[1] The particulate backscattering coefficient b bp is an inherent optical property that plays a central role in studies of ocean color remote sensing. Because of practical difficulties associated with measurements of the volume scattering function (VSF) over the whole backward hemisphere, b bp is currently derived using fixed-angle backscattering sensors and applying a conversion factor for particulate backscattering, referred to as c p . The underlying assumptions of the fixed-angle approach are as follows: (1) in the green band, c p is fairly constant in the angular range 100°-150°and (2) for a fixed scattering angle, c p is wavelength-independent. In this study we investigated the variability of c p based on spectral measurements of the full VSF, both in situ and for algal culture in the laboratory. The in situ data used in our study were acquired in a coastal environment outside of phytoplankton blooms, whereas the laboratory data were representative for phytoplankton bloom conditions in oceanic waters. At 555 nm, c p was found to vary significantly in the angular range 100°-130°, and at 140°, c p was found to be weakly variable in nonblooming waters only. The spectral variability of c p was studied for the first time, and the spectral slopes of c p , measured in situ, were found to vary within ±6%. Under the assumption that c p (140°) is wavelength-independent, the induced error in the estimates of b bp was found to be lower than 10%. The algal culture showed a much higher spectral variability in c p (±20%), which induced an error in the estimates of b bp up to ±25.8%.Citation: Chami, M., E. Marken, J. J. Stamnes, G. Khomenko, and G. Korotaev (2006), Variability of the relationship between the particulate backscattering coefficient and the volume scattering function measured at fixed angles,
Oceans cover about 70% of the surface of the Earth and represent an important source of food. To obtain information about primary production, propagation of phytoplankton blooms, nutrient status, and temperature, the marine environment is continuously monitored both in situ and from satellites. In this context, in situ optical measurements are indispensable because they are fast and suited for automated routine operation. Among the most commonly measured parameters in situ are the absorption and attenuation coefficients, the diffuse attenuation coefficient for downwelling irradiance, and natural fluorescence. From satellite measurements, one can estimate the water-leaving radiance, and in the laboratory, the chlorophyll concentrations of water samples can be measured by fluorometric or spectrophotometric methods.The volume-scattering function (VSF) is an important optical property that is not routinely measured. Thus, the measurements of Petzold (1977), which date back to the 1970s, are still commonly used to represent the VSF for seawater. Among the laboratory measurements of the VSF relevant for our study, the data obtained by Privoznik et al. (1978) and Morel and Bricaud (1986)
AbstractWe present an improved technique for measurements of the volume-scattering function (VSF) for marine particles, which include added spectral information, a novel optical design of the sample container, and two new ways of eliminating unwanted reflections. The novel optical design enables us to measure an angular range comparable to the largest angular range previously reported. Our improved setup eliminates the need for an empirically based data correction and reveals interesting characteristics of the VSF for different phytoplankton species. Measurements with our improved setup provide information that is important to understand and simulate radiative transfer in the ocean.
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