Optical closure in highly absorptive coastal waters: significance of inelastic scattering processes

In hydrological optics, "optical closure" means consistency between the apparent optical properties (AOPs) determined from radiometric measurements and those derived from radiative transfer modelling based on concurrently measured inherent optical properties (IOPs) and boundary conditions (sea and sky states). Good optical closure not only provides confidence in the data quality but also informs on the adequacy of the radiative transfer parameterization. Achieving optical closure in highly absorptive coastal waters is challenging due to the low signal-to-noise ratio of radiometric measurements and uncertainties in the measurements of IOPs, namely the spectral absorption and backscattering coefficients. Here, we present an optical closure assessment using a comprehensive set of in situ IOPs acquired in highly absorptive coastal waters optically dominated by chromophoric dissolved organic matter (CDOM). The spectral remote sensing reflectance, R_rs(λ), was modeled using the software HydroLight (HL) with measured IOPs and observed boundary conditions. Corresponding in-water in situ R_rs(λ) was derived from radiometric measurements made with a Compact Optical Profiling System (C-OPS; Biospherical). The assessment revealed that the inclusion of inelastic scattering processes in the model, specifically sun-induced CDOM fluorescence (f_DOM) and sun-induced chlorophyll fluorescence (SICF) from Chlorophyll-a ([chl]), significantly improved the optical closure and led to good agreement between measured and modeled R_rs (i.e., for 440 ≤ λ ≤ 710 nm with no inelastic processes: R²=0.90, slope=0.64; with inelastic processes: R²=0.96, slope=0.90). The analysis also indicated that f_DOM and SICF contributed a substantial fraction of the green-red wavelength R_rs in these waters. Specifically, f_DOM contributed ∼18% of the modeled R_rs in the green region and SICF accounted for ∼20% of the modeled R_rs in the red region. Overall, this study points out the importance of accounting for f_DOM in remote sensing applications in CDOM-dominated waters.