Emission inventory

Emission inventory

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An emission
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

is an accounting of the amount of pollutants discharged into the atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

. An emission inventory usually contains the total emissions for one or more specific greenhouse gases or air pollutants, originating from all source categories in a certain geographical area and within a specified time span, usually a specific year.

An emission inventory is generally characterized by the following aspects:
  • Why: The types of activities that cause emissions,
  • What: The chemical or physical identity of the pollutants included,
  • Where: The geographic area covered,
  • When: The time period over which emissions are estimated,
  • How: The methodology to use.

Emission inventories are compiled for both scientific applications and for use in policy processes.

Use of Emission Inventories

Emissions and releases to the environment are the starting point of every environmental pollution problem. Information on emissions therefore is an absolute requirement in understanding environmental problems and in monitoring progress towards solving these. Emission inventories provide this type of information.

Emission inventories are developed for a variety of purposes:
  • Policy use: by policy makers to
    • track progress towards emission reduction targets,
    • develop strategies and policies or;
  • Scientific use: Inventories of natural and anthropogenic emissions are used by scientists as inputs to air quality models.

Policy use

Two more or less independent types of emission reporting schemes have been developed:
  • Annual reporting of national total emissions of greenhouse gases and air pollutants in response to obligations under international conventions and protocols; this type of emissions reporting aims at monitoring the progress towards agreed national emission reduction targets;
  • Regular emission reporting by individual industrial facilities in response to legal obligations; this type of emission reporting is developed to support public participation in decision-making .

Examples of the first are the annual emission inventories as reported to the United Nations Framework Convention on Climate Change
United Nations Framework Convention on Climate Change
The United Nations Framework Convention on Climate Change is an international environmental treaty produced at the United Nations Conference on Environment and Development , informally known as the Earth Summit, held in Rio de Janeiro from June 3 to 14, 1992...

 (UNFCCC) for greenhouse gases and to the UNECE Convention on Long-Range Transboundary Air Pollution
Convention on Long-Range Transboundary Air Pollution
The Convention on Long-Range Transboundary Air Pollution, often abbreviated as Air Pollution or CLRTAP, is intended to protect the human environment against air pollution and to gradually reduce and prevent air pollution, including long-range transboundary air pollution.-Overview:The convention...

 (LRTAP) for air pollutants.

Examples of the second are the so-called Pollutant Release and Transfer Registers.

Policy users typically are interested in annual total emission only.

Scientific use

Air quality models
Atmospheric dispersion modeling
Atmospheric dispersion modeling is the mathematical simulation of how air pollutants disperse in the ambient atmosphere. It is performed with computer programs that solve the mathematical equations and algorithms which simulate the pollutant dispersion...

 need input to describe all air pollution sources in the study area. Air emission inventories provide this type of information. Depending on the spatial and temporal resolution of the models, the spatial and temporal resolution of the inventories frequently has to be increased beyond what is available from national emission inventories as repoprted to the international conventions and protocols.

Compilation of Emission Inventories

For each of the pollutants in the inventory emissions are typically estimated by multiplying the intensity of each relevant activity ('activity rate') in the geographical area and time span with a pollutant dependent proportionality constant ('emission factor
Emission factor
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...


Why: the source categories

To compile an emission inventory, all sources of the pollutants must be identified and quantified. Frequently used source categorisations are
Both source categorisations make a clear distinction between sources related to the combustion of (fossil) fuels and those that are not caused by combustion. In most cases the specific fuel combusted in the former is added to the source definition. Source categories include:
  1. Energy
    1. Fuel combustion
      1. Stationary combustion
        1. Industrial combustion
        2. Residential heating
      2. Mobile combustion (transport)
    2. Fugitive emissions
      Fugitive emissions
      Fugitive emissions are emissions of gases or vapors from pressurized equipment due to leaks and various other unintended or irregular releases of gases, mostly from industrial activities. As well as the economic cost of lost commodities, fugitive emissions contribute to air pollution and climate...

       from (fossil) fuel use
  2. Industrial Processes
  3. Solvent and other product use
  4. Agriculture
  5. LULUCF (Land Use, Land Use Change and Forestry)
  6. Waste

Many researchers and research projects use their own source classifications, sometimes based on either the IPCC or the SNAP source categories, but in most cases the source categories listed above will be included.

What: the pollutants

Emission inventories have been developed and still are being developed for two major groups of pollutants:
  • Greenhouse gases:
    • carbon dioxide
      Carbon dioxide
      Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

    • methane
      Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

    • nitrous oxide
      Nitrous oxide
      Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...

       (N2O) and
    • a number of fluorinated gaseous compounds (HFCs, PFCs, SF6)
    • other greenhouse gases, not included in the United Nations Framework Convention on Climate Change (UNFCCC)
  • Air pollutants:
    • Acidifying pollutants
      Acid rain
      Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...

      : sulphur dioxide (SO2), nitrogen oxides (NOx, a combination of nitrogen monoxide, NO and nitrogen dioxide
      Nitrogen dioxide
      Nitrogen dioxide is the chemical compound with the formula it is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent...

