A hybrid method to estimate suspended particle sizes from satellite measurements over Bohai Sea and Yellow Sea

摘要

Particle-size distribution (PSD), a measure of particle concentrations at different sizes, is of great importance to the understanding of many biogeochemical processes in coastal marine ecosystems. Here, a hybrid method, including analytical, semianalytical, and empirical steps, is developed to estimate PSD through the median diameter of suspended particles (D_v~(50)). Four cruise surveys were conducted to measure optical scattering properties, particle concentrations, spectral reflectance, and particle-size distributions (obtained with a LISST instrument covering a size range of 2.5–500 μm) in coastal waters of Bohai Sea, Yellow Sea, and Jiangsu coastal region. Based on the Mie scattering theory, D_v~(50) is closely related to massspecific backscattering coefficient of suspended particles (b_(bp)~*), and their relationship is calibrated through a power model (R~2=0.796, n=67, p<0.001) for the D_v~(50) range of 23.5–379.8 μm. The model is shown to perform better than the previously used inverse-proportion model. The retrieval of b_(bp)~* is through a bio-optical model that links remote sensing reflectance just beneath the surface to inherent optical properties, where a close empirical relationship is established between particulate backscattering and particle concentration. The hybrid method shows high degree of fitting (R~2=0.875, n=46, p<0.001) between the measured and estimated D_v~(50) for the size range of 17.2–325.2 μm used in the model calibration, while validation using two independent data sets shows mean absolute percentage errors of 46.0% and 64.7%, respectively. Application of the hybrid method to MODIS (Moderate Resolution Imaging Spectroradiometer) data results in spatial distributions of D_v~(50) that are generally consistent with those from in situ observations, suggesting potential use of the method in studying particle dynamics through time series of remote sensing observations. However, its general applicability to other regions still requires further research.