Role of Organic Carbon in Heterogeneous Reaction of NO₂ with Soot

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 n-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 (yHONO). 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 (OCR). A linear dependence of the NO2 uptake coefficient and yields of HONO and NO on the OCR content of the soot was established, with γ(initial)(NO2) = (1.54 ± 1.39) × 10(-6) + (1.96 ± 0.35) × 10(-7) × OCR, yHONO = (11.6 ± 16.1) + (1.3 ± 0.40) × OCR, and yNO = (13.1 ± 1.9) - (0.2 ± 0.05) × OCR, respectively.