      , NO2) and ammonia
      Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

    • Photochemical smog precursors: again nitrogen oxides and non-methane volatile organic compounds (NMVOC
      NMVOC is the abbreviation for non-methane volatile organic compounds.It is a generic term for a large variety of chemically different compounds, like for example, benzene, ethanol, formaldehyde, cyclohexane, 1,1,1-trichloroethane or acetone....

    • Particulates
      Particulates – also known as particulate matter , suspended particulate matter , fine particles, and soot – are tiny subdivisions of solid matter suspended in a gas or liquid. In contrast, aerosol refers to particles and/or liquid droplets and the gas together. Sources of particulate matter can be...

       and particulate precursors
      Precursor (chemistry)
      In chemistry, a precursor is a compound that participates in the chemical reaction that produces another compound. In biochemistry, the term "precursor" is used more specifically to refer to a chemical compound preceding another in a metabolic pathway....

    • Toxic pollutants like heavy metals
      Heavy metals
      A heavy metal is a member of a loosely-defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic weight,...

       and persistent organic pollutants
    • Carbon Monoxide (CO)

Where: geographical resolution

Typically national inventories provide data summed at the national territory only. In some cases additional information on major industrial stacks ('point sources') is available.

In scientific applications, where higher resolutions are needed, geographical information such as population densities, land use or other data can provide tools to disaggregate the national level emissions to the required resolution, matching the geographical resolution of the model.

When: temporal resolution

Similarly, national emission inventories provide total emissions in a specific year, based on national statistics. In some model applications higher temporal resolutions are needed, for instance when modelling air quality problems related to road transport. In such cases data on time dependent traffic intensities (rush hours, weekends and working days, summer and winter driving patterns, etc.) can be used to establish the required higher temporal resolution.

How: methodology to compile an emission inventory

The European Environment Agency
European Environment Agency
European Environment Agency is an agency of the European Union. Its task is to provide sound, independent information on the environment. It is a major information source for those involved in developing, adopting, implementing and evaluating environmental policy, and also the general public...

 updated in 2007 the third edition of the inventory guidebook. The guidebook is prepared by the UNECE/EMEP Task Force on Emission Inventories and Projections and provides a detailed guide to the atmospheric emissions inventory methodology.
Especially for Road Transport the European Environment Agency
European Environment Agency
European Environment Agency is an agency of the European Union. Its task is to provide sound, independent information on the environment. It is a major information source for those involved in developing, adopting, implementing and evaluating environmental policy, and also the general public...

 finances COPERT
COPERT is an MS Windows software program aiming at the calculation of air pollutant emissions from road transport. The technical development of COPERT is financed by the European Environment Agency , in the framework of the activities of the European Topic Centre on Air and Climate Change...

 4, a software program to calculate emissions which will be included in official annual national inventories.

Quality of Emission Inventories

The quality of an emission inventory depends on its use. In policy applications, the inventory should comply with all what has been decided under the relevant convention. Both the UNFCCC and LRTAP conventions require an inventory to follow the quality criteria below (see ):
Criterium Description
Transparent: the assumptions and methodologies used for an inventory should be clearly explained to facilitate replication and assessment of the inventory by users of the reported information. The transparency of inventories is fundamental to the success of the process for the communication and consideration of information
Consistent: an inventory should be internally consistent in all its elements with inventories of other years. An inventory is consistent if the same methodologies are used for the base and all subsequent years and if consistent data sets are used to estimate emissions. Under certain circumstances referred to in the chapter on time series consistency (Time Series Consistency chapter of the General Guidance part of this Guidebook), an inventory using different methodologies for different years can be considered to be consistent if it has been recalculated in a transparent manner, taking into account any good practices
Comparable: estimates of emissions reported by Parties in inventories should be comparable among Parties. For this purpose, Parties should use the methodologies and formats agreed within the convention for estimating and reporting inventories
Complete: an inventory covers all sources, as well as all pollutants, included in the Convention and Protocols , as well as other existing relevant source categories which are specific to individual Parties, and therefore may not be included in the Guidebook. Completeness also means full geographic coverage of sources and sinks of a Party.
Accurate: a relative measure of the exactness of an emission estimate. Estimates should be accurate in the sense that they are systematically neither over or under true emissions, as far as can be judged, and that uncertainties are reduced as far as practicable. Appropriate methodologies conforming to guidance on good practices should be used to promote accuracy in inventories

A well constructed inventory should include enough documentation and other data to allow readers and users to understand the underlying assumptions and to assess its usability in an intended application.

See also

  • Emission factor
    Emission factor
    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...

  • Emissions & Generation Resource Integrated Database (eGRID)
    Emissions & Generation Resource Integrated Database (eGRID)
    The Emissions & Generation Resource Integrated Database is a comprehensive source of data on the environmental characteristics of almost all electric power generated in the United States. eGRID is issued by the U.S...

  • Greenhouse gas inventory
    Greenhouse gas inventory
    Greenhouse gas inventories are a type of emission inventory that are developed for a variety of reasons. Scientists use inventories of natural and anthropogenic emissions as tools when developing atmospheric models. Policy makers use inventories to develop strategies and policies for emissions...

Sources and further reading