"Our goal is to understand aerosol growth in Titan’s atmosphere. This requires the description of the initial energy deposition and subsequent ionization processes in the upper atmosphere to the chemical formation and sedimentation of aerosols on the surface. This implies sharing knowledge from various fields of research, from physics to chemistry and planetary sciences. We propose to extend an ongoing collaboration between two European institutions, Imperial College, London, UK and Laboratoire de Planétologie de Grenoble, France, and an American partner, University of Arizona, Tucson. Six permanent researchers, four post-docs as well as graduate students are involved in the project. Titan, the largest moon of Saturn, is a fascinating object. Solar radiation and energetic particles induce a complex chemistry in its atmosphere, leading to the formation of hydrocarbons and nitrogen-bearing species. The Cassini-Huygens space mission has recently revealed the presence in the upper atmosphere of positively and negatively charged ions with a mass-to-charge ratio up to a few thousand amu. This is an exceptional discovery, as these extremely complex particles are most likely the precursors of the aerosols detected at lower altitude. However, aerosol formation and composition are still poorly known. The Cassini-Huygens mission provides a wealth of observational constraints on ionization and chemical processes in the atmosphere. In order to interpret the data, it is necessary to develop laboratory experiments as well as complex models of the atmosphere of Titan whose predictions can be confronted to the observations. Specific objectives include: A) Investigate the photochemistry of the atmosphere including formation of odd N species and the links with ionospheric chemistry B) Determine the dynamics of the upper atmosphere and its role in the distribution of neutral and ion species C) Understand the formation and growth of complex molecules and aerosols"