Historical data from rural and Alpine sites in Europe and selected recent measurements show how the ozone concentration increased and how its seasonal distributions vary at different locations. The coincidence of the spring maximum at Mauna Loa in Hawaii an at Alpine stations in Europe indicates that the underlying phenomenon affects the entire northern hemisphere. The observed trends and differences between seasonal distributions can be explained with the help of results from recent chemical modeling calculations. Such a heuristic interpretation provides further insight into the processes through which anthropogenic emissions of nitrogen oxides and hydrocarbons influence tropospheric ozone. ¿ American Geophysical Union 1989