Quality Assessment of the Sentinel-3 Land Surface Reflectivity (LSR) Auxiliary Product from OLCI and SLSTR for Improved Aerosol and Cloud Retrievals

The Copernicus Sentinel-3 mission, carrying the Ocean and Land Colour Instrument (OLCI) and the Sea and Land Surface Temperature Radiometer (SLSTR), provides critical observations for monitoring atmospheric composition. Operational Level-2 algorithms for aerosol and cloud retrievals rely on accurate information from the Sentinel-3 Land Surface Reflectivity (LSR) Auxiliary Product to stabilize the inversion process. To address this requirement, this static LSR product is being developed at 1–2 km spatial resolution using a hybrid GRASP retrieval approach.The hybrid GRASP approach is designed to provide stable full BRDF retrievals by reducing the number of parameters to be inverted. In this framework, aerosol properties at coarse resolution are not retrieved but are instead reused from existing multi-year datasets, such as PARASOL/GRASP and VIIRS. Furthermore, coarse-resolution surface products from the PARASOL/POLDER-3 satellite are utilized as a priori information for the high-resolution retrieval. Within this framework, OLCI benefits from its dense spectral sampling in the visible to near-infrared range, which is well suited for characterizing surface reflectance and BRDF spectral dependence. SLSTR complements this capability through its dual-viewing geometry, which provides additional constraints on surface anisotropy and helps reduce ambiguities between atmospheric and surface contributions. In addition, SLSTR’s short-wave and thermal infrared bands enhance cloud and snow screening and support more robust atmospheric correction.In this work, we evaluate the generated LSR auxiliary product derived from Sentinel-3 OLCI and SLSTR measurements. The assessment follows a hierarchical strategy: first, the impact of the generated LSR is demonstrated by validating retrieved aerosol properties (AOD, Ångström exponent) globally against AERONET ground observations. Second, the retrieved surface BRDF is validated regionally against the GROSAT reference dataset, which provides synergetic AERONET and OLCI-A/B retrievals. Third, the products are compared with MODIS MCD43A3 surface albedo at the global scale, demonstrating strong spatial and radiometric consistency. The impact of the generated LSR dataset on aerosol property retrievals, aerosol layer height (ALH), water vapor (WV) and cloud properties is also discussed.