Photosensitized reactions at the Air-Sea interface

The significant presence of organic compounds at the surface of the ocean form the link between ocean biogeochemistry through the physico-chemical processes near the water-air interface with primary and secondary aerosol formation and evolution in the air aloft and finally to the climate impact of marine boundary layer aerosols. However, their photochemistry and photosensitizer properties have only been suggested and discussed but never fully addressed. 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. Taking chlorophyll as an accessible proxy for the organic fraction of this surface layer, we will demonstrate that photochemical process may be central to the deposition and chemistry of the sea surface. Especially, we will address the kinetics of the heterogeneous reaction between gas-phase ozone and chlorophyll present at the air-salt water interface. We will demonstrate that these kinetics are altered in the presence of actinic radiation. 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. These results together suggest that Cl atoms are formed in this system, via the reduction of the photo-formed chlorophyll cation by chloride anions. The yield of Cl is enhanced in the presence of ozone, through the oxidation of Cl2. 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 also has implications for gas phase oxidative chemistry in the marine boundary layer.