CityZen aims at determining the air pollution distribution and change in and around hotspots over the last decade from extensive satellite and in-situ observations and employs a series of different scale models to analyze the impacts of air pollution hot spots on regional and global air quality, including potential future changes for various climate scenarios. The focus is on ozone and particulate matter (chemical and physical characterization), and their precursors. The Eastern Mediterranean (Istanbul, Athens, Cairo), the Po Valley, the BeNeLux region, the Pearl River Delta in China (including Guangzhou and Hong Kong) and the hot and polluted European summers 2003 and 2007 are chosen for case studies. The consortium includes experts on observations, emission data and modeling. A set of chemical transport models connecting the most important spatial and temporal scales is further developed and used to quantify how the observed air pollution arises. Models and emission inventories are evaluated, errors identified and improved on the urban, regional and global spatial scales. Climate change may cause changes in air pollution in and around hotspots, and hotspot pollution can change precipitation and temperature/albedo. These feedbacks are studied in scale-bridging model systems based on global climate model scenarios, and in a coupled high resolution chemistry-climate model. The model systems evaluated in the project are applied to analyse mitigation options in and around hotpots, also taking into account climate change. Best available technologies and sectoral changes are studied. Several partners have key roles in the technical underpinning of policy. They will ensure that the improved emission inventories, scale-bridging model systems and the systematic observational evidence has a significant, broad and lasting impact.