United States

Environmental Protection

Agency

Air and Radiation EPA420-R-98-009

August 1998

Emissions of Nitrous Oxide

from Highway Mobile

Sources

 Printed on Recycled Paper

Comments on the Draft Inventory of

U.S. Greenhouse Gas Emissions and

Sinks, 1990-1996 (March 1998)
EPA420-R-98-009

August 1998

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)

Harvey Michaels

Assessment and Modeling Division

Office of Mobile Sources

U.S. Environmental Protection Agency

NOTICE

This technical report does not necessarily represent final EPA decisions or positions.

It is intended to present technical analysis of issues using data which are currently available.

The purpose in the release of such reports is to facilitate the exchange of

technical information and to inform the public of technical developments which

may form the basis for a final EPA decision, position, or regulatory action.
Page ii

TABLE OF CONTENTS

1 INTRODUCTION .................................................. Page 1

2 AN ANALYSIS OF THE DATA SOURCES AND METHODS USED TO OBTAIN

THE EMISSION FACTORS FOR NITROUS OXIDE FROM GASOLINE HIGHWAY

VEHICLES IN THE RECENT DRAFT INVENTORY

................................................................. Page 3

3 IMPROVED ESTIMATES OF EMISSION FACTORS FOR GASOLINE HIGHWAY

VEHICLES ....................................................... Page 5

4 OTHER ISSUES .................................................. Page 11

5 EFFORTS THAT WOULD IMPROVE FUTURE INVENTORIES .......... Page 14

6 CONSOLIDATED LIST OF SPECIFIC CHANGES TO THE DRAFT INVENTORY

................................................................ Page 16

7 REFERENCES ................................................... Page 19

APPENDIX A

A DETAILED DISCUSSION OF THE ORIGIN OF THE EMISSION FACTORS FOR

 NITROUS OXIDE FOR GASOLINE HIGHWAY VEHICLES IN THE DRAFT

INVENTORY .................................................... Page A-1

APPENDIX B

NVFEL TESTING PROGRAM ..................................... Page B-1

APPENDIX C

ASSUMED FUEL ECONOMIES WHOSE RATIOS WERE USED TO GENERATE

EMISSION FACTORS FOR VEHICLES FOR WHICH THERE WERE NO DATA

............................................................... Page C-1
Page 1

EMISSIONS OF NITROUS OXIDE FROM MOBILE SOURCES:

COMMENTS ON THE DRAFT INVENTORY OF U.S. GREENHOUSE GAS EMISSIONS

AND SINKS , 1990-1996 (MARCH 1998)

August 13, 1998

1 INTRODUCTION

The estimate of the contribution of nitrous oxide from mobile sources to total U.S. emissions of

greenhouse gases went from one-half percent in the last official inventory, published in 1997

(U.S. EPA) to three percent in the March 10, 1998, draft Inventory of U.S. Greehouse Gas

Emissions and Sinks 1990-1996 (U.S. EPA), which will be referred to in these comments as the

Draft Inventory. The primary reason for this change is the use of much larger emission factors

for gasoline highway vehicles, rather than increases in vehicle miles traveled. OMS believes that

these emission factors are considerably larger than they should be. Therefore, these comments

will focus primarily on the origin and validity of the emission factors used in the Draft Inventory

and on the development of better ones.

The emission factors for passenger vehicles from the last official Inventory, from the 3/10/98

Draft Inventory, and from OMS’s proposed revisions that are developed in this document, are

listed in the following table.

Control

Technology

N2O Emission Factors for Passenger Vehicles

Last Official Inventory

(g/km)

3/10/98 Draft Inventory

(g/km)

OMS Revision

(g/km)

 LEV 0.040 0.018

Advanced 3 Way (Tier 1) 0.019 0.170 0.029

Early 3-way (Tier 0) 0.046 0.170 0.051

Oxidation Catalyst 0.027 0.075 0.032

Non-Catalyst 0.005 0.020 0.010

Uncontrolled 0.005 0.020 0.010

The Draft Inventory adopted the emission factors for U.S. vehicles from the Revised 1996 IPCC

Guidelines for National Greenhouse Gas Inventories (IPCC 1997), which are referred to in these

comments as the IPCC Guidelines. They list emission factors for European cars that are between

four and thirty-four times lower than for similar U.S. vehicles:
Page 2

Comparison of estimated emission factors in the IPCC Guidelines

between U.S. and European passenger vehicles

Control

Technology

Emission Factors (g/mi)

U.S. Passenger

Vehicles 

European Passenger

Vehicles 

LEV 0.064

Advanced 3 Way 0.274 0.08

Early 3-way 0.274 0.008

Oxidation Catalyst 0.121 0.008

Non-Catalyst 0.032 0.008

Uncontrolled 0.032 0.008

1.1 Control technology terminology

For U.S. vehicles, the following control technology designations are more appropriate than those

used in the Draft Inventory:

% For "Early three-way catalyst," substitute "Tier 0."

% For "Three-way catalyst" or "Advanced 3 Way," substitute "Tier 1."

Tier 0, Tier 1 and LEV (Low Emission Vehicle) are not control technologies per se, but

emissions regulations. They do, however, correspond to combinations of control technology and

engine design. Tier 0 refers to standards earlier than Tier 1 that applied to vehicles equipped

with three-way catalysts (TWCs). Tier 1s and LEVs both have TWCs, but the data show that

their more stringent NOX standards are associated with lower nitrous oxide emissions as well. 

The introduction dates for "early three-way catalysts" and "advanced three-way catalysts" in the

Draft Inventory correspond approximately to the introduction of Tier 0 and Tier 1 emissions

regulations (see Table C-7 in the Draft Inventory or Section 4.2 below). The assignments of

control technologies to model years are revised in Section 4.2.

1.2 Purposes and overview

The purposes of these comments are 1) to review the data supporting the nitrous oxide emission

factors used in the Draft Inventory, 2) to provide revised emission factors, 3) to recommend

changes in other factors affecting nitrous oxide emissions, and 4) to recommend changes in

future inventories.

Section 2 reviews the data sources and methods supporting the nitrous oxide emission factors

used in the Draft Inventory. Section 3 presents the development of revised emission factors. 

This development is based on a review of the literature (Section 3.1) and on recent tests

conducted at EPA’s National Vehicle and Fuel Emissions Laboratory (NVFEL) (Section 3.2). 

Section 4 discusses other issues that affect the calculation of U.S. emissions of nitrous oxide

from mobile sources. These include diesel emission factors (Section 4.1), assignment of control
Page 3

technology by model year (Section 4.2), distribution of model years in each calendar year

(Section 4.3), and uncertainty (Section 4.4). Section 5 discusses further work to better evaluate

the contribution of mobile sources to U.S. emissions of nitrous oxide. Section 6 is a consolidated

list of the specific changes recommended for the Draft Inventory.

2 AN ANALYSIS OF THE DATA SOURCES AND METHODS USED TO OBTAIN

THE EMISSION FACTORS FOR NITROUS OXIDE FROM GASOLINE

HIGHWAY VEHICLES IN THE RECENT DRAFT INVENTORY

The trail of references from the Draft Inventory back to the original data sources is described

briefly below. A more detailed analysis is provided in Appendix A.

% The grams/mile emission factors for U.S. mobile sources used in the Draft Inventory

were taken from the IPCC Guidelines. 

% The grams/mile emission factors for U.S. vehicles in the IPCC Guidelines come from a

report prepared by Weaver and Chan (1996): "Mobile source emission factors for global

warming gases."

% Weaver and Chan (1996) obtained their grams/mile emission factors from the last column

of Table 7 in Ballantyne et al. (1994). The heading of this column is "Current Canadian

Estimates: EPS Inventory."

% The reference for the last column of Table 7 in Ballantyne et al. is to Jaques (1992),

Canada’s Greenhouse Gas Emissions: Estimates for 1990, published by the Canadian

Government. Ballantyne et al. obtained the grams/mile emission factors (for aged TWCs,

new TWCs, and oxidation catalysts) in this column from the grams/kilogram emission

factors presented in Jaques, by assuming fuel economies of 9.4, 11.9, and 6 km/L

respectively and a standard value for the density of gasoline. The fuel economies are

from Jaques’s Table 16 and the gasoline density from Table 32. It is not clear where

Ballantyne et al. obtained their grams/mile emission factor for non-catalyst vehicles,

since it is roughly half the value that would be derived from Jaques by the method

described.

