This book presents the proceedings of a NATO Advanced Research Workshop which was also financially supported by the National Research Council of Italy. The Workshop was held from October 9 to 15, 1994, at the Centro Ettore Maiorana in Erice, Italy. Over 40 researchers from a wide variety of fields attended the Workshop, which brought to attention the ongoing research on various phenomena related to urban air pollution. The presence of high levels of atmospheric pollutants in the air of several urban centres of developed and developing countries causes a great concern among authorities and public opinion. Some 20% of the European population live in cities of more than 500,000 inhabitants and about 40% in cities of more than 50,000. Since exceedance of the Air Quality Guidelines has been observed to occur worldwide, a great effort has been addressed to the control of primary pollutants, but many problems related to secondary pollutants such as nitrogen containing species (nitrogen oxides, nitric and nitrous acid, ni trates) and photochemical oxidants (ozone, PAN and others) are far from being solved. The importance of atmospheric chemistry in understanding the processes occurring in urban atmospheres has been well recognised, thus there is a strong need to exchange experiences and results from urban centres in different Countries. Indeed, atmospheric pollution is very much dependent on the type of emissions which are very different according to the economic development of the urban centre under consideration.