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Accueil du site > Thèmes de recherche > Atmosphère & environnement > 2.2 Interactions espèces gazeuses -surfaces > 2.2.1 Fate of Polycyclic Aromatic Hydrocarbons (PAH) in atmospheric soot particles

2.2.1 Fate of Polycyclic Aromatic Hydrocarbons (PAH) in atmospheric soot particles

Two types of processes determining the atmospheric fate of soot-bound PAHs have been studied : PAH desorption from soot surface in relation with the PAHs partitioning between gas and particulate phases in the atmosphere and their heterogeneous oxidation reactions. Thermal desorption of PAH from soot surface
Kinetics of thermal desorption of three to six-ring PAH from kerosene soot surface was studied over the temperature range 260-320 K in a flow reactor combined with an electron-impact mass spectrometer. The soot was prepared in the laboratory under controlled conditions. Two methods were used to measure the desorption rate constants : monitoring of the surface-bound PAH decays by off-line HPLC measurements of their concentrations in soot samples, and monitoring of the desorbed molecules in the gas phase by in situ mass spectrometry. The results obtained with the two methods were in good agreement and yielded the Arrhenius expressions for desorption kinetics of 16 PAH. The derived desorption enthalpies are close to the corresponding sublimation enthalpies, which is consistent with the structural similarity of PAH molecules and soot. The obtained experimental data were applied to the calculations of PAH partitioning in the atmosphere using available theoretical models (ACL 90, 102, 109, 105). Heterogeneous reactions of soot surface-bound PAH with atmospheric oxidants O3, NO2, OH
Kinetic studies were performed in the flow reactor with off-line particulate PAH analysis by HPLC. No measurable decay of PAHs due to the reaction with NO2 was observed under experimental conditions which allowed to determine the upper limits of the first-order rate constants for the heterogeneous reactions of 17 soot-bound PAHs with NO2 : k < 5.0×10-5 s-1. Regarding the heterogeneous reactions of particulate PAHs with ozone, the first-order rate constants measured for individual PAHs ranged from 0.004 to 0.008 s-1 and were found to be independent of the ozone concentration and temperature. The rate of the heterogeneous reaction of PAHs with OH radicals was found to be in the range (0.02 - 0.04) s-1 at T = 290 K, independent of the OH concentration and of the molecular structure of the PAH. The results of this work indicate that the reactions of PAHs adsorbed on soot surface with O3 and OH can be important pathways of particulate PAHs degradation in the atmosphere (ACL99, 104, 106).