Office of Transportation

and Air Quality

EPA420-P-04-016

November 2004

Update of Methane and

Nitrous Oxide Emission

Factors for On-Highway

Vehicles
EPA420-P-04-016

November 2004

Update of Methane and Nitrous Oxide

Emission Factors for On-Highway Vehicles

Assessment and Standards Division

Office of Transportation and Air Quality

U.S. Environmental Protection Agency

Prepared for EPA by

ICF Consulting

EPA Contract No. 68-W-99-054

Work Assignment No. WA4-37
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Update of Methane and Nitrous Oxide Emission Factors for On-Highway Vehicles Draft Report 

CONTENTS 

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

1.1 Purpose............................................................................................................................1

1.2 Previous Emission Factors ............................................................................................1

1.3 Definitions of Emission Control Technologies and Standards...................................1

2 METHODOLOGY................................................................................................ 4

3 RESULTS ........................................................................................................... 6

3.1 Nitrous Oxide Emission Factors....................................................................................6

3.1.1 N2O Federal Test Procedure Results ...............................................................................................6

3.1.2 N2O Running and Start Emissions ..................................................................................................7

3.1.3 N2O HR505 Comparisons ...............................................................................................................8

3.2 Methane Emission Factors.............................................................................................9

3.2.1 CH4 Federal Test Procedure Results ...............................................................................................9

3.2.2 CH4 Running and Start Emissions.................................................................................................11

3.2.3 CH4 HR505 Comparisons .............................................................................................................12

4 EMISSION ESTIMATES FOR OTHER CATERGORIES.................................. 15

4.1 Nitrous Oxide Emission Factor Estimates..................................................................15

4.1. N2O Estimates for Light-Duty Gasoline Vehicles.........................................................................15

4.1. N2O Estimates for Light-Duty Gasoline Trucks............................................................................15

4.1. N2O Estimates for Heavy-Duty Gasoline Trucks..........................................................................17

4.1. N2O Estimates for Motorcycles.....................................................................................................17

4.1. N2O Estimates for Heavy-Duty Diesel Vehicles...........................................................................18

4.1. N2O Estimates for Light-Duty Diesel Cars and Trucks.................................................................18

4.2 Methane Emission Factor Estimates...........................................................................18

4.2. CH4 Estimates for Light-Duty Gasoline Vehicles.........................................................................19

4.2. CH4 Estimates for Light-Duty Gasoline Trucks............................................................................19

4.2. CH4 Estimates for Heavy-Duty Gasoline Trucks..........................................................................19

4.2. CH4 Estimates for Motorcycles.....................................................................................................20

4.2. CH4 Estimates for Heavy-Duty Diesel Vehicles ...........................................................................20

4.2. CH4 Estimates for Light-Duty Diesel Cars and Trucks.................................................................20

5 RECOMMENDED EMISSION FACTORS FOR ON-HIGHWAY VEHICLES .... 22

APPENDICES ......................................................................................................................... 23

A Description of Test Data ...............................................................................................23 

B Response To Peer Review Comments From Thomas Durbin...................................24 

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Update of Methane and Nitrous Oxide Emission Factors for On-Highway Vehicles Draft Report 

LIST OF TABLES 

Table 1. Emission Factors for N2O and CH4 for Highway Vehicles..............................................2

Table 2. Driving Cycles ................................................................................................................4

Table 3. N2O Emission Factors for the Federal Test Procedure..................................................6

Table 4. N2O Emission Factors Comparisons against previous IPCC values .............................6

Table 5. N2O Running and Start Emission Factors using Equations 2 and 3 ..............................7

Table 6. N2O Running and Start Emission Factors using Equations 5 and 6 ..............................7

Table 7. HR505 Comparison with Bag 2 Running emissions in grams per mile..........................8

Table 8. HR505 Comparison with Bag 2 Running emissions in grams per hour .........................8

Table 9. CH4 Emission Factors for the Federal Test Procedure ................................................10

Table 10. CH4 Emission Factors Comparisons against previous IPCC values..........................10

Table 11. CH4 Running and Start Emission Factors using Equations 2 and 3 ..........................11

Table 12. CH4 Running and Start Emission Factors using Equations 5 and 6 ..........................11

Table 13. HR505 Comparison with Bag 2 Running emissions in grams per mile......................13

Table 14. HR505 Comparison with Bag 2 Running emissions in grams per hour .....................13

Table 15. CO2 Emissions in grams per mile for Vehicle Types and Emission Control 

Technologies ..............................................................................................................16

Table 16. Estimated N2O Emission Factors for Light-Duty Gasoline Vehicles ..........................17

Table 17. Estimated N2O Emission Factors for Light-Duty Gasoline Trucks .............................17

Table 18. Estimated N2O Emission Factors for Heavy-Duty Gasoline Trucks...........................17

Table 19. Estimated N2O Emission Factors for Motorcycles .....................................................18

Table 20. Estimated N2O Emission Factors for Heavy-Duty Diesel Vehicles ............................18

Table 21. Estimated N2O Emission Factors for Light-Duty Diesel Vehicles and Trucks............18

Table 22. Estimated CH4 Emission Factors for Light-Duty Gasoline Vehicles...........................19

Table 23. Estimated CH4 Emission Factors for Light-Duty Gasoline Trucks .............................19

Table 24. Estimated CH4 Emission Factors for Heavy-Duty Gasoline Trucks ...........................20

Table 25. Estimated CH4 Emission Factors for Motorcycles......................................................20

Table 26. Estimated CH4 Emission Factors for Heavy-Duty Diesel Vehicles ............................20

Table 27. Estimated CH4 Emission Factors for Light-Duty Diesel Vehicles and Trucks ............21

Table 28. Recommended Values for N2O and CH4 Emission Factors.......................................22

LIST OF FIGURES 

Figure 1. Comparison of Start Emissions using two methods .....................................................8 

Figure 2. Comparison of Running Emissions in grams per mile ..................................................9

Figure 3. Comparison of Running Emissions in grams per hour .................................................9

Figure 4. Comparison of Start Emissions for light-duty vehicles using two methods.................12

Figure 5. Comparison of Start Emissions for light-duty vehicles using two methods.................12

Figure 6. Comparison of Running Emissions in grams per mile ................................................14

Figure 7. Comparison of Running Emissions in grams per hour ...............................................14

Contract 68-W-99-054, WA 4-37 ii ICF Consulting
Update of Methane and Nitrous Oxide Emission Factors for On-Highway Vehicles Draft Report 

1 INTRODUCTION 

1.1 Purpose 

The U.S. Environmental Protection Agency Office of Transportation and Air Quality (OTAQ) are 

currently developing a new mobile source emissions factor model called MOVES. This new model will 

estimate greenhouse gas (GHG) emissions for highway vehicles and will be incorporated into 

transportation GHG inventory development. Besides other improvements in the methodology, the model 

will use updated emission factors for nitrous oxide (N2O) and methane (CH4). While MOVES is 

somewhat behind schedule, data to update N2O and CH4 emission factors are available for this year’s 

inventory. These revised emission factors will be incorporated into the model itself. 

1.2 Previous Emission Factors 

Emission factors used in the US EPA Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-

20011

 are listed in Table 1 and are taken from Annex E of that document. It states “The EPA does not 

systematically track emissions of CH4 and N2O; therefore, estimates of these gases were developed using 

a methodology similar to that outlined in the Revised1996 IPCC Guidelines2

” Many of these values will 

be updated with new information detailed in this report. In addition, MOVES specifies separate running 

and start emissions, which are combined in the emission factors shown in Table 1. 

1.3 Definitions of Emission Control Technologies and Standards 

The N2O and CH4 emission factors used depend on the emission standards in place and the corresponding 

level of control technology for each vehicle type. The definitions of these control technologies are listed 

in Annex E of the EPA Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-20011 and 

reproduced here: 

Uncontrolled (Unc) -- Vehicles manufactured prior to the implementation of pollution control 

technologies are designated as uncontrolled. Gasoline light-duty cars and trucks (pre-1973), gasoline 

heavy-duty vehicles (pre-1984), diesel vehicles (pre-1983), and motorcycles (pre-1996) are assumed to 

not have significant control technologies in place. 

Non-catalyst (Ncat) -- These emission controls were common in gasoline passenger cars and light-duty 

gasoline trucks during model years (1973-1974) but phased out thereafter, in heavy-duty gasoline vehicles 

beginning in the mid-1980s, and in motorcycles beginning in 1996. This technology reduces hydrocarbon 

(HC) and carbon monoxide (CO) emissions through adjustments to ignition timing and air-fuel ratio, air 

injection into the exhaust manifold, and exhaust gas recirculation (EGR) valves, which also helps meet 

vehicle NOx standards. 

Oxidation catalyst (Ocat) -- This control technology designation represents the introduction of the 

catalytic converter, and was the most common technology in gasoline passenger cars and light-duty 

gasoline trucks made from 1975 to 1980 (cars) and 1975 to 1985 (trucks). This technology was also used 

1

 EPA. “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2001,” Report EPA 430-R-03-004, April 2003. 2 Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories, Paris: Intergovernmental Panel on 

Climate Change, United Nations Environment Programme, Organization for Economic Co-Operation and 

Development, International Energy Agency. 

