D. VAN VELZEN Commission of the European Communities, Joint Research Centre Ispra, Environment Institute, 1-21020 Ispra (Varese) ITAL Y 1. Introduction Worldwide, there is an ever increasing interest and concern about the destructive effects of air pollution on man's ecosystem. The growing awareness of these effects has revealed the need to take adequate measures to minimize the emission of air polluting products. The two most important contaminants, occurring in the largest concentrations and quantities, are sulphur dioxide and nitrogen oxides. Both pollutants are formed mainly during the...