The determination of refractive index distributions for oceanic particulates

Graduation date: 1975The refractive index of particulate matter suspended in seawater\ud is nonuniform and potentially is an indicator of composition and\ud origin. A method is described for determining the refractive index\ud distribution. The method requires the measurement of the volume\ud scattering function and assumes the total size distribution to be\ud capable of being reproduced as a linear superposition of hyperbolic\ud size distributions. The procedure utilizes 40 theoretical volume\ud scattering functions computed for a range of refractive indices and\ud particle size distributions. Each one represents a particular combination\ud of index and distribution. The eight values for the refractive\ud index (relative to that of water) range from 1.02 to 1.15. The\ud distribution of refractive index and the total particle size distribution\ud are determined by finding the best possible representation of the\ud measured volume scattering function as a linear superposition of the\ud 40 computed functions. An iterative descent method is used to\ud minimize the error between the measured curve and the superposition\ud of component scattering functions. If the particle size distribution is\ud known beforehand, the determination can be restricted to reproduce\ud the measured distribution to within limits set by a weighting factor.\ud A similar technique is given for finding the refractive index composition\ud for phytoplankton populations using measured scattering and\ud size-distribution data.\ud Twelve volume scattering functions measured in oceanic\ud environments (Sargasso Sea, Mediterranean Sea and Baltic Sea) and\ud three measured in cultured phytoplankton populations (the dinoflaggelate\ud Amphidinium carterae, and unidentified unicellular alga labeled\ud T-24, and a centric diatom of the genus Biddulphia) were analyzed.\ud Determinations for the Mediterranean Sea were performed at a series\ud of depths extending from near surface to 2OO m. All the determinations\ud showed the refractive index distributions to be essentially\ud bimodal with components of refractive indices near 1.05 and 1.15.\ud The method reproduced measured scattering curves with an average\ud absolute error less than the experimental uncertainty.\ud Suggestions for obtaining better results using the same technique\ud are given