Climatology of ozone transport from the free troposphere into the boundary layer south of the Alps during North Foehn

[1] In the Swiss Alpine foothills south of the Alps, on the northern edge of the Po Basin of Italy, 10 years of ozone data were analyzed to deduce the climatological influence of North Foehn on the boundary layer concentrations of ozone. It was found that northerly winds are connected with low relative humidity and small differences in potential temperature between the Jungfraujoch (3600 m above sea level (asl)) and Mendrisio (350 m asl) and therefore are primarily caused by North Foehn. This confirms that North Foehn transports air masses from high altitudes above the Alpine crest down into the lower boundary layer. On days with North Foehn the yearly cycles of ozone are very similar at Jungfraujoch and Mendrisio, around 40 ppb in winter and 60 ppb in summer. Compared to the whole data set in Mendrisio, these values are, at comparable temperatures, high in winter and very low in summer, when ozone concentrations often exceed 120 ppb during daytime. In contrast, during nighttime the North Foehn causes an enhancement of the ozone concentration near the ground during the whole year. The frequencies of the North Foehn events are 4–6% from May to October and 6–10% from November to April. Thus North Foehn is an important process for the ozone concentrations in the Po Basin.