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Comparative study of wintertime NO and NO2 measured by DOAS near a motorway in the Inn Valley

A comparative study of wintertime NO and NO2 concentrations and the investigation of the influences of meteorological conditions upon air quality in a valley near a motorway was performed in the Inn valley near Innsbruck, Austria. A DOAS with emitter/receiver unit and three retroreflectors was used for this study. One retroreflector was installed at a mast on the other side of the motorway (120 m path length) so that the path was about 10 m above motorway level. Another retroreflector was set up for a path parallel to the motorway and the third retroreflector was used to measure at a path perpendicular and away from the motorway. The path across the motorway was directly above the air quality monitoring station Vomp of Land Tyrol which is only three meters away from the motorway. The first measurement campaign was performed during winter 2005 / 2006 and the second one during the winter 2007 / 2008. The concentrations above the motorway are clearly dominated by the traffic volume. Higher concentration values are found during week days than during the weekend. The diurnal differences in air pollution e.g. due to temporal variations of motorway emissions (10 times higher during peak hours in the morning and afternoon compared to night hours) and meteorological conditions (stable from late afternoon until mid-morning) are investigated. The mean NO2 / NOx ratio from these DOAS measurements is 0.3 which can be caused by the high amount of heavy duty vehicles at that motorway. During synoptically undisturbed winter periods persistent inversion conditions determined by a SODAR as mixing layer heights lead to relatively high air pollutant concentrations from late afternoon until mid-morning. In this context the influence of the quasi-regular mountain wind system (valley and slope winds) is studied. The colder winter 2005 / 2006 with a persistent snow cover is characterised by higher concentration of pollutants (e.g. NO2).