Absorption Coefficient and Chlorophyll Concentration of Oceanic Waters Estimated from Band Difference of Satellite-Measured Remote Sensing Reflectance
Zhongping Lee, Longteng Zhao, Chuanmin Hu, Daosheng Wang, Junfang Lin, Shaoling Shang
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 ( R rs ). 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 R rs 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 R rs 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/m 3 ) 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 R rs in oligotrophic oceans in order to minimize the discrepancies observed in satellite Chl products derived using the same algorithm concept but different empirical approaches.