Remote Sensing of Dispersed Oil Pollution in the Ocean-The Role of Chlorophyll Concentration

Abstract

In the contrary to surface oil slicks, dispersed oil pollution is not yet detected or monitored on regular basis. The possible range of changes of the local optical properties of seawater caused by the occurrence of dispersed oil, as well as the dependencies of changes on various physical and environmental factors, can be estimated using simulation techniques. Two models were combined to examine the influence of oceanic water type on the visibility of dispersed oil: the Monte Carlo radiative transfer model and the Lorenz-Mie model for spherical oil droplets suspended in seawater. Remote sensing reflectance, R_rs, was compared for natural ocean water models representing oligotrophic, mesotrophic and eutrophic environments (characterized by chlorophyll-a concentrations of 0.1, 1 and 10 mg/m³, respectively) and polluted by three different kinds of oils: biodiesel, lubricant oil and crude oil. We found out that dispersed oil usually increases R_rs values for all types of seawater, with the highest effect for the oligotrophic ocean. In the clearest studied waters, the absolute values of R_rs increased 2-6 times after simulated dispersed oil pollution, while R_rs band ratios routinely applied in bio-optical models decreased up to 80%. The color index, CI, was nearly double reduced by dispersed biodiesel BD and lubricant oil CL, but more than doubled by crude oil FL.

Keywords: chlorophyll-a; color index; dispersed oil detection; oil pollution; radiative transfer; remote sensing reflectance.

Figure 1

Inherent optical properties for oceanic water model characterized by chlorophyll-a concentrations of 0.1, 1 and 10 mg/m³: (a) total absorption coefficients a_tot for three types of oceanic waters and the absorption contribution coming from chlorophyll particles, a_particle, color-dissolved organic matter, a_CDOM, and pure water, a_water, for chl-a of 10 mg/m³; (b) total scattering coefficients for three types of oceanic waters b_tot and for pure water b_water; (c) log-linear plot of corresponding scattering phase functions at 555 nm.

Figure 2

(a) Real part of refractive indices of oils measured using multispectral refractometer DSR-λ; (b) imaginary part of refractive indices of oils obtained from absorption measurements by spectrophotometer Perkin Elmer Lambda 850; (c) oil droplet size distributions measured using LISST-100X.

Figure 3

Scheme of the model of radiative transfer in seawater polluted by dispersed oil: yellow boxes mark the input boundary conditions, blue boxes illustrate the input optical properties of natural seawater; data related to dispersed oil droplets are in brown boxes, and model output data are in the green box.

Figure 4

The modeling concept and boundary conditions setup in three types of oceanic waters.

Figure 5

Percentage impact of (a) absorption and (b) scattering coefficients of dispersed oils to the total absorption and scattering of oligotrophic ocean water (Chl-a = 0.1 mg/m³) obtained as a result of Mie modeling.

Figure 6

Scattering phase functions from Mie calculations for three kinds of dispersed oils: (a) biodiesel BD, (b) lubricant oil CL and (c) crude oil FL, within the borders of visible spectral range (400 and 700 nm).

Figure 7

Remote sensing reflectance spectra obtained for three types of unpolluted natural ocean water models (marked by blue and green lines) and polluted by three kinds of dispersed oil: (a) oligotrophic ocean water characterized by Chl-a = 0.1 mg/m³, (b) mesotrophic ocean water characterized by Chl-a = 1 mg/m³ and (c) eutrophic ocean characterized by Chl-a = 10 mg/m³.

Figure 8

R_rs band ratios for polluted water; fold change in relation to unpolluted natural water for (a) oligotrophic ocean water characterized by Chl-a = 0.1 mg/m³, (b) mesotrophic ocean water characterized by Chl-a = 1 mg/m³ and (c) eutrophic ocean water characterized by Chl-a = 10 mg/m³.