Photosensitized production of halogenated radicals at the sea-water surface
Abstract
Recently, increasing attention has been given to the physical and chemical processes occurring at the uppermost layer of the oceans, especially in the sea-surface microlayer. This region is found at the uppermost tens to hundreds of micrometres of the water surface, and exhibits physical, chemical and biological properties that differ from those of the underlying water. Chemical analysis of the organics at the sea surface has shown that amphiphiles derived from oceanic biota (fatty acids, fatty alcohols, sterols, amines, and more complex colloids and aggregates exuded by phytoplankton mainly constituted by lipopolysaccharides), can be highly enriched in this microlayer. We have used chlorophyll as a proxy for the photochemically active organic fraction of this surface layer. In salt solutions a transient absorption feature assignable to Cl2- is observed following laser excitation of chlorophyll. As well, absorptions due to the chlorophyll cation and / or triplet state and also solvated electrons are seen in illuminated salt- and fresh- water chlorophyll solutions. When chlorophyll present at the water surface is illuminated using visible light, it is removed via chemical reaction at a rate proportional to the concentration of added salt (NaCl, NaBr or NaNO3). These results together suggest that halogen atoms are formed in this system, via the reduction of the photo-formed chlorophyll cation by halide anions. Transient absorption experiments indicate that the yield of Cl is enhanced in the presence of ozone, perhaps through the oxidation of Cl2-, or the hydrolysis of O3- to OH radicals. The chlorine atoms thus formed are responsible for the enhanced chlorophyll loss at the surface of illuminated salt-water substrates. The formation of Cl atoms and potentially small chlorinated organic compounds also has implications for gas phase oxidative chemistry in the marine boundary layer.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.A44B..03G
- Keywords:
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- 0312 Air/sea constituent fluxes (3339;
- 4504);
- 0315 Biosphere/atmosphere interactions (0426;
- 1610);
- 0317 Chemical kinetic and photochemical properties