Reactivity of NO2 and H2O on soot generated in the laboratory: a diffusion tube study at ambient temperature

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Abstract
In this work we have obtained the kinetic parameters and the product spectrum of NO2 and H2O separately interacting with soot samples in terms of surface residence times τs of the interacting molecules and uptake rate constant expressed as uptake coefficients (γ). The NO2 interaction on soot from either a decane or a toluene flame results in HONO as the main product in contrast to acetylene soot. NO was observed to be a minor product for all soot substrates investigated. Residence times up to 70 ms for NO2 interacting with acetylene soot and up to 50 ms with decane and toluene soot were obtained as well as initial uptake coefficients γ0 MC ranging from 0.26 for NO2 on acetylene soot to an average value of 0.1 for NO2 on decane and toluene soot. All experiments were subject to significant saturation at typical doses of 1015 molecules. A Monte-Carlo (MC) trajectory model was applied to extract τs and γ0 MC values, as well as the average number of adsorption sites ns. The interaction of NO2 with toluene and decane soot was modeled using a simple MC-model using a single adsorption site, whereas acetylene soot required a two-site model. For the interaction of H2O on soot, a residence time τs ranging from 0.1 ms on decane soot to 5 ms for acetylene soot has been determined using the single-reaction-site MC-model with τs values between 2 and 3 ms for toluene and diesel soot, respectively. For H2O an upper limit for the uptake coefficient γ0 MC of 2 × 10−3 has been measured for all soot samples.

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