Mineral Oxides Change the Atmospheric Reactivity of Soot: NO₂ Uptake under Dark and UV Irradiation Conditions

The heterogeneous reactions between trace gases and aerosol surfaces have been widely studied over the past decades, revealing the crucial role of these reactions in atmospheric chemistry. However, existing knowledge on the reactivity of mixed aerosols is limited, even though they have been observed in field measurements. In the current study, the heterogeneous interaction of NO₂ with solid surfaces of Al₂O₃ covered with kerosene soot was investigated under dark conditions and in the presence of UV light. Experiments were performed at 293 K using a low-pressure flow-tube reactor coupled with a quadrupole mass spectrometer. The steady-state uptake coefficient, γ_ss, and the distribution of the gas-phase products were determined as functions of the Al₂O₃ mass; soot mass; NO₂ concentration, varied in the range of (0.2–10) × 10¹² molecules cm^–3; photon flux; and relative humidity, ranging from 0.0032% to 32%. On Al₂O₃/soot surfaces, the reaction rate was substantially increased, and the formation of HONO was favored compared with that on individual pure soot and pure Al₂O₃ surfaces. Uptake of NO₂ was enhanced in the presence of H₂O under both dark and UV irradiation conditions, and the following empirical expressions were obtained: γ_ss,BET,dark = (7.3 ± 0.9) × 10^–7 + (3.2 ± 0.5) × 10^–8 × RH and γ_ss,BET,UV = (1.4 ± 0.2) × 10^–6 + (4.0 ± 0.9) × 10^–8 × RH. Specific experiments, with solid sample preheating and doping with polycyclic aromatic hydrocarbons (PAHs), showed that UV-absorbing organic compounds significantly affect the chemical reactivity of the mixed mineral/soot surfaces. A mechanistic scheme is proposed, in which Al₂O₃ can either collect electrons, initiating a sequence of redox reactions, or prevent the charge-recombination process, extending the lifetime of the excited state and enhancing the reactivity of the organics. Finally, the atmospheric implications of the observed results are briefly discussed