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Table 1. Previous Emission Factors for N2O and CH4 for Highway Vehicles 

Vehicle Type/Control Technology 

N2O 

(g/mi) 

CH4 

(g/mi) 

Gasoline Passenger Cars (LDGV) 

Low Emission Vehicles 0.0283 0.0402 

EPA Tier 1 a 0.0463 0.0483 

EPA Tier 0 a 0.0816 0.0644 

Oxidation Catalysts 0.0518 0.1126 

Non-Catalyst 0.0166 0.1931 

Uncontrolled 0.0166 0.2173 

Gasoline Light-Duty Trucks (LDGT) 

Low Emission Vehicles 0.0354 0.0483 

EPA Tier 1 a 0.0581 0.0563 

EPA Tier 0 a 0.1022 0.1126 

Oxidation Catalysts 0.0649 0.1448 

Non-Catalyst 0.0208 0.2253 

Uncontrolled 0.0208 0.2173 

Gasoline Heavy-Duty Vehicles (HDGV) 

Low Emission Vehicles 0.1133 0.0708 

EPA Tier 1 a 0.1394 0.0966 

EPA Tier 0 a 0.1746 0.1207 

Oxidation Catalysts b 0.1109 0.1448 

Non-Catalyst 0.0354 0.2012 

Uncontrolled 0.0354 0.4345 

Diesel Passenger Cars (LDDV) 

Advanced 0.0161 0.0161 

Moderate 0.0161 0.0161 

Uncontrolled 0.0161 0.0161 

Diesel Light Duty Trucks (LDDT) 

Advanced 0.0322 0.0161 

Moderate 0.0322 0.0161 

Uncontrolled 0.0322 0.0161 

Diesel Heavy Duty Vehicles (HDDV) 

Advanced 0.0483 0.0644 

Moderate 0.0483 0.0805 

Uncontrolled 0.0483 0.0966 

Motorcycles (Mot) 

Non-catalysts Control 0.0071 0.2092 

Uncontrolled 0.0071 0.4184 

Sources: IPCC/UNEP/OECD/IEA (1997), EPA (1998) a The categories “EPA Tier 0” and “EPA Tier 1” were substituted for the early three-way catalyst and advanced 

three-way catalyst categories, respectively, as defined in the Revised 1996 IPCC Guidelines. Detailed descriptions 

of emissions control technologies are provided at the end of this annex. b The methane emission factor was assumed based on the oxidation catalyst value for gasoline light-duty trucks. 

in some heavy-duty gasoline vehicles between 1982 and 1997. The two-way catalytic converter oxidizes 

HC and CO, significantly reducing emissions over 80 percent beyond non-catalyst-system capacity. One 

reason unleaded gasoline was introduced in 1975 was due to the fact that oxidation catalysts cannot 

function properly with leaded gasoline. 

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EPA Tier 0 (T0) --This emission standard from the Clean Air Act was met through the implementation of 

early “three-way” catalysts, therefore this technology was used in gasoline passenger cars and light-duty 

gasoline trucks sold beginning in the early 1980s, and remained common until 1994. This more 

sophisticated emission control system improves the efficiency of the catalyst by converting CO and HC to 

CO2 and H2O, reducing NOx to nitrogen and oxygen, and using an on-board diagnostic computer and 

oxygen sensor. In addition, this type of catalyst includes a fuel metering system (carburetor or fuel 

injection) with electronic “trim” (also known as a “closed-loop system”). New cars with three-way 

catalysts met the Clean Air Act’s amended standards (enacted in 1977) of reducing HC to 0.41 g/mile by 

1980, CO to 3.4 g/mile by 1981 and NOx to 1.0 g/mile by 1981. 

EPA Tier 1 (T1) -- This emission standard created through the 1990 amendments to the Clean Air Act 

limited passenger car NOx emissions to 0.4 g/mile, and HC emissions to 0.25 g/mile. These bounds 

represent a 60 and 40 percent reduction, respectively, from the EPA Tier 0 standard set in 1981. For 

light-duty trucks, this standard set emissions at 0.4 to 1.1 g/mile for NOx and 0.25 to 0.39 g/mile for HCs, 

depending upon the weight of the truck. Emission reductions were met through the use of more advanced 

emission control systems, and applied to light-duty gasoline vehicles beginning in 1994. This advanced 

emission control systems included advanced three-way catalysts, electronically controlled fuel injection 

and ignition timing, EGR, and air injection. 

Low Emission Vehicles (LEV) -- This emission standard requires a much higher emission control level 

than the Tier 1 standard. Applied to light-duty gasoline passenger cars and trucks beginning in small 

numbers in the mid-1990’s, LEV includes multi-port fuel injection with adaptive learning, an advanced 

computer diagnostics systems and advanced and close coupled catalysts with secondary air injection. 

LEVs as defined here include transitional low-emission vehicles (TLEVs), low emission vehicles, ultralow emission vehicles (ULEVs) and super ultra-low emission vehicles (SULEVs). In this analysis, all 

categories of LEVs are treated the same due to the fact that there are very limited CH4 or N2O emission 

factor data for LEVs to distinguish among the different types of vehicles. Zero emission vehicles (ZEVs) 

are incorporated into the alternative fuel and advanced technology vehicle assessments. 

Moderate control (Mod) -- Improved injection timing technology and combustion system design for 

light- and heavy-duty diesel vehicles (generally in place in model years 1983 to 1995) are considered 

moderate control technologies. These controls were implemented to meet emission standards for diesel 

trucks and buses adopted by the EPA in 1985 to be met in 1991 and 1994. 

Advanced control (Adv) -- EGR and modern electronic control of the fuel injection system are designated 

as advanced control technologies. These technologies provide diesel vehicles with the level of emission 

control necessary to comply with standards in place from 1996 through 2003. 

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2 METHODOLOGY 

Data obtained from EPA3 included testing on many of the vehicle type/control technology categories 

listed in Table 1. This data has been used to develop new overall emission factors as well as running and 

start emission profiles for MOVES. 

Overall emissions which compare directly to those listed in Table 1 were determined using the U.S. 

Federal Test Procedure (FTP). The FTP incorporates three driving segments in which the vehicle’s 

exhaust is captured in separate “bags,” one for each driving segment. Each bag is analyzed separately and 

then combined to calculate composite emissions. The formula to calculate composite emissions for the 

FTP emissions test is given below:4 

Composite = Bag1* 0.43 + Bag2 + Bag3 * 0.57 Equation 1

FTP dis tan ce 

Where FTP distance is approximately 7.44 miles 

The federal test procedure includes both starts and running emissions. The bag 1 segment starts from a 12 

hour soak at approximately 75oF and is driven over a transient driving cycle for 505 seconds with an 

average speed of 25.55 mph. It contains cold start emissions and running emissions. The bag 2 segment 

has no start and represents running emissions. Its length is 867 seconds with an average speed of 16.02 

mph. The third bag segment is a repeat of the bag 1 segment, but after only a 10 minute soak. This 

contains both hot start emissions and running emissions. In addition, some of the data included a hot 

running 505 second (HR505) driving cycle. This cycle contains only running emissions (no starts) during 

the same cycle used for Bag 1 and Bag 3. It can be used to calculate cold and hot start emissions from the 

Bag 1 and Bag 3 segments of the FTP. Since the HR505 cycle has an average speed of 25.55 mph and the 

Bag 2 driving cycle has an average speed of 16.02 mph, the two cycles could be used to determine speed 

factors at low speed. In this report, however, they are used to determine whether grams per mile or grams 

per hour are more constant over the low speed range. Details of the cycles are given in Table 2. 

Table 2. Driving Cycles 

Cycle 

Length Average

Speed

(mph) Start 

Time 

(seconds) 

Distance 

(miles) 

FTP a 1372 7.44 19.53 Cold/Hot 

Bag 1 505 3.58 25.55 Cold 

Bag 2 867 3.86 16.02 No 

Bag 3 505 3.58 25.55 Hot 

HR505 505 3.58 25.55 No a While the FTP actually lasts 1877 seconds, the bag 1 and bag 3 

results are multiplied by 43% and 57% respectively to represent cold 

start activity 43% of the time and hot start activity 57% of the time. 

3 The datasets received from EPA represented 13,277 FTP tests on 6,950 vehicles for methane emissions and 95 

FTP tests on 64 vehicles for nitrous oxide emissions. It also included 14,636 non-FTP tests on 2,963 vehicles for 

methane emissions and 232 non-FTP tests on 74 vehicles for nitrous oxide. The non-FTP tests included a hot 

running 505 as well as several other driving cycles not utilized in this report. Methane tests were performed in 

various U.S. locations during the period between April 1982 and June 2000. Nitrous oxide tests were performed in 

various U.S. locations during the period between January 2000 and June 1998. 4 Code of Federal Regulations Title 40: Protection of the Environment, Chapter 1, Part 600, Section 1134-78. 

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Since there are many more FTP tests than HR505 tests, FTP bag 2 emissions in grams per mile are being 

used in this report to calculate running emissions. To calculate start emissions, the running emissions 

were subtracted from the FTP emissions in grams per mile and multiplied by the length in miles of the 

FTP (approximately 7.44 miles). This provided average start emissions which combined both cold and 

hot start emissions. These are shown in Equations 2 and 3 below. Running emissions were then 

compared against the HR505 emission rate for vehicles in which both an FTP and HR505 were run. 

Running Emissions (g/mi) = Bag2 Emissions (g/mi) Equation 2 

Start Emissions (g/start) = (FTP Emissions – Bag 2 Emissions) x Actual FTP Distance Equation 3 

Another approach to calculate running emissions is to calculate them in grams per hour using the average 

speed of each cycle. Start emissions can then be calculated from the FTP emissions in grams per hour 

and the running emissions in grams per hour as shown in Equations 4 through 6. 

FTP Emissions (g/hr) = FTP Emissions (g/mi) 

x Actual FTP distance x 3600 second/hr / 1372 seconds Equation 4 

Running Emissions (g/hr) = Bag2 Emissions (g/mi) 

x Bag 2 distance x 3600 sec/hr / 867 seconds Equation 5 

Start Emissions (g/start) = (FTP Emissions (g/hr) – Running Emissions (g/hr)) 

x (1372/3600) hrs Equation 6 

Because the distribution of each set of tests varied, an arithmetic mean was used to determine the average 

of all tests. In addition to the arithmetic mean (Average), a standard deviation (SD) and a 95% 

confidence interval (95% CI) were also calculated for each set of data. Only data taken at the FTP 

temperature range (68oF to 86oF) were used in this analysis. Temperature correction factors using 

additional data at higher temperatures might be part of a later report. 

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3 RESULTS 

Emission factor results for nitrous oxide and methane for on-highway vehicles are discussed in this 

section. Only some of the vehicle type/emission tier categories produced statistically significant results 

within the 95% confidence interval. Those data are discussed here. Those categories that did not have 

enough data to produce statistically significant results within the 95% confidence interval are discussed in 

Section 4. Recommended emission factors for all categories are given in Section 5. 

3.1 Nitrous Oxide Emission Factors 

Emission factors for nitrous oxide are presented in this subsection. Emission results for the federal test 

procedure are discussed in Section 3.1.1, running and start emissions are discussed in Section 3.1.2, and 

comparisons of running emissions from Bag 2 with HR505 emissions are discussed in Section 3.1.3. 

3.1.1 N2O Federal Test Procedure Results 

Emission factors for N2O for on-highway vehicles are given in Table 3 for the federal test procedure. 

FTP emissions include both start and running emissions. Emission factors are displayed in both grams 

per mile and grams per hour. 

