United States Air and Radiation EPA420-R-98-009
Environmental Protection August 1998
Agency
&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)
> Printed on Recycled Paper
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EPA420-R-98-009
August 1998
of
Comments on the Draff inventory of U.S. Emissions and Sinks,
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.
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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-l
APPENDIX B
NVFEL TESTING PROGRAM Page B-l
APPENDIX C
ASSUMED FUEL ECONOMIES WHOSE RATIOS WERE USED TO GENERATE
EMISSION FACTORS FOR VEHICLES FOR WHICH THERE WERE NO DATA
Page C-l
Page ii
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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
LEV
Advanced 3 Way (Tier 1)
Early 3-way (Tier 0)
Oxidation Catalyst
Non-Catalyst
Uncontrolled
N2O Emission Factors for Passenger Vehicles
Last Official Inventory
(g/km)
0.019
0.046
0.027
0.005
0.005
3/1 0/98 Draft Inventory
(g/km)
0.040
0.170
0.170
0.075
0.020
0.020
OMS Revision
(g/km)
0.018
0.029
0.051
0.032
0.010
0.010
The Draft Inventory adopted the emission factors for U. S. vehicles from the Revised 1996IPCC
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:
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Comparison of estimated emission factors in the IPCC Guidelines
between U.S. and European passenger vehicles
Control
Technology
LEV
Advanced 3 Way
Early 3-way
Oxidation Catalyst
Non-Catalyst
Uncontrolled
Emission Factors (g/mi)
U.S. Passenger
Vehicles
0.064
0.274
0.274
0.121
0.032
0.032
European Passenger
Vehicles
0.08
0.008
0.008
0.008
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 Is 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
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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.
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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
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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
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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
All ages
New
Aged
Parameter
mg/mi
n
std. err. of mean
mg/mi
n
std. err. of mean
mg/mi
n
std. err. of mean
Catalyst Type
All
56.
50
6.5
42.7
29
4.8
74.2
21
13.2
None
17.
3.
13.
17.
3.
13.
Oxidation
51.7
11
19.1
37.8
4
12.
59.7
7
29.7
3-Way
60.5
36
6.8
47.2
22
5.4
81.5
14
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.
All
Without Ballantyne et al.
Ballantyne et al. only
New TWC
50.4 (25)
47.2 (22)
74. (3)
Aged TWC
97.7 (22)
81.5(14)
126. (8)
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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 Is
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 low-
sulfur 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
All
Passenger vehicles
Light trucks and SUVs
Emission
factor
(mg/mi)
63.6
46.3
108.9
Number
of vehicles
17
12
5
Number
of samples
29
21
8
Std. err.
mean
(mg/mi)
7.1
5.0
11.8
Range
(mg/mi)
24-167
24-124
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.
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LEV emission factors from NVFEL program
Fuel
CAAB Fuel
Indolene
Emission
factor
(mg/mi)
77.8
28.3
Number of
vehicles
3
4
Number of
samples
6
8
Std. err.
mean
(mg/mi)
14.7
2.5
Range
(mg/mi)
32.-116.
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.
N2Ovs.
0.15 -,
| 0.10-
O)
£5 0.05 -
0.00
(
Tierl
4
) 10000 20000
Mileage for CAAB Fuel
Passenger Vehicles
•
:. • *• ; *
30000 40000 50000 60000 70000 80000
Mileage
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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
NVFEL
Ballantyne et al.
(1994)
Barton and
Simpson (1994)
Age
Old
All
Old
New
All
Old
New
Emission factors
(mg/mi) (number of vehicles)
Light-duty trucks
109 (5)
188 (3)
93(1)
236 (2)
163 (3)
300(1)
95(1)
Passenger
vehicles
46 (12)
111(11)
126 (8)
74(3)
75(11)
80(11)
55(2)
Average
Trucks/PVs
(ratio)
2.4
1.7
0.73
3.2
2.2
3.8
1.7
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 MOBILES
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
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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
Non-catalyst
Oxidation catalyst
TierO
Tier 1
LEVs on standard fuel
LEVs on low-S fuel
Emission
Factor
(mg/mi)
16.6
51.7
81.5
46.3
77.8
28.3
n
O
11
12
21
6
8
Std. Err.
