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

Identification and Quantification of Aromatic Hydrocarbons Adsorbed on Soot from Premixed Flames of Kerosene, Synthetic Kerosene, and Kerosene–Synthetic Biofuels

In the current study, the chemical characterization of polycyclic aromatic hydrocarbons (PAHs) adsorbed on soot from premixed flames of synthetic paraffinic kerosene (SPK), conventional kerosene (Jet A-1), and Jet A-1/synthetic biofuel blends was carried out. Jet A-1 and SPK liquid fuels were analyzed with NMR to provide supporting information on their chemical composition. The analytical procedure used to characterize PAHs fraction in soot samples includes the following: (i) filtration of the soot samples diluted into <i>n</i>-hexane through PTFE filters, (ii) automated solid-phase extraction (A-SPE) for fractioning and cleaning-up the soot extracts, and (iii) chromatographic analysis of every fraction by reverse high-performance liquid chromatography (RPLC) with photodiode array (PDA) detection. Application of the aforementioned methodology allowed the identification of 78 compounds including indene, toluene, and 76 PAHs. Moreover, the relative abundance of five-membered-ring PAHs and alkyl PAHs was evaluated, and 19 PAHs (16 EPA PAHs, 1-methylnaphthalene, 2-methylnaphthalene, and coronene) were quantified. The PAH characterization should contribute to improve our understanding of atmospheric reactivity of soot and other environmental aspects of aromatic compounds adsorbed on soot

Photodegradation of Pyrene on Al₂O₃ Surfaces: A Detailed Kinetic and Product Study

In the current study, the photochemistry of pyrene on solid Al₂O₃ surface was studied under simulated atmospheric conditions (pressure, 1 atm; temperature, 293 K; photon flux, <i>J</i>_NO₂ = 0.002–0.012 s^–1). Experiments were performed using synthetic air or N₂ as bath gas to evaluate the impact of O₂ to the reaction system. The rate of pyrene photodegradation followed first order kinetics and was enhanced in the presence of O₂, <i>k</i>_d(synthetic air) = 7.8 ± 0.78 × 10^–2 h^–1 and <i>k</i>_d(N₂) = 1.2 ± 0.12 × 10^–2 h^–1 respectively, due to the formation of the highly reactive O₂^•– and HO^• radical species. In addition, <i>k</i>_d was found to increase linearly with photon flux. A detailed product study was realized and for the first time the gas/solid phase products of pyrene oxidation were identified using off-line GC-MS and HPLC analysis. In the gas phase, acetone, benzene, and various benzene-ring compounds were determined. In the solid phase, more than 20 photoproducts were identified and their kinetics was followed. Simulation of the concentration profiles of 1- and 2-hydroxypyrene provided an estimation of their yields, 33% and 5.8%, respectively, with respect to consumed pyrene, and their degradation rates were extracted. Finally, the mechanism of heterogeneous photodegradation of pyrene is discussed

Investigation of the Photochemical Reactivity of Soot Particles Derived from Biofuels Toward NO₂. A Kinetic and Product Study

In the current study, the heterogeneous reaction of NO₂ with soot and biosoot surfaces was investigated in the dark and under illumination relevant to atmospheric conditions (<i>J</i>_NO₂ = 0.012 s^–1). A flat-flame burner was used for preparation and collection of soot samples from premixed flames of liquid fuels. The biofuels were prepared by mixing 20% v/v of (i) 1-butanol (CH₃(CH₂)₃OH), (ii) methyl octanoate (CH₃(CH₂)₆COOCH₃), (iii) anhydrous diethyl carbonate (C₂H₅O)₂CO and (iv) 2,5 dimethyl furan (CH₃)₂C₄H₂O additive compounds in conventional kerosene fuel (JetA-1). Experiments were performed at 293 K using a low-pressure flow tube reactor (<i>P</i> = 9 Torr) coupled to a quadrupole mass spectrometer. The initial and steady-state uptake coefficients, γ₀ and γ_ss, respectively, as well as the surface coverage, <i>N</i>_s, were measured under dry and humid conditions. Furthermore, the branching ratios of the gas-phase products NO (∼80–100%) and HONO (<20%) were determined. Soot from JetA-1/2,5-dimethyl furan was the most reactive [γ₀ = (29.1 ± 5.8) × 10⁻⁶, γ_ss(dry) = (9.09 ± 1.82) × 10⁻⁷ and γ_ss(5.5%RH) = (14.0 ± 2.8)⁻⁷] while soot from JetA-1/1-butanol [γ₀ = (2.72 ± 0.544) × 10^–6, γ_ss(dry) = (4.57 ± 0.914) × 10^–7, and γ_ss(5.5%RH) = (3.64 ± 0.728) × 10^–7] and JetA-1/diethyl carbonate [γ₀ = (2.99 ± 0.598) × 10^–6, γ_ss(dry) = (3.99 ± 0.798) × 10^–7, and γ_ss(5.5%RH) = (4.80 ± 0.960) × 10^–7] were less reactive. To correlate the chemical reactivity with the physicochemical properties of the soot samples, their chemical composition was analyzed employing Raman spectroscopy, NMR, and high-performance liquid chromatography. In addition, the Brunauer–Emmett–Teller adsorption isotherms and the particle size distributions were determined employing a Quantachrome Nova 2200e gas sorption analyzer. The analysis of the results showed that factors such as (i) soot mass collection rate, (ii) porosity of the particles formed, (iii) aromatic fraction, and (iv) pre-existence of nitro-containing species in soot samples (formed during the combustion process) can be used as indicators of soot reactivity with NO₂