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OCR for page 15
1
Overview of
Mobile-Source Emissions
AN ACCURATE ASSESSMENT of motor-vehicle emissions is essential for an
effective air-quality improvement program. The U.S. Environmental Pro-
tection Agency's (EPA's) Mobile Source Emissions Factor MOBILE model
is the primary tool used by air-quality planners at national, state, and
local levels to estimate on-road mobile-source emissions, and hence is key
to assessing associated environmental impacts. Because of the model's
importance in assessing air-quaTity control programs and because of con-
cerns about weaknesses in the accuracy and reliability of the model, Con-
gress asked the National Academy of Sciences to review and evaluate the
MOBILE model. The National Research CounciT's Committee to Review
EPA's Mobile-Source Emissions Factor Model was formed in response to
that request. Specifically, the committee was to consider the adequacy of
the model's input data, assumptions, structure, and results and recom-
mend ways to improve the reliability of its mobile-source emissions esti-
mates.
This chapter reviews mobile-source emissions within the context of the
total air-quality problem in the United States, provides an overview of the
categories and estimated levels of mobile-source emissions, introduces the
legislative and regulatory initiatives used to control such emissions, de-
tails the committee's charge, and describes the structure of this report.
75
OCR for page 16
6 M ODE[ING MOB/LE-SOURCE EMISSIONS
AIR-QUALITY PROTECTION
Air-quality policy in the United States is directed largely towards pro-
tecting public health. It also addresses public-welfare issues, such as visi-
bility in national parks and wilderness areas and environmental damage
due to acid deposition. The National Ambient Air Quality Standards
(NAAQS) sets a primary standard for ambient concentrations of criteria
pollutants to protect public health with "an adequate margin of safety",
and a secondary standard to protect public welfare against environmental
and property damage. The attainment of NAAQS for the six criteria pol-
lutants, ozone, carbon monoxide (CO), nitrogen dioxide (NO2), lead, sulfur
dioxide (SO2), and particulate matter (PM), is one measure of air quality.
Another category of pollutants consists of the hazardous air pollutants
or "air tonics," including carcinogens, neurotoxins, teratogens, allergens,
and other harmful compounds. Control of air tonics is aimed at ensuring
that any health risks are less than one in a million from lifetime exposure
to a particular toxic.
Other air-quality protection programs are directed towards acid-rain
and visibility concerns. Acid-deposition regulations are designed to reduce
the negative impacts on humans and the environment from the deposition
of nitric and sulfuric acids. Visibility regulations are designed to protect
the visibility in federal Class I areas, which are primarily national parks
and wilderness areas.
Pollutants of Interest
The pollutants of interest for air quality are both primary and second-
ary pollutants. Primary pollutants are those directly emitted to the atmo-
sphere and include CO, SO2, and lead. Ambient concentrations of such
pollutants are directly related to their sources. Secondary pollutants are
those formed by atmospheric processes, including chemical reactions and
condensation. Ozone is a secondary pollutant, formed by the action of sun-
light and chemical reactions involving volatile organic compounds (VOCs)i
iAn organic compound is a compound containing carbon combined with atoms of
other elements, commonly hydrogen, oxygen, and nitrogen. Simple carbon-contain-
ing compounds such as carbon monoxide and carbon dioxide are usually classified
as inorganic compounds. A volatile organic compound is a compound that can exist
as a gas under typical atmospheric conditions. This report, unless otherwise noted,
will refer to the general class of gaseous organic compounds. as VOCs. Appen-
dix B describes the differences among the terms used to refer to gaseous
organic compounds.
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OVERVIEW OF MOBI[E-SOURCE EMISSIONS 7 7
and nitrogen oxides (NOX). Airborne PM and air taxies are combinations of
primary and secondary pollutants.
In urban areas, motor vehicles generally are the dominant emissions
sources of VOCs, NOX, and CO and their control is critical for reducing ur-
ban antipollution problems caused by these emissions. The adverse effects
of ozone and CO are well understood, whereas those of PM and most tonics
are less well understood. As more is known about the specific aspects of
the toxicity of PM and air tonics, a better understanding of the contribu-
tion of motor vehicles to these pollutants becomes important.
