B5a. Air pollution
i. Area affected by acidity
ii. Area affected by nitrogen
Type: Pressure Indicator
Introduction
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 livestock waste and from burning fossil fuels in industry and road transport. More than a third (36%) of UK land area (91,000 km2) is sensitive to acidification, and 38% (94,000 km2) is sensitive to nutrient nitrogen (eutrophication); many areas (almost 72,000 km2) are 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. To reduce the effects of variation in meteorology, exceedance statistics are reported as the mean of three years of data and time periods are referred to using the middle year of the three. For example, “2019” means the period 2018 to 2020.
Key results
There have been two major changes to the methodology used to prepare the 2022 update of this indicator (see the ‘Background’ section and ‘Technical Background Document’ for details of these changes). Applying these new methods there was a 16% decrease in the percentage of sensitive terrestrial habitat areas in the UK exceeding the critical load for acidification between 2010 and 2019, and a 9% decrease in the percentage of areas exceeding the critical load for nutrient nitrogen deposition. In 2019, acid deposition exceeded critical load in 53% of sensitive terrestrial habitats and nitrogen deposition exceeded critical load in 68% of sensitive habitats.
Figure B5ai. Percentage area of sensitive terrestrial UK habitats exceeding critical loads for acidification and eutrophication, 2010 to 2019.
Notes:
- Each bar represents a rolling three-year average of deposition data. To reduce the effects of year-to-year 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, “2019” means the period 2018 to 2020.
- Changes to the methodology mean that the time-series could only be extended back to 2010, rather than to 1996 as in previous publications (see ‘Background’ section). These changes also mean that the chart presented here cannot be directly compared to those presented in previous publications of this indicator.
Source: UK Centre for Ecology & Hydrology.
Assessment of change in area of sensitive terrestrial UK habitat exceeding critical loads
| Long term | Short term | Latest year | |
| Area affected by acidity |
|
|
No change (2019) |
| Area affected by nitrogen |
|
|
No change (2019) |
Note: It has not been possible to calculate a long-term assessment for this indicator in the 2022 update. A change in methods meant that the time-series could only be extended back to 2010, rather than to 1996 as in previous publications. 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 91,000 km2 of UK terrestrial habitats are sensitive to acid deposition. About 94,000 km2 is sensitive to eutrophication; many areas (almost 72,000 km2) are sensitive to both.
In 2010, acid deposition exceeded critical loads in 63.3% of the UK area of sensitive terrestrial habitats. This declined to 52.9% in 2019. The short-term trend between 2014 and 2019 showed a 17.3% decrease in the area affected by acidity. In 2019, nitrogen deposition exceeded the critical load for eutrophication in 67.7% of sensitive habitats. This was a decrease from a level of 74.4% in 2010. In the short term, the area where nitrogen deposition exceeded critical load decreased by 10.5% between 2014 and 2019.
Based on these figures, the habitat areas at risk from acid and nitrogen deposition have declined over the longer term (2010 to 2019). 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.
Relevance
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. These species and habitats tend to be of high conservation value and are often the first ones to be affected. This in turn can lead to loss of ecosystem function.
Background
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 acidification and nutrient nitrogen (eutrophication effects). The pollutants causing acidification and eutrophication mainly arise as a result of emissions from livestock waste and from burning fossil fuels in industry and road transport.
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.
While these three main steps remain valid, there have been two major changes to the underlying methodology for the 2022 update of this indicator.
First, the habitat maps used to assess broad-scale sensitivity to air pollution have been updated. This has resulted in some changes to the 14 habitat categories used in the indicator (see Table B5ai) and a 28% increase in the total mapped area of habitats sensitive to nutrient nitrogen in the UK (see section 1.1.1 of the Trends Report 2022 for further details).
Second, the atmospheric chemistry and transport model used to estimate ammonia concentration and deposition loads of nitrogen and sulphur has been updated. This new European Monitoring and Evaluation Programme (EMEP) model is considered to provide more accurate estimates of ammonia concentration and therefore allow more accurate calculations of critical load exceedances than the previous UK Fine Resolution Atmospheric Multi-pollutant Exchange (FRAME) model (see sections 1.2 and 4.2 of the Trends Report 2022 for further details).
Values have been recalculated for previous years back to 2010 (2009 to 2011) using the new maps and modelled chemistry, to allow time series to be reported with consistent methodology; however, it will not be possible to present a long-term assessment until the 2020 (2019 to 2021) data are available. The values of the metrics reported in the 2022 update of this indicator have changed, in some cases considerably, from those reported in the 2021 update, but the trends in air pollution pressure over time are similar using the old and new methods. For example, in the UK as a whole, exceedance statistics for acidification reported under the new methodology are between 27% and 44% higher than those reported under the old methodology, and exceedance statistics for eutrophication are between 14% and 23% higher. Country-level data highlight disparity in the changes to figures reported under the old and new methodologies, with individual exceedance statistics for acidification increasing by up to 88% in Scotland and decreasing by up to 9% in Wales (see Tables 2 and 3 in the technical background document for a full summary of changes).
Table B5ai. The 14 habitats considered sensitive to acidification and/or eutrophication for which critical loads are calculated.
| Habitat | Critical loads calculated for acidification | Critical loads calculated for eutrophication |
| Acid grassland | Yes | Yes |
| Calcareous grassland | Yes | Yes |
| Dwarf shrub heath | Yes | Yes |
| Bog | Yes | Yes |
| Montane | Yes | Yes |
| Coniferous woodland | Yes | Yes |
| Beech woodland | No | Yes |
| Oak woodland on acid soil | No | Yes |
| Scots pine | No | Yes |
| Dune grassland | No | Yes |
| Saltmarsh | No | Yes |
| Mixed Woodland | No | Yes |
| Freshwaters (1752 sites) | Yes | No |
| Broadleaved and mixed woodland | Yes | No |
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 in the technical background document and on the Critical Loads and Dynamic Modelling website.
Table B5aii. Percentage area of sensitive UK habitats exceeding critical loads for acidification and eutrophication by country for 2019.
| Acidification (%) | Eutrophication (%) | |
| UK | 52.9 | 67.7 |
| England | 72.9 | 97.0 |
| Wales | 77.8 | 93.1 |
| Scotland | 38.2 | 47.0 |
| Northern Ireland | 84.2 | 98.6 |
Critical loads for acidification and nutrient nitrogen have also 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 in the Trends Report 2022 and 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
| Reference | Title |
| Defra | |
| 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) |
| European Environment Agency | Annual air quality report (PDF, 18.4 Mb) |
Downloads
Download the Datasheet and Technical background document from JNCC's Resource Hub.
Last updated: December 2022
Latest data available: 2019 (average of 2018 to 2020)
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