IN-SITU MEASUREMENTS OF INELASTIC LIGHT-SCATTERING IN MONTEREY BAY USING SOLAR FRAUNHOFER LINES

The contribution of inelastic scattering processes (scattering with a change in wavelength) to the light field in the ocean can be measured directly by observing the apparent decrease in absorption (line filling) of the Fraunhofer absorption lines in the solar spectrum. An instrument possessing very high spectral resolution has been developed to directly measure the Fraunhofer line filling in the upwelling and downwelling irradiances as a function of depth. Measurements made in Monterey Bay show line filling similar to that predicted on theoretical grounds when Raman scattering by water itself is assumed to be the only inelastic process; however, the lines are filled more rapidly with depth than predicted. We believe the more rapid filling results from solar-induced fluorescence of dissolved organic matter. A simple model including both fluorescence and Raman scattering, with the fluorescence parameters (quantum efficiency, excitation spectrum, and emission spectrum) taken from the literature, provides a closer but not precise agreement with the observations. Direct measurement bf the fluorescence parameters is likely required to achieve complete agreement with theory.