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 overor 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 R rs (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 R rs (λ) simulated for natural seawater to R rs (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 R rs while the heavy Romashkino reduces R rs for 30-50%.

Modelling Remote Sensing Reflectance to Detect Dispersed Oil at Sea

“…Assuming that such a multispectral device could be part of the sensor system for detecting dispersed oil in the sea, numerical simulations of light transfer in water containing oil contamination were conducted. The radiative transfer model was applied which is based on the Monte Carlo method of simulating a large number of solar photons penetrating the sea surface (some of which return to the atmosphere and are registered by virtual photon detector [13][14][15]). As the input data for the model, the spectral distribution of both absorption and scattering coefficients, as well as angular distribution of light scattered on water density fluctuations and particles suspended in the water were used.…”

The effect of dispersed Petrobaltic oil droplet size on photosynthetically active radiation in marine environment

“…Figure 1 shows the width parameter σ dependence on the most common droplet radius (after Otremba 2007). Analyzing these data we have (Majchrowski and Ostrowska 2000) b In vivo data (Roy et al 2011;Bricaud et al 2004) Fig. 2 Log-normal size distributions of oil droplets suspended in seawater at the concentration of 1 ppm for different peak diameters concluded, the width parameter for the Petrobaltic oil droplets can be described by the equation:…”

Light penetration in seawater polluted by dispersed oil: results of radiative transfer modelling

“…In order to determine the scattering phase function for natural seawater we run a set of radiative transfer (RT) simulations using our model described in [25] with different Fournier-Forand (F-F) phase functions parameterized by different backscattering ratios, b b /b (see [26,27]). We compared the remote sensing reflectance (R rs ) from the model with in situ measurement performed by a set of Ramses Trios devices.…”

“…For radiative transfer modelling we applied our previously described model based on Monte Carlo code [25,36]. The boundary conditions were chosen as follows: 10% sky overcast, actual sun elevation (zenith angle) of 58 • , wind speed of 5 ms −1 , actual seabed at 11 m, lambertian bottom reflectance of 10% including 2% of specular reflection and 8% of diffuse reflection.…”