Kinetic Study of the Gas-Phase Reactions of Nitrate Radicals with Methoxyphenol Compounds: Experimental and Theoretical Approaches

Abstract

The gas-phase reactions of five methoxyphenols (three disubstituted and two trisubstituted) with nitrate radicals were studied in an 8000 L atmospheric simulation chamber at atmospheric pressure and 294 ± 2 K. The NO3 rate constants were investigated with the relative kinetic method using PTR-ToF-MS and GC-FID to measure the concentrations of the organic compounds. The rate constants (in units of cm(3) molecule(-1) s(-1)) determined were: 2-methoxyphenol (guaiacol; 2-MP), k(2-MP) = (2.69 ± 0.57 × 10(-11); 3-methoxyphenol (3-MP), k(3-MP) = (1.15 ± 0.21) × 10(-11); 4-methoxyphenol (4-MP), k(4-MP) = (13.75 ± 7.97) × 10(-11); 2-methoxy-4-methylphenol, k(2-M-4-MeP) = (8.41 ± 5.58) × 10(-11) and 2,6-dimethoxyphenol (syringol; 2,6-DMP), k(2,6-DMP) = (15.84 ± 8.10) × 10(-11). The NO3 rate constants of the studied methoxyphenols are compared with those of other substituted aromatics, and the differences in the reactivity are construed regarding the substituents (type, number and position) on the aromatic ring. This study was also supplemented by a theoretical approach of the methoxyphenol reactions with nitrate radicals. The upper limits of the NO3 overall rate constants calculated were in the same order of magnitude than those experimentally determined. Theoretical calculations of the minimum energies of the adducts formed from the reaction of NO3 radicals with the methoxyphenols were also performed using a DFT approach (M06-2X/6-31G(d,p)). The results indicate that the NO3 addition reactions on the aromatic ring of the methoxyphenols are exothermic, with energy values ranging between -13 and -21 kcal mol(-1), depending on the environment of the carbon on which the oxygen atom of NO3 is attached. These energy values allowed identifying the most suitable carbon sites for the NO3 addition on the aromatic ring of the methoxyphenols: at the exception of the 3-MP, the NO3 ipso-addition to the hydroxyl group is one of the favored sites for all the studies compounds.

Rate constant and secondary organic aerosol formation from the gas-phase reaction of eugenol with hydroxyl radicals

“…The reactivity of eugenol toward OH radicals is slightly higher than those of guaiacol and 2,6dimethylphenol, but slightly slower than that of syringol. The presence of two methoxyl groups (-O-CH 3 ) in syringol activates the electrophilic addition of OH radicals to the benzene ring by donating electron density through the resonance effect (Lauraguais et al, 2016). The activation effect of the methoxyl group is much larger than that of alkyl groups (Mc-Murry, 2004).…”

“…Methoxyphenols are semivolatile aromatic compounds with low molecular weight, and many of them are found to mainly exist in the gas phase at typical ambient temperature (Schauer et al, 2001;Simpson et al, 2005). Thus, methoxyphenols can be chemically transformed through gas-phase reactions with atmospheric oxidants (Coeur-Tourneur et al, 2010a;Lauraguais et al, 2012Lauraguais et al, , 2014aLauraguais et al, , b, 2015Lauraguais et al, , 2016Yang et al, 2016;Zhang et al, 2016;El Zein et al, 2015). The corresponding rate constants control their effectiveness as stable tracers for wood combustion and atmospheric lifetimes.…”

“…The corresponding rate constants control their effectiveness as stable tracers for wood combustion and atmospheric lifetimes. In recent years, the rate constants for the gas-phase reactions of some methoxyphenols with hydroxyl (OH) radicals (Coeur-Tourneur et al, 2010a;Lauraguais et al, 2012Lauraguais et al, , 2014bLauraguais et al, , 2015, nitrate (NO 3 ) radicals (Lauraguais et al, 2016;Yang et al, 2016;, chlorine atoms (Cl) (Lauraguais et al, 2014a), and ozone (O 3 ) (El Zein et al, 2015) have been determined. Some studies have indicated significant SOA formation from 2,6-dimethoxyphenol (syringol) and 2-methoxyphenol (guaiacol) with respect to their reactions with OH radicals (Sun et al, 2010;Lauraguais et al, 2012Lauraguais et al, , 2014bAhmad et al, 2017;Yee et al, 2013;Ofner et al, 2011).…”

“…Some studies have indicated significant SOA formation from 2,6-dimethoxyphenol (syringol) and 2-methoxyphenol (guaiacol) with respect to their reactions with OH radicals (Sun et al, 2010;Lauraguais et al, 2012Lauraguais et al, , 2014bAhmad et al, 2017;Yee et al, 2013;Ofner et al, 2011). Although biomass burning emissions have been indicated to have great SOA formation potential via atmospheric oxidation (Bruns et al, 2016;Gilardoni et al, 2016;Li et al, 2017;Ciarelli et al, 2017;Ding et al, 2017), SOA formation and growth from methoxyphenols are still poorly understood. Besides, the observed SOA levels in the atmosphere cannot be explained well by the present knowledge of SOA formation, which reflects the fact that a large number of precursors are not taken into account in the SOA formation reactions included in atmospheric models (Lauraguais et al, 2012).…”

Enhancement of Secondary Organic Aerosol Formation and its Oxidation State by SO<sub>2</sub> during Photooxidation of 2-Methoxyphenol

“…Its emission factor of wood burning is in the range of 172−279 mg kg -1 fuel (Schauer et al, 2001). In recent years, the reactivity of gas-phase guaiacol toward OH radicals (Coeur-Tourneur et al, 2010a), NO3 radicals (Lauraguais et al, 2016;Yang et al, 2016), chlorine atom (Lauraguais et al, 2014a), and O3 (El Zein et al, 2015) has been investigated, suggesting that its degradation by OH radicals and NO3 radicals might be predominant Atmos. Chem.…”