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OCR for page 167
5
Alternative Mobile-Source
Emissions Mocleling Techniques
IN ADDITION TO MOBILE, numerous other computer models that estimate
vehicle emissions have been developed over the years. Several of these
models are regional in nature, simulating emissions over large areas, and
others are far more microscale, simulating emissions along a corridor, at
an intersection, or from an individual transportation project. This chapter
briefly describes these alternative mobile- source emissions models, with a
focus on defining key differences between MOBILE and the alternatives.
Further, this chapter builds upon the transportation and emissions-model
integration issues introduced in Chapter 2, defining how mobile-source
emissions inventories can be generated at different levels of detail.
CALIFORNIA AIR RESOURCES BOARD MOTOR-VEHICLE
EMISSIONS INVENTORY SUITE
The Clean Air Act allows California to adopt more restrictive automo-
bile and fuel standards. This motivated the California Air Resources
Board (CARB) to develop a mobile-source emissions model that more close-
ly reflected their standards. The California model also integrates data
sets for region-specific fleet characteristics and travel, allowing it to con-
tain the activity data necessary to estimate both emissions factors as well
as inventories. Below is a brief description of their motor-vehicle emis-
767
OCR for page 168
7 68 MODELING MOBILE-SOURCE EMISSIONS
signs inventory (MVEI) suite, as well as a discussion of the differences be-
tween the California approach and MOBILE.
Overview and Recent History of MVEI
The current version of the CARB mobile-source emissions inventory
model is designated as MVEI7G (CARB 1996b). This is actually a suite of
models consisting of the following components, their relationship is shown
in Figure 5-1:
· CALIMFAC is used to compute the basic emissions rates for light-
and medium-duty gasoline-powered vehicles. The output of CALIMFAC is
a set of regression equations giving the emissions rates as a function of
calendar year for these vehicles.
· WEIGHT calculates the distribution of vehicles, starts, and vehicle
miles traveled MAT) by model year and vehicle category.
· EMFAC calculates all emissions rates (exhaust, evaporative, and tire
and brake wear) for a specified calendar year for all vehicle types. These
rates are computed as a function of vehicle speed and temperature.
· BURDEN calculates the emissions inventory (tons/ day) for a speci-
fied county, air basin, or the entire state.
One significant difference between the structure of the MOBILE pro-
gram and the CARB model is the addition of an emissions inventory mod-
ule, BURDEN. The U.S. Environmental Protection Agency's (EPA's)
MOBILE model is designed to compute the emissions rates from vehicles.
This is the function performed by CARB's EMFAC module. MOBILE has
default distributions for the fraction of VMTs by various vehicle classes
and model years.
The new version of the CARB emissions factor mode] is EMFAC2000.
This model has a large amount of area-specific data to compute vehicle
activity in various areas of the state. Preliminary results from this model
show significant increases in emissions, with statewide inventories for on-
road vehicles increasing 68% for CO, 78% for VOCs, and 93% for NOX.
Emissions Categories
MVEI provides estimates of gaseous and particulate emissions. Gas-
eous species are volatile organic compounds (VOCs), carbon monoxide
(CO), oxides of nitrogen (NOX), and sulfur dioxide. VOCs can be expressed
as total hydrocarbons (HC), nonmethane hydrocarbons, or reactive organic
OCR for page 169
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OCR for page 170
~ 70 MODELING MOBILE-SOURCE EMISSIONS
gases. Only total hydrocarbons are actually computed; adjustment factors
are used to obtain the other VOCs measures. (The differences among the
various terms used for classifying organic compounds is shown in Appen-
dix B.) MVEI computes emissions of lead, total exhaust p articulate s, and
particulate emissions from tire wear and brake dust. It also computes fuel
consumption. It does not compute refueling emissions. In California, refu-
eling emissions are considered a stationary source and are handled as a
separate part of the inventory. EMFAC2000 will include computations of
carbon dioxide emissions.
