United States Air and Radiation EPA420-R-01-039
Environmental Protection July 2001
Agency
svEPA Fuel Sulfur Effects on
Exhaust Emissions
Recommendations for
MOBILE6
$5b Printed on Recycled
Paper
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EPA420-R-01-039
July 2001
Sulfur on
Recommendations for MOBILE6
Report Number M6.FUL.001
Venkatesh Rao
formerly of
Office of Transportation and Air Quality
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 that 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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Introduction
This document describes EPA's effort to estimate empirical relationships between fuel
sulfur content and exhaust emissions of hydrocarbons (HC), non-methane hydrocarbons
(NMHC), nitrogen oxides (NOx), and carbon monoxide (CO) as a function of vehicle technology
and vehicle emitter classification for gasoline-powered vehicles. MOBILE6 will use these
relationships to adjust exhaust emission rates in response to varying fuel sulfur. The vehicle
technologies addressed in this analysis include: Tier 0, Tier 1, Low-Emitting Vehicles (LEVs),
and Ultra Low-Emitting Vehicles (ULEVs). Where possible, the vehicle technology data are
further stratified by passenger car and light light-duty trucks (LDV) , Light-Duty Truck Class 1
(LDT1), and heavier Light-Duty Truck classifications (LDT2/LDT3/LDT4). Table 1 below
defines the weight classifications for the different truck classes.
Table 1
Light-Duty Truck Weight Definitions
Truck Category
LDT1
LDT2
LDT3
LDT4
Gross Vehicle Weight in Ibs.
(GWVR)
0-6000
0-6000
6001-8500
6001-8500
Loaded Vehicle Weight in Ibs
(LVWR)
0-3750
3751-5750
3751-5750
5751-8500
Wherever possible, normal and high emitters are also addressed and analyzed separately. Diesel-
powered and heavy-duty vehicles are not considered in this analysis as MOBILE6 is not set up to
compute fuel sulfur effects for these vehicles.
The structure of this report is as follows:
• Introduction
Respone to Comments and Changes from Draft Report
• Technical Background
Brief overview of EPA's previous sulfur adjustment proposal made
at 10/1/97 MOBILE6 workshop
Summary of major comments received on the proposal made at
10/1/97 workshop
• EPA's proposed final methodology for MOBILE6
• Data sources
Valid sulfur range
• Analysis of short-term sulfur data
• Definition of emitter classes
• Regression methodology and mathematical fits
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Start, running, and FTP emissions
• Analysis of Tier 0 normal emitters
• Analysis of Tier 1 normal emitters
• Analysis of LEV and ULEV normal emitters
• Analysis of high emitters
Long term effects
Irreversibility
Response to Comments and Changes from Draft Report
A draft report on the proposed methodology for MOBILE6 sulfur effects was posted on
the MOBILE6 web page for stakeholder comment in May 1999. Because much of this sulfur
analysis was used in the Tier 2 rulemaking, stakeholder comments on the analysis were addressed
as part of the Tier 2 regulation package. Readers are referred to the Tier 2 docket for detailed
response to comments on the sulfur analysis detailed in this report.
Also, as part of MOBILE6's review process, this report (excluding the last section on
"Revisions to Short-Term Effects and Inclusion of Long-Term and Irreversibility Effects") was
sent our for independent, external peer review. No major comments were received on the
methodologies and conclusions reached in the report.
This final report reflect the stakeholder and peer review comments, as well as additional
research completed too late for incorporation in the draft report. This includes changes to take
into account long-term sulfur effects and the "irreversibility" of some high sulfur effects. The
new analysis was reviewed as part of the Tier 2 rulemaking process.
Technical Background
Since the early 1970s it has been recognized that gasoline sulfur levels impact the
conversion efficiency of automotive three-way catalysts. A significant amount of test data has
been generated in the recent past to investigate this phenomenon. Data were collected first
during the development of EPA's Complex Model, and more recently in response to questions
about the in-use performance of advanced technology (i.e., low-emission and Tier 2) vehicles.
The Complex Model was constructed in response to the Clean Air Act's Reformulated Gasoline
(RFG) requirements and is used to certify reformulated gasolines. The Complex Model
(completed as part of the Reformulated Gasoline Regulation in December 1994) is an empirical
model designed to predict emissions as a function of fuel properties. The exhaust portion of the
Complex Model is based on data from a number of different emission testing programs and
includes the effect of aromatics, olefins, RVP, distillation characteristics, sulfur, and oxygen
content on emissions of VOC, NOx, and air toxics. Also, the exhaust Complex Model only
applies to "1990 technology" vehicles (which are, in general, vehicles of Model Year 1987-
1992). The reader is referred to the RFG web site (http://www.epa.org/oms/reformulated
gasoline) for further information on the RFG regulations and the Complex Model. In this report,
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any reference to the "Complex Model" will refer to the exhaust portion of the model only.
Many studies have been conducted on the effects of fuel sulfur levels on vehicle exhaust
emissions. In 1991, the US Auto/Oil Air Quality Improvement Research Program (AQIRP)
published the first of a series of studies on the effects of sulfur.1 The first study concluded that
reducing sulfur levels from 450 to 50 ppm reduced exhaust emissions in 1990 Tier 0 technology
vehicles by 16, 13, and 9% respectively, for total hydrocarbon (THC), carbon monoxide (CO),
and nitrogen oxide (NOx) emissions. A subsequent study was published by the US AQIRP in
1993 2 jj^g stu(jy confirmed the previous study's results for emission benefits accrued from
reducing sulfur from 450 to 50 ppm and also investigated effects of reducing sulfur down to 10
ppm. In 1995, the US AQIRP published a study investigating sulfur's effect on emissions from
vehicles certified to Tier 1 standards.3 In addition, EPA has conducted several studies to
investigate sulfur's effect on emissions.4'5'6 Recently, the American Petroleum Institute also
sponsored a "Sulfur Reversibility" study7 which examines sulfur's effect on exhaust emissions
from newer vehicles. All data sources used in this analysis are described in more detail in the
"Data Sources" section below.
Based on much of this data, EPA will include an adjustment for in-use gasoline sulfur
levels in the newest version of its highway motor vehicle emission factors model, MOBILE6.
Consistent with the new start/running methodology11 being implemented in MOBILE6, this
report will present separate estimates for the impacts of gasoline sulfur levels on running exhaust
emissions and start emissions. MOBILE6 will use these start and running emission rates to
calculate composite effects when necessary. However, to facilitate comparisons to results from
previous studies and other existing emission models, estimates have been developed for
FTP/composite (the composite emissions calculated from appropriate weighting of bag data)
emissions directly from the data. FTP (Federal Test Procedure) /composite emissions will be
identified by "FTP emissions" in the remainder of this report. It should be noted that all FTP
emissions correlations shown and discussed in this paper are for illustrative purposes only: sulfur
correction factors in MOBILE6 will be solely based on the correlations developed for start and
running emissions.
