The Pan-European Gas-AeroSOls-climate interaction Study (PEGASOS) European large scale integrating project brings together most of the leading European research groups, with state-of the-art observational and modeling facilities to:(1) Quantify the magnitude of regional to global feedbacks between atmospheric chemistry and a changing climate and to reduce the corresponding uncertainty of the major ones.(2) Identify mitigation strategies and policies to improve air quality while limiting their impact on climate change.The project is organized into four scientific Themes designed to optimize the integration of methodologies, scales, and ultimately our understanding of air quality and climate interactions:(I) Anthropogenic and biogenic emissions and their response to climate and socio-economy(II) Atmospheric interactions among chemical and physical processes(III) Regional and global links between atmospheric chemistry and climate change(IV) Air quality in a changing climate: Integration with policyPEGASOS will bridge the spatial and temporal scales that connect local surface-air pollutant exchanges, air quality and weather with global atmospheric chemistry and climate. Our major focus for air quality will be Europe including effects of changes in pollutant emissions elsewhere and the time horizon for the study will be the next 50 years. During the project we will provide improved process understanding in areas of major uncertainty for better quantification of feedbacks between air quality and a changing climate. We will present, for the first time, a fully integrated analysis of dynamically changing emissions and deposition, their link to tropospheric chemical reactions and interactions with climate, and emerging feedbacks between chemistry-climate and surface processes. We will target both local and regional scales, taking into account chemistry and climate feedbacks on the global scale.