Emission factor

An emission factor can be defined as the average emission rate of a given pollutant for a given source, relative to the intensity of a specific activity. Emission factors are used to derive estimates of air pollutant or greenhouse gas emissions based on the amount of fuel combusted, the number of animals in animal husbandry, on industrial production levels, distances traveled or similar activity data.

Estimating emissions

Emission factors assume a linear relation between the intensity of the activity and the emission resulting from this activity:

"Emission_pollutant = Activity * Emission Factor_pollutant"

The level of uncertainty of the resulting estimates depends significantly on the source category and the pollutant. Some examples:
*Carbon dioxide (CO₂) emissions from the combustion of fuel can be estimated with a high degree of certainty regardless of how the fuel is used as these emissions depend almost exclusively on the carbon content of the fuel, which is generally known with a high degree of precision. The same is true for sulphur dioxide (SO₂), since also sulphur contents of fuels are generally well known. Both carbon and sulphur are almost completey oxidized during combustion and all carbon and sulphur atoms in the fuel will be present in the flue gases as CO₂ and SO₂ respectively.
*In contrast, the levels of other air pollutants and non-CO₂ greenhouse gas emissions from combustion depend on the precise technology applied when fuel is combusted. These emisisons are basically caused by either incomplete combustion of a small fraction of the fuel (carbon monoxide, methane, non-methane volatile organic compounds) or by complicated chemical and physical processes during the combustion and in the smoke stack or tailpipe. Examples of these are particulates, NO_x, a mixture of nitric oxide, NO, and nitrogen dioxide, NO₂).
*Nitrous oxide (N₂O) emissions from agricultural soils are highly uncertain because they depend very much on both the exact conditions of the soil, the application of fertilizers and meteorological conditions.

Emission Factors for Greenhouse Gas Inventory Reporting

One of the most important uses of emission factors is for the reporting of national greenhouse gas inventories under the United Nations Framework Convention on Climate Change (UNFCCC). The so-called Annex I Parties to the UNFCCC have to annually report their national total emissions of greenhouse gases in a formalized reporting format, defining the source categories and fuels that must be included.

UNFCCC has accepted the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories [http://www.ipcc-nggip.iges.or.jp/public/gl/invs6.htm] , developed and published by the Intergovernmental Panel on Climate Change (IPCC) as the emission estimation methods that must be used by the parties to the convention to ensure transparency, completeness, consistency, comparability and accuracy of the national greenhouse gas inventories [http://unfccc.int/resource/docs/2004/sbsta/08.pdf] . These IPCC Guidelines are the primary source for default emission factors. Recently IPCC has published the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. These and many more greenhouse gas emission factors can be found on IPCC's Emission Factor Database [http://www.ipcc-nggip.iges.or.jp/EFDB/main.php] .

Particularly for non-CO₂ emissions, there is often a high degree of uncertainty associated with these emission factors when applied to individual countries. In general, the use of country-specific emission factors would provide more accurate estimates of emissons than the use of the default emission factors. According to the IPCC, if an activity is a major source of emissions for a country ('key source'), it is 'good practice' to develop a country-specific emission factor for that activity.

Emission Factors for Air Pollutant Inventory Reporting

National Air Pollution Emission Inventories are required annually under the provisions of the UNECE Convention on Long-Range Transboundary Air Pollution (LRTAP). Emission estimation methods and the associated emission factors for air pollutants have been developed by the [http://www.emep.int/ EMEP] Task Force on Emission Inventories and Projections ( [http://tfeip-secretariat.org/unece.htm TFEIP] ) and are published in the [http://reports.eea.europa.eu/EMEPCORINAIR5/en/page002.html EMEP/CORINAIR Emission Inventory Guidebook] .

ources of emission factors

Greenhouse gases

* [http://www.ipcc-nggip.iges.or.jp/public/2006gl/index.htm 2006 IPCC Guidelines for National Greenhouse Gas Inventories ]
* [http://www.ipcc-nggip.iges.or.jp/public/gl/invs6.htm Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (reference manual)] .
* [http://www.ipcc-nggip.iges.or.jp/EFDB/main.php IPCC Emission Factor Database]
* [http://www.greenhouse.gov.au/workbook/index.html Australian Greenhouse Office factors and methods workbook] .
* [http://www.ec.gc.ca/pdb/ghg/inventory_report/2005_report/tdm-toc_eng.cfm NationalInventory Report: Greenhouse Gas Sources and Sinks in Canada] .

Air Pollutants

* [http://www.epa.gov/ttn/chief/ap42/index.html AP 42, Compilation of Air Pollutant Emission Factors] US Environmental Protection Agency
* [http://reports.eea.europa.eu/EMEPCORINAIR5/en/page002.html EMEP/CORIMAIR 2007 Emission Inventory Guidebook] .
* [http://files.harc.edu/Projects/AirQuality/Projects/H005.2002/H5FinalReport.pdf Fugitive emissions leaks from ethylene and other chemical plants] .

ee also

*Emission inventory
*Air pollution
*AP 42 Compilation of Air Pollutant Emission Factors
* [http://www.naei.org.uk/emissions/index.php United Kingdom's emission factor database] .
*Emission standard
*Greenhouse gas and Greenhouse effect
*IPCC list of greenhouse gases
*Mobile Emission Reduction Credit (MERC)
*Radiative forcing