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Abstract:
Previous research has demonstrated the formation of Reactive Oxygen Species (ROS), including hydroxyl radical (•OH) and singlet oxygen (1O2) during sunlight illumination of dissolved organic matter (DOM) in freshwater. Comparatively little research has evaluated photochemical radical generation in seawater. Halides (e.g., Cl-, Br-) are known to serve as important scavengers of •OH, leading to the production of Reactive Halogen Species (RHS), including Br2•- and ClBr2•-. This talk discusses the photochemical formation of RHS during sunlight illumination of DOM in seawater. Specifically, our research has demonstrated greater RHS production than expected based upon halide scavenging of •OH. Accordingly, we find that triplet excited states of DOM are able to directly oxidize halides, particularly Br-, leading to RHS. RHS are more selective oxidants than •OH. Within complex mixtures, •OH attacks constituents without discrimination, while RHS focus their oxidizing power on the most reactive constituents. Many organic contaminants of concern in estuaries exhibit high reactivity, as do chromophores within DOM. As a result, the photochemical formation of RHS can result in higher degradation rates of contaminants in seawater than observed in freshwater, and higher bleaching rates of DOM. Enhanced bleaching is important, since sunlight absorption by chromophores initiates photochemical reactivity. Lastly, our recent research evaluates the implications of these findings for the phototoxicity of sunscreens (which are chromophores) to corals.