Table 3. N2O Emission Factors for the Federal Test Procedure 

Vehicle 

Type 

Emission 

Tier 

No of 

Test Pts 

FTP Emissions (g/mi) FTP Emissions (g/hr) FTP Dist 

Average SD 95% CI Average SD 95% CI (mi)

LDGV 

LEV 

T1 

T0 

7 

12 

12 

0.012 

0.030 

0.054 

0.009 

0.012 

0.050 

0.007 

0.007 

0.028 

0.245 

0.582 

1.057 

0.179 

0.243 

0.987 

0.133 

0.138 

0.559 

7.491 

7.472 

7.494 

LDGT 

LEV 

T1 

5 

16 

0.009 

0.067 

0.007 

0.061 

0.006 

0.030 

0.178 

1.321 

0.135 

1.193 

0.118 

0.584 

7.489 

7.466 

HDDV Adv 6 0.005 0.001 0.001 0.096 0.016 0.012 7.470 

The gram per mile emission factors are compared against Table 1 IPCC emission factors in Table 4. As 

can be seen from this table, the newly calculated emission factors are in most cases lower than previous 

values. This is most likely because newer technologies are represented in the dataset versus those used to 

derive the IPCC factors. The newer values better represent the current vehicle fleet. 

Table 4. N2O Emission Factors Comparisons against previous IPCC values 

Vehicle 

Type 

Emission 

Tier 

Emission Factors (g/mi) 

IPCC This Study 

LDGV 

LEV 

T1 

T0 

0.028 

0.046 

0.082 

0.012 

0.030 

0.054 

LDGT 

LEV 

T1 

0.035 

0.058 

0.009 

0.067 

HDDV Adv 0.048 0.005 

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3.1.2 N2O Running and Start Emissions 

Running emissions in grams per mile and start emissions in grams per start, calculated using Equations 2 

and 3, are provided in Table 5 along with standard deviations (SD) and 95% confidence intervals (95% 

CI). Running emissions in grams per hour and start emissions in grams per start, calculated using 

Equations 5 and 6, are provided in Table 6. 

Table 5. N2O Running and Start Emission Factors using Equations 2 and 3 

Vehicle 

Type 

Emission 

Tier 

Running Emissions (g/mi) Start Emissions (g/start) 

Average SD 95% CI Average SD 95% CI 

LDGV 

LEV 

T1 

T0 

0.000 

0.015 

0.042 

0.001 

0.014 

0.044 

0.001 

0.008 

0.025 

0.090 

0.113 

0.092 

0.063 

0.056 

0.107 

0.046 

0.032 

0.060 

LDGT 

LEV 

T1 

0.001 

0.041 

0.002 

0.052 

0.002 

0.025 

0.059 

0.200 

0.036 

0.154 

0.032 

0.076 

HDDV Adv 0.005 0.001 0.001 -0.002 0.003 0.003 

Table 6. N2O Running and Start Emission Factors using Equations 5 and 6 

Vehicle 

Type 

Emission 

Tier 

Running Emissions (g/hr) Start Emissions (g/start) 

Average SD 95% CI Average SD 95% CI 

LDGV 

LEV 

T1 

T0 

0.007 

0.235 

0.671 

0.018 

0.218 

0.716 

0.013 

0.123 

0.405 

0.091 

0.132 

0.147 

0.064 

0.051 

0.134 

0.047 

0.029 

0.076 

LDGT 

LEV 

T1 

0.019 

0.652 

0.035 

0.835 

0.031 

0.409 

0.061 

0.255 

0.039 

0.186 

0.034 

0.091 

HDDV Adv 0.083 0.019 0.015 0.005 0.002 0.001 

Comparisons of start emissions using the two methods (g/mi and g/hr) are shown in Figure 1. As can be 

seen from this figure, the start emissions calculated using the two methods are statistically similar within 

the 95% confidence interval, except for the heavy-duty diesel vehicle. In that case assuming no start 

emissions is a good assumption. 

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Figure 1. Comparison of Start Emissions using two methods 

0.40

g/mi method 

0.30 g/hr method 

0.20 

0.10 

0.00 

LEV T1 T0 LEV T1 Adv 

LDGV LDGT HDDV 

3.1.3 N2O HR505 Comparisons 

Several of the vehicles tested in the dataset provided by EPA also included hot running 505 test emissions 

along with the FTP emissions tests. Since not all vehicles in the dataset also included HR505 tests, 

comparisons were made for those vehicles that did have both tests. Since the HR505 test contains no 

starts, it is equivalent to a running emission. The HR505 emissions are compared in terms of grams per 

mile in Table 7 and grams per hour in Table 8 to the FTP Bag 2 running emissions presented in Section 

3.1.2. 

Table 7. HR505 Comparison with Bag 2 Running emissions in grams per mile 

Start E

missio

n

s (g/start) 

Vehicle 

Type 

Emission 

Tier 

No of 

Test Pts 

FTP Bag 2 Emissions (g/mi) HR505 Emissions (g/mi) 

Average SD 95% CI Average SD 95% CI 

LDGV T1 9 0.018 0.014 0.009 0.022 0.016 0.010 

LDGT T1 12 0.052 0.056 0.032 0.059 0.054 0.031 

Table 8. HR505 Comparison with Bag 2 Running emissions in grams per hour 

Vehicle 

Type 

Emission 

Tier 

No of 

Test Pts 

FTP Bag 2 Emissions (g/hr) HR505 Emissions (g/hr) 

Average SD 95% CI Average SD 95% CI 

LDGV T1 9 0.295 0.219 0.143 0.574 0.400 0.261 

LDGT T1 12 0.827 0.898 0.508 1.508 1.397 0.791 

Running emissions are compared on a grams per mile basis in Figure 2 and a grams per hour basis in 

Figure 3. As can be seen in those figures the Bag 2 running emissions in either grams per mile or grams 

per hour are statistically similar to the HR505 running emissions. In absolute value, however, the bag 2 

emission levels in grams per mile are closer to the HR505 emission levels in grams per mile than the 

comparison in grams per hour. Thus, it is suggested that emission rates in grams per mile be used for the 

low speed case. Conversion to grams per hour should be done at FTP speed, which is somewhere 

between the Bag 2 and HR505 speeds. 

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Figure 2. Comparison of Running Emissions in grams per mile 

0.10 

0.08 

0.06 

0.04 

0.02 

0.00 

LDGV LDGT 

Bag 2 

HR505 

Figure 3. Comparison of Running Emissions in grams per hour 

2.50 

2.00 

1.50 

1.00 

0.50 

0.00 

LDGV LDGT 

Bag 2 

HR505 

3.2 Methane Emission Factors 

Emission factors for methane are presented in this section. Emission results for the federal test procedure 

are discussed in Section 3.2.1, running and start emissions are discussed in Section 3.2.2, and 

comparisons of running emissions from Bag 2 against HR505 emissions are discussed in Section 3.2.3. 

3.2.1 CH4 Federal Test Procedure Results 

Emission factors for CH4 for on-highway vehicles are given in Table 9 for the federal test procedure. The 

federal test procedure combines both start and running emissions. Emission factors are displayed in both 

grams per mile and grams per hour. 

Running Emissions (g/hr) Running E

missions (

g/mi) 

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Table 9. CH4 Emission Factors for the Federal Test Procedure 

Vehicle 

Type 

Emission 

Tier 

No of 

Test Pts 

FTP Emissions (g/mi) FTP Emissions (g/hr) FTP Dist 

Average SD 95% CI Average SD 95% CI (mi)

LDGV 

LEV 

T1 

T0 

Ocat 

Ncat 

7 

131 

9504 

690 

20 

0.013 

0.020 

0.066 

0.133 

0.162 

0.006 

0.010 

0.087 

0.129 

0.130 

0.005 

0.002 

0.002 

0.010 

0.057 

0.254 

0.383 

1.294 

2.609 

3.199 

0.121 

0.193 

1.711 

2.536 

2.552 

0.089 

0.033 

0.034 

0.189 

1.118 

7.434 

7.453 

7.457 

7.475 

7.509 

LDGT 

LEV 

T1 

T0 

Ocat 

10 

80 

1666 

455 

0.017 

0.034 

0.071 

0.143 

0.016 

0.018 

0.067 

0.112 

0.010 

0.004 

0.003 

0.010 

0.327 

0.672 

1.396 

2.804 

0.306 

0.358 

1.308 

2.185 

0.190 

0.078 

0.063 

0.201 

7.443 

7.441 

7.449 

7.455 

HDGV 

T1 

T0 

Ocat 

36 

101 

90 

0.047 

0.218 

0.209 

0.018 

0.115 

0.076 

0.006 

0.022 

0.016 

0.904 

4.230 

4.018 

0.361 

2.213 

1.453 

0.118 

0.432 

0.300 

7.401 

7.393 

7.354 

HDDV Adv 8 0.004 0.003 0.002 0.081 0.049 0.034 7.471 

The gram per mile emission factors are compared against the Table 1 IPCC emission factors in Table 10. 

As can be seen from this table, the newly calculated emission factors are in most cases lower than 

previous values. This is most likely because newer technologies are represented in the dataset versus 

those used to derive the IPCC factors. The newer values better represent the current vehicle fleet. In the 

HDGV case, however, the results from the data analysis were higher than those developed by IPCC, 

except for the EPA Tier 1 case. Since the IPCC HDGV values were estimated from the light-duty 

gasoline vehicle values based upon fuel economy, it is suggested that the newer values be used as these 

represent real test data. 

Table 10. CH4 Emission Factors Comparisons against previous IPCC values 

Vehicle 

Type 

Emission 

Tier 

Emission Factors (g/mi) 

IPCC This Study 

LDGV 

LEV 

T1 

T0 

Ocat 

Ncat 

0.040 

0.048 

0.064 

0.113 

0.193 

0.013 

0.020 

0.066 

0.133 

0.162 

LDGT 

LEV 

T1 

T0 

Ocat 

0.048 

0.056 

0.113 

0.145 

0.017 

0.034 

0.071 

0.143 

HDGV 

T1 

T0 

Ocat 

0.097 

0.121 

0.145 

0.047 

0.218 

0.209 

HDDV Adv 0.064 0.004 

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3.2.2 CH4 Running and Start Emissions 

Running emissions in grams per mile and start emissions in grams per start, calculated using Equations 2 

and 3, are provided in Table 11 along with standard deviations (SD) and 95% confidence intervals (95%

CI). Running emissions in grams per hour and start emissions in grams per start, calculated using 

Equations 5 and 6, are provided in Table 12. 