EPA Preliminary Proposal for MOBLE6 on 10/1/97
As a first-cut approach to model the impacts of fuel sulfur content on exhaust emissions,
EPA first segregated all available vehicle data on the effects of sulfur on exhaust emissions
(mostly Auto/Oil and EPA testing data) into normal and high emitter categories. High emitters
were defined, as they were in the Complex Model, as vehicles emitting more than two times the
particular vehicle's HC standard on a base fuel (0.82 grams/mile for Tier 0 vehicles). Vehicles
were not grouped by fuel injection technology as the type of fuel injection did not greatly affect
sulfur's impact on emissions. For each data set, an average gram/mile value was calculated at
each individual sulfur level. Then, a nonlinear curve of the following form was fit through the
resulting averages:
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Emissions = A* In(Sulfur) + B
This fit was used for all vehicle technologies, emitter classifications, emission categories, and
pollutants. In this initial analysis for the 10/1/97 MOBILE6 workshop, the effects of sulfur on
start emissions were found to be small in magnitude and statistically insignificant in most cases
when compared to FTP emissions and running emissions effects. So, the effect of sulfur on start
emissions was assumed to be zero in all cases. The reader is referred to the 10/1/97 workshop
presentation materials and handouts for the exact correlations developed based on the
relationship between sulfur and exhaust emissions shown above. (Add footnote) Table 2 below
illustrates the effects calculated from the correlations developed using the above equation:
Table 2
Sulfur Reductions Based on Correlations Presented at 10/1/97 MOBILE6 Workshop
Emis-
sions
Mode
FTP
running
Vehicle
Tech
TierO
Tier 1
RFG
Model*
TierO
Tier 1
Percent Reduction in HC
Emissions when Sulfur (in
ppmW) Changed From:
700->
400
4.8
2.8
14.0
8.6
10.1
400->
200
6.0
3.4
9.9
11.7
13.4
200->
50
12.8
7.1
7.5
26.3
32.5
Percent Reduction in NOx
Emissions when Sulfur (in
ppmW) Changed From:
700->
400
1.64
1.87
2.90
2.66
1.84
400->
200
2.08
2.40
5.70
3.38
2.30
200->
50
4.20
4.80
7.40
7.00
4.71
Percent Reduction in CO
Emissions when Sulfur (in
ppmW) Changed From:
700->
400
5.28
7.50
14.4
7.80
7.20
400->
200
6.92
5.23
9.60
10.5
9.72
200->
50
14.8
11.1
7.26
23.3
21.4
High Emitters
FTP
running
TierO
RFG
Model*
TierO
0.55
-1.60
4.00
0.72
-1.00
5.04
1.43
-0.80
10.8
0.45
7.00
2.91
0.55
4.80
3.70
1.10
3.60
7.66
0.85
11.8
5.37
1.10
6.90
6.97
2.20
6.10
15.2
* "RFG Model" refers to EPA's Complex Model for certifying reformulated gasolines. The Complex Model applies
strictly only to "1990 technology" vehicles. The numbers in the "RFG model" rows are for comparison purposes only.
After the workshop, EPA received comments indicating that the methodology used to compute
the correlations and obtain the results shown in Table 2 may not be the most accurate way to
model sulfur's effect on emissions for this set of data. The following concerns were identified:
modeling sulfur impacts as a non-linear effect for all pollutants may not be appropriate in
all cases;
a simpler approach should be used for modeling higher emitters since the data available is
so sparse and comes from different test programs;
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statistical techniques that adjust for the different fleets and base fuels used in the testing
programs must be employed;
the definition of a high emitter should be re-examined (it is not clear that the Complex
Model definition of 0.82 g/mi HC is appropriate for MOBILE6);
an approach should be developed that allows sulfur effects to be estimated for both
conventional gasoline and reformulated gasoline (RFG) areas.
Based on these comments, and on additional test data from Tier 1, LEV, and ULEV vehicles that
became available after the workshop, EPA revised the methodology to estimate sulfur's effect on
exhaust emissions for gasoline-powered vehicles.
EPA's Final Methodology for MOBILE6
Data Sources
EPA's sulfur analysis relied on the following data sources:
Auto/Oil Phase I Sulfur Study1-!! this portion of this extensive testing program, ten 1989 model
year light-duty gasoline vehicles (representing a subset of the "Current" fleet (Tier 0 vehicles)
tested in all the other Auto/Oil studies) were tested using two fuels with sulfur levels of 466 and
49 ppm (other fuel parameters were held constant). The results of that testing indicated that
overall HC, CO, and NOx emissions were reduced by approximately 16%, 13% and 9%,
respectively, when fuel sulfur content was reduced from 466 to 49 ppm.
Auto/Oil Phase n Sulfur Study2-This portion of the testing expanded on the Phase I study by
testing the same "Current" fleet vehicles over a wider range of sulfur levels with more
intermediate points. This was done to determine non-linear trends in the data. Two fuel sets
were used. The first, termed "Part F, was a five-fuel set ranging from a nominal sulfur level of
450 ppm down to 50 ppm in increments of 100 ppm. The second, termed "Part II", was a three-
fuel set having sulfur levels of 50 ppm to 10 ppm in increments of 20 ppm. This study confirmed
the results of the Phase I study and further found that reducing fuel sulfur from 50 ppm to 10
ppm, resulted in a reduction in HC of 6% and CO of 10%; there was no statistically significant
effect on NOx emissions in this range.
T50/T90/Sulfur Study3-Testing in this part of the Auto/Oil program was designed to investigate
possible non linear impacts of the fuel distillation parameter T90, interactive impacts of fuel
distillation parameters T50 and T90, and sulfur on emission from light-duty vehicles. Three
vehicle fleets were tested: the Current fleet assessed in the Phase I and Phase II Studies (10
vehicles), a Federal Tier 1 fleet (consisting of six vehicles), and an Advanced Technology fleet
(six production type LEV and ULEV vehicles). Only the Current and Tier 1 fleets were tested
for their response to changes in sulfur levels. Two fuel sets tested in this program can be used to
investigate the impact of fuel sulfur on exhaust emissions: a low T90 set and a high T90 set with
approximate sulfur levels of 33 and 317 ppm.
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API Extension Fuel Set8-In this program, the 10-vehicle "Current" fleet from the Auto/Oil
program was tested at sulfur levels of 450 and 900 ppm to investigate the impact of the higher
levels of fuel sulfur observed in U.S. gasoline. Results from this program showed very modest
emission reductions as a result of reducing sulfur from 900 to 450 ppm (5% HC, 2% CO, and 3%
NOx).
EPA RFG Phase I Study4-Phase I was an initial investigation of the impacts of oxygenates,
volatility, distillation properties, and sulfur on emissions. Vehicles included in this program
represented 1990 model year or equivalent technology(Tier 0 vehicles). Two fuels examined in
this program had differing sulfur levels (112 ppm and 371 ppm) with other fuel parameters at
approximately constant levels. Results indicated that decreasing sulfur from 371 ppm to 112
ppm caused an approximate 5% reduction in HC emissions, a 7% reduction in NOx emissions,
and a 9% reduction in CO emissions in the fleet tested.
EPA RFG Phase n Studys-Phase n was a continuation of Phase I, investigating further the
effects of oxygen content, oxygenate type, volatility, sulfur, olefins, and distillation parameters.
Relevant testing included fuels with sulfur levels of 59 and 327 ppm. Again, vehicles with 1990
model year or equivalent technology were tested. For the fleet tested, the results indicated that a
reduction in sulfur from 327 to 59 ppm caused an approximate 7% reduction in HC, a 5%
reduction in NOx emissions, and a 8% reduction in CO emissions.
API "Reversibility" Study7- API tested a series of vehicles in response to the issue of sulfur
reversibility in LEV and advanced technology vehicles. Sulfur "reversibility" refers to the ability
of a vehicle to return to low emissions on low sulfur fuel after temporary use of high sulfur fuel.
When this MOBILE6 analysis was being drafted, only very few vehicles had finished testing.
Only one of the vehicles tested that had accumulated 100K mileage (Ford Taurus-VEST #). This
vehicle was used in this analysis as part of the LEV emissions data set (all of which had
approximately 100K mileage). The other vehicles in this test program were not included in the
analysis either because: 1) they did not have the 100K mileage accumulation or, 2) the testing
was not completed at the time the analysis for this report was completed. See discussion below
on why only vehicles with 100K mileage are thought to be most appropriate for the purposes of
this EPA report.
