The ozone patterns in the aerological basin of Milan (Italy)

Abstract In the last years, photochemical pollution has been recognised as a critical environmental problem. Summer smog episodes occur over large parts of Europe and also in the North of Italy high ozone concentrations are measured. A working group is investigating the ozone distribution and estimating the contribution of breeze circulations transport to the actual ozone levels measured in the territories of the provinces of Milan, Bergamo, Varese and Como in Northern Italy. In this paper the principal results of this outstanding study are presented. Times series of ozone concentrations measured in the last years by the four provincial air pollution networks, including 18 monitoring sites, have been analysed by comparing trends and frequency of exceedances of air quality standard limits. Moreover, a statistical analysis has been made to evaluate similarity among measured ozone concentrations in terms of levels and temporal variability (frequency and peaks distribution, daily shape etc.). This study covers the summer period from June to September 1992 and uses the measured 1-h ozone concentrations in two different test cases. The first case considers day-time ozone data (9:00–20:00); the second one also includes night-time ozone data (0:00–24:00). The method developed is based on hierarchical Cluster Analyses techniques. Every group has been well characterized by an averaged hourly concentration and relative variance. The statistical analyses has shown a summer time gradient of surface ozone concentrations going from south to north of the area under study and interesting similarities among monitoring sites in the day-time maximum peak occurrence.