In EMFAC2000, emissions are computed for passenger cars, eight
weight classes of trucks, school buses, urban buses, motorcycles, and
motor homes. Vehicle classes are subdivided into gasoline-fueled, diesel-
fueled, and electric. Gasoline-fueled vehicles are further subdivided into
catalyst and noncatalyst.
Basic Operation of CARB Moclels
The overall approach for EMFAC2000, MOBILES, and MOBILES is
very similar. All these models rely upon regression analyses of data sets
to get basic emissions rates and correction factors. There is some sharing
of data between the two models, but generally the main data sets for
exhaust emissions of light-duty vehicles (LDVs) have been kept separate
because of the differences in emissions standards between California
vehicles and those sold in the rest of the country.
Because the approaches of the California model are similar to MOBILE,
the potential accuracy limitations are the same. The Coordinating Re-
search Council has sponsored a detailed study of the accuracy of the
EMFAC module in MVEI7G (Pollack et al. l999b). That review has noted
several data and analysis limitations, which are similar to the criticisms
leveled against the MOBILE models. Those include the need for better
data on high emitters, heavy-duty vehicles (HDVs), start emissions, partic-
ulate emissions, and evaporative emissions. It also noted the need to im-
prove estimates of the effects of air-conditioning, inspection and mainte-
nance (I/M), and on-board diagnostic (OBD) on emissions.
Key Technical Differences Between EMFAC2000 and MOBILES
Starting with EMFAC2000, the CARE model for light-duty vehicles
(LDVs) will be based on a new cycle called the "unified" or LA92 cycle
(Carlock 19991. The data for this cycle have been obtained on recent sur-
OCR for page 171
ALTERNATIVE MOB/[E-SOURCE EMISSIONS MODELING TECHNIQUES ~ 7 ~
veys, which have measured emissions on both the unified cycle and the
Federal Test Procedure (FTP) cycle. Users of EMFAC2000 will have the
option of obtaining emissions based on the FTP or the unified cycle. The
unified cycle emissions should be closer to real-world driving results.
MOBILES will continue to report FTP emissions, adjusted by IM240 data.
The new CARB model will continue to use five emissions categories
(normal, moderate, high, very high, and super) for light- and medium-duty
vehicles. This contrasts with the decision to reduce the emissions catego-
ries from four in MOBILES to two in MOBlLE6. The emissions category
boundaries for MOBILES and EMFAC2000 are compared in Table 5-1.
The category definitions in EMFAC2000 provide more distinction among
emissions. However, these definitions increase the data requirements for
getting a reasonable sample in each category.
Much of the development of EMFAC2000 has been devoted to the inclu-
sion of area-specific activity data for various regions of California. Area-
specific vehicle registration data, mileage accumulation data, vehicle age
distributions, and data on VTM are provided for different areas of the
state. Area-specific data on temperature and humidity profiles are also
provided.
EMFAC2000 will use trip-based speed-correction factors (SCFs) rather
than facility-specific speed correction factors that are planned for
MOBILES. The trip-based SCFs are appropriate for an emissions inven-
tory, but link-based SCFs, which should be given by the facility-specific
cycles in MOBILES, are more appropriate for applications such as confor-
mity determinations (Neimeier et al. 1998~. The use of trip-based SCFs
can lead to reduced emissions estimates because the VMT distribution for
a complete trip, at an overall average speed, is different from the VMT
distribution for a particular link at the same overall average speed.
There are various other differences in the detailed implementation of
EMFAC2000 and MOBILE6. These can lead to significant differences in
the emissions estimates from the two models. However, the two models
share the same overall approach and neither model provides any guidance
for an innovative approach to the estimation of on-road mobile-source
emissions.
