standard and all the associated regulatory 

requirements remain in place. States and nonattainment areas are expected to continue 

their plans for implementing the 1997 standard. EPA will address transition issues from 

the 1997 standards to the 2008 standard in a separate future rulemaking. 

Currently, through summer 2008, eight monitors along the DMA/NFR violate the new 

0.075 ppm 8-hour ozone standard. This proposed Attainment SIP is not intended to 

address attainment of the 0.075 ppm 8-hour ozone standard. However, the Regional Air 

Quality Council and the Colorado Department of Public Health and Environment 

(CDPHE) will continue to consider measures that move the region toward attainment of 

the new 8-hour ozone standard as expeditiously as practical. Provisions in the 2008 

Ozone Action Plan are intended to begin moving the region to compliance with the new 

standard. 

2008 OZONE ACTION PLAN

After several months of analysis and evaluation and after more than 40 stakeholder and 

public meetings, the Regional Air Quality Council proposed an Ozone Action Plan to 

reduce ozone levels in the Denver/North Front Range area by 2010. In addition, the Air 

Pollution Control Division proposed a revised Interstate Transport SIP, in response to 

comments from the Environmental Protection Agency, demonstrating that the State does 

not contribute significantly or interfere with maintenance by any other state of the 0.08 

ppm ozone NAAQS, The overall action plan includes elements that will be included in 

the federally-enforceable State Implementation Plan (SIP), elements that are included as 
 vi 

state-only enforceable measures in state regulation, and elements that need further 

evaluation for a possible SIP amendment in the near future. These elements are 

discussed briefly below and the emissions control strategies for ozone are summarized 

in the Table at the end of this section. 

Measures adopted for the federally-enforceable 8-Hour Ozone Attainment 

Plan – A Revision to the State Implementation Plan (SIP) 

 (See attached SIP document for more details) 

The following measures were adopted for inclusion in the Ozone State Implementation 

Plan. In addition to being adopted and enforced by the State of Colorado, these 

measures will also be federally-enforceable upon approval of the State Implementation 

Plan revisions by EPA. 

1. Increase the system-wide control requirements for all condensate tanks to 

85% by May 1, 2010 and 90% by May 1, 2011

 

 This SIP measure will increase the condensate tank emission control 

requirement to 81% system-wide by May 1, 2009, 85% system-wide by May 1, 

2010 and 90% by May 1, 2011 from the current 75% system-wide control 

requirement in Regulation No. 7. The current 30 tpy system-wide exemption will 

remain in place. Auto-igniters will be required for all condensate tank control 

devices with uncontrolled VOC emissions greater than 50 tpy by May 1, 2009 

and all tanks greater than or equal to 2 tpy by May 1, 2010. These controls are 

expected to reduce VOC emissions in 2009 by 24 tpd, in 2010 by 34 tpd and 

2011 by a total of 49. By February 2009, condensate tanks serving newly drilled, 

re-completed or stimulated wells must control emissions during the first 90 days 

of production. After that, the control may be removed if emissions do not 

exceed/will not exceed 2 tons per year. 

2. Remove current exemptions contained in Regulation No. 3 for selected 

small sources required to file air pollution emission notices and obtain 

permits 

Regulation No. 3 currently contains exemptions for many small source 

categories. Many of these exemptions pertaining to VOC sources will be 

removed by the AQCC in revisions to Regulation No. 3 in December 2008 and 

become effective in February 2009. This change will result in the identification of 

more sources of VOCs and potentially additional control requirements. The 

impact of these revisions is difficult to quantify since it is unknown how many 

sources will be affected and the control levels that will be required. 
 vii 

3. Require general application of permit requirements in Regulation No. 3 and 

reasonably available control technology (RACT) for all VOC stationary 

sources greater than two tons per year and NOx stationary sources greater 

than five tons per year in the entire nonattainment area 

Revisions to Regulation No. 3 implementing these changes were adopted by the 

AQCC in February 2008. The impact of these revisions is difficult to quantify 

since it is unknown how many sources will be affected and the control levels that 

will be required. 

Measures adopted as state-only measures in state regulation

 

The following measures will not be included in the federally-enforceable State 

Implementation Plan at this time, but will be adopted and enforced exclusively under 

state authority. These measures will provide additional reductions of ozone-causing 

emissions, which will give the region an additional margin of safety to maintain 

compliance with the 1997 8-hour ozone standard and will help the region make further 

progress towards meeting the new EPA standard. 

1. Implement a motor vehicle inspection/maintenance program in the North 

Front Range (Larimer and Weld counties)

 The North Front Range Transportation and Air Quality Planning Council has 

endorsed a proposal to extend the inspection/maintenance (I/M) program 

structure that currently exists in the Denver metro area to portions of Larimer and 

Weld counties. The program includes IM240 testing, remote-sensing clean 

screen, gas cap checks, and advisory On-Board Diagnostics (OBDII) checks. 

Revisions to Regulation No. 11 implementing this change in the former basic I/M 

program area in Larimer and Weld counties will be proposed to the AQCC in 

September 2008 for adoption in December 2008. The program will become 

effective on July 1, 2010. Changes to the boundary of the North Front Range 

program area to include the entire urbanized portion of Larimer and Weld 

counties will likely be considered by the General Assembly during the 2009 

session. Conservatively, this program is expected to reduce mobile source VOC 

emissions by at least one tpd, NOx emissions by at least one tpd, and CO 

emissions by at least 17 tpd. 

The North Front Range Transportation and Air Quality Planning Council also 

endorses an evaluation of the I/M program structure by 2013 that includes 

consideration of expanded OBDII testing and high-emitter identification. 
 viii 

2. Implement more stringent cut-points for inspection/maintenance program 

in 7-county Denver metro area

Lower cut-points will identify more high-emitting vehicles that will result in repairs 

to reduce emissions. The Air Quality Control Commission approved revisions to 

Regulation No. 11 implementing these cut-points in March 2008 and the changes 

took effect in May 2008. These revisions are expected to reduce mobile source 

VOC emissions by one ton per day (tpd), NOx emissions by three tpd, and 

carbon monoxide (CO) emissions by 13 tpd. 

3. Continue implementing the high-emitter pilot program in the Denver metro 

area 

A mandatory pilot program using remote sensing technology began January 1, 

2008. The pilot program will continue through July 2009 after which the results 

from the program will be analyzed. This analysis may lead to implementation of 

a full-scale high-emitter program in the future. Since this pilot program is still 

underway, the emission reduction potential of this program has not yet been 

identified. However, it is a well-established fact that high-emitting vehicles 

contribute a disproportionate amount of pollution to our air. 

 

4. Tighten up collector plate requirements in state law 

Collector plate requirements in current state statute limit emission tests on 

vehicles more than 25 years old. The RAQC and CDPHE are working with 

stakeholders to develop legislation that will limit collector plates to true collector 

vehicles and close the emissions testing loophole for old, non-collector vehicles. 

The impact from these old, non-collector vehicles is difficult to quantify, but it is 

expected the VOC reduction could be around one tpd and the CO reduction 

could be around seven tpd. 

5. Implement control requirements for reciprocating internal combustion 

engines (RICE) statewide 

The control requirements will mirror requirements currently in place in the 

Denver/North Front Range nonattainment area. Revisions to Regulation No. 7 

making these requirements apply statewide will be adopted by the AQCC in 

December 2008 and will become effective by May 1, 2010. The emission 

reduction impact from these statewide controls has not yet been quantified. 
 ix 

6. Require low-bleed control devices on all new and existing pneumatic 

valves in oil and gas operations by 2009 

The AQCC will adopt revisions to Regulation No. 7 in December 2008 effective in 

May 2009 that require low-bleed controllers on valves. Exemptions will be 

granted for operations that require high-bleed controllers on valves for safety 

reasons. These controls are expected to reduce VOC emissions between 19 

and 23 tpd. 

7. Expand current requirements in Regulation No. 7 for Volatile Organic 

Compound (VOC) controls to the entire nonattainment area 

 Control requirements for VOC stationary sources currently pertain only to the 

former one-hour ozone attainment/maintenance area (most of the 7-county 

Denver area). These reasonably available control technology (RACT) 

requirements in Regulation No. 7 will now apply to specific new and existing 

listed source categories and all new and existing major (greater than 100 tons 

per year (tpy)) stationary sources of VOCs in portions of Larimer and Weld 

counties and eastern portions of Adams and Arapahoe counties. These 

revisions to Regulation No. 7 will become effective in February 2009. The impact 

of these revisions is difficult to quantify since it is unknown how many sources 

will be affected and the control levels that will be required. 

Potential strategies requiring additional evaluation for a near-term ozone 

plan/SIP amendment 

During the development of the Ozone Action Plan and SIP revision, the RAQC, CDPHE, 

and interested stakeholders considered several measures that hold considerable 

promise for further reducing ozone levels in the future. However, the RAQC and 

CDPHE concluded these measures need additional evaluation and analysis because of 

the potential impacts and complexities of the strategies. 

The RAQC and CDPHE will initiate additional technical and modeling analysis of these 

strategies during the fall of 2008 and will conduct a stakeholder involvement process to 

consider these strategies through the first part of 2009. The RAQC and AQCC will 

consider these strategies for a possible state-only plan and/or SIP amendment in the 

latter half of 2009. The timeline for the implementation of these potential strategies will 

be considered during the stakeholder and regulatory processes. 

The additional strategies that will undergo further analysis and evaluation by the RAQC 

and CDPHE include, but are not limited to, the five below. These strategies are included 
 x 

in the Ozone Action Plan for information purposes only. The RAQC and CDPHE did not 

request the AQCC to act on or approve these strategies. 

1. Evaluate potential ozone fuels strategies 

Fuels strategies include 7.0 RVP base gasoline, federal reformulated gasoline, 

and eliminating the one-pound psi RVP waiver for ethanol blended gasoline. The 

evaluation will also address any national fuels strategies that may be coming 

from EPA in the future. 

2. Evaluate emission controls for large industrial sources of NOx 

Recent modeling in the Denver/North Front Range area indicates additional 

reductions of NOx emissions in the area may be beneficial for ozone reductions. 

This evaluation will include analysis of control options for power plants, large 

industrial boilers, cement kilns, and other potential sources. Future modeling 

analysis will also evaluate the impact of further NOx reductions from motor 

vehicles and non-road engines that will result from federal standards already in 

place. 

3. Evaluate statewide control requirements for new oil and gas condensate 

tanks and pneumatic valves 

Other areas in Colorado also have concerns about the impacts of oil and gas 

development on air quality in these regions. CDPHE and AQCC will consider 

statewide control requirements for condensate tanks, pneumatic valves, and 

other potential oil and gas sources patterned, in part, after requirements in effect 

in the Denver/North Front Range nonattainment area. 

4. Evaluate the feasibility of adopting California requirements for paints, 

solvents and consumer products 

EPA is considering adopting more stringent formulation requirements for a range 

of paints, solvents, and other household consumer products. Other states and 

regions have also adopted more stringent regulations for these products than the 

pending federal rule. RAQC and CDPHE will evaluate the benefits, impacts, and 

technical feasibility of adopting more stringent regulations for these products in 

Colorado. 
 xi 

5. Evaluate 95% system-wide controls and surveillance systems for 

condensate tanks 

Industry representatives have raised questions concerning the viability of 

increasing the required system-wide control to 95% from the 90% system-wide 

required in 2011. The industry states a 95% system-wide control will nullify any 

flexibility currently available to meet the requirement, eliminate potential for overcontrol currently provided, and create an unnecessary burden for the Division 

and the industry in terms of enforcement actions. The RAQC and the CDPHE 

will evaluate impacts of 95% system-wide controls for condensate tanks in terms 

of the added benefit to ozone reduction and the industry’s concerns. The 

anticipated additional VOC reductions are approximately 12 tpd. 

