"Despite its importance for human health and climate change organic aerosol (OA) remains one of the least understood aspects of atmospheric chemistry. We propose to develop an innovative new framework for the description of OA in chemical transport and climate models that will be able to overcome the challenges posed by the chemical complexity of OA while capturing its essential features.The objectives of ATMOPACS are: (i) The development of a new unified framework for the description of OA based on its two most important parameters: volatility and oxygen content. (ii) The development of measurement techniques for the volatility distribution and oxygen content distribution of OA. This will allow the experimental characterization of OA in this new “coordinate system”. (iii) The study of the major OA processes (partitioning, chemical aging, hygroscopicity, CCN formation, nucleation) in this new framework combining lab and field measurements. (iv) The development and evaluation of the next generation of regional and global CTMs using the above framework. (v) The quantification of the importance of the various sources and formation pathways of OA in Europe and the world, of the sensitivity of OA to emission control strategies, and its role in the direct and indirect effects of aerosols on climate.The proposed work involves a combination of laboratory measurements, field measurements including novel “atmospheric perturbation experiments”, OA model development, and modelling in urban, regional, and global scales. Therefore, it will span the system scales starting from the nanoscale to the global. The modelling tools that will be developed will be made available to all other research groups."