4. Air

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).

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