e-Digest Statistics about: Air Quality
Emissions Inventories
Emissions of greenhouse gases and air pollutants arise from a wide variety of sources. The National Atmospheric Emissions Inventory (NAEI) is prepared annually for the Government and the devolved administrations by AEA Energy & Environment, with the work being coordinated by Defra.
The inventory is updated annually and any developments in methodology are applied retrospectively to earlier years. Adjustments in the methodology are made to accommodate new technical information and to improve international comparability. The first results from the 2005 air pollutant inventory were published in March 2007. A full report on the 2005 Inventory is published by AEA Energy & Environment and released on the National Atmospheric Emissions Inventory web site. Reports of previous annual inventories may also be found there. As well as air pollutants, which are discussed in this topic, the inventory also includes the main greenhouse gases, which are reported on in more detail in the Climate Change section.
How emissions are estimated
Information from direct monitoring of emissions is available for a range of point sources, including the most significant polluters. However, a different approach has to be taken for diffuse sources such as transport and domestic emissions where this type of information is not available. Consequently, the remainder of the NAEI is compiled as estimates derived from statistical information and from research on emission factors for stationary and mobile sources.
For most pollutants, the main sources of emissions are from fossil-fuel combustion for electricity generation, heating or road vehicles. These emissions are estimated by using the official UK fuel consumption data published annually in the Digest of UK Energy Statistics. These data are then combined with the most appropriate emission factors. In most cases the methodology is complex - for example some pollutants emissions from road vehicles vary with speed, and therefore information about road usage and speed distributions in the UK must be incorporated into the emission calculation.
Other analyses of emissions on this web site
For many of the pollutants shown here, emissions are broken down by fuel type and end user, as well as by source. For estimates of emissions by end user, a simple pro rata method is used to re-allocate estimated emissions from power stations and other fuel processing industries to final users of delivered energy. This method does not take into account higher emissions from increased coal and oil-fired generation used to meet peak domestic demand for electricity. Emissions by end user are therefore subject to more uncertainty than those by source and should only be used to give a broad indication by sector.
Emissions by fuel and by end user for 2005 are shown in some tables.For nitrogen oxides and sulphur dioxide, total emissions from sources covered by the EC Large Combustion Plants (LCPs) Directive are also shown. Nitrogen dioxide emissions are based on reported emissions from 1991 onwards, and estimated from the NAEI for 1990 and earlier by assuming all power station emissions from coal, fuel oil and orimulsion stations are included, plus 69 per cent of refineries emissions, 10 per cent of iron and steel emissions and 30 per cent of other industrial combustion in fuel extraction and transformation. Sulphur dioxide emissions for LCPs for 1990 and earlier are calculated similarly and include all power station emissions from coal, fuel oil and orimulsion stations but include 47 per cent of refineries' emissions, 56 per cent of iron and steel emissions and 37 per cent of other industrial combustion in fuel extraction and transformation.
Other analyses of emissions available
Information about emissions by economic sector, on a consistent basis with the economic data held in the National Accounts, is published each year in the UK National Accounts Blue Book and can be found on the UK environmental accounts website. These estimates are based upon the NAEI but use other sources of information to allocate emissions from sources such as road transport to the sector which owns the transport. The estimates also follow as far as possible National Accounts conventions on the scope of the emissions included - this means that emissions from the international operations of UK airline, shipping and other transport operators are included, whereas they are excluded under IPCC conventions, and largely excluded under UNECE. Adjustments are also made for emissions from energy either purchased or used overseas by UK residents.
Emissions inventories are also compiled for the Devolved Administrations and emissions estimates from the National Inventory disaggregated to 1km x 1km grid squares are published on the NAEI web site.
The European Environment Agency compiles inventory information for the European Union as a whole from the inventories provided by individual member states and candidate countries.
Accuracy of emissions estimates
Quantitative estimates of the uncertainties in the emission estimates are based on calculations made using a direct simulation technique, which corresponds to the IPCC Tier 2 approach recommended for greenhouse gases, and also the methodology proposed in draft guidance produced by the UN ECE Taskforce on Emission Inventories and Projections. The methods used for estimation of uncertainties, the assumptions made, and discussion of the results have been given in detail elsewhere [13-15]. The results of the analysis are summarised in Table 4.
The uncertainty in inventories, when expressed relative to the mean value, differ widely, from ±3 per cent for sulphur dioxide to -80 per cent to +300 per cent for gamma HCH. In general the inventories for gaseous pollutants are less uncertain than the inventories for particulate matter and metals, which, in turn, are less uncertain than the inventories for persistent organic pollutants. The differences in the uncertainty for the various inventories can be explained by the level of resource used to compile the inventory and the volume and quality of activity data and emission factors which are then available. For example, the inventories for carbon dioxide, sulphur dioxide, oxides of nitrogen and NMVOC have been developed over many years and are based on large sets of data, and therefore have relatively low levels of uncertainty.
Although for any given year considerable uncertainties surround the emission estimates for each pollutant, it is important to note that trends over time are likely to be much more reliable.
Further Information:
- Data Tables:
- 1 Air pollutant types (including greenhouse gases) XLS
- 4 Uncertainty in estimates of UK total emissions of air pollutants: 2005 inventory XLS
- References, further reading and links to other resources:
- [13] Passant NR, (2003) Estimation of Uncertainties in the National Atmospheric Emissions Inventory. AEA Technology
- AEA Technology: Search the NAEI Reports Database
- [14] Salway AG, (2002) Treatment of Uncertainties for National Estimates of Greenhouse Gas Emissions. AEA Technology
- [15] Eggleston, (1998) Treatment of Uncertainties for National Estimates of Greenhouse Gas Emissions. AEA Technology
- Internet Links:
- National Atmospheric Emissions Inventory (NAEI)
- National Statistics: UK environmental accounts website
- Digest of United Kingdom Energy Statistics
- European Environment Agency
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Page last modified: 3 March 2008
Page published: 29 March 2007