Mean
(mg/mi)
13.0
19.1
13.8
5.0
14.7
2.5
Range
(mg/mi)
2-42
8-233
6-190
24-124
32-116
14-36
Emission
Factor
(mg/km)*
10.3
32.2
50.7
28.8
48.4
17.4
* Extra precision has been included so conversion between units does not introduce a
significant difference.
Summary of Sources:
Control Technology
Non-catalyst
Oxidation catalyst
TierO
Tier 1
LEVs on standard
fuel
Data Source
Prigent and de Soete (1989), Dasch (1992), and Urban and
Garbe (1979)
Smith and Carey (1982), Urban and Garbe (1979)
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).
NVFEL (1998). CAAB fuel, both A/C modes.
NVFEL (1998). CAAB fuel, both A/C modes.
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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 MOBILES, 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 HDD Vs.
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
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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
Diesel Passenger Cars
Control Technology
Advanced
Moderate
Uncontrolled
Diesel Light Trucks
Control Technology
Advanced
Moderate
Uncontrolled
Diesel Heavy-Duty Vehicles
Control Technology
Advanced
Moderate
Uncontrolled
Draft Inventory
(g/km)
0.0070
0.0100
0.0140
0.0240
0.0630
0.0310
0.0250
0.0250
0.0310
Fuel-specific based on
Dietzmann et al. (1980)
(g/km)
0.0068
0.0071
0.0091
0.0094
0.0095
0.0119
0.0283
0.0289
0.0314
European
(g/km)
0.0100
0.0100
0.0100
0.0200
0.0200
0.0200
0.0300
0.0300
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."
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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.
VMT splits by vehicle age, in Draft Inventory
(mobile96.xls)
Note: LDGV=LDDV and LDGT1=LDDT
Age (years, 1=current model year)
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 13
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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. Second-
by-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 14
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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 15
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"Early" and "Advanced" TWCs, we use the terms "Tier 0" and "Tier 1".
Vehicle type and control technology
Gasoline Passenger Cars
Control Technology
Low Emission Vehicles*
Tierl
TierO
Oxidation Catalyst
Non-Catalyst
Uncontrolled
Nitrous Oxide Emission
Factors
g/mi
0.0283
0.0463
0.0815
0.0517
0.0166
0.0166
* Applicable to California VMT only
Gasoline Light-Duty Trucks
Control Technology
Low Emission Vehicles*
Tierl
TierO
Oxidation Catalyst
Non-Catalyst
Uncontrolled
* Applicable to California VMT only
Gasoline Heavy-Duty Vehicles
Control Technology
TierO
Oxidation Catalyst
Non-catalyst
Uncontrolled
Diesel Passenger Cars
Control Technology
Advanced
Moderate
Uncontrolled
Diesel Light Trucks
Control Technology
Advanced
Moderate
Uncontrolled
Diesel Heavy-Duty Vehicles
Control Technology
Advanced
Moderate
Uncontrolled
Motorcycles
Control Technology
Non-Catalyst Control
Uncontrolled
0.0400
0.0643
0.1362
0.0673
0.0188
0.0190
0.2781
0.1400
0.0412
0.0432
0.0161
0.0161
0.0161
0.0322
0.0322
0.0322
0.0483
0.0483
0.0483
0.0068
0.0087
g/km
0.0176
0.0288
0.0507
0.0322
0.0103
0.0103
0.0249
0.0400
0.0846
0.0418
0.0117
0.0118
0.1729
0.0870
0.0256
0.0269
0.0100
0.0100
0.0100
0.0200
0.0200
0.0200
0.0300
0.0300
0.0300
0.0042
0.0054
6.2 Revised technology assignments by model year for gasoline highway vehicles except
Page 16
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motorcycles
For Gasoline Passenger Cars (light duty gas vehicles. LDGVX except California:
