Dispersion of Vehicle Emissions within the Urban Roughness Layer

In the framework of the German Atmospheric Research Program AFO-2000 a system of consistent coupled numerical models is under development. The purpose of the model system is to serve as tools for the execution of European urban air quality regulations. A consortium with the acronym VALIUM was formed, which consists of German research institutes, environmental consultancies and an environmental agency. A substantial part of the VALIUM program is devoted to the generation of a set of high quality data for the validation of the numerical model system. The validation data will be based on a combination of field experiments and corresponding wind tunnel experiments. The field experiments were carried in and around a street canyon in a city district of Hanover /Germany.

The objectives of the VALIUM wind tunnel project were: 1. Support of the field measurement campaigns with corresponding flow visualization studies, which help to determine optimal measurement positions.

2. Generalization and enhancement of the field data through systematic laboratory experiments in order to close gaps in meteorological situations not met during the field campaigns.

3. Replication of selected field conditions and variation of certain parameters in a systematic way in order to improve the understanding of the governing flow and dispersion processes.

4. Quantification of the inherent scatter within the field data caused by the unsteadiness of meteorological conditions during the time interval of an individual experiment.

A detailed wind tunnel model of the field site was built at a scale of 1:250, covering an area of about 1 km². The core model area with a radius of about 500 m was mounted on the turn table of Hamburg Universities large boundary layer wind tunnel 'WOTAN' in order to enable any wind direction to be tested in the wind tunnel.