Role of Organic Carbon in Heterogeneous Reaction of NO₂ with Soot

Abstract

A large uncertainty among the reported uptake coefficients of NO₂ on soot highlights the importance of the composition of soot in this reaction. Soot samples with different fractions of organic carbon (OC) were prepared by combusting <i>n</i>-hexane under controlled conditions. The heterogeneous reaction of NO₂ on soot was investigated using a flow tube reactor at ambient pressure. The soot with the highest fuel/oxygen ratio showed the largest uptake coefficient (γ_initial) of NO₂ and yield of HONO (<i>y</i>_HONO). Compared to fresh soot samples, preheated samples exhibited a great decrease in uptake coefficient of NO₂ and HONO yield due to the removal of OC from soot. Ozonized soot also showed a lower reactivity toward NO₂ than fresh soot, which can be ascribed to the consumption of OC with a reduced state (OC_R). A linear dependence of the NO₂ uptake coefficient and yields of HONO and NO on the OC_R content of the soot was established, with γ_initial(NO₂) = (1.54 ± 1.39) × 10^–6 + (1.96 ± 0.35) × 10^–7 × OC_R, <i>y</i>_HONO = (11.6 ± 16.1) + (1.3 ± 0.40) × OC_R, and <i>y</i>_NO = (13.1 ± 1.9) – (0.2 ± 0.05) × OC_R, respectively