Real-time Source Apportionment of Organic Aerosols in Three European Cities. GANG CHEN, Francesco Canonaco, Jay G. Slowik, Kaspar R. Daellenbach, Iasonas Stavroulas, Nikolaos Mihalopoulos, Evangelos Gerasopoulos, Jean-Eudes Petit, Olivier Favez, Urs Baltensperger, André S. H. Prévôt, Paul Scherrer Institute