The Sources That Give Rise to the Backscattering and Its Spectral Variation

While the color of the ocean mainly results from the spectral absorption by particle assemblage in the ocean, it is the volume scattering, particularly, the backscattering that reflect the light, thus making the color observable. In addition, the backscattering and its spectral variations also carry information about the particles that cannot be retreived from the absorption alone. While the backscattering coefficients and its spectral signature have been empirically related to a variety of biogeochemical quantities, such as PIC, POC, SPM, or mean sizes of particulates, theoretical studies show the small particles, most in the operationally dissolved domain, are the main contributor. Thus, the question remains as to the sources of backscattering and its spectral variations, the answer to which is not only important to the sicence of ocean color but can also lead to improvements in its algorithm development and data interpretation. The challenges in addressing this question include, among many, the accurate measurements of backscattering coefficients, the interpretation of angular VSFs, and simulation of scattering by non-spherical and -homogeneous particles. Overcoming some of these challenges, we have measured the spectral VSFs in three coastal waters around US and during the SABOR experiment in North Atlantic ocean. The spectral backscattering coefficients were derived from the VSFs in a varietry of water types, ranging from turbit estuarine to open ocean, from mineral dominant tidal zone to algal bloom. We also applied the inversion technique to partition the measured VSFs into contributions by particle populations of a wide ranges of sizes and composition that are representative of oceanic particles, living and nonliving. This inversion approach allows us to examine the relative importance of various particle populations to the backscattering. Furthermore, we applied forward modeling using non-spherical particles to simulate the spectral backscattering by particle populations that have been inferred from the inversion to examine their contribution to the observed spectral behavior of the backscattering. Finally, we found that the backscattering can exhibit dramatic changes in its spectra during, hence be useful in detecting, spontaneous events such as dinoflagellate blooms.