"Air quality in the urban environment is gaining increasing attention by city authorities and residents due to rising urbanization and growing individual traffic. One important aspect in today's urban design and planning is to make cities favorable for the natural ventilation enabling a fast dispersal and removal of air pollutants emitted by traffic and buildings. Green spaces play an important role in these considerations as they may enhance, e.g. by wide open parks, or inhibit, e.g. by avenue-like tree plantings in urban street canyons, the natural ventilation.The influence of vegetation on pollutant dispersion and concentrations in the urban environment will be investigated by Computational Fluid Dynamics (CFD) employing Reynolds-averaged Navier Stokes (RANS) turbulence models and Large Eddy Simulations (LES). A central point in this research project is to develop new parameterizations (vegetation modules) accounting for the effects of vegetation on flow and turbulence characteristics. The CFD simulations will be evaluated against available wind tunnel data, e.g. the CODASC database (Concentration Data of Street Canyons, www.codasc.de), wind tunnel studies to perform at collaborating institutions and full scale experiments in the Dutch cities of Arnhem and Nijmegen (leader of these street case studies: Dr. ir. Bert Blocken, TU Eindhoven, period 2011/12).The project's outcomes will strongly contribute to an improved air quality modelling in the urban environment by taking the important implications of vegetation on flow and turbulence characteristics into account. CFD simulations are becoming gradually more important in urban air quality modelling with increasing computer performance in the next years. Therefore, validated CFD models with vegetation modules will become an indispensable tool for considerably improved predictions and assessments of urban air quality, to achieve an overall better, healthier and more comfortable living environment."