Validation of photosynthetically active radiation by OLCI on Sentinel-3 against ground-based measurements in the central Mediterranean and possible aerosol effects

ABSTRACT

Instantaneous determinations of photosynthetically active radiation (PAR) over the sea from the Ocean and Land Color Instrument (OLCI) on Sentinel-3 are compared with in-situ measurements at the island of Lampedusa in the central Mediterranean Sea. Radiative transfer calculations show that the PAR measured at the island site is representative for open ocean conditions. Satellite data show a good agreement (5.2% positive bias, R² = 0.97) with in-situ data, in line with similar analyses for other satellite sensors. Larger satellite-in situ differences are found during summer, and the possible role of aerosols in degrading PAR estimate has been investigated by comparing AOD values measured at Lampedusa and derived by OLCI. The relative difference between OLCI and in-situ PAR appears to be negatively correlated with the relative differences between OLCI and in-situ AOD, suggesting that a more accurate determination of AOD, in particular, for cases with AOD > 0.2, mostly related to Saharan dust, may lead to improved satellite PAR estimates.

KEYWORDS:

• PAR
• ocean
• OLCI
• satellite
• aerosol
• Mediterranean

Acknowledgments

Contributions by Pamela Trisolino, Fabrizio Anello, and Salvatore Piacentino are gratefully acknowledged.

The authors thank EUMETSAT and the EU Copernicus service for making available the OLCI data and the University of Valladolid and AERONET staff for the provision of Cimel photometer calibration and processing.

Mirko Nobili is thanked for the Lampedusa aerial picture shown in Figure 1.

The International Space Station picture of Figure 1 is courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center (https://eol.jsc.nasa.gov).

The reviewers are thanked for their valuable suggestions and contributions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data available on request from the authors.

Additional information

Funding

This study has been supported by the Italian Ministry for University and Research through the Marine Hazard Project (PON03PE_00203_1) and contributes to the Italian Integrated Environmental Research Infrastructures System (ITINERIS) Project. Measurements of aerosol optical depth at Lampedusa contribute to the Aerosol, Clouds, and trace gases Research Infrastucture (ACTRIS).This study is also part of a project that is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2019-2, Grant Agreement number: 871115 (ACTRIS-IMP).This analysis also contributes to the European Space Agency-funded Instrument Data quality Evaluation and Assessment Service - Quality Assurance for Earth Observation (IDEAS-QA4EO) framework contract (n. 4000128960/19/I-NS) under WP 2640: “Carbon dioxide fluxes estimation merging satellite and in-situ data in the Mediterranean Sea” (QA4EO/SER/SUB/35).