Use Of The Specific Beam Attenuation Coefficient For Identification Of Suspended Particulate Material

Abstract

Many researchers have measured the beam attenuation coefficient of monochromatic light in seawater in conjunction with the concentration (volume or mass) of suspended material. The ratio of the beam attenuation coefficient to suspended load (known as the specific beam attenuation coefficient) has been shown to vary considerably for different oceanic areas and depths. However, the specific beam attenuation coefficient has also been shown to be a reliable identifier of water masses and tracer of particles in many marine systems. The factors which control the value of the specific beam attenuation coefficient are the size distribution, refractive index distribution and shape of the suspended particles. The theoretical response of the beam attenuation coefficient to changes in the size and refractive index of a particle is well known. Consequently, the response of the beam attenuation coefficient to changes in suspended load may be quantified and predicted and the specific beam attenuation coefficient may provide a high resolution measure of the refractive index and particle size distribution of the suspended load. Calibration curves for the specific beam attenuation coefficient as a function of the particulate refractive index and the slope of the hyperbolic particle size distribution are presented. The curves have been derived for a light wavelength of 660 nm; this is the spectral line most commonly used for optical studies of the marine suspended load. The application of the curves to existent data sets supports their utility and demonstrates the resolution of the optical measurements in defining suspended particulate material. For a known particle load (i.e. approximate bulk refractive index and particle size distribution), measurements of the beam attenuation coefficient may be used to accurately estimate suspended volume. Similarly, measurements of the specific beam attenuation coefficient may be used to determine the particulate bulk refractive index and particle size distribution.