Model Year
<1972
1973-1974
1975
1976-1977
1978-1979
1980
1981
1982
1983
1984-1993
1994
1995
1996
Percentage of 49 States LDGV with each control technology
Uncontrolled
100
Non-catalyst
control
100
20
15
10
5
Oxidation
80
85
90
88
15
14
12
TierO
7
85
86
88
100
60
20
Tier 1
40
80
100
Page 17
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For Gasoline Light Duty Trucks (LDGTX except California:
Model Year
<1972
1973-1974
1975
1976
1977-1978
1979-1980
1981
1982
1983
1984
1985
1986
1987-1993
1994
1995
1996
Percentage of 49 States LDGT with each control technology
Uncontrolled
100
Non-catalyst
control
100
30
20
25
20
Oxidation
70
80
75
80
95
90
80
70
60
50
5
TierO
5
10
20
30
40
50
95
60
20
Tier 1
40
80
100
For Gasoline Heavy-Duty Vehicles (heavy-duty gas vehicles. HDGV):
Model Year
<1981
1982-1984
1985-1986
1987
1988-1989
1990-2003
2004
Percentage of national HDGV with each control technology
Uncontrolled
100
95
Non-catalyst
control
95
70
60
45
Oxidation
5
5
15
25
30
TierO
15
15
25
100
Page 18
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For California Gasoline Passenger Cars and Light-Duty Trucks (light duty gas vehicles and
trucks. LDGV and LDGTI:
Model
Year
<1972
1973-1974
1975-1979
1980-1981
1982
1983
1984-1991
1992
1993
1994
1995
1996
Percentage of California LDGV and LDGT fleet with each control
technology
Uncontrol
led
100
Non-
catalyst
control
100
Oxidation
100
15
14
12
Tier 0
85
86
88
100
60
20
Tier 1
40
80
90
85
80
LEV
10
15
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 19
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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 InternationalIPCC
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 20
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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 //, 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 //, 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 Three-
Way 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 21
-------
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 22
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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:
Low Emission Vehicles
Advanced 3 Way
Early 3 -way
Oxidation Catalyst
Non-Catalyst
Uncontrolled
U.S. Draft
Inventory
, Calca
g/km . .
g/mi
0.04 .064
0.17 .274
0.17 .274
0.075 .121
0.02 .032
0.02 .032
IPCC Guidelines
g/kg
0.453
1.892
1.81
0.622
0.125
0.13
km/L g/km
8.5
8.3
8
6.2
4.5
4.7
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
0.040
0.170
0.170
0.075
0.020
0.020
km/mi
Calcb
g/km
0.04
0.171
0.17
0.075
0.021
0.021
Calcc g/mi
0.064
0.275
0.273
0.121
0.034
0.033
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-l
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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
Catalyst Type
New 3 -way
Aged 3 -way
Oxidation
None
Current
Canadian
Estimates:
EPS Inventory
(mg/mi)
60
280
120
32
Units
conversion to
(g/km)
.037
.174
.075
.020
Weaver and Chan (1996)
Control
Technology
LEV
Early 3 -Way,
Three-way
Oxidation
Uncontrolled,
Non-Catalyst
Emission
Factor
(g/km)
0.040
0.170
0.075
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-2
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Jaques (1992)
Catalyst
Type Table 3 1
(g/kg)
New 0.6
Aged 2.2
Ox 0.6
None 0.31
Table 16
„ , Table 3 1/
Fuel „ , ,
„ Table 16
Economy , ...
(km/L) (mg/mi)
11.9 61
9.4 282
6 121
6 62
Ballantyne et al. (1994)
Table 7
Current Canadian Estimates
(mg/mi)
60
280
120
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
New
Aged
Ox
None
Ballantyne et al.
