The reactivity of NO2 and HONO on flame soot at ambient temperature: The influence of combustion conditions

Abstract

The heterogeneous interaction of NO₂ on decane and hexane soot generated in the laboratory was examined in a Knudsen reactor. Two different types of soot were produced using both fuels: Soot originating from a rich flame at a high fuel/oxygen ratio (‘grey’ soot) and soot generated from a lean flame at a low fuel/oxygen ratio (‘ black’ soot). The fuel/oxygen ratio is a key parameter influencing the conversion yields of NO₂ into nitrous acid (HONO) and NO. For both types of soot a reaction mechanism was developed in which NO₂ is first converted into HONO by a redox mechanism. For soot originating from a rich flame HONO is released at yields of up to 100% whereas decomposition of HONO on the surface of soot generated in a lean flame leads to significantly reduced HONO yields. HONO decomposes to NO which is instantaneously released and NO₂ formed on a longer timescale. In addition, there is an adsorption pathway for both types of soot which leads to irreversible removal of NO₂. Soot from a rich flame showed a HONO production of 4 × 10¹⁶ molecule mg⁻¹ for both hexane and decane soot for a NO₂ concentration of 7.6 × 10¹² molecule cm⁻³ integrated over 20 min. In contrast, hexane and decane soot generated in a lean flame do not form significant amounts of HONO. Instead, 2.7 × 10¹⁷ molecule mg⁻¹ of NO at a NO₂ concentration of 1.5 × 10¹⁴ molecule cm⁻³ are observed. Initial uptake coefficients (γ₀) of up to 0.1 were measured for both types of soot. However, with increasing amounts of NO₂ taken up, γ quickly decreases owing to saturation. After an uptake of 8 × 10¹³ molecule cm⁻² of NO₂, γ drops to 3 × 10⁻⁷ (‘grey’ decane soot) and 6 × 10⁻⁷ (‘black’ decane soot), respectively. A Soxhlet extraction of decane soot from a rich flame using tetrahydrofuran as a solvent also produced HONO on interaction with NO₂ after evaporation of the solvent. This observation suggests that the compounds that are responsible for HONO production are part of the transferable organic fraction that is condensed on the elemental carbon backbone structure of soot.