AERONET-OC reference measurements to assess earth observation data :Towards a revised data reduction protocol

The Ocean Color component of the Aerosol Robotic Network (AERONET-OC) was established in 2002 as a joint JRC-NASA effort to provide highly accurate and standardized in situ reference measurements for the validation of satellite data from sensors designed to monitor the aquatic environment at the visible and near-infrared wavelengths. Notably, the assessment of radiometric data products delivered by the Copernicus Marine Service relies for more than 80% on AERONET-OC measurements. The data reduction process of AERONET-OC in situ measurements is based on theoretical values of the sea-surface reflectance 𝜌-factors estimated as a function of the wind speed and sun elevation. Acknowledging that the atmospheric properties and the water refractive index are responsible for spectral changes in the 𝜌-factors, these dependencies were analyzed relying on the Monte Carlo code for Ocean Color Simulations (MOX) and the in-house Finite Element Method radiative transfer code (FEM). The measurement geometry supported by protocols for the validation of satellite ocean color (OC) data and commonly applied for operational field measurements, was considered. Simulations indicated that the values of 𝜌, as well as their dependence on wind speed and sun elevation, decrease with atmospheric turbidity and increase with wavelength. For wind speeds lower than 4 m s-1 and sun zenith angles greater than 20o, the variability of 𝜌 in the 400-709 nm spectral interval fluctuates between −8% and +3% with respect to the values determined at 551 nm for typical atmospheric conditions. Conversely, when relaxing the constraints on sun elevation and wind speed, the variability spans from −63% to more than six folds. The impact of this variability on field measurements is addressed with a specific focus on European AERONET-OC sites managed by the JRC in collaboration with Member States.