The industry representatives have raised concerns about requirements for 

electronic surveillance systems in terms of costs and benefits. The Division will 

participate with the industry in a pilot program regarding implementation of 

electronic surveillance systems. 
 xii 

Denver Metro Area/North Front Range 

Nonattainment Area 
xiii

Emission Control Strategies for Ozone

2008 Ozone Action Plan 

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

Potenti

Additional 

Pla

Measures Adopted 

for Federally-Enforceable 

State Implementation Plan (SIP)

Potential 

Emission 

Reduction

Measures 

Adopted and Enforced as Stateonly Measures

Potential 

Emission 

Reduction

3⁄4 Inspection/maintenance program in 

 North Front Range – eff. July 1, 2010

~ 1 tpd VOC, ~1 tpd NOx, ~17 tpd CO 

Ozone Fuels St

3⁄4 7.0 RVP gaso

3⁄4 More stringent Reg. 11 I/M cut-points 

 (Denver area) – adopted, effective  May 1, 2008 

~ 1 tpd VOC, ~3 tpd NOx, ~13 tpd CO 

3⁄4 Mandatory high-emitter pilot program 

(Denver area) – began January 1, 2008 

Pilot program results are not available 

3⁄4 Federal Refo

Gasoline 

3⁄4 Tighten up collector plate 

requirements for older vehicles (statewide)-legislation being pursued 

~ 1 tpd VOC ~ 7 tpd CO 3⁄4 Eliminate eth

3⁄4 Increase system-wide condensate tank 

control requirements to 85% in 2010 and 90% in 2011 for all tanks greater than or equal to 2 tpy

Total from 2010 base ~ 34 tpd (2010)/ 49 tpd (2011) VOC

3⁄4 Statewide Oil & Gas regulations -- 

Controls on existing reciprocating internal combustion engines

~4 tpd VOC ~16 tpd NOx 

3⁄4 Statewide O

regulations – requirements condensate tpneumatic va

3⁄4 Pneumatic valves controls - require 

 low/no bleed valves on all new and  existing valves by 2009 

~ 23 tpd VOC 

3⁄4 Increase syste

condensate tarequirements tanks 

3⁄4Remove current exemptions in Reg. 3 

for selected small sources required to file air pollution emission notices and obtain permits

3⁄4Require Reasonably Available 

 Control Technology (RACT) for minor  sources in NAA (Reg. 3)

Emission reductions are difficult to quantify at this time, but are expected to be small in the short-term

3⁄4 Expand Reg. 7 (VOC control 

requirements) to entire NAA

Emission reductions are difficult to quantify at this time, but are expected to be small in the short-term

3⁄4 Emission con

 NOx sources 

 power plan boilers  cement kiln

3⁄4 California Pa

Consumer Pr

TOTAL 

EMISSION 

REDUCTIONS 

VOC 

NOx 

CO

~34 tpd(2010) 

49 tpd (2011) 

VOC 

NOx 

CO

~30 tpd ~20 tpd >37 tpd
 

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Denver Metro Area & North Front Range 

8-Hour Ozone 

Attainment Plan 

A Revision to the State Implementation Plan 

Approved by: 

Colorado Air Quality Control Commission 

December 12, 2008 

 
 

This page left blank intentionally.
Table of Contents 

8-Hour Ozone Attainment Plan 

CHAPTER I: FEDERAL REQUIREMENTS FOR OZONE ATTAINMENT SIP 

CHAPTER II: OZONE MONITORING INFORMATION 

CHAPTER III: BASE CASE EMISSIONS INVENTORIES 

CHAPTER IV: CONTROL MEASURES 

CHAPTER V: PHOTOCHEMICAL MODELING AND OTHER WEIGHT OF 

EVIDENCE ANALYSES FOR ATTAINMENT 

DETERMINATION 

CHAPTER VI: VOC AND NOx MOTOR VEHICLE EMISSIONS BUDGETS 

ATTACHMENT A: SIP RULE LANGUAGE 

 Revisions to Regulation No. 7 

 Revisions to Regulation No. 3 

 Revisions to Ambient Standards Regulation 

Documents referenced in Attachment A are contained in separate electronic files. 
This page left blank intentionally.
 I -1

CHAPTER I 

FEDERAL REQUIREMENTS FOR PREPARING 8-HOUR 

OZONE ATTAINMENT SIP REVISION 

Clean Air Act Requirements

The Clean Air Act Amendments (CAA) of 1990 established a classification system for 

ozone nonattainment areas based on the severity of the area’s ozone problem as 

measured by the area’s ozone design value. In April 2004 the United States 

Environmental Protection Agency (EPA) issued a rule classifying all the areas 

designated nonattainment for the 8-hour standard. However, this rule was vacated by 

the U.S. Court of Appeals in December 2007 and EPA has not yet issued a new rule 

consistent with the Court’s decision. 

EPA has indicated the Denver/North Front Range nonattainment area, based on its 

2005-2007 design value, will likely be classified as a Marginal area and subject to the 

provisions of Section 181 and 182(a) of the Clean Air Act Amendments of 1990. In 

addition, as a former Early Action Compact (EAC) area, the Denver/North Front Range 

nonattainment area is subject to 40 CFR 81.300(e)(3)(ii)(D) that requires a new 

attainment demonstration with photochemical air quality modeling. 

The core elements that EPA has indicated necessary for an approvable revised 

attainment plan for the Denver/North Front Range nonattainment area under the 

Marginal classification are as follows: 

• Photochemical grid modeling based on the latest EPA modeling guidance 

• Emissions inventories for the base and future modeling years 

• Modeled attainment demonstration for summer 2010 (3 years after designation) 

• Required controls must be effective no later than prior to the beginning of the 

2010 summer ozone season (May 1, 2010) 

• Mobile vehicle emissions budgets for the attainment year (2010) 

• Reasonably Available Control Measures - demonstration that controls needed for 

attainment have been achieved as expeditiously as possible 

• New Source Review applicable to volatile organic compounds (VOC) and 

nitrogen oxide (NOx) major sources of 100 tons per year (tpy) with offsets of 1.1 

to 1 

• Construction permits required for new and modified major stationary sources 

• Requirements of section 182(a)(3) including: 

o Submission of periodic inventories every three years until the area is 

redesignated to attainment; 
 I -2

o Annual submission of a statement of actual VOC and NOx emissions 

from stationary sources; and, 

o Offset requirements of 1.1 to 1 as noted above. 

Additional elements that EPA has indicated are not necessary for an approvable revised 

state implementation plan for the Denver/North Front Range nonattainment area under 

the Marginal classification are as follows: 

• Contingency measures are not required; however, upon failure to attain the area 

would be reclassified to a higher classification and additional control 

requirements may be required (Section 182(a)); 

• A Reasonably Available Control Technology pre-1990 fix-up is not required 

because it was achieved with redesignation of the Denver metro 1-hour ozone 

area to attainment-maintenance (Section 182(a)(2)(A)); and 

• Corrections to the pre-1990 Inspection/Maintenance program are not required 

because it was achieved with redesignation of the Denver metro 1-hour ozone 

area to attainment-maintenance (Section 182(a)(2)(B)). 

Photochemical Grid Modeling

As a former EAC area, an attainment demonstration using photochemical grid dispersion 

modeling is required and was performed for the revised 8-hour Ozone Attainment State 

Implementation Plan (SIP). All modeling is based on “Guidance on the Use of Models 

and Other Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, 

particulate matter (PM2.5) and Regional Haze" (EPA-454/B-07-002, April 15, 2007). The 

modeling follows the guidance as facilitated by EPA Region 8 technical staff. The 

modeling is within EPA's accepted margin of accuracy; is fully documented; sufficiently 

accounts for projected future growth in ozone precursor emissions; and was used to 

determine the effectiveness of NOx and/or VOC reductions. The 2010 base case was 

tested with 16 sensitivity tests to determine the relative effectiveness of different 

emission reduction controls and to aid in the selection of appropriate emission reduction 

strategies. 

Emissions Inventories

Emissions inventories used in this revised 8-hour ozone attainment SIP were developed 

for a typical summer episode day for the years 2006 and 2010 using EPA’s MOBILE6 

emissions model and the latest transportation information, area sources using a 

combination of EPA’s non-road model data, and latest demographics information, area 

source data, and local survey and information data, as well as the latest stationary 

sources emissions information, as required. Future year inventories will sufficiently 
 I -3

account for projected future growth in ozone precursor emissions through 2010 

particularly from stationary, area, and mobile sources. Emissions inventories were 

compared and analyzed for trends in emission sources over time. Inventories included 

in the photochemical modeling were also characterized by time of day, day of week, 

speciation, location, temperature, and other factors. 

Modeled Attainment Demonstration

The EPA Model Attainment Test Software (MATS) was used with the 2006 and 2010 

base case photochemical grid modeling results to project 2010 8-hour ozone attainment. 

The 2010 base case modeling provides the basis for this SIP’s demonstration of 

attainment. Note that the 2010 base case modeling only takes credit for measures that 

are “on-the-books or on-the-way”; additional control measures described in this 

document (see Chapter IV) are not included in the 2010 base case modeling but their 

emission reduction benefits are instead included as part of a weight of evidence (WOE) 

analysis (a WOE is required to support the modeled attainment demonstration). 

Emission Reduction Strategies

All adopted Federal and SIP emission reduction strategies that have been or will be 

implemented by the November 20, 2010 attainment date are included in all emissions 

inventories. The strategies included in the federally-enforceable SIP will be 

implemented as soon as practical, but no later than May, 2010. The emission reduction 

strategies will be specific, quantified, permanent and enforceable. The strategies will 

also include specific implementation dates and detailed documentation and reporting 

processes. 

Conformity and Motor Vehicle Emissions Budgets

Transportation conformity provisions of section 176 (c)(2)(A) of the CAA require regional 

transportation plans and transportation improvement programs to demonstrate that 

“...emissions expected from implementation of plans and programs are consistent with 

estimates of emissions from motor vehicles and necessary emissions reductions 

contained in the applicable implementation plan...” 

Mobile Source Vehicle Emissions Budgets for VOC and NOx in the 2010 attainment year 

are established as both subarea and regional budgets for future conformity for the two 

metropolitan planning organizations (Denver Regional Council of Governments and 
 I -4

North Front Range Transportation and Air Quality Planning Council) serving the 

Denver/North Front Range nonattainment area. 

New Source Review and Construction Permits

The State of Colorado currently performs New Source Review in nonattainment areas 

applicable to VOC and NOx major sources of 100 tpy with offsets of 1.1 to 1. The State 

also maintains a Construction Permits program for new and modified major stationary 

sources.
II-1 

CHAPTER II 

OZONE MONITORING INFORMATION 

A. Ozone Monitoring Network 

 

The 2007 ozone ambient air monitoring network in the Denver area and along the 

northern Front Range consists of 13 stations operated by the Colorado Air Pollution 

Control Division (APCD) and one station operated by the National Park Service (NPS) in 

Rocky Mountain National Park. There have been other stations that have operated in 

the past. The geographical distribution of the Front Range monitors is presented in 

Figure 1. 

