Importance of the parallel polarization radiance for estimating inorganic particle concentrations in turbid waters based on radiative transfer simulations

Contemporary ocean colour radiometers and inversion algorithms to derive optical properties and oceanic constituents rely only on the scalar treatment of water-leaving radiance (Lw). However, Lw polarization components, which contain embedded hydrosol information, have been largely neglected. Here, we examine Lw polarization characteristics, based on vector radiative transfer (RT) simulation, for determining suspended particulate matter in highly turbid waters. The RT simulations show that parallel polarization radiance (PPR) can improve the retrieval of Lw, with a higher relative fraction of ocean colour signal to total radiance (Ioc/It) than the total intensity (I). Moreover, the reflectance for PPR (ρp), compared with that for I, is more sensitive to inorganic particle concentration (IPC) variations, particularly those in the red and NIR bands. Additionally, ρp displays significant directional and spectral variations with respect to geometrical conditions, and the maximum ρp (~10%) is highly peaked at 555 nm in the solar plane. Furthermore, the superiority of PPR to retrieve IPCs, based on the back propagation neural network, was discussed with a very high determination coefficient. Significant improvements in inversion accuracy were observed for PPR with a lower relative deviation (0.901%) than I (3.740%). This study highlights that PPR might be used as an alternative approach to retrieve Lw and thus to derive biogeochemical parameters, particularly in turbid coastal waters.