International Corporation 

 Air Sciences sierra 

research

Final Report 

WINTERTIME GASOLINE 

FUEL SPECIFICATIONS STUDY 

Prepared for 

Randy White 

Robert Tekniepe 

Clark County Department of 

Air Quality and Environmental Management 

500 S. Grand Central Parkway 

Las Vegas, NV 89106 

Prepared by 

Chris Emery 

Susan Kemball-Cook 

Sneha Jagtap 

Gary Wilson 

ENVIRON International Corporation 

101 Rowland Way, Suite 200 

Novato, California 94945

Siona Delaney 

James Lyons 

Sierra Research, Inc. 

1801 J Street 

Sacramento, CA 95814 

January 17, 2008 

 101 Rowland Way, Suite 220, Novato, CA 94945 415.899.0700 
Final / January 17, 2008 

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TABLE OF CONTENTS 

Page 

1. INTRODUCTION....................................................................................................................1 

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

1.2 Project Concept..............................................................................................................1 

2. HISTORICAL CARBON MONOXIDE TRENDS...............................................................3 

2.1 The Data.........................................................................................................................3 

 2.2 Results............................................................................................................................3 

3. WINTERTIME GASOLINE OPTIONS .............................................................................10 

3.1 Background..................................................................................................................10 

3.2 Fuel Scenarios..............................................................................................................11 

3.3 Analysis Methodology.................................................................................................11 

3.4 Results..........................................................................................................................12 

4. EVALUATION OF TRANSITIONAL PERIOD GASOLINE OPTIONS ......................14 

5. CO EMISSIONS INVENTORY AND UAM MODELING ................................................16 

5.1 CO Emissions Processing Approach ...........................................................................16 

 5.2 CO Emission Results ...................................................................................................17 

 5.3 UAM Modeling Results...............................................................................................17 

6. REGULATORY IMPACTS OF WINTER GASOLINE OPTIONS .................................22 

 

6.1 Regulations on Clark County Gasoline .......................................................................22 

 6.2 Regulatory Impacts of Gasoline Options.....................................................................23 

 

REFERENCES.............................................................................................................................24 

APPENDICES 

Appendix A: NAC 590.065 Gasoline: Adoption of specification guides by reference; 

exemption from strict compliance with standards; limitations on vapor pressure; 

limitations on contents 

Appendix B: Clark County Air Quality Regulations: Section 53 - Oxygenated Gasoline 

Program 
Final / January 17, 2008 

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TABLES 

Table 2-1. Annual 1st maximum 8-hour ambient CO concentrations 

 from Clark County, NV. ..........................................................................................5 

Table 2-2. Annual 2nd maximum 8-hour ambient CO concentrations 

 from Clark County, NV. ..........................................................................................5 

Table 3-1. Wintertime gasoline fuel requirements for Clark County .....................................10 

Table 3-2. Inputs for Clark County (winter 2006 gasoline)....................................................12 

Table 3-3. Approximate increase in CO g/mi emissions associated with 

 Clark County winter gasoline fuel scenarios .........................................................13 

Table 4-1. Wintertime Gasoline Fuel Requirements for Clark County ..................................14 

Table 5-1. Domain-wide on-road CO emission components and 

 totals (TPD) by future year ....................................................................................18 

Table 5-2. Domain-wide and central sub-domain total on-road CO 

 emissions (TPD) by future year. Only values for Monday, 

 December 9 are shown...........................................................................................18 

Table 5-3. Total domain-wide CO emissions (TPD) by future year used 

 for UAM modeling of each fuel scenario ..............................................................18 

Table 5-4. Peak predicted 8-hour CO (ppm) from the UAM CO SIP 

 modeling system. The “SIP” column shows the replicated 

 CO SIP values. The 8-hour CO NAAQS is 9 ppm...............................................20 

Table 5-5. Peak 8-hour average CO concentrations (ppm) predicted by 

 UAM, CAL3QHC, and UAM+CAL3QHC for each of the Five 

 Points intersections. .............................................................................................21 

Table 5-6. Peak total UAM, EDMS, and total UAM+EDMS 8-hour CO 

 concentrations (ppm) at all three airports evaluated. ...........................................21 

FIGURES 

Figure 2-1. Upper panel: Las Vegas Valley carbon monoxide monitoring 

 site as of the year 2000. ...........................................................................................6 

Figure 2-2. Time series of 1st and 2nd maximum 8-hour ambient CO 

 concentrations for Clark County, NV ......................................................................7 

Figure 2-3. Monthly 1st maximum 8-hour CO concentration in 

 Clark County, Nevada..............................................................................................8 

Figure 2-4. Monthly 2nd Maximum 8-Hour CO Concentration in 

 Clark County, Nevada..............................................................................................8 

Figure 2-5. First maximum 8-hour ambient CO concentration at each 

 monitor in the Las Vegas Valley over the time period 1990-2006..........................9 

Figure 2-6. Number of exceedances of the 8-hour CO standard at monitors 

 in the Las Vegas Valley during the period 1990-2006 ............................................9 

Figure 3-1. On-road mobile CO effects of Clark County winter gasoline 

 fuel reformulation ..................................................................................................13 

Figure 5-1. Total domain-wide CO emissions (TPD) by future year used 

 for UAM modeling of each fuel scenario. Weekday values are shown...............19 
Final / January 17, 2008 

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1. INTRODUCTION 

1.1 BACKGROUND 

The U.S. Environmental Protection Agency (EPA) has established two National Ambient Air 

Quality Standards (NAAQS) for carbon monoxide (CO): 35 parts per million (ppm) averaged 

over one hour; and 9 ppm averaged over eight hours. CO cannot exceed the 8-hour NAAQS at 

any monitoring site more than once per year, meaning that the annual second-highest 

concentration above the NAAQS defines an exceedance and is used to establish the design value 

(DV) for the area. Because of the rounding convention used by EPA, a concentration of 9.5 ppm 

is needed to violate the 8-hour standard. 

The Las Vegas Valley has historically violated the 8-hour CO standard during the winter 

months. The 1-hour CO standard has never been exceeded. During cold, clear, stagnant periods, 

pollutants emitted in the evening and morning hours are trapped within the valley in very 

shallow layers near the ground, and slowly move with overnight drainage flows into the lowest 

areas of the basin. Subsequent to the Clean Air Act Amendments of 1990, the EPA designated 

the valley as a “moderate” nonattainment area for 8-hour CO. Ultimately, the area was 

reclassified as a “serious” area in the mid-1990’s after continued exceedances of the CO 

standard. 

The resulting 2000 CO State Implementation Plan (SIP) for Clark County contained local 

measures for attaining the standards, and in June 2005 the EPA found that the valley had attained 

the standards by the applicable date of December 31, 2000. The 2000 CO SIP includes three 

control measures that affect Clark County gasoline specifications during the CO season (October 

– March); oxygenated fuel using 10% ethanol by volume and at least 3.5% oxygen content by 

weight; 9.0 psi Reid Vapor Pressure (RVP) requirement with a 1.0 psi waiver for >9% ethanol; 

and Cleaner Burning Gasoline (CBG) specifications to reduce emissions of sulfur and aromatic 

hydrocarbons. 

Later, Clark County submitted a 2005 CO SIP Revision, which included revised and improved 

methodologies for estimating on-road mobile source emissions (the chief component of CO 

emissions in the valley), and extended future year estimates of CO concentrations out to 2030 to 

demonstrate maintenance of the standards and to establish conformity emission budgets for 

multiple “out” years. No changes in actual control regulations were incorporated into the 2005 

CO SIP Revision. 