Table 11. CH4 Running and Start Emission Factors using Equations 2 and 3 

Vehicle 

Type 

Emission 

Tier 

Running Emissions (g/mi) Start Emissions (g/start) 

Average SD 95% CI Average SD 95% CI 

LDGV 

LEV 

T1 

T0 

Ocat 

Ncat 

0.009 

0.012 

0.062 

0.132 

0.155 

0.006 

0.011 

0.102 

0.155 

0.151 

0.004 

0.002 

0.002 

0.012 

0.066 

0.032 

0.055 

0.034 

0.009 

0.059 

0.024 

0.034 

0.192 

0.300 

0.298 

0.018 

0.006 

0.004 

0.022 

0.131 

LDGT 

LEV 

T1 

T0 

Ocat 

0.011 

0.023 

0.062 

0.130 

0.017 

0.019 

0.073 

0.125 

0.011 

0.004 

0.003 

0.012 

0.046 

0.082 

0.072 

0.099 

0.015 

0.040 

0.148 

0.250 

0.009 

0.009 

0.007 

0.023 

HDGV 

T1 

T0 

Ocat 

0.024 

0.194 

0.179 

0.020 

0.105 

0.066 

0.007 

0.020 

0.014 

0.163 

0.183 

0.215 

0.060 

0.263 

0.178 

0.020 

0.051 

0.037 

HDDV Adv 0.006 0.004 0.003 -0.011 0.011 0.008 

Table 12. CH4 Running and Start Emission Factors using Equations 5 and 6 

Vehicle 

Type 

Emission 

Tier 

Running Emissions (g/hr) Start Emissions (g/start) 

Average SD 95% CI Average SD 95% CI 

LDGV 

LEV 

T1 

T0 

Ocat 

Ncat 

0.139 

0.196 

0.989 

2.125 

2.500 

0.096 

0.173 

1.648 

2.500 

2.443 

0.018 

0.030 

0.033 

0.187 

1.071 

0.044 

0.071 

0.116 

0.184 

0.266 

0.024 

0.029 

0.143 

0.207 

0.231 

0.008 

0.005 

0.003 

0.015 

0.101 

LDGT 

LEV 

T1 

T0 

Ocat 

0.169 

0.374 

0.989 

2.090 

0.271 

0.303 

1.164 

2.015 

0.168 

0.066 

0.056 

0.185 

0.060 

0.114 

0.155 

0.272 

0.017 

0.042 

0.140 

0.216 

0.011 

0.009 

0.007 

0.020 

HDGV 

T1 

T0 

Ocat 

0.390 

3.074 

2.831 

0.320 

1.652 

1.026 

0.105 

0.322 

0.212 

0.196 

0.441 

0.452 

0.056 

0.313 

0.218 

0.018 

0.061 

0.045 

HDDV Adv 0.090 0.062 0.043 -0.003 0.006 0.004 

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Comparisons of start emissions using the two methods (g/mi and g/hr) are shown in Figure 4 for lightduty vehicles and Figure 5 for heavy-duty vehicles. As can be seen from these figures, the start emissions 

calculated using the two methods do not result in statistically the same value within the 95% confidence 

interval, except for a few cases. As described in Section 3.2.3 below, the grams per mile method 

produced running emission values that are closer to the HR505 values, so it is recommended to use start 

emissions based upon the grams per mile method. 

Figure 4. Comparison of Start Emissions for light-duty vehicles using two methods 

0.30 

g/mi method 

g/hr method 

Start E

missio

n

s (g/start) Start E

missio

n

s (g/start) 

0.25 

0.20 

0.15 

0.10 

0.05 

0.00 

LEV T1 T0 Ocat Ncat LEV T1 T0 Ocat 

LDGV LDGT 

Figure 5. Comparison of Start Emissions for heavy-duty vehicles using two methods 

0.50 

g/mi method 

0.40 g/hr method

0.30 

0.20 

0.10 

0.00 

-0.10 

T1 T0 Ocat Adv 

HDGV HDDV 

3.2.3 CH4 HR505 Comparisons 

Several of the vehicles tested in the dataset provided by EPA also included hot running 505 test emissions 

along with the FTP emissions tests. Since not all vehicles in the dataset also included HR505 tests, 

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comparisons were made for those vehicles that did have both tests. Since the HR505 test contains no 

starts, it is equivalent to a running emission. The HR505 emissions are compared in terms of grams per 

mile in Table 13 and grams per hour in Table 14 to the FTP Bag 2 running emissions presented in Section 

3.2.2. 

Table 13. HR505 Comparison with Bag 2 Running emissions in grams per mile 

Vehicle 

Type 

Emission 

Tier 

No of 

Test Pts 

FTP Bag 2 Emissions (g/mi) HR505 Emissions (g/mi) 

Average SD 95% CI Average SD 95% CI 

LDGV 

T1 

T0 

Ocat 

31 

51 

7 

0.015 

0.085 

0.097 

0.013 

0.119 

0.105 

0.005 

0.033 

0.077 

0.014 

0.062 

0.069 

0.009 

0.074 

0.050 

0.003 

0.020 

0.037 

LDGT 

LEV 

T1 

T0 

Ocat 

2 

43 

33 

6 

0.040 

0.023 

0.130 

0.430 

0.019 

0.018 

0.109 

0.426 

0.026 

0.005 

0.037 

0.341 

0.033 

0.022 

0.096 

0.302 

0.022 

0.015 

0.076 

0.267 

0.030 

0.005 

0.026 

0.214 

HDGV 

T1 

T0 

Ocat 

22 

76 

74 

0.042 

0.196 

0.189 

0.046 

0.100 

0.067 

0.019 

0.022 

0.015 

0.050 

0.156 

0.175 

0.049 

0.078 

0.085 

0.021 

0.017 

0.019 

Table 14. HR505 Comparison with Bag 2 Running emissions in grams per hour 

Vehicle 

Type 

Emission 

Tier 

No of 

Test Pts 

FTP Bag 2 Emissions (g/hr) HR505 Emissions (g/hr) 

Average SD 95% CI Average SD 95% CI 

LDGV 

T1 

T0 

Ocat 

31 

51 

7 

0.248 

1.366 

1.556 

0.212 

1.918 

1.671 

0.075 

0.526 

1.238 

0.354 

1.598 

1.758 

0.221 

1.891 

1.278 

0.078 

0.519 

0.947 

LDGT 

LEV 

T1 

T0 

Ocat 

2 

43 

33 

6 

0.630 

0.376 

2.070 

6.886 

0.304 

0.295 

1.738 

6.855 

0.421 

0.088 

0.593 

5.485 

0.829 

0.572 

2.450 

7.708 

0.558 

0.393 

1.934 

6.862 

0.773 

0.117 

0.660 

5.490 

HDGV 

T1 

T0 

Ocat 

22 

76 

74 

0.643 

3.108 

2.984 

0.758 

1.565 

1.044 

0.324 

0.352 

0.238 

1.266 

3.965 

4.407 

1.247 

1.959 

2.115 

0.521 

0.440 

0.482 

Running emissions are compared on a grams per mile basis in Figure 6 and a grams per hour basis in 

Figure 7. As can be seen in those figures the Bag 2 running emissions in either grams per mile or grams 

per hour are statistically similar to the HR505 running emissions within the 95% confidence interval. In 

absolute value, however, the bag 2 emission levels in grams per mile are closer to the HR505 emission 

levels in grams per mile than the comparison in grams per hour. Thus, it is suggested that emission rates 

in grams per mile be used for the low speed case. Conversion to grams per hour should be done at FTP 

speed, which is somewhere between the Bag 2 and HR505 speeds. 

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Figure 6. Comparison of Running Emissions in grams per mile 

0.90 

0.80 

0.70 

0.60 

0.50 

0.40 

0.30 

0.20 

0.10 

0.00 

T1 T0 Ocat LEV T1 T0 Ocat T1 T0 Ocat 

Bag 2 

HR505 

LDGV LDGT HDGV 

Figure 7. Comparison of Running Emissions in grams per hour 

14 

12 

10 

8 

6 

4 

2 

0 

T1 T0 Ocat LEV T1 T0 Ocat T1 T0 Ocat 

Bag 2 

HR505 

LDGV LDGT HDGV 

R

u

n

nin

g E

missi

o

ns (

g/hr) R

unning E

mis

sions (

g/mi) 

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4 EMISSION ESTIMATES FOR OTHER CATERGORIES 

Several of the vehicle types/control technology categories either contained no data or statistically 

insignificant data within the 95 percent confidence interval. For these data, estimation of emission factors 

is needed. This section discusses how the IPCC data was derived and makes recommendations for newer 

estimates. 

4.1 Nitrous Oxide Emission Factor Estimates 

Nitrous oxide emissions data was either non-existent or produced statistically insignificant results in the 

case of most of the older control technologies for gasoline light-duty vehicles. In addition, there were 

either no or statistically insignificant data for heavy-duty gasoline vehicles and light duty diesel vehicles 

and trucks. For heavy-duty diesel vehicles, only the most advanced control technology had any data and 

that was only for sanitation trucks. There were no motorcycle data. 

The data shown in Table 4 indicates that the new data produces statistically significant values that are 

lower than the previous IPCC values. Most of the values used currently in the Inventory of U.S. 

Greenhouse Gas Emissions and Sinks: 1990-2001 come from an EPA report by Harvey Michaels.5 In 

this report, Michaels examined limited data for gasoline passenger cars and developed new N2O emission 

factors based upon that data. For other vehicle types, he used the ratio of fuel economies to produce new 

values. For diesel vehicles, he suggested using the European values listed in the IPCC guidelines. 

A similar method is suggested here. To estimate the emission factors of N2O for other vehicle types, but 

the same emission tier, it is suggested that the ratio of CO2 emissions be used. CO2 emissions per mile 

were extracted from the Harvey Michaels report and converted to grams per mile. These values are 

shown on Table 15. Estimates of CO2 emissions for LEV and Tier 1 heavy-duty gasoline vehicles were 

estimated from the CO2 emissions for Tier 0 heavy-duty gasoline vehicles and the ratio of CO2 emissions 

between a Tier 1 and LEV light-duty truck and a Tier 0 light-duty truck. 

4.1.1 N2O Estimates for Light-Duty Gasoline Vehicles 

The datasets provided by EPA produced statistically significant emission factors for LEV, Tier 1 and Tier 

0 vehicles. There was no data for any of the earlier technologies such as oxidation catalyst, non-catalyst, 

and uncontrolled. To estimate emissions for the oxidation catalyst category, the Tier 0 emission levels 

were multiplied by the ratio of CO2 emissions for the Tier 0 light-duty gasoline vehicle divided by the 

oxidation catalyst CO2 emissions for the same vehicle type. This ratio was applied to the FTP, running 

and start emissions. For the non-catalyst and uncontrolled, previous IPCC values were used for the FTP 

values, and the ratio of FTP to running and start emissions for the oxidation catalyst category was used to 

determine the running and start emissions for the non-catalyst and uncontrolled levels. Estimated values 

for these three control technologies are shown in Table 16. 

