Sources of Ozone

5. Ozone is not emitted directly from any man-made source in any significant quantities, but arises from chemical reactions driven by sunlight in the atmosphere. In the stratosphere, ozone is produced by the direct action of ultraviolet light on oxygen molecules; the balance between ozone and oxygen is currently being disturbed by migration upwards of chemicals such as chlorofluorocarbons which remove ozone and therefore may increase the amount of ultraviolet light that penetrates to the earth's surface.
6. In the lower layers of the atmosphere, the troposphere, while some ozone arises from periodic winter intrusions of air from the stratosphere, it is primarily generated as a pollutant by a complicated series of chemical reactions driven by sunlight. In these, oxides of nitrogen and hydrocarbons, derived mainly from vehicle exhausts, react in the atmosphere to produce ozone (Figure 1). In normal circumstances ozone is present in the atmosphere in balance with the oxides of nitrogen - a so-called photostationary equilibrium. This equilibrium can be disturbed by the presence of other chemicals in the atmosphere such as carbon monoxide, methane and many volatile organic compounds. These substances are produced by combustion and other industrial processes, but also arise naturally from animals and plants. In particular, the volatile organic compounds produced by vehicles and other sources of combustion may react in the atmosphere to form reactive chemicals which combine with nitric oxide to form nitrogen dioxide. The more nitrogen dioxide and volatile organic compounds in the air, the more ozone will be produced in the presence of sunlight.
7. These chemical reactions do not take place instantaneously, but over several hours or even days, and once ozone has been produced it may persist for several days. In consequence, ozone produced at one site may be carried for considerable distances in the air, and maximum concentrations usually occur away from the source of the primary pollutants, oxides of nitrogen and volatile organic compounds. Indeed, in urban areas, where concentrations of traffic exhaust gases may be high, nitric oxide from exhaust emissions may react with ozone to form nitrogen dioxide, thus actually reducing ground level ozone concentrations. However, as air movement carries the primary pollutants away, more ozone is generated and concentrations rise over suburban and rural areas. From this, and bearing in mind the importance of sunlight in the reactions, it can be understood that ozone pollution occurs in summer more than in winter, in southern more than in northern areas of the United Kingdom, and in the country more than in cities. Itcan also be seen that the problem is an international one, in that ozone precursors generated in countries with large traffic and industrial emissions may affect less polluted ones. Figure 2 illustrates the geographical distribution of ozone concentrations in the United Kingdom, while Figure 3 shows a typical gradient of ozone levels from city centre to rural areas.
8. The highest concentrations of ozone occur during hot, sunny and relatively windless days in summer, when the chemical reactions described above are promoted by heavy production and poor dispersion of traffic fumes. Such circumstances are well recognised by the public around cities such as Athens and Los Angeles, but also occur, albeit less frequently, in more northerly climates such as that of the United Kingdom. These are discussed in the next section.