Mobile-Source Contributions
Total mobile source emissions (on-road plus off-road emissions) contrib-
ute significantly to overall air pollution in the United States.2 Figure 1-1
shows estimates for 19973 of the contribution of mobile sources (on-road
and off-road) to emissions of criteria air pollutants and their precursors.
For example, mobile sources in that year contributed over 75% of CO emis-
sions (about 67,000 thousand tons), almost half of the NOX (about 11,600
thousand tons), and 40% of the VOCs (about 7,700 thousand tons) (see
Figure 1-1) (EPA 1998b). The elimination of lead from gasoline has great-
ly reduced mobile-source emissions of this pollutant to just over 500 tons
(13% of total lead emissions) in 1997, compared with mobile-source lead
emissions in 1970, which were over 180,000 tons (EPA 1998a). According
to the EPA (1998b), mobile-source exhaust is a less important source of
PM-10 (those particles smaller than 10 mm in diameter) and sulfur diox-
ide. Mobile-source contribution to fine particles (those smaller than 2.5
mm in diameter and referred to as PM-2.5) is an area of continuing study.
One recent study reported higher than expected PM-2.5 emissions from
light-duty vehicles (LDVs) at higher elevations (Cadre et al. 1998~. It
should be noted that, for a given location, the fraction of emissions inven-
tories contributed by mobile sources varies greatly.
2The principal sources of emissions are stationary sources (fuel combustion by
utilities, industry and residential sources), industrial process sources (chemical
manufacturing, petroleum refining, solvents and waste disposal), on-road vehicles
(light- and heavy-duty gas and diesel-powered vehicles), non-road engines and
vehicles (recreational, industrial, and commercial vehicles and engines, trains,
marine vessels, and aircraft), and other (biogenic emissions from natural, agricul-
tural and forestry sources, and other combustion) (EPA 1998a).
3To the extent that the mobile source emissions estimates are obtained by using
the MOBILE model, they are subject to the types of uncertainties discussed
throughout this report.
OCR for page 18
78 MODEL/NO MOBI[E-SOURCE EMISSIONS
CARBON MONOXIDE
956~481 7
e_
I_
f _
16755- ~
LEAD
503~ _~n
.~
19~:
/ t.
/ I:.
~ ,
~ i
__ ~ "~w
I,
..:,,
.'
7~6052
, ., , , .
: .! . . : : . : . .
_ ~ : , : : , , , , i ,
_ rr , . ,
_ : . :
l , ,
_ , . . :
. , . ,
. , : ,
: . : :
. .
, , ,
. ~ , , , , ,
K, , ,
: . . .
_ , _ .
, i ~
50257
;2897
OXIDES OF NITROGEN
346
7035)
, a.;;;;.;;
A:.:.::.:.:
\' 917
LEGEND
VOLATILE ORGANIC CARBON
s230~
PARTICULATE MATTER (PM-10)
1101 - 1277,268
new—
SULFUR DIOXIDE
~-~ :~
- - :)
~17260
STATIONARY INDUSTRIAL ~1 ON-ROAD 1~ NON-ROAD ·OTHER
FIGURE 1-1 Sources of Criteria Air Pollutants. Estimated total annual
emissions of criteria pollutants from stationary, industrial process, and
mobile (on-road and non-road), and other sources for 1997. Emissions are
shown in thousands of tons except for lead, which is shown in tons.
Source: EPA l99Sb.
Human Health Concerns
Total emissions from mobile sources contribute significantly to the det-
rimental health effects resulting from exposure to ambient ozone, CO, PM,
OCR for page 19
OVERVIEW OF MOBIIE-SOURCE EMISSIONS 7 9
and air tonics. Urban ozone has been one of the most persistent health con-
cerns. The current 1-fur primary NAAQS for ozone is 0.12 parts per million
(ppm), which is the daily maximum not to be exceeded more than once per
year on average. (This is averaged over 3 years, so the fourth highest 1-fur
value over 3 years is the one that is used to compare with the standard.)