Current
Canadian
Estimates
(mg/mi)
60
280
120
32
(1994), Table 7
de Soete, 1989
Range
(mg/mi)
60-170
260-355
120
8-32
de Soete's (1989)
Actual Ranges
trom lable XIV
all points
lines 2,4-7, 11-19
(mg/mi)
54-141
50-1000
112-257
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-3
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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
New
Aged
Ox
None
Ballantyne et al. (1994)
Current Canadian Estimates
(mg/mi)
60
280
120
32
Jaques (1992)
assuming 8.5km/L
(mg/mi)
85
312
85
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:
line 1, Table XXIX (gN/km)
line 2, Table XXIX (gN/km)
line 1, Table XXIX (gN2O/km)
line 2, Table XXIX (gN2O/km)
line 1, Table XXIX (gN2O/mi)
line 2, Table XXIX (gN2O/mi)
avg of lines 1 and 2 (gN2O/km)
avg of lines 1 and 2 (gN2O/mi)
Assuming 8.5km/L,.75kg/L (gN2O/kg)
Jaques (1992), Table 31 (gN2O/kg)
Uncontrolled
0.026
0.008
0.041
0.013
0.066
0.021
0.027
0.044
0.307
0.31
New
TWC
0.03
0.037
0.048
0.059
0.077
0.095
0.053
0.086
0.604
0.6
Aged
TWC
0.137
0.11
0.216
0.173
0.347
0.278
0.194
0.313
2.201
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-4
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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
(gN2OasN/km)
Averaged over all cycles: ECE cold, ECE
hot, EUDC and SDC (Table XIV, lines 2,4
to 7, 11 to 19)
Averaged over all cycles: ECE cold, ECE
hot and EUDC (Table XIV, lines 1 1
through 19)
Uncontrolled
(without catalyst)
0.0261
0.0084
With Three-Way Catalyst
New
0.0304
0.0374
Aged
0.1373
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
2,4 to 7, 11 to 19
11 to 19
2,4 to 7
Lines 2 & 3 of this
table
Uncontrolled
(without catalyst)
0.0236
0.0085
0.0284
0.0184
With Three-Way Catalyst
New
0.0355
0.0374
0.0292
0.0333
Aged
0.1177
0.11
0.1486
0.1293
We have succeeded in replicating line 2 of Table XXIX, but not line 1.
Page A-5
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This table shows the data sources contributing to Jaques (1992). Table XIV is in de Soete
(1989).
Data Source
Table XIV,
line 2
Table XIV,
lines
4-7
Table XIV,
lines 11-19
Total
Data Points for3
no
cat.
6
24%
1
13
52%
2
6
24%
1
25
new
TWC
15
24%
48
76%
63
aged
TWC
6
20%
1
24
80%
30
ox
cat.
0
Reference13
42. Lindskog, A., data presented at the EPA/IFP
Workshop 1988, report on that meeting p. 103 and
tables 5-18 and 5-19.
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.
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.
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-6
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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 1.4m 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 A-7
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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
PV
MV
PV
MV
MV
PV
PV
PV
PV
MV
PU
PU
PU
PU
PU
MV
PV
PV
SUV
PV
MV
PV
MV
Odometer
(miles)
75698
39539
48690
23914
27491
47461
38766
75083
24086
23838
26262
41549
20585
16319
19251
32818
28935
41896
20949
4959
169311 *
5038*
5038*
Emissions
Control
TierO
TieM
Tierl
TieM
Tierl
Tierl
Tierl
Tierl
Tierl
Tierl
Tierl
Tierl
Tierl
Tierl
Tier 1
Tierl
Tierl
Tierl
Tierl
LEV
LEV
LEV
LEV
CAAB Fuel
75° F
A/COff
0.018
0.046
0.059
0.038
0.028
0.027
0.036
0.042
0.024
0.027
0.227
0.167
0.082
0.087
**
**
**
95° F
A/C On
0.053
0.067
0.124
0.033
0.049
0.035
0.054
0.052
0.081
0.029
0.203
0.145
0.082
0.102
0.080
0.043
0.033
0.046
0.126
**
**
**
Indolene Fuel
75° F
A/C Off
0.115
0.063
**
**
**
**
95° F
A/C On
0.039
**
**
**
**
* Catalyts bench-aged to 100,000 miles.