CRC Sulfur/LEV Study9- This testing involved 6 LDV models certified for sale in California as
LEVs in 1997. Two vehicles from each model type were tested on 7 fuels. Two fuel sets were
investigated: one fuel set was a California RFG with two sulfur levels (nominally 40 ppm and
150 ppm); the other set of five fuels consisted of a base, conventional fuel with five different
sulfur levels (nominally 40, 100, 150, 330, and 600 ppm). The same base gasoline was used for
all five of these fuels and the sulfur levels were varied by adding representative sulfur-containing
hydrocarbons. The vehicles were first tested in an "as-received" condition (average vehicle
mileage of 10,000 miles) and with the catalysts bench-aged to simulate 100,000 miles of
operation (although the oxygen sensors were original, low mileage sensors). The 10,000 mile
emissions data will hereafter be referred to as the "10K data" and the 100,000 mile data will be
referred to as the "100K data." The conclusions from this study included:
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For the 10,000-mile catalysts, reducing sulfur from 600 to 40 ppm resulted in a fleet
emission FTP composite emission reduction in NMHC of 46% , in NOx of 63%, and in
CO of 57%.
• For the aged 100,000-mile catalysts, reducing sulfur from 600 to 40 ppm resulted in a
fleet emission FTP composite emission reduction in NMHC of 32%, in NOx of 61%, and
in CO of 46%.
• The fleet response to fuel sulfur changes was found to be linear for the 10,000-mile
catalysts and nonlinear for the 100,000-mile catalysts. With the aged catalysts, the effect
of sulfur change was more pronounced at lower sulfur levels.
In this EPA analysis, only the 100K data was used (since the other major LEV/ULEV testing
program only tested vehicles with aged components to simulate 100,00 miles of driving-see the
next section below). Emissions data from both fuel sets (conventional and RFG gasoline) were
used in this analysis.
AAMA/AIAM Sulfur/LEV Studv10-This study tested 21 vehicles, each of different design: 9
LEV LDVs, 1 LEV LDT1, 7 LEV LDT2s, and 4 ULEV LDVs. Some of the vehicle designs have
been certified for sale in California, while others were designs which were deemed ready for
certification and production. The vehicles were equipped with emission control components that
were aged to mimic 100,000 miles of on-road driving. The base fuel used in the program was a
California RFG with a nominal sulfur level of 40 ppm. The base fuel was then doped with sulfur
compounds to obtain nominal sulfur levels of 100, 150, 330, and 600 ppm. Based on the 21-
vehicle fleet, AAMA/AIAM reached the following conclusions:
• The emissions benefits of low-emission vehicle hardware are diminished as fuel sulfur
level is increased above 40 ppm;
The LEVs and ULEVs tested in this program showed a larger detrimental effect from fuel
sulfur increases than the Tier 0 or Tier 1 vehicles tested in the Auto/Oil program; and
• The emissions response of LEVs and ULEVs to fuel sulfur is non-linear for all pollutants
and is more pronounced at lower sulfur levels.
Valid Sulfur Range—The range of sulfur data available for this analysis varied from 10 ppm to
900 ppm. Table 3 summarizes the range of actual data available to estimate sulfur's effect on
emissions by vehicle category:
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Table 3
Range of Available Sulfur Data by Vehicle Technology and Type
Vehicle Standard/Emitter
Type//Class
Tier 0/Normals/LDVs
Tier 1/Normals/LDVs
LEV/Normals/LDVs
LEV/Normals/T rucks
High Emitters (Data
Available only for Tier 0
Vehicles)
Studies Available
Auto/Oil, EPA RFG, and
API Extension-Set Studies
Auto/Oil T50/T90 Study
AAMA/AIAM , CRC, and
API Reversibility Studies
AAMA/AIAM Study
EPA RFG Studies
Approximate Sulfur* Range for which Data is Available
and Over which Regressions were Based
10->900 ppm
30->350 ppm
30->600 ppm
30->600 ppm
40->450 ppm
All sulfur values in the databases are "actual" sulfur values (as opposed to "nominal" values)
While the regressions were based on all the data, the valid range for MOBILE6 will be limited to
30 ppm on the low end and 600 ppm on the high end. The main reasoning for these limits is that
LEV emissions data is only available over that range. Because LEV emissions are most
sensitive to sulfur fluctuations, it was thought that extrapolations would be both speculative and
possibly very inaccurate, especially below 30 ppm. For consistency within MOBILE6 and for
ease of use, in MOBILE6 this valid range will apply to all vehicle technologies and emitter
classifications.
Analysis of Sulfur Data
Definition of Emitter Classes
Several comments since the October 1, 1997 MOBILE6 workshop have indicated that
sulfur's effect on emissions is a strong function of emission levels of individual vehicles and that
the Complex Model definitions of emitter classes are too broad and are based on hydrocarbon
emissions only. The comments further suggested that predictive equations based on normal
emitters with relatively low emissions (e.g., vehicles in the Auto/Oil program) may not be
directly applicable to the entire fleet of vehicles with HC emissions below the Complex model
normal-emitter definition of < 0.82 grams/mile HC. Some of the suggestions further indicated
that a "moderate emitter" category be established12. While this type of approach has merit in
estimating the effect of sulfur control on emissions, MOBILE6 is structured with only two
emitter classes, normals and highs. Thus, for the current sulfur analysis, emitter categories are
defined in the following manner and are slightly different than the original proposal made at the
October 1997 workshop:
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Table 4
Emitter Categories
Normal
Highs
< Two times the
> Two times
emission standard for NOx, and HC, and < Three times the emissions
standard for CO
the emission standard for either NOx, or HC, or > Three times the
emission standard for CO
Table 5 below shows the number of vehicles in each category that are normal and high emitters
according to the definition in Table 4. Note that there are no Tier 1 or LEV high emitters (as
defined in Table 4) in the database.
Table 5
Distribution of Number of Vehicles in Each of the Emitter Categories
Defined in Table 4 for Studies Used in this Analysis
Study
All Auto/Oil
(all Tier 0 Vehicles)
EPA RFG Phase I
(all Tier 0 Vehicles)
EPA RFG Phase II
(all Tier 0 Vehicles)
Tier 1 T50/T90 Study
(all Tier 1 Vehicles)
CRC Sulfur/LEV Study
(LEV and ULEV Vehicles)
AAMA/AIAM Sulfur/LEV Study
(LEV and ULEV Vehicles and
Trucks)
TOTALS:
Number of Normal Emitters
10
20
24
6
12
21
93
Number of High Emitters
0
19
15
0
0
0
34
Regression Methodology and Mathematical Fits
Unless otherwise specified, all data sets were analyzed using the following regression
methodology. Individual fuel/vehicle data points were analyzed using a regression procedure in
the SAS statistical software package called "ABSORB." The SAS manual provides details on
this procedure. Dummy variables were used to absorb the vehicles' effect on emissions thereby
allowing the fuel sulfur effect to be isolated and better approximated. This approach is rather
similar to the approach used in the development of the reformulated gasoline Complex model in
which a "dummy" variable was created for each vehicle in the data set. Repeat tests on vehicles
(and for the same vehicle(s) used in different testing programs) at a given sulfur level were
averaged to represent one data point. Emissions were regressed against raw ("as-reported"
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values) sulfur values (all sulfur values in this report are in ppmW).
In all cases, two different mathematical fits were used to represent the emissions vs.
sulfur data as shown below. The final decisions on which fit to use in the different sulfur
regimes were based on accuracy of fit, previous knowledge of how sulfur affects emissions in the
regime being considered, quantity of data available, and other published work. Some of the
comments on the methodology proposed at the October 1997 workshop suggested use of a
polynomial (quadratic) fit to represent the emissions data. However, quadratic fits and the non-
linear logarithmic fits shown above yielded nearly the same level of accuracy and fit, thus
polynomial regressions were not included in the final analysis. Whenever a tabular entry in this
report indicates "Ln-Ln" fit, it refers to the following relationship:
ln(Emissions (g/mile)) = [(Regression Coefficient) * In (S (in ppm))]+ C
Whenever an entry indicates a "Ln-Linear" fit, the corresponding mathematical relationship is:
ln(Emissions (g/mile)) = [(Regression Coefficient) *S (in ppm)]+ D
Note that C and D in the above equations are constants but are never used in the calculations.
The equations in this report are used only for comparing emission effects (i.e., percent change in
emissions resulting from sulfur variation) and not for estimating absolute or relative g/mile
numbers. Thus, the values of C and D are irrelevant.