Figure 1: 

.
II-2 

This section shall not be construed to establish a monitoring network in the federallyenforceable State Implementation Plan (SIP). The United States Environmental 

Protection Agency (EPA) has already approved a monitoring SIP for the State of 

Colorado and this description of the ozone monitoring network shall not be construed to 

amend such monitoring SIP. 

B. Quality Assurance Program 

Ozone monitoring data for the Denver area have been collected and tested for quality 

assurance in accordance with 40 CFR Part 58 Appendix A, EPA’s “Quality Assurance 

Handbook for Air Pollution Measurement Systems, Vol. II - Ambient Air Quality 

Monitoring Program”, the APCD’s Quality Management Plan and Quality Assurance 

Project Plan documents, and Colorado’s Monitoring SIP which EPA approved in 1993. 

The data are recorded in EPA’s Air Quality System (AQS) and are available for public 

review at the APCD and through EPA’s AQS database. Table 1 presents the data 

recovery rates for each monitoring site in the Denver and North Front Range area. 

Percent data recovery is the number of valid sampling days occurring within the "ozone 

season" divided by the total number of days encompassing the "ozone season". For 

Colorado, the “ozone monitoring season” has been designated by EPA to be March 01 

through September 30. A valid sampling day is one in which at least 75% of the hourly 

averages are recorded. 

Table 1: Ozone Data Recovery Rates for Each Monitoring Site 

(Based on EPA designated ozone season of 3/1 – 9/30) 

 

Welby 

08-001-3001 

Highland 

08-005-0002

S. Boulder 

Creek 

08-013-0011

CAMP 

08-031-0002

Carriage 

08-031-0014 

Chatfield 

Reservoir *

08-035-0002

Year Data Data Data Data Data Data 

 Recovery Recovery Recovery Recovery Recovery Recovery 

2000 99% 99% 99% --- 90% 95% 

2001 96% 91% 98% --- 95% 91% 

2002 95% 96% 97% --- 97% 93% 

2003 95% 96% 99% --- 99% 87% 

2004 94% 99% 96% --- 99% --- 

2005 99% 97% 97% 98% 96% --- 

2006 97% 98% 98% 99% 97% --- 

2007 99% 99% 98% 98% 97% --- 
II-3 

Table 1 (continued): 

Ozone Data Recovery Rates for Each Monitoring Site 

(Based on EPA designated ozone season of 3/1 – 9/30) 

 

Chatfield 

Park * 

08-035-0004 

Arvada 

08-059-0002

Welch 

08-059-0005

Rocky Flats 

North 

08-059-0006

NREL 

08-059-0011 

Rock Mtn. 

NP 

08-069-0007

Year Data Data Data Data Data Data 

 Recovery Recovery Recovery Recovery Recovery Recovery 

2000 --- 98% 94% 98% 99% 89% 

2001 --- 99% 97% 98% 98% 95% 

2002 --- 99% 98% 96% 99% 95% 

2003 --- 98% 98% 99% 99% 94% 

2004 92% 99% 99% 99% 98% 95% 

2005 99% 95% 98% 94% 95% 90% 

2006 97% 97% 99% 99% 99% 96% 

2007 97% 99% 99% 96% 99% 97% 

 

Fort Collins 

West 

08-069-0011

Fort Collins 

CSU 

08-069-1004

Greeley ** 

08-123-0007

Weld 

County 

Tower ** 

08-123-0009 

Year Data Data Data Data 

 Recovery Recovery Recovery Recovery 

2000 --- 99% 98% --- 

2001 --- 92% 99% --- 

2002 --- 87% 96% 99% 

2003 --- 97% --- 97% 

2004 --- 98% --- 96% 

2005 --- 91% --- 97% 

2006 99% 98% --- 99% 

2007 99% 97% --- 99% 

*The Chatfield Reservoir seasonal monitor was moved from the campground registration building to the 

Chatfield Park office yard as a year-round monitor in 2004. 

**The Greeley monitor was moved from 811 15th Street to the Weld County Tower site at 3101 35th Avenue 

in 2002. 

C. Monitoring Network/Verification of Continued Attainment 

The APCD has and will continue to operate an appropriate air quality monitoring network 

of State/Local Air Monitoring System monitors (SLAMS) in accordance with 40 CFR Part 

58 to verify the attainment of the 8-hour-hour ozone National Ambient Air Quality 

Standard (NAAQS). If measured mobile source parameters (e.g., vehicle miles traveled, 
II-4 

congestion, fleet mix, etc.) change significantly over time, the APCD will perform the 

appropriate studies to determine whether additional and/or re-sited monitors are 

necessary. Annual review of the SLAMS air quality surveillance system will be 

conducted in accordance with 40 CFR Part 58.10 to determine whether the system 

continues to meet the monitoring objectives presented in Appendix D of 40 CFR Part 58. 

D. Monitoring Data

Tables 2 and 3 below present the monitoring data for the APCD’s Denver and North 

Front Range monitoring sites and the NPS Rocky Mountain National Park monitoring 

site. For each site, the fourth maximum 8-hour ozone concentrations along with the 3-

year averages of the 4th maximum concentrations at each site are presented. 

Table 2: 4th Maximum 8-Hour Ozone Values 

Site Name AQS #

2000

8-hr. 

4th 

Max. 

(ppm) 

2001

8-hr. 

4th 

Max. 

(ppm) 

2002

8-hr. 

4th 

Max. 

(ppm) 

2003

8-hr. 

4th 

Max. 

(ppm) 

2004

8-hr. 

4th 

Max. 

(ppm) 

2005

8-hr. 

4th 

Max. 

(ppm) 

2006

8-hr. 

4th 

Max. 

(ppm) 

2007

8-hr. 

4th 

Max. 

(ppm) 

2008*

8-hr. 

4th 

Max. 

(ppm) 

Welby 08-001-3001 0.062 0.064 0.068 0.066 0.066 0.073 0.069 0.070 0.076 

Highland 08-005-0002 0.076 0.077 0.076 0.091 0.072 0.080 0.081 0.075 ** 

S. Boulder 

Creek 08-013-0011 0.072 0.071 0.078 0.082 0.068 0.076 0.082 0.085 0.076 

CAMP 08-031-0002 --- --- --- --- --- 0.051 0.062 0.057 --- 

Carriage 08-031-0014 0.071 0.072 0.073 0.085 0.066 0.074 0.072 0.076 0.072 

Chatfield 

Reservoir 08-035-0002 0.080 0.077 0.083 0.095 --- --- --- --- --- 

Chatfield Park 08-035-0004 --- --- --- --- 0.075 0.084 0.086 0.082 0.080 

Arvada 08-059-0002 0.076 0.074 0.073 0.083 0.065 0.078 0.082 0.079 0.074 

Welch 08-059-0005 0.068 0.064 0.069 0.077 0.062 0.064 0.081 0.080 0.073 

Rocky Flats 

North 08-059-0006 0.081 0.082 0.088 0.091 0.073 0.077 0.090 0.090 0.079 

NREL 08-059-0011 0.083 0.081 0.081 0.095 0.074 0.079 0.083 0.085 0.076 

Rocky 

Mountain NP 08-069-0007 0.078 0.070 0.087 0.086 0.073 0.075 0.076 0.078 0.076 

Fort Collins 

West 08-069-0011 --- --- --- --- --- --- 0.087 0.085 0.076 

Fort Collins 

CSU 08-069-1004 0.069 0.067 0.072 0.075 0.064 0.076 0.078 0.069 0.067 

Greeley 08-123-0007 0.069 0.074 --- --- --- --- --- --- --- 

Weld Co. 

Tower 08-123-0009 --- --- 0.080 0.083 0.069 0.078 0.082 0.074 0.073 

* 2008 data is current through September 30, 2008. The post season quality assurance review has taken 

place for all monitors except Rocky Mountain National Park. 

** The Highland monitor was out of service much of the season due to nearby construction. 
II-5 

Table 3: 8-Hour Ozone 

Three-Year Average 4th Maximum Ozone Values 

Site Name

2000-02

3-yr. Avg. 

4th Max. 

Value 

(ppm) 

2001-03

3-yr. Avg. 

4th Max. 

Value 

(ppm) 

2002-04

3-yr. Avg. 

4th Max. 

Value 

(ppm) 

2003-05

3-yr. Avg. 

4th Max. 

Value 

(ppm) 

2004-06

3-yr. Avg. 

4th Max. 

Value 

(ppm)

2005-07

3-yr. Avg. 

4th Max. 

Value 

(ppm)

2006-08*

3-yr. Avg. 

4th Max. 

Value 

(ppm) 

Welby 0.064 0.066 0.066 0.068 0.069 0.070 0.071 

Highland 0.076 0.081 0.079 0.081 0.077 0.078 0.071 

S. Boulder 

Creek 0.073 0.077 0.076 0.075 0.075 0.081 0.081 

CAMP --- --- --- --- --- 0.056 --- 

Carriage 0.072 0.076 0.074 0.075 0.070 0.074 0.073 

Chatfield 

Reservoir 0.080 0.085 --- --- --- --- --- 

Chatfield Park --- --- --- --- 0.081 0.084 0.082 

Arvada 0.074 0.076 0.073 0.075 0.075 0.079 0.078 

Welch 0.067 0.070 0.069 0.067 0.069 0.075 0.078 

Rocky Flats 

North 0.083 0.087 0.084 0.080 0.080 0.085 0.086 

NREL 0.081 0.085 0.083 0.082 0.078 0.082 0.081 

Rocky Mountain 

NP 0.078 0.081 0.082 0.078 0.074 0.076 0.076 

Fort Collins 

West --- --- --- --- --- --- 0.082 

Fort Collins CSU 0.069 0.071 0.070 0.071 0.072 0.074 0.071 

Greeley --- --- --- --- --- --- --- 

Weld Co. Tower --- --- 0.077 0.076 0.076 0.078 0.076 

* 2008 data is current through September 30, 2008. The post season quality assurance review has taken 

place for all monitors except Rocky Mountain National Park. 
This page left blank intentionally.
 III-1 

CHAPTER III 

BASE CASE EMISSIONS INVENTORIES 

This section presents emissions inventories for this Ozone State Implementation Plan 

(SIP) for the 8-hour ozone Denver Metro Area/North Front Range (DMA/NFR) 

attainment area, 2006 base case, and the 2010 base case used in the modeling 

scenarios. Inventories for the 8-hour ozone control area 2010 control case modeling are 

presented later in this document and include the additional control measures that are 

included in the attainment demonstration for the 8-hour ozone National Ambient Air 

Quality Standard (NAAQS). All of the base and control case inventories are for the 8-

hour ozone nonattainment area (NAA), which includes the counties of Denver, Jefferson, 

Douglas, Broomfield, Boulder, Adams, Arapahoe, and portions of Weld and Larimer 

counties. These inventories in tons per summer day (tpsd) represent emissions 

estimates for an average episode day during the peak summer ozone season (May 

through September). Details of the inventories can be found in the Technical Support 

Documents (TSD) Appendix C at www.colorado.gov/airquality/documents/deno308/. 

 

The emission estimates were developed based on the most recent vehicle miles traveled 

(VMT) estimates contained in: 1) Denver Regional Council of Government’s (DRCOG) 

conformity analysis for the updated fiscally constrained element of the 2035 Regional 

Transportation Plan; 2) North Front Range Transportation and Air Quality Planning 

Council’s (NFRTAQPC) 2035 Regional Transportation Plan; 3) the Air Pollution Control 

Division (APCD) estimates of VMT derived from data provided by the Colorado 

Department of Transportation (CDOT); and 4) Population estimates from the State 

Demographer. Table 4 presents this information. 

Table 4: Demographic Data

 2006 2010 

DRCOG VMT 69,548,803 76,551,505 

NFRTAQPC VMT 10,537,341 11,753,832 

NON-DRCOG/NFR VMT 1,715,579 1,835,149 

TOTAL NAA VMT 81,801,723 90,140,486 

NAA Population 3,118,439 3,357,009 

The 2006 and 2010 base case inventories incorporate the control measures in place at 

that time. Control measures in place in 2006 and assumed for 2010 include: 

1. Federal tailpipe standards and regulations, including those for small engines and 

non-road mobile sources. Credit is taken for these federal requirements but they 
 III-2 

are not part of the Colorado SIP. The credits change from 2006 to 2010 as the 

United States Environmental Protection Agency (EPA) Tier II and low sulfur 

gasoline standards become effective. 

2. Air Quality Control Commission Regulation No. 11--covering the Automobile 

Inspection and Readjustment (A.I.R.) program in place during the 2006 ozone 

season, which includes an enhanced Inspection/Maintenance (I/M). For 2006, a 

maximum of 10% fleet coverage is assumed, and for 2010, a maximum of 50% 

fleet coverage is assumed for the remote sensing clean screen program in the 

Denver metro area (DMA) based on Regulation No. 11. 

3. Air Quality Control Commission Regulations No. 3, No. 6, No. 7, and Common 

Provisions--covering gasoline station and industrial source control programs. 

The Common Provisions, Parts A and B of Regulation No. 3, and the volatile 

organic compounds (VOC) control requirements of Regulation No. 7 are already 

included in the approved SIP. Regulation No. 6 and Part C of Regulation No. 3 

implement the federal standards of performance for new stationary sources and 

the federal operating permit program. This reference to Regulation No. 6 and 

Part C of Regulation No. 3 shall not be construed to mean that these regulations 

are included in the SIP. 

4. Since 2004, gasoline sold in the Denver metro area during the summer Reid 

Vapor Pressure (RVP) ozone season (June 1 to September 15) has been subject 

to a national RVP limit of 7.8 pounds per square inch (psi) to reduce fuel 

volatility. For ethanol-blended fuels, the RVP limit is 8.8 psi due to the federal 

1.0 psi RVP waiver for ethanol. 

Since 1991, gasoline sold in the Larimer and Weld area during the summer 

ozone season (June 1 to September 15) has been subject to a national RVP limit 

of 9.0 psi to reduce fuel volatility. For ethanol-blended fuels, the RVP limit is 10.0 

psi due to the federal 1.0 psi RVP waiver for ethanol. 

For 2006, the RVP of gasoline for the Denver metropolitan portion of 

nonattainment area was determined by survey to be at 8.2 psi, with an ethanol 

market share of 60%, and for the Larimer and Weld portion of the nonattainment 

area the RVP was determined to be 8.4 psi with the same ethanol share of 60%. 

 For purposes of the base case 2010 mobile source inventory, the RVP of the 

base gasoline is assumed to be 7.8 psi for the Denver metropolitan portion of 

nonattainment area, with an ethanol market share of 85%, and for the Larimer 

and Weld portion of the nonattainment area the RVP was assumed to be 9.0 psi 

with an ethanol share of 25%. Explanation of assumptions found in the 

Emissions Inventory TSD, Section 5.1. 
 III-3 

5. The EPA approved the EAC Ozone Action Plan (OAP) on August 19, 2005. The 

OAP included an amendment to Regulation No. 7 requiring the reduction of flash 

emissions of volatile organic compounds from condensate collection, storage, 

processing and handling operations by May 1, 2005. This initial rule required the 

installation of air pollution control technology to achieve a system-wide 47.5% 

reduction from uncontrolled emissions of volatile organic compounds from new 

and existing oil and gas exploration and production operations located within the 

8-hour ozone nonattainment area designated by EPA for operators with total 

emissions greater than 30 tons per year. The 2006 base case estimate was 

developed from actual reported emissions based on the system-wide 47.5% 

reduction requirement. 

In February 13, 2008, the EPA approved revisions to Regulation No. 7 to require 

the system-wide reduction of condensate tank flash VOC emissions of 75% for 

the 2007 ozone season by May 1, 2007 and 78% reduction for the 2012 ozone 

season, with technology that achieves a 95% reduction in VOC emissions. The 

2010 base case emissions estimate assumes the 75% system-wide reduction 

requirement. 

6. The effect of EPA final locomotive Tier 3 standards were considered and 

included, where appropriate, in the 2010 area source estimates. Tier 4 

locomotive standards do not go into effect until 2015 and therefore were not 

included in the 2010 inventories. 

 

All of the inventories in this 8-hour Ozone Attainment SIP were developed using EPAapproved emissions modeling methods, including EPA’s MOBILE6 model and local VMT 

data for on-road mobile source emissions, EPA’s non-road model and local demographic 

information for area and off-road sources, and reported actual emissions for point 

sources. Estimates for future emissions are based on the above-mentioned tools and 

the EPA’s Economic Growth and Analysis System (EGAS) model for estimating future 

point sources activity, VMT growth for on-road mobile sources, and 2010 and 2012 

demographic data for off-road and area sources. The technical support document 

contains detailed information on model assumptions and parameters for each source 

category. 

Highway mobile source emissions are from the ENVIRON Consolidated Community 

Emissions Processing Tool (CONCEPT) model inventory, which is based on DRCOG 

VMT data and MOBILE6 input data provided by APCD and expanded to the entire NAA 

based on VMT from the North Front Range Transportation and Air Quality Planning 

Council and CDOT. 

 
 III-4 

Non-road source emissions are from the EPA Non-Road Model. This model includes 

the impact of future controls on non-road engines, which is used in equipment such as 

lawn and garden equipment and construction equipment. 

Oil and gas source emissions are from the revised Independent Petroleum Association 

of Mountain States (IPAMS) inventory, and were projected to 2010 using the 

methodology in the IPAMS projection methodology document. The IPAMS inventory 

was sponsored by the IPAMS and is Phase III of a regional oil and gas emission 

inventory for the Inter-Mountain West jointly with the Western Regional Air Partnership 

(WRAP). 

Non-oil and gas area source emissions (including heating, consumer solvent use, 

aircraft and railroads, etc.) are from the 2002 EPA National Emissions Inventory (NEI), 

grown to 2006 and 2010 by population growth from data from the State Demographer. 

Consumer solvent emission reductions based on 75% of the per-person reductions listed 

in the EPA May 30, 2007 Emission Reduction Credit Memo were applied to the 

projected 2010 non-oil and gas area source inventory. A check on the non-oil and gas 

area sources estimates comparing the recently available 2005 NEI emissions data is 

shown in the Emissions Inventory TSD, Section 7.1. An inventory completed in 2005 for 

Denver International Airport (DIA) was used for aircraft and airport non-road source 

emissions from DIA for both 2006 and 2010. 

Non-oil and gas point source emissions were grown to 2010 by the EPA EGAS 

economic model, and by adding sources for which permits have been issued. 

Emissions of VOC and nitrogen oxides (NOx) from biogenic sources have been 

generated by the Model of Emissions of Gases and Aerosols from Nature (MEGAN) 

Biogenic Emissions Model using land cover data base of biomass type and density and 

hourly meteorology data. The National Center of Atmospheric Research (NCAR) has 

produced a global data base of land use data, the MEGAN Driving Variable Database 

Version 1.2, for use with MEGAN. Surface temperatures are provided by the Mesoscale 

Meteorological Model (MM5) modeling. 

Summaries of the VOC and NOx base case inventories for the nonattainment area for 

2006 and 2010 are presented in Table 5. Emissions of NOx and VOCs are in tons per 

average episode day. Additional detail on the categories of emissions can be found in 

the TSD. 
 III-5 

Wildfire Emissions Estimates

Wildfire emissions, though not included in Tables, have been considered for the 

background ozone concentrations in the modeling effort. Wildfire emissions can vary 

significantly on a day-to-day basis depending on conditions. 
 III-6 

Table 5: 8-Hour Ozone DMA/NFR NAA Base Case Inventories 

(tons per average episode day) 

2006 2010 

Source Category NOx VOC NOx VOC 

Point Sources 

Electric Generation Units (EGU) 55.6 0.7 58.5 1.6

External Combustion Boilers 9.5 0.4 10.0 0.5

Industrial Processes 12.5 10.2 14.0 11.0

Petroleum and Solvent Evaporation 0.3 19.0 0.3 22.0

Other 3.1 1.8 3.6 2.0

Point Sources Subtotal 81.0 32.1 86.4 37.0

 

Oil & Gas Point & Area Sources 

Condensate Tanks 126.5 129.6

Other O&G Point Sources 22.6 6.8 23.6 8.6

Pneumatic Devices (Area Source) 24.8 31.1

Unpermitted Fugitives (Area Source) 16.2 20.4

Other Area Sources 17.1 10.8 22.5 13.7

O&G Point & Area Sources Subtotal 39.7 185.2 46.2 203.3

 

Area Sources 

Personal Care Products 7.1 7.0

Household Products 21.4 17.9

Automotive Aftermarket Products 11.9 13.0

Architectural Coatings 20.1 16.8

Aircraft 7.4 1.3 8.2 1.5

Railroad 12.8 0.5 13.8 0.6

Other Coatings/Pesticides/Cooking/ Miscellaneous. 3.9 4.1

Area Source Subtotal 20.2 66.3 22.1 61.0

 

Non-Road Mobile Sources 

Agricultural Equipment 7.0 0.9 6.3 0.7

Airport Equipment 0.7 0.1 0.6 0.1

Commercial Equipment 5.3 6.2 5.1 7.0

Construction and Mining Equipment 35.7 5.5 31.2 4.5

Industrial Equipment 10.5 2.4 6.9 1.4

Lawn and Garden Equipment (Commercial) 9.4 35.9 8.9 28.1

Lawn and Garden Equipment (Residential) 1.2 7.5 1.2 11.8

Boats/Recreational Equipment/Miscellaneous 0.7 6.9 0.8 7.8

Non-Road Mobile Source Subtotal 70.5 65.3 61.0 61.3

 

On-Road Mobile Sources 

On-Road Mobile (including vehicle refueling) 165.5 129.7 122.9 109.2

On-Road Mobile Subtotal 165.5 129.7 122.9 109.2

 

Anthropogenic Total 376.8 478.6 338.5 471.8

Biogenic Total 53.0 694.0 53.0 694.0

Anthropogenic & Biogenic Total 429.8 1172.6 391.5 1165.8
 IV-1 

CHAPTER IV 

SIP CONTROL MEASURES 

This section of the 8-Hour Ozone Attainment State Implementation Plan (SIP) lists the 

additional control measures, above and beyond those assumed in the 2010 base case 

inventory described in Chapter III, that are incorporated in this attainment demonstration 

SIP for the 1997 0.08 parts per million (ppm) 8-hour Ozone National Ambient Air Quality 

Standard (NAAQS) by 2010. For purposes of this 8-Hour Ozone Attainment SIP and for 

inclusion of such control measures in the State Implementation Plan, the term "8-hour 

ozone nonattainment area" shall mean the area designated by the United States 

Environmental Protection Agency (EPA) as a nonattainment area for the 8-hour ozone 

standard in 2004 (69 FR 23857, April 30, 2004). 

