Acid rain damage, Slaska Poremba, Poland

Source: C Martin, The Environmental Picture Library

Atmospheric emissions of acidifying substances such as sulphur dioxide (SO₂) and nitrogen oxides (NO_x), mainly from the burning of fossil fuels, can persist in the air for up to a few days and thus can be transported over thousands of kilometres, when they undergo chemical conversion into acids (sulphuric and nitric). The primary pollutants sulphur dioxide, nitrogen dioxide and ammonia (NH₃), together with their reaction products, lead after their deposition to changes in the chemical composition of the soil and surface water. This process interferes with ecosystems, leading to what is termed 'acidification'. The decline of forests in Central and Eastern Europe and the many 'dead' lakes in Scandinavia and Canada are examples of damage which are, in part, due to acidification. Modern forestry and agriculture contribute to but can also be affected by acidification. Acidifying substances also play a role in the greenhouse effect (see Chapter 27). Furthermore, nitrogen oxides contribute to the ozone problems (build-up of tropospheric ozone, depletion of stratospheric ozone; see Chapters 32 and 28), and, together with ammonia, contribute to the nitrogen fertilisation of natural terrestrial ecosystems; with phosphate they contribute to eutrophication in water (see Chapter 33).

By the end of the 1970s, acidification was widely recognised as a major threat to the environment. As a result large research programmes were set up to investigate the chain from emission to effects of acidifying substances and to indicate possible policy measures in this field. This has led to a much better understanding and modelling of the processes involved, which in turn has helped to formulate international agreements with explicit objectives for reducing emissions of pollutants leading to acidification.

The effects of these measures are now being evaluated in order to define sustainable releases of acidifying substances. This is based on studies which indicate that there are deposition loadings below which no harmful damage is observed, that is, the concept of 'critical loads'.

Anthropogenic sulphur dioxide emissions are due largely to the combustion of sulphur-containing fuels (oil and coal) used in power stations, other stationary combustion activities and process industries (refineries). Nitrogen oxides are emitted by combustion processes; transport, power generation and heating are the most important sources. Most of the ammonia in the atmosphere is due to the production and spreading of animal manure (see Chapter 22). Although ammonia is a base gas, it may lead to acidification after it reaches the soil and is nitrified. Nitrification is the bacteriological conversion in the soil of ammonium with oxygen to nitric acid. The acidifying substances (nitrogen oxides and sulphur dioxide), their conversion products and ammonium can generally be transported and spread over relatively long distances (thousands of kilometres), whereas ammonia is deposited relatively close to its source of emission (100 to 500 km).

Emissions of compounds leading to acidification increased considerably in Europe after the industrial revolution and especially after World War 2 (see Figure 4.3). For instance, sulphur dioxide emissions doubled between 1950 and 1970, but have generally levelled off since the early 1970s after the first oil crisis. Since 1980, European sulphur dioxide emissions in particular have been considerably reduced. Increasing oil prices following the oil crisis led to a full restructuring of the petroleum refining industry during the early 1980s, drastically reducing the output of heavy, residual fuel oil. Motor vehicles account for a considerable proportion of the total emissions of nitrogen oxides to the atmosphere in Europe (see Figure 31.1). Because of the general growth in private car use and road transport of goods, there are indications that emissions of nitrogen oxides will continue to increase. Increased emissions have been accompanied by increased deposition levels in Europe, with the relative contribution of nitrate increasing during the past 30 years.

The effects of emitted pollutants and the resulting transboundary air pollution on the environment within Europe have been assessed within the European Monitoring and Evaluation Programme (EMEP) under the UNECE Convention (see Table 31.1).

European ecosystems are sensitive to the deposition of sulphur and nitrogen. Sulphur dioxide and nitrogen oxides exert both a direct and an indirect influence on organisms and materials. In the Nordic countries, acidification of soils and surface waters is attributed mainly to sulphur deposition. However, nitrogen is estimated to contribute 10 to 30 per cent of the acidification in Denmark, southern Sweden and southern Norway. In some parts of Central Europe, nitrogen is responsible for a major part of acidification, and it seems likely that it will increase in importance as a source of acidification. This is explained by more stringent reduction of sulphur emissions as a consequence of international protocols, while nitrogen emissions seem to be continuously increasing due to insufficient measures to achieve reductions, which are largely counteracted by a relentless expansion in the numbers and use of the motor car.

