Contact: Thomas Trickl
The Alpine wind systems strongly influence the vertical distribution of air pollution. Since 1996, three of the IFU lidar systems have been applied in investigations of the ozone and aerosol transport in and across he Alps (VOTALP, EARLINET and AFO2000 project VERTIKATOR). Important topics have been trans-Alpine transport during Föhn events, occasionally associated with elevated ozone from northern Italy or with an observation of dust from the Sahara desert, the transport of urban air pollution (e.g., Milano, Munich) into Alpine valleys. The main effort has been to study the upward transport of the polluted air masses in and above these valleys illustrated in the following diagram:
As shown in the diagram an upvalley air flow (valley wind) forms in the morning that ascends in the upper part of the valley and typically reaches altitudes of 1 to 1.5 km above the neighbouring summits. This can be quite conveniently visualized by lidar sounding of aerosol (see both figures). The influence of the valley wind is seen in the growing aerosol extinction coefficient α below 2 km. Under favourable conditions of the synoptic wind aloft a reverse air stream (anti-valley wind) may form that arrives above the lidar system with short delay (above 3.5 km in the example). As one sees from a comparison of the extinction coefficients in the two layers the upward transport is very efficient, which underlines the importance of the Alps in the export of air pollution from the boundary layer to the free troposphere.