B5a. Air pollution
i. Area affected by acidity
ii. Area affected by nitrogen
Type: Pressure Indicator
The air pollutants sulphur dioxide, nitrogen oxides and ammonia can contribute to acidification; nitrogen oxides and ammonia can also contribute to terrestrial eutrophication. These pollutants arise mainly from burning fossil fuels and from livestock waste. Around a third of UK land area (78,000 km2) is sensitive to acidification, and a third (73,000 km2) to eutrophication (much of this area is sensitive to both). Critical loads are thresholds for pollutant load above which significant harmful effects may occur on sensitive habitats; statistics on critical load exceedance indicate the risk of damage.
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The percentage of sensitive terrestrial habitat areas in the UK exceeding the critical load for acidification has continued to decline since 1996, but there has been less change in the percentage of areas exceeding the critical load for nutrient nitrogen deposition (eutrophication). In 2018, acid deposition exceeded critical load in 40% of sensitive terrestrial habitats and nitrogen deposition exceeded critical load in 59% of sensitive habitats.
Figure B5ai. Percentage area of sensitive terrestrial UK habitats exceeding critical loads for acidification and eutrophication, 1996 to 2018.
- Each bar represents a three-year average of deposition data. To reduce the effects of variation in meteorology, exceedance statistics are reported as the mean of three years of data. In Figure B5ai, time periods are referred to using the middle year of the three. For example, “1996” means the period 1995 to 1997.
- Since 2002, nitric acid has been included in the estimates of nitrogen deposition. This additional deposition led to some increases in critical load exceedance compared with earlier periods.
- There are a few inconsistencies between years due to changes in the methods used to derive deposition estimates, and some minor alterations to the acidity critical loads. This information should be taken into account when interpreting the trends' results.
Source: UK Centre for Ecology & Hydrology.
Assessment of change in area of sensitive habitat exceeding critical loads
|Long term||Short term||Latest year|
|Area affected by acidity||
|Area affected by nitrogen||
Note: Long- and short-term assessments are based on a direct comparison of the two relevant data points, using a 3% rule of thumb. See Assessing Indicators.
Critical loads are thresholds for the deposition of pollutants causing acidification and/or eutrophication above which significant harmful effects on sensitive habitats may occur. Approximately 70,000km2 of UK terrestrial habitats are sensitive to acid deposition. About 73,000km2 is sensitive to eutrophication; much of this is sensitive to both.
In 1996, acid deposition exceeded critical loads in 77% of the UK area of sensitive terrestrial habitats. This declined to 40% in 2018. The short-term trend between 2013 and 2018, showed a 14% decrease in the area affected by acidity. In 2018, nitrogen deposition exceeded critical loads in 59% of sensitive habitats. This was a decrease from a level of 75% in 1996. In the short term, the area where nitrogen deposition exceeded critical load decreased by 5% between 2013 and 2018.
Based on these figures, the habitat areas at risk from acid and nitrogen deposition have declined over the long term (1996 to 2018). However, reducing deposition below the critical loads does not necessarily mean that ecosystems will recover immediately, as there can be a time-lag before the chemical environment and the flora and fauna recover.
The air pollutants sulphur dioxide, nitrogen oxides and ammonia can contribute to acidification, and nitrogen oxides and ammonia can contribute to terrestrial eutrophication, both of which adversely affect semi-natural ecosystems. Exceeding the critical load for acid deposition is likely to cause low soil pH and high aluminium availability, making the habitat unsuitable for many species. Excess nitrogen as a nutrient can also affect species composition, for example, by triggering accelerated growth of some species at the expense of others.
Critical loads are thresholds above which significant harmful effects on sensitive habitats may occur, according to current levels of scientific understanding. Critical loads have been established separately for nutrient nitrogen (eutrophication effects) and for acidification. The pollutants causing acidification and eutrophication mainly arise as a result of emissions from burning fossil fuels in industry and road transport, and from livestock waste.
There are three main steps in the assessment of the area of sensitive habitat that exceeds critical loads:
- calculation of critical loads for each of the sensitive habitats;
- mapping of the habitats; and
- identification of the area of habitat where deposition exceeds the critical load.
