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

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 O₃ and H₂O on the particles followed by a slower surface reaction between adsorbed O₃ 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 O₃ 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 H₂O 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: O₃ and H₂O Langmuir adsorption equilibrium constants K_o3 = (2.8 ± 0.2) × 10^-13 cm³ and K_H2O = (2.1 ± 0.4) × 10^-17 cm³, 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) × 10¹⁴ cm^-2. On the basis of these values, a second-order BaP-O₃ surface reaction rate coefficient k_2,s = (2.6 ± 0.8) × 10^-17 cm² s^-1 can be calculated, and estimates for the mean surface residence times and adsorption enthalpies of O₃ and H₂O have been derived: τ_o3 5-18 s; τ_H2o ≈ 3 ms, Δ_ads,o3 ≈ -(80-90) kJ mol^-1, Δ_Hads,H,o ≈ -50 kJ mol^-1. The results and their atmospheric implications are discussed in view of related studies.