Absorption and scattering coefficient estimation with asymptotic apparent optical properties

In deep, homogeneous waters with no internal sources at a particular wavelength, the vertical profiles of thc reflectance R(z) and the downward diffuse attenuation coefficient K-d(z) approach asymptotic values that are inherent optical properties (IOP's) of the water. The apparent optical properties R(z) and K-d(z) are obtained from the upward and downward monospectral irradiance measurements E(u)(z) and E(d)(z) that are commonly available to optical oceanographers. Given a specific scattering phase function, there are unique correlations between these asymptotic IOP's and the absorption and scattering coefficients a and b that can be derived from the radiative transfer equation. Here we evaluate a method for first determining the asymptotic IOP's from E(u)(z) and E(d)(z) and then using the correlations to estimate the absorption and scattering coefficients a and b. At depths near the asymptotic radiance regime, both R(z) and K-d(z) can be fitted to a three-parameter model that sometimes helps in the determination of the asymptotic IOP's. A good estimation of a can be obtained from the asymptotic IOP's even when the scattering phase function is unknown; however, estimates of b are highly dependent on the assumed phase function.