Vertical Pollutant Transport over Alpine Foothills

During the VOTALP II (Vertical ozone transports over the Alps) European Union project, aircraft measurements were performed in northern Italy (May to June, 1998) and southern Germany (May, 1999) in the Alpine foothills. The altitudes in the observation area ranged from 200 to 1600 m MSL in Italy and 600 to 2000 m MSL in Germany. The flight path involved a box-like pattern at different altitudes around the hilly areas covering some 100 km². This allowed to derive air mass and pollutant flux budgets at these altitudes. For days with high insolation and high photochemical activity, convergence in the boundary layer and divergence at altitudes well above the hill crests were found. The divergences at altitudes of around 500 meters above crest height were accompanied by substantial increases of water vapor, aerosol number, NOy and HCHO concentrations downwind of the hilly areas compared to the inflow concentration at the same altitude. These concentration increases were caused by injections of polluted air masses from below induced by the foothills. In former work, we have shown that deep Alpine valleys can act as very efficient pumps for vertical air transport. This work shows that Alpine foothills also play an important role in topography induced vertical air mass and pollutant exchange.