Interaction of Ozone and Water Vapor with Spark Discharge Soot Aerosol Particles Coated with Benzo[a]pyrene:  O<INF>3</INF> and H<INF>2</INF>O 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 O<INF>3</INF> and H<INF>2</INF>O on the particles followed by a slower surface reaction between adsorbed O<INF>3</INF> 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<INF>3</INF> 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<INF>2</INF>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<INF>3</INF> and H<INF>2</INF>O Langmuir adsorption equilibrium constants K<INF>O</INF><INF></INF><INFINF>3</INFINF> = (2.8 ± 0.2) × 10^-13 cm³ and K<INF>H</INF><INF></INF><INFINF>2</INFINF><INF>O</INF> = (2.1 ± 0.4) × 10^-17 cm³, maximum pseudo-first-order BaP decay rate coefficient k<INF>1,4</INF> = (0.015 ± 0.001) s^-1, adsorption site surface concentration [SS]<INF>S</INF> = (5.7 ± 1.7) × 10¹⁴ cm^-2. On the basis of these values, a second-order BaP−O<INF>3</INF> surface reaction rate coefficient k<INF>2,s</INF> = (2.6 ± 0.8) × 10^-17 cm² s^-1 can be calculated, and estimates for the mean surface residence times and adsorption enthalpies of O<INF>3</INF> and H<INF>2</INF>O have been derived:  τ<INF>O</INF><INF></INF><INFINF>3</INFINF> ≈ 5−18 s; τ<INF>H</INF><INF></INF><INFINF>2</INFINF><INF>O</INF> ≈ 3 ms, ΔH<INF>ads,O</INF><INF></INF><INFINF>3</INFINF> ≈ −(80−90) kJ mol^-1, ΔH<INF>ads,H</INF><INF></INF><INFINF>2</INFINF><INF>O</INF> ≈ −50 kJ mol^-1. The results and their atmospheric implications are discussed in view of related studies.