Overview of air pollutants 

Emissions of air quality pollutants in the UK contribute to both local and transboundary air pollution. 'Transboundary air pollution' refers to pollution transported in the atmosphere from one country or region to another and often undergoing chemical transformation in the process.  It is not generally possible to distinguish the contribution of individual emission sources or groups of sources to their impacts. Acidification, eutrophication and ground level ozone are types of transboundary air pollution, arising from the emissions of the following air quality pollutants:

  • Nitrogen oxides
  • Sulphur dioxide
  • Hydrogen chloride
  • Non-methane volatile organic compounds
  • Ammonia
  • Hydrogen fluoride

Concerns about acidifying pollutants arise as the deposition of acidifying species can have adverse effects on buildings and vegetation, as well as acidifying streams and lakes and damaging the aquatic environment. Eutrophying pollutants enrich soils or rivers and lakes through rain with higher levels of nitrogen, thereby disturbing the natural balance of nutrient levels and diversity of species in sensitive environments.  Ground level ozone precursors play a key role in ozone formation and ozone can affect human health and damage plants and crops.

Particulate matter (PM) is also formed from the long-range transport of precursor gases forming PM through atmospheric processes.

Local air quality can also be affected by emissions of air quality pollutants. High concentrations of air pollutants are known to be harmful to human health and the environment. In addition to the pollutants which affect transboundary pollution, the concentration of the following pollutants are monitored under the Air Quality Directive and Air Quality Strategy for England, Scotland, Wales and Northern Ireland (PDF):

  • Particulate Matter
  • Lead
  • Benzene
  • Carbon monoxide
  • Ozone
  • Polycyclic Aromatic Hydrocarbons (PAHs)
  • Cadmium
  • Arsenic
  • Nickel
  • Mercury
  • 1,3 – Butadiene

Important sources and trends

Sources which account for high shares of emissions differ from pollutant to pollutant. However, in general terms the combustion of solid and liquid fuels (coal, wood, oil) tend to produce the highest emissions per unit of fuel burned as well as specific industrial processes for specific pollutants. Agriculture is the dominant source of ammonia emissions.  Over the years, emissions from solid fuel combustion have declined significantly, e.g. emissions from coal combustion in the residential sector. In contrast the strong increase in the use of transportation and road traffic, an important source for many air quality and transboundary pollutants, has resulted in the increase in emissions from the combustion of petrol and diesel, although this has been counteracted by improvements in emissions from individual vehicles due to tighter vehicle emission regulations.

Emissions from all pollutants have decreased since the earliest year the data is available from (1970, 1980, 1990 and 2000, depending on pollutant) partly as a result of policies put in place to control certain emissions and a decline in the use of solid and liquid fuels in the domestic and power generation sectors. Emission trends of selected pollutants (NOx, SO2, NMVOC, NH3, PM10, and CO) from key source categories since 1980 are summarised in the latest UK Informative Inventory Report in the Reports section. Further information and analysis on the emission trends of all pollutants reported under the CLRTAP are available in the latest NAEI Annual Report also found in the Report section.

Acidification and eutrophication pollutants

In the UK, sulphur dioxide (SO2) and nitrogen oxides (NOx) emissions arise primarily as a result of fuel combustion. For SO2 emissions the two main sources are combustion of solid fuel and petroleum products. Power stations are a key source, typically responsible for up to 50%. Hydrogen fluoride and hydrogen chloride emissions also primarily arise from coal combustion in power generation. Road transport is currently the largest source of NOx in the UK with stationary combustion and power generation also forming significant sources. Approximately one third of the UK NOx emissions arise from road transport, with diesel vehicles contributing most.

Compared to NOx and SO2, the contribution of emissions from combustion sources is much lower for Non-Methane Volatile Organic Compounds (NMVOC) and ammonia (NH3). The diversity of processes which emit NMVOCs is huge, covering not only many branches of industry using solvents and through losses of fuel (evaporation), but also transport, agriculture and domestic sources. Emissions from solvent and other product use both in industrial and domestic application are responsible for the largest share, typically around 40%. Ammonia emissions are dominated by agricultural sources with emissions from livestock production and their wastes comprising two-thirds of the total emission. These emissions derive mainly from the decomposition of urea in animal wastes and uric acid in poultry wastes.

Ground level ozone precursors

Ozone is not emitted directly, but is formed in the atmosphere over a large spatial scale by the sunlight-initiated oxidation of VOCs and CO in the presence of NOx. Estimating ozone concentrations in the troposphere requires the use of atmospheric models and information on emissions of ozone precursors. The key emission sources of the ozone precursor pollutants (NOx, NMVOCs and CO) are discussed in the above and later sections.

Local air quality pollutants

Concentrations of particulate matter (PM) in the atmosphere arise from direct emissions of PM from a wide range of sources such as fuel combustion, surface erosion and windblown dusts and mechanical break-up processes, for example, in quarrying and construction sites. Further concentrations of PM are produced from the the interaction of NOx, SO2, NH3 and NMVOCs in the atmosphere.  These are important precursors to secondary PM.

In the UK, road transport, production processes and residential combustion plants are the most important sources of primary PM10 - each typically contributing between 15-20% to total emissions.

The most important source of CO is road transport and in particular petrol driven vehicles under slow, urban conditions.

Emissions of 1,3-butadiene arise from the combustion of petroleum products (which are predominantly used in transport) and their manufacture and use in the chemical industry. 1,3-butadiene emissions also arise from certain processes in the chemical and manufacturing industries.

Benzene emissions arise predominantly from the evaporation and combustion of petroleum products. Emissions from road transport are the most significant source of benzene, accounting for approximately a third of the UK total, whilst typically <15% comes from residential combustion.

Heavy metals

Heavy metal emissions arise from a number of different sources, but in general fuel combustion and certain industrial processes that produce dust are the main contributors. Emissions also arise from trace concentrations in the fuels or in the case of industrial processes, the raw materials. In combustion, metals are emitted either as vapour or particulate matter or both. Volatile metals such as mercury and selenium are mostly emitted as vapour. Metals such as cadmium and lead are emitted as both with some of the vapour condensing onto the ash particles. Other metals such as chromium do not vaporise and may be emitted in the ash particles.

Polycyclic Aromatic Hydrocarbons (PAHs)

Residential combustion is currently the largest source of PAH emissions contributing to nearly two thirds of the emissions of the 16 PAH species included in the inventory. The second largest source is the road transport sector.

More information on important emissions sources for each pollutant and the latest figures can be found  in the NAEI Annual report in the Reports section.

Targets

The UK is committed to reducing transboundary air pollution through emission targets set under the UNECE's Convention on Long-Range Transboundary Air Pollution (CLRTAP) and the National Emissions Ceilings Directive (NECD).

Page last modified: 28 January 2013