Introduction

In the United Kingdom, research over the last 20 years into the environmental effects caused by emissions of sulphur and nitrogen compounds to the atmosphere has provided quantitative estimates of the effects of acidification on soils and freshwaters. This has given a basis for the development of effects based emission control policies through the formulation of the critical loads and levels approach, which is a rigorous scientific method of linking air pollutant emission reductions on both national and international scales to environmental benefits. Its application requires the definition of sensitive receptor ecosystems or elements of the built environment and an understanding of pollutants which may adversely affect them.

In 1991, the Department of the Environment set up the Critical Loads Advisory Group (CLAG) to develop a national critical loads and levels programme. Individual sub-groups were set up within CLAG to review the impacts of air pollutants on specific parts of the environment including soils, freshwaters, vegetation (trees, semi-natural vegetation and crops), buildings and materials and to provide information on input fluxes and estimates of pollutant exposure. In addition, a mapping and modelling group was set up to integrate the output from the different sub-groups. Full reports from each sub-group, providing the detailed arguments and data to support the conclusions drawn in this summary, will be published during 1994. This report is a compilation of the main findings of the sub-groups.

Critical loads, levels and exceedance

• The definition of critical loads adopted by the United Nations Economic Commission for Europe (UNECE), is 'a quantitative estimate of exposure to one or more pollutants below which significant harmful effects on sensitive elements of the environment do not occur according to present knowledge'. The term critical load refers only to the deposition of pollutants. Threshold gaseous concentration exposures are termed critical levels and are defined as 'the concentrations in the atmosphere above which direct adverse effects on receptors such as plants, ecosystems or materials, may occur according to present knowledge'.
  
• Critical loads and levels are conveniently represented on maps and this allows combined effects of physical, chemical, ecological, geological and hydrological factors on sensitivity to pollutant inputs to be quantified. Once a critical loads or levels map is available for a particular receptor-pollutant combination, comparison with current deposition loads or exposure levels for that pollutant provides the spatial distribution of areas of exceedance, which may also be mapped. Such maps are called critical load or level exceedance maps.
  

• Inputs of acidic substances to terrestrial ecosystems include wet deposition, dry deposition of gases and particles and cloud droplet deposition. The compounds which make a significant contribution to acidic deposition in the UK include the gases sulphur dioxide, nitrogen dioxide, nitric acid, the major ions in precipitation such as sulphate, nitrate and hydrogen ion, and sulphur and nitrogen containing aerosols.
  
• Reduced nitrogen species such as gaseous ammonia and ammonium in rain, cloud and aerosols also contribute to acidification following biological transformation in soil or plants. As current understanding is inadequate to quantify net acidic inputs to ecosystems following the deposition of reduced nitrogen compounds, the input of dry deposited ammonia is excluded from this summary report. The subject is considered however in some detail in the report of the sub-group on fluxes.
  
• The critical load of acidity for an ecosystem is determined by net input of acidity, making it necessary to estimate the atmospheric inputs of base cations, and in particular calcium and magnesium.
  
• The comparison of precipitation has been shown to vary with altitude at sites where the precipitation amount increases with altitude as a consequence of washout of hill cloud by falling rain (seeder-feeder scavenging). The maps of wet deposition presented here incorporate these effects and are therefore referred to as seeder-feeder corrected wet deposition maps.
  
• The dominant acidifying anion in precipitation is sulphate and areas with largest deposition are the high rainfall areas of north and west England, west central Scotland and North Wales. There is a clear north west to south east gradient in the UK of decreasing wet deposition with the low rainfall areas of East Anglia receiving only 30 to 40% of their annual acidic inputs through this process, while the high rainfall areas receive 80% or more of their annual inputs in precipitation.
  
• Cloud droplet deposition refers to the direct capture of cloud droplets by terrestrial surfaces. Hill cloud observed throughout the UK has been shown to contain much larger concentrations of ions than rain. Areas of upland forest in the Scottish borders receive up to 30% of their input through this process.
  
• The direct deposition of gases and particles to terrestrial surfaces is referred to as dry deposition. Dry deposition processes dominate the inputs of sulphur and nitrogen in central and southern England.
  
• The contribution of dry deposition of particles has generally been neglected as a minor and poorly understood component of the acid input. However, for base cation inputs this is not strictly correct. In this section therefore, the input of calcium by dry deposition has been estimated although the mechanisms used make the estimates uncertain.
  

• The objective in setting critical loads for soil-plant systems is to determine the maximum load of a given pollutant which will not produce significant changes in the structure or function of the system. A formal and agreed international procedure has been developed to calculate the critical load. This so-called Level I approach involved (1) identifying a biological indicator, the health of which is indicative of the health of the whole soil-plant system and (2) establishing a critical chemical value, in terms of a given parameter above which the indicator organism will show an adverse response such as a reduction in growth. This critical chemical value is then used in the calculation of the critical load.
  
• The most widely used biological indicator is 'fine roots' of trees. The currently used parameter for the critical chemical value is the calcium + magnesium:aluminium ratio (Ca+Mg/Al) in soil solution below which significant damage to fine roots will occur.
  
• Because of the lack of detailed scientific information, the provisional critical load map for soils of GB did not follow the Level I approach but utilised critical loads which were based on mineralogy and which were set to prevent soil acidification (Level 0). A separate methodology was developed to calculate critical loads for peats. The resulting map shows that a) soils with small critical loads are widespread in the west and south of Britain where relatively shallow soils derived from acidic base poor rocks are dominant; b) soils of much of the south and east of England have large critical loads, being largely formed on thick calcareous and/or clay rich glacial deposits or material derived from calcareous rocks; c) some areas of soil in East Anglia, the New Forest and the Weald have small critical loads as a consequence of their mineralogy, being formed on sands with few weatherable minerals.
  
