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Un.u.'d s-titfi _ Office of Transportation EPA420-F-05-001
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Emission
Facts
Average Carbon Dioxide Emissions
Resulting from Gasoline and Diesel
Fuel
The U.S. Environmental Protection Agency (EPA) developed this series
of four fact sheets to facilitate consistency of assumptions and practices
in the calculation of emissions of greenhouse gases from transportation
and mobile sources. They are intended as a reference for anyone
estimating emissions benefits of mobile sources air pollution control
programs.
Carbon content in motor vehicle fuels
One of the primary determinants of carbon dioxide (CO2) emissions
from mobile sources is the amount of carbon in the fuel. Carbon content
varies, but typically we use average carbon content values to estimate
CO2 emissions.
The Code of Federal Regulations (40 CFR 600.113) provides values for
carbon content per gallon of gasoline and diesel fuel which EPA uses in
calculating the fuel economy of vehicles:
Gasoline carbon content per gallon: 2,421 grams
Diesel carbon content per gallon: 2,778 grams
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Note that for the "Inventory of U.S. Greenhouse Gas Emissions and
Sinks," EPA estimates CO2 emissions from fuel from the heat content of
the fuel and carbon content coefficients in terms of carbon content per
quadrillion BTU (QBTU), using data from the Energy Information
Administration (EIA). EIAs numbers are derived from carbon content
by mass, and equate to roughly the same carbon content per gallon of
fuel as the values provided in 40 CFR 600.113. EPA uses heat content
data from Energy Information Administration's (EIA) "Annual Energy
Outlook 2003" and carbon content from EIAs "Emissions of Green-
house Gases in the United States, 2000."
Note also that these estimates are based only on an average carbon
content of conventional gasoline and diesel fuel, and do not specifically
address the impact of fuel additives such as ethanol or methyl tertiary-
butyl ether (MTBE) that may depend on the feedstock.
Calculating CO2 emissions
The Intergovernmental Panel on Climate Change (IPCC) guidelines for
calculating emissions inventories require that an oxidation factor be
applied to the carbon content to account for a small portion of the fuel
that is not oxidized into CO2. For all oil and oil products, the oxidation
factor used is 0.99 (99 percent of the carbon in the fuel is eventually
oxidized, while 1 percent remains un-oxidized.)1
Finally, to calculate the CO2 emissions from a gallon of fuel, the carbon
emissions are multiplied by the ratio of the molecular weight of CO2
(m.w. 44) to the molecular weight of carbon (m.w. 12): 44/12.
CO2 emissions from a gallon of gasoline = 2,421 grams x 0.99 x (44/12)
= 8,788 grams = 8.8 kg/gallon = 19.4 pounds/gallon
CO2 emissions from a gallon of diesel = 2,778 grams x 0.99 x (44/12) =
10,084 grams = 10.1 kg/gallon = 22.2 pounds/gallon
Note: These calculations and the supporting data have associated
variation and uncertainty. EPA may use other values in certain circum-
stances, and in some cases it may be appropriate to use a range of
values.
1 Based on emissions data, EPA's Office of Transportation and Air Quality (OTAQ) is
currently examining whether this fraction is higher (closer to 100 percent) for gaso-
line.
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For information
You can access documents on greenhouse gas emissions on the Office of
Transportation and Air Quality Web site at:
www.epa.gov/otaq/greenhousegases.htm
For further information on calculating emissions of greenhouse gases,
please contact Ed Coe at:
U. S. Environmental Protection Agency
Office of Transportation and Air Quality
1200 Pennsylvania Ave., NW (6406J)
Washington, DC 20460
202-343-9629
E-mail: coe.edmund@epa.gov
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