Health effects associated with exposures above this standard are well doc-
umented and are summarized by Lippmann (1989~. They range from
short-term consequences such as chest pain, decreased lung function, and
increased susceptibility to respiratory infection, to possible long-term con-
sequences, such as premature lung aging and chronic respiratory illnesses.
In 1997, approximately 48 million U.S. residents in 77 counties, primarily
urban and suburban regions, lived in areas where the second highest daily
maximum concentration exceeded 0.12 ppm (EPA 1998b).Due to concerns
about prolonged exposures to lower levels of ozone, EPA adopted an 8-fur
standard of 0.08 ppm, which would put a much greater area and popula-
tion into nonattainment with the standard (Wolff 1996; Chameides et al.
1997~. Based on the period from 1993 to 1995, EPA estimated that 248
counties with a population of 83 million people would have violated the 8-
hr ozone standard (EPA 1997a). Prospects for the implementation of the
new standards are uncertain because a U.S. Court of Appeals in May 1999
remanded them for further consideration by EPA.
Attaining CO standards has been far more successful. There are two
primary standards for CO, a 1-fur average of 35 ppm and an 8-fur average
of 9 ppm. The health effects from exposures to concentrations exceeding
these standards are also documented. CO enters the blood stream and
links to hemoglobin, reducing the amount of oxygen the blood can carry
and causing mental and physical impairment. In 1997, approximately 9
million people in three urban counties lived in areas that exceeded these
standards (EPA 1998b). This number has been declining over time.
Standards for PM and air tonics are currently changing due to increased
knowledge about their health effects. Current regulations for ambient con-
centrations of PM focus on particles less than or equal to 10 mm. New
regulations to control smaller particles (less than or equal to 2.5 mm), in-
tended to address some preliminary findings that these particles have the
greatest impact on human health (Dockery et al.1993), were remanded for
consideration back to EPA by a U.S. Court of Appeals in May 1999. Air
tonics include a wide class of emissions and effects. Few of these sub-
stances have been studied to examine possible health effects. In 1998, the
California Air Resources Board (CARB) identified diesel PM as a toxic air
contaminant.
An issue unique to mobile sources is the proximity of sources to recep-
tors. Air vents on cars can scoop up exhaust from a vehicle just ahead so
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20 MODELING MOBILE-SOURCE EMISSIONS
that exposures in the cab of a car or truck are higher than at the roadside.
A jogger at a roadside, breathing deeply and rapidly, might inhale much
larger amounts of contaminants than a sedentary individual sitting out-
doors away from the road. Urban canyons can retain, and thus concen-
trate, contaminants emitted by motor vehicles. Figure 1-2 shows how ex-
posure to CO varied for one individual during typical commutes. Because
health effects are a function of what is actually inhaled, this variation in
exposure is a critical feature of mobile-source pollutants.
Environmental Concerns
Mobile sources contribute significantly to the risk of detrimental envi-
ronmental effects of ozone, acidic deposition, PM, and air tonics. However,
environmental impacts have received much less attention than health ef-
fects. For most criteria pollutants, the secondary NAAQS, set to protect
the environment and susceptible outdoor structures, is the same as the
primary standard set to protect human health. Ozone concentrations can
damage crops and other vegetation (Heck et al. 1982; EPA 1986), exhaust
emissions are toxic to roadside flora and fauna (particularly evident when
lead was used (EPA 1998b), acid can damage human artifacts (e.g., stain-
ing of buildings and corrosion of outdoor statuary) (NAPAP 1990), and
p articulate s contribute to haze and poor visibility (NRC 1993~.
