The concentrations of pollutants in the air vary over the course of the day, and over the seasons of the year. Occasionally these typical patterns are interrupted by a period of high air pollution - called an air pollution episode. This can last up to several days and may extend over a large geographical area. In some cases there are high concentrations of many pollutants at the same time, but in others only one pollutant may be affected.
Variation in Pollution Levels with Time
Episodes of High Air Pollution
Episodes are caused in different ways: during the coldest winter months, stable weather conditions can trap pollutants close to their sources and prevent their dispersion. This leads to high concentrations building up over several days. In the 1950's and 1960's, domestic coal burning was responsible for the 'pea-souper' fogs with their high levels of black smoke and sulphur dioxide. Today, however, a major source of many air pollutants in most UK cities is road traffic, and high concentrations of nitrogen oxides, particles (PM10) and hydrocarbons such as benzene are usually observed during such mid-winter episodes.
In summer, a completely different type of episode may occur. Hot and sunny weather can produce high levels of ozone, sometimes accompanied by nitrogen dioxide or particles (PM10). The species that cause the episode can often travel long distances - sometimes from other parts of Europe. During this large-scale air movement they undergo a complex series of chemical reactions, driven by sunlight, which produce high levels of ozone and other pollutants.
Site prepared by the National Environmental Technology Centre, part of AEA Technology, on behalf of the UK Department of the Environment, Transport and the Regions
