Heterogeneous Reaction of NO₂ with Hydrocarbon Flame Soot

Abstract

The reaction of NO2 with toluene, kerosene, and hexane flame soot was studied over the temperature range 240−350 K using a low-pressure (a few Torr) flow reactor coupled to a modulated molecular beam mass spectrometer. A flat-flame burner was used for the preparation and deposition of soot samples from premixed flames of liquid fuels under well-controlled and adjustable combustion conditions. The values of (5.0 ± 2.0) × 10-5 and (2.9 ± 1.2) × 10-5 (calculated using the specific surface area of soot) at T = 298 K and the value of (4.0 ± 1.6) × 10-5 independent of temperature in the range 240−350 K were determined for the initial uptake coefficient (γ0) of NO2 on kerosene, hexane, and toluene soot, respectively. The process of soot aging (deactivation) was parametrized, the uptake coefficient (γ) being expressed as a function of time and gas-phase NO2 concentration: γ = γ0/(1 + γ0k[NO2]t), with k = (1.0 ± 0.4) × 10-9 and (1.9 ± 0.7) × 10-9 cm3 molecule-1 s-1 at T = 298 K for kerosene and hexane soot, respectively, and k = (7.3 ± 2.5) × 10-10 cm3 molecule-1 s-1 independent of temperature in the range 240−350 K for toluene soot. HONO was observed as a product of NO2 interaction with soot, with a yield of 30% ± 5% independent of the type of soot, mass of the soot, conditions of its preparation, initial concentration of NO2, and time of exposure to NO2 under the experimental conditions of this study. Experiments on soot aging confirmed that soot deactivation occurs under real ambient conditions. The present results support current considerations that heterogeneous reaction of NO2 with soot does not significantly influence the oxidative capacity of the atmosphere by producing HONO and consequently OH radicals.