AUTHOR=González Vilas Luis , Brando Vittorio Ernesto , Di Cicco Annalisa , Colella Simone , D’Alimonte Davide , Kajiyama Tamito , Attila Jenni , Schroeder Thomas 

TITLE=Assessment of ocean color atmospheric correction methods and development of a regional ocean color operational dataset for the Baltic Sea based on Sentinel-3 OLCI

JOURNAL=Frontiers in Marine Science

VOLUME=10

YEAR=2024

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2023.1256990

DOI=10.3389/fmars.2023.1256990

ISSN=2296-7745

ABSTRACT=<p>The Baltic Sea is characterized by large gradients in salinity, high concentrations of colored dissolved organic matter, and a phytoplankton phenology with two seasonal blooms. Satellite retrievals of chlorophyll-<italic>a</italic> concentration (chl-<italic>a</italic>) are hindered by the optical complexity of this basin and the reduced performance of the atmospheric correction in its highly absorbing waters. Within the development of a regional ocean color operational processing chain for the Baltic Sea based on Sentinel-3 Ocean and Land Colour Instrument (OLCI) full-resolution data, the performance of four atmospheric correction processors for the retrieval of remote-sensing reflectance (<italic>Rrs</italic>) was analyzed. Assessments based on three Aerosol Robotic Network-Ocean Color (AERONET-OC) sites and shipborne hyperspectral radiometers show that POLYMER was the best-performing processor in the visible spectral range, also providing a better spatial coverage compared with the other processors. Hence, OLCI <italic>Rrs</italic> spectra retrieved with POLYMER were chosen as input for a bio-optical ensemble scheme that computes chl-<italic>a</italic> as a weighted sum of different regional multilayer perceptron neural nets. This study also evaluated the operational <italic>Rrs</italic> and chl-<italic>a</italic> datasets for the Baltic Sea based on OC-CCI v.6. The chl-<italic>a</italic> retrievals based on OC-CCI v.6 and OLCI <italic>Rrs</italic>, assessed against <italic>in-situ</italic> chl-<italic>a</italic> measurements, yielded similar results (OC-CCI v.6: <italic>R</italic><sup>2 =</sup> 0.11, bias = −0.22; OLCI: <italic>R</italic><sup>2 =</sup> 0.16, bias = −0.03) using a common set of match-ups for the same period. Finally, an overall good agreement was found between chl-<italic>a</italic> retrievals from OLCI and OC-CCI v.6 although differences between <italic>Rrs</italic> were amplified in terms of chl-<italic>a</italic> estimates.</p>