Mixing of free-tropospheric air with the lowland boundary layer during anabatic transport to a high altitude station

High altitude stations are the only platforms allowing for continuous measurements of the free-troposphere composition, and monitoring of trends away from pollution sources. However, they are influenced by mountain breezes and convection that bring air from the lowland boundary layer up to the summits. In summer 2005, a field campaign involving in situ measurements and ozone lidars was organized in the Pyrenees to investigate the impact of such processes on in situ measurements at the Pic du Midi (PDM) high altitude station (2875 m a.s.l.). On June 17 and 19, a plain-to-mountain thermal circulation developed during the day. Observations show that direct transport of lowland air masses to PDM cannot account for ozone measurements at the station. Also, according to measurements, the PDM station did not directly sample the free troposphere. These two days were further investigated using a Lagrangian box model combining transport, photochemistry and mixing with the background troposphere. It was possible to reproduce and analyze ozone time series recorded at PDM, and quantify the partial mixing with free tropospheric air during the transport. A large fraction (43 to 86%) of air from the lower free troposphere was found to contribute to the gas melange sampled at PDM, with the best agreement found for fractions 57% (resp. 74%) on June 17 (resp. June 19).