Urban air pollution is nothing new--it has existed, in one form or another, for as long as people have lived together in substantial communities. But it was during the 19th century, with the rise of coal-powered industrial steam and increased domestic coal use, that its impact really began to be felt. Dickens atmospherically painted this London landscape in the opening of Bleak House: "Smoke lowering down from chimney-pots, making a soft black drizzle, with flakes of soot in it as big as full-grown snow-flakes--gone into mourning, one might imagine, for the death of the sun."
The concentration of air pollution in cities is a consequence of more than just the volume of people and industrial activity. It's also the result of cities being founded along the original arteries of commerce--rivers. The walls of a river valley, however low, will help to capture pollution. The cities' placement on rivers also often meant that there were no ready sources of fuel nearby, so it had to be transported from far afield. This, in turn, meant that energy-rich fuels, such as coal, were preferred. The coal that arrived in London was often quite 'dirty'--contaminated with sulphur--and, when burnt, it produced large amounts of particulate pollution, as well as sulphur dioxide (S[O.sub.2]).
For most of the year, turbulence in the atmosphere helps to disperse air pollution. But climatic conditions often cause a concentration of emissions at ground level. This typically occurs in winter, when high-pressure (anticyclone) weather systems settle over the country. A reduction in the amount of wind reduces the lateral movement of air, and vertical escape is cut off by what's known as a temperature inversion, when a layer of cold air close to the ground is trapped by a layer of warmer air above. This situation is compounded by the cold--more fuel is burnt for heating and the temperature inversion seals the resultant smog into the heart of the city and the lungs of its inhabitants.
The Great Smog of London (see box, below left) is credited with kick-starting a serious move to combat urban air pollution. But, as Professor Peter Brimblecombe of the University of East Anglia points out, "The move to using cleaner fuels, especially gas, and the decline of heavy industry were both more important in securing clearer air in the metropolis than the highly praised Clean Air Act."
There is a fascinating complexity to efforts to control pollution. In the 1960s and '70s, control of industrial emissions of smoke and S[O.sub.2] in urban areas was mainly achieved through the construction of large new power stations well outside urban areas. These successfully limited local ground-level pollution by dint of tall chimneys. But the pollution had to go somewhere. The S[O.sub.2] and nitrous oxides caught the prevailing winds and travelled east, falling as acid rain that destroyed thousands of hectares of forest in northern Europe. Now, European legislation has brought this problem under control through the use of 'scrubbing' technology that removes the pollutants at the site of production. However, this technology consumes energy, which contributes to climate change through the release of greenhouse gases. …