% Jaques’s emission factors for vehicles without catalysts, vehicles equipped with aged

TWCs and vehicles equipped with new TWCs, are the averages of the first two lines of

de Soete’s (1989) Table XXIX. Jaques converts these averages, which are in units of

g/km, to units of g/kg by assuming a uniform fuel economy of 8.5 km/L and a gasoline

density of 0.75 kg/L. Jaques’s emission factor for oxidation catalyst vehicles is the same

as that for new TWC vehicles. Since none of his references support this emission factor

for oxidation catalysts, it is possible that he simply adopted the emission factor for new

TWCs. The average emission factor for oxidation catalysts from de Soete’s Table XIV

was 70% higher than the one Jaques uses.
Page 4

% Lines 1 and 2 of De Soete’s Table XXIX are averages of emission factors in his Table

XIV. Line 1 is the average of Table XIV lines 2, 4-7, and 11-19, and represents the data

from three studies which measured emission factors on a total of five cars tested without

catalysts, with new TWCs, and with aged TWCs on various European dynamometer test

cycles. Line 2 is the average of Table XIV lines 11-19, and represents the data from a

single car tested without a catalyst, with eight new TWCs and with the same eight TWCs

bench aged. Therefore, Jaques’s average of lines 1 and 2 of de Soete’s Table XXIX

double weights lines 11-19 from Table XIV. Since the averages are of individual data

points, and approximately 80% of the new and aged TWC data come from lines 11-19,

Jaques’s emission factors for TWCs are derived approximately 90% from a single study

involving one car and eight non-production catalysts.

% De Soete’s Table XIV lines 11-19 refer to one study, Prigent et al. (1991), in which one

car was tested without a catalyst, with eight different non-production catalysts, and then

with the same eight catalysts bench-aged. The catalysts were located 1.4 m from the

engine.

% Table XIV line 2 refers to Lindskog (1988), in which one non-catalyst car and one car

equipped with a TWC were tested on the Swedish driving cycle.

% Table XIV lines 4-7 refer to Prigent et al. (1989), in which two cars were each tested 

with and without new TWCs.

In summary:

 All the emission factors originate from testing done on five cars using European

test cycles. Fuel sulfur content for these tests was unspecified.

 The new and aged TWC emission factors are based 90% on a single study using a

single car with eight non-production catalysts, new and bench-aged, with the

catalysts located 1.4 m from the engine. The other 10% of the data for the TWC

emission factors came from two studies and three more cars, all tested on

European driving cycles only.

 The non-catalyst emission factors were derived from four cars.

 The emission factor for oxidation catalyst vehicles does not appear to be based on

testing, but is instead the same emission factor used for new TWCs.

3 IMPROVED ESTIMATES OF EMISSION FACTORS FOR GASOLINE

HIGHWAY VEHICLES

Compared to regulated tailpipe emissions, there exist relatively few data that can be used to
Page 5

estimate nitrous oxide emission factors for gasoline highway vehicles. Nitrous oxide is not a

criteria pollutant, and measurements of it in automobile exhaust are not routinely collected. 

Many of the recent measurements have been part of research efforts attempting to understand

why and under what conditions TWCs produce nitrous oxide, rather than trying to characterize

the U.S. fleet.

OMS determined emission factors for Tier 0 and earlier vehicles primarily from the published

literature (Section 3.1). For Tier 1 vehicles and for LEVs, data was used from the recent testing

program at NVFEL (Section 3.2). Section 3.3 discusses the limited data that we have for trucks. 

Section 3.4 summarizes our recommendations for emission factors by vehicle type and control

technology.

3.1 Emission factors for Tier 0 and earlier passenger cars

In looking for a better estimate of emission factors, OMS has decided to review only published

values for the composite of the standard FTP driving cycle, since it is the standard driving cycle

for the U.S. To do otherwise would require reconciling alternative test cycles, tunnel studies,

and remote sensing studies&an effort beyond the scope of this review.

To determine emission factors for Tier 0 and earlier vehicles, the following published studies

were included in the analysis:

Prigent and de Soete (1989)

Dasch (1992)

Smith and Carey (1982)

Smith and Black (1980)

Urban and Garbe (1979)

Urban and Garbe (1980)

Ballantyne et al. (1994)

Barton and Simpson (1994)

Braddock (1981)

Also included were two measurements of one Tier 0 vehicle that the NVFEL included in its

recent study of nitrous oxide emissions from Tier 1 vehicles and LEVs.

Light trucks are analyzed separately, since their emissions are significantly higher than passenger

vehicles. The above studies that included light trucks also treated them separately from

passenger vehicles. Emission factors for trucks are addressed in Section 3.3 below.

Some authors distinguish "dual bed" catalysts from TWCs, but the distinction is not clear, and

we have followed most authors in considering dual-bed catalysts as a form of TWC.

There is evidence that aged TWCs emit more nitrous oxide than new ones. For this reason, we

have separated the data into "new" and "aged" (or "old"). "New" means a vehicle that was
Page 6

supplied by a manufacturer for testing and has less than a few thousand miles on the odometer. 

Everything else is aged or old.

The results are summarized in the following table:

Catalyst age Parameter

Catalyst Type

All None Oxidation 3-Way

All ages mg/mi 56. 17. 51.7 60.5

n 50 3. 11 36

std. err. of mean 6.5 13. 19.1 6.8

New mg/mi 42.7 17. 37.8 47.2

n 29 3. 4 22

std. err. of mean 4.8 13. 12. 5.4

Aged mg/mi 74.2 59.7 81.5

n 21 7 14

std. err. of mean 13.2 29.7 13.8

The study by Ballantyne et al. has been excluded from our averaging, because the fuel they used

contained 700 ppm sulfur, roughly double what might be expected in U.S. gasoline. The sulfur

content of the fuel used in Braddock (1981) was 250 ppm. It was 290 ppm in Urban and Garbe

(1979 and 1980), Smith and Carey (1982), and Smith and Black (1980). It was 500 ppm in

Barton and Simpson. Sulfur in fuel has been shown to degrade catalyst performance with respect

to conventional emissions (see, e.g., Lindhjem 1995 and Monroe et al. 1991). Newly acquired

data at NVFEL, discussed below, indicates that emissions of nitrous oxide were significantly

higher using Clean Air Act Baseline (CAAB) fuel, a fuel intended to represent a "normal"

commercial fuel and which contained 285 ppm sulfur, than when using Indolene, a fuel used in

vehicle certification and which contained 24 ppm sulfur. We believe that the higher nitrous

oxide emissions were due to the higher sulfur content of CAAB fuel. The fuel analyses and our

reasons for believing that the differences in nitrous oxide emissions were due to differences in

sulfur content rather than to differences in other fuel parameters are detailed in Appendix B.

For comparison, the following table presents emission factors for new and aged TWCs for all

data, for data excluding Ballantyne et al. (1994), and for Ballantyne et al. alone. Units are

mg/mi, with the number of data points in parentheses.

New TWC Aged TWC

All 50.4 (25) 97.7 (22)

Without Ballantyne et al. 47.2 (22) 81.5 (14)

Ballantyne et al. only 74. (3) 126. (8)
Page 7

Including Ballantyne would increase the aged TWC emission factor from 0.08 to 0.1 g/mi.

3.2 Emission factors for Tier 1 and LEV vehicles: recent measurements by the NVFEL

A measurement program was undertaken during June and July, 1998, to determine nitrous oxide

emissions from aged Tier 1 and LEV vehicles using commercial fuels. 23 vehicles were tested:

18 Tier 1 vehicles, 4 LEVs, and one Tier 0 vehicle that was recruited in error. One of the Tier 1s

was recruited specifically to verify the results for a single high-emitting pickup truck. Tier 1

odometers ranged from 16,000 to 75,000 miles. All four LEVs were obtained from their

manufacturers. Three of the four were equipped with TWCs that had been bench-aged to

100,000 miles. Three of the odometers read about 5,000 miles; the fourth read about 169,000

miles. Vehicles were tested with air conditioning (A/C) off at 75(F and on at 95(F. All vehicles

except one LEV and one Tier 1 were tested using CAAB fuel, a commercial fuel containing 285

ppm sulfur. All of the LEVs and three of the Tier 1 vehicles were tested with Indolene, a lowsulfur fuel used in vehicle certification. The testing schedule and fuel analyses are in Appendix

B. The schedule included 23 vehicles and 50 samples.

In order to estimate the emission factors for Tier 1 vehicles, we averaged only tests run with

CAAB fuel, and we omitted the second high-emitting pickup truck that was recruited specifically

to verify the first one. The following table shows these results:

Tier 1 emission factors from NVFEL program 

Vehicles included in

average

Emission

factor

(mg/mi)

Number

of vehicles

Number

of samples

Std. err.

mean

(mg/mi)

Range

(mg/mi)

All 63.6 17 29 7.1 24-167

Passenger vehicles 46.3 12 21 5.0 24-124

Light trucks and SUVs 108.9 5 8 11.8 80-167

The emission factor of 46 mg/mi for these Tier 1 passenger vehicles compares favorably with the

emission factor of 82 mg/mi for Tier 0 vehicles equipped with TWCs.

