Kinetics of surface-bound benzo[a]pyrene and ozone on solid organic and salt aerosols

An aerosol flow tube apparatus was developed to perform the first kinetic study of the oxidation of particulate-bound BaP on solid organic and salt aerosols by gas-phase ozone. The studies on azelaic acid aerosols were performed with submonolayer coatings of BaP under both dry (RH < 1%) and high relative humidity (RH similar to 72%) conditions. The reaction exhibited pseudo-first-order kinetics for BaP loss and the pseudo-first-order rate coefficients displayed a Langmuir-Hinshelwood dependence on gas-phase ozone concentration. Under high relative humidity conditions the kinetics were faster but also displayed a similar functional dependence on the gas-phase ozone concentration. By assuming Langmuir-Hinshelwood behavior, the following parameters were obtained: ozone-surface equilibrium constant K-O3(< 1% RH) = (1.2 +/- 0.4) x 10(-15) cm(-1), K-O3(72% RH) = (2.8 +/- 1.4) x 10(-15) cm(-3), the maximum pseudo-first-order rate coefficient k(max)(I) (< 1% RH) = (0.048 +/- 0.008) s(-1), k(max)(I)(72% RH) = (0.060 +/- 0.018) s(-1). Uptake coefficients were extracted from the pseudofirst-order rate coefficients and a slight trend of decreasing uptake coefficients with increasing ozone concentration was observed. In contrast to the behavior on azelaic acid aerosols, no reaction was observed between ozone and BaP adsorbed to solid NaCl particles. These results are compared to previous studies, which have been performed on different substrates, and their atmospheric implications are discussed. We conclude that a strong substrate effect prevails in this reaction with the kinetics proceeding faster on surfaces best able to adsorb ozone.