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%.

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The demand to control the amount of oil discharged to the seawater is the consequence of increasing shipping activities. Oil products enter marine environment on daily basis in the form of fuels, engine oils or crude oils. Each of them have a significant impact on marine life and the coastal water management. Oil content influences many environmental factors, like water quality and bio-optical parameters (e.g. water-leaving radiance, inherent optical properties, seawater fluorescence). It should be measured on regular basis to avoid over or underestimation of those parameters. Currently there are several methods used to estimate the total hydrocarbon content in seawater, mostly based on fluorescence measurements. We present a unique method suitable to determine the concentration of oil products in two forms: the amount of dissolved oil and the amount of oil-in-water emulsion. The method consist of several steps: seawater sampling, vacuum filtering, extracting of oil in n-hexane, fluorescence measurements and calculation of oil concentration in comparison to the reference samples, i.e. the solutions of different types of crude oil in n-hexane. The results have been measured for the samples collected in Southern Baltic Sea during several ship cruises in 2012. We discuss the advantages and disadvantages of presented method in the context of radiative transfer modelling and potential remote detection of dispersed oil.