Optical and microphysical properties of Arctic aerosols have been studied at single sites, like Barrow, Alaska (Bodhaine and Dutton, 1993; Polissar et al., 1999), Ny-Ålesund, Svalbard (Herber et al., 2002), at several Russian Arctic sites (Shahgedanova and Lamakin, 2005), and during international campaigns, like ASTER (Treffeisen, 2004, Yamanouchi et al., 2005). Over the last decade, data from a large number of satellite sensors suitable for aerosol studies (e.g., MODIS, MISR) have become available. Due to the special conditions with high surface albedo, large solar zenith angle, long path through the atmosphere, and low background aerosol concentrations, the determination of aerosol properties from satellites is still a challenge in the Arctic region. Only limited knowledge of the vertical structure of aerosol properties in the European Arctic is available. A real step forward can be expected from a lidar on the CALIPSO satellite (launched in April 2006), which will provide high-resolution vertical profiles of aerosols and clouds (Winker et al., 2002). Nevertheless, more advanced ground-based Raman lidars are required for CALIPSO validation. Due to mesoscale variability of tropospheric aerosols (Anderson et al., 2003), a combination of quasi-continuously operating ground-based and satellite born instruments is needed to study their optical and microphysical properties.

Vertically integrated optical and microphysical properties will be studied from observations at four sites located on Svalbard (Ny-Ålesund and Hornsund), and in Northern Scandinavia (ALOMAR, Sodankylä). The work will be performed in close collaboration with AWI and NIPR, who operate photometer/lidar in Ny-Ålesund, FMI, who run a PFR in Sodankylä, and AERONET (instrument at Hornsund). Through participation in the IPY project POLAR-AOD homogenized AOD measurements from other Arctic sites will be available for joined interpretation of the results. By comparison with ground-based measurements we will determine the best suitable satellite data for the European Arctic. The synthesized data set will be used to classify aerosol properties on Svalbard and Northern Scandinavia. Vertical aerosol profiles will be retrieved from ground-based lidar at ALOMAR (on the island of Andøya) and compared to results from Ny-Ålesund. Intercomparison of measurements from the ground-based Raman lidars and the CALIPSO lidar will help narrow down uncertainties in the description of the aerosol profiles. In collaboration with the outcomes from transport models (WP 1), the potential impacts of natural and anthropogenic contributions to the aerosol concentration will be estimated. Another aim of our study is to improve the knowledge of the ageing processes of the aerosols. During the spring 2007 campaign, closure studies will combine ground-based measurements with aircraft and satellite observations. During the 2008 summer campaign data obtained within WP 6 will contribute to the study of long-range transported aerosols from boreal fires (WP 4). Our studies will be complemented with data from the EARLINET-ASOS project, which is useful for tracking aerosols from central Europe. An important issue is, not only to observe the polluted Arctic, but also to determine the background aerosol concentrations and properties.