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U.S. Environmental Protection Agency

Office of Transportation and Air Quality

February 2004

Description and History of the MOBILE Highway Vehicle Emission Factor Model

 MOBILE is an EPA model for estimating pollution from highway vehicles. MOBILE

calculates emissions of hydrocarbons (HC), oxides of nitrogen (NOx) and carbon monoxide (CO)

from passenger cars, motorcycles, light- and heavy-duty trucks. (Additional pollutants are now

being added.) MOBILE is based on emissions testing of tens of thousands of vehicles. The

model accounts for the emission impacts of factors such as changes in vehicle emission

standards, changes in vehicle populations and activity, and variation in local conditions such as

temperature, humidity and fuel quality. 

 MOBILE results are used to calculate current and future inventories of these emissions at the

national and local level. These inventories are used to make decisions about air pollution policy

and programs at the local, state and national level. Inventories based on MOBILE are also used

to meet the federal Clean Air Act's state implementation plan (SIP) and transportation

conformity requirements, and are sometimes used to meet requirements of the National

Environmental Protection Act (NEPA). 

 The MOBILE model was first developed as MOBILE1 in the late 1970s, and has been

updated periodically to reflect improved data, changes in vehicle, engine, and emission control

system technologies, changes in applicable regulations and emission standards and test

procedures, and improved understanding of in-use emission levels and the factors that influence

them.

 MOBILE estimates emissions of both exhaust and evaporative emissions. The output from

the model is in the form of emission factors expressed as grams of pollutant per vehicle per hour

(g/hr), or per vehicle mile traveled (g/mi). Thus, emission factors from MOBILE can be

combined with estimates of total vehicle miles traveled (VMT) to develop highway vehicle

emission inventories (in terms of tons per day, per month, per season, per year). The change in

emission factors for a given vehicle category over time are a reflection of the impacts of fleet

turnover. (Over time, older vehicles built to less stringent emission standards are replaced in the

fleet by newer vehicles built in compliance with more stringent standards.) 

 Each generation of the MOBILE model has become more sophisticated in its approach to

modeling average in-use emissions and has provided the user with additional options for tailoring

emission factor estimates to specific times and geographic locations. A brief history of the

MOBILE model versions follows. The current official release, MOBILE6, was released for use

in January 2002. 

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For more information about MOBILE, or to download the MOBILE6 model, see the EPA's

MOBILE6 web page: www.epa.gov/otaq/m6.htm

 Brief History of the MOBILE Highway Vehicle Emission Factor Model

MOBILE1 (1978)

The first model for highway vehicle emission factors. Previously, all factors were tabulated in

"look-up tables" published as part of AP-42 "Compilation of Air Pollutant Emission Factors,

Volume II: Mobile Sources." Modeled exhaust emission rates as functions of age and mileage.

MOBILE2 (1981)

Updated with new data on emission-controlled vehicles (i.e., catalytic converters, model years

1975 and later) at higher ages/mileages. Provided additional user control of input options.

MOBILE3 (1984)

Updated with new in-use data. Eliminated California vehicle emission rates. Continued to model

low- and high-altitude emissions. Added tampering rates and associated emissions impacts and

anti-tampering program benefits. Adjusted non-exhaust emissions to account for "real world"

fuel volatility as measured by Reid vapor pressure (RVP).

MOBILE4 (1989)

Updated with new in-use data. Added evaporative running losses as a distinct emission source

for gasoline powered vehicles. Modeled fuel volatility (RVP) effects on exhaust emission rates.

Expanded user-controlled options for input data.

MOBILE4.1 (1991)

Updated with new in-use data. Added features allowing user control of more parameters

affecting in-use emission levels, including more inspection/maintenance (I/M) program designs.

Added the effects of various new emission standards and related regulatory changes (e.g., test

procedures). Included the impact of oxygenated fuels (e.g., gasohol) on CO emissions.

MOBILE5 and MOBILE5a (1993)

Updated with new in-use data. Based new basic emission rate equations on much larger database

derived from State-implemented IM240 test programs. Included effects of new evaporative

emission test procedure. Added effects of reformulated gasoline (RFG). Added the impact of

oxygenated fuels on HC emissions. Added effect of light duty Tier 1 emission standards and

new NOx standard of 4.0 g/bhp-hr for heavy-duty engines. Added July 1 evaluation option.

Allowed modeling of low-emitting vehicle (LEV) programs patterned after California

regulations. Revised speed corrections. MOBILE5a was issued about 4 months after MOBILE5

to correct a number of minor errors detected under certain specific conditions.

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MOBILE5b (1996)

Updated to reflect impacts of new regulations promulgated since release of MOBILE5 and

MOBILE5a, including: onboard refueling vapor recovery systems, detergent gasoline additives,

and Phase II reformulated gasoline (RFG) requirements. Reactivated calculation of idle emission

factors and expanded calendar year range for which emission factors can be calculated from 2020

to 2050. Increased flexibility of modeling of inspection/maintenance (I/M) programs, providing

easier modeling of retest-based hybrid I/M programs, evaporative emission system pressure and

purge tests, technician training and certification (TTC) credits, and acceleration simulation mode

(ASM) tests (ASM1 and ASM2). Corrected phase-in of emission benefits for first cycle of I/M

program operation.

MOBILE6.0 (2002)

Updated with new and improved data in many areas, including in-use deterioration of 1981-andnewer vehicles, light duty speed effects, gasoline sulfur effects, and evaporative emissions. 

RevisedI/M benefit algorithm. Removed calculation of purge test benefit. Revised algorithms

for air conditioning and high acceleration driving. Added the effects of Tier 2 and new heavy

duty engine and diesel fuel rules. Substantially revised input and output structures, adding "plain

English" input commands and database output. Expanded number of vehicle sub-classes from

eight to 28. Added hourly calculation of emissions and emission estimates by roadway type. 

Separated start and running exhaust emissions. Removed calculation of idle emissions. Allowed

user entry of more detailed vehicle activity information.

MOBILE6.2 (draft 2002, final 2004)

Added ability to model emission factors for particulate matter and six air toxics (Benzene,

Methyl Tertiary Butyl Ether, 1,3-Butadiene, Formaldehyde, Acetaldehyde, and Acrolein). Added

ability to model additional air toxics with user-supplied emission factors. Added spreadsheet

output option. Improved carbon monoxide emission factors.

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