Modelling the influence of oil content on optical properties of seawater in the Baltic Sea

Abstract

The accuracy and correct interpretation of optical parameters of seawater depend on the complete information osn the interactions between seawater components and the light field. Among components influencing the radiative transfer, the droplets of oil can cause over- or underestimation of modelled and measured optical quantities, especially in closed seas and coastal zones. Oil content in the Baltic Sea varies from several ppb in the open sea to several ppm in estuaries or ship routes. Oil droplets become additional absorbents and attenuators in seawater causing changes in apparent optical properties. These changes can potentially enable remote optical detection of oil-in-water emulsion in visible bands. To demonstrate potential possibilities of such optical remote sensing, a study of inherent optical properties of two types of crude oil emulsion was conducted, i.e. high absorptive and strongly scattering Romashkino, and low absorptive and weakly scattering Petrobaltic. First, the calculations of spectral absorption and scattering coefficients as well as scattering phase functions for oil emulsions were performed on the basis of Lorentz-Mie theory for two different oil droplets size distributions corresponding to a fresh and 14-days aged emulsions. Next, radiative transfer theory was applied to evaluate the contribution of oil emulsion to remote sensing reflectance Rrs(λ). Presented system for radiative transfer simulation is based on Monte Carlo code and it involves optical tracing of virtual photons. The model was validated by comparison of Rrs(λ) simulated for natural seawater to Rrs(λ) from in situ measurements in Baltic Sea. The deviation did not exceed 10% for central visible wavelengths and stayed within 5% for short and long wavelengths. The light Petrobaltic crude oil in concentration of 1 ppm causes typically a 10-30% increase of Rrs while the heavy Romashkino reduces Rrs for 30-50%.