Emission intensity

Graph of UK figures for the carbon intensity of biodiesels and fossil fuels. This graph assumes that all biodiesels are burnt in their country of origin. It also assumes that the diesel is produced from pre-existing croplands rather than by changing land use^[1]

An emission intensity is the average emission rate of a given pollutant from a given source relative to the intensity of a specific activity; for example grams of carbon dioxide released per megajoule of energy produced, or the ratio of greenhouse gas emissions produced to GDP. Emission intensities 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. Emission intensities may also be used to compare the environmental impact of different fuels or activities. The related terms emission factor and carbon intensity are often used interchangeably, but "factors" exclude aggregate activities such as GDP, and "carbon" excludes other pollutants.

An air pollution emission source

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 emissions 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.

Note: 3.6 MJ = megajoule(s) == 1 kW·h = kilowatt-hour(s), thus 1 g/MJ = 3.6 g/kW·h.
Legend: B = Black coal (supercritical)–(new subcritical), Br = Brown coal (new subcritical), cc = combined cycle, oc = open cycle, T_L = low-temperature/closed-circuit (geothermal doublet), T_H = high-temperature/open-circuit, W_L = Light Water Reactors, W_H = Heavy Water Reactors, #Educated estimate.

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[1], 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[2]. 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[3].

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.

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 EMEPTask Force on Emission Inventories and Projections (TFEIP) and are published in the EMEP/CORINAIR Emission Inventory Guidebook.

Intensity targets

Greenhouse gas intensity in 2000 including land-use change

The U.S. plans to cut carbon intensity by 18% by 2012.^[5] This has been criticised by environmentalists as it can be achieved by increasing the GDP as well as by reducing carbon output.^[6]

From 1990 to 2000 the carbon intensity of the U.S. economy declined by 17 percent yet total emissions increased by 14 percent.^[7] In 2002, the U.S. National Environmental trust labelled carbon intensity, "a bookkeeping trick which allows the administration to do nothing about global warming while unsafe levels of emissions continue to rise."^[8]

Sources of emission factors

Greenhouse gases

• 2006 IPCC Guidelines for National Greenhouse Gas Inventories
• Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (reference manual).
• IPCC Emission Factor Database
• Australian Greenhouse Office factors and methods workbook.
• National Inventory Report: Greenhouse Gas Sources and Sinks in Canada.
• United Kingdom's emission factor database.

Air Pollutants

• AP 42, Compilation of Air Pollutant Emission Factors US Environmental Protection Agency
• EMEP/CORIMAIR 2007 Emission Inventory Guidebook.
• Fugitive emissions leaks from ethylene and other chemical plants.

References

• Washington Post article with an example of change in carbon intensity
• A Note On Variations in UK Grid Electricity CO2 Intensity with Time
• IPCC Special Report on Emissions Scenarios

See also

• Energy intensity
• Carbon footprint
• List of countries by ratio of GDP to carbon dioxide emissions
• Low carbon economy
• Low-carbon fuel standard
• Emission inventory
• Air pollution
• AP 42 Compilation of Air Pollutant Emission Factors
• Emission standard
• Greenhouse gas and Greenhouse effect
• IPCC list of greenhouse gases
• Mobile Emission Reduction Credit (MERC)
• Radiative forcing

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