Critical loads for acidity and nutrient nitrogen are calculated for 14 broad habitats (Table B5ai) considered sensitive to acidification and/or eutrophication. Different methods have been used to calculate critical loads, based either on empirical (observational or experimental) evidence or on mass-balance (input/output) data. To identify the area exceeding critical loads, deposition maps based on a 5 km x 5 km grid covering the UK are produced based on the sum of wet deposition, dry deposition and cloud deposition. These deposition data are overlain on maps of critical loads for each habitat to calculate critical load exceedances and the areas of habitat exceeded. Critical loads data for freshwaters (not reported here; see Trends Report 2021 are available for 1,752 sites selected across the UK where water samples have been collected and analysed – these data do not provide complete UK coverage. The critical loads data for all the other habitats listed are based on national-scale habitat distribution maps.
Table B5ai. The 14 habitats considered sensitive to acidification and/or eutrophication for which critical loads are calculated
|Dwarf shrub heath|
|Coniferous woodland (managed)|
|Broadleaved woodland (managed)|
|Beech woodland (unmanaged)|
|Oak woodland on acid soil (unmanaged)|
|Scots pine (unmanaged)|
|Other unmanaged woodland|
|Dune grassland (eutrophication only)|
|Saltmarsh (eutrophication only)|
|Freshwaters (acidification only)|
In general, the areas of sensitive habitat where critical loads are exceeded for acidity and for eutrophication are lower in Scotland than elsewhere in the UK (Table B5aii); this is because levels of deposition are generally lower in Scotland. Further information on how critical loads are calculated and detailed critical load exceedance maps are available on the Critical Loads and Dynamic Modelling website.
The trends in critical loads exceedances use deposition maps based on the CBED Concentration Based Estimated Deposition (CBED) methodology. Since 2002 (2001 to 2003), the inclusion of nitric acid deposition in the assessment has increased the area of estimated critical load exceedance compared to earlier periods. The deposition values from 2003 (2002 to 2004) additionally include aerosol deposition of ammonium (NH4+), nitrates (NO3-), and sulphates (SO4-). In all years, the three-year average deposition is used to smooth substantial year-to-year variability.
Table B5aii. Percentage area of sensitive UK habitats exceeding critical loads for acidification and eutrophication for 2018 (2017 to 2019)
|Acidification (%)||Eutrophication (%)|
As new research data become available, critical loads are reviewed and updated periodically. Critical loads for nutrient nitrogen were established in 2003, and revised in 2011. For all years, exceedance is calculated using the 2011 values for nutrient nitrogen critical loads. Details of the revision can be found in the 2011 UK Status Report and the 2015 Methods Report, available on the Critical Loads and Dynamic Modelling website. The results for exceedance of acidity critical loads remain unchanged from those published earlier.
Critical loads for acidification and nutrient nitrogen have been applied to interest features of protected sites (Special Areas of Conservation, Special Protection Areas and Areas/Sites of Special Scientific Interest). Further information on critical load exceedance on protected sites is available on the Air Pollution Information System (APIS) website.
Goals and Targets
Aichi Targets for which this is a primary indicator
Strategic Goal B. Reduce the direct pressures on biodiversity and promote sustainable use.
Target 8: By 2020, pollution, including from excess nutrients, has been brought to levels that are not detrimental to ecosystem function and biodiversity.
Aichi Targets for which this is a relevant indicator
Strategic Goal B. Reduce the direct pressures on biodiversity and promote sustainable use.
Target 10: By 2015, the multiple anthropogenic pressures on coral reefs, and other vulnerable ecosystems impacted by climate change or ocean acidification are minimized, so as to maintain their integrity and functioning.
Web links for further information
|UK Centre for Ecology & Hydrology||Critical Loads and Dynamic Modelling|
|UK conservation bodies/environment agencies/UK CEH||Air Pollution Information System|
|United Nations||Convention on Long Range Transboundary Air Pollution|
|German Environment Agency (UBA)||Critical Loads Coordination Centre for Effects (CCE)|
|Natural Environment Research Council (NERC)||2015 Development of a new model DELTA sampler and assessment of potential sampling artefacts in the UKEAP AGANet DELTA system: summary and technical report|
|European Environment Agency||Annual air quality report (PDF, 18.4 Mb)|
Download the Datasheet and Technical background document from JNCC's Resource Hub.
Last updated: October 2021
Latest data available: 2018 (i.e. 2017–2019)
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