Photosensitized Heterogeneous Chemistry of Ozone on Organic Films

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 (γss) of ozone on benzophenone films ranged from below 10-6 in dark conditions to ∼4 × 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 (γi) which varied from ca. 1.5 × 10-6 in the dark to ∼7 × 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, ∼2.9 × 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 ∼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.