Heterogeneous kinetics of methoxyphenols in the OH-initiated reactions under different experimental conditions

Methoxyphenols as the potential tracers for wood smoke emissions, are emitted into the atmosphere in large quantities but their atmospheric chemical behaviors have not been well characterized. In this work, heterogeneous kinetics of methoxyphenols in the OH-initiated reactions was investigated using a flow reactor under different experimental conditions. The average second-order rate constants (k₂) of vanillic acid (VA), coniferyl aldehyde (CA), and syringaldehyde (SA) were (4.72 ± 0.51) × 10^-12, (10.59 ± 0.50) × 10^-12, (12.25 ± 0.60) × 10^-12 cm³ molecule^-1 s^-1, respectively, obtained at relative humidity (RH) and temperature of 40% and 25 °C. In addition, the results showed that high temperature played a positive role in promoting these reactions while high RH had an inhibiting impact. The k₂ values of VA, CA, and SA at 40% RH and different temperature followed the Arrhenius expressions, i.e., k₂ = (2.45 ± 0.40) × 10^-10exp [-(1170.73 ± 47.35)/T], k₂ = (6.40 ± 0.26) × 10^-10exp [-(1516.16 ± 13.71)/T], and k₂ = (1.02 ± 0.13) × 10^-9exp [-(1310.79 ± 36.75)/T], respectively. Based on the determined rate constants, the atmospheric lifetimes of these three methoxyphenols ranged from 0.54 to 2.18 d under different conditions. The experimental results indicate that OH radicals might play an important role in controlling the atmospheric lifetimes of methoxyphenols, and also help to further cognize the chemical behaviors of methoxyphenols in the atmosphere.