The following summarizes the LEV emission factors under our test program. All the LEVs were

obtained from their manufacturers. Three had catalysts bench-aged to 100,000 miles. 
Page 8

N2O vs. Mileage for CAAB Fuel

Tier 1 Passenger Vehicles

0.00

0.05

0.10

0.15

0 10000 20000 30000 40000 50000 60000 70000 80000

Mileage

N20 (g/mi)

LEV emission factors from NVFEL program 

Fuel

Emission

factor

(mg/mi)

Number of

vehicles

Number of

samples

Std. err.

mean

(mg/mi)

Range

(mg/mi)

CAAB Fuel 77.8 3 6 14.7 32.-116.

Indolene 28.3 4 8 2.5 14.-36.

LEVs are currently running only in California on low-sulfur fuel, so the emission factor using

Indolene is the applicable one.

Emissions were always higher with CAAB Fuel than with Indolene.

In 8 cases, tests were repeated with both fuels. Six of the tests were with LEVs, and two with

Tier 1 vehicles. All showed higher emissions with CAAB than with Indolene. The ratio of

nitrous oxide emissions using CAAB to those using indolene ranged from 1.2 to 4.4 and

averaged 2.6. The mean of the ratio was significantly larger than 1 (p<.01). We believe that the

basis for this difference is fuel sulfur content. The fuel analyses and some modeling results

supporting this belief are in Appendix B.

Emissions were usually higher with A/C On at 95(F than with A/C off at 75(F

In 22 cases, tests were repeated under both A/C modes. In seventeen cases emissions were

higher with A/C on, in five cases with A/C off. The ratio of nitrous oxide emissions with A/C on

to those with A/C off ranged from 0.9 to 3.4 and averaged 1.5. The mean of the ratio was

significantly larger than 1 (p<.01).

Nitrous Oxide was unrelated to the mileage of the vehicles.

A regression of nitrous oxide emission factors against mileage for Tier 1 passenger vehicles

yielded a slight positive slope not significantly different from zero (p<0.25). R2 was 0.06. 
Page 9

Barton and Simpson (1994) similarly did not find a significant relationship between nitrous

oxide emissions and mileage. Their slope was negative.

Light-duty trucks had higher emissions than passenger vehicles. This result is in agreement with

Ballantyne et al. (1994) and with Barton and Simpson (1994).

3.3 Emission factors for gasoline highway vehicles other than passenger cars

Only three of the reviewed studies include data on vehicles other than passenger vehicles. All

the non-passenger vehicles were light duty trucks equipped with TWCs. The results are

summarized in the following table:

Study Age

Emission factors

(mg/mi) (number of vehicles) Trucks/PVs

Light-duty trucks (ratio) Passenger

vehicles

NVFEL Old 109 (5) 46 (12) 2.4

Ballantyne et al.

(1994) All 188 (3) 111(11) 1.7

Old 93 (1) 126 (8) 0.73

New 236 (2) 74 (3) 3.2

Barton and

Simpson (1994) All 163 (3) 75 (11) 2.2

Old 300 (1) 80 (11) 3.8

New 95 (1) 55 (2) 1.7

Average 2.2

While the data are limited and not without exception, they are fairly convincing that light-duty

trucks emit more nitrous oxide per mile than passenger vehicles.

In the absence of a better alternative, we recommend that emission factors for passenger vehicles

be applied to other gasoline highway vehicles in proportion to their fuel economy, which is the

same practice employed in the Draft Inventory. For this purpose, we have used the fuel

economies specified by Weaver and Chan (1996) and incorporated into the IPCC Guidelines. 

They are listed in Appendix C. According to Chan (1998), they were obtained from MOBILE5

and then reduced by 15%. The use of fuel-consumption ratios to determine emission factors

should be considered a temporary measure only, to be replaced as soon as real data are available.

3.4 Recommended emission factors for gasoline highway vehicles
Page 10

Passenger vehicles

A list of the revised emission factors is presented in Section 6.1. Except for LEVs as specified, it

is assumed that these vehicles are being operated on a standard commercial fuel containing about

300 ppm sulfur. Aged TWCs emit more than new TWCs, but we believe aging happens fairly

early, so we assume most of the fleet is aged. There are no data to assign a mileage to this

transition.

Control Technology

Emission

Factor

(mg/mi)

n

Std. Err.

Mean

(mg/mi)

Range

(mg/mi)

Emission

Factor

(mg/km)*

Non-catalyst 16.6 3 13.0 2-42 10.3

Oxidation catalyst 51.7 11 19.1 8-233 32.2

Tier 0 81.5 12 13.8 6-190 50.7

Tier 1 46.3 21 5.0 24-124 28.8

LEVs on standard fuel 77.8 6 14.7 32-116 48.4

LEVs on low-S fuel 28.3 8 2.5 14-36 17.4

* Extra precision has been included so conversion between units does not introduce a

significant difference.

Summary of Sources:

Control Technology Data Source

Non-catalyst Prigent and de Soete (1989), Dasch (1992), and Urban and

Garbe (1979)

Oxidation catalyst Smith and Carey (1982), Urban and Garbe (1979)

Tier 0 Smith and Carey (1982), Barton and Simpson (1994), and

NVFEL (1998) (one car). Only old cars were included. 

Ballantyne et al. (1994) was excluded because of high fuel sulfur

content (700 ppm).

Tier 1 NVFEL (1998). CAAB fuel, both A/C modes.

LEVs on standard

fuel

NVFEL (1998). CAAB fuel, both A/C modes.
Page 11

LEVs on low-sulfur

fuel

NVFEL (1998). Indolene fuel, both A/C modes.

Gasoline highway vehicles other than passenger vehicles

A list of the revised emission factors is presented in Section 6.1. We have used fuel-specific

emission factors, as was done in the Draft Inventory. That is, we use the preceding emission

factors for passenger vehicles, adjusted by the ratio of the fuel economies of passenger vehicles

and the other vehicle type. The data that support this practice are that light trucks emit more

nitrous oxide than passenger vehicles (see Section 3.3). The data are not good enough to say

how much more, but fuel-specific emission factors seem an appropriate estimate at this time. 

The increasing proportion of light trucks in the U.S. fleet emphasizes the need to collect

additional data. 

We have used the fuel economies in the IPCC Guidelines for calculating fuel-specific emission

factors. These fuel economies came from MOBILE5, reduced by 15% (Chan 1998). While it is

likely that these estimates of fuel economy can be improved, it is only their ratios that are being

used in this context. The use of fuel-consumption ratios to determine emission factors should be

considered a temporary measure only, to be replaced as soon as real data are available.

Note that for Gasoline Heavy-Duty Vehicles the emission factors in Table C-8 of the Draft

Inventory specified as Catalyst and Non-Catalyst Control were actually the fuel-specific values

for Advanced 3-Way and Early 3-Way. This error is also present in the IPCC Guidelines and in

Weaver and Chan (1996).

4 OTHER ISSUES

4.1 Diesel emission factors

Weaver and Chan (1996) cite Dietzmann et al. 1980 (SAE 801371) as the basis for nitrous oxide

emission factors for heavy-duty diesel trucks, saying that they averaged the Dietzmann et al.

values for heavy-duty trucks and estimated emission factors for lighter duty vehicles by

assuming fuel-specific emission factors. Four engines were studied in Dietzman et al., one from

1977 and three from 1979. The 1979 engines were required to meet more stringent emissions

standards. The average nitrous oxide emission factors for Dietzman et al.’s three 1979 engines

were 31, 55, and 40 mg/mi. The 1977 engine emitted 76 mg/mi. The average of the four values

is 50.5 mg/mi = 31.4 mg/km, which is the value Weaver and Chan use for uncontrolled HDDVs. 

Fuel-specific emission factors seem to have been applied inconsistently to other diesel classes. 

For example, 63 mg/km is assigned to light-duty diesels with moderate control. Application of

fuel-consumption proportionality yields emission factors of about 10 mg/km for light-duty trucks

and 8 mg/km for passenger vehicles. The IPCC Guidelines values for European diesels (Tables

1-37 to 1-39) are 30, 20, and 10 mg/km for heavy-duty, light-duty, and passenger vehicles
Page 12

respectively. The values in Dietzmann, Weaver and Chan, and the European tables in the IPCC

Guidelines are all quite low and in the same range. Because of very limited data and greater

European experience with diesel, OMS recommends taking the European values from the IPCC

Guidelines: 30, 20, and 10 mg/km for heavy-duty, light-duty, and passenger vehicles

respectively. 