Mobile sources also contribute to greenhouse gas emissions. Approxi-
mately one-third of the total U.S. anthropogenic emissions of CO2 comes
from the transportation sector (Energy Information Administration 1997),
including about one-quarter of the total from light-duty vehicles and
heavy-duty vehicles (Heinz Center 1998~. Also, mobile-source emissions of
chlorofluorocarbons and hydrochiorofluorocarbons from air-conditioning
systems contribute to stratospheric ozone depletion. Although chlorofluoro-
carbons are now banned from use in the United States, existing systems
containing these compounds will continue to leak them to the atmosphere
(Holmes and Ellis 1997), and significant illegal use has been reported
(Vallette 1995~.
ESTIMATING EMISSIONS FROM MOBILE SOURCES
Importance of Source Identification and Quantification
An effective air-quality improvement program requires the identifica-
tion, inventory, and control of emissions sources, including mobile sources.
OCR for page 21
OVERVIEW OF MOB`LE-SOURCE EM/SS!ONS 2 7
Sol
40
-
E
At
0 30
z
us
8
20
10
Morning
commute
PASS "S Lo
— ROUTE 66]
RourE 215
S5AS,.
IVA _D
Evening
commute
WALK - IN EPA GARAGE
DRIVE- I N GAR A GE
COLLEC TION 800TH
SW FREEWAY
· I 495
I ROUTE 66
IN OFFICE ~ HARASSES, VA
~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1
5 6 7 a 9 10 al 12 13 14 15 16 17 18 19 20
HOUR OF toy
FIGURE 1-2 Daytime exposure to carbon monoxide during peak-hour
commutes. Source: Ott 1985.
This requires not only a broad understanding of which pollutants are de-
rived from which sources, but also details about their spatial and temporal
variation, the contributions of subsets of sources, the chemical and physi-
cal characteristics that determine their propensity to form secondary pol-
lutants, their levels of exposure and toxicity, and the actual effectiveness
of strategies to control emissions. The large number of individual sources,
the large variability of emissions characteristics among these sources, and
the need for emissions estimation methods to fulfill many applications cre-
ates daunting challenges. To aid in identifying, estimating, and reducing
risks of motor-vehicle emissions, EPA has developed a series of models
referred to as the MOBILE model. The current (MOBILE5b) and upcom-
ing (MOBILES) versions of MOBILE provide some but not all of the
required information. MOBILE estimates emissions factors from broad
vehicle classes using average vehicle speed to represent highway condi-
tions. However, MOBILE provides only limited information on the physi-
cal and chemical characteristics of emissions, and it does not simulate
emissions related to dynamic traffic-Row conditions.
The MOBILE model deals only with the on-road vehicle emissions of
CO, VOCs, and NOx. Related models cover other mobile-source emissions
(e.g., NONROAD for off-road emissions, PARTS for PM, and MOBTOX for
air tonics). These related models are often discussed in the same context
as MOBILE because they are needed to represent the full suite of emis-
sions from mobile sources and often use the same input data as MOBILE.
OCR for page 22
22 MODELING MOBI[E-SOURCE EMISSIONS
On-Road Vehicle Emissions Categories
Regulations and the MOBILE model divide on-road vehicles into two
broad categories, light-duty (LDVs) and heavy-duty (HDVs) vehicles. The
size boundary between LDVs and HDVs historically has been 8,500
pounds, gross vehicle weight (the weight of the vehicle plus the weight of
the rated load-hauling capacity). LDVs are fueled primarily by gasoline.
HDVs are fueled by diesel and gasoline with the heavy HDVs (those with
gross vehicle weights greater than 26,000 pounds) fueled with diesel.
These heavy HDVs are important because, though they represent a small
fraction of the total number of vehicles, they represent a significant frac-
tion of total vehicle miles traveled, fuel consumption, and emissions (Davis
1999; EPA 1998a). Figure 1-3 displays the separation of emissions by fuel
use.
