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Open Access Publications from the University of California

Measurements of angle-resolved polarized light scattering by seawater as a tool to characterize natural assemblages of marine particles

  • Author(s): Koestner, Daniel Warren
  • Advisor(s): Stramski, Dariusz
  • et al.
Abstract

The light scattering properties of seawater play important roles in oceanic radiative transfer and optically-based methods for characterizing marine suspended particles from in situ and remote sensing measurements. In order to realize the full gamut of potential applications associated with light scattering in the ocean, advancements in the fundamental understanding of the effects of particle size and compositional characteristics on variability in scattering across various marine environments must be made.

The recently commercialized LISST-VSF instrument measures the volume scattering function, Beta_p , the degree of linear polarization, DoLP_p , and scattering matrix element p_{22} associated with particle scattering at a single light wavelength (532 nm) with high angular resolution over the range ~0.1° to 155°. This thesis presents the first independent and thorough evaluation of LISST-VSF performance, including the development of necessary corrections for improved results and validation of such corrections using measurements and Mie scattering calculations for polystyrene bead suspensions.

Seventeen seawater samples representing contrasting natural assemblages of particles from coastal environments near San Diego, California have been comprehensively characterized with laboratory measurements of angle-resolved polarized light scattering, particle size distribution (PSD), and particle composition in terms of various metrics derived from mass concentration and particulate absorption. Measurements of angle-resolved light scattering and PSD were made on original (unfiltered) seawater samples and particle size-fractionated samples obtained using 5 μm and 20 μm mesh filters.

Although the effects of particle size and composition are complex, small particles (< 5 μm in size) consistently produced a major or dominant contribution (~50–80%) to the particulate backscattering coefficient, b_{bp}, in both phytoplankton and non-algal dominated organic samples regardless of significant variations in PSD. The notable exception was a sample dominated by large-celled diatoms from microphytoplankton size range, which exemplifies a scenario when large particles (> 20 μm) can produce a considerable contribution (~40%) to b_{bp}. Samples dominated by inorganic material, by contrast, consistently exhibited weaker contributions (~30−40%) of small particles to b_{bp}. The maximum value of DoLP_p, DoLP_{p,max}, was found to be weakly dependent on particle composition, but exhibited negative correlation with the proportion of relatively large sized particles in samples. The scattering matrix element p_{22} exhibited similiar trends as DoLP_{p,max} at 100º. In contrast, p_{22}(20º) was relatively unaffected by the presence of large sized particles but showed negative correlation with inorganic content of particulate assemblages. Finally, simple optically-based proxies for the estimation of particle size and compositional parameters which rely on polarized light scattering measurements at only one or two angles were developed.

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