1. Methodology background

 National Statistic   
 Survey name 
 Frequency  Annual
 How compiled  Based on third party data
 Geographic coverage  UK
 Sample size 
 Last revised  28 February 2010

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2. Executive summary

This Quality and Methodology Information report relates to statistics for air emissions, which is an important topic within the UK Environmental Accounts and aims to provide users with information on the usability and fitness for purpose of these estimates.

For further information regarding the methods and terminology used in UK Environmental Accounts, please refer to the following publications:

This document contains the following sections:

  • Output quality
  • About the output
  • How the output is created
  • Validation and quality assurance
  • Sources for further information or advice
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3. Output quality

This document provides a range of information that describes the quality of the data and details any points that should be noted when using the output.

We have developed Guidelines for measuring statistical quality; these are based upon the five European Statistical System (ESS) quality dimensions. This document addresses these quality dimensions and other important quality characteristics, which are:

  • relevance
  • timeliness and punctuality
  • coherence and comparability
  • accuracy
  • accessibility and clarity

More information is provided about these quality dimensions in the following sections.

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4. About the output


(The degree to which the statistical product meets user needs for both coverage and content.)

Air emissions data published in UK Environmental Accounts provide an analysis of air pollutants broken down by emission type and industry. The maximum resolution of industrial disaggregation used for air emissions data is 93 industries.

The following air emissions datasets are published as part of the UK Environmental Accounts release:

  • emissions, atmospheric, summary
  • greenhouse gas emissions and acid rain precursors by 13 industries
  • greenhouse gas emissions by 93 industries
  • acid rain precursors by 93 industries
  • heavy metals by 93 industries
  • other air pollutants by 93 industries
  • road transport emissions by pollutants
  • 68 sectors; energy, emissions and output
  • air emissions bridge table, converting from a national accounts basis to United Nations Framework Convention on Climate Change (UNFCC)
  • air emissions bridge table, converting from a national accounts basis to United Nations Economic Commission for Europe (UNECE)

These estimates are updated annually each spring to include the most recent data published by national accounts and the National Atmospheric Emissions and Greenhouse Gas Emissions Inventories (NAEI, GHGI). The data are compiled by contractors AEA Energy and Environment, who also compile the NAEI and GHGI for the Department for Energy and Climate Change (DECC) under the UK’s Kyoto obligations.

The air emissions accounts are compiled in accordance with SEEA 2003, which closely follows national accounting standards set out in the System of National Accounts 1993 (SNA 1993).

The data cover the UK as a whole; there are currently no estimates at a regional level.

These data are used to provide information for policy makers and analysts. The main users of these air emissions data, in government, is the Department for Environment, Food and Rural Affairs (Defra) although other departments such as the Environment Agency, Department for Transport, and DECC have also used this data in their analyses. Outside government, research organisations and academia use the data in their work on environmental impacts and sustainable development.

Timeliness and punctuality

(Timeliness refers to the lapse of time between publication and the period to which the data refer. Punctuality refers to the time lag between the actual and planned dates of publication.)

Air emissions data are published each spring with an e18 month lag. For instance, 2007 data were first published in spring 2009. This allows estimates to be based on most recent supply and use tables, which incorporate latest national accounts information regarding the composition of the economy and latest AEA Energy and Environment data used in the NAEI and GHGI.

Notification of the exact date on which this data are published each year is made public approximately 3 months beforehand via the National Statistics Release calendar. To date, publication of these data has occurred without delay. In the unlikely event of a change to the pre-announced release schedule, public attention will be drawn to the change and the reasons for the change will be explained fully at the same time, as set out in the Code of Practice for Official Statistics.

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5. How the output is created

Atmospheric emissions are measured using a variety of methods dependant on the availability of data. These methods include point source emissions, which are directly measured such as those recorded by road side measuring instruments. Emissions are also estimated using conversion factors, which link fuel use with its associated emissions; in other circumstances emissions data may be modelled using information from other sources such as economic supply and use tables, and input and output tables.

