Variability of Marine Particle Size Distributions and the Correlations with Inherent Optical Properties in the Coastal Waters of the Northern South China Sea

Abstract

Particle size distribution (PSD), which is an important characteristic of marine suspended particles, plays a role in how light transfers in the ocean and impacts the ocean’s inherent optical properties (IOPs). However, PSD properties and the correlations with IOPs are rarely reported in coastal waters with complex optical properties. This study investigated the PSD variabilities both for the surface water and the water in vertical planes, and the correlations between PSD and the backscattering coefficient (bbp), scattering coefficient (bp), and attenuation coefficient (cp), based on in situ PSD observations (within a size range of 2.05–297 μm) and IOPs in the coastal northern South China Sea. The results show a large variety of PSDs, with a range of 41.06–263.02 μm for the median particle diameter (Dv50) and a range of 2.61–3.74 for the PSD slope. In addition, the predominance of small particles is most likely to appear in the nearshore shallow water and estuaries with a large amount of sediment discharge, and vice versa. For the variabilities of IOPs, the particle concentration in a cross-sectional area (AC) is the first driving factor of the variations of bbp, bp, and cp, and the product of the mean particle diameter (DA) and the apparent density (ρa) can explain most variations of the mass-specific bbp (bbp/SPM), bp (bp/SPM), and cp (cp/SPM). In this study, we found that particle size is strongly correlated with volume-specific bbp (bbp/VC), bp (bp/VC), and cp (cp/VC), and the 10th percentile diameter of the accumulated volume concentration (Dv10) can better explain the variations of bbp/VC. These findings suggest a potential PSD retrieval method utilizing the bbp or bp, which may be determined by remote sensing observations.