Light scattering by particles suspended in the sea: The role of particle size and density

Abstract

Optical scattering and particle size measurements are reported at 78 stations along the south and west coast of Britain. The composition of the particles is mainly mineral (70% by mass on average) and the light scattering per unit concentration of suspended particles, the specific scattering coefficient, was observed to vary by over an order of magnitude, from less than 0.1 to over 1 m 2 g −1. More than 3 4 of this variance is explained by changes in the apparent density (dry weight/‘wet’ volume) and mean size of the particles. For this data set, the specific scattering coefficient can be written b* M =(2.35/ ρD A ) where b* M is the scattering coefficient per unit concentration of mineral particles, ρ the apparent density and D A the mean particle diameter by area. The standard error of the numerator in this equation is 0.08. In fact, most (64%) of the variance in b* M is explained by differences in the apparent density of the particles, with variations in particle diameter alone accounting for only 15% of the variance. Because particles in coastal waters flocculate into aggregates containing mineral and organic material as well as water, the apparent density of the particles is variable and considerably less than solid mineral material. Provided a suitable atmospheric correction for visible-band imagery over shelf seas can be applied, it may be possible to estimate b* using satellite remote-sensing measurements of reflectance at two wavelengths. This work has shown that maps of b* produced in this way will be sensitive to variations in near-surface particle density as well as size. It may, however, be possible to use site-specific empirical relationships between particle size and density to derive both of these parameters from space.