Start. Running, and FTP-Composite Emissions
Start and running emissions were calculated from bag data using the methodology
outlined in the MOBILE6 EPA report entitled "The Determination of Hot Running and Start
Emissions from FTP Bag Emissions."11. Though this report used only Tier 0 vehicle data to
generate the bag-to-emissions mode correlations, the correlations were used for Tier 0, Tier 1,
and LEV vehicles and trucks (for both normal and high emitters) in this analysis. For regression
purposes, FTP-composite emissions were used as reported in the individual databases.
In most cases, the effect of sulfur on start emissions was statistically insignificant.
However, since MOBILE6 will use start and running emissions to recalculate FTP emissions
whenever necessary, it is very important that these start effects be included. Thus, regressions
for start emissions will be developed and used for all vehicle technologies and pollutants. It
should be noted that, in a few cases, the sulfur effect on start emissions is negative. While this is
counterintuitive, it is supported by the data and assures that the composite emissions are not too
high.
Analysis of Tier 0 Normal Emitters
The sulfur impacts for normal-emitting Tier 0 vehicles are based on analysis of the entire
Auto/Oil database, the API extension fuel set, and the EPA Phase I and Phase IIRFG data sets.
The SAS "ABSORB" procedure was applied to the Tier 0 data and regressed using the two non-
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linear schemes discussed above. It was found that the log-log fit was consistently better than the
log-linear fit. The resulting correlations are shown below in Table 6 and the emission effects
resulting from these correlations are shown in Table 7:
Table 6
Regression Analysis for Tier 0 Normal Emitting Vehicles
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
Type of Regression
Fit
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Regression
Coefficient
0.06126
0.05502
0.07596
0.03077
0.15262
0.15187
0.19086
0.02083
0.0027436
0.0037181
-0.01792
0.04772
R2
0.963
0.959
0.950
0.939
0.947
0.918
0.886
0.944
0.959
0.961
0.860
0.862
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Table 7
Emission Effects from Varying Sulfur for Tier 0 Normal Emitting Vehicles
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
% Increase in Emissions when Sulfur is Increased from 30 ppm to:
75
5.77
5.17
7.21
2.86
15.0
14.9
19.1
1.93
0.25
0.34
-1.63
4.47
150
10.4
9.26
13.0
5.08
27.8
27.7
36.0
3.41
0.44
0.60
-2.84
7.98
330
15.8
14.1
20.0
7.66
44.2
43.9
58.0
5.12
0.66
0.90
-4.21
12.1
600
20.1
17.9
25.6
9.66
58.0
57.6
77.1
6.44
0.83
1.12
-5.23
15.4
The Tier 0 analysis summarized in Tables 6 and 7 will apply to all normal emitters of Tier 0 or
earlier (pre-Tier 0) categorization (all vehicles equipped with a catalyst) since very little data is
available to support an evaluation of sulfur's effect on pre-Tier 0 vehicles. Pre-catalyst vehicles
are treated separately because sulfur will have no direct effect on exhaust emissions from those
vehicles.
For comparison, Table 8 shows estimated emission effects of reducing sulfur from 450->50 ppm
using the regressions listed in Table 6 for Tier 0 normal emitters and the effects computed from
the Complex Model12 for normal emitters. The results are similar for CO but the HC and NOx
effects estimated in this EPA analysis are smaller when compared to the NOx and HC effects
predicted by the Complex Model. This is most likely due to inclusion of the T50/T90 heavy-
hydrocarbon Auto/Oil data set in the current analysis. The T50/T90 heavy-hydorcarbon Auto/Oil
data was not available at the time the Complex Model was constructed. Inspection of the
T50/T90 heavy-hydrocarbon data shows somewhat muted HC effects and much lower NOx
effects for sulfur variations. Thus, the overall HC and NOx effects of reducing sulfur are much
lower in this analysis than those estimated by the Complex Model.
-13-
-------
Table 8
Comparison of Composite Emission Effects for Normal Emitting
Tier 0 vehicles estimated from this Analysis to those Estimated from the Complex
Model when Sulfur is Reduced from 450 to 50 ppm
Approach
This EPA Analysis
Complex Model*
Percent Reduction in HC
13.0
19.0
Percent Reduction in NOx
6.6
13.6
Percent Reduction in CO*
15.4
18.5
CO emissions were not part of the original RFG Complex Model. The CO model estimates are based on the CO model
developed separately (using the same statistical techniques used to construct the RFG Complex Model) from the RFG
rulemaking and discussed in SAE paper 96121413.
Analysis of Tier 1 Normal Emitters
Only one set of data3 has examined the effects of sulfur on emissions from certified Tier 1
vehicles. Two sulfur data points were tested in this analysis (~ 30 ppm and ~ 330 ppm) at high
and low levels of T90. Because only two sulfur data points were available, the log-linear version
of the fits were chosen to represent the data. Log-linear regressions were run using the
procedures outlined earlier and the regression coefficients and effects obtained are shown
respectively in Tables 9 and 10. It is interesting to note that for Tier 1, percent change benefits
from reducing sulfur are generally larger than the benefits for Tier 0 for CO and HC, and are
about the same for NOx.
-14-
-------
Table 9
Regression Analysis for Tier 1 Normal Emitting Vehicles
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
Type of Regression
Fit
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Regression
Coefficient
8.053E-4
7.223E-4
6.295E-4
3.181E-4
2.457E-3
2.897E-3
1.746E-3
6.337E-4
9.516E-5
9.172E-5
-2.338E-4
8.023E-4
R2
0.765
0.748
0.907
0.903
0.818
0.785
0.911
0.853
0.941
0.936
0.820
0.692
-15-
-------
Table 10
Emission Effects from Varying Sulfur for Tier 1 Normal Emitting Vehicles
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
% Increase in Emissions when Sulfur is Increased from 30 ppm to:
75
3.69
3.30
2.87
1.44
11.7
13.9
8.17
2.90
0.43
0.41
-1.05
3.68
150
10.1
9.05
7.85
3.89
34.3
41.6
23.3
7.90
1.15
1.11
-2.77
10.1
330
27.3
24.2
20.8
10.0
109.0
138.5
68.8
20.9
2.90
2.79
-6.77
27.2
600*
34.8
30.7
26.6
12.6
143.0
181.7
91.4
26.3
3.65
3.47
-8.41
34.6
* Please see explanation below about how the effects at 600 ppm were estimated.
Since the available Tier 1 data only extends to 330 ppmW sulfur, it would be inaccurate to use
the log-linear regression equations listed in Table 6 all the way out to 600 ppm, which will be the
valid high end of the sulfur range in MOBILE6. Instead, the equations listed in Table 6 will be
applicable only for sulfur values between 30 and 330 ppm; for any sulfur level above 330 ppm,
the following equation will be used to estimate Tier 1 effects for a given pollutant and a given
emissions mode (start vs. running) and emitter classification:
Tier 1 Effect at any sulfur level "X" above 330 ppm = [(TierOx)/(Tier0330)]*(Tierl330)
where,
TierOx = Tier 0 percent emission change at level X using a 30 ppm as baseline
(can be estimated from Table 6)
Tier0330 = Tier 0 percent emission change at 330 ppm using 30 ppm as baseline
(available in Table 7)
Tierl330 = Tier 1 percent emission change at 330 ppm using 30 ppm as baseline
(available in Table 10)
-16-
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For example, according to the above equation, the Tier 1 effect of increasing sulfur to 600 ppmW
from 30 ppmW on running HC emissions would be: (58.0%/44.2%)* 109.0% = 143.0 %. The
values 58.0% and 44.2% were obtained from Table 7 and 109.0% was obtained from Table 10.
A graph of the emission effects from the regression equations in Table 9.
