e-Digest Statistics about: Air Quality
Acid deposition
The main precursors of acid deposition are emissions of SO2, NOx and NH3. This deposition consists of both wet processes (through polluted rainfall) and dry processes (by removal of gases and particles from the atmosphere at the land surface) and can occur hundreds of kilometres away from the source of emissions. Some acid deposition is neutralized by the deposition of base cations, principally calcium and magnesium, and so any assessment of effects in an ecosystem has to determine the balance between acidic and neutralizing atmospheric inputs.
In the UK, wet deposition is monitored at 38 sites across the country through the collection and analysis of rainfall samples using bulk collectors. Measurements from five of these sites form the UK contribution to the European Monitoring and Evaluation Programme (EMEP) network. The rainfall sample is used to estimate rain ion concentrations at each site and also, for sulphur and base cations, to identify the proportion of the concentration coming from sea salt as opposed to other, generally anthropogenic, pollutant emissions. Along with rainfall maps for the UK, these data are used to estimate the wet deposition of non-seasalt sulphate (SO42-), nitrate (NO3-), ammonium (NH4+) and non-seasalt base cations (Ca2+, Mg2+) across the country. Cloud droplet deposition, the removal by vegetation of fine mist droplets (for example when a high elevation forest plantation is covered by hill fog), is generally a small component of acid deposition but is also estimated from the rain ion concentrations.
Dry deposition of sulphur and nitrogen to vegetation and other surfaces is a more complex process, with the possibility of uptake of gases by several pathways such as through stomatal pores, onto plant surfaces and by absorption in leaf water films. Gas concentrations (SO2, NO2, NH3, HNO3) are monitored at varying numbers of rural sites across the UK and these data are input to models of surface-atmosphere exchange to derive estimates of the dry deposition of sulphur and nitrogen. A similar procedure is used to map the deposition of base cation particles.
Deposition across the UK is currently mapped on a 5km X 5km grid square basis by interpolating maps from the measured pollutant concentrations and using models that also incorporate spatial information on meteorology and land use [44]. These maps do not provide deposition estimates to point locations or small areas, which will often be substantially different from the average value in the 5km square, as the major causes of this local variation are not represented within the national mapping programme.
Maps of annual mean total acid deposition (i.e. non-marine sulphur plus oxidised and reduced nitrogen) to specific ecosystems for the years 2002-2004 are shown in Figure 17a and Figure 17b. The mapped values are the sum of wet, dry and cloud droplet deposition. Figure 17a shows the total acid deposition assuming moorland land cover across the UK and Figure 17b shows the total acid deposition assuming woodland land cover across the UK. The deposition to woodland is greater than the deposition to moorland in the same area, because the increased atmospheric turbulence associated with trees provides a route to deliver larger amounts of pollutant gases and particles from the atmosphere to the vegetation surfaces. Total acid deposition is generally greatest over England with higher deposition occurring particularly over the Pennines, Lake District, and Snowdonia. However, the areas of high acid deposition are often not the areas where there are substantial areas of critical load exceedance (see below).
The critical loads approach is an effects-based method of assessing and controlling the impacts of acid deposition on ecosystems or species (receptors). The critical load for a particular combination of receptor and pollutant is defined as the highest deposition load of the pollutant that the receptor can withstand without long-term damage occurring.
Figure 17c shows the critical loads of acidity for the dwarf shrub heath broad habitat. These critical loads are empirically derived and based on the dominant soil type in each 1km square for which the habitat is mapped nationally [45, 46]. The acidity critical load for the managed coniferous woodland broad habitat is mapped in Figure 17d. This map is also based on the dominant soil type in each 1km grid square, but the critical load values are derived using a simple mass balance equation [45, 46]. Critical loads are required to protect these habitats and also to protect the land under managed coniferous woodland for future non-forest use and possible reversion to semi-natural land uses.
The excess deposition above the critical load is called the exceedance. The maps in Figure 17e and Figure 17f show the exceedance of the acidity critical loads (Figures 17c and 17d) by the total acid deposition (Figures 17a and 17b) for 2002-2004 for dwarf shrub heath and managed coniferous woodland respectively. The neutralizing effects of base cation deposition and the removal of base cations and nitrogen by vegetation and soil processes have been taken into account [45, 46, 47].
The summary in Figure 30 shows sensitive habitats where critical loads for acidification and eutrophication were exceeded. It is a sustainable development indicator used to review progress toward the goal of sustainable development set out in the UK Government Sustainable Development Strategy in 2005.
Further information on the methods used to calculate and map critical loads in the UK can be found on the UK National Focal Centre website.
In Detail:
- Nitrogen dioxide
- Sulphur dioxide
- Inland Water topic: Acidification
Further Information:
- Key Facts:
- Acidification and eutrophication exceedances
- References, further reading and links to other resources:
- [44] NEGTAP 2001 Transboundary Air Pollution: Acidification, Eutrophication and Ground-level Ozone in the UK
- [45] Status of UK critical loads: Critical loads methods, data and maps. February 2003. Report to Defra (Contract EPG 1/3/185). Hall, J., Ullyett, J., Heywood, L., Broughton, R., Fawehinmi, J. & 31 UK experts. 2003. http://critloads.ceh.ac.uk
- [46] Status of UK critical loads: Critical loads methods, data and maps. February 2004. Report to Defra (Contract EPG 1/3/185). Hall, J., Ullyett, J., Heywood, L. & Broughton, R. & 12 UK experts. 2004. http://critloads.ceh.ac.uk
- [47] Addendum to Status of UK critical loads: The status of UK critical load exceedances. April 2004. Report to Defra (Contract EPG 1/3/185). Hall, J., Ullyett, J., Heywood, L., Broughton, R. & Fawehinmi, J. 2004. http://critloads.ceh.ac.uk
- Stoma (description)
- In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. Air containing carbon dioxide and oxygen enters the plant through these openings where it gets used in photosynthesis and respiration. Waste oxygen produced by photosynthesis in the chlorenchyma cells of the leaf interior exits through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration. The opening and closing of a stoma is controlled by guard cells that surround the opening and involves cellular sodium-potassium pumps. (wikipedia)
- Internet Links:
- United Kingdom National Focal Centre (UK NFC) for critical loads modelling and mapping
- National Expert Group on Transboundary Air Pollution (NEGTAP)
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Page last modified: 14 August 2007
Page published: 14 April 2005