Research paperTheoretical study on the atmospheric oxidation reaction of 2-furanaldehyde initiated by NO3 radicals
Graphical abstract
Introduction
Furanaldehydes have attracted environmental attention due to their potential to promote secondary organic aerosol formation [1], [2], which may influence the radiation balance of the planet. In addition, furanaldehydes exacerbate the health risks brought by photochemical smog. This provided that furanaldehydes are toxic as O containing aromatic compounds [3], [4], [5] and the source of reactive OHx and nocuous peroxyacyl nitrates (PANs) [6], [7]. Furanaldehydes are present in the atmosphere as primary pollutants from both anthropogenic sources including fossil fuel [8], [9] and biomass burning [10], in company with biogenic sources from tropical forests [11] and wood smoke [12]. Furthermore, the degradation processes of alkyl furans with Cl atom [13], [14] contribute to the formation of furanaldehydes in the atmosphere.
Many experimental studies have been carried out to investigate the fate of furanaldehydes in the troposphere. Gas-phase reactions of furanaldehydes with OH and photolysis in day-time as well as reactions with NO3 in night-time are expected to be important removal pathways for furanaldehydes [15], [16]. The reactions with Cl in coastal and marine areas are also a possible removal route during the day-time [17]. Despite the removal rates for furanaldehydes were previously determined by several experimental studies, the ring-opening products of the reactions initiated by NO3 remain unknown [15]. This impairs the understanding for the fate of furanaldehydes in the atmosphere. Owing to the limitation of experimental methods, certain reaction intermediates and transition states with short lifetimes are difficult to detect, and distinguishing complex products lacking of standard spectrum are nontrivial. Therefore, quantum chemical calculation can be considered as complementary to the experimental studies in the aspect of delineating the reaction mechanism. Wang et al. carried out several computational studies to elucidate the atmospheric reaction mechanism for furan derivatives including furfural [18] and methyl-substituted furans [19] initiated by OH. The reaction rate constants were also estimated under different conditions.
In this study, we have proposed the degradation pathways of 2-furanaldehydes initiated by NO3 by meanings of quantum chemical calculation. The rate constants of concerning elementary reactions were calculated by implementing Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The probable products were analyzed to reach a more plausible predication for the fate of 2-furanaldehyde in the atmosphere.
Section snippets
Computational methods
Utilizing the Gaussian 09 software [20], all the quantum chemical calculations were carried out to elucidate the mechanism of the gas-phase removal process for 2-furanaldehyde initiated by NO3. As per our previous successful appliance [21], we opted the M06-2X functional [22] to optimize the geometrical structures involved in the reactions with the 6–31 + G(d,p) basis set. To verify whether the structures obtained are local minima or first-order saddle points, the harmonic vibrational
NO3 addition reactions of 2-furanaldehyde
Cis- and trans-isomers of 2-furanaldehyde were observed in previous experimental studies [27], [28]. Herein, we compared the reaction energies and potential barriers for NO3 addition to cis-2-furanaldehyde (Fig. S1) with that to trans- furanaldehyde (Fig. 1). The differences of reaction energies between trans- and cis-2-furanaldehyde at the corresponding positions (C2 to C5) are less than 1 kcal mol−1. In comparison with NO3 addition to the furan ring, NO3 addition to the C atom of the
Environmental implications and conclusions
The 24 h average concentration of NO3 was 5.0 × 108 molecule cm−3 in the tropospheric at 298 K [33]. The calculated overall rate constant for trans-2-furanaldehyde with NO3 is 1.04 × 10−12 cm3 molecule−1 s−1 at 298 K and 1 atm. According to the overall rate constant calculated in this study, the atmospheric lifetime is determined as 0.53 h based on the equation below:
the reactions of furan derivatives with O3 are not suggested to be significantly faster [34].
Conflict of interest
Author declares that there is no conflict of interest.
Acknowledgments
The work was financially supported by NSFC (National Natural Science Foundation of China, project Nos. (21876102), Nos. (21477066), Taishan Scholars (No. ts201712003) and Shenzhen Science and Technology Research and Development Funds (No. JCYJ20160510165106371).
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