Heterogeneous Reaction of NO2 with Hydrocarbon Flame Soot

Abstract

The reaction of NO 2 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. Aflat-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) x 10 - 5 and (2.9 ′ 1.2) x 10 - 5 (calculated using the specific surface area of soot) at T = 298 K and the value of (4.0 ′ 1.6) x 10 - 5 independent of temperature in the range 240-350 K were determined for the initial uptake coefficient (γ 0 ) of NO 2 on kerosene, hexane, and toluene soot, respectively. The process of soot aging (deactivation) was parametrized, the uptake coefficient (y) being expressed as a function of time and gas-phase NO 2 concentration: y = γ 0 /(1 + γ 0 k[NO 2 ]t), with k = (1.0 ′ 0.4) x 10 - 9 and (1.9 ′ 0.7) × 10 - 9 cm 3 molecule - 1 s - 1 at T = 298 K for kerosene and hexane soot, respectively, and k = (7.3 ′ 2.5) x 10 - 1 0 cm 3 molecule - 1 s - 1 independent of temperature in the range 240-350 K for toluene soot. HONO was observed as a product of NO 2 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 NO 2 , and time of exposure to NO 2 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 NO 2 with soot does not significantly influence the oxidative capacity of the atmosphere by producing HONO and consequently OH radicals.