Boundary layer structure in the Inn Valley during high air pollution (INNAP)

The subject of this observational study is the wintertime boundary layer of the Inn Valley (Austria) during episodes of high air pollution. The aim is to investigate meteorological processes which are responsible for the spatial and temporal variability of air pollutants in an Alpine valley. In our cases these processes are transport and redistribution by valley and slope winds as well as local dilution through convective and shear-induced turbulent mixing.

The study is based on the analysis of airborne observations of aerosol backscatter intensity collected with the DLR TropOLEX lidar on board of the DLR Cessna C-208B Grand Caravan aircraft. These measurements were made within the framework of the research project “Boundary layer structure in the Inn Valley during high air pollution (INNAP)”. The target of the lidar observations were small dust particles (particulate matter). Such aerosols are typically emitted by anthropogenic sources, e.g. heavy traffic as well as industrial and domestic burning. Morning as well as afternoon flights were conducted on four selected days between January and February 2006. Repeated vertical cross-sections of backscatter intensity, aligned parallel and perpendicular to the valley axis, reveal the temporal evolution and the spatial structure of aerosols within the valley.

Especially in the morning hours, before sunrise, the stably stratified wintertime boundary layer shows strong vertical gradients of air pollutant concentrations with highest concentrations near the surface. This typical structure is well known and documented in other studies. Less well known is the impact of dynamical processes which modify this vertical profile. Our lidar observations highlight such processes: Two cases are documented for which the across-valley circulation, characterized by thermally driven up-slope winds, redistributes pollutants both horizontally and vertically and therefore reduces concentrations at the valley bottom. In a third case, a very shallow and stably stratified polluted layer persists throughout the day. On top of this layer the stratification is neutral and dynamically driven down-valley winds are observed. Turbulent mixing and down-valley transport of pollutants appear to play an important role in reducing the pollutant concentrations in this upper layer. The lower layer is significantly less affected by theses processes.