The pollutants we monitor

Nitrogen Dioxide
Particulates
Ozone (O3)
Sulphur Dioxide

Nitrogen Dioxide

Nitrogen oxides are formed during high temperature combustion processes from the oxidation of nitrogen in the air or fuel. The principal source of nitrogen oxides (NOx) is road traffic, which is responsible for approximately half the emissions in Europe. Nitric oxide (NO)and nitrogen dioxide(NO2) concentrations are therefore greatest in urban areas where traffic is heaviest. Other important sources are power stations, heating plants and industrial processes.

Nitrogen oxides are released into the atmosphere mainly in the form of nitric oxide, which is then readily oxidised to nitrogen dioxide by reaction with ozone (O3). Elevated levels of nitrogen oxides occur in urban environments under stable meteorological conditions, where the air mass is unable to disperse.

For the purposes of local air quality management, nitrogen dioxide is the pollutant of concern as it has a variety of environmental and health impacts. It is a respiratory irritant, may exacerbate asthma and possibly increase susceptibility to infections.

Particulates

PM10 are those particles, defined as less than ten microns in size (one micron being one millionth of a metre), which are able to penetrate deep into the lungs. These are determined as a weight per volume of air.

Particles are often classed as either primary (those emitted directly into the atmosphere) or secondary (those formed or modified in the atmosphere from condensation and growth).

Primary PM10 is produced from combustion processes, in particular diesel combustion, where transport of hot exhaust vapour into a cooler tailpipe or stack can lead to spontaneous nucleation of carbon particles before emission. Other sources of primary particles include mineral operations, industry, construction as well as many natural sources such as salt from the sea or fragments of pollen grains and fungal spores.

It is less easy to ascribe the original sources of secondary particles. They comprise mainly ammonium sulphate and nitrate, originating from the oxidation of sulphur and nitrogen oxides to acids, which are then neutralised by atmospheric ammonia derived from agricultural sources.

Concerns about the potential health impacts of PM10 have increased very rapidly over recent years and there is a lot of research being carried out to ascertain the exact nature and extent of these impacts, particularly in relation to respiratory and cardiovascular diseases.

Ozone (O3)

Ozone is not emitted directly from any man-made source in any significant quantities. It arises from chemical reactions driven by sunlight in the atmosphere. In the upper layers of the atmosphere (the stratosphere), ozone is produced by the direct action of ultraviolet light on oxygen molecules. This is the layer of ozone which is currently being disturbed by certain chemicals, mainly as a result of mans activities. This may increase the amount of ultraviolet light that penetrates to the earth's surface, which may have serious consequences for human health.

In the lower layers of the atmosphere, the troposphere, ozone is a pollutant produced by a complicated series of chemical reactions driven by sunlight. Pollutants, mainly derived from vehicle exhausts, react in the atmosphere to produce ozone. These reactions take several hours, even days to take place and, once formed, ozone may persist for several days. This means that high concentrations of ozone may occur considerable distances away from the source.

The effects of high concentrations of ozone can cause irritation of the eyes and nose.

Sulphur Dioxide

Sulphur dioxide is one of the gases released into the atmosphere by volcanic activity, which is the main natural source of sulphur dioxide. However, in the UK, the predominant source of sulphur dioxide is from the combustion of sulphur-containing fossil fuels, principally coal and heavy oils.

It was one of the components of the dense fogs that occurred in industrial cities in the late nineteenth and early twentieth century. The combination of this gas and sooty particles was derived from the widespread burning of coal in homes and factories, and it gave rise to the term smog. Public awareness of the effects on health was increased in the 1950s following the major smog episode in London in 1952. Such episodes were shown to be associated with increased numbers of episodes of chest illness and large numbers of excess deaths, mainly among elderly people with heart and lung disease. People suffering from asthma may be especially susceptible to the adverse effects of sulphur dioxide.