Air pollution

Source: Gamma


INTRODUCTION

Good air quality is a prerequisite for the health and well-being of humans and ecosystems. Polluted air will affect human health, ecosystems and materials in a variety of ways. The atmosphere can act as a means for transporting local pollution emissions to other locations, even long distances away and to other media (land and water). In this chapter, the mechanisms behind the build-up of atmospheric pollution at different scales are described, to explain the causal chain between anthropogenic pollutant emissions and the status of air quality in Europe. Present and future trends of air quality are discussed in relation to implemented and planned policy measures and technological developments. The consequences and impacts of changed air quality are described and assessed using common and consistent procedures at three spatial scales extending across national borders. There are, however, important issues which cut across these scales, where, for example, control policies optimal at one scale may not be optimal at another, and air pollution emission reductions targeted in one area may lead to pollution of other parts of the environment.

It was for long believed that air pollutants, once released, were eventually diluted to negligibly low concentrations in the atmosphere. However, measurements taken during the last 20 to 30 years have shown this belief to be erroneous and incomplete. There are three main reasons for this:

  1. Not all of the troposphere is available to dilute released pollutants. Most pollutant emissions occur at or close to the Earth's surface in the lowest layer of the atmosphere, the so-called 'mixing layer' (Box 4A). Depending on the meteorological conditions, and especially when the mixing layer corresponds to a temperature inversion, pollutants can accumulate in locally restricted zones leading to high concentrations and 'smog'. Constructing stacks higher than temperature inversions was a short-sighted solution that brought some local relief, but shifted pollution problems to different areas and even to larger scales.
  2. Emitted pollutants undergo changes in the atmosphere: the many different anthropogenic and natural compounds (see Box 4B) disperse, mix, are transported and undergo chemical and physical reactions. Sooner or later, nearby or remote to the original release, ingredients of this pollution 'cocktail' are returned to the Earth's surface in one of a number of forms, where they can have adverse effects on ecosystems, humans and buildings.
  3. Compounds remain in the atmosphere for differing lengths of time. This duration, the residence time, is determined by the processes of deposition and chemical conversion (see also Box 4B). If atmospheric residence times are of the order of 30 days, vertical mixing may extend to the whole troposphere, and hemispheric transport will be important. Only if residence times are considerably longer, between 6 and 12 months, will exchange between the northern and southern hemispheres take place; after 12 months, exchange between the troposphere and the stratosphere becomes important. It is in this way that European emissions of so-called 'greenhouse gases' and of CFCs contribute to the global problems including the 'enhanced greenhouse effect' and stratospheric ozone depletion.

The fate of airborne pollutants is determined mainly by the release height and the prevailing weather. Thus, the scale of distribution patterns and effects will range from local (up to a few tens of kilometres), through regional (up to a few hundred kilometres), to continental (up to a few thousand kilometres) and global. Concentrations of air pollutants will vary greatly with time (daily, weekly, and seasonally) and in space.

High concentrations of primary pollutants, including enhanced deposition, can occur within and around emission areas. Practically all large particles are deposited locally. Local weather is an important factor determining short-term pollution levels; in Southern Europe, systems of local air circulation (such as land­sea breezes) are particularly influential. Continental-size weather patterns (cyclones and anticyclones), usually lasting a few days, can suddenly increase pollution loads on the regional scale, resulting in 'pollution episodes'. The transfer of pollutants from the mixing layer to the upper troposphere and stratosphere increases residence times which may have far-reaching global impacts on the properties of the atmosphere. (CFCs emitted at the surface but which destroy the ozone layer at an altitude of 20 to 30 km are an important example of this phenomenon.)

Local and national emissions of air pollutants may thus have implications at regional (transboundary) and global scales. The assessment of air pollution cannot therefore be limited to the local or the national level. Three levels of assessment are considered appropriate:

  1. the local level (time-scale less than one day) with the related exposure of population and materials;
  2. the regional level (between a day and a week) with the related atmospheric deposition input into terrestrial, marine and surface water systems in relation to critical loads;
  3. the European contribution to global air pollution (time-scale longer than a month).
URBAN AND LOCAL AIR POLLUTION

Europe today is a highly urbanised continent with more than 70 per cent of Europeans living in urban areas (see Chapter 10). Many urban activities (eg, traffic, combustion processes, industrial production) are accompanied by emissions into air yielding elevated concentrations of pollutants. This is especially significant when a large number of activities are concentrated together, as in an urbanised area.

Air pollution on the urban scale is the source of a range of problems: health risks mostly associated with inhalation of gases and particles, accelerated deterioration of building materials, damage to historical monuments and buildings, and damage to vegetation within and near the cities. In order to tackle these problems it is necessary to understand: what characterises urban air quality; what are the sources of urban air pollutants; and which exposures are associated with the high concentrations of air pollutants occurring in cities.

The actual occurrence and frequency of increased air pollution concentrations depends primarily on the magnitude and the distribution of emission sources, on local topography (eg, flat terrain or basin or valley) and local meteorology (eg, average wind speed, frequency of calm weather conditions, occurrence of inversion layers). The significance of any air pollution depends ultimately on the type of pollutants, the resulting exposure and the health and other effects associated with this exposure. In this section, the World Health Organization air quality guidelines (WHO-AQGs) have been taken as reference values to assess where ambient concentrations may possibly cause effects on human health and where further study may be necessary (WHO, 1987). The AQGs are given in Table 4.2, together with the effects they aim to prevent. As a precaution, some national limit values are even lower than WHO-AQG values.

A considerable safety margin is often built into AQGs with respect to the lowest observed effect level, to help protect the sensitive part of the population. However, AQGs have only been specified for a limited number of the thousands of possible constituents of urban air and for a limited number of averaging times. The number of constituents covered by AQGs, and the number of averaging times specified for each, increase as evidence of possible adverse effects accumulates.

Table 4.3 gives the main categories of sources that contribute to the total emissions of the components mentioned in Table 4.2 (see also Chapter 14). In non-industrial cities the largest contributions come from local traffic and domestic heating when oil, coal or wood is used.

Most large cities in Europe operate monitoring networks in order to assess the air quality in their city. However, the design and the technical procedure (components measured, methods used, number and location of the stations) of the monitoring activities vary widely within Europe. The most advanced networks have incorporated air dispersion models and emission inventories in order to determine the geographical distribution of air pollution in the city and to determine the contribution from the different sources. In such cities the authorities are able to use this information to decide on the measures needed to reduce air pollution to an acceptable level, and to assess the cost of these measures (cost-efficient abatement measures).

Consistent information about air quality in large European cities is not yet collected systematically except by the European Commission under Council Decision 82/459/EEC (see also CEC, 1990) and WHO/UNEP (UNECE, 1992). In general, air pollution problems may occur in all the 2000 cities in Europe with more than 50 000 inhabitants (WHO, in press). For this report, only a limited number of cities were evaluated. Questionnaires were sent out to all cities or conurbations in Europe with more than 500 000 inhabitants, as well as to the largest city in each country (105 cities in all). (See Map 10.1 for city locations.) Approximately 22 per cent of the European population, or 150 million people, live in these cities. A summary of the information from these questionnaires covering emissions, air concentrations and exposure situations in these European cities is presented in this chapter, and the details elsewhere (RIVM/NILU, 1994).

Air quality in large European cities

An overview of the most relevant air quality indices in the selected European cities is given in Table 4.4. The table gives five groups of indices derived according to the methodology of RIVM/NILU (1994). The indices are:

These indices are presented to help guide the interpretation and comparison of the data in order to obtain some understanding of the relative severity of three major air pollution situations that may occur in the selected European cities:

  1. winter-type smog by sulphur dioxide (SO2) and particulate matter (PM) measured by the black smoke or gravimetric methods;
  2. summer-type smog by ozone (O3) as resulting from emissions of volatile organic compounds (VOCs) and nitrogen oxides (NOx);
  3. high annual average concentration levels (including benzene, benzo(a)pyrene (BaP), and lead, in addition to SO2 and PM).

The indices for the exceedance and exposure situations are based mainly on results from air pollution measurements in each city. The indices for environmental pressure, emissions and dispersion may be used to explain the differences in air quality from city to city. The quality of the data can differ considerably between cities and many of the data are uncertain (as marked). However, this is the best comparison that can be made with existing data and those collected specially for this exercise. Table 4.4 is based mainly on measurement data from 1990, although for some cities data were given for other years, between 1984 and 1991.(See Statistical Compedium).

Winter smog

Results in Table 4.4 indicate that winter smog concentrations exceed WHO-AQGs in 61 of the 87 cities with data on SO2 and/or PM. Extrapolation to all 105 cities gives an estimate of exceedance of the AQG for winter smog in 70 per cent of the surveyed cities. Such exceedances occur in general over a large part of the city during periods (days) with poor dispersion conditions. In about 24 of the 87 cities (28 per cent), concentrations around or higher than twice the AQG have been measured during recent years. Among these cities are: Ljubljana, Lód´z, Sofia, Istanbul, Milan, Turin, Stuttgart, Belfast, Dublin and Prague. Urban winter smog can be found in all parts of Europe.

According to the reported measurements, the eight cities of Istanbul, Leipzig, Berlin, Donetsk, Krivoj Rog, Odessa, Riga and Yaroslavl have the highest measured concentrations of winter smog, because of high emission densities and the topographical situation of the cities (valley or basin). Coal is widely used for domestic heating in many of these cities, and they experience the same type of pollution as in London's notorious smog episodes of the 1950s, but fortunately the present concentration levels in the affected cities are lower than in those London smogs.

High concentration of suspended particles is a more widespread urban air pollution problem than high SO2 concentrations. Most of the surveyed cities have significantly higher PM than SO2 concentrations. Exceptions are the 'coal cities' already mentioned, and also Antwerp, Lille, Lyons, Bremen, Leeds, Liverpool, Sheffield and Oporto, where SO2 is higher than PM.

