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 O3 concentrations, the average summer O3 concentrations at the high mountain station of Mt. Cimone (MTC—2165m a.s.l.) have been analyzed. In particular, at this remote station unusually high O3 concentrations were recorded during August 2003, when an intense heatwave (the ‘‘August heatwave’’) affected Europe. During this heatwave, the highest O3 concentrations were recorded at MTC in connection with air masses coming from continental Europe and the Po basin boundary layer as shown by threedimensional air mass back-trajectory and mixing height analyzes. However, high O3 concentrations were also recorded in air masses coming from the middle troposphere (above 3000m a.s.l.), thus suggesting the presence of O3-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 O3 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 O3 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 CO2 measurements used as non-conventional tracer for fire emissions.