B5a. Air pollution
1. Area affected by acidity
2. 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 is sensitive to acidification, and a third to eutrophication (with some areas 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.
- Key results
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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(n1), but there has been less change in the percentage of areas exceeding the critical load for nutrient nitrogen deposition (eutrophication). In 2017, acid deposition exceeded critical load in 39% of sensitive terrestrial habitats and nitrogen deposition exceeded critical load in 58% of sensitive habitats.
Figure B5ai. Percentage area of sensitive terrestrial UK habitats exceeding critical loads for acidification and eutrophication, 1996 to 2017 (n1).
- Each bar represents a 3-year average of deposition data, to reduce year-to-year variability.
- 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.
- The method for calculating acid-sensitive habitat area has changed since the last edition of this publication. The area of acid-sensitive habitats now excludes catchments above acid-sensitive freshwater locations, which had led to overlaps. Numbers for all preceding years have been recalculated, so results and trends presented here are internally consistent but may differ from those in previous reports.
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 2 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 is 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 39% in 2017. The short-term trend between 2012 and 2017 (using a 3-year average for years 2011 to 2013 and comparing with 2017), showed an 18% decrease in the area affected by acidity. In 2017, nitrogen deposition exceeded critical loads in 58% of sensitive habitats. This was a decrease from a level of 75% in 1996. In the short term, nitrogen deposition decreased by 8% (using a 3-year average for years 2011 to 2013 and comparing with 2017).
Based on these figures, the habitat areas at risk from acid and nitrogen deposition have declined over the long term (1996 to 2017). 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 5km x 5km 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 2020) 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 area of sensitive habitat where critical loads are exceeded for both acidity and eutrophication is 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) methodology. Since 2002 (2001-2003), 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 (2002-2004) additionally include aerosol deposition of ammonium (NH4+), nitrates (NO3-), and sulphates (SO4-). In all years, the 3-year average deposition is used to smooth substantial year-to-year variability. The deposition data sets for 2004 to 2013 were updated in 2015 following research by NERC, UK CEH and Defra and the report can be viewed on the NERC website. The research assessed the current DELTA sampler configuration’s specificity for HNO3 measurement and showed additional sampling of other atmospheric oxidised nitrogen species (HONO, N2O5, ClNO2). From the research a correction factor has been obtained and applied to the HNO3 concentrations used in the CBED mapping. The trends in critical loads exceedances for the period 2004-2006 to 2011-2013 have therefore also been updated.
Table B5aii. Percentage area of sensitive UK habitats exceeding critical loads for acidification and eutrophication for 2017 (2016 to 2018)
|Acidification (%)||Eutrophication (%)|
As new research data become available, critical loads are reviewed and updated periodically. New and revised critical loads for nutrient nitrogen were established in 2010/11. The results for all years for exceedance of nutrient nitrogen critical loads were updated in 2011 using the new/revised 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
|Defra||Trends Report 2020: Trends in critical load and critical level exceedances in the UK|
|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)|
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Last updated: October 2020
Latest data available: 2017 (i.e. 2016–2018)
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(n1) For ease of reference, time periods are usually referred to using the middle year of the 3 years used to calculate the mean. For example, “1996” refers to the time period 1995 to 1997. In figure B5ai “1996 to 2017” refers to the time period 1995-1997 to 2016-18.