Levels of all air pollutants vary markedly across the country. Data from the national air monitoring networks have shown that patterns vary from pollutant to pollutant, depending on how they are formed and where their major sources are located.
Primary Pollutants
Primary pollutants are those emitted directly into the atmosphere. They tend to have highest concentrations around their sources, which are usually in urban and industrial areas.
Sulphur dioxide provides a good example of this. Domestic or industrial fuel burning are the major sources of this pollutant. The highest levels are often found in Belfast, where solid fuel is still used extensively for domestic heating. Short-term levels of high sulphur dioxide may also be seen from time to time around power stations when prevailing winds and atmospheric conditions combine to give occasional episodes in nearby locations.
Motor vehicles are now a major source of air pollution in many large cities. In particular, most of the carbon monoxide, nitrogen dioxide, and hydrocarbons (e.g. benzene and 1,3-butadiene) are emitted from traffic, together with a significant proportion of the particles (PM10). Concentrations of all these pollutants are therefore usually highest in built-up urban areas.
Secondary pollutants
In general, patterns of secondary pollutants such as ozone - which are formed by chemical reaction in the atmosphere - are different from those of primary pollutants; they are less dependent on emission patterns, and tend to be more strongly influenced by meteorology.
UK-wide concentrations of two of the most important secondary pollutants, ozone and nitrogen dioxide are shown in Figure 12 and Figure 13.
Ozone is formed by a series of chemical reactions involving oxygen, oxides of nitrogen and hydrocarbons - so called precursor pollutants. UV radiation drives these reactions and, as a result, ozone production is highest in hot, sunny weather. This process takes from hours to days to complete. As a result, ozone can often be formed considerable distances downwind of the original oxides of nitrogen and hydrocarbon sources.
Patterns of ozone concentrations are also influenced by other factors. Concentrations in urban areas, especially where there is a lot of traffic are often lower than in the surrounding countryside. This is because the emissions from vehicle exhausts react very quickly with ozone (O3) to turn it into oxygen (O2). The concentrations of ozone in areas with high traffic density therefore decrease rapidly as this reaction occurs. This effect is usually called ozone 'scavenging'.
Because ozone is very reactive, it is also readily removed from the atmosphere by being deposited onto the ground or onto vegetation. This process is more important in sheltered, lowland areas than exposed higher altitudes. As a result, ozone levels can often be higher at elevated locations.
The net result of all these effects is shown in Fig. 12. The highest summer ozone concentrations are seen in the rural parts of South East England and Wales; these areas are either sunny or elevated, or both, and are often downwind of polluted areas of Northern Europe. By contrast, lower levels are found in low-lying sheltered areas, or in cities where ozone scavenging is important.
The UK-wide pattern of nitrogen dioxide concentration in Fig.13 offers an interesting contrast. Although some nitrogen dioxide is emitted directly from vehicles or other sources, most is formed by rapid chemical reaction. Nitric oxide is emitted from vehicles in significant quantities, and this is oxidised to nitrogen dioxide by reaction with ozone. Concentrations of nitrogen dioxide therefore tend to be highest in urban areas where traffic densities are high, such as in London.
Particles
Particles are not a chemical species like the other pollutants measured in the automatic networks, but is material from many sources that is classified on the basis of its size, rather than its chemical composition. In the automatic networks, particles of material that are 10mm across, or smaller (mm - a micron, which is a thousandth of a millimetre) are measured. The species is therefore called PM10 - for Particulate Matter less than 10 microns.
The sources of particles (PM10) are particularly diverse. They are produced from motor vehicles, fuel burning, building work, industrial emissions, soil and road dust and quarrying. These are the primary sources of particles, because they are emitted directly into the air.
A significant proportion of particles (PM10) are secondary particles, formed by the reaction of gases in the air. Particles of ammonium sulphate and nitrate are produced by the same photochemical reactions which give rise to ozone. Like ozone, secondary particles can therefore be formed a long way from the sources of the gases that take part in the reactions - even in different countries. These sources vary in importance geographically and seasonally, and their relative importance is still not fully understood.
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