Vehicle type and control

technology

Draft Inventory

(g/km)

Fuel-specific based on

Dietzmann et al. (1980)

(g/km)

European

(g/km)

Diesel Passenger Cars

Control Technology

Advanced 0.0070 0.0068 0.0100

Moderate 0.0100 0.0071 0.0100

Uncontrolled 0.0140 0.0091 0.0100

Diesel Light Trucks

Control Technology

Advanced 0.0240 0.0094 0.0200

Moderate 0.0630 0.0095 0.0200

Uncontrolled 0.0310 0.0119 0.0200

Diesel Heavy-Duty Vehicles

Control Technology

Advanced 0.0250 0.0283 0.0300

Moderate 0.0250 0.0289 0.0300

Uncontrolled 0.0310 0.0314 0.0300

4.2 Control technologies and their assignment by model year.

A small section of Table C-7 of the Draft Inventory is shown below:

U.S. Greenhouse Gas Emissions and Sinks: 1990-1996 Page 152

Table C-7: Control Technology Assignments for Highway Mobile

Sources

Vehicle Type/Technology Model Years

Gasoline Passenger Cars and Light-Duty Trucks

Uncontrolled 1966-1972

Non-catalyst controls 1973-1977

Oxidation catalyst 1978-1982

Early three-way catalyst 1983-1995

Three-way catalyst 1996

Low emission vehicle* 1996

The following control technology designations are more appropriate for U.S. vehicles:

% For "Early three-way catalyst," substitute "Tier 0."

% For "Three-way catalyst," which is referred to in Table C-8 (Emission Factors) as

"Advanced 3 Way," substitute "Tier 1."
Page 13

VMT splits by vehicle age, in Draft Inventory 

(mobile96.xls)

Note: LDGV=LDDV and LDGT1=LDDT 

0

0.02

0.04

0.06

0.08

0.1

147

1013161922

Age (years, 1=current model year)

VMT

LDGV

LDGT1

HDGV

HDDV

See Section 1.1 above for additional discussion of this issue.

Our revised assignment of technologies by model year are detailed in the tables in Section 6.2. 

Our principal source for this data is the "Compilation of Air Pollutant Emission Factors, Volume

II: Mobile Sources" (U.S. EPA 1998), commonly referred to as AP42. Additional information

concerning the phase-in of Tier 1 and LEV technologies and schedules for California have been

provided by our MOBILE team.

A significant change from the way the Draft Inventory technology assignments were done is the

splitting of a single model year between more than one technology. We felt it was especially

important to do this for later model years, which make up a large proportion of the fleet. The

effect of our revisions is to introduce technologies earlier than they were introduced in the Draft

Inventory.

4.3 Distribution of VMT by vehicle age for each calendar year

The table of fraction of VMT by vehicle age that was used for all calendar years in the Draft

Inventory is plotted in the figure below. Each vehicle type is plotted with a separate line.

The irregularity of the plot indicates that these values represents data for a particular year. 

However, the spreadsheet used in the Draft Inventory applies this table to all years from 1990 to

1996. The table has a large peak for vehicles that are eleven years old, reflecting large purchases

of new vehicles in that model year. When this table is applied to other years than the one for

which the data apply, this peak will be incorrectly associated with other model years. As a

matter of documentation, the year from which the data for this table were taken and the source of

the data should be specified.
Page 14

4.4 Uncertainty estimates

Various places in the Draft Inventory contain discussions of uncertainty, but the Executive

Summary and Annex C do not. The discussion of uncertainty on p. 27 should be repeated in

both the Executive Summary and Annex C. The data in these locations otherwise give an

impression of far greater precision than is warranted.

5 EFFORTS THAT WOULD IMPROVE FUTURE INVENTORIES

5.1 Measure the nitrous oxide emissions of in-use vehicles

There is a great need for additional data. Nitrous oxide emissions from in-use vehicles should be

measured in as many testing programs as possible. In programs where an FTIR is being used,

adding the analysis of nitrous oxide should be relatively simple.

Heavier gasoline vehicles should be tested to determine their emission factors. The light truck

fleet is becoming a larger proportion of the U.S. total and therefore needs to be well

characterized. The current stratagem of using fuel-specific emission factors is suitable only as a

temporary measure.

The effect of sulfur on nitrous oxide emissions should be studied, on different vehicle types, with

and without catalysts. It appears that sulfur has a strong effect on nitrous oxide emissions. 

Emission factors for vehicles with TWCs may prove to be a strong function of the sulfur content

of the fuel used. 

Diesel vehicles of all weight classes should be tested. Routine testing should include nitrous

oxide. We need data on in-use vehicles, and, as new control technologies are developed, we will

need data on how those technologies affect nitrous oxide emissions.

The large variability in nitrous oxide emissions should be understood. Such knowledge might

lead to changes in catalyst design and configuration that would eliminate high emitters. Secondby-second studies of low and high emitters would probably yield good insight into the problem,

and provide some productive hypotheses for further testing.

5.2 Refine estimates of fleet composition and activity

Separate tables of VMT fraction by vehicle age should be developed for each historical calendar

year for which an inventory is prepared.

VMT estimates could benefit from close scrutiny and comparison between sources.

VMT and fuel-sales-based estimates should be reconciled.
Page 15

5.3 Analyze additional sources in the literature

While only further testing will provide the real data we need, some additional value can be

obtained by a more exhaustive review of the literature.

 Authors who tested vehicles using the FTP, but did not report the composite number we

need for consistency, might be willing to supply that data if requested. For example:

% Laurikko and Paivi (1995) tested five cars of different mileages at different

temperatures on the FTP cycle, but only reported bags 1 and 3.

% Joumard et al. (1996) tested 25 private cars, some with and some without

catalysts, on a variety of driving cycles, including the FTP, but nitrous oxide was

not reported for the FTP.

 Careful analysis of European and Japanese driving cycles could possibly yield data

comparable to those from the FTP cycle.

5.4 Develop estimates of uncertainty

Estimates of uncertainty should be developed in future Inventories.

5.5 Include nitrous oxide as part of a future version of MOBILE

Incorporating nitrous oxide into MOBILE would assure that our knowledge of nitrous oxide

emissions by mobile sources is represented in a way consistent with other mobile emissions. It

would also simplify the generation of an annual inventory.

5.6 Integrate with the Trends process

The estimates of nitrous oxide emissions from mobile sources should be integrated with the

process by which OAQPS produces the National Air Pollutant Emission Trends Data Base. This

approach would avoid duplication of effort and improve consistency across EPA.

6 CONSOLIDATED LIST OF SPECIFIC CHANGES TO THE DRAFT

INVENTORY

6.1 Revised nitrous oxide emission factors for highway mobile sources

The following table lists the revised emission factors. It corresponds to Table C-8 in Annex C of

the Draft Inventory. The rationale for these emission factors is detailed in the body and

appendices of these comments. We have included more significant figures than is warranted by

their uncertainty to assure consistent calculations when using different units. Note that instead of
Page 16

"Early" and "Advanced" TWCs, we use the terms "Tier 0" and "Tier 1".

Vehicle type and control technology Nitrous Oxide Emission

Factors

g/mi g/km

Gasoline Passenger Cars

Control Technology

Low Emission Vehicles* 0.0283 0.0176

Tier 1 0.0463 0.0288

Tier 0 0.0815 0.0507

Oxidation Catalyst 0.0517 0.0322

Non-Catalyst 0.0166 0.0103

Uncontrolled 0.0166 0.0103

* Applicable to California VMT only

Gasoline Light-Duty Trucks

Control Technology

Low Emission Vehicles* 0.0400 0.0249

Tier 1 0.0643 0.0400

Tier 0 0.1362 0.0846

Oxidation Catalyst 0.0673 0.0418

Non-Catalyst 0.0188 0.0117

Uncontrolled 0.0190 0.0118

* Applicable to California VMT only

Gasoline Heavy-Duty Vehicles

Control Technology

Tier 0 0.2781 0.1729

Oxidation Catalyst 0.1400 0.0870

Non-catalyst 0.0412 0.0256

Uncontrolled 0.0432 0.0269

Diesel Passenger Cars

Control Technology

Advanced 0.0161 0.0100

Moderate 0.0161 0.0100

Uncontrolled 0.0161 0.0100

Diesel Light Trucks

Control Technology

Advanced 0.0322 0.0200

Moderate 0.0322 0.0200

Uncontrolled 0.0322 0.0200

Diesel Heavy-Duty Vehicles

Control Technology

Advanced 0.0483 0.0300

Moderate 0.0483 0.0300

Uncontrolled 0.0483 0.0300

Motorcycles

Control Technology

Non-Catalyst Control 0.0068 0.0042

Uncontrolled 0.0087 0.0054

6.2 Revised technology assignments by model year for gasoline highway vehicles except
Page 17

motorcycles

For Gasoline Passenger Cars (light duty gas vehicles, LDGV), except California:

Model Year

Percentage of 49 States LDGV with each control technology

Uncontrolled Non-catalyst

control

Oxidation Tier 0 Tier 1

1972 100

1973-1974 100

1975 20 80

1976-1977 15 85

1978-1979 10 90

1980 5 88 7

1981 15 85

1982 14 86

1983 12 88

1984-1993 100

1994 60 40

1995 20 80

1996 100
Page 18

For Gasoline Light Duty Trucks (LDGT), except California:

Model Year

Percentage of 49 States LDGT with each control technology

Uncontrolled Non-catalyst

control Oxidation Tier 0 Tier 1

1972 100

1973-1974 100

1975 30 70

1976 20 80

1977-1978 25 75

1979-1980 20 80

1981 95 5

1982 90 10

1983 80 20

1984 70 30

1985 60 40

1986 50 50

1987-1993 5 95

1994 60 40

1995 20 80

1996 100

For Gasoline Heavy-Duty Vehicles (heavy-duty gas vehicles, HDGV):

Model Year

Percentage of national HDGV with each control technology

Uncontrolled Non-catalyst

control Oxidation Tier 0

1981 100

1982-1984 95 5

1985-1986 95 5

1987 70 15 15

1988-1989 60 25 15

1990-2003 45 30 25

2004 100
Page 19

For California Gasoline Passenger Cars and Light-Duty Trucks (light duty gas vehicles and

trucks, LDGV and LDGT):

Model

Year

Percentage of California LDGV and LDGT fleet with each control

technology

Uncontrol

led

Noncatalyst

control

Oxidation Tier 0 Tier 1 LEV

1972 100

1973-1974 100

1975-1979 100

1980-1981 15 85

1982 14 86

1983 12 88

1984-1991 100

1992 60 40

1993 20 80

1994 90 10

1995 85 15

1996 80 20

6.3 Document distribution of VMT by vehicle age for each calendar year

The existing table of VMT by vehicle age is for a particular but unspecified year. As a matter of

documentation, the year from which the data for this table were taken and the source of the data

should be specified.

6.4 Include a discussion of uncertainty in the Executive Summary and Annex C

Various places in the Draft Inventory contain discussions of uncertainty, but the Executive

Summary and Annex C do not. The discussion of uncertainty on p. 27 should be repeated in

both the Executive Summary and Annex C. The data in these locations otherwise give an

impressions of far greater precision than is warranted.

7 REFERENCES

Ballantyne, Vera F., Peter Howes, and Leif Stephanson. 1994. "Nitrous Oxide Emissions from

Light Duty Vehicles." SAE Paper 940304.
Page 20

Barton, Peter and Jackie Simpson. 1994. "The effects of aged catalysts and cold ambient

temperatures on nitrous oxide emissions." Mobile Sources Emissions Division (MSED),

Environment Canada, MSED Report #94-21.

Braddock, James N. 1981. "Impact of low ambient temperature on 3-way catalyst car

emissions." SAE paper 810280.

Chan, Lit-Mian. 1998. Phone call July 23.

Dasch, Jean Muhlbaler. "Nitrous Oxide Emissions from Vehicles." January, 1992. Journal of

the Air and Waste Management Association, 42(1):63-67.

De Soete, Gerard G. 1993. "20. Nitrous oxide from combustion and industry: chemistry,

emissions and control." In A. R. van Amstel (ed.) Proceedings of an International IPCC

Workshop on Methane and Nitrous Oxide : Methods in National Emissions Inventories

and Options for Control. RIVM Report No. 481507003, Bilthoven, The Netherlands.

De Soete, G. 1989. "Updated evaluation of nitrous oxide emissions from industrial fossil fuel

combustion," draft final report prepared for the European Atomic Energy Community,

Institut Francais du Petrole, Ref 37-559.

Dietzmann, Harry E., Mary Ann Parness, and Ronald L. Bradow. 1980. "Emissions from

Trucks by Chassis Version of 1983 Transient Procedure." SAE Paper 801371.

IPCC/UNEP/OECD/IEA. 1997. 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.

Jaques, A. P. 1992. Canada’s Greenhouse Gas Emissions: Estimates for 1990. Environmental

Protection Series, Report EPS 5/AP/4, December 1992. Environmental Protection,

Conservation and Protection, Environment Canada.

Joumard, Robert, Robert Vidon, Laurent Paturel, and Gerard de Soete. 1996. "Changes in

pollutant emissions from passenger cars under cold start conditions." SAE Paper 961133.

Laurikko, Juhani and Paivi Aakko. 1995. "The effect of ambient temperature on the emissions of

some nitrogen compounds: a comparative study on low-, medium- and high-mileage

three-way catalyst vehicles." SAE paper 950933.

Lindhjem, Christian E. 1995. "The effect of gasoline reformulation and sulfur reduction on

exhaust emissions from post-1983 but pre-1990 vehicles." SAE paper 950778.

Lindskog, A. 1988. Data presented at the EPA/IFP European Workshop on the Emission of
Page 21

Nitrous Oxide from Fossil Fuel Combustion, Figures 5-18 and 5-19. Rueil-Malmaison,

France, June 1-2, 1988. EPA Report EPA/600/13.

Monroe, David R., Martin H. Krueger, Donald D. Beck and Michael J. D’Aniello, Jr. 1991. 

"The effect of sulfur on three-way catalysts." in A. Crucq (Editor), Catalysis and

Automotive Pollution Control II, proceedings of the Second International Symposium

(CAPoC 2), Brussels, Belgium, September 10-13, 1990. New York: Elsevier Science

Publishers.

Neitzert, Frank. 1998. Personal communication.

Prigent, M., G. de Soete, and R. Doziere. 1991. "The effect of aging on nitrous oxide N2O

formation by automotive three-way catalysts," in A. Crucq (Editor), Catalysis and

Automotive Pollution Control II, proceedings of the Second International Symposium

(CAPoC 2), Brussels, Belgium, September 10-13, 1990. New York: Elsevier Science

Publishers.

Prigent, Michel and Gerard De Soete. 1989. "Nitrous oxide N2O in engines exhaust gases&a

first appraisal of catalyst impact." SAE Paper 890492.

Prigent, M. and G. G. De Soete. 1992. "The increase of N2O emissions caused by three-way

automotive catalysts." NIRE/IFP/EPA/SCEJ 5th Interrnational Workshop on N2O

Emissions, 1-3 July 1992, Tsukuba, Japan, paper 10-3. We do not have this document. 

Data from it is referenced in de Soete (1993).

Smith, L. R. and F. M. Black. 1980. "Characterization of exhaust emissions from passenger cars

equipped with three-way catalyst control systems." SAE paper 800822.

Smith, Lawrence R. and Penny M. Carey. 1982. "Characterization of exhaust emissions from

high mileage catalyst-equipped automobiles." SAE paper 820783.

Urban, Charles M. and Robert J. Garbe. 1979. "Regulated and Unregulated Exhaust Emissions

from Malfunctioning Automobiles." SAE Paper 79696.

Urban, Charles M. and Robert J. Garbe. 1980. "Exhaust Emissions from Malfunctioning ThreeWay Catalyst-Equipped Automobiles." SAE Paper 800511.

U.S. EPA. 1997. The 1997 U.S. Climate Action Report, Submitted by the United States of

America Under the United Nations Framework Convention on Climate Change.

U.S. EPA. 1998. Inventory of U.S. Greehouse Gas Emissions and Sinks 1990-1996. Draft,

3/10/98. U.S. EPA, Office of Policy, Planning, and Evaluation. EPA-230-R-97-002.

U.S. EPA. 1998. "Compilation of Air Pollutant Emission Factors, Volume II: Mobile Sources,"
Page 22

commonly referred to as AP-42, pending 5th edition, last updated: 06 April 1998. 

Accessed from the web site: http://www.epa.gov/omswww/ap42.htm.

U.S. EPA. 1989. "EPA/IPF European Workshop on the emission of nitrous oxide from fossil

fuel combustion (Rueil-Malmaison, France)," Prepared by Air and Energy Engineering

Research, Research Triangle Park, Raleigh, North Carolina, Report No. EPA-600/9-89-

089.