4.1.2 N2O Estimates for Light-Duty Gasoline Trucks 

The datasets provided by EPA produced statistically significant emission factors for LEV and Tier 1 

vehicles only. Estimated emission factors for the other control technologies for light-duty trucks were 

estimated from the emission factors for light-duty gasoline cars based upon the ratio of the light-duty 

truck CO2 emission rate versus the light-duty gasoline car CO2 emission rate. Estimated values for the 

5 EPA, “Emissions of Nitrous Oxide from Highway Mobile Sources: Comments on the Draft Inventory of U.S. 

Greenhouse Gas Emissions and Sinks, 1990-1996 (March 1998),” Report No. EPA420-R-98-009, August 1998. 

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Table 15. CO2 Emissions in grams per mile for Vehicle Types and Emission Control Technologies 

Vehicle/Control Technology CO2 (g/mi) 

Gasoline Passenger Cars 

Low Emission Vehicles 451 

Tier 1 459 

Tier 0 480 

Oxidation Catalyst 616 

Non-Catalyst 855 

Uncontrolled 814 

Gasoline Light-Duty Trucks 

Low Emission Vehicles 637 

Tier 1 637 

Tier 0 801 

Oxidation Catalyst 801 

Non-Catalyst 967 

Uncontrolled 932 

Gasoline Heavy-Duty Vehicles 

Low Emission Vehicles 1,301 

Tier 1 1,301 

Tier 0 1,637 

Oxidation Catalyst 1,667 

Non-Catalyst Control 2,124 

Uncontrolled 2,124 

Diesel Passenger Cars 

Advanced 381 

Moderate 399 

Uncontrolled 513 

Diesel Light Trucks 

Advanced 531 

Moderate 533 

Uncontrolled 668 

Diesel Heavy-Duty Vehicles 

Advanced 1,588 

Moderate 1,627 

Uncontrolled 1,765 

Motorcycles 

Non-Catalyst Control 352 

Uncontrolled 428 

various emission control categories where there were not statistically significant data are shown in Table 

17. 

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Table 16. Estimated N2O Emission Factors for Light-Duty Gasoline Vehicles 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

Oxidation Catalyst 

Non-Catalyst 

Uncontrolled 

0.042 

0.017 

0.017 

0.032 

0.013 

0.013 

0.072 

0.028 

0.028 

Table 17. Estimated N2O Emission Factors for Light-Duty Gasoline Trucks 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

Tier 0 

Oxidation Catalyst 

Non-Catalyst 

Uncontrolled 

0.090 

0.054 

0.019 

0.019 

0.069 

0.042 

0.015 

0.015 

0.153 

0.093 

0.032 

0.032 

4.1.3 N2O Estimates for Heavy-Duty Gasoline Trucks 

The datasets provided by EPA did not produce any statistically significant emission factors for heavyduty trucks [because N2O emissions were measured for only a single heavy-duty gasoline truck]. 

Emission factors for heavy-duty gasoline trucks were estimated from light-duty gasoline trucks based 

upon the ratio of CO2 emissions for each control technology. Estimated values for the various emission 

control categories where there were not statistically significant data are shown in Table 18. 

Table 18. Estimated N2O Emission Factors for Heavy-Duty Gasoline Trucks 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

LEV 

Tier 1 

Tier 0 

Oxidation Catalyst 

Non-Catalyst 

Uncontrolled 

0.019 

0.138 

0.183 

0.113 

0.041 

0.043 

0.002 

0.083 

0.142 

0.088 

0.032 

0.033 

0.120 

0.409 

0.313 

0.194 

0.070 

0.074 

4.1.4 N2O Estimates for Motorcycles 

The datasets provided by EPA did not contain any test data for motorcycles. Emission factors for 

motorcycles were estimated from light-duty gasoline cars based upon the ratio of CO2 emissions for the 

two control technologies. Estimated values for motorcycles are shown in Table 19. 

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Table 19. Estimated N2O Emission Factors for Motorcycles 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

Non-Catalyst 

Uncontrolled 

0.007 

0.009 

0.005 

0.007 

0.012 

0.015 

4.1.5 N2O Estimates for Heavy-Duty Diesel Vehicles 

The datasets provided by EPA only produced statistically significant emission factors for the advanced 

control technology. While the data only represented the light heavy-duty trucks (i.e., with GVWR 

between 8,500 and 10,000 pounds), the same engine technology would apply to other heavy-duty truck 

types. Also since the advanced technology does not include any aftertreatment devices, it is assumed that 

the N2O emissions from the other categories are the same as those for the advanced technology. Estimated 

values for the various emission control categories where there were not statistically significant data are 

shown in Table 20. 

Table 20. Estimated N2O Emission Factors for Heavy-Duty Diesel Vehicles 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

Moderate 

Uncontrolled 

0.005 

0.005 

0.005 

0.005 

-0.002 

-0.002 

4.1.6 N2O Estimates for Light-Duty Diesel Cars and Trucks 

The datasets provided by EPA did not produce any statistically significant emission factors for either 

light-duty diesel cars or trucks. Emission factors for light-duty cars and trucks were estimated from 

heavy-duty diesel vehicles based upon the ratio of CO2 emissions for the various control technologies. 

Estimated values for the light-duty diesel cars and trucks are shown in Table 21. 

Table 21. Estimated N2O Emission Factors for Light-Duty Diesel Vehicles and Trucks 

Vehicle 

Type 

Light-Duty 

Diesel 

Vehicles 

Emission Control 

Technology 

Advanced 

Moderate 

Uncontrolled 

FTP 

(g/mi) 

0.001 

0.001 

0.001 

Running 

(g/mi) 

0.001 

0.001 

0.002 

Start 

(g/start) 

0.000 

0.000 

-0.001 

Light-Duty 

Diesel Trucks 

Advanced 

Moderate 

Uncontrolled 

0.002 

0.002 

0.002 

0.002 

0.002 

0.002 

-0.001 

-0.001 

-0.001 

4.2 Methane Emission Factor Estimates 

While significantly more data were available for the methane emission factor analysis, there were still 

several categories of vehicle/control technology combinations for which there was either no or not enough 

data to produce statistically significant results within the 95% confidence interval. This included most 

diesel vehicles, motorcycles and earlier technologies of gasoline vehicles (non-catalyst and uncontrolled). 

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The data in Table 10 indicate that the IPCC LEV and T1 emission factors for light duty gasoline vehicles 

and trucks were higher than the new data suggests. This is most likely because the newer data represents 

advancements in emission control above that used to determine the IPCC values. The IPCC values were 

determined from EPA’s earlier mobile source emission factor model MOBILE5. 

In order to estimate methane emissions for other vehicles, the ratio of CO2 emissions were used to 

extrapolate values similar to that for N2O emission factors. 

4.2.1 CH4 Estimates for Light-Duty Gasoline Vehicles 

Statistically significant data were available for all but the uncontrolled light-duty gasoline vehicle 

category. This was estimated from the non-catalyst emission factors based upon the ratio of CO2 

emission rates. Estimated methane emission factors for uncontrolled light-duty gasoline cars are given in 

Table 22. 

Table 22. Estimated CH4 Emission Factors for Light-Duty Gasoline Vehicles 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

Uncontrolled 0.171 0.162 0.062 

4.2.2 CH4 Estimates for Light-Duty Gasoline Trucks 

The datasets provided by EPA produced statistically significant emission factors for all but the noncatalyst and uncontrolled categories. Estimated emission factors for these control technologies were 

estimated from the emission factors for light-duty gasoline cars based upon the ratio of the light-duty 

truck CO2 emission rate versus the light-duty gasoline car CO2 emission rate for the given control 

technology. Estimated values for the various emission control categories where there were not 

statistically significant data are shown in Table 23. 

Table 23. Estimated CH4 Emission Factors for Light-Duty Gasoline Trucks 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

Non-Catalyst 

Uncontrolled 

0.184 

0.195 

0.175 

0.186 

0.067 

0.071 

4.2.3 CH4 Estimates for Heavy-Duty Gasoline Trucks 

The datasets provided by EPA produced statistically significant emission factors for Tier 1, Tier 0, and 

oxidation catalyst heavy-duty gasoline trucks. Emission factors for the other categories were estimated 

from light-duty gasoline trucks based upon the ratio of CO2 emissions for each control technology. 

Estimated values for the various emission control categories where there were not statistically significant 

data are shown in Table 24. 

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Table 24. Estimated CH4 Emission Factors for Heavy-Duty Gasoline Trucks 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

LEV 

Non-Catalyst 

Uncontrolled 

0.034 

0.403 

0.445 

0.022 

0.384 

0.423 

0.094 

0.147 

0.162 

4.2.4 CH4 Estimates for Motorcycles 

The datasets provided by EPA did not contain any test data for motorcycles. Emission factors for 

motorcycles were estimated from light-duty gasoline cars based upon the ratio of CO2 emissions for the 

two control technologies. Estimated values for motorcycles are shown in Table 25. 

Table 25. Estimated CH4 Emission Factors for Motorcycles 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

Non-Catalyst 

Uncontrolled 

0.067 

0.090 

0.064 

0.085 

0.024 

0.033 

4.2.5 CH4 Estimates for Heavy-Duty Diesel Vehicles 

The datasets provided by EPA only produced statistically significant emission factors for the advanced 

control technology. Since the advanced technology does not include any aftertreatment devices, it is 

assumed that the CH4 emissions from the other categories are the same as those for the advanced 

technology. Estimated values for the various emission control categories where there were not statistically 

significant data are shown in Table 26. 

Table 26. Estimated CH4 Emission Factors for Heavy-Duty Diesel Vehicles 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

Moderate 

Uncontrolled 

0.004 

0.004 

0.006 

0.006 

-0.011 

-0.011 

4.2.6 CH4 Estimates for Light-Duty Diesel Cars and Trucks 

The datasets provided by EPA did not produce any statistically significant emission factors for either 

light-duty diesel cars or trucks. Emission factors for light-duty cars and trucks were estimated from 

heavy-duty diesel vehicles based upon the ratio of CO2 emissions for the various control technologies. 

Estimated values for the various emission control categories for light-duty diesel cars and trucks are 

shown in Table 27. 

