Desorption of polycyclic aromatic hydrocarbons from a soot surface: three- to five-ring PAHs.

The kinetics of the thermal desorption of a set of three- to five-ring polycyclic aromatic hydrocarbons (PAHs) from a laboratory-generated kerosene soot surface was studied over the temperature range 250-355 K in a low-pressure flow reactor combined with an electron-impact mass spectrometer. Two methods were used to measure the desorption rate constants: monitoring of the surface-bound PAH decays due to desorption using off-line HPLC measurements of their concentrations in soot samples and monitoring of the desorbed molecules (anthracene and phenanthtrene) in the gas phase using in situ mass spectrometric detection. The Arrhenius parameters (A factors and activation energies) for the desorption rate constants of 10 soot-bound PAHs were determined. The PAH-soot binding energies were found to be similar for PAHs with the same number of carbon atoms and to increase with increasing number of PAH carbon atoms. The experimental data are discussed in the frame of the existing theoretical gas to particle partitioning model.