Role of the Aerosol Substrate in the Heterogeneous Ozonation Reactions of Surface-Bound PAHs

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Abstract
To probe how the aerosol substrate influences heterogeneous polycyclic aromatic hydrocarbon (PAH) oxidation, we investigated the reaction of surface-bound anthracene with gas-phase ozone on phenylsiloxane oil and azelaic acid aerosols under dry conditions in an aerosol flow tube with offline analysis of anthracene. The reaction exhibited pseudo-first-order kinetics for anthracene loss, and the pseudo-first-order rate coefficients displayed a Langmuir−Hinshelwood dependence on the gas-phase ozone concentration on both aerosol substrates. The following parameters were found: for the reaction on phenylsiloxane oil aerosols, KO3 = (1.0 ± 0.4) × 10-13 cm3 and kImax = (0.010 ± 0.003) s-1; for the reaction on azelaic acid aerosols, KO3 = (2.2 ± 0.9) × 10-15 cm3 and kImax = (0.057 ± 0.009) s-1, where KO3 is a parameter that describes the partitioning of ozone to the surface and kImax is the maximum pseudo-first-order rate coefficient at high ozone concentrations. The KO3 value for the reaction of surface-bound anthracene and ozone on azelaic acid aerosols is similar to the KO3 value that we obtained in our previous study for the reaction of surface-bound benzo[a]pyrene and ozone on the same substrate. This finding supports our earlier hypothesis that the substrate influences the partitioning of ozone to the surface irrespective of the organic species (i.e., PAH) adsorbed to it. Preliminary ab initio calculations were performed to investigate whether there is a relationship between the relative binding energies of the ozone−substrate complex and the KO3 values for the different substrates studied. A comparison between kinetic results obtained on aerosol substrates and thin films is presented.

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