The optical properties of mineral suspended particles: A review and synthesis

Small mineral particles suspended in the sea are excellent at reflecting light and show up well in visible band satellite images. In order to make quantitative estimates of the particle concentration, and its effect on the penetration of sunlight into the sea, it is necessary to know how the absorption, scattering and backscattering coefficients of these inorganic particles change with concentration, the nature of the particles, and with wavelength. In this paper, observations from the literature are supplemented with a data set from the Irish Sea. The concentration-specific absorption coefficient of mineral particles am∗ is generally found to decrease exponentially with wavelength towards (in our data) a constant non-zero value in the red. Specific scattering coefficients show a tendency to decrease from the open ocean into energetic shelf seas and estuaries, but then to increase again within shelf seas as turbulent energy increases. The variation of specific scattering with turbulent energy in the Irish Sea is consistent with particle size scaling with the Kolmogorov microscale. Colour ratios (the ratio of two reflection coefficients) are less sensitive to variations in scattering, and we suggest that a combination of satellite measurements of brightness and colour in water with high mineral suspended sediment content will produce (1) a better estimate of concentration and (2) information on the variation of specific scattering.