In situ variability of mass-specific beam ttenuation and backscattering of marine particles with respect to article size, density, and composition
Résumé
This study analyzes relationships between concentration of suspended particles represented by dry mass, [SPM], or area, [AC], and optical properties including particulate beam attenuation (cp), side scattering (bs), and backscattering (bbp), obtained from an intensive sampling program in coastal and offshore waters around Europe and French Guyana. First-order optical properties are driven by particle concentration with best predictions of [SPM] by bbp and bs, and of [AC] by cp. Second-order variability is investigated with respect to particle size, apparent density (dry weight-to-wet-volume ratio), and composition. Overall, the mass-specific particulate backscattering coefficient, bmbp ( = bbp : [SPM]), is relatively well constrained, with variability of a factor of 3-4. This coefficient is well correlated with particle composition, with inorganic particles having values about three times greater (bmbp = 0.012 m2 g−1) than organic particles (bmbp = 0.005 m2 g−1). The mass-specific particulate attenuation coefficient, cmp ( = cp : [SPM]), on the other hand, varies over one order of magnitude and is strongly driven (77% of the variability explained) by particle apparent density. In this data set particle size does not affect cmp and affects bmbp only weakly in clear (case 1) waters, despite size variations over one order of magnitude. A significant fraction (40-60%) of the variability in bmbp remains unexplained. Possible causes are the limitation of the measured size distributions to the 2-302-µm range and effects of particle shape and internal structure that affect bbp more than cp and were not accounted for.