Abstract : 1B.4
Spatial distributions of aerosols in the Inn Valley atmosphere during wintertime -- a detailed view

Florian Harnisch, Alexander Gohm, Andreas Fix, Ralf Schnitzhofer, Armin Hansel, Bruno Neininger
Institute of Meteorology and Geophysics, University of Innsbruck

This study investigates the boundary layer structure and the spatial and temporal aerosol distribution in the lower Inn Valley (Austria) during high air pollution in the winter. The analysis is essentially based on data from two different airborne measurement systems, on one hand, an aerosol backscatter lidar system, and on the other hand, a set of instruments for in situ observations of chemical and meteorological variables.

On the basis of case studies, mean vertical profiles of lidar backscatter intensity will be compared with in-situ concentrations, measured with a motor glider. The qualitative aerosol structure, detected with these two instruments agrees well. Layers of high and low aerosol concentration are detected by both systems. However, the computation of quantitative correlations between the values of backscatter intensity and aerosol concentration is problematic. Hygroscopic growth of aerosol particles at high values of relative humidity and the sometimes poor matching of the time and location between the measurements cause discrepancies between the two profiles.

The role of thermally driven upslope winds as a ventilating mechanism during stable conditions and high air pollution is examined. In one of our cases, a strong asymmetry in the thermal circulation is found, which is caused by the asymmetry in solar insolation and snow coverage between the sun-exposed and the shaded valley slope. Slope winds on the sun-exposed slope only transport a few amount of aerosols through the capping inversion. Most aerosols are trapped below the inversion and spread horizontally over the whole valley atmosphere.