Weaver, Christopher S., Lit-Mian Chan. 1996. "Mobile source emission factors for global

warming gases." Draft Final Report, June 24, 1996, submitted to ICF, Inc., 1850 K Street,

NW, Suite 1000, Washington DC by Engine, Fuel, and Emissions Engineering, Inc.,

9812 Old Winery Place, suite 22, Sacramento, CA 95827.

Weaver, Christopher S. 1998. Telephone conversation on July 10.
Page A-1

APPENDIX A

A DETAILED DISCUSSION OF THE ORIGIN OF THE EMISSION FACTORS FOR

 NITROUS OXIDE FOR GASOLINE HIGHWAY VEHICLES IN THE DRAFT

INVENTORY

The following sections detail the number trail backward from the Draft Inventory to original

sources, supporting the more limited description in the body of the comments.

A. The Draft Inventory, the IPCC Guidelines, Weaver and Chan (1996).

The Draft Inventory lists emission factors that are identical to those of the IPCC Guidelines, 

which in turn are identical to those of Weaver and Chan (1996), who are the source of these

values (Weaver 1998). For light-duty passenger gasoline highway vehicles, the emissions factors

are as follows:

U.S. Draft

Inventory IPCC Guidelines

g/km Calca

g/mi g/kg km/L g/km Calcbg/km Calcc

 g/mi

Low Emission Vehicles 0.04 .064 0.453 8.5 0.040 0.04 0.064

Advanced 3 Way 0.17 .274 1.892 8.3 0.170 0.171 0.275

Early 3-way 0.17 .274 1.81 8 0.170 0.17 0.273

Oxidation Catalyst 0.075 .121 0.622 6.2 0.075 0.075 0.121

Non-Catalyst 0.02 .032 0.125 4.5 0.020 0.021 0.034

Uncontrolled 0.02 .032 0.13 4.7 0.020 0.021 0.033

a

 Calculated from g/km by conversion factor for km/mi

b

 Calculated from g/kg using km/L and 0.75kg/L

c

 Calculated from the calculated g/km by conversion factor for km/mi

The calculations in this table have been done to verify the internal consistency of the emission

factors expressed in different units in the IPCC Guidelines and to examine how the precision

shown affects the interconversion of units. The emission factors listed as g/km in the Draft

Inventory are identical to the factors listed as g/km in the IPCC Guidelines. The tables of

emission factors in the IPCC Guidelines are identical to those in Weaver and Chan (1996).

B. From Ballantyne et al. (1994) to Weaver and Chan (1996)

Weaver and Chan (1996) got their emission factors from the last column of Table 7 in Ballantyne
Page A-2

et al. (1994), headed "Current Canadian Estimates: EPS Inventory:"

Comparison of control-technology terminology and emission factors between Ballantyne et al.

(1994) and Weaver and Chan (1996) 

Ballantyne (1994) Table 7 Weaver and Chan (1996) 

Catalyst Type Current

Canadian

Estimates:

EPS Inventory

(mg/mi)

Units

conversion to

(g/km)

Control

Technology

Emission

Factor

(g/km)

New 3-way 60 .037 LEV 0.040

Aged 3-way 280 .174 Early 3-Way,

Three-way 0.170

Oxidation 120 .075 Oxidation 0.075

None 32 .020 Uncontrolled,

Non-Catalyst 0.020

Of the four sets of estimates in Ballantyne et al.’s Table 7, the reason for choosing this one,

according to Weaver (1998), was the wide range of the estimates, the difficulty of reconciling

them, and the fact that the Canadian emission factors appeared to be official government figures.

Weaver and Chan (1996) assumed both advanced and early TWCs to have the same nitrous oxide

emissions properties. Weaver (1998) indicated that since catalyst aging occurs relatively

quickly, all TWCs were assumed to be aged. He further indicated that he and Chan reasoned that

LEVs would behave like new TWCs, because part of LEV technology was fine tuning catalyst

placement and other factors to insure quick light-off, and that the aging effect was likely due to

delayed light-off. Therefore LEVs would behave like new TWC vehicles.

C. From Jaques (1992) to Ballantyne et al. (1994)

The last column of Ballantyne et al.’s Table 7 originated, with some modification, from

Canada’s Greenhouse Gas Emissions: Estimates for 1990 (Jaques 1992). Ballantyne et al.

appear to have converted Jaques’s units (g/kg) into g/mi by using the fuel economies in Jaques’s

Table 16 and assuming a fuel density of 0.75 kg/L (Jaques’s Table 32), as we show in the

following table:
Page A-3

Catalyst

Type

Jaques (1992) Ballantyne et al. (1994) 

Table 7

Current Canadian Estimates

(mg/mi)

Table 31

(g/kg)

Table 16

Fuel 

Economy

(km/L)

Table 31/

Table 16

(mg/mi)

New 0.6 11.9 61 60

Aged 2.2 9.4 282 280

Ox 0.6 6 121 120

None 0.31 6 62 32

A problem with this derivation is that it produces 62 mg/mi for non-catalyst vehicles, but

Ballantyne et al. list 32 mg/mi. The table below shows the column of Ballantyne’s Table 7

giving the range of de Soete’s (1989) estimates. We have produced the last column below

(Actual Ranges) directly from de Soete’s (1989) Table XIV.

Catalyst

Type

Ballantyne et al. (1994), Table 7 de Soete's (1989)

Actual Ranges

from Table XIV

all points

lines 2,4-7, 11-19

(mg/mi)

Current

Canadian

Estimates

(mg/mi)

de Soete, 1989

Range

(mg/mi)

New 60 60-170 54-141

Aged 280 260-355 50-1000

Ox 120 120 112-257

None 32 8-32 13-151

Ballantyne et al.’s listing of de Soete’s (1989) ranges suggest that they took what they

understood to be the high end of the non-catalyst range rather than the value provided by Jaques

(1992), and mis-attributed it to Jaques. However, the last column of the above table shows de

Soete’s (1989) actual ranges as well as we have been able to determine them. One could suppose

that Ballantyne et al. excluded outlier values, but the high end of Ballantyne et al.’s (1994) new

TWC range and the low end of their non-catalyst range lie outside the actual ranges. Ballantyne,

in an email through Stephanson (one of Ballantyne’s co-authors), was unable to recall the origin

of the non-catalyst emission factor.

Another problem in using the last column in Ballantyne et al.’s Table 7 to represent the Current

Canadian Estimates is that Jaques derived his emission factors from g/km data in de Soete (1989)

and converted them to g/kg by assuming a uniform fuel economy of 8.5km/L (Neitzert 1998). 

Ballantyne et al. then converted these numbers (except for the non-catalyst case) to units of g/mi
Page A-4

by assuming a different set of fuel economies. The following table compares Ballantyne et al.’s

"Current Canadian Estimates" to Jaques’s, using the same fuel economy he assumed in deriving

them.

Catalyst

Type

Ballantyne et al. (1994)

Current Canadian Estimates

(mg/mi)

Jaques (1992)

assuming 8.5km/L

(mg/mi)

New 60 85

Aged 280 312

Ox 120 85

None 32 44

D. From de Soete (1989) to Jaques (1992) 

The official Canadian estimates for 1990 (Jaques 1992) have been derived primarily from de

Soete (1989). Neitzert (1998) said that the emission factors in Jaques (1992) for new TWC, aged

TWC and non-catalyst vehicles were obtained by averaging lines one and two of Table XXIX (de

Soete 1989) and assuming a fuel economy of 8.5 km/L and a fuel density of 0.75kg/L. The

following table demonstrates this derivation and also shows emission factors in g/km and g/mi:

Uncontrolled New

TWC

Aged

TWC

line 1, Table XXIX (gN/km) 0.026 0.03 0.137

line 2, Table XXIX (gN/km) 0.008 0.037 0.11

line 1, Table XXIX (gN2O/km) 0.041 0.048 0.216

line 2, Table XXIX (gN2O/km) 0.013 0.059 0.173

line 1, Table XXIX (gN2O/mi) 0.066 0.077 0.347

line 2, Table XXIX (gN2O/mi) 0.021 0.095 0.278

avg of lines 1 and 2 (gN2O/km) 0.027 0.053 0.194

avg of lines 1 and 2 (gN2O/mi) 0.044 0.086 0.313

Assuming 8.5km/L,.75kg/L (gN2O/kg) 0.307 0.604 2.201

Jaques (1992), Table 31 (gN2O/kg) 0.31 0.6 2.2

Comparison of the last two lines shows that we have successfully reproduced Jaques’s emission

factors, except for the oxidation catalyst case.