1.2 PROJECT CONCEPT 

The Clark County Department of Air Quality and Environmental Management (DAQEM) 

commissioned an independent analysis of the current wintertime gasoline specifications that 

apply in the Las Vegas Valley, as well as an evaluation of possible changes to those 

specifications, and/or their period of applicability. The DAQEM contracted with ENVIRON 

International Corporation and Sierra Research, Inc. to assist in this wintertime gasoline study. 
Final / January 17, 2008 

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ENVIRON and Sierra performed the following analyses, and results are documented in this 

report as noted: 

 Summarized the historical CO trends in the Las Vegas Valley from 1990 through 2005 

(Section 2); 

 Evaluated various wintertime gasoline options available to Clark County in light of 

EPA’s regulations limiting “boutique” fuels (Section 3); 

 Evaluated various options for addressing the transitions to and from winter gasolines 

available to Clark County (Section 4); 

 Estimated on-road mobile source CO emission totals and assessed future year CO 

concentration peaks using the Urban Airshed Model (UAM) for two gasoline options in a 

manner consistent with the 2005 SIP Revision (Section 5); 

 Summarized the regulatory impacts associated with each of the identified wintertime 

gasoline options (Section 6). 
Final / January 17, 2008 

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2. HISTORICAL CARBON MONOXIDE TRENDS 

This section presents a summary of ENVIRON’s analysis of ambient CO trends in the Las Vegas 

Valley between 1990 and 2006. This analysis was based on 8-hour CO data provided by the 

Clark County DAQEM. 

2.1 THE DATA

CO data presented below were measured by the Las Vegas Valley carbon monoxide airmonitoring network that was initially administered by the Air Quality Division of the Clark 

County Health District, and later by the DAQEM. For this analysis, continuous daily 8-hour CO 

data were extracted from the EPA Air Quality System (AQS) by DAQEM for the entire period 

of January 1990 through Spring 2006, and provided to ENVIRON. Daily maximum 8-hour CO 

concentrations were extracted from these data over each CO season, which extends from October 

through March. Each monitor in the network continuously measures ambient CO levels using a 

Dasibi Carbon Monoxide Analyzer (Model 3003) which employs the Gas Filter Correlation 

technique. Figure 2-1 shows the Las Vegas Valley CO monitoring system as of 2000 (upper 

panel) and 2005 (lower panel). These figures are taken from the Las Vegas CO State 

Implementation Plans (Clark County Board of Commissioners, 2000; 2005) and do not show all 

monitors that recorded data during the 1990-2005 period. Note in particular the location of the 

City Center monitor, as this is the monitor with the longest continuous ambient CO time series. 

A full accounting of all stations and the times during which they were active is given in Tables 

2-1 and 2-2. 

2.2 RESULTS 

The first and second CO concentration maxima for each monitor for each year during the time 

period 1990-2006 are shown in tabular form in Tables 2-1 and 2-2. While CO concentrations are 

measured year-round, ambient data were analyzed over the October-March CO season each year, 

and the year label indicates the October 1 start date. For example, the year 1995 indicates the 

October 1995 – May 1996 period. The 1st and 2nd maxima (taken over all monitors) for each 

year are shown in graphical form in Figure 2-2. The 8-hour NAAQS of 9 ppm1 is shown in red 

in Figure 2-2. Both the first and second maxima show a steadily declining trend during the 

1990-2006 period. The first and second maxima have not exceeded the 8-hour CO standard 

since 1998. The most recent value of the first maximum (5.3 ppm) is well below the 8-hour 

standard, as is the most recent value of the 2nd maximum (5.0 ppm).

Figures 2-3 and 2-4 show the monthly 1st and 2nd maximum CO concentration taken over all Las 

Vegas monitors as a function of time and month. Only the months of the CO season (OctoberMarch) are shown. The 1st (Figure 2-3) and 2nd (Figure 2-4) maxima are each represented by a 

surface. The surfaces are domed across the months, indicating that yearly maximum CO 

concentrations generally occurred during December-January. This is consistent with the cold 

temperatures and strong atmospheric stability frequently occurring during these months, as these 

 

1 Note that when rounding is taken into account, the CO concentration required to exceed the 8-hour standard is 9.5 

ppm. 
Final / January 17, 2008 

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conditions are conducive to high CO concentrations. Note that the only exceedance of the 8-

hour CO standard during the months of March or October came in March of 1996. During the 

last 10 years, then, there has not been a single exceedance of the 8-hour standard during October 

or March. The downward slope of the 1st and 2nd maximum surfaces as time progresses from 

1990-2006 indicates the overall reduction in CO concentrations in the Las Vegas Valley. The 1st

and 2nd maximum CO values have declined for all months from 1990 to 2006. 

Figure 2-5 shows the first maximum 8-hour ambient CO level by monitor as a function of year. 

By examining these time series, we can evaluate whether there are shifts in the locations of peak 

CO concentrations over the years. The only monitors that exceeded the 8-hour CO standard are 

the East Charleston and the Sunrise Acres monitors. Comparison of Figure 2-5 and Figure 2-2 

shows that the highest 1st maximum 8-hour CO times series over all monitors (Figure 2-2) is 

simply the 1st maximum CO at East Charleston from 1990-1996 and the 1st maximum CO at 

Sunrise Acres from 1997-2005. There are no significant shifts in the location of the peak CO 

concentrations over the years. Monitors with relatively high maximum CO (Sunrise Acres, E. 

Sahara) at the beginning of the period are the monitors with highest maximum CO 

concentrations throughout, and the monitors with relatively low CO maxima (Pittman, Craig 

Road) at the start of their operation show relatively low CO maxima throughout. 

There is only one monitor with a continuous time series extending from 1990-2005. The City 

Center monitor shows a trend of decreasing CO over the period of interest. This decreasing 

trend is apparent in nearly all of the other monitors. The only monitor to show an increase in CO 

over the lifetime of the monitor is Craig Road, and its first maximum 8-hour value never rises 

above 2 ppm, which is well below the 8-hour standard. 

Several of the monitors in the Las Vegas Valley showed an increase in 1st maximum CO 

concentrations during the 1996-1998 period (e.g. Sunrise Acres, City Center, S. LV Blvd.), but 

all of these returned to a decreasing trend in the 1999-2005 period. 

Figure 2-6 shows the number of exceedances of the 8-hour CO standard across all monitors in 

the Las Vegas Valley monitoring network during the period 1990-2006. From a high value of 13 

in 1990, the number of exceedances has shown an overall declining trend, with some interannual 

variability likely due to weather conditions. Since 1998, there have been no exceedances of the 

8-hour CO standard. 

In summary, the 8-hour CO trend data show that the 1st and 2nd maxima CO concentrations in the 

Las Vegas Valley have decreased significantly during the 1990-2005 time period. This result is 

not sensitive to the choice of location within the Valley or to the month of the CO season 

examined. Peak CO values generally occurred during the months of December and January. No 

exceedances of the 8-hour CO standard have occurred since 1998. 
Final / January 17, 2008 

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Table 2-1. Annual 1st maximum 8-hour ambient CO concentrations from Clark County, NV. 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