Initial Analysis of LEV. ULEV. and Cleaner Normal Emitters
This section describes our initial analysis, as presented in our draft report. The estimated short-
term effect of sulfur on LEV and ULEV emissions was revised in the time after the draft report
was published, as described in the following section. EPA also revised the analysis of emission
effects for the newest vehicles to account for long-term and irreversibility effects, as discussed
later in this report.
As discussed in the "Data Sources" section above, 100K data from the recently completed
AAMA/AIAM and CRC testing programs were used to develop sulfur impacts for LEVs and
ULEVs. Emissions from both the conventional and RFG set of fuels in the CRC testing program
were used in this analysis. The CRC 10K data was omitted because: (1) it was felt that is was
not representative of true in-use conditions and, (2) there was no accurate way to combine the
10K data with the 100K data common to both testing programs. Because the AAMA/AIAM
testing program contained data on trucks, the impacts were stratified into light-duty vehicles
(passenger cars and light trucks) and LDT2, LDT3, and LDT4 trucks. The combination of
passenger cars and LDT1 trucks will be referred to as Light-Duty Vehicles (LDVs) hereafter.
Emissions data from passenger cars and LDT1 trucks were combined due to the technical
similarities in their catalyst systems that result in similar emission responses. Past EPA
analyses14 have also traditionally combined these two categories of vehicles. As suggested by
the authors of the CRC and AAMA/AIAM reports, log-log fits were found to be better for most
of the pollutants and emission modes. Tables 11 and 12 show the regression statistics and the
emission effects of changing sulfur for LDVs, respectively:
-17-
-------
Table 11
Initial Regression Analysis for LEV and ULEV Normal Emitting LDVs
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
Type of Regression
Fit
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Regression
Coefficient
0.16845
0.13992
0.23746
0.35392
0.42809
0.49561
0.48626
0.57085
0.05067
0.05552
0.04847
0.11240
R2
0.947
0.944
0.917
0.889
0.879
0.859
0.915
0.904
0.958
0.954
0.941
0.723
* Note, for the final equations used in MOBILE6, see Table 17
-18-
-------
Table 12
Initial Emission Effects from Varying Sulfur for LEV & ULEV Normal Emitting LDVs
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
% Increase in Emissions when Sulfur is Increased from 30 ppm to:
75
16.7
13.7
24.3
38.3
48.0
57.5
56.1
68.7
4.75
5.22
4.54
10.8
150
31.1
25.3
46.5
76.8
99.2
122.0
118.7
150.6
8.50
9.35
8.11
19.8
330
49.8
39.9
76.7
133.6
179.1
228.2
220.9
293.1
12.9
14.2
12.3
30.9
600
65.6
52.1
103.6
188.7
260.5
341.4
329.2
453.0
16.4
18.1
15.6
40.0
* Note, these are not the final effects used in MOBILE6.
Note that Table 12 shows ULEV and LEV normal emitting vehicles have a much stronger
emissions response to sulfur changes than did Tier 1 vehicles (Table 10) or Tier 0 vehicles (Table
7).
While the Tier 0 and Tier 1 analysis is based only on LDV data, the AAMA/AIAM study also
provided truck data. A total of 7 LDT2 trucks were tested in the AAMA/AIAM program in
addition to the testing conducted on LDVs. These data were analyzed in the same manner as
described above using "ABSORB" in SAS to arrive at the regression analysis and emission
effects shown in Tables 13 and 14, respectively.
-19-
-------
Table 13
Regression Analysis for LEV and ULEV Normal Emitting LPT2 Trucks
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
Type of Regression
Fit
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Ln-Ln
Regression
Coefficient
0.12549
0.08956
0.15084
0.14625
0.31818
0.25326
0.38379
0.29491
0.02551
0.02846
0.07030
0.04130
R2
0.985
0.983
0.980
0.951
0.939
0.960
0.887
0.934
0.990
0.989
0.968
0.901
-20-
-------
Table 14
Emission Effects from Varying Sulfur for LEV & ULEV Normal Emitting LPT2 Trucks
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
% Increase in Emissions when Sulfur is Increased from 30 ppm to:
75
12.2
8.55
14.8
14.3
33.8
26.1
42.1
31.0
2.36
2.64
6.65
3.86
150
22.4
15.5
27.5
26.5
66.9
50.3
85.5
60.7
4.19
4.68
12.0
6.88
330
35.1
24.0
43.6
42.0
114.5
83.5
151.0
102.8
6.31
7.05
18.4
10.4
600
45.6
30.8
57.1
55.0
159.4
113.5
215.7
141.9
7.94
8.89
23.4
13.2
Note that the sensitivity of emissions to changes in sulfur is much lower for LEV trucks than for
LEV LDVs.
Revisions to LEV and ULEV calculations
Since the draft report on sulfur was completed, several additional LEV-type light-duty
vehicles were tested for sulfur sensitivity. Those that were completed in time for inclusion in
this analysis (9/11/99) are listed below in Table 15. The emissions data from these 11 new
vehicles and from the six CRC-10K vehicles were included in the LEV database (which
previously consisted only of 100K data) and revised composite regression coefficients were
estimated (using the exact same procedures used to develop regression coefficients for "short-
term" effects) using SAS. The emissions data from these 17 vehicles were added to the existing
LEV database and SAS was used to conduct regressions as before. The resultant composite
emission regression coefficients are summarized (for comparison purposes the old regression
coefficients are also listed) in Table 16.
-21-
-------
Table 15
Additional LEV/ULEV vehicles tested for Sulfur Sensitivity
Make/Model
ALTIMA
TAURUS
ACCORD
AVALON
TOWN CAR
TAURUS
ESCORT
HONDA
NISSAN
TOYOTA
GEO
ACCORD
CAVALIER
TAURUS
WTNDSTAR
TAURUS
EXPLORER
Test Program
API
API
API
API
API
CRC-1
CRC-1
CRC-1
CRC-1
CRC-1
CRC-1
EPA
EPA
ATL
ATL
FORD
FORD
Catalyst Aging (~ miles)
100k
4k
4k
4k
4k
10k
10k
10k
10k
10k
10k
50k
50k
53k
43k
4k
4k
Only short-term exposure
or short-term and long-
term exposure data?
BOTH
BOTH
BOTH
BOTH
Only Short-Term
Only Short-Term
Only Short-Term
Only Short-Term
Only Short-Term
Only Short-Term
Only Short-Term
BOTH
BOTH
Only Short-Term
Only Short-Term
Only Short-Term
Only Short-Term
Table 16
Revised Composite Regression Coefficients based on original and additional LEV data
Pollutant
HC
NMHC
CO
NOx
Previous Regression Coefficient
0.168
0.140
0.237
0.354
Revised Regression Coefficient
0.168
0.160
0.236
0.351
-22-
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The composite regression coefficients listed in Column 3 of Table 16 are used as the MOBILE6
coefficients to determine sulfur's short-term emission effects on LEV and cleaner vehicles. Note
that we were unable to get the bag data from the new testing programs to determine start and
running coefficients directly from the new data. Instead, we revised the LEV running and start
emission regression coefficients based on the existing LEV composite regression coefficients, as
described in the next section..
Revision of LEV Running and Start Regression Coefficients
In general, start and running emissions and corrections to them are calculated using bag data and
correlations between the bags and start and running emissions. However, the data that was used
to develop these correlations, consisted entirely of Tier 0 emissions data. There is some question
as to whether those correlations can be directly applied to LEV data. At this time, we feel the
most appropriate approach for LEV and cleaner vehicles (and trucks) is to apply the composite
regression coefficients to both running and start emissions.
The final regression coefficients to use for determining sulfur's short term impacts on
LEV (and cleaner technology) emissions are summarized in Table 17. While unrevised, the LEV
truck, Tier 0 (in Table 18), and Tier 1 (in Table 18) coefficients are also listed for completeness.
The mathematical fits relating the regression coefficients to emissions for each set of vehicles
remains as outlined in the draft report (i.e., as discussed in the sections preceeding this one).
