Transport studies with the IFU three-wavelength aerosol lidar during the VOTALP Mesolcina experiment

The IFU three-wavelength lidar was operated during part 1 of the VOTALP valley campaign in the Swiss Mesolcina valley near Grono. The measurements yielded information on both the three-dimensional distribution of the aerosol and the pollution transport in and above the valley. In the afternoon, a pronounced asymmetry of the transverse aerosol distribution was found which may be due to a difference in heating by the sun or to the bending of the valley near Grono. With very short delay with respect to the arrival time of the air pollution at the bottom of the valley a second step with enhanced aerosol formed every day which is ascribed to the build-up of a reverse (anti-valley) wind. During the second half of the investigations this reverse flow is expected to have been influenced by the almost parallel synoptic wind. For the final day of the lidar measurements wind data are available obtained during aircraft flights. This allows an estimate of the upward transport efficiency to be made. The flux ratio of the flow above and in the valley in the afternoon of that day was found to be roughly 90%. This high efficiency demonstrates the importance of the vertical exchange processes in the Alps for the pollution export out of the boundary layer over Central Europe.