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Photoenhanced Reaction of Ozone with Chlorophyll at the Seawater Surface

2008; American Chemical Society; Volume: 113; Issue: 6 Linguagem: Inglês

10.1021/jp805167d

1932-7455

Dorea I. Reeser, A. Jammoul, Daniel Clifford, Marcello Brigante, Barbara D’Anna, C. George, D. J. Donaldson,

Air Quality Monitoring and Forecasting

The kinetics of the heterogeneous reaction between gas-phase ozone and chlorophyll present at the air−salt water interface are altered in the presence of actinic radiation. Under dark conditions, a Langmuir−Hinshelwood mechanism is followed, but actinic illumination brings about a large enhancement in the rate and a change to a linear dependence on ozone concentration. This change is observed through the entire near-UV and visible spectrum but reverts to that seen in darkness with illumination >800 nm. The excitation spectrum of chlorophyll on the salt-water surface is only somewhat different than that observed with a pure-water substrate. However, the photochemical loss rate of chlorophyll in the absence of ozone is much greater on the surface of salt solutions than upon pure water. In salt solutions, a transient absorption feature assignable to Cl2− is observed following laser excitation of chlorophyll. Also, absorptions due to the chlorophyll cation or triplet state and due to solvated electrons are seen in illuminated salt- and fresh-water chlorophyll solutions. Altogether, these results suggest that Cl atoms are formed in this system via the reduction of the photoformed chlorophyll cation by chloride anions. The yield of Cl is enhanced in the presence of ozone, perhaps 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.