Collaborative Research: Inferring Marine Particle Properties from Polarized Volume Scattering Functions (IMPROVE)
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Understanding biogeochemical cycles and their interaction with the climate system requires quantifying the various forms of materials in the global ocean. In particular, the chemical composition and size distribution of living and non-living particles in the ocean have an enormous impact on marine ecosystems, including the dynamics of marine food chains, the vertical transmission of solar energy, and the transport of organic matter and trace elements. These particle characteristics not only affect but reflect changes in many biogeochemical processes in the ocean. In this project, a team of scientists will launch a set of technical and numerical modeling innovations that will allow them to determine key biogeochemical quantities from optical observations of the angular distribution of scattered light. The project will involve an interdisciplinary team of experts, who will theoretically model the optics of marine particles, develop an innovative advanced mathematical modeling scheme, and then laboratory- and field-test and validate the approach for observing marine particle properties. When perfected, the technique should see a broader application, as it will be designed to be amenable to operation on ship-borne and autonomous platforms with the potential to provide estimates of the particle size distribution and composition at high temporal and spatial resolutions. It should thus benefit different fields requiring detailed knowledge of aquatic particles (biogeochemistry, biology, optics). The project will also provide for the training and support of graduate and undergraduate students as well as public educational outreach, including a special effort to reach Native Americans in North Dakota. The goal of this project is to derive water-column quantitative particle size and composition information from in situ unobtrusive volume scattering function (VSF) measurements to characterize marine biogeochemical particulate stocks. The angular patterns of the scattered intensity and polarization state of the scattered light by particles can be described in terms of a 4x4 Mueller matrix (S) that is intrinsically determined by the sizes, shapes, composition, and structures of the particles. The particle properties can be, therefore, potentially inferred from measurements of S. Unfortunately, the complete Mueller matrix of oceanic waters has been seldom measured. Even the most commonly measured component, the volume scattering function (element S11) representing the angular distribution of unpolarized light, was scarcely measured until recently, followed by development of an inversion technique to derive size distributions and composition of particles from the VSF. Recently, a commercial product, LISST-VSF, became available for measuring the Mueller matrix components S11 (VSF), S12 (linear polarization), and S22 (cross-polarization), potentially providing an avenue to obtain a much more detailed characterization of particles. This project will incorporate the additional information provided by S12 and S22 using recent advances in scattering modeling to further constrain the inversion with the following: (i) better knowledge in particle shapes using S22 (spherical vs. non-spherical); (ii) reduced uncertainty using both S11 and S12; and (iii) further improved capability to characterize particles in the size range of 0.02 to 200 um. The study should greatly enhance our ability to quantify size distributions and refractive indices (closely linked to particle densities) for particle groups such as phytoplankton cells, detrital particles, organic particles, mineral particles, bubbles, and emulsified oil (if present).