MOBILE-SOURCE EMISSIONS MODELING
IN THE FEDERAL REPUBLIC OF GERMANY
The Federal Republic of Germany has developed a set of mobile-source
emissions models for a number of purposes (UBA/SAEFL 1999~. There are
three primary models that have different levels of complexity:
OCR for page 172
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OCR for page 173
A1TERNAT/VE MOB/IE-SOURCE EMISSIONS MODEI/NG TECHNIQUES 7 73
Hand book of Emissions Factors (Hand book)7 This is a detailed
model for calculating mobile-source emissions factors for a variety of driv-
ing conditions and vehicle types; the level of detail for this model is high
compared with other German models. This model serves as the foundation
of many of the emissions assessments made.
CITAIR This model is used to predict emissions levels and pollut-
ant concentrations for a variety of microscale control measures; essen-
tially, this is a dispersion model combined with a set of emissions factors
(not too dissimilar from CALINE4 or CAL3QHC).
.
.
TREMOD This is more of an aggregate model that is used to calcu-
late the total emissions inventory for the entire country based on emis-
sions from the entire transportation sector (i.e., road traffic, rail, air, and
ship) (IFEU 19971.
The three different models listed above are highly interrelated, with
data files shared between the different models. For example, many of the
emissions factors used in TREMOD come directly from the Handbook.
This set of emissions models is used for a number of purposes, including
assisting in the development of standards of emissions protection;
performing environmental assessment studies;
road planning; and
establishing permits for construction;
.
Over the years, the German mobile-source estimation techniques have
become more and more refined (similar to the incremental improvements
to the MOBILE series of models). It is expected in future years to be even
more sophisticated. Of most relevance to the MOBILE model are the
Handbook and TREMOD models, which are briefly described below.
Handbook of Emissions Factors
The Handbook of Emissions Factors (UBA/SAEFL 1999) is essentially a
database program that is capable of accepting a number of user inputs,
combining appropriate data sets, and predicting emissions factors for sev-
eral situations. The Handbook is programmed in Microsoft ACCESS, a
iThe Handbook was developed with participation from Switzerland and Austria;
thus, it is used in all three countries.
OCR for page 174
~ 74 MODELING MOBIIE-SOURCE EMISSIONS
flexible, programmable database application and is contained entirely
within a single CD-ROM.
The Handbook is used to provide emissions factors for a variety of appli-
cations. It is often used in conjunction with traffic-simulation models in
which emissions are estimated for different roadway sections in a trans-
portation network. The Handbook also provides key emissions factors for
the other emissions models, CITAIR and TREMOD.
The Handbook database contains information on several aspects of
mobile-source emissions: (1) different vehicle categories, (2) different traf-
fic compositions of those vehicle categories, (3) different traffic scenarios,
(4) cold- and warm-start emissions factors, (5) evaporative emission fac-
tors, (6) different years of reference, (7) ambient temperature profiles, and
(8) different functions for different species of emissions.
In creating the Handbook, two primary components were developed: an
emissions-behavior component, and a driv~ng-behavior component (see
Figure 5-2~. To characterize driving behavior, a number of instrumented
vehicles were used to measure real-world driving patterns. A large set of
velocity-time profiles were collected for a wide range of driving conditions,
ranging from high-speed Autobahn conditions to stop-and-go traffic in ur-
ban centers.
Statistical analysis was then performed on the large driving-behavior
data set, resulting in a number of representative driving cycles for differ-
ent road types and different congestion conditions. All together, 43 repre-
sentative driving cycles were created: 13 urban, 3 rural, 14 highway, and
13 special driving conditions.
In the parallel component, continuous (i.e., second-by-second) tailpipe
emissions measurements were made during various chassis dynamometer
tests. These dynamometer tests were applied to a wide variety of vehicle
types, using several standard driving cycles (e.g., the FTP, the New Euro-
pean Driving cycle (NEDC), the U.S.-Highway cycle, and the German Au-
tobahn cycles).
Approximately 300 LDVs were tested in the basic program, represent-
ing 15 different gasoline-fueled and 6 diesel-fueled vehicle types. The
second-by-second emissions data for these different categories were
matched with their corresponding instantaneous velocity and acceleration
values, and velocity-acceleration indexed lookup tables were created to
represent the emissions.2 These lookup tables were Fred out using inter-
polation techniques. For HDVs such as trucks, a similar methodology was
applied, using the European transient cycle and steady-state emissions
measurements made on engine dynamometers.
