A spectral model of the beam attenuation coefficient in the ocean and coastal areas

A large set (100 data points at each wavelength) of multispectral beam attenuation, c(X), data at nine wavelengths (440, 450, 490, 520, 535, 550, 565, 630, and 670 nm) is used to develop a spectral model of the beam attenuation coefficient. The relationship c(X) cW(X) = [c(490 nm) cW(490 nm)](1.563 1.149 x 1O-3 X) describes the spectral variation of c(X) where cW(X) is the pure water beam attenuation and X is the wavelength in nm. From a subset of the data a relationship of chloropyll (Chl) to ~(490) was found to be ~(490) = 0.39 Ch1°.57; however there is significant scatter in this relationship. The spectral c model was tested with independent data sets and the average percent difference of the measured to predicted values ranged from 0.4 to 5% for the different spectral bands. The spectral beam attenuation coefficient c is an important property in optical oceanography. Because c (units given in list of notation) is an inherent optical property (Preisendorfer 1976) it is an important measurement in the optical characterization of a water sample. The inverse of the beam attenuation coefficient, the attenuation length, is important as a scaling parameter for problems in imaging and radiative transfer. When calibrated correctly, c can be used to determine the suspended particulate load in a water sample. Reported measurements of c have predominantly been in a single spectral band. Most often the photopic band has been used (with an instrument such as the Martek transmissometer) or more recently in a spectral band at 660 nm (with an instrument such as the SeaTech transmissometer). Comparisons between data sets of c in different spectral bands require knowledge of the spectral nature of c(X). Previous studies, in which measurements at multiple waveAcknowledgments I thank Dennis Clark for allowing me to use the NOAA spectral attenuation data. I also thank Roswell Austin, Albert Chapin, Gerald Edwards, and Jeffrey Nolten for collecting the Vislab data set, and Leonard Lopez for collating the data. I am also grateful to Andre Morel and Howard Gordon for their many suggestions. This work was supported by the Applied Physics Laboratory/Johns Hopkins University and the Ocean Optics program of the Office of Naval Research contract NO00 14-90-J1505. lengths were reported, were of phytoplankton cultures (Bricaud et al. 1983) or for cases of limited geographic extent (Kitchen et al. 1982). The goal of this work was to investigate a large, geographically diverse, data set and empirically determine the spectral characteristics of c(X). c(X) will be shown to have a simple wavelength dependence that appears to be broadly applicable. This simple dependence allows measurements at one wavelength to be extrapolated to other spectral bands.