Spectral particulate attenuation and particle size distribution in the bottom boundary layer of a continental shelf

Spectral attenuation and absorption coefficients of particulate matter and colocated hydrographic measurements were obtained in the Mid-Atlantic Eight during the fall of 1996 and the spring of 1997 as part of the Coastal Mixi ng and Optics experiment. Within the bottom boundary layer (BBL) the magnit ude of the beam attenuation decreased and its spectral shape became steeper with distance from the bottom. Concurrently, the slope of the particulate size distribution (PSD) was found to increase with distance from the bottom . Changes in the PSD shape and the magnitude of the beam attenuation as fun ctions of distance from the bottom in the BBL are consistent with particle resuspension and settling in the BBL, two processes that are dependent on p article size and density. For particles of similar density, resuspension an d settling would result in a flattening of the PSD and an increase in the b eam attenuation toward the bottom. In both fall and spring the magnitude of the particle attenuation coefficient correlates with its spectral shape, w ith a flatter shape associated with higher values of the attenuation. This observation is consistent with idealized optical theory for polydispersed n onabsorbing spheres. According to this theory, changes in the steepness of the particle size distribution (particle concentration as a function of siz e) will be associated with changes in the steepness of the attenuation spec tra as a function of wavelength; a flatter particle size distribution will be associated with a flatter attenuation spectrum. In addition, the observe d ranges of the beam attenuation spectral slope and the PSD exponent are fo und to be consistent with this theory.