Heterogeneous reactions of particulate methoxyphenols with NO₃ radicals: kinetics, products, and mechanisms.

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.