Emissions of air pollutants in the UK - Background

Question 1

1.  Why quantify UK emissions of air pollutants?

Accepted Answer

Poor air quality is a local, national and international problem caused by high concentrations of air pollutants. These pollutants can either be emitted directly from a source or formed in the atmosphere through chemical reactions. Poor air quality causes damage to human health and ecosystems.

There are many sources of air pollution from human activity, including, but not limited to, power stations, transport, household heating, agriculture and industrial processes. There are also natural sources of air pollution, such as volcanic eruptions, sea spray and waste from wild animals. Air pollutants which come directly from an emission source are referred to as ‘primary pollutants’. Sometimes, pollutants can react together in the atmosphere to form further pollutants, these are referred to as ‘secondary pollutants’. Knowledge of emissions sources aids the development of strategies to reduce air pollution and its impact on the environment and our health.

The data for this publication are calculated for the National Atmospheric Emissions Inventory (NAEI). The NAEI provides estimates of the amount of the different primary pollutants emitted to the air from human activity each year in the UK.

This publication covers UK emissions of:

particulate matter (PM10 and PM2.5)
nitrogen oxides (NOx)
ammonia (NH3)
non-methane volatile organic compounds (NMVOCs)
sulphur dioxide (SO2)

The publication includes emissions estimates of all these pollutants since 1990. As improvements in estimating emissions are made, they are applied to each year of the inventory from 1990 onwards. Emissions estimates prior to 1990 are not routinely updated because there is greater uncertainty in the sources of information used before this year. However, emissions for these pollutants from prior to 1990 can be seen in the tables accompanying this publication.

Reporting on UK emissions of these pollutants is consistent with that required by the UK National Emission Ceilings Regulations (2018), which are transposed from the EU National Emission Ceilings Directive. and the Gothenburg Protocol to the UNECE Convention on Long-range Transboundary Air Pollution. Data on emissions of further air pollutants (including carbon monoxide, heavy metals and persistent organic pollutants (POPs)) are published each year on the NAEI website (see "UK Air Quality Pivot Table").

Further information on the scope of the data used in this publication, and the methodology used to estimate emissions is available in Methods and quality processes for UK air pollutant emissions statistics which is published as part of this release.

Question 2

2.  The impacts of air pollution on health and the environment

Accepted Answer

Poor air quality is the greatest environmental threat to human health and has been linked to a number of the most serious and costly challenges for our health service, including chronic obstructive pulmonary disease (COPD), heart disease and strokes. It is a particular threat to vulnerable groups, including the elderly and young children. PM2.5 is the most harmful pollutant to health, and, as well as being emitted directly, it can be formed in the atmosphere from reactions between other pollutants such as SO2, NOx, NMVOCs and NH3.

Ground-level ozone also has an impact on health. This is formed when emissions of NOx and NMVOCs react together in the atmosphere and, at higher concentrations, can cause breathing problems, trigger asthma, reduce lung function and cause lung diseases. For more information about the effects of ozone see the relevant section in the latest edition of ‘Air quality statistics in the UK’.

More detail on the health impacts of other pollutants is provided within each pollutant section of this release.

Air pollution also damages ecosystems through:

acidification (SO2, NOx and NH3) - where chemical reactions involving air pollutants create acidic compounds which when deposited on land and aquatic systems can cause harm to soils, vegetation and buildings.
eutrophication (NOx and NH3) - where nitrogen is deposited in soils and water, affecting nutrient levels and reducing the diversity of species in sensitive environments.
ground-level ozone (NOx and NMVOCs) – ozone is a toxic gas which can damage wild plants, crops, forests and some materials. Ozone is also a greenhouse gas and therefore contributes to global warming.

As each pollutant can have different impacts on health and the environment, they should not be compared to each other simply based on the relative emissions values. For example, if NOx emissions are ten times greater than PM2.5 emissions, this does not mean that NOx is ten times more important. The government guidance on Air Quality Appraisal uses damage costs to enable comparison between pollutants.

Question 3

3.  The links between emissions of air pollutants and air quality in the UK

Accepted Answer

Throughout this document ‘emissions’ is used to refer to the release of air pollutants into the atmosphere as a result of human related activity. Air quality is defined by the concentrations of air pollutants, which are driven by a number of local, national and international factors including emissions, human behaviour, weather patterns, natural sources of air pollutants and atmospheric chemical processes.

Whilst reducing UK emissions of air pollutants helps reduce atmospheric concentrations in the UK, the level of reduction in atmospheric concentrations is not always proportionate to the reduction in emissions. This is in part because of the transboundary nature of air pollution. For example, emissions of the pollutants that lead to ozone formation have reduced substantially in the UK, but this is not reflected in the long-term trend in ozone concentrations. One reason for this difference is that some of the ozone precursor pollutants in the UK come from mainland Europe and further afield. Trends in global hemispheric background concentrations of ozone also contribute to ozone concentrations in the UK.

The transboundary nature of air pollution is a key reason the UK became a founding member of the UNECE Convention on Long-Range Transboundary Air Pollution in 1979. More recently, in 2021, we co-founded the Forum for International Cooperation on Air Pollution with the Swedish Government. This forum aims to help all regions drive down poor air quality by sharing relevant scientific, technical, and policy information.

