Mie-Theory Models of Light Scattering by Ocean Particulates

Abstract

The general application of Mie theory to the study of the volume-scattering function, VSF, for oceanic particles is discussed in detail. The problem examined is the combination of Coulter counter measurements and the theory in order to enhance our understanding of the size-refractive index distributions of the sea water particles, First single-component models (one scattering species) are examined and compared with the Sargasso Sea VSF. These models yield an average refractive index m̄ relative to water. Questions concerning the meaning of m̄ require the investigation of simple two-component models consisting of low-index organic particles, and high-index inorganics. These models are systematically studied and result (again for the Sargasso Sea) in the conclusion that the inorganic particles occupy large size ranges while the organic particles are confined to small sizes. Similar studies in the Tongue of the Ocean in the Bahama Islands indicate the necessity of three-component models, which place organic particles in small and large size ranges and inorganic particles in midsizes. This model predicts a VSF in good agreement with observation, and indicates that most of the scattering is due to the inorganic particles, with the large-size organic particles (phytoplankton) contributing only at small angles. This suggests the possibility of monitoring phytoplankton populations in coastal areas through simultaneous measurement of small- and large-angle scattering, for which a preliminary experimental verification is presented. A relaxation of the constraints on the three-component model consistent with the observed VSF has been effected and provides very general limitations on the size distribution in size ranges where it cannot be measured. A simple method for determination of the average refractive index of particles is presented and its limitations are discussed in detail.