Air Pollution Trends Reports
Air Pollutions Trends Reports track how air pollution policies have reduced the risk of damage to UK habitats.
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Impact of air pollution on UK habitats
Air pollution can harm habitats and reduce biodiversity. This affects the ability of habitats to:
- capture and store carbon (carbon sequestration)
- prevent flooding
In the annual Air Pollution Trends Report, Defra uses models to show the impact of ammonia, oxides of nitrogen and sulphur dioxide on UK habitats.
Models used in the report
The EMEP4UK model is used to convert official UK air pollution emissions (published in the National Atmospheric Emissions Inventory) into concentrations.
The CBED model is used to calculate deposition. Deposition is when pollution in the air is deposited onto land, water or vegetation. The model calculates deposition of:
- acidifying pollutants (which make the environment more acidic)
- eutrophying pollutants (which cause excess nitrogen in soil or water)
Modelling results are verified with monitoring data from Defra’s UK Eutrophying and Acidifying Pollutants (UKEAP) monitoring networks.
Critical loads and critical levels
Critical loads
Critical loads are thresholds that identify when deposition of air pollution is significantly harmful to habitats or species.
UK habitats have been assigned a critical load for:
- nutrient nitrogen deposition – caused mainly by ammonia and oxides of nitrogen
- acidity deposition – caused mainly by ammonia, oxides of nitrogen and sulphur dioxide
Critical levels
Critical levels are thresholds that identify when air pollution concentrations affect biodiversity.
Two critical levels have been set for ammonia concentrations.
Mosses and lichens (lower plants) can be more sensitive. An annual average concentration of 1 ug/m3 has been set as the lowest critical level.
Vascular (higher) plants are generally less sensitive and have a higher critical level of 3 ug/m3.
Mapping values for critical loads and critical levels
The Convention on Long Range Transboundary Air Pollution (CLRTAP) reviews critical loads and critical levels.
The latest review for nitrogen critical loads defined a range for several sensitive habitats.
Within this range, UK experts then assigned a critical load value for each habitat to map nitrogen deposition exceedance. These values are outlined in the Empirical critical loads for nutrient nitrogen: defining mapping values for UK habitats report.
Tracking reduced emissions
Emissions of ammonia, oxides of nitrogen and sulphur dioxide have reduced in Europe since the 1990s. This is because of statutory emissions ceilings agreed by each country through either or both the:
- National Emissions Ceilings Directive (for EU Member States)
- Gothenburg Protocol (for signatory countries to CLRTAP)
UK emissions ceilings are set by the Gothenburg Protocol.
The Air Pollution Trends Reports track how policies implemented to meet these targets are reducing pressures on ecosystems. Data helps identify the most effective ways to improve air quality and reduce impacts on UK habitats.
Previous reports
The Air Pollution Trends Report was not published in 2025.
Previous reports: