*EPA
    Un.tud SwtL-s          Office of Transportation                                EPA420-F-05-002
    AuoncC""""1''' P'"tl>rt ""   and Air Quality                                      February 2005

                                   for
                      77?e L/.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.
                      Global warming potentials and the use of a common
                      metric
                      Emissions of greenhouse gases are typically expressed in a common
                      metric, so that their impacts can be directly compared, as some gases are
                      more potent (have a higher global warming potential or GWP) than
                      others.

                      The international standard practice is to express greenhouse gases in
                      carbon dioxide (CO2) equivalents. Emissions of gases other than CO2 are
                      translated into CO2 equivalents using global warming potentials. The
                      Intergovernmental Panel on Climate Change (IPCC) recommends using
                      100 year potentials.

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These potentials1 are:

                                            GWP
Carbon dioxide (CO2)                            1
Methane (CH4)                                 21
Nitrous oxide (N2O)                            310
Hydrofluorocarbon (HFC)-134a
(used in mobile source air conditioning)        1,300
Carbon dioxide equivalents vs. carbon equivalents
While the international standard is to express emissions in CO2 equiva-
lents (CO2e), many U.S. sources have expressed emissions data in terms
of carbon equivalents (CE) in the past. In particular, EPA has used the
carbon equivalent metric in the past for budget documents. The benefits
of voluntary programs have often been expressed in million metric tons
of carbon equivalent (MMTCE).  Essentially, this practice accounts for
the carbon in the CO2 molecule, as opposed to counting the entire mol-
ecule.

EPA's Office of Transportation and Air Quality  (OTAQ) uses the CO2
equivalent metric for calculating the national inventory for greenhouse
gas emissions, and is moving towards using the CO2 metric to character-
ize the benefits of its voluntary programs to be consistent with interna-
tional practice. Emission reductions from voluntary programs will
generally be expressed in million metric tons of CO2 equivalent. For the
purposes of national greenhouse gas inventories, emissions are expressed
as teragrams of CO2 equivalent (Tg CO2 Eq). One teragram is equal to
1012 grams, or one million metric tons.
1 These estimates are from the IPCC's "Second Assessment Report" (1996). These are
the values used internationally for reporting greenhouse gas (GHG) emissions to the
United Nations. (EPA also uses them for the "Inventory of U.S. Greenhouse Gas
Emissions and Sinks.") The IPCC has since published the "Third Assessment Report"
(2001) and has updated GHG potential values. These are 23 for CH4, 296 for N2O, and
1,300 for HFC-134a. These values, however, have not yet been adopted by EPA for
the purpose of developing inventories.

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Converting           carbon dioxide equivalents and
carbon equivalents
The conversion between CO2e and CE is directly related to the ratio of
the atomic mass of a carbon dioxide molecule to the atomic mass of a
carbon atom (44:12).

To convert from CE to C(Xe. multiply by 44/12
For example, 6 million metric tons of carbon equivalent (6 MMTCE) =
6 x (44/12) = 22 million metric tons of CO2 equivalent.
1 metric ton carbon equivalent = 3.667 metric tons of CO2 equivalent.

To convert from C(Xe to CE. multiply by 12/44
For example, 11 million metric tons of CO2 equivalent = 11 x (12/44) =
3 million metric tons of carbon equivalent (3 MMTCE).
1 metric ton of CO2 equivalent = 0.2727 metric tons of carbon equiva-
lent.
For More 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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