Next, we must ask where Jaques’s oxidation catalyst emission factor of 0.6 g/kg originated. De

Soete’s Table XXIX is a summary table for his Table XIV, which lists five emission factors for
Page A-5

vehicles equipped with oxidation catalysts. The average of these is 0.092 g/km, which converts

to 1.04 g/kg and does not match Jaques’s emission factor. We believe the most plausible

explanation of Jaques’s oxidation catalyst emission factor is that, in the absence of data lower

than the new TWC data, it was simply taken to be identical to the new TWC emission factor. 

Neitzert (1998) did not know how the oxidation catalyst emission factor was derived.

In the next section, we examine the original data sources.

E. De Soete’s (1989) original sources

The averages in lines 1 and 2 of Table XXIX are referenced to lines 2, 4-7, and 11 of Table XIV. 

Each of the Table XIV lines referenced is actually a series of individual data points. Below are

lines 1 and 2 of de Soete’s Table XXIX:

Emission factor

(g N2O as N/km)

Uncontrolled

(without catalyst)

With Three-Way Catalyst

New Aged

Averaged over all cycles: ECE cold, ECE

hot, EUDC and SDC (Table XIV, lines 2,4

to 7, 11 to 19)

0.0261 0.0304 0.1373

Averaged over all cycles: ECE cold, ECE

hot and EUDC (Table XIV, lines 11

through 19)

0.0084 0.0374 0.1099

Note that the values in this table are averaged over European driving cycles only. There are a

few FTP cycles included in Table XIV, but they are not included in the averages in Table XXIX.

Below, we have attempted to replicate Table XXIX by averaging the specified lines in Table

XIV:

Lines of Table XIV

used in average

Uncontrolled

(without catalyst)

With Three-Way Catalyst

New Aged

2,4 to 7, 11 to 19 0.0236 0.0355 0.1177

11 to 19 0.0085 0.0374 0.11

2,4 to 7 0.0284 0.0292 0.1486

Lines 2 & 3 of this

table

0.0184 0.0333 0.1293

We have succeeded in replicating line 2 of Table XXIX, but not line 1.
Page A-6

This table shows the data sources contributing to Jaques (1992). Table XIV is in de Soete

(1989).

Data Source Data Points fora Referenceb

no

cat.

new

TWC

aged

TWC

ox

cat.

Table XIV,

line 2

6

24%

1

6

20%

1

42. Lindskog, A., data presented at the EPA/IFP

Workshop 1988, report on that meeting p. 103 and

tables 5-18 and 5-19.

Table XIV,

lines

4-7

13

52%

2

15

24%

43, see also 44 and 45.

43. Prigent, M., Doziere, R. and De Soete, G.,

unpublished document of the French Petroleum

Institute, 1988.

44. Prigent, M., data presented at the EPA/IFP

Workshop 1988, p. 103-109 and figs 5-7 and 5-8

45. Prigent, Michel and Gerard De Soete. 1989. 

"Nitrous oxide N2O in engines exhaust gases&a

first appraisal of catalyst impact." SAE Paper

890492.

Table XIV,

lines 11-19

6

24%

1

48

76%

24

80%

47. Prigent, M.F., de Soete, G.G. and Doziere, R.,

"The effect of catalyst aging on nitrous oxide

emissions from automobiles with a three-way

catalyst," to be presented at the CAPoC Meeting,

Brussels, 10-13 September 1990.

Total 25 63 30 0

a. Top number is the number of data points. The next number is the percentage of total data

points that reference comprises for the control category. The third number is the number of

vehicles tested. If there is only one vehicle number in a row, the same vehicle or vehicles

were used to test all control technologies.

b. The numbers are those used in Table XIV to indicate the references.

42. Lindskog 1988 reported on two cars on the SDC (Swedish driving cycle), a Volvo 240

without a catalyst, and a Volvo 260 with 10,000 km with a TWC.

45. Prigent and de Soete. 1989. "Nitrous oxide N2O in engines exhaust gases&a first appraisal of

catalyst impact." SAE Paper 890492 is apparently the same material presented at the EPA/IFP

Workshop in 1988. Two cars were tested, a Citroen BX19GT and a Renault Fuego U.S. version,
Page A-7

with and without TWCs.

47. Prigent, M., G. de Soete, and R. Doziere. 1991. "The effect of aging on nitrous oxide N2O

formation by automotive three-way catalysts," in A. Crucq (Editor), Catalysis and Automotive

Pollution Control II, proceedings of the Second International Symposium (CAPoC 2), Brussels,

Belgium, September 10-13, 1990. New York: Elsevier Science Publishers. These tests were

performed using 8 different catalysts and two similar 2.2L 4 cylinder engines. One was for aging

the catalysts, the other, mounted in an unspecified chassis, was used for running the tests. The

catalysts were mounted 1.4 m from the engine. They were not production catalysts but

apparently were fabricated for these tests. This reference gives measurements in g/test

graphically and ratios between catalyst and non-catalyst vehicles numerically. However, the

individual test results in g/km are given in Table XIV of de Soete (1989). The test vehicle and

engine are unidentified in the reference, but are identified in Table XIV as a "Fuego, 2.21 L

engine equipped with electronic fuel injection + oxygen sensor, closed loop."

A subsequent publication by the same researchers apparently involving a different vehicle and

the same or a similar set of new and aged catalysts, gives sharply lower nitrous oxide emission

factors: the averages were 12 mg/mi for non-catalyst vehicles, 29 mg/mi for new TWCs, and 42

mg/mi for aged TWCs (Prigent and de Soete 1992). These are lower than their previously

reported values by factors of approximately 4, 3, and 7, respectively.

F. Summary of the data sources

 All the emission factors originate from testing done on five cars using European

test cycles. Fuel sulfur content for these tests was unspecified.

 The new and aged TWC emission factors are based 90% on a single study using a

single car with eight non-production catalysts, new and bench-aged, with the

catalysts located 1.4 m from the engine. The other 10% of the data for the TWC

emission factors came from two studies and three more cars, all tested on

European driving cycles only.

 The non-catalyst emission factors were derived from four cars.

 The emission factor for oxidation catalyst vehicles does not appear to be based on

testing, but is instead the same emission factor used for new TWCs.
Page B-1

APPENDIX B

NVFEL TESTING PROGRAM

SUMMARY TABLE OF PRELIMINARY NVFEL TESTING RESULTS.

Nitrous oxide emissions are shown in grams per mile for the composite FTP cycle. CAAB is the

Clean Air Act Baseline Fuel, which contained 285 ppm sulfur. Indolene contained 24 ppm

sulfur.

PV = passenger vehicle

MV = mini-van

PU = pickup truck

SUV = sport utility vehicle

Vehicle

Type

Odometer

(miles)

Emissions

Control

CAAB Fuel Indolene Fuel

75( F

A/C Off

95( F

A/C On

75( F

A/C Off

95( F

A/C On

PV 75698 Tier 0 0.018 0.053

MV 39539 Tier 1 0.046 0.067

PV 48690 Tier 1 0.059 0.124

MV 23914 Tier 1 0.038 0.033

MV 27491 Tier 1 0.028 0.049

PV 47461 Tier 1 0.027 0.035

PV 38766 Tier 1 0.036 0.054 0.039

PV 75083 Tier 1 0.042 0.052

PV 24086 Tier 1 0.024 0.081

MV 23838 Tier 1 0.027 0.029

PU 26262 Tier 1 0.227 0.203 0.115

PU 41549 Tier 1 0.167 0.145

PU 20585 Tier 1 0.082 0.082

PU 16319 Tier 1 0.087 0.102

PU 19251 Tier 1 0.080

MV 32818 Tier 1 0.043

PV 28935 Tier 1 0.033

PV 41896 Tier 1 0.046

SUV 20949 Tier 1 0.126 0.063

PV 4959 LEV **   ** 

MV 169311 * LEV **   ** 

PV 5038 * LEV ** **

MV 5038 * LEV **   ** 

* Catalyts bench-aged to 100,000 miles.

