Large scale episodes of high concentrations of photochemical oxidants occur every summer in Europe. Ozone concentrations of the order of 200-300 Myg/m3 may be monitored during these episodes, i.e. far above ambient quality standards and proposed critical levels. The reason for these episodes has been an area of research during the last two decades and strategies for the control of the episodes have been the subject of study by different international bodies. At present the oxidant problem is studied by the UN ECE. The objective of this report is to give an overview of the knowledge on the occurrence of photochemical oxidants. The oxidant episodes in Europe were discovered in the early 70 s and it was early recognized that they were connected with stable high pressure situations on the European continent. The areas affected depended on the localisation of the center of the high pressure cell. The episodes are superimposed on the increased backtround in tropospheric ozone observed over large regions of the northern hemisphere. The formation mechanisms for ozone in Europe are discussed in terms of the role of organic species and nitrogen oxides. The importance of a specific organic compound for the formation of ozone depends on its reactivity and on the number of ozone molecules that can be formed from the organic molecule. These factors may be of importance for the development of control strategies. Manmade and natural emissions are discussed in the report. The only significant natural emissions besides methane are isoprene and monoterpenes. In areas with large biogenic emissions isoprene may be an important contributor to ozone formation, while terpenes seem to be a relatively unimportant source of oxidants. Source-receptor relationships are necessary tools for the development of control strategies. Such relationships are presently developed in terms of theoretical models. The principles of such models and their status in predicting oxidant episodes are presented. Finally, in the last chapter of the report the principles of control strategies are discussed.
