Skip to Content

B5a. Air pollution

i. Area affected by acidity

ii. Area affected by nitrogen

Type: Pressure Indicator

This indicator was updated in 2023.

Introduction

This indicator shows the proportion of sensitive habitat that is affected by two major impacts of air pollution: (i) acidification; and (ii) the excessive richness of nitrogen (eutrophication). 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, "2020" means the period 2019 to 2021.

Key results

There have been two changes to the methods used to prepare the 2023 update of this indicator (see the ‘Background’ section and ‘Technical Background Document’ for details of these changes). Applying these new methods, in 2020, acid deposition exceeded critical load in 45% of sensitive terrestrial habitats, a 32% decrease from 2003. Also in 2020, nitrogen deposition exceeded critical load in 86% of sensitive habitats, an 8% decrease from 2003.

Figure B5ai. Percentage area of sensitive terrestrial UK habitats exceeding critical loads for acidification and eutrophication, 2003 to 2020

Figure B5ai is a bar chart showing the percentage of area of sensitive habitats in the UK where critical loads for acidification and eutrophication (nutrient nitrogen) were exceeded, on an annual basis between 2003 and 2020. For acidification, the percentage has fallen from 66% in 1996  to 46% in 2020. During the same period, the percentage for eutrophication has fallen from 94% to 86%.

Notes about Figure B5ai:

  1. 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, “2020” means the period 2019 to 2021.
  2. Changes to the methodology mean that the time-series could be extended back to 2003, 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

Improving

2003–2020

Improving

2015–2020

Decreased

(2020)

Area affected by nitrogen

Improving

2003–2020

Improving

2015–2020

Little or no change

(2020)

Notes for Assessment of Change table:

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 2003, acid deposition exceeded critical loads in 65.6% of the UK area of sensitive terrestrial habitats. This declined to 44.5% in 2020. The short-term trend between 2015 and 2020 showed a 26.2% decrease in the area affected by acidity. In 2020, nitrogen deposition exceeded the critical load for eutrophication in 85.9% of sensitive habitats. This was a decrease from a level of 93.8% in 2003. In the short term, the area where nitrogen deposition exceeded critical load decreased by 8.8% between 2015 and 2020.

Based on these figures, the habitat areas at risk from acid and nitrogen deposition have declined over the longer term (2003 to 2020). 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.

Top

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.

Top

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 changes to the underlying methodology for the 2023 update of this indicator.

  1. Critical loads for nutrient nitrogen were reviewed and revised in 2022, and the new values have been applied in 2023.
  2. A calibration step has been introduced to ensure that outputs from the atmospheric chemistry and transport model used to estimate ammonia concentration are closely matched to measured concentrations from the UK Ammonia Monitoring Network.

Values have been recalculated for previous years back to 2003 (2002 to 2004) using the revised methods, to allow time series to be reported with consistent methodology.

The values of the metrics reported in the 2023 update of this indicator have changed, in some cases considerably, from those reported in the 2022 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 approximately 4% lower than those reported under the old methodology. Exceedance statistics for eutrophication are approximately 20% higher, due the more precautionary values for empirical critical loads agreed in the 2022 review.

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 2020

 

Acidification (%)

Eutrophication (%)

UK

44.5

85.9

England

66.5

99.9

Wales

70.1

99.1

Scotland

28.9

75.8

Northern Ireland

72.8

100.0

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 Air Pollution Trends Report 2022 and on the Air Pollution Information System (APIS) website.

Top

Goals and Targets

The UK and England Biodiversity Indicators are currently being assessed alongside the Environment Improvement Plan Targets, and the new Kunming-Montreal Global Biodiversity Framework Targets, when this work has been completed the references to Biodiversity 2020 and the Aichi Global Biodiversity Framework Targets will be updated.

Aichi Targets for which this is a primary indicator

Strategic Goal B. Reduce the direct pressures on biodiversity and promote sustainable use.

Aichi Target 8 iconTarget 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.

Aichi Target 10 iconTarget 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.

Top

Top

Downloads

Download the Datasheet and Technical background document from JNCC's Resource Hub.

Top

Last updated: November 2023

Latest data available: 2020 (average of 2019 to 2021)

 

This content is available on request as a pdf in non-accessible format. If you wish for a copy please go to the enquiries page.

Categories:

UK Biodiversity Indicators 2023

Published: .

Back to top