Unraveling Pathways of Guaiacol Nitration in Atmospheric Waters: Nitrite, A Source of Reactive Nitronium Ion in the Atmosphere

The tropospheric aqueous-phase aging of guaiacol (2-methoxyphenol, GUA), a lignocellulosic biomass burning pollutant, is addressed in this work. Pathways of GUA nitration in aqueous solution under atmospherically relevant conditions are proposed and critically discussed. The influence of NaNO2 and H2O2, hydroxyl radical scavenger, and sunlight was assessed by an experimental-modeling approach. In the presence of the urban pollutant, nitrite, GUA is preferentially nitrated to yield 4- and 6-nitroguaiacol. After a short lag-time, 4,6-dinitroguaiacol is also formed. Its production accelerates after guaiacol is completely consumed, which is nicely described by the model function accounting for NO2 center dot and NO2+ as nitrating agents. Although the estimated second-order kinetic rate constants of methoxyphenol nitration with NO2 center dot are substantially higher than the corresponding rate constants of nitration with NO2+, nitration rates are competitive under nighttime and liquid atmospheric aerosol-like conditions. In contrast to concentrations of radicals, which are governed by the interplay between diffusion-controlled reactions and are therefore mostly constant, concentrations of electrophiles are very much dependent on the ratio of NO2- to activated aromatics in solution. These results contribute substantially to the understanding of methoxyphenol aging in the atmospheric waters and underscore the importance of including electrophilic aromatic substitution reactions in atmospheric models.