1Swiss Federal Laboratories for Materials Testing and Research (EMPA), Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland
2Remote Sensing Laboratories, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
3Institute of Atmospheric Science and Climate (ISAC-CNR), Via Gobetti 101, I-40129 Bologna, Italy
4Institute of Environmental Physics, University of Bremen, P.O. Box 33 04 40, 28334 Bremen, Germany
Abstract. High tropospheric NO2 amounts are occasionally detected by satellite-borne spectrometers even though clouds shield the highly polluted boundary layer. We present a method to investigate such events and apply the model to the high NO2 vertical tropospheric column densities (VTCs) over middle Europe observed from the Global Ozone Monitoring Experiment (GOME) instrument on 17 February 2001. Our case study shows that pollution originally residing near the ground has been advected to higher tropospheric levels by a passing weather front. With backward trajectories, the NO2 source region is located in central Germany, the Ruhr area and adjacent parts of the Netherlands and Belgium. The highly polluted air masses are traced by forward trajectories starting from the GOME columns to move further to the Alpine region. Their impact on the air quality there is modeled by combining the NO2 VTCs observed by GOME with the trajectory calculations and a given NO2 lifetime. Considering ground-based in-situ measurements in the Alpine region we conclude that for this episode, 50% to 90% of the NO2 concentration recorded at the sites can be attributed to transboundary transport during the frontal passage.