Large-Eddy Simulation (LES) models are so far barely applied to dispersion and chemical transformation of pollutants in urban air quality studies. Within the joint project MOSAIK (Modellbasierte Stadtplanung und Anwendung im Klimawandel/Model-based city planning and application in climate change) a new LES based state-of-the-art microscale urban climate model PALM-4U, has been developed. The new model includes both gas phase and aerosol chemistry. For practical applications, our approach is to go beyond the simulation of single street canyons to chemical transformation, advection and deposition of air pollutants in the larger urban canopy. First LES results of a test case for an urban quarter of Berlin (Germany) are presented.