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Table 27. Estimated CH4 Emission Factors for Light-Duty Diesel Vehicles and Trucks 

Vehicle 

Type 

Emission Control 

Technology 

FTP 

(g/mi) 

Running 

(g/mi) 

Start 

(g/start) 

Light-Duty 

Diesel 

Vehicles 

Advanced 

Moderate 

Uncontrolled 

0.001 

0.001 

0.001 

0.001 

0.001 

0.002 

-0.003 

-0.003 

-0.003 

Light-Duty 

Diesel Trucks 

Advanced 

Moderate 

Uncontrolled 

0.001 

0.001 

0.002 

0.002 

0.002 

0.002 

-0.004 

-0.004 

-0.004 

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5 RECOMMENDED EMISSION FACTORS FOR ON-HIGHWAY 

VEHICLES 

Table 28 presents previous IPCC values used in the 2001 US Inventory and newly recommended 

emission factors for methane and nitrous oxide emissions for all categories. Values in bold represent 

those derived using statistically significant data within the 95% confidence interval. 

Table 28. Recommended Values for N2O and CH4 Emission Factors 

Nitrous Oxide Methane 

Vehic IPCC Run Start IPCC Run Start le Type

Control Technology g/mi g/start g/mi g/start 

Gasoline Passenger Cars 

Low Emission Vehicles 0.028 0.012 0.040 0.013 

Tier 1 0.046 0.030 0.048 0.020 

Tier 0 0.082 0.054 0.064 0.066 

Oxidation Catalyst 0.052 032 0.133 

Non-Catalyst 0.017 0.162 

Uncontrolled 0.017 013 171 

Gasoline Light-Duty Trucks 

Low Emission Vehicles 0.035 0.009 0.048 0.017 

Tier 1 0.058 0.067 0.056 0.034 

Tier 0 0.102 0.090 0.071 

Oxidation Catalyst 0.065 042 0.143 

Non-Catalyst 0.021 015 184 

Uncontrolled 0.021 015 195 

Gasoline Heavy-Duty Vehicles 

Low Emission Vehicles 0.113 0.019 0.034 

Tier 1 0.139 0.138 0.047 

Tier 0 0.175 0.183 0.218 

Oxidation Catalyst 0.111 088 0.208 

Non-Catalyst Control 0.035 0.041 0.403 

Uncontrolled 0.035 033 445 

Diesel Passenger Cars 

Advanced 0.016 000 -0.003 

Moderate 0.016 000 -0.003 

Uncontrolled 0.016 -0.001 0.016 -0.003 

Diesel Light Trucks 

Advanced 0.032 -0.001 0.001 0.002 

Moderate 0.032 -0.001 0.001 0.002 

Uncontrolled 0.032 -0.001 0.016 -0.004 

Diesel Heavy-Duty Vehicles 

Advanced 0.005 -0.002 0.064 0.004 -0.011 

Moderate 0.048 -0.002 0.004 0.006 

Uncontrolled 0.048 -0.002 0.097 -0.011 

Motorcycles 

Non-Catalyst Control 0.007 0.007 0.067 

Uncontrolled 0.007 007 090 

FTP FTP 

g/mi g/mi g/mi g/mi 

0.000 0.090 0.009 0.032 

0.015 0.113 0.012 0.055 

0.042 0.092 0.062 0.034 

0.042 0. 0.072 0.113 0.132 0.009 

0.017 0.013 0.028 0.193 0.155 0.059 

0.017 0. 0.028 0.217 0. 0.162 0.062 

0.001 0.059 0.011 0.046 

0.041 0.200 0.023 0.082 

0.069 0.153 0.113 0.062 0.072 

0.054 0. 0.093 0.145 0.130 0.099 

0.019 0. 0.032 0.225 0. 0.175 0.067 

0.019 0. 0.032 0.217 0. 0.186 0.071 

0.002 0.120 0.071 0.022 0.094 

0.083 0.409 0.097 0.024 0.163 

0.142 0.313 0.121 0.194 0.183 

0.113 0. 0.194 0.145 0.179 0.215 

0.032 0.070 0.201 0.384 0.147 

0.043 0. 0.074 0.435 0. 0.423 0.162 

0.001 0.001 0. 0.016 0.001 0.001 

0.001 0.001 0. 0.016 0.001 0.001 

0.001 0.002 0.001 0.002 

0.002 0.002 0.016 -0.004 

0.002 0.002 0.016 -0.004 

0.002 0.002 0.002 0.002 

0.048 0.005 0.006 

0.005 0.005 0.081 -0.011 

0.005 0.005 0.004 0.006 

0.005 0.012 0.209 0.064 0.024 

0.009 0. 0.015 0.418 0. 0.085 0.033 

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Appendix A 

Description of Test Data 

EPA provided its contractor (ICF) with datasets with test results containing methane measurements: 

-- 13,277 FTP tests on 6,950 vehicles and 

-- 14,636 non-FTP tests on 2,963 vehicles, 

and with datasets with test results containing nitrous oxide measurements: 

-- 95 FTP tests on 64 vehicles and 

-- 232 non-FTP tests on 74 vehicles. 

The FTP tests that measured nitrous oxide emissions were primarily those performed by EPA, 

supplemented by tests performed by the University of California at Riverside CE-CERT, Southwest 

Research Institute (SwRI), and CARB. 

The non-FTP tests included a hot running 505 which were used in this study to validate the approach used 

to separate the start and running emissions as well as several other driving cycles not utilized in this 

report. Methane tests were performed in various U.S. locations during the period between April 1982 and 

June 2000. Nitrous oxide tests were performed in various U.S. locations during the period between June 

1998 and May 2002. 

The analyses performed by ICF were limited to the FTP tests that were performed within the temperature 

range of 68 degrees to 86 degrees Fahrenheit (i.e., at a nominal temperature of 75° F). 

Since the goal of ICF's analyses was to develop separate emission rates for both the running operation and 

engine starts, the analyses focused on the FTP tests since they contained both of those two types of 

vehicle operation. 

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Appendix B 

Response to Peer Review Comments from Thomas Durbin 

This report was formally peer reviewed by Thomas Durbin, Ph.D., Associate Research 

Engineer with the College of Engineering-Center for Environmental Research and 

Technology (CE-CERT) at the University of California-Riverside. In this appendix, 

comments from Thomas Durbin are reproduced in plain text, and EPA’s responses to those 

comments are interspersed in indented italics. 

************************************ 


October 10, 2004 

The following is a review of the IFC Consulting document “Update of Methane and Nitrous Oxide 

Emission Factors for On-Highway Vehicles” and the underlying datasets used in developing this 

document. This document is being used by the United States Environmental Protection Agency (US EPA) 

in the development of emissions factors for the EPA MOVES model. This review covers several relevant 

areas including the dataset completeness, methodology, and report clarity. The suggestions given in this 

review are to provide EPA guidance in moving forward and improving the emission factors for methane 

(CH4) and nitrous oxide (N2O) from vehicles. 

Overall, the report appears to be satisfactory in characterizing CH4 and N2O emission factors based on the 

information provided in the EPA database. The primary concern with the updated emission factors is that 

there are still some gaps in the EPA database and there is also a need to develop emission factors for some 

categories by extrapolating data from more broadly tested categories using comparisons of CO2 

emissions. In reviewing the EPA datasets for CH4 and N2O, it was found that a number of studies with 

CH4 and/or N2O emissions measurements have not been included. It is suggested that the next step in 

improving the EPA emission factors for CH4 and N2O is to augment the current database with additional 

information from the literature, especially in under populated categories such as diesel vehicles. In the 

larger context of greenhouse gases (GHG), the contribution of CH4 and N2O emissions is still less than 

5% of the total GHG contribution from mobiles sources; therefore, improvements in CO2 estimates from 

vehicles should probably remain a higher priority. 

RESPONSE: The analyses for the MOVES2006 version will make use of all the available data. 

For the report itself, some description of how the emission factors will be implemented in the MOVES 

model would be useful. To provide additional detail to the report, it would be useful to include number of 

test points available in each vehicle category and a brief discussion of the datasets. A discussion of the 

criteria used in judging the statistical significance of the available data in particular categories could also 

be added. Finally, it is suggested that as the emissions factors are improved through the years that the 

potential effects of other parameters on CH4 and N2O emissions be considered. These could include fuel 

sulfur level, different driving cycles, vehicle mileage/age, and ambient temperature, with fuel sulfur level 

being one of the most important of these parameters. 

RESPONSE: The implementation of these emission factors is discussed in more detail in the 

report entitled “MOVES2004 Energy and Emissions Inputs. 

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A more detailed commentary on different aspects of the report/underlying datasets is provided below. To 

address the EPA’s primary areas of interest, this review is broken down into four main topics: 

completeness of data sources, overall methodology and analysis, additional factors to be considered in 

developing emission factors, and presentation and report clarity. 

Completeness of datasets selected. The robustness of the underlying datasets in developing emission 

factors is essential in the overall accuracy of emissions inventories. The datasets used appear to be 

obtained from a larger database of emissions testing results maintained by EPA. While this database is 

sufficient CH4 emissions for some of the larger categories, some additional data sources need to be 

considered in other N2O and CH4 categories. This is one of the most significant weaknesses of the 

methodology. Emissions results for CH4 and N2O from vehicles are reviewed by Lipman and Delucchi 

(2002) and in the discussion below. 

The available emissions data for CH4 emissions appears to be large enough for some of the more 

important categories (i.e., T1, T0, Ocat). The CH4 data for LEV vehicles is relatively limited, however (7 

LDGV and 10 LDGT). Is it possible that information on CH4 emissions for late model vehicles can be 

obtained from certification data using the difference between THC and NMHC emissions? Other studies 

of fuel properties for LEV certified vehicles may also provide information on THC and NMHC emissions 

for late model vehicles (AAM/AIAM, 2001; Durbin et al., 2003), again using differences in THC and 

NMHC emissions to get CH4. In the motorcycle category, The California Air Resources Board (CARB) 

has done testing on a series of 100 1966-1999 motorcycles (Jones, 2000). This report does not include 

CH4 directly, but the CH4 contribution to THC for motorcycles could be estimated from data of other 

sources to provide a better emission factor CH4 for motorcycles. 

