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 arm). 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 H2O 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 (similar to 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 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: Oi and H2O Langmuir adsorption equilibrium constants K-O3 = (2.8 +/- 0.2) x 10(-13) cm(3) and K-H2O = (2.1 +/- 0.4) X 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) x 10(14) cm(-2). On the basis of these values, a second-order BaP-O-3 surface reaction rate coefficient k(2.8), = (2.6 +/- 0.8) x 10(-17) cm(2) s(-1) can be calculated, and estimates for the mean surface residence times and adsorption enthalpies of O-3 and H2O have been derived: tau (O3) approximate to 5-18 s; tauH(2)O approximate to 3 ms, DeltaH(ads.O3) approximate to -(80-90) kJ mol(-1). DeltaH(ads.H2O) approximate to -50 kJ mol(-1). The results and their atmospheric implications are discussed in view of related studies.