Kinetics of NO2 uptake on methane flame soot

Abstract

The dependences of the initial NO₂ uptake coefficient γ on a methane soot coating at 255 K on the NO₂ concentration (1.3 × 10¹²–3.3 × 10¹³ cm^–3) and time t were studied using a flow reactor with a mobile insert and mass spectrometric recording of gaseous reagents and products: 1/γ(t) = 1/γ₀ + at, where γ₀ and a are the parameters that depend on the NO₂ concentration; γ₀ = γ₀ⁱⁿⁱ/(1 + k_L [NO₂], a= k [NO₂], with the constants γ₀ⁱⁿⁱ = (4.8 ± 2) × 10^–4, K_L = (8.2 ± 3) × 10^–13 cm³, and k = (2.3 ± 0.1) × 10^–10 cm³ s^–1. The elementary parameters that determine the uptake were evaluated on the basis of the Langmuir adsorption model: desorption rate constant k_d = (52 ± 20) s^–1, adsorption heat Q_ad = (38 ± 8) kJ mol^–1, rate constant of the unimolecular heterogeneous conversion of NO₂ into the product k_r = (2.5 ± 1.3) × 10^–2 s^–1, and its activation energy E_a = (19 ± 10) kJ mol^–1. The laboratory data were extrapolated, based on the uptake coefficient, to the limiting tropospheric NO₂ concentrations in remote marine (1 ppb) and polluted industrial regions (40 ppb).