Many of the processes resulting from the emission, transport and deposition of pollutants, the acidifying effects on soil and surface-water chemistry, and the link to biological damage are now understood, particularly with regard to surface-water acidification. The effects of acidification result from indirect influences:

• organisms in water are affected when the hydrogen-ion concentration increases (pH decreases) and when toxic metals, leached from the ground, come into circulation;
• plants are affected when the chemistry and biology of the soil become altered by an increased concentration of hydrogen ions;
• people are affected through the consumption, in particular, of drinking water (surface and groundwater) which, due to the decreased pH, may have elevated metal concentrations.

The effects of acidification depend on the combination of two factors: the magnitude of deposition (wet and dry), and the natural, inherent sensitivity of the receiving media (soil and water) to acidification. Acidification effects can therefore occur both in the immediate vicinity of an emission source and at great distance from it. The earliest effects become apparent where both deposition and sensitivity are high. Most precipitation reaching land runs into watercourses or percolates through the soil and becomes groundwater. In most cases precipitation comes into contact with both the ground and vegetation, and can therefore have effects on both.

In Scandinavia and North America lakes provided the first warning signals of acidification (see Chapter 5): the water became peculiarly clear; beds were gradually covered with ever thicker growths of species of Sphagnum mosses; and there was an increase in fish deaths and a decline in numbers of feeding fish and other aquatic animals. European rainwater unaffected by human activity should have a pH between 5 and 6. Today, increasing acidity has been observed over wide areas with notably lower pH ­ between 4 and 4.5, and even individual values as low as 3.

The process of surface water acidification essentially takes place in three stages:

1. Neutralisation of runoff water occurs in areas where the surrounding land and bedrock is easily weathered due to the generally high bicarbonate content. In due course, the content of bicarbonate ions in runoff declines, thus lowering the buffering capacity. There are no noticeable changes in lake biology during this stage.
2. At sufficiently low bicarbonate concentrations, large influxes of hydrogen ions can no longer be neutralised and the pH value begins to decrease. Periods of heavy rain are able to upset drastically the current balance, giving way to 'acid surges'. Large pH variations will cause direct and large-scale biological damage, such as mass death among fish and/or disruption to the reproductive capacity of fish species. The greater the influx of hydrogen ions at this stage, the more pronounced and prolonged will be the acidic periods of the lake. The second stage sets in if the pH of the water at any time during the year is lower than about 5.5, leaving the lake or watercourse moderately acidified.
3. The pH value stabilises around 4.5, even if precipitation is more acidic and the influx of hydrogen ions continues. At this point, humus substances and aluminium begin to buffer against further acidification. Aluminium in ion form increases drastically in acid surface water. This element has a strongly toxic effect on many organisms, and aluminium poisoning is becoming increasingly apparent as the real cause of the mass death of fish. This third stage has been reached in many Nordic lakes (see Chapter 5) and is marked by an entirely new ecosystem where fish life has for the most part completely disappeared. A severely acidified lake has characteristically unusually clear water, with the depth of visibility (Secchi depth) ranging from 4­5 m to 15­20 m, and Sphagnum moss spreading across its bed

At present, severe acidification of freshwater is occurring over large areas of southern Norway, Sweden, Finland and some sites in Denmark (Brodin and Kuylenstierna, 1992). The most serious acidification of surface waters has been experienced by the Nordic countries, parts of Scotland and Germany and in Austria and the former Czechoslovakia (see Chapter 5). Less serious effects have also been observed in the Italian Alps and the French Vosges. In the Nordic countries, lake ecosystems seem to be in general more sensitive to acidification than forest ecosystems, forests generally being considered the most sensitive to nitrogen enrichment (see Chapter 34).

Compared with surface waters, soil has great buffering capacity. It requires a relatively heavy input of acid to produce appreciable acidification of soil and groundwater. However, the chemical stability of the soil system varies, and sensitivity to acidification is related to the type of bedrock, the kind of soil, landuse and the proximity to major emission sources. Regions with old lime-poor rocks resistant to weathering and overlaid by thin soil layers (such as Scandinavia and eastern North America) are the most sensitive to acidification. Over the greater part of Europe rocks have a higher content of lime (calcium carbonate), thereby providing a greater capacity to neutralise acid deposition or acids formed in the soil.

