Mechanism Development and Evaluation of the Gas-Phase Photooxidation of Guaiacol: Insights from a Chamber Study

Guaiacol (2-methoxyphenol) is an oxygenated aromatic volatile organic compound (VOC) emitted from biomass burning during lignin pyrolysis with significant potential to impact air quality through tropospheric ozone (O3) production and secondary organic aerosol (SOA) formation. In this work, a near-explicit mechanism for the photooxidation of guaiacol was developed for the inclusion into the Master Chemical Mechanism (MCM) framework, to improve the mechanistic understanding of the chemistry occurring in biomass burning plumes. The mechanism is evaluated using an outdoor smog chamber experiment conducted at the EUropean PHOtoREactor in Valencia, Spain. Model-measurement comparisons replicate chamber experiment profiles of first- and second-generation products reasonably well. Simulations also indicated that the hydroxyl radical (OH) production was sensitive to the ratio of ring-retained and ring-opened products from first- and second-generation products, with increased formation from the ring opening pathways. However, despite optimization of the mechanism, several deficiencies remained in the model, which require further constraining: (1) overestimation of the O3 concentration, (2) underestimation of guaiacol loss from reaction with OH, and (3) overestimation of the rate of conversion from nitric oxide (NO) to nitrogen dioxide (NO2). An investigation into the composition of particle-phase guaiacol products was also evaluated using the CHemistry with Aerosol Microphysics in Python (PyCHAM) chamber box model, which predicted a significant contribution of nitroaromatics to early-stage guaiacol SOA formation with potentially important implications for air quality and climate from biomass burning.