Average In-Use Emissions from
Urban Buses and School Buses
The amount of pollution that a vehicle emits is dependent on many
factors. The U.S. Environmental Protection Agency (EPA) has
developed a series of computer models that estimate the average emis-
sions for different types of highway vehicles. This fact sheet is one of
a series on highway vehicle emission factors. It presents average in-use
emission rates for urban (transit) and school buses. It also presents
idle emission rates for urban and school buses.
I-L.
Introduction
There are a number of factors that affect the rate at which any vehicle emits air
pollutants. Some of the most important are:
• vehicle type/size (cars, light-duty trucks, heavy-duty trucks, urban and school
buses, motorcycles)
• vehicle age and accumulated mileage
• fuel used (gasoline, diesel, others)
• ambient weather conditions (temperature, precipitation, wind)
• maintenance condition of the vehicle (well maintained, in need of maintenance,
presence and condition of pollution control equipment)
• type of driving (e.g., long cruising at highway speeds, stop-and-go urban
congestion, typical urban mixed driving)
The most current version of the computer model that EPA uses to estimate average
in-use emissions from highway vehicles is MOBILE6.2. EPA, the States, and others
use this model to estimate total emissions of pollutants generated by highway ve-
hicles in various geographic areas and over specific time periods. The emission rates
(or "emission factors") presented in this fact sheet are based on national average data
representing the in-use fleet as of July 2008,
United States
Environmental Protection
Agency
Office of Transportation and Air Quality
EPA420-F-08-026
October 2008
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The emission rates for hydrocarbons (for both volatile organic compounds [VOCs] and Total
Hydrocarbons [THC]), carbon monoxide (CO), and nitrogen oxides (NOx) are presented in the
following tables. These emission factors are for both diesel and gasoline urban buses and school
buses, although there are very few urban or school buses using gasoline engines. In addition, the
emission rates of particulate matter (PMjQ and PM2 5) are provided for the diesel buses only
because the MOBILE model does not include PM for gasoline buses.1
These estimates assume an average, properly maintained bus, operating on typical diesel and
gasoline fuel, on a warm summer day. Emission rates can be higher in very hot weather (espe-
daily HC) or very cold weather (especially CO),
The emission factors produced by MOBILE6.2 are based on national average data on the frac-
tion of total vehicle miles traveled (VMT) accrued on each of four major roadway types, and
national average traffic speeds associated with each of these facility types. The four roadway
types are:
limited access highways (freeways, expressways)
ramps (entrance and exit ramps to and from limited access highways)
arterials (primary surface roadways)
local and collector roads (local
streets and minor surface roadways)
These emission rates account for the fact
that a single value of average speed is
not adequate for the characterization of
real-world driving patterns. For example,
driving patterns associated with an aver-
age speed of 40 miles per hour (mph) on a
limited access highway are not the same as
driving patterns associated with an average
speed of 40 mph on an arterial route; in
the first case, 40 mph implies heavy traffic
with some congestion and varying speeds,
while in the latter case 40 mph represents
near free-flow conditions. The emission
factors developed for the four roadway
types include hard acceleration and de-
celeration rates (up to 6.9 mph/second),
which result in significantly higher emis-
sion rates for short periods of time, and
maximum speeds up to 75 mph for parts of
the limited access highway driving pat-
terns.
National average values are used for regis-
tration distributions by age (what fraction
Abbreviations and Acronyms Used
CO: Carbon monoxide; a regulated pollutant
g/hr: grams per hour
g/min: grams per minute
HC: Hydrocarbons; molecules formed of
hydrogen and carbon that constitute
gasoline, diesel, and other petroleum-based
fuels; a regulated pollutant
miles per hour
Nitrogen oxides; a regulated pollutant
Particulate matter under 10 microns
diameter; a regulated pollutant
Particulate matter under 2.5 microns
diameter, sometimes referred to as "fine
particulate"
ppm: parts per million
THC: Total hydrocarbons
VMT: Vehicle miles traveled
VOC: Volatile organic compounds; equivalent to
THC plus aldehydes minus both methane
and ethane
mph:
NOx:
PM10:
PM
2.5
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of all buses in use today are of the current model year, are one to two years old, two to three
years old, and so forth) and annual mileage accumulation rates by age (newer buses tend to be
driven more miles per year than do older buses). Some of the other primary assumptions inccr-
porated in these emission factors are:
Ambient temperature: 72 to 92° F day time range
Nominal gasoline volatility: 9.0 psi RVP
Weathered fuel volatility: 8.6 psi RVP
11 ppm
30 ppm
Diesel sulfur content
Gasoline sulfur content
I/M program:
Reformulated gasoline:
No
No
Average Emission Rates
Table 1 presents average in-use emission rates for urban buses and school buses while being
driven,
Table 1: Average Emission Rates for Urban Buses and School Buses*
(in grams per mile)
Pollutant
voc
THC
CO
NOx
PM25
PM10
Urban Diesel Buses
0.349
0.353
3.376
14.793
.0274
0.297
School Diesel Buses
0.642
0.653
2.312
10.536
0.556
0.604
School Gasoline
Buses
7.580
7.791
89.600
7.477
0.104
0.145
*See Endnotes
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Table 2 presents average idle emission factors expressed as grams per hour (g/hr) and grams per-
minute (g/min) of idle time,
Table 2: Average Idle Emission Rates for Urban Buses and School Buses*
Pollutant
voc
THC
CO
NOx
PM25
PM10
Urban Diesel Buses
g/hr
2.700
2.735
37.430
61.113
1.069
1.161
g/min
0.045
0.046
0.624
1.019
0.018
0.019
School Diesel Buses
g/hr
4.968
5.055
25.630
43.505
1.401
1.523
g/min
0.083
0.084
0.427
0.725
0.023
0.025
School Gasoline Buses
g/hr
48.903
52.140
1,036.000
13.373
N/A*
N/A*
g/min
0.815
0.869
17.267
0.228
N/A*
N/A*
*See Endnotes
For More Information
The other fact sheets in this series and additional information are available on the Office of
Transportation and Air Quality's Web site at:
Emission factor fact sheets: www.epa.gov/otaq/consumer.htm
Modeling and estimating vehicle emissions: www.epa.gov/otaq/models.htm
Clean School Bus USA Program: www.epa.gov/cleanschoolbus
Idle Reduction Technologies: www.epa.gov/cleandiesel/idle-ncdc.htm
SmartWay Idle Reduction: www.epa.gov/otaq/smartway/transport/what-
smartway/idling-reduction.htm
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Endnotes
1. Although the MOBILE model includes idling PM estimates for heavy-duty diesel vehicles,
it does not include idling PM estimates for gasoline-fueled vehicles. There has been insufficient
emission testing of PM from gasoline-fueled vehicles of the type necessary to develop emission
rates for these vehicles at idle, since the PM contribution to ambient air quality from gasoline
vehicles is normally negligible,
2. Figures presented in this fact sheet are averages only. Individual buses can differ substantially
in the amount of pollution emitted per mile from the values indicated here. Values shown may
differ slightly from original sources due to rounding,
3. All of the emission estimates provided in this document are consistent, in terms of assump-
tions made and modeling methodology, with those provided in the other fact sheets in this
series: "Average Annual Emissions and Fuel Consumption for Gasoline-Fueled Passenger Cars
and Light Trucks" (EPA420-F-08-024), "Idling Vehicle Emissions" (EPA420-F-08-025), and
"Average In-Use Emissions from Heavy-Duty Trucks" (EPA420-F-08-027).
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