0007 S.E. Valley --- --- --- --- --- --- --- --- 2.1 1.8 1.5 1.4 1.4 1.1 --- --- ---

0016 City Center 8.6 7.4 7.4 7.4 7.6 6.9 6.8 5.7 7.7 5.7 5.1 4.4 4.0 3.7 3.7 3.7 2.9

0557 East Charleston 15.9 12.6 12.1 11.9 10.9 10.3 10.3 6.7 --- --- --- --- --- --- --- --- ---

0561 Sunrise Acres --- --- --- --- --- --- 8.3 10.0 10.3 8.6 7.3 6.5 6.6 5.5 5.8 5.3 5.3

0020 Craig Road --- --- --- --- --- 0.9 1.7 1.7 2.0 1.2 1.5 1.4 1.8 0.9 --- --- ---

0043 Paul Meyer Park --- --- --- --- --- --- --- --- 1.7 2.1 1.8 1.1 1.9 1.2 --- --- ---

0107 Pittman --- --- --- --- --- 2.7 3.0 2.7 2.5 2.5 2.3 2.5 2.3 --- --- --- ---

0298 Green Valley --- --- --- --- --- --- --- --- 2.1 2.0 1.8 1.6 1.4 1.3 --- --- ---

0538 Winterwood --- --- --- --- --- --- --- --- 6.3 5.7 4.1 4.5 3.6 3.9 3.3 3.7 2.9

0539 E. Sahara --- --- --- --- --- --- --- --- 7.9 7.0 5.8 6.3 4.9 5.1 5.2 4.6 4.2

0562 Crestwood --- --- --- --- --- --- 6.5 6.0 6.4 6.3 5.2 4.5 3.8 --- --- --- ---

0601 Boulder City --- --- --- --- --- --- --- --- 0.7 0.6 1.2 0.7 0.7 0.3 --- --- ---

1022 E. Flamingo --- --- --- --- --- 4.6 5.0 4.5 5.3 5.7 5.1 4.3 3.5 --- --- --- ---

1023 S. LV Blvd. --- --- --- --- --- --- 4.8 5.2 5.6 5.0 3.9 3.7 3.0 3.0 2.9 2.3 2.0

2002 J.D. Smith --- --- --- --- --- --- --- --- 6.8 6.1 4.7 5.6 4.7 4.9 3.8 4.1 3.7

0563 Freedom Park --- --- --- --- --- --- --- --- --- --- 5.5 5.3 5.2 5.0 4.9 4.6 ---

0021 Shadow Lane --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---

1021 --- --- --- --- --- --- --- --- --- --- --- --- 4.0 3.6 4.1 4.2 3.1

1st Maximum 8-Hour Concentration (ppm) Monitor Site

10.0 8.6 6.5 6.6 5.5 5.8 Maximum 1st Max 8-Hour 

Concentration 5.3

Annual 1st Maximum 8-Hour CO Concentrations in Clark County, Nevada

(Based on October to March Data)

15.9 12.6 12.1 11.9 10.9 10.3 10.3 10.3 7.3 5.3

Table 2-2. Annual 2nd maximum 8-hour ambient CO concentrations from Clark County, NV. 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

0007 S.E. Valley --- --- --- --- --- --- --- --- 2.0 1.7 1.5 1.3 1.3 1.0 --- --- ---

0016 City Center 7.8 7.1 6.1 7.2 7.2 6.4 6.6 5.5 7.3 5.7 4.7 4.3 3.8 3.6 3.5 3.6 2.9

0557 East Charleston 14.2 12.1 11.0 11.1 10.6 9.4 10.1 6.5 --- --- --- --- --- --- --- --- ---

0561 Sunrise Acres --- --- --- --- --- --- 8.1 9.0 10.1 8.2 7.1 6.3 6.0 5.3 5.1 5.2 5.0

0020 Craig Road --- --- --- --- --- 0.8 1.6 1.5 1.8 0.8 1.4 1.3 1.0 0.9 --- --- ---

0043 Paul Meyer Park --- --- --- --- --- --- --- --- 1.7 1.9 1.6 1.0 1.5 1.2 --- --- ---

0107 Pittman --- --- --- --- --- 2.6 2.8 2.6 2.5 2.4 2.1 2.3 2.2 --- --- --- ---

0298 Green Valley --- --- --- --- --- --- --- --- 1.8 1.9 1.7 1.5 1.3 1.3 --- --- ---

0538 Winterwood --- --- --- --- --- --- --- --- 5.1 5.4 3.7 3.9 3.5 3.8 3.1 3.2 2.5

0539 E. Sahara --- --- --- --- --- --- --- --- 6.8 6.9 5.7 5.6 4.8 4.9 4.7 4.5 3.9

0562 Crestwood --- --- --- --- --- --- 5.4 5.9 6.2 5.8 4.7 4.3 3.8 --- --- --- ---

0601 Boulder City --- --- --- --- --- --- --- --- 0.7 0.6 1.0 0.6 0.7 0.3 --- --- ---

1022 E. Flamingo --- --- --- --- --- 4.5 4.4 4.3 5.0 5.4 4.7 4.1 3.2 --- --- --- ---

1023 S. LV Blvd. --- --- --- --- --- --- 4.6 4.9 5.1 4.4 3.7 3.4 2.9 3.0 2.6 2.3 1.9

2002 J.D. Smith --- --- --- --- --- --- --- --- 6.6 5.8 4.7 5.1 4.5 4.8 3.7 3.9 3.5

0563 Freedom Park --- --- --- --- --- --- --- --- --- --- 5.2 5.5 4.8 4.5 4.8 4.1

0021 Shadow Lane --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---

1021 Katie Ave. --- --- --- --- --- --- --- --- --- --- --- --- 3.6 3.5 3.7 3.8 3.0

Maximum 2nd Max 8-Hour 12.1 11.0 11.1 10.6 9.4 10.1 9.0 10.1 8.2 7.1 6.3 6.0 5.3 5.1 5.2 5.0

Concentration

Monitor Site

14.2

Annual 2nd Maximum 8-Hour CO Concentrations in Clark County, Nevada

(Based on October to March Data)

2nd Maximum 8-Hour Concentration (ppm)
Final / January 17, 2008 

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Figure 2-1. Upper panel: Las Vegas Valley carbon monoxide monitoring site as of the year 

2000. Figure from the Carbon Monoxide State Implementation Plan for the Las Vegas Valley 

(Clark County Board of Commissioners, 2000). Lower panel: Las Vegas Valley carbon 

monoxide monitoring site as of the year 2005. Figure from the Carbon Monoxide State 

Implementation Plan for the Las Vegas Valley (Clark County Board of Commissioners, 2005). 
Final / January 17, 2008 

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1st and 2nd Maximum 8-Hour CO Levels in Clark County, Nevada 

(Based on October to March Data)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Calendar Year

CO Concentration (ppm)

8-Hour CO Standard (9.5 ppm)

First Maximum

Second Maximum

Figure 2-2. Time series of 1st and 2nd maximum 8-hour ambient CO concentrations for Clark 

County, NV. Data are analyzed during the October-March CO season, and the year label 

indicates the October start date. For example, the year 1995 indicates the October 1995- May 

1996 time period. 
Final / January 17, 2008 

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1990

1992

1994

1996

1998

2000

2002

2004

2006

October

November

December

January

February

March

1.5

3.5

5.5

7.5

9.5

11.5

13.5

15.5

Monthly 1st Maximum 8-Hour CO 

Concentration in Clark County, NV

13.5-15.5

11.5-13.5

9.5-11.5

7.5-9.5

5.5-7.5

3.5-5.5

1.5-3.5

Figure 2-3. Monthly 1st maximum 8-hour CO concentration in Clark County, Nevada. 

1990

1992

1994

1996

1998

2000

2002

2004

2006

October

November

December

January

February

March

1.5

3.5

5.5

7.5

9.5

11.5

13.5

15.5

Monthly 2nd Maximum 8-Hour CO Concentration 

in Clark County, NV

13.5-15.5

11.5-13.5

9.5-11.5

7.5-9.5

5.5-7.5

3.5-5.5

1.5-3.5

Figure 2-4. Monthly 2nd Maximum 8-Hour CO Concentration in Clark County, Nevada. 
Final / January 17, 2008 

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First Maximum 8-Hour Ambient CO Levels by Monitor

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

19901991199219931994199519961997199819992000200120022003200420052006

Year

CO (ppm)

CO 8-Hour NAAQS

S.E. Valley

City Center

East Charleston

Sunrise Acres

Craig Road

Paul Meyer Park

Pittman

Green Valley

Winterwood

E. Sahara

Crestwood

Boulder City

E. Flamingo

S. LV Blvd.

J.D. Smith

Freedom Park

Shadow Lane

Figure 2-5. First maximum 8-hour ambient CO concentration at each monitor in the Las Vegas 

Valley over the time period 1990-2006. 

