Spectral radiative properties of seawater-in-oil emulsions in visible-infrared region
Introduction
Oil slicks are common marine pollutants [1], [2]. Oil slicks on sea surface can hinder light and heat exchange between the ocean and the atmosphere and affect the absorption, transmission and reflection of electromagnetic waves in the ocean, thus affecting photosynthesis of plants and respiration of organisms inhabiting the sea floor. With rapid developments of the maritime industry, coastal city economies, and offshore oil and gas exploitation technology, oil spills occur frequently and cause considerable harm to the marine environment and coastal economic activities. As more attention is paid to the marine environment, offshore oil spills and submarine hydrocarbon leakages have become one of the most important directions in offshore oil and gas resource exploration, offshore oil pollution monitoring, and remote sensing of the marine environment [3], [4], [5]. Oil slick formation on the sea surface occurs as follows. Spilled oil first forms a thick black oil slick floating on the sea surface, the thickness of which is difficult to estimate as it is difficult for sunlight and other active detection incident light to penetrate. Black oil slicks are easily detected by optical sensors [1]. The thickness of oil slicks decreases with diffusion, and oil-slick films with different thicknesses and light transmittance are gradually formed, the thickness of which can reach 0.1 mm or even thinner [6]. Under the actions of wind, waves and other marine forces, oil slicks on the sea surface mix with seawater to form seawater-oil mixtures (emulsions) [5], [6]. Due to the incorporation of seawater droplets, large areas of crude oil will form seawater-in-oil emulsions in the case of relatively lighter waves and weaker wind. Under the actions of stronger waves and wind, crude oil can be dispersed into small oil droplets and mixed with seawater to form oil-in-seawater emulsions. In the field of remote-sensing detection, oil slicks can be divided into the following categories: black oil slick, oil-slick film and oil-water emulsions.
Various remote-sensing technologies, such as optical remote sensing, laser fluorescence, synthetic aperture radar and thermal remote sensing [7], [8], [9], [10], [11], [12], [13], [14], have been used in the study of oil slicks on the sea surface. For example, Palmer et al. [15] analyzed the oil spill in the Shetlands Islands in 1993 and found that floating crude oil exhibited different reflectivity at different concentrations of 700, 740 and 800 nm. Their studies indicate that 400–900 nm is an effective waveband for oil-film extraction. Lu et al. [1], [9], [16], [17], [18] systematically designed and performed laboratory experiments on oil films and oil emulsions. They measured and analyzed the remote-sensing reflectance and absorption of water-in-oil and oil-in-water emulsions with different volumetric concentrations and film thicknesses in the spectral range of 400–2500 nm. In terms of theoretical simulations, Haule et al. [19] performed radiative transfer simulations in the visible (VIS) region for natural seawater polluted by dispersed crude oil with various droplet-size distributions and discussed the influence of oil droplets on remote sensing. Otremba et al. [20], [21], [22] simulated the bidirectional reflection distribution function (BRDF) of pure seawater, oil films and oil-in-water emulsions at 350–750 nm by Monte Carlo (MC) method and studied the effects of wavelength and oil droplet size. Baszanowska et al. [23] modeled remote-sensing reflectance of dispersed oil on the sea surface using the MC method in the range of 412–676 nm and discussed the effects of wind speed. Evidently, a majority of previous research on oil slicks has been limited to VIS and near-infrared (NIR) regions.
Infrared multispectral remote sensing which is based on aircraft and satellites has been widely used for the detection of oil slicks [10], [24], [25]. In 2014, Xiong et al. [26] studied the characteristics and mechanism of the thermal infrared emissivity spectrum of artificial crude oil slicks with different thicknesses. Under the geometric optics approximation, Pinel et al. [27], [28] derived the infrared emissivity of oil films to study oil film detectability.
