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An air pollution emission source

An emission factor can be defined as the average emission rate of a given pollutant for a given source, relative to units of activity. Emission factors can be used to derive estimates of gas emissions (for instance, greenhouse gas emissions) based on the amount of fuel combusted or on industrial production levels. The level of precision of the resulting estimates depends significantly on the activity in question. 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 certainty. In contrast, the levels of non-CO₂ emissions from combustion depend on the precise nature of the activity in which the fuel is being combusted. For instance, methane (CH₄) emissions from transport depend on a vehicle's type, whether the vehicle has been fitted with emissions controls and so on. Therefore, because of the uncertainties involved, the estimates of non-CO₂ emissions from transport using emission factors are much less precise than estimates of CO₂ emissions.

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. The Intergovernmental Panel on Climate Change (IPCC) publishes default emission factors covering a range of different activities and fuels for use in national inventories. The primary source for IPCC default emission factors is the 'Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories'.

Particularly for non-CO₂ emissions, there is often a very high degree of uncertainty surrounding these emission factors when they are applied to individual countries. In general, the use of country-specific emission factors provides 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, it is 'good practice' to develop a country-specific emission factor for that activity.

• Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (reference manual).
• AP 42, Compilation of Air Pollutant Emission Factors US Environmental Protection Agency
• United Kingdom's emission factor database.
• European Environment Agency's 2005 Emission Inventory Guidebook.
• Fugitive emissions leaks from ethylene and other chemical plants.

• Air pollution
• AP 42 Compilation of Air Pollutant Emission Factors
• Emission standard
• Greenhouse gas and Greenhouse effect
• IPCC list of greenhouse gases
• Radiative forcing