Aerosol Profile Retrieval Algorithm Development and Validation for Sentinel-4
Stammes, Pieter1; Vieitez, Ofelia1; Sanders, Bram1; Apituley, Arnoud1; Veihelmann, Ben2; Meijer, Yasjka2; Koopman, Rob2

In the ESA AEROPRO project we develop and validate an oxygen A-band aerosol profile retrieval concept for ESA's Sentinel-4 mission, to be launched in 2019. Oxygen A-band spectra from the Japanese GOSAT Fourier Transform Spectrometer are being used as proxy for the Sentinel-4 data. The spectral resolution of GOSAT is downgraded to 0.12 nm, which is the Sentinel-4 resolution. This also helps to improve the Signal-to-Noise ratio of GOSAT. The AEROPRO O2 A-band algorithm, with heritage from the Sentinel-5P/Tropomi algorithm, is based on a spectral fit of reflectance using optimal estimation and a forward model with full multiple scattering, using a simple aerosol microphysics model. The basic set of fit parameters comprises aerosol optical thickness, aerosol layer pressure and surface albedo. For vegetated land, we also fit fluorescence emission. Aerosol cases for which the algorithm will be specifically suited include elevated, optically thick aerosols, such as volcanic ash plumes, desert dust and biomass burning aerosols. Sensitivity studies show that moderate to large aerosol optical thicknesses are needed for reliable aerosol height retrievals, but that the retrieval is fairly independent of the aerosol microphysics assumption. Since the altitude of aerosols is difficult to determine from space using passive sensors, validation of the AEROPRO algorithm is essential. During the ESA CarbonExp aircraft campaign, which took place over Greece and Turkey in August-September 2011, a GOSAT special pointed observation request was executed. This campaign offered a unique chance to validate the O2 A-band aerosol profile retrievals from GOSAT, since the aircraft lidar data give the detailed vertical structure of the atmosphere. Additional ground-based lidar data from the Earlinet network are used to validate the aerosol height retrievals from GOSAT for selected cases of strong aerosol events in 2011-2012. First results from the AEROPRO study will be shown.