Interaction of Ozone and Water Vapor with Spark Discharge Soot Aerosol Particles Coated with Benzo[a]pyrene: O3 and H2O Adsorption, Benzo[a]pyrene Degradation, and Atmospheric Implications

Abstract

The interaction of ozone and water vapor with spark discharge soot particles coated with the five-ring polycyclic aromatic hydrocarbon benzo[ a ]pyrene (BaP) has been investigated in aerosol flow tube experiments at ambient temperature and pressure (296 K, 1 atm). The investigated range of ozone volume mixing ratio (VMR) and relative humidity (RH) was 0−1 ppm and 0−25%, respectively. The observed gas-phase ozone losses and pseudo-first-order BaP decay rate coefficients exhibited Langmuir-type dependencies on gas-phase ozone concentration and were reduced in the presence of water vapor, which indicates rapid, reversible and competitive adsorption of O 3 and H 2 O on the particles followed by a slower surface reaction between adsorbed O 3 and BaP. At low ozone VMR and RH, the half-life of surface BaP molecules was found to be shorter than previously reported (∼ 5 min at 30 ppb O 3 under dry conditions). At higher RH and for multilayer BaP surface coverage, however, a strong increase of BaP half-life was observed and can be attributed to competitive H 2 O adsorption and to surface/bulk shielding effects, respectively. From four independent sets of ozone loss and BaP decay measurement data the following parameters have been derived: O 3 and H 2 O Langmuir adsorption equilibrium constants K O 3 = (2.8 ± 0.2) × 10 -13 cm 3 and K H 2 O = (2.1 ± 0.4) × 10 -17 cm 3, maximum pseudo-first-order BaP decay rate coefficient k 1,4 = (0.015 ± 0.001) s -1, adsorption site surface concentration [SS] S = (5.7 ± 1.7) × 10 14 cm -2 . On the basis of these values, a second-order BaP−O 3 surface reaction rate coefficient k 2,s = (2.6 ± 0.8) × 10 -17 cm 2 s -1 can be calculated, and estimates for the mean surface residence times and adsorption enthalpies of O 3 and H 2 O have been derived: τ O 3 ≈ 5−18 s; τ H 2 O ≈ 3 ms, Δ H ads,O 3 ≈ −(80−90) kJ mol -1, Δ H ads,H 2 O ≈ −50 kJ mol -1 . The results and their atmospheric implications are discussed in view of related studies.