The natural production of acids from coniferous forest litter is normally neutralised by the buffering capacity of the soil. However, in places with poor buffering capacity and where the atmospheric deposition of acids exceed the natural acid production, complete neutralisation of acids can no longer take place. This leads to lower pH-values, increased leaching of base cations (for example, potassium, calcium, magnesium) and decreasing base saturation. At a later stage acidification can also lead to leaching of aluminium (Al³⁺), which is harmful to organisms (eg, fine plant roots and aquatic organisms). Sulphur deposition is the key factor for soil acidification in the Nordic countries. Acid soils do not retain sulphur which is being leached to the groundwater as sulphate with equivalent amounts of base cations, thus further contributing to acidification. The critical load for sulphur deposition in areas with sensitive granite and gneiss bedrock has been estimated at 300 to 800 mg sulphur/m² per year (Christensen et al, 1993).

For nitrogen, the sequence is more complicated. Most forest ecosystems are nitrogen deficient and nitrogen addition (as atmospheric deposition or fertiliser application) will to a certain level act as a nutrient and increase tree biomass production. Beyond this level production will not increase and nitrate will start leaching, leading to acidification. Before this stage is reached other nutrients (eg, potassium, phosphorus, magnesium and micro-nutrients) can become growth limiting, leading to the well-known forest degradation symptoms such as discolouration of needles, decreased resistance to drought and frost, and increased sensitivity to insect plagues (see Chapter 34). Soil acidification caused by nitrogen leaching and associated problems are, in particular, a problem in Central Europe and The Netherlands (Grennfelt et al, 1993).

Evidence of anthropogenic acidification of soils is sparse due to lack of historical data; however, increase of soil acidity has been observed in southern Sweden (Tamm and Hallbäcken, 1988).

In agriculture, the increasing dependence on fertilisers over the last few decades (see Chapter 22) has entailed an increasingly serious acidification, especially with the use of ammonium-containing fertilisers and direct application of liquid ammonia. As the soil pH drops, there is an increase in the rate at which most trace nutrients and heavy metals are released from the soil and absorbed by plants. This can limit crop growth.

The susceptibility of forests (which generally are not fertilised ­ see Chapter 23) to acidification is dependent upon a number of factors such as: tree species, soil type, soil texture, soil depth and the thickness of the humus layer, the occurrence of bare rocks and of peatland, and soil moisture. The combined stress from acid deposition and photochemical oxidants may increase the potential for forest damage.

Nitrogen deposited in forest systems may have three main fates: uptake in stems and bark, accumulation in the living biomass or as organic compounds in soil, and leaching from the soil. The accumulation of nitrogen in the forest soil will change the availability of nitrogen in relation to other nutrients and to organic carbon. This may lead to increased nitrate leaching. Although the accumulation of nitrogen in soils is a natural process in northern forests, the deposition of nitrogen is far above the natural rate in Central Europe and Scandinavia. Nitrogen plays an important role in the acidification of soils in Central Europe but is of minor importance in Scandinavia, where sulphur is the main acidifying component. In areas with insignificant nitrate leaching, sulphur is also the main cause of lake acidification. The situation in Scandinavia may, however, change since the critical load for nitrogen is exceeded in many areas, and this will in the long run lead to nitrate leaching. The shift from a situation of minor to substantial nitrate leaching can occur over a very short time.

In coniferous forests of mountainous areas of Central Europe, particularly in the Czech Republic, Germany, Poland and the Slovak Republic, forest decline has occurred. This decline is believed to be caused by soil acidification and high concentrations of ozone and sulphur dioxide in ambient air (see Chapter 34). In the Nordic countries, no large-scale forest decline that can be attributed to anthropogenic acidification has been reported. However, substantial needle loss has been recorded in the coniferous forests of some areas, particularly in southern Sweden. In fact, Nordic forests are more productive than ever, but future effects that may be caused by long-term acidification cannot be discounted. Indeed, the areas now suffering in Central Europe also underwent an increase in yield prior to decline.

Besides damage to ecosystems, acidification may in the long term reduce the quality of groundwater by leaching sulphur and nitrogen compounds and the mobilisation of aluminium and heavy metals in the soil. It may also damage materials (eg, buildings and fabrics) and crops. Acidification of the soil or the soil water may have a corrosive effect on materials, such as steel and cast-iron pipes, steel posts, concrete foundations and lead-jacketed cables buried in the ground. However, there are few data available to quantify the significance of these effects.