A. Condensate Tank Emissions Controls 

The approved EAC Ozone Action Plan (OAP) included an amendment to Regulation No. 

7 to require the reduction of flash emissions of volatile organic compounds from 

condensate collection, storage, processing and handling operations. The initial rule 

required the installation of air pollution control technology to achieve a system-wide 

47.5% reduction from uncontrolled emissions of volatile organic compounds from new 

and existing oil and gas exploration and production operations located within the 8-hour 

ozone nonattainment area designated by EPA. The rule includes an exemption if total 

emissions from an operator are less 30 tons per year. 

In 2006 the AQCC approved changes to Regulation No. 7 to require the system-wide 

reduction of condensate tank flash VOC emissions of 75% for the 2007 ozone season, 

and 78% reduction for the 2012 ozone season, with technology that achieves a 95% 

reduction in VOC emissions. These two system-wide thresholds are proposed to remain 

as part of the Federal State Implementation plan. 

This revision to the State Implementation Plan further amends Regulation No. 7 (See 

Attachment A SIP Rule Language, Regulation No.7 Section XII) requiring the systemwide reduction of condensate tank flash VOC emissions of 81% for tanks ≥ 2 tons per 

year (tpy) for the 2009 ozone season, 85% for tanks ≥ 2 tpy for the 2010 ozone season 

and 90% for tanks ≥ 2 tpy for the 2011 ozone season with technology that achieves a 

95% reduction in VOC emissions and installation and operation of auto igniters on all 

tanks ≥ 2 tpy effective May 1, 2010. By February 2009, condensate tanks serving newly 

drilled, re-completed or stimulated wells must control emissions during the first 90 days 

of production. After that, the control may be removed if emissions do not exceed/will not 

exceed 2 tpy. 
 IV-2 

The reduction from these strategies is estimated at 34 tons per day (tpd) reduction in 

VOC for 2010 and 49 tpd reduction in VOC for 2011 based on an assumed Rule 

Effectiveness adjustment of 0.83 applied to the estimated potential emissions reduction. 

Justification of the rule effectiveness for Condensate Tanks is presented in the Rule 

Effectiveness document as part of TSD Appendix C. 

 

B. Additional Revisions to Regulation No. 3 and No. 7 

Regulation No. 3 Exemptions

Regulation No. 3 currently defines a wide variety of sources that are exempt from 

providing Air Pollutant Emission Notices (APEN) and/or permitting because by 

themselves or cumulatively as a category they are deemed to have a negligible air 

quality impact. 

APEN and permitting exemptions will be removed or revised to develop an inventory of 

emissions from source categories where actual emissions are anticipated to exceed 

reporting thresholds or where there are equity issues and in the case of condensate 

tanks, to exclude categorical exemptions over the new 1 ton per year APEN threshold in 

the nonattainment area. Proposed exemptions for removal/revision are as follows: 

• Removal of APEN and permit exemptions for oil and gas condensate storage 

tanks, but may make use of the generic APEN exemption when actual emissions 

fall below defined de minimis levels. 

• Remove APEN exemptions, but retain permit exemptions for the following: 

o petroleum industry flares 

o crude oil truck loading 

o oil production wastewater 

o crude oil storage tanks 

• Revise APEN/permitting exemptions for surface water impoundments and 

chemical storage tanks to exclude the exemption for the following: 

o oil and gas production wastewater 

o commercial facilities’ operations 

• Revise APEN/permitting exemptions for fuel storage dispensing to expand the 

applicability all nonattainment areas for equity purposes. 

Regulation No. 3 and No. 7 RACT Clarification 

This revision clarifies how both Regulation No. 3 minor source RACT requirements and 

Regulation No. 7 RACT requirements apply in an ozone nonattainment area as follows: 
 IV-3 

• All new and modified sources having VOC ≥ 2 tpy or NOx ≥ 5 tpy emissions are 

required to complete a RACT analysis, unless subject to a general RACT (based 

on adopted control technique guidelines (CTGs)) in Regulation No. 7, and 

implement RACT 

(For Rule Language on Regulations No. 3 and No. 7 see Attachment A - SIP Rule 

Language, Regulation No.7 and Regulation No.3) 
This page left blank intentionally. 
 V-1 

CHAPTER V 

PHOTOCHEMICAL MODELING AND 

OTHER WEIGHT OF EVIDENCE ANALYSES 

FOR ATTAINMENT DEMONSTRATION 

A. Photochemical Modeling for the 2006 and 2010 Base Case Scenarios 

As a former Early Action Compact (EAC) area, the United States Environmental 

Protection Agency (EPA) regulation requires a photochemical grid modeling attainment 

demonstration as part of the revised 8-Hour Ozone Attainment State Implementation 

Plan (SIP). The goal of the attainment plan’s 8-hour ozone modeling analysis is to 

conduct a comprehensive photochemical modeling study for the Denver Metro 

Area/North Front Range (DMA/NFR) nonattainment area that can be used as the 

technical basis for demonstrating attainment with the 8-hour ozone National Ambient Air 

Quality Standard (NAAQS). 

The photochemical model “Comprehensive Air Quality Model with Extensions” (CAMx) 

(as applied by consultants ENVIRON International Corporation and Alpine Geophysics 

Atmospheric Sciences Group) was used for this study. Meteorological fields for input 

into CAMx were produced using the Mesoscale Meteorological Model (MM5). Model 

ready emissions data for the 2006 and 2010 base case were processed through the 

emissions processing systems, Consolidated Community Emissions Processing Tool 

(CONCEPT) for the DMA on-road mobile, Model of Emissions of Gases and Aerosols 

from Nature (MEGAN) for biogenic emissions and Sparse Matrix Operating Kernel 

Emissions (SMOKE) for the other emissions categories. The photochemical modeling 

study was conducted in accordance with EPA modeling guidance for ozone (“Guidance 

on the Use of Models and Other Analyses for Demonstrating Attainment of Air Quality 

Goals for Ozone, PM2.5 and Regional Haze" (EPA-454/B-07-002, April 15, 2007) and a 

prepared modeling protocol. The modeling protocol was specifically designed to identify 

the processes responsible for 8-hour ozone exceedances in the region and to develop 

realistic emissions reduction strategies for the ozone exceedances. 

Several technical documents are available that detail the meteorological, emissions, and 

photochemical modeling and are included in the Technical Support Document 

Appendices for this plan at www.colorado.gov/airquality/documents/deno308/. Technical 

support documentation for modeling includes: 

• Modeling Protocol, Episode Selection, and Domain Definition (Appendix A) 

• Evaluation of MM5 Simulations of the June-July 2006 Denver Ozone Season 

(Appendix B) 

• Development of the 2006 & 2010 base case Modeling Inventory (Appendix C) 
 V-2 

• Model Performance Evaluation June-July 2006 Denver Ozone Season, 

Diagnostic Testing and Analysis (Appendix D) 

• Air Quality Modeling for the 2006 & 2010 base case, ozone projections, 

sensitivity analysis, and source attribution (Appendix E)

• 2010 Ozone Attainment Demonstration Modeling for the Denver 8-Hour Ozone 

State Implementation Plan Control Strategy (Appendix F )

• Weight of Evidence Analysis to Support the Modeled Attainment Demonstration 

(Appendix G)

• Ambient Data (Appendix H)

• Final 2010 Ozone SIP Control Measure Modeling for the Denver 8-Hour Ozone 

State Implementation Plan (Appendix I)

It should be noted that the suite of mathematical models used to evaluate current and 

future air quality possesses 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. To put the air quality model results in full perspective, the technical 

support document contains model performance evaluations for the meteorological and 

photochemical model. 

A very brief summary of photochemical model performance is offered 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 inability of the meteorology model to push the ozone concentrations far 

enough into the foothills; 

o inability to replicate retention of ozone aloft for a sustained period of days; 

and, 

o some days possibly understating 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. 

• 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 meets EPA guidance performance requirements which require that 

most of the matched pairs near the monitor be within ± 20% of the observed 

value. This model’s performance for the matched pairs is as follows: 

o Maximum modeled daily maximum = 76% within ± 20% of the observed 

value; 
 V-3 

o Closest modeled daily maximum = 91% within ± 20% observed value; 

and 

o Spatial paired modeled daily maximum = 82% within ± 20% observed 

value. 

Even though all models and modeling protocols have inherent limitations, photochemical 

grid modeling is the best tool available to assess progress in reducing ozone 

concentrations and to integrate the hourly variable inputs of emissions, meteorology and 

chemistry data over a two-month modeled ozone season. To mitigate the limitations of 

the modeling platform, the results are not used in an absolute sense, but rather are used 

in a relative sense as discussed in the next section. In addition, EPA modeling guidance 

requires a Weight of Evidence (WOE) analysis that uses other objective air quality 

measures and modeling parameters to supplement the modeling results. 

B. Base Case Relative Response Factors (RRF) 

The modeling produces base case relative response factors (RRF) for receptors in the 

modeling domain where ozone monitors are located. In general, the RRF for each 

monitor is equal to the mean 2010 base case modeled 8-hour ozone concentration 

divided by the mean 2006 base case modeled 8-hour concentration. The RRF is 

essentially the percentage change in modeled ozone concentrations between 2006 and 

2010. Specifically, each RRF is the mean of at least 10 daily 8-hour predicted maximum 

concentrations in 2006 greater than 0.075 ppm "nearby" (within 15 kilometers) a monitor 

during a given episode divided by the mean of similar 2010 daily 8-hour predicted 

maximum concentrations during a given episode as shown below. (Based on EPA’s 

“Guidance on the Use of Models and Other Analyses for Demonstrating Attainment of 

Air Quality Goals for Ozone, PM2.5 and Regional Haze" (EPA-454/B-07-002, April 15, 

2007). 

 

Relative Response Factor (RRF) =

Mean 2010 Base Case Modeled 

8-hour Ozone Concentrations (ppm)

Mean 2006 Base Case Modeled 

8-hour Ozone Concentrations (ppm) 

 

An RRF for each monitoring site for modeled (predicted) days greater than 0.075 parts 

per million (ppm) is presented in Table 6. 
 V-4 

C. Estimated Future (2010) Base Case Design Value 

Once the RRFs are developed, the RRF for each monitoring site is multiplied by the 

monitoring site’s base case design value to determine a future case design value for 

each site, as shown below, indicating if attainment is demonstrated at each site. 

Table 6 presents the current (2005-2007) base case design values (DVC) for each 

monitoring site, the modeled base case RRFs for modeled days greater than 0.075 ppm, 

and the future base case design values (DVF) for each site. If the future (2010) base 

case design values are less than 0.085 ppm (85 parts per billion (ppb)), then attainment 

is demonstrated. However, when there are DVFs in the range of 82-87 ppb EPA 

guidance requires a WOE analysis to supplement the attainment demonstration. 

EPA guidance indicates that base design values, which are the three year average of 

the 4th maximum values at each monitor, are to be presented to three places in ppm, 

truncating the 4th place right of the decimal point. When projecting future design values 

in ppm, similarly rounding to the 4th place and ultimately truncating the 4th place for 

comparison with the NAAQS is required. In Table 6, the future design values have been 

calculated to the 4th place in ppm and presented with the 4th place truncated for 

comparison with the 1997 8-hour standard of 0.084 ppm. 

