Absorption Coefficient and Chlorophyll Concentration of Oceanic Waters Estimated from Band Difference of Satellite-Measured Remote Sensing Reflectance

Abstract

Absorption coefficient and chlorophyll concentration (Chl) are important optical and biological properties of the aquatic environment, which can be estimated from the spectrum of water color, commonly measured by the remote sensing reflectance (Rrs). In this study, we extended the band-difference scheme for Chl of oceanic waters developed a decade ago to the estimation of absorption coefficient at 440 nm (a(440)). As demonstrated earlier for the estimation of Chl, a(440) product from the band difference of Rrs showed much smoother spatial pattern than that from a semianalytical algorithm. More importantly, it is found that the upper limit of using band difference of Rrs can be extended from −0.0005 sr−1 (the upper limit set a decade ago for the estimation of Chl) to ~0.0005 sr−1 (corresponding to a(440) ~0.08 m−1), which covers ~91% of the global ocean. We further converted a(440) to Chl based on the “Case-1” water assumption and found that the standard Chl product of oligotrophic waters (Chl ~ 0.1 mg/m3) distributed by NASA is generally ~20% higher than Chl converted from a(440), possibly a result of different datasets used to determine the algorithm coefficients. These results not only extended the application of the band-difference scheme for more oceanic waters but also highlighted the need of more accurate field measurements of Chl and Rrs in oligotrophic oceans in order to minimize the discrepancies observed in satellite Chl products derived using the same algorithm concept but different empirical approaches.