Measurements of Aquatic Particle Volume Scattering Function up to 178.5 degrees in the East China Sea

Particulate volume scattering function (VSF), especially at angles larger than 170 degrees, is of particular importance for interpreting ocean optical remote sensing signals and underwater imagery. In this study, a laboratory-based VSF instrument (VSFlab) adopting the periscopic optical system was developed to obtain VSF measurements from 1 degrees-178.5 degrees. In the VSFlab, a new prism design that simply combines a single prism and a neutral density filter was proposed to more efficiently reduce the stray light in the backward direction, while a detailed calibration procedure was given. A full validation based on standard beads of various sizes and a comparison with the results from LISST-VSF and POLVSM indicated that the VSFlab can provide reliable results from 1 degrees to 178.5 degrees. VSFlab measurements in the East China Sea (ECS) exhibited a moderate increase (not more than 5 times) in VSF from 170 degrees to 178.5 degrees rather than a sharp increase of more than one order of magnitude presented in other instrument results measured in other coastal regions. The estimates of the particulate backscattering coefficient using single angle scattering measurements near 120 degrees or 140 degrees and suitable chi(p) were justified. Two types of the VSFs with different size distribution and shape parameters in the ECS can be distinguished based on the variability of chi(p) after 155 degrees. The measured VSF could provide a basis for the parameterization of VSF in the radiative transfer model and the variability of chi(p) in the backward direction had the potential to be used to characterize the particles in the coastal region of the ECS.