Emission Facts
 

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 emissions 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. 

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 vehicles 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. 

Office of Transportation and Air Quality 

EPA420-F-08-026 

October 2008 
Emission Facts
 

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 (PM10 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 (especially HC) or very cold weather (especially CO). 

The emission factors produced by MOBILE6.2 are based on national average data on the fraction 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)
 Abbreviations and Acronyms Used 

These emission rates account for the fact CO: Carbon monoxide; a regulated pollutant that a single value of average speed is g/hr: grams per hour not adequate for the characterization of g/min: grams per minute real-world driving patterns. For example, 

HC: Hydrocarbons; molecules formed of driving patterns associated with an average speed of 40 miles per hour (mph) on a hydrogen and carbon that constitute 

limited access highway are not the same as gasoline, diesel, and other petroleum-based 

driving patterns associated with an average fuels; a regulated pollutant speed of 40 mph on an arterial route; in mph: miles per hour the first case, 40 mph implies heavy traffic NOx: Nitrogen oxides; a regulated pollutant with some congestion and varying speeds, 

while in the latter case 40 mph represents PM10: Particulate matter under 10 microns 

near free-flow conditions. The emission diameter; a regulated pollutant 

factors developed for the four roadway PM2.5: Particulate matter under 2.5 microns 

types include hard acceleration and de- diameter, sometimes referred to as “fine celeration rates (up to 6.9 mph/second), particulate” which result in significantly higher emisppm: parts per million sion rates for short periods of time, and 

maximum speeds up to 75 mph for parts of THC: Total hydrocarbons 

the limited access highway driving pat- VMT: Vehicle miles traveled 

terns. VOC: Volatile organic compounds; equivalent to 

THC plus aldehydes minus both methane National average values are used for regis­ and ethane tration distributions by age (what fraction 

2 
 

 

 

Emission Facts
 

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 incorporated 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 

•	 Diesel sulfur content: 11 ppm 

•	 Gasoline sulfur content: 30 ppm 

•	 I/M program: No 

•	 Reformulated gasoline: 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 Urban Diesel Buses School Diesel Buses School Gasoline 

Buses 

VOC 0.349 0.642 7.580 

THC 0.353 0.653 7.791 

CO 3.376 2.312 89.600 

NOx 14.793 10.536 7.477 

PM2.5 .0274 0.556 0.104 

PM10 0.297 0.604 0.145

 *See Endnotes 


 
 

 

 

 

 

Table 2 presents average idle emission factors expressed as grams per hour (g/hr) and grams perminute (g/min) of idle time. 

Table 2: Average Idle Emission Rates for Urban Buses and School Buses* 

Pollutant 

Urban Diesel Buses School Diesel Buses School Gasoline Buses 

g/hr g/min g/hr g/min g/hr g/min 

VOC 2.700 0.045 4.968 0.083 48.903 0.815 

THC 2.735 0.046 5.055 0.084 52.140 0.869 

CO 37.430 0.624 25.630 0.427 1,036.000 17.267 

NOx 61.113 1.019 43.505 0.725 13.373 0.228 

PM2.5 1.069 0.018 1.401 0.023 N/A* N/A* 

PM10 1.161 0.019 1.523 0.025 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/whatsmartway/idling-reduction.htm

Emission Facts
 

4 
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 assumptions 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). 

Emission Facts