Abstract : B.7
On boundary layer processes determining the spatial distribution of air pollutants in an Alpine valley

Alexander Gohm, Florian Harnisch, Ralf Schnitzhofer, Armin Hansel, Andreas Fix, Bruno Neininger, Friedrich Obleitner, Johannes Vergeiner, Esther Griesser
Institute of Meteorology and Geophysics, University of Innsbruck

The subject of our investigation is the wintertime boundary layer of the lower Inn Valley (Austria) during episodes of high air pollution. We investigate physical processes which are responsible for tranport and redistribution of pollutants. Our study illuminates the role of thermally driven winds (valley and slope winds), dynamically driven flows (e.g., foehn winds, low level cold-air advection) and related mechanically and thermally induced turbulence in determing the spatial structure of air pollutants.

Our analysis is based on airborne and ground-based measurements collected on several winter days in 2007 during the joint field experiment of the three research projects INNAP, INNOX and ALPNAP. The dataset consists of airborne aerosol backscatter lidar data as well as airborne and ground-based in situ measurements of chemical and meteorological parameters. Our study highlights the strong case-to-case variability of the distribution of pollutants due to different meteorological background conditions. Remarkably, for winter conditions, some cases show thermally induced circulations with vertical transport of pollutants by upslope winds. Other cases are dominated by high concentrations of pollutants near the surface and low concentrations at mid-valley levels. In such cases the trapping effect of a near-surface temperature inversion keeps pollutants at low levels. Here, weak surface winds and high stability imply weak mixing at low

levels. Low concentrations at mid-valley levels on top of the inversion are partly explained by turbulent mixing due to enhanced (foehn-related) downvalley winds.