Table 17
LEV Normal Emitters: Revised regression coefficients for the effects of sulfur on composite,
start, and running. Emissions from LEV-and-cleaner vehicles & trucks
Pollutant
HC
NMHC
CO
NOx
Light -Duty Vehicles
For Compos.
Emissions
0.168
0.160
0.236
0.351
For Running
Emissions
0.168
0.160
0.236
0.351
For Start
Emissions
0.168
0.160
0.236
0.351
LDT2,3,4 Trucks
For Compos.
Emissions
0.125
0.090
0.151
0.146
For Running
Emissions
0.125
0.090
0.151
0.146
For Start
Emissions
0.125
0.090
0.151
0.146
-23-
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Table 18
Tier 0 and Tier 1 Normal Emitters: Regression coefficients for the effects of sulfur on
composite, start, and running. Emissions from Tier 0 and Tier 1 Vehicles
Pollutant
HC
NMHC
CO
NOx
All Tier 0 Vehicles
For Compos.
Emissions
0.0613
0.0550
0.0760
0.0308
For Running
Emissions
0.1526
0.1519
0.1909
0.0208
For Start
Emissions
0.0027
0.0037
-0.018
0.0477
All Tier 1 Vehicles
For Compos.
Emissions
8.05e-4
7.22e-4
6.30e-4
3018e-4
For Running
Emissions
2.46e-3
2.90e-3
1.75e-3
6.34e-4
For Start
Emissions
9.52e-5
9.17e-5
-2.34e-4
8.02e-4
Analysis of High Emitters
The emissions criteria for high emitters are listed in Table 4. Actual data on the effects of
sulfur on emission from high emitters is available only for Tier 0 vehicles as indicated in Table 5.
These data were used to determine regression coefficients for high emitters. A log-linear fit was
used since the amount of high emitter data available was small and only two sulfur levels were
tested in the EPA RFG programs. The regression coefficients for high emitters are shown in
Table 19 and the corresponding emission effects are shown in Table 20.
-24-
-------
Table 19
Regression Analysis for Tier 0 High Emitting Vehicles
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
Type of Regression
Fit
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Ln-Linear
Regression
Coefficient
3.727E-5
3.727E-5
6.317E-6
3.046E-4
1.138E-4
9.614E-5
1.111E-4
2.848E-4
-2.227E-4
-1.824E-4
-5.336E-4
2.519E-4
R2
0.997
0.997
0.997
0.996
0.996
0.996
0.993
0.998
0.985
0.989
0.962
0.889
-25-
-------
Table 20
Emission Effects from Varying Sulfur for High Emitting Tier 0 Vehicles
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
Emissions Mode
Composite
Composite
Composite
Composite
Running
Running
Running
Running
Start
Start
Start
Start
% Increase in Emissions when Sulfur is Increased from 30 ppm to:
75
0.17
0.17
0.03
1.39
0.51
0.43
0.50
1.29
-1.00
-0.82
-2.37
1.14
150
0.45
0.45
0.08
3.72
1.37
1.16
1.34
3.48
-2.64
-2.17
-6.20
3.07
330
1.12
1.12
0.19
9.57
3.47
2.93
3.39
8.92
-6.46
-5.32
-14.8
7.85
600
2.15
2.15
0.37
19.0
6.70
5.63
6.54
17.6
-11.9
-9.87
-26.2
15.4
The effects in Table 20 are in good agreement with the Complex Model which showed that NOx
effects were much more sensitive than HC effects to sulfur variation in Tier 0 high emitting
vehicles14. As an example, the Complex Model indicates that the effect of reducing sulfur f
450 to 50 ppm on high emitting vehicles to be an approximate 10% decrease in NOx emissk
This EPA analysis shows the same effect to be approximately 11%. Table 21 is a comparison of
the emission effects estimated in this EPA report to the Complex Model estimates for high
emitters
: emissions.
Table 21
Comparison of Composite Emission Effects for High Emitting
Tier 0 vehicles estimated from this Analysis to those Estimated from the Complex
Model when Sulfur is Reduced from 450 to 50 ppm
Tool
This EPA Analysis
Complex Model
Percent Reduction in HC
1.5
-5.0
Percent Reduction in CO*
0.3
1.4
Percent Reduction in NOx
11.2
10.0
CO emissions were not part of the original RFG Complex Model. The CO model estimates are based on the CO model
developed separately (using the same statistical techniques used to construct the RFG Complex Model) from the RFG
rulemaking and discussed in SAE paper 96121413.
-26-
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Note that in some cases Table 20 shows that reducing sulfur may actually increase start
emissions. While this result was unexpected, MOBILE6 will apply the start effects as reported in
Tables 19 and 20. We considered applying manual adjustments to the start correction factors,
(such as "zeroing out" the counterintuitive effects), but we rejected this option because
MOBILE6 will combine running and start emissions to estimate composite emissions whenever
necessary and any changes to the start effects would skew the composite emission estimates.
The regression coefficients shown in Table 19 will be used for Tier 0 vehicles. However, no
high emitter test data relating sulfur to emissions exists for any other category of vehicles. Thus,
a simple algorithm is required to estimate the emission sensitivity to sulfur changes in high
emitters certified to Tier 1 and cleaner standards. Analysis of the Complex Model15 indicates
that the NOx sensitivity of high emitters is approximately 60 percent of the sensitivity for normal
emitters. Thus, for vehicles and trucks certified to Tier 1 and cleaner standards (LEV, ULEV,
Tier 2, etc.), a 60 percent correction factor will be applied to estimate a NOx effect for high
emitting vehicles. For example, if the normal emitter NOx emissions effect of reducing sulfur
from 150 to 50 ppm is 25% for Tier 1 (or LEV or ULEV) vehicles, then high emitters in this
same category would get a NOx benefit of (0.6)*(25%), or 15%.
The Complex Model does not show nearly as great or as consistent a CO or HC effect13'14 for
normal emitters or a conistent sensitivity to changes in sulfur from high emitting vehicles; thus,
the regression coefficients listed for HC and CO in Table 19 will be used as is for all vehicle
categories. Table 22 summarizes the high emitter effects to be used in MOBILE6:
-27-
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Table 22
Summary of High-Emitter Effects
Vehicle Category
Tier 0-LDVs
Tier 0-LDVs
Tier 0-LDVs
Tier 0-LDVs
Tier 1-LDVs
Tier 1-LDVs
Tier 1-LDVs
Tier 1-LDVs
LEV & ULEV-LDVs
LEV & ULEV-LDVs
LEV & ULEV-LDVs
LEV & ULEV-LDVs
LEV & ULEV-LDT2s
LEV & ULEV-LDT2s
LEV & ULEV-LDT2s
LEV & ULEV- LDT2s
Pollutant
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
HC
NMHC
CO
NOx
High Emitter Effect
Use Appropriate Regression
Coefficient listed in Table 19
Use Appropriate Regression
Coefficient listed in Table 19
Use Appropriate Regression
Coefficient listed in Table 19
Use Appropriate Regression
Coefficient listed in Table 19
Use HC Regression Coefficient
listed in Table 19
Use NMHC Regression Coefficient
listed in Table 19
Use CO Regression Coefficient
listed in Table 19
Use (0.60* Normal Emitter Tier 1
Effect (calculated from Table 9))
Use HC Regression Coefficient
listed in Table 19
Use NMHC Regression Coefficient
listed in Table 19
Use CO Regression Coefficient
listed in Table 19
Use (0.60* Normal Emitter LEV
LDV Effect (calculated
from Table 17))
Use HC Regression Coefficient
listed in Table 19
Use NMHC Regression Coefficient
listed in Table 19
Use CO Regression Coefficient
listed in Table 19
Use (0.60* Normal Emitter LEV
LDT2 Effect (calculated
from Table 17))
-28-
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Long-Term and Irreversibility Effects
After the draft report was completed, additional sulfur effect data collection and analysis was done
to support the EPA's Tier 2 rulemaking, including work to address the long-term effects of fuel
sulfur levels and to the effects of temporary exposure to high sulfur fuels. We felt it was
important to incorporate this analysis into MOBILE6.