Table 6: 2010 Base Case Design Values for Each Monitoring Site 

for Modeled Days greater than 0.075 ppm 

* FCW has only 2 years of complete data available, 2006 and 2007 

Estimated Future 

Design Value (ppm) = RRF * Current Design Value (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 
 V-5 

As can be noted attainment at all of the monitors is demonstrated (design values less 

than 85 ppb) for the 2010 base case for the 8-hour ozone nonattainment area as a result 

of the reductions expected from existing programs and regulations. However, since 

there are four monitors with design values between 82-87 ppb a WOE analysis is 

required. 

 

D. Weight of Evidence (WOE) Analysis 

EPA’s 8-hour ozone modeling guidance suggests a weight of evidence analysis (a set of 

supplemental analyses) be provided 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 NREL) 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. 

The supplemental analyses used in a weight of evidence will help to determine whether 

attainment is likely when modeled attainment test results indicate future air quality levels 

are near the NAAQS. Additional detail has been provided in the TSD Appendices F and 

G. As previously noted the TSD Appendices F, 2010 Attainment Demonstration 

Modeling and G, Weight of Evidence Analysis to Support the Modeled Attainment 

Demonstration are found at www.colorado.gov/airquality/documents/deno308/.

Recent Air Quality Related Trends 

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 is 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 
 V-6 

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 adopted state-only controls will continue to reduce emissions in 

the oil and gas industry and mobile sources beyond 2010. 

The total estimate of VOC and NOx emissions from all sources in Tables 7 and 8 below 

demonstrate an overall reduction in emissions between the 2006 base case and the 

2010 base case and additional SIP and state-only measures case. The 2010 additional 

SIP and state only measures modeled in this analysis include: 

• State–only revisions to Regulation No. 11 (March 2008) that tighten tailpipe 

emissions standards, 

• State-only revisions to Regulation No. 7 (December 2008) requiring low-bleed 

devices for new and existing pneumatic controllers. 

• Revisions to Regulation No. 7 requiring increased system-wide control of 

condensate emissions from 75% to 81% in 2010 which reduced condensate tank 

emissions by 24 tpd from the 2010 base case, and 

• An estimate of the impact of 7.8 psi RVP in the NFR area considering local 

survey data of ethanol penetration 

AQCC action on December 12, 2008 adopted a federally enforceable SIP control 

measure revising Regulation No. 7, noted above, requiring increased system-wide 

control of condensate VOC emissions from 75% to 85% in 2010 which reduced 

condensate tank emissions by 34 tpd from the 2010 base case. The analysis in Table 9 

below is therefore considered conservative since the adopted SIP control measure 

provides 10 tpd more reduction than originally modeled. 

In Table 7, the reduction in total anthropogenic VOC from 2006 base case to 2010 base 

case is about 7 tpd. The 2010 additional control case will net 46 tpd in reductions. The 

total VOC reduction from 2006 base case to 2010 additional control case is around 53 

tpd, which is about an 11% reduction. Including the emissions from the adopted SIP 

control measures results in a 63 tpd reduction from the 2006 base case, this is about a 

13% reduction. In Table 8, the reduction in total anthropogenic NOx from 2006 base 

case to 2010 base case is about 38 tpd. The 2010 additional control case will net 4 more 

tons per day. The total NOx reduction from 2006 base case to 2010 additional control 

case is around 42 tpd, which is about an 11% reduction. Table 9 reflects the ozone 

benefits of the emission reductions presented in Tables 7 & 8. 
 V-7 

For an analysis of the ozone benefits of the adopted SIP control measures (34 tpd of 

VOC emissions) alone see Final 2010 Ozone SIP Control Measure Modeling for the 

Denver 8-Hour Ozone State Implementation Plan in Appendix I of the TSD. 
 V-8 

Table 7: VOC Base Case and Control Case Emission Inventory 

(tons per average episode day) 

2006 Base 2010 Base 2010 Additional 

 Control* 

Source Category VOC VOC VOC 

Point Sources 

Electric Generation Units (EGU) 0.7 1.6 1.6

External Combustion Boilers 0.4 0.5 0.5

Industrial Processes 10.2 11.0 11.0

Petroleum and Solvent Evaporation 19.0 22.0 22.0

Other 1.8 2.0 2.0

Point Sources Subtotal 32.1 37.0 37.0

 

Oil & Gas Point & Area Sources 

Condensate Tanks 126.5 129.6 105.6

Other O&G Point Sources 6.8 8.6 8.6 

Pneumatic Devices (Area Source) 24.8 31.1 12.0 

Unpermitted Fugitives (Area Source) 16.2 20.4 20.4

Other Area Sources 10.8 13.7 13.7

O&G Point & Area Sources Subtotal 185.2 203.3 160.1 

 

Area Sources 

Personal Care Products 7.1 7.0 7.0

Household Products 21.4 17.9 17.9

Automotive Aftermarket Products 11.9 13.0 13.0

Architectural Coatings 20.1 16.8 16.8

Aircraft 1.3 1.5 1.5

Railroad 0.5 0.6 0.6

Other Coatings/Pesticides/Cooking/Miscellaneous 3.9 4.1 4.1

Area Source Subtotal 66.3 61.0 61.0

 

Non-Road Mobile Sources 

Agricultural Equipment 0.9 0.7 0.7

Airport Equipment 0.1 0.1 0.1

Commercial Equipment 6.2 7.0 7.0

Construction and Mining Equipment 5.5 4.5 4.5

Industrial Equipment 2.4 1.4 1.4

Lawn and Garden Equipment (Commercial) 35.9 28.1 28.1

Lawn and Garden Equipment (Residential) 7.5 11.8 11.8

Boats/Recreational Equipment/Miscellaneous 6.9 7.8 7.8

Non-Road Mobile Source Subtotal 65.3 61.3 61.3

 

On-Road Mobile Sources 

On-Road Mobile (including vehicle refueling) 129.7 109.2 106.0

On-Road Mobile Subtotal 129.7 109.2 106.0

 

Anthropogenic Total 478.6 471.8 425.4 

Biogenic Total 694.0 694.0 694.0

Anthropogenic & Biogenic Total 1172.6 1165.8 1119.4 
 V-9 

Table 8: NOx Base Case and Control Case Emission Inventory 

(tons per average episode day) 

2006 Base 2010 Base 2010 Additional

Control* 

Source Category NOx NOx NOx 

Point Sources 

Electric Generation Units (EGU) 55.6 58.5 58.5

External Combustion Boilers 9.5 10.0 10.0

Industrial Processes 12.5 14.0 14.0

Petroleum and Solvent Evaporation 0.3 0.3 0.3

Other 3.1 3.6 3.6

Point Sources Subtotal 81.0 86.4 86.4

 

Oil & Gas Point & Area Sources 

Condensate Tanks 

Other O&G Point Sources 22.6 23.6 23.6 

Pneumatic Devices (Area Source) 

Unpermitted Fugitives (Area Source) 

Other Area Sources 17.1 22.5 22.5

O&G Point & Area Sources Subtotal 39.7 46.2 46.2 

 

Area Sources 

Personal Care Products 

Household Products 

Automotive Aftermarket Products 

Architectural Coatings 

Aircraft 7.4 8.2 8.2

Railroad 12.8 13.8 13.8

Other Coatings/Pesticides/Cooking/Miscellaneous 

Area Source Subtotal 20.2 22.1 22.1 

 

Non-Road Mobile Sources 

Agricultural Equipment 7.0 6.3 6.3

Airport Equipment 0.7 0.6 0.6

Commercial Equipment 5.3 5.1 5.1

Construction and Mining Equipment 35.7 31.2 31.2

Industrial Equipment 10.5 6.9 6.9

Lawn and Garden Equipment (Commercial) 9.4 8.9 8.9

Lawn and Garden Equipment (Residential) 1.2 1.2 1.2

Boats/Recreational Equipment/Miscellaneous 0.7 0.8 0.8

Non-Road Mobile Source Subtotal 70.5 61.0 61.0

 

On-Road Mobile Sources 

On-Road Mobile (including vehicle refueling) 165.5 122.9 118.9

On-Road Mobile Subtotal 165.5 122.9 118.9

 

Anthropogenic Total 376.8 338.5 334.6 

Biogenic Total 53.0 53.0 53.0

Anthropogenic & Biogenic Total 429.8 391.5 387.6 
 V-10 

* The 2010 additional control measures modeled in this analysis include: 

• State–only revisions to Regulation No. 11 (March 2008) that tighten tailpipe emissions standards, 

• State-only revisions to Regulation No. 7 (December 2008) requiring low-bleed devices for new and existing 

pneumatic controllers. 

• Revisions to Regulation No. 7 requiring increased system-wide control of condensate emissions from 75% to 

81% in 2010 which reduced condensate tank emissions by 24 tpd from the 2010 base case, and 

• An estimate of the impact of 7.8 psi RVP in the NFR area considering local survey data of ethanol penetration. 

Table 9 presents the current (2005-2007) base case design values (DVC) for each 

monitoring site, the 2010 modeled control case RRFs for modeled days greater than 

0.075 ppm, and the 2010 additional control case design values (DVF) for each site 

calculated per EPA Guidance with the 2005-2007 base case design values and the 

modeled control case RRFs. 

Table 9: 2010 Additional Control 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 

Control 

Case 

Relative 

Reduction 

Factors 

Calculated 

8-Hour Ozone 

Future (2010) 

Control Case 

Design Values 

(ppm) 

Truncated 

8-Hour Ozone 

Future (2010) 

Control Case 

Design Values 

(ppm) 

Welby 0.070 1.0039 0.0702 0.070 

Arvada 0.079 1.0022 0.0791 0.079 

NREL 0.082 1.0027 0.0822 0.082 

Rocky Flats North 0.085 0.9981 0.0848 0.084 

S. Boulder Creek 0.081 0.9963 0.0807 0.080 

Fort Collins 0.074 0.9853 0.0729 0.072 

Fort Collins West* 0.086 0.9852 0.0847 0.084 

Carriage 0.074 1.0015 0.0741 0.074 

Welch 0.075 1.0002 0.0750 0.075 

CAMP 0.056 1.0009 0.0560 0.056 

Weld County Tower 0.078 0.9925 0.0774 0.077 

Highland 0.078 0.9900 0.0772 0.077 

Chatfield Res. 0.084 0.9921 0.0833 0.083 

Rocky Mtn. N.P. 0.076 0.9892 0.0751 0.075 

* FCW has only 2 years of complete data available, 2006 and 2007 

Attainment at all of the monitors continues to be achieved (design values less than 85 

ppb) in 2010 for the 8-hour ozone nonattainment area with an additional (0.1 to 0.2 ppb) 

margin of safety as a result of the additional control measures (SIP and state-only 

measures discussed above in this Section D.. This analysis of ozone benefit is 

conservative since the final SIP measures add 10 more tpd of VOC reduction than was 

estimated in the above analysis. 
 V-11 

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

0.100

2002 2003 2004 2005 2006 2007 2008

NREL R. Flats-N Chatfield FC-W

Ozone Trends 

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 Denver Metro Area (DMA) 

1-hour ozone attainment maintenance area prior to the 2004 ozone season. 

