FTIR Spectroscopic Investigation of the Mechanism and Kinetics of the Heterogeneous Reactions of NO₂ and HNO₃ with Soot

Soot samples from a spark generator, a flame, and a diesel passenger car were either collected on a Teflon filter and transferred to an IR-transparent window or deposited directly from a flame onto the window and investigated by Fourier transform infrared (FTIR) spectroscopy. The soot-covered windows were mounted in a 10 cm vacuum cell connected to a standard flow system with He as carrier gas. Reactive gases, such as NO 2 and HNO 3, were added to the carrier gas flow at a concentration of (0.016 to 2.5) × 10 14 molecule cm -3 . FTIR spectra of soot samples before and after exposure to HNO 3, NO 2, and O 3 are presented. Formation of IR absorption bands was analyzed as a function of exposure time. IR bands attributable to soot surface oxidation products and nitrogen containing species, e.g. −C O, R−NO 2, R−ONO 2, and R−ONO were observed. The observed time dependence of the absorption bands of the spark generator soot can be fitted by two parallel reactions, a slow and a fast process. Both processes have a reaction order of n ≈ 0.2 (±0.3) for the NO 2 + soot reaction and n ≈ 0.5 (±0.6) for the HNO 3 + soot reaction. The number of active sites, N max = 2.2 × 10 14 molecules cm -2 soot surface, has been estimated from saturation experiments. Surface reaction probabilities depend on reactant concentration and reaction time and were in the range of γ ≈ 10 -6 to 10 -8 for the slow, and γ ≈ 10 -3 to 10 -6 for the fast processes. The reaction probability on diesel engine soot was nearly 1 order of magnitude slower. It is concluded that the reaction of NO 2 with soot cannot account for the HONO levels observed in urban air.