Current nitrogen deposition also causes environmental effects other than acidification. Nitrogen enrichment of soils by atmospheric deposition in large parts of Europe has caused substantial changes in natural vegetation (eg, heaths changed into grasslands). Direct deposition and leaching from soils are important sources of nitrogen to waterbodies, giving rise to, for example, eutrophication of coastal waters (see Chapter 6). The effects of acidification can also be enhanced by climate change: increased soil temperatures can increase mineralisation and therefore leaching of nitrogen and sulphur from soils.

Regional pollution effects differ across Europe with the relative importance of the different precursors (see Table 31.2).

Assessments of deposition loads to environmental media (surface waters, soil, vegetation) have led to the development of the 'critical-load' approach. The critical load to a medium is defined as a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to our present knowledge. Critical loads vary considerably from ecosystem to ecosystem and from site to site (see Chapter 4) such that specific methods of evaluation are required. Models developed to determine critical loads can differ as to their underlying assumptions.

The use of the critical-load concept was established by the UNECE nitrogen oxides protocol (see below) as the basis for future pollution control. Critical loads may be compared with current deposition rates to indicate the degree to which acidic deposition has to be reducted to maintain or reinstate sustainable environmental conditions. Previous attempts to reduce depositions have been based on only the vague understanding that damage was occurring and that emissions needed to be reduced, usually by suggesting national reductions.

The critical-load approach offers a useful method to allow a more rational abatement strategy to be carried out, based on the carrying capacity of ecosystems. Ecosystems are complex and therefore detailed dose­response relationships are difficult to obtain, but thresholds offer a simpler concept which may be defined and estimated from a broad appreciation of ecology. Ecosystem-based abatement strategies are an effective way to ensure long-term sustainable use of the environment.

In many areas of Europe, sulphur and nitrogen depositions currently exceed the critical-load values and in some areas are many times greater (see eg, Maps 4.10 and 4.11, and Chapters 5 and 7). In certain regions of Europe, estimated emission reductions of up to 90 per cent for sulphur dioxide and nitrogen oxides are needed to reach critical loads for nitrogen and sulphur, and more than 50 per cent for ammonia (Lövblad et al, 1992). For technical, social and economic reasons the emission reductions required to achieve critical-load values cannot realistically be implemented in a single step within a few years. The concept of 'target loads' is therefore used to indicate goals which countries may aim for within a given time frame. Target loads are nationally set values, which may vary within a country. They include a stepwise implementation of abatement strategies which may be set as a series of aims of which the ultimate objective is to reduce emissions to match critical loads.

Control measures need to take into account the regional variation of the contribution to acidification from sulphur and nitrogen throughout Europe. In acidified soils with substantial nitrate leaching, control measures need to be directed towards nitrogen as well as sulphur deposition. In soils without nitrate leaching, but with ongoing sulphur acidification, a decrease in sulphur deposition may increase the resistance to nitrate leaching. In areas where sulphur and nitrogen both contribute substantially to acidification, it might be possible to use a general acidification concept (such as total potential acid deposition) where the contributions from the compounds are added. However, the relevance of the concept for all regions remains to be demonstrated scientifically.

The critical-load approach has clarified understanding of soil acidification and to a large extent also of freshwater acidification, eutrophication and direct impact on vegetation. Through the identification of receptors and damaging pollutants or pollutant loads, it is now possible to attribute deposition excess to emission sources in each European country with a significant level of confidence. The critical-loads approach will also improve quantification of environmental benefits in the future, especially through the widening of the scope of future protocols from single pollutants to a suite of pollutants and from single issues of concern to include several issues on a variety of scales. In terms of the cost­benefit comparison, each country can then estimate the cost of controls and evaluate the benefits of any improvement within its own territory. In the long-range transport context, the question is whether the benefits in other countries, when added up over a range of effects and a suite of pollutants, become significant (Grennfelt et al, 1993). Emission reduction of nitrogen oxides, ammonia and sulphur dioxide will be necessary to reduce exceedance above the critical loads for acidification and nitrogen deposition. Control of nitrogen oxides would reduce acidification but also eutrophication and photo-oxidant formation on both European and global scales; ammonia control would reduce acidification and eutrophication, while control of sulphur dioxide would reduce acidification on the regional scales.

If critical loads for acidification and nitrogen deposition can be reached within the next few decades, there will be substantial potential for:

• a considerable improvement in environmental conditions in coastal sea and brackish water areas in Europe;
• a considerable decrease in effects of ozone on human health, materials, crops and wild plant
• a considerable reduction in erosion of buildings, metal constructions and other man-made artefacts; and
• an improvement of air quality on a local scale.