2This type of instantaneous or "modal" emissions modeling is described in more
detail in a following section.
OCR for page 175
Representative driving
cycles for different
road categories
1 1
ALTERNATIVE M OBILE-SOURCE EMISSIONS M ODELING TECHNIQUES 7 75
Real-world-driving Modal emissions
behavior measurements measurements over
(highways, arterials, etc.) various driving cycles
r: ~
Instantaneous
Corrective
functions
emissions model
(velocitylacceleration)
1
Emissions factors
for different applications
FIGURE 5-2 Database development for the Handbook of Emissions Fac-
tors. Source FRG-FEPA 1993.
The representative driving patterns derived as part of the driving-be-
havior component were then combined with the instantaneous emissions
functions (i.e., lookup tables) representing the different vehicle types. For
every second in a representative driving cycle, the emissions for a particu-
lar vehicle type can simply be looked up, and all the second-by-second
emissions values for the specific driving cycle can then be summed to-
gether to represent an emissions factor.
In addition to the hot-stabilized emissions factors, supplementary test-
ing was performed to provide additional correction factors for changes in
road grade and for cold- and warm-start effects. By combining the driving-
behavior database, the emissions-function database, and the added correc-
tion factors, emissions of CO, HO, NOx, particulate matter (PM), CO2, and
a few other emissions species can be predicted.
Compared with the MOBILE model, Germany's Handbook of Emissions
OCR for page 176
7 76 MODELING MOBl[E-SOURCE EMISSIONS
Factors is somewhat more detailed and disaggregated in its emissions pre-
dictions. One of the key differences is that the emissions factors repre-
sented in the Handbook have been derived from the ground-up, using in-
stantaneous emissions models developed specifically for producing emis-
sions inventories for a wide variety of vehicles. MOBILE, in comparison,
derives its emissions factors from integrated certification emissions test-
ing, with additional correction factors for speed. Instead of using a global
set of speed correction factors for all types of driving, the Handbook has
also derived and established a wide range of representative driving cycles,
something that MOBILE is now attempting to do in MOBILES with its
facility and congestion cycles (see Chapter 3~. Other key differences are
that the German Handbook also has corrections for road grade, something
MOBILE does not have.
TREMOD "The Traffic Emissions Estimation Model"
In addition to the detailed Handbook, another macroscale emissions
model was created for the entire transport sector of Germany. The
TREMOD model was developed in 1993; it also uses the Microsoft AC-
CESS program. TREMOD was designed to compute emissions of CO,
VOCs, NOX, PM, and other species of emissions from all vehicles in Ger-
many, including motor bikes, cars, trucks, airplanes, ships, buses, tractors,
and trains. In addition, fuel consumption is also computed. The model is
capable of predicting the transport sector emissions inventory for base
years ranging from 1980 to 2020. The model uses extensive fleet charac-
teristics and activity patterns (for past, present, and future years) for all
transport modes.
TREMOD has been validated by comparing its overall predicted fuel
consumption with collected fuel sales data. For gasoline, TREMOD predic-
tions match very well. For diesel fuel, the match was not as good, primar-
ily because diesel fuel is used in many different parts of the transport sec-
tor (e.g., military, agriculture, and stationary generators) where it is par-
ticularly difficult to estimate fuel consumption.
FUEL-BASED EMISSIONS INVENTORIES
The majority of regional emissions models in the United States, such as
MOBILE and EMFAC, use travel-based models that combine gram-per-
mile emissions factors with activity data in the form of VMT to estimate
emissions. In contrast, fuel-based emissions inventories can also be calcu-
lated by normalizing emission factors to fuel consumption rather than
VMT. Typically, fuel-based emissions factors are calculated from on-road
emissions measurements (e.g., from remote sensors and tunnel studies).
OCR for page 177
ALTERNATIVE MOB
