A Four-Step Method for Estimating Suspended Particle Size Based on In Situ Comprehensive Observations in the Pearl River Estuary in China

Abstract

The suspended particle size has great impacts on marine biology environments and biogeochemical processes, such as the settling rates of particles and sunlight transmission in marine water. However, the spatial–temporal variations in particle sizes in coastal waters are rarely reported due to the paucity of appropriate observations and the limitations of particle size retrieval methods, especially in areas with complex optical properties. This study proposed a remote sensing-based method for estimating the median particle size Dv50 (calculated with a size range of 2.05–297 μm) that correlates Dv50 with the inherent optical properties (IOPs) retrieved from in situ remote sensing reflectance above the water’s surface (Rrs(λ)) in the Pearl River estuary (PRE) in China. Rrs(λ) was resampled to simulate the Multispectral Instrument (MSI) onboard Sentinel-2A/B, and the wavebands in 490, 560, and 705 nm were utilized for the retrieval of the IOPs. The results of this method had a statistical performance of 0.86, 18.52, 21.28%, and −1.85 for the R2, RMSE, MAPE, and bias values, respectively, in validation, which indicated that Dv50 could be estimated by Rrs(λ) with the proposed four-step method. Then, the proposed method was applied to Sentinel-2 MSI imagery, and a clear difference in Dv50 distribution which was retrieved from a different time could be seen. The proposed method holds great potential for monitoring the suspended particle size of coastal waters.

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

“…The particle scattering (b p (λ), in m −1 ) was calculated as c pg (λ) minus a pg (λ) [33]. The backscattering measurements were conducted using the BB9 instrument with nine bands [21], and the particle backscattering coefficient (b bp (λ), in m −1 ) was obtained after the correction on raw BB9 data following the BB9 protocol [34]. b p (λ) and b bp (λ) were resampled at 5 m-depth intervals by averaging the data within a 0.5 m depth up and down, to match with the PSD and SPM datasets.…”

“…The V(D) for each size bin was calculated using the random shape model in this study, and the total volume concentration (VC) within the measured size range could be derived by summing the V(D) of each size bin. It should be noted that the VC was calculated by summing the V(D) within the size bin of 4-33 (i.e., size range of 2.05-297 µm), as studies have shown that the data were not stable for the initial and final bins [21,38]. The particle number concentration N(D), which could be regarded as the number of spherical particles with the same diameter for each size bin, was derived by dividing V(D) by the volume of a sphere of the same diameter (Equation ( 1)).…”

“…Such a shortage may be overcome with the help of satelliteborne platforms coupled with accurate retrieval algorithms [18]. In recent years, the success of PSD remote sensing retrieval has been shown in case studies [10,[19][20][21], most of which were realized based on the relationship between PSD and the inherent optical properties (IOPs).…”

“…It is reported that the cross-sectional area of particles, which is closely related to the numbers and sizes of the particles in the unit volume of seawater, plays a key role in the scattering of light in natural waters [23]. On the basis of the Mie theory [24], the particle size makes a significant contribution to the concentration-specific scattering in marine water, based on which the particle size has been estimated using the scattering or backscattering of particles in marine water [19,21]. Therefore, it is of great interest to explore the relationships between PSD and IOPs.…”