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Abstract. The dispersion of pollutants from naturally ventilated underground parking garages hasbeen studied in a boundary layer wind tunnel. Two idealized model setups have been analysed,one was simulating pollutant dispersion around an isolated rectangular building and one was repre-senting dispersion in a finite array of idealized building blocks. Flow and dispersion close to mod-elled ground level emission sources was measured. The results illustrate the complexity of theflow around buildings and provide insight in pollutant transport from ground level sources locateddirectly on building surfaces. As a result, areas critical with respect to high pollutant concentra-tions could be visualized. Particularly, the results show high concentration gradients on the sur-face of the buildings equipped with modelled emission sources. Inside the boundary layers on thebuilding walls, a significant amount of pollutants is transported to upwind locations on the surfaceof the building. The paper documents the potential of physical modelling to be used for the simu-lation and measurement of dispersion close to emission sources and within complex buildingarrangements.Key words: pollutant dispersion, car exhaust, underground parking garage, wind tunnel modelling,concentration measurements, flow measurements1. IntroductionPredicting the dispersion of air pollution in urban areas is one of the most chal-lenging problems environmental research is facing. Especially in urban areas, thedispersion of pollutants is dominated by a number of complex boundary condi-tions like different types of emission sources, constantly changing meteorologi-cal conditions and complex arrangements of structures. Specific air pollutionproblems arise from toxic or carcinogenic pollutants released for instance fromparking garages.63442
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Since many of the big cities experience a general lack of suffi-cient parking spaces, an increasing number of new office and residential build-ings is provided with underground parking garages to solve the permanentparking problems. In small building units, a natural ventilation of the parkingarea is often preferred to other solutions for economical reasons. Several build-ing codes have been developed in Germany in order to ensure a safe air qualityin parking spaces as well as to avoid health risks in closed parking areas. Car-bon monoxide (CO) was chosen as leading pollutant for the assessment of airquality in underground garages because it can be measured easily. Besides, areliable and permanent CO-monitoring can be used for controlling mechanicalemergency ventilation systems in situations that are critical with respect to airquality and pollutant concentrations. However, newer studies give evidence that there is a possibility of reaching or even exceeding the threshold values for othercritical air pollutants inside and outside of the parking area though the CO-basedair quality demands are satisfied inside a naturally ventilated parking garage.Benzene (C6H6) represents another air pollutant critical to human health, whichis used for assessing the impact of the exhaust from underground parking ga-rages on air quality outside the parking area. Benzene was selected as secondleading component for two reasons. First, benzene was found to be a carcino-genic air pollutant, which requires low threshold values and a special assessmentof exposure time to avoid risks for human health. Secondly, benzene is perma-nently emitted from underground garages. Driving vehicles release benzene asone component of the vehicles exhaust. Significant amounts of benzene are alsoreleased by parking vehicles because of evaporation of fuel and the effect of‘tank breathing’. In a recent study (TÜV Nord, 1996), maximum sourcestrengths of up to 75 µg/s benzene per ventilation outlet of underground garageswith natural ventilation were estimated. In a similar study by BAUCH (1993),average concentrations of 9.5 µg/m³ benzene with peak concentrations up to 18.1µg/m³ and a 90th-perzentile of 15 µg/m³ were measured in the exhaust gas of anunderground parking area. Considering typical mean background concentrationsof 5 to 7 µg/m³ benzene in large cities and a threshold value of 10 µg/m³ ben-zene defined in Germany, it becomes evident that even small amounts of ben-zene emitted by naturally ventilated parking garages can cause exceedances ofthe threshold value given by environmental guidelines.In order to study the flow and pollutant concentrations patterns close to theair outlets of naturally ventilated underground parking garages, a set of illustra-tive wind tunnel experiments has been set up in the BLASIUS wind tunnel of theMeteorological Institute at Hamburg University. The primary goal was to visu-alize critical emission situations near simplified building structures as well as onbuilding surfaces. In addition, it was intended to provide a set of high qualityexperimental data, which can be used for testing the quality of numerical model-ling, applied to a particular urban-type dispersion problem.2.