For N2O emissions, the database appears to be limited to tests conducted directly by EPA, some of the 

earlier work from the University of California at Riverside CE-CERT, and smaller number of tests from 

the Southwest Research Institute (SwRI) and CARB. Several more recent studies should be considered 

for inclusion, including those by Durbin et al. (2003) and Huai et al. (2002, 2003, 2004) that include 

approximately 20 LEV LDGVs and 10 LEV LDGTs. The limitations of the LEV N2O emissions 

estimates are evident in comparing the emission factors for LDGV and LDGT. Specifically, on the basis 

of 5 test points, the emission factor for the LDGT is found to be less than that of the LDGVs, contrary to 

what is found with a more complete review of the literature (Huai et al., 2003). CARB has also 

characterized N2O for a fleet of in-use vehicles (Behrentz et al. 2004). The individual vehicle results are 

not presented in this study, but may be available through CARB. Environment Canada has also collected 

N2O emissions for a fleet of 21 1978-1996 vehicles (Graham, 1999). Becker et al. (1999) and Baronick et 

al. (2000) have conducted studies of 1996 and newer vehicles, although specific vehicle information is 

not included in the work by Becker et al. Michaels et al. (1998) also reviewed some earlier data sets that 

are not included in the current N2O dataset used for this study, although in some cases the previous IPCC 

values based on these data are still used. Huai et al. (2003b) conducted a more recent review including 

these data sets as well as some more current information. 

RESPONSE: The analyses for the MOVES2006 version will make use of all the available data. 

The data on CH4 and N2O for diesel vehicles is limited to a small number of tests on medium-duty diesel 

trucks conducted at SwRI. There are some additional sources of data that should be considered for diesel 

vehicles, although even a more comprehensive literature review yields only a limited number of diesel 

test records for these specific emissions. Merritt (2003) of SwRI conducted a comprehensive literature 

review of diesel emissions data as part of Coordinating Research Council’s (CRC) project No. AVFL10A. They identified approximately 10 studies that include either CH4 and/or N2O diesel emissions data 

from vehicles or engines. For CH4 emissions, several organizations have made measurements on heavyduty diesel vehicles including West Virginia University (Gautam et al., 2003; Gautam et al., 1996), SwRI 

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(Ullman et al., 2003) and Ecotraffic of Sweden (Ahlvik and Brandberg, 2000). For light- to medium-duty 

vehicles, Schaurer et al. (1999) and researchers from Ford (Siegl et al., 1999) and CE-CERT (Durbin et 

al., 1999) have all made speciation hydrocarbon measurements, although methane emissions are not 

reported in all of these publications. For light-duty diesel vehicles, N2O emissions have been measured by 

Ahlvik (2002), Fanick et al. (2001) of SwRI [which may be in the database], and Oyama and Kakegawa 

(2000). 

RESPONSE: The analyses for the MOVES2006 version will make use of all the available data. 

In examining the datasets, it is also useful to consider the stratification of the vehicle technology binning 

structure. The current stratification appears to be sufficient at the present time. As newer LEV II vehicles 

are introduced into the fleet, these categories should be reflected in the stratification structure. It may also 

be worthwhile in conducting some sensitivity studies on the older Tier 0 vehicles, since there was 

considerable evolution of vehicle technology over this time period (early 1980s to mid-1990s). While 

these vehicles will compose a progressively smaller fraction of the fleet over time, they can still represent 

a large fraction of the total emissions inventory. It would be useful to further break the Tier 0 group down 

into roughly 5 year periods based on model year (1980-1985, 1986-1990, and 1990 and newer) and 

compare the results with the composite emission factor. 

RESPONSE: As more data become available, we shall revisit the selection of model year 

groupings. 

Analysis and Overall Methodology - The methodologies used in this document are reasonable given the 

limitations of the datasets provided. As discussed above, there are areas where the EPA database is 

incomplete and additional data are available. This additional data should be used instead of extrapolating 

data from CO2 for certain categories. 

RESPONSE: As more data become available, we shall use those data rather than relying on 

extrapolations. 

Separating the FTP emissions into start and running emissions. The separation of emissions into 

start and running emissions is a good idea since each represents an important segment of the emissions 

inventory. The comparisons of hot running 505s (HR505) and the bag 2 emissions indicate that the 

subtraction of bag 2 emissions is reasonable for determining start emissions, at least for CH4 and N2O. 

Presently, the methodology characterizes start emissions as a combination of cold and hot starts. Some 

analysis should be performed to evaluate the differences between cold and hot start emissions for these 

pollutants to better understand whether the contribution of these emissions should be considered 

separately in the model. A quick review of the N2O dataset by this reviewer indicated that the bag 1 

emissions were very similar to those for bag 3 averaged over the entire fleet, but varied considerably from 

vehicle to vehicle. On this basis, using combined hot and cold start emissions for N2O is probably 

adequate for fleet wide emissions. Similar analysis was not done for CH4 emissions. 

RESPONSE: In estimating the HC, CO, and NOx emissions in the MOVES2006 version, we 

shall distinguish between hot-start and cold-start emissions. We shall use that 

opportunity to revisit our estimates of CH4 and N2O start emissions to determine 

whether to also distinguish between hot-start and cold-start for these emissions 

as well. 

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Extrapolation of emission estimates from untested strata using CO2 emissions. While CO2 and fuel 

use can be successfully used in some situations to predict emission rates of various pollutants, this 

relationship depends on a variety of factors such as the vehicle and emissions control technology, the 

operating conditions, and the emissions standard to which the vehicle is certified to. While relationships 

between fuel economy and particular emissions may be found under aggressive or off-cycle conditions, 

under FTP conditions emissions would primarily be related to the control technology required to meet the 

applicable emissions standards (which are not directly related to fuel economy). Nevertheless, there are 

some real differences in emissions control technology and emissions standards between passenger cars 

and trucks and vehicles in different weight classifications that have trends directionally consistent with 

increasing CO2 emissions. It is uncertain, however, if these differences would best be obtained from 

straight comparisons of emissions standards or using CO2 emissions. 

RESPONSE: In developing the MOVES2006 version of the model, we shall estimate the HC, 

CO, and NOx emissions. We shall use that opportunity to revisit our estimates of 

CH4 and N2O start emissions to determine whether to base those extrapolations 

on those estimates of HC, CO, or NOx emissions. 

N2O emissions for a specific vehicle are not expected to correlate with CO2 over a range of operating 

conditions. In fact, N2O emissions tend to have higher formation rates at intermediate catalyst temperature 

ranges [250-450°C] that occur when the catalyst is warming up to its operation temperature. Under higher 

speeds or more aggressive driving conditions, where fuel use would be at a maximum, catalyst 

temperature would also be at a maximum and N2O emissions would be low. This has been observed in 

several studies (Pringent and DeSeote, 1989; Hirano et al., 1992; Odaka et al., 1998; Koike et al., 1999, 

Huai et al., 2003). 

For heavy-duty gasoline vehicles, only one N2O test record for a Tier 0 truck was found in the database. 

In the almost complete absence of emissions test data, it is reasonable to expect that these vehicles would 

have higher N2O emissions than lighter vehicles due to differences in emissions controls. As such, it 

seems reasonable N2O emissions would increase in some proportionally to CO2 emissions. It is worth 

noting that the single Tier 0 test record is lower than the emission factor given in Table 18 [55 vs. 183 

mg/mi]. A limited number of tests on fairly old technology vehicles are also provided in Dietzmann et al. 

(1981). These data should probably be considered for comparison or possible inclusion. 

For the non-catalyst technologies, it is agreed that previous IPCC N2O emission factors should be used 

instead of extrapolating from CO2 emissions for catalyst-equipped vehicles, since the formation of N2O is 

more directly related to the catalyst than combustion conditions. Overall, the values for non-catalyst 

vehicles may be a little high, since the primary mechanism for forming N2O is over the catalyst. In 

deriving the previous IPCC values for non-catalyst vehicles, Michaels et al. (1998) used results from three 

primary studies (Pringent and De Soete 1989; Dasch, 1992; Urban and Garbe, 1979). Of these studies, 

Pringent and De Soete (1989) reported FTP emission rates of approximately 50 mg/mi, considerably 

higher than the results observed in the other studies that were below 5 mg/mi (Dasch, 1992; Urban and 

Garbe, 1979). Perhaps the Pringent and De Soete (1989) data are outliers. Other studies have reported 

N2O emission rates of 15-20 mg/mi for non-catalyst vehicles (Warner-Selph and Harvey, 1990; Robinson, 

1991), while Huai et al. (2003) found an emission rate below 10 mg/mi for non-catalyst light-duty truck. 

The oxidation catalyst N2O emission factors are extrapolated from the Tier 0 results using CO2 emission 

ratios since the data available in the EPA database was not statistically significant. Earlier estimates by 

Michaels et al. (1998) in the category, however, indicate that 11 vehicles records were available in the 

oxidation catalyst category (mostly LDGVs). It seems like using the direct N2O measurements from 

oxidation catalysts for at least the LDGVs would be more appropriate than extrapolating the results from 

only 12 Tier 1 test points using CO2. 

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The diesel section is somewhat confusing when the database is cross-referenced with the applicable text 

on N2O emissions estimates. The database itself appears to only have 6 records for light-heavy-duty pickup trucks. The text in section 4.1.5 mentions sanitation truck data that did not appear to be present in the 

database examined by this reviewer. Since diesel engines are typically not put in pick-up trucks smaller 

than those in the database, having a separate category for light-duty diesel trucks may not be needed. For 

light-duty diesel vehicles, Oyama and Kakegawa (2000) found emission rates of about 4-8 mg/mi. Ahlvik 

(2002) found much higher rates [on the order of 40 mg/mi], but expressed skepticism that these results 

were too high and could not rule out a mix-up in samples. The experimental values for light-duty diesel 

vehicles are higher than those obtained from extrapolating CO2 measurements. For a complete inventory 

standpoint, however, N2O emissions from light-duty diesel passenger cars are expected to make a small 

contribution. 

RESPONSE: The reviewer is correct about the sanitation trucks. There were none in that 

database. We revised the text by replacing "sanitation" truck with "light heavyduty" truck. 

For CH4 emissions, there is statistically significant data for all but the oldest light-duty vehicles, lightduty truck non-catalyst and uncontrolled trucks, and heavy-duty gasoline trucks. These vehicles probably 

make relatively small contribution to the inventory so estimates based on CO2 emissions should be 

sufficient to provide factors in these categories. As discussed above, CARB has conducted some testing 

on motorcycles (Jones, 2000). It is suggested the THC emissions from this study be extrapolated to obtain 

the CH4 estimates for motorcycles as opposed to estimates based on CO2 emissions from light-duty 

gasoline cars. 

For diesel vehicles, CH4 emissions generally comprise a small portion of the overall THC emissions. 