Local factors such as proximity to pollution sources also partially explain the differences in trends between emissions and concentrations of air pollutants. For example, emissions of nitrogen oxides at a national level have been decreasing in the long-term, this trend may not be observed at some roadside locations because of significant local emissions of nitrogen oxides.

Another reason that trends in emissions and concentrations may differ is that the emissions estimates only capture primary emissions (that is direct emissions from a source). However, there may be significant secondary pollutant formation (when pollutants react with each other in the atmosphere to create further pollutants) that contribute to concentrations measured at a location. For example, primary emissions of ammonia can react in the atmosphere with other substances to form secondary particulate matter.

Defra produce accredited official statistics annually in April on air quality, using pollutant concentration data as measured by the main national network of monitoring stations. This statistical release contains analysis of national levels and trends in air pollution, including the prevalence of short-term air pollution episodes. In addition, near real-time measurements of air pollutant concentrations and air quality forecasts for the UK are available from the UK-AIR website. For further information on air quality data and information, please refer to the air quality and emissions statistics GOV.UK webpage.

Question 4

4.  Emission Reduction Commitments

Accepted Answer

The amended Gothenburg Protocol to the UNECE Convention on Long Range Transboundary Air Pollution (CLRTAP) sets emission reduction commitments compared to emissions in 2005 applicable in ‘2020 and beyond’ for PM2.5, nitrogen oxides, ammonia, NMVOCs, and sulphur dioxide. The revised EU National Emission Ceilings Directive (2016/2284/EU) (NECD) implemented these emission reduction commitments for EU Member States from 2020 to 2029. The emission reduction commitments described in the NECD were implemented in UK legislation through the National Emission Ceilings Regulations (2018) (NECR). This legislation also sets out further emission reduction commitments for these pollutants which will take effect from 2030.

Nitrogen oxides (NOx)

For emissions of NOx, there are two ceilings applicable between 2020 and 2029 inclusive. The NECR ceiling is 765 thousand tonnes (which excludes emissions from the agricultural sector and is set as a 55 per cent reduction from 2005 levels) and the CLRTAP ceiling is 779 thousand tonnes of NOx emissions (which includes emissions from the agricultural sector and is set as a 55 per cent reduction from 2005 levels). In 2023 the UK was compliant with both of these commitments, with 578 thousand tonnes of emissions excluding agricultural sources and 602 thousand tonnes of emissions including agricultural sources.

Ammonia

Between 2020 and 2029, the NECR and CLRTAP ceiling for ammonia is 258 thousand tonnes, set as a reduction of 8 per cent from adjusted 2005 levels. This adjustment to the inventory, approved by the independent CLRTAP Executive Body, is applied when reporting against these commitments. Adjustments are allowed where non-compliance with the commitments results from applying improved emission inventory methods updated in accordance with scientific knowledge. Since non-manure digestate spreading was not included in the inventory when these commitments were set, emissions from this source are excluded for compliance purposes. The unadjusted annual total emissions of ammonia was 265 thousand tonnes in 2023, but when the adjustment is taken into account there were 249 thousand tonnes of ammonia emissions in 2023. Therefore, emissions decreased by 11 per cent since 2005, which means the UK was compliant with the 2020-2029 emission reduction commitments in 2023.

NMVOCs

For emissions of NMVOCs, there are two ceilings applicable between 2020 and 2029 inclusive. The NECR ceiling is 772 thousand tonnes (which excludes agricultural sources and is set at a 32 per cent reduction from 2005 levels) and the CLRTAP ceiling is 852 thousand tonnes (set at a 32 per cent reduction from 2005 levels, including agricultural sources). In 2023, the UK was compliant with both these emission reduction commitments with 751 thousand tonnes of total emissions and 623 thousand tonnes excluding agriculture.

SO2

For emissions of sulphur dioxide, there is a single annual ceiling of 321 thousand tonnes between 2020 and 2029, applicable to both the NECR and the CLRTAP (set at a 59 per cent reduction from 2005 levels). In 2023, there were 111 thousand tonnes of emissions of sulphur dioxide. Therefore, the UK was compliant with the 2020-2029 commitments under the NECR and the CLRTAP.

PM2.5

For PM2.5, there is a single annual ceiling of 74 thousand tonnes of emissions between 2020 and 2029 (set at a 30 per cent reduction from 2005 levels), applicable to both the NECR and the CLRTAP. In 2023, the UK was compliant with this commitment, with 56 thousand tonnes of PM2.5 emissions.

A table summarising the Emission Reduction Commitments (ERCs) discussed here can be found in the Summary section.

Question 5

5.  How air pollutant emissions are estimated

Accepted Answer

It is not practical, except for a limited number of large industrial processes, to measure emissions from all sources directly. Therefore, the amount of each air pollutant generated from different activities and the level of that activity in the UK are estimated based on a combination of different methods, with variable degrees of uncertainty. Where available, measured emissions are used. If there is good understanding of the activity and its emissions, highly detailed calculation methods have been developed. In other cases, the emissions from an activity are based on assumptions or representative measurements informed by expert scientific opinions. These methods and assumptions are subject to continuous review and improvement as our understanding improves and new sources of information become available. They follow internationally agreed guidance and are frequently reviewed by experts from other countries.

A quality and methods report has been produced with this release to explain how the emissions figures are estimated by each pollution source. A more detailed version, the Informative Inventory Report, is available on the NAEI website and updated annually in March. These reports also contain an assessment of uncertainty in estimating emissions from each source.