Simulation And Analysis Of The Optical Properties Of Seawaters Polluted By Polydisperse Oil Droplets

After the occurrence of oil pollution,oil pollutants float on the sea surface in the form of oil slicks,then they would experience spreading,advection,dispersion,entrainment,Langmuir circulation,sinking,over-washing,partitioning and sedimentation when they present in marine environment,and exist in water column as oil droplets with different diameters.Oil slick has been widely concerned by public and academics,however,oil droplets dispersed in water columns are also harmful to marine life and marine environment.Hence,this work only focuses on the oil pollution caused by polydisperse oil droplet population(PODP)in waters,rather than oil slick,to simulate the optical properties of oil droplets dispersed in seawater and study their influences on the optical properties of seawater body.These are the foundations of remote sensing oil droplet pollution within marine water column.First of all,the attenuation coefficient,scattering coefficient and absorption coefficient of PODP for Petrobaltic,Romashkino,yellow diesel fuel and gasoline oil pollutions were calculated by Mie theory and the relations among the efficiencies of single oil droplet,the optical properties of equal-sized oil droplet population(EODP)and PODP.By analyzing the spectra of optical properties of PODP,we know that the contribution of absorption to attenuation is less,the attenuation of light is mainly caused by the scattering of oil droplets,and the two key factors which can determine the optical properties of PODP are the efficiencies and the cross sections of oil droplets.Secondly,the optical properties of PODP calculated above were used to simulated the underwater light field of waters which contains phytoplankton,suspended sediment and PODP by Hydrolight,in order to analyze the characteristics of the remote sensing reflectance(Rrs)spectrum and the impacts of PODP on the spectrum of waters.The simulation shows that redshift of Rrs peak values are mainly caused by the discrepancies of the relative complex refractive indices among the four kinds oil droplets.Compared with the strong absorption feature and the decrease effects on Rrs spectra for Romashkino PODPs,Petrobaltic and yellow diesel fuel PODPs featured by low absorption feature and strong backscattering capacity,in most cases,can rise the spectra of Rrs dramatically,especially for PODPs with small peak diameters.Furthermore,the absorption and backscattering of gasoline PODP are less than Romashkino,Petrobaltic and yellow diesel fuel PODPs,so the influence of gasoline PODPs with small peak diameters on the Rrs spectrum is less than other kinds of PODPs either.Thirdly,the Rrs spectra of waters contains phytoplankton,suspended sediment and PODP show that the spectra are dominated by PODP in the case of low concentration of phytoplankton and suspended sediment.On the contrary,in the case of high concentration of phytoplankton and suspended sediment,phytoplankton or suspended sediment dominate the Rrs spectra as the content of phytoplankton or suspended sediment increasing,respectively,whereas the influence of PODP on the spectra becomes less.Specifically,suspended sediment plays a major role on the spectra of Rrs when the content of suspended sediment is high enough,and the spectra of waters which contain high concentration of phytoplankton and low concentration of suspended sediment are similar with those spectra of waters only contain phytoplankton.Last,the influence of PODPs on the Rrs spectra is diverse and complicated,the diversity of the influence makes the establishment of inversion model of waters polluted by PODP possible.However,the peak bands of Rrs spectra for waters containing PODPs are similar to or overlapped with wavelengths used in conventional water color inversion model,hence,the primary task of constructing inversion model of waters polluted by polydisperse oil droplets is to solve the similarity or overlapping problem.