Aerosol Optical Depth Retrieval over Boreal Forests using AATSR - Case Studies

Climate is significantly influenced by the distribution of aerosol particles in the atmosphere. The characteristics which influence climate must be considered on a global scale. Satellites provide a way to obtain information on atmospheric composition on regional and global scales. In particular the spatial distribution of aerosol parameters such as aerosol optical depth (AOD) at several wavelengths as well as some of the microphysical parameters can be retrieved from satellite observations. Satellite data are complementary to ground-based measurements which have a local character with the advantage of high accuracy. The retrieval of aerosol properties can only be applied to cloud-free pixels. It is based on the comparison of the measured and modeled reflectance at the top of the atmosphere (TOA). The AATSR dual view (ADV) algorithm for application over land uses the AATSR IR and visible wavebands for cloud detection and the visible wavebands (0.555, 0.659, 0.865, and 1.6 μm) for aerosol retrieval. The resolution of these instruments is 1x1 km2 at nadir view and the swath width is 512 km, resulting in a return time of three days at mid-latitudes. The retrieval is based on minimizing the error function between modeled and measured TOA reflectances, using all available wavelengths. The algorithm uses look-up-tables (LUTs) to compute the modeled TOA reflectance. For AOD retrieval, an aerosol in the atmosphere is assumed to be an external mixture of fine and coarse mode particles. The two aerosol types are mixed such that the spectral behavior of the reflectance due to aerosol best fits the measurements. We applied ADV to investigate the AOD distribution over the boreal forest in North America, Europe and Siberia. We retrieved AOD for the selected areas which we considered representative for the boreal forest area. Background AOD for boreal forests is rather low. For North America, Scandinavia and Central Siberia aerosol optical depth is 0.05-0.2; for the European part of Russia, which is more polluted as compared with the other areas, AOD is higher (0.3-0.6). Besides the anthropogenic emissions, biomass burning can significantly increase background AOD. Retrieved AOD was evaluated by comparison with ground-based AOD measurements using data from the AERONET (Aerosol Robotic NETwork), network of ground-based sun photometers. Three locations were selected which we considered representative for the areas of interest - Pickle Lake (56°26'N, 90°26'W) in North America, Hyytiälä (60°50'N, 24°17'E) in northern Europe and Yekaterinburg (57°02'N, 59°32'E) in Northern Asia. The satellite retrieved AOD is in a good agreement with the AERONET data. More detailed analysis of the satellite retrieved AOD is planned to investigate the AOD spatial distribution over the boreal forest areas in relation to local production and to long-range transport of aerosols.