Number of Exceedances of the 8-Hour CO 

Standard in the Las Vegas Valley

13

6

2

3

4

1

3

1

2

00000000

0

3

6

9

12

15

19901991199219931994199519961997199819992000200120022003200420052006

Exceedances

Figure 2-6. Number of exceedances of the 8-hour CO standard at monitors in the Las Vegas 

Valley during the period 1990-2006. 
Final / January 17, 2008 

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3. WINTERTIME GASOLINE OPTIONS

Sierra Research assessed the on-road vehicle carbon monoxide (CO) emissions impacts of 

possible reformulations of the Clark County wintertime gasoline specifications. After a review 

of recently promulgated federal regulations regarding “boutique” fuels and consultation with 

DAQEM, several agreed-upon fuel scenarios were analyzed. In this analysis, the impacts of the 

different fuel scenarios on fleet average CO emission factors generated by MOBILE6.2 were 

investigated. Based on these results, DAQEM selected two fuel scenarios for a more detailed 

evaluation using the methodology documented in the 2005 CO SIP Revision for air quality 

planning purposes in Clark County. 

3.1 BACKGROUND 

Winter gasoline sold in Clark County is currently subject to RVP limits as well as specifications 

for oxygenate, sulfur, and aromatic content. The RVP specification is imposed by Nevada 

statute, while oxygenate, aromatic, and sulfur content specifications are imposed by sections 53 

and 54 of the Clark County Air Quality Regulations. The current wintertime gasoline fuel 

requirements in Clark County are summarized in Table 3-1. 

Table 3-1. Wintertime gasoline fuel requirements for Clark County. 

Fuel Property Standard 

Reid Vapor Pressure (RVP) Max 9.0 psi (10.0 psi for fuel with 9+ vol % EtOH) 

Sulfur Content Max 80 ppmw; Flat limit of 40 ppmw 

Aromatic Content Max 30 vol %; Flat limit of 25 vol % 

Oxygen Content Min 3.5 wt % 

According to recently promulgated federal regulations (Federal Register, 2006), winter gasoline 

sold in Clark County is considered to be a “boutique” fuel due to the sulfur and aromatic limits 

imposed by sections 53 and 54. These regulations, which are intended to restrict the creation of 

new boutique fuels, preclude any changes to the Clark County winter aromatic and sulfur limits 

except for their elimination. Further, once eliminated these limits cannot be reinstated. 

Given the restrictions on boutique fuels, Clark County has limited options with respect to 

changes in its wintertime gasoline specifications, which are summarized below. 

1. With respect to the sulfur and aromatics limits, the only choices are retention of the 

existing limits or their elimination. It should be noted that the existing sulfur limit has 

been rendered obsolete by the more stringent federal Tier 2 gasoline limit on sulfur 

content. 

 

2. Because Clark County is still designated as a “serious” nonattainment area with respect 

to attainment of the eight-hour CO NAAQS, winter gasoline is still required by the Clean 

Air Act to contain at least 2.7% oxygen by weight, which limits the changes that can be 

made to the existing 3.5 wt% oxygen requirement. However, it is possible that 
Final / January 17, 2008 

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redesignation of Clark County to attainment with respect to the CO NAAQS could allow 

the elimination of the oxygenate requirement. 

3. Finally, there are no impediments to changes in the winter RVP limit for Clark County 

gasoline other than the necessity of demonstrating, via a SIP revision or as part of 

maintenance plan if redesignation occurs, that such changes will not lead to 

nonattainment of the CO NAAQS. 

3.2 FUEL SCENARIOS 

Based on the above, the following matrix of fuel scenarios was developed for analysis: 

1. Elimination of the 9 psi winter RVP requirement in favor of enforcement of the 

ASTM limit for gasoline sold in Clark County during the winter months, which can 

go as high as 15 psi (ASTM, 2006); 

2. Relaxation of the 3.5 wt% oxygen limit to 2.7 wt%; 

3. Elimination of the vapor pressure requirement and relaxation of the oxygenate 

requirement (combination of scenarios 1 and 2); 

4. Elimination of the aromatic and sulfur content requirements; 

5. Elimination of the aromatic, sulfur, and RVP requirements (combination of scenarios 

1 and 4); 

6. Elimination of the aromatic and sulfur requirements and relaxation of the oxygenate 

requirement to 2.7 wt% (combination of scenarios 2 and 4); 

7. Elimination of the RVP, aromatic, and sulfur requirements with relaxation of the 

oxygenate requirement (combination of scenarios 1, 2, and 4); and 

8. Elimination of all wintertime gasoline requirements (i.e., using no oxygenates in the 

gasoline along with eliminating the aromatic, sulfur and RVP requirements). 

3.3 ANALYSIS METHODOLOGY 

The different fuel scenarios were analyzed for winter conditions during the years 2006, 2010, 

2015, 2020, 2025, and 2030. Each winter year assumes a fleet distribution equivalent to January 

of the following year (e.g., winter 2006 = January 2007). This analysis focused on effects of the 

fuel scenarios on fleet-average CO emission factors, which have units of grams per mile. 
Final / January 17, 2008 

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For changes in RVP and oxygenate content, CO impacts were assessed using EPA’s 

MOBILE6.2 emission factor model2 using Clark County specific input data derived from the 

latest CO SIP (Clark County Board of Commissioners, 2005). The runs reflected area-specific 

winter temperatures, registration distribution, distribution of vehicle miles traveled (VMT) by 

vehicle class and hour of day, distribution of vehicle starts by hour of day, and inspection and 

maintenance (I/M) parameters. 

This was done by inputting the 2006 winter fuel properties for Clark County and estimating the 

CO effects of revising the aromatics content to reflect the 2006 nationwide average. Winter 

gasoline fuel properties for Clark County and the nationwide average aromatic level were taken 

from Winter 2006 Alliance of Automobile Manufacturers (AAM) Fuel Survey Data (AAM, 

2006). The Clark County gasoline properties are shown in Table 3-2. The average nationwide 

aromatic content was 23.1%. 

Table 3-2. Inputs for Clark County (winter 2006 gasoline). 

Fuel Property Clark County Gasoline 

MTBE (wt% oxygen) 0 

ETBE (wt% oxygen) 0 

Ethanol (wt% oxygen) 3.47 

TAME (wt% oxygen) 0 

Sulfur (ppm) 31 

RVP (psi) 8.8 

E200 (%) 46.4 

E300 (%) 78.7 

Aromatics (vol%) 20.2 

Olefins (vol%) 6.7 

Benzene (vol%) 0.60 

3.4 RESULTS 

Figure 3-1 shows a graphical illustration of the on-road vehicle CO emission impacts for all the 

fuel scenarios, and Table 3-3 summarizes the percentage increase in emissions associated with 

all the fuel scenarios. As one would expect, eliminating all wintertime gasoline fuel 

requirements in Scenario 8 leads to the highest CO emissions, which are 20% to 30% higher than 

the baseline depending on the year. 

The largest single impact on CO emissions is associated with eliminating the RVP requirement, 

which leads to an increase of 16% to 18%. It should be noted that the MOBILE6 algorithm for 

modeling RVP impacts on CO emissions is quite dated and has been shown to be of questionable 

validity for the later model-year vehicles that dominate this analysis (Sierra Research, 2005). 

Therefore, the impact of elimination of the RVP requirement on CO emissions based on 

MOBILE6 is likely to be considerably overstated. 

