Box 4C Pollutants leading to regional air pollution

Primary pollutants with residence times corresponding to the regional scale include the acidifying species sulphur dioxide (SO2), nitrogen oxides (NOx), and ammonia (NH3), and aerosol-bound pollutants (such as dust, heavy metals, and persistent organic pollutants). Secondary pollutants that are formed through chemical reactions in the atmosphere on these time-scales have to be considered as well. Examples are the oxidation products of SO2, NOx and NH3, sulphate, nitrate and ammonium respectively and photochemical oxidants such as ozone, aldehydes and peroxides (eg, PAN) resulting from atmospheric reactions of emissions of volatile organic compounds (VOCs) and NOx. These air pollutants lead to the following typical regional air pollution problems: atmospheric deposition of sulphur and nitrogen compounds leading to acidification of soils and surface waters; photochemical oxidant formation; and occurrence of summer and winter type pollution episodes. Air pollution problems (including visibility degradation) are also associated with aerosol particles, metals and persistent organic pollutants (including their deposition in marine waters).

Sulphur dioxide, nitrogen oxides and ammonia

Anthropogenic emissions of SO2 are due largely to combustion of sulphur-containing fossil fuels (oil, coal and lignite). Nitrogen oxides are also emitted mainly from combustion sources: especially transport, power generation and domestic heating. Agriculture, particularly the storage and spreading of animal manure, is the main source of ammonia emissions to the atmosphere. In Europe, anthropogenic emissions outweigh emissions from natural sources: for SO2 and NOx natural emissions are less than about 4 per cent of total emissions (Tarrason, 1991; Builtjes and Hulshoff, 1991), and for NH3 the natural contribution is estimated with a large uncertainty to be approximately 9 per cent (Asman, 1992).

Volatile organic compounds

Road transport and industry (in particular, combustion of fossil fuels, solvent use and evaporation losses) are the most important sources of VOC. Natural sources are vegetation (especially forests), soil and oceans. VOC emissions consist of a mixture of many organic species, each having its own reactivity and time-scale in ozone formation. VOC emissions, both total amount and speciation over the various chemical reactivity classes, are less well known than SOx or NOx emissions. The potential of the individual VOC components in ozone formation varies widely. Although ozone formation depends on the actual weather situation and the physico-chemical conditions (eg, concentrations of VOC and NOx), in general terms the ozone-creation potential decreases in the order: aromatics (excluding benzene), olefins, aliphatics, methane.

Aerosol particles

Aerosol particles are microscopic solid or liquid particles suspended in air. This includes wind-blown sand and dust and a host of air pollutants. The chemical and physical properties of atmospheric aerosols vary widely. The size of the aerosol varies from smaller than 0.1 up to a few hundred microns (µm) in diameter. Aerosol particles smaller than about 10 µm are considered to be mainly responsible for health effects. Typical examples of aerosol-bound pollutants that have adverse effects are heavy metals, persistent organic pollutants with a high molecular mass (such as PAHs and dioxins), and soot or black smoke. Atmospheric aerosols consist partly of primary (directly emitted) aerosols, and partly of aerosols formed in the atmosphere by chemical conversion of primary pollutants such as SO2, NOx, NH3 and VOCs. These secondary aerosols generally have a fine particle size of less than 2.5 µm aerodynamic diameter (standard borderline between large and small/fine particles). No official data for European emissions of aerosols exist, although several estimates are available (RIVM, 1992; 1993). The most important source categories for aerosol emission are the combustion of (fossil) fuels by industry, road transport and households. Reduced visibility is one of the most obvious effects of air pollution by secondary aerosols.

Atmospheric visibility is limited by the attenuation of light. In the case of fog, this attenuation is effected by atmospheric water droplets. In clean, dry air, the molecules of air limit visibility to about 300 km, but the presence of small particles generally reduces visibility to much shorter distances, particularly in areas affected by air pollution. Studies, in both North America and Europe, indicate a strong causal relationship between the concentration of small particles and visibility (WMO, 1983). In urban air, soot and other primary particles are important contributors to visibility impairment, whereas in rural areas the visibility reduction is caused mainly by sulphates and nitrates (Leaderer et al, 1979; Barnes and Lee, 1978) (see also Regional and transboundary air pollution).

Both sulphates and nitrates take up water and transform to liquid droplets at relative humidities above 70 to 80 per cent. This water uptake increases the particles' effect on visibility. However, for all relative humidities the optical attenuation is nearly proportional to the air's content of sulphates and nitrates. The relative contribution of elemental carbon, or soot, to visibility reduction in rural areas can be estimated at 10 to 20 per cent on the basis of estimated emissions and limited measurements (Heintzenberg and Meszaros, 1985).

Metals

Metal emissions result predominantly from combustion (coal, oil and waste incineration), the metal industry and transport (lead and copper) (Pacyna and Munch, 1988). However, emission data for metals are rather limited and uncertain. Metal emissions pose a direct health hazard to people through inhalation. When deposited they can be taken up by vegetation and may enter the food chain, resulting in exposure to humans and animals.

Trends

The emissions of all these pollutants have strongly increased in Europe since the beginning of this century, although the increase has been much more dramatic for NOx and SO2 than for ammonia (see Figure 4.3). During this century, the 'hot spots' of SO2 emissions have moved from West to East and South: from the Black Country in England to the Ruhr area and thence to the 'Black Triangle' ­ the border area between Germany, Poland and the Czech Republic (Mylona, 1993). Due to the growing industrialisation of Southern Europe, emissions in this area are expected to become relatively more important. The largest NOx emission densities have until now been mostly located in Western Europe (UK, Germany, Benelux); these are expected to shift eastwards in the immediate future. For ammonia, the largest emission densities are in The Netherlands and Denmark as a result of intensive cattle and pig farming.


Figure 4.3 - Temporal evolution of European emissions of acidifying compounds since the beginning of the century

Source: RIVM