title: Photoenhanced ozone loss on solid pyrene films.
abstract: This work presents the results of two complementary studies of the heterogeneous reaction of gas-phase ozone with solid pyrene films. In the first study, ozone uptake by the pyrene film was determined using a coated-wall flow tube system. In the second, pyrene loss within the film upon exposure to ozone was monitored using a laser-induced fluorescence technique. The dependence of the reactive loss rate on ozone concentration observed in both methods suggests that the reaction proceeds via a Langmuir-Hinshelwood-type surface mechanism. At a mixing ratio of 50 ppb, the steady-state reactive uptake coefficient of ozone by pyrene films increased from 5.0x10(-6) in the dark to 3.7x10(-5) upon exposure to near-UV radiation (300-420 nm). The uptake coefficient increased linearly as a function of UV-A spectral irradiance and decreased markedly with increasing relative humidity. The loss of surface pyrene upon exposure to ozone also displayed a light enhancement: analysis of Langmuir-Hinshelwood plots for the light and dark reactions revealed a small increase in the two-dimensional reaction rate in the presence of light (lambda>or=295 nm). This modest enhancement, however, was less significant than the corresponding enhancement in the loss of gas-phase ozone. In order to explain these observations, we present an integrated mechanism whereby the light-enhanced ozone uptake arises from the reaction of ozone with O2(1Sigmag+) formed via energy transfer from excited-state pyrene and the enhanced pyrene loss occurs via the formation of a charge-transfer complex between excited-state pyrene and adsorbed ozone. The disparity between surface- and gas-phase results underscores the important role that multifaceted strategies can play in elucidating the mechanisms of heterogeneous atmospheric reactions.
authors.affiliation: Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6.
authors.forename: Sarah A
authors.initials: SA
authors.lastname: Styler
authors.forename: Marcello
authors.initials: M
authors.lastname: Brigante
authors.forename: Barbara
authors.initials: B
authors.lastname: D'Anna
authors.forename: Christian
authors.initials: C
authors.lastname: George
authors.forename: D J
authors.initials: DJ
authors.lastname: Donaldson
mesh_terms: D000336:Aerosols / Q000737:chemistry* / Q000528:radiation effects; D001272:Atmosphere / Q000032:analysis / Q000737:chemistry*; D005740:Gases / Q000737:chemistry / Q000528:radiation effects; D008027:Light; D008956:Models, Chemical; D010126:Ozone / Q000737:chemistry* / Q000528:radiation effects*; D010316:Particle Size; D052638:Particulate Matter / Q000737:chemistry* / Q000528:radiation effects; D010777:Photochemistry / Q000379:methods*; D011721:Pyrenes / Q000737:chemistry* / Q000528:radiation effects
chemical_list: D000336:Aerosols; D005740:Gases; D052638:Particulate Matter; D011721:Pyrenes; D010126:Ozone