Sources, Sinks, and Oxidation Pathways of Phenolic Compounds in South Korea Constrained Using KORUS-AQ Airborne Observations

Aromatics are an important class of volatile organic compounds with impacts on human health. The impacts of aromatics and their oxidation products vary. While the chemistry and major pathways of the precursor aromatics are relatively well understood, the same is not true for their phenolic oxidation products. Here, we use new observations of aromatic oxidation products collected during the Korea-United States Air Quality aircraft campaign to evaluate the aromatic chemical mechanism in the GEOS-Chem v13.4.0 chemical transport model. Based on these results, we implement changes to emissions, add ethylbenzene chemistry, and introduce phenol production from ethylbenzene and toluene oxidation. These changes improve simulation of benzene (reducing normalized mean bias from 24% to −9%) and phenol (−71% to −42%). Model biases increase for toluene, xylene, and cresol, but simulated mixing ratios remain within measurement uncertainties and observed interquartile ranges. We identify potential toluene emission overestimates from petrochemical complexes in Ulsan and Daesan and underestimates from the Daegu dyeing industrial complex, and underestimates of benzene emissions from China. Using the updated model, we find benzene and toluene contribute equally to phenol production in the boundary layer (accounting for 40% of phenol production each), and that toluene and ethylbenzene are atmospherically relevant precursors of phenol. Phenol and cresol loss is found to be dominated by OH oxidation (73% for both phenol and cresol). We find that benzaldehyde is the dominant source of nitrophenol production (67%), although phenol dominates nitrophenol production at night.