Since only LEV data were used to evaluate the irreversibility and long-term exposure effects, the
long-term exposure effects will only apply to LEV and cleaner categories of vehicles and trucks.
Irreversibility effects will only be applied to 2004-and-later vehicles since the data used were
SFTP-compliant data. Both the long term exposure and irreversibility effects will be applied in
conjunction with the short term effects estimated in the sections preceeding this one in this report.
Background for Long-Term and Irreversibility Effects
Fuel sulfur impacts vehicle emissions in three basic ways: 1) short-term effects due to
sulfur's adsorption onto the catalyst surface, 2) longer-term effects due to sulfur's penetration into
the precious metal layer of the catalyst and oxygen-storage material in the catalyst, and 3) a more
lasting impact, referred to as irreversibility. Generally, items 1 and 2 are referred to as "sulfur
sensitivity" and item 3 is referred to as "sulfur irreversibility."
The immediate impact (or "short-term" effects) of sulfur on emissions (item 1 above) is
discussed in the previous sections of this report. From the "short-term" effects it was shown that
operation on typical conventional gasoline containing 330 ppm sulfur increases exhaust VOC and
NOx emissions from LEV and Tier 2 vehicles, on average, by 40% and 139% respectively
compared to operation on 30 ppm sulfur fuel. New data generated since the draft version of this
report was completed on similar LEVs and ULEVs show that when these vehicles were driven on
high sulfur (330 ppm) fuel for a few thousand miles, the NMHC and NOx emission increase due
to high sulfur fuel increased by a greater margin than what was estimated with the "short-term"
data; these effects are referred to as "long-term" effects. These new data will be used to generate
new estimates (both for "short-term" and "long-term" effects) for sulfur-sensitivity of LEV,
ULEV, and cleaner vehicles.
Sulfur's irreversibility on LEV/ULEV vehicles also affects emissions. Sulfur
"irreversibility" refers to the decreased ability of a vehicle to return to low emissions on low sulfur
fuel after temporary use of high sulfur fuel. Sulfur has an almost immediate effect on catalyst
performance, with the sulfur level of the fuel primarily impacting the speed with which the
catalyst is affected. One tankful of fuel containing high levels of sulfur will inhibit catalyst
performance to essentially the same degree as several tankfuls of fuel with somewhat lower sulfur
content. However, the return of catalyst performance upon refueling on low sulfur fuel is not as
prompt with the higher sulfur fuel. This could have substantial consequences for the design of a
commercial sulfur control program. For this reason API carried out a study to begin to investigate
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this phenomena. Our analysis of the API data and the approach that will be used to incorporate
these findings into MOBILE6 are described in the final section of this report.
Long-term Sulfur Effects
As discussed above, in addition to adsorbing onto the surface of the catalyst and acting as
a poison, sulfur can also penetrate into the precious metal layer, especially into palladium (the
metal of choice for LEV catalysts), and into the oxygen storage material and poison the catalyst
further. Full penetration may not have occurred during the very few miles of operation prior to
short term emission testing on high sulfur fuel. The short-term exposure in the test programs
(evaluated previously ) typically consisted only of running several emission tests (FTP or LA4).
Since each FTP is approximately 18 miles in length, short-term exposure usually amounted to just
under 100 miles of operation, all of which was in a controlled laboratory environment.
To address this concern, API and EPA conducted test programs on a total of six light-duty
vehicles (see Table 15 for listing of the vehicles) for sulfur sensitivity after both short-term and
long-term exposure to sulfur. The long-term exposure consisted of between 1,500 and 4,000
miles of in-use operation over urban, rural, and highway roads. Two of the vehicles were 1999
models, while the other four were all 1998 models. All six were either LEV or ULEV vehicles.
As listed in Table 15, three of the vehicles were equipped with catalyst systems aged to either
50,000 or 100,000 miles. The other three vehicles had low mileage catalyst systems aged to only
about 4,000 miles.
All of the vehicles were tested for short-term exposure prior to the long-term testing. Each
vehicle was tested using a FTP baseline tested on low sulfur fuel (30 or 40 ppm). The number of
tests used to establish the baseline varied from two to four. The vehicles were then tested with the
high sulfur fuel (EPA @ 350 ppm, API @ 540 ppm). Sulfur sensitivity was determined by
calculating the percent increase in average emissions with the high sulfur fuel compared to the
average emissions with the low sulfur fuel. Table 23 lists both the short-term and the long-term
sulfur sensitivity data for all six vehicles.
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Table 23
Vehicle-by-Vehicle Short-Term vs. Long-Term Sulfur Sensitivity
Vehicle
Accord
Cavalier
Altima
Taurus
Accord
Avalon
Sulfur
Aging
Short
Long
Short
Long
Short
Long
Short
Long
Short
Long
Short
Long
Sulfur
Level
30
350
30
350
30
350
30
350
40
540
40
540
40
540
40
540
40
540
40
540
40
540
40
540
Exhaust Tailpipe Emissions (g/mi)
NMHC
0.031
0.035
0.033
0.040
0.070
0.105
0.070
0.223
0.041
0.059
0.041
0.057
0.033
0.051
0.033
0.073
0.029
0.032
0.029
0.041
0.040
0.061
0.040
0.060
CO
0.351
0.478
0.330
0.731
1.778
4.048
1.778
7.224
0.788
1.058
0.788
0.987
0.522
0.832
0.522
1.310
0.285
0.299
0.285
0.465
0.406
0.541
0.406
0.734
NOx
0.092
0.155
0.09
0.234
0.068
0.303
0.068
0.324
0.061
0.112
0.061
0.132
0.075
0.101
0.075
0.117
0.100
0.192
0.100
0.245
0.068
0.116
0.068
0.142
Sulfur Sensitivity (%)
NMHC
12.0
21.7
49.3
216.
43.9
39.0
54.5
121.2
10.3
41.4
52.5
50.0
CO
36.3
121.1
127.7
306.4
34.3
25.3
59.4
151.0
4.9
63.2
33.3
80.8
NOx
69.4
158.5
347.0
411.8
83.6
116.4
34.7
56.0
92.0
145.0
70.6
108.8
In order to quantify the difference between short-term and long-term exposure, a fleet average
emission rate was determined for both low and high sulfur fuels for each pollutant, for both long-
term and short-term exposure. The percent change in emissions between low and high sulfur fuels
was calculated, and the ratio of long-term sensitivity to the short-term sensitivity was then
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determined. Table 24 shows the percent increases from short-term to long-term were quite large,
especially for hydocarbon emissions. Statistical tests to determine whether this observed increase
in sulfur sensitivity was significant are discussed in Appendix B of the Tier 2 Regulatory Impact
Analysis.
Table 24
Percent Difference Between Short-Term vs. Long-Term Sulfur Sensitivity
Average
Short-Term
Long-Term
Sulfur Sensitivity (%)
NMHC
40.2
100.3
CO
75.7
178.7
NOx
111.3
163.4
Ratio of long-term to short-term sensitivity
NMHC
2.50
CO
2.36
NOx
1.47
The ratio of long-term to short-term sensitivity is was then multiplied by the short term
sensitivities from the larger vehicle database to arrive at a total sulfur sensitivity effect. Numbers
and ratios for HC emissions will be assumed to be the same as those for NMHC.
For MOBILE6, the most important numbers for long-term sulfur effects are the ratios developed
above, so they are re-state in Table 25 below (for easy reference).
Table 25
Ratio of long-term to short-term Sulfur Sensitivity for LEV and Cleaner Vehicles
Pollutant
NMHC
HC
CO
NOx
Ratio to be Applied
2.50
2.50
2.36
1.47
To calculate an overall sulfur sensitivity for LEV and cleaner vehicles in MOBILE6, the revised
LEV regression coefficients in Table 17 is used to determine short-term percent increase in
emissions when sulfur is increased from one level to another. The short-term percent change is
then multiplied by the ratio in Table 25. The product is the total sulfur sensitivity for a given
emission model for increasing sulfur from one level to another, except for 2004-and-later model
year vehicles, where an irreversibility effect is also applied.
The following should be remembered when applying these new factors:
• The long-term exposure effects applies only to LEV and cleaner vehicles and trucks. Tier
0 and Tier 1 vehicles and trucks only have short-term sulfur effects.
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For high emitting LEV vehicles and trucks the long-term factors listed in Table 25 are
used as written, but as explained in the discussion of high emitters, above, the short-term
effects are reduced.
Sulfur Irreversibility
In addition to the sulfur sensitivity (short and long term emission effects) issue discussed above,
fuel sulfur can also impact vehicle emissions in a more lasting fashion, ranging from 20 or more
miles to potentially permanent. This lasting effect of sulfur on emissions is termed irreversibility,
referring to the fact that the emission impact of high sulfur fuel does not reverse when low sulfur
fuel is used. The EPA Staff Paper on Gasoline Sulfur Issues (U.S. EPA, May 1998, EPA420-R-
98-005) summarizes conditions required to remove sulfur from the catalyst once a vehicle has
been exposed to high sulfur fuel.
In particular, the results of a number of studies have shown that generally high temperatures (in
excess of 700 F) are required to remove sulfur from both the surface of the catalyst and from the
internal catalyst matrix. In addition to high temperature, a rich exhaust (absence of oxygen
coupled with presence of HC and CO, or a low air-to-fuel ratio) or an alternating sequence of rich
and lean exhaust is often needed to fully regenerate the catalyst.
However, the two changes in conditions necessary to reverse sulfur poisoning—hotter catalyst
temperatures and variable air-to-fuel ratios—both run counter to other design criteria aimed at
achieving stringent emission standards in-use. Thus, EPA believes that sulfur reversibility effects
should be included when assessing sulfur's total impact on exhaust emissions in MOBILE6.
The incorporation of sulfur irreversibility effects into the MOBILE6 model is similar to the steps
used in the Tier 2 model16. As in the Tier 2 model, MOBILE6 applies a sulfur correction to 2004-
and-later vehicles that is based on the vehicle's maximum sulfur level exposure, as well as the
short- and long-term corrections for the current sulfur level. However, for MOBILE6, the Tier 2
methodology was simplified, as follows:
• In the Tier 2 model, there were a total of 7 fuel sulfur phase-in categories. It was felt that
three major categories will suffice for most of the fuel-sulfur modeling scenarios that
MOBILE6 will be used for. These three regions are: East Conventional Gasoline, West
Conventional Gasoline, and Reformulated Gasoline. The average and cap (or
"maximum") sulfur levels for these three different fuel categories can be found in
reference 16 (for both the Tier 2 and no Tier 2 cases). Maximum levels are determined by
whichever (summer vs. winter) is the maximum for a given year.
• For calendar years 2000 through 2015, average and cap sulfur levels were developed for
each region with and without Tier 2. The sulfur levels are based on the methodology in
reference 16, with the additional step of aggregating the SBREFA and non-SBREFA
sulfur levels for the Conventional Gasoline areas. The aggregations were done based on
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projected fuel volumes16.
For a given 2004+ model year light-duty vehicle or truck, two sulfur levels are calculated
in MOBILE6: "current" (or "average") and "maximum" exposure.
The emission impacts of the "current" sulfur effects are calculated using the "current"
sulfur level in the modeled calendar year by using the "short-term" sulfur coefficients, and,
as necessary, multiplying the result by the long-term factors (discussed above).
The emission impacts of the "maximum" sulfur effects are calculated using the short-term
sulfur correction coefficients with the "maximum" sulfur level for the vehicle. Because
sulfur levels are decreasing with time, the maximum sulfur level for a vehicle is the
maximum sulfur level allowed in the year the vehicle was first driven, ie, the vehicle's
model year. Long-term effects are not applied
The average sulfur effect, including short-term, long-term and irreversibility is then
calculated as a weighted average of the "current" and "maximum" based on the
irreversibility factors (IR) developed in reference 16. The irreversibility factors are 0.15
(or 15%) for LEVs and 0.425% (or 42.5%) for Tier 2. The equation used to determine the
average sulfur effect is:
Average Factor = [IR * Maximum Effect] + [(1-IR) * Current Effect]
Note that in MOBILE6, emissions for all post-2004 cars and trucks are calculated with
irreversibility effects. However, in cases where post-2004 trucks are not subject to LEV or
Tier 2 standards, the emissions for post-2004 trucks are calculated to have no long-term
sulfur effects.
References
1. Benson, J. D., et al., "Effects of Gasoline Sulfur Level on Mass Exhaust Emissions-
Auto/Oil Air Quality Improvement Research Program, SAE Paper No. 912323, 1991.
2. Koehl, W. J., et al., "Effects of Gasoline Sulfur Level on Exhaust Mass and Speciated
Emissions: The Question of Linearity-Auto/Oil Air Quality Improvement Research
Program," SAE Paper No. 932727, 1993.
3. Rutherford, J. A., et al., "Effects of Gasoline Properties on Emissions of Current and
Future Vehicles-T50, T90, and Sulfur Effects-Auto/Oil Air Quality Improvement Research
Program," SAE Paper No. 952510, 1995.
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4. Mayotte, S. C., et al., "Reformulated Gasoline Effects on Exhaust Emissions: Phase I:
Initial Investigation of Oxygenate, Volatility, Distillation and Sulfur Effects," SAE Paper
No. 941973, 1994.
5. Mayotte, S. C., et al., "Reformulated Gasoline Effects on Exhaust Emissions: Phase II:
Continued Investigation of the Effects of Fuel Oxygenate Content, Oxygenate Type,
Volatility, Sulfur, Olefm and Distillation Parameters," SAE Paper No. 941974, 1995.
6. Korotney, D. J., et al., "Reformulated Gasoline Effects on Exhaust Emissions: Phase IE:
Investigation on the Effects of Sulfur, Olefms, Volatility, and Aromatics and the
Interactions Between Olefms and Volatility or Sulfur," SAE Paper No. 950782, 1995.
7. Sulfur "Reversibility" Study on LEV-certified Vehicles. Private communication with
David Lax of the American Petroleum Institute.
8. Sulfur "Extension" study conducted by American Petroleum Institute. Data Transmitted
to EPA in electronic format.
9. "Summary: CRC Sulfur/LEV Program," Coordinating Research Council Report, CEC
Project No. E-42, December 22, 1997.
10. "AAMA/AIAM Study on the Effects of Fuel Sulfur on Low Emission Vehicle Criteria
Pollutants," December 1997.
11. EPA Report Number M6-STE-002 "The Determination of Hot Running and Start
Emissions from FTP Bag Emissions." Available on MOBILE6 web site
(www.epa.gov/otaq/m6.htm).
1 la. Rao, Venkatesh. "Effects of Sulfur on Exhaust Emissions: A Proposal for MOBILE6,"
October 1, 1997. Available on MOBILE6 web site (www.epa.gov/otaq/m6.htm)
12. EPA's Complex Model for certifying Reformulated Gasolines. This model is available
on the web at http://www.epa.gov/otaq/rfg.htnrfmodels.
13. Rao. V., "Development of an Exhaust Carbon Monoxide Emissions Model" SAE Paper
No. 961214, 1996.
14. EPA Final Regulatory Impact Analysis for Reformulated Gasoline, December 13, 1993.
15. Private communication with EPA's Rick Rykowski, National Expert, Engine Programs
and Compliance Division, August 1998.
16. Memorandum to Air Docket A-97-10, "Development of Light-Duty Emission Inventory
Estimates in the Final Rulemaking for Tier 2 and Sulfur Standards," John W. Koupal,
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QMS, December 15, 1999.
17. EPA Staff Paper on Gasoline Sulfur Issues (U.S. EPA, May 1998, EPA420-R-98-005)
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