High-temperature oxidation of dispersed soot particles by O atoms

The reaction of O atoms with soot particles dispersed in 10 to 20 ppm N2O/Ar mixtures was studied at high temperatures behind reflected shock waves. Under the present conditions (T > 1900 K), N2O represents a fast thermal O-atom source, because it decomposes rapidly to form 0 atoms which in turn react with the dispersed soot particles. The progress in the heterogeneous reaction was determined by time-resolved measurements of O-atom concentration using the highly sensitive atomic resonance absorption spectroscopy. Besides the absorption of the OI radiation (lambda = 130.5 nm) by O atoms, the absorption signal includes also particle light extinction. A separation of both influences is possible by performing additional experiments on pure soot/Ar aerosols. Laser light extinction measurements using an Ar+ laser (A = 488.0 nm) were performed simultaneously providing a relationship between the particle extinction at both wavelengths, which allows us to determine the pure O-atom absorption in aerosols. The reaction conditions were chosen such that the total amount of carbon is much higher than the total amount of atomic oxygen; that is, the reaction conditions are pseudo-first-order. A rate coefficient for the consumption of O atoms due to soot particle oxidation was determined. CO was identified to be the main reaction product. To determine the heterogeneous reaction probability from the O-atom consumption rate, it is essential to measure the reactive particle surface. This quantity can be determined from the soot volume fraction available from the laser light extinction.