Emissions from a coal-fired power plant. Credit: Steve Hise

An emission factor is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant.

These factors are usually expressed as the weight of pollutant divided by a unit weight, volume, distance, or duration of the activity emitting the pollutant (e.g., kilograms of particulate emitted per megagram of coal burned). Such factors facilitate estimation of emissions from various sources of air pollution. In most cases, these factors are simply averages of all available data of acceptable quality, and are generally assumed to be representative of long-term averages for all facilities in the source category (i.e., a population average).

Emissions factors have long been the fundamental tool in developing national, regional, state, and local emissions inventories for air quality management decisions and in developing emissions control strategies. More recently, emissions factors have been applied in determining site-specific applicability and emissions limitations in operating permits by federal, state, and local agencies, consultants, and industry. These users rely on emissions factors and other emissions quantification tools (e.g., emissions testing and monitoring, mass balance techniques) in developing permits that are practical in their enforcement.

Calculation of emission factors

The general equation for emissions estimation is:

E = A x EF x (1-ER/100)

where:

E = emissions;
A = activity rate;
EF = emission factor, and
ER =overall emission reduction efficiency, %

One source of information recently published by the National Renewable Energy Lab (NREL) estimates the total emissions resulting from energy use, tracing these back to the actual ultimate energy source to extract, convert, and transmit that energy to a user in the United States. Table 1 shows the emissions factors for various pollutants expressed as kilograms of material generated per kilowatt-hour of electricity delivered to the end user. The data represent a national average of all forms of electric power generation.

 Note: TNMOC = total non-methane organic compounds; PM10 = particulate matter 10 micrometers in diameter and smaller; and GWP = global warming potential.

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, 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. 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.

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 emissions 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.

Table 2 lists some CO₂ emission factors for electricity generation as calculated by the Energy Information Administration of the U.S. Department of Energy.

Sources

• Lawrence, Thomas (Lead Author); Environmental Protection Agency (Content source); Cutler J. Cleveland (Topic Editor). 2007.  Emissions factors.  In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth August 29, 2006; Last revised February 4, 2007; Retrieved June 2, 2009].
• U.S. Department of Energy, Energy Information Administration, Fuel emission factors, Accessed 2 June 2009.