Air pollution and wind comfort are both important indicators of quality of life in urban environments. This study examines the influence of a tree area (Green Park) on the wind patterns and air pollution concentrations over a downtown urban area (1 km x 1 km) in Madrid (Spain) with the CFD PALM4U model (5 m spatial resolution) coupled with the WRF/Chem mesoscale chemical model. Two numerical experiments have been performed: a) with trees (BAU) and b) without trees (NOTREE) to investigate the impacts of trees on the wind patterns and air pollution concentrations. Nitrogen dioxide and ozone concentrations in the BAU scenario were compared with the values at the pollution monitoring station in the area to validate the simulations. The impacts (BAU-NOTREE) indicate that the pedestrian wind comfort and NO2 concentrations is greatly affected by the trees far away of the Green Park area. In some specific locations in the area the NO2 concentrations are found to increase up to 10% and in other locations, reductions of up to 20% are found. This results demonstrate the efficiency of using sophisticated CFD numerical models, such as PALM4U, for conducting air quality and micro-climate simulations in urban areas.