 

2

 Version 6.2.03 dated September 2003. 
Final / January 17, 2008 

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0

2

4

6

8

10

12

14

2006 2010 2015 2020 2025 2030

Calendar Year

Fleet Average CO Emissions (g/mi)

Baseline Case

Fuel Scenario 1

Fuel Scenario 2

Fuel Scenario 3

Fuel Scenario 4

Fuel Scenario 5

Fuel Scenario 6

Fuel Scenario 7

Fuel Scenario 8

Figure 3-1. On-road mobile CO effects of Clark County winter gasoline fuel reformulation. 

Table 3-3. Approximate increase in CO g/mi emissions associated with Clark County winter 

gasoline fuel scenarios.

Scenario 2006 2010 2015 2020 2025 2030 

1. Eliminate RVP Requirement 18% 18% 17% 16% 16% 16% 

2. Relax Oxy Requirement 2% 1% 1% 1% 1% 1% 

3. Eliminate RVP + Relax Oxy Requirement 20% 19% 18% 17% 17% 17% 

4. Eliminate Aromatics + Sulfur Requirement 1% 1% 1% 1% 1% 1% 

5. Eliminate Aromatics + Sulfur + RVP 

Requirement 19% 19% 18% 18% 18% 18% 

6. Eliminate Aromatics + Sulfur Requirement 

and Relax Oxy Requirement 3% 3% 2% 2% 2% 2% 

7. Eliminate RVP + Aromatics + Sulfur and 

Relax Oxy Requirement 22% 21% 19% 19% 18% 18% 

8. Eliminate All Requirementsa 29% 25% 22% 21% 20% 20% 

a Clark County will need to be redesignated to attainment for CO during the winter to allow for the use 

of no oxygenates. 

As shown in Figure 3-1 and Table 3-3, eliminating the aromatics and sulfur limits would have a 

very small impact on CO emissions, as would relaxing the 3.5 wt% oxygen content requirement 

to 2.7 wt%. Based on the data presented in Table 3-3, complete elimination of the oxygenate 

requirement would increase CO emissions by about 10% in 2006, but that would drop to about 

2% by 2030 due to turnover of the fleet. 
Final / January 17, 2008 

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4. EVALUATION OF TRANSITIONAL PERIOD GASOLINE OPTIONS 

The availability of gasoline in Clark County during the winter is affected by the specifications 

imposed on that fuel by state and local requirements. At present, winter gasoline sold in Clark 

County is subject to controls on RVP and oxygenate, sulfur, and aromatic content. The current 

specifications and control periods are summarized in Table 4-1. 

Table 4-1. Wintertime Gasoline Fuel Requirements for Clark County. 

Fuel Property Standard 

Control 

Period 

RVP Max 9.0 psi (10.0 psi for fuel with 9+ vol % EtOHa) 10/1 – 3/31 

Oxygen Content Min 3.5 wt % 10/1 – 3/31 

Sulfur Content Max 80 ppmw; Flat limit of 40 ppmw 11/1 – 3/31 

Aromatic Content Max 30 vol %; Flat limit of 25 vol % 11/1 – 3/31 a

 Nevada statutes impose a limit of 10% ethanol by volume for on-road gasoline. 

The winter RVP specification in Clark County is imposed by Nevada statute3 between October 1 

and March 31, and the statute provides no transition period. The minimum oxygen content 

requirement is set forth in Section 53 of the Clark County Air Quality Regulations and applies 

between October 1 and March 31; the aromatic and sulfur content specifications are set forth in 

Section 54 of the Clark County Air Quality Regulations and apply between November 1 and 

March 31. Both Sections 53 and 54 have enforcement provisions that effectively require 

gasoline being delivered to retail stations to meet the requirements 15 days before the actual 

effective date. 

Based on a review of the regulations, examination of AAM fuel survey data for Los Angeles, 

Phoenix, and Las Vegas, discussions with fuel suppliers, and review of the Kinder Morgan 

pipeline system in the Pacific Region, it is the winter fuel specifications themselves, particularly 

the RVP and oxygenate requirements, rather than the transition periods, that present the greatest 

issues with respect to fuel supply in Clark County. 

Beginning with the RVP requirement, Clark County and Phoenix are the only two areas that 

enforce a low wintertime RVP requirement. In addition, both Clark County and Phoenix are 

generally supplied with gasoline from refineries in southern California. The low RVP 

requirement leads to tightened gasoline supply in Clark County (and Phoenix) because it 

precludes the sale in Clark County of the conventional and reformulated gasoline sold 

throughout the rest of the U.S., including California, during the winter. The fact that most of the 

low RVP gasoline sold in Clark County and Phoenix comes from southern California refineries 

leaves gasoline supply to those areas vulnerable to refinery upsets. 

Clark County’s high oxygen content requirement and relatively long winter oxygenate period 

increase the amount of ethanol that has to be available for the production of compliant gasoline. 

To the extent that ethanol supplies are limited or become disrupted, the supply of gasoline in 

Clark County will also be similarly impacted. 

 

3

 Nevada Administrative Code § 590.065 
Final / January 17, 2008 

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As described in Section 3, the Clark County winter sulfur content requirement became 

superfluous with the implementation of the federal Tier 2 gasoline sulfur limits and the winter 

aromatic content requirements do not appear to impose any significant burden on fuel suppliers. 

Therefore, these requirements are not a significant issue with respect to the supply of gasoline to 

Clark County. 

As discussed in Section 3, the emissions impacts of eight potential winter gasoline scenarios in 

Clark County were analyzed, and the potential impact of all eight scenarios on the availability of 

gasoline in Clark County were reviewed. Because each of the scenarios relaxes current 

constraints on the specifications of gasoline sold in Clark County, there should in general be no 

adverse impacts relative to current gasoline supply if any of the scenarios are selected for 

implementation. 

As discussed above, elimination of the winter RVP requirement is the scenario that is likely to 

have the greatest positive impact on gasoline supply to Clark County. There are several reasons 

for this. First, it would allow C4 and C5 hydrocarbons to be blended into gasoline destined for 

Clark County, thereby increasing the volume of available gasoline. Secondly, it would greatly 

increase the supply pool of gasoline that could be shipped to Clark County during the winter 

months. 

As also discussed above, elimination, rather than relaxation, of the oxygen content requirement 

would also be expected to remove a potential limitation on the supply of gasoline to Clark 

County. Note that eliminating the requirement would not prohibit the sale of oxygenated 

gasoline in Clark County, it would merely not require it, which would provide suppliers with 

greater flexibility. It should also be noted that while Clark County cannot completely eliminate 

the winter oxygenate requirement without being redesignated as in attainment with the NAAQS 

for CO and demonstrating the ability to maintain compliance with the NAAQS without 

oxygenated gasoline, the number of months when oxygenated gasoline is required could be 

reduced. Based on the analysis of winter CO concentrations in Section 2, it appears that, at a 

minimum, oxygenate requirements could be eliminated during the months of October, February, 

and March. The Clean Air Act requires a minimum of four months of oxygenate use during the 

CO nonattainment period, but would allow less time if “the State can demonstrate that because 

of meteorological conditions, a reduced period will assure that there will be no exceedances of 

the carbon monoxide standard outside of such reduced period.” 
Final / January 17, 2008 

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5. CO EMISSIONS INVENTORY AND UAM MODELING 

This section presents an assessment of CO emissions and UAM modeling for two of the eight 

wintertime gasoline options presented to the Clark County (DAQEM) as described in Section 3. 

ENVIRON ran the MOBILE6 on-road emission factor model for the current wintertime gasoline 

and two fuel options for the following years: 2006, 2010, 2015, 2020, and 2030. These three sets 

of emissions data were combined with year-specific traffic pattern data developed as part of the 

2005 CO SIP revision (referred to hereafter as the “CO SIP”) to derive several sets of UAM 

model-ready basin-wide gridded on-road emission input files per future year. The on-road 

inventories were then combined with pre-existing model-ready stationary and non-road CO 

emissions files (also from the CO SIP) so that the UAM could be run to project the resulting CO 

emissions into CO concentration distributions. Following the CO SIP, UAM was run for the 

various future year emission scenarios using meteorological inputs representative of the 

historical December 8-9, 1996 CO episode. 