** Data not shown here, but included in averages.
Page B-2

ANALYSES OF THE FUELS USED IN NVFEL TESTING

Tested at NVFEL

CAAB Dispen. #2: tests conducted between 6/4 and 6/15/98

tank #21 cert (Indolene): tests conducted between 2/26 and 3/16/93

Test 

Code Test method CAAB Indolene UNITS

552 MTBE by OFID 0.099 0 Oxy Percent

562 ETBE by OFID 0 0 Oxy Percent

534 Ethanol by OFID 0 0 Oxy Percent

572 TAME by OFID 0 0 Oxy Percent

421 Sulfur in Gasoline by ASTM D 2622 285 24 Parts Per Million

62 Vapor Pressure by Appendix E Method 3 7.44 9.02 PSIA

65 Percent Evaporated at 200 Degrees F 39.5 36.4 Volume Percent

66 Percent Evaporated at 300 Degrees F 80.5 88.3 Volume Percent

48 Aromatics in Gasoline MSD D5769 38.261 49.437 Volume Percent

49 Olefins in by FIA D-1319-93 8.524 1.053 Volume Percent

64 Benzene in Gasoline by ASTM D 3606 1.22 0.2409 Volume Percent

46 Aromatics by FIA D-1319-93 30.6 30.1 Volume Percent

69 Specific Gravity @ 60 Degrees F 0.75352 0.74397 60/60F

692 Degrees API 56.28 58.69 Degrees API

691 Density @ 60 deg F 0.75278 0.74324 g/cm-03 @ 60 deg F

101 D 86 Initial Boiling Point 98.8 92.8 Degrees F

110 10 Percent 139.69 132.39 Degrees F

150 50 Percent 222.6 219.8 Degrees F

190 90 Percent 338.89 315.79 Degrees F

200 End Point 413.89 381.7 Degrees F

201 Residue 1.89 1 mL

202 Total Recovery 96.8 97.69 mL

203 Loss 1.29 1.29 mL

592 Volume Percent Oxygenates by MSD 0.55 0 Volume Percent

541 Methanol by MSD (Screen) 0 0 Volume Percent

591 Weight Percent Oxygen by MSD 0.1 0 Weight Percent

543 Methanol by OFID 0 0 Volume Percent

585 t-Butanol by OFID 0 0 Volume Percent

589 Isobutanol by OFID 0 0 Volume Percent

5802 n-Butanol by OFID 0 0 Volume Percent

593 Volume Percent Oxygenates by OFID 0.55 0 Volume Percent

59 Weight Percent Oxygen by OFID 0.09 0 Weight Percent

30 Lead in Gasoline by ASTM D 3237 0.001 na Grams Pb per Gallon

32 Weight Fractioin Carbon ASTM D 3343-

95 0.8673 0.8661 Weight Fraction

991 Phosphorus in Gasoline by ASTM D

3231 0 na Grams per Gallon

73 Net Heat of Combustion ASTM D 3338-92 18428 na BTU per Pound

221 Motor Octane 82.59 na Motor Octane Number

220 Research Octane 92.09 na Research Octane

Number

218 Sensitivity 9.5 na RON-MON

na = not analyzed
Page B-3

WHY WE BELIEVE THE NITROUS OXIDE DIFFERENCES BETWEEN THE TWO

FUELS ARE DUE TO DIFFERENCES IN SULFUR CONTENT

In the NVFEL testing program, emissions of nitrous oxide average 2.5 times higher using CAAB

fuel than using Indolene. There are many differences between these two fuels, as the previous

table shows. In this section, we make our case&suggestive, rather than conclusive&that the

difference in nitrous oxide emissions is primarily due to the difference in the sulfur content of the

fuels.

The argument may be summarized that TWCs emit nitrous oxide when they are performing less

efficiently than normal (e.g., at lower than normal operating temperatures or after aging), when

they also emit more NOX. Sulfur decreases the efficiency of TWCs and increases NOX

emissions. Therefore, sulfur is also likely to increase nitrous oxide emissions. Of the differences

between CAAB fuel and Indolene, modeling shows that only sulfur accounts for the increased

NOX emissions of CAAB fuel. Therefore, that same difference probably also accounts for the

increased nitrous oxide emissions.

We know of no published testing on the effects of sulfur content or other fuel parameters on

nitrous oxide emissions. However, existing data suggest that for cars equipped with TWCs,

conditions that enhance NOX emissions (i.e., decrease the effectiveness of the catalyst) also

enhance nitrous oxide emissions. For example:

1. Nitrous oxide emissions are maximal at around the light-off temperature of catalysts,

when NOX conversion is suboptimal (Prigent et al. 1991). 

2. For the same cars equipped with TWCs, tuneups decrease both NOX and nitrous oxide

emissions (Smith and Carey 1982).

3. Running the FTP at a lower than normal temperature in two cases increased NOX

emissions and in two cases decreased them. In all cases the change in nitrous oxide

emissions was in the same direction as the change in NOX emissions (Braddock 1981).

Sulfur has been shown to decrease the effectiveness of NOX conversion (Lindhjem 1995, Monroe

et al. 1991).

Also, we have received an email from Matthias Tappe of the German Federal Environmental 

Agency - Environment and Traffic as follows: "Regarding the sulfur content your idea that the

sulfur level in gasoline influences the N2O emissions has been confirmed by industry data

available to us."

EPA’s Complex Model shows that the sulfur difference between CAAB and Indolene is the

only difference that strongly affects NOX emissions

EPA’s Complex Model was developed in conjunction with petroleum refiners and gasoline

formulators to provide guidance as to how emissions would change when various gasoline

components were altered. The model predicts that our CAAB fuel will emit 13% more NOX than
Page B-4

our Indolene fuel. The first column in the following table lists the components used by the

Complex Model that are different between CAAB and Indolene. The second column shows the

percentage change in NOX emissions that results when the component in the first column is

changed from its value in Indolene to its value in CAAB. The third column shows the

percentage change in NOX emissions when all components except the one in the first column are

changed from their Indolene to their CAAB values.

Parameter

Percentage change in NOX emissions from Indolene when:

Only this parameter is

changed to its CAAB value

All parameters except this

one are changed to their

CAAB values

All parameters 13 0

MTBE 0 13

Sulfur 12 1

RVP 0 13

E200 0 13

E300 0 12

Aromatics 0 13

Olefins 1 12

Benzene 0 13

Sulfur is the parameter of greatest importance in diminishing the catalytic reduction of NOX, and,

therefore, we suspect, also the parameter of greatest importance in enhancing the production of

nitrous oxide.
Page C-1

APPENDIX C

ASSUMED FUEL ECONOMIES WHOSE RATIOS WERE USED TO GENERATE

EMISSION FACTORS FOR VEHICLES FOR WHICH THERE WERE NO DATA

The following of fuel economies and carbon dioxide emission factors are from Tables 1-27

through 1-33 of the IPCC Guidelines. The source of these tables is Weaver and Chan (1996). 

Chan (1998) said that their source for fuel economies was MOBILE5 reduced by 15%. The use

of fuel-consumption ratios to determine emission factors should be considered a temporary

measure only, to be replaced as soon as real data are available. In calculating emission factors,

we used the ratios of carbon dioxide emission factors, rather than of fuel economies, because

they were listed with more significant digits. The two are equivalent within rounding error, as is

shown by the third column of the table below, which was obtained by multiplying the first two

columns together.

Vehicle type and control

technology

Fuel Ec

(km/L)

CO2

(g/km)

Test

CO2*Fuel Ec

(g/L)

Gasoline Passenger Cars

Control Technology

Low Emission Vehicles* 8.5 280 2380

Tier 1 8.3 285 2366

Tier 0 8 298 2384

Oxidation Catalyst 6.2 383 2375

Non-Catalyst 4.5 531 2390

Uncontrolled 4.7 506 2378

* Applicable to California VMT only

Gasoline Light-Duty Trucks

Control Technology

Low Emission Vehicles* 6 396 2376

Tier 1 6 396 2376

Tier 0 4.8 498 2390

Oxidation Catalyst 4.8 498 2390

Non-Catalyst 4 601 2404

Uncontrolled 4.1 579 2374

* Applicable to California VMT only

Gasoline Heavy-Duty Vehicles

Control Technology

Tier 0 2.3 1017 2339

Oxidation Catalyst 2.3 1036 2383

Non-catalyst 1.8 1320 2376

Uncontrolled 1.8 1320 2376

Diesel Passenger Cars

Control Technology

Advanced 10 237 2370

Moderate 9.6 248 2381

Uncontrolled 7.5 319 2393

Diesel Light Trucks

Control Technology
Vehicle type and control

technology

Fuel Ec

(km/L)

CO2

(g/km)

Test

CO2*Fuel Ec

(g/L)

Page C-2

Advanced 7.2 330 2376

Moderate 7.2 331 2383

Uncontrolled 5.7 415 2366

Diesel Heavy-Duty Vehicles

Control Technology

Advanced 2.4 987 2369

Moderate 2.4 1011 2426

Uncontrolled 2.2 1097 2413

Motorcycles

Control Technology

Non-Catalyst Control 10.8 219 2365

Uncontrolled 8.9 266 2367