The heavily industrialised Po Basin in Italy is a region with high ozone concentrations during the summer months. Ozone accumulates in the atmospheric boundary layer especially when fair weather conditions prevail for several days or weeks, and extremely high levels of ozone can occur towards the end of such periods. Often a significant change in the weather regime is responsible for the termination of high ozone episodes, bringing a change in air mass or precipitation. Another cause for the drop of the ozone levels in the Milano area is the influence of north foehn, a regional wind of the Alps that may transport air from the Alpine crest down into the Po Basin. Feeding from a less polluted reservoir higher in the atmosphere, the north foehn advects air containing not only less ozone, but also less primary pollutants for the formation of ozone. Consequently the original highly polluted Po Basin air with ozone concentrations of 70 – 170 ppb is replaced by cleaner free tropospheric air. with a low potential for ozone formation. During north foehn cases the ozone concentration in the area is substantially reduced to about 60 ppb. Evidence for the origin of this free tropospheric air is given by a similar annual variation at the Jungfraujoch (3454 m MSL) and in Mendrisio near the Po Basin, for foehn days, and vanishing potential temperature gradients between the two stations in these cases. This cleansing effect is evident in the region between the southern Alpine foothills and the urban area of Milano. North foehn occurs on 4 - 6 % of the summer days and on 6 - 8 % of winter days.