ASTAR, Arctic Study of Tropospheric Aerosol and Radiation is a a joint German (AWI Potsdam) - Japanese (NIPR Tokyo) campaign with participation from NASA LaRC Hampton, VA (USA). In addition to AWI, NIPR, and NASA LaRC the following institutions contributed to the project: Hokkaido University (Japan), Nagoya University (Japan), Norwegian Polar Institute Tromsoe/ Longyearbyen (Norway), NILU Kjeller (Norway), MISU Stockholm (Sweden), NOAA-CMDL Boulder, CO (USA) and Max Planck Institute for Aeronomy Katlenburg-Lindau (Germany). The campaign is based on simultaneous airborne measurements from the German research aircraft POLAR 4 and ground-based measurements in Ny-Ålesund. The main goals of the project are - to measure aerosol parameters of climate relevance, like extinction coefficient, absoprtion coefficients and phase function. - to create an Arctic Aerosol Data Set for climate impact investigation by using the regional climate model HIRHAM. - to carry out comparison measurements with the SAGE II (Stratospheric Aerosol and Gas Experiment) and the ground based Raman-Lidar.