Kinetic and product studies of the heterogeneous reactions of surface-bound polycyclic aromatic hydrocarbons with selected atmospheric oxidants

The heterogeneous reaction of surface-bound polycyclic aromatic hydrocarbons (PAHs) and gas-phase ozone was used as a model system to investigate the factors that influence the heterogeneous reactions of organic compounds. The heterogeneous reactions of surface-bound anthracene and benzo[a]pyrene with ozone on different aerosol substrates were determined using an aerosol flow tube apparatus. The kinetics of these reactions displayed pseudo-first order behaviour with respect to ozone. The non-linear dependence of the pseudo-first order rate constant as a function of ozone concentration was consistent with reactions that proceed by the Langmuir---Hinshelwood mechanism for the substrates and PAHs investigated. The variability in the kinetics from substrate to substrate was influenced more by the partitioning of ozone to the aerosol surface than by the surface-phase reaction rate.;The product yield of anthraquinone, one of the known products of the reaction between anthracene and ozone, as a function of ozone concentration yielded a non-linear functional dependence that was similar to a Langmuir adsorption profile thus providing the first direct evidence that ozone is involved in the rate-limiting step in the formation of this product. Investigations of the heterogeneous oxidation of anthracene, pyrene and n-hexane soot with the NO3 radical, using a suite of analytical techniques indicated the formation of nitro functional groups on the surface. Carbonyl functionalities were observed in addition to nitro functional groups on the surface of soot surfaces following exposure to a nitrating flow. Absorption experiments indicate that the nitration of PAHs alter the optical properties of the particles to which they are adsorbed, giving rise to absorption intensity in the near UV and visible portions of the spectrum.;The experimental results of the kinetic studies were used in a multimedia model that was the first to assess the importance of these heterogeneous reactions in an urban environment. The kinetic, product and modeling studies illustrate that PAHs found on particle surfaces may be lost at a rate greater than by gas-phase loss mechanisms. Further, these studies suggest that the rate of formation of the toxic oxidized PAHs may also be significant in the urban environment.