The chart below 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, NREL, 

Rocky Flats-N, Chatfield and Fort Collins West, are shown in the following chart. 

Chart 1: Time Series of Monitored 4th Maximum 8-Hour Ozone Values (ppm) 

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 Chart 2, shows what appears to be 

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

) of the regression is 

very small and additional analysis shows that this trend does not pass the standard 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 inter-annual 

variation in the 4th high values at Rocky Flats North to conclude from the observations 
 V-12 

that the 4th high value in the 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 discernible trends during 

this period. 

 

Chart 2: Linear Regression of Monitored 4th Maximum Ozone Values 

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

Weather-Corrected Ozone Time Series 

The correspondence between 500-millibar heights and ozone, noted later in this section, 

can be used to correct ozone time series for the effects of weather. These 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 maximum 8-hour ozone for Fort Collins 

and Greeley is shown in Chart 3. 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. A similar analysis for Rocky Flats, NREL, Chatfield, Carriage, South 

Boulder Creek, and Arvada is shown in Chart 4. Gradual decreases through 2004 are 

replaced by apparent steep 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. While many factors may 
 V-13 

-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 in ppb

(Residuals are differences between actual and weather

predicted ozone.)

have contributed to this increase, it is outside the scope of this process to complete a 

thorough analysis of the causes. 

Chart 3: Trend in Weather-Corrected July Mean Daily Max 8-Hour Ozone 

for Fort Collins and Greeley 

Chart 4: Trend in Weather-Corrected July Mean Daily Max 8-Hour Ozone 

for Denver Metro Area Sites

-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.)
 V-14 

-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 in ppb

Correcting all eight of the Front Range annual fourth maximum time series for weather 

leads to the pattern shown in Chart 5. The corrected time series 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. 

Chart 5: Trend in Weather-Corrected Annual Fourth Max 8-Hour Ozone 

for Front Range Sites

Days Greater than 75 ppb and 84 ppb 

The following chart 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 above 75 ppb and 84 ppb. 

Excluding the year, 2004, the years 2005 through 2008 show a modest reduction in 

number of total days of elevated ozone when compared with the years 2000 through 

2003. 
 V-15 

Chart 6: Days Greater than 75 ppb and 84 ppb 8-Hour Ozone 

0

10

20

30

40

50

2000 2001 2002 2003 2004 2005 2006 2007 2008

Number of Days

>75 ppb >84 ppb

Trends Analysis Summary 

Monitored data time series and weather corrected monitored data time series indicate 

that ozone levels have been relative flat. This suggests that without additional emissions 

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

reductions are provided in the 2010 base case inventory, the attainment case. Additional 

emissions reductions are provided for in SIP/State-only controls. Various sensitivity 

analyses of precursor reductions have noted that VOC and NOx reductions will reduce 

ozone levels. The modeling for the base (attainment) case and the controls cases with 

the emissions reduction noted indicate attainment, and attainment with a slight margin.

Review of Ozone Conceptual Models for the 8-Hour Nonattainment Area (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 and emission inventory data becomes available, along with a better 

understanding of the chemical processing that takes place in the nonattainment area, 
 V-16 

there will be an opportunity to review the current understanding of local ozone formation 

in the DMA/NFR region as it presents itself. 

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

photochemical reactive volatile organic compounds (VOCs) and nitrogen oxides (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. 

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 

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. This 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 
 V-17 

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 

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 the single best predictor for this value at sites along 

the Front Range urban corridor.

Back Trajectory Analysis 

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 
 V-18 

for RFN can show the influence of sources within the Denver metro area and the Platte 

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 Mountain Daylight Time (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. 

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. Please see Appendix G for greater detail of these analyses. 
 V-19 

Conceptual Model Summary 

A review of the Conceptual model for the Front Range reveals the complexity of the 

meteorological, emissions 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 of the Front Range. 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. 

Review of Modeled Metrics 

Modeled metrics assess the changes in ozone levels at grid cells in the NAA from 2006 

base case to 2010 base case to 2010 additional control cases. In the charts that follow, 

“Cntrl1” represents the SIP and state-only strategies presented in previous Tables 7, 8 & 

9, while “Cntrl2” represents additional SIP and state-only strategies adopted or 

considered that can provide additional reductions in the DMA/NFR area. The additional 

SIP and state-only strategies adopted or considered in Control 2 include: 

• Inspection/maintenance program in the Forth Front Range 

• Tighten up collector plate requirements for older vehicles (statewide) 

• Increase condensate control to 90% system-wide by 2011 ozone season 

• Increase condensate control to 95% system-wide by 2012 ozone season 

• Statewide Oil & Gas regulations – Controls on existing reciprocating internal 

combustion engines. 
 V-20 

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

The 95% system wide control on condensate tanks was not adopted by the AQCC as 

SIP or state-only measure. The 90% system-wide control by 2011 was adopted as a SIP 

strategy. Tightening of collector plates is being pursued but was not adopted as a stateonly measure. The remaining strategies were adopted as state-only. 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 SIP and state-only controls. 

Relative change in grid cells ≥ 85 ppb, ≥ 80 ppb, ≥ 75 ppb and ≥70 ppb 

As can be seen in the Chart 7 below, 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 SIP strategies. Further reduction of emissions 

(through state-only or voluntary measures) continues to demonstrate reduction 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 SIP and state-only controls. 

Chart 7: Relative Change in Grid Cells 

Relative change in grid cells-hours ≥ 85 ppb, ≥ 80 ppb, ≥ 75 ppb and ≥70 ppb 

As can be seen in Chart 8 below, 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 SIP strategies. Further 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 
 V-21 

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

2010 base case and show ≥ 1% increased reduction in cell-hours due to the SIP and 

state-only controls. 

Chart 8: Relative Change in Grid Cell-hours

Relative change in total 8-hour ozone ≥ 85 ppb, ≥ 80 ppb, ≥ 75 ppb and ≥70 ppb 

As can be seen in Chart 9 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 SIP strategies. Further 

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 SIP and state-only controls. 
 V-22 

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

Chart 9: Relative Change in Total 8 hour Ozone

Modeled Metrics Summary

The three additional modeling metrics (grid cells, grid cell hours and total ozone) indicate 

that the emissions reductions in the base case show decreases of 11% to 22% in peak 

elevated ozone ≥ 85 ppb and ≥ 80 ppb from emissions reductions due to existing 

controls and regulations along the Front Range. Continued decreases in emissions due 

to SIP and state-only controls provide additional decreases in peak elevated ozone ≥ 85 

ppb and ≥ 80 ppb of 2% to 11% along the Front Range. 

Review Alternative Attainment Test Methodology

EPA’s Recommended Methodology for Determining Base Year Design Value 

(DVB) for Modeling Purposes 

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).” 
 V-23 

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. Data from the 2008 

ozone season (currently formally quality assured by the EPA for all monitors except the 

Rocky Mountain National Park monitor) is now mostly available and the average of the 

three design values in the 2004-2008 period can now be calculated. 

In this WOE 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, all as shown in the following 

table: 

 

Table 10: 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 0.0706 1.0042 0.0709 0.070 

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** 0.082 0.9874 0.0810 0.081 

Carriage 0.0727 1.0022 0.0729 0.072 

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. 0.0828 0.9934 0.0823 0.082 

Rocky Mtn. N.P. 0.0759 0.9903 0.0752 0.075 

* thru September 30, 2008. 2008 data have been fully quality assured at this timefor all monitors 

except Rocky Mountain National Park (Rocky Mtn. N.P).; 

** FCW only has three years of data and is presented as a Design Value to three places 
 V-24 

As can be seen in Table 10, using EPA’s recommended base year design value 

calculation approach results in achieving the standard at all the monitor sites with more 

safety margin than the results previously presented in Tables 6 using the three-year 

average (2005-07) base case design values for the 2010 base case. 

Table 11 presents a modeling analysis of the future year design values from the 2010 

additional control case (referenced above) using EPA’s recommended DVB calculation 

approach. This approach results in achieving the standard at all the monitor sites with 

more safety margin than the results previously presented in Table 9 using the three-year 

average (2005-07) base case design values. 

Table 11: 2010 Additional 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 0.0706 1.0039 0.0708 0.070 

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** 0.082 0.9852 0.0807 0.080 

Carriage 0.0727 1.0015 0.0728 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. 0.0828 0.9921 0.0814 0.081

Rocky Mtn. N.P. 0.0759 0.9892 0.0751 0.075 

* thru September 30, 2008. 2008 data have been fully quality assured at this time for all monitors 

except Rocky Mtn. N.P.; 

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

Alternative Attainment Test Summary 

The alternative attainment test using the EPA recommended approach for establishing 

the base case design value design indicates that the 2010 base case will likely achieve 
 V-25 

attainment in the Denver region of the 0.08 ppm 8-hour ozone NAAQS. Additionally, 

there will be more certainty that the Denver region will achieve 8-hour ozone attainment 

in 2010 under the 2010 additional control case emission scenario.

Assess the Efficacy of SIP, State-Only and Voluntary Control Strategies 

The reduction in emissions from the 2006 base case to the 2010 additional control 

scenarios, which include reductions from current state and federal regulations and newly 

proposed state regulations for inclusion in the SIP, reduce VOC and NOx emissions by 

11% from the 2006 base case. Photochemical grid modeling has shown that these 

reductions will 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 

Reid Vapor Pressure (RVP) in the North Front Range (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,” 

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 to repair or salvage high-emitting vehicles that 

are identified on the road by remote-sensing technology. 
 V-26 

• Pollution Prevention programs implemented by local business and industry to 

reduce their loss of product and to prevent emissions of ozone-causing 

precursors. 

• 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 this agreement will be signed by the 

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. 
 V-27 

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 as follows: 

• 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. 

• 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. Analysis of the weekend-weekday effect for the 

Front Range shows a strong effect in Central Denver and weaker effect in 

outlying areas. This points to the possibility for NOx control disbenefits in Central 

Denver due to the role of NOx quenching there. The spatial pattern of the 

weekend effect is consistent with the localized NOx disbenefit identified in the 

photochemical modeling. 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 suggest that 

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

concentrations have been highly variable. This suggests that without additional 

emission reductions (controls) the region will remain at or near the level of the 

standard. 

• 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 15-30% for thresholds of 85 ppb and 80 ppb from the 2006 

base case through the 2010 base case, the proposed 2010 additional control 

case (Control 1) and the expected state-only additional control case (Control 2). 

• A comparison ambient and modeled data (shown in the TSD Appendix G) 

indicates that VOC emissions from the oil and gas sector may be 

underestimated. The consequence of underestimating VOC emissions is that 

effectiveness of controls is under estimated on a mass basis. 
 V-28 

• 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 additional Control 1(SIP), and the 2010 additional Control 2 (State-only) 

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 base case, 2010 additional Control 1, and 2010 additional Control 2 

emissions scenarios. The preponderance of evidence suggests that the region will 

attain the standard in 2010 under the base case, additional Control 1, and additional 

Control 2 scenarios, but the safety margin is small. 

