Air Pollution
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Critical Loads


Critical loads have been defined as: "the highest load that will not cause chemical changes leading to long-term harmful effects in the most sensitive ecological systems". Critical loads are the maximum amount of pollutants that ecosystems can tolerate without being damaged. The definition has been redrafted in order to fit specialist areas of interest, most particularly the acidification of freshwater, vegetation and soils.

The Concept

Since 1988 the critical loads approach has been given much attention by the United Nations Economic Commission for Europe (UN-ECE), and new strategies in relation to acid rain are being developed along these lines. In 1986 the Nordic countries, at a workshop in Norway, defined the term "critical load", and values were given for the critical loads of sulphur and nitrogen on different ecosystems. These values are subject to change, and Parties are asked to submit revised maps on a twice-yearly basis. The concept behind critical loads is based upon a dose-response relationship where the threshold of harmful response is triggered by a certain load of pollutant - the critical load. However, it is not always easy to apply without careful consideration of the pollutant receptor and the threshold effects of harmful pollutants. In order for critical loads to be used, target loads need to be set for different areas in order to try and halt the acidification processes. Target loads have been defined as "the permitted pollutant load determined by political agreement". Therefore, target loads can be either higher or lower than the critical load values. For example, the target load may be lower so as to give a safety margin or the target load may be higher for economic reasons. The reasoning behind this is that critical loads only show where there is a problem and to what degree damage is occurring. Target loads are used in order that emissions can be reduced accordingly to meet the targets and limit the amount of damage.

Methods for Calculating Critical Loads

The critical loads for total acidity of sulphur and nitrogen need to be determined so that a coherent international agreement can be reached with regard to abatement policies. There are numerous methods that are available for obtaining critical loads. The National Centre for Critical Loads at the Centre for Ecology and Hydrology can supply further information if required. In order to obtain values for the critical loads, an ecosystem has to be chosen and then a suitable indicator species is selected to represent the ecosystem. A chemical limit is subsequently defined as the concentration at which the indicator species will die. In forests the indicators are trees, and in fresh waters they are fish.

The UN-ECE Convention on Long-Range Transboundary Air Pollution (LRTAP), signed in 1979, was devised to limit and find solutions to cross border air pollution problems within Europe. It was agreed to promote protocols whereby reductions could be made in the deposition of sulphur and nitrogen. To achieve this it has been agreed by the member countries of the UN-ECE that the critical loads approach provides an effective scientific approach for devising strategies for the abatement of air pollution.

The strategy for a Europe-wide reduction in the deposition of sulphur follows a five point basic framework.

  • Each country has to produce maps depicting critical loads for national areas, receptors and pollutants.

  • The resulting data are then assembled to produce Europe-wide maps showing critical loads.

  • Using this data and data on the deposition of pollutants, maps can be produced showing where critical loads are being exceeded (exceedence maps).

  • Countries then set target loads which can be regarded as intermediate objectives towards bringing down deposition so that critical loads are not exceeded.

  • Finally, there are negotiations to arrive at new agreements on emissions reductions.

This strategy, known as the optimised approach, allows for the reduction in emissions to be applied to the most effective geographical areas at the least overall cost. This strategy was implemented with the signing of the Second Sulphur Protocol in 1994.

The Second Sulphur Protocol

The Second Sulphur Protocol of the UN-ECE was signed by the then Secretary of State for the Environment, John Gummer, in June 1994 in Oslo. This followed the original Protocol adopted in 1985 which agreed for the reduction of a minimum of 30% in sulphur dioxide (SO2) emissions by 1993 from a baseline level of 1980 emissions. This was adopted by many countries throughout Europe, but there were some notable exceptions including the UK. However, the UK did begin the introduction of Flue Gas Desulphurisation (FGD) technology into large combustion plants in 1986 and subsequently signed the 1988 EC directive on large combustion plants which committed the UK to reducing SO2 by 60% from these sources by 2003 based on 1980 levels. With the signing of the Second Sulphur Protocol, the UK has agreed to secure a reduction of at least 80% of its national annual emissions of sulphur dioxide by 2010 compared with 1980 levels. This is to be achieved through transitional reductions of at least 50% by 2000 and 70% by 2005.

UK Critical Load Maps

The UK Government accepted that the critical loads approach was the best way to establish abatement strategies in relation to SO2 and NOx emissions. It was recognised that critical loading maps are essential in providing information on the geographical distribution of pollutant sensitive locations and in their ability to allow comparison with pollution distribution maps. This comparison allows identification of areas where the critical load is exceeded. As part of its commitment to the UN-ECE Convention on Long-Range Transboundary Pollution, the Critical Loads Advisory Group (CLAG) was set up by the Department of the Environment to produce critical load maps for the United Kingdom. The National Centre for Critical Loads Mapping was subsequently established at the Institute for Terrestrial Ecology, now the Centre for Ecology and Hydrology.

Critical load maps of soil and vegetation acidity have been produced for the UK, including Northern Ireland at a grid resolution of 1 km squares. Maps for the critical acidity of freshwater environments are based on a single water sample from a single site in each of the 10 km squares used, assumed to be the most sensitive surface water within the grid square.

The critical loads maps, when combined with deposition values, produce exceedance maps which show where and by how much the critical loads are being exceeded. Maps are available for soils, vegetation and freshwater in the UK relating to acidity and sulphur deposition showing areas that are sensitive to acidification. These correspond to areas where there have been reports of acidification. In the UK a national target load map for the year 2005 has been produced for soils on a 20km by 20km grid system, showing the target loads that need to be met for such areas. These maps have been submitted to the UN-ECE, and with the maps produced for other countries, will provide the basis for discussions on new agreements under the LRTAP.

In 1997, critical loads for acidification were exceeded in 71% of UK ecosystems. As sulphur deposition continues to fall, this value is expected to fall to below half by 2010, when nitrogen deposition will dominate. Critical loads for eutrophication (nutrient depletion) in 1997 were exceeded in about a quarter of UK 1km by 1km squares with sensitive grasslands and a little over half with heathland. Again, this is expected to decline over the next 10 to 15 years.


"Critical loads" provide a useful approach for dealing with the problem of acid rain. However, its use does have limitations. One of the main problems with the approach is the way in which areas are assigned critical load values. In the UK the maps for freshwater ecosystems are calculated on a 10km by 10km grid. This can mean that in some squares the critical load value may be too high to protect some of the ecosystems present. This is also a problem where only the dominant ecosystem is taken into account, resulting in maps which are not representative of the area.