Urban air quality

Since 1950 the world population has more than doubled, and the global numbe r of cars has increased by a factor of 10. In the same period the fraction of people living in urban areas has increased by a factor of 4. In year 200 0 this will amount to nearly half of the world population. About 20 urban r egions will each have populations above 10 million people. Seen over longer periods, pollution in major cities tends to increase durin g the built up phase, they pass through a maximum and are then again reduce d, as abatement strategies are developed. In the industrialised western wor ld urban air pollution is in some respects in the last stage with effective ly reduced levers of sulphur dioxide and soot. In recent decades however, t he increasing traffic has switched the attention to nitrogen oxides, organi c compounds and small particles. In some cities photochemical air pollution is an important urban problem, but in the northern part of Europe it is a large-scale phenomenon, with ozone levels in urban streets being normally l ower than in rural areas. Cities in Eastern Europe have been (and in many c ases still are) heavily polluted. After the recent political upheaval, foll owed by a temporary recession and a subsequent introduction of new technolo gies, the situation appears to improve. However, the rising number of priva te cars is an emerging problem. In most developing countries the rapid urba nisation has so far resulted in uncontrolled growth and deteriorating envir onment. Air pollution levels are here still rising on many fronts. Apart from being sources of local air pollution, urban activities are signi ficant contributors to transboundary pollution and to the rising global con centrations of greenhouse gasses. Attempts to solve urban problems by intro ducing cleaner, more energy-efficient technologies will generally have a be neficial impact on these large-scale problems. Attempts based on city plann ing with a spreading of the activities, on the other hand, may generate mor e traffic and may thus have the opposite effect. (C) 1999 Elsevier Science Ltd. All rights reserved.