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

Abstract

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) × 10(12) 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.

Nitrous acid (HONO) emissions under real-world driving conditions from vehicles in a UK road tunnel

“…A number of laboratory studies have supported this reaction, with HONO yields varying between 20-100 % depending on the fuel type used to generate the soot, initial NO2 concentrations and soot coatings (Arens et al, 2001;Aubin and Abbatt, 2007;Gerecke et al, 1998;Guan et al, 2017;Khalizov et al, 2010;Kleffmann et al, 1999;Lelièvre et al, 2004;Romanias et al, 2013;Stadler and Rossi, 2000). An observation that is consistent across many laboratory experiments is that the uptake of NO2…”

Nitrous acid (HONO) emissions under real-world driving conditions from vehicles in a UK road tunnel

“…It is likely that wet surfaces inside the tunnel, as a result of spray from tyres, resulted in a loss of HONO. The Henry's law constant (in water) of HONO (k H = 4.8 × 10 −1 mol m −3 Pa −1 ) is approximately 4800 times greater than that of NO 2 (k H = 9.9 × 10 −5 mol m −3 Pa −1 ) (Sander, 2015), and therefore HONO is likely to be washed out more rapidly than NO 2 on wet surfaces, resulting in a reduction in the HONO/NO x ratios. As a result, this precipitation event was excluded from the final dataset for calculation of emission ratios.…”

Photodegradation of Pyrene on Al2O3Surfaces: A Detailed Kinetic and Product Study

“…Note that similar absorption spectra were recorded in the literature. 8,22 Al 2 O 3 is not a semiconductor material. Thus, it cannot directly initiate a sequence of redox reactions, such as TiO 2 and Fe 2 O 3 ; however, it may assist and enhance a photochemical process.…”

“…6,7 In particular, Kim et al have observed that mineral dust originated from North China after long-range transportation over polluted areas, was enriched with substantial concentrations of elemental and organic carbon. 7 It has been proposed that this "mixing process" can affect the optical properties 7,8 and hygroscopicity of the aerosol particles. 9 Hence, they could affect the radiation balance of the planet.…”