The last of these effects may form a basis for more stringent control than regional effects.

A decrease in sulphur emissions may result in an intensification of the greenhouse effect (via less solar reflection) and speed up the rate of temperature increase, whereas decreases in nitrogen emissions counteract this effect (see Chapter 27). This effect of reducing sulphur emissions should not constitute an excuse to prevent sulphur abatement, but instead increase the necessity to reduce greenhouse-gas emissions.

The growing awareness of current and possible future effects of anthropogenic emissions of acidifying compounds and their long-range transport from country to country has already resulted in EC legislation and international negotiations on emissions reductions. A series of EC Directives have been developed to tackle emissions of acidifying substances from transport (70/220/EEC and 72/306/EEC) and industry, especially from large combustion plants (eg, 84/360/EEC, 88/609/EEC, 89/369/EEC and 89/429/EEC). At the international level, a sulphur protocol was developed within the framework of the convention on Long-Range Transboundary Air Pollution (LRTAP) of the UNECE and adopted in 1985 in Helsinki. The protocol states that the signatories should reduce their sulphur dioxide emissions to at least 30 per cent below 1980 levels by 1993. By 1994, the sulphur protocol had been ratified by 20 European countries and Canada. In many Western countries, larger reductions have already been achieved, while the economic recession in Central and Eastern Europe has resulted in temporarily lower emissions. Total European emissions are expected to decrease by some 30 per cent during the period 1990 to 2000, if current policies are implemented.

The new sulphur protocol on further reductions of sulphur emissions was signed in 1994 by 25 European countries, the EC and Canada. Critical loads form an integral part of the new protocol, a significant new departure for such international agreements. The basic obligations require that sulphur emissions be controlled and reduced to protect human health and the environment and to ensure, in the long term, that deposition of oxidised sulphur compounds does not exceed critical loads.

In 1988 in Sofia, a further UNECE protocol was drawn up on the freezing of nitrogen oxide emissions. The Sofia protocol states that emissions of nitrogen oxides should not increase from the 1980­85 level until 1995. It has now been ratified by 22 European countries, the EC, the USA and Canada. Most Western European countries have declared that instead of freezing they will decrease by 30 per cent their nitrogen oxide emissions. However, the general growth in private car use and in road transport suggests that only a few countries will be able to reduce their national emissions by the announced 30 per cent.

All these agreements have the force of international law, and require their parties to monitor pollution levels, compute emissions, devise models for the dispersion of emissions, perform evaluations, compile statistics and report on results. A separate agreement, the Protocol on Long-Term Financing of the Co-operative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe (EMEP), was concluded in 1984 and was ratified by 30 countries. Today, some 100 monitoring stations in 25 countries are involved in the EMEP monitoring system.

Most SO₂ emission reductions which took place in Western Europe since 1980 were due to increased use of nuclear power, a switch from coal to oil and natural gas and technological improvements such as desulphurisation of petroleum products in the refining process, as well as a lower sulphur content in the crude petroleum intake. This development has also occurred partly because of increasing demand for low-sulphur products, following national and EC regulations. In Western countries especially, further reduction of sulphur dioxide emissions can be achieved not only by flue-gas desulphurisation, but also by use of advanced combustion technologies, energy conservation, increased energy efficiency of installations and increasing the share of non-fossil fuel power production. In Central and Eastern Europe, significant reductions of sulphur dioxide emissions are technically possible by switching to gas and low-sulphur coal, flue-gas desulphurisation, energy saving and implementation of energy-efficient installations.

According to petroleum companies, more than half of the sulphur content of crude oil is now removed in the refining process (and does not enter into the fuels). Accordingly, the sulphur content of residual fuel oil and middle distillates (diesel oil and light fuel oil) has also been reduced. Reduced output of heavy residual fuel oil from refineries also reflects the substitution of this fuel with lighter fuel products. A substitution of residual fuel oil containing 2.4 per cent sulphur with hard coal containing 1.6 per cent sulphur would lead to only a very slight reduction in sulphur dioxide emissions because of the lower energy content of the coal.

Currently, more than 70 per cent of total sulphur dioxide emissions stems from coal combustion in thermoelectric power plants. Measures to reduce sulphur from this source include the removal of sulphur dioxide from stack gas (based on absorption in lime slurry or some other added absorbent) or recovery of sulphur as either sulphuric acid (the Wellman­Lord process) or gypsum. If not recovered, the absorbent may have to be disposed of in landfills.

