La Polucion En El Aire

Atmospheric Environment 33 (1999) 4029}4037

Air pollution in cities
Helmut Mayer
Meteorological Institute, University of Freiburg, D-79085 Freiburg, Germany

Abstract Air quality in cities is the result of a complex interaction between natural and anthropogenic environmental conditions. Air pollution in cities is a serious environmental problem } especially in the developing countries. Theair pollution path of the urban atmosphere consists of emission and transmission of air pollutants resulting in the ambient air pollution. Each part of the path is in#uenced by di!erent factors. Emissions from motor tra$c are a very important source group throughout the world. During transmission, air pollutants are dispersed, diluted and subjected to photochemical reactions. Ambient airpollution shows temporal and spatial variability. As an example of the temporal variability of urban air pollutants caused by motor tra$c, typical average annual, weekly and diurnal cycles of NO, NO2, O and O are presented for an o$cial urban air-quality station in Stuttgart, southern Germany. They are supplemented  V by weekly and diurnal cycles of selected percentile values of NO, NO2, and O3. Timeseries of these air pollutants give information on their trends. Results are discussed with regard to air pollution conditions in other cities. Possibilities for the assessment of air pollution in cities are shown. In addition, a qualitative overview of the air quality of the world's megacities is given. 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Emissions for urban air pollution;Cycles and trends of urban air pollutants; Assessment of air pollution

1. Introduction Most cities world-wide su!er from serious air-quality problems, which have received increasing attention in the past decade. A major probable reason for the air-quality problems is urban population growth, combined with change in land use due to increasing urban areas. The urban population growth is caused by(1) drift to the cities and (2) excess of births over deaths in the cities themselves } especially due to high birthrates in the developing countries. Mainly responsible for the migration to the cities is a deep structural change, especially in non-industrialised countries. This structural change is the consequence of (1) economic opening-up, (2) new trading partners, and (3) change of politicalconditions, e.g. democratisation. Structural change takes a rapid course in some countries, dubbed `tigersa. It is not surprising that the expected urban population growth from 1992 until 2010 is much higher for Lagos, Bombay or Dhaka than for Tokyo or New York (Table 1). Urban population growth has many consequences. One of them is higher emission of air pollutants. Even though for most airpollutants,

the emission rate per inhabitant is at present higher in industrialised countries, the tendency is obvious that this rate will in future be higher in the so-called developing countries. 2. Air pollution path in the atmosphere Emission of air pollutants is caused by di!erent anthropogenic processes which can be categorised into the source groups motor tra$c, industry, power plants, trade,and domestic fuel. In industrialised countries like Germany, emissions of `classica air pollutants are decreasing (Fig. 1). This trend is pronounced for carbon monoxide (CO), sulphur dioxide (SO) and total suspended particulate (TSP), and is weakly evident for nitrogen oxides (NOV) and non-methane volatile organic compounds (NMVOC). In Germany, emissions of CO and NOV caused by motor tra$c amountto more than half the total emissions of these pollutants, and NMVOC emissions of motor tra$c are just under half the total emission of NMVOC. Hence, it follows that motor vehicle tra$c seems to be the most important source group for air pollution,

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