Atmospheric influences and local variability of air pollution close to a motorway in an Alpine valley during winter

Air quality is mainly determined by traffic, industrial and urban emissions as well as by specific meteorological and topographic conditions. Focussing on nitrogen oxides and particulate matter, the temporal and spatial differences in air pollution in a deep Alpine valley with a major motorway were studied by a measurement campaign between November 2005 and February 2006 near Schwaz, Austria. The variation of air pollutant concentrations near the motorway was determined on the basis of path-averaged (DOAS) and in situ measurements. The layering of the valley atmosphere and the mixing-layer height were observed continuously by SODAR and ceilometer measurements. The winter-time weather conditions were characterized by exceptionally low temperature and high amount of snow in the valley. This resulted in the development of very stable cold pools with low wind speeds and an accordingly high air pollution burden. During several periods the air pollutants accumulated from day to day and frequent exceedances of NO2 and PM10 thresholds were detected. Furthermore it could be concluded that the emission situation was not unusually high during these periods, at least concerning road traffic as the dominant source for nitrogen oxides. Analysis confirmed that the typical weather phenomena associated with stable high-pressure regimes were the main factor for the observed pollution burden. During foehn and cold front passages, which interrupted these high pressure periods, concentration levels generally decreased at the valley ground and increased at the slope. Analysis revealed that this is a consequence of vertical mixing due to the influence of mountain specific wind systems, like thermally driven and quasi-periodic valley or slope winds. NO2 concentration levels are a matter of concern regarding annual mean values as well as - previously not recorded - exceedanecs of the half-hourly threshold in severe winter conditions. Compared to previous investigations an increased NO2/NOx ratio was observed during the measurement campaign which is related to enhanced NO2 emissions from road traffic in consequence to fleet composition and emission changes.