For nitrogen oxides, a 60 per cent emission reduction could be achieved through the introduction of low-NO_x burners and modified combustion chambers, by new modes of operation (such as catalytic reduction techniques) in combustion installations for stationary combustion sources, as well as by Europe-wide implementation of improved catalysts in cars and improved diesel engines. Further reduction of emissions requires reaction of nitrogen oxides with ammonia to form elementary nitrogen, usually in the presence of a catalyst (selective catalytic removal, or SCR). Both flue-gas desulphurisation and denitrification are costly, and have only been implemented for a few large power plants and boilers.

The advanced combustion technologies of fluidised bed combustion and pressurised fluidised bed combustion (FBC and PFBC) are applicable for medium-sized boilers only. The fuel is mixed with lime to increase the retention of sulphur dioxide and the low combustion temperature reduces nitrogen oxide emissions. This technology is attractive for district heating and co-generation plants since construction costs are less than for conventional boiler designs. Drawbacks include waste disposal and possible N₂O formation because of the low combustion temperature.

Tables 31.3 and 31.4, which are based on information supplied to the ECE in connection with the Long-Range Transboundary Air Pollution (LRTAP) Convention, sum up typical performance data (emissions) and costs of these control options. The tables indicate that removal of sulphur and nitrogen oxides from the flue gases is technically feasible. However, only 50 000 MW electricity generating capacity in Europe applies the flue-gas desulphurisation and secondary denitrification control options. One of the problems associated with flue-gas desulphurisation is the disposal of waste products: gypsum or sulphuric acid. With the limited capacity now installed, gypsum is utilised in building materials, whereas the market for sulphuric acid is limited.

Motor vehicles' emissions of nitrogen oxides are the main cause of photochemical oxidant and ozone formation on a regional scale in Europe. Planned restrictions and introduction of technical control devices are expected to reduce total European nitrogen oxide emissions in 2000 by about 20 to 30 per cent with respect to 1990 emission levels. However, because of the general growth in private car use and in road transport of goods, there are indications that the actions taken will not be sufficient to reduce nitrogen oxide emissions (see Chapter 32).

Ammonia emissions from agriculture can in general be reduced by careful storage and application of manure and balanced animal feeding. These measures limit volatilisation of ammonia from manure and reduce the nitrogen content in animal fodder. This is difficult to achieve in areas of intensive animal husbandry, and may incur considerable investment costs for manure storage and handling equipment. European ammonia emissions can be expected to stabilise or even increase slightly during the next ten years, and the relative contribution of ammonia emissions to acidic deposition is expected to increase. So far, no internationally coordinated policy exists for this compound.

In summary, acid deposition in Europe is expected to decrease following reductions in sulphur and nitrogen emissions. However, these reductions are still insufficient, and in more than half of the European area the deposition loads are expected to remain in excess of critical loads, resulting in prolonged risk for ecosystems.

Given a regional air pollution problem, the problem of emission reduction raises two important questions: which emissions should be reduced, and where should they be reduced, in order to provide the most cost-effective solution to the problem? The protocols to the LRTAP Convention specify national reductions as a percentage of national emissions in a given base year. While this approach has been practical as a first attempt to share the burden of reducing emissions equally between parties, it may not be the most cost-effective solution. Costs of reducing the emissions of a given pollutant may vary widely between countries, and the environmental effects of the releases may also be quite different. Both of these arguments are being addressed in the renegotiation of the protocols to the LRTAP Convention. A first assessment of the achievements of the LRTAP Convention under the sulphur protocol will be made in 1995.

There has been rapid acceptance of critical loads for use in negotiations because they form a bridge between scientists, policy makers and the public. Integrated assessment models have been developed that use information on emissions, atmospheric transport, costs of abatement, and critical or target loads to study the implication of different abatement strategies. These models can indicate the best abatement strategies necessary to achieve objectives such as reduction in the areas that exceed critical loads or reduction of the total deposition in excess of critical loads.

Studies indicate that very strong emission reductions of all acidifying compounds are needed to avoid exceedance of critical loads in Europe (about 90 per cent for sulphur dioxide and nitrogen oxides and more than 50 per cent for ammonia). Implementing critical loads for environmentally optimised and cost-effective abatement strategies means that some countries, for example in Central and Eastern Europe, may have to reduce emissions by more than 85 per cent. This has raised the question of creating a European environmental fund with the major task of distributing economic support and technology to Central and Eastern European countries where the economic situation is less favourable.