** Data not shown here, but included in averages.
PageB-1
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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
552
562
534
572
421
62
65
66
48
49
64
46
69
692
691
101
110
150
190
200
201
202
203
592
541
591
543
585
589
5802
593
59
30
32
991
73
221
220
218
Test method
MTBE by OFID
ETBE by OFID
EthanolbyOFID
TAME by OFID
Sulfur in Gasoline by ASTM D 2622
Vapor Pressure by Appendix E Method 3
Percent Evaporated at 200 Degrees F
Percent Evaporated at 300 Degrees F
Aromatics in Gasoline MSD D5769
Olefinsin by FIA D-1319-93
Benzene in Gasoline by ASTM D 3606
Aromatics by FIA D-1319-93
Specific Gravity @ 60 Degrees F
Degrees API
Density @ 60 deg F
D 86 Initial Boiling Point
1 0 Percent
50 Percent
90 Percent
End Point
Residue
Total Recovery
Loss
Volume Percent Oxygenates by MSD
Methanol by MSD (Screen)
Weight Percent Oxygen by MSD
Methanol by OFID
t-Butanol by OFID
Isobutanol by OFID
n-Butanol by OFID
Volume Percent Oxygenates by OFID
Weight Percent Oxygen by OFID
Lead in Gasoline by ASTM D 3237
Weight Fractioin Carbon ASTM D 3343-
95
Phosphorus in Gasoline by ASTM D
3231
Net Heat of Combustion ASTM D 3338-92
Motor Octane
Research Octane
Sensitivity
CAAB
0.099
0
0
0
285
7.44
39.5
80.5
38.261
8.524
1.22
30.6
0.75352
56.28
0.75278
98.8
139.69
222.6
338.89
413.89
1.89
96.8
1.29
0.55
0
0.1
0
0
0
0
0.55
0.09
0.001
0.8673
0
18428
82.59
92.09
9.5
Indolene
0
0
0
0
24
9.02
36.4
88.3
49.437
1.053
0.2409
30.1
0.74397
58.69
0.74324
92.8
132.39
219.8
315.79
381.7
1
97.69
1.29
0
0
0
0
0
0
0
0
0
na
0.8661
na
na
na
na
na
UNITS
Oxy Percent
Oxy Percent
Oxy Percent
Oxy Percent
Parts Per Million
PSIA
Volume Percent
Volume Percent
Volume Percent
Volume Percent
Volume Percent
Volume Percent
60/60F
Degrees API
g/cm-03 @ 60 deg F
Degrees F
Degrees F
Degrees F
Degrees F
Degrees F
mL
mL
mL
Volume Percent
Volume Percent
Weight Percent
Volume Percent
Volume Percent
Volume Percent
Volume Percent
Volume Percent
Weight Percent
Grams Pb per Gallon
Weight Fraction
Grams per Gallon
BTU per Pound
Motor Octane Number
Research Octane
Number
RON-MON
na = not analyzed
Page B-2
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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
etal. 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-3
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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
All parameters
MTBE
Sulfur
RVP
E200
E300
Aromatics
Olefins
Benzene
Percentage change in NOX emissions from Indolene when:
Only this parameter is
changed to its CAAB value
13
0
12
0
0
0
0
1
0
All parameters except this
one are changed to their
CAAB values
0
13
1
13
13
12
13
12
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 B-4
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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 MOBILES 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
Gasoline Passenger Cars
Control Technology
Low Emission Vehicles*
Tierl
TierO
Oxidation Catalyst
Non-Catalyst
Uncontrolled
* Applicable to California VMT only
Gasoline Light-Duty Trucks
Control Technology
Low Emission Vehicles*
Tierl
TierO
Oxidation Catalyst
Non-Catalyst
Uncontrolled
* Applicable to California VMT only
Gasoline Heavy-Duty Vehicles
Control Technology
TierO
Oxidation Catalyst
Non-catalyst
Uncontrolled
Diesel Passenger Cars
Control Technology
Advanced
Moderate
Uncontrolled
Diesel Light Trucks
Control Technology
Fuel EC
(km/L)
8.5
8.3
8
6.2
4.5
4.7
6
6
4.8
4.8
4
4.1
2.3
2.3
1.8
1.8
10
9.6
7.5
C02
(g/km)
280
285
298
383
531
506
396
396
498
498
601
579
1017
1036
1320
1320
237
248
319
Test
C02*Fuel EC
(9/L)
2380
2366
2384
2375
2390
2378
2376
2376
2390
2390
2404
2374
2339
2383
2376
2376
2370
2381
2393
Page C-l
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Vehicle type and control
technology
Advanced
Moderate
Uncontrolled
Diesel Heavy-Duty Vehicles
Control Technology
Advanced
Moderate
Uncontrolled
Motorcycles
Control Technology
Non-Catalyst Control
Uncontrolled
Fuel EC
(km/L)
7.2
7.2
5.7
2.4
2.4
2.2
10.8
8.9
CO2
(g/km)
330
331
415
987
1011
1097
219
266
Test
C02*Fuel EC
(9/L)
2376
2383
2366
2369
2426
2413
2365
2367
Page C-2
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