The interactions of ozone with benzophenone and phenol solid films have been investigated under simulated atmospheric conditions with respect to relative humidity, pressure, temperature, and O3 concentration using a coated flow tube reactor. The steady-state reactive uptake coefficients (gammass) of ozone on benzophenone films ranged from below 10(-6) in dark conditions to approximately 4 x 10(-6) under UV-A irradiation and decreased with increasing O3 concentration in the range 28-320 ppbv. A similar trend was observed for the initial uptake coefficient (gammai) which varied from ca. 1.5 x 10(-6) in the dark to approximately 7 x 10(-6) under UV-A irradiation. The uptake coefficients under irradiation were strongly dependent on the relative humidity (from 5 to 70%), with their lowest values at high humidity (70% RH). The ozone uptakes for multilayer coverage turned out to be independent of the deposited mass of the organic compound. The benzophenone-phenol mixture also showed photoenhanced uptake with a larger steady-state uptake under visible irradiation, approximately 2.9 x 10(-6). Contact angle measurements showed an increase of the organic film hydrophobicity for the benzophenone-phenol mixture upon combined exposure to light and ozone. A linear dependence of the kinetic values on the photon flux has been demonstrated and when extrapolated to the solar spectral irradiance would lead to uptake coefficients of approximately 10(-5). UV-vis analysis and contact angle measurements of the organic film after irradiation and ozone exposure showed relevant changes only in the mixture, with an increase in the hydrophobic character of the film and the appearance of a new absorption band up to 450 nm.

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 Cl 2is 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 Cl 2 -. 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. † Part of the special section "Physical Chemistry of Environmental Interfaces".