Variability of the relationship between the particulate backscattering coefficient and the volume scattering function measured at fixed angles

[ 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, chi(p) is fairly constant in the angular range 100 degrees - 150 degrees and ( 2) for a fixed scattering angle, chi(p) is wavelength-independent. In this study we investigated the variability of chi(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, chi(p) was found to vary significantly in the angular range 100 degrees - 130 degrees, and at 140 degrees, chi(p) was found to be weakly variable in nonblooming waters only. The spectral variability of chi(p) was studied for the first time, and the spectral slopes of chi(p), measured in situ, were found to vary within +/- 6%. Under the assumption that chi(p)(140 degrees) 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 chi(p)( +/- 20%), which induced an error in the estimates of b(bp) up to +/- 25.8%.