A New Inversion Model to Estimate Bulk Refractive Index of Particles in Coastal Oceanic Waters: Implications for Remote Sensing

An inversion model is developed to estimate bulk refractive index (η_bp relative to water) for understanding the particle assemblages and dynamics in coastal oceanic waters. The model is based on an inversion of Mie theory combined with parameterizations such as the backscattering ratio, hyperbolic slope of the particle size distribution, and bulk density (relative to water). With the advent of high-frequency in-situ spectral attenuation and fluorescence-turbidity sensors, these parameters can be easily measured and used in the new model to estimate the η_bp values. To test the robustness of this model, the η_bp values estimated from the new model are verified with the Mie input refractive index values (relative to water) and those produced by an existing model. The new model is also applied to spatial, temporal, and vertical insitu profile data measured from turbid coastal waters off Point Calimere and clear waters off Chennai, southeast part of India. The η_bp values estimated by the present model are generally agreeable with the previously reported η_bp values (1.02-1.28) for these waters. By contrast, the existing model tends to provide relatively high η_bp values (1.10) for clear waters and low η_bp values (1.215) for relatively clear and sediment-laden waters. Application of these models to time-series in-situ data from moderately turbid and highly turbid waters reveals that the vertical distribution patterns of η_bp from the new model correspond better with turbidity patterns (with an increasing η_bp trend in sediment-laden bottom waters) that display large variations depending on the tidal cycles of the day. However, the existing model produces a very narrow range of η_bp values displaying nearly homogenous patterns regardless of the turbidity variation along the depth. The new mo- el enables more reliable estimates of η_bp for living cells (1.02-1.07 because of higher water content) in clear waters, detrital particles (1.07-1.15) and minerals/mineral-detrital particles (>1.15 because of lower water content). These results suggest that the new model will have important implications for studies of the particle assemblages in coastal oceanic waters, with the feasibility of remote estimation of η_bp with the proof-of-concept approaches which will inspire further research into the nature of particles in the ocean and their variability on regional and global scales.