The modeling reported here was performed to show that the two wintertime fuel options selected 

for analysis will not result in exceedances of the EPA 8-hour CO NAAQS in any of the future 

years listed above, given the projected roadway network and traffic activity projections assumed 

in the CO SIP. 

5.1 CO EMISSIONS PROCESSING APPROACH 

Sierra Research presented the DAQEM with eight wintertime gasoline options as part of its work 

described in Section 3. In consultation with DAQEM staff, two of these options were selected 

for the development of UAM CO emission inventories and air quality model projections: 

#1: Elimination of the 9 psi winter RVP requirement in favor of enforcement of the 

13.5 psi ASTM limits for gasoline sold in Clark County during the winter months; 

and 

#8: Elimination of all wintertime gasoline requirements. 

ENVIRON quantified emission impacts of each option. The starting point for this analysis was 

the on-road mobile source emissions processing tools, scripts, and programs developed by Clark 

County and ENVIRON during the development of the CO SIP. The current wintertime gasoline 

and each of the two gasoline options were first defined in the inputs to the EPA’s MOBILE6.2 

on-road emission factor model using the various fuel parameters listed in Section 3. The current 

wintertime gasoline was re-run through the standard EPA MOBILE6.2 model to replicate the CO 

SIP numbers. For both fuel scenarios #1 and #8, two versions of the MOBILE6.2 model were 

run; the standard version from EPA, and a modified version from Sierra Research that improved 

the estimates of RVP effects for late model vehicles (Sierra Research, 2003). The modified 

model uses alternative RVP correction factors for later model vehicles (post 1990 model year 

vehicles) derived from available “paired” test data at the time (i.e., test data from the same 

vehicle tested on two different fuels with different RVP levels while holding other fuel 

parameters constant to the extent possible). On-road emission inventory results from both 

models are shown to provide a direct comparison of the effect. 
Final / January 17, 2008 

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The MOBILE model was run in a manner exactly equivalent to the approach used in developing 

the SIP emissions inventory, with each run at 1-degree temperature intervals. All other inputs 

remained identical to the assumptions and parameters defined in the SIP processing. The output 

from the matrix of MOBILE runs were then combined with link-specific traffic activity, trip 

lengths, volumes, and speeds derived from pre-existing output from the Regional Transportation 

Commission’s TRANSCAD traffic demand model to generate gridded, hourly on-road mobile 

source emission rates on the CO SIP modeling grid. The DAQEM and ENVIRON staff worked 

together closely to develop a very detailed and complex methodology to apply MOBILE 

emission factors to the various data provided by TRANSCAD. Hence, ENVIRON already 

possessed all of the necessary TRANSCAD output, and processing tools/scripts from the 

previous work, from which to develop the mobile source emissions in a manner that was 

completely consistent with the emission budgets reported in the CO SIP. 

5.2 CO EMISSION RESULTS 

Mobile emission totals relative to the SIP on-road emissions inventory were tabulated for each of 

the future years: 2006, 2010, 2015, 2020, and 2030. Table 5-1 compares the domain-wide 

emissions for just the on-road components for: (1) the re-generated CO SIP case (“SIP”) using 

EPA’s MOBILE6.2; (2) scenario #1 using EPA’s MOBILE6.2; (3) scenario #1 using Sierra’s 

MOBILE6.2; (4) scenario #8 using EPA’s MOBILE6.2; and (5) scenario #8 using Sierra’s 

MOBILE6.2. Table 5-2 compares the total on-road CO inventory for the entire domain and the 

total on-road CO inventory for just the central portion of the modeling domain that covers the 

most urbanized portion of the grid4. We refer to this smaller area of the grid as the central subdomain. 

Comparison of the re-generated “SIP” on-road emission values in Table 5-1 against the actual 

CO SIP values5 shows that the re-generated start emissions on Monday, December 9 are 

approximately 5 TPD higher than the CO SIP start emissions. We tracked this back to a small 

error in the emissions processing conducted for the CO SIP modeling application; therefore, the 

values in Table 5-1 are correct. Table 5-3 shows the total (on-road + non-road + stationary) CO 

inventories for the entire UAM modeling domain. Figure 5-1 shows these results graphically for 

Monday, December 9. 

5.3 UAM MODELING RESULTS 

ENVIRON conducted a “roll-forward” projection of future CO concentrations that would result 

from the two gasoline options. The future years of 2006, 2010, 2015, 2020, and 2030 were 

analyzed. Specifically, we identified how each option would impact the forecasts of the 

 

4

 See Section 4.4 of the 2005 CO SIP Technical Support Document for a definition of this sub-domain (Clark 

County Board of Commissioners, 2005). 

5

 Table 2-15, Section 2.3.6 of the 2005 CO SIP Technical Support Document (Clark County Board of 

Commissioners, 2005). 
Final / January 17, 2008 

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Table 5-1. Domain-wide on-road CO emission components and totals (TPD) by future year. 

SIP EPA M6 Fuel#1 EPA M6 Fuel#1 Sierra M6 Fuel#8 EPA M6 Fuel# 8 Sierra M6 

Year Category 8-Dec 9-Dec 8-Dec 9-Dec 8-Dec 9-Dec 8-Dec 9-Dec 8-Dec 9-Dec 

2006 Intrazonal 0.56 0.89 0.65 1.05 0.58 0.94 0.74 1.18 0.66 1.06 

Starts 125.22 241.6 134.38 260.57 129.38 249.94 149.7 290.31 144.35 278.93 

Links 150.64 204.61 193.78 265.65 160.59 219.65 215.85 295.93 179.75 245.88 

Total 276.42 447.1 328.81 527.27 290.55 470.53 366.29 587.42 324.76 525.87 

2010 Intrazonal 0.83 1.35 0.97 1.58 0.86 1.4 1.06 1.72 0.93 1.52 

Starts 136.04 262.55 145.31 281.71 138.75 268 154.49 299.46 147.67 285.17 

Links 150.26 205.29 195.65 267.98 157.66 215.44 212.54 291.14 171.83 234.84 

Total 287.13 469.19 341.93 551.27 297.27 484.84 368.09 592.32 320.43 521.53 

2015 Intrazonal 0.67 1.08 0.78 1.27 0.68 1.1 0.84 1.35 0.72 1.16 

Starts 138.56 267.28 147.36 285.42 139.62 269.35 152.84 295.94 144.83 279.3 

Links 137.32 187.7 178.62 244.79 139.36 190.48 192.27 263.52 150.24 205.4 

Total 276.55 456.06 326.76 531.48 279.66 460.93 345.95 560.81 295.79 485.86 

2020 Intrazonal 0.72 1.16 0.83 1.35 0.72 1.16 0.88 1.42 0.76 1.22 

Starts 139.35 268.58 147.88 286.13 139.77 269.41 150.67 291.37 142.35 274.24 

Links 133.15 182 173.12 237.18 134.03 183.21 184.39 252.67 142.89 195.37 

Total 273.22 451.74 321.83 524.66 274.52 453.78 335.94 545.46 286 470.83 

2030 Intrazonal 0.71 1.14 0.82 1.34 0.71 1.14 0.86 1.39 0.74 1.19 

Starts 152.68 294.3 161.83 313.08 152.68 294.3 163.94 317 154.6 297.84 

Links 142.66 194.67 185.4 253.52 142.66 194.67 196.75 269.09 151.55 206.85 

Total 296.05 490.11 348.05 567.94 296.05 490.11 361.55 587.48 306.89 505.88 

Table 5-2. Domain-wide and central sub-domain total on-road CO emissions (TPD) by future 

year. Only values for Monday, December 9 are shown. 

