Meteorol Atmos Phys 103, 223–235 (2009) DOI 10.1007/s00703-008-0318-3 Printed in The Netherlands 1 Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria 2 Institut f€ ur Physik der Atmosph€ are, Deutsches Zentrum f€ ur Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany 3 Institute of Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria 4 MetAir AG, Switzerland Spatial distribution of aerosols in the Inn Valley atmosphere during wintertime F. Harnisch 1 , A. Gohm 1 , A. Fix 2 , R. Schnitzhofer 3 , A. Hansel 3 , B. Neininger 4 With 7 Figures Received 28 September 2007; Accepted 14 March 2008 Published online 22 October 2008 # Springer-Verlag 2008 Summary This study analyzes the structure of the wintertime boundary layer in an Alpine valley (Inn Valley, Austria) for a case of high air pollution. We present airborne aerosol observations collected with particle counters and a back- scatter lidar. The effect of upslope winds on the spatial distribution of pollutants is investigated. An asymmetry in the aerosol distribution is observed in the cross-valley direction which presumably is related to differences in orientation and albedo of the two valley slopes. A one-sided thermal circulation, which develops above the sun-exposed slope, is most likely responsible for the observed re- distribution of aerosols during daytime. Elevated aerosol layers form at the height of shallow inversion layers. Despite this vertical transport of pollutants by slope winds, no effective vertical venting of the polluted air mass into the free atmosphere can be achieved. 1. Introduction With the increase of heavy goods vehicle (HGV) traffic over the last years, air pollution has be- come a hotly debated subject. HGV traffic along major Alpine transport routes, such as the auto- bahn through the Inn Valley and over the Brenner Pass, has strongly contributed to the air pollu- tion problem in these densely populated areas (Beauchamp et al. 2004). For the period between 2002 and 2012 an increase in total traffic volume of more than 35 percent has been predicted (Thudium 2003). Coupled with this increase of traffic and air pollution the impact on human health – for example the risk of lung cancer mor- tality (Pope et al. 2002) – increases as well. The vertical structure (stability) of the atmospheric boundary layer has a major impact on the dis- tribution of air pollutants (see e.g., Hanna and Strimaitis 1990; Chazette et al. 2005). Particularly in wintertime, Alpine valleys are favored loca- tions for high air pollution due to the formation of cold pools characterized by strong surface inversions which frequently decouple the valley atmosphere from the background synoptic flow. As long as convective mixing is not dominant (e.g., during nighttime), valley walls limit plume spread and the volume of air through which the plume can be dispersed (Whiteman 2000). These effects reduce the dispersion of pollutants and Correspondence: Alexander Gohm, Institute of Meteorology and Geophysics, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria (E-mail: alexander.gohm@uibk.ac.at)