Abstract
Estimation of daily photosynthetically active radiation (PAR) is of primary importance for monitoring the ocean primary production and the subsequent production of carbon by phytoplankton at global scale from remote sensing ocean color sensors. On the other hand, aerosol abundance and composition play a critical role in the modulation of PAR. In this study, an original algorithm, so-called OLCIPAR, is proposed for routinely determining the daily PAR from optical satellite sensors such as the OLCI sensor aboard Sentinel-3 (ESA). The OLCIPAR algorithm has been developed to overcome some of the limitations of the current existing methods. In particular, multiple scattering effects induced by the atmospheric layer are taken into account based on exact radiative transfer calculations. Another advantage of OLCIPAR method is to consider a great variety of aerosol models to better account for their optical variability as observed in real world conditions. The OLCIPAR algorithm was applied to the archive of MERIS data, whose sensor is similar to OLCI. The validation of the retrieved daily PAR was carried out based on comparison with the time series acquired by the BOUSSOLE oceanographic buoy moored in the Mediterranean Sea. Results show a regression slope of 1% and an accuracy within 10% which confirms the robustness of the algorithm. The comparison of OLCIPAR retrievals with the products routinely distributed by NASA shows that estimates of PAR differ by up to 20% in the subtropical Atlantic Ocean where important amounts of dust aerosols are present. The improvements brought by OLCIPAR method for deriving the daily PAR could thus permit to better assess the impact of aerosols on reduction of PAR with implications on the estimation of oceanic primary production.
Highlights
The distribution of phytoplankton biomass is controlled by the availability in nutrients within the water column and by the amount of light entering the ocean
Since the incident irradiance at the sea surface is directly related to the atmospheric transparency, which is dependent on the gaseous absorption, cloud cover and aerosol load, an accurate estimation of daily photosynthetically active radiation (PAR) requires a good characterization of the optical properties of the atmosphere [7,8,9,10]
To better quantify the discrepancies between OLCIPAR and the standard NASA algorithm, the absolute and relative differences between the daily PAR retrieved by both methods, noted ΔPAR, are examined as a function of the Aerosol Robotic Network (AERONET) aerosol optical thickness (AOT) at 550 nm for all the MERIS images acquired over the region offshore the Capo Verde AERONET site (Fig. 7)
Summary
The distribution of phytoplankton biomass is controlled by the availability in nutrients within the water column (e.g., nitrate, phosphate, iron) and by the amount of light entering the ocean. Anthropogenic aerosols are thought to be capable of significantly affecting SST and the thickness of the mixed layer even under non-cloudy conditions [15] Despite this recognized impact on ocean waters, the radiative effects of aerosols are still poorly assessed in terms of light availability for marine organisms and potential modification of the intensity of the primary production [16]. Recent studies based on theoretical modeling and SeaWiFS satellite observations showed that desert dust aerosols induce a mean reduction of daily PAR by 15% in the subtropical Atlantic Ocean, with a maximum decrease of daily PAR of 30% off West Africa equatorial coast [17,18]. The objective of this study is to propose a robust algorithm, which could overcome some of the limitations of Frouin’s algorithm, to estimate the daily PAR over the global ocean from MERIS/OLCI-like satellite sensors for clear sky conditions.
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