Studies at SwRI on light-heavy-duty and heavy-heavy-duty diesel vehicles have shown CH4 emissions to 

generally be below 10 mg/mi, and near background levels compared with the THC (Fanick et al., 2001; 

Ullman et al., 2003). Gautam et al. (2003) reported higher emission rates of ~40 to 140 mg/mi for a small 

set of tests on the UDDS in the E-55/59 program, with emissions going up to over 2 grams/mi for the 

“creep” portion of the CARB heavy-duty cycle. For light- and medium-duty vehicles, values reported by 

Siegl et al. (1999), Oyama and Kakegawa (2000), and Durbin et al. (1999) range from 1-20 mg/mi. 

Statistical Significance – It is mentioned throughout section 4 that statistically significant emission factors 

could not be obtained for particular emission/technology categories. An examination of the N2O database 

by this reviewer, however, indicated that in some of these categories, there was actually either no 

available data or only a single data point (i.e., N2O emissions for HDGVs). A better explanation why data 

were classified as not statistically significant should be given (i.e., number of data points, variability of 

the data, etc.). 

RESPONSE: We revised the text to include that the reason for the lack of statistical 

significance was that only a single gasoline-fuel heavy-duty truck produced N2O 

FTP emissions. 

Parameters used to characterize emissions. 

The main parameters used in the IFC Consulting document for characterizing the emission factors for 

N2O and CH4 include the vehicle technology stratification, running emissions, and start emissions. There 

are several additional parameters that could be important to characterize for N2O and CH4 emission 

factors in future efforts. 

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Fuel S Effects 

As fuel sulfur levels continue to be reduced nationally, it is important that these effects be evaluated or 

included in the estimates. A number of studies have shown that decreasing fuel sulfur level leads to 

significant reductions in N2O emissions (Baronick et al. 2000; Huai et al, 2002; Michaels et al. 1998; 

Durbin et al. 2003). In reducing fuel sulfur levels from levels near 300 to levels closer to 30, reductions in 

N2O emissions of more than 50% are often found. These changes should be considered in characterizing 

N2O emissions in going forward. If possible, it would be useful to add the fuel sulfur level as a parameter 

in the N2O database. These fuel sulfur effects would likely be stronger and more important when 

considering older data sets where higher sulfur fuels were more likely used. For example, measurements 

made by Ballantyne et al. (1994) on a fleet of Canadian vehicles were performed with a fuel with a 700 

ppm sulfur level. 

A number of studies have shown that for gasoline vehicles hydrocarbons can increase with fuel sulfur in 

the range of 5-800 ppm (AAM/AIAM, 2001; Korotney et al., 1995; Rutherford et al., 1995; Benson et al., 

1991; Durbin et al., 2003). It is not known, however, if the effects of fuel sulfur on CH4 have specifically 

been characterized. It is possible that CH4 emissions may also increase with fuel sulfur. Since CH4 

emissions are more difficult to oxidize over the catalyst than other hydrocarbons, catalysts are generally 

not as effective in controlling CH4 emissions compared to other hydrocarbons. As such, it is anticipated 

that the impact of fuel sulfur on CH4 emissions would be smaller than its impact on THC emissions. 

RESPONSE: In estimating the HC, CO, and NOx emissions in the MOVES2006 version, we 

shall consider the effects on those emissions of the sulfur content of the fuel. We 

shall use that opportunity to revisit our estimates of CH4 and N2O emissions to 

determine if they are also sensitive to the sulfur content of the fuel. 

Driving cycles 

There is no discussion about how emissions factors for different or off-cycle operating conditions would 

be implemented. For N2O, emissions are expected to be a stronger function of catalyst temperature as 

opposed to operating condition. As such, start emissions that are already included are the most critical 

operating condition in characterizing N2O emissions. Measurements of N2O emissions for higher speed or 

more aggressive driving have generally shown that N2O emission rates are low under these conditions 

(Dasch, 1992; Sasaki and Kameoka, 1992; Huai et al., 2002, 2003, 2004). Since there are not significant 

increases in N2O emissions under aggressive operating conditions, the influence of operating conditions 

can be a lower priority parameter. The effects of different driving cycles would most likely track the effects 

of driving cycles on THC emissions, hence, these effects can probably be estimates based on THC 

emissions. 

RESPONSE: This analysis (by ICF) was limited to emissions produced over the standard FTP 

/ LA-4 driving cycle. Speed / cycle adjustment factors are being studied for the 

MOVES2006 version of the model. 

Vehicle Mileage/Catalyst Age 

Several studies have shown that vehicles with older catalysts can have higher emission rates of N2O. 

These include studies where direct comparisons between older and newer catalysts were made on the 

same vehicle or under the same operating conditions and other studies where the comparisons were 

made between sets of newer vs. older vehicles. Jobson et al. (1994) and Odaka et al. (1998, 2000) both 

showed in laboratory studies that aged catalysts can result in increased N2O emissions. Odaka et al. 

(2000) suggested that this could be attributed more to a decline in the ability of the catalyst to decompose 

N2O than a reduction in the generation of N2O. Odaka et al. (1998) found that the effect of catalyst age on 

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N2O emission levels depended on the catalyst composition, with Pt/Rh and Pd catalysts with high metal 

contents showing little differences with catalyst age while Pt/Rh catalysts with low metal contents showed 

dramatic increases with catalyst age. Some studies have shown that N2O formation also occurs at higher 

temperatures with aged compared to new catalysts (Odaka et al., 1998). It has been suggested that this 

could lead to higher actual N2O vehicle emissions because a greater proportion of the driving cycle may 

occur in the higher temperature “window” of formation (Lipman and Delucchi, 2002). 

In actual vehicle applications, the effects of catalyst age on N2O emissions have been mixed. Odaka et al. 

(1998) found that N2O emissions for a low Pt/Rh content passenger car increased under stabilized 

operating conditions after 30,000 miles of operation, but decreased under start conditions. De Soete 

(1993) found that N2O emissions increased comparing a catalyst aged to 15,000 miles with a new 

catalyst. Durbin et al. (2003), on the other hand, found that catalyst age did not have a statistically 

significant effect on N2O emissions in comparing 12 vehicles operated with new and aged catalysts. 

These vehicles were all late model and in the LEV category. In other studies, vehicles with higher 

mileage/older catalysts also represented older technologies so these data are more difficult to interpret 

(Ballantyne et al., 1994; Laurikko and Aakko, 1995). 

It is expected that CH4 emissions would also show deterioration with age. Lipman and Delucchi (2002) 

characterized CH4 emissions from a variety of sources and observed that most data showed an increase in 

CH4 emissions with catalyst age. They found that modern vehicles and fuels (e.g., 1990s vintage vehicles 

operating on reformulation fuels) showed emissions levels of 50 mg/mi under new conditions, rising to 

150 mg/mi when the catalyst was significantly aged. For older three-way catalyst vehicles, they found 

these vehicles in a “new” condition had emission rates of approximately 100 mg/mi, increasing to 300 

mg/mi with higher age. 

RESPONSE: In estimating the HC, CO, and NOx emissions in the MOVES2006 version, we 

shall consider the effects on those emissions of vehicle age and/or mileage 

accumulation. We shall use that opportunity to revisit our estimates of CH4 and 

N2O emissions to determine if they are also sensitive to vehicle age and/or 

mileage accumulation. 

5.1 Ambient Temperature Effects 

Ambient temperature is known to have impacts on regulated emissions, with emissions increasing at 

colder temperatures. Few studies have directly looked at ambient temperature effects on CH4 and N2O 

emissions. Ahlvik (2002) looked at the effects of temperature between –7 and 22°C on N2O emissions for 

2 light-duty gasoline and 2 light-duty diesel vehicles. The lower temperature results only showed a large 

increase for one of the gasoline vehicles, with slight changes for two other vehicles. Stump et al. (1989, 

1990) looked at temperature and oxygenated fuel effects on CH4 emissions. THC emissions decreased 

slightly as the temperature was increased from 40 to 90°F, with CH4 emissions proportionally changing 

with THC. These results suggest the effects of temperature on CH4 emissions can be estimated from the 

effects of temperature on THC. 

RESPONSE: This analysis (by ICF) was limited to emissions produced within the standard 

FTP temperature range (68 to 86 degrees Fahrenheit). Temperature adjustment 

factors will be developed for the MOVES2006 version of the model. 

Presentation and Report Clarity. Some additional details and information would help to clarify some 

of the steps of the methodology and how the emission factors will be implemented. 

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Update of Methane and Nitrous Oxide Emission Factors for On-Highway Vehicles Draft Report 

The level of detail used in describing the data sources is very limited. It would be useful to have some 

description of the data sources to give the reader an idea of what data may have been included/excluded. 

In the EPA’s “Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990-2001” document (US EPA, 

2003), a short discussion of data sets is provided in appendix E. Something similar could be added here. 

For the CH4 data, it would be useful to include descriptions of some of the larger data sources, since 

considerably more records are available for CH4. For N2O, since data is limited, the specific sources could 

even be listed as done in the US EPA (2003) document. It would also be useful to have a table of the 

number of vehicles used in each technology for each of the pollutants, instead of just the information 

provided in footnote 3 on page 4. 

RESPONSE: Appendix A has been added to this report to provide a brief description of the test 

data used by ICF to produces these estimated emission rates. 

It would be useful to provide a brief one or two paragraph description of the how the emission factors will 

be implemented in to the EPA MOVES model. If not, a reference to where more details on the model can 

be found. How are running and start emissions going to be implemented into the model? How will the 

running emissions be implemented in terms of the MOVES modal binning structure? Finally, emissions 

are broken down into g/mi and g/hour, but there is no discussion on how the g/hour would be 

implemented into the model. 

RESPONSE: The discussion of how these emission rates are incorporated into the MOVES 

model appears in the MOVES technical report entitled “MOVES2004 Energy 

and Emissions Inputs.” 

A description of the technology categories is provided, but no information is provided on the different 

weight categories used (i.e., light-duty vs. heavy-duty). 

Contract 68-W-99-054, WA 4-37 31 ICF Consulting 
Update of Methane and Nitrous Oxide Emission Factors for On-Highway Vehicles Draft Report 

GRAMMAR

1. p 4 – paragraph below equation 1. line 9. There is an extra period after - segments of the FTP. 

2. p. 19 – Table 23. Is the first entry under the emission control technology supposed to be noncatalyst instead of moderate? 

RESPONSE: The reviewer is correct on both points. The text has been corrected. 

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