Summer smog

The occurrence of photochemical oxidants, as indicated by the ozone concentration, is a regional phenomenon in Europe (see below). In most cases this regionally elevated ozone concentration will be partly suppressed in urban areas due to the emission of nitrogen oxide (NO), which instantaneously reacts with ozone to produce nitrogen dioxide (NO2). Further reactions, involving VOCs and nitrogen oxides, will result in production and build-up of ozone and oxidants in the city 'plume', but these are slow processes and the additionally produced ozone will usually be found downwind of the area where the emissions occurred. An exception is when cities are located in confined valleys, or when the polluted air is trapped in land­sea breeze circulation systems, where the residence times are long enough for significant photo-oxidation to take place. For example, particularly high ozone concentrations, up to 400 µg/m3, are measured in Athens and Barcelona.

Long-term exposure

Long-term exposure to SO2 and PM exceeds WHO-AQGs in 24 of the 61 cities with winter smog problems, particularly in the city centres of Prague, Turin, Donetsk, Krivoj Rog, Krasnodar, Lvov, Samara, Togliatti and Istanbul.

For long-term exposure to benzene and BaP, a systematic overview of the concentrations is not possible as these are measured in only a few cities, except for BaP in the former USSR. These compounds are indicators of organic compounds associated with volatile organic hydrocarbons and with soot and polycyclic aromatic hydrocarbons (PAHs) from small-scale combustion processes, respectively. From available data and model calculations, benzene will probably exceed the lifetime cancer risk level of 10-4 in most cities, and BaP probably even up to a lifetime risk level of 10-3 (RIVM, 1992), especially in those cities which still use coal or oil for domestic heating or which are unfavourably located (see Table 4.4).

Countermeasures

Successful measures for reducing air pollution in cities in Europe over the past 20 to 30 years have included the regulation of fuel for domestic heating (low sulphur content in oil and coal, introduction of natural gas instead of coal and oil), development of district heating, reduced lead content in petrol and restriction on the use of cars in city centres (eg, Cologne, Dortmund, Zurich). Furthermore, dust removal from industrial sources and large boilers has resulted in major improvements. These measures have been particularly successful in reducing SO2, particulate and lead emissions, and hence daily and annual average urban atmospheric concentrations, by significant amounts in many cities. However, there has been little or no evidence of similar downward trends in NOx concentrations. During smog conditions, emergency actions are taken by some cities (eg, Rome, Milan, Athens, Prague) involving, for example, restrictions on car traffic and fuel substitution in power plants/industries. Cost-effective energy conservation measures and substitution of fossil fuels with renewable energy resources have a large potential for reduction of emissions.

'Hot spot' air pollution

The term 'hot spot' pollution may be used to describe the high short-term pollution concentrations to which the population may be exposed when located close to pollution sources. 'Hot spot' pollution includes the case of urban streets with busy traffic, and the pollution impact from industrial stacks in cities.

Air pollution from road transport

Urban street pollution is monitored in many cities, and the measurements show that short-term maximum concentrations of carbon monoxide (CO), NO2 and particles may exceed AQGs by a factor of 2 to 4, depending upon the actual traffic and dispersion conditions of the street. It has been estimated (Mage and Zali, 1992) that there are between 9.5 and 18 million people in Europe who spend a considerable part of their working day in and near roadside settings (eg, commuters, those working in roadside buildings), although only some of them experience exceedances of AQGs. However, this population exposure is important, since, undoubtedly, for many regular commuters in city centres this exposure constitutes the main contribution to their total exposure to air pollution. Such exposure occurs in all cities, but high concentrations occur more frequently in cities with unfavourable winter smog dispersion conditions (Table 4.4).

In all European cities, smog occurrences and long-term average concentrations of harmful compounds such as lead, benzene, PM and BaP receive a significant contribution from emissions from road transport. Road transport contributes on average more than half to the nitrogen oxides emissions in Europe, and about 35 per cent of the VOC emissions. Within most cities, the relative contribution from road transport to the overall city emissions is considerably higher. Moreover, most of the summer smog formation in Europe, both in individual cities and on the regional scale in Europe, originates from the potential ozone-forming capacity of VOCs together with relatively high concentrations of nitrogen oxides, both being emitted from road transport. Diesel-engine vehicles also produce very fine particulate material, less than 10 µm aerodynamic diameter (PM10), although their per cent contribution has not yet been quantified.

While NOx emissions from stationary sources have decreased in many urban areas, emissions from motor vehicles have increased because the growth in the number of vehicles and the distance travelled per vehicle has been much larger than the reduction in emission factors. The fitting of three-way catalyst exhaust-cleaning devices is a major improvement, but is limited to new vehicles. Since the lifetime of vehicles is typically more than 10 years and in some countries more than 20 years, reductions in emissions will be slow even if vehicle numbers and distances travelled were to stop increasing. Further improvements are possible, particularly with respect to reductions of VOC emissions, for example through new EC regulations for reducing emissions of petrol vapours from service stations. More rapid improvements in air quality in the short term can only be achieved by limitations on road transport in urban areas.

A particular and significant air pollution problem in Nordic countries results from the use of studded tyres during the winter. These cause major wear of the road surface, producing grit which is suspended in the air through the action of car turbulence. This results in high levels of suspended particulate material close to roads and generally in city-centre air during dry winter conditions.

Urban pollution from industry

In some cities, air pollution from road transport and domestic heating is supplemented by that from local industry. However, depending on the height of the stack and the prevailing wind directions, the impacted area is usually a few kilometres from the emission sources. In many European cities, monitoring stations are operated in areas where principal industries are expected to have a large impact. Only in some cities (eg, Bratislava, Duisburg, Rotterdam, Athens, Antwerp, Turin, Ljubljana and Helsinki)do these 'hot spot' stations indicate a possible significant impact from industrial emissions, ie, air concentration of SO2 and/or PM are considerably higher than in the city centre. Other large European cities with important sources of industrial air pollution and exposure include: Sofia, Bucharest, Zagreb, Perm, Togliatti, Budapest, Cracow, Lille, Lyons, Pozna´n, Lisbon, Donetsk, Odessa and Valencia.

The data collected specifically for this report do not allow an evaluation of the actual magnitude of 'hot spot' industrial pollution exposure in large European cities. (Local industrial pollution in areas outside the largest cities is treated below.)

Population exposure to air pollution

Health effects which arise from exposure to air pollution can be classified as: irritation and annoyance, loss of organ functions (eg, reduced lung capacity), morbidity and mortality (see Chapter 11). Some of these effects can be acute and reversible, while others develop gradually into irreversible chronic conditions. The respiratory system and the eyes are the main organs affected by air pollution, while systemic effects may also be evoked. Table 4.2 also shows expected effects upon exposure to given levels of the pollutants (WHO, 1991; 1992). For the carcinogens, risk estimates are given for lifetime exposure to the indicated concentrations.

The actual exposure of the urban population to air pollutants is difficult to estimate. Besides estimating the spatial distribution and time variation of the pollutant concentration, the location and physical activity level (in relation to inhalation volume rate) of the population should be known. Since detailed data about the activity and actual location of the population are not available, the description of exposure has been limited here to the description of ambient air concentrations in relation to population density.

The role of indoor air pollution to the overall population exposure must not, however, be overlooked. This is a complex problem which involves the entrapment of outdoor air pollutants inside, as well as the indoor release of compounds, such as combustion products, formaldehyde, solvents and tobacco smoke (see Chapter 11 and WHO, in press).

As a rough indication of air pollution exposure, pollutant concentrations have been compared to WHO-AQGs as given in Table 4.2. Qualitative results of exposure estimates are presented in Table 4.4. Data were made available for cities which contained about 116 million of the 150 million people covered by the questionnaires. Of this population, it is estimated that about 56 million people (48 per cent) live in surroundings where at least once a year (actually in 1990) the concentration level is above a short term AQG level for SO2 and/or PM (winter smog conditions). Extrapolation to all 105 cities gives an estimate of 71 million people exposed to winter smog conditions. There is not sufficient city data to calculate exposure to high ozone concentrations. Regional background estimates (see below) suggest that more than 80 per cent of the people of these cities are exposed to high ozone concentrations (above the AQG) at least once a year.

Exposure of materials, buildings and cultural heritage to air pollution

Air pollution in urban and industrial areas increases the deterioration of many buildings and construction materials. For structural metals such as steel, zinc, copper and aluminium, quantitative dose­response relations are available describing the corrosion rate as a function of, inter alia, sulphur dioxide concentrations, chloride deposition rates, and climatic factors such as time of wetness. Pollution effects on other materials have been documented, but general quantitative assessment may still be difficult to express. Extensive documentation exists of the effect of acid pollutants, particularly SO2, on the deterioration of marble, and other calcareous stone used in buildings and monuments (Coote et al, 1991; ECOTEC, 1986; Kucera et al, 1992).

There is a strong correlation of the weight decrease of calcareous sandstone with ambient sulphur dioxide concentrations (Figure 4.2). This deterioration depends on exposure, particularly wetting of surfaces and also on the mineralogical quality of the stone material. Different types of marble and limestone have quite different deterioration characteristics.

In sheltered positions, a surface layer known as 'black crust' forms on calcareous materials. Black crust does not act as a protection patina and it will often conceal advanced stages of decay until larger parts begin to peal away leaving a surface with a bad visual appearance. On several materials, nitrogen oxides and ozone increase the effects of sulphur dioxide attack; moreover, nitrogen compounds are thought to encourage the growth of mosses, lichens and algae on buildings.

Various attempts have been made to estimate the costs of material degradation and maintenance due to air pollution. Extrapolation of the data from one study suggests that costs for damage by sulphur dioxide to buildings and construction materials might be in the order of ECU 10 billion per year for Europe as a whole (Kucera et al, 1992).

Many historic monuments and buildings are affected by air pollutants, especially those made from marble, calcareous sandstone, or other materials susceptible to damage. Many of these objects are situated in heavily or moderately polluted areas and are thus subject to serious deterioration. Examples are the Acropolis in Athens, Cologne Cathedral, and whole cities, such as Cracow and Venice, on the UNESCO cultural heritage list.

The deterioration of historic buildings and monuments may be seen as something to be avoided at all costs. However, materials cannot be expected to last for ever; repairs, and therefore renovation and protective measures, would be necessary even if air pollution levels were reduced. Cost figures are not available. Much of the restoration work which is now being carried out is due to damage which has occurred in the past 50 to 100 years and the accumulated need for repair and restoration is not known.

Exposure to toxic air pollution from industry

Some of the most severe exposure to harmful air pollutants in Europe occurs near individual industrial plants or industrial areas, usually located outside large centres of population. Metal smelters, oil- and coal-fired power plants, chemical industry (eg, production of fertiliser, pesticides) and oil refineries/petrochemical industries are industrial sectors responsible for some of the major local air pollution problems in Europe (RIVM/NILU, 1994). The concentration level of harmful air constituents is obviously dependent upon the type of industry, the level of process technology and emission control, as well as emission conditions, mainly the height of stacks. The technological level of process and emission control is less developed in Eastern Europe, and in parts of Central and Southern Europe as well, than in Western and Northern Europe. This is probably due to a multiplicity of factors including differences in economic development and perhaps also the rigour of national policies on air pollution management and abatement.

Sulphur dioxide, particles and heavy metals are overall the most significant pollutants emitted from industrial sources. Indeed, SO2 and PM (measured by the black smoke or other methods) are the compounds most often monitored near industrial sites. Measurements of heavy metals, fluoride, ammonia and other compounds have also been reported, but this information has not been assessed in this study.

High air pollution exposure from industrial stacks typically occurs under unstable atmospheric dispersion conditions (ie, high insolation and strong vertical mixing) with moderate winds, as opposed to high general urban air pollution which occurs under calm stable weather. An exception is the situation near industries with low-level diffuse emissions (eg, iron and steel works, nickel or aluminium smelters), where air pollution is also highest under calm weather conditions. For both low-level emissions and stack emissions, the exposure situation is the most severe in valleys, when residential areas are located downwind from the plant in the prevailing wind direction(s).

The principal air pollution impact near SO2-emitting industrial sources is typically short term (one to a few hours, although industrial plants may also give extremely high long-term impact), as opposed to general urban air pollution, where the 24-hour average concentrations are often more significant, relative to AQGs.

Comprehensive data on air pollution concentrations and exposure near industrial plants and areas in Europe do not exist. Data have been collected unsystematically, and often not at all. However, selected data on pollution in industrial areas in EU countries have been collected as part of the Council Decision 82/459/EEC. For the present report, information on local industrial air pollution was collected through the return, by more than 20 countries, of a simple questionnaire (RIVM/NILU, 1994).

Data provided by this questionnaire are not complete regarding the number of industrial areas, and data from individual areas are often not very detailed. Nevertheless, an overview is presented in Table 4.5, giving, for each country from which reliable information is available, maximum reported concentrations, location of polluting site, and the types of industries involved.

By far, the most severe exposures to high pollution concentrations occur in Eastern European countries, where, in several industrial areas, largely uncontrolled industrial emissions from old-type processes result in severe exposure of nearby population and vegetation to harmful air pollution. In most of the EU and EFTA countries, efforts to clean up industrial emissions have substantially reduced such problems, but here also industrial areas with air pollution exposure exceeding WHO-AQGs still exist. The following countries indicated no significant local industrial air pollution problems: Denmark, Iceland, Ireland, Sweden and Switzerland. No specific information was available from seven countries.

Table 4.5 shows that high exposure to SO2, PM and other compounds associated with specific industries occurs in all parts of Europe. Sulphur dioxide concentrations above 1000 µg/m3, one-hour average, are not uncommon. Extreme SO2 exposure occurs, for example, in Zlatna and Baia Mare (Romania), Asenovgrad (Bulgaria), Sokolov and Teplice regions (Czech Republic) and in Toru´n (Poland), with annual averages around 500 µg/m3 (Toru´n, Asenovgrad) and 24-hour averages up to 8000 µg/m3 (Baia Mare). Non-ferrous metal (cadmium and aluminium) industries and coal-fired power plants are those industries most often responsible for very high local industrial pollution in Europe. An estimate of the size of the population affected by local industrial emissions in Europe cannot be made at this time.

Conclusions

Considerable improvements in local air quality in recent decades have been achieved in many cities through substitution of coal and heavy fuel oils by cleaner fuels such as gas oil and natural gas, and by electricity and heat supplied from large electric power plants and district heating plants. Emission controls have been particularly successful in the reduction of SO2, dust and fly ash emissions to the atmosphere, and in the reduction of the emissions of various gases from the process industries.

On the other hand, it is estimated that in 70 to 80 per cent of European cities with more than 500 000 inhabitants, air pollution levels of one or more pollutants exceed the WHO-AQGs at least once in a typical year. Similar conditions may occur in many smaller cities. These exceedances are taken in this report as indicators of where possible human health risks and health effects due to air pollution in these cities may occur and where further study may be necessary. This is part of a wider problem concerning the urban environment (see Chapter 10). The main pollution sources are combustion and industry in many Central and Eastern European countries, and road transport, particularly in Western cities. The deterioration of historic monuments and buildings due to air pollution may be seen as a destruction of priceless value. Urban air pollution is therefore regarded as a prominent environmental problem of concern to Europe (see also Chapter 37).

In many cities, consistent information on pollution sources, emissions, air concentrations, exposure and health effects, and their interconnections is not available. To rectify this it will be necessary to collect, harmonise and evaluate data on urban air pollution, human exposure and health effects on a European scale. Data for smaller cities and data on exposure to indoor air pollution, occupational exposure and exposure to pollutants in food, soil and water should also be considered (see Chapter 11 and WHO, in press).

REGIONAL AND TRANSBOUNDARY AIR POLLUTION

Air pollutants can often be transported over considerable distances, affecting air quality and deposition of pollutants in adjacent or distant countries. Such effects can extend over hundreds or several thousands of kilometres. The pollutants involved have residence times in the air of between half a day to one week (see Box 4B). The effects from such regional or continental scale transportation of pollutants have received considerable attention, both scientifically and politically, and have led to the UNECE convention on Long-Range Transboundary Air Pollution (LRTAP), an important framework for environmental assessment and policy in Europe.

Atmospheric processes important on the regional scale are, in general, confined to the lowest few kilometres of the troposphere. However, processes at higher levels of the troposphere may affect concentrations at ground level (see global air pollution, below). Specific weather conditions or increased emissions and often both simultaneously can result in enhanced levels of air pollution. Such pollution episodes may last for several days or even weeks. During winter smog episodes concentrations of a variety of pollutants, including sulphur dioxide and aerosol particles, are high, while during summer episodes concentrations of ozone and other photochemical oxidants may be increased over large areas in Europe. The combination of meteorological patterns and spatial distribution of air pollutant sources (Map 4.1a, 4.1b, 4.1c, and 4.1d) explains to a great extent the deposition and concentration fields in Europe. An overview of these emissions in European countries is presented in Chapter 14.

The presentation below of regional scale air pollution in Europe is based on a discussion of the pollutants introduced in Box 4C grouped under eight different headings. Air pollutants important at this scale constitute a major input/source of pollution to other media (freshwaters, seas, soils; see Chapters 6 and 7) and are summarised at the end of this section.

To help characterise the impacts of acid deposition on surface waters and terrestrial ecosystems (notably forests), and of air pollutants on natural and agricultural ecosystems, 'critical loads' and 'critical levels' have been formulated. Critical loads are defined as those deposition loads below which no adverse effects are to be expected (Downing et al, 1993). Critical levels are defined as those concentrations above which adverse effects on sensitive receptors are expected.

The concept of critical load was developed during the 1980s. Values for critical loads of sulphur and nitrogen for sensitive receptors have been developed and values for critical levels of the atmospheric pollutants SO2, NOx, O3, NHx are under discussion. Although it was realized that the definition of critical loads and levels is an iterative process and that further research was still needed, it was believed that mapping areas where the values presented are currently exceeded would be useful in a first estimate of areas at risk in different regions in Europe. This approach is adopted below. It should be stressed that the exceedance areas thus mapped do not, at present, necessarily correspond to those with recognised adverse effects.

Acid deposition

When the air pollutants SOx (SO2 and sulphate), NOy (NOx, nitric acid and nitrate) and NHx (NH3 and NH4, ammonium) are scavenged from the atmosphere and deposited at the surface, a series of problems result, often collectively referred to as acidification. The recognition of acid deposition as a threat to ecosystems has resulted in major research projects and international negotiations and agreements to reduce emissions. Present understanding emphasises that it is not the acidity of the precipitation itself that matters (as suggested by the familiar term 'acid rain'). Rather, what is important is the acidification of soils and waters, due to the total deposition of sulphur and nitrogen compounds by precipitation (rain, snow and mist) and dry deposition, in relation to the soil's or the ecosystem's capacity to accommodate and utilise these compounds, as well as the direct exposure of vegetation to atmospheric pollution. In the past decade, it has been shown that, next to sulphur and nitrogen oxides and their oxidation products, ammonia (NH3) and its atmospheric conversion products may act as acidifying agents due to conversion to nitric acid in soils and waters. The combined deposition of SOx, NOy and NHx is therefore hereafter referred to as 'potential acid deposition', representing the maximum atmospheric acid load that could lead to acidification.

The actual potential acid deposition pattern in Europe derived from model calculations is shown in Map 4.2 (Sandnes and Styve, 1992; Sandnes, 1993). The highest deposition values are found in the industrialised, densely populated zone that runs from Poland and the Czech Republic, over Germany and the Benelux countries to the UK. Maximum values of more than 10 000 eq/ha/year are found here. This translates into exceedances of critical loads in these areas by several hundred per cent. Map 4.2 is based on model calculations using the EMEP model; the spatial resolution in the model is 150 km by 150 km. The model results are corroborated by deposition measurement. Considerable variation can occur within this grid size, and local deposition may be much higher.

Acid deposition in Europe originates mainly from air pollutants coming from European emissions. Emissions of acid components result largely from combustion of fossil fuels (SO2 and NOx) and agricultural activities (NH3) (see Box 4C). Today, more than 70 per cent of total atmospheric SO2 emissions stem from coal combustion in thermoelectric power plants while motor vehicles account for about 50 per cent of total atmospheric NOx emissions in Europe. Because the spatial distribution of the emissions and the chemical properties of the compounds involved are different, the relative contribution of the three components to the total acid deposition varies over Europe. In Central and Eastern Europe sulphur dominates, whereas in Western and Southern Europe NOx may be relatively more important. In countries

where there is intensive cattle breeding (such as Denmark, The Netherlands, and parts of the UK) ammonia makes a significant contribution. Model calculations (Tarrason, 1992) indicate that anthropogenic and biogenic emissions from North America contribute less than 5 per cent to total SOx deposition on Europe's west coast. The influence of Asian sources is estimated as less than 1 per cent. European emissions from power plants contribute most to total acid deposition (30 to 55 per cent) (RIVM, 1992). Next most important is the contribution of NOx emissions from transport; the range is from 7 per cent in Poland to about 30 per cent in Finland and Norway (see Chapter 31). In the UNECE LRTAP convention, 'budget matrices' have been constructed from which the contribution of emissions in each of 36 European countries and areas of Europe to deposition in these countries and areas can be derived.

Long-term trends in total acid deposition are not yet available because of the lack of 'historical' data on the dry deposition contribution. However, measurements of concentrations in air and precipitation indicate a growing contribution of NOx in environmental acidification. Long- term measurements of the nitrate and sulphate contents of the atmospheric aerosol in southern England show an increasing nitrate to sulphate ratio through a 30-year period (Atkins and Law, 1984). Precipitation networks continue to record an upward trend in the nitrate content of European precipitation. Although the analysis of ice cores from the south Greenland ice sheet is showing a decreasing trend in sulphate, the nitrate content is growing (Mayewski et al, 1986). The nitrate concentrations of over 300 lakes in Norway have shown a doubling over the period 1974 to 1986, despite there being little change in sulphate and pH levels (Henriksen et al, 1988). The impacts of acid deposition are discussed in Chapters 5 (Inland waters), 7 (Soil), 31 (Acidification) and 34 (Forest degradation).

Photo-oxidants and photochemical smog episodes

Under conditions of high insolation and low winds, concentrations of ozone and photochemical oxidants can build up to high levels which are damaging both to human health and to vegetation. Usually such situations take several hours to develop. 'Photochemical smog' is formed particularly in land basins, and in coastal areas where air pollutants may be contained in a land­sea breeze circulation system, as in Barcelona or Athens. However, under high-pressure conditions, ozone concentrations often reach serious levels over much larger areas in Europe, and may reach Scandinavia and other more remote areas following transport of ozone and its precursors from the more central source areas. Because of the strong link to particular meteorological situations, the occurrence of 'ozone episodes' in Europe is very variable from year to year, in both space and time.

NOx and VOC emissions from motor vehicles are the main cause of photochemical oxidant and ozone formation on a regional scale. Volatile organic compounds (VOCs) present in the air from anthropogenic and natural emissions are oxidised in the atmosphere in the presence of sunlight. The process generally starts through reaction with the very reactive hydroxyl (OH) radical, by reaction with ozone, or by direct action of sunlight. The oxidation processes are complex, and NOx play an important role as necessary catalysts. Oxidation takes place on time-scales ranging from a few hours to several months. A host of secondary products is formed during this oxidation; they are usually called photo-oxidants. Ozone is by far the most important of these in terms of the adverse effects it has on human health and ecosystems. The role of less reactive, relatively long-lived VOCs such as methane in tropospheric chemistry, and their contribution to the global background levels of ozone, is discussed below with other 'global' pollutants. Here, only the more reactive organic compounds which lead to photo-oxidant formation will be considered.

Several international bodies such as WHO, UNECE and EC have discussed, proposed or set guideline values for ozone for the protection of human health and/or vegetation (Table 4.6). These guideline values indicate levels combined with exposure times during which no adverse effects are currently expected. However, progress in scientific understanding may lead to revisions of these guideline values.

The 75 ppb hourly concentration level, chosen here as a reference, was exceeded in summer 1989 at all stations in the EUROTRAC-TOR and ECE-EMEP networks, except for a few stations in Scandinavia. In Switzerland and Austria this level was exceeded on more than 20 per cent of days during April to September (Beck and Grennfelt, 1993). These networks of background measurements have a fair coverage in Northern and Western Europe but the information is incomplete and largely lacking in Southern, Central and Eastern Europe. Therefore, model estimates are presented here in order to give a European picture of the problem.

In order to quantify exposure to high levels, excess ozone is defined as the sum of the concentrations minus a given limit value (here 75 ppb), summed over all hours in a period. The modelled values of excess ozone show a large variation over Europe; for summer 1989 the highest values were calculated in Western Europe (Map 4.3) (Simpson and Styve, 1992). Excess ozone depends strongly on number and severity of summer smog episodes in a particular year; at individual stations this quantity can vary by a factor of 100 or more. In heavily populated areas the ozone concentrations may be lower due to chemical scavenging by local nitrogen oxide emissions.

Calculations with a variety of atmospheric transport models indicate that emissions from Europe are the main cause of the occurrence of high ozone peak values (Simpson and Styve, 1992; de Leeuw and Van Rheineck Leyssius, 1991). Model calculations show that implementation of the 1991 VOC protocol under the UNECE LRTAP convention would result in most areas in a 40 to 60 per cent reduction in ozone levels in excess of 75 ppb. The effect on the annual average ozone concentration is much less: 4 to 8 per cent in northwest Europe, but 1 to 4 per cent on average (Simpson and Styve, 1992).

Winter smog episodes

High levels of SO2 and suspended particulate matter (SPM) lead to winter air pollution episodes when low winds and a strong temperature inversion impede the vertical mixing and dilution of pollutants in the lowest atmospheric layers. Low temperatures increase the demand for energy, resulting in increased emissions and the further accumulation of pollutants. During these episodes, emissions related to domestic heating can be up to 70 per cent higher than the winter season average (de Leeuw and Van Rheineck Leyssius, 1990).

As well as SO2 and PM, the winter smog mixture contains compounds such as carbon monoxide (CO), nitric acid, nitrogen dioxide (NO2), and inhalable particles of variable content such as soot, sulphate, nitrate, ammonium, metals and organic compounds, such as polycyclic aromatic hydrocarbons (PAH). The concentrations of strong oxidants such as ozone are low during winter episodes.

Winter smog episodes occur most frequently and are most severe in Central Europe. For the densely populated parts of the Czech Republic, eastern Germany and southern Poland, model calculations, supported by measurements, show yearly averaged concentrations up to 10 times higher than yearly averaged values in Western Europe. Based on these data it is expected that, under winter smog conditions, daily average SO2 concentrations in rural areas will be well above 400 µg/m3. Even higher episodic values can be measured in cities.

As an illustration, a recent winter smog episode from 2 to 13 February 1993 in Northern Bohemia is considered (J Benes Czech Ecological Institute, personal communication) (Figure 4.4). During this episode, daily average SO2 and PM concentrations reached maximum values of 825 and 480 µg/m3 respectively. These values may be compared to the WHO-AQG for the evaluation of ambient concentrations with respect to their probable effects on human health of 125 µg/m3 for SO2 and 120 µg/m3 for PM as 24-hour averages, and EC guideline values of 100 to 150 µg/m3 for SO2 and 100 to 150 µg/m3 for PM. Maximum 30-minute average concentrations were 1850 µg/m3 for SO2, 2600 µg/m3 for PM and 760 µg/m3 for NOx. Similar or even more severe episodes are still expected to occur 1 to 2 times a year in the future. The most severe smog episode ever reported was in London in December 1952 when SO2 and PM daily average concentrations reached values of about 5000 µg/m3 each (see eg, Brimblecombe, 1987). In the two-week period during and immediately after this smog episode, a total of approximately 4000 excess deaths were observed compared to a similar period in previous years (Ministry of Health, 1954).

Winter smog episodes often extend across several European countries due to long range transport across the continent. This is evident from measurements and model calculations. Map 4.4 shows the transboundary spread of an episode in January 1987, with maximum 24-hour SO2 and PM concentrations of 900 and 700 µg/m3 in northwest Europe. Instead of the normal West-to-East transport in Europe, East-to-West transport was important during some of these episodes (see also Chapter 2).

For continental Western Europe (western part of Germany, Benelux countries, France) it has been estimated that during the 1985 and 1987 episodes at least 50 per cent ­ but probably up to 75 per cent ­ of the SO2 was of Eastern European origin. For sulphate, and probably also PM, the Eastern European contribution is probably of the order of 80 to 90 per cent. In contrast, the contribution of Eastern Europe to the annual average sulphur deposition in northwest Europe is of the order of only 10 per cent (Lübkert, 1989; de Leeuw and Van Rheineck Leyssius, 1990).

Aerosol particles

Atmospheric aerosols can be primary ­ directly emitted ­ or secondary ­ formed in the atmosphere by chemical conversion of primary pollutants (see Box 4C). Aerosol particle measurements available at present are insufficient to prepare a European map of aerosol concentration. Only a few of the national monitoring stations report their data to international data collecting programmes. As for other pollutants, particulate measurements are concentrated in urban areas and, being influenced by local sources, are often not representative for larger areas. Moreover, the variety of measuring techniques applied makes it difficult to interpret the data.

A first estimate of average large-scale aerosol concentrations for Europe has been made by means of the TREND atmospheric dispersion model (Map 4.5) (RIVM, 1992). It is estimated that, in the whole of Europe, secondary aerosols form a substantial contribution to total aerosol concentrations in rural areas. In urban/industrial areas the contribution of primary aerosol is high. In the Upper Silesia area, yearly averaged PM concentrations of 180 to 230 µg/m3 are measured, with episodic background values of the order of 500 µg/m3 or higher. For the Czech Republic, PM yearly average values up to 220 µg/m3 PM are reported. The WHO-AQG for total suspended particulates both for maximum daily values (120 µg/m3) and for yearly average values (100 µg/m3) are probably exceeded in these areas.

Sulphate and nitrate aerosol levels are expected to decrease by 20 to 30 per cent between 1990 and 2000. For aerosol particles, including both emitted dust and soot, present emissions are not well known and future developments are uncertain.

Concurrent with technological changes and shifts in fuel use, primary dust emissions from power generation and industry in Central and Eastern Europe are expected to decrease in the next 20 years by at least 30 per cent. Strong emission reductions for dust and aerosol particles can be achieved by implementing various filtering techniques (such as electrostatic precipitation) on power plants and baghouse filters on industrial installations, fuel change from coal to gas in power production and residential heating, and energy conservation. While industrial particulate emissions are expected to decrease, the trend in future transport emissions is uncertain, particularly with respect to soot, since traffic volume is expected to rise strongly in Central and Eastern Europe.

Metals and persistent organic pollutants

Heavy metals ­ mostly aerosol-bound ­ result from combustion processes, industry and transport. The long-range atmospheric transport of heavy metals is well documented, and the influence of anthropogenic sources has been observed as far as the polar regions. Measurements in southern Scandinavia indicate that the concentrations of many trace elements may be an order of magnitude higher in air masses that have passed the European continent than in air masses coming from the North Atlantic region (Lannefors et al, 1983).

Since the 1970s the deposition and concentration of heavy metals have fallen. The decrease is most pronounced for lead, due to a reduction of the lead content in petrol, as illustrated for rural (Cottered), suburban (North Tyneside), urban (Cardiff) and motorway edge (Manchester) measurement stations in the UK in Figure 4.5 (McInnes, 1991).

The deposition of metals in Europe is generally much less than the European emissions, indicating that Europe contributes to deposition outside its borders, to the oceans and to the metal content of the atmosphere. For instance, in a recent study (Petersen, 1992), the deposition of mercury in Europe is estimated at 120 to 300 tonnes/year, while European emissions are 730 tonnes/year. Europe is probably contributing to the background concentration level of 2 to 4 ng/m3. The accumulation of mercury in soils from atmospheric deposition is considered to be the dominant source of high contents of mercury in freshwater fish from lakes in Scandinavia (Chapter 5).

Adverse effects of lead and cadmium are known from a number of specific studies carried out in both West and East Europe (WHO, 1987). Only a few air quality standards have been defined for some metals (for example lead, cadmium, zinc) by national governments and by international bodies. Generally, these are exceeded only at the local scale, close to sources.

For heavy metals, few target or limit values have been defined for atmospheric deposition to prevent accumulation in soil, groundwater and food. Research is ongoing to assess exceedances of target values for soil and sea and related critical deposition load values in Europe for various heavy metals and organic pollutants. The modelled spatial distribution of the deposition of cadmium (Map 4.6) is generally in agreement with the available measurements (RIVM, 1992). The model resolution is not sufficient to resolve the contributions of sources or the precipitation patterns on the local scale. Measurements of heavy metals in mosses in Scandinavia indicate various areas where deposition is increased by local emissions (see Maps 7.5 and 7.6 for lead and cadmium respectively) (Rühling et al, 1992). Deposition maps are available for other metals as well. Uncertainties in emissions, particle size distribution and measurements contribute to the remaining discrepancies between model estimates of deposition and depositions inferred from measurements.

Organic pollutants which have a long environmental residence time are generally called persistent organic pollutants (POPs). These include polycyclic aromatic hydrocarbons (PAHs) and a host of chlorinated organic compounds. Among these, the chlorinated dibenzodioxins and dibenzofurans (PCDD/PCDFs) and the polychlorinated biphenyls (PCBs) are of special concern because of their toxicity and their accumulation in food-chains. These pollutants are present in the atmosphere owing to their use and release in a wide variety of activities. Major sources of PCDD/PCDF are waste incinerators, metal recycling, wood preservatives, transport and the metal industry. Polychlorinated biphenyls have been extensively used in electric equipment such as transformers and capacitors, and as fire-resistant hydraulic fluid.

Concentration levels of dioxins in ambient air are of the order of 1 to 100 femtogrammes (fg) i-TEQ/m3 (fg=10-15 g; i-TEQ = international 2,3,7,8 TCDD toxic equivalents). Model studies indicate deposition loads in northwestern Europe of the order of 1 to 20 ng i-TEQ/m2 per year. There is no doubt that there is a significant transboundary transport of these and related compounds (Van Jaarsveld and Schutter, 1993).

Important sources for PAH are combustion, including transport. For benzo(a)pyrene (BaP), emissions in Europe are dominated by domestic fuel combustion, especially of coal and wood. Highest concentrations are found in Central and Eastern European cities in wintertime. High concentration levels of BaP at more remote locations due to long-range transport have been documented (Lunde and Bjorseth, 1977). Map 4.7 shows the deposition of BaP over Europe from model calculations (Van Jaarsveld, 1994).

Concentrations of PCB in air in Europe are of the order of 10 to 10 000 picogrammes (pg)/m3 (pg=10-12 g). Regulations on the production and use of PCBs were introduced already in the 1970s (OECD, 1973) because of their recognised harmful effects. However, due to releases both before and after these regulations were passed, PCBs are still present in the atmosphere, and their concentrations are being reduced only slowly (Jones et al, 1992).

Visibility degradation by air pollution

Visibility degradation can be caused by primary pollutants ­ soot and other particles ­ emitted directly into the atmosphere, and by secondary pollutants ­ mainly sulphates and nitrates ­ formed in the atmosphere by reactions of primary pollutants (see Box 4C). Using measured sulphate aerosol concentrations from the EMEP network, 'average' visibility under dry conditions may be quantified, and is usually in the range 50 to 100 km. Relative humidities of more than 80 per cent will reduce expected visibility to less than 50 km, depending on the concentration of sulphates and other aerosol particles.

Even in dry conditions, visibility will be reduced greatly if, for example, aerosol concentrations are as high as those shown for 21 July 1990 in Map 4.8. The high concentrations of sulphate aerosols in this case were caused by a combination of rapid oxidation of SO2 and NOx, due to photochemical oxidant formation, and low wind speeds. The reported visibilities shown in Map 4.8b indicate reductions down to 5 km for wide areas across Europe including the UK, the Benelux countries and the Po Valley on this day.

Radioactivity

Airborne artificial radioactivity originates from the operation of nuclear facilities and the past testing of nuclear weapons in the atmosphere. Until the Chernobyl accident (see Box 18E) the fall-out resulting from the airborne testing of nuclear weapons was the largest source of artificial radioactivity in air across Europe, forming a background concentration in air in the order of 10-6 Bq/m3 for caesium-137 (Cs-137, the most significant radionuclide of interest in the long term). This level is many orders of magnitude below atmospheric radiation levels of natural origin predominated by radon (1 to 2 Bq/m3 in outdoor air). Other natural radionuclides present in aerosol particles include beryllium -7 (7Be) and potassium-40 (40K) (see Chapter 16).

The occurrence of airborne radioactivity levels above natural levels and of significance from the point of view of health is always the result of an event, an accident or an unauthorised release. Radioactive decay and the same natural scavenging processes which act on other airborne pollutants usually mean that elevated air concentrations last only over relatively short periods of time (days to weeks). However, this can be sufficient for significant levels of radiation exposure to be experienced and, as a result of the scavenging processes, can lead to deposition and elevated radiation levels in soil and water (see Chapters 5 and 7).

Authorised routine emissions from nuclear installations are based on the exposure limits recommended by the ICRP (International Commission on Radiological Protection) and, more significantly, within these limits on the ALARA (as low as reasonably achievable) principle. In addition to the on-site control of the quantities of radioactivity actually emitted, routine monitoring of radioactivity is carried out in the vicinity of nuclear facilities. This monitoring acts as a check that these limits are adhered to, giving a direct measure of the environmental consequences of the authorised routine emissions, as well as a warning of any unexpected increase in ambient radioactivity, from whatever source.

Away from nuclear facilities, regional scale levels of radioactivity are monitored by networks of radioactivity measuring stations. Data for the period 1984 to 1991 for 12 of the 18 stations across Europe, where highly accurate measurements are continuously performed (Map 4.9), clearly shows the influence of the 1986 Chernobyl accident superimposed on the normal background levels. In the weeks following the accident, levels in air diminished very rapidly through atmospheric dilution, washout of the radioactive plumes and dry deposition. However, the data clearly indicate that it took several years for concentrations to fall back to the pre-Chernobyl level of 10-6 Bq/m3, mainly due to resuspension of deposited activity and to the early dispersion of the caesium-contaminated particles in the upper atmosphere, which acted thereafter as a reservoir from which subsequent leakage occurred.

Atmospheric inputs to other media

Input to coastal seas

Deposition of pollutants from the air plays an important role in polluting coastal seas in Europe. It is estimated that of the total anthropogenic emissions of heavy metals in Europe about 1 to 15 per cent is deposited on the North Sea and 4 to 20 per cent is deposited over the northwestern Mediterranean (see Chapter 5). For parts of the Mediterranean Sea (the Ligurian Sea) atmospheric inputs of heavy metals may exceed the input of rivers, especially for lead (Migon et al, 1991).

About 90 per cent of all the lead deposited annually in the North Sea (some 1120 tonnes in 1991) originates in Belgium, France, western Germany, The Netherlands and the UK. Relative to the total pollution load to the North Sea the atmosphere contributes about 35 per cent for lead, 14 per cent for cadmium and 5 per cent for zinc (Warmenhoven et al, 1989).

Model calculations for the Baltic Sea reveal that, for the second half of the 1980s, the total deposition of lead appears to be approximately 1300 tonnes/year, with 70 per cent of this coming from countries directly surrounding the Baltic Sea; the remainder is due to long-range transport from other regions in Europe (HELCOM, 1991).

The total nitrogen deposition (sum of reduced nitrogen NHx, and oxidised nitrogen NOx) originates mainly from surrounding countries as well. It contributes about 60 to 70 per cent of the total atmospheric nitrogen deposition of 380 to 510 kilotonnes/year to the North Sea (Van Jaarsveld, 1994). For the Baltic Sea, adjacent countries contribute about 65 per cent to the total annual atmospheric nitrogen flux of about 300 kilotonnes of nitrogen (HELCOM, 1991). This accounts for one third of the total load of nitrogen to the Baltic Sea, which has increased by a factor of four since the beginning of the century. This has resulted in increased biological production followed by sedimentation and biological destruction, giving rise to reductions in the levels of dissolved oxygen in the sea (see Chapter 6).

Deposition in coastal areas is substantially greater than in more remote areas. The influence of distance from major pollution sources is illustrated by measurements from the Baltic Sea, where annual nitrogen deposition decreases from about 1100 kg/km2 in the south to around 650 kg/km2 in the northern parts (HELCOM, 1991).

Inputs to forest soils, surface waters and ecosystems

Critical loads for acidity and for sulphur and nitrogen are shown in Map 4.10 (Downing et al, 1993). Comparing these with actual atmospheric deposition over Europe (Sandnes and Styve, 1992; Sandnes, 1993) by calculating the difference gives the exceedance map (Map 4.11). The results show that the greatest excess (more than 2000 acid equivalents per hectare per year) occur in Central and northwestern Europe. Present loads of acidic deposition are higher than critical loads in roughly 60 per cent of Europe. Almost all European countries have areas where current levels of acidic deposition exceed critical loads, and many countries have areas where critical loads are significantly exceeded (by 1000 to 2000 acid equivalents per hectare per year). Central parts of Europe receive 20 or more times acidity than the ecosystems' critical loads, thus affecting the long-term sustainability of these ecosystems at the present levels of deposition (see Chapter 7).

Maps of critical levels are still under development. Analysis of direct effects of atmospheric concentrations of particular pollutants is complicated by the difficulty of pollution measurements in the field and of realistic manipulation of air pollution in laboratory experiments. This situation is further complicated by the fact that, although critical levels are set for individual pollutants (eg, SO2, O3, etc), in reality vegetation is exposed to a complex mixture of potentially damaging substances in parallel with climatic and biological stresses; all these interact with one another in poorly understood ways (but which can be additive, synergistic or antagonistic).

More attention is now being paid to the critical level for ozone with the introduction of a VOC emission protocol under the UNECE LRTAP convention. The original critical level for chronic exposure to ozone for vegetation (see Table 4.6) is now considered unsatisfactory, since it did not take account of exposure outside the 0900 to 1600 'daytime' period, nor does it give greater weight to episodic concentrations. At a UNECE workshop on critical levels for ozone held in Bern in 1993, new advances in scientific understanding, stemming especially from the European Open-Top Chamber Programme, led to two new considerations for critical ozone values. It was recommended that critical ozone levels be defined separately for forest trees and crops, and that those for crops be based on significant crop damage (at least 10 per cent) and those for trees on significant decreased biomass growth. The workshop recommended for crops a critical level of 5300 ppb-hours excess ozone above a reference level of 40 ppb (at daylight, during 3 months), and for forests 10 000 ppb-hours above 40 ppb (24 hours during 6 months).

Whether the critical level for ozone is expressed as an average or as a cumulative dose makes little difference to the situation with regard to exceedances. For most of Europe, ozone concentrations, particularly in recent years, are well above the values set by either method. Even taking into account the uncertainties mentioned above, this suggests that present levels of ozone in the lower atmosphere are damaging forests, crops and natural vegetation, and that efforts are required to reduce these effects by reducing emissions of precursors.

Conclusions

Concentrations of acidifying compounds in the atmosphere lead, after deposition, to acidification problems in soils and freshwaters as well as eutrophication in fresh and marine waters. Acidification is a major problem in Europe. The deposited acidifying compounds originate from emissions of sulphur and nitrogen oxides and of ammonia. The highest depositions are found in the highly populated and industrialised zone extending between Poland and the UK. In 60 per cent of Europe's area, critical loads for acidification are being exceeded and are expected to continue to be exceeded, resulting in prolonged risk for ecosystems. In many European countries these critical loads are exceeded by several hundreds of per cent. Studies indicate that considerable emission reductions of all acidifying compounds are needed to avoid exceedance of critical loads in Europe (reductions of about 90 per cent for SO2 and NOx and more than 50 per cent for ammonia).

In summary, in the next ten years, acid deposition in Europe is expected to decrease following the expected decrease in European emissions of SO2 and NOx (while ammonia emissions will probably not change significantly). But deposition will still exceed critical loads in some areas.

Concentrations of ground-level ozone have exceeded the WHO air quality guideline level of 75 ppb one-hour average for human exposure at nearly all existing European stations. In view of their transboundary character and harmful effects to natural and agricultural ecosystems, as well as to human health, activities leading to acidification and tropospheric photochemical oxidant formation are addressed as prominent environmental problems in Chapters 31 (Acidification) and 32 (Tropospheric photochemical oxidants).

Smog episodes continue to occur during the winter when weather conditions prevent the dispersion of pollutants and emission rates are increased. These episodes occur most frequently and are most severe in Central Europe, where maximum daily concentrations of SO2 and PM can exceed the WHO-AQGs by factors of 4 or more. Both local sources and long-range transport can contribute to these episodes.

Information on atmospheric aerosol concentrations, particularly secondary aerosols and rural concentrations, is limited. Emission reductions have been achieved for some dust and aerosol particles, particularly from large stationary sources, but the trend in transport emission is not yet certain.

Heavy metal depositions and concentrations have decreased since the 1970s, particularly airborne lead concentrations following reductions in the lead content of petrol. Heavy metals such as lead, cadmium, mercury and zinc have been a problem mainly in the immediate vicinity of industrial sources and road transport but are also a subject for concern in terms of long-range transport into the Arctic region and beyond Europe.

Persistent organic pollutants, including dioxins and PAHs, are released from a number of diverse sources and, as for metals, are of concern on the local and transboundary scale.

Visibility reduction is caused by both primary and secondary particulate pollutants. Secondary aerosols produced during photochemical oxidant formation at low wind speeds can be particularly effective in reducing visibilities.

The occurrence of airborne radioactivity levels significantly above the natural background is always the result of an event, an accident or an unauthorised release. Data from a series of monitoring stations across Europe clearly show the accidental release from Chernobyl. The initially high airborne Cs-137 levels from the accident diminished rapidly through washout and deposition in the weeks following, but took several years to fall back to pre-Chernobyl levels because of resuspension and the initial dispersion into the upper layers of the atmosphere.

Atmospheric emissions can contribute significant proportions of the heavy metals reaching the North and Baltic seas, with most coming from countries bordering the seas but some also arriving after long-range transport.

Improved monitoring and emission inventories are required for assessing these mechanisms consistently:

GLOBAL AIR POLLUTION

Scientific understanding and public concern have recently been rising about potential global impacts such as increased exposure to ultraviolet solar radiation, sea level rise, more frequent droughts and storms with all the subsequent economic and societal damage. It is becoming increasingly clear that human activities can now disturb or alter processes at the planetary level. In effect, long-lived gaseous releases from anthropogenic activities can accumulate in the atmosphere, thus destabilising the prevailing equilibrium beyond Europe, and affecting global-scale processes. The main effects known so far are the enhanced greenhouse effect, the depletion of stratospheric ozone and the changing composition of the atmosphere.

Long-lived air pollutants

Air pollutants (eg, CO2, CH4, N2O, CO, O3 and CFCs) with residence times longer than one week have a significance on a global scale. In terms of population or surface area, Europe contributes disproportionately to the global budget of most of these pollutants (Table 4.7).

The main sources of carbon dioxide (CO2) in Europe are combustion of fossil fuel in power generation, road transport and landuse changes. CFC emissions originate from industrial and household applications of refrigeration, packaging and cleaning. In the 1992 Copenhagen Amendments to the Montreal Protocol it was agreed that CFC emissions should be phased out by 1996. Anthropogenic emissions of nitrous oxide (N2O) are probably connected to agriculture, landuse change and combustion; the sizes of these emissions are quite uncertain. Major anthropogenic sources for methane (CH4) in Europe are ruminants (cows, goats, sheep, etc), agriculture (wetland rice cultivation), landfills and energy systems. Nitrogen oxides and carbon monoxide (CO) are emitted predominantly by road transport, power production and industry. Volatile organic compounds (VOCs) are emitted mainly by road transport and industry, with important contributions from industrial and domestic uses of solvents and evaporative losses from the distribution and transport of motor spirit (IPCC, 1990b; 1992; RIVM, 1992).

The changing troposphere

The chemical composition of the global troposphere, the lowest 10 to 15 km of the atmosphere, is changing. Concentrations of both the greenhouse gases that are chemically inert in the troposphere, such as CO2, N2O and the CFCs, and those of more reactive gases such as CH4, CO and O3 are increasing (see Table 4.1). The largest increase (4 per cent per year) has been noted for CFCs, and there have been even greater increases for some individual CFCs (WMO-UNEP, 1991). Recent data verify that the rate of increase of methane has fallen from about 1 per cent in the late 1970s to about 0.6 per cent per year at present (Steele et al, 1992). Carbon monoxide concentrations are increasing by 1 per cent per year in the northern hemisphere. Ozone concentration is increasing in the northern hemispheric troposphere at a rate of 0.5 to 1 per cent per year (Figure 4.6). The increased concentration of all these trace gases contributes to the global warming potential.

Ozone, as already mentioned, is not directly emitted, but is formed in the troposphere by photochemical reactions involving methane, carbon monoxide, VOCs and NOx. A higher average ozone concentration in the troposphere has led to higher background levels of ozone at the Earth's surface. In Europe, the average ozone concentration at ground level has risen from 10 ppb at the end of the last century to about 25 ppb today (Volz and Kley, 1988). Regional episodes of ozone in Europe are superimposed on these higher background levels, with the result that higher episodic levels of ozone occur, aggravating ozone's adverse impacts on human health and vegetation.

Ozone concentrations in the northern hemispheric troposphere are expected to keep rising at a rate of about 0.5 to 1.0 per cent per year, as a consequence of the expected increase during the next decade of global emissions of carbon monoxide, methane, and nitrogen oxides by 0.9, 0.9, and 0.8 per cent per year, respectively. This will lead to increased exposure of humans and ecosystems to ozone.

Another large-scale change of importance is the decline of the self-cleansing ability of the troposphere because of a lowering of the level of the hydroxyl radical concentration. Hydroxyl levels have probably decreased by 5 to 20 per cent since 1900 as a consequence of the build-up of methane and carbon monoxide stemming from anthropogenic sources (Thompson, 1992). Although hydroxyl is present in very small concentrations in the atmosphere, it reacts with virtually all trace gases, including those that would otherwise be inert. As a result, lower concentrations of hydroxyl increase the atmospheric residence times and hence the concentrations of trace gases. The very complex chemistry of the troposphere provides some compensation mechanisms (eg, increased concentration of hydrogen peroxide concurrent with lower hydroxyl levels could speed up oxidation of SO2 in clouds). Generally, however, this lowered reactivity of the troposphere may have important impacts on trace gas concentrations ­ including, for example, methane, which contributes to the global warming potential, and substitutes for CFCs such as HCFCs, which could reach the stratosphere and deplete the ozone layer.

For trace gases with very long residence times, such as CO2, N2O and CFCs, the European contributions to tropospheric change are approximately those given in Table 4.7. This does not apply to methane, for which the observed concentration trend may be towards higher levels than can be explained from the increased global emissions, as a consequence of the reduced hydroxyl concentration (IPCC, 1992). Simplified model calculations indicate that if global emissions of methane and carbon monoxide were reduced by 10 per cent ­ about equal to Europe's share ­ methane levels would stabilise rather than increase (Rotmans et al, 1992). For trace gases such as ozone, CO, NOx and VOCs the residence time is less than the time needed for global mixing, and Europe's contribution to tropospheric change cannot be derived simply from Table 4.7. Model calculations indicate that European contributions to tropospheric ozone concentrations are of the order of 10 to 15 per cent of present levels, with considerable uncertainty (Roemer and Van den Hout, 1989).

Aircraft emissions play an important role in changes in the upper parts of the troposphere, the level at which aircraft cruise. Model calculations suggest that NOx emissions from aircraft are responsible for 5 to 12 per cent of the observed ozone concentrations in the northern hemisphere's upper troposphere (Beck et al, 1992).

Recently, other greenhouse gases have also been discussed, especially fluoridised compounds such as carbon tetrafluoride (CF4) and sulphur hexafluoride (SF6). Globally the contribution from these gases to the global warming potential is modest. Nationally, however, such emissions contribute significantly to total greenhouse gas emissions.

Depletion of stratospheric ozone

The observed depletion of the stratospheric ozone layer during the last decades is a problem of global scope (WMO-UNEP, 1991). The major concern is that certain wavelengths of ultraviolet solar radiation penetrate the atmosphere down to the Earth's surface, damaging human health and ecosystems. Another important consequence could be a change in global circulation and global climate, as absorption of solar radiation by ozone leads to heat formation, which is a key factor in stratospheric climate. It is now evident that the principal cause of this problem is the substantial increase of industrially produced unreactive CFCs and halons which have found their way to the upper atmosphere and are expected to cause ozone depletion up to at least the year 2100.

The annual average depletion of beneficial ozone in the stratosphere, which provides protection against excess ultraviolet-B (UV-B) radiation, is not compensated by the average increase in destructive tropospheric ozone, since the stratospheric ozone layer contains about 90 per cent of all the ozone in the atmosphere (Figure 4.7).

The annual average ozone column abundance (the sum of stratospheric and tropospheric ozone) has now been decreasing on a global scale for more than a decade, apparently at an increasing rate. The trend is larger nearer the poles, with no detectable change at the tropics. In the northern hemisphere the largest depletion has been at mid-latitudes at an average rate of 0.4 to 0.5 per cent per year over the period 1979­90 (Stolarski et al, 1991). WMO has reported that seasonal average values in winter 1991­92 over the entire 50°­60° N latitudinal belt, including Northern Europe, were 11 per cent lower than multi-year averages, an unprecedented minimum in 35 years of observations. Preliminary evaluations (Bojkov et al 1993) show even larger depletions (13 per cent) in winter 1992­93. These recent large ozone depletions coincide with high stratospheric aerosol loadings arising from the eruption of the Pinatubo volcano in 1991.

The decrease is dramatically manifest in spring at polar latitudes. The Antarctic ozone hole that started to appear after 1975 tends to become deeper, last longer and extend to a larger area each year. In 1992, WMO reported up to 65 per cent ozone destruction in the Antarctic spring. For the first time, inhabited parts of South America experienced a 50 per cent reduction in total ozone amount, causing approximately doubled UV-B radiation levels (WMO press releases, 1992). In the Arctic region, a recent European research campaign has demonstrated strongly perturbed atmospheric chemistry characterised by high levels of active chlorine compounds (Pyle, 1992).

Stratospheric levels of chlorine have increased from a pre-industrial level of less than 1 ppb to more than 3 ppb, and are not expected to decrease before the end of the century, even if emissions could be halted now (RIVM, 1992). In the 12-year period 1976 to 1987, the EU countries produced 3.9 Mtonnes of CFCs, 38 per cent of world production. Europe consumes about 0.4 Mtonnes per year of CFCs and halons, some 34 per cent of the world's total. Consumption in Eastern Europe is only 1 per cent of the world's total (RIVM, 1992). Assuming CFC emissions to be the dominant cause of ozone depletion, Europe's present contribution is probably close to one third of the global total.

The Copenhagen Amendment to the Montreal Protocol will put a stop to global emissions of CFCs, carbon tetrachloride and methyl chloroform before 1996. The EC has already adopted a faster mandatory reduction scheme aiming at phase-out before 1995.

There are many alternatives for replacement of the CFCs in such uses as aerosol propellants, foam blowing agents and cleaning of electronic equipment. Replacement of refrigerating, freezing, and air conditioning equipment will have to be made in stages. However, substitute products do still contain chlorine or bromine atoms and, moreover, have an elevated global warming potential and thus should be replaced themselves rather rapidly.

As a consequence of ozone layer depletion, an increase of harmful UV-B radiation is expected to occur in Europe. Since adequate and well calibrated UV-B trend measurements are in short supply, it has not yet been possible to detect a positive trend in UV-B radiation in Europe, possibly also because of other compensating atmospheric factors. Figure 4.8 shows an example where a positive trend is indeed evident. An increase in UV-B radiation levels, if it does occur, is likely to produce increases in skin cancers, effects on the human immune response system and effects on aquatic and terrestrial ecosystems, although these effects cannot yet be quantified. The effects on elements of aquatic ecosystems, such as phytoplankton, could have severe consequences for biomass production and ocean food-chains (UNEP, 1991).

Given the global consequences and the significant European contribution to this problem, stratospheric ozone depletion is considered as a prominent environmental problem (see Chapter 28).

Global climate change

Levels of greenhouse gases regulate the temperature of the Earth and its atmosphere. The most important of these gases are water vapour and carbon dioxide; of lesser importance but still significant are CFCs, methane, nitrous oxide, and ozone. According to climate change theory, an increase in the concentration of these gases should cause an increase in global average surface temperatures (IPCC, 1990b;1992). Indeed, palaeoclimatological data provide some support for this theory (Schneider, 1989). However, it should be noted that emissions of yet another group of gases, SO2 and dimethylsulphide (DMS), lead to the build-up of sulphate aerosols in the atmosphere, which may affect cloud structure and earth albedo (reflectivity). In this way, these aerosols may partly offset the effect of greenhouse gases by reflecting solar radiation back into space (Charlson et al, 1992).

The growing interest in the greenhouse effect has led to more intense study of historical and current climatic data. Some major findings have emerged (see, eg: ECGB, 1992):

Although these observations are broadly consistent with theoretical predictions of climate models, it is still rather uncertain when and how much global warming to expect. This is because large gaps remain in our knowledge of the global climate system, as in the strength of feedbacks to climate from the atmosphere, ocean, and biosphere (the role of clouds, for example), or in the influence of particles on the Earth's energy balance and global warming potential. The assessment (derived from global climate models) of the Intergovernmental Panel on Climate Change (IPCC) is that a doubling of pre-industrial CO2 would lead to an increase of global average temperature of 1.5 to 4.5°C (IPCC, 1992).

To evaluate the consequences of climate change, it is common to take as a reference case the doubling of CO2 (or an equivalent amount of total greenhouse gases) in the atmosphere relative to its pre-industrial level (before the years 1750­1800). As to the likely timing of a doubling of CO2 in the atmosphere, the IPCC scenario (IS92a) suggests this will occur by about 2050 (IPCC, 1992).

Model predictions of climate change are that the temperature increases will be greater at high latitudes than low latitudes. Hence the extent of temperature increases in Nordic countries is expected to be larger on average than those in the Mediterranean area. The IPCC reports predictions for Southern Europe from pre-industrial times to the year 2030, under the assumption that average global warming would be 1.8°C by that year. Temperature was estimated to be 2°C warmer in winter and 2 to 3°C warmer in summer, precipitation 0 to 10 per cent greater in winter and 5 to 15 per cent lower in summer, with a ­5 to +5 per cent change in soil moisture in winter and a ­15 to ­25 per cent change in summer (IPCC, 1990a).

The main biophysical consequences of climate change in Europe will be (see Chapter 27): a rise in the global mean sea level creating threats to coastal areas, and risks to ecosystems in connection with rapid changes of temperature and hydrological conditions.These in turn will alter agricultural potential, hydrological processes leading to increased land degradation and threats to the food supply. The overall European contribution to the processes of global climate change is significant, although unevenly distributed across the continent. Annual CO2 emissions are estimated to be about 2200 million tonnes of carbon (Table 4.7) from fossil fuel combustion and cement manufacturing, which is equivalent to approximately 30 per cent of the world's total. Carbon dioxide emissions from landuse changes are relatively minor compared with other world regions. European nations also emit 55 million tonnes a year of methane, another important greenhouse gas (Table 4.7); this is about 16 per cent of the world's total anthropogenic emissions of methane. Carbon dioxide taken up by Europe's forests may partly offset emissions from the region (see Box 23F).

An effective CO2-equivalent doubling of greenhouse gas concentrations is expected around 2050 and will result in rapid changes in surface temperature (about 0.3°C/decade), precipitation and sea level (3 to 10 cm/decade), leading to high risks and to substantial costs. For most trace gases contributing to climate change, the global emissions are expected to rise in the next decade: carbon dioxide at 2.10 per cent/year, methane at 0.9 per cent/year, and nitrous oxide at 0.6 per cent/year. Tropospheric ozone concentrations are also expected to keep on increasing. In contrast, CFC emissions are expected to decrease to near zero around 2000.

Apart from the obvious way in which Europe affects climate change by emitting and absorbing greenhouse gases, there are less obvious linkages between the European continent and the regional and global climate. These linkages are sometimes called climate feedbacks, with one example being the feedback between land-cover and climate. In the future, a combination of economic development and climate changes may alter the proportion of Europe's land that is covered by forests, crops, settlements, and so on. Moreover, the amount of snow and ice coverage will also be modified by changes in the region's climate patterns. Together, the changes in land-cover and ice will alter the amount of solar radiation reflected by Europe, as well as the amount of heat and moisture that it gives off or absorbs. This in turn will affect the Earth's heat balance, which will lead to further changes in the region's climatic patterns. Hence, a change in land-cover and ice brought about by global warming or other factors will feed back to the climate system, possibly accelerating (or dampening) the rate of climate change. (For further detail see Chapter 27.)

Conclusions

Long-term changes to the composition of the atmosphere may potentially lead to changes in the world's climate. Europe plays important direct and indirect roles in these changes. Europe's contribution to most anthropogenic emissions of greenhouse gases and ozone depleting compounds is proportionately far larger than its geographical extent and population size relative to the rest of the world. At the same time, Europe may suffer consequences of global changes from non-European sources of emissions.

If fully realised, impacts could be far-reaching with: increased occurrence of skin cancer due to exposure to enhanced UV-B radiation; flooding and salinisation of groundwater in coastal areas due to sea level rise; alterations to ecosystems, land degradation, and changed runoff patterns leading to modification of agricultural potential. Regional predictions of these global effects are still very uncertain.

From the perspectives of both cause and effect, climate change and stratospheric ozone depletion have been ranked as prominent environmental problems facing Europe as a threat to sustainability, and a risk to human health and ecosystems.

SUMMARY AND CONCLUSIONS

Urban air pollution

Urban and local air pollution is a potentially important factor affecting human health, especially in Central and Eastern Europe. It was estimated that in 70 to 80 per cent of the 105 European cities with more than 500 000 inhabitants studied for this report, levels of one or more pollutants exceed the WHO air quality guidelines (WHO-AQG) at least once in a typical year. Long-term exposure to SO2 and particulate matter exceeds WHO-AQG in 24 of the 61 cities with winter smog problems. Especially in Eastern Europe, estimated long-term exposure to benzo(a)pyrene and particulates is above agreed risk levels, in some cases by a factor of 100 or more.

Short-term peak levels of ozone during summer smog episodes are estimated to exceed AQG values in 60 large cities, and sulphur dioxide and particulates or black smoke levels in 79 cities, in each case concerning over 100 million inhabitants.The main pollution sources are combustion and industry in many Central and Eastern European countries, and road transport, particularly in western cities (see Chapter 10).

Some local improvements in urban air quality have occurred since the 1970s as a consequence of technical and structural improvements. These improvements have been particularly dramatic for SO2, particulate material and lead in many areas. Pollution peaks in urban/industrial areas can, however, occur unnoticed by existing monitoring networks due to: the sparsity of the network, unsampled pollutants, inappropriate location and obsolete or ill-maintained sampling apparatus. Monitoring systems need improvement. In many cities, consistent information on pollution sources, emissions, air concentrations, exposure and health effects, and their interconnections is not available. Assessing urban air quality throughout Europe for this report has required a new collection of data. To obtain a consistent and regular assessment of air quality in Europe there is a need to monitor, collect and evaluate data on urban air pollution, human exposure and health effects in a harmonised manner at the European scale.

A desire to reduce the health effects of air pollution and the associated costs can be considered as one of the major reasons for air pollution emission reductions. Furthermore, annual maintenance costs due to air pollution are of the order of ECU 10 billion. For these reasons urban air pollution is recognised as an important environmental problem in Europe. The processes leading to urban air pollution are described in Chapter 10. Its links with the wider issue of urban stress are addressed in Chapter 37.

Regional air pollution

Pollutants originating from sulphur and nitrogen oxides and ammonia emissions can be transported long distances downwind and deposited onto natural surfaces, causing major problems including acidification and contamination of soils (Chapter 7) and surface waters (Chapter 5). This has dramatic consequences for the diversity and conditions of ecosystems including forests and crops. Fish stock die-back is a serious problem in regions susceptible to freshwater acidification. The highest deposition levels are found in the highly populated and industrialised zone extending beween Poland and the UK. In 60 per cent of Europe's area, critical loads for acidification are being exceeded, and in many European countries these critical loads are exceeded by several hundreds of per cent, for example in Norway and Sweden. Acidification has already been identified as a priority problem for which a European abatement strategy has been developed in the UNECE Convention on Long-Range Transboundary Air Pollution. Because of the effects of acidification on forest and aquatic ecosystems it is also addressed as a prominent environmental problem in Chapter 31 of this report. It has been estimated that very significant emission reductions of all acidifying compounds are needed to avoid exceedance of critical loads in Europe (about 90 per cent for SO2 and NOx and more than 50 per cent for ammonia).

Photo-oxidant formation from increased NOx and VOC emissions has led to elevated ozone concentration values exceeding the one-hour 75 ppb maximum level proposed by the WHO at nearly all existing European stations. Photochemical ozone, both in the free troposphere and at ground level, has been recognised as a priority problem by the EU and UNECE, in view of its transboundary character and its harmful effects on human health and on natural and agricultural ecosystems. Photochemical ozone is also considered as a prominent environmental problem (see Chapter 28).

Severe extensive winter smog episodes creating health hazards occur frequently in Central and Eastern Europe, and less frequently also in other parts of Europe. In the densely populated parts of the Czech Republic, eastern Germany and southern Poland, levels of sulphur dioxide and particulate matter can exceed short-term WHO-AQGs by a factor of four or more. Combustion processes, including residential heating, and power production and industry are the main sources for this problem. Concentrations of lead and other heavy metals in air have decreased since the 1970s (lead decreases were mostly in Western Europe following the introduction of unleaded petrol).

The total input of pollutants, especially of heavy metals and nutrients, to the Baltic Sea, the North Sea or the Mediterranean Sea is estimated to have a significant atmospheric contribution (5 to 70 per cent). Most of this atmospheric input is coming from adjacent countries. These estimates of atmospheric inputs into regional seas are rather preliminary, but still have been valuable for supporting protection measures for the North Sea and the Baltic Sea.

For many compounds, emission inventories are uncertain (eg, for VOCs and ammonia), not up to date (heavy metals) or even missing (aerosols and persistent organic compounds). Future assessments of atmospheric pollution and reduction measures will need improved and quality assured European emission inventories (see Chapter 14). For pan-European transboundary air pollution problems the UNECE LRTAP convention and the EMEP network have formed the most important source of data and assessments. Once fully operational, the CORINAIR system, available to all countries in the UNECE area, will provide more detailed data relating to the type and structure of sources and their emissions.

Air quality monitoring coverage is still insufficient for ozone and VOCs but especially for toxic substances such as aerosols, metals and persistent organic pollutants for which no European network exists. No formal European network for supporting smog alert systems is available yet. The EMEP network for acidifying compounds and photo-oxidants is still too sparse to cover regional air pollution and deposition problems in certain regions, especially southwestern and Eastern Europe. Impacts of air pollutants on natural and agricultural ecosystems are starting to be monitored, for example, through integrated ecological monitoring. Critical levels for a number of air pollutants are under discussion and conclusions are still evolving.

Global air pollution

The build-up of long-lived pollutant compounds in the atmosphere alter the composition, chemistry and dynamics of the atmosphere with possible climatic changes and depletion of the protecting shield against solar ultraviolet radiation provided by the stratospheric ozone layer. Europe's contribution to most anthropogenic emissions of greenhouse gases and ozone-depleting compounds is disproportionately large in relation to its geographical extent and population size.

Tropospheric ozone concentration in the northern hemisphere is increasing continuously at a rate of 0.5 to 1.0 per cent per year. European emissions contribute some 10 to 15 per cent to the present levels. High ground level concentrations contribute to effects on humans and ecosystems. There is concern about the concurrent decline of the self-cleansing ability of the atmosphere that may lead to longer residence times of many trace gases, enhanced greenhouse effects and more extensive stratospheric ozone depletion.

While 'bad' tropospheric ozone is increasing, 'good' stratospheric ozone has been depleted in the northern hemisphere at an average rate of 0.4 to 0.5 per cent per year over the period 1979 to 1990. Winter 1991/92 average values over the entire 50­60°N latitudinal belt were 12 per cent lower than multi-year averages. The contribution of European emissions to this problem is around 35 to 40 per cent. Exposure to enhanced UV-B radiation in Europe would lead, according to model calculations, to increased occurrence of skin cancer and eye cataract, and depletion of sea plankton, thus affecting the food-chain.

Observations of climate changes are broadly consistent with theoretical predictions of climate models, but it is still rather uncertain how much global warming can be expected in Europe. If realised, impacts could be far-reaching:

Europe's contribution to climate change includes emission of greenhouse gases, but also absorption by its forests. Changes in landuse may also affect climate change.

Tropospheric change, stratospheric ozone depletion and climate change are global problems with a relatively large contribution from Europe, and possibly large impacts in Europe. These characteristics combined with the risk to human health and the damage they inflict on ecosystems is the basis for treating them as prominent environmental problems (see Chapters 27, 28 and 32).

Overall

Some local improvements in urban air quality and regional deposition levels have been achieved in Europe as a consequence of international conventions and technical improvements, as well as economic and societal incentives. However, urban air concentrations exceeding AQG, and hence potentially detrimental to human health, or causing damage to material and buildings, are still observed in many places. Moreover, deposition of acidifying compounds and photo-oxidants exceeds critical loads for protection of natural ecosystems over large parts of Europe. Technological possibilities exist for greatly reducing many urgent air pollution problems. However, following past attention on reducing the impact from stationary combustion emission sources, emissions from mobile sources ­ in particular from road transport ­ are now increasing in absolute terms or as a proportion of total emissions.

There is a need to establish harmonised monitoring networks and assessment methodologies (analysis, models, inventories, description of uncertainty) across countries to assist in optimising the control strategies which are required.

Finally, scenario studies are needed to assess the impact of economic development and implementation of new technologies on pollutant emissions (RIVM 1992, 1993). Such studies have not been incorporated here since they go beyond the scope of this report.