Reflection of light from the air/water interface covered with sea-surface microlayers

The sea-surface microlayer as a boundary between the ocean and the atmosphere, where important physicochemical, biological, and photochemical processes take place, plays a major role in the exchange processes of gases, materials, and energy. However, its chemical composition, structure, and involved interactions are very complex and yet not fully understood. Here, we have applied optical/spectroscopic methods and fractal analysis to gain better insight and information regarding composition, structure, and processes in sea-surface microlayer. Regarding the optical properties of sea surface, the material known as chromophoric dissolved organic matter (CDOM) represents the predominant light-absorbing part of dissolved organic material. The enrichment of light-absorbing material in the microlayers has significant effect on transformation processes at the inter-face and on the properties of interface itself. Original and reconstructed sea-surface microlayer samples have been characterized and visualized by reflection spectroscopy and Brewster angle microscopy (BAM). Due to the presence of chromophores at the air/water interface, the spectrum of reflected light is changed within the spectral range of chromophore absorption, depending on the chromophore density and orientation at the interface. On the other hand, BAM provides information about the homogeneity of the film, existence and formation of domains, phase transitions, and adsorption of material from the aqueous phase to the interface. In this study we have analyzed original and ex-situ reconstructed sea-surface microlayer samples from the middle Adriatic (salt lake Rogoznica) and from Norwegian fjords in the vicinity of Tromso. The reflection spectra and BAM images of microlayers have been taken at different surface pressures. Also, the film formation on initially clean surface was monitored by BAM. It was found that the mechanisms responsible for initial film formation and its later development are diffusion from the bulk and adsorption on the interface followed by subsequent aggregation. Fit of experimental data with theoretical results indicates that the adsorption obeys Frumkin-type isotherm with attractive lateral interaction. Fractal analysis applied to the BAM images detected and identified a second-order phase transition during reconstructed layer compression and provided information on mechanisms involved in spontaneous formation and structural changes of the sea-surface microlayer sample. (c) 2005 Elsevier B.V. All rights reserved.