G. Commitment to Conduct Periodic Assessment of Growth 

Assumptions 

The State of Colorado will periodically evaluate the growth assumptions used to develop 

this plan and will evaluate the need for additional control measures necessary to remedy 

unanticipated emission increases. Specifically, the APCD will periodically evaluate the 

data and growth assumptions used in the SIP’s attainment demonstration for new point 

source growth and future transportation patterns and their impact on air quality. If the 

review of growth demonstrates that adopted control measures are inadequate to 

address growth in emissions, additional measures will be considered and added to the 

plan. 
VI-1

CHAPTER VI 

VOC AND NOx MOTOR VEHICLE EMISSIONS BUDGETS

A. Transportation Conformity 

Transportation conformity provisions of section 176 (c)(2)(A) of the Clean Air Act (CAA) 

require regional transportation plans and transportation improvement programs to 

demonstrate that “...emissions expected from implementation of plans and programs are 

consistent with estimates of emissions from motor vehicles and necessary emissions 

reductions contained in the applicable implementation plan...” 

The Environmental Protection Agency’s (EPA) transportation conformity rule requires 

that control strategy implementation plans, which are defined in 40 CFR 93.101 as 

reasonable further progress plans and attainment demonstrations, contain motor vehicle 

emissions budgets. Because this State Implementation Plan (SIP) is an 8-hour ozone 

attainment demonstration, motor vehicle emissions budgets for volatile organic 

compounds (VOC) and nitrogen oxides (NOx) are proposed for 2010, which is the area’s 

attainment year. Once these budgets are found adequate or are approved, the 

metropolitan planning organizations (MPOs) in the nonattainment area will use the 

budgets to demonstrate that projected emissions that would result from implementation 

of their transportation plans and transportation improvement programs are less than or 

equal to the adequate or approved emissions budgets. 

The 8-hour ozone nonattainment area encompasses multiple MPOs and transportation 

planning regions. The Denver Regional Council of Governments (DRCOG) is the 

metropolitan planning organization responsible for transportation planning in the 7-

county Denver metropolitan area and a portion of southwest Weld County. Likewise, the 

North Front Range Transportation and Air Quality Planning Council (NFRTAQPC) is the 

metropolitan planning organization responsible for transportation planning in the 

urbanized portions of Larimer and Weld counties. Finally, the Upper Front Range (UFR) 

Transportation Planning Region (TPR), not a designated metropolitan planning 

organization, is responsible for transportation planning in the rural portions of Larimer, 

Weld, and Morgan counties. 

Because of the different institutional arrangements and different schedules and timelines 

for transportation plans and programs development, this SIP establishes both VOC and 

NOx subarea motor vehicle emission budgets, and budgets for VOCs and NOx for the 

entire nonattainment area for purposes of transportation conformity in the Denver/North 

Front Range 8-hour ozone nonattainment area. The two subareas are described below. 

Also described below are the procedures to be followed when switching between using 

subarea budgets and the nonattainment area-wide budgets. 
VI-2

B. Motor Vehicle Emissions Budgets

According to EPA regulations and guidance, the SIP may establish a budget or budgets 

that apply to the entire nonattainment area, and/or subarea budgets for each 

metropolitan planning organization or subarea within the nonattainment area. 

For purposes of this SIP, Motor Vehicle Emissions Budgets for VOC and NOx are 

established for the 2010 attainment year. Budgets are specifically established for two 

sub-regional areas and for the entire nonattainment area for purposes of transportation 

conformity. The two subareas are defined as follows and shown in Figure 2: 

• Southern Subarea--Area denoted by the ozone nonattainment area south of the 

Boulder County northern boundary and extended through southern Weld County 

to the Morgan County line. This area includes the nonattainment portion of 

DRCOG’s’ regional planning area and the southern Weld County portion of the 

Upper Front Range TPR. 

• Northern Subarea--Area denoted by the ozone nonattainment area north of the 

Boulder County northern boundary and extended through southern Weld County 

to the Morgan County line. This area includes the North Front Range 

Transportation and Air Quality Planning Council transportation planning area as 

well as the northern ozone nonattainment area portion the Upper Front Range 

TPR in Larimer and Weld counties. 

When subarea budgets are created in the SIP, the sum of the subarea budgets must 

equal the total allowable emissions the entire nonattainment area can have from the 

transportation sector and still lead to attainment of the standard. This SIP expressly 

allows the MPOs the flexibility to demonstrate conformity with either the established 

subarea budgets or the nonattainment area-wide Motor Vehicle Emission Budgets. 

Therefore, if each subarea meets its motor vehicle emission budgets or if the total 

emissions for the entire nonattainment area (the sum of the subareas) are less than or 

equal to the budget for the entire nonattainment area, then the entire area will meet the 

total SIP’s purpose of attaining the relevant standard. 

Proposed 2010 Emissions Budgets for the Denver Metro and North Front Range 8-

Hour Ozone Subareas

Table 12 indicates the separate Motor Vehicle Emissions Budgets for the ozone 

precursors VOC and NOx for the two subareas discussed above and the VOC and NOx 

budgets for the entire nonattainment area. 
VI-3

Table 12: Subarea and Nonattainment Area-wide 

2010 Motor Vehicle Emissions Budgets 

2010 Motor Vehicle 

Emissions Budget Subareas VOC 

(tpd) 

NOx 

(tpd) 

Southern Subarea Budget 

(DRCOG & UFR TPR Subarea) 89.7 102.4 

Northern Subarea Budget 

(NFRTAQPC & UFR TPR Subarea) 19.5 20.5 

Total Nonattainment Area Budget 

(Entire Nonattainment Area) 109.2 122.9 

The 2010 VMT estimates were used with 2010 emission factors obtained from the EPA 

MOBILE6.2 Emission Factor Model to calculate emissions. The two subarea budgets 

presented in the table add to the sum of the total 2010 motor vehicle emissions for the 

entire nonattainment area in the 2010 base case inventory (See Table 5 in Chapter III), 

which demonstrates attainment of the standard. 

For the underlying transportation modeling, the roadway and transit links in DRCOG’s 

2005 and 2015 Cycle 2 (2007) networks were truncated to include only the portion of the 

network within the 8-hour ozone nonattainment area. Vehicle miles traveled (VMT) 

estimates from these networks were interpolated to obtain 2006 and 2010 baseline VMT 

estimates for purposes of developing the SIP emissions inventories. Likewise, the 2005 

and 2015 (2007) networks from the North Front Range MPO were truncated to include 

only the portion of the network within the 8-hour ozone nonattainment area. The VMT 

estimates were interpolated to obtain 2006 and 2010 baseline VMT estimates. Where 

there was overlap between the North Front Range (NFR) and DRCOG networks in Weld 

County, the DRCOG network was used. In areas where there was no MPO network, the 

Federal Highway Authority (FHWA) Highway Performance Management System 

(HPMS) and Colorado Department of Transportation (CDOT) networks, plus a growth 

factor, were used to calculate VMT. 

The following table summarizes the VMT estimates for each of the budget subareas. 

The total VMT is identical to the 2010 base case VMT estimates in Table 3 (See Chapter 

III). 
VI-4

Table 13: Distribution of VMT between the Budget Subareas (2010) 

Southern Subarea 

DRCOG Network VMT 76,551,505 

Upper Front Range VMT 777,910 

Total Southern Area VMT 77,329,415 

Northern Subarea 

NFR MPO Network VMT 11,753,832 

Upper Front Range VMT 1,057,239 

Total Northern Area VMT 12,811,071 

Total Nonattainment Area VMT 90,140,486 

The 2006 and 2010 VMT estimates were used with emission factors obtained from the 

EPA Mobile 6.2 Emission Factor Model to calculate emissions. Emissions were 

calculated on a link-by-link basis. Speeds were obtained from the MPO transportation 

networks and the roadway speed limit was used for CDOT links. The ambient 

temperatures for the regional emissions analysis were derived from the meteorological 

modeling performed for the attainment demonstration for a typical ozone episode period. 

The motor vehicle mix was obtained from the CDOT automated traffic counters. 

Process for Considering Subarea Motor Vehicle Emissions Budgets in MPO 

Conformity Determinations

The nonattainment area-wide and subarea motor vehicle emission budgets, once 

approved by the Air Quality Control Commission (AQCC) and determined adequate by 

the EPA, will be used to measure the conformity of plans and programs for the 

respective areas. Through an agreement between the affected agencies, DRCOG has 

agreed to perform transportation forecasts and conformity determinations for the entire 

Southern Subarea, while the North Front Range Transportation and Air Quality Planning 

Council has agreed to perform transportation forecasts and conformity determinations 

for the entire Northern Subarea. The nonattainment-area wide budgets shall be used for 

the initial conformity determination; however, consistent with EPA regulations and 

guidance, the MPOs may use the subarea budgets for subsequent conformity 

determinations. 

The subarea budgets will allow for independent conformity determinations based on the 

applicable subarea motor vehicle emissions budgets by the two MPOs, whose frequency 

and timing needs for conformity determinations differ substantially. With subarea 

budgets, the affected MPOs can make independent conformity determinations for their 

plans and programs as long the other subarea in the nonattainment area has conforming 

transportation plans and programs in place at the time of each MPO’s and United States 

Department of Transportation’s (DOT) plan/transportation improvement program (TIP) 
VI-5

determination. If conformity lapses for one subarea (i.e., the conformity determination 

for a plan or program has expired), the existing plans and TIPs in the other subarea 

continue to be valid and the MPO can continue to implement transportation projects in its 

currently conforming plans and programs. However, the MPO cannot make new plan 

and TIP conformity determinations until the lapse in the other subarea is resolved and 

conformity is determined in the lapsed subarea. 

Initial Conformity Determination 

Once nonattainment area-wide and subarea budgets in this plan have been found 

adequate or approved by EPA, DRCOG and the North Front Range Transportation and 

Air Quality Planning Council must initially make a conformity determination of their 

respective transportation plans and programs within two years after EPA’s adequacy 

finding and/or SIP approval (40 CFR 93.104(e)). Under provisions of this SIP, the MPOs 

must make an initial concurrent conformity determination using the nonattainment areawide Motor Vehicle Emissions Budgets for VOCs and NOx. The MPOs must ensure the 

sum of their subarea emissions is less than or equal to the established total 

nonattainment area-wide budgets. 

Subsequent Conformity Determinations 

Once the initial joint conformity determination using the nonattainment area-wide 

budgets has been made and approved, the MPOs must continue to make future 

conformity determinations using the established nonattainment area-wide budgets 

whenever either MPO is required to make a new conformity determination for a 

transportation plan or program. However, since this SIP expressly allows the MPOs the 

flexibility to demonstrate conformity with either the established subarea motor vehicle 

emissions budgets or the nonattainment area-wide motor vehicle emissions budgets, the 

MPOs may revert to demonstrating conformity by meeting their respective subarea 

emission budgets if the MPOs initially make concurrent conformity determinations that 

demonstrate consistency of their respective plans and programs with their individual 

subarea budgets. Thereafter, an MPO can make independent conformity determinations 

for their plans and TIPs as long as the other subarea in the nonattainment area 

continues to have a conforming transportation plan and TIP in place at the time of the 

conformity determination. 

Likewise, at any time in the future, the MPOs may switch from using subarea budgets to 

using nonattainment area-wide budgets as long as they once again perform a joint 

conformity determination and the sum of their subarea motor vehicle emissions is equal 

to or less than the established nonattainment area-wide budgets, and continue to make 

joint conformity determinations until they again decide to revert to the subarea budgets. 
VI-6

Throughout this process of determining conformity with the budgets in this plan, the 

MPOs shall consult with federal, state, and local air quality and transportation agencies 

through the normal interagency consultation process established by Air Quality Control 

Commission Regulation No. 10. 

Figure 2: 8-Hour Ozone Emission Budget Subarea