• The provisional map was further modified to make allowance for land use factors such as the liming of agricultural areas. The main effect of the modification is an increase in critical loads in some areas in the south and east of England, western England and the margins of the Welsh uplands.
  
• The provision critical load map sets the critical load to equal the annual release of base cations by weathering. It makes no allowance for the acidity produced within the soil-plant system as a result of the net uptake of base cations, nor of the potential neutralising effect of atmospheric inputs of calcium and magnesium.
  
• An alternative approach (Level II) has been developed using a mass balance equation, which allows these factors to be taken into account. This approach follows more closely the formal procedure set out for the calculation of critical loads. The three approaches will be described in some detail in the Soils Sub-group report.
  
• The mass balance equation can be solved for any soil-plant system, providing that a critical chemical value can be specified for the chosen biological indicator species. To illustrate the principles, the critical load maps for Norway spruce and for acid grass dominated communities are derived. These maps are consistent with the main geographical pattern of critical loads provided by the provisional map.
  

• In the UK freshwaters programme, two different but complementary approaches have been used to set critical loads for individual water bodies. These are the diatom model and the steady-state chemistry model. Diatoms are freshwater organisms which demonstrates a broad range of sensitivity to acid stress. Analysis of lake sediment records for species composition of diatoms provides evidence for present and historical acidification in the lake. The diatom model is used to set what may be regarded as the basic critical load for the site whilst the steady-state chemistry model which relates to current acidity is used to set critical loads to protect individual species or groups of species. In addition, a modelling approach using a dynamic model such as MAGIC, has been used in a predictive capacity at selected sites to explore the effect of time lags, land-use and other factors on critical loads at different sites.
  
• By subtracting the critical load for a site from the actual load of acidity received, critical load exceedance values have been calculated. So far in the UK, only sulphur deposition has been used for this work, but models are currently being modified to take into account water acidity associated with nitrogen deposition. Future critical loads exceedance maps will be produced for total acid (sulphur plus nitrogen) deposition.
  
• The main areas of exceedance identified coincide with those areas where acidification problems are already well know. These include the Cairngorms, Rannoch Moor and the western Scottish Highlands, Arran, the Trossachs, Galloway, North Wales, Central Wales, Cumbria, the Pennines, Dartmoor and the New Forest, and the mountains of Mourne in Northern Ireland. However, there are many other sites and regions where exceedance is indicated and either not expected (eg the far north of Scotland) or not previously studied (eg the North Yorkshire moors, the Sperrins of Northern Ireland and the acid heathlands of southern England).
  
• As acid deposition decreases in the future, it will be important to quantify the rate of chemical and biological recovery of streams and lakes. The critical loads maps for freshwaters have already been used to calculate the approximate reductions in deposition required for recovery to take place. They show that the level of emissions reductions for sulphur dioxide currently being proposed will not allow the restoration of aquatic ecosystems in major portions of upland Britain.
  

• Over the last two years, the Critical Levels Sub-group has made an important contribution to redefining critical levels for direct effects on vegetation and has begun to explore the implication of these critical levels.
  
• Mapping of critical levels exceedances in the UK is at a preliminary stage. However, it is clear that critical levels for the most sensitive vegetation types are currently exceeded over a considerable part of the UK, in the case of all pollutants except ammonia. It is planned to define the areas involved more precisely in 1994.
  
• It is important to recognise that critical levels are set 'according to present knowledge', and that knowledge is limited with little data close to the critical level and the relative sensitivity of different plant species being poorly defined. More monitoring and impacts data are required to improve the scientific basis for defining and mapping critical levels. In the case of ozone, the recent Berne workshop has provided new critical levels and a stronger bases for them (5.3).
  

• The response of building materials and the natural environment to pollutants differ in that building material deteriorate in the absence of pollutants while natural systems can tolerate certain environmental conditions without damage.
  
• The absence of a threshold has necessitated the definition of an acceptable level of deterioration or extent of damage where pollution enhanced degradation does not exceed a defined multiple of the natural background degradation.
  
• Damage to buildings is related more to the combination of total pollutant 'dose' and the quality of rainfall on a long term basis rather than to specific pollution episodes. Deterioration rates have been calculated using does-response functions for a number of calcareous stones and ferrous and non-ferrous materials, using data from the UK National Materials Exposure Programme funded by the Department of the Environment.
  
• Exceedance maps have been produced to show where the predicted deterioration rate for Portland limestone and steel is above the acceptable level, and also to show the areas which will continue to be at risk assuming a 70% reduction in emissions.
  
• By combining these maps with distribution maps of sensitive materials, it will be possible in the future to quantify the spatial distribution of damage costs.
  
• The buildings approach, by using the concept of dose (or exposure), is consistent with the approach used to determine critical levels for ozone effects on crops and forests agreed at an international workshop in Berne (November 193).
  

• The Mapping-Modelling Sub-group was the last group to be established to integrate the work of the other CLAG sub-groups. An additional role is to provide full documentation and validation wherever possible, of all models, procedures and data sets involved in the production of critical loads and levels maps.
  
• A centralised database for UK critical loads and levels maps has been established by the Department of Environment at the Institute of Terrestrial Ecology, Monkswood. This enables all data sets to be fully documented and ensures that the most appropriate environmental information is applied and is readily available for use by all the other CLAG sub-groups.
  
• The maps and models used in implementing the critical loads approach and their input databases are described. One example of the environmental consequences for soils and freshwaters of specific emission control policies shows that an 86% reduction in sulphur dioxide emissions results in reductions in critical loads exceedances for soils and freshwaters of 92% and 72% respectively.

   Other reports prepared for DETR