Gas-Phase Oxidation of Guaiacol by NO₃ Radicals: Kinetic Measurements and Implications

Methoxyphenols, critical tracers of biomass burning emissions, are emitted in large quantities during lignin pyrolysis. Gas-phase reaction of methoxyphenols with nitrate radicals (NO3) yields to important secondary products like nitrophenols, yet their atmospheric kinetics are far from adequate and consistent. This study investigates the gas-phase reaction kinetics of guaiacol, a key representative methoxyphenol, with NO3 by using a controlled 30 m3 indoor chamber. Both relative rate and absolute rate methods were employed, yielding rate constants of (3.1 ± 0.2) × 10–11 and (2.43 ± 0.19) × 10–11 cm3 molecule–1 s–1, respectively, at 298 ± 2 K under dry conditions. The rate constant decreased with an increase in humidity, highlighting the inhibitory effect of water vapor on NO3-initiated reactions. The calculated nocturnal atmospheric lifetime of guaiacol due to NO3 reactions is less than 2 min at typical nocturnal NO3 levels, underscoring the significance of this pathway in the budget of methoxyphenols during nighttime or within a 24 h cycle. To the best of our knowledge, this study represents the first determination of the NO3-guaiacol rate constant using the absolute rate method and provides insights into the role of relative humidity in methoxyphenol oxidation. These findings offer critical data for understanding the atmospheric fate of biomass burning emissions and their contributions to nighttime chemistry and air quality.