CLAiR-City will apportion air pollution emissions and concentrations, carbon footprints and health outcomes by city citizens’ behaviour and day-to-day activities in order to make these challenges relevant to how people chose to live, behave and interact within their city environment. Through an innovative engagement and quantification toolkit, we will stimulate the public engagement necessary to allow citizens to define a range of future city scenarios for reducing their emissions to be used for supporting and informing the development of bespoke city policy packages out to 2050.Using six pilot cities/regions (Amsterdam, NL; Bristol, UK; Aveiro, PT; Liguria, IT; Ljubljana, SI; and Sosnowiec, PO), CLAiR-City will source apportion current emissions/concentrations and carbon emissions not only by technology but by citizens’ activities, behavior and practices. CLAiR-City will explore and evaluate current local, national and international policy and governance structures to better understand the immediate policy horizon and how that may impact on citizens and their city’s future. Then, working with the new methods of source apportionment to combine both baseline citizen and policy evidence, CLAiR-City will use innovative engagement methods such as Games, an App and Citizen Days to inform and empower citizens to understand the current challenges and then subsequently define their own visions of their city’s future based on how their want to live out to 2050. The impact of these citizen-led future city scenarios will analysed, to develop city specific policy packages in which the clean-air, low-carbon, healthy future, as democratically defined by the city citizens, is described and quantified. The results of the CLAiR-City process will be evaluated to provide policy lessons at city, national and EU levels. Additionally, the toolkit structure will be developed for all EU cities with more than 50,000 citizens establishing a basis to roll out the CLAiR-City process across Europe.