Observation-based modeling of ozone chemistry in the Seoul metropolitan area during the Korea-United States Air Quality Study (KORUS-AQ)

Abstract

The Seoul Metropolitan Area (SMA) has a population of 24 million and frequently experiences unhealthy levels of ozone (O₃). In this work, measurements taken during the Korea-United States Air Quality Study (KORUS-AQ, 2016) are used to explore regional gradients in O₃ and its chemical precursors, and an observationally-constrained 0-D photochemical box model is used to quantify key aspects of O₃ production including its sensitivity to precursor gases. Box model performance was evaluated by comparing modeled concentrations of select secondary species to airborne measurements. These comparisons indicate that the steady state assumption used in 0-D box models cannot describe select intermediate species, highlighting the importance of having a broad suite of trace gases as model constraints. When fully constrained, aggregated statistics of modeled O₃ production rates agreed with observed changes in O₃, indicating that the box model was able to represent the majority of O₃ chemistry. Comparison of airborne observations between urban Seoul and a downwind receptor site reveal a positive gradient in O₃ coinciding with a negative gradient in NO_x, no gradient in CH₂O, and a slight positive gradient in modeled rates of O₃ production. Together, these observations indicate a radical-limited (VOC-limited) O₃ production environment in the SMA. Zero-out simulations identified C₇₊ aromatics as the dominant VOC contributors to O₃ production, with isoprene and anthropogenic alkenes making smaller but appreciable contributions. Simulations of model sensitivity to decreases in NO_x produced results that were not spatially uniform, with large increases in O₃ production predicted for urban Seoul and decreases in O₃ production predicted for far-outlying areas. The policy implications of this work are clear: Effective O₃ mitigation strategies in the SMA must focus on reducing local emissions of C₇₊ aromatics, while reductions in NO_x emissions may increase O₃ in some areas but generally decrease the regional extent of O₃ exposure.