Light-Duty Vehicles
Historically, LDVs were sub-divided into two main categories4: passen-
ger cars and light-duty trucks (LDTs), with the latter used primarily for
commercial purposes. Two different sets of exhaust emissions standards
applied. Differences between the two categories of vehicles have dimin-
ished and the regulatory distinction will disappear in 2007. Tailpipe emis-
sions standards were applied in 1968 and now call for a reduction of more
than 90% from 1968 levels for all pollutants.
Passenger cars, as the name implies, refer to personal vehicles used pri-
marily to transport people. A single set of emissions standards applies to
all passenger cars, regardless of size, passenger occupancy, or use. Emis-
sions are regulated on the basis of grams of pollutant per mile (g/mi) and
vehicles are certified on a chassis dynamometer test.5 Table 1-1 shows
historical categories of standards for passenger cars for up to 50,000 miles
or from 50,001 to 100,000 miles.6
4Motor cycles also form a category of on-road vehicles. However, their emissions
tend to be small relative to passenger cars and light-duty trucks. Thus, they are not
discussed here.
sin a chassis dynamometer test, the whole vehicle is mounted on a dynamometer
for testing. In contrast, in an engine dynamometer test only the engine, rather than
the whole vehicle, is mounted on a dynamometer.
Manufacturers are allowed to certify compliance using low mileage cars and an
agreed-upon deterioration assumption. They are not required to recall and test in-
service vehicles. (Recalls may be required if emissions control systems are shown
to be faulty.)
OCR for page 23
OVERVIEW OF MOB/LE-SOURCE EMISSIONS 23
OXIDES OF NITROGEN
1 933 ~
/
. 2::~
:,:,.:::, -
.:.:::.:::.: ::-:::,.::.:.: :..\
·:-:i-::-i:-: ::.- :ii:-:-:-\
::-::-::-:: ::::-
.~ :.-::.-:.-.:-:
,~,~ . - , -.i. ,-
·. ~ ::.:-.-.: .:-
.;: .:.i: :-: -:-: : -: -: ii: :-:: -:-:-:.i: :-: :-:-.
~~ :- -,-.
03
VOLATILE ORGANIC CARBON
238~
^:..N
\
'- i.~ ".2 .-i-.- . ,2
;-: :-:-:-::: i:-:-:-i:;-i:-i
:~ :.-:.-:.-.-:.-::.-.:~
. : . ~
_ ~
, : : - 2.i.
'i,..i.i. i.,..,i..,i.,.i...-...
:- i: ::-i-: - -i:--:- -- :-:-i:-
~ . ' §.i -:.-.i .- ~ §'~
it- .2 .2.- ii.~ ii. i- i. .- :. i. ~ A non
~ ~: ~ ~ -~0~
PARTICULATE MATTER (PM-10)
162
CARBON MONOXIDE
84-
SULFUR DIOXIDE
48749
PARTICULATE MATTER (PM-2.5)
A. . .:-.:-:.
.- ..:.. .:. ..-.:...~105
:: :.-.:.:: ::::: :.:::::~::::
.:.:-..:.:.-.:....,:.::....:.:.:..-:
·:-:-:-:-i:-:-:---:-:~ :-:-:-:-:- 1
: -: :-- -.::: . 2.:~ : :-.: 1
I.. :.
1 \ ~
LEGEND
I ~
.. .. GAS _ D I ESEL
FIGURE 1-3 Estimated mobile-source emissions by fuel type. MOBILES
and PARTS estimates of 1997 emissions from the on-road motor-vehicle
fleet. It is likely that MOBILES underestimates gasoline VOC and diesel
NOx emissions. Emissions are shown in thousands of tons. Source: EPA
l99Sa.