Year 

SIP 

EAP M6 

Fuel #1 

EPA M6 

Fuel #1 

Sierra M6 

Fuel #8 

EPA M6 

Fuel #8 

Sierra M6 

2006 Total 447.10 527.27 470.53 587.42 525.87 

Sub-domain 352.43 415.63 370.94 463.08 414.61 

2010 Total 469.19 551.27 484.84 592.32 521.53 

Sub-domain 350.36 412.19 362.18 443.01 389.68 

2015 Total 456.06 531.48 460.93 560.81 485.86 

Sub-domain 323.73 378.27 327.23 399.30 345.09 

2020 Total 451.74 524.66 453.78 545.46 470.83 

Sub-domain 311.48 363.14 312.94 377.83 324.92 

2030 Total 490.11 567.94 490.11 587.48 505.88 

Sub-domain 320.50 373.17 320.50 386.39 331.11 

Table 5-3. Total domain-wide CO emissions (TPD) by future year used for UAM modeling of 

each fuel scenario. 

SIP EPA M6 

Fuel #1 

EPA M6 

Fuel #1 

Sierra M6 

Fuel #8 

EPA M6 

Fuel #8 

Sierra M6 

Year 8-Dec 9-Dec 8-Dec 9-Dec 8-Dec 9-Dec 8-Dec 9-Dec 8-Dec 9-Dec 

2006 392.65 587.26 446.04 667.42 407.79 610.69 483.53 727.57 441.90 666.03 

2010 415.89 622.46 470.69 704.54 426.03 638.10 496.85 745.59 449.19 674.80 

2015 417.84 624.20 468.06 699.62 420.94 629.07 487.22 728.95 437.07 654.00 

2020 427.79 635.99 476.42 708.91 429.10 638.03 490.51 729.71 440.59 655.07 

2030 477.39 706.83 529.39 784.66 477.38 706.83 542.89 804.22 488.23 722.60 
Final / January 17, 2008 

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Domain-Wide Total CO Emissions

Monday, December 9

0

100

200

300

400

500

600

700

800

900

2006 2010 2015 2020 2030

Year

TPD

SIP EPA M6 Fuel #1 EPA M6 Fuel #1 Sierra M6 Fuel #8 EPA M6 Fuel #8 Sierra M6

Figure 5-1. Total domain-wide CO emissions (TPD) by future year used for UAM modeling of 

each fuel scenario. Weekday values are shown.

historical exceedance-level CO episode in each future year as modeled in the CO SIP. The 

mobile source inventories for each gasoline option were first combined with the other stationary 

and non-road emission components, and the resulting total CO inventory was then run through 

the UAM. The modeled impact on simulated peak CO relative to the NAAQS is reported in 

Table 5-4. Note that the ~5 TPD error found for start emissions on December 9 has no impact on 

the peak CO reported for the “SIP” scenario, as these numbers are identical to the CO SIP 

values. 

As this task was originally intended to demonstrate, in a general manner, the relationship 

between projected on-road emissions and resulting peak CO concentrations over the entire 

domain, hotspot and airport modeling were not regenerated for the fuel scenarios analyzed here. 

Furthermore, the procedure of separately scaling up on-road emissions within and outside of the 

central sub-domain was not performed as it was in the CO SIP6. 

The 2006 and 2010 “sub-domain” emission totals shown in Table 5-2 for Scenario #8 using the 

EPA MOBILE6 model are higher than the maximum scaled-up CO sub-domain budgets shown 

in Table 4-8 of the 2005 CO SIP Technical Support Document. One might expect that sub- 

 

6

 Increasing on-road emissions within and outside of the central sub-domain was performed in the modeling 

conducted for the SIP revision to demonstrate continued maintenance of the CO standard while allowing for larger 

separate emission budgets within the sub-domain and in outer areas (see Table 4-8 of the 2005 CO SIP Technical 

Support Document). 
Final / January 17, 2008 

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Table 5-4. Peak predicted 8-hour CO (ppm) from the UAM CO SIP modeling system. The 

“SIP” column shows the replicated CO SIP values. The 8-hour CO NAAQS is 9 ppm. 

Year 

SIP 

EPA M6 

Fuel #1 

EPA M6 

Fuel #1 

Sierra M6 

Fuel #8 

EPA M6 

Fuel #8 

Sierra M6 

2006 7.4 7.8 7.7 8.6 8.5 

2010 7.2 7.6 7.4 8.1 7.9 

2015 6.5 6.8 6.6 7.2 6.9 

2020 6.7 7.1 6.7 7.2 6.8 

2030 8.0 8.3 8.0 8.4 8.0 

domain emissions that exceed the budgets established in Table 4-8 should result in peak CO 

exceeding the 9 ppm standard. Table 4-8 is based on the simple upward linear scaling of mobile 

source CO emissions according to the individual factors listed for each future year. However, 

the fuel scenarios run in the current assessment changed the spatial characterization of CO 

emissions. This is because removing the RVP limit leads to different effects to start and running 

exhaust emissions, and different start distributions are defined within the modeling domain (one 

for Las Vegas Boulevard, and one for everywhere else). Since the spatial start distribution 

causes a change in the spatial increase of CO emissions, it is possible to achieve higher subdomain CO emissions than shown in Table 4-8 that result in peak CO concentrations below the 9 

ppm standard. In other words, peak CO responds not only to emission increases, but also to 

where the CO is increased. 

The higher sub-domain emissions shown in Table 5-2 for 2006 and 2010 under fuel Scenario #8 

using EPA’s MOBILE6 model also has ramifications for the hot-spot and airport modeling 

results reported in the 2005 CO SIP.7 As stated above, we did not re-run the hot-spot and airport 

models for this analysis. Instead, we developed a simpler approach in which we scaled the 2006 

and 2010 hot-spot and airport results from Tables 4-4 and 4-5 in the Technical Support 

Document by the relative increase in the on-road sub-domain emissions shown in Table 5-2 of 

this report. The resulting Scenario #8 scaling factors are 1.31 for 2006 and 1.27 for 2010, based 

on the weekday (December 9) emissions growth. For the hot-spot intersections, we scaled both 

the UAM and hot-spot model results since they are almost entirely associated with on-road CO 

emissions. For the airport results, we scaled only the UAM component, since the airport model 

results are dominated by CO emissions from aircraft and stationary CO sources that would not be 

impacted by a change in gasoline as in Scenario #8. Note that this scaling approach is 

conservative since it is based on on-road emissions growth only, while developing a scaling 

approach based on the total sub-domain emissions (to include stationary and area sources) would 

reduce the scaling factors.

Table 5-5 displays the scaled hot-spot modeling results and Table 5-6 displays the scaled airport 

model results to reflect Scenario #8 with the EPA MOBILE6 model. With the scaling applied, 

peak 8-hour CO concentrations remain well below the 9 ppm standard for both years. 

 

7

 See Tables 4-4 and 4-5 in the 2005 CO SIP Technical Support Document. 
Final / January 17, 2008 

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Table 5-5. Peak 8-hour average CO concentrations (ppm) predicted by UAM, CAL3QHC, and 

UAM+CAL3QHC for each of the Five Points intersections. Note that peaks reported for each of 

the models and their combined effects occur over different 8-hour periods. Results are based 

on scaling factors developed from the sub-domain on-road emissions differences shown in 

Table 5-2 for fuel Scenario #8 using EPA MOBILE6. 

Eastern/Charleston Eastern/Fremont Fremont/Charleston Year UAM CAL3QHC CAL3QCH+UAM CAL3QHC CAL3QHC+UAM CAL3QHC CAL3QHC+UAM 

2006 6.41 2.15 8.04 1.68 7.41 0.93 6.67 

2010 5.87 1.69 7.12 1.45 6.76 0.88 6.11 

Table 5-6. Peak total UAM, EDMS, and total UAM+EDMS 8-hour CO concentrations (ppm) at 

all three airports evaluated. Values shown for McCarran airport occur at the peak publicly 

accessible receptor. Results are based on scaling factors developed from the sub-domain onroad emissions differences shown in Table 5-2 for fuel Scenario #8 using EPA MOBILE6. 