In summary, the reflection and emission characteristics of seawater-in-oil emulsions are essential for remote-sensing identification and classification of oil slicks on sea surface. Previous theoretical studies on the radiative properties of oil emulsions are limited to VIS-NIR regions [19], [21], [23] and are conducted using the traditional Lorenz-Mie theory (TLMT) in which the absorption of host medium is neglected. However, in the VIS region, the absorption of heavy crude oil is so large that they cannot be ignored. Besides, less research has been conducted to systematically study multispectral emission and reflection characteristics of oil-water emulsions in mid-infrared (MIR) regions. In this study, the effects of wavelength, droplet size, droplet volume fraction, and oil emulsion thickness on the spectral directional-hemispherical emittance and reflectance of seawater-in-oil emulsions were systematically studied in the VIS-IR regions. The complex refractive indices of crude oil and seawater were precisely measured, after which the radiative properties, including the absorption efficiency factor , scattering efficiency factor , extinction efficiency factor , and far-field phase function Φp of seawater droplets embedded in crude oil were calculated on account of Lorenz-Mie theory. The directional-hemispherical emittance and reflectance for an oil layer filled by spherical seawater droplets were calculated using the MC method. In the VIS-IR region, the absorption indices of both seawater and crude oil are so large that they cannot be ignored. Thus, the applicable conditions of TLMT are no longer satisfied. Mishchenko and Yang [29], [30] recently proposed an improved Lorenz-Mie theory (ILMT) in which the absorption of host medium is considered when studying the scattering properties of spherical particles. Ma et al. [31], [32] has studied the radiative properties of dispersed media neglecting and considering the absorption of host medium, and shows that neglecting the effects of the absorption of host medium will result in some errors in scattering properties. In order to illustrate the effects of the absorption of host medium, the radiative properties of seawater droplets embedded in crude oil and seawater-in-oil emulsions obtained on account of TLMT and ILMT are compared and analyzed.
Section snippets
Combined ellipsometry-transmission method
The complex refractive indices (optical constants) of liquid crude oil were measured using the combined ellipsometry-transmission method proposed before [33], [34]. The combined ellipsometry-transmission method was verified in our previous paper by comparing the measured optical constants of deionized water and methanol with paper data [33], [34]. The schema of ellipsometry measurements is presented in Fig. 1(a). Zinc selenide (ZnSe) was used as the prism material for the infrared waveband.
Modeling methodology
To theoretically study the radiative properties (i.e., optical remote sensing characteristics) of seawater-in-oil emulsions floating on seawater, we conduct the radiative energy transfer modeling of a crude oil film filled by spherical seawater droplets, as shown in Fig. 4. The oil layer with thickness d is floated on seawater. To accurately describe the Fresnel reflections and transmissions at the boundaries, the pre-measured complex refractive indices of air, crude oil, and seawater are used
Comparison of the traditional and improved Lorenz-Mie theory
To theoretically obtain the hemispherical reflectance and emittance of seawater-in-oil emulsions, the radiative properties of seawater droplets embedded in crude oil should be calculated using Lorenz-Mie theory. The TLMT doesn't consider the effect of the absorption of host medium. Mishchenko and Yang [29], [30] just proposed an ILMT in which the absorption of host medium is considered when studying the scattering properties of spherical particles. Considering the absorption indices of crude
Conclusion
In this study, the complex refractive indices of crude oil are precisely measured using the combined ellipsometry-transmission method. The complex refractive index data are provided in the supplemental material. The radiative properties of seawater droplets embedded in crude oil are calculated considering the absorption of crude oil host medium, and the effects of droplet size distribution are discussed. 〈Qext〉r shows an increasing trend with increasing droplet size except in the spectral range
Supplementary materials
Supplementary material associated with this article can be found in the online version.
CRediT authorship contribution statement
Chengchao Wang: Conceptualization, Investigation, Formal analysis, Validation, Data curation, Writing – original draft, Funding acquisition. Lanxin Ma: Methodology, Software, Data curation, Project administration, Writing – review & editing, Funding acquisition. Linhua Liu: Methodology, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is supported by the National Natural Science Foundation of China [Grant No. 51906127, 52076123, 51806124], China Postdoctoral Science Foundation [Grant Nos. 2019M662353, 2019M662354, 2020T130365] and Young Scholars Program of Shandong University. We sincerely thank M.I. Mishchenko and P. Yang for the far-field Lorenz-Mie scattering theory in an absorbing host medium. We also sincerely thank Prof. Dexin Liu of China University of Petroleum for the crude oil samples.
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