OCR for page 24
24 MODEL/NO MOBILE-SOURCE EMISSIONS
TABLE 1-1 Passenger-Car Exhaust Gaseous Em
50,000 miles
issions Standards (g/mi)
100,000 miles
NOX NMHCa CO NOx
THC (NMHC)a CO
Mode! Year
Pre-control 10.6 84.0 4.1
1968-71 4.1 34.0 3/4 3/4 3/4 3/4
1972-74 3 0 28.0 3.1 3/4 3/4 3/4
1975-76 1.5 15.0 3.1 3/4 3/4 3/4
1977-79 1.5 15.0 2.0 3/4 3/4 3/4
1980 0.41 7.0 2.0 3/4 3/4 3/4
Category
Tier 0 (1981-93) 0.41 3.4 1 0 3/4 3/4 3/4
Tier 1 (beginning 0.41 (0.25) 3.4 0.4 0.31 4.2 0.6
with model year
1994- ~
NLEV (beginning 3/4 3/4 3/4 0.09 4.2 0.3
with model year
1999- )
Tier 2 - Default set 3/4 3/4 3/4 0.125 1.7 0.2
in CAAA9O
(beginning with
model year 2004- )
Tier 2- Current 3/4 3/4 3/4 >o.o9b >4.2b 0.07
proposed standards
(beginning with
model year 2004- )
Note: Emissions standards were originally written for total hydrocarbons
(THC) and later for nonmehtane hydrocarbons (NMHCs). Appendix B describes
the differences among the terms used to refer to gaseous organic compounds. This
report, unless otherwise noted, will refer to the general class of gaseous organic
compounds as VOCs.
b Note: The proposed Tier 2 standards are a corporate average standard with a
focus on NOX emissions. This allows NMHCs and CO emissions standards to
"float", in that fleet emissions rates depend on the mix of vehicles used to meet the
NOx standard. The emissions standards shown for NMHCs and CO are those that
would result given the mix assumed in the Notice of Final Rulemaking (EPA
1999a) to meet the NOx standard.
Sources: EPA 1998c,d, 1999a; Davis 1997; Chrysler Corp. 1998.
OCR for page 25
OVERVIEW OF M OBILE-SOURCE EMISSIONS 25
The LDT category originally described vehicles designed for Toad-haul-
ing rather than for passenger transportation, and was further divided into
weight categories. Some of these LDTs, and some passenger cars, have
evolved into vans and sport-utility vehicles. As a result, differences in size
and function among passenger cars, LDTs, vans, and sport-utility vehicles
have diminished. As a consequence EPA in its Final Rule for Tier 2 emis-
signs standards (EPA 1999a) mandates the same emissions standards to
passenger cars and LDTs.7 As with passenger cars, pollutant emissions
limits for LDTs are expressed in grams of pollutant per mile and vehicles
are certified on a chassis dynamometer. The MOBILE model retains the
historical distinction between passenger cars and LDTs because current
emissions standards and control technologies differ among vehicle classes.
Heavy-Duty Vehicles
Different exhaust emissions standards apply to the two broad categories
of HDVs: trucks and buses, with further differentiation made between gas-
oline and diesel engines. Emissions are regulated on grams of pollutant
per brake-horsepower-hour (BHP-hr) because of the difficulty of devising
reasonable grams per mile limits for the broad range of vehicles covered
and the difficulty in developing a practical chassis dynamometer test. En-
gines are certified on an engine dynamometer. Emissions standards were
applied later than for LDVs and are less stringent than for LDVs.
Because certification is for engines rather than vehicles, measurements
are of engine emissions that are then used to estimate vehicle emissions.
To obtain emissions in grams of pollutant per mile, which is what MO-
BILE calculates, vehicle efficiency (BHP-hr/mile) must be estimated. The
problem of converting engine dynamometer emissions to on-road emissions
is the same for both buses and trucks. Engines used in buses must meet
more stringent PM standards than engines used in trucks.
Evaporative Emissions
Evaporative emissions, including those resulting from leaks of liquid
fuel, can be classified into five categories for modeling purposes: diurnal,
7EPA has also introduced a new category of vehicle in the Tier 2 proposal, the
medium-duty passenger vehicle for passenger vans and sport-utility trucks between
8,500 and 10,000 pounds gross vehicle weight. This category of vehicle will also be
required to meet the Tier 2 emission standard.
OCR for page 26
26 M ODE[`NG MOB