Airport 2006 2010 

McCarran Total (CO SIP) 7.47 7.14 

 EDMS (CO SIP) 6.21 5.89 

 UAM (CO SIP) 1.26 1.25 

 UAM (Scenario #8) 1.65 1.58 

 Scenario #8 Total 7.86 7.48 

Henderson Executive (CO SIP) 1.12 1.36 

 EDMS (CO SIP) 0.42 0.42 

 UAM (CO SIP) 0.70 0.94 

 UAM (Scenario #8) 0.91 1.20 

 Scenario #8 Total 1.34 1.61 

North Las Vegas (CO SIP) 5.01 5.04 

 EDMS (CO SIP) 0.40 0.36 

 UAM (CO SIP) 4.61 4.68 

 UAM (Scenario #8) 6.04 5.94 

 Scenario #8 Total 6.44 6.30 
Final / January 17, 2008 

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6. REGULATORY IMPACTS OF WINTER GASOLINE OPTIONS 

After discussions with DAQEM on the preferred winter and transitional period gasoline options 

for Clark County, the regulatory impacts of select winter gasoline scenarios were evaluated. All 

relevant federal, state, and county regulations were reviewed, and the required modifications to 

address the winter gasoline scenarios and remove overlaps and conflicts between the laws, 

statutes, and regulations were identified. The following winter gasoline scenarios were selected 

for evaluation: 

• Elimination of the 9 psi winter RVP requirement (Fuel Scenario 1); 

• Elimination of RVP and oxygenate, aromatic and sulfur content requirements (Fuel 

Scenario 8); and 

• Elimination of the RVP, aromatic and sulfur content requirements (Fuel Scenario 8) and 

a shortening of the winter oxygenate period to November through January. 

The existing statutes and regulations and the required modifications associated with the 

aforementioned winter gasoline scenarios are discussed below. 

6.1 REGULATIONS ON CLARK COUNTY GASOLINE 

The current requirements imposed on winter gasoline in Clark County are summarized in Table 

4-1. Following is a discussion of each fuel property standard on the federal, state, and local 

levels. 

Reid Vapor Pressure (RVP): The RVP specification in Clark County is imposed by Nevada 

statute8

 between October 1 and March 31. The RVP limit of 9 psi (10.0 psi for summertime 

ethanol gasoline) is more stringent than the ASTM performance standards in Nevada during the 

winter, and no federal standards exist for winter conventional gasoline. Upon elimination of the 

RVP limit in the Nevada Administrative Code, the ASTM limits for southern Nevada would 

apply. 

Oxygen Content: The minimum oxygen content of 3.5% by weight for Clark County winter 

gasoline is required under Section 53 of the Clark County Air Quality Regulations during the 

months of October through March. This requirement is more stringent than the federal winter 

oxygenate requirement for Clark County. Because the Las Vegas Valley within Clark County is 

designated under the Clean Air Act (CAA) as a “serious” nonattainment area for the eight-hour 

CO National Ambient Air Quality Standard (NAAQS) during the winter, a federal minimum 

oxygen content requirement of 2.7% by weight is mandated.9 Within the same CAA section, a 

minimum four-month period of oxygenate use is required during the CO nonattainment period, 

unless “the State can demonstrate that because of meteorological conditions, a reduced period 

 

8

 Nevada Administrative Code § 590.065 

9

 Clean Air Act Title § 211(m)(2) 
Final / January 17, 2008 

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will assure that there will be no exceedances of the carbon monoxide standard outside of such 

reduced period.” 

Clark County has had no violations of the CO standard NAAQS since 1998 and it appears that 

the County could seek redesignation to attainment. Redesignation could allow the elimination of 

the winter oxygenate requirement. 

Aromatics and Sulfur Content: The aromatic and sulfur content specifications for Clark County 

winter gasoline are found in Section 54 of the Clark County Air Quality Regulations, and apply 

between November 1 and March 31. According to recently promulgated federal regulations 

(Federal Register, 2006), winter gasoline sold in Clark County is considered to be a “boutique” 

fuel due to the local controls on sulfur and aromatics. These federal regulations, which are 

intended to restrict the creation of new boutique fuels, preclude any changes to the Clark County 

winter aromatic and sulfur limits except for their elimination. Further, once eliminated, these 

limits cannot be reinstated. The Clark County sulfur specification, however, has been rendered 

redundant by the more stringent federal Tier 2 sulfur standard. 

6.2 REGULATORY IMPACTS OF GASOLINE OPTIONS 

The regulatory impacts of the winter gasoline options affect the one Nevada statute and two 

Clark County Air Quality Regulations identified above. Any additional federal, state, or local 

statutes or regulations that would be impacted were not identified. The required changes to 

existing statues and regulations necessary to implement the selected scenarios are discussed 

below. 

Elimination of 9 psi RVP Limit: The amendments to the Nevada Administrative Code (NAC) 

necessary to eliminate the existing 9 psi winter RVP limit in Clark County are shown in track 

changes in Appendix A. In addition, the language of the statute was updated to reflect the most 

recent version of the ASTM gasoline standards. 

Elimination of Other Winter Gasoline Requirements: As noted above, the oxygenate, sulfur, and 

aromatic content requirements for winter gasoline in Clark County are specified in Sections 53 

and 54 of the Clark County Air Quality Regulations. Elimination of these requirements could be 

achieved by the elimination of these two sections of the Clark County Air Quality Regulations. 

In addition to repeal of Sections 53 and 54, it is important to note that the Nevada State 

Implementation Plan (SIP), which was approved by EPA to include winter Cleaner Burning 

Gasoline, will need to be revised in accordance with Title 40 of the Code of Federal Regulations 

(CFR) Part 51 in order to completely eliminate the sulfur and aromatic requirements. 

Shortening of the Winter Oxygenated Gasoline Period: Shortening of the winter oxygenated 

gasoline period to the months of November through January would require modifying Section 53 

of the Clark County Air Quality regulations. The necessary modifications are shown in track 

changes in Appendix B. 

 
Final / January 17, 2008 

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REFERENCES 

Alliance of Automobile Manufacturers. 2006. “Alliance of Automobile Manufacturers North 

American Fuel Survey Motor Gasoline Winter 2006.” 

ASTM. 2006. “Standard Specification for Automotive Spark-Ignition Fuel.” ASTM Standard 

D 4814-06, ASTM International (November 2006). 

Department of Comprehensive Planning, Clark County Board of Commissioners, 2000. Carbon 

Monoxide State Implementation Plan, Las Vegas Valley Nonattainment Area, Clark 

County, Nevada. August. 

Department of Air Quality and Environmental Management, Clark County Board of 

Commissioners, 2005. Carbon Monoxide State Implementation Plan Revision, Las 

Vegas Valley Nonattainment Area, Clark County, Nevada. October. 

Federal Register. 2006. “Boutique Fuels List Under Section 1541(b) of the Energy Policy Act.” 

Notice, Federal Register, Volume 71, Number 249, Pages 78192-78199 (December 28, 

2006). 

Sierra Research. 2003. “Review of Current and Future CO Emissions from On-Road Vehicles 

in Selected Western Areas.” Prepared for Western States Petroleum Association, by 

Sierra Research, Inc., Report No. SR03-01-01 (January 28, 2003). 

Sierra Research. 2005. “Impacts of Eliminating Maricopa County Wintertime Gasoline 

Standards on Emissions and Ambient Concentrations of CO in February and March.” 

Prepared for the Arizona Department of Environmental Quality by Sierra Research, Inc. 

(January 24, 2005).