There are a wide range of pollutants that contribute emissions to the atmosphere. They include greenhouse gases and substances that are directly toxic such as heavy metals. These pollutants can be grouped according to their contribution to environmental themes such as climate change and acid rain.

Greenhouse gases

The UK is required to report emissions under different international agreements for main air pollutants covered by National Atmospheric Emissions Inventory (NAEI) and greenhouse gases (GHG) covered by the UK GHG inventory. The National Environmental Technology Centre (AEA Energy and Environment) maintains the NAEI.

Changes in air emissions are compared to 1990 as this is the base year used in the Kyoto Protocol.

Although most greenhouse gas occurs naturally, some are man-made. Since the industrial revolution, human activity has led to an increase in both the natural and man-made gases, especially carbon dioxide. There is growing consensus that the rise in greenhouse gas emissions has led to changes in the global atmosphere, so called global warming. The greenhouse gases included in the air emissions accounts are those covered by the Kyoto Protocol.

Greenhouse gases included are:

  • carbon dioxide
  • methane
  • nitrous oxide
  • hydrofluorocarbons
  • perfluorocarbons
  • sulphur hexafluoride

To aggregate the greenhouse gases covered in the accounts, a weighting based on the relative Global Warming Potential (GWP) of each of the gases is applied, using the effect of carbon dioxide over a 100-year period as a reference. This gives methane a weight of 21 relative to carbon dioxide and nitrous oxide a weight of 310 relative to carbon dioxide. Sulphur hexafluoride has a GWP of 23,900 relative to carbon dioxide. The GWP of the other fluorinated compounds varies according to the individual gas.

ONS air emissions accounts show greenhouse gas emissions in terms of carbon equivalent rather than carbon dioxide equivalent. To convert from carbon dioxide equivalent to carbon equivalent it is necessary to multiply by 12/44. This is the ratio of the atomic weights of carbon and carbon dioxide. Conversion factors for greenhouse gas emissions are recommended by the Intergovernmental Panel on Climate Change (IPCC) in their second assessment report.

Acid rain precursors

The term “acid rain” describes the various chemical reactions acidic gases and particles undergo in the atmosphere before being deposited as wet or dry deposition. When deposited the hydrogen ions may be released causing acidification. These dilute acids damage ecosystems and buildings.

The emissions are weighted together using their relative acidifying effects. The weights, given relative to sulphur dioxide, are 0.7 for nitrogen oxides and 1.9 for ammonia. This is a simplification of the chemistry involved and there are a number of factors that can affect the eventual deposition and effect of acid rain. There may be an upward bias on the weights of the nitrogen- based compounds in terms of damage to ecosystems.

The gases covered are sulphur dioxide, nitrogen oxides and ammonia.

Heavy metals

Emissions of heavy metals can affect human and animal health. In particular, the immune, metabolic and nervous systems are affected. Some heavy metals are suspected or have been proven to be carcinogenic. Heavy metals impact on the environment in areas such as aquatic and terrestrial ecosystems, metals may accumulate in mesofauna and macrofauna such as birds and mammals and agricultural products through heavy metal input to arable soils.

Heavy metals included are:

  • arsenic
  • cadmium
  • chromium
  • copper
  • mercury
  • lead
  • nickel
  • selenium
  • zinc

Other pollutants

Other pollutants included are benzene, non-methane volatile organic compounds (NMVOC) and 1,3 butadiene, which are considered carcinogenic. Carbon monoxide is toxic in high concentrations and PM10s. PM10s are smoke particles, less than 10 microns in diameter that have been linked to physiological damage.

Attributing emissions to industrial sectors

NAEI projections of future emissions are an increasingly important requirement for UK government policy-making. National estimates of emissions are calculated across all economic sectors, for example, industry, domestic use. The disaggregation of national estimates of emissions to industrial sectors is based upon an initial disaggregation provided by AEA Energy and Environment.

Emissions are estimated by multiplying fuel consumption by emissions factors and adding releases unrelated to fuel use such as methane arising from landfill and collieries.

The NAEI data are used to identify the main processes and industries responsible for the emissions. These are then allocated to individual sectors on the basis of information from a variety of sources. For example, emissions from diesel use by Heavy Goods Vehicles are allocated to sectors using vehicle mileage data from Department of Environment, Food and Rural Affairs (Defra). Expenditure information is also used, for example, emissions arising from the use of various industrial coatings (such as general industrial, heavy duty and vehicle refinishing) are allocated to relevant sectors in proportion to each sector’s expenditure on paints, varnishes and similar coatings, printing ink and mastics, using national accounts input and output supply and use tables as the main source.

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6. Validation and quality assurance


(The closeness between an estimated result and the (unknown) true value.)

ONS air emissions data are produced by contractors AEA Energy and Environment who use inventory data and a range of other sources, including national accounts supply and use tables, as a framework for allocating emissions to industries. Therefore, two factors impact on the accuracy of UK Environmental Accounts air emissions data – economic (that is, the accuracy of the allocation of emissions to particular industries) and the accuracy of the emissions factors used.

No work has been done to collate the impact of these different factors on the accuracy of estimates of energy use. However, one dimension of accuracy is reliability, which can be estimated by measuring revisions to previously published data. Table 1 shows the extent to which new information and methodology changes have caused data revisions between more recent and earlier publications.

Coherence and comparability

(Coherence is the degree to which data that are derived from different sources or methods, but refer to the same topic, are similar. Comparability is the degree to which data can be compared over time and domain, for example, geographic level.)

Each year the UK Environmental Accounts present estimates of pollutants directly emitted to the atmosphere by each industrial sector. The figures are on a National Accounts basis – they include emissions generated by UK households and businesses in the UK and emissions from UK residents' transport and travel activities abroad. They exclude emissions generated by non- residents transport and travel in the UK.

These data are therefore on a different basis from estimates published by Department of Energy and Climate Change (DECC) under the UK's Kyoto Protocol obligations. The Kyoto basis covers emissions from UK territory only and excludes emissions from international aviation and shipping. Two bridge tables, available on the UK Environmental Accounts page, show the transformation process from National Accounts to territorial basis.

Methodological improvements are made to the data each year as part of an ongoing process of review and are then applied to the whole time series. Therefore, data are consistent within each publication but not from year to year. These improvements involve data relating to use of specific emissions factors and sources, as well as incorporation of National Accounts and other data used to establish the distribution of air emissions across the industries.

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7. Source for further information or advice

Accessibility and clarity

(Accessibility is the ease with which users are able to access the data, also reflecting the format(s) in which the data are available and the availability of supporting information. Clarity refers to the quality and sufficiency of the metadata, illustrations and accompanying advice.)

Air emissions data and text are presented in as clear a way as possible to ensure access to as wide a range of readers as possible. Plain English definitions are given where technical terms are unavoidable.

An overview of UK Environmental Accounts can be found at Focus on Environmental Accounts.

Tables, analysis and documentation are published in the spring and autumn editions of the UK Environmental Accounts.

Tables, analysis and documentation are also published in UK National Accounts, The Blue Book.

Our recommended format for accessible content is a combination of HTML web pages for narrative, charts and graphs, with data being provided in usable formats such as CSV and Excel. We also offer users the option to download the narrative in PDF format. In some instances other software may be used, or may be available on request. Available formats for content published on our website but not produced by us, or referenced on our website but stored elsewhere, may vary. For further information please contact us by email to environment.accounts@ons.gov.uk.

General queries on air emissions data, compilation methods, quality information or difficulties finding the latest figures can be emailed to environment.accounts@ons.gov.uk.

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Contact details for this Methodology