United States
Environmental Protection
Agency
Mav 1995
wEPA Highway Vehicle Emission
Estimates -- II
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DISCLAIMER
This paper has not been peer reviewed.
It is intended to present the current thinking of
the Office of Mobile Sources with respect to a number
of issues pertaining to the accurate modeling of in-use
emission factors for highway vehicles, and to facilitate
discussion of these issues among interested parties.
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June 9, 1995
The paper "Highway Vehicle Emission Estimates - II" (May 1995) is an
update to an earlier paper of the same title released in July 1992. The
earlier paper, as well as this update, were prepared in response to
concerns regarding potential underestimation of in-use highway vehicle
emissions by highway vehicle emission factor models. A number of
studies had provided indications that then-current estimates of total
highway vehicle emissions may be too low. Among these were tunnel
studies (such as the 1987 Van Nuys tunnel study), roadside emission
measurements (including the use of remote sensing devices), and
ambient concentration comparisons. Each provided suggestive but not
definitive evidence that the total emissions contribution of highway
vehicles is understated by current emission inventory development
procedures, which include the use of emission factor models (such as
MOBILESa) to estimate average in-use per vehicle emission rates in
grams per mile.
EPA was aware of a number of areas in which then-current practice,
including the collection of in-use vehicle emission data, could be
improved. Some of these improvements are reflected in MOBILESa,
released in 1993, while others will take longer to implement. The EPA
approach to estimating highway vehicle emission factors prior to 1990,
the known limitations of that approach, issues that have been identified
as possibly contributing to the underestimation of in-use emission
levels by the model, and the approaches EPA has undertaken and is
planning for the future to address these issues, are discussed in these
papers. This second paper provides updated information on a number
of issues relevant to estimating highway vehicle emission factors and
emission inventory contributions.
This paper is intended to provide an overview and to facilitate
discussion; it is not a statement of official EPA policy. Comments on the
paper and the issues discussed therein are welcome, and should be
directed to:
Mr. Terry Newell (AQAB)
U. S. Environmental Protection Agency
National Vehicle and Fuels Emission Laboratory
2565 Plymouth Road
Ann Arbor, MI 48105
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Summary
In July 1992, EPA's Office of Mobile Sources released "Highway Vehicle
Emission Estimates." a document that was prepared in response to heightened
attention to and concern about the accuracy of emission inventory estimates
for highway mobile sources (sometimes referred to as the "QMS White Paper"
on vehicle emissions).1 Highway mobile sources (cars, light and heavy trucks,
buses, and motorcycles) account for a significant portion of overall emission
inventories for volatile organic compounds (VOC), carbon monoxide (CO), and
oxides of nitrogen (NOx). With a substantial part of the nation's population
living in areas that fail to attain the National Ambient Air Quality Standards
(NAAQS) for ozone and/or CO, the need for additional reductions in emissions of
these pollutants is clear. In order for officials to make the best choices in
achieving the needed reductions, accurate emission inventory estimates are
crucial.
In the late 1980s, the results from a number of different studies along
with other evidence suggested that the inventory contribution of highway
vehicles was being underestimated by the methods in use for calculating such
estimates at that time. The July 1992 paper provided an overview of some the
more important studies that suggested this, including studies of ambient
emission ratios and the 1987 Van Nuys tunnel study. The July 1992 paper also
summarized many of the known weaknesses in the methods used to develop
these estimates, from the process of collecting data on in-use vehicle emissions
performance, to the calculation of average fleetwide emission factors under a
range of conditions and driving, to the estimation of the total vehicle miles
traveled (VMT). Finally, it outlined some of the significant improvements that
EPA was implementing and planning to implement to improve the accuracy of
the process.
Since the release of the July 1992 paper on highway vehicle emission
estimates, considerable activity has occurred on a wide range of issues
discussed therein. EPA has revised the highway vehicle emission factor model
(the latest version, MOBILESa, was released in May 1993) with the net effect of
a number of changes being an increase in the estimated average in-use
emission rates for light-duty gas vehicles under most conditions. Additional
tunnel studies have been performed, using tunnels with different
characteristics and in other parts of the country. An evaluation of various
ambient emission studies conducted in recent years has been performed. EPA
has implemented a number of new regulations under the provisions of the
Clean Air Act Amendments of 1990 that are designed to further reduce in-use
emissions from new cars, trucks, and buses. Studies intended to address other
concerns, such as the representativeness of the driving cycle long used to
certify new vehicles and as a baseline for other emission estimates from those
vehicles, have made considerable progress in determining improvements and
changes that are necessary in order to increase the accuracy of highway
vehicle emission inventory estimates.
This second OMS "White Paper" on highway vehicle emission estimates
provides an update on these topics. Some background information provided in
the July 1992 paper is repeated below. Following that, the revisions that have
been made to EPA's highway vehicle emission factor model and the effects of
these changes on emission factor estimates are summarized in Section 1. A
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summary of the findings of an evaluation of various ambient emission studies
is given in Section 2, and in Section 3 more recent tunnel studies are discussed.
A comparison of emission factors estimated by MOBILE5a to those calculated by
the 1987 Van Nuys tunnel study, as well as the 1992 Fort McHenry and
Tuscarora tunnel studies, is presented in Section 4. Section 5 provides a recap
of many of the issues concerning the accuracy of highway vehicle emission
factor and inventory estimates, in the format used in the first July 1992 paper.
Finally, Section 6 outlines some of EPA's plans for the next major revision to
the model (which will be MOBILE6), and other ongoing research is briefly
discussed.
The next section (Background) is largely repeated from the July 1992
paper, and is included here as refresher and for the benefit of readers that
may not be familiar with that document. Readers unfamiliar with the first
White Paper are encouraged to obtain a copy by contacting QMS.2
Background
The three primary pollutants from motor vehicles and engines for
which EPA has established emission standards are hydrocarbons (HC), carbon
monoxide (CO), and oxides of nitrogen (NOx). All of these pollutants are emitted
from vehicle tailpipes when the engine is running (exhaust emissions).
Vehicles also emit HC through evaporation of fuel from the engine and fuel
system when they are not running: Diurnal ("breathing") emissions, trip-end
("hot soak") emissions, continuous resting loss emissions (as can occur due to
porous tubing in the evaporative emission control system), and refueling
emissions (vapor in the partially filled fuel tank being displaced by the
addition of new fuel), and from sources other than the tailpipe when in
operation (running loss emissions). Some vehicles with disabled or
disconnected hoses also exhibit crankcase emissions of HC ("blowby" losses).
One key to the accurate assessment of air quality problems and to
estimating reductions in air pollution is the development of reliable emissions
inventories, which quantify the total amount of a given pollutant under a
specified set of conditions. Emission inventories, usually expressed in tons of
pollutant per year, are the product of two factors: emission factors and
activity levels. An emission factor expresses the amount of pollution emitted
per unit of activity (i.e., grams of carbon monoxide emitted per vehicle mile
traveled). An activity level represents the amount of the given activity that
occurs over a specified period of time (i.e., vehicle miles traveled by highway
vehicles in a metropolitan area on a typical summer ozone season day). The
sum of the products of the emission factors and activity levels for all sources of
a given pollutant constitute the emission inventory for that pollutant.
For highway vehicles (light-duty vehicles, light-duty trucks, heavy-
duty trucks, and motorcycles, both gasoline and diesel), emission factors are
most often expressed in grams of pollutant emitted per mile driven (grams per
mile, or g/mi). The activity level for highway vehicles is generally vehicle
miles traveled (VMT). Emission factors and VMT can be estimated for each
individual vehicle type, or for all highway vehicles as a group. The scope of a
highway vehicle emission inventory can be as small as a single link of a given
roadway for a specific hour, or as large as an entire Consolidated Metropolitan
Statistical Area (CMSA), State, or the whole country for an entire year. In any
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event, development of an accurate emission inventory for highway mobile
sources requires the best possible estimates of the emission factors.
Since the late 1960s EPA has issued and periodically tightened emission
standards applicable to highway motor vehicles. However, the emission
standards applicable to new vehicles do not represent the emission factors
applicable to those vehicles once they are actually in use. Emissions from
vehicles vary over the entire range of conditions that vehicles operate under:
ambient temperature, traffic conditions (represented by average speed in the
MOBILE emission factor model), operating mode (the mix of cold or hot starting
and warmed-up vehicle operation), fuel volatility and. composition, types and
condition of emission control equipment and other vehicle or engine
components that affect emission levels (maintenance), expected deterioration
in emission control performance with increasing age/mileage of the vehicle,
and other variables all affect the emissions actually produced by vehicles in
everyday use. In addition, the in-use vehicle fleet is composed of several
generations of vehicles and emission control technology, each of which
behaves differently in terms of in-use emission levels and how these levels
change over time (as the vehicle ages and accumulates higher mileage). Thus
average in-use highway vehicle emission factors are estimated using
computer models, which allow emission factors for different vehicle types to
be estimated under conditions specified by the user of the model and combined
into an overall estimate of emission factors for the in-use vehicle fleet.
Direct measurement of emission levels from all in-use vehicles under
all possible conditions is clearly impossible. To estimate in-use emission levels,
EPA conducts surveys in the form of vehicle test programs, collecting emission
data from as many vehicles as is practical over as wide a range of conditions
affecting emissions as possible. Time and funding are the constraints that
determine what is practical in terms of adding to the emission factor data
bases. Over the last 25 years, EPA has collected emissions data from Federal Test
Procedure (FTP) tests on tens of thousands of in-use vehicles under the
emission factors program (EFP). The EFP has also accumulated data on
thousands of vehicles tested over non-FTP cycles (e.g., speed correction cycles.
Highway Fuel Economy Test cycle) and conditions (e.g., different
temperatures, fuels).
Direct measurements of emissions from highway vehicles operating on
the road have become available through the results of several on-road
emission measurement projects and tunnel emission studies. One well
publicized example of such a study is the Van Nuys, CA tunnel study, sponsored
by the Coordinating Research Council (CRC) and conducted in 1987. The results
of this study were widely interpreted as indicating that the highway vehicle
mobile source emission factor models developed by the California Air
Resources Board (ARB) and by EPA may underestimate hydrocarbon (HC) and
carbon monoxide (CO) emissions from highway vehicles. (California has had
more stringent motor vehicle emission controls than the rest of the country
since the 1970s. Emission factor models have been developed by both EPA and
ARB. The current versions of these models are known as MOBILESa and
EMFAC7F, respectively.) That study initiated increased scrutiny of in-use
emission factor and inventory estimates; work inspired by that study and its
reported results continues.
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Since then, two other major tunnel studies have been conducted in the
U. S., one in the Fort McHenry tunnel under Chesapeake Bay (Baltimore) and
another in the Tuscarora Mountain tunnel in Pennsylvania. The results of the
tunnel studies and their implications for highway vehicle emission factor
estimation are also discussed in a later section of this paper.
L Revisions to the MOBILE mnHpl
Since the release of "Highway Vehicle Emission Estimates" in July 1992,
EPA has extensively revised the model used to estimate average in-use emission
factors for highway vehicles. At the time that the 1987 Van Nuys tunnel study
was performed, the current EPA model was MOBILE3 (1984). The July 1992
paper presented a comparison of emission factors calculated by MOBILE4.1
(1991) to those measured in the Van Nuys tunnel study, with the MOBILE4.1
emission factors corrected to account for the differences in microscale and
area-wide weightings of the contribution of vehicles of each specific model
year to the total fleetwide average emissions. In March 1993, EPA released
MOBILE5a, a corrected version of MOBILES (December 1992). MOBILESa is the
current model at the time of this writing, and is used for the comparisons to
tunnel study-measured emission factors later in this paper.
EPA has continued to collect emission factor data from in-use vehicles,
and has expanded its data collection efforts to include IM240 program lanes
(see "Recruitment Bias in Emission Factor Testing" in the section on
implemented and planned improvements to the model). This section presents a
brief summary of the most significant changes that have been made to the
MOBILE model since 1992. For a more thorough discussion of these changes,
see Chapter 1 of the "User's Guide to MOBILES."
Updated Basic Emission Rates
The basic emission rate equations describe emissions as a function of
increasing odometer mileage, for properly maintained non-tampered vehicles.
The rates consist of zero-mile levels (ZMLs) and deterioration rates (DRs),
which indicate how much emissions are predicted to increase with increasing
age/mileage. For light-duty gas vehicles and light-duty gas trucks, there are
two distinct DRs, with one being applied to the first 50,000 miles of accumulated
mileage and the second (higher) DR being applied to mileage in excess of
50,000.
The data used to develop the basic emission rates for use in the model are
collected under the Emission Factors Program (EFP). Historically, such data
have been collected primarily through mail solicitation of owners selected
from vehicle registration lists. Under this method of recruitment, privately
owned vehicles are recruited by mail (invitation postcards) to loan their
vehicles to EPA (or an EPA contractor) for testing, and incentives for
participation (e.g., use of late model leaner vehicle) are provided to the
vehicle owner. Typical response rates (the fraction of those owners contacted
about participation in the program that agree to participate) have been low,
raising concerns about biases in the samples of in-use vehicles used to develop
the BERs. For example, one might expect that an owner who has knowingly
tampered with his vehicle, or one who is aware that the vehicle has not been
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maintained properly, would be reluctant to provide that vehicle to EPA for
emissions tests.
In the last several years, EPA has expanded the EFP to include collecting
data from centralized inspection and maintenance (I/M) program lanes. In a
centralized I/M program, all vehicles subject to the program requirements
must visit one of a relatively few state-operated inspection locations for the
test. EPA took over operation of one lane at the centralized I/M test site in
Hammond. IN for two two-year periods, separated by a one-year interruption.
Recently, data collection at the Hammond I/M lane ceased, and was replaced by
data obtained from a lane at a test site in Chicago Heights, IL. A similar
program of data collection is currently operating at one lane of an I/M site in
Mesa, AZ. The required nature of these tests means that there is a far lower
degree of self-selection bias evidenced in the vehicles that pass through such
stations. (There is still some residual bias, in that most programs exempt
vehicles above a specified age, and sometimes also those under another
specified age, from the program requirements, and no programs achieve 100%
compliance in practice.)
The I/M programs in Hammond and Phoenix utilize the IM240 test, a
short dynamometer-based transient driving cycle test. This test cycle, which
was derived from the Federal Test Procedure (FTP) driving cycle, measures
emissions over a range of vehicle speed and load. Such measurements provide
a much more realistic set of emission measurements than do simpler idle-based
tests. In addition to the substantial reduction in potential sample bias
described above, EPA has used the data collected from operating one of the
lanes in these programs to improve the basic emission rates used in the
MOBILE model in two other significant ways.
First, EPA obtained IM240-based emissions measurements of HC, CO, and
NOx from all of the vehicles that passed through the lane being operated by
EPA. This provided emission results for thousands of vehicles annually at each
lane, a considerable increase over the hundreds of vehicles tested annually
under the more traditional EFP. Second, EPA has randomly recruited a
subsample of the vehicles that pass through the lane for additional testing
(including full FTP tests) in a laboratory setting. Using the IM240 lane results
and the lab-based FTP results for those vehicles has enabled EPA to develop a
statistical correlation between the IM240 and FTP emission results. This
correlation was then used to "predict" the FTP emission results for all of the
vehicles that passed through the EPA-operated lane. Thus, this approach both
vastly increased the sample sizes available for development of basic emission
rates and greatly reduced the bias, particularly with respect to self-selection,
of the samples.
Another important benefit of data collection at the I/M lanes follows
from the far higher volume of vehicles that are tested. With so many more
vehicles, and (over time) access to a cross-section of almost all vehicles in the
I/M area, EPA was able to obtain significantly more data from vehicles that
have "aged." This refers to the ability of testing at such sites to include
vehicles that are both "old" (measured by age) aM have very high mileages
(75.000 to 100,000 and greater). Much of the data on high-mileage vehicles
that has been collected in recent years came from tests of vehicles that had
accumulated mileage at far above average rates (e.g., a vehicle is only 4 years
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old but has 80,000 miles). These data provided an opportunity to examine
whether the degree of in-use deterioration in emission levels is the same for
all vehicles that have high mileage (those that have high mileage because
they are used more frequently and/or for longer trips, and those that have
high mileage because they have been in use for a longer number of years.
The most significant difference, found when new basic emission rates
were calculated from the IM240 lane data, relative to the previous basic
emission rates based only on data collected in the mail-solicitation part of the
EFP, was an increase in estimated deterioration rates. This may be due in part
to the inclusion of substantially more ail, high mileage vehicles in the test
samples, as noted above. Relatively little difference was found in the zero-mile
level (ZML) estimates. Table 1 summarizes the basic emission rates for all
three pollutants used in MOBILE4.1 and those used in MOBILES for light-duty
gas vehicles. The differences in the estimates for future vehicles (model years
1994 and later), particularly in the ZMLs, are primarily the result of the
Federal Tier I tailpipe emission standards mandated by the Clean Air Act
Amendments of 1990, which were not yet legislated at the time of MOBILE4.1.
These changes in the basic emission rates, when translated to changes
in average fleetwide in-use emissions as estimated by the model, resulted in
increases on the order of 20 to 30 percent. EPA is continuing to collect IM240
data for further expansion of the in-use data base and refinement of the basic
emission rate equations.
Revisions to Speed Correction Factors
The bulk of emission data available for light-duty vehicles and light-
duty trucks is measured during Federal Test Procedure (FTP) tests, which
include a driving cycle with an average speed of 19.6 mph. This cycle, which
is used for certification of new vehicles for sale and in measuring compliance
with standards in use as part of EPA's recall program, is intended to represent
urban commuting driving. Exhaust emission factors calculated by the MOBILE
model are trip based, with the speed provided by the user of the model
representing average trip speeds. In other words, MOBILE exhaust emission
factors represent transient operation, and include the effects of speed
variability (acceleration and deceleration), time at idle (such as at red traffic
lights), and some cruising at relatively stable speeds.
In order to more accurately account for vehicle emissions in emission
inventory calculations, emission factors representing a broader range of
driving behaviors than are included in the FTP are necessary for many areas.
While the emissions impacts of some of these behaviors are not yet
characterized in the model (very high rates of acceleration, for example - see
the discussion of the Non-FTP Study in section 6), the model does include the
ability to model emission factors at other average speeds in the range of 2.5 to
65 mph. This is done in MOBILE5a through the use of speed correction factors,
which are applied to the basic emission factor estimates and are specific for
each pollutant and vehicle type.
The speed correction factors in the model are developed for three bands
of average speeds: "low" speeds, defined as average trip speeds under 19.6 mph
down to 2.5 mph; "mid-range" speeds, from 19.6 to 48 mph; and "high" speeds,
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from 48 to 65 mph. In MOBILES, all available data on vehicle emissions over
driving cycle tests w,th average speeds over 19.6 mph were analyzed, and as a
result the speed correction factors for mid-range and high average speeds
applicable to light-duty vehicles and light-duty trucks were revised.
For VOC and CO emissions, the effect of this change is that emissions
modeled at average speeds over 19.6 mph are generally greater relative to
emissions modeled at 19.6 mph, than was true when MOBILE4.1 was used to
estimate emissions. Average emission factors estimated by MOBILESa in grams
per mile (g/mi). decrease with increasing average speed in the range 19 6-48
mph, are constant from 48-55 mph, and increase with further average speed
increases to 65 mph. This is similar to the behavior of emissions as a function
of average speed in MOBILE4.1, although the decline in emission factors
between 19.6 and 48 mph is less than, was modeled previously. For NOx
emissions, the new speed correction factors show slight increases in NOx as
average speed increases from 19.6 to 48 mph, and more pronounced increases
as average speed further increases to 65 mph.
The new speed correction factors more accurately describe the behavior
of emissions as a function of average trip speed seen in the available test data.
For areas that develop highway vehicle emission estimates using emission
factors for different roadway facility classifications with their associated
variation in average speed, the new speed correction factors will result in
higher emission factor estimates than would be obtained using the previous
model.
Fleet Characterization Data
MOBILES models average fleetwide emission factors for each vehicle
type by first estimating the average emission for vehicles of age X in calendar
year Y, then considering the relative population of vehicles of each age X and
how many miles they are typically driven in terms of annual averages. The
relative population of vehicles of each age is described by registration
distribution fractions by age, a set of values that sum to 1.0 for each vehicle
type and give the fraction of all vehicles of that type that are in their first,
second, third (and so on) year of operation. MOBILE accounts for the most
recent 25 years' vehicles, with all vehicles over age 25 being considered in the
"25+" age category. The average annual mileage accumulation rate varies with
the age of the vehicle, with newer vehicles driven more and older vehicles
less. By combining data on the relative population of vehicles of each age and
how frequently they are driven, MOBILESa calculates a travel fraction for
vehicles of each age, representing the fraction of all vehicle miles traveled
(VMT) by a given vehicle type accounted for by vehicles of that age. These
travel fractions are then used to weight the average emission rates for
vehicles of each age X in a given calendar year, with the result being the
average in-use emission factor.
The registration distribution and annual mileage accumulation rates by
age are together described as fleet characterization data. Although the user of
the model has the ability to alter either or both of these in MOBILES to account
for local variations, adequate data is not always available, particularly for
annual mileage accumulation rates. Thus the values for these data that are
included in the MOBILE model, which represent nationwide averages, are
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important. For MOBILES, EPA updated both the registration distributions by
age and the annual mileage accumulation rates by age. The registration
distribution data in MOBILES (and MOBILE5a) are national values for calendar
year 1990. The annual mileage accumulation rates used in MOBILE5/5a have
been adjusted upward by about 10%; this adjustment was based on the increase
in total vehicle miles traveled (VMT) since the previous rates were developed,
accounting for the increase in total registrations (total vehicles on the road)
that also occurred over that time.
Relative to the values used in MOBILE4.1, these data show that the fleet
of in-use vehicles is older on average, and that vehicles of all types and ages
are driven more miles, than before. The net effect of both of these changes is
to further increase the average in-use emission factors calculated for any
given set of conditions.
As always, EPA encourages States and others estimating vehicle
emissions to use locality-specific registration data to calculate registration
distributions by age when it is available, as variation in the average age of the
in-use fleet modeled will affect the emission factors. However, for areas
without such data (and with respect to mileage accumulation rates by age, for
most areas), these revisions will better describe the emissions of the in-use
fleet by better characterizing its age and use patterns.
Evaporative Emission Estimates
Beyond exhaust emissions of VOC, CO, and NOx, gasoline-fueled vehicles
are responsible for considerable evaporative emissions. These "non-tailpipe"
emissions consist primarily of gasoline vapor, and are emitted from cars and
trucks in a number of ways: At the end of a trip, when the vehicle, engine,
and emission control and fuel systems are hot and the vehicles is turned off,
there are emissions of evaporating gasoline. These are known as hot soak (or
trip-end) emissions. When a vehicle sits idle during a period of rising ambient
temperatures, such as in a parking lot all day, the heating of the day results in
heating of the fuel tank and fuel, and the resulting evaporation of gasoline is
termed diurnal emissions. When vehicles are being driven, the heat
generated and transferred to the fuel will result in vapor generation; if the
vapor generated is not captured by the evaporative control carbon canister, or
if the canister becomes saturated with vapor which is not properly purged and
routed to the engine to be burned, the resulting running loss emissions are
another evaporative emission source. When gasoline-fueled vehicles are
refueled, evaporative hydrocarbon emissions termed refueling emissions are
released; these occur as liquid fuel displaces the vapor in the "empty part" of
the. vehicle fuel tank (displacement losses) and as a result of small amounts of
fuel being spilled via dripping fuel dispenser nozzles or splashbacks (spillage
losses). All gasoline-fueled vehicles also leak small amounts of vapor through
the fuel tank, canister and hoses, and other parts of the fuel handling system;
these are termed resting loss emissions.
The non-tailpipe emissions of VOC from the sources outlined above can
equal or exceed the tailpipe (exhaust) emissions of VOCs, particularly in very
warm weather or when higher than recommended volatility gasoline is used.
To improve the accuracy of the estimated non-tailpipe emissions from cars and
light trucks, EPA has collected substantial data on emission rates from in-use
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vehicles that pass both, fail one or the other, or fail both of the functional
evaporative emission system tests recommended for use in enhanced
inspection and maintenance (I/M) programs. These tests are referred to as the
pressure test and the purge test.
In the pressure test, the integrity of the vehicle evaporative emission
control system is tested. If the system fails to maintain a specified pressure
when ostensibly sealed, then it is concluded that one or more leaks exist in the
system. Under such conditions, at least some of the gasoline vapor that should
be trapped and routed to the carbon canister for storage and later combustion
is being released as emissions. In the purge test, the evaporative control
system is tested to determine if vapors once captured in the canister are being
properly purged and routed to the engine to be burned. If a vehicle fails the
purge test, then vapor may be routed to the canister, but once the canister
capacity has been reached, additional vapor will be released as emissions. As
might be expecied, given the above, vehicles that fail one or both of these
functional tests are found to have considerably higher evaporative emissions
(except for resting losses).
MOBILE5a evaporative emission factor estimates have been improved by
accounting for the rates of failure on one or both of these tests as a function of
vehicle age (the older the vehicle, the higher the probability that it fails one
or both of the tests), and by accounting for the emissions impacts of such
failures on the various types of evaporative emissions. The estimated effect on
evaporative emissions due to failure of the pressure and/or purge tests has
been improved as a result of having significantly more test data from vehicles
that failed one or both tests. (EPA performed the pressure and purge tests
before selecting vehicles for further testing in the laboratory. This allowed
data to be obtained from more "failing" vehicles than would otherwise have
been feasible, and with more data the emissions impacts of failing these tests
could be estimated with greater confidence.) Fleetwide average evaporative
emission factors that account for the rates of pass/fail on the functional
pressure and purge tests, and for the emissions increases associated with
failure on one or both tests, are more representative of actual in-use
conditions, and are generally higher than previous estimates that did not take
these factors into account.
Another improvement in the MOBILE5/5a evaporative emission factors
results from accounting for "real-time" effects on diurnal emissions. Data
describing emissions that occur over 8-hour periods of rising ambient
temperature have been collected, and were used to modify the emission factor
estimates based on 1-hour simulated diurnal events. Data have also been
analyzed from hot soak and diurnal emissions generated using fuels of lower
volatility (down to 7 psi RVP), which has improved the accuracy of these
emission factors when fuel volatility under 9.0 psi RVP is specified.
Fuel Effects
Historically, EPA certification of new vehicles and most other vehicle
test programs (including the EFP) have used a specifically formulated .
gasoline. This fuel (formerly known as Indolene) is blended to regulatory
specifications, and has certain specific properties (e.g., 9.0 psi RVP volatility).
While much attention has been focused on the effects of fuel volatility (as
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measured by RVP) on emissions, it is known that other fuel properties also
have an impact on emission levels and that commercially available fuels do not
necessarily correspond to EPA's test fuel with respect to these additional
properties.
In MOBILESa, EPA has included adjustments to emission levels to account
for differences between EPA test fuel and industry-average commercial fuel
These adjustments are independent of RVP effects, which are controlled by the
modeler as part of the model input, and of other fuel programs (such as
oxygenated fuel or reformulated gasoline requirements). When the modeler
specifies the inclusion of fuel programs, such as reformulated gasoline the
necessary adjustments to the emission factors are made from the starting point
of industry-average commercial fuel emissions.
The net effect of the industry-average fuel properties being accounted
for in the emission factors produced by MOBILESa, for current technology
(model year 1990, three-way catalyst, fuel-injected) vehicles at standard FTP
conditions, are increases of about 13.6% for VOC, 8% for CO, and 13.8% for NOx.
(The NOx effect is applied for all three-way catalyst vehicles only. The VOC and
CO effects are also applicable to oxidation catalyst vehicles.) These increases
are independent of, and in addition to, increases in the emission factors
resulting from the revised basic emission rate equations and other revisions to
the model. This adjustment to the modeled emission factors is another
significant step toward making the modeled emissions more representative of
actual in-use conditions.
The revisions to the MOBILE model discussed above all had the effect, to greater
or lesser extents, of increasing the estimated average in-use emission factors
calculated by the MOBILE model for any calendar year after 1980. There were
also other revisions made to the MOBILE model that had only limited effects on
current or past (e.g., 1990) emission factor estimates, in terms of changing the
overall level of emissions, but are included in order to improve the accuracy of
estimated future year emission factors. These are briefly described below:
July I Evaluation Option
MOBILE4.1 and earlier versions of the model allowed the user to specify
the calendar year of evaluation (the year for which average in-use emission
factors are to be calculated), but allowed no choice as to the date within the
specified year: All emission factors were estimated as of January 1 of the
specified calendar year. When emission factors for summer conditions were
required, the guidance was to evaluate two consecutive calendar year sets of
emission factors and interpolate (i.e., if July 1, 1995 emission factors were
desired, the modeler would obtain MOBILE emission factor estimates for
January 1, 1995 and January 1, 1996, and interpolate between them).
In MOBILESa, the provision to have the model directly calculate
emission factors as of either January 1 or July 1 of the specified year has been
added. This allows the model to account for the benefits of an additional six
months of fleet turnover (sales of new vehicles meeting the latest standards
and retirement from service of older, more polluting vehicles). In addition to
allowing summer. season emissions to be estimated from a single run, rather
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than from interpolation of two runs, this feature is used in the modeling of
reformulated gasoline effects (see below). If January is specified, then
"winter" RFG rules are assumed, while if July is specified, "summer" RFG rules
are assumed.
If emission factors as of a specific date other than January 1 or July 1
are needed, then interpolation between the appropriate January and July
evaluation dates is still required.
Oxygenated Fuel Effects
The effects of oxygen content in the fuel on emissions has been revised
and expanded in MOBILESa. This feature is important for regulatory analyses
(many areas have mandatory oxygenated fuel requirements in the winter
season, during which most ambient CO violations occur), and for more accurate
assessment of base year (1990) emission levels, as many areas have significant
market penetration of oxygenated fuels even in the absence of regulatory
requirements, particularly in the Midwest.
The largest impact of oxygenated fuels is on CO emissions. Due to the
tighter time constraints for inventory and State Implementation Plan (SIP)
development imposed on CO nonattainment areas (relative to ozone
nonattainment areas) in the Clean Air Act Amendments of 1990, EPA included
the effects of oxygenated fuels on CO emissions in MOBILE4.1 (1991).4 In
MOBILE5a, the effects of oxygenated fuels on emissions have been updated
using the revised Tech5 model, and the effects of oxygenated fuels on VOC
emissions have been incorporated in the model. These effects are smaller than
the impacts on CO emissions. On the basis of all available data, MOBILES
continues to model no impact of oxygenated fuels on NOx emissions.
Reformulated Gasoline
One of the requirements of the 1990 Clean Air Act Amendments for
highway mobile sources is the use of reformulated gasoline (RFG). The use of
such fuel is mandated for the nine worst ozone nonattainment areas of the
country (Los Angeles, Chicago, Houston, Milwaukee, New York City, Baltimore,
Philadelphia, San Diego, and "Greater Connecticut"). Other areas not included
in this list are permitted to "opt in" to the RFG requirements, and some areas
have done so.
Section 211(k) of the amended Clean Air Act specifies two levels, or
phases, of RFG requirements. In Phase I, RFG that results in at least a 15%
reduction in VOC emissions from "baseline vehicles" is required. Phase I RFG
requirements begin in 1995. For Phase II, this requirement is strengthened to
at least a 25% reduction in VOC emissions, under the same provisions (on the
basis of emissions from "baseline vehicles," which has been interpreted to
mean model year 1990, current technology light-duty gas vehicles). Other
requirements of the RFG sections of the CAAA include that there be no
negative impact on (increase in) NOx emissions from RFG and that the
maximum benzene content of the fuel be no greater than 1.0 percent.
The details of RFG effects on emissions depend on the season (summer or
winter). In summer, when temperatures are higher and emissions of ozone
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precursors are the primary concern, the focus of the RFG rules is on
reductions in fuel volatility. In winter, when the weather is colder and CO
emissions are generally more of a concern, the focus of RFG requirements is
on oxygen content of the fuel.
Both summer and winter Phase I RFG requirements can be modeled
using MOBILE5a. The effects are based on the month specified by the user (if
January emission factors are requested, winter RFG rules are applied, and if
July emission factors are requested, then summer RFG rules are applied). Only
the VOC and CO effects of Phase II RFG are included in MOBILESa, and are based
on the "simple model" used in the development of the RFG regulations. In the
next update to the model, Phase II RFG effects on NOx emissions will be
included, and the estimates of the emission impacts of both Phase I and II RFG
will be updated on the basis of the "complex model" used in the regulatory
development process. The additional requirement that NOx emissions be
reduced (by 6.8% for "current technology" 1990 model year, three-way
catalyst, fuel-injected vehicles), rather than simply not being allowed to
increase, will also be reflected in the next model update.
Federal (Tier 1 and other) Exhaust Emission Standards
The Clean Air Act Amendments of 1990 included the first new
tightening of tailpipe (exhaust) emission standards to be implemented since
model year 1981. These standards are known as the Federal Tier 1 emission
standards, and the compliance phase-in period began with the 1994 model
year. The Act specified the levels of both the Tier 1 standards (section 202(g))
and intermediate standards (section 207(c)), with the complete phase in of the
final Tier 1 standards not complete until model year 1998.
MOBILESa includes the estimated in-use impact of these emission
standards. EPA has assumed that these standards will reduce emissions from
"normal emitter" vehicles, but will not have a significant impact the in-use
emissions of "high" and "super" emitters, nor the rates at which vehicles
migrate from the "normal" to the "high" to the "super" emitter categories.
Thus, the reduction in average in-use emission levels projected to result from
implementation of these standards is less than the reduction in the levels of
the standards. For example, the HC standard is being reduced from 0.41 to 0.25
g/mi, a decrease of about 40 percent, but the reduction in .the fleet average
emission factor will be less than that, even after the phase-in is completed
(after 1998) and essentially all vehicles on the road are certified to the new
standards (25 years later, or after 2023).
While not defined as part of the Federal Tier 1 emission standards under
the 1990 Clean Air Act Amendments, the 4.0 gram per brake horsepower-hour
(g/bhp-hr) standard for NOx emissions from heavy-duty diesel trucks that is
set to take effect for the 1998 model year is also included in MOBILESa.
California LEV Program
Under the Clean Air Act and its amendments, California has long been
granted the right to establish its own regulations regarding highway vehicle
emissions, so long as the regulations promulgated by California are no less
stringent than those promulgated by EPA for the other 49 states (Federal
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standards). .With the severe and persistent air quality problems experienced
in most of the populated portions of the state, California cars and light trucks
have been required to meet more stringent standards- than Federal standards
for many years. The Act also provides other States the option of remaining
subject to the requirements of the Federal Motor Vehicle Control Program, or
of implementing the California motor vehicle emission control program.
The most significant difference between the programs being
implemented by California and those being implemented by EPA in the 1990s is
that California's low emission vehicle (LEV) program, which includes a zero-
emission vehicle (ZEV) sales mandate beginning in the. 1998 model year.
Section 177 of the Act allows other states to adopt California vehicle emission
control programs and standards, which several northeastern states have
already done. In response to a petition from the Ozone Transport Commission
(OTC), EPA published a final rule in December 1994 mandating adoption of the
California LEV program, without a ZEV sales mandate, in the Northeast Ozone
Transport Region (OTR). In addition, the states and the auto industry have
been negotiating a possible "49-state LEV" program without a ZEV sales
mandate, which might ultimately replace proposed LEV programs in the OTR
and the Federal Tier 1 program in the rest of the country. EPA has included
the ability to model the effects of all of these options in MOBILES a. Model users
can specify the initial model year, the phase-in schedule, and the type of in-
use inspection and maintenance (I/M) program applicable (which has a
strong effect on the level of benefits achieved by implementation of an LEV
program) for whichever LEV program option is applicable. The reader is
referred to the User's Guide to MOBILE5 for additional details on modeling of
LEV program options.5
/
The discussion above summarizes the most important revisions that have been
made to EPA's MOBILE model since 1992. In the next section, an overview is
provided of a report prepared for EPA that evaluates various ambient emission
studies and their conclusions. Section 5 includes comparisons of emission
factors modeled by MOBILESa to those measured in the 1987 Van Nuys tunnel
study, as well as to the 1992 Fort Me Henry and Tuscarora tunnel studies.
2. Ambient Studies
As discussed in the first "Highway Vehicle Emissions Estimates"
document, tunnel studies (which are discussed in the next section) were not
the only evidence that highway mobile source emissions may have been
underpredicted by the models and methodologies in use in the late 1980s. Other
types of studies, collectively referred to as "ambient studies," also provided
indications that the mobile source contribution to overall emissions was being
underestimated.
There are four main types of studies that are considered "ambient
studies" in this context: receptor modeling studies, source fingerprint studies,
ambient ratio studies, and mass balance studies. Ambient studies as a
descriptive term means that the studies are in some way based on
measurements of ambient pollution concentrations. That is, rather than
working from estimated emission rates and activity levels for various sources
in an effort to estimate overall pollutant inventories and/or ambient
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concentrations, the studies discussed in this section start from measurable
ambient aspects of the air pollution problem (such as pollutant concentrations
or the ratios of concentrations of various pollutants) and work back to
examine whether the ambient (measured) data are compatible with estimated
(modeled) emissions.
In 1994, a critical review of a number of such ambient studies was
performed by Systems Applications International (SAI), under contract to EPA.
The final report of that work, "Evaluation of Ambient Species Profiles.
Ambient Versus Modeled NMHCNOx and CO:NOx Ratios, and Source-Receptor
Analyses,"6 provides an extensive review of 25 ambient studies performed
between the early 1980s and 1994. The studies were each reviewed from two
viewpoints independently: once from an "objective" stance and again from a
"skeptical" stance. These reviews were performed by different reviewers, who
did not see the other review until their own review had been completed. The
objective review briefly summarizes the study in question and more resembles
a typical peer review, while the skeptical review is developed from the
viewpoint of being eager to find things about the study, its logic and
conclusions, and its underlying premises that can be challenged. As stated in
the final report, "the extent to which the two reviews are similar, both
unfavorable, divided, or both favorable ... is an indication of the perceived
strength of the study."
The discussion in this section only deals with the overall conclusions of
the report and the implications of those conclusions for the accuracy of
highway vehicle emission estimates. The reader interested in more details of
this review, particularly with respect to the review of specific studies among
the 25 evaluated, is encouraged to obtain a copy of the SAI report.
The ambient studies examined in this work can be categorized into four
basic types, as noted above. In each type of study, there are assumptions that
must be met (inherently or explicitly), although in practice it is rarely
possible to precisely meet all assumptions. The extent to which the validity of
the study results is affected by deviations in the assumptions is discussed, and
the degree to which the applicable assumptions are met by the individual
studies examined is stated. The relative strengths and weaknesses of each type
of study are also discussed.
Receptor- modeling, techniques >includc:.- chemical< mass:? balance -(CMB),
tracer analysis; and factor analysis methods. The SAI report concludes that-
one of the'strengths of receptor modeling techniques is that small deviations
from the assumptions may be tolerated, although large deviations will
invalidate the. results of the study. Thus, although strict adherence to all
necessary assumptions is rarely achieved in actual practice (as is true of all
ambient studies), useful results can still be obtained from such studies if the
assumptions are reasonably well met.
Discussion of the limitations specific to each type of ambient study, and
the degree to which various specific studies that were evaluated by SAI met the
necessary assumptions, is beyond the scope of this paper. Again, the
interested reader is referred to the SAI final report for more details. In
general, however, a few points can be made that are relevant to the current
discussion:
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There is considerable disagreement in the conclusions of the various
studies. For example, of the 13 receptor modeling studies examined, only one
reported a motor vehicle exhaust emission contribution that was significantly
higher than inventory-based values. Nine of the 13 studies reported evidence
of underestimation of emissions from one or more source categories. SAI notes
that "although there is no consistent [emphasis added] conclusion of motor
vehicle exhaust underprediction in the studies reviewed, there is a consistent
result of low gasoline vapor contribution." The question of the accuracy of
speciation profiles assumed for gasoline vapor (evaporative) emissions, and
the degree to which the speciation profile of gasoline vapor corresponds to
that of whole gasoline, requires additional investigation before firm
conclusions can be drawn. SAI also states in the report that, based on their
reviews, emissions from petroleum refineries may be underestimated in
emission inventories.
Of the five ambient ratio studies that were examined, only one provided
evidence for a discrepancy between (measured) ambient and (modeled)
inventory ratios of NMHCNOx and CO:NOx. As noted, although the evidence in
that study for a discrepancy in these ratios was substantial, the fact that a
discrepancy exists does not necessarily imply that a bias in the emission
inventory is the cause of the discrepancy. In general, it is not prudent to
conclusively state, on the basis of an ambient emission study, that emissions
from any one particular source category (including highway vehicles) are
underestimated. However, the evidence of the studies reviewed when taken
together is sufficient to conclude that further improvements in emission
inventory estimates for all major contributors is warranted.
The 25 studies examined in this report range from very recent (1994) to,
by the standards of this field, quite old (1980-81). Other ambient studies
became available, either published or in pre-publication form, after the list of
25 for inclusion in this work assignment was finalized, and others were
recommended for inclusion after it was too late to do so. More work in these
areas is certain to continue, and EPA will consider the results of newer studies
as they become available.
At this time, it does seem clear that some (but by no means all) ambient
studies provide indications that emission inventory estimates for highway
vehicles require additional improvement. The varying conclusions of the
studies examined also make clear that disagreement between two methods of
estimating the same thing (for example, measuring ambient emission ratios
and calculating them from modeled emission factors and estimated activity
levels) is insufficient to draw a firm conclusion that one or the other methods
must be right and the other wrong. Measurement errors, which are a factor
in any experimental work (including laboratory emission tests of in-use
vehicles), can lead to inappropriate conclusions being drawn. EPA believes
that caution must be used in interpreting the results of ambient studies; in
particular, citing of only one or two statements from such a study, taken
without the context and caveats provided in other portions of the report, is not
appropriate for drawing generalized conclusions.
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3. Tunnel Studie^
The July 1992 paper discussed the tunnel study that was conducted in
Van Nuys, CA in 1987. This study became the focus of considerable attention,
in part due to the fact that among its conclusions was the statement that
existing highway vehicle emission factors and inventories were being
underestimated (at least for HC and CO emissions). As outlined in Section 1, the
EPA model for estimating average in-use emission factors for highway
vehicles has undergone two major revisions since then; for a variety of
reasons, the emission factors estimated by MOBILESa are generally
significantly higher than those estimated by MOBILE4 and MOBILE4.1 for the
same conditions. Also since then, more tunnel studies have been performed.
In particular, the Coordinating Research Council (CRC), under whose
sponsorship the 1987 Van Nuys tunnel study was performed, sponsored two
tunnel studies that were performed in 1992, one at Fort McHenry (Interstate 95
tunnel under Chesapeake Bay, near Baltimore) and the other at Tuscarora
Mountain in south-central Pennsylvania.
The next section on the Van Nuys study is partly taken from the July
1992 paper. Following are similar overviews of the Fort McHenry and
Tuscarora tunnel studies and their results. Section 4 presents updated
comparisons of measured vs. modeled emission factors, with the latest EPA
emission factor model MOBILESa used to develop the modeled emission factors
for comparison with the 1987 Van Nuys measurements and the 1992 Fort
McHenry and Tuscarora measurements.
Van Nuvs Tunnel (1987)
In 1987, the Coordinating Research Council (CRC) sponsored what is
usually referred to as the "Van Nuys tunnel study," in which a highway tunnel
in Van Nuys, CA was used to measure the total emissions produced by all of the
traffic passing through the tunnel during a total of 22 one-hour sampling
periods. By measuring all of the emissions exiting the mouth of the tunnel
(accounting for winds and total air flow) and counting the number and types
of vehicles that passed through the tunnel during each sample period,
estimates of gram per mile (per vehicle) emissions of HC, CO, and NOx were
derived.
The details of the design and execution of the Van Nuys tunnel study are
beyond the scope of the current discussion. For additional information on the
study, the reader is referred to the final report "Measurement of On-Road
Vehicle Emissions in the California South Coast Air Basin, Volume I: Regulated
Emissions."3
While not the original design purpose of the tunnel study, one use of
the emission factor estimates measured in the study is validation of emission
factor estimates produced by the EPA and California Air Resources Board
(CARB) emission factor models. After fleet average gram per mile (g/mi)
emissions for each tunnel study sample period were calculated, they were
compared to emission factors modeled by the then-current version of the
California model, EMFAC7C. These comparisons reflected good agreement for
NOx emissions, but suggested that HC and CO emissions were being
underpredicted by EMFAC7C. Over the range of conditions (average trip speeds
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and ambient temperatures) occurring during the tunnel study, the apparent
underproductions were by ratios of between 1.4 and 6.9 (average 3.8) for HC,
and between 1.1 and 3.6 (average 2.7) for CO. The results of these comparisons
led to much questioning of the accuracy of the EMFAC7C model and its
successors. Since EMFAC7C was, in many ways, similar to the EPA emission
factor model (MOBILE3 at the time), and relied upon some of the same in-use
vehicle test data, the accuracy of the emission factors calculated by MOBILE3
(and succeeding versions of the model) were called into question as well.
In the 1991 technical memorandum7 and the July 1992 paper, EPA
provided a comparison of the emission factors calculated by MOBILE4 for the
tunnel study sample periods to those measured in the study. While the
evidence clearly indicated that HC and CO emissions from highway vehicles
were likely being underestimated, the degree to which the measured emissions
exceeded the modeled emissions was much less than was cited in the tunnel
study final report. The ratios of the tunnel study emission factors to those
calculated by MOBILE4 ranged from 0.6 to 3.2 with an average of 1.9 (the same
comparisons using EMFAC7C showed rations ranging from 1.4 to 6.9 with an
average of 3.8). Thus, the discrepancy between measured and modeled HC
emission factors was only about one-half that reported in the final report on
the tunnel study.
Similar results were found in the case of CO emissions: the ratio of
tunnel study to MOBILE4 CO emission factors ranged from 0.5 to 2.4 and
averaged 1.7 (the EMFAC7C comparisons gave ratios of 1.1 to 3.6 with an
average value of 2.7). The NOx emissions comparison changed in the same
direction; however, since the tunnel study report showed reasonably good
agreement between measured and modeled NOx emissions, the lowered ratios
resulting from comparing MOBILE4 to the tunnel study results indicated less
agreement. The tunnel study to EMFAC7C ratios for NOx emissions ranged from
0.6 to 1.4 and averaged 1.0 (on average, the modeled and measured NOx emission
factors were the same), but the tunnel study to NOx ratios for NOx ranged from
0.4 to 0.9, with 0.7 as the average of the ratios.
The problems inherent in using emission factor models (such as EPA's
MOBILE model) to estimate emission factors for microscale situations (such as
tunnel studies), when the model was developed to estimate average area-wide
emission factors over the course of an entire day, were discussed in the
technical memo and the July 1992 paper. The conclusions drawn at that time
included (i) that the discrepancy between tunnel study measurements and
MOBILE modeling of in-use emission factors was reduced, but not eliminated,
when the comparisons were carefully constructed to account for some of the
differences in microscale and area-wide modeling (such as the inapplicability
of "travel fractions," as defined in the MOBILE model, to microscale modeling
situations), (ii) that this fact did not negate the evidence that the models were
underestimating average in-use vehicle emissions, (iii) that there were
aspects of the tunnel study that could have contributed to errors in the
measured emission factors, and thus in the comparisons of measured to
modeled emission factors, and (iv) that additional work underway would likely
improve the model estimates of in-use emissions, which would be expected to
increase their accuracy and the degree of agreement between measured and
modeled emissions.
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Two major tunnel studies were conducted in 1992, as discussed below
The 1992 tunnel studies in 1992 were performed at Fort McHenry (Baltimore)
and Tuscarora Mountain (Pennsylvania) and are very briefly described below.
The combination of data obtained from more recent tunnel studies, and the
considerable revisions that have been made to the MOBILE model since that
time, provide an opportunity to examine the extent to which discrepancies still
exist between measured and modeled emissions.
Comparisons of the results of the Van Nuys tunnel study to emission
factor estimates calculated using MOBILESa are summarized in Section 5. Note
that the revisions and corrections made to MOBILES (04 Dec 92) that are
reflected in MOBILESa (26 Mar 93) are to aspects of the model that would not
affect emission estimates developed for comparison to the tunnel study
measurements. Most of the changes between MOBILES and MOBILESa affect
future calendar years only (i.e., would not affect the emission factor estimates
for calendar years 1987 or 1992), and several others affect only evaporative
emission factors (not applicable to tunnel studies) or emission factors under
certain specific sets of assumptions about operating inspection and
maintenance (I/M) programs. For more details on the differences between
MOBILES and MOBILESa, the reader is referred to the Federal Register notice
announcing the release and availability of MOBILESa (58 FR 29409, May 20,
1993).
Fort McHenry and Tuscarora Mountain Tunnels (1992)
The two tunnel studies conducted in 1992 were performed by Desert
Research Institute (DRI). The work was sponsored by the Coordinating
Research Council (CRC), which sponsored the Van Nuys study, as well as the
Auto/Oil Air Quality Improvement Program, the Department of Energy's
National Renewable Energy Laboratory, the Southern Oxidant Study, and the U.
S. EPA. At the time of this writing, the final report of these two studies was not
available. Information provided here is taken from the draft final report
"Real-World Automotive Emissions - Results of Studies in the Fort McHenry and
Tuscarora Mountain Tunnels" (March 1994). 8
The Fort McHenry tunnel carries eight lanes of Interstate 95 traffic
under the Baltimore (MD) Harbor, with four separate bores each carrying two
lanes of traffic. The tunnel is 7200 ft long. Eastbound traffic was sampled in
the tunnel emissions study. As an interstate freeway segment, it is
characterized by higher average speeds (generally 50 mph or more) and less
speed variability than an urban tunnel, such as Van Nuys. One major
difference between Fort McHenry and most other tunnel studies of this type is
that, the Fort McHenry tunnels contain significant grades, with both uphill
and downhill portions of the tunnel having ±. 3.76% maximum grades. The
average grade in the eastbound bores used in the study is -1.8% downhill
(entrance at west portal to bottom of tunnel) and +3.3% uphill (bottom of
tunnel to exit at east portal). As none of the existing highway vehicle
emission factor models (MOBILES, EMFAC7F, and earlier versions of each)
account for the effects of grade, the results from this study were of particular
interest in assessing the importance of roadway grade on emission levels.
The Tuscarora Mountain Tunnel is a two-bore tunnel, 5325 ft long,
carrying Interstate 76 traffic through Tuscarora Mountain at elevations of
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approximately 1000 feet (303 to 306 m) above sea level. Emissions
measurements were conducted using the two-lane eastbound bore of the
tunnel. The tunnel segment of the roadway is essentially flat, which made it a
better test of the emission factor model, since MOBILES and other such models
do not account for the impacts of roadway grade on emissions. As in :he case
of Fort McHenry, traffic through this tunnel was characterized by relatively
high average speeds and relatively little variation in speed.
In each of these studies, emissions measurements were made over
eleven one-hour sampling periods. The sample periods were selected so as to
span a range of vehicle fleet compositions (light- and heavy-duty vehicles,
gasoline-fueled and diesel powered). Overall, the average traffic volume for
the 11 samples at Fort McHenry was 2424 vehicles per hour at an average speed
of 49 mph; at Tuscarora, the average volume was 539 vehicles per hour at an
average speed of 59 mph.
Due to the locations of these two tunnels, it could safely be assumed that
virtually all vehicles were operating in stabilized mode. That is, the
contributions of cold-start (or hot-start) engine operation should be near zero,
as each tunnel is a link of interstate freeway with the nearest accesses
(entrance ramps) being from other arterial roadways and more than one mile
away (Fort McHenry) or at least 6 mi (10 km) away (Tuscarora). Based on
observations and some interviewing of drivers, the average ages of the light-
duty vehicle fleets during the tunnel study samples was somewhat newer than
the national average, and average mileage accumulations (odometer readings)
were somewhat higher than would be expected for a fleet of those average
ages. DRI notes that the average vehicle age was lower and the condition of
vehicles in the tunnel study sample periods was better (i.e., better
maintenance, fewer gross problems with engines or emission controls) than
would be seen in urban areas taken as a whole.
Beyond comparing the measured emission results and derived emission
factors to those produced by the MOBILES model, each of these studies examined
a number of other related issues. Remote sensing data, speciation of
hydrocarbon (HC) emissions, apportionment of tailpipe (exhaust) and non-
tailpipe (running loss and evaporative) HC emissions, separation of light-
fro m heavy-duty vehicle emissions, and characterization of compositional
differences in hydrocarbon emissions from light- and heavy-duty vehicles
were also examined. For additional details on these two studies, see the report
prepared by DRI.8
DRI included in their report on the Fort McHenry and Tuscarora tunnel
studies the results of comparisons between measured emission factors and
those modeled using MOBILES. Although the comparisons presented in the
draft final report were based on MOBILES, rather than the corrected MOBILESa,
this should not have a significant impact on the results of the comparison, as
the changes between MOBILES and 5a did not affect emission estimates for
calendar year 1992 in nearly all cases. (For details on the differences between
MOBILES and 5a, see the Federal Register notice of May 20, 1993, 58 FR 29410).
The use of observed vehicle registration distributions by age, rather than
travel fractions by age, as weighting factors for the model year-specific
emission factors in order to obtain fleet average estimates, has also been
correctly accounted for in the comparisons presented by DRI. Thus, detailed
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comparisons (for each individual sample period and for each study) were not
re-performed for inclusion in this paper. The results of the comparisons
presented by DRI are summarized in the next section.
4, Comparison Of MOBIT.ESa and Tunnel Study Emission Fartnrc
After the release of the Van Nuys tunnel study reports, a widespread
perception that the MOBILE model was grossly underestimating in-use
emissions of HC, CO, and NOx took hold. This perception was based on the fact
that the final report on the tunnel study results indicated that California's
then-current emission factor model, EMFAC7C, was underestimating in-use
emission levels by as much as a factor of 7 (the worst case comparison of
measured to modeled emission factors, for HC in one of the 21 sampling periods,
showed a measured-to-modeled emission factor ratio of 6.7). Since California's
EMFAC model borrowed considerably from both the data sources underlying
and the modeling algorithms developed for EPA's emission factor model, then
MOBILES, the conclusions drawn with respect to EMFAC were also assumed
applicable to the MOBILE model.
In the July 1992 paper on vehicle emissions and in the technical memo
analyzing the comparison of tunnel study measurements of emissions to
emission factors estimated by the MOBILE model, EPA drew the conclusion that
while the evidence pointed to the possibility that MOBILE3 was
underestimating emissions, the problem was not nearly so severe as would be
assumed by extension of the worst-case performance of EMFAC to the MOBILE
model. The reader is referred to the July 1992 paper and the technical memo
for additional details in this area. Essentially, when a later update of the model
(MOBILE4, 1989) was used and the contribution of vehicles of each model year
was accurately reflected, the apparent discrepancies between the measured
and modeled emissions estimates were reduced by roughly SO percent.
However, all indications were still in the direction of concluding that MOBILE
was in fact underestimating emissions.
In the intervening time, EPA has extensively updated and revised the
emission factor model (MOBILE4.1, 1991; MOBILES, 1992; MOBILESa, 1993). In
each of these updates, the estimated emission factors for a given set of
conditions was increased (in almost all cases) over the corresponding estimate
of the preceding version of the model. This is most particularly true of
MOBILE4.1 and MOBILES, where the basic emission rate equations were
increased dramatically over previous estimates on the basis of extensive
testing of vehicles in conjunction with IM240 tests in Hammond, IN. The first
section of this paper discussed model revisions since 1989 in more detail.
Additional tunnel studies have been conducted since 1989, as described
above. Before examining the performance of MOBILES and 5a relative to the
tunnel studies conducted in 1992, one item of interest is the re-examination of
the performance of the latest model against the measurements taken during
the 1987 Van Nuys tunnel study. This comparison is summarized in Table 3
(there are three tables, as in the cases of Tables 1 and 2: Table 3a for HC, 3b for
CO, and 3c for NOx). This comparison is discussed in the following paragraphs.
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The July 1992 paper and the accompanying technical memorandum
described the use of the MOBILE model to estimate emission factors for the
tunnel study sample periods and the adjustments that must be made to the
MOBILE emission factors in order to better represent fixed-length, microscale
modeling domains. That information is not repeated in detail here, but is
briefly summarized in the next paragraph:
The MOBILE model is designed with area-wide (e.g., metropolitan area,
state, region, nation), daily emission factor estimates in mind. For this reason,
the model accounts for both the registration-distributions by age (fraction of
all vehicles of a given type that are of a given age) and the average annual
mileage accumulation rates by age (newer vehicles on average are driven
more than older vehicles) in developing travel fractions (the fraction of all
vehicle miles traveled (VMT) by vehicles of a given type represented by
vehicles of each age). In a microscale situation, such as the tunnel studies, the
use of such travel fractions is inappropriate: Each vehicle contributes
mileage (i.e., the length of the tunnel) in direct proportion to its
representation in the fleet of vehicles that drove through the tunnel.
Table 3 shows, in summary, that predictions of average HC and CO
emissions from MOBILESa are in generally good agreement with the measured
emissions from the Van Nuys tunnel study, while NOx emissions would appear
to be over-predicted by the model. In general, this result is not surprising, for
the following reasons: The previous comparisons between the measured
emissions and emission factors from MOBILE4 indicated some under-prediction
of HC and CO but generally good agreement for NOx; and, emission factors
produced by MOBILESa are generally greater than corresponding estimates
from MOBILE4, as discussed above and illustrated by Tables 1 and 2. Thus the
increased basic emission rate estimates and other model revisions since
MOBILE4 have had the net effect of improving the agreement between
measured and modeled HC and CO while degrading the extent to which
measured and modeled NOx emissions are in agreement.
Specifically, when the ratio of measured (tunnel study) to modeled
(MOBILE) emission factor estimates is examined for each of the sample periods
of the Van Nuys study, measured HC emissions averaged 86% higher than
modeled emissions when MOBILE4 was used, but average only 22% higher
when MOBILESa is used. Over the 20 sample periods for which valid HC
emission measurements were reported, the range of this ratio was 0.64 to 3.24
when the comparison is based on MOBILE4, and 0.54 to 2.36 when based on
MOBILESa. Most of the ratios were actually much closer to 1.0, which would be
indicative of exact agreement; of the 20 samples, the ratio of measured to
MOBILESa-modeled emissions was between 0.7 and 1.5 in 15 cases.
For CO emissions, the degree to which measured and modeled emissions
are in reasonably good agreement was even better: The average of 19 valid
samples' ratios of measured to modeled emissions was 1.67 using MOBILE4, and
0.94 using MOBILESa; the range of these ratios was 0.52 to 2.43 using MOBILE4,
and 0.48 to 1.25 using MOBILESa. Most of the emission factors from MOBILESa
were within ± 25% of the measured values (true for 17 of 19 cases).
The situation in the case of oxides of nitrogen (NOx) emissions is not as
clear. Since the original comparisons of MOBILE4 emission factors to those
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from the tunnel study showed excellent agreement, the increase in NOx
emissions modeled by MOBILESa relative to MOBILE4 had the effect of
increasing the discrepancies between measured and modeled emissions. This
can be seen by comparing the results presented in Table 3c. Reasons for this
cannot be stated with certainty.
The results of the newer tunnel studies at Fort McHenry and Tuscarora,
in terms of comparison with emission factors estimated by MOBILESa for the
same conditions, generally show that the models are performing well to the
extent that the inherent assumptions of the model are met. DRI reports that
predictions were generally within ±.50%, although MOBILES had a tendency to
over-predict emissions, especially at Tuscarora. This can be partly explained
by the fact that speed variability in these two tunnels was quite low, while one
of the inherent assumptions of the MOBILE model is that "average speed" is
descriptive of entire trips rather than of essentially free-flowing links;
variability of speed implies accelerations, which result in higher emissions,
and decelerations, which in some cases also can result in higher emissions.
Also worth noting with respect to the 1992 tunnel studies is that the
emission, ratios (CO to NOx, and NMHC to NOx) measured in the tunnel studies is
within the range of ratios predicted by the model. The separation of exhaust
from non-tailpipe NMHC emissions showed a split similar to that predicted by
MOBILE5a, with less than 20% of NMHC from light-duty vehicles being
evaporative running loss emissions. At Fort McHenry, non-tailpipe NMHC
emissions (in the form of running losses and resting losses) were estimated to
be 15 + 2% of total NMHC based on use of MOBILESa; at Tuscarora, the
corresponding estimate was 13 ± 2%. These model estimates agree reasonably
well with the estimates of the fraction of NMHC emissions from non-tailpipe
sources derived from the tunnel study work (15 ±.3% and 16 ±. 4% at Fort
McHenry and Tuscarora, respectively).
DRI concludes that with respect to the emission factor models, the Fort
McHenry and Tuscarora tunnel studies indicate that the models are
performing reasonably well, particularly to the extent that the assumptions
concerning average speed and other conditions are met. While this is
undoubtedly not the final word regarding real-world verifications of modeled
emission factors, EPA is encouraged by the indication that the revisions that
have been made to the MOBILE model over the last few years appear to be
improving the model's accuracy at predicting in-use average emissions.
The Fort McHenry tunnel, as noted, is characterized by significant
grade (up to 3.76% both uphill and downhill). Since the MOBILE models
include an "inherent assumption" of level roadway, this was of particular
interest in assessing the importance of grade on emissions. The results
indicated that grade has a substantial effect (up to a factor of 2) on emissions
expressed in grams per mile, but a negligible effect on fuel-specific emissions
(e.g., grams per gallon gas consumed). This indicates both the importance of
efforts to include some means of correcting emissions for grade in g/mi
models, such as MOBILES, and the potential that models based on fuel
consumption rather than distance traveled could have significant advantages.
The latter is a longer-range issue; in the nearer term, EPA will be considering
methods by which model users could account for grade effects when running
the MOBILE model.
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L Update on Specific Issues (since .Tulv 199jj
This section of the paper presents an update on specific issues
concerning recognized shortcomings in EPA's historical (1990 and before)
approaches to in-use vehicle emission data collection, highway vehicle
emission factor modeling (MOBILE), and highway vehicle emission inventory
development (e.g., traffic models, VMT estimation). The statements of the
issues are the same as those presented in the July 1992 paper, while the
"action" sections have been updated to reflect changes implemented and
progress made since then. Many of the activities that have occurred or are
now occurring have been discussed in the preceding sections, while longer-
range projects are mostly covered in the last section.
ISSUE: Recruitment bias in emission factor testing.
Traditional recruitment programs are based on initial contacts by mail
and generally have not had high positive response (participation) rates. Are
the vehicle samples developed by EPA's emission factor testing programs
really representative of the in-use fleet?
Action toward resolution:
When the July 1992 paper was written, EPA was performing emission
testing of vehicles at a centralized I/M program test stations in Hammond, IN
and was preparing to start a similar program near Phoenix, AZ. Testing at the
Hammond site ran from September 1989. through February 1992, and after a
temporary interruption testing recommenced in September 1993 and
continued through March 1995. EPA has since started operation of a new lane
testing program at Chicago Heights, IL. Testing at Mesa, AZ was performed
from November 1991 through September 1994, when it was temporarily
stopped. Testing at the Mesa site may recommence in fiscal 1996 (after October
1, 199S), depending on the availability of contract funding.
Since all vehicles are required to obtain these tests, and the lane
operated by EPA would be expected to obtain a cross-section of all vehicles
reporting for testing at the site, the resulting emission data has much less
selection bias, relative to mail-solicitation vehicle recruitment. (To the extent
that vehicle owners make "temporary" repairs or other vehicle/engine
adjustments in anticipation of this required testing, it is possible that vehicle
selection itself could be relatively unbiased, while emission levels from tested
vehicles could still be biased toward lower emissions. The extent to which such
behaviors may have influenced emissions data obtained at the I/M lane
programs is unclear.)
A subsample of vehicles participating in the lane testing is then
recruited for additional laboratory testing. The recruitment of vehicles for
laboratory testing has been much more successful (higher participation rates)
at these lanes -- potential participants recruited at the lanes have the
immediate opportunity to see their vehicles tested on a dynamometer and to see
the loaner vehicles provided for their use while EPA has their vehicle for
testing, decreasing reluctance to participate that may be based in part on
uncertainty about what .will be done with their vehicle during emission factor
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testing. The personal face-to-face contact between the vehicle owner and the
recruiter also appears to have a positive impact on participation rates.
The I/M lane testing programs allow far more vehicles to be tested and
added to the data base within time and resource constraints, greatly increasing
the number of vehicles represented in the data base and thus increasing
confidence in the resulting estimates. In recent years, the "traditional"
emission factor program has tested between 500 and 1000 vehicles annually,
depending on the number and complexity of the tests performed on each
vehicle. To date, IM240 data has been obtained from about 12,000 vehicles in
the Hammond program, at a rate of about 70 vehicles per week. The Mesa
program is testing about 70 vehicles per week, with a total of about 6500
vehicles as of November 1994 . Of the 12,000 Hammond vehicle tests,
corresponding laboratory FTP results were obtained for about 640 vehicles; of
6500 Mesa vehicle tests, corresponding FTP tests have been performed on about
480 vehicles.
I/M lane testing also* provides the opportunity for EPA to focus on
recruitment and laboratory testing of higher-emitting vehicles. The
emissions behavior of properly maintained, non-tampered vehicles is
supported by more data than that of various higher-emitting vehicles
(whether due to tampering, malmaintenance, or other causes), thus this
ability to focus on recruitment and testing of high emitting vehicles provides
useful data for inclusion in the model. The large proportion of all vehicles
registered in the I/M area that pass through a centralized test location over
the course of an inspection cycle (i.e., annual, biennial) mean that EPA has
also been able to obtain more data from old vehicles at high mileages, an area
that has lacked data in the past. Generally, high mileage vehicles tested in the
EFP have been relatively new vehicles that accumulated mileage at greater
than average rates, while relatively few high age and high mileage vehicles
have been tested. The data from vehicles that reached high mileages at more
average rates of mileage accumulation have been used in revising the basic
emission rate equations in MOBILE5, as discussed earlier in this paper.
In the discussion of revisions to the MOBILE model in Section 1, the
development of new basic emission rate equations using data from IM240
programs (Hammond) and correlations between IM240 and FTP emission
results was outlined. For MOBILE5a, IM240 and FTP emissions from 590 vehicles
were used to develop the correlations for HC and CO emissions. For NOx
emissions, an additional 55 vehicles specifically selected for use in
characterizing NOx emissions were also used, for a total of 645 vehicles. All of
the data used in MOBILESa for this purpose was collected at the Hammond lane.
In the next revision of the BERs, additional Hammond and Chicago Heights data
plus data obtained at Mesa, AZ will be also be used.
The basic emission rates used in MOBILE4.1 and in MOBILESa, for 1981
and later model year light-duty gas vehicles and for all three pollutants, are
presented in Table 1. The biggest difference observed in these rates is in the
in-use deterioration rates (rates of increase in emissions with increasing
age/mileage of the vehicle). Relatively little change in the zero-mile level
emission estimates was supported by these data. For the reasons cited above,
EPA believes that the sample of vehicles from the Hammond program is
comparatively free of selection bias, particularly relative to the mail-
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solicitation EFP. As data continue to be collected in the Mesa area, those results
will be added to the emission factor data base, and the complete data will be
used for future updates to the basic emission rates.
Table 2 presents another way of viewing the impact of the use of the
IM240 data in the modeling of basic non-tampered emission rates in MOBILESa.
These tables (one for each pollutant) show the predicted emissions in grams
per mile, by model year, at 50,000 (50K) and 100,000 (IOOK) miles accumulated
mileage. There are also columns indicating the ratio of the MOBILE5a to
corresponding MOBILE4.1 emissions estimates. Table I showed that the zero-
mile levels for various model year/pollutant combinations were, for the most
part, not significantly different when revised to account for the IM240 testing
data. The bigger differences were in the deterioration rates, most notably in
the estimated values of "DR2" (the deterioration rate applied to accumulated
mileage above 50,000 miles).
In the case of HC emissions. Table 2 shows that MOBILESa estimates of
non-tampered vehicle emissions at 50,000 miles tend to be around 5-10%
higher than the corresponding MOBILE4.1 estimates for early 1990s vehicles.
In some model years (1981-2, 1984, 1987-88, and 1997+), the MOBILESa estimates
are slightly lower than the MOBILE4.1 estimates, although generally by only I
or 2%. This is not the case at 100,000 miles, where MOBILESa emissions are
much higher than MOBILE4.1 emissions for all model years 1980 and later,
often double the earlier estimates. This notably includes all future vehicles,
where even with the implementation of Tier 1 tailpipe standards, the available
data on current late-model vehicles indicates that average in-use HC emissions
exceed applicable standards at 100,000 miles.
Basically similar conclusions can be drawn from Table 2 for CO
emissions. At 50,000 miles, the MOBILESa estimates (relative to the
corresponding MOBILE4.1 estimates) range from a decrease of 4% (for MY
1981) to an increase of 63% (MY 1980), and are generally in the range of 10 to
35% increases. At 100,000 miles, the impact of the new basic emission rate
equations based on the IM240 data is to roughly double the average CO emission
factor for most model years after 1980. Note that the Federal Tier 1 exhaust
emission standards, which are phased in from MY 1994 through 1998, do not
include any change in the current 3.4 g/mi CO exhaust emission standard.
Finally, in the case of NOx emissions, similar statements can be made.
The major difference for the new basic emission rate equations for NOx,
relative to the changes made in the HC and CO equations, is that the best
estimates for both deterioration rates (DR1, applicable to the first 50,000 miles,
and OR2, applicable to mileage accumulation over 50,000 miles) are
significantly higher than previous estimates. For HC and CO, as noted, the
largest contributor to the increase in emission factors was the substantial
increase in the estimates for DR2. For NOx, on the other hand, deterioration
throughout the life of the vehicle is now estimated as substantially higher
than in the past. As can be seen in Table 2c, the pre-50,000-mile deterioration
rates are 2 to 3 times earlier estimates for model years 1984 and later. The
deterioration rates for 50,000+ miles are also considerably higher than
previous estimates.
There is some indication from tunnel study results that the model's
estimates of in-use NOx emissions may be too high. The reasons behind this
May 1995
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are not yet understood. The same sources of data were used to revise the basic
emission rate equations for all three pollutants. One possible explanation is
that the vehicles in the 1992 tunnel studies (Fort McHenry and Tuscarora)
tended to be newer and better maintained than the overall population of
vehicles, particularly those in urban areas. EPA will continue to collect in-use
emission data under both the mail-solicitation and IM240 lane portions of the
emission factor test program, and will make changes as warranted by that data.
In the future, the utility of collecting data from operating I/M
programs for the derivation of basic non-tampered emission rate equations
will have to be reconsidered. As vehicles built to compliance with the newest
levels of exhaust emission standards (Federal Tier 1 standards, LEV standards)
are introduced over the next decade, EPA anticipates that all areas of the
country required to operate I/M programs will in fact be doing so. This will
prevent EPA from collecting the wide range of in-use test data from vehicles
in their first I/M cycle, as has been obtained in Hammond and is being
obtained in Mesa. Since vehicles in areas that have had an I/M program in
effect for longer than one full cycle (e.g., a year for annual programs or two
years for biennial programs) will, as a fleet, exhibit the benefits of that I/M
program in the results of their emissions tests, EPA will no longer be able to
use data collected under these conditions to represent "basic, non-tampered"
emission rates, which by definition do not include any I/M benefits. Other
approaches to using data collected from I/M programs, and other means of
collecting in-use data from the widest and most bias-free samples available,
will be considered by EPA for future updates to the basic emission rate
equations.
ISSUE: Are "super" emitting vehicles correctly represented in the
model?
Within each model year, MOBILE4.1 [and MOBILESa] assume a
distribution of vehicles among four emitter categories: normal, high, very
high, and super emitters. Normal emitters have hydrocarbon (HC) emissions
of no more than twice the then-current 0.41 g/mi standard, or 0.82 g/mi, and
carbon monoxide (CO) emissions of no more than three times the standard, or
10.2 g/mi. Super emitters are vehicles emitting at least 10 g/mi HC at at least
150 g/mi CO. The relative proportion of vehicles in each of these emitter
categories, the growth in the fractions of "high, "very high," and "super"
emitters over time, and the emission rates of the "super" emitters are estimated
on the basis of emission factor program data. The fraction of vehicles modeled
as super emitters and their modeled in-use emission rates may not accurately
reflect the occurrence and behavior of these vehicles in the real world.
Action toward resolution:
In the real world vehicle emission levels fall along a continuum, from
low (normal) levels, through "high" and "very high" emitters, to the extreme
values characterized in the model as "super" emitters. The grouping of
vehicles into four emitter categories in the model, with vehicles slowly
migrating from the lower to the higher emitter categories with increasing
mileage, may underestimate emissions in two ways: By underestimating the
number of vehicles that are in the higher emitter categories at any given
May 1995
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point in time or accumulated mileage, and/or by underestimating the average
in-use emission levels associated with those categories.
However, the same data collected in the Hammond lane program
(discussed in Section 1 and in the first "Issue" above) was used to derive
estimates of the relative in-use populations of vehicles in each of the four
emitter categories (normal, high, very high, super) used in the MOBILE model.
What was found was that there were considerably more vehicles in the "verv
high" emitter category than was assumed in MOBILE4.1. The increase in this'
category was approximately 100% (there were about twice as many very high
emitters as was assumed in MOBILE4.1). The estimate of the fraction of vehicles
in the high emitter category derived from the Hammond data was slightly
lower than that in MOBILE4.1, and the estimated fraction of super emitters was
slightly higher than in MOBILE4.1, but the biggest difference by far was in
the very high emitter category.
EPA has included these updated estimates of the fractions of vehicles in
each emitter category in MOBILESa. The large increase in the very high
emitter category, and the smaller increase in the super emitter category, more
than outweigh the slight decrease in the high emitter category, such that the
net effect of including this revised emitter category distribution in MOBILESa
was to further increase the average in-use emission factors over the range of
conditions included in the model.
What was not found in the Hammond data was evidence that the
emission rates associated with vehicles in each of the above-normal emitter
categories needed to be significantly changed. This is partly due to the
definition of the emitter categories by their emission levels (the average
emissions of all vehicles that are defined as having emissions between X and Y
g/mi is not likely to change significantly, regardless of the number of
vehicles considered or the source of the data). The super emitter category is
also defined by emission levels, but is open-ended (anything over 10 g/mi HC
or over 150 g/mi CO). The Hammond data did not demonstrate that the average
emission rate of super emitters used in MOBILE4.1 needed to be increased.
EPA will continue to examine the distribution of vehicles among the
defined emitter categories as additional lane data are collected. To the extent
that remote sensing data meeting the caveats listed above become available for
analysis, such data will also be included in future updates to the emitter
distribution. There is also a need for remote sensing data from non-l/M areas.
Thus, this is an issue that will continue to draw attention over the longer term.
A number of investigators (EPA, State and local agencies, motor vehicle
manufacturers and oil companies, and other researchers) have conducted
studies using remote sensing devices (RSD) for HC and CO emission
measurements, and to a much lesser extent NOx emission measurements. Only
some of these studies have also obtained an independent measurement of
emissions (such as from an IM240 or an FTP test) to relate instantaneous RSD
emission measurements to overall emissions over a driving cycle. The
California Bureau of Automotive Repair has conducted the most extensive RSD
study, which included RSD readings on about I million vehicles; IM240
measurements were obtained on about 3000 of those vehicles (0.3%). Analysis
is underway to determine how to incorporate the use of RSD readings into an
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I/M program so as to focus on the identification of high-emitting vehicles for
more complete testing and repairs.
ISSUE: Representativeness of the FTP and other driving cycles.
The Federal Test Procedure (FTP), used to represent urban traffic, and
the other driving cycles used in emission testing (i.e., cycles of differing
average speeds used to develop speed correction factors) used to represent
other modes of driving behavior, may not be representative of all of the types
of actual vehicle operation important to determining overall highway vehicle
emission levels.
The FTP was developed during the early 1970s in southern California.
Traffic patterns and typical driving patterns may have changed significantly
since then, calling into question the utility of the FTP in characterizing
typical urban driving behavior. Even if the FTP is still representative of
average urban driving in terms of such variables as operating mode, average
speed, and average trip length, EPA is aware that accelerations in real-world
driving conditions at time exceed the maximum acceleration rates included in
the FTP, and that emission rates at higher accelerations can be much greater
than those at lower rates of acceleration. Due to the sensitivity of emissions to
the amount and severity of acceleration, the various speed cycles used in the
development of speed correction factors (SCFs), and thus the SCFs themselves,
may not be adequately representative of the emissions behavior of vehicles at
those speeds in real-world conditions.
Action toward resolution:
The issues of representativeness of the FTP to characterize typical
urban driving, the effects of "off-cycle" driving patterns (defined as types of
driving, such as higher speeds and acceleration rates, not reflected in the
existing FTP), and the representativeness of the various speed cycles used in
emission factor testing to actual traffic at various average speeds are closely
related. Since the July 1992 paper, EPA and the California Air Resources Board
(CARB) have conducted programs involving both instrumented vehicles and
chase cars to evaluate the degree to which the FTP is representative of today's
driving, and the prevalence of driving behaviors not included in the FTP, are
as required by the 1990 Clean Air Act Amendments. Additional information on
the study design, including survey methods, data collection, and results based
on preliminary analyses, can be found in the report "Federal Test Procedure
Review Project: Preliminary Technical Report."8
Although the results of these studies are not yet available for use
directly in the MOBILE emission factor model, they are providing the
information necessary for EPA to develop new driving cycles for use in
emission factor testing that will better capture the range of driving patterns
actually encountered in use. The information obtained from such studies on
the frequency of occurrence of, and the emissions impacts of, various
conditions not currently included in the FTP will culminate in revisions in the
FTP itself. [EPA published a Notice of Proposed Rulemaking (NPRM) initiating
the formal process of revising the FTP on February 7, 1995 (60 FR 7404). A
public hearing on the NPRM was held in Ann Arbor on April 19,1995.]
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A report has been published by EPA's Certification Division that
summarizes the findings of research into the frequency of occurrence and
emissions impacts of various "off-cycle" driving behaviors.9 This report
identified a number of significant ways in which the existing FTP fails to
capture situations that occur under actual in-use conditions and may
significantly affect in-use emission levels. Among those identified:
• Commanded enrichment - Vehicles accelerations greater than the
maximum rate included in the current FTP, particularly when
demanded at moderate to high travel speeds, result in the vehicle's
computer controls "commanding" enrichment of the air/fuel mixture
reaching the engine in order to supply the power demanded by the
driver. Such events can result in emission rates much greater than
are seen in testing over the existing FTP.
• Transient enrichment - In addition to enrichment events that are
"commanded" by the vehicle/engine computer controls, the in-use
driving survey revealed that brief periods of fuel/air mixture
enrichment occur during driving that is otherwise similar to FTP
driving. It appears that this may be the result of minor but frequent
variations in throttle position (how far the gas pedal is depressed)
during otherwise ordinary driving. This type of enrichment also
results in "spikes" of higher emissions that are generally not
encountered in EPA vehicle tests, during which vehicles are operated
by technicians that are trained to follow speed/time traces.
• Air conditioning use - Although the FTP includes provisions for
increasing the dynamometer load during vehicle testing, in order to
simulate the added power demand of air conditioning (A/C), FTP and
other emissions tests historically are not performed with the vehicle
air conditioner turned on. Measurements obtained as part of the in-
use driving surveys and subsequent emission testing in the laboratory
indicate that use of air conditioners can result in substantial increases
in NOx emissions.
After some initial testing conducted by EPA demonstrated an
unexpectedly large increase in NOx emissions with the air conditioner
on, the vehicle manufacturers tested seven (7) model year 1994
vehicles in a sophisticated environmental test chamber. Under fairly
extreme temperature conditions (95°F ambient temperature, 40%
relative humidity, simulation of sunny mid-afternoon radiant heating,
and 135°F road surface temperature, and using HCF-134a refrigerant
in the A/C), average emission increases of 25% for NMHC (0.09 to 0.11
g/mi), 51% for CO (1.0 to 1.5 g/mi), and 92% for NOx (0.21 to 0.41 g/mi)
were observed over the full FTP. A survey of in-use A/C operation
conducted by EPA in Phoenix, AZ demonstrated that A/C compressor
operation decreases rapidly at lower ambient temperatures and with
lowered solar (radiant) heat loading, although data to quantify the
associated emissions impact do not yet exist. Another complicating
factor is that the NOx increase due to A/C operation appears to be
dependent on vehicle speed. The same vehicles, when operated over a
high-speed driving cycle, had average increases in NOx emissions of
only 44% (0.22 to 0.32 g/mi). Thus, not only is the emission increase
associated with A/C usage highly dependent on ambient temperatures
May 1995
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and solar heat loading, but adjustments in the algorithms used to
generate speed correction factors may be required as well in order to
more accurately quantify the effects of A/C on in-use emission levels.
Trip patterns - Beyond the speed vs. time trace that is used to define all
driving cycles for laboratory testing, including the FTP, there is an
issue regarding the pattern of trips taken. The FTP and its associated
operating mode fractions assume that 43% of all vehicle starts are
'cold starts," where the engine and the catalyst have cooled off
completely, and that the remaining 57% of starts are "hot starts," in
which the engine and catalyst have only cooled slightly from
operating temperatures and therefore will regain operating
efficiency temperatures very quickly. The data obtained by EPA in
Baltimore indicated that almost 40% of starts occur after engine-off
times of between 10 minutes and two hours (see next paragraph). The
trip length of the FTP is 7.5 miles; the average trip length in Baltimore
was only 4.9 miles, and the median length (half of all trips longer and
half shorter) was only 2.5 miles. This implies that a greater fraction
of total VMT is accumulated in "start" modes, when engine and catalyst
temperatures have yet to reach the high levels required for efficient
operation, than is reflected in the operating mode fractions associated
with the FTP. Other differences in trip patterns, such as the average
distance traveled between stops, were also observed.
EPA recently received the final report from a contractor on the
analysis of trip pattern data.10 This analysis focused on trips per day,
miles per day, and other aspects of in-use travel descriptors, and was
initiated by QMS for the purpose of improving the handling of these
parameters in the model. The results of the analysis indicated that
there are in fact differences between current EPA modeling
assumptions and real-world experience.
The results of these analyses will be considered in the next major
revision to the MOBILE model, MOBILE6, in which OMS will also make
an effort to include some form of "trip-based emission factors" option.
It is also possible that MOBILE6 will include the option to model hot
soak evaporative emissions as a function of soak time (see following
paragraph).
Intermediate soak times • "Soak time" refers to the amount of time that
the vehicle is not running before a trip begins. The FTP as currently
structured includes the emission impacts of a 10-minute engine-off
period, representing "hot start" emissions (the hot start bag 3 of the
FTP is preceded by a 10-minute soak after the completion of the
stabilized bag 2). The emissions impacts of a cold-start are measured
during the bag 1 portion of the test, following a soak time of 12 hours.
Data on in-use trip patterns and follow-up emissions testing show that
the extent to which additional emissions are associated with start-ups
is a continuous function of the soak (engine off) time immediately
preceding the trip. Additional testing is required to adequately
characterize incremental start-up emissions over a wide range of soak
times.
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Simulation of actual road load - The road load setting is used in
emission testing to set the dynamometer power absorption at a level
intended to equal the power required for on-road driving over a range
of vehicle speeds. To the extent that current procedures for
determining road load (e.g., coastdowns) are inadequate in measuring
actual road load, the dynamometer settings will be in error and the
resulting emissions test will not accurately reflect the conditions
being simulated. The advent of new electric dynamometers (replacing
the water brake dynamometers that have - been used since the early
1970s) should enable EPA to better match actual road load, over the
entire range of speeds included in the test cycles, to actual on-road
road loads, and hence should improve the accuracy of the emissions
data obtained in these tests.
• Road grade • The FTP, and all other laboratory driving tests, assume
level roadways. That is, the additional power required of a vehicle to
climb a grade is not accounted for in any emissions testing. That
many roads are not in fact level is well known. Quantifying the
impact of grade on emissions, however, has not been investigated
until recent years (see the discussion of the Fort McHenry tunnel
study above). Preliminary indications are that road grade has a
substantial impact on emissions, but questions on how best to model
such effects require additional investigation. •
The task of determining the impacts of each of these aspects of in-use
driving behavior on emissions, the quantities and variety of data collection
required to attempt to incorporate the effects of each item into the MOBILE
model, and the potential interactions among items that are listed distinctly
above, make it clear that all concerns about the representativeness of driving
cycles cannot be resolved to the satisfaction of all interested parties in the
short term. EPA continues to collect and analyze data, and recognizes that a
great deal of effort will need to be expended in these areas for the foreseeable
future, which the Agency intends to pursue.
It should be pointed out again here that the current MOBILES model and
its predecessor versions do not and did not rely only on FTP emissions test
results in estimating in-use emission factors for highway vehicles. Much
testing has been performed outside of the FTP and its specified conditions (e.g.,
with different fuels, at different temperatures, and over different driving
cycles), and that information has long been used in estimating average in-use
emissions over a wider range of conditions than are represented in the
current FTP. Also, no matter how the revised FTP rulemaking is finalized, no
single driving cycle will be capable of capturing all of the driving behaviors
and other variables that affect in-use emissions.
ISSUE: Actual freeway (limited access highway) speeds exceed 55
mph in many areas.
MOBILE4 and earlier versions of the EPA emission factor model estimated
emission factors only up to a maximum speed of 55 mph. EPA guidance was to
use the 55 mph emission factors for situations where vehicle miles traveled
(VMT) were accumulated at higher speeds. The increase in the speed limit to
65 mph on many limited access highways and the frequent violation of the 55
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mph limit in areas where it applies has led to requests for emission factors at
higher speeds to be calculated by the model.
Action toward resolution:
MOBILE4.1 used data collected by the California Air Resources Board
from driving cycles with average speeds as high as 65 mph to revise the speed
correction factors used in MOBILE4 and previous versions of the model and to
extend them to a maximum average speed of 65 mph. Since that time, EPA has
re-examined the available data on emissions as a function of speed, as discussed
previously. In MOBILES, this led to further revision of the speed correction
factors, for all three pollutants, for all average speeds over 19.6 mph (the
average speed of the existing FTP driving cycle). These differences, as
described in section 1, included changing the modeled emissions behavior of
vehicles in the "high speed range", which for the purposes of speed
correction factors is considered to be average speeds of 48-65 mph. Where in
MOBILE4.1 emissions were modeled as increasing with increases in average
speed from 48 to 65 mph, MOBILES models g/mi emissions as being constant
over the range of 48-55 mph average speed, then increasing as speed
increases from 55 to 65 mph. Both EPA and CARB are continuing to obtain
emission data from vehicle tests over driving cycles having high average
speeds, and additional data will be considered in the next revision to the model.
This does not address the problems inherent in using average trip
speeds, and emission correction factors based on testing over transient driving
cycles of differing average speeds, to model emissions for applications not well
characterized by such driving cycles (i.e., link-based estimates). This is the
next "Issue" discussed. Nor does it address the fact that a non-zero fraction of
travel, particularly in certain areas, occurs at speeds of 70, 75, or 80 mph (or
more). The "high-speed" cycles currently being used for emission factor
testing have average speeds up to 60-65 mph; these cycles include some
operation at speeds in excess of 70 mph. As more data from testing of vehicles
over high-speed cycles becomes available, it will be taken into account in the
development of revised speed correction factors for use in later versions of the
MOBILE model. In the interim, EPA recommends that VMT at speeds over 65
mph continue to be assigned the maximum-speed 65 mph emission factors
from MOBILES.
ISSUE: The use of average trip speed to estimate emissions as a
function of speed in the model.
The speed correction factors (SCFs) used in the models to correct
emissions to average speeds other than the FTP average speed of 19.6 mph are
derived from testing over a series of driving cycles, having average speeds
ranging from 2.5 to 65 mph. Each of these driving cycles represents a trip, in
that each cycle begins and ends at idle and includes a mix of accelerations,
decelerations, and driving at different speeds, such that the total distance of
the cycle divided by elapsed time gives the average speed of the cycle. Many
users of the MOBILE model apply the emission factors to individual or
aggregated highway links (for example, a given length of an arterial roadway,
or all arterial roads taken together), where emission factors based on
"cruising" at the given speed (steady-state, or modal, emission factors) would
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be more appropriate than the trip-based average speed emission factors
produced by the model.
Action toward resolution:
The emission factors as a function of average speed produced by
MOBILE5a are the best that can be developed at present, given the driving
cycles used to develop the speed correction factors and the available data.
Although the application of emission factors estimated on the basis of data
collected from transient speed cycles to highway links has been criticized,
there is no immediate solution that would resolve all concerns. It should be
noted that most highway "links," or segments, are der'ined as including either
the beginning or ending intersection of the link. Thus, the speed estimates
for links (as might be generated from the output of a traffic demand model)
would reflect the inclusion of at least one stop, idle, and acceleration, and so a
link modeled in this way is actually closer to being a "trip" (in the sense that
much of the MOBILE modeling is based ) than generally has been considered.
As data obtained from recent driving characterization studies continue
to be analyzed and made available, EPA will be considering ways in which
emission factors can be refined to represent trips and highway links more
accurately. One possibility would be to generate driving cycles of different
average speeds for different roadway types, reflecting differences in traffic
congestion and other conditions (reflected by differences in frequency of
stops and intensity of acceleration/deceleration rates), then to develop
corrections to the emission factors that would account for both average speed
and roadway type. The California Air Resources Board (CARB) has developed
some "facility-specific" driving cycles using the newest data, and CARB and
EPA are collecting emissions data from tests over these cycles. The speed
correction cycles currently used include cycles that begin and end at
relatively high speeds; some of these might be used to develop emission factors
more representative of highway links as well.
EPA and CARB continue to test vehicles at different speeds, and to
evaluate the suitability of the SCFs used in the emission factor models. Since
speed is one of the most critical determinants of emission levels, improvements
in the characterization of in-use speeds and speed variability, and in the
methodology used to convert speed-dependent emission data into speed
correction factors used in the model, will lead to further improvements in the
accuracy of the emission factors. EPA is also working to develop more and
different driving cycles, accounting for information obtained from
instrumented vehicles and other work in support of the Revised FTP
rulemaking discussed earlier. This continuing work will be taken into
consideration by EPA in future revisions to the SCFs used in MOBILE.
ISSUE: Accuracy of the characterization of in-use emission
deterioration rates in the model.
If vehicles overall exhibit significantly greater emission deterioration
with increasing age and/or accumulated mileage than is modeled on the basis
of currently available data (based on FTP testing at different mileages), then
the basic emission rate equations understate emissions for all but relatively
new vehicles. There are two aspects to the issue of modeling in-use emission
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deterioration rates: extrapolating emissions as a function of mileage to verv
high mileages (very few vehicles with extreme odometer mileages have been
tested in mail-solicitation emission factor testing programs), and separation of
the emission effects of vehicle age and vehicle mileage.
Action toward resolution:
EPA believes that the premise of the first sentence above is true:
Vehicles overall do exhibit greater emission deterioration rates with
increasing age and/or accumulated mileage than has been modeled in the past.
The updating of the basic emission rate equations for light-duty vehicles for
all three pollutants on the basis of thousands of IM240 tests and the
correlations developed between IM240 and FTP emission rates, as already
discussed and illustrated in Tables la, Ib, Ic, showed that in-use deterioration
rates are considerably greater than had been estimated in the past. This is
particularly true for vehicles after they have accumulated 50,000 miles; the
post-50K deterioration rates used in MOBILE5a are double and triple the
previous estimates for HC and CO, and have increased by more than a factor of
10 (from previously very low estimates) in the case of NOx emissions from late
model vehicles. As noted previously, these increases in the in-use
deterioration rates account for much of the overall increase in emission factor
estimates in MOBILES relative to MOBILE4.1.
The increased deterioration rates developed from the data collected at
the IM240 program lane and used in MOBILES were estimated on the basis of
the emission test results from the highest mileage vehicles tested in the
program thus far. Most of the very high mileage vehicles tested were also
relatively old vehicles (early 1980s model years, mostly carbureted), and the
data from those vehicles was used to estimate the emissions at very high
mileages from later vehicles (for which direct data are not generally
available, since most such vehicles have yet to attain those very high
mileages, and which are primarily fuel-injected).
This approach to estimating in-use deterioration rates at high mileages
(over 50,000 miles) has been criticized as inappropriate. However, in the
absence of late model year vehicles having high odometer mileages for testing
(a problem that always exists for the newest, latest technology vehicles in the
first years of production and sale), there is no readily apparent alternative to
reasonable extrapolations. With respect to the specific case noted in the
preceding paragraph, EPA notes that while data from tests of mostly
carbureted vehicles was used in estimating high-mileage deterioration rates
from newer, mostly fuel-injected vehicles, there is no evidence that
carburetor problems or failures were the underlying cause of the increases
observed in emissions. Thus use of such data for estimating deterioration rates
for fuel-injected vehicles is not inappropriate. As emission factor testing
continues, an increasing fraction of vehicles of mid- and late-1980s model
years will reach very high mileages and will become available for testing, and
the estimated deterioration rates applicable to mileages over 50,000 miles for
those later vehicles may require further revision.
The revisions made so far to the basic emission rate equations and in
particular to the deterioration rates only address one of the two aspects of the
issue identified above. While the extensive IM240 data has allowed EPA to
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estimate emissions deterioration at high mileages with more confidence, it so
far has not been used to attempt to distinguish between the effects of high
mileage and the effects of age. While a strong correlation exists between age
and mileage, it remains unclear whether the increase in emissions due to
mileage is the same for very high and very low rates of mileage accumulation.
Consider, for example, two MY 1991 vehicles that are each tested at 100,000
miles, one as a high-average mileage three-year-old vehicle in 1994 and
another as a low-average mileage 10-year-old vehicle in 2001: Not only is it
not possible at this time to predict on the basis of data whether the emissions
increases experienced by these two vehicles are comparable, it is unclear how
such information would be reflected in the model.
EPA suspects that a real distinction exists between in-use deterioration
in emissions performance as a result of high accumulated mileage, and that
resulting from advancing age of the vehicle (whatever the accumulated
mileage on the older vehicle). These are not independent effects, and
resolution of this issue can only be reached by collecting substantial in-use
emission data from vehicles spanning a range of both ages and mileages over
a long period.
ISSUE: Evaporative emissions, particularly diurnal emissions, may
not be the same under test conditions as in real-world experience.
The diurnal portion of the evaporative emission test includes a
temperature increase, in the vehicle fuel tank only, of 60° to 84°F over one
hour. This temperature increase would typically occur over eight or more
hours outdoors and would affect the entire vehicle, not just the fuel tank;
hence the resulting emissions may not be the same. EPA in the past has only
measured hot soak emissions for one hour after the end of operation, while
such emissions do not just cease after an hour, and are in part dependent on
the nature of the trip immediately preceding the hot soak (i.e., a long or a
short trip, at high or low speeds).
Action toward resolution:
As required by the 1990 Amendments to the Clean Air Act, EPA has
revised the evaporative emissions test procedure used for. new vehicle
certification to more accurately reflect real-world conditions. This new test
procedure, compliance with which will be phased in over the next few years
beginning with model year 1996, will result in better control of evaporative
emissions under a wider range of in-use conditions not reflected in the
current evaporative test procedure. The benefits of the new evaporative
emission test procedures on in-use emissions from vehicles certified under
those procedures is included in MOBILESa.
To more accurately measure evaporative emissions from in-use vehicles
certified under the current procedures, EPA implemented changes in the
evaporative test procedures used for emission factor testing to cover more
realistic conditions, including "real-time" diurnal tests (e.g., measure
emissions over an 8-hour period during which temperatures increase from
60° to 84°F, rather than forcing the 24F° rise in temperature to occur during a
I-hour test), measurements of hot soak emissions in the second and third
hours following engine operation (rather than for only one hour) and after
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trips of varying lengths, speeds, and duration (rather than only after the FTP).
and functional purge and pressure testing of evaporative emission control
systems (see next item).
As was noted in the discussion in section 1, EPA has already taken one
step toward improving the diurnal evaporative emission estimates by
accounting for observed differences in emissions over 8-hour diurnal
temperature rises (rather than relying solely on 1-hour simulations of such
events). EPA has also been testing vehicles using different fuels and at
different temperatures, over longer diurnal periods (e.g., 33 hours, 72 hours).
These longer real-time measurements will be used in updating the estimated
emissions from "multiple diurnal" events, in which the vehicle is not driven
for 2, 3, or more consecutive days (and so does not have an opportunity to
purge the evaporative control canister of accumulated vapor). Hot soak
emissions are being measured after actual on-road trips, rather than only
after dynamometer simulations.
All of the information gathered through these testing programs will be
considered in the development of MOBILE6. Indications are that higher
emissions will be observed in most cases, which would lead to revisions in the
emission estimates produced by the model. Results from such testing to date
have been used, and further results will continue to be used, to revise the
evaporative emission factors to better represent actual in-use conditions.
ISSUE: Many vehicles, particularly those more than five years old,
have functional problems with evaporative emission control
systems.
Recent data have shown that many vehicles, particularly those greater
than five years of age, demonstrate functional problems with their
evaporative emission control systems. Such problems fall into two categories,
"purge" and "pressure" failures. Vehicles with either or both of these
functional problems will exhibit much higher levels of evaporative emissions.
Action toward resolution:
As part of the Hammond IN, Chicago Heights IL, and Mesa AZ programs
already discussed, EPA has been conducting functional pressure and purge
tests of vehicle evaporative control systems. This provides data on both the
rate of failure of these tests observed in a large random sample of vehicles,
and on the emissions impact of vehicles exhibiting these problems.
QMS has revised the evaporative emissions algorithms in the MOBILE
model to account for the in-use rates of pressure and purge test failures, based
on the data available for use in MOBILES, and the associated emissions impact.
As additional data on both failure rates over the two tests and the emissions
impact of such failures is collected, we will continue to analyze those data and
the results of such analyses will be reflected in future model revisions. As
noted earlier, examination of ambient and modeled emission ratios seems to
suggest that evaporative emissions are not adequately accounted for; this work
assists in better quantifying both the relative and absolute contribution of
evaporative emissions to overall hydrocarbon emissions from motor vehicles.
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EPA is also investigating ways in which the functional pressure and
purge tests of evaporative emission control systems can be made more
practical. While the tests currently being performed are useful in obtaining
data on the rates and emissions impacts of failure on one or both of these tests,
the tests themselves are somewhat complex and are difficult to perform on
some vehicles.
ISSUE: Commercial and test fuels differ in properties (e.g.,
volatility, sulfur level, distillation curves, composition) that have
an impact on vehicle emission levels.
The fuel used in new vehicle. certification and standard emission factor
testing programs (excluding tests designed to characterize the impact .of fuel
parameters, such as volatility) is blended to a specified formula. Commercial
fuels differ, both from this test fuel and in different areas of the country and
at different times, and fuel parameters other than volatility affect emissions.
Such parameters include, but may not be limited to, the sulfur content of the
fuel, the distillation curve (i.e., what fraction of the fuel evaporates at various
temperatures), and the composition. of the fuel in terms of fractions of
paraffins, olefins, and aromatics.
Action toward resolution:
MOBILE4 incorporated fuel volatility as measured by Reid vapor
pressure (RVP) as a user input, and adjusted emission factors to account for
volatility effects. These correction factors will be updated as more data are
available for analysis. In MOBILES, EPA also has adjusted upward the "base
fuel" (9.0 psi RVP) emission factors to reflect the effects of average
commercially available fuel (defined using other fuel parameters, such as
average sulfur content) based on a large body of recent testing performed as
part of the Auto/Oil Air Quality Improvement Research Program.
The other significant fuels issue affecting estimated average in-use
emission levels concerns the reformulated gasoline (RFG) requirements.
Under the 1990 Clean Air Act Amendments, the nine worst ozone areas of the
country are required to have RFG during the summer ozone season, and many
other areas of the country have "opted in" to this program. The estimates of
the impact of RFG on emissions included in MOBILE were first based on the
"simple model" developed as part of the RFG rule making, and later updated in
MOBILES to reflect the results of analyses based on use of the "complex model,"
which accounts for more fuel parameters. EPA plans to include the complex
model or its results, in some form, in a future update to the MOBILE model. This
would increase the accuracy of the estimates and make the emission factors
produced by MOBILE better reflect the conclusions drawn from the "complex
model," particularly for the second phase of the RFG requirements.
ISSUE: Accuracy of vehicle miles traveled (VMT) estimates and
traffic modeling; potential use of operating hours rather than
VMT as the activity level for highway mobile sources.
While not related to emission factor modeling per se, this issue is
important to the development of accurate highway mobile source emission
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inventories. The accuracy of VMT estimates has a directly proportional effect
on the accuracy of mobile source emission inventories (i.e., a 20 percent
underestimate of VMT results in emissions being underestimated by 20
percent). Interest has been expressed in the idea of using vehicle operating
hours, rather than VMT, as the activity level for highway mobile sources on
the grounds that operating hours might be more readily and objectively
measured.
Action toward resolution:
The development of accurate VMT estimates is a local/State
responsibility. EPA is working with the U. S. Department of Transportation
(DOT) and other parties to develop and demonstrate better methods for
estimating VMT and modeling traffic flow. EPA is requiring that States tie
their estimates of VMT to actual observations of traffic volumes in a more
comprehensive way than has been required in the past, which is intended to
improve the accuracy of these estimates. The use of traffic counts, such as in
the Highway Performance Monitoring System (HPMS), as part of this process
is an important component of efforts to improve the accuracy of this aspect of
the inventory process.
Researchers at Georgia Technological University (Ga Tech), working
with EPA's Office of Research and Development (ORD), have examined estimates
for total VMT in the metropolitan Atlanta region using a variety of
approaches. The methods included estimates from the Georgia DOT and from
HPMS, estimates derived from estimated fuel efficiency data and from fuel tax
revenues, and estimates developed from odometer data collected as part of the
Atlanta area's inspection and maintenance (I/M) program. When minor
adjustments to account for known or suspected biases are included in these
estimates (e.g., the DOT estimate may be higher than other methods since it
includes "external" or through trips, fuel consumption-based estimates may be
high since some fuel allocated to Atlanta is shipped to rural areas; fuel tax
revenue-based estimates may be low due to some use of agricultural and
construction fuels for personal transportation), the results show that all
estimates were in reasonable agreement. This implies that VMT estimates may
be more accurate, and more consistent across methodologies, than is generally
considered to be the case.. However, additional work is required to confirm
these conclusions and their applicability to other areas.
EPA's ORD is also investigating the potential for using vehicle operating
hours as the activity level for highway mobile sources, as a substitute for VMT.
This is considered a long-range research effort, as EPA does not believe that
States or others are in any better position to accurately measure operating
hours than to estimate VMT at this time. The suggestion has been made that
emission factors based on seconds of operation would be less sensitive to
average speed and driving cycle; if proven true, this would reduce the
importance of uncertainties in these areas. The Office of Mobile Sources is
skeptical that this will prove to be the case, but will continue to examine new
information as it becomes available.
EPA is also involved in other efforts to improve emission factors and
VMT estimates that do not fit neatly into the problem/action format of the
preceding paragraphs. QMS is participating in the HPMS Steering Committee,
which is working to better coordinate and integrate the work, including
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development of models and guidance, of the emissions/air quality and
transportation communities. Part of this effort is aimed at improving the
accuracy and consistency of VMT estimates developed for different purposes.
EPA is also working to improve and enhance our cooperation and coordination
with efforts being undertaken by the California Air Resources Board to
improve emission factor and activity level estimates for'use in developing
accurate emission inventories.
The list above, while not exhaustive, indicates some of the most
important efforts underway and planned to improve the estimates produced by
the emission factor model.
6. Plans for MOBILE6 and Other Ongoing Studies
Many of the issues that have been discussed in this paper will require
considerable resources (time, money, testing capability) to be thoroughly
addressed. It is the nature of this type of work that some issues will never be
resolved in a final manner, no matter how much information becomes
available, and no matter how many resources are devoted to addressing the
issue by EPA or others, the situation will change over time, and so an answer
deemed "final" one day will be rendered incomplete, or even incorrect, by
future developments. With this in mind, this final section of the paper
addresses some of the studies, outreach initiatives, and other activities that are
aimed at helping better estimate real-world in-use emissions from highway
vehicles.
As noted in the last "Issue/Actions" segment above, the MOBILE model
and the best estimate of in-use emission factors is only one aspect (although
arguably the most important single aspect) of the overall process of
quantifying total in-use emissions from highway vehicles. Beyond
continuing to collect emissions test data and revise the model to reflect the
latest and most complete information available, EPA wants to make the model
more responsive to the needs of its users, particularly those at the State and
local/regional government levels with responsibilities for air quality under
the Clean Air Act. This is discussed in the following paragraphs. This section
concludes with a brief overview of some of the other (outside of OMS) efforts
underway to improve the accuracy of highway vehicle emission factors and
inventories.
Increasing Participation in the Model Development Process
One of the criticisms that EPA has received concerning the MOBILE model
concerns the processes by which in-use emissions data are collected and analyzed
and decisions made as to the best means of modeling in-use emission factors on
the basis of such data, Although EPA has always held public workshops during
the development stage of each major new revision to the model (for example,
during the development of MOBILES public workshops were conducted on March 5
and July 8. 1992), these workshops have in fact been attended primarily by
representatives of the automotive and, in recent years, petroleum industries.
Relatively few representatives of EPA Regional Offices or of State or local air
quality agency officials have attended these workshops. Further, though EPA has
held these workshops in order to present preliminary findings, proposed model
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revisions, and related information, the Agency has been criticized for not taking
sufficient time to educate audiences, and for not adequately considering the input
of parties outside the Agency in making its final decisions about revisions to the
MOBILE model.
In response to these criticisms and to the perception that the process of
developing and revising the model is "closed," two changes are being
implemented in the model development process. The first of these actually has
been started already, while the second will be applicable to the development and
release of the next major revision to the model (MOBILE6, which is specifically
discussed below). These changes are (1) to conduct "user workshops" during the
development of new models and (2) to submit the model and its underlying
analyses to a more focused outside review process.
(1) Emissions modeling workshops.
On June 28-29, 1994, the Office of Mobile Sources hosted the first Mobile
Source Emissions Modeling Workshop. This workshop was specifically aimed at
the users of the MOBILE model at the regional. State, and local level. The technical
workshops mentioned above, which tended to be aimed more at the regulated
industries as the audience, mostly focused on OMS presentations of new
information, data, and preliminary analyses and results to the audience. This
workshop, by contrast, was intended for those parties that are required to develop
analyses, such as State Implementation Plans (SIPs), that use MOBILE emission
factors. Rather than having OMS present information, the invitees were asked to
make presentations on topics such as their experiences in the use of the model,
the guidance for mobile source emission inventory development provided by EPA
and its utility, and their perceptions of EPA's strengths and weaknesses in
developing the model and related guidance and providing of support to States and
local or regional air quality planners.
The workshop spanned two days. On the first day, there were four panel
discussions:
• Problems Modeling SIP Strategies for Mobile Sources
• Experiences with Transportation Plans and TIPs
• Emission Effects Not Included in the Current MOBILE Model
States' Needs and Requirements: Planning a New MOBILE Model
Each panel discussion included three or four presentations from workshop
attendees, and provided some opportunity for questions and discussion. On the
second day, "break-out" sessions corresponding to each of the panel discussions
were conducted. In these break-out sessions, participants were asked to
brainstorm and develop lists of the most significant items requiring EPA attention
under each topic. The workshop then concluded with a general session at which
the results from each break-out session, in the form of a list of the items most
urgently requiring attention, was presented.
The workshop provided OMS with insights into the processes required of
and the problems faced by those officials at the State and local levels attempting to
accurately quantify mobile source emissions and their relative contribution to air
quality problems, particularly nonattainment status for the National Ambient Air
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Quality Standards (NAAQS) for criteria pollutants. While many of the problems
identified and discussed were known to QMS, others were not, and this workshop
was extremely helpful to QMS in its efforts to identify the areas of greatest need
from the perspective of the users of the model. This will assist QMS in focusing
attention and resources in the right areas.
The feedback received from workshop attendees was generally very
positive, and the desire to maintain the ability to present issues and problems to
QMS for consideration in its future work was strong. Thus, although many
problems identified in the June 1994 workshop sessions have yet to be addressed,
or at least will require significant resources (both time and financial) to address
thoroughly, QMS is committed to periodically conducting such user-oriented
workshops in the future. At least one more workshop of this nature will be held
before the next major revision to the model (MOBILE6, see below). State and local
air quality agencies and metropolitan planning organizations that are on the
MOBILES Mailing List will be notified directly of the next user workshop.
(2) Outside review of model revisions.
In the past, QMS has conducted two kinds of workshops relating to the
MOBILE model, excluding the workshop discussed immediately above.
Developmental, or pre-release, workshops were held in order to present
preliminary results of analyses and to provide information on the types of
revisions planned for the model. As noted above, the audiences at these
workshops tended to be composed more of automotive and petroleum company
technical staff dealing with emissions modeling and air quality issues, with
relatively little representation of many model users (States, local and regional
governments, EPA Regional Office staff)- Although these workshops generally
led to the submission of comments, additional data, and other suggestions from
those attending, OMS has been criticized in the past for not thoroughly
considering all of the information submitted in making the final decisions on
how to revise the MOBILE model.
What OMS has termed "user workshops" have been conducted after the
release of most of the major revisions to the model. These workshops are aimed
almost entirely at the air quality staff of State, regional, and local governments
and EPA Regional Office staff, and are focused on explaining the changes to the
model since the last revision and the presentation and discussion of guidance on
how best to use the model for various applications. While these workshops have
proved useful to the modeling community, they do not provide a forum for
reviewing how the model was revised or for suggestion of alternatives. Thus,
neither the developmental or user workshops have been perceived as providing
sufficient opportunity for parties outside of OMS to influence the choices made in
revising the model.
Due to the size, complexity, and long history of the MOBILE model, a full
formal peer review of the entire model does not appear to be the best or most
efficient means of obtaining the outside review and input needed. However, the
need for some review and input is apparent to parties both inside and outside EPA.
Thus, beginning now, in the earliest stages of work on MOBILE6, OMS has decided
on an approach to providing significantly more opportunities for more parties to
have the chance to comment on components of the model and to provide relevant
information at a much earlier stage of the model development process.
May 1995
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While further changes and additions to this approach are still possible, the
basic outline can be stated. In addition to basic emission rate equations developed
for each pollutant and each vehicle type, for different model years or model year
groupings and for different technologies (e.g., non-catalyst, oxidation catalyst,
three-way catalyst, three-way+oxidation catalysts; carburetor, throttle-body fuel
injection, multi-point fuel injection), MOBILES contains algorithms for modeling
the effects of many parameters on emissions. Thus, the model is some ways can be
viewed as a large set of interrelated components. QMS will be actively seeking
review and comment on analyses that may be used in considering revisions to
these many components.
This may be best illustrated by an example. One of the many variables
affecting exhaust emissions is the ambient temperature in which vehicles are
operating. Carbon monoxide (CO) emissions, for example, are much greater on a
grams per mile (g/mi) basis at cold winter temperatures (e.g., 25°F) than at
warmer summer temperatures (e.g., 80°F). MOBILE calculates exhaust emission
factors initially assuming the nominal FTP test temperature of 75°F, then corrects
these estimates to the temperature specified by the modeler on the basis of
temperature correction factors (TCFs). Like the basic emission rates, there are
distinct TCFs for each vehicle type, pollutant, model year or model year group, and
for different technologies. The form of the current correction factor equations
includes the effects of fuel volatility (as- measured by RVP) and the interaction of
temperature and RVP, for temperatures of 75°F and higher. The equations
generally are in the form of exponential functions (e.g., TCP = exp[a * (temp-75) +
b * (RVP-9) + c * (temp-75) * (RVP-9)], where a, b, c are coefficients specific to a
combination of vehicle type/pollutant/model years/technology), and the
resulting TCP is applied as multiplicative correction to the estimated emissions at
75°F and 9.0 psi RVP fuel.
Recently, an EPA contractor completed an analysis of the correction factors
for the effects of temperature and fuel volatility (RVP) on emissions from light-
duty gas vehicles and trucks. Using all of the available test data on emissions at
different temperatures and/or fuel volatilities, including new data that were not
yet available the last time these temperature correction factors (TCFs) were
revised (MOBILE4.1, 1991), the contractor was asked to examine the suitability of
the current algorithm as well as alternatives (e.g.. different equational forms,
additive vs. multiplicative corrections) and to present statistical summaries of the
ability of each alternative analyzed to best predict the effects of temperature and
RVP on exhaust emissions.
Since the results of this report might well find their way into MOBILE6 in
some form as part of updating the model, this could be termed the temperature
correction "component" of the model as described above. QMS will shortly be
sending copies of this contractor report to a group of 10 to 20 individuals or
organizations thought to be in a position to critically review and comment on the
report, its methodology, and its results. QMS will establish a central file,
analogous to the dockets established in support of rulemaking activities, to
maintain all of the review comments received. These comments then will be
available for consideration and further analysis when attention is turned to
modeling the effects of temperatures on emissions in MOBILE6.
Over time, this informal docket-style file should grow to the point that it
can serve as a central repository of model documentation and related information.
OMS will include in this file other documents and data relating to estimating of in-
May 1995
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use emission levels, developed by parties outside the Agency. This would include
the reports developed by Sierra Research and Systems Applications International
for the American Petroleum Institute, reports and data generated by the Auto/Oil
Air Quality Improvement Research Program on real-world emissions behavior,
reports and data generated by the North American Research Strategy for
Tropospheric Ozone (NARSTO) group and by the Southern Oxidants Study (SOS),
and other similar information.
Such a central repository of information and data would be a big step
toward addressing concerns that have been expressed by both those who argue
that the revision development process in the past has ..been too "closed," and those
who feel that the lack of overall documentation for the model serves as a
hindrance to thorough understanding and correct use and application of the
model's results. As noted, this process is not finalized, and could be expanded to
include more and wider reviews than would be indicated by this outline.
As now envisioned, this process would not provide the opportunity for
every component of the model to be subject to outside review before the next
release of a major model update. This is due, in part, to the fact that none of the
revisions to the model, going back to the replacement of MOBILE 1 by MOBILE2 in
1980, have included revisions to every component of the model. Sufficient new
data is not generally available to support revisiting every aspect of the model
every time that it is being updated, and there are often cases in which enough
new data exists to virtually demand that some component of the model be revisited
even though little or no additional data exists to support revisions to many other
parts of the model. However, with targeted reviews of each component (or, each
study or analysis affecting a component) being done as such components or
analyses are prepared, and with the significantly greater involvement of outside
reviewers at early stages of the model revisions process, OMS should be provided
with much more useful information from outside the Agency in time for such
information to be carefully considered before decisions on final approaches to
specific issues must be made.
Another source of outside-EPA input to the model development process is
the information gathered at the "Emissions Modeling Workshop" held in June 1994
(preceding section) and other similar workshops planned in the future. While
OMS has yet to thoroughly absorb all of the information provided by attendees at
the June 1994 workshop, the information is still in hand and will be used in pan
to determine where our resources (for in-house and contracted emission factor
testing programs and for data analyses) should be directed for the maximum
benefits.
On a broader scale, OMS is in the process of developing a standing technical
review panel of some type, the focus of which will be to provide outside review
and input on OMS technical projects. This would include the MOBILE model and
revisions and updates to the model. A plan for involving this technical review
panel in the development of MOBILE6 is being prepared. OMS also continues with
efforts to more closely coordinate with the California Air Resources Board (CARB)
on emission factor testing and modeling issues.
Uay
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MOBILE6 Plans
Exactly what changes will be incorporated into MOBILE6, the next major
revision to EPA's highway vehicle emission factor 'model, will be determined in
part by when the model is released, and by other events that cannot now be
foreseen. However, some things can be said about MOBILE6 with reasonable
certainty. In addition to the items that have been discussed in the preceding
section, QMS believes that these revisions and updates are likely to be included in
MOBILE6.
• General update of some underlying data: In every major iteration of the
development of the model (MOBILE1, MOBILE2, and so on), EPA updates the basic
emission rate equations on the basis of additional emission factor testing that has
been performed since the previous update. In particular, as has always been the
case, the modeled behavior of emissions of vehicles of a specific model year and
technology type as the vehicle reaches advanced age/ accumulated mileage levels
will need to be updated as data from those specific vehicles, at advanced
ages/mileages, become available. Thus, while emission rate equations for pre-
1980 vehicles are unlikely to change significantly in the future, those for later
model vehicles will likely require revision as directly applicable data become
available and are used in place of extrapolations based on results from older
vehicles. Furthermore, potential problems with continuing to base development
of basic emission rate equations on IM240 data in the future, as was done for
MOBILE5a, will need to be addressed. These potential complications, as described
in the discussion of "Recruitment Bias in Emission Factor Testing" (page 27), may
require EPA to find other means of enhancing the representativeness of the
vehicles tested in the emission factor testing program.
• Effects of temperature and fuel volatility on exhaust emissions:
Substantially more data on the effects of "high" temperatures (over 75°F) and of
fuel volatility (as measured by RVP) have become available since the correction
factors currently used were developed. These data are the subject of an analysis
performed under contract to EPA in 1994, which was discussed in the section on
"outside review of model development" above. On the basis of the results of that
analysis and the input EPA obtains from outside review of the contractor's final
report, revisions to the "high temperature and fuel volatility" exhaust emission
factor corrections will be included in MOBILE6.
• Evaporative emissions under "real world" conditions: The evaporative
emission factors for diurnal (daily temperature rise while vehicles are not being
driven) and hot soak (trip end) emissions will be updated on the basis of new test
data obtained from conditions more closely approximating real-world conditions.
In the case of diurnal emissions estimates, EPA has collected data from "real time"
diurnal testing (i.e., measurement of evaporative emissions over the 6-12 hour
periods of increasing ambient temperatures during which such emissions
actually occur, rather than from a 1-hour test in which the temperature rise
characteristic of an entire day is forced to occur within one hour). Diurnal
emissions under real-time temperature rises have been measured over a range of
diurnal temperature rises and using fuels of different volatilities. As has been
described, MOBILES includes an adjustment to diurnal emission estimates to
account for the differences in 8-hour real-time temperature rises and I-hour
forced-heating simulations of such events, on the basis of the data available at
May 1995
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1
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that time. The results of additional tests will be used to further improve the
diurnal emission estimates produced by MOBILE6.
In the case of hot soak emissions, data have been collected reflecting
variation in two other parameters affecting the levels of such emissions in the
real world: characteristics (including length/duration) of the trip(s) preceding
the hot soak, and the duration over which emissions are measured. Historically,
the test procedure for measuring hot soak emissions involved an FTP test
preceding the hot soak and emissions being measured for one hour after the
engine is turned off. Obviously, not all trips resemble the FTP in terms of length,
speeds, duration, and so on; perhaps less obviously, such emissions do not simply
cease after one hour. By collecting hot soak data after different types of
"immediately preceding" trips (e.g., longer durations, significantly higher or
lower speeds), and collecting emissions for analysis until they emission
generation has effectively stopped, EPA is obtaining data that will be used to
revise the hot soak emission estimates produced by MOBILE6.
• Inspection and maintenance (I/M) program modeling: EPA has been
working for some time at improving the ability of the model to calculate emission
reduction credits associated with I/M program options other than the "standard"
set of options (test only vs. test-and-repair, annual vs. biennial, and test type).
The recent release of the MOB5a_H "hybrid I/M program credits" version of
MOBILESa is a first step toward providing the States the ability to assess the likely
impacts of various "hybrid" program options. Work in this area
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vehicle age implicitly defines average miles per trip as well. The need for better
temporal and spatial resolution of motor vehicle emissions, as required for in the
Urban Airshed Model for example, has increased the demand for emission factors
to be expressed in terms that can be associated with specific activity levels beyond
VMT: diurnals in grams per vehicle per event, hot soaks in grams per trip end,
refueling emissions in grams per gallon of fuel dispensed, and so forth.
The recent activity in the areas of non-FTP driving behavior studies,
instrumented vehicle data collection efforts, and other related areas are
providing substantial new data on the characteristics of the "average trip." These
are the first recent major efforts to obtain data on travel/trip characteristics. It
is too soon to determine exactly how this information will be incorporated into the
model or what effect such updated trip characteristics data are likely to have on
the emission factors produced by the model. However, enough is known at this
point to indicate that changes in the modeling of average trip characteristics may
be warranted, and EPA plans to develop ways to reflect this new information in
the model.
• Fuels: The impact of fuels on vehicle emissions is dependent on many fuel
parameters, and the model does not yet reflect the full complexity of the issue.
The earliest versions of MOBILE assumed that in-use fuels, everywhere, were the
same as "Indolene" (the specially formulated, 9.0 psi RVP test fuel used in
laboratory tests). In MOBILES, this assumption was updated to reflect the increase
in average in-use fuel volatility, for evaporative emissions only (exhaust
emission factors were based on 9 psi RVP and evaporative emission factors on
11.5 psi RVP). In MOBILE4, RVP was added to the list of user-defined parameters
for estimating emissions, and both exhaust and evaporative emissions were
modeled as a function of user-specified RVP in the range 9.0-11.5 psi.
MOBILE4.1 included extension of the range of RVP for which exhaust
emissions could be modeled, down to 7.0 psi RVP, for scenarios where the ambient
temperature exceeded 75°F. The ability to model the impacts of oxygenated fuels
(ether blends and alcohol blends) was introduced in 1988 as a post-processing
algorithm; this was later included in the MOBILE model, and revised in MOBILES to
extend the effects from carbon monoxide emissions (which are most impacted by
oxygenate content) to hydrocarbon and oxides of nitrogen emissions. In MOBILES,
a correction to account for differences in laboratory test fuel and commercially
available fuels, in terms of fuel composition parameters other than RVP (such as
sulfur content) was added. Each of these changes represents an incremental step
toward better handling of the impacts of fuel composition on in-use emissions.
The increasing regulation of in-use fuels (Phase I and Phase II volatility
limits during the summer ozone season, Phase I and Phase II reformulated
gasoline requirements, and mandated oxygenated fuel programs), and increasing
understanding of the effects of fuel parameters on in-use emissions, point to the
need for additional model revisions in this area. EPA is considering the addition
of more fuel parameters to the list of parameters affecting emissions that can be
specified by the model user. EPA is also considering development of a means of
incorporating the so-called "complex model" used in the reformulated gasoline
rulemaking analyses, or at least the results from that model, into the MOBILE
model. Collection and analysis of data on the impacts of fuels on emissions will
continue, and MOBILE6 will include updates and revisions based on that work.
May 1995
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An issue somewhat related to the impacts of fuels is the increasing need for
speciation data for HC emissions. While incorporation of speciation profiles into
the MOBILE model is not likely, QMS is aware of the need to further refine the
speciation profiles for exhaust and evaporative emissions, for highway vehicles
and for non-road mobile sources. The introduction of reformulated gasoline
(RFG) in many areas also highlights the need for more and better emission
speciation data. QMS plans for work in this area over the next few years include:
Updating of the exhaust, evaporative, and running loss emission profiles for
light-duty vehicles operated on industry average gasoline and on oxygenated
gasoline blends; development of speciation profiles for non-road mobile sources
(such as agricultural and construction equipment); development of speciation
profiles for alternative fuels; updating of the profiles for light-duty diesel
vehicles; and development of profiles for heavy-duty diesel vehicles.
• Non-FTP driving: In the "Issues/Actions" section of this paper, the fact
that the FTP clearly does not represent all driving and in fact fails to account at
all for certain driving behaviors (e.g., high acceleration rates) that have a major
impact on emission levels, was discussed. That work, and a final rulemaking to
revise the FTP, may not be complete before the next revision of the MOBILE model
is developed. However, that does not imply that the information already available
will not be included in the next model update. The effects on emissions of some of
the "off-cycle" driving behaviors is too great to be ignored, or to wait for the final
estimates; EPA plans to include some means of accounting for non-FTP driving
behaviors in MOBILE6. This will be one of the areas in which more outside input
during the model development process will be crucial to providing the best
feasible interim approach for modeling.
• Trip-Based Emission Estimates: This would involve providing, as an option
for users of the MOBILE model, the separation of trip-start emissions (for both
cold- and hot-starts) from running emissions. Trip-start emission factors would
be estimated in units of grams per vehicle per starting event (g/veh-start) for
cold- and hot-starts separately. Such emissions would then be excluded from the
exhaust emission factors in g/mi produced by MOBILE, which would represent
only stabilized operation. This would enable modelers using tools such as Urban
Airshed, which require that emissions be finely resolved both spatially and
temporally, to assign all start-related emission increments to the locations and
times of vehicle starts (as if all of the start-related increase in emissions occurred
instantaneously at the location and moment of vehicle start), and to exclude such
emission effects from the emissions modeled for traffic links and intersections.
The data required to estimate trip-start emissions in this way are implicitly
included in the basic emission rate equations, but an algorithm to perform the
required calculations has not been programmed for use in the model. EPA is
considering the inclusion of such an option in MOBILE6. If such an option were
exercised, the operating mode fraction inputs to the model would not be relevant
to the resulting emission factors, in that all starts could be accounted for by use of
the cold- and hot-start g/veh-start emission factors, and all "running" emissions
would be assumed to be in stabilized operating mode.
In-use emissions from heavy-duty vehicles: Emissions from heavy-duty
engines (HDEs) are regulated on the basis of mass pollutant per unit work
performed, rather than per mile traveled, due to the wide variety of applications
in which such engines are used. In order to estimate emissions in grams per mile
from heavy-duty vehicles (HDVs, defined as vehicles over 8500 Ib Gross Vehicle
""""""""" May
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Weight. and equipped with HDEs), MOBILES and earlier versions of the model use
conversion factors to convert gram per brake horsepower-hour (g/bho-hr)
emissions to grams per mile. These conversion factors take into account average
fuel economy, m-use loads, and other variables affecting how much work is
necessary to move the vehicles per mile. This necessarily involves making a
number of assumptions about the use patterns of such vehicles as well as the
weighting together of information applicable to different subcategories of HDVs
on the basis of their relative populations and use profiles. Furthermore the
expense and complexity of. testing heavy-duty vehicles on chassis dynamometers
relative to testing heavy-duty engines on engine dynamometers, and the inabilitv
tor various reasons to conduct an in-use emission factor testing program for HDVs
analogous to that used for light-duty vehicles and trucks, mean that the emission
estimates in the model for HDVs are not based on actual in-use data in the way that
light-duty vehicle and truck emission factors are estimated. Rather, they are
based primarily on HDE certification data, engineering judgment, and
extrapolations.
The two most important improvements that could be made to emission
factors for HDVs in the MOBILE model would be (1) collection of data from chassis
dynamometer tests of HDVs, and/or from instrumented HDVs in actual use, and (2)
subdivision of the HDV vehicle categories (particularly diesel) into more distinct
groups. For example, the current heavy-duty diesel vehicle emission factors
account for transit buses, inter-city buses, and freight-haulers; while
characteristics of each of these are reflected in the various conversion factors
and weighting factors used to develop one set of emission factors for all HDDVs,
these emission factors are clearly not the best estimates of in-use emissions for
any one of those subcategories. EPA anticipates having some chassis
dynamometer test data from in-use HDDVs available for analysis before MOBILE6 is
released (see NARSTO discussion below). EPA is also considering division of at
least the HDDV vehicle category into subcategories (e.g., urban buses, Class VIII
freight haulers). The extent to which useful data can be obtained on the various
subclasses of HDDVs will determine exactly what refinements can be made for
MOBILE6, and what may have to wait until a later revision.
While other changes and improvements may also be included in MOBILE6, those
outlined above and suggested by the "Issue/Actions" in the preceding section are
the most important of those currently planned.
Other Ongoing Studies
NARSTO
A major research initiative now underway, the North American Research
Strategy for Tropospheric Ozone (NARSTO), is expected to provide significant new
information for use in the improvement of emission factor and inventory
estimation for highway vehicles. Much of the highway mobile source research
being sponsored by NARSTO is being performed by researchers at Georgia
Tecnological University (Ga Tech) in Atlanta under a cooperative agreement with
EPA's Office of Research and Development.
May 1995
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One of the first areas that is likely to provide data for near-term
consideration in the process of revising the MOBILE model is the plan for NARSTO
to sponsor chassis testing of heavy-duty diesel vehicles. This will provide data
that can be used in evaluating the validity of the conversion factors (g/bhp-hr to
g/mi) used in MOBILE as well as the assumptions that have been made regarding
the relationship of certification test emission levels to in-use emission levels,
presumably increasing the accuracy of these emission estimates and the
confidence that can be placed in them by policy makers and air quality analysts.
NARSTO is also planning to sponsor work in which HDDVs'will be instrumented
while in actual real-world use; as noted above, this will be most useful in
evaluating and improving the estimates of in-use emissions from such vehicles.
A number of other efforts are planned or under consideration by the
emissions modeling subgroup within NARSTO that could provide valuable data for
improvements in and verification of the highway vehicle emission factors and
inventories. These include:
• Evaluation of the population of "super emitting" vehicles by region
• Studying the effects of I/M programs through use of remote sensing
data
• Evaluation of the effects of alternative fuels on in-use vehicle
emissions through use of remote sensing data
• Evaluation of the "real-world" impacts of transportation control
measures on reducing in-use emissions from highway vehicles
• Investigation of the variability of emission rates for in-use highway
vehicles
• Work on reconciliation of mobile source emission inventory
methodologies and results on the State and national levels
• Development of a modal emission factor model for highway vehicles
• Development of temporally and spatially resolved activity data for
highway vehicles, for use in conjunction with modal emission factor
models
While many of these efforts may not be completed in time for use in
MOBILE6, EPA will consider whatever data is available at that time, and will
continue to work with NARSTO to ensure that the information collected is useful
and relevant to the most important uncertainties to be addressed.
As has been pointed out elsewhere, EPA (and the rest of the community) are
far from being able to say that all uncertainties regarding in-use emissions from
highway vehicles have been resolved, and MOBILE6 will not reflect the "last
word" on these issues. Longer-range work, such as that being sponsored and
evaluated by NARSTO, will be critical to continued improvement in the estimates
of actual in-use emission levels and quantification of the uncertainties in those
estimates.
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Vnlpe Center
EPA has also been working with personnel at the Volpe Transportation
Center in their efforts to develop a modal emission model. Unlike the MOBILE
model, which could be termed a "transient" or "trip-based" emission factor model,
a modal model provides a series of emission factors for each vehicle type and
modeling scenario. For example, a modal model would include distinct emission
factors for vehicles at stabilized operating temperatures for acceleration mode (at
various rates of acceleration), deceleration model(at various rates of
deceleration), cruising (steady-state operation at a range of speeds), trip-start
emissions in grams per vehicle start for both cold- and hot-starts, trip-end
emissions in grams per vehicle per trip-end, and so on. Such emission factors
would be particularly useful for cases, such as Urban Airshed Modeling, where
emissions must be allocated on an hour-by-hour basis and for each of numerous,
relatively small "grid cells." Emission factors could be calculated for each link of
roadway, with other non-driving emissions allocated spatially on the basis of
where they occur (i.e., hot soaks at employer parking lots in the morning,
refueling emissions at service stations, and so forth).
Considerable resources would need to be expended on data collection and
vehicle testing in order for such a model to be developed correctly for use in the
development of emission inventories for State Implementation Plans and other
regulatory applications. This is a longer-range effort that, while not likely to
provide sufficient information to influence the development of MOBILE6,may
provide useful information for future modeling improvement efforts.
Los Alamos
Researchers at the Los Alamos National Laboratory are investigating the
potential for development of a model that would encompass all major aspects of
estimating highway vehicle emissions and their environmental impact. Such a
model would incorporate, in a single integrated package, a transportation model
(to provide activity levels on the basis of mode of operation), a modal emission
factor model (to provide emission factors for each activity level), and air quality
models (to model the impacts of emissions on ambient air quality). The inclusion
of exposure models might also be considered at a later stage. This is a long-range
research effort. Los Alamos researchers are not planning, to actually test vehicles
or otherwise collect data that are necessary for such a project; rather, this effort
is aimed at integrating the various modeling tools needed for emission inventory
development and air quality analyses. The type of model being planned, and the
enormous data requirements of such a model, would require the use of super-
computer type systems to operate. As such, these efforts must be considered to be
research efforts at present, with practical use and application being more distant
May 1995
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7. Co nc I usion
The preceding discussion is not exhaustive, in that many researchers in
other EPA offices, other government agencies at various levels (Federal, State,
local/regional), the automotive and petroleum industries, and academia
(primarily state universities) are at work on studies and projects that address one
or more aspects of the issue of accurately quantifying highway vehicle emission
estimates. This subject is expected to continue to be the focus of considerable
work by all of the aforementioned parties. As results from various studies and
other research become available, EPA will continue to reexamine the approaches
used in modeling highway vehicle emission estimates, and will continue to make
revisions supported by the data to improve those estimates. This paper was
intended to provide an update on the issues raised in the July 1992 paper, and to
facilitate discussions and productive interactions among the various parties
involved in such work.
May 1995
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Table la
MQBILE4.1 and MQBILESa Basic Emission Rate Equations for Light-Dut
Vehicles
Hydrocarbons (HO
Pre-1968
1968-1969
1970-1971
1972-1974
1975-1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998+
MOBILE4
ZML
7.250
4.430
3.000
3.380
1.060
0.360
0.287
0.287
0.257
0.240
0.249
0.247
0.248
0.253
0.256
0.262
0.263
0.264
0.264
0.264
0.264
0.264
0.264
0.264
DET1
0.180
0.250
0.370
0.160
0.280
0.100
0.118
0.110
0.068
0.076
0.070
0.071
0.073
0.071
0.067
0.062
0.061
0.060
0.060
0.060
0.060
0.060
0.060
0.060
j
DET2
0.180
0.250
0.370
0.160
0.280
0.100
0.111
0.106
0.078
0.090
0.085
0.089
0.092
0.092
0.087
0.084
0.084
0.083
0.083
0.083
0.083
0.083
0.083
0.083
ZML
7.250
4.430
3.000
3.380
1.060
0.360
0.287
0.286
0.241
0.247
0.249
0.253
0.253
0.257
0.258
0.260
0.261
0.261
0.261
0.247
0.233
0.210
0.193
0.185
MOBILESa
DET1
0.180
0.250
0.370
0.160
0.280
0.205
0.101
0.105
0.089
0.073
. 0.077
0.071
0.070
0.070
0.073
0.075
0.075
0.076
0.076
0.074
0.073
0.072
0.072
0.072
DET2
0.180
0.250
0.370
0.160
0.280
0.205
0.285
0.271
0.274
0.282
0.284
0.282
0.271
0.265
0.277
0.280
0.281
0.283
0.283
0.279
0.275
0.273
0.273
0.273
Where:
ZML = Zero-mile level, in grams per mile (g/mi)
DET1 = Deterioration rate, in (g/mi)/10,000 miles accumulated mileage,
applicable to mileage accumulation up to 50,000 miles
DET2 = Deterioration rate, (g/mi)/10K mi, applicable to mileage accumulated
in excess of 50,000 miles
May 1995
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Table Ib
MQBILE4.1 and MQBILE5a Basic Emission Rate
for Light-Dutv Gas
Vehicles
Carbon Monoxide (CO)
Pre-1968
1968-1969
1970-1971
1972-1974
1975-1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992+
ZML
MOBILE4.1
DET1
78.270
56.340
42.170
40.940
17.720
6.090
3.071
3.107
2.842
2.574
2.629
2.576
2.644
2.704
2.652
2.669
2.669
2.665
2.250
2.550
3.130
2.350
2.460
0.730
1.757
1.634
1.039
1.200
1.065
1.081
1.097
1.043
0.971
0.875
0.857
0.844
2.250
2.550
3.130
2.350
2.460
0.730
1.683
1.584
1.153
1.352
1.211
.248
.277
.236
.148
.049
1.030
1.015
ZML
MOBILESa
DET1
DET2
78.270
56.340
42.170
40.940
17.720'
6.090
3.069
3.105
3.255
3.184
2.920
2.740
2.704
2.490
2.424
2.203
2.166
2.147
2.250
2.550
3.130
2.350
2.460
1.958
1.663
1.727
1.549
1.193
1.331
1.240
1.242
1.289
1.343
1.423
1.439
1.448
2.250
2.550
3.130
2.350
2.460
1.958
3.609
3.318
3.345
3.604
3.547
3.554
3.403
3.286
3.423
3.407
3.419
3.434
Where:
ZML = Zero-mile level, in grams per mile (g/mi)
DET1 = Deterioration rate, in (g/mi)/10,000 miles accumulated mileage,
applicable to mileage accumulation up to 50,000 miles
DET2 = Deterioration rate, (g/mi)/10K mi, applicable to mileage accumulated
in excess of 50,000 miles
May 1995
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-54-
Table Ic
MOBILE4.1 and MOBILESa Basic Fmission Rate Equation. far Light-Dutv Gas
Vehicles
Oxides of Nitrogen (NOx)
Pre-1968
1968-1972
1973-1974
1975-1976
1977-1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996+
Where:
ZML
MOBILE4.1
DET1
MQBILESa
DFT2
ZML
3.440
4.350
2.860
2.440
1.790
1.500
0.648
0.635
0.632
0.652
0.656
0.459
0.442
0.442
0.470
0.489
0.494
0.498
0.498
0.498
0.498
0.498
0.000
0.000
0.050
0.040
0.110
0.070
0.068
0.072
0.051
0.049
0.042
0.038
0.039
0.034
0.031
0.025
0.024
0.024
0.024
0.024
0.024
0.024
0.000
0.000
0.050
0.040
0.110
0.070
0.068
0.072
0.051
0.049
0.042
0.038
0.039
0.034
0.031
0.025
0.024
0.024
0.024
0.024
0.024
0.024
3.440
4.350
2.860
2.440
1.790
1.500
0.648
0.635
0.578
0.465
0.469
0.425
0.442
0.483
0.478
0.464
0.465
0.467
0.467
0.365
0.240
0.178
0.000
0.000
0.050
0.040
0.110
0.102
0.063
0.066
0.067
0.079
0.078
0.082
0.078
0.077
0.080
0.082
0.082
0.083
0.083
0.083
0.083
0.083
0.000
0.000
0.050
0.040
0.110
0.102
0.190
0.190
0.199
0.224
0.210
0.214
0.213
0.204
0.198
0.189
0.188
0.186
0.186
0.189
0.193
0.195
ZML = Zero-mile level, in grams per mile (g/mi)
DET1 = Deterioration rate, in (g/mi)/10,000 miles accumulated mileage,
applicable to mileage accumulation up to 50,000 miles
DET2 = Deterioration rate, (g/mi)/10K mi, applicable to mileage accumulated
in excess of 50,000 miles
May 1995
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-55-
Table 2a
MOBILE4.1 and MQBILESa Basic (Non-Tampered^ Emission Levels
(Note that the basic emission estimates for pre-1980 model year vehicles were not changed:
hence these values are the same in both MOBILE 4.1 and MOBILESa.)
Hydrocarbons (HO
al SOK miles (s/ml)
at 1QOK miles (a/mi}
Model Year(s)
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998*
M4.1
0.860
0.877
0.837
0.597
0.620
MSa
1.385
0.792
0.811
0.686
0.612
Ratio*
1.610
0.903
0.969
1.149
0.987
0.599
0.602
0.613
0.608
0.591
0.634
0.608
0.603
0.607
0.623
1.058
1.010
0.984
0.998
1.054
0.572
0.568
0.564
0.564
0.564
0.564
0.564
0.564
0.564
0.635
0.636
0.641
0.641
0.617
0.598
0.570
0.553
0.545
1.110
1.120
1.137
1.137
1.094
1.060
1.011
0.980
0.966
M4.1
0.992
0.988
0.979
0.979
0.979
0.979
0.979
0.979
0.979
M5a
2.035
2.041
2.056
2.056
2.012
1.973
1.935
1.918
1.910
Ratip*
1.360
1.432
1.367
0.987
1.070
2.410
2.217
2.166
2.056
2.022
1.772
1.548
1.584
2.083
1.890
1.024
1.047
1.073
1.068
1.026
2.054
2.018
1.958
1.932
2.008
2.006
1.927
1.825
1.809
1.957
2.051
2.066
2.100
2.100
2.055
2.015
1.977
1.959
1.951
* 'Ratio" is the MOBILESa emission factor estimate divided by the MOBILE4.1
emission factor estimate. For example, MOBILE5a estimated average, non-
tampered HC emissions for MY 1980 LDGVs at 50,000 miles are 61% greater than the
corresponding MOBILE4.1 estimate; at 100,000 miles, the MOBILESa estimate is
77.2% higher than the corresponding MOBILE4.1 estimate.
May 1995
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-56-
Table 2b
MQBILE4.1 and MQBILE5a Basic (Non-Tampered) Emission Levels
(Note that the basic emission estimates for pre-1980 model year vehicles were not changed:
hence these values are the same in both MOBILE 4.1 and MOBILESa.)
Carbon Monoxide (CO)
at 50K miles (g/mi)
at 100K miles (g/mil
Model Year(s)
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992+
M4.1
9.740
11.856
11.277
8.037
8.574
7.954
7.981
8.129
7.919
7.507
7.044
6.954
6.885
M5a
15.880
11.384
11.740
11.000
9.149
9.575
8.940
8.914
8.935
9.139
9.318
9.361
9.387
Ratio*
1.630
0.960
1.041
1.369
1.067
1.204
1.120
1.097
1.128
1.217
1.323
1.346
1.363
M4.1
13.390
20.271
19.197
13.802
15.334
12.289
12.104
11.960
M5ja
25.670
29.429
28.330
27.725
27.169
14.009
14.221
14.514
14.099
13.247
27.310
26.710
25.929
25.365
26.254
26.353
26.456
26.557
Ratio*
1.917
1.452
1.476
2.009
1.772
1.949
1.878
1.786
1.799
1.982
2.144
2.186
2.220
* "Ratio" is the MOBILESa emission factor estimate divided by the MOBILE4.1
emission factor estimate. For example, MOBILESa estimated average, non-
tampered CO emissions for MY 1980 LDGVs at 50,000 miles are 63% greater than the
corresponding MOBILE4.1 estimate; at 100,000 miles, the MOBILESa estimate is
91.7% higher than the corresponding MOBILE4.1 estimate.
-------
-57-
Table 2c
MQBILE4.1 and MQBILE5a Basic (Non-Tampered Emission Levels
(Note that the basic emission estimates for pre-1980 model year vehicles were not changed;
hence these values are the same in both MOBILE 4.1 and MOBILESa.)
Oxides of Nitroen
at 50K miles (a/mi)
at 100K miles
Model Year(s^
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1993
1994
1995
1996+
M4.1
1.850
0.988
0.995
0.887
0.897
0.866
0.649
0.637
0.612
0.625
0.614
0.614
0.618
0.618
0.618
0.618
M5a
2.010
0.963
0.965
0.913
0.860
0.859
0.835
0.832
0.868
0.878
0.874
0.875
0.882
0.780
0.655
0.593
RatiQ*
1.086
0.975
0.970
1.029
0.959
0.992
1.287
1.306
1.418
1.405
1.423
1.425
1.427
1.262
1.060
0.960
M4.1
2.200
1.328
1.355
1.142
1.142
1.076
0.839
0.832
0.782
0.780
0.739
0.734
0.738
0.738
0.738
0.738
MSa
2.520
1.913
1.915
1.908
1.980
1.909
1.905
1.897
1.888
1.868
1.819
1.815
1.812
1.725
1.620
1.568
Ratio'
1.145
1.441
1.413
1.671
1.772
1.774
2.271
2.280
2.414
2.395
2.461
2.473
2.455
2.337
2.195
2.125
* "Ratio" is the MOBILESa emission factor estimate divided by the MOBILE4.1
emission factor estimate. For example, MOBILESa estimated average, non-
tampered NOx emissions for MY 1980 LDGVs at 50,000 miles are 8.6% greater than
the corresponding MOBILE4.1 estimate; at 100,000 miles, the MOBILESa estimate is
14.5% higher than the corresponding MOBILE4.1 estimate.
-------
-58-
Table 3a
Comparison of Emission Factors as Estimated in the Van Nuys Tunnel Study (1987)
to Emission Factors Estimated by MQBILE4 (1989) and MOBILE5a (1993)
Hydrocarbons (HC)
Ambient Average
Sample
Period
B-l
B-2
B-3
B-4
B-5
B-6
B-7
B-8
B-9
B-10
B-ll
B-12
B-13
B-14
B-15
B-16
B-17
B-18
B-19
B-20
B-22
Temp
(°F)
78
81
65
72
66
65
64
66
71
65
58
64
66
58
72
73
68
72
65
62
54
Speed
(mph)
42
42
44
43
43
45
40
43
40
44
11
40
39
15
40
36
41
39
40
41
41
Van Nuys
EF
3.79
*
1.35
2.61
2.81
2.30
3.01
2.32
2.78
2.59
3.88
3.24 .
2.39
6.12
4.68
1.28
2.59
2.29
1.58
2.21
1.62
M4
EF
1.17
1.28
1.06
1.10
1.08
1.04
1.85
1.51
1.46
1.32
6.10
1.45
1.54
4.60
1.39
1.57
1.28
1.32
1.22
1.12
1.14
Ratio
TVN/M4}
3.24
1.27
2.37
2.60
2.21
1.63
1.54
1.90
1.96
0.64
2.23
1.55
1.33
3.37
0.82
2.02
1.73
1.30
1.97
1.42
M5a
EF
1.91
1.97
1.92
1.86
1.94
1.89
2.10
1.94
2.00
1.92
7.12
2.10
2.12
5.39
1.98
2.18
1.99
2.03
2.08
2.09
2.24
Ratio
(VN/MSal
1.98
0.70
1.40
1.45
1.22
1.43
1.20
1.39
1.35
0.54
1.54
1.13
1.14
2.36
0.59
1.30
1.13
0.76
1.06
0.72
Mean ratios:
1.86
1.22
Value deemed outlier by SwRI and not reported.
May 1995
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-59-
Table 3b
Comparison of Emission Factors as Estimated in the Van Nuys Tunnel Study (1987)
Carbon Monoxidf
Sample
Period
B-l
B-2
B-3
B-4
B-5
B-6
B-7
B-8
B-9
B-10
B-ll
B-12
B-13
B-14
B-15
B-16
B-17
B-18
B-19
B-20
B-22
Ambient
Temp
(°f)
78
81
65
72
66
65
64
66
71
65
58
64
66
58
72
73
68
72
65
62
54
Average
Speed
fmph}
42
42
44
43
43
45
40
43
40
44
11
40
39
15
40
36
41
39
40
41
41
(CO)
Van Nuys M4
EF
(a/mi)
20.38
*
23.04
18.43
21.64
19.91
21.15
21.53
18.44
17.80
37.56
28.08
19.30
44.11
24.75
*
21.17
18.27
26.20
15.95
19.15
EF
(g/mil
9.86
10.97
9.47
9.19
9.52
9.21
19.76
14.77
13.39
12.35
72.05
13.90
15.06
53.76
12.23
14.35
11.67
11.68
11.36
10.40
11.52
Ratio
(VN/M4>
2.07
2.43
2.01
2.27
2.16
1.07
1.46
1.38
1.44
0.52
2.02
1.28
0.82
2.02
1.81
1.56
2.31
1.53
1.66
M5a
EF
fg/mi )
19.64
20.65
20.83
19.24
20.85
20.57
22.53
20.85
20.53
20.83
78.12
22.53
22.38
58.33
20.25
21.85
20.97
20.66
22.23
22.72
25.23
Ratio
(VN/M^a^
1.04
1.11
0.96
1.04
0.97
0.94
1.03
0.90
0.85
0.48
1.25
0.86
0.76
1.22
1.01
0.88
1.18
0.70
0.76
Mean ratios:
1.67
0.94
* Value deemed outlier by SwRI and not reported.
May 199!
-------
-60-
Table 3c
Comparison of Emission Factors as Estimated in the Van Nuys Tunnel Study (1987)
Oxides of Nitroeen
Sample
Period
B-l
B-2
B-3
B-4
B-5
B-6
B-7
B-8
B-9
B-10
B-ll
B-12
B-13
B-14
B-15
B-16
B-17
B-18
B-19
B-20
B-22
Ambient
Temp
(°F)
78
81
65
72
66
65
64
66
71
65
58
64
66
58
72
73
68
72
65
62
54
Average
Speed
(mph)
42
42
44
43
43
45
40
43
40
44
11
40
39
15
40
36
41
39
40
41
41
Van Nuys
EF
1.20
41
1.79
1.73
1.57
1.34
1.47
2.00
1.39
1.65
1.15
1.85
1.22
1.36
1.65
1.08
1.82
1.38
1.28
1.57
41
M4
EF
1.99
1.96
2.10
2.07
2.09
2.12
2.53
2.51
2.38
2.39
2.75
2.44
2.42
2.64
2.37
2.33
2.25
2.19
2.14
2.12
2.23
(NOx)
Ratio
( VN/M41
0.60
0.85
0.84
0.75
0.63
0.58
0.80
0.58
069
0.42
0.76
0.50
0.52
0.70
0.46
0.81
0.63
0.60
0.74
,woii^->q i i
M5a
EF
2.57
2.50
2.79
2.68
2.76
2.81
2.75
2.76
2.66
2.79
2.54
2.75
2.71
2.51
2.65
2.58
2.71
2.63
2.74
2.79
2.90
77-M
Ratio
( VN/M5a1
0.47
0.64
0.65
0.57
0.48
0.53
0.72
0.52
0.59
0.45
0.67
0.45
0.54
0.62
0.42
0.67
0.52
0.47
0.56
Means:
0.66 '
0.55
* Value deemed outlier by SwRI and not reported.
May 1995
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-61-
References
1. "Highway Vehicle Emission Estimates," U. S. EPA Office of Mobile Sources June
1992.
2. The July 1992 paper "Highway Vehicle Emissions Estimates" (Ref. I) is available
through EPA's Technology Transfer Network computer Bulletin Board System
(BBS) in the "QMS - Mobile Sources" section. To obtain a single paper copy, send
your request to: White Paper (AQAB), U. S. EPA Office of Mobile Sources,
National Vehicle and Fuel Emission Laboratory, 2565 Plymouth Road, Ann Arbor
MI 48105.
3. Ingalls, Melvin N., et al., "Measurement of On-Road Vehicle Emission
Factors in the California South Coast Air Basin, Volume I: Regulated
Emissions," Final Report, June 1989.
4. "User's Guide to MOBILE4.1," U. S. EPA Office of Mobile Sources, July 1991, EPA-
AA-TEB-91-01; 56 FR 42053, August 26, 1991.
5. "User's Guide to MOBILES," U. S. EPA Office of Mobile Sources, May 1994, EPA-AA-
AQAB-94-01; 58 FR 7780 (February 9, 1993) and 58 FR 29409 (May 20, 1993).
6. "Evaluation of Ambient Species Profiles, Ambient Versus Modeled NMHCNOx and
CO:NOx Ratios, and Source-Receptor Analyses," Systems Applications
International, September 1994, SYSAPP94-94/081; prepared for U. S. EPA Office
of Mobile Sources (under Contract No. 68-C1-0059, Work Assignment No. 2-03).
7. "On-Road Emission Measurements in South Coast Air Basin (Tunnel Study'),"
EPA Memorandum from Terry Newell to Charles L. Gray, Jr., August 22, 1991.
8. Pierson, William, et al., "Real-World Automotive Emissions - Results of Studies in
the Fort McHenry and Tuscarora Mountain Tunnels," Draft Final Report, DRI
Document No. 6480.1D1, March 1994.
9. Enns, Phil, et al., "EPA's Survey of In-Use Driving Patterns: Implications for
Mobile Source Emission Inventories," U. S. EPA Office of Mobile Sources,
Certification Division; from "The Emission Inventory: Perception and Reality,"
Air & Waste Management Association Special Publication VIP-38, 1994.
10. Carlson, Thomas R., et al., "Travel Trip Characteristics Analysis," Sierra
Research, Inc., September 1994; prepared for U. S. EPA Office of Mobile Sources
(under Contract No. 68-C1-0079, Work Assignment No. 2-05).
May 1995
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