Methoxyphenols, tracers for wood smoke, are emitted into the atmosphere in large quantities, but their chemical degradation in the atmosphere has not been well characterized. In this study, heterogeneous kinetics of particulate syringaldehyde (SA), vanillic acid (VA), and coniferyl aldehyde (CA) with NO3 radicals is investigated with a mixed-phase relative rate method. A vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer and an atmospheric gas analysis mass spectrometer are used to monitor online the decays of particulate methoxyphenols and gas-phase isoprene synchronously. The reactive uptake coefficients of NO3 radicals on SA, VA, and CA particles are calculated to be 0.33, 0.31, and 0.28, respectively, according to the measured methoxyphenol loss ratios and the average NO3 concentrations. The effective rate constants for heterogeneous reactions of particulate SA, VA, and CA with NO3 radicals measured under experimental conditions are 5.7 × 10–12, 5.2 × 10–12, and 3.5 × 10–12 cm3 molecule–1 s–1, respectively. In addition, oxalic acid, 2,6-dimethoxybenzoquinone, 5-nitro-VA, 4,6-dinitrogaiacol, protocatechuic acid, vanillin, 5-nitrovanillin, VA, and 5-nitro-CA are identified as the reaction products by gas chromatography–mass spectrometry analysis. On the basis of the identified products, the reaction mechanisms of methoxyphenols with NO3 radicals are proposed. The main transformation pathway of methoxyphenols is the NO3 electrophilic addition, followed by H-abstraction and nitro-substituted processes. The experimental results might shed light on the chemical behaviors of methoxyphenols at night.
h i g h l i g h t s• Reaction products of NO 3 radical with three methoxyphenols are obtained.• Nitro-substituted derivatives are observed as major transformation products.• Reaction mechanisms of methoxyphenols with NO 3 radicals are proposed.• The rate constants of three methoxyphenols with NO 3 radicals are measured. a b s t r a c t Methoxyphenols, lignin pyrolysis products, are major biomass combustion components and are considered potential tracers for wood smoke emissions. Their atmospheric reactivity, however, has not been well characterized. Guaiacol, creosol, and syringol are three typical methoxyphenols generated in relatively high concentrations in fresh wood smoke. In this study, the gas-phase reactions of NO 3 radicals with these methoxyphenols were investigated using a laboratory-built vacuum ultraviolet photoionization gas time-of-flight mass spectrometer (VUV-GTOFMS) and off-line GC-MS. By combining experimental and theoretical methods, 4-nitroguaiacol, 6-nitroguaiacol, and 4,6-dinitroguaiacol were determined as the primary degradation products for guaiacol; similarly, 6-nitrocreosol and 3-nitrosyringol were identified for creosol and syringol, respectively. Using the relative rate method, rate constants at 298 K and 1 atm for the gas-phase reactions of guaiacol, creosol, and syringol with NO 3 radicals were measured to be 3.2 × 10 −12 , 2.4 × 10 −13 , and 4.0 × 10 −13 cm 3 molecule −1 s −1 , respectively. At a typical tropospheric concentration of NO 3 radicals (5 × 10 8 molecule cm −3 ), atmospheric lifetimes for guaiacol, creosol, and syringol toward NO 3 radicals were 0.2, 2.3, and 1.4 h, respectively. These results indicate that the reaction with NO 3 radicals can be a major sink for methoxyphenols at night. a r t i c l e i n f o
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