Anomalous high ozone concentrations recorded at a high mountain station in Italy in summer 2003

In order to evaluate the possible effects of heatwave phenomena on background O 3 concentrations, the average summer O 3 concentrations at the high mountain station of Mt. Cimone (MTC—2165 m a.s.l.) have been analyzed. In particular, at this baseline station unusually high O 3 concentrations were recorded during August 2003, when an intense heatwave (the “August heatwave”) affected Europe. During this heatwave, the highest O 3 concentrations were recorded at MTC in connection with air masses coming from continental Europe and the Po basin boundary layer as shown by three-dimensional air mass back-trajectory and mixing height analyzes. However, high O 3 concentrations were also recorded in air masses coming from the middle troposphere (above 3000 m a.s.l.), thus suggesting the presence of O 3-rich atmospheric layers over Europe. This could be due to the large extension of the mixing layer which favoured the transport of high concentrations of O 3 and its precursors to altitudes that would usually be in the free troposphere. Other than from traffic and industrial activities, a contribution to the high O 3 concentrations recorded at MTC during the August heatwave could derive from fires in the North of Italy, as suggested by a well-documented episode and supported by in situ CO 2 measurements used as non-conventional tracer for fire emissions.