Aerosol optical depth over the Baltic Sea derived from AERONET and SeaWiFS measurements

Three years of data on aerosol optical depth (τa) from the Aerosol Robotic Network (AERONET) station on the island of Gotland in the Baltic Sea have been analysed and compared with Sea‐viewing Wide Field of view Sensor (SeaWiFS) data. Normally, the atmosphere over Gotland could be considered as clear, with a daily median value of τa at 500 nm of about 0.08. The median value of Ångström's wavelength exponent (α) for wavelengths between 440 nm and 870 nm was 1.37, indicating that the dominant aerosol is more of a continental than of a pure maritime type. SeaWiFS GAC level 2 data on τa and α were compared to data from the ground‐based AERONET station. For the 37 cases of simultaneous satellite and ground‐based measurements under cloud‐free skies it was found that, on average, the value of τa(865 nm) from SeaWiFS was about 0.02 higher than τa(870 nm) from the AERONET station. At the same time approximate τa(440 nm) from SeaWiFS was about 0.03 lower than τa(440 nm) of AERONET. α(510, 865) from SeaWiFS was, on average, 1.2 lower than α(500, 870) from the AERONET measurements. These results cannot explain the very frequent occurences of negative values of normalized water